"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.

"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.
 

"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.

"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.

"Genetic method to optimize binary dithering technique for high-quality fringe generation," Opt. Lett. ,(2013)

[50] W. Lohry* and S. Zhang, "Genetic method to optimize binary dithering technique for high-quality fringe generation," Opt. Lett. 38(4), 540-542, 2013; doi: 10.1117/1.OE.51.11.113602

Abstract

The recently proposed dithering techniques could substantially improve measurement quality when fringes are wide, but offer limited improvement when fringes are narrow. This Letter presents a genetic algorithm to optimize the dithering technique for sinusoidal structured pattern representation. We believe both simulation and experimental results show that this proposed algorithm can substantially improve fringe quality for both narrow and wide fringe patterns. 

"Fourier transform profilometry using a binary area modulation technique," Opt. Eng., (2012)

W. Lohry* and S. Zhang, "Fourier transform profilometry using a binary area modulation technique," Opt. Eng. 51(11),113602, 2012; doi: 10.1117/1.OE.51.11.113602

A recent study found that it is very difficult to use the squared binary defocusing technique to eliminate the influence of third-order harmonics without compromising fringe quality, and thus it is challenging to utilize Fourier transform profilometry to achieve high-quality three-dimensional measurement. A novel approach is presented to effectively eliminate the third-order harmonics by modulating the squared binary structured patterns. Both simulation and experiments are presented to verify the performance of the proposed technique.

"3D shape measurement with 2D area modulated binary patterns," Opt. Laser Eng.,(2012)

W. Lohry* and S. Zhang, "3D shape measurement with 2D area modulated binary patterns," Opt. Laser Eng. 50(7), 917-921, 2012; doi: 10.1016/j.optlaseng.2012.03.002

This paper presents a novel area-modulation technique for three-dimensional (3D) shape measurement with binary defocusing. Specifically, this technique modulates local 2*2 pixels to create five grayscale values to enhance fringe quality when the projector is not perfectly in focus. With this novel technique, we will show that the phase error is approximately 1/3 of the square binary method when fringe pattern is dense and the projector is nearly focused.