International projectionist (Jan-Dec 1950)

Record Details:

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• LOOKING BACK-TO THE FUTURE* " This department, a regular IP feature, is a review of fundamental technical data. The Language of Lighting i THE use of the candle in the definition of light units is a natural outcome of the fact that measurements of light were first seriously undertaken at the time when the newer light sources began to replace the candle. A similar situation led to the introduction of the term "horsepower" when steam engines began to replace the horse. It was soon found that in order to use a candle as a standard, it had to be made according to strict specifications regarding size and ingredients, and burned under prescribed conditions. The light in a horizontal direction would then have a certain intensity which could be taken as a standard. A value for the standard of luminous intensity was established in 1909 by an agreement effected among the three National Standardizing Laboratories of France, Great Britain, and the United States, and is now maintained in these laboratories by means of incandescent lamps. Luminous Intensity in C.P. This unit may be used to specify the luminous intensity in candlepower of any light source in a given direction and represents the light density in that direction. However, the candle-power measured in one direction gives no indication of the FIGURE 1 total amount of light produced by the illuminant. Candlepower read in one direction is analogous to the depth of a pool of water at one given point — a measurement which is useful for certain purposes, but which is of no value in determining the total quantity of water in the pool. Just as it is necessary to know the dimensions of a pool and the depths at all points before its total contents can be established, so it is necessary to know the candlepower of an illuminant in all directions before its total light output can be determined. This fact that candlepower in one direction does not indicate total amount of indicated in Fig. 2. In A the candlepower of light radiating in only one direction is measured. When a number ot readings are taken at uniform intervals in a horizontal plane, as indicated in B, and then averaged, the result is the mean horizontal candlepower of the light source. Instead of taking a large number of individual readings, this result is obtained in ordinary practice by rotating the illuminant rapidly about its vertical axis while a single reading is taken. The intensity of light in all directions may be A — Horizontal candlepower -Mean horizontal candlepower C — Mean spherical candlepower FIGURE 2. MEASUREMENT OF CANDLEPOWER light produced is illustrated in Fig. 1. In each instance the candlepower in the direction of P is measured by means of an instrument known as a photometer. The reading obtained from the bare candle as shown in A is one candlepower. Photometric Readings In B the same candle is surrounded by a sphere having a moderately large opening. Assuming that none of the light rays are reflected from the inside walls of the sphere, the photometer still will indicate one candlepower despite the fact that a large portion of the total light from the candle has been absorbed. In C a sphere with a much smaller opening is illustrated; and still more of the light is consumed by the sphere; but even in this case the light in the direction of the photometer is 1 candlepower. In fact, the reading will be 1 candlepower irrespective of the size of the opening and regardless of light allowed to be emitted, provided the direct rays from the candle to the photometer are not obstructed. Three ways in which candlepower measurements are ordinarily made are ascertained as indicated in C. by measuring the candlepower of uniform intervals around the light source. An average of these readings will give the mean spherical candlepower of the illuminant. In the past, it was quite common to rate light sources in terms of this unit, since it is directly related to the total light output of the lamp. At the present time, however, a unit known as the lumen is much more commonly used for this purpose. The Term 'Lumen' Light is not a concrete object which may be weighed or touched, but is rather a form of energy in motion. For this .eason, an amount of light cannot be strictly measured in the usual manner of measuring quantities. But light (more correctly, light flux) coming from a source may be considered to do so at a constant rate of speed. Therefore, for all practical photometric measurements, the element of time may be neglected, and light may be considered as a definite quantity. The unit of this light flux or light quantity is the lumen. It may be defined INTERNATIONAL PROJECTIONIST January 1950 15