physics of color alej garcia dept. physics sjsu
TRANSCRIPT
Physics Physics of Colorof Color
Alej Garcia
Dept. Physics SJSU
www.algarcia.org
Spectrum of Visible Light
Wavelengths & Photons
Red Photon
Blue Photon
Green Photon
Yellow Photon
PrismSpectrum
Particles of light, called photons, each have a wavelength.
Additive Color Wheel
SpectralColors
There are No
Photons of These Colors
R
Y
B G
M
C
RedYellowGreenCyanBlueMagenta
Adding Color Lights
Stream ofred & green photons looks same as yellow photons(metamerism)
Theatrical lighting
or
YELLOW
Eye toBrain
Notice overlap of red, green, & blue is seen as whitewhite 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 TheoryYellow &
Redphotons
excite me
Yellow, Green &
Cyanphotons
excite me
Cyan &Blue
photons excite me
Inside your eye there are three receptors
MOECURLY
LARRY
Trichromatic: Seeing YellowYellow &
Redphotons
excite me. I’M
EXCITED
Yellow, Green &Cyan photons
excite me.I’M EXCITED
Cyan & Bluephotons excite
me.Yawn.
Yellow seen when Curly and Larry excited, either by yellow photons or red & green photons.
MOE
CURLYCURLY
LARRYOR
Seeing YellowSodium 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 MagentaYellow &
Redphotons
excite me. I’M
EXCITED
Yellow, Green &Cyan photons
excite me.Yawn.
Cyan & Bluephotons excite me.
I’M EXCITED
Magenta is seen by eye when Moe and Larry excited, which no single type of photon can achieve.
MOE
CURLYCURLY
LARRY
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 WhiteWhiteYellow &
Redphotons
excite me. I’M
EXCITED
Yellow, Green &Cyan photons
excite me.I’M EXCITED
Cyan & Bluephotons excite me.
I’M EXCITED
White seen when all three very excitedGray seen when all three less excited
MOE
CURLYCURLY
LARRY
Trichromatic: Color BlindnessRed , Yellow,
Green &Cyan photons
excite me.I’M EXCITED
Cyan & Bluephotons excite me.
Yawn.
MOE
CURLYCURLY
LARRY
Color blindness occurs if the eye is missing one of the three receptors. Other receptors try to compensate but cannot distinguish some colors.
Do I see red or green?
OR
Color Blindness
Classification
Incidence (%)
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?
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.
Trichromatic: After-Image
First stare at RED
I’M EXCITED!Yawn.
Yawn.
MOE
CURLYCURLYLARRY
Me too, but tired.
ME TOO!
I’M EXCITED!
MOE
CURLYCURLYLARRY
Then stare at WHITE
Moe and Curly are excited so what color is seen?CYAN (light blue-green)
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 ComplementsLARRY
MOE CURLY
After-image of red is cyan because Larry gets tired so when white light excites all three Stooges, Moe & Curly stronger than Larry.
R
C
CyanCyan = White - Red
Trichromatic: OpponencyYellow &
Redphotons
excite me. I’M
EXCITED
Yellow, Green &Cyan photons
excite me.I’M EXCITED
Oh, Shut The F*@% Up!
Yellow seen when Curly and Larry excited, which can annoy Moe, who then opposes them.
MOE
CURLYCURLY
LARRYShine Red & Green photons (or Yellow photons)
Simultaneous Contrast
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
Does the gray bar look slightly bluish?
Color Vision in the Eye
Three types of cones (color)One type of rod (B/W only)
CIE Hue-Saturation Diagram
Eye is not a perfectoptical instrument.
Color “wheel” isactually distortedcone shape.
Rim is full saturation,center is white
LARRY %
CU
RL
Y %
50% Larry50% Curly0% Moe
33% Larry33% Curly33% Moe
Hue, Saturation, Value
Color wheel is nota single wheel butstack of wheels that range invalue.
Why Yellow & Orange are Special
Blue
RedGreen
Con
e S
ensi
tivi
ty
Peak sensitivities of green and red cone are close together, so we easily separate colors in this range, probably to spot ripe fruit and …
CIE color “wheel”
Wavelength
La Victoria Hot SauceLa Victoria Hot Sauce
Spectral Reflectance Curves
When white light shines on a colored object, some photonsabsorbed, others reflected by the object’s surface.
White Red
Name That Pigment
Titanium White
Cadmium Red
Burnt Sienna
BLUE GREEN RED
0%
100%BLUE GREEN RED BLUE GREEN RED
Name That Pigment
Phthalocyanine Green
Cadmium Yellow
Cobalt Blue
BLUE GREEN RED
0%
100%BLUE GREEN RED BLUE GREEN 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 MixturesBLUE 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.
NOTE: This only gives approximate results; full theory more complex!
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%.)
Cadmium Red
Cobalt Blue
400 500 600 700Nanometers
Mixture
100%
80%
60%
40%
20%
Mixing to a Color: Near vs. Far
Target
A
B
C
D
Difficult to hit a target color by mixing two distant colors (pigments A and B).
Easier to hit a target color by mixing two nearby colors (pigments C and D).
Ideal mix
ActualMix
Green vs. Yellow as Primary
Dots indicate pure pigment (Dana poster)Curved lines are mixtures of pigments.Vertical bars indicate value.
BLUE
GREEN
RED
YELLOWYELLOW
CYAN
MAGENTA
Note that mixing green & red passes near the white/black spot.
WHITE
Mixing Pigments with White
Mixing paint pigments with Titanium White can cause shifts in hue
BLUE
GREEN
RED
YELLOWYELLOW
CYAN
MAGENTA Some pigments even become more saturated when mixed with a bit of white
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