Using this technique and a CCD camera with direct display of the images on the screen, the positions of all the dots is defined. Then the neighborhood of each dot is analyzed to define the strain of each element of the sheet during stamping. These values are then graphically displayed and set in comparison with the limit specified for the sheet. ESPI , deformation measurements with submicrometer resolution Measurement of deformation and strain in the micrometer range. ESPI is a laser based deformation measuring system.
Laser light is returned from the object and interferes, producing a speckle pattern image.
Due to small deformations of the object, this speckle pattern moves and fringes are produced on a monitor. These fringes can be evaluated to deformation and surface strain of the object. The method allows measurements with high resolution, but big displacements will reduce the fringe contrast and reduce therefore the maximum acceptable deformation.