Measurement of straylight for glare assessment and driving.

In this review an overview of straylight as method to assess glare sensitivity for driver licensing will be given. Glare is an important safety issue while driving. It is well known that people stop driving at night because of glare when early cataract develops. It has been found to derive from the physical process of light scattering in the eye, causing a veil of straylight over the road scene. This veil lessens visibility, potentially up till complete blinding. By international agreement, glare is to be quantified by straylight based on the “equivalent luminance” concept. Normal standards have been defined, including age and cataract dependence. Straylight can be measured accurately with the psychophysical 2AFC “compensation comparison” method, including a reliability check. A commercial instrument called C-Quant is marketed. As straylight is a basic quality of the eye, also other aspects of quality of vision, such as face recognition, contrast sensitivity, etc. depend on it. Straylight is used in ophthalmology as early indication for cataract surgery. It is to a large degree independent from acuity. Practical aspects of straylight measurement and limit values for occupational testing will be discussed. Normal young values are around log(s)=0.9. For demanding professions a limit elevation of 2x (0.3 log, corresponding to 3 standard deviations) is proposed. For normal driving a limit elevation of 4x is proposed, corresponding to log(s)=1.5. Cataract surgery is indicated at log(s)=1.4.