Newton: color depends on “imagination
and fantasy and invention.”
Vector nature of color:
“Spectral” (rainbow)
colors are those
perceived from light of
narrow band of w,l.
Response per intensity unit
Vector nature of color:
Color information 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.
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].
COLOR SYSTEMS
Additive: dashed lines
Subtractive: solid lines
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
380
405
430
455
480
505
B
530
G
555
R
580
C
605
M
630
Y
655
680
705
730
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
What is white in this RGB model?
black?
gray?
The brain can distinguish between about 10 million
colors.
How many colors are there in this color model?
2563=224 =108
2-D representation:
A color’s complement is…
______________________________
One color system (HSV)
Hue: basic “color”: from differences in R, G, B
Value (brightness) based on context
Saturation: degree differences stand out above “white”
Hue: Yellow
Yellow, 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!
I dipole _ radiation
Blue sky
w
4
1
l4
Why is it often “pastel” (unsaturated) blue?
Why is it deeper blue higher up?
Why are sunsets red and orange?
Sky seen through polarizer
Bees have polarized eyes
for navigation
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,
Scattering of E/M waves by a polarizable sphere vs freq
Mie scattering
Resonant Mie
scattering
Rayleigh
scattering
d=l
d = 10l
Paper fibers
Tobacco smoke is bluish, yet when he
blows it out, it’s white. What’s happened?
uses dyes, inks, and pigments to absorb
some wavelengths of light and not others.
Subtractive
coloring
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
380
405
430
455
480
505
530
555
580
C
605
M
630
655
680
705
730
Y
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colviscon.html#c1
Subtractive
coloring
colored paper, and fabric:
white-scattering fibers coated
with absorbing dyes.
paint: white-scattering
spheres (TiO2) mixed with
absorbing pigments
“Reflection” vs transmission, scattering:
For a physicist, “reflection” is “specular”  r  i at a
smooth interface between two materials. “Scattering”
is what puts rays out at random angles from small
objects.
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 light
b) transmitting red light
c) reflecting red light
Remember this?
In a glass that
absorbs green light
the index , real index
n is probably
greatest for ______
light.
a) red
b) yellow
c) green
d) blue
e) violet
 n  i  nAir 
R

 n  i  nAir 
Reflection colors
are different from
transmission
colors
2
What color(s) do the surfaces of these red plastic
objects reflect? What color(s) does the one on the
right scatter?
From an optics perspective, 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 animals
have layer of cells behind retina with multilayer
mirrors that reflect unabsorbed light back through it!
Cameleon
http://www.youtube.com/watch?v=7Yxq_G05cHU
Change colors for
camouflage, mood
1. 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)
Chameleon colours 'switched by crystals' - http://www.bbc.co.uk/news/science-environmen
Latest
Chameleon colours 'switched
by crystals' http://www.bbc.co.uk/news/sc
ience-environment-31819588