In order to answer a practical question as to the most effective red light to be used in military situations requiring dark adaptation, an experiment was undertaken to compare the effects of four “red” filters at constant brightness levels. It was obvious that with traditional methods of measuring dark adaptation, these effects could not be measured with sufficient accuracy to make comparisons. An application of the method of constant stimuli was developed which gave adequate accuracy of measurement.
The results show differences in the effects of adapting to light having dominant wavelengths of 601, 626, 640, 675, 690 mµ and neutral. The maximum sensitivity differences occurred early in dark adaptation (before two minutes). Dark adaptation proceeded most rapidly following adaptation to 626 mµ. The maximum difference between neutral and 626 mµ was 0.5 logµµL and differences between the various “red” conditions ranged from zero to 0.22 logµµL.
This application of the constant stimuli method gives results which may be more sensitive than the calibrations of standard instruments.
STANLEY W. SMITH, A. MORRIS, and FORREST L. DIMMICK, "Effects of Exposure to Various Red Lights upon Subsequent Dark Adaptation Measured by the Method of Constant Stimuli," J. Opt. Soc. Am. 45, 502-504 (1955)