Dual Phase-shifting Moiré Projection with Tunable High Contrast Fringes for Three-Dimensional Microscopic Surface Profilometry

A new dual phase-shifting moiré projection method is proposed for microscopic three-dimensional surface profilometry. One of the most difficult technological challenges encountered in employing moiré triangulation projection for automatic optical inspection (AOI) is to detect high contrast projected fringes from an object's surface having vast variation of surface reflectance. Surface reflectance from different surface regions could vary significantly due to diverse surface characteristics, such as texture and roughness, in which returning light may be arbitrary combination of specular reflection, diffusion and scattering. Undesired measurement results are common when deformed moiré fringes cannot be satisfactorily detected by an imaging detector. To resolve this, the proposed method generates a moiré fringe formed by employing a combination of the deformed fringe detected from the object's surface and a virtual sinusoidal grating self-generated by the computer. A spatial averaging approach is applied to phase-shifted moiré interferograms formed between one deformed fringe and virtual fringes having an equal shifted phase, such as π/2. High-contrast phase-shifted interferograms can be generated for more accurate 3-D surface reconstruction via phase wrapping and unwrapping. One great advantage of the method is its tunable control over the moiré fringe period for reaching more measurable step heights without encountering phase ambiguity problem. From the experimental analysis using standard targets and industrial PCB, it is verified that the method can significantly improve measurement accuracy with a measuring standard deviation less than 0.14% of the overall measuring depth range when measuring a low reflected industrial object surface.