Moire Interferometry Near The Theoretical Limit

The theoretical upper limit of moire interferometry is approached as the reference grating pitch approaches /2 and its frequency approaches 2/. This work demonstrates the method at 97.6% of the theoretical limit. A virtual reference grating of 4000 lines/mm (101,600 lines/in.) was used in conjunction with a phase type reflection grating of half of that frequency on the specimen. Sensitivity was 0.25 _m/fringe (9.8 _in./fringe). In-plane displacement fringes of excellent definition were obtained throughout the 76 _ 51-mm (3 _ 2-in.) field of view. They were very closely packed, exhibiting a maximum fringe density of 24 fringes/mm (610 fringes/in.). Effectiveness of moire interferometry near the theoretical limit was proved.