The testing of a cutting-edge geometry of a tool takes place during time intervals that comprise time instants at which it is to be expected that a region, to be tested, of the tool enters a measuring beam. A locking ring is used to secure the retaining sleeve to the mandrel. First supporting surface is close to other end of the cover. The source and drain are then formed. There are further provided a distance calculating section that calculates a distance to the subject based on the current feed quantity of a focus lens, when the distance measuring unit has failed in the measurement of the distance to the subject, and a light intensity determining section that determines the intensity of light to be emitted from the stroboscope based on the calculated distance.