Application and Optimisation of the Spatial Phase Shifting ...

6.7 Extensions of SPS by temporal unwrapping 169
Fig. 6.26: White-light image (left) and deformation map (right) of fresh chestnut.
The changes of the chestnut's surface were monitored over some days from shortly after its fastening in
the interferometer (which was the set-up of Fig. 5.1) until the deformation had settled somewhat. Besides
the matched storage of phase maps ϕ(x, y) whenever the threshold of m=5 fringes was reached, additional
ones were stored at the steady rate of 1 frame per 10 min to study possible performance differences
between the methods. Fig. 6.27 provides an overview of the deformation dynamics. The black curves (left
ordinate) show the courses of the matched and static storage intervals ∆t versus time after the beginning of
the observation. The white curves (right ordinate) show the corresponding courses of the hard disk space
required for storing the phase maps.
∆t /min
35
30
25
20
15
10
5
static storage
HD usage/ MB/d
700
600
500
400
300
200
100
0
0 12 24 36
48 60 72 84
0
168 180 192 204
time/h
Fig. 6.27: Course of the matched and static storage interval ∆t (black curves, left ordinate) during several days; for
static storage ∆t is fixed to 10 min. White curves: hard disk space required in MBytes/day (right ordinate).
The matched data storage went through several phases: in the first 3 hours, the chestnut appeared to settle
in its spring-loaded holder and short storage intervals ∆t were necessary. After 15 h, the deformation
slowed down; the matched ∆t were incidentally similar to the static ones in the time period between
25 h and 60 h. After 60 h, a distinct slowing down of the shrinkage took place, and the matched ∆t
remained around 20 – 25 min for the rest of the observation. Hence, temporal unwrapping was able to
avoid undersampling (in the sense of appropriate data storage) initially and to save disk space later on.
To illustrate the value of this approach, we shall consider images from the two situations. In Fig. 6.28, a
comparison of a 10-minutes' deformation measurement at t 7¾ h is shown. Since the automatic routine
determined the instant of saving by itself, the initial and final object states are only by chance very nearly,
but not exactly, the same for the two storage series.