Helmet mounted display systems

Abstract: This invention relates to a helmet mounted display system of the kind comprising: a helmet having a transparent spherical visor (5) and an optical system (1, 7) which directs rays of light from a display produced by a display source (1) onto the visor to provide by reflection at the visor, at a helmet wearer's eye position (7), a collimated virtual image of the display on a line of sight of a wearer of the helmet through the visor. In order to facilitate adjustment of the eye position with respect to the helmet the visor is rotationally symmetrical about an axis about which the optical system is rotatable. The visor is preferably also rotationally symmetric about a second axis about which the visor is arranged to be rotatable between inoperative and operative positions. The invention finds particular application to binocular helmet mounted display systems, the two optical systems being then suitably arranged to be rotated relatively about a common axis with a scissors action, thereby to vary the spacing between the eye positions of the two display systems.

Description: This invention relates to helmet mounted display systems. More particularly the invention relates to helmet mounted display systems of the kind by which a wearer of the helmet on which the system is mounted is provided with a collimated virtual image of a display produced by a display source superimposed on the helmet wearer's view of a scene through a combiner element of the system. A known form of such a helmet mounted display system, as disclosed in WO-A-91-04508, comprises a helmet carrying a visor having a transparent spherical region with a centre of curvature and an optical system which is adjustably mounted to the helmet and is arranged to project an image produced by a display source onto the spherical region of the visor to provide, by reflection at a concavely curved surface of the visor, a collimated virtual image at a helmet wearer's eye position, the system allowing adjustment of the position of the virtual image to coincide with the wearer's eye position. Such a helmet mounted display system is hereinafter referred to as a display system of the kind specified. It will be appreciated that in a display system of the kind specified the visor serves both as a combiner element and a collimating element to produce the required virtual image. The use of the visor as combiner and collimating elements has the advantage that it avoids the use of a relatively heavy eyepiece combiner and collimating element commonly used in helmet mounted display systems. However, use of the visor as the combiner and collimating elements makes it no longer possible to adapt the system to suit a helmet wearer's eye position by movement of the whole display system. It is an object of the present invention to provide a display system of the kind specified wherein this problem is overcome. According to the present invention in a display system of the kind specified the adjustable mounting permits rotation of the optical system about an axis which passes through the centre of curvature. In a display system according to the present invention the optical system preferably has an optical axis lying in a plane which contains the axis. With such an arrangement, whilst the light rays are subject to off-axis reflection at the visor in the plane, the rays are on-axis in a direction orthogonal to the plane, thereby simplifying off-axis correction. In a display system according to the invention the visor is preferably arranged to be rotatable between inoperative and operative positions about a further axis of rotation. In such an arrangement the axis and the further axis preferably intersect orthogonally at the centre of curvature. The invention finds particular application in a binocular helmet mounted system. Accordingly there are preferably two optical systems, one for each eye of the helmet wearer. By arranging for the two optical systems to be mounted for simultaneous rotational movement in opposite senses about the axis thereby to vary a spacing between each eye position to accommodate helmet wearers of different interpupillary dimensions. In one particular display system according to the invention each optical system comprises a relay lens arrangement incorporating a fold mirror and is arranged to direct rays of light from the display source downwardly onto a forward facing mirror surface mounted above the face aperture of the helmet, which serves to reflect rays from the relay lens arrangement forwardly and downwardly onto the visor. Where two optical systems of this particular kind are used in a binocular display system, a single forward facing mirror surface is preferably common to both optical systems used. In a display system according to the invention the components of each optical system, the forward facing mirror surface and the visor are preferably mounted on a frame mounted on the helmet which serves to hold the components and visor in the required positions relative to one another against flexure of the helmet. Preferably the frame is also designed so as to compensate for thermal expansion of at least one component of the optical system. The frame may fit around a face aperture of the helmet, the frame may be mounted on the helmet by means of a single spigot arrangement and may be secured within a helmet casing member on which the frame is mounted. One helmet mounted display system in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic view of the system from the front; Figure 2 is a diagram illustrating the light ray paths in an optical system of the display system; Figure 3 is a perspective view of a frame member which supports various components of the display system; and Figures 4A, 4B and 4C are side, rear and front views of the frame member. The display system is a binocular system and utilises two display systems according to the invention, one of each eye of a user of the system. Referring to Figure 2, the display system for each eye comprises a miniature cathode ray tube (CRT) 1 at the screen 3 of which there is produced a real image of the display it is desired to present to the wearer of the helmet (not shown), superimposed on his view through a spherical visor 5 mounted on the helmet. Light rays from the screen 3 pass first through a relay lens arrangement 7 comprising a lens group 9, a plane fold mirror 11, and a lens 13. Light rays exiting the lens 13 are directed in a general rearwards and downwards direction towards a forwards facing plane mirror 15 mounted at a central brow position on the helmet, i.e. centrally above the helmet face aperture. The mirror 15 is disposed in a generally vertical plane so as to reflect the light rays forwards and downwards, toward a region of the internal, concavely curved surface of the visor 5, for reflection thereat to the left or right eye position 17 of the helmet wearer. The lens arrangement 7 and lens 13 are positioned and designed to produce a real image of the display on the screen 3 at the principal wavefront 19 of the concave reflecting surface constituted by the internal surface of the visor 5, which image contains equal and opposite optical aberrations to those produced by subsequent reflection at the visor 5. Due to the close proximity of the wavefront 19 to the eye position 17, the helmet wearer is provided at each eye with a large instantaneous field of view of a collimated virtual image of the display on the screen 3, superimposed on the forward scene viewed through the visor 5. As is apparent from Figure 2, the optical axis of the optical system of Figure 2 lies in a plane. This plane is arranged to contain the centre of curvature X of the visor 5. As a result, whilst the light rays reflected at visor 5 are subject to off-axis aberration in the plane of the optical axis, they are on-axis in planes orthogonal to the optical axis plane. It will be appreciated that whilst depicted as geometrically flat in Figure 2, the plane is in fact folded by the mirror 11. The purpose of the mirror 11 is to allow the components of the system, more particularly the lens group 9 and CRT 1, to be positioned closely around the helmet wearer's head. Referring particularly to Figure 1, the planes 21A and 21B of the optical axes of the optical systems for the helmet wearer's left and right eyes, the components of which systems are differentiated by suffixes A and B respectively, intersect along an axis P passing through the centre of curvature X of the visor 5. Hence, as shown in Figure 1, the exit pupils 23A, 23B of the two systems lie on a circle 25 centred on the axis P. To allow for variation of the distance D between the centres of the exit pupils 23A and 23B, the planes 21A, 21B are arranged to be rotatable about the axis P. Hence, by moving the planes 21A, 21B relative to one another i.e. with a scissors action, the distance D can be varied between a maximum value D Max and a minimum value D Min to suit the interpupillary distance of the helmet wearer. By virtue of the rotational symmetry of the visor 5, and by arranging for the two optical systems to share a common brow mirror 15 at a fixed position, i.e. aligned with the axis P along which the optical planes 21A and 21B intersect, it is only necessary to arrange for the components 1 and 7 of each optical system to be rotatable about the axis P to obtain the required rotation of the planes 21A and 21B for variation of the distance D. As illustrated in Figure 1, the variation of distance D is accompanied by a variation of the vertical positions of the exit pupils 23A, 23B. This variation is suitably compensated by using liners of different thickness in the helmet to adjust the vertical position of the helmet relative to the wearer's eyes. As further described below, the distance D is arranged to be variable through several presettable different positions, e.g. seven, as illustrated in Figure 1. The exit pupils 23A, 23B in adjacent positions are arranged to overlap, and the exit pupils 23A, 23B are made sufficiently larger than a user's eye pupil to allow for small movements of the helmet relative to a wearer's head. To correct for rotation of the images of the display presented to the helmet wearer as the planes 21A and 21B are rotated relatively, the images of the display on the CRT screens 3A and 3B are rotated electronically between the different presettable positions. Electronic correction of other undesired minor differences in the display in the different presettable positions, e.g. in boresight alignment between the different positions, may also be effected electronically. The required electronic corrections may be determined empirically and stored in a look-up table. It will be appreciated that for satisfactory operation of the display system the relative positions of the various components of the system have to be accurately maintained against helmet flexures arising from vibration, acceleration, donning and doffing of the helmet etc. and against temperature variations. This problem arises particularly from the fact that visors capable of surviving ejection windblast loading are made from materials such as polycarbonate which exhibit low stiffness and a high thermal coefficient of expansion. To maintain their positions the various components of the system are mounted on a frame member 27 (see Figures 3 and 4) which is, in turn, secured to the outside of the shell (not shown) of the helmet around the face aperture, as further described below. The frame member 27 is made of a rigid material which is capable of holding the optical components in their required relative positions against vibration etc. and is designed to have thermal expansion characteristics which compensate for the thermal expansion of the visor 5. A suitable material is a hybrid composite of carbon, aramid and glass fibre bound in an epoxy resin, and the member 27 is suitably constructed of laminated sections, at orientations selected and arranged to give the required thermal and mechanical performance. The frame member 27 provides, on its rear side, three kinematic locating surfaces 29A or 29B for the CRT and relay lens arrangements 1A, 7A or 1B, 7B of each optical system, which components 1 and 7 of each system constitute a unit housed in a casing (not shown). The different positions of each such unit 1, 7 are defined by slots at different positions on the unit casing which co-operate with a lug 31A or 31B on the frame member 27. The brow mirror 15 is also mounted on the frame member 27, on its forward side. To this end the mirror 15 is accurately pre-positioned on a frame 33 which is secured accurately in position on the frame member 27 at three fixing points 35. The visor 5 is mounted on two pivots 37 on the frame member 27 positioned so that the axis of rotation of the visor 5 between operative and inoperative positions passes through the point X, typically orthogonally to the axis P. In consequence, and by virtue of the rotational symmetry of the visor 5, the need for the operative position of the visor 5 to be accurately defined by stops 41 is avoided. Positional accuracy of the optical component mounting surfaces and points on the frame member 27 is achieved by first manufacturing the frame member 27 per se, and ageing. Once stability has been confirmed by process control and measurement etc. the mounting surfaces 29 are bonded with an adhesive into position using a precise jig to hold them in position whilst the adhesive cures. The frame member 27 is mounted on the helmet shell by way of a single spigot (not shown) secured to the helmet shell at a position just above the brow area which fits into an aperture 39 in the member 27. The frame member 27 is also embedded in a so-called helmet module, i.e. a casing fitting around the helmet shell and member 27, the helmet module being in turn secured to the helmet shell by special purpose latches at positions aft of the helmet wearer's ear positions. This arrangement structurally isolates the frame member 27 from helmet shell and module flexures, but fully arrests the helmet module, and hence the frame member 27, in the event of windblast or frontal impact. It will be understood that whilst in the embodiment of the invention described above by way of example the visor 5 is of overall spherical form, in other ambodiments of the invention only a region of the visor on a line of sight of the helmet wearer through the visor is spherical, eg the region through which the helmet wearer views the forward scene.

Claims: A display system comprising a helmet carrying a visor (5) having a transparent spherical region with a centre of curvature (X), and an optical system (7) which is adjustably mounted to the helmet and is arranged to project an image produced by a display source (1) onto the spherical region of the visor (5) to provide, by reflection at a concavely curved surface of the visor (5), a collimated virtual image at a helmet wearer's eye position (17), the system allowing adjustment of the position of the virtual image to coincide with the wearer's eye position characterised in that the adjustable mounting permits rotation of the optical system (7) about an axis (P) which passes through the centre of curvature (X). A display system, according to Claim 1, wherein the optical system (7) has an optical axis (21A or 21B) lying in a plane which contains the axis (P). A display system, according to Claim 1 or Claim 2, wherein the visor (5) is arranged to be rotatable between inoperative and operative positions about a further axis of rotation (37). A display system, according to Claim 3, wherein the axis (P) and the further axis (37) intersect orthogonally at the centre of curvature (X). A display system, according to any preceding claim, wherein the optical system (7) comprises a relay lens arrangement (9, 13) incorporating a fold mirror (11) and is arranged to direct rays of light from the display source (1) downwardly onto a forward facing mirror surface (15) mounted above a face aperture of the helmet which serves to reflect rays from the relay lens arrangement (9, 13) forwardly and downwardly onto the visor (5), A display system, according to any preceding claim, having two optical systems (7), one for each eye of the helmet wearer. A display system, according to Claim 6, arranged so that the two optical systems are mounted for simultaneous rotational movement in opposite senses about the axis (P) thereby to vary a spacing (D) between each eye position. A display system, according to Claim 6 or Claim 7 when dependent on Claim 5, wherein a single forward facing mirror surface (15) is common to both optical systems used. A display system, according to any of Claims 5 to 8, wherein the components of each optical system (7), the forward facing mirror surface (15) and the visor (5) are mounted on a frame (27) mounted on the helmet which serves to hold the components and visor (5) in the required positions relative to one another against flexure of the helmet. A display system, according to Claim 9, wherein the frame (27) is designed so as to compensate for thermal expansion of at least one component of the optical system (7). A display system, according to Claim 9 or Claim 10, wherein the frame (27) fits around a face aperture of the helmet. A display system, according to Claim 11, wherein the frame (27) is mounted on the helmet by means of a single spigot arrangement (39). A display system, according to Claim 11 or Claim 12, wherein the frame (27) is secured within a helmet casing member which is in turn attached to a helmet shell member on which the frame (27) is mounted.

Inventors: FREEMAN JONATHAN PAUL, INGLETON MARTYN, STAVELEY CHRISTOPHER BARRY, FREEMAN, JONATHAN PAUL, INGLETON, MARTYN, STAVELEY, CHRISTOPHER BARRY
Applicants: MARCONI GEC LTD, GEC-MARCONI LIMITED
Application year: 1993
Main classification: G02B27
IPC codes: A42B3, G02B27