Helmet mounted display systems

Abstract: A head-up display comprises a headgear mounted visor (12) having a viewing area (54) which, when the headgear (38) is being worn is in, or is movable to be in, the line of sight of the wearer, and image generating means (42, 44) arranged to direct light first to a parabolic mirror (48), then to first (50) and second (52) fold mirrors, hand then to the viewing area (54) and into an eye (56) of the wearer. Such an arrangement enables the size of the visor to be smaller than those of the prior art.

Description: This invention relates in general to head mounted displays which produce images visible to the wearer of the display, and, more particularly, to a helmet mounted display that operates in conjunction with a partially reflective visor to superimpose the display on an exterior scene that is being viewed by the wearer. Helmet mounted display apparatus finds utility in many areas, such as training, machine control or entertainment. In recent years, such devices have found particular utility in military applications where information must be supplied to vehicle operators and weapons controllers. Aircraft pilots find such devices useful to furnish information relating to aircraft operation, radar presentations, maps, weapons aiming and other things that would be useful or desirable in their mission. The images, which are superimposed on a surface in the wearer's line of sight, can be derived from a variety of sources including cathode ray tubes (CRTs) that can be mounted in a helmet. Prior art devices have utilized fibre optics displays, flat screen liquid crystal devices with image enhancers, and even photographic projectors operating with an appropriate optical relay system, all of which might supply the wearer with an informational image. Helmet mounted systems for displaying images are well known in the art. US-A-3059519 shows a helmet mounted display in which a CRT display is directed through an optical system that included a plurality of reflecting mirrors which ultimately places the image in the eyes of the observer, slightly above the normal line of sight. However, an alternative embodiment includes a partially reflective mirror in combination with a parabolic reflector. Another alternative embodiment replaces one of the mirrors with a semi-reflective mirror so that the pilot's view would not be completely obstructed by the helmet and mirrors. U.S.-A-3787109 discloses a display where the helmet is provided with a paraboloidal visor with a partially reflective coating on the inner surface as the primary optical element in transmitting the image from the generating device to the eye of the observer. Such an innovation eliminates the need for separate combiner screens or beam splitters near the eyes. A more complex optical path that still utilizes a visor is disclosed in U.S.-A-3923370. In this patent, limited areas of the inner surface of the visor are coated to be partially or totally reflective. An image generating system mounted at the upper portion of the helmet projects an image to a first reflective area which is high on the visor. The image is relayed to a planar mirror mounted on the front of the helmet and from where to a second reflective area which is just above the normal line of sight of the observer. As a result, all of the reflective elements are deemed to be substantially out of the field of view of the helmet wearer. A similar optical arrangement is also shown in U.S.-A-4465347. However, the image which is observed in the disclosed system is derived from a telescopic optical system which projects the image on the inner surface of the visor. The image is reflected to a planar mirror on the helmet and then to the visor again where it is directed into the eye of the viewer and appears to be a part of the exterior scene. A substantially more complex helmet display system is disclosed in U.S.-A-4361384. That device combines a thin film flat panel display and a wafer diode image intensifier tube. A plurality of semi-transparent optical elements direct the image from the flat panel display into the eye of the observer. The image intensifier, using some of the same optics, also directs an image into the eye where it can be super-imposed over the images received from the exterior scene. The wearer thus views the exterior scene through both the visor and the flat panel display images. The visor, for reflecting purposes, can be holographic, toric, aspherical or spheric. A toric combination is disclosed in U.S.-A-4026641. The toric reflector does not appear to be a part of a visor, but rather, appears to be adapted to be mounted on the helmet. A rather straightforward helmet mounted system is disclosed in U.S.-A-4153913, which utilizes a CRT in combination with a semi-transparent combiner screen. The patent suggests that the combiner plate be worn as goggles or spectacles. U.S.-A-4081209 utilizes a more complex optical system in combination with a spherical, semi-reflective combiner surface which may be part of a visor. A prism transmits the image from the source and directs it to the combiner surface which returns the image to the eye of the observer. A refractive interface at an inclined plane with respect to the image source compensates for distortion of the image due to different optical path lengths of different rays. As will be seen, most prior art helmet visors are surfaces of revolution, usually paraboloids which results in a rather bulky visor. Such a design has been necessitated by the requirements of tooling for the moulding or forming of such visors. A modification to the design of the prior art paraboloidal visor is described in U.S.-A-3870405 which provides for a visor whose inner and outer surfaces are sections of cofocal paraboloids of revolution. Because earlier paraboloidal visors were of substantially constant thickness, images reflected off of the inner and outer surfaces did not have the same focal point and would therefore result in "ghost" images. The improvement of this patent results in both images being collimated with no angular difference in the pointing direction, and, as a result, the images appeared to be superimposed on the exterior scene with no loss in sharpness or clarity to the viewer. A problem with each of the prior art references noted above and presently available helmet mounted display devices has been their size and bulk. Because the helmets member of the helmet mounted display system must be worn by an individual with many complex tasks to perform, usually under great stress and with little spare time in which to accomplish the tasks, the helmet should be as light and compact as possible while still providing superior optical performance. Specifically, bulky and large helmets cause their wearers to tire rapidly, thus shortening the effectiveness of the wearer. Similarly, poor images cause miscalculation as well as hinder the wearer's decision and reaction capabilities. Consider, for example, the system disclosed in US-A-3923370, mentioned above. The system includes a pair of visor mounted parabolic mirror sections working with a helmet mounted planar mirror to achieve the proper projection of the optical image into the observer's eye. However, because the parabolic mirror sections are located high on the visor for optical symmetry, the resultant helmet-visor combination tends to be rather large and bulky, requiring a substantial volume to accommodate the optical display system. To solve the problem of bulk and weight, the present invention envisages that by replacing the single, helmet-mounted planar mirror, which is positioned on the front of the helmet above the wearer's eyes and substantially midway between the parabolic mirrors, with a pair of "fold" mirrors which rotate the images to be displayed to maintain a symmetric, although folded, arrangement of the parabolic mirror surfaces, the volume and size of the helmet and visor can be substantially reduced with improved optical performance and less physical demand on the helmet wearer. The applicant's invention, as defined in Claim 1, attempts to overcome the disadvantages of the prior art by providing a display apparatus for use with headgear including, in combination, visor means which are adapted to attach to the headgear, and have at least one concave semi-reflecting curved surface directed inwardly of the headgear. When the headgear is being worn, the visor means is at, or is movable to, a position at which it intercepts the forward line-of-sight of an eye of the wearer. The visor means is also further characterised in that the concave semi-reflecting surface of the visor means is part of a paraboloid having an axis of rotation. Above the level of the wearer's eyes, there is provided an optical projector by means of which luminous data is directed at the semi-reflecting surface of the visor means for reflection thereby to the wearer's eye so that the wearer is presented with a virtual image of the luminous data within a predetermined field of view centred on the wearer's forward line-of-sight. The optical projector is adapted to be attached to the headgear. Parabolic mirror means are interposed in the optical path from the optical projector to the semi-reflecting surface of the visor means. The shape, dimensions and position of the parabolic mirror means are such that rays from the optical projector are reflected in the parabolic mirror means to a fold mirror means. While the fold mirror means of the present invention can include more than two fold mirrors to reflect rays received from the parabolic mirror means to the semi-reflecting surface of the visor means, for simplicity of example, the fold mirror means described herein includes only first and second fold mirrors. The first fold mirror has a shape, dimension and position that enable it to receive the rays from the parabolic mirror means and to direct the rays to the second fold mirror. The second fold mirror, in turn, has a shape, dimension and position that enable it to direct rays received from the first fold mirror to the semi-reflecting surface of the visor means. According to the invention, there is provided a head-up display comprising a headgear mounted visor having a viewing area, which viewing area, when the headgear is being worn, is in, or is movable to be in, the line of sight of the wearer, image generating means, a parabolic mirror and a first fold mirror, the image generating means being arranged to direct light first to the parabolic mirror, then to the first fold mirror, and then to the viewing area and into an eye of the wearer when the viewing area is in the line of sight of the wearer, characterised in that a second fold mirror is arranged in the optical path between the first fold mirror and the viewing area, in that the image generating means is mounted to one side of the headgear not substantially above the level of the wearer's eyes and in that light from the image generating means is arranged to pass to the parabolic mirror along a path substantially adjacent the wearer's temple. The invention may be carried into practice in various ways, and one specific embodiment will now be described, with reference to the drawings, in which: Figure 1 is a frontal view of a typical helmet-mounted display system of the prior art; Figure 2 is a side section view of a helmet mounted display system according to an embodiment of the present invention; and Figure 3 is an idealised front view of the helmet mounted display system of Figure 2. In order better to understand the preferred embodiment of the invention described below, it is useful first to examine in general a typial prior art method and device. Turning first to Figure 1, there is shown a typical prior art helmet mounted display such as is disclosed in Figure 3 of the above identified patent US-A-3923370. A helmet 10 has a transparent visor 12 mounted thereon. A cathode ray tube (CRT) 14 and associated projection optics 16 are mounted on the left side of helmet 10 by means which are not shown. A field lens 18 is mounted so as to transmit the images from the CRT 14 to a first mirror 20. The optical path of the images transmitted by the CRT 14 and associated projection optics 16 can be represented by a central ray 22 which reflects from the first mirror 20, through a projection lens system 24, to a second mirror 26. The projected images are reflected by the second mirror 26 to a reflective area 28 on the paraboloidal inner surface of the visor 12. The projected images are then reflected back to a plane mirror 30 mounted on the front of the helmet 10. The reflective area 28 is part of a paraboloidal surface with an axis 32 that is just below the mirror 30 and is perpendicular to its plane. The aperture of the mirror 30 is centred significantly above the focal point of the paraboloid forming the paraboloidal surface of the visor 12. Light striking the mirror 30 is reflected to a second area 34 on the surface of the visor 12. The second area 34 is treated to be partially reflective. While the reflective mirror areas 28, 34 are shown as squares, their shapes are not critical and in some embodiments are generally somewhat rectangular. This prior art arrangement has been designed to keep the mirror 30 and the reflective area 28 well above the observer's line of sight and preferably outside of his range of vision. Further, this apparatus is intentionally arranged to be non-symmetrical with respect to the axis 32 of the paraboloid. Further, the reflective area 34 is positioned to be slightly above the eye 36 of the observer. Recent experiments, however, have determined that in some applications, it is desirable to have a reflective area positioned directly in the line-of-sight of the observer. Further, considering the space limitations frequently found in the cockpits of military vehicles, including aircraft, where such helmet mounted displays are most useful, tests have shown that helmets and visors such as those of the prior art described above require headgear and visors that were higher and bulkier, that is, require greater volume and weight, than was comfortable or convenient for the average wearer. Figures 2 and 3, show in side and frontal view, an improved helmet mounted display and helmet visor combination embodying the present invention. A helmet 38 has a visor 40 with a generally curved shape. A cathode ray tube (CRT) 42 with associated optics 44 is mounted on the upper side 46 of the helmet 38. A first parabolic mirror 48 may be integral with the visor 40 or may be attached thereto. In the preferred embodiment, the first parabolic mirror 48 can be a local parabolic section of the visor 40 that has been treated with a coating to be either fully or partially reflective to the images projected from the CRT 42 and associated projection optics 44, since it is substantially out of the field of view and line-of-sight of the wearer of the helmet 38. A pair of "fold" mirrors 50, 52 are positioned adjacent the first parabolic mirror 48 and act to redirect the image transmitted by the CRT 42 and associated projection optics 44 to a parabolic segment 54 of the visor 40 which is treated with a coating to be reflective. The parabolic visor segment 54 is of an appropriate shape, curvature, dimension and position to direct images transmitted by the CRT 42 and associated projection optics 44 to the eye 56 of the wearer of the helmet 38. The spatial locations of the parabolic mirror 48 and the fold mirrors 50, 52, are optimized to accomplish a number of goals. First, the locations are chosen to minimize the volume of the helmet 10 by following the contour of the head with the optical path from the CRT 42 to the parabolic segment 54 of the visor 40. Second, the locations are chosen to reduce the overall height of the centre of gravity of the associated projection optics 44. The optical path from the CRT 42 to the parabolic visor segment 54 is routed to the side of the head of the helmet wearer, for example past the temple, and downward to the rear of the helmet wearer. For high-gravity manoeuverability on tactical aircraft, it is preferred to have an aft centre of gravity for the associated projection optics 44. Third, the locations are chosen to collimate properly the light rays carrying the images generated by the CRT. Parallel light rays entering into the parabolic mirror 48 then can exit as parallel light rays from the fold mirrors 50, 52. Conventional technology may be employed to produce a generally paraboloidal visor 40 which may be treated to be partially reflective in selected, desired areas, in accordance with the teachings of the co-pending European Application 87309476.7, filed 27th October, 1987 (EP-A-0 269 259, published on 01/06/88). The visor may also be specially configured so that the correct paraboloidal shape for reflective purposes can be located in only those areas in which reflection is necessary, and that any other desired shape or curvature can be employed for the remainder of the visor. In general, the visor viewing area should be parabolic in shape and located in the line of sight of the helmet wearer, either centred or displaced. Nevertheless, the invention could also include embodiments that have other aspherical shapes than parabolic for optical correction purposes. For example, one possibility is a spherical visor with a buried hologram that contributes power such that the overall effect is that of a parabolic surface at a sharply defined wavelength of interest. Another possibility would be a visor surface that is a higher order asphere; for example, for large fields of view, the symmetric parabolas of the preferred embodiment described herein are inadequate from a performance standpoint, and a more complex surface would be required at large angular distances from the line of sight. While the preferred embodiment described herein includes a CRT as its image generating means, other optical image generating means such as fibre image generators, liquid crystal displays, electrolumin-escence, laser diodes with scanners, etc., may also be used in this invention as optical image generating means. Further, through the use of additional mirrors and/or CRT displays similar to that described above, images can be brought to both eyes of the helmet wearer.

Claims: A head-up display comprising a headgear mounted visor (40) having a viewing area (54), which viewing area, when the headgear (38) is being worn, is in, or is movable to be in, the line of sight of the wearer, image generating means (42,44), a parabolic mirror (48) and a first fold mirror (52), the image generating means (42,44) being arranged to direct light first to the parabolic mirror (48), then to the first fold mirror, and then to the viewing area (54) and into an eye (56) of the wearer when the viewing area (54) is in the line of sight of the wearer, characterised in that a second fold mirror (50) is arranged in the optical path between the first fold mirror (52) and the viewing area, in that the image generating means (42,44) is mounted to one side of the headgear not substantially above the level of the wearer's eye and in that light from the image generating means (42,44) is arranged. to pass to the parabolic mirror (48) along a path substantially adjacent the wearer's temple. A head-up display as claimed in Claim 1 in which the inner surface of the viewing area (54) is substantially paraboloidal. A head-up display as claimed in Claim 1 in which the viewing area (54) includes a tuned holographic filter. A head-up display as claimed in Claim 3 in which the effect of the filter is to cause the inner surface of the viewing area (54) to operate as a parabolic reflecting element. A head-up display as claimed in any one of the preceding claims in which the parabolic mirror (48) is attached to the inner surface of the visor (40). A head-up display as claimed in any one of the preceding claims in which the parabolic mirror (48) is integral with the visor (40). A head-up display as claimed in any one of Claims 1 to 4 in which the parabolic mirror (48) consists of a locally parabolic inner surface of the visor (40) having enhanced reflectivity. A head-up display as claimed in any one of the preceding claims in which at least one of the fold mirrors (50,52) is part of or is attached to a forwardly-facing surface of the headgear (38). A head-up display as claimed in any one of the preceding claims in which at least one of the fold mirrors (50,52) is part of or is attached to the inner surface of the visor (40). A head-up display as claimed in any preceding claim in which the image generating means (42,44) is located on the same side as the eye which views the image generated by the image generating means. A head-up display as claimed in any one of the preceding claims in which the light from the image generating means (42,44) is substantially collimated as it leaves the second fold mirror (50). A head-up display as claimed in any one of the preceding claims in which the light from the image generating means (42,44) is arranged to impinge directly upon the parabolic mirror (48), then upon the first fold mirror (52), then upon the second fold mirror (50), then upon the viewing area (54), without intermediate reflection.

Inventors: EVANS CHARLES DEWAYNE, LARKIN ERIC WALTER, MELZER JAMES EDWARD, TIRUMS ANDREW THEODORE, EVANS, CHARLES DEWAYNE, LARKIN, ERIC WALTER, MELZER, JAMES EDWARD, TIRUMS, ANDREW THEODORE
Applicants: KAISER AEROSPACE & ELECTRONICS, KAISER AEROSPACE & ELECTRONICS CORPORATION
Application year: 1988
Main classification: G02B27
IPC codes: A42B1, G02B27, A42B3