Infrared radiation detecting systems

Claim 1

An infrared radiation detecting system comprising an optical imaging system for receiving radiation from a field of view and delivering said radiation to an image surface, a detector unit having a detecting surface formed by one or more detector elements located in a cold-shield housing having a cooled aperture which limits the radiation acceptance angle of the detector unit whereby the detector unit is background limited, and a transfer lens system intermediate said image surface and said detector unit for transferring infrared radiation therebetween and focussing the transferred radiation at the detecting surface of said detector unit, wherein said transfer lens system is composed of four lens elements that one of the lens elements adjacent said image surface being negatively powered and the other three of said lens elements being positively powered, the relative powers of said four lens elements being selected with respect to the curvature of said image surface to provide a field curvature of radiation focussed at the detecting surface coincident with the physical curvature of said detecting surface, and the respective axial dimensions of the air space between the two lens elements proximal the image surface and between the two lens elements proximal the detecting surface being substantially greater than that of the remaining air space of the transfer lens system, said axial dimensions being selected to locate the pupil formed by the transfer lens system coincident with said cooled aperture, the overall power of the transfer lens system being selected to dimension the pupil formed by the transfer lens system to match the dimension of the cooled aperture and the curvature of the refractive surfaces of said lens elements being selected to provide low field and pupil aberrations.