Physics of Color Alej Garcia Dept. Physics SJSU

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