SCIENCE15 - Color Perception
Pat suggested that I write a blog about color perception after she read an article that proposed that women may have an advantage over men. So, I looked into the subject, and found that color perception is a very complicated subject. But it was also a very fascinating subject, so I decided to go ahead, trying not to make it too technical.
After a short introduction, I
will talk about the nature of color, how we see color, why individuals may not
see the same color, and end with the importance of color to our lives today.
My principal sources include: “Color
Vision,” Wikipedia.com; “Primary Colors - What Are the Primary Colors in Color Theory?”
artincontext.org; “Why We Don’t See the Same Colors,” psychologytoday.com; “The
Science of How We See Color,” datacolor.com; “Colour,” Britannica.com; “Color
is in the eye, and brain, of the beholder,” knowablemagazine.org; “Color
Blindness: The Most Common, Uncommon Eye
Condition,” downtownvisionnv.com; plus, numerous other online sources.
Introduction
Color vision is a feature of visual perception that
allows us perceive a multitude of different colors.
In humans and other primates, color vision may have
evolved under selective pressure for a variety of visual tasks including
foraging for nutritious young leaves, ripe fruit, and flowers, as well as
detecting predator camouflage and emotional states in other primates.
Today, color helps us remember
objects, influences our purchases, and sparks our emotions.
Objects do not possess color. They reflect wavelengths of light that are
seen as color by the human brain. When
white light strikes a white object, it appears white to us because it absorbs
no color and reflects all color equally.
When it strikes a colored object, this color light is reflected back to
our eyes. A black object absorbs all
colors equally and reflects none, so it looks black to us.
A person can see in dim light without
being able to distinguish colors. Only
when more light is present do colors appear.
Light of some critical intensity, therefore, is necessary for color
perception. Finally, the way the brain
responds to visual stimuli must also be considered. Even under identical conditions, the same
object may appear red to one observer and orange to
another. Clearly, the perception of
color depends on vision, light, and individual interpretation.
The Nature of Color
Aristotle viewed color to be the
product of a mixture of white and black, and this was the prevailing
belief until 1666, when Isaac Newton’s prism experiments
provided the scientific basis for the understanding of color. Newton showed that a glass prism could break
up white light (like sunlight) into a range of colors, which he called
the spectrum, and that the recombination of these spectral colors
re-created the white light.
Although he recognized that the spectrum was continuous, Newton used the
seven color names violet, indigo, blue, green, yellow, orange, and red for
segments of the spectrum (some say by analogy with the seven notes of
the musical scale).
 |
| In 1666, Isaac Newton showed that a prism could break up white light into a range of colors. |
The Visible Light Spectrum. Today, we know that color is associated
specifically with electromagnetic radiation of a certain range
of wavelengths visible to the human eye. This range of wavelengths is known
as the visible spectrum - i.e., light.
Electromagnetic radiation is energy that travels at the speed
of light and spreads out as it goes. From
short wavelengths to long wavelengths, the types of electronic radiation that
make up the electromagnetic spectrum include:
cosmic rays, X-rays, ultraviolet light, visible light, infrared light,
microwaves, radar, radio, and the broadcast band consisting of TV and AM and FM
radio.
The figure below shows the electromagnetic
spectrum and the narrow range of visible light, extending left to right, from
violet light to red light.
The visible light spectrum is a narrow range of the electromagnetic spectrum.
The visible light
spectrum for humans ranges from about 380 to 740 nanometers (a nanometer
is one billionth of a meter). Newton’s spectral colors, with cyan
(blue-green) replacing indigo, can be found in this range.
These spectral colors do not refer to
a single wavelength, but rather to a set of wavelengths: violet, 380 - 450 nm;
blue, 450 - 485 nm; cyan, 485 - 500 nm; green, 500 - 565 nm; yellow, 565 - 590
nm; orange, 590 - 625 nm; red, 625 - 740 nm.
Wavelengths just shorter and longer
than this visible range are ultraviolet light and infrared light,
respectively. Humans cannot generally
see these wavelengths, but some animals can.
Spectral Colors. A spectral color can be precisely specified by
its hue, saturation, and brightness - three attributes
sufficient to distinguish it from all other possible perceived colors.
Hue is the attribute of colors that permits them to be
classed as red, yellow, green, blue, or an intermediate between any contiguous pair of these colors. Hue is the wavelength within the
visible-light spectrum at which the reflected energy output from a viewed
object is greatest.
Saturation refers to relative purity. When a pure, vivid, strong shade of red is
mixed with a variable amount of white, weaker or paler reds are produced, each
having the same hue but a different saturation.
As saturation increases, colors appear sharper or
purer. As saturation decreases, colors
appear more washed-out or faded.
Finally, light of any given combination of hue and saturation
can have a variable brightness (also called intensity), which depends on the
total amount of light energy present. The intensity of the color grows with increased brightness,
but the color itself does not change.
Note:
Many photo processing computer programs have adjustable settings for
these parameters.
Non-Spectral Colors. There are a variety of colors in addition to
spectral colors. These
include grayscale colors, shades of colors obtained by mixing grayscale
colors with spectral colors, violet-red colors, and metallic colors.
Grayscale colors include white, gray,
and black.
Shades include colors such as pink or Navy.
Pink is obtained from mixing red and white.
Navy is obtained from mixing blue and black.
Violet-red colors include hues and
shades of magenta. The light spectrum is
a line on which violet is one end and the other is red, and yet we see hues of
purple that connect those two colors.
A metallic color is a color that appears to be that
of a polished metal. The visual
sensation usually associated with metals is its metallic shine.
Now that I’ve talked about the nature of color, I want to
talk about how we see color.
How We See Color
We see colors thanks to photoreceptor
cells in the retinas of our eyes that transmit signals to our brains.
Rods and Cones. Light enters the human eye, where the lens
focuses the light on the retina, the layer of nerve cells in the back of
the eye. The retina is covered by millions of light-sensitive cells,
some shaped like rods and some like cones.
These receptors process the light into nerve impulses and pass them
along to the cortex of the brain via the optic nerve.
We see colors thanks to photoreceptor rods and cones in our eyes that transmit signals to our brains.
Rods work at very low levels of
light. We use these for night vision
because only a few bits of light (photons) can activate a rod. Rods don't help with color vision, which is why
at night, we see everything in a gray scale.
Cones require a lot more light, and
they are used to see color. We have
three types of cones: blue (short wavelengths), green (medium wavelengths), and
red (long wavelengths), that contain different forms of opsin - a pigment
protein - that has different sensitivities to light.
As shown in the graph below, each cone
type is able to detect a range of colors (wavelengths). Even though each
cone type is most sensitive to a specific color of light (where the line
peaks), they also can detect other colors (shown by the stretch of each
curve).
It is the overlap of the cone types,
and how the brain integrates and interprets the signals sent from them, that
allows us to see millions of different colors.
Each of the three human cone types is able to detect a range of colors.
Note: This is the so-called RGB color model,
associated with the wavelengths of visible light. The primary colors are red, green, and blue,
from which all other spectral colors can be obtained with appropriate mixing. (See
the figure at the beginning of this article.)
The traditional color theory we all learned
in school, tells us that the primary colors are red, yellow, and blue. That color theory only involves the use of
pigments in paint, and does not take into consideration the way light blends
color.
Cones are concentrated in the middle
of the retina, with fewer on the periphery.
Six million cones in each eye transmit the higher levels of light
intensity that create the sensation of color and visual sharpness.
Dimension. There is another property of color perception
called “dimension,” which has to do with the number of different cone
types.
Most humans have three cone types,
which absorb maximally in different regions of the wavelength spectrum. So, most humans are trichromats. However, eight percent of males (and an
insignificant number of females) have only two cone types. They are dichromats
(color-deficient).
The complete list of possible
dimensions, based on the number of different cone types present, is:
Monochromacy -
lack of any color perception.
Color-blind. Many mammals, such as cetaceans
(whales, dolphins, porpoises), the owl monkey, and
the Australian sea lion are monochromats.
Dichromacy - most
mammals and a quarter of color-blind humans have this color vision deficiency.
Trichromacy -
most humans. All perceptible colors can
be formed with different combinations of the primary colors: red, green, and blue.
Tetrachromacy -
most birds, reptiles, and fish.
Pentachromacy - rare in vertebrates. Some birds, notably pigeons; some lampreys
(jawless fish).
Some insects, especially bees, can see ultraviolet colors
invisible to the human eye.
Color Blindness. Color blindness doesn’t have anything to do
with how sharp our vision is or how much light we see, but it does mean the
cone cells process colors differently. It affects 1 in 12 men throughout their life,
and is much rarer in women, where only 1 in 200 will have some kind of color
deficiency. It’s often inherited
genetically through the mother but has been known to develop over time with age
or diseases like diabetes and multiple sclerosis. Color blindness can be separated into three
different categories: red-green color blindness, blue-yellow color blindness,
and the much rarer complete color blindness.
Why Individuals Don’t See the
Same Color
A lot of factors feed into how people
perceive and talk about color, from the biology of our eyes to how our brains
process that information, to the words our languages use to talk about color
categories. There’s plenty of room for
differences, all along the way.
For example, most people have three
types of light receptor cones in the eye that are optimized to detect different
wavelengths or colors of light. But
sometimes, a genetic variation can cause one type of cone to be different, or
absent altogether, leading to altered color vision. Some people are color-blind. Others may have color superpowers (see below).
Our sex can also play a role in how we
perceive color, as well as our age, and even the color of our irises. Our perception can change depending on where
we live, when we were born, and what season it is.
It would be rather surprising if there
were no variation in how we experience colors.
The number of cones in the human retina is not the same for everyone. Sometimes individuals have many cones, and
sometimes they are barely present. This
difference has been observed in so-called normal individuals who react in the
same way to color stimuli.
The fact that the number of cones in
our eyes varies considerably suggests that the brain must be able to
automatically adjust the input from the retina. So, individual variations in color perception
may not purely be a matter of the nature and number of the cones in the
retina. It can also be a result of the
fact that people with different numbers of cones calibrate the input from the
retina in different ways.
Recent studies indicate significant
variance in a gene located on the X chromosome which codes for a
protein that detects light in the long-wavelength (red/orange) regions of the
color spectrum. Since women have two
copies of the X chromosome, it is possible for them to have two different
versions of this gene, and hence it is possible for them to have a more
fine-grained ability to discriminate light in the long-wavelength regions of
the color spectrum. Women are thus
potentially able to perceive a broader spectrum of colors (color vision
superpower) in the long-wavelength regions than men.
Importance of Color to Our Lives
Today
Color symbolism serves important roles
in art, religion, politics, and ceremonials, as well as in everyday life.
The most important aspect of color in
daily life is probably the one that is least defined and most variable. It involves aesthetic and
psychological responses to color and influences art, fashion, commerce, and
even physical and emotional sensations.
One example of the link between color and emotion is the common
perception that red, orange, yellow, and brown hues
are “warm,” while the blues, greens, and grays are “cold.” The red, orange, and yellow hues are said to
induce excitement, cheerfulness, stimulation, and aggression; the blues and
greens induce security, calm, and peace; and the browns, grays, and blacks
induce sadness, depression, and melancholy.
Many psychologists believe that
analyzing an individual’s uses of and responses to color can reveal information
about the individual’s physiological and psychological condition. It has even
been suggested that specific colors can have a therapeutic effect on
physical and mental disabilities.
Although medical benefits are still in
question, color has been shown to cause definite physical and emotional
reactions in humans and in some animals.
Rooms and objects that are white or in light shades of “cool”
colors may appear to be larger than those that are in intense dark or “warm”
colors; black or very dark colors have a slimming, or shrinking, effect, as is
well known to designers and decorators.
A “cool” room decorated in a pale blue requires a higher thermostat
setting than a “warm” room painted a pale orange to achieve the same sensation
of warmth. People who view a display of
unusual colors produced by special illumination may experience
headaches and nervous disorders; tasty wholesome food served under such conditions
appears repulsive and may even induce illness.
Some colors induce a feeling of pleasure in the observer.
The effect of combinations of colors
on an observer depends not only on the individual effects of the colors but
also on the harmony of the colors combined and the composition of the
pattern. Artists and designers have been
studying the effects of colors for centuries and have developed a multitude of
theories on the uses of color. The
number and variety of these theories demonstrate that no universally accepted
rules apply; the perception of color depends on individual experience.
Colors even play a vital role in our
safety. Like the yellow school bus. Why
is it important that we see it, even in our periphery? For safety, of course. Many colors are used to depict important
safety messages without words. Red stop
signs and green traffic lights are universal.
Comments
Post a Comment