Directional backlight having a rear reflector

• US 10,054,732 B2
• Filed: 02/21/2014
• Issued: 08/21/2018
• Est. Priority Date: 02/22/2013
• Status: Active Grant

• Current Assignee: Reald Spark, LLC
• Original Assignee: Reald Spark, LLC
• Inventors: Robinson, Michael G., Woodgate, Graham J.
• Primary Examiner(s): Alexander, William R
• Assistant Examiner(s): Duong, Henry
• Application Number:
  US14/186,862
• Publication Number:
  US 20140240828A1
• Time in Patent Office: 1,642 Days
• Field of Search: 359465, 362 19, 362606, 362607, 362611, 362613
• CPC Class Codes: G02B 30/25 using polarisation techniques
• CPC Class Codes: G02B 6/0048 with stepwise taper
• CPC Class Codes: G02B 6/0056 for producing polarisation effects, e.g. by a surface with polarizing properties or by an additional polarizing elements
• CPC Class Codes: G06F 3/012 Head tracking input arrangements

Abstract

A directional display may include a waveguide. The waveguide may include light extraction features arranged to direct light from an array of light sources by total internal reflection to an array of viewing windows and a reflector arranged to direct light from the waveguide by transmission through extraction features of the waveguide to the same array of viewing windows. The brightness of the directional display can be increased. An efficient and bright autostereoscopic display system can be achieved.

Claims

1. A directional backlight comprising:

a waveguide comprising an input end;

an array of light sources disposed at different input positions in a lateral direction across the input end of the waveguide,

the waveguide further comprising first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting the input light back through the waveguide, the first guide surface being arranged to guide light by total internal reflection and the second guide surface having a stepped shape comprising;

a plurality of facets oriented to reflect light from the light sources, after reflection from the reflective end, through the first guide surface into optical windows in output directions distributed in a lateral direction to the normal to the first guide surface in dependence on the input positions, and

intermediate regions between the facets that are arranged to guide light through the waveguide; and

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,

wherein the light sources are configured to output light that is directed into selectable viewing windows at different locations, the selectable viewing windows comprising at least one of the optical windows.

2. A directional backlight according to claim 1, further comprising a diffuser disposed to receive light output through the first guide surface.

3. A directional backlight according to claim 2, wherein the diffuser is an asymmetric diffuser that is arranged to provide greater angular dispersion in a direction orthogonal to said lateral direction than in said lateral direction.

4. A directional backlight according to claim 2, further comprising a Fresnel lens having optical power in at least said lateral direction, disposed between the first guide surface of the waveguide and the diffuser.

5. A directional backlight according to claim 1, further comprising a Fresnel lens having optical power at least in said lateral direction disposed to receive light output through the first guide surface.

6. A directional backlight according to claim 1, 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.

7. A directional backlight according to claim 6, wherein the facets of the waveguide are inclined at an angle (90°

−

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

>

90°

−

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.

8. A directional backlight according to claim 1, wherein the rear reflector is spaced from the waveguide such that the light from au 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.

9. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector have an irregular pitch.

10. A directional backlight according to claim 9, wherein the reflective facets of the rear reflector have an irregular, randomised pitch.

11. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector have an inclination that varies across the array of reflective facets.

12. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector are linear.

13. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector are elongate and are curved along their length.

14. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector are concave, and arranged in alignment with the facets of the waveguide.

15. A directional backlight according to claim 1, wherein the reflective facets of the rear reflector are undulating along their length.

16. A directional backlight according to claim 1, wherein the rear reflector further comprises, a rough diffusing surface on at least some of the reflective facets.

17. A directional backlight according to claim 1, wherein the facets of the rear reflector are disposed behind respective ones of the facets of the waveguide and arranged to reflect all of the light incident thereon from the light sources that is transmitted through the respective one of the facets of the waveguide.

18. A directional backlight according to claim 1, wherein the reflective end has positive optical power in a lateral direction across the waveguide.

19. A directional backlight according to claim 1, wherein the rear reflector formed by at least two polarized reflector sheets arranged to reflect light that is polarized in respective polarization directions that are orthogonal to each other, the polarized reflector sheets being shaped to form said linear array of reflective facets.

20. A directional backlight according to claim 1, wherein the facets of the waveguide are elongate and are curved along their length.

21. A directional display device comprising:

a directional backlight comprising;

a waveguide comprising an input end;

an array of light sources disposed at different input positions in a lateral direction across the input end of the waveguide,

the waveguide further comprising first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting the input light back through the waveguide, the first guide surface being arranged to guide light by total internal reflection and the second guide surface having a stepped shape comprising;

a plurality of facets oriented to reflect light from the light sources, after reflection from the reflective end, through the first guide surface into optical windows in output directions distributed in a lateral direction to the normal to the first guide surface in dependence on the input positions, and

intermediate regions between the facets that are arranged to guide light through the waveguide; and

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; and

a transmissive spatial light modulator arranged to receive the light output through the first guide;

wherein the light sources are configured to output light that is directed into selectable viewing windows at different locations, the selectable viewing windows comprising at least one of the optical windows.

22. A directional display device according to claim 21, wherein the spatial light modulator is arranged to modulate a first polarization component of that light having a first polarization, and

the directional display device further comprises 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 comprises 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.

23. A directional display device according to claim 22, further comprising a polarization rotator disposed between the reflective polarizer and the spatial light modulator and arranged to rotate the first polarization component.

25. A display apparatus according to claim 23, wherein the display apparatus is an autostereoscopic display apparatus wherein the control system is further arranged to control the display device to display temporally multiplexed left and right images and synchronously to direct the displayed images into optical windows in positions corresponding to left and right eyes of an observer.

24. A display apparatus comprising:

a directional backlight comprising;

a waveguide comprising an input end;

an array of light sources disposed at different input positions in a lateral direction across the input end of the waveguide,

the waveguide further comprising first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting the input light back through the waveguide, the first guide surface being arranged to guide light by total internal reflection and the second guide surface having a stepped shape comprising;

a plurality of facets oriented to reflect light from the light sources, after reflection from the reflective end, through the first guide surface into optical windows in output directions distributed in a lateral direction to the normal to the first guide surface in dependence on the input positions, and

intermediate regions between the facets that are arranged to guide light through the waveguide; and

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;

a transmissive spatial light modulator arranged to receive the light output through the first guide; and

a control system arranged to selectively operate the light sources to direct light into optical windows corresponding to said output directions;

wherein the light sources are configured to output light that is directed into selectable viewing windows at different locations, the selectable viewing windows comprising at least one of the optical windows.

26. A display apparatus according to claim 24, wherein

the control system further comprises a sensor system arranged to detect the position of an observer across the display device, and

the control system is arranged to direct the output light into optical windows selected in dependence on the detected position of the observer.

27. A display apparatus according to claim 24, wherein

the spatial light modulator is arranged to modulate a first polarization component of that light having a first polarization, and

the directional display device further comprises 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 comprises 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.

28. A display apparatus according to claim 27, further comprising a polarization rotator disposed between the reflective polarizer and the spatial light modulator and arranged to rotate the first polarization component.