Color as a signal Photoreceptors (some review) Three-dimensionality of color vision

1. Color as a signal • Photoreceptors (some review) • Three-dimensionality of color vision • Characteristics of cones • Spectra, organization, genetics • Opponency (basics) • Comparative color vision • Evolution of color vision • Color blindness jcarroll@cvs.rochester.edu Meliora 252

4. What is Color? • Color is a sensation which occurs when light energy, incident on the retina is interpreted by the brain. • In other words, the perception of color lies in the ability of our visual system to transform light energy into a neural signal.

5. The color signal sent to the brain is a function of the surface properties of the object and the spectral composition of its illuminant...

7. Variations in the illuminant change the incident light on the retina, but don’t always affect the appearance of objects...

15. The perception of color lies in the ability of our visual system to transform light energy into a neural signal Let’s review how this is accomplished...

18. 200 femtoseconds !!

19. Neitz & Neitz (1998)

21. Isomerization Opsin - transducin Transducin - PDE PDE - cGMP to GMP Low cGMP closes ion channels hyperpolarized Phototransduction(vertebrates)

22. Phototransduction(vertebrates)

23. Photopigments can be characterized according to the efficiency with which they absorb light...

24. Principle of Univariance: Photoreceptors cannot register the wavelength of the photons they catch - i.e., “the output depends on quantum catch, but not upon which quanta are caught”.

25. l1 l2 .100 .075 Fraction of incident light absorbed .050 .025 0.00 400 450 500 550 600 650 700 Wavelength (nm) Monochromat with 1 cone type Incident Photons 1000 1000 Pigment Absorptance 0.10 .050 l1 l2 Number Absorbed 100 50

26. l1 l2 .100 .075 Fraction of incident light absorbed .050 .025 0.00 400 450 500 550 600 650 700 Wavelength (nm) Monochromat with 1 cone type Incident Photons 1000 2000 Pigment Absorptance 0.10 .050 l1 l2 Number Absorbed 100 100 A Perfect Match!

27. Monochromat with 1 cone type Incident Photons 1000 1000 Pigment Absorptance .050 .050 l2 l1 l2 Number Absorbed 50 50 Unable to discriminate! l1 l2 .100 .075 Fraction of incident light absorbed .050 .025 0.00 400 450 500 550 600 650 700 Wavelength (nm)

28. l2 l1 l1 l2 .100 .075 Fraction of incident light absorbed .050 .025 0.00 400 450 500 550 600 650 700 Wavelength (nm) Dichromat with 2 cone types No Match Possible

29. Color Mixing • Additive: spectrum of light is the result of addition of individual spectra • CRT color mixing • LCD projectors • Subtractive: color resulting from the selective absorption of light wavelengths • paints • dyes

30. Color Primaries Subtractive Color Mixing Additive Color Mixing

31. CIE 1931 Color Matching Experiments • Fixed 3 primaries, R, G & B: • r = 700nm, g = 546.1nm, b = 435.8nm • Power ratio R:G:B = 72.1:1.4:1.0 • Field of view fixed to 2, subjects assumed to be light-adapted • Subjects matched monochromatic stimuli from 380nm to 780nm in 5nm increments by varying intensity of 3 primary stimuli

32. CIE RGB Tristimulus Curves

33. CIE XYZ matching functions

35. Three fundamental chromatic attributes: Hue (color - red, green, etc.) Saturation (measure of purity - pink vs red) Brightness (visual effectiveness)