| 摘要 |
PURPOSE:To detect a focus position with high precision without any influence of the drift of a position sensor, etc., by making reference light incident directly on the reference position of a photodetecting elements which photodetects reflected light from a surface to be measured without passing through the reflection path of the surface to be measured. CONSTITUTION:A position sensor 9 is irradiated with reference light from a reference light source 8 directly not through the reflection path of the surface 4 to be measured prior to focus position detection. The current output of the position sensor 9 is specified as a reference value to know the quantity of output drift of the position sensor 9, and the surface 4 to be measured is irradiated slantingly with luminous flux from a light source 1; and the position of reflected light from the surface 4 to be measured is detected by the position sensor 9 and the quantity of output drift of the position sensor 9 which is measured previously is subtracted from the image formation position signal of the sensor to eliminate the influence of the drift of the position sensor 9. |
