The importance of the optical turbulence effect along a slant path downward on probability of exceeding the maximum permissible exposure level (MPE) from a laser is discussed. The optical turbulence is generated by fluctuations (variations) in refractive index of the atmosphere. These fluctuations are caused in turn by changes in atmospheric temperature and humidity. The structure function of refractive index, C-n(2), is the single most important parameter in the description of turbulence effects on the propagation of electromagnetic radiation. In the boundary layer, the lowest part of the atmosphere where the ground directly influence the atmosphere, is the variation of C-n(2) in Sweden between about 10(-17) and 10(-12) m(-2/3), see Bergstrom et al. [5]. Along a horizontal path is the C-n(2) often assumed to be constant. The variation of the C-n(2) along a slant path is described by the Tatarski model as function of height to the power of -4/3 or -2/3, depending on day or night conditions. The hazard of laser damage of eye is calculated for a long slant path downward. The probability of exceeding the maximum permissible exposure (MPE) level is given as a function of distance in comparison with nominal ocular hazard distance (NOHD) for adopted levels of turbulence. Furthermore, calculations are carried out for a laser pointer or a designator laser from a high altitude and long distance down to a ground target. The used example shows that there is an 10% risk of exceeding the MPE at a distance 2 km beyond the NOHD, in this example 48 km, due to turbulence level of 5(.)10(-15) m(-2/3) at ground height. The turbulence influence on a laser beam along horizontal path on NOHD have been shown before by Zilberman et al. [4]
