Color Perception
Introduction to Color Perception
At this point, you're familiar with the nature of the visual system, including the perception of basic visual phenomena (such as lightness) and also shape and depth. Although we've talked a bit about how the brain processes color information, we will now explore this topic in greater detail.
Link - Webvision on Color - A good brief introduction to many topics in color vision.
Link - DIY Calculator on Color Vision - Another overview of color vision, including useful links.
Nature of Color
In this section, we turn again to the distinction between characteristics of
the physical stimulus and the nature of the perceptual phenomena that are
associated with the stimulus. The varying wavelengths of the light stimulus
are depicted below, and these wavelengths are closely associated with the
perceptual phenomenon of hue (the color names seen below the wavelengths).
Subtractive Mixtures
Additive Mixtures
- Demonstration 7.1: Predicting Additive Mixtures
What happens if we make an additive mixture of equal amounts of green and violet? To gain a rough sense of the hue that would emerge, first draw a straight line between green and violet, as seen in the figure below. Then place a dot on the line halfway between the two colors. Next, draw a straight line from the middle of the color circle to the dot. As you can see below, the result is blue, tending slightly toward green, with intermediate saturation (given the location of the dot). To predict an additive mixture with a lot of violet and relatively little green, you would place the dot closer to violet. Notice that the resulting hue would be close to blue (though not fully saturated). Similarly, try a mixtrue of mostly orange, with just a little green. Notice that the result is yellowish-orange. Finally, try an equal mixture of blue and a slightly orange yellow.
Trichromatic Theory and the Photoreceptors
Update 7/06 [Scientific American]: What Birds See by Timothy H. Goldsmith
Evolution has endowed birds with a system of color vision that surpasses that of all mammals, including humans. The typical human has three cone photopigments, while birds typically have four cone photopigments. In addition, birds' retinas contain some cones with tiny oil droplets (red, yellow, and colorless) that filter the light hitting the cones, thereby allowing the birds to distinguish more colors. Birds can even see into the near ultraviolet portion of the spectrum! Thus, birds that may be largely indistinguishable to humans probably look quite different to other birds, which probably plays an important role in mate selection.
Opponent-Process Theory and Mechansisms Beyond the Photoreceptors
Achromatopsia and the Color Center
Kinds of Color Deficiencies
• Color vision deficiencies typically arise from differences in cone photopigments and lead to differences in color experience. For dichromats such as protanopes and deuteranopes either the L- cone system or the M-cones system is missing, which leads these people to have difficulty discriminating reds and greens. For the other type of dichromat -- the tritanope -- the S-cone system is missing, which leads these people to have difficulty discriminating yellows and blues.
Link - To get a sense of the visual experience of people with color vision deficiencies, try out the Vischeck site.
Link - Jay Neitz has provided an interesting illustration of the processing of color information in the retina, including the impact of color vision deficiencies.• Color vision deficiencies (e.g., protanopia and deuteranopia) lead to difficulties distinguishing red and green. As you might imagine, that distinction is particularly important when one encounters a stoplight when driving. Most stoplights have been standardized such that the top light is red and the bottom light is green. That way, even a person who might have difficulty distinguishing red and green knows to stop when the top light is lit. The placement of the lights on stoplights was not always consistent. In fact, you can still find at least one stoplight with green on top. In the Tipperary Hill area of Syracuse, NY (an area that was once predominantly Irish) one can find the stoplight seen below. Although some people may be happy that the Irish green color is on top, the light may be a bit confusing to a person with color-vision deficiencies used to stopping when the top light is lit.
Photo by H. Foley
Diagnosing Color Deficiences
Link - Try out a simulation of the Farnsworth color arrangement test, along with useful links for color vision deficiencies. (As the author notes, this exercise is purely a simulation because the colors cannot be easily calibrated from monitor to monitor.)
Link - You can try out a web-based color vision test developed at City University London.
Simultaneous Color Contrast
Successive Color Contrast
Color Constancy
Subjective Colors
Purkinje Shift
Color Stereopsis
Memory Color
Categorization of Colors
Lafayette Instrument Co. is one resource for color demonstrations. For example, they offer a color vision deficiency test (Hardy, Rand, and Ritter (HRR) Pseudoisochromatic Subset Book), an additive color mixing box, a stand perimeter, etc. You can use their Illusionator to produce subjective colors using the Benham's Top disk.
Link - Mark Newbold has produced some online demonstrations that illustrate subjective colors (Fechner colors).
Thomson Higher Education has published two very useful CD-ROMs. John Baro (Polyhedron Learning Media) has developed Insight: A Media Lab in Experimental Psychology [see Color Arrangement Test and Color Mixing] and Colin Ryan (James Cook University) has developed Exploring Perception [see Module 3].
Link - Eugene Vishnevsky developed an Introduction to Color to help explain color perception and various color spaces, color mixing, etc.
Link - Dale Purves (Duke) has provided a number of compelling color demonstrations.
Link - Donald Kline (University of Calgary) has a number of useful tutorials on his web pages, including a section on color perception.
Link - Hans Irtel has produced an amazing teaching device for perception called PXLab (including Vision Demonstrations). You'll find a number of relevant demonstrations for color perception.
Lammens, J. M. G. (1994). A computational model of color perception and color naming. (Dissertation)
Purves, D. & Lotto, R. B. (2003). Why we see what we do: An empirical theory of vision. Sinauer.
