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Physics of Color. Alej Garcia Dept. Physics SJSU www.algarcia.org. Spectrum of Visible Light. Red Photon. Wavelengths & Photons. Particles of light, called photons, each have a wavelength. Yellow Photon. Green Photon. Blue Photon. Prism Spectrum. There are No Photons of These Colors.

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physics of color

Physics of Color

Alej Garcia

Dept. Physics SJSU

www.algarcia.org

wavelengths photons

Red Photon

Wavelengths & Photons

Particles of light, called photons, each have a wavelength.

Yellow Photon

Green Photon

Blue Photon

Prism

Spectrum

additive color wheel

There are No Photons of These Colors

Spectral

Colors

Additive Color Wheel

R

M

Y

Red

Yellow

Green

Cyan

Blue

Magenta

G

B

C

adding color lights
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

newton s color wheel
Newton’s Color Wheel

Prism spectrum is a straight line, so why did Isaac Newton describe color using a circular wheel?

trichromatic theorists
Trichromatic Theorists

James Clerk Maxwell

(1831-1879)

Scottish physicist

Thomas Young

(1773-1829)

English physicist

  • Hermann von Helmholtz
  • (1821-1894)
  • German physicist
simple trichromatic theory
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

trichromatic seeing yellow
Trichromatic: Seeing Yellow

Yellow &

Red

photons excite me. I’M EXCITED

Yellow, Green &

Cyan photons excite me.

I’M 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.

seeing yellow
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
Trichromatic: Seeing Magenta

Yellow &

Red

photons excite me. I’M EXCITED

Cyan & Blue

photons excite me.

I’M 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.

maxwell color disk
Maxwell Color Disk

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

mixing blue red paint
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
Trichromatic: Seeing White

Yellow &

Red

photons excite me. I’M EXCITED

Cyan & Blue

photons excite me.

I’M EXCITED

Yellow, Green &

Cyan photons excite me.

I’M EXCITED

CURLY

MOE

LARRY

White seen when all three very excited

Gray seen when all three less excited

trichromatic color blindness
Trichromatic: Color Blindness

Red , Yellow, Green &

Cyan photons excite me.

I’M 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?

color blindness
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?

trichromatic after image

I’M EXCITED!

Yawn.

Yawn.

Then stare at WHITE

MOE

LARRY

CURLY

Me too, but tired.

I’M EXCITED!

ME TOO!

First stare at RED

MOE

LARRY

CURLY

Trichromatic: After-Image

Moe and Curly are excited so what color is seen?

CYAN (light blue-green)

negative after image
Negative After-image

Stare, unfocused, at the red cross for 10 seconds then look at white wall

negative after image2
Negative After-image

Stare, unfocused, at the flag for 10 seconds then look at white wall

negative after image3
Negative After-image

Yellow

Cyan

Magenta

additive complements
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

trichromatic opponency
Trichromatic: Opponency

Yellow &

Red

photons excite me. I’M EXCITED

Yellow, Green &

Cyan photons excite me.

I’M EXCITED

Oh, Shut The F*@% 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.

simultaneous contrast
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
Color Vision in the Eye

Three types of cones (color)

One type of rod (B/W only)

cie hue saturation diagram
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 %

hue saturation value
Hue, Saturation, Value

Color wheel is not

a single wheel but

stack of wheels

that range in

value.

why yellow orange are special
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

spectral reflectance curves
Spectral Reflectance Curves

White

Red

When white light shines on a colored object, some photons

absorbed, others reflected by the object’s surface.

name that pigment

BLUE GREENRED

100%

0%

Name That Pigment

BLUE GREENRED

BLUE GREENRED

Titanium White

Burnt Sienna

Cadmium Red

name that pigment1

BLUE GREENRED

100%

0%

Name That Pigment

BLUE GREENRED

BLUE GREENRED

Phthalocyanine Green

Cobalt Blue

Cadmium Yellow

artist s handbook
Artist’s Handbook

These spectral reflectance curves and those of many other standard pigments are found in Mayer’s book.

pigment value light source
Pigment Value & Light Source

Shine colored lights on pigments to see how values change

pigment value light source1
Pigment Value & Light Source

Paint your color grid under bright, natural light (no lava lamps)

reflectance of pigment mixtures

Mixture

Reflectance of Pigment Mixtures

BLUE GREENRED

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!

mixing to a color near vs far

Ideal mix

C

D

Actual

Mix

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

green vs yellow as primary
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

mixing pigments with white
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

why paint a color grid
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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