Surface Reflectance Properties Research Articles

Simulation and analysis of adjacency effects in coastal waters: a case study

A methodology has been developed and applied to accurately quantify and analyze adjacency effects in satellite ocean color data for a set of realistic and representative observation conditions in the northern Adriatic Sea. The procedure properly accounts for sea surface reflectance anisotropy, off-nadir views, coastal morphology, and atmospheric multiple scattering. The study further includes a sensitivity analysis on commonly applied approximations. Results indicate that, within the accuracy limits defined by the radiometric resolution of ocean color sensors, adjacency effects in coastal waters might be significant at both visible and near-infrared wavelengths up to several kilometers off the coast. These results additionally highlight a significant dependence on the angle of observation, on the directional reflectance properties of the sea surface, and on the atmospheric multiple scattering.

The inscrutability of colour similarity

This paper presents a new response to the colour similarity argument, an argument that many people take to pose the greatest threat to colour physicalism. The colour similarity argument assumes that if colour physicalism is true, then colour similarities should be scrutable under standard physical descriptions of surface reflectance properties such as their spectral reflectance curves. Given this assumption, our evident failure to find such similarities at the reducing level seemingly proves fatal to colour physicalism. I argue that we should dispense with this assumption, and thus endorse the inscrutability of colour similarity. This strategy is inspired by parallels between the colour similarity argument and the explanatory gap between mind and body made vivid by Jackson’s (1986) knowledge argument, and in particular by type-B physicalist responses to that argument. This inscrutability response is further motivated by cases in chemistry and biochemistry in which analogous scrutability theses fail to hold. Along the way, I present a challenge to standard formulations of the colour similarity argument based on the extreme context sensitivity of the similarity relation. Most presentations of the argument fail to control for such contextual variation, which raises the distinct possibility that the argument equivocates on the similarity relation across its premises. Although ultimately inconclusive, this context challenge forces a significant reformulation of the colour similarity argument, and highlights the need for much greater care in handling claims about colour similarity.

Defect inspection system for steel wire rods produced by hot rolling process

A vision-based inspection system has been investigated in order to improve the quality of products and processes found in various industries. In this paper, we propose a new defect detection algorithm for steel wire rods produced by the hot rolling process. Because the steel wire rods are long cylinder rods with a circular cross section, the brightness at the sides and center is inconsistent. Moreover, the various types of steel wire rods and the presence of scales affect the reflection properties of the rod surface. In order to resolve the abovementioned difficulties, the use of dynamic programming and a discrete wavelet transform are proposed. An adaptive local binarization method is used to further reduce the effects of scale. The effectiveness of the proposed method is shown by means of experiments conducted on images of steel wire rods that were obtained from an actual steel production line.

Novel silicon surface passivation by porous silicon combined with an ultrathin Al2O3 film

We report on a dual passivation approach of monocrystalline-silicon (c-Si), which combines porous silicon (PS) treatment and pulsed laser deposition (PLD) of Al2O3 ultrathin layer. Al2O3 ultrathin films were deposited, under an oxygen background pressure of 10 mTorr. In this work we demonstrate that the dual treatment Al2O3/PS provide excellent passivation quality of c-Si surfaces. Surface passivation and reflectivity properties are investigated before and after heat treatment at 700 °C. The level of surface passivation is quantified using photoconductance and Fourier-transform infra-red absorption techniques. X-ray diffraction analysis suggested that the heat treatment leads to an average crystallite size decrease which help the diffusion of the Al2O3 NPs along the wafer surface. As a result, the effective minority carrier lifetime increases from 2 to 7 μs. Our results demonstrate that this dual treatment not only provides strong passivation of the c-Si substrate but decreases also the total surface reflectivity at 500 nm (from 28 % for untreated c-Si samples to ∼7 % for Al2O3/PS treated ones).

Real-time three-dimensional fingerprint acquisition via a new photometric stereo means

A real-time means for three-dimensional (3-D) fingerprint acquisition is presented. The system is configured with only one camera and some white light-emitting diode lamps. The reconstruction is performed based on the principle of photometric stereo. In the algorithm, a two-layer Hanrahan–Krueger model is proposed to represent the finger surface reflectance property instead of the traditional Lambert model. By the proposed lighting direction calibration and the nonuniform lighting correction methods, surface normal at each image point can be accurately estimated by solving a nonlinear optimization problem. Finally, a linear normal transformation is implemented for the enhancement of 3-D models. The experiments are implemented with real finger and palm prints, and the results are also compared with traditional means to show its feasibility and improvement in the reconstruction accuracy.

Optimal design of optical system for LED road lighting with high illuminance and luminance uniformity

A new method is proposed to achieve high illuminance and luminance uniformity of the road surface in LED road lighting. Based on the reflection properties of the road surface, the illuminance and luminance are analyzed simultaneously with the least-square method; meanwhile, energy efficiency and glare requirements are considered. Through the analysis and calculations, the optimal light distribution of a luminaire is obtained, and then a freeform lens with this light distribution is designed. For a 2-lane C1 class road illuminated by LED luminaires mounted with these lenses, the overall illuminance and luminance uniformity on the road surface can reach over 0.9 and 0.85, respectively, and the glare factors less than 10%.

Low-budget transient imaging using photonic mixer devices

Transient imaging is an exciting a new imaging modality that can be used to understand light propagation in complex environments, and to capture and analyze scene properties such as the shape of hidden objects or the reflectance properties of surfaces. Unfortunately, research in transient imaging has so far been hindered by the high cost of the required instrumentation, as well as the fragility and difficulty to operate and calibrate devices such as femtosecond lasers and streak cameras. In this paper, we explore the use of photonic mixer devices (PMD), commonly used in inexpensive time-of-flight cameras, as alternative instrumentation for transient imaging. We obtain a sequence of differently modulated images with a PMD sensor, impose a model for local light/object interaction, and use an optimization procedure to infer transient images given the measurements and model. The resulting method produces transient images at a cost several orders of magnitude below existing methods, while simultaneously simplifying and speeding up the capture process.

Retrieving atmospheric dust opacity on Mars by imaging spectroscopy at large angles

We propose a new method to retrieve the optical depth of Martian aerosols (AOD) from OMEGA and CRISM hyperspectral imagery at a reference wavelength of 1μm. Our method works even if the underlying surface is completely made of minerals, corresponding to a low contrast between surface and atmospheric dust, while being observed at a fixed geometry. Minimizing the effect of the surface reflectance properties on the AOD retrieval is the second principal asset of our method. The method is based on the parametrization of the radiative coupling between particles and gas determining, with local altimetry, acquisition geometry, and the meteorological situation, the absorption band depth of gaseous CO2. Because the last three factors can be predicted to some extent, we can define a new parameter β that expresses specifically the strength of the gas–aerosols coupling while directly depending on the AOD. Combining estimations of β and top of the atmosphere radiance values extracted from the observed spectra within the CO2 gas band at 2μm, we evaluate the AOD and the surface reflectance by radiative transfer inversion. One should note that practically β can be estimated for a large variety of mineral or icy surfaces with the exception of CO2 ice when its 2μm solid band is not sufficiently saturated. Validation of the proposed method shows that it is reliable if two conditions are fulfilled: (i) the observation conditions provide large incidence or/and emergence angles (ii) the aerosols are vertically well mixed in the atmosphere. Experiments conducted on OMEGA nadir looking observations as well as CRISM multi-angular acquisitions with incidence angles higher than 65° in the first case and 33° in the second case produce very satisfactory results. Finally in a companion paper the method is applied to monitoring atmospheric dust spring activity at high southern latitudes on Mars using OMEGA.

Estimating Photometric Properties from Image Collections

We address the problem of jointly estimating the scene illumination, the radiometric camera calibration and the reflectance properties of an object using a set of images from a community photo collection. The highly ill-posed nature of this problem is circumvented by using appropriate representations of illumination, an empirical model for the nonlinear function that relates image irradiance with intensity values and additional assumptions on the surface reflectance properties. Using a 3D model recovered from an unstructured set of images, we estimate the coefficients that represent the illumination for each image using a frequency framework. For each image, we also compute the corresponding camera response function. Additionally, we calculate a simple model for the reflectance properties of the 3D model. A robust non-linear optimization is proposed exploiting the high sparsity present in the problem.

Signal processing for hemispherical measurement data

To realistically model the sound propagation in rooms, a detailed knowledge of the reflection properties of the surrounding surfaces is required. In this context, the reflection properties include both the sound absorption as well as scattering. In order to be able to measure the angle-dependent reflection properties of surfaces in-situ, a hemispherical microphone array was recently designed and built. For a reduction of the required hardware an efficient, rotationally symmetric sampling was chosen, so that 28 microphones on two concentric semicircles are employed to measure a total of over 3000 positions on a hemisphere in just over 15 minutes. This contribution will give an overview over the required signal processing steps to process the measurement data from such a microphone array. Special emphasis will be placed on the determination of the microphone positions and the special case of data available on a hemispherical surface. Also, the sound field model used to determine the impedance on the surface will be explained. Preliminary results will be presented.

Retrieval of remotely sensed LAI using Landsat ETM+ data and ground measurements of solar radiation and vegetation structure: Implication of leaf inclination angle

A time series of leaf area index (LAI) of a managed birch forest in Germany (near Dresden) has been developed based on 16-day normalized difference vegetation index (NDVI) data from the Landsat ETM+ sensor at 30m resolution. The Landsat ETM+ LAI was retrieved using a modified physical radiative transfer (RTM) model which establishes a relationship between LAI, fractional vegetation cover (fC), and given patterns of surface reflectance, view-illumination conditions and optical properties of vegetation. In situ measurements of photosynthetically active radiation (PAR) and vegetation structure parameters using hemispherical photography (HSP) served for calibration of model parameters, while data from litter collection at the study site provided the ground-based estimates of LAI for validation of modelling results. Influence of view-illumination conditions on optical properties of canopy was simulated by a view angle geometry model incorporating the solar zenith angle and the sensor viewing angle. Effects of intra-annual and inter-annual variability of structural properties of the canopy on the light extinction coefficient were simulated by implementing variability of the leaf inclination angle (LIA), which was confirmed in the study site. The results revealed good compatibility of the produced Landsat ETM+ LAI data set with the litter-estimated LAI. The results also showed high sensitivity of the LAI retrieval algorithm to variability of structural properties of the canopy: the implementation of LIA dynamics into the LAI retrieval algorithm significantly improved the model accuracy.

Effects of low-salinity and high-turbidity waters on empirical ocean colour algorithms: An example for Southwestern Atlantic waters

The spectral reflectance of the sea surface recorded using ocean colour satellite sensors has been used to estimate chlorophyll-a concentrations for decades. However, in bio-optically complex coastal waters, these estimates are compromised by the presence of several other coloured components besides chlorophyll, especially in regions affected by low-salinity waters. The present work aims to (a) describe the influence of the freshwater plume from the La Plata River on the variability of in situ remote sensing reflectance and (b) evaluate the performance of operational ocean colour chlorophyll algorithms applied to Southwestern Atlantic waters, which receive a remarkable seasonal contribution from La Plata River discharges. Data from three oceanographic cruises are used, in addition to a historical regional bio-optical dataset. Deviations found between measured and estimated concentrations of chlorophyll-a are examined in relation to surface water salinity and turbidity gradients to investigate the source of errors in satellite estimates of pigment concentrations. We observed significant seasonal variability in surface reflectance properties that are strongly driven by La Plata River plume dynamics and arise from the presence of high levels of inorganic suspended solids and coloured dissolved materials. As expected, existing operational algorithms overestimate the concentration of chlorophyll-a, especially in waters of low salinity (S<33.5) and high turbidity (Rrs(670)>0.0012sr−1). Additionally, an updated version of the regional algorithm is presented, which clearly improves the chlorophyll estimation in those types of coastal environment. In general, the techniques presented here allow us to directly distinguish the bio-optical types of waters to be considered in algorithm studies by the ocean colour community.

CLARA-SAL: a global 28 yr timeseries of Earth's black-sky surface albedo

Abstract. We present a novel 28 yr dataset of Earth's black-sky surface albedo, derived from AVHRR instruments. The dataset is created using algorithms to separately derive the surface albedo for different land use areas globally. Snow, sea ice, open water and vegetation are all treated independently. The product features corrections for the atmospheric effect in satellite-observed surface radiances, a BRDF correction for the anisotropic reflectance properties of natural surfaces, and a novel topography correction of geolocation and radiometric accuracy of surface reflectance observations over mountainous areas. The dataset is based on a homogenized AVHRR radiance timeseries. The product is validated against quality-controlled in situ observations of clear-sky surface albedo at various BSRN sites around the world. Snow and ice albedo retrieval validation is given particular attention using BSRN sites over Antarctica, Greenland Climate Network stations on the Greenland Ice Sheet (GrIS), as well as sea ice albedo data from the SHEBA and Tara expeditions. The product quality is found to be comparable to other previous long-term surface albedo datasets from AVHRR.

Simulation of light transport in scintillators based on 3D characterization of crystal surfaces

In the development of positron emission tomography (PET) detectors, understanding and optimizing scintillator light collection is critical for achieving high performance, particularly when the design incorporates depth-of-interaction (DOI) encoding or time-of-flight information. Monte-Carlo simulations play an important role in guiding research in detector designs and popular software such as GATE now include models of light transport in scintillators. Although current simulation toolkits are able to provide accurate models of perfectly polished surfaces, they do not successfully predict light output for other surface finishes, for example those often used in DOI-encoding detectors. The lack of accuracy of those models mainly originates from a simplified description of rough surfaces as an ensemble of micro-facets determined by the distribution of their normal, typically a Gaussian distribution. The user can specify the standard deviation of this distribution, but this parameter does not provide a full description of the surface reflectance properties. We propose a different approach based on 3D measurements of the surface using atomic force microscopy. Polished and rough (unpolished) crystals were scanned to compute the surface reflectance properties. The angular distributions of reflectance and reflected rays were computed and stored in look-up tables (LUTs). The LUTs account for the effect of incidence angle and were integrated in a light transport model. Crystals of different sizes were simulated with and without reflector. The simulated maximum light output and the light output as a function of DOI showed very good agreement with experimental characterization of the crystals, indicating that our approach provides an accurate model of polished and rough surfaces and could be used to predict light collection in scintillators. This model is based on a true 3D representation of the surface, makes no assumption about the surface and provides insight on the optical behaviour of rough crystals that can play a critical role in optimizing the design of PET detectors. This approach is also compatible with existing simulation toolkits and next steps include the implementation in GATE.

INTERPRETATION OF TOPOGRAPHIC DATA FROM "SHAPE FROM SHADING" METHOD, APPLICATION IN VILLAGARCÍA DE CAMPOS CASTLE

Abstract. Generating digital products allowing the assessment and interpretation of Cultural Heritage is a key task in conservation processes. Thus a variety of (Shape from X) methods have been developed through photogrammetry with that aim in mind, which in the digital era widen their scope on account of independence of physical resources as well as high geometrical and radiometric qualities of their results. A method based on virtual 3D reconstruction of objects (scenarios) and the further 2.5 D maps generation by means of the Shape from Shading method for simulating the reflectance properties of surfaces within 2D images is being proposed. This alternative, seldom used when studying Urban-Architectural Heritage, is aimed at overcoming inadequacies when analyzing surfaces from the bi-dimentional view -or the three-dimensional one itself- over areas with little variations in elevation (relevant to urbanarchitectural context). The methodology is applied to Villagarcía de Campo Castle, Spain, stemming from the reconstruction of the property by the General Photogrammetry Method until achieving "Images by Reflectance Transformation", and ending by interpreting its topography for the inspection of un-documented historical traces.

Fiber Optic Displacement Sensor Using Multimode Plastic Fiber Probe and Tooth Surface

The principle of operation, experimentation, and performance of a fiber optic displacement sensor on a tooth surface is presented. The performances of this sensor are investigated by correlating the detector output with different probe configurations and various teeth samples with different surface reflectivity properties. With a multimode plastic bundled fiber probe, the sensitivities for the molar, canine, hybrid composite resin, and acrylic surfaces are obtained at 0.9667, 0.775, 0.5109, and 0.25 mV/mm, respectively, with a good linearity of more than 99% based on the analysis of the front slope. With a multimode plastic coupler, the sensitivities for the molar, canine, hybrid composite resin, and acrylic surfaces are obtained at 0.3538, 0.2951, 0.2069, and 0.1579 mV/mm, respectively. This proves that the sensor is capable of discriminating between different teeth surfaces. Such results are useful as guidelines for tooth surface related research, such as tooth surface profiling and measurement of tooth surface roughness.

Resistor Loaded EBG Surfaces for Slot Antenna Design

We present a resistor loaded EBG surface (REBG) for antenna design. Specifically, lumped resistors are embedded between periodic metal patches of mushroom like EBG structures. Surface wave propagation along the REBG surface is analyzed as a periodically loaded transmission line model and low transmission bandgap is identified and experimentally verified. The reflection properties of REBG surface to the incident electromagnetic waves are also studied. Slot antenna is placed within the REBG surface. The REBG surface is used to suppress surface waves across the antenna aperture, resulting in radiation performance improvement. Concurrently, the REBG surface can also absorb electromagnetic energy to reduce antenna radar cross section (RCS).

Thin Film Heater for Removable Volatile Protecting Coatings

Freshly coated aluminum mirrors have excellent reflectivity at far ultraviolet wavelengths. However, reflectivity rapidly degrades when the mirror surfaces are exposed to atmosphere. In order to avoid this problem, freshly coated aluminum surface can be protected by over-coating of a removable volatile protecting coating. This protecting coating can be re-evaporated by controlled heating or by some other methods when required. This type of removable coating has immediate application in UV space astronomy. The purpose of this paper is to demonstrate the feasibility of re-evaporation of removable volatile Zn protecting coating using a NiCr thin film heater without affecting the reflection properties of Al mirror surfaces.

Effect of temperature in electrochemical texturising of multi-crystalline silicon in KOH solution

In this paper we present the study of electrochemical etching of silicon in KOH solutions to improve texturisation and reflection properties of p–type multi–crystalline silicon. The influence of etching temperature on surface morphology and reflection properties of the silicon wafers has been investigated. The results of the study showed that etching at high temperatures formed isotropic textured concaves on the silicon surface which is similar to the effect of acidic etching. Etching at room temperature resulted in formation of a uniform nano–porous texture and lowest reflectance. With optimised etching conditions, the total reflectance of the textured surface reduced below 25%. These results can be utilised in further studies to improve the efficiency of solar cells based on multi–crystalline silicon.

DEM Densification Using Perspective Shape From Shading Through Multispectral Imagery

Although photogrammetry has long been used as the primary method to produce digital elevation models (DEMs), there are still some locations that are not covered by multiangle images because of various limitations. Moreover, traditional interpolation methods for producing denser DEMs often cause oversmoothing, particularly over rough terrain. To formulate a robust procedure that is capable of reconstructing the terrain surface using sparse ground control points in heterogeneous areas, this letter describes an improved spatial enhancement method combining perspective shape from shading (SFS) using a single optical image. This method is called SFS-based densification using multispectral information (SDMI). First, the image irradiance equation based on the perspective Lambertian model is formulated as a static Hamilton–Jacobi equation and is solved using a fast sweeping strategy. We then reconstruct the relative surface shape and apply an edge-preserved iterative interpolation method to generate a higher resolution DEM grid. Multispectral information is used to reveal the actual surface reflection properties, and the land surface is classified into several types to better estimate surface reflectance. Experiments indicate that SDMI is effective for the interpolation of a sparse grid DEM over heterogeneous terrain.