Showing 1–28 of 28 results for abstract: skin spectral reflectance

A 2D autocorrelation-based frequency estimator reflecting spatial tissue distribution to improve Ultrasound H-scan tissue characterization

Authors: Jihye Baek, Thurston Brevett, Dongwoon Hyun, Ahmed El Kaffas, Kevin J. Parker, Jeremy J. Dahl

Abstract: …a promising quantitative ultrasound technique that estimates the frequency content of backscattered signals and maps the estimated frequencies onto a red/blue color scale to reflect underlying tissue properties. Although it relies on matched filters tuned to different frequencies, the broad… ▽ More H-scan is a promising quantitative ultrasound technique that estimates the frequency content of backscattered signals and maps the estimated frequencies onto a red/blue color scale to reflect underlying tissue properties. Although it relies on matched filters tuned to different frequencies, the broad spectral bandwidth of ultrasound produces noisy, granular displays. Here, we introduce an adaptive frequency estimator designed to suppress the noise within homogeneous regions while preserving sharpness across tissue boundaries. The method combines 2D autocorrelation with a matched filter. In the first stage, a matched-filter-based estimation yields an a priori map of the spatial distribution of frequencies. The local heterogeneity of these estimates then defines a 2D weighting function that guides a second estimation stage. Drawing on the concept of Loupas's blood velocity estimator, we apply autocorrelation over a 2D spatial kernel to recover the axial frequency components, employing a weighted summation that accounts for the spatial frequency distribution within the kernel. We benchmarked the proposed estimator against conventional approaches, including the short-time Fourier transform, the H-scan matched filter, and standard autocorrelation, using both Field II simulations and in vivo data from human subjects with hepatic steatosis. In simulation, our adaptive estimator reduced the noisy texture in homogeneous regions while retaining clear boundary delineation, whereas the other estimators could achieve only one of these objectives. Applied to the in vivo human liver, the estimator improved H-scan image quality by lowering noise and enhancing the discrimination of steatotic liver from adjacent gallbladder and skin layers. △ Less

Submitted 24 September, 2026; originally announced September 2026.

Melanin- and linewidth-corrected pulse-oximetry

Authors: Giles Blaney, Jodee Frias, Ravi Durbha, Sergio Fantini, Valencia Koomson

Abstract: Current pulse oximeters tend to overestimate arterial saturation in darkly pigmented patients, increasing occult hypoxemia rates. One proposed mechanism is spectral coloring: melanin contributes to reshaping the detected spectrum of a broad-linewidth source, making the wavelength-dependent parameters behind calibration… ▽ More Current pulse oximeters tend to overestimate arterial saturation in darkly pigmented patients, increasing occult hypoxemia rates. One proposed mechanism is spectral coloring: melanin contributes to reshaping the detected spectrum of a broad-linewidth source, making the wavelength-dependent parameters behind calibration skin-tone-dependent. Diffuse optics theory allows for a melanin- and linewidth-corrected calibration equation, which to our knowledge has never been tested in vivo. In this work, we fit the empirical linear equation, the diffuse optics theoretical equation, and its corrected form to the public OpenOximetry Repository---considering 98 patients with paired arterial blood-draws, raw photoplethysmograms, and skin reflectance spectra. Accuracy is practically indistinguishable between the three approaches, yet the saturation error's dependence on melanin falls from +2.8 to +1.2 to +0.1 percentage points per unit melanin volume fraction---consistent with the spectral-coloring theory. None of these dependencies on melanin are statistically resolved here---a cohort roughly ten times larger is needed---so the correction is backed by theory and its expected direction. All three calibration approaches carry two degrees of freedom: the theoretical equation drops into existing calibration procedures, and its melanin-aware form needs a per-patient skin-tone measurement. This work demonstrates how the theoretical pulse oximetry model can be applied in calibration. △ Less

Submitted 24 September, 2026; originally announced September 2026.

CardiacMamba: Fair and Robust RGB-RF Fusion for Remote Heart Rate Estimation via State Space Modeling

Authors: Bo Zhao, Zheng Wu, Yiping Xie, Zitong YU

Abstract: …photoplethysmography (rPPG) enables non-contact heart rate (HR) monitoring from facial videos, but RGB-only methods are vulnerable to illumination changes, motion artifacts, and skin-tone-dependent optical… ▽ More Remote photoplethysmography (rPPG) enables non-contact heart rate (HR) monitoring from facial videos, but RGB-only methods are vulnerable to illumination changes, motion artifacts, and skin-tone-dependent optical reflectance. We propose CardiacMamba, a fair and robust RGB-RF fusion framework that integrates optical facial cues and radio-frequency cardiac motion cues through state space modeling. CardiacMamba introduces a Temporal Difference Mamba Module (TDMM) to enhance subtle RF temporal variations, a bidirectional SSM-based interaction mechanism to align heterogeneous RGB-RF dynamics, and a Channel-wise Fast Fourier Transform (CFFT) module for channel-domain spectral refinement. On the EquiPleth dataset, CardiacMamba achieves state-of-the-art performance with 0.96 bpm MAE, 3.06 bpm RMSE, and 0.97 Pearson correlation, while reducing the observed light-dark skin-tone MAE gap to 0.26 bpm and maintaining robustness under RGB degradation and RF-missing conditions △ Less

Submitted 16 August, 2026; originally announced August 2026.

Infrared Imaging Empowered by Artificial Intelligence for Pediatric Skeletal Triage: A Narrative Review and Future Perspectives

Authors: Sajad Amiri, Pardis Afshar, Elham Anjomshoa

Abstract: …with deep-learning cross-modal translation to generate clinically interpretable synthetic-radiograph reconstructions from non-ionizing data. Approach. We review five IR spectral windows spanning 650 nm to 1 mm - NIR-I, NIR-II, SWIR, MIR/LWIR, and THz - and how dual-geometry (transmission/… ▽ More Background. Pediatric musculoskeletal trauma represents up to 18% of pediatric ED visits, yet diagnosis still depends on ionizing radiography. Cumulative low-dose radiation in early life raises lifetime leukemia and brain malignancy risk, motivating radiation-free triage alternatives. Objective. To synthesize evidence for a hybrid framework coupling broad-spectrum infrared (IR) imaging with deep-learning cross-modal translation to generate clinically interpretable synthetic-radiograph reconstructions from non-ionizing data. Approach. We review five IR spectral windows spanning 650 nm to 1 mm - NIR-I, NIR-II, SWIR, MIR/LWIR, and THz - and how dual-geometry (transmission/reflection) acquisition exploits wavelength-specific tissue depth and biochemical sensitivity. We summarize image-to-image translation networks (Pix2Pix, CycleGAN, Swin-Unet) and feature-matching algorithms (SuperPoint, SuperGlue, ALIKED, LightGlue) used to align and fuse IR data into radiograph-equivalent reconstructions. Implications. Pediatric anatomy - smaller cross-sections, thinner cortical bone - favors IR penetration, enabling compact, portable, non-ionizing triage hardware. Feasibility is grounded in fNIRS and transcranial photobiomodulation evidence: near-infrared light passes through skin, skull, and cortex with sufficient signal for hemodynamic monitoring - a longer, more attenuating path than through a pediatric forearm or distal leg. Key barriers: paired IR/X-ray dataset construction, AI-as-medical-device regulatory pathways, generalization across body habitus and skin pigmentation, and acquisition-protocol standardization. Conclusions. Integrated multi-spectral IR+AI imaging is a promising radiation-free complement to pediatric skeletal radiography. Progress requires multi-center paired datasets, externally validated models, and IR source safety qualification under IEC 60825-1. △ Less

Submitted 27 July, 2026; originally announced July 2026.

Comments: 20 pages, 4 figures

Pseudospectral phenomena and the origin of the non-Hermitian skin effect

Authors: J. Sirker

Abstract: The non-Hermitian skin effect (NHSE), characterized by a macroscopic accumulation of eigenstates at the edge of a system with open boundaries, is often ascribed to a non-trivial point-gap topology of the Bloch Hamiltonian. We revisit this connection and separate the question of the NHSE as a… ▽ More The non-Hermitian skin effect (NHSE), characterized by a macroscopic accumulation of eigenstates at the edge of a system with open boundaries, is often ascribed to a non-trivial point-gap topology of the Bloch Hamiltonian. We revisit this connection and separate the question of the NHSE as a spectral reconstruction effect in clean systems from the question of stable topological protection. For a Hatano-Nelson ladder, where point-gap winding and non-normality can be varied independently, we demonstrate that, in a clean translationally invariant multiband setting, the NHSE can occur without point-gap winding and, conversely, that point-gap winding can persist without the NHSE. These results establish that in the clean case the connection between point-gap winding and the NHSE only holds in the scalar one-band case but, in general, not in the multiband case. Even more importantly, the eigenspectrum of non-normal operators is generically highly sensitive to boundary conditions and perturbations, and therefore does not constitute a stable object encoding topological information. Instead, topological properties are reflected in the splitting of the singular-value spectrum for finite systems and, in the semi-infinite limit, correspond to boundary-localized kernel modes implied by the index of the corresponding Toeplitz operator. △ Less

Submitted 6 July, 2026; v1 submitted 23 March, 2026; originally announced March 2026.

Comments: Significantly extended version, clearly delineating the question of boundary reconstruction in clean systems from the question what the stable topological objects are

Spectral Subsurface Scattering from RGB via Biophysical Skin Inversion

Authors: Carlos Aliaga, Adrian Jarabo

Abstract: In this paper we present a spectral optical inversion for… ▽ More In this paper we present a spectral optical inversion for skin for path tracing-based rendering of subsurface scattering. Skin is a complex multilayered medium, with appearance determined by the mixture of biophysical chromophores. However, current methods rely on medium homogeneization, with optical parameters obtained via albedo inversion from a reflectance texture and hand-tuned scattering distance and anisotropy. This results into significant art-skilled manual labor for authoring, and an inaccurate scattering profile for skin. To solve these problems, we generalize existing albedo inversion techniques, and propose a framework that predicts full-spectral skin scattering parameters from a single RGB diffuse albedo. Our method builds upon a new mixture-of-media representation, that approximates the aggregated multilayered appearance of skin by mixing the aggregated scattering of three uncorrelated media. We train a chained neural decoder that maps RGB diffuse albedo to the optical properties of the mixture of media, including anisotropy, scattering radius and scattering albedo. Then, we show this mixture can be used in a random-walk-based path tracer with minimal modifications, by simply randomly selecting the medium to traverse. △ Less

Submitted 25 June, 2026; originally announced June 2026.

Comments: 14 pages, 9 figures

ACM Class: I.3.7; I.4.8

Anomalous Localization and Mobility Edges in Non-Hermitian Quasicrystals with Disordered Imaginary Gauge Fields

Authors: Guolin Nan, Zhijian Li, Feng Mei, Zhihao Xu

Abstract: …with a Bernoulli imaginary gauge field and quasiperiodic onsite modulation. In the nearest-neighbor limit, we identify an anomalous transition from a fully erratic non-Hermitian skin effect (ENHSE) phase to a fully localized phase. Although the fractal dimension vanishes in both regimes, the Lyapunov exponent and the fluctuation of the eigenstate center of m… ▽ More Localization in non-Hermitian quasicrystals can differ fundamentally from its Hermitian counterpart when non-reciprocity is spatially disordered. Here we study a one-dimensional non-Hermitian Aubry-André-Harper chain with a Bernoulli imaginary gauge field and quasiperiodic onsite modulation. In the nearest-neighbor limit, we identify an anomalous transition from a fully erratic non-Hermitian skin effect (ENHSE) phase to a fully localized phase. Although the fractal dimension vanishes in both regimes, the Lyapunov exponent and the fluctuation of the eigenstate center of mass sharply distinguish them. For generic finite-size realizations, this transition is further accompanied by a complex-to-real spectral change under periodic boundary conditions and a change of spectral winding from nontrivial to trivial. With weak next-nearest-neighbor hopping, we uncover an anomalous mobility edge at the same location as in the Hermitian generalized Aubry-André-Harper model, but separating Anderson-localized states from ENHSE-type macroscopic-accumulation states rather than extended states. We further show that this anomalous localization structure is reflected in spectral winding and wave-packet dynamics: single realizations exhibit winding-dependent drift, winding-resolved averaging preserves opposite directional responses, and full disorder averaging largely restores Hermitian-like transport. Our results establish practical diagnostics of anomalous localization and mobility edges in non-Hermitian quasicrystals. △ Less

Submitted 19 April, 2026; v1 submitted 21 January, 2026; originally announced January 2026.

Comments: 22 pages, 13 figures

When and why non-Hermitian eigenvalues miss eigenstates in topological physics

Authors: Lucien Jezequel, Loïc Herviou, Jens Bardarson

Abstract: Non-Hermitian systems exhibit a fundamental spectral dichotomy absent in Hermitian physics: the eigenvalue spectrum and the eigenstate spectrum can deviate significantly in the thermodynamic limit. We explain how non-Hermitian Hamiltonians can support eigenstates completely undetected by eigenvalues, with the unidirectional Hatano-Nelson model serving as bot… ▽ More Non-Hermitian systems exhibit a fundamental spectral dichotomy absent in Hermitian physics: the eigenvalue spectrum and the eigenstate spectrum can deviate significantly in the thermodynamic limit. We explain how non-Hermitian Hamiltonians can support eigenstates completely undetected by eigenvalues, with the unidirectional Hatano-Nelson model serving as both a minimal realization and universal paradigm for this phenomenon. Through exact analytical solutions, we show that this model contains not only hidden modes but multiple macroscopic hidden exceptional points that appear more generally in all systems with a non-trivial bulk winding. Our framework explains how the apparent bulk-edge correspondence failures in models like the non-Hermitian SSH chain instead reflect the systematic inability of the eigenvalue spectrum to detect certain eigenstates in systems with a skin-effect. These results establish the limitation of the eigenvalue spectrum and suggest how the eigenstate approach can lead to improved characterization of non-Hermitian topology. △ Less

Submitted 8 January, 2026; originally announced January 2026.

Broad Iron Line as a Relativistic Reflection from Warm Corona in AGN

Authors: P. P. Biswas, A. Różańska, F. H. Vincent, D. Lančová, P. T. Zycki

Abstract: …illuminated by X-ray radiation from a lamp located above a central SMBH. Thermal lines from highly ionized iron ions (FeXXV and FeXXVI), caused by both internal heating and reflection from the warm corona, can be integrated into an observed broad line profile due to the close vicinity of the SMBH. We investigate the dependence of the broad line profile by va… ▽ More We present that the broad feature usually observed in X-ray spectra can be explained by a ray-traced emission from a two-slab system containing a dissipative, warm corona on top of an accretion disk in an AGN. Such an accretion flow is externally illuminated by X-ray radiation from a lamp located above a central SMBH. Thermal lines from highly ionized iron ions (FeXXV and FeXXVI), caused by both internal heating and reflection from the warm corona, can be integrated into an observed broad line profile due to the close vicinity of the SMBH. We investigate the dependence of the broad line profile by varying the SMBH spin parameter, viewing angle, lamp height, and dissipation factor. Our results introduce a new method to probe properties of the warm corona using high-resolution spectroscopic measurements. We use the photoionization code TITAN to compute local ion populations and emission line profiles, and the ray-tracing code GYOTO to include relativistic effects on the outgoing X-ray spectrum. In our models, the temperature of the inner atmosphere covering the disk can reach values of 10^7 - 10^8 K due to internal warm corona dissipation and external illumination, which is adequate for generating the highly ionized iron lines. These lines can undergo significant gravitational redshift near the black hole, leading to a prominent spectral feature centered around 6.4 keV. For all computed models, the relativistic corrections shift highly ionized iron lines to the X-ray region, usually attributed to fluorescent emission from the illuminated skin of an accretion disk. Hence, in the case of a warm corona covering the inner disk regions, the resulting theoretical line profile under strong gravity is a sum of different iron line transitions, and those originating from highly ionized iron contribute the most to the observed total line profile in AGN. △ Less

Submitted 5 November, 2025; originally announced November 2025.

Comments: 18 pages, 19 figures, 3 tables. After first round of revision in A&A

Journal ref: A&A 707, A156 (2026)

A Telecentric Offset Reflective Imaging System (TORIS) for Terahertz Imaging and Spectroscopy

Authors: Pouyan Rezapoor, Aleksi Tamminen, Juha Ala-Laurinaho, Dan Ruan, Zachary Taylor

Abstract: …face limitations such as slow mechanical scanning, restricted field of view, and poor telecentricity. To overcome these challenges, we introduce the Telecentric Offset Reflective Imaging System (TORIS), a novel dual-mirror scanning design optimized for high-speed, distortion-free imaging. The system employs a telecentric f-theta lens and is validated using r… ▽ More Terahertz (THz) imaging has emerged as a promising technology in medical diagnostics due to its non-ionizing radiation and high sensitivity to water content. However, conventional THz imaging systems face limitations such as slow mechanical scanning, restricted field of view, and poor telecentricity. To overcome these challenges, we introduce the Telecentric Offset Reflective Imaging System (TORIS), a novel dual-mirror scanning design optimized for high-speed, distortion-free imaging. The system employs a telecentric f-theta lens and is validated using ray tracing and physical optics simulations. It achieves uniform resolution across a 50 mm x 50 mm field of view without the need for mechanical translation stages. Broadband spectral imaging of a USAF resolution test target across WR-2.2 (325-500 GHz) and WR-1.5 (500-700 GHz) frequency bands demonstrates consistent beam focus and minimal distortion, with a maximum deviation of 2.7 degrees from normal incidence and a beam waist of 2.1 lambda at the field edge in the WR-1.5 band. The system's sensitivity to hydration dynamics is further validated through imaging of wet tissue paper, capturing temporal changes in water content. In vivo imaging of human skin after capsaicin patch application reveals localized hydration variations due to biochemical response and adhesive removal. These findings confirm the system's potential for real-time hydration sensing and dermatological evaluation. TORIS sets a new benchmark in THz imaging, with applications in clinical diagnostics, wound assessment, and material characterization. △ Less

Submitted 7 April, 2025; originally announced April 2025.

Spreading dynamics in the Hatano-Nelson model with disorder

Authors: Jinyuan Shang, Haiping Hu

Abstract: The non-Hermitian skin effect is the accumulation of eigenstates at the boundaries,… ▽ More The non-Hermitian skin effect is the accumulation of eigenstates at the boundaries, reflecting the system's nonreciprocity. Introducing disorder leads to a competition between the skin effect and Anderson localization, giving rise to the skin-Anderson transition. Here, we investigate wave packet spreading in the disordered Hatano-Nelson model and uncover distinct dynamical behaviors across different regimes. In the clean limit, transport is unidirectionally ballistic (Δx ~ t) due to nonreciprocity. For weak disorder, where skin and Anderson-localized modes coexist, transport transitions from ballistic at early times to superdiffusive (Δx ~ t^{2/3}) at long times. In the deeply Anderson-localized regime, initial diffusion (Δx ~ t^{1/2}) eventually gives way to superdiffusive spreading. We examine how these scaling behaviors emerge from the system's spectral properties and eigenstate localization behaviors. Our work unveils the rich dynamics driven by nonreciprocity and disorder in non-Hermitian systems. △ Less

Submitted 6 April, 2025; originally announced April 2025.

Comments: 6 pages, 4 figures

Polarimetric BSSRDF Acquisition of Dynamic Faces

Authors: Hyunho Ha, Inseung Hwang, Nestor Monzon, Jaemin Cho, Donggun Kim, Seung-Hwan Baek, Adolfo Muñoz, Diego Gutierrez, Min H. Kim

Abstract: 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 ch… ▽ More 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. △ Less

Submitted 29 December, 2024; originally announced January 2025.

ACM Class: I.3.7

Journal ref: ACM Transactions on Graphics 43, 6, Article 275 (December 2024)

Optimizing Curved EM Skins for Opportunistic Relaying in Vehicular Networks

Authors: Reza Aghazadeh Ayoubi, Silvia Mura, Dario Tagliaferri, Marouan Mizmizi, Umberto Spagnolini

Abstract: Electromagnetic skins (EMSs) are recognized for enhancing communication performance, spanning from coverage to capacity. While much of the scientific literature focuses on reconfigurable intelligent surfaces that dynamically adjust phase configurations over time, this study takes a different approach by considering low-cost static passive curved EMS (CEMS)s.… ▽ More Electromagnetic skins (EMSs) are recognized for enhancing communication performance, spanning from coverage to capacity. While much of the scientific literature focuses on reconfigurable intelligent surfaces that dynamically adjust phase configurations over time, this study takes a different approach by considering low-cost static passive curved EMS (CEMS)s. These are pre-configured during manufacturing to conform to the shape of irregular surfaces, e.g., car doors, effectively transforming them into anomalous mirrors. This design allows vehicles to serve as opportunistic passive relays, mitigating blockage issues in vehicular networks. This paper delves into a novel design method for the phase profile of CEMS based on coarse a-priori distributions of incident and reflection angles onto the surface, influenced by vehicular traffic patterns. A penalty-based method is employed to optimize both the average spectral efficiency (SE) and average coverage probability, and it is compared against a lower-complexity and physically intuitive modular architecture, utilizing a codebook-based discrete optimization technique. Numerical results demonstrate that properly designed CEMS lead to a remarkable improvements in average SE and coverage probability, namely when the direct path is blocked. △ Less

Submitted 15 May, 2024; originally announced May 2024.

Polarization-sensitive PHASR Scanner and calibration technique for accurate mapping of the Stokes vectors in terahertz frequencies

Authors: Zachery B. Harris, Kuangyi Xu, M. Hassan Arbab

Abstract: …been in rapid development for various applications ranging from non-destructive testing to biomedical imaging and sensing. For instance, we have deployed our Portable Handheld Spectral… ▽ More In recent years, handheld and portable terahertz instruments have been in rapid development for various applications ranging from non-destructive testing to biomedical imaging and sensing. For instance, we have deployed our Portable Handheld Spectral Reflection (PHASR) Scanners for in vivo full-spectroscopic imaging of skin burns in large animal models in operating room settings. In this paper, we debut the polarimetric version of the PHASR Scanner, and describe a generalized calibration technique to map the spatial and spectral dependence of the Jones matrix of an imaging scanner across its field of view. Our design is based on placement of two orthogonal photoconductive antenna (PCA) detectors separated by a polarizing beam splitter in the PHASR Scanner housing. We show that at least three independent measurements of a well-characterized polarimetric calibration target are sufficient to determine the polarization state of the incident beam at the sample location, as well as to extract the Jones propagation matrix from the sample location to the detectors. We have tested the accuracy of our scanner by validating polarimetric measurements obtained from a birefringent crystal rotated to various angles, as compared to the theoretically predicted response of the sample. This new version of our PHASR scanner can be used for high-speed imaging and investigation of heterogeneity of polarization-sensitive samples in the field. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Comments: 13 pages, 6 figures

IR radiometer sensitivity and accuracy improvement by eliminating spurious radiation for emissivity measurements on highly specular samples in the 2 to 25 μm spectral range

Authors: Telmo Echániz, Raúl Benjamín Pérez-Sáez, Manuel José Tello

Abstract: …radiation. This spurious radiation must be taken into account in the calibration processes since it can be of the same order of magnitude as that of the sample in highly reflective surfaces. Its presence may also be the cause of the inability to detect small surface emissivity changes induced by any surface or bulk properties (anomalous… ▽ More This paper indicates that the sensitivity and accuracy of an infrared radiometer for emissivity measurements depends not only on its design and the measurement method, but also on the spurious radiation. This spurious radiation must be taken into account in the calibration processes since it can be of the same order of magnitude as that of the sample in highly reflective surfaces. Its presence may also be the cause of the inability to detect small surface emissivity changes induced by any surface or bulk properties (anomalous skin effect, phase transitions, etc.). In this paper, the analysis of the spurious radiation is performed for a T form radiometer and a measurement method where the surroundings are considered to emit as if they were black bodies. However, the results and conclusions that are obtained can be extended without difficulty to any type of radiometer and to all measurement methods. Our research shows that if the sample is placed normal to the emitted radiation optical path, two different spurious radiation sources are detected. However, for low emitting and highly specular materials, they can be eliminated by tilting the sample between 6 and 20° without modifying the spectral emissivity. △ Less

Submitted 1 February, 2024; v1 submitted 23 January, 2024; originally announced January 2024.

Journal ref: T Echániz, RB Pérez-Sáez, MJ Tello, Measurement 110 (2017) 22-26

Observation of dynamical degeneracy splitting for the non-Hermitian skin effect

Authors: Tuo Wan, Kai Zhang, Junkai Li, Zhesen Yang, Zhaoju Yang

Abstract: The non-Hermitian skin effect is a distinctive phenomenon in non-Hermitian systems, which manifests as the anomalous localization of bulk states at the boundary. To understand the physical origin of the non-Hermitian… ▽ More The non-Hermitian skin effect is a distinctive phenomenon in non-Hermitian systems, which manifests as the anomalous localization of bulk states at the boundary. To understand the physical origin of the non-Hermitian skin effect, a bulk band characterization based on the dynamical degeneracy on an equal frequency contour is proposed, which reflects the strong anisotropy of the spectral function. In this paper, we report the experimental observation of both phenomena in a two-dimensional acoustic crystal, and reveal their remarkable correspondence by performing single-frequency excitation measurements. Our work not only provides a controllable experimental platform for studying the non-Hermitian physics, but also confirms the correspondence between the non-Hermitian skin effect and the dynamical degeneracy splitting, paving a new way to characterize the non-Hermitian skin effect. △ Less

Submitted 21 March, 2023; v1 submitted 20 March, 2023; originally announced March 2023.

Comments: 14+3 pages, 4+4 figures

Journal ref: Science Bulletin, 2023, 68(20)

Topological spin excitations in non-Hermitian spin chains with a generalized kernel polynomial algorithm

Authors: Guangze Chen, Fei Song, Jose L. Lado

Abstract: Spectral functions of non-Hermitian Hamiltonians can reveal the existence of topologically non-trivial line gaps and the associated topological edge modes. However, the computation of… ▽ More Spectral functions of non-Hermitian Hamiltonians can reveal the existence of topologically non-trivial line gaps and the associated topological edge modes. However, the computation of spectral functions in a non-Hermitian many-body system remains an open challenge. Here, we put forward a numerical approach to compute spectral functions of a non-Hermitian many-body Hamiltonian based on the kernel polynomial method and the matrix-product state formalism. We show that the local spectral functions computed with our algorithm reveal topological spin excitations in a non-Hermitian spin model, faithfully reflecting the non-trivial line gap topology in a many-body model. We further show that the algorithm works in the presence of the non-Hermitian skin effect. Our method offers an efficient way to compute local spectral functions in non-Hermitian many-body systems with tensor-networks, allowing to characterize line gap topology in non-Hermitian quantum many-body models. △ Less

Submitted 9 March, 2023; v1 submitted 12 August, 2022; originally announced August 2022.

Comments: 6 pages, 3 figures + supplemental material (6 pages, 5 figures)

Journal ref: Phys. Rev. Lett. 130, 100401 (2023)

Experimental Realization of Geometry-Dependent Skin Effect in a Reciprocal Two-Dimensional Lattice

Authors: Wei Wang, Mengying Hu, Xulong Wang, Guancong Ma, Kun Ding

Abstract: Recent studies of non-Hermitian periodic lattices unveiled the non-Hermitian skin effect (NHSE), in which the bulk modes under the periodic boundary conditions (PBC) become… ▽ More Recent studies of non-Hermitian periodic lattices unveiled the non-Hermitian skin effect (NHSE), in which the bulk modes under the periodic boundary conditions (PBC) become skin modes under open boundary conditions (OBC). The NHSE is a topological effect owing to the non-trivial spectral winding, and such spectral behaviors appear naturally in non-reciprocal systems. Hence prevailing approaches rely on non-reciprocity to achieve the NHSE. Here, we report the experimental realization of the geometry-dependent skin effect (GDSE) in a two-dimensional (2D) reciprocal system, in which the skin effect occurs only at boundaries whose macroscopic symmetry mismatches with the lattice symmetry. The role of spectral reciprocity and symmetry is revealed by connecting reflective channels at given boundaries with the spectral topology of the PBC spectrum. Our work highlights the vital role of reciprocity and macroscopic geometry on the NHSE in systems with dimensions larger than one and opens new routes for wave structuring using non-Hermitian effects. △ Less

Submitted 13 February, 2023; originally announced February 2023.

Comments: 14 pages, 3 figures

Journal ref: Phys. Rev. Lett. 131, 207201 (2023)

R-DMFT study of a non-Hermitian skin effect for correlated systems: analysis based on a pseudo-spectrum

Authors: Tsuneya Yoshida

Abstract: We analyze a correlated system in equilibrium with special emphasis on non-Hermitian topology inducing a skin effect. The pseudo-spectrum, computed by the real-space dynamical mean-field theory, elucidates that additional pseudo-eigenstates emerge for the open boundary condition in contrast to the dependence of the density of states on the boundary condition… ▽ More We analyze a correlated system in equilibrium with special emphasis on non-Hermitian topology inducing a skin effect. The pseudo-spectrum, computed by the real-space dynamical mean-field theory, elucidates that additional pseudo-eigenstates emerge for the open boundary condition in contrast to the dependence of the density of states on the boundary condition. We further discuss how the line-gap topology, another type of non-Hermitian topology, affects the pseudo-spectrum. Our numerical simulation clarifies that while the damping of the quasi-particles induces the non-trivial point-gap topology, it destroys the non-trivial line-gap topology. The above two effects are also reflected in the temperature dependence of the local pseudo-spectral weight. △ Less

Submitted 10 November, 2020; v1 submitted 9 November, 2020; originally announced November 2020.

Comments: 8 pages, 3 figures. References are updated

Journal ref: Phys. Rev. B 103, 125145 (2021)

Point-gap topology with complete bulk-boundary correspondence in dissipative quantum systems

Authors: Jian-Song Pan, Linhu Li, Jiangbin Gong

Abstract: The spectral and dynamical properties of dissipative quantum systems, as modeled by a damped oscillator in the Fock space, are investigated from a topological point of view. Unlike a physical lattice system that is naturally under the open boundary condition, the bounded-from-below nature of the Fock space offers a unique setting for understanding and verify… ▽ More The spectral and dynamical properties of dissipative quantum systems, as modeled by a damped oscillator in the Fock space, are investigated from a topological point of view. Unlike a physical lattice system that is naturally under the open boundary condition, the bounded-from-below nature of the Fock space offers a unique setting for understanding and verifying non-Hermitian skin modes under semi-infinity boundary conditions that are elusive in actual physical lattices. A topological characterization based on the complex spectra of the Liouvillian superoperator is proposed and the associated complete set of topologically protected skin modes can be identified, thus reflecting the complete bulk-boundary correspondence of point-gap topology generally absent in realistic materials. Moreover, we discover anomalous skin modes with exponential amplification even though the quantum system is purely dissipative. Our results indicate that current studies of non-Hermitian topological matter can greatly benefit research on quantum open systems and vice versa. △ Less

Submitted 28 October, 2020; originally announced October 2020.

Comments: 9 pages and 3 figures (including supplementary material)

Journal ref: Phys. Rev. B 103, 205425 (2021)

Tilted black-Si: ~0.45 form-birefringence from sub-wavelength needles

Authors: Darius Gailevicius, Meguya Ryu, Reo Honda, Stefan Lundgaard, Tai Suzuki, Jovan Maksimovic, Jingwen Hu, Denver P. Linklater, Elena P. Ivanova, Tomas Katkus, Vijayakumar Anand, Mangirdas Malinauskas, Yoshiaki Nishijima, Mangirdas Malinauskas, Vijayakumar Anand, Soon Hock Ng, Kestutis Staliunas, Junko Morikawa, Saulius Juodkazis

Abstract: …m width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si $\simλ/4$ for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is… ▽ More The self-organised conical needles produced by plasma etching of silicon (Si), known as black silicon (b-Si), create a form-birefringent surface texture when etching of Si orientated at angles of $θ_i < 50 - 70^\circ$ (angle between the Si surface and vertical plasma E-field). The height of the needles in the form-birefringent region following 15~min etching was $d\sim 200$ nm and had a 100 $μ$m width of the optical retardance/birefringence, characterised using polariscopy. The height of the b-Si needles corresponds closely to the skin-depth of Si $\simλ/4$ for the visible spectral range. Reflection-type polariscope with a voltage-controlled liquid-crystal retarder is proposed to directly measure the retardance $Δn\times d/λ\approx 0.15$ of the region with tilted b-Si needles. The quantified form birefringence of $Δn = - 0.45$ over $λ= 400-700$~nm spectral window was obtained. Such high values of $Δn$ at visible wavelengths can only be observed in the most birefringence calcite or barium borate as well as in liquid crystals. The replication of b-Si into Ni-shim with high fidelity was also demonstrated and can be used for imprinting of the b-Si nanopattern into other materials. △ Less

Submitted 31 March, 2020; originally announced April 2020.

Comments: 14 pages, 13 figures

BioFaceNet: Deep Biophysical Face Image Interpretation

Authors: Sarah Alotaibi, William Smith

Abstract: …we present BioFaceNet, a deep CNN that learns to decompose a single face image into biophysical parameters maps, diffuse and specular shading maps as well as estimating the spectral power distribution of the scene illuminant and the… ▽ More In this paper we present BioFaceNet, a deep CNN that learns to decompose a single face image into biophysical parameters maps, diffuse and specular shading maps as well as estimating the spectral power distribution of the scene illuminant and the spectral sensitivity of the camera. The network comprises a fully convolutional encoder for estimating the spatial maps with a fully connected branch for estimating the vector quantities. The network is trained using a self-supervised appearance loss computed via a model-based decoder. The task is highly underconstrained so we impose a number of model-based priors. Skin spectral reflectance is restricted to a biophysical model, we impose a statistical prior on camera spectral sensitivities, a physical constraint on illumination spectra, a sparsity prior on specular reflections and direct supervision on diffuse shading using a rough shape proxy. We show convincing qualitative results on in-the-wild data and introduce a benchmark for quantitative evaluation on this new task. △ Less

Submitted 13 September, 2019; v1 submitted 28 August, 2019; originally announced August 2019.

Comments: Accepted to BMVC2019

Decomposing multispectral face images into diffuse and specular shading and biophysical parameters

Authors: Sarah Alotaibi, William A. P. Smith

Abstract: We propose a novel biophysical and dichromatic reflectance model that efficiently characterises spectral skin reflectance. We show how to fit the model to multispectral face images enabling high quality estimation of diffuse and specular sh… ▽ More We propose a novel biophysical and dichromatic reflectance model that efficiently characterises spectral skin reflectance. We show how to fit the model to multispectral face images enabling high quality estimation of diffuse and specular shading as well as biophysical parameter maps (melanin and haemoglobin). Our method works from a single image without requiring complex controlled lighting setups yet provides quantitatively accurate reconstructions and qualitatively convincing decomposition and editing. △ Less

Submitted 18 February, 2019; originally announced February 2019.

A Novel Light Source Design for Spectral Tuning in Biomedical Imaging

Authors: Chandrajit Basu, Sebastian Schlangen, Merve Meinhardt-Wollweber, Bernhard Roth

Abstract: We propose a novel architecture with a remote phosphor based modular and compact light source in a non-contact dermoscope prototype for skin cancer screening. The spectrum and color temperature of the output light can easily and significantly be changed depending on… ▽ More We propose a novel architecture with a remote phosphor based modular and compact light source in a non-contact dermoscope prototype for skin cancer screening. The spectrum and color temperature of the output light can easily and significantly be changed depending on spectral absorption characteristics of the tissues being imaged. The new system has several advantages compared to state-of-the-art phosphor converted ultra-bright white LEDs, used in a wide range of medical imaging devices, which have a fixed spectrum and color temperature at a given operating point. In particular, the system can more easily be adapted to the requirements originating from different tissues in the human body which have wavelength dependent absorption and reflectivity. This leads to improved contrast for different kinds of imaged tissue components. The concept of such a lighting architecture can be vastly utilized in many other medical imaging devices including endoscopic systems. △ Less

Submitted 4 June, 2014; originally announced June 2014.

Comments: 10 pages, 8 figures

Report number: Basu_June_2014

Reprocessing of X-rays in the outer accretion disc of the black hole binary XTE J1817--330

Authors: Marek Gierlinski, Chris Done, Kim Page

Abstract: …3 in the soft state, which should produce a marked change in the reprocessing efficiency. The easiest way to match the observed constant reprocessed fraction is for the disc skin to be highly ionized (as suggested 30 years ago by van Paradijs), so that the bulk of the disc flux is… ▽ More We build a simple model of the optical/UV emission from irradiation of the outer disc by the inner disc and coronal emission in black hole binaries. We apply this to the broadband Swift data from the outburst of the black hole binary XTE J1817--330 to confirm previous results that the optical/UV emission in the soft state is consistent with a reprocessing a constant fraction of the bolometric X-ray luminosity. However, this is very surprising as the disc temperature drops by more than a factor 3 in the soft state, which should produce a marked change in the reprocessing efficiency. The easiest way to match the observed constant reprocessed fraction is for the disc skin to be highly ionized (as suggested 30 years ago by van Paradijs), so that the bulk of the disc flux is reflected and only the hardest X-rays heat the disc. The constant reprocessed fraction also favours direct illumination of the disc over a scattering origin as the optical depth/solid angle of any scattering material (wind/corona) over the disc should decrease as the source luminosity declines. By contrast, the reprocessed fraction increases very significantly (by a factor ~6) as the source enters the hard state. This dramatic change is not evident from X-ray/UV flux correlations as it is masked by bandpass effects. However, it does not necessarily signal a change in emission e.g. the emergence of the jet dominating the optical/UV flux as the reflection albedo must change with the dramatic change in spectral shape. △ Less

Submitted 6 November, 2008; v1 submitted 29 August, 2008; originally announced August 2008.

Comments: Accepted by MNRAS, this version has a new title

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

Authors: A. Janiuk, B. Czerny, G. M. Madejski

Abstract: …which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emissi… ▽ More We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class. △ Less

Submitted 21 April, 2001; originally announced April 2001.

Comments: 22 pages, 10 figures, accepted to ApJ

Journal ref: Astrophys.J. 557 (2001) 408-420

Observational signatures of X-ray irradiated accretion disks

Authors: C. Done, S. Nayakshin

Abstract: Reflection of X-rays from cool material around a black hole is one of the few observational diagnostics of the accretion flow geometry. Models of this… ▽ More Reflection of X-rays from cool material around a black hole is one of the few observational diagnostics of the accretion flow geometry. Models of this reflected spectrum generally assume that the accretion disk can be characterized by material in a single ionization state. However, several authors have recently stressed the importance of the classic ionization instability for X-ray irradiated gas in hydrostatic balance. This instability leads to a discontinuous transition in the vertical structure of the disk, resulting in a hot ionized skin above much cooler material. If the Compton temperature of the skin is high then even iron is completely ionized, and the skin does not produce any spectral features. These new models, where the ionization structure of the disk is calculated self-consistently, require an excessive amount of computing power and so are difficult to use in directly fitting observed X-ray spectra. Instead, we invert the problem by simulating X-ray spectra produced by the new reflection models, and then fit these with the old, single zone reflection models, to assess the extent to which the derived accretion geometry depends on the reflection model used. We find that the single zone ionization models can severely underestimate the covering fraction of the ``cold'' material as seen from the X-ray source if the optical depth in the ionized skin is of order unity, and that this can produce an apparent correlation between the covering fraction and the X-ray spectral index similar in nature to that reported by Zdziarski, Lubinski and Smith (1999). △ Less

Submitted 18 August, 2000; originally announced August 2000.

Comments: 13 pages, 5 figures, accepted for publication in ApJ

On the X-ray heated skin of Accretion Disks

Authors: Sergei Nayakshin

Abstract: We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretion disks valid at any radius and for a broad range of… ▽ More We present a simple analytical formula for the Thomson depth of the X-ray heated skin of accretion disks valid at any radius and for a broad range of spectral indices of the incident X-rays, accretion rates and black hole masses. We expect that this formula may find useful applications in studies of geometry of the inner part of accretion flows around compact objects, and in several other astrophysically important problems, such as the recently observed X-ray ``Baldwin'' effect (i.e., monotonic decrease of Fe line's equivalent width with the X-ray luminosity of AGN), the problem of missing Lyman edge in AGN, and line and continuum variability studies in accretion disks around compact objects. We compute the reflected X-ray spectra for several representative cases and show that for hard X-ray spectra and large ionizing fluxes the skin represents a perfect mirror that does not produce any Fe lines or absorption features. At the same time, for soft X-ray spectra or small ionizing fluxes, the skin produces very strong ionized absorption edge and highly ionized Fe lines that should be observable in the reflected spectra. △ Less

Submitted 21 December, 1999; originally announced December 1999.

Comments: Accepted to ApJ; 7 pages including 3 figures

Journal ref: AIP Conf.Proc.510:440-444,2004