Fresnel lens with aspheric grooves

Abstract

A Fresnel lens formed of an infrared transmitting material has a grooved surface. The grooves all extend to the same depth, and the distances between successive grooves vary across the lens. In a circular lens, the grooves are concentric, and the distances between grooves nearer the center are longer than the distances between the grooves nearer the circumference. A substantial portion of the grooves have an aspherical surface.

I claim: 
     
       1. A Fresnel lens formed of a material capable of transmitting infrared energy rays, .Iadd.in the wavelength range from 8 to 14 micrometers, .Iaddend.the lens comprising: a body, having a grooves surface;   the grooves surface having a plurality of grooves, which extend to the same depth;   the grooves being of different widths; and   a substantial portion of the grooves each individually having a surface formed thereon which is entirely aspherical and differs in contour from the surfaces of the other grooves.   
     
     
       2. A Fresnel lens formed of a material capable of transmitting infrared energy rays, .Iadd.in the wavelength range from 8 to 14 micrometers, .Iaddend.the lens comprising: a body, having a substantially flat grooved surface;   the grooved surface having a plurality of grooves, each of which extend to the same depth;   the grooves being of different widths; and   a substantial portion of the grooves each individually having a surface formed thereon which is entirely aspherical and differs in contour from the surfaces of the other grooves. .Iadd.3. The Fresnel lens according to claim 1 wherein the material of the lens is a plastic. .Iaddend..Iadd.4. The Fresnel lens according to claim 1 wherein the material of the lens is   
     
     
        polyethylene. .Iaddend..Iadd.5.  A Fresnel lens, comprising in combination: A body formed of a polyethylene material capable of transmitting infrared energy rays in the wavelength range from 8 to 14 micrometers, the body having a grooved surface;   the grooved surface having a plurality of grooves which extend to the same depth;   the grooves being of different widths; and   a substantial portion of the grooves each individually having a surface formed thereon which is entirely aspherical and differs in contour from the surface of the other grooves. .Iaddend.