Color Perception

Color vision is defined as the ability to discriminate differences in the wavelengths of light. Yet, like all vision, the perception of color is ultimately not about seeing wavelength or light but about seeing the objects that light illuminates. Color provides powerful cues to object recognition. For example, it can help reveal what sort of surface we are viewing (e.g., leaf or bark) and the particular state that the material is in (e.g., dry or lush foliage, ripe or unripe fruit, or a pallid or glowing complexion). To make these judgments, the color percept must be tied to properties of the object (how it reflects light) rather than simply to the light reaching the eye (which can change as the illumination changes). The strategies and extent to which the visual system achieves this perceptual goal is the subject of color constancy.

Color can tell us what we are looking at and where it is by helping to segment different parts of the scene based on their colors (see color insert, Figure 18). Black and white photography shows that color is not necessary for seeing the layout of a scene, and many studies have found that color cues on their own do not support the perception of fine spatial details nor as strong a sense of motion as luminance cues. Indeed, the chromatic equivalent of a black and white image—where only differences in color but not in luminance are shown—is often unrecognizable. Yet color is nevertheless one of the most salient cues for segregating objects. A ripe fruit pops out from green leaves, and the leaves in turn appear grouped together to form trees, because differences in color set objects apart whereas similarities in color bind them. Thus, color vision plays an important role in spatial vision. Chromatic cues may be especially important for seeing shapes in the presence of shadows. Cast shadows produce much larger changes in brightness than in color, and thus the ability to link together parts of the scene that have a common color allows the underlying objects to be delineated more clearly. Individuals with color deficiencies often find spatial tasks, where items must be located or grouped by their color, most difficult. This entry discusses color appearance and contextual influences on color appearance

Figure 18 Color Perception—Image Decomposed Into Its Luminance or Chromatic Components (1)

Note: The grayscale image (left) retains much of the fine spatial detail, depth, and shading information in the scene. The chromatic images (center and right) captures the material differences between the objects much better and more clearly delineates the objects in terms of these differences and despite variations in shadows and shading. Thus, adding color helps viewers perceive both what the objects are and where they are. See the Color Perception entry for additional information (pp. 266–270).