We would like to thank NSF grant CMMI #1126862 for funding the equipment and development of the synthetic aperture algorithms at BYU, In-house Laboratory Independent Research (ILIR) funds (monitored by Dr. Tony Ruffa) for funding the equipment and development at NUWC Newport, and NIH/NIDCD grant R01DC009616 for funding SLT, DJD and JRN and data relating to the vocal fold experiments and the University of Erlangen Graduate School in Advanced Optical Technologies (SAOT) for partial support of SLT. Finally, the Rocky Mountain NASA Space Grant Consortium for funding JRN.

1. 3D Light Field Imaging Setup
<ol>
 <li>Start by determining the size of the measurement volume as well as the temporal and spatial resolution required for investigating the fluid flow experiment being studied. </li>
 <li>Estimate the optical density that will be present in the experiment in order to determine the number of cameras required to generate refocused images with good signal-to-noise ratio (SNR) 1, 2 (e.g. for PIV one should calculate particles per pixel). For the 3D SAPIV experiment with the synthetic vocal folds presented herein, we use 8 cameras and expect to achieve a seeding density of 0.05-0.1 particles per pixel (ppp). This number increases with increasing number of camera with diminishing returns reached around 13 cameras; the SNR decreases rapidly below 5 cameras. </li>
 <li>Mount the cameras in an array configuration on a frame such that each camera can view the measurement volume from different viewpoints. </li>
 <li>Attach the cameras to a central computer for data capture and viewing. </li>
 <li>Select lenses with focal lengths appropriate for the desired magnification and optical working distances. Typically, the same type of fixed focal length lens is mounted to each camera to generate similar magnification in each image. </li>
 <li>Place a visual target (such as a calibration grid) in the center of the measurement volume. </li>
 <li>Using the image from the center camera of the array as a reference, move the entire camera array frame closer to or farther from the measurement volume to achieve the desired magnification. </li>
 <li>Next, separate the remaining cameras in the array. Spacing the cameras farther apart from each other improves the spatial resolution in the depth dimension at the cost of total resolvable depth 1. Note: we use depth to refer to the Z-dimension, which is positive toward the cameras (see Figure 1). The ratio of depth to in-plane resolution is given approximately by <img alt="Equation 1" fo:src="/files/ftp_upload/4325/4325eq1highres.jpg" src="/files/ftp_upload/4325/4325eq1.jpg" />, where Z is the depth in the volume, so is the distance of the cameras to the front of the volume, and D is the ratio of camera spacing to so. </li>
 <li>Angle all cameras such that the visual target in the center of the measurement volume is approximately centered in each camera image.</li>
 <li>With the apertures completely open on each camera lens, focus each camera on the visual target. </li>
 <li>Place a calibration target at the back of the measurement volume. Ensure that the target is in the view of each camera; if it is not, then the distance between the cameras and measurement volume and/or camera spacing needs adjustment (steps 1.7-1.8). </li>
 <li>Close the aperture of each camera until the target is in focus in each camera. </li>
 <li>Repeat steps 1.11-1.12 with the target at the front of the measurement volume. The calibration target should appear similar to Figure 2 after each camera is adjusted. </li>
</ol>
2. Volume Illumination Setup
<ol>
 <li>Determine the appropriate method for illuminating the measurement volume based on the specific measurement method being applied to the flow field. For particle image velocimetry (PIV) a laser volume is used.</li>
 <li>Select a laser with a pulse rate that can achieve the desired temporal resolution of the measurement. The laser may be single pulsed for time-resolved or double pulsed for frame-straddling 3. </li>
 <li>Use optical lenses to form the laser beam into a light volume that covers the measurement volume.</li>
 <li>Seed the volume with tracer particles suitable for PIV measurements 3. The concentration of particles in the fluid should be large enough to achieve the desired spatial resolution, but not so large as to reduce the SNR in SA refocused images below an acceptable level. Reference 1 contains a thorough study of achievable seeding density, but as a rule of thumb an image density of 0.05-0.15 particles per pixel (ppp) is appropriate for most experiments with 8 or more cameras. For a fixed number of cameras, the particles per pixels decreases for larger volume depth dimensions. </li>
</ol>
3. Camera Array Calibration
<ol>
 <li>Calibration requires capturing a series of images in each camera with a calibration target (e.g. a checkerboard grid, see Figure 2) in multiple locations throughout the measurement volume. First, choose between two types of calibration: either a multi-camera self-calibration method or imaging a known calibration target that is precisely moved through the field of interest.</li>
 <li>Establish a reference coordinate system in the measurement volume. This coordinate system is often chosen in a manner that is relevant for the experiment (e.g. aligned with the axis of a cylinder, originating at the leading edge of a flat plate, etc). Here we have chosen to place our grids in the X-Y plane aligned on points along the Z-axis (Figure 1).</li>
 <li>If using a multi-camera self-calibration algorithm 4, 5 the calibration target locations can be random, except for one location that is precisely located in the reference coordinate system. The location of calibration points on this precisely located target must be known with high accuracy. In each camera, capture an image of the target in each location similar to Figure 2. </li>
 <li>If not using a multi-camera self-calibration algorithm, then the calibration target must be precisely placed in several locations in the measurement volume such that the orientation of the target in the reference coordinate system is known with high accuracy. In each camera, capture an image of the target in each location.</li>
 <li>Identify points on the target in each camera for each image. For self-calibration, image point correspondences across all cameras are required 5, but explicit reference-to-image point correspondences are only required for the points generated by the precisely located target. For the precisely traversed calibration method, explicit reference-to-image point correspondences are required for all points in all cameras. </li>
 <li>Apply the chosen calibration algorithm to calibrate all cameras. Here we have chosen to utilize a multi-camera self-calibration algorithm 4, 5 (open source<a href="http://cmp.felk.cvut.cz/~svoboda/SelfCal/" target="_blank"> http://cmp.felk.cvut.cz/~svoboda/SelfCal/</a>) and the resulting camera locations relative to the planes of interest are shown in Figure 3.</li>
</ol>
4. Timing, Triggering and Data Collection
<ol>
 <li>Quantitative, time-resolved light field imaging requires all cameras and illumination sources to be accurately synchronized, often to a relevant experimental event.</li>
 <li>An external pulse generator is used to trigger the camera exposures and illumination sequences. Program the appropriate timing pulse sequences on the pulse generator. For the vocal fold experiment, we use a frame-straddling sequence, whereby the laser is pulsed close to the end of one camera exposure and near the beginning of the next 3.</li>
 <li>If triggering from an experimental event, ensure that an appropriate signal is generated and input to the pulse generator.</li>
 <li>If manually triggering, make provisions for triggering the pulse generator.</li>
 <li>Begin the experimental data capture by initiating the camera capture and illumination sequence via the chosen triggering method.</li>
 <li>Although it sounds trivial, when acquiring the large amount of data associated with a multi-camera light field-imaging experiment a good naming convention is crucial. It is helpful to consider how the data will be used from capture to final analysis when developing the naming convention.</li>
</ol>
5. Synthetic Aperture Refocusing
<ol>
 <li>We will now generate a 3D focal stack of images to produce a synthetically refocused volume. First, define the spacing between focal planes and overall refocusing depth to be used in the refocused volume <img alt="Equation 1" fo:src="/files/ftp_upload/4325/4325eq1highres.jpg" src="/files/ftp_upload/4325/4325eq1.jpg" /> 1, 7. Typically, the focal plane spacing is set to half the depth resolution and the total refocusing depth is governed by the region where all camera fields of view overlap. The focal planes will be perpendicular to the Z-axis of the reference coordinate system.</li>
 <li>Define the scale to apply to the images upon reprojection into the measurement volume. The scale should be consistent with the magnification of the raw images in order to avoid significant over-sampling or under-sampling of the reprojected images.</li>
 <li>Establish transformations between each camera image plane and each synthetic focal plane.</li>
 <li>Perform image preprocessing to remove background noise and accommodate for differences in intensity between images 1, 7.</li>
 <li>Reproject images onto the synthetic focal planes, apply the scale and re-sample the images. A set of built-in Matlab functions (image processing toolbox a ) can handle these tasks given the plane-to-plane transformations.</li>
 <li>On each synthetic focal plane, apply either the additive or multiplicative SA refocusing algorithm 1, 7. For 3D SAPIV applications, we have had good success with additive SA (as applied to the vocal folds here). For backlit bubble images, the multiplicative SA has yielded superior results. As a check apply the refocusing to one plane of the calibration images to see if the reconstruction appears as expected.</li>
</ol>
6. Volume Post-processing
<ol>
 <li>To estimate the original objects in the volume that generated the light field requires a processing step known as reconstruction. Several algorithms exist ranging from simple intensity thresholding 1 to gradient-based focus metrics 7 to more complex 3D deconvolution 8. Choose a reconstruction algorithm appropriate for the application. For PIV, we have had success with both intensity thresholding and 3D deconvolution. We use intensity thresholding here to form a focal stack. Two focal stacks from time 1 (t1) and time 2 (t2)are cross-correlated to form a vector field. The 3D Light Field Imaging method inherently results in objects that are elongated in the depth dimension, which can affect PIV accuracy; a good reconstruction algorithm attempts to mitigate this elongation. </li>
 <li>After the reconstruction step, features in the volume may need to be or extracted to allow for measurement of size, shape, etc. The algorithms used for feature extraction are varied and depend on the application 7. To extract bubbles, for example, requires a means of localizing bubble features and defining their size. For PIV applications, we do not explicitly extract particles and this step can be skipped. </li>
 <li>For 3D SAPIV applications, parse the reconstruction volume into smaller interrogation volumes and apply a suitable cross-correlation based PIV algorithm to measure the vector field 1, 3. </li>
</ol>
 a maketform: constructs a plane to plane transformation &amp; imtransform: maps and resamples an image based on the transformations from maketform.

<ol>
	<li>Belden, J., Truscott, T. T., Axiak, M., Techet, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+synthetic+aperture+particle+imaging+velocimetry.">Three-dimensional synthetic aperture particle imaging velocimetry.</a> Measurement Science and Technology. 21 (12), 125403 (2010).</li><li>Wilburn, B., Joshi, N., Vaish, V., Talvala, E. -. V., Antunez, E., Barth, A., Adams, A., Horowitz, M., Levoy, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+performance+imaging+using+large+camera+arrays.">High performance imaging using large camera arrays.</a> ACM Trans. Graph. 24, 765-776 (2005).</li><li>Raffel, M., Willert, C., Wereley, S., Kompenhaus, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Particle image velocimetry - A Practical Guide. , (2007).</li><li>Belden, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Auto-Calibration+of+Multi-Camera+Systems+with+Refractive+Interfaces.">Auto-Calibration of Multi-Camera Systems with Refractive Interfaces.</a> Experiments in Fluids. , (2013).</li><li>Svoboda, T., Martinec, M., Pajdla, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+convenient+multi-camera+self-calibration+for+virtual+environments.">A convenient multi-camera self-calibration for virtual environments.</a> PRESENCE: Teleoperators and Virtual Environments. 14 (4), 407-422 (2005).</li><li>Vaish, V., Garg, G., Talvala, E., Antunez, E., Wilburn, B., Horowitz, M., Levoy, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthetic+aperture+focusing+using+a+shear-warp+factorization+of+the+viewing+transfor.">Synthetic aperture focusing using a shear-warp factorization of the viewing transfor.</a> 3, 129 (2005).</li><li>Belden, J., Ravela, S., Truscott, T. T., Techet, A. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-Dimensional+Bubble+Field+Resolution+Using+Synthetic+Aperture+Imaging:+Application+to+a+Plunging+Jet.">Three-Dimensional Bubble Field Resolution Using Synthetic Aperture Imaging: Application to a Plunging Jet.</a> Experiments in Fluids. , (2012).</li><li>Levoy, M., Ng, R., Adams, A., Footer, M., Horowitz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light+field+microscopy.">Light field microscopy.</a> ACM Transactions on Graphics. 25 (3), (2006).</li><li>Triep, M., Br&#252;cker, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+nature+of+the+glottal+jet.">Three-dimensional nature of the glottal jet.</a> Journal of the Acoustical Society of America. 127, 1537-1547 (2008).</li><li>Elsinga, G., Scarano, F., Wieneke, B., van Oudheusden, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tomographic+particle+image+velocimetry.">Tomographic particle image velocimetry.</a> Experiments in Fluids. 41, 933-947 (2006).</li></ol>

We have nothing to disclose.

Several steps are critical for proper execution of a Light Field Imaging experiment. Lens selection and camera placement should be carefully chosen to maximize the resolution within the measurement volume. Calibration is perhaps the most critical step, as the SA refocusing algorithms will fail to produce sharply focused images without accurate calibration. Fortunately, multi-camera self-calibration facilitates accurate calibration with a relatively low level of effort. Uniform illumination in all images that provides good contrast between the objects of interest and the background is also necessary, although image processing can normalize the images to a degree. 
Timing is also important when performing SA on volumes that have moving objects. If each camera is not triggered to take an image at the same time, the image reconstruction will obviously be inaccurate. For the experiments in this paper we utilized the timing sequence shown in Figure 7.
The 3D Light Field Imaging applications presented herein involve a spatial resolution trade-off. For example, 3D SAPIV can reconstruct particle volumes from optically dense particle images, but the particles are distributed throughout a (potentially large) volume. For 2D PIV, the particles are distributed over a thin sheet, and thus images with the same particle density correspond to a much larger density in the measurement volume. Nonetheless, the 3D SAPIV method allows for much larger seeding densities that other 3D PIV methods 1. Another potentially limiting consideration is the relatively large computational intensity associated with Light Field Imaging methods; computational complexity is typical for image-based 3D reconstruction methods such as tomographic-PIV 10.
For this experiment we used 8 Photron SA3 cameras fitted with Sigma 105 mm macro lenses and a Quantronix Dual Darwin Nd:YLF laser (532 nm, 200 mJ). The cameras and laser were synched together via a Berkley Nucleonics 575 BNC digital delay/pulse generator. The fluid flow was seeded with Expancel helium filled glass microspheres. The microspheres had an average diameter of 70 &mu;m with a density of 0.15 g/cc. We offer open source versions of the codes used herein for the academic community via our website <a href="http://www.3dsaimaging.com/" target="_blank">http://www.3dsaimaging.com/</a> and we encourage users to give us feedback and participate in improving and supplying useful codes for the quantitative light field community.

determining 3d flow fields via multi-camera light field imaging

In the field of fluid mechanics, the resolution of computational schemes has outpaced experimental methods and widened the gap between predicted and observed phenomena in fluid flows. Thus, a need exists for an accessible method capable of resolving three-dimensional (3D) data sets for a range of problems. We present a novel technique for performing quantitative 3D imaging of many types of flow fields. The 3D technique enables investigation of complicated velocity fields and bubbly flows. Measurements of these types present a variety of challenges to the instrument. For instance, optically dense bubbly multiphase flows cannot be readily imaged by traditional, non-invasive flow measurement techniques due to the bubbles occluding optical access to the interior regions of the volume of interest. By using Light Field Imaging we are able to reparameterize images captured by an array of cameras to reconstruct a 3D volumetric map for every time instance, despite partial occlusions in the volume. The technique makes use of an algorithm known as synthetic aperture (SA) refocusing, whereby a 3D focal stack is generated by combining images from several cameras post-capture 1. Light Field Imaging allows for the capture of angular as well as spatial information about the light rays, and hence enables 3D scene reconstruction. Quantitative information can then be extracted from the 3D reconstructions using a variety of processing algorithms. In particular, we have developed measurement methods based on Light Field Imaging for performing 3D particle image velocimetry (PIV), extracting bubbles in a 3D field and tracking the boundary of a flickering flame. We present the fundamentals of the Light Field Imaging methodology in the context of our setup for performing 3DPIV of the airflow passing over a set of synthetic vocal folds, and show representative results from application of the technique to a bubble-entraining plunging jet.

High-quality raw PIV images contain uniformly distributed particles appearing with high contrast against the black background (Figure 4a). To compensate for non-uniform illumination across the image, image pre-processing can be performed to remove bright regions, adjust contrast and normalize the intensity histograms across all the images from all cameras (Figure 4b). When the experiment is seeded to an appropriate density and an accurate calibration is performed, the SA refocused images will reveal in focus particles on each depth plane (Figure 5). If the measurement volume is over seeded, the SNR in the refocused images will be low making it difficult to reconstruct the particles. SA refocused images with good SNR can be thresholded to retain in focus particles on each depth plane. Figure 6 shows two thresholded images from two time steps at the Z = -10.6 mm depth plane. The thresholded volume is then parsed into interrogation volumes that contain an adequate number of particles for performing PIV 3. Applying a 3DPIV algorithm to the parsed volume yields a fluid velocity field shown in Figure 7; in this case, the flow field is that induced by a model vocal fold. The velocity of the flow field outside the jet is very small, thus very few vectors can be seen outside this region. At t = 0 msec the vocal fold is closed and very little velocity in the field is present. The largest speed in the jet at t = 1 msec moves in the positive y direction and reduces in intensity from t = 2 to 4 msec. The fold closes at t = 5 msec reducing the jet velocity and the cycle is repeated. These images do not have the same smoothness as many previous authors 9 who present up to 100 averaged images as each velocity field presented represents a single snapshot in time. As a point of reference, previous simulations have shown typical errors on calculated velocities to be on the order of 5-10% on each velocity component, which includes error from the PIV algorithm itself 1; for the algorithm we are using (MatPIV 11 adapted for 3D), this error is known to be large relative to other codes.
Bubbly flows are another area of scientific interest that can benefit from the 3D capabilities of Light Field Imaging. The SA technique can be similarly applied to bubbly flow fields, where the laser light is replaced with diffuse white backlighting, which results in images such as that shown in Figure 8a where the bubbles edges appear dark against the white background. After self-calibration, the multiplicative variant of the SA algorithm can be applied to yield a focal stack with bubbles sharply focused on the depth plane corresponding to the depth of the bubble and blurred from view on other planes, as shown in Figure 8b-d 7. Simple thresholding is not an adequate method for extracting the bubbles, instead a series of advanced feature extraction algorithms are utilized as detailed in 7.
<img src="/files/ftp_upload/4325/4325fig1.jpg" alt="Figure 1" /> 
Figure 1. Image of cameras and vocal folds with labels and coordinate system.
<img src="/files/ftp_upload/4325/4325fig2.jpg" alt="Figure 2" /> 
Figure 2. Calibration grid at Z = 0 mm as seen from all 8 cameras.
<img src="/files/ftp_upload/4325/4325fig3.jpg" alt="Figure 3" fo:content-width="2.5in" fo:src="/files/ftp_upload/4325/4325fig3highres.jpg"/> 
Figure 3. Topview of camera setup from multi-camera self calibration output. Cameras 1-8 are located with numbers and circles, with their general viewing direction indicated by a line. The red blob near the origin is actually 400+ points from the calibration grid at each Z depth plotted in 3D relative to the cameras.
<img src="/files/ftp_upload/4325/4325fig4.jpg" alt="Figure 4" /> 
Figure 4. Raw images of the particle field viewed from camera #6 at t1 and t2 (a &amp; b). Same images after pre-processing (c &amp; d).
<img src="/files/ftp_upload/4325/4325fig5.jpg" alt="Figure 5" /> 
Figure 5. From left to right: Raw refocused SAPIV images at depths (a) Z = -5.9 mm, (b) -10.6 mm and (c) -15.3 mm.
<img src="/files/ftp_upload/4325/4325fig6.jpg" alt="Figure 6" /> 
Figure 6. Thresholded images at time steps (a) t1 and (b) t2 at Z = -10.6 mm. 
<img src="/files/ftp_upload/4325/4325fig7.jpg" alt="Figure 7" fo:content-width="5.25in" fo:src="/files/ftp_upload/4325/4325fig7highres.jpg"/> 
Figure 7. Three-dimensional vector field of the jet created by synthetic vocal folds for 6 time steps. The left hand side shows an isometric view of the entire 3D velocity field. Cuts of the x-y and y-z planes are made through the center of the vocal fold as indicated above each column.
<img src="/files/ftp_upload/4325/4325fig8.jpg" alt="Figure 8" /> 
Figure 8. From left to right: Raw image of bubbly flow field from camera array and refocused images at depths (b) Z = -10 mm, (c) 0 mm and (d) 10 mm. The circle highlights a bubble that lies on the Z = -10 mm depth plane, and disappears from view on other planes. Details of the bubble experiments can be found in 4.

Watch this Scientific Journal Video about Determining 3D Flow Fields via Multi-camera Light Field Imaging at JoVE.com

Determining 3D Flow Fields via Multi-camera Light Field Imaging

A technique for performing quantitative three-dimensional (3D) imaging for a range of fluid flows is presented. Using concepts from the area of Light Field Imaging, we reconstruct 3D volumes from arrays of images. Our 3D results span a broad range including velocity fields and multi-phase bubble size distributions.

In the field of fluid mechanics, the resolution of computational schemes has outpaced experimental methods and widened the gap between predicted and ...

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16.5K Views.  Brigham Young University. Una tecnica per eseguire quantitativa tridimensionale (3D) per una serie di flussi di fluidi viene presentato. Utilizzando i concetti della zona di Imaging campo di luce, abbiamo ricostruire volumi 3D da array di immagini. I nostri risultati 3D abbracciano una vasta gamma tra cui campi di velocità e multi-fase distribuzione delle dimensioni bolla.

Video: Determining 3D Flow Fields via Multi-camera Light Field Imaging

High-resolution, High-speed, Three-dimensional Video Imaging with Digital Fringe Projection Techniques

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses triangulation between matching points in two views of the same scene at different angles to compute depth.&#160;However, unlike a stereo-based method, DFP uses a digital video projector to replace one of the cameras1. The projector rapidly projects a known sinusoidal pattern onto the subject, and the surface of the subject distorts these patterns in the camera&#8217;s field of view. Three distorted patterns (fringe images) from the camera can be used to compute the depth using triangulation.
Unlike other 3D measurement methods, DFP techniques lead to systems that tend to be faster, lower in equipment cost, more flexible, and easier to develop.&#160;DFP systems can also achieve the same measurement resolution as the camera.&#160;For this reason, DFP and other digital structured light techniques have recently been the focus of intense research (as summarized in1-5). Taking advantage of DFP, the graphics processing unit, and optimized algorithms, we have developed a system capable of 30&#160;Hz 3D video data acquisition, reconstruction, and display for over 300,000 measurement points per frame6,7.&#160;Binary defocusing DFP methods can achieve even greater speeds8.
Diverse applications can benefit from DFP techniques. Our collaborators have used our systems for facial function analysis9, facial animation10, cardiac mechanics studies11, and fluid surface measurements, but many other potential applications exist.&#160;This video will teach the fundamentals of DFP techniques and illustrate the design and operation of a binary defocusing DFP system.

This video describes the fundamentals of digital fringe projection techniques, which provide dense 3D measurements of dynamically changing surfaces.&#160;It also demonstrates the design and operation of a high-speed binary defocusing system based on these techniques.

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses ...

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

The robust device design of fringing-field electrostatic MEMS actuators results in inherently low squeeze-film damping conditions and long settling times when performing switching operations using conventional step biasing. Real-time switching time improvement with DC-dynamic waveforms reduces the settling time of fringing-field MEMS actuators when transitioning between up-to-down and down-to-up states.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input ...

Data Acquisition Protocol for Determining Embedded Sensitivity Functions

The effectiveness of many structural health monitoring techniques depends on the placement of sensors and the location of input forces. Algorithms for determining optimal sensor and forcing locations typically require data, either simulated or measured, from the damaged structure. Embedded sensitivity functions provide an approach for determining the best available sensor location to detect damage with only data from the healthy structure. In this video and manuscript, the data acquisition procedure and best practices for determining the embedded sensitivity functions of a structure is presented. The frequency response functions used in the calculation of the embedded sensitivity functions are acquired using modal impact testing. Data is acquired and representative results are shown for a residential scale wind turbine blade. Strategies for evaluating the quality of the data being acquired are provided during the demonstration of the data acquisition process.

The data acquisition procedure for determining embedded sensitivity functions is described. Data is acquired and representative results are shown for a residential scale wind turbine blade.

The effectiveness of many structural health monitoring techniques depends on the placement of sensors and the location of input forces. Algorithms for ...

Hand Controlled Manipulation of Single Molecules via a Scanning Probe Microscope with a 3D Virtual Reality Interface

Considering organic molecules as the functional building blocks of future nanoscale technology, the question of how to arrange and assemble such building blocks in a bottom-up approach is still open. The scanning probe microscope (SPM) could be a tool of choice; however, SPM-based manipulation was until recently limited to two dimensions (2D). Binding the SPM tip to a molecule at a well-defined position opens an opportunity of controlled manipulation in 3D space. Unfortunately, 3D manipulation is largely incompatible with the typical 2D-paradigm of viewing and generating SPM data on a computer. For intuitive and efficient manipulation we therefore couple a low-temperature non-contact atomic force/scanning tunneling microscope (LT NC-AFM/STM) to a motion capture system and fully immersive virtual reality goggles. This setup permits &#34;hand controlled manipulation&#34; (HCM), in which the SPM tip is moved according to the motion of the experimenter&#39;s hand, while the tip trajectories as well as the response of the SPM junction are visualized in 3D. HCM paves the way to the development of complex manipulation protocols, potentially leading to a better fundamental understanding of nanoscale interactions acting between molecules on surfaces. Here we describe the setup and the steps needed to achieve successful hand-controlled molecular manipulation within the virtual reality environment.

We demonstrate the precise manipulation of individual organic molecules on a metal surface with the tip of a scanning probe microscope driven in 3D by the experimenter's hand using a motion capture system and fully immersive virtual reality goggles.

Considering organic molecules as the functional building blocks of future nanoscale technology, the question of how to arrange and assemble such building ...

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...

Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is not well understood. The flow visualization technique is a promising non-intrusive method to illustrate the complex flow field spatially and temporally. A basic flow visualization setup consists of a high-powered laser and optic lenses to generate the laser sheet, a camera, a tracer particle generator, and a data processor. The wind tunnel setup, the specifications of devices involved, and the corresponding parameter settings are dependent on the flow features to be obtained.
Normal smoke wire flow visualization uses a smoke wire to demonstrate the flow streaklines. However, the performance of this method is limited by poor spatial resolution when it is conducted in a complex flow field. Therefore, an improved smoke flow visualization technique has been developed. This technique illustrates the large-scale global LEV flow field and the small-scale shear layer flow structure at the same time, providing a valuable reference for later detailed particle image velocimetry (PIV) measurement.
In this paper, the application of the improved smoke flow visualization and PIV measurement to study the unsteady flow phenomena over a delta wing is demonstrated. The procedure and cautions for conducting the experiment are listed, including wind tunnel setup, data acquisition, and data processing. The representative results show that these two flow visualization methods are effective techniques for investigating the three-dimensional flow field qualitatively and quantitatively.

Here, we present a protocol to observe unsteady vortical flows over a delta wing using a modified smoke flow visualization technique and investigate the mechanism responsible for the oscillations of the leading-edge vortex breakdown locations.

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is ...

Synchrotron X-ray Microdiffraction and Fluorescence Imaging of Mineral and Rock Samples

In this report, we describe a detailed procedure for acquiring and processing&#160;x-ray microfluorescence (&#956;XRF), and Laue and powder microdiffraction two-dimensional (2D) maps at beamline 12.3.2 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Measurements can be performed on any sample that is less than 10 cm x 10 cm x 5 cm, with a flat exposed surface. The experimental geometry is calibrated using standard materials (elemental standards for XRF, and crystalline samples such as Si, quartz, or Al2O3 for diffraction). Samples are aligned to the focal point of the x-ray microbeam, and raster scans are performed, where each pixel of a map corresponds to one measurement, e.g., one XRF spectrum or one diffraction pattern. The data are then processed using the in-house developed software XMAS, which outputs text files, where each row corresponds to a pixel position. Representative data from moissanite and an olive snail shell are presented to demonstrate data quality, collection, and analysis strategies.

We describe a beamline setup meant to carry out rapid two-dimensional x-ray fluorescence and x-ray microdiffraction mapping of single crystal or powder samples using either Laue (polychromatic radiation) or powder (monochromatic radiation) diffraction. The resulting maps give information about strain, orientation, phase distribution, and plastic deformation.

In this report, we describe a detailed procedure for acquiring and processing&#160;x-ray microfluorescence (&#956;XRF), and Laue and powder ...

Simultaneous Measurement of Turbulence and Particle Kinematics Using Flow Imaging Techniques

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine micro-organisms, and/or sediments in the ocean, or fluidized bed reactors and combustion processes in engineered systems. In order to study the effect of turbulence on the kinematics of particles in such flows, simultaneous measurements of both the flow and particle kinematics are required. Non-intrusive, optical flow measurement techniques for measuring turbulence, or for tracking particles, exist but measuring both simultaneously can be challenging due to interference between the techniques. The method presented herein provides a low cost and relatively simple method to make simultaneous measurements of the flow and particle kinematics. A cross section of the flow is measured using a particle image velocimetry (PIV) technique, which provides two components of velocity in the measurement plane. This technique utilizes a pulsed-laser for illumination of the seeded flow field that is imaged by a digital camera. The particle kinematics are simultaneously imaged using a light emitting diode (LED) line light that illuminates a planar cross section of the flow that overlaps with the PIV field-of-view (FOV). The line light is of low enough power that it does not affect the PIV measurements, but powerful enough to illuminate the larger particles of interest imaged using the high-speed camera. High-speed images that contain the laser pulses from the PIV technique are easily filtered by examining the summed intensity level of each high-speed image. By making the frame rate of the high-speed camera incommensurate with that of the PIV camera frame rate, the number of contaminated frames in the high-speed time series can be minimized. The technique is suitable for mean flows that are predominantly two-dimensional, contain particles that are at least 5 times the mean diameter of the PIV seeding tracers, and are low in concentration.

The technique described herein offers a low cost and relatively simple method to simultaneously measure particle kinematics and turbulence in flows with low particle concentrations. The turbulence is measured using particle image velocimetry (PIV), and particle kinematics are calculated from images obtained with a high-speed camera in an overlapping field-of-view.

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine ...

Label-Free Imaging of Single Proteins Secreted from Living Cells via iSCAT Microscopy

We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living cells in real time. In this protocol, we cover the fundamental steps to realize an iSCAT microscope and complement it with additional imaging channels to monitor the viability of a cell under study. Following this, we use the method for real-time detection of single proteins as they are secreted from a living cell which we demonstrate with an immortalized B-cell line (Laz388). Necessary steps concerning the preparation of microscope and sample as well as the analysis of the recorded data are discussed. The video protocol demonstrates that iSCAT microscopy offers a straightforward method to study secretion at the single-molecule level.

We present a protocol for the real-time optical detection of single unlabeled proteins as they are secreted from living cells. This is based on interferometric scattering (iSCAT) microscopy, which can be applied to a variety of different biological systems and configurations.

We demonstrate interferometric scattering (iSCAT) microscopy, a method capable of detecting single unlabeled proteins secreted from individual living ...

O-cresol Concentration Online Measurement Based On Near Infrared Spectroscopy Via Partial Least Square Regression

Unlike macroscopic process variables, near-infrared spectroscopy provides process information at the molecular level and can significantly improve the prediction of the components in industrial processes. The ability to record spectra for solid and liquid samples without any pretreatment is advantageous and the method is widely used. However, the disadvantages of analyzing high-dimensional near-infrared spectral data include information redundancy and multicollinearity of the spectral data. Thus, we propose to use partial least squares regression method, which has traditionally been used to reduce the data dimensionality and eliminate the collinearity between the original features. We implement the method for predicting the o-cresol concentration during the production of polyphenylene ether. The proposed approach offers the following advantages over component regression prediction methods: 1) partial least squares regression solves the multicollinearity problem of the independent variables and effectively avoids overfitting, which occurs in a regression analysis due to the high correlation between the independent variables; 2) the use of the near-infrared spectra results in high accuracy because it is a non-destructive and non-polluting method to obtain information at microscopic and molecular scales.

The protocol describes a method of predicting o-cresol concentration during the production of polyphenylene ether using near-infrared spectroscopy and partial least squares regression. To describe the process more clearly and completely, an example of predicting the o-cresol concentration during the production of polyphenylene is used to clarify the steps.

Unlike macroscopic process variables, near-infrared spectroscopy provides process information at the molecular level and can significantly improve the ...