Pupil Size as Determined by Adapting Luminance*

Available measurements by different investigators on the relationship between pupil diameter and luminance of large adapting fields were compiled and weighted for the number of observers used. The equation for the curve that best fits eight sets of data is logd=0.8558−0.000401 (logB+8.1)3, where d is the diameter of the natural pupil in millimeters and B is the luminance of the adapting field in millilamberts. This is the best estimate that can be made to date of the pupil size—luminance relationship. To make the data of more general use they were corrected for the Stiles-Crawford effect and expressed so that retinal illuminance, adjusted for average pupil size, may be obtained directly from knowledge of the luminance of the stimulus field. The expression for this is logE(trolands)=logB+1.8614–0.000986 (logB+6.5)3, where E is the retinal illuminance in trolands and B is luminance in millilamberts. It is unfortunate that no extensive measurements have been made in this important area.