Glare Points Research Articles

Spatio-temporal reconstruction of droplet impingement dynamics by means of color-coded glare points and deep learning

Abstract The present work introduces a deep learning approach for the three-dimensional reconstruction of the spatio-temporal dynamics of the gas-liquid interface on the basis of monocular images obtained via optical measurement techniques.
The method is tested an evaluated at the example of liquid droplets impacting on structured solid substrates.
The droplet dynamics are captured through high-speed imaging in an extended shadowgraphy setup with additional glare points from lateral light sources that encode further three-dimensional information of the gas-liquid interface in the images.
A neural network is trained for the physically correct reconstruction of the droplet dynamics on a labelled dataset generated by synthetic image rendering on the basis of gas-liquid interface shapes obtained from direct numerical simulation.
The employment of synthetic image rendering allows for the efficient generation of training data and circumvents the introduction of errors resulting from the inherent discrepancy of the droplet shapes between experiment and simulation. 
The accurate reconstruction of the three-dimensional shape of the gas-liquid interface during droplet impingement on the basis of images obtained in the experiment demonstrates the practicality of the presented approach.
The introduction of glare points from lateral light sources in the experiments is shown to improve the reconstruction accuracy, which indicates that the neural network learns to leverage the additional three-dimensional information encoded in the images for a more accurate depth estimation.
By the successful reconstruction of obscured areas in the input images, it is demonstrated that the neural network has the capability to learn a physically correct interpolation of missing data from the numerical simulation.
Furthermore, the physically reasonable reconstruction of unknown gas-liquid interface shapes for drop impact regimes that were not contained in the training dataset indicates that the neural network learned a versatile model of the involved two-phase flow phenomena during droplet impingement.

An Appraisal Of Large-Scale Particle Tracking With Uncrewed Aerial Vehicles

Camera technology is rapidly enhancing within the last years, which led to the availability of sophisticated technical features within consumer cameras, that could only be found in scientific set-ups earlier. The present work explores the possibilities of utilizing the potentials of commercially-available camera drones as moving cameras for large-scale particle tracking velocimetry. We made use of the glare-point particle tracking approach introduced by Kaiser & Rival (2023), where only a single camera and natural illumination, e.g. the sun, could be used. The set-up demonstrated full suitability for large volumes in the order of 10-100 m³. The frame-to-frame camera movement, caused by the slight movement of the drone could be quantified and corrected by an imaged-based approach. The calibration strategy could be simplified due to the fixed set-up in the camera drone. The limits introduced through the tracer size and uncertainty caused by the glare-point approach are discussed accordingly. For the presented magnification and camera set-up given by the DJI Mini 3 pro, the limit in height can be determined when the two most dominant glare points collapse to one, which happened well above 10 m for the present set-up. The equipment together with the glare-point particle tracking approach is best suited for flow information extraction at large-scale facilities or difficult accessible terrain, where the current set-up could depict its strengths.

Particle Detection in Free-Falling Nanoliter Droplets.

Sorting and dispensing distinct numbers of cellular aggregates enables the creation of three-dimensional (3D) in vitro models that replicate in vivo tissues, such as tumor tissue, with realistic metabolic properties. One method for creating these models involves utilizing Drop-on-Demand (DoD) dispensing of individual Multicellular Spheroids (MCSs) according to material jetting processes. In the DoD approach, a droplet dispenser ejects droplets containing these MCSs. For the reliable printing of tissue models, the exact number of dispensed MCSs must be determined. Current systems are designed to detect MCSs in the nozzle region prior to the dispensing process. However, due to surface effects, in some cases the spheroids that are detected adhere to the nozzle and are not dispensed with the droplet as expected. In contrast, detection that is carried out only after the droplet has been ejected is not affected by this issue. This work presents a system that can detect micrometer-sized synthetic or biological particles within free-falling droplets with a volume of about 30 nanoliters. Different illumination modalities and detection algorithms were tested. For a glare point projection-based approach, detection accuracies of an average of 95% were achieved for polymer particles and MCF-7 spheroids with diameters above 75 μm. For smaller particles the detection accuracy was still in the range of 70%. An approach with diffuse white light illumination demonstrated an improvement for the detection of small opaque particles. Accuracies up to 96% were achieved using this concept. This makes the two demonstrated methods suitable for improving the accuracy and quality control of particle detection in droplets for Drop-on-Demand techniques and for bioprinting.

Spatially dependent veiling glare degrades image quality in medical extended reality

Spatially dependent veiling glare in medical extended reality (MXR) degrades image quality and needs to be characterized across technologies. Measurement methods of veiling glare on virtual and augmented reality (VR and AR) head-mounted displays (HMDs) have not been established in regulatory evaluation of MXR devices or display measurement standards. We describe an experimental bench setup enabling highly flexible translations and rotations of the light measuring device in five degrees of freedom within the eye box. Glare point spread functions (PSFs) of multiple VR and AR headsets are extracted and compared across the display field of view (FOV) in dark environment. At the center, the evaluated VR headsets (HTC VIVE Pro and VIVE Pro 2) demonstrate reduced long-range glare compared to the tested AR HMDs (Microsoft HoloLens 2 and Epson Moverio BT-300). The measured PSFs at multiple locations are spatially invariant for the HoloLens 2. However, veiling glare on the evaluated VR HMDs substantially increases toward the periphery of the FOV primarily due to optical aberration. For VR devices in medical use, increased peripheral glare can lead to image quality degradation and poor visualization of anatomical details.

Three-dimensional encoding of a gas–liquid interface by means of color-coded glare points

The present work introduces an extension of the shadowgraphy method by differently colored oblique light sources for the observation of the three-dimensional spatio-temporal dynamics of gas–liquid interfaces. The proposed expanded approach is tested and elaborated with the example of a droplet during impingement. Particularly, it is elaborated in a combined experimental/theoretical approach, how well glare points from differently colored oblique light sources can be used to encode additional 3D information of the droplet shape within a single shadowgraph image. Narrow-banded LEDs with distinct spectra and maxima in the visible light illuminate the droplet from different angles in red, green and blue light, respectively, while a high-speed RGB camera captures the images produced by each light source in the corresponding image channel, therefore creating three unique views of the droplet. In order to compensate for the mutual perturbation of the images resulting from cross-talk between the channels and the polychromatic light of the LEDs, a color correction is introduced, which is based on the transfer function between the light sources and the channels of the RGB camera. In experiments with the proposed measurement setup of a water droplet impinging onto a flat substrate it is successfully demonstrated that three unique and independent grayscale images can be reconstructed with this color correction function. The optimal illumination angles for the lateral light sources are determined experimentally, which lead to consistent glare points on the deforming gas–liquid interface throughout the dynamic process of the drop impact. An ellipsoidal droplet is considered to derive information on orientation and three-dimensional shape of a non-axisymmetrical droplet from the relative positions of the glare points and the shadowgraph contour. Thereby it is successfully demonstrated that the additional three-dimensional information encoded in the glare points can lay the groundwork for the volumetric reconstruction of the deforming gas–liquid interface during the impingement of a droplet.

Three-Dimensional Detection Of A Gas-Liquid Interface By Means Of Oblique Colored Light Sources

The present work introduces an extension of the shadowgraphy method by differently colored oblique light sources for the observation of the three-dimensional dynamics during drop impingement. The wetting of a flat or structured surface by an impinging droplet is relevant for many technical processes and therefore a further understanding of the drop impact dynamics is essential for an optimization of these processes. The most widely used optical measurement technique for drop impact experiments is the shadowgraphy method, which produces a clear two-dimensional representation of the gas-liquid interface. However, drop impact dynamics – especially on structured surfaces – are inherently three-dimensional. Therefore, two lateral light sources are introduced, in order to produce glare points on the gas-liquid interface that reveal additional three-dimensional information of the droplet shape. Narrow-banded LEDs with distinct spectra and maxima in the visible light illuminate the droplet from different angles in red, green and blue light, respectively, while a high-speed RGB-camera captures the images produced by each light source in the corresponding image channel, therefore creating three unique views of the droplet. Thereby the shadowgraphy image resulting from the blue backlight illumination is recorded in the blue image channel, while the glare points from the green and red lateral light sources are captured in the accordingly remaining channels. In order to compensate for the mutual perturbation of the images resulting from cross-talk between the channels and the polychromatic light of the LEDs, a color correction is introduced, which is based on the transfer function between the light sources and the channels of the RGB-camera. The transfer function is determined by a spectral integration over the power distribution of the individual light sources and sensitivities of the single camera channels likewise, with the assumptions of fully specular reflection and that the spectral distribution of the reflected light equals the incoming light. In experiments with the proposed measurement setup of a water droplet impinging on a flat substrate it is successfully demonstrated, that three unique and independent grayscale images can be reconstructed with this color correction function, which successfully removes the mutual perturbations. The presented method is a pure addition to the shadowgraphy method, as it produces a clear shadowgraphy image and optionally provides two further unique views by glare points on the gas-liquid interface within a single RGB-camera setup.

Polarization-dependent LIF/Mie ratio for sizing of micrometric ethanol droplets doped with Nile red.

The present study deals with droplet sizing based on laser-induced fluorescence (LIF) and Mie scattering for varied polarization of the utilized laser (parallel or perpendicular). The polarization-dependent LIF/Mie ratio is studied for micrometric droplets (25-60 µm) produced with a droplet generator. The investigations were carried out with the dye Nile red dissolved in ethanol and ethanol/iso-octane mixtures. A spectral absorption and fluorescence characterization at various dye and ethanol concentrations is carried out in a cuvette in order to identify reabsorption effects. The LIF|| droplet images (index ||: parallel polarization) show a more homogeneous intensity distribution in the droplets and slightly stronger morphology-dependent resonances (MDRs) in comparison to LIF⊥ (index ⊥: perpendicular polarization). The spectral LIF emissions reveal a dependence of the MDR on the ethanol admixture. The larger the ethanol content, the lower the MDR peak, which is also shifted further to the red part of the spectrum. The Mie droplet signal images are mainly characterized by two distinct glare points, one at the entrance of the laser light (reflection) and one at the exit (first-order refraction). The Mie⊥ images show a more pronounced entrance glare point, in comparison to Mie||, where the exit glare point is more pronounced. These observations are in accordance with the theory. The calibration curve of the micro droplet signals revealed a volumetric trend of the LIF signals and a slightly higher LIF⊥ signal and sensitivity in comparison to LIF||. The signal Mie⊥ follows roughly a quadratic trend on average, while Mie|| follows a linear trend. Consequently, the calculated LIF⊥/Mie⊥ ratio shows a linear trend, whereas the LIF||/Mie|| ratio shows a quadratic trend, which confirms theoretical calculations. A numerical simulation of the Mie signal at various detection angles shows a good agreement with the experimental data at large apertures.

Quality estimation of the nozzle spray by measuring the brightness of the reflected light

This paper presents the results of the laboratory and numerical experiments performed to measure the sizes of transparent liquid droplets sprayed in air. The results of the laboratory experiments were mainly obtained using the Glare Point Technique (GPT) which gave information about the droplet size and the brightness of the light reflected by drops. The relationship between the brightness of the light reflected from the surface of droplets and their sizes was analyzed. Theoretically, the brightness of light scattered by a single spherical drop is proportional to the drop surface area and, accordingly, to the square of the drop diameter. It has been observed experimentally and verified numerically that the theoretical dependence obtained is relevant only for the brightest droplets because of nonuniform illumination. The results of the numerical experiments with a random sample of drops indicated the dependence of the total brightness of reflected light on the effective droplet size. It is shown that, for a fixed total volume, the total brightness of light reflected by drops is proportional to the droplet Sauter mean diameter.

A novel single-camera approach to large-scale, three-dimensional particle tracking based on glare-point spacing

A novel method for three-dimensional particle tracking velocimetry (PTV) is proposed that enables flow measurements in large volumes [ $$V=\mathcal {O}(10$$ m $${^3}$$ )] using a single camera. Flow is seeded with centimeter-sized soap bubbles, when combined with suitable illumination, produce multiple glare points. The spacing between the two brightest glare points for each bubble is then utilized to reconstruct its depth. While the use of large soap bubbles comes at the expense of non-Stokesian behaviour, the excellent ray optics allow for large volume illumination when coupled with for instance pulsed LED banks. Possible error sources and the out-of-plane accuracy are discussed before the feasibility of the method is tested in an industrial-scale wind tunnel facility (test section of cross section 9.1 m $$\times$$ 9.1 m). In particular, the vortical structure in the wake of a 30%-scale tractor-trailer model at a $$9^{\circ }$$ yaw angle is captured in a 4.0 m $$\times$$ 1.5 m $$\times$$ 1.5 m measurement volume. Long tracks of up to 80 time steps are extracted in three-dimensional space via a single perspective. The successful proof-of-concept confirms the potential of the novel approach for three-dimensional measurements in volumes of industrial scale.

Determination of spray droplet size by wavelet analysis of interferometric images

The problem of size droplet determining by the Interferometric Particle Imaging (IPI) in a spray of an optically transparent liquid is considered. The measurements were carried out using the example of a fuel injector flame. A new method for analyzing the IPI image of a droplet using a continuous wavelet transform is proposed. The spatial distribution of wavelet coefficients is used to introduce an integral measure similar to the spectral density of the signal. As a reliable estimate of the droplet size, we considered the results by the Glare Point Technique (GPT), which were applied simultaneously. A comparative analysis of the sizes obtained using GPT and IPI showed that the application of the wavelet transform to IPI images, in comparison with the Fourier method, significantly reduces the frequency of occurrence of a systematic error in determining the size, which is most specific for large droplets. It was found that the false maxima in the Fourier spectrum causing these errors are due to the appearance of distortions at the boundary of the interferometric image of the droplet. The wavelet transform allows one to separate these distortions from the main maximum in the vicinity of the droplet center.

Dual-view tomographic particle holography: Modeling and light scattering effects

Dual-view tomographic particle holography, combining tomography and holography, is a promising approach to yield high accurate 3D measurement of particle field. A standard model of particle hologram has been developed in the framework of generalized Lorenz-Mie theory to model and investigate the light scattering effects on three configurations of dual-view tomographic particle holography, that is, dual-beam dual-view Gabor inline holography, single beam dual-view combining Gabor inline holography with off-axis scattering holography, and single beam dual-view off-axis scattering holography. Dual-beam dual-view Gabor inline holography is dominated by the traditional Gabor inline hologram, but the two light scattering fields bring about small amplitude off-axis scattering inline recording hologram, negligible self-reference hologram and lensless Fourier hologram. The off-axis scattering inline recording hologram can produce additional glare points in the reconstructed particle image. The off-axis scattering holography reconstructs multiple glare light, and generates more than one intersections in the merged 3D reconstructed field in tomographic particle holography, leading to possible ghost particle image and implying new algorithm for particle detection to be developed. This model can provide standard hologram to processing algorithm development.

Imaging through a homogeneous circular cylinder: the role of virtual caustics, rainbow glare points, and image fragmentation.

Small air bubbles on the rear inside surface of a water-filled cylinder, near its edges, appear horizontally elongated, joined in pairs, and take on color. Similarly, if an extended object is sufficiently close to the water-filled cylinder, three images of the object are seen when looking through the cylinder. The center image joins onto the left or right image as the observer moves his or her head back and forth in front of the cylinder. The first observation is explained in terms of glare points of light, and the real and virtual parts of the external caustic of the light transmitted through the water-filled cylinder. The second observation is explained as an example of Berry's caustic touching theorem which describes the topological method of fragmentation of an object's image into multiple images. For the situation studied here, an imaginary cylindrical aberration caustic of the water-filled cylinder decomposes object space into a three-ray region sandwiched between two one-ray regions. As an extended object crosses the caustic boundary from one of the one-ray regions into the three-ray region, an image-pair creation event occurs, which is followed by an image-pair disconnection event producing the three images. Similarly, when the extended object crosses the caustic boundary from the three-ray region into one of the one-ray regions, an image-pair merging event occurs, which is followed by an image-pair annihilation event producing the one remaining image.

Optimization of Natural Lighting Design for Visual Comfort in Modular Classrooms: Temuco Case

The emergency situations generated by the earthquakes in Chile have revealed the country’s lack of preparation to face these kinds of disasters. Many educational buildings were destroyed by these natural events. In response, a system of modular schools was implemented. Although these have been a temporary fast solution, they have not met the needs that a classroom demands.This article focuses on studying daylighting strategies to achieve the visual comfort of students, as many investigations indicate that this is one of the key environmental factors in their wellbeing and performance. The article also focuses on achieving a correct distribution of horizontal illuminance by implementing design strategies, looking to achieve uniformity inside classrooms, avoiding annoying glare points. The implementation of these strategies fosters the use of daylight in order to reduce the energy consumption of artificial lights and generate optimal conditions for students to perform their school activities.The studied classroom is a modular prototype located in Temuco, Chile, and the evaluated strategies were defined based on the “Guía de Eficiencia Energética para Establecimientos Educacionales (Energy Efficiency Guide for Educational Facilities or GEEEduc)”. These were evaluated independently in order to compare, starting from the results obtained from computer simulations, the compliance of the lighting comfort standards set out in said guide, focusing on the light uniformity and illuminance level indicators. The strategies with the best results were later evaluated in detail, ultimately reaching the case with the best performance.The study proved there are several ways to respond to the presented need, allowing concluding from a set of evaluated strategies, which one of them better serves the required standards. Likewise, implementing strategies of the “GEEEduc” as well as its indicators, results in a practical application of this guideline, allowing the identification of potential errors and good options for future applications.

Third glare point effect in 90°Interferometric Laser Imaging for Droplet Sizing

A detailed analysis of Interferometric Laser Imaging for Droplet Sizing is performed at the scattering angle 90°, in order to identify the influence of a third glare point on the measurements performed. Experimentally, p = 3 scattering, which involves surface waves, is shown to have an important impact on droplet sizing.

Analysis of the LIF/Mie Ratio from Individual Droplets for Planar Droplet Sizing: Application to Gasoline Fuels and Their Mixtures with Ethanol

In this work, the possibility of using planar droplet sizing (PDS) based on laser-induced fluorescence (LIF) and Mie scattering was investigated within the framework of measuring the droplet Sauter mean diameter (SMD) of direct-injection spark-ignition (DISI) spray systems. For this purpose, LIF and Mie signals of monodisperse fuel droplets produced by a droplet generator were studied at engine relevant diameters (20–50 µm). The surrogate gasoline fuel Toliso (consisting of 65 vol. % isooctane, 35 vol. % toluene) and the biofuel blend E20 (consisting of 80 vol. % Toliso, 20 vol. % ethanol) were used and which were doped with the fluorescence dye “nile red”. The effects of ethanol admixture, dye concentration, laser power, and temperature variation on the LIF/Mie ratio were studied simultaneously at both macroscopic and microscopic scale. The deduced calibration curves of the LIF and Mie signals of both fuels showed volumetric and surface dependent behaviors, respectively, in accordance with the assumptions in the literature. The existence of glare points and morphology-dependent resonances (MDRs) lead to slightly higher LIF and Mie exponents of E20 in comparison to Toliso. In principle, these calibration curves enable the determination of the SMD from LIF/Mie ratio images of transient fuel sprays.

Droplet Sizing in the Spray of a Fuel Injector Using Wavelet Analysis

The paper discusses the applied methods for studying the structure of the two-phase flow. Measurements of the size of liquid droplets in a spray of fuel injector were carried out using two methods: Interferometric Particle Imaging (IPI) and Glare Point Technique (GPT). The combined use of these methods allows to significantly expand the range of measured droplet sizes, and also provides an opportunity for cross-validation of methods and determining the level of errors. In this paper, we tested the IPI method using wavelet processing of interference drop images. As a result, it was shown that IPI images can have a highly inhomogeneous structure. This leads to a considerable variation in the period of the IPI-image and, as a consequence, to an increase of the error in determining the drop size. The proposed error estimation algorithm allows one to introduce an additional selection criterion and obtain statistically comparable results obtained by the IPI and GPT methods.

Drawbacks of GPT and IPI measurements in dense sprays

Two different techniques IPI (interferometric particle imaging) and GPT (glare point technique) for droplet sizing were applied for measurements in nozzle sprays. New procedures that substantially improve the quality of GPT measurements are developed and described. Measurements were carried out for different flow rates of water and intensity of air blowing in a purger. The comparison of IPI and GPT measurements in dense sprays revealed intrinsic problem of these techniques. It was shown that reconstructed droplet size distribution strongly depends on parameters of the optical system and post-processing such as laser intensity, focal ratio, brightness threshold and others.

Interferometric out-of-focus imaging of freezing droplets

Abstract Interferometric out-of-focus images of freezing droplets are presented, and compared to simultaneous in-focus images. In these experiments, the freezing process is characterized by a breaking of the initial droplet’s glare points into a wide number of neighbor bright spots. The 2D-Fourier transforms of the interferometric patterns show an elliptical contour, with a central spot and eventually symmetric spread spots more or less intense, located around the position of the initial glare points of the liquid droplet (depending on the degree of freezing).

Diffraction glare points: principle and application in particle sizing.

A novel particle sizing method based on diffraction glare points is proposed, which is independent of the refractive index of particle. With geometric optics approximation, the relationship between distance of diffraction glare points and particle size is obtained. In analysis of measurement parameters, we find that 4° is the optimal central scattering angle for measuring wide size range particles based on diffraction glare points. With an experimental system at this angle, diffraction glare points of four kinds of standard particles were imaged. The relative deviations between measured and nominal particle size were not greater than 2%, less than that using reflection glare points, demonstrating validity and advantage of the particle sizing method based on distance of diffraction glare points.

Sizing of sand and ash particles using their speckle pattern: influence of particle opacity

A speckle pattern is an interference pattern produced by coherent light scattered from an irregular particle. This pattern is observed in the out-of-focus plane of the particle and it can be used to obtain information about the particle size. When the particle is observed in focus, several bright spots known as glare points are observed on its surface. They correspond to the points from which the light is scattered in the direction of observation. Previous studies using the speckle pattern to obtain the size of irregular particles are based on the hypothesis that the glare points are distributed homogeneously over the whole surface of the particle. The research presented in this paper shows that in the case of opaque particles (such as ash particles), only the areas illuminated by the laser light are covered with glare points. This results in an underestimation of the particle size when characterizing opaque particles using their speckle pattern. In this paper, the speckle pattern is used to perform sizing measurements of translucent and opaque particles, and the results are compared with the ones obtained by image processing of their in-focus images.