Contrast and color

	Contrast and color   |   Flashing light sensitivity   |   Spectral sensitivity   |   Color considerations

Spectral sensitivity
The visual system is not equally sensitive to all wavelengths of visible light and is most sensitive to wavelengths of about 545 nm, with a secondary peak of sensitivity at about 610 nm. These values are different from the commonly quoted CIE (Commission Internationale de l'Eclairage) photopic sensitivity curve, as shown in Figure 23.

The CIE standard relative spectral sensitivity curve (CIE, 1924).

The CIE material was proposed in 1924 and adopted as an international standard in 1931.

Weighted spectral sensitivity function for the fovea (Ripps and Weale, 1976).

If relative spectral sensitivity is of importance, however, it would be better to consider the relationships shown in Figure 24, which is a compromise by Ripps and Weale (1976) of several different studies. Those studies all used stimuli limited to the fovea, and the results of that work appear to be a better representation of the visual system's spectral sensitivity response than is the CIE standard.

The lens of the eye becomes yellowish with age. When that happens, the lens is less able to transmit blue light. In fact, the amount of blue light transmitted by the lens drops by about 30% from age 20 to age 50. This has the effect of shifting the sensitivity curve toward the longer wavelengths for older people.

Loss of transmission of blue light (460nm) through the lens of the eye due to aging (Weale, 1973).

Color Vision
Colors may be specified in several ways. One of the more common technical methods is to use the CIE color diagram, which may be used to specify the relative values of the three primary colors (red, green and blue) that, when mixed will duplicate any given color. This system gives three proportional values to describe a particular color, and although the system is useful for technical applications, it is not useful for the average person. Accordingly, names of colors have been associated with various segments of the visual spectrum. The system described by LeGrand (1957) is as follows:

There are three types of color receptors in the human eye. Each type is differentially sensitive to the visible wavelengths of light.

A continuous range of colors may be perceived by the variation in the relative rates at which the three receptor systems send signals to the brain.

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