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The study of the smoothest reflectance functions under a given illuminant is extended to the domain outside the principal domain. The functions are identically zero or unity on certain intervals determined by their tristimulus values. The domains of validity are obtained. There result 12 additional types of reflectance function. An algorithm for the generation of the most important ones from a practical point of view is described.
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C. van Trigt (1990) studied this question.
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