La rugosité est une propriété clé des sols qui contrôle de nombreux processus de surface et influence la fonction de diffusion du rayonnement incident, alias sa BRDF (Bidirectional Reflectance Distribution Function). Bien qu’elle dépende fortement de l’échelle spatiale, la rugosité est souvent considérée comme stationnaire dans les modèles photométriques de réflectance de surfaces. En particulier, celui de Hapke l’inclut sous la forme d’un angle de pente moyen, intégré sur toutes les échelles variant de la taille d’un grain du régolithe à celle de la topographie locale. Le sens physique de ce paramètre de rugosité moyenne est largement débattu car l’échelle n’est pas clairement définie. Cette thèse a pour objectifs de comprendre comment la rugosité moyenne peut décrire un phénomène multi-échelle et d’investiguer l’influence des échelles spatiales de rugosité sur la BRDF d’une surface. On teste notamment la capacité du modèle de Hapke à restituer par inversion de la BRDF une rugosité moyenne compatible avec la réalité terrain. La topographie de terrains volcaniques et sédimentaires du Piton de la Fournaise (île de La Réunion) et du rift d’Asal-Ghoubbet (République de Djibouti) a été mesurée par photogrammétrie haute résolution sur une large gamme de résolutions à partir de données multi-instrumentales : images satellite, drone et acquises manuellement. Leur BRDF a été mesurée en parallèle par Pléiades et par un spectro-goniomètre (appelé Chamelon), et simulée numériquement par tracé de rayon sur les MNT réalisés. Une analyse multi-échelle par transformée en ondelettes révèle le comportement multi-modal de la rugosité des surfaces étudiées et permet de montrer que les structures sub-centimétriques dominent à la fois le paramètre de rugosité intégré et la forme de la BRDF. La rugosité estimée par inversion sur les données simulées avec une version simplifiée du modèle de Hapke coïncide avec celle déterminée sur les modèles de surface lorsque les hypothèses du modèle sont respectées et l’albédo connu à priori. L’adéquation n’est pas systématique mais reste bonne dans le cas de terrains à rugosité modérée avec une version complète du modèle de Hapke / Surface roughness is a key property of soils that controls many surface processes and influences the scattering function, or BRDF (Bidirectional Reflectance Distribution Function), of incident radiation. While it is strongly scale-dependent, it is often considered as a stationnary parameter in photometric models. In particular, it is included in the Hapke model as a mean slope angle, integrated over all scales from the grain size to the local topography. Yet its physical meaning is still a question at issue, as the scale at which it occurs is undefined. This thesis aims at understanding how this mean parameter can describe a multiscale phenomenon and to investigate the role of spatial scale on surface BRDF. Finally, we investigate the ability of the Hapke model to retrieve a roughness parameter which is consistent with the ground truth. The topography of volcanic and sedimentary terrains in the Piton de la Fournaise (Réunion Island) and the Asal-Ghoubbet rift (Republic of Djibouti) has been measured using high resolution photogrammetry at a wide range of resolutions thanks to multi-instrumental data : satellite, drone and handheld images. Simultaneously, the BRDF has been numerically simulated, and measured by satellite and a spectrogoniometer (named Chamelon). A multiscale analysis by the means of the wavelet transform reveals the multi-modal behavior of roughness and shows that sub-centimeter surface features dominate both the integrated parameter and the shape of the BRDF. The roughness estimated by inversion of a simplified version of the Hapke model matches the roughness determined over surfaces when the assumptions of the model are verified, with a priori knowledge on surface albedo. The match is not systematic, but remains good for moderately rough terrains using the full Hapke model

The atmosphere distorts the spectrum of remotely sensed data, negatively affecting all forms of investigating Earth's surface. To gather reliable data, it is vital that atmospheric corrections are accurate. The current state of the field of atmospheric correction does not account well for the benefits and costs of different correction algorithms. Ground spectral data are required to evaluate these algorithms better. This dissertation explores using cameras as radiometers as a means of gathering ground spectral data. I introduce techniques to implement a camera systems for atmospheric correction using off the shelf parts. To aid the design of future camera systems for radiometric correction, methods for estimating the system error prior to construction, calibration and testing of the resulting camera system are explored. Simulations are used to investigate the relationship between the reflectance accuracy of the camera system and the quality of atmospheric correction. In the design phase, read noise and filter choice are found to be the strongest sources of system error. I explain the calibration methods for the camera system, showing the problems of pixel to angle calibration, and adapting the web camera for scientific work. The camera system is tested in the field to estimate its ability to recover directional reflectance from BRF data. I estimate the error in the system due to the experimental set up, then explore how the system error changes with different cameras, environmental set-ups and inversions. With these experiments, I learn about the importance of the dynamic range of the camera, and the input ranges used for the PROSAIL inversion. Evidence that the camera can perform within the specification set for ELM correction in this dissertation is evaluated. The analysis is concluded by simulating an ELM correction of a scene using various numbers of calibration targets, and levels of system error, to find the number of cameras needed for a full-scale implementation.

In the development of autonomous vehicles, replacing the driver and its perceptive abilities is one of many technical challenges. As a part in solving these challenges, Light Detection And Ranging (LiDAR) is a promising technology. In short, LiDAR works by using lasers to detect objects in its vicinity by detecting the light that reflects on them. With knowledge of the reflective properties of an object, a prediction can be made regarding whether a certain LiDAR unit will be able to detect the object or not. When making this prediction, the common description of reflectance is often insufficient. Instead, a more complete description is given by the Bidirectional Reflectance Distribution Function (BRDF) of a surface, which describes reflection on the surface while taking the incident and reflected direction into consideration. In this thesis, an experimental setup was built with the capabilities of measuring the BRDF while taking incident and reflected polarization into account. Program software was written in Python and integrated with the hardware, providing a user interface for simple control of the setup. The BRDF was measured on a total of 6 samples; 2 reference samples and 4 samples taken from the hood of 4 different cars. Conclusively, the setup provided useful information about the reflective and polarization properties of the samples. These measurements can help in predicting whether or not a surface can be detected by a given LiDAR unit, and can also be helpful when designing new LiDAR units by providing useful information about the surfaces they are required to detect.

Gold-black has emerged as a popular absorptive coating for thermal radiation detectors in aerospace applications. The performance and accuracy of thermal radiation detectors largely depends on the surface optical properties of the absorptive coating. If the absorptivity of the layer is directional or wavelength dependent, then so will be the detector gain itself. This motivates our interest in the manufacture, physical characterization, and study of the wavelength and polarization sensitivity of the directional spectral absorptivity of gold-black. A first-principle model based on lossy antenna theory is presented to predict the polarization dependent directional spectral absorptivity of gold-black in the visible and near infrared. Results for normal spectral absorptivity are in good agreement with measurements reported in the literature. However, suitable experimental data were not available to validate the theory for directional spectral absorptivity. Therefore, an experimental campaign to fabricate and measure the directional spectral behavior of gold-black had to be undertaken to validate the first-principle model. New in-plane bidirectional reflectance distribution function (BRDF) measurements for two thicknesses (~4 μm and ~8 μm) of gold-black laid down on a gold mirror substrate are reported in the visible (532 nm) and near-infrared (800 and 850 nm) for p- and s-polarizations. The investigation is then extended to a three-layer sample, which is shown to exhibit off-specular reflectivity. Described are processes for laying down gold-black coatings and for measuring their in-plane BRDF as a function of thickness, wavelength, and polarization state. A novel method for retrieving the directional absorptivity from in-plane BRDF measurements is presented. The influence of polarization on directional absorptivity is shown to follow our earlier theory except at large incident zenith angles, where an unanticipated mirage effect is observed. / Doctor of Philosophy / Instruments called thermal radiation detectors play an important role in monitoring the global climate from space. Gold-black is often used as an absorptive coating to enhance the performance of these instruments. Users need to know how gold-black coatings influence instrument performance. In general, coating properties depend on the wavelength and direction of incident radiation, as well as on an optical phenomenon called polarization. This dissertation investigates the relationship between the creation of gold-black coatings and their performance. A physical model is postulated for predicting the optical behavior of gold-black in the visible and near infrared. The model produces results that are in good agreement with measurements reported in the literature. However, suitable directional measurements were not available to validate the theory. Therefore, an experimental campaign was mounted to fabricate gold-black coatings and measure their optical behavior in order to validate the mathematical model. We observed the optical behavior of several of our gold-black samples of various thickness and over a range of wavelengths. We also studied a three-layer sample which was found to exhibit an unexpected behavior called off-specular reflectivity. Described are processes for creating gold-black coatings and for measuring and explaining their optical performance. During the course of this investigation an unanticipated mirage effect was observed for the first time.

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Approved for public release, distribution is unlimited / ine discrimination of similar soil classes was produced by the BRDF variations in the high-spatial resolution panchromatic image. Texture analysis results depended on the directionality of the gray level co-occurrence matrix (GLCM) calculation. Combining the different modalities of analysis did not improve the overall classification, perhaps illustrating the consequences of the Hughes paradox (Hughes, 1968) / Flight Lieutenant, Royal Thai Air Force

Das Monitoring von Landwirtschaftsflächen ist eines der Kernthemen der zukünftigen EnMAP Mission, einem deutschen, hyperspektralen Fernerkundungssensor, dessen Start für 2015 geplant ist. In Vorbereitung dieser Mission gehören die Erweiterung und Entwicklung objektiver, robuster sowie zuverlässiger Methoden zur Ableitung biophysikalischer Parameter zu den Hauptaufgaben. Die für das Monitoring von Vegetation notwendige hohe zeitliche Auflösung wird durch ein stufenloses Schwenken von bis zu +/-30° quer zur Flugrichtung erreicht. Daraus resultiert, dass die Daten durch variierende Ein- und Ausstrahlungsgeometrien stark beeinflusst werden. Daher ist eine detaillierte Kenntnis der bidirektionalen Reflexionsfunktion (engl. bidirectional reflectance distribution function, BRDF) der beobachteten Oberflächen notwendig, um diese Einflüsse auf das Reflexionssignal zu identifizieren und anschließend zu korrigieren. Zu diesem Zweck wurde in dieser Arbeit eine Methodik entwickelt, die es ermöglicht, auf der Basis von simulierten Spektren realistische BRDF Szenarien zu modellieren und oberflächenspezifische Korrekturfunktionen abzuleiten. Die Methodik besteht aus drei aufeinander aufbauenden Komponenten. Im ersten Schritt erfolgt die Modellierung der BRDF von landwirtschaftlichen Vegetationsbeständen. Im zweiten Schritt wird der Einfluss der Bestandsarchitektur auf die BRDF analysiert. Darüber hinaus wird untersucht, inwiefern sich Variationen in der BRDF auf die quantitative Ausprägung von Vegetationsindizes auswirken. Solche Indizes sind eine häufig genutzte Möglichkeit zur Quantifizierung biophysikalische Parameter im Rahmen empirischer Verfahren. Aufbauend auf den gewonnenen Erkenntnissen wurden im dritten Schritt Korrekturfunktionen für ausgewählte Vegetationsindizes entwickelt, um Schrägblickbeobachtungen in Nadirbeobachtungen zu transformieren. Abschließend wurde die entwickelte Methodik auf simulierte, sensorspezifische Spektren übertragen. / Monitoring of arable crops is one of the core applications of the upcoming spaceborne EnMAP mission, a German hyperspectral imaging spectrometer scheduled for launch in 2015. During the present preparatory phase one of the primary tasks is the development of accurate, robust and reliable retrieval methods for biophysical canopy parameters. Monitoring of crop canopies requires a frequent temporal coverage. In case of EnMAP, this will be realised by an off-nadir pointing of the sensor up to +/-30° across to the flight direction. The off-nadir pointing leads to data strongly influenced by varying acquisition geometry. Therefore, detailed knowledge of bidirectional reflectance distribution functions (BRDF) of the observed surfaces is necessary to identify and to correct BRDF influenced reflectance signals. For this purpose, a methodology was developed that allows modelling of realistic BRDF scenarios and of surface-specific correction functions. This methodology consists of three consecutive parts. In the first part, modelling of the BRDF of crop canopies was performed. In the second part, the influence of canopy architecture on the BRDF was analysed. Additionally, the BRDF related dependencies of vegetation indices were investigated. Such indices are widely used to quantify biophysical canopy parameters based on empirical methods. In the third part, correction functions were developed for selected vegetation indices to enable a transformation from off-nadir into nadir observations. These correction functions incorporate results obtained in the second part of the methodology. Finally, the developed methodology was applied on sensor-specific simulated spectra.

Multi-Angle, Multi-Date, Hyperspectral imagery of forests have been used to provide accurate estimates of the canopy characteristics. This thesis investigated the influence of various forest attributes on the spectral reflectance over time and view direction. The Compact High Resolution Imaging Spectrometer (CHRIS) is aboard the ESA PROBA satellite. The revisits of the CHRIS multi-angle images have been used to improve the accuracies of forest species recognition and stand densities compared to a nadir view only. Multi-angle data for CHRIS analysis of forest species produced higher accuracy and were easier to obtain than multi-date date. 5-Scale, a radiative transfer model, and CHRIS data have been compared as inputs into Partial Least Squares (PLS), a fullspectrum analytical method that offers relations between forest stand parameters and the resulting spectra. The resulting coefficients highlight where (view angle and spectral regions) within the multi-angle spectra contributed to estimating the various forest parameters. Methodology of collecting spectral calibration data in the field and the unique pre-processing challenges have been described.

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For producing realistic images, reflection is an important visual effect. Reflections of the environment are important not only for highly reflective objects, such as mirrors, but also for more common objects such as brushed metals and glossy plastics. Generating these reflections accurately at real-time rates for interactive applications, however, is a difficult problem. Previous works in this area have made assumptions that sacrifice accuracy in order to preserve interactivity. I will present an algorithm that tries to handle reflection accurately in the general case for real-time rendering. The algorithm uses a database of prerendered environment maps to render both the original object itself and an additional bidirectional reflection distribution function (BRDF). The algorithm performs image-based rendering in reflection space in order to achieve accurate results. It also uses graphics processing unit (GPU) features to accelerate rendering.