SCI 200 Physical Science Lecture 9 Color & Color Vision

1. SCI 200 Physical Science Lecture 9Color & Color Vision Rob Daniell July 21, 2011

2. Psychological Color Subjective Indirectly measurable Based on the response of cones and subsequent processing Ganglions Brain Physical color Objective Directly measurable Based on wavelength Any “color” can be defined by the relative intensity of light at each wavelength Physical vs. Psychological Color NEiA SCI 200 Lecture 9

3. Physical Color • Electromagnetic Spectrum • Color vs. wavelength NEiA SCI 200 Lecture 9

4. Physical Color • Electromagnetic Spectrum • Intensity vs. wavelength NEiA SCI 200 Lecture 9

5. Physical Color • Spectroscope • Light source • Entrance slit • Dispersive element (prism or grating) • Screen or detector NEiA SCI 200 Lecture 9

6. Physical Color • Simplified Grating Spectroscope • Project STAR Spectrometer • Transmission grating • Adjustable scale • Do not point directly at sun NEiA SCI 200 Lecture 9

7. Physical Color • Diffraction Grating NEiA SCI 200 Lecture 9

8. Physical Color • Simplified Grating Spectroscope • Project STAR Spectrometer • Transmission grating • Adjustable scale • Do not point directly at sun NEiA SCI 200 Lecture 9

9. Physical Color • Wavelength spectra of various light sources • Intensity units are relative • Gilbert and Haeberli [2007] Am. J. Phys., 75, 313-319. NEiA SCI 200 Lecture 9

10. Physical Color • Wavelength spectra of fluorescent light bulbs • As seen through Project STAR Spectrometer NEiA SCI 200 Lecture 9

11. Physical Color • Discrete spectrum • Helium NEiA SCI 200 Lecture 9

12. Physical Color • Discrete spectrum: more examples • Hydrogen, Sodium, Helium, Neon, Mercury NEiA SCI 200 Lecture 9

13. Physical Color • Continuous spectrum • White light, sunlight, etc. NEiA SCI 200 Lecture 9

14. Monochromatic vs.Non-monochromatic Colors • Monochromatic colors: • Consist of a single wavelength • Sometimes called “spectral colors” • Non-monochromatic colors: • 1. A discrete spectrum • several discrete wavelengths • 2. A continuous spectrum • Most colors in nature are non-monochromatic • Example: sunlight NEiA SCI 200 Lecture 9

15. Psychological Color • Physical color as perceived by the human eye and brain • Color perception is mediated by the cones in the retina • There are (usually) three kinds of cones operating • Each cone type responds differently to a specific physical color • The signals from the cones are processed in a non-intuitive way to produce the sensation of color NEiA SCI 200 Lecture 9

16. Psychological Color • Color specification systems: • HSV: Hue, Saturation, Value • Also: • HSL (hue, saturation, lightness) • HSB (hue, saturation, brightness) • Corresponds most closely to human color perception • Preferred by many artists • RGB: (Red, Green, Blue) • Used in additive color systems • Used in many digital graphics applications • Displays • Software • CMYK: (Cyan-Magenta-Yellow-blacK) • Used in subtractive color systems • Used for printing inks, etc. • “Four Color Printing” NEiA SCI 200 Lecture 9

17. Color Vision • HSV: cylinder • Hue: • perceived color • 0°-240° • 240°-360° (“purples”) • Saturation: • Purity of color • 0-1 • Value: • Light intensity • 0-1 or black to white (brightest) NEiA SCI 200 Lecture 9

18. Color Vision Another representation of HSL NEiA SCI 200 Lecture 9

19. Trichromacy • History • Thomas Young (1773-1829) • Observed that it only takes three quantities (Hue, Saturation, Value) to specify a color • Three output quantities require three input quantities • Postulated three kinds of photoreceptors NEiA SCI 200 Lecture 9

20. Trichromacy • History (continued) • Hermann von Helmholtz (1821-1894) • Suggested that Young’s three photoreceptors were • Short wavelength • Intermediate wavelength • Long wavelength • Must overlap • Monochromatic light of different wavelengths have different colors NEiA SCI 200 Lecture 9

21. Response 700 600 400 500 Trichromacy • Suppose there were no overlap: • Monochromatic light would appear to consist of exactly three colors S I L • For example, (above) any monochromatic light source between 400 and 500 nm would appear blue. • Yet we know that 450 nm light is a very different shade of blue than 475 nm light NEiA SCI 200 Lecture 9

22. A. Overlap of Response Curves • Example: six monochromatic emission lines from atomic Helium • Each a different color • Conclusion: There must be at least two overlapping cones at each wavelength in the visible region • Line spectrum of helium (He) • Blue-violet: 447.1 nm • Blue: 471.3 nm • Green: 501.5 nm • Orange: 587.5 nm • Red-orange: 706.5 nm • Dark red: 728.1 nm NEiA SCI 200 Lecture 9

23. Trichromacy • Where do the curves cross? • This requires exploring the properties of psychological color NEiA SCI 200 Lecture 9

24. Trichromacy • Complementary colors: • R + C W • G + M W • B + Y W • White can be produced by • Broadband light (e.g., sunlight) • Pairs of complementary colors • Stimulate the three kinds of photoreceptors “equally” • An infinite variety of other combinations C = Cyan, M = Magenta, Y = Yellow W = White NEiA SCI 200 Lecture 9

25. Color Perception Mechanisms • If Helmholtz is right, how can we determine the actual response curves? • A. Overlap of response curves • B. Spectral complementaries • C. Hue discrimination • D. Microspectrophotometry NEiA SCI 200 Lecture 9

26. Trichromacy • Where do the curves cross? • Consider a monochromatic color at about 430 nm. • Stimulates S with a little I • Another monochromatic color near 610 nm could stimulate some I and more L to produce white. • Also, vice versa NEiA SCI 200 Lecture 9

27. Trichromacy • Where do the curves cross? • Note that in the region where the Intermediate photoreceptors dominate, no single complementary spectral (monochromatic) color exists • No one spectral color can stimulate both the S and the L photoreceptors equally. • Empirically, this is the region from 490 nm to 565 nm NEiA SCI 200 Lecture 9

28. Trichromacy • So 490 nm and 565 nm represent the crossover points between S and I and between I and L, respectively • Between these wavelengths, it takes two additional monochromatic sources to combine with a “green” source to produce white • A “blue” source and a “red” source - hence “purple” (or magenta) • This has consequences for color mixing (Lecture 10) NEiA SCI 200 Lecture 9

29. Trichromacy • Hue discrimination: • The difference in wavelength (Δλ, pronounced “delta lambda”) at which two monochromatic sources are barely distinguishable • Varies with wavelength • Where Δλis small, the photoreceptor response must be changing rapidly • Further Details of the spectral response curves required microspectrophotometry • The physical measurement of the amount of light of each wavelength absorbed by each kind of cone • Although many cones have been measured this way only three basic types have been found NEiA SCI 200 Lecture 9

30. Trichromacy • Cone Mosaic: • Simulation based on measured cone densities • No “blue” cones in the central fovea! • Visual acuity in blue light is less than in green and red light • Over the entire retina • There are about 100 “red” and “green” cones for every “blue” cone • There are about 150 “red” cones for every 100 “green” cones • However: Much variation among individuals NEiA SCI 200 Lecture 9

31. Trichromacy • Spectral sensitivity of the three types of cones in the human eye • Intensity of each wavelength is the same • There is considerable overlap among the three cone types • Type II & Type III cones have the same sensitivity at about 560 nm • Figure 6.4 from text NEiA SCI 200 Lecture 9

32. Trichromacy • Spectral sensitivity of Type II (green) cones • Two different wavelengths can produce the same response • Figure 6.5 from text • Using all three types of cones, the four colors can be distinguished. • Figure 6.6 from text NEiA SCI 200 Lecture 9

33. Trichromacy • Color vision: • : 3 kinds of cones • Type I: Short (S), beta (β), or blue (B) • Type II: Intermediate (I), gamma (γ), or green (G) • Type III: Long (L), rho (ρ), or red (R) Note that the three kinds of cones do not actually correspond to blue, green, and red. The RGB model is merely a convenient means of representing color. NEiA SCI 200 Lecture 9

34. Color Vision • RGB color system: • Based (loosely) on the three cones of the human eye • Z ~ blue, Y ~ green, X ~ red (even though it peaks shortward of red) NEiA SCI 200 Lecture 9

35. Color Vision • Additive color rules: • R + G + B =W • R + G =Y • G + B =C • R + B =M • Complementary colors: • R + C =W • G + M =W • B + Y =W • Can any 3 colors be combined to produce any other color? • Can R, G, & B be combined to produce any other color? C = Cyan, M = Magenta, Y = Yellow W = White NEiA SCI 200 Lecture 9

36. Color Vision • Red, Green, & Blue can be combined to produce most colors, but some saturated (or nearly saturated) colors cannot be reproduced. • Will be considered in more detail in Lecture 10 C = Cyan, M = Magenta, Y = Yellow W = White NEiA SCI 200 Lecture 9

37. Color Vision • Subtractive color combination: • Filters that absorb or block light of certain colors • Ink or pigments that reflect only certain colors and absorb the others • Primary Subtractive Colors: • Cyan, Magneta, Yellow • Supplemented by Black in “four color printing” • Will be considered in more detail in Lecture 10 C = Cyan, M = Magenta, Y = Yellow K = Black NEiA SCI 200 Lecture 9

38. Trichromacy • Where does “yellow” come from? NEiA SCI 200 Lecture 9

39. Trichromacy or Opponent Colors? • Statements: • Magenta looks like a mixture of Red & Blue • Cyan looks like a mixture of Green & Blue • Yellowlooks nothinglike a mixture of Red & Green NEiA SCI 200 Lecture 9

40. Trichromacy or Opponent Colors? • Based on the trichromacy theory • We should expect an additive mixture of red and green to give a reddish green (or a greenish red). • Instead it gives yellow • In fact, it takes fourpsychological primaries to verbally describe any color • Blue, green, yellow, and red • Orange looks yellowish red • Cyan looks bluish green • Purple looks reddish blue • Etc. NEiA SCI 200 Lecture 9

41. Opponent Processing • When asked to name the color of a spot of spectral (i.e., monochromatic) light, most people give responses similar to those at right • Note that there is no “reddish green” or “yellowish blue” NEiA SCI 200 Lecture 9

42. Opponent Processing • Yellow and blue seem to oppose each other • Red and green also seem to oppose each other • How can the three kinds of cones be wired together to produce this kind of color opposition? NEiA SCI 200 Lecture 9

43. Opponent Processing • S inhibits y-b and stimulates r-g & w-bk • I inhibits r-g and stimulates y-b & w-bk • L stimulates all three opponent systems NEiA SCI 200 Lecture 9

44. Opponent Processing • Net stimulation of y-b makes the light appear yellowish; net inhibition, bluish • Net stimulation of r-g makes the light appear reddish; net inhibition, greenish • The w-bk channel conveys brightness information NEiA SCI 200 Lecture 9

45. Opponent Processing • There are at least two rival theories for the details of how the three kinds of cones get processed to produce the opponent activity. • One theory makes use of lateral inhibition in the form of center-surround antagonism among the various cones • Another assumes some kind of filter that narrows the wavelength range accessible to some cones but not others. NEiA SCI 200 Lecture 9

46. Genetics of Color Vision • Review: basics of human genetics • Each cell in the human body contains 23 pairs of chromosomes • The chromosomes are numbered 1 through 22 plus the X and/or Y chromosome • In each pair, one comes from the mother, the other from the father. • The gender is (mostly) determined by the X and Y chromosomes • Females have 2 X chromosomes, one from each parent • Males have an X chromosome from their mothers and a Y chromosome from their fathers • Other primate species have differing numbers of chromosomes • The Great Apes all have 24 pairs • Gender is generally determined in the same was as for humans • The genes controlling color vision differ among primate species NEiA SCI 200 Lecture 9

47. Genetics of Color Vision • Color Vision • The number of cone types varies dramatically throughout the Animal Kingdom • Mammals • Most mammals have only two types of cones – dichromats • Short vs. long wavelength: Yellow vs. Blue • Red-Green color blind • Primates • All new world primates are dichromats • But see next slide • Many old world primates are trichromats • Especially monkeys, apes, and humans NEiA SCI 200 Lecture 9

48. Genetics of Color Vision • Scientific American, April 2009, The Evolution of Primate Color Vision, pp. 56-63. • Some Old World primates (including humans) are trichromats • Gene for the short wavelength (“blue”) cone resides on chromosome 7 • Genes for the medium wavelength (“green”) cone and the long wavelength (“red”) cone both reside on the X chromosome NEiA SCI 200 Lecture 9

49. Genetics of Color Vision • Scientific American, April 2009, The Evolution of Primate Color Vision, pp. 56-63. • New World primates are mostly dichromats • Gene for the short wavelength (“blue”) cone resides on chromosome 7 • Gene for one of the longer wavelength (“green”, “yellow”, or “red”) cones resides on the X chromosome NEiA SCI 200 Lecture 9

50. Genetics of Color Vision • Scientific American, April 2009, The Evolution of Primate Color Vision, pp. 56-63. • Some female New World primates are trichromats • One X chromosome has one of the green, yellow, or red cones • The other X chromosome has a different “long wavelength” cone • These females can distinguish colors that their dichromat brothers and sisters cannot NEiA SCI 200 Lecture 9