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1949 EXPOSURE METER 547
First, if accurate reregistration of a small area of light on a large cell is impossible, the output of the cell may be different at various spots on the cell and second, the unilluminated area of the cell becomes resistive in nature and the total series circuit resistance thus created is directly proportional to the area from the illuminated spot to the power take-off point or current-collecting ring of the cell. It was found, however, that the use of a properly selected white opacous material in front of the cell would satisfactorily diffuse the y2-hich masked light spot over the complete cell area, thus disposing of the registration and resistance problems. Since the small exploring area is required only when using relatively high-level nonuniform artificial source illuminants, the transmission losses through the aperture and opacous diffusion screen could well be tolerated.
In order to achieve maximum camera speed, it is quite likely that banks of lamps of the General Electric type will be used focusing all of the lamps on the same area. Under such conditions, the filament patterns have a tendency to wash out, but the light rises to rather extreme levels. Several hundred thousand foot-candles of illumination may result from the use of a bank of eight lamps and at the same time substantial amounts of radiant-heat energy may be transmitted. In order to change the range or multiply the range of a photoelectric type of light-measuring instrument, it is common practice to shunt away from the measuring instrument a large portion of the total current developed by the light-sensitive cell. This is accomplished by means of a suitable resistor network ahead of the meter. However, the continual exposure of a barrier-layer type of cell to excessively high levels of illumination may, under certain conditions, contribute to cell deterioration. Therefore, it seemed desirable to depart from the usual electrical range-switching arrangement and resort to the use of suitably calibrated neutral-density filters for range multiplication. By the simple process of controlling accurately the density of the white opacous diffusion plate, any desired per cent of transmission loss could be achieved, thus multiplying the basic range of the instrument by ten or higher as needed. By facing the cell multiplier with polished aluminum and using the white materials throughout, the effects of radiant heat could be minimized.
The final design of a suitable light-measuring instrument for this application, therefore, has taken on the following form.
The light-sensitive cell is mounted in a small paddle equipped with an extension cable to permit more comfortable reading of the