Color Vision - All humans perceive color in a similar manner, but not all perceive them identically.

• Light enters the eye through the lens and is focused on to the retina. There an array of cells known as photoreceptors respond to light by specialized nerve cells, which pass the signals on to the brain.

• The light receptors are called rods and cones. Rods are not sensitive to color and work best at low levels of illumination. (An SEM of these is shown in Fig. 2-7)

• They contain a light sensitive pigment called rhodopsin (visual purple).

• The cone photoreceptors perceive the color. They need higher illumination than rods. They contain a related pigment called iodopsin which acts as a color filter.

• In humans there are three kinds of cones corresponding to long, medium and short wavelengths of visible light. These correspond to the Red, Green and Blue regions of the spectrum.

• This is essentially why CRTs can display all visible colors.

• Other animals can “see” in the UV or IR regions of the spectrum because of different cone structure.

• The impulse from the cones is processed in the ganglion cells behind the retina and the sent to the optic nerve to the visual cortex where image processing takes place.

• There are about 6 million cones and about 100 neurons in the visual cortex for each photoreceptor cell.

• The human visual system is analogous to the charge-couple devices (CCDs) used in video recorders, digital cameras and desktop scanners. They both respond to light of different wavelengths and intensities by sending out signals. They both require optical systems for focus and use red green and blue filters to analyze the image.

• The analogy ends at this point since the human visual system is dynamic, more than just a photographic image of the world. This dynamic response is more in analogy with the method of compression of motion pictures by encoding only the differences between successive frames.

• The response of the rod and cone cells is not uniform. The response in monotonic but is closer to logarithmic than linear.

• There is considerable overlap between the responses of the red, green and blue cones, which causes some problems in color reproduction. (See figure below.)

• The cones are not evenly distributed being in the ratio of roughly 40:20:1 for RGB respectively. The green cones are most sensitive and the blue the least.

• Thus, it is easier to discriminate between colors in the red-yellow-green-cyan regions of the spectrum than in the blue region.

• Thus blue should be avoided in text and other graphic elements where recognition is very important.

• There is a .4 second delay in response from the time the image falls on the retina. The sensation may persist for up to 2 minutes. This after image effect is exploited by film, TV and computer monitors which display a rapid succession of still images, which are refreshed before the perceptual image has decayed.

• About one in ten men and one in a hundred women experience some form of divergent color perception. The most common is confusion between red and green. This can cause some problems in color matching.

• The ability to discriminate between different colors is crucial to color reproduction. It makes us sensitive to colors which are similar but not identical, especially to neutral colors such as white and grays or near grays.

• Thus, we are likely to notice slight differences between near-neutrals than between intense saturated colors. We are more sensitive to changes in lightness (owing to a larger number of rods dedicated to lightness), but find changes in color more objectionable (i.e., clashing colors).