Speckle Pattern Interferometry (SPI) determines the difference between two small deformation states of the same inspected object. Speckle-pattern-interferometry shows a fringe pattern, which can be interpreted as contour lines of the same state of deformation. The distance between two fringes corresponds to half the laser wavelength. Therefore SPI allows sensitive and non-destructive detection of defects.

It makes use of speckle pattern interferometry along with CCD imaging techniques and image processing software for studying the static and dynamic conditions of the test objects. HeNe laser is used as the light source. A disc specimen loading system is provided with the instrument for calibration and laboratory experimentation.