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Holographic interferometry is a measurement technique that studies changes in an object's surface structure. Laser light reflected from an object is recorded in a hologram as a three-dimensional record later reconstructed in exact detail. This record may be compared to the original object or to other holographic exposures of that object. When compared, change in the surface will be indicated by fringe, or striped, patterns of interference. These patterns are produced when the light waves reflected by an object differ because of surface displacement, interfering with one another.
The use of holograms gives this a unique advantage over conventional methods of optical interferometry. Structural changes can be studied in a direct comparison between holograms made of the object at different times under varying conditions. The information recorded is of an object's entire surface. Displacement effects the object as a whole can then be observed.
There are three basic methods of holographic interferometry. These include real time, multiple exposure and time average methods. Lasers of nearly any wavelength can be used. Continuous lasers are typically used for real time examination of surface changes and motion. Pulsed lasers are best used in the study of rapidly changing phenomena.
Real time holographic interferometry allows the immediate observation of tiny changes in an object as it is subjected to stress. A hologram of the object to be studied is superimposed over the object itself. If the object is now subjected to stress factors, any deformation in the surface will be observed as fringe interference patterns. Measurement of these patterns reveal the magnitude and direction of deformation in precise detail.
Multiple exposure techniques make use of two or more holographic exposures. The initial hologram is of the object at rest. Additional exposures are made and recorded in the same image, as the object under study is subjected to a particular stress factor. The final holographic image depicts the change in surface displacement over the course of testing. Pulsed lasers can be timed to record critical test intervals or an object's change in state.
In the time-average method, a hologram is created while an object is subjected to periodic rather than continual stress. The result is an image of the object's vibration pattern. Holographic interferometry allows for a very precise measurement of complex vibrational patterns.
Each of these methods shows the shape, size and direction of surface displacement. The precision of holographic interferometry allows meaningful data to be gathered from non-destructive testing. This makes the technique particularly suited for quality control inspection. The exact preservation of data in holographic form allows for easy conversion to digital format and computerized examination.