Colors and the perception of colors
Visible light is only a “small member” of the “family” of electromagnetic
(EM) waves. The wavelengths of EM waves that we can observe using
many different devices span from tens of kilometers (long radio waves)
to picometers (gamma rays, i.e., EM radiation produced by radioactive
nuclei).
The range of wavelengths
we can see by our eyes is
relatively narrow, spanning
from about 700 nm (red color to about 400 nm (violet
light). If light is a “mixture”
of all wavelengths from this
range, we see it as white.
White light can be split into
constituent wavelengths
(or colors) using a prism or
a grating.
We can see colors because the retina – i.e.,
the light-sensitive organ in our eyes – contains photosensitive cell (called “cones”) of
three types: one is most sensitive to red
light, another to green light, and the third
type to blue light.
Most of the cones
are located in the
retina’s central
area (the macula).
In addition to the
cones, in retina
there are also
cells called “rods”.
Many rods are
located in the macula, and even
more in the
peripheral regions
of the retina.
Rods and cones in the retina:
Rods are the “night-vision” cells. They are activated in low light conditions.
However, thy are not sensitive to colors. Therefore, a landscape viewed in
moonlight seems gray. Actually, the colors are the same as in daylight.
We cannot see
them, but
cameras can!
This picture
was taken at
10 pm in February 2007, at
full moonlight,
using a very
long exposure
time. Tiny
Specs In the
sky are stars.
Another picture takes the same night as that in the preceding slide. The
peak right from the center is Mary’s Peak. The spots in the lower part are
bright windows of residential houses.
What causes the well-known “red-eye effect” in flash pictures?
This is nothing else than the color of the retina! The retina needs
much blood, which is supplied to it by a dense web of tiny blood
vessels. Therefore, the red color.
However, there is no “red-eye” effect in flash pictures of many animals.
In contrast, their eyes seem to “backreflect” the flash. This is the same
effects as the “eyeshine” you can see if you drive on a rural highway in
the night, and a cat or a dog caught in your headlights looks toward the
approaching car.
The “eyeshine effect” is caused by an
extra layer of special tissue called
tapetum lucidum that is located behind
the retina of many animals – especially,
nocturnal animals. The tapetum lucidum
act as a backreflecting mirror.
Most primates – i.e., members
of the biological order we belong
to – do not have tapetum lucidum.
Lemurs – small “cat-like” nocturnal
primates, unique to the island of
Madagascar, are an exception.
Humans and diurnal animals do not have
tapetum lucidum in their eyes. Most bird
species do not have it – owls are an exception. But nocturnal animals – carnivores in particular -- need to see well their
prey in low light condition, and tapetum
lucidum does enhance their nightime
vision – can you explain how? (if you
missed the lecture at which we talked
about that, you may find an explanation
in this webpage).
The sensitivity of the three types of human retina cones to light of
different wavelengths from the visible region. Note that there is
another smaller maximum for the “red” cones in the violet region.
It causes that violet light looks “somewhat reddish” to us.
From the curves one can read the relative strength of the signal passed to
the brain for a given wavelength. A triad of such numbers is called the
tristimulus values.
The RGB color scheme – fundamentals: Let’s take three
pure (monochromatic) colors corresponding to the
maximum sensitivity of the three cone types:
600 nm
546 nm
When projected on one screen, the area where all
three colors merge appears white. The fusion of red
and blue produces magenta, blue and green – cyan,
and green and red – yellow. One can also say:
red + cyan = white, blue + yellow = white, green
+ magenta = white. Such pairs: red-cyan, blue-yellow,
green-magenta are called complementary colors
(they are not the only complementary color pairs).
436 nm
The preceding slide showed the results of mixing (or adding) primary colors, each of which had 100% intensity.
But one can add colors with any intensity proportions!
(e.g., 80% red, 45% green, and 23% blue). This is the
idea of a color scheme, known as RGB, widely used in
computer graphics.
However, in the practical RGB scheme, instead of
0 – 100% scale, one uses a 0-255 scale (corresponding
to a Byte, i.e., a binary number of 8 Bits). So, for instance,
80% translates to 255x(80/100) = 204. The color in the
above example is then encoded as 204, 115, 59.
The best way of demonstrating how it works is to use
one of the many available Web on-line RGB generators.
Link to an on-line generator of RGB color schemes
In the simple figure with the mixing of the three primary colors
(a.k.a. the “primes” -- red, green and blue) there are only eight
colors altogether (the three primes, three complementary to the
primes, white, and black. In the RGB scheme white is 255, 255, 255,
black is 0, 0, 0, cyan is 0, 255, 255, and so on. But since we can
use the 0-255 scale, the total number of available colors is (256)3
= 16.7 million!
However, RGB is only one of the possible schemes of describing
colors. Another, no less popular, is the one called “HSB” or “HSV”,
where H stands for “Hue”, S for “Saturation”, and B for “brightness”
(or V for “Value”). The idea of the HSB (HSV) scheme is explained
in the next slide.
Begin with the familiar figure:
Next, arrange the three “primes”
in a circle. Then, insert
the complementary
colors in between.
Next, merge the
primes with the
adjacent complementary colors to
obtain more “pie
chunks” (red plus
yellow yield orange,
and so on) – keep
doing that until you
get a continuous distribution
of colors. Such a figure is
called “the RGB Color Wheel”.
Now, in order to describe the colors, one has to introduce a numerical scale. Most often a 0 – 360° scale
is used, with 0 at the top red, and the angle incrementing
clockwise. So, e.g., yellow is 60°, pure green 120°, cyan
180°, and so on. And this angular value is called “the hue” of a given color.
Saturation: at the center
of the color wheel, all
colors merge to form
white. So, the closer to
the center you go, the
more white component
is “admixed” to the color –
or, one can say, the more
The color is “de-saturated”.
Colors on the rim have 100%
saturation, white = 0% saturation.
Brightnes (or Value):
you can think of it as
Brightness, in the HSB color scheme:
an “admixture of gray”
to your color described
by H and S. 0% is total
0%
100%
blackness. It’s an analog of the brightness in
Brightness, in grayscale (“black and white”) scheme:
grayscale graphics
(“black and white
pictures”) for a color
100%
scale.
0%
RGB to HSB on-line converter
Fancy on-line HSB “color picker” with convenient mouse-operated
number inputs
RGB  HSB generator/converter
A sophisticated HSB tool, with the names of colors created – the
angle changes counterclockwise