Chapter 5 Overview
The Perception of Color
Are the jellybeans in the image at right “colored”? Not really. Although lemon jellybeans appear “yellow” and cherry jellybeans appear “red,” our experience of color is created by our visual systems, not inherent to the jellybeans themselves. The jellybeans each have certain reflectance characteristics—the patterns of light wavelengths that are reflected and absorbed—that are physical properties of those objects. However, color is our subjective perception of the wavelengths of light that end up bouncing off the object and onto our retinas. So how do we get from the physical properties of light wavelengths to our perceptual experience of colors? That is the subject of this chapter.
Photoreceptors in the retina begin the process by selectively responding to different wavelengths. A single type of photoreceptor alone cannot accomplish color vision since it cannot distinguish between enough wavelengths, as demonstrated in the activity on The Principle of Univariance. We overcome this difficulty by using three different types of cone photoreceptors to code for color, a concept known as trichromacy. One of the key observations in developing the trichromatic theory of color vision was that lights can be added together to form mixtures that look identical to other, single light wavelengths. Another activity illustrates additive and subtractive forms of Color Mixing.
Later downstream in the visual system, the output of the three types of cones is translated into Opponent Process mechanisms, such as red versus green and blue versus yellow. These opponent pairs are responsible for an interesting and fun phenomenon called the negative Afterimage.
The purpose of our visual systems is to tell us what is out there rather than about the particular wavelengths of light that strike our eyes. For instance, the light reflected off of a red shirt under the fluorescent lighting of a department store might be very different than the light reflected from the same shirt outside in the sun. Yet, in both cases we see the shirt as red. In the end, we want to know about the surface qualities of the objects the light reflects off of, not about the light itself. Therefore, our visual systems take into account the luminance characteristics of a scene to tell us what the actual color of objects are in the world, regardless of the color of light illuminating them. This ability, known as Color Constancy, is trickier than you might think; a related concept, lightness constancy, is illustrated by a set of Illusions of Lighting.
Most peoples’ experience of color is similar, even in cultures that use fewer color words than we do in English. However, loss of one or more types of cones is fairly common, and leads to a deficit known as color blindness. The activity on Trichromacy illustrates the physiology of color blindness, and the essay on experiencing Color Blindness provides a hint about what the world looks like to a color blind individual.
Once you’ve read the chapter in the textbook and done the activities here, use the study aids (Study Questions, Flashcards, and Chapter Summary) to review.