Patent ID: 7041952

Claim:
A method for automatic focusing an imaging optical system on the surface of a sample, comprising the steps of: illuminating the sample by a measurement light beam that strikes the surface of the sample at an incidence angle differing from 0°; detecting light reflected from the surface of the sample by a position-sensitive receiving surface; allocating intensity values of the reflected light as to positions on the receiving surface; imaging the opening of a field stop onto the receiving surface, thereby generating an image that is smaller than the receiving surface; allocating a value of “light” or “dark” to each intensity value depending on whether each intensity value exceeds or falls below a defined brightness threshold; determining a smallest rectangle that encloses all positions on the receiving surface whose intensity value is “light”; determining a geometric center point of said rectangle; comparing a position of the geometric center point with a position on the receiving surface that corresponds to the geometric center point of the image generated by the field stop in the focused state; and effecting a change in the distance between the sample and the imaging optical system corresponding to the distance between the two points.