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The size (spatial coherence) and monochromaticity (temporal coherence) of the light
source must satisfy certain minimum requirements that depend on the geometry of
the system, as described by Hansen (1955, 1984) and by Birch (1979). It is interesting
to know that if the optical element under test has very steep reflections, the state of
polarization of the light may change in the reflection, introducing changes in the
contrast (Ferguson, 1982). However, in most of the cases, the important factor in the
contrast is the coherence of the light source.
2.3.1. Spatial Coherence
The light source for interferometry must satisfy some minimum requirements of
spatial as well as temporal coherence, depending on the interferometer configuration
and the specific application and needs. As described in Chapter 1, Section 1.1.2.,
some gas or vapor lamps can be used in conjunction with a small pinhole to
illuminate an interferometer. These lamps with the pinhole do not have perfect
spatial and temporal coherence. A gas laser, however, has perfect spatial coherence
and can have almost perfect temporal coherence. We might think at first that this is
the ideal light source for interferometry, but this is not always the case. The
coherence length is, in...
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