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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 Laboratories. Luminosity values were obtained with a Brace spectrophotometer. The step was made by moving the collimator slit. The ratios of luminosities were measured with a Brodhun variable sector. The photometric field was the divided circle type subtending an angle of 3 degrees . Brightnesses were safely above the Purkinje region, except perhaps at the ends of the spectrum. Detailed comparisons are made between the individual and average results of this investigation and those of previous investigations. There is no certain difference between the average values of visibility obtained by the flicker and equality-of-brightness methods provided the former is used under the experimental conditions recommended by Ives and the latter does not depart too widely from these conditions. A revision of the I.E.S. mean curve is proposed, which results in better agreement with the average experimental visibility data, and still gives the same wave-length centre of gravity for light of a colour temperature of 2077 degrees K., as is given by Ives' physical photometer solution. It is considered that the step-by-step method of measuring visibility is a reliable method, with a precision comparable with that of the flicker method, and much superior to the simple equality-of-brightness method involving large hue differences.
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<br>Subjects:</div>
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Gibson, K.S. Tyndall, E.P.T.</div>
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