Application and Optimisation of the Spatial Phase Shifting ...

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1 Introduction
The present era of high technology, with its enormous production capacities and ever-increasing rate of
invention, has generated a great need for tools to make new solutions reliable and safe. It is indispensable
to test prototypes experimentally to find design flaws, improve concepts and increase outputs. In serial
production of delicate and expensive items, it is desirable to distinguish faulty pieces quickly from good
ones without subjecting them to excessive stress and possibly destroying them; consequently, these
methods are referred to as non-destructive.
While non-destructive testing (NDT) supports industrial development, it is also suitable to deal with some
consequences of industrialisation, which has given rise to environmental pollution, changing into
destruction in the past few decades. Due to air pollution, the decay of historical buildings and monuments
has accelerated in a disquieting way since about the middle of the 20 th century. In exact opposite to serial
production, the role of NDT in this context is to assist in valuation of measures to preserve unique works
of art. The Applied Optics workgroup at the Carl von Ossietzky University of Oldenburg has been
working in this field for more than two decades.
Interferometry is an elegant way to accomplish these contradictory tasks, with the additional benefit of
being non-contacting, in contrast to, e.g., strain gauges. The sensitivity of interferometric methods
depends largely on the wavelength of the used radiation; for the optical wavelength range, the sub-µm
scale is therefore easily accessible, and with some care, even the nm scale can be reached. Since the
invention of strong sources of coherent light [Mai60], interferometric methods can be conveniently
utilised for a multitude of measuring problems.
However, with the advent of masers and lasers, the so-called speckle effect, known since the 19 th century,
became very important. As opposed to classical interferometry with polished parts like lenses and mirrors,
optically smooth surfaces are generally rare; they seldom occur in industrial processes, and almost never
in studies of historical objects. The wavefront coming back from a scattering object has a random
intensity and phase structure, the speckle pattern; therefore, a general approach to interferometry requires
comparing such a wavefront with itself.
This was initially done by holographic interferometry, where a hologram of the object provides the
reference. By viewing the object through the hologram, a real-time interference of the reference and the
slightly different momentary wavefront is observed. Depending on whether the wavefronts are locally in
phase or out of phase, the object appears covered by a pattern of bright and dark fringes that can easily be
interpreted as iso-lines of equal object deformation. Thanks to the high spatial resolution of holographic
silver halide emulsions, these fringes are very clear for the most part and very small speckles are
allowable.
A significant disadvantage of holographic interferometry is the necessity of relatively long exposure
times, typically about a second; therefore great stability, most probably in a laboratory, is required, or