WO1998010323A1

WO1998010323A1 - Display apparatus

Info

Publication number: WO1998010323A1
Application number: PCT/GB1997/002353
Authority: WO
Inventors: Richard Holmes; Ian Marshall
Original assignee: Retinal Display Cayman Ltd.
Priority date: 1996-09-03
Filing date: 1997-09-03
Publication date: 1998-03-12
Also published as: AU4124197A; GB9618331D0

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'), after that light has passed through the beamsplitter device and has been reflected back by a concave mirror (16). The split light is reflected by respective fold mirrors (17, 17') and magnified and/or collimated by respective optical devices (18, 18') for viewing at the exit pupils (22, 22'). The beamsplitter device (13), the concave mirror (16), the fold mirrors (17, 17') and the optical devices (18, 18') are all mounted on a common housing (23), whilst the display screen (10) is mounted on a further housing (12). The housings (12, 23) engage one another by means of location elements (29, 30) which allow the position of the further housing (12) relative to the first-mentioned housing (23) to be adjusted longitudinally of the optical axis.

Description

Title: Display Apparatus

This invention relates to display apparatus of the type which permits binocular viewing of an image.

In display apparatus of this type, it is desirable that magnified collimated images are projected to a user's eyes over a wide field of view, and that the lines of sight at each eye from any one point on the object being viewed are substantially parallel. Deviations from this condition are referred to as parallax. Natural vision has a specific form of parallax (called convergence) where the eyes rotate inwards to view close objects. For comfortable viewing over long periods of time, parallax errors must be kept small unless they have the same geometry as convergence .

It is an object of the present invention to provide display apparatus in which parallax errors can be kept to a minimum whilst allowing relaxed manufacturing tolerances in the component parts .

According to the present invention, there is provided display apparatus comprising

an image generator (such as a display screen) operative to generate an image for viewing by a 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,

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, and

for each magnifying/collimating device, at least one fold mirror operative to deflect light received from the beamsplitter device towards said magnifying/collimating device,

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 components of the optical system being mounted on a first housing structure,

the image generator being mounted on a second housing structure which engages the first housing structure by means of location elements, the position of the second housing structure relative to the first housing structure being adjustable longitudinally of the optical axis.

Preferably, the position of the second housing structure relative to the first housing structure is pre-set during manufacture of the device. Advantageously, this is achieved by means of a jig which holds the first and second housing structures square and in the correct lateral positions relative to each other, whilst allowing adjustment in the longitudinal direction. In this regard, the term "longitudinal" is intended to mean a direction parallel to the axes X, Y and Y¹, the term "lateral" is intended to indicate directions transverse to those axes, and the term "square" is intended to signify alignment with respect to the longitudinal and transverse directions.

Desirably, the fold mirrors are adjustably mounted on the first housing structure, such that the linear position of each fold mirror can be varied along an axis non-parallel to its optical surface. Conveniently, the fold mirrors are adjustably mounted on the first housing structure, such that the angular position of each fold mirror can be varied about two orthogonal axes which lie substantially within a plane containing its optical surface. In this way, the linear and angular positions of the fold mirrors can be adjusted in relation to the intersection of optical axes defined by the magnifying/collimating devices on the one hand, and the light reflected by the beamsplitter elements on the other hand .

In a particular embodiment, each fold mirror is adjustable relative to the first housing by means of an adjustable pivot element, at least one first adjusting element located on a common horizontal line with the pivot element, and at least one second adjusting element located on a common vertical line with the pivot element.

In one arrangement, the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings, vertical parallax is adjusted by adjusting the angular position of each fold mirror about a horizontal axis, and horizontal parallax is adjusted by adjusting the angular position of each fold mirror about a vertical axis.

In a second arrangement, the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings, vertical parallax is adjusted by adjusting the angular position of each fold mirror about a horizontal axis, and horizontal parallax is adjusted by moving each fold mirror by means of the respective pivot element.

In a third arrangement, the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings, vertical parallax is adjusted by adjusting the angular position of each fold mirror about a horizontal axis, and horizontal parallax is adjusted by moving each fold mirror by means of combined adjustment of the pivot element and the respective first adjusting element (s) .

In a fourth arrangement, vertical parallax of the projected images is set by adjusting the relative longitudinal positions of the first and second housings, collimation is adjusted by moving each fold mirror by means of the respective pivot element, and horizontal parallax is adjusted by adjusting the angular position of each fold mirror about a vertical axis.

In a fifth arrangement, collimation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings whilst moving each fold mirror by means of the respective pivot element, and vertical and horizontal parallax is adjusted by moving each fold mirror about horizontal and vertical axes, respectively, by means of the respective first and second adjusting elements.

In a sixth arrangement, the projected images are rendered parallel in a horizontal direction by moving one of the fold mirrors about a horizontal axis by means of the respective second adjustment element (s) , the height difference between the projected images is adjusted by effecting relative movement between the first and second housing structures in the longitudinal direction, and the projected images are rendered parallel in a vertical direction by moving one of the fold mirrors about a vertical axis by means of the respective first adjustment element, s).

Advantageously, lens elements of the magnifying/collimating devices are formed in laterally spaced relation as a single unit with an integral bridging piece interconnecting the lens elements .

Conveniently, each magnifying/collimating device includes a lens element which is cut-away to accommodate the user's nose, the cut-away portions of the lens elements being asymmetrically arranged with respect to a plane parallel to the axes of the magnifying/collimating devices and disposed mid-way therebetween.

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; and

Figures 8 and 9 are perspective views of a focussing jig for use in the manufacture of the display apparatus.

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 sub- assembly 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 left- hand 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 Y¹, the three axes X, Y and Y¹ 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. 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 fold mirrors 17, 17¹ are accommodated within respective wings 25 and 25¹ of the housing 23, each of the mirrors 17, 17¹ being supported by a respective backing member or mount 26, 26¹ which is secured to the respective wing 25,2s¹ 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.

For each of the mirrors 17 and 17¹, four screws 27, 27¹ are provided. Two of these screws are located on a common horizontal line with screw 28,2s¹ and are disposed on opposite sides thereof. The other two screws 27, 27¹ are located on a common vertical line with screw 28,2s¹ and are disposed respectively above and below the latter. By combined adjustment of the screws 27, 27¹, 28, 28¹ each of the mirrors 17, 17¹ can be adjusted linearly along an axis non-parallel to its optical surface, and can also be adjusted angularly about two orthogonal axes which lie substantially within a plane containing the said optical surface .

The lens elements of the lens systems 18, 18¹ 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 head- engaging 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 Y¹ and disposed mid-way therebetween, and about an axis perpendicular to the axes Y and Y¹ 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.

The optical system per se is generally as described in our International patent application PCT/GB95/01891. In that application, it is explained how the vignetting effect of the beamsplitter elements 14 and 15 is compensated by inclining the axis of the optical system relative to an imaginary line connecting the centres of the user's pupils. To allow for this, 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 Y¹ and disposed mid-way therebetween (see Figure 5) .

In devising the above-described structure, the critical features of the geometry of the optical system have been analysed to define the necessary accuracies to which the component parts must be positioned for correct functioning of the display apparatus. The three most critical features, but not the only ones, are (1) the squareness of the display screen to the optical axes, (2) the squareness of the beamsplitter elements 14,15 to the horizontal plane that contains the optical axes X, Y and Y¹, and (3) the form of the concave mirror 16. Feature (1) is accommodated by means of a separate focussing jig (shown in Figures 8 and 9) which keeps the housing 12 and the housing 23 aligned whilst the housing 12 is moved longitudinally for focus. Feature (2) is accommodated by means of the accuracy to which mountings for the beamsplitter elements 14,15 within housing 23 are moulded, and feature (3) by means of the accuracy to which the concave mirror 16 is moulded.

Referring now specifically to feature (1) , the arrangement of the tabs 29 and the recess 30 enables the housing 12 to move in a longitudinal direction to focus the projected images. The tabs 29 are in fact a loose fitting in the recesses 30 in order to allow for manufacturing tolerances in the housings 12 and 23. It is well-known that such complex structures cannot be moulded cheaply without material shrinkage of a complex nature. In this case, the relative positions of the tabs to the other features on the components cannot be controlled to the necessary optical accuracy. Therefore, in order to provide low-cost manufacture with adequate performance, the housings 12 and 23 are designed to locate off more accurately controlled features on the focussing jig.

More particularly, the jig comprises generally a base 100 which carries two slide rails 101 in laterally spaced relation, a carriage 102 being carried by the rails 101 for sliding movement therealong. The carriage 102 in turn supports a further pair of slide rails 103, with first and second jig components 104,105 being carried by the rails 103 for sliding movement therealong. The direction of sliding of the jig components 104,105 on the rails 103 is orthogonal to the direction of sliding of the carriage 102 on the rails 101. Each jig component 104,105 comprises a pair of planar members 106,107 arranged at right- angles to one another, and a corner piece 108 connected between the members 106 and 107. The surfaces of the parts 106,107 and 108 are all mutually orthogonal.

A stand 109 is located at one end of the base 100, and carries horizontal mounting members 110. In use, the mounting members 110 receive therebetween the housing 23 containing the concave mirror 16, etc. The housing 23 has moulded on its exterior a series of lugs^' 111 and apertures 112 (see Figure 6) which are accurately positioned relative to the optical components contained within the housing 23, and these lugs and apertures are engaged with accurately-positioned engagement features on the mounting members 110. Reference numeral 113 denotes a pivotable clip which serves to retain the housing 23 in position on the stand 109.

In order to secure the housings 12 and 23 together, the housing 12 containing the display screen 10 and the field lens 11 is mounted on the jig components 104,105 so as to be sandwiched therebetween. The housing 12 also has moulded on its exterior a lug 114 (see Figures 6 and 7) which is generally T-shaped in plan and which is accurately positioned relative to the display screen 10, and this lug is engaged by accurately-positioned features on the jig components 104 and 105. By moving the jig components 104,105 along the rails 103, the lateral position of the housing 12 relative to the housing 23 can be adjusted until it is correct. However, in a preferred arrangement the location tabs 29 on housing 23 are designed to be a close fit within the recesses 30 in the housing 12 in a vertical direction, so that alignment of the housings 12 and 23 in that direction does not have to be adjusted by means of the jig. The longitudinal position of the housing 12 relative to the housing 23 is then adjusted by moving the carriage 102 along the rails 101. Once the correct positioning has been achieved, the location tabs 29 on housing 23 are cemented in position within the recesses 30 in housing 12. As depicted in Figure 9, the jig can include a pair of video cameras 115 and 116 carried for sliding movement in unison along a rail 117 disposed above and accurately aligned with the slide rails 101. The cameras 115 and 116 effectively perform the function of a user's eyes, and are employed to facilitate the correct alignment of the various optical components within the housings 12 and 23.

The jig thus controls the relative alignment of housings 12 and 23 during assembly and focussing adjustment, the two housings being cemented in place relative to each other whilst in position in the jig. Upon removal from the jig, the relative positions of the housings 12 and 23 are fixed, and the optical system is aligned to a higher accuracy than the tolerances of the component parts .

Because of the offset nature of the exit pupils to the optical axes of lenses 18, 18¹ (i.e. because one exit pupil is high and the other is low) , varying the longitudinal position of the housing 12 alters the vertical parallax between the two projected images as well as their collimation. This results from a well- known effect in systems that are not telecentric.

Turning now to feature (2), in order to relax the optical tolerances on the components, and in particular to relax tolerances on the positions of the beamsplitter elements 14 and 15, adjustments have been designed into the optical assembly. Errors in the positioning of components can cause optical defects that can be analysed as perturbations from the nominal design. The form of these perturbations can be split into first and higher order effects. The first order defects will change the focus and relative alignment of the two images projected from the lens systems 18. Higher order effects will generally increase the aberrations in the optical system, and in particular increase parallax errors. Generally, first order defects are larger than the higher order defects, so that adjustment of the optics to compensate the first order defects will usefully allow the optical tolerances to be relaxed.

The adjustments are designed to allow the compensation of first order parallax and focus for each eye. This is achieved by first setting test equipment to measure the two projected images at a geometry equivalent to a fixed IPD, and then using it to measure and control the following processes:

1. The positions of the plane mirrors 17, 17¹ are set using jigs to their nominal relationship to a datum surface on housing 23. This removes some of the moulding errors in the housing 23 and in the plane mirror mounts 26, 26¹.

2. The housing 12 is moved to focus both projected images to infinity. Some residual focus errors will remain as differences between the two images, but the overall error will be reduced.

3. The plane mirrors 17, 17¹ are adjusted by means of the screws 27, 27¹ in both horizontal and vertical angles about the points generated by the interaction of the screws 28, 28¹ with the mounts 26, 26¹. These adjustments serve to compensate the parallax errors between the two projected images .

If the adjustments described in processes 2 and 3 above are linked in a non-linear way to the first order optical defects, then processes 2 and 3 may be reiterated in sequence to converge on a compensated solution.

It is possible to vary the above-described adjustment sequence.

In a first variation, the plane mirrors 17, 17¹ are moved in a substantially linear movement, i.e. by a combined adjustment of the screws 28,2s¹ and screws 27, 27¹, to adjust the horizontal parallax. The collimation and vertical parallax are compensated as before by adjustment of the screws 27, 27¹ and by longitudinal movement of the housing 12, respectively.

In a second variation, the plane mirrors 17, 17¹ are moved by an adjustment of the screws 28, 28¹ to adjust the horizontal parallax. The collimation and vertical parallax are compensated as before by adjustment of the screws 27, 27¹ and by longitudinal movement of the housing 12, respectively.

In a third variation, the housing 12 is moved longitudinally to compensate the vertical convergence, the plane mirrors 17, 17¹ are moved in a substantially linear movement, i.e. by a combined adjustment of the screws 28, 28¹ and screws 27, 27¹, to collimate the projected images, and the screws 27, 27¹ are separately adjusted to rotate the plane mirrors 17, 17¹ about the said horizontal axis only to compensate horizontal convergence. This sequence will converge on a single setting of the screws 28,2s¹ and screws 27, 27¹ if repeated in sequence, despite the screws 27, 27¹ being used to set more than one optical parameter.

In a fourth variation, the housing 12 is moved longitudinally to compensate the vertical convergence, the screws 28, 28¹ are adjusted to collimate the projected images, and the screws 27, 27¹ are adjusted in the said horizontal axis only to compensate horizontal convergence. This sequence will converge on a single setting of the screws 28, 28¹ and screws 27, 27¹ if repeated in sequence, despite the adjustment screws 27, 27¹ being used to set more than one optical parameter.

In a fifth variation, the housing 12 is fixed in its nominal position, the plane mirrors 17, 17¹ are moved in a substantially linear movement, i.e. by a combined adjustment of the screws 28, 28¹ and screws 27, 27¹ to adjust the collimation of the projected images, and the screws 27, 27¹ are adjusted to rotate the plane mirrors 17, 17¹ about the said vertical and horizontal axes to compensate both horizontal and vertical parallax.

In a sixth variation, the projected images are rendered parallel in a horizontal direction by moving one of the fold mirrors 17, 17¹ about a horizontal axis by means of the adjustment screws 27, 27¹, the height difference between the projected images is adjusted by moving the housing 12 relative to the housing 23, and the projected images are then rendered parallel in a vertical direction by moving one of the fold mirrors 17, 17¹ about a vertical axis by means of the adjustment screws 27, 27¹. Under these circumstances, no specific action is taken to adjust collimation: instead, reliance is placed on the manufacturing tolerances to keep the collimation within a reasonable band of tolerance .

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 diftractive 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

1. Display apparatus comprising

an image generator, such as a display screen (10), operative to generate an image for viewing by a user, and

a crossed beamsplitter device (13) which directs light from the image generator (10) into two separate paths for viewing of respective images at left and right exit pupils (22.22'),

a concave mirror (16) disposed optically on the opposite side of the beamsplitter device (13) to the image generator (10) and operative to relay said image towards the beamsplitter device (13),

a pair of magnifying/collimating optical devices (18,18') for magnifying and/or collimating the relayed image for viewing at the left and right exit pupils, (22,22'), respectively, and

for each magnifying/collimating device (18,18'), at least one fold mirror (17,17') operative to deflect light received from the beamsplitter device (13) towards said magnifying/collimating device (18,18'),

the crossed beamsplitter device (13) comprising two semi-reflecting beamsplitter elements (14,15) disposed one above the other and mutually inclined, such that the light from the image generator (10) passes through the beamsplitter elements (14, 15), is reflected by the concave mirror (16) back towards the beamsplitter device (13), and is then deflected by the beamsplitter device (13) such that part of said light is reflected by one of the beamsplitter elements (14) towards one of the magnifying/collimating devices (18), and such that part of said light is reflected by the other beamsplitter element (15) towards the other magnifying/collimating device (18'),

the components of the optical system being mounted on a first housing structure (23),

the image generator (10) being mounted on a second housing structure (12) which engages the first housing structure (23) by means of location elements (29,30), the position of the second housing structure (12) relative to the first housing structure (23) being adjustable longitudinally of the optical axis.

2. Display apparatus according to claim 1 , wherein the position of the second housing structure (12) relative to the first housing structure (23) is pre-set during manufacture of the apparatus. Advantageously, this is achieved by means of a jig.

3. Display apparatus according to claim 2, wherein the relative positioning of the first and second housing structures is pre-set by means of a jig which holds the first and second housing structures (23,12) square and in the correct lateral positions relative to each other, whilst allowing adjustment in the longitudinal direction.

4. Display apparatus according to claim 1 , 2 or 3, wherein the fold mirrors (17,17') are adjustably mounted on the first housing structure (23), such that the linear position of each fold mirror (17,17') can be varied along an axis non-parallel to its optical surface.

5. Display apparatus according to claim 4, wherein the fold mirrors (17,17') are adjustably mounted on the first housing structure (23), such that the angular position of each fold mirror (17,17') can be varied about two orthogonal axes which lie substantially within a plane containing its optical surface.

6. Display apparatus according to claim 4 or 5, wherein each fold mirror (17,17') is adjustable relative to the first housing (23) by means of an adjustable pivot element (28), at least one first adjusting element (27) located on a common horizontal line with the pivot element (28), and at least one second adjusting element (27) located on a common vertical line with the pivot element (28).

7. Display apparatus according to claim 4, 5 or 6, wherein the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings (23,12), vertical parallax is adjusted by adjusting the angular position of each fold mirror (17,17') about a horizontal axis, and horizontal parallax is adjusted by adjusting the angular position of each fold mirror (17,17') about a vertical axis.

8. Display apparatus according to claim 6, wherein the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings (23,12), vertical parallax is adjusted by adjusting the angular position of each fold mirror (17,17') about a horizontal axis, and horizontal parallax is adjusted by moving each fold mirror (17,17') by means of the respective pivot element (28).

9. Display apparatus according to claim 6, wherein the focus/accommodation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings (23,12), vertical parallax is adjusted by adjusting the angular position of each fold mirror (17,17') about a horizontal axis, and horizontal parallax is adjusted by moving each fold mirror (17,17') by means of combined adjustment of the pivot element (28) and the respective first adjusting element(s) (27).

10. Display apparatus according to claim 6, wherein vertical parallax of the projected images is set by adjusting the relative longitudinal positions of the first and second housings (23,12), collimation is adjusted by moving each fold mirror (17,17') by means of the respective pivot element (28), and horizontal parallax is adjusted by adjusting the angular position of each fold mirror (17,17') about a vertical axis.

1 1. Display apparatus according to claim 6, wherein collimation of the projected images is set by adjusting the relative longitudinal positions of the first and second housings (23,12) whilst moving each fold mirror (17,17') by means of the respective pivot element (28), and vertical and horizontal parallax is adjusted by moving each fold mirror (17,17') about horizontal and vertical axes, respectively, by means of the respective first and second adjusting elements (27).

12. Display apparatus according to claim 6, wherein the projected images are rendered parallel in a horizontal direction by moving one of the fold mirrors (17,17') about a horizontal axis by means of the respective second adjustment element(s) (27), the height difference between the projected images is adjusted by effecting relative movement between the first and second housing structures (23,12) in the longitudinal direction, and the projected images are rendered parallel in a vertical direction by moving one of the fold mirrors (17,17') about a vertical axis by means of the respective first adjustment element(s) (27).

13. Display apparatus according to any preceding claim, wherein lens elements (20) of the magnifying/collimating devices (18,18') are formed in laterally spaced relation as a single unit with an integral bridging piece (37') interconnecting the lens elements.

14. Display apparatus according to any preceding claim, wherein each magnifying/ collimating device (18,18') includes a lens element (38) which is cut-away to accommodate the user's nose, the cut-away portions (39) of the lens elements (38) being asymmetrically arranged with respect to a plane parallel to the axes of the magnifying/ collimating devices (18,18') and disposed mid-way therebetween.