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This type of test was adopted in 1932 in the Joint I.E.S.–S.A.E. Standard Specification for Laboratory Tests of Reflex Reflectors for Motor Vehicles, obtainable from the Illuminating Engineering Society, 51 Madison Avenue, New York City. This specification requires that the reflector shall be tested at 100 feet from a headlamp of 10,000 candlepower, thus providing an illumination of 1 foot-candle on the reflector. The apparent candlepower of the reflector is then measured by an observer situated 7 inches (one-third of a degree) above the center of the headlamp.
An “electrocell” was used in this apparatus, coupled directly to a 0–20 microammeter. Care was taken in drilling the hole to avoid any burr which might short-circuit the cell. The suggestion to drill the photo-cell in this manner was made to the author by Mr. Clifton Tuttle.