Wide angle imaging directional backlights
View Patent ↗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 backward direction may be refracted, diffracted, or reflected by the features to provide discrete illumination beams exiting from the top surface of the waveguide. In operation, luminance streaks and bright illumination regions may be formed due to undesirable imaging characteristics from the structure of the Fresnel mirror. Fresnel mirror draft facets and reflective facet microstructures are provided that achieve reduction of visibility of light streaks and bright illumination regions.
1. A directional waveguide comprising:
an input end;
first and second opposed, laterally extending guide surfaces for guiding light along the waveguide; and
a reflective end facing the input end for reflecting the input light back along the waveguide, the second guide surface being arranged to deflect the reflected input light through the first guide surface as output light, and the waveguide being arranged to direct the output light into optical windows in output directions that are distributed in a lateral direction in dependence on the input position of the input light,
wherein the reflective end is a Fresnel reflector comprising alternating reflective facets and draft facets, each reflective facet having a plurality of convex, curved sub-facets, the reflective facets providing the Fresnel reflector with positive optical power laterally toward the input end of the waveguide.
2. The directional waveguide according to claim 1 , wherein the pitch of the reflective facets laterally across the reflective end is constant.
3. The directional waveguide according to claim 1 , wherein the width of the reflective facets laterally across the reflective end is at most 0.5 mm.
4. The directional waveguide according to claim 1 , wherein the depth of each of the draft facets is at least 0.5 μm.
5. The directional waveguide according to claim 1 , wherein the height of the reflective end between the first and second guide surfaces has a profile that is flat.
6. The directional waveguide according to claim 5 , wherein the second guide surface has a stepped shape in which said light extraction features are facets between the intermediate regions.
7. The directional waveguide according to claim 5 , wherein the light extraction features have positive optical power in the lateral direction.
8. The directional waveguide 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 guided along the waveguide 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.
9. A directional backlight comprising:
a directional waveguide according to claim 1 ; and
an array of input light sources arranged at different input positions in a lateral direction across the input end of the waveguide and arranged to input light into the waveguide.
10. A directional display device comprising:
a directional backlight according to claim 9 ; and
a transmissive spatial light modulator arranged to receive the output light from the waveguide and to modulate it to display an image.
11. A directional display apparatus comprising:
a directional display device according to claim 10 ; and
a control system arranged to control the light sources.