COLOR VISION

1. COLOR VISION

2. Wavelength and Color • Recall that light is electromagnetic radiation

3. Wavelength and Color • Recall that light is electromagnetic radiation • Light waves have a frequency/wavelength

4. Wavelength and Color • Recall that light is electromagnetic radiation • Light waves have a frequency/wavelength • Frequency/wavelength is the physical property that corresponds (loosely) to the perception called color

5. Color Vision Wavelength and Color • Different wavelengths correspond roughly to the “colors” of the spectrum

6. Color Vision Wavelength and Color • White light is a mixture of wavelengths • prisms decompose white light into assorted wavelengths

7. Color Vision Wavelength and Color • White light is a mixture of wavelengths • prisms decompose white light into assorted wavelengths • likewise, adding all wavelengths together recovers white light What happens if you mix several different paints together?

8. Color Vision Wavelength and Color • Objects have different colors because they reflect some but not all wavelengths of light • the surfaces of objects are like filters that selectively absorb certain wavelengths

9. Color Vision Perceiving Color • Primary colors What are the primary colors?

10. Color Vision Perceiving Color • Primary colors RedGreenBlue

11. Color Vision Perceiving Color • Primary colors What makes them primary?

12. Color Vision Perceiving Color • Primary colors • Every color (hue) can be created by blending light of the three primary colors in differing proportions

13. Color Vision Perceiving Color • Primary colors • Every color (hue) can be created by blending light of the three primary colors in differing proportions • Led to prediction that there must be three (and only three) distinct color receptor types

14. Color Vision Perceiving Color • Four absorption peaks in retina: 3 cone types plus rods Absorption/Cone response

15. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” Blue “Green” “Red”

16. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” “Green” Green “Red”

17. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” “Green” “Red” Red

18. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” Equal Parts Red and Green = “Green” Yellow “Red”

19. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” Equal Parts Red and Green = “Green” Yellow “Red”

20. Color Vision Theories of Color Vision Signal to Brain Wavelength Input Cone “Blue” Equal Parts Red and Green = “Green” Yellow “Red”

21. Color Vision Theories of Color Vision • Trichromatic theory of color vision: • brain interprets the relative amounts of signaling from each of these cone types

22. Color Vision Theories of Color Vision • Trichromatic theory of color vision: • brain interprets the relative amounts of signaling from each of these cone types • This means that some colors can be matched by a pair of wavelengths • metamers: colors that have no definite single wavelength (e.g. yellow)

23. Color Vision Theories of Color Vision • Trichromatic theory of color vision: • brain interprets the relative amounts of signaling from each of these cone types • This means that some colors can be matched by a pair of wavelengths • metamers: colors that have no definite single wavelength (e.g. yellow) • This also means that any color can be matched by mixing (not more than) three different wavelengths

24. Color Vision Theories of Color Vision • Trichromatic Theory can explain colorblindness: • most of us are trichromats • someone missing one of the three cone types is a dichromat • someone missing two is a monochromat • someone missing all cone types is called a rod monochromat (very poor vision!)

25. Color Vision Theories of Color Vision • Trichromatic Theory can explain colorblindness: • dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths that they are sensitive to

26. Color Vision Theories of Color Vision • Trichromatic Theory can explain colorblindness: • dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths that they are sensitive to • most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones

27. Color Vision Theories of Color Vision • Trichromatic Theory can explain colorblindness: • dichromats have only two primaries: any color they can see can be matched with differing proportions of the two wavelengths that they are sensitive to • most common is deuteranopia (~3% of men, <1% of women) - missing “green” cones • cannot see difference between reds and greens

28. Color Vision Theories of Color Vision Ishihara Color Plates can indicate color blindness

29. Color Vision Theories of Color Vision DON’T DO THIS ! …~3% of male readers will have trouble seeing it!

30. Color Vision Theories of Color Vision But this is OK.

31. Color Vision Theories of Color Vision So is this.

32. Color Vision Theories of Color Vision Even this is good.

33. Color Vision Theories of Color Vision But this probably isn’t!

34. Theories of Color Vision • Problem with Trichromatic Theory:

35. Theories of Color Vision • Problem with Trichromatic Theory: YELLOW

36. Theories of Color Vision • Problem with Trichromatic Theory: • most people categorize colors into four primaries: red, yellow, green, and blue • some colors simply cannot be perceived as gradations of each other • redish green !? • blueish yellow !?

37. Theories of Color Vision • Opponent-Process Theory • color is determined by outputs of two different continuously variable channels: • red - green opponent channel • blue - yellow opponent channel

38. Theories of Color Vision • Opponent-Process Theory • Red opposes Green • (Red + Green) opposes Blue • Opponent-Process Theory explains color afterimages