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546
HlGGONS
November
high-intensity artificial-light sources. Operating under these highintensity-light conditions, the human eye is incapable of any reasonable accuracy. Quite often, the light levels are far beyond the illumination levels with which the eye is commonly familiar, in fact, the eye actually may approach the point of paralysis at these extreme levels. Therefore, the result is a complete or partial failure of the first two methods of exposure determination.
The use of an illumination meter or exposure meter thus becomes a necessity in order to obtain correct photographic exposures. To meet the above requirement, a direct-reading photoelectric illumination meter has been designed, the development and construction of which is the principal purpose of this paper. (See Fig. 1.)
A rather dependable source of high-intensity illumination can be obtained from the General Electric 750watt, 115-volt intermittent-service photographic lamps. At 18 inches, this lamp produces an average value of 16,000 foot-candles of light in an 8-inch circle. However, when using a single lamp of this type, the 8-inch pattern of light is not completely uniform throughout its entire area and hot spots result from the filament pattern. When very small areas are to be studied with the camera, it is, therefore, desirable to be able to explore relatively small areas of the illumination to be sure that the actual subject area illumination is being measured. A Vs-inch exploring area appeared to be a satisfactory limit and, as a result, a Vz-inch-diameter light-sensitive cell seemed to be dictated, but since this small-size cell was not commercially available, a standard 1.75-inch cell was used equipped with a suitable mask having a V2-inch-diameter aperture. This immediately introduced a problem. For most consistent accuracy, it is desirable to illuminate the entire active area of a barrier-layer type of cell because of two conditions.
Fig. 1 — High-speed light meter.