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

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-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

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-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

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-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