Showing papers on "Fresnel zone published in 1970"

Computational Reconstruction of Scattering Objects from Holograms

Abstract: This paper reports the successful computational determination of structural detail in a simple transparent object through holographic measurement of scattered monochromatic light. The complex disturbance of the scattered light is measured in amplitude and phase, along a line transverse to the illumination in the Fresnel zone of the object. The scattering potential of the object is then calculated along a parallel line using the field data and a new inverse scattering theory. The results agree well with the known parameters of the two test objects, a high-quality and a low-quality right parallelepiped aligned with two faces normal to the illumination. This experiment is believed to be the first which includes the quantitative reconstruction of structure in a physical object from measurement of scattered light. The technique is somewhat similar to that employed in connection with reconstruction of crystal structures from x-ray diffraction experiments.

Application of microwave holography in the study of the field from a radiating source

Abstract: Microwave holography is applied to the investigation of the radiation mechanism of a monopole. It is shown that a radiating monopole can be described mathematically as two point sources located at each end of the monopole. A microwave hologram was recorded using liquid crystals which upon optical reconstruction yielded two points as predicted by the theory. Using optical techniques, one can visualize the field at the antenna plane, at a plane in the Fresnel zone, and at a plane in the far field. This technique is, however, only one example of microwave holography in the investigation of a radiator. It can be very useful in designing any type of antenna.

Electro-optic variable fresnel zone plate

Abstract: Apparatus is disclosed including a variable electro-optic Fresnel zone plate having an electro-optic medium with a characteristic that varies as a function of an applied electric field, means for applying to that medium an electric field whose intensity varies in a pattern of alternate high intensity and low intensity bands, and means for demodulating radiation modulated by that characteristic of the medium to produce a Fresnel zone plate pattern of radiation

Particle-Size Measurement Using Forward-Scatter Holography*

Abstract: This study is an experimental and theoretical investigation of the application of forward-scatter Fresnel holography to water droplets. The theoretical holograms were constructed on a digital computer by determining the irradiance of the interference pattern formed by the light scattered from a droplet and a reference source, at discrete points on a hologram. The forward-scattered light is evaluated by assuming that the droplet is an opaque disk and utilizing the Rayleigh–Sommerfeld theory of diffraction. The reconstructed wave front is evaluated by performing the Fresnel transform on the hologram matrix. The corresponding experimental holograms were made with a helium–neon continuous-wave laser and using glass beads from 80 to 250 μm in diameter as models of the water droplets. The half-radiance width of the reconstructed wave-front radiance distribution is shown theoretically to increase linearly as the distance of the object from the hologram is decreased. Also, the half-radiance width increases logarithmically with hologram area. The theoretical and experimental radiance distributions in the plane of reconstruction were compared. Proper alignment of the hologram during reconstruction is necessary for the reconstruction of the original radiance distribution. Size can be measured by noting edge enhancement (radiance peak at the edge of the droplet) in the reconstructed image. A relationship between the diameter of the peak and the actual droplet diameter is determined for sphere diameters from 20 to 250 μm and object distances from 10 to 20 cm. Experimental accuracy to within 4% is achieved with this technique.

Fresnel zone plate solved by the boundary-diffraction-wave theory

Abstract: The Fresnel zone plate is studied by the boundary-diffraction-wave theory (BDWT). Expressions are given for the fields in the transverse planes and along the axis; in particular, the wave amplitudes at the foci are considered in some detail. The BDWT provides a clear physical picture of the problem. The wave amplitude at any point is given by the sum of a finite number of rays determined by the number of zones in the plate. The foci are the points where the singly diffracted rays are in phase.

Clutter fence for suppression of electromagnetic energy

Abstract: A conducting or absorbing ground clutter fence at least partly encircles an antenna to block electromagnetic energy in a horizontal band having a vertical height equal to one-half the diameter of the first Fresnel zone of the antenna and, furthermore, having a vertical portion thereof completely within the first Fresnel zone