Pupil expander integrity

1. A waveguide pupil expander comprising:

first and second substantially planar reflective surfaces arranged in parallel having an optically transparent material therebetween, and

an input port for receiving input light;

wherein the first reflective surface is fully reflective and the second reflective surface is partially reflective such that input light is guided from the input port to an output port at the second partially reflective surface by a series of internal reflections;

wherein the waveguide is formed by a layered glass structure arranged to maintain the integrity of the waveguide in the event of breakage of glass; and

wherein the layered glass structure comprises a polymer-based layer comprising a polymer-based turning film arranged to change an angle of light propagating therethrough; a polymer-based louvre, or a polymer-based polariser.

2. The waveguide pupil expander as claimed in claim 1 wherein the layered glass structure comprises a glass layer and an optically transparent non-glass layer.

3. The waveguide pupil expander as claimed in claim 1 wherein the layered glass structure comprises a glass layer laminated with the polymer-based layer.

4. The waveguide pupil expander as claimed in claim 1 wherein the layered glass structure comprises a glass layer and a resin layer.

5. The waveguide pupil expander as claimed in claim 4 wherein the layered glass structure forms the optically transparent material, wherein the layered glass structure has low-birefringence and/or the resin is a resin material that is refractive index-matched to the glass.

6. The waveguide pupil expander as claimed in claim 4 wherein the layered glass structure comprises a resin interlayer between internal major surfaces of first and second glass layers.

7. The waveguide pupil expander as claimed in claim 6 comprising a fully reflective coating on the external major surface of the first glass layer and a partially reflective coating on the external major surface of the second glass layer surface.

8. The waveguide pupil expander as claimed in claim 4 wherein the resin material is arranged to maintain parallelism between the external major surfaces of the first and second glass layers.

9. The waveguide pupil expander as claimed in claim 8 wherein the resin material comprises polyvinyl butyral.

10. The waveguide pupil expander as claimed in claim 1 wherein the layered glass structure comprises two or more layers of tempered glass.

11. The waveguide pupil expander as claimed in claim 10 wherein the layered glass structure forms the optically transparent material.

12. The waveguide pupil expander as claimed in claim 10 wherein the layered glass structure comprises a first layer of glass under compressive strain and a second layer of glass under tensile strain.

13. The waveguide pupil expander as claimed in claim 12 wherein the layered glass structure further comprises a third layer of glass under compressive or tensile strain.

14. A system comprising:

a display device comprising a spatial light modulator arranged to output spatially modulated light to form an image, and

the waveguide pupil expander as claimed in claim 1 configured to receive spatially modulated light from the display device at an input port thereof and to expand the viewing window of the system.

15. The system as claimed in claim 14 wherein the display device further comprises a controller configured to control the spatially modulated light output by the display device.

16. The system as claimed in claim 15 wherein the controller is configured to control a light source of the display device in response to a signal indicating detection of the breakage of glass.

17. The system as claimed in claim 14 further comprising a viewer tracking system arranged to monitor a viewer's face to detect stray light incident thereon.

18. The system as claimed in claim 17 wherein the viewer tracking system comprises a light detector directed at the viewer's face to detect stray visible light, such as visible laser light, on the viewer's face.

19. The system as claimed in claim 14 comprising a holographic system, wherein:

the display device comprises a spatial light modulator arranged to display a diffractive pattern of the image and to output spatially modulated light in accordance with the diffractive pattern to form a holographic reconstruction corresponding to the image.