Electronic Speckle Interferometry (ESPI) allows surface deformations to be measured accurate to the nanometer. This rapid and spatially resolved measurement technique has long been in use for measuring minimal deformations which may be produced as the result of component vibrations, thermal loading or also mechanical tensile and shearing stresses.
The method involves illuminating an object with an expanded laser beam. The resulting camera images contain a speckle pattern. Any deformation of the object by a fraction of the wavelength leads to a detectable change in this speckle pattern. This allows the measurement of minute motions and surface deformations due to internal stress. Special computer algorithms compute these deviations at high speed, providing real-time deformation measurement.
The great advantage of speckle interferometry is its accuracy: the measuring method has thus long been in use for measuring minimal deformation. Classic ESPI methods frequently use temporal phase-shift methods. It is always necessary in this case to take a sequence of camera images to register all required information on the current state of deformation. The position and shape of the object to be measured must be absolutely stable during the recording time since otherwise, measurement is not possible. This means that the measurements can be conducted only in thermally and mechanically stable conditions. The total time required for a series of tests to examine the mechanical deformation as the result of thermal loading would be extremely long because of this. Examining deformation as the result of dynamic, thermal or mechanical processes is virtually impossible. The
The system from Fraunhofer IPM conducts measurements 500 times per second – commercial units require a few seconds for a single comparable measurement.