A VISUAL DISPLAY SYSTEM HAVING A LARGE FIELD OF VIEW

Abstract: A visual display system (2) having a large field of view, which visual display system (2) comprises at least one projector (12) which has at least one exit pupil (14) and which in use is positioned remote from a head of a user of the visual display system, optical imaging means (6) for imaging the exit pupil (14) of the projector (12) onto eye pupils of the user (4), tracking means (18) for tracking the head (10) of the user (4), and control means (20) for positioning and orientating the projector (12) such that the exit pupil (14) of the projector (12) remains imaged on the eye pupils of the user (4) as the head (10) of the user (4) moves throughout the field of view and viewing volume.

Description: A VISUAL DISPLAY SYSTEM HAVING A LARGE FIELD OF VIEW This invention relates to a visual display system and, more especially, this invention relates to a visual display system having a large field of view. The visual display system may be used for simulators for use in training, research, leisure or entertainment. Simulators used in training, research, leisure and entertainment are usually required to provide a highly realistic visual representation of the user's environment. Thus, for example, a simulator used in aircraft training is required to provide a highly realistic visual representation of the view from an aircraft cockpit. Irrespective of whether the simulator is used for training, research, leisure or entertainment, the required highly realistic visual representation should ideally cover as large a field of view as the real world original, with image resolution approaching that of the human eye. The visual display system used in the simulators comprises a database of information describing the simulated world, an image generator for converting the database of information into signals suitable for feeding to a display, and the display itself which turns the signals into a form which can be perceived by the human eye/brain. The database is usually stored in digital form, the image generator is usually a powerful graphics computer, and the display may take one of several known forms as described below. One known type of display is such as to surround the user with a screen on to which the image is projected. For field of views with a large angular subtense, both horizontally and vertically, the screen naturally takes the form of a spherical dome or partial dome. If the required field of view is limited in one dimension, for example the restricted field of view of a ship's bridge simulator, then a truncated cone or cylinder may also be appropriate. In order that the image on the screen is not perceived by the user to be too close, the domes tend to have a radius of several meters. Thus the domes are physically large and they give rise to consequent problems of facility space, transport and mass. This is especially so if the domes are placed on a movable platform to simulate motion. The large surface area of the domes results in a dim image because of the limited light output of available projection devices, and the integrating sphere effect severely reduces image contrast. Practical limits to the radius of domes means that the image is still too close to provide correct perspective to two users simultaneously over a large horizontal and vertical field of view. The projectors used to illuminate the screen have a limited number of resolvable pixels. To simultaneously cover a large field of view at adequate resolution requires a number of such projectors. Alternatively, at least one projector can be steerable such that its image is concentrated where it is required at a particular time. The image can follow a target in a scene, the user's head direction, eye direction or some combination. These area of interest displays have an instantaneous field of view within a larger total field of view or what is called field of regard. The area of interest approach addresses the problem of providing good image resolution over a large field of view, but does not remove the disadvantage associated with the dome screen. Another known technique which overcomes the size problems associated with domes is the use of collimation. The image is projected on to a small radius screen and viewed via an optical component, usually a concave mirror, such that the final image is virtual and nominally at optical infinity. The image distance is now considerably greater than the physical extent of the display. The smaller screen radius also reduces screen area for a given field of view, giving a somewhat brighter image. The difficulty of placing the screen in an acceptable position relative to the mirror for good collimation, without it blocking the field of view, means that in practice the field of view of these displays is limited in at least one direction, that is either horizontally or vertically. Both of the dome and the collimated techniques described above require a screen to hold the image. The function of the screen is to scatter light into the viewing volume of the display. The viewing volume is the space around the design eye point from which a quality image is visible. In a flight simulator, for example, the viewing volume should be most or all of the cockpit, or at least that part of the cockpit which the head of the or each pilot in the cockpit can occupy. The screen surface is covered with a light- diffusing coating. An important parameter of this coating is its gain. A perfectly diffusing (Lambertian) coating on a dome screen would have a gain of unity and would scatter light evenly in all directions. It would appear equally bright from any eye point. Higher gain coatings can be used to concentrate the scattered light into the viewing volume. This increases the image brightness as seen from the design eye point and improves image contrast (as less light is scattered into unwanted directions) but tends to degrade uniformity of brightness across the field of view. However, it is not possible to formulate or apply a coating that scatters light only into the viewing volume. Moreover, since in practice the eye point is off the illumination axis of the projector, a minimum scatter is required in order to get enough light into the viewing volume. Thus much light is inevitably wasted, resulting in low image brightness and contrast, for both the dome and the collimated case. A further known technique, which overcomes the problem of display size and portability is to have a helmet mounted display. The helmet mounted display is sometimes referred to as a head mounted display. With a helmet mounted display, one or more usually both of the eyes of a user view a display device through collimating optics mounted on the helmet. The display device may be a small cathode ray tube, a liquid crystal display, or similar. The display device may alternatively be a large projector coupled to the helmet optics by fibre optic bundles. The helmet mounted display is inherently an area of interest display. Since the field of view of the helmet mounted display moves with the head of the user, the helmet mounted display has a very large field of regard. It is also possible to inset a small eye-tracked or centrally fixed high resolution field of view within the main helmet mounted display field of view. If images with the correct disparity are fed to each eye, then a stereoscopic display is possible. In the absence of a large diffusing screen, image brightness and contrast are much improved. However, the helmet becomes non-standard, bulky and heavy. The mass and inertia of the helmet are unrepresentative of the original, thereby producing unrealistic head loading. In small cockpits, the bulk of the helmet mounted display restricts the head movements of the user. A helmet mounted display also puts severe demand on the image generator. More specifically, because the head of the user can move at such high rates and the helmet mounted display is directly coupled to the head of the user, the image is prone to instability due to image generator time lags. This is a more difficult problem than for a head tracking projector that is mounted off the head. In order not to further load the image generator by having it generate a simulated view of the cockpit interior, the helmet mounted display also has to be see-through. The image generator is still required to blank those parts of the image that would otherwise overlay the view of the cockpit, to avoid the appearance of a transparent cockpit . So the helmet mounted display is also prone to some spill of its image onto the cockpit during rapid head movements. In some applications where a helmet is not worn in the real world, for example in the case of civil airline pilots, a helmet mounted display is unacceptable. Major deficiencies of the prior art as described above may therefore be summarised as size/brightness/contrast problems for domes, limited field of view/brightness/contrast problems for collimators, and helmet/image generator problems for helmet mounted displays. It is an aim of the present invention to provide a visual display system able to provide an image with high brightness and contrast over a large field of view. The present invention also aims to provide the image without display hardware on the head of the user. Accordingly, in one non-limiting embodiment of the present invention, there is provided a visual display system having a large field of view, which visual display system comprises at least one projector which has at least one exit pupil and which in use is positioned remote from a head of a user of the visual display system, optical imaging means for imaging the exit pupil of the projector onto eye pupils of the user, tracking means for tracking the head of the user, and control means for positioning. and orientating the projector such that the exit pupil of the projector remains imaged onto the eye pupils of the user as the head o£ the user moves throughout the field of view and viewing volume. The visual display system of the present invention may be one in which the optical imaging means is a concave mirror. Other optical imaging systems may be employed. There may be two pupils per user providing a binocular display. If it is desired that the binocular display is stereoscopic, then each pupil should contain an image with correct disparity. The image of the pupil of the projector may be large enough to cover both eyes of the user. The field of vision may have at least one high resolution inset. The visual display system may comprise a first projector or projectors for a first user of the visual display system, a second projector or projectors for a second user of the visual display system, and a common mirror which is shared by the first projector or projectors and the second projector or projectors. The visual display system may include optical replication means for enlarging the pupil of the projector. The projector may be a multi-channel pupil projector or a pupil-switching projector. The visual display system may include corrector means in front of the projector or the optical imaging means. The visual display system may be apparatus which is such that the image is of moving or static scenes. Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a visual display system having a large field of view, and for use by a single users; Figure 2 shows a visual display system having a large field of view, and for use by two users; Figures 3 and 4 are plan and side views of a visual display system utilising a multi-channel pupil projector; Figure 5 shows a visual display system utilising a pupil-switching projector; Figure 6 shows a visual display system with corrector means in front of a mirror; and Figure 7 shows a visual display system with corrector means in front of a projector. Referring to Figure l, there is shown a visual display system 2 having a large field of view and for use by a single user 4. The user 4 may be, for example, a trainee pilot. The visual display system 2 comprises a large concave mirror 6 positioned in front of the user 4 such that the centre of curvature 8 of the mirror 6 is somewhat above the head 10 of the user . The angular subtense of the mirror 6 at the eye of the user 4 is substantially equal to the total field of view of the display. A movable projector 12 having two exit pupils 14 is positioned with its exit pupils 14 somewhat above the centre of curvature θ of the mirror 6. Relay optics 16 are employed as shown. The mirror 6 has the inherent property that an object near the centre of curvature 8 of the mirror 6 is imaged by the mirror 6 at unit magnification to a substantially symmetrical position on the other side of the centre of curvature 8. If the object is the exit pupil 14 of the projector 12, and the eye of the user 4 is placed at the image of the exit pupil 14, then a bright view of the projected display will be seen. Thus, as will be appreciated from Figure l, the two exit pupils 14 of the projector 12 are imaged by the mirror 6 onto the eyes of the user 4. Tracking means in the form of a head position sensor 18 is provided to measure the position and orientation of the head 10 of the user 4. The tracking means may be a known tracking means using, for example, either magnetic fields, ultrasonics or video cameras. The head 10 wears a helmet 22. As the head 10 of the user 4 moves, either in translation, or in rotation to look around the total field of view, signals from the head position sensor 18 are fed to control means in the form of a servo system 20. The servo system moves the exit pupils 14 of the projector 12 such that the exit pupils 14 remain imaged on to the eyes of the user 4. The effect is of looking into the optics of the projector 12 so the image is very bright. For example, a cathode ray tube projector capable of producing a brightness of a few ftL on a dome screen will have a phosphor brightness of more than 10,000 ftL. When looking into the projector 12, the user sees the phosphor directly, minus any transmission losses in the projection optics. Making reasonable estimates of transmission loss (50%) and Mirror reflection loss (10%) displayed brightness should be greater than 4000 ftL. The above described principle of operation works with any suitable and appropriate kind of projector 12, for example a cathode ray tube projector 12, a liquid crystal display projector 12, or a light valve projector 12. The principle does not depend on a particular source or illumination arrangement in the projector 12. In addition, because the projected light is imaged onto the pupils of the user 4, very little light is wasted in unwanted directions and so image contrast is also high. Since the user 4 has no helmet mounted display optics, the user 4 has a clear view of his or her environment, for example a cockpit. If the projector 12 is focused onto the focal surface of the mirror 6, then the image seen by the user 4 will be collimated. The display is inherently of the area of interest type. Previously developed area of interest display techniques can be applied. Thus, for example, the projector 12 may have a high resolution area inset within the main instantaneous head tracked field of view. Alternatively, the main field of view may have variable acuity, the resolution falling off away from the centre in a way that mimics eye resolution fall off. Referring now to Figure 2, there is shown a visual display system 24 which is for use by two users 4 having heads 10. The heads 10 of the users 4 are symmetrically disposed about the centre of curvature 8 with their respective projectors 12. For clarity of illustration, the mirror 6 has been omitted. The users 4 share a common mirror 6 and the principle of operation of the visual display system 16 is the same as described above for the visual display system 2 shown in Figure 1. The significant difference is that in Figure 2, the heads 10 of the users 4 and the projectors 12 must be displaced both horizontally and vertically around the centre of curvature 8. In a third embodiment of the invention (not shown) , the two exit pupils 14 of the projector 12 may contain images with the correct relative disparity. In this case, a stereoscopic display results. A further embodiment of the invention is shown in Figures 3 and 4 in which the projector is a multi¬ channel pupil projector. The projector's field-of- view is covered by a number of channels of projection optics 26, each with its own pupil 28. The exit pupils 28 are optically overlapped by, for example, an arrangement of flat mirrors 30, to form an apparent composite pupil 32. The smaller field-of-view per projector allows the overall field-σf-view to be increased, or alternatively the projection optics to be simpler, or the exit pupil to be larger. The latter is particularly useful for a biocular display, where the exit pupil must be large for its image to cover both eyes of the user simultaneously. Multiple channels also provide an alternative to a high-resolution inset as a way to increase the resolution of the display. A further embodiment of the invention is shown in Figure 5 in which the projector is a pupil-switching projector. In a display with a large field of view and separate projectors for each eye, the exit pupils may get in the way of each other, that is the optics for one pupil may block some of the light emitted by the other. This clash may be avoided by a single projector with a single exit pupil 34, the pupil being sequentially switched between the locations required for the left and right eyes. Pupil switching optics 36 are employed and the switching would take place too fast to be visible to the human eye. A further embodiment of the invention is shown in Figure 6 in which corrector means in front of a main mirror 40 are employed. Aberrations of the main mirror 40, which may limit the field of view or require large exit pupils, may be corrected to some extent by an additional optical element 42 in front of the mirror 40. This may be a thin concave element with a lenticular structure 44 on its surface to re¬ direct rays 46 from the exit pupil 48 that would otherwise miss the eye 50, after reflecting off the mirror 40. A further embodiment of the invention is shown in Figure 7 in which corrector means in front of a projector 52 are employed. Aberrations of a main mirror 54, which may limit the field of view or require large exit pupils, may be corrected to some extent by an additional optical element 56 in front of the projector 52. This may be, for example, a thin cylindrical optical element. It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, the embodiments of the invention described above with reference to Figures 1 and 2 are binocular insofar as they have provided a separate exit pupil and image for each eye. If the exit pupil 14 of the projector 12 is made large enough such that its image encompasses both eyes of the user 4, then a binocular display results. If desired, in order to reduce the load on projector servos, only the latter stages of the projection optics need to be fully mobile, allowing the bulk of the or each projector 12 to be stationary or of a restricted movement. The apparatus of the invention may be such that the optical imaging means images the exit pupil of the projector onto the eye pupils of two or more users. If desired, the visual display system of the invention may utilise a high gain screen. A mirror with a certain amount of surface scatter can be thought of as a very high gain screen. Thus the invention may use a high gain screen in place of a mirror. The scatter at the screen surface effectively enlarges the projector exit pupil, increasing the viewing volume of the display. If the projector is focussed on the equivalent location to the mirror's focal surface, the scatter also blurs the image. This blurring is removed if the projector is re-focussed onto the screen surface. The visual display system no longer simply images the exit pupil or pupils onto the viewer as for the pure reflective case, but the high brightness and contrast advantages of the display are retained. The projector still tracks the viewer's head as it moves, maintaining the high brightness which is lost in conventional displays with high gain screens as soon as the viewer moves away from the design eyepoint. By correcting the picture geometry as the head moves, it is also possible to retain the same parallax as in a collimated display. The visual display system would look like the visual display system shown in Figure 1 except for the mirror 6.

Claims: CLAIMS 1. A visual display system having a large field of view, which visual display system comprises at least one projector which has at least one exit pupil and which in use is positioned remote from a head of a user of the visual display system, optical imaging means for imaging the exit pupil of the projector onto eye pupils of the user, tracking means for tracking the head of the user, and control means for positioning and orientating the projector such that the exit pupil of the projector remains imaged on the eye pupils of the user as the head of the user moves throughout the field of view and viewing volume. 2. A visual display system according to claim 1 in which the optical imaging means is a concave mirror. 3. A visual display system according to claim 1 or claim 2 in which there are two pupils per user for providing a binocular display. 4. A visual display system according to any one of the preceding claims in which the image of the pupil of the projector is large enough to cover both eyes of the user. 5. A visual display system according to any one of the preceding claims in which the field of view has at least one high resolution inset. 6. A visual display system according to any one of the preceding claims and comprising a first projector or projectors for a first user of the visual display system, a second projector or projectors for a second user of the visual display system, and a common mirror which is shared by the first projector or projectors and the second projector or projectors. 7. A visual display system according to any one of the preceding claims and including optical replication means for enlarging the pupil of the projector. 8. A visual display system according to any one of the preceding claims in which the projector is a multi-channel pupil projector. 9. A visual display system according to any one of claims 1-7 in which the projector is a pupil-switching projector. 10. A visual display system according to any one of the preceding claims and including corrector means in front of the projector or the optical imaging means.

Inventors: WYNN OWEN JOHN WILLIAMS, WYNN, OWEN, JOHN, WILLIAMS
Applicants: SEOS DISPLAYS LTD, WYNN OWEN JOHN WILLIAMS, SEOS DISPLAYS LIMITED, WYNN, OWEN, JOHN, WILLIAMS
Application year: 1997
Main classification: G09B9
IPC codes: G09B9