Application and Optimisation of the Spatial Phase Shifting ...

6.1 Optimisation of experimental parameters 137
Fig. 6.3: Interferogram power spectra (log scale). Upper row, d s = 3 d p ; lower row, d s = 2 d p ; left, α x =90°/column;
right, α x =120°/column. Irregularities in the spectra are due to the fibre guide obscuring part of the
aperture. The contrast of the images has been enhanced to make the speckle halo visible.
It is also clear that a decrease of the speckle size, as shown in the lower row, will shift the advantage even
more towards α x =90°/sample because this minimises "crosstalk" of the sidebands around both ν=0 and
ν=ν N , as discussed in Chapter 3.4.4. On the other hand, the sidebands have less overlap with the speckle
halo for larger phase shifts; but evidently, the issue of speckle size is more important.
6.1.3 Speckle aspect ratio
In Chapter 5.6, we saw what improvement a change to an elliptical aperture can bring about when the
available illumination power is critical. However it is by no means necessary to choose a 1:3-elliptical
aperture. For instance, an aspect ratio of, say, 1:2 means less anisotropy, at the cost of light, while an
aspect ratio of, say, 1:4 improves the light gain but generates elongated speckles, and accordingly, a
distinct anisotropy of measurement. The change in performance need not be restricted to the spatial
direction in which the speckle size is reduced: the finer overall phase structure of the speckle pattern
could increase the noise in the whole measurement, which would diminish the advantage gained by the
larger aperture.
This subsection attempts to answer the question what speckle aspect ratios can be used and at what gain or
expense. Since the course of σ d as a function of object illumination is similar for TPS and SPS (cf. Fig.
5.18), we just retain here that the gain in accuracy may be related to the gain in light as before, only now