Patent Description:
Optical apparatus, such as exit pupil expanders, can be used in display systems and devices such as near eye displays, augmented and/or virtual reality headsets and head up displays for example.

Certain features and views of the figures may be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Similar reference numerals are used in the figures to designate similar features. For clarity, all reference numerals are not necessarily displayed in all figures.

<FIG>, and <FIG> illustrate embodiments of the claimed invention. All other figures illustrate examples which do not unambiguously feature all the features recited in independent claim <NUM>.

<FIG> shows an example light guiding means <NUM> that can be used in examples of the disclosure. The light guiding means <NUM> can be formed on a waveguide, an optical substrate, a transparent plate or any other suitable material.

In this example the light guiding means <NUM> comprises an exit pupil expander. The exit pupil expander is configured to increase the size of an exit pupil from a light engine or other optical arrangement. The light engine could be a display means such as a light engine, projection engine, or a picture generating unit.

The light guiding means <NUM> comprises in-coupling diffractive means <NUM>, expanding means <NUM>, and out-coupling diffractive means <NUM>, located on the surfaces or inside the volume of the light guiding means <NUM>.

The in-coupling diffractive means <NUM> comprise any means that is configured to in-couple one or more beams of light from a light engine into the light guiding means <NUM>. The in-coupling diffractive means <NUM> is positioned within the light guiding means <NUM> so that, in use, the in-coupling diffractive means <NUM> can be positioned adjacent to the light engine.

The in-coupled beam of light travels inside the light guiding means <NUM> via total internal reflection. The refractive index of the material that is used for the light guiding means <NUM>, the wavelength of the in-coupled beam, and the parameters of the in-coupling diffractive means <NUM> determine the total internal reflection angles.

The expanding means <NUM> is positioned within the light guiding means <NUM> so that the in-coupled beam of light is provided from the in-coupling diffractive means <NUM> to the expanding means <NUM>.

The expanding means <NUM> comprise any means that is configured to expand the in-coupled beam of light in at least one dimension. The expanding means <NUM> can comprise a diffractive means such as a diffraction grating or any other suitable means. In the diffraction grating the in-coupled beam of light is split into two with every interaction with the diffraction grating. The interaction could be, for example, an internal reflection. The two split sections of the beam travel in different directions and continue splitting and so expand the exit pupil of the light engine. In the example shown in <FIG> the expanding means <NUM> has a grating which expands the beam in a horizontal direction.

The out-coupling diffractive means <NUM> is positioned within the light guiding means <NUM> so that the horizontally expanded beam of light is provided from the expanding means <NUM> to the out-coupling diffractive means <NUM>.

The out-coupling diffractive means <NUM> comprises any means that is configured to out-couple the horizontally expanded light beam out of the light guiding means. The out-coupling diffractive means <NUM> can function in a similar manner to the expanding means <NUM> so that the expanded beam of light is split into two with every interaction with the diffraction grating. The out-coupling diffractive means <NUM> can also be configured to expand the horizontally expanded beam of light in a second dimension. In the example shown in <FIG> the out-coupling diffractive means <NUM> comprises horizontal grating lines which expand the horizontally expanded beam in the vertical direction.

The light guiding means <NUM> is configured so that the out-coupled expanded beam of light can be viewed by a user. The out-coupled expanded beams of light provide a virtual image that can be observed by a user. The out-coupled beam of light therefore provides an expanded exit pupil.

It is to be appreciated that the variations in the size, shape, position, and expansion direction of the different diffractive means are examples and that other variations could be used in other examples of the disclosure. For example, the expanding means <NUM> could expand the light beam in some other direction than horizontal. As another example, the out-coupling diffractive means <NUM> could expand the beam in some other direction than vertical. As a third example, the exit pupil of the beam of light in-coupled by the in-coupling diffractive means <NUM> would be expanded in the first expansion dimension already prior to in-coupling the beam into the light guiding means <NUM>. In this example there would be no expanding means <NUM> positioned within the light guiding means <NUM>. As a fourth example, beams would be in-coupled by the in-coupling means <NUM> into two different directions inside the light guiding means <NUM>. In this example there could be two separate expanding means <NUM> and either one shared or two separate out-coupling means <NUM> provided for these two directions.

The diffractive means that are used for the in-coupling diffractive means <NUM>, expanding means <NUM>, and out-coupling diffractive means <NUM> can comprise any means that can be configured to diffract the input beams of light. The diffractive means can comprise any one or more of a diffractive optical element, diffractive structure, diffraction gratings, holographic gratings, Bragg gratings, rulings, ridges, surface relief diffractive gratings, volume holograms, or any suitable optical component or feature having a periodic structure that splits and diffracts light into several beams travelling in different directions.

<FIG> shows how the light guiding means <NUM> shown in <FIG> can result in uneven brightness within the exit pupil.

In <FIG> an image <NUM> is provided to the in-coupling diffractive means <NUM>. The image <NUM> in this example comprises a single dot <NUM> in the centre of the image <NUM>. This corresponds to a beam of light perpendicular to the light guiding means <NUM>, being projected from the light engine.

<FIG> shows how the in-coupled beam of light comprising the image <NUM> is expanded in a horizontal direction by the expanding means <NUM> and then in a vertical direction by the out-coupling diffractive means <NUM>. This provides an expanded horizontal exit pupil <NUM> and an expanded vertical exit pupil <NUM> for the out-coupled beams of light.

An example exit pupil gate <NUM> that out-couples the beam of light into the exit pupil is shown and a representation of the light energy distribution of the out-coupled beam in the exit pupil <NUM> that would be provided within this exit pupil gate <NUM> is shown. The energy distribution of the exit pupil <NUM> is not uniform. The energy distribution of the exit pupil <NUM> has dark regions towards the lower right-hand corner of exit the pupil gate <NUM>. This corresponds to the regions of the out-coupling diffractive means <NUM> that are furthest away from the expanding means <NUM> and the in-coupling diffractive means <NUM>. Therefore, the beams of light that are out-coupled in this region of the exit pupil gate <NUM> have travelled further through the light-guiding means <NUM> and so have a lower intensity. This results in un-even brightness within any images that are provided by these light guiding means <NUM>.

It is to be appreciated that <FIG> is a schematic diagram and is not shown to scale. In implementations of the disclosure the in-coupling diffractive means <NUM> would likely be much larger so that the exit pupils would be larger and have more overlap. This could increase the uniformity of the out-coupled beam. Also, the images in <FIG> have been shown for a single wavelength of light. It is to be appreciated that different wavelengths of light would be in-coupled to slightly different angles also partly adding to the increased beam uniformity.

<FIG> shows how the uneven brightness within the exit pupil can be, at least partially, corrected by providing different diffractive efficiencies within the light guiding means <NUM>.

In the example shown in <FIG> the thickness of the expanding means <NUM> increases along the horizontal length of the expander means. Plot <NUM> schematically shows how the thickness of the expanding means <NUM> gradually increases along the length of the expanding means <NUM> in the direction indicated by arrow <NUM>. The thickness of the out-coupling diffractive means <NUM> can also increase along the vertical length of the out-coupling diffractive means <NUM> in the direction indicated by arrow <NUM>.

The variations in thickness can provide variable diffraction efficiencies within different parts of the light guiding means <NUM>. This can help to reduce unevenness in the brightness of the exit pupil. As shown in <FIG> the energy distribution of the exit pupil <NUM> provided within the exit pupil gate <NUM> has a more even brightness compared to the energy distribution of the exit pupil <NUM> provided by the light guiding means <NUM> shown in <FIG>.

It is to be appreciated that other means for providing variable diffraction efficiencies can be used in other examples. For instance, a different grating line fill factor, or grating line profile can be used for different parts of the diffractive means within the light guiding means <NUM>. It is also possible to combine multiple different means for providing more subtle variations in diffraction efficiencies. For example, different thickness levels as well as grating line fill factor levels can simultaneously be used in different parts of the diffractive means within the light guiding means <NUM>.

<FIG> shows another example of how the light guiding means <NUM> shown in <FIG> can result in uneven brightness within images. This shows how the arrangement of the diffractive means within the light guiding means <NUM> can result in bright corners within the exit pupil.

<FIG> shows another example image <NUM> that can be provided to the in-coupling diffractive means <NUM>. This example image <NUM> comprises a single dot <NUM> in the lower right-hand corner of the image <NUM>.

An example exit pupil gate <NUM> is shown in the lower right hand corner of the out-coupling diffractive means <NUM>. The energy distribution of the exit pupil <NUM> corresponding to this exit pupil gate <NUM> is also shown.

This shows a bright region <NUM> in the lower right-hand corner of the energy distribution of the exit pupil <NUM>. This is caused by the section of the light guiding means <NUM> indicated within the dashed lines <NUM>. This section is provided between the expanding means <NUM> and the out-coupling diffractive means <NUM>. This section does not comprise any diffractive grating and so the rays of light that travel through this section do not reduce in intensity as much as the rays of light that travel through the out-coupling diffractive means <NUM>. This therefore causes the bright bottom corner in the energy distribution of the exit pupil <NUM> of the out-coupled image <NUM>.

<FIG> shows a light guiding means <NUM> according to examples of the disclosure. The light guiding means <NUM> comprises an in-coupling diffractive means <NUM>, an expanding means <NUM> and an out-coupling diffractive means <NUM>.

In the example shown in <FIG> the out-coupling diffractive means <NUM> comprises different sections <NUM>, <NUM> that are configured to out-couple the expanded beams of light with different efficiencies. The different sections <NUM>, <NUM> are configured to provide a more uniform brightness for exit pupils of the light guiding means <NUM>. In particular the different sections <NUM>, <NUM> can be configured to address problems with the bright bottom corners of the exit pupils as shown in <FIG>.

The out-coupling diffractive means <NUM> comprises a first section <NUM> which is configured to out-couple the one or more expanded beams of light with a first efficiency. The out-coupling diffractive means <NUM> also comprises one or more second sections <NUM> which are configured to out-couple the one or more expanded beams of light with a second efficiency that is different to the first efficiency.

The second efficiency is lower than the first efficiency so that the second sections <NUM> of the out-coupling diffractive means <NUM> out-couple the expanded beams of light with a lower efficiency than the first sections <NUM> of the out-coupling diffractive means <NUM>.

In some examples the efficiency with which beams of light are outcoupled can vary across the respective sections <NUM>, <NUM> so that the out-coupling efficiency varies across the respective areas. The overall efficiency of the coupling from the second section <NUM> would be lower than the overall efficiency of the first section <NUM>.

The one or more second sections <NUM> are positioned within the out-coupling diffractive means <NUM> so as to reduce bright sections from appearing within the exit pupils. The one or more second sections <NUM> are therefore configured to control the brightness of an optical output provided by an apparatus comprising the light guiding means <NUM>.

The relative positions and arrangements of the different sections <NUM>, <NUM> within the out-coupling diffractive means <NUM> can depend on the configuration of the light guiding means <NUM>. For example, it can depend upon the size, shape and relative positions of the respective diffractive means within the light guiding means <NUM>.

The out-coupling diffractive means <NUM> shown in <FIG> has a different size and shape to the out-coupling diffractive means <NUM> shown in <FIG>. In <FIG> the out-coupling diffractive means <NUM> has a rectangular shape while in <FIG> the out-coupling diffractive means <NUM> has a trapezium shape.

In the example of <FIG> the out-coupling diffractive means <NUM> is sized and shaped to align with a size and shape of the preceding diffractive means to enable extreme rays of the preceding diffractive means to be outcoupled by the out-coupling diffractive means <NUM>. In this example the preceding diffractive means is the expanding means <NUM>. In other examples the preceding diffractive means could be an in-coupling diffractive means <NUM> or other intervening diffractive means could be comprised within the light guiding means <NUM>.

The extreme rays of the preceding diffractive means are the rays of light that are diffracted on the edges of the preceding diffractive means. In the example shown in <FIG> the extreme rays comprise the rays that pass through the section indicated by the dotted line <NUM> that does not comprise any diffractive grating. However, in the example of <FIG> the out-coupling diffractive means <NUM> extends far enough in the horizontal direction that these extreme rays are incident on the out-coupling diffractive means <NUM>.

In examples of the disclosure the outcoupling diffractive means <NUM> can be sized so that the edge of the out-coupling diffractive means <NUM> extends at least as far as the edges of the expanding means <NUM> so that the extreme rays of the expanding means <NUM> are incident on the out-coupling diffractive means <NUM>. In the example of <FIG> the side edges of the out-coupling diffractive means <NUM> are aligned with the edges of the expanding means <NUM>. Other configurations could be used in other examples of the disclosure.

In the example of <FIG> the longer base of the trapezium is positioned closer to the expanding means <NUM> and the shorter base of the trapezium is positioned further away from the expanding means <NUM>.

In the example of <FIG> the trapezium shape of the out-coupling diffractive means <NUM> is formed from a rectangular first section <NUM> and two triangular second sections <NUM>. The two triangular second sections <NUM> are positioned on either side of the rectangular first section <NUM>. The first section <NUM> of the out-coupling diffractive means <NUM> has the same size and shape as the standard out-coupling diffractive means <NUM> shown in <FIG> and the second sections <NUM> are provided as additional sections on the side of the first section <NUM>. Other arrangements for the respective sections <NUM>, <NUM> could be used in other examples of the disclosure. For instance, in other examples a second section <NUM> might only be provided at one of the side edges of a rectangular first section <NUM>.

The triangular second sections <NUM> have a lower diffractive efficiency than the first section <NUM>. The different efficiencies of the different sections <NUM>, <NUM> of the out-coupling diffractive means <NUM> can be achieved using any suitable means. For instance, the diffractive grating in the second section <NUM> could have a different fill factor for the diffractive grating, a different diffractive grating depth, a different grating profile, or any other suitable different properties. <FIG> shows an example of a light guiding means where the different sections <NUM>, <NUM> have different configurations for the diffractive gratings.

In other examples the different efficiencies of the different sections <NUM>, <NUM> of the out-coupling diffractive means <NUM> can be achieved by using absorptive means to absorb some of the light in the second section <NUM>. <FIG> shows an example light guiding means <NUM> comprising absorptive means.

<FIG> shows an example of a light guiding means <NUM> where the different sections <NUM>, <NUM> achieve different diffractive efficiencies by having different configurations for the diffractive gratings. In this example the out-coupling diffractive means <NUM> is provided in a trapezium shape with triangular shaped second sections <NUM> provided at the edge of a rectangular shaped first section <NUM> as shown in <FIG>. Different shapes and configurations for the out-coupling diffractive means <NUM> and the respective sections of the out-coupling diffractive means <NUM> could be used in other examples of the disclosure.

The first section <NUM> of the out-coupling diffractive means <NUM> comprises a diffractive grating having a first periodicity. The diffractive grating has the same periodicity across all of the first section <NUM> of the out-coupling diffractive means <NUM>. The diffractive grating is present across all of the first section <NUM> of the out-coupling diffractive means <NUM>.

The second section <NUM> of the out-coupling diffractive means comprises a diffractive grating that is not present across all of the area covered by the second section. In the example shown in <FIG> the second section <NUM> comprises sub-sections <NUM> that comprise a diffractive grating and sub-sections <NUM> that do not comprise a diffractive grating. The beams of light are not out-coupled in the subsections <NUM> that do not comprise the diffractive grating. This reduces the amount light out-coupled in the second section <NUM> sections compared to the light out-coupled by the first section <NUM>.

In the example shown in <FIG> each of the sub-sections <NUM> that comprise a diffractive grating are shown as comprising two slits. It is to be appreciated that this is not shown to scale and that any number of grating lines or grating structures could be provided in each of these sub-sections.

The diffractive grating that is provided in sub-sections <NUM> of the second section <NUM> has the same periodicity as the diffractive gratings that is provided in the first section.

The sub-sections <NUM> with diffractive gratings and sub-sections <NUM> without diffractive gratings are provided in alternating sub-sections in the second sections <NUM>. The sub-sections <NUM> having no diffractive grating are positioned in-between sub-sections <NUM> that comprise diffractive gratings. The periodicity of the subsections <NUM>, <NUM> within the second section <NUM> of the out-coupling diffractive means <NUM> can be selected to support the manufacturing process in order to improve the manufacturing efficiency. For example, the periodicity can be based on the stitched pattern generation size of the electron-beam lithography equipment. The periodicity of the subsections <NUM>, <NUM> within the second section <NUM> of the out-coupling diffractive means <NUM> is configured to reduce phasing within the expanded beam of light. For example, non-repeating patterns are created so that phase diffraction due to the periodicity of the subsections is minimized.

As shown in <FIG> the extreme rays from the edges of the expanding means <NUM> are incident on the second section <NUM> of the out-coupling diffractive means <NUM>. This controls the brightness of these rays of light so that all of the rays out-coupled within the exit pupil gate <NUM> have a similar brightness level.

When the image <NUM> comprising the dot <NUM> in the lower right-hand corner is provided to the light guiding means <NUM> the second sections <NUM> of the out-coupling diffractive means <NUM> control the brightness of the beams of light that pass through the second sections <NUM> and out-couple from the exit pupil gate <NUM> towards the exit pupil. This enables the brightness of these beams after out-coupling to be matched to the brightness of the beams in the rest of the exit pupil. The energy distribution of the exit pupil <NUM> shown in <FIG> has an even distribution and so shows a high level of uniformity.

The diffractive efficiency of the second sections <NUM> can be controlled by controlling the relative proportions of the areas of the sub-sections <NUM> comprising diffractive gratings with the areas of the sub-sections <NUM> that do not comprise diffractive gratings. If the proportion of the sub-sections <NUM> with diffractive gratings is too high then this would reduce the brightness of the beams of light energy distribution of the exit pupil <NUM>. Conversely if the proportion of the sub-sections <NUM> with diffractive gratings is too low then this would not reduce the brightness of the beams of light enough. This would result in a brighter area in the lower right-hand corner of the energy distribution of the exit pupil <NUM>.

<FIG> shows an example of a light guiding means <NUM> where the different sections <NUM>, <NUM> achieve different brightness of the beams of light by using an absorptive means in the second sections <NUM>. In this example the out-coupling diffractive means <NUM> is provided in a trapezium shape with triangular shaped second sections <NUM> provided at the edge of a rectangular shaped first section <NUM> as shown in <FIG>. Different shapes and configurations for the out-coupling diffractive means <NUM> and the respective sections of the out-coupling diffractive means <NUM> could be used in other examples of the disclosure.

In this example the absorptive means <NUM> comprises an absorptive coating. The absorptive coating is provided in the second sections <NUM> of the out-coupling diffractive means <NUM>. The absorptive coating can be provided as a surface treatment on the second sections <NUM> of the out-coupling diffractive means <NUM>.

In some examples the absorptive coating can be provided covering all of the second sections <NUM> of the out-coupling diffractive means <NUM>. In other examples the absorptive coating could be provided on just part of the second sections <NUM>.

As shown in <FIG> the extreme rays from the edges of the expanding means <NUM> are incident on the second section <NUM> of the out-coupling diffractive means <NUM>. The absorptive means <NUM> controls the brightness of the beams of light so that the out-coupled beam within the exit pupil gate <NUM> is uniform, or substantially uniform.

When the image <NUM> comprising the dot <NUM> in the lower right-hand corner is provided to the light guiding means <NUM> the absorptive means <NUM> in the second sections <NUM> of the out-coupling diffractive means <NUM> control the brightness of the beams of light that pass through the section sections <NUM>. This enables the brightness of these beams after out-coupling within the exit pupil gate <NUM> towards the exit pupil to be matched to the brightness of the beams in the rest of the exit pupil. The energy distribution of the exit pupil <NUM> shown in <FIG> has an even distribution and so shows a high level of uniformity.

The brightness of the beams of light travelling through the second sections <NUM> can be controlled by controlling the absorptive means <NUM> used in the second sections. For example, the brightness can be increased by providing a smaller amount of absorptive means <NUM> or can be decreased by provided a larger amount of absorptive means <NUM>.

<FIG> show different layout examples of light guiding means <NUM> comprising different arrangements for the diffractive means and the sections <NUM>, <NUM> within the out-coupling diffractive means <NUM>. In these examples the second section <NUM> of the out-coupling diffractive means <NUM> has a lower diffractive efficiency than the first section <NUM>. The differences in the diffractive efficiencies could be achieved by using different arrangements for the diffractive gratings as shown in <FIG>, absorptive coatings <NUM> as shown in <FIG> or any other suitable means.

In the example of <FIG> the light guiding means <NUM> comprises two expanding means <NUM>. The in-coupling diffractive means <NUM> is positioned between the two expanding means <NUM> so as to provide a symmetrical arrangement.

In the example of <FIG> out-coupling diffractive means <NUM> comprises a trapezium shape. The first section <NUM> of the out-coupling diffractive means <NUM> comprises a rectangular section in the centre of the out-coupling diffractive means <NUM> and the second sections <NUM> of the out-coupling diffractive means <NUM> are provided as two triangles at the sides of the rectangular first portion <NUM>.

In the example of <FIG> the light guiding means <NUM> comprises an in-coupling diffractive means <NUM> and an out-coupling diffractive means <NUM>. In this example the beam of light has been expanded in a first direction before it is provided to the light guiding means <NUM>. In this example the in-coupling diffractive means <NUM> has an elongate shape and no additional expanding means <NUM> is provided.

In this example the in-coupling diffractive means <NUM> is the preceding diffractive element for the out-coupling diffractive means <NUM>.

The out-coupling diffractive means <NUM> comprises two second sections <NUM> configured to out-couple the expanded beams of light with a lower level of efficiency compared to the first section <NUM> of the out-coupling diffractive means <NUM>. In the example of <FIG> the second sections <NUM> comprise triangular portions provided at the side edges of trapezium shaped out-coupling diffractive means <NUM>.

In the example of <FIG> the in-coupling diffractive means <NUM>, the expanding means <NUM> and the out-coupling diffractive means <NUM> are provided in a sequence that extends along the horizontal direction so that the in-coupling diffractive means <NUM> is positioned to the left of the expander means <NUM> and the out-coupling diffractive means <NUM> is positioned to the right of the expander means <NUM>.

In the example of <FIG> the in-coupling diffractive means <NUM> is provided above the expanding means <NUM> so that the in-coupling diffractive means <NUM>, expanding means <NUM> and out-coupling diffractive means <NUM> are provided in an L-shape.

In the example of <FIG> the in-coupling diffractive means <NUM> and the expander means <NUM> are provided above the outcoupling diffractive means <NUM> so that the in-coupling diffractive means <NUM>, expanding means <NUM> and out-coupling diffractive means <NUM> are provided in an inverted L-shape.

In each of the examples of 8B to 8E the out-coupling diffractive means <NUM> comprises two second sections <NUM> configured to out-couple the expanded beams of light with a lower level of efficiency compared to the first section <NUM> of the out-coupling diffractive means <NUM>. In each of these examples the second sections <NUM> comprise triangular portions provided at the side edges of trapezium shaped out-coupling diffractive means <NUM>.

In the example shown in <FIG> the light guiding means <NUM> comprises two in-coupling diffractive means <NUM>, two expanding means <NUM> and a central out-coupling diffractive means <NUM>. The first in-coupling diffractive means <NUM> and the first expander means <NUM> are provided above the shared out-coupling diffractive means <NUM> so that the first in-coupling diffractive means <NUM>, first expanding means <NUM> and shared out-coupling diffractive means <NUM> are provided in an inverted L-shape. The second in-coupling diffractive means <NUM> and second expanding means <NUM> are provided below the shared out-coupling diffractive means <NUM> so that the shared out-coupling diffractive means <NUM>, the expander means <NUM> and the in-coupling means <NUM> are provided in an L shape.

The shared out-coupling diffractive means <NUM> comprises a rectangular first portion <NUM> provided in the centre of the out-coupling diffractive means <NUM>. The second sections <NUM> are provided on the sides of the rectangular first portion <NUM>. In this example each second section has a shape formed from two right angled triangles connected at the apex of the triangles. Other shapes of the sections <NUM>, <NUM> of the out-coupling diffractive means <NUM> can be used in other examples of the disclosure.

<FIG> shows another example the light guiding means <NUM>. In this example the variations in thickness of the expanding means <NUM> and the out-coupling diffractive means <NUM> can cause an unevenness in brightness of the exit pupil.

In the example shown in <FIG> the thickness of the expanding means <NUM> increases along the horizontal length of the expanding means <NUM>. The expanding means <NUM> is shallowest closest to the in-coupling diffractive means <NUM> and deepest furthest away from the in-coupling diffractive means <NUM>. Plot <NUM> schematically shows how the thickness of the expanding means <NUM> increases along the length of the expanding means <NUM>. In this example the thickness of the expanding means <NUM> increases in step wise increments. This target variation in depth may be intended to be a smooth curve however due to manufacturing tolerances the actual variation in thickness could be provided in the step wise increments.

The thickness of the out-coupling diffractive means <NUM> can also increase along the vertical length of the out-coupling diffractive means <NUM>. The out-coupling diffractive means <NUM> is shallowest closest to the expanding means <NUM> and deepest furthest away from the expanding means <NUM>. The variation in thickness of the out-coupling diffractive means <NUM> can also increase in stepwise increments.

The stepwise variations in thickness of the expanding means <NUM> and the out-coupling diffractive means <NUM> can cause uneven brightness in the exit pupil of the light guiding means <NUM>. In <FIG> an image <NUM> is provided to the in-coupling diffractive means <NUM>. The image <NUM> in this example comprises a single dot <NUM> in the centre of the image <NUM>.

An example exit pupil gate <NUM> is shown and a representation of the energy distribution of the exit pupil <NUM> that would be provided within this exit pupil gate <NUM> is shown. The energy distribution of the exit pupil <NUM> is non-uniform and has a plurality of dark regions corresponding the stepped increases in the depth of the expanding means <NUM> and the out-coupling diffractive means <NUM>.

<FIG> shows another example light guiding means <NUM> that is configured to correct the unevenness in brightness that is caused by the step-wise variation in thickness of the expanding means <NUM> and the out-coupling diffractive means <NUM>.

In this example the out-coupling diffractive means <NUM> comprises a first section <NUM> and a second section <NUM> where the second section <NUM> has a lower diffractive efficiency to account for the variations in thickness of the respective components of the light guiding means <NUM>.

In this example the out-coupling diffractive means <NUM> has a rectangular shape. The out-coupling diffractive means is positioned so that the length of the rectangle extends parallel to, or substantially parallel to, the length of the expanding means <NUM>.

In this example the first section <NUM> of the out-coupling diffractive means <NUM>, that has the higher diffractive efficiency, is provided as a rectangular portion. The first section <NUM> is provided in the region of the out-coupling diffractive means <NUM> that is furthest away from the expanding means <NUM>. The second section <NUM> of the out-coupling diffractive means <NUM>, that has the lower diffractive efficiency, is also provided as a rectangular portion. This second section <NUM> is provided in the region of the out-coupling diffractive means <NUM> that is closest to the expanding means <NUM>.

In the example shown in <FIG> the second section <NUM> of the out-coupling diffractive means <NUM> is provided across the out-coupling diffractive means <NUM> so that all of the light incident on the out-coupling diffractive means <NUM> passes through the second section <NUM> of the out-coupling diffractive means <NUM>. The second section <NUM> extends along length of the out-coupling diffractive means <NUM>. The second section <NUM> can extend along all of the length of the out-coupling diffractive means <NUM>.

In this example the out-coupling diffractive means <NUM> also comprises a first section <NUM> and a second section <NUM> where the second section <NUM> has a lower diffractive efficiency to account for the variations in thickness of the respective components of the light guiding means <NUM>.

In this example the out-coupling diffractive means <NUM> has a trapezium shape. The out-coupling diffractive means is positioned so that the bases of the trapezium extend parallel to, or substantially parallel to, the length of the expanding means <NUM>.

In this example the first section <NUM> of the out-coupling diffractive means <NUM>, that has the higher diffractive efficiency, is provided as a trapezium shaped portion. The first section <NUM> is provided in the region of the out-coupling diffractive means <NUM> that is furthest away from the expanding means <NUM>. The second section <NUM> of the out-coupling diffractive means <NUM>, that has the lower diffractive efficiency, is also provided as a trapezium shaped portion. This second section <NUM> is provided in the region of the out-coupling diffractive means <NUM> that is closest to the expanding means <NUM>.

The out-coupling diffractive means <NUM> are sized so that the edge of the out-coupling diffractive means <NUM> extends at least as far as the edges of the expanding means <NUM>. This enables extreme rays of the expanding means <NUM> to be incident on the out-coupling diffractive means <NUM>. In the example of <FIG> the side edges of the out-coupling diffractive means <NUM> are aligned with the edges of the expanding means <NUM>. Other configurations could be used in other examples of the disclosure.

In the example shown in <FIG> the second section <NUM> of the out-coupling diffractive means <NUM> is provided across the out-coupling diffractive means <NUM> so that all of the light incident on the out-coupling diffractive means <NUM> passes through the second section <NUM> of the out-coupling diffractive means <NUM>. The second section <NUM> extends along the length of the out-coupling diffractive means <NUM>. The second section <NUM> can extend along all of the length of the out-coupling diffractive means <NUM>.

The different diffractive efficiencies of the second sections <NUM> shown in <FIG> can be achieved using any suitable means. In some examples the different diffractive efficiencies can be achieved by providing a different fill factor for a diffractive grating, a different diffractive grating depth, a different grating profile, a different refractive index profile or any other suitable variations of the diffractive gratings. Using a plurality of different means together to control the diffraction efficiency can be used to achieve a uniform exit pupil. In some cases not all of these means are available due to for example manufacturing limitations. <FIG> show examples of different configurations for the diffractive grating within the second section <NUM> of the out-coupling diffractive means <NUM>. These configurations are examples how the diffraction efficiency could be controlled to achieve better uniformity for the exit pupil. These examples could also be combined with other means of controlling the diffraction efficiencies.

<FIG> shows another example light guiding means <NUM>. In the example shown in <FIG> the out-coupling diffractive means <NUM> has a rectangular shape. The out-coupling diffractive means is positioned so that the length of the rectangle extends parallel to, or substantially parallel to, the length of the expanding means <NUM>.

In this example the first section <NUM> of the out-coupling diffractive means <NUM>, that has the higher diffractive efficiency, is provided as a rectangular portion and the second section <NUM> of the out-coupling diffractive means <NUM>, that has the lower diffractive efficiency, is also provided as a rectangular portion in a similar arrangement to that shown in <FIG>.

In the example shown in <FIG> second section <NUM> of the out-coupling diffractive means <NUM> comprises a diffractive grating that is not uniform across all of the area covered by the second section <NUM>. In the example shown in <FIG> the second section <NUM> comprises sub-sections <NUM> that comprise a diffractive grating and sub-sections <NUM> that do not comprise a diffractive grating. The sizes and the positions of the respective sub-sections <NUM>, <NUM> are positioned so as the control the brightness of the beams of light to provide even brightness levels across the exit pupil.

The diffractive grating that is provided in sub-sections <NUM> of the second section <NUM> has the same periodicity as the diffractive gratings that is provided in the first section <NUM>. The diffraction grating can be uniform across all of the first section <NUM>.

The sub-sections <NUM> with diffractive gratings and sub-sections <NUM> without diffractive gratings are provided in alternating sub-sections in the second section <NUM>. The sub-sections <NUM> having no diffractive grating are positioned in-between subsections <NUM> that comprise diffractive gratings. The periodicity of the subsections <NUM>, <NUM> within the second section <NUM> of the out-coupling diffractive means <NUM> is configured to reduce phasing within the expanded beam of light.

In the example shown in <FIG> the sub-sections <NUM>, <NUM> extend across the whole of the length of the rectangle that forms the out-coupling diffractive means <NUM>. The sub-sections <NUM>, <NUM> therefore comprise elongate rectangular shapes that extend along the length of the out-coupling diffractive means <NUM>. In this example the sub-sections <NUM> that comprise diffractive grating portions extend along the whole length of the out-coupling diffractive means <NUM> and the sub-sections <NUM> that do not comprise any diffractive gratings also extend along whole length of the out-coupling diffractive means <NUM>.

In the example shown in <FIG> the sub-sections <NUM> that comprise a diffractive grating are shown as comprising two or three slits. It is to be appreciated that this is not shown to scale and that any number of diffractive grating lines or diffractive structures could be provided in each of these sub-sections <NUM>. It is to be appreciated that different numbers of diffractive grating lines or diffractive structures could be provided in the different sub-sections <NUM>.

When the image <NUM> comprising the dot <NUM> in the centre is provided to the light guiding means <NUM> the second sections <NUM> of the out-coupling diffractive means <NUM> control the brightness of the beams of light that pass through the section sections <NUM>. This enables the brightness of these beams after out-coupling to be matched to the brightness of the beams in the rest of the exit pupil. The energy distribution of the exit pupil <NUM> shown in <FIG> is has an even distribution and so shows a high level of uniformity.

<FIG> shows another example light guiding means <NUM> that is configured to correct for variations in brightness caused by the different thicknesses of the components of the light guiding means <NUM>. In the example shown in <FIG> the out-coupling diffractive means <NUM> has a rectangular shape. The out-coupling diffractive means is positioned so that the length of the rectangle extends parallel to, or substantially parallel to, the length of the expanding means <NUM>.

In the example shown in <FIG> second section <NUM> of the out-coupling diffractive means <NUM> comprises a diffractive grating that is not uniform across all of the area covered by the second section <NUM>. In the example shown in <FIG> the second section <NUM> comprises sub-sections <NUM> that comprise a diffractive grating and sub-sections <NUM> that do not comprise a diffractive grating. The sizes and the positions of the respective sub-sections <NUM>, <NUM> are positioned so as the control the brightness of the rays of light to provide even brightness levels across the exit pupil.

The diffractive grating that is provided in sub-sections <NUM> of the second section <NUM> has the same periodicity as the diffractive gratings that is provided in the first section <NUM>.

In the example of <FIG> the sub-sections <NUM> that comprise diffractive gratings and the sub-sections <NUM> that do not comprise diffractive gratings are provided in an alternating arrangement along the length of the out-coupling diffractive means <NUM>. The sub-sections <NUM> that comprise diffractive gratings and the sub-sections <NUM> that do not comprise diffractive gratings are provided in an interdigitated arrangement along the length of the out-coupling diffractive means <NUM>. The sub-sections <NUM> that comprise diffractive gratings have a tapered shape where the narrowest end of the shape is closest to the expanding means <NUM> and the widest end is adjacent to the first section of the out-coupling diffractive means <NUM>. The sub-sections <NUM> that do not comprise diffractive gratings have a tapered shape which is inverted compared to that of sub-sections <NUM> that comprise diffractive gratings. The narrowest end of the sub-sections <NUM> that do not comprise diffractive gratings is therefore provided at the end adjacent to the first section <NUM> of the out-coupling diffractive means <NUM> and the widest end is provided closest to the expanding means <NUM>.

When the image <NUM> comprising the dot <NUM> in the centre is provided to the light guiding means <NUM> the second sections <NUM> of the out-coupling diffractive means <NUM> control the brightness of the beams of light that pass through the section sections <NUM>. This enables the brightness of these beams after out-coupling to be matched to the brightness of the beams in the rest of the exit pupil. The energy distribution of the exit pupil <NUM> shown in <FIG> has an even distribution and so shows a high level of uniformity.

<FIG> shows an example apparatus <NUM> comprising a first light guiding means 101A and a second light guiding means 101B. In the apparatus <NUM> the second light guiding means 101B is provided overlaying the first light guiding means 101A. <FIG> also shows the first light guiding means 101A and second light guiding means 101B separately so as to illustrate the components of the respective light guiding means 101A, 101B.

Each of the first light guiding means 101A and second light guiding means 101B comprises an in-coupling diffractive means 103A, 103B, an expanding means 105A, 105B and an out-coupling diffractive means 107A, 107B.

In the example shown in <FIG> the in-coupling diffractive means 103A, 103B, expanding means 105A, 105B and out-coupling diffractive means 107A, 107B have the same sizes and shapes in each of the different light guiding means 101A, 101B so that when the second light guiding means 101B is provided overlaying the first light guiding mean 101A the respective components of the different light guiding means 101A, 101B are aligned with each other.

The out-coupling diffractive means 107A, 107B are configured to reduce interference between the light out-coupled from the different light guiding means 101A, 101B when they are stacked over each other. In the example of <FIG> the out-coupling diffractive means 107A of the first light guiding means 101A comprises an alternating sequence of first section <NUM> and second sections <NUM>. The first sections <NUM> can be diffractive sections and the second sections <NUM> can be non-diffractive sections so as to reduce the interference of the out-coupled light from the different light guiding means 101A, 101B.

In the example shown in <FIG> the first sections <NUM> and second sections <NUM> form elongate sections <NUM>, <NUM> that extend away from the expanding means <NUM>. Other configurations could be used in other examples.

The sequence of the first sections <NUM> and second sections <NUM> of the out-coupling diffractive means 107B of the second light guiding means 101B is configured so that, when the second light guiding means 101B is provided overlaying the first light guiding means 101A the first sections <NUM> of the second light guiding means 101B overlay the second sections <NUM> of the first light guiding means 101A. Similarly, the second sections <NUM> of the second light guiding means 101B overlay the first sections <NUM> of the first light guiding means 101A.

The alternating sequences of first section <NUM> and second sections <NUM> extends across all of the area of the out-coupling diffractive means 107A, 107B. The first sections <NUM> and the second sections <NUM> are configured so that approximately half of the out-coupling diffractive means 107A, 107B comprises diffractive sections and approximately half of the out-coupling diffractive means 107A, 107B comprises non-diffractive sections.

In the example shown in <FIG> the first sections <NUM> have a similar width to the second sections <NUM>. Other configurations of the respective sections <NUM>, <NUM> could be used in other examples of the disclosure.

In the example shown in <FIG> the second light guiding means 101B has the same configuration as the first light guiding means 101A so that when they are in the stacked configuration each of the components of the second light guiding means 101B overlay corresponding components of the first light guiding means 101A. <FIG> shows another example in which the second light guiding means 101B has a different configuration to the first light guiding means 101A.

In the example of <FIG> the second light guiding means 101B is configured so that the expanding means 105B is provided underneath the in-coupling diffractive means 103B rather than to the side of the in-coupling diffractive means. When the second light guiding means 101B is positioned overlaying the first light guiding means 101A the respective expanding means 105A, 105B are not overlapping however the respective in-coupling diffractive means 103A, 103B would still be overlaying each other, and the out-coupling diffractive means 107A, 107B would still be overlaying each other.

The first sections <NUM> and second sections <NUM> of the first out-coupling diffractive means 107A form elongate sections <NUM>, <NUM> that extend away from the expanding means <NUM> similar to the examples shown in <FIG>. However, in <FIG> the first sections <NUM> and second sections <NUM> of the second out-coupling diffractive means 107B form elongate sections <NUM>, <NUM> that extend parallel to the expanding means 105B. This ensures that when the second light guiding means 101B is provided in an apparatus <NUM> overlaying the first light guiding means 101A the first sections <NUM> of the second light guiding means 101B overlay the second sections <NUM> of the first light guiding means 101A. Similarly, the second sections <NUM> of the second light guiding means 101B overlay the first sections <NUM> of the first light guiding means 101A.

In the examples shown in <FIG> the respective out-coupling diffractive means 107A, 107B are configured so that they are completely overlapping each other. In some examples there might be only a partial overlap between the respective out-coupling diffractive means 107A, 107B.

<FIG> shows another example apparatus <NUM> comprising a first light guiding means 101A and a second light guiding means 101B. <FIG> shows an example light guiding means <NUM> comprising an in-coupling diffractive means <NUM> an expanding means <NUM> and an out-coupling diffractive means <NUM>. Two of these light guiding means 101A, 101B can be stacked to form an apparatus <NUM> as shown in <FIG>.

In this example the out-coupling diffractive means <NUM> is also configured to reduce interference between the light out-coupled from the different light guiding means when they are stacked over each other to form an apparatus <NUM>. In the example of <FIG> the out-coupling diffractive means <NUM> also comprises an alternating sequence of first section <NUM> and second sections <NUM> in which the first sections <NUM> can be diffractive sections and the second sections <NUM> can be non-diffractive sections.

In the example shown in <FIG> the first sections <NUM> and second sections <NUM> form elongate sections <NUM>, <NUM> that extend away from the expanding means <NUM>. In the example of <FIG> the first sections <NUM> increase in size across the out-coupling diffractive means <NUM>. In the example of <FIG> the first sections <NUM> closest to the expanding means <NUM> are smaller than the first sections <NUM> that are further away from the expanding means <NUM>. In the example of <FIG> the second sections <NUM> decrease in size across the out-coupling diffractive means <NUM>. In the example of <FIG> the second sections <NUM> closest to the expanding means <NUM> are larger than the second sections <NUM> that are further away from the expanding means <NUM>.

When the first light guiding means 101A is stacked with the second light guiding means 101B the second light guiding means 101B is configured in mirror image of the first light guiding means 101A as though the first light guiding means 101A has been reflected about a horizontal axis.

The size and spacings of the first sections <NUM> and second sections <NUM> of the out-coupling diffractive means 107B of the second light guiding means 101B is configured so that, when the second light guiding means 101B is provided overlaying the first light guiding means 101A the first sections <NUM> of the second light guiding means 101B overlay the second sections <NUM> of the first light guiding means 101A. Similarly, the second sections <NUM> of the second light guiding means 101B overlay the first sections <NUM> of the first light guiding means 101A.

Examples of the disclosure therefore provide various light guiding means that can be configured to provide more even brightness in an exit pupil and so can provide improved image quality.

Apparatus comprising light guiding means <NUM> as described above can be comprised within a module, a device, a display, a stereoscopic display, an auto stereoscopic display, a head-up display, a display unit of a vehicle and/or a vehicle or any other suitable entity.

Claim 1:
An apparatus comprising:
a light guiding means (<NUM>) comprising at least; in-coupling diffractive means (<NUM>) configured to in-couple one or more input beams of light into the light guiding means from a light engine, expanding means (<NUM>) configured to expand the one or more input beams of light, and out-coupling diffractive means (<NUM>) configured to out-couple the one or more expanded beams of light from the light guiding means;
wherein the out-coupling diffractive means comprises at least a first section (<NUM>) configured to out-couple the one or more expanded beams of light with a first efficiency and at least a second section (<NUM>) configured to out-couple the one or more expanded beams of light with a second efficiency, where the second efficiency is lower than the first efficiency; and
characterized in that the second section of the outcoupling diffractive means comprises absorptive means (<NUM>) configured to absorb at least some of the expanded beam of light.