Graphics Reference
In-Depth Information
Figure 8.1.
A horizontal cross-section
of the eye.
been made relatively recently. Even today there is no complete answer. One needs to
know more about the eye and the brain.
Some objects like the sun and a burning ember radiate light and are said to be
luminous
. Most objects that we see are
nonluminous
. We see them because light from
some other source has reflected off them. Here are the basic events that happen when
we look at an object ([AgoG87]). See Figure 8.1.
(1) The object's light enters the eye through the
cornea
.
(2) It passes through a clear liquid (
aqueous humor
), the
pupil
, the
lens
, and some
jelly-like material (
vitreous humor
) before it falls on the
retina
.
(3) In the retina it passes through several layers of
nerve cells
before it is absorbed
by
receptor cells
. There are two types of receptor cells, called
rods
and
cones
.
There are three varieties of cones.
(4) The absorption causes some chemical changes leading to electrical changes
that are then transmitted by
optic nerve
fibers from the eye to the opposite
half of the brain.
(5) Finally, the brain responds by producing various sensations such as color, size,
position, etc.
For an overview of the basic ingredients of the human visual processing system see
[Ferw01].
We return to our original question: What is color? There are two possible answers:
(1) It is a sensation
produced
by the brain (the “perceived” color) in response to
light received at the retina, so that one would say, for example, “the color pro-
duced by some given light is red or whatever.”
(2) It is an
arbitrary
definition by specialists in colorimetry, the measure of color.
The next two sections attempt to explain these answers. We rely heavily on [AgoG87].
8.3
Perceived Color
Because a color is often influenced by surrounding colors, how can one judge a color
from a nonluminous object in isolation? One way that this can be done is to illumi-