Calculation of strain from an object grating on a reseau film

A correlation technique for the automatic calculation of grating coordinates in digital images is presented yielding displacements and strain on plane surfaces with an accuracy of 100 to 300 microstrain. It is shown that a large grating area can be investigated when the object grating is filmed by a reseau camera. Each segment of the negative can be digitized with a high-resolution CCD-camera resulting in a global resolution of about 6000^*6000 pixels in the whole image. To improve the accuracy of the grating coordinates and to avoid segmentation errors at the overlapping edges, the distortion of the CCD-camera and of the digitizing setup is corrected by a two-dimensional polynomial function. This is derived from the distortion of a digitized image of a high-precision reference grating. The strain calculation considers large deformations as well as rigid-body rotation.