Color

The methods of Color Reproduction are undergoing radical transformations. All movement is towards a total digital base.

The current methods are unrecognizable to someone involved only a decade ago. In addition the there is significant growth in short-run totally digital color some at the expense of traditional color printing methods. The move from proprietary systems to open platforms is increasing.

Macs, Windows and Unix will coexist in the in the marketplace if not in the same workplace. The success of open systems is highly dependent on standards such as PostScript, TrueType etc.

Color prepress work, traditionally carried out by trained specialists will be done increasingly in small shops and even homes. Competition in color reproduction is intensifying. Because of the explosion in digital communication, one can buy “print” anywhere in the world with instant delivery.

Competition continues to drive down prices.

Fundamentals of Color

Light - Radiated when electrons in molecules move from a higher energy level to a lower one. A photon is produced to maintain conservation of energy and momentum.

If some of this radiated energy falls within the visible wavelength range it is detected by the human eye. Visible light is a small part of the electromagnetic spectrum, which includes X-rays, ultraviolet (UV), microwaves, Infrared (IR), radar and radio waves.

Visible light lies in the 400 and 700 nm range of wavelengths. X-rays and UV have shorter wavelengths and microwave, IR, radar and radio waves have longer wavelengths than visible light. Some mammals can perceive into the IR and UV range.

White light - Composed of many colors, essentially all of the visible colors in the visible range with equal intensities of each wavelength. Isaac Newton showed that white light could be split into all colors of the rainbow by passing through a prism.

Visible light appears to be split up into three predominant bands of light, red, green and blue. This results from how the human eye perceives light in terms of red, green and blue.

Thus, these are called the primary colors of light. Balanced amounts of R, G and B produce white light. Computer monitors and TV screens use RGB to display color.

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 at low levels of illumination.

The cone photoreceptors perceive the color and need higher levels of illumination than rods. Each cone contains a pigment called rhodopsin which acts as a color filter. In humans there are three versions corresponding to long (Red), medium (Green) and short (Blue) wavelengths of visible light. This is essentially why CRTs can display essentially all visible colors.

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 cone photoreceptor cells. There are about 100 neurons in the visual cortex for each photoreceptor cell. 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 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.

For the same reason blue is a good background color. 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 animations which display a rapid succession of still images, which are updated 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 as seen from the spectral responses. 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 cells dedicated to lightness), but find changes in color more objectionable (clashing colors).