Physics 1230: Light and Color
•  Color Perception
–  Trichromacy
–  Color response
–  Perception
http://www.colorado.edu/physics/phys1230
HOW DO WE “SEE” COLOR?
FOUR PRIMARY COLOR GROUPS
Additive colors are
created with light.
You project a red
light through a red
screen or on a
monitor or
television. The
additive primaries
are red, green and
blue (RGB). If you
combine red, green
and blue light you
get white.
Subtractive colors apply
to print and pigment. In
this case color exists
because the pigment
absorbs some light rays
and reflects others. If
we use red paint the
pigment absorbs or
subtracts all light rays
other than red, which it
reflects back to the eye.
The subtractive
primaries are magenta,
yellow, cyan and black
(CMYK). If you combine
M, Y & C you get black.
Artist’s primaries
consist of red, yellow,
and blue. From these
primary colors the
secondary colors
orange, green and
purple can be mixed
from combinations of
two primaries. The
result when all three
primaries are mixed is
black
Psychological colors
consist of red, yellow,
blue, green and the
achromatic pair, black
and white. This is the
group we were taught
as children. All colors
can be described
verbally as a mixture
of these four
psychological
primaries.
http://www-cvrl.ucsd.edu/
COLOR AFTERIMAGES
What do these colored afterimages tell us about the way our eyes perceive
color?
http://www.yorku.ca/eye/toc-sub.htm
COLOR VISION
What is color?
Color fills our world and we marvel at the shades of flowers and sunsets. Color has
almost an emotional quality, yet it is difficult to describe in simple language. In fact, we
can not describe red, for example, without saying its like something. So, we say red like
blood, green like grass or leaves, and blue like the sky. Some colors go together. Thus,
we can imagine blue/green, which we call cyan or sea green according to the shade.
Combinations of red and blue make purple. But, there are no blends of red and green or
blue and yellow. There are no reddish greens or bluish yellows and there are no names
for these color mixtures. Red and green are opponent and so are blue and yellow. This is
the first clue that some colors are processed differently and there are two separate
pathways by which our visual systems process color.
What is color vision?
Color vision is the ability to differentiate between colors. In its simplest form, this is
the ability to tell the difference between say red and green, though we have the ability
to discriminate among many subtle shades. The fundamental basis of color vision is the
presence of three photoreceptor types, each containing different but closely related
pigments, which preferentially absorb three different wavelengths of light. We refer to
these as red, green and blue cones. The signals generated by the three cone types are
somehow sorted or combined by neuronal circuits in the retina.
http://eye.med.uth.tmc.edu/MasseyLab/color%20vision/colorvision.htm
Color Perception - History
By experimenting with prisms as early as 1672, Isaac Newton made the fundamental discovery
that ordinary "white" light is really a mixture of lights of many different wavelengths, as seen
in a rainbow. Objects appear to be a particular color because they reflect some wavelengths
more than others. A red apple is red because it reflects rays from the red end of the
spectrum and absorbs rays from the blue end. A blueberry, on the other hand, reflects the
blue and absorbs the red.
Thinking about Newton's discovery in 1802, the physician Thomas Young, who later helped
decipher the hieroglyphics of the Rosetta Stone, concluded that the retina could not possibly
have a different receptor for each of these wavelengths, which span the entire continuum of
colors from violet to red. Instead, he proposed that colors were perceived by a three-color
code.
As artists knew well, any color of the spectrum (except white) could be matched by judicious
mixing of just three colors of paint. Young suggested that this was not an intrinsic property
of light, but arose from the combined activity of three different "particles" in the retina,
each sensitive to different wavelengths.
We now know that color vision actually depends on the interaction of three types of cones—
one especially sensitive to red light, another to green light, and a third to blue light. In 1964,
George Wald and Paul Brown at Harvard and Edward MacNichol and William Marks at Johns
Hopkins showed that each human cone cell absorbs light in only one of these three sectors of
the spectrum. Wald went on to propose that the receptor proteins in all these cones were
built on the same plan as rhodopsin. Each protein uses retinal, a derivative of vitamin A, to
absorb light; and each tunes the retinal to absorb a different range of wavelengths.
http://www.yorku.ca/eye/toc-sub.htm
COLOR VISION
Who has it?
Among mammals, only primates have true, trichromatic color vision. Most mammals, such
as rodents, rabbits, cats, and dogs have no red cones. They have mostly green cones with
a small fraction of blue cones. We could say they have a blue/green color system. This
represents a fundamental division into two separate streams for the processing of color
signals. The blue system is unusual in many respects and clearly forms a different
pathway. The blue cone pigment is more distantly related than the red and green opsins
which were formed in primates by a more recent gene duplication. The gene for blue
cone opsin is located on a separate chromosome while, in primates, the red and green
genes are found together on the X chromosome. This explains why red/green color
blindness is most prevalent among males who only have one copy of the X chromosome.
Why do we have color vision?
We have color vision as a mechanism to increase our visual discrimination, as a means to
increase contrast. One theory holds that color is a secondary development arising from
the relentless pursuit of high acuity which finished with the evolutionary development of
the primate midget system. This retinal circuit has reached the ultimate precision with
one cone connected to one midget bipolar cell which, in turn, is connected to one midget
ganglion cell. This circuit then is color coded because it is connected to only one cone.
Color provides an extra variable by which to discriminate detail in our richly textured
world. It lends immeasurably to our aesthetic enjoyment but presumably color
perception also enhanced our survival rate by improving the performance of the human
visual system.
http://eye.med.uth.tmc.edu/MasseyLab/color%20vision/colorvision.htm
COLOR VISION
Qualitative differences in vertebrate, mammalian
and primate color vision
http://www.handprint.com/HP/WCL/color1.html#eye
TRICHROMACY OF COLOR VISION
•  We have 3 types of cones - B , G and
R (blue, green, and red).
•  These are also known as S, I, and L
(for short, intermediate, and long
wavelength sensors).
•  All three cones are sensitive to
almost all spectral regions, but are
most sensitive in their respective
optimized color regions.
•  Thus each color produces a unique
set of responses from the S, I, and L
cones, and is therefore interpreted
correctly by the brain.
COLOR Vision
The distribution of photoreceptor cells varies
considerably across different parts of the
retina (diagram at right). The fovea contains a
"central bouquet" of only two types of cones (R
and G); the B cones and rods are entirely
excluded. This allows a tight packing of
anywhere from 160,000 to 250,000 cones per
square millimeter — an average cell spacing
(about 2.5 to 2 microns) that equals the eye's
maximum possible optical resolution. Outside
the fovea, the R and G cones are mixed with B
cones to provide the complete range of color
vision; here color and lightness, rather than
edges, determine the perception of shapes. All
the cones become fatter and less densely
packed, and the spacing between them
increases, along the sides of the eye. This
reduces optical resolution and light sensitivity
where the curved surface of the retina makes
it impossible for the lens to provide a crisply
focused image.
http://www.handprint.com/HP/WCL/color1.html#eye
SENSITIVITY OF
SIGHT
Rod cells, containing only the photopigment rhodopsin, have a peak sensitivity to
blue-green light (wavelength of about 500 nanometers), although they display a
broad range of response throughout the visible spectrum. They are the most
common visual receptor cells, with each eye containing about 125-130 million rod
cells. The light sensitivity of rod cells is about 1,000 times that of cone cells.
However, the images generated by rod stimulation alone are relatively unsharp
and confined to shades of gray, similar to those found in a black and white softfocus photographic image. Rod vision is commonly referred to as scotopic or
twilight vision because in low light conditions, shapes and the relative brightness
of objects can be distinguished, but not their colors. This mechanism of dark
adaptation enables the detection of potential prey and predators via shape and
motion in a wide spectrum of vertebrates.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
COLOR SENSITIVITY of Human Photoreceptors
• These data are from human photoreceptors. Bowmaker & Dartnall (1980) projected a
known amount of light directly through the outer segments of photoreceptors and
measured how much light was absorbed by the photopigment molecules (using
microspectphotometry).
• They found four classes of photopigments as shown in the above graph. The colors of the
curves do not represent the colors of the photopigments. The wavelength of maximum
absorbance is indicated at the top of each curve. The 420 curve is for the short wavelength
(S) cones, the 498 curve is for the rods, and the 534 and 564 curves are for the middle (I)
and long wavelength (L) sensitive cones respectively.
http://www.yorku.ca/eye/toc-sub.htm
COLOR SENSITIVITY – Human Photoreceptors
Rhodopsin
• These data are from human photoreceptors. Bowmaker & Dartnall (1980) projected a
known amount of light directly through the outer segments of photoreceptors and
measured how much light was absorbed by the photopigment molecules (using
microspectphotometry).
• They found four classes of photopigments as shown in the above graph. The colors of the
curves do not represent the colors of the photopigments. The wavelength of maximum
absorbance is indicated at the top of each curve. The 420 curve is for the short wavelength
(S) cones, the 498 (rhodopsin) curve is for the rods, and the 534 and 564 curves are for
the middle (I) and long wavelength (L) sensitive cones respectively.
http://www.yorku.ca/eye/toc-sub.htm
Rhodopsin
VISION SENSITIVITY
The human visual system response is logarithmic, not linear, resulting in the ability to
perceive an incredible brightness range (interscene dynamic range) of over 10 decades. In
broad daylight, humans can visualize objects in the glaring light from the sun, while at night
large objects can be detected by starlight when the moon is dark. At threshold sensitivity,
the human eye can detect the presence of about 100-150 photons of blue-green light (500
nanometers) entering the pupil. For the upper seven decades of brightness, photopic vision
predominates, and it is the retinal cones that are primarily responsible for photoreception.
In contrast, the lower four decades of brightness, termed scotopic vision, are controlled by
the rod cells
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
Rhodopsin
VISION SENSITIVITY
Cones consist of three cell types, each "tuned" to a distinct wavelength response maximum
centered at either 430, 535, or 590 nanometers. They contain three different
photopigments, each with a characteristic visible light absorption spectrum. The
photopigments alter their conformation when a photon is detected, enabling them to react
with transducin to initiate a cascade of visual events. Transducin is a protein that resides in
the retina and is able to effectively convert light energy into an electrical signal. The
population of cone cells is much smaller than rod cells, with each eye containing between 5
and 7 million of these color receptors. True color vision is induced by the stimulation of
cone cells. The relative intensity and wavelength distribution of light impacting on each of
the three cone receptor types determines the color that is imaged (as a mosaic), in a
manner comparable to an additive RGB video monitor or CCD color camera.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
Rhodopsin
VISION SENSITIVITY
Here are the absorption spectra of the four human visual pigments, which display maxima in
the expected red, green, and blue regions of the visible light spectrum. When all three
types of cone cell are stimulated equally, the light is perceived as being achromatic or
white. For example, noon sunlight appears as white light to humans, because it contains
approximately equal amounts of red, green, and blue light. There are shifts in color
sensitivity with variations in light levels, so that blue colors look relatively brighter in dim
light and red colors look brighter in bright light. This effect can be observed by pointing a
flashlight onto a color print, which will result in the reds suddenly appearing much brighter
and more saturated.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
Rhodopsin
VISION SENSITIVITY
In recent years, consideration of human color visual sensitivity has led to changes in the
long-standing practice of painting emergency vehicles, such as fire trucks and ambulances,
entirely red. Although the color is intended for the vehicles to be easily seen and
responded to, the wavelength distribution is not highly visible at low light levels and appears
nearly black at night. The human eye is much more sensitive to yellow-green or similar hues,
particularly at night, and now most new emergency vehicles are at least partially painted a
vivid yellowish green or white, often retaining some red highlights in the interest of
tradition.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
Rhodopsin
VISION SENSITIVITY
When only one or two types of cone cells are stimulated, the range of perceived colors is
limited. For example, if a narrow band of green light (540 to 550 nanometers) is used to
stimulate all of the cone cells, only the ones containing green photoreceptors will respond to
produce a sensation of seeing the color green.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
Rhodopsin
COLOR VISION
Human visual perception of primary subtractive colors, such as yellow, can arise in two ways.
If the red and green cone cells are simultaneously stimulated with monochromatic yellow
light having a wavelength of 580 nanometers, the cone cell receptors each respond almost
equally because their absorption spectral overlap is approximately the same in this region
of the visible light spectrum. The same color sensation can be achieved by stimulating the
red and green cone cells individually with a mixture of distinct red and green wavelengths
selected from regions of the receptor absorption spectra that do not have significant
overlap. The result, in both cases, is simultaneous stimulation of red and green cone cells to
produce a sensation of yellow color, even though the end result is achieved by two different
mechanisms. The ability to perceive other colors requires the stimulation of one, two, or all
three types of cone cells, to various degrees, with the appropriate wavelength palette.
http://micro.magnet.fsu.edu/primer/lightandcolor/humanvisionintro.html
COLOR SENSITIVITY
•  So red excites the L cones the most, while blue excites the S cones the most.
•  Magenta excites both S and L cones.
•  The photoreceptors tell you what the color is, but not the purity. For example, 600nm
looks yellow, and so does a mixture of 500, 600, and 700 nm.
•  Color discrimination is highest in between two curves (where a small change in color gives a
large change in response).
http://www.yorku.ca/eye/toc-sub.htm