CIE System of  Color Specification:

·        Standard observer

·        CIE primaries.

Another specification of color that is also popular was generated by CIE (commission International de L' E clairage) an international body of color scientists in 1931.

·  Standard Observer

The CIE defined a “standard observer” by averaging the color matching data of a large number of observers having normal color vision. This standard observed data consists of color matching functions for primary stimuli of wavelengths 700 (R_o), 546-1(G_o) and 435-8(B_o) nm with units normalized in the standard way i.e. equal amounts of the three primaries are required to match the light from the equal energy illuminant E. Using these curves shown in Fig(2.9) and given the spectral distribution of any color, we can use equation(1) to calculate the tristimulus values required by the standard observer to match that color.

CIE defined three new primaries x, y, and z in which standard observer results can be expressed. It is possible to calculate the amounts of X,Y,Z needed to match any color, given its tristrimulus values corresponding to any other primaries such as R_o, G_o and B_o. In order to do this, CIE has defined the transformation equations relating two primary systems as:

· Properties of CIE Coordinate System:-

(1)   The trisimulus values X, Y, Z are normalized to equal energy white.

(2) The Y tristimulus value corresponds to the luminance of the color.The color matching function for Y is proportional to the relative luminous efficiency shown earlier.

(3) Unlike R, G, B system, where sometimes certain tristimulus values must be negative for match, the tristimulus value  and the color matching functions in CIE-XYZ system are always positive as shown is Fig (2.12).

Figure (1.12) Color matching functions x(λ), y(λ), z(λ), for the 20 Standard Observer

This positivity makes X, Y, Z primaries non-real or imaginary i.e. they cannot be realized by any actual color stimuli. In X, Y, Z tristimulus vector space, the primaries are represented by vectors outside the domain representing real colors. This will be clear from the following section.