Partial beam focus sensing in an optical recording system

Focus sensing using a light detector array is accomplished in an optical recording system using the light beam returned from the recording media through the objective focusing lens by passing the beam through a spherical focusing lens using a mask such as a knife edge which only passes a portion of the beam to the light detector array. The light detector array is matched to the mask and detects the different partial image patterns formed as a result of the close to focus condition with the recording media too far away from the objective lens and the near to focus condition with the media too close to the objective lens. In the focused condition, the light detector array receives a comparatively fine focused spot of light. Alternative embodiments of the present invention may use a cylindrical lens which in the focused condition creates a longitudinal bar of light. Further alternative embodiments may be made with a spherical Fresnel lens or a cylindrical Fresnel lens which create larger areas of light, round and bar shaped, respectively, in the focused condition.