spectral luminous efficiency, <for a specified photometric condition>

V(λ), <for photopic vision>; V′(λ), <for scotopic vision>; V_mes;m(λ), <for mesopic vision>; V₁₀(λ), <for the CIE 10° photopic photometric observer>; V_M(λ), <for the CIE 1988 modified 2° spectral luminous efficiency function for photopic vision>

quotient of the radiant flux at wavelength λ_m and that at wavelength λ, such that both produce equally intense luminous sensations for a specified photometric condition and λ_m is chosen so that the maximum value of this quotient is equal to 1

Note 1 to entry: The spectral luminous efficiency of the human eye depends on a number of factors, particularly the state of visual adaptation and the size and position of the source in the visual field. The photometric condition should be specified (e.g. photopic, scotopic, mesopic). If it is not specified, photopic vision is assumed and the symbol V(λ) is used. The values of the V(λ) function have been internationally agreed in 1924 by the CIE (Compte Rendu 6^e session, p.67), completed by interpolation and extrapolation (ISO 23539:2005(E)/CIE S 010/E:2004 Photometry – The CIE System of Physical Photometry), and recommended by the International Committee of Weights and Measures (CIPM) in 1972.

Note 2 to entry: For scotopic vision, the symbol V′(λ) is used. The values of the V′(λ) function were adopted by the CIE in 1951, were published in Compte Rendu 12^e session, Vol. 3, p. 37 and in ISO 23539:2005(E) / CIE S 010/E:2004 Photometry – The CIE System of Physical Photometry, and ratified by the CIPM in 1976.

Note 3 to entry: For mesopic vision, the spectral luminous efficiency function is denoted by V_mes;m(λ), where m is a coefficient determined by the visual adaptation level. The values of the V_mes;m(λ) function at representative values of m are given in CIE 191:2010 Recommended System for Mesopic Photometry based on Visual Performance.

Note 4 to entry: The V(λ) function applies at all luminance levels for foveal conditions, i.e. for all on-axis visual tasks (where objects seen by the eye are in a narrow field of view). For visual tasks that are not on-axis, the following specifications are given by CIE 191:2010 Recommended System for Mesopic Photometry based on Visual Performance: Scotopic photometric quantities are applicable to the condition where the eye is adapted to an average luminance of < 0,005 cd·m⁻²; mesopic photometric quantities are applicable to the condition where the eye is adapted to average luminance levels between 0,005 cd·m⁻² and 5 cd·m⁻²; photopic photometric quantities are applicable to the condition where the eye is adapted to an average luminance greater than 5 cd·m⁻².

Note 5 to entry: Considering that the spectral luminous efficiency function of the human eye changes with visual angle, in 2005 CIE adopted the "CIE 10° photopic photometric observer", V₁₀(λ), and recommended it for situations where the eye is fully light adapted and the visual target has an angular subtense larger than 4° or is seen off-axis (see CIE 165:2005 CIE 10 Degree Photopic Photometric Observer).

Note 6 to entry: Considering the discrepancies between the average human spectral luminous efficiency and the V(λ) function, in 1990 CIE adopted the "CIE 1988 Modified 2° Spectral Luminous Efficiency Function for Photopic Vision", V_M(λ), and recommended it for applications in visual sciences (see CIE 86-1990 CIE 1988 2° Spectral Luminous Efficiency Function for Photopic Vision).

Note 7 to entry: Photometric quantities are calculated by integrating the product of the radiometric quantity by the spectral luminous efficiency function and then multiplying by the maximum of the stated spectral luminous efficacy function, with the integral being taken across the full optical radiation spectrum. For example, for the CIE standard observer for photopic vision, the luminous flux of a source with spectral radiant flux Φ_e,λ(λ) is expressed by


where K_m ≈ 683 lm⋅W⁻¹ (see also "maximum luminous efficacy").

Note 8 to entry: The spectral luminous efficiency has unit one.

Note 9 to entry: This entry was numbered 845-01-22 in IEC 60050-845:1987.

Note 10 to entry: This entry was numbered 17-1222 in CIE S 017:2011.