Opaque Objects Research Articles

Polarimetric BSSRDF Acquisition of Dynamic Faces

Acquisition and modeling of polarized light reflection and scattering help reveal the shape, structure, and physical characteristics of an object, which is increasingly important in computer graphics. However, current polarimetric acquisition systems are limited to static and opaque objects. Human faces, on the other hand, present a particularly difficult challenge, given their complex structure and reflectance properties, the strong presence of spatially-varying subsurface scattering, and their dynamic nature. We present a new polarimetric acquisition method for dynamic human faces, which focuses on capturing spatially varying appearance and precise geometry, across a wide spectrum of skin tones and facial expressions. It includes both single and heterogeneous subsurface scattering, index of refraction, and specular roughness and intensity, among other parameters, while revealing biophysically-based components such as inner- and outer-layer hemoglobin, eumelanin and pheomelanin. Our method leverages such components' unique multispectral absorption profiles to quantify their concentrations, which in turn inform our model about the complex interactions occurring within the skin layers. To our knowledge, our work is the first to simultaneously acquire polarimetric and spectral reflectance information alongside biophysically-based skin parameters and geometry of dynamic human faces. Moreover, our polarimetric skin model integrates seamlessly into various rendering pipelines.

Criteria for objects suitable for reconstruction from holograms and diffraction patterns

In this study, quantitative criteria for reconstruction of objects from their hologram and diffraction patterns, and in particular for the phase objects in digital holography, are derived. The criteria that allow distinguishing the hologram and diffraction pattern are outlined. Gabor derived his criterion for objects suitable for holography based on the condition that the background in the reconstructed object’s distribution should be nearly flat so that its intensity contrast does not exceed 0.05. According to Gabor, an opaque object is suitable for holographic reconstruction if it occupies no more than 1% of the imaged area, and a phase-shifting object cannot be reconstructed in principle. We revisit these criteria and show that both amplitude-only and phase-only objects can be reconstructed when the object occupies less than 1% of the total illuminated area. In addition, a simplified derivation of the criteria is provided that is based on Parseval’s theorem. It is shown that for objects (including amplitude-only and phase-only) reconstructed from their holograms and the twin image treated as noise, a signal-to-noise ratio of 10 or higher can be achieved provided the object occupies less than 0.5% of the total illuminated area. When a hologram is reconstructed by applying iterative algorithms, the requirement for the object size is much more generous and identical to that applied in coherent diffraction imaging: any type of object (amplitude-only, phase-only, or amplitude-and-phase mixed properties) is suitable for holography when the object’s size in each dimension is less than half of the probed region’s extent (or the field of view).

Imaging of microcrack propagation in 3D nanostructures applying laboratory nano-XCT

Abstract Laboratory X-ray microscopy and nano X-ray computed tomography (nano-XCT) have the unique capability to combine sub-100nm resolution and high object penetration. Therefore, these are appropriate non-destructive inspection techniques for the detection of flaws with a size of 100 nm and below in opaque objects and bulk materials. Another advantage of X-ray microscopy – as opposed to destructive failure analysis methods – is that kinetic processes such as microcrack evolution can be imaged. The unique combination of micromechanics and high-resolution 3D imaging allows to study degradation and failure mechanisms in opaque 3D nanopatterned structures, and it allows to provide essential information for fracture mechanics in small dimensions. The high-resolution in-situ/operando imaging of microcrack propagation in microelectronic products and in battery electrodes is demonstrated.

Remote sensing of high energy charged particle current (HEC) for megavoltage therapeutic electron beams

Objective. We demonstrate detection of high energy particle current (HEC) for MeV therapeutic electron beams. Detection of HEC comprises of remote sensing or acquiring information about HEC inside radiation transport medium from a distance outside of the medium. Approach. HEC is self-propelled motion of charged particles through a radiation transport medium. Remote sensing of HEC is embodied in an experimental setup, which includes homogeneous and heterogeneous phantoms irradiated with 4–15 MeV electron beams and two large area parallel-plane electrodes extraneous to the phantoms providing two-parameter detection. We also introduce a new type of scanning method (depth-scan) for probing object properties along the beamline axis. Main Results. Deterministic radiation transport simulations and measurements agree, considering differences in simulation vs experimental geometry and experimental uncertainties. Significance. This method may be suitable for range detection of charged particle beams, or for probing of radiation opaque objects in non-destructive testing.

Breaking Biomphalaria black box by in situ revelation of fluorescent Schistosoma mansoni parasites

Tissue clearing is an old-fashioned method developed in the 1900's and used to turn an opaque biological object into a 3D visualizable transparent structure. Developed and diversified over the last decade, this method is most of the time applied to mammals' tissues, and especially mouse and human tissues for cytological, histological and pathophysiological studies. Through autofluorescence, immunofluorescence, in situ hybridization, intercalating agents, fluorescent transfection markers or fluorescent particle uptake, optically cleared samples can be monitored to discover new biological structures and cellular interactions through 3D-visualization, which can be more challenging in some extend through classical histological methods. Most of the tissue clearing procedures have been developed for specific applications like endogenous fluorescence visualization, immunolabeling or for revealing specific organs. Thus, choosing the adapted protocol may be empirical for non-model species, especially for mollusks for which very little related literature is available. Herein, we suggest an effective optical tissue clearing procedure for the freshwater snail Biomphalaria glabrata, known as the intermediate host of the human parasite Schistosoma mansoni. This clearing procedure involves solvents with a minimal toxicity, preserves the endogenous fluorescence of labeled parasites inside snail tissues and is compatible with an immunolabeling procedure.

NeuralTO: Neural Reconstruction and View Synthesis of Translucent Objects

Learning from multi-view images using neural implicit signed distance functions shows impressive performance on 3D Reconstruction of opaque objects. However, existing methods struggle to reconstruct accurate geometry when applied to translucent objects due to the non-negligible bias in their rendering function. To address the inaccuracies in the existing model, we have reparameterized the density function of the neural radiance field by incorporating an estimated constant extinction coefficient. This modification forms the basis of our innovative framework, which is geared towards highfidelity surface reconstruction and the novel-view synthesis of translucent objects. Our framework contains two stages. In the reconstruction stage, we introduce a novel weight function to achieve accurate surface geometry reconstruction. Following the recovery of geometry, the second phase involves learning the distinct scattering properties of the participating media to enhance rendering. A comprehensive dataset, comprising both synthetic and real translucent objects, has been built for conducting extensive experiments. Experiments reveal that our method outperforms existing approaches in terms of reconstruction and novel-view synthesis.

Validity of Thin Element Approximation in diffractometry of opaque thick objects

The Thin Element Approximation (TEA) is widely used in diffraction analyses since it can be applied in efficient plane-to-plane propagation algorithms. While refinements have been developed for dielectric objects to obtain more accurate results, TEA is assumed precise for opaque objects. However, through numerical simulations presented in this work, based on Wave Propagation Method, we analyze the tendency of TEA to overestimate the dimensions of opaque objects in diffractometry. This effect was acknowledged in the case of cylinders, and we obtain similar results for the case of rectangular thick strips. On the other hand, TEA demonstrates a high level of accuracy when applied to objects with thickness concentrated at the center and thin edges, such as the isosceles triangular obstacle. Finally, we analyze objects whose shape is defined using the superellipse function, which is chosen for its versatility in generating objects of equivalent width and maximum thickness but with different shapes just changing a single parameter. Our results highlight the importance of considering the object geometry when employing this approximation in diffraction studies of opaque three-dimensional objects.

Self-healing propagation of a longitudinally varying vector optical beam for distance measurement.

Based on the longitudinal manipulation of polarization, a special vector optical beam (VOB) with customized polarization variation in propagation direction can be generated, whose properties and applications remain to be studied. Here, the self-healing propagation behaviors of the longitudinally varying VOB after an opaque object are investigated, and the localized polarization responses on the object distance are revealed. On this basis, characteristic parameters are defined to measure the distance of object, achieving a minimum relative error of 0.63% in a longitudinal range of 300 mm. Besides, the correlations and uncoupling methods of object distance and size are discussed. Our studies open new ways to use the structural properties of VOB and may be instructive for laser measurement.

A New Dataset and a Distractor-Aware Architecture for Transparent Object Tracking

Performance of modern trackers degrades substantially on transparent objects compared to opaque objects. This is largely due to two distinct reasons. Transparent objects are unique in that their appearance is directly affected by the background. Furthermore, transparent object scenes often contain many visually similar objects (distractors), which often lead to tracking failure. However, development of modern tracking architectures requires large training sets, which do not exist in transparent object tracking. We present two contributions addressing the aforementioned issues. We propose the first transparent object tracking training dataset Trans2k that consists of over 2k sequences with 104,343 images overall, annotated by bounding boxes and segmentation masks. Standard trackers trained on this dataset consistently improve by up to 16%. Our second contribution is a new distractor-aware transparent object tracker (DiTra) that treats localization accuracy and target identification as separate tasks and implements them by a novel architecture. DiTra sets a new state-of-the-art in transparent object tracking and generalizes well to opaque objects.

Laboratory X-ray Microscopy of 3D Nanostructures in the Hard X-ray Regime Enabled by a Combination of Multilayer X-ray Optics.

High-resolution imaging of buried metal interconnect structures in advanced microelectronic products with full-field X-ray microscopy is demonstrated in the hard X-ray regime, i.e., at photon energies > 10 keV. The combination of two multilayer optics-a side-by-side Montel (or nested Kirkpatrick-Baez) condenser optic and a high aspect-ratio multilayer Laue lens-results in an asymmetric optical path in the transmission X-ray microscope. This optics arrangement allows the imaging of 3D nanostructures in opaque objects at a photon energy of 24.2 keV (In-Kα X-ray line). Using a Siemens star test pattern with a minimal feature size of 150 nm, it was proven that features < 150 nm can be resolved. In-Kα radiation is generated from a Ga-In alloy target using a laboratory X-ray source that employs the liquid-metal-jet technology. Since the penetration depth of X-rays into the samples is significantly larger compared to 8 keV photons used in state-of-the-art laboratory X-ray microscopes (Cu-Kα radiation), 3D-nanopattered materials and structures can be imaged nondestructively in mm to cm thick samples. This means that destructive de-processing, thinning or cross-sectioning of the samples are not needed for the visualization of interconnect structures in microelectronic products manufactured using advanced packaging technologies. The application of laboratory transmission X-ray microscopy in the hard X-ray regime is demonstrated for Cu/Cu6Sn5/Cu microbump interconnects fabricated using solid-liquid interdiffusion (SLID) bonding.

Transparency-Aware Segmentation of Glass Objects to Train RGB-Based Pose Estimators.

Robotic manipulation requires object pose knowledge for the objects of interest. In order to perform typical household chores, a robot needs to be able to estimate 6D poses for objects such as water glasses or salad bowls. This is especially difficult for glass objects, as for these, depth data are mostly disturbed, and in RGB images, occluded objects are still visible. Thus, in this paper, we propose to redefine the ground-truth for training RGB-based pose estimators in two ways: (a) we apply a transparency-aware multisegmentation, in which an image pixel can belong to more than one object, and (b) we use transparency-aware bounding boxes, which always enclose whole objects, even if parts of an object are formally occluded by another object. The latter approach ensures that the size and scale of an object remain more consistent across different images. We train our pose estimator, which was originally designed for opaque objects, with three different ground-truth types on the ClearPose dataset. Just by changing the training data to our transparency-aware segmentation, with no additional glass-specific feature changes in the estimator, the ADD-S AUC value increases by 4.3%. Such a multisegmentation can be created for every dataset that provides a 3D model of the object and its ground-truth pose.

David Brewster’s and William Herschel’s experiments on inflection that delivered the coup de grâce to Thomas Young’s ether distribution hypothesis

ABSTRACT In his ‘Theory of Light and Colours’, presented to the Royal Society in November 1801, Thomas Young defended a mechanical explanation of the coloured fringes observed outside of the shadow of an opaque object – the so-called ‘colours by inflection’ – that was based on the hypothesis of an ethereal density gradient surrounding all material bodies. However, two years later, he publicly rejected that hypothesis, without giving much detail of his reasons. Although Geoffrey Cantor has demonstrated the crucial role of mechanical and astronomical arguments in explaining the withdrawal of this fundamental hypothesis long ago, the purpose of this article is to draw deeper attention to a set of experiments performed by David Brewster on the inflection of light, described in a letter he addressed to the Royal Society in January 1802, but was retained by William Herschel, and finally was neither read before the Society, nor published in its Philosophical Transactions. For the hypothesis that will be evaluated here is that these unpublished experiments of Brewster’s were eventually known to Young through the mediation of Herschel, and eventually played a significant role in Young’s rejection of his ether distribution hypothesis.

Double-light-sheet, consecutive-overlapping particle image velocimetry for the study of boundary layers past opaque objects

Double-light-sheet, consecutive-overlapping particle image velocimetry for the study of boundary layers past opaque objects

Ingestion of a row of artificial dentures in an adult: A case report and review of the literature.

Foreign body (FB) ingestion is a common clinical emergency, although in most cases, the FB can pass safely through the entire gastrointestinal tract without causing any damage. However, ingestion of large dentures is very rare and alarming, as it can threaten the intestinal mucosa and cause perforation of the gastrointestinal tract, among other complications. A 64-year-old Chinese male was referred to our hospital for removal of a FB, which was a large denture. Clinical symptoms included chest and upper abdominal pain. He had no cough or dyspnea. Medical history included a recent cerebral infarction, craniocerebral surgery, and being bedridden for a long term. We initially suspected a single and smooth denture, complicated by pharyngeal and esophageal mucosal injury. Radiographic examination however showed a 70-mm long opaque object located in the middle and upper esophagus, close to the trachea and aorta. Multiple dentures and metal hooks were removed via endoscopy using a net, grasping forceps, and rubber jacket. The patient recovered well and experienced no postoperative complications. The patient was discharged 5 days after endoscopic therapy. Our case showed that endoscopy was effective for the retrieval of an esophageal FB. For sharp FBs, the use of a net and rubber jacket is a good choice. However, we advocate for appropriate surgery in patients in whom endoscopy is not possible after an accurate diagnosis or those with severe complications.

Surface topography detection based on an optical differential metasurface.

Surface topography detection can extract critical characteristics from objects, playing an important role in target identification and precision measurement. Here, an optical method with the advantages of low power consumption, high speed, and simple devices is proposed to realize the surface topography detection of low-contrast phase objects. By constructing reflected light paths, a metasurface can perform spatial differential operation via receiving the light directly reflected from a target. Therefore, our scheme is experimentally demonstrated as having remarkable universality, which can be used not only for opaque objects, but also for transparent pure phase objects. It provides a new, to the best of our knowledge, application for optical differential metasurfaces in precise detection of microscale surface topography.

Controlling self-healing of optical field based on moiré dual-microlens arrays

Optical self-healing is a repairing phenomenon of a beam in the propagation, as it is perturbed by an opaque object. In this work, we demonstrate experimentally and theoretically that the moiré distributed dual-microlens array enables to generate optical fields with better healing ability to withstand defects than their counterparts of a single microlens array. By utilizing the double parameter scanning method, the self-healing degree of the optical field is significantly affected by both the interval distance and the relative angle of the dual-microlens arrays. The self-healing level is decreased significantly by lengthening the interval between the two microlens array with a small twist angle, while increasing the angle enhances the self-healing degree. Further study manifests the self-healing process with respect to the size and central location of the obstacle. The research results provide a simple and effective method to generate self-healing optical wave fields, which have potential applications including optical communication, assisted imaging technology, and even intense laser physics.

Comparative evaluation of the effects of lighting conditions on the shade selection of maxillary central incisor using visual methods

Background: The selection of shade of a tooth is a routine practice for a dentist, especially a prosthodontist, to make esthetically pleasing prostheses. The phenomenon of colour is a matter of perception by the eye. Every opaque object receives light, absorbs some of it and reflects the rest. A tooth being an opaque object, reflects some amount of light. The dominant wavelength of this reflected light is discerned by us as the colour of the tooth. Light has variable properties that influence our perception. Daylight is regarded as the standard source for the selection of the colour of a tooth. Conversely, the lack of optimal circumstances during different times of the day necessitates using artificial illuminants to select shade of a tooth. This study was carried out to compare the accuracy of a shade-matching light (SMILE LINE) to that of standard daylight. Methods: This study followed the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines for cross-sectional studies. The study comprised 125 participants of ages ranging from 20 to 30 years. Five observers from the Department of Prosthodontics, Sharad Pawar Dental College (SPDC), Wardha selected the shade of the maxillary central incisor of 125 subjects under sunlight, dental operatory light, and shade-matching light. The statistical analysis was performed using the SPSS software, and Kappa statistics were used to compare the shades selected by the shade-matching light and dental operatory light with sunlight. Results: A strong correlation was discovered between the shades of tooth selected under the shade matching light and shades selected under sunlight for four out of five observers. Conclusions: The shade correcting device gives shades comparable to those selected under sunlight in ideal conditions making it useful as an alternative to sunlight for shade selection.

Sensing the position of a single scatterer in an opaque medium by mutual scattering.

We investigate the potential of mutual scattering, i.e., light scattering with multiple properly phased incident beams, as a method to extract structural information from inside an opaque object. In particular, we study how sensitively the displacement of a single scatterer is detected in an optically dense sample of many (up to N = 1000) similar scatterers. By performing exact calculations on ensembles of many point scatterers, we compare the mutual scattering (from two beams) and the well-known differential cross-section (from one beam) in response to the change of location of a single dipole inside a configuration of randomly distributed similar dipoles. Our numerical examples show that mutual scattering provides speckle patterns with an angular sensitivity at least 10 times higher than the traditional one-beam techniques. By studying the "sensitivity" of mutual scattering, we demonstrate the possibility to determine the original depth relative to the incident surface of the displaced dipole in an opaque sample. Furthermore, we show that mutual scattering offers a new approach to determine the complex scattering amplitude.

Polarization-Maintaining Diffractive Elements for Off-Axis Terahertz Digital Holography

Off-axis digital holography in the terahertz waves allows three-dimensional visualizations of optically opaque volumetric objects in a single exposure and with angular separation of the zero-order component. Forming the tilted reference beam usually requires a set of mirrors, which can disturb the polarization, causing the loss of contrast in fringes at the detector. Here we present a mirror-less approach with 3D-printed transmissive diffractive optical elements, providing unaltered polarization and successful holographic measurements at 0.1 THz without any pre- or postprocessing of interference fringes.

Triplet Fusion Upconversion for Photocuring 3D-Printed Particle-Reinforced Composite Networks.

High energy photons (λ<400nm) are frequently used to initiate free radical polymerizations to form polymer networks, but are only effective for transparent objects. This phenomenon poses a major challenge to additive manufacturing of particle-reinforced composite networks since deep light penetration of short-wavelength photons limits the homogeneous modification of physicochemical and mechanical properties. Herein, the unconventional, yet versatile, multiexciton process of triplet-triplet annihilation upconversion (TTA-UC) is employed for curing opaque hydrogel composites created by direct-ink-write (DIW) 3D printing. TTA-UC converts low energy red light (λmax =660nm) for deep penetration into higher-energy blue light to initiate free radical polymerizations within opaque objects. As proof-of-principle, hydrogels containing up to 15wt.% TiO2 filler particles and doped with TTA-UC chromophores are readily cured with red light, while composites without the chromophores and TiO2 loadings as little as 1-2wt.% remain uncured. Importantly, this method has wide potential to modify the chemical and mechanical properties of complex DIW 3D-printed composite polymer networks.