By using three noncoplanar coherent laser beams intersecting one another at angles larger than 30°, three sets of interferometric gratings are generated. By placing a specimen coated with a photosensitive material inside this grating field and exposing it before and after load, one can obtain three distinctive moire fringe patterns, from which principal surface strains can be calculated by using the strain rosette formula.
F. P. Chiang,
C. C. Kin,
"Three-Beam Interferometric Technique For Determination Of Strain Of Curved Surfaces," Optical Engineering 23(6), 236766 (1 December 1984). https://doi.org/10.1117/12.7973379