Luminous efficiency function

It is not an absolute reference to any particular individual, but is a standard observer representation of visual sensitivity of a theoretical human eye.

The CIE photopic luminous efficiency function y(λ) or V(λ) is a standard function established by the Commission Internationale de l'Éclairage (CIE) and standardized in collaboration with the ISO, [1] and may be used to convert radiant energy into luminous (i.e., visible) energy.

The following equation calculates the total luminous flux in a source of light: where Formally, the integral is the inner product of the luminosity function with the spectral power distribution.

[2] In practice, the integral is replaced by a sum over discrete wavelengths for which tabulated values of the luminous efficiency function are available.

The small excess fractional value comes from the slight mismatch between the definition of the lumen and the peak of the luminosity function.

Stockman & Sharpe has subsequently produced an improved function in 2011, taking into account the effects of chromatic adaptation under daylight.

The standard scotopic luminous efficiency function or V′(λ) was adopted by the CIE in 1951, based on measurements by Wald (1945) and by Crawford (1949).

[18] For older people with normal color vision, the crystalline lens may become slightly yellow due to cataracts, which moves the maximum of sensitivity to the red part of the spectrum and narrows the range of perceived wavelengths.

Photopic (black) and scotopic (green) luminous efficiency functions. [ c 1 ] The photopic includes the CIE 1931 standard [ c 2 ] (solid), the Judd–Vos 1978 modified data [ c 3 ] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [ c 4 ] (dotted). The horizontal axis is wavelength in nm .
Fluorescence in beer. The one watt laser appears much dimmer than the fluorescence it produces, because the camera, like the human eye, is much more sensitive between 500 and 600 nm than at the laser's 450 nm wavelength.
Protanopic (red, dotted) and deuteranopic (green, dashed) luminosity functions. [ 17 ] For comparison, the standard photopic curve is shown (black, solid).