Showing papers on "Fresnel zone published in 1972"

Zone Plates and Their Aberrations

Abstract: The zone plate is an optical device that depends on interference, not reflection or refraction, for its image-forming properties. Although the irradiance at the focus is only 110 that of a lens with the same aperture, a zone plate with 100 or more zones is capable of resolution equal to that of the lens. Zone plates can be used in spectral regions where conventional optics are unavailable and for special applications in the visible spectrum. This paper derives the third-order and chromatic aberrations of the zone plate. The image may be diffraction limited only if the illumination is relatively monochromatic. Like the pinhole camera, the zone plate turns out not to suffer from linear distortion, even at very wide fields.

Use of scintillations to measure average wind across a light beam

Abstract: We report the successful construction and testing of an optical wind sensor that uses the motion of the scintillation pattern to measure the transverse component of wind blowing across a laser beam. As is done for measuring ionospheric and interplanetary winds, we use a correlation method. However, in our application, the slope at zero lag of the time-lagged correlogram proves to be more useful than the more commonly used delay to the peak. The reason is that, in the atmosphere, irregularities are distributed along the entire propagation path. We use a detector spacing of 0.33 of the diameter of the first Fresnel zone to obtain a nearly uniform weighting function along the path, though the center of the path is still more effective than the ends. The sensor has been used extensively over 1-km and 15-km paths, and field tests of various applications are planned.

On deducing the form of surfaces from their diffracted echoes

Abstract: Kirchhoff diffraction theory is set up for the echo received back at the source when a spherical outgoing pulse of radiation is reflected from a rough surface. The formulation derived here permits a clear separation between those features of the echo that depend only on the form of the incident pulse, and those involving only the properties of the surface. The surface function depends on the contours of constant distance from the source which can be drawn in the surface; these are the appropriate analogues of Fresnel zone boundaries. Geometrical optics gives the singularities and discontinuities of the surface function. The problem of determining the surface function from echo data which are incomplete in the sense that they do not contain all radiation frequencies is discussed. For the special case of one dimensional roughness, the problem of how far the surface function can be deconvoluted to obtain the shape of the surface is considered.

Spatially modulated imaging system

Abstract: Noncoherent radiation, such as X-rays, is spatially coded, directed through an object and spatially detected to form a spatially coded pattern, from which an image of the object may be reconstructed The X-ray source may be formed by X-ray fluorescence and subtraction of the holographic images formed by two sources having energy levels predominantly above and below the maximum absorption range of an agent in the object may be used to enhance contrast in the reproduced image Constant spatial frequency coding of the source may be used to produce a spatial carrier frequency to enhance detection of low spatial frequency components of the spatially coded pattern so that images may be a more easily reconstructed such as the optical reconstruction of spatially code formed by the off-center section of a Fresnel zone pattern by converting the detected intensity pattern to a diffraction or phase pattern

Fresnel Diffraction by a Circular Aperture Illuminated with Partially Coherent Light. I

Abstract: A formulation is developed for investigating the intensity distribution of Fresnel diffraction by a circular aperture illuminated with partially coherent light. The theory formulated is applied to obtain the intensity distribution along the normal axis to the aperture in the Fresnel diffraction field when the spatially mutual coherence function of the illumination over the aperture takes three different forms. It is shown that the Fresnel diffraction is largely influenced by the coherence condition across the diffracting aperture.

Fresnel-Zone-Plate Spectrometer with Central Stop*

Abstract: The marked chromatic aberration of a Fresnel-zone-plate lens can be used as a variable narrow-band optical filter, and thus a holographic spectrometer. The filtering properties of the zone plate are analyzed, with particular emphasis on the effect of a central stop over the low-order rings. It is shown that the central stop can markedly improve the rejection of frequencies outside the pass band without significant degradation of the spectrometer resolution.

Evidence for a Continuous, Power Law, Electron Density Irregularity Spectrum

Abstract: The spectral form of the irregularities in electron density that cause interplanetary scintillation (IPS) of small angular diameter radio sources is discussed. The intensity scintillation technique always yields an irregularity scale size, which is of the order of the first Fresnel zone for the wavelength at which the observations are taken. This includes not only the radio wavelength measurements of the structure of the interplanetary medium, but also radio wavelength measurements of the irregularity structure of the ionosphere and interstellar medium, and optical wavelength measurements of the irregularity structure of the atmosphere.

Generalized radar equations and Doppler shift in the fresnel zone

Abstract: Generalizd radar equations are derived for single and multiple point targets. Expressions for the multiple spectra indicate that the frequency shift, as well a the variance due to target movement in a direction perpendicular to the antenna axis, diminishes with the distance from the antenna aperture.