The New CHROMATICITY DIAGRAM for Research provides a convenient method of describing the capability of various image reproduction systems to excite the human eye.

Beginning with the CHROMATICITY DIAGRAM, it is important to note that the colors used to illustrate the diagram on this site are not true. No display device or conventional "process" printing technique can generate the full range of colors perceivable by the human eye. To properly portray the entire chromatic range of the eye on a cathode ray tube type of monitor requires that one of the phosphors have a peak irradiance near 400 nm., a second have a peak irradiance near 655 nm. and the third have a peak near 532 nm.

To print a test sample using conventional process color that would properly represent the entire perceptual spectrum of the eye would be quite difficult. High quality printing designed to reproduce paintings typically employs six different inks instead of the conventional three colored and one black inks of CYMK printing. These inks include a violet and a red along with the conventional cyan, yellow, magenta and black. To reproduce a particular color properly, recourse is frequently made to what is called "spot color." Spot color printing introduces an additional ink with the specific spectral characteristics required to reproduce a particular color such as in a trademarked logo.

The following figure describes conceptually the color reproduction capability of various systems in common use. The specific wavelengths achievable by a particular monitor depend on the particular phosphors used.

It should be noted that the phosphors used in conventional television sets are different in the United States and in Europe. Similarly, industrial and computer monitors may use specialized phosphors. It is critically important in vision research that the specific phosphors used in experiments be noted.

As in the case of monitors, the chromatic range achievable by a particular process color press depends on the specific inks used. In the case of both monitors and presses, the capability is generally less than that which the eye can perceive.

Recently, great efforts have been made to standardize the color reproduction capability of computer generated color imagery, whether destined for a display device or a printed page. This is a difficult problem. The printing industry has proceeded in one direction and the display device based industry has proceeded in another.

The activity of the Microsoft Corporation, and many standardization organizations that have chosen to follow their lead, must be noted. Until recently, it has been conventional to assign digital values associated with each of the chromatic outputs of a cathode ray tube monitor ranging from zero to 255 (0 to FF in hexadecimal notation), with the higher number associated with full saturation of the output in that color. Using the conventional sequence of defining colors as RGB and the convention of additive color mixing, white is defined as the hexadecimal value FFFFFF. Because this convention has produced inappropriate amounts of green in many pictures, Microsoft has defined saturated green as an intensity level of 80 in hexadecimal or 128 in decimal. They have assigned the name "lime" to the color corresponding to the value of FF, or full saturation, in the green channel. This procedure is somewhat counterproductive in that it ties the reproductive capability of most computer programs to a limited range of vision represented by three unspecified phosphors. In the long term, it would be better to associate the computer values associated with a given color to include the extreme perceptual capabilities of the human eye.

As a result of the different approaches taken by the printing and the display device oriented industries, great care must be taken in vision research to understand what color palletes are used by a given set of equipment.