Slide 1

John Redman, TI physicist, imaging and color expert:

Color only exists in our minds, i.e., it is strictly a perceptual attribute.

Newton stated that color depends on “imagination and fantasy and invention.”

Response per intensity unit

Vector nature of color:

“Spectral” (rainbow) colors are those perceived from light of narrow band of w, . l

Color is determined by the relative stimuli to three cone cells. [Sr, Sg, Sb].

Orange “color”: [Sr, Sg, Sb] =

Your eye perceives this color for narrow-band 600 nm light.

Vector nature of color:

Infinite number of other spectra give the “orange” stimulus ratios. Which approx combination of 550 nm and 650 nm light intensities will give the same as 600 nm pure light? 550 to 650 ______

a) 1:1 b) 1:2 c) 2: 1 d) 1:6 e) 6: 1

Vector nature of color:

Explain why brown e.g. [4, 2, 1] can’t be in the rainbow

Can you come up with a stimulus ratio [Sr, Sg, Sb] that isn’t a color (no

combination of light can make this ratio in our eyes)? e.g [0, 1, 0].

Slide 6

COLOR SYSTEMS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

380 405 430 455 480 505 530 555 580 605 630 655 680 705 730

B G R C M Y

Additive: dashed linesSubtractive: solid lines

Additive coloring :

(light emission, screens, projection)Can create any color by superimposing three primary colors with varying intensities.  

Requirements for primary spectral colors?

A color model is a way to quantify colors using standards

The human eye can distinguish between about 10 million colors.

How many colors are there in this color model?

What is white in this RGB model?

gray?

black?

2563=224 =108

2-D representation:

A color’s complement is… ______________________________

One color system (HSV)Hue: basic “color”: from differences in R, G, BValue (brightness) based on contextSaturation: degree differences stand out above “white”

Hue: YellowYellow, fully saturated: (R,G,B) = Yellow, less saturated (pastel): (R,G,B) = Yellow, less bright: (R,G,B) =

Play online

Rayleigh scattering occurs when particle size << l(air molecules)This scattering is most efficient at ________ frequencies or _________ wavelengths!

4_ 4

1dipole radiationI

Blue sky

Why is it often “pastel” blue?

Why is it deeper blue higher up?

Why are sunsets red and orange?

Sky seen through polarizer

Bees have polarized eyes for navigation

Scattering of E/M waves by a polarizable sphere vs freq

Rayleigh scattering

Mie scattering

Resonant Mie scattering

d = d = 10

Mie scattering:Why do so many things look white?

Salt

When any non absorbing material (glass, water, salt) is broken into pieces of size >l they scatter all wavelengths equally,

Paper fibers

Tobacco smoke is bluish, yet when he blows it out, it’s white. What’s happened?

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colviscon.html#c1

Subtractive coloring

uses dyes, inks, and pigments to absorb some wavelengths of light and not others.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

380 405 430 455 480 505 530 555 580 605 630 655 680 705 730

C M Y

Subtractive coloring

paint: white-scattering spheres (TiO2) mixed with absorbing pigments

colored paper, and fabric: white-scattering fibers coated with absorbing dyes.

“Reflection” vs transmission, scattering:

You write with red ink on white paper, and now the paper looks red. This is because the red ink itself is good at a) scattering red lightb) transmitting red lightc) reflecting red light

For a physicist, “reflection” is “specular” at a smooth interface between two materials. “Scattering” is what puts rays out at random angles from small objects.

r i

“Reflection” vs transmission, scattering:

From , and where n and k vary most, at what colors do we expect the surface of a smooth pool of red ink to reflect well?

a) colors transmitted by red inkb) colors absorbed by red ink

1

1

Nr

N

A yellow piece of paper “scatters” yellow light. The yellow ink transmits yellow light (absorbing reds and blues), and the white paper base scatters white light. The paper does not “reflect” yellow light.

What color(s) do these red plastic objects reflect?

From an optics point of view, what’s the biggest difference between what’s inside the two red plastics?

Irridenscence in nature

http://www.webexhibits.org/causesofcolor/15A.html

Interference/diffraction (“structural”) colors in nature: iridescence

http://www.webexhibits.org/causesofcolor/15A.html

Opals are natural 3-d diffraction gratings or “photonic crystals”

What’s missing in the albino peacock?

Opals

Photonic Crystals: 3-D diffraction grating

Nocturnal animalshave layer of cells behind retina with multilayer mirrors that reflect unabsorbed light back through it!

Cameleonhttp://www.youtube.com/watch?v=7Yxq_G05cHU

Change colors for camouflage,

mood1. Outer layer: cells that can concentrate red or yellow pigment in center (cells almost clear), or spread them over cell (colored)

2. Middle layer: sacks of photonic crystal plates, reflecting blue

3. Inner layer: cells that can concentrate melanin (black) in center (cells almost clear), or spread them over cell (black)

Only 1 and 3 controlled by nerves?

Chromatophore organ

Iridophores (photonic crystals) and chromatophores (pigment organs)