Showing papers on "Spatial filter published in 1973"

Radial- and circular-fringe interferograms

Abstract: New types of interference-fringe configurations in displays of interferograms are introduced. In particular, two types that show the data directly in polar coordinates appear attractive. These are radial-and circular-fringe interferograms. Several ways are described to transform the reference wave in a conventional interferometer so that a new type of interferogram is obtained. Feasibility experiments have been promising. Spiral-fringe interferograms have also been realized.

Photosensor aperture shaping to reduce aliasing in optical-mechanical line-scan imaging systems.

Abstract: Optical-mechanical scanning techniques are generally employed in instruments specifically designed to characterize variations in scene brightness spectrally or radiometrically. The effect of aliasing, which can be caused by line-scan sampling, on the spatial detail of the reconstructed image has therefore been of little concern. Emphasis of some recent applications of optical-mechanical scanning techniques is, however, on the spatial characterization of the scene. As is shown here, such images can be severely degraded by aliasing. Photosensor aperture shaping and line-scan spacing are investigated as a means for reducing this degradation.

Method for determining a specific characteristic of fluid suspended particles

Abstract: A method for determining a specific characteristic of fluid suspended particles such as determining the total volume of particulate, as in stack gases. A laser beam is passed through the stack gases to produce diffraction patterns which are spatially filtered so that the total diffracted light flux passing through the filter is proportional to the total volume and hence the mass of the particles. A central opaque region is used on the filter to intercept the incident laser beam. The portion of diffracted light which is intercepted by that opaque region is compensated for by a region of enhanced transmission next to the center to restore response relating to the large particles. The spatial filter transmits a fraction of the diffracted light proportional to the reciprocal of the diffraction angle so that the total flux will be proportional to the cube of the diameter of the particulate and hence the particulate volume. When a long path is involved the particle information which is lost is compensated for by an enhanced transmission of the diffracted light from nearby small particles. Thus, in an outer region of the filter the transmission factor is held constant to provide the necessary compensation for the lost light flux.

Increasing the Dynamic Range of Coherent Optical Filters by Means of Modulating Gratings.

Abstract: We report on a new method for increasing the linear dynamic range of coherent-optical spatial filters, with special reference to holographic image-deblurring filters. In the filter-generating step, a transmission grating is placed in contact with the transparency representing the spread function of the blur. The spectrum of the spread function then appears in a series of equally spaced islands of different intensities, each occupying a different region of the nonlinear characteristic of the recording medium. Each spatial frequency can then be properly exposed somewhere on the filter, even with a film of limited dynamic range. In the image -processing step, the grating is superposed on the blurred image, the spectrum of which then falls on all the islands of the filter. In this way, the filtering operation is done in parallel channels, each with a different nonlinear characteristic. The channels are combined in the output plane by superposing a grating on the filtered aerial image and viewing this plane through an optical system that does not resolve the grating. Theory, computer simulations, and experimental results are presented. The results show in particular that a good transfer function can be achieved with a hologram-only filter.

Pattern recognizing optical apparatus

Abstract: A pattern recognizing apparatus having a spatial filter manufactured by exposing a surface of a hologram dry plate to a pair of light beams simultaneously directed to a point on the hologram dry plate surface with one of the light beams extending along an optical axis which is perpendicular to the surface while the other of the light beams makes a predetermined angle with respect to the optical axis. These light beams are directed through a mask which contacts the dry plate surface during exposure thereof to the light beams, this mask having a transmission function which will determine the function of the spatial filter formed by the hologram after it is developed subsequent to exposure thereof. A series of these masks which are respectively of different transmission functions are sequentially used during sequential exposures to the light beams where the light beam which makes an angle with the optical axis has different angles for the different exposures, so that in this way a multiple-point hologram is achieved. This spatial filter is then used in an optical reader where a photoelectric detecting structure is arranged along each of the axes along which the light beams which make an angle with the optical axis were directed during exposure of the dry plate.

New Method of Optical Processing Using a Random Diffuser

Abstract: In this article we describe a new method of optical processing which is one of the principal topics of study in our laboratory. This consists of the detection of the difference between two images, the multiplexing of images, the contrast reversal of the reconstructed images in carrier frequency photography, the coding and decoding of a message and the reconstruction of colour images from black and white recordings. The signal, modulated by a random diffuser with high spatial frequencies, is recorded at least twice on the same photographic plate with a small displacement of the plate between the exposures. The required information can be extracted by spatial filtering of the spectrum of the recorded signals. This spectrum is modulated by a system of interference fringes which determines the nature of the filter. The recording can be done in white light whereas the reconstruction is made in the laboratory using coherent light.

Dimensional measurement and defect detection using spatial filtering

Abstract: A new method is described that utilizes coherent bandpass spatial filtering and subsequent superposition to form filtered images in which small differences in size and geometry of the original object are readily detected. The theoretical basis is discussed and experiments described in which signal ratios of about 10:1 are obtained for a diameter change of 2.5 percent of a clear circular disc. The method is used to process in parallel a 57-mm evaporation mask containing 12 000 holes, each being 0.1 mm in diameter. The size of each hole is accurately gauged and small imperfections are indicated in the filtered image.

Restoration of degraded images by composite gratings in a coherent optical processor.

Abstract: A study has been made on restoration of degraded images utilizing spatial filtering techniques in a coherent optical processor. This study differs from other coherent approaches in that the spatial filters employed contain only composite gratings that are quite simple to produce. A general method of synthesizing the appropriate composite gratings for linearly smeared as well as multiply exposed and out-of-focus images will be presented in some detail with experimental results. The extension of the method of synthesis to other types of degradation is briefly discussed.

Additional Filters Enhancing the Sensitivity of an Optical Correlator

Abstract: The influence of additional filters upon the output of an optical correlator is studied for the case of a spatial filter measuring the size of rectangles. Addtional filters leading to increased sensitivity are derived by means of a series expansion. One type of additional filter, consisting of several transparent slits, yields a linear relation between output signal and the deviation from the desired size. The other additional filter, effecting a quadratic output characteristic, is a properly dimensioned bandpass. Experimental results demonstrate the enhancement of sensitivity achieved with additional filters. As this method can easily be extended to a correlator measuring the diameter of circular apertures, additional filters may be of interest in the field of automatic production control.

Photo-detector for optical inspection system

Abstract: The photodetector in an optical inspection system is provided with a variable spatial filter which is defined with a nonuniform light-receiving elongated slot to compensate for the nonuniformity of light energy on the material under inspection as a scanning beam moves through a predetermined angle in scanning across the material.

Application of spatial filtering subtraction to thin film and integrated circuit mask inspection.

Abstract: Studies have been performed to investigate the application of spatial filtering subtraction to mask inspection. By using a grating at the spatial frequency plane, images can be subtracted to give a display of errors on a black background. Results indicate that this technique could have application for integrated circuit and thin film mask inspection provided there are two or more repeated patterns on the mask.

Applications of the speckle pattern techniques to the visualization of modulation transfer functions and quantitative study of vibrations of mechanical structures.

Abstract: Some statistical properties of a speckle pattern can be characterized by its autocorrelation function and the intensity distribution in its Fraunhofer diffraction pattern, which depends on the granularity of the diffused wave and the contour of the pupil that limits the diffusing surface. It is shown that this property can apply to the visualization of modulation transfer functions, namely, those of synthetic apertures. A method for measurement of tilt angle for diffusing elementary surfaces is also derived. The information that can be extracted from a time-average photographic record of the intensity distribution scattered from a vibrating rough surface is discussed. It is shown that a complete mapping of the surface can be carried out by a suitable spatial filtering of the speckle pattern. As an example, in vibration mode analysis areas of singularity are displayed on the filtered image of a plate.

An optical radar for airborne use over natural waters

Abstract: An optical radar for detecting targets in natural waters was built and tested in the Gulf of Mexico. The transmitter consists of a Q-switched neodymium-glass laser, with output amplified and doubled in KDP to 0.53 micrometer wavelength. The receiver incorporates a novel optical spatial filter to reduce the dynamic range required of the photodetector to a reasonable value. Detection of targets to a depth of 84 feet was achieved with a considerable sensitivity margin. The sensitivity of the radar is highly dependent on the optical attenuation coefficient. In general, measured returns fell between the values predicted on the basis of monopath and multipath attenuation. By means of simple physical arguments a radar equation for the system was derived. To validate this theoretical model, measurements of optical attenuation and of water surface behavior were also instrumented, and some of these results are given. Volumetric backscatter was measurable over the entire depth range; such radars should therefore be useful for monitoring natural water quality. Airborne bottom-profiling is another application.

Spatial filtering for noise reduction in a multirecording telecine system.

Abstract: The purpose is to determine the transfer function of the two-dimensional Wiener filter in a color telecine system using multirecorded monochrome photographic film in which three-color information of an object is recorded separately with different spatial frequency regions. According to theory, the S/N of a high frequency carrier signal is improved approximately 10 dB with the Wiener filter, A Wiener filter is produced by introducing a coherent spatial filtering optical system into the telecine system. Satisfactory results were obtained in the filtering experiments on actual production of the Wiener filter.

Objective-pinhole unit for coherent light

Abstract: A lens-pinhole unit which is prefocussed for use as a beam expanding and spatial filtering device for coherent light comprising an objective lens with a support secured to one end thereof and carrying a pinhole diaphragm, through the intermediary of a hardened synthetic resin layer, at such a position that the pinhole is at the focus of the lens. The unit is mounted in a support and the diaphragm is adjusted on the still soft resin until the pinhole is set at the focus after which the resin hardens in place.

Extended field depth holographic method and apparatus

Abstract: A hologram is formed by simultaneously exposing a photographic plate to light reflected from an object illuminated by a laser and to a composite reference beam derived from a laser. The composite beam includes two components which have traveled through different path lengths from the laser to the photographic plate but which coincide with one another and emanate from the same spatial filter so as to have the same curvature at the plate. The composite beam is formed by passing the laser beam through an interferometer which divides the beam into two components, and recombines them after they have gone through different path lengths. Since the components have traveled through different distances to arrive at the plate they will be coherent with reflections from separated areas on the object, thereby allowing the photographic plate to record interference patterns with reflections emanating from larger areas of the object than would be possible if only a single component reference beam were employed. After development of the plate, the image may be reconstructed by appropriately illuminating the hologram with any coherent light.

Single step detection of blurred images in a coherent optical processor.

Abstract: Two important operations that can be performed by coherent optical processing systems are image detection and image restoration. These operations require complex spatial filters, and if both operations are to be performed, cascaded processing stages employing two separate spatial filters are usually needed. Spatial filters are described that will perform in a single step both operations simultaneously, thus eliminating the need of extra processing stages while providing the advantages of higher efficiency and smaller space requirements. The filters are characterized by their simplicity and are easily fabricated using conventional techniques. Experimental results are presented.

Simple Methods For High‐Resolution Scanning of Laser Beams And Alignment of Small Spatial Filtered Beams

Abstract: A 50‐μ glass filament from a fiber‐optics bundle, bent at 90° near the lower end and fastened to an adjustable support, proves to be a better light pickup than the usual pinhole aperture for high‐resolution scanning of a laser beam. Scans of a 50‐mW laser beam were used to develop a spatial filter combined with a short‐focus collimating lens which yields a uniform, Gaussian‐shaped beam with no increase in size.

Coherent optical scanning analog processor

Abstract: The design of a new type of coherent optical processor is presented. The image is scanned with a small collimated beam in order to reduce the diameter of the optics and to reduce artifacts caused by coherent light. If the shape of the beam is Gaussian, the scan line pattern will not appear on the recording film. Use of an optical circulator allows simultaneous propagation of the input and output beams of the processor in the same space. By using a mirror adjacent to the transparency, the film gamma can be set equal to 1 rather than 2.

In-flight detection of small hypervelocity particles.

Abstract: A technique is described in which small (25-micron) hypervelocity (10-km/sec) in-flight particles can be detected in the presence of high background noise. The system is based on a spatial filtering principle whereby spurious noise effects are reduced by use of a beam stop in the entrance aperture of the system and a bandpass filter in the transform plane. A theoretical analysis of the system is presented, and some experimental results are obtained by detecting in-flight hypervelocity particles generated by an exploding lithium wire electrothermal accelerator.

Matched-Filter Detection of Mode-Locked Laser Signals

Abstract: The passive Fabry-Perot cavity is shown to be a good practical approach to the match-filter optimization for the sensitive detection of mode-locked laser signals. Doppler measurements of relative motion over a wide range of velocities are possible simply by measuring the cavity length for a peak output.

Prefocused objective-pinhole unit for beam expanding and spatial filtering.

Abstract: A beam-expanding and spatial-filtering device, the prefocused objective-pinhole unit (POP unit), is presented The design is primarily aimed at greater simplicity in handling and construction than the commercially available lens-pinhole spatial filters (LPSF), for once the pinhole is fixed in the correct position with respect to the objective, the alignment of the whole unit can be made an easy matter

Ferroelectric, conductivity and secondary emission characteristics of a KD 2 PO 4 light valve

Abstract: An on-line electrical to optical input transducer capable of spatially modulating a collimated laser beam is a major element in a real-time coherent optical processor. The electron-beam addressed light valve has received considerable attention as such an image transducer. Real time correlation, spatial filtering and spectrum analysis have been demonstrated using this device. This paper emphasizes ferroelectric and conductivity measurements obtained for an electron-beam addressed KD 2 PO 4 crystal over the operating range from room temperature through the transition temperature to the crystal's Curie point. Since the device stores information at its -52°C operating temperature, real-time operation necessitates charge removal by secondary emission using a flood gun. Experimental data on the crystal's secondary emission curve, crossover potentials, and sticking potential will also be presented, and the effects of these device parameters on system operation, design, performance and lifetime will be discussed.

Optimization of coherence properties of thermal sources by spatial filtering

Abstract: Light of perfect spatial coherence may be generated by a finite system of passive filters. The propagator for the mutual intensity must be the product of two functions, each depending on the coordinates of just one image point. Particular cases are discussed, and the theory is illustrated using an apodized form of a grating system due to Lohmann.