"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. 

"Towards superfast three-dimensional optical metrology with digital micromirror device (DMD) platforms," Opt. Eng., (2015)

[67] T. Bell* and S. Zhang, "Towards superfast three-dimensional optical metrology with digital micromirror device (DMD) platforms," Opt. Eng., 53(11), 112206, 2014; doi: 10.1117/1.OE.53.11.112206

Decade-long research efforts toward superfast three-dimensional (3-D) shape measurement leveraging the digital micromirror device (DMD) platforms are summarized. Specifically, we will present the following technologies: (1) high-resolution real-time 3-D shape measurement technology that achieves 30 Hz simultaneous 3-D shape acquisition, reconstruction, and display with more than 300,000 points per frame; (2) superfast 3-D optical metrology technology that achieves 3-D measurement at a rate of tens of kilohertz utilizing the binary defocusing method we invented; and (3) the improvement of the binary defocusing technology for superfast and high-accuracy 3-D optical metrology using the DMD platforms. Both principles and experimental results are presented.

"High-resolution, real-time three-dimensional shape measurement on graphics processing unit," Opt. Eng., (2014)

[64] N. Karpinsky*, M. Hoke*, V. Chen*, and S.  Zhang, "High-resolution, real-time three-dimensional shape measurement on graphics processing unit," Opt. Eng. 53(2), 024105, 2014;  doi: 10.1117/1.OE.53.2.024105

Abstract

A three-dimensional (3-D) shape measurement system that can simultaneously achieve 3-D shape acquisition, reconstruction, and display at 30 frames per second (fps) with 480,000 measurement points per frame is presented. The entire processing pipeline was realized on a graphics processing unit (GPU) without the need of substantial central processing unit (CPU) power, making it achievable on a portable device, namely a laptop computer. Furthermore, the system is extremely inexpensive compared with similar state-of-art systems, making it possible to be accessed by the general public. Specifically, advanced GPU techniques such as multipass rendering and offscreen rendering were used in conjunction with direct memory access to achieve the aforementioned performance. The developed system, implementation details, and experimental results to verify the performance of the proposed technique are presented.

"Digital micromirror transient response influence on superfast 3D shape measurement," Opt. Laser Eng., (2014)

[63] Y. Wang*, B.  Bhattacharya, E. H. Winer, P. Kosmicki, W. H. El-Ratal, and S. Zhang, "Digital micromirror transient response influence on superfast 3D shape measurement," Opt. Laser Eng. 58, 19-26, 2014; doi: 10.1016/j.optlaseng.2014.01.015

Abstract

Nowadays, the high speed (e.g., kilo-Hertz) refreshing rate of the digital micro-mirror device (DMD) has enabled superfast 3D shape measurement using the binary defocusing technique. This research finds that when the system reaches its extreme binary pattern refreshing rate, the transient response of the DMD induces a coupling effect (i.e., two neighboring patterns blend together) that may cause substantial measurement error. Since this transient response repeats itself, this systematic measurement error is substantially reduced to a negligible level when the timing between the projector and the camera is properly adjusted. Experimental results are presented to demonstrate the observed phenomena, and the success of utilizing the proposed method to overcome the problems associated with the transient response of the DMD.

"Three-dimensional shape measurement with dual reference phase maps, " Opt. Eng. ,(2014)

[58] J. Dai, C. Gong*, and S. Zhang, "Three-dimensional shape measurement with dual reference phase maps, " Opt. Eng. 53(1), 014102, 2014; doi: 10.1117/1.OE.53.1.014102

Abstract

Single reference-phase-based methods have been extensively utilized in digital fringe projection systems, yet they might not provide the maximum sensitivity given a hardware system configuration. This paper presents an innovative method to improve the measurement quality by utilizing two orthogonal phase maps. Specifically, two reference phase maps generated from horizontal and vertical (i.e., orthogonal) fringe patterns projected are combined into a vector reference phase map through a linear combination for depth extraction. The experiments have been conducted to verify the superiority of the proposed method over a conventional single reference-phase-based approach.