(Received December 14, 1978)
A method for obtaining a highly-sensitive in-plane strain contour as a two-beam interferogram is devised. Using an optical setup in which an object is illuminated from two directions, deformation of the object is recorded in a form of a holographic modulated diffraction grating (MDG) modulated by the in-plane (x) and out-of-plane (z) components of displacement. Two identical MDGs are introduced into a system which connects two coherent double diffraction systems in series. Two spatial filtering processes are incorporated so that the z-component of displacement is cancelled and the difference between the two x-components inherent in the two MDGs, slightly shifted, forms an interferogram. Then the derivative of the in-plane displacement, i.e. the strain, is contoured in the image plane. In a simple experiment with a bent rectangular beam, the resultant interference pattern agreed well with the theoretical pattern and the amount of strain per fringe space was 3.6×10-5.