Abstract: Head mounting apparatus which can accommodate different head sizes and shapes comprises a substantially rigid member for supporting equipment, such as night vision equipment, to be used by the wearer, and a flexible member which can flex to adapt to the head of the wearer. The rigid and flexible members are connected by pivotal means at pivot positions along spaced pivotal axes. The flexible member has buttons for attachment of straps by which it is in use secured to the wearer's head. The position of the rigid member relative to the flexible member may be adjustable by manual rotation of knobs.

Abstract: An "eye-piece" system for an infra-red afocal telescope comprises a back element of positive power having a convex front surface and a curved back surface, and a pair of elements closely spaced to define a gas lens therebetween, the front element of the pair having a concave front surface whose radius of curvature is equal to or greater than that of the convex back surface of the back element of the pair, the pair of elements in combination with the gas lens therebetween being of positive power. The "eye-piece" system can be used in combination with an objective lens system, which may be catadioptric or all refractive, to provide a collimated magnified view of a scene or object, from which infra-red radiation is received, at a real exit pupil where a scanner operates.

Abstract: A security system having a mesh-like intruder detection structure for use as or with a security fence 2 or for detecting intruders jumping down from a fence or wall. The structure is made up from a number of elongated members (4, 6, 8, 10, 12, 14) each in the form of or including a fibre-optic waveguide, the elongated members being secured together at a number of spaced jointing points (20) spaced 20 cms or less apart. Portions of the elongated members at the jointing points are held in a position fixed relative to one another by encapsulating the jointing points in a plastics material (40), welding the portions together and/or surrounding them with a metal ferrule (38).

Abstract: In a head-up display system by which visual information from a display is superimposed by means of a combiner on a pilot's view of the outside scene, the pilot's view is recorded by a head mounted camera to which light from the outside scene and superimposed display is reflected by a head-mounted mirror. The arrangement is particularly useful for pilot training purposes.

Abstract: Optical apparatus for biocular viewing of relatively low cost and light weight comprises a solid Cassegrain configuration in a solid meniscus element having a negative lens element cemented to its convex face and an associated corrector lens element which provide a magnified collimated image of an object for viewing by a pair of telescopes having light folding means to provide an exit pupil separation suitable for two eye viewing. At least some of the lens elements may be of plastics material and aspheric surfaces may be employed. The apparatus is particularly useful for night vision goggles having a single image intensifier tube.

Abstract: An "eye-piece" system for an infra-red afocal telescope comprises a positive back element having a concave or convex back surface and a convex front surface, and a pair of elements closely spaced to define a gas lens therebetween, the back element of the pair having a convex back surface and the front element of the pair having a concave front surface, the pair of elements in combination with the gas lens therebetween being of positive power. The "eye-piece" system can be used in combination with an objective lens system to provide a collimated magnified view of a scene or object, from which infra-red radiation is received, at a real exit pupil where a scanner operates, and can provide a wide field of view of around 70 degrees or more in the scanner space.

Abstract: Low light level or night vision apparatus, and particularly goggles, having an objective lens to focus incident light on to an image intensifier and a magnifier enabling an observer to view a magnified version of the intensified image includes a wavelength selective filter with a hole or aperture through which light of other wavelengths can pass, the wavelength selective filter or means associated with the hole or aperture being arranged to have a focussing effect so that light from a distant scene or object of a wavelength passed by the filter can be properly focussed on the image intensifier, and light from a near scene or object passing through the hole or aperture can simultaneously also be properly focussed on the image intensifier, so that either or both images can be viewed in a focussed condition without adjustment of the apparatus.

Abstract: A system for biocular viewing of a distant scene or object especially under daylight conditions comprises an objective lens which produces an initial image, one or two relay lenses with associated reflectors which produce from that initial image a pair of tilted images, and a biocular magnifying eyepiece lens through which an observer views so that each eye receives light from a respective one of the tilted images. The system can be incorporated in a combined day and night viewing system employing common objective lens and eyepiece lens components for the day and night viewing paths.

Abstract: A transfer lens comprises a first field-flattening component of negative power, a second component of positive power providing overcorrect astigmatism, a third component of positive power consisting of a plurality of positive members providing most of the power of the overall lens and whose undercorrect astigmatism is corrected by the overcorrect astigmatism of the second component, and a fourth component of negative power which increases the field angle. The lens is particularly useful in a head-down display having two primary displays from which light travels to a combiner so that the transfer lens produces a combined image. A Fresnel field lens is preferably located at the image position.

Abstract: The invention provides an objective lens with a front lens group and a rear lens group spaced apart from the front group. The front group comprises at least two doublet components one of which is a crown positive element cemented to a flint negative element which has a higher refractive index and dispersion values than those of the crown glass. The other doublet component of the front group is a crown positive element and a flint negative element in which the flint and crown glasses have similar refractive indices but dissimilar dispersions with the flint glass having a positive red relative partial dispersion deviant.

Abstract: Optical apparatus for biocular viewing which is small and of light weight comprises a solid optical element having a face which internally reflects light from an image surface to internally concave reflecting areas which reflect the light back to the face to emerge as two beams travelling towards respective eye positions via a corrector lens. The apparatus can be used in night vision goggles having a single image intensifier tube providing the image surface.

Abstract: Optical apparatus, such as head-up display apparatus, for superimposing visual information on an observer's view of a scene or object comprises a substantially flat transmission hologram through which the observer views the scene or object, the hologram being disposed substantially orthogonally to and midway along an axis between the observer's eye position and projection optics so as to deviate light from an image produced by the projection optics to the observer's eye position with minimal field aberrations.

Abstract: A collimating magnifier lens which may be used in a head-up display has a focal length of F. The lens has a front part comprising at least one positive element with a focal length between +1 F and +3 F and a negative element with a focal length between -1.2 F and infinity, the focal length of the front part as a whole being between +1 F and +6 F and the negative element of the front part not being cemented to any other element, a rear part comprising a single positive element with a focal length between +0.4 F and +1.2 F, and a field flattener behind the rear part and comprising a single negative element with a focal length between -0.4 F and -1.1 F.The negative element of the front part may have a refractive index less than or equal to 0.13 greater than that of any positive element of the front part and may be meniscus in shape having a focal length of at least 2.8 times that of the positive component and which produces overcorrect spherical aberration.

Abstract: The invention provides a portable reader for reading, by way of a magnified image, microform material such as microfiche or microfilm. The reader has a support for the microform material and a light source to pass light through the material. A projection lens is provided to project an image of the material on to a screen and there is also a biocular magnifier lens through which an observer can view a magnified version of the image. A partial reflector is disposed in the light paths between the projection lens and the screen and between the screen and the biocular magnifier lens so that light travels from the projection lens to the screen via the partial reflector and from the screen to the biocular magnifier lens via the partial reflector. In a convenient arrangement light travelling from the projection lens is reflected by the partial reflector towards the screen, and light travelling from the screen is transmitted through the partial reflector to the biocular magnifier lens.

Abstract: The "eye-piece" for a non-Gallilean afocal infra-red telescope comprises a front element which is meniscus with a convex back surface of greater radius of curvature than its concave front surface but which has a thickness such that it is of positive power, and a back element of positive power with a convex front surface and a convex or concave back surface. Such an "eye-piece" can be used in combination with a variety of objective lens systems.

Abstract: The invention provides a telephoto lens with a positive front group of lens elements, a negative middle group of lens elements and a rear positive part comprising a lens element or group of lens elements. The front group of elements includes at least one positive doublet consisting of a positive element of a crown glass type in front of a negative element of a flint glass type having a positive red relative partial dispersion coefficient deviation. The middle group of elements includes a positive lens element of a crown glass type and a negative lens element of a flint glass type having a positive red relative partial dispersion coefficient deviation. The rear element or group of elements may consist of a doublet component comprising a negative bi-concave element and a positive bi-convex element, the rear part of the telephoto lens being well separated from the negative middle part.

Abstract: A biocular magnifying lens suitable for use as an eyepiece in a night vision device comprises from front to back a positive element which is bi-convex or meniscus convex towards the front, a negative meniscus element concave towards the front, a positive singlet or doublet element which is convex towards the front, a positive element which is bi-convex or meniscus convex towards the front, and a positive meniscus element convex towards the front.

Abstract: An objective lens particularly for use in a low light level or night vision system designed to introduce a controlled amount of barrel distortion at the edges of its field of view to permit use with a flat input faced imaging tube.

Abstract: Apparatus for biocular viewing comprises a collimating magnifier lens and a prismatic assembly providing inclined partially transmitting/partially reflecting interfaces between solid parts and reflecting faces parallel to the interfaces which give multiple light paths by which an observer can view with each eye a magnified image produced by the lens.

Abstract: This invention provides a head up display apparatus particularly for use by the pilot of an aircraft including a cathode ray tube to display visual information to the pilot, means for mounting the cathode ray tube on the head of the pilot and a combiner also mounted on the pilot's head to superimpose an image of the displayed information on the pilots view. The apparatus includes a visual information relay system with a visual information source and display means arranged to view the visual information source and operatively connected with the cathode ray tube to display the visual information so viewed. The relay system includes a component arranged to be mounted at a fixed position and a component arranged to be mounted on the pilot's head so that a change of angular position of the pilots head produces a change of position of the visual information relative to the display means and a corresponding change of position of the display of the information on the cathode ray tube.