Industrial adoption of whole-field optical metrology has been hindered by the
interpretation of the resulting wrapped phase data using traditional spatial unwrapping
algorithms. These algorithms typically require long computation times and are often
unable to provide unique solutions. An alternative approach described by Huntley and
Saldnert simplifies the data analysis by using a time-series of wrapped phase maps to
unwrap the data temporally, and has the advantage that errors due to noise and
specimen boundaries do not propagate spatially. However, viability of the algorithm
is restricted by the need to store and subsequently analyse very large datasets. This
thesis describes the development and application of an instrument that implements the
temporal phase unwrapping algorithm (TPUA) in real-time to overcome these
problems and allows whole-field optical techniques to become more intuitive by
enabling quasi-live results to be displayed.
Tradeoffs across the multiple domains of algorithm, hardware and software are
discussed, including decomposition of the algorithm onto particular architectures and
implementation using a commercial pipeline image processing system.
In the first application, the surface profile of discontinuous objects is measured using
a Digital Mirror Device spatial light modulator (SLM) to project an optimised
sequence of sinusoidal white light fringes onto the object surface at 60 frames s-'.
Less than 0.5 s is required to measure and display approximately 250,000 co-ordinates
with a precision of better than one part in 5,000 of the field of view.
Issues affecting the performance of white light projected fringe profilometers
implemented using SLMs are investigated. Defocusing of the projector is shown to be
a critical limiting factor, with a precision of better than one part in 20,000 of the field
of view being achieved when optimised. A speckle interferometer is used in the second application to measure object
displacement. Quasi-live unwrapped speckle interferograms are displayed at
15 frames s-' using either a piezoelectric transducer-mounted prism or a Pockels cell
as the phase-stepping device. The reference speckle interferogram is automatically
updated at regular intervals allowing arbitrarily large deformations to be measured.
The signal-to-noise ratio of the calculated displacement fields can be improved by
performing real-time temporal least-squares fitting.
A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.