Passively athermalized hybrid objective for a far-infrared uncooled thermal imager

The development of a Petzval objective lens which is passively athermalized over the temperature range minus 20 degrees Celsius to plus 50 degrees Celsius is described. The lens is compatible with a latest generation uncooled staring array imager operating in the far infrared band, currently under development at the UK Defence Research Agency (DRA). In order to minimize the number of lens components which are required, a diamond turned hybrid refractive-diffractive element is employed. Design options are presented and the manufacturing issues relating to the diffractive surface are described. The goal is to produce a cost effective solution rather than placing the emphasis on achieving the ultimate in performance. Results from optical performance tests are given, including interferometry at a range of temperatures and broadband MTF.