726 and 727 for example additional fig7 scatter losses are represented at 728 and 730 where the entrance and exit laser beams intercept materials of differing index of refraction in this regard the canopy 700 is presumed to have an index of refraction n 2 706 of magnitude 1 5 and this is interfaced on either side by air of index n 1 of magnitude of 1 0 as represented at 704 and 708 if the canopy 700 in fig7 were perfect with no defects there would be no scatter of the input energy and no generation of the interfering glow i e there would be no angle at which the laser beam 702 could enter the canopy material that could cause internal reflection since however there are defects present in the canopy material the beam 702 can enter at angles that result in reflection depending upon these scatter angles entering light is either reflected or totally internally reflected with the resultant canopy glow which causes degradation of the night vision goggles operation snell 39 s law equation 1 shown below describes factors governing the fig7 a example and determines whether beam losses are totally internally reflected or fresnel reflected in nature the critical value u03b8 c for the angle u03b8 2 at 734 is determined from equation 1 using a u03b8 1 value of 90 degrees when the angle u03b8 2 at 734 equals or exceeds the critical angle as determined by equation 2 the beam will be totally internally reflected at the next point of incidence n 2 index of refraction for aircraft canopy plastics u2248 1 49 to 1 56 u03b8 c the critical value for the u03b8 2 angle of refraction inside the medium the canopy glow resulting from such mechanisms.