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Disturbance of visual functions as a result of temporary blinding from low power lasers

Although it is well-known that dazzle, flash-blindness and afterimages may be caused by bright optical radiation, only sparse quantitative data are available with regard to the effects arising from low power laser products. Indirect effects like temporary blinding might result in serious incidents or even accidents due to the alteration of visual functions like visual acuity, contrast sensitivity and color discrimination. In order to determine the degree and duration of impairment resulting from dazzle, flash-blindness and afterimages, caused by a laser beam, an investigation has been performed with the goal to improve the current knowledge as far as especially the visual acuity recovery duration is concerned. Two different test set-ups were designed and applied in order to determine the afterimage duration and the recovery time for visual acuity after temporary blinding from a laser, respectively. In order to get the desired information a helium-neon laser was mounted on a movable assembly where the respective beam position and direction could be set up on a semicircle. In addition the mount could be inclined in a vertical plane in order to increase the variability of feasible settings. The power was adjusted in several steps in order to investigate the respective dependence of the afterimage. The investigations were relatively time consuming, since re-adaptation of about half an hour was necessary after every exposure in order not to falsify the results. The trials have been done with several volunteers in the laboratory. After the experimental mapping of the local afterimage duration for the various sites on the retina the foveal afterimage duration taf,fv produced by a red laser beam was determined. The investigations have shown a strong dependence on the angle between the line of sight and the beam direction. Besides a maximum of 300 s the dose relationship taf,fv/s ≈ 50.6•ln[(P•texp)/μJ] - 13.4 for laser output powers P between 10 μW and 30 μW with exposure durations texp from 0.25 s up to 10 s was found. The inability to read due to the disturbance produced by afterimages lasts for about 20 s even if the exposure is not more than 0.25 s from a laser with about 0.8 mW optical output power. In addition to the physiological effects of temporary blinding the psychological glare was determined as a function of the angle between line of sight and beam direction. It was found that already at output powers below 30 μW of a He-Nelaser the subjects reported intense glare effects and felt uncomfortable due to the high brightness. Instead of the standard visual acuity measurement used by eye care professionals, a reading test on a computer monitor was applied after laser irradiation. In this case two different lasers were used as a dazzling light source, one with a wavelength of 632.8 nm and the other with 532 nm. The maximum applied optical power in a 7-mm aperture, which is equivalent to the pupil diameter of a dark adapted eye, was 0.783 mW. The exposure durations were chosen as 0.25 s, 0.5 s, 1 s, 5 s, and 20 s.