Composite fresnel lens having array of lens segments providing long

Abstract

A composite Fresnel lens useful in an infrared passive detection system includes an array of Fresnel lens segments having a given vertical centerline and a curvature of a given radius. The Fresnel lens segments fall in first and second groups. Each Fresnel lens segment in the first group has a long detection range and a width less than one-third of the diameter of an original whole Fresnel lens from which the segment was taken. Also, each lens segment of the first group has a geometrical center located horizontally from the vertical centerline of the array along the curvature of the array through a circumferential distance less than one-fourth of the radius of curvature of the array. Each Fresnel lens segment in the second group has a short detection range less than the long detection range by a predetermined amount and a width less than one-third of the diameter of an original whole Fresnel lens from which the segment was taken. Also, each lens segments in the second group has a geometrical center located horizontally from the vertical centerline of the array along the curvature of the array through a circumferential distance less than one-half of the radius of curvature of the array. The Fresnel lens segments have respective optical centers which are aligned along the vertical centerline of the array.