Data visualization: the end of the rainbow

A rainbow colormap is often supplied as a default in visualization software. In this kind of colormap, red is mapped to the highest data value, blue to the lowest, and the other data values are interpolated along the full extent of the spectrum. But there is more to color than meets the eye. Color is a perceptual as well as physical phenomenon. What is commonly called color-hue-is only one of three parameters. Another is the brightness of the signal-intensity. The third is the admixture of white -saturation. Change any one parameter enough, and the color looks different. (The hue-intensity-saturation model of color is one of a several explored through the years, and captures some of the basic characteristics of basic color perception.) To make matters worse, the parameters' relationship to what is perceived is nonlinear. At the same intensity, for example, yellow appears brighter than blue. Some of the perceptual principles involved have been implemented in software developed at IBM Corp.'s Thomas J. Watson Research Center, Yorktown Heights, NY. The module runs with IBM's visualization package Data Explorer and is called Pravda (for perceptual rule based architecture for visualizing data accurately).