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At least 1,387 records · Page 77

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NASA NTRS1988

Measurements of reflectance spectra of ion-bombarded ice and application to surfaces in the outer solar system

The first results of laboratory measurements of the wavelength dependence of the alternation of the visible reflectance of H2O ice irradiated by keV ions are presented. When the implanted species is chemically neutral, absorption is slightly enhanced below 0.55 micron. For an incident species containing sulfur, a strong absorption feature is produced at 0.4 micron, probably corresponding to S3. This occurs at too large a wavelength to account for the absorption feature observed at Europa by Voyager and therefore casts doubt on the recent interpretation of the reflectance data of Europa.

O'Shaughnessy, D. J.↗

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 NTRS1988

Estimation of canopy parameters for inhomogeneous vegetation canopies from reflectance data. III - TRIM: A model for radiative transfer in heterogeneous three-dimensional canopies

A model for radiative transfer in heterogeneous three-dimensional canopies such as those found in forests is proposed. Its use in estimating important biophysical variables such as leaf area index and canopy architecture from bidirectional canopy reflectance data is discussed. The model and its use in estimating canopy parameters through its inversion are validated with measured canopy reflectance data for corn canopies.

Goel, Narendra S.↗

NASA NTRS1988

The spectral bidirectional reflectance of snow

A radiative transfer model for the bidirectional reflectance-distribution function (BRDF) shows that snow is moderately anisotropic in the near-infrared wavelengths. Although the directional-hemispherical albedo of snow decreases as the grains become larger, the forward scattering also increases, with the result that the illumination and viewing geometry must be considered when interpreting physical properties of the surface layer of the snow pack from remote sensing data. Measurements of the BRDF for a variety of snow conditions were made throughout the winter and spring seasons with a spectroradiometer, for wavelengths from 0.38 to 1.11 microns. Coincident with these the surface grain properties were analyzed by stereological methods. The data show that Mie scattering calculations using an equivalent spherical radius match the directional-hemispherical reflectance, but the BRDF usually shows a small peak in the backscattered direction that would not occur from spherical grains. The sphere with the same surface-to-volume ratio as the ice grains is used as the equivalent sphere.

Dozier, J.↗

NASA NTRS1988

Sensor fusion of range and reflectance data for outdoor scene analysis

In recognizing objects in an outdoor scene, range and reflectance (or color) data provide complementary information. Results of experiments in recognizing outdoor scenes containing roads, trees, and cars are presented. The recognition program uses range and reflectance data obtained by a scanning laser range finder, as well as color data from a color TV camera. After segmentation of each image into primitive regions, models of objects are matched using various properties.

Kweon, In SO↗

NASA NTRS1988

Short- and long-term determination of the TDRS solar reflectivity parameter using the Goddard trajectory determination system

Two aspects of the orbit determination of the Tracking and Data Relay Satellite-East (TDRS-E) are considered. The TDRS-E solar reflectivity parameter, as it affects the accuracy of the determination and propagation of the TDRS-E geosynchronous orbit, varies seasonally due to the changing declination of the sun. Orbit solutions are carried out using varying tracking data arc lengths to ascertain the minimum arc length needed for an accurate determination of the TDRS-E orbital elements and the solar reflectivity parameter.

Bobrowsky, M.↗

NASA NTRS1988

Invertible canopy reflectance modeling of vegetation structure in semiarid woodland

The Li-Strahler canopy reflectance model, driven by Landsat Thematic Mapper (TM) data, provided regional estimates of tree size and density in two bioclimatic zones in West Africa. This model exploits tree geometry in an inversion technique to predict average tree size and density from reflectance data using a few simple parameters measured in the field (spatial pattern, shape, and size distribution of trees) and in the imagery (spectral signatures of scene components). The model was tested in sparse woodland and wooded grassland in the Sahelian and Sudanian bioclimatic zones in West Africa.

Franklin, Janet↗

NASA NTRS1988

The effect of the near earth micrometeoroid environment on a highly reflective mirror surface

A resurgence of interest in placing large solar concentrator solar dynamic systems in space for power generation has brought up again a concern for maintaining the integrity of the optical properties of highly specular reflecting surfaces in the near earth space environment. One of the environmental hazards needing evaluation is the micrometeoroid environment. It has been shown that highly reflective polished metals and thin film coatings degrade when exposed to simulated micrometeoroids in the lab. At NASA-Lewis, a shock tube was used to simulate the phenomenon of micrometeoroid impact by accelerating micron sized particles to hypervelocities. Any changes in the optical properties of surfaces exposed to this impact were then evaluated. The degradation of optical properties of polished metals and thin metallic films after exposure to simulated micrometeoroids was determined as a function of impacting kinetic energy area of the particles. A calibrated sensor was developed to not only detect the micrometeoroid environment, but also to evaluate the degradation of the optical properties of thin aluminum films in space. Results of the simulation are presented and discussed.

Mirtich, Michael J.↗

NASA NTRS1988

An efficient high-frequency analysis of modal reflection and transmission coefficients for a class of waveguide discontinuities

A high-frequency analysis of electromagnetic modal reflection and transmission coefficients is presented for waveguide discontinuities formed by joining different waveguide sections. The analysis uses an extended version of the concept of geometrical theory of diffraction based equivalent edge currents in conjunction with the reciprocity theorem to describe interior scattering effects. If the waveguide modes and their associated modal rays can be found explicitly, general two- and three-dimensional waveguide geometries can be analyzed. Expressions are developed for two-dimensional reflection and transmission coefficients. Numerical results are given for a flanged, semi-infinite parallel plate waveguide and for the junction between two linearly tapered waveguides.

Pathak, P. H.↗

NASA NTRS1988

Acquisition of spectral reflectance data using an artificial source of hemispherical illumination

An integrating hemisphere illumination system has been developed to facilitate the collection of spectral reflectance factor measurements of targets of interest in a laboratory environment. One of the most significant advantages associated with such an illumination source is that repeated measurements can be made over an extended period of time, for a variety of targets, under nearly identical illumination and viewing angle conditions. The illumination system consists of a 76 cm (30 in.) aluminum hemisphere coated internally with barium sulfate paint. Illumination is provided by sixteen 62 watt quartz halogen bulbs with tungsten filaments. A simple metal structure has been developed to hold the hemisphere and all peripheral equipment, such as spectrometers, radiometers, and cameras, in place during data collection. The entire set up can be easily disassembled and packed in airline-approved shipping cases to facilitate transportation to laboratory facilities located near any study area. The illumination system is described briefly, and numerous plots of radiance and spectral reflectance are provided to illustrate the performance and utility of the apparatus.

Williams, Darrel L.↗

NASA NTRS1988

Tersail - A numerical model for combined analysis of vegetation canopy bidirectional reflectance and thermal emissions

A modification of the Tergra model (Soer, 1977) is presented, which incorporates the scattering from arbitrarily inclined leaves canopy reflectance model (Verhoef and Bunnik, 1981) for the calculation of albedo and canopy resistance. The combined model, known as Tersail, is capable of simulating the relationship between the bidirectional reflectance and the thermal response of a canopy. The accuracy of the model is tested using data over wheat canopies in Phoenix, Arizona, showing that the model is a good simulator of canopy temperatures under a variety of conditions.

Hope, Allen S.↗

NASA NTRS1989

Optically controlled reflection modulator using GaAs-AlGaAs n-i-p-i/multiple-quantum-well structures

An optically controlled reflection modulator has been demonstrated that consists of a combination of a GaAs-AlGaAs n-i-p-i doping structure with a multiple-quantum-well structures on top of a distributed Bragg reflector, all grown by MBE. A modulation of approximately 60 percent is obtained on the test structure, corresponding to a differential change of absorption coefficient in the quantum wells of approximately 7500/cm. Changes in reflectance can be observed with a control beam power as low as 1.5 microW. This device structure has the potential of being developed as an optically addressed spatial light modulator for optical information processing.

Law, K.-K.↗

NASA NTRS1988

Vegetation reflectance features in AVIRIS data

Equations for converting Airborne Visible-IR Imaging Spectrometer (AVIRIS) digital numbers to percent reflectance were developed using a set of three calibration targets. When converted to reflectance the following spectral features of vegetation can be observed in AVIRIS data: details in the pigment absorption region, the red edge, the NIR plateau, leaf water absorptions on the NIR plateau, leaf water absorptions, and lignocellulose absorptions. These results indicate that AVIRIS data will be quite useful in geobotanical remote sensing.

Elvidge, Christopher D.↗

NASA NTRS1989

The reflectance of Ames 24E, Infrablack, and Martin black

Results are reported from measurements of the specular reflectances (SRs) and bidirectional reflectance distribution functions (BRDFs) of three black optical coatings in the FIR wavelength range. The nonspecular reflectometer apparatus described by Smith (1984) is employed, and the data are presented in tables and graphs and discussed in detail. It is found that Ames 24E has an FIR SR one order of magnitude lower than that of Martin black (MB), with BRDF values characteristic of a nearly Lambertian surface, while Infrablack has SR two orders lower than MB and a specular-diffuse surface; MB itself has a very specular surface.

Smith, Sheldon M.↗

NASA NTRS1989

Comparison of the bidirectional reflectance distribution function of various surfaces

This paper describes the development and use of a system to measure the bidirectional reflectance distribution function (BRDF) of various surfaces. The BRDF measurements are to be used in the analysis and design of optical measurement systems such as laser anemometers. An Ar-ion laser (514 nm) was the light source. Preliminary results are presented for eight samples: two glossy black paints, two flat black paints, black glass, sand-blasted Al, unworked Al, and a white paint. A BaSO4 white reflectance standard was used as the reference sample throughout the tests.

Fernandez, Rene↗

NASA NTRS1989

Transmission and reflection studies of thin films in the vacuum ultraviolet

Both the transmittance and reflectance of 2 mm thick MgF2 substrates and of thin films of BaF2, CaF2, LaF3, MgF2, Al2O3, HfO2, and SiO2 deposited on these substrates were measured for the wavelength range 120 nm to 230 nm. Results for BaF2, LaF2 and MgF2 show promise as being good materials from which interference filters can be made. The software and related hardware needed to take large amounts of data automatically in future measurements of the transmittance and reflectance was developed.

Peterson, Lennart R.↗

NASA NTRS1990

Multiwavelength pyrometry to correct for reflected radiation

Computer curve fitting is used in multiwavelength pyrometry to measure the temperature of a surface in the presence of reflected radiation by decomposing its radiation spectrum. Computer-simulated spectra (at a surface temperature of 1000 K; in the wavelength region 0.3 to 20 microns; with a reflected radiation-source temperature of 700 to 2500 K; and reflector emissivity from 0.1 to 0.9) were generated and decomposed. This method of pyrometry determined the surface temperatures under these conditions to within 5 percent. The practicability of the method was further demonstrated by the successful analysis of a related problem--decomposition of the real spectrum of an infrared source containing two emitters to determine their temperatures.

Ng, Daniel L. P.↗

NASA NTRS1989

Response to reflected-force feedback to fingers in teleoperations

Reflected-force feedback is an important aspect of teleoperations. The objective is to determine the ability of the human operator to respond to that force. Telerobotics operation is simulated by computer control of a motor-driven device with capabilities for programmable force feedback and force measurement. A computer-controlled motor drive is developed that provides forces against the fingers as well as (angular) position control. A load cell moves in a circular arc as it is pushed by a finger and measures reaction forces on the finger. The force exerted by the finger on the load cell and the angular position are digitized and recorded as a function of time by the computer. Flexure forces of the index, long and ring fingers of the human hand in opposition to the motor driven load cell are investigated. Results of the following experiments are presented: (1) Exertion of maximum finger force as a function of angle; (2) Exertion of target finger force against a computer controlled force; and (3) Test of the ability to move to a target force against a force that is a function of position. Averaged over ten individuals, the maximum force that could be exerted by the index or long finger is about 50 Newtons, while that of the ring finger is about 40 Newtons. From the tests of the ability of a subject to exert a target force, it was concluded that reflected-force feedback can be achieved with the direct kinesthetic perception of force without the use of tactile or visual clues.

Sutter, P. H.↗