Light consists of particles called photons, each one of which can be
regarded as a packet of electromagnetic waves. For a beam of electromagnetic
energy to be light, and not X-rays or radio waves, is a matter of the wave-
length—the distance from one wave crest to the next—and in the case of light
this distance is about 5 X 10 to the -7 meters, or 0.0005 millimeter, or 0.5 micrometer, or 500 nanometers.
Light is defined as what we can see. Our eyes can detect electromagnetic
energy at wavelengths between 400 and 700 nanometers. Most light reaching our eyes consists of a relatively even mixture of energy at different wave-
lengths and is loosely called white light. To assess the wavelength content of a
beam of light we measure how much light energy it contains in each of a series
of small intervals, for example, between 400 and 410 nanometers, between 410
and 420 nanometers, and so on, and then draw a graph of energy against
wavelength. For light coming from the sun, the graph looks like the left illus-
tration on this page. The shape of the curve is broad and smooth, with no very
sudden ups or downs, just a gentle peak around 600 nanometers. Such a broad
curve is typical for an incandescent source. The position of the peak depends
on the source’s temperature: the graph for the sun has its peak around 600
nanometers; for a star hotter than our sun, it would have its peak displaced
toward the shorter wavelengths—toward the blue end of the spectrum, or the
left in the graph—indicating that a higher proportion of the light is of shorter
wavelength. (The artist’s idea that reds, oranges, and yellows are warm colors
and that blues and greens are cold is related to our emotions and associations,
and has nothing to do with the spectral content of incandescent light as related
to temperature, or what the physicists call color temperature.)
If by some means we filter white light so as to remove everything but a
narrow band of wavelengths, the resulting light is termed monochromatic (see
the graph at the right on this page).
PIGMENTS
When light hits an object, one of three things can happen: the light
can be absorbed and the energy converted to heat, as when the sun warms
something; it can pass through the object, as when the sun’s rays hit water or
glass; or it can be reflected, as in the case of a mirror or any light-colored
object, such as a piece of chalk. Often two or all three of these happen; for example, some light may be absorbed and some reflected. For many objects,
the relative amount of light absorbed and reflected depends on the light’s
wavelength. The green leaf of a plant absorbs long- and short-wavelength
light and reflects light of middle wavelengths, so that when the sun hits a leaf,
the light reflected back will have a pronounced broad peak at middle wave-
lengths (in the green). A red object will have its peak, likewise broad, in the
long wavelengths, as shown in the graph on this page.
An object that absorbs some of the light reaching it and reflects the rest is
called a pigment. If some wavelengths in the range of visible light are absorbed
more than others, the pigment appears to us to be colored. What color we see, I
should quickly add, is not simply a matter of wavelengths; it depends on
wavelength content and on the properties of our visual system. It involves
both physics and biology.
VISUAL RECEPTORS
Each rod or cone in our retina contains a pigment that absorbs
some wavelengths better than others. The pigments, if we were able to get
enough of them to look at, would therefore be colored. A visual pigment has
the special property that when it absorbs a photon of light, it changes its
molecular shape and at the same time releases energy. The release sets off a
chain of chemical events in the cell, described in Chapter 3, leading ultimately
to an electrical signal and secretion of chemical transmitter at the synapse. The
pigment molecule in its new shape will generally have quite different light-
absorbing properties, and if, as is usually the case, it absorbs light less well
than it did before the light hit it, we say it is bleached by the light. A complex
chemical machinery in the eye then restores the pigment to its original confor-
mation; otherwise, we would soon run out of pigment.
Our retinas contain a mosaic of four types of receptors: rods and three types
of cones, as shown in the illustration at the top of the facing page. Each of
these four kinds of receptors contains a different pigment. The pigments differ
slightly in their chemistry and consequently in their relative ability to absorb
light of different wavelengths. Rods are responsible for our ability to see in
dim light, a kind of vision that is relatively crude and completely lacks color.
Rod pigment, or rhodopsin, has a peak sensitivity at about 510 nanometers, in
the green part of the spectrum. Rods differ from cones in many ways: they are
smaller and have a somewhat different structure; they differ from cones in
their relative numbers in different parts of the retina and in the connections
they make with subsequent stages in the visual pathway. And finally, in the
light-sensitive pigments they contain, the three types of cones themselves dif-
fer from each other and from rods.
The pigments in the three cone types have their peak absorptions at about
430, 530, and 560 nanometers, as shown in the graph on this page; the cones
are consequently loosely called “blue”, “green”, and “red”, “loosely” because
(i) the names refer to peak sensitivities (which in turn are related to ability to
absorb light) rather than to the way the pigments would appear if we were to
look at them; (2) monochromatic lights whose wavelengths are 430, 530, and
560 nanometers are not blue, green, and red but violet, blue-green, and
yellow-green; and (3) if we were to stimulate cones ofjust one type, we would
see not blue, green, or red but probably violet, green, and yellowish-red in-
stead. However unfortunate the terminology is, it is now widely used, and efforts to change embedded terminology usually fail. To substitute terms such
as long, middle, and short would be more correct but would put a burden on
those of us not thoroughly familiar with the spectrum.
With peak absorption in the green, the rod pigment, rhodopsin, reflects blue
and red and therefore looks purple. Because it is present in large enough
amounts in our retinas that chemists can extract it and look at it, it long ago
came to be called visual purple. Illogical as it is, “visual purple” is named for the
appearance of the pigment, whereas the terms for cones, “red”, “green”, and
“blue”, refer to their relative sensitivities or abilities to absorb light. Not to
realize this can cause great confusion.
The three cones show broad sensitivity curves with much overlap, espe-
cially the red and the green cones. Light at 600 nanometers will evoke the
greatest response from red cones, those peaking at 560 nanometers, but will
likely evoke some response, even if weaker, from the other two cone types.
Thus the red-sensitive cone does not respond only to long-wavelength, or red,
light; it just responds better. The same holds for the other two cones.
So far I have been dealing with physical concepts: the nature of light and
pigments, the qualities of the pigments that reflect light to our eyes, and the
qualities of the rod and cone pigments that translate the incoming light into
electrical signals. It is the brain that interprets these initial signals as colors. In
conveying some feel for the subject, I find it easiest to outline the elementary
facts about color vision at the outset, leaving aside for the moment the three-
century history of how these facts were established or how the brain handles
color.
It may be useful to begin by comparing the way our auditory sys-
tems and our visual systems deal with wavelength. One system leads to tone
and the other to color, but the two are profoundly different. When I play a
chord of five notes on the piano, you can easily pick out the individual notes
and sing them back to me. The notes don’t combine in our brain but preserve
their individuality, whereas since Newton we have known that if you mix two
or more beams of light of different colors, you cannot say what the compo-
nents are, just by looking.
A little thought will convince you that color vision has to be an impover-
ished sense, compared with tone perception. Sound coming to one ear at any
instant, consisting of some combination of wavelengths, will influence thou-
sands of receptors in the inner ear, each tuned to a slightly different pitch than
the next receptor. If the sound consists of many wavelength components, the
information will affect many receptors, all of whose outputs are sent to our
brains. The richness of auditory information comes from the brain’s ability to
analyze such combinations of sounds. Vision is utterly different. Its information-handling capacity resides largely
in the image’s being captured by an array of millions receptors, at every in-
stant. We take in the complex scene in a flash. If we wanted in addition to
handle wavelength the way the ear does, the retina would need not only to
have an array of receptors covering its surface, but to have, say, one thousand
receptors for each point on the retina, each one with maximum sensitivity to a
different wavelength. But to squeeze in a thousand receptors at each point is
physically not possible. Instead, the retina compromises. At each of a very
large number of points it has three different receptor types, with three differ-
ent wavelength sensitivities. Thus with just a small sacrifice in resolution we
end up with some rudimentary wavelength-handling ability over most of our
retina. We see seven colors, not eighty-eight (both figures should be much
higher!), but in a single scene each point of the many thousands will have a
color assigned to it. The retina cannot have both the spatial capabilities that it
has and also have the wavelength-handling capacity of the auditory system.
The next thing is to get some feel for what it means for our color vision to
have three visual receptors. First, you might ask, if a given cone works better
at some wavelengths than at others, why not simply measure that cone’s out-
put and deduce what the color is? Why not have one cone type, instead of
three? It is easy to see why. With one cone, say the red, you wouldn’t be able to
tell the difference between light at the most effective wavelength, about 560
nanometers, from a brighter light at a less effective wavelength. You need to
be able to distinguish variations in brightness from variations in wavelength.
But suppose you have two kinds of cones, with overlapping spectral sensi-
f tivities—say, the red cone and the green cone. Now you can determine wave-
length simply by comparing the outputs of the cones. For short wavelengths,
the green cone will fire better; at longer and longer wavelengths, the outputs
‘ will become closer and closer to equal; at about 580 nanometers the red sur-
passes the green, and does progressively better relative to it as wavelengths get
still longer. If we subtract the sensitivity curves of the two cones (they are
logarithmic curves, so we are really taking quotients), we get a curve that is
independent of intensity. So the two cones together now constitute a device
that measures wavelength.
Then why are not two receptors all we need to account for the color vision
that we have? Two would indeed be enough if all we were concerned with was
monochromatic light—if we were willing to give up such things as our ability
to discriminate colored light from white light. Our vision is such that no
monochromatic light, at any wavelength, looks white. That could not be true
if we had only two cone types. In the case of red and green cones, by progress-
ing from short to long wavelengths, we go continuously from stimulating just
the green cone to stimulating just the red, through all possible green-to-red
response ratios. White light, consisting as it does of a mixture of all wave-
lengths, has to stimulate the two cones in some ratio. Whatever monochro-
matic wavelength happens to give that same ratio will thus be indistinguish-
able from white. This is exactly the situation in a common kind of color
blindness in which the person has only two kinds of cones: regardless of
which one of the three pigments is missing there is always some wavelength of
light that the person cannot distinguish from white. (Such subjects are color
defective, but certainly not color-blind.)
To have color vision like ours, we need three and only three cone types. The
conclusion that we indeed have just three cone types was first realized by
examining the peculiarities of human color vision and then making a set of
deductions that are a credit to the human intellect.
We are now in a better position to understand why the rods do not mediate
color. At intermediate levels of light intensity, rods and cones can both be
functioning, but except in rare and artificial circumstances the nervous system
seems not to subtract rod influences from cone influences. The cones are com-
pared with one another; the rods work alone. To satisfy yourself that rods do
not mediate color, get up on a dark moonlit night and look around. Although
you can see shapes fairly well, colors are completely absent. Given the simplic-
ity of this experiment it is remarkable how few people realize that they do
without color vision in dim light.
Whether we see an object as white or colored depends primarily (not en-
tirely) on which of the three cone types are activated. Color is the consequence
of unequal stimulation of the three types of cones. Light with a broad spectral
curve, as from the sun or a candle, will obviously stimulate all three kinds of
cones, perhaps about equally, and the resulting sensation turns out to be lack
of color, or “white”. If we could stimulate one kind of cone by itself (some-
thing that we cannot easily do with light because of the overlap of the absorp-
tion curves), the result, as already mentioned, would be vivid color—violet,
green, or red, depending on the cone stimulated. That the peak sensitivity of
what we call the “red cone” is at a wavelength (560 nanometers) that appears
to us greenish-yellow is probably because light at 560 nanometers excites both
the green-sensitive cone and the red-sensitive cone, owing to the overlap in the
green- and red-cone curves. By using longer wavelength light we can stimu-
late the red cone, relative to the green one, more effectively.
The graphs on the facing page sum up the color sensations that result when
various combinations of cones are activated by light of various wavelength
compositions.
The first example and the last two should make it clear that the sensation
“white”—the result of approximately equal stimulation of the three cones—
can be brought about in many different ways: by using broad-band light or by
using a mixture of narrow-band lights, such as yellow and blue or red and
blue-green. Two beams of light are called complementary if their wavelength
content and intensities are selected so that when mixed they produce the sensa-
tion “white”. In the last two examples, blue and yellow are complementary, as
are red at 640 nanometers and blue-green.
Source: http://hubel.med.harvard.edu
