Abstract: Methods and apparatus for reducing surface discontinuities, resulting from the formation of a three-dimensional object out of a plurality of stacked layers, during layer by layer stereolithographic formation of the three-dimensional object. The discontinuities between layers are reduced by utilizing formation and solidification of thin fill layers near the edges of thicker structural layers or by utilizing surface tension effects to smooth discontinuities between the structural layers by formation of meniscuses of building material and solidifying the meniscuses.

Abstract: A process of fabricating a three-dimensional object from a light curable liquid resin includes irradiating a light to a surface of the light curable liquid resin so as to form successive cross-sectional layers of the cured resin at that surface and to superimpose the successive layers on each other. An improvement resides in that one or more of the successive layers is made to have different portions made through different curing conditions. By providing such cross-sectional layers having different curing conditions, the resulting three-dimensional object can be made into accurate outer configuration without leaving any critical residual stress therein.

Abstract: A method and apparatus for making an object by stereolithography from layers of a medium solidifiable upon exposure to synergistic stimulation by selecting an area element of a first layer of medium. The depth of the medium within the object underlying the area element is determined and compared to the depth to the minimum solidification depth of the medium. The area element is exposed to solidifying synergistic stimulation only if the depth of the medium equals or exceeds the minimum solidification depth. A next layer is created over the first layer without exposing the first layer to solidifying synergistic stimulation, if the depth is less than the minimum solidification depth. The layers may have a thickness selected such that the minimum solidification depth is a integer multiple of the layer thickness.

Abstract: An improved stereolithographic apparatus (SLA) and an improved method for generating a part from curable material. The invention utilizes control and/or knowledge of depths of penetration of actinic radiation into a vat of photopolymer to determine and/or control and/or produce desirable characteristics associated with the creation of parts. From a predictive point of view, these desirable characteristics may include determination of cure depth from a given exposure, determination cure width, determination of required minimum surface angle (MSA), determination of optimum skin fill spacing, the strength of cross sections of partially polymerized material, amount of curl type distortion, and necessary overcure to attain adhesion between layers, etc. These determinations can lead to the use of particular building techniques to insure adequate part formation.

Abstract: An improved method for stereolithographically making an object by alternating the order in which similar sets of vectors are exposed over two or more layers. In another method, a pattern of tightly packed hexagonal tiles are drawn. Each tile is isolated from its neighboring tiles by specifying breaks of unexposed material between the tiles. Using an interrupted scan method, vectors are drawn with periodic breaks along their lengths. In another method, modulator and scanning techniques are used to reduce exposure problems associated with the acceleration and deceleration of the scanning system when jumping between vectors or changing scanning directions.

Abstract: Apparatus and methods for stereolithographically forming a three-dimensional object includes a vessel for holding a building material and a smoothing member for forming a uniform coating over a previously formed layer of the object. The smoothing member has a plurality of blades. The smoothing member is swept over a previously formed layer of the object, in at least two directions. Different clearances between the lower surface of the smoothing member and the upper surface of the previously formed layer are used to provide a uniform coating for a subsequent layer over the previously formed layer. The sweeping velocity of the smoothing member can be varied. Retractable needles are attached to the smoothing member for adjusting a blade gap between the lower surface of the smoothing member and the surface of the building material.

Abstract: An organic thin film formed of molecules of at least one dye compound selected from the compounds represented by the following general formulae: ##STR1## wherein X is a hydrogen atom, a methyl group, or a halogen atom, R.sup.1 is an electron attractive group substituted with a hydrophobic group having 12 or more carbon atoms, Z is either=0 or=NR.sup.2, and R.sup.2 is an electron attractive group or an electron attractive group substituted with an organic group having 1 to 50 carbon atoms; andR--(DS) (III)where R is an organic hydrophobic group having terminated with two long chain alkyl groups or an organic hydrophobic group having a steroid carbon skeleton, and DS is a dyestuff group having a dye skeleton of tetracyanoquino dimethane, N, N'-dicyanoquinonediimine, N-cyanoquinoneimine, benzoquinone, pheylenediamine, tetrathiafulvalne, tetraselenavalene, ferrocene, phthalocyanine, or porphyrin.

Abstract: Recording and erasing optical information can be done by using an alloy film capable of forming two stable crystalline states differing in crystal texture and optical characteristics by being irradiated with optical energies under different conditions. A thin memory film preferably including 60 to 90 atom % of Indium (IN) and 10 to 40 atom % of Bismuth (Bi) is formed on a substrate.

Abstract: A stereolithographic method for constructing an object from a medium capable of solidification upon exposure to synergistic stimulation (e.g. a laser beam). Stacked layers of solidified medium are constructed by exposing the medium to synergistic stimulation in magnitude and pre-selected pattern, such that each layer is formed with external boundaries and up and down facing surfaces as necessary. At least a portion of a layer which is neither upfacing nor downfacing is also skinned, e.g., provided with a continuous skin formed by overlapping skin vectors or traces of the laser beam. Alternatively, all intermediate or internal cross-sectional layers are provided with skin and cross-hatch. Upfacing and downfacing features, and intermediate layers may be provided with a skin created by scanning in a first pass using non-consecutive skin vectors followed by scanning in at least one additional pass that completes the exposing process by filling in between the originally drawn vectors.

Abstract: An optical disk structure for an erasable optical disk for use with an erasable optical disk drive using electron trapping optical memory media is disclosed in which several distinct layers in additon to the electron trapping optical memory media are utilized in order to minimize light scatter between adjacent tracks on the disk and to carry permanent format and guidance information for use in accomplishing the write, read and focusing and tracking functions of the erasable opticl disk drive with which the disk structures are utilized. The use of absorbing layers as part of the optical disk structure for attenuating reflections of light within the disk structure and eliminating the spreading of marks is also disclosed.

Abstract: An apparatus and a process for storing and retrieving information comprising an optical disk having a substrate and a recording layer of a predetermined optical thickness disposed on the substrate. The indices of refraction of the substrate and recording layer are unequal, causing Fresnel reflections from the surface of the layer and the boundary between the substrate and the layer. The optical thickness of the layer is selectively changeable to a plurality of distinct values different from the predetermined value, thereby changing the intensity reflectance of the optical disk to one of a plurality of values. Differences in the intensity of light reflected or transmitted by the disk are detected and serve to represent information according to a multi-level rather than binary digital system.

Abstract: An improved sterolithography system for generating a three-dimensional object from object defining information, the information defining the object being structurally specified to reduce curl, stress and distortion in the ultimately formed object is described. A stereolithographic distortion known as curl is reduced by the system, and several techniques to eliminate or reduce curl are utilized by the system, including dashed line, bent line, secondary structure, rivets, and multi-pass techniques.

Abstract: The invention provides an optical recording medium and a method of recording, reading, and erasing information to an optical recording medium, where information is recorded, read, and erased to the recording medium by applying beams projected from a light source capable of varying wavelength of beams. The recording medium contains at least two kinds of organic compounds each presenting photochromic phenomenon and being dispersed in binder. By projecting beams onto the optical recording medium, recording, reading, and erasure of information making use of a photochromic phenomenon can be executed independently against each organic compound dispersed in the recording medium. As a result, the optical recording medium promotes to increase the information recording density in proportion to the number of the kind of organic compounds.

Abstract: An optical location system having a beam projection device (11, 51, 12, 13) to illuminate the field of view, and a detector array (29) to detect reflected light (21) from the field of view. The projected beam (13) is incrementably progressed across the field of view and for each increment the reflected points of light (61) detected by the detector (29) are recorded to build a map of the field of view, which is stored as a sparse array having as its first coordinate the angle of projection (A) of the illuminating beam (13) and as its second coordinate the azimuth angle (B) of the reflected ray detected by the detector. The value stored in each element of the array is the angle of the incidence (C) of the horizontal component of the detected ray with the detector.

Abstract: Pump laser light is focused into a solid state laser material. A resonator is positioned around the laser material. The resonator is comprised of reflectors which have high reflectivity at both the pump light frequency and the solid state laser light frequency. The resonator is impedance matched to the pump laser. The pump light resonates inside the solid state laser material until practically all of it is absorbed.

Abstract: Recording and erasing optical information can be done by using an alloy film capable of forming two stable crystalline states differing in crystal texture and optical characteristics by being irradiated with optical energies under different conditions. The thin memory film preferably includes not more than 60 atom % of Gallium (Ga) and not less than 40 atom % of Antimony (Sb).

Abstract: An optoelectronic signal recording and storage medium including a base layer, a conductive layer, a photoconductive layer and storage layer has a coherent crystal morphology throughout, even though the chemical and electrical properties of its layers are by choice dramatically different. The base layer is preferably made of monocrystalline sapphire grown in a manner as to allow the growth of the other layers directly on a surface of the base layer without the need to grind and polish that surface, thereby minimizing internal defects in the medium. The monocrystalline base layer also allows the acceptance of exeptionally uniformly distributed charges over wide areas of the medium, thereby enabling the medium to locally record and store minutely differing optoelectronic signals on a background of minimal noise, thus facilitating low light level electronic or optical recording and long term storage of signals and minimal energy readout of those stored signals.

Abstract: Recording and erasing optical information can be done by using an alloy film capable of forming two stable crystalline states differing in crystal texture and optical characteristics by being irradiated with optical energies under different conditions. The memory film includes 35-45 atom % of indium (In) and 55-65 atom % of antimony (Sb). The memory film may include an additional element M selected from the group consisting of Al, Si, P, S, Zn, Ga, Ge, As, Se, Ag, Cd, Sn, Te, Tl, Pb, Bi, and a combination of these elements, the composition of the memory film being expressed by the formula (In.sub.x Sb.sub.1-x).sub.1-y M.sub.y.

Abstract: An optical memory cell constructed from an optical combiner, a 1.times.2 optical swtich and optical fibers. One input port of the optical combiner serves as the input to the memory cell. The output port of the optical combiner is connected to the input of the optical switch by an optical fiber. A first output port of said switch is connected by an optical fiber to the second input of the optical combiner, such that when said switch is in the straight-through state, an optical loop is formed through which an optical pulse can circulate. The second output of the optical switch serves as the output of the memory cell when said optical switch is in the cross-over state. Control signals are provided by a clock.

Abstract: A system and method for erasing data on an optical storage disk of the type which is responsive to light of first selected characteristics to produce a detectable physical change in the disk and is further responsive to light of second selected characteristics to reverse the physical change. Erasure is effected by rotating the disk proximate a linear light source capable of projecting a curtain of light having said second selective characteristics onto the disk. Uniform erasure of the data may be obtained by modifying the light flux which is delivered to the disk as a function of distance from the center of the disk. In particular, by increasing the flux or flux density which strikes the disk proximate its periphery relative to that which strikes near the center, the unit dosage received by any portion of the disk may be equalized.

Abstract: A method for selectively applying a voltage to selected points on a surface is disclosed. The method utilizes a layer of photo-conductive material in contact with the surface in question. The layer of photo-conductive material is illuminated in the region immediately above the selected points so as to cause the voltage on an electrode bonded to one surface of the photo-conductive region to be electrically connected to the selected points.

Abstract: An erasable electro-optic storage disk for use in focused laser beam operated data handling systems comprises a semiconductor substrate with a storage layer arranged on top of it that is capable of trapping charge carriers. Electric contacts are connected to the semiconductor substrate and to the storage layer, respectively. An energy barrier with associated depletion zone is formed in the substrate near the interface with the storage layer. Upon simultaneous application of a control voltage to the contacts and of a focused laser beam directed at a storage site, electron pairs are generated in the depletion zone, and charge carriers are transferred towards the interface. This causes a reduction of the depletion zone and thus of the voltage over the depletion zone, and an increase of the field in the storage layer, whereby charge carriers are injected into and trapped in the storage layer; method of recording, reading data and apparatus therefor.

Abstract: A photorefractive device is disclosed comprised of a first and second electrodes for establishing a potential gradient between first and second spaced locations and, interposed between the first and second electrodes, intermediate means capable of producing in a readout beam of electromagnetic radiation an image pattern corresponding to that present in a spatially intersecting writing beam of electromagnetic radiation when a potential gradient is applied to the intermediate means by said first and second electrodes. The intermediate means consists of a photorefractive layer capable of internally storing the image pattern of the writing beam created by its interference with an intersecting reference beam of electromagnetic radiation, the photorefractive layer being comprised of a homogeneous organic photoconductor containing organic noncentrosymmetric molecular dipoles capable of imparting to the photorefractive layer a second order polarization susceptibility of greater than 10.sup.-9 esu.

Abstract: The disclosed liquid crystal electrooptic memory device uses a ferroelectric liquid crystal sandwiched by a pair of molecular oriented films attached to opposite surfaces thereof, and a pair of transparent conductive films are attached to the outside surfaces of the molecular oriented films, respectively. An impedance having a constant electrostatic capacitance and a variable resistance is connected to one of the transparent conductive films. Further, an electric pulse source is connected across the transparent conductive films through the impedance. When a pulse from the pulse source is applied to the transparent conductive films, light transmittance of the ferroelectric liquid crystal is set at different levels depending on the resistance of the impedance. Thus, the resistance of the impedance is memorized in the form of the light transmittance of the ferroelectric liquid crystal. The impedance can be in the form of a photoconductive film attached to the molecular oriented film.

Abstract: A photographic contact printing process is disclosed having application in the mass production of replicate video discs from a master disc, and other applications wherein it is desired to replicate micro-detail over a relatively large area. A problem with conventional contact printing from a mask to a photographic medium is one of maintaining intimate contact over a relatively large area since dust, dirt, etc., are almost impossible to completely eliminate in any practical manner. In accordance with the present invention, a contact printing process is provided wherein intimate contact is not necessary for making high quality contact prints. The present invention recognizes that in contact printing information from a master disc to a replicate disc, the contact printing process is significantly less sensitive to imperfect contact between the master disc and the replicate disc if one employs a replicate disc comprising a photosensitive material having a certain optical properties.

Abstract: A three-dimensional memory element comprises a multilayer tunnel switch portion formed by alternately stacking conductive films and insulating films, both the ends of the switch portion consisting of insulating films, a write electrode formed on the insulating film as one end of the multilayer tunnel switch portion, a read electrode formed on the insulating film as the other end of the multilayer tunnel switch portion, and charge accumulating capacitors respectively connected to the conductive films of the multilayer tunnel switch portion.

Abstract: The storage medium for an optical data storage system is a contiguous laminate formed by a plurality of layers of photochemical hole burning material, such layers individually exhibiting inhomogeneous absorption spectrum characteristics spanning different, successive wavelength ranges. This arrangement greatly expands the cumulative or overall absorption band of the storage medium available for laser hole burning to implement data bit writing, and thus increases the storage density at each memory site.

Abstract: A wavelength selective optical recording and reproducing device has a reference wavelength in a marker hole of a medium. The optical detector is adapted to detect light which is reflected from or transmitted through the marker hole. A phase detector detects the phase of an output of the optical detector by using a high frequency signal as a reference to obtain a signal indicative of the deviation of the wavelength of the output from a desired wavelength. A wavelength regulator is responsive to the wavelength deviation to control the wavelength of the light source to be the desired wavelength.

Abstract: Recording and erasing optical information can be done by using an alloy film capable of forming two stable crystalline states differing in crystal texture and optical characteristics by being irradiated with optical energies under different conditions. The memory film includes 35-45 atom % of Indium (In) and 55-65 atom % of antimony (Sb).

Abstract: A system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction, successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, being automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process.

Abstract: A monolithically integrated MOS circuit includes a memory area having electrically programmable storage cells (E.sup.2 PROM) with outputs, a potential source, at least one blocking circuit for connecting at least one of the outputs to the potential source, and a radiation sensitive sensor connected to the blocking circuit for controlling the connection of the outputs to the potential source.

Abstract: The invention relates to an apparatus for reversible optical data storage using polymeric liquid crystals with mesogenic side groups as a storage medium, arranged to store information by means of selective variation of the order of the polymeric liquid crystals be means of a heat source. The macroscopically oriented film made from the liquid crystal polymer is held in the solid form retaining condition of the liquid crystal polymer in the temperature range below the glass temperature T.sub.g >T.sub.a (room remperature) for the purpose of storing the information. The polymer is then selectively and locally heated by means of the heat source into the isotropic liquid condition and the local information is fixed in the glass condition of the polymer after the heat source is turned off.

Abstract: This invention is an optical memory device in which data is written, stored and read out in optical form without any conversion to electronics. The functions performed such as readout, reset and the like can be via electronic means, optical means, or a combination of both. The inherent bistability/hysteresis which is required for the memory function is obtained with a laser structure which includes a saturable absorber and, as a portion of the cavity, a waveguide.

Abstract: Information is recorded by creating recording hole at a predetermined wavelength position on a memory element with a characteristic of wavelength selective optical data storage. Because the memory element is pit-like, it causes diffraction of a light beam transmitted through or reflected by it. The diffracted beams are detected for positioning a beam spot right on the selected memory element.

Abstract: A first memory element and a second memory element have characteristics of wavelength selective optical data storage. Information is recorded on the first memory element by creating recording holes by a narrow band light beam at specific wavelength positions within a broad inhomogenous absorption line. The second element has at least one prerecorded hole at a specific wavelength position thereof for detecting the wavelength positions of the recording holes.

Abstract: A recording medium for wavelength selective optical data storage has at least one memory element on which information is recorded by creating recording holes on the absorption spectrum thereof. The spectrum has at least one prerecorded hole for identifying the wavelength positions of the recording holes.

Abstract: An optical memory system and associated method uses electron trapping materials. Data may be written into an optical memory made of the electron trapping material by application of a visible light source to the material. The visible light source causes electrons to be trapped in the material, which electrons will be released and emit visible light upon application of a read beam infrared source to the material. An arrangement realizes logical functions by parallel memories each memory having stripes of memory material disposed upon a plastic substrate.

Abstract: An optoelectronic signal recording and storage medium including a base layer, a conductive layer, a photoconductive layer and storage layer has a coherent crystal morphology throughout, even though the chemical and electrical properties of its layers are by choice dramatically different. The base layer is preferably made of monocrystalline sapphire grown in a manner as to allow the growth of the other layers directly on a surface of the base layer without the need to grind and polish that surface, thereby minimizing internal defects in the medium. The monocrystalline base layer also allows the acceptance of exeptionally uniformly distributed charges over wide areas of the medium, thereby enabling the medium to locally record and store minutely differing optoelectronic signals on a background of minimal noise, thus facilitating low light level electronic or optical recording and long term storage of signals and minimal energy readout of those stored signals.

Abstract: A rewritable information recording medium which has a recording layer containing a Group I transition element and a Group IV representative elements of the Periodic Table as two principal element and a support substrate for physically supporting the recording layer, and an information-write, -read, and erase method using this recording medium. When the recording layer is immediately cooled after it is heated up to near a eutectic temperature of the two principal elements, two metastable phases having different energy levels appear. A state in the first metastable phase of the higher energy level has a reflectivity sufficiently higher than that of a state of a mixed phase including the second metastable phase of the lower energy level, or that of the equilibrium state. The state in the first metastable phase can be obtained by heating a recording layer in another state by light beam irradiation.

Abstract: An optical recording system which includes a recording medium of which the light absorption spectrum changes when exposed to light; a first transparent substrate provided on a first side of the recording medium; a plurality of first parallel transparent elongated electrodes provided on the first substrate; a second substrate provided on a second side of the recording medium opposite to the first side; a plurality of second parallel elongated electrodes provided on the second substrate in the direction perpendicular to the first elongated electrodes so as to form a plurality of recording spots at their intersections; a light source for directing a light beam to a plurality of the recording spots on the recording medium; and an electric field applying device for controlling the field strength of an electric field applied to a plurality of the recording spots to control the degree of change in the light absorption spectrum so that multiplex information recording is made in a dimension of electric field.

Abstract: An information writing method of an optical recording device selectively irradiates light on an optical recording medium, having a thin organic film containing donor and acceptor molecules, and a pair of electrodes for applying a voltage to the thin organic film, in order to cause charge transfer between the donor and acceptor molecules, and records a change in optical or electric characteristics of the thin organic film by the charge transfer as information. Selective light irradiation is performed while a voltage is applied to the electrodes.

Abstract: Two electro-optical computer interface embodiments provide for one-way read only and two-way optical read and write. The two-way embodiment includes a main module having a shared memory, and a processor/controller and a main bus. A plurality of local processor modules each includes a local memory, a local processor, and a local bus. The processors are electrically joined by control conductors which provide for coordination and timing between local processors and the main processor. Each memory array has a film deposited on it by the Langmuir/Blodgett technique. The memory arrays are each illuminated by a pulsed laser or Q-switched laser. The film is responsive to the electric fields in the memory array cells for modulating the illumination light. The image is then read onto other memory arrays which are responsive to the illumination for transferring the data between memories.

Abstract: A three-dimensional tunnel memory device includes a multilayer Langmuir-Blodgett film wherein each layer can store or carry an electric charge. Charges are introduced into one side of the film in a time sequence corresponding to the information to be carried. An electric field is applied between the faces of the film to cause the charge stored by any layer to be transferred to the adjacent layer, and for thus reading out the sequence of charges stored by the film. The multilayer Langmuir-Blodgett film includes memory unit cells each comprising Langmuir-Blodgett films formed, respectively, of different kinds of organic compounds and contacting each other. Electric fields of different magnitudes are applied, respectively, to the film constituting each memory unit cell thereby allowing the stored charge in each film constituting the memory unit cell to hop the tunnel barrier.

Abstract: A data card having an optical recording medium formed by an erasable layer atop a polymer layer with prerecorded indicia, all atop a card base. The polymer layer has surface contours which constitute prerecorded indicia. The erasable layer disposed over the polymer layer displays the prerecorded indicia for reading, together with information on the erasable strip. The erasable material may be magnetooptical material, amorphous-crystalline material or liquid crystal material.

Abstract: A memory medium for information storage and reproduction and an apparatus for reading information from the memory medium are set forth herein. The memory medium includes a solid storage layer formed of a pyroelectric and electro-optic material, and information is stored in the medium through local residual polarization of the layer owing to the pyroelectrical property of the material.

Abstract: In an optical memory of phase transition type such that information is recorded or erased by reversibly changing the phase of a thin recording film between equilibrium phase and non-equilibrium phase in response to a laser beam, the memory is formed with a recording film comprising as the main component an intermetallic compound with a melting point between 300.degree. to 800.degree. C. in accordance with a binary target co-sputtering technique. Since the phase transition temperature of the recording film is higher than room temperatures, the recorded film can stably be kept in non-equilibrium phase state for a long time. Further, since the melting point thereof is lower than 800.degree. C., it is possible to record or erase information in or from the medium in response to a relatively small power laser beam.

Abstract: An image recording and/or reproducing apparatus including a recording unit having a recording driving mechanism for recording a plurality of images in the form of electrostatic pattern on different regions of a photosensitive member, a reading unit having a beam source and capable of scanning the photosensitive member by an electron beam so as to convert the recorded pattern into electric signals, and a connecting member for connecting the recording unit and the reading unit without causing any change in the states of these units. This apparatus makes it possible to record and/or to reproduce images with a high image quality. The connecting member includes a container accommodating the recording unit and the reading unit. Alternatively, the connecting member is adapted to keep a portion of the recoding unit and a portion of the reading unit in close contact with each other.

Abstract: A multilinear charge transfer array is provided formed by N lines of P photosensitie detectors. Each photosensitive detector is connected directly by a connection to a demultiplexing and reading system, the signals obtained at the output of the array being fed to a processing device external to the array.The demultiplexing system comprises a charge transfer shift register with N.times.P stages, the connections between each detector and the corresponding input of the register being provided so that the detectors of the same rank are connected to contiguous inputs.

Abstract: Disclosed is an optical data storage device and system, and a method of doing one or more of entering data into, reading data out of, or erasing data from the device. The optical data storage device is characterized by an encapsulated structure including an optically non-transmissive, chalcogenide, phase changeable layer. The phase changeable layer is sufficiently thick such that the impinging vitrifiying energy pulse does not vitrify all the way through the layer.

Abstract: An associative (content addressable) optical memory system is comprised by a matched optical holographic filter (10, 18, 21, L1, L2) coupled to a digital computing system (14, 15, 16) having a memory (15, 16). In order to search the computing system memory for occurrences of an item, a hologram of a binary representation of the item is formed in the Fourier transform plane (18) employing laser (11) as the light source. A page of the memory to be searched is subsequently displayed at the input plane (10), which for example is comprised by a liquid crystal over silicon display, and illuminated by the laser (11). The light is filtered at the Fourier transform plane (18). Any occurrences in the input plane display of the item result in a respective correlation spot at the output plane, which spot output is decoded and supplied to the processor (14), thus providing correlation information to the computing system as to the location in its memory of the occurrences of the item.