Method and unit for focus detection

Abstract

Abstract PURPOSE:To obtain an accurate focus error signal with a small-size optical system, by making the reflection polarized light incident onto the prism face before and after the critical angle and by detecting light quantity distribution change of the reflection light, in the optical reader of a video disc and so on. CONSTITUTION:The light reflected by disc 6 is incident to detecting prism 10 through objective lens 5, 1/4 wave length plate 4, and polarization prism 3 having a polarization film. The light flux reflected by reflection mirror 11 is detected by detector 12. Reflection face 11 is set to be the approximately critical angle for the incident light (parallel light flux) under the focusing state of disc 6. When disc 6 is displaced to the (a) direction, light quantity distribution of photodetector 12 is dark and bright in photo detection area 12A and photo detection area 12B respectively; and when disc 6 is displaced to the (b) direction, light quantity distribution becomes reverse to the above. Consequently, a focus error signal indicating slippage quantity and the direction of disc 6 can be obtained by detecting the output difference between photo detection areas 12A and 12B.

Inventors

• 👤 Toru Musha

Bibliographic Details