New methods in order to determine the extent of temporary blinding from laser and LED light and proposal how to allocate into blinding groups

Abstract

Indirect effects arising from bright artificial optical sources like temporary blinding might result in serious incidents or even accidents due to accompanying alteration of visual functions like visual acuity, contrast sensitivity and color discrimination. In order to determine the degree and duration of impairment resulting from glare, dazzle, flash-blindness and afterimages, caused by a beam from a laser or lamp product, particularly under low ambient light conditions, an investigation has been performed with the goal to improve the current knowledge as far as especially recovery duration of visual acuity is concerned. For this two different test set-ups were designed and engineered in order to be able to determine the time duration after which visual acuity returns to its previous value after temporary blinding with a laser or an LED and in addition to search for functional relations as far as wavelength, optical power and exposure duration are concerned. Instead of normal visual acuity measurement, which is the standard test done by eye care professionals, and which has been applied in order to determine the recovery time after irradiation with a high brightness LED (HB-LED) with the aid of a modified commercially available binoptometer with Landolt-C rings as optotypes, a special reading test on a computer monitor was developed for the case of laser irradiation. Two different laser were applied, one with a wavelength of 632.8 nm and the other with 532 nm. Red, green, royal blue and white HB-LEDs were used as stimulating light sources. 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 (laser) and 3 mW (LED). The exposure durations were chosen as 0.25 s, 0.5 s, 1 s, 5 s, and 20 s in the case of laser irradiation and 0.25 s, 1 s, 5 s, and 10 s for LEDs, respecting maximum permissible exposure (MPE) and/or limit exposure levels (ELVs) in all exposure situations. The visual acuity recovery time tVA has been found to obey the dose relationship: t_VA /s ≈ 3.7•ln(energy/μJ) - 16.2 in the case of a green HB-LED in the power range 0.12 mW to 1.5 mW and for exposure durations between 1 s and 8 s. Further investigations were performed with other LED colors especially as far as threshold values for temporary blinding are concerned. The afterimage duration t_{afterimage,fv} produced by a red laser beam was determined to be: t_{afterimage,fv}/s ≈ 50.6•ln[(P•t_exp)/μJ] - 13.4, for laser output powers P between 10 μW and 30 μW with exposure durations t_exp from 0.25 s up to 10 s, when the beam hits the fovea. Additional results have been achieved with a green laser at a wavelength of 532 nm and compared with the respective values at 632.8 nm. The results of the research project suggest classifying light sources like laser and LEDs into so-called blinding groups. In total 3 different groups which reflect the obtained results and are proposed in order to fulfil the requirements of special classification and might be regarded as an appropriate assistance to perform a risk analysis.