Abstract: A field emission device which may be used, for example, as a surge arrester, comprises two electrode structures each comprising a substrate from which project tapered electrically-conductive emitter bodies. The structures are bonded together, face-to-face, so that the emitters all project into a sealed space formed between the substrates. The space may be evacuated or gas-filled. The emitters are formed by depositing a conductive layer on each substrate, forming masking pads on the layer at the required emitter positions, and etching the conductive layer to leave a tapered body beneath each pad. The dimensions of the emitter bodies and the spacing between the substrates are preferably of the order of a few microns.

Abstract: In a video signal coder/decoder arrangement or codec for an image transmission system motion of picture elements between frames is encoded with reference to any one of eight preceding frames, so that picture information which repeats within eight frames does not need to be encoded again, and recovery after errors in transmission can commence with reference to the last-received uncorrupted frame of the eight.

Abstract: An echo canceller (1) for full duplex digital data transmission in a transmitter/receiver unit when connected via a hybrid circuit (2) to one end of a two wire line (3). In order to provide adequate cancellation at a line data rate of 160 kbit/s over 7-8 km, a data bit history of at least 9 bits is used to address a memory having at least 3 partitions (41-44) with the first partition (41) being addressed by a sequence of at least 3 data bits. This partitioning achieves a small memory size, less than 2 kbits, and a small total chip area for integrated circuit realization of the canceller together with a clocking speed for moderate power consumption is maintained by using serial arithmetic (61-63) for adding the partition outputs together with pipelining (D1, D21-D24). An exponentially quantitizing analog-to-digital converter error signal circuit (9), shown in detail in FIG. 2, is used which has wider application to echo cancellers other than the partitioned memory compensation type.

Abstract: Apparatus for recognizing a target such as a ship comprises a radar for successively sweeping the target to obtain input data representing the return amplitude from different target angles; a filter for selecting from the input data those significant ranges at which there is a significantly high amplitude; and a range difference compiler for identifying the frequency of occurrence of each of several range differences between pairs of the significant ranges, the most common range differences (expressed as a histogram) being representative of the distances between major reflectors on the target and thus distinctive of the target.

Abstract: In the production of micron-size pyramid emitters for field emission devices, very sharp emitter points are achieved by providing a layer of suitable metal, metal compound or semiconductor, forming masking pads over the required emitter positions, etching the layer so that column-like structures are formed beneath the pads, removing the pads, and then subjecting the columns to dry etching, such as plasma etching, reactive ion etching, ion beam milling or reactive ion beam milling. The dry etching process shapes the columns into pyramids with a tip size of the order of 0.03 microns.

Abstract: In a synthetic aperture radar system the radar pulse echo return signals are sample, which samples are correlated with a replica of the original signal to produce range line samples with increased resolution in the range direction. To increase resolution in the azimuth direction, a series of range lines are correlated or combined in some other way with Doppler or other reference-coefficents a problem with this is that there is relative movement between any particular point on the ground and the radar system, and the samples in the range lines corresponding to that position lie on a slant line such as 103 (skew axis y). To utilize the incoming data, the skew axis is arranged in a saw-tooth form. To facilitate processing data is fed into an input sequence of data storage location in for example a buffer 51, at a variable starting position which moves corresponding to each discontinuity in the zig-zag, so that the discontinuities are transparent to the processor.

Abstract: An authenticator for digital data is derived by generating a numerical array, using a secret key to controllably shift the relative positions of the elements of the array, and applying the scrambled array to a message to create an authenticator for that message. The method allows a high level of security and minimizes the risk of a system, such as an intelligent portable token and reader system, being compromised.

Abstract: An active noise control system for controlling noise propagated through a duct comprises a first microphone located to receive the propagated noise at a point in a plane perpendicular to the length of the duct. A loudspeaker is located substantially at that plane for introducing sound into the duct. An analog first control circuit is operative in response to the noise received by the first microphone to feed a signal to the loudspeaker such that the sound introduced thereby destructively interferes with the noise at that point. A second microphone is located further downstream to receive residual noise propagated along the duct despite the destructive interference. A second control circuit, responsive to the residual noise received by the second microphone, controls parameters of the analog first control circuit so as to minimize the residual noise. In particular, the first control circuit includes an integrator which has a transfer function which is electrically controlled by the second control circuit.

Abstract: A high current drive integrated circuit amplifier in which the quiescent currents in the output transistors are controlled by control current signals which are applied to respective input signal amplifiers and which are of values determined by currents flowing in respective current sensing transistors associated with the output transistors. The control arrangement ensures that the output transistors can not be driven to a non-conducting condition.

Abstract: A thermal imaging device comprises a non-pixellated detector (3) which includes a component (12), such as a liquid crystal element, having optical properties which vary with temperature. An infra-red lens (1) forms a focused image of a thermal scene on the non-pixellated detector. A visible light source (17) is arranged to illuminate the non-pixelled detector such that visible light from the source is modulated by the image formed on the non-pixellated detector. A second detector (7), such as a television camera chip, detects the modulated visible light to give an output which is representative of the image of the thermal scene. A visible light zoom lens (5) is interposed in the optical path between the non-pixellated detector and the second detector so that variation of the focal length of the zoom lens varies the field of view of the thermal imaging device.

Abstract: An acousto-optic tunable filter, for laser tuning, comprises an acoustic transducer connected to one face of an anisotropic crystal or Bragg cell which is cut at such an angle relative to its optic axis that an acoustic wave projected into the crystal normal to the face causes wavelength-specific diffraction of an incident optic wave (from a laser amplifying element) which nearly, but not completely, satisfies the parallel-tangents phase matching condition, thus preserving a substantial angular field of view, the angle also being such that the diffraction is polarization-insensitive.

Abstract: In apparatus for carrying out a method of manufacturing optical fibre preforms by causing a chemical reaction to take place in a gaseous reaction mixture so as to form a coating on a glass substrate wherein the reaction is promoted by the production of a plasma, a non-volatile component of the reaction mixture is held within a dispenser tube so that when it is evaporated by applying heat, it travels through perforations in the dispenser tube and reacts with the other components of the gaseous mixture.

Abstract: A radiation meter for measuring the intensity of incident ultraviolet radiation in a predetermined wavelength band against a background of broadband radiation, such as sunlight, includes a sheet of a material incorporating a fluorescent dye. The dye is selected to absorb radiation in the predetermined wavelength band and consequently to emit fluorescent radiation. The fluorescent radiation is internally reflected within the sheet and emerges from an edge of the sheet, where it impinges on a photodiode. The photodiode produces an output which is dependent upon the intensity of the ultraviolet radiation. This output is used to drive a suitable display. The unwanted background radiation passes through the sheet without causing fluorescence of the dye, and is absorbed in an absorbent backing layer. The photodiode is so positioned and screened that the background radiation cannot impinge upon it.

Abstract: An electric power measuring device wherein input analog signals representative of instantaneous values of current and voltage are applied to a multiplier (23) and the product applied to a converter (35, 37, 41, 43, 45, 47) to provide a pulse output signal having a frequency representative of the product and hence of the level of power consumption. Compensation for offset errors in the multiplier is effected by periodically reversing the polarity of one of the inputs to the multiplier and effecting corresponding reversals at the input to the converter to avoid irregularities in the output pulse rate when the level of power consumed is low.

Abstract: An antenna includes an array of electrically conductive cells each having one open side and containing a radiating/receiving element. The conductive cells reduce mutual coupling between elements.

Abstract: An optical component comprises a thin film of an electrically controllable birefringent material confined between substantially planar surfaces. A respective electrode structure is provided on each of the surfaces, each electrode structure being patterned such that an electric field applied across the film by means of electrode structures, when a voltage is applied therebetween, causes modulation of the refractive index of the material such that the wavefront of electromagnetic radiation incident of the component and transmitted through the thin film is divided into Fresnel zones.

Abstract: An integrated optic device comprises a strip-like waveguide having a Z-bend to provide a lateral offset. Weak lateral guiding in the waveguide is provided by giving the waveguide a higher effective refractive index than that of the remaining material in that layer. In accordance with the invention, an edge guide region is formed in the same layer just outside each bend, and has an even greater effective refractive index. The edge guide regions minimize power loss at the bends.

Abstract: A light sensor for use to detect only light above a given frequency uses as its light sensitive element a light emitting diode. The L.E.D. is one which, if it were arranged to emit light, would emit light having a maximum frequency equal to the said given frequency.In combination with such a sensor another L.E.D. can be used having a maximum emission frequency below the said given frequency to illuminate an area without affecting the light sensor.

Abstract: Liquid crystal polymers of general formula ##STR1## wherein the ratio b : a is less than 3 and greater than O, X is a mesogenic group of formula ##STR2## where A and B are cyclic groups, preferably phenyl, which may carry lateral substituents, preferably methyl, R may be F, CH.sub.3, CN, R.sup.1 , OR.sup.1 or COOR.sup.1 where R.sup.1 is alkyl, Y is COO.sub.1 OOC, CH.sub.2 CH.sub.2, c is 0 or 1. A preferred group X is ##STR3## The polymers may be used for optical information storage either alone or in mixture with other liquid crystals for example with smectic materials of formula ##STR4## where R" is alkyl or alkoxy and r is 0 or 1.

Abstract: A housing arrangement for equipment, such as telephone exchange equipment or transmission equipment, in which individual circuit units are formed on printed circuit boards mounted side-by-side on a shelf unit, wherein the shelf unit forms a first electromagnetic screening enclosure, and interconnections between said circuit units, external connections to those units, are made by way of a second electromagnetic screening enclosure having one face in common with the first enclosure.