Moire fringe projection techniques are gaining popularity due to their non-contact nature and high accuracy in measuring the surface shape of many objects. The fringe patterns seen when using these instruments are similar to the patterns seen in traditional interferometry but differ in that the spacing between consecutive fringes in traditional interferometry is constant and equal to the wavelength of the source. In moire fringe projection, the spacing (equivalent wavelength) between consecutive fringes may not be constant over the field of view and it depends on the geometry (divergent or parallel) of the set-up. This variation in the equivalent wavelength causes the surface height measurements to be inaccurate. This paper looks at the aberrations that are caused by this varying equivalent wavelength and a calibration process to determine the equivalent wavelength map.
Optical techniques are used for non contact, high precision measurement tools. The most common optical technique is classical laser interferometry. Although laser interferometers offer high resolution, they suffer from limited dynamic range since the range is related to the wavelength of light. Other optical techniques like scanning white light interferometry and holography overcome this limitation. In this paper we propose a technique to enhance the vertical measurement range of a fringe projection system without reduction in its vertical resolution. It is based on the principle of inverse fringe projection, where the surface form is first measured by projecting a low frequency straight grating, and then used to create high frequency fringes with the proper inverse profile to project back on the surface and measure the surface finish without the impact of the form. The proposed technique is modeled, simulated and tested to measure the form, waviness and roughness of surfaces.