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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)).

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.

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 application of a planar density measurement technique for compressible flowfields based on acetone planar laser-induced fluorescence is presented. An error analysis indicates a minimum inherent uncertainty of ∼2.5% in density measurements due to uncertainty in local pressure and a total experimental uncertainty of 8%, primarily driven by shot noise due to low-signal levels. The technique is demonstrated through the visualization of the separated shear layer and turbulent wake of a wall-mounted hemisphere at a freestream Mach number of 0.78 and a Reynolds number of approximately 900,000. The flow is marked by a large-scale flapping motion of the wake and low-density vortex cores, where density drops of up to 50% of the freestream density are detected. In addition, closeup images of the shear layer near the separation point reveal the formation of lambda shocks. The density fields are used to perform aerooptic distortion calculations through spatial integration of the density field. A correlation is fou...

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.

A high-speed three-dimensional flow visualization system has been developed and is described. The technique is based on the high-speed scanning and subsequent imaging of a two-dimensional laser sheet through the flowfield. A three-dimensional image is then reconstructed from the stack of two-dimensional slices. The technique achieves high speeds using a home-built megahertz-rate pulse-burst laser system, a galvanometric scanning mirror, and a high-speed intensified charge-coupled-device camera capable of 500,000 frames per second. These components allow for the acquisition of three-dimensional image data with a resolution of 220 x 220 x 68 volumetric elements (voxels) in 136 μs. The speed of the technique is limited by the available camera speed and can be increased substantially using a higher-speed camera. The technique is demonstrated through three-dimensional visualization a turbulent round jet (Re = 6700) seeded with small water droplets for light scattering. Three-dimensional flow visualization images display numerous three-dimensional features of the jet, including ring vortices, azimuthal modes and counterrotating streamwise vortex pairs. Future work will focus on the development of a high-speed three-dimensional laser-induced fluorescence technique.

This research effort addresses the use of numerical simulations to predict the flow physics over an airborne turret, represented by a hemisphere. Computational fluid dynamics codes, Loci-Chem, Kestrel and OVERFLOW were used in this study with different high fidelity turbulence models, including DES, DDES, D-MS and hybrid RANS/LES, to determine their capabilities of predicting these complex flow features. These numerical simulations were compared against the experiment conducted at Auburn University, in terms of wall-pressure data, instantaneous density contour images, and optical path difference (OPD). Among the three codes, Loci-Chem using the hybrid RANS/LES turbulence model was able to capture the unsteady flow structures, such as shear-layer fluctuations, and the rolling motions of the vortex shedding. Both Kestrel using DDES+SST and OVERFLOW using the DES, DDES, and D-MS turbulence models were able to predict certain levels of shear-layer oscillations; however the presence of the vortex motions behind the wake region were not fully captured. Therefore, the results from Kestrel and OVERFLOW predicted limited OPD values as compared to the experimental data. Since the solution from Loci-Chem was able to produce the vortex shedding, the OPD analysis was able to follow the most of the trends, except that under-predicted the vortex motion shown in the experiment.

A pulse burst laser and either one or two high-speed charge-coupled-device cameras were used to perform onecomponent time-resolved planar-Doppler-velocimetry (PDV) measurements in a rectangular Mach 2.0 jet. The measurements were carried out on a streamwise plane passing through the jet centerline and covering approximately 6‐12 jet heights downstream of the jet exit. The pulse burst laser operated at 0.532-µ mw avelength and produced 28 pulses at 250 kHz with approximately 9 mJ/pulse energy. Velocity image sequences consisting of 28 frames showed dynamics of the velocity field over a time span of 108 µs (approximately 4.5 convective timescales). A typical sequence of images is presented, which demonstrates the process of entrainment of low-speed fluid into the high-speed region of the jet. Mean and standard deviation statistics of the velocity calculations produced expected trends and showed good agreement between the single- and two-camera experiments. An error analysis revealed speckle as the predominant source of noise, as in a conventional PDV technique. At a transmission ratio of 0.5, the estimated total error is 13 m/s for the single-camera experiment and 15 m/s for the two-camera experiment. I. Introduction P LANAR Doppler velocimetry is a powerful optical diagnostic technique that can be used to measure all three components of instantaneous velocity over a two-dimensional plane within a flowfield with high spatial resolution. This is accomplished by using an atomic or molecular vapor filter to measure the frequency shift of light as it is scattered by particles contained in the flowfield. The �

Light field imaging with a plenoptic camera provides the ability to capture three dimensional information about a given volume or flow with a single camera. The single camera nature of this technique allows for imaging in environments with restricted optical access. Such environments are often associated with chemically reacting flows, which often have minimal external access due to high temperature and pressure. The plenoptic camera shows promise as an emerging tool for investigation of these flows, although its application in these environments is still largely unexplored. An overview of the basics of light field imaging with a plenoptic camera is given with a focus on the ability of the camera to refocus an image after it has been acquired. Three dimensional deconvolution is discussed as a method for enhancing the spatial resolution of refocused two dimensional images in environments possessing significant out-of-plane signal from a flame or other translucent source. Preliminary results showing the application of 3D deconvolution for image enhancement are presented with a summary of future work.

Work was carried out as part of an ongoing effort to relate instantaneous large-amplitude features of the fare eld acoustic to the dynamic evolution and interaction of large-scale structures within the mixing layer of ideally expanded, high-speed, high-Reynolds-number jets. It is believed that such information is essential for a better understanding of jet noise sources and jet aeroacoustic modeling and control. The acoustic measurements were taken with a four-microphone inline array placed 30 deg to the jet axis. Conditional sampling of the data from the microphones was used to create characteristic waveforms for the large-amplitude, far-e eld sound pressure peaks. The frequency content of these phase-averaged waveforms compare very well with those of the overall acoustic far e eld at 30 deg. A vast majority of the large-amplitude sound events originated between four and nine jet exit diameters downstream of the nozzle. The acoustic measurements were taken simultaneously with real-time e ow visualizations to determine the mechanisms that were responsible for the creation of individual far-e eld acoustic peaks,and theseresultshaveidentie ed threenoise-generation mechanisms:intensecross-mixing-layerinteraction, the tearing of large turbulent structures, and the rollup of large turbulent structures. The simultaneous data also showed that large structures entrain more ambient air into the jet and the mixing layer extends farther into the jet core during intense noise production than during periods of relative quiet.

A newly developed MHz rate imaging system that provides real-time flow visualization is described. The technique utilizes a custom-built Nd:YAG pulse burst laser and an ultra high-speed digital camera and is capable of capturing 17 images over 150 microseconds. The system was used to visualize a Mach 1.3 (M c =0.6) axisymmetric jet. Sample results indicate the potential of the technique to provide detailed information on the dynamic characteristics of large-scale structures. A two-dimensional cross-correlation technique was used to calculate the convective velocity of large-scale structures. Present results generally agree with the findings of earlier investigations that indicate a significant deviation of the convective velocity from theoretical predictions.

Plenoptic imaging is a 3D imaging technique that has been applied for quantification of 3D particle locations and sizes. This work experimentally evaluates the accuracy and precision of such measurements by investigating a static particle field translated to known displacements. Measured 3D displacement values are determined from sharpness metrics applied to volumetric representations of the particle field created using refocused plenoptic images, corrected using a recently developed calibration technique. Comparison of measured and known displacements for many thousands of particles allows for evaluation of measurement uncertainty. Mean displacement error, as a measure of accuracy, is shown to agree with predicted spatial resolution over the entire measurement domain, indicating robustness of the calibration methods. On the other hand, variation in the error, as a measure of precision, fluctuates as a function of particle depth in the optical direction. Error shows the smallest variation within the predicted depth of field of the plenoptic camera, with a gradual increase outside this range. The quantitative uncertainty values provided here can guide future measurement optimization and will serve as useful metrics for design of improved processing algorithms.

The combination of the background oriented schlieren (BOS) technique with the unique imaging capabilities of a plenoptic camera, termed plenoptic BOS, is introduced as a new addition to the family of schlieren techniques. Compared to conventional single camera BOS, plenoptic BOS is capable of sampling multiple lines-of-sight simultaneously. Displacements from each line-of-sight are collectively used to build a four-dimensional displacement field, which is a vector function structured similarly to the original light field captured in a raw plenoptic image. The displacement field is used to render focused BOS images, which qualitatively are narrow depth of field slices of the density gradient field. Unlike focused schlieren methods that require manually changing the focal plane during data collection, plenoptic BOS synthetically changes the focal plane position during post-processing, such that all focal planes are captured in a single snapshot. Through two different experiments, this work demonstrates that plenoptic BOS is capable of isolating narrow depth of field features, qualitatively inferring depth, and quantitatively estimating the location of disturbances in 3D space. Such results motivate future work to transition this single-camera technique towards quantitative reconstructions of 3D density fields.

As with any flow diagnostic imaging system, plenoptic PIV must be able to capture images under a wide variety of imaging conditions. One common problem is the change in magnification caused by a change in the index of refraction between a flow volume and the camera, often seen in water tunnel imaging. This work seeks to analytically derive the effect of refraction on the imaging equations and to find a correction factor for the depth of particles in a tomographic reconstruction of the volume. This solution was tested by a series of simulated images that were generated under three different conditions: no refractive interfaces, a flat refractive interface between the volume and the camera, and a flat refractive interface between the main lens and the microlens array. Results demonstrated that the depth of a particle is shifted with regard to the focal point of the system by a factor of the index of refraction. For an internal interface, this scaling remains true for all particles focused behind the interface, but there is significant deviation in particle depth for particles focused in front of this plane.

This paper presents a detailed description of a scanning 3D flow visualization technique which was used to compare and contrast the large scale features in a zero pressure gradient (ZPG) turbulent boundary layer with those in two adverse pressure gradient (APG) turbulent boundary layers. Ramps were installed in a wind tunnel to produce an adverse pressure gradient on the wind tunnel wall. The boundary layer flow was seeded with smoke from a reservoir that was drawn into the tunnel. A high-repetition-rate laser formed into a light sheet was scanned in the wall-normal direction through the boundary layer. The 2D images were captured using a high-speed camera and reconstructed into a nearly instantaneous 3D flow visualization volume of the intensity of the smoke in the turbulent boundary layer. Static pressure measurements and 2D particle image velocimetry (PIV) were taken to characterize the flow with and without the ramps. Observations about the boundary layer structures were made by investigating the flow visualization volumes, the power spectral density of the images, the features from binary image slice, and conditionally averaged volumes around a structure. In general, the ZPG structures were larger and spaced farther apart compared to the APG cases. The structures in the APG cases were more elongated in the streamwise direction and occurred at a steeper angle from horizontal compared to the ZPG case.

Light field imaging with a plenoptic camera offers a straightforward method for capturing three-dimensional information about a flow with a single camera. Chemically reacting flows are a subject of great interest in the aerospace industry; however, there is little research regarding the imaging of combustion processes or flames with a plenoptic camera. A preliminary investigation into the viability of three-dimensional flame measurements with a plenoptic camera is conducted. A high-level overview of the key concepts undergirding the plenoptic camera is given. Three-dimensional deconvolution is described as a promising volume reconstruction technique. Some initial results are presented. Preliminary conclusions are shared with a brief discussion of the direction of future work.

Simultaneous particle image velocimetry (PIV) and flow visualization measurements were performed in a turbulent boundary layer in an effort to better quantify the relationship between the velocity field and the image intensity typically observed in a classical flow visualization experiment. The freestream flow was lightly seeded with smoke particles to facilitate PIV measurements, whereas the boundary layer was densely seeded with smoke through an upstream slit in the wall to facilitate both PIV and classical flow visualization measurements at Reynolds numbers, Re θ , ranging from 2,100 to 8,600. Measurements were taken with and without the slit covered as well as with and without smoke injection. The addition of a narrow slit in the wall produces a minor modification of the nominal turbulent boundary layer profile whose effect is reduced with downstream distance. The presence of dense smoke in the boundary layer had a minimal effect on the observed velocity field and the associated proper orthogonal decomposition (POD) modes. Analysis of instantaneous images shows that the edge of the turbulent boundary layer identified from flow visualization images generally matches the edge of the boundary layer determined from velocity and vorticity. The correlation between velocity deficit and smoke intensity was determined to be positive and relatively large (>0.7) indicating a moderate-to-strong relationship between the two. This notion was extended further through the use of a direct correlation approach and a complementary POD/linear stochastic estimation (LSE) approach to estimate the velocity field directly from flow visualization images. This exercise showed that, in many cases, velocity fields estimated from smoke intensity were similar to the actual velocity fields. The complementary POD/LSE approach proved better for these estimations, but not enough to suggest using this technique to approximate velocity measurements from a smoke intensity image. Instead, the correlations further validate the use of flow visualization techniques for determining the edge and large-scale shape of a turbulent boundary layer, specifically when quantitative velocity measurements, such as PIV, are not possible in a given experiment.

The particle reconstruction capabilities of a novel 3-D, 3-C PIV technique, based on volume illumination and a plenoptic camera to measure a velocity field, are tested. This technique is based on light-field photography, which uses a dense microlens array mounted near a camera sensor to sample the spatial and angular distribution of light entering the camera. Tomographic algorithms (MART) are then used to reconstruct a volumetric intensity field after the image is taken, and cross-correlation algorithms extract the velocity field from the reconstructed volume. This paper provides an introduction to the concepts of light fields and describes the tomographic algorithms used to reconstruct the measurement volume. A preliminary test on the accuracy of single particle reconstructions is presented, showing that laterally we can expect errors to be less than a voxel for most cases. A test on the reconstruction quality is presented for a volume of particles as a function of particle density and is shown to vary little as the particle density is increased. Finally a simulated Gaussian ring vortex is presented showing a full simulation as well as the velocity accuracy.

This paper describes current calibration methods for plenoptic cameras and introduces a new method of calibration that seeks to estimate the position and orientation of the microlens array based on the camera geometry and on a calibration image. A geometrical model was formulated to relate the position of a microlens to the location on the image sensor where the lens was focused. The location of the focal points were determined by stopping down the aperture of the main camera lens such that only a small beam of light was incident on each microlens. The position and orientation of the microlens array was assumed, and the predicted focal points were compared with the known points determined from the calibration data. This process was repeated until the root-mean-square difference between the expected and predicted results was minimal. The geometrical method was shown to provide a reasonable estimate for the orientation of the microlens array, as each translational parameter converged to less than one pixel and each rotational parameter converged to within 0.00034 radians. Preliminary results show that the best estimate for the image distance is obtained from a measurement of the magnification by imaging a ruler.

The commercial release of the Lytro camera, and greater availability of plenoptic imaging systems in general, have given the image processing community cost-effective tools for light-field imaging. While this data is most commonly used to generate planar images at arbitrary focal depths, reconstruction of volumetric fields is also possible. Similarly, deconvolution is a technique that is conventionally used in planar image reconstruction, or deblurring, algorithms. However, when leveraged with the ability of a light-field camera to quickly reproduce multiple focal planes within an imaged volume, deconvolution offers a computationally efficient method of volumetric reconstruction. Related research has shown than light-field imaging systems in conjunction with tomographic reconstruction techniques are also capable of estimating the imaged volume and have been successfully applied to particle image velocimetry (PIV). However, while tomographic volumetric estimation through algorithms such as multiplicative algebraic reconstruction techniques (MART) have proven to be highly accurate, they are computationally intensive. In this paper, the reconstruction problem is shown to be solvable by deconvolution. Deconvolution offers significant improvement in computational efficiency through the use of fast Fourier transforms (FFTs) when compared to other tomographic methods. This work describes a deconvolution algorithm designed to reconstruct a 3-D particle field from simulated plenoptic data. A 3-D extension of existing 2-D FFT-based refocusing techniques is presented to further improve efficiency when computing object focal stacks and system point spread functions (PSF). Reconstruction artifacts are identified; their underlying source and methods of mitigation are explored where possible, and reconstructions of simulated particle fields are provided.