"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.

"Development of a mobile tool mark characterization/comparison system," J. Forensic Sci., (2017)

L. S. Chumbley,  M. Morris, R. Spotts, and C. Macziewski, "Development of a mobile tool mark characterization/comparison system," J. Forensic Sci., 62(1), 83-91 (2017), doi: 10.1111/1556-4029.13233

Since the development of the striagraph, various attempts have been made to enhance forensic investigation through the use of measuring and imaging equipment. This study describes the development of a prototype system employing an easy-to-use software interface designed to provide forensic examiners with the ability to measure topography of a toolmarked surface and then conduct various comparisons using a statistical algorithm. Acquisition of the data is carried out using a portable 3D optical profilometer, and comparison of the resulting data files is made using software named “MANTIS” (Mark and Tool Inspection Suite). The system has been tested on laboratory-produced markings that include fully striated marks (e.g., screwdriver markings), quasistriated markings produced by shear-cut pliers, impression marks left by chisels, rifling marks on bullets, and cut marks produced by knives. Using the system, an examiner has the potential to (i) visually compare two toolmarked surfaces in a manner similar to a comparison microscope and (ii) use the quantitative information embedded within the acquired data to obtain an objective statistical comparison of the data files. This study shows that, based on the results from laboratory samples, the system has great potential for aiding examiners in conducting comparisons of toolmarks.

"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.

"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.

"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.

"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.

"A self-recalibration method based on scale-invariant registration for structured light measurement systems," Opt. Laser Eng., (2017)

[92] R. Chen, J. Xu, S. Zhang, H. Chen, Y. Guan, and K. Chen, "A self-recalibration method based on scale-invariaant registration for structured light measurement systems," Opt. Laser Eng., 88, 75-81 (2017); doi:10.1016/j.optlaseng.2016.07.003

Abstract

The measurement accuracy of structured light measurement depends on delicate offline calibration. However, in some practical applications, the system is supposed to be reconfigured so frequently to track the target that an online calibration is required. To this end, this paper proposes a rapid and autonomous self-recalibration method. For the proposed method, first, the rotation matrix and normalized translation vector are attained from the fundamental matrix; second, the scale factor is acquired based on scale-invariant registration such that the actual translation vector is obtained. Experiments have been conducted to verify the effectiveness of our proposed method, and the results indicate a high degree of accuracy.

“Fast registration methodology for fastener assembly of large-scale structure,” IEEE Trans Industrial Electronics (2017)

[86] J. Xu, R. Chen, H. Chen, S. Zhang, and K. Chen, " Fast registration methodology for fastener assembly of large-scale structure," IEEE Trans Industrial Electronics, 64(1),  717-726 (2017); doi:10.1109/TIE.2016.2599140

Abstract

Fastener assembly is a tedious and time-consuming work because operators have to check assembly manuals and find right fastener for each hole. Hence, this article aims to develop a 3D projection system which projects assembly instruction onto the work piece surface directly to guide operators to assemble. However, in order to project the instruction accurately, the corresponding part of the CAD model of the physical scanned area needs to be attained through the rapid and accurate registration. In order to achieve this goal,firstly, a high-accuracy and rapid 3D measurement system is developed; secondly, a fast registration method based on local multi-scale geometric feature vector is proposed to accelerate the registration speed and improve the registration reliability. Experimental results demonstrate the measurement accuracy of the developed system, and verify the feasibility of the proposed registration method. Hence the proposed method can lead to improved assembly efficiency and decreased error probability, making great contributions to large-scale structure assembly.

"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.

 

"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.

"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.

"High quality 3D shape measurement using saturated fringe patterns,'' Opt. Laser Eng. (2016)

[81] B. Chen and S. Zhang, "High quality 3D shape measurement using saturated fringe patterns,'' Opt. Laser Eng. (2016) (doi:10.1016/j.optlaseng.2016.04.012)

Abstract

This paper proposes a method to potentially conquer one of the challenges in the optical metrology community: optically measuring three-dimensional (3D) objects with high surface contrast. We discover that  for digitally equally phase-shifted fringe patterns, if the fringe period P is an even number, the N = P/2 x k, (k = 1, 2, 3, ...) step algorithm can accurately recover phase even if the fringe patterns are saturated; and if P is an odd number, N = P x k step algorithm can also accurately recover phase even if the fringe patterns are saturated.  This finding leads to a novel method to optically measure shiny surfaces, where the saturation due to surface shininess could be substantially alleviated. Both simulations and experiments successfully verified the proposed method.

"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.

 

 

“Method for out-of-focus camera calibration,” Appl. Opt., (2016)

[79] T. Bell, J. Xu, and S. Zhang, "Method for out-of-focus camera calibration," Appl. Opt., 55(9), 2346-2352, 2016; doi: 10.1364/AO.55.002346

Abstract

State-of-the-art camera calibration methods assume that the camera is at least nearly in focus, and thus fail if the camera is substantially defocused. This paper presents a method which enables the accurate calibration of an out-of-focus camera. Specifically, the proposed method uses a digital display (e.g., liquid crystal display monitor) to generate fringe patterns which encode feature points into the carrier phase; these feature points can be accurately recovered even if the fringe patterns are substantially blurred (i.e., the camera is substantially defocused). Experiments demonstrated that the proposed method can accurately calibrate a camera regardless of the amounts of defocusing: the focal length difference is approximately 0.2% when the camera is focused compared to when the camera is substantially defocused.

Technical Paper

“Multiwavelength depth encoding method for 3D range geometry compression,” Appl. Opt., (2015)

[78] T. Bell and S. Zhang, “Multiwavelength depth encoding method for 3D range geometry compression,” Appl. Opt., 54(36), 10684-10961, 2015; doi: 10.1364/AO.54.010684

Abstract

This paper presents a novel method for representing three-dimensional (3D) range data within regular two-dimensional (2D) images using multiwavelength encoding. These 2D images can then be further compressed using traditional lossless (e.g., PNG) or lossy (e.g., JPEG) image compression techniques. Current 3D range data compression methods require significant filtering to reduce lossy compression artifacts. The nature of the proposed encoding, however, offers a significant level of robustness to such artifacts brought about by high levels of JPEG compression. This enables extremely high compression ratios while maintaining a very low reconstruction error percentage with little to no filtering required to remove compression artifacts. For example, when encoding 3D geometry with the proposed method and storing the resulting 2D image with Matlab R2014a JPEG80 image compression, compression ratios of approximately 935:1 versus the OBJ format can be achieved at an error rate of approximately 0.027% without any filtering.

Technical Paper

"Comparative study on passive and active projector nonlinear gamma calibration," Appl. Opt., (2015)

S. Zhang, "Comparative study on passive and active projector nonlinear gamma calibration," Appl. Opt. 54(13), 3834-3841, 2015; doi:10.1364/AO.54.003834

Abstract

This paper compares the active and passive projector nonlinear gamma compensation methods for phase error reduction. The active method modifies fringe patterns before their projection to ensure sinusoidality; the passive method, in contrast, compensates for the phase error after capturing those distorted sinusoidal fringe patterns. Our study finds that the active method tends to provide more consistent high-quality fringe patterns regardless of the amount of projectors defocusing; yet the effectiveness of the passive method is diminished if the measurement condition deviates from the calibration condition. Experimental results will be presented to demonstrate the differences between these two nonlinear gamma compensation methods.

"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.