A commercial laser interferometer for length measurement by fringe counting

The advantages of length measurement by fringe counting are described. The instrument employs a stabilized frequency source which maintains the frequency to ± 1 part in 107, for a temperature range of at least 26 deg C. The error rate due to amplifier noise in the fringe circuits is negligible, and 20% drift can be tolerated. A measuring accuracy of 3 parts in 106 in average atmospheric conditions is achieved with an economical wavelength to inch and millimetre conversion method.