Infrared objective lens systems

Abstract

An infrared objective lens system (15) comprises a primary lens group (13) air spaced from a secondary lens group (14) by means of a support assembly (16). The primary lens group (13) and secondary lens group (14) are aligned on a common optical axis (10) and are arranged to form a real image (9) at an external image surface from infrared radiation entering the system (15) through a pupil .0. in object space (11). Primary lens group (13) is self-compensated for chromatic aberrations, is positively powered and is made of material the refractive index of which is relatively temperature insensitive such as arsenic triselenide and/or zinc selenide. The secondary lens group (14) is positively powered, introduces minimal chromatic aberrations and is formed by a positively powered lens (C) and a negatively powered lens (D). Lens (C) is made of material the refractive index of which is relatively temperature insensitive such as arsenic triselenide. Lens (D) is made of a material the refractive index of which is relatively temperature sensitive such as germanium and the thermal aberration introduced by lens (D) is arranged to compensate for thermal aberrations introduced by the primary lens group (13) and secondary lens group (14) caused by thermal expansion of the support assembly (16) as a result of which the system (15) is rendered substantially passively athermal.