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The CIE 1924 photopic V(λ) luminosity function, [cie 3] which is included in the CIE 1931 color-matching functions as the y (λ) function, has long been acknowledged to underestimate the contribution of the blue end of the spectrum to perceived luminance. There have been numerous attempts to improve the standard function, to make it more ...
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 10 8 cd/m 2). In humans and many other animals, photopic vision allows color perception , mediated by cone cells , and a significantly higher visual acuity and temporal resolution than available with scotopic vision .
A comparison between a typical normalized M cone's spectral sensitivity and the CIE 1931 luminosity function for a standard observer in photopic vision. In the CIE 1931 model, Y is the luminance, Z is quasi-equal to blue (of CIE RGB), and X is a mix of the three CIE RGB curves chosen to be nonnegative (see § Definition of the CIE XYZ color space).
Photopic (daytime-adapted, black curve) and scotopic (darkness-adapted, green curve) luminosity functions. The photopic includes the CIE 1931 standard (solid), the Judd-Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The horizontal axis is wavelength in nm. Photometry is a branch of optics that ...
There are two luminous efficiency functions in common use. For everyday light levels, the photopic luminosity function best approximates the response of the human eye. For low light levels, the response of the human eye changes, and the scotopic curve applies. The photopic curve is the CIE standard curve used in the CIE 1931 color space.
However, the overall luminosity function (which in fact is a weighted sum of these three curves) is subjective, since it involves asking a test person whether two light sources have the same brightness, even if they are in completely different colors. Along the same lines, the relative magnitudes of the X, Y, and Z curves are arbitrarily chosen ...
The mesoscopic luminosity function at wavelength can be written as the weighted sum, [12] = ′ + (),where () is the standard photopic luminosity function (peaking at 683 lm/W at 555 nm) and ′ is the scotopic luminosity function (peaking at approx. 1700 lm/W at 507 nm), and standardized by CIE and ISO. [13]
The three coordinates of CIELAB represent the lightness of the color (L* = 0 yields black and L* = 100 indicates white), its position between red and green (a*, where negative values indicate green and positive values indicate red) and its position between yellow and blue (b*, where negative values indicate blue and positive values indicate yellow).