Objective lens and imaging optical system for optical information recording and readout device

Claim 1

An objective lens for an optical memory device designed as a single-element lens having a first aspherical surface and a second aspherical surface expressed by the formula shown below when the direction along the optical axis is taken as the x axis and the direction perpendicular to the optical axis is taken as the y axis: ##EQU3## wherein the reference symbol r represents radius of curvature on the vertex of the aspherical surface, the reference symbol P designates the conic constant and the reference symbol A 2i denotes the coefficient of aspherical surface of the 2i'th order, and so designed as to satisfy the following conditions (1) through (4): (1) n<1.65 (2) 0.25<d/nf<0.4 (3) -0.5<P 1 <0.5 (4) P 2 <-20 wherein the reference symbol n represents refractive index of the objective lens, the reference symbol f designates focal length of the objective lens, the reference symbol d denotes thickness of the objective lens, and the reference symbols P 1 and P 2 represent the conic constants of the first surface and the second surface respectively.