luminous efficacy (of radiation) [K]

quotient of the luminous flux, Φv, by the corresponding radiant flux, Φe


Unit: lm·W-1

NOTE 1 Luminous efficacy depends on a number of factors, particularly the state of visual adaptation and the size and position of the source in the visual field. For this reason it is possible to define a number of spectral luminous efficacy functions, for specific visual conditions. Unless otherwise indicated, the luminous flux referred to in the definition above is that determined using the CIE standard photometric observer i.e. using the V(λ) and V'(λ) functions for photopic and scotopic vision respectively.

NOTE 2 For any spectral luminous efficacy function, K(λ),the luminous efficacy for monochromatic radiation at a frequency 540×1012 Hz, which corresponds to the wavelength λ = 555,016 nm in standard air, is defined as 683 lm·W-1.

NOTE 3 The maximum value of K(λ) is denoted by the symbol Km.
For photopic vision Km = 683 V(555 nm) / V(555,016 nm) lm·W-1 = 683,002 lm·W-1 ≈ 683 lm·W-1
and for scotopic vision
For other wavelengths: .

See also "spectral luminous efficiency

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