Digital Color Management: Encoding Solutions, 2nd Edition

3

The Human Color-Imaging System

In Chapter 2, color-imaging systems were described in terms of the three basic functions they must perform. In this chapter, those three functions will be used to examine the most important color-imaging system of all: the visual system of the human observer. In this discussion, and throughout the book, the visual system will be dealt with in the context shown in Figure 3.1.

The figure is meant to illustrate that both the original and its reproduction are to be viewed, and that a human observer will judge the color quality of the reproduction. This is a common situation, and it conforms to the basic rule established in the introduction to this section, but it does not seem like a very scientific method of assessing color quality. Would it not be better to replace the observer with some type of objective measurement instrument? And, if so, what kind of instrument should be used?

Colorimeters directly measure CIE XYZ tristimulus values of color stimuli, so a colorimeter would certainly seem an obvious choice as a substitute for the observer. It could be used to measure the XYZ values of both the original and the reproduction (Figure 3.2). The quality of the reproduction then could be quantified in terms of how well its XYZ values (or other colorimetric values, such as CIELAB L^*a^*b^* values, derived from the XYZ values) match those of the original.

As logical as this approach might seem, it is fundamentally flawed. To understand why, one has to consider ...