Device for accurately focussing a laser beam

Claim 1

Device for focussing a laser beam onto an object, provided with a laser source, optical components for directing the laser beam to the object, mechanical and electronic means for controlling and correcting adjusting organs for the optical components, which electronic means comprise a quadrant detector which receives part of the reflected laser beam and detects form changes as a result of faulty focussing and also comprise a regulating circuit controlled by a focussing signal formed by the difference of the sums of the output signals of quadrants of the quadrant detector facing one another diagonally, assuming (A+C)-(B+D); A, B, C and D being the output signals of each of the quadrants of the quadrant detector, when said quadrants are passed through clock-wise starting at the first top left quadrant, characterized thereby that electronic means are provided which by means of the four output signals of the quadrant detector generate a correction signal for the fault arising on focussing as a result of the faulty centering of the quadrant diode and the laser beam proportional to the value "-V c H c /Σ" wherein V.sub.c =(A+B)-(C+D), H.sub.c =(A+D)-(B+C) and Σ=A+B+C+D; which correction signal is added to the focussing signal for controlling the regulating circuit.