GB902535A - Improvements in or relating to lenses - Google Patents

Abstract

902,535. Fresnel lenses. JOBLING & CO. Ltd., JAMES A. April 6, 1960 [April 6, 1959], No. 11597/59. Class 97 (1). A Fresnel lens has a plurality of concentric prisms which each lie in the position of minimum deviation with respect to rays incident on a prism from a source located at the principal focus of the lens and the corresponding refracted rays. In this lens the hypothetical surface containing the points of intersection of said incident and refracted rays is defined by the relationship r=Kq, where r (not shown) is the distance of any given point on the surface from the principal focus, K (not shown) is the distance from the principal focus to a point where said surface is cut by the optical axis and q is a function of the angle of deviation D (not shown) at said point and the refractive index of the lens material, such function being given in the Specification. One example is given in the Specification of a method of designing such a lens from a table of data giving the values of q (not shown) for corresponding values of the angle D taking the refractive index of the lens material to be 1.51. From these values, the value of K is found and any number of corresponding values of r and D could then be calculated so that the sectional shape of the hypothetical surface could be drawn in. The smooth surface on the side of the lens away from the principal focus is then drawn parallel to the hypothetical surface and displaced therefrom by a distance equal to half the desired mean thickness of the lens. The inner surface of each prism P1 to P4 is then drawn in at an inclination to the vertical plane through the optical axis which is necessary to give minimum deviation for rays incident thereon from the principal focus F. The lens as so designed may be made by pressing hot fluid glass in suitable moulding equipment. One example of a finished lens is given in the Specification.