Automatic gauge block measurement by phase stepping interferometry with three laser wavelengths

An evaluation system for automatic measurements of short gauge blocks by interferometry is described. The system is based on a commercially available Twyman-Green interferometer which has been modified so that phase stepping interferometry can be applied. Three stabilized lasers are used to determine the gauge block length by the method of exact fractions. The data of the evaluated interference phase have to be processed to enable automatic identification of the gauge block's measuring face in the interference pattern and to determine characteristic quantities of the gauge block under test. The uncertainty contribution of the interference evaluation for the central length is approximately 1,5 nm which is sufficiently small compared with other uncertainty contributions of the measurement process.