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Chapter 2:
Principles of Digital Speckle Pattern Interferometry
The invention of the laser in 1960 allowed for the availability, as never before, of light sources with a high degree of coherence. Researchers working with this kind of light source noticed that a strange phenomenon was produced: when a rough surface is illuminated with laser light, a high-contrast and finescale granular pattern is observable.1 This effect was called the “speckle effect,” characterized by the random distribution of scattered light. Although the random distribution of the optical field can be deterministically modified by displacement or rotation of the diffuser where the light is reflected or transmitted, this effect was considered as a mere nuisance, especially for the first holography techniques developed at that time. In addition to this property, the distribution is also modified by changes in the illumination and observation geometry, in the wavelength of the laser light, and in the refractive index of the medium through which the laser travels. Thus, speckle distribution can be used to measure features of the surface under investigation, such as (a) out-of-plane and in-plane components of the surface deformation of a rough object, (b) 3D surface shapes (by generating contours of the constant depth of an object), and (c) derivatives of the surface displacements.
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