Color Matching Functions

When viewing an object, people interpret the mixture of light frequencies as a specific color. The light may consist of a monochromatic (single frequency) source or a mixture or multiple frequencies. Any perceived color can be reduced to just three variables. This property of human color vision, which is referred to as trichromacy, has been demonstrated in a number of color matching experiments.

Imagine two lights lying side by side: One is illuminated by a mixture of the three colors (red, green and blue), while the other can be of any arbitrary monochromatic color and intensity. A person with normal color vision is able to make the two lights appear identical, simply by adjusting the relative intensities of the red, green and blue lights. Note that "perceived" equivalent brightness was matched, not the total energy.

The color matching functions or CMFs are obtained from a series of such matches, in which the subject sets the intensities of the three colors required to match a series of monochromatic (single wavelength) lights of equal energy that traverse the visible spectrum. Sometimes the value of one of the , and CMFs is negative, which indicates that that particular primary light had to be removed from the mixture and added to the monochromatic light to complete the match.

There are three major derivations of the color matching functions:

1. The CIE 1931 2 deg CMFs and its corrections by Judd (1951) and Vos (1978).
2. The Stiles-Burch 2 deg pilot CMFs measured in 10 observers (Stiles & Burch, 1955).
3. The Stiles-Burch 10 deg CMFs measured in 49 observers (Stiles & Burch, 1959) on which the CIE 1964 10-deg CMFs are primarily based.

Bleaching

The human eye can undergo a loss of spectral sensitivity at moderate and high light intensities which is referred to as bleaching. The amount of pigment bleached is proportional to the energy incident on the eye. When the eye is exposed to high energy levels the effective pigment concentration and therefore the spectral sensitivity of the eye is temporarily reduced. Although the spectral band is narrower, the l of peak sensitivity is unchanged.

This is usually a short term effect, but exposure to very high levels, especially to the shorter wave lengths, can lead to permanent loss including blindness.

The effects of bleaching on pigment concentration can have large effects on spectral sensitivity and may need to be taken into account when evaluating chromatic adaptation and color-matching. The following may be used to approximate the effect of bleaching;

A troland is defined as the level of retinal illuminance resulting when a surface with a luminance of 1 candle/m² is viewed through a pupil with an area of 1 mm².