Intraocular light scattering and glare testing
Abstract: Aims: To analyze the parameters that are used in psychophysical glare testing and the validity of the light scattering factor (LSF). The purpose is also to investigate the light scattering properties in excised eye lenses, and to compare these properties with light transmission for different wavelengths in aging and cataractous lenses. Finally, the purpose was to investigate whether a small angle glare test in a clinical setting can produce and detect a significant decrease in contrast sensitivity with age in normal young and elderly eyes.Materials & Methods: The importance of glare testing parameters was studied through theoretical analysis and comparison with earlier studies. Stimulus geometry and contrast definition were studied through optical modeling. Adaptation was modeled according to the laws of Weber and DeVries-Rose. The method for the light scattering measurements used an integrating sphere, an adjustable diaphragm, and a lens holding unit. The lens collimated light from one of five laser-lines (458 to 633 nm). By using a sphere with an extra port, the collimated directly transmitted light could be separated from the scattered light. The diaphragm changed the spheres angle of acceptance during angular measurements. In the small angle glare test, low contrast Landolt-C test targets was presented on a computer monitor. Glare was induced using a fiberoptical bundle mirrored into the center of the Landolt-C test target. Results: The choice of contrast definition may corrupt the result by a factor of 2. At background luminance levels above ~10 cd/m2 the Paulsson-Sjöstrand equation agrees well with theory. At lower luminance levels correction factors must be used. The results with our method for light scattering measurements showed a wavelength dependence for excised lenses with low light scattering levels, but when the light scattering increases the wavelength difference tends to level out. Despite the higher light scattering percentage for shorter wavelengths, when calculated as an "effective light scattering" (compensated for light transmission), more scattered light actually falls towards the retina at longer wavelengths. The angular distribution of the scattered light was found to have no relation with wavelength. However, when compared to the total light scattering a significant shift toward lesser angular dependence was found for lenses with pronounced light scattering. Contrast sensitivity data (CS) in the small angle glare test produced low correlation with age. However, the data showed large correlation with age when calculated as glare factors (CS/CSglare). The relative increase in glare factor with age was about 2.7 (max range) in the tested group (20-59 yrs).Conclusions: Glare testing using the LSF was found to be valid at high luminance levels, with targets of low spatiotemporal frequencies and with the use of a properly chosen definition of contrast. The results from the light scattering measurements indicate that our method is useful in studies of excised eye lenses. The method is very sensitive for early lens changes and can monitor the change in light scattering during various phases of cataract formation. Using the effective light scattering we can explain the disagreements whether the intraocularly scattered light has a wavelength dependence or not. A wavelength dependency with opposite sign was found in agreement with psychophysical findings. Correlation between angular distribution and wavelength was found only when compared to total light scattering. The results indicate that our glare method is useful in studies of glare in normal and elderly eyes. Glare factors show much smaller individual variation than data of contrast and glare measurements analysed separately.
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