Specular Research Articles

SpecReFlow: an algorithm for specular reflection restoration using flow-guided video completion.

Specular reflections (SRs) are highlight artifacts commonly found in endoscopy videos that can severely disrupt a surgeon's observation and judgment. Despite numerous attempts to restore SR, existing methods are inefficient and time consuming and can lead to false clinical interpretations. Therefore, we propose the first complete deep-learning solution, SpecReFlow, to detect and restore SR regions from endoscopy video with spatial and temporal coherence. SpecReFlow consists of three stages: (1)an image preprocessing stage to enhance contrast, (2)a detection stage to indicate where the SR region is present, and (3)a restoration stage in which we replace SR pixels with an accurate underlying tissue structure. Our restoration approach uses optical flow to seamlessly propagate color and structure from other frames of the endoscopy video. Comprehensive quantitative and qualitative tests for each stage reveal that our SpecReFlow solution performs better than previous detection and restoration methods. Our detection stage achieves a Dice score of 82.8% and a sensitivity of 94.6%, and our restoration stage successfully incorporates temporal information with spatial information for more accurate restorations than existing techniques. SpecReFlow is a first-of-its-kind solution that combines temporal and spatial information for effective detection and restoration of SR regions, surpassing previous methods relying on single-frame spatial information. Future work will look to optimizing SpecReFlow for real-time applications. SpecReFlow is a software-only solution for restoring image content lost due to SR, making it readily deployable in existing clinical settings to improve endoscopy video quality for accurate diagnosis and treatment.

Empirical BRDF model for goniochromatic materials and soft proofing with reflective inks.

The commonly used analytic bidirectional reflectance distribution functions (BRDFs) do not model goniochromatism, that is, angle-dependent material color. The material color is usually defined by a diffuse reflectance spectrum or RGB vector and a specular part based on a spectral complex index of refraction. Extension of the commonly used BRDFs based on wave theory can help model goniochromatism, but this comes at the cost of significant added model complexity. We measured the goniochromatism of structual color pigments used for additive color printing and found that we can fit the observed spectral angular dependence of the bidirectional reflectance using a simple modification of the standard microfacet BRDF model. All we need to describe the goniochromatism is an empirically-based spectral parameter, which we use in our model together with a specular reflectance spectrum instead of the spectral complex index of refraction. We demonstrate the ability of our model to fit the measured reflectance of red, green, and blue commercial structural color pigments. Our BRDF model enables straightforward implementation of a shader for interactive preview of 3D objects with printed spatially and angularly varying texture.

EGMM: removal of specular reflection with cervical region segmentation using enhanced Gaussian mixture model in cervix images

EGMM: removal of specular reflection with cervical region segmentation using enhanced Gaussian mixture model in cervix images

A Reflective Object Reconstruction Method Based on Neural Radiance Field

We propose a neural rendering-based 3D reconstruction method for reconstructing the geometry and BRDF of reflective objects from multi-view images captured in unknown environments. Multi-view reconstruction of reflective objects is extremely challenging because specular reflections are view-dependent and therefore violate multi-view consistency, which is the cornerstone of most multi-view reconstruction methods. Recent neural rendering techniques can model the interaction between ambient light and object surfaces to adapt to view-dependent reflections, making it possible to reconstruct reflective objects from multi-view images. However, accurately modeling ambient light in neural rendering is difficult. We propose a two-step approach to solve this problem. First, by introducing view-dependent photometric losses, our method accurately reconstructs the geometry of reflective objects. Then, with the object geometry fixed, we use more accurate sampling to recover the BRDF of the ambient light and object. Experiments show that our method is able to accurately reconstruct the geometry and BRDF of reflective objects from only RGB images without knowing the ambient light and object masks.

Durability Assessment of Silvered Glass Mirrors Exposed in Two Different Desert Sites Suitable for CSP Installations

The durability of thick solar glass mirrors has been evaluated in this study by exposing samples at two potential exposure sites. Samples have been exposed for a period of 18 months at different orientations (North, South, East, and West) to evaluate the impact of orientation on the durability. The samples performance has been evaluated by measuring the specular reflectance of the glass samples before and after an appropriate cleaning process. In addition, the contact angle and the surface energy have been analysed. Obtained results show that mirror durability is very specific to the environmental conditions of the exposure site.

Spectroscopic and electronic investigations on tin(II) 2,3-naphthalocyanine/p-Si heterojunction for optoelectronic applications

The spectroscopic and electronic properties of small-molecular organic semiconductors in thin films are a significant aspect that controls the performance of their optical/optoelectronic devices. A heterojunction of thermal vacuum deposited film of tin(II)2,3-naphthalocyanine (SnNc) dye onto a p-type Si wafer (SnNc/p-Si) was fabricated. The morphology of SnNc thin film onto Si wafer was explored using the Field Emission Scanning Electron Microscope (FE-SEM). The specular reflectance and the fluorescent properties of SnNc thin film onto Si wafer were investigated. The PL emission spectra of SnNc/p-Si emission bands are related to the B and Q absorption bands for SnNc. The spectral behavior of the refractive index and the specific polarizability for both SnNc/Glass and SnNc/p-Si were extracted. Under varying temperatures (298–378 K), the electronic properties of the SnNc/p-Si heterojunction were analyzed in dark conditions. The diode parameters of SnNc/p-Si heterojunctions, which exhibited temperature-dependent behavior, were extracted from the current–voltage (I-V) procedure. According to the Cheung-Cheung and Norde procedure, the essential electrical diode parameters of the SnNc/p-Si device, such as the ideality factor, the series resistance, and barrier height, were estimated. The photoconductivity parameters, such as the photocurrent, the sensitivity, and the responsivity of the SnNc/p-Si devices, were estimated. These findings can be helpful and utilized in optoelectronic and organic electronic applications.

Technical note: Visual, rapid, scintillation point dosimetry for in vivo MV photon beam radiotherapy treatments.

While careful planning and pre-treatment checks are performed to ensure patient safety during external beam radiation therapy (EBRT), inevitable daily variations mean that in vivo dosimetry (IVD) is the only way to attain the true delivered dose. Several countries outside the US require daily IVD for quality assurance. However, elsewhere, the manual labor and time considerations of traditional in vivo dosimeters may be preventing frequent use of IVD in the clinic. This study expands upon previous research using plastic scintillator discs for optical dosimetry for electron therapy treatments. We present the characterization of scintillator discs for in vivo x-ray dosimetry and describe additional considerations due to geometric complexities. Plastic scintillator discs were coated with reflective white paint on all sides but the front surface. An anti-reflective, matte coating was applied to the transparent face to minimize specular reflection. A time-gated iCMOS camera imaged the discs under various irradiation conditions. In post-processing, background-subtracted images of the scintillators were fit with Gaussian-convolved ellipses to extract several parameters, including integral output, and observation angle. Dose linearity and x-ray energy independence were observed, consistent with ideal characteristics for a dosimeter. Dose measurements exhibited less than 5% variation for incident beam angles between 0° and 75° at the anterior surface and 0-60 at the posterior surface for exit beam dosimetry. Varying the angle between the disc surface and the camera lens did not impact the integral output for the same dose up to 55°. Past this point, up to 75°, there is a sharp falloff in response; however, a correction can be used based on the detected width of the disc. The reproducibility of the integral output for a single disc is 2%, and combined with variations from the gantry angle, we report the accuracy of the proposed scintillator disc dosimeters as ±5.4%. Plastic scintillator discs have characteristics that are well-suited for in vivo optical dosimetry for x-ray radiotherapy treatments. Unlike typical point dosimeters, there is no inherent readout time delay, and an optical recording of the measurement is saved after treatment for future reference. While several factors influence the integral output for the same dose, they have been quantified here and may be corrected in post-processing.

Andreev reflections in deformed semi-Dirac material superconducting junctions

Besides the strong anisotropy, another unique feature of the semi-Dirac materials (SDMs) is the evolution of its dispersion with gap parameter from a merging semi-Dirac point to two Dirac points. In this study, we study the superconducting coherent transport at the SDM superconducting junction and focus on the influence of evolution of the SDM dispersion on the Andreev reflection. We found that the coexistence of peculiar quadruple reflections, including double specular normal reflection and double retro Andreev reflection, can occur within a wide range of parameters. We in detail discuss the dependence of reflection probabilities on the incident angle, incident energy, and gap parameter. By modulating these parameters, we also observe rich double reflection scenarios. These results enrich our understanding of the superconducting coherent transport and facilitate the probing of the intrinsic gap parameter in SDMs experimentally.

Role of tumbling in bacterial scattering at convex obstacles.

Active propulsion, as performed by bacteria and Janus particles, in combination with hydrodynamic interaction results in the accumulation of bacteria at a flat wall. However, in microfluidic devices with cylindrical pillars of sufficiently small radius, self-propelled particles can slide along and scatter off the surface of a pillar, without becoming trapped over long times. This nonequilibrium scattering process has been predicted to result in large diffusivities, even at high obstacle density, unlike particles that undergo classical specular reflection. Here, we test this prediction by experimentally studying the nonequilibrium scattering of pusherlike swimmers in microfluidic obstacle lattices. To explore the role of tumbles in the scattering process, we microscopically tracked wild-type (run and tumble) and smooth-swimming (run only) mutants of the bacterium Escherichia coli scattering off microfluidic pillars. We quantified key scattering parameters and related them to previously proposed models that included a prediction for the diffusivity, discussing their relevance. Finally, we discuss potential interpretations of the role of tumbles in the scattering process and connect our work to the broader study of swimmers in porous media.

Friction tensor for an ellipsoid enclosed in a rarefied gas

This research renders calculations of rotranslational friction and diffusion tensors in a rarefied gas, considering both specular and diffuse reflection off ellipsoidal particles. Calculations were conducted in a spherical polar coordinate system, revealing the dynamic nature of these off-diagonal tensors. The tensor elements were determined analytically, intending to compute the surface integrals. All diagrams, and pictures of two-dimensional surfaces were obtained by the numerical integration process in MATLAB. Additionally, we performed calculations without using the time parameter and time derivatives in the relevant relations. Deviations from the state of spherical symmetry were observed in the results for an ellipsoid and show that the tensors are dynamic quantities. Therefore, it is necessary to provide average values. We established that the non-zero elements of the friction tensor for an ellipsoid represent the directions of linear momentum transfer. Also, the non-zero components of the diffusion tensor signify the extent of fluctuations in relevant momentum transfers in corresponding directions.

Molecular dynamics simulations of interfacial resistance of gases transport through MOF HKUST-1

The mass transfer resistances of CH4 and CO2 transport through MOF HKUST-1 were intensively studied by molecular simulations. The corrected diffusivities increase as thickness increases in finite HKUST-1, and converge to the maximum in infinite crystalline. The interfacial resistance plays an essential role in total one, and weakens as HKUST-1 thickness increases. Under the same conditions, CO2 molecules suffer from stronger transport resistance than CH4 counterparts due to enhanced energy barriers, larger molecular weight, stronger interactions and freedom degree loss. The transport resistances decrease with loadings, opposite to trend of gas–gas interactions. The collisions of CH4 and CO2 molecules at high loadings are inclined to specular reflection, and enhance molecular collision, resulting in stronger permeability. Compared to CH4, CO2 exhibits 40% higher interface resistance under identical loading conditions. Such similar quantitative relationship also appears on critical membrane thickness, both of which are on the order of several tens of nanometers.

Large-scale aerial scene perception based on self-supervised multi-view stereo via cycled generative adversarial network

Unmanned aerial vehicle (UAV) has the characteristics of strong maneuverability and wide field of vision, and its application in real-time terrain perception and target capture is one of the active frontier topics in the field of UAV cooperative situation awareness. LiDAR-based method has unique advantages, but data acquisition is difficult and costly. In comparison, multiple view stereo-based (MVS) perception method contains rich image information with low cost, which is relatively easy to be collected. MVS also has potential application value in human–computer interaction and robot positioning (unstructured environment perception, visual servo). To reduce the dependence on costly ground truth, the current self-supervised MVS works calculate the training loss by assuming that the same space points projected into multiple views will share the same RGB information. However, this gap occurs mainly because in the real scene, interference factors such as specular reflection, illumination, and noise will cause large color changes in the source images, which will make it difficult to restrict the photometric consistency of different perspectives with ideal self-supervised loss. To realize more accurate scene perception, a novel self-supervised MVS integrated with generative adversarial network (GAN) is proposed. The overall framework is stacked twice by inputting real images and synthetic images into the network in turn, and adversarial learning is introduced to jointly distinguish the images. In addition, the consistency loss of predicted depth maps from real images and synthetic images is designed to improve the robustness of feature matching and color anti-noise ability. To the best of our knowledge, the proposed work is the first end-to-end depth inference model in MVS field that introduces the adversial training based on GAN, which is an effective complement to self-supervised scene reconstruction. Comprehensive experiment results on the public datasets show that the proposed work is comparable to the mainstream self-supervised MVS.

Evaluation of the influence of distortions of the digital object model on the result of synthetic aperture radar image modelling

An essential problem in radar image processing is the synthesis of optimal and quasi-optimal algorithms for the formation of a vector of sufficient statistics and, as a consequence, for obtaining the features used to classify an object of observation. For this purpose it is required to consider the influence of distortions of digital three-dimensional models of objects on the results of modelling images obtained for synthetic aperture radar. Aim of article – to develop and evaluate the possibilities of the method of geometrical optics in the process of application of ray tracing apparatus for the cases of diffuse and specular reflection of waves from the surface of a reflecting object while evaluating the influence of angular distortions of a digital 3D model of an object on the results of modelling. The method of taking into account the theoretical relationship between the magnitude of coordinate distortions of the digital model and the magnitude of distortion of the received radar image is proposed. The paper evaluates the influence of distortions of digital models of three-dimensional (3D) objects on the results of modelling images obtained for synthetic aperture radar. The presented method of taking into account the influence of distortions of the digital 3D model of the object on the results of radar image modelling allows to estimate the conditions of applicability of the ray tracing method.

Self-supervised monocular depth estimation on water scenes via specular reflection prior

Monocular depth estimation from a single image is an ill-posed problem for computer vision due to insufficient reliable cues as the prior knowledge. Besides the inter-frame supervision, namely stereo and adjacent frames, extensive prior information is available in the same frame. Reflections from specular surfaces, informative intra-frame priors, enable us to reformulate the ill-posed depth estimation task as a multi-view synthesis. This paper proposes the first self-supervision for deep-learning depth estimation on water scenes via intra-frame priors, known as reflection supervision and geometrical constraints. In the first stage, a water segmentation network is performed to separate the reflection components from the entire image. Next, we construct a self-supervised framework to predict the target appearance from reflections, perceived as other perspectives. The photometric re-projection error, incorporating SmoothL1 and a novel photometric adaptive SSIM, is formulated to optimize pose and depth estimation by aligning the transformed virtual depths and source ones. As a supplement, the water surface is determined from real and virtual camera positions, which complement the depth of the water area. Furthermore, to alleviate these laborious ground truth annotations, we introduce a large-scale water reflection scene (WRS) dataset rendered from Unreal Engine 4. Extensive experiments on the WRS dataset prove the feasibility of the proposed method compared to state-of-the-art depth estimation techniques.

S-LIGHT: Synthetic Dataset for the Separation of Diffuse and Specular Reflection Images.

Several studies in computer vision have examined specular removal, which is crucial for object detection and recognition. This research has traditionally been divided into two tasks: specular highlight removal, which focuses on removing specular highlights on object surfaces, and reflection removal, which deals with specular reflections occurring on glass surfaces. In reality, however, both types of specular effects often coexist, making it a fundamental challenge that has not been adequately addressed. Recognizing the necessity of integrating specular components handled in both tasks, we constructed a specular-light (S-Light) DB for training single-image-based deep learning models. Moreover, considering the absence of benchmark datasets for quantitative evaluation, the multi-scale normalized cross correlation (MS-NCC) metric, which considers the correlation between specular and diffuse components, was introduced to assess the learning outcomes.

Three-dimensional reconstruction of polarized ambient light separation in complex illumination

In current research, it is still a hot topic for 3D reconstruction under complex illumination. This paper uses a polarization camera combined with a coding technique to propose a new 3D reconstruction method for polarized ambient light separation. Based on the polarization camera, a specific separation model is established to analyze the relationship between the polarization characteristics of polarized and natural light. Specular reflections were filtered first and then analyzed based on the stocks vector and muller matrix. A specific calculation process was used to calculate different polarization azimuths according to the polarization characteristics, and finally, the polarized light and ambient light were separated. The experimental results show that the use of this polarization camera approach reduces the number of steps required to rotate the polarizer multiple times. This not only reduces the shooting time but also improves the efficiency. Moreover, after separating the ambient light, polarization imaging suppresses the interference of the ambient light, which helps to highlight the complete point cloud image more clearly in the 3D reconstruction. The standard deviation of 3D reconstruction was improved to 0.1675 mm by using this method in indoor and outdoor experiments.

Investigation of Excitation Modes in Al2O3-YAG Phosphor Ceramics for High-Power Laser-Driven Lighting

The performance of laser-driven white light can vary significantly depending on the excitation mode. To attain desirable application performance of color converters in laser lighting, the excitation mode must be selected carefully. In this work, to understand the effect of the excitation mode on the performance of color converters, a detailed study was conducted on the heat dispersion, luminescence, and color quality of Al2O3-YAG:Ce phosphor ceramics in both transmissive and reflective modes. The relationship between ceramic porosity and scattering performance has also been analyzed by studying the beam expansion ratio. The results demonstrate that the phosphor ceramics in reflective mode exhibit superior heat dissipation capabilities and can withstand higher power laser excitation compared to the transmissive mode, while the light uniformity is relatively poorer due to the specular reflection. Therefore, in situations where a uniform light is not necessary, reflective color converters are more suitable for achieving superior luminance and luminous efficacy. Finally, a laser-driven white light device with a luminous flux of 1206 lm, a luminous efficacy of 241.2 lm W−1, and a CCT of 5749 K has been realized using Al2O3-YAG:Ce phosphor ceramic in reflective mode, which can be applied for automotive headlights.

Photodegradation of Phenol under Visible Light Irradiation Using Cu-N-codoped ZrTiO<sub>4</sub> Composite as a High-Performance Photocatalyst

Codoping of nitrogen and copper into zirconium titanate composite (Cu-N-codoped ZrTiO4) was carried out through a sol-gel process. This study aimed to investigate the effect of copper and nitrogen dopants on the photocatalytic activity of ZrTiO4 composite in degrading phenol. To prepare the composite, an aqueous suspension of zirconia (ZrO2) alongside a fixed amount of urea and various amount of copper sulfate was added dropwise into diluted titanium(IV) tetraisopropoxide (TTIP) in ethanol. The composites were calcined at temperatures of 500, 700, and 900 °C. Fourier-transform infrared spectrophotometry (FTIR), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray (SEM-EDX) mapping, and specular reflectance UV-visible spectrophotometry (SR UV-vis) were used for their characterization of composite. The photocatalytic activity was evaluated by adding the composite into a 10 mg L−1 phenol solution for various irradiation time spans. The remaining concentration of phenol solution was determined by absorption at 269 nm. Cu-N-codoped ZrTiO4 composite containing 5% Cu calcined at 500 °C demonstrated the highest observed rate constant and a significant band gap decrease from 3.13 to 2.68 eV.

Terahertz reflective imaging systems with improved angle compatibility realized by the designed quasi-scattering screen

Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure.

Stimulated emission does not radiate in a pure dipole pattern

Stimulated emission (StE) remains relatively unused as an image-forming signal despite having potential advantages over fluorescence in speed, coherence, and ultimately resolution. Several ideas for the radiation pattern and directionality of StE remain prevalent, namely, whether a single molecule would radiate StE itself in a pure dipole pattern, or whether its emission direction depends on the driving field. Previous StE imaging has been carried out in transmission, which would collect signal either way. Here, we introduce the StE driving field (the probe) at an angle, using total internal reflection to avoid incident probe light and its specular reflections in our detection path. In this non-collinear detection configuration that also collects some fluorescence from the sample, we observe fluorescence depletion even in the spectral window where an increase in detected signal from StE would be expected if StE radiated like a simple classical dipole. Because simultaneous direct measurement of the fluorescence represents a calibration of the potential size of StE were it spatially patterned like a classical dipole emitter, our study clarifies a critical characteristic of StE for optimal microscope design, optical cooling, and applications using small arrays of emitters.