THE SAFETY ASPECTS OF ATMOSPHERIC TRANSMISSION OF LASERS *

The safety implications of atmospheric scintillation of laser beams have long been recognized. With the testing of military lasers in the field, the “hot spots” created by atmospheric turbulence were looked upon as an uncertain variable in determining a laser’s hazardous range ( F I G U R E I ) . l In the past ten years several studies have been performed, largely with He-Ne lasers, in an effort t o quantify this e f f e ~ t . ~ ~ These studies resulted in statistical probabilities of finding an irradiance hot spot a certain factor above the mean. Deitz made the first significant contribution in this area when he developed a nomograph ( F I G U R E 2) for such a purpose.2 There has been some debate among safety researchers, however, as to how useful such statistics are in establishing safe viewing distances and whether they tell the complete story. In numerous tests of field lasers by myself and my associates at the US Army Environmental Hygiene Agency over the past decade, we have found that during periods of strong turbulence the probability of occurrence of significant hot spots above the beam irradiance measured during quiet conditions is less on an absolute scale than reported in the other studies. Our explanation has been that the other studies did not take into account the beam spreading, which is always present during periods of strong turbulence. This spreading of the beam reduces the average beam irradiance such that the excursions of localized beam irradiance above the average are not as serious as they appear t o be at first g l a n ~ e . ~ ~ Most studies were performed by atmospheric physicists who were looking only at the irradiance profile of the laser beam, i.e., the beam “cross section.” They did not look at the projected radiance of the laser. The latter parameter is, however, of significance from an eye-safety standpoint. The radiance or “brightness” of an extended light source determines the irradiance falling on the retina.’ ’Ihe parallel rays from a point source however are always imaged on the retina as a minimal image. A source such as a laser is effectively a point source when the intrinsic divergence of the light rays entering the pupil is less than 0.3 mrad. When “turbulons” in the atmosphere tend t o focus the collimated rays in a laser beam, the focused rays occasionally can have a divergence greater than 0.3 mrad, and the resulting retinal image is enlarged. The safety question that must be answered when the eye is located in a hot spot is whether the retinal irradiance will be significantly increased. For an extended source we can apply the law of conservation of radiance. A glass lens (or a turbulon) or a telescope cannot increase the source radiance, and therefore cannot increase the retinal irradiance of a searchlight or other light source that is already resolved by the eye. If, however, the source cannot be resolved by either the unaided eye or a telescope because the adjacent rays in the