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At least 451 records · Page 25

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NASA NTRS1973

Heterodyne readout for read-write holographic memories

A heterodyne readout technique for read-write holographic memory systems that reconstruct a virtual image wavefront is described and demonstrated. The conventionally recorded hologram is illuminated simultaneously with a suitable combination of temporally modulated reference and modified object waves for readout. Best performance is obtained for temporal phase modulation. The coupling of the illuminating wavefronts by the hologram is analyzed.

Mezrich, R. S.↗

NASA NTRS1973

Photoelectrons and lunar limb shocks

It is suggested that boundary conditions for solar wind/lunar limb interactions are active. The 'whole-moon' limb does not evoke a shock cone because warm solar wind electrons are replaced by cool photoelectrons that are ejected from the generally smooth areas of the lunar terminator illuminated at glancing angles by the sun. A localized volume of low thermal pressure is created in the solar wind by these cool photoelectrons. The solar wind expands into this volume without shock production. Directly illuminated highland areas exchange hot photoelectrons for warm solar wind electrons. The hot electrons generate a localized pressure increase in the adjacent solar wind flow which evokes a shock streamer in the solar wind. Shock streamers are identifiable by a coincident increase in the magnitude of the solar wind magnetic field. Shock occurrence is controlled by lunar topography, solar activity in the hard ultraviolet, solar wind electron density and thermal velocity, and the density of the solar wind magnetic field.

Criswell, D. R.↗

NASA NTRS1974

S-193 scatterometer transfer function analysis for data processing

A mathematical model for converting raw data measurements of the S-193 scatterometer into processed values of radar scattering coefficient is presented. The argument is based on an approximation derived from the Radar Equation and actual operating principles of the S-193 Scatterometer hardware. Possible error sources are inaccuracies in transmitted wavelength, range, antenna illumination integrals, and the instrument itself. The dominant source of error in the calculation of scattering coefficent is accuracy of the range. All other ractors with the possible exception of illumination integral are not considered to cause significant error in the calculation of scattering coefficient.

Johnson, L.↗

NASA NTRS1975

Holographic system for nondestructive testing

A description is given of a holographic system for nondestructive testing. The system is comprised of a mirror which illuminates the test object surface; the mirror is positionable to direct illumination on an object at varying angles with respect to a line normal to the surface of the object. In this manner holograms may be produced with varying degrees of sensitivity enabling optimum observation of dimensions of deformation of an object occurring between test exposures.

Kurtz, R. L.↗

NASA NTRS1975

The 7.5 kW solar array simulator

A high power solar array simulator capable of providing the input power to simultaneously operate two 30 cm diameter ion thruster power processors was designed, fabricated, and tested. The maximum power point is set to between 150 and 7500 watts representing an open circuit voltage from 50 to 300 volts and a short circuit current from 4 to 36 amps. Illuminated solar cells are used as the control element to provide a true solar cell characteristic and permit the option of simulating changes in this characteristic due to variations in solar intensity and/or temperature of the solar array. This is accomplished by changing the illumination and/or temperature of the control cells. The response of the output to a step change in load closely approximates that of an actual solar array.

Robson, R. R.↗

NASA NTRS1975

MTF and point-spread function for a large-area CCD imager

The MTF degradation due to lateral diffusion is calculated for a back illuminated CCD imager for typical device parameters. The discrete nature of the CCD and finite size of the photosensitive elements result in an additional MTF degradation. The Fourier transform approach is utilized to calculate the effective point spread function for these processes in the time domain. Experimental data are presented on the point spread function for a three phase, double level anodized aluminum 160 x 100 thinned and back illuminated CCD imager and compared with the theoretical results. A simple modification of the Crowell and Labuda model suggested by these results is presented.

Ando, K. J.↗

NASA NTRS1974

Photomechanical vibration of thin crystals of polar semiconductors

It was found that thin crystals of polar (non-centrosymmetric) semiconductors constitute a new type of photosensitive system in which incident illumination is converted into mechanical energy: thus, illumination-induced elastic deformation (bending) was observed on thin (00.1) CdS and (111) GaAs crystals; furthermore, by employing chopped light the crystals were excited to their resonant vibration (photomechanical vibration); the dependence of the amplitude of this vibration on the energy of the incident radiation was found to be similar to the dependence of the surface photovoltage on the energy of the incident radiation (surface photovoltage spectrum). The present findings are consistent with a model based on light-induced modulation of the piezoelectric surface stresses.

Lagowski, J.↗

NASA NTRS1975

Galilean satellite mutual occultation data processing

Results of processing seven mutual occultation lightcurves are presented. The lightcurves were obtained using the 60-in. telescope (152 cm) at Mt. Wilson to observe six J1 occulting J2 events and one J3 occulting J2 event. Using a uniformly illuminated disk model, local satellite Jovicentric longitude corrections of 675 plus or minus 150 km, 275 plus or minus 240 km, and 1175 plus or minus 350 km for J1, J2, and J3, respectively, were determined. These corrections enabled the event midpoint times to be computed to within 5 sec of the observed midpoint times for all seven events. These longitude corrections have been verified by Pioneer 10 and recent (1973 and 1974) conventional Jovian eclipse observations. A relative J1:J2 out-of-plane error of less than a few hundred kilometers has been indicated; however, it appears that the relative J3:J2 out-of-plane error is larger than 600 km. Deficiencies in both the uniformly illuminated disk model and Sampson's theory of the Galilean satellite motions for the reduction of mutual event data are described.

Duxbury, T. C.↗

NASA NTRS1976

Application of LANDSAT data to delimitation of avalanche hazards in Montane, Colorado

The author has identified the following significant results. Photointerpretation of individual avalanche paths on single band black and white LANDSAT images is greatly hindered by terrain shadows and the low spatial resolution of the LANDSAT system. Maps produced in this way are biased towards the larger avalanche paths that are under the most favorable illumination conditions during imaging; other large avalanche paths, under less favorable illumination, are often not detectable and the smaller paths, even those defined by sharp trimlines, are only rarely identifiable.

Knepper, D. H.↗

NASA NTRS1976

Subresolution imaging - Use of wavelength and angular diversity techniques

The paper demonstrates the possibility of using an illumination system tunable in angle or wavelength to determine the amplitude and height structure of an object within the resolution cell defined by the resolving power of some imaging system. It is noted that since the value of the electric field at a point on the image of an object varies with the wavelength or angle of illumination on the object, the Fourier transform of this variation provides directly the amplitude and phase structure of the object within the resolution cell corresponding to the image point of interest. This Fourier transform is calculated, and it is shown that the transform of the electric-field intensity with respect to the inverse wavelength dimension is simply the autocorrelation of the electric-field variation with wavelength. Practical techniques that can be employed to measure the subresolution detail of an object are briefly discussed.

Jain, A.↗

NASA NTRS1976

Computer-aided analysis of CCD linear image sensors

Special test equipment and techniques to collect and process image information from charge coupled devices (CCDs) by digital computer were reviewed. The video channel was traced from the CCD to the direct memory access bus of the Interdata Computer. Software was developed to evaluate and characterize a CCD for (1) dark signal versus temperature relationship, (2) calculation of temporal noise magnitude and noise shape for each pixel, (3) spatial noise into the video chain due to dark signal, (4) response versus illumination relationship (gamma), (5) response versus wavelength of illumination (spectral), (6) optimization of forcing functions, and (7) evaluation of an image viewed by a CCD. The basic software differences and specific examples of each program operating on real data are presented.

Prince, S. S.↗

NASA NTRS1976

A five-frequency radiometer antenna system feed horn

The feasibility of operating a five frequency feed as a radiometer for use on board a Nimbus type satellite was investigated. Five narrow band frequencies (6.6, 10.7, 18.5, 21.5 and 33 GHz) were multiplexed onto a single receive feed employing dual linear polarization at each frequency with a better than 25 db isolation between each polarization. The production of a horn design which can illuminate a parabolic reflector segment with an f/D ratio of approximately 0.4, producing an edge illumination of approximately -20 db is described. The final overall design is presented with results showing that this design concept has an acceptable electrical performance.

Source record↗

NASA NTRS1977

Sun direction detection system

One of the detectors is an illumination detector consisting of two spaced apart elongated strips with a strip of cadmium sulphide (Cds) deposited therebetween. Whenever the line image impinges the CdS strip, the resistance between the two other strips is relatively low, while being high when the line image is outside the field of view of the illumination detector. Also included is a sun angle detector which consists of a vapor deposited resistor strip connected at one end to plus 10v and at the other end to minus 10v. Spaced apart from the resistor strip is an elongated strip of low resistance material acting as an output strip, with a CdS strip between the two strips. When the line image is within the field of view of the sun angle detector, the output voltage at the output strip depends on the position of the line image across the sun angle detector.

Schmidt, L. F.↗

NASA NTRS1975

Photoconductivity of high voltage space insulating materials: Measurements with metal electrodes

The electrical conductivities of high voltage insulating materials were measured in the dark and under various intensities of illumination. The materials investigated included FEP Teflon, Kapton-H, fused quartz, and parylene. Conductivities were determined as functions of temperature between 22 and 100 C and light intensity between 0 and 2.5 kW/m2. The thickness dependence of the conductivity was determined for Teflon and Kapton, and the influence of spectral wavelengths on the conductivity was determined in several cases. All measurements were made in a vacuum to simulate a space environment, and all samples had metallic electrodes. The conductivity of Kapton was permanently increased by exposure to light; changes as great as five orders of magnitude were observed after six hours of illumination.

Coffey, H. T.↗

NASA NTRS1977

A theoretical analysis of the MSFC holographic correlation system

A theoretical analysis was completed for the correlation output signal for the MSFC holographic correlation filtering system. Under appropriate assumptions, the correlation output was derived as a function of the roughness of the tested surface, the displacement of the illuminated area of the test surface, the characteristics of the optical components used in the system, and the system configuration. In addition, an approximate relationship between the displacement of the detected signal (which is focused on the photomultiplier tube) and the displacement of the illuminated area on the test surface was also derived.

Liu, H. K.↗

NASA NTRS1977

Active and passive multispectral scanner for earth resources applications: An advanced applications flight experiment

The development of an experimental airborne multispectral scanner to provide both active (laser illuminated) and passive (solar illuminated) data from a commonly registered surface scene is discussed. The system was constructed according to specifications derived in an initial programs design study. The system was installed in an aircraft and test flown to produce illustrative active and passive multi-spectral imagery. However, data was not collected nor analyzed for any specific application.

Hasell, P. G., Jr.↗

NASA NTRS1976

Silicon solar cell testing in concentrated sunlight and simulated sunlight

A method is described for testing silicon solar cells in concentrated sunlight and simulated sunlight. Concentrated sunlight is obtained by using an acrylic Fresnel lens; the simulated sunlight source is a short arc Xenon lamp. Average illumination levels during the tests are inferred from an assumed linear relationship between short circuit current and illumination. The linearity assumption is investigated for 0.3 alpha cm base resistivity silicon cells and found to be valid. Some typical results are presented to illustrate the type of information obtained during the testing.

Burgess, E. L.↗

NASA NTRS1976

Spectral response measurements with white light bias

The spectral response of solar cells such as the CdS/Cu2S cell is non-linear with distinct quenching and enhancement bands. One possible technique to produce standardized solar efficiencies is to fold in spectral response with a standard solar spectrum. The spectral response of a cell was measured in a way which matched cell behavior under white light illumination. A technique was developed to measure the response of a cell to low intensity chopped monochromatic light while the cell is also illuminated with a white light bias corresponding to AMI.

Devaney, W.↗