Showing papers on "Spatial filter published in 1968"

Computer generated spatial filters for coherent optical data processiing.

Abstract: The setup used in many optical data processing schemes is a coherent optical image forming system. The most important lement in this setup is the complex spatial filter. It can perform a large variety of linear operations upon the object or input. In general, it is difficult to produce complex filters, since both amplitude transmission and phase delay may vary across the filter plane in a complicated manner. Our own filters which are very similar to binary holograms, consist of many little transparent rectangles on opaque background. They can easily be drawn on a large scale by a computer-guided plotter, and then photographically reduced in size. We show that our filters, despite containing only amplitude values zero and one, can perform any data processing operation which could be performed by any complex filter. After explaining the principle, we present three groups of applications. First, we describe new versions of some classical methods: schlieren observation and phase contrast. Next, we report on spatial which perform differential operations upon the object in order to enhance gradients or corners. Finally, we use our binary filters for signal detection.

A Review of Optical Data-processing Techniques

Abstract: Various optical data-processing techniques are reviewed in this article Both coherent and incoherent optical systems are considered Coherent systems are described in terms of spatial filtering concepts and two principal methods for implementing a general linear integral operator are discussed: one using space-domain reference function masks and one using frequency-domain spatial filters Incoherent systems are described in terms of geometrical optics

Long-period signal processing results for the large aperture seismic array.

Abstract: Processing of seismic data from LASA can be accomplished by simple delay and sum or beam forming, weighted delay and sum or by maximum‐likelihood filtering. It is found that band‐pass prefiltering the data has a considerable effect on the results. The extra gain produced by maximum likelihood filtering does not justify use of this process on‐line, but could justify off‐line processing in special cases. The spurious precursor introduced by maximum‐likehood processing, and shown to be caused primarily by the signal amplitude scatter within a subarray, can be effectively reduced by using amplitude equalization. The signal amplitude scatter within a subarray, and between subarrays, does not degrade seriously the performance of the signal processing. The effectiveness of the processing depends not only on the array spatial filtering ability, but on differences in absolute level of the noises in the various sensors. Our experience in processing a weak event whose apparent magnitude, as averaged over the LASA wa...

Methods of Increasing Discrimination in Optical Filtering

Abstract: Optical spatial filtering techniques for pattern detection and several methods of increasing the discrimination between similar patterns are described. The use of high signal spectrum-to-reference beam intensities is found to be the most useful of these methods, and the properties of filters of alphanumeric characters made in this way are compared with linear filters.

The Effect of Spatial Averaging on Spectrum Estimation

Abstract: The effect of instrument baseline averaging on the estimated spectrum density of a homogeneous random field of a scalar variable in turbulent flow is investigated. The spectrum transfer function of the spatial filter is evaluated for all orientations of the baseline to the vector mean wind. It is shown that the influence of baseline averaging on the spectrum increases as the angle between the baseline and the vector mean wind decreases and as the ratio of the baseline length to the wavelength of the fluctuations increases. These results can be used to correct the computed spectra of spatially averaged components of the velocity field in turbulent flow and of spatially averaged conservative passive additives in a turbulent flow.

Multiple imaging by means of point holograms.

Abstract: Multiple imaging devices are capable of producing simultaneously a number of images of one object. Several methods have been suggested to realize such an optical system. They make use of the imaging properties of a fly's-eye lens or of an array of pinholes, or they utilize birefringence. More recently, multiple image generation has been achieved by forming the convolution of the object with an array of δ functions in an ordinary spatial filtering arrangement. The complex filter is realized by the Fourier transform hologram of the cor­ responding array of point sources. The second Fourier trans­ form lens, however, has to transmit more than twice the full information of the multiplied image array. Since the most im­ portant application of multiple imaging is semiconductor device fabrication as has been discussed earlier, the resolving power has to be very high, so that up to now no lenses are available that are capable of imaging the multiple array of images of a usual etching mask. I t is for this reason that to date, images of etching masks are transferred to the semiconductor wafer by contact printing. In this communication, we propose to use an arrangement dif­ ferent from that in Ref. 4, such that the multiplied images do not have to be transmitted by any additional optical component. Par t of this communication has already been mentioned in a paper presented at a holography meeting organized by the Battelle Institute at Frankfurt /M, Germany, on 2 December 1967. The principle of operation is illustrated by Fig. 1. The hologram of the array of point sources is produced in the usual way by recording the interference pattern caused by coherent superposition of the spherical waves emanating from the ref­ erence point and the signal points the spatial positions of which correspond to the desired positions of the images to be formed. Multiple imaging is then performed in the reconstruction process by replacing the reference point source by the real image to be multiplied. This can be achieved in the well-known way by means of a lens that images the object through the hologram into the surrounding of the reference point. In this case, an additional real image of the object is formed in the surrounding of each signal point. The behavior of the hologram can be easily understood by thinking of a point hologram as of a generalized Fresnel zone plate with its well-known lenslike properties. Indeed, this hologram of the distribution of point sources may be considered as some special kind of a fly's-eye lens, in which the lenses are replaced by Fresnel zone plates. But there is a rather important difference. Contrary to lenses, the single point holograms may overlap completely without disturbing each other. For this reason, the aperture of each point hologram determining the diffraction limited size of the image point can be made equal to the aperture of the hologram of the whole point distribution provided that suitable spherical waves are used in the recording process. There is no relationship between the diameter of this aperture and the distances of the images to be formed. Limitations to the resolution of this multiple imaging device are mainly introduced by aberrations. In the special case of small plane objects, which are of dominant interest in semicon-

Holographic data reduction.

Abstract: Very large bandwidths are required for the transmission of holographic data for systems such as TV. This paper presents a technique in which the large bandwidths normally required are traded off for either increased noise or decreased resolution in the image. The light radiated from the object is diffracted by an intermediate dispersion medium and collected at the hologram aperture. Correct illumination of this hologram provides an image beam that passes back through the intermediate medium and comes to focus in the space originally occupied by the object. By proper selection of the dispersion medium, the hologram aperture can be made extremely small, thus representing a large data reduction. The three dimensionality of the image and the original viewing angles are maintained. Included in the paper are experimental results that show reconstructed images after a data reduction of as much as 3600.

Optical Filtering to Compensate for Degradation of Radiographic Images Produced by Extended Sources

Abstract: A method of optical filtering in order to provide for enhancement of x-ray images whose quality has been degraded because of the use of extended x-ray sources (penumbra effect) is considered. This method employs spatial filtering techniques in a coherent optical system. A description of the penumbra effect follows from the mathematical description of imaging in incoherent optical systems. With the use of this model the degradation can be described in terms of the effect of the extended source on the frequency content of the resulting image. This leads to a precise definition of the ideal filter in terms of the spatial intensity distribution of the source and a general description of the filtering operations which will be required in all cases, independent of the exact source distribution. It is shown that the form of the ideal filter is such that the same optical system used in the filtering process is ideally suited for generating filters with, approximately, the required transmission characteristics. Experimental results are presented.

Spatial filter color encoding and image reproducing apparatus and system

Abstract: A filter is provided in the path of light from a scene, either live or on film, ultimately to be displayed as on a television tube. The filter has the property of encoding color information in the intensity pattern of light transmitted. The encoded intensity pattern may be used directly or recorded and regenerated. The encoding medium of the filter comprises at least three grids superimposed one upon another and disposed at different angles relative to a reference whereby at least four different bands of frequencies are generated when the encoded intensity pattern is scanned, as by a photocathode. That is, a first band of frequencies is generated including a waveform (a video signal) proportionate to variations in light intensity, two individual bands of frequencies are generated each in separate individual waveforms (video signals) modulated in accordance with intensity variations of a different one selected component color, and a fourth individual band of frequencies is generated in a waveform modulated in accordance with intensity variations in color components including still another selected component color. The frequency bands are separated by filters and used to generate a color picture on a color receiver by applying the waveform incorporating the intensity band of frequencies (the first band) to the receiver to give the general picture luminance information, applying the waveforms containing the second, third, and fourth bands of frequencies to a processor (e.g. the matrix of a colorplexer) for developing color difference signals that are applied to the receiver along with the first band of frequencies, thereby to develop a color balanced picture.

Laser Light Diffraction, Spatial Filtering, and Reconstruction of Medical Radiographic Images: A Pilot Study

Abstract: Using a laser light source, an optical bench, transform lenses, and spatial filters in a system called LaserScan, 1 we are studying laser light diffraction, spatial filtering, and reconstruction of medical radiographic images, with the help of GTS Corporation, whose laboratories house the laser computer Medical radiographs are minified to a 35 mm second-generation transparency, then transilluminated by monochromatic, coherent, collimated light from a helium-neon gas laser A first transform lens produces a Fourier Transform of the minified image in the form of a diffraction pattern that is photographed on Polaroid film, and/or viewed with a CCTV system Spatial filtering is accomplished in the plane of the diffraction pattern by using various opaque elements of appropriate geometric form to filter undesired spatial frequencies Spatial filters take the form of ``wires,'' ``sector-wedges,'' and circular apertures, as well as other empirically determined shapes A second lens performs an inverse Fourier transform on the diffraction pattern producing a reconstructed, filtered, real image of the minified film This reconstructed image is also photographed on Polaroid film or viewed on the CCTV screen One of our goals is to remove certain unnecessary pattern detail from particular radiographic images so that characteristic patterns of diagnostic picture detail might be more readily identified and diagnosed Experimental results, including photographs of diffraction patterns and some filtered reconstructed images, are presented

Multiple Frequency Wavefront Recording

Abstract: The effects of multiple frequencies in spatially recording two wavefronts are analysed to describe the fringes recorded in holography as a function of the number of frequencies used, and to relate the changes in the fringe structure to the quality of the reconstructed image. The analysis is applicable to spatial filtering or to any process where a coherent wavefront is recorded by interference with another wavefront. A possibility of doppler shift of the frequencies in one wave is included to describe the effects of recording a hologram of a moving object. It is shown that, if the multiple frequencies are due to laser multimoding, the fringe visibility is a maximum for path differences equal to even multiples of the laser cavity length, but that the locations of the minima depend upon the relative amplitudes of the different frequencies. The effects of the changes in the fringe structure upon the resulting image or correlation are discussed.

An electronic shutter for holographic applications.

Abstract: Precise control of exposure is generally required in holography and optical spatial filtering. The experimental results of Friesem et al. show that, for Kodak 649-F film, the diffraction efficiency changes rapidly with small changes in bias because of the high contrast nature of the emulsion. For example, the maximum allowable change in bias is about one half stop on either side of the optimum value. Even over this restricted range, the diffraction efficiency at the extremes is only one-third the maxi­ mum value. In the past, holographic exposures of many seconds were not uncommon, and exposure control was relatively simple without recourse to mechanical shutters. With the advent of more powerful lasers and of faster high resolution emulsions such as Agfa-Gevaert Scientia 14C70, exposure times have been signifi­ cantly reduced. Standard, readily available mechanical shutters are largely inadequate because fine exposure control between the fixed stops is not generally possible. In this letter, we describe an electronic shutter suitable for the described application, which provides continuously variable exposure times in the range from 1 msec to many seconds.

Temporal light modulator using deflectable membrane

Abstract: A system for modulating the amplitude of a beam of radiant energy which includes a reflective surface, a lens positioned to form a Fraunhofer diffraction pattern of a beam reflected from the surface, and a spatial filter located at the focal point of the lens. Means are provided for moving the reflective surface through a range of positions, so that the intensity at the center of the pattern varies from a maximum to a minimum. The filter is arranged to select only the central region as the output beam.

Eliminating a perturbation when forming a holagraphic spatial filter

Abstract: The present invention relates to a method of eliminating perturbation between a reference beam and a source beam during the formation of a hologram. A Fraunhofer diffraction pattern of the subject of the hologram is placed in front of a recording medium in which a hologram of the subject is to be formed. Perturbation between the reference beam and the source beam is thus eliminated. A hologram, such as is used in a character recognition system, may be improved by the method of the present invention.

Fourier transform representation of an ideal lens in coherent optical systems

Abstract: Fourier transform of focussed diffraction pattern to describe coherent optical systems with lenses