Effect of yellow filters on pupil size.

The purpose of this study was to investigate the size of the pupil while viewing through yellow and neutral density (ND) filters. Previous studies have shown that the pupil of the human eye is relatively more sensitive to short wavelengths than indicated by the photopic luminosity curve. We first measured the consensual horizontal pupil diameter of 11 observers as a function of luminance (0.144 to 18,150 cd/m2) to establish the luminance-response function for each observer. We then measured the pupil diameter while the observer viewed through a Corning Photochromic Filter (CPF) 550 lens and two ND filters (ND 0.5 and 1.0). The pupil diameters obtained with each filter were compared to the diameters at an equivalent luminance based upon each observer's luminance-response function. Our results show that the pupil diameter is larger with the yellow lens than when viewing a broad spectrum white field at an equivalent luminance. We speculate that our results may explain some, but not all, of the well-known subjective brightness enhancement that occurs when viewing through yellow filters.