On-machine Measurement of Mirror Profile-Measurement Principle and Basic Performance of the Developed Sensor-

This paper presents a new differential method named mixed-method, which is combined with the laser autocollimation and 2-point method. This method is developed to measure the profile accurately under the circumstance of on-machine measurement where large datum errors are introduced by scanning stage, vibration and thermal drift. An optical sensor using parallel beams with incident angle of 45° was designed and made to realize the mixed-method. Estimating from the SN ratio, the sensor can measure the displacement and angle of two points on a mirror surface simultaneously with the resolution of 0.1 μmand 1 second. Experiments with the sensor have shown that it can reduce datum errors by 30 dB. Using this sensor, measurements of the profile of flat mirrors and the straightness of cylindrical mirrors have been tried and effectiveness of the method especially for detection of high spatial frequency of profiles has been confirmed.