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There are many applications of interferometry where the specimen cannot be transported to the laboratory, i.e., where an appropriate interferometer cannot be installed in a quiet and isolated environment. In these cases, the interferometer should be brought to the specimen. The specimen is sometimes located in a harsh environment where the interferometer must operate correctly. This chapter analyzes the primary harsh agents and the requirements that make it possible for an interferometer to successfully and consistently work in a harsh environment. A basic definition of a harsh environment is anywhere not “indoors.” However, this definition is not complete. It can be extended to a place where any condition of extremes relative to the human condition applies: temperature, humidity, environment (including pressure), radiation, shock and vibration, erosive flows, and corrosive media, whether indoors or not. Figure 4.1 shows two examples of harsh environments that fulfill the above definition because users are exposed to temperature, atmospheric variations, humidity, sun radiation, vibration, wind, and dust. The right part of this figure is a photograph of a measurement performed with a DSPI interferometer in a very aggressive environment with conditions far from temperature and humidity laboratory settings. In this case, residual stresses are being measured in a buried pipeline by using a portable digital speckle pattern interferometer combined with the incremental hole-drilling method. The device is firmly attached to the pipeline surface by using strong rare-earth magnets and an appropriate isotactic clamping system. It is evident from the figure that the interferometer should “survive” in the harsh environment, but it is also important to note that it must also give reliable results.
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