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Speckle Interferometer

Speckle Interferometer


Figure 1: The speckle pattern formed by a HeNe laser illuminating an aluminum plate.

    When an object is illuminated by a laser, the rough surface of the object causes the reflected light to interfere with itself. The consequence is that a "speckle pattern" is formed, such as the one shown in Figure 1, with small areas of constructive interference (high intensity) and destructive interference (low intensity). A speckle interferometer is a device that makes use of speckle patterns to detect displacements, deformations, and path length differences on the order of several microns. Such a device works by splitting a laser beam into an image beam, which illuminates an object, and a reference beam. The two beams are recombined before being captured by in a photo. 
    Image subtraction is performed to see small displacements of the object being viewed. First, a photo of the object with no displacement or deformation is taken. A second photo is then taken with the displacement. The two images are subtracted using image processing software. In the subtracted image, regions where the displacement causes light's phase to be shifted by an even multiple of pi, the total intensity subtracts to zero, leaving a dark fringe[1]. Similarly, when the displacement causes the phase to be shifted by an odd multiple of pi, a bright fringe is formed[1].
    A speckle interferometer can be used to observed a variety of phenomena. For example, when a vibrating plate is looked at using a speckle interferometer, fringes would appear at the object's nodes. Furthermore, such an interferometer can detect deformations such as when the center of plate is pushed forward by only a few microns. In fact, thermodynamics and fluid dynamics can also be investigated using this device as changes in the index of refraction can be observed using speckle interferometry.

Speckle Interferometer Setup
A guide to assembling a speckle interferometer.


[1] Thomas R. Moore, "A simple design for an electronic speckle pattern interferometer," American Journal of Physics 72, 1380-1384