Possible changes in laser exposure limits for extended source pulsed lasers

Since the first ANSI Z136.1 standard (1973), there have been virtually no changes made to the minimal-spot size (small-source) Maximum Permissible Exposure (MPE) limits for the human eye that normally apply to intrabeam viewing of a laser for pulse durations ranging from 1 ns to 10 s and for wavelengths from 400 nm to 1200 nm. This is also true for these MPEs for wavelengths greater than 3000 nm. Considering that well over 90 percent of potentially hazardous laser exposures probably result from this range of conditions, this is a remarkable 30-year record. However, the MPEs for extended sources (currently referred to as “large sources”) have been revised on several occasions. This is surprising in several respects, since biological studies of larger retinal image sizes should be less challenging. Prior to 1993, alpha-ruin (α_min) varied from 8 to 11.3 mrad at 1 ns to 24 mrad at 10 s, with a minimal value of 1.6 mrad at 18 μs. In 1993, the extended-source MPEs were changed again and a scaling factor C_E (C₆ in IEC) was introduced for angular subtense values ranging up to 100 mrad (α_max) and with constant values for α_min = 1.5-mrad for t < 0.7 s. However, by 2001, the treatment of extended-source MPEs were changed still again to provide a constant value of α_min = 1.5 mrad for all exposure durations. Now, it appears that biological research supports another change to the MPEs for extended sources. There appears now to be no retinal spot-size dependence of injury thresholds for pulse durations between 1 ns and 18 μs (the thermal confinement time). If home out by further study, there would be no need to change α_min and extended-source MPEs could be expressed (as in 1973) as a constant pulsed integrated radiance for these short pulse durations. However, the value of α_max must increase from 1.5 mrad for durations greater than 18 μs to at least 24 mrad, but not as large as 100 mrad at 10 s. The exact function for α_max depends on radial heat flow in the retina. It is interesting that in some limited respects, the 1973 MPEs for extended sources had greater accuracy that those presently published!