HOW DO WE SEE COLORS? (AND HOW
By David Mallory, M.D.
Normal perception of color is something most of us take for
granted. Seeing green grass or red sunsets, however, only
has specific meaning to people with normal color vision.
Eight percent of males and one in 200 females are
"colorblind," or have faulty color perception from birth.
This sex linked defect is usually in the discrimination of
red and green colors.
Men have just one X chromosome, and if it is defective, they
will have abnormal vision ranging from slight difficulty in
recognizing shades of color to complete loss. Women have two
X chromosomes, and both would have to be defective to have
this color loss.
The retina, which lines the back of our eyes, has special
neuroreceptor cells, called cones, for day and color vision,
and others, called rods, for night vision. There are three
types of cone cells, each of which has a different pigment
to "catch" the red, green, or blue wavelengths of light.
Acting together, they allow us to perceive a magnificent
range of color from deep indigo to blazing red. The specific
contribution of each of the cone cells in the retina is
"added up" in the eye and brain to produce this myriad of
hues and shades.
Color television relies on the ability of the eye to add up
tiny adjacent points of light. If you look at a color
television from six inches away, you would see tiny dots of
only three colors: red, green and blue.
If you then back away, the full range of colors becomes
apparent, and you can no longer distinguish the tiny dots.
The eye adds up the adjacent colors to produce different
shades (e.g. tiny dots of red and blue = purple, red and
green = yellow, red, green and blue = white, and so on.)
There is a reason why color we see "changes" in artificial
light. Daylight from the sun contains equal amounts of light
of all wavelengths. A purple dress, therefore, appears
redder under incandescent light. Fluorescent lighting, on
the other hand, emits more light in the blue green
wavelengths. A shopper who picks out drapes in a store with
fluorescent lights may be surprised to find the material
looks quite different at home.
Many different eye problems, such as cataracts, diabetes,
and glaucoma can cause a slow unnoticed loss of color
vision. By far, the most common treatable cause is a
cataract. Cataracts can, even before noticeable loss of
vision, cause a profound defect in color perception. People
are sometimes more pleased with how bright and clear colors
are after their cataract has been removed than they are with
their actual improvement in visual acuity.
David Mallory, M.D. is a cataract, intraocular lens and
laser specialist in practice at the Southwest Eye Clinic.