This report presents an analysis of retinal irradiances from He-Ne gas CW lasers. Equations are developed for retinal irradiance as a function of viewing distance for typical He-Ne lasers with peak output powers of 0.5 to 100 milliwatts. The various viewing conditions are described and irradiance equations developed for each. Based on extrapolations from theoretical retinal burn models and available experimental data several conclusions are stated. (1) Continuous exposure of the eyes to a He-Ne laser of any power in the laboratory cannot be tolerated. (2) Typical alignment He-Ne lasers in the range of 0.5 to 5 milliwatts present no retinal hazard from a momentary accidental exposure at distances beyond 300 meters and are probably safe for continuous exposure at distances beyond 1,000 meters.
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Paul W. Lappin (1968) studied this question.