Displacement measurement apparatus (1) shown in FIG. 1 measures the position of a reading means (4) relative to a coded track (2) which carries a sequence of code elements (5). The track is encoded with a binary sequence such that reading a binary word at any position along the track uniquely identifies...http://www.google.ca/patents/US5187546?utm_source=gb-gplus-sharePatent US5187546 - Displacement measurement apparatus with dual wedge interferometers
