Imaging system aberrations through optical windows with nonuniform laser heating.

An optical window is a critical component of an imaging system. When operating in harsh environments with extreme heating, nonuniform temperature changes occur throughout the window and cause nonuniform refractive index changes and mechanical deformations due to thermal expansion, which can degrade the imaging system's performance. In this paper, we present results collected from an experimental setup developed to characterize these aberrations. This setup includes a laser for sample heating, an infrared camera for measuring front and back surface temperatures, and a visible imaging system and a wavefront sensor for measuring degradations of a collimated beam from a point source transmitted through the heated window. Sapphire samples are laser heated with a Gaussian profile to temperatures in excess of 500 K with surface temperature gradients in excess of 15 K/mm. These measurements are compared with first principles models, which show quantitative agreement for window temperatures and qualitative agreement with the transmitted wavefront and imaged point source.