"Three-dimensional shape measurement using a structured light system with dual projectors", Appl. Opt., (2018)

[113] C. Jiang, B. Lim and S. Zhang, "Three-dimensional shape measurement using a structured light system with dual projectors," Appl. Opt.,  57(14), 3983-3990(2018); doi:10.1364/AO.57.003983

This paper introduces a structured light system with two projectors and one camera for three-dimensional (3D) shape measurement to alleviate problems created by a single projector such as the shadow problem. In particular, we developed (1) a system calibration framework that can accurately calibrate each such camera-projector system; (2) a residual error correction method based on the system error function; and (3) a data fusion method utilizing the angle between the projection direction and surface normal. Experimental results demonstrate that the proposed dual-projector structured light system improves the measurement accuracy besides extending the measurement range of a single projector system.

"Double-pattern triangular pulse width modulation technique for high-accuracy high-speed 3D shape measurement," Opt. Express (2017)

Y. Wang, C. Jiang, and S. Zhang, "Double-pattern triangular pulse width modulation technique for high-accuracy high-speed 3D shape measurement," Opt. Express 25(24), 30177-30188 (2017); doi:10.1364/OE.25.03177

Abstract

Using 1-bit binary patterns for 3D shape measurement has been demonstrated advantageous over using  8-bit sinusoidal patterns in terms of achievable speeds. However, the phase quality generated by binary pattern(s) typically  is not  high if only a small number of phase-shifted patterns is used. This paper proposes a method to improve the phase quality  by representing each pattern with the difference of two binary patterns:  the first binary pattern is generated by triangular pulse width modulation (TPWM) technique, and the second being $\pi$ shifted from the first pattern is also generated by TPWM technique. The phase is retrieved by applying a three-step phase-shifting algorithm to the difference patterns. Through optimizing the modulation frequency of the  triangular carrier signal, we demonstrate that high-quality phase can be generated for a wide range of fringe periods (e.g., from 18 to 1140 pixels) with only six binary patterns. Since only 1-bit binary patterns are required for 3D shape measurement, this paper will present a real-time 3D shape measurement system that can achieve 30 Hz.

“Absolute three-dimensional shape measurement with two-frequency square binary patterns,” Appl. Opt., (2017)

C. Jiang and S. Zhang, “Absolute three-dimensional shape measurement with two-frequency square binary patterns,” Appl. Opt., 56(31), 8710-8718 (2017); doi:10.1364/AO.56.008710

Abstract

This paper presents a novel method to achieve absolute three-dimensional (3D) shape measurement solely using square binary patterns. This method uses six patterns: three low-frequency phase-shifted patterns and three phase-shifted high-frequency patterns. The phase obtained from low-frequency phase temporally unwraps the phase obtained from high-frequency patterns. The projector is defocused such that the high-frequency patterns produce high-quality phase, but the phase retrieved from low-frequency patterns has large harmonic error that fails two-frequency temporal phase unwrapping process. In this paper, we develop a computational framework to address the challenge. The proposed computational framework includes four major approaches to alleviate the harmonic error problem: i) use more than one period of low-frequency patterns enabled by geometric constraint-based phase unwrapping method; ii) artificially apply a large Gaussian filter to low frequency patterns before phase computation; iii) create an error lookup table (LUT) to compensate for harmonic error; and iv) develop a boundary error correction method to alleviate problems associated with filtering. Both simulation and experimental results demonstrated the success of the proposed method.

“Absolute phase unwrapping for dual-camera system without embedding statistical features,” Opt. Eng., (2017)

C. Jiang and S. Zhang, “Absolute phase unwrapping for dual-camera system without embedding statistical features,” Opt. Eng. 56(9), 094114 (2017), doi: 10.1117/1.OE.56.9.094114.

Abstract

This paper proposes an absolute phase unwrapping method for 3D measurement that uses two cameras and one projector. On the left camera image, each pixel has one wrapped phase value which corresponds to multiple projector candidates with different absolute phase values. We use geometric relationship of the system to map projector candidates into right camera candidates. By applying a series of candidate rejection criteria, a unique correspondence pair between two camera images can be determined. Then the absolute phase is obtained by tracing the correspondence point back to projector space. Experimental results demonstrate that the proposed absolute phase unwrapping algorithm can successfully work on both complex geometry and multiple isolated objects measurement.

"Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., (2017)

C. Jiang,  B. Li, S. Zhang, "Pixel-by-pixel absolute phase retrieval using three phase-shifted fringe patterns without markers," Opt. Laser Eng., 91, 232-241 (2017);  doi:10.1016/j.optlaseng.2016.12.002

This paper presents a method that can recover absolute phase pixel by pixel without embedding markers on three phase-shifted fringe patterns, acquiring additional images, or introducing additional hardware component(s). The proposed
three-dimensional (3D) absolute shape measurement technique includes the following major steps: 1) segment the measured object into different regions using rough priori knowledge of surface geometry; 2) artificially create phase maps at different z planes using geometric constraints of structured light system; 3) unwrap the phase pixel by pixel for each region by properly referring to the artificially created phase map; and 4) merge unwrapped phases from all regions into a complete absolute phase map for 3D reconstruction. We demonstrate that conventional three-step phase-shifted fringe patterns can be used to create absolute phase map pixel by pixel even for large depth range objects. We have successfully implemented our proposed computational framework to achieve absolute 3D shape measurement at 40 Hz.

"High dynamic range real-time 3D shape measurement," Opt. Express, (2016)

[80] C. Jiang, T. Bell, and S. Zhang, "High dynamic range real-time 3D shape measurement," Opt. Express., 24(7), 7337-7346, 2016(Cover feature); doi: 10.1364/OE.24.00733 

Abstract

This paper proposes a method that can measure high-contrast surfaces in real-time without changing camera exposures. We propose to use 180-degree phase-shifted (or inverted) fringe patterns to complement regular fringe patterns. If not all of the regular patterns are saturated, inverted fringe patterns are used in lieu of original saturated patterns for phase retrieval, and if all of the regular fringe patterns are saturated, both the original and inverted fringe patterns are all used for phase computation to reduce phase error. Experimental results demonstrate that three-dimensional (3D) shape measurement can be achieved in real time by adopting the proposed high dynamic range method.