Athermalization of an infrared zoom lens system for target detection

A preliminary investigation has been conducted into the feasibility of athermalizing an infrared zoom lens system for target detection in the eight to twelve micrometer region of the wavelength spectrum. The starting point for this study is an all-germanium 3 to I zoom lens operating at 20 degrees Centigrade that has been previously described in the literature. In the current study, this system has been athermalized over a large temperature range by means of active compensation of one of the zooming components. Another approach that has also been conducted in this study is to utilize the glass substitution capability in ZEMAX Global Search as an aid in selecting suitable optical materials for athermalization. ZEMAX has recently been modified to allow for simultaneous computer optimization of a zoom lens over an extended thermal range of operation. with the help of Global Search, certain optical materials have been selected with lower dn/dt than germanium. A solution using these materials has been obtained by this method which provides good image quality with passive compensation. These results are presented and compared with the starting solution.