Application and Optimisation of the Spatial Phase Shifting ...

170 Improvements on SPS
Fig. 6.28: Comparison of matched vs. static data acquisition. Upper row: sequence of sawtooth images calculated
from 5 automatically saved phase maps ϕ f (x, y) (matched ∆t), leading to the resulting grey-scale height
map on the left in the lower row when spatially unwrapped, converted to heights and added. Lower row,
centre: sawtooth image for almost the same displacement calculated from only one phase map ϕ f (x, y)
(static ∆t); right: resulting height map.
The deformation is decomposed into five parts (upper row) by the matched phase map acquisition, and the
corresponding sawtooth images indeed show m 5 fringes each. The incremental sawtooth images can
all be spatially unwrapped with no problems, and the corresponding height data can be added to yield a
flawless deformation map (lower row, left). Depending on the individual phase gradients, the sum of these
incremental phase maps may contain well below 25 fringes, but not more. The single sawtooth image
(lower row, centre) from the static data storage indeed contains only 19 fringes. Their strongly
fluctuating density causes problems in spatial unwrapping, so that some height assignments are faulty in
the result (lower row, right). While one would not lose track of the course of displacement in this
example, there may be cases where only a higher image rate can ensure getting safely through the process.
After t 65 h, the situation is reversed: the deformation is oversampled by the static acquisition, which
generates a large amount of superfluous data. Fig. 6.29 gives an example from t 79 h.
Fig. 6.29: Sawtooth image for matched data storage (left) and corresponding sequence of sawtooth images from
static storage interval (right).