A computer model that predicts thresholds for laser induced skin injury was used to systematically analyze wavelength, pulse duration and beam diameter dependencies. The thresholds were compared with the respective maximum permissible exposure (MPE) values promulgated by ANSI Z136.1-2014, ICNIRP 2013 and IEC 60825-1:2014.
Due to discontinuities in the MPEs, the reduction factor between the predicted threshold and the MPEs varies widely. For some wavelengths, for beam diameters of 7 mm, the reduction factor is between 2 and 3 in the pulse regime of roughly 1 ms to 100 ms. For other wavelengths, the reduction factor is above 10.
The effect of the limiting aperture to reduce the radiant exposure is accounted for by increasing the MPE for beam diameters smaller than 3.5 mm. For small beam diameters, for the case that there is no relative movement between the beam and the skin (which has to be assumed for exposure to pulsed emission), this greatly reduces the margin between the predicted injury threshold and the MPE. Due to the averaging effect of the limiting aperture, for beam diameters of 1 mm, we found reduction factors considerably less than 1, particularly in the visible wavelength range, but also for wavelengths approaching 1400 nm. As a worst case, the MPE permits a factor 3 higher exposure levels than the predicted injury threshold. Since the reduction factors are particularly low in the regime between 1 ms and 100 ms, it is not possible to justify the 3.5 mm limiting aperture by relative movements of the laser beam and the skin. It is also questionable if scattering in the tissue is sufficient to generally justify the 3.5 mm limiting aperture. It appears prudent to consider an exposure-duration varying limiting aperture in the same way as for the cornea above 1400 nm, where for the pulsed regime the limiting aperture has a diameter of 1 mm.