Abstract: The thermo-optic behavior of an optical path over a range of temperatures &Dgr;T is controlled by determining a figure-of-merit (FOM) for the optical path and including in the path a body of crystalline material that enables the conditions specified by the FOM to be satisfied. The crystalline material is highly transparent at a wavelength &lgr; of radiation propagating in the path, and it has a coefficient of thermal expansion (CTE) and a refractive index n such that the CTE and dn/dT are mutually adapted to satisfy the FOM over the range &Dgr;T. In one embodiment, the CTE and dn/dT of an etalon compensate one another so as to perform frequency discrimination that is essentially temperature insensitive over the range &Dgr;T. In a preferred embodiment of the optical etalon the crystalline material comprises LiCaAlF6.

Abstract: Waveguides according to this invention have a chalcogenide glass core that comprises Se and one or both of As and Ge, and further comprises one or more dopant elements selected from the elements that cause a change in optical energy gap Eg of the glass core. The amount of dopant is selected such that Eg is equal to or less than a 2-photon energy 2h&ngr;, where h is Planck's constant and &ngr;=c/&lgr;, where c is the speed of light in vacuum and &lgr; is an operating wavelength. The dopant elements furthermore are selected such that the chalcogenide glass core has a nonlinear refractive index n2 greater than 200n2 (SiO2), where n2 (SiO2) is the nonlinear index of silica at &lgr;. Typically, &lgr; is about 1.55 &mgr;m. Exemplary, the dopant elements are selected from Sb, Bi, Sn, Pb, In, Tl, Cu, Ag and S. Currently preferred dopants are Te, Sb, Tl, Cu and Ag.

Abstract: A night vision viewing system (13a or 13b) suitable for mounting on an observer's headgear (11) wherein light rays from a forward scene pass to the observer's eye (E) via a path comprising a forward looking objective lens (19), a first inclined reflector (23) which directs the rays upwardly onto the input face of a downward-looking image intensifier (25). Intensified rays from the image intensifier pass, in turn, via a fibre optic stub image rotator (27), a second inclined reflector (29), a relay lens arrangement (31) and third and fourth inclined reflectors (33) and (35), the fourth reflector directing the rays downwardly into an eyepiece (15) which is in the observer's forward line of sight. The eyepiece (15) incorporates an internal surface (39) having both light reflective and light transmissive properties which directs rays entering the eyepiece (15) from the intensifier (25) to the observer's eye.

Abstract: A helmet sight system for use in an aircraft in which the angular displacement of the helmet in the reference frame of the aircraft is measured by measurement of the angular displacement of a set of three non-colinear radiation sources, fixed with respect to the helmet, whose angular positions are sensed by a sensor which is fixed with respect to the coordinate reference frame of the aircraft. The radiation sources may be light emitting diodes and the sensor may comprise an aperture having two mutually inclined rectilinear slits and an array of light sensitive charge coupled devices.

Abstract: A display unit for a vehicle head up display system in which the optical data to be displayed is obtained from a projector having a generally rectangular output aperture and the projected optical data is reflected to an observer by partially light reflective, partially light transmissive films embedded in a body of light transmissive, refractive material through which the observer views the distant real world scene, each film providing the observer with a different field of view of the projected data.

Abstract: An angular position sensor having a housing with an aperture in the form of two mutually inclined slits. Behind the aperture is a linear array of light sensitive elements which intercepts laminar light beams developed by the slits from light from a small light source. The positions of interception of the light on the array constitute a measure of the angular position of the light source in elevation and azimuth.

Abstract: Headgear incorporating an optical display system comprising, an optical data source mounted within the headgear above the level of the wearer's eyes, an image combiner having a spherical reflecting surface directed inwardly of the helmet and intercepting the wearer's forward line-of-sight, and a prism positioned and dimensioned so that a collimated, aberration free virtual image of the data source is presented to the wearer as a result of reflection of rays transmitted from the data source by way of the prism to the spherical reflector. Such aberration as is introduced by the prism compensates for aberration introduced by the spherical reflector.

Abstract: This invention relates to a head-up display unit having at least a part so disposed, when in use in a vehicle, as to intercept the line of sight of an observer using the unit to view a scene lying ahead of him. The display unit employs a light deflecting arrangement to enable the observer to see the part of the scene obscured by the unit and includes optical means for projecting light rays from a luminous signal source into the light deflecting arrangement thence to the observer, who sees the luminous signals superimposed on his view of the scene ahead.

Abstract: A topographical map display device for a vehicle, such as an aircraft, wherein a map in miniaturized form is carried on a carriage coupled to a frame by way of a mechanism which permits translational planar movement only of the carriage with respect to the frame and an image of an area of the map is projected onto a display surface on which a fixed datum represents the vehicle position, the carriage being moved with respect to the frame by actuator means in dependence on the range and bearing of the vehicle with respect to a ground-fix position so that the image of the map projected onto the display device changes progressively with vehicle movement whilst maintaining a fixed orientation.

Abstract: A head-up display unit for use for example in an aircraft has (a) a porthole part of a source of collimated luminous signals disposed directly in the field of view of an observer when looking at a scene ahead of him from an observation position thus obscuring part of his view and (b) a combining optical system disposed adjacent the porthole part and having plane optical surfaces which transmit to the observer's eyes light from the porthole directly, and light from the scene ahead by reflection, whereby the observer sees in superposed relationship the scene ahead and a display of collimated luminous signals. In a preferred arrangement the plane optical faces are constituted by plane boundary faces of prisms cemented together to form a stable assembly.