GAUGE-BLOCK INTERFEROMETER BASED ON ONE STABILIZED LASER AND A WHITE-LIGHT SOURCE

An automated interferometer equipment is described which can be used for calibration of gauge blocks with lengths up to 1 000 mm. The main advantage of the new type of interferometer is that no prior knowledge on the length of the gauge block is required. The measurement method is based on scanning and it 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 indicate the distance to be measured with the laser. The final length is determined from the average laser-signal phase shift between the gauge and the reference flat. Environmental conditions are measured and the necessary corrections are made automatically. An integrating sphere used for evaluation of the surface roughness correction is described. Results of the various test measurements with both short and long gauge blocks are given. The estimated overall uncertainty (2sigma) is 30 nm for a 100 mm gauge block and 160 nm for a 1 000 mm gauge block. Repeatability of wringing is the main source of uncertainty with short gauge blocks. With long gauge blocks, the accuracy is limited by uncertainty in the temperature of the gauge and in the air refractive index.