Focusing error detection apparatus

Claim 1

In a disk reproducing apparatus, a light beam focusing apparatus comprising: a light beam source; a light reception member divided, with respect to a first axis, into three light receiving portions, including a first light receiving portion having a first width a measured along a second axis perpendicular to said first axis, and second and third light receiving portions, one on either side of said first light receiving portion in a direction of said second axis, and having second and third widths b and c, respectively, measured along said second axis, said light reception member being positioned so as to intersect an optical axis of the light being emitted by said light beam source, first, second, and third photodetector outputs respectively being output in response to said light beams striking said first, second and third light receiving portions, said light beam having a radius r at a point of contact with said light reception surface; and focus error signal producing means for combining said first, second, and third outputs to provide a focus error signal indicative of focus error, wherein said first, second, and third widths are determined in accordance with the following formula: 2r+dc>a+b+c, wherein dc is a maximum distance that said light beam can deviate along said second axis from a center of said first light receiving portion while said level of said focus error signal remains below a predetermined reference value.