# 17-21-090

luminous efficacy of radiation, <for a specified photometric condition>

K, <for photopic vision>; K′ , <for scotopic vision>; Kmes;m , <for mesopic vision>; K10 , <for the CIE 10° photopic photometric observer>; K, <for the CIE 1988 modified 2° spectral luminous efficiency function for photopic vision>

quotient of luminous flux and the corresponding radiant flux for a specified photometric condition

Note 1 to entry: The photometric condition should be specified (e.g. photopic, scotopic, mesopic). If it is not specified, photopic vision is assumed and the symbol K is used. For other photometric conditions the respective symbol for identification shall be used.

Note 2 to entry: Luminous efficacy of radiation for photopic vision is expressed by

where Φv is luminous flux and Φe is radiant flux.

Note 3 to entry: The value of the luminous efficacy of radiation for photopic vision for monochromatic radiation of frequency νcd = 540 × 1012 Hz is defined as 683 lm·W−1 and denoted as Kcd. The corresponding wavelength, (where n is refractive index and c0 is the speed of light in vacuum) in standard air, i.e. dry air at 15 °C and 101 325 Pa, containing 0,045 % of carbon dioxide by volume (see P.E. Ciddor: Refractive index of air: new equations for the visible and near infrared, Appl. Opt. Vol 35, No. 9, 1996, pp. 1566-1573), is accepted to be 555,017 nm (rounded from 555,017 069 nm). This is very close to 555 nm, the wavelength where V(λ) reaches its maximum. For photometric measurements made in air under real environmental conditions the influence of the variation of the refractive index on λcd with respect to standard air is typically in the range of a few picometres and can be neglected in most cases.