Photopic luminous efficiency function

In Eqs. (7)—(10), 5(A) is the spectral power distribution of the illuminant, and R A) is the spectral reflectance factor of the object. Jc(A), y(A), and 5(A) are the color-matching functions of the observer. In the usual practice, k is defined so that the tristimulus value, Y, for a perfect reflecting diffusor (the reference for R A)) equals 100. Using the functions proposed by the CIE in 1931, y(A) was made identical to the spectral photopic luminous efficiency function, and consequently its tristimulus value, Y, is a measure of the brightness of objects. The X and Z values describe aspects of color that permit identification with various spectral regions. 

Photopic (right) and scotopic (left) luminous efficiency functions. 

International Commission on Illumination 1990 CIE 1988 2° Spectral luminous Efficiency Function for Photopic Vision. Technical Report 86, International Commission on Illumination. 

The human eye responds to electromagnetic radiation in the wavelength range from about 360 nm (violet) to 820 nm (red), with a peak sensitivity near 555 nm (green). While the detailed shape of this response curve depends on the individual person, studies on representative samples of human subjects have led to adoption of a standard function relating the perceived brightness (luminous flux) to the actual power of the spectral radiation. This function is referred to as V(X), the photopic spectral luminous efficiency function, and it plays an important role in photometry. 

FIGURE 13 The luminous efficiency of the human eye as a function of wavelength. Photopic bright light, typically outdoor illumination. Scotopic near darkness. 

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