Radiometric and photometric quantities

Units, Symbols, and Equations for Radiometric and Photometric Quantities 77 Ed. Removed (this information is contained in 90 Ed., p. 2-1 and 2-29)... 

UNITS, SYMBOLS, AND EQUATIONS FOR RADIOMETRIC AND PHOTOMETRIC QUANTITIES... 

Units, Symbols, and Equations for Radiometric and Photometric Quantities... 

Table 1.2-3 summarizes the names, definitions, and SI units for the most frequently used radiometric and photometric quantities in radiation physics. 

Table1.2-3 Radiometric and photometric quantities in radiation physics...

A summary of symbols and units that are used in radiometry is shown in Table B.l. A summary of symbols and units that are used in photometry is shown in Table B.2. The photometric quantities use the same symbol except that the subscript v is added. A good introduction to the theory of radiometry is given by Poynton (2003), Jahne (2002), Horn (1986), and Levi (1993). See also International Commission on Illumination (1983). The radiometric units refer to physical quantities whereas the photometric units refer to perceptual quantities. In photometry, the physical quantities are weighted by the spectral sensitivity of the human visual system. Let Q(X) be the energy at wavelength X. Then, the corresponding photometric quantity is given by... 

Note The symbols for photometric quantities (see following table) are the same as those for the corresponding radiometric quantities (see above). When it is necessary to differentiate them the subscripts r and e respectively should be used. e.g.. Q, and Q,. 

In addition to these two radiometric assessments of the beam intensity, for beams of visible light there is a third possibility, which is to quantify the intensity of the beam by the intensity of visual perception by the human eye. Physical quantities connected with this physiological type of assessment are called photometric quantities, in contrast to the two radiometric quantities described above. In photometry, the intensity of the beam is called the luminous intensity 7y. The subscript v stands for visual . The luminous intensity 7v is an ISO recommended base quantity the corresponding SI base unit is the candela (cd). The luminous flux is determined as the product of the luminous intensity and the solid angle. Its dimensions therefore are luminous intensity times solid angle, so that the SI unit of the luminous flux < v turns out to be candela times steradian (cd sr). A derived unit, the lumen (Im), such that 1 Im = 1 cd sr, has been introduced for this product. 

The quantity Fg can be any of the above radiometric quantities photometric quantity is obtained by replacing the term radiant with the term luminous (e.g. luminous flux luminous intensity /, luminance L, illuminance E etc.). The crucial criterion for the assessment of a CMS is the final impact on the (human) driver. Therefore, the physical quantities to be considered are the photometric ones. However, there are significant differences between the luminosity functions for photopic vision (high intensity, daylight) and scotopic vision (low intensity, night scenes)—see Fig. 1. 

In the absence of an integrating sphere, radiometric/photometric quantities, such as radiance/luminance (i), are also used to quantify the performance of SSL devices. Radiance (W m sr ) measures the quantity of radiation that passes through or is emitted from a surface and fells within a given solid angle in a specified direction. When radiance is weighted by the sensitivity of human eyes, luminance values (hn m sr ) are reported. The main parameters that are used to characterize the quality of a light source are the following ... 

CIE stands for Commission Internationale de L Eclairage (International Commission on Illumination). The nomenclature and notation for radiometric, photometric, and quantum calculations evolved independently, and early workers in those fields developed their own vocabularies. This has been a stumbling block for newcomers. Many workers attempted to establish one common vocabulary, using modifiers for the different disciplines - see, for example, Jones (1963), Nicodemus (1971), Geist andZalewski (1973), Geist (1976). This met with some success, but even now words for two quantities (radiance/luminance, and irradiance/illuminance) defy that attempt at commonality. 

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