One of the widely recognised limitations of moire topography is its inability to provide unambiguous data from objects which have discontinuities in their shape. Similar limitations arise when two parts of an object are separated by a region which is in shadow. This feature of moire topography has limited its use to the measurement of objects which have continuous surfaces. With the development of phase measuring techniques for fringe pattern analysis, moire fringe contour patterns can now be analysed to high levels of accuracy, the contour interval being subdivided into, perhaps, 200 increments of measurement by the phase calculation procedure. This paper describes a modified version of moire topography in which the contour interval has been greatly increased so that, in some cases, the entire depth of the object lies within one fringe. By applying high accuracy phase measuring techniques to the analysis of these moire fringe patterns, sub-millimetre accuracy can be obtained in measurements of objects which would normally be thought of as unmeasurable by moire topography.
G. T. Reid,
R. C. Rixon,
"Moire Topography With Large Contour Intervals", Proc. SPIE 0814, Photomechanics and Speckle Metrology, (17 February 1987); doi: 10.1117/12.941713; https://doi.org/10.1117/12.941713