Worst-case conditions for threshold injury on direct viewing of a cw He-Ne laser are defined. A transpupillary power of 50-1OC µW for threshold injury to the eye is calculated on the basis of a heat conduction model and experimental observations.
W. P. Hansen, L. Feigen, and S. Fine, "A Worst-Case Analysis of Continuous Wave He-Ne Laser Hazards to the Eye," Appl. Opt. 6, 1973-1975 (1967)