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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Measurement and models of bent KAP(001) crystal integrated reflectivity and resolution (invited)

The Advanced Light Source beamline-9.3.1 x-rays are used to calibrate the rocking curve of bent potassium acid phthalate (KAP) crystals in the 2.3-4.5 keV photon-energy range. Crystals are bent on a cylindrically convex substrate with a radius of curvature ranging from 2 to 9 in. and also including the flat case to observe the effect of bending on the KAP spectrometric properties. As the bending radius increases, the crystal reflectivity converges to the mosaic crystal response. The X-ray Oriented Programs (XOP) multi-lamellar model of bent crystals is used to model the rocking curve of these crystals and the calibration data confirm that a single model is adequate to reproduce simultaneously all measured integrated reflectivities and rocking-curve FWHM for multiple radii of curvature in both 1st and 2nd order of diffraction.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Porous Nanomaterials for Ultrabroadband Omnidirectional Anti‐Reflection Surfaces with Applications in High Concentration Photovoltaics

Materials for nanoporous coatings that exploit optimized chemistries and self‐assembly processes offer capabilities to reach ≈98% transmission efficiency and negligible scattering losses over the broad wavelength range of the solar spectrum from 350 nm to 1.5 µm, on both flat and curved glass substrates. These nanomaterial anti‐reflection coatings also offer wide acceptance angles, up to ±40°, for both s‐ and p‐polarization states of incident light. Carefully controlled bilayer films have allowed for the fabrication of dual‐sided, gradient index profiles on plano‐convex lens elements. In concentration photovoltaics platforms, the resultant enhancements in the photovoltaics efficiencies are ≈8%, as defined by experimental measurements on systems that use microscale triple‐junction solar cells. These materials and their applications in technologies that require control over interface reflections have the potential for broad utility in imaging systems, photolithography, light‐emitting diodes, and display technologies.

Yao, Yuan↗
NASA NTRS1984

Inversion of vegetation canopy reflectance models for estimating agronomic variables. III - Estimation using only canopy reflectance data as illustrated by the suits model. IV - Total inversion of the SAIL model

The possibility of estimating agronomic and spectral parameters for a vegetation canopy from the canopy reflectance (CR) data in the infrared region is investigated for a set of solar/view directions. It is shown that such an estimation is possible, in principle, for the Suits (1972) model for a homogeneous canopy. The technique is then applied to a more complex model, the SAIL (scattering by arbitrarily inclined leaves) model which explicitly includes the leaf angle distribution in the computation of the canopy reflectance. It is concluded that, given the expected accuracy of CR measurements and the accuracy of the SAIL model in representing CR in the infrared region, the agronomic parameters, leaf area index, and leaf angle distribution can be estimated fairly accurately using ancillary data on spectral parameters.

Goel, N. S.↗
NASA NTRS1985

A test of the Suits vegetative-canopy reflectance model with LARS soybean-canopy reflectance data

The Suits vegetative-canopy reflectance model is tested with an extensive set of field reflectance measurements made by the Laboratory for Application of Remote Sensing (LARS) for soybean canopies. The model is tested for the full hemisphere of observer directions as well as the nadir direction. The results show moderate agreement for the visible channels of the Landsat MSS and poor agreement in the near-infrared channel of Landsat MSS. An analysis of errors is given.

Chance, J. E.↗
NASA NTRS1984

Estimation of leaf area index from bidirectional spectral reflectance data by inverting a canopy reflectance model

A technique for estimating the leaf area index from bidirectional canopy reflectance (CR) data, in the infrared region, e.g., in band 4 of a Thematic Mapper (TM), is described. It involves inversion of a CR model which accurately represents the reflectance from the canopy. A method for remotely collecting this CR data using an aircraft based TM is described. The bidirectional CR's, for a black spruce (picea mariana) canopy, for 7 solar/view directions, as measured using this technique, are given. A very preliminary analysis of the data from a point of view of estimating LAI by inversion of a CR model is given. This analysis suggests that for an acceptably accurate estimation of LAI, one will require bidirectional CR's for many more than 7 solar/view directions.

Goel, N. S.↗
NASA NTRS1986

Colors of reflection nebulae. III - Ultraviolet scattering by small particles in reflection nebulae associated with 17, 20, and 23 Tauri

New surface brightness measurements in the reflection nebulae associated with the Pleiades stars 17, 20, and 23 Tau have been carried out with the IUE satellite. Each nebula was observed at two angular offsets south of the respective illuminating star. At the larger offsets (40, 40, and 60 arcsec respectively) each nebula exhibits a flux distribution which rises much more steeply with decreasing wavelengths than the corresponding stellar spectrum. At the smaller offsets (20 arcsec) the nebular spectra are more similar to those of the stars and fail to show the steep rise at wavelengths less than 2200 A. Independent observations of interstellar absorption lines as well as theoretical arguments by several previous authors lead to a model wherein the reflection nebulosity near the Pleiades stars is caused by a thin sheet of interstellar matter in front of the stars, at an approximate distance of a few x 10 to the -2nd pc. In the context of such a model the nebular spectra obtained near 17, 20, and 23 Tau can be understood only if the phase-function asymmetry of scattering declines with decreasing wavelengths throughout the spectral range. This result indicates that the role of scattering in the UV by particles in the 10 to the -6th cm size range appears more important than is generally recognized by current models.

Witt, A. N.↗
NASA NTRS1991

Estimation of shortwave hemispherical reflectance (albedo) from bidirectionally reflected radiance data

An equation for estimating albedo from bidirectional reflectance data is proposed. The estimates of albedo are found to be greater than values obtained with simultaneous pyranometer measurements. Particular attention is given to potential sources of systematic errors including extrapolation of bidirectional reflectance data out to a view zenith angle of 90 deg, the use of inappropriate weighting coefficients in the numerator of the albedo equation, surface shadowing caused by the A-frame instrumentation used to measure the incoming and outgoing radiation fluxes, errors in estimates of the denominator of the proposed albedo equation, and a 'hot spot' contribution in bidirectional data measured by a modular multiband radiometer.

Starks, Patrick J.↗
NASA NTRS2003

[Technology Development for X-Ray Reflection for the Constellation-X Reflection Grating Spectrometer (RGS)]

This Grant covers MIT support for the technology development of x-ray reflection gratings for the Constellation-X Reflection Grating Spectrometer (RGS). Since the start of the Grant MIT has extended its previously-developed patterning and super-smooth, blazed grating fabrication technology to ten-times smaller grating periods and ten-times larger blaze angles to demonstrate feasibility and performance in the off-plane grating geometry. In the past year we successfully developed several nanoimprint grating replication methods that achieved very high fidelity replication of master silicon gratings. Grating geometry on the nano and macro scales were faithfully replicated, demonstrating the viability of the process for manufacturing the thousands of gratings required for the RGS. We also successfully developed an improved metrology truss for holding test grating substrates during metrology. The flatness goal of grating substrates is under 500 nm. In the past, grating holders would cause non-repeatable distortion of >> 500 nm to the substrates due to friction and gravity sag. The new holder has a repeatability of under 50 nm which is adequate for the proposed RGS grating substrates.

Schattenburg, Mark L.↗
NASA NTRS2009

System and method for determination of the reflection wavelength of multiple low-reflectivity bragg gratings in a sensing optical fiber

A system and method for determining a reflection wavelength of multiple Bragg gratings in a sensing optical fiber comprise: (1) a source laser; (2) an optical detector configured to detect a reflected signal from the sensing optical fiber; (3) a plurality of frequency generators configured to generate a signal having a frequency corresponding to an interferometer frequency of a different one of the plurality of Bragg gratings; (4) a plurality of demodulation elements, each demodulation element configured to combine the signal produced by a different one of the plurality of frequency generators with the detected signal from the sensing optical fiber; (5) a plurality of peak detectors, each peak detector configured to detect a peak of the combined signal from a different one of the demodulation elements; and (6) a laser wavenumber detection element configured to determine a wavenumber of the laser when any of the peak detectors detects a peak.

Moore, Jason P.↗