Interferometry
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Interferometry Christopher J. Evans Mechanical Engineering and Engineering Science, The William States Lee College of Engineering, UNC Charlotte, Charlotte, NC, USA
Synonyms Interferometric metrology
measurement;
Interferometric
Definition Interferometry is a process in which the superposition of two or more waves (beams) of sufficiently coherent electromagnetic radiation provides information about the waves and/or their differences.
Theory and Application In modern manufacturing, interferometry is commonly encountered in: • Definition, realization, and dissemination of standards of length and form
• Measurement of displacement for the control or calibration of production and measuring machines • Measurement of the surface form of optics and precision machined metal and other components • Measurement of surface texture Before considering each of these areas in turn, the fundamentals of interferometry will be very briefly reviewed. Most applications of interferometry in manufacturing use “light” ranging from the UV to the infrared (i.e., wavelengths from 350 nm to 10.6 mm); in principle everything discussed here applies at any wavelength for which appropriate beam splitters, reflectors, sources, and detectors exist. Each of the major sections of this article takes the Michelson interferometer as its point of departure. It is, conceptually, an easily understood interferometric configuration and is used to some extent in each of the main areas identified above. Basic Concepts The fundamental ideas of interferometry are treated, at various levels of mathematical rigor, in standard optical texts (Hecht 2003; Born et al. 1999). Conceptually, interferometry is most easily understood in terms of the wave nature of light, an electromagnetic wave (following Maxwell’s equations) with a phase that changes by 2p radians every wavelength. A wavefront is a
# CIRP 2016 The International Academy for Production Engineering et al. (eds.), CIRP Encyclopedia of Production Engineering, DOI 10.1007/978-3-642-35950-7_16700-3
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Interferometry
surface of constant phase (e.g., the peak, valley, or any other phase in Fig. 1). The Michelson is a conceptually simple interferometer (Fig. 2). Collimated light from a source is divided into two beams, each of which reflects off mirrors and returns through the beam splitter to a light-sensitive detector. The time-averaged intensity depends on the phase difference between the two beams. If the path lengths of the two beams is equal (modulo 2p), the waves interfere constructively (intensity increases); minimum intensity is observed when the beams are exactly out of phase (destructive interference). As the optical path difference (OPD) varies, the detected intensity (phase) varies. The Michelson is a common form of interferometer in production engineering applications. Like most interferometer configurations, it splits the amplitude from the light source. A more limited number of interferometer configurations use wavefront division (see, e.g., Hecht 2003).
Two other critical factors need to be cons
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