spectral luminous efficiency (of a monochromatic radiation of wavelength λ) [V(λ) for photopic vision; V'(λ) for scotopic vision]

ratio of the radiant flux at wavelength, λm, to that at wavelength, λ, such that both produce equally intense luminous sensations under specified photometric conditions and λm is chosen so that the maximum value of this ratio is equal to 1

Unit: 1

NOTE 1 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. For this reason it is possible to define a number of spectral luminous efficiency functions, for specific visual conditions.

Unless otherwise indicated, the values used for the spectral luminous efficiency in "photopic vision" are the values agreed internationally in 1924 by the CIE (Compte Rendu 6e session, p.67), completed by interpolation and extrapolation (ISO 23539:2005(E)/CIE S 010/E:2004), and recommended by the International Committee of Weights and Measures (CIPM) in 1972.

For "scotopic vision", the CIE in 1951 adopted, for young observers, the values published in Compte Rendu 12e session, Vol. 3, p. 37 and in ISO 23539:2005(E) / CIE S 010/E:2004, and ratified by the CIPM in 1976.

These values define respectively the V(λ) and V'(λ) functions for photopic and scotopic vision.

NOTE 2 CIE, considering the discrepancies between the average human spectral luminous efficiency and the V(λ) function, adopted in 1990 (see CIE 86-1990) the "CIE 1988 Modified 2° Spectral Luminous Efficiency Function for Photopic Vision”, VM(λ), and recommended it for applications in visual sciences.

NOTE 3 CIE, considering that the spectral luminous efficiency function of the human eye changes with visual angle, adopted in 2005 (see CIE 165:2005) the “CIE 10° photopic photometric observer”, V10(λ),and recommended it for applications in visual sciences where the eye is fully light adapted and the visual target has an angular subtense larger than 4° or is seen off-axis.

NOTE 4 Other spectral luminous efficiency functions can be used to characterise visual performance under other specified photometric conditions, such as for specified adaptation conditions in the mesopic range (see CIE 191:2010).

NOTE 5 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 Φe,λ(λ) is given by


where Km = 683,002 1 lm·W-1 ≈ 683 lm·W-1.

See also "luminous efficacy

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