16 Moiré and Fringe Projection Techniques

The term “moiré” is not the name of a person; in fact, it is a French word referring to “an irregular wavy finish usually produced on a fabric by pressing between engraved rollers” (Webster's 1981). In optics it refers to a beat pattern produced between two gratings of approximately equal spacing. It can be seen in everyday things such as the overlapping of two window screens, the rescreening of a half-tone picture, or with a striped shirt seen on television. The use of moiré for reduced sensitivity testing was introduced by Lord Rayleigh in 1874. Lord Rayleigh looked at the moiré between two identical gratings to determine their quality even though each individual grating could not be resolved under a microscope. Fringe projection entails projecting a fringe pattern or grating on an object and viewing it from a different direction. The first use of fringe projection for determining surface topography was presented by Rowe and Welford in 1967. It is a convenient technique for contouring objects that are too coarse to be measured with standard interferometry. Fringe projection is related to optical triangulation using a single point of light and light sectioning where a single line is projected onto an object and viewed in a different direction to determine the surface contour (Case et al. 1987). Moiré and fringe projection interferometry complement conventional holographic interferometry, especially for testing optics to be used at long wavelengths. Although two-wavelength holography (TWH) can be used to contour surfaces at any longer-than-visible wavelength, visible interferometric environmental conditions are required. Moiré and fringe projection interferometry can contour surfaces at any wavelength longer than 10-100 μm with reduced environmental requirements and no intermediate photographic recording setup.