"Phase unwrapping error reduction framework for a multiple-wavelength phase-shifting algorithm," Opt. Eng., (2009)

[19] S Zhang, "Phase unwrapping error reduction framework for a multiple-wavelength phase-shifting algorithm," Opt. Eng.,48(10), 105601, 2009 (Accepted without revision); doi:10.1117/1.3251280

We address a framework to reduce the unwrapping errors of the measurement system using a digital multiple-wavelength phaseshifting algorithm. In particular, the following framework is proposed: 1 smooth the raw phase by smoothing the sine and cosine images of the phase computation of the inverse tangent function; 2 locate and remove the incorrectly unwrapped points by the monotonicity condition of the phase map; 3 obtain the unwrapped phase map for the shortest wavelength without smoothing; 4 detect holes and fill them to preserve as much useful information as possible. Experiments demonstrated that the proposed framework significantly alleviated the measurement errors caused by the phase noise. 

"Flexible 3-D shape measurement using projector defocusing," Opt. Lett., (2009)

[20] S Lei* and S Zhang, "Flexible 3-D shape measurement using projector defocusing," Opt. Lett. 34(20),3080-3082, 2009;doi: 10.1364/OL.34.003080

We present a 3-D shape-measurement technique using a defocused projector. The ideal sinusoidal fringe patterns are generated by defocusing binary structured patterns, and the phase shift is realized by shifting the binary patterns spatially. Because this technique does not require calibration of the gamma of the projector, it is easy to implement and thus is promising for developing flexible 3-D shape measurement systems using digital video projectors. 

"High dynamic range scanning technique," Opt. Eng., (2009)

[18] S Zhang and S-T Yau, "High dynamic range scanning technique," Opt. Eng. 48(3), 033604, 2009; doi: 10.1117/1.3099720

Measuring objects with a high variation range of surface reflectivity is challenging for any optical method: This paper addresses a high dynamic range scanning technique that can measure this type of object. It takes advantage of one merit of a phase-shifting algorithm: pixel-by-pixel phase retrieval. For each measurement, a sequence of fringe images with different exposures are taken: the brightest ones have good fringe quality in the darkest areas while the darkest ones have good fringe quality in the brightest areas. They are arranged from brighter to darker i.e., from higher exposure to lower exposure. The final fringe images, used for phase retrieval, are produced pixel-by-pixel by choosing the brightest but unsaturated corresponding pixel from one exposure. A phase-shifting algorithm is employed to compute the phase, which can be further converted to coordinates. Our experiments demonstrate that the proposed technique can successfully measure objects with high dynamic range of surface reflectivity variation.