Showing papers on "Spatial filter published in 1972"

Displacement Measurement from Double-exposure Laser Photographs

Abstract: Surface displacement can be measured by recording a double-exposure photograph of the laser-illuminated object, followed by optical processing of the recorded speckle-pattern image. The analysis can either be on a point-by-point basis, or by a spatial filtering technique which resolves the motion in any desired direction orthogonal to the line of sight. The limits within which the technique may be used to measure lateral translations and rotations of the surface are examined theoretically and experimentally, and the effect of lens aberrations and surface scattering properties are discussed. Surface tilt may also be measured by recording a defocused double exposure image and analysing its optical transform. The photographic techniques described can be extended to measuring surface vibration, by recording a single ‘time-averaged’ exposure and examining the modified optical transform fringe pattern.

Spatial sampling and filtering in near-field measurements

Abstract: A sample spacing criterion and a data minimization technique for measurements made over the surface of a plane in the near field of an antenna are presented. The sample spacing is shown to depend on the distance from the antenna to the measurement plane, and on the extent to which evanescent waves can be neglected. The near-field data minimization technique utilizes two-dimensional spatial filtering to effect a significant reduction in computational effort required to calculate selected portions of the far-field pattern. Far-field patterns of an X band antenna calculated from near-field measurements are presented and compared with those measured on a standard far-field range. The far-field calculations are repeated for several near-field sample spacings and for various post-filter sample rates.

Talbot Interferometer for Radial and Lateral Derivatives

Abstract: The theory and experimental evidence of a shearing interferometer based on the Talbot effect are presented. Multiple-shearing interferences are obtained that can be reduced to triple-shearing or doubleshearing interferences by the addition of simple spatial filtering. When the shear is less than the width of the details in the object, these interferences become either the second or first derivative of the object under test, respectively. Either lateral or constant radial shear can be introduced by choosing Ronchi rulings or circular gratings. Thus both lateral and radial derivatives are easily obtained. If white light is used as a source, color fringes of high contrast are observed.

Surface inspection with scanned focused light beams

Abstract: A surface inspection system uses a highly focused spot of light moved rapidly over the surface to perform an inspection This permits a ready location of the defects, gives some indication as to their size, and permits a high resolution inspection to be performed in a relatively short time An optical arrangement is employed in which a scanned beam is brought back through the same lens system used in generating the scan to produce an immobilized return signal beam A stop or spatial filter is used on the return beam to permit only the light scattered from defects to reach a detector

Method of forming and cooling pinhole spatial filter for high power laser

Abstract: An electrostatically cooled spatial filter for high powered pulsed lasers wherein the filter comprises a first lens which focuses a laser beam on a pinhole, and a second lens positioned behind the pinhole to receive the beam. To prevent the laser beam from destroying the pinhole an electric field is applied across the pinhole. This field cools the pinhole and as a result the size of the pinhole is preserved.

Spatial filtering system utilizing compensating elements

Abstract: A spatial filtering system for inspecting integrated circuit photomasks, and the like. The system includes a spatial filter comprising a matrix-like array of opaque regions on a transparent field, as well as one pair of opaque compensating elements for each element edge orientation in the photomask to be inspected. The opaque regions block the d.c. and lower spatial frequencies of the periodic photomask feature information, while the compensating elements block the corresponding higher spatial frequencies (i.e., the edge information). Use of the system significantly improves the signal-to-noise ratio of the filtered image.

Synthesis of Complex Optical Wavefronts

Abstract: For any coding scheme designed for the synthesis of an optical wavefront by a computer-generated, amplitude-transmittance hologram, two types of synthetic holograms may be defined: the first is the encoded form of the optical wavefront itself; the second is the encoded form of the Fourier transform of the wavefront. The extent of the optical wavefront produced by the first type varies inversely, while that of the second type varies directly, with resolution. Five coding schemes are characterized by two important quantities: the first is the minimum number of samples that must be numerically evaluated and drawn; the second is the product of the minimum resolution (lines/unit length) and length (per dimension) required for the drawing. These quantities are a measure of the required computer and drawing device capabilities, respectively, and knowledge of them permits comparison of the coding schemes.

Method and apparatus for recording and projecting holograms

Abstract: A method and apparatus for recording and projecting single color or multiple color encoded image-plane carrier holograms are described. An object is illuminated with a coherent collimated laser beam. The parallel spatially modulated rays are focused on a monochrome recording medium located in a recording plane with the rays maintained essentially parallel. A collimated reference beam, which is coherent with the object beam, is directed at the recording medium to form an image-plane hologram having a single discrete spatial carrier frequency in case of a single color laser or additional spatial carrier frequencies for as many colors as are in the laser. The processed image-plane hologram is projected by directing a generally collimated beam of incoherent white light upon the hologram to form separated wavefront reconstructions of parallel rays. A lens projects the parallel rays in a viewing plane after their convergence at a focal spot in a focal plane. A spatial filter located in the focal plane provides selection of desired color images and blocks undesired ghosts. A control for moving and adjustment of the spatial filter is provided to attain hue and saturation control as well as brightness of the projected image. An embodiment for recording of image-plane holograms of diffuse objects or diffusely illuminated objects as well as three-dimensional objects is described.

Intensity spatial filter having non-uniformly spaced filter elements

Abstract: A spatial filtering system for inspecting integrated circuit photomasks, and the like. The system employs a spatial filter comprising a matrix-like array of opaque regions on a transparent field. Unlike prior art systems where the region-to-region spacing of the filter is uniform, in the instant invention the region-to-region spacing steadily increases from the centermost element outward according to a precise mathematical formula.

Optical pattern processing utilizing nematic liquid crystals.

Abstract: This paper describes a new type of optical processing based on the dynamic scattering effect in liquid crystals. It is possible to realize a binary spatial filter with nematic liquid crystals and to control the optical modes by voltage signals. A two-dimensional Walsh function is used for extracting properties of the input patterns. The realization of an 8 × 8 Walsh function filter with nematic liquid crystals is demonstrated. The contrast ratio is over 30 to 1. The variation in transparency is within ±3%. Experimental results show that it is possible to extract the properties of handwritten characters by detecting the intensity of light transmitted through both the input pattern and the liquid crystal filter.

Spatial filtering in optical data-processing

Abstract: Optical data-processing is generally considered to have begun with Abbe's theory of image formation in the microscope. Since then the subject has expanded greatly until at this point in time it includes a plethora of techniques ranging from the simplest one-lens Fourier analysers and multipliers through coherently and incoherently illuminated correlators and processors to electro-optic devices and computer processing of optically logged data. To review the complete range of techniques and applications adequately is not possible within this article which is restricted, therefore, to a discussion of optical imaging techniques employing coherent illumination that are designed to process the input signal by the inclusion of masks and spatial frequency filters that control the system transfer function. Emphasis will be put on the characteristics of those spatial frequency filters and the processed images produced with them. The article will introduce the basic theory of coherently illuminated optical systems in terms of the Fourier transform relationships that exist between specific planes of those systems, including a discussion in general terms of the influence of masks and filters on the imaging characteristics of those systems. The major portion of the article will present a detailed examination of the properties of specific spatial frequency filters. They will be discussed firstly under a classification of filter types where their individual data processing characteristics will be elaborated and then, in summary, they will be reviewed within a classification of data processing function. Finally an appendix is included which further elaborates the theory of coherently illuminated optical systems, detailing the exact relationships between the complex amplitude distributions occurring in the principle and other planes of such optical systems.

Intensity spatial filtering applied to defect detection in integrated circuit photomasks

Abstract: An intensity spatial filter has been devised and used for the semiautomatic photoelectric detection of 2.5-µ defects in 5-cm2photolithography masks for silicon integrated circuits. The filter is based on a simple geometric approximation to the form factor or envelope function for the intensity in the Fourier-transform plane, and permits small-area diffraction-limited illumination. This approach complements the Watkins' method.

Methods and apparatus for inspecting workpieces by spatial filtering subtraction

Abstract: Integrated circuit and thin-film photomasks are inspected by a spatial filtering subtraction technique. The photomask to be inspected is illuminated by coherent, collimated radiation from a laser. An apertured mask is placed in front of the photomask to block light from all except two, spaced-apart columns in the array of features on the photomask. A lens forms Fraunhoffer diffraction patterns of the two apertured portions of the photomask at the focal plane of the lens, and these patterns are passed through an asymmetrically positioned optical grating. The diffraction patterns are then displayed on a screen. The grating produces a central image and two side images of each apertured portion of the photomask. The centermost side images of each aperture are 180* out of phase and cancel, because the columns of the photomask are identical. Differences between the two columns, which include nonperiodic errors in the photomask, are not cancelled and, hence, are readily detected.

Spatial-frequency filter for a kell-type color camera.

Abstract: There has been interest recently in a color television system using a single camera tube in a modified Kell system in which the color information is modulated to high frequencies by dichroic spatial modulators placed in the image plane. Because the total allowable bandwidth for the signal is restricted, the overlap of the luminance and chrominance bands produces interference in the form of moire patterns and color misinformation. This problem can be eliminated by including in the camera optics a low-pass filter that rejects the spatial frequencies causing the interference. In this paper, we discuss several types of phase filters and show that the rectangular wave phase grating, with a grating period of about 1 mm, offers the best performance. The optical transfer function and the filter characteristics of this optical element are discussed in terms of the grating parameters, and subjective tests are made on a prototype television system.

Method and apparatus for sensing surface displacement orthogonal to the line of sight

Abstract: A novel method and apparatus for sensing surface displacement of an object orthogonal to the line of sight is described. The object is illuminated from a single laser source and imaged by a common lens using two laterally displaced apertures. Two successive images are obtained in this manner and are superimposed. The displacement is displayed as a pattern of moire'' fringes or bands over the image. Increasing the moire'' contrast is achieved by spatial filtering. The invention has application to measuring object displacement or distortion under both static and vibratory conditions.

Linear Restoration of Incoherently Radiating Objects

Abstract: Light from an incoherently radiating object and background light are focused onto a photosensitive mosaic, the currents from whose elements constitute the data on which is based a least-squares linear estimate of the radiance at points in the object. By comparison of the mean-square error with that given by Wiener filtering theory, the equivalent noise spectral density for use in the latter is shown to consist of a shot-noise term and a term due to the random fluctuations of the incoherent light; the former predominates under most circumstances. Turbulent distortion of the image after passage of the rays through a random-phase screen is also treated from this standpoint.

A method of and apparatus for analysing objects with two or three dimensional shapes and/or patterns

Abstract: 1293348 Photo-electric investigation of geometrical characteristics of objects SIRA INSTITUTE 10 Dec 1969 [10 Dec 1968] 58649/68 Heading G1A [Also in Division G2] The geometrical characteristics of an object e.g. shape or the pattern within its external shape, are analysed by a Fourier transform technique involving irradiating the object with a beam of electromagnetic radiation to form a diffraction pattern, particular areas of which are investigated to derive electrical signals representative of the object characteristics. The object may be a diffraction grating, target, scale, graticule, photographic transparency, or mechanical component, and a typical application is to compare the object with a standard object to detect differences therebetween (e.g. faults). Fixed objects may be analysed or a series of photographs of an article or articles on a production line may be taken on a moving, progressively developed, film which is passed through the apparatus for analysis. Optical apparatus. In one embodiment, Fig. 1, light from a laser 10, is chopped at 9, split into two beams by a prism or half-silvered mirror 12, and one beam used to illuminate object 4 and the other a similar (e.g. reference) object B. Each beam comprises a microscope objective 15 (or 23), a pinhole 16 (or 24), a collimator 17 (or 25) and a lens 18 (or 26) focusing the light diffracted by the object in the plane of a spatial filter member 19 (or 27), a detector e.g. photo-multiplier or vidicon tube 20 (or 28), picking up light transmitted thereby. A comparator 29 detects any difference in the detector output signals as indicative of differences between the spatial frequencies introduced into the respective beams by the two objects A, B. Thus a zero difference indicates identity of the objects. In a second embodiment, Fig. 7 (not shown), only one light beam and detector is used, with the two objects placed side by side in the collimated beam. A chopper member in the path cuts off the light reaching each object alternately. In a third embodiment, Fig. 8 (not shown) - another single beam arrangement - a single test object A in the collimated beam can give an absolute measurement value or a comparison measurement if a spatial filter 19 in the form of a photographic mask prepared by photographing the diffraction pattern formed by a standard object is also introduced. Devices for scanning different areas of the diffraction pattern are described and can be used, if required, in combination with a spatial filter 19. In one arrangement, Fig. 4 (not shown), a cone-shaped lens (30) is placed in the plane of the or each diffraction pattern, the spherical aberration of the lens causing spreading along the optical axis of the beam the foci of the different annular bands in diffraction pattern. An aperture (31), moving along the axis, thus allows different areas of the diffraction pattern to be passed to the detector (32) which moves with it. In Fig. 5 (not shown) an arrangement of annular photo-cells of increasing diameter is used, the outputs thereof being switched so as to scan continuously or incrementally annular bands in the diffraction pattern. The arrangement of Fig. 6 (not shown) as the same, but uses an annular aperture of continuously increasing diameter formed by a variable iris and a moving cone. An integrating sphere collects light passing through for detection by photo-cell(s).

Enhancement of Patterns in Geologic Data by Spatial Filtering

Abstract: The structural configuration of the state of Kansas is well known and has been the subject for the application of both polynomial and Fourier series trend analyses. It is therefore an ideal area for reanalysis using spatial filtering techniques. The filtering method permits selective display of features of desired spatial dimensions which are relatively free from obscuring effects of large-scale trends and small-scale fluctuations. Directional and nondirectional filters were applied to a set of Kansas maps and the output maps were contoured, then Z-trended for optimum display of contained trends. Known trends were presented with a distinction and precision that confirmed previous analyses. In addition, several weak and poorly defined cross-trends were clarified. North-trending features, emphasized by a north-south directional filter, may be more important in interpreting the structural evolution of Kansas than previously realized. These considerations suggest the need for some reevaluation of the tectonic...

Efficient Generation of Laser Beams with an Elliptic Cross Section

Abstract: There frequently exists a need in holographic and other coherent-optic research for a laser beam that does not have a circular cross section. For example, if rectangular holograms are to be recorded or if rectangular object transparencies are to be illuminated, the usual circular cross section laser beam, e.g., TEM00 mode, cannot be efficiently utilized. This note describes a simple, straightforward technique for generating a clean laser beam with an elliptic cross section. A single prism can convert a light beam with a circular cross section into one with an elliptic cross section. Referring to Fig. 1, one sees that if an incoming beam of width w is normally incident upon the first surface of a prism of refractive index n and apex angle 0, after refraction at the second surface, it will have a new reduced width w', where

X-ray image enhancement by least-mean-square estimation.

Abstract: The x-ray penumbra effect, caused by the finite focal spot of an x-ray tube, and inherent sources of random noise are the primary limiting factors regarding radiographic image quality. An attempt has been made to devise an optimum spatial filter, which, when realized in binary form, provides substantive image improvement.

Color camera system having complete spectral characteristics

Abstract: Improved color reproduction is made possible by a television camera exhibiting a complete predetermined camera spectral sensitivity characteristic having both the positive and negative lobes. Materials having transmission characteristics which are responsive to both the positive and inverted negative lobes of the predetermined characteristics are combined to form a single striped optical spatial filter at the camera, and the resultant composite video signal is demodulated to yield a chrominance output signal proportional to the difference between the light transmission provided by the filter stripes so that the prescribed spectral sensitivity characteristic is produced for each output channel. The technique permits a camera to exhibit complete spectral characteristics and to generate by direct optical means any color signal, including I or Q, and it is applicable to single or multiple tube cameras using spatial modulation techniques for color discrimination.

Ray Analysis of Image Transfer through Interferometrically Produced Fiber-Optics Structures

Abstract: Ray optics is used to analyze the image-transfer properties of interferometrically prepared fiber-optics structures. A previously demonstrated image-transfer plate formed by a crossed pair of sinusoidal-refractive-index gratings is analyzed to lowest order in index variations. Results include ray-guiding criteria, trajectories and their oscillation periods, the efficiency of image transfer, and an over-all picture of image formation and the effects of unguided light on fidelity and contrast. A model of a high-resolution array of discrete fibers is analyzed for comparison. The crossed-sinusoid structure is found to be more efficient under narrow-angle illumination. Adaptation of the fiber model as a limiting interferometric index profile yields a general idea of the range of image-transfer capabilities of interferometric structures and an assessment of the effects of large index variations.

Spatial Filtering Considerations in Bragg Diffraction Imaging

Abstract: Many of the techniques and effects observed in images from systems using Bragg diffraction imaging can be explained using concepts of optical spatial filtering. This is possible because the Bragg diffraction phenomenom can be thought of as an interaction of a plane-wave of light with a plane-wave of sound to produce a plane-wave of diffracted light. Changing the incident light causes a change in the diffracted light. Hence, the input light field of the imaging system plays an analogous role to the filter transparency of an optical spatial filtering system. The plane-wave components of the diffracted light that form the image of the sound field can be selectively changed by modification of the incident light field. When the imaging method is analyzed as a plane-wave: plane-wave interaction, simple explanations can be given for such diverse effects as the observed astigmatic resolution, the dependence of the resolution on the semi-apex angle of the incident light wedge, dark field imaging, and reflection imaging.

Encoding and Decoding of Color Information Using Two-Dimensional Spatial Filtering

Abstract: Spatial frequency encoding is described as it applies to both photographic and television systems. In each case, color separations amplitude modulate spatial frequency carriers. The photographic system separates the carriers in the two-dimensional Fourier transform plane using coherent optics; the television system employs a two-dimensional electrical filter to perform the same separation. These filters are synthesized so that the two-dimensional bandwidths of the television camera are used efficiently. Nonlinear processing techniques are also described to minimize crosstalk between channels.

Optical correlation of scanned real-time signals

Abstract: A light beam is generated by means of a narrow band collimated light source, and this light beam is modulated in accordance with an electrical input signal to be identified. The light beam is then optically scanned at a predetermined scanning rate to produce an optical image of the input signals against a time base. Light from this optical image is passed through a holographic reference transparency, which is a spatial filter corresponding to the Fourier transform of the image of known electrical signals, the light passing through the transparency being fed to a detector. Successive reference transparencies are utilized until correlation between the transparency and the input signal to be identified is indicated by a peaking of the detector output which is sensed by a correlation indicator.