Charles M. Bachmann

Wavelength dependence of the bidirectional reflectance distribution function (BRDF) of beach sands.

The wavelength dependence of the dominant directional reflective properties of beach sands was demonstrated using principal component analysis and the related correlation matrix. In general, we found that the hyperspectral bidirectional reflectance distribution function (BRDF) of beach sands has weak wavelength dependence. Its BRDF varies slightly in three broad wavelength regions. The variations are more evident in surfaces of greater visual roughness than in smooth surfaces. The weak wavelength dependence of the BRDF of beach sand can be captured using three broad wavelength regions instead of hundreds of individual wavelengths.

Modeling geophysical properties of the Algodones Dunes from field and laboratory hyperspectral goniometer measurements using GRIT and comparison with G-LiHT imagery

We measure and describe the angular dependence of field and laboratory hyperspectral reflectance measurements of sediments from the Algodones Dunes, CA using the Goniometer of the Rochester Institute of Technology (GRIT) and compare with NASA G-LiHT hyperspectral imagery. G-LiHT imagery was acquired concurrently during a joint field experiment in March 2015 conducted by NASA Goddard, South Dakota State University, University of Arizona, University of Lethbridge, and Rochester Institute of Technology (RIT). Radiative transfer models1 and our own observations10 demonstrate that the angular dependence observed in the bidirectional reflectance distribution (BRDF)1,2,3,4,5,6 is strongly influenced by factors such as density, grain size distribution, moisture content, and surface roughness.5,6,7,8,9 Hapke’s model applied to a uniform sediment predicts increasing reflectance as density increases, however, we have observed that multiple scattering and the presence of optically contrasting mineral fractions can lead to the opposite trend.9,10 The degree of multiple scattering is influenced by incident illumination zenith angle, which determines whether the Hapke prediction is observed or the opposite trend.10 To better match observations, modifications of the model are necessary.10 In this paper, we consider some initial work showing the relationship between NASA G-LiHT hyperspectral imagery and GRIT10 field and laboratory BRDF and GRIT-Two (GRIT-T)11 laboratory BRDF. We also discuss preliminary work using this data for retrieval of geophysical properties of the sediment such as density from multi-angular measurements.

A next generation field-portable goniometer system

Various field portable goniometers have been designed to capture in-situ measurements of a materials bi-directional reflectance distribution function (BRDF), each with a specific scientific purpose in mind.1-4 The Rochester Institute of Technologys (RIT) Chester F. Carlson Center for Imaging Science recently created a novel instrument incorporating a wide variety of features into one compact apparatus in order to obtain very high accuracy BRDFs of short vegetation and sediments, even in undesirable conditions and austere environments. This next generation system integrates a dual-view design using two VNIR/SWIR pectroradiometers to capture target reflected radiance, as well as incoming radiance, to provide for better optical accuracy when measuring in non-ideal atmospheric conditions or when background illumination effects are non-negligible. The new, fully automated device also features a laser range finder to construct a surface roughness model of the target being measured, which enables the user to include inclination information into BRDF post-processing and further allows for roughness effects to be better studied for radiative transfer modeling. The highly portable design features automatic leveling, a precision engineered frame, and a variable measurement plane that allow for BRDF measurements on rugged, un-even terrain while still maintaining true angular measurements with respect to the target, all without sacrificing measurement speed. Despite the expanded capabilities and dual sensor suite, the system weighs less than 75 kg, which allows for excellent mobility and data collection on soft, silty clay or fine sand.

Improved modeling of multiple scattering in hyperspectral BRDF of coastal sediments observed using the Goniometer of the Rochester Institute of Technology (GRIT)

Approximate solutions to the Radiative transfer equation for granular media have been previously developed1. To apply these models to coastal sediments, modifications are needed to account for observed phenomenology. This study uses a new hyperspectral goniometer system, the Goniometer of the Rochester Institute of Technology (GRIT), designed for both field and laboratory settings, to compare observed bidirectional reflectance distribution function (BRDF) measurements with outcomes predicted by the approximate radiative transfer solutions. In previous laboratory studies,2 using a more limited hyperspectral goniometer observing in the principle plane, we had seen that the degree of optical contrast between coastal sand constituents was indicative of whether these models accurately predict the observed BRDF dependence on sediment density. Our earlier measurements using another field hyperspectral goniometer also demonstrated results consistent with the laboratory measurements as well as with CASI- 1500 airborne hyperspectral measurements3,4. In our earlier work,2 the presence of highly contrasting constituents (translucent quartz and more opaque fractions composed of minerals such as magnetite) led to greater reflectance as density decreased, exactly the opposite of what was anticipated from radiative transfer models for a more uniform sand. The present study shows that the illumination zenith angle also plays a significant role in whether or not BRDF dependency exhibits behavior predicted by current radiative transfer theory, and this distinction is directly related to the degree of multiple scattering, which depends on the illumination zenith angle. We also investigate a novel sampling paradigm that constrains the measurements to constant phase angle and reveals when the multiple scattering component of models departs from the assumptions of current theory. For the multiple scattering term, we also propose and analyze a simple modification which removes the isotropic assumption and provides a better match to BRDF observations under the constrained sampling paradigm.

Soil signature simulation of complex mixtures and particle size distributions

Abstract. Soil reflectance signatures were modeled using the digital imaging and remote sensing image generation model and Blender three-dimensional (3-D) graphic design software. Using these tools, the geometry, radiometry, and chemistry of quartz and magnetite were exploited to model the presence of particle size and porosity effects in the visible and the shortwave infrared spectrum. Using the physics engines within the Blender 3-D graphic design software, physical representations of granular soil scenes were created. Each scene characterized a specific particle distribution and density. Chemical and optical properties of pure quartz and magnetite were assigned to particles in the scene based on particle size. This work presents a model to describe an observed phase-angle dependence of beach sand density. Bidirectional reflectance signatures were simulated for targets of varying size distribution and density. This model provides validation for a phenomenological trade space between density and particle size distribution in complex, heterogeneous soil mixtures. It also confirms the suggestion that directional reflectance signatures can be defined by intimate mixtures that depend on pore spacing. The study demonstrated that by combining realistic target geometry and spectral measurements of pure quartz and magnetite, effects of soil particle size and density could be modeled without functional data fitting or rigorous analysis of material dynamics. This research does not use traditional function-based models for simulation. The combination of realistic geometry, physically viable particle structure, and first-principles ray-tracing enables the ability to represent signature changes that have been observed in experimental observations.

Modeling and intercomparison of field and laboratory hyperspectral goniometer measurements with G-LiHT imagery of the Algodones Dunes

Abstract. We compare field hyperspectral bidirectional reflectance distribution function (BRDF) measurements acquired by a hyperspectral goniometer system known as the goniometer of the Rochester Institute of Technology (GRIT) during an experiment in the Algodones Dunes system in March 2015 with NASA Goddard’s light detection and ranging, hyperspectral, and thermal imagery of the site acquired during the experiment. We augment our field spectral data collection with laboratory hyperspectral BRDF measurements of samples brought back from the Algodones Dunes site using GRIT and our second-generation goniometer GRIT-two (GRIT-T). In these laboratory experiments, we vary geophysical parameters such as sediment density and grain size distribution of the sediments that would typically impact observed BRDF with the goal of extending the range of applicability of our resulting BRDF spectral libraries. Geotechnical measurements on site confirm the variability of geophysical parameters such as density and grain size distributions within the dune system, and measurements with GRIT and GRIT-T demonstrate the impact on observed spectral variation. By augmenting field spectral libraries with laboratory BRDF, we show that a greater proportion of the dune system is more faithfully represented in the expanded spectral library. Beyond developing appropriate calibration data for airborne and satellite imagery of the Algodones Dunes, laboratory and field studies also support goals to develop reliable retrieval methods for geophysical quantities such as sediment density directly from spectral imagery. We consider approaches based on the Hapke model. Our approaches use the invariance of the observed functional forms of the single scattering phase function, which must be invariant to differences in the illumination geometry. Fill factor is retrieved and correlates with expected direct measurements of sediment density in a laboratory setting.

Overview of the 2015 Algodones Sand Dunes field campaign to support sensor intercalibration

Abstract. Several sites from around the world are being used operationally and are suitable for vicarious calibration of space-borne imaging platforms. However, due to the proximity of these sites (e.g., Libya 4), a rigorous characterization of the landscape is not feasible, limiting their utility for sensor intercalibration efforts. Due to its accessibility and similarities to Libya 4, the Algodones Sand Dunes System in California, USA, was identified as a potentially attractive intercalibration site for space-borne, reflective instruments such as Landsat. In March 2015, a 4-day field campaign was conducted to develop an initial characterization of Algodones with a primary goal of assessing its intercalibration potential. Five organizations from the US and Canada collaborated to collect both active and passive airborne image data, spatial and temporal measurements of spectral bidirectional reflectance distribution function, and in-situ sand samples from several locations across the Algodones system. The collection activities conducted to support the campaign goal is summarized, including a summary of all instrumentation used, the data collected, and the experiments performed in an effort to characterize the Algodones site.

Manifold alignment with Schroedinger eigenmaps

The sun-target-sensor angle can change during aerial remote sensing. In an attempt to compensate BRDF effects in multi-angular hyperspectral images, the Semi-Supervised Manifold Alignment (SSMA) algorithm pulls data from similar classes together and pushes data from different classes apart. SSMA uses Laplacian Eigenmaps (LE) to preserve the original geometric structure of each local data set independently. In this paper, we replace LE with Spatial-Spectral Schoedinger Eigenmaps (SSSE) which was designed to be a semisupervised enhancement to the to extend the SSMA methodology and improve classification of multi-angular hyperspectral images captured over Hog Island in the Virginia Coast Reserve.

How many spectral bands are necessary to describe the directional reflectance of beach sands

Spectral variability in the visible, near-infrared and shortwave directional reflectance factor of beach sands and freshwater sheet flow is examined using principal component and correlation matrix analysis of in situ measurements. In previous work we concluded that the hyperspectral bidirectional reflectance distribution function (BRDF) of beach sands in the absence of sheet flow exhibit weak spectral variability, the majority of which can be described with three broad spectral bands with wavelength ranges of 350-450 nm, 700-1350 nm, and 1450-2400 nm.1 Observing sheet flow on sand we find that a thin layer of water enhances reflectance in the specular direction at all wavelengths and that spectral variability may be described using four spectral band regions of 350-450 nm, 500-950 nm, 950-1350 nm, and 1450-2400 nm. Spectral variations are more evident in sand surfaces of greater visual roughness than in smooth surfaces, regardless of sheet flow.

A hyperspectral vehicle BRDF sampling experiment

Hyperspectral imagery was taken of four vehicles from a roof at the Rochester Institute of Technology (RIT) at various vehicle orientations in illumination conditions dominated by direct solar radiation in order to explore and model the in-scene bidirectional reflectance distribution functions (BRDFs) of 3D objects. The four vehicles were rotated and imaged through the span of six hours resulting in many combinations of vehicle orientation, source azimuth, and source zenith. In addition to the general sampling of vehicle BRDFs, three experiments were designed and executed in order to understand the contributions of vehicle shape, vehicle color, and background on the observed in-scene BRDFs.