| Definition: | ratio of the radiant flux at wavelength λm to that at wavelength λ such that both radiations 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
NOTE – Unless otherwise indicated, the values used for the spectral luminous efficiency in photopic vision are the values agreed internationally in 1924 by the CIE, completed by interpolation and extrapolation, 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 ratified by the CIPM in 1976.
These values define respectively the V (λ) or V ′ (λ) functions represented by the V (λ) or V ′ (λ) curves (see the figure below).
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