# CIE photopic luminous efficiency function

[3.10.2]$V(\lambda )$

NOTE: This term and its definition were changed in RP-16-17 Addendum 1. Deletions have a strike-through. New text is bold.

Previous name: values of spectral luminous efficiency for photopic vision

Values at 10-nm intervals were adopted by the International Commission on Illumination (CIE) in 1924 and by the International Committee for Weights and Measures (CIPM) in 1933 as a basis for the establishment of photometric standards of types of sources differing from the primary standard in spectral distribution of radiant flux. Definitive values at one-nanometer intervals were obtained by interpolation, extrapolation,and smoothing, and were adopted by the CIE in 1970* and by the CIPM in 1970. These values are identical to those of the color-matching function y-bar (ȳ) of the CIE 1931 Standard Colorimetric Observer.

Note: These standard values of spectral luminous efficiency were determined by observations with a two-degree photometric field having a moderately high luminance; consequently, photometric evaluations based upon them do not apply exactly to other conditions of observation. Watts weighted in accord with these standard values are often referred to as light-watts.

The spectral luminous efficiency function for photopic vision.  It was adopted in 1924 by the International Commission on Illumination (CIE) and in 1970* by the International Committee of Weights and Measures (CIPM).  It is based on a 2° viewing field and principles of heterochromatic photometry and was incorporated into the CIE 1931 Colorimetric Observer as the ȳ function.

Unless otherwise indicated, the values used for spectral luminous efficiency in photopic vision are the values agreed upon internationally in 1924 by the CIE, completed by interpolation and extrapolation, and recommended by the CIPM in 1972.

* CIE No. 86, CIE 1988 2o Spectral Luminous Efficiency Function for Photoptic Vision. Vienna, Austria: International Commission on Illumination; 1990.

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