Confirmation of the validity of the psychophysical light scattering factor.

PURPOSE To reevaluate the validity of the light scatter factor (LSF) formula of Paulsson and Sjöstrand, LSF = L/E (M2/M1-1), where L is the target luminance, E is the illuminance of the glare source at the eye, and M2 and M1 are modulation contrast thresholds measured with and without the glare source, respectively. This equation has recently been deemed invalid by Yager, Yuan, and Mathews. METHOD Ratios of contrast thresholds with and without glare were measured for three glare illuminance levels for each of three stimulus luminances. This results in five different ratios of L/E, spanning a range of 1.60 log units. RESULTS The data show an excellent fit to the Paulsson and Sjöstrand equation, and the LSF scores conform well to previously published normative values. CONCLUSION The light scatter factor equation of Paulsson and Sjöstrand is confirmed as valid without resorting to the need for correction factors based on variables such as pupil size.