INFRARED WINDOWS IN INDUSTRIAL APPLICATIONS I a Andres E. Rozlosnik a SI Termografía Infrarroja -- Buenos Aires -- Argentina
ABSTRACT
This paper is a slightly modified version the one presented orally at THERMOSENSE XXVI April 13-15 2004 SPIE [5405-46] Session 8: Research & Development.
Many industrial applications exist for which, due to the lack of access or for other reasons, thermal images or thermometry measurements cannot be generated. This happens especially when the target is inside an enclosure and cannot be viewed, for safety - process reasons, except through a window transparent to visible energy only. The traditional materials transparent to visible radiation ~ 380nm – ~780nm are not, in general, transparent to infrared radiation. It is clear there is a more specific exception in the NIR band. This lack of IR transmission in traditional glass / crystals has required the search for other alternatives in order to carry out measurements or generate images with instruments that work in the thermal bands (MWIR -LWIR) of the electromagnetic spectrum. This paper includes infrared windows basics and a study of attenuation with LWIR sensor of two windows two colors transmission (MWIR / LWIR) commonly installed in medium voltage electrical enclosures.
Keywords: Infrared windows / Theory / Thermometry / Transmission / Attenuation
1 – INTRODUCTION
Gas, liquid and solids can absorb / emit, transmit (+ scatter) and reflect electromagnetic radiations. For example the terrestrial atmosphere, which is a gas, is not transparent to all wavelengths and when it transmits, it does not transmit 100%. Some absorption, scattering and small reflections do exist. There are two basic atmospheric windows in the so-called thermal band, and they are approximately at λ = 3-5 µm (MWIR) and λ = 8-14µm. (LWIR). The level of transparency in these two bands depends mainly on: altitude, humidity, path length, pressure, soot/dust/aerosol concentration and temperature of the medium where we are carrying out the measurement. In other words: in which part of the planet are we doing the measurement? Is our target in the horizontal or vertical path? What is the distance to the target? What are the weather conditions? Is the atmosphere clean? etc. The two major constituents in the atmosphere that vary with temperature and altitude are ozone and water vapor. CO2 is part of the permanent composition of dry atmosphere. CO2 and water vapor (H2O) are the major IR absorbers.
Figure N°1 shows atmospheric transmission curves for vertical and horizontal paths corresponding to the two thermal bands: MWIR (~ 2.5µm -- ~ 5µm) LWIR (~ 7µm - ~ 14µm). The vertical path length is 100 km and the horizontal path length is 1 km at sea level. (Source: Raytheon Vision Systems)
The basic law that governs atmospheric transmission is based on the Beer –Lambert study: the exponential decrease in transmitted IR intensity. In the MWIR region it is an example of the problem of applying Beer’s law to an entity as complex as the earth’s atmosphere. Sometimes the extinction coefficient is very wavelength dependent. - γ R Φ = Φ0 . e Φ0 = radiant flux (watt) - γ R Atmospheric Transmission = τ = e (the negative sign indicate a loss of mechanism -- vacuum is the only perfectly transparent medium) Where: R = Path length γ = Extinction coefficient γ = σ (λ) + K (λ) ≡ (in the visible sometimes call turbidity) σ = Scattering component -- air molecules, aerosol, (suspended solid particles smoke dust) hydrometeors= suspended water droplets clouds and fog K = Absorptive component = gaseous lines /continues absorption (CO2 //H2O.etc) Normally we use infrared cameras in civil--industrial applications through relatively short media path lengths (camera-to-target through terrestrial atmosphere) and, depending on the camera we use, there is little or no correction necessary for atmospheric absorption (transmission is considered to be unity). Often environmental conditions can be neglected when they are constant and only qualitative measurements are required. Now, when we consider long path-length applications such as military or meteorological, atmospheric transmission becomes of greater importance and affects the SNR. (signal-to-noise-ratio) much more. A special case is when we look through the atmosphere in a furnace in order to see the walls and pipes. Inversely, in these cases, we need use filters to avoid undesirable radiation of the hot gases and flame.
Horizontal 1 km path
Horizontal Vertical 100 km path Vertical Wavelength (µm) Figure N°1—Atmospheric transmission MWIR–LWIR (Transmission values taken from Raytheon Vision Systems Chart)
There is mathematical analogy (both exponential) between the atmospheric transmission and the transmission through a semitransparent solid like an infrared window: - α b The transmission through a semitransparent solid is: τ = e
In which b is the thickness and α the absorption coefficient of the substrate respectively. Larger b and α means more attenuation, loss of transmission. On the other hand the absorption coefficient (α) is temperature dependant and varies with the purity of the material. (See Figure N° 2) When do we need to install or use an IR Window? When, for some reason (security… environment) a medium or specific atmosphere needs to be isolated, or it is already isolated and we want to know the thermal status inside it. The IR window should be transparent at wavelengths compatible with those of the IR sensor we will use. Additionally, we would use a window (UV-Visible-Infrared) when two different media or environments exist and electromagnetic radiation at specified wavelengths pass between the two. The attenuation effect of the window itself should be added to the atmospheric transmission phenomenon. (sometimes this is negligible) As an analogy, windows that we use in daily life (houses, offices, industrial buildings, cars etc) permit the visible & NIR infrared radiation go in, which allows us to view the external world and at the same time keep us isolated from the outside (temperature, winds, snow… dust etc). The Cambridge Dictionary defines a glass window in this way: “space usually filled with glass in the wall of a building or in a vehicle, to allow light and air in and to allow people inside the building to see out.” So IR window are used to isolate, or maintain the isolation between two media with different parameters: Medium 1 ≠ Medium 2. Between the two any of these situations can occur:
-- Pressures differential (example vacuum & high pressure differential/survive flight environment) -- Temperature differential -- Different atmosphere - gas constituent -- Medium safety banned (example high voltage electrical &-nuclear applications) -- Protect systems from dust or other particles like salt spray, raindrops or effluents and excessive moisture.(In the infrared for example: IR windows in surveillance cameras enclosures-missiles domes, etc)
Why can’t we use standard or reinforced glass in IR Inspections or IR Systems? Glass does not transmit MWIR or LWIR radiation! The cutoff wavelengths are around 0.32µm (UV) --2.5 µm (IR) (depending in thickness & type) Glass consists mostly of molecules of silicon dioxide, (SiO2) although there are usually also impurities present, even a certain amounts of water. This means that there are several different ways in which light passing through the glass can give up energy to the glass. Infrared photons have energies of about 0.4 ev⇒ 3 µ-- ~90 mev.⇒ 14µ (E=h.ν) This is approximately the energy corresponding to a rotational or vibrational state of a silicon dioxide (SiO2) molecule or a water molecule. When infrared photons pass through the glass, they tend to cause the molecules to rotate and vibrate. If one molecule starts to rotate or vibrate, it can share its extra
2
IR Window - α b Fresnel reflection losses equation: good approximation for tilt angles up to 45° Internal Transmission = e = X2 / X1 = τi b =:Thickness 2 The emitted (εw) flux its is External Transmission = X / X0 (total) n1 - n2 determined by a series of R = expression accounting for the n1 + n2 multiple reflections and transmittance losses. The total α = α absorption + α scatter = X2 - X1 / X0 α is temperature dependant! 2R radiance from surface I given as infinite series (1) 4πk Total reflectance (TR) = K= Extinction coefficient 1 + R α = X0 X1 X2 X n. λ 2 n n2 If an Infrared windows reach high T = 1 - R = n 2 + 1 temperature depending the material n1 Refraction at interface: n1 it can oxidize and dramatically 2 increased the emissivity. (εw) In - α b Do not change radiation (1 – R ) e frequency (ν) order to recover the transmission properties probably the substrate Ta = Surfaces finish: should be polish again. 2 - 2 αb α = Absorption 1 - R e Machining R = One surfaces reflection (Fresnel) The thickness variation Grinding εw (1) = (1-ρ w) (1-τ w) / (1- τ w ρ w) = TR = Total reflection produce curvature into Polishing T = Transmission non abs. IR Window emerging wavefront - α b Ta = Transmission abs. IR Window (could lose of focus if not (1-R) . ( 1 - e ) εw = Emissivity IR Window optically flat) = τi = Internal transmission - α b n = Refraction index Acceptable 1 – R . e -1 Fourier Transform IR (FTIR) spectrometers can be used to determine α (cm ) λ/4=thickness variations FTIR spectrometers typically can achieve transmission accuracies of better than 0.1 %. for IR applications Figure N° 2 --- Transmission of radiation through substrates -classical formulas - energy with nearby molecules, and soon the initial energy of the photon is distributed amongst many molecules. The result is that the photon has been absorbed, and the glass is a little warmer. Another characteristics of glass that is that it is an amorphous material (vitreous). This feature it is not directly related to the transmission itself. There are some amorphous materials that transmit in the infrared. (AMTIR: Amorphous Material Transmitting Infrared). AMTIR-1, for example, is an amorphous IR transmission material. It was originally produced for night vision systems, but it has other applications including optical elements and optical sensors for remote temperature sensing. To be amorphous (no regular crystalline structure) means, basically, that it can be molded and that the mechanical strength is not as good as crystalline and polycrystalline solids. Also, crystallines and polycrystallines have unique optical properties compared with glasses, which give them a wider choice of applications.
Molecular arrangement a--Molecular arrangement in a crystal b--Molecular arrangement in a glass (amorphous)
Single crystals form only under special conditions. The normal solid form of an element or compound is polycrystalline. As the name suggests, a polycrystalline solid or polycrystal is made up of many crystals. The properties of a polycrystal are different from those of a single crystal. The individual component crystallites are often referred to as grains and the junctions between these grains are known as grain boundaries. Special note: Conventional liquids, when cooled, have a freezing point at which they suddenly become solid. Liquid glasses, by contrast, become firm gradually as they cool. At room temperature the rate of flow is so slow that it would take a very long time to change shape. For most practical purposes, glass may be treated as a solid despite its appearance. Glass is really a highly viscous liquid rather than a solid. Infrared transparent windows for different applications (industrial included) can be manufactured as: Crystals, Polycrystals, Amorphous and Plastics. Most of them are polycrystalline materials.
Structure summary:
Crystalline solids (a) molecules are ordered in a regular lattice
Polycrystalline compact mass randomly oriented of crystallines
Glasses (b) molecules are disordered but are rigidly bound Fluids molecules are disordered and are not rigidly bound Plastics semi crystalline –amorphous
2 – INFRARED WINDOWS BASICS
2.1 Mathematic of transmission of radiation through substrates
As we mentioned before, Figure N° 2 shows the basic formulas that involve reflection, transmission and emission through a real piece of infrared material.
2.2 Balance of energy of a substrate Figure N° 3 shows the scheme of the balance of energy. The irradiation incident to infrared window surfaces can be reflected, scattered, absorbed or transmitted. The reflection and/or the scatter might be as the result of irregularities in the surface of the window or within the material.
E = Irradiance (watt /m2) = 100% = α + τ + ρ + s (α-Absorption + ρ-reflection + τ-transmission + s-scattering)
Opaque: e + ρ = 1 ---- Most thermography applications ----- τ = 0 (non – transmitting material) In general, transmitting materials can be classified as either transparent or translucent. The transparent materials are homogeneous.
IR Window bulk material Thickness = b Coating
IR photon can be scatter from incident direction: Target side n1 Light passes into inhomogeneous region of the Sensor side n1 First surface Sample: grain boundaries, impurities and voids Second surface
(s) First surface (s) Second surface
backscatter / forward scatter (s) Bulk backscatter / forward scatter backscatter / forward scatter Scattering (s) = striae Surfaces pitting = scattering and (birefringence), inclusions, transmission loss. Energy it’s absorbed at the bubbles, strain, digs, scratches surfaces of the substract as well α E = Irradiance inside the bulk of the material (watt . m ²) α τ
L = Radiance (watt . m ² . sr ¹) ρ(1) Parallelism: To avoid multiple reflections between the faces, a small wedge angle may Windows surfaces could frost over actually be specified. (*) under certain conditions: wire mesh E = 100% = α + τ + ρ (1+2) + s (1+2) = /external/ internal reflection screen, thin film or semiconductor coating on the window for heat ρ(2) Self-emission: windows surfaces. It is a volumetric phenomenon –semitransparent material. In some applications in order to avoid the self -emission due to surrounding n2 temperature and the self- absorption, Scattering at the IR window edges the crystal could be refrigerated in their can increase the apparent IR perimeter with water or by air injection. window emissivity.
Figure N° 3 --- Balance of energy at the infrared window (physically flat) Transmission of Radiation
STANDARD LAY OUT Lb (λ, T) = Spectral radiance emittance (Watt . m¯ ² . microns¯¹ . Sr¯¹) ⇒ blackbody Background Radiant flux per unit wavelength interval at the wavelength λ ,per unit projected area and per unit solid angle incident on or passing through, or emerging in specific direction from a εb. (1 - ε t) Lback1 (t1) specified point in specified surfaces. Graybody –non graybody + Spatial geometrical dependence or directional nature (x,y,θ,φ)-// dω-- Solid angles steradian (sr).
Target Medium 1 (opaque material) εt L (Tx) (std. atmosphere) (1-τ1) Lm 1 (t1) (k) Tx IR Sensor Rd (λ)
L (λ, T, x ,y ,θ ,φ) Vdet (λ)
Self-emission: obscure the target / detector saturated / small SNR. Self-emission IR WINDOW LAY OUT is caused by: emission from gray body or non-gray body radiator window, (lack IR Windows of 100% transmissions) and gain strength with the ~ fourth power of its absolute M = εt σ T⁴ = (Semitransparent material) temperature. Emittance (gray body) Background
εb. (1 - ε t) Lback2 (t2) ε w Lw (tw) (1-τ1) Lm 1(t1)
(1-τ2) Lm2 (t2) Medium 2 Tw (λ, T) (k) Rd (λ) Target Medium 1 IR Sensor (opaque material) εt L (Tx) Tx εt. (1 - εw -τ w) L (Tx)
(+ Lm2 (t2)) Vdet (λ) εt: target emisivity Background L (λ, T, x ,y ,θ ,φ) εb: background emissivity ε w Lw (tw) εb. (1 - εw -τ w) L Back1 (t1) εw: window emissivity (could be inside a furnace) τw: window transmission (Tw) Could be a camera enclosure. τ1/2: mediums transmissions (components with worthless values) Rd: responsivity of the detector K : constant of the system If necessary window can be tilted (~10°) to avoid Narcissus. Vdet: output detector (cold spot / hot spot) t : different temperatures Figure N° 4 ---Thermometry general lay out
7 The translucent materials are optically inhomogeneous and any incident radiation is scattered and loses all identity. Objects cannot be seen through them. The flux transmitted by a translucent material is complete scattered. The transparent materials always have some scattering although the flux lost is usually negligible. 2.3 Thermometry general lay out -- IR window infrared scenario
Figure N°4 shows the scheme of the infrared radiation scenario without an IR window (standard lay out) and with an IR window. (IR Window lay out) Depending on the specific case, some of the IR radiation components shown in these two drawings can be ignored, as being insignificant when compared to the target radiation component of interest. The combination of all of the factors (target emission, target reflection, atmospheric transmission, window transmission and detector responsivity) produces the detector signal, which can be summarized as: output of the detector (Vdet-λ-- also see 3.5.1). Figures N° 2/3/4 are interrelated and supplemental to each other. Keep in mind that these layouts could be presented of different ways.
2.4 Infrared window specifications
Industrial Infrared Windows are precision optical components that must be selected, specified, and designed prior to construction and usage. Which are the basic parameters to consider in the design of a specific IR window? Which aspects or topics are more relevant in an IR window material selection and design? Figure N° 5 show six fields that are involved in the design of certain type of IR windows: mechanical, thermal, chemical, optical, crystallography and dimensions. Below, in alphabetical order, are listed basic guidelines, or memory aids for consideration in the process of specifying an infrared window: The starred (*) items are usually most relevant in our opinion --Atmospheric constituents of the two media (fluids –solids) (*) --Budget --Dual mode System (Radar –(RF)-- Infrared) –Military applications --FOV--IFOV Target size & distances (depth –width etc.) (*) --Lifetime expected for IR window --Media abrasion, humidity and thermal cycle requirements --Outdoors or indoors application-space --Pressure differential between media (*) --Qualitative or quantitative application --Target estimated temperature and emissivity (*) --Target geometry --Temperature expected for the window (Self emission :obscure the target / detector saturated / small SNR) --Temperature variation between the two media (Changes = transmission /index refraction) (*) --Transient heat loads between surfaces – (IR Window: Thermal shock exposure / thermal conductivity -- diffusivity) (*) --Thermal references near the target --Wavelength (relative spectral response) of the IR sensor or future potential IR sensors to be used in the application (*)
2.5 Infrared materials – coatings
Once the application and parameters have been specified the next step is to select the appropriate material to fulfill the specification. On the table of Figure N° 6 are listed 24 (twenty four) well know infrared transparent materials. The list is in alphabetical order following their chemical composition and describing only the basic parameters: bandwidth transmission wavelength (λ) and approximately index of refraction for that bandwidth. Refraction index changes with wavelength, temperature and purity of the material. The values for transmission and refraction shown in the table are at ambient temperature. Temperature rises in some materials cause dramatic drops in transmission. Germanium, which is a very popular material for infrared applications, is a clear example of that phenomenon. A 3mm thick germanium window (which does not transmit in the visible) at 300 Kelvin’s transmits approximately 47% (without coating) starting from at 2µm of wavelength. At 500 Kelvin’s the transmission is practically zero, with the loss more marked toward to the LWIR region. The cutoff transmissions wavelengths on the table of Figure N° 6 are representatives and can also change with temperature, thickness and purity of the material. Be aware that there are more infrared transmitting materials than those listed. A more complete list can be found, for example, in the Infrared & Electro-Optical Systems Handbook ERIM –SPIE Press or in the Infrared Hand Book. Coatings may be applied under certain conditions 1--Mechanical Properties 4--Optical Properties
Strength Mpa Transmission range & % λ Wavenumber 1000 cm ¯¹ = 10(µm) -- dt/dt Elastics constant limit MPa Transmission - Reflection - Absorption τ -- ρ -- α ( ≡ ε) Fracture toughness ------Absorption coefficient cm ¯¹ Poisson’s Ratio ------Refractive index (n) -Dispersion Thermorefractive coefficient= dn(λ) /dt Young’s Modulus σ/ ε = Stress (Mpa) / strain Scattering (surfaces + bulk material) Degrades image sharpness (S) Density gr/cm³ Optical homogeneity (ppm)--Birefringence Isotropic -anisotropic RF Properties (Radar) dual mode Shear Modulus GPa Dielectric constant/Loss tangent /Microwave (Kg / mm²)- Moh: Talc 1- Diamond Protective –Antireflection -- Filtering Hardness Coating 10 5--Crystal Structure - Crystallographic 2--Thermal Properties Syngony Cubic --Hexagonal --Tetragonal Thermal conductivity K (w/cm K) Symmetry glass Thermal diffusivity m² /seg Cleavability (Highly perfect -perfect --imperfect) Thermal expansion / ºC Lattice constants (Angstrom) ------Melting point ºC Amorphous Glass Specific Heat Capacity J/(kg•ºC) Thermal shock –Temperature Figure of merit 6--Dimensions Devitrification Temperature ºC Diameter (aperture) / width/ Length mm ------+ / − Annealing Temperature ºC Thickness Mm ------+ / − Stagnation point ºC Parallelism minute of arc Interferometry (µm) Exam /10 or /4 3--Chemical Properties Flatness Surface finish / quality Peak to valley nm / scratch-dig Chemical Composition Major components Bow -Concave /Convex Units are generally micrometers Erosion Rain --Sand Solubility --water / acids/organic gram/100 cm³ Figure N°5 --- Infrared windows specifications Chemical name ---Trade Name Example IRTRAN (Kodak) IR windows material properties that should be examined in terms of the intended systems applications or requirements
Index of Index of Transmission Transmission Chemical Material refraction Chemical Material refraction Range (#) Range (#) (#) (#) Silver Potassium AgCl 0.5 - 20µm 2.0 / 1.90 KBr 0.25 - 25µm 1.58 / 1.46 Chloride Bromide Al2O3 Potassium Sapphire 0.3 – 5 µm 1.79 / 1.66 KCl 1.10 - 20µm 1.47 / 1.39 (n0 / ne) Chloride Thallium AMTIR GeAsSe --Glass 0.75 to 14µm 2.60 / 2.48 KRS-5 0.5 - 35µm 2.60 / 2.25 Bromide- Barium 1.45 / 1.32 BaF 0.2 - 11.5µm 1.52 / 1.38 LiF Lithium Fluoride 0.1 – 5 µm 2 Fluoride 1.53 / 1.48 MgF Magnesium BK7 (*) BK7 Schott Glass 0.3—2.32 µm 2 0.11 - 7.5µm 1.42 / 1.31 (n0 / ne) Fluoride Calcium CaF 0.15 - 9µm 1.47 / 1.31 NaCl Rock Salt 0.25 - 15µm 1.73 / 1.44 2 Fluoride Cadmium 2.68 / 2.59 Oxy-Fluoride CdTe 0.9 -30µm OFG IR (1-4 um) ----- Telluride Glass Cesium CsI 1.5 - 50µm 1.80 / 1.61 Si Silicon 1--10µm 3.50 / 3.41 Iodide Infrasil Diamond Carbon (C ) 0.25->100 (5µm) 2.70 / 2.36 SiO 0.4 – 3.5µm 1.46 / 1.41 2 Quartz 3.34 / 3.13 Zero Expansion Glass ceramic 1,5544 Gallium arsenide 1--10µm 0.4--3 GaAs Glass Zerodur ® (435,8nm) Irtran-2 Ge Germanium 2 - 11.5µm 4.07 / 4.00 ZnS 1 - 14µm 2.29 / 2.12 Zinc Sulfide Infrared ~1.53 Zinc HDPE / TPX Discontinuous ZnSe 1 - 18µm 2.48 / 2.33 Plastics Selenide
Figure N° 6 --- Infrared materials (single or two colors windows—MWIR / LWIR)--- (*) Visible & NIR material-- (#) approximately values at ambient temperature --(~ 3mm/thickness) ---- Birefringence materials (n0 /ordinary rays -----ne/ extraordinary rays) such as when it is necessary to avoid reflection in order to improve transmission (coatings can be a few microns thick with no emission), for protection to increase durability (coatings can be more than 10 microns: emission concerns) and for filtering IR radiation. The increase of the transmissivity of the optical elements is accomplished by reducing the index of refraction on its boundary layers. The following is a more complete list of benefits or application of the coatings: (in alphabetical order)
--Endurance for rain particle impact --Endurance for sand impact and general erosion --Enhance mechanical strength (thin coating) --Filter unwanted IR radiation (instead of using a IR filter or a blocker) --Protection from scratches --Protect the window from any type of contamination --Reduce reflection --Reduce unwanted radio frequency wavelength (radar /military)
3 – PRELIMINARY PRACTICAL TEST OF ATTENUATION IN INDUSTRIAL INFRARED WINDOWS (Fluoric IR Crystals 80 & 85)
3.1 Radiation attenuation test goals
In order to show how infrared windows can affect non-contact temperature measurements (and contrast) we are going to develop a practical attenuation analysis for a typical current industrial application. The final result of the test will be directly related to the spectral commonality among Target, IR window (+atmosphere) and Camera. (could be an infrared thermometer -non imaging) In others words, how the target emits, how the IR window (+ atmospheres) transmits and how the IR Sensor (camera) responds. To develop this test we will select IR windows that normally are used in electrical panels. These windows allow performing the IR electrical inspections in non-intrusive way, safely and with no downtime. Spectral transmission of them changes (as non gray body) with variations in bandwidth of the sensor we use. (LWIR). They don’t have flat transmission. The following table details approximately the spectral values of transmission for two 3.5mm thick sample windows called 80 and 85, at room temperature. (see Figures N° 2 for transmissions definitions and refraction)
See plotted charts Figures N° 8 a / b --11 b
transmission Indices of transmission Indices of Wavelength % refraction % refraction (λ) microns (85) (n) 85 (80) (n) 80
7 94 1.4357 94 1.3693 8 94 1.4258 91 1.3498
9 92 1.4144 79 1.3268 10 90 1.4014 50 1.3002
~LWIR 11 87 1.3865 17 1.2676 12 70 1.3696 1 1.2299 13 45 1.3499 0 14 18 1.3289 0 15 5 1.3050 0
Why are these two windows good samples to analyze? In the 90's MWIR cameras were very popular in the PPM field, so companies installed IR Windows for electrical Inspection's with very good total transmission in the MWIR but weak in the LWIR. (cheaper material on example 80 above) In the 2000's the LWIR microbolometer camera become popular. The question is, what happens if measurements are made at different target temperatures with a LWIR camera through the IR window that we call 80 (MWIR)? What is the difference in attenuation if we use another IR window (more expensive: material on example 85 above) for which the total transmission is more closely matched to the LWIR band microbolometer responsivity? What we are looking for with this test is the following:
Attenuation test Spectral commonality LWIR MWIR ~ Simulated sensor No Window No window responsivity (varies from manufacturer to Heat source: Hot water inside -- Starting manufacturer) Visibl e LWIR from ~ 90 cooling naturally to 30 Celsius The responsivity of LWIR
cameras doesn't embrace or LWIR Camera 80 coincide spectrally exactly with the atmospheric window: ~8/~14 (LWIR) and its µ relative strength is wavelength Visible MWIR ~ 3µm- 5.3µm LWIR ~ 8 µm-14µm dependant. High High Transmission MWIR MWIR Both IR windows analyzed has very high transmission in the MWIR 85 Transmission 85 80
Visibl e LWIR Visible LWIR Figure N° 8 a ---Three alternatives overlap with a simulated sensor responsivity
Figure N° 8 b --- Transmission: three alternatives tested
Detail Lay Out attenuation test Lay Out attenuati on test φ= 3” – b=3.5 mm -- 80 & 85 Fluoric IR Crystals φ= 3” – b=3.5 mm 80 & 85 Fluoric IR Crystals
Heat source: Hot water inside -- Starting from ~ 90 cooling naturally to 30 Celsius
LWIR Camera Hot water inside –Starting Target fro m ~ 90 to L (λ, T) = Spectral radiance emittance (Watt. m¯ ² . sr ¯ ¹ microns¯¹) Tx No window ~ 30 Ce lsius (L=Radiance (watt .mֿ². sr ֿ¹ 40° IR Window 85 LWIR Camera Target Target Hot water inside –Starting LWIR Camera Tx Tx fro m ~ 90 to HT -- IR window -- 85 (opaque) b’ b High transmission in LWIR ~ 30 Ce lsius (L=Radiance (watt .mֿ². sr ֿ¹
80 Thickness b = 3.5 mm b Thickness b < b’ = ~ 4.56 mm L = Radiance (watt .m¯ ² . sr ¯ ¹) LWIR Camera Hot water inside –Starting Target fro m ~ 90 to Tx LT -- IR window -- 80 ~ 35 cm 2.90E+01 2.80E+01 4 2.70E+01 W = εt σ T LWIR Microbolometer 2.60E+01 2.50E+01 2.40E+01 εt b ~ 30 Ce lsius =1 ~363.15 K = 90 Celsius 2.30E+01 2.20E+01 2.10E+01 (2.00E+01 Low transmission in LWIR L=Radiance (watt .mֿ². sr ֿ¹ 1.90E+01 0,95 1.80E+01 1.70E+01 4 1.60E+01 ~303.15 K = 30 Celsius 1.50E+01 1.40E+01 0.70 1.30E+01 40° 1.20E+01 The same optical path is used 1.10E+01 t 1.00E+01 M 0.50 = ε σ T 9.00E+00 8.00E+00 εt = 1 7.00E+00 6.00E+00 0.95 5.00E+00 (no window –gray body) 0.70 4.00E+00 3.00E+00 for the 5 spot at each4 / 5- 0.30 0.50 0.10
Spectral radiance emittance (Watt. m¯ ² . microns¯¹) ² m¯ emittance (Watt. radiance Spectral 2.00E+00 0.30 1.00E+00 0.00E+00 0.10 -1.00E+00 123456789101112131415 -2.00E+00 Celsius step. Wavelength (microns) Upper View 40° Figure N° 8 d - Test set up Figure N° 8 c - Test set up --IR Windows Tilted 1—How radiation is attenuated (temperature drop) when we use a microbolometer (~8µm -12µm) to measure a target of defined emissivity viewed perpendicularly through both IR Windows (80 and 85) 2—The effect on attenuation of varying the camera angle from the normal optical axis in situation point 1. 3—How must we adjust the emissivity setting in order to compensate for lack of blackbody radiation of the target plus the loss of transmission for both IR Windows (80 and 85). 4—What is the loss of contrast due to the use of both IR windows (80 and 85) looking a target with a microbolometer. 5—How do these results compare with the measurements using no IR windows.
3.2 Test set up
This preliminary evaluation was carried out with instruments and information (camera responsivity) that were not as good as we had wanted. To develop a more detailed and deeper evaluation of these IR windows more sophisticated instruments are needed. Nonetheless we will try to give the thermographer some idea of what should be their concerns when dealing with IR Windows an how to select them. A small office laboratory was assembled in order to be able to control the variation of the radiation transmitted through an IR window installed between the camera (microbolometer) and the target. The target was a container filled with near-boiling hot water (~ 95 Celsius) at atmospheric pressure. Figures N° 7/ 8 a/b/c/d) Measurements were made for around ~ 4 /5 degree increment of reduction from approximately 90 (~363 Kelvin) to 30 (~303 Kelvin) degrees Celsius. In other words, the water was left to cool normally until it reached 30 Celsius.) A larger ∆t would have been more appropriate in order to have a wider profile of the behavior of the windows, but the limitations in using the water container prevented this. An extended area blackbody source with low-temperature capabilities in a controlled lab environment would be the best choice for any future analysis, particularly with additional accessories in order to obtain MTF, MDT. Also new NEDT of cameras installed in sealed enclosure with fixed IR windows, etc. The following is the sequence of the IR Camera measurements (5 spots) each ~4 /5 Celsius drops (See Figures N°7 below and 8 d) In this process each 4/5-degree of temperature decrease we record a thermogram for the 5 (five) different alternatives: Spot 1 -- No window in between the camera and the container. Spot 2-- φ (window diameter) = 3 inches, 85-window material in between the camera and the container installed normal to the optical axis of the camera. Spot 3-- φ =3 inches, 85-window material in between the camera and the container turned 40 degrees from the plane normal to the optical axis of the camera. (increases the effective thickness by ~1mm Figure N° 8c) Spot 4--- φ=3 inches, 80-window material in between the camera and the container installed normal to the optical axis of the camera. Spot 5-- φ =3 inches, 80-window material in between the camera and the container turned 40 degrees from the plane normal to the optical axis of the camera. (increases the effective thickness by ~1mm Figure N° 8c)
Spot 1 Spot 2 / 4
Perpendicular to optical axis
Spot 3 / 5
Figure N° 7--- Lay out photos φ= 3” – b=3.5 mm --Fluoric IR Crystals (80/85) Angle 40 ° --optical axis More details of test set up are shown in Figure N°8a and in the superimposed chart. This includes atmospheric transmission, IR window transmission (80/85) and sensor responsivity (which varies from manufacturer to manufacturer). Figure N°8b charts transmission through the atmosphere (LWIR), and through both IR windows (85 and 80). The purpose of tilting the IR window (Figure N° 8c-- like spot 3 and 5) 40 degrees from normal is to note how attenuation varies by increasing the thickness of the windows by ~ 1mm (~ +30%). The idea was to simulate the attenuation that we get when we inspect the inside of a medium voltages electrical panel through an IR window, turning the camera right and left, up and down and just keeping it perpendicular to the IR window. In practice, thermographers move the camera in front of the window to increase the FOV inside the panel. It is a way of minimizing the number of windows required. It is important to remark that both windows are isotropic materials :optical properties are the same in every direction.
3.3 Radiation curves for real IR window’s spectral transmission values
As we said, one of our goals is to analyze how the windows (85/80) behave at different levels of radiation coming from the container (our target) and to compare them. In this process there are many variables involved: Temperature of the target, temperature of the window (during test was the same as the surrounding/ambient), window transmission (≠ flat), window reflection and emission, camera relative spectral responsivity and background temperature. We assume that the target is a gray body with an emissivity of 0.95.
The following paragraphs describe the different radiation curves plotted in Figures N° 9a-- 9b--9c--10a --10b. These graphics help to visualize and to better understand how the Planck Blackbody theoretical curves vary in different situations when essential parameters like emissivity change at the highest an lowest temperatures of the test and how window transmission changes the Planck function itself.
--Figure N° 9a shows approximate Planck blackbody curves in linear coordinates (including our reference 0.95) for the theoretical highest and lowest analyzed test points of temperature (~363 and 303 K-- Spot 1--No window) and several curves for lower constant target emissivities are plotted as well (including our reference ~ 0.95).
--Figure N° 9b shows approximate Planck blackbody curves (including our reference 0.95) in linear coordinates corresponding to the theoretical highest test point of temperature (~363 K Spot 1--No window, and the equivalent curve modified after the radiation passes through 85 window. (~363 Spot 2) In both cases curves are plotted for lower target constant emissivities (including our reference ~ 0.95).
--Figure N° 9c shows approximate Planck blackbody curves (including our reference 0.95) in linear coordinates corresponding to the theoretical highest test point of temperature (~363 K Spot 1--No window) and the equivalent curve modified after the radiation passes through 80 window. (~363 K Spot 4) In both cases curves are plotted for lower target constant emissivities. (including our reference ~ 0.95)
-- Figure N° 10a shows approximate gray body curves in linear coordinates corresponding to ~0,95 emissivity (surface of the container-black paint)) without an infrared window for highest and lowest test analyzed temperature (~363/303 K Spot 1--No window. Also overlap shown are the equivalent theoretical curves (non- graybody) after the radiation passes through both types of infrared windows, again plotted for the highest and lowest points of temperature of the test. (~363and 303 Spot 2/4)
--Figure N° 10b shows approximate radiation gray body curves in linear coordinates corresponding emission of a surface of ~ 0,95 emissivity (container-black paint) without an infrared window corresponding to the highest test point of temperature (~363 Spot 1--No window) and the corresponding curves modified after the radiation passes through both types of infrared windows for the highest test point of temperature of the test. (~363 K Spot 2 and 4) The dashed curve below represents the curve of radiation from 0.95 emissivity surfaces for the highest test temperature after it passes through a theoretical flat 60% transmission IR window (~ gray body). This is in order to visually compare the shape and slope of the radiation curve resulting after passing an IR window (as non gray body--80/85) with transmission changing through the band pass of the detector and an IR window with constant transmission (60% -as ~ gray body) through the same band pass. All these charts represent theoretical target radiation curves with and without the two windows. These charts do not consider the self-emission of the windows, (80/85) reflections from the windows (very low) and target background reflections.
14 2,90E+01 2,80E+01 2,70E+01 ~ LWIR Microbolometer 2,60E+01 Cooling down ε = 1 2,50E+01 2,40E+01 0,95 ~363.75 K = 90 Celsius 2,30E+01
) 2,20E+01 εt= 1 2,10E+01 2,00E+01 1,90E+01 1,80E+01 0.70 1,70E+01 ~303.15 K = 30 Celsius 1,60E+01 = 1 1,50E+01 εt 1,40E+01 1,30E+01 0.50 1,20E+01 1,10E+01 1,00E+01 9,00E+00 0.95 8,00E+00 7,00E+00 0.70 6,00E+00
L = Spectralradiance emittance (Watt. m¯ ² . microns¯¹ 5,00E+00 0.30 0.50 4,00E+00 3,00E+00 0.30 2,00E+00 0.10 1,00E+00 0.10 0,00E+00 -1,00E+00 123456789101112131415 -2,00E+00 Wavelength (microns) λ µm
Figure N° 9 a - Planck Blackbody + different emissivity - Linear coordinates ~363 / 303 3,00E+01
~LWIR
2,50E+01 ε = 1
0,95 (85) 2,00E+01
1,50E+01 0,50 (nw) 0,95(nw)
1,00E+01 0,50 (85)
L = Spectral radiance emittance(Watt. m¯² . microns¯¹) 5,00E+00
0,10 (nw) 0,10 (85) µm 0,00E+00 123456789101112131415 Wavelength (microns) Figure N° 9 b - Planck Linear coordinates-- No window & IR windows 85 - ~363 Kelvin’s
3,00E+01
~LWIR
2,50E+01 ε = 1
0,95 (80)
2,00E+01 0,95 (nw)
1,50E+01
1,00E+01 0,50 (80)
0,50 (nw)
L = Spectral radiance emittance (Watt. m¯ ² . microns¯¹)
5,00E+00
0,10 (80) 0,10 (nw) 0,00E+00 µm 1 2 3 4 5 6 7 8 9 101112131415 Wavelength (microns) Figure N° 9 c - Planck Linear coordinates-- No window & IR windows 80 - ~363 Kelvin’s
15
2,8E+01 2,7E+01 Planck Distribution LWIR-- Microbolometer ~ (detector responsivity - sensitivity) 2,6E+01 λ max 363 = ~ 7.89 µm 2,5E+01 Cooling down 85 window 363 K 2,4E+01 2,3E+01 2,2E+01
2,1E+01 ² . microns¯¹) m¯ (Watt. 80 window 363 K
2,0E+01 emittance L radiance = Spectral 1,9E+01 1,8E+01 1,7E+01 No window 363 K 1,6E+01 1,5E+01 1,4E+01 λ max 303 = ~ 9.56 µm 1,3E+01 1,2E+01 85 window 303 K 1,1E+01 No window 303 K 1,0E+01 9,0E+00 8,0E+00 7,0E+00 6,0E+00 85 window 303 K 5,0E+00 4,0E+00 3,0E+00 80 window 303 K 2,0E+00 1,0E+00 Wavelength (microns) 0,0E+00 1 2 3 4 5 6 7 8 9 10 11 12 13 14λ 15µm
Figure N°10 a ---Planck-- Linear coordinates No window + 85 window + 80 window ----Highest and lowest test temperature
2,8E+01 LWIR-- Microbolometer 2,6E+01 ~ (detector responsivity - sensitivity)
2,4E+01
2,2E+01 85 -- window 363 K (non-graybody) 2,0E+01 (Watt.microns¯¹) m¯ ² . No window --363 K Spectral = L radiance emittance (target assume graybody) 1,8E+01
1,6E+01 Window with a simulated 60% flat 1,4E+01 transmission--363 k (graybody) 1,2E+01
1,0E+01 80 window --363 K 8,0E+00 (non-graybody)
6,0E+00
4,0E+00
2,0E+00
0,0E+00 123456789101112131415 Wavelength (microns) λ µm Figure N° 10 b -Planck- Linear coordinates simulated 60 % window flat transmission together with no window --+ 85 window + 80 window ----Highest test temperature
16 In other words, the curves do not include all the radiosity components from the scene that may reach the camera lens. (lay out Figure N° 4) Depending the case some of them could be worthless.
3.4 Test precautions
Since we are using an elementary source in an improvised and uncontrolled laboratory, we must take the following precautions: 1-The 5(five) measurements must be carried out at the same physical point of the container and quickly enough such that the container water maintains the same temperature for the five spots, but slowly enough to allow the temperature reading of the camera to stabilize after each change in transmission scenario. 2-It is necessary to thermally isolate the windows and the camera from the water container between readings (as the water cools) to prevent them from being effected thermally by absorbing radiative energy from the water. (The distances from the camera to the container is about 35 cm. --Figures N°7 ,8c, 8d ). If the windows warm up and begin to emit consistently (particularly window material 80) it could result in an erroneous camera reading. The camera it is set very near to the window as used in the real world. At the same time we avoid reflection or emission of the anodized aluminum frame. 3-The container with hot water is tilted 5 to10 degrees physically back such that ambient (and operator) reflections will not reach the camera. The surroundings are isolated with styrofoam and incandescent lamps are turned off to avoid undesirable point-source reflections. (shielded from extraneous radiation + convection) 4-Contact temperature sensors are set within the water as well as the surface of the container to the height of the optical axis of camera. Although there is some difference between water temperatures and container surface temperature. (lost by conduction through the wall and by convection) the measurements are relative rather than absolute and absolute calibration is not necessary to evaluate the different transmission scenarios. 6-Although the container is low emissivity metal, the surface is painted with black strips to generate a zone of high emissivity (~0.95). The idea is not only to be able to observe the values of attenuation of each window for a real target at different temperatures, but also to be able to evaluate the loss of contrast due to the windows. 7-The cooling process of the container with hot water inside it is slow (since water has a high thermal capacitance) but continuous and stable. (Little radiation noise = measurement uncertainties, which increase as temperature decreases). The cooling process it is much slower as container temperature approach to the ambient temperature. (this due thermal gradient container-background decreases)
3.5 Test considerations and results
3.5.1 Temperature drops
Figure N° 11a shows the plot of all temperature values taken for the target (paint ~0.95 emissivity) during the cooling test corresponding to the three conditions; no window (Spot 1), IR window 80 (Spot 2) and IR window 85 (Spot 4) Also plotted in the charts are container water and surface temperature (non-optically recorded). Figure N° 13 a shows the 5 (five) spots (thermograms) corresponding to the highest temperature of the test. (~363 Kelvin) It can be seen that, for both windows, the difference in contrast between the thermograms taken perpendicular and at 40 degrees are almost imperceptible. We confirmed this for all temperature readings in the cooling process. We found that increasing the effective thickness by ~1mm resulted in a barely perceptible. (1 to 2% additional drop in temperature). In order to evaluate a further increase in window thickness we performed an additional test with the same source and camera, but we added to the original test (no window /85/80) a perpendicular sample of 85 material [b = 7 mm thick & φ = 130 mm (~5 inches)]. This doubled the thickness of 85 from the original test and the increase in attenuation between the 3.5mm and 7mm thick (85) window was clearly detected. Figure N°12a shows the results in temperature drop for this new test as the container cooled down. A picture of this bigger 85 crystal is shown there. Below of Figure N°11a can be found Figure N° 11b, the radiation curves for no window (Spot 1), IR window 80(Spot 2) and IR window 85 (Spot 4) detailed in the previous figures with corresponding energy areas (approximately to scale) from both highest and lowest end points of temperature (~ 363 K - ~ 303 K) These curves are the same as the ones shows all together in Figure N° 10a. The radiation areas in the interior of the curves represent the energy passing through the atmosphere or/and window and reaching the lens of the camera. It is clear to see in both Figures how camera calibration is broken up with the windows (80/85) and not yet continuing the radiation profile of a gray body. (T ⁴ ) In other words the areas in Figure N°11 b represent the LWIR (8-14) photons that are going to reach the microbolometer sensor in the different situations. (the radiosity that implies the reflection and the self-emission from the windows is not included)
17 100
97 ~Water temperature 94 94 The temperature values optically
91 obtained in this graph have been 90 90 ~Container surface temperature corrected for to a 0.95 emissivity. 88 87.8 Camera reading with no window Temperature obtains through both 85 85 85 (all spot's 1) 83 windows are uncorrected for lack of 82 80 80 transmission. Extra radiosity (low) that 79 79.3 79.3 implies the reflection and the self- 76 76 74.7 74.5 75 emission from windows it is also 73 72 uncorrected. 71.3 70 70.3 70
67 67.4 67 65.9 Curves taken for angles 40 ° are 65 64 63.7 discards - spot’s 3 / 5 63 61 61.2 61.3 59.8 60 59 58 58.3 57.2 56.2 55 54.7 55 53.4 53.1 ~303 Kelvin 52 52.5 50.7
Thermodynamic Temperature (Celsius) 50 49 48.2 48.6 48.2 46 ⊥ / Camera reading with IR window 85 45.5 44.8 45 (all spot's 2) 44.1 43 43 41.3 40 40.4 40 ⊥ / Camera reading with IR window 80 39.2 37.8 37 ~363 Kelvin (all spot's 4) 37.3 35.7 3635.7 35 34 34 33.1 31 Target emissivity = ~ 0.95 31.2 3 Camera sensitivity = ~ 8-14 microns (long wave camera) Microbolometer 2 28 Cooling down Thermal equilibrium Background / window temperature = ~ 27 Celsius 25 1234567891011121314SPOT Figure N° 11a ---Container temperature drop --different spots It is clear to see how camera calibration is broken up with the windows
L (λ, T) = Spectral radiance emittance (Watt . m¯ ² . microns¯¹ . Sr¯¹) Wavelength
Spot 1 (nw) Spot 2 (85) Spot 4 (80)
363 Kelvin (nw) 363 Kelvin (85) 363 Kelvin (80)
Spot 1 (nw) Spot 2 (85) Spot 4 (80) 303 Kelvin (nw) 303 Kelvin (85) 303 Kelvin (80)
Figure N° 11 b – In these charts are represented the radiation of the target with no window (spot 1) and after passing both windows (spot 2/4) at ~ 303K and 363 K. Is clear that relative small changes in temperature can I cause large changes in L (λ T).
Transmission
No window 85 80
Wavelength
18 Some of those photons that reach the camera won't be read simply because they are not exactly within spectral band of the camera. Although microbolometers have a flat spectral response, microbolometer modules or cameras do not. (Figure N° 8a) The chart that represents response is call relative spectral response and as we said varies from manufacturer to manufacturer. The camera we used during the test was the TVS 700 and we did not have the opportunity to get the spectral response for this paper. Some of the arriving photons from sources (plus reflections & windows self emission) after passing through the window are going to have a better chance than others of being accepted or their contribution will be more important to signal generation. The temperature (radiation temperature) value came from considering the total spectral response of the detector weighted by the combination of the spectral distribution of the source (+background ρ) and the total spectral transmittance of the IR Window. (+background & window self emission) ⇒ spectral commonality. The output voltage at the detector can be approximately expressed theoretically with these two integrals:
~14 Target side ~14 Sensor side
Vdet = K Tw (λ,T). Rd (λ). Lt(λ, T). dλ + K Rd (λ). Ls (λ, T). dλ
~8 ~8 Where: Ad . To --Vdet = output detector (voltage) K = --K = constant of the systems 4. f# ² Ad = Area of the detector (fill factor.. etc) --Tw (λ T) = transmission of the window To = Transmission of the camera optics --Rd (λ) = responsivity of the detector f # = F Number (lens) --Lt (λ,T) & Ls (λ,T) = spectral radiance emittance Strictly we should divide the calculus in two parts considering the radiation target side of the window (direct affected by window transmission) and on the other hand the radiation sensor side of the window. (indirect affected by window transmission) ------In our practical test atmosphere of target side = sensor side. --Lt (λ, T) = spectral radiance emittance target side = emission container + background reflection at container + emission atmosphere-t = ε cont. Lcont + εback . (1 - ε cont). Lback (ta-t) + (1-τatm). Latm (ta-t)
--Ls (λ, T) =spectral radiance emittance sensor side = emission at window + background reflection at the window + emission atmosphere-s = (1-τw). (1-ρw) / (1- τw ρw). Lw (tw) + ρw . Lback (ta-s). + (1-τatm). Latm (ta-s) Since all components of Lt and Ls are not blackbodies are also spatial - geometrical dependant (x, y,θ,φ). Some of the detailed radiation above components could be worthless depending the case. We have not considered all of them in the practical test. Regarding the self-emission of the crystals, the two curves in Figure N°12 b are the transmission (dotted curve) shown previously in Figures N°8a / b and 11 b but also added is the approximate absorption (emission solid curve) curve and approximately the very low reflection curve shown below of the two crystals (80 and 85). In some cases, when the temperature of the crystal increases, the windows emission should be considered and subtracted from the total that arrives at the camera (especially 80 windows). Also, when measuring the temperature of an object that is below ambient temperature (not the case in electrical applications) with the window at ambient temperature, the self-emission could be important compared with target itself. In the LWIR the self-emission will be much greater for 80 than 85 windows. Any how in our test for both the relative importance of self-emission increases as the container cools down. The signal from the target becomes slowly weaker (no contrast) and is almost equal the intensity of the emission of the IR window reaching thermal equilibrium. In order to verify that self-emission exists, we ran some extra experiments with these two windows. We filled the container with ice, obtaining a target close to 5 –10 degrees Celsius (~278— ~283 Kelvin’s). We found that the reading with no window gave lower temperature (radiation) values, and with both windows separately between the camera and the container the radiation went up, especially with the window type 80. The windows upon not being totally transparent and upon having higher temperature than the container they had a stronger signal. Another test that we ran to confirm this was irradiating the windows with a heat source (the same container) from 60 to 50 Celsius (this means between 9 and 8.7 µm peak by Wien’s law) for about 30 minutes. We then compared, with the camera, the emission of both windows to that of a uniform wall at ambient temperature. Again we confirmed that window 80 has much more LWIR absorption (emission) than the 85 window. The next formula shows an approximation /generalization of how SNR (signal-to-noise-ratio) of a system can be diminished if window emittance increases and how higher window transmission and ∆t between parts of the target & background can lead to an improvement in the SNR.
19 95
Camera reading with no window 85.9 (all spot's) 85
80.6 85 material 77 b = 7 mm 76.2 75 75.1 φ =130 mm 72 72.1 69.7 68.4 68.3 66.4 65 64.8 63.1 63.8 61.5 59.6 59.8 57.4 57.6 Cooling down 56.3 56.1 55 53.9 54.3 52.9 51.6 51.3 52 49.4 49.7 ~ 303 K 47.7 47.7 47 46.6 45 ⊥/ Camera reading with IR window 85 --- b= 3.5mm 44.9 Thermodynamic Temperature (Celsius) (all spot's) 44 43.7 42.7 43.1 40.6 40.7 39.8 ⊥ / Camera reading with IR window 85 ---b =7mm 38.6 39 ~ 363 K (all spot's) 36.2 35.936.6 35 ⊥ / Camera reading with IR window 80 34.1 34 33 (all spot's) 32 32 Emissivity = 1--comparing windows specially 7mm thick (85) 30 Camera sensitivity = ~ 8-14 microns (long wave camera) Microbolometer Thermal Background / window temperature = ~ 27 Celsius Container temperature (Celsius) (Spot each 4/5 Celsius) Equilibrium 25 94 90 85 80 75 70 65 60 55 50 45 40 35
Figure N° 12 a---Additional test Included 7 mm (85) IR Window
100 99100 98 98.5 96.9 94 91 90 90 Crystal 80 80 80.26 80 79
70 Transmission 70
60 60
50 50 50 46.66 40 40 Absortion 30 30 Transmission(%)- Absorption(%) - Reflection(%) - Absorption(%) Transmission(%)- (%) Reflection -- -Absorption(%) Transmission(%) 20 Reflection 20 17.14 17 10 10 4.67 4.74 4.33 1.26 3.86 3.34 1 0 2.74 2.1 2 0000 1.5 1 7 8 9 10 11 12 13 14 15 Wavelength 100 100
94 94 LWIR 92 91.56 90 90 90 87 Crystal 85 Transmission 80 80 78.09
70 70 70
60 60
50 50.67 50 45 40 40
30 30 25.25 Reflection Transmission(%)- Absorption(%) - Reflection(%) - Absorption(%) Transmission(%)- Transmission(%) -Absorption(%) Reflection (%) -Absorption(%) -- Transmission(%) Absortion 20 20 18 10 10 7.89 4.57 5 65.98 5.72 5.43 5.11 2.28 4.75 4.33 3.91 3.44 0 0 0.02 0 7 8 9 10 11 12 13 14 15 Wavelength
Figure N° 12 b---Approximately spectral transmission- absorption -reflection windows 80 /85
(windows at room temperature)
20 Mw = windows emittance ⇒ noise Path radiance (we take window internal transmission----total transmission is lower) does not contain any target information - α b and can be considered as noise. Noise is a contaminant of the e window . ∆t signal. SNR = t = between target-background and parts of the target. ∆
√ Mw
3.5.2 Emissivity dial setting
How we can correct the apparent temperature values taken through both windows (show in Figure N° 11a) and so obtain in the camera the real temperature value corresponding to the approximate container surface temperature? Since the container is not a blackbody, (Planckian radiator) we need to correct the apparent temperature value by assigning the correct emissivity (~0.95) in order to obtain the true temperature values with no window When either of the IR windows is placed between the target and the camera, the signal is further attenuated so an extra correction it is needed. This is accomplished approximately by multiplying the emissivity of the target by the total transmission of the window in the bandwidth of the sensor. Now which value of transmission we should choose? The problem here is that both windows (85/80) have spectrally non- flat transmission which means that transmission changes with wavelength. We can find the approximately setting by adjusting the emissivity setting on the camera with the windows in place until we repeat temperature reading with no window. Can we use the transmission value so obtained for all future work? Yes but only if the estimated target temperature and surroundings (windows included) temperatures are the same as those used for the transmission test. In this type of window the transmission varies with temperature of the source, (relative) varies like in any IR window with its own temperature (α =absorption is temperature dependant) and also the transmission can differ using different instruments in the same bandwidth (different responsivities). Transmission here it is a relative issue. The transmission changes with temperature because the radiation profile of the container, as it cools shifts to longer wavelength maintaining the function of a gray body, while the spectral transmission of the window remains the same (it its temperature remains constant) as a non-gray body and the response of the detector also stays the same. When using an IR window with flat transmission. (as shown in Figure N° 10b), however, the correction of the apparent temperature can be approximately obtained for any target temperature (considering background and window at same temperature etc) by multiplying the target emissivity by the flat transmission of the IR window, in both cases corresponding to the sensor bandwidth (εdial setting (NIST) = εtarget x τwindow). If the IR windows transmission changes with wavelength (within the sensor response bandwidth), however, the procedure for inferring surface target temperature from the total radiance temperature becomes complicated. In this case the transmission of the window will change as target temperature changes. In Figure N° 13b are plotted emissivity dial setting (EDS) values which need to be set in the camera to correct the apparent temperature and obtain, for both windows (80 and 85), the real container temperature. It can be seen clearly that the correction varies with the temperature of the container and especially in window 80. The 80 slope is much more marked than the 85 slope. The 80 window is much more a short wave window. It has a very abrupt reduction (plunge to cero) in transmission in the band that we are considering. For that reason, when a target is warming up, which means it emits more in the shorter wavelength, the transmission recovers more quickly than a window of 85 material. The percentage of reduction of energy that Figure N° 13 b shows for both windows take into account: a) the loss because the target is not a blackbody (ε = 1 and our reference is 0.95), and b) the loss in transmission in the LWIR due to the installation of any of the windows between the camera and the target. The 0.95 line(dots) shows that for any temperature the emissivity of the target it is constant if it is a gray body. We assume that it is a gray body. If it is not, the problem becomes much more complicated. The 80 window requires a much greater EDS correction than the 85 window. The 85 has very high EDS and the transmission dispersion (0.77- 0.72 = 0.05) is low considering the ∆t of the test. The dispersion is within the error that we permit normally in fixing the emissivty for any industrial infrared service. With some caution we can assume that the 85 window is close a graybody. The 80 window, however, has a much greater transmission dispersion (0.44- 0.35 = 0.09) and much lower over all transmission in the LWIR. The values of EDS for the 80 are too low for use in practical thermography. We should also consider the fact that we have made our analysis with a reference target of quite high emissivity (0.95). In practice the values of electrical component in panels are not always this high and, therefore, the emissivity dial will need to be set at lower values and so EDS. Can we use another transparent infrared material for this specific application? Yes, but there are two points to be considered for this application. First, any other material transparent to the infrared probably will be much more expensive particularly if coated for flat and higher transmission. Second, not all IR-transmitting materials transparent in the visible as well, which is necessary for visual observation within a closed electrical panel.
21 No Window IR Window perpendicular to optical axis IR Window with an angle 40 ° to optical axis
No window
LWIR TVS Microbolometer ------Heat Source LWIR TVS Microbolometer ------Heat Source LWIR TVS Microbolometer ------Heat Source 85 ⊥ 85 / 40 ° No window Thermogram
Scale fixed for the five spots 8080 ⊥ ⊥ 80 / 40 °
Themograms -85 crystal -3.5 mm high transmission in the LWIR
Themograms - 80 crystal -3.5 mm low transmission in the LWIR
FigureFigure N°9 ---N° Measurement 13 a --5 Thermogramsand Results (five spots) corresponding to the highest temperature of the test.
1 0,950,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 0,95 ~ Target emissivity 0,9 Emissivity Dial Setting IR window 85--b=3.5 mm (for a target ~ 0.95 emissivity) 0,8 0,77 0,77 0,76 0,76 0,76 0,76 0,76 0,76 0,76 0,75 0,73 0,73 0,72 0,72 0,7 g 0,6
Emissivity Dial Setting IR window 80--b=3.5 mm (for a target ~ 0.95 emissivity) 0,5
0,445 0,44 0,44 0,43
Emissivity Dial Settin Dial Emissivity 0,42 0,42 0,42 0,42 0,4 0,4 0,4 0,38 0,38 0,37 0,35 0,3
0,2
Cooling down process 0,1 ~303 K (near room temperature) ~363 K 0 100 90 80 70 60 50 40 30 20 10 0 Container Temperature (Celsius) Figure N° 13 b---The emissivity dial setting (EDS) corresponding a 3.5mm thick 80/85 IR Windows at room temperature --Target temperature from ~ 363 Kelvin’s cooling till near room temperature
22 It should be clarified that both crystals 80 and 85 have an excellent transmission in the visible 0.4µm—0.7µm (total~ 93%) better than standard commercial glass of 6 mm of thickness (total~ depending on material ~ 87%) Might we improve the transmission of the windows type 80 by applying a coating? Probably not. The coating will reduce only the reflection loss, which is minimum in both cases, and not absorption loss that is much more important. The application of a coating will, however, increase the cost significantly and unnecessarily. Selecting a window we don’t have just to focus our attention in transmission issues. All other requirements (mechanical, thermal etc) for this application are also important. Probably new materials to come or materials that are unknown for us can meet all requirements, so installed in electrical panel. (transmission etc..) Returning to the practical arena, we need to perform infrared services through IR transmitting windows. What will determine the emissivity dial setting (EDS) we use on the camera? As previously discussed, we need to estimate (or find it) total emissivity of the target in the LWIR (estimated temperature) and multiply it by the total transmission of the window in the LWIR for defined target temperature. How can we know the temperature of the target if it is, in fact, what we are looking for? Estimating as we do with emissivity? It is one of the ways but not the best. Too much uncertainty! On active electrical panels we can’t do the test both with a window and without. The best approach here would be to generate charts using previously plotted data for sample windows and sample component materials for anticipated variations in sample target temperatures. This would allow us to estimate readings at different temperature with and without windows. Curves could be traced for different typical target emissivities, for different background temperatures (assume same of the window) and for different LWIR cameras or infrared thermometers. We have generated some curves, at least for the two windows with the information obtained in the cooling test but it is premature to publish them in this paper. Ideally, tests would be run using the windows to be installed, the camera to be used and a large ∆t. In practice, then, the thermographer’s could look up the approximate temperature through window installed and could look up on the curve the approximately value of the target temperature.
3.5.3 Loss of contrast -- Contrast Transmittance
How is the contrast transferred through an IR window from the source to the observer? How is contrast between the target and the background and between different parts of the target affected by an IR window? For thermographers the loss of contrast should be the biggest concern, even more than absolute temperature measurement accuracy. Thermographers most often make a qualitative analysis of the scene, and then, when a fault is found, they perform a quantitative analysis. Generally, if you miss a fault indication at first look you are at a high risk of missing it altogether. Both target spatial resolution and contrast are involved here. Looking through both 80 and 85 windows we should be able to detect any small temperature rises on a target that are on an area bigger than the IFOV (instantaneous field of view /miliradians) of the camera and with target and background surfaces of different emissivities values. We should keep in mind that many of these industrial IR windows are installed in medium voltage enclosures, electrical junction boxes, transformers, MCC, generators, switchgear, switchboards and motor boxes connections. A manuscript by Anthony La Rocca (University of Michigan –Erim /USA) defines Contrast Transmittance as “the ratio of contrast under the influence of atmosphere to that which would be obtained if there were no atmospheric influence.” La Rocca’s calculations establish that, for both resolved and unresolved targets, the atmospheric transmission is equal to the contrast transmittance of the scene through the path. (some assumptions were made for simplification & non-spectrally related) He first relates contrast with atmosphere and without atmosphere then reaches the conclusion mathematically that the contrast transmittance (relation between contrast: with and without atmosphere) can be found knowing the atmospheric radiance, atmospheric transmission and background radiance of the target-to sensor path. He concludes that the contrast transmittance is equal to transmittance of the scene. Following the above ideas we can consider that the Contrast Transmittance is the ratio of contrast under the influence of a specific IR Window to that which we would obtain if there were no IR window. (contrast values compare with the same camera responsivity and target temperature) With this definition we are considering the atmospheric influence on both sides of the window negligible. Warning: Strictly speaking, even though the atmosphere attenuates like a window, the way they are related to the environment and the target itself is not exactly the same. (see figure N° 4-Thermometry general lay out) One is a semitransparent gas in contact with the target and the other is a semitransparent solid surrounded by a semitransparent gas both sides (target side /sensor side: could or not be the same), immersed, in both cases, in a background with continuous radiation exchange. (+ conduction /convection involve) We now undertake to develop a mathematical model to confirm that transmittance is equal, or near equal, to the contrast transmittance in infrared windows in the same bandwidth.
23
We did a brief analysis with radiance values obtained during the test in order to check the practical relation between transmission and transmittance contrast. A way to represent contrast is with the following formula:
Lpaint - Lbright Lpaint - Lbright C (τ NW) = C (τ IRW)80/85 = Lpaint +Lbright Lpaint +LBright
Contrast with no window Contrast with IR window
L (λ, T) = Spectral radiance emittance (Watt . m¯ ² . microns¯¹ . Sr¯¹)
Both C (τ NW) and C (τ IRW) 80/85 could be calculated using radiance values obtained during the test. (with emissivity = 1.00) We can compare the radiation coming from a point on the opaque paint (Lpaint-- our first references for EDS spot 1 through 5) on the container to the radiation coming from a point (always the same references) on the bright surface (LBright) of the container (same temperature different emissivity) for each test temperature and for no window and windows 80 and 85. It should be noted that variations in emissivity between various elements in the scene (example: inside electrical enclosure) may contribute as much to the radiation contrast as do temperature differences. Then, linking the results, and with the following formula, we can find the transmittance contrast for each temperature. The transmittance contrast, as we said before, is the relation between the contrast with window and without window, and it is represented by the following formula:
Tc = Contrast Transmittance
C (τ IRW) 80/85 Contrast with IR window Tc = C (τ NW) Contrast with no window
We didn’t do the calculation for all spot of the test, nor did we evaluate different emissivities of the container or the background against the container. We just calculated for the extreme values of the test, for paint-bright surfaces and they are shown in the table below. We use the radiance values obtained in the second test Figure N° 12a which included a thicker 85 window. That chart shows apparent temperature values obtained with emissivity at one. Again adjusting (reducing) the emissivity in all the spots with windows in order to see the same temperature with no windows we obtain the approximately transmission value for each window at that container temperature. In order to compare with Tc we just want the transmission and not the EDS. Below table shows the obtained approximate transmission and contrast transmittance values for the two windows. (window’s and background at room temperature) TW = windows transmission --Tc= contrast transmittance through an IR windows
(both change on the temperature of the target)
~ 363 Kelvin’s ~ 303 Kelvin’s Infrared Window ~ TW (%) ~ Tc (%) ~ TW (%) ~ Tc (%) 80------3.5mm 43.0 45.2 38.3 43.9 85----- 3.5mm 81.3 82.4 79.3 82.7 85 -----7.0mm 77.3 78.6 72.0 75.1
The results match closely as was defined. At lower temperatures (~ 303K) the dispersion between Tw and Tc is higher. Also Figure N° 13a confirms visually how contrast is lost, especially with a type 80 window. All five (5)-test spots were taken with the same camera temperature range (scale) for all tests performed. It can be seen very clearly how several radiance values assigned false colors that appear with no window disappears with the spots taken with 80 windows. We must conclude that the loss of contrast is very important in the 80 window, and for both it is around the same percentage of the transmission loss for certain target temperatures. So it is necessary to be very cautious when infrared inspection is performed through an 80 window with a LWIR camera. Again TW ≈ Tc should be confirmed mathematically and also in practice comparing different values of radiance, (temperature / emissivity)
24
Figure N° 14 ---Infrared windows photos 4 – PHOTOS OF INFRARED WINDOWS
25 4 – PHOTOS OF INFRARED WINDOWS
Photos of real infrared windows are shown in Figure N°14. These include infrared windows installed in electrical panels, (such as 80 and 85 that we tested), windows for vacuum chambers, windows used to protect lenses of infrared cameras, windows for applications in furnaces, windows made of sapphire (Al2 03) and air- cooled calcium fluoride (CaF2), windows for spectrometry and other Industrial and lab uses and, for microelectronics inspection, zinc selenide (ZnSe) windows.
5 – CONCLUSIONS
The conclusions are presented in two parts; Attenuation test and General conclusions.
5.1 Attenuation test
The attenuation analysis tasks reported in this paper need more work and with more sophisticated instruments in a controlled lab environment, especially for the lower temperatures of the test. Also, although testing window transmission near room temperature using a simple container as a source is not the best way, and results can have high level of uncertainty, the real world is not far from this scenario. In general medium voltage hot spots do not generate significant ∆ts against background so the indication of the fault is low. An interesting pending task could be to analyze these two windows with a MWIR camera. That could bring to the table a point of very important comparison since both windows have a flat transmission in the MWIR band.
The installation of an IR window in an electrical panel is to ensure safety, without danger of electric shock while allowing visual and IR inspection. If an IR window of 80 or similar material had been installed in a panel that is currently being inspected with a LWIR camera, it is advisable to replace the window with one with better spectral transmission in the LWIR. (85 or equivalent). Economic reasons may preclude alternative. If there is no other choice, the following consideration should be taken into account when inspecting with a LWIR camera through an 80 or similar window:
First consider having the inspection performed by a highly skilled thermographer with very good visual acquisition capabilities. The professional must consider, among other aspects:
--The high attenuation of the LWIR energy of the 80 or similar crystal --The loss in contrast using this window in the LWIR such that a small rise in temperature or emissivity difference between different parts of the target and background might be missed. --The emissivity dial setting (EDS) may turn out to be very low result in much dispersion when the target temperature changes (MWIR 80 material can not be considered a spectrally flat window, in any sense) --The high self-emission that the 80 or similar IR window can have under certain circumstances
Results obtained with the 85 crystals seemed near to those with no window (close to a gray body in the LWIR) although care should still be taken when measuring through them. Any how the error that we could have with 85 window are not no more no less that those that we could have assigning an incorrect value of emissivity to the LWIR camera o thermometer or the object that we are looking at is not really a gray body The two crystals (80 and 85) tested differently, not only in transmission-emission. There are other differences, but the analysis of those differences is outside the scope of this paper.
Other general recommendations:
1-Before purchasing an infrared window ask the manufacturer for transmission data and other specification data. Then test and evaluate them with your IR sensor for at least four diverse temperatures, simulating the anticipated temperature range of the target. 2-If it is possible perform a Modulation Transfer Function (MTF) on the IR system with the IR window in between camera and defined heat source. (MTF is a measure of the contrast reduction imposed by imaging systems) 3-Also take into consideration the fact that optical properties of IR windows can differ from tabulated values due to manufacturing variations. Optical systems are seldom perfect.
26 4-Consider that optical properties of a window may vary over the time due to film deposition, fogging and scratches. IR windows should be cleaned periodically with an appropriate cleaner. Be aware that any of the window surfaces can be altered by external abrasions in the form of scratches. 5-Non-optical recommendations: Although the transmission curve and other optical properties are of primary importance when evaluating an industrial infrared window, it is important to take into in account other parameters as well. IR windows used in industrial applications and installed in electrical panels should also meet several another requirements. They should be able to tolerate the mechanical efforts, thermal efforts and tests of air tightness. The entire window assembly, (crystal, steel frame, screws, subjection washers and O- ring) must be designed and tested to tolerate potential thermal expansion as the result of relative motion between the frame and crystal. The assembly must also be capable of withstanding other mechanical stresses, such as the flash of an electrical arc.
5.2 General conclusions
Industrial infrared thermography is generally performed on opaque targets through high transmission media. In some instances, however, it is sometimes difficult to obtain precise measurements (or enough contrast) of target temperature. This may be due to diverse factors such as background radiation, emissivity, low signal in relation to noise and geometric factors. When we add a semitransparent material like an infrared window other factors are added that must be considered. Here the challenges are bigger but not impossible. The installation and use of infrared window however, can bring great advantages to users in the process, security and economy. Applications and use of the infrared window in the industrial arena is going to be extended in the future. Non- contact temperature measurement by a thermometer or IR cameras will become more and more popular. There are many hidden places or inaccessible media in industries where the only way to make a thermal evaluation is installing an appropriate crystal transparent to infrared radiation. This isn’t limited to medium voltage electric panels. There are, without doubt, a wide variety of industrial applications for infrared windows. It is important, when looking into the future to try to simplify for the industry the parameters that should be considered when an infrared window is going to be used, and how to deal with them in the field. Some of the theory and cautions reviewed in this paper may not be applicable from the practical point of view, depending on the specific application but the thermographers should know that they exist. Thermometry using semitransparent materials is like chess game: when a piece is moved you expose several others.
6 – ACKNOWLEDGEMENTS
Special acknowledgements to Herbert Kaplan Honeyhill Technical Co. USA for his contribution in the revision of the English grammar of this paper and to Mr. Brun / José Fernandez from SOREM – France. I also thank (in alphabetical order):
ACP--Autoridad del Canal de Panamá - Roger Hernández -Enrique Sosa—Panama Amorphous materials USA Mr. Ray Hilton – USA CEDIP Infrared Systems - France CNEA Dra. Ana María Fortis -Buenos Aires – Argentina Infrared Fiber Systems, Inc - USA JCD Publishing -Jerry Holst – USA ICON Tecnologia / Macsym - São Paulo- Brazil INVAP - Ricardo Sagarzazu – Miguel Albero -- S.C. Bariloche Argentina ISP Optics Corporation Mr Mark Lifshotz -USA Loma Negra -- Planta L´Amalí - Argentina Naval Air Warfare Center China Lake -Daniel C. Harris --USA Petrobras –Replan - Felipe Leonardo Gomes- Brazil Repsol-YPF –Petroquímica La Plata - Federico Balboni –Argentina SIDERAR - Mariana Viale –Omar Martin - San Nicolás -Argentina Square D / Schneider Electric - Joy Sonn -USA
7– REFERENCES
De Beers - South Africa Direct Emissivity Measurements of IR Materials (Yanina Kisler Lenn Kupferberg, Gordon MacKenzei Chia Ming Chen) Dynamic Infrared Scene Projection - (Owen Williams) Elements of Infrared Instruments Design - (Marija Strojnik Scholl)
27 FSI - (Glossary) Fundamentals of Heat and Mass Transfer - (Incropena- Dewitt) Halliday - Resnik - Krane (Physics) Infrared & and Electro-Optical Systems Handbook - (SPIE—ERIM) Infrared Methodology and Technology –X. Malgague/ J.L Beaudoin -C. Bissieux Infrared Systems Engineering - (Richards Hudson) Infrared Technology Fundamentals - (Monroe Schlessinger) Introduction to Infrared System Design - (William L Wolfe) Introduction to Radiometry -(William L Wolfe) ISP Optics Corporation –General Characteristics of the materials Laser Beam Propagation in the Atmosphere (Hugo Weichel) Materials for infrared Windows and Domes - (Daniel C. Harris) NIST Thermometry short course Photonics dictionary - (Laurin Publishing) Practical Applications of infrared Thermal Sensing and Imaging Equipment (Herbert Kaplan) Practical thermal measurements techniques - (Gary L. Orlove) Radiant Energy and the eye - (Sidney Lerman, M.D.) Radiation Thermometry - (Dewitt-Nutter) Science of Vision - (K. N. Leibovic) Testing and Evaluation of Infrared Imaging Systems (Gerald C. Holst) The infrared &Electro-Optical System Handbook The University of Michigan -Advance Infrared Technology (George Zissis) Visionary (Lars Liden) Windows and domes: past, present, and future -(W. j. Tropf, M. E. Thomas, R. K. Frazer) ______. a Further author information: SI Termografía Infrarroja ----Andres E. Rozlosnik: Phone / fax 54-11-4 825 3408 / 54-11-4 772 4829 email: [email protected]
USEFUL GLOSSARY RELATED TERMS ABBE NUMBER: A numeric value describing the dispersive ability of a medium for a given spectral range - the ratio of refraction to dispersión. ABERRATIONS: Are the deviations from perfection of the optical systems. Aberrations are inherent to the design of the optical system, even when perfectly manufactured. Manufacturing error can bring additional aberrations. ACCURACY: A measure of the similarity of an instrument reading to the actual value for that reading. Instrument drift cause by: environment, temperature, time, operator expertise, emissivity etc. (only radiometric camera) AMBIENT TEMPERATURE: Room temperature or the existing temperature of the surroundings This parameter is used to compensate for the radiation reflected in the object and the radiation emitted from the atmosphere between the camera and the object. Sometimes call background temperature. The background temperature creates the radiation level available as reflected energy. AMORPHOUS: Having no definite form. Having no real or apparent crystalline form. Shapeless. Without the regular, ordered structure of crystalline solids. ANISOTROPIC: having different properties in different directions (Non isotropic) ATMOSPHERIC TEMPERATURE: Is the temperature of the atmosphere between the object and the scanner. Some of the radiation that reaches the scanner emanates from the object and some from atmosphere that it is not perfectly transparent to the infrared radiation and therefore also radiates and absorb. ATMOSPHERIC TURBULENCE: Refers to the density fluctuations arising from atmospheric temperature fluctuations. (Not fluid dynamics turbulence) ATMOSPHERIC WINDOWS: Spectral radiation regions not absorbed by atmospheric gasses. These windows are transparent to radiation at those wavelengths. The most obvious window is the visible light window - If the smog is not too bad we can see through the atmosphere forever. BANDWIDTH: The range o frequencies over which a particular instrument is designed to function within specified limits BANPASS: The range of frequencies that will pass through a filter or other device. BIREFRINGENCE: The separation of a light beam, as it penetrates a doubly refracting object, into two diverging beams, commonly known as ordinary and extraordinary beams. A birefringence crystal is a transparent crystalline substance that is anisotropic relative to the velocity of light. Doubly refracting crystal BLACKKBODY: Is defined as an object, which absorbs all radiation that impinges on it at any wavelength. BLOCKER: inverse of a filter, instead of blocking everything like the filter does, it lets everything else pass and blocks only the specified wavelength. BRIGHTNESS: A term to indicate the relative amount of light intensity available. Brighter is more light, dimmer is less light. In an infrared system, the brightness control may affect actual image intensity or it may change the temperature range displayed. Either effect will change the brightness of a given temperature. CELCIUS º C (Centigrade): A scale for measuring temperature where absolute cold is -273.2 °C, the melting point of water (ice point) is 0 º C, and the boiling point of water is 100° C. COATING: To cover something with a layer of a particular substance. Layers over optics serve to reduce or increase reflection and to protect the surfaces from fumes or abrasion. CONDUCTION: The transfer of energy through a solid without motion of the conducting solid as a whole. Steady-State conduction calls if the temperature at each point is independent of time. Unsteady or transient conduction situations changes with
28 CONVECTION: The transfer of energy through a liquid or gas due to the motion of the medium. CRYSTAL: A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal consists of identical structural units, consisting of one or more atoms, which are regularly arranged with respect to each other in space. CRYSTALLOGRAPHY: The analysis of the atomic structures within crystals by means o X-ray diffraction. DIFFUSE REFLECTION (surface scatter): The reflection from a rough surface that emerges at almost any angle DYNAMIC RANGE: Is the ratio of the maximum signal radiance and the minimum signal radiance that can be controllably projected. DISPERSION (index of refraction): the rate of change of index with wavelength. ELECTROMAGNETIC RADIATION: The field effects given off by accelerating a charged particle in a magnetic field. Depending on field strength and speed of acceleration, many types of electromagnetic radiation are created. EMISSIVITY: The ability of an object to radiate and absorb energy from its surroundings measured as a ratio of the actual object emission to the blackbody equivalent emission FIELD OF VIEW (FOV): The total field measured in angle within which objects can be imaged or measured and displayed by an infrared system. FILTER: (optical) An optical device, which modifies the characteristics of radiation, which is passed through it. Usually filters either attenuate all wavelengths of radiation a certain controlled amount or modify the optical pass band of the radiation - eliminating selected wavelengths or bands while allowing others to pass. FLUX: Time rate of propagation of the energy (Watt = Joule/seg) FRACTURE TOUGHNESS: Resistance to extension of a crack. FRECUENCY (υ): A property of a wave that describes how many wave patterns or cycles passes by in a period of time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second. GRAY SURFACE: (gray body) A temperature radiator whose spectral emissivity at all wavelengths is in constant and less than unity. HARDNESS: How firm and stiff; not easy to bend, cut or break. A measure of resistance to indentation. INFRARED (IR): Electromagnetic radiation which occupies the band from 0.7 microns to 100 microns. Infrared radiation is between the visible spectrum and microwave radiation. Emission of energy as electromagnetic waves in the portion of the spectrum just beyond the limit of the red portion of visible radiation. INFRARED CAMERA: A camera or instrument that uses infrared optics to image and focus infrared radiation onto a recording medium sensitive to its wavelengths. INFRARED WINDOW: A thin parallel plate of material unlike glass that transmits in the infrared region or spectral region (of the infrared) in which the atmosphere transmits. INSTANTANEOUS FIELD OF VIEW (IFOV): The angle in milliradians derived by dividing the active detector element's size by the system's effective focal length. IRRADIANCE: Flux per unit area incident on a surface. IRTRAN: Is a registered trademark of the Eastman Kodak Company. Irtran-1 ( MgF2) Irtran-2 ( ZnS) Irtran-3 ( CaF2) Irtran-4 (ZnSe)Irtran-5 ( MgO) Irtran-6 (CdTe) ISOTROPIC: Having uniform physical properties, such as elastic constants, in all directions. LASER: Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It's a device that produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energy levels they emit energy. KELVIN: (Lord Kelvin---1824 - 1907) Thermodynamic temperature scale. The zero Kelvin is - 273,16 ºC (Celsius or centigrade) or - 459,7 ºF (Fahrenheit). A temperature scale often used in sciences such as astronomy. MEASUREMENTS UNCERTAINTY: parameter related with the results of a measurement that defines the dispersion of the values that could be attributed to the measurand. MICRON (µ): A measurement of length in the metric system appropriate for measuring infrared radiation wavelengths. 1,000,000 microns equals one meter MICROBOLOMETER: sensor made by materials that measure heat by changing resistance at each pixel MID INFRARED (MWIR): The middle infrared spectrum, (window) usually from 2.4 to 7.0 microns. MILLIRADIANS: A measure of small angles. Two thousand-pi milliradians can be measured in a complete circle. There are 17.4 mrads per degree of angle. MRTD: Minimum resolvable temperature difference. Smallest change in blackbody equivalent temperature that can be detected. MTF: Modulation Transfer function: The ability of an optical system to reproduce (transfer) various level of detail from the object to the image NANOMETER: Is a unit of measurement of light wavelength. A nanometer is one millionth of a millimeter. NOISE; The random fluctuations that are always associated with a measurement that is repeated many times over. These fluctuations do not represent any real sources of infrared radiation of target, but rather are caused by the imperfections of the system. NOISE EQUIVALENT TEMPERATURE DIFFERENCE (NEDT): Is the temperature difference at which the signal amplitude equals the total noise. That it is when SNR = 1. NONSELECTIVE RADIATOR: See Gray surface OPAQUE: Transmittance (τ) equals zero OPTICAL AXIS: The line between the centers of curvature of the two surfaces POLISHING: The optical process, following grinding, that puts highly finished, smooth and apparently amorphous surface on a lens or a mirror POLYCRYSTAL: A structure that consists of many crystals joining one another on their boundaries is described as polycrystalline. The crystals are ordered in planes, called lattices. QUANTITATIVE ANALYSIS: An analysis of objects or processes, which is concerned with measuring temperatures or radiant energy levels by assigning numerical values to the characteristics of the displayed scene. RADIANCE TEMPERATURE: is the temperature of an equivalent blackbody having the same radiance over the prescribed spectral band as the surfaces having the spectral emissivity ε in the same spectral band. RAW MATERIALS: The starting materials for an industrial process RAINBOW: Dispersion of he white light in raindrops RADIATION: Energy radiated in the form of waves or particles; photons. RADIOSITY: Accounts for all of the radiant energy leaving a surface. Target exitance --- (Wε + Wρ + Wτ) REFLECTION: Return of radiation by a surface, without change in wavelength REFRACTION: The bending incident rays as they pass from a medium having one refractive index into a medium with a different refractive index. RENDERING: Process in which two-dimensional image it is obtaining from three-dimensional target
29 RESOLUTION: Defines the smallest resolvable object in the target plane at a given range-to-target, atmospheric condition and target signal level ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance RESPONSIVITY: Output per unit input of a radiation detector ROOM TEMPERATURE: At or near 74 º F (24 º C). SCATTERING: Describes a change in the direction of motion of a particle because of a collision with another particle. SELECTIVE RADIATOR: Radiator, which emits radiation in different amounts as a function of wavelength. A radiator whose spectral emissivity changes with wavelengths and is less than unity. (Non-graybody) SENSITIVITY: Defines the smallest signal detectable in the presence of systems noise SIGNAL-TO-NOISE RATIO: (SNR) Is the ratio of the signal amplitude to the total noise SPECTRAL RADIANCE TEMPERATURE: The spectral radiance temperature of a surface is the temperature of an equivalent blackbody having the same spectral radiance as the surface with emissivity ε. SPECULAR REFLECTION: (Lambertian surface) Reflection from smooth surface in which angle of incidence is equal to angle of reflection. The surface radiance it is independent of the angle. SPEED OF LIGHT: The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (~186,000 miles/second). TARGET: An object in the object plane, which the system can focus on and analyze. TEMPERATURE: An expression of thermal energy density. How hot or cold an object is. THERMAL CONDUCTIVITY: A property of materials, which measures the ability of a material to conduct heat. It can be expressed as power per degree length (watts/meter-°C). Metals have a high thermal conductivity (conduct well) while air fibers and plastics have much poorer conductivities. TRANSLUCENT: Pertaining to materials having the property of reflecting a part and transmitting a part of the incident radiation TRANSMISSION: The process by which incident flux passes through matter without change of frequency and is exitent from a surface other than the incident surface. THERMOGRAM: A two-dimensional hard copy record of the apparent scene temperatures displayed on an IR system. ULTRAVIOLET: Electromagnetic radiation at wavelengths shorter than the violet end of visible light VISIBLE LIGHT: Electromagnetic radiation at wavelengths which the human eye can see. WAVELENGTH: The length of distance between cycles on a repetitive event. WAVENUMBER: The reciprocal of wavelength 1/ λ cm - ¹ …………………………………………………………………………………………………………………………………..
30
Trabalho apresentado no encontro:
Defense and Security Symposium 2005 Thermosense XXVII
Liberado pelo autor para publicação no site Termonautas.
O Autor:
Andres Esteban Rozlosnik
Engenheiro Industrial pela Universidade Católica de Buenos Aires – Argentina.
Desenvolveu estudos de pós-graduação em Qualidade, Meio Ambiente, Exportação- Importação, Construção Industrial e Ensaios não Destrutivos.
Sua experiência profissional está relacionada com a construção industrial, aplicações em meio ambiente, ensaios não destrutivos e, especificamente, nas diversas aplicações da Termografia Infravermelha, técnica na qual se certificou.
Participa ativamente em congressos internacionais relacionados com o tema.