White-light moire phase-measuring interferometry

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.