In the fundamental colorimetry of lights and objects a single standard is used for each class of specimen. Opaque surfaces are referred to the ideal perfect diffuser, or to physically realizable near-perfect diffusers, such as a sufficiently thick layer of magnesium oxide deposited from the smoke of magnesium turnings or ribbon [108] or layers of barium sulfate [141, 6]. Transparent objects, such as gelatin films and crystal or glass plates, are referred to the equivalent thickness of air; transparent solutions, to the equivalent thickness of distilled water or solvent. Self-luminous objects, such as fluorescent lamps, cathode-ray tubes, and incandescent lamps, are measured relative to one of the standard sources, usually source A [19]. The colors of specimens closely resembling the respective standards can be evaluated quite precisely and accurately; those differing radically in spectral composition, only with relative uncertainty. That is, near-white specimens, nearly clear glass plates, and incandescent lamps nearly equivalent to source A present the simplest colorimetric problem; highly selective absorbers and emitters, like the rare-earth glasses and gaseous discharge tubes, present difficult measurement problems. In general, the greater the deviation in spectral composition between the unknown specimen and the standard, the greater the uncertainty of the result obtained by a visual or a photoelectric colorimeter.
Automatic spectrophotometry has greatly extended the application of both visual and photo-electric colorimetry. It has supplied a practical way to calibrate working standards of color. If a fairly large group of specimens is at hand to be measured, say twenty or more, all of similar spectral composition, the most satisfactory way to measure them in the present state of colorimetric science is to evaluate one or two of them carefully by means of the spectrophotometer to serve as working standards, then obtain the color specifications of the remainder by visual or photoelectric determination of the difference between specimen and standard.
In the interpretation of the importance of chromaticity differences based upon separation of the points representing the two chromaticities in the ()-diagram a warning is necessary. This diagram is considerably expanded in the green portion relative to the other portions, much as the Mercator projection of the earth's surface is expanded near the poles. Thus, two points separated by a given distance in the green portion of this diagram correspond to chromaticities that are
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Fig. 7.Chromaticity spacing in the (x,y)-diagram.Between any center and any point on the corresponding ellipse there are approximately 100 just noticeable chromaticity steps [67].
much harder to distinguish under ordinary viewing conditions than two chromaticities separated by the same amount in other portions of the diagram. Furthermore, the bluish purple portion of the diagram is correspondingly compressed. The system of ellipses shown on figure 7 serves to indicate approximately the metric properties of the () -diagram. Under moderately good observing conditions, the distances from the central point of each ellipse to any point on its boundary correspond approximately to one hundred times the chromaticity difference just perceptible with certainty. These ellipses were drawn from a review of the literature in 1936 [66, 67], and subsequent extensive work published by Wright [160, 161] and by MacAdam [92, 94] has corroborated the essential correctness of the indicated chromaticity spacing. Figure 7 not only indicates the extent to which the green portion of the diagram is expanded, and the bluish purple compressed, but also indicates that, in general, the chromatic importance of a distance on the ()-diagram is a function both of the position of the central point and the direction of the deviation from it.
When sets of primaries other than those of the CIE standard observer system are expressed by transformations of the form of eq (3) , the chromaticity spacings in the resulting Maxwell triangle may be made to vary widely. There have been several attempts to select primary sets that yield uniform chromaticness scales in which the chromaticity spacing corresponds to perceptibility [14, 55, 66, 90, 144]. The transformation equations for the chromaticity coordinates, r,g, of the uniform-
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