Principles of Radiation Measurement This report presents a comprehensive summary of the terminology and units used in radiometry, photometry, and the measurement of photosynthetically active radiation (PAR). Measurement errors can arise from a number of sources, and these are explained in detail. Finally, the conversion of radiometric and photometric units to photon units is discussed. In this report, the International System of Units (SI) is used unless noted otherwise.9 Radiometry component of sunlight plus the diffuse component of skylight received together on a horizontal surface). This Radiometry1 is the measurement of the properties of physical quantity is measured by a pyranometer such as radiant energy (SI unit: joule, J), which is one of the many the LI-200R. Unit: W m-2. interchangeable forms of energy. The rate of flow of radi- ant energy, in the form of an electromagnetic wave, is Direct Solar Radiation is the radiation emitted from the called the radiant flux (unit: watt, W; 1 W = 1 J s-1). Radi- solid angle of the sun’s disc, received on a surface per- ant flux can be measured as it flows from the source (the pendicular to the axis of this cone, comprising mainly sun, in natural conditions), through one or more reflect- unscattered and unreflected solar radiation. This physical -2 ing, absorbing, scattering and transmitting media (the quantity is measured by a pyrheliometer. Unit: W m . Earth’s atmosphere, a plant canopy, etc.) to the receiving Diffuse Solar Radiation (sky radiation) is the downward surface of interest (e.g. a photosynthesizing leaf).8 scattered and reflected radiation coming from the whole hemisphere, with the exception of the solid angle sub- Terminology and Units tended by the sun’s disc. Diffuse radiation can be mea- sured by a pyranometer mounted on a shadow band, or Radiant Flux is the amount of radiation coming from a calculated using global solar radiation and direct solar source per unit time. Unit: watt, W. radiation. Unit: W m-2. Radiant Intensity is the radiant flux leaving a point on the source, per unit solid angle of space surrounding the Photosynthetically Active point. Unit: watts per steradian, W sr -1. Radiation Radiance is the radiant intensity emitted by a unit area In the past, there has been disagreement concerning of a source or scattered by a unit area of a surface. Unit: units and terminology used in radiation measurements W m-2 sr-1. in conjunction with the plant sciences. It is LI-COR’s pol- Irradiance is the radiant flux incident on a receiving icy to adopt the recommendations of the international surface from all directions, per unit area of surface. committees, such as the Commission Internationale de Unit: W m-2. I’Eclairage (CIE), the International Bureau of Weights and Measures, and the International Committee on Radiation Absorptance is the fraction of the radiant flux that is Units. The International System of Units (SI) should be absorbed by a medium. used wherever a suitable unit exists.9 Reflectance and Transmittance are equivalent terms Units for the fractions of radiant flux that are reflected or transmitted. The SI unit of radiant energy flux is the watt (W). There is no official SI unit of photon flux. A mole of photons Spectroradiometry: All the properties of the radiant flux is commonly used to designate Avogadro’s number of depend on the wavelength of the radiation. The prefix photons (6.022 × 1023 photons). The einstein (E) has spectral is added when the wavelength dependency is been used in the past in plant science, however, most being described. Thus, the spectral irradiance is the irra- societies now recommend the use of the mole since diance at a given wavelength, per unit wavelength inter- the mole is an SI unit. When either of these definitions val. The irradiance within a given waveband is the is used, the quantity of photons in a mole is equal integral of the spectral irradiance with respect to wave- to the quantity of photons in an einstein (1 mole = length.8 Unit: W m-2 nm-1. Spectral measurements can 1 einstein = 6.022 × 1023 photons). be made using the LI-1800 Portable Spectroradiometer. Global solar radiation is the solar irradiance received on a horizontal surface (also referred to as the direct 2 | Principles of Radiation Measurement Note: The einstein has also been used in books on photo- chemistry, photobiology and radiation physics as the quan- 1.00 tity of radiant energy in Avogadro’s number of photons.5 This definition is not used in photosynthesis studies. Ideal 0.75 Quantum LI-190R Response Response Terminology LI-COR continues to follow the lead of the Crop Science 0.50 Society of America, Committee on Terminology10 and Response (Arbitrary Units) 11 other societies until international committees put forth 0.25 further recommendations. ULTRAVIOLET VIOLET BLUE GREEN YELLOW RED INFRARED 0.00 Photosynthetically Active Radiation (PAR) is defined 300 400 500 600 700 800 as radiation in the 400 to 700 nm waveband. PAR is the Wavelength (nm) 7 general radiation term which covers both photon terms Figure 1. Typical LI-190R quantum response and and energy terms. the ideal quantum response displayed in energy units. Photosynthetic Photon Flux Density (PPFD) is defined as the photon flux density of PAR, also referred to as 120 Quantum Flux Density. This is the number of photons Ideal Quantum Response in the 400–700 nm waveband incident per unit time on 100 a unit surface. The ideal PPFD sensor responds equally LI-190R Response to all photons in the 400–700 nm waveband and has a 80 cosine response. This physical quantity is measured by a cosine (180˚) quantum sensor such as the LI-190R or 60 LI-192SA. The LI-191R Line Quantum Sensor also mea- sures PPFD. Figures 1 and 2 show the ideal quantum Response (%) Relative 40 response curve and the typical spectral response curve of LI-COR quantum sensors. Units: 1 µmol s-1 m-2 ≡ 1 20 µE s-1 m-2 ≡ 6.022 x 1017 photons s-1 m-2. ULTRAVIOLET VIOLET BLUE GREEN YELLOW RED INFRARED 0 300 400 500 600 700 800 Photosynthetic Photon Flux Fluence Rate (PPFFR). Wavelength (nm) LI-COR introduced this term which is defined as the Figure 2. Typical LI-190R quantum response and photon flux fluence rate of PAR, also referred to as the ideal quantum response displayed in photon Quantum Scaler Irradiance or Photon Spherical Irradi- units. ance. This is the integral of photon flux radiance at a point over all directions about the point. The ideal PPFFR sensor has a spherical collecting surface which Ø=0° exhibits the properties of a cosine receiver at every 0.8 point on its surface (Figure 3) and responds equally to 0.6 all photons in the 400–700 nm waveband. This physical 0.4 0.2 quantity is measured by a spherical (4π collector) quan- Ø=270° Ø=90° tum sensor such as the LI-193SA. Units: 1 µmol s-1 m-2 ≡ 1 µE s-1 m-2 ≡ 6.022 × 1017 photons s-1 m-2 ≡ 6.022 × 17 -1 -2 Ideal 10 quanta s m . LI-193SA Quantum Response Response Note: There is no unique relationship between the PPFD Ø=180° and the PPFFR. For a collimated beam at normal inci- dence, they are equal; while for perfectly diffuse radia- tion, the PPFFR is 4 times the PPFD. In practical situations the ratio will be somewhere between 1 and 4. Figure 3. Typical Angular Response of the LI-193SA Spherical Quantum Sensor. Principles of Radiation Measurement | 3 Photometry containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal Photometry refers to the measurement of visible radia- projection of the element of the surface on a plane per- tion (light) with a sensor having a spectral responsivity pendicular to the given direction. Unit: cd m-2; also, lm curve equal to the average human eye. Photometry is sr -1 m-2. This unit is also called the nit. used to describe lighting conditions where the eye is the primary sensor, such as illumination of work areas, Illuminance is defined as the density of the luminous interior lighting, television screens, etc. Although pho- flux incident at a point on a surface. Average illumi- tometric measurements have been used in the past, nance is the quotient of the luminous flux incident on a PPFD and irradiance are the preferred measurements surface by the area of the surface. This physical quan- in plant science. The use of the word “light” is inappro- tity is measured by a cosine photometric sensor such -2 priate in plant research. The terms “ultraviolet light” and as the LI-210R. Unit: lux, lx. One lux is one lm m . “infrared light” clearly are contradictory.8 Measurement Errors The spectral responsivity curve of the standard human eye at typical light levels is called the CIE Standard At a Controlled Environments Working Conference in Observer Curve (photopic curve), and covers the wave- Madison, Wisconsin, USA (1979), an official from the band of 380–770 nm. The human eye responds U.S. National Bureau of Standards stated that one could differently to light of different colors and has maximum not expect less than 10 to 25% error in radiation mea- sensitivity to yellow and green (Figure 4). In order to surements made under non-ideal conditions. In order to make accurate photometric measurements of various clarify this area, the sources of errors which the colors of light or from differing types of light sources, researcher must be aware of when making radiation a photometric sensor’s spectral responsivity curve must measurements have been tabulated.