Application and Optimisation of the Spatial Phase Shifting ...

6.7 Extensions of SPS by temporal unwrapping 171
At that stage of the experiment, the automatic storage interval had expanded to ∆t 32 min.
Consequently, the fringe density in the images from the fixed-rate series is unnecessarily low, disk space
is wasted and the data evaluation gets more laborious.
In Fig. 6.29, we also find a hint that our cautious decision to regularly reset Φ(x, y, t i ) after each storage is
justified. As the object deformation grows slower, ∆t becomes larger, more noise is accumulated in the
temporally unwrapped data, reduces the accuracy and also triggers storage too early: in the automatically
saved image, we find less than 4 fringes instead of m 5.
The shown experiment demonstrates that fringe counting by means of temporal unwrapping is suitable to
adapt the data storage rate to the actual displacements. This is helpful not only for long-term observations:
in any experiment where no assumptions about the object's dynamics can be made, its motion can reliably
be tracked by the approach proposed here.
6.7.3 Relative displacements of discontinuous object
Especially in the investigation of historical material, one frequently encounters cracks in the surface under
inspection [Gül96] and it is important to know the relative motion of neighbouring sub-areas of the
object. As a realistic specimen of an aged material, a slice of a historical brick (2 cm thick) was
observed under temperature changes. The interferometer was again the out-of-plane assembly of Fig. 5.1,
only the test object had been replaced by the brick slice in upright position. The heat source was an
infrared radiator positioned some 30 cm behind the object. Fig. 6.30 shows a white-light image of the
measuring field.
Fig. 6.30: White-light image of historical brick.
When this sample is subjected to cycles of alternately 15 min of heating from the backside and 15 min of
cooling, the resulting deformations reveal 9 separately moving portions with rather different fringe
densities and complicated boundaries, as Fig. 6.31 demonstrates. The dashed line does not mark a
cleavage; but the fringes slightly change their orientation, as may be verified by viewing them along the
black-white edges at a small angle to the paper. The shown displacements each have evolved in time
intervals of 10 min. For the heating period, ϕ O (x, y, t i ) was stored at an ambient temperature T 1 when