Color Perception

Two and only two elements are required to perceive color: an illuminant and an observer—an object is not required. To perceive the color of an object, an illuminant and an observer must still be involved, as well as the object itself.

The observer is the human eye/brain combination. In fact, color perception is a physiological phenomenon. From this perspective, the retina of the human eye contains only three kinds of photoreceptors, cones roughly covering the long wavelength (red), medium wavelength (green), and short wavelength (blue) regions of the visible spectrum. The specific region of the spectrum to which each cone is sensitive is represented by a spectral-response curve. The information passed to the brain from each photoreceptor represents the amount of light energy absorbed within its spectral response curve.

Processing in the brain starts with these three stimulus values. The brain processes these values into a single color perception. Since the brain only receives one value from each cone, even the most complex spectral stimulation is reduced to three color responses, which are then combined into the single perceived color. Therefore, the human eye may be unable to differentiate between certain combinations of wavelength energies.

A review of the typical spectral-response functions of the three types of cones in the human retina yields some interesting results. Consider this example: stimulations at 510 nm (green) and 575 nm (yellow) will yield about the same response on the green cones; however, the 575 nm stimulation will yield a much higher response on the red cones. It is the brain’s job to interpret this differential response as the difference between colors. The brain, sometimes using previous (memory) color experience, smoothly converts each possible combination of the three cone responses into the proper perceived color.

If we stimulate the observer with light energy in a narrow band at 575 nm, both the green and red cones roughly provide equal responses. Since we can see only a single color at a single time and place, we do not see green and red; rather, we combine the responses of the red and green cones and perceive...yellow. There is a very interesting consequence of this combinatorial effect. Suppose another illuminant is emitting light in two narrow bands, one primarily stimulating the green cones and one primarily stimulating the red ones. The same yellow color may be perceived!

Because of the limitations of the human observer, it is possible that two very different spectral energy distributions will result in the same perceived color. If we could stimulate the red and green cones (but not blue), a yellowish color would be perceived; if we were to stimulate the green and blue cones (but not red), a cyan would be perceived. In both cases, the perceived color corresponds to that of an intermediate wavelength. But what if we were to stimulate the blue and red cones, but not the green ones? The perceived color, a magenta, does not correspond to any intermediate wavelength; in fact, there is no wavelength anywhere on the spectrum that corresponds to the perceived color of magenta! Nevertheless, we are able to perceive magenta, a color that is, in some sense, the absence of green.

Interestingly, the maximum response of the short wavelength cones is only one-tenth that of the medium wavelength and long wavelength cones. This means that violet and blue are harder to differentiate from black than are red or green. And yellow, stimulating both the sensitive long wavelength and medium wavelength cones, is the brightest color—harder to distinguish from white than are cyan or magenta. The overall response is centered in the spectrum on green.

By the way, most new fire engines are lime-yellow—they’re easier to see against a dark background because they stimulate the medium wavelength cones as well as the long wavelength cones, eliciting a higher natural response.