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<p>By the visibility of radiant energy is meant the ratio of luminosity to radiant power at the various wave-lengths in the spectrum. The American Bureau of Standards, in co-operation with the Nela Research Laboratories, has made a new determination of the visibility of radiant energy by the step-by-step method, an equality-of-brightness method with little or no hue difference in the two parts of the photometric field. Between 490 and 680 /b m/mu inclusive, measurements were made upon 52 observers, some of them common to previous investigations. For 38 of these observers, measurements were continued to include 430 and 740 /b m/mu. Energy values were based upon radiometric and spectrophotometric measurements made at the bureau, checked by an independent colour temperature measurement at the Nela Research
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