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Plenoptic particle image velocimetry and surface pressure measurements were used to analyse the early development of leading-edge vortices (LEVs) created by a flat-plate wing of aspect ratio 2 rolling in a uniform flow parallel to the roll axis. Four cases were constructed by considering two advance coefficients, $J=0.54$ and 1.36, and two wing radii of gyration, $R_g/c=2.5$ and 3.25. In each case, the wing pitch angle was articulated such as to achieve an angle of attack of $33^{\circ }$ at the radius of gyration of the wing. The sources and sinks of vorticity were quantified for a chordwise rectangular control region, using a vorticity transport framework in a non-inertial coordinate system attached to the wing. Within this framework, terms associated with Coriolis acceleration provide a correction to tilting and spanwise convective fluxes measured in the rotating frame and, for the present case, have insignificant values. For the baseline case ( $J=0.54, R_g/c=3.25$ ), three distinct spanwise regions were observed within the LEV, with distinct patterns of vortex evolution and vorticity transport mechanisms in each region. Reducing the radius of gyration to $R_g/c=2.5$ resulted in a more stable vortex with the inboard region extending over a broader spanwise range. Increasing advance ratio eliminated the conical vortex, resulting in transport processes resembling the mid-span region of the baseline case. Although the circulation of the LEV system was generally stronger at the larger advance coefficient, the shear-layer contribution was diminished.

The three-dimensional (3-D) shock-wave/boundary-layer interaction (SBLI) between a fin-generated shock and a turbulent boundary layer is examined using plenoptic particle image velocimetry (PIV): a...

Rotating 3D velocimetry (R3DV) is a single-camera PIV technique designed to track the evolution of flow over a rotor in the rotating reference frame. A high-speed (stationary) plenoptic camera capable of 3D imaging captures the motion of particles within the volume of interest through a revolving mirror from the central hub of a hydrodynamic rotor facility, a by-product being an undesired image rotation. R3DV employs a calibration method adapted for rotation such that during MART reconstruction, voxels are mapped to pixel coordinates based on the mirror’s instantaneous azimuthal position. Interpolation of calibration polynomial coefficients using a fitted Fourier series is performed to bypass the need to physically calibrate volumes corresponding to each fine azimuth angle. Reprojection error associated with calibration is calculated on average to be less than 0.6 of a pixel. Experimental uncertainty of cross-correlated 3D/3C vector fields is quantified by comparing vectors obtained from imaging quiescent flow via a rotating mirror to an idealized model based purely on rotational kinematics. The uncertainty shows no dependency on azimuth angle while amounting to approximately less than 0.21 voxels per timestep in the in-plane directions and correspondingly 1.7 voxels in the radial direction, both comparable to previously established uncertainty estimations for single-camera plenoptic PIV.

In this work, a high-speed spectral plenoptic camera was used for three-dimensional (3D) simultaneous particle tracking and pyrometry measurements of hot spatter particles ejected during the metal additive manufacturing process. Additive manufacturing (AM) has an increasing role in the aerospace, energy, medical and automotive industry (DebRoy et al., 2018). While this new technology enables the production of highly advanced parts, research on the fundamental mechanisms governing the laser-matter interactions are an ongoing challenge because of the spatial and temporal resolution inherent to the AM process. One challenge is the characterization of spatter particles ejected from the melt pool, as these particles can be incorporated into the final part affecting the mechanical properties (Deng et al., 2020). One potential solution for simultaneously measuring velocity and temperature of the spatter particles is the spectral plenoptic camera.

Time of Flight (ToF) cameras are a type of range-imaging camera that provides three-dimensional scene information from a single camera. This paper assesses the ability of ToF technology to be used for threedimensional particle tracking velocimetry (3D-PTV). Using a commercially available ToF camera various aspects of 3D-PTV are considered, including: minimum resolvable particle size, environmental factors (reflections and refractive index changes) and time resolution. Although it is found that an off-the-shelf ToF camera is not a viable alternative to traditional 3D-PTV measurement systems, basic 3D-PTV measurements are shown with large (6mm) particles in both air and water to demonstrate future potential use as this technology develops. A summary of necessary technological advances is also discussed.

This paper investigates the effect of smoothing operation in 3D reconstruction using a plenoptic camera. A plenoptic camera - also known as light field camera - features a commercial off the shelf camera with added microlens array (MLA) behind the imaging lens, directly in front of the sensor. The main lens focuses the light to the MLA plane, where each microlens then re-directs the light to small regions of pixels behind, each pixel corresponding to different angle of incident (T. Fahringer (2015)) (Adelson and Wang (1992)). Thus, MLA encodes angular information of incident light rays into the recorded image that assist to acquire 4D information (u,v,s,t) of light-field including both position and angular information of light rays captured by the camera (Ng et al. (2005)) (Adelson and Wang (1992)).

Flow evolution over helicopter rotors, wind turbine blades, and insect wings are unsteady, three-dimensional (3D), and influenced by phenomena unique to the rotating frame of reference (FoR), e.g., Coriolis and centrifugal forces. Conventional 3D-PIV techniques are unable to fully characterize these rotating FoR physics, since the measurements are limited to a fixed FoR of a relatively small volume through which the rotor blade or wing traverses intermittently. In this paper, a new “Rotating Three-Dimensional Velocimetry (R3DV)” technique is proposed to address these gaps. R3DV consists of 3D measurements made with a single stationary plenoptic camera in combination with a hub-mounted mirror that aligns the camera’s field of view with a rotating wing. In post-processing R3DV data, a rotational volumetric calibration method is developed to account for image acquisition through a rotating mirror. Rotating FoR volumes are then reconstructed using the Multiplicative Algebraic Reconstruction Technique (MART) algorithm with the adapted calibration scheme and subsequently cross-correlated to derive a 3D velocity field. R3DV was experimentally demonstrated in a study of 3D unsteady flow over an impulsively rotated flat-plate wing. Prominent flow features like the formation and shedding of the primary and secondary leading-edge vortices (LEVs) were observed, which corroborate well with the existing literature on rotating wings. The time-resolved variation of LEV velocity profiles and circulation with azimuthal angle exhibited expected trends. The ability to quantify 3D and time-resolved velocity fields in the rotating FoR demonstrates the feasibility of adopting R3DV as a technique to investigate rotating flows.

Vortex topology is analyzed from measurements of flow over a flat, rectangular plate with an aspect ratio of 2 which was articulated in pitch and roll, individually and simultaneously. The plate was immersed into a $$\text {Re} = 10,000$$ flow (based on chord length). Measurements were made using a 3D–3C plenoptic PIV system to allow for the study of complete vortex topology of the entire wing. The prominent focus is the early development of the leading-edge vortex (LEV) and resulting topology. The effect of the wing kinematics on the topology was explored through a parameter space involving multiple values of pitch rate and roll rate at pitch and roll angles up to $${50}^{\circ }$$ . Characterization and comparisons across the expansive data set are made possible through the use of a newly defined dimensionless parameter, $${\textit{k}_{\text {Rg}}}$$ . Termed the effective reduced pitch rate, $${\textit{k}_{\text {Rg}}}$$ , is a measure of the pitch rate that takes into account the relative rolling motion of the wing in addition to the pitching motion and freestream velocity. This study has found that for a purely pitching wing, increasing the reduced pitch rate $${\textit{k}}$$ delays the vortex evolution with respect to $$\alpha _\mathrm {eff}$$ . For a purely rolling wing, as the advance coefficient $${\textit{J}}$$ is increased, the vortex evolution is advanced with respect to nondimensionalized time and the bifurcation point of the LEV shifts inboard. For a pitching and rolling wing, the addition of roll stabilizes and delays the evolution of the LEV in both nondimensionalized time and effective angle of attack.

The novel 3D imaging and reconstruction capabilities of plenoptic cameras are extended for use with continuous scalar fields relevant to reacting flows. This work leverages the abundance of perspective views in a plenoptic camera with the insertion of multiple filters at the aperture plane. The aperture is divided into seven regions using off-the-shelf components, enabling the simultaneous capture of up to seven different user-selected spectra with minimal detriment to reconstruction quality. Since the accuracy of reconstructed features is known to scale with the available angular information, several filter configurations are proposed to maintain the maximum parallax. Three phantoms inspired by jet plumes are simulated onto an array of plenoptic cameras and reconstructed using ASART+TV with a variety of filter configurations. Some systematic challenges related to the non-uniform distribution of views are observed and discussed. Increasing the number of simultaneously acquired spectra is shown to incur a small detriment to the accuracy of reconstruction, but the overall loss in quality is significantly less than the gain in spectral information.

The development of a tomographic background oriented schlieren implementation system utilizing up to four plenoptic cameras is presented. A systematic set of experiments was performed using a pair of solid dimethylpolysiloxan cylinders immersed in a nearly refractive index matched gylcerol/water solution to represent discrete flow features with known sizes, shapes, separation distances, and orientation. A study was conducted to assess the influence of these features on the accuracy of 3D reconstructions of the refractive index field. It was determined that the limited angular information collected by a single plenoptic camera is insufficient for single-camera 3D reconstructions. In multi-camera configurations, the additional views collected by a plenoptic camera were shown to improve the overall reconstruction accuracy compared to an equivalent single view per camera reconstruction, potentially reducing the number of overall cameras needed to achieve a desired accuracy. For the imaging of two cylinders, three or more cameras are generally needed to avoid significant ghosting artifacts in the reconstruction. Quantitative results are presented that show that: (1) two separate cylinders will be individually resolved as long as measurements from one camera are able to observe separation between the cylinders; (2) the error in the reconstructed 3D refractive index field increases as the size of the feature decreases; and (3) the use of volumetric masking within the reconstruction algorithm is critical in order to improve the accuracy of the solution.

Light-field cameras in conjunction with computational refocusing can be used to produce volumetric estimates of an imaged scene. However, these estimates are often dominated by image blur in the depth direction from objects not in each synthesized focal plane. Tomographic algorithms have been shown to be effective in creating volumetric estimates from plenoptic data but are often prohibitively slow. Deconvolution would be an attractive solution due to processing speed, but existing image synthesis equations are shift-variant. This research proposes an alternate refocusing transformation that makes the core problem described in continuous coordinates shift-invariant so that deconvolution is a viable solution. Shift-invariance of the new refocusing transform is demonstrated mathematically. Furthermore, the discretization involved in the imaging system and refocusing algorithm are characterized with respect to shift-variance in order to identify potential sources of artifacts and to propose potential mitigating steps where possible. While the sampled light field is not directly invertible, experimental data are used to demonstrate that regularized deconvolution using the derived synthesis equations produces improved results compared to the base focal stack in both synthetic examples and actual camera data.

Plenoptic particle image velocimetry was used to perform instantaneous three-dimensional velocity measurements in the near wake of a wall-mounted hemispherical roughness element at a Reynolds numbe...

A novel algorithm was developed to estimate scene depth by uniquely exploiting plenoptic image data. Plenoptic images allow multiple perspectives to be extracted from a single exposure from a singl...

This paper presents a methodology to obtain three-dimensionally (3D) resolved measurements of temperature, soot volume fraction, and excited state radical concentration in a volume of luminous combustion gases using light field imaging with a plenoptic camera. The technique involves measurement of emission intensities at multiple wavelength bands selected to enable two-color pyrometry of soot particles and chemiluminescence of CH* and C2* radicals. The feasibility of this method was tested using laboratory flames and reconstruction algorithms that were developed previously for 3D imaging of discrete particles. The results show that the existing algorithms reproduce the basic structural features of the flame but with limited spatial resolution, demonstrating feasibility but also revealing the need for improved algorithms for volumetric reconstruction. Quantitative measurements of temperature and soot volume fraction were obtained that were in good agreement with independent measurements using conventional techniques and distributions of CH* and C2* were obtained that were consistent with expected trends.

Several volumetric reconstruction algorithms are surveyed and evaluated using plenoptic images of a burner flame. Automatic generation of static and dynamic masks is implemented to limit the scope of the reconstruction problem. Volumetric reconstructions of the flame are performed using all surveyed algorithms with a number of plenoptic cameras ranging from one to four. Simultaneous Algebraic Reconstruction Technique (SART), a reconstruction algorithm widely used in medical imaging, is identified as the most promising algorithm of those surveyed. When combined with inter-iteration minimization of L1 Total Variation (TV-L1), SART-TV reconstructions of a burner flame are shown to be smooth, circular, and uniform. Two-camera reconstructions recover the approximate shape of the flame, yielding qualitative information about the flame structure. Three-and four-camera yield the full three-dimensional flame structure.

The recent unexpected discovery of thrombosis in transcatheter heart valves (THVs) has led to increased concerns of long-term valve durability. Based on the clinical evidence combined with Virchow's triad, the primary hypothesis is that low-velocity blood flow around the valve could be a primary cause for thrombosis. However, due to limited optical access in such unsteady three-dimensional biomedical flows, measurements are challenging. In this study, for the first time, we employ a novel single camera volumetric velocimetry technique to investigate unsteady three-dimensional cardiovascular flows. Validation of the novel volumetric velocimetry technique with standard planar particle image velocimetry (PIV) technique demonstrated the feasibility of adopting this new technique to investigate biomedical flows. This technique was used to quantify the three-dimensional velocity field in the vicinity of a validated, custom developed, transparent THV in a bench-top pulsatile flow loop. Large volumetric regions of flow stagnation were observed in the neo-sinus throughout the cardiac cycle, with stagnation defined as a velocity magnitude lower than 0.05 m s −1 . The volumetric scalar viscous shear stress quantified via the three-dimensional shear stress tensor was within the range of low shear-inducing thrombosis observed in the literature. Such high-fidelity volumetric quantitative data and novel imaging techniques used to obtain it will enable fundamental investigation of heart valve thrombosis in addition to providing a reliable and robust database for validation of computational tools.

This work details the development of an algorithm to determine 3D position and in plane size and shape of particles by exploiting the perspective shift capabilities of a plenoptic camera combined with stereo-matching methods. This algorithm is validated using an experimental data set previously examined in a refocusing based particle location study in which a static particle field is translated to provide known depth displacements at varied magnification and object distances. Examination of these results indicates increased accuracy and precision is achieved compared to a previous refocusing based method at significantly reduced computational costs. The perspective shift method is further applied to fragment localization and sizing in a lab scale fragmenting explosive.

This paper describes the development of a Modular Plenoptic Adaptor (MPA) for rapid and reversible conversion of high-speed cameras into plenoptic imaging systems, with the primary goal of enabling single-camera, time-resolved 3D flow-measurements. The MPA consists of a regular imaging lens, a microlens array, a tilt-adjustable microlens mount and an optical relay, which are collectively installed onto a high-speed camera through a standard lens mount. Each component within the system is swappable to optimize for specific imaging applications. In this study, multiple configurations of the MPA were tested and they demonstrated the ability to refocus and shift perspectives within high-speed scenes after capture. Additionally, the MPA demonstrated 3D reconstruction of captured scenes with

Plenoptic particle image velocimetry (PPIV) has been demonstrated in the past as a viable single-/dual-camera technique for 3D flow measurements. Compared to established four-camera tomographic-PIV and 3D-PTV, PPIV has the advantages of lower cost, a simpler setup with a smaller footprint, a deeper depth-of-field for a given aperture and potential for access to otherwise optically restricted facilities. However, because camera bodies must be significantly modified to accommodate an embedded plenoptic microlens array (MLA), past PPIV implementations have been limited to

Plenoptic background oriented schlieren imaging has recently been introduced as a single-camera technique used to observe three-dimensional density gradients in a flow field. With the ability to generate focused BOS images, the signature of density gradients produced at different depth locations can be distinguished from one another. Two experiments demonstrate the capabilities of this technique. The first experiment visualized the rising plumes produced from two simple flames placed at different depths in a low magnification configuration. The second experiment used a high magnification configuration with long working distance to visualize shock waves in a 6.35 millimeter diameter underexpanded jet. These experiments demonstrate plenoptic BOS as a simple and convenient three-dimensional visualization technique that can be applied in facilities with limited optical access.

The Light-Field Imaging Toolkit (LFIT) is a collection of MATLAB functions designed to facilitate the rapid processing of raw light field images captured by a plenoptic camera. An included graphical user interface streamlines the necessary post-processing steps associated with plenoptic images. The generation of perspective shifted views and computationally refocused images is supported, in both single image and animated formats. LFIT performs necessary calibration, interpolation, and structuring steps to enable future applications of this technology. Keywords: Plenoptic, Light field, MATLAB

Large-scale features in a zero-pressure-gradient turbulent boundary layer are compared with those in two adverse-pressure-gradient turbulent boundary layers using a scanning three-dimensional flow-visualization technique. The boundary-layer flow was formed on the wall of a wind tunnel and seeded with smoke introduced through a slit. A ramp was mounted on the opposite wall to create an adverse pressure gradient (β=8.1 and 9.7). A high-repetition-rate laser formed into a light sheet was scanned in the wall-normal direction, and the two-dimensional images were reconstructed into a nearly instantaneous three-dimensional flow-visualization volume. Observations about the boundary-layer structures for Reynolds numbers Reθ between 7400 and 8400 are made by investigating the instantaneous flow-visualization volumes, the features from binary-image slices, and conditional averaged volumes around a structure. In general, structures in a zero-pressure-gradient boundary layer are larger and spaced farther apart compare...

The projection of an externally-mounted microlens array (MLA) onto the sensor via relay lenses is oft adopted by the scientific community in lieu of traditional plenoptic cameras with embedded on-sensor MLA. In most cases a relayed design better supports prototyping and temporary setups, while in other cases such as intensified plenoptic imaging the extended projection distance is necessary due to physical constraints. However, relaying the MLA inevitably blurs the raw image, often by 1.5px or more. We observed an unusual behavior where fixed blurring at the raw image level produced blurs that are worsened by depth within refocused images- enough to reduce the plenoptic system's depth-of-field by 60% in most cases. To address this problem, we hereby put forth a model to elucidate the mechanism of this depth-dependent blurring, and propose a Lucy-Richardson Deconvolution (LRD) procedure for mitigation. Disccusions are made in the contexts of regular and intensified plenoptic photography, as well as application in 3D particle image velocimetry for fluid dynamics. Results were validated on synthetic as well as experimental images.

The motivation of this work is to compare results from plenoptic and conventional background oriented schlieren (BOS) systems through two experimental configurations using a buoyant thermal plume to produce the inhomogeneous density field. The first set of experiments fixed the nominal focal plane and varied the plume position along the optical axis. The second set of experiments fixed the position of the plume while varying the nominal focal plane. These experiments were used to observe how measurement performance was influenced by (1) spatial resolution, (2) plume position, (3) focal plane position, (4) depth of field, (5) exposure time, and (6) aperture size. It was determined that both imaging systems follow the trend that the signal is dependent on the distance between the background and the plume but relatively independent of the nominal focal plane position. Conventional BOS images result in high-resolution, single line-of-sight measurements with a high signal-to-noise ratio. These images typically require a small aperture with a longer exposure time in order to achieve an appropriate depth of field. Focused BOS images are a result of lower resolution, multiple line-of-sight measurements with lower signal-to-noise ratios compared to that of the conventional system. Images from this system use a larger aperture to more efficiently collect light while still maintaining an extended depth of field.

The volumetric calibration of a plenoptic camera is explored to correct for inaccuracies due to real-world lens distortions and thin-lens assumptions in current processing methods. Two methods of volumetric calibration based on a polynomial mapping function that does not require knowledge of specific lens parameters are presented and compared to a calibration based on thin-lens assumptions. The first method, volumetric dewarping, is executed by creation of a volumetric representation of a scene using the thin-lens assumptions, which is then corrected in post-processing using a polynomial mapping function. The second method, direct light-field calibration, uses the polynomial mapping in creation of the initial volumetric representation to relate locations in object space directly to image sensor locations. The accuracy and feasibility of these methods is examined experimentally by capturing images of a known dot card at a variety of depths. Results suggest that use of a 3D polynomial mapping function provides a significant increase in reconstruction accuracy and that the achievable accuracy is similar using either polynomial-mapping-based method. Additionally, direct light-field calibration provides significant computational benefits by eliminating some intermediate processing steps found in other methods. Finally, the flexibility of this method is shown for a nonplanar calibration.

A three-color pyrometer has been developed based on plenoptic imaging technology. Three bandpass filters placed in front of a camera lens allow separate 2D images to be obtained on a single image sensor at three different and adjustable wavelengths selected by the user. Images were obtained of different black- or grey-bodies including a calibration furnace, a radiation heater, and a luminous sulfur match flame. The images obtained of the calibration furnace and radiation heater were processed to determine 2D temperature distributions. Calibration results in the furnace showed that the instrument can measure temperature with an accuracy and precision of 10 Kelvins between 1100 and 1350 K. Time-resolved 2D temperature measurements of the radiation heater are shown.