handprint : the geometry of color perception
The previous pages have explained three very different methods for defining a color: (1) the measurement of the color stimulus as a spectral emittance or reflectance curve in colorimetry; (2) the proportional responses to the stimulus by the L, M and S cones, represented as a chromaticity diagram; and (3) the subjective description of the color sensation in terms of lightness, hue and hue purity, the three colormaking attributes. These methods focus on the physical stimulus, receptor outputs or color sensation, respectively.
This page describes how these different aspects of color perception fit together. For example, how is the physical stimulus (luminance) related to a measure of sensation (brightness)? How can we show that red is more similar to blue than to green? Does the brightness of a color change as its chroma increases?
Mapping the quantities of a physical stimulus onto color sensations is called psychophysics, and it was the earliest form of color specification. This geometrical approach was innovated by the 17th century naturalist Isaac Newton, who summarized his experiments in light and pigment mixing as a hue circle, the first geometrical model of color perception. This evolved into the painters' many color wheels and is incorporated into all modern color models.
