How do we see color? There is only one type of rod. It can only tell the intensity of the light, not its color. Because the cones can differentiate colors, there must be more than one type of cone. Thomas Young (1801) ( this is the Young of double slit experiment fame) postulated three types of cones --- Trichromacy, based on there being three attributes of color: hue, saturation, lightness. ( need three inputs to get three outputs)

If we just had rods… How would we tell a small amount of light at 1 from a large amount of light at 2 ? We would see in black and white - as we do under scotopic conditions

Helmholtz three cone types Hermann von Helmholtz ( ) postulated three response curves for the three types of cones to explain Young’s trichromacy: –S-cones: has its best response to short wavelengths of light –L-cones: …. to long wavelengths of light –I-cones: ….. to the intermediate wavelengths of light. Different colors correspond to different patterns of response in these cone types.

Determine the response curves Since every wavelength of the visible light is perceived as a different color, the response curves of the three cone types must overlap. Why is this true? For white light, all cones respond equally. From the region of spectral colors without spectral complementaries (greens from 495 to 565 nm), we can determine where L & I and S & I responses cross.

C.I.E. Chromaticity Diagram

I > S and L then complement must have S & L > I (because green + purple (magenta) = white I green + I purple = S green +S purple =L green +L purple ) Can’t be done with a single wavelength Purple (magenta) is a double humped distribution S IL

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

The cone responses can be measured directly from microspectrophotometry. –Measure the amount of light absorbed by the cones as a function of wavelength. –Only three types of responses have been found! Corresponds to three types of cones (trichromacy).

(short) (intermediate)

(long)

Response curves and color mixing The information contained in the chromaticity diagram is consistent with that of the response curves. Pick two colors that are complements. When they combine to produce white, they must be exciting the S, I, and L cones about equally, just like broadband white.

Bees are Trichromatic also

Chromaticity Diagram for Bees

Four Psychological Primaries When we additively mix red and green, we don’t see reddish green; we see yellow. A subtractive mixture of cyan and yellow gives green, not yellowish cyan. Thus to tell what colors look like, we need four primaries: blue, green, yellow and red. Any hue can be 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 ( ) and was in competition with trichromacy.

Color Afterimage ++ Stare at the plus on the left side for 30 s. Then quickly move your gaze to the plus on the right side. You should see yellow and magenta circles.

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.

Opponents: Green:red and Blue:yellow

Which is correct : trichromacy or opponent processing? Biology : trichromacy Psychology: opponent processing Answer: 3 cone types are wired in the retina to give opponent processing!

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-bk + + +

Opponent Processing S I L W-BlackY-Blue R-G to brain - Luminance channel Chromatic channel

Opponent Process Curves

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.

Simultaneous Color Contrast

Trichromacy and Opponent Color Theory We now appear to have a consistent picture of how signals from the three types of cones are combined into r-g and y-b opponent chromatic visual channels. The two competing theories of color vision both give part of the story.

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 a white area. When you look at yellow for a while, the L and I cones are excited and become tired. So when you avert your gaze to white, the S cones respond more than L and I cones. This gives you an impression of blue! Illusion: Bidwell’s disk.

Greener than Green

Positive afterimage Movies and TV. The positive afterimage retains its original color. –Since the cones recover at different rate, you may experience a change of the color with time.