"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.

"High-speed high-accuracy three-dimensional shape measurement using digital binary defocusing method versus sinusoidal method," Opt. Eng., (2017)

[99] J. -S. Hyun, B. Li, and S. Zhang, "High-speed high-accuracy three-dimensional shape measurement using digital binary defocusing method versus sinusoidal method," Opt. Eng. 56(7), 074102 (2017).

Abstract

This paper presents our research findings on high-speed high-accuracy 3D shape measurement using digital light processing (DLP) technologies. In particular, we compare two different sinusoidal fringe generation techniques using the DLP projection devices: direct projection of 8-bit computer generated sinusoidal patterns (a.k.a., the sinusoidal method), and the creation of sinusoidal patterns by defocusing binary patterns (a.k.a., the binary defocusing method). This paper mainly examines their performance on high-accuracy measurement applications under precisely controlled settings. Two different projection systems were tested in this study: the commercially available inexpensive projector, and the DLP development kit. Experimental results demonstrated that the binary defocusing method always outperforms the sinusoidal method if a sufficient number of phase-shifted fringe patterns can be used.

"Superfast 3D absolute shape measurement using five binary patterns," Opt. Laser Eng., (2017)

[93] J. -S. Hyun and S. Zhang, "Superfast 3D absolute shape measurement using five binary patterns," Opt. Laser Eng., 90, 217-224, 2017; 10.1016/j.optlaseng.2016.10.017

Abstract

This paper presents a method that recovers high-quality 3D absolute coordinates point by point with only five binary patterns. Specifically, three dense binary dithered patterns are used to compute the wrapped phase; and the average intensity is combined with two additional binary patterns to determine fringe order pixel by pixel in phase domain. The wrapped phase is temporarily unwrapped point by point by referring to the fringe order. We further developed a computational framework to reduce random noise impact due to dithering, defocusing and random noise. Since only five binary fringe patterns are required to recover one 3D frame, extremely high speed 3D shape measurement can be achieved.  For example, we developed a system that captures 2D images at 3,333Hz, and thus performs 3D shape measurement at 667 Hz.

"Pixel-wise absolute phase unwrapping using geometric constraints of structured light system," Opt. Express, (2016)

[87] Y. An, J. -S. Hyun, and S. Zhang, "Pixel-wise absolute phase unwrapping using geometric constraints of structured light system", Opt. Express, 24(15), 18445-18459, 2016; doi: 10.1364/OE.24.018445

This paper presents a method to unwrap phase pixel by pixel by solely using geometric constraints of the structured light system without requiring additional image acquisition or  another camera. Specifically, an artificial absolute phase map, Φ_{min},  at a given virtual depth plane z = z_{min}, is created from geometric constraints of the calibrated structured light system; the wrapped phase is pixel-by-pixel unwrapped by referring to Φ_{min}. Since Φ_{min} is defined in the projector space, the unwrapped phase obtained from this method is absolute for each pixel.  Experimental results demonstrate the success of this proposed novel absolute phase unwrapping method.

"Enhanced two-frequency phase-shifting method," Appl. Opt. (2016)

[82] J. -S. Hyun, and S. Zhang, "Enhanced two-frequency phase-shifting method," Appl. Opt., 55(16), 4395-4401, 2016; doi: 10.1364/AO.55.004395

Abstract

One of the major challenges of employing a two-frequency (or -wavelength) phase-shifting algorithm for absolute three-dimensional (3D) shape measurement is its sensitivity to noise. Therefore, three- or morefrequency phase-shifting algorithms are often used in lieu of a two-frequency phase-shifting algorithm for applications where the noise is severe. This paper proposes a method to use geometric constraints of digital fringe projection (DFP) system to substantially reduce the noise impact by allowing the use of more than one period of equivalent phase map for temporal phase unwrapping. Experiments successfully verified the enhanced performance of the proposed method without increasing the number of patterns.