CIE Color System

The CIE system developed by the Commission Internationale de l’Eclairage of France is the universally accepted standard for color specification and measurement. The CIE system is based on the appearance of color to the theoretical standard observer. The CIE definition followed experiments with a sample of 17 human observers. The results have since been confirmed with larger samples. The CIE definition of the standard observer is based on three specific wavelengths of light in the RGB regions respectively (435.8, 546.1 and 700nm).

The device independent tristimulus values (X,Y,Z) are derived from the relative amounts of these characteristic wavelengths present in a color. The tristimulus values can be determined with a colorimeter or a spectrophotometer. In particular, they are given by an integral transform X = òPlx(l)dl where Pl is the spectral power curve for the color being evaluated and x(l) is the color matching function for the x tristimulus color. Similar expressions for Y and Z are given in terms of y(l) and z(l). x(l), y(l) and z(l) have been determined empirically.

Plots of these quantities are given in the Figure above. The chromaticity coordinates (Y,x,y) are derived from the tristimulus coordinates by (x =X/(X + Y + Z), y = Y/(X + Y + Z))

The x and y chromaticity coordinates can be plotted on the two dimensional chromaticity diagram at the right. The observable colors define the horseshoe shaped region shown in the figure.

This region is similar to the color wheel in that each point represents hue and saturation values.The X, Y and Z values are usually measured approximately on a 0-100 scale. (The luminance, Y, of the chromaticity scale is measured 0-100 by construction) Higher numbers are lighter, lower darker. The CIE system underlies all color measuring and encoding systems. However, x,y values may be difficult to interpret as they don’t relate directly to the perceptual attributes of color. In addition the chromaticity diagram is not uniform, i.e. equal distances do not represent equal shifts in color appearance.

The 1976 CIELAB system - The CIELAB coordinates L* (lightness), a* (red-green) and b* (blue-yellow) are defined in terms of the X, Y and Z tristimulus values. L* = 116(Y/Yn)1/3 - 16 a* = 500[f(X/Xn) - f(Y/Yn)] b* = 200[f(Y/Yn) - f(Z/Zn)] f(r) = r1/3 r > .008856 f(r) = 7.787r +16/116 r = .008856 The Xn, Yn, Zn are the nominal color values of the nominally white color stimulus. CIELAB is the native model for Photoshop.