Coherent Correlator Research Articles

Color object identification by monochromatic binary correlation

A real-time polychromatic image correlator that uses a magnetooptic (MO) spatial light modulator (SLM) device for pattern recognition based on both the color and shape of an input object is presented. The proposed system utilizes a multichannel spectral matched spatial filter employed in a binary coherent optical correlator. Input color images are transformed into binary color coded coherent images by a color grating. The color encoded images are read out by a charge coupled device interfaced with a MO SLM. The color encoded binary images are then processed by a multichannel joint spectral matched spatial filter synthesized by monochromatic light. Pattern recognition experiments for naturally illuminated real color objects are presented.

Real-time pattern recognition using a modified liquid crystal television in a coherent optical correlator

La television a cristal liquide est utilisee comme modulateur spatial dans un correlateur optique

Aperture tapering in a coherent optical correlator

Sharp edges located in an optical correlator input beam cause unwanted edge effects. A 1-D analysis is used to demonstrate this effect and how edge tapers can reduce this effect. Then an aperture made on film approximating the best taper is used to reduce the spurious signals in a matched filter correlator.

Hybrid optical–digital pattern recognition

This paper describes the design of a hybrid optical–digital image-processing system and the development of methods to perform a statistical analysis of the correlation signals. The optical setup is based on a coherent correlator using binary-coded filters. The digital part is based on a microprocessor image-processing system. Six different algorithms for correlation signal evaluation are investigated. Feature reduction is achieved by multivariate analysis. High classification results have been obtained for alphanumerical patterns even in the presence of noise, rotation, change of scale, and shearing.

Optical processor with a liquid crystal light valve

A liquid crystal light valve (LCLV) is used as an input plane of a coherent optical correlator. The matched filters are analog or computer generated holograms. The image quality and the diffraction spectrum are studied when the LCLV is used as an amplitude or phase modulator.

Portable real-time coherent optical correlator

A real-time coherent optical correlator is mounted inside a 15- × 23- × 38-cm enclosure and features laser diode light sources and a liquid crystal incoherent-to-coherent image converter. Each of four reference filters is illuminated by a separate laser diode, real-time operation is achieved by the image converter, and a zoom lens forming the incoherent image allows a scale search. The correlator can be mounted on a tripod and performs correlations indoors or outdoors.

Two-Dimensional Magneto-Optic Spatial Light Modulator For Signal Processing

This electrically alterable magneto-optic device can be used as a two-dimensional spatial light modulator in an optical image processor or a display system. A thin magnetic garnet film is epitaxially deposited on a transparent nonmagnetic garnet substrate, in the manner of magnetic bubble memory films. Semiconductor photolithographic techniques are used to etch the film into a two-dimensional array of mesas and to deposit X-Y drive lines for matrix-addressed current switching of the mesa magnetization. Electromagnetic switching provides higher speed switching than a previously reported thermal switching method. The axis of polarization of polarized light transmitted through the film is rotated by the Faraday effect in opposite directions where opposite magnetic states have been written, and a polarization analyzer converts this effect into image brightness modulation. The resulting high speed random access light modulator is applicable to all three planes of the classic coherent three-plane correlator. Although the basic effect is binary, there are at least four possible configurations which achieve gray scale rendition.

Light utilization in optical correlators

An analysis is made of the overall light efficiency in a coherent optical correlator. The results are applied to a matched filter and an inverse filter. Kogelnik's coupled wave theory is applied to analyze the diffraction efficiency of a filter recorded on a high-efficiency phase medium such as dichromated gelatin. Experimental results are presented for a matched filter and out-of-focus spatial filter, and the former is compared to the theory with good agreement.

Registration fiducials for automated alignment with optical processing

Since automated assembly, as well as automated inspection and sorting, usually require prealigned work-pieces, the availability of rapid and simple automatic alignment is a prerequisite to widespread acceptance of such systems. Coherent optical correlators potentially offer the necessary speed, but they have required mechanically coupled image rotation prisms and spatial light modulators and are, therefore, quite complicated and cumbersome. We describe here an alternative approach in which circularly symmetric fiducials are incorporated into the object itself and detected with an incoherent optical correlator. With incoherent correlators there is no need for spatial light modulators; also, the fiducials are so chosen that the correlation is not rotation sensitive. Since several (typically three) identical fiducials are used, their location can provide sufficient information on the object orientation. The approach was tested in the laboratory, and some experimental examples are included.

Real-time screen-aided multiple-image optical holographic matched-filter correlator

The basic principle and experimental results of applying a specific contact screen for performing multiplex holographic filtering in a real-time coherent optical correlator are presented. The specific design and fabrication of a 1- and 2-D gray density contact screen as well as a 2-D dichromated gelatin phase screen are described. The 2-D density-type screen can yield nearly an equal intensity 5 x 5 array of spectrum islands at the Fourier plane of a Fourier transform lens in a coherent optical image processor. The main drawback of the density-type screen is the lack of required light efficiency at the Fourier plane because of the blocking of light by the silver particles in the screen. Dichromated gelatin phase screens can be made to alleviate this problem, but it is very difficult to fabricate a phase screen exactly according to our design due to inherent experimental limitations. Nevertheless a functional phase screen has been made for the desired correlation functions. Experimental results have demonstrated that the phase screen is reasonably efficient for the multiplex holographic filtering in the real-time correlator.

Scanning optical correlator

The operation of a scanning optical correlator is described and demonstrated. The device is shown to combine the advantages of a coherent correlator, which makes preprocessing operations easy to realize, with the advantages of direct-image-casting correlators, which are insensitive to setup misalignment and imperfections and do not require the preparation of any filter. For some applications, these attributes may compensate for the fact that scanning optical systems are inherently slower than parallel, frequency plane processors.

Coherent intensity correlator

A diffraction intensity correlation with a holographic filter recorded with a random phase mask is considered. The linearity of processing in intensity is achieved by averaging the coherent light in the output plane. As compared to amplitude coherent correlators, in this correlator there is no influence of the input image phase structure and pulse response, the requirements for adjustment accuracy are allowed, parallelism and multichannelity of high resolution processing being preserved in this case. The random phase mask application for filter recording made it possible to reduce the required dynamic range of a hologram and refuse the use of continuous scanning deflectors. Comparative analysis of this correlator with the diffraction intensity correlator, in which spatial radiation coherence is distorted immediately before the filtering process, is made. The results of investigations on such characteristics as SNR, light loss coefficient, the degree of spatial invariance of processing are presented.

Construction of multicapacity matched filters using birefringent object film

Target objects used to construct matched filters for correlator applications are generally recorded on photographic film. Most commercial emulsions of interest are coated on birefringent substrate. The authors address the effects of this birefringence upon the interference pattern recorded as a matched filter in coherent optical correlators. Results are then expanded to include several types of multiple matched filters. Finally, techniques are discussed that reduce the deleterious effects of input film birefringence.

Space-blur bandwidth product in correlator performance evaluation

The effects of space-invariant blurring in the frequency plane of a coherent optical correlator on the system’s performance are considered. The correlator performance criteria used include loss in output correlation peak amplitude, output signal-to-noise ratio, and variation in the probability of a false alarm for a fixed probability of detection. A statistical approach is used to derive general expressions for the mean m and variance var of the output correlation. Expressions for the three correlator performance criteria are then obtained in terms of m and var. When these results are applied to several specific cases, we find that the performance of a correlator can be described and controlled by a new parameter, the space-blur bandwidth product.

Optical processing for adaptive phased-array radar

A multichannel coherent optical correlator and a noncoherent optical vector-matrix processor are described and their application to the processing of data from an adaptive phased-array radar are discussed.

Shortening the length of an optical correlator by the use of a lens array

The very close positioning tolerances of the Fourier transform lens (FTL) in a coherent optical correlator with respect to the filter, can be a handicap for its use in an industrial environment. Replacing the single FTL by a lens array, this distance can be reduced by an order of magnitude, leading to constructions which are less sensitive to mechanical vibration. To each array-lens, a corresponding object field is related. This permits a splitting up of information, which facilitates the interpretation of variations between the objects under test. The optical properties of the correlation system and their influence on the autocorrelation function and cross talk are discussed.

Reconnaissance d'objets tridimensionnels de taille variable: Etude et realisation d'un filtre-hologramme adapte

The principle and the mathematical description of a hologram filter for the recognition of 2 and 3-dimensional signal were presented in a previous paper. This hologram is to be used as a multiple filter in a conventional coherent correlator, and the advantages of such a simple device are compared to that of more complex systems. Two different recording setups are proposed. Some possibilities of this hologram - such as information storage - are also introduced.

Spatially coherent white light correlator

A spatial-temporal correlator in which the spectral components of light behave as independent information carriers working simultaneously is described. The use of spatially coherent light permits to process the amplitude and the phase of an input data through a chromatic Fourier Hologram acting as a matched filter. A rough estimation of the S/N ratio shows a great increase compared to that obtained in conventional monochromatic correlators. It could be applied in pattern recognition to reduce the so-called false alarms.

Birefringence effects in estar base reconnaissance film in an optical correlator

Coherent optical correlators using Fourier transform matched filters are generally constructed using the interference between a transformed signal beam and an equipolarized collimated coherent reference beam. Polarization effects of any system elements must be accounted for in using such a correlator, or in any system using holographic techniques, particularly if analysers are used. Birefringence effects of estar base film, used almost exclusively in aerial reconnaissance work, have been examined by the authors when such film is placed in the signal path. Methods of compensation for such effects are suggested.

Filter plane resolution requirements for coherent optical correlators

Filter plane resolution requirements for coherent optical correlators