Color Vision

Color is pleasing
Color is useful
- color is ubiquitous in nature: essentially all objects - animate and inanimate - have characteristic "colors"
- color causes objects to stand out
- animals deploy color to draw attention to themselves

- some species deploy color to signal sexual receptivity (why do humans wear make-up?)
- color distinguishes objects that would be indistinguishable based only overall intensity
- in effect, color perception can defeat camouflage, by promoting figure/ground segregation
- some animals deploy color to blend into their surroundings

- color signals the ripeness of fruits and vegetables and the freshness of meat
- color plays a role in social communication - e.g., facial color (embarassment, anger, fright, illness)
\Color is elusive
- size, shape, distance, movement - these are aspects of vision that lend themselves to verbal description: we can imagine communicating these ideas about these dimensions to a blind person
- color defies description independent of itself or representative objects; it is inconceivable how to communicate the experience of color to a blind person
- color is subjective, as Isaac Newton acknowledged: "... rays are not colored. There is nothing else in them but a certain power ... to produce in us the sensation of this or that color."
- "Unweaving the rainbow" -Newton's great discovery - and great heresy!

Do not all charms fly

At the mere touch of cold philosophy?

There was an awful rainbow once in heaven:

We know her woof, her texture; she is given

In the dull catalogue of common things.

Philosophy will clip an Angel's wings,

Conquer all mysteries by rule and line,

Empty the haunted air, and gnomed mine -

Unweave the rainbow ...

From "Lamia" by John Keats

- White light of the sun can be decomposed into 7 basic colors (Newton was inclined to look for seven basics because of his knowledge of the 7 tones of the musical scale)

- Colored light could be recombined to create white light (process was reversible)


- A basic color is one that cannot further be decomposed upon passing through a prism

- Basic colors can be recombined to create new colors, which can be again decomposed into the basics

- the color produced by isolating light at 570 nm is indistinguishable from the color produced by adding light at 520 nm and 680 nm. We say the two different physical stimuli are metamers?Why do they appear indistinguishable?
What, exactly, is color - a perceived quality of surfaces determined by:
- spectral content of ambient illumination
-- ambient light from the sun or from electric bulbs contains energy over the entire visible spectrum of wavelengths (think about what happens when light is passed through a prism), but in differing proportions; different light sources are composed of differing extents of light from the visible spectrum
-- suppose you take a picture indoors with the lights on, using daylight color film in your camera? what does the picture look like?
- pigmentation of reflecting surface
-- molecular structure of surface material determines the percentage of incident light absorbed by that surface and the percentage reflected from the surface to the eye
-- a surface's color depends on the light reflected (i.e., not absorbed); in a sense, the skin of a "red" apple is everything but red. Some examples:
lemon

tomato

white paper

number of distinct cone photopigments (i.e., light absorbing molecules) in the eye, and the spectral sensitivities of those pigments
normal human eye contains 3 distinct cone photopigments

people with deficient color vision have either: three pigments with abnormal spectral sensitivities, or fewer than three pigments (two pigments = dichromate; one pigment = monochromat)
perceived color starts with the pattern of absorptions within the three cone types: color information, in other words, is transmitted from the eye to the brain as a trio of 'values' (xs, xm and xl
"tomato" reflectance function convolved with cone spectral functions:

X




Question: in a 3-pigment cone system, is there a single wavelength of light that can give rise to the perception of "white"?
Question: in a 2-pigment cone system, is there a single wavelength of light that can give rise to the perception of "white"?
state of visual adaptation of the eye (color adaptation)
- color of neighboring regions in the visual scene (color contrast)
- healthy brain (recall the colorblind painter)
Our childhood introduction to color: the paintbox, the prism and the color television
- in middle school, we were shown how white light passed through a prism fanned out into a rainbow of colors - in fact, this prism effect is the basis of rainbows (rain particles behave like tiny prisms). Light that cannot be further decomposed into finer gradations of color is called "monochromatic" or "pure"
- in elementary school, we were taught how to mix two colors to obtain a third
- paints contain pigments that absorb certain wavelengths of light (e.g., "red" paint absorbs all wavelengths but the very longest ones, and these long wavelengths are reflected from the surface to an observer's eye; mix the "red" paint with another "color" paint and the reflected light will be further limited - this is subtractive color mixture - the greater the amount of paint added, the darker the color (which is why interior decorators avoid painting walls with saturated colors - the room becomes too dark).

- Most colors we perceive in the natural environment are the result of subtractive color principles, because surfaces are naturally "painted" (i.e., they have pigments that absorb light energy)
- from infancy, we've enjoyed the pictures displayed on a color tv, which operates on the principle of additive color mixture
- projected light

- color television screens are composed of tiny (approx. 0.2 mm) dots that are red, green or blue; the dots are so small that their emitted light effectively adds together before reaching our eyes; by varying the proportion of energy produced by each of the three, the television is able to reproduce most of the colors of the spectrum
- pontillist paintings
Some further observations on color additivity
- G + R --> Y, when G &R are roughly equal in intensity; varying the relative amounts of R + G will vary the product from pure green, through olive, yellow, orange and, finally, red.
- the yellow produced by mixing red and green is indistinguishable from pure yellow associated with 570 nm light. The two are said to be metameric (physically different but psychologically identical). Why is this so?
- G + R + B --> white; these are known as "primary" colors
- by reasoning, B + Y --> ??? (think about what primaries comprise Y)
- any two colors that produce white (or an achromatic color) are called complementary colors
Color phenomena to be explained
- color mixture: white light can be produced by selected combinations of three colors (homework exercise)
- colored afterimages: a white surface appears colored after prolonged exposure to a colored surface (demonstration)
- color contrast - perceived color depends on colors in immediate surroundings; color shadows provide one example of color contrast (demonstration)
- color constancy - perceived color remains unchanged despite large changes in the wavelength composition of the incident (and, hence, reflected) light
- color catgeories - all cultures have red, yellow, green, and blue (as well as black and white)
- acute color discrimination - we are very good at discriminating subtle differences in hue (homework assignment, to turn in)
- color deficiency
- color vision in other species