EXAM 2 !!!

1. EXAM 2 !!! Monday, Tuesday, Wednesday, Thursday of NEXT WEEK

2. EXAM 2 !!! Will cover everything from: taste/touch/smell/balance To Colour Vision (Thursday this week)

3. COLOR VISION

4. Color is an illusion • What color is this box?

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

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

7. 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

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

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

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

11. Color Vision Perceiving Color • Primary colors RedGreenBlue

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

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

14. 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

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

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

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

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

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

20. Color Vision Trichromatic Theory 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 • 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 • 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 • 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 Mixing • What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength?

25. Color Mixing • What color can only exist as a metamer (an additive mixture of wavelengths)? In other words, what color cannot be made with a single wavelength? Magenta Think about why!

26. Color Mixing • Subtractive mixing is different! • Subtractive mixing is commonly used in color printers

27. Color Mixing • Subtractive mixing is different! • Subtractive mixing is commonly used in color printers • In subtractive mixing, pigments are used to REMOVE wavelengths • The resulting colour is a mixture whatever wavelengths don’t get absorbed by the pigment

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

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

30. Theories of Color Vision: Trichromatic Theory • 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 !? • It is as if these colors are opposites

31. Theories of Color Vision: Opponent-Process Theory • Opponent-Process Theory • color is determined by outputs of two different continuously variable channels: • red - green opponent channel • blue - yellow opponent channel • “yellow” = red + green • Your visual system balances input to either end of each channel

32. Color is an illusion • Everything you’ve learned so far is wrong.

33. Color is an illusion • Everything you’ve learned so far is wrong. • Well, not really wrong, just far from complete.

34. L A N D Everthing you thought you knew about color is wrong...

35. What Newton Found (and everyone believed) • White light can be split into all wavelengths by a prism • According to previous theories: two wavelengths combine to yield intermediate color and no others Red Light Green Light Red + Green = YELLOW

36. What Newton Found (and everyone believed) • White light can be split into all wavelengths by a prism • According to previous theories: two wavelengths combine to yield intermediate color and no others • Red + Green light can never yield blue • Blue + Green light can never yield red

37. What Land found: • Two bands (colors) of the spectrum recombine to produce all the possible colors • provided the appropriate relative amount of each wavelength is projected Red Light Green Light transparency slides

38. How did Land project the “appropriate” ratio of wavelengths?

39. Projector combines “longer” and “shorter” wavelengths using the maps to get the appropriate amounts of each long/“red” light medium/ “green” light Viewer perceives desaturated hues including blues

40. Why would the visual system have evolved this way? • Hint: “Within broad limits, the actual values of the wavelengths make no difference, nor does the over-all available brightness of each”

41. Color Constancy • The “color” of objects is independent of the ambient light – even though light can vary dramatically Sunlight Incandescent Light Relative Intensity Relative Intensity Wavelength

42. Color Constancy • Because of our mechanism of color constancy we can even use completely artificial spectra

43. Next Time • ATTENTION!