The present invention relates to a display for augmented reality and virtual reality applications.

In an augmented reality headset a transparent waveguide is provided in front of a user’s eye or eyes. A light projector transmits light towards the waveguide. Light can be coupled into the waveguide by an input diffraction grating. Light then propagates within the waveguide by total internal reflection and an output diffraction grating couples light out of the waveguide and towards a viewer. In use, a viewer can see light from their external environment, transmitted through the transparent waveguide, as well as projected light from the projector. This can provide an augmented reality experience. A virtual reality headset works in a similar way, except that the user can only see projected light, and cannot see any light from their external environment.

One challenge in the field of augmented reality and virtual reality displays is to provide wide field-of-view polychromatic images. It is also desirable to provide augmented reality displays that provide augmented reality images at the edges of the user’s field-of-view so that these images do not distract from real world activity in the centre of the user’s vision. It is particularly desirable to provide below the horizon viewing, since this mimics conventional display panels and dashboards used in the automotive and aviation industries. Further, it is desirable to position projectors and electronics in positions where they provide minimum interference to a user’s vision and minimum obscuration of a user’s face.

US 2012/0127577 describes an optical device that can provide wide field of view polychromatic images. One problem with this system is that it cannot easily be adapted to provide below-the-horizon viewing in a convenient headset design.

An object of the present invention is to address and overcome some of these issues.

According to an aspect of the invention there is provided an augmented reality or virtual reality display device, comprising: a projector; a waveguide; an input diffractive optical element positioned in or on the waveguide configured to receive light from the projector and to couple the light into the waveguide; and an output diffractive optical element positioned in or on the waveguide configured to couple light out of the waveguide towards a notional viewing position; wherein the projector is configured to provide light to the input diffractive optical element in a direction that is at an angle to a waveguide normal vector at the position of the input diffractive optical element, and wherein the output diffractive optical element is configured to couple light out of the waveguide in a direction that is angled towards the waveguide normal vector at the position of the input diffractive optical element.

In this way, light is coupled out of the waveguide in a direction that is inclined back towards the position of the input diffractive optical element. This can enable injection of light at one side of a waveguide, and outcoupling of light from the other side of the waveguide in a direction that is inclined back towards the input position. In one arrangement this can facilitate injection of light at a top of a waveguide and below-the-horizon viewing from the output diffractive optical element which is at the bottom of the waveguide.

Preferably the projector provides light in a direction that is angled away from the position of the output diffractive optical element. In one configuration the projector may be provided on the same side of the waveguide as the notional viewing position. In an alternative configuration it would be possible for the projector to be positioned on the opposite side of the waveguide to the notional viewing position.

Preferably the angle between light from the projector and the waveguide normal vector is less than 25 degrees. Preferably the angle is greater than around 5 degrees.

In use, the projector is preferably provided in a position that is above the notional viewing position and the output diffractive optical element is provided at a position that is below the notional viewing position. This can provide below-the- horizon viewing for a user, using a projector that is mounted above the user’s eyes. This can provide a convenient mounting for a projector on a frame that is above the user’s eye, perhaps adjacent the user’s brow or within a motorcycle helmet, while still providing below-the-horizon viewing. This can be desirable for many augmented reality applications which seek to provide information at the edges of a user’s field of view, without distracting the user from real world activity occurring at the centre of vision. In one example, below-the-horizon augmented reality images may be able to provide a‘dashboard’ in a motorcycle helmet visor. In this way, the images can provide helpful augmented reality information without impairing the user’s vision of the road ahead.

In an alternative (presently less preferred) configuration the projector may be provided in a position that is below the notional viewing position and the output diffractive optical element may be provided at a position that is above the notional viewing position. This would provide above-the-horizon viewing for a user.

The display device may comprise a plurality of waveguides with respective input diffractive optical elements and output diffractive optical elements which have different properties in the respective waveguides. Thus, a stack of waveguides can be used together to provide different respective images. Preferably the colour dispersion in each waveguide is different so that the stack of waveguides provide a complementary effect.

The respective input diffractive optical elements and output diffractive optical elements may have different pitches in the respective waveguides. Preferably the pitches are less than or at least substantially equal to the shortest wavelength of the polychromatic light. In one example the pitches are preferably less than 400nm.

According to another aspect of the invention there is provided an augmented reality or virtual reality display device, comprising: a projector configured to project light at a plurality of wavelengths; a first waveguide and a second waveguide; a first input diffractive optical element positioned in or on the first waveguide configured to receive light from the projector and to couple the light into the first waveguide; a second input diffractive optical element positioned in or on the second waveguide configured to receive light from the projector and to couple the light into the second waveguide; a first output diffractive optical element positioned in or on the first waveguide configured to couple light out of the first waveguide towards a notional viewing position; a second output diffractive optical element positioned in or on the second waveguide configured to couple light out of the second waveguide towards the notional viewing position; wherein the projector is configured to provide light to the first and second input diffractive optical elements in a direction that is at an angle to a waveguide normal vector for the first and second waveguides at the respective positions of the first and second input diffractive optical elements, and wherein the first and second output diffractive optical elements are configured to couple light out of the first and second waveguides respectively in directions that are angled towards the waveguide normal vector at the positions of the first and second input diffractive optical elements, wherein the angular output of light for a selected wavelength from the first output diffractive optical element is different to the angular output of light at the selected wavelength from the second output diffractive optical element.

In this way, the output from each waveguide can provide a complementary effect so that, together, a broader range of angles is output than can be provided from a single waveguide. Preferably, for the selected wavelength, the first output diffractive optical element provides a range of angular outputs, in two- dimensions or in a polar plot. For the same selected wavelength, the second output diffractive optical element preferably provides a partially overlapping range of angular outputs. In this way, a larger range of angles can be provided from the pair of waveguides than is possible from one of the waveguides on its own.

The projector is preferably configured to provide red, green and blue light. At least two of these colours are preferably coupled into each of the first and second waveguides by the first and second input diffractive optical elements respectively. In some arrangements, wavelengths of red, green and blue light are respectively coupled into the first and second waveguides. In this way, each waveguide can provide a different range of angular outputs in red, green and blue wavelengths in order to provide a complementary effect from the perspective of a viewer. This is achieved by careful selection of the angular tilt of the waveguides and the pitches of the diffractive optical elements.

According to another aspect of the invention there is provided a method of operating an augmented reality or virtual reality display device, comprising the steps of: providing light from a projector towards an input diffractive optical element positioned in or on a waveguide in a direction that is at an angle to a waveguide normal vector at the position of the input diffractive optical element; coupling light into the waveguide at the input diffractive optical element; totally internally reflecting light within the waveguide from the input diffractive optical element towards an output diffractive optical element; coupling light out of the waveguide from the output diffractive optical element towards a notional viewing position in a direction that is angled towards the waveguide normal vector at the position of the input diffractive optical element.

According to another aspect of the invention there is provided a method of operating an augmented reality or virtual reality display device, comprising the steps of: providing a plurality of wavelengths of light from a projector towards a first input diffractive optical element positioned in or on a first waveguide in a direction that is at an angle to a waveguide normal vector at the position of the first input diffractive optical element; providing a plurality of wavelengths of light from the projector towards a second input diffractive optical element positioned in or on a second waveguide in a direction that is at an angle to a waveguide normal vector at the position of the second input diffractive optical element; coupling light into the first waveguide at the first input diffractive optical element; coupling light into the second waveguide at the second input diffractive optical element; totally internally reflecting light within the first waveguide from the first input diffractive optical element towards a first output diffractive optical element; totally internally reflecting light within the second waveguide from the second input diffractive optical element towards a second output diffractive optical element;coupling light out of the first waveguide from the first output diffractive optical element towards a notional viewing position in a direction that is angled towards the waveguide normal vector at the position of the first input diffractive optical element; coupling light out of the second waveguide from the second output diffractive optical element towards a notional viewing position in a direction that is angled towards the waveguide normal vector at the position of the second input diffractive optical element, wherein the angular output of light for a selected wavelength from the first output diffractive optical element is different to the angular output of light at the selected wavelength from the second output diffractive optical element.

Embodiments of the invention are now described, by way of example, with reference to the drawings, in which:

Figure 1 is a schematic side view of an optical set up in an augmented reality display in an embodiment of the invention;

Figure 2 is a schematic plan view of an optical set up in an augmented reality display in an embodiment of the invention;

Figures 3A-C provides three plots showing the angular output from an output element in a first waveguide in red, green and blue;

Figures 4A-C provides three plots showing the angular output from an output element in a second waveguide in red, green and blue;

Figure 5 is an example of an augmented reality image produced by a display in an embodiment of the invention; and

Figure 6 is an exploded schematic side view of a stack of waveguides in an embodiment of the invention. Figure 1 is a schematic side view of an optical set up in an augmented reality display. The display comprises an input projector 2 and a waveguide 4. The waveguide 4 comprises an input grating 6 and an output structure 8, which may be a photonic crystal or crossed gratings, as described in WO2016/020643. The output structure 8 is provided within a viewing window 10 which represents the normal field of view of a user, when viewed from a notional viewing position. The output structure 8, in this example, is provided in the lower half of the viewing window 10.

The waveguide 4 is, in fact, provided as a stack of separate waveguides. In some embodiments two or more waveguides may be provided within the stack.

The waveguide 4 is provided with a surface normal vector, n ₀ , at a position close to the projector 2 and the input grating 6. The projector 2 is configured to direct light so that the beam of light from the projector subtends an angle a ₀ to the waveguide normal vector, n ₀ . The angle a ₀ is within the range of 5-25 degrees. Of course, this effect could be achieved by projecting light horizontally, and tilting the waveguide 4 away from the projector 2, relative to the vertical, by an angle in the range of 5-25 degrees.

The input grating 6 receives and diffracts light from the projector 2. The diffracted light travels within the waveguide 4 by total internal reflection towards the output structure 8. Light is coupled out of the waveguide 4 by the output structure 8 in order to provide augmented reality or virtual reality images.

In this configuration the viewing position is on the same side of the waveguide 4 as the projector 2. However, it would be equally possible for these to be provided on opposite sides of the waveguide 4.

The output structure 8 couples light out of the waveguide 4 in a beam that subtends an angle to a waveguide normal vector at the position of the output structure 8. The outcoupled light is provided in a direction that is angled towards the location of the projector 2, and towards the normal vector n ₀ at the position of the input grating 6. The output structure 8 is provided at a position on the waveguide 4 that is below the horizon from the perspective of a viewer. The outcoupled light is angled upwards so that it appears to emanate from below the horizon from the viewer’s perspective. Advantageously this can be used to provide augmented reality images. Figure 5 is an example of such an augmented reality image that can be produced in this way in which information such as speed, speed limit and directions can be overlaid on the real world perceived by a user. The overlaid augmented reality images are presented in the lower half of the user’s field of view so that they do not distract from objects at the centre of the user’s vision.

A full colour display can be provided by using a stack of waveguides 4. In one example two waveguides 4 can be provided in the stack, both waveguides 4 made of BK7 glass. Each waveguide 4 in the pair is similar in structure, but the properties of the respective input gratings 6 and output structures 8 are different. In the first waveguide the input grating 6 and the output structures 8 are provided with a pitch of around 340nm. In the second waveguide the input grating 6 and the output structures 8 are provided with a pitch of around 420nm. The pitch of a grating corresponds to the separation of diffractive features. This corresponds to the separation of grooves in a blazed grating or the separation of refractive index structures in the case of a photonic crystal. In the first and second waveguides 4 the pitch of diffractive structure is less than the shortest wavelength of the polychromatic light received from the projector 2.

Each waveguide 4 in the stack receives polychromatic light which is diffracted at the input grating 6 and coupled out of the waveguide 4 at the output element 8. Colour dispersion occurs during this process, and there are angular differences in the output for different wavelengths. Figure 3A includes plots showing the angular output from the output element 8 of the first waveguide in red, green and blue. Figure 3B shows the angular output from the output element 8 of the second waveguide in the different colours.

As is clear from Figures 3A-C, the angular output from the output element 8 is shifted downwards and to the sides of the output element 8 as the wavelength of light increases. For blue light the optical output is central within the output element 8 and is muffin-shaped with sides that are angled inwardly towards the base. For green light the optical output is similar in shape, but is shifted downwards and has a wider field of view. For red light an optical output is only present in the extreme lower corners of the output element 8. Figures 4A-C show the optical output from the second waveguide, where the output structures are provided with a pitch of around 420nm. In this arrangement blue light is provided centrally within the user’s field of view. From a viewer’s perspective blue light from the first waveguide cannot be distinguished from blue light from the second waveguide, and therefore these combine to provide a good coverage of the lower half of the output element 8. Green light is provided from the output element 8 in the second waveguide centrally within the user’s field of view, and with a muffin shape. Green light from the first and second waveguides is combined from a viewer’s perspective to provide good coverage, down to the lower corners of the output element 8. In this way, green light from the first waveguide can be provided in regions where no green light is provided from the second waveguide, and vice- versa. Red light is provided from the output element 8 in the second waveguide at a position that is low within the user’s field of view. Red light from the first and second waveguides is combined from a viewer’s perspective to provide good coverage, down to the lower corners of the output element 8.

Figure 6 is an exploded schematic view of a stack of waveguides in an embodiment of the invention. A projector 112 provides a full colour image using first, second and third primary colours which are red, green and blue. Light from the projector 112 is received at an optical structure which includes a first waveguide 110 and a second waveguide 112. The first waveguide 110 is sometimes referred to as the“blue waveguide” 110, since it is predominantly for carrying blue light. The second waveguide 112 is sometimes to as the “red waveguide” 112, since it is predominantly for carrying red light.

The first and second waveguides 110, 120 each comprise two major, flat, parallel faces and are made of a transparent medium, such as glass having a refractive index, n, of around 1.7. Light from the projector 112 is transmitted through a front surface of the first waveguide 110 and is incident on a first input diffraction grating 102 on a rear surface. In some embodiments the first input diffraction grating 102 is a blazed reflection grating having a period of around 340nm.

A second input diffraction grating 106 is provided on a rear surface of the second waveguide 120. The second input diffraction grating 106 in this embodiment is a blazed reflection grating having a period of around 420nm. In different embodiments it is envisaged that different types of grating may be used for the first and second input diffraction gratings 102, 106 such as binary gratings or gratings with a sinusoidal profile.

The first input diffraction grating 102 is configured to diffract incident light from the projector 102. In particular, because of its period, the first input diffraction grating 2 diffracts a large proportion of the blue wavelengths, some of the green wavelengths, and a small proportion of the red wavelengths. The diffracted light travels within the first waveguide 110 by total internal reflection towards an expansion grating 104 or output element. The expansion grating 104 diffracts the light that is totally internally reflected within the first waveguide 110 so that it is coupled out of the first waveguide 110 and towards a viewer. The expansion grating 104 also provides a one or two-dimensional expansion of the light so that it can provide a large eye box for a viewer. Light that is output from the expansion grating 104 is angled towards the first waveguide normal vector nO, at the position of the first input diffraction grating 102. From a viewer’s perspective this provides below-the-horizon images to create an effect similar to that shown in Figure 5.

Light that is not diffracted by the first input diffraction grating 102 is transmitted and continues to propagate in the same direction as it was output from the projector 112. The transmitted light from the first input diffraction grating 102 includes a large proportion of the red wavelengths, around half of the green wavelengths, and a small proportion of the blue wavelengths.

A filter 114 is optionally positioned between the first and second waveguides 110, 120. In this example embodiment the filter 114 is a blue cut-off filter made of plastic which is designed substantially to block blue wavelengths of light and to allow red and green wavelengths to propagate towards the second waveguide 120. In this embodiment, glue spots 116 are provided between the edges of the filter 114 and the second waveguide 120. This is sometimes referred to as a glue gasket. The glue spots 116 have a width of around 50pm. In this way, the filter 114 is separated from the front surface of the second waveguide 120 by an air gap of approximately 50pm. The filter 114 has a thickness of around 0.5mm. Light is transmitted directly through the filter 114. Therefore, precise alignment of the filter 114 with the first and second waveguides 110, 120 is not strictly necessary. Light will be transmitted out of the filter 114 in the same direction as it is received from the projector 112, even if there is a slight misalignment between the filter 114 and the first and second waveguides 110, 120. This reduces manufacturing tolerances and means that the optical arrangement can be produced more easily.

The filter 114 is positioned at one end of the second waveguide 120, adjacent the second input grating 106. A shim 118 is provided at the other end of the second waveguide 120, adjacent the second expansion grating 108. The shim 118 has a thickness of around 0.5mm, which is the same as the thickness of the filter 114. The shim 118 is fixed to the second waveguide 120 by glue spots 122 which have a thickness of around 50pm, respectively. In this way, the shim 118 can space the first and second waveguides 110, 120 apart by the same amount as the filter 114. This can ensure that the spacing of the first and second waveguides 110, 120 is even along their respective lengths. Glue 111 is provided between the edges of the first waveguide 110 and the filter 114 and shim 118 so that the first and second waveguides 110, 120 can be connected together.

In another embodiment the filter 114 and the shim 118 may be affixed to the rear surface of the first waveguide 110, rather than the front surface of the second waveguide 120. In a further alternative, the filter 114 may be provided as a dielectric film on the front surface of the second waveguide 120. In this embodiment the film may be provided directly on the surface of the second waveguide 120 so that there is no air gap. The shim 118 may be omitted where the filter is provided as a dielectric film.

Light that passes through the filter 114 is transmitted through a front surface of the second waveguide 120 and is then incident on the second input diffraction grating 106 on the rear surface. The second input diffraction grating 106 diffracts the filtered light and couples it into the second waveguide 120 to be totally internally reflected within the second waveguide 120. The diffracted light then travels within the second waveguide 120 under total internal reflection towards an expansion grating 108 which diffracts the light again and couples it out of the second waveguide 120 towards a viewer. Light that is output by the first and second expansion gratings 104, 108 in the first and second waveguides 110, 120 respectively is combined so that a full colour augmented reality image can be formed and experienced by a viewer.

A tinted cover 130 is provided adjacent the second waveguide 120. Glue 132 is provided between the respective edges of the tinted cover 130 and the second waveguide 120. The tinted cover 130 provides protection for the second waveguide 120 since otherwise the second input diffraction grating 106 and the second expansion grating 108 would be exposed and accessible to damage. By providing a tint it is possible to reduce the brightness of light from the outside world and increase the contrast for augmented reality light. This can improve the efficiency by reducing the amount of power that needs to be supplied to the projector 112 to achieve a desired level of contrast. In other embodiments it is possible to provide a cover 130 that only provides protection and does not have any tint.

The first and second waveguides 110, 120 have respective thicknesses of around 1mm. The tinted cover 30 also has a thickness of around 1 mm. In this example embodiment the shim 118 and the filter 114 have respective thicknesses of around 0.5mm and there are three layers of glue, each with a thickness of around 50pm.

Therefore, the overall thickness of the stack may be around 3.65mm.

The light that is output from the first and second expansion gratings 104, 108 has different angular properties at different wavelengths, as shown in Figures 3A-C and 4A-C for an embodiment which does not include the filter 114, where red, green and blue light are coupled into the first and second waveguides 110, 120. In particular, the angular output from each waveguide can provide a complementary effect so that, together, at any given wavelength a broader range of angles is output than is provided by either waveguide. For some wavelengths the angular range of outputs from one waveguide are partially overlapping with the range of angular outputs from the other waveguide.