[Dim the light! Glare problems in a world with ever increasing demands on visual acuity]. / Blända av! Bländningsproblemen ökar i ett samhälle som ställer allt större krav på synen.

Ocular tissue transparency is dependent on the regular lattice configuration of lens and corneal fibres of uniform diameter. Ageing is associated with degeneration of both lens and cornea, which lose some of their structural order and eventually their transparency, though this process is not uniform. The structural changes are local and result in ocular media opacities. When the opacities increase in number and extension, they begin to affect visual acuity. As the loss of acuity becomes clinically significant, we speak of clinically relevant cataract. Retinal exposure to light diffused by intraocular light scattering induces optical glare, one of two forms of glare. The other form is transient glare--i.e., glare due to adaptation problems in an environment with rapidly changing ambient luminance. Contemporary society is characterised by increasing emphasis on visual information in such forms as texts, icons, signs and symbols. The computer revolution has been accompanied by further stress on the importance of the detection and interpretation of written instructions.

The 'light scattering factor'. Importance of stimulus geometry, contrast definition, and adaptation.

PURPOSE: Paulsson and Sjöstrand have suggested that the light scattering factor (LSF) can be estimated by using the equation: LSF = L/E (M2/M1-1). Here L is the space average luminance of the target, E is the illuminance of the glare source, and M2 and M1 are modulation contrast thresholds in the presence and absence of the glare source. To compensate for change of adaptation. Abrahamsson and Sjöstrand later modified the above equation by introducing a correction factor (CF): LSF = L/E ((CF) (M2/M1-1). The purpose of this study is to analyze the validity of the above equations. METHODS: The importance of stimulus geometry, contrast definition, background luminance, and glare illumination is studied through theoretical analysis and comparison with earlier studies. Stimulus geometry and contrast definition are studied through optical modeling. Adaptation is modeled according to the laws of Weber and DeVries-Rose. RESULTS: The choice of contrast definition may corrupt the result by a factor of 2. At background luminance levels above approximately 10 cd/m2, the Paulsson-Sjöstrand equation agrees well with theory. At lower background levels, the Abrahamsson-Sjöstrand equation is used with correction factors derived from adaptation measurements. Using this equation and earlier published data from glare testing performed at 2 cd/m2, the results are found to be in fair agreement with the light scattering theory. CONCLUSIONS: Glare testing using the Paulsson-Sjöstrand equation is found to be valid as long as the measurements are performed at high luminance levels (above 10 cd/m2), with targets of low spatiotemporal frequencies (e.g., 2 cpd and 1 Hz) and with the use of a properly chosen definition of contrast. At lower luminance levels, the Abrahamsson-Sjöstrand equation may be used with well-derived correction factors.