There are first and second relatively movable plates. On a face of each of the first and second plates, there are first and second alignment marks, each being a linear grating of parallel lines of uniform spatial period, the spatial periods being different from each other. There is a light source for...http://www.google.de/patents/US5414514?utm_source=gb-gplus-sharePatent US5414514 - On-axis interferometric alignment of plates using the spatial phase of interference patterns
