Showing papers on "Fresnel zone published in 1978"

The effect of the outer scale of turbulence and wavelength on scintillation fading at millimeter wavelengths

Abstract: Amplitude scintillation measurements made on two-millimeter microwave links at 36 GHz and 110 GHz along a common 4.1 km path across central London are deseribed. The variances of the logarithmic amplitude scintilliations at these two frequencies are compared for different ranges of the outer scale of turbulence. A comparison is made between the experimental results and the theoretical predictions of Tatarski. Good agreement has been found for the two cases when the outer scale of turbulence is greater than and smaller than the Fresnel zone size.

Wave propagation in optical systems with large apertures

Abstract: The Kirchhoff-Huygens equation is used to investigate wave propagation in optical systems, with large propagation Fresnel numbers NF and aperture-to-length ratios (a/L) which are not small. The limit of applicability of the Fresnel approximation is analytically established for a thin rectangular aperture. It is shown that the error introduced by the Fresnel approximation to the Kirchhoff integral is comparable to the effects of diffraction, computed by the approximation, times the dimensionless parameter πNF(a/2L)2.

Large area parabolic concentrating solar collector

Abstract: An apparatus for the concentration of solar radiation by directing it upon Fresnel zone sections of axially short, electroplated surfaces of spin cast parabolic mirrors. The short focal length optical system is composed of off axis parabolic surfaces of revolution and secondary reflectors. These reflectors concentrate the suns rays to produce heat with which to generate steam, and to melt high temperature salts.

Comparison of Fresnel zone plates and uniformly redundant arrays

Abstract: Several imaging systems in laser fusion, a-beam fusion, and astronomy employ a Fresnel zone plate (FZP) as a coded aperture. The recent development of uniformly redundant arrays (URAs) promises several improvements in these systems. The first advantage of the URA is the fact that its modulation transfer function (MTF) is the same as the MTF of a single pinhole, whereas the MTF of an FZP is an erratic function including some small values. This means that if inverse filtering is used, the URA will be less susceptible to noise. If a correlation analysis is used, the FZP will produce artifacts whereas the URA has no artifacts (assuming planar sources). Both the FZP and URA originated from functions which had flat MTFs. However, practical considerations in the implementation of the FZP detracted from its good characteristics whereas the URA was only mildly affected. The second advantage of the URA is that it better utilizes the available detector area. With the FZP, the aperture should be smaller than the detector in order to maintain the full angular resolution corresponding to the thinnest zone. The cyclic nature of the URA allows one to mosaic it in such a way that the entire detector area collects photons from all of the sources within the field of view while maintaining the full angular resolution. If the FZP is as large (or larger) than the detector, all parts of the source will not be resolved with the same resolution. The FZP does have some advantages, in particular its radial symmetry eases the alignment problem; it has a convenient optical decoding method; and higher diffraction order reconstruction might provide better spatial resolution.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Micro-Fresnel zone plates for coded imaging applications.

Abstract: Design considerations and fabrication procedures for Fresnel zone plates appropriate for high-resolution coded imaging of x-ray and particle emission from laser produced plasmas are presented. Fabrication results for free standing zone plate structures of high Z material (gold), large zone number (100

Intensity mapping optical aberrations.

Abstract: Intensity mapping aberrations for high average power laser beams are examined from the point of view of diffraction theory. For the Fresnel ripple aberrations, it was determined that indeed they behave like a circular diffraction grating where energy is diffracted into the far wings of the far-field intensity distribution without significantly affecting the width of the central diffraction lobe, only its energy. It was further established that the effect of such aberrations on the far field may well be characterized by techniques where the aberration is randomized, and on-axis intensities are obtained through approximate methods.

Calculation of aperture and far-field distributions from measurements in the Fresnel zone of large reflector antennas

Abstract: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers.

Optical Methods of Information Processing for Some Radio Physical Problems

Abstract: The solution of a number of radio physical problems requires that multichannel integral transformations should be fulfilled.Such transformations are comparatively simply fulfilled by the optical methods of information processing.

Analysis and interpretation of soft X-ray photographs of coronal active regions taken with Fresnel zone plates

Abstract: Soft X-ray photographs of the Sun taken at O vii 21.6 A and in a spectral band ranging from 13.2 to 22.1 A have been analysed in order to establish spatially resolved maps of temperature and emission measure for several active regions in the corona. The photographs were taken on 11 March, 1971, and on 2 March, 1972, with Fresnel zone plate cameras which were flown on ESRO and NRL sounding rockets.

ELF propagation in a non-stratified earth-ionosphere waveguide

Abstract: : This report analyzes extremely low-frequency (ELF) propagation under conditions where the properties of the earth-ionosphere waveguide change markedly over transverse distances comparable with the width of a Fresnel zone. An integral equation formulation is presented that can be used to obtain numerical results for most types of daytime ionospheric disturbances. Approximate solutions are given for ionospheric disturbances of the type that would occur in single-burst nuclear environments. These fullwave results are compared with results calculated from the widely used two-dimensional WKB approximation, which neglects transverse ionospheric gradients. It is shown that this WKB approximation gives good results for burst-heights above about 100 km, but that fullwave theory that accounts for transverse gradients must be used for lower burst altitudes. For these lower burst-heights, the WKB method seriously overstates the propagation anomaly caused by an on-path burst and understates the anomaly caused by an off-path burst. (Author)