AA&A spring 20021
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
AA&A spring Atoms and photons: reminders 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
AA&A spring Atoms and photons: reminders 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
AA&A spring ***Atoms and photons: questions*** 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
AA&A spring 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)
AA&A spring Spectral intensity: continuous source Wavelength (nm) spectral intensity
AA&A spring Spectral intensity: line source Spectral intensity of hydrogen discharge source Wavelength (nm) intensity
AA&A spring 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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AA&A spring Red filter: transmission spectrum Wavelength (nm) intensity spectral transmission Wavelength (nm)
AA&A spring Three filters Wavelength (nm) 1 0 transmission 0.5 red + blue = ??
AA&A spring ****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
AA&A spring R+B, R+G, G+B
AA&A spring Subtractive combination 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
AA&A spring Color synthesis Physicist: specify spectral intensity at each wavelength
AA&A spring 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 Wavelength (nm) 1 0 intensity 0.5 NOW red + blue = ?
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AA&A spring 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
AA&A spring 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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AA&A spring Metamerism/metamers Spectral intensity of hydrogen discharge source intensity Spectral intensity of R,G, B match intensity
AA&A spring 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
AA&A spring magnifications of screen image d
AA&A spring Liquid crystal color display screen 0.01”
AA&A spring 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)
AA&A spring Rules for CIE chromaticity diagramchromaticity diagram Pure spectral colors along arc-like boundary line Purples (R + B) along bottom— NON-spectral
AA&A spring 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
AA&A spring 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!
AA&A spring 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
AA&A spring 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
AA&A spring 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
AA&A spring ****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
AA&A spring But I don’t see any brown Closest I find is a dirty orange
AA&A spring But I don’t see any brown Closest I find is a dirty orange Just use less of everything! (physiology, not physics)
AA&A spring Chromaticity diagram 2/3 R, 1/3 G1/3 R, 2/3 G
AA&A spring Chromaticity diagram 1/3 R, 2/3 G 1/6 R, 1/3 G, 1/2 B
AA&A spring 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
AA&A spring *****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
AA&A spring 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
AA&A spring UNUSED SLIDES
AA&A spring Red filter: absorption spectrum Wavelength (nm) intensity absorption Wavelength (nm)
AA&A spring Three filters Wavelength (nm) 1 0 Absorption or transmission?? 0.5
AA&A spring Three filters Wavelength (nm) 1 0 Absorption 0.5
AA&A spring Three filters Wavelength (nm) 1 0 Absorption or transmission?? 0.5 red + blue = ??
AA&A spring ??????????? Hue, dominant wavelength Saturation, chroma, purity Value, brightness, luminance Resource site on vision and colorResource site Color wheel