GB2472444A - Head up display - Google Patents

Abstract

A head up display 100, the display comprising a virtual image generation system 1010 (fig. 1) to generate a virtual image. The virtual image generation system has output optics including a partially reflecting optical surface 22, wherein an optical axis 28 of light exiting from the image generation system is tilted with respect to a normal to the optical surface. This defines a tilt angle of greater than zero degrees between the optical axis and the normal. The optical surface has an angular filter on an output side of the optical surface to attenuate external light 32 reflected 34 from the optical surface at an angle α greater than a threshold angle to the optical axis. The filter may be a baffle adjacent said optical surface, the baffle comprising an arrangement of tubes extending longitudinally along said optical axis of light exiting the image generation system.

Description

I

HEAD UP DISPLAYS

FIELD OF THE INVENTION

This invention relates to Head Up Displays and more particularly to tight shields for such displays, for inhibiting both reflections from incoming light such as sunlight and damaging injection of light into the projection optics.

BACKGROUND TO THE INVENTION

Head Up Displays (HUD5) producing a distant virtual image are based on projection optics. Depending upon the implementation, these optics may be composed of glass/plastic lenses, mirrors, or a combination of both these. Two common problems observed in existing systems are sunrelated damage to the HUD, and sunlight reflections from inside the system.

Sunlightrelated damage is typically caused by sunlight entering the optical system and ending up concentrated at the location of an image generation device such as a spatial light modulator (SLM). The concentration of the spot of tight depends upon the level of collimation of the system and can be high enough to permanently damage the imaging system.

The problem of sunlight reflections from an HUD occurs especially in HUD systems employing mirrors the sunlight can then be reflected out of the HUD by one of the mirrors of the optical combination and cause light pollution or worse inside the cockpit, for example causing flares on the windshield (windscreen) of a road vehicle such as a car. However, the problem of reflected sunlight is not exclusive to systems using mirrors as just a few percent reflection of sunlight from a glass surface without an anti-reflection coating can be sufficient to tblind" a driver. We will describe techniques which address both these problems and which, in so doing, help to reduce the integration constraints on a HUD by reducing the effects of solar exposure.

A range of solutions already exists to mitigate solar exposure problems, applied depending on the use case.

To reduce sun-related damage by restricting sunlight entering the system and potentially damaging the imager, existing solutions include: 1. Preventing the sun entering the system by a system of shutters.

2. Filtering the light inside the system (HUD light can be monochromatic and polarized) to minimize the actual part of the spectrum hitting the imager.

3. De-collimate the HUD to increase the spot size of the sunlight at the imager's level (reducing the pick exposure).

4. Using a heat drain layer at the display level to avoid hot spots cause by solar exposure.

5. Introducing a combiner with optical power (non flat) to cause the sun entering directly the system (i.e. without reflecting on the combiner) to be significantly non-collimated.

The solutions implemented in an HUD with solar exposure problems are normally a combination of these. For example, Fujitsu has a number of patents in the HUD field including a patent relating to the use of a folding shutter for an HUD. Nissan, in JP61238015A describe an arrangement including a transparent plate with plural light shield plates arranged in a transparent resin film which transmit only light which is incident within a narrow range of angles to the perpendicular to the film surface; a polarising plate is also employed to cut off polarised external light (the windshield is at the Brewster angle so that light transmitted through this is relatively polarised). Many examples of background prior art can also be found in Head Up Display patents held by Nippon Seiki Co Limited.

Further examples of background prior art can be found in: JP7261674 (Takiron Co Ltd), JP9185011 (Denso Corp.), JP2004/196020 (Denso Corp.) and JP2006/011168 (Nippon Seiki Co Ltd).

An apparently similar approach to that described in JP'015 was employed in a Jaguar fighter HUD from Smiths Aerospace, using a black honeycomb structure on top of the projection optics in a plane separate from an image plane of the HUD. This arrangement prevented sunlight at a shallow angle, for example at sunrise, from entering the HUD. Smiths have a substantial number of patents to Head Up Displays, to which reference may also be made.

The problem of avoiding light pollution resulting from light reflected out of an HUD system is mainly a problem for mirrorbased HUD systems, including automotive HUD systems. In such systems, because the freedom of movement of the vehicle is reduced there is a limited range of different possible sun positions and the orientation of the HUD in the dashboard can be selected to minimise problems from sunlight reflection from the HUD. In general it is not necessary to block all sunlight reflections, merely those which cause particular problems by, for example, reflecting sunlight onto the windshield some reflected sunlight on, for example, the internal roof of the car may be tolerated. Nonetheless this approach puts significant constraints on the integration of an HUD into a dashboard (where space is generally very limited).

Moreover the design of the HUD must typically incorporate significant light-absorbing surfaces to attenuate sunlight reflected by internal mirrors, for example the last mirror of the projector. As HUDs are becoming increasingly common in cars the constraints imposed by these solutions are becoming an important obstacle to the implementation of a low-cost, high-performance HUD product policy by manufacturers.

The inventors have previously described new techniques for expanding the exit pupil of a head up display, in particular in GB0902468.8, "Optical Systems", filed on 16 February 2009, hereby incorporated by reference in its entirety. These techniques employ a parallel sided waveguide into which light is injected at an angle and which multiply the exit pupil of an HUD by providing a plurality of output beams, tiling the exit pupils, the output beams emerging substantially parallel to one another and tilted with respect to a normal to the parallel sided waveguide. The inventors have recognised that such an exit pupil expander enables new techniques to be employed for inhibiting reflected sunlight and reducing sun-related damage and that, moreover, these new techniques are not limited to an exit pupil expander of the type previously described, although they are particularly useful when employed with such an exit pupil or eye box expander.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided a head up display, the display comprising a virtual image generation system to generate a virtual image for presentation to an optical combiner to combine light exiting said image generation system bearing said virtual image with light from an external scene, for presentation of a combined image to a user, wherein said virtual image generation system has output optics including a partially reflecting optical surface, wherein an optical axis of said light exiting said image generation system is tilted with respect to a normal to said optical surface, defining a tilt angle of greater than zero degrees between said optical axis and said normal to said optical surface, and wherein said partially reflecting optical surface has an angular filter on an output side of said optical surface to attenuate external light reflected from said partially reflecting optical surface at greater than a threshold angle to said optical axis.

In embodiments by tilting the partially reflecting optical surface with respect to an optical axis of the light exiting the system a (maximum) field of view of the head up display can be preserved whilst attenuating reflected sunlight. Thus, in embodiments, light entering the system along the optical axis is reflected and substantially blocked from exiting the system, although light entering at an angle closer to the normal to the output optical surface than the optical axis may not be blocked, depending upon the degree of angular filtering and also on the type of angular filter employed. (In the baffle example described later whether or not a ray is blocked depends, in part, on spatial location of the ray with respect to the baffle, more particularly whether or not is close to a side of a tube of the baffle).

The output side of the optical surface, that is the surface adjacent to which the angular filter is located to selectively inhibit reflected light is, in embodiments, an output surface of an exit pupil expander of the head up display (in a direction of propagation of light from the image generator towards the viewer). Thus in some preferred embodiments the partially reflecting optical surface comprises a partially transmissive, planar mirror surface, in embodiments with a reflectance which has a reflectance which is at least 80% or 90% at a wavelength at in the visible region of the spectrum, more particularly between 400nm and 700nm; more particularly which has a reflectance which is at least 80% or 90% at one or more wavelengths used by the image source. However, as previously mentioned, even low reflectance surfaces can cause significant problems with reflected sunlight and embodiments of the above described approach are useful even when the output optical surface is, for example, a simple uncoated glass surface.

In general the optical surface to which the angular filter is applied will be a final optical surface of the optical surface of the head up display (apart from the combiner), but nonetheless some benefit can be obtained from the technique by employing a tilting optical surface and angular filter at an internal optical surface of the display although this can be less effective at inhibiting sunlight reflections (and may require a larger volume assembly), it can still be useful in reducing sunrelated damage. In embodiments employing our planar, waveguiding type pupil expander the rear or internal optical surface of the waveguide generally has a very high reflectivity, for example greater than 95% or 98%, and hence even if the front surface is not mirrored reflection will result from the internal, rear surface of the waveguide.

In embodiments of the head up display the threshold angle is substantially equal to the aforementioned tilt angle that is the angle between the optical axis and the perpendicular to the output optical surface defines the cut off angle of the angular filter (a skilled person will appreciate that the angular filter may not have a sharp cutoff, in which case the cutoff angle may be defined, for example, as a 3dB point on the attenuation angle curve). In embodiments the tilt angle of the optical surface is at least 30, 5 o, 10 0 or 15 0; more typically the tilt angle is in the range 1545°, again particularly where our parallel plate pupil expander is employed (in principle, however, an additional optical surface could be included in the head up display after the last optical element (apart from the combiner), merely for the purpose of sunlight attenuation by angular filtering.

In embodiments of the system the threshold angle is substantially equal to half a maximum field of view (FOV) of the head up display (more precisely, of the head up display without the angular filter). This angle will be less than the tilt angle for a pupil expander of the type we describe. In practice, whether or not it is desirable to entirely block reflections of light from the system depends, in part, on the type of angular filter employed as described further below.

The skilled person will appreciate that many different types of angular filter may be employed. For example the angular filter may comprise a dielectric stack coating (such coatings have an acceptance angle which, in effect, operates as an angular filter).

Alternatively a reflective polariser may be employed (for example of the type available from Moxtek mc, USA), or a diffractive optical element, or microprisms, or a TIR (totally internally reflecting) light trap may be employed in front of the reflecting surface, or a multilayer (volume) hologram may be used. In some particularly preferred embodiments, however, the angular filter comprises an array of tubes, in particular, each extending longitudinally along the optical axis. As described in more detail later, such an arrangement is able to attenuate substantially reflections at all angles above a threshold angle, but also the degree of blocking depends upon the point of incidence of a ray of light on the array of tubes. Similarly, for light exiting the head up display through the array of tubes, for a ray incident just inside the edge of a tube, effectively half the field of view is blocked by the outer side of the tube. Because of this it can be desirable to pass more light than the field of view of the head up display, to avoid losing light at these points of incidence. Thus in embodiments where the angular filter comprises an array of tubes it can be desirable not to entirely block or trap light outside a field of view of the display, for improved light output efficiency (to avoid the field of view dimming towards the edge). One advantage of employing an array of tubes as the angular filter is that this is inexpensive and easy to fabricate, as well as being effective.

According to a related aspect of the invention there is therefore provided a head up display, the display comprising a virtual image generation system to generate a virtual image for presentation to an optical combiner to combine light exiting said image generation system bearing said virtual image with light from an external scene, for presentation of a combined image to a user, wherein said virtual image generation system has output optics including a partially reflecting optical surface, wherein an optical axis of said light exiting said image generation system is tilted with respect to a normal to said optical surface, defining a tilt angle of greater than zero degrees between said optical axis and said normal to said optical surface, and wherein said partially reflecting optical surface has a baffle adjacent said optical surface, said baffle comprising an arrange of tubes each extending longitudinally along said optical axis of said light exiting said image generation system.

In embodiments a tube has a longitudinal length (h) which is sufficiently long for light entering the HUD along the optical axis at the edge of a tube (parallel to a side wall of the tube) to be substantially blocked by the (opposite) side wall of the tube. It will be appreciated that light parallel to the optical axis at the edge of a tube is a worst case for this given incidence incoming light at the centre of a tube imposes less of a constraint on the tube height (length) h. More particularly the constraint is that a ratio of a longitudinal length of the tube to a maximum lateral internal dimension of the tube is sufficiently large for incoming light parallel to the optical axis at the edge of the tube, which is reflected at the tilt angle, to be blocked by the opposite side wall of the tube.

This defines a minimum longitudinal length or height of a tube. Still more particularly a ray of light parallel to the optical axis incident anywhere along the edge of a tube should be blocked (depending upon the shape of the tube crosssection and orientation with respect to the reflecting surface this may include a corner-4ocorner reflection within a tube: a ray as previously described at the edge of a tube, in a corner, if present, should also be blocked). In embodiments, therefore, a longitudinal length h, of a (each) tube satisfies the constraint: h>dmax1 +tana tan 2a Where dmax is a maximum internal lateral dimension of the tube and a is the tilt angle.

In embodiments at least some light off the optical axis, more particularly at an angle to the optical axis equal to or greater than the tilt angle which is incident at the centre of a tube is reflected such that it is substantially blocked by a side wall of the tube. Thus, in embodiments, light incident at the centre of a tube at greater than a tilt angle is blocked. Preferably the tubes are long enough such that at least some light incident at the centre of the tube at greater than a half field of view angle of the HUD is blocked.

In embodiments the tubes may be sufficiently long to block substantially all reflections from the output surface of the HUD (though this is a much more stringent condition than the previous inequality and reduces the optical transmission of the system). In embodiments the length of a tube may thus satisfy the further constraint that: d max cosa *sina In embodiments a tube has a minimum lateral internal dimension which is sufficiently large for a field of view of the head up display to be substantially unrestricted by the baffle. More particularly a ratio of the minimum lateral internal dimension to the length of a tube is sufficiently large for a (maximum) field of view of the HUD to be substantially unrestricted (the FOV may be different in different directions). Thus in embodiments the FOV is effectively unrestricted by the baffle. In embodiments, therefore, the minimum lateral internal dimension dmjn satisfies the constraint: h�=--.I'--tana 2 tan(FOV/2) The baffle is not located at an image plane, so that it is not directly perceptible when observing a virtual image significantly further in the distance. However it may, nonetheless, have a perceptible effect on the viewed image. For this reason a non-rectangular tube cross-section is preferable as having a different symmetry to the rectangular symmetry of the display helps reduce the perceptibility of any artefacts arising from the baffle. In embodiments the cross-section of a tube may therefore be substantially hexagonal, and the tubes may be substantially close-packed. In other embodiments, however, the cross-section of a tube may be substantially square or rectangular.

As previously mentioned, in embodiments the partially reflecting surface is a final output optical surface of the output optics of the HUD (the output optics here not being considered as including the combiner, that is a combining optical surface, such as a vehicle windscreen, which combines the image from the HUD with an external scene).

This is advantageous for inhibiting sunlight reflections from the HUD. As previously mentioned, in preferred embodiments the output optics comprise exit pupil expander optics.

The exit pupil expander optics preferably comprise image replication optics comprising a pair of substantially planar reflecting optical surfaces defining substantially parallel planes spaced apart in a direction perpendicular to the parallel planes, a first, front optical surface and a second, rear optical surface. The image generation system is configured to launch a collimated beam into a region between the parallel planes. A small divergence, for example up to 3°, may be tolerated, especially if the image replication optics is located relatively close to the spatial light modulator (in a holographic image display system). The beam is launched at an angle to the normal to the parallel, reflecting planes, for example at greater than 15 degrees, 30 degrees, 45 degrees or more to this normal, such that the reflecting optical surfaces waveguide the beam in a plurality of successive reflections between the surfaces. The front optical surface is a partially transmitting mirrored surface, to transmit a proportion of the collimated beam when reflecting the beam such that at each reflection at the front optical surface a replica of the image is output from these optics. The rear optical surface is a coated, mirrored surface.

The front optical surface may either transmit a first polarisation and reflect an orthogonal polarisation, or transmit a proportion of the incident light substantially irrespective of polarisation. In the first case a phase retarding layer is included between the reflecting optical surfaces such for each reflection from the rear surface (two passes through the phase retarding layer) a component of light at the first polarisation is introduced, which is transmitted through the front optical surface. In the second case the transmission of the partially transmitting mirror depends on the number of replicas desired for example for four replicas, the mirror transmission is typically between 10% and 50%, but for ten or more replicas the range is typically in the range 0.1% to 10%.

Increased optical efficiency can be achieved by stacking two (or more) sets of image replication optics one above another so that a replicated beam from a first set of image replication optics provides an input beam to a second set of image replication optics (the latter preferably with a smaller spacing between the planar reflectors). This can be used to replicate beams in one dimension or in two dimensions.

In preferred embodiments the image generation system is a laser-based system comprising a laser light source illuminating image generating optics comprising a spatial light modulator (SLM), preferably a reflective SLM for compactness. There are many advantages of using a laser-based image generation system, especially when combined with a holographic image generation technique. However special problems are presented by laser-based image display systems because of the small etendue of laser sources. The etendue is preserved in a geometrical optical system and if a laser is employed to generate the light from which the image is produced, absent other strategies the etendue will be small, but in a laser-based image display system for a head-up display it is desirable to increase the etendue to increase the size of the region over which the displayed imagery may be viewed. An image replicator of the type we describe here is particularly useful to achieve this with a laser-based head up display.

In preferred embodiments the laser-based image generation system comprises a holographic image generation system, illuminating a spatial light modulator (SLM) with the laser light to generate a substantially collimated input beam for the pupil expander replication optics. Thus in embodiments a hologram generation processor drives the SLM with hologram data for the desired image. The processor converts input image data to target image data prior to converting this to a hologram, for a colour image compensating for the different scaling of the colour components of the multicolour projected image for replication when calculating this target image.

In some particularly preferred embodiments the processor is coupled to memory storing processor control code to implement and OSPR (One Step Phase Retrieval) type procedure. Thus in embodiments an image is displayed by displaying a plurality of temporal holographic subframes on the SLM such that the corresponding projected images (each of which has the spatial extent of a replicated output beam) average in a viewer's eye to give the impression of a reduced noise version of the image for display.

(It will be appreciated that for these purposes, video may be viewed as a succession of images for display, a plurality of temporal holographic subframes being provided for each image of the succession of images). We have previously described such techniques in, for example: WO 2005/059660 (Noise Suppression Using One Step Phase Retrieval), WO 2006/1 34398 (Hardware for OSPR), WO 2007/031797 (Adaptive Noise Cancellation Techniques), WO 2007/110668 (Lens Encoding), WO 2007/141 567 (Colour Image Display), and WO 2008/1 20015 (Head Up Displays), all hereby incorporated by reference.

In a related aspect the invention provides a method of inhibiting reflections of incoming light in a head up display, the method comprising generating a substantially collimated light beam comprising a virtual image for display, said virtual image having a field of view, said light beam defining an optical axis; passing said light beam through a tilted partially reflective optical surface, a normal to said optical surface having a greater than zero angle to said optical axis; passing said light beam exiting said tilted optical surface through an optical angular filter to attenuate light at greater than a threshold angle to said optical axis; wherein light in said collimated beam within said field of view is substantially unattenuated by said angular filter, and wherein at least some incoming light incident on said tilted partially reflective optical surface through said optical angular filter is partially reflected back towards said angular filter at greater than said threshold angle and attenuated.

In embodiments the threshold angle is selected such that reflections of incoming light, in particular sunlight, from the partially reflective optical surface, where these reflections are at greater than the threshold angle to the optical axis, are trapped by the angular filter. In embodiments reflections at an angle greater than the angle of the normal to the optical surface to the optical axis are trapped. Thus in embodiments light entering the head up display along the optical axis is trapped by the angular filter.

There is a special situation where light exiting along the optical axis of the head up display is directed towards a mirror or a substantially reflecting surface. In such a case absent angular filtering light reflected from this external mirror can be reinjected into the head up display and replicated by the reflecting surfaces of the optics, causing the appearance of a ghost or echo image. In this situation the angular filter should at least block incoming light at an angle of twice the tilt angle of the system (that is twice the angle between the optical axis and the normal to the optical surface), since this is the angle at which incoming light reflected from the mirror arrives. In a similar way, in the previously described aspects and embodiments of the invention, in some implementations a threshold angle for attenuation or cutoff of reflections from the front optical surface of the head up display is twice the tilt angle of the optical surface.

In a further related aspect the invention provides a head up display including means for inhibiting reflections of incoming light, the head up display comprising means for generating a substantially collimated light beam comprising a virtual image for display, said virtual image having a field of view, said light beam defining an optical axis; wherein an optical path for said light beam in said device includes (passes through) a tilted partially reflective optical surface, a normal to said optical surface having a greater than zero angle to said optical axis; wherein, in an output direction, said optical path exits said tilted optical surface through an optical angular filter to attenuate light at greater than a threshold angle to said optical axis; and wherein light in said collimated beam within said field of view is substantially unattenuated by said angular filter, and wherein at least some incoming light incident on said tilted partially reflective optical surface through said optical angular filter is partially reflected back towards said angular filter at greater than said threshold angle and attenuated.

Embodiments of the above described aspects of the invention are particularly applicable to head up displays for road vehicles such as cars.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described by way of example only, with reference to the accompanying figures in which: Figure 1 shows a headup display (HUD) incorporating a holographic image display system using an optical image replicator for an exit pupil expander; Figure 2 shows a functional representation of the pupil expansion based HUD of Figure 1; Figure 3 shows a functional representation of the pupil expansion based HUD of Figure 1 incorporating a reflected light shield according to an embodiment of the invention; Figure 4 shows a ray diagram illustrating reflection of light beams entering the system of Figure 3 within the angular filtering of the field of view; Figures 5a and 5b show an example of a shutter or baffle-based light shield according to an embodiment of the invention comprising an array of square base oblique (a=30°) tubular prisms; Figure 6 shows a ray diagram for determining a condition that the full field of view should at least be visible from the centre of each cell of a shutter or baffle of the type shown in Figure 5 when employed in a HUD as illustrated in Figure 3; Figures 7a and 7b show a ray diagrams for determining, respectively, a condition that incoming rays parallel to the optical axis are fully blocked, and a condition that no incoming light can escape the optical system after reflection from the front reflecting surface; Figures 8a and 8b show, respectively, a simplified ray diagram for the HUD of Figure 3, and a characterisation of the angular filtering for a generalised HUD of type shown in Figure 3 in which a generalised angular filter is employed; Figures 9a to 9c show, respectively, a ray diagram for reflection of an incoming ray for the HUD of Figure 3, a characterisation of the possible range of angles of the emerging reflected rays given a generalised angular filtering applied on the incoming rays, and a diagrammatic illustration of a condition on the angular filtering for no reflected incoming ray to emerge from the HUD; and Figure 10 illustrates a use-case of the HUD of Figure 3 where the HUD projects an image towards a mirror.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

To aid in understanding background and context for the description of preferred embodiments of the light shields we describe, it is helpful first to outline an example of a preferred implementation of our head up display. This uses a laser-based system to generate an image for display, more particularly an image generator which generates an image by calculating a hologram for the image and displaying this on an SLM.

Preferred implementations use an OSPR (One Step Phase Retrieval)-based hologram generation procedure. The skilled person will, however, appreciate from the later description that the techniques we describe are applicable to any type of head up display, albeit they have particular advantages for automotive HUDs.

OSPR-based hologram generation It will be appreciated that the techniques we describe are not limited to HUDs employing a laser or hologram-based image generation procedure. However in our preferred method of image generation the SLM is modulated with holographic data approximating a hologram of the image to be displayed. This holographic data defines a displayed image made up of a plurality of temporal sub-frames, each generated by modulating the SLM with a respective sub-frame hologram, the displayed image sub-frames spatially overlapping in the replay field. Each sub-frame when viewed individually would appear relatively noisy because noise is added, for example by phase quantisation by the holographic transform of the image data. However when viewed in rapid succession the replay field images average together in the eye of a viewer to give the impression of a low noise image. The noise in successive temporal subframes may either be pseudo-random (substantially independent) or the noise in a subframe may be dependent on the noise in one or more earlier subframes, with the aim of at least partially cancelling this out. Such a system can provide a visually high quality display even though each sub-frame, were it to be viewed separately, would appear relatively noisy.

The basic procedure is a method of generating, for each still or video frame I = sets of N binary-phase holograms h. We refer loosely to the procedure as One Step Phase Retrieval (OSPR). An example is shown below (more detailed, preferred implementations are described in the patent applications we have previously mentioned above): Let = i exp (;q)) where is uniformly distributed between 0 and 2 for I < <N/2 d 1, 2. Let = FI [] where F1 repnents the two-dimensional inverse Fourier transform operator for 1 it �= N/2 3. Let = for I <n <N/2 4. Letm = 9{g?} for I < N/2 5, Let = <. whei Q(fl) = median 1 itm > Q(fl) and I n < N Step 1 forms N targets equal to the amplitude of the supplied intensity target I,, with random or quasi-random phase. Step 2 computes the N corresponding full complex Fourier transform holograms g?. Steps 3 and 4 compute the real part and imaginary part of the holograms, respectively. Binarisation of each of the real and imaginary parts of the holograms is then performed in step 5; thresholding around the median of m? (or zero) aims to ensure approximately equal numbers of -1 and I points are present in the holograms for DC balance.

In embodiments the displayed hologram may encode a plurality of substantially two-dimensional images at different focal plane depths such that these appear at different distances from the observer's eye (each 2D image may be encoded with a different lens power, the hologram encoding a combination or sum of each of these). In this way the HUD is able to display multiple, substantially two-dimensional images at different effective distances from the observer's eye, all encoded in the same hologram. The image planes may have different colours or combinations of colours, by using two different holograms to encode the differently coloured images at different depths, displaying these successively on the SLM and controlling a colour of the light source in synchrony. Alternatively a more sophisticated, multicolour, three-dimensional approach may be employed. The ability to display images in different colours and/or at different visual depths is useful since more important imagery (symbology) can be placed in the foreground and/or emphasised using colour.

Head-Up DispHays Referring now to Figure 1, this shows an example of a head-up display (HUD) 1000 comprising a preferred holographic image projection system 1010 in combination with image replication optics 1050 and a final, semi-reflective optical element 1052 to combine the replicated images with an external view, for example for a cockpit display for a car driver 1054. As illustrated the holographic image projection system 1010 provides a polarised collimated beam to the image replication optics (through an aperture in the rear mirror), which in turn provides a plurality of replicated images for viewing by user 1054 via a combiner element 1052 which may comprise, for example, a chromatic mirror or the windscreen of a car (where the element is curved the hologram may be calculated for distortion introduced by reflection from this element).

The back optical surface of the image replication optics 1050 typically has a very high reflectivity, for example better than 95%.

In the example holographic image projector 1010 there are red R, green G, and blue B lasers and the following additional elements: SLM is the hologram SLM (spatial light modulator). In embodiments the SLM may be a liquid crystal device. Alternatively, other SLM technologies to effect phase modulation may be employed, such as a pixellated MEMS-based piston actuator device.

LI, L2 and L3 are collimation lenses for the R, G and B lasers respectively (optional, depending upon the laser output).

e Ml, M2 and M3 are corresponding dichroic mirrors.

PBS (Polarising Beam Splitter) transmits the incident illumination to the SLM.

Diffracted light produced by the SLM naturally rotated (with a liquid crystal SLM) in polarisation by 90 degrees is then reflected by the PBS towards L4.

Mirror M4 folds the optical path.

Lenses L4 and L5 form an output telescope (demagnifying optics), as with holographic projectors we have previously described. The output projection angle is proportional to the ratio of the focal length of L4 to that of L5. In embodiments L4 may be encoded into the hologram(s) on the SLM, for example using the techniques we have described in W02007/I 10668, and/or output lens L5 may be replaced by a group of projection lenses.

A system controller 1012 performs signal processing, in either dedicated hardware, or in software, or in a combination of the two, to generate hologram data from input image data. Thus controller 1012 inputs image data and touch sensed data and provides hologram data 1014 to the SLM. The controller also provides laser light intensity control data to each of the three lasers to control the overall laser power in the image.

An alternative technique for coupling the output beam from the image projection system into the image replication optics employs a waveguide 1056, shown dashed in Figure 1. This captures the light from the image projection system and has an angled end within the image replication optics waveguide to facilitate release of the captured light into the image replication optics waveguide. Use of an image injection element 1056 of this type facilitates capture of input light to the image replication optics over a range of angles, and hence facilitates matching the image projection optics to the image replication optics.

The arrangement of Figure 1 illustrates a system in which symbology (or any video content) from the headup display is combined with an external view to provide a head-up display within a vehicle. The eye-box is expanded to provide a larger exit pupil using a pair of planar, parallel reflecting surfaces to provide an image replicator located at any convenient point after a final optical element of the virtual image generation system, as previously described in our patent application number GB 0902468.8 filed 16 Feb 2009.

Light shields for head-up displays The output stage of the head-up display architecture shown in Figure 1 can be represented as illustrated in Figure 2, which shows a pupil expander 20 comprising substantially parallel front 22 and rear 24 reflecting surfaces into which a collimated input beam 26 bearing an image for display is injected at an angle a to the normal to the (planar) reflecting surfaces. The angle a defines a tilt angle of the pupil expander and the direction of the input beam 26 defines an optical axis 28 for the system. At successive reflections from the back reflecting surface the input beam is replicated 30a, b, c..., to provide an expanded exit pupil for the system.

In terms of its behaviour with respect to external solar illumination, this architecture has two important characteristics: the last surface (front reflecting surface 22) is reflective and the image formed by the HUD is formed by a light beam passing through this surface, and the image is projected off-axis to this last surface. This latter point means that there is a non-null angle a between the optical axis 28 of the projection optics and the front mirror 22 (typically, a 300). Thus with this architecture the vast majority of the incoming visible external light is reflected by the front reflective surface 22. For this reason, if we apply an angular selection on the useful angles coming out of the HUD the projected image can be almost unaffected whereas the incoming rays can be trapped by the light shield. More particularly the reason that the incoming rays can be trapped is that the mirror surface 22 reflects these rays off surface 22 with a significantly changed angle.

A practical embodiment of the pupil expander 20 of Figure 2 incorporating a light shield or baffle 50 is illustrated in Figure 3. In this figure incoming sunlight 32 is reflected from a front surface 22 as illustrated by cross-hatched arrows 34. The light shield or baffle comprises a set of tubes (shown in cross-section in Figure 3), the tubes being longitudinally aligned along the optical axis 28 and aligned at an angle to the perpendicular to the front reflecting surface 22. This light trap is effective especially where the reflectivity of the front reflecting surface 22 is high, and where the field of view of the HUD is reasonably small and in proportion to (of a similar order of magnitude size as) the tilt angle a of the pupil expander. This latter statement can be formalised into an approximate first order relation between the maximum field of view (FOV) and the angle a: if we assume that the light shield ideally passes the maximal viewing angles and that this same light shield ideally blocks all the reflected light entering through these angles, then we can formalise the condition that these two domains do not overlap: referring to Figure 4, this shows the geometry of the system, the rectangular cross-hatching 36 showing the allowed output angles according to the field of view of the HUD, the diagonal cross-hatching 38 illustrating angles of blocked reflected light from surface 22. In Figure 4 the field of view angular filtering selects the angles ranging from l3 to -I around the optical axis (where 213 is the field of view).

This filtering allows some incoming light to be reflected on the mirror surface. The incoming light beams with incident angles from +13 to -13 around the optical axis get reflected along the mirror's normal axis and appear emerging from the mirror within a certain range of angles.

A condition to realise to block this light is to ensure that none of the emerging angles are in the acceptance region of the angular filtering (i.e. from +13 to -f3 around the optical axis).

This condition can be expressed as follows: MaxFOV a> This condition links the tilt of the optical axis with regard to the mirror's normal with the maximum field of view (FOV) of the HUD. This is a necessary but not sufficient condition to formalise that the two aforementioned domains do not overlap although, as previously mentioned, in a practical system it may not always be desirable to impose this condition.

Figure 3 schematically illustrates an angular filter comprising an array of tubes.

However there are many other ways in which the angular filtering could be implemented including, 1. Dielectric angular filtering layers, 2. Microstructures (based on metallic layers or on diffractive optical element, 3. Index variations (total internal reflection trap), potentially limited by the index differences, 4. Holograms, 5. Other shutter structures.

The applicability of these different techniques depends upon the type of headup display and, for example, on whether or not coherent light, or polarised light, or multi colour light is employed. For example a hologram or other diffractive optical element is a potentially useful option as this may be configured to pass a range of angles for one or more of a set of colours. Alternatively if polarised light is employed a reflective polariser, for example of the type available from Moxtek lnc, USA may be employed as an angular filter since such materials (for example their ProFlux (TM) line) can have an angledependent response. In another approach a TIRbased angular trap may be provided as a thin layer in front of the front reflecting surface 22. In a still further approach microprisms may be employed, although these are less preferable because they can introduce artefacts. In yet another approach a pair of microlens arrays may be positioned to either side of a mask, again these elements lying across the front of the front reflecting surface 22 (see, for example, US5,351,151 which describes an optical filter device arranged along these lines). The skilled person will appreciate that an appropriate angular filter may be selected based upon, for example, the type of head-up display employed and upon cost. However, a particularly advantageous, and inexpensive, structure comprises an array of hollow prisms.

In more detail a preferred shutter or baffle structure comprises an array of hollow, oblique, tube-like prisms, preferably fabricated from or coated with a light-absorbing material. These tubes or prisms are oriented with an axis along the optical axis 28 and can be used in one or more layers having a defined height. Figures 5a and 5b show an example of such a structure which uses square base oblique prisms, with a tilted lower open end angled to match the tilt angle of the pupil expander (in the illustrated

example, 300).

Such an elementary structure can be made easily out of plastic or any light absorbing material structured in thin layers. It is preferable that the sides of the prisms are as thin as possible (within mechanical requirements) to avoid unnecessarily blocking light.

There is no specific requirement for the base of the prisms to be a square. A hexagonal base (honeycomb type structure) can be a good solution for regularity and symmetry for ease of fabrication of the structure, as well as for perception (breaking the usual square angle geometry).

One important design choice of the shutter structure is the height of the prisms. This height is preferably selected based on Tilt angle of the optical axis with reference to the mirror's normal axis, Viewing angles of the HUD, Prisms' base dimension.

A dimensioning procedure for a simple square base case is described hereafter.

Referring to Figure 6, assume the following notation: -a the tilt angle of the optical axis with reference to the mirror's normal axis, MaxFOV.

2 the half angle of the maximal field of view,

d the dimension of the elementary cell of the shutter, h the height (along the optical axis) of the shutter.

A preferable condition to fulfill is that the complete field of view is visible from the centre of each cell. This formalises as follows: d( 1 -.1 --tana �=h 2 tan/3) It is also preferable that at least the incoming rays parallel to the optical axis are fully blocked.

Referring to Figure 7a, this condition can be expressed as follows: h>d.I 1 +tana tan2a Practically, if we consider the following example case: a=30° d = 5mm Thenwe have: 5.8mm <h <27mm It can be appreciated that this leaves significant design freedom. The final selection of the height of the cell can be made based on the practical sun positions (in the intended application, for example position on a car dashboard) and bearing in mind that the height is preferably kept minimal to optimise light transmission in the complete angular range.

In addition to this, it is possible to calculate the condition that no incoming light (whether or not parallel to the optical axis) can escape the optical system after reflecting on the reflecting surface 22.

Referring to Figure 7b this can be expressed as follows: d h> cosa *sina which in the numerical example case above gives: 11.6mm <h <27mm.

We now consider a theoretical analysis of potential requirements for a generalised angular filter. This analysis assumes that the angular filtering performed on top of the reflecting surface is a perfectly sharp filtering forming a Heavyside step function.

We first explain the conditions under which no incoming light can emerge from the optical system after a reflection on the reflecting surface (condition for total light extinction).

Referring to the configuration of Figure 8a, if we consider an emerging ray forming an angle y with the optical axis (counter clockwise-positive notation), the angular filtering can be characterised as shown in Figure 8b.

Figure 8b shows that only the emerging rays with an angle in the range [-!3max +max] around the optical axis would be allowed out. This filtering is assumed to be equally true for the incoming rays meaning that only the incoming rays forming an angle in the range [-I3max: +I3max] around the optical axis would be allowed in.

Now consider an incoming ray reflected on the front reflecting surface, as shown in Figure 9a: This ray would emerge from the system with an angle ci+(a-7)2a-y.

Knowing the filtering on incoming rays, we can identify the possible range of emerging rays, as shown in Figure 9b.

Now these emerging rays need to pass again through the angular filtering which means that the filtering function on an incoming ray would be as shown in Figure 9c. Hence, an incoming ray cannot escape from the system when: 2 -flinax > flrnax a > flrnax This is the condition for total extinction of incoming light, assuming the angular filtering is perfect.

Referring now to Figure 10, this shows a special use case of a head-up display 30 incorporating a light shield as previously described, where the HUD projects an image towards a mirror in a particularly penalizing orientation. In the example of Figure 10, the pupil expander directs light towards a reflecting surface which is angled so as to direct image-carrying light from the head-up display back into the head-up display the incoming light is a reflection of the outgoing light. The reflecting surface could be, for example, a mirror placed inside the car or a portion of a windshield (if the windshield is curved there is a greater risk of a portion of the windshield having the orientation shown in Figure 10, reflecting light back into the head-up display). Light reflected back in can be reflected by the surface of the pupil expander and cause an echo image (viewable in a different direction to the main image). As can be seen from the geometry shown in Figure 10, incoming light is at an angle 2a to the optical axis and thus a light shield of the type previously described can effectively inhibit such light from re-entering the head-up display.

Broadly speaking we have described a light shield for systems producing virtual images through a significantly reflective surface non-normal to the projection axis. The virtual nature of the image allows the light shield to be placed in a plane distinct from the image plane so that it is not visible (and generates few artefacts). The reflective nature of the optical surface contributes to the filtering of the incoming light by reflection (in part, the origin of the problem). The off-optical axis nature of the system enables the system to work as we have described because this allows the reflecting surface to deflect the incoming light towards the shield. Thus the light shield may comprise a straight forward angular filter applied on top of the reflecting surface such that it acts not only as an angular filter, but also as a light trap.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims (22)

1. CLAIMS: 1. A head up display, the display comprising a virtual image generation system to generate a virtual image for presentation to an optical combiner to combine light exiting said image generation system bearing said virtual image with light from an external scene, for presentation of a combined image to a user, wherein said virtual image generation system has output optics including a partially reflecting optical surface, wherein an optical axis of said light exiting said image generation system is tilted with respect to a normal to said optical surface, defining a tilt angle of greater than zero degrees between said optical axis and said normal to said optical surface, and wherein said partially reflecting optical surface has an angular filter on an output side of said optical surface to attenuate external light reflected from said partially reflecting optical surface at greater than a threshold angle to said optical axis.
2. 2. A head up display as claimed in claim I wherein said threshold angle is substantially equal to said tilt angle.
3. 3. A head up display as claimed in claim I or 2 wherein said threshold angle is substantially equal to half a maximum field of view of said head up display.
4. 4. A head up display as claimed in any preceding claim wherein said tilt angle is greater than half a maximum field of view of said head up display.
5. 5. A head up display as claimed in any preceding claim wherein said angular filter comprises an array of tubes each extending longitudinally along said optical axis.
6. 6. A head up display, the display comprising a virtual image generation system to generate a virtual image for presentation to an optical combiner to combine light exiting said image generation system bearing said virtual image with light from an external scene, for presentation of a combined image to a user, wherein said virtual image generation system has output optics including a partially reflecting optical surface, wherein an optical axis of said light exiting said image generation system is tilted with respect to a normal to said optical surface, defining a tilt angle of greater than zero degrees between said optical axis and said normal to said optical surface, and wherein said partially reflecting optical surface has a baffle adjacent said optical surface, said baffle comprising an arrange of tubes each extending longitudinally along said optical axis of said light exiting said image generation system.
7. 7. A head up display as claim in claim 5 or 6 wherein light entering said head up display along said optical axis at an edge of a said tube is reflected off said partially reflecting surface at substantially said tilt angle, and wherein a said tube has a longitudinal length which is sufficiently long for said light reflected at said tilt angle at said edge of said tube to be substantially blocked by a side waU of said tube.
8. 8. A head up display as claimed in claim 7 wherein a longitudinal length of a said tube, h, satisfies: (1 h > dinax* + tan a tan 2a where dmax is a maximum internal lateral dimension of said tube and a is said tilt angle.
9. 9. A head up display as claimed in any one of claims 5 to 8 wherein light entering said head up display at an angle to said optical axis equal to or greater than said tilt angle and incident on said optical surface at a centre of a said tube is reflected from said output optical surface and substantially blocked by a side wall of said tube.
10. 10. A head up display as claimed in any one of claims 5 to 9 wherein light entering said head up display at an angle to said optical axis equal to or greater than half a maximum field of view of said head up display and incident on said optical surface at a centre of a said tube is reflected from said output optical surface and substantially blocked by a side wall of said tube.
11. 11. A head up display as claimed in any one of claims 5 to 10 wherein a longitudinal length of a said tube, h, satisfies: h> dinax cosa sina where dmax is a maximum internal lateral dimension of said tube and a is said tilt angle.
12. 12. A head up display as claimed in any one of claims 5 to 11 wherein a said tube has a minimum lateral internal dimension which is sufficiently large for a field of view of said head up display to be substantially unrestricted by said baffle.
13. 13. A head up display as claimed in any one of claims 5 to 12 wherein a minimum internal lateral dimension of said tube, d111 where length of said tube, h satisfies: d. ( 1 -tana 2 tan(FOVI2) a is said tilt angle and FOV is a maximum field of view of said display in the absence of said baffle.
14. 14. A head up display as claimed in any one of claims 5 to 13 wherein said array of tubes comprises a close packed array of substantially hexagonal cross-section tubes.
15. 15. A head up display as claimed in any preceding claim wherein said partially reflecting surface has a reflectance of at least 80% at a wavelength in the range 400nm to 700nm.
16. 16. A head up display as claimed in any preceding claim wherein said partially reflecting surface is a final output optical surface of said output optics.
17. 17. A head up display as claimed in any preceding claim wherein said output optics comprise exit pupil expander optics.
18. 18. A head up display as claimed in any preceding claim wherein said output optics comprise at least one set of substantially planar parallel optical surfaces having an output optical surface comprising said partially reflecting optical surface and a rear reflecting optical surface, wherein said planar parallel optical surfaces define substantially parallel planes spaced apart in a direction perpendicular to said parallel planes, and wherein said substantially planar optical surfaces define optical surfaces of a waveguide such that light launched into said waveguide parallel to said optical axis is reflected along said waveguide and escapes through said output optical surface when reflected at said output optical surface.
19. 19. A head up display as claimed in claim 18 wherein said virtual image generation system includes an image production system to generate a beam of substantially collimated light carrying said virtual image, and wherein said virtual image generation system is optically coupled to said output optics and configured to launch said collimated light into said waveguide along a direction substantially parallel to said optical axis.
20. 20. A head up display as claimed in claim 18 or 19 wherein said virtual image generation system is a laserbased image generation system.
21. 21. A method of inhibiting reflections of incoming light in a head up display, the method comprising: generating a substantially collimated light beam comprising a virtual image for display, said virtual image having a field of view, said light beam defining an optical axis; passing said light beam through a tilted partially reflective optical surface, a normal to said optical surface having a greater than zero angle to said optical axis; passing said light beam exiting said tilted optical surface through an optical angular filter to attenuate light at greater than a threshold angle to said optical axis; wherein light in said collimated beam within said field of view is substantially unattenuated by said angular filter, and wherein at least some incoming light incident on said tilted partially reflective optical surface through said optical angular filter is partially reflected back towards said angular filter at greater than said threshold angle and attenuated.
22. 22. A head up display including means for inhibiting reflections of incoming light, the head up display comprising: means for generating a substantially collimated light beam comprising a virtual image for display, said virtual image having a field of view, said light beam defining an optical axis; wherein an optical path for said light beam in said device passes through a tilted partially reflective optical surface, a normal to said optical surface having a greater than zero angle to said optical axis; wherein, in an output direction, said optical path exits said tilted optical surface through an optical angular filter to attenuate light at greater than a threshold angle to said optical axis; and wherein light in said collimated beam within said field of view is substantially unattenuated by said angular filter, and wherein at least some incoming light incident on said tilted partially reflective optical surface through said optical angular filter is partially reflected back towards said angular filter at greater than said threshold angle and attenuated.