FOCAL POSITION DETECTING DEVICE FOR CONDENSING LENS

PURPOSE:To efficiently calculate the accurate focal position from the convergent diameters at the front and rear points of the focal position and the shift quantity of a condensing lens by providing the shift quantity detecting means of the condensing lens and a sensor arranged with light receiving elements in a matrix shape. CONSTITUTION:A motor 9 is driven to move a condensing lens 4 to the first measuring position, and the shift quantity of the condensing lens 4 is applied to a focal position calculating device 13 from a position detecting device 11. A laser oscillator 1 is operated, a laser beam 2 is radiated on a sensor 5 via a reflecting mirror 3 and the condensing lens 4, and the voltage proportional to the light reception quantity of photoelectric transfer elements 6 is generated. The voltage is compared with the threshold value by a convergent diameter detecting device 12 and binarized, then the number N1 (convergent diameter) of the elements 6 receiving the laser beam 2 is obtained in the X-direction or the Y-direction of the sensor 5. The shift quantity L2 of the lens 4 and the convergent diameter N2 are likewise obtained at the next measurement position, the focal position is calculated 13 from these values, and the focal position is obtained accurately and efficiently.