This is an optical method of measuring surface strains. A grating is deposited on the surface of the specimen using photolithography. This grating, as shown below, typically has a pitch of 5 microns for large strain measurements and can be much smaller for elastic strain measurements. Two laser beams are then used to illuminate the specimen surface. Two of the diffracted beams from the grating are collected and interfered, this produces a fringe pattern. By using a grid, that is, two perpendicular gratings, and collecting a fringe pattern for both gratings, all three components of the surface strain can be measured. See publication  on the previous page for more details.
Microscopic view of the grating on a specimen surface. The dots are 2.5 microns in diameter and spaced 5.0 microns apart.
Typical fringe pattern obtained using the moire microscope.