Patent ID: 7042817

Claim:
An aberration detection device in which a light beam emitted from a light source is focused on an information recording layer of an optical recording medium through a dual lens objective lens constituted by a first lens element and a second lens element, and in which a light beam reflected from the information recording layer is incident on light beam separation means through the dual lens objective lens so as to separate the light beam into a first light beam, which includes a light axis of the light beam, and a second beam, which does not include the light axis of the light beam, and thereby detect a spherical aberration of the dual lens objective lens, said aberration detection device comprising: spherical aberration detection means for detecting the spherical aberration in accordance with at least one of focus positions of the two light beams separated by said light beam separation means, said light beam separation means including a first region for separating the first light beam, and a second region for separating the second light beam, and said first and second regions are separated by a boundary, and said boundary is formed as a circle or an arc centered on the light axis of the light beam, having a radius virtually 70 percent of an effective radius of the light beam.