Page:NBS Circular 553.djvu/16

 These constant-value charts show at a glance for all Munsell chromas what ranges of Munsell hue are referred to by the various hue names of this system; this information is of course obtainable, though less conveniently, from the color-name charts themselves.

6. Color Designations From the Munsell Book of Color for Diverse Types of Specimens

The procedure for comparing a specimen with the Munsell Book of Color depends importantly on specimen type, whether the specimen is solid or liquid, opaque or transparent, large or small, flat or of irregular shape, and so on.

6.1. Dry Opaque Powders (Grain Size Less Than 1 mm)

6.1.1. Preparation of Sample. The sample is placed slightly heaped up in a clean holder (see fig. 3) at least 2 mm deep. Over this is placed a clear- colorless cover-glass about 1 mm in thickness which is pressed down with a rotary motion and held hr place by friction between the holder and the cover- glass mount.

For very fine powders, the pressure exerted upon the sample by the holder becomes critical and may require that a special holder be designed. In the measurement of the colors of cosmetic powders, for instance, color changes many times greater than the allowable tolerances may result from inadvertent pressure on the sample.

6.1.2. Lighting and Viewing Conditions. Since the color designations conform to usage only for specimens viewed in daylight, this source is to be used in the color-comparison work. A table placed by a window so that light reaches the table top from the operator’s left or right chiefly from the sky and chiefly at angles centering on 45° from the horizontal is recommended. A north window is best because no special precautions are usually required to elimi- nate direct sunlight. A canopy of black cloth, preferably black velvet, should be hung above the sample on the side opposite the operator hi such a position as to be imaged in the mirror surface of the cover glass; such an arrangement eliminates errors from unwanted admixture of ceiling light reflected from the cover glass. The sample and standard placed on the table top are viewed nearly along the perpendicular to the surfaces, that is, just enough off perpendicular to avoid having the operator’s face mirrored in the cover glass. Illumination at 45° and perpendicular viewing are recommended by the CIE [53]. A skylight or source of artificial daylight (fig. 4) located above the sample may also be used, but in such an arrangement the angle of view should be approximately at 45° from the horizontal, and the black cloth should be hung vertically beside the sample opposite the observer. Perpendicular illu- mination with viewing at 45° gives results equivalent to the recommended CIE method.

It is important that the illumination of sample and working standards be closely the same both in amount and quality; otherwise different Munsell notations will be found by interchanging them. Even with closely uniform illumination it is good practice to make this interchange as a check during the comparison.

In any computations involving the spectral energy distribution of the source, that of standard source