FIELD OF THE INVENTION

The invention relates to diffractive display technology. In particular, the invention relates to lightguide-based diffractive displays comprising an in-coupling grating, an exit pupil expander grating and an out-coupling grating. In addition, the invention relates to a method of displaying an image on a diffractive display and a novel use. Specifically, the invention relates to personal displays, such as near-to-eye displays (NEDs) utilizing diffractive gratings.

BACKGROUND OF THE INVENTION

The exit pupil of diffractive lightguide-based displays can be expanded using an additional grating, so called exit pupil expander (EPE) grating between the in-coupling grating and out-coupling grating of the display. Typically, as described e.g. in U.S. Pat. No. 6,580,529 B1, in-coupling gratings of diffractive NEDs utilize a single reflective or transmissive diffraction order, which is guided via an exit pupil expander to an out-coupling grating. To get high in-coupling efficiency, complex grating structures such as slanted or overhanging structures needs to be used to get all light into the diffraction order employed. Some of such advanced grating structures, as well as NED display element geometries, are disclosed in US 2016/0231568 A1.

Maximum field of view (FOV) of a single lightguide with a 2D exit pupil expander depends on the refractive index of the lightguide and the wavelength band of the in-coupled light. When the wavelength band is 460-630 nm and lightguide refractive index is 2.0, the maximum FOV is around 33-35 degrees. Currently, the refractive index is limited to 2.0 as glass materials with higher refractive index absorb too much blue light to be usable in lightguides. Typical approach to increase FOV is to stack multiple lightguides on top of each other so that in-coupling gratings of the lightguides are laterally coincident. One approach is to use a single light for each primary color (red, green, blue). In this approach, an RGB image requires three lightguides.

Even with the stacking approach, the prior-art-solutions like those referred to above utilize only a single diffraction order in the in-coupling lead to physically large EPE gratings. This prevents their usage in high FOV (>40 degrees) wearable NED devices. To exemplify the problem, the display design disclosed in US 2016/0231568 A1 is applied for a monochrome (50-100 nm) wavelength band and a 50 degrees FOV NED display with 15 mm×10 mm (height×width) eyebox and 20 mm eyerelief, a structure shown in FIG. 1 is obtained. The structure comprises a lightguide 11, an in-coupling grating 12, an exit pupil expander grating 13 and an out-coupling grating 14 on the lightguide 11. Due to the large FOV, the EPE grating 13 is vertically and horizontally bigger than the out-coupling grating 14. The total size of the lightguide is 77 mm×52 mm (height x width) which is too large for compact eyewear-like NEDs.

Thus, there is a need for improved solutions that allow for producing smaller NEDs with large FOVs.

SUMMARY OF THE INVENTION

It is an aim of the invention to overcome at least some of the problems of prior art and to provide a novel display element for personal displays. A particular aim is to provide a display element that requires a smaller area than prior art solutions. A specific aim to achieve the same eyebox size and FOV using a smaller lightguide.

An additional aim is also to provide a display element that allows for achieving a higher FOV than conventional solutions.

Additional aims are to provide a personal display device employing such display element, an improved method of displaying images on personal displays and a novel use.

The invention is based on the idea of using more than one diffraction order of the incident light and coupling and guiding the diffraction orders into a waveguide and therein along different paths via different exit pupil expander gratings. This allows for manufacturing small-size display elements without compromising the field of view or even increasing the maximum field of view of the element.

According to one aspect, the present display element for a personal display system comprises a lightguide capable of guiding light by total internal reflections, a diffractive in-coupling grating, and a diffractive out-coupling grating. The in-coupling grating is adapted on the lightguide so that is capable of coupling light directed to the in-coupling grating to the lightguide, where the light propagates to the out-coupling grating for producing a viewable image. According to the invention, the in-coupling grating is adapted to split the incoming light to at least two, in particular two, diffraction wavefronts. The display element further comprises at least two, in particular two, different exit pupil expander gratings, which are positioned on the lightguide laterally displaced with respect to each other so as to guide said wavefronts respectively to the out-coupling grating along different paths. The final viewable image is formed on the single out-coupling grating as a sum of the two wavefronts that propagate via different exit pupil expanders.

The present personal display device, such as near-to-eye display device, comprises an image source for projecting an image and at least one diffractive display element according to any of the preceding claims for displaying the image projected to said in-coupling grating thereof on said out-coupling grating. There may be provided two such display elements, one for each eye of the user. The device can be e.g. an augmented reality display, such as an eyewear-integrated display.

The present method for producing an image on a personal display comprises directing light to an in-coupling grating arranged on a lightguide capable of guiding light laterally by total internal reflections, whereby the light is coupled to the lightguide as two different diffraction wavefronts propagating in different directions. The diffraction wavefronts are guided to two different exit pupil expander gratings for extending the exit pupil of the display. Finally, light is guided from the exit pupil expander gratings to a single out-coupling grating for producing a final viewable image. Typically, light is directed to the in-coupling grating essentially in the normal direction of the plane of the lightguide.

The present use comprises using of diffraction order -based splitting of incident light and recombination of the splitted light for producing an image on a diffractive display element.

More specifically, the invention is characterized by what is stated in the independent claims.

The invention offers significant benefits. First, the invention allows for reducing the lateral size of diffractive display elements. This is because the two exit pupil expanders can be positioned more efficiently on the lightguide than a single bigger exit pupil expander that would provide a comparable FOV. Using embodiments of the invention, the maximum field of view can in fact be further increased compared with traditional gratings and grating layouts. More specifically, the present invention allows even 15% larger field of view (FOV) to be guided in the lightguide than the state-of-the art approaches.

In particular, with the presented in-coupling and exit pupil expander structure, both the first plus and minus orders of the in-coupled light can be utilized, which allows the use of binary gratings that can be manufactured relatively easily without sacrificing efficiency.

On the out-coupling grating, the different diffraction order wavefronts are recombined. In comparison to the state-of-the-art approaches, the present invention requires smaller total lightguide area, which in the end enables a better form factor for a NED device.

The dependent claims are directed to selected embodiments of the invention.

In some embodiments, the element comprises two exit pupil expander gratings located laterally essentially on opposite sides of the in-coupling grating. In one embodiment, the gratings are located as fan-shaped zones symmetrically with respect to the in-coupling grating, and preferably in its immediate vicinity, therefore producing a bowtie-like geometry.

In some embodiments, the out-coupling grating is located symmetrically with respect to the exit pupil expander gratings. The grating lines of the gratings are arranged such that the exit pupil expander gratings guide light to the out-coupling grating where the final image is formed.

In some embodiments, the lightguide has a tapering shape, where the in-coupling grating and exit pupil expander gratings are located on the wider end thereof the out-coupling grating on the narrower end thereof. This way, the space of the lightguide is optimally used.

In some embodiments, the in-coupling grating is adapted to couple to the substrate two wavefronts by diffracting light into a positive and a corresponding negative diffraction order, respectively. These diffraction orders may comprise the first positive and negative transmission order or the first positive and negative reflection order, depending on the positioning of the in-coupling gratin with respect to the lightguide and the image source.

In some embodiments, the in-coupling grating comprises a doubly periodic grating. In a typical embodiment, the grating is periodic in two orthogonal directions one of which is aligned with the symmetry axis of the two EPE gratings, which may also be the symmetry axis of the in-coupling grating and/or the out-coupling grating.

In some embodiments, the in-coupling grating is periodic both orthogonal lateral directions so that it in-couples light mainly to the first positive and negative orders in the first orthogonal direction and in the first positive or negative order in the other orthogonal direction.

In some alternative embodiments, the in-coupling grating is comprised of a singly periodic grating, or comprises different in-coupling zones with singly periodic gratings. In some embodiments the in-coupling grating comprises at least two portions arranged laterally with respect to each other and having different grating line orientations and/or profiles for performing coupling of the two wavefronts simultaneously to the lightguide on different paths.

In some embodiments, the grating vector or vectors of the in-coupling grating, exit pupil expander gratings and out-coupling grating are chosen such that when said wavefronts are out-coupled from the lightguide by the out-coupling grating, the wavefronts have the same orthogonal wave vector components as when incident on the in-coupling grating.

In some embodiments, the wavefronts are each adapted to carry a partial image of a total image directed to the in-coupling grating for increasing the maximum field of view that can propagate on the out-coupling grating. This way, the maximum FOV of the display device can be significantly increased. In some embodiments the field of view of the display element is at least 40 degrees in at least one direction and in particular in the direction in which the diffractive order splitting of the incident light is carried out.

In some embodiments, each of the gratings is arranged on one side of the lightguide only or within the lightguide. However, in some embodiments, at least one of the gratings is formed of two gratings arranged on opposite sides of the lightguide aligned with each other. This way, the brightness uniformity of the element can be improved. In typical embodiments, the in-coupling grating is, however, arranged on one side of the lightguide only.

In some embodiments at least one of the gratings, preferably all of the abovementioned gratings of the display element are binary gratings.

Next, embodiments of the invention and advantages thereof are discussed in more details with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a lightguide-based diffractive display comprising a conventional grating arrangement applied to achieve a large FOV.

FIG. 2 shows a lightguide-based diffractive display comprising a grating arrangement according to one embodiment of the invention applied to achieve a large FOV.

FIGS. 3A and 3B show a grating arrangement according to another embodiment of the invention.

FIGS. 4A and 4B show a grating arrangement according to still another embodiment of the invention.

FIGS. 5A and 5B illustrate as wave vector graphs the improvement of FOV with the aid of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, various implementations of a lightguide based diffractive near-to-eye display (NED) element are described that employ a bowtie-shaped EPE as two grating zones placed symmetrically around the in-coupling grating.

FIG. 2 shows a display element according to one embodiment. The lightguide 21 contains a doubly periodic in-coupling grating 22, two EPE gratings 23, 24 and the out-coupling grating 25. The out-coupling gratings 14, 25 in FIG. 1 and FIG. 2 have exactly the same physical size for easy comparison. The embodiment exhibits a much smaller total EPE grating area with respect of the out-coupling grating size than in the solution in FIG. 1. This enables more compact design for a NED device. It can be seen that the total lightguide area required with the arrangement of FIG. 2 is much smaller.

The total footprint of the lightguide can be e.g. 75 mm by 50 mm or less, such as 50-70 mm by 25-45 mm. With the same design parameters than referred to above with respect to FIG. 1 (50 degrees FOV NED display with 15 mm×10 mm (height×width) eyebox and 20 mm eyerelief) a 62 mm×36 mm (height x width) lightguide is sufficient.

The gratings can be located on the first or on the second main surface of the lightguide. When gratings are on the lightguide surface on which light is first incident when coming out from a projector, the in-coupling grating 12 is a transmissive grating. When gratings are on the other main surface, the in-coupling grating 12 is a reflective grating.

In the embodiment of FIG. 2, the in-coupling grating 22 is periodic both in x- and y-directions and its nanostructure is designed so that it in-couples light mainly to +/−1 orders in the x-direction and -1 order in the y-direction. Grating vectors are ideally designed so that when a light ray out-couples from the out-coupling grating 25, the ray has the same x- and y-direction vector components as when incident on the in-coupling grating 22.

FIGS. 3A and 3B shows another embodiment of the present invention. The first main surface 31A of the lightguide, illustrated in FIG. 3A, contains an in-coupling grating 32 that is periodic in the x-direction. The in-coupling grating couples light to the plus and minus first transmissive diffraction order (T−1, T+1) (when light comes from the side of the grating as herein assumed). The second main surface 31B of the lightguide contains the EPE gratings 34, 35 and the out-coupling grating 36.

Rays incident on the in-coupling grating have a direction vector {circumflex over (k)}=(sin(θ_x) , sin(θ_y), √{square root over (1−sin²θ_x−sin²θ_y)}). The rays with negative θ_xtravel to the eyebox, i.e. out-coupling grating, from the in-coupling grating mainly through the right EPE grating 35, while rays with positive θ_xtravel through the left EPE grating 34.

In one embodiment (not shown) there are two laterally arranged sections in the in-coupling grating that are adapted to provide the splitting of the incoming light to the opposite diffraction orders.

As understood by a skilled person, in all embodiments referred to herein, the orientations the gratings with respect to each other and their other properties are suitably chosen such that the diffraction of desired wavelengths and propagation of diffracted rays as described takes place.

It should be noted that in the prior art solutions that utilize only a single in-coupling diffraction order, the entire FOV needs to propagate through a single EPE grating. This inherently limits the maximum FOV supported by a single lightguide. With the present invention, higher total FOV can propagate through the two EPE gratings than through a single conventional EPE. This effect is illustrated in the wave vector diagrams of FIGS. 5A (conventional single diffraction order coupling) and 5B (present opposite diffraction order coupling according to one embodiment).

Rays that propagate with so high angles inside the lightguide that the distance between the two adjacent hit points on the same light guide surface is bigger than the in-coupling grating diameter may cause areas with lower brightness into the final image perceived by a human eye. This effect can be reduced by having EPE gratings and out-coupling gratings on both sides of the lightguide. This is illustrated in FIGS. 4A and 4B that show an embodiment in which the first lightguide surface 41A contains a transmissive in-coupling grating periodic in the x-direction, first EPE gratings 43A, 44B and an out-coupling grating 45A periodic in the y-direction. The second surface 41B contains second EPE gratings 43B, 44B and an out-coupling grating 45B.

Embodiments of the present invention with different grating vectors can be stacked on top of each to further increase the field of view or operational wavelength band of the entire di splay element.