Showing papers on "Spatial filter published in 1976"

Position, rotation, and scale invariant optical correlation

Abstract: A new optical transformation that combines geometrical coordinate transformations with the conventional optical Fourier transform is described. The resultant transformations are invariant to both scale and rotational changes in the input object or function. Extensions of these operations to optical pattern recognition and initial experimental demonstrations are also presented.

Strain analysis by one-beam laser speckle interferometry. 2: Multiaperture method.

Abstract: A detailed analysis of using multiple apertures to record laser speckles for strain analysis is presented. It is shown that the basic fringe-forming mechanism is no different from that of single-aperture recording, except that at the Fourier filtering stage the diffracted light energy is concentrated at the spatial frequencies admitted by the apertures. As a result, better isothetic fringes at higher frequencies are obtainable. The concept of moire is not utilized in the analysis. Indeed, it is shown that some of the observed phenomena cannot be explained using the moire concept. Multiaperture arrangements studied include two, three, and four apertures.

Theory of speckle dependence on surface roughness

Abstract: The theory of speckle in partially coherent, monochromatic light is extended to cover the entire range of object roughness. For “almost white-noise” objects it is shown how the speckle statistics are related to a Gaussian approximation of the object statistics. Within this approximation, general relations are derived for the spatial speckle correlation and the spatial Wiener spectrum. The theory is applied to calculate rms speckle contrast as function of object roughness, coherence of the illumination, and receiver plane defocusing.

A method of incoherent optical-image processing using synthetic holograms*

Abstract: Under coherent illumination, the interference between signal and noise produces an amplification of the noise. That is why image processing by optical methods may be done better with incoherent light than with coherent light. A method is described where image deblurring is performed in spatially incoherent illumination. The two main features are an additional step of illuminance subtraction to simulate negative intensities, and the use of computer-generated holograms of a low number of cells.

A new coherent optical pseudo-color encoder

Abstract: A new and simple method for pseudo-coloring a photograph in a coherent optical processing system, utilizing a newly developed technique of half-tone screens, is described. By selective spatial filtering of the high diffraction orders in the Fourier plane of a coherent optical system, images with up to ten contours of constant brightness have been generated on a picture from a single half-tone copy of that picture. The pseudo-coloring of the picture is achieved by mixing the high-diffraction-order outputs generated by lasers of different wavelengths. In the experiment described herein, an argon laser and a He-Ne laser were used. The experimental results demonstrate that the method has considerable flexibility. A discussion of the theory, the method, and potential applications is given.

Water pollution monitoring using matched spatial filters

Abstract: Presented are some results obtained from the application of matched spatial filtering techniques to the identification of biological specimens called diatoms (i.e., water algae). A prototype semiautomatic optical processor has been developed that utilizes the Vander Lugt type complex spatial filters. We describe the optical filter averaging technique employed and discuss our semi in situ developing method for the filters. The advantage of this method with regards to the critical position requirements for time sharing optical filters is also presented. The filter holder is mounted on X, Y stages and precision positioned under the control of a PDP-11-40 computer.

Image processing system using incoherent radiation and spatial filter hologram

Abstract: A linear optical processing system in which a spatial filter in the form of a Fourier transform hologram is included with a lens for processing incoherent electromagnetic radiation. Such a system provides a given real, two-dimensional transfer function for spatially filtering incoherent radiation within a range of spatial frequencies 0 ≦ω≦Ω. The filter is made by forming a mask of a graph of a point spread function which is the inverse transform of the desired transfer function, and employing the mask in a coherent system to produce a Fourier transform hologram of the mask. Spatial filters are designed for providing the resultant systems with various transfer functions, including (a) a directional response for passing substantially higher spatial frequency components in a first direction than are passed in a second direction; (b) a selective attenuator response; (c) a notch filter response; (d) a correctional filter response; and (e) a linear minimum mean square error response. The systems are useful for recognizing specified patterns from a plurality of patterns and for determining the concentration of a species of particulate matter having a predetermined specific pattern within a sample of particles.

Synthesis of spatial filters with Chebyshev characteristics

Abstract: A spatial filtering technique using layered dielectrics for the reduction of antenna grating lobes or sidelobes is introduced and developed. The filters are used as a radome at the array face, and can be synthesized to have Chebyshev behavior in the spatial domain. Data are given for the design of two, three, and four layer filters with various spatial passband ranges and ripple levels.

Auto focus with spatial filtering and pairwise interrogation of photoelectric diodes

Abstract: Apparatus for photoelectrically determining the position of at least one focal plane of an image inside an optical instrument comprising an optics for imaging at least one object on at least one spatial frequency filter of an optical image correlator and further comprising a measurement display of the light fluxes filtered through the spatial frequency filter. The apparatus 1-45 for receiving the light fluxes concentrated through optical components 3,4 comprises at least one photoelectric detector system 5, 6, 31, 32. This detector system has at least one row of several detectors 11-15, . . . n, and electrical means 16, 17, 26-28, 33-38 are provided for pairwise interrogation of the photoelectric detectors 11-15, . . . n and also for the analysis of the photoelectric signals generated during the interrogation.

Mode conversion in bent step index multimode fibers

Abstract: Multimode fibers with large cores are expected to be used as transmission media of optical communication systems because they are more easily excited and connected than single mode fibers. However, transmission characteristics, such as impulse response and total loss, vary complicatedly according to the mode conversion phenomena caused by bends in the fibers, core-cladding interface roughness, refractive index distribution fluctuation, etc. To clarify the mode conversion phenomenon mechanism, it is necessary to investigate the mode power distribution in the fibers. However, few methods to measure the mode power distribution have been reported. The light emitting angle from the endface of the fiber is related to the propagation angle in the guides, in the case of multimode step index fibers with many guided modes. Therefore, the mode power distribution can be obtained by measuring the light power separately at each emitting angle from the endface of the fiber. This Letter describes the mode conversion in a step index multimode fiber by measuring mode power distribution under various bending conditions. Figure 1 (a) shows the apparatus for measuring the fiber mode conversion phenomena. The laser beam is launched into the short fiber F1, which is connected to the long fiber F2 by a manipulator. The short fiber F1 is used to maintain

Equidensitometry by coherent optical filtering

Abstract: A new and simple method has been developed for fabrication of multilevel halftone screens that have proved useful for generation of equidensity contours by means of coherent optical filtering. Up to ten contours of constant brightness have been generated on a picture by coherent optical filtering of a single halftone copy of that picture. The possible application of this method to optical analog-to-digital conversion using a single halftone photograph is discussed.

Perturbed Gaussian modes of an unbalanced antiresonant-ring laser cavity*

Abstract: We present an analysis of the perturbed Gaussian transverse modes of an antiresonant-ring laser cavity useful for cavity dumping, mode locking, and other applications. A plane-wave analysis of the antiresonant ring with a 50-50 beam splitter indicates that the coupling out the output port of the ring should be identically zero. A perturbed Gaussian mode analysis of a stable antiresonant-ring cavity shows, however, that if the ring is not optically symmetric (in the sense that the paraxial ray matrix is not symmetric in the two directions around the ring) then the modes of the cavity consist of a perturbed mixture of stable Gaussian modes, with a significant interference output or leakage from the output port of the ring. The existence of this leakage output has also been confirmed experimentally and by numerical Fox and Li calculation of the lowest mode of the unbalanced ring. This represents an unusual situation in which there is transverse mode discrimination without there being any diffractive output coupling.

Quality control of small mechanical pieces using optical correlation techniques

Abstract: A holographic matched filtering technique is used to control the contour shape of small mechanical elements. The optimization of the spatial filter leads to a well-defined response curve which is used to calibrate the output of the optical correlator in terms of shape error. It is shown that the techniques of computer holograms offer the widest possibilities to perform this optimization. The basis of the technique is presented together with a few simple examples showing the possibility of detecting shape errors of a few microns (less than 1% of the controlled dimension).

Determining optimal matched filter parameters.

Abstract: The coherent optical frequency plane correlator is poten­ tially one of the most powerful pattern recognition systems. Since the development of the holographic matched spatial filter, many optical image correlation demonstrations using this optical configuration have appeared. In this Letter we report on a simple method for determining the optimal pa­ rameters used in the synthesis of a holographic matched spatial filter. These parameters include exposure, beam balance ratio K (the ratio of the intensities of the reference and signal beams), and the choice of the spatial frequency band in which K is set. The effect of these parameters on the diffraction efficiency (η) and signal-to-noise ratio (SNR) of the correlations and cross-correlations are best determined experimentally on representative imagery. We will shortly publish the theoretical considerations and detailed experi­ mental results for these issues. At present we report only on a convenient experimental technique for data collection using a wedge-ring detector array in the optical Fourier transform plane. The issue of automatic control of the beam balance ratio has been proposed earlier. The detector array (or diffraction pattern sampling unit) consists of sixty-four separate photodiodes on a common 2.5-cm diam silicon substrate. The array contains thirty-two semiannular ring-shaped elements in one semicircle and thirty-two wedge-shaped elements in the other semicircle, with all sixty-four outputs available in parallel on separate leads. The detector elements are operated in the photovoltaic Fig. 1. SNR of the correlation peak vs the spatial frequency band in which the beam balance ratio K was sent equal to 2.

Signal noise filtration for laser beam recording system

Abstract: The invention provides a system for optically recording information on a recording medium. A light source emits a laser light beam and an optical system, including a lens, focuses the laser beam spot on the recording medium. The light intensity of the laser beam is modulated responsive to an information signal. The light modulator is in a path between the light source and the optical system. A noise component is detected in the laser light beam passing through the light path and removed therefrom. The optical system records the information on the recording medium by the laser light beam, in which the noise component has been removed, and on which the information has been applied by the light modulator.

Method of producing contour mapped and pseudo-colored versions of black and white photographs

Abstract: A method of making multilevel equidensity contour mappings and pseudo-colored versions of a photograph. First a half-tone transparency of the photograph is made by the method described in copending patent application entitled "Method of Making Half-Tone Screens", Ser. No. 708,539 filed 26 July, 1976 by Liu, now abandoned. The half-tone photograph transparency is placed at the object plane of a first lens. A spatially filtered collimated light beam is directed through the transparency and the lens such that a multitude of diffraction orders appear in the focal plane of the lens. A particular non-zero order of diffraction is singled out by placing a thin slit spatial filter at the Fourier plane of the lens. Reimaging of the diffraction order by another lens produces a filtered image which contains multilevel equidensity contours of the original photographic image. In one embodiment the light beam is generated by lasers of different wavelengths. A colored version of the photograph results from the mixing of high-diffraction order outputs.

Optimized spatial filter for defect detection

Abstract: An apparatus for detecting the defects composed of non-linear components of a subject pattern, includes a laser source for illuminating the subject pattern by a laser beam, a Fourier-transform lens for projecting an information light from the pattern through a spatial filter onto a screen. The filter is placed on the Fourier-transform plane of the Fourier-transformed information light for intercepting the coherent light having information of the linear components, the filter having arm sections extending correspondingly to the linear components of said normal pattern, the outer periphery of said arm sections including curved sections protruding toward the intersecting point of the arms.

Mathematical model of conventional tomography.

Abstract: A Fourier decomposition approach is used to study the imaging properties of conventional tomography.Spatial frequency response curves (MTFs) are calculated for both linear and axial transverse tomography. These curves depend on the product of the spatial frequency of the sinusoidal density variations in thin layers parallel to the tomographic plane and the distance between such layers and the tomographic plane. Based on the spatial frequency response curves, a quantitative definition of tomographic layer thickness is given. Furthermore, the spatial frequency response curves suggest that unattenuated low‐frequency information from outside the tomographic layer limits the resolution in conventional tomograms and that high‐pass spatial filtering of the image may substantially improve the diagnostic quality of tomographic images, particularly in the identification of boundaries.

Method and apparatus for measuring the sum of the radii of particles in a collection

Abstract: There is disclosed a Fraunhofer plane spatial filter design for use in filtering the forward scattered light from a collection of particles to obtain an indication of the sum of the radii of the particles as a direct function of the total diffracted light flux passed by the filter.

Theoretical intensity uniformity from an anti-Gaussian collimating lens.

Abstract: A plano-convex absorption glass lens proposed earlier by Ih [ Appl. Opt.11, 694 ( 1972)] for transforming a Gaussian intensity beam into a uniform collimated beam was analyzed theoretically on the basis of the actual paths of off-axis light rays through the lens. An analysis provided by Ih did not take into account the actual light paths. The present analysis shows that a simple spherical convex curvature results in a transmitted intensity varying less than 3% for an f/15 lens. The analysis was applied to the geometry of a lens constructed by Ih, and the corresponding theoretical maximum intensity variation is in agreement with intensity measurements reported by him.

Real-time optical signal processors employing optical feedback: amplitude and phase control.

Abstract: The development of real-time coherent optical signal processors has increased the appeal of optical computing techniques in signal processing applications. A major limitation of these real-time systems is the fact that the optical processing material is generally of a phase-only type. The result is that the spatial filters synthesized with these systems must be either phase-only filters or amplitude-only filters. The main concern of this paper is the application of optical feedback techniques to obtain simultaneous and independent amplitude and phase control of the light passing through the system. It is shown that optical feedback techniques may be employed with phase-only spatial filters to obtain this amplitude and phase control. The feedback system with phase-only filters is compared with other feedback systems that employ combinations of phase-only and amplitude-only filters; it is found that the phase-only system is substantially more flexible than the other two systems investigated.

Polychromatic partial coherence: space-time filtering and Fourier transform

Abstract: The transfer of the partial coherence by an optical system is studied, within the framework of a polychromatic light. The method relies on the fundamental fact that the coherence is transformed by a linear filter univocally connected to the filter acting on the electric field. It results a particular relationship of structure between the transformations assumed by the field and the coherence: space-time filtering as much as spatial Fourier transform, which are successively analysed in the case of transfer by direct diffraction. The results obtained are specified in the particular case of a narrow-band radiation or yet an incoherent object; which allows, among others, to situate Zernike theorem in a quite general context.

Mechanisms of the plasma spatial filter for high‐power lasers

Abstract: We describe mechanisms operating in the plasma spatial filter. Curvature of the critical density surface of the blow‐off plasma causes high and intermediate spatial frequencies on the incoming laser beam to be rejected. In addition, time‐dependent plasma motions result in the masking of high spatial frequency information on a time‐integrated near‐field photograph of the beam. Effects on the spectrum of the reflected laser light are evaluated.

Slotted Apertures for Multiplying Grating Frequencies and Sharpening Fringe Patterns in Moire Photography

Abstract: Slotted aperture filter masks can be used to "tune" a photographic system to an integral multiple of a fundamental grating frequency. Such a procedure can be used in moire photography for multiplying sensitivity and for obtaining improved rendering of fringe patterns. Depth of field is increased, and a camera lens of poorer quality than is normally required for high resolution photography of moire gratings is adequate. Also gained is flexibility in optical data processing of moire photographs and in the use of two-dimensional grid and dot specimen arrays.