Angular Reflectance Research Articles

Ratio of physical model parameters can retrieve aggregate size from different types of soil in cultivated regions

Soil aggregate size, which is a proxy used to guide agricultural decisions and tillage management, can be estimated using optical remote sensing techniques. However, limited investigation has been conducted into the potential of using a physical model to retrieve soil aggregate size (< 2 mm) from different types of soil. This study is based on the multi-angular spectral measurements of seven soil samples from five soil types with 14 aggregate size fractions collected in Northeast China, three versions of the Hapke model were inverted using the Bayesian method. The findings confirmed that all three versions of the Hapke model can well characterize the angular reflection characteristics of all soil samples with different aggregate sizes. The inverted photometric parameters such as single scattering albedo ω, shape parameter b, and asymmetry parameter c were found to be sensitive to soil aggregate size, but the relationships rely on soil types because of the dependence of parameters related to soil composition. In order to obtain a general model that can be applied to different types of soil, the ratio of parameters (RoP) = (b + c)/ω, which is controlled by the external structure of soil aggregates, was proposed to retrieve the aggregate size from different soil types. Results show that the RoP can robustly capture the aggregate size of the soil with high accuracy and is insensitive to soil types. The combination of photometric parameters related to soil aggregate size provides a new method for retrieving the structural properties of the soil by eliminating interfering factors.

Conformal Antireflective Multilayers for High‐Numerical‐Aperture Deep‐Ultraviolet Lenses

AbstractPrecise surface reflectance control at specific deep‐ultraviolet (DUV) wavelengths across wide angles is crucial for semiconductor inspection and lithography tools. The inherent challenges in designing DUV antireflective multilayers stem from limited transparent materials and the resultant fabrication complexity owing to numerous interfaces. Here, wide‐angle antireflective multilayers finely tuned to 248 nm designed using an active learning scheme is presented. The active learning scheme employing factorization machines (FM) identifies the optimal configurations for binary‐material‐based multilayers (AlF3/LaF3, AlF3/MgF2, and AlF3/Al2O3) with varying index contrasts, achieving minimal figure‐of‐merit (i.e., average angular reflectance) values at predetermined total thicknesses. High‐index‐contrast AlF3/Al2O3 multilayers are fabricated via atomic layer deposition, thus enabling the conformal coating of high‐numerical‐aperture (NA) lenses with atomic precision. An optimized AlF3/Al2O3 tri‐layer with a total thickness of 180 nm results in an average (0°–45°) reflectance of 0.4% on a CaF2 planar substrate and 0.6% on a CaF2 convex lens (NA = 0.47), similar to the performance of an ideal single‐layer coating requiring a practically unavailable refractive index. Phasor analysis, which considers only first‐order reflections between adjacent layers, supports the benefits of high‐index‐contrast binary materials and the use of the FM‐based active learning scheme in antireflective multilayer design.

Retro-reflective plaster coatings after outdoor aging and soiling: an in-lab optical performance characterization

The potential benefits of highly reflective and retro-reflective (RR) materials on urban context have been performed by several studies through their optical characterization. During their lifetime, outdoor aging and soiling can affect the RR optical behaviour. To this aim, this study investigates the performance of RR plaster coatings after outdoor aging and soiling from May to October 2022 in Perugia, Italy. The same RR plaster coatings in pristine conditions were already characterized in lab in terms of optical performance. In particular, RR samples obtained by using glass beads with different diameters (i.e., 40 ÷ 70 μm and 70 ÷ 110 μm) distributed in three superficial density ranges (i.e., 0.30 ÷ 0.40 kg/m2, 0.20 ÷ 0.30 kg/m2 and 0.10 ÷ 0.20 kg/m2) have been investigated in this study through spectrophotometric and angular reflectivity analyses. In all cases, the post-aging RR samples exhibit lower global reflectance values with respect to the same RR samples in pristine condition. Concerning the angular reflection distribution analysis, a stronger relative RR component was found for the post-aging RR sample with an average diameter of 70 ÷ 110 µm and superficial density distribution equal to 0.10 ÷ 0.20 kg/m2 for each investigated incident direction.

Laser-Induced Backward Transfer of Light Reflecting Zinc Patterns on Glass for High Performance Photovoltaic Modules.

Commercially available photovoltaic (PV) modules typically consist of individual silicon half-cut cells that are electrically interconnected. This interconnection method results in gaps between the cells, which do not contribute to the overall PV output power. One approach to enhance the cell-to-module power ratio is the placement of white, diffuse reflecting plastic material within these gaps. Conventionally, the process of generating reflective patterns involves several discrete steps, including film deposition, resist patterning, etching, and resist stripping. This study presents an innovative single-step procedure for the direct deposition of zinc reflective patterns onto glass substrates using laser-induced backward transfer (LIBT) and a nanosecond pulsed laser system. The process successfully produced lines and squares, demonstrating its versatility in achieving diverse geometric patterns under ambient atmospheric pressure and room temperature conditions. The evaluation of the transferred patterns included an examination of geometric dimensions and surface morphology using a 3D microscope and scanning electron microscopy (SEM) analysis at the air/Zn interface. Additionally, the thickness of the zinc film and its adhesion to the glass substrate were quantified. The angular reflectance at a wavelength of 660 nm for both the glass/Zn and air/Zn interfaces was measured.

Three-dimensional angular deviation and diffraction efficiency of a grating in Littrow-configuration ECDL

We consider in this paper the angular deviation and diffraction efficiency of the reflection gratings in Littrow-configuration for applications of external cavity diode laser using the rigorous coupled-wave analysis method. We consider the three-dimensional diffraction case in general, where the incidence plane is un-parallel with the grating vector, i.e. conical diffraction. The angular tolerance of arbitrary gratings under plane and conical diffraction are thus derived and presented. A typical blazed grating is chosen as an example to calculate its diffraction efficiency using the rigorous coupled-wave analysis method. Furthermore, we point out that the angular tolerance and reflection efficiency can be improved if the appropriate parameter settings are selected for Littrow-configuration devices, including incidence angle, diffraction order, grating period and blazed angle. Otherwise, a tiny slanting angle of the grating vector deviated from the incidence plane will deviate the feedback light away from entering the laser-diode-chip and halt laser oscillation in the external cavity. Finally, a general rule for the parameter settings in Littrow-configuration is provided as a benchmark.

Proposal of Technique for Analysis of Complementary Frequency Selective Surfaces

AbstractA specific class of frequency selective surface (FSS) is the complementary frequency selective surface (CFSS) that has interesting characteristics such as high angular stability, multiple transmission and/or reflection bands, and the possibility of miniaturization. The analysis and design of this sort of structure are commonly performed using commercial software, which demands a high computational effort, impacting a longer optimization time. The equivalent circuit model combined with a cascading technique emerges as an alternative method to the use of these softwares, in the optimization of the physical dimensions of these structures, as they model the behavior of a CFSS with low computational effort and optimization time, in addition to being able to be implemented in various programming languages. Thus, this work proposes a CFSS analysis technique that combines the equivalent circuit method with the ABCD matrix. To the best of our knowledge, this is the first reported research on approximate techniques for CFSS analysis. The chosen geometry was the circular ring, due to high angular stability and polarization independence. The modeling of the equivalent circuit for patch and aperture geometries is presented. Some structures are simulated, and the results are compared with results obtained with the HFSS software. Finally, two prototypes are built to validate the analyses performed.

Real-time hybrid angular-interrogation surface plasmon resonance sensor in the near-infrared region for wide dynamic range refractive index sensing

Herein, we integrated angle-scanning surface plasmon resonance (SPR) and angle-fixed SPR as a hybrid angular-interrogation SPR to enhance the sensing performance. Galvanometer-mirror-based beam angle scanning achieves a 100-Hz acquisition rate of both the angular SPR reflectance spectrum and the angle-fixed SPR reflectance, whereas the use of near-infrared light enhances the refractive index (RI) sensitivity, range, and precision compared with visible light. Simultaneous measurement of the angular SPR reflectance spectrum and angle-fixed SPR reflectance boosts the RI change range, RI resolution, and RI accuracy to 10–1–10–6 RIU, 2.24 × 10−6 RIU, and 5.22 × 10−6 RIU, respectively. The proposed hybrid SPR is a powerful tool for wide-dynamic-range RI sensing with various applications.

From design to realization of high diffraction efficiency Littrow configuration diffraction gratings based on multilayer dielectric mirrors patterned with electron beam lithography

Diffraction efficiency (DE) and laser-induced damage threshold of Chirped pulse amplification (CPA) system diffraction gratings (DGs) are one of the main limiting factors for the anticipated progress of high peak power lasers. Optimization of a high reflectivity dielectric multilayer ZrO2/SiO2 stack mirror with a DG etched in the top SiO2 layer was carried out employing the Rigorous coupled wave analysis method. Two families of solutions possessing DE > 0.99 over the bandwidth of the 1030 nm wavelength ultrashort laser and withstanding different fabrication margins were identified for the Littrow configuration. DGs were produced and verified experimentally. Structures permitting variation in both the depth of the DG and filling factor without loss in DE were obtained for the 690 nm thick SiO2 layer. While the 770 nm thick layer permitted separation of the two parameters requiring either rigorous control of the DG depth or filling factor. DG realization technology was proposed employing electron beam lithography and inductively coupled plasma reactive ion etching. Measurements of the fabricated structures demonstrated 0.91 DE for the first diffraction order over the 60 nm bandwidth and under a 6° mounting error from the Littrow configuration. Spectral angular reflectance DE simulations and respective measurements indicated quantitative agreement. The work provides an integrated overview of high DE periodic structure numerical optimization and manufacturing roadmap which could be applied for the high-peak-power laser CPA system development.

Full-angle high-reflection ultrathin composite film realizing high spatial resolution of scintillation crystal array

The performance of current nuclear medicine imaging systems is largely limited by the performance of detectors, and high spatial resolution detectors require high optical yield scintillator arrays. In this work, we simulated and designed for the first time a distributed Bragg reflector (multilayer dielectric film) that covers the entire lutetium yttrium oxyorthosilicate emission spectral band and consists of three 1/4 wavelength (λ/4) primary film systems centered at 420, 500, and 575 nm. In order to achieve ultrahigh reflectivity at the full incidence angle of the scintillator emitting surface, we propose a master optical configuration combining the dielectric film with a metal film/diffuse reflection adhesive. To explain this mechanism, we also simulated the change in reflectivity of the actual inner surface light collection. Experimental results show that a combination of a highly reflective reflector can achieve full-angle high reflectance at the total angle of incidence. We find that the dielectric film does not change the total reflection structure inside the crystal, while the light-blocking layer changes and increases the angular reflection of the dielectric film about the angle. These findings provide important insights into surface treatment as well as the design of scintillation crystal arrays, with far-reaching implications for high spatial resolution optical imaging systems.

Video-based Elevated Skin Temperature Detection.

In this work, we propose a non-contact video-based approach that detects when an individual's skin temperature is elevated beyond the normal range. The detection of elevated skin temperature is critical as a diagnostic tool to infer the presence of an infection or an abnormal health condition. Detection of elevated skin temperature is typically achieved using contact thermometers or non-contact infrared-based sensors. The ubiquity of video data acquisition devices such as mobile phones and computers motivates the development of a binary classification approach, the Video-based TEMPerature (V-TEMP) to classify subjects with non-elevated/elevated skin temperature. We leverage the correlation between the skin temperature and the angular reflectance distribution of light, to empirically differentiate between skin at non-elevated temperature and skin at elevated temperature. We demonstrate the uniqueness of this correlation by 1) revealing the existence of a difference in the angular reflectance distribution of light from skin-like and non-skin like material and 2) exploring the consistency of the angular reflectance distribution of light in materials exhibiting optical properties similar to human skin. Finally, we demonstrate the robustness of V-TEMP by evaluating the efficacy of elevated skin temperature detection on subject videos recorded in 1) laboratory controlled environments and 2) outside-the-lab environments. V-TEMP is beneficial in two ways; (1) it is non-contact-based, reducing the possibility of infection due to contact and (2) it is scalable, given the ubiquity of video-recording devices.

Common-mode plasmon sensing scheme as a high-sensitivity compact SPR sensor.

A deep metal grating enables quasi-phase-matched simultaneous excitation of two counterpropagating surface plasmon modes by means of its +1st and -2nd diffraction orders. The resulting angular reflection spectra of the scattered -1st and zeroth orders exhibit three interleaved zeros and maxima in a range centered around the Littrow angle. The spectra differ thoroughly from the usual reflection dip resulting from single-order plasmon coupling that produces strong absorption. The zeroth and -1st orders exhibit two crossing angles enabling high-sensitivity common-mode detection schemes designed to reject variations in source power and environmental noise. The proof of concept and experimental assessment of this new surface plasmon resonance (SPR) sensing scheme are demonstrated by monitoring gases in a pressure-controlled chamber. A limit of detection (LOD) of 2 × 10-7 refractive index unit (RIU) was achieved.

The impact of outdoor aging and soiling on the optic features of glass beads retro-reflective coatings

The latest research suggests the use of retro-reflective (RR) materials as innovative coating solutions for building envelopes and urban surfaces. This study aims at investigating the performance of RR materials after outdoor aging and soiling during summer 2021 in Perugia, Italy. Different glass beads RR coatings were realized with three substrate typologies, characterized by various roughness, and they were investigated in terms of their optic performance. Also diffusive (DIFF) coatings were made for comparison purposes. Variations in terms of solar reflectance and relative angular reflection distribution were compared with the same coatings in pre-aging conditions. Pre-aging optic data were presented and discussed in a previous work by the Authors. In all cases, the aged samples exhibit lower global reflectance values compared to their original conditions: the RR coating over a smooth pine wood substrate shows the highest reduction value equal to 14.4%. A stronger relative RR component was found for all the RR samples in post-aging conditions. Future developments may involve the identification of an optimal protective layer to be applied on RR materials to prevent the detachment of glass beads in RR post-aging coatings.

Shadow is related to roughness but MODIS BRDF should not be used to estimate lateral cover

A simple geometric-optical model is used to show that shadow in optical remote sensing imagery contains information about surface roughness. The use of MODIS albedo products for the purpose of estimating surface roughness so that these products may aid in dust emission modeling has been proposed in previous studies (Chappell et al. 2010; Chappell and Webb 2016; Chappell et al. 2018). This approach is evaluated through geometric-optical modeling and by analysis of existing MODIS data. Little support is found for the utility of this approach for dust emission modeling. However, because there is clearly information about surface roughness in optical satellite imagery, some suggestions for the proper use of optical data to estimate surface roughness are made.Chappell, A., Van Pelt, S., Zobeck, T., & Dong, Z. (2010). Estimating aerodynamic resistance of rough surfaces using angular reflectance. Remote Sensing of Environment, 114, 1462–1470.Chappell, A., & Webb, N.P. (2016). Using albedo to reform wind erosion modeling, mapping and monitoring. Aeolian Research, 23, 63–78.Chappell, A., Webb, N.P., Guerschman, J.P., Thomas, D.T., Mata, G., Handcock, R.N., Leys, J.F., & Butler, H.J. (2018). Improving ground cover monitoring for wind erosion assessment using MODIS BRDF parameters. Remote Sensing of Environment, 204, 756–768.

Wide-angle deep ultraviolet antireflective multilayers via discrete-to-continuous optimization

Abstract To date, various optimization algorithms have been used to design non-intuitive photonic structures with unconventional optical performance. Good training datasets facilitate the optimization process, particularly when an objective function has a non-convex shape containing multiple local optima in a continuous parametric space. Herein, we developed a discrete-to-continuous optimization algorithm and confirmed its validity by designing and fabricating deep-ultraviolet antireflective MgF2/LaF3 multilayers. For discrete optimization, a multilayer was encoded into a binary vector with multiple bits; a 10 nm thick MgF2 or LaF3 layer was assigned a binary digit of 0 or 1, respectively. Using the binary-based training datasets, a factorization machine formulated a surrogate function, which discovered the ground binary vector representing a near-optimal figure of merit. Then, the figure of merit was refined through continuous optimization (e.g., using an interior-point method) of the ground binary vector. MgF2/LaF3 multilayers with a variety of bit levels were created to attain a minimum average angular (0°–45°) reflectance at 193 nm. A MgF2/LaF3 multilayer optimized at ten bits (i.e., a total thickness of approximately 100 nm) yielded an average reflectance of 0.2%, which agreed well with the experimental results. Moreover, an integrated ray-wave optics simulation predicted that a single CaF2 plano-convex lens coated with the optimized multilayer could exhibit a transmittance of 99.7%. The developed optimization approach will be widely applicable to any photonic structures that can represent a binary vector with multiple bits, such as microwave metasurfaces, in addition to being useful for producing ideal optical multilayers.

MART-soil: A modified analytical radiative transfer mode for simulating multi-angular reflection of soils with different particle size

Understanding the angular reflection properties of soils and quantifying their particle size distribution through physical models is valuable for ecological and agricultural applications in arid and semi-arid regions. An analytical radiative transfer (ART) model has considered the absorptivity, k, as a constant to simulate the angular reflection of soil at two measurement directions and invert the soil particle size. However, the limited viewing directions cannot reflect the key role of ART in characterizing the effects of particle size on angular reflection. In this study, an ART model was first assessed based on a multi-angular soil reflection dataset from six types of dry soil samples with different particle size. The absorptivity, k, an inverted model parameter, was found to be strongly dependent on the angular distribution of soil reflection for different soil types. Subsequently, we coupled the ART model with a semiempirical function that can characterize the angular distribution of k, to improve the model’s ability in simulating the multi-angular reflection of soil. The modified ART model (defined as MART-Soil) made it possible to generate satisfying simulated multi-angular soil reflections with different particle sizes from all six soil types. Moreover, the model parameters of MART-Soil have the potential for representing different soil types in the simulation. These results indicate that MART-Soil model can quantify angular reflection properties of soils with different particle size and may also be useful for characterizing the radiative transfer process in land surfaces.

Mapping Arctic Sea-Ice Surface Roughness with Multi-Angle Imaging SpectroRadiometer

Sea-ice surface roughness (SIR) is a crucial parameter in climate and oceanographic studies, constraining momentum transfer between the atmosphere and ocean, providing preconditioning for summer-melt pond extent, and being related to ice age and thickness. High-resolution roughness estimates from airborne laser measurements are limited in spatial and temporal coverage while pan-Arctic satellite roughness does not extend over multi-decadal timescales. Launched on the Terra satellite in 1999, the NASA Multi-angle Imaging SpectroRadiometer (MISR) instrument acquires optical imagery from nine near-simultaneous camera view zenith angles. Extending on previous work to model surface roughness from specular anisotropy, a training dataset of cloud-free angular reflectance signatures and surface roughness, defined as the standard deviation of the within-pixel lidar elevations, from near-coincident operation IceBridge (OIB) airborne laser data is generated and is modelled using support vector regression (SVR) with a radial basis function (RBF) kernel selected. Blocked k-fold cross-validation is implemented to tune hyperparameters using grid optimisation and to assess model performance, with an R2 (coefficient of determination) of 0.43 and MAE (mean absolute error) of 0.041 m. Product performance is assessed through independent validation by comparison with unseen similarly generated surface-roughness characterisations from pre-IceBridge missions (Pearson’s r averaged over six scenes, r = 0.58, p &lt; 0.005), and with AWI CS2-SMOS sea-ice thickness (Spearman’s rank, rs = 0.66, p &lt; 0.001), a known roughness proxy. We present a derived sea-ice roughness product at 1.1 km resolution (2000–2020) over the seasonal period of OIB operation and a corresponding time-series analysis. Both our instantaneous swaths and pan-Arctic monthly mosaics show considerable potential in detecting surface-ice characteristics such as deformed rough ice, thin refrozen leads, and polynyas.

Optical properties and thermal degradation of NbB2/Nb(BNO)/Al2O3 spectrally selective tandem absorber

The development of any new ceramic absorber demands the evaluation of different performance-limiting properties. In a recently published paper from our research group, we reported the development of NbB2-based new tandem absorber. In our efforts to investigate further potentiality for concentrated solar power applications, this study reports the optical and angular dependent reflectance properties of the NbB2/Nb(BNO)/Al2O3 spectrally selective absorber. The optical properties (extinction coefficient and refractive index) and the dielectric constants of the coatings were determined using spectroscopic ellipsometry analysis. Such analysis together with SCOUT simulation of the reflectance spectrum, confirms the refractive index gradient from the substrate to the top layer. The angular-dependent reflectance measurement demonstrates good solar absorptance (from 8 to 48°). More importantly, the coatings exhibited good thermal stability up to 200 h in vacuum at 500 °C. Extensive transmission electron microscopy analysis reveals the traces of oxygen in the layers. The moderate decrease in absorptance can be rationalised due to the compaction and formation of an oxide layer. Based on the obtained results and earlier published studies, the potential of the NbB2-based coatings as solar selective absorber for concentrated solar power applications is established.

Reflectance Transformation Imaging Visual Saliency: Local and Global Approaches for Visual Inspection of Engineered Surfaces

Reflectance Transformation Imaging (RTI) is a non-contact technique which consists in acquiring a set of multi-light images by varying the direction of the illumination source on a scene or a surface. This technique provides access to a wide variety of local surface attributes which describe the angular reflectance of surfaces as well as their local microgeometry (stereo photometric approach). In the context of the inspection of the visual quality of surfaces, an essential issue is to be able to estimate the local visual saliency of the inspected surfaces from the often-voluminous acquired RTI data in order to quantitatively evaluate the local appearance properties of a surface. In this work, a multi-scale and multi-level methodology is proposed and the approach is extended to allow for the global comparison of different surface roughnesses in terms of their visual properties. The methodology is applied on different industrial surfaces, and the results show that the visual saliency maps thus obtained allow an objective quantitative evaluation of the local and global visual properties on the inspected surfaces.

An investigation towards the optimum design of retro-reflective materials as building envelopes for the enhancement of optical performance

Retro-reflective (RR) materials may represent an advanced solution for building envelopes, as they could contribute to Urban Heat Island mitigation. This study could bring novelty to the current state of the art on this topic, as it proposes to investigate the optimum design of glass beads RR materials for building application. To this aim, the impact of glass beads superficial density and diameter on the optical performance of RR materials has been further explored. In particular, 15 RR glass beads samples were developed, using a highly reflective paint as substrate and varying both glass beads superficial density and diameter. Also a diffusive sample, without RR glass beads, was made for comparison. Three different glass beads superficial density ranges and five diameter ranges were investigated. Samples were tested through spectrophotometric and angular reflection distribution analysis. All RR samples exhibited an overall lower solar reflectance compared to the diffusive sample, whereas RR samples with 200 ÷ 300 µm diameter exhibited the highest RR capability. By reducing glass beads superficial density, generally a reduction of RR performance was observed.

Instructive errors of Bouguer, Lambert, and Arago in the first determinations of angular reflectances on flat surfaces: discussion.

The first photometric measurements performed in the eighteenth century were based on brightness matching between two illuminated surfaces. In 1760, Bouguer and Lambert proposed the first methods to measure the angular reflectance of a flat surface, and Arago proposed a third one in the mid-nineteenth century. These pioneering experiments provided rather good estimates of the values we can predict or measure much more accurately today, considering that the human visual system was the only available light detector at that time. We show that the errors made in their measurements come not only from experimental uncertainties but also from incomplete knowledge of the physical properties of light, leading to incorrect assumptions in their models. The main errors are (i) the fact that light is totally reflected at grazing incidence, (ii) the glass plates they used were not perfectly clear, and (iii) light is partially polarized after transmission across the surface. By highlighting the impact of these three errors, we can better understand the state of knowledge in optics at that time and question our current practices in radiometric measurements and calculations.