Showing papers on "Spatial filter published in 1984"

Optical-Image Correlation With A Binary Spatial Light-Modulator

Abstract: The use of a binary, magneto-optic spatial light modulator as an input device and as a spatial filter in a VanderLugt correlator is investigated. The statistics of the correlation that is obtained when the input image or the spatial filter is thresholded are estimated. Optical correlation using the magneto-optic device at the input and Fourier planes of a VanderLugt correlator is demonstrated experimentally.

Computer-generated holographic component with optimum light efficiency

Abstract: A filter combination consisting of a lens and two pure phase filters, situated in the two focal planes, is discussed. This element, which we call a tandem component, apparently does not absorb any light; in other words, the tandem component is capable of shaping wave fronts with 100% efficiency independent of the object function. We describe a basic configuration and outline its space-variant system properties. The tandem component can be used for many of the standard applications of computer-generated holograms and possibly for some new types as well in view of its space variance.

Image gathering and processing for enhanced resolution

Abstract: Traditional approaches to optical resolution enhancement have involved either the design of appropriate image-formation systems or some type of postprocessing of an image that has already been formed. Results presented in this paper suggest that improved images can be obtained if the image-gathering system is designed specifically to enhance the performance of the image-restoration algorithm to be used. We consider a class of problems in which the total available spatial bandwidth is fixed but the location of this bandwidth along the spatial-frequency axis is to some extent under our control. For example, we might consider either a low-pass system or a bandpass system of the same total bandwidth. We show that system performance can be substantially improved by proper allocation of the available bandwidth in the spatial-frequency domain. The optimum allocation is shown to be a function of the signal-to-noise ratio. We also describe coherent and incoherent optical image-gathering systems that can achieve the desired spatial-frequency passbands.

Observation of optical chaos in a phase-conjugate resonator.

Abstract: The outcoupled power of a phase-conjugate resonator that uses four-wave mixing in BaTiO(3) is observed to undergo transitions from steady to periodic and from periodic to chaotic behavior as a function of time. Period doubling, period-three cycles, and period-five cycles are observed. Control parameters for the transitions are the diameter of an intracavity pinhole aperture, the length of the resonator, and the radius of curvature of the outcoupling mirror.

Fizeau wavemeter for pulsed laser wavelength measurement.

Abstract: A Fizeau wavelength meter optimized for use with pulsed laser sources has been developed and characterized which demonstrates a CW resolution better than 2 parts in 10 to the 7th and a pulsed resolution better than 1 part in 10 to the 6th. The static optical design is based on a Fizeau wedge interferometer, which together with spatial filtering and collimating optics is used to produce a pattern of parallel fringes which is imaged on a linear photodiode array and analyzed by a minicomputer. A series of CW and pulsed measurements of various narrowband laser sources are described, and particular difficulties involved in pulsed laser measurements with the wavemeter are examined.

Multiple scattering effects in spatial frequency filtering

Abstract: The existing theory of imaging through an aerosol medium based on the small-angle approximation to radiative transfer is extended to the general case of multiple scattering with an arbitrary degree of anisotropy. By applying the discrete-ordinates finite-element radiation transport code twotran, we compute the modulation transfer function for a medium characterized by optical depth, single-scattering albedo, and asymmetry parameter. To partially suppress the ray effect in the discrete-ordinates solution to the transfer equation, the first-collision source approach is employed. The results of our modeling which apply to the case of incoherent imaging indicate a strong dependence on the angular divergence of an isotropic source of radiation. Distinctions between the coherent and incoherent illumination cases are stressed, and the earlier small-angle approximation results are rederived.

Position measurement by laser beam

Abstract: A laser beam from a source (20) is used to provide a reference in space adjacent a portion of a structure movements of which are monitored. The beam is received by a receiver (30). A number of targets (300) may be mounted to move with parts of the structure extending alongside the beam path. Target constructions (Figs. 6-8) are described which allow for normal passage of the beam past the target but which enable the position of the beam relative to a given target to be measured by intercepting the beam. In a variation (Fig. 13) for civil engineering use, a single target is selectively mountable to one of a number of supports. Another variation (Fig. 11) uses graduated mesh targets on which the beam impinges on each mesh but is alo transmitted through the mesh. Dynamic movement of a part of structure such as a bridge is measured with a single target attached to the part in question and comprising a barred reflective assembly (Fig. 3), a fibre optic array (Fig. 4) or a position sensing photo-diode (Fig. 5). For use in such systems there is also described a spatial filter for a laser receiver (Fig. 10), a laser assembly adapted for precise re-positioning (Fig. 9) and a dust excluding laser housing (Fig. 12).

Investigations using an X-ray image intensifier and a TV camera for imaging transverse sections in humans.

Abstract: A method for computed tomography using equipment available on some radiotherapy simulators has been investigated. An offset narrow fan beam of X rays passes transversely through the object and is continuously detected by an image intensifier and TV camera system while the beam and detector complete a rotation about the object in 30 seconds. Several TV lines of the images obtained at equal angular increments are digitised and summed to produce transmission projections from which a transverse section is formed by filtered back-projection. The dynamic range required of the camera is reduced by modifying the beam with a shaped metal filter. Images are produced without logarithmic processing or X-ray output calibration. Digital control circuitry is used for digitisation and spatial filtering, and a high-speed arithmetic array processor and digital frame store are used for back projection and image storage (Cynosure Imaging Systems Ltd.). Phantom studies using an object on a turntable with a stationary X-ray tu...

Optical differentiation of quasi-periodic patterns using Talbot interferometry

Abstract: Two techniques for implementing the concept of grating shearing interferometry to optical differentiation of information-encoded quasi-periodic patterns are described. Two identical copies of the investigated pattern replace diffraction gratings used in the Talbot interferometer configuration. Two methods, one using moire fringe detection and the other spatial filtering, are presented and compared with other differentiation techniques.

Improved interpretation of gated cardiac images by use of digital filters.

Abstract: The authors describe a digital filter that greatly enhances the quality of gated cardiac blood-pool images. Spatial filtering is accomplished with a minimum-mean-square-error (Wiener) filter incorporating measured camera blur and Poisson noise statistics. A low-pass temporal filter is then applied to each pixel, with the cutoff frequency determined from measurements of frequency spectra in 20 patients. This filter was evaluated in routine clinical use for nearly one year and found to significantly improve chamber definition, delineate wall motion abnormalities better, and reduce noise. To quantitatively assess the effect of the filter on image interpretation, four experienced observers evaluated wall motion in a series of mathematically simulated left ventricular images. ROC analysis revealed that accuracy in assessing wall motion was significantly greater with the filtered images.

Adaptive acoustooptic filter

Abstract: A new adaptive filter utilizing acoustooptic devices in a space integrating architecture is described. Two configurations are presented; one of them, suitable for signal estimation, is shown to approximate the Wiener filter, while the other, suitable for detection, is shown to approximate the matched filter.

Optical spatial frequency filtering using interferences

Abstract: Several recent publications describe optical image-processing experiments in which the spatial-filtering operation is performed by an interference phenomenon. In this paper we derive the general conditions allowing interferences to play the role of a spatial filter. It is shown that, although a spatially coherent illumination is necessary, the interferometer must be able to produce localized fringes, and that the localization locus must coincide with the Fourier spectra formed through the two (or more) arms of the interferometer. The filtering operation may be understood either as an interference between these Fourier spectra or as an interference between the images of the object formed by each of the arms of the interferometer. In the case of most usual interferometers, these images are simply geometrical images of the object that are shifted with respect to each other, and the shift is responsible for the filtering effect. Two classes of interferometric setups can be distinguished, depending on whether the shift is perpendicular or parallel to the image plane. Examples are given, and experimental results are presented.

A High-Resolution Target-Tracking Concept Using Spectral Estimation Techniques

Abstract: : A target-tracking concept based on modern spectral estimation techniques combined with adaptive array processing has been investigated to attain superresolution tracking performance. The goal is to achieve satisfactory monopulse track angle estimates in the presence of strong multiple interference sources which are located within a beamwidth of the target. This application area was addressed via an all-digital receive system concept whereby adaptive spatial filter weights are periodically updated on the basis of source estimates, and the filter output residue signals are then sifted for targets. This report includes several radar-simulation examples to illustrate some of the processing step outputs, tracking beam distortions, corrections for target wavefront distortions, and tracking in the presence of closely spaced interference sources. It also contains a section devoted to the algorithm theory, analysis, and development which will help to understand the various processing steps involved. (Author)

Optical image processor

Abstract: @ An optical processing system based on noncoherent light processing employs both geometrical and diffraction optical systems. One portion of the optical system (25, 26) preforms a subtraction operation of the image from itself where one of the subtracted images is relatively defocused and one is delayed in time from the other. The subtracted image is edge- enhanced (32) and then applied to a spatial filter employing an interferometer (56) in which spatial filters (61, 62) are contained in the interferometer iris plates (59, 60). Additional spatial filtering (72) is performed on the interference image and the processed image is displayed in an appropriate display (78, 81) which can selectively view the processed image, the unprocessed image, or combinations of the two. Light storage elements are employed for storing the processed images in appropriate buffers (23, 24, 64) over a given length of time. The buffers are read out by a flashlamp. Suitable shutters (15, 16, 80, 82) are closed during the readout process.

Spatial filter investigation of the distribution of power between transverse modes in a fiber waveguide

Abstract: An experiment was carried out in which a method suggested earlier by the present authors was used to investigate the transverse mode composition of multimode coherent light beams. A study was made of multimode beams in a graded-index waveguide excited by a Gaussian beam. The experimental dependences of the coefficients describing the excitation of four modes on the Gaussian beam radius were in agreement with the theory.

Incoherent spatial filtering with a scanning heterodyne system.

Abstract: Imaging systems with two pupils in the receiving optics have been used to synthesize bipolar point spread functions in incoherent image processing. We describe a two-pupil system using a scanning illumination technique and compare its attributes with more conventional techniques. Experimental examples using two-pupil interaction and an acoustooptic frequency offset for the direct 2-D bandpass filtering of diffusely reflecting objects are presented.

Statistical analysis of whole-field filtering of specklegram and its upper limit of measurement

Abstract: The process of optical whole-field filtering of a double-exposure specklegram has been analyzed from a statistical point of view. It is shown that the size of the filtering aperture plays a vital role in the range of visible fringes, and this in turn determines the upper limit of measurement.

Edge-oriented spatial filtering of images with application to post-processing of vector quantized images

Abstract: An important application of spatial filtering techniques is in the post-processing-of images degraded by coding. Linear post-processing methods are inadequate to reduce the degradations in block-coded images. In this paper, a new nonlinear, space-variant filtering algorithm is proposed, which sharpens jagged edges without blurring them, and smoothes out small abrupt changes in the grey-level in monotone areas. It is not much more complex to implement than simple linear filtering. We show that this kind of post-processing significantly reduces the subjective effects of quantization noise in decoded images with almost no degradation of the signal.

Spatial and temporal properties of a tunable picosecond dye-laser oscillator-amplifier system

Abstract: In this paper, we describe a high-power picosecond dye-laser system providing tunable 1.5-mJ pulses with a uniform wave front and Gaussian spatial profile. With the aid of a computer-controlled data acquisition system, we have analyzed for the first time the temporal coherence properties of such a system.

Method for modulating a carrier wave

Abstract: A method for externally modulating an optical carrier wave by propagating the wave through a single channel waveguide structured to have an output whose optical power in the near field is distributed in a predetermined spatial pattern, selectively changing the optical power distribution pattern in correspondence with an information signal, and spatially filtering the power distribution pattern so that only the power available within selected region of the pattern is transmitted beyond the point of spatial filtering where it is available as an optical carrier wave whose intensity is modulated in correspondence with the information signal.

Multi-function acousto-optic signal processor

Abstract: An acousto-optic architecture is provided for simultaneously obtaining time integration correlation and power spectrum analysis. A laser beam is expanded and split into first and second beams. The first beam is diffracted by a first acousto-optic Bragg cell, and the second beam is diffracted by a second acousto-optic Bragg cell. The diffracted first beam is split into third and fourth beams, and the diffracted second beam is split into fifth and sixth beams. A first Fourier transform lens system is placed in the path of the third beam, and a first photodiode detector array is placed at the back focal plane of the lens system. A second Fourier transform lens system is placed in the path of the sixth beam, and a second photodiode detector array is placed at the back focal place of the lens system. The fourth and fifth beams are combined and the combined beam is split into seventh and eighth beams. A first Schlerin spatial filter is disposed in the path of the seventh beam for filtering undiffracted light from it. A second Schlerin spatial filter is disposed in the path of the eighth beam for filtering undiffracted light from it. A third photodiode detector array is disposed in the output of the first Schlerin spatial filter, and a fourth detector is disposed in the output of the second Schlerin spatial filter. The outputs of the first and second photodiode detector arrays are proportional to the power spectral density of the signals S 1 (t) and S 2 (t), which are respectively applied to the first and second Bragg cells. These outputs are resolved to a limit determined by the time aperture of the Bragg cells and are time averaged over the integration period of the array. The output of the third photodiode detector array is proportional to the correlation of the bandpass signals S 1 (t)cos ω a t offset by the frequency ω o and in a compressed, shifted time frame. The output of the fourth photodiode detector array is proportional to the correlation of the bandpass signals S 1 (t)cos ω a t and S 2 (t)cos ω a t offset by the frequency ω o , but in a more restricted delay range than that of the third photodiode detector array.

Focusing elliptical laser beams

Abstract: The spot formed by focusing an elliptical laser beam through an ordinary objective lens can be optimized by properly filling the objective lens. Criteria are given for maximizing the central irradiance and the line-spread function. An optimized spot is much less elliptical than the incident laser beam. For beam ellipticities as high as 2:1, this spatial filtering reduces the central irradiance by <14%.

Increased light efficiency of coherent-optical matched filters

Abstract: The light efficiency of the coherent-optical matched filter is determined by the integral intensity transmittance of the spatial filter; in other words, by the integral power spectrum of the target object. We increase the light efficiency by multiplying the object with a spatially fixed phase mask which disperses the light more evenly in the Fourier domain. The mask has to be designed to achieve a reasonable balance between increase of light efficiency and decrease of SNR. The SNR will decrease in general, since the target object is at an arbitrary position but the phase mask is at a fixed position. In special cases, however, the light efficiency can be increased considerably while the SNR remains almost the same as in the case of an ordinary matched filter.

Median and spatial low-pass filtering in ultrasonic computed tomography

Abstract: To reduce noise‐ and reconstruction‐related artifacts in ultrasoundcomputed tomography, the use of combined median and Hamming‐weighted spatial low‐pass filtering is evaluated. The evaluation of the reconstruction filters uses both computer‐generated projections of a known object with the least mean‐square error criterion as well as a more subjective evaluation of conventional ultrasonic attenuation and speed of soundimages.

Spatial frequency pseudocoloring by interference: application to texture

Abstract: Interferometric setups can be used as spatial frequency filters The use of a polychromatic point source allows spatial filtering to be converted to color coding of spatial frequency The effect of interferometers on various kinds of object including textured objects in polychromatic illumination is investigated, using a classification of interferometers according to whether the image to be processed is split axially or transversally when seen through the arms of the interferometer Experimental examples are shown