The infrared zoom lenses are of prime interest for thermal imagery, IR systems simulation or missiles guiding. They are composed of germanium chiefly, which main drawback is the important variation of its refraction index according to the temperature. Generally, this problem can be solved while refocusing the image by displacement of front or rear lens-elements of the zoom lens. Therefore, this involves new translation guidings, not necessary to the zooming function. There appears the possibility and interest of using a converter element already in translation for the zooming function, to perform the functions of thermal compensation and focusing. Then the displacement of the considered element takes the focal-length, the focusing and the temperature into account.
Pierre Nory, Pierre Nory,
"Use Of A Zoom Converter To Perform Focusing And Thermal Compensation", Proc. SPIE 0590, Infrared Technology and Applications, (1 May 1986); doi: 10.1117/12.951962; https://doi.org/10.1117/12.951962