SPECTROPHOTOMETRY AND DESCRIPTION OF COLOR IN CIE 1931 SYSTEM
|Publication Date:||1 January 1966|
|ICS Code (Colours and measurement of light):||17.180.20|
This recommended practice is concerned with the spectral characteristics of light-transmitting, -diffusing, or -reflecting nonluminescent materials: reflectance, transmittance, and color, in the CIE'2 system. Measurement of color difference is not of direct concern, but see Footnote 6. The colors of luminescent (phosphorescent or fluorescent) materials are frequently commercially important enough to require measurement; specialized techniques have been developed for this purpose, but they are beyond the scope of this recommended practice. Due to the frequent occurrence of low levels of fluorescence, and of its effect on measurements, general principles for treating this factor are included. The materials are without limitation as to composition but must be homogeneous to the unaided eye over portions large enough for measurement.
If a test specimen is homogeneous, 1 Under the standardization procedure of the Society. this recommended practice is under the jurisdiction of the ASTM Committee E-12 on Appearance of Materials. A list of members may be found in the ASTM Year Book. Accepted Sept. 20. 1966. measurements of portions of it may characterize the material of the specimen; thus, a specimen consisting of an opaque sheet of plastic may be shown to represent adequately this specific material. But if the specimen is such that measure-ment depends on its geometry (such as thickness) or if the specimen comprises two or more materials so combined that the measurement depends upon their arrangement, the measurement characterizes not a material, but the specimen itself. Thus, a specimen consisting of an incompletely hiding paint-film on a ground coat represents not a material, but simply an object made from the superposition of a particular finishing coat on a particular ground coat.
The experimental method is based upon spectrophotometry for the visible spectrum, 380 to 780 m,u. (Nanometers, nm, may be preferred to millimicrons as a unit of wavelength.) Sources of error are numerous3 and demands are very great upon the skill of the operator of the instruments used. and upon the interpretation of results obtained. Reflectance values are obtained by comparison with a previously established standard which may be calibrated by measurements of absolute reflectance. The results obtained are also frequently dependent upon the specific instrumentation employed, especially upon spectral and geometric conditions of illuminating and viewing. Thus, the method considers two classes of instrumentation, depending upon whether the separation and selection of desired wavelengths occur in the light irradiating the specimen or in the light reflected or transmitted by the specimen. For these reasons, data obtained on the same type instrument will be more comparable. Even in this case, satisfactory interlaboratory agreement will be reached only when instruments are in good working order and technique has been agreed upon. Conversely, correlations of data obtained from different types of instruments are nearly always only empirical and limited to specific instruments, and to specific data measured on identical specimens. Further, there are color problems which can be solved only by using unusual experimental conditions of illuminating and viewing. Correspondence of results with carefully determined visual evaluations can be achieved but more so with color differences than with absolute identification of specific colors. One reason for this is the variance in human response with level of illumination and with details in field of view, in addition to variation of real observers from the Standard Observer, and variation of real sources from standard sources. These factors are not explicitly treated in the CIE system.
2 Originally recommended in 1931 by the International Commission on Illumination and designated in English-speaking countries generally as the ICI system. the system is now designated by the official abbreviation CIE adopted in 1951 from the French name. Commission Internationale de I·Eclairage. The method is applied primarily to spectrophotometry. but its principles are used in visual or photoelectric colorimetry. See Reference Footnote 15 for detailed description. Significant to this practice. the CIE has recently adopted, a supplementary set of color matching functions for use in calculations related to visual color matching of fields of large angular subtense (more than 4 deg at the eye of the observer). Since the color matches on which this set of color matching functions was based were obtained from observing fields subtending 10 deg at the eye of the observer. these supplementary functions are designated x10(λ), y10(λ).
These functions are published in the Comptes Rendue. XIV Session. Brussels. Belgium. June, 1959. Vol. A. p. 95; by D. B. Judd and G. Wysecki. Color in BUsiness. Science. and Industry. 2nd Edition. John Wiley and Sons. Inc . New York. N. Y . 1963. Table 27. p. 144; by W. D. Wright. and The Measurement of Colour. 3rd Edition, D. Van Nostrand Co . Inc . Princeton. N. J . 1964. Table II. p. 281.