"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.

"Method for large-range structured light system calibration," Appl. Opt., (2016)

[91] Y. An, T. Bell, B. Li, J. Xu and S. Zhang, "Method for large range structured light system calibration", Appl. Opt., 55(33), 9563-9572 (2016); doi:10.1364/AO.55.009563

Structured light system calibration often requires the usage of a calibration target with a similar size as the field of view (FOV), which brings challenges to large range structured light system calibration since fabricating large calibration targets is difficult and expensive. This paper presents a large range system calibration method that does not need a large calibration target. The proposed  method includes two stages: 1) accurately calibrate intrinsics  (i.e. focal lengths, and principle points) at a near range where both the camera and projector are out of focus; and 2) calibrate the extrinsic parameters (translation and rotation) from camera to projector with the assistance of a low-accuracy large range 3D sensor (e.g., Microsoft Kinect). We have developed a large-scale 3D shape measurement system with a FOV of (1120 × 1900 × 1000) mm^3. Experiments demonstrate our system can achieve measurement accuracy as high as 0.07 mm with a standard deviation of 0.80 mm by measuring a 304.8 mm diameter sphere. As a comparison, Kinect V2 only achieved mean error of 0.80 mm with a standard deviation of 3.41 mm for the FOV of measurement.

"High-accuracy, high-speed 3D structured light imaging techniques and potential applications to intelligent robotics," Int. J. Intell. Robot. Applic. (2016)

[90] B. Li, Y. An, D. Cappelleri, J. Xu and S. Zhang, "High-accuracy, high-speed 3D structured light imaging techniques and potential applications to intelligent robotics," Int. J. Intell. Robot. Applic. 1(1), 86–103, (2016).

Abstract

This paper presents some of the high-accuracy and high-speed structured light 3D imaging methods developed in the optical metrology community. These advanced 3D optical imaging technologies could substantially benefit the intelligent robotics community as another sensing tool. This paper mainly focuses on one special 3D imaging technique: digital fringe projection (DFP) method because of its numerous advantageous features comparing to other 3D optical imaging methods in terms of accuracy, resolution, speed, and flexibility. We will discuss technologies that enabled 3D data acquisition, reconstruction, and display at 30 Hz or higher with over 300,000 measurement points per frame. This paper intends to introduce the DFP technologies to the intelligent robotics community, and casts our perspectives on potential applications that such sensing methods could be of value.

Motion induced error reduction by combining Fourier transform profilometry with phase-shifting profilometry, Opt. Express, (2016)

[88] B. Li, Z. Liu and S. Zhang, "Motion induced error reduction by combining Fourier transform profilometry with phase-shifting profilometry," Opt. Express 24(20), 23289-23303 2016; doi: 10.1364/OE.24.023289

We propose a hybrid computational framework to reduce motion induced measurement error by combining the Fourier transform profilometry (FTP) and phase-shifting profilometry (PSP). The proposed method is composed of three major steps: Step 1 is to extract continuous relative phase maps for each isolated object with single-shot FTP method and spatial phase unwrapping; Step 2 is to obtain an absolute phase map of the entire scene using PSP method, albeit motion induced errors exist on the extracted absolute phase map; and Step 3 is to shift the continuous relative phase maps from Step 1 to generate final absolute phase maps for each isolated object by referring to the absolute phase map with error from Step 2. Experiments demonstrate the success of the proposed computational framework for measuring multiple isolated rapidly moving objects.

"Microscopic structured light 3D profilometry: binary defocusing technique VS sinusoidal fringe projection," Opt. Laser Eng., (2016)

[92] B. Li and S. Zhang, "Microscopic structured light 3D profilometry: binary defocusing technique VS sinusoidal fringe projection, " Opt. Laser Eng. 96, 117–123, (2017); doi: 10.1016/j.optlaseng.2016.06.009

Abstract

This paper compares the binary defocusing technique with conventional sinusoidal fringe projection under two different 3D microscopic profilometry systems: 1) both camera and projector use telecentric lenses, and 2) only camera uses a telecentric lens. Our simulation and experiments found that the binary defocusing technique is superior to the traditional sinusoidal fringe projection method by improving the measurement resolution approximately 19%. Finally, by taking the speed advantage of the binary defocusing technique, we presented a high-speed (500 Hz) and high-resolution (1600 X 1200) 3D microscopic profilometry system that could reach kHz.

"Single-shot absolute 3D shape measurement with Fourier transform profilometry", Appl. Opt., (2016)

[83] B. Li, Y. An and S. Zhang, "Single-shot absolute 3D shape measurement with Fourier transform profilometry," Appl. Opt., 2016; (accepted)

Abstract

Fourier transform profilometry (FTP) is one of the frequently adopted three-dimensional (3D) shape measurement methods owing to its nature of single-shot 3D shape recovery, yet it is challenging to retrieve the absolute phase map solely from one single grayscale fringe image. This paper presents a computational framework that overcomes this limitation of FTP with digital fringe projection (DFP). By using geometric constraints, an absolute phase map can be retrieved point-by-point from one single grayscale fringe image. Experiments demonstrate the success of our proposed framework with single-shot absolute 3D shape measurement capability.

"Flexible calibration method for microscopic structured light system using telecentric lens," Opt. Express, (2015)

[77]  B. Li and S. Zhang, “Flexible calibration method for microscopic structured light system using telecentric lens,” Opt. Express, 23(20), 25795-25803, 2015; doi:10.1364/OE.23.025795

Abstract

This research presents a novel method to calibrate a microscopic structured light system using a camera with a telecentric lens. The pin-hole projector calibration follows the standard pin-hole camera calibration procedures. With the calibrated projector, the 3D coordinates of those feature points used for projector calibration are then estimated through iterative Levenberg-Marquardt optimization. Those 3D feature points are further used to calibrate the camera with a telecentric lens. We will describe the mathematical model of a telecentric lens, and demonstrate that the proposed calibration framework can achieve very high accuracy: approximately 10 μm with a volume of approximately 10(H) mm × 8(W) mm × 5(D) mm.

"Structured light system calibration method with optimal fringe angle," Appl. Opt., (2014)

B. Li* and S. Zhang, "Structured light system calibration method with optimal fringe angle," Appl. Opt., 53(13), 7942-7950, 2014 (Cover feature); doi: 10.1364/AO.53.007942

Abstract

For structured light system calibration, one popular approach is to treat the projector as an inverse camera. This is usually performed by projecting horizontal and vertical sequences of patterns to establish one-to-one mapping between camera points and projector points. However, for a well-designed system, either horizontal or vertical fringe images are not sensitive to depth variation and thus yield inaccurate mapping. As a result, the calibration accuracy is jeopardized if a conventional calibration method is used. To address this limitation, this paper proposes a novel calibration method based on optimal fringe angle determination. Experiments demonstrate that our calibration approach can increase the measurement accuracy up to 38% compared to the conventional calibration method with a calibration volume of 300
H mm × 250
W mm × 500
D mm. 

"Superfast 3D optical sensing using fiber interference", SPIE Newsroom, 2014

B. Li*, S. Zhang and P. Ou, "Superfast 3D optical sensing using fiber interference," SPIE Newsroom, doi: 10.1117/2.1201408.005584, 2014 (invited)

Incorporating a lithium niobate electro-optic phase modulator into an interferometric fringe projection system could permit imaging at rates of megahertz or even gigahertz.

 

"Novel calibration method for structured light system with an out-of-focus projector," Appl. Opt. 2014

B. Li*, N. Karpinsky*, and S. Zhang, "Novel calibration method for structured light system with an out-of-focus projector,"Appl. Opt. 53(13), 3415-3426, 2014; doi: 10.1364/AO.53.003415

A structured-light system with a binary defocusing technique has the potential to have more extensive application due to its high speeds, gamma-calibration-free nature, and lack of rigid synchronization requirements between the camera and projector. However, the existing calibration methods fail to achieve high accuracy for a structured-light system with an out-of-focus projector. This paper proposes a method that can accurately calibrate a structured-light system even when the projector is not in focus, making it possible for high-accuracy and high-speed measurement with the binary defocusing method. Experiments demonstrate that our calibration approach performs consistently under different defocusing degrees, and a root-mean-square error of about 73 μm can be achieved with a calibration volume of 150
H mm × 250
W mm × 200
D mm. 

"Intensity-optimized dithering technique for high-quality 3d shape measurement," Opt. Laser Eng., 2014

J. Dai, B. Li*, and S. Zhang, "Intensity-optimized dithering technique for high-quality 3d shape measurement," Opt. Laser Eng. 53, 79-85, 2014; doi: 10.1016/j.optlaseng.2013.08.015

Our previously proposed phase-based optimization method [1] has proven successful in improving the measurement quality when a dithering technique is used. This paper presents an intensity-based optimization method for 3D shape measurement with binary dithering techniques. Both simulations and experiments find that the phase-based optimization method can generate high-quality phase under a given condition, but it is sensitive to the amount of defocusing. In contrast, the proposed intensity-based optimization method can consistently generate high-quality phase with various amounts of defocusing.

"Some recent advances on superfast 3D shape measurement with digital binary defocusing techniques," Opt. Laser Eng. 2014

B. Li*, Y. Wang*, J. Dai, and W. Lohry*, and S. Zhang, "Some recent advances on superfast 3D shape measurement with digital binary defocusing techniques," Opt. Laser Eng. 54, 236-246, 2014 (invited); doi:10.1016/j.optlaseng.2013.07.010

The digital binary phase-shifting technique has been demonstrated for its merits over the conventional sinusoidal phase-shifting method in terms of measurement speed and simplicity. Yet, the measurement depth range is small when a squared binary method is used. Our recent research focuses on improving its measurement accuracy without sacrificing measurement speed, and increasing its depth range without losing measurement quality. This paper will summarize our recent work on the following three major areas: (a) realization of kHz 3D shape measurement with binary phase-shifting methods; (b) binary pattern improvement with pulse width modulation and binary dithering/halftoning techniques; and (c) applications of superfast 3D shape measurement techniques. Principle of each technique will be presented, and experimental results will be shown to verify its performance.

"High-quality fringe pattern generation using binary pattern optimization through symmetry and periodicity," Opt. Laser Eng., 2014

J. Dai, B. Li*, and S. Zhang, "High-quality fringe pattern generation using binary pattern optimization through symmetry and periodicity," Opt. Laser Eng., 52, 195-200, 2014; doi: 10.1016/j.optlaseng.2013.06.010

This paper presents a novel method to construct binary patterns for high-quality 3D shape measurement. The algorithm generates small patches using symmetry and periodicity, randomly initializes each pixels, optimizes the small patches through mutations, and finally tiles the optimized patches into full size patterns using again symmetry and periodicity. We will demonstrate that the proposed method can achieve substantial phase quality improvements over the dithering techniques for different amounts of defocusing.

"Flexible real-time natural 2D color and 3D shape measurement," Opt. Express, 2013;

P. Ou, B. Li*, Y. Wang*, and S. Zhang, "Flexible real-time natural 2D color and 3D shape measurement," Opt. Express,21(14), 16736-16741, 2013; doi: 10.1364/OE.21.016736

The majority of existing real-time 3D shape measurement systems only generate non-nature texture (i.e., having illumination other than ambient lights) that induces shadow related issues. This paper presents a method that can simultaneously capture natural 2D color texture and 3D shape in real time. Specifically, we use an infrared fringe projection system to acquire 3D shapes, and a secondary color camera to simultaneously capture 2D color images of the object. Finally, we develop a flexible and simple calibration technique to determine the mapping between the 2D color image and the 3D geometry. Experimental results demonstrate the success of the proposed technique.