Physics
of Color
Spectrum of Visible Light
Wavelengths & Photons
Particles of light, called photons, each have a wavelength.
Red Photon
Yellow Photon
Green Photon
Blue Photon
Prism
Spectrum
Newton’s Color Wheel
Prism spectrum is a
straight line, but Isaac
Newton described
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
Additive Color Wheel
R
M
Y
B
G
C
Spectral
Colors
Red
Yellow
Green
Cyan
Blue
Magenta
Hue, Saturation, Value
Color wheel is not
a single wheel but
stack of wheels
that range in
value.
Color Vision in the Eye
Three types of cones (color)
One type of rod (B/W only)
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?
Incidence (%)
Classification
Males
Females
Anomalous
Trichromacy
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
CIE color “wheel”
Cone Sensitivity
Why Yellow & Orange are Special
Green
Red
Blue
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 …
Hot Sauce!!!
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.
circles overlapping and indicate the areas
producing the primary and secondary colours.
MAGENTA
BLUE
RED
YELLOW
CYAN
WHITE
GREEN
http://lectureonline.cl.msu.edu/~mmp/applist/RGBColor/c.htm
Adding Color Lights
Stream of
red & green photons
looks same as
yellow photons
(metamerism)
or
Theatrical lighting
Eye to
Brain
Notice overlap of red, green, &
blue is seen as white light
How does one get a complimentary
color?
• Give some examples.
• - mix any two primary colors which
becomes a complimentary color to the
remaining primary color
• - e.g. yellow (redprimary + greenprimary)
is complimentary to blueprimary
Yellow and blue would make white light
Additive Color Mixing
• Colors can be created by adding colors.
• e.g.
red and green = yellow
•
(2 primary colors = secondary color)
•
e.g. red and blue = magenta
Review-Color
•
•
•
•
•
•
•
Visible light is made up of:
Wavelengths of the electromagnetic spectrum from
 = 400 nm (4.00 x 10–7 m)to  = 700 nm (7.00 x 10–7 m)
Newton called the ordered arrangement of colors from violet to
red a spectrum.
A spectrum can be produced by passing a light beam through a
prism.
White light can be produced by adding the colors, red, green
and blue which are also known as the primary colors.
Secondary colors are formed by:
mixing any two of the primary colors
Secondary colors include yellow, cyan and magenta.
The main spectral Colors
red
orange
yellow
green
blue
indigo
violet
R
•
•
•
•
•
O Y
Goes Bowling
Venezuela
In
lowest frequency
highest frequency
longest wavelength
shortest wavelength
least refracted
most refracted
lowest energy
highest energy
We see a specific color because we see a particular
frequency.
• Think of frequency as the number of crests per second
hitting the retina of your eye.
• So, if 4.29 x 1014 crests hit your retina in one second, then
you see a visible part of the electromagnetic spectrum
known as a color called red.
The Color of Light and Refraction
• When light waves pass from a less dense into a
more dense medium, such as from air to water,
the speed of the light wave decreases.
• Yet, we see the same color under water as in air,
therefore we still have 4.29 x 1014 crests hitting
our retina per second.
• This means that as the speed of the wave slows
down, the wavelength gets shorter (crunched).
• This way, the number of crests hitting the retina
per second remains constant, ensuring that the
color remains unchanged.
Subtractive Process
Subtractive Process
Colors can be created by the subtractive process using filters.
e.g.
R O Y G
B
I V
yellow
filter
We See Mostly Yellow
Subtractive Process
e.g.
R
O
Y
G
B
I
V
blue filter
We See Mostly Blue
Subtractive Process
e.g.
R
O
Y
G
blue filter
red filter
We See Black ( No Color)
B
I
V
Artist’s Handbook
These spectral reflectance
curves and those of many
other standard pigments are
found in Mayer’s book.
Subtractive Process
e.g.
R
O
Y
green filter
yellow filter
We See MostlyYellow
G
B
I
V
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
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
Spectral Reflectance Curves
When white light shines on a colored object, some photons
absorbed, others reflected by the object’s surface.
Name That Pigment
100%
0%
BLUE
GREEN
Titanium
White
RED
BLUE
GREEN
Cadmium
Red
RED
BLUE
GREEN
Burnt
Sienna
RED
Name That Pigment
100%
0%
BLUE
GREEN
RED
Phthalocyanine
Green
BLUE
GREEN
Cadmium
Yellow
RED
BLUE
GREEN
Cobalt
Blue
RED
Reflectance of Pigment Mixtures
100%
80%
BLUE
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.
Cadmium Red
60%
40%
Cobalt Blue
20%
Mixture
400
500
600
Nanometers
700
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%.)
NOTE: This only gives approximate results; full theory more complex!
Pigments are opaque (not transparent) paints or inks placed
onto opaque surfaces. These pigments absorb and reflect
different amounts of color from white light.
Most of the colored objects we see on earth are made up of
combinations of reflected wavelengths. Surfaces or objects
illuminated by white light absorb differing proportions of
visible wavelengths and reflect the remainder.
The principle of reflected color is illustrated in the following
series of graphics:
•
a. Sunlight, as described above, is composed of equal
amounts of all spectral hues (i.e., the colors of the
rainbow). When sunlight, or artificial white light is
shone on an opaque surface, certain wavelengths of
light (colors) will be reflected off the surface while
others will be absorbed by the surface. The "color" we
see depends on the type and amount of reflectance of
the wavelengths we identify as the spectral hues. White,
as shown here, is created by the equal, full-strength
(100%) reflectance of all three main spectral hues, that
is 100% reflectance of blue, green and red.
b. Medium Grey results from the 50% absorption, 50%
reflectance of blue, green and red. Darker grey happens when
more than 50% of each primary color is being absorbed and
less is reflected. Light grey results when more than 50% of each
primary color is being reflected and less is absorbed.
c. Black is the absence of color, or the absence of reflected
light, thus the total (100%) absorption of all three primary
colors.
d. Other colors can be created by the combined reflectance
of certain of the spectral hues. A full strength cyan, for
example, is comprised of 100% reflectance of blue and green,
and 100% absorption of magenta. A medium strength cyan
would result from 50% reflectance of blue and green. More or
less than 50% reflectance of blue and green would form
darker or lighter cyan, respectively. Note that there must be
equal reflectance of blue and green in order for a true cyan to
be formed. If more blue were reflected, the resulting color
would be blue-green. If more green were reflected, the
resulting color would be green-blue.
e. Magenta (full strength) is formed by the equal reflectance
of blue and red. Thus 100% reflectance of blue and red, and
100% absorption of green, forms a full strength magenta.
f. As with cyan and magenta, a full strength yellow is formed
by the equal (100%) reflectance of red and green, and 100%
absorption of blue.
Pigment Color Wheel
http://lectureonline.cl.msu.edu/~mmp/applist/CYMColor/c.htm
Red
Green
Blue
• The Subtractive Primary Colors, created by
the mixing of the Additive Primary Colors,
are Cyan, Magenta, Yellow, and also
Black, as shown here
• Note that the subtractive primaries are those
formed by the mixture of additive primaries.
This is how paints and inks are mixed and
used in painting and printing onto opaque
surfaces. Note that the Subtractive Primary
Colors mix to recreate the additive primaries:
•
•
•
•
Magenta + Cyan = Blue
Cyan + Yellow = Green
Yellow + Magenta = Red
Cyan + Magenta + Yellow = Black (absence
of color)
Examples for the Subtractive
primaries are shown below:
• Red (Magenta + Yellow) = White minus Green (Yellow + Cyan),
minus Blue (Magenta + Cyan); in other words, Cyan is subtracted from
both Green and Blue, leaving Yellow and Magenta which, when
mixed, forms Red. Another way to look at it is that Green and Blue are
subtracted from White, leaving only Red.
• Blue (M + C) = White - Green (Y + C) - Red (M + Y);
the subtraction of Yellow from both Green and Red leaves M +
C which mixes to form Blue.
• Green (Y + C) = White - Red (M + Y) - Blue (M + C)
subtract magenta from both, leaves Y + C which together forms Green.
• Black = White - Red - Blue - Green (absence of color)
• Transparency example 4 color printing process
How Polarizing filters work
How Polarizing filters work
only transverse waves can be polarized.
- only perpendicular transverse waves are cancelled
Malus’s Law
http://www.colorado.edu/physics/2000/polarization/polarizationI.html
Fun with Colors and messing with
your brain
•
•
•
How anaglyph works
Viewing anaglyphs through appropriately colored glasses results
in each eye seeing a slightly different picture. In a red-blue
anaglyph, for instance, the eye covered by the red filter sees the
red parts of the image as "white", and the blue parts as "black"
(with the brain providing some adaption for color); the eye
covered by the blue filter perceives the opposite effect. True
white or true black areas are perceived the same by each eye. The
brain blends together the image it receives from each eye, and
interprets the differences as being the result of different distances.
This creates a normal stereograph image without requiring the
viewer to cross his or her eyes.
http://www.jessemazer.com/3Dphotos.html
More Fun
• http://dogfeathers.com/java/spirals.html
• http://www.qualitytrading.com/illusions/
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
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