1. Ch. 3 The Human Color-Imaging System
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.
Fig. 3.1 An original scene and its reproduction, where both are viewed and evaluated by a human observer.
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.

2. Colorimeters directly measure CIEXYZ tristimulus values of color stimuli, so a colorimeter would certainly seem an obvious choice as a substitute for an observer. It could be used to measure the XYZ values of both the original and the reproduction.
Fig 3.2 An original scene and its reproduction. Both are measured by a colorimeter, and the resulting colorimetric values are compared.

3. Fig. 3. 3 shows the human visual system in terms of those functions
Fig. 3.3 shows the human visual system in terms of those functions. The image capture process is performed by the trichromatic responses of the eye. The image formation process corresponds to the formation of a perception of a color image in the observer’s mind. The formation of this mental image is influenced by intermediary visual signal process, which can be broadly divided into two types psychological and psychophysical.
Fig. 3.3 The human visual system can be described in terms of the same basic functions used to describe all other color-imaging systems.

4. Psychological signal processing includes effects due to color memory, which generally is not colorimetrically accurate. It also includes color preference, which can be different from color memory, and various cognitive effects that cause the observer to perceive colors somewhat according to expectation and experience.
Psychophysical signal processing includes a variety of effects that are due to both physiological and mental processes. These effects result from various forms of visual adaptation.

5. Adaptation
Adaptation refers to the process by which the visual mechanism adjusts to the conditions under which the eyes are exposed to radiant energy.
General-brightness adaptation refers to the adjustments of the visual mechanism in response to the overall level of the stimulus to which the eyes are exposed. For example, when the eyes are exposed for a sufficient length of time to a low level of illumination, the visual receptors compensate by becoming relatively more sensitive.

6. Lateral adaptation refers to changes induced in the sensitivities of adjacent areas of the retina. For example, the sensitivity of a particular retinal receptor may be increased or decreased depending on the amount of light being received by neighboring receptors. Lateral effects help the visual system to discriminate objects by making their edges more apparent. Another manifestation of lateral-brightness adaptation is that the apparent luminance contrast of an image is lowered when areas immediately surrounding the image are relatively dark.

8. Chromatic adaptation refers to adjustments of the visual mechanism in response to the chromaticity of the stimulus to which the eyes are exposed. For example, when exposed sufficiently long to a reddish-yellow stimulus, such as a tungsten light, the eye’s longer-wavelength sensitive receptors become somewhat desensitized and its shorter-wavelength receptors become relatively more sensitive