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Nov., 1931] GLOSSARY OF TECHNICAL TERMS 821
more desirable transmission characteristic at all frequencies within a certain range.
Back focus. — The distance from the second principal focus of a lens to the vertex of the nearest refracting surface.
Beat. — A complete cycle of amplitude variations resulting from the interference of two or more periodic waves of different frequencies.
Bel. — The bel is the unit of measure of the ratio of two amounts of power, the number of bels corresponding to any given ratio of two amounts of power being the common logarithm of this ratio.
Blooping patch. — A black section, approximately triangular in shape, introduced over a splice on a positive sound track to prevent noise which the splice would otherwise cause during reproduction. The patch effects a gradual diminution of the light transmitted through the sound track, followed by gradual restoration to the original value. The patch may be applied with black lacquer or may be a triangle of black paper or film cemented on the track. The same result can be accomplished by punching a triangular hole in the negative before printing.
Brightness. — The luminous intensity (candle power) per unit of projected area of a radiating or reflecting surface, the projection being made on a plane normal to the direction in which the brightness is observed.
Calibrate. — To determine the correct scale intervals for any measuring device or to ascertain the errors of the scale, such as marking the focusing scale of a camera lens, or determining the errors in the readings of a thermometer.
Camera angle. — The angle of view subtended at the lens by the portion of the subject included within the picture area. It usually refers to the horizontal angle.
Candle (international). — The unit of luminous intensity. The standard candle has been maintained since 1909 by means of incandescent lamps at the National Standardizing Laboratories of France, Great Britain, and the United States.
Candle power. — The luminous intensity of a source, expressed in candles.
Change-over. — In projection, the act of changing from one projector to another, preferably without interrupting the continuity of projection.
Channel. — (See Recording channel.)