Patent ID: 6236501
Filing Date: 2001-05-22
Classification: G02B

Abstract:
An objective lens system for use with a detection system in the 8-12 micron thermal infra red waveband, said objective lens system being light in weight, utilising only three lens elements each of which is made of Germanium, and having high aperture to provide sensitivity at a focal plane of the system, wherein the lens system comprises a single-element positively-powered objective lens (A), a single element positively-powered intermediate lens (B), and a single element negatively-powered field lens (C) which is located close to the focal plane of the system, the lenses being mutually spaced apart along a common optical axis (9), and wherein the field lens (C) is arranged to flatten the field curvature of the lens system, the intermediate lens (B) is arranged to correct astigmatism, and the intermediate lens (B) has an aspheric surface (3) to balance oblique sagittal, tangential spherical aberration and high order astigmatism.