"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.

“Quantification of transient behavior of wind-driven surface droplet/rivulet flows by using a digital fringe projection technique,” Journal of Visualization, (2015)

[74]  H. Hu, B. Wang, K. Zhang, W. Lohry* and S. Zhang, “Quantification of transient behavior of wind-driven surface droplet/rivulet flows by using a digital fringe projection technique,” Journal of Visualization, 18(4), 705-718, 2015

Abstract

Due to historical legal challenges, there is a driving force for the development of objective methods of forensic toolmark identification. This study utilizes an algorithm to separate matching and nonmatching shear cut toolmarks created using fifty sequentially manufactured pliers. Unlike previously analyzed striated screwdriver marks, shear cut marks contain discontinuous groups of striations, posing a more difficult test of algorithm applicability. The algorithm compares correlation between optical 3D toolmark topography data, producing a Wilcoxon rank sum test statistic. Relative magnitude of this metric separates the matching and nonmatching toolmarks. Results show a high degree of statistical separation between matching and nonmatching distributions. Further separation is achieved with optimized input parameters and implementation of a “leash” preventing a previous source of outliers—however complete statistical separation was not achieved. This paper represents further development of objective methods of toolmark identification and further validation of the assumption that toolmarks are identifiably unique.

"The status, challenges, and future of additive manfuacturing in engineering," Computer Aided Design, (2015)

[75]  W. Gao, Y. Zhang, D. Ramanujana, K. Ramani, Y. Chen, C. B. Williams, C. Wang, Y. Shin, S. Zhang, and P. D. Zavattieri, "The status, challenges, and future of additive manfuacturing in engineering," Computer Aided Design, 69, 65-89, 2015; doi:10.1016/j.cad.2015.04.001

Abstract

Additive manufacturing (AM) is poised to bring about a revolution in the way products are designed, manufactured, and distributed to end users. This technology has gained significant academic as well as industry interest due to its ability to create complex geometries with customizable material properties. AM has also inspired the development of the maker movement by democratizing design and manufacturing. Due to the rapid proliferation of a wide variety of technologies associated with AM, there is a lack of a comprehensive set of design principles, manufacturing guidelines, and standardization of best practices. These challenges are compounded by the fact that advancements in multiple technologies (for example materials processing, topology optimization) generate a ‘‘positive feedback loop’’ effect in advancing AM. In order to advance research interest and investment in AM technologies, some fundamental questions and trends about the dependencies existing in these avenues need highlighting. The goal of our review paper is to organize this body of knowledge surrounding AM, and present current barriers, findings, and future trends significantly to the researchers. We also discuss fundamental attributes of AM processes, evolution of the AM industry, and the affordances enabled by the emergence of AM in a variety of areas such as geometry processing, material design, and education. We conclude our paper by pointing out future directions such as the ‘‘print-it-all’’ paradigm, that have the potential to re-imagine current research and spawn completely new avenues for exploration.

"Quantification of transient behavior of wind-driven surface droplet/rivulet flows by using a digital fringe projection technique," Journal of Visualization, (2015)

[74]  H. Hu, B. Wang, K. Zhang, W. Lohry* and S. Zhang, “Quantification of transient behavior of wind-driven surface droplet/rivulet flows by using a digital fringe projection technique,” Journal of Visualization, 18(4), 705-718, 2015;doi:10.1007/s12650-014-0264-8

Abstract

A digital fringe projection (DFP) system is developed to achieve non-intrusive thickness measurements of wind-driven water droplet/rivulet flows over a test plate to quantify the unsteady surface water transport process pertinent to various atmospheric icing phenomena. The DFP technique is based on the principle of structured light triangulation in a similar manner as a stereo vision system but replacing one of the cameras for stereo imaging with a digital projector. The digital projector projects line patterns of known characteristics onto the test specimen (i.e., a water droplet/rivulet on a test plate for the present study). The pattern of the lines is modulated from the surface of the test object. By comparing the modulated pattern and a reference image, the 3D profile of the test object with respect to the reference plane (i.e., the thickness distribution of the water droplet/rivulet flow) can be retrieved quantitatively and instantaneously. The feasibility and implementation of the DFP system is first demonstrated by measuring the thickness distribution of a small flat-top pyramid over a test plate to evaluate the measurement accuracy level of the DFP system. After carefully calibrated and validated, the DFP system is applied to achieve time-resolved thickness distribution measurements of a water droplet/rivulet to quantify the transient behavior of a water droplet/rivulet flow driven by a boundary layer air flow over a test plate. The dynamic shape changes and stumbling runback motion of the wind-driven water droplet/rivulet flow were measured in real time in terms of film thickness distribution, contact line moving velocity, wet surface area and droplet evaporation rate.

"Angular determination of toolmarks using a computer generated virtual tool," J. Forensic Sci., (2015)

[73]  R. Spotts, L. S. Chumbley,  L. Ekstrand*, S. Zhang, and J. Kreiser, "Angular determination of toolmarks using a computer generated virtual tool," J. Forensic Sci., 60(2), 303-315, 2015;doi:10.1111/1556-4029.12759

Abstract

A blind study to determine whether virtual toolmarks created using a computer could be used to identify and characterize angle of incidence of physical toolmarks was conducted. Six sequentially manufactured screwdriver tips and one random screwdriver were used to create toolmarks at various angles. An apparatus controlled tool angle. Resultant toolmarks were randomly coded and sent to the researchers, who scanned both tips and toolmarks using an optical profilometer to obtain 3D topography data. Developed software was used to create virtual marks based on the tool topography data. Virtual marks generated at angles from 30 to 85° (5° increments) were compared to physical toolmarks using a statistical algorithm. Twenty of twenty toolmarks were correctly identified by the algorithm. On average, the algorithm misidentified the correct angle of incidence by
6.12°. This study presents the results, their significance, and offers reasons for the average angular misidentification. 

"Absolute three-dimensional shape measurement using coded fringe patterns without phase unwrapping or projector calibration," Opt. Express, (2014)

[65] W. Lohry*, V. Chen*, and S.  Zhang,  "Absolute three-dimensional shape measurement using coded fringe patterns without phase unwrapping or projector calibration," Opt. Express 22(2), 1287-1301, 2014; doi:10.1364/OE.22.001287

Abstract

This paper presents a novel stereo-phase-based absolute three-dimensional (3D) shape measurement that requires neither phase unwrapping nor projector calibration. This proposed method can be divided into two steps: (1) obtain a coarse disparity map from the quality map; and (2) refine the disparity map using wrapped phase. Fringe patterns are modified to encode the quality map for efficient and accurate stereo matching. Experiments demonstrated that the proposed method could achieve high-quality 3D measurement even with extremely low-quality fringe patterns.
 

"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. 

"High-Speed 3-D Optical Metrology and Applications," Opt. Eng., (2014)

S. Zhang, R. Liang, and L. Yang, "High-speed 3D optical metrology and applications," Opt. Eng. 53(11), 112201, 2014 (Special Section Guest Editorial); doi:10.1117/1.OE.53.11.112201

"High-speed absolute three-dimensional shape measurement using three binary dithered patterns," Opt. Express, (2014)

[69]  W. Lohry* and S. Zhang, "High-speed absolute three-dimensional shape measurement using three binary dithered patterns," Opt. Express 22(22), 26752-26762, 2014 (Cover feature) (Image of the week of Nov. 22, 2014, OSA Optics InfoBase); doi: 10.1364/OE.22.026752

Abstract

This paper describes a method to reconstruct high-speed absolute three-dimensional (3D) geometry using only three encoded 1-bit binary dithered patterns. Because of the use of 1-bit binary patterns, high-speed 3D shape measurement could also be achieved. By matching the right camera image pixel to the left camera pixel in the object space rather than image space, robust correspondence can be established. Experiments demonstrate the robustness of the proposed algorithm and the potential to achieve high-speed 3D shape measurements.

"High-speed absolute three-dimensional shape measurement using three binary dithered patterns," Opt. Express, (2014)

[69]  W. Lohry* and S. Zhang, "High-speed absolute three-dimensional shape measurement using three binary dithered patterns," Opt. Express 22(22), 26752-26762, 2014 (Cover feature) (Image of the week of Nov. 22, 2014, OSA Optics InfoBase); doi: 10.1364/OE.22.026752

Abstract

This paper describes a method to reconstruct high-speed absolute three-dimensional (3D) geometry using only three encoded 1-bit binary dithered patterns. Because of the use of 1-bit binary patterns, high-speed 3D shape measurement could also be achieved. By matching the right camera image pixel to the left camera pixel in the object space rather than image space, robust correspondence can be established. Experiments demonstrate the robustness of the proposed algorithm and the potential to achieve high-speed 3D shape measurements.

"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.

 

"Virtual tool mark generation for efficient striation analysis," J. Forensic Sci., (2014)

[62] L. Ekstrand*, S. Zhang, T. Grieve, L. S. Chumbley, and J. Kreiser, "Virtual tool mark generation for efficient striation analysis," J. Forensic Sci., 59(4), 950-959, 2014; doi:10.1111/1556-4029.12435

"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.