Applications of optical interferometry to engineering measurements at different scales

Optical interferometry combines two or more light waves in such a way that interference occurs between them. This principle is widely utilized in engineering applications to carry out high-sensitivity full-field measurements. The information on the quantity to be determined - spatial coordinates, displacements, strains, etc. - are encoded in a pattern of interference fringes. These fringes form because a reference wavefront interferes with a modulated wavefront experiencing some change in optical path with respect to the reference beam. From the interference pattern, one can obtain a full-field map of phase related to the difference in optical path between object and reference wavefronts. For each point of the specimen, the value of the quantity to be measured depends linearly on the corresponding value of phase. Sensitivity is the main parameter characterizing interferometric techniques and identifies the measurement scale range: it represents the change in the measured quantity between points located on two adjacent interference fringes. Optical interferometry techniques allow us to perform measurements at very different scales, ranging from nanometers to millimeters. It is worthy noting that the level of accuracy achieved in the measurements does not depend on the size of the investigated specimen. Coherent-light interferometry and white-light interferometry differ by the nature of the light used for illuminating the investigated object. Coherent illumination (for example, a laser source) allows sensitivity to be reduced down to a fraction of the wavelength of the light. White-light interferometry allows to carry out measurements on rather large objects without spending the large amount of energy which would instead be required in the case of coherent illumination. Moiré, speckle and holography are the interferometric techniques most widely used in engineering measurements. The chapter reviews the theory behind those methods and presents five cases taken from disparate fields: (i) calibration of gauge blocks; (ii) determination of residual stresses in thin films; (iii) monitoring of failure modes in electronic components subject to thermal cycling; (iv) mechanical characterization of composite laminates for aeronautical use; (v) measurement of deflections and determination of buckling loads for large-scale aircraft panels. The examples illustrated in the chapter cover a wide range of scales both in terms of magnitude of the different quantities to be measured and specimen sizes. Results demonstrate clearly the reliability, robustness and versatility of optical interferometry in engineering applications