The Rasnik 3-point optical alignment system

The Rasnik alignment system was developed initially in 1983 for the monitoring of the alignment of the muon chambers of the L3 Muon Spectrometer at CERN. Since then, the development has continued as new opto-electronic components become available. Rasnik systems are 3-point optical displacement monitors and their precision ranges from below nanometers to several micrometers, depending on the design and requirements of the systems. A result, expressed in the range/precision ratio of 2×106, is presented. According to the calculations of the Cram'er-Rao limit, and by means of MonteCarlo simulations, a typical Rasnik image should have enough information to reach deep sub-nanometer precision. This paper is an overview of the technological developments and achievements since Rasnik was applied in high energy physics experiments.