Color Constancy Essay

Chapter 5 Study Questions

Basic Principles of Color Perception

1. Briefly describe the three steps to color perception.

Answer: The three steps to color vision are 1) Detection during which wavelengths of light are first registered by the visual system; 2) Discrimination during which different wavelengths of light are distinguished from each other; and 3) Appearance during which viewing conditions are taken into account while producing the final percept of a scene.

Step 1: Color Detection

2. Describe the three types of cones in the human visual system and explain the differences between them.

Answer: The three types of cones in the human visual system are: S-cones, M-cones, and L-cones. They are all collectively responsible for discriminating between different colors. The S-cones are preferentially sensitive to short wavelengths (e.g., “blue”), the M-cones are preferentially sensitive to middle wavelengths (e.g., “green”), and the L-cones are preferentially sensitive to long wavelengths (e.g., “red”).

Step 2: Color Discrimination

3. What is the principle of univariance?

Answer: The principle of univariance is the fact that an infinite set of different wavelength–intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make accurate color discriminations by itself.

4. What does the trichromatic theory of color vision tell us about color perception?

Answer: The trichromatic theory of color vision tells us that the color of any light is defined in our visual system by the relationships between the outputs of the three cone types.

5. Why do metamers produce the same perceived color?

Answer: Metamers are different mixtures of wavelengths that nonetheless look identical. Even though the wavelength mixtures are different, they produce the same response from the cones in our visual system, which in turn causes the colors to appear identical.

6. What is the trichromatic theory of color vision?

Answer: The trichromatic theory of color vision, developed in the nineteenth century by both Young and Helmholtz, proposes that any light is defined in our visual system by the relationships between a set of three numbers, which we now know to be the outputs of the three cone types.

7. What is an additive color mixture?

Answer: An additive color mixture is when two sources of illumination combine to make a new color, as when mixing lights. If light A and light B are both reflected from a surface to the eye, the colors of those two lights add together.

8. What is a subtractive color mixture?

Answer: A subtractive color mixture is when one source of illumination is subtracted from another, as when two color filters are placed in front of a light source or when pigments are mixed. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remaining light that was not absorbed by either A or B contributes to the perception of color.

9. What happens if you shine “blue” and “yellow” lights on the same patch of paper?

Answer: If you shine “blue” and “yellow” lights on the same patch of paper, the wavelengths will add, producing an additive color mixture. Since “yellow” is equivalent to a mix of long and medium wavelengths, and “blue” consists of short wavelengths, the two lights will produce a mixture of short, medium, and long wavelengths. The resulting mixture will therefore look “white.”

10. Explain why the LGN is important in color perception.

Answer: The LGN is a structure in the thalamus of the brain that receives input from retinal ganglion cells and has input and output connections to the visual cortex. Some of its cells are maximally stimulated by spots of light in a center–surround architecture, which is critical to color perception. The LGN contains cone-opponent cells that essentially subtract one type of cone input from another in a center–surround manner.

Step 3: Color Appearance

11. Describe the idea of color space.

Answer: Color space is a three-dimensional representation of all possible colors. The color space has three dimensions because color perception is based on the outputs of three cone types.

12. What does a color with zero saturation look like?

Answer: A color with zero saturation looks white.

13. What are the opponent color sets in opponent color theory?

Answer: The opponent color sets in opponent color theory are red vs. green, blue vs. yellow, and black vs. white.

14. What is a unique hue? Provide an example.

Answer: A unique hue is a color that can be described with only a single color term. Red is an example of a unique hue, as opposed to orange, which can be described as a compound (reddish yellow).

15. Describe the method of “hue cancellation.”

Answer: The method of hue cancellation is used to demonstrate opponent color theory. In this method, the experimenter might start with a light that appears to be a yellowish green. The experimenter then cancels the yellowness by adding its opponent color, blue. The experimenter then measures the amount of blue light needed to remove all traces of yellow.

16. What is a negative afterimage?

Answer: A negative afterimage is a type of afterimage whose polarity is the opposite of the original stimulus. For instance, light stimuli produce dark negative afterimages. Colors are complementary: red produces green afterimages and yellow produces blue afterimages. The negativity of the afterimages arises from the cone-opponent cells.

17. What is a double-opponent cell?

Answer: A double-opponent cell is a neuron whose output is based on a difference between sets of cones and is more complicated than a cone-opponent cell. In double-opponent cells, the center region is excited by one cone type and inhibited by another (e.g., R+/G–) and the surround has the opposite arrangement (e.g., G+/R–).

18. What is achromatopsia?

Answer: Achromatopsia is an inability to perceive colors that is caused by damage to the central nervous system.

Individual Differences in Color Perception

19. What is cultural relativism?

Answer: Cultural relativism is the idea that basic perceptual experiences such as color perception may be determined in part by the cultural environment.

20. What are basic color terms?

Answer: Basic color terms are single color words (e.g., “blue,” not “sky blue”), used with high frequency, and have meanings that are agreed upon by speakers of a language.

21. In what way are color-anomalous individuals and cone monochromats color-blind?

Answer: Color-anomalous individuals are people who can make discriminations based on wavelength, but the discriminations are different from the normal. Cone monochromats are individuals with only one cone type, and therefore they cannot discriminate different colors, meaning that they are truly color-blind.

From the Color of Lights to a World of Color

22. Describe the idea of color constancy.

Answer: Color constancy is the tendency of a surface to appear to be the same color under a fairly wide range of illuminants.

23. Describe two physical constraints that make constancy possible.

Answer: Luminance tends to change abruptly between surfaces and gradually within surfaces, so surface boundaries are an important physical constraint for achieving constancy. The fact that shadow boundaries change the brightness and not the chromatic properties of a surface is also an important physical constraint for constancy.

What is Color Vision Good For?

24. How might color vision aid animals in finding food?

Answer: Color vision helps animals locate and identify food that would not otherwise stand out for a creature with monochromatic vision. For instance, red berries in a green bush are much easier to locate with color vision, and it is also easier to tell whether the berries are ripe or not if you can perceive the difference between green and red.

25. How do some birds and reptiles achieve color vision without having photoreceptors with different spectral sensitivities?

Answer: Some animals have evolved a system for color vision in which small droplets of colored oils sit on top of photoreceptors and filter the light coming into them. This has the same function as having photoreceptors with different spectral sensitivities.

Essay 5.3 Color Constancy in the Lab

McCann, McKee, and Taylor (1976) brought color constancy into the lab (Image 1). They created a setup in which the left eye was looking at a set of color patches under one light source while the right eye looked at other patches under an independent light source in a different part of the visual world. With this setup they could ask observers to look at a patch shown only to the left eye and match it to the appearance of a patch shown to the right eye. These collections of patches are often called “Mondrians” in honor of Piet Mondrian, the twentieth-century artist whose abstract works look a little like these collections of rectangular patches (you decide; see Image 2).

Image 1

In panel 1 of Image 1, the critical patches are the gray and green ones at the center. Under a “white” light, the gray patch reflected equal amounts of S-, M-, and L-wavelength light. The patches in the other eye were illuminated with white light of the same composition. Naturally enough, observers matched the gray patch in the left eye to the gray patch in the right, and they matched green to green.

In panel 4 of Image 1, the illumination of the Mondrian has been changed; it has been made redder. In the illustration, you can see this. In the experiment, the change might not have been noticed, just as you might not notice a change from sunlight to skylight. Under the white light, the green patch produced 120 units of S-cone excitation, 150 units of M-cone, and 70 units of L-cone. If the illuminant were made a bit redder, the same green patch could now be made to produce 100 units each of S-, M-, and L-cone excitation, exactly the same as what the gray patch produced under the white light.

If seen in isolation, the “green” patch would now appear gray, because the three cone types would all be firing at approximately equal rates in response to the light reflecting off the patch. In the context of the Mondrian, however, the patches kept their original, “true” colors. Green looked green, and gray looked gray. The presence of the other colors allowed the colors of the test patches to remain constant over a fairly dramatic change in the nature of the illumination. How is this possible?

This feat of color constancy is possible because the visual system does not just judge colors by the amount of S-, M-, and L-cone stimulation they produce on their own, but by how much stimulation they produce within the context of the lighting of the scene. In panel 4, all of the color patches in the Mondrian appear somewhat reddish, so the visual system makes the assumption of red lighting in the scene and discounts (or subtracts away) some redness from the patches to determine their true color. Thus, even though the “green” patch produced 100 units of S-, M-, and L-cone activation in panel 4, the visual system discounts some of the red (L-cone) lighting, leading to the impression that the patch is green, not gray.

For more information on this topic, refer to the subsection of your textbook called “Physical Constraints Make Constancy Possible” on page 149.

Image 2

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