Physics of Color

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Physics of Color

• Alej Garcia
• Dept. Physics SJSU
• www.algarcia.org

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Spectrum of Visible Light
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Wavelengths Photons
Particles of light, called photons, each have a
wavelength.
Yellow Photon
Green Photon
Blue Photon
Prism Spectrum
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Additive Color Wheel
R
Y
M
Red Yellow Green Cyan Blue Magenta
G
B
C
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Adding Color Lights
Stream of red green photons looks same as
yellow photons (metamerism)
YELLOW
Eye to Brain
or
Notice overlap of red, green, blue is seen as
white light
Theatrical lighting
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Newtons Color Wheel
Prism spectrum is a straight line, so why did
Isaac Newton describe color using a circular
wheel?
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Trichromatic Theorists
James Clerk Maxwell (1831-1879) Scottish
physicist
Thomas Young (1773-1829) English physicist

• Hermann von Helmholtz
• (1821-1894)
• German physicist

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Simple Trichromatic Theory
Yellow Red photons excite me
Yellow, Green Cyan photons excite me
Cyan Blue photons excite me
MOE
LARRY
CURLY
Inside your eye there are three receptors
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Trichromatic Seeing Yellow
Yellow Red photons excite me. IM EXCITED
Yellow, Green Cyan photons excite me. IM
EXCITED
Cyan Blue photons excite me. Yawn.
CURLY
MOE
OR
LARRY
Yellow seen when Curly and Larry excited, either
by yellow photons or red green photons.
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Seeing Yellow
Sodium lamps emit pure yellow photons
Color monitor can only emit red, green, and blue
(RGB) creates other colors by selectively
turning RGB pixels on or off.
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Trichromatic Seeing Magenta
Yellow Red photons excite me. IM EXCITED
Cyan Blue photons excite me. IM EXCITED
Yellow, Green Cyan photons excite me. Yawn.
CURLY
MOE
LARRY
Magenta is seen by eye when Moe and Larry
excited, which no single type of photon can
achieve.
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Maxwell Color Disk

• Disk painted half red, half blue looks magenta
  when rapidly spinning.

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Mixing Blue Red Paint

• Mixing paint or ink is different from adding
  colors together by light.

Mix of blue and red paint produces a blackish
brown
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Trichromatic Seeing White
Yellow Red photons excite me. IM EXCITED
Cyan Blue photons excite me. IM EXCITED
Yellow, Green Cyan photons excite me. IM
EXCITED
CURLY
MOE
LARRY
White seen when all three very excited Gray seen
when all three less excited
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Trichromatic Color Blindness
Red , Yellow, Green Cyan photons excite me. IM
EXCITED
Cyan Blue photons excite me. Yawn.
Color blindness occurs if the eye is missing one
of the three receptors. Other receptors try to
compensate but cannot distinguish some colors.
CURLY
MOE
OR
LARRY
Do I see red or green?
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Color Blindness
Weakness or absence of one of the three types of
cones is the cause of color blindness, leading to
a reduced ability to distinguish colors.
Classification Incidence () Incidence ()
Classification Males Females
AnomalousTrichromacy 6.3 0.37
Protanomaly(Red-cone weak) 1.3 0.02
Deuteranomaly(Green-cone weak) 5.0 0.35
Tritanomaly(Blue-cone weak) 0.0001 0.0001
Dichromacy 2.4 0.03
Protanopia(Red-cone absent) 1.3 0.02
Deuteranopia(Green-cone absent) 1.2 0.01
Tritanopia(Blue-cone absent) 0.001 0.03
Rod Monochromacy(no cones) 0.00001 0.00001
29 or 70?
21 or 74?
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Trichromatic After-Image
Moe and Curly are excited so what color is
seen? CYAN (light blue-green)
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Negative After-image
Stare, unfocused, at the red cross for 10 seconds
then look at white wall
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Negative After-image
Cyan
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Negative After-image
Stare, unfocused, at the flag for 10 seconds then
look at white wall
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Negative After-image
Yellow
Cyan
Magenta
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Additive Complements
LARRY
After-image of red is cyan because Larry gets
tired so when white light excites all three
Stooges, Moe Curly stronger than Larry.
R
CURLY
MOE
C
Cyan White - Red
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Trichromatic Opponency
Yellow Red photons excite me. IM EXCITED
Yellow, Green Cyan photons excite me. IM
EXCITED
Oh, Shut The F_at_ Up!
CURLY
MOE
LARRY
Shine Red Green photons (or Yellow photons)
Yellow seen when Curly and Larry excited, which
can annoy Moe, who then opposes them.
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Simultaneous Contrast
Does the gray bar look slightly bluish?
Are the two gray bars the same shade of gray?
Yes, the presence of a nearby color affects
perception of both hue and value, shifting both
towards complement
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Color Vision in the Eye
Three types of cones (color) One type of rod (B/W
only)
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CIE Hue-Saturation Diagram
Eye is not a perfect optical instrument. Color
wheel is actually distorted cone shape. Rim is
full saturation, center is white
CURLY
50 Larry 50 Curly 0 Moe
33 Larry 33 Curly 33 Moe
LARRY
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Hue, Saturation, Value
Color wheel is not a single wheel but stack of
wheels that range in value.
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Why Yellow Orange are Special
Red
Green
CIE color wheel
Blue
Cone Sensitivity
Wavelength
Peak sensitivities of green and red cone are
close together, so we easily separate colors in
this range, probably to spot ripe fruit and
La Victoria Hot Sauce
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Spectral Reflectance Curves
White
Red
When white light shines on a colored object, some
photons absorbed, others reflected by the
objects surface.
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Pigment Reflectance Curves
Quinacridone rose (PV19)
Quinacridone magenta (PR122)
Manganese violet (PV16)
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Name That Pigment
BLUE GREEN RED
BLUE GREEN RED
Titanium White
Burnt Sienna
Cadmium Red
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Name That Pigment
BLUE GREEN RED
BLUE GREEN RED
Phthalocyanine Green
Cobalt Blue
Cadmium Yellow
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Artists Handbook
These spectral reflectance curves and those of
many other standard pigments are found in Mayers
book.
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Pigment Value Light Source
Shine colored lights on pigments to see how
values change
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Pigment Value Light Source
Paint your color grid under bright, natural light
(no lava lamps)
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Reflectance of Pigment Mixtures
BLUE GREEN RED
100 80 60 40 20
Take a mixture of equal parts cadmium red and
cobalt blue. The mixture reflectance profile is
defined as the geometric mean (square root of the
product) of their separate reflectances for every
wavelength in the spectrum.
Cadmium Red
Cobalt Blue
For example, if cobalt blue reflects 20 of a
specific blue wavelength (say 500nm), and cadmium
red reflects only 5, then their mixture will
reflect roughly 10 of the 500nm light. (The
product 20 x 5 100, the square root of 100
is 10.)

• 500 600 700
• Nanometers

NOTE This only gives approximate results full
theory more complex!
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Mixing to a Color Near vs. Far
Difficult to hit a target color by mixing two
distant colors (pigments A and B).
A
Target
Easier to hit a target color by mixing two nearby
colors (pigments C and D).
B
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Reflectance of Pigment Mixtures
Take a mixture of equal parts ultramarine blue
and cadmium red deep. The mixture reflectance
profile is defined as the geometric mean (square
root of the product) of their separate
reflectances for every wavelength in the spectrum.
For example, if ultramarine blue reflects 80 of
a specific blue wavelength (say 480nm), and
cadmium red deep reflects only 8, then their
mixture will reflect roughly 25 of the 480nm
light. (The product 8 x 80 640, the square
root of 640 is 25.)
NOTE This only gives approximate results full
theory more complex!
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Green vs. Yellow as Primary
Dots indicate pure pigment (Dana poster) Curved
lines are mixtures of pigments. Vertical bars
indicate value.
GREEN
YELLOW
WHITE
Note that mixing green red passes near the
white/black spot.
RED
CYAN
MAGENTA
BLUE
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Mixing Pigments with White
Mixing paint pigments with Titanium White can
cause shifts in hue
GREEN
YELLOW
WHITE
CYAN
RED
Some pigments even become more saturated when
mixed with a bit of white
MAGENTA
BLUE
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Why Paint a Color Grid?
The results of mixing colored paints are
sufficiently complicated so that no fully
reliable theory has yet been developed. For the
artist, there is no choice but to be fully
familiar with the mixing properties of the paints
on the palette. Light and Color in Nature and
Art S. Williamson and H. Cummins