Background Oriented Schlieren Imaging Research Articles

Investigating the Influence of Wavelength-Specific Textured Backgrounds in Background Oriented Schlieren (BOS) Imaging

This study investigates the influence of wavelength-specific textured backgrounds on the effectiveness of Background-Oriented Schlieren (BOS) imaging, focusing on wavelengths from 400 nm to 670 nm at intervals of 30 nm intervals and multiple captured recordings for each background wavelength interval. By analyzing the signal-to-noise ratio (SNR) computationally, and the image gradient magnitude, we aimed to determine the optimal wavelengths for capturing turbulence and determine the effectiveness of colored backgrounds in natural external environments for schlieren. The SNR, calculating the ratio of mean signal intensity to noise standard deviation, revealed the highest value at 550 nm (SNR = 22.8), indicating maximized clarity. Similarly, image gradient magnitude, computed using the Sobel operator to assess spatial intensity changes, peaked at 550 nm (G=52.3), confirming effective turbulence visualization. Our findings align with the Bayer color filter trend, suggesting that the green spectrum is particularly advantageous for BOS imaging. Deviations at 490 nm and 580 nm, characterized by lower SNR and gradient magnitude, could be attributed to atmospheric scattering, refractive index overlap, or slight digital video capture differences., highlighting environmental factors that can influence imaging performance and value variation. These insights emphasize the importance of wavelength selection and background design in real-world BOS applications, suggesting that while 550 nm provides optimal results, further refinement may enhance the effectiveness of other wavelengths.

Background-oriented schlieren sensitivity in terms of geometrical parameters of measurement setup

Background-oriented schlieren imaging is a recently proposed noninvasive optical method for imaging of full ultrasound fields. In this work, the impact of uncertainty in geometrical parameters of a background-oriented schlieren measurement setup for imaging of full ultrasound fields is studied using numerical simulations. The studied parameters are focal length of the camera and positions and orientations of the camera, water tank, and ultrasound field. The results demonstrate that the most sensitive parameters affecting the accuracy of the reconstructed ultrasound fields are the orientations of the camera that change the direction of an effective optical axis. Other sensitive parameters are the focal length of the camera and the position of the ultrasound field in perpendicular directions of an optical axis. This synthetic study demonstrates the accuracy requirements for calibrating the geometrical parameters of a measurement setup that would be required to achieve accuracy comparable to that of hydrophone measurements using the background-oriented schlieren imaging. Explicitly, limits of the variation ranges of the geometrical parameters resulting in relative error ranges of 5% and 10% are given. The results of this study may contribute to help design future background-oriented schlieren measurement setups intended for measurement of full ultrasound fields.

An Inexpensive, Portable Shield to Improve Healthcare Worker Safety During Intubation Procedures.

Healthcare workers (HCW) are exposed to risk of infection during intubation procedures, in particular, in the prehospital setting. Here, we demonstrate a novel shield that can be used during intubation to block aerosols and droplets from reaching the HCW. The device is mounted on the patient's head and provides a barrier between patient and HCW. It incorporates a self-sealing port through which an endotracheal tube can be inserted. The port "floats" in the plane of the shield to facilitate maneuvering of the endotracheal tube. The shield is fabricated from transparent materials to enable the HCW to visualize the procedure. Using two complementary imaging methods, background oriented Schlieren imaging and laser sheet droplet imaging, we show that the device prevents detectable transmission of gas flow and droplets through the shield both before and after endotracheal tube insertion.Clinical Relevance- This device has the potential to protect HCWs from infections during intubation procedures, especially in the prehospital setting.

Qualitative visualization of butane and helium tracers in air using a digital background oriented schlieren method

Fluid flow around turbine blades, inside burners or on aerofoil is responsible for operation and overall design of the fluid energy machinery or airplane. Stall effects and turbulence behind blades are responsible for losses in efficiency. Because of this, exact knowledge of separation behaviour and vortex development of fluid flow around profile geometries or inside burners is mandatory. For this reason, aerodynamic research focuses on investigations and qualitative visualization of turbulence.The aim of this work is to develop a digital camera filter to visualize turbulence and stall effects directly around the object. One possibility is to use a digitally generated background pattern together with a filtered video image to visualize differences in diffraction coefficient of the gas. This so-called Background Oriented Schlieren (BOS) imaging can be used to visualize strains for comparison against Computational Fluid Dynamics (CFD) [1]. The BOS is based on the index of refraction of different gas phase species and temperatures. A qualitative evaluation of the flow field was also carried out, as an adjustment experiment for further turbulence models. It allows an evaluation of the flow resulting from different Reynolds numbers [2]. The 2-dimensional, dynamic image of the flow field allows to develop further approaches for mathematical models which are needed for validation. Different background patterns are generated and tested for their suitability for BOS imaging. Also, the experimental setup is explained, and different real time software filter methods are tested to get better quality of the visual results. Several images of different blade geometries are presented, and streams of hot air, butane and helium have been visualized. A filter from a Particle Image Velocity (PIV) method (here: PIVlab) was used to calculate velocities from the flow fields [3,4]. This enables further evaluation and visual representation i.e., of Reynolds numbers or a vector field [2].

Shedding Light on Gas-Dynamic Effects in Laser Beam Fusion Cutting: The Potential of Background-Oriented Schlieren Imaging (BOS)

In laser beam fusion cutting of metals, the interaction of the gas jet with the melt determines the dynamics of the melt extrusion and the quality of the resulting cutting kerf. The gas-dynamic phenomena occurring during laser beam cutting are not fully known, especially regarding temporal fluctuations in the gas jet. The observation of gas and melt dynamics is difficult because the gas flow is not directly visible in video recordings and access to the process zone for observation is limited. In this study, the problem of imaging the gas jet from the cutting nozzle is addressed in a novel way by utilizing the striation pattern formed at the cutting kerf as a background pattern for background-oriented Schlieren imaging (BOS). In this first feasibility study, jets of different gas nozzles were observed in front of a solidified cutting kerf, which served as a background pattern for imaging. The results show that imaging of the characteristic shock diamonds of cutting nozzles is possible. Furthermore, the resulting shock fronts from an interaction of the gas jet with a model of a cutting front can be observed. The possibility of high-speed BOS with the proposed method is shown, which could be suitable to extend the knowledge of gas-dynamic phenomena in laser beam fusion cutting.

Application of reference-free natural background–oriented schlieren photography for visualizing leakage sites in building walls

Air leakage in buildings can cause health and comfort concerns for occupants and can contribute to mold growth on building materials, or in extreme conditions, rot of building materials. Unwanted air leakage through the building envelope also contributes to approximately 4 quadrillion Btu (1172 TWh) of energy consumption per year in the building sector in the United States. Locating and sealing leakage sites can improve the energy efficiency, comfort, air quality, and moisture durability of the building stock. Typical methods of finding leakage sites, such as infrared imaging and smoke tracing, rely on concurrent blower door operation, which can also measure the total leakage rate of the building. Smoke tracing can be disruptive to occupants, and infrared imaging and smoke tracing cannot measure the contribution of individual leaks to prioritize sealing efforts. In this work, an optical fluid flow imaging technique, reference-free natural background–oriented schlieren imaging, was adapted to visualize air exfiltration. This is the first step in developing a method to noninvasively locate and measure exfiltration or infiltration sites so that sealing efforts can be prioritized. Experimental results of this technique are presented, demonstrating the method's applicability to visualizing exfiltration through three common building claddings in an outdoor environment. Key variables impacting the performance of this technique when applied to building leakage are also discussed.

Nonlinear estimation of pressure projection of ultrasound fields in background-oriented schlieren imaging.

Background-oriented schlieren imaging is a recently proposed method for measuring projections of ultrasound fields. The method is based on observing deflection of light in a heterogeneous refractive index field that is induced by ultrasound via an acousto-optic effect. The deflection of light manifests as apparent perturbations in an imaged target, forming a potential flow estimation problem. In this work, the potential flow approach is formulated as a nonlinear regularized least-squares approach to alleviate limitations of approaches that linearize the problem. The nonlinear approach is shown to outperform the linear one when estimating projections of medically relevant ultrasound fields.

Evaluation of Respiratory Emissions During Labor and Delivery: Potential Implications for Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2).

To characterize respiratory emissions produced during labor and vaginal delivery vis-à-vis the potential for transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Observational study of three women who tested negative for SARS-CoV-2 and had uncomplicated vaginal deliveries. Using background-oriented schlieren imaging, we evaluated the propagation of respiratory emissions produced during the labor course and delivery. The primary outcome was the speed and propagation of breath over time, calculated through processed images collected throughout labor and delivery. In early labor with regular breathing, the speed of the breath was 1.37 meters/s (range 1.20-1.55 meters/s). The breath appeared to propagate faster with a cough during early labor at a speed of 1.69 meters/s (range 1.22-2.27 meters/s). During the second stage of labor with Valsalva and forced expiration, the propagation speed was 1.79 meters/s (range 1.71-1.86 meters/s). Labor and vaginal delivery increase the propagation of respiratory emissions that may increase risk of respiratory transmission of SARS-CoV-2.

Natural Thermal Convection Field Measurement Technology Based on Background Oriented Schlieren Technology

Background oriented schlieren technology is developed from the combination of schlieren technology and PIV technology. It is an emerging non-contact flow field quantitative measurement method. The development and principle of background oriented schlieren technology are introduced in detail. A set of background oriented schlieren imaging system suitable for natural thermal convection field is built. The quantitative measurement research is carried out on the natural thermal convection field above the candle flame, and the density field is calculated. Distorted vector displacement map. The results show that the background oriented schlieren technology can be better applied to the quantitative measurement of natural thermal convection field, and it has broad development prospects.

Background-Oriented Schlieren Imaging of Supersonic Aircraft in Flight

This paper describes the development and use of background-oriented schlieren imaging of a full-scale supersonic jet in flight. A series of flight tests was performed in April 2011, October 2014, February 2015, and December 2018 using the flora of the desert floor in the supersonic flight corridor near the Edwards Air Force Base as a background. Flight planning was designed based on the camera resolution, the mean size and color of the predominant plants, and coordination of several aircraft. Image processing was done with both cross correlation and optical flow algorithms. The planning proved to be effective, and the vast majority of the passes of the target aircraft were successfully recorded. Results were obtained that are the most detailed schlieren imagery of an aircraft in flight to date.

Three-dimensional shock wave reconstruction using multiple high-speed digital cameras and background-oriented schlieren imaging

A methodology to perform three-dimensional reconstruction of an explosively driven shock wave’s position and shape as a function of time is developed here. A series of explosive tests are performed where the explosive process is imaged by multiple high-speed digital cameras spread over a wide area. The high-speed images are processed using the background-oriented schlieren method to visualize the shock wave. The data from the multiple camera views are then merged into a single three-dimensional point cloud representing the locations on the shock wave. The propagation of the shock wave is measured and fit to the Dewey equation. Analysis of the shock wave position and propagation allows identification of asymmetries on the shock front due to an asymmetrical explosion process. The techniques developed here are shown to be useful tools that can be implemented to augment the traditional point-wise instrumentation of current explosives research testing and provide an enhanced characterization of an explosion over traditional arena test methods.

A direct comparison between conventional and plenoptic background oriented schlieren imaging

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.

Instantaneous 3D flame imaging by background-oriented schlieren tomography

We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise “deflections” between a reference and distorted image, caused by variations in the refractive index along the path between the camera and background. Multiple simultaneous BOS images, each from a unique perspective, are combined with a tomography algorithm to reconstruct the refractive index distribution (or optical density) in the probe volume. This quantity identifies the edges of the wrinkled turbulent flame surface. This is the first application of BOS imaging to flame tomography, setting the stage for low-cost 3D flame thermometry. Tomography is carried out within the Bayesian framework, using Tikhonov and total variation (TV) priors. The TV prior is more compatible with the abrupt spatial variation in the refractive index field caused by the flame front. We demonstrate the suitability of TV regularization using a proof-of-concept simulation of BOS tomography on an LES phantom. The technique was then used to reconstruct the instantaneous 3D refractive index field of an unsteady natural gas/air flame from a Bunsen burner using a 23-camera setup. Our results show how BOS tomography can capture and visualize 3D features of a flame and provide benchmark data for simulations.

Background-oriented schlieren imaging of flow around a circular cylinder at low Mach numbers

The background-oriented schlieren (BOS) imaging method has, for the first time, been applied in the investigation of the flow around a circular cylinder at low Mach numbers ( $$M<0.1$$ ). The measurements were conducted in the high pressure wind tunnel, Gottingen, with static pressures ranging from 0.1 MPa to 6.0 MPa and covered a range of the Reynolds numbers of $$0.1\times 10^6 \le Re \le 6.0\times 10^6$$ . Even at ambient pressure and the lowest Reynolds number investigated, density gradients associated with the flow around the cylinder were recorded. The signal-to-noise ratio of the evaluated gradient field improved with increasing stagnation pressure. The separation point could easily be identified with this non-intrusive measurement technique and corresponds well to simultaneous surface pressure measurements. The resulting displacement field is in principle of qualitative nature as the observation angle was parallel to the cylinder axis only in a single point of the recorded images. However, it has been possible to integrate the density field along the surface of the cylinder by successive imaging at incremental angular positions around the cylinder. This density distribution has been found to agree well with the pressure measurements and with potential theory where appropriate.

Ultrasound field characterization using synthetic schlieren tomography.

Synthetic schlieren imaging, also known as background oriented schlieren imaging, is used to determine the acoustical field of a focused ultrasound transducer operating at 1.01 MHz frequency with peak pressure amplitude of 0.97 MPa. The measurement setup is composed of a commercial off-the-shelf digital single-lens reflex (DSLR) camera with an ordinary objective, a high power light-emitting diode driven in pulsating mode, water tank, ultrasound transducer, rotation stage, and driving electronics. Measurements are performed in tomographic fashion by rotating the ultrasound transducer within the water tank and photographing an imaged target behind the ultrasound field. The photographs are processed with a Horn-Schunck-type algorithm, commonly used in optical flow analysis, in order to determine the deflection of light rays as caused by ultrasound field induced acousto-optic effect. Inverse Radon transform is then used, with the deflection data, to obtain three-dimensional spatial distribution of the pressure field gradient, from which an approximation of the ultrasonic pressure field is computed. The pressure field obtained with synthetic schlieren tomography is then compared to hydrophone measurements mainly qualitatively.

On the influence of wind on cavity receivers for solar power towers: Flow visualisation by means of background oriented schlieren imaging

In previous studies the influence of wind on convective losses of a solar cavity receiver has been investigated experimentally and numerically. It was found that in the backward facing cavity a reduction of convective losses with increasing wind speed can occur. In this short communication we give an explanation for these results by analysing the flow structure with the background oriented schlieren (BOS) method. For image analysis a gradient-based approach following Lucas and Kanade (1981) has been implemented. With a coarse to fine iterative strategy density gradients resulting in pixel shifts of several pixels in the image plane are determinable. The results show good agreement with theoretical analyses. The developing plume is well defined and with increasing wind speed disturbances of the plume structure are clearly resolved. Moreover, in the natural convection cases the transition from laminar to turbulent flow has been made visible.

Background-oriented schlieren imaging and tomography for rapid measurement of FUS pressure fields: initial results

FUS pressure field mapping is important for dosimetry, quality assurance, and other uses. Hydrophone measurements are the current standard, and are accurate but costly and slow. As a simple low-cost alternative, background-oriented schlieren (BOS) imaging of ultrasound fields has been proposed.[1] In that technique, a predetermined image (usually a grid of lines or a random dot pattern) is placed on one side of a water tank and viewed from the other side, through the water and FUS pressure field. When the FUS is on, spatial variations in the water’s index of refraction are created that blur the image. Subtracting images with and without the FUS field provides a rapid visualization of it. The method has also been used to tomographically reconstruct air flow density.[2] The overall goal of the present work is to develop a low-cost BOS hardware system and BOS tomography acquisitions and reconstructions to enable rapid and cheap volumetric measurements of continuous-wave FUS fields. Here we present our current progress towards that goal.

Background-Oriented Schlieren Imaging for Full-Scale and In-Flight Testing

Background-oriented schlieren (BOS) methods suited for large-scale and in-flight testing are presented with special emphasis on the detection and tracing of blade tip vortices in situ. Retroreflective recording and photogrammetric epipolar analysis for the computation of the vortices' spatial coordinates in the wind tunnel are described. Feasibility and fidelity of reference-free BOS in conjunction with natural formation backgrounds and related evaluation methods are discussed, additionally, illustrating their simplicity and robustness. Results of successful image acquisition from a chaser aircraft are presented allowing vortex wakes to be identified at a wide range of flight attitudes, including complex maneuvers.