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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Geometric-optical bidirectional reflectance modeling of the discrete crown vegetation canopy - Effect of crown shape and mutual shadowing

Many natural vegetation covers can be regarded as assemblages of plant crowns, on a background, which interact with light as discrete objects. On this basis, geometric optics furnish an approach to the modeling of vegetation canopies that captures the most important features of such growths' bidirectional measurements. Attention is presently given to models which approximate these phenomena and relate the size, shape, and count density of plant crowns to viewing and illumination positions and to crown-background reflectance contrasts.

Li, Xiaowen↗
NASA NTRS1991

Application of Hapke photometric model to three geologic surfaces using PARABOLA bidirectional reflection data

The Geologic Remote Sensing Field Experiment (GRSFE) was conducted in July and September of 1989 to collect data with both ground and airborne instrumentation. A major objective of GRSFE was to collect data which could be used to test radiative transfer models for the extraction of composition and textural surface properties from remotely acquired data. Reported here are the initial results from an application of the Hapke photometric model, using data from the Portable Apparatus for Remote Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA), a ground based radiometer with three spectral channels. PARABOLA data was collected in the Lunar Crater Volcanic Field in Nevada, specifically from the region of Lunar Lake, a playa. The Hapke model was found to be inadequate for three relatively common geologic surfaces (a clay-rich, hard packed surface with decimeter sized mudcracks; a cobble site, similar to a playa site, but strewn with basaltic cobbles and pebbles; and a surface mantled basalt lava flow). The model is not at fault; rather, the complexity of most geologic surfaces is not accounted for in the initial assumptions.

Shepard, Michael K.↗
NASA NTRS2002

Using Multi-Angle Imaging SpectroRadiometer (MISR) bidirectional reflectances for land surface studies

MISR imagery enables new methods for cloud screening and retrieval of aerosol properties over a wide range of surface types, and these data are used to compensate for atmospheric effects in the surface retrievals. The derived surface angular signatures are governed, at least in part, by macroscopic texture at the subpixel scale. Such information complements spectral data in distinguishing different types of surface cover, retrieving land surface hemispherical reflectance,and determining biogeophysical properties such as leaf area index and fractional absorbed photosynthetically active radiation. Regional and global MISR data products will be presented to illustrate the capabilities of the instrument andassociated retrieval methodologies.

MISR bidirectional surface studies spectr↗
NASA NTRS1986

A note on Hapke's bidirectional reflectance spectroscopy. Part 3: Correction for macroscopic roughness

The multiple-scattering theory of K. Lumme and E. Bowell (1981) was criticized by B. Hapke by stating, in particular, that energy is not conserved. It is shown that Hapke's treatment is, in this respect, inferior to that of Lumme and Bowell, and itself violates the principal concepts of radiative transfer theory. Hapke's additinal claim that, in Lumme and Bowell's work, the reflectance tends to zero at the limb is also refuted. Comment is made on the deduction of surface physical properties by modeling photometric observations.

Lumme, K.↗
NASA NTRS1987

Bidirectional reflectance properties of planetary surface materials

Laboratory measurements using a spectrogoniometer to separate the effects of surficial texture and albedo in the characterization of planetary surface materials are discussed. An investigation of the surface of Io is discussed. A number of technical improvements to the goniometer are summarized.

Buratti, B.↗
NASA NTRS1987

Bidirectional reflectance spectroscopy

An analytical model is developed for the opposition effects (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a bright surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes. To illustrate the theory it is fitted to observations of the Moon, an asteroid, and a satellite of Uranus; Europa is also discussed.

Hapke, Bruce↗
NASA NTRS1986

Bidirectional reflectance spectroscopy. IV - The extinction coefficient and the opposition effect

An analytical model is developed for the opposition effect (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a brightness surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes.

Hapke, B.↗
NASA NTRS1988

Response to soil moisture of spectral indexes derived from bidirectional reflectance in Thematic Mapper wavebands

Laboratory reflectance measurements of 10 soils were used to determine the relationship between soil moisture and three spectral indices: the TM5/7 ratio and the Wetness(R) and Brightness(R) features of the reflectance factor TM Tasseled Cap transformation. Response of the indices to dry mass water percentage was approximately linear for individual soils, except for Wetness(R) and Brightness(R) at high moisture content. Soil differences in the slopes of the Wetness(R)- and Brightness(R)-moisture content relationships were almost entirely eliminated by expressing water content as the percentage of water retained at 0.1 bar (10 kPa) tension (relative water content). The resultant soil lines were offset from one another by the differences in dry soil index value. Slope of the TM5/7 response was not completely normalized by expressing moisture status as relative water content, because slope appeared to vary with dry soil ratio value. Sensitivity to the effects of illumination angle was negligible for the TM5/7 ratio, somewhat greater for Wetness(R) and greatest for Brightness(R).

Musick, H. Brad↗
NASA NTRS1989

The Tsarev meteorite - Petrology and bidirectional reflectance spectra of a shock-blackened L chondrite

The Tsarev meteorite is a highly shocked black L5 ordinary chondrite. Meteorites of this class may have been subjected to regolith processes on or near the surface of ordinary chondrite bodies. The study of their optical alteration can provide valuable support in the spectral search for ordinary chondrite parent bodies. Although Tsarev is very dark in a cut-surface hand sample, it is characterized by subtle variations in apparent darkness, called in this work grey and black areas. Both areas are substantially optically altered from normal ordinary chondrites and are characterized by a much lower albedo and strongly suppressed absorption features. Although both areas are altered, they show differences in albedo and the strength of absorption features. Particulate samples from the black area have a slightly higher albedo and stronger absorption features, while the particulate samples from the grey area show a systematic suppression of spectral features and albedo. Chemical, mineralogical, and spectral analyses suggest that the spectral differences can be the result of one or a combination of several factors, including shock effects on the crystallographic structure of the minerals and differences in the size, distribution, and amount of metal and troilite grains.

Britt, D. T.↗
NASA NTRS1989

Observation and analysis for the bidirectional reflectance of stratus clouds at near-infrared wavelengths

Results are presented from observations of the spectral reflectance of marine stratocumulus clouds acquired during the First ISCCP Regional Experiment in June and July, 1987. The data were obtained with aircraft-mounted cross track scanning radiometers. The radiance data show persistent and significant scattering features. The characteristics of the observed NIR reflectance patterns are examined and compared with model calculations based on in situ particle measurements.

Spinhirne, J. D.↗
NASA NTRS1991

A pointable, helicopter-based remote sensing data acquisition system for collecting bidirectional reflectance data

Discussed here is a new configuration of a helicopter-borne remote sensing system to support field measurements programs such as studies of forest decline damage associated with atmospheric deposition and the Forest Ecosystems Dynamics project. In this new configuration, the sensor payload is mounted externally on a hinged pallet located on the starboard side of the Bell Iroquois UH-1B (HUEY) helicopter. The sensor payload includes a set of bore-sighted instruments consisting of a Barnes Modular Multiband Radiometer (MMR), a Spectron Engineering (SE) 590 spectroradiometer, an Everest infrared temperature sensor, and a Sony CCD video camera with a 10x zoom lens. The system was tested and excellent data was acquired in 1990.

Williams, Darrel L.↗
NASA NTRS1991

The bidirectional reflectance spectra of five gas-rich ordinary chondrites

In general, the spectra of the dark areas of gas-rich ordinary chondrites show reductions in both reflectance and band depth relative to the spectra of the light areas and the spectra of normal ordinary chondrites. The dark areas of these samples show spectral alteration despite having substantial fractions of light material intimately mixed with the dark, gas-rich grains. If the spectra of the dark gas-rich grains represent the spectral characteristics of a regolith soil on an ordinary chondrite body, then that surface would display a dark and strongly subdued spectrum.

Britt, Daniel T.↗
NASA NTRS1997

Measurement and Validation of Bidirectional Reflectance of Space Shuttle and Space Station Materials for Computerized Lighting Models

Task illumination has a major impact on human performance: What a person can perceive in his environment significantly affects his ability to perform tasks, especially in space's harsh environment. Training for lighting conditions in space has long depended on physical models and simulations to emulate the effect of lighting, but such tests are expensive and time-consuming. To evaluate lighting conditions not easily simulated on Earth, personnel at NASA Johnson Space Center's (JSC) Graphics Research and Analysis Facility (GRAF) have been developing computerized simulations of various illumination conditions using the ray-tracing program, Radiance, developed by Greg Ward at Lawrence Berkeley Laboratory. Because these computer simulations are only as accurate as the data used, accurate information about the reflectance properties of materials and light distributions is needed. JSC's Lighting Environment Test Facility (LETF) personnel gathered material reflectance properties for a large number of paints, metals, and cloths used in the Space Shuttle and Space Station programs, and processed these data into reflectance parameters needed for the computer simulations. They also gathered lamp distribution data for most of the light sources used, and validated the ability to accurately simulate lighting levels by comparing predictions with measurements for several ground-based tests. The result of this study is a database of material reflectance properties for a wide variety of materials, and lighting information for most of the standard light sources used in the Shuttle/Station programs. The combination of the Radiance program and GRAF's graphics capability form a validated computerized lighting simulation capability for NASA.

Fletcher, Lauren E.↗
NASA NTRS1998

Bidirectional Reflectance Factors used in Lidar Calibration

Many atmospheric remote sensing applications of infrared lidar systems require that the backscatter signal be radiometrically calibrated. These systems are usually monostatic, consequently the reflected radiance from hard targets used in calibration will include the diffuse and the retroreflectance characteristics of the material. Radiometers which are part of orbiting sensors frequently contain onboard calibration subsystems. The onboard calibrator often utilizes onboard reflectance panels which are deployed at particular orbital locations to use the reflected sun light to calibrate the radiometers. To meet the need for correlative calibration of these remote sensing techniques a physically derived model could be used to extend the angular range and to improve the angular resolution of the measured reflectance properties of the calibration materials. A modified version of the B. Hake and E. Wells model has been adapted and will be described.

Haner, David A.↗
NASA NTRS2004

Laboratory Measurement of Bidirectional Reflectance of Radiometric Tarps

This experiment measured the reflectance of tarps with ground instruments in order to check radiometric calibration, validate atmospheric correction, and predict at-sensor radiance for satellite instruments. The procedure of this experiment is as follows: 1) Assemble laboratory apparatus to duplicate ground reference measurement geometry and satellite measurement geometry; 2) Measure spectral radiance with Optronics OL 750 double monochromator/spectroradiometer; 3) Measure radiance of NIST-calibrated Spectralon panel irradiated by collimated light at incidence angle of calibrated reflectance (20 deg, 30 deg, 40 deg, or 50 deg), viewing normal to panel surface; 4) Measure radiance of Spectralon panel irradiated at incidence angle equal to solar zenith angle at time of overpass; 5) Calculate reflectance of Spectralon panel irradiated at solar zenith angle, viewing normal to panel surface (ground geometry).

Knowlton, Kelly↗