The Secretary of State for Defence in Her Britannic Majesty's Government

Qinetiq Limited

Herbert A French

Philip Sutton

Arthur Maitland

William Dawber

Andrew P Shaw

Colin J Flynn

A filter for removing coherent radiation from a source in a field of view, substantially independent of the size of the source, comprises a first reticlelocated in the path of received light, a first lensproducing an optical transform of the first reticleat a second reticlelocated in the image plane of the first lens, a second lensproducing an optical transform of the second reticleand a third reticlelocated in the image plane of the second lens. The arrangement is such that the spatial transmittance of the third reticleis selected to block at least part of the diffracted image of the first reticleproduced in the image plane of the second lensand characteristic of the coherent radiation. Preferably the optical transforms are Fourier Transforms. A monochromatic coherent source in the field of view produces a pattern of diffracted energy in the image plane of the second lens which is independent of the size of the source. Thus, by providing a suitable reticlein the image plane of the second lens light from a coherent source in the field of view can be blocked while polychromatic light is transmitted. The first and second reticles may be periodic picket-fence reticles or different spatial frequencies may be used for the first and third reticles so as to vary the stop-band characteristics of the filter.

Spectral filter

A passive rangefinder for determining the range to an object such as an aircraft, rocket or missile which emits electromagnetic radiation makes use of spectral measurements over a broad band of frequencies. The rangefinder includes means 202 for forming a real image of a field of view, spectrophotometer means 201 including an entrance aperture 27 placed to receive at least a part of said real image, dispersion means (15, 28, 29) for separating radiation of different wavelengths and detector means 24 for measuring the spectrum of radiation received from distinct portions of the entrance aperture. The detector output is connected to data processing means 204 arranged to deconvolve the received spectral profile with a stored representation (205) of a range-dependent atmospheric transmission spectral profile to obtain a representation of the spectral emission profile of radiation from the object. From the emission spectral profile a characteristic temperature of emission (206) is derived and hence a Planck emission spectrum for the object. The Planck spectrum is used to determine the range from the calculated emission spectrum and the observed spectrum (205, 208). Thus by deconvolving the measured spectrum with the atmospheric transmission spectrum the range can be determined. The invention can work at any range of e.m. radiation for which suitable components are available. Advantageously the ranging is done on two different temperature portions of the same object so that different Planck profiles are used in the signal processing. The atmospheric transmission spectrum can be measured prior to operation in the prevailing conditions by use of a laser rangefinder.

Passive rangefinder

Wide bandwidth RF spectrum analyser

An interference device for discriminating between radiation sources of differing coherence length comprises means to divide received radiation from a source into two components. A path difference, defining a coherence length cut-off, is introduced into the path of one component and the components are brought together for interference. The recombined light passes through a reticle with alternate opaque and tranparent bars and an optical band-pass filter to a detector. Interference fringes present in the plane of the reticle are swept across the reticle by the action of the collection optical system of the device which includes a scanning rotating mirror. Two similar devices can be arranged for band-pass coherence length filtering and when used in conjunction with a light soruce whose coherence is modulated the device can be used for signalling.

Radiation interference devices

A remote sensor for detecting gas, vapor or aerosol comprises means to measure the change in temporal coherence of light of a selected narrow waveband when it interacts with the gas etc. The light can be provided by a laser source or spectrally filtered sun light etc. Received radiation is split in to two beams by a Fresnel biprism and then detected by a detector sensitized by a modulating reticle to interference fringes. A glass delay plate of suitable thickness is placed in the path of one of the beams such that only received radiation having a temporal coherence greater than a minimum determined by the plate thickness produces an output signal from detector. The minimum temporal coherence is set higher than the temporal coherence of the illuminating radiation. In alternative arrangements a band-pass temporal coherence filter may be used and the sensor can be made to spectrally scan the field of view by using a tunable laser or a variable center frequency band-pass optical filter.

Apparatus and method for remote sensing of gases, vapors or aerosols

The invention relates to a pulsed laser viewing system adapted to view range intervals in a scene by gating the detector signal; according to the invention, the viewing system also includes a switching device for repetitive interchange of the range interval viewed so that the image contrast of a viewed target may be repetitively alternated between positive and negative. The switching device may operate to interchange either the detector gating time periods or the detector signal channel. Alternatively, the switching device may operate to interchange time delays applied to the detector signal before gating.

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Emsworth, United Kingdom

Herbert A French