• Homework Set 8:

Due Monday, Nov. 18

From Chapter 9:

P10, P22, P26, P30, PH3, From Chapter 10:

P4, P5, P9

How do we see color?

There is only one type of rods. It can only tell the intensity of the light, not its color.

One cannot see color under scotopic conditions

Three types of cones?

Because the cones can differentiate colors, there must be more than one type of cones.

Thomas Young (1801) postulated three types of cones --- Trichromacy, based on the three attributes of color: hue, saturation, lightness. need three inputs to get three outputs

Helmholtz three cones

Von Helmholtz postulted three response curves for the three types of cones:S-cones: has the best response to short

wavelength of lightL-cones: …. to long wave length of light

I-cones: ….. to the intermediate wavelength of light.

Different colors correspond to different patterns of responses in these cones.

No two colors produce the same response from the three cones.

Helmholtz hypothetical spectral response curves of humanphotoreceptors.

Determine the response curves

Since every wavelength of the visible light has different color, the response curves must overlap.The response curves are generally broad and

cover a range of frequencies If there were a region of the spectrum where

cones of only one type responded, one cannot discriminate the colors based on the wave length along

Determine the response curves

For white light, all cones respond equally.All wavelengths contribute equally to broad-

band white light.An additive mixture of two complementary

lights can also yield white. From the region of spectral colors without

spectral complementary, we determine where L & I and S & I responses cross.

The ability for hue discrimination depends on the wavelength. From this, one can find the steep rise and falling segments of the response curves.

The cone responses can be measured directly from microspectrophotometry. Measuring the amount of light absorbed by

the cones when it passes through it. Only three types of responses have been

found!

Spectral absorptions by 3-cone types

Consistency between Response curves and color mixing Consider matching 490-nm spectral cyan

which excites the S and I-cones equally and a little bit of L-cones.

Use 460-nm blue and 530-nm green and 650-nm red to match cyan. The blue and green must be roughly the same amount. However, this mixture excite the L-cones too much, we need anti-red to de-excite the L-cones.

The information contained in the chromaticity diagram is consistent with that of the response curves.

Four Psychological Primaries

When we mix red and green, we don’t see reddish green; we see yellow.

When we mix cyan and yellow, we don’t see yellowish cyan, we see green.

Thus to tell what colors look like, we need four primaries: blue, green, yellow and red.

Any hue can verbally described as a combination of them.

Color Opponents Using the psychological primaries, we can

name all spectral colors. Red and green are opposite colors; they never

appear at the same wavelength. So are yellow and blue.

We call them opponents. The opponent color theory was put

forward by Ewald Hering, was in competition with trichromacy.

Color Cancellation

If a color is too bluish, it can be made less bluish by adding yellow. The amount of yellow that one adds to cancel the blue entirely determines the blueness of the original color.

The yellowness and blueness, and redness and greenness can be used to describe a color quantitatively.

Opponent Processing

The responses of the L,I,S cones are processed by three opponent channels: yellow-blue, red-green, and white-black. It is the latter information which is passed to the brain.

S I L type of cones

Y-b - + + contribution to the signal

R-g + - +

W-bl + + +

Simultaneous Color Contrast

Imbed a gray region among yellow and green. The yellow region makes the gray look bluish and the green region makes the gray reddish. Thus a region of the retina contains spatial opponency---lateral inhibition of color. The center is r-g, the surrounding is g-r. Used widely in impressionists’ paintings.

Color Constancy

Objects tend to retain the same perceived color even though the coloration of the overall illumination may change. A biological necessity.Comes from lateral inhibition.

The color constancy is nor perfect. It depends on the state of adaptation.

Negative afterimage

When you are adapted to yellow, you will see blue in the white.

When you see too much yellow, the L and I cones excited and tired. So when you see the white, the S cones respond more than L and I cones. This gives you a feeling of blue!

Illusion: Bidwell’s disk.

Positive afterimage

Movies and TV. The positive afterimage retains its original

color. Since the cones recover at different rate, you

may experience the change of the color with time.