"Three-dimensional absolute shape measurement by combining binary statistical pattern matching with phase-shifting methods," Appl. Opt., (2017)

Y. An and S. Zhang, "Three-dimensional absolute shape measurement by combining binary statistical pattern matching with phase-shifting methods," Appl. Opt., (2017); (accepted)

Abstract

This paper presents a novel method that leverages the stereo geometric relationship between projector and camera for absolute phase unwrapping on a standard one-projector and one-camera structured light system. Specifically, we use only one additional binary random image and the epipolar geometric constraint to generate a coarse correspondence map between projector and camera images. The coarse correspondence map is further refined by using the wrapped phase as a constraint. We then use the refined correspondence map to determine a fringe order for absolute phase unwrapping. Experimental results demonstrated the success of our proposed method.

"Evaluation of pixel-wise geometric constraints based phase unwrapping method for low signal-to-noise-ratio (SNR) phase," Advanced Optical Technologies, (2016)

[91] Y. An, Z. Liu and S. Zhang, "Evaluation of pixel-wise geometric constraints based phase unwrapping method for low signal-to-noise-ratio (SNR) phase," Advanced Optical Technologies, 5(5-6), 423–432, (2016); doi: 10.1515/aot-2016-0048

This paper evaluates the robustness of our recently proposed geometric constraints based phase unwrapping method to unwrap low signal-to-noise ratio (SNR) phase.  Instead of capturing additional images for absolute phase unwrapping, the new phase unwrapping algorithm uses geometric constraints of the digital fringe projection (DFP) system to create a virtual reference phase map to unwrap the phase pixel by pixel. Both simulation and experimental results demonstrate that this new phase unwrapping method can even successfully unwrap low SNR phase maps that brings difficulties for conventional multi-frequency phase unwrapping methods.

"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.

"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.

"High-resolution, real-time simultaneous 3D surface geometry and temperature measurement," Opt. Express, (2016);

[85] Y. An and S. Zhang, "High-resolution, real-time simultaneous 3D surface geometry and temperature measurement," Opt. Express, 24(13), 14552-14563, 2016; doi: 10.1364/OE.24.014552

This paper presents a method to simultaneously measure three-dimensional (3D) surface geometry and temperature in real time. Specifically, we developed 1) a holistic approach to calibrate both a structured light system and a thermal camera under exactly the same world coordinate system even though these two sensors do not share the same wavelength; and 2) a computational framework to determine the sub-pixel corresponding temperature for each 3D point as well as discard those occluded points. Since the thermal 2D imaging and 3D visible imaging systems do not share the same spectrum of light, they can perform sensing simultaneously in real time: we developed a hardware system that can achieve real-time 3D geometry and temperature measurement at 26 Hz with 768 X 960 points per frame.
 

"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.