Display system

Abstract: A helmet mounted display system comprising an eyepiece (3) through which a wearer of the helmet directly views a forward scene (13). The eyepiece also serves to combine an intensified image of the forward scene with the wearer's direct view of the scene. The intensified image is produced by an optical arrangement including an image intensifier (6) from which light is passed to the eyepiece via a prism means (2). The prism means serves to compensate for orientation of the intensified image in other parts of the optical arrangement so that the intensified image is in the same orientation as the wearer's direct view of the forward scene. In addition the prism means allows the various components in the system light path to conform to the profile of the helmet thereby making the helmet more comfortable for a wearer.

Description: This invention relates to a display system, more particularly, to a helmet mounted display system of the kind which provides a wearer of the helmet with a direct view of a scene forward the helmet wearer on which is superimposed an intensified image of said scene, e.g. for night vision. In such a system it is desirable for the components of the system to be relatively positioned so as to distribute the weight of the system around the helmet in a balanced manner, and thereby make the helmet comfortable in use for the wearer. This is difficult to achieve whilst at the same time obtaining an intensified image of the forward scene which is in the same orientation as the wearer's direct view of the forward scene. From EP-A-0 366 467, EP-A-0 077 193 and EP-A-0 380 035 it is known for a helmet mounted display system to comprise an eyepiece through which a wearer of the helmet can directly view a forward scene, and an optical arrangement arranged to provide an intensified image of the forward scene, the system being arranged so that the eyepiece combines the intensified image of the forward scene with the wearer's direct view of the scene, the system further comprising a prism arranged to compensate for the orientation of the intensified image so that the orientation of the intensified image viewed through the eyepiece is the same as the orientation of the direct view of the forward scene. Known arrangements typically use a so-called Peckham prism which has five internal faces at which reflection of light takes place, and consequently is bulky and has an inconveniently long equivalent length. It is an object of the present invention to provide a display system wherein the components of the system are relatively positioned to distribute the weight of the system around the helmet in a balanced manner and at the same time to obtain an intensified image of the forward scene in the same orientation as the wearer's direct view of the forward scene. According to the present invention a known helmet mounted display system further comprises a relay lens group and a mirror arranged to direct light representing the intensified image to the eyepiece along a light path substantially conforming to a profile of the helmet and the prism is arranged separate from the eyepiece and is positioned in the light path between the optical arrangement and the relay lens group to distribute the weight of the optical components on the helmet. The prism may comprise a first surface through which the light representing the intensified image passes to be incident on a second surface at which the light is reflected towards a third surface that is co-planar with the first surface to direct the light by internal reflection towards a fourth surface through which the light exits the prism. Preferably the light representing the intensified image may pass to the first surface via a wedge portion adjacent to the prism. The optical arrangement may comprise an image intensifier arranged to view the scene through an objective lens group including an objective lens, and to transmit the intensified image through a second lens group to the first surface of the prism. The objective lens group may incorporate a prism to turn the optical path. This objective lens group may include lenses before and after the prism. The second surface may be partially reflective and the system may include a display channel for introducing light representing a further display into the prism through the second surface for combination with the light representing the intensified image. Preferably, the light representing the further display passes to the second surface through a wedge portion adjacent to the prism. The light representing the further display may pass to the prism through a further prism to turn the optical path between a source of the light representing the further display and the second surface. The system may include further lenses on either side of the further prism. The eyepiece may incorporate an internal curved surface which possesses both light transmitting and reflecting properties and serves as a collimating reflector for the light representing the intensified image. The internal curved surface may lie between flat parallel fore and aft faces of the eyepiece and is concavely curved towards the aft face, the light representing the intensified image being directed towards the eyepiece through a face for internal reflection at the aft face forwardly onto the curved surface from which it is reflected rearwardly through the aft face and to the user. In order that the invention may be fully understood, one example of a display system in accordance therewith will now be described with reference to the accompanying drawings in which:- Figure 1 is a side view of the system; Figure 2 is a top view of the system; Figure 3 is a view of the system in the direction of the arrow A of Figure 1; Figure 4 is a view of the system in the direction of the arrow B in Figure 3; and Figure 5 is an optical schematic view of the system showing the arrangements of the lenses and prisms and indicating the light path therein. Referring now to the drawings, confornity of the display system to the general helmet profile 29 (Figure 2) is achieved by a number of reflections internally of the system to steer the optical path in the desired direction. The number and angular relationship of the reflections are critical to ensure that an intensified image of a night scene forward of the user, as viewed by an objective lens 1, is synchronised with a user's direct view of the scene through an eyepiece 3. The opto-mechanical arrangement to achieve this orientation synchronisation is arranged to provide an even number of reflections, six, which have sufficient separation and size to accommodate a lens system necessary to achieve optical performance. The enabling component is a 2-reflection prism means 2 shown schematically in Figure 5, hereinafter referred to as the de-rotation prism. Compound reflections between the input and exit surfaces 17 and 18 of a main portion 2A of the de-rotation prism 2 provide the necessary compensating image rotation to ensure that the intensified forward scene image regarded by the user is in the same orientation as the directly viewed scene through the eyepiece 3. The reflection surface 18 of the de-rotation prism 2 is optically coated so as to be partially reflective, thus allowing an introduction of additional display information into the intensified forward scene image display channel. Selection of the position and size of the de-rotation prism 2 allows sufficient separation and a clear optical diameter for the eyepiece 3, a main relay lens group 4, an intensified image relay group 5, an image intensifier 6 and an additional display information channel 7, keeping within the physical constraints of the 3-dimensional system. An objective lens group 8 of which the objective lens 1 constitutes the first element couples to the input of the image intensifier 6 and completes the 3-dimensional optical system. On-axis spherical lenses (not shown) constitute the active optical elements within the objective lens assembly 8. These lenses are grouped into two areas, one before and one after a fold prism 9. This prism 9 causes the optical path to turn through approximately 90°, the folding reflection being made by surface 20. A bright display of the forward night scene, provided by the image intensifier 6, is viewed by the user via the combination of the eyepiece 3, the relay lens groups 4 and 5 and the de-rotation prism 2, light exiting the lens group 4 being directed into the eyepiece 3 by a folding reflection at a mirror surface 15. The eyepiece 3 comprises two plastic prismatic elements 10 and 11 which mate at a common off-axis spherical surface 12. The off-axis surface 12 has a partially reflective coating to allow the user to view the forward outside world scene 13 through parallel planar aft and fore external faces 21, 22 and internal surface 12 of the eyepiece 3, and the images generated by either or both the image intensifier 6 and the additional display channel 7 by reflection at surface 12. The surface 12 is concavely curved towards the aft external face 21 so that the images viewed by the user are collimated. The functions of the eyepiece 3 and other elements of the system will now be described, as is conventional, working from a user's eye 14 back to the display sources. From the user's eye 14 light is reflected back from the off-axis spherical surface 12 to the aft surface 21 of the rear prism 11 of the eyepiece 3. This surface 21 is arranged at a sufficiently shallow angle to ensure the light is again reflected by total internal reflection (TIR) through an input face 30 of the prism 11 of the eyepiece 3 to the planar mirror surface 15. An intermediate image plane 16 of distorted nature is formed before the mirror 15; the distortion being a result of the off-axis spherical reflection at surface 12. Light couple between the intermediate image plane 16 and the two display sources 6 and 7 is achieved by the relay lens group 4 and the de-rotation prism 2. The relay group 4 serves partially to correct the eyepiece abberations. Light then passes to the de-rotation prism 2 via exit face 28 and meets a surface 24 of portion 2A of the prism 2 at a shallow angle to cause a reflection by TIR towards the partially reflective surface 18, the surface 24 being coplanar with surface 17 in the embodiment described. At the surface 18 the two displays are combined according to the ratio of reflection to transmission. Depending on which display is considered, the final eyepiece intermediate image correction is achieved by lens group 5 or in additional display channel 7, the relayed image planes being formed at the image intensifier 6 or at additional display source 19, illustrated in Figure 5 as the screen of a cathode ray tube 23. Surfaces 17 and 18 are coupled to lens groups 5 and 7 via respective wedge potions 2B and 2C of de-rotation prism 2, the input faces of wedge portions 2B and 2C being parallel to the output faces of lens group 5 and channel 7 respectively. In addition to cathode ray tube 23, the channel 7 includes a pathfolding prism 25 and lens groups 26 and 27 on either side of prism 25. It will be appreciated that the user at 14 is able to see not only the scene ahead 13 but also to have it intensified by the image intensifier 6, which is particularly useful for night time vision. In addition to this the additional display source 19 enables further information simultaneously to be fed through the de-rotation prism 2 so that it can be viewed together with the night scene so as to further inform the user. The complex faceted prism means 2 between image intensifier 6 and eyepiece 3 provides compensating image rotation with high transmission efficiency, short optical path length and a large clear diameter together with the ability to combine displays. The system is able to be mounted on a helmet so as to conform to the profile of the helmet as shown in Figure 2.

Claims: A helmet mounted display system comprising, an eyepiece (3) through which a wearer (14) of the helmet can directly view a forward scene (13), and an optical arrangement (5, 6, 8) arranged to provide an intensified image of the forward scene (13), the system being arranged so that the eyepiece combines the intensified image of the forward scene (13) with the wearer's direct view of the scene (13), the system further comprising a prism (2) arranged to compensate for the orientation of the intensified image so that the orientation of the intensified image viewed through the eyepiece (3) is the same as the orientation of the direct view of the forward scene, characterised in that a relay lens group (4) and a mirror (15) are arranged to direct light representing the intensified image to the eyepiece (3) along a light path substantially conforming to a profile (29) of the helmet, and the prism (2) is arranged separate from the eyepiece (3) and is positioned in the light path between the optical arrangement (5, 6, 8) and the relay lens group (4) to distribute the weight of the optical components on the helmet. A system, according to Claim 1, wherein the prism (2) comprises a first surface (17) through which the light representing the intensified image passes to be incident on a second surface (18) at which the light is reflected towards a third surface (24) that is co-planar with the first surface (17) to direct the light by internal reflection towards a fourth surface (28) through which the light exits the prism (2). A system, according to Claim 2, wherein the light representing the intensified image passes to the first surface (17) through a wedge portion (2B) adjacent to the prism (2). A system, according to Claim 2 or 3, wherein the optical arrangement (5,6,8) comprises an image intensifier (6) arranged to view the scene (13) through an objective lens group (8) including an objective lens (1), and to transmit the intensified image through a second lens group (5) to the first surface (17) ofthe prism (2). A system, according to Claim 4, wherein the objective lens group (8) incorporates a prism (9) to turn the optical path. A system, according to Claim 5, wherein the objective lens group (8) includes lenses before and after the prism (9). A system, according to any of Claims 2 to 6, wherein the second surface (18) is partially reflective and the system includes a display channel (7) for introducing light representing a further display into the prism (2) through the second surface (18) for combination with the light representing the intensified image. A system, according to Claim 7, wherein the light representing the further display passes to the second surface (18) through a wedge portion (2C) adjacent to the prism (2). A system, according to Claims 7 and 8, wherein the light representing the further display passes to the prism (2) through a second prism (25) to turn the optical path between a source (19) of the light representing the further display and the second surface (18). A system, according to Claim 9, including further lenses (26, 27) on either side of the second prism (25). A system, according to any preceding claim, wherein the eyepiece (3) incorporates an internal curved surface (12) which possesses both light transmitting and reflecting properties and serves as a collimating reflector for the light representing the intensified image. A system, according to Claim 11, wherein the internal curved surface (12) lies between flat parallel fore and aft faces (22, 21) of the eyepiece (3) and is concavely curved towards the aft face (21), the light representing the intensified image being directed towards the eyepiece (3) through a face (30) for internal reflection at the aft face (21) forwardly onto the curved surface (12) from which it is reflected rearwardly through the aft face (21) and to the user (14).

Inventors: INGLETON MARTIN, INGLETON, MARTIN
Applicants: MARCONI ELECTRONIC SYST LTD, MARCONI ELECTRONIC SYSTEMS LIMITED
Application year: 1993
Main classification: G02B27
IPC codes: G02B27, G02B5