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13 RHEOSTAT CALIBRATE POSITION KEASUnlNG PO SITIO N FIG. 1 (a) FIG. 1 (b) In operation, the sample to be measured is placed under the port and the lamp house placed as shown in Fig, 1(a)* The rheostat is then adjusted to produce a full scale or 100 reading on the meter. The lamp house is then swivelled around to the position shown in Fig. 1(b)* The meter then reads per-cent reflectance directly. Two things are important when such a device is used for calibration. The cells must have the same color response as the image orthicon and the results obtained will only hold good under conditions of flat light. For the readings to be interpreted, they must be related to the standard RETKA gray scale and this has been done on the attached chart. The gray scales on the lithographed studio test pattern should not be used as a reference as they are somewhat inaccurate* Accurately calibrated sets of four small gray scales are available from RETKA and may be cemented over those on the test pattern. THE MUNSELL SYSTEM The reflectometer is a valuable device for checking samples of costumes, drapes and other properties but the problems of calibrating the thousands of color combinations possible with paint mixes becomes a difficult one. Various systems for the description and cataloging of color have been devised such as the Munsell system, the Ostwald system and others. Ihe most useful one from the TV viewpoint is the Munsell system, where the relationship to the reflectance is obtainable. In the Munsell system color is defined in three ways. (1) Hue - which is the visible color such as red, green, blue, etc. and is designated R, G, B, etc, (2) Value - which is the relative brightness of samples and may be translated into reflectance. It is designated by numbers from one to ten written thus 1/ 7/ 9/ and so on. The gradations in value represent equal visual differences.