Exposure Meter for Precision Light Dosage

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Abstract
The exposure meter is an electronic device of the type in which a photo‐tube charges a capacitor to the firing voltage of a trigger tube, whose pulse is amplified by a thyratron. The device has been adapted to high intensity sources and is used for the regulation of carbon arc light dosage in the standardization of light‐sensitive paper at the National Bureau of Standards. The device, including a modification for automatic extinction of the source when the desired light dosage is obtained, is described, and is shown to be capable of very high reproducibility. Two instruments simultaneously viewing a gradually aging monochromatic source gave count values whose ratios showed a coefficient of variation of 0.32 percent in two weeks of continuous measurement. One instrument, viewing a controlled tungsten lamp, showed a coefficient of variation of 0.18 percent in counts per watt hour over a period of nine days, after which the values changed—the result, presumably, of changes in the lamp. The instrument was tested for suitability of application by comparing counts with reflectance changes of light‐sensitive paper under arc exposure. The results showed that the instrument could control light dosage adequately in spite of large changes in arc voltage, glass filters, and time of exposure. The instrument was used to test the variability of the uranyl oxalate actinometer, frequently used in photo‐chemical work, by simultaneous exposure to a carbon arc. It was found that the actinometer was capable of higher precision than previous investigators had found, in that the coefficient of variation was 1.4 percent, less than ¼ as large as formerly believed.

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