Wide angle imaging directional backlights

• US 10,228,505 B2
• Filed: 05/26/2016
• Issued: 03/12/2019
• Est. Priority Date: 05/27/2015
• Status: Active Grant

• Current Assignee: Reald Spark, LLC
• Original Assignee: Reald Spark, LLC
• Inventors: Robinson, Michael G., Ramsey, Robert A., Woodgate, Graham J.
• Primary Examiner(s): Truong, Bao Q
• Application Number:
  US15/165,960
• Publication Number:
  US 20160349444A1
• Time in Patent Office: 1,020 Days
• CPC Class Codes: G02B 6/0016 Grooves, prisms, gratings, scattering particles or rough surfaces
• CPC Class Codes: G02B 6/0038 Linear indentations or grooves, e.g. arc-shaped grooves or meandering grooves, extending over the full length or width of the light guide
• CPC Class Codes: G02B 6/0048 with stepwise taper
• CPC Class Codes: G02B 6/0055 Reflecting element, sheet or layer
• CPC Class Codes: G02B 6/0068 Arrangements of plural sources, e.g. multi-colour light sources
• CPC Class Codes: G02B 6/0091 of the light source relative to the light guide G02B6/0021 takes precedence

Abstract

An imaging directional backlight apparatus including a waveguide, a light source array, for providing large area directed illumination from localized light sources. The waveguide may include a stepped structure, in which the steps may further include extraction features optically hidden to guided light, propagating in a first forward direction. Returning light propagating in a second deflected direction may be refracted, diffracted, or reflected by the features to provide discrete illumination beams exiting from the top surface of the waveguide. Viewing windows are formed through imaging individual light sources from the side of the waveguide and hence defines the relative positions of system elements and ray paths. A directional backlight with small footprint and low thickness may be provided.

Claims

1. A directional backlight comprising a directional waveguide and light sources,

the directional waveguide comprising:

a reflective end that is elongated in a lateral direction;

first and second opposed guide surfaces extending from laterally extending edges of the reflective end for guiding input light along the waveguide towards the reflective end and for guiding light reflected by the reflected end away from the reflective end, the second guide surface being arranged to deflect light reflected from the reflective end through the first guide surface as output light; and

side surfaces extending between the first and second guide surfaces,

wherein

the light sources include an array of light sources arranged along a side surface to provide said input light through that side surface, and

the reflective end comprises first and second facets alternating with each other in the lateral direction, the first facets being reflective and forming reflective facets of a Fresnel reflector having positive optical power in the lateral direction, the second facets forming draft facets of the Fresnel reflector, the Fresnel reflector having an optical axis that is inclined towards the side surface in a direction in which the Fresnel reflector deflects input light from the array of light sources into the waveguide, the waveguide being arranged to direct the output light from the light sources into respective optical windows in output directions that are distributed laterally in dependence on the positions of the light sources.

2. A directional backlight according to claim 1, wherein

the light sources include two arrays of light sources each arranged along one of the side surfaces, and

the first facets and the second facets are reflective, the first and second facets forming, respectively, reflective facets of first and second Fresnel reflectors each having positive optical power in the lateral direction, the second and first facets forming, respectively, draft facets of the first and second Fresnel reflectors, the first and second Fresnel reflectors having optical axes inclined towards respective side surfaces in directions in which the Fresnel reflectors deflect input light from the array of light sources arranged along the respective side surface into the waveguide.

3. A directional backlight according to claim 2, wherein the first and second facets have mirror symmetry about a plane of symmetry extending from the reflective end, the optical axes of the first and second Fresnel reflectors being inclined from the plane of symmetry towards the respective side surfaces.

4. A directional backlight according to claim 1, wherein the light sources are arranged with mirror symmetry about the plane of symmetry.

5. A directional backlight according to claim 1, wherein the first guide surface is arranged to guide light by total internal reflection and the second guide surface comprises a plurality of light extraction features oriented to direct light reflected by the reflected end in directions allowing exit through the first guide surface as the output light and intermediate regions between the light extraction features that are arranged to guide light along the waveguide.

6. A directional backlight according to claim 5, wherein the light extraction features have positive optical power in the lateral direction.

7. A directional backlight according to claim 6, wherein the light extraction features have an optical axis to which the optical axis of the Fresnel reflector is inclined.

8. A directional backlight according to claim 5, wherein the light extraction features and the intermediate regions alternate with one another in a stepped shape.

9. A directional backlight according to claim 1, further comprising a rear end facing the reflective end.

10. A directional backlight according to claim 9, further comprising input sources arranged along at least part of the rear end adjacent the side surface.

11. A directional backlight according to claim 9, wherein at least part of the rear end is reflective.

12. A directional backlight according to claim 9, wherein at least part of the rear end is non-reflective.

13. A directional backlight according to claim 1, wherein the ratio between (a) height of the rear end between the first and second guide surfaces and (b) the height of the reflective end between the first and second guide surfaces has a profile across the lateral direction that is greatest at the side on which the light source array is arranged and reduces in the lateral direction.

14. A directional backlight according to claim 13, wherein the height of the rear end between the first and second guide surfaces has a profile across the lateral direction that is greatest at the side on which the light source array is arranged and reduces in the lateral direction.

15. A directional backlight according to claim 6, wherein the ratio between (a) height of the rear end between the first and second guide surfaces and (b) the height of the reflective end between the first and second guide surfaces has a profile across the lateral direction that is least at the optical axis of the light extraction features and increases towards each side of the optical axis.

16. A directional backlight according to claim 15, wherein the height of the rear end between the first and second guide surfaces has a profile across the lateral direction that is lowest at the optical axis of the light extraction features and reduces towards each side of the optical axis.

17. A directional backlight according to claim 16, wherein the edge of the rear end at the first guide surface is curved and the edge of the rear end at the second guide surface is straight to provide said profile.

18. A directional backlight according to claim 1, wherein the waveguide includes a tapered region adjacent the side surface within which the height of the waveguide between the first and second guide surfaces increases along a direction from the remainder of the waveguide towards the side surface.

19. A directional backlight according to claim 18, wherein the width of the tapered region in the lateral direction increases in the direction away from the reflective end.

20. A directional backlight according to claim 1, wherein the optical axis of the Fresnel reflector is inclined towards the side surface in a direction such that the output light from a light source of the array of light sources arranged along a side surface is directed into the central optical window of the distribution of optical windows.

21. A directional backlight according to claim 1, wherein the side surface along which the array of sources is arranged comprises recesses including input facets facing the reflective end, the light sources being arranged to provide said input light through the input facets.

22. A directional backlight according to claim 1, wherein the side surface opposite the surface along which the array of sources is arranged comprises side reflective facets wherein said side reflective facets provide a side Fresnel reflector with substantially the same positive optical power in the lateral direction and optical axis direction as the Fresnel reflector arranged at the reflective end.

23. A directional backlight according to claim 1, further comprising a rear reflector comprising a linear array of reflective facets arranged to reflect light from the light sources, that is transmitted through the plurality of facets of the waveguide, back through the waveguide to exit through the first guide surface into said optical windows.

24. A directional backlight according to claim 23 wherein the facets of the waveguide and the reflective facets of the rear reflector are inclined in the same sense in a common plane orthogonal to said lateral direction.

25. A directional backlight according to claim 23 wherein the facets of the waveguide are inclined at an angle (π

/2−

α

) to the normal to the first guide surface and the reflective facets of the rear reflector are inclined at an angle β

to the normal to the first guide surface, and 2β

>

π

/2−

sin<sup>−

1</sup> (n·

sin (α

−

θ

_c)), θ

_c being the critical angle of the facets of the waveguide and n being the refractive index of the material of the waveguide.

26. A directional backlight according to claim 23, wherein the rear reflector is spaced from the waveguide such that the light from an individual facet of the waveguide is incident on plural reflective facets of the rear reflector, the rear reflector further comprising intermediate facets extending between the reflective facets of the rear reflector, the intermediate facets being inclined in an opposite sense from the reflective facets of the rear reflector at an angle such that said light from the light sources that is transmitted through the plurality of facets of the waveguide is not incident on the intermediate facets.

27. A directional backlight according to claim 23, wherein the reflective facets of the rear reflector have an irregular pitch.

28. A directional backlight according to claim 23, wherein the reflective facets of the rear reflector have an irregular, randomized pitch.

29. A directional backlight according to claim 23, wherein the reflective facets of the rear reflector have an inclination that varies across the array of reflective facets.

30. A directional backlight according to claim 23, wherein the reflective facets of the rear reflector are linear.

31. A directional backlight according to claim 23, wherein the reflective facets of the rear reflector are curved.

32. A directional display device comprising:

a directional backlight according to claim 1; and

a transmissive spatial light modulator arranged to receive the output light from the waveguide and to modulate it to display an image.

33. A directional display device comprising:

a directional backlight according to claim 1;

a transmissive spatial light modulator arranged to receive the light output through the first guide surface and arranged to modulate a first polarization component of that light having a first polarization; and

a reflective polarizer disposed between the first guide surface of the waveguide and the spatial light modulator and arranged to transmit the first polarization component and to reflect a second polarization component of the output light having a polarization orthogonal to the first polarization as rejected light,

the rear reflector further comprising intermediate facets extending between the reflective facets of the rear reflector and inclined in opposite senses from the reflective facets of the rear reflector in a common plane, so that pairs of a reflective facet and an intermediate facet together form corner facets arranged to reflect rejected light for supply back to the spatial light modulator, the pairs of a reflective facet and an intermediate facet being inclined in a plane that is oriented around the normal to the spatial light modulator so that the rear reflector converts the polarization of rejected light supplied back to spatial light modulator into the first polarization on reflection.

34. A directional display device according to claim 33, further comprising a polarization rotator disposed between the reflective polarizer and the spatial light modulator and arranged to rotate the first polarization component.

35. A directional display device according to claim 32, wherein the spatial light modulator has a rectangular shape having a first side aligned with the reflective end of the directional waveguide, the optical axis of the Fresnel reflector being inclined with respect to an axis of the rectangular shape of the spatial light modulator that is perpendicular to its first side.

36. A directional display apparatus comprising:

a directional display device according to claim 32; and

a control system arranged to control the light sources.