Improving spatial resolution of background-oriented Schlieren based on directional rays

The upper part of the seismic section is studied using refracted waves, as well as surface waves using the MASW method during engineering seismic surveys. This work is devoted to consideration of a relatively new near-surface approach, which is actively used in seismology for studying the upper mantle and deep part of the earth's crust, the method of surface wave tomography. This method is of great practical interest, since it allows to obtain 3D subsurface models and conduct remote researches; also it potentially has better spatial resolution than the widely used MASW method. Within the framework of this work, tests of the developed algorithm for surface-wave tomography using direct rays were carried out on modeled data. The performance of the algorithm was assessed and the resolution of the method was estimated. Also the optimal observation schemes were considered as well as the influence of the regularization parameter value on the inversion result. Basing on the results of the current study, it can be concluded that the method of surface-wave tomography and its realization via the developed algorithm can be effectively used to solve engineering-geological problems.

Density measurements have been performed using Background Oriented Schlieren (BOS) and differential interferometry, in the case of supersonic flows around a cone-cylinder body, a hemisphere profile and two spike-tipped models for Mach numbers 2 and 3, and no angle of attack. Experiments have been driven in the blow-down wind tunnel of the FrenchGerman Research Institute of Saint-Louis (ISL). For BOS technique a specific Abel transform algorithm has been developed to rebuild density field from ray deflections. For differential interferometry an original and simple calibration and filtering process allows to obtain quantitative results in spite of experimental noise during the blow-down. Performances of these two very different techniques – accuracy, spatial resolution, limitations – have been evaluated and compared in the basic cases of cone and hemisphere. BOS measurements have been 2% accurate and spatial resolution has been 1 millimeter in a 100*100mm2 flowfield. In the same flowfield, differential interferometry has given 2% accurate measurements and spatial resolution has been 0.25 mm. Pretty good agreement has been found with RANS numerical simulations outside from boundary layers and bow shock regions. The latter is the main limitation to both of methods. In a bow shock region, BOS experimental set up is not “stigmatic”, while a “parasite” shadowgraph appears in differential interferometry. The lack of information leads to under-estimation of density in such a region. Having knowledge about performances and limitations of these two techniques, Background Oriented Schlieren and Differential Interferometry have been used to measure density in the case of a supersonic flow around a spike-tipped body: a disk-spike and a biconical-spike for Mach numbers 2 and 3 have been under study. Outside bow shock regions, a pretty good agreement has been found between measurements by BOS and by Differential Interferometry, which allow them to be used later for CFD validation.

The concept of multichannel array projection is generalized in order to realize an ultraslim, highly efficient optical system for structured illumination with high lumen output, where additionally the Kohler illumination principle is utilized and source light homogenization occurs. The optical system consists of a multitude of neighboring optical channels. In each channel two optical freeforms generate a real or a virtual spatial light pattern and furthermore, the ray directions are modified to enable Kohler illumination of a subsequent projection lens. The internal light pattern may be additionally influenced by absorbing apertures or slides. The projection lens transfers the resulting light pattern to a target, where the total target distribution is produced by superposition of all individual channel output pattern. The optical system without absorbing apertures can be regarded as a generalization of a fly’s eye condenser for structured illumination. In this case light pattern is exclusively generated by freeform light redistribution. The commonly occurring blurring effect for freeform beamshaping is reduced due to the creation of a virtual object light structure by means of the two freeform surfaces and its imaging towards the target. But, the remaining blurring inhibits very high spatial frequencies at the target. In order to create target features with very high spatial resolution the absorbing apertures can be utilized. In this case the freeform beamshaping can be used for an enhanced light transmission through the absorbing apertures. The freeform surfaces are designed by a generalized approach of Cartesian oval representation.

The Compton camera is used for imaging the distributions of γ ray direction in a γ ray telescope for astrophysics and for imaging radioisotope distributions in nuclear medicine without the need for collimators. The integration of γ rays on a cone is measured with the camera, so that some sort of inversion method is needed. Parra found an analytical inversion algorithm based on spherical harmonics expansion of projection data. His algorithm is applicable to the full set of projection data. In this paper, six possible reconstruction algorithms that allow image reconstruction from projections with a finite range of scattering angles are investigated. Four algorithms have instability problems and two others are practical. However, the variance of the reconstructed image diverges in these two cases, so that window functions are introduced with which the variance becomes finite at a cost of spatial resolution. These two algorithms are compared in terms of variance. The algorithm based on the inversion of the summed back-projection is superior to the algorithm based on the inversion of the summed projection.

Development of organ-specific dual-head single-photon emission computed tomography using variable pinhole collimator

Augmented Reality 3D head-up displays use a autostereoscopic 3D display as a panel. The 3D optical unit of autostereoscopic 3D displays controls the direction of the light rays in each pixel, allowing the users enjoy 3D world without glasses. However, these 3D optics cause image quality degradation. Deterioration of resolution has a serious impact on 3D image quality. Therefore, it is important to properly measure the 3D resolution according to 3D optics and analyze its impact. In this study, a method for measuring spatial resolution in 3D displays using contrast modulation is proposed. We describe a conventional 2D resolution measurement methods that are standardized. Based on the existing 2D resolution methods, we propose a 3D resolution method. The spatial and frequency signal responses of 3D displays were investigated. The first method is determined by the predominant frequency series. The second method is conducted by contrast modulation. Through experiments with 3D displays, 3D resolution was measured using the proposed method, and the relationship between the parameters and resolution of 3D optics was examined.

Correlation plenoptic imaging (CPI) is emerging as a promising approach to light-field imaging (LFI), a technique for concurrently measuring light intensity distribution and propagation direction of light rays from a 3D scene. LFI thus enables single-shot 3D imaging, offering rapid volumetric reconstruction. The optical performance of traditional LFI, however, is limited by a micro-lens array, causing a decline in resolution as 3D capabilities improve. CPI overcomes these limitation by measuring photon number correlations on two photodetectors with spatial resolution, in a lenslet-free design, so that the correlation function can be decoded in post-processing to reconstruct high-resolution images. In this paper, we derive the analytical expression of CPI images reconstructed through refocusing, addressing the previously unknown mathematical relationship between object shape and its final image. We show that refocused images are not limited by numerical aperture-induced blurring, as in conventional imaging. Rather, the image features of CPI can be explained through an analogy with imaging systems illuminated by spatially coherent light.

The importance of pre-clinical research of single photon emission computed tomography(SPECT) imaging is now widely recognized. For the demand of high resolution and high detection efficiency, SPECT has been developed in several ways and it is simultaneously making demand for high quality imaging. This paper introduces the newly proposed dual-head L-SPECT system and investigates the initial performance of the system having an array of pinholes as a collimator. The proposed L-SPECT system is based on the concept of light field imaging allowing the refocusing of images after exposure. A microlens array is placed just before the imaging sensor to simultaneously record the direction of incident light rays and their intensities. We are proposing a detector module with 48mm by 48mm of active area behind an array of 100×100 pinholes for gamma rays instead of microlenses. The system is based on a pixelated array of NaI crystals (10×10×10 mm elements) coupled with an array of position sensitive photomultiplier tubes (PSPMTs). The basic characteristics of this system were evaluated with pinhole radii of 50µm, 60µm and 100µm. The measurements of system sensitivity, system spatial resolution, energy resolution, volume sensitivity and uniformity were evaluated for 99mTc (140keV) solution where reconstructed images are well visualized. Monte Carlo simulation studies using the Geant4 Application for Tomographic Emission (GATE) software package validate the performance of this novel dual head L-SPECT where a general cylindrical water phantom is used to evaluate its performance. The analysis results show the combination of excellent spatial resolution and high detection efficiency over an energy range between 20-160 keV.

Two quantitative schlieren methods are assessed and compared: calibrated color schlieren (CCS) and background oriented schlieren (BOS). Both methods are capable of measuring the light deflection angle in two spatial directions, and hence the projected density gradient vector field. Spatial integration using the conjugate gradient method returns the projected density field. To assess the performance of CCS and BOS, density measurements of a two-dimensional benchmark flow (a Prandtl-Meyer expansion fan) are compared with the theoretical density field and with the density inferred from PIV velocity measurements. The method’s performance is also evaluated a priori from an experiment ray-tracing simulation. The density measurements show good agreement with theory. Moreover, CCS and BOS return comparable results with respect to each other and with respect to the PIV measurements. BOS proves to be very sensitive to displacements of the wind tunnel during the experiment and requires a correction for it, making it necessary to apply extra boundary conditions in the integration procedure. Furthermore, spatial resolution can be a limiting factor for accurate measurements using BOS. CCS suffers from relatively high noise in the density gradient measurement due to camera noise and has a smaller dynamic range when compared to BOS. Finally the application of the two schlieren methods to a separated wake flow is demonstrated. Flow features such as shear layers and expansion and recompression waves are measured with both methods.

Light-field raw data captured by a state-of-the-art light-field camera is limited in its spatial and angular resolutions due to the camera's optical hardware. In this paper, we propose an all-software algorithm to synthesize light-field raw data from a single RGB-D input image, which is driven largely by the need in the research area of light-field data compression. Our synthesis algorithm consists of three key steps: (1) each pixel of the input image is regarded as a spot lighting source that emits directional light rays with an equal strength; (2) the optical path of each directional light ray through the camera's main lens as well as the corresponding micro lens is considered as accurately as possible; and (3) the occlusion of light rays among objects at different distances within the input image is handled with the depth information. The spatial and angular resolutions of our synthesized light-field data can be scaled up when the input RGB-D image has a higher and higher spatial resolution. Meanwhile, for a given input image with a fixed size, we pay a special attention to what would be the extreme we can push the parameters involved in our synthesis algorithm, such as the number of rays emitted from each pixel, the number of micro lenses, and the number of sensors associated with each micro lens. The usefulness of our synthesized data is validated by refocusing, all-in-focus, and sub-aperture reconstructions. In particular, all-in-focus images are evaluated objectively by computing the structural similarity (SSIM) index, which allows us to reach the goal of pushing to the extreme through selecting various parameters mentioned above.

Light field angular super-resolution based on intrinsic and geometric information

Proposed in the mid-seventies as a tool to retrieve the direction of gamma rays emerging from tumors marked by a radio-tracer, the Compton Camera (CC) was also proposed more recently to localize radioactivity for nuclear dismantling operations. For the TEMPORAL project, CeBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> scintillation crystals coupled with SiPM arrays will be used both for the scatterer and the absorber. The sensitivity of the CC, as well as its angular resolution, depend on the distance between the scatterer and the absorber, on their sizes and on the energies of the detected gamma rays. Its angular resolution also depends on the scatterer and the absorber spatial and energy resolutions. Monte Carlo simulations have shown that CeBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is suitable for detection of energies from 200 keV to 2 MeV with a scatterer thickness of 1.5 cm and a distance between the scatterer and absorber front faces of 7.5 cm. To reconstruct source positions and activities, the most commonly used algorithm is the list-mode Maximum Likelihood Expectation Maximization (MLEM), which is limited to image reconstruction of mono-energetic gamma rays. We also propose an extension of the list-mode MLEM algorithm in order to allow poly-energetic imaging of radioactive sources (PE-LM-MLEM).

A small anisotropy in the arrival directions of comic rays has been consistently observed by ground detectors based on very large sample of events. The Dark Matter Particle explorer (DAMPE) has so far accumulated more than 10 billion events above GeV with relatively high spatial and energy resolution during its 5-year plus on-obit operation, providing a good data sample for probing the anisotropy of cosmic rays. We introduce in this proceeding our optimizations in the direction measurement, data sampling and anisotropy analysis. The anisotropy predicted by the east-west effect due to the Earth magnetic field and the Compton-Getting effect due to the Earth revolution are then applied to the validation of our analysis.

Light field cameras can capture the radiance and direction of light rays by a single exposure, providing a new perspective to photography and 3D geometry perception. However, existing sub-aperture based light field cameras are limited by their sensor resolution to obtain high spatial and angular resolution images simultaneously. In this paper, we propose an inference-reconstruction variational autoencoder (IR-VAE) to reconstruct a dense light field image out of four corner reference views in a light field image. The proposed IR-VAE is comprised of one inference network and one reconstruction network, where the inference network infers novel views from existing reference views and viewpoint conditions, and the reconstruction network reconstructs novel views from a latent variable that contains the information of reference views, novel views, and viewpoints. The conditional latent variable in the inference network is regularized by the latent variable in the reconstruction network to facilitate information flow between the conditional latent variable and novel views. We also propose a statistic distance measurement dubbed the mean local maximum mean discrepancy (MLMMD) to enable the measurement of the statistic distance between two distributions with high-resolution latent variables, which can capture richer information than their low-resolution counterparts. Finally, we propose a viewpoint-dependent indirect view synthesis method to synthesize novel views more efficiently by leveraging adaptive convolution. Experimental results show that our proposed methods outperform state-of-the-art methods on different light field datasets.

For prompt gamma ray imaging for biomedical applications and environmental radiation monitoring, we propose herein a multiple-scattering Compton camera (MSCC). MSCC consists of three or more semiconductor layers with good energy resolution, and has potential for simultaneous detection and differentiation of multiple radio-isotopes based on the measured energies, as well as three-dimensional (3D) imaging of the radio-isotope distribution. In this study, we developed an analytic simulator and a 3D image generator for a MSCC, including the physical models of the radiation source emission and detection processes that can be utilized for geometry and performance prediction prior to the construction of a real system. The analytic simulator for a MSCC records coincidence detections of successive interactions in multiple detector layers. In the successive interaction processes, the emission direction of the incident gamma ray, the scattering angle, and the changed traveling path after the Compton scattering interaction in each detector, were determined by a conical surface uniform random number generator (RNG), and by a Klein-Nishina RNG. The 3D image generator has two functions: the recovery of the initial source energy spectrum and the 3D spatial distribution of the source. We evaluated the analytic simulator and image generator with two different energetic point radiation sources (Cs-137 and Co-60) and with an MSCC comprising three detector layers. The recovered initial energies of the incident radiations were well differentiated from the generated MSCC events. Correspondingly, we could obtain a multi-tracer image that combined the two differentiated images. The developed analytic simulator in this study emulated the randomness of the detection process of a multiple-scattering Compton camera, including the inherent degradation factors of the detectors, such as the limited spatial and energy resolutions. The Doppler-broadening effect owing to the momentum distribution of electrons in Compton scattering was not considered in the detection process because most interested isotopes for biomedical and environmental applications have high energies that are less sensitive to Doppler broadening. The analytic simulator and image generator for MSCC can be utilized to determine the optimal geometrical parameters, such as the distances between detectors and detector size, thus affecting the imaging performance of the Compton camera prior to the development of a real system.