Moire is a common technique for contouring large scale features on diffuse surfaces. Typically, this is accomplished by
projecting a high-frequency line grating onto the object and viewing its image through another high-frequency line
grating. The resultant pattern, produced by aliasing between the two gratings, shows lines of surface contour. The
surface height between these contour lines is determined by triangulation between the illumination and observation
beams. Although moire is really a geometric ray technique, it is typically referred to as interferometry because its
contour patterns resemble interferograms;
Moire contouring can be classified as either shadow or projection moire. Shadow moire uses the same grating for
both illumination and observation, while projection moire uses separated gratings. Another surface contouring
technique is projected fringe. This approach is similar to moire in that it projects a line grating and uses triangulation
to measure surface height, however, there is no second grating or aliasing. Rather, the projected pattern is.viewed
directly. Although it is not really moire (because it does not use aliasing with a second grating), projected fringe is
typically considered a moire technique.
This paper has four parts. First, it reviews the theory behind moire contouring. Second, it outlines how to phase
modulate a moire 'interferogram'. Third, it discusses practical considerations associated with performing moire
contouring such as projection and observation geometry, imaging, and illumination. And fourth, it presents two
examples of objects that were contoured using a white-light moire phase-measuring interferometer.