1. AA&A spring 20021

2. 2 Today Atomic absorption and emission Quantifying a spectrum What does a filter (colored glass) do? Combining filters Color matching experiments How to summarize it all (A website for further exploration—also lots of optical illusions.)website

3. AA&A spring 20023 Atoms and photons: reminders 0 -20 -9 -13 -7 Absorption of light photon absorbed IF E photon = E final - E initial Electronic energy levels discrete set lowest ones occupied 0 —> electron leaves atom Spectral absorption lines at E photon = 7, 11, 13 eV E photon > 20 eV —> ionization

4. AA&A spring 20024 Atoms and photons: reminders 0 -20 -9 -13 -7 Emission of light photon emitted with E photon = E initial - E final Electronic energy levels discrete set a higher one occupied Spectral emission lines at E photon = 2, 6, 13 eV

5. AA&A spring 20025 ***Atoms and photons: questions*** 0 -20 -9 -13 -7 What energy photons could this atom emit? What energy photons could this atom absorb? What energy photon might this atom have absorbed to get into this “excited state” from its lowest possible energy state? 4 or 11 eV; 2, 7, 11, or 13 eV; 11 eV

6. AA&A spring 20026 Physics versus physiology What is color? For the physicist— –of a source: how much light at each wavelength –of a filter: how much light transmitted at each wavelength For you and me— –What color is it? What’s the connection (if any)

7. AA&A spring 20027 Spectral intensity: continuous source 400500600700 Wavelength (nm) spectral intensity

8. AA&A spring 20028 Spectral intensity: line source Spectral intensity of hydrogen discharge source 400500600700 Wavelength (nm) intensity

9. AA&A spring 20029 Characterizing filters Observe effect of red, green and blue filters (colored glass) What colors do they let through? What colors do they remove? “How” do they absorb the light? –Exciting electrons from lower to higher states –Electron states in glass smeared out into broad bands How to describe filter quantitatively?

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11. AA&A spring 200211 Red filter: transmission spectrum 400500600700 Wavelength (nm) intensity spectral transmission 400500600700 Wavelength (nm) 1 0.5 0

12. AA&A spring 200212 Three filters 400500600700 Wavelength (nm) 1 0 transmission 0.5 red + blue = ??

13. AA&A spring 200213 ****Filtered hydrogen source**** What would be the intensity spectrum of the hydrogen source (slide 8) after going through each of the filters indicated schematically in slide 12? Red: single line at about 620 nm Green: nothing Blue: the three lines at 375, 405 and 455 nm

14. AA&A spring 200214 R+B, R+G, G+B

15. AA&A spring 200215 Subtractive combination 400500600700 Wavelength (nm) 1 0 transmission 0.5 Transmission spectrum of light going through both is product of the individual spectra R + G B + G blue + green or red + green

16. AA&A spring 200216 Color synthesis Physicist: specify spectral intensity at each wavelength

17. AA&A spring 200217 Simpler idea—just three Mixtures of three different filtered lights: R, G, and B NOT on top of each other but variable amounts of each superpose from separate sources Additive combination 400500600700 Wavelength (nm) 1 0 intensity 0.5 NOW red + blue = ?

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19. AA&A spring 200219 Two different ways to combine colors Our first way: one light through two filters –Called subtractive combination –Transmission spectrum of pair is product of individual transmission spectra –Mixing inks or dyes together Our second way: combine several, separately filtered lights –Called additive combination –Intensity spectrum of combined sources is sum of the two individual intensity spectra –computer monitor screen

20. AA&A spring 200220 Matching experiment Let’s try a color matching experiment Can we match the color of the hydrogen lamp with additive mixture of light from the three filtered lights, R, G and B?

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22. AA&A spring 200222 Metamerism/metamers Spectral intensity of hydrogen discharge source 400500600700 intensity Spectral intensity of R,G, B match 400500600700 intensity

23. AA&A spring 200223 More on color matching Can we match ANY color with additive combination of light from R, G and B sources? Yes and no-- –In principle, with the “right” R, G and B we could –With any real set of R, G and B we can’t Can see a familiar attempt to do this— photos of computer screen

24. AA&A spring 200224 3 magnifications of screen image d

25. AA&A spring 200225 Liquid crystal color display screen 0.01”

26. AA&A spring 200226 How to quantify Long series of color matching experiments Match to variety of colors with various pairs and triples of individual spectral lines (not just R, G and B) Results neatly summarized in CIE chromaticity diagram (Commission Internationale d’Éclairage)

27. AA&A spring 200227 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral

28. AA&A spring 200228 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral Get ANY color on line by mixing two ends

29. AA&A spring 200229 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral Get ANY color on line by mixing two ends Many ways to get a given color!

30. AA&A spring 200230 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral Get ANY color on line by mixing two ends Many ways to get a given color! On line through “white point,” right mixture of two ends is white—complementary colors

31. AA&A spring 200231 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral Get ANY color on line by mixing two ends Many ways to get a given color! On line through “white point,” right mixture of two ends is white—complementary colors With three colors at corners of triangle, can match ANY color within triangle

32. AA&A spring 200232 Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral Get ANY color on line by mixing two ends Many ways to get a given color! On line through “white point,” right mixture of two ends is white—complementary colors With three colors at corners of triangle, can match ANY color within triangle ANY color can be matched by a color from diagram if adjust brightness

33. AA&A spring 200233 ****Chromaticity diagram**** What spectral color would I need to use, in combination with a source of wavelength 490 nm, to create the color of the “magic point” on the previous slide? What is the wavelength of the spectral color which is the complement of the color at the position of the green star on the previous slide? 590 nm; 530 nm

34. AA&A spring 200234 But I don’t see any brown Closest I find is a dirty orange

35. AA&A spring 200235 But I don’t see any brown Closest I find is a dirty orange Just use less of everything! (physiology, not physics)

36. AA&A spring 200236 Chromaticity diagram 2/3 R, 1/3 G1/3 R, 2/3 G

37. AA&A spring 200237 Chromaticity diagram 1/3 R, 2/3 G 1/6 R, 1/3 G, 1/2 B

38. AA&A spring 200238 Gamut = available range of color Given 3 colors, can match only to interior of triangle = the gamut avail- able using those three “primaries” What to use for corners R, G, B seem good! NOT perfect Typical monitors—black triangle Can’t make spectral colors (can’t make a lot of colors) Why don’t monitor colors match printer colors? A B C

39. AA&A spring 200239 *****Gamut***** The previous slide shows the gamuts of two different devices, black and white; and three different colors, A, B, and C. Which of the three colors could be displayed by either device? Which of the three colors could be displayed by neither device? C; A

40. AA&A spring 200240 Today Electrons and photons Spectral intensity Transmission spectra of filters Subtractive/additive combination of colors There’s more than one way to make a color CIE chromaticity diagram

41. AA&A spring 200241 UNUSED SLIDES

42. AA&A spring 200242 Red filter: absorption spectrum 400500600700 Wavelength (nm) intensity absorption 400500600700 Wavelength (nm) 1 0.5 0

43. AA&A spring 200243 Three filters 400500600700 Wavelength (nm) 1 0 Absorption or transmission?? 0.5

44. AA&A spring 200244 Three filters 400500600700 Wavelength (nm) 1 0 Absorption 0.5

45. AA&A spring 200245 Three filters 400500600700 Wavelength (nm) 1 0 Absorption or transmission?? 0.5 red + blue = ??

46. AA&A spring 200246 ??????????? Hue, dominant wavelength Saturation, chroma, purity Value, brightness, luminance Resource site on vision and colorResource site Color wheel