Crosstalk suppression in a directional backlight
View Patent ↗Disclosed is a light guiding valve apparatus including a light valve, a two dimensional light emitting element array and an input side arranged to reduce light reflection for providing large area directional illumination from localized light emitting elements with low cross talk. A waveguide includes a stepped structure, in which the steps may include extraction features hidden to guided light propagating in a first forward direction. Returning light propagating in a second backward direction may be refracted or reflected by the features to provide discrete illumination beams exiting from the top surface of the waveguide. Stray light falling onto a light input side of the waveguide is at least partially absorbed.
1. A directional backlight for a transmissive spatial light modulator, comprising:
a waveguide having an input end, first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting light from the input light back through the waveguide;
an array of light sources arranged to output light predominantly in an emission band and in a conversion band, the light sources being disposed at different input positions in a lateral direction across the input end of the waveguide, the waveguide being arranged to direct input light from light sources at the different input positions across the input end after reflection from the reflective end as output light through the first guide surface for supply through a transmissive spatial light modulator into respective optical windows in output directions distributed in the lateral direction in dependence on the input positions; and
a reflection reduction element arranged to reduce reflections of light incident on the input end after reflection from the reflective end, wherein each of the light sources comprise a light generation element arranged to generate light in the emission band and a wavelength conversion material arranged to convert at least some of the light in the emission band generated by the light generation element into light in the conversion band.
2. The directional backlight according to claim 1 , wherein the reflection reduction element is a light diffusing element.
3. The directional backlight according to claim 2 , wherein the light diffusing element has asymmetric light diffusing properties.
4. The directional backlight according to claim 1 , wherein the reflection reduction element extends across the entirety of the input end.
5. The directional backlight according to claim 1 , wherein the reflection reduction element comprises a light absorptive layer extending across parts of the input end outside the light sources.
6. The directional backlight according to claim 1 , wherein the light generation element comprises a semiconductor diode.
7. The directional backlight according to claim 1 , wherein the wavelength conversion material is a phosphor.
8. The directional backlight according to claim 1 , wherein the emission band is blue light and the conversion band is yellow light.
9. The directional backlight according to claim 1 , wherein the reflection reduction element comprises a filter, disposed between the input end and the array of light sources, and arranged to absorb light in the conversion band preferentially over light in the emission band.
10. The directional backlight according to claim 1 , wherein the reflection reduction element comprises, disposed between the input end and each of the light sources, at least one light absorbing element covering part of the area of the respective light source and arranged to absorb light after reflection from the reflective end.
11. The directional backlight according to claim 10 , wherein the at least one light absorbing element comprises plural light absorbing elements covering part of the area of the respective light source.
12. The directional backlight according to claim 10 , wherein the at least one light absorbing element comprises at least two layers including a layer facing the waveguide arranged to absorb light after reflection from the reflective end and a layer facing the light respective light source arranged to reflect light incident thereon.
13. The directional backlight according to claim 10 , wherein the light sources comprise a light generation element arranged to generate light in the emission band and a wavelength conversion material arranged to convert at least some of the light in the emission band generated by the light generation element into light in the conversion band.
14. The 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 reflect light guided through the waveguide in directions allowing exit through the first guide surface as the output light.
15. The directional backlight according to claim 14 , wherein the light extraction features are facets of the second guide surface.
16. The directional backlight according to claim 15 , wherein the second guide surface has a stepped shape comprising said facets and intermediate regions between the facets that are arranged to direct light through the waveguide without extracting it.
17. The 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 is substantially planar and inclined at an angle to reflect light in directions that break the total internal reflection for outputting light through the first guide surface,
the display device further comprising a deflection element extending across the first guide surface of the waveguide for deflecting light towards the normal to the spatial light modulator.
18. The directional backlight according to claim 1 , wherein the reflective end has positive optical power in the lateral direction.
19. A display device comprising:
a directional backlight for a transmissive spatial light modulator, comprising:
a waveguide having an input end, first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting light from the input light back through the waveguide;
an array of light sources arranged to output light predominantly in an emission band and in a conversion band, the light sources being disposed at different input positions in a lateral direction across the input end of the waveguide, the waveguide being arranged to direct input light from light sources at the different input positions across the input end after reflection from the reflective end as output light through the first guide surface for supply through a transmissive spatial light modulator into respective optical windows in output directions distributed in the lateral direction in dependence on the input positions; and
a reflection reduction element arranged to reduce reflections of light incident on the input end after reflection from the reflective end, and
a control system arranged to selectively operate the light sources to direct light into viewing windows corresponding to said output directions;
wherein the transmissive spatial light modulator is arranged to receive the output light from the directional backlight and comprising an array of pixels arranged to modulate light passing therethrough.
20. The display device according to claim 19 , being an autostereoscopic display device 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 viewing windows in positions corresponding to left and right eyes of an observer.
21. The display device according to claim 20 , 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 displayed images into viewing windows in positions corresponding to left and right eyes of an observer, in dependence on the detected position of the observer.
22. A directional backlight for a transmissive spatial light modulator, comprising:
a waveguide having an input end, first and second, opposed guide surfaces for guiding light along the waveguide, and a reflective end facing the input end for reflecting light from the input light back through the waveguide;
an array of light sources arranged to output light predominantly in an emission band and in a conversion band, the light sources being disposed at different input positions in a lateral direction across the input end of the waveguide, the waveguide being arranged to direct input light from light sources at the different input positions across the input end as output light through the first guide surface after reflection from the reflective end for supply through a transmissive spatial light modulator into respective optical windows in output directions distributed in the lateral direction in dependence on the input positions,
wherein the light sources have respective light emitting regions having a width, in a direction along the input end in which the light sources are arrayed, which is at or below 50% of the pitch of the light sources, wherein the light emitting regions of the light sources each comprise a light generation element arranged to generate light in the emission band and a wavelength conversion material arranged to convert light in the emission band generated by the light generation element into light in the conversion band,
wherein the light generation element comprises a semiconductor diode,
wherein the wavelength conversion material is a phosphor, and
wherein the emission band is blue light and the conversion band is yellow light.
23. The directional backlight according to claim 22 , 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 reflect light guided through the waveguide in directions allowing exit through the first guide surface as the output light,
wherein the light extraction features are facets of the second guide surface,
and further wherein the second guide surface has a stepped shape comprising said facets and intermediate regions between the facets that are arranged to direct light through the waveguide without extracting it.
24. The directional backlight according to claim 22 , wherein the first guide surface is arranged to guide light by total internal reflection and the second guide surface is substantially planar and inclined at an angle to reflect light in directions that break the total internal reflection for outputting light through the first guide surface,
the directional backlight further comprising a deflection element extending across the first guide surface of the waveguide for deflecting light towards the normal to the spatial light modulator.
25. The directional backlight according to claim 22 , wherein the reflective end has positive optical power in the lateral direction.