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Product Code: ILSC2007_401

Local Susceptibility of the Retina, Formation and Duration of After-images in the Case of Class 1 Laser Products and Disability Glare Arising from HB-LEDs
Hans-Dieter Reidenbach, University of Applied Sciences Cologne; Koeln Germany
Presented at ILSC 2007

According to the new European Directive on Artificial Optical Radiation (2006/25/EC) the employer has to determine the exposure and the assessment of risks, i.e. workers shall not be exposed above the exposure limit values, which are based on various ICNIRP guidelines. In addition, the employer shall give particular attention, when carrying out the risk assessment, to any indirect effects amongst others such as temporary blinding. 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 of the effects arising from class 1 laser products. Due to the fact that the accessible emission limits of Class 1 laser products and thus the maximum permissible exposure values have been increased in the younger past the new edition of the international standard IEC 60825-1 mentions in the description of the respective laser classes that intrabeam viewing of e.g. Class 1 laser products, which emit visible radiant energy, may produce dazzling visual effects, particularly in low ambient light. This might be regarded as a proper step into an adequate direction, but solely for Class 2 laser beams, additional information is available, which states that indirect general safety implications might result from temporary disturbance of vision or from startle reactions. It was the goal of the current investigations to get some more information with regard to the duration of afterimages as a function of the respective area and site irradiated on the human retina and as a function of exposure duration and beam power. 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. The investigations have shown a strong dependence on the angle between the line of sight and the beam direction and in addition a dose-relationship holds concerning the duration of an afterimage as a function of exposure duration in the time interval between 0,5 s and 10 s. Afterimage durations up to 300 seconds were found if the fovea of the human retina is irradiated from a class 1 laser beam, whereas much lower values are valid in the parafoveal region and in the periphery.
In addition to investigations with a laser an extensive research program has been started which deals with the formation of afterimages and the impairment of visual acuity and colour contrast sensitivity in the case of extended optical sources like high brightness LEDs, not at least in order to search for the respective differences between both modern artificial light sources. First results concerning disability glare will be reported although the total project is still not completed.

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