Tutorials - Page 157 Table of Contents

I Introduction II Properties and Concepts of and Color II.1 The wavelength range of optical radiation II.2. Velocity, amplitude, wavelength, and - the measures of a wave II.3. Spectra of various light sources II.4. Basic radiometric quantities II.4.a. Definition of II.4.b. Radiant power or radiant Φe II.4.c. Radiant Ie II.4.d. Le II.4.e. Ee II.4.f. Me II.4.g. Spectral radiant power Φλ(λ), spectral Iλ(λ), spectral radiance Lλ(λ), spectral irradiance Eλ(λ), and spectral radiant II.5. Calculation of radiometric quantities - Examples II.5.a. Example 1: Isotropic point source II.5.b. Example 2: Spot source II.5.c. Example 3: The Lambertian surface II.6. Spectral sensitivity of the human eye II.7. Basic photometric quantities II.7.a. Luminous flux Φv II.7.b. Iv II.7.c. Lv II.7.d. Illuminance Ev II.7.e. Luminous exitance Mv II.7.f. Conversion between radiometric and photometric quantities II.8. , Transmission, and Absorption II.8.a. ρ, τ, and α II.8.b. Radiance coefficient qe, Bidirectional reflectance distribution function (BRDF) II.9. The perception of color II.9.a. Physiological background II.9.b. Color addition II.9.c. Color subtraction II.10. II.10.a. RGB and XYZ color matching functions II.10.b. The (x,y)- and (u’,v’)-chromaticity diagrams II.10.c. Correlated color temperature III. Measurement of light with integral detectors III.1. The detector’s input optics and its directional sensitivity III.1.a. Integrating spheres used with integral detectors III.1.b. Measurement of radiant power and luminous flux III.1.c. Measurement of irradiance and illuminance III.1.d. Measurement of radiant and luminous intensity III.1.e. Measurement of radiance and luminance III.1.f. Measurement of reflection and transmission properties III.2. Spectral sensitivity of an integral detector III.2.a. Monochromatic III.2.b. Polychromatic radiometry III.2.c. III.2.d. Colorimetry III.3. The detector’s time behaviour III.4. The detector’s dynamic range III.5. Calibration of integral Detectors III.5.a. Traceability: an Unbroken Chain of Transfer Comparisons III.5.b. ISO/IEC/EN 17025 (formerly ISO Guide 25 and EN45001) III.5.c. Calibration Quantities III.5.d. Calibration Standards IV. Detector Signal Measurement V. Theory and applications of integrating spheres V.1. Theory of the ideal integrating sphere V.2. Real integrating spheres VI. Applications for Light Measurement in Medicine, Technology, Industry and Environmental Science VI.1. Phototherapy and Radiation Protection VI.2. Plant physiology VI.3. UV-Disinfection and Lamp Control VI.4. UV Curing and UV Processing VI.5. Colorimetry VI.6. Photostability VI.7. Telecommunication VI.8. Lasers & LEDs Measurements VI.9. Nondestructive testing VII. Appendix VII.2. Summary of radiometric and photometric quantities VII.3. Sources and references for figures VII.4. Most relevant CIE- DIN- and ISO-publications and regulations VII.4.a. DIN Publications VII.4.b. CIE Publications VII.5. National Calibration Laboratories Page 158 - Tutorials Introduction, Properties and Concepts of Light and Color

I. Introduction Light, or the visible part of the ments in image processing and optics. tions, detectors, electronics and electromagnetic radiation spec- pattern recognition. In fact a major Radiometry deals with the meas- calibration are included. A list of trum, is the medium through which part of the information flow from urement of all optical radiation reference sources is provided for human beings receive a major external stimuli to our brain is inclusive of the visible portion of future study. portion of environ- transferred visually. Photometry this . SI (Système International) units are mental information. Evolution has deals with the measurement of this used throughout these tutori- optimized the human eye into a visible light energy. This tutorial is an introduction als. Many international organiza- highly sophisticated sensor for However, optical radiant en- to the basic nature of light and tions including the CIE electromagnetic radiation. Joint ergy not only encompasses visible color, radiometric, photomet- (Commission Internationale de performance between the human 'light' but radiation invisible to the ric, colorimetric, reflection and l'Eclairage) have adopted this sys- eye and visual cortex, a large part human eye as well. The term opti- transmission principles, quantities, tem of units exclusively. The termi- of the human brain, dwarfs recent cal is used because this radiation symbols and units. Sections cover- nology used follows that of the CIE technical and scientific develop- follows the laws of geometrical ing a sampling of current applica- International Lighting Vocabulary.

II. Properties and Concepts of Light and Color Thorough knowledge of the physi- is limited to the range between 400 cal nature of light and light percep- nm and 800 nm (1 nm = tion provides the foundation for a 1 nanometer = 10-9 m). In the opti- comprehensive understanding of cal range of the electromagnetic optical measurement techniques. spectrum, wavelength is sometimes Yet, from a practical point of view also given in Ångstrøm (Å= 10-10 there is little necessity to fully m). Outside this range, our eye is understand formation and propaga- insensitive to electromagnetic tion of light as an electromagnetic radiation and thus we have no wave as long as the reader accepts perception of ultraviolet (UV, wavelength as the most important below 400 nm) and infrared (IR, parameter describing the quality of above 800 nm) radiation. light. The human eye perceives light with different wavelengths as Fig. II.1. Monochromatic electromagnetic radiation of different wave- different colors (figure II.1.), as lengths between 400 nm and 800 nm causes the impression of different long as the variation of wavelength colors. Outside this wavelength range, the human eye is insensitive.

II.1. The wavelength range of optical radiation types of skin can- of the UVA range between 315 nm cer. As the strength and 400 nm. of radiation effects Spectral sensitivity functions, such on matter does not as the CIE photopic response, have change abruptly also been defined in other biologi- with wavelength, cal effects of optical radiation, such different authors as DNA damage, formation of define UVA and erythema (sunburn) and of non- UVB ranges melanoma skin cancer, tanning of slightly different. the human skin and the photosyn- As an example, the thesis process in green plants, have US Food and Drug been studied and quantified by Fig. II.2. Wavelength ranges of electromagnetic radiation. Administration spectral sensitivity functions According to DIN 5031, the term motivated by the effects of the (FDA) and the US Environmental (Fig. ). Especially when related to „optical radiation“ refers to elec- electromagnetic wave on matter. Protection Agency (EPA) define certain biological reactions, the tromagnetic radiation in the wave- For instance, the UV-B range cov- the UV-A range between 320 nm term “action spectrum” is often length range between 100 nm and 1 ers the wavelengths in the solar and 400 nm. However, two of the used instead of “spectral sensitiv- mm. The terms „light“ and „visible spectrum which is particularly main standardization authorities, ity”. radiation“ (VIS) refer to the wave- responsible for DNA damage and the CIE and the DIN, length range between 400 nm and thus causes melanoma and other agree with their definition 800 nm, which can be perceived by the human eye. Optical radiation with wavelengths shorter than 400 nm is called ultraviolet (UV) radiation and is further subdivided in UV-A, UV-B and UV-C ranges. Similarly, infrared (IR) radiation covers the wavelength range above 800 nm and is subdivided in IR-A, IR-B and IR-C ranges (DIN 5031, part 7). It must be emphasized that this classification of electromagnetic radiation is a matter of convention and is not based on qualitative properties of the electromagnetic wave itself. Instead, it is largely Fig. II.3. Action spectra for various biological reactions to UV radiation Tutorials - Page 159 Properties and Concepts of Light and Color

II.2. Velocity, amplitude, wavelength, and frequency - the measures of a wave Like all other waves (waves in a • The amplitude is the maximum obvious that the period of a wave As the frequency of a wave does string, water waves, sound, earth- disturbance of the medium from completely defines its frequency not depend on the medium the quake waves ...), light and electro- its equilibrium. In the case of a and vice versa. The relation wave is passing, it is more conven- magnetic radiation in general can wave in a horizontal string (Fig. between these quantities is given ient to use frequency instead of be described as a vibration (more II.4.a), this value is identical with by wavelength to characterize the general: a periodical change of a half of the vertical distance be- wave. In acoustics, this is common certain ) that tween the wave’s crest and its ν = 1 / T practice – in most cases the pitch of propagates into space (Fig. II.4.a). trough. In the case of a pressure sound is characterized by its fre- The propagation is caused by the wave in air („sound“, Fig. II.4.b), If we look at a wave as a process quency instead of its wavelength in fact that the vibration at a certain the amplitude is half of the pres- that is periodical in space and in a certain medium (for example air). location influences the region next sure difference between rarefac- time, we can regard the wavelength In optics, the situation is different: to this location. For example in the tion and compression. λ as the distance between two In most cases wavelength is used case of sound, the alternating rare- • The wavelength λ is the distance repetitions of the process in space instead of frequency, although this faction and compression of air between two adjacent crests (or and the period T as the „distance“ leads to a certain complication: For molecules at a certain location troughs) and is given in meters. between two repetitions of the example, green light has a wave- results in periodic changes in the • The period T of a wave is the process in time. length of 520 nm in vacuum, but in local pressure, which in turn causes time that elapses between the A basic relation between wave- water its velocity is smaller by a the movement of adjacent air mole- arrival of two consecutive crests length, frequency and velocity factor of 1.33 and thus, in water the cules towards or away from this (or troughs) at a certain location results from the following consid- same green light has a wavelength location (Fig. II.4.b). X. This definition is eration: of only 520 / 1.33 = 391.0 nm. Hence, if we want to characterize a wave by its wavelength, we also have to state for which medium the actual wavelength value is given. According to CIE regulations, which are also applied throughout this tutorial, the term “wavelength” refers to “wavelength in air” unless otherwise stated. However, when applying the given wavelength figures to light passing through a medium other than vacuum, one should keep in mind that the light’s wavelength changes according to the relation

λMedium = λVacuum / nMedium = λAir ⋅ Fig. II.4 (a) Formation and propagation of a wave nAir / nMedium in a string. (b) Formation and propagation of a compression wave in air, a phenomenon with colloquially called sound nAir = cVacuum / cAir and nMedium = In the case of an electromagnetic During the time span a crest needs cVacuum / cMedium wave, the mechanism of propaga- identical with the statement that the period is the time the vibra- to travel the distance of one wave- tion involves mutual generation of length λ from location X to loca- tion at X takes to complete a full nMedium is called the medium’s periodically varying electric and tion Y (Fig. II.4.a), the next crest index of refraction and is more magnetic fields and is far more cycle from crest to trough to crest. The period of a wave is arrives at location X. Thus, this commonly used to specify the difficult to understand than sound. time span is identical with the optical properties of a material than Yet, the result still can be described given in seconds. • ν wave’s period T. But when a crest cMedium. as a periodic change of a physical The frequency of a wave is the number of vibration cycles per needs the time span T to travel the quantity (the strength of the electric distance λ, its velocity c amounts and the magnetic field) propagating second at a certain location X. The unit of frequency is to into space. The velocity of this When a wave passes from one propagation is generally abbrevi- (Hz) and 1 Hz is the reciprocal of λ λ ν ated with the letter c (unit: meters 1 second. As an example, a wave c = / T = . Equ. II.1 per second, m/s) and depends on with a period T = 0.25 s takes ¼ of a second to complete a full medium to another, its frequency the nature of the wave and on the remains the same. If the velocities medium (see Table II.Ibelow). vibration cycle (crest - trough - crest) at a certain location and of the wave in the two media differ, In order to describe the basic prop- the wavelengths in the two media erties of a wave, the following thus performs four vibrations per second. Hence its frequency is f also differ as a consequence of quantities have been defined for all equation II.1.. kinds of waves: = 4 Hz. From this example it is Sound Optical (electromagnetic) radiation at λҏ= 434 nm at λҏ= 589 nm at λҏ= 656 nm in vacuum - 299792 km/s (n = 1) in air 340 m/s 299708 km/s (n = 1.000280) 299709 km/s (n = 1.000277 299710 km/s (n = 1.000275) in water 1500 m/s 299708 km/s (n = 1.340) 224900 km/s (n = 1.333) 225238 km/s (n = 1.331) Table II.I Velocities of sound and light in air and in water. For optical radiation, the respective index of refraction is given in parenthesis Page 160 - Tutorials Properties and Concepts of Light and Color

II.3. Spectra of various light sources A spectrum generally describes the narrow bandwidths. variation of a certain physical Typical sources of quantity as a function of wave- near monochromatic length. The term “spectrum” with- radiation are light out any further specifications refers emitting diodes and to the quantification of the mono- band pass filtered chromatic intensity as a function of sources. wavelength (the term “spectrum” is If a mixture of radia- also used for other (physical) quan- tion covers a rela- tities apart of intensity, but is then tively large range of always used with a specific prefix. wavelengths without As an example, the strength of a gaps, this radiation biological reaction (for example has a continuous erythema – see § VI.1) to light with spectrum. Typical different wavelengths is described examples of continu- by an “action spectrum”). As an ous radiation spectra example, the next figure shows are direct and diffuse spectra of an incandescent bulb, sunlight and light natural sunlight and two types of emitted by incandes- discharge lamps. cent bulbs. On the When examining spectral intensity other hand, in a band distributions of various light spectrum there are sources, it is possible to distinguish gaps separating four significant types. These are: individual regions of • Monochromatic radiation radiation. If a spec- • Near monochromatic radia- trum has a number Fig. II.5. Emission spectra of natural light from the sun and the sky and of tion of lines of mono- artificial light from incandescent bulbs at different temperatures, from • Continuous spectra chromatic intensity, a mercury vapor lamp and from a multi-vapor lamp. • Band spectra it is called a line Typical sources of monochromatic spectrum. Typical sources emitting xenon lamps, and metal vapor are used to achieve a more uniform radiation are lasers and the output a line spectrum are gaseous dis- lamps, such as the mercury vapor spectral distribution (see Fig. II.5.). signal from monochromators with charge lamps, such as helium or lamp. Multi-vapor discharge lamps

II.4. Basic radiometric quantities The whole discipline of optical the units of all radiometric quanti- stood without a basic comprehen- defined by dλ, dA and/or dΩ.

measurement techniques can be ties are based on (W). Ac- sion of differential quantities. For Similarly, dΦe, dIe, dLe and dEe can roughly subdivided into the two cording to CIE regulations, sym- an intuitive understanding of these be regarded as small portions areas of photometry and radiome- bols for radiometric quantities are quantities, which is the main pur- which add up to the total value of try. Whereas the central problem denoted with the subscript “e” for pose of this paragraph, the differen- the respective quantity. In para- of photometry is the determination “energy”. Similarly, radiometric tial quantities dλ, dA and dΩ can graph II.5., the concept of differen- of optical quantities closely related quantities given as a function of be regarded as tiny intervals or tial quantities and integral calculus to the sensitivity of the human eye wavelength are labeled with the elements ∆λ, ∆A and ∆Ω of the is briefly explained for spectral (see § II.6.), radiometry deals with prefix “spectral” and the subscript respective quantity. As a conse- radiometric quantities. the measurement of energy per “λ” (for example spectral radiant quence of the fact that these inter- time ( = power, given in watts) power Φλ). vals or elements are very small, emitted by light sources or imping- Remark: The definitions of radio- radiometric quantities can be con- ing on a particular surface. Thus, metric quantities cannot be under- sidered constant over the range

II.4.a. Definition of solid angle The geometric quantity of a solid angle Ω quantifies a part of an ob- server’s visual field. If we imagine an observer located at point P, his full visual field can be described by a sphere of arbitrary radius r (see Fig.II.). Then, a certain part of this full visual field defines an area A on the sphere’s surface and the solid angle Ω is defined by

Ω = A / r² Equ. II.2 As the area A is proportional to r², this fraction is independent of the actual choice of r. If we want to calculate the solid angle determined by a cone, as shown in Fig.II.area A is the area of a spherical calotte. However, as the solid angle is not only defined for conical parts of the full visual field, area A can be any arbitrary shape on the sphere’s surface. Although Ω is dimensionless, it is common to use the unit (sr). The observer’s total visual field is described by the whole surface of the sphere, which is given by 4πr², and thus covers the solid angle Fig.II.6. The solid angle Ω quantifies a certain part of the visual field, seen Ω π π total = 4 r² / r² = 4 sr = 12.57 sr by an observer located at P Tutorials - Page 161 Properties and Concepts of Light and Color

II.4.b. Radiant power or Φ e

Radiant powe r Φ e is defined by netic radiation only by a single the total power of radiation emitted number and does not contain any by a source (lamp, light emitting information on the spectral distri- diode, etc.), transmitted through a bution or the directional distribu- surface or impinging upon a sur- tion of the lamp output. face. Radiant power is measured in watts (W). The definitions of all other radiometric quantities are based on radiant power. If a light source emits uniformly in all direc- tions, it is called an isotropic light source. Radiant power characterizes the output of a source of electromag- Fig. II.7. The radiant power of Φe of a light source is given by its total emitted radiation.

II.4.c. Radiant intensity Ie

Radiant intensity I e describes the In general, radiant intensity de- radiant power of a source emitted pends o n spa tia l d irec tio n. The unit in a certain direction. The source’s of radiant intensity is W / sr.

(differential) radiant power d Φ e emitted in the direction of the (differential) solid angle element d Ω is given by

dΦe = Ie dΩ and thus

Φ = I dΩ e “ e 4π Fig. II.8. Typical directional distribution of radiant intensity for an incandescent bulb.

II.4.d. Radiance Le

Radiance Le describes the intensity the (differential) solid angle ele- From the definition of radiant whereby ϑ is the angle between the of optical radiation emitted or ment dΩ is given by intensity Ie it follows that the dif- emitting surface element dA and reflected from a certain location on ferential radiant intensity emitted the direction for which Ie is ca lc u- an emitting or reflecting surface in dΦe = Le cos(ϑ) dA dΩ by the differential area element dA lated. a particular direction (the CIE in a certain direction is given by definition of radiance is more gen- Equ.II3 The unit of radiance is W/(m2.sr). eral. Within the frame of this tuto- dIe = Le cos(ϑ) dA rial, the most relevant application In this relation, ϑ is the angle be- of radiance describing the spatial tween the direction of the solid Thus, emission characteristics of a source angle element dΩ and the normal is discussed ). The radiant power d I = L ⋅cos(ϑ)dA of the emitting or reflecting surface e “ e Φe emitted by a (differential) sur- element dA. emitting surface face element dA in the direction of Equ.II.4

II.4.e. Irradiance Ee

Irradiance Ee describes the Note that the corresponding area amount of radiant power impinging element dA normal, whic h is o r ie nted upon a surface per unit area. In perpendicular to the incident beam, detail, the (differential) radiant is given by dΦe power d Φ e upon the (differential) surface element dA is given by dA normal = cos(ϑ) dA ϑ d Φ = E dA e e with ϑ denoting the angle between the beam and the normal of dA, we dA Generally, the surface element can get be oriented at any angle towards the direction of the beam. How- dAnorma l ever, irradiance is maximised when Ee = Ee,normal cos(ϑ) Equ.II.5 the surface element is perpendicu- lar to the beam: The unit of irradiance is W/m².

Fig. II.9 – Irradiance is defined as incident radiant power dΦe per surface d Φ e = Ee,normal dA normal area element dA.at P Page 162 - Tutorials Properties and Concepts of Light and Color

II.4.f. Radiant exitance Me

Radiant exitance Me quantifies the From the definition of radiance and consequently between the respective direction radiant power per area, emitted or follows that the (differential) and the surface’s normal. reflected from a certain location on amount radiant exitance dMe emit- M = L cos(ϑ) dΩ The unit of radiant exitance is W/ e “ e a surface. In detail, the ted or reflected by a certain loca- 2π sr m². In some particular cases, Me= (differential) radiant power dΦe tion on a surface in the direction of Ee (see § II.8.a). emitted or reflected by the surface the (differential) solid angle ele- The integration is per- Equ.II6 element dA is given by ment dΩ is given by formed over the solid angle of 2π steradian correspond- ing to the directions on one side of dΦe = Me dA dMe = Le cos(ϑ) dΩ the surface and ϑ denotes the angle

II.4.g. Spectral radiant power Φ λ(λ), spectral radiant intensity Iλ(λ), spectral radiance Lλ(λ), spectral irradiance Eλ(λ), and spectral radiant exitance Mλ(λ)

The radiometric quantities dis- area describes the contribution of UVA irradiance is defined as as the UVA range is defined from cussed above are defined without this very wavelength interval to the 400nm λ= 315 nm to λ = 400 nm. any regard to the wavelength(s) of total value of radiant power Φ e, E = Eλ (λ )dλ the quantified optical radiation. In which is graphically represented by e,UVA “ 315nm order to quantify not only the abso- the total area under the graph of lute amount of these quantities but spectral radiant power Φλ(λ).

also the contribution of light from Mathematically, this can be ex- R different wavelengths, the respec- pressed by the integral 14 tive spectral quantities are defined. 12 Spectral radiant power is defined ∞ Φ = Φ (λ)dλ 10 dλ as a source’s radiant power per e “ λ wavelength interval as a function 0 8 of wavelength. In detail, the 6 source’s (differentia l) rad iant The unit of spectral radiant power

Φ [mW/nm] power d e emitted in the is W/nm or W/Å 4 (differential) wavelength interval The other spectral quantities are λ λ λ 2 between and +d is given by defined correspondingly and their units are given by the unit of the 0 SPECTRAL RADIANT POWE RADIANT SPECTRAL λ d Φ e = Φλ(λ) d λ respective quantity, divided by nm 300 400 500 600 700 800 900 1000 or Å (see This equation can be visualised Table VII.IV and Table VII.V). WAVELENGTH [nm] geometrically (see Fig. II.10.). As Generally, a radiant quantity can be d λ is infinitesimally small, spectral calculated from the respective Fig.II.10. Relation between spectral radiant power Φλ(λ) and radiant radiant power Φλ(λ) is approxi- spectral quantity by integration Φ mately constant in the interval over wavelength from λ=0 to λ=∞. power e, visualized at a hypothetical example. Radiant power emitted in between λ and λ+d λ. Thus, the However, this integration is often the wavelength interval between λ and λ+dλ is given by the area of the Φ λ λ product Φλ(λ) d λ equals the area restricted to a certain wavelength shaded rectangle, which amounts to λ( ) d . The total amount of radiant range, which is indicated by the Φ under the graph of Φλ(λ) in the power e emitted over the whole spectrum is given by the area under the Φ λ interval between λ and λ+d λ. This respective prefix. For instance, curve describing λ( ), which is mathematically expressed by an integral.

II.5. Calculation of radiometric quantities - Examples II.5.a. Example 1: Isotropic point source

A small source emits light equally b/ An infinitesimal surface element passes through the surface of a for distances much larger than the in all directions (spherical symme- dA at distance r and perpendicular sphere with radius r, which is given geometric dimensions of the try). Its radiant power equals Φ to the beam occupies the solid by 4r²π. As the light source emits source, which allows the assump- e,source=10 W. angle light symmetrically in all direc- tion of a point source. In other If we are interested in the charac- dΩ = dA / r² tions, the irradiance has the same cases, a source with considerable teristics of this source in a distance value at every point of this sphere. geometric dimensions might possi- r that is much larger than the geo- and thus the infinitesimal radiant Thus, irradiance E of a surface at a bly be replaced by a “virtual” point metric dimensions of the source power dΦe,imp impinging onto dA certain distance r and oriented source, and then the “inverse itself, we can neglect the actual can be calculated by perpendicular to the beam can be square law” still holds true when size of the source and assume that calculated from its definition: distance r is measured from this

the light is emitted from a point. As dΦe,imp = I dΩ = Φe,source / 4π sr · virtual point source (see Example a rule of thumb, this approximation dA / r² = Φe,source / 4πr² · dA Ee = radiant power impinging upon 2). However, when the source is justified if distance r is at least a surface / area of this surface = Φ cannot be assumed point like and

10 times larger than the dimensions Thus, the irradiance at distance r e,source / 4πr² every point of the source emits of the light source. amounts to light in more than a single direc- which is identical with the result tion , the “inverse square law” no a/ As the source emits light sym- lo nger ho lds tr ue. As a n exa mp le, Ee = Φe,sou rce / 4πr² above. metrical in all directions, its radiant this is the case for fluorescent tubes. intensity is equal for all directions This result can also be obtained by Remark: The fact that E is propor- Φ π and amounts to Ie = e,source / 4 sr the following argument: tional to r-² is generally known = 10 W / 4π sr = 0.796 W / sr. At distance r, all the radiant power under the name “inverse square Φe,source emitted by the source law”. However, it only holds true Tutorials - Page 163 Properties and Concepts of Light and Color

II.5.b. Example 2: Spot source In a simple flashlight, a concave b/ In order to determine the flash- Thus, the cone defines a solid angle Remark: As a virtual point source reflects light from a small bulb light’s radiant intensity, we have to given by located at the cone’s vertex pro- (radiant power Φ = 200 mW) into a determine the solid angle deter- duces the same spatial radiation divergent cone (see figure below). mined by the cone. Following the Ω π distribution as the flashlight’s bulb Assuming that the mirror reflects with- = ACircle / r² = 0.05² / 0.357² = out any losses and uniform distribution definition of solid angle and ap- 0.0616 sr together with its concave mirror, of power over the cone, proximating the area of the spheri- the “inverse square law” holds true cal calotte by the area of a circle and the flashlight’s radiant inten- for this configuration. However, Note that the flashlight does not emit with a radius of 5 cm (= 0.05 m), sity amounts to the distance r which the law relates light symmetrically in all directions, we get to has to be measured from the therefore the equations derived in Exam- Ω = ACircle / r² I = Φ / Ω = 0.2 / 0.0616 W / sr = position of the virtual point source. ple 1 cannot be used. 3.25 W / sr. a/ In a distance of 25 cm from the flash- with r describing the distance of the light’s front window, the whole radiant circle form the cone’s vertex. power of 200 mW ( = 0.2 W) impinges on a circle with a radius of 0.05 m. If we From Fig. II.11, we get assume that irradiance is constant all over this circle and we neglect the fact r = x + 0.25 m that the surface is not everywhere strictly perpendicular to the beam, we and can calculate the irradiance at a distance x / x+0.25 = 0.03 / 0.10 of 25 cm from the flashlight’s front window: from which we calculate E = radiant power impinging upon a surface / area of this surface x = 0.107 m == 0.2 / 0.05² π W / m² and E § 25 W / m² r = 0.357 m Fig. II.11 - Calculating the irradiance caused by a flashlight. II.5.c. Example 3: The Lambertian surface

By definition, a Lambertian by use of the relation dΩ = sin(ϑ) d geometries for radiant power, exi- all directions of a solid angle of 4π surface either emits or reflects ϑ dϕ, we get tance and irradiance (or luminous or 2π. Lambertian reflection is radiation into all directions of a π flux, luminous especially desired for the coating of 2π 2 hemisphere with constant radiance = ϑ Ω = ϑ ϑ ϑ ϕ = ⋅ π exitance and integrating spheres, which are Me Le cos( ) d Le cos( ) sin( ) d d Le Le (Fig.II.12). From Equ.II.4 in “ ““ illuminance), widely used for detector input 2π sr 00 paragraph II.4.d. follows that the which have to be optics or for output optics of radi- directional distribution of radiant Equ.II7 determined by an integration over ance or luminance standards. intensity is given by The respective relations for photo- Ie(ϑ) = Ie,0 · cos(ϑ) metric quantities (see § II.6.) char- acterizing a Lambertian surface can with be derived by replacing the index “e” by the index “v”. I = L cos(0)dA = L dA e,0 “ e “ e emitting emitting Reflecting Lambertian surfaces are surface surface widely used in light measurement

whereby Ie,0 denotes radiant inten- for well defined, perfectly diffuse sity emitted in the direction perpen- scattering fully independent of the direction of the incoming beams. dicular to the surface and Ie(ϑ) denotes radiant intensity emitted in Thus, the radiance reflected from a Ideal Diffuse reflection with a direction enclosing the angle ϑ certain location on the surface in a (lambertian surface) directional components with the surface’s normal. Calcu- certain direction is proportional to the total radiant power impinging lating the surface’s exitance Me from Equ.II.6 in paragraph II.4.f. onto the reflecting surface. This Fig.II.12. Constant spatial distribution of radiance Le after ideal diffuse allows the realization of detector reflection at a Lambertian surface II.6. Spectral sensitivity of the human eye The sensitivity of the human eye to defined by the CIE spectral lumi- to 20% of its sensitivity at 555 nm. impression of same “brightness” to light of a certain intensity varies nous efficiency function V(λ). As a consequence, when a source the human eye. strongly over the wavelength range Only in very rare cases, the spectral of monochromatic light at between 380 and 800 nm. Under sensitivity of the human eye under 490 nm emits five times as daylight conditions, the average dark adapted conditions (scotopic much power (expressed in normal sighted human eye is most vision), defined by the spectral watts) than an otherwise sensitive at a wavelength of 555 luminous efficiency function V’(λ), identical source of mono- nm, resulting in the fact that green becomes technically relevant. By chromatic light at 555 nm, light at this wavelength produces convention, these sensitivity func- both sources produce the the impression of highest tions are normalized to a value of 1 “brightness” when compared to in their maximum. Fig. II.13. Spectral lumi- light at other wavelengths. The As an example, the photopic sensi- nous efficiency functions V λ spectral sensitivity function of the tivity of the human eye to mono- ( ) for photopic vision and average human eye under daylight chromatic light at 490 nm amounts V’(λ) for scotopic vision, conditions (photopic vision) is as defined by the CIE. Page 164 - Tutorials Properties and Concepts of Light and Color

II.7. Basic photometric quantities

One of the central problems of metric quantities represent a the eye’s sensitivity under daylight replacing V(λ) by V’(λ) and re- optical measurements is the quanti- weighted sum with the weighting conditions, thus being relevant for placing Km ( = 683 lm / W ) by K’m fication of light sources and light- factor being defined by either the the vast majority of lighting situa- (= 1700 lm / W) (see definition in § ning conditions in numbers directly photopic or the scotopic spectral tions (photopic vision takes place VII.2). related to the perception of the luminous efficiency function (see § when the eye is adapted to lumi- As the definition of photometric human eye. This discipline is called II.6). Thus, the numerical value of nance levels of at least several quantities closely follows the corre- “Photometry”, and its significance photometric quantities directly per square meters, sponding definitions of radiometric leads to the use of separate physical relates to the impression of scotopic vision takes place when quantities, the corresponding equa- quantities, which differ from the “brightness”. Photometric quanti- the eye is adapted to luminance tions hold true – the index “e” just respective radiometric quantities ties are distinguished from radio- levels below some hundredths of a has to be replaced by the index “v”. only in one respect: Whereas radio- metric quantities by the index “v” per square meter. For Thus, not all relations are repeated. metric quantities simply represent a for “visual”, and photometric quan- mesopic vision, which takes place Instead, a more general formulation total sum of radiation power at tities relating to scotopic vision are in the range in between, no spectral of all relevant relations is given in various wavelengths and do not denoted by an additional prime, for luminous efficiency function has the Appendix. account for the fact that the human example Φv’. The following expla- been defined yet). However, the eye’s sensitivity to optical radiation nations are given for the case of respective relations for scotopic depends on wavelength, the photo- photopic vision, which describes vision can be easily derived when

II.7.a. Luminous flux Φv

Luminous flux Φv is the basic power. The unit of luminous flux is exactly 683 lm. The value of 683 where the human eye is far less photometric quantity and describes lumen (lm), and at 555 nm, where lm / W is abbreviated by the sym- sensitive and V(λ) = 0.107, has a the total amount of electromagnetic the human eye has its maximum bol Km (the value of Km = 683 lm/ luminous flux of 0.107*683 lm = radiation emitted by a source, sensitivity, a radiant power of 1 W W is given for photopic vision. For 73.1 lm. For a more detailed expla- spectrally weighted with the human corresponds to a luminous flux of scotopic vision, Km’=1700 lm/W nation of the conversion of radio- eye’s spectral luminous efficiency 683 lm. In other words, a mono- has to be used). However, a mono- metric to photometric quantities, function V(λ). Luminous flux is the chromatic source emitting 1 W at chromatic light source emitting the see paragraph II.7.f. photometric counterpart to radiant 555 nm has a luminous flux of same radiant power at 650 nm,

II.7.b. Luminous intensity Iv

Luminous intensity Iv quantifies flux dΦv emitted in the direction of and thus The unit of luminous intensity is the luminous flux emitted by a the (differential) solid angle ele- lumen per steradian (lm / sr), which source in a certain direction. It is ment dΩ is given by Φ = I dΩ is abbreviated with the expression therefore the photometric counter- v “ v “candela” (cd): part of the radiometric quantity π dΦv = Iv · dΩ 4 “radiant intensity” Ie. In detail, the 1 cd = 1 lm / sr source’s (differential) luminous

II.7.c. Luminance Lv

Luminance Lv describes the meas- ticular direction (the CIE definition In detail, the (differential) lumi- with θ denoting the angle between urable photometric brightness of a of luminance is more general. nous flux dΦv emitted by a the direction of the solid angle certain location on a reflecting or Within the frame of this tutorial, (differential) surface element dA in element d Ω and the normal of the emitting surface when viewed from the most relevant application of the direction of the (differential) emitting or reflecting surface ele- a certain direction. It describes the luminance describing the spatial solid angle element dΩ is given by ment dA. luminous flux emitted or reflected emission characteristics of a source The unit of luminance is from a certain location on an emit- is discussed). dΦv = Lv cos(θ) · dA · dΩ ting or reflecting surface in a par- 1 lm m-2 sr-1 = 1 cd m-2

II.7.d. Illuminance Ev Illuminance E describes the lumi- with arbitrary orientation is related v dΦv = Ev · dA nous flux per area impinging upon to illuminance Ev,normal upon a with ϑ denoting the angle between a certain location of an irradiated Generally, the surface element can surface perpendicular to the beam the beam and the surface’s normal. surface. In detail, the (differential) be oriented at any angle towards by The unit of illuminance is lux (lx), Φ luminous flux d v upon the the direction of the beam. Similar and (differential) surface element dA is to the respective relation for irradi- Ev = Ev,normal cos(ϑ) given by ance, illuminance Ev upon a surface

II.7.e. Luminous exitance Mv

-2 Luminous exitance Mv quantifies element dA is given by lm m , which is the same as the the luminous flux per area, emitted unit for illuminance. However, the or reflected from a certain location abbreviation lux is not used for dΦv = Mv · dA on a surface. In detail, the luminous exitance. Φ (differential) luminous flux d v The unit of luminous exitance is 1 emitted or reflected by the surface Tutorials - Page 165 Properties and Concepts of Light and Color

II.7.f. Conversion between radiometric and photometric quantities

Monochromatic radiation: In the case of monochromatic radiation at a Polychromatic radiation: If a source emits polychromatic light described by

certain wavelength λ, a radiometric quantity Xe is simply transformed to its the spectral radiant power Φλ(λ), its luminous flux can be calculated by photometric counterpart Xv by multiplication with the respective spectral spectral weighting of Φλ(λ) with the human eye’s spectral luminous effi- luminous efficiency V(λ) and by the factor Km = 683 lm / W. Thus, ciency function V(λ), integration over wavelength and multiplication with with Km = 683 lm / W, so X = X * V(λ) * 683 lm / W v e Φ = K Φ (λ)⋅V(λ) dλ v m “ λ with X denoting one of the quantities Φ, I, L, or E. λ

Example: An LED (light emitting As V(λ) changes very rapidly in In general, a photometric quantity Xv is calculated from its spectral radio- λ diode) emits nearly monochromatic this spectral region (by a factor of 2 metric counterpart Xλ( ) by the relation radiation at λ = 670 nm, where V within a wavelength interval of 10 X = K X (λ)⋅ V(λ) dλ (λ) = 0.032. Its radiant power nm), for accurate results the LED’s v m “ λ amounts to 5 mW. Thus, its lumi- light output should not be consid- λ nous flux equals ered monochromatic. However, using the relations for monochro- with X denoting one of the quantities Φ, I, L, or E. Φv = Φe * V(λ) * 683 lm / W = 0.109 lm = 109 mlm matic sources still results in an approximate value for the LED’s luminous flux which might be

II.8. Reflection, Transmission, and Absorption Reflection is the process by which In this case, we speak about diffuse of the radiation unchanged. Excep- Absorption is the transformation electromagnetic radiation is re- reflection and diffuse transmis- tion: The Doppler effect causes a of radiant power to another type of turned either at the boundary be- sion (Fig. II.14). When no diffu- change in frequency when the energy, usually heat, by interaction tween two media (surface reflec- sion occurs, reflection or transmis- reflecting material or surface is in with matter. tion) or at the interior of a medium sion of an unidirectional beam motion. (volume reflection), whereas trans- results in an unidirectional beam mission is the passage of electro- according to the laws of geometri- magnetic radiation through a me- cal optics (Fig. II.15). In this case, dium. Both processes can be ac- we speak about regular reflection companied by diffusion (also (or ) and regu- called scattering), which is the lar transmission (or direct trans- process of deflecting a unidirec- mission). Reflection, transmission tional beam into many directions. and scattering leave the frequency

a b c

d ef g h i Fig. II.15 - When directly reflected or directly transmitted, an unidirec- tional beam follows the laws of geometrical optics: direct reflection (left): Fig. II.14 - a-c: Direct, mixed and diffuse reflection d-f: direct, mixed and αin = αout, direct transmission (right): n1 ⋅ sin(αin) = n2 ⋅ sin(αout) with n1 diffuse transmission and n2 denoting the respective medium’s index of refraction

II.8.a. Reflectance ρ, Transmittance τ, and Absorptance α

In general, reflection, transmission ratio of reflected radiant power to or Me = ρ Ee transmittance τr and diffuse and absorption depend on the incident radiant power. For a cer- transmittance τd, which are given wavelength of the affected radia- tain area element dA of the reflect- Total reflectance is further subdi- by the ratios of regularly (or di- tion. Thus, these three processes ing surface, the (differential) inci- vided in regular reflectance ρr rectly) transmitted radiant power can either be quantified for mono- dent radiant power is given by the and diffuse reflectance ρd, which and diffusely transmitted radiant chromatic radiation (in this case, surface’s irradiance Ee, multiplied are given by the ratios of regularly power to incident radiant power. the adjective “spectral” is added to with the size of the surface ele- (or specularly) reflected radiant Again, the respective quantity) or for a ment, thus power and diffusely reflected radi- τ = τr + τd Φ certain kind of polychromatic d e,incident = Ee dA ant power to incident radiant radiation. For the latter, the spectral power. From this definition, it is The absorptance α of a medium is distribution of the incident radia- and the (differential) reflected obvious that defined by the ratio of absorbed tion has to be specified. In addition, radiant power is given by the exi- ρ = ρr + ρd radiant power to incident radiant reflectance, transmittance and tance Me, multiplied with the size power. absorptance might also depend on of the surface element: The transmittance τ of a medium Being defined as ratios of radiant polarization and geometric distri- dΦ = M dA e,reflected e is defined by the ratio of transmit- power values, reflectance, transmit- bution of the incident radiation, Thus, ted radiant power to incident radi- tance and absorptance are dimen- which therefore also have to be sionless. dΦ MdA⋅ M ant power. Total transmittance is specified. ρ=e, reflected = e = e ρ Φ ⋅ further subdivided in regular The reflectance is defined by the d eincident, EdAe Ee Page 166 - Tutorials Properties and Concepts of Light and Color

Quantities such as reflectance and are not a constant since they are • temperature The measurement of optical prop- transmittance are used to describe dependent on many parameters • the spectral composition of the erties of materials using integrating the optical properties of materials. such as: radiation (CIE standard illumi- spheres is described in DIN 5036-3 The quantities can apply to either nants A, B, C, D65 and other and CIE 130-1998. complex radiation or to monochro- • thickness of the sample illuminants D) Descriptions of the principle meas- matic radiation. • surface conditions • polarization effects urements are presented in para- The optical properties of materials • angle of incidence graph III.1.f below.

II.8.b. Radiance coefficient qe, Bidirectional reflectance distribution function (BRDF)

The radiance coefficient qe char- difference is that the BRDF is a radiance coefficient generally is acterizes the directional distribu- function of the directions of the valid for just one specific direc- tion of diffusely reflected radiation. incident and the reflected beam tional distribution of incident radia- In detail, the radiance coefficient (Fig. ). In detail, the (differential) tion. depends on the direction of the irradiance dEe impinging from a The unit of radiance coefficient and reflected beam and is defined by certain direction causes the re- BRDF is 1/steradian. The BRDF is the ratio of the radiance reflected in flected radiance dLe in another often abbreviated by the Greek this direction to the total incident direction, which is given by letter ρ, which bears the danger of irradiance. In general, the reflected dLe = BRDF · dEe mixing the BRDF up with reflec- radiance is not independent from tance (see foregoing paragraph). the directional distribution of the This BRDF depends on more argu- incident radiation, which thus has ments than the radiance coefficient. Fig. II.16 - Geometry used for the definition of the bidirectional to be specified. However, its advantage is the si- reflectance distribution function (BRDF). The BRDF depends on In the USA, the concept of Bidi- multaneous description of the the directions of incident and reflected radiation, which are given ϑ ϑ rectional reflectance distribution material’s reflection properties for by the angles i and r, which are measured relative to the reflect- ϕ ϕ function BRDF is similar to the all possible directional distributions ing surface’s normal, and the azimuth angles i and r, which are radiance coefficient. The only of incident radiation, whereas the measured in the plane of the reflecting surface. II.9. The perception of color Color sensations are human sen- distinguished from all the other moving object with an unmoving pends not only on physical laws, sory perceptions, and color meas- impressions received when seeing. eye does not allow for the percep- but also on the physiological proc- urement technology must express The insertion of the term tion of gloss, the evaluation of essing of the radiation in the sense them in descriptive and compre- ”unstructured” into this definition gloss is excluded from the percep- organs. Visual conditions, lumi- hensible quantities. DIN 5033, Part also separates the texture of ob- tion of color. nance (brightness) and the state of 1, defines color as follows: served objects from the sensation In general, unlike mass, volume or the eye’s adaptation are amongst of color. temperature, color is not merely a the contributory factors. ”Color is the visual sensation, Thus the texture of a textile, for of an object. It is, Color manifests itself in the form associated with a part of the field instance, is not included in the rather, a sensation triggered by of light from self-emitting light of view that appears to the eye to color. radiation of sufficient intensity. sources, surface colors (of non-self be without structure, through The definition also calls for obser- This can be the radiation of a self- emitting light sources) and in the which this part can be distin- vation with a ”single” eye which is emitting light source, or it can be intermediate form of the lumines- guished from another unstructured ”unmoving”, conditionally exclud- reflected from a surface. This ra- cent colors of dyestuffs such as neighbouring area when observed ing other factors such as spatial diation enters the eye, where recep- optical brightening agents and day- with a single, unmoving eye”. sensation, the perception of the tive cells convert it into nervous glow paints that absorb location of objects, their direction, stimulation, which is in turn trans- from a short wavelength part of the This rather complicated but unam- and even their relative movement mitted to the appropriate part of the spectrum and emit the energy in a biguous definition of color allows from the perception of color. Since brain, where it is experienced as part of the spectrum with longer the visual sensation of ”color” to be single-eyed observation of an un- color. The sensation of color de- wavelengths. II.9.a. Physiological background From the fact that spectral decom- are called cones and are responsi- wavelength. The brain interprets Therefore, these cones are called position of white light produces the ble for photopic vision under day- signals from only this type of cones „red cones“. Similarly, the two perception of different colors, it light conditions. Scotopic (night) - in the absence of a signal from the other types of cones are called can be deduced that color percep- vision is caused by photoreceptors other cones - as the color „red“. „blue cones“ and “green cones”. tion is closely connected to the called rods, which are much more wavelength of light (Fig. II.17.). As sensitive than cones. As there is an example, light with a wave- only one kind of rods, night vision length of 650 nm wavelength is is colorless. perceived as „red“ and light with a The three different kinds of cones wavelength of 550 nm is perceived differ in their spectral sensitivity to as „green“. However, there are electromagnetic radiation, which is colors, such as purple, which can- shown in Fig. for the average not be directly related to a certain normal sighted human eye. If wavelength and therefore do not monochromatic radiation irradiates occur in the spectral decomposition the eye, as it is the case with spec- of white light. tral decomposition of white light, The perception of color is formed the wavelength determines which in our brain by the superposition of types of cones are excited. For the neural signals from three differ- instance, monochromatic light at ent kinds of photoreceptors which 680 nm exclusively excites one Fig. II.17 - Relative spectral sensitivity of all four types of the human eyes are distributed over the human type of cones, whereas the two retinal light receptors. The three types of cones are responsible for pho- eye’s retina. These photoreceptors other types are insensitive at this topic vision, whereas the rods are responsible for scotopic (night) vision Tutorials - Page 167 Properties and Concepts of Light and Color

II.9.b. Color addition

As discussed above, monochro- of blue and red cones (Fig. II.17). Fig. II.18 - The matic light of a certain wavelength Simultaneous stimulation of all effect of color might predominately excite a single three types of cones results in the addition demon- type of cones, thus producing the perception of “white”. strated with color perception of “blue”, “green” This fact has an important conse- white light from or “red”. Depending on the actual quence: Consider a light source an overhead wavelength, monochromatic light consisting of three single sources projector before might also excite two types of with the colors red, green and blue. (top) and after cones simultaneously, thus produc- If it is possible to vary the intensi- (bottom) passing ing the perception of another color. ties of the three single sources through a ma- Red and green cones, for instance, individually, all possible colors can genta filter. To the left, the respective spectral decomposition is shown, are both excited by monochromatic be produced. This is the main idea whereas the circle to the right shows the resulting color impressions. It can light of 580 nm, and a signal from of color cathode ray tubes com- be clearly seen that the filter strongly absorbs light from the green part of these two types of cones – with the monly used in TV and computer the visual spectrum, whereas blue and red light pass the filter with low simultaneous absence of a signal monitors - every pixel (a point on attenuation. The impression of magenta is produced by simultaneous pres- from blue cones – produces the the monitor) consists of three ence of light from the blue and red regions of the visible spectrum, whereas color perception of “yellow”. smaller individual spots in the light from the green region is missing. However, our visual system cannot colors red, green and blue (Figure discriminate between monochro- II.18). As these individual spots are matic and broadband radiation as so close together, the human eye long as the excitation of the three cannot resolve them. Instead, they types of cones remains the same. produce the perception of a certain Thus, the perception of “yellow” color by superposition of their can also be produced by a broad- respective intensities. For instance, band spectrum between 550 nm the pixel appears yellow when only and 700 nm as long as green and the red and the green spot are emit- yellow cones are comparably ting light, and the pixel appears stimulated and blue cones are not white when all three spots are stimulated at all. Similarly, the emitting light. The entirety of perception of “cyan” is produced colors produced by color addition by simultaneous stimulation of blue forms the RGB color space, as and green cones, whereas the per- they are based on the three Fig. II.19 - An RGB monitor consists of tiny red, green and blue spots. ception of “magenta” (or purple) is (additive) primary colors red, green Variation of their brightness produces the impression of different colorsby caused by simultaneous stimulation and blue. color addition. II.9.c. Color subtraction Whereas color addition describes and reflecting wavelengths above white light. So, the combination of genta and black form the so called the perception of different colors appears yellow when irradiated yellow and cyan pigments results CYMK color space. caused by a superposition of red, with white light. So, when irradi- in green reflected light. Similarly, green and blue light sources, the ated with white light filters (or the combination of yellow and concept of color subtraction is pigments) absorbing blue light magenta pigments results in red based on the absorption of white appear yellow, filters (or pigments) and the combination of cyan and light by filters or pigments. As an absorbing green light appear ma- magenta results in green reflected example, a yellow filter absorbs genta and filters (or pigments) light. In next figure, the effect of wavelengths below about 500 nm, absorbing red light appear cyan. As color subtraction is demonstrated corresponding to blue light, but the effect of filters on transmitted for filters. transmits longer wavelengths cor- light is the same as the effect of Ideally, a combination of yellow, responding to green and red light. pigments on reflected light, the cyan and magenta pigments should Thus, when irradiated with white following conclusions derived for result in total absorption of the light the filter transmits only wave- pigments are also valid for filters. whole visible wavelength range lengths which stimulate the green What happens if two pigments are and thus in the perception of a and red cones, whereas the blue combined ? The combination of a black surface. cones are not stimulated. As dis- yellow pigment, which absorbs However, in reality the absorption Fig. II-20 - Overlapping arrange- cussed above, this results in the short (blue) wavelengths with a properties of these pigments are ment of yellow, cyan and magenta perception of the color “yellow”. cyan pigment, which absorbs long never ideal, thus four-color- color filters on an overhead projec- Similarly, a surface (better: pig- (red) wavelengths, leaves only printing uses a black pigment in tor. In the overlapping regions, ments on a surface) absorbing medium (green) wavelengths to be addition. Colors produced by a color subtraction results in green, wavelengths below about 500 nm reflected when irradiated with combination of cyan, yellow, ma- red and blue light. II.10. Colorimetry

The basic problem of colorimetry quantity, such as radiant intensity coefficient or its spectral transmit- Illuminant D56 is representative of is the quantification of the physio- or radiance). When the color of a tance. As colors of reflecting or average daylight with a correlated logical color perception caused by reflecting or transmitting object transmitting objects depend on the color temperature of 6500 K (for a certain spectral color stimulus (for example a filter) has to be object’s illumination, the CIE has the definition of color temperature, ϕ λ function λ( ). When the color of characterised, ϕλ(λ) equals the defined colorimetric standard illu- see below). a primary light source has to be incident spectral irradiance imping- minants. The CIE Standard Illumi- characterised, ϕλ(λ) equals the ing upon the object’s surface, mul- nant A is defined by a Planckian source’s spectral radiant power Φλ tiplied by the object’s spectral blackbody radiator at a temperature (λ) (or another spectral radiometric reflectance, its spectral radiance of 2856 K, and the CIE Standard Page 168 - Tutorials Properties and Concepts of Light and Color

II.10.a. RGB and XYZ color matching functions According to the tristimulus theory, of view of 10°, the CIE specifies constant k depends on the colori- every color which can be perceived another set of colour matching 100 metric by the normal sighted human eye x (λ) y (λ) z (λ) func- k = task: 10 10 10 λλλ can be described by three numbers tions “ Eydλ () () When which quantify the stimulation of This set defines the CIE 1964 the λ spectral red, green and blue cones. If two supplementary standard colori- color stimulus ϕλ(λ) describes a color stimuli result in the same metric observer, which has to be spectral radiometric quantity of a values for these three numbers, used for fields of view larger than primary light source, k = 683 lm/W they produce the same color per- 4°. The choice of the normalisation and consequently Y yields the ception even when their spectral Although RGB and XYZ color distributions are different. Around matching functions can be equally 1930, Wright and Guild performed used to define three parameters experiments during which observ- which numbers uniquely describe a 2 ers had to combine light at 435.8 certain color perception, the XYZ Y nm, 546.1 nm and 700 nm in such a color matching functions are pre- way that the resulting color percep- ferred as y(λ) they have positive 1,5 tion matched the color perception values for all wavelengths (Fig. z(λ) produced by monochromatic light II.21). In addition, is equal to the 1 λ at a certain wavelength of the visi- CIE spectral luminous efficiency y( ) ble spectrum. Evaluation of these function V(λ) for photopic vision. λ experiments resulted in the defini- The XYZ tristimulus values of a 0,5 x( ) tion of the standardised RGB color cer- tain λ λ = ϕ λ ⋅ λ λ matching functions r( ) g ( ) X k“ λ ( ) x( )d spec- RELATIVE SENSITIVIT and b(λ) which have been trans- tral λ color 0 formed into the CIE 1931 XYZ = ϕ λ ⋅ λ λ 350 400 450 500 550 600 650 700 750 color matching functions x(λ) Y k“ λ ( ) y( )d λ y(λ)z(λ) These colour matching WAVELENGTH [nm ] functions define the CIE 1931 Z = k ϕ (λ)⋅z(λ)dλ standard colorimetric observer “ λ λ and are valid for an observer’s field Fig. II.21 - XYZ color matching functions as defined by the CIE 1931 stan- of view of 2°. Practically, this stimulus function ϕλ(λ) are calcu- dard colorimetric observer. x (λ ) (solid black line) consists of a observer can be used for any field lated by short- and a long-wavelength y(λ) part, and (solid gray line) is of view smaller than 4°. For a field identical with the CIE spectral luminous efficiency function V(λ).

II.10.b. The (x,y)- and (u’,v’)-chromaticity diagrams

Although the XYZ tristimulus its coordinates defined by coordinates u’ and v’ does not the CIE (x, y) chromaticity dia- values define a three-dimensional provide a strict correspondence gram. 4X 9Y color space representing all possi- u'= v'= between geometric distances and + + + + ble color perceptions, for most X 15Y 3Z X 15Y 3Z perceived color differences, there applications the representation of Although this definition of the are far less discrepancies than in color in a two-dimensional plane is sufficient. One possibility for a two-dimensional representation is the CIE 1931 (x, y) chromaticity diagram with its coordinates x and y calculated from a projection of the X, Y and Z values: X Y x = y = X + Y + Z X + Y + Z Although widely used, the (x, y) chromaticity diagram has a major disadvantage of non-uniformity as geometric distances in the (x, y) chromaticity diagram do not corre- spond to perceived color differ- ences. Thus, in 1976 the CIE de- fined the uniform (u’, v’) chroma- ticity scale (UCS) diagram, with Fig. II.22 - The CIE 1931 (x,y) chromaticity diagram and the CIE 1976 (u’, v’) chromaticity diagram. II.10.c. Correlated color temperature The correlated color temperature coarse. In detail, the correlated spectral distributions dominated by incandescent lamps has a correlated is used to characterize the spectral color temperature is given in Kel- long (reddish) wavelengths corre- color temperature of about 2800 K, distribution of optical radiation vin (K) and is the temperature of spond to a low correlated color average daylight has a correlated emitted by a light source. This the blackbody (Planckian) radiator temperature whereas spectral distri- color temperature of about 6500 K characterisation corresponds to the whose received color most closely butions with dominated by short and the bluish white from a Cath- projection of a two-dimensional resembles to that of a given color (bluish) wavelengths correspond to ode Ray Tube (CRT) has a corre- chromaticity diagram onto a one- stimulus. a high correlated color temperature. lated color temperature of about dimensional scale and thus is very As a (simplified) rule of thumb, As an example, the warm color of 9000 K. Tutorials - Page 169 Measurement of Light

III. Measurement of light with integral detectors Spectroradiometry – the measure- the output signal of an integral matched to the CIE spectral lumi- • The detector’s spectral sensitiv- ment of radiation intensity as a detector is proportional to the nous sensitivity function V(λ), and ity function of wavelength – is the wavelength integral over the meas- detectors for solar UV irradiance • The dynamic range, over which only way to provide full spectral ured quantity’s spectral distribu- potentially harmful to the human the detector’s output is propor- information about optical radiation tion, multiplied with the detector’s skin are matched to the CIE ery- tional to the input signal’s inten- emitted by a light source or im- spectral sensitivity (see § III.2)) thema action spectrum. sity pinging upon a surface. Obtaining offer an economical and user As integral detectors provide just a • The detector’s time behaviour this information has its price: Spec- friendly alternative: In most cases, single output signal (usually volt- troradiometers are highly sophisti- it is not necessary to determine the age or photocurrent), they are much Refer to § III.5. for more info cated optical measurement devices exact spectral distribution of the easier to characterise than spectro- about calibration laboratory. and in general, their proper calibra- measured quantity, and it is suffi- radiometers. The main parameters More information is also avail- tion, operation and maintenance is cient to use a detector especially determining the usability and the able about our calibration ser- rather time consuming. However, designed to match a certain prede- quality of an integral detector are vices and calibration standards for the vast majority of applica- fined spectral sensitivity function. • The detector’s input optics, as well as Uniform Light Sources. tions, integral detectors (the term As an example, the spectral sensi- which determines its directional „integral“ describes the fact that tivity of photometric detectors is sensitivity III.1. The detector’s input optics and its directional sensitivity Basically, the design of a detector’s 4π steradian or over the hemi- tor’s directional sensitivity pro- function of solid angle, and input optics is determined by its spherical solid angle of 2π stera- portional to the cosine of the therefore the detector’s field of desired directional sensitivity, dian. This is achieved by an angle of incidence, which can be view has to be limited to a small which in turn depends on the radio- integrating sphere with the light achieved either by a flat field angle. This can be achieved by metric or photometric quantity to source placed either inside the detector or by the entrance port baffles and/or lenses arranged in be measured: sphere or directly at the sphere’s of an integrating sphere. Refer a tube. • The determination of a light entrance port. Refer also to § also to § III.1.c. source’s radiant and luminous III.1.b. • Radiant and luminous intensity, Refer also to § III.1.d and III.1.e. flux requires constant directional • The determination of irradiance radiance and luminance are sensitivity over the solid angle of and illuminance requires a detec- quantities which are defined as a III.1.a. Integrating spheres used with integral detectors In an ideal case, the inner surface Due to its geometry, an ideal inte- tion is far from being ideally re- detector’s sensitivity to the po- of an integrating is a perfect dif- grating sphere is characterised by flected. For these reasons, the larisation of incident radiation. fuse Lambertian reflector (see § constant irradiance (or illumi- quality of measurements performed • For the characterisation of pow- II.4.c). Thus, the directional distri- nance) ) at all locations of it’s inner with integrating spheres strongly erful light sources, an integrating bution of reflected radiation is surface. Furthermore, the level of depends on the sphere’s coating sphere can be used for attenua- independent from the directional this irradiance (illuminance solely material, on the exact position of tion in order to prevent saturation distribution of incident radiation, depends on the total amount of baffles and on the size of the ports effects of the detector. As this and no specular reflection occurs. radiant power (luminous flux) in relation to the sphere’s diameter. attenuation results in an increase entering the sphere and is inde- As a general rule of thumb, the of internal temperature, the light pendent from its directional distri- total area of entrance and exit ports source’s maximum power is bution. should not exceed 5% of the limited by the sphere’s tempera- However, real surfaces do not show sphere’s internal surface. Numer- ture range of operation. perfect Lambertian reflection prop- ous standard setups are used for the • In general, geometric alignment erties. Although minimised by the determination of radiometric and of source and integrating sphere properties of the respective mate- photometric quantities, defining the is not very critical, which simpli- rial, a certain amount of specular arrangement of the sphere’s en- fies calibration and measurement reflection still occurs. Baffles, trance and exit ports and internal procedures. placed at specific locations inside baffles (see § III.1.b. to III.1.f). For more detailed information the sphere, are used to prevent Apart from their directional sensi- about the theory and application of major measurement errors by tivity, integrating spheres offer integrating spheres, see chapter V. Fig. III.1. Ideal multiple Lam- specular reflection. Moreover, at additional advantages: bertian reflections inside an the input and exit ports, where the • The high number of internal integrating sphere coating material is missing, radia- reflections generally eliminates a III.1.b. Measurement of radiant power and luminous flux Radiant power and luminous flux tions. of lasers and spot sources The entrance port has to be large detector enough to ensure that all radiation Lasers, LEDs, spot lamps, endo- from the source enters the sphere. scopes, optical fibres and other baffle A baffle is necessary to shield the sources emit radiation with various detector from direct irradiation by directional distributions. As long the source. the emission is limited to a hemi- laser beam spherical (2π steradian) solid angle, the source can be attached to the entrance window of an integrating sphere and does thus not interfere with the sphere’s internal reflec- integrating sphere Fig. III.2. Integrating sphere used for laser power measurements Page 170 - Tutorials Measurement of Light

• Φ consequence, the lamp itself and its the integrating sphere. It must be X : luminous flux of the test detector accessories interfere with the baffled against direct irradiation by lamp baffle • Φ sphere’s internally reflected radia- the lamp. N : luminous flux of the calibra- tion and thus causes a source of For precise measurements, the tion lamp spot measurement error, which can be lamp must be aged before testing. • YX : measurement signal of the source accounted for by use of an auxil- The burn-in time depends on the test lamp (with auxiliary lamp iary lamp (see below). lamp type. The burn-in time for switched off) Integrating spheres used to measure integrating tungsten lamps should be 2-5 hours • YN : measurement signal of the sphere the radiant power or luminous flux (IEC 64) and for arc lamps about calibration lamp (with auxiliary of lamps must be well suited for 100 hours (IEC 81) is recom- lamp switched off) Fig. the lamp under test to reduce meas- mended. • YHN : measurement signal of the III.2. Integrating sphere used for urement uncertainty. One important radiant power and luminous flux In precise luminous flux measure- auxiliary lamp (with calibration design parameter is that the diame- ment applications an auxiliary lamp lamp switched off) ter of the hollow sphere should be with baffle(s) is recommended. The • YHX : measurement signal of the As an alternative, radiant power about ten times (twice for tube diffuse illumination generated by auxiliary lamp (with test lamp and luminous flux of collimated lamps) the maximum dimension of the auxiliary lamp can be used to switched off) (parallel) beams can be directly the lamp. For example, an integrat- reduce the negative effects of the measured by flat field detectors as ing sphere set-up to measure the lamp under test and its accessories long as the detector’s active area luminous flux of fluorescent lamps according to the relation exceeds the beam’s cross section. with 120 cm (47 in) length should Y Y Despite the simple measurement be at least 2 m (79 in) in diameter. Φ = Φ ⋅ X ⋅ HN setup, this method has significant Furthermore, the diameter of the X N YN YHX disadvantages in comparison to the sphere limits the maximum power use of an integrating sphere: of the lamp. • The detector might be possibly In actual measurements, the lamp sensitive to the beam’s polarisa- must be placed in the centre of the tion. hollow sphere. This is typically • The detector’s active area might accomplished using a tube holder, be possibly inhomogeneous in its which carries the power and meas- test lamp sensitivity. In this case, it is urement leads into the sphere. A important to ensure equal illumi- socket at the end of the tube holds detector nation during calibration and and connects the lamp. To get the baffle measurement. lamp in the centre position, hinged auxiliary lamp • Alignment of the detector rela- integrating spheres that open and tive to the beam is critical. have large diameters of more than 50 cm (20 in) are used. Spheres with smaller diameters may offer a integrating sphere Radiant power and luminous flux large diameter port to mount the of lamps lamp in the centre of the sphere. Fig. III.3. Experimental setup for radiant power and luminous flux meas- Lamps emit radiation in all direc- The port is normally closed with a urements of a lamp. The auxiliary lamp is used to reduce measurement tions of the full (4π steradian) solid cap during the measurement. The uncertainties caused by the interference of the lamp under test and its angle. Therefore, a lamp has to be port cap‘s inside surface should be accessories with the sphere’s internally reflected radiation. placed inside an integrating sphere coated with the same diffuse coat- in order to determine its total radi- ing as the hollow sphere surface. ant power or luminous flux. As a The detector is placed at a port on

III.1.c. Measurement of irradiance and illuminance

S(ϑ) According to Equ.II.5 in paragraph cosine response can only be ap- − cos(ϑ) entrance optics at an angle ș, meas- S(0) S(ϑ) − S(0) cos(ϑ) II.4.e., a detector for irradiance or proximately achieved. Deviations cos ineerror(ϑ) = = ured relative to the normal (see Fig. illuminance of a surface has to of a real detector’s directional cos(ϑ) S(0) cos(ϑ) II.9). S(0) denotes the detector's weight the incident radiation ac- response from the ideal cosine In this equation, S(ș) denotes the signal caused by the same ray of cording to the cosine of its angle of response are quantified by the detector's signal caused by a ray of light impinging vertically upon the incidence. This can be either detector’s cosine error function, light impinging upon the detector's detector's entrance optics. achieved by which is given by • an integrating sphere espe- Fig. III.5 - Irradiance detector cially designed for irradiance (or heads with cosine diffuser and illuminance) measurements (see waterproof version for underwa- figure below) or ter and outdoor use • a cosine diffuser, an optical element which shows purely dif- fuse transmission regardless of the directional distribution of incident Fig. III.4. Integrating sphere de- radiation (Fig. III.5). detector sign for measurement of irradiance In both cases, the ideal directional port or illuminance of a horizontal surface. The baffle prevents direct Light Source Approximate Average Illuminance (lx) illumination of the detector, and baffle overcast night 0,0001 the knife edges at the sphere’s full moon 0,1 entrance port prevent shading by the sphere’s wall, which would office light 500 distort the detector’s cosine re- clear bright sky 70000 - 85000 sponse.sources. Table III.1 – Some avarage illuminance values integrating sphere Tutorials - Page 171 Measurement of Light

III.1.d. Measurement of radiant and luminous intensity Radiant and luminous intensity stant. As the source’s directional of radiance and luminance (see care has to be taken to minimize describe the directional distribution characteristics often depends on its Equ. II.4), the emitting source has reflections at the lamp’s surround- of a source’s emitted radiation. For internal temperature distribution to be completely in the detector’s ing (walls, ceiling, the goniopho- determination of this directional and thus on its position relative to field of view. Ideally, both quanti- tometer itself) in the direction of distribution, the relative position the vertical, for accurate measure- ties have to be determined with a the detector. Blackening of the between source and detector has to ments of radiant and luminous setup that allows the source to be surrounding, the use of additional be varied. The goniophotometer is intensity it is not recommended to considered point like. As a crude baffles and the reduction of the a mechanical setup allowing the rotate the source around a horizon- rule of thumb, the distance between detector’s field of view are proper variation of the source’s orientation tal axis. detector and source should be at precautions. and/or the detector’s position, least ten times the largest geomet- whereby the distance between As radiant and luminous intensity ric dimension of the source. source and detector is kept con- are defined by the surface integral For precise measurements, special

III.1.e. Measurement of radiance and luminance

Radiance and luminance describe surface and the detector’s field of Light Source Approximate Average Luminance (cd/m²) the directional distribution of the view is limited to a few degrees. -3 radiance emitted or reflected by a Thus, only radiation from a small self-luminous paints 0,02 10 certain area element. Similar to part of the source’s surface enters Candle flame 1 radiant and luminous intensity, the detector (Fig. III.6). computer screen 100 radiance and luminance can be determined with a goniophotome- overcast daytime sky 1000 ter, but the detector is placed much clear bright sky 5000-6000 closer to the emitting or reflecting Table III.2 – Some average luminance values

III.1.f. Measurement of reflection and transmission properties Reflectance ρ (for incident radia- collimated or conical radiation tion of given spectral composition, beam. detector baffle irradiation polarization and geometrical distri- The signals of the detector will be beam bution) is used to describe the calculated as follow: optical properties of materials (see o § II.8). I(X)-I(stray) 8 o Ratio of the reflected radiant or ȡ = ------ȡN 8 luminous flux to the incident flux I(N)– I(stray) in the given conditions. The measurement of reflectance is I(X): signal with sample irradiation made in comparison to a reflection I(N): signal with standard irradia-

standard (reflectance ρN) with a tion Fig. III.7. Integrating Sphere Total Reflection Measurement Set-up

Diffuse Reflectance rd is used to I(X) - I(stray) - ȡҏ[I(mi) - I(stray)] detector baffle irradiation describe the optical properties of ȡd = ------ȡN materials (see § II.8). I(N) - I(stray) - ȡN [I(mi) - I(stray)] beam Ratio of the diffusely reflected part

o of the (whole) reflected flux, to the I(X): signal with sample irradiation sample 8

incident flux. I(N): signal with standard irradia- o The measurement of diffuse reflec- tion 8 tance is made in comparison to a I(stray): signal with open measure- reflection standard (reflectance rN) ment port regular with a collimated or conical radia- I(mi): signal with irradiance of a eflected tion beam. mirror beam The signals of the detector will be calculated as follow: Fig. III.8. Integrating Sphere Diffuse Reflectance Measurement Set-up

Transmittance τ (for incident ra- cal radiation beam. The signals of diation of given spectral composi- the detector will be calculated as detector baffle tion, polarization and geometrical follow: distribution) is used to describe the optical properties of materials (see τ = I(X) / I(open) sample § II.8). I(X): signal with sample irradiation irradiation Ratio of the transmitted radiant or I(open): signal with open measure- beam luminous flux to the incident flux ment port in the given conditions.

The measurement of transmittance is made with a collimated or coni- Fig. III.9. Integrating Sphere Total Transmittance Measurement Set-up Page 172 - Tutorials Measurement of Light

Diffuse Transmittance τd is used to describe the optical properties of I(X)- t.I(stray) detector baffle materials (see § II.8). IJd = ------I(open)- I(stray) Ratio of the diffusely transmitted sample part of the (whole) transmitted flux, I(X): signal with sample irradiation to the incident flux. I(open): signal with open measure- regular ment port and close output port irradiation transmitted beam The measurement of transmittance I(stray): signal with open measure- beam is made with a collimated or coni- ment port and open output port cal radiation beam. The signals of the detector will be calculated as follow: Fig. III.10. Integrating Sphere Diffuse Transmittance Measurement Set-up III.2. Spectral sensitivity of an integral detector Within the normal range of opera- to correct for them. An example of spectral sensitivity function s(λ) is ing relation containing the respec- tion of an integral detector, the a nonlinearity effect is the satura- described by the product of a refer- tive biological action spectrum. For relation between the input signal tion of a detector’s output signal at ence value sm and the relative instance, the erythemal action (the spectral radiometric quantity to high radiation levels, which poses spectral sensitivity sr(λ): spectrum is used for definition of be measured) Xλ(λ) entering the the upper limit of a detector’s range Sunburn Unit, which is used for

detector and its corresponding of operation. sr(λ) = sm · sr(λ) quantification of erythemally active output signal Y has to fulfil the When nonlinearity effects can be solar UV irradiance (see § VI.1).

following condition of linearity: neglected, the detector’s output In many cases, sm is given by the This correspondence allows the signal under arbitrary polychro- maximum of s(λ), thus s (λ) is direct determination of photopic Let Y be the detector’s response to r 1 matic radiation can be regarded as normalised to a value of 1 in its quantities or biologically active the input signal Xλ (λ) and Y the 1 2 a superposition of the detector’s maximum. Another possibility is radiation by an especially designed detector’s response to the input output signals under monochro- integral detector. In particular, the the normalisation of sr(λ) to a total signal Xλ (λ). Then, the detector’s 2 matic radiation. This leads to the wavelength integral value 1, which detector’s relative spectral sensitiv- response to the superimposed input concept of spectral sensitivity. ity s (λ) has to be matched closely λ λ is achieved by the definition of r signal Xλ1( )+Xλ2( ) is given by to the CIE spectral luminous effi- In detail, the CIE defines a detec- s = s(λ) dλ Y1+Y2. Moreover, the detector’s m “ ciency function V(λ) or to the tor’s spectral sensitivity (also: λ response is proportional to the respective action spectrum. For the spectral responsivity) s(λ) by input signal and therefore, the In terms of relative spectral sensi- determination of chromaticity response to the input signal a·Xλ1 tivity the detector’s output signal Y coordinates or correlated color λ = 1 dY (λ) is given by a·Y1 (whereby a s( ) temperature, it is necessary to X (λ) dλ is given by denotes an arbitrary positive num- λ simultaneously use three detectors Y = s X (λ)⋅s (λ)dλ ber). A detector might possibly m “ λ r with their spectral sensitivities whereby Xλ(λ) denotes the spectral λ show a certain dark signal Y0 especially adapted to the color (usually dark current or dark volt- radiometric quantity defining the matching functions defined by the detector’s input signal and dY This integral relation is equivalent age), which is a nonzero output to the definition of photopic quanti- CIE 1931 standard colorimetric signal even when the detector is not denotes the (differential) increase observer (see § II.10.a). of the output signal caused by the ties, whereby the detector’s relative exposed to any radiation at all. In λ input radiation in the (differential) spectral sensitivity sr( ) corre- Gigahertz Optik uses different this case, Y, Y1 and Y2 have to be wavelength interval between λ and sponds to the CIE spectral lumi- combinations of photodiodes and replaced by Y-Y0, Y1-Y0 and Y2-Y0. nous efficiency function V(λ) and Deviations from this behaviour are λ+dλ. When linear behavior of the filters to achieve proper spectral detector can be assumed, the detec- sm corresponds to Km = 683 lm/W sensitivities for detectors used in called nonlinearities and cause (see § II.7.f). Similarly, the calcula- measurement errors. However, it is tor’s signal Y is given by photometry, radiometry and col- tion of effective radiation doses orimetry. possible to experimentally deter- = λ ⋅ λ λ relevant for certain biological mine a detector’s nonlinearities and Y “ Xλ ( ) s( )d O f -λ ten, the reactions is based on a correspond-

III.2.a. Monochromatic radiometry For radiometric characterisation of lasers with collimated beams ) or

monochromatic or near monochro- with an integrating sphere (for 200nm 400nm 600nm 800nm 1000nm 1200nm 1400nm 1600nm 1800nm matic radiation of a known wave- lasers with non-collimating length, a detector’s spectral sensi- beams and LEDs), tivity does not necessarily have to • integrating spheres equipped match a certain predefined shape. small area photodiodes, whose Silicon Carbid

Thus, a photodiode can be used low capacitance results in a UV-enhanced Silicon Photodiode without any spectral correction detector time constant in the filters as long it is sensitive at the order of nanoseconds. Thus, Silicon Photodiode respective wavelength. these detectors are perfectly Typical tasks of monochromatic suitable for laser pulse analysis Germanium Photodiode, InGaAs Photodiode radiometry are the laser power with high time resolution. Fig. III.11. Sensitivity ranges of various types of photodiodes measurements, characterisation of • detectors equipped with integrat- LEDs with near monochromatic ing spheres with unique baffle light output and power measure- design for measurements in fibre ments in fibre optical telecommuni- optics telecommunication. Addi- cation. Gigahertz Optik offers tional adapters for standard fibre • laser power meters equipped optic connectors are available. with a flat field detector (for Tutorials - Page 173 Measurement of Light

III.2.b. Polychromatic radiometry The determination of total radiation power over a certain spectral range requires the detector’s spectral 1 sensitivity function to closely match a rectangular shape. Gigahertz Optik offers absolutely calibrated irradiance and radiant power meters equipped with a 0,5 cosine diffuser or an integrating rel. sensitivity sphere, whose spectral sensitivity is optimised for UVA, UVB, UVC, visible (VIS) and near infrared Fig. III.12. Spectral sensitivity of Giga- 0 (NIR) ranges. hertz Optik’s RW-3703 VISIBLE400-800 380 430 480 530 580 630 680 730 780 830 nm Irradiance detector closely matches the ideal rectangular shape. wavelength ( nm ) III.2.c. Photometry

∗ λ ≤ For photometric measurements, the sVd()λλλ− () where SA( ) is the spectral distribu- 3% for “Class A” instruments detector’s relative spectral sensitiv- “ r ≤ ′= λ tion of the CIE standard Illuminant and f1’ 6% for “Class B” instru- λ f1 ity sr( ) has to match the CIE spec- “ Vd()λλ A (see § II.10.), which is the rec- ments. Gigahertz Optik offers high tral luminous efficiency function V λ ommended photometric calibration quality illuminance, luminance and (λ) as close as possible. In order to * source. High quality photometric luminous flux detectors meeting where s r(λ) is given by quantify a detector’s inevitable detectors show a value for f1’ be- Class A level (f1’ = 3%) and, as an spectral mismatch, the CIE recom- SVd()λλλ () low 3%, whereas a value of f1’ economical alternative, detectors “ A mends the evaluation index f1’, ∗ λ =⋅λ λ above 8% is considered as poor meeting class B level (f1’ = 5%). sr () sr () which is defined by Ssd()λλλ () quality. The DIN 5032, part 7 “ Ar λ requires a spectral mismatch of f1’

III.2.d. Colorimetry For the determination of a color ing function y(Ȝ) is identical with stimulus X, Y and Z values as the CIE spectral luminous effi- λ defined by the CIE 1931 standard ciency function V( ), the respec- x short colorimetric observer, the same tive detector can be absolutely 1,5 stimulus has to be measured by calibrated for simultaneous photo- x long three different detectors, whose metric measurements. y spectral sensitivity functions have 1 z to be adapted to the CIE 1931 XYZ color matching functions (see §

II.10.a). However, as the x(Ȝ) color rel. sensitivity matching function consists of two 0,5 separated regions of sensitivity, the X value is often determined by two detectors. In this case, altogether Fig. III.13. Spectral sensitivity 0 four detectors are needed for the functions used for colorimetric 380 430 480 530 580 630 680 730 780 determination of a stimulus’ X, Y measurements with Gigahertz wavelength ( nm ) and Z values. As the color match- Optik’s CT-3701 High Precision

III.3.The detector’s time behaviour

A detector’s time resolution is percentage (usually 90%) to a t laser signal’s time characteristics. − limited by its response to an instan- certain low percentage (usually Yt()=+ Y ( Y − Y ) ⋅ eτ For this application, Gigahertz taneous change of the input signal. 10%) of the maximum value when 00f Optik offers the LPPA-9901 detec- Due to electrical capacities of the a steady input is instantaneously Gigahertz Optik’s integral detec- tor, which uses a photodiode with light sensitive element and the removed. tors use photodiodes, which are an especially small capacity. This electronics, the output signal does Typically, the detector’s response typically characterised by time allows to reduce the LPPA-9901’s not change instantaneously as well to an instantaneous change of the constants of µs. As most variable time constant to a value of 5 ns. but gradually increases or de- input signal exponentially ap- light sources change their intensity creases until it reaches its final proaches the final value. In that levels in significantly longer time value. The detector’s rise time is case, the detector’s time behaviour scales, the detector’s time constant defined by the time span required is best described by the time con- is not really an issue for most ap- for the output signal to rise from a stant τ, which is the time span plications. However, especially certain low percentage (usually required for the output signal to lasers are often pulsed with a fre- 10%) to a certain high percentage vary from its initial value by 63% quency in the order of 109 Hz (for (usually 90%) level of the maxi- of its final change (the value of example in telecommunication), mum value when a steady input is 63% is derived from 1 – 1/e, which which corresponds to signals peri- instantaneously applied. Accord- equals 0.63). The temporal change ods in the order of 1 ns. In that ingly, fall time is defined by the of the output signal Y(t) from its case, the relatively slow response time span required for the output initial value Y0 to its final value Yf of normal photodiodes prevents the signal to drop from a certain high is then given by accurate characterisation of the Page 174 - Tutorials Measurement of Light

III.4. The detector’s dynamic range In general, a detector fulfils the noise and does no longer quan- longer detector integration times measurement uncertainties at condition of linearity (see § III.2.) tify the physical quantity which or by averaging subsequent high levels of the input signal. only for a limited range of the input should be determined. measurements of the same input signal level. There are two effects The lower limit of the measure- signal. The detector dynamic range de- which define the boundaries of this ment range, which is posed by • At high levels of the input signal, pends on the photodiode type The dynamic range: noise, is quantified by the noise the detector’s output signal no overall measurement system dy- equivalent input. The CIE de- longer increases proportional to namic range will depend on both • At very low levels of the input fines the noise equivalent input its input signal, and thus the the detector and electronic meter’s signal, the detector’s output is as the value of the respective detector no longer fulfils the range capabilities. For example, a largely dominated by noise. physical quantity (radiant power condition of linearity (see § typical silicon photodiode is capa- Noise is a random temporal or luminous flux, irradiance or III.2.). Instead, physical limits of ble of measuring more than 2 mA fluctuation of the output signal illuminance, ...) that produces an the light sensitive element and / of current before saturating, how- which occurs even when the output signal equal to the root or the electronics cause satura- ever the upper current measure- input signal is constant. The mean square noise output. As the tion of the output signal, which ment range of the meter may be absolute level and the frequency shape of the noise signal depends increases less than proportional limited to 200 µA. distribution of these variations on the temporal resolution that to the input signal and finally This range covers extremely low depends on the physical proper- can be achieved of the recording reaches a constant level. To a intensity levels, for instance the ties of the detector and the subse- electronics (often characterised certain extent, subsequent correc- quantification of erythemally active quent electronics. For many by the electronics’ time con- tion of the detector’s output UV radiation, or very high intensity detectors, noise is largely inde- stant), the noise equivalent input signal can account for the effects levels, which are used for industrial pendent from the absolute level is defined for a stated frequency of saturation and thus extend the UV curing processes. of the input signal and can be and bandwidth. Unless otherwise detector’s dynamic range. This neglected for input signals above stated, a 1 Hz bandwidth is usu- correction has to be based on a a certain minimum level. How- ally considered. Depending on thorough laboratory investigation ever, for very low input signals, the detector’s characteristics, its of the detector’s dynamic behav- the output signal is dominated by noise level can be reduced by iour and still leads to higher

III. 5. Calibration of integral Detectors Calibration is the determination of (better international) standards. But (the German national standard the correlation between an input this in itself does not guarantee laboratory) offer an accreditation and an output quantity. All meas- accuracy or the ability of the lab to service for industrial calibration urement instruments, such as volt- meet its calibration uncertainty laboratories where the lab‘s cali- meters, manometers, vernier cali- claims. Since calibration standards bration standards, calibration pro- pers, etc., must be calibrated to are subject to change with age and cedures and the stated re- determine the variation in reading use, a means to periodically check calibration intervals are subject to from the absolute quantity. In the calibration of the standards audit. This accreditation ensures radiometry, the input quantity is themselves must be in place. Occa- that the traceability of the calibra- provided by standard lamps and sionally the standards must be tion is on an absolute level. The optical radiation detector standards. replaced in order to maintain the DKD also ensures the international Because of the many different quality of calibration and traceabil- acceptance of its accredited calibra- measurement quantities involved, ity. The end user of the calibrated tion laboratories. Refer to Appen- calibration standards for each quan- product may be required to audit dix VI.6 for more information on tity are required if an optical radia- the calibration facility to ensure its national calibration organizations. tion calibration laboratory hopes to competency and traceability. In cover the whole range of possible Germany, the Deutsche Kalibrier- calibrations. A 'traceable' lab cali- dienst – DKD (the German accredi- bration standard should show an tation institution) and the Physi- Fig. III.14. unbroken chain of links to national kalisch-Technische Bundesanstalt GO Calibration Engineer

III.5.a Traceability: an Unbroken Chain of Transfer Comparisons C a libration is the most important corded and certified. laboratory, primary standard (A) is Each transfer device in the chain prerequisite for accuracy in meas- used to calibrate the reference should be subjected to periodic urement instrumentation. It is the Since the reading of a meter-under- standard (B) at an accredited cali- examination to ensure its long-term foundation upon which subsequent test is directly compared against bration laboratory. This reference stability. The lab performing the measurements are based upon. that of the transfer standard, the standard is used to calibrate the calibration typically sets the time Optical radiation calibration is qualification of this standard is of laboratory work standard (C) to be span between examinations and is typically done by the transfer the highest importance. used daily by the cal lab. This work self audited. method where the relationship standard is then used to calibrate between the value indicated by a A qualified standard should display the final product (D) or device measuring instrument and the value an unbroken chain of transfer com- under test. Accordingly, the cali- represented by a calibration stan- parisons originating at a national bration path is A-B-C-D. This path dard is compared with the former standards laboratory. For example, is called the chain of traceability. reading adjusted as needed, re- the transfer standard of the national Tutorials - Page 175 Measurement of Light

III.2.b. Polychromatic radiometry

SI-Uni t ( defini tion ) nat. institut for metrology National Labora tory Primary Primary Standard national Stan dard ( A ) standard ( realization )

accredited Calibration Labora tory calibration laboratory Reference Standard Reference Standard Reference Standard Reference Standard (1. Level ) ( B )

Internal Calibration Laboratory

Company Standard for daily use Reference Standard Control Standar d ( 2. Level )

Testing Equipment of the Company

Calibration Laboratory Measurement Infra-structure Trade and Industry Infra-structure Work Standard (C) Fig. III.15 Calibration Hierarchy from Primary Standard to Test Equipment

Testing Equipment (D) Accredited calibration laboratories guarantee recalibration cycle times for their standards plus a review of their calibration procedures since they are subject to review by an official accreditation authority. Fig. III.16 - Hierarchy of Standards

III.5.b. ISO/IEC/EN 17025 (formerly ISO Guide 25 and EN45001) The aims of the General Require- Without exception, DKD accred- and ISO/IEC Guide 25, known as ISO/IEC/EN 17025 is compatible ments for the Competence of Cali- ited calibration laboratories fulfill ANSI/NCSL Z540-1 in the United with A2LA and NVLAP require- bration and Testing Laboratories the requirements of the European States, is identical. This is the basis ments. are to provide a basis for use by standard EN 45001 (general criteria for the mutual appreciation be- *ISO 17025 web page : http:// accreditation bodies in assessing to operate a testing laboratory, May tween the European cooperation for www.fasor.com/iso25 competence of laboratories; estab- 1990). Outside of Europe this Accreditation (EA) and its extra- lish general requirements for dem- standard is not compulsory. Instead European partners. In 1999 ISO/ onstrating laboratory compliance to of this the ISO/IEC Guide 25 IEC 17025 took the place of EN carry out specific calibrations or (General requirements on the com- 45001 and ISO/IEC Guide 25 test; and assist in the development petence of testing and calibration which eliminated any formal differ- and implementation of a labora- laboratories, December 1990) is ences. tory’s quality system. * recognized. In content, EN 45001

III.5.c Calibration Quantities Spectral Irradiance optical unit used to calibrate optical length range from 250 to 1100 nm length may be all that’s required W cm-2 nm-1 radiation sources. Calibration is at a set increment. Or a GaAsP for some applications. Calibrations Irradiance (W/m2) measured as a normally performed with a spectral photodiode from 250 to 700 nm. are performed by transfer compari- function of wavelength (nm), is measurement system or spectrora- The increment setting could span son and certified against qualified known as spectral irradiance. This diometer equipped with a radiance from 1 to 50 nm depending on the reference standards. type of source calibration is per- lens assembly that has been cali- required resolution. Also, single formed with a spectral measure- brated with an integrating sphere point response at a particular wave- ment device or spectroradiometer based radiance standard. The spec- as compared to a reference stan- tral range of calibration will de- dard. The spectral range of calibra- pend on the source and the spectral 1 tion depends on the source and range of interest. A full spectral spectral zone of interest. A typical scan may cover from 350 to 2500 QH lamp may be spectrally nm. scanned from 200 to 2500 nm using a fixed wavelength increment Spectral Responsivity or variable bandwidth depending Optical radiation detectors, photo- 0,5 on the required resolution. detectors, photodiodes, exhibit changes in sensitivity at different responsivity rel. Spectral Radiance wavelengths. This spectral respon- W cm-2 sr-1 nm-1 sivity can be measured as relative Radiance (W/cm2·sr) measured as responsivity in percent (%) versus function of wavelength is called wavelength (nm) across the active 0 spectral radiance. Radiance in a wavelength bandpass of the photo- 800 925 1025 1125 1250 1375 1500 1625 1750 given direction, at a given point of device. For example a silicon de- wavelength ( nm ) a real or imaginary surface is the vice scan could cover the wave- Fig. III.18. InGaAs Detector with sphere. Relative Spectral Response Plot Page 176 - Tutorials Measurement of Light

Illuminance Sensitivity detector spectral response does not to a narrow angle so that the detec- Irradiance Sensitivity lux / foot-candles match the CIE photopic curve too a tion area is overfilled with a sample W/m2 & W/cm2 Calibration of the illuminance high degree, measurement errors of the uniform luminance field. Broadband irradiance detector response of photopic detectors is will occur when measuring differ- Luminance detectors are calibrated calibrations are performed by trans- normally performed as a transfer ent type sources. to measure in the optical units of fer comparison to reference stan- comparison from a photopic refer- candela per square meter and foot- dards with consideration to the ence standard. The photometric Luminance Sensitivity lamberts. spectral characteristics of the detec- responsivity of the reference stan- cd/m2 & fL tor to be tested. Reference detectors dard can be qualified through ra- Luminance responsivity of pho- Color Sensitivity are calibrated against spectral diometric measurement using red, topic detectors equipped with field Broadband colorimetric detectors are irradiance measurements using a blue and green filtered photodetec- limiting input optics is accom- calibrated by comparison to reference double monochromator spectrora- tors. However this is a complicated plished by comparison to a lumi- standards based on CIE tristimulus values using a light source of known diometer, itself calibrated using procedure left to advance radiome- nance reference standard detector. color temperature. Color temperature, traceable spectral irradiance stan- try labs. Illuminance sensitivity A uniform field of luminance is luminance and illuminance calibrations dards. Irradiance detectors are calibrations allow direct reading of produced as the calibration source may be included depending on the color calibrated to read out in the optical the photopically corrected detector using an integrating sphere or a meters capability. The color meter is units of watts per square meter or in lux or foot-candles. Very often a source with an optically diffuse calibrated to display the color chroma- watts per square centimeter over a tungsten source is used for illumi- material in front of it. Luminance ticity coordinates x,y and /or u’, v’ of specific wavelength range. nance calibrations. If the photopic detector’s field of view is confined the light source under test.


0,9 1 x short 0,8 1,5

0,7 x long

0,6 y

0,5 1 z 0,5 0,4 rel. responsivity rel. responsivity 0,3 rel. sensitivity 0,5 0,2

0,1 0 0,0 380 430 480 530 580 630 680 730 780 0 300 320 340 360 380 400 420 440 460 480 500 520 540 wavelength (nm) 380 430 480 530 580 630 680 730 780 wavelength ( nm ) wavelength ( nm ) Fig. III.21. BLUE Spectral Response Irradi- Fig. III.19. CIE Scotopic and Photopic Function Fig. III.20. Color Detector Tristimulus Functions ance Detector Spectral Reflectance tometer measures the normal Spectral Transmittance regular reflectance component only. Calibration of the spectral reflec- specular reflectance component Calibration of the spectral transmit- Calibration is performed as percent tance of materials is accomplished only. tance of materials is accomplished transmission versus wavelength. by comparison to reference reflec- by comparison to reference trans- tance standards which themselves mission standards which themselves are used to set-up calibration of the are used to set-up calibration of the spectrophotometric instrument spectrophotometric instrument which actually performs the meas- which actually performs the meas- urement. Single or double beam urement. Single or double beam spectrophotometers can spectrally spectrophotometers can spectrally range from 250 to 2500 nm, with range from 250 to 2500 nm, with adjustable wavelength increments. adjustable wavelength increments. When coupled to an integrating When coupled to an integrating sphere; total hemispherical, diffuse sphere; total hemispherical, diffuse and specular reflectance can be and regular (specular) transmittance separately measured with the spec- can be separately measured with the trophotometer. Without the sphere spectrophotometer. Without the the in-line set-up of the spectropho- Fig. III.22. 8 Degree Reflectance sphere the in-line set-up of the Fig. III.23. Transmission Meas- Measurement Set-up spectrophotometer measures the urement Set-up

III.5.d Calibration Standards Calibration standards are employed be used for both, photometric and dependent on the calibration hierar- to generate an input quantity for the Photometric – Radiometric Quan- radiometric calibration, if the cali- chy of the standard. equipment used in the calibration. tity: bration data is available. Since the calibration transfer is a Since the calibration standard Luminous Flux – Radiant Power For very precise or close tolerance real hardware transfer of the stan- supplies a signal of known quan- Luminous Intensity – Radiant calibrations specially selected dard itself, careful handling and tity, the difference of the output Intensity calibration standards are needed. operation of the calibration stan- signal of the equipment to the Luminance – Radiance Typically calibration standards are dard is extremely important. signal of the calibration standard Illuminance – Irradiance used as transfer standards, meaning In imaging applications the uni- can be evaluated. From these dif- Equivalent photometric and radio- they transfer the values of the formity of response or transmit- ferences calibration correction metric quantities exist where the primary standard to a lab standard tance is critical. So light sources factors can be calculated allowing measurement and therefore calibra- for subsequent transfer to a device with a uniform luminous area are absolute readings when the output tion geometry is the same. The under test. For traceable calibra- needed to determine the non- signal is corrected by these values. only difference is the radiometric tions an unbroken chain of transfer uniformity of a lens system or For photometric and radiometric or photometric responsivity of the comparisons back to the national visual imaging detection system measurement quantities different detection system. This means most primary standard is certified. Cali- calibration standards are needed: calibration reference standards can bration uncertainty is of course Tutorials - Page 177 Measurement of Light

Source Based Standards Every optical radiation detection or since its emission spectrum is close if changes in the spectral emission radiance meters with a limited measurement system needs to be to a Planckian radiator (blackbody characteristics are not critical. field-of-view, a diffuse transmitting calibrated in reference to an optical source). Other sources where opti- If the tungsten halogen lamp is screen can be used at the sphere radiation source. There are two cal radiation is produced by means used as a spot source the exact output. In imaging applications the possible ways to handle the calibra- of an element heated to incandes- location on the filament used du r- uniformity and the diffuse function tion: cence by electrical current are also ing the calibration of the source, of currently available screen mate- • The source may be calibrated in use. Filament position and stabil- must be subsequently used. rials are not precise enough so the in the required quantity and ity of the tungsten halogen lamp is In luminance, radiance and imag- open output port of the sphere is the difference between the critical plus it has a limited life- ing uniformity calibrations, tung- typically used. input signal, generated by the time. Therefore the lamp should sten halogen lamps must be fitted If the intensity of the tungsten source, and the output signal only be operated in the position with a diffuse screen or pla- halogen lamp is not high enough, of the detection system can be specified in the calibration certifi- ced within an integrating which happens especially in the determined using the calibra- cate. Comparing the lamp output sphere. Sphere based luminance UV range, arc lamps such as xenon tion data of the source against other in-house reference and radiance standards offer higher lamps may be used. But the in- • The uncalibrated source may standard sources or against suitable uniformity and a better diffuse crease in intensity can affect cali- be operated under stable reference detection systems must function. bration uncertainty. conditions and the calibration be periodically done to check For calibrating luminance and done by comparing the read- stated lamp calibration uncertainty. ing of the detection system In radiometric applications, where with a calibrated detection the spectral characteristic of the system (reference standard). source is used, the source should be The reference standard must operated in current controlled have the same measurement mode, to ensure the stability of the geometry and the same spec- spectral characteristic. The mini- tral response as the unit to be mum specification for current calibrated. stability should be held at 10-4 A. In photometric applications or The most common optical radiation radiometric applications with source used for calibration stan- broadband detectors, intensity dards is the tungsten halogen lamp controllable standards can be used Fig. III.24. Diffuse Screen & Integrating Sphere Detector Based Standards Due to the high levels of mainte- radiometric applications such as the mostly limited to photometric tions a monochromatic radiation nance and care required to operate calibration of laser power meters applications where detectors with a source is needed. If a detector‘s optical radiation source based for telecommunication testing. But precise filter corrected photometric spectral responsivity is to be meas- calibration standards, detector because of surface reflections, spectral response are available. ured over its entire active bandpass, based standards are very attractive polarization effects, beam mis- Calibration is performed by com- a tungsten halogen lamp with a alternatives. The detector has the alignment and beam ‘bounce-back’ parison of the output signal of the monchromator attached to it can be advantage of long-term absolute errors, the use of detector based reference detector to that of the used to create monochromatic and mechanical stability, especially calibration standards must be care- device to be calibrated. The same radiation at all of the different true of semiconductor detectors. fully considered. stable source of optical radiation is required wavelengths. The use of detector-based standards The use of spectrally broadband used during the calibration proce- is very common in monochromatic detectors as calibration standards is dure. For monochromatic calibra- Spectral Irradiance Standards Tungsten Halogen lamps are the ceramic or other material which ‘workhorse’ of spectral irradiance ensures long-term stability and standards. protection. Filament targeting aids 1,0 FEL type lamps with a filament may be provided for best measure- support arm are recommended for ment accuracy in the calibration high intensity BLUE light and UV laboratory. applications. Since lifetime is somewhat limited,

Sylvania calibration grade lamps lamp power supplies with on/off 0,5

are recommended for visible and ramping functions are recom- rel irradiance near IR applications where the best mended for use with these sources. long term stability is required. In order to qualify for calibration as a standard, each lamp must un- dergo a minimum 15-hour burn-in 0,0 procedure and must display a satis- 250 500 750 1000 1250 1500 1750 2000 2250 2500 factory burn-in data trend during wavelength ( nm ) this period. The calibrated tungsten Fig. III.26. Lamp Spectral Distribution reference source provides spectral irradiance data from 250 to 2500 nm covering many typical UV-Vis- IR radiometric and photometric applications. The lamp is normally provided in a housing and socket made from Fig. III.25. FEL Calibration Standard Lamp Page 178 - Tutorials Measurement of Light

Luminance Standards Reference sources of luminance are chined from optically diffuse plas- used to calibrate the uniformity of tics. Seasoned tungsten halogen 1,01 imaging systems and the luminance sources are typically used with output of luminance meters, spot lamp power supplies and tempera- meters and other photo- ture stabilized photometric refer- metric equipment. ence detectors to form the complete 1,00 The standard is constructed around system. Control feedback loop the integrating sphere of various techniques control the luminance

diameters which provide the highly output intensity and help prolong luminance rel uniform diffuse luminance at the the useable lifetime of the system. 0,99 exit port required for these types of Any change in ambient and sphere calibrations. The spheres may be body temperature affecting the coated with barium sulfate or ma- output signal is eliminated through the temperature stabilized reference 0,98 detector. This also reduces system -50 -40 -30 -20 -10 0 10 20 30 40 50 warm-up time. luminous area diameter ( mm ) An optimally designed sphere layout is capable of less than ± Fig. III.28. Typical Luminance Uniformity Response Plot 0.7% non-uniformity over 90% the port opening which can be as large as 100 mm in diameter. Angular dards may offer a variable lumi- tion facility. Luminance output, uniformity of less than ± 5% within nance output requiring more so- uniformity and angular uniformity ±40° enables luminance output phisticated electronics, multiple must be measured and certified. calibration of detection systems lamps and exhaust fans. with wide acceptance angles. Lu- In order to qualify as a calibration minance outputs can range from standard the system itself must be Fig. III.27. Luminance Standard 0.5 to 35000 cd/m2. Some stan- calibrated by a competent calibra- Spectral Radiance Standards Reference sources of radiance are calibrations. form the complete system. Control stabilized reference detector. This used to calibrate radiance detectors The spheres may be coated with feedback loop techniques control also reduces system warm-up time. and other radiometric equipment. barium sulfate or machined from the luminance output intensity and An optimally designed sphere The standard is constructed around optically diffuse plastics. Seasoned help prolong the useable lifetime of layout is capable of less than ± the integrating sphere of various tungsten halogen sources are typi- the system. Any change in ambient 0.7% non-uniformity over 90% the diameters which provide the highly cally used with lamp power sup- and sphere body temperature af- port opening which can be as large uniform diffuse radiance at the exit plies and temperature stabilized fecting the output signal is elimi- as 100 mm in diameter. Angular port required for these types of photometric reference detectors to nated through the temperature uniformity of less than ± 5% within ±40° enables luminance output calibration of detection systems

100 with wide acceptance angles. Some standards may offer a variable 80 radiance output requiring more sophisticated electronics, multiple 60 lamps and exhaust fans. In order to qualify as a calibration 40 standard the system itself must be calibrated by a competent 20

spectral radiance (mW/(m2/srnm) radiance spectral calibration facility. Radiance out-

0 put, uniformity and angular uni- 380 430 480 530 580 630 680 730 780 formity must be measured and wavelength ( nm ) certified.

Fig. III.29. Variable Radiance Standard Fig. III.30. Typical Spectral Radiance Plot

Spectral Responsivity Standards

Due to their long term stability and characteristics during test sessions. Fig. III.31. Detector Calibration Standard broad spectral coverage, silicon Or to ensure best measurement photodiodes are used as reference uncertainty, temperature stabiliza- spectral standards by national and tion using cooling jackets that 1,0 private calibration laboratories maintain the device temperature to worldwide. within ± 0.5°C are employed. These photodiodes with active UV enhanced Si devices offer 0,5 areas as large as 100 mm2, are spectral coverage from 250 to

mounted into machined housings to >1100 nm. Calibration with certifi- rel. responsivity

protect and precisely fix the detec- cation from an accredited traceable 0,0

tor in a calibration set-up in concert calibration facility is required to 250 350 450 550 650 750 850 950 1050 1150

with targeting aids. Some housings qualify the device for use as a wavelength ( nm ) may include an integral tempera- reference standard. ture sensor to monitor thermal Fig. Tutorials - Page 179 Measurement of Light

Reflectance Standards White optically diffuse reflectance tance over the spectral range of standards traceably calibrated for interest. Processed PTFE machined spectral reflectance over a spectral and cut into various shapes and range from 250 to 2500 nm are thicknesses is currently used for used in the calibration of reflec- reflectance standards. High reflec- tance meters, optical distance tance white, black and gray shades measurement systems, densitome- at varying reflectance values are ters, spectrophotometers and other available. optical and imaging systems. To maintain the quality of a cali- Qualifications of a reflectance brated standard it is normally standard include light and tempera- mounted into a protective housing ture stability and durability, near with a removable lid to keep the Lambertian diffuse reflectance and material clean and covered when up to 98% spectrally neutral reflec- not in use.

IV. Detector Signal Measurement A typical light detector or photo- are used with colorimetric detectors tion of the device can be accom- display, digital interface, datalog- active device converts impinging to display multiple quantities. The plished through a logical menu ging) separate the different models. photons into a current or voltage optometer is a term used to indicate structure with user input via front- Usually the application determines proportional to the incoming sig- that the meter can be used with panel keyboard or through com- what specific capabilities are im- nal. either radiometric or photometric puter control via RS232 or IEEE portant to have in the radiometer The detector connects to an elec- type detector heads. Microproces- computer interface. system. For example, in a UV tronic meter for amplification, sor controlled units capable of The quantity or optical unit meas- curing production process where possible conversion from an analog measuring currents down to tenths ured will depend on the detector multiple stations must be moni- to digital signal (ADC), calibration of picoamperes up to a few milli- type, how it’s configured in the tored, a multi-channel radiometer and display of the measurement amperes are available. This allows way of filtering and input optic, with settable min/max reading result. Together, the meter, full utilization of the sensitivity and its calibration. Radiometers are feature, RS232 or IEEE interface photodetector and accessory com- range of most photosensitive de- available in hand-held mobile or and remote multiplexed detectors ponents form an optometer, radi- vices. Measurement methodology bench-top models for laboratory would be desirable. ometer, , color, laser or might employ 16-bit signal digiti- use. Self-contained cordless models The following is a list of various optical power meter and reflection/ zation by means of an analog to are used for remote dynamic moni- features, modes of operation and transmission measurement systems. digital converter (A/D) with sam- toring where a standard detector specifications offered in current A radiometer consists of a voltage pling rates in the microseconds. that connects to the meter via a light meters. Note that available or current meter coupled with a Selectable averaging calculation of cable might foul. Capabilities such features and functions will vary radiometric type detector. Pho- the sampled results from microsec- as dynamic measurement range, depending on the type of meter and tometers employ the same meters onds to seconds provides more operating modes (example: CW, manufacturer. used with photometric type detec- measurement flexibility for fast dose, pulse energy) and features tors. Multi-channel color meters events or low-level signals. Opera- (example: auto-ranging, backlit Operating Modes & Features:

CW: Continuous wave is a run of continuous type measurements. The Color: Chromaticity coordinates x,y and u‘,v‘ and the correlated color measurement frequency depends on the integrating time and the max. temperature are calculated from the ratio of the detector‘s signals. sample rate. of the meter. Peak Maximum: Each CW measurement interval consists of a certain CW Min/Max: CW measurement where the min. or max. value that oc- number of samples (number depends on integration time and sampling curred during the measurement run will be displayed. The min. or max. rate). Peak Maximum is the most positive sample of one measurement value can be reset with the RESET switch. interval. A new Peak Maximum is calculated and displayed for each meas- CW Level Check: CW measurement where the measurement values are urement interval. compared against min.-max. threshold values. The threshold values are Peak Minimum: Each CW measurement interval consists of a certain entered into the meter by the user. number of samples (number depends on integration time and sampling CW Level Minimium / Maximum: Menu to adjust the threshold values rate). Peak Minimum is the most negative sample of one measurement for CW Level Check. interval. A new Peak Minimum is calculated and displayed for each meas- Run/Hold: To freeze a measurement value on the display and stop the urement interval. continuous measurement. Peak to Peak: Each CW measurement interval consists of a certain num- Relative Ratio (%): Measurement value as the relative ratio of a reference ber of samples (number depends on integration time and sampling rate). value (stored in the optometer) or a reference measurement value (2- Peak to Peak is the difference of the most positive to the most negative channel optometer required). sample of one measurement interval. A new Peak to Peak value is calcu- Relative Ratio Factor: Measurement value as the relative ratio factor of a lated and displayed for each measurement interval. reference value (stored in the optometer) or a reference measurement value I-Effective: Measures and calculates the energy of light pulses based on (2-channel optometer required). the Schmidt-Clausen formula. The input signal is sampled with the max. Attenuation (dB or dBm): Measurement value as the logarithmic ratio sampling rate for one measurement interval (Pulse Measurement Time). factor (attenuation) of a reference value e.g. dBm (stored in the optometer) First the pulse-energy is calculated by integrating the samples. I-Effective or a reference measurement value e.g. dB (2-channel optometer required). is calculated by using the pulse-energy and the peak-value of the measure- Dose: CW measurement values integrated over the dose measurement time. ment interval using the following formula: A preset dose measurement time or a max. dose value will stop the meas- I-effective = peak-value * pulse-energy / (peak-value * C + pulse-energy) urement. C = IF-Time Constant (between 0.1s and 0.2s, depending on application) Data Logger: Each measurement value of a CW measurement will be IF Time Constant: Factor C for calculation of I-Effective (Schmidt- stored individually in the optometer‘s memory. Each measurement may be Clausen). manually or automatically initiated by a preset measurement cycle time. Pulse Energy: Measures and calculates the energy of light pulses. The Measurement data can be outputted through computer interface. input signal is sampled with the max. sampling rate for one measurement Page 180 - Tutorials Measurement of Light

interval (Pulse Measurement Time). The energy is calculated by integrating Manual Range: with autorange disabled, the measurement range can be these samples. manually fixed to a certain value. The device is not allowed to switch meas- Pulse Measurement-Time: Measurement interval for I-Effective and urement ranges automatically. Manual range adjustment can be useful in Pulse Energy measurements. cases where input signals are changing rapidly. Remote RS232: enables RS232 interface of the device. RS232 is a stan- Calibration Factor: Optical sensors transform optical signals into current. dard for Asynchronous Transfer between computer equipment and accesso- This current is measured by the device. The calibration factor determines the ries. Data is transmitted bit by bit in a serial fashion. The RS232 standard relationship between the measured current and the calculated and displayed defines the function and use of all 25 pins of a DB-25 type connector. measurement result (optical signal). Minimally configured, 3 pins (of a DB-9 type connector) are used, namely: Offset: The Offset value is subtracted from the measured signal to calculate Ground, Transmit Data and Receive Data. On PCs, the RS-232 ports la- the result. Offset can be set to zero or to the measured CW-value. Offset is beled as "serial" or "asynch" and are either 9 or 25 pin male type. useful to compensate for the influence of ambient light or if the measure- Remote IEEE488: Interface IEEE488 of the device enabled. IEEE488 is a ment value is very small relative to the adjusted measurement range. standard for Parallel Transfer between computer equipment and measure- Integration Time: Time period for which the input signal is sampled and ment instruments. Data is transmitted in parallel fashion (max. speed the average value of the sampled values is calculated (>CW). Integration time 1MByte/s). Up to 31 devices (with different addresses) can be connected to should be selected carefully. For example, if multiples of 20 ms (50 Hz) are one computer system. selected as the integration interval, errors produced by the influence of a USB: a communication standard that supports serial data transfers between 50Hz AC power line can be minimized. a USB host computer and USB-capable peripherals. USB specifications Sampling Rate: The rate which specifies how often the input signal is meas- define a signaling rate of 12 Mbs for full-speed mode. Theoretically 127 ured (sampled). The CW-value is calculated using the average value of all USB-capable peripherals are allowed to be connected to one USB host samples of one measurement interval (integration time). A sampling rate of computer. The connected devices may be powered by the host computer. 100ms means that 10000 samples per second are taken. If the measurement Auto Range: when activated, the measurement range is switched by the interval (integration time) is 0.5 s, there are 5000 samples used to get the CW device automatically to the optimal value (depending on the input signal). value.

Specifications: Slew-rate: how fast a signal changes. For example, a rate of 5 Volt/ms Some errors cannot be compensated because they are produced by the means that the signal changes with a value of 5 Volts every ms. nonlinearity of the ADC (Analog Digital Converter) and the display resolu- Rise-time: Time needed for a signal to change from 10% to 90% of its tion. final value. Maximum Detector Capacitance: The input current-to-voltage ampflifier Fall-time: Time needed for a signal to change from 90% to 10% of its is sensitive to input capacitance. If the input capacitance is too large, the start value. amplifier may oscillate. The maximum detector capacitance is the largest Input Ranges / Measurement Range: To achieve a dynamic measure- value of capacitance for which the amplifier will remain out of oscillation. ment capability greater than six decades, different levels of measurement Measurement Range: The measurement range is typically specified by the ranges (Gains) for the “current to voltage input amplifier” are necessary. resolution and the max. reading value. But the user should note that for a Gains can span from 1V/10pA to 1V/1mA (depending on the device). measurement with a max. measurement uncertainty of 1%, the min. meas- Linearity: The linearity of an optometer can be described as follows: urement value should be a factor of 100X higher than the resolution. On Reading a value of 10nA, with a max. gain error of 1%, the possible error the other hand, the max. value may be limited by the detector specifica- is +/-0.1nA. Together with an additional offset error of 0.05nA, the total tions such as max. irradiation density, max. operation temperature, detector measurement uncertainty would be 10nA +/-0.15nA or 1.5%. saturation limits, etc., and therefore the manufacturer‘s recommended At a reading of only 1nA in the same gain range, the gain error would be measurement values should be adhered to. 1% of 1nA or 0.01nA. The offset error would still be 0.05%. The total measurement uncertainty would be 1nA +/-0.06nA or 6%. The offset error is minimal with our optometers since these meters offer an internal offset compensation or allow an offset zero setting from the menu. Here the only offset error is from the display resolution or the nonlinearity of the analog- digital converter (ADC). Measurement Accuracy / Linearity: The max. possible error of a meas- urement result can be calculated as follows: Total Error: Gain Error + Offset Error Gain Error: Displayed (or readout) result X (Gain Error (in percent) / 100 ) Offset Error: Constant value depending on measurement range The Offset Error can be nearly eliminated be using offset compensation. Fig. IV.1. Radiometer Schematic Tutorials - Page 181 Integrating Spheres

V. Theory and applications of integrating spheres Integrating spheres are very versa- surface is proportional to the tile optical elements, which are total radiant power either emitted lamp designed to achieve homogenous by a source inside the sphere or distribution of optical radiation by entering the sphere through its means of multiple Lambertian entrance port. Geometrical and baffle reflections at the sphere’s inner directional distribution of the large field-of-view surface. The primary radiation primary source’s radiation do not source can be located either inside influence irradiance levels as the sphere or in front of the long as direct illumination of the baffle source’s entrance port. In the latter respective location is prevented. case, only the optical radiation This property becomes especially lamp entering the sphere is relevant for important when an integrating the sphere’s internal radiation sphere is used as the input optical distribution. element of a detector for radiant As long as we restrict ourselves to power (see § III.1.b). Fig. V.1. Integrating sphere used as a standard source for optical radia- those regions which are shielded tion. Multiple Lambertian reflections inside the sphere result in a homoge- from direct irradiation by the pri- • Radiance reflected by a region of nous radiance and exitance distributions at the sphere’s exit port. mary source and are thus only the sphere’s inner surface illuminated by reflections at other shielded from direct illumination sphere’s exit port can be used as distributions. This property be- of the inner surface, the theory of is constant in its directional an ideal Lambertian source as comes especially important when the ideal integrating sphere leads to distribution and independent optical radiation leaving the a sphere is used as a standard two important conclusions: from the specific location where sphere is characterised by ho- calibration source. • Irradiance of the sphere’s inner the reflection occurs. Thus, the mogenous radiance and exitance

V.1. Theory of the ideal integrating sphere The ideal integrating sphere, a ... denote the irradiance caused by and is further related to the ele- by theoretical construction which light from the source after one, ment’s direct irradiance E0 by cos2 (ε )dA ' allows the explanation of the two, ... reflections. Total irradiance LdAdLdAcos(ε )Ω= ' d 2 sphere’s basic principle of opera- is then given by the infinite sum M1 = ρ E0 tion, is characterised by the follow- and dividing this expression results ing properties: Etotal = E0 + E1 + E2 + ... whereby ρ denotes the total reflec- in the (infinitesimal) irradiance dE1 • Its entrance and exit ports are tance of the sphere’s inner surface. of the sphere’s inner surface at the infinitesimally small. For convenience, the index „e“, location of dA’ which is caused by • All objects inside the sphere, denoting radiometric quantities, is As a consequence, a single reflection of direct radia- omitted. However, if the reflec- tion from the source at the area light sources and baffles, are also ρ E ρ = 0 Equ.V.1 element dA: infinitesimally small and their tance of the sphere’s coating L π influence on optical radiation material is independent from wave- cos2 (ε ) E ρ 1 ==0 after its first reflection at the length, the derived relations also dE1 L 2 dA 2 dA Although L does not depend on the d π 4 R sphere’s inner surface can be hold true for photometric quanti- direction relative to the surface neglected. ties, which would be denoted by element dA, it still depends on the using Equ. V.1 and the relation d = 2 R ε • Its inner surface is a perfectly the index „v“. location at the sphere’s inside, cos( ), which can be easily seen from Fig.V.2 homogenous Lambertian reflec- Lets consider an ideal integrating which is a consequence of the ρ sphere of radius R, consisting of a tor, and its reflectance is inde- generally irregular direct illumina- In order to obtain the irradiance E hollow perfect Lambertian reflector 1 pendent from wavelength. For a tion by the light source. at the location of dA’, caused by a with infinitesimally small entrance more detailed discussion of If we want to calculate the radiant single reflection of the source’s and exit ports. An inhomogeneous reflective materials largely ful- power emitted by the area element radiation at the whole inner surface radiation source produces direct filling these properties, see § dA and impinging upon another of the sphere, the above expression irradiance levels E (the term II.5.c. and III.1.a. 0 area element dA’, we have to cal- for dE has to be integrated over „direct irradiance“ refers to the fact 1 culate the solid angle of dA’, as the sphere’s inner surface: that E is directly caused by the During the following considera- 0 seen from dA (Fig.V.2). As dA’ is Formula A tions, the symbol index describes source without any reflections) tilted by an angle ε relative to the whereby Φ0 denotes the total radi- the order of reflection. So, E0 de- which depend on the respective line of sight between the two area location at the sphere’s inner sur- ant power emitted by the source notes the irradiance caused directly elements, dA’ occupies the solid and impinging upon the sphere’s face (Fig.V.2). As a first step, we Ω by the light source, whereas E1, E2, angle d ’, as seens from dA: inner surface. want to calculate the irradiance E1 cos(ε )dA ' Note that the irradiance of the inner of the sphere’s inner surface, pro- Ω = d ' 2 surface after the first reflection is duced by the radiance L after the d 1 independent from the actual loca- first reflection. Due to the Lamber- with d denoting the distance be- tian reflection property of the tion on the sphere, which holds true E0 tween dA and dA’. sphere’s material, the radiation despite the inhomogeneous direct irradiance caused by direct illumi- reflected by a certain area element According to Equ.II.3 in paragraph dA is characterised by a constant nation from the source. Deriving II.4.d., the radiant power emitted the irradiance E caused by the R directional radiance distribution L. Ω 2 by dA into the solid angle d ’ and source’s radiation after two reflec- dA dA` According to thus impinging upon dA’ is given Equ. II.7 in paragraph II.5.c, the d E ρ 1 ρ ρ Φ area element’s exitance M1 is re- A: E ===0 dA EdA 0 1 ““π 44R 220π R 4π R 2 lated to the reflected radiance L1 by inner inner surface surface E ρ 1 ρ ρ ρ2 Φ π B: E ===⋅==1 dA EdA ERE4 πρ2 0 Fig.V.2. Geometry of an ideal M1 = L1 2 ““π 22121π π 1 π 2 inner 44R R inner 4R 4 R integrating sphere of radius R. surface surface Page 182 - Tutorials Integrating Spheres

tions in the same way, we get In this expression, only E0 actually ance is proportional to the total reflecting sphere, it is called depends on the respective location amount of radiant power Φ0 reach- „sphere multiplier“ and, for an Formula B on the sphere’s inner surface. As a ing the sphere’s inner surface di- ideal sphere, solely depends on the consequence, Etotal is independent rectly from the source: coating material’s reflectance ρ. Generally, the irradiance of the from the actual location of the Φ ρ Φ sphere’s inner surface caused by sphere’s inner surface as long as E =⋅00=⋅K total − ρ the source’s radiation after k reflec- we assure that E0 = 0 at this loca- AAsphere1 sphere tions is given by tion. This means that no direct radiation from the source reaches ρk Φ As the constant K describes the E = 0 the location, which can be obtained enhancement of irradiance relative k 4 π R2 by baffles. In this case, total irradi- to the average irradiance of a non- ∞ ρk ΦΦ∞ Φρ and the total irradiance is thus Formula C: EEEE=+++=+... E 0 =+E 0 ρk =+E 0 ⋅ total 012 0ƒƒπ 2 0 0 − ρ given by Formula C k=0 41R Asphere k=0 Asphere

V.2. Real integrating spheres Due to the simplifications assumed area of the respective port. As a strong wavelength dependency of traditional coating material Barium for an ideal integrating sphere, the result, these ports might consid- the sphere multiplier. sulphate (BaSO4) ages significantly relations derived in paragraph IV.1 erably reduce the amount of light when exposed to UV radiation. cannot be directly used in practical reflected at the sphere’s inner • Objects inside the sphere, for Optically diffuse material applications. Instead, they have to surface, which can be accounted example the light source itself, (OP.DI.MA) is an optical grade be altered for the following rea- for by a modified sphere multi- cannot generally be neglected in plastic especially designed to work sons: plier: their influence on the reflected as a volume reflector. It has been • The reflectance ρ might depend Φ ρ Φ optical radiation. A possible designed to replace barium sulfate =⋅00=⋅ as a coating for integrating spheres on wavelength. This results in a Etotal K solution is the determination of A []11−−ρ ()a A wavelength dependent sphere sphere sphere the light source’s influence by in UV and high temperature appli- multiplier K and thus in a spec- means of an auxiliary lamp (see cations. Its reflective properties tral distortion of the primary with paragraph III.1.b). depend on its thickness, generally source’s output. Thus, the rela- ρ • specified at 10 mm, which is the K = Baffles inside the sphere and recommended minimum thickness tions for the ideal sphere, which []11−−ρ ()a deviations of the coating mate- for lighting engineering. have been formulated for radio- rial’s reflectance properties from metric quantities, can no longer In these relations, a denotes the Apart from its temporal stability, relative share of the area of all perfect Lambertian reflection OP.DI.MA offers additional advan- directly be applied. Instead, the cause further deviations of the sphere’s behaviour for mono- ports and other non-reflecting areas tages. Using different additives, its on the sphere’s total inner surface: sphere’s behaviour from the reflection factor can be adjusted to chromatic radiation has to be relations derived in chapter 0. determined by the respective any value between 3% (deep black) sum of all non− reflecting areas Their influence can only be relations for spectral radiometric a = and 99% (brilliant white), whereby A simulated by numerical Monte uniform reflectance over a wide quantities. If desired, radiometric sphere Carlo simulations, which basi- quantities describing the sphere’s spectral range and over large geo- below shows the dependence of cally use ray tracing techniques metrical areas can be achieved. radiation output can be deter- to follow the paths of a large mined by subsequent wavelength the sphere multiplier on reflectance Like other plastics, it can be proc- ρ number of individual photons. integration of the respective for different values of a. It can be essed by turning, drilling, sawing spectral radiometric quantities. clearly seen that even a small varia- and milling and is available in raw tion of reflectance might cause Apart from these factors, integrat- blocks, plates and foils in various • Intensity considerations pose a significant change in the sphere ing spheres are also subject to sizes for this purpose. lower limit for the size of the multiplier. temporal variations of their optical entrance and exit ports, as the properties, which are primarily radiant power entering or exiting For this reason, a slight wavelength caused by degradation of their a sphere is proportional to the coating material. Especially the dependency of ρ may result in a


80 a=1% 60 a=3% 40 a=5% 20 a=10%

Sphere Multiplier 0 0,8 0,82 0,84 0,86 0,88 0,9 0,92 0,94 0,96 0,98 1 Reflectance

Fig. V.3. Dependence of the sphere multiplier K on reflectance ρ for different values of the share of non-reflecting areas on the sphere’s total inner surface. Tutorials - Page 183 Light Measurement Applications

VI. Applications for Light Measurement in Medicine, Technology, Industry and Environmental Science For most technical applications of measurement instruments. Many of States or the Physikalisch- based on customer requirements. light, authorities like the Interna- these instruments must be specially Technische Bundesanstalt (PTB) in Gigahertz Optik’s accredited cali- tional Commission on Illumination designed and manufactured for the Germany. bration facility provides accurate, (CIE) or the Deutsche Industrienor- specific application. Moreover, Gigahertz Optik not only offers a state of the art absolute calibration men (DIN) have developed well- these instruments must be cali- wide variety of absolutely cali- of instruments and secondary stan- defined standards regarding its brated against national standardiza- brated light detectors, but also dard light sources (see § III.5). measurement. In virtually all areas tion authorities, such as the Na- offers its experience in light meas- connected with light, there is a tional Institute of Standards and urement technology for the devel- strong demand for high quality Technology (NIST) in the United opment of specialized solutions VI.1. Phototherapy and Radiation Protection 85% of all sensory perceptions are controlled through accurate meas- vices like sun creams, UV blocking Network for Ultraviolet Measure- optical in origin but optical radia- urements. These measurements are fabrics and sunglasses are the sub- ments“ funded by the Standards, tion is not only involved in the typically performed with a spec- jects of study. Measurements and Testing pro- process of human vision, it has trally and spatially qualified UV-A, Photobiologists, industrial hygien- gram of the Commission of the many other biological effects as UV-B and UV-B311 radiometer. ists, health and safety officers European Communities, the detec- well. However, optical radiation also measure UV irradiance (W/m2) and tor and instrument designs are at poses a potential health hazard for irradiance dose (J/m2) of solar and the highest available level. The The photobiological effects of both human skin and eyes. For artificial light sources in the lab, CIE, Commission Internationale de optical radiation, especially in the example, overexposure to ultravio- field and in the work place in order l‘Eclairage, is reviewing many of ultraviolet and blue (400 to 500nm) let and blue ‘light’ can cause com- to study both the harmful and help- the concepts put forth by the Euro- spectral regions, can be therapeutic. mon sunburn, photokeratitis ful effects of light and establish pean Commission in an effort to For example, it is used in photo- (welder‘s eye) and burning of the safe guidelines for its use. It is internationally standardize the therapy to treat a variety of skin retina or cornea. important to note that UV levels evaluation of UV radiometric diseases and in postnatal treatment Because of the dramatic increase in and subject exposure times typi- measurement instrumentation much of Hyperbilirubinemia. For proper global UV radiation and the cumu- cally vary so datalogging over like the way photometric instru- dosimetry, irradiance (W/m2) and lative nature of the harmful effects, some time period is commonly ments are characterized now. irradiance dose (J/m2) delivered by the additional risk of UV exposure employed. UV sources in phototherapy proc- by artificial sources is a concern. Because Gigahertz-Optik is ac- esses need to be monitored and The efficiency of protective de- tively involved in the „Thematic

Incoherent Optical Radiation Protection Even though there are many wide for this are to be found in the rising ranging and highly positive effects exposure to radiation from of light there are also negative sunlight, particularly in the UV effects to consider. Naturally oc- range, and the growing use of high curring optical radiation, especially powered lamps in radiation ther- in the UV range of the solar spec- apy, radiation cosmetics, UV radia- trum, poses a potential health risk tion curing, UV sterilization, vehi- to outdoor workers and others who cle headlamps, lighting equipment, spend a significant amount of time etc. The high proportions of UV outdoors. The most serious long- and blue light in the emission spec- term consequence of UV exposure tra of these lamps can, in addition is the formation of malignant mela- to their desired effects, also result noma of the skin, a dangerous type in radiation damage through both of cancer. In the US, skin cancer is direct and indirect contact if the the most frequently contracted type maximum permitted exposure of cancer, and since the 1970s, the levels are exceeded. incidence rates of malignant mela- The shallow depth of penetration of noma have more then doubled. As optical radiation restricts the health a similar development can be found hazards primarily to the eye and Fig.VI.1. Incidence rates of malignant melanoma in the US since 1973. for other countries, national and skin. supranational networks of solar UV detectors have been established Health risks from incoherent optical radiation recently to monitor solar UV levels and the World Meteorological Organization is currently preparing guidelines for their characteriza- Skin Eye tion, calibration and maintenance.

In simple terms, incoherent optical radiation is optical radiation in the Acute effect Chronic effect Acute effect Chronic effect range of wavelengths between ø Erythema ø Skin cancer ø Photochemical and thermal ø Cataract development 100 nm and 1 mm, other than that ø Burning ø Skin ageing retinal damage ø Degeneration of the retina emitted by lasers. The effect of ø Photokeratitis incoherent optical radiation on the skin and the eye is being afforded increasing attention. The reasons Fig.VI.2. Optical Radiation Health Risks Page 184 - Tutorials Light Measurement Applications

Relevant Radiation Quantities

When evaluating the harm that ∞ the radiation sources Elimit, then the following conditions might be caused by incoherent should be maintained: L = L λ ()λ * s ()λ * dλ optical radiation, it is the effective biol “ e biol,rel Photobiologically effective expo- 0 2 radiance (or the time integral of the sure (dose, J/m ) Ebiol ≤ Elimit or Lbiol ≤ Llimit radiance) that is critical for the with Leλ(λ): spectral radiance of the retina, whereas for the skin, cornea t1 radiation sources H = E * dt If the exposure vaues are given as and lens of the eye the critical biol “ biol the time integral of the radiance Li 0 quantity is the effective irradiance Photobiologically effective irradi- or as the exposure (dose), H, then 2 λ (or the exposure, also known as the ance (W/m ) with s( )biol,rel stands for the rele- the maximum permissible exposure dose) that can, for instance, arise at vant spectral response functions of duration, t, can be calculated: a workplace or in some other loca- ∞ the skin and eye. tion where time is spent. E = E ()λ * s ()λ * dλ biol “ eλ biol,rel t = Li / Lbiol or t = H / Ebiol 0 If exposure limits are given in Photobiologically effective radi- guidelines as effective radiance, 2 with E λ(λ): spectral irradiance of ance (W/(m sr)) e Limit, or as effective irradiance,

ACGIH / ICNIRP Spectral Weighting Functions for Assessing UV Radiation Hazards The spectral weighting function for of wavelengths from 315 to 400 nm spectral weighting function) for UV-A rich sources. the acutely harmful effects of UV (UV-A) corresponds to a rectangle radiation, was developed by the function representing total UV-A. American Conference of Govern- Threshold Limit Values given for mental Industrial Hygienists the maximum permissible exposure (ACGIH) and the International of the skin define the range of Commission on Non-Ionising Ra- wavelengths as 200 (180) to diation Protection (ICNIRP) 400 nm in reference to the ACGIH- If one examines the spectral curve ICNIRP function. The limits of describing this function, it is seen maximum permissible exposure for that the spectral effectiveness in the the eye in the range 200 (180) to UV-C and UV-B ranges is very 400 nm and 315 to 400 nm (UV-A) high, and that it falls drastically in are defined separately. By defini- the UV-A range. The reason for tion ACGIH-ICNIRP UV-C/B is this is that the function is derived measured in effective irradiance from the functions relating the according to the spectrally radiation to erythema (skin redden- weighted function and the UV-A ing) and photokeratoconjunctivitis level is assessed by measurement (corneal inflammation). The range of the total UV-A irradiance (no Fig. VI.3. ACGIH Spectral Function

Blue-Light Hazard Photochemical Risks to the Retina If optical radiation with wave- radiance of the BLH function: -2 -1 lengths between 380 and 1400 nm LBLH * t ≤ 100 J*cm * sr for t ≤ of sufficient intensity reaches the 10.000s 1 -2 -1 retina it can cause photochemical LBLH ≤ 10 mW * cm * sr for t > and thermal injury. Radiation in the 10.000s "blue" part of the spectrum from LBLH = effective radiance 380 to 700 nm (effectively 380 to t =duration of exposure 550 nm) triggers photochemical 0,5

reactions, if the energy in The blue light hazard function rel. sensitivity the radiation is high enough, con- generally applies to exposure peri- verting chemically unstable mole- ods of more than 10 s. For shorter cules into one or more other mole- exposure times, the thermal retinal cule types. The spectral curve of injury function applies. the blue light hazard response 0 function is shown in the following 300 400 500 600 700 800 900 1000 1100 1200 diagram. ICNIRP 1997 gives the waverlength ( nm ) following limits for the effective Fig. VI.4. Blue Light Hazard Spectral Function

Thermal Injury to the Eye -(RTH - Retina Thermal Hazard) If the retina is exposed during short thus on the size of the image of the function by a factor of 10. Whereas text it is quite adequate to measure periods to high radiation intensi- radiation source on the retina. The the latter rapidly falls to zero above the range up to 1200 nm, since ties, a temperature rise to 45°C diagram above illustrates the spec- 500 nm, the thermal function con- various light sources exhibit no leads to hyperthermia, to 60°C tral response function for thermal tinues on to 1400 nm. Since no more than 4% difference in the causes coagulation, and to over damage to the retina according to industrially useable radiation sen- integrated totals to 1200 nm and to 100°C results in vaporization. ICNIRP. sors with spectral sensitivity from 1400 nm. This statement is also Removing the heat depends for the In the spectral range between 380 380 to 1400 nm exist, an accept- confirmed by ICNIRP in their most part on the capacity of the and 500 nm, the effect of the RTH able simulated match using silicon working paper /1/. irradiated zone to transfer heat, and function is larger than the BLH photodiodes is in use. In this con- Tutorials - Page 185 Light Measurement Applications

For radiation sources whose emis- Radiance is the quantity relevant to sions lie primarily in the near infra- the evaluation of BLH and RTH red range (IR-A) between 780 and hazards. The latest draft standards Light source α 1400 nm, and that generate a visual (IEC 825-1, November 1998), and luminance of less than 10 cd/m2, ICNIRP (printed in Health Physics the visual stimulus is so weak that 1999) express views as to the angle Retina the aversion reflex is not activated. of the measurement field of radi- In such applications the measure- ance meters. The applicable figures ment of radiance must, according related to exposure durations are: Fig. VI.5. Light Source - Subtended Angle – Retina to ICNIRP, take place exclusively in the IR-A region. t<10s an α of 1.7mrad 1) ; L(Ȝ): spectral radiance of the radia- t=10s...100s an α of 11mrad 2) tion source being measured, RTH t= 100s...10000s an α of 1.1*t 0,5 (Ȝ) : Retina Thermal Hazard Func- 2), 100 tion, Į = apparent radiation source. t> 10000s an α of 100mrad 2)

Limits are also prescribed for the 1) Dominance of thermal damage to 10 RTH function. Thus, for the case the retina where 2) Dominance of blue light hazard 10µ ≤ t ≤ 10s 1 For RTH IR-A evaluation ANSI/ 0,25 -2 -1 Lhaz ≤ 50/( α*t ) (kW*m *sr )

IESNA RP-27.1-96 recommends a rel. sensitivity Lhaz = effective radiance for the field of view of 11 mrad, and of 0,1 RTH function, 100 mrad for very large radiation a = size of the light source ex- sources. pressed in radians /1/ ICNIRP: Guidelines of limits of 0,01 For t < 10 µs the limit must not be exposure to broad-band incoherent 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 any greater than Lhaz for t= 10 µs. optical Radiation (0,38µm to 3 For t > 10 µs the limit must not be µm) (September 1997)(0,38µm to wavelength any greater than Lhaz for t= 10 s. 3 µm) (September 1997) Metrological Considerations Fig. VI.6. Retinal Thermal & Blue Light Hazard Spectral Functions

UV-Erythema The typical symptom of UV ery- received in the first years of life, thema is acute skin inflammation and this can be an important factor 1 caused by UV radiation (sunburn). in the development of skin tumors It used to be thought that erythema in later years. was only caused by radiation com- Sunburn occurs in fair-skinned ponents in the UV-B range of people (skin type 2) with a UV wavelengths. Present opinion is dose of as little as 250 J/m². Our that UV-A plays a part in causing table (following F. Greiter: Sonne 0,5 erythema because there is so much und Gesundheit, (Sun and Health), sensitivity rel. more of it present. Medical investi- published by Gustav Fischer Ver- gations have shown that intensive lag 1984) lists the various exposure exposure to UV in leisure time and duration's for minimal skin redden- at work increases the risk of skin ing for different skin types. 0 cancer. Children in particular 200 215 230 245 260 275 290 305 320 335 350 should be protected from strong UV radiation, as the skin stores the wavelength ( nm ) information about the UV dose Fig. VI.7. Erythemal Spectral Function Skin type Description Identification Reaction to the sun Exposure duration Sunburn Tanning ( min ) I Skin: noticeably light; Freckles: strong; Celtic type(2 Only very painful No reddeningwhite after 5 to 10 Hair: reddish; Eyes: blue, rarely brown; percent) 1 to two days, skin peels Nipples: very pale II Skin: somewhat darker than I Light skinned Only very painful HardlySkin peels 10 to 20 Freckles: rarely; Hair: blonde to brown; European (12 Eyes: blue, green and grey; Nipples: percent) light III Skin: light to light brown, fresh Dark skinned Moderate Average 20 to 30 Freckles: none; Hair: dark blonde, European (78 brown; Eyes: grey, brown; Nipples: percent) darker IV Skin: light brown, olive; Freckles: none; Mediterranean Hardly Fast and deep 40 Hair: dark brown; Eyes: dark; Nipples: type (8 per- dark cent) Table VI.1. Skin Type Categories Page 186 - Tutorials Light Measurement Applications

Phototherapy; UV-A, UV-B and UV-B311 Phototherapy UV is widely used by dermatolo- normally administered without a mulated over time, is normally any variation in output through gists in the treatment of certain skin photosensitizing agent. It is consid- measured in phototherapy applica- lamp or delivery system degrada- diseases like Psoriasis and Vitiligo. ered safer than UV-A for wave- tions. tion but most of today’s photother- Whole body exposure booths and lengths between approx. 290 to 315 /cm² = watts/cm² x sec- apy equipment is equipped with hand and foot units employing light nm, since it does not penetrate as onds sensors and electronics which sources which emit broadband UV- deeply into the skin and is more (dose/energy) = irradiance x allow delivery of pre-selected A, UV-B, narrowband 311nm energetic allowing shorter overall time doses of UV. UVB and combinations of UV-A exposure times. However, it is Third party checks of these internal and UV-B are used to irradiate the generally accepted that wave- In the research & development dosimeters by qualified UV radi- patient. lengths below 290 nm produce stage or field service, direct irradi- ometers is recommended to ensure In PUVA phototherapy, also called more erythema which can actually ance may be monitored to discern proper dosimetry and safety. photochemotherapy, UV-A is inhibit the therapeutic effects of the applied in combination with a longer wavelengths. photosensitizing agent which is As a result, narrowband UV-B taken in pill form or applied topi- sources emitting at predominantly cally to the skin. This medication 311-312 nm, have been developed. called psoralen, giving rise to the They emit right in the wavelength acronym PUVA, makes the skin zone of most effectiveness while more sensitive and responsive to producing less erythemal interfer- the UV-A (315-400nm) wave- ence than broadband UV-B lengths. sources. Due to the risks of premature skin This is generally known as a TL-01 ageing and skin cancer from pro- source. A TL-12 UV-B source with longed exposures, also with consid- a slightly wider band of emittance eration to skin type, PUVA is only between 280-350 nm, peaking at recommended for moderate to about 305 nm is also in use. For severe cases of Psoriasis. As a side more information contact the Na- note, psoralen is also being used as tional Psoriasis Foundation and the a photosensitizer in UV steriliza- American and European Acad- tion of blood. emies of Dermatology. UV-B broadband treatment is Dose, used here as irradiance accu- Fig. VI.8. Narrowband 311nm & Broadband UV-B Source Spectra Bilirubin Phototherapy Newborn jaundice or neonatal in a ‘bilibed’ or protected isolette hyperbilirubinaemia, a yellowish and exposed to fluorescent appearance of the skin and whites designed or filtered to emit in the 1 of the eyes, is present to some blue spectrum. A recent develop- degree in almost all newborn in- ment is the ‘biliblanket’ that deliv- fants. This is caused by an elevated ers blue light through fiber optics level of bilirubin molecule in the and can be wrapped around the blood which results from immatur- infant. Radiometric measurements ity of the liver function combined of bililights are important in order 0,5 with the destruction of red blood to ensure proper dosimetry. cells present. When these levels are Efforts to standardize an action rel. sensitivity very high, one method of clearing spectral function and measurement the jaundice is by exposing the procedures for bilirubin are in newborn to light in the blue spec- process. Due to early work in this 0 tral region between 400 to 550 nm. field, the units of microwatts/cm²/ 405 430 455 480 505 530 The light interacts with the nm were wrongly adopted for bilirubin, converts it to a substance radiometric measurement of wavelength ( nm ) excreted back into the bloodstream bililights. To be technically correct which can then be excreted in the the units of watts/cm² should apply. feces. The newborn is placed nude Fig. VI.9. Bilirubin Spectral Function VI.2. Plant physiology The study and understanding of the cal factors require a precise and tween 300 nm and 930 nm initiates Photosynthesis, Phototropism, interrelation of optical radiation predictable measurement technol- photochemical reactions in plants Photomorphogenesis and plants, seeds and soil is criti- ogy. that are essential for plant growth. cally important for our existence. The absorption of optical radiation The three most important reactions Research and control of biochemi- in the range of wavelengths be- of plants to optical radiation are: Photosynthesis Photosynthesis is one of the most radiation energy to build sugar, The occurrence of photosynthesis tion energy in the chlorophyll important biochemical processes releasing oxygen and water into the in plants is characterized by the molecules raises the electrons to a on the planet. In the process of atmosphere. This process can bede- green color of their leaves. This is higher energy state. As they return photosynthesis green plants absorb scribed by the following assimila- due to chlorophyll which is ab- to their initial state, the energy carbon dioxide from the atmos- tion formula: sorbed with the photosynthetically released is converted into chemical phere and water from the soil, h.v active radiation. Accordingly, the energy. 6CO +12 H O → C H O + 6O + 6 H O combining them with the aid of 2 2 674 kal 6 12 6 2 2 absorption of the quanta of radia- Tutorials - Page 187 Light Measurement Applications

Photosynthesis In general plant physiology, the morphogenetic effect thetic process, and therefore is of Classical investigations into plant term Photosynthetically Active - 510 nm to 610 nm: weak light minor importance. Just the contrary physiology have indicated that Radiation (PAR) refers to the absorption by chlorophyll, no has been demonstrated by experi- photosynthetic bacteria possess radiation in the range of wave- morphogenetic effect ment. It is precisely this green special pigments with strong ab- lengths between 400 nm and - 610 nm to 720 nm: strong light radiation that yields the greatest sorption bands in vivo at 750 nm 720 nm. This is the energy that is absorption by chlorophyll, high productivity and efficiency in (chlorobium chlorophyll in the absorbed by the assimilation pig- morphogenetic and ontogenetic densely populated arrangements of green chlorobacteria) or at 800, ments in blue-green algae, green effect plants or in thick suspensions of 850, 870 and 890 nm. In contrast to algae and higher order plants. The micro-organisms. This discovery is the blue-green algae, green algae wavelengths for the lower limit This response function can be important for investigations into and the higher plants, the absorp- (400 nm) and an upper limit considered as a mean spectral the yields of plants in the lower tion spectrum of the photosynthetic (720 nm) are not entirely rigid. response function. A number of layers of wooded areas or of green- bacteria also extends into the UV Photosynthetic reactions have, for different investigations have shown house stocks, or in deep water (e.g. region as far as about 300 nm. example, been established in some that the spectral absorption spectra in sea plants). algae at wavelengths shorter than of various plant types can be very 400 nm. In general, the lower limit different. These differences can depends on the structure and the also occur, in a single plant, e.g. in 1 Phototropism thickness of the leaf as well as on leaves of different ages or with the chlorophyll content. Some different thicknesses, chlorophyll research projects have shown content, etc.. It should also be 700 nm as the upper wavelength noted that the spectral response limit. function for photosynthesis is 0,5 In DIN 5031, Part 10 (currently in defined with avoidance of mutual

the draft phase) the spectral re- cell shading, experimenting with a rel. sensitivity Photosynthesis sponse function for photosynthesis young, thin leaf or with a thin layer is defined, and this is illustrated of algae suspension. graphically below. For plant physi- The spectral distribution of the ology, this range can be divided response function for photosynthe- 0 into three narrower bands: sis might give the impression that 380 420 460 500 540 580 620 660 700 740 visible radiation in the green range wavelength ( nm ) - 400 nm to 510 nm: strong light centered around 550 nm contrib- absorption by chlorophyll, high utes very little to the photosyn- Fig. VI.10. Phototropism (blue) & Photosynthesis (green) Ultraviolet Radiation Effects The weakening of the ozone layer that in general an increase in UV-B ence of an ozone reduction of 25%, linear growth, the cell division rate has been discussed in public for radiation must be expected. the UV-sensitive soy bean variety and other factors. So in the fields of more than a decade. It presents a UV-B radiation penetrates the Essex, unlike the insensitive Wil- plant physiology and crop cultiva- serious challenge to plant physiol- tissues and leads to molecular liams variety, undergoes a reduc- tion it is necessary not only to ogy. Even above Europe, a reduc- changes in DNA, proteins, lipids tion in photosynthesis, and there- investigate the positive photosyn- tion in the total amount of ozone of and phytohormones. At high levels fore of yield, of up to 25%. This thetic effects of optical radiation, 3-6% per decade (since 1978) has of UV radiation, oxygen radicals effect of UV-B radiation (280 nm- but also the negative effects, been established, which correlates are formed, leading to oxidation of 313.3 nm) is internationally known mostly due to UV radiation, if the with a measured increase of up to proteins and lipids. The result of as "generalized plant damage", and activity and protection mechanisms 7% in the level of UV-B radiation this is that growth, photosynthesis, is evaluated using the UV-B re- of plants are to be understood and in high alpine areas with clean air. and finally productivity and yield sponse function according to Cald- manipulated. In March of 1993 a 15% disappear- are impaired. Some field trials have well. This GPD response function ance of ozone was observed, so provided evidence that in the pres- incorporates the degree of damage, Phototropism Phototropism describes the effect of plant growth. The regions of range between 380 nm and 520 nm the effect of causing parts of plants of optical radiation on the direction maximum effect lie in the blue (see Fig.VI.10). Radiation can have to move. Photomorphogenesis Photomorphogenesis describes the of the spectrum encourages linear intensities in the range of wave- 680 nm (short wavelength red) is of way in which plants are formed growth, while blue radiation yields lengths from 690 nm to 780 nm great importance for the plant's under the influence of optical ra- small, strong plants. To be more (long wavelength red) to the range biological processes. diation. Radiation in the red region precise, the ratio of the radiation of wavelengths from 560 nm to Measurement Aim, Measurement Methods The photochemical processes in- photosynthesis (yield factor) radiation and high levels of heat diation input quantity. volved in plant physiology are when exposed to radiation radiation (infrared radiation) These quantities are: understood as quantum processes. sources having different emis- • the irradiance in W/m2, i.e. the Associated measuring techniques sion spectra The effect of radiation of various radiant power per unit area of the should also treat them as such. The • Comparison of the rate of photo- wavelengths on the growth proc- irradiated object most important measurements for esses taking place within plants can synthesis in various plant types • or, alternatively, the photosyn- plant physiology are: cultivated under various radia- be represented in a number of • ways. The rate of photosynthesis is thetic photon irradiance Ep,sy. Analysis of the efficiency of tion conditions This magnitude is also frequently energy conversions in photosyn- • defined as the ratio of the quantity Determination of the protective of assimilated carbon dioxide referred to as the quantum flux thesis mechanism and the stress proc- density. (CO2) molecules to a suitable ra- • Determination of the rate of esses of plants in relation to UV Page 188 - Tutorials Light Measurement Applications

Photosynthetic Photon Irradiance E p,sy The radiation conditions used in growth, which implies that it is The unit of photosynthetic photon measuring head must, however,

determining the rate of photosyn- quantum magnitudes effective in irradiance, E p,sy, is defined as satisfy two important conditions: thesis and the photosynthetic po- plant biology that need to be meas- follows: tential of various plant or algae ured. The most important magni- − − − − [Ε ]=1Ε.s 1.m 2 =1Mol.s 1.m 2 ø The incident radiation must be types are not the same in all re- tude is the photosynthetic photon p,sy 23 evaluated in accordance with the search institutions. Results ob- irradiance E p,sy. where 1 E = 1 Mol = 6.02* 10 cosine of the angle of incidence, tained under very different radia- The photosynthetic photon irradi- photons (the most commonly used -1 -2 i.e. using a cosine diffuser tion conditions, using detector ance E p,sy is defined as follows: unit is µMol s m ). heads with non-uniform rectangu- where: The limits of integration need to be ø The spectral sensitivity of the measuring head must be adapted lar (radiometric) characteristics, 1 specified for integration according Ε = Ε ()λ λ = Ε()λ λ λ to the l/lr function. lr is the refer- and then relating them to one an- p,sy p,λ d . .d to the formula. If, for example, the “ h.c “ other, may lead to false conclu- photosynthetic photon irradiance is ence wavelength, and for the sions. This is because the varying E(λ) is the spectral irradiance of to be measured in the range of 400-700 nm, 320-500 nm and spectra of the radiation sources in the light source wavelengths between 400 nm and 590-900 nm ranges it is always the upper limit wavelength, i.e. use are ignored in obtaining the λ is the wavelength of the radiation 700 nm, 320 nm and 500 nm and 700 nm, 500 nm and 900 nm. measurement. E p,sy (λ) is the spectral photon 590 nm to 900 nm, the integration The solution is to evaluate the irradiance = number of photons per is carried out in the corresponding The spectral sensitivity of the sen- irradiance with a sensor with an second, per unit area and wave- spectral segments. sor should be zero outside the appropriate spectral response func- length. This numerical integration can be responsive spectral zone of inter- tion. It is presently assumed that h is Planck's constant performed implicitly by means of est. the number of light quanta ab- c is the velocity of light. an integral measuring head. Such a sorbed is responsible for plant

VI.3. UV-Disinfection and Lamp Control UV radiation can have harmful bacteria, mold and fungus. Its use spectral range from 100 to 280 nm low pressure mercury is the light effects on human skin and eyes, is becoming more widespread than is used in the germicidal/ source of choice in these applica- especially during indoor applica- traditional water treatment tech- bactericidal sterilization of air and tions. Medium and high pressure tion of high-energy UV irradiators. niques employing chlorine and water. UV-C at 253.7 nm is also Hg as well as metal halide and There is however a positive aspect ozone for reasons of cost and envi- employed in eprom erasure and the other broadband UV sources are to UV’s hazardous effect on living ronmental factors. cleaning of sensitive surfaces in the also used, especially in UV curing. organisms. Ultraviolet treatment of semiconductor industry. UV curing Light Source Life-time drinking and wastewater is a well- Recently, a field study performed is another area where UV-C is The useful lifetime of high power established, economical and effi- in homeless shelters in New York, applied. UV-C sources is limited. UV-C cient method for killing germs, Birmingham and New Orleans (TB UV-C Light Sources intensity must be monitored to UV Shelter Study, TUSS) has Due to its high and pre-dominantly ensure process control. shown that UV treatment of room monochromatic output at 253.7 nm, air with upper room irradiators leads to a drastic reduction of tu- berculosis infection rate.

The CIE divides ultraviolet optical radiation into three ranges:

• UV-A: 315 to 400 nm (skin pigmentation) • UV-B: 280 to 315 nm (vitamin D synthesis, erythema) • UV-C: 200 to 280 nm (germicidal action, absorption maximum of DNA). Below 230 nm, UV radiation has enough energy to break chemical bonds.

Short wavelength high energy Fig. VI.11. Fig.VI.12. ultraviolet radiation in the UV-C UV treatment of wastewater Upper room UV irradiators help lower tuberculosis infection rates.

VI.4. UV Curing and UV Processing UV curing is a process in which In the curing application, dose is linear ageing, UV output needs to Therefore, optical bandpass filters photocureable chemicals applied to used to describe the amount of be continuously monitored and are used to limit the detector‘s substrates are irradiated with high energy delivered to the target prod- controlled. sensitivity to the spectral range of energy UV or Visible radiation for uct. It is defined as radiant expo- These high UV levels place special interest. curing. This energy accelerates sure (energy per unit area) and demands on the measurement Conventionally designed UV irra- polymerization (cross-linking) and typically measured as irradiance devices used in this application. diation detectors show drift and consequently the hardening or over time. That component of Ultraviolet instability over time due to the drying process. The irradiated energy useful for curing makes up hostile ambient conditions found in energy needs to be controlled, since J/cm² = W/cm² x seconds only a small part of the spectral the UV-curing process. Solariza- too low a dose will not cure the bandwidth within the lamp’s total tion, ’fogging” effects and even de- product, whereas too high a dose High-power UV sources are used emission spectrum and the bare lamination of the filter elements will damage it. in this process. Because of non- detector‘s spectral sensitivity. and other optical components can Tutorials - Page 189 Light Measurement Applications

occur. These effects not only can Active Range It should be noted that information change the detector’s absolute for on the spectral response function of sensitivity but can also change its UV-Curing the detector in use should be pro- spectral sensitivity. On recalibra- vided along with any statement of tion a change in absolute sensitivity measured magnitude to properly may be noted and adjusted but ‘frame’ the results. UV detectors unless a complete spectral test is from different manufacturers can perfomed a change in spectral have very different spectral re- sensitivity can go undetected. So sponses.This means that they will

what is thought to be a newly re- 1,6µm not read the same under the same 200nm calibrated detector very often will test conditions. produce erroneous readings when Fig. VI.13. UV Curing Spectral Region Due to the many errors involved returned to the end user. with UV measurement, even two A new detector design has been Thermal isolation by detectors from the same manufac- developed based on the integrating a flexible light guide. turer can read much differently. element RADIN™, which is not Detector Normally in the field, readings only able to withstand the high UV low profile within ± 10% are considered ac- and temperature conditions of the ceptable in the UV-A range. UV - Filter UV-curing process but also main- High UV attenuation Uncertainties get progressively tain stability and measurement worse as you move to the shorter accuracy over long term use. Criti- wavelengths. cal components in the detector are Temperature and UV Low UV- and thermal-radiation not exposed to direct irradiation but stable radiation integrator intensity at the detector reduces It is important to remember that the only see a fraction of it. (RADIN) drift, ageing, and saturation effects UV meter is after all a scientific RADIN is a trade name of Giga- Fig. VI.14. Horizontal UV Detector Design instrument which is asked to per- hertz-Optik. form reliably and repeatably in Temperature and UV very hostile environments. The detector response which best stable radiation integrator Maintaining calibration cycles at matches the absorption spectrum of (RADIN) the manufacturer recommended the photocureable chemical in use interval is necessary. If unaccept- is selected. This way the detector Thermal isolation by able levels of change are seen on spectrally emulates the product to a flexible light guide. recalibration, the cycle time should be cured. low profile be shortened (staircase method). The lamp(s) used in the system This way you end up with a recali- were selected by the equipment bration program tailored to your manufacturer for optimal curing specific requirements. within this active bandpass. Also consider having a second When lamp replacement becomes Low UV- and thermal-radiation instrument on hand which is used necessary the replacement lamps intensity at the detector reduces only for an in-house calibration should be the same in spectral and drift, ageing, and saturation effects check of the working production absolute output as the old ones so unit(s). that the established process pa- rameters are not invalidated. Fig. VI.15. UV Curing Detector

VI.5. Colorimetry Color is the attribute of visual brown, red, pink, green, blue, pur- Perceived color depends on the on the person’s experience of pre- perception consisting of any com- ple, etc., or by achromatic color spectral distribution of the color vailing and similar situations of bination of chromatic and achro- names such as white, grey, black, stimulus, on the size, shape, struc- observation. For more details about matic content. This attribute can be etc., and qualified by bright, dim, ture and surroundings of the stimu- theory, see paragraph II.9 of this described by chromatic color light, dark or by combinations of lus area, on the state of adaptation tutorials. names such as yellow, orange, such names. of the observer’s visual system, and Color and Illuminance Measurement In many applications involving the more sources of radiation be satis- 7.5 % for precise measurements. measurement of color or of the fied. Detectors used to determine Devices of class B, with a total color temperature of self-emitting absolute illuminance must there- uncertainty of measurement of light sources, the same measure- fore have a cosine spatial function 10 % for operating measurements. ment geometry is used as for illu- as their measurement geometry. If minance. Appropriate absolute the incident radiation is not paral- Since there are no equivalent regu- calibration of the measuring system lel, the accuracy of the cosine lations for colorimeters, some of allows the illuminance of a refer- function is critically important to the regulations in DIN-5032 can ence plane in Lux (lx) to be deter- the result of the measurement. In also be usefully applied to illumi- mined, in addition to the colorimet- Germany, DIN 5032, Part 7 classi- nance measuring colorimeters. ric parameters. If the incident light fies the quality of devices for meas- is falling diffusely, this measure- uring illuminance (luxmeters/ ment requires the measuring sys- ) according to the tem to have a field of view adapted accuracy of their measurement to the cosine function. Only in this into: way can the laws for the incidence Devices of class A, with a total of diffuse radiation from one or uncertainty of measurement of Gigahertz-Optik’s HCT-99 Page 190 - Tutorials Light Measurement Applications

Color and Luminous Flux Measurement Luminous flux is the quantity used beam it is possible to measure the often used for highly divergent Such ”integrating spheres” are to define all of the emitted radia- luminous flux with a photodetector sources is a hollow body, ideally known in Germany as Ulbricht tion in all directions by a light assuming that the diameter of the formed as a hollow sphere, with a spheres. The figure below illus- source in the photometric unit, beam is less than that of the detec- diffusely reflecting interior wall. trates the principle of its construc- Lumens (lm). One of its purposes tor measurement aperture. If the tion. is to reference the efficiency of light beam is highly divergent, or if incandescent lamps, arc lamps, a 4π radiation characteristic must Fig. VI.16. light emitting diodes, etc., as this is be considered, a measurement Color and Lumi- derived from the relationship be- geometry must be used that ensures nous Flux Measure- tween the input electrical power that all of the radiated light is and the luminous flux. evaluated, regardless of the direc- ment Instrument In cases where the light source tion in which it is emitted. with a 150 mm Inte- emits in an approximately parallel The measurement geometry most grating Sphere Color and Luminous Intensity Measurement The quantity of luminous intensity system must be calculated. that attach to the front end of the specifies the light flux emitted by a In order to measure luminous in- detector can be used for this pur- light source in a particular direction tensity, the field of view of the pose. within a specified solid angle in the color detector must be restricted to photometric units of Candela (cd). the desired solid angle. This is One area where it is applicable is usually accomplished using stera- when lamps and projectors are used dian adapter tubes that limit the in imaging systems (lens systems, detector head’s field of view. It is reflectors), and the subsequent important that the inner walls of distribution of luminous intensity these tubes are designed to exhibit from the illumination or spotlight low reflectance. Steradian tubes Fig. VI.17. Typical Measurement Geometries Color and Luminance Measurement The quantity of luminance is used to evaluate the intensity of light from surface emitters in the photo- metric units of Candela/square metre (cd/m2). A defined angular field of view for the color measur- ing device is needed in order to measure luminance. This can be accomplished using either steradian Fig. VI.18. Color Detector Head for Measurement of Luminous tubes or lens systems. Intensity and Luminance, Color Temperature Measurement Color temperature is a simplified "size" of one kelvin is the same as • 3400 K, Tungsten lamp of the same color type as that of the way to characterize the spectral that of one degree Celsius, and is • 5500 K, sunny daylight around radiator being characterized. The properties of a light source. While defined as the fraction 1/273.16 of noon color temperature is calculated and in reality the color of light is deter- the thermodynamic temperature of For many color measurement tasks displayed by the meter. The calcu- mined by how much each point on the triple point of water, which it is important to determine the lation of color temperature is per- the spectral curve contributes to its positions 0° Celsius at 273.16 K. color temperature of luminous formed using an algorithm accord- output, the result can still be sum- ing to Qiu Xinghong, which en- Light sources are sometimes de- objects. According to DIN 5031- marized on a linear scale. ables very good research results for scribed by their correlated color P.5, the color temperature tc of a Low color temperature implies the color temperature range from temperatures (CCT). The correlated radiator requiring characterization warmer (more yellow/red) light 1667°K to 25000°K to be obtained. color temperature of a source is the is the temperature of a Planckian while high color temperature im- temperature of a radia- radiator at which it emits radiation plies a colder (more blue) light. tor that is most similar to the Daylight has a rather low color source. A blackbody radiator is an temperature near dawn, and a ideal surface that absorbs all en- higher one during the day. There- ergy incident upon it, and re-emits fore it can be useful to install an all this energy. The spectral output electrical lighting system that can distribution of an incandescent supply cooler light to supplement (tungsten) lamp approximates a daylight when needed, and fill in blackbody at the same temperature. with warmer light at night. This Correlated color temperature is also correlates with human feelings typically presented using the abso- towards the warm colors of light lute centigrade scale, degrees Kel- coming from candles or an open fireplace at night. vin (K). Standard unit for color temperature Some typical color temperatures is Kelvin (K). are: The kelvin unit is the basis of all • 1500 K, Candlelight temperature measurement, starting • 3000 K, 200 W incandescent with 0 K (= -273.16° C) at the lamp absolute zero temperature. The • 3200 K, sunrise / sunset Tutorials - Page 191 Light Measurement Applications

VI.6. Photostability The current ICH (International Gigahertz-Optik ‘flat’ UV-A detec- equipped with internal light sensors Without proper protection engi- Conference for Harmonization) tor is only 14%. to continuously monitor the light neered into the detector, changes guidelines specify that drug and The guidelines also state that to and UV-A output. due to solarization, temperature drug products must be phototested ensure spectral conformity of the Maintaining accuracy and reliabil- effects and ensuing calibration drift to ensure that exposure to light light source(s) a phototester ‘may ity in on-line continuous monitor- can occur. It is advisable to do a does not cause photochemical rely on the spectral distribution ing of UV applications is a daunt- third party check using a qualified degradation of the product or pack- specifications of the light source ing challenge. radiometer/photometer. aging. The product under test must manufacturer’. It has been found in receive a measured dose of both actual practice that either the spec- UV-A (200 -hours per square tral data is not available or typical meter) and Visible (1.2 million lux- data is not reliable due to ageing hours) optical radiation exposure. effects of the source and other This requires both radiometric and factors. This is another important photometric measurements in terms reason for using photodetectors of illuminance in lux and UV-A with the best spectral match to the (315 to 400 nm) irradiance in W/m² ideal functions. multiplied by exposure time in hours. Most often the phototesting is It is important to note that total or performed in a photostability absolute UV-A is implied. No chamber with long fluorescent light effective UV-A spectral function is sources mounted above the prod- specified. Ideally for total UV-A ucts under test. For larger profile measurements, the perfect broad- products, light sources may also be band UV-A detector would have a mounted along the sides of the flat square-wave spectral shape chamber to fully immerse the tar- starting at 315 to 400 nm for 100% get. Since this is an extended response at each wavelength across source type of measurement rather this spectrum with no response than a point source configuration, outside this bandpass. Most cur- the detector angular responsivity rently available UV-A detectors should be cosine corrected using a Fig. VI.19. Typical UV-A Spectral Function have a ‘bell’ shaped spectral re- diffuser. This way the incoming sponse which, if uncorrected light signals are properly weighted through calibration or redesign of according to the cosine of the angle spectral function, will read >25% of incidence. too low on the UV-A fluorescent Then the detector properly emu- source, and >40% too low for lates the target in the way the light Xenon + glass ID65 type light signal is received. sources. Note that UV-A fluorescent and Profiling the photostability cham- Xenon or Metal Halide simulated ber for uniformity over the expo- ID65 light sources are the only sure plane is an important proce- sources specified in the ICH guide- dure since products placed in dif- lines. ferent areas inside the chamber A closer approximation to an ideal should be uniformly exposed to the UV-A broadband detector has same light levels. Moving the recently been developed for photo- detector or using multiple detectors biological and photostability appli- in multiplex mode maps the expo- cations by Gigahertz-Optik. sure levels at various locations Compared to the ideal UV-A spec- across the exposure plane. tral function the typical detector Some of the photostability cham- total area error is 34% while the bers manufactured today are Fig. VI.20. Flat UV-A Spectral Function

VI.7. Telecommunication An ongoing revolution occurring in the tion, signal ‘bounce-back’ and of the incoming signal is captured field of telecommunications is the beam misalignment. Also, the use and reflected inside the sphere development of small laser diodes and of large size photodiodes required multiple times before reaching the high capacity optical fibres. Without to reduce source to detector mis- optical fibre telecommunication devices baffled detector mounted to it, the the highly convenient availability of alignment, increases cost. adverse effects of polarization, huge amounts of information at com- A welcome alternative is the inte- local saturation, signal ‘bounce- paratively low costs, as provided by the grating sphere which is able to back’ and beam misalignment are Internet, would not be possible. collect all of the source optical reduced. The measurement of the power radiation output independent of output of laser diodes or fibers is a beam geometry. In the world of daily routine in the field of tele- photonics, the integrating sphere is Fig. VI.21. During the last two communication components test- well known for its ability to relia- decades, optical fibre capacity has ing. Optical power meters using a bly and accurately measure total increased by a factor of about bare detector claim high sensitivity flux from fibers, laser diodes, la- 1000. For comparison, commercial but at a cost of potential measure- sers, LEDs and any other optical state of the art wire connections ment inaccuracies caused by the radiation or light source. Since all range in the region of about 0.1 effects of polarization, local satura- Gigabit per second (Gb/s). Page 192 - Tutorials Light Measurement Applications

VI.8. LEDs Measurements Presently, a slow but steady large 90%. Low power consumption and LEDs high-energy efficiency and scale technological change is tak- a typical LED lifetime of 100,000 low operating voltage leads to their ing place: The traditional incandes- hours (compared to about 1000 use when power consumption is a cent bulb is being replaced more hours for incandescent bulbs), critical parameter as when batteries and more by special semiconductor drastically reducing maintenance, or solar cells must be used to devices called light emitting di- often equates to significant overall power the lamps. odes or LEDs. Over the past dec- cost reduction when LEDs replace ade, LEDs have caught up in effi- traditional lighting. As an exam- ciency and now offer an economi- ple, the city of Denver, which cal alternative to incandescent recently has replaced some 20000 bulbs even for bright signal lamps incandescent bulbs in traffic light such as traffic and automotive signals with LED devices, esti- lighting. A high percentage of an mates the total savings to about incandescent bulb’s total light $300 per signal, calculated for the output is lost when passed through lifetime of an LED device. a coloured filter. In contrast, an At the current level of technology, LED emits light of only the desired the broad application of LED de- color, thus no filtering is necessary vices as sources of artificial light is Fig. VI.22. Efficiency of red, green, and consumption of electrical restricted by comparatively high white and blue LEDs has dramati- energy can be reduced by up to production costs. However, the cally increased during the last V1.9 LASER Measurements

The most commonly encountered Because of the monochromatic • Lasers with collimated (parallel) tions, the sphere offers high monochromatic source is the laser. emission spectrum and fixed output beams are typically measured attenuation for high power meas- Because of its monochromatic and wavelength, detectors used to with a flat-field detector, whose urements. The max. power is coherent radiation, high power measure laser power do not need a active size is larger than the laser limited by the sphere’s upper intensity, fast modulation fre- radiometric broadband characteris- beam diameter. Because of the operating temperature limit. quency and beam orientated emis- tic. This means that the typical high power of lasers, the respon- Also through multiple internal sion characteristics, the laser is the spectral sensitivity characteristic of sivity of the detector may have to reflections, measurement errors primary source used in fiber optic Si or InGaAs photodiodes can be be reduced by an attenuation caused by polarization effects communication systems, range used without requiring spectral filter. But there is a risk of meas- with flat-field detectors do not finders, interferometers, alignment correction. urement errors due to polariza- occur systems, profile scanners, laser For absolute power measurements tion effects, surface reflections The sphere port diameter can be scanning microscopes and many the bare detector’s spectral re- from optical surfaces in the light enlarged by increasing the sphere other optical systems. sponse can be calibrated at a single path and misalignment of the diameter which allows measure- Traditional monochromatic radio- wavelength or over its complete beam on the detector. ment of larger diameter beams metric applications are found in the spectral range (typically done in 10 • • Laser Stray-light: Although very range of optical spectroscopy with nm increments). Lasers with non-collimated (divergent) beams cannot be useful, laser radiation can pose a narrow band-pass filtered detectors The corresponding calibration health risk to the human eye. and scanning monochromators factor for that specific wavelength measured with a flat-field detec- tor because of the different an- Even stray-light from lasers may used as monochromatic detection is selected when making the laser be hazardous due to the typically systems or monochromatic light power measurement. Some meters gles of incidence. The power output of these lasers is typically high power levels found. The EN sources. offer the capability of selecting a 60825 standard describes the risk Optical radiation describes the wavelength by menu on the dis- measured with detectors com- bined with an integrating sphere and measurement methods for segment of electromagnetic radia- play. The meter then calculates the risk classification. Laser stray- tion from l=100 nm to l=1 mm. reading by applying the calibration which collects all incoming radiation independent of the light can be assessed with the use Most lasers used in measurement factor for the wavelength selected of a detector head with a 7 mm instrumentation and fiber optic and displays the measurement angle of incidence. • dia. free aperture to mimic the telecommunications systems work result. Here are more unique features open pupil. predominantly in the 200 to 1800 There are two typical measurement offered by the integrating sphere: nm wavelength range. strategies for laser power detection: Through multiple internal reflec- V1.10 Non-Destructive Testing The American Society for Nonde- (VI) are test methods used to detect the detection of surface defects in For highest sensitivity, a fluores- structive Testing defines NDT as defects in materials with the aid of nonporous metal and non-metal cent dye is used as the penetrating the examination of an object with optical radiation or light. materials. Two different methods liquid and the test is carried out technology that does not affect the The light levels used in these op- are in use: under ultraviolet lighting. UV-A object’s future usefulness. The erations are critical to the integrity Dye Penetration Process: sources known as ‘blacklights’ are term NDT includes many methods of the inspection process so radio- A colored liquid or dye is applied most commonly used. To reliably that can: metric and photometric measure- to the surface of the test object test with fluorescent agents, an • Detect internal or external imper- ments are made for quality control which, through capillary action, adequate level of UV-A irradiance fections purposes. penetrates into any existing surface containing a very low proportion of • American Society for Testing and defect(s). After removing any white (visible) light must be gener- Determine structure, composition Materials (ASTM), MIL and DIN excess, an absorptive white layer is ated at the object under test. or material properties standard practices exist to help applied, drawing the colored liquid Magnetic Particle Testing and of • Measure geometric characteris- ensure uniformity in these exami- out of the defect making it visible. course Visual and Optical Testing tics nations. Adequate illumination of the test both rely on ensuring adequate Liquid Penetrant Testing (PT), Liquid Penetrant Testing using object with white light is critical to light levels for quality control of Magnetic Particle Testing (MT) the dye penetration examination create good contrast. the examination. and Visual and Optical Testing process is a widely used method for Fluorescent Penetrating Agent: Tutorials - Page 193 Light Measurement Applications / Appendix

DIN EN 1956, ASTM and MIL Many of the UV-A radiometers Spectral errors when measuring lux a flat surface does, so in effect the Standards exist that describe the used in this application are cali- or foot-candles can occur when detector emulates the sample under general conditions and standard brated at a single point at the peak testing light sources different from test. If the detector spatial response practices for the penetrant test of the detector spectral response, the source used for calibration. A does not closely match the true examinations, including the proce- typically 360 nm or simply ad- detector that very closely matches cosine function, significant errors dures to be followed. The mini- justed to some reading on a particu- the CIE photopic function is re- in readings will result mum requirements for the illumina- lar light source. To reduce meas- quired for accurate photometry. tion or irradiation conditions, test urement errors due to light sources The Gigahertz-Optik photopic UV sources pose a potential procedures to be used for checking with different spectral outputs, sensor’s spectral function is within health hazard risk to the skin these levels and suitable measure- Gigahertz-Optik uses the integral DIN Class B limits of <6% fidelity and eye. UV-A sources used in PT ment equipment specifications are calibration method where the de- to the CIE photopic curve. emit some levels of the most harm- also covered. tector is calibrated to a measured The detector spatial response ful UV-C and UV-B rays. The UV- It is particularly emphasized in UV-A integrated spectral irradi- (angular response) is another im- A rays are considered less of a risk, these standards that the calibration ance standard. portant factor and potential error but the ACGIH/ICNIRP guidelines of the radiometer and photometer To reduce spectral errors even source. do state Threshold Limit Values for used to measure the illuminance further, the Gigahertz-Optik UV-A Since the detector is fully im- UV-A at 1 mW/cm² for an eight- and irradiance must be carried out detector exhibits a nearly flat re- mersed in light from all directions, hour exposure period. For UV-B, with the aid of calibration stan- sponse across the UV-A bandpass including any ambient contribution, the TLV is much less at 0.1 Effec- dards that can be traced back to with a sharp cut-off at 400 nm to it should be cosine corrected, using tive W/cm². So UV exposure of national standards. The test certifi- eliminate visible stray light from a diffuser. This way the incoming workers in PT environments should cate must document the calibration contaminating the UV reading. light signals are properly weighted be tested to ensure safety as well as testing. The spectral response function of according to the cosine of the inci- for quality control.. The calibration method must also the photometric sensor is very dent angle. The detector receives be considered. important for the same reasons. the light signals in the same way as



TEST OBJECT TEST OBJECT Fig. V.23. Penetrant Testing Schematic VI.11. Transmission & Reflection ‘Measurement with Light’ refers to this type of instrument. defined V(λ) photometric action with a minimum diameter of 50 cm the use of optical radiation as a tool Light describes optical radiation function applies to any application for the measurement of photomet- for performing a measurement task. visible to the human eye over the where light must be evaluated for ric material properties such as light Reflectance and transmittance wavelength range from 380 to 780 visual purposes including reflection transmittance/reflectance, diffuse meters employ both light sources nm. The eye’s sensitivity to light and transmission. transmittance/reflectance and regu- and a detection system. A spectro- varies over this spectral range DIN 5036-part 3 and CIE 130-1998 lar transmittance/reflectance. photometer is a good example of peaking at 555 nm. This same CIE recommend an integrating sphere

VII.1. Relevant quantities, their symbols and units Quantity Symbol Unit(s) Wavelength λ 1 nanometer = 1 nm = 10-9 m / 1 Ångstrøm = 1 Å = 10-10 m Power P 1 Watt = 1 W Solid angle Ω 1 steradian = 1 sr

Radiant power or radiant flux Φe 1 Watt = 1 W -1 Radiant intensity Ie 1 W sr -1 -2 Radiance Le 1 W sr m -2 Irradiance Ee 1 W m -2 Radiant exitance Me 1 W m

Luminous flux Φv 1 lumen = 1 lm photopic: 1 lm corresponds to Φe = 1/683 W at λm = 555 nm scotopic: 1 lm corresponds to Φe = 1/1700 W at λ’m = 507 nm

Luminous intensity Iv 1 candela = 1 cd = 1 lm / sr -1 -2 -2 Luminance Lv 1 lm sr m = 1 cd m = 1 nit 1 stilb = 1 sb = 1 cd m-2 1 apostilb = 1 asb = 1/π cd m-2 1 lambert = 1 L = 104/π cd m-2 1 footlambert = 1 fl = 3.426 cd m-2 -2 Illuminance Ev 1 lux = 1 lx = 1 lm m 1 phot = 1 ph = 104 lx 1 footcandle = 1 fc = 1 lm ft-2 = 10.764 lx Page 194 - Tutorials Appendix

Quantity Symbol Unit(s) -2 Luminous exitance Mv 1 lm m -1 Spectral radiant power Φλ(λ) 1 W nm -1 -1 Spectral radiant intensity Iλ(λ) 1 W sr nm -1 -2 -1 Spectral radiance Lλ(λ) 1 W sr m nm -2 -1 Spectral irradiance Eλ(λ) 1 W m nm -2 -1 Spectral radiant exitance Mλ(λ) 1 W m nm Table VII.IV. Units in italic are not SI units, not consistent with CIE regulations and should not be used ! VII.2. Summary of radiometric and photometric quantities

Quantification of electromagnetic Radiometric Spectral quantity Photometric quantity quantity radiation ... quantity depends on

... emitted by a source in total radiant power spectral radiant power luminous flux -

Φe Φλ(λ) Φv W W nm-1 lm (lumen)

... emitted in a certain direction radiant intensity spectral radiant intensity Iλ(λ) luminous intensity direction -1 -1 Ie W sr nm Iv W sr-1 lm / sr = cd ... emitted by a location on a surface radiant exitance spectral radiant exitance luminous exitance position on source’s surface Me Mλ(λ) Mv W m-2 W m-2 nm-1 lm m-2 ... emitted by a location on a surface radiance spectral radiance luminance position on source’s surface in a certain direction Le Lλ(λ) Lv and direction W sr-1 m-2 W sr-1 m-2 nm-1 lm sr-1 m-2 = cd m-2 ... impinging upon a surface irradiance spectral irradiance illuminance position on irradiated surface Ee Eλ(λ) Ev W m-2 W m-2 nm-1 lm m-2 = lx Table VII.V – Radiometric and photometric quantities Radiometric quantities: In the following relations, X has to be replaced by one of the symbols Φ, I, L or E:

∞ λ 2

XXd= ()λλ or XXd= λ ()λλ e λ e, range “ “ λ 0 1

with λ1 and λ2 denoting the lower and the upper limit of the respective wavelength range (for instance, UVA) Photometric quantities: In the following relations, X has to be replaced by one of the symbols Φ, I, L or E: ∞ ∞ =⋅λλλ ⋅ Photopic vision: =⋅λλλ ⋅ with Km = 683 lm / W Scotopic vision: XKXvm'()'()λ V d with K’m = 1700 lm / W XKvm Xλ () Vd () “ “ 0 0

Basic integral relations between radiometric and photometric quantities: In the following, x has to be replaced either by e (denoting radiometric quantities) or v (denoting photometric quantities). ΦΩ= Id IL= cos(ϑ ) dAML= cos(ϑ ) dΩ xx“ xx“ xx“ 4π emitting 2π surface VII.3. Sources and references for figures Fig. II.1: http://www.cameraguild.com/technology/colorimetry.htm Fig. II.19: http://www.cs.princeton.edu/courses/archive/fall99/cs426/ Fig. II.3: http://sedac.ciesin.org/ozone/docs/AS.html lectures/raster/img013.gif Fig. II.5: adapted from http://www.salsburg.com/lightcolor/lightcolor.html Fig. II.22: http://home.wanadoo.nl/paulschils/10.02.htm Fig.II.6: adapted from http://whatis.techtarget.com/ Fig. VI.1: http://www.coolibar.com/skin-cancer-in-the-us.html definition/0,,sid9_gci528813,00.html Fig. VI.11: http://www.mindfully.org/Water/UV-Disinfection- Fig. II.11: http://omlc.ogi.edu/classroom/ece532/class1/ Wastewater.htm intensity_flashlight.html Fig. VI.12: http://www.news.ucf.edu/FY2001-02/011205.html Fig. II.13: adapted from http://www.cameraguild.com/technology/ Fig. VI.22: http://www.bell-labs.com/history/physicscomm/images/ colorimetry.htm br_v_t6w.gif Fig. II.16: http://math.nist.gov/~FHunt/appearance/brdf.html Fig. VI.23: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/leds.html Fig. II.17: http://lsvl.la.asu.edu/askabiologist/research/seecolor/ rodsandcones.html Tutorials - Page 195 Appendix

VII.4. Most relevant CIE- DIN- and ISO-publications and regulations VII.4.a. DIN Publications ISO 31-6, Ausgabe:1992-09 65-1985: Electrically calibrated ters DIN 4512-8, Ausgabe:1993-01 Größen und Einheiten; Teil 6: thermal detectors of optical radia- 114/2: International intercompari- Photographische Sensitometrie; Licht und verwandte elektromagne- tion (absolute radiometers) son on transmittance measurement Bestimmung der optischen Dichte; tische Strahlung 69-1987: Methods of characteriz- - Report of results and conclusions Geometrische Bedingungen für ISO 8599, Ausgabe:1994-12 ing illuminance meters and lumi- 114/3: Intercomparison of lumi- Messungen bei Transmission Optik und optische Instrumente - nance meters: Performance, charac- nous flux measurements on HPMV DIN 4512-9, Ausgabe:1993-01 Kontaktlinsen - Bestimmung des teristics and specifications lamps Photographische Sensitometrie; Spektral- und Licht- 70-1987: The measurement of 114/4: Distribution temperature Bestimmung der optischen Dichte; Transmissionsgrades absolute luminous intensity distri- and ratio temperature Spektrale Bedingungen butions 114/5: Terminology relating to DIN 5030-2, Ausgabe:1982-09 VII.4.b. CIE Publications 75-1988: Spectral luminous effi- non-selective detectors Spektrale Strahlungsmessung; 13.3-1995: Method of measuring ciency functions based upon bright- 114/6: Photometry of thermally Strahler für spektrale Strahlungs- and specifying colour rendering of ness matching for monochromatic sensitive lamps messungen; Auswahlkriterien light sources New edition point sources, 2° and 10° fields 116-1995: Industrial colour differ- DIN 5031 Beiblatt 1, Ausga- (including Disk D008) 76-1988: Intercomparison on ence evaluation be:1982-11 15.2-1986: Colorimetry, 2nd ed. measurement of (total) spectral 118-1995: CIE Collection in colour Strahlungsphysik im optischen 16-1970: Daylight radiance factor of luminescent and vision Bereich und Lichttechnik; Inhalts- 17.4-1987: International lighting specimens 118/1: Evaluation of the attribute verzeichnis über Größen, Formel- vocabulary, 4th ed. (Joint publica- 78-1988: Brightness-luminance of appearance called gloss zeichen und Einheiten sowie Stich- tion IEC/CIE) relations: Classified bibliography 118/2: Models of heterochromatic wortverzeichnis zu DIN 5031 Teil 18.2-1983: The basis of physical 82-1989: CIE History 1913 - 1988 brightness matching 1 bis Teil 10 photometry, 2nd ed. 84-1989: Measurement of lumi- 118/3: Brightness-luminance rela- DIN 5031-2, Ausgabe:1982-03 19.21-1981: An analytic model for nous flux tions Strahlungsphysik im optischen describing the influence of lighting 85-1989: Solar spectral irradiance 118/4: CIE guidelines for co- Bereich und Lichttechnik; Strah- parameters upon visual perform- 86-1990: CIE 1988 2° spectral ordinated research on evaluation of lungsbewertung durch Empfänger ance, 2nd ed., Vol.1.: Technical luminous efficiency function for colour appearance models for DIN 5033-7, Ausgabe:1983-07 foundations photopic vision reflection print and self-luminous Farbmessung; Meßbedingungen für 19.22-1981: An analytic model for 87-1990: Colorimetry of self- display image comparisons Körperfarben describing the influence of lighting luminous displays - A bibliogra- 118/5: Testing colour appearance DIN 5037 Beiblatt 1, Ausga- parameters upon visual perform- phy models: Guidelines for co- be:1992-08 ance, 2nd ed., Vol.2.: Summary 95-1992: Contrast and visibility ordinated research Lichttechnische Bewertung von and application guidelines 96-1992: Electric light sources - 118/6: Report on colour difference Scheinwerfern; Vereinfachte Nutz- 38-1977: Radiometric and photo- State of the art - 1991 literature lichtbewertung für Film-, Fernseh- metric characteristics of materials 98-1992: Personal dosimetry of 118/7: CIE guidelines for co- und Bühnenscheinwerfer mit rotati- and their measurement UV radiation ordinated future work on industrial onssymmetrischer Lichtstärkever- 39.2-1983: Recommendations for 101-1993: Parametric effects in colour-difference evaluation teilung surface colours for visual signal- colour-difference evaluation 121-1996: The photometry and DIN 5037 Beiblatt 2, Ausga- ling, 2nd ed. 105-1993: Spectroradiometry of goniophotometry of luminaries be:1992-08 40-1978: Calculations for interior pulsed optical radiation sources 124-1997: CIE Collection in colour Lichttechnische Bewertung von lighting: Basic method 106-1993: CIE Collection in photo- and vision, 1997 Scheinwerfern; Vereinfachte Nutz- 41-1978: Light as a true visual biology and photochemistry 124/1: Colour notations and colour lichtbewertung für Film-, Fernseh- quantity: Principles of measure- (1993): order systems und Bühnenscheinwerfer mit zu ment 106/1: Determining ultraviolet 124/2: On the course of the disabil- einer oder zwei zueinander senk- 44-1979: Absolute methods for action spectra ity glare function and its attribution rechten Ebenen symmetrischer reflection measurements 106/2: Photokeratitis to components of ocular scatter Lichtstärkeverteilung 46-1979: A review of publications 106/3: Photoconjunctivitis 124/3: Next step in industrial col- DIN 5039, Ausgabe:1995-09 on properties and reflection values 106/4: A reference action spectrum our difference evaluation - Report Licht, Lampen, Leuchten - Begrif- of material reflection standards for ultraviolet induced erythema in on a colour difference research fe, Einteilung 51.2-1999: A method for assessing human skin meeting DIN 5042-1, Ausgabe:1980-10 the quality of daylight simulators 106/5: Photobiological effects in 125-1997: Standard erythema dose Verbrennungslampen und Gas- for colorimetry (with supplement plant growth 127-1997: Measurement of LEDs leuchten; Einteilung, Begriffe 1-1999) 106/6: Malignant melanoma and 130-1998: Practical methods for DIN 5043-1, Ausgabe:1973-12 52-1982: Calculations for interior fluorescent lighting the measurement of reflectance and Radioaktive Leuchtpigmente und lighting: Applied method 106/7: On the quantification of transmittance Leuchtfarben; Meßbedingungen für 53-1982: Methods of characteriz- environmental exposures: limita- 134-1999: CIE Collection in photo- die Leuchtdichte und Bezeichnung ing the performance of radiometers tions of the concept of risk-to- biology and photochemistry, 1999 der Pigmente and photometers benefit ratio 134/1: Standardization of the terms DIN 19010-1, Ausgabe:1979-03 55-1983: Discomfort glare in the 106/8: Terminology for photosyn- UV-A1, UV-A2 and UV-B Lichtelektrische Belichtungsmes- interior working environment thetically active radiation for plants 134/2: UV protection of the eye ser; Skalen, Kalibrieren 59-1984: Polarization: Definitions 108-1994: Guide to recommended 134/3: Recommendations on DIN 58141-5, Ausgabe:1993-11 and nomenclature, instrument practice of daylight measurement photobiological safety of lamps. A Prüfung von faseroptischen Ele- polarization (including disk) review of standards menten; Bestimmung der Faser- 60-1984: Vision and the visual 109-1994: A method of predicting 135-1999: CIE Collection in vision bruchrate von Licht- und Bildlei- display unit work station corresponding colours under differ- and colour and in physical meas- tern 63-1984: The spectroradiometric ent chromatic and illuminance urement of light and radiation, DIN 58141-10, Ausgabe:1997-02 measurement of light sources adaptations 1999 Prüfung von faseroptischen Ele- 64-1984: Determination of the 114-1994: CIE Collection in pho- 135/1: Disability glare menten - Teil 10: Bestimmung der spectral responsivity of optical tometry and radiometry 135/2: Colour rendering (TC 1-33 Beleuchtungsstärke und des effek- radiation detectors 114/1: Survey of reference materi- closing remarks) tiven Öffnungswinkels von Kalt- als for testing the performance of 135/3: Supplement 1-1999 to CIE lichtquellen spectrophotometers and colorime- Page 196 - Tutorials Appendix

51-1981: Virtual metamers for retinal hazard 142-2001: Improvement to indus- CIE Draft Standard DS assessing the quality of simulators 138/2: Action spectrum for photo- trial colour difference evaluation 012.2:2002: Standard method of of CIE illuminant D50 carcinogenesis (non-melanoma 148:2002: Action spectroscopy of assessing the spectral quality of 135/4: Some recent developments skin cancers) skin with tunable lasers daylight simulators for visual ap- in colour difference evaluation 138/3: Standardized protocols for 149:2002: The use of tungsten praisal and measurement of colour 135/5: Visual adaptation to com- photocarcinogenesis safety testing filament lamps as secondary stan- CIE Draft Standard DS plex luminance distribution 138/4: A proposed global UV dard sources 013.2:2002: International standard 135/6: 45°/0° spectral reflectance index 151:2003: Spectral weighting of global UV index factors of pressed polytetrafluoro- 139-2001: The influence of day- solar ultraviolet radiation CIE Draft Standard DS ethylene (PTFE) powder light and artificial light on diurnal CIE Draft Standard DS 010.3- 015:2002: Lighting of work places 138-2000: CIE Collection in and seasonal variations in humans - 2002: Photometry - The CIE sys- - outdoor work places Photobiology and Photochemistry, a bibliography (also available as tem of physical photometry 2000 disk) 138/1: Blue light photochemical

VII.5 National Calibration Laboratories

DKD – German Accreditation Institution

The German accreditation institu- tem. The qualification of the trace- Europe this standard is not compul- Rotterdam, which was founded out tion DKD (Deutscher Kalibrierdi- ability to national standards is the sory. Instead of this the ISO/IEC of the Western European Calibra- enst) was founded by German trade job of the Physikalisch-Technische Guide 25 (General requirements on tion Cooperation (WECC) and the and industry and the German state Bundesanstalt (PTB), the German the competence of testing and Western European Laboratory represented by the Physikalisch- national standards laboratory. The calibration laboratories, December Accreditation Cooperation Technische Bundesanstalt (PTB), PTB will define, realize, keep and 1990) is recognized. In content, EN (WELAC) in 1994. Within the the German national standards transmit the physical quantities of 45001 and ISO/IEC Guide 25 are EAL different national accredita- laboratory. The basic idea of the the SI-system, such as a meter, a identical. This is the basis for the tion institutes cooperate with the DKD is to transfer as many PTB second, a kilogram, a candela, etc. mutual appreciation between the goal of international acceptance of responsibilities to industry as possi- To ensure objective results, equal European cooperation for Accredi- calibration certificates of the EAL- ble, including the calibration of standards must be used. The cali- tation (EA) and its extra-European calibration laboratories. In Novem- measurement and testing equip- bration of measurement and testing partners. In 1999 ISO/IEC 17025 ber 2000, 34 accreditation institu- ment. The DKD ensures the trace- arrangements based on SI-units is a took the place of EN 45001 and tions from 28 countries, including ability of measurement and testing basis for correct, comparable, ISO/IEC Guide 25 which elimi- the PTB, the accreditation institu- equipment to national standards by recognizable and therefore measur- nated any formal differences. tion of the DKD, signed a Mutual the accreditation and continuous able values, which can be audited. Recognition Arrangement (MRA) auditing of industrial calibration Within the DIN ISO 9000 ff. stan- Existing DKD calibration of the International Laboratory laboratories. Therefore, calibrations dard the relationship between qual- laboratories automatically Accreditation Cooperation (ILAC). carried out by DKD accredited ity management and calibration are qualify for ISO/IEC/EN More information about this ar- laboratories offer a secured trace- intertwined in part for continuous 17025 conformance. rangement and the participating able and well-documented link to control of measurement and testing DKD homepage: http:// countries is available online at national calibration standards. An equipment. Without exception, www.dkd.info/ uninterrupted traceable chain of DKD accredited calibration labora- http://www.ilac.org. calibration links to national stan- tories fulfill the requirements of the The European position of the DKD dards is absolutely necessary for European standard EN 45001 is noted by its membership in the acceptance of measurement devices (general criteria to operate a testing European Cooperation for Accredi- by any quality management sys- laboratory, May 1990). Outside of tation of Laboratories (EAL) in

PTB – Physikalisch-Technische Bundesanstalt

The Physikalisch-Technische cial accreditation institution for on Traceability of Measurement and PTB for the SI Units of Lumi- Bundesanstalt (PTB) is the highest DKD calibration laboratories for Standards was signed between the nous Intensity and Luminous Flux technical authority for metrology in optical radiation measurement Physikalisch-Technische Bunde- was officially recognized in April Germany. The PTB define, realize, quantities such as Gigahertz-Optik. sanstalt (PTB) and the National 1999. keep and transmit the physical The PTB is also actively working Institute of Standards and Technol- PTB homepage: quantities of the SI-system, such as on bilateral acceptance on national ogy (NIST) USA. The Equivalence http://www.ptb.de/ a meter, a second, a kilogram, a standards. Because of their activi- of the National Standards of NIST candela, etc. The PTB is the offi- ties in 1995 a Statement of Intent

NRC – National Research Council Canada

The NRC’s Institute for National ric standards, and provides associ- turing, telecommunications, public dated: 2001-07-18 Measurement Standards Photome- ated, high-accuracy measurement health and safety, and the environ- (http://www.nrc.ca/inms/ try and Radiometry Group main- services to industry, university, and ment.NRC INMS Photometry & phot_rad/prei.html) tains photometric, radiometric, government clients involved with Radiometry Home Page Last Up- spectrophotometric and colorimet- lighting, transportation, manufac- Tutorials - Page 197 Appendix

NIST – U.S. National Institute of Standards and Technology

The Optical Technology Division luminous intensity, the candela. As disseminates these standards by mentation, and in application of of NIST's Physics part of its responsibilities the Divi- providing measurement services to optical technologies in nanotech- Laboratory has the mandate to sion: Develops, improves, and customers requiring calibrations of nology, biotechnology, optoelec- provide a high quality national maintains the national standards for the highest accuracy and contrib- tronics, and in diverse industries measurement infrastructure to radiation thermometry, spectroradi- utes to the intellectual reservoir of reliant upon optical techniques. support industry, government, and ometry, photometry, colorimetry, technical expertise by publishing NIST Physics Laboratory Optical academia who are reliant upon and ; provides descriptions of NIST developed Technology Division Home Page, optical technologies for their com- National measurement standards advances in appropriate scientific Last updated: April 2002 petitiveness and success. As a part and support services to advance the journals and books; conducts basic, of this mandate, the Division has use and application of optical tech- long term theoretical and experi- (http:// the institutional responsibility for nologies spanning the ultraviolet mental research in photophysical www.physics.nist.gov/ maintaining two SI base units: the through microwave spectral re- and photochemical properties of unit for temperature, the kelvin, gions for diverse industrial, gov- materials, in radiometric and spec- Divisions/Div844/ above 1234.96 K and the unit of ernmental, and scientific uses; troscopic techniques and instru- about_otd.html)

NPL – National Physical Laboratory UK

The NPL is UK’s National Stan- industrial and academic measure- ñ All types of optical radiation the highest accuracy optical meas- dards Laboratory for Physical ment requirements throughout the sources urement references in the world as well as to enable the fostering of Measurements. NPL's Optical IR, Visible, and UV spectra, pro- ñ Optical radiation detectors and Radiation Measurement (ORM) viding a comprehensive range of new ideas and techniques. Areas in associated devices Group provides services which are Measurement and Calibration which NPL is a recognized world the backbone for optical radiation Services, Instrumentation Products, ñ Optical properties of materials leader include the development of measurements in the UK and inter- Training and Consultancy. and components the first cryogenic radiometer and nationally. ñ Aspects of appearance including the use of lasers for radiometry. Here the UK's Primary Standards Some of the range of Measurement colour, haze and gloss NPL ORM Introduction Web Page: and scales are maintained, and and Calibration Services, traceable http://www.npl.co.uk/ pioneering research in measure- to national standards, available in The development of NPL's Primary optical_radiation/ ment science is carried out. this field, includes the characteriza- Standards and Measurement © Crown Copyright 2002 ORM anticipates and responds to tion and calibration of: Scales, enables the UK to maintain Page 198 - Tutorials Appendix

VII.5 National Calibration Laboratories See chapter Calibration Services in this catalogue or on Gigahertz-Optik’s homepage