The laser interferometer is attractive as a position-sensing device for microlithography because of its high resolution, high accuracy, ease of installation, and non-contacting measurement capability. The usual confiqu ration for x-y coordinate sensing is the plane mirror interferometer which minimizes Abbe offset errors. The laser's disadvantages are its relatively high cost, susceptibility to environmental disturbances, non-standard output units, and incremental rather than absolute measurement. However, all of these disadvantages may be reduced or avoided by proper design and installation techniques. There are several approaches to electronically interfacing the laser interferometer to a machine controller, and each approach has its own tradeoffs among cost, speed, and hardware and software complexity.