Visibility of radiant energy

In cooperation with the Nela Research Laboratories a new determination of the visibility of radiant energy has been made by the step-by-step method, an equality- of-brightness method with little or no hue difference in the two parts of the photo- metric field. Between 490 and 680 mᵗ, inclusive, measurements were made upon 52 observers, some of them cormnon to previous investigations; for 38 of these observers measurements were continued to include 430 and 740 m/t. Energy values were based upon radiometric and spectrophotometric measurements made at the bureau, checked by an independent color temperattu-e meastu-ement 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 Brodhuu variable sector. The photometric field was the divided circle type subtending an angle of 3°. Brightnesses were safely above the Piu-kinje 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 tmder 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 center of gravity for light of a color temperature of 2, 077°^-^ⁱs given by Ives's physical photometer solution. CONTENTS. Page. I. Introduction 132 II. Auxiliary work 136 1. Apparatus. 136 2. Energy measiu-ements 138 3. Spectral transmission of the Brace spectrophotometer 140 4. Field brightness 142 5. Stray light 143 6. Determination of the step 145 III. Luminosity and visibility 148 1. Procedure 148 2. Computations 149 3. Restilts 151 IV. Comparisons with other investigations 161 1. Flicker determination of Coblentz and Emerson 161 2. Step-by-step determination of Hyde, Forsythe, and Cady 166 3. Flicker determinations of Ives and of Nutting 169 4. General compilation of visibility data 171 5. Peculiarities of individual data 178 V. Precision of the measurements 179 1. Energy measurements 180 2. Luminosity measiu-ements 182 (a) Observational errors 183 (6) Errors in the step 185 3. General reliability of the step-by-step method 187 VI.