The original rationale for a nominal ocular hazard distance

The current ANSI Z136 definition of the "nominal ocular hazard distance" (NOHD) is "the distance along the axis of the unobstructed beam from a laser, fiber end, or connector to the human eye beyond which the irradiance or radiant exposure is not expected to exceed the applicable MPE." Although this definition of NOHD is seemingly definite, this was never the original intention. Today, most LSOs are unaware of the extensive debate that ensued in the laser safety community decades ago that lead to the addition of "nominal" in front of "ocular hazard distance." The term, "nominal" was applied to take into account that there was never an absolute ocular hazard distance, but there was always a vanishingly small risk of a hazardous retinal exposure because of atmospheric scintillation for laser pulses, etc. However, for CW beams, scintillation could never add risk because of retinal radiant heat flow and thermal averaging within the moving (saccadic) retinal image. There is also a significant attenuation of the axial beam irradiance by small-angle Mie scattering that is not accounted for in gross atmospheric visibility extinction coefficients, nor from beam spreading during strong turbulence. Thus "exceeding the MPE," might happen theoretically, but the probabilistic risk analysis performed by the ANSI Z136 Committee and a NATO committee in the 1970s resolved the issue and allowed the calculation of a "NOMINAL" OHD which was quite practical. Most importantly, the strong turbulence is a problem of daylight conditions when the atmospheric-index-structure coefficient C_N is large, but this is when the pupil size is 2-3 mm in diameter and the actual hazard (not reflected in the MPE derived for a 7-mm pupil) is 4-6 times less - thus nullifying the apparent increased risk that some more recent papers have calculated. There is a marginal increased risk of injury in late twilight near turbulence lapse conditions. Recent safety publications have suggested that the NOHD is a "deterministic" calculation not based on extensive probabilistic risk analysis, which cannot be explicitly stated in a published safety "standard." Nevertheless, the definite definition could be revised to state that it would be extremely remote for the MPE to be exceeded.