Surface Roughness Spectrum Research Articles

Test of CaF2 glass sub-surface damage using angle-resolved ellipsometry

Calcium fluoride (CaF2), as a typical brittle optical material, has been applied in various fields due to its excellent optical properties. However, an existing challenge is how to non-destructively characterize the surface integrity of brittle materials, which may easily suffer from subsurface damage (SSD) during machining processes. This paper explores a novel snapshot surface-integrity test method based on single-exposure back-focal-plane (BFP) imaging of CaF2 within ellipsometric parameter spectra corresponding to an extensive range of incident angles. The relationship between the layer thicknesses of surface roughness, SSD, and ellipsometric spectra was analyzed by establishing a four-layer-subsurface optical model combined with quasi-Brewster angle technology. Through polishing CaF2 under different conditions and combining the measurement results of angle-resolved ellipsometry (ARE), it has been demonstrated that the study based on ARE is a successful non-destructive testing method for surface integrity, which could help to further elucidate the surface evolution mechanism of brittle material polishing processes.

Instantaneous measurement of surface roughness spectra using white-light scattering projected on a spectrometer.

Following on from previous studies on motionless scatterometers based on the use of white light, we propose a new, to the best of our knowledge, experiment of white-light scattering that should overtake the previous ones in most situations. The setup is very simple as it requires only a broadband illumination source and a spectrometer to analyze light scattering at a unique direction. After introducing the principle of the instrument, roughness spectra are extracted for different samples, and the consistency of results is validated at the intersection of bandwidths. The technique will be of great use for samples that cannot be moved.

Sub-regional polishing and machining trajectory selection of complex surface based on K9 optical glass

With the rapid progress of science and technology, complex optical surfaces have been widely used in various devices. Due to the complexity of their geometric features, machining with a single trajectory inevitably has an impact on the accuracy of the machined surface. Sub-regional milling has been studied intensively by researchers worldwide. However, few studies have focused on sub-region polishing. Therefore, sub-regional polishing is not widely used in the machining field. This paper investigates the method of sub-regional polishing of complex surfaces and focuses on the selection of polishing trajectories for different processing areas. The method takes into account the curvature characteristics of each point on the surface and combines the Freeman code method to divide the surface into multiple processing regions. The selection of polishing trajectories for each processing area is analyzed from the viewpoint of surface roughness and power spectrum density analysis. The correctness of the trajectory selection is verified by polishing experiments on K9 optical glass. The results show that all three trajectories used in the experiments can reduce the surface roughness of the workpiece by more than 90%. Using the Hilbert trajectory can effectively reduce the medium-frequency and high-frequency errors of the machined surface and improve the optical properties of the surface. This work is expected to provide significant guidance for efficient and high-quality processing of complex optical surfaces compared with traditional machining methods using a single trajectory.

Evolution of Surface Topography and Microstructure in Laser Polishing of Cold Work Steel 1.2379 (AISI D2) Using Quadratic, Top-Hat Shaped Intensity Distributions.

Within the scope of this study, basic experimental research was carried out on macro-laser polishing of tool steel 1.2379 (D2) using a square intensity distribution and continuous wave laser radiation. The influence of the individual process parameters on surface topography was analyzed by a systematic investigation of a wide range of process parameters for two different, square laser beam diameters. Contrary to a typical laser polishing approach, it was shown that short interaction times (high scanning velocity and small laser beam dimensions) are required to reduce both micro-roughness and meso-roughness. A significant reduction of surface roughness of approx. 46% was achieved from Raini = 0.33 ± 0.026 µm to Ramin = 0.163 ± 0.018 µm using a focused square laser beam with an edge length of dL,E = 100 µm at a scanning velocity of vscan = 200 mm/s, a laser power PL = 60 W and n = 2 passes. However, characteristic surface features occur during laser polishing and are a direct consequence of the laser polishing process. Martensite needles in the micro-roughness region, undercuts in the meso-roughness region, and surface waviness in the macro-roughness region can dominate different regions of the resulting surface roughness spectrum. In terms of mechanical properties, average surface hardness was determined by hundreds of nano-indentation measurements and was approx. 390 ± 21 HV0.1 and particularly homogeneous over the whole laser polished surface.

How surface stress transforms surface profiles and adhesion of rough elastic bodies.

The surface of soft solids carries a surface stress that tends to flatten surface profiles. For example, surface features on a soft solid, fabricated by moulding against a stiff-patterned substrate, tend to flatten upon removal from the mould. In this work, we derive a transfer function in an explicit form that, given any initial surface profile, shows how to compute the shape of the corresponding flattened profile. We provide analytical results for several applications including flattening of one-dimensional and two-dimensional periodic structures, qualitative changes to the surface roughness spectrum, and how that strongly influences adhesion.

Martian Topographic Roughness Spectra and Its Influence on Bistatic Radar Scattering

There are few studies on predicting fully bistatic scattering from the rough surface of Mars, though some bistatic radar observations have been made, such as in the MARS EXPRESS mission. To better understand the interaction of radar signals with a planetary surface in bistatic radar observations, the topographic-scale roughness of Mars, characterized by a two-dimensional power spectrum density (2D-PSD), is examined in view of its global roughness variations and scale dependence on geological units. The analysis shows that the Martian 2D-PSD is strongly dependent on the geological units and that it lies between Gaussian and exponential functions, with a power index equal to 1.9. The bistatic scattering coefficients are calculated by an advanced integral equation model (AIEM) with the 2D-PSD as the input. It shows that the specific surface roughness spectrum and the dielectric inhomogeneity should be taken into account in interpreting the bistatic radar scattering response.

Visible-Light Active Photocatalytic Dual Layer Hollow Fiber (DLHF) Membrane and Its Potential in Mitigating the Detrimental Effects of Bisphenol A in Water

The presence of bisphenol A (BPA) in various water sources has potentially led to numerous adverse effects in human such as increased in blood pressure and derangement in liver function. Thus, a reliable treatment for the removing BPA is highly required. This present work aimed to study the efficiency of visible light driven photocatalytic dual-layer hollow fiber (DLHF) membrane for the removal of BPA from water and further investigated its detrimental effects by using an in-vivo model. The prepared membranes were characterized for their morphology, particles distribution, surface roughness, crystallinity and light absorption spectra. The removal of 81.6% and 86.7% in BPA concentration was achieved for N-doped TiO2 DLHF after 360 min of visible and UV light irradiation, respectively. No significant changes for all three groups were observed in liver function test meanwhile the rats-exposed to untreated BPA water shows significance blood pressure increment contrary to rats-exposed to treated BPA water. Similarly, the normal morphology in both jejunum and ileum were altered in rats-exposed to untreated BPA water group. Altogether, the presence of N-doped TiO2 in DLHF are shown to significantly enhance the photocatalytic degradation activity under visible irradiation, which effectively mitigates the effect of BPA in an in-vivo model.

Characterization of rain impact on L-Band GNSS-R ocean surface measurements

Earth remote sensing using reflected GNSS signals is currently an emerging trend especially in ocean surface wind measurements. Unlike the existing scatterometer missions, GNSS-R uses L-Band navigation signals that can penetrate through clouds and rain. Rain may have a negligible impact on the transmitted signal in terms of path attenuation at this wavelength. However, there are other effects due to rain, such as changes in surface roughness and rain induced local winds, which can significantly alter the measurements. Currently, there is no observation-based characterization of all possible impacts of rain on radar forward scatter, which is the nature of operation of GNSS-R missions. In this study, we propose a 3-fold rain model which accounts for attenuation, surface effects of rain and rain induced local winds. We utilize the large dataset of measurements made by the CYGNSS mission to separate these different effects of rain. The attenuation model suggests that a total of at least 96% transmissivity exists at L-Band up to a rain rate of 30 mm/h. A perturbation model is used to characterize the other rain effects. It suggests that rain is accompanied by an overall reduction in the scattering cross-section of the ocean surface and, most importantly, this effect is observed only up to surface wind speeds of 15 m/s, beyond which the gravity capillary waves dominate the scattering in the quasi-specular direction. Observations also suggest that, at very low wind speeds, the lower bound on wavenumber of the portion of the surface roughness spectrum that influences the measurements deviates from the geometric optics approximation normally used. This work binds together several rain-related phenomena and enhances our overall understanding of rain effects on GNSS-R measurements.

Impact on Sea-Surface Electromagnetic Scattering and Emission Modeling of Recent Progress on the Parameterization of Ocean Surface Roughness, Drag Coefficient, and Whitecap Coverage in High Wind Conditions

Ocean surface wind stress is the force generating ocean surface roughness and whitecaps. The surface wind stress is related to the surface wind speed by the drag coefficient. Consequently, the magnitude of roughness and whitecaps calculated from wind speed input depends on the applied drag coefficient formula, which remains uncertain especially for high wind conditions. Because roughness and whitecaps are two major components of the ocean surface response in microwave remote sensing, a clear understanding of the drag coefficient, ocean surface roughness, and whitecap coverage is critical to interpreting the microwave signals emitted or scattered from the ocean surface. In reverse, the microwave signals represent precious data sources of these ocean surface properties [surface wind stress (or equivalently the drag coefficient), surface roughness, and whitecaps] that are difficult to measure with traditional oceanographic instruments, especially in severe weather conditions. This article describes the improvement of ocean surface properties achieved by incorporating microwave remote sensing measurements. In essence, the global data of microwave monostatic and bistatic radar cross sections contribute to the revision of ocean surface roughness spectrum model, and microwave radiometer measurements in tropical cyclones provide clarification regarding the dependence on wind speed of the drag coefficient and whitecap coverage. In return, the calculated excess emissivities and scattering radar cross sections are improved from implementing the updated results of drag coefficient, whitecap coverage, and surface roughness in microwave emission and scattering models.

INFLUENCE OF EMBEDDED TUNGSTEN PARTICLES ON MECHANICAL BEHAVIORS OF CVD DIAMOND COATING

Diamond coating has gained intensive attraction in the tribological field due to its high hardness. However, its weak flexibility always gives rise to the fragile crack, which causes the delamination and peeling off from substrate. In this work, a novel deposition method combining the conventional hot filament chemical vapor deposition (HFCVD) and particles doping technique is proposed to balance the hardness and flexibility of diamond coating, by which the diamond coating with tungsten particles is deposited on the co-cemented tungsten carbide substrate. The as-deposited diamond coating is characterized by scanning electron microscopy (SEM) analysis, surface roughness and Raman spectrum. The indentation tests are conducted to evaluate the crack propagation of diamond coating. Tribological behavior is examined on a reciprocating ball-on-plate tribometer. The results indicate that tungsten carbide may be formed between tungsten particles and diamond coating. The W–WC–amorphous carbon–diamond structural coating can validly inhibit the crack propagation and decrease the friction coefficient. Hence, adding embedding particles into the diamond coating may provide a useful way in enhancing the mechanical properties of diamond coating.

High-Wind Drag Coefficient and Whitecap Coverage Derived from Microwave Radiometer Observations in Tropical Cyclones

AbstractOcean surface roughness and whitecaps are driven by the ocean surface wind stress; thus, their values calculated from the wind speed input may vary significantly depending on the drag coefficient formula applied. Because roughness and whitecaps are critical elements of the ocean surface response in microwave remote sensing, the extensive microwave remote sensing measurements contain the information of the drag coefficient, surface roughness, and whitecap coverage. The scattering radar cross sections from global measurements under calm to tropical cyclone conditions have been used effectively to improve the formulation of the surface roughness spectrum. In this paper, the microwave radiometer measurements in tropical cyclones are exploited to extract information of the drag coefficient and whitecap coverage in high winds. The results show that when expressed as a wind speed power function, the exponent in high winds (greater than about 35 m s−1) is about −1 for the drag coefficient, 0.5 for the wind friction velocity, and 1.25 for the whitecap coverage.

Assessment of Wind Speed Estimation From C-Band Sentinel-1 Images Using Empirical and Electromagnetic Models

Surface wind speed estimation from synthetic aperture radar (SAR) data is principally based on empirical (EP) approaches, e.g., CMOD functions. However, it is necessary and significant to compare radar backscattering modeling based on EP and electromagnetic (EM) approaches for enhancing the understanding of the physical processes between radar signal and sea surface, which is important for the design of radar sensors (e.g., cyclone global navigation satellite system). Indeed, through comparisons, it is worth noticing that the scattering of wave breaking is not taken into account in the physical modeling of radar backscattering. Surface wind speed is selected here as a reference parameter for investigating the difference between EP and EM models, due to its important role in radar backscattering modeling. In addition, wind speed estimates can be easily compared to in situ measurements. For EP approach, CMOD5.N and Komarov’s model are selected for wind speed estimation from Sentinel-1 images. The CMOD5.N can offer wind speed estimates up to 25–35 m/s, while wind speed estimation based on Komarov’s model does not require wind direction input. For EM approach, the asymptotic models, i.e., composite two-scale model, small-slope approximation (SSA), and resonant curvature approximation (RCA), are investigated for wind speed retrieval. They are studied with two models of surface roughness spectrum: semi-EP spectrum and EP model. In general, normalized radar cross section (NRCS) calculated by CMOD5.N and SSA/RCA is quite similar for incidence angles below 40° in vertical polarized and below 30° in horizontal polarized. For larger ones, significant NRCS deviations between two approaches are demonstrated, due to the lack of wave breaking scattering in EM models. As a result, wind speed estimates by CMOD5.N and SSA/RCA are very close for low and moderate incidence angles, while SSA-/RCA-based wind speeds are overestimated for larger ones.

Depolarized Backscattering of Rough Surface by AIEM Model

This paper presents a new expression for multiple scattering by including the upward and downward propagation waves in the medium 1 and medium 2. Unlike the single scattering, the multiple scattering accounts for the interactions, up to second order, among all the spectral components of surface roughness spectrum. Though the derivation is mathematically intricate, but yet manageable, the final expression is compact and easy for numerical implementation, which only involves a series of two-dimensional integration. Some of special cases in depolarized backscattering are also derived and compared with known analytical model to partly validate the update AIEM model. Then, extensive comparisons with numerical simulations and field measurements are conducted to illustrate the model accuracy.

A note on a critical wind speed for air–sea boundary processes

Wind and wind-generated waves were measured in a wind-wave tank. A clear transition was found in the relation between the wind speed U10 and the wind friction velocity u* near u* = 0.2 m/s, where U10 is the wind speed at 10 m height extrapolated from the measured wind profile in a logarithmic layer, and u* = 0.2 m/s corresponds roughly to U10 = 8 m/s in the present measurement. Quite a similar transition was found in the relation between the spectral density of high frequency wind waves and u*. These results suggest the existence of the critical wind speed for air–sea boundary processes, which was proposed by Munk (J Marine Res 6:203–218, 1947) more than half a century ago. His original idea of the critical wind speed was based on the discontinuities in such phenomena as white caps, wind stress, and evaporation, which commonly appear at a wind speed near 7 m/s. On the basis of the results of our present study and those of earlier studies, we discuss the phenomena which are relevant to the critical wind speed for the air–sea boundary processes. The conclusion is that the critical wind speed exists and it is attributed to the start of wave breaking rather than the Kelvin–Helmholtz instability, but the air–sea boundary processes are not discontinuous at a particular wind speed; because of the stochastic nature of breaking waves, the changes occur over a range of wind speeds. Detailed discussions are presented on the dynamical processes associated with the critical wind speed such as wind-induced change of sea surface roughness and high frequency wave spectrum. Future studies are required, however, to clarify the dynamical processes quantitatively. In particular, there is a need to further examine the gradual change of breaking patterns of wind waves with the increase of wind speed, and the associated change of the structure of the wind over wind waves, such as separation of the airflow at the crest of wind waves, the turbulent stress, and wave-induced stress. Studies on the dynamical structure of the high frequency wave spectrum are also needed.

Application of AMSR-E and AMSR2 Low-Frequency Channel Brightness Temperature Data for Hurricane Wind Retrievals

We present a method to retrieve wind speeds in hurricanes from spaceborne passive microwave radiometer data. Brightness temperature $(T_{B})$ observations acquired at the 6.9-GHz horizontal polarization channel by the AMSR-E and AMSR2 onboard the Earth Observing System Aqua and Global Change Observation Mission-Water 1 satellites are selected for wind retrieval due to the fact that the signal at this frequency is sensitive to high wind speeds but less sensitive to rain scatter than those acquired at other higher frequency channels. The AMSR-E and AMSR2 observations of 53 hurricanes between 2002 and 2014 are collected and collocated with stepped-frequency microwave radiometer (SFMR) measurements. Based on the small slope approximation/small perturbation method model and an ocean surface roughness spectrum, the wind speeds are retrieved from the $T_{B}$ data and validated against the SFMR measurements. The statistical comparison of the entire data set shows that the bias and root-mean-square error (RMSE) of the retrieved wind speeds are 1.11 and 4.34 m/s, respectively, which suggests that the proposed method can obtain high wind speeds under hurricane conditions. Two case studies show that the wind speed retrieval bias and RMSE are 1.08 and 3.93 m/s for Hurricane Earl and 0.09 and 3.23 m/s for Hurricane Edouard, respectively. The retrieved wind speeds from the AMSR-E and AMSR2 continuous three-day observations clearly show the process of hurricane intensification and weakening.

Electromagnetic scattering and Doppler spectrum simulation of time-varying oil-covered nonlinear sea surface

Based on the model of Lombardini et al. [J. Atmos. Ocean. Technol.6(6), 882–890 (1989)], which can predict the hydrodynamic damping of rough sea surfaces in the presence of oil films, the influence of sea slicks on the sea surface roughness spectrum and sea surface geometrical structure is examined briefly in the present study. On this basis, the influence of sea slicks on the angular distribution of the bistatic scattering coefficient of sea surfaces and the Doppler spectrum signature of backscattered radar sea-echo is investigated in detail based on a frequency-domain numerical method of the parallel fast multiple method. Simulation results show that Doppler spectrum signatures including Doppler shift and spectral bandwidth of radar sea-echo are significantly affected by sea slicks, which are qualitatively consistent with wave-tank or open sea measurements. Moreover, simulation results indicate that the Doppler spectrum signature is a promising technique for remote sensing of oil films floating on sea surfaces.

夏用および冬用紳士服地の摩擦変動曲線と表面粗さ曲線の相関解析

In this paper, fabric friction curve measured by KES-FB4 surface tester is investigated based on spectrum analysis. Friction spectrum is analyzed compared to surface roughness spectrum obtained simultaneously along the same direction. Samples used are 25 kinds of men’s suitings made of wool including summer and winter use. Results are obtained as follows. (1) Surface roughness of fabrics is divided into “rough” and “smooth” surface of which threshold is 4-5 μm for SMD and -7 for log Pmax, where Pmax is peak value of surface spectrum. (2) The behavior of friction spectrum is divided into three types as follows, (a) uneven surface type, (b) stick-slip (SS) type and (c) mixed type. (3) For the uneven surface type, wave number (1/cm) where the peak value of friction spectrum appears is the same as that of surface roughness spectrum. (4) For the SS type, wave number where the peak value of friction spectrum appears is 20, 40 (1/cm) and low frequency around 0, and does not coincide with that of surface roughness spectrum. (5) For the mixed type, both behaviors of uneven and SS types appear in friction spectrum. (6) The energy of friction spectrum is divided into two parts, i.e. component arised from uneven surface and component arised from stick-slip behavior.

Metocean Comparisons of Jason-2 and AltiKa—A Method to Develop a New Wind Speed Algorithm

As well as range, the AltiKa altimeter provides estimates of wave height, Hs and normalized backscatter, σ0, that need to be assessed prior to statistics based on them being included in climate databases. An analysis of crossovers with the Jason-2 altimeter shows AltiKa Hs values to be biased high by only ˜0.05m, with a standard deviation (s.d.) of ˜0.1m for seven-point averages. AltiKa's σ0 values are 2.5–3 dB less than those from Jason-2, with a s.d. of ˜0.3 dB, with these relatively large mismatches to be expected as AltiKa measures a different part of the spectrum of sea surface roughness. A new wind speed algorithm is developed through matching a histogram of σ0 values to that for Jason-2 wind speeds. The algorithm is robust to the use of short durations of data, with a consistency at roughly the 0.1 m/s level. Incorporation of Hs as a secondary input reduces the assessed error at crossovers from 0.82 m/s to 0.71 m/s. A comparison across all altimeter frequencies used to date demonstrates that the lowest wind speeds preferentially develop the shortest scales of roughness.

Reverberation modeling approximately accounting for three-dimensional forward scattering effects

A transport theory approach has been developed for modeling shallow water propagation and reverberation at mid-frequencies with emphasis at 1–3 kHz. With this approach, sea surface forward scattering can be taken into account in a 2-D (range-depth) approximation. While the effects of surface forward scattering on transmission loss are found to be modest (1–2 dB), the corresponding effects on reverberation level for typical conditions can be significant (~10 dB), even though bottom backscatter dominates reverberation in shallow water. An attempt to make data/model comparisons brought out the issue of the proper way of using the 2-D (in horizontal plane) surface roughness spectrum to model forward scattering with a 2-D (range-depth) propagation model. For reverberation modeling, we are investigating the approximation of preserving the distribution in vertical scattering angles in reducing the full 3-D problem to a 2-D (range-depth) problem when using the traditional N ' 2-D approach. A description of the method will be given, followed by data-model comparisons for TREX-13. [Work supported by ONR Ocean Acoustics.]

Doppler Spectrum Analysis of Time-Evolving Sea Surface Covered by Oil Spills**Supported by the National Science Foundation for Distinguished Young Scholars of China under Grant No 61225002, and the Aeronautical Science Fund and Aviation Key Laboratory of Science and Technology on AISSS of China under Grant No 20132081015.

Based on the model of a contaminated sea surface that was proposed by Lombardini et al., the influence of the damping effect of oil films on the sea surface roughness spectrum and the geometrical structure of the sea surface is examined in detail by comparing with a clean sea surface. Furthermore, based on a quasi-stationary algorithm, a time series of backscattered echoes from a time-evolving sea surface covered by oil slicks is obtained by utilizing the frequency-domain numerical method of the parallel fast multiple method. Then, the Doppler spectrum is evaluated by performing a standard spectral estimation technique. Finally, the influence of the oil film damping effect on the Doppler spectrum of the backscattered echoes from time-evolving sea surface is investigated in detail by making a comparison of the Doppler spectrum of an oil-covered sea surface with the Doppler spectrum of a clean sea surface. The numerical simulations show that the damping effect of oil films has an influence on the Doppler spectrum signature for both horizontal-to-horizontal and vertical-to-vertical polarizations.