invention enables the position of mirrors especially rotating mirrors to be quickly measured optically using a simple optical construction the measuring beam is focussed onto the light detector by an optical system a signal corresponding to the geometric center or the maximum of the intensity distribution of the focussed measuring spot is determined based on the measured values obtained by the light detector application_number US 19807702 A description 0031 fig1 shows an apparatus according to the invention comprising a light source 2 that is preferably constructed as a laser and or a diode laser a measuring beam 6 is produced by an aperture 4 with a predefined aperture diameter a and reaches an optical imaging system 8 in the simplest case this optical imaging system 8 comprises a lens with a positive focal length or a convex lens possibly a system of several lenses e g an achromatic lens or objective as may be seen the measuring beam from optical system 8 reaches a mirror 10 to be measured and is reflected by the mirror onto a position sensitive light detector 12 at a predetermined and preferably small angle to the perpendicular the mirror 10 is configured as a rotating mirror which can be rotated about an axis 14 orthogonal to the drawing plane in accordance with arrow 16 the measuring beam is focused with a long focal length by optical system 8 with light detector 12 at the focal point the position sensitive light detector 12 also.