in the same way to map out vibration modes. In-plane measurement The object is illuminated by two beams derived from the same laser beam which are incident on the object from opposite sides. When the object is displaced or deformed in the direction normal to the viewing direction (i.e. in it its own plane), the phase of one beam increases, while that of the other decreases, so that the relative phase of the two beams changes. When this change is a multiple of 2π, the speckle pattern coincides with itself (remains the same), while elsewhere it changes. When the subtraction technique described above is used, fringes are obtained which represent in-plane displacement contours. In-plane displacement gradient measurement The object is illuminated by two beams derived from the same laser which are incident on the object from the same side but at different angles. When the object is displaced or deformed within its own plane, the relative phases of the two beams change in proportion to the gradient of the in-plane displacement. Again, subtraction of the two images is used to display the fringes. Holographic interferometry has no equivalent to in-plane measuring ESPI. Acoustic interferometry, among with electromagnetic acoustic transducers, is capable of measure the two polarizations of in-plane vibrations. See also Holographic interferometry Interferometry Speckle pattern References External links 3D-ESPI Systems for material testing Tech On-line ESPI isi-sys optonor – optical testing and metrology Residual stress measurement based on hole-drilling and ESPI (stresstech group) Shearing ESPI for composites inspection Interferometry