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Physics of Color

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Physics
of Color
Alej Garcia
Dept. Physics
SJSU
www.algarcia.org
Spectrum of Visible Light
Wavelengths & Photons
Particles of light, called photons, each have a wavelength.
Red Photon
Yellow Photon
Green Photon
Blue Photon
Prism
Spectrum
Additive Color Wheel
There are
No
Photons
of These
Colors
R
M
Y
B
G
C
Spectral
Colors
Red
Yellow
Green
Cyan
Blue
Magenta
Adding Color Lights
Stream of
red & green photons
looks same as
yellow photons
(metamerism)
or
Theatrical lighting
Eye to
Brain
Notice overlap of red, green, &
blue is seen as white light
Newton’s Color Wheel
Prism spectrum is a
straight line, so why
did Isaac Newton
describe color using
a circular wheel?
Trichromatic Theorists
Thomas Young
(1773-1829)
English physicist
Hermann von Helmholtz
(1821-1894)
German physicist
James Clerk Maxwell
(1831-1879)
Scottish physicist
Simple Trichromatic Theory
Cyan &
Blue
photons
excite me
MOE
Yellow &
Red
photons
excite me
Yellow,
Green &
Cyan
photons
excite me
LARRY
CURLY
Inside your eye there are three receptors
Trichromatic: Seeing Yellow
Cyan & Blue
photons excite
me.
Yawn.
Yellow, Green &
Cyan photons
excite me.
I’M EXCITED
CURLY
OR
Yellow &
Red
photons
excite me.
I’M
EXCITED
MOE
LARRY
Yellow seen when Curly and Larry excited,
either by yellow photons or red & green
photons.
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.
Trichromatic: Seeing Magenta
Cyan & Blue
photons excite me.
I’M EXCITED
Yellow, Green &
Cyan photons
excite me.
Yawn.
CURLY
Yellow &
Red
photons
excite me.
I’M
EXCITED
MOE
LARRY
Magenta is seen by eye when Moe and Larry
excited, which no single type of photon can
achieve.
Maxwell Color Disk
Disk painted half red, half blue looks magenta
when rapidly spinning.
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
Trichromatic: Seeing White
Cyan & Blue
photons excite me.
I’M EXCITED
Yellow, Green &
Cyan photons
excite me.
I’M EXCITED
CURLY
Yellow &
Red
photons
excite me.
I’M
EXCITED
MOE
LARRY
White seen when all three very excited
Gray seen when all three less excited
Trichromatic: Color Blindness
Cyan & Blue
photons excite me.
Yawn.
OR
Red , Yellow,
Green &
Cyan photons
excite me.
I’M EXCITED
CURLY
MOE
Color blindness
occurs if the eye is
missing one of the
three receptors.
Other receptors try
to compensate but
cannot distinguish
some colors.
LARRY
Do I see red or green?
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.
29 or 70?
21 or 74?
Incidence (%)
Classification
Males
Females
Anomalous
Trichromacy
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
Trichromatic: After-Image
Yawn.
I’M
EXCITED!
Yawn.
MOE
CURLY
LARRY
First stare at RED
Moe and Curly are excited
so what color is seen?
CYAN (light blue-green)
Then stare at WHITE
I’M
EXCITED!
ME
TOO!
MOE
CURLY
Me too,
but tired.
LARRY
Negative After-image
Stare, unfocused, at the red cross for 10 seconds then look at white wall
Negative After-image
Cyan
Negative After-image
Stare, unfocused, at the flag for 10 seconds then look at white wall
Negative After-image
Cyan
Magenta
Yellow
Additive Complements
LARRY
R
CURLY
MOE
C
After-image of
red is cyan
because Larry
gets tired so
when white light
excites all three
Stooges, Moe &
Curly stronger
than Larry.
Cyan = White - Red
Trichromatic: Opponency
Oh, Shut The
F*@% Up!
Yellow, Green &
Cyan photons
excite me.
I’M EXCITED
CURLY
Yellow &
Red
photons
excite me.
I’M
EXCITED
MOE
Shine Red
& Green
photons (or
Yellow
photons)
LARRY
Yellow seen when Curly and Larry excited,
which can annoy Moe, who then opposes them.
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
Color Vision in the Eye
Three types of cones (color)
One type of rod (B/W only)
Eye is not a perfect
optical instrument.
CURLY %
CIE Hue-Saturation Diagram
50% Larry
50% Curly
0% Moe
Color “wheel” is
actually distorted
cone shape.
Rim is full saturation,
center is white
33% Larry
33% Curly
33% Moe
LARRY %
Hue, Saturation, Value
Color wheel is not
a single wheel but
stack of wheels
that range in
value.
CIE color “wheel”
Cone Sensitivity
Why Yellow & Orange are Special
Red
Green
Blue
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
Spectral Reflectance Curves
When white light shines on a colored object, some photons
absorbed, others reflected by the object’s surface.
Name That Pigment
100%
0%
BLUE
GREEN
Titanium
White
RED
BLUE
GREEN
Cadmium
Red
RED
BLUE
GREEN
Burnt
Sienna
RED
Name That Pigment
100%
0%
BLUE
GREEN
RED
Phthalocyanine
Green
BLUE
GREEN
Cadmium
Yellow
RED
BLUE
GREEN
Cobalt
Blue
RED
Artist’s Handbook
These spectral reflectance
curves and those of many
other standard pigments are
found in Mayer’s book.
Pigment Value & Light Source
Shine colored lights on pigments to see how values change
Pigment Value & Light Source
Paint your color grid under bright, natural light (no lava lamps)
Reflectance of Pigment Mixtures
100%
80%
BLUE
GREEN
RED
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
60%
40%
Cobalt Blue
20%
Mixture
400
500
600
Nanometers
700
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%.)
NOTE: This only gives approximate results; full theory more complex!
Mixing to a Color: Near vs. Far
Difficult to hit a
target color by
mixing two distant
colors (pigments A
and B).
A
C
Target
D
Actual
Mix
B
Easier to hit a
target color by
mixing two nearby
colors (pigments C
and D).
Green vs. Yellow as Primary
GREEN
YELLOW
Dots indicate pure
pigment (Dana poster)
Curved lines are
mixtures of pigments.
Vertical bars indicate
value.
WHITE
RED
CYAN
MAGENTA
BLUE
Note that mixing
green & red
passes near the
white/black spot.
Mixing Pigments with White
Mixing paint
pigments with
Titanium White
can cause
shifts in hue
GREEN
YELLOW
WHITE
CYAN
RED
MAGENTA
BLUE
Some pigments
even become
more saturated
when mixed with
a bit of white
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
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