| 摘要 |
PURPOSE:To discriminate focus errors with use of a refraction member which has simple constitution and can be produced at low cost, by using two photodetectors set inside and outside a circular boundary to obtain a photodetecting member and setting this member at a place close to the back of the refraction member produced by putting two types of materials on each other with a cylindrical boundary surface formed on the basis of the center of an optical axis. CONSTITUTION:A refraction member 21 has a double structure, i.e., an inside material 21a and an outside material 21b. The refractive index n1 of the material 21a is set larger than the refractive index n2 of the material 21b. The detection beam reflected by a recording surface Da of an optical disk D is reflected again by a beam splitter 14 and made incident on the material 21a together while being condensed. The incident beams within a range alpha larger than the critical angle theta in terms of the incident angle are reflected totally on a boundary surface A and detected as alpha' by an inside photodetector 22a of a photodetecting member 22. Then the beams within a range beta smaller than the angle theta pass through the surface A and are made incident on the material 21b to be detected as beta' by an outside photodetector 22b of the member 22. |
