Colorimetry[1]
I. Nimeroff
National Bureau of Standards
Washington, D.C. 20234
The definition of color, as a characteristic of light, and the basic principles of its measurement are given. The reduction of spectrophotometric data to three chromaticity coordinates by means of the three-function CIE standard observer system for colorimetry is described. Various methods of direct colorimetry, and visual and photoelectric methods of colorimetry by differences from material standards are treated. The most useful collections of material color standards are described and the most widely used one-dimensional color scales are explained. The limitations of several colorimetric methods: spectrophotometric, photoelectric, and visual, are discussed.
| Key Words: | Color dictionaries, color vision, colorimeters, photoelectric tristimulus, spectrophotometric colorimetry, visual colorimetry. |
It is common practice to regard color as a property of objects, and in a limited sense this view is justified. We have color comparators for solutions in which the color is taken as an index of the composition of the solution; and in applying a suitably prepared set of color standards in a color comparator, the color of the unknown behaves as if it were a property of the solution itself, just as the concentrations of the constituents which it indicates. However, this document deals with color for its own sake; and, for this purpose, a broader view is useful. Lights have colors as well as objects. The flame of a Bunsen burner can be changed from bluish purple to orange by the introduction of sodium. And since even objects lose their colors and become invisible unless they reflect, scatter, or transmit radiant energy, or form a part of an illuminated scene, this broader view is that color is a property of light, and of light alone.
As it is possible to measure with a spectrophotometer the spectral energy distribution of any light beam, and as the color oi a light correlates closely with its spectral composition, some of the more physically minded people have contended that color is a physical property of radiant energy; but this is not the most useful view. The color change of the Bunsen flame from bluish purple to orange can be shown by a purely physical measurement to be caused by a change in the spectral composition of the emitted energy, but it takes more than physics to decide whether this flame has the same color as the light reflected from the peel of a given citrus fruit. Application of the spectrophotometer to the orange peel will show that the spectral composition of the light reflected from it under daylight illumination is radically different from that emitted by the sodium flame. It has a continuous spectrum relatively strong in the long-wave (550 to 770 nm)[2] portion of the visible spectrum (380 to 770 nm) . The visible energy of the sodium flame is nearly all confined to two narrow bands (589.0 and 589.6 nm) . Physically, therefore, the two lights are different, but they have closely the same color. The two lights must therefore be identical in some other respect. This identity consists in some aspect of the response made by a normal observer to the sodium flame being the same as the corresponding aspect of the response to the peel of the citrus fruit. The broader view of color must, therefore, include not only the spectral composition of the radiant energy reaching the eye of the observer, but the properties of the observer as well. These properties have been evaluated by finding equivalent stimuli, like the energy of the sodium flame and that reflected from an orange peel, which have different spectral compositions but still manage to stimulate the same color response to the normal observer. Such equivalent stimuli are called metamers. In this chapter there will be presented the standard method for
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