GB2317022A - Display apparatus with pivotable optical system - Google Patents

Abstract

Light from a display screen 10 is split by a crossed beamsplitter device 13 into two separate paths for viewing by a user at left and right exit pupils (22,22',Fig.1), after that light has passed through the beamsplitter device 13 and has been reflected back by a concave mirror 16. The split light in the two paths is magnified and/or collimated by respective optical devices 19,20,21,19',20',21', for viewing at the exit pupils. The optical system comprising the beamsplitter device 13, the concave mirror 16 and the optical devices 19-21' is movable bodily relative to a frame which houses it, about an axis which is perpendicular to a line connecting the left and right exit pupils and substantially equi-distant from those exit pupils using pivot pin 36. Pivot pin 33 may allow the optical system to move about a perpendicular axis.

Description

Title: Display AnDaratus This invention relates to display apparatus of the type which permits binocular viewing of an image, and has as its object the provision of such apparatus in which collimated images are projected to a user from a single object over a wide field of view, and in which the lines of sight from any point on the object enter each of the user's eyes substantially parallel.

According to the present invention, there is provided display apparatus comprising a frame whose position in use is fixed relative to the head of a user, an image generator (such as a display screen) operative to generate an image for viewing by the user, and an optical system for projecting said image to the user's eyes, the optical system being composed of a crossed beamsplitter device which directs light from the image generator into two separate paths for viewing of respective images at left and right exit pupils, a concave mirror disposed optically on the opposite side of the beamsplitter device to the image generator and operative to relay said image towards the beamsplitter device, and a pair of magnifying/collimating optical devices for magnifying and/or collimating the relayed image for viewing at the left and right exit pupils, respectively, the crossed beamsplitter device comprising two semi-reflecting beamsplitter elements disposed one above the other and mutually inclined, such that light from the image generator passes through the beamsplitter elements, is reflected by the concave mirror back towards the beamsplitter device, and is then deflected by the beamsplitter device such that part of said light is reflected by one of the beamsplitter elements towards one of the magnifying/collimating devices, and such that part of said light is reflected by the other beamsplitter element towards the other magnifying/collimating device, the optical system being movable bodily relative to the frame angularly about an axis which is perpendicular to a line connecting the left and right exit pupils and substantially equidistant from the left and right exit pupils.

By means of such an apparatus, a substantially collimated image can be projected through the two exit pupils which overlay the user's eyes, so that the user can clearly see the whole field of view.

The image generator can be mounted so as to be angularly movable relative to the frame along with the optical system, in which case the image as produced by the image generator is rotated by a corresponding amount to compensate. Alternatively, the image generator can be fixed relative to the frame, in which case it is preferably disposed substantially on said axis.

Preferably, an optical element (such as a lens) is included in the optical path between the image generator and the beamsplitter device in order to magnify the image. In this case, the optical element is configured to alter the vignetting effect of the beamsplitter elements.

Desirably, the optical system also includes, for each magnifying/collimating device, at least one fold mirror operative to deflect light received from the beamsplitter device towards the magnifying/collimating device.

Conveniently, the fold mirror or at least one of the fold mirrors for each magnifying/collimating device is positioned so that an end thereof is located closely adjacent to an outer portion of the concave mirror. In this case, to allow for reduced separation of the fold mirrors where the device is intended for use by a user having a relatively small inter-pupillary distance, said ends of the fold mirrors can terminate at respective points which do not obstruct light transmitted to and reflected from said outer portion of the concave mirror.

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view of an optical system which forms part of display apparatus according to the present invention; Figure 2 is a perspective view of a physical embodiment of the display apparatus; Figure 3 is a sectional side view of the display apparatus; Figure 4 is a sectional plan view of the display apparatus; Figure 5 is a front view of the display apparatus; Figures 6 and 7 are exploded perspective views of the display apparatus; Figures 8, 8A and 8B are diagrams illustrating the dividing of exit pupils of the apparatus between two optical paths; Figure 8C is a diagram illustrating the effect on fixed exit pupils of varying inter-pupillary distance of users; Figures 9, 9A and 9B are diagrams illustrating the vignetting effects in the optical system; Figure 10 is a diagram illustrating the effect of a lens incorporated into the optical system in order to improve vignetting in the system; and Figure 11 is a diagram illustrating a modified form of display apparatus, also according to the present invention.

The illustrated display apparatus comprises generally a frame (not shown) which is mounted on the head of a user in use.

Disposed within the frame is a sub-assembly as depicted in Figures 2 to 7. The sub-assembly comprises an image generator in the form of an LCD display screen 10, and a field lens 11, both of which are mounted in a box-like housing 12. The subassembly also includes an optical system for relaying to the user's eyes an image as displayed on the screen 10, the components of the optical system being shown to advantage in Figure 1.

The optical system comprises generally a crossed beamsplitter device 13 composed of two semi-reflecting beamsplitting elements 14 and 15 disposed one above the other and mutually inclined at an angle of substantially 90". Disposed optically on the opposite side of the beamsplitter device 13 to the display screen 10 is an aspheric concave mirror 16. Positioned to one side of the beamsplitter device 13 is a plane fold mirror 17 and a magnifying/collimating lens system 18 composed of three lens elements 19, 20 and 21. Similarly, positioned to the other side of the beamsplitter device 13 is a further plane fold mirror 171 and a magnifying/collimating lens system 181 composed of three lens elements 191, 201 and 211.

In use, light from the display screen 10 passes through the beamsplitter device 13 and is reflected back towards the latter by the concave mirror 16. Part of the light is then reflected by the beamsplitter element 14 towards the fold mirror 17 and thence through the lens system 18 towards a left-hand exit pupil 22, whilst part of the light is reflected by the beamsplitter element 15 towards the fold mirror 171 and thence through the lens system 181 towards a right-hand exit pupil 221. The terms right- and left-hand are here used in the sense as viewed by a user of the apparatus, rather than as actually depicted in Figure 1. The concave mirror 16 serves to relay to the beamsplitter device 13 an image as displayed on the display screen 10. The beamsplitter device 13 then directs the light into two separate paths for viewing of respective images at the right- and lefthand exit pupils 22 and 221, with the lens systems 18 and 181 serving to magnify and/or collimate these images.

The display screen 10, the field lens 11, the beamsplitter device 13 and the concave mirror 16 are disposed on a common axis X, whilst the fold mirror 17 and lens system 18 are disposed on an axis Y and the fold mirror 171 and the lens system 181 are disposed on an axis yl, the three axes X, Y and yl all being substantially parallel to one another and disposed in a common plane.

Referring now particularly to Figures 2 to 7, the beamsplitter device 13, the concave mirror 16 and the fold mirrors 17 and 171 are mounted on a housing 23. The fold mirrors 17,171 are accommodated within respective wings 25 and 251 of the housing 23, each of the mirrors 17,17l being supported by a respective backing member or mount 26,261 which is secured to the respective wing 25,251 by means of respective adjustment screws 27, 271 and a respective pivot screw 28,281. The position of each of the mirrors 17 and 171 relative to the housing 23 is adjustable by means of these screws 27, 271, 28 and 281. A pair of location tabs 29 extend forwardly from the open front of the housing 23 at the top and bottom thereof, and are received by respective recesses 30 in the housing 12 for the display screen 10.

The lens elements of the lens systems 18,18l are mounted within a common housing 31 which is fixedly secured to the housing 23.

The housing 31 contains a bore 32 which accommodates an upright pivot pin 33, the pin 33 being carried by a bearing element 34.

The bearing element 34 has a bore 35 which receives a horizontal pivot pin 36 fixedly mounted relative to the aforesaid headengaging frame, the pin 36 being disposed on the optical axis X.

The two pivot pins 33 and 36 allow the whole sub-assembly to be adjusted relative to the head-engaging frame both about an axis parallel to the optical axes Y and yl and disposed mid-way therebetween, and about an axis perpendicular to the axes Y and yl and disposed mid-way therebetween.

The lens elements 19 and 191 are formed as separate components due to their being mounted respectively on opposite sides of the housing 12 for the display screen 10. However, the lens elements 20 and 201 are formed in laterally spaced relation as a single, integral unit 37 with an integral bridging piece 371. This not only reduces the number of component parts of the system, but also ensures at the manufacturing stage that the individual lens elements are fixed at the correct lateral distance apart. The lens elements 21,211 are similarly formed as a single unit 38 with an integral bridging piece 381. However, in the unit 38 the lens elements are cut-away to form a generally triangular recess 39 to accommodate the user's nose.

An optical system of the general type depicted in Figure 1 is described in our pending International patent application PCT/GB95/01891. A feature of the system is that, because of the vignetting effect of the crossed beamsplitter device 13, the image as viewed at the left-hand exit pupil 22 is projected through (say) a lower half of that exit pupil (denoted by crosshatching at 40 in Figure 8), whilst the image as viewed at the right-hand exit pupil 221 is projected through an upper half of that exit pupil (denoted by cross-hatching at 401). To facilitate viewing of both of these images simultaneously by both of the user's eyes, the whole optical system is tilted at an angle so that an imaginary line A connecting the user's eyes passes comfortably through both of the areas 40 and 401. Under these circumstances, the plane (referenced P in Figure 8) containing the axes X, Y and yl is tilted at a small angle to the horizontal.

For ease of use, it is necessary to design the optical system so that users with differing inter-pupilliary distances (IPDs) can view the image through the exit pupils with their eyes in a horizontal plane. Nominally, this would necessitate the extension of the exit pupil size vertically as shown in Figure 8C. A user with a narrow IPD will place his or her eyes closer to plane P than a user with a wider IPD. Since the centres of the exit pupils 22,221 are projected to lie on plane P, the shaded areas 40,401 of the exit pupils would have to be higher to accommodate a range of short to wide IPDs.

With reference to Figures 9, 9A and 9B, vignetting occurs in the beamsplitting device 13. This is because light from the display screen 10 undergoes two passes of the beamsplitter device 13 in its passage to the exit pupils 22,221, so the apertures of the beamsplitter elements 14,15 are traversed at two different longitudinal conjugates. Therefore it is not possible for the lens system 18,181 to form accurately focused images of the vertical edges of the beamsplitter elements 14,15 at the exit pupils 22,221 for both conjugates. As a result, the vertical edges of beamsplitter elements 14,15 that almost touch will cause vignetting of the off-axis beams either in the first pass, second pass, or both passes depending on the conjugate relationship of the beamsplitter elements 14,15 and the exit pupils 22,221. The best compromise is to focus the aperture of the concave mirror 16 onto the exit pupils 22,221. The resultant vignetting is minimal for beams that are projected substantially parallel to the axes Y,Y1, but as the field angle being viewed increases towards the top or bottom of the displayed image, the vignetting restricts the vertical height of the exit pupils 22,221.

Referring to Figure 9, for points on the image (designated I) displayed on the screen 10 which lie on a line 42 parallel to the plane P and passing through the axis X, the vignetting effect caused by the beamsplitter elements 14 and 15 is the same at each pass through the beamsplitter device 13, with the cut-off line lying along a centreline 43 of the exit pupil image. However, for points on the image I lying off axis (i.e. displaced from the line 42), the vignetting effect is different during the two passes of the beamsplitter device 13.

For example, as depicted in Figure 9A, for a point 44 located above the line 42, the vignetting occurs in the first pass at a line 45 which is disposed above the centreline 43, and in the second pass at a line 46 which is disposed below the centreline 43. Correspondingly, as shown in Figure 9B, for a point 47 located below the line 42, the vignetting occurs in the first pass at a line 48 which is displaced above the centreline 43, and in the second pass at a line 49 displaced below the centreline 43. Areas in the exit pupil that are hatched in both halves of the diagram are where the user's eyes can be placed to see the displayed images without vignetting in either pass. The combined effect is always to reduce the area of the exit pupils for field positions that are either higher or lower than the horizontal centreline 42. For this reason, the exit pupils are effectively reduced in height for the user to see the whole vertical field of view.

According to the present invention, an additional feature has been added to the arrangement described in International Patent Application PCT/GB95/01891 by arranging for the optical system to be rotated relative to the user's head about a central horizontal axis. This enables the device to be adjusted by means of a simple tilting action so that exit pupils smaller in height overlay the user's eyes. Thus, Figure 8 shows the system set up for a user having an IPD of x, wherein centres of the user's pupils are denoted by crosses 41 and 411 respectively. Figure 8A shows the system adjusted for a user having a larger IPD of y, wherein the plane P is inclined at a smaller angle to the line A. Correspondingly, Figure 8B shows the system adjusted for a user having a smaller IPD of z, with plane P inclined at a larger angle to line A.

Such adjustment of the tilt is achieved by mounting the whole of the optical system so that it can be angularly rotated about a horizontal axis relative to the frame (not shown) which engages the user's head, this axis being substantially (but not necessarily exactly) coincident with the aforesaid axis X. In the illustrated embodiment, this is achieved by tilting the whole of the sub-assembly as depicted in Figures 2 to 7 relative to the head-engaging frame about the aforesaid pivot pin 36. In this arrangement, the display screen 10 is tilted along with the remaining components of the optical system. The displayed image is rotated with the optical system, which is acceptable because the range of movement is acceptably small to most users.

However, to allow for the initial tilt in the system, the display screen 10 is set in a position in which it is rotated about axis X so that the average orientation of the displayed image is horizontal. In an alternative arrangement (not shown), the orientation of the display screen is fixed relative to the headengaging frame so that it does not move as the optical system is tilted, and the displayed image thus remains at all times in the correct orientation for viewing by the user.

To allow for tilting of the optical system, the cut-away portions of the lens elements 21,211 are asymmetrically arranged with respect to a plane Z parallel to the optical axes Y and yl and disposed mid-way therebetween (see Figure 5).

In conventional display systems, the exit pupils are adjusted to the user's eyes by varying the geometry of the optical system, particularly by altering the separation of the exit pupils. It is often the case that the geometry of the system has to be altered in other respects as well, e.g. to compensate for changes in optical path length introduced by the IPD adjustment.

Moreover, the movement of parts must not introduce significant parallax. Accordingly, the mechanism needed for IPD adjustment can add significant bulk and cost to the system. In this invention, the projected exit pupils are wide enough to cover a suitably larger range of user IPDs, and the optical system can be rotated by the user to achieve optimum vertical alignment of the exit pupils to his or her eyes. It has been found that, for an exit pupil width of 18 mm, simple tilting of the optical system can typically accommodate a range of IPDs from 45 mm to 70 mm.

According to another aspect of this invention, the field lens 11 is included between the display screen 10 and the beamsplitter device 13. A raytrace through the optical system is illustrated in Figure 10, where rays are traced in reverse direction from the exit pupils 22,221 to the field lens 11. The vignetting (or bottom) of the exit pupils 22,221 are defined by the ray 10/2, which just passes through the beamsplitter device 13 above the bottom of the beamsplitter element 14. With the field lens 11 in place, this ray 10/2 is closer to the principal ray 10/3 for the field point 10/4 in question. Because ray 10/2 is closer to the principal ray 10/3, and ray 10/2 defines the unvignetted edge of the left exit pupil 22, the effective unvignetted height of the exit pupil 22 is increased.

According to a further aspect of the invention, a compromise between the width of the exit pupils and vignetting can be optimised to allow users with narrow IPDs to use the display with acceptable imagery. This is achieved by reducing the length of the plane mirrors 17,171 as shown in the raytrace in Figure 11, i.e. so that their ends closest to the concave mirror 16 do not overlap an outer portion of the latter. The design of the device is constrained in space by the requirement that the average spacing between the optical axes Y and yl sets the centres of the exit pupils close to the average IPD of the range of target users, which is normally around 60 mm. The actual distance between the axes Y and yl is about 58 mm, i.e. less than 60 mm, to allow for the vertical offset between the used portions of the exit pupils, as shown in Figure 8. A conflict results between rays traced backwards from the exit pupils 22,221 towards an inner field point (marked 11/2 in Figure 11, e.g. to the right from the left exit pupil 22), and rays traced from the inner portion of the exit pupils 22 (marked 11/3 in Figure 11). The plane mirror 17 should be fabricated with a larger inner aperture to reflect the rays 11/2, but that would vignette the reflected rays 11/3 by preventing them reaching the concave mirror 16.

This is fundamentally a result of the optical axes Y, X and yl being spaced to suit the user's IPDs. A useful design development is to deliberately vignette the rays 11/2 by cutting back the aperture of the plane mirror 17, and allowing the rays 11/3 to pass unhindered. Further rays 11/4 that are also aimed towards an inner portion of the field of view, but at a reduced aperture, are sufficiently within the aperture of the plane mirror 17 to be reflected and seen. The result is a compromise between the outer dimension of the exit pupils and the inner.

For example, the unmodified design will typically illuminate a range of user IPDs from 55 to 75 mm, whereas the modified design will illuminate a range from 45 mm to 70 mm.

Various modifications can be made to the apparatus as described above. For example, additional optical elements can be included in the optical path between the beamsplitter device 13 and the concave mirror 16, the lens systems 18,181 can include diffractive elements in addition to refractive elements, and more than one fold mirror 17,171 can be provided. Also, the display screen 10 can be replaced by an alternative type of image generator, such as a laser scanner.

Claims (7)

Claims

1. Display apparatus comprising a frame whose position in use is fixed relative to the head of a user, an image generator (such as a display screen), operative to generate an image for viewing by the user, and an optical system for projecting said image to the user's eyes, the optical system being composed of a crossed beamsplitter device which directs light from the image generator into two separate paths for viewing of respective images at left and right exit pupils a concave mirror disposed optically on the opposite side of the beamsplitter device to the image generator and operative to relay said image towards the beamsplitter device and a pair of magnifyingJcollimating optical devices for magnifying and/or collimating the relayed image for viewing at the left and right exit pupils, respectively, the crossed beamsplitter device comprising two semi-reflecting beamsplitter elements disposed one above the other and mutually inclined, such that light from the image generator passes through the beamsplitter elements, is reflected by the concave mirror back towards the beamsplitter device, and is then deflected by the beamsplitter device such that part of said light is reflected by one of the beamsplitter elements towards one of the magnifying/collimating devices and such that part of said light is reflected by the other beamsplitter element towards the other magnifying/collimating device.

the optical system being movable bodily relative to the frame angularly about an axis which is perpendicular to a line connecting the left and right exit pupils and substantially equi-distant from the left and right exit pupils.

2. Display apparatus according to claim 1, wherein the image generator is mounted so as to be angularly movable relative to the frame along with the optical system, and in which the image as produced by the image generator is rotated by a corresponding amount to compensate.

3. Display apparatus according to claim 1, wherein the image generator is fixed relative to the frame, and is preferably disposed substantially on said axis.

4. Display apparatus according to claim 1, 2 or 3, wherein an optical element, such as a lens, is included in the optical path between the image generator and the beamsplitter device in order to magnify the image, the optical element being configured to alter the vignetting effect of the beamsplitter elements.

5. Display apparatus according to any preceding claim, wherein the optical system also includes, for each magnifying/collimating device, at least one fold mirror operative to deflect light received from the beamsplitter device towards the magnifying/collimating device.

6. Display apparatus according to claim 5, wherein the fold mirror or at least one of the fold mirrors for each magnifying/collimating device is positioned so that an end thereof is located closely adjacent to an outer portion of the concave mirror.

7. Display apparatus according to claim 6, wherein , to allow for reduced separation of the fold mirrors where the device is intended for use by a user having a relatively small inter-pupillary distance, said ends of the fold mirrors terminate at respective points which do not obstruct light transmitted to and reflected from said outer portion of the concave mirror.