Speckle and Speckle Metrology

Abstract

Speckles are a three-dimensional self interference pattern formed by a large number of random coherent waves. This results in a granular structure in free space or can be observed to be superposed on the image of the object and sets a fundamental limitation in high-resolution hologram microscopy and coherent imagery. Essentially, speckles are a randomly coded pattern that carries information about the object surface, and also has the rare distinction of being a nuisance and simultaneously a useful tool for measurement. Emergence of better measurement capabilities, newer recording media, faster computers and interfaces, and software has revitalized speckle metrology and has resulted in many fascinating new developments. Scores of books and review articles cover the myriads of applications and developments, and this chapter is intended to provide a concise and precise account of speckles starting from its infancy to the latest developments such as temporal speckle pattern interferometry or digital holography. Attractiveness of speckle methods lies in their ability to measure: large deformations, displacements with variable sensitivity to in-plane and out-of-plane, displacement derivatives, shape of a three-dimensional object, surface roughness, and dynamic displacement. Speckle techniques can nowadays be used to measure a wide range of displacement, sensitivities ranging from 0.25 µm to 150 µm for in-plane and 0.05 to 150 µm for out-of-plane displacements and with shearing techniques to over a 1000 µm. Speckle and Speckle MetrologyKeywords speckle; speckle photography; speckle interferometry; electronic speckle pattern interferometry; digital holography.