Transactions of the Society of Motion Picture Engineers (1916)

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a fair degree of accuracy. It is very simple in operation, so light that it can be easily carried about, and so small that readings can be taken in very restricted spaces. The instrument is shown in Fig. 9. In operation, it is placed upon or adjacent to the surface on which a measurement of the foot-candle intensity is desired. A lamp within the box illuminates the under side of the photometric screen to a much higher intensity at one end than at the other. The illumination which it is desired to measure is, of course, practically uniform over the entire scale. Closely spaced translucent dots, which serve the same purpose as the grease spot in the simple bar photometer, line the scale from end to end. If the illumination on the scale from the outside falls within the measuring limits of the meter (0.5 — 25 foot-candles) the spots will appear brighter at one end of the scale than at the other, and at the point where the spots are neither brighter nor darker than the white paper scale the illuminations from within and from without are equal. The scale is accurately calibrated with the lamp within the Fig. 9 Foot Candle Meter box burning at a certain definite voltage. A voltmeter and rheostat permit the operator to adjust the lamp voltage to that at which the instrument was originally calibrated. The energy is supplied from small dry cells. This instrument is proving very serviceable for "checking up" installations to insure, for example, that the illumination is ample when the projection equipment is in first-class shape and to see that it is not allowed to fall below a desirable value due to improper care and attention being given to the system. This instrument is of special value in securing data which can be applied in the designing of future installations. The three units which have been discussed at length, the candle, the lumen and the foot-candle, will find wider and wider usage in the motion picture industry as their meanings become better understood. A discussion of light sources, for example, involves a discussion of intrinsic brilliancy (which means candle-power per square inch of luminous area) for upon the intrinsic brilliancy of a source depends in large measure its effectiveness in light projection. It involves a dis 83