Wide aperture objective lens

Claim 1

A light-weight, wide-aperture objective lens for use in an optical disc recording system, said lens comprising: a first lens element of the double-convex type; a second lens element being of the negative meniscus type, having its concave surface facing said first lens element and spaced therefrom; a third lens element, being of the double-convex type, and spaced from said second lens element; a fourth lens element, being of the negative meniscus type and having its concave surface in surface contact with said third lens element; and a fifth lens element, being of the positive meniscus type, having its convex surface toward said fourth lens element and spaced therefrom; wherein, provided said fifth element is spaced by a preselected distance from a plastic layer having uniform thickness, of approximately 1.2 mm, light from said objective lens is refracted through the layer and focused substantially at the point of exit from the layer; wherein said lens has radius, spacing and thickness dimensions approximately as follows: Where: The elements are numbered from left to right, the light entering element 1 first; a positive radius means the center of curvature is to the right of the element; a negative radius means the center of curvature is to the left of the element; R L = radius of left face of lens; R R = radius of right face of lens; T = lens thickness at the central axis; and D = distance to next lens element, at the axis; and wherein said lens provides a numerical aperture in the range from 0.7 to 0.8, and is color-corrected to accommodate a spectral range including light of wavelengths 478.5 nm and 496.5 nm.