The Effects of Thermal Radiation on Selected Optical Components

The materials studied were (1) Schott glass absorptive filters, (2) General Electric´s (GE) polycarbonate thermoplastic Lexan, (3) single and polycrystalline Ge, and (4) Eastman Kodak Irtran 1, sintered MgF2. The response to thermal radiation of the optical materials investigated ranged from no discernible damage for Irtran 1 to catastrophic breakage at fairly short irradiance times (tmax of 0.2 to 0.45 s) in the lower intensity (40-cal/cm2s) focus for the Schott glass absorptive filters. The observed damage levels and their variation with sample size was satisfactorily explained by our thermal stress model. The lack of damage in Irtran 1 was also explained by the same model. It was probably differential thermal expansion between the anti-reflection layers and the polycrystalline Ge substrate that caused the plastic flow that occurred on this sample at irradiation time of tmax = 1.00 s in the 300-cal/cm2s focus. The optical transmission was not affected, but the surface polish was destroyed. An uncoated sample of single crystal germanium irradiated under similiar conditions showed evidence of surface melting, but no plastic flow. Lexan showed a continuum of damage beginning with the generation of small bubbles beneath the surface and ending with charring and complete destruction of the surface.