When the brain processes information sent by the eye, it
analyzes changes in information, not the actual information itself.
Though you may not know it, your eye is constantly moving, or scanning. It
notices the words on this web page because the signals from the eye change
as the eye scans the black text and the light background. If this page was
yellow text on a yellow background, you would not be able to read it because
there would be no change.
Brightness is a term
used to describe how much light is striking your retina. Something's brightness
is completely based on the difference of light intensity between an
object and the objects surrounding it. For example, a light in a dark room may
seem extremely bright. That same light outside on a sunny day will appear dim.
The eye adapts to light changes across time as well as
space. The process of the eye becoming used to a certain amount of light is
called adaptation. A dim light after a long period of time will appear
just as bright as a bright light over a long period of time.
Wow, is that Really True?
Take an unmarked pingpong ball and cut it in half. (Make sure that the half you use for this experiment is completely uniform and unmarked!) Close one eye and put your hand over it to block all light entering through your eyelid. Cover your other eye with the ping pong ball.
Have someone shine a bright light at the ping pong ball. Notice that your vision will soon recover to a stable gray. Try a brighter or dimmer light. Try a colored light. Notice that after a few seconds the ping pong ball always appears to be moderately bright gray. What's going on?
If the ping pong ball is uniform, the light striking it will scatter across the ball. No matter what you shine at the ping pong ball, everything in you visual field will have the same color and intensity. Your eye adapts to this constancy, so everything becomes neutral gray.
Often confused with brightness is the
concept of lightness. Unlike
brightness, lightness is independent of the amount of light striking your
eye. The lightness of an object is determined by the amount of light
reflecting off an object relative to the amount of light reflecting off
of objects surrounding it. A piece of grass will look dark green in the
evening sunlight. That same piece of grass will still look dark green
under the noon sun even though there is actually much more light reflected
off the blade. This is because more light is also reflected off
the the other things (more grass, trees, and buildings) so even though
your eye is receiving a stronger signal it is receiving the same relative
signal at both times.
Lightness is caused by a process
called lateral inhibition. When light
is received by a rod or cone, the cell will release a chemical that
reduces the strength of all surrounding signals. If all cells were to
receive the same amount of light, the cells would cancel out each other's
signals. The only lightness we can observe is relative lightness: a piece
of grass is green because it is more green than the objects around it.