Abstract: Detection of remote acoustic signals is achieved using a pulse to pulse coherent laser beam which is projected from a telescope to a remote region that contains acoustic signals from a nearby source. Light from the beam is scanned in the region and a resultant signal collected by a telescope. The signal and a reference are applied to an interferometer and the resultant interference pattern applied to a photoreceiver. An alternating current is produced across the photoreceiver which corresponds to the scattered light from the beam caused by the periodic movement of acoustic signals through a region. The detector can be used to determine the curvature of an acoustic signal wavefront and to detect the location of a source producing the acoustic signals.

Abstract: A transmitter (41) is arranged to produce and focus a beam (54) on a remote focal point (55). A small proportion of resultant light (58) is back-scattered from the focal point (55) by particles or aerosols in air flowing through point (55) and returns to a receiver (42) where it is collected A detector combines resultant light (58) and reference light (72), splits the resultant light (58) and reference light (72) equally between two arms and detects which arm leads the other in phase thereby indicating the flow direction of air through focal point (55).

Abstract: In a laser radar system, in which pulses from a laser are focused by a telescope onto a target, the back-scattered radiation being deflected by polarizing beam splitter onto a detector in conjunction with a reference beam, the reference beam being produced from pulses derived from the pulse laser by the beam splitter, the pulses making repeated journeys around a cavity and a proportion of the pulse being emitted each time so that each input pulse produces a multiplicity of output pulses to form the reference beam, thereby avoiding the need for a separate laser for the reference beam.

Abstract: A receiver-transmitter for a target identification system having a particular but not necessarily an exclusive application for the identification of aircraft which utilizes laser vibrometry and a coherent array receiver including an array of photodetectors adapted to receive signals from different parts of the target and to use a corresponding array of frequency-offset reference beams to provide coherent detection of each of the received signals.

Abstract: A sensor, for use in determining physical properties of fluid, comprises a flexible diaphragm having a plane. Two tines depend from the diaphragm substantially at right angles to the plane. First and second substantially planar transducers are each bonded to the diaphragm so that the plane of the transducers are parallel to that of the diaphragm. A first one of the transducers is operative, in use, to flex between in its plane and to couple an alternating driving force into the plane of the diaphragm such that the tines vibrate in an oscillatory motion. Second of the transducers is operative to detect vibrations of the diaphragm.

Abstract: A holographic objective system (3) produces an intermediate real image (4) of information displayed on an electrically addressable spatial light modulator or SLM (1), the real image (4) being collimated by a powered holographic reflector (5) constructed as part of the visor of a helmet worn by the viewer (9). The SLM (1) is illuminated by laser light from a remote laser (8) via a single optical fibre (6) and beam splitter (2). A diffuser (7) is provided either between the laser and the beam splitter or in the plane of the intermediate image (4). The objective system (3) may alternatively include, or consist entirely of, conventional optical elements.

Abstract: The orientation of a helmet is monitored by illuminating the cockpit volume by a moving interference pattern produced by three beams. The sensors detect the varying phase to produce outputs which are compared to determine the orientation of the helmet.

Abstract: A position measurement system transmits moving interference pattern and these are received by a sensor fixed relative to the transmitter and a mobile sensor, by comparing the phases of the interference pattern at the two sensors the position of the mobile sensor relative to the transmitter can be calculated.

Abstract: A fluxgate magnetometer includes a body of magnetic material, a drive coil and a sensing coil. At least one of the drive coil and the sensing coil is composed of two portions, each of the two portions being disposed in generally parallel planes which are spaced apart and respectively include an electrically insulating substrate and a spiral shaped portion which is electrically conductive supported by the electrically insulating substrate, the two portions being electrically connected such that electric current flows in the respective spiral shaped portions in opposite rotational senses relative to the body of magnetic material.

Abstract: A sensor comprises a beam, e.g. of silicon, which resonates at a frequency dependent on the force imposed on the beam. Light on a line induces resonance of the beam by means of the photothermal effect. The light reflected from the beam is amplitude modulated at the resonance frequency, and returns along the line. It is reflected by a semi-reflecting plane mirror onto a lens which focusses it onto a photodetector. The photodetector produces an output voltage modulated at the oscillation frequency, and is thus representative of the force imposed on beam 1.

Abstract: A flowmeter has a vane which is caused to vibrate at one of its resonant frequencies and a flexure standing wave set up along it. Two piezoelectric sensors are used to monitor characteristics of the standing wave and hence the mass flow rate of a fluid flowing past the vane along a pipe.The vane comprises a ceramic planar member on which a conductive ceramic ink is laid down to form tracks. The ink also bonds a ceramic piezoelectric transducer to the member to drive and maintain it in vibration. A non-conductive layer is laid down on the surface of the member covering the conductive tracks surrounding the transducer. A metal layer forms the external surface of the vane and acts as a ground plane for the transducer.

Abstract: A piezoelectric beam structure for a vibrating beam force sensor in which three coplanar beams of piezoelectric material are spaced side by side and supported between respective common mountings at either end. The two outer counterbalancing beams in operation flex in opposite phase to the center main beam, with each beam flexing in a direction normal to the beam plane. A force sensor which includes this piezoelectric beam structure.

Abstract: A device for measuring the mass flow of a fluid consists of a body which is arranged to resonate under the influence of a travelling flexure wave. Various embodiments of the invention are described. In certain embodiments in which the resonating body is in the form of a cylinder, the frequency of resonance is related to the mass of fluid flowing over the surface of the cylinder. In other embodiments, the wave velocity of the flexure wave is modified by a fluid flowing over the surface of the cylinder and amplitude modulation is introduced into the resonating flexure wave. The amplitude modulation is related to the mass of fluid flowing and an output indicative of the mass flow is produced. In further embodiments the body is in the form of a flat vane. The wave velocity of the flexure wave is modified by the fluid over the surface of the vane, and the wave velocity is monitored by comparing the relative phases of the flexure wave at different points along the vane.

Abstract: A liquid-level measuring device including, in effect, two similar elongate members each adapted to be partially submerged in a liquid, the two elongate members being displaced lengthwise relative to one another by an odd number of quarter wavelengths of a predetermined wavelength of a chosen flexurally resonant mode of the elongate members. Each elongate member has flexure wave transmitting elements in contact therewith for launching flexure waves along the member and flexure wave receiving elements in contact with the member for receiving flexure waves launched by the transmitting elements. Also, associated with each elongate member is a phase detection arrangement connected both to the transmitting elements and to the receiving elements for detecting and comparing the phase of the flexure waves at the transmitting elements with the phase at the receiving elements so as to obtain a phase difference measurement, the magnitude of which provides an indication of the resonance frequency of the elongate member.

Abstract: A device for measuring the density of a liquid relies on the generation of resonant modes in a torsional vibration set up in a paddle which is immersed in the liquid. The frequency at which a resonance mode, usually a fundamental, is produced is a function of the density of the liquid. A feedback circuit is described which varies the frequency of an oscillator producing the vibrations so as to enable the frequency at which the resonance mode exists to be automatically found. Transducers for generating and detecting the torsional vibration are located at one end of a rod above the surface of the liquid, and the paddle is mounted on the other end of the rod.

Abstract: A liquid level sensing device consists of an elongate member in which flexure waves are induced. By partially submerging the elongate member in a fluid the depth of immersion can be determined from characteristics of the flexure waves. In one mode of operation the transit time for a flexure wave to travel along the elongate member is measured, and in another mode the nature of resonant modes in the flexure wave is determined.