GB2295026A - Off axis projection optics for head mounted display - Google Patents

Abstract

A head mounted virtual display system uses an optical system comprising projection optics 14 and viewing mirror 15 which have an optical axis 17 passing through or very close to the use's head 13. The projection optics are used off-axis so that no paraxial rays form any part of the image. A display may be a notebook computer display panel 11 or a smaller display 12. The system may be used independent to each eye or increased in size so that a single projection lens or two segments of a single projection lens may be located above the head and a single mirror positioned in front of the eyes so that the axis of the system passes between and above the eyes. At regions 19, 20 optional mirrors may be placed for displays in other locations. <IMAGE>

Description

HEAD MOUNTED DISPLAY SYSTEM This invention concerns head mounted display systems especially suitable for virtual reality presentations and also suitable for military and other head-mounted display applications where the user may wish to retain a partial view of the real world, a condition sometimes known as augmented reality. The system is required to provide to the eyes of the user a wide angle field of view of a computer driven or similar electronic display such as a Liquid Crystal or Gas Discharge panel.

The present invention provides a head mounted display system having a general axis connecting a viewing mirror and a display source which passes through or close to the head of a user and off-axis or partially off-axis projection optics which utilise light passing above and/or around the head so that the head itself does not block the view and where the use of small display sources is facilitated by auxiliary collimating optics close to the off-set projection optics so that the display source has an apparently larger size.

The optical concept utilises an optical system, the main axis of which passes through or very close to the head so that the electronic display may be mounted behind the head and so counterbalance the weight of the optics. The wide angle field of view to the eyes can be provided by a concave mirror which may be spherical or aspheric of any type or may be compound as in the case of a Mangin mirror. The light from the electronic display is projected into the focal region of the mirror by an optical system which is used substantially off-axis. This means that only a part of the normal circular aperture of a lens system is used. This may be further sub-divided into two discrete parts which the concave mirror projects into the region of the two eyes.

Such a system may be folded by the use of mirrors at locations in front of or behind the projection lens. In particular, a mirror behind the projection lens can be used to re-position the electronic display to be above the head or at the side of the head. If the mirror has a semi-reflecting capability it is possible to use two display panels optically at the same position for the projection lens but provide different colours or different levels of resolution. If two mirrors behind the projection lens position are used to redirect light for a pair of discrete projection lenses, two display panels can be used at or near the sides of the head to provide for possible stereoscopic viewing by, for example, having the computer modify the image on each to introduce the disparity associated with two-eye viewing.

Alternatively the two eyes may be treated individually. A reduced scale version of the optical concept may be positioned with its axis passing through the head between the eyes and either side of the head so that the off-axis projection optics are located substantially at the side of the head. Such an arrangement lends itself to the use of small display sources which may be made apparently larger by the use of auxiliary optics close to the projection optics. The auxiliary optics may be relatively standard optical designs using lenses in a conventional symmetrical construction. There also exists laser scanning displays which, if used with this invention, take the place of both the display source and the auxiliary optics.

When two optics systems are used for the two eyes of the user it is not necessary that their axes remain parallel to each other. This freedom can allow the fields of view of the two displays to overlap only partially so than both eyes see a common area, the binocular field of view while each eye sees an extra area specific to that eye, the monocular field of view.

A centred lens system is a collection of optical components which have symmetry about a common axis of revolution. This is the most common type of optical system and is sometimes known as a symmetrical optical system (see W T Welford's book "Aberrations of the Symmetrical Optical System").

Decentred lens systems are those where one or more of their components are not symmetric about a common axis. These are much more difficult to analyse and design.

An important component of any optical system is the location of the limiting aperture often called the "aperture stop" because it stops unwanted light rays reaching the image. A symmetrical lens system as defined above normally has its aperture stop located symmetrically about the common axis.

In this invention an "off-axis" system is defined as a symmetrical system where the components are symmetrically disposed about a common axis except for the aperture stop which is displaced from the axis so that a light ray coincident with the common axis does not pass through the system. This is directly contrary to standard optical design methods as this ray and others close to it are called paraxial rays and normally provide the region of best optical quality. In an off-axis system these rays are specifically excluded from the imaging process. It is expected that these systems will make substantial use of aspheric surfaces.

This use of an off-axis stop is not limited to otherwise symmetrical lens systems. In a decentred lens system the components do not lie on a common axis. However, it is very difficult to design such systems to have good optical quality across a wide image field if the decentration is extreme. Nevertheless small decentrations can be very valuable.

Figure 1 illustrates a simple lens and an aperture plate to illustrate theory; Figure 2 is for a similar purpose illustrating off-axis aperture stops; Figure 3 is a schematic illustration of one embodiment of the present invention; Figure 4 is a schematic illustration of a second embodiment of the present invention; and Figure 5 is a schematic illustration of a manufacturing method.

Considering the simple lens 1 shown in cross section in Figure 1 with two spherical surfaces 2 and 3, its axis 7 is defined as the line joining the centres of rotation 4 and 5 of those surfaces. In normal optics this lens would have a circular stop 6 formed deliberately or by the mounting structure holding the lens. The axial view 6 of this lens is naturally circular as shown in Figure 2. All the area within the circle is available to pass light to the image. An off-axis lens is defined as a component where an aperture stop is displaced from the axis so that no light ray to any part of the image passes through the component along its axis. The oval aperture 8 might be a suitable shape for single eye viewing while the segment aperture 9 would be suitable for two eyes.

This segment aperture might be further subdivided into two 'discrete' apertures 10 to save weight or bulk in the system. In an overhead virtual reality system the segment or discrete segments would be above the axis and therefore above the head.

Figure 3 gives a general description of one embodiment of the system. A notebook computer display panel 11, or a smaller display 12, or any other convenient display of reasonable size is located behind a head 13 of a user; two head sizes are shown, one size covering half of all possible users and the other 95%. Light from this display is projected by an off-axis projection lens 14 towards a curved mirror 15 which redirects the light into the eyes at 16. The curved mirror does not have to reflect 100% and may therefore allow light from the outside real world to reach the eyes. The overall optical system: display, projection lens and mirror has a general axis 17 which passes through or very close to the head of the observer so that optical rays close to this axis, the so-called paraxial region cannot be used.

The off-axis projection lens 14 has an optical axis 17 passing through the user's head and thus has to be shaped as a chordal segment of the notational lens. Lens stops 18 are provided to ensure that light passes through these chordal segments.

The regions 19 and 20 indicate the general locations where mirrors may be located to redirect the light from display sources in different locations to that shown. If the mirrors have a semi-reflecting facility more than one display source can be used providing a combined display for the projection lens or lenses to project the information into the eyes.

Figure 4 gives a general description of a second embodiment of the system. A small display source 21 such as a LCD or miniature CRT is magnified by the auxiliary optical system 22. The emerging light is as if from a large and relatively distant display.

These units are located to the side of the user's head. The off-axis optics 23 are centred about an axis 24 which passes through the user's head, 30. The reflecting system 25 is substantially centred on the same axis and may reflect the light at its central surface 27. Surface 26 refracts the light, in this case, both before and after reflection. If the observer is to view the outside world without distortion surface 27 must be partially transparent and a further optical element must be added so that surface 28may compensate for the refractive effects of surface 26. The material between surfaces 26 and 27 may be the same as the material between surfaces 27 and 28 so that no refractive effects occur at surface 27 on the light arriving from the outside world. Alternatively surface 28 may be different from surface 26 in a way that compensate for any effects at surface 27.If surface 17 or 16 is made fully reflecting no forward view of the outside world is possible.

The reflected light originating from the display 21 is directed into the user's eye 29.

An analogous system may be used to directed light into the other eye, 31, of the user.

The axis of the second system does not have to be parallel to axis 24 and in some applications, it is preferable to tilt them so that they cross in front of the user's face.

It is also possible to tilt the axis of the auxiliary lens system 22 so that it is not necessary parallel to the main axis 24. This changes the used area of the reflecting system 25 which may then give different areas of view to the two eyes so that the area viewed instantaneously by both eyes is less than the total area viewed. Alternatively, axis 24 can be tilted to compensate or partially compensate for the tilted axis of the auxiliary system.

Maximum optical independence between the display source/auxiliary lens 21/22 and the projection optics/combiner 23/25 is achieved when the light beams between them are collimated or substantially collimated. For most applications the light entering the eye is also collimated or substantially collimated. This means that the projection optics 23 and reflection system 25 comprise an afocal system in infinity or near-infinity adjustment.

If this afocal system has a magnification greater than unity the effect is to locate the projection optics 23 somewhat behind the line of the eyes. In some, especially military, applications the need for good sideways viewing of the outside world is important and this leads to a magnification between 1.3X and 1.9X. By way of example, the magnification shown in Figure 4 is 1.6X. Larger magnifications are likely if the display source and auxiliary lens are replaced by a scanned laser or similar display generator.

With the magnification of 1.6X a display field of 450 or more can be directed to the eye over a 12mm or more pupil at the eye using aspheric surfaces 26 and 27 together with off-axis projection optics comprising an inner lens of high-index low to medium dispersion glass between aspheric lenses of a plastic material housing relatively low index and high dispersion. Over the quoted field size the distortion may be 1% or less.

In standard optical systems the best imagery is formed near the axis of the system by light which passes through the system close to the axis. This new concept chooses to locate this axis in an impossible or uncomfortable location with respect to the user's head and use off-axis segments of the lenses the axial areas of which would otherwise interfere with the user's head.

This concept uses a symmetrical or substantially symmetrical arrangement of projection and reflection optics together with a display source either directly or indirectly via auxiliary optics or a scanning system where the axis of the projection optics and reflection optics passes through or very close to the head of the user and the projection optics are used off-axis so that any axial part of the projection optics actually existing does not direct light into the user's eyes.

The projection optics given by way of example in Figure 4 has a high index glass lens with spherical surfaces 'encased' in plastics lenses with aspheric outer surfaces. Only a small part of the glass lens is needed. In manufacture the glass lens would be 'edged' on its outer rim to provide an accurate location surface as is normally done during manufacture by mounting the lens on a rotating mandrel and ensuring it is axially symmetrical with the mandrel by adjusting it while the hot wax sets to secure it to the mandrel. This process is known as centring. In this concept an extra process is carried out whereby a central hole is cut in the lens and this is also 'edged' to provide a surface accurately located with respect to the axis.

The plastics lens elements may have their spherical surfaces cut on a diamond turning lathe so that they match the glass lens not only in surface curvature but also to match the overall diameter and central hole. This means these plastics lenses can encase the glass lens before their outer aspheric surfaces are produced by diamond turning. This means that their centre thickness, that is their thickness at the axis of the system, can be very small or even negative. The diamond turning process can then provide new accurate location surfaces at the edge and in the central aperture so that mechanical mounting is straightforward. Figure 5 shows in schematic form how the glass lens 32 is encased in the plastic lenses 33 and 34. From such a circular triplet lens some six or eight segments may be cut to provide the projection lenses needed for the headset systems. A single segment 35 is shown in Figure 5 with location surfaces 36 and 37.

The manufacture as described above uses high-precision diamond turning as the enabling technology. This needs no polishing of the cut surfaces which therefore retain their mechanical precision. In the past this method has been expensive but recent lathe developments notably at Rank Pneumo Ltd have seen the capital cost of the lathes reduced together with the required level of operator skill.

The off-axis concept is not, of course, restricted to this manufacturing method or to designs incorporating a spherical glass element. Continuing developments in optical manufacture are improving the quality and reducing the costs of aspheric surfaces on glass materials as well as plastics materials.

Claims (1)

1. Claim 1 An optical system for head-mounted displays which has a general axis connecting a viewing mirror and a display source which passes through or close to the head of a user and off-axis or partially off-axis projection optics which utilise light passing above or around the head so that the head itself does not block the view.

   Claim 2 An optical system as in claim l where the display source is small and auxiliary optics are provided close to the off-set projection optics so that the display source has an apparently larger size.

   Claim 3 An optical system for head-mounted displays comprising a reflecting optical subsystem and an off-axis projection optical sub-system which are generally centred about an axis which passes through or close to the users head and auxiliary magnifying optics which are not centred on said axis and which magnify a small electronic display.

   Claim 4 A substantially afocal optical system for head-mounted displays comprising a reflecting optical sub-system and an off-axis projection optical sub-system which are generally centred about an axis which passes through or close to the head of a user.

   Claim 5 An optical system for head-mounted displays substantially as described herein with reference to Figures 3 and 4 of the accompanying drawing.

   .Claim 6 A head-mounted display system comprising a substantially afocal optical system comprising a reflecting optical sub-system and an off-axis projection optical sub-system generally centred about an axis which passes through or close to the head of a user and an electronic display with or without magnifying optics.

   Claim 7 A head-mounted display system comprising a substantially afocal optical system comprising a reflecting optical sub-system and an off-axis projection optical sub-system generally centred about an axis which passes through or close to the head of a user and a scanned laser electronic display.