Abstract: An infra-red optical system, such as an afocal telescope, having a beam-splitter plate located in a beam position having one valve of numerical aperture and tilted with respect to the optical axis has a single compensating plate located in a beam position having a different value of numerical aperture and tilted differently with respect to the optical axis so as to compensate for aberrations introduced by the tilted beam-splitter plate. The beam-splitter and compensating plates are preferably tilted about respective first and second axes orthogonal to the optical axis and lying on mutually orthogonal planes.

Abstract: The invention is concerned with a weapon aiming system for use in a fire control system such as those used in a military tank. The aiming system includes a linkage which connects a mirror with an angularly movable weapon (a gun). Upon angular movement of the gun its movement is transferred through the linkage to the mirror. The ratio of angular movement imparted between the weapon and the mirror is 2:1. Two exemplary embodiments are described and in each the linkage includes two parallelogram linkage sets each set comprising a connecting link joining two parallel arms. The system also includes a slider crank positioned in the system to form a first side of a variable triangle of links in the system. The second side of the variable triangle is formed by one parallel arm of one of the parallelogram linkage sets and the third side is formed by at least part of the connecting link.

Abstract: A display system which can be used inside a vehicle such as a military tank to monitor at least part of the surrounding panorama. The display system has a display generator and an optical system for viewing the display. Electronic means are provided to predistort the display to compensate for distortion present in the optical system. The system includes individual display units mounted side-by-side, each having a respective television camera associated therewith so that a substantially continuous picture of at least part of the surrounding panorama can be reconstructed and viewed by an observer without any apparent mismatch between adjacent displays.

Abstract: A reflection hologram is recorded in a photosensitive film on a substrate held in a support by interference between an illuminating beam and its return from a back reflector. A scanning device moves the beam from a laser angularly about a fixed apparent beam source position which remains stationary relative to the film to give improved hologram formation with the back reflector spaced from the film.

Abstract: An apochromatic catadioptric lens is provided which can operate in the 3 to 5 and the 8 to 12 micron wavebands. The lens comprises a primary mirror having an opening and a Mangin secondary mirror for directing light of the required wavebands through the opening. In the light paths between the primary mirror and Mangin secondary mirror are a pair of refracting lens elements of optically transmissive materials different from each other and from that of the Mangin secondary mirror. Preferably, the Mangin secondary mirror is of germanium, the refracting lens element of the pair nearer the Mangin secondary mirror is of zinc sulphide and the refracting lens element of the pair further from the Mangin secondary mirror is of zinc selenide. The invention also extends to an afocal telescope of non-Galilean form incorporating such a lens.

Abstract: A zoom lens cam mechanism has two cam follower bearings mounted on a common shaft and arranged so that they engage against and roll on opposite sides of a cam groove or slot so as to reduce backlash.

Abstract: An optically athermalized infra-red lens has a group of a least three closely spaced lens elements of different respective materials having positive thermal coefficients of refractive index. Two of the elements are of materials whose refractive indices are relatively temperature insensitive, one, e.g. of zinc selenide or chalcogenide glass, being positively powered and the other, e.g. of zinc sulphide, being negatively powered to effect achromatization and correct spherical aberration. The third element is negatively powered and of a relatively temperature senitive refractive index material, e.g. germanium, of lower dispersion and higher refractive index than the other two. The lens may be the objective of an afocal telescope having an eyepiece and associated scanner and detector arrangement and may effectively athermalize the whole system.

Abstract: An infra-red objective zoom lens has a fixed front positive singlet, a movable negative component, a movable positive singlet and a fixed rear component. The zoom lens can be relatively short and can be used with an "eye-piece" lens to form an infra-red afocal telescope. The system can be further shortened by using a rear group of telephoto form. The fixed front positive singlet is shaped so that at high magnification it gives undercorrected third and fifth order spherical aberration, and the moveable negative component has at least one lens element shaped so that at high magnification the negative component gives compensating overcorrected third and fifth order spherical aberration.

Abstract: Lack of versatility and efficiency in optical multiplexing/demultiplexing is countered by progressing incident light along a wavelength selective holographic reflection filter which reflects different wavelengths at different positions along the filter. Light from a fibre optic input cable is directed by mirrors to make successive passes at the filter at respective different positions therealong and detectors receive the respectively reflected wavelengths. The filter preferably varies the reflected wavelength continuously and progressively as the incident light progresses along the filter.

Abstract: A well defined passband for an optical reflection filter is achieved by causing light to be reflected at least twice from a holographic wavelength selective reflector. Preferably the wavelength/reflectivity characteristics differ at the two reflections to give relatively shifted, but overlapping, spectral passbands. The apparatus may comprise a fibre optic input cable which directs light at the holographic reflector and a corner cube reflector which returns the reflected light for a second reflection prior to receipt by a detector.

Abstract: A contact or spectacle or implant lens has positive diffractive power which introduces negative longitudinal chromatic aberration that more than counteracts, and is preferably greater than twice, the natural positive longitudinal chromatic aberration of the eye. The residual negative longitudinal chromatic aberration enables the eye/brain system to perform tasks by concentrating on the appropriate color component at the different respective distances without need to accommodate.

Abstract: An ophthalmic lens incorporates a transmission hologram which has negative diffractive power and introduces positive longitudinal chromatic aberration to add to the natural longitudinal chromatic aberration of the human eye, thereby reducing the need to accommodate. The lens also has refractive power to balance the diffractive power or to give a desired overall or residual power.

Abstract: A contact lens has a transmission hologram which provides diffractive power on a wavelength and/or amplitude selective basis whereby light from near and distant objects can be focussed on the retina of a presbyopic wearer. Similarly an implant lens can have a transmission hologram to correct for non-accommodative vision. The invention is particularly useful in providing an artificial eye lens with a bi-focal action without need for distinct near and far vision zones.

Abstract: A bifocal contact lens has diffractive power added to the basic refractive power provided by the material of the lens and the basic curvature of its front and rear surfaces. The diffractive power arises from concentric zones, each providing an asymmetric retardation of light across the zone width to direct design wavelength light predominantly into a required order and sign of diffraction, while other wavelength light is predominantly transmitted at zero order. Design wavelength light from an object at one distance can then be focused by way of diffraction of that order and sign, and other wavelength light from an object at another distance can be focused by way of zero order transmission. Preferably the zones are defined, and the asymmetric retardation provided, by a rear surface contour having steps but which approximates to the required basic curvature.

Abstract: An eyeguard for a low light level or night vision rifle sight which does not rely on uniform axial pressure for proper opening comprises a moulded rubber body having a front portion which fits on the viewing end of the sight, a back cup portion angled with respect to the front portion, and a middle thin walled deformable portion to which a single flap is secured. The flap is biased towards a closed position by the resilience of the body but is opened by angular pressure applied by an observer's eye socket to the cup portion.

Abstract: To facilitate correct orientation of a solar cell cover to locate its coated face away from the solar cell member with which it is assembled, its other face is provided with an orientation indicative surface relief mark which becomes invisible when the cover is secured to the member by an adhesive of matching refractive index.

Abstract: An `eye-piece` system for a non-Galilean afocal infra-red telescope designed to contribute to astigmatism correction, pupil match, and true afocality without undue increase in objective aperture requirements comprises a positively powered back group and a negatively powered front group consisting of a rear lens element with a convex rear surface and a front lens element with a concave front surface, there being a negatively powered gas lens between the rear and front elements of the front group. The objective system may be catadioptric.

Abstract: In order to prevent ambient light severely reducing the contrast of a display on a tricolor cathode ray tube, a holographic element having reflection wavelength peaks between the emission wavelength peaks of the cathode ray tube is disposed in the light path from the display to the observer. The holographic plane of reflection is preferably inclined to the substrate plane of the element so as to direct unwanted light away from the observer. The display may be head-down or head-up.

Abstract: A relatively simple dual display system, e.g. a map or stand-by graticule superimposed on a radar or main display, is provided by use of holography. A holographic element when illuminated with a reconstruction beam from a light source provides a holographic image of visual information at a postiion behind and spaced from the element itself. The element is mounted an appropriate distance in front of a cathode ray tube such that the holographic image appears at the phosphor surface superimposed on and coincident with the CRT display.

Abstract: A composite security panel having at least two fibre optic elements contained within a block of material, the elements being arranged with portions emerging from the panel at three or more spaced locations. Translatory movement, rotation or tilting of the panel when assembled in a wall or housing causing translatory movement of an emergent portion of an element resulting in the interruption of a transmission line through the wall of housing. The elements may be firmly embedded within the panel, the fibre optic being carried on a strip of mateial, or the elements may be formed into a mesh which can be loosely housed between interconnectable sections of the panels.