Method of detection null of foreign material coming in contact to the object glass of

摘要

PURPOSE:To detect a foreign material such as tar of a cigarette or water drop deteriorating the image forming performance of an objective lens by detecting a focusing error signal to apply a prescribed drive signal to a lens actuator. CONSTITUTION:The light of a light source 8 is collimated by a collimator 7, collected onto a disk 1 by an objective lens 3, the reflected light from the disk 1 is collected by the lens 3, reflected on an optical path conversion means 6, collected by a cylindrical lens 9, made incident in a 4-split photodetector 10 to detect a focusing error signal. Normally, a switch 12 is thrown to the position of the circuit 11 to apply focusing control and in detecting the adhesion of a foreign material onto the lens 3, the switch 12 is thrown to the position of the circuit 5, a sinusoidal drive signal is fed to a lens actuator 4 to change the distance between the disk 1 and the lens 3, an error signal is generated by an astigmatism method using the lenses 3, 9 and the detector 10 and the adhesion of tar of a cigarette or water drop onto the lens 3 is detected by a focus error signal level detector 13.