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2AA&A spring 2002

RGB complements

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CMY complements

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Mixing paints and subtractive filters

C-Y pair—only green is transmitted

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Mixing paints and subtractive filters

Y-M pair—only red is transmitted

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Mixing paints and subtractive filters

M-C pair—only blue is transmitted

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*****Subtractive mixing*****• A paint (A) is made using a mixture of two pigments, yellow and blue. The blue

transmits light only from 400 nm to 510 nm, while the yellow transmits only from 520 nm to 600 nm. What will be the color of the mixture?

• A paint (B) is made using a mixture of two pigments, yellow and blue. The blue transmits light only from 420 nm to 540 nm, while the yellow transmits only from 530 nm to 650 nm. What will be the color of the mixture?

• A paint (C) is made using a mixture of two pigments, yellow and cyan. The blue transmits light only from 420 nm to 560 nm, while the yellow transmit only from 510 nm to 650 nm. What will be the color of the mixture?

• Which of the three is the brightest? Which of the three is the most saturated?

• black, green, green, C, B

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Rest of electromagnetic spectrum?

• Near infra-red (IR)—wavelength > 800 nm

• Near ultra-violet (UV)—wavelength < 400 nm

• X-ray—wavelength ~ 0.01-1 nm

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Infra-red (IR)—index of refraction

• Index of refraction n is less in IR than in visible

• Implies less reflection, less scattering, deeper penetration of light into paint

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IR—particle size and wavelength

• In addition to wavelength dependence of index n• What if particle size same or less than wavelength:

– New physics

– Scattering falls off rapidly with longer wavelength

– (blue scattered more strongly than red and IR)

Scatteringstrength

wavelengthParticle

size

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Wavelength dependence of scattering

• What happened to the sky in the IR photo?• Blue sky, red sunsets• Yellow fog-lights on cars• Again, deep penetration of IR into paint layer

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IR—selective absorption

• Brick—reflects more in the red than in the IR

• Leaves—reflect more in IR than in the red

• Charcoal absorbs everywhere! (Wednesday)

link to photos

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IR-summary• IR light penetrates more deeply into paint layer

than visible light: both– Wavelength dependence of index n

– Particle size small compared with wavelength

– —>Possibility of seeing “what’s underneath”

• Selective absorption– Different image contrast

• Details depend critically on variety of factors: – specific pigments

– binders

– particle sizes

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UV—selective reflection

Links to flowers and rugs

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UV—fluorescence0

-5.0

-6.5

-2.5

UVabsorption

Visibleemission

Examples: recall Friday’s lecture

Energyin eV

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X-Ray fluorescence

0

-5.0

-6.5

-2.5

X-rayabsorption

X-rayemission

Energyin keV

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*****Fluorescence***** • Based on the level scheme in the UV-fluorescence slide, what

other energies might I expect for fluorescence excited by the 4 eV photon? (Note that it’s possible to get two, or sometimes more, photons out for one photon in!)

• Based on the level scheme in the X-ray fluorescence slide, what other energies might I expect for fluorescence excited by a high energy (enough to knock any electron out of the atom) x-ray photon? (Note that the photon may knock any one of the electrons out.)

• 1.5, 2.5, or 4 eV; 1.5, 2.5, or 4 keV

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X-rays—spectroscopy

• Fluorescence—XRF– Another element specific spectroscopy– Excitation of inner shell electrons by

• X-rays in—X-rays (longer wavelength) out

• Electron beam in—X-rays out

• Proton beam in—X-rays out

Excitation beam: X-rays, electrons, ions, protons

X-ray energyanalyzer

sample

X-rays

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XRF in action

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X-Ray radiography

Lighter = less exposure

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How does it work?

X-rayabsorption

• X-ray photo is a shadowgraph• X-ray absorption ~ # of electrons = Z• H, C, N, O: Z = 1, 6, 7, 8• Ca, Fe: Z = 20, 26• lead (82), titanium (22): could you tell PbO from TiO2?

X-raysource

guts

spine

film

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IR—UV—X

• IR—smaller index n, longer wavelength, selective absorption– Penetration reveals deeper material– Modified images

• UV—selective reflection, fluorescence– Modified images– Enhanced visible intensities– Pigment identification

• X—fluorescence, Z dependent absorption– XRF spectroscopy– X-radiography