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Chapter 1:
Tristimulus Specification
Excerpt Colorimetry is the method of measuring and evaluating colors of objects. The term “color” is defined as an attribute of visual perception consisting of any combination of chromatic and achromatic contents. This visual attribute has three components: it can be described by chromatic color names such as red, pink, orange, yellow, brown, green, blue, purple, etc.; or by achromatic color names such as white, gray, black, etc.; and qualified by bright, dim, light, dark, etc., or by combinations of such names. The Commission Internationale de l'Éclairage (CIE) was the driving force behind the development of colorimetry. This international organization is responsible for defining and specifying colorimetry via a series of CIE Publications. The foundation of colorimetry is the human visual color sensibility, illuminant sources, and spectral measurements that are described in the context of a color space. The backbone of colorimetry is the tristimulus specification. 1.1 Definitions of CIE Tristimulus Values The trichromatic nature of human color vision is mathematically formulated by CIE to give tristmulus values X, Y, and Z. The CIE method of colorimetric specification is based on the rules of color matching by additive color mixture. The principles of additive color mixing are known as the Grassmann's laws of color mixtures: (1) Three independent variables are necessary and sufficient for specifying a color mixture. (2) Stimuli, evoking the same color appearance, produce identical results in additive color mixtures, regardless of their spectral compositions. (3) If one component of a color mixture changes, the color of the mixture changes in a corresponding manner. The first law establishes what is called “trichromacy”—that all colors can be matched by a suitable mixture of three different stimuli under the constraint that none of them may be matched in color by any mixture of the others. If one stimulus is matched by the other two, then the stimulus is no longer independent from the other two. The second law means that stimuli with different spectral radiance distributions may provide the same color match.
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