Showing papers on "Spatial filter published in 1974"

Coherent optical processing

Abstract: The principles of communication theory were applied in the 1950's to optical imaging systems and to the analysis of images. Optical systems were analyzed in much the same way as linear systems (modulation transfer functions and channel capacities) and images were characterized in ways analogous to time signals (space-bandwidth products, spatial frequency content, etc.). Both coherently and incoherently illuminated optical systems can be treated using these concepts. Coherently illuminated systems are most useful for performing operation such as convolution, cross correlation, and spectral analysis because the Fourier transform of an optical signal physically exists and can, therefore, be measured or modified. The basic Fourier transform relationship for coherently illuminated systems is developed in this paper. It can be detected directly and used to estimate the distribution of spatial frequencies contained in the signal. Methods for constructing complex-valued spatial filters are described; these filters can be used to realize such operations as convolution or cross correlation, addition or subtraction, and differentiation or integration. Experimental results are given to illustrate the concepts and to susgest potential applications. To extend the range of applications, interface devices are needed to allow optical processing of two-dimensional raster-scanned time signals, wide bandwidth electrical signals, and incoherent optical signals. Interface devices are often needed to convert the output optical signal to an electrical signal for post-processing by a digital computer. For some applications, interface devices are needed to construct spatial filters in real time, so different operations can be performed on a given signal. The desired characteristics of these three interface devices and the current state of their development are briefly reviewed.

Method and apparatus for pattern analysis

Abstract: A technique and apparatus for two dimensional pattern analysis utilizing a transform of the pattern enables the extraction of desired pattern information by means of spatial filtering in accordance with known human visual system processing. Two dimensional spatial frequencies resulting from the transform are acted on by either anisotropic or uniquely used conventional filters to extract one, two and three dimensional pattern information from spatial frequency subsets to determine general form, edge, texture and depth information for detection, identification and classification of objects in simple or complex scenes.

Holographically generated lens.

Abstract: A holographic lens has been made that utilizes some of our earlier work on holographic aberration correction [ J. E. Ward D. C. Auth F. P. Carlson , Appl. Opt.10, 896 ( 1971)]. The lens properties are obtained by interferometrically recording a plane wave and a pinhole generated spherical wave. The theoretical analysis is followed by the results of the Ronchi lens test, Fourier transformation of a grating, and imaging of a test picture.

Remote probing using spatially filtered apertures

Abstract: The use of spatial filters, or weighting functions, on transmitting and receiving apertures used on line-of-sight propagation paths is a powerful technique for probing the velocity and refractivity structure of the transmission medium. Provided that the weak-scattering approximation is appropriate, simple and inexpensive techniques offer on the order of 100 resolution elements along the path, with little or no data processing required. A simple optical experiment has been conducted over a 61 m path, with a resolution-cell size of about 0.7 m long, 0.1 m3 volume, with results that compare well with anemometer measurements.

Mathematical operations by optical processing

Abstract: Some recent developments in coherent optical processing are described in this paper. Specifically, the use of one or more elementary gratings to perform useful processing operations and computer-generated spatial filters to obtain generalized transforms are discussed. Applications incorporating nonlinear optical elements and optical feedback into the processing system are also presented.

Measurement of surface displacement normal to the line of sight

Abstract: Two methods for measuring the displacement of a surface normal to the line of sight are described and experimental results presented. The displacement in either case may consist of a simple lateral or rotational shift of the body in total, or it may consist of small local displacements due to some type of mechanical stress. The displacement is displayed as a system of interference fringes over the image of the surface. The first method permits observation of the displacement fringes in real time; or, if a double exposure is recorded, the fringes can be observed directly on the doubly exposed photograph in white light. The second method requires spatial filtering of the image after recording a double exposure of the surface before and after displacement. However it permits selection of the component of displacement in any direction normal to the line of sight with a continuously variable sensitivity from the single (doubly exposed) transparency. Both methods are noncontacting, use only a single laser beam for illuminating the object and require no previously constructed grids or rulings.

Color image intensification and projection using deformable mirror light valve

Abstract: A color image intensifier and projector includes a reflective light valve lamination having, in order, a flexible conductive mirror, a deformable layer, a photoconductive layer, a transparent conductive layer, and a plurality of optical diffraction gratings each for a different color An electric potential is applied across the flexible conductive mirror and the transparent conductive layer to bias the photoconductive layer When an input colored image is projected through the gratings to the photoconductive layer, the resulting electric fields cause the deformable layer and flexible conductive mirror thereon to be deformed in accordance with the colored image as modified by the diffraction gratings A Schlieren optical system is included to direct light from a source to the flexible conductive mirror, and to collect reflected light and direct it through a fourier transform plane to a utilization plane A color-decoding spatial filter is located in the fourier transform plane to recreate the color image at the utilization plane

Coherent optical processor apparatus with improved fourier transform plane spatial filter

Abstract: A line pattern or fingerprint ridge pattern identification apparatus employs coherent optical processing techniques wherein the line or ridge orientations and spacings in a plurality of preselected finite areas of the fingerprint are inspected by means of a rotating spatial filterdisposed in the Fourier transform plane of the optical processor for cyclically selecting distinct components of the Fourier transform for transmission to the image plane of the processor in which is disposed a plurality of photodetectors each corresponding to a discrete sampled area of the print. The time delays between a reference orientation of the spatial filter and the blocking and unblocking of light transmitted therethrough toward each photodetector are noted. These values are processed to provide proportional representations thereof for storage and for subsequent comparison with similarly obtained signals representative of ridge line orientation and separation of a fingerprint presented for identification.

Reduction of the number of samples in computer holograms for image processing.

Abstract: Institut d'Optique, Université de Paris XI, Centre d'Orsay, 91405 Orsay, France. Received 14 December 1973. Computer generated holograms improved significant­ ly the possibilities of optical data processing by spatial fil­ tering. They allow an easy synthesis of several inter­ esting filter functions for the purpose of deconvolution and other image enhancement methods. Nevertheless, a practical limitation appears when the processed image has to contain a large number of degrees of freedom, i.e., when a large space-bandwidth product is involved. Then, in order to avoid overlapping of the vari­ ous orders in the image plane, the number of sampling cells of the computer generated Fourier hologram used for filtering must be at least equal to the number of degrees of freedom of the image to be processed. As an example, let us consider the case of a 24 × 36 mm image recorded with an f/18 lens at a wavelength of 0.5 μm: it contains a number N0 = 10 degrees of free­ dom. The processing hologram has to contain this num­ ber of sampling cells and needs a large computing and plotting time. Moreover, the plot is not easy to perform because, even with a cell size as small as 1 mm, the plot area is 10 m or more. In addition, the requirements upon the optical reduction system are very severe. The trouble is that these conditions hold even when the im­ pulse response of the hologram contains only a small number of degrees of freedom, say 10. This means that a large oversampling is necessary, not to perform the desired processing but only to avoid image overlapping. A typical example is pattern recognition of a small detail in a large image. Another example is relat­ ed to images degraded by some small blurring function. Then the deblurring function is in general of the same physical size as the blurring function, i.e., it contains many less degrees of freedom than the image. Our aim is to elude this difficulty and to compute and plot a binary hologram with only the minimum number of resolution cells that allow reconstruction of a given im­ pulse response with a given bandwidth, irrespective of the number of image resolution cells. To this end, we pro­ pose a two-step method. 1. A binary computer hologram (C.H.) with a small num­ ber of cells is realized. The space-bandwidth of this hologram is just large enough to prevent overlap of ad­ jacent impulse response orders. 2. A natural ordinary hologram (N.H.) is recorded using one order of (C.H.) as the object. By adjusting proper-

Spatial Filtering to Improve Transverse Tomography

Abstract: A new transverse tomographic apparatus is described that enables the tomogram to be related to the ideal image by a linear blurring operation. It is shown that digital and optical linear spatial filtering techniques may be employed to remove the blurring that is present in the tomographic image obtained with the new device. Experimental verification of both restoration methods is presented.

Coherent semiconductor injection laser array

Abstract: Disclosed is a spatially coherent semiconductor injection laser array. The system comprises a plural element laser diode array placed symmetrical in respect to an optical axis. An external resonating cavity formed of spherical lenses and plane mirrors is disposed about the array with its axis of symmetry coincident with the optical axis. A spatial filter in the Fourier plane of the laser diode array selects the TEM 00 mode generated from each of the elements in the array. In operation, the system generates spatially coherent radiation from each of the elements of the array. The radiations are coherently coupled between all of the elements to provide a high optical power output in a spatially coherent beam.

Two-dimensional frequency domain filtering

Abstract: This specification discloses an automatic computer-implemented technique for filtering a two-dimensional array of data in the frequency domain. The program converts the array of numbers by Fourier transformation into the real and imaginary part of the frequency domain. In this domain, undesired waves present in the original array can be completely removed by the elimination from the frequency domain of the components of the Fourier-transformed array which fall within (or, if desired, outside of) a convex polygon of arbitrary shape. The system then re-transforms the remaining data back to the time domain, but the undesired components have been filtered. The program can cause the computer to produce a visual reproduction of the filtered array, or it may simply re-format the data of this array on a tape for further computer processing. Accordingly, this invention permits precision spatial filtering of undesired components from the original array. The original array may represent the digitized data from a seismic section, or the amplitude and frequency response of a two-dimensional array of geophones to a steady state alternating seismic wave from a source. A rapid version of large automatic Fourier transformation and re-transformation is included.

Interferometric method of measuring plotter distortion.

Abstract: The authors are with the Itek Corporation, Lexington, Massachusetts 02173. Received 25 September 1973. In this letter an interferometric method of measuring plotter distortion is described. A common method of measuring plotter distortion is to draw a known pattern, such as straight lines, and scan this pattern with a microdensitometer. This is a precise method of measuring distortion; however, it is a time consuming process; and it gives distortion only along the microdensitometer scan lines. The technique described below gives a contour map for the whole plot showing lines of constant distortion in a particular direction. By combining two contour maps for distortion in two orthogonal directions, the distortion in any arbitrary direction can be found. If the plotter distortion is repeatable and the plotter is computer controlled, once the distortion is known it can be eliminated as described by Fercher et al. As will now be shown, if a plotter draws straight lines the plot can be thought of as a hologram produced by interfering two plane wavefronts. The plotter distortion produces the same effect on the plot as aberration in one of the plane wavefronts would produce on the hologram. If a plotter were to draw perfectly straight lines perpendicular to the x direction, the equation of. the straight lines would be (x / ∆ x) = M, where ∆ x is the line spacing and M is an integer, 0, 1, 2, and so forth. Because of plotter distortion, the lines are never perfectly straight. Let the distortion in the x direction be δ x (x, y). Thus a point that should have the x coordinate x will have the x coordinate x + δ x (x, y). Therefore, the equation of the lines drawn by the plotter will be

Unstable split mode laser resonator

Abstract: An unstable laser resonator system which provides an output beam with enhanced power, energy distribution and optical characteristics is disclosed. Two separate regions of gain medium each of which has the maximum dimensions permitted by the physical constraints of superfluorescent and excited population decay characteristics resonate in a split mode optical cavity having a common interlocked region of resonance which incorporates the contributions of each gain medium and combines the composite intracavity mode prior to output coupling from the resonator. The resonator system produces a coherent, phase-locked, high power, symmetric output beam. Optical cavities based on both spherical and cylindrical optics are discussed.

Optimum Fourier-transform division filters with magnitude constraint

Abstract: Fourier-transform division filters are discussed. Using integrated squared error as a fidelity criterion with the magnitude of the filter transfer function subject to a constraint, the optimum spatial filter is developed. By consideration of the problem of image transformation, various methods for improving image reconstruction by altering image phase are discussed. The input-image phase and the desired output-image phase may be chosen to improve the performance of the filtering system. The problem is to shape the spectra of the input and the desired output. An algorithm, previously used for computing kinoforms, effectively determines an image phase that significantly improves the image reconstructed from the spatial filters.

Spatial filtering for error detection

Abstract: Fourier transform imaging microscope for examining parts carrying repetitive stripe pattern having a source of light with means forming a slit through which the light passes. Means including the part carrying the repetitive stripe pattern is used for forming a Fourier transform image of the slit after the light beam passes through it. Slit means is provided for suppressing the image of the stripe pattern itself while enhancing any defects and non-uniformities in the pattern.

Optical Filtering of Aeromagnetic Maps

Abstract: An optical processor has been used to filter aeromagnetic contour maps in order to obtain information on underground magnetic ore deposits. This was accomplished by directional filtering of the spatial Fourier transform of the contour map. The directional filtering yields maps of gradients having given directions. A digital analysis was also performed on the data so that the feasibility of the optical technique could be evaluated. The results obtained so far suggest that a systematic filtering of aeromagnetic maps can be carried out at low cost and that the filtered maps can yield useful information to the interpreter.

On Optimal Holographic Filters

Abstract: Vander Lugt (Ref. 1) introduced the use of holograms as frequency plane masks in spatial filtering applications. The technique produces the convolution of an input spatial scene s(x) with a convolving function a(x) by placing a hologram in the Fourier transform plane of a coherent optical processor. The hologram is a photograph of a reference beam and the Fourier trans form- H(f) of a spatial mask function h(x). The filtered scene is realized as an off-axis spatial signal. See Figure 1.© (1974) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Automatic focus apparatus

Abstract: A focus control system for a still camera includes a photoelectric amplifier and a high frequency spatial filter. The filter is arranged to scan the image region of a fixed focus lens as the taking lens of the camera is moved from an extended position toward the infinity position thereof. During a first portion of the scanning movement the filter provides an initial signal representative of the brightness of the imaging light bundles for automatic gain control. The gain of a photosensitive detector is initially set during this period. During a second portion of the scanning movement, the filter moves through the image space and continues to provide a signal for adjusting the gain of the detector. This signal has a maximum amplitude as the filter passes through the actual image. A change of slope detector senses this position and actuates a mechanical stop to freeze the position of the taking lens at that position. It is contemplated that the shutter of the camera would then be released to initiate the exposure.

Focussed image hologram projector using a long narrow light source

Abstract: A filamentary light source and condenser system provide one or more light beams for illuminating a frequency-encoded focussed image hologram, the light diffracted therefrom being decoded by a slit-type spatial filter and projected onto a viewing screen to display a full color reconstruction.

Reduction of multimode pulse dispersion by intentional mode coupling

Abstract: Guidelines for the design of multimode, step-index fibers with intentional fluctuations of the refractive index of the core are given, with the aim of reducing multimode pulse dispersion. It appears possible to engineer a fiber with carefully designed refractive index fluctuations, the azimuthal variation of which is governed by the function cos ϕ and the z dependence of which has a spatial Fourier spectrum with a sharp cutoff frequency. By limiting the location of the index fluctuations to a region below a certain radius r max , coupling to modes with large azimuthal mode numbers can be avoided and power loss via coupling to radiation modes can be held to a minimum.

Spatial Filtering By Digital Holography

Abstract: The construction of digital holographic spatial filters using a high precision multiple gray-level plotting device is demonstrated. Examples of the use of such filters in a coherent optical system are presented, and analogous results obtained by computer processing are shown for comparison. The theory of sampled holograms and spatial filters is discussed in some detail.

A Fourier approach to optical shadowing with a transmission electron microscope

Abstract: SUMMARY Optical shadowing offers the possibility of combining the high resolution of the transmission electron microscope with the study of surface detail, commonly investigated with the scanning electron microscope. Image formation in this mode of operation is treated according to the formalism of Fourier optics, where we consider the objective aperture to be a spatial filter in a coherent imaging system. For a simple specimen it is shown theoretically as well as experimentally that the image appears to be a shadowed representation of the object, when the objective aperture is moved off centre so that half of the zero-order diffraction spot is blocked. A computer model is presented that gives a faithful description of the theoretical and experimental image intensity profile for the simple test specimen. The computer model is then used to predict image profiles for more complicated (or more realistic) specimen profiles. The calculations show that a certain ambiguity arises when one attempts to interpret the optically-shadowed image profiles: it would be difficult to distinguish the three different edge contours based only on their appearance in this mode of operation. As with any other EM technique, then, results obtained with optical shadowing must be compared with the results obtained using the others.

Optical Spatial Filtering of Radiographic Images with Binary Filters

Abstract: An optical spatial filtering technique was used to remove grid lines by eliminating the transmission of the corresponding Fourier spectrum in the Fourier transform plane. Studies of the effect of band-pass filtering on cerebral angiograms showed that a recognizable image of the blood vessels is produced even when the filter is placed in a spatial frequency range beyond that at which the Fourier spectrum of the radiographic image would presumably be essentially zero. These results indicate that radiographic mottle carries image information, supporting an image-dependent (multiplicative) noise theory.

Problems in the use of selective optical spatial filtering to obtain enhanced information from electron micrographs

Abstract: SUMMARY Possible hazards in the use of optical spatial filtering applied to the analysis of electron micrographs are illustrated. Two experiments show how information is either lost or spuriously introduced into reconstructed images of micrographs when (a) only one part of a two-part micrograph image is reconstructed and (b) when noise is eliminated from the reconstruction.

Coherent optical noise suppression device.

Abstract: In optical data processing the quality of the output image is usually degraded by diffraction noise generated by the optical components of the system. The suitability of previously suggested techniques to a dc spatial filtering processor is discussed. A new system is proposed that overcomes the problems of the previous noise suppression techniques. Experimental results of the new system are presented along with a determination of the resolution of the system. The errors and limits of the new coherent noise suppression system are discussed.

Restoration Of Radiologic Images By Optical Spatial Filtering

Abstract: Finite-size X-ray sources degrade the radiologic image by attenuating the high-frequency components of the image and by phase distorting a significant number of these frequencies. Coherent-optical spatial filtering was employed to correct these distortions in the Fourier transform of the radiologic image. This included the use of amplitude-only and amplitude-and-phase (holographic) filters. The contrast and resolution of X-ray images were improved.

Spatial filter for Q-switched lasers

Abstract: A spatial filtered Q-switched laser system is disclosed having means to prevent ionization of air in close proximity to the aperture of the spatial filter A compound lens system having an astigmatic focus is disposed between the laser and the spatial filter for defocusing the light beam emanating from the laser in the vicinity of the aperture of the spatial filter to an intensity below that which produces ionization of air The preferred construction of the compound lens system as viewed from the laser comprises a cylindrical lens and a pair of positive lenses

Versatile Flexure Mount for Precise Positioning of Optical Elements

Abstract: The purpose of the present communication is to describe a versatile flexure mount that has been developed for precise angular and/or translational positioning of optical elements. The devices described below are compact, sensitive, and stable, yet, due to their basic simplicity, they are easily fabricated with a minimum of equipment and at very little cost. The basis for the present design is an adaptation of a universal joint mechanism developed and patented by NASA, which has subsequently been utilized in the design of a laser velocimeter system. The basic concept upon which the flexure mount works is illustrated by the cubic flexure in Fig. 1. With surface B fixed, any element affixed to surface A may be independently rotated about mutually perpendicular axes xx and yy by adjusting the two sets of screws on adjacent sides of the block. As discussed below, other geometries are often advantageous; however, we will describe the fabrication procedures in terms of the cubic device in Fig. 1 where only minor changes need be made for the other geometries mentioned. To construct a flexure mount, we start with a cube (or rectangular block) of a suitable material. The flexure webs defined by axes xx and yy are generated by drilling two pairs of parallel, closely spaced holes through the block at right angles to each other, lying in a single plane. Parallel saw cuts are then made from opposite ends into each pair of parallel holes, as illustrated. Precision adjusting screws are provided through opposite sides of the lever arms formed by the saw cuts, which, when driven against the center blocks, provide two independent rotational motions about the mutually perpendicular axes xx and yy. By using two screws for each axis, adjustments may be made in either direction from center, where the

Color control system for optical image apparatus

Abstract: In an optical apparatus of the Fourier transform type employing a plurality of individual light sources arrayed about the optical axis of the apparatus, a control subsystem for all the light sources samples light from the light sources selectively. In the illustrated embodiment, a pellicle deflects a minor fraction of the source light to a spatial filter which passes a selected sample or samples to a group of spectrally filter photodiodes, which respond with currents that are fed to a photometric circuit and means to display measured quantities to an operator. The apparatus includes operator controllable means to adjust the intensity of each supervised light source independently of the others. A primary light source, from which the individual light sources are derived, is also supervised, to provide a basis for taking into account variations in energizing voltage, and dimming of the primary source with age.