Interferometric length measurements

Automated interferometer equipments are described which can be used for calibration of gauge blocks and line scales with lengths up to 1000 mm. For the gauge blocks, the scanning measurement method utilizes a white-light source and one stabilized 633-nm laser. White-light interference signals from the reference flat and from the front surface of the gauge block mark the distance to be measured with the laser. The final length is determined from the laser-signal phase shifts between the gauge and the reference flat. Environmental conditions are measured and the necessary corrections are made automatically. The repeatability of the measurements is ± 3 nm for a 400-mm gauge block and the estimated relative uncertainty (1s) is about 10^-7. The variation in length of the gauge block is obtained by computer-aided analysis of the digitized interference fringe patterns. For the line scales, a new measurement method has been developed which can utilize the same electronic units as used for gauge-block calibration: Images of the graduation lines replace the white-light interferences. The two fields constituting a video frame (image) are recorded by a moving microscope and by a ccd camera with 1-ms charging time interval. The exact distance between the microscope positions at the time of charging the ccd pixels is deduced from the interference and video synchronization signals. It is expected that a total uncertainty of about 100 nm for a 1000-mm scale will be reached.