The present invention is related to a process of high sensitivity for measuring the amount of fine deformation of a test object at the material test, non-destructive test or the like.
As for conventional processes for measuring fine deformation of test objects in material tests, non-destructive test or the like have hitherto been known. For instance, the process is carried out by measuring strain based the variation of resistance value caused by deformation, or in a process, so called Moire process, equidistant gratings are formed on a test object by about 20-50 per/mm, and the detected gratings are lined up for reproducing visible structural patterns formed by the two gratings so as to measure the amount of deformation in the pattern.
In the former conventional process which is carried out by strain measurement, the detected amount of one portion to be detected is indicated as an average value of the whole portions to be detected. Therefore, it is necessary to arrange a lot of small portions to be detected adjacent to one another when a local deformation over an extensive range is requested for detection, and thus an increased number of indicators for the detected amount are also indispensable to be installed. Even in this case the maximum dimension of the portion which can be detected is only about 0.5 mm.sup.2.
On the other hand, according to the latter Moire process, although the whole area for measurement can be detected uniformly and extensively it is necessary that the gratings of fine pitches are equipped for detecting the amount of fine deformation. This causes difficulties in manufacturing the original plate as well as in printing treatment of the original plate on the test object through photographing.
In order to eliminate the foregoing drawbacks, the prior Patent Application was filed in U.S.A. on Oct. 6, 1971 by the same applicant of the subject application as U.S. Patent application Ser. No. 186,927, now abandoned, entitled with "A METHOD FOR MEASURING DISPLACEMENT OR DEFORMATION" in which a periodic structure having diffractive function is illuminated with beam, and waves of diffraction order in conjugate relation or in similar relation are selected among the diffracted wave fronts projected from the periodic structure by the illumination with beam so as to cause mutual interference, thus, the amounts of deformation occurring at the basic displacement in the period of the periodic structure or deformation in the periodic structure are measured by means the interference fringe formed by the interference.