Patent ID: 7564617

Claim:
An infrared lens system comprising: a focus section that focus infrared radiation at an image plane; and a zoom section that adjusts a desired focal length of said infrared radiation, said focus section and zoom section comprising a plurality of lenses which are located along an optical axis in optical communication with each other, said focus section and zoom section each comprising a fixed lens fixed with respect to the optical axis and a movable lens arranged for movement along the optical axis, wherein the lenses of said zoom section satisfy the following conditions: 1.5 >T/f >1.3 0.7 >Ts/f >0.5 1.8>( D*FN )/ f >1.2 6>X>3 Wherein T—is a total length of said lens system, measured from a first surface of said fixed lens of said zoom section to the image plane along the optical axis f—is the longer focal length of said zoom section at a fully extended position of said zoom section Ts—is the distance between the first surface of said fixed lens of said zoom section to an aperture stop D—is the diameter of said fixed lens of said zoom section FN—is the F number of the infrared lens system X—is the focal length ratio between the longer focal length f and the shorter focal length at a fully retracted position of said zoom section.