Design of infrared afocal zoom system

The design method and result are described for an infrared afocal zoom system. Its zoom ratio is 4, and the corresponding field of view is 3°-12°. The working waveband is from 7.5μm to 10.5μm, and its total length is required to be no more than 350mm. The final optical system consists of 7 elements, with aspheric surfaces and a diffractive optical element. It achieves diffraction-limited imaging at the far infrared waveband. Two conclusions can be drawn from the design. (1) Using diffractive optical elements in the design of an infrared optical system is an effective approach to control color aberrations, which eliminates the need to use exotic and expensive materials and helps to reduce the cost of the system; (2) Functions such as changing the field-of-view, focusing for objects at finite distances and athermalizing the system can be achieved by axially moving a single element in the system.