Optical fingerprinting with the laser for engineering measurement

A survey is made of metrological techniques that have been developed using the laser speckle pattern as a means of 'fingerprinting', or optically characterising a particular surface. Speckle interferometers have the high sensitivity of the conventional instrument, but can measure displacements and strain of a surface without preliminary preparation, and can detect surface vibration patterns. The simpler technique of laser photography relies upon imaging the speckle pattern, with subsequent analysis of its diffraction spectrum, to measure surface displacement and vibration to a lower order of accuracy. The advantages and limitations of these techniques are discussed, and examples given of their engineering application. Methods for detecting surface changes, and for assessing surface roughness quantitatively, are described.