Search NASA⌕ Search

SEARCH · Search NASA

Results for “absorption coefficient”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,027 records · Page 57
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

A model function for ocean microwave brightness temperatures

A relatively simple, yet accurate, relationship between the microwave brightness temperature of the ocean and conventional oceanographic and meteorological parameters is derived. The equation for the brightness temperature upwelling from the sea surface through the intervening atmosphere is obtained, considering radiative emission and scattering by the sea surface along with radiative absorption and emission by the atmosphere. A number of approximations are applied to the integral brightness temperature equation and its supporting equations in order to obtain a simple equation for the brightness temperature that does not contain integrals. Values for a number of atmospheric parameters are determined, including temperature sensitivities, oxygen opacity, water vapor and liquid water normalized absorption coefficients, and effective columnar height. The sea surface emissivity model is then considered, modelling the sea surface as a composite of foam-free rough water and foam patches.

Wentz, F. J.↗
NASA NTRS1983

Radiative equilibrium model of Titan's atmosphere

The present global radiative equilibrium model for the Saturn satellite Titan is restricted to the two-stream approximation, is vertically homogeneous in its scattering properties, and is spectrally divided into one thermal and two solar channels. Between 13 and 33% of the total incident solar radiation is absorbed at the planetary surface, and the 30-60 ratio of violet to thermal IR absorption cross sections in the stratosphere leads to the large temperature inversion observed there. The spectrally integrated mass absorption coefficient at thermal wavelengths is approximately constant throughout the stratosphere, and approximately linear with pressure in the troposphere, implying the presence of a uniformly mixed aerosol in the stratosphere. There also appear to be two regions of enhanced opacity near 30 and 500 mbar.

Samuelson, R. E.↗
NASA NTRS1983

Composite thin-foil bandpass filter for EUV astronomy Titanium-antimony-titanium

Thin metallic foils of antimony and titanium have been investigated in an attempt to develop an EUV filter with a bandpass from 350 to 550 A. A composite filter has been developed composed of antimony sandwiched between two titanium foils. The transmissions of sample composite foils and of pure titanium foils from 130 to 1216 A are presented. The absorption coefficients of anatimony and titanium and the effect of titanium oxide on the transmission are derived. The composite filter has been found to be quite stable and mechanically rugged. Among other uses, the filter shows substantial promise for EUV astronomy.

Jelinsky, P.↗
NASA NTRS1983

Assessment of relative error sources in IR DIAL measurement accuracy

An assessment is made of the role the various error sources play in limiting the accuracy of infrared differential absorption lidar measurements used for the remote sensing of atmospheric species. An overview is presented of the relative contribution of each error source including the inadequate knowledge of the absorption coefficient, differential spectral reflectance, and background interference as well as measurement errors arising from signal fluctuations.

Menyuk, N.↗
NASA NTRS1982

The effects of vegetation cover on the radar and radiometric sensitivity to soil moisture

The measured effects of vegetation canopies on radar and radiometric sensitivity to soil moisture are compared to emission and scattering models. The models are found to predict accurately the measured emission and backscattering for various crop canopies at frequencies between 1.4 and 5.0 GHz, especially at theta equal to or less than 30 deg. Vegetation loss factors, L(theta), increase with frequency and are found to be dependent upon canopy type and water content. In addition, the radiometric power absorption coefficient of a mature corn canopy is 1.75 times that calculated for the radar. Comparison of an L-band radiometer with a C-band radar shows the two systems to be complementary in terms of accurate soil moisture sensing over the extreme range of naturally occurring soil moisture conditions.

Ulaby, F. T.↗
NASA NTRS1983

Multiwavelength analyzer for the determination of diffusion lengths

The minority carrier diffusion length Ln in the base or substrate region is an important parameter which governs a solar cell's performance. The present investigation is concerned with the development of a multiwavelength analyzer (MWA) technique for the nondestructive spatial testing of polycrystalline solar cells. The MWA method is based on the utilization of the short-circuit current generated by two or more light-emitting diodes (LEDs) operating at different wavelengths and modulated 180 deg out-of-phase. For a determination of Ln by the MWA technique, it is necessary to know the value of the absorption coefficient.

Stafsudd, O. M.↗
NASA NTRS1984

Effect of external pressure environment on the internal noise level due to a source inside a cylindrical tank

A small cylindrical tank was used to study the effect on the noise environment within a tank of conditions of atmospheric (sea level) pressure or vacuum environments on the exterior. Experimentally determined absorption coefficients were used to calculate transmission loss, transmissibility coefficients and the sound pressure (noise) level differences in the interior. The noise level differences were also measured directly for the two exterior environments and compared to various analytical approximations with limited agreement. Trend study curves indicated that if the tank transmission loss is above 25 dB, the difference in interior noise level between the vacuum and ambient pressure conditions are less than 2 dB.

Clevenson, S. A.↗
NASA NTRS1983

Emission, absorption, and polarization of gyrosynchrotron radiation of mildly relativistic particles

Approximate analytic expressions for the emissivity and absorption coefficient of synchrotron radiation of mildly relativistic particles with an arbitrary energy spectrum and pitch angle distribution are given. From these, an expression for the degree of polarization is derived. To accomplish this, previously developed methods of integration are used. The analytic results are compared with numerical results for both thermal and nonthermal (power law) distributions of particles. Previously announced in STAR as N82-34197

Petrosian, V.↗
NASA NTRS1983

Stratospheric aerosol light absorption before and after El Chichon

Direct in-situ measurements of aerosol light absorption before, during, and after the eruption of Mt. St. Helens and El Chichon reveal the presence of light absorbing species at all times. Large enhancements which appear to drop rapidly with time are found at certain locations immediately following volcanic events. Although a general patchiness is present in the aerosol and its associated absorption coefficient, post eruption values appear to remain elevated in the mean when compared to early 1979 data prior to the El Chichon eruption. From the data collected for this period, lower bounds for the single scatter albedo are presented along with implications for associated radiative effects.

Clarke, A. D.↗
NASA NTRS1982

Effects of indirect bandgap top cells in a monolithic cascade cell structure

The effect of having a slightly indirect top cell in a three junction cascade monolithic stack is calculated. The minority carrier continuity equations are utilized to calculate individual junction performance. Absorption coefficient curves for general III-V compounds are calculated for a variety of direct and indirect gap materials. The results indicate that for a small excursion into the indirect region, (about 0.1 eV), the loss of efficiency is acceptably small (less than 2.5 percent) and considerably less than attempting to make the top junction a smaller direct bandgap.

Curtis, H. B.↗
NASA NTRS1984

Two-beam interferometer for optical constants measurements at near-millimeter wavelengths

A simple and versatile instrument for the measurement of optical constants in the near millimeter spectral region is described. Studies of quasi-optical components used in this instrument for effective beam transmission and control are reported. These studies include transmission characteristics of dielectric waveguides and performance of a variable coupler based on frustrated total internal reflection. Operation of this instrument as a two-beam interferometer for determining the index of refraction or as a device to measure transmission for determining the absorption coefficient are described. Values measured with this instrument at 245 GHz, using an optically pumped molecular laser as the source, are reported for several low-loss materials. Appropriate corrections and error estimates are discussed. The potential for improvement and increased accuracies are discussed.

Jones, C. R.↗
NASA NTRS1984

Saturn's rings - 3-mm low-inclination observations and derived properties

To determine a more precise brightness temperature and more accurate properties for Saturn's rings, 3.3 mm low inclination observations have been made at 90 GHz with the Aerospace 4.6 m radio telescope. A mean brightness temperature of 17 plus or minus 4 K has been determined by comparing the data with the variation of the inclination of the total flux from the planet and rings predicted by a simple model with uniformly bright A and B rings. Variation of the normal optical depth from 0.4 to 1.0 resulted in a total variation of about 1.5 K in A and B brightness. A portion of the brightness attributed to ring particle thermal emission has been determined to be at a temperature of 11 plus or minus 5 K. If the maximum particle radius (approximately 5 m) deduced from Voyager bistatic radar observations is correct, results indicate a particle distribution ranging between 1 cm and several meters radius of the form r exp -s with s = 3.3-3.6, or a material absorption coefficient ranging between 3 and 10 times lower than that of pure water ice at 85 K, or both. An increase in the porosity of the ice particles through a decrease in their density will not produce the observed particle albedo. If the rocky material is uniformly distributed, low ring brightness temperature would allow a silicate upper limit of approximately 10 percent by mass; however, the silicate material could be more abundant if it is separated from the icy material.

Epstein, E. E.↗
NASA NTRS1984

Thermal emission of nonspherical particles

The thermal emission of nonspherical particles is studied by applying the fluctuation dissipation theorem to calculate the Stokes' vector source terms in radiative transfer theory. Generally, all the four Stokes' parameters in the source term are nonzero and are proportional to the absorption coefficient in the backward direction. The measurements of active and passive remote sensing are related by showing that within the framework of radiative transfer theory, the emissivity of vertically and horizontally polarized waves can be expressed in terms of the bistatic scattering coefficients in the usual manner. In addition, the emissions of the third and fourth Stokes' parameters are also expressed in terms of bistatic scattering coefficients.

Tsang, L.↗
NASA NTRS1984

High-resolution spectra of isotopic methanes near 1645 nm

It is demonstrated that the coincidence between Er:YAG laser emission and CH4 absorption is associated with the R(8) multiplet of C-13H4 instead of the R(6) of C-12H4. The Er:YAG emission was characterized as a laser emission at 6079.4/cm with a bandwidh of 2.2/cm and overlapping CH4 emission at 6078.7/cm. The R(6) component of C-12H4 closest to 6078.7/cm is calculated to be 6077.06/cm, corresponding to an absorption coefficient (AC) of 1.12/cm per atm. The F2-prime component of R(8) of C-13H4 has an AC of 0.308/cm per atm, which corresponds well with a range of 1.2-30.30/cm per atm based on a partial pressure of 1.6/89 ppm. The Er:YAG laser is a candidate for use in spectroscopic studies of CH4 atmospheric pollutants, natural gas movement and storage and gas accumulation in coal mines.

Fox, K.↗
NASA NTRS1985

Identification and properties of molecular systems of potential use in solar-pumped lasers

The concepts and computational tools of theortical chemistry are used to investigate molecular properties needed in direct solar-pumped lasers. Compounds of the type RR'CXY, with R and R' organic groups, and X and Y halide atoms were identified as likely candidates because of their highly enhanced absorption coefficients over compounds with a single halide atom. The use of a combination of vibrational excitation followed by electronic excitation to enhance quantum yields at certain wavelengths is indicated. A self-consistent eikonal approximation to state-to-state transitions was tested for CH3I and is useful for other problems involving electronic energy and charge transfer. An approach to calculate potential energy surfaces and transition dipoles was developed which is based on the generation of eigenstates of the nonrelativisitc Hamiltonian followed by incorporation of the spin-orbit coupling by configuration interaction.

Micha, D. A.↗
NASA NTRS1985

Phenomena Simulation for Heavy Doping and Surface Recombination Velocity

The theoretical models now available that characterize heavily doped (highly conducting) regions in silicon are survyed. Analytical and numerical approaches that determine the influence of such regions on the conversion efficiency of solar cells are examined. Although dilutely doped silicon is well characterized except for some disagreement about optical absorption coefficients, what exists now for heavily doped silicon and its interplay with adjoining regions is an incomplete theory in which not all contributers to transport, recombination, generation, and trapping are defined. Further, the parameters relating to these mechanisms and their values as determined by experiment are subject to various interpretations. The characterization of heavily doped silicon is treated not as a theory but rather as an imperfectly articulated and incompletely formalized body of experience. This view is intended to help point the way toward the attainment of a more complete of heavily doped silicon and thereby toward more informed designs of solar cells. Because computer programs constitute tools both for design and for estimating performance limits, the review includes some remarks pertinent to existing and developing programs.

Lindholm, F. A.↗
NASA NTRS1983

Analysis of 12-700 micron reflectance spectra of three optical black samples

Normal incidence, specular reflectance spectra of three samples of optical black coatings have been measured in 10 filter passbands at effective wavelengths between 12 and 700 microns. Strong absorption, due to amorphous silicate material in the coatings, is found in the 15-50 micron region of each spectrum. Reflecting-layer model calculations are fitted by a nonlinear least-squares routine to the spectra at wavelengths beyond the absorption band to determine far-infrared optical parameters. It is found that, with a similar wavelength dependence, the absorption coefficient of a new black coating (ECP 2200) made by the Minnesota Mining and Manufacturing Company (3M) is less than 1/10 that of the discontinued coating, 3M Black Velvet Nextel. It is also shown that the extinction effect of a very rough substrate is negligible compared with the effects of increased upper surface roughness and coating thickness attendant upon fully covering a very rough substrate.

Smith, S. M.↗