AA&A spring 20021. 2 Today Atomic absorption and emission Quantifying a spectrum What does a filter...
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Transcript of AA&A spring 20021. 2 Today Atomic absorption and emission Quantifying a spectrum What does a filter...
AA&A spring 2002 1
AA&A spring 2002 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.)
AA&A spring 2002 3
Atoms and photons: reminders0
-20
-9
-13
-7
Absorption of lightphoton absorbed IFEphoton = Efinal - Einitial
Electronic energy levelsdiscrete setlowest ones occupied0 —> electron leaves atom
Spectral absorption lines at Ephoton = 7, 11, 13 eV Ephoton > 20 eV —> ionization
AA&A spring 2002 4
Atoms and photons: reminders0
-20
-9
-13
-7Emission of lightphoton emitted withEphoton = Einitial - Efinal
Electronic energy levelsdiscrete seta higher one occupied
Spectral emission lines at Ephoton = 2, 6, 13 eV
AA&A spring 2002 5
***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
AA&A spring 2002 6
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 2002 7
Spectral intensity: continuous source
400 500 600 700
Wavelength (nm)
spectralintensity
AA&A spring 2002 8
Spectral intensity: line source
• Spectral intensity of hydrogen discharge source
400 500 600 700
Wavelength (nm)
intensity
AA&A spring 2002 9
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?
AA&A spring 2002 11
Red filter: transmission spectrum
400 500 600 700
Wavelength (nm)
intensity
spectraltransmission
400 500 600 700
Wavelength (nm)
1
0.5
0
AA&A spring 2002 12
Three filters
400 500 600 700
Wavelength (nm)
1
0
transmission
0.5
red + blue = ??
AA&A spring 2002 13
****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 2002 15
Subtractive combination
400 500 600 700
Wavelength (nm)
1
0
transmission
0.5
Transmission spectrum of light going through both is product of the individual spectra
R + GB + G
blue + green or red + green
AA&A spring 2002 16
Color synthesisPhysicist: specify spectral intensity at each wavelength
AA&A spring 2002 17
Simpler idea—just threeMixtures of three different filtered lights: R, G, and B
NOT on top of each other butvariable amounts of eachsuperpose from separate sources
Additive combination
400 500 600 700
Wavelength (nm)
1
0
intensity
0.5
NOW red + blue = ?
AA&A spring 2002 19
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 2002 20
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?
AA&A spring 2002 22
Metamerism/metamers • Spectral intensity of hydrogen discharge source
400 500 600 700
intensity
• Spectral intensity of R,G, B match
400 500 600 700
intensity
AA&A spring 2002 23
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 2002 24
3 magnifications of screen image
d
AA&A spring 2002 25
Liquid crystal color display screen
0.01”
AA&A spring 2002 32
Rules for CIE chromaticity diagramPure spectral colors along arc-likeboundary 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 2002 33
****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 2002 35
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 2002 38
Gamut = available range of colorGiven 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 matchprinter colors?
A B C
AA&A spring 2002 39
*****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 2002 40
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