Koshel, R. John Photon Engineering, LLC, Tucson, Arizona.
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The photometric intensity of visible radiation in the electromagnetic spectrum after taking into consideration the response of the human visual system to the radiant intensity. The International Commission on Illumination (abbreviated CIE, due to its French form: Commission Internationale de l'Éclairage) maintains standards for the sensitivity of the human eye to visible light as a function of wavelength and viewing conditions. The term, light, typically is used for the visible spectrum, but it is often extended into the infrared and ultraviolet regions. The standard visible spectrum is based on the photopic curve, which has been refined since its establishment by the CIE in 1924. The visible spectrum is defined to extend over a range of wavelengths from 380 to 780 nm. Luminous intensity typically is represented by the symbol Iν, where the subscript ν denotes a photometric quantity, and its unit is the candela, an SI base unit (1 cd = 1 lumen/steradian, where the steradian is a measure of solid angle). An antiquated term for the candela is candlepower. Luminous intensity is equal to the differential luminous flux (Φν, with units of lumen) per differential solid angle (Ω, a dimensionless quantity expressed in steradians), as in Eq. (1).
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