The theoretical requirements to be met by a photoelectric colorimeter capable of yielding I.C.I, tristimulus values, and relationships between tristimulus values and instrumental reflectances are given. For practical reasons, the correct photoelectric colorimeter must be equipped with four filters. It is commonly believed that a three-filter photoelectric colorimeter would be generally suitable for color specification and color matching; this is tested from the theoretical standpoint, and it is shown that the best physically realizable three-filter photoelectric colorimeter would suffer a fundamental chromaticity limitation. Relationships between the displacements of instrumental points on the chromaticity diagram and errors in the “red,” green, and blue reflectances are given. Especial attention is given to errors originating in the assumption that the blue reflectance may be used in the evaluation of both the X and Z tristimulus values; maximal displacements associated with this assumption are given for a number of points on the diagram, and it is shown that actual chromaticity errors would be objectionably great for many colors of importance. The use of a three-filter photoelectric colorimeter as a color comparator is discussed.
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