SCI 200 Physical Science
Lecture 11
Sources of Color
Rob Daniell
August 4, 2011
Sources of Color
Part I: Self-luminous objects
Objects that emit their own light
Part II: Non-luminous objects
Object that a visible due to reflected light
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Sources of Color
Part I: Self-luminous objects
Thermal radiators
Non-thermal sources
Sources:
Malacara, Daniel: Color Vision and Colorimetry:
Theory and Applications,SPIE Press,
Bellingham, 2001
 Especially Chapter 2.
Various Wikipedia articles, particularly “Liquid
Crystal Display” and “Backlight”
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Sources of Color: Self-luminous objects
Thermal radiation
Any object with a finite temperature
radiates electromagnetic (EM) waves
“free” electrons vibrate (oscillate) due to
thermal energy
 Oscillating electrons emit EM waves
 Light consists of EM waves between 400-700 nm
An ideal thermal radiator is termed a “black
body”
 If it radiates EM waves (light), why is it “black”?
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Sources of Color: Self-luminous objects
Black body
A good emitter and a good absorber
Because of the “free” electrons
A black body not only emits EM waves,
It absorbs EM waves.
 No reflection
At low temperatures
 little radiation
 good absorption
 So a black body looks black
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Sources of Color: Self-luminous objects
Black body radiation
Thermal equilibrium
Characterized by a definite temperature
 Usually expressed in kelvins (K)
 TK = 273 + TC
 TK = 273 + (5/9)(TF –32)
Characterized by a specific spectrum
Intensity of radiation as a function of
wavelength
First use of “quantum hypothesis”
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Sources of Color: Self-luminous objects
Visible wavelengths:
400-700 nm
 Black body spectrum
 Wavelength of peak emission intensity
lmax = 2.9 T´10
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nm K
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Sources of Color: Self-luminous objects
 Black body spectrum in the visible range
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Sources of Color: Self-luminous objects
Real (as opposed to ideal) radiators
Sun
Surface temperature about 5500 K
Incandescent light bulbs
Filament (usually tungsten) heated by electric
current
Bulb filled with inert gas (e.g., argon) to prevent
oxidation
About 90% of the power is emitted as heat
Approximate black body spectrum
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Sources of Color:
Incandescent light bulbs
 Incandescent light
bulbs
 Filament (usually
tungsten) heated by
electric current
 Bulb filled with inert
gas (e.g., argon) to
prevent oxidation
 Clear or frosted
glass
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Sources of Color: Self-luminous objects
 Solar spectrum & equivalent black body spectrum
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Sources of Color: Self-luminous objects
Color temperature
Temperature of a black body that would
have the same color as the emitter
For daylight
Color temperature varies through the day
Color temperature varies with cloud cover etc.
For incandescent bulbs
Color temperature is very close to actual
temperature
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Sources of Color: Self-luminous objects
 Color temperature & chromaticity
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Incandescent light sources
Incandescent light sources glow from their own heat and
emit a “black body spectrum.”
Incandescent light sources
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Sources of Color: Self-luminous objects
Color temperature
Note that lower color temperatures
correspond to redder colors
“Cooler” and “warmer” colors refer to
psychological perception
To produce “warmer” (redder) colors, lower the
color temperature
To produce “cooler” (bluer) colors, raise the
color temperature
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Sources of Color: Self-luminous objects
Non-thermal sources
Wide variety
Color temperature usually has little to do
with the operating temperature
Color temperature is often a poor
approximation to the actual color of the
emitted light
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Sources of Color: Self-luminous objects
 Gas discharge lamps
 Glass tube filled with
 A low pressure gas such as hydrogen, argon, neon
 A vaporized metal such as sodium or mercury
 When an electric current is passed through the
gas or vapor, the individual atoms are caused to
emit light.
 Atoms absorb discrete amounts of energy from the
electrons
 Atoms reemit the energy as photons, i.e., discrete
amounts of energy
 Discrete “lines” (wavelengths) rather than thermal
continuum
 Spectrum details depend on the pressure of the gas or
vapor
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Sources of Color: Self-luminous objects
 Fluorescent lamps
 A mercury vapor lamp with the interior of the glass
tube coated with a fluorescent powder.
 A fluorescent material
 Absorbs short wavelength radiation
 Emits longer wavelength radiation
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Sources of Color: Self-luminous objects
 Fluorescent lamps
 A mercury vapor lamp with the interior of the glass tube
coated with a fluorescent powder.
 A fluorescent material
 Absorbs short wavelength radiation
 Emits longer wavelength radiation
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Applications of self-luminous
sources
• Television, computer monitors,
electronic displays
• Cathode Ray Tube (CRT)
• Largely obsolete
• Liquid Crystal Display (LCD)
• Many variations on the basic design
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Applications of self-luminous
sources
• Cathode Ray Tubes
• Date back to the 19th century
• Vacuum tube
• Electron “gun”
• Electrons were first known as “cathode rays”
• Screen coated with phosphors
• Chemicals that glow when struck by electrons
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Applications of self-luminous
sources
• Cathode Ray Tubes
• Vacuum tube
• Electron “gun”
• Designed to produce
a narrow beam
• Electromagnet
deflection coils to aim
the beam
• Screen coated with
phosphors
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Applications of self-luminous
sources
• Cathode Ray Tubes
• Red, Green, & Blue
signals
• Each signal controls an
electron beam
• Screen devided into Red,
Green, & Blue pixels
• Phosphors continue to
glow even after the
electron beam has moved
on
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Applications of self-luminous
sources
• Liquid Crystal Displays
• Based on the optical activity of Liquid
Crystals
• Liquids of long molecules, usually organic
• Rotate the plane of polarization of light
• Amount of rotation can be controlled through
the applied voltage
• Do not emit light themselves
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Applications of self-luminous
sources
• Liquid Crystal Displays
• 1-linear polarizer (vertical)
• 2-glass substrate with
electrodes
• Shape of electrodes
determines what can be
displayed
• 3-Liquid crystal
• 4-glass substrate with common
electrode
• 5-linear polarizer (horizontal)
• 6-reflecting screen or backlight
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Applications of self-luminous
sources
• Liquid Crystal Displays
• Backlight
• White lights plus colored filters
• Fluorescent light
• “white” LEDs (Light Emitting Diodes)
• Colored LEDs (RBG)
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Sources of Color: Self-luminous objects
 A note on units related to brightness
 candela (cd) = the luminous intensity, in a given
direction, of a source that emits monochromatic
radiation of frequency 540x1012 hertz (555.6 nm)
and that has a radiant intensity in that direction of
1/683 watt per steradian
 A common candle emits roughly 1 cd
 A 100 W incandescent light bulb emits about 120 cd
 Luminous flux
 1 lumen = 1 cd sr = flux from 1 cd into 1 sr
 Illuminance (light received per unit surface area)
 SI unit: 1 lux (lx) = 1 lumen m-2
 English unit: 1 foot-candle (fc) = 1 lumen ft-2 = 10.76 lx
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Sources of Color: Self-luminous objects
 Illuminance (light
received per unit
surface area)
 SI unit:
 1 lux (lx) = 1 lumen m-2
 English unit:
 1 foot-candle (fc)
 = 1 lumen ft-2 = 10.76 lx
 Can be measured by
commercially available
lux meters:
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Sources of Color: Self-luminous objects
 Another definition of
“foot-candle”
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Sources of Color
Part II: Non-luminous objects
Various causes of color:
Pigments
Structural elements
Atomic, molecular, and crystalline properties
Sources:
Chapter 9 of textbook (Gilbert & Haeberli,
Physics in the Arts)
Misc sources (mostly Wikipedia)
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Sources of Color: Non-luminous sources
Pigments
Chemical compounds that absorb light of
different wavelengths
Reflected (or scattered) light is colored
Appearance depends on
 Source of illumination
 Thickness of pigment coating
 Mixtures of various pigments
Natural: “Animal, vegetable, and mineral”
Synthetic
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Sources of Color: Non-luminous sources
Pigments
Animal pigments include
Melanin
 Eumelanin: skin color and dark hair colors
 Pheomelanin: red hair
Carotenoids - not strictly animal
 Absorbed by animals from plants, algae, and
photosynthetic bacteria
 Responsible for pinks of flamingos and salmon
Blue eyes are due to lack of pigment
 Rayleigh scattering (as in the atmosphere) causes
more blue light to be scattered
 Longer wavelengths are absorbed at the back of the
iris
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Sources of Color: Non-luminous sources
Pigments
“Vegetable” (i.e., plant) pigments include
Chlorophyll (green)
 Participates in photosynthesis
Carotenoids (yellow, orange, pink, or red)
Tannin (black or brown)
Anthocyanins (red or bluish purple)
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Sources of Color: Non-luminous sources
Pigments
Mineral pigments are extremely varied
Often oxides of metals
Each chemical compound has a unique
absorption and emission spectrum
 Discrete wavelengths
 Bands of wavelengths
Used in paints, dyes, makeup, etc.
Usually sold as powders
Final colors depend on the liquid or paste base
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Sources of Color: Non-luminous sources
 Structural colors: Iridescence
 If a material layer’s thickness is similar to the
wavelength of visible light
 Interference effects produce color
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Sources of Color: Non-luminous sources
 Structural colors: Iridescence
 If a material layer’s thickness is similar to the
wavelength of visible light
 Interference effects produce color
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Sources of Color: Non-luminous sources
Structural colors: Iridescence
If a material layer’s thickness is similar to
the wavelength of visible light
Interference effects produce color
Relative phase depends on angle of incidence
Colors vary with relative locations of source,
layer, and observer
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Sources of Color: Non-luminous sources
 Structural colors:
Iridescence
 Examples:
 Soap bubbles
 Oil films
 Bird feathers
 Butterfly wings
 Beetle shells
 Pearls, mother of
pearl
 Opals
 Fossils shells
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Sources of Color: Non-luminous sources
Gemstones
Chemical composition
Sometimes impurities can affect colors
Ruby is corundum [Al2O3] with chromium
impurities
Emerald is beryl [Be3Al2(SiO3)6] with chromium
impurities
Beryl with other impurities takes on other
colors:
 Aquamarine is beryl with Fe2+
 Heliodor is beryl with Fe3+
 Marganite is beryl with Mn2+
 Etc., etc.
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Sources of Color: Non-luminous sources
Gemstones
Active participation
Blue sapphire: corundum [Al2O3] with iron (Fe)
and titanium (Ti) impurities.
 When light hits the mineral it causes electrons to
jump between the Fe and the Ti. Red and green
light is absorbed in the process, leaving blue to be
reflected back to the observer
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Sources of Color: Non-luminous sources
Gemstones
Color centers
Defects in the crystal structure trap electrons
which can absorb light
 Often only certain wavelengths
 Other wavelengths are scattered, giving color to the
gem
Defects can be natural
 Occurring when the crystal forms
Defects can be artificial
 Produced by exposure of the crystal to various forms
of radiation
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Sources of Color: Non-luminous sources
Gemstones
Semiconductor band gaps
In order to conduct electricity, electrons must
gain a minimum amount of energy
 Recall that light of shorter wavelengths has higher
energy
There is a maximum wavelength that can be
absorbed
 Threshold is different for different semiconductors
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Sources of Color: Non-luminous sources
 Gemstones
 Semiconductor band
gaps
 Threshold longer than
700 nm: opaque
 Threshold shorter
than 400 nm:
transparent
 Threshold between
400 nm and 700 nm:
colored
 Red, orange, or
yellow
 Not blue
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Sources of Color: Non-luminous sources
Gemstones
Impurities can cause multiple band gaps
Permits blue color
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Summary
• Self-luminous sources
• Thermal
• Non-thermal
• Non-luminous sources
• Pigments
• Structural color (iridescence)
• Mineral color
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Summary
• Self-luminous sources
• Thermal
• Approximately a black body spectrum
• Wavelength of peak emission varies with
temperature
• Non-thermal
• Atomic or molecular process
• Excitation by electrons or other non-luminous
processes
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Summary
• Non-luminous sources
• Pigments
• Chemicals whose composition determines
which wavelengths are absorbed and which are
reflected or transmitted
• Structural color (iridescence)
• Relies on destructive and constructive
interference effects
• Depends on relative location of source, object,
and observer
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Summary
• Non-luminous sources
• Mineral color
• Wide variety of mechanisms
• Color is frequently caused by
• Impurities
• Structural defects
• Peculiarities of the electronic structure of the material
(semi-conductors)
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Homework Assignment
• No Homework Packet
• Lab 8: Color Subtraction
– Thursday, August 4
• Make-up Lab
– Thursday, August 11
• Test #3, Thursday, August 11
– Chapters 7, 8, & 9
– Lectures 9, 10, & 11
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