Low Surface Reflectance Research Articles

Design and implementation of a Cube satellite mission for Antarctic glacier and sea ice observation

The research for global climate changes calls for high quality satellite data and imageries regarding the Polar Regions. In recent years, the emerging Earth-Observation micro/nano satellite technology provides new data sources for polar region observations. The STU-2A, also named TW-1A, is such a nano satellite designed for polar region observation activities. It is a 3U CubeSat of 2.9 kg with a size of 30 × 10 × 10 cm carrying an Earth observation camera, launched into a Sun Synchronous Orbit (SSO) at 481 km with an inclination of 97.3°, on September 25, 2015. During the Antarctic summer of 2015/16, it has acquired visible-light true color images with a resolution of 94 m, covering different sea and coastal regions including Amundsen Sea, Ross Sea and Vincennes Bay. These images were used to analyze the change of glacier and sea ice, compared and calibrated with reference to the publically available MODIS images with a resolution of 250 m. As the camera was specially designed for the Polar regions which have an environment of low solar elevation angle and high surface reflectance, it eliminates the oversaturation problem of the MODIS sensors and can provide high quality images. Based on data analysis and assessment, it is confirmed that this satellite data can meet the demand of glacier and sea ice observation. This paper presents the Cubesat system design and configuration, the payload camera design, and its application in Antarctic glacier and sea ice observation.

Study on silicon nanopillars with ultralow broadband reflectivity via maskless reactive ion etching at room temperature

Nanostructured silicon with low surface reflectivity has important application prospects in a wide range. We fabricate crystalline silicon nanopillars with ultralow broadband reflectivity via maskless reactive ion etching at room temperature. The measurements show that the silicon nanopillars are in nano-scale diameter and micron-scale height with tip ends, which show extremely powerful antireflection capability. The silicon nanopillars with the heights of 5μm and 7μm demonstrate ultralow weighted average reflectivity of 0.80% and 0.40%, respectively, in a wide wavelength range of 300–1030nm. Additional SiNx coating reduces the reflectivity to 0.09%. The two-dimensional weighted average reflectivity as function of angle of incidence and wavelength is measured, which shows low sensitivity to angle of incidence in the range of 0°–70°. The calculated wavelength dependent reflectance is in good agreement with the measured result in the range of 300–1030nm.

Investigation of Simultaneous Effects of Aerosol Properties and Aerosol Peak Height on the Air Mass Factors for Space-Borne NO2 Retrievals

We investigate the simultaneous effects of aerosol peak height (APH), aerosol properties, measurement geometry, and other factors on the air mass factor for NO2 retrieval at sites with high NO2 concentration. A comparison of the effects of high and low surface reflectance reveals that NO2 air mass factor (AMF) values over a snowy surface (surface reflectance 0.8) are generally higher than those over a deciduous forest surface (surface reflectance 0.05). Under high aerosol optical depth (AOD) conditions, the aerosol shielding effect over a high-albedo surface is revealed to reduce the path-length of light at the surface, whereas high single scattering albedo (SSA) conditions (e.g., SSA = 0.95) lead to an increase in the aerosol albedo effect, which results in an increased AMF over areas with low surface reflectance. We also conducted an in-depth study of the APH effect on AMF. For an AOD of 0.1 and half width (HW) of 5 km, NO2 AMF decreases by 29% from 1.36 to 0.96 as APH changes from 0 to 2 km. In the case of high-AOD conditions (0.9) and HW of 5 km, the NO2 AMF decreases by 240% from 1.85 to 0.54 as APH changes from 0 to 2 km. The AMF variation due to error in the model input parameters (e.g., AOD, SSA, aerosol shape, and APH) is also examined. When APH is 0 km with an AOD of 0.4, SSA of 0.88, and surface reflectance of 0.05, a 30% error in AOD induces an AMF error of between 4.85% and −3.67%, an SSA error of 0.04 leads to NO2 VCD errors of between 4.46% and −4.77%, and a 30% error in AOD induces an AMF error of between −9.53% and 8.35% with an APH of 3 km. In addition to AOD and SSA, APH is an important factor in calculating AMF, due to the 2 km error in APH under high-SZA conditions, which leads to an NO2 VCD error of over 60%. Aerosol shape is also found to have a measureable effect on AMF under high-AOD and small relative azimuth angle (RAA) conditions. The diurnal effect of the NO2 profile is also examined and discussed.

Imaging Mercury's Polar Deposits during MESSENGER's Low-altitude Campaign.

Images obtained during MESSENGER's low-altitude campaign in the final year of the mission provide the highest-spatial-resolution views of Mercury's polar deposits. Images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign. All of these regions of permanent shadow were found to have low-reflectance surfaces with well-defined boundaries. Additionally, brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions, supporting the conclusion that multiple volatile organic compounds are contained in Mercury's polar deposits, in addition to water ice. A recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to Mercury. Either scenario is consistent with the distinctive reflectance properties and well-defined boundaries of Mercury's polar deposits and the presence of volatiles in all available cold traps.

Study on the effect of ambient gas on nanostructure formation on metal surfaces during femtosecond laser ablation for fabrication of low-reflective surfaces

Nanostructure formation on bulk metals (silver, gold, copper and titanium) by femtosecond Ti-sapphire laser irradiation (775nm, 150fs) is studied aiming the production of low-reflectivity surfaces and the better understanding of the development process. The experiments were performed in nitrogen, air, oxygen and helium environments at atmospheric pressure. The samples were irradiated with fluences in the 0.1–2J/cm2 range and an average pulse number of 100 falling over a given area. The reflectivity of the treated surfaces was determined by a microspectrometer in the 450–800nm range and their morphology was studied by scanning electron microscopy. The gas ambience influenced the results via two effects: formation processes and the chemically-induced modifications of the nanostructures. In case of He the nanoparticle aggregates–otherwise generally present–are predominantly missing, which leads to a lower darkening efficiency. The presence of oxygen enhances the darkening effect for copper mostly at lower fluences, while causes a slow increase in reflectivity in the case of titanium (in case of pure oxygen) in the high fluence range. The surface morphology in case of nitrogen and air were quite similar probably due to their close molecular mass values.

0.76% absolute efficiency increase for screen-printed multicrystalline silicon solar cells with nanostructures by reactive ion etching

In this study, the reactive ion etching in combination with alkaline etching was employed to texture the surface of a 156×156mm2 multi-crystalline silicon wafers to reduce their solar reflectivity and improve conversion efficiency. Reactive gases comprising chlorine (Cl2), sulfur hexafluoride (SF6) and oxygen (O2), were activated in the radio-frequency (rf) plasma to fabricate pyramids in an reactive ion etching (RIE) system. The multi-crystalline Si substrates were etched in various compositions of gases to form the nanostructures of various shapes. Besides the nanoscale features, the high density of nanostructures formed on the multi-crystalline Si surface was also required to significantly reduce the reflectivity. A low reflectivity surface was successfully fabricated with the average reflectivity significantly reduced down to <2% for the wavelength range of 300–850nm. The short wavelength spectral response (blue response) improvement is observed in RIE textured solar cells compared to the standard textured cells. The RIE textured surfaces in combination with optimized emitter resistance result in a remarkable enhancement of short circuit current density. Compared with the acidic textured solar cells, the absolute conversion efficiency of the alkaline+RIE textured cells was improved by 0.76% in average.

Double-pulse 2-μm integrated path differential absorption lidar airborne validation for atmospheric carbon dioxide measurement.

Field experiments were conducted to test and evaluate the initial atmospheric carbon dioxide (CO₂) measurement capability of airborne, high-energy, double-pulsed, 2-μm integrated path differential absorption (IPDA) lidar. This IPDA was designed, integrated, and operated at the NASA Langley Research Center on-board the NASA B-200 aircraft. The IPDA was tuned to the CO₂ strong absorption line at 2050.9670 nm, which is the optimum for lower tropospheric weighted column measurements. Flights were conducted over land and ocean under different conditions. The first validation experiments of the IPDA for atmospheric CO₂ remote sensing, focusing on low surface reflectivity oceanic surface returns during full day background conditions, are presented. In these experiments, the IPDA measurements were validated by comparison to airborne flask air-sampling measurements conducted by the NOAA Earth System Research Laboratory. IPDA performance modeling was conducted to evaluate measurement sensitivity and bias errors. The IPDA signals and their variation with altitude compare well with predicted model results. In addition, off-off-line testing was conducted, with fixed instrument settings, to evaluate the IPDA systematic and random errors. Analysis shows an altitude-independent differential optical depth offset of 0.0769. Optical depth measurement uncertainty of 0.0918 compares well with the predicted value of 0.0761. IPDA CO₂ column measurement compares well with model-driven, near-simultaneous air-sampling measurements from the NOAA aircraft at different altitudes. With a 10-s shot average, CO₂ differential optical depth measurement of 1.0054±0.0103 was retrieved from a 6-km altitude and a 4-GHz on-line operation. As compared to CO₂ weighted-average column dry-air volume mixing ratio of 404.08 ppm, derived from air sampling, IPDA measurement resulted in a value of 405.22±4.15 ppm with 1.02% uncertainty and 0.28% additional bias. Sensitivity analysis of environmental systematic errors correlates the additional bias to water vapor. IPDA ranging resulted in a measurement uncertainty of <3 m.

In-vacuum scattered light reduction with black cupric oxide surfaces for sensitive fluorescence detection.

We demonstrate a simple and easy method for producing low-reflectivity surfaces that are ultra-high vacuum compatible, may be baked to high temperatures, and are easily applied even on complex surface geometries. Black cupric oxide (CuO) surfaces are chemically grown in minutes on any copper surface, allowing for low-cost, rapid prototyping, and production. The reflective properties are measured to be comparable to commercially available products for creating optically black surfaces. We describe a vacuum apparatus which uses multiple blackened copper surfaces for sensitive, low-background detection of molecules using laser-induced fluorescence.

Adaptive digital fringe projection technique for high dynamic range three-dimensional shape measurement.

It is a challenge for any optical method to measure objects with a large range of reflectivity variation across the surface. Image saturation results in incorrect intensities in captured fringe pattern images, leading to phase and measurement errors. This paper presents a new adaptive digital fringe projection technique which avoids image saturation and has a high signal to noise ratio (SNR) in the three-dimensional (3-D) shape measurement of objects that has a large range of reflectivity variation across the surface. Compared to previous high dynamic range 3-D scan methods using many exposures and fringe pattern projections, which consumes a lot of time, the proposed technique uses only two preliminary steps of fringe pattern projection and image capture to generate the adapted fringe patterns, by adaptively adjusting the pixel-wise intensity of the projected fringe patterns based on the saturated pixels in the captured images of the surface being measured. For the bright regions due to high surface reflectivity and high illumination by the ambient light and surfaces interreflections, the projected intensity is reduced just to be low enough to avoid image saturation. Simultaneously, the maximum intensity of 255 is used for those dark regions with low surface reflectivity to maintain high SNR. Our experiments demonstrate that the proposed technique can achieve higher 3-D measurement accuracy across a surface with a large range of reflectivity variation.

Low-coherence interferometer for contact lens surface metrology

Contact lens performance depends on a number of lens properties. Many metrology systems have been developed to measure different aspects of a contact lens, but none test the surface figure in reflection to subwavelength accuracy. Interferometric surface metrology of immersed contact lenses is complicated by the close proximity of the surfaces, low surface reflectivity, and instability of the lens. An interferometer to address these issues was developed and is described here. The accuracy of the system is verified by comparison of glass reference sample measurements against a calibrated commercial interferometer. The described interferometer can accurately reconstruct large surface departures from spherical with reverse raytracing. The system is shown to have residual errors better than 0.05% of the measured surface departure for high slope regions. Measurements made near null are accurate to λ/20. Spherical, toric, and bifocal soft contact lenses have been measured by this system and show characteristics of contact lenses not seen in transmission testing. The measurements were used to simulate a transmission map that matches an actual transmission test of the contact lens to λ/18.

Low-reflectance laser-induced surface nanostructures created with a picosecond laser

Using high-speed picosecond laser pulse irradiation, low-reflectance laser-induced periodic surface structures (LIPSS) have been created on polycrystalline silicon. The effects of laser fluence, scan speed, overlapping ratio and polarization angle on the formation of LIPSS are reported. The anti-reflective properties of periodic structures are discussed, and the ideal LIPSS for low surface reflectance is presented. A decrease of 35.7 % in average reflectance of the silicon wafer was achieved over the wavelength range of 400–860 nm when it was textured with LIPSS at high scan speeds of 4000 mm/s. Experimental results of broadband reflectance of silicon wafers textured with LIPSS have been compared with finite difference time domain simulations and are in good agreement, showing high predictability in reflectance values for different structures. The effects of changing the LIPSS profile, fill factor and valley depth on the surface reflectance were also analyzed through simulations.

Femtosecond laser nanostructuring of titanium metal towards fabrication of low-reflective surfaces over broad wavelength range

We investigated experimentally the formation of laser induced periodic surface structures (LIPSS) on titanium (Ti) metal upon irradiation with linearly polarized Ti:Sapphire femtosecond (fs) laser pulses of ∼110fs pulse width and 800nm wavelength in air and water environments. It is observed that initially formed random and sparsely distributed nano-roughness (nanoholes, nanoparticles and nanoprotrusions) gets periodically structured with increase in number of laser pulses. In air at lower fluence, we observed the formation of high spatial frequency-LIPSS (HSFL) oriented parallel to the laser polarization direction, whereas at higher fluence formation of low spatial frequency-LIPSS (LSFL) were observed that are oriented perpendicular to the incident laser polarization. In water two types of subwavelength structures were observed, one with spatial periodicity of ∼λ/15 and oriented parallel to laser polarization, while the other oriented perpendicular to laser polarization with feature size of λ/4. The optimal conditions for fabricating periodic sub-wavelength structures are determined by controlling the fluence and pulse number. The fs laser induced surface modifications were found to suppress the specular reflection of the Ti surface over a wide wavelength range of 250–2000nm to a great extent.

Allowance for polarization in passive space sensing of reflective properties of the Earth’S surface

The problem of allowance for polarization in statistical simulation of satellite imaging of the surface of the Earth in the optical wavelength range is considered. It is shown that the neglect of polarizing properties of radiation can lead to significant errors when retrieving the reflection coefficients of low-reflectivity surfaces.

Infrared thermography for assessing moisture related phenomena in building components

In this work the potential of infrared thermography (IRT) to assess moisture related phenomenon was tested. Initially, several set-ups and boundary conditions were created to compare IRT with other techniques and to understand the ideal conditions for accurate IRT. Afterwards, IRT was used to assess capillary absorption on a full-scale laboratory model and the drying process of two exterior walls. The main conclusions were that smaller differences are obtained for low reflectance surfaces and during the night, IRT can detect invisible moisture problems and can be used to assess the drying process, although the phenomenon complexity raises some difficulties if a quantitative approach is intended.

Wave optical simulation of the light trapping properties of black silicon surface textures.

Due to their low reflectivity and effective light trapping properties black silicon nanostructured surfaces are promising front side structures for thin crystalline silicon solar cells. For further optimization of the light trapping effect, particularly in combination with rear side structures, it is necessary to simulate the optical properties of black silicon. Especially, the angular distribution of light in the silicon bulk after passage through the front side structure is relevant. In this paper, a rigorous coupled wave analysis of black silicon is presented, where the black silicon needle shaped structure is approximated by a randomized cone structure. The simulated absorptance agrees well with measurement data. Furthermore, the simulated angular light distribution within the silicon bulk shows that about 70% of the light can be subjected to internal reflection, highlighting the good light trapping properties.

Wet/dry etching combined microtextured structures for high‐efficiency solar cells

A solar cell texturing process using a two-step process that includes wet etching and dry etching has been developed. The surface reflectance and fill factor (FF) of the pyramid structure by general wet etching texturing process were 5.834 and 70.597%, respectively; the surface reflectance and FF of the two-step pyramid structure by wet etching and dry etching texturing process were 3.69 and 65.013%; and the surface reflectance and FF of the two-step pyramid structure by rounded pyramid and dry etching texturing process were 4.533 and 70.727%. The process of the two-step pyramid structure is as follows. First, the pyramid structure formed by the wet etching process was etched again with tetramethylammonium hydroxide solution. Then, the crest and the trough of the pyramid structure was fine etched, and rounded pyramid structures with a uniform n+ layer was formed. Secondly, a pyramid structure with a high angle on the rounded pyramid structure was formed by using reactive-ion etching with a metal mesh. The two-step pyramid structure by the wet etching and dry etching texturing process has a lower surface reflectance and higher FF than the pyramid structure formed of the wet etching texturing process. Therefore, the etching process is suitable for producing high-efficiency solar cells.

Propagation of structured light beams after multiple reflections in a spiral phase plate

This work presents propagation dynamics of structured light (complex light) containing optical vortices after it has undergone multiple reflections in a spiral phase plate (SPP) device having a nonzero surface reflection. In the calculations, the thick-plate approximation is assumed as it is expected to give a more accurate representation of the standard geometry of an SPP device from a low-surface reflection to a high-surface reflection. Calculations showing the propagation of counter-rotating optical vortices are presented, and the effect of the statistical nature of photons on the observation of the angular intensity modulation of the beam is discussed.

High‐efficiency black silicon interdigitated back contacted solar cells on p‐type and n‐type c‐Si substrates

AbstractThis work demonstrates the high potential of Al2O3 passivated black silicon in high‐efficiency interdigitated back contacted (IBC) solar cells by reducing surface reflectance without jeopardizing surface passivation. Very low reflectance values, below 0.7% in the 300–1000 nm wavelength range, together with striking surface recombination velocities values of 17 and 5 cm/s on p‐type and n‐type crystalline silicon substrates, respectively, are reached. The simultaneous fulfillment of requirements, low reflectance and low surface recombination, paves the way for the fabrication of high‐efficiency IBC Si solar cells using black silicon at their front surface. Outstanding photovoltaic efficiencies over 22% have been achieved both in p‐type and n‐type 9‐cm2 cells. 3D simulations suggest that efficiencies of up to 24% can be obtained in the future with minor modifications in the baseline fabrication process. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Comparison of the Production of Nanostructures on Bulk Metal Samples by Picosecond Laser Ablation at Two Wavelengths for the Fabrication of Low-reflective Surfaces

Comparison of the Production of Nanostructures on Bulk Metal Samples by Picosecond Laser Ablation at Two Wavelengths for the Fabrication of Low-reflective Surfaces

Structural and Optical Characterization of Electrodeposited Black Chrome-Graphite Encapsulated FeCo Nanoparticles Composite Solar Selective Coatings

Black Chrome-graphite encapsulated FeCo nanoparticles (BC-GE FeCo NPs) composite based solar selective coatings were prepared on substrates such as stainless–steel, Cu and Ni-coated Cu employing electrodeposition technique. Electrodeposition was done by addition of three different weight proportions of GE FeCo NPs (0.025, 0.05, and 0.1) in the electrolyte bath. X-ray diffraction, Atomic Force Microscopy, UV-Visible and FTIR spectroscopy were used to characterize the structural and optical properties of these coatings. The major content of the deposited film is contributed by Cr (ICDD 74-7045), Fe-Cr (ICDD 34-0396), Fe2AlCr (ICDD 54-0387) and FeCr0.29Ni0.16C0.06 (ICDD 33-0397) as deduced from XRD. Spectral reflectance of the selective surface is observed to be low in UV-Visible region even after the addition of GE FeCo NPs. The low surface reflectance in the near infrared region due to the addition of GE FeCo NPs indicates increase in the absorptance of the selective surface in near infrared region. Atomic Force Microscopy indicated that film smoothness increases and grain size decreases with decrease in the weight percentage of GE FeCo NPs. Hardness and Young’s modulus were found to decrease with decreasing weight percentage of GE FeCo NPs. Keywords: Electrodeposition; Black Chrome; Graphite encapsulated FeCo NPs, solar selective coatings