Position determination system for determining the position of one relatively moveable part relative to another relatively movable part

摘要

A measurement scale ( 20 ) is movable in plane x relative to a mask ( 10 ) and photodetector ( 30 ). When a pattern of apertures in the mask ( 10 ) are in correlation with a pattern of apertures e.g. a reference mark in the scale ( 20 ) then light bands a b and c may propagate to the detector ( 30 ). The relative sizes of the patterns is such that the light bands converge at the detector ( 30 ). Other embodiments show light divergent and light inversion correlation of incongruent patterns.