How ESPI Works
The component under investigation must have an optically rough surface so that when it is illuminated by an expanded laser beam, the image formed is a subjective speckle pattern. The light arriving at a point in the speckled image is scattered from a finite area of the object, and its phase, amplitude and intensity, which are all random, are directly related to the microstructure of that area in the object.
A second light field, known as the reference beam, is derived from the same laser beam and is superimposed on the video camera image (different configurations enable different measurements to be made). The two light fields interfere and the resulting light field has random amplitude, phase and intensity, and is therefore also a speckle pattern. If the object is displaced or deformed, the distance between object and image will change, and hence the phase of the image speckle pattern will change. The relative phases of reference and object beam change, and therefore the intensities of the combined light field changes. However, if the phase change of the object light field is a multiple of 2π, the relative phases of the two light fields will be unchanged, and the intensity of the overall image will also be unchanged.
To visualise this effect, the image and reference beams are combined on a video camera and recorded. When the object has been displaced/deformed, the new image is subtracted point by point from the first image. The resulting image is a speckle pattern with black 'fringes' representing contours of constant 2nπ.
Read more about this topic: Electronic Speckle Pattern Interferometry
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