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

${\displaystyle K=\frac{{\Phi }_{\text{v}}}{{\Phi }_{\text{e}}}}$

where Φ_v is luminous flux and Φ_e is radiant flux.

Note 3 to entry: The value of the luminous efficacy of radiation for photopic vision for monochromatic radiation of frequency ν_cd = 540 × 10¹² Hz is defined as 683 lm·W⁻¹ and denoted as K_cd. The corresponding wavelength, ${\lambda }_{\text{cd}} =n^{-1}·c_0·{\nu }_{\text{cd}}^{-1}$ (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 λ_cd with respect to standard air is typically in the range of a few picometres and can be neglected in most cases.

Note 4 to entry: See also "spectral luminous efficacy".

Note 5 to entry: The luminous efficacy of radiation is expressed in lumen per watt (lm·W⁻¹).

Note 6 to entry: This entry was numbered 845-01-56 in IEC 60050-845:1987.