Abstract

All common infrared materials suffer from a relatively large change of refractive index with temperature (dn/dt); the most common material - germanium - also being the worst having a value more than two orders of magnitude higher than that for a typical optical glass. The effect of the resultant thermal change of optical power is that an all-germanium FLIR optic will rapidly defocus as temperature changes. The tolerable temperature range before this defocus results in unacceptable imagery (Strehl Ratio <0.8) can be calculated as being only approximately $\pm 3\frac{1}{2}°\text{C}$ for a typical all-Ge optic of 100 millimetre diameter : thus some form of athermalization is required for the successful operation of most FLIR optics.

© 1990 Optical Society of America