THE LIGHT-SCATTERING FACTOR - IMPORTANCE OF STIMULUS GEOMETRY, CONTRAST DEFINITION, AND ADAPTATION

Purpose. Paulsson and Sjostrand have suggested that the light scatteri ng factor (LSF) can be estimated by using the equation: LSF = L/E (M(2 )/M(1) - 1). Here L is the space average luminance of the target, E is the illuminance of the glare source, and M(2) and M(1) are modulation contrast thresholds in the presence and absence of the glare source. To compensate for change of adaptation, Abrahamsson and Sjostrand late r modified the above equation by introducing a correction factor (CF): LSF = L/E ((CF)(M(2)/M(1) - 1). The purpose of this study is to analy ze the validity of the above equations. Methods. The importance of sti mulus geometry, contrast definition, background luminance, and glare i llumination is studied through theoretical analysis and comparison wit h earlier studies. Stimulus geometry and contrast definition are studi ed through optical modeling. Adaptation is modeled according to the la ws of Weber and DeVries-Rose. Results. The choice of contrast definiti on may corrupt the result by a factor of 2. At background luminance le vels above similar to 10 cd/m(2), the Paulsson-Sjostrand equation agre es well with theory. At lower background levels, the Abrahamsson-Sjost rand equation is used with correction factors derived from adaptation measurements. Using this equation and earlier published data from glar e testing performed at 2 cd/m(2), the results are found to be in fair agreement with the light scattering theory. Conclusions. Glare testing using the Paulsson-Sjostrand equation is found to be valid as long as the measurements are performed at high luminance levels (above 10 cd/ m(2)), with targets of low spatiotemporal frequencies (e.g., 2 cpd and 1 Hz) and with the use of a properly chosen definition of contrast. A t lower luminance levels, the Abrahamsson-Sjostrand equation may be us ed with well-derived correction factors.