"Motion induced phase error reduction using a Hilbert transform," Opt. Express (2018)

 [117] Y. Wang, Z. Liu, and C. Jiang, and S. Zhang, "Motion induced phase error reduction using a Hilbert transform," Opt. Express 26(26), 34224-34235 (2018); doi:10.1364/OE.26.034224

Abstract

The motion of object could introduce phase error and thus measurement error for phase-shifting profilometry. This paper proposes a generic motion error compensation method based on our finding that the dominant motion introduced phase error doubles the frequency of the projected fringe frequency, and Hilbert transform shifts the phase of a fringe pattern by $\pi/2$. We apply Hilbert transform to phase-shifted fringe patterns to generate another set of fringe patterns, calculate one phase map using the original fringe patterns and another phase map using Hilbert transformed fringe patterns, and then use the average of these two phase maps for 3D reconstruction. Both simulation and experiments demonstrated the proposed method can substantially reduce motion-introduced measurement error.

"Depth-driven variable-frequency sinusoidal fringe pattern for accuracy improvement in fringe projection profilometry," Opt. Express (2018)

 [116] G. Rao, L. Song, and S. Zhang, X. Yang, K. Chen, and J. Xu, "Depth-driven variable-frequency sinusoidal fringe pattern for accuracy improvement in fringe projection profilometry," Opt. Express 26(16), 19986-20008 (2018); doi:10.1364/OE.26.019986

Abstract

Sinusoidal fringe pattern is widely used in optical profilometry; however, the traditional constant-frequency sinusoidal fringe pattern reduces 3D measurement accuracy in the defocus region. To this end, this paper presents a variable-frequency sinusoidal fringe pattern method that is optimized by the measurement depth. The proposed method  improves the pixel matching accuracy and thus increases measurement accuracy. This paper  1) theoretically determines the optimal frequency by analyzing the pixel matching error caused by intense noise in a captured image; 2) presents the online frequency  optimization along abscissa and ordinate axes in the sinusoidal fringe patterns; 3) details the encoding and decoding to use variable-frequency  fringe patterns for 3D profilometry. Simulations and experiments demonstrate that that our proposed method can improve the 3D measurement accuracy and increase measurement robustness.

"Motion-induced error compensation for phase shifting profilometry," Opt. Express (2018)

[114] Z. Liu, P. Zibley and S. Zhang, "Motion-induced error compensation for phase shifting profilometry," Opt. Express, 26(10), 12632-12637 (2018); doi:10.1364/OE.26.012632

Abstract

This paper proposes a novel method to substantially reduce motion-introduced phase error in phase-shifting profilometry. We first estimate the motion of an object from the difference between two subsequent 3D frames. After that, by leveraging the projector’s pinhole model, we can determine the motion-induced phase shift error from the estimated motion. A generic phase-shifting algorithm considering phase shift error is then utilized to compute the phase. Experiments demonstrated that proposed algorithm effectively improved the measurement quality by compensating for the phase shift error introduced by rigid and nonrigid motion for a standard single-projector and single-camera digital fringe projection system.

"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.

"High-speed 3D shape measurement with structured light methods: a review," Opt. Laser Eng. (2018)

[111] S. Zhang, "High-speed 3D shape measurement with structured light methods: a review," Opt. Laser Eng. 106, 119-131 (2018)

Abstract

High-speed 3D shape measurement (or imaging) has seen tremendous growths over the past decades, especially the past few years due to the improved speed of computing devices and reduced costs of hardware components. 3D shape measurement technologies have started penetrating more into our daily lives than ever before with the recent release of iPhone X that has an built-in 3D sensor for Face ID, along with prior commercial success of inexpensive commercial sensors (e.g., Microsoft Kinect).  This paper overviews the primary state-of-the-art  3D shape measurement techniques based on structured light methods, especially those that could achieve high measurement speed and accuracy. The fundamental principles behind those technologies will be elucidated,  experimental results will be presented to demonstrate capabilities and/or limitations for those popular techniques, and finally present our perspectives on those remaining challenges to be conquered to make advanced 3D shape measurement techniques ubiquitous.

"Absolute phase retrieval methods for digital fringe projection profilometry: A review," Opt. Laser Eng. (2018)

[112] S. Zhang, "Absolute phase retrieval methods for digital fringe projection profilometry: A review," Opt. Laser Eng. 107, 28-37 (2018); doi:10.1016/j.optlaseng.2018.03.003

Abstract

This paper provides a review for absolute phase recovery methods that are applicable for digital fringe projection (DFP) systems. Specifically, we present two conventional absolute phase unwrapping methods: multi-frequency or -wavelength phase-shifting methods, and hybrid  binary coding and phase-shifting methods; and also introduce some non-conventional methods that are specific for DFP systems: multiview geometry methods with additional camera(s) or projector(s), DFP system geometric constraint-based phase unwrapping method, and pre-knowledge (e.g., computer-aided-design, CAD, model) based phase unwrapping method. This paper also briefly overviews hybrid methods including phase coding, composite, and pre-defined markers based absolute phase unwrapping methods. This paper explains the principle behind each individual absolute phase unwrapping method; and finally offers some practical tips to handle common phase unwrapping artifact issues.

"High-speed and high-accuracy 3D surface measurement using a mechanical projector," Opt. Express (2018)

J. -S. Hyun, George T. -C. Chiu and S. Zhang, "High-speed and high-accuracy 3D surface measurement using a mechanical projector," Opt. Express, 26(2), 1474-1487 (2018); doi:10.1364/OE.26.001474

Abstract

This paper presents a method to achieve high-speed and high-accuracy 3D surface measurement using a custom-designed mechanical projector and two high-speed cameras. We developed a computational framework that can achieve absolute shape measurement in sub-pixel accuracy through: 1) capturing precisely phase-shifted fringe patterns by synchronizing the cameras with the projector; 2) generating a rough disparity map between two cameras by employing a standard stereo-vision method using texture images with encoded statistical patterns; and 3) utilizing the wrapped phase as a constraint to refine the disparity map. The projector can project binary patterns at a speed of up to 10,000 Hz, and the camera can capture the required number of phase-shifted fringe patterns with 1/10,000 second, and thus 3D shape measurement can be realized as high as 10,000 Hz regardless the number of phase-shifted fringe patterns required for one 3D reconstruction. Experimental results demonstrated the success of our proposed method.

"Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology (2018)

[109] B. Li and S. Zhang, "Novel method for measuring dense 3D strain map of robotic flapping wings," Measurement Science and Technology, 29(4), 045402 (2018);

Abstract

Measuring dense 3D strain map of inextensible membranous flapping wings of robots is of vital importance to the field of bio-inspired engineering. Conventional high-speed 3D videography method typically reconstructs the wing geometries through measuring sparse points with fiducial markers, and thus cannot obtain full-field mechanics of the wings in details.  In this research, we propose a novel system to measure dense strain map of the  inextensible membranous flapping wings by developing a superfast 3D imaging system and a computational framework for strain analysis. Specifically, first, we developed a 5,000 Hz 3D imaging system based on the digital fringe projection technique using the defocused binary patterns to precisely measure the dynamic 3D geometries of rapidly flapping wings. Then, we developed a geometry-based algorithm to perform point tracking on the precisely measured 3D surface data. Finally, we developed a dense strain computational method using the Kirchhoff-Love shell theory. Experiments demonstrate that our method can effectively perform point tracking and measure highly dense strain map of the wings without many fiducial markers.