Optical stack for switchable directional display
A privacy display comprises a spatial light modulator and a compensated switchable liquid crystal retarder arranged between first and second polarisers arranged in series with the spatial light modulator. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without loss, whereas off-axis light has reduced luminance. The visibility of the display to off-axis snoopers is reduced by means of luminance reduction over a wide polar field. In a wide angle mode of operation, the switchable liquid crystal retardance is adjusted so that off-axis luminance is substantially unmodified.
1. A display device comprising:
a spatial light modulator;
a display polariser arranged on a side of the spatial light modulator;
an additional polariser arranged on the same side of the spatial light modulator as the display polariser; and
plural retarders arranged between the additional polariser and the display polariser, there being no further polarisers arranged between the plural retarders;
wherein the plural retarders comprise:
a switchable liquid crystal retarder comprising a layer of liquid crystal material and two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof, each of the surface alignment layers being arranged to provide homogeneous alignment in the adjacent liquid crystal material and each of the surface alignment layers having a pretilt having a pretilt direction with a component in a plane of the liquid crystal layer that is parallel or anti-parallel or orthogonal to an electric vector transmission direction of the display polariser, the layer of liquid crystal material of the switchable liquid crystal retarder having a positive dielectric anisotropy and having a retardance for light of a wavelength of 550nm in a range from 500nm to 1000nm; and
at least one passive compensation retarder having its optical axis perpendicular to a plane of the retarder, the at least one passive compensation retarder having a retardance for light of a wavelength of 550nm in a range from −300nm to −700nm.
2. The display device according to claim 1 , wherein the display polariser and the additional polariser have electric vector transmission directions that are parallel.
3. The display device according to claim 1 , wherein the at least one passive compensation retarder comprises two passive retarders having an optical axis perpendicular to the plane of the passive retarders, and the switchable liquid crystal retarder is provided between the two passive retarders.
4. The display device according to claim 3 , further comprising a transparent electrode and a liquid crystal alignment layer formed on a side of each of the two passive retarders adjacent the switchable liquid crystal retarder.
5. The display device according to claim 3 , further comprising first and second substrates between which the switchable liquid crystal retarder is provided, the first and second substrates each comprising one of the two passive retarders.
6. The display device according to claim 1 , wherein the retardance of the at least one passive compensation retarder is equal and opposite to the retardance of the switchable liquid crystal retarder.
7. The display device according to claim 1 , wherein the at least one passive compensation retarder is arranged to introduce no phase shift to polarisation components of light passed by one of the display polariser and the additional polariser on an input side of the plural retarders along an axis along a normal to the plane of the at least one passive compensation retarder.
8. The display device according to claim 1 , wherein the at least one passive compensation retarder is arranged to introduce a phase shift to polarisation components of light passed by one of the display polariser and the additional polariser on an input side of the plural retarders along an axis inclined to a normal to the plane of the at least one passive compensation retarder.
9. The display device according to claim 1 , wherein the switchable liquid crystal retarder is arranged to introduce no phase shift to polarisation components of light passed by one of the display polariser and the additional polariser on an input side of the plural retarders along an axis along a normal to a plane of the switchable liquid crystal retarder.
10. The display device according to claim 1 , wherein the switchable liquid crystal retarder is arranged to introduce a phase shift to polarisation components of light passed by one of the display polariser and the additional polariser on an input side of the plural retarders along an axis inclined to a normal to a plane of the switchable liquid crystal retarder in a switchable state of the switchable liquid crystal retarder.
11. The display device according to claim 1 , wherein the plural retarders are arranged to not affect a luminance of light passing through the display polariser, the additional polariser and the plural retarders along an axis along a normal to a plane of the plural retarders.
12. The display device according to claim 1 , wherein the plural retarders are arranged to reduce a luminance of light passing through the display polariser, the additional polariser and the plural retarders along an axis inclined to a normal to a plane of the plural retarders.
13. The display device according to claim 1 , wherein the switchable liquid crystal retarder further comprises electrodes arranged to apply a voltage for controlling the layer of liquid crystal material.
14. The display device according to claim 13 , wherein the electrodes are on opposite sides of the layer of liquid crystal material.
15. The display device according to claim 13 , wherein the electrodes are patterned to provide at least two pattern regions.
16. The display device according to claim 13 , further comprising a control system arranged to control the voltage applied across the electrodes of the switchable liquid crystal retarder.
17. The display device according to claim 16 , wherein the control system is configured to determine a location of a snooper with respect to the display device and the control system is arranged to adjust the voltage applied across the electrodes of the switchable liquid crystal retarder in response to the snooper location.
18. The display device according to claim 1 , further comprising a backlight arranged to output light, wherein the spatial light modulator is a transmissive spatial light modulator arranged to receive output light from the backlight.
19. The display device according to claim 18 , wherein the backlight comprises:
an array of light sources;
a directional waveguide comprising:
an input end extending in a lateral direction along a side of the directional waveguide, the light sources being disposed along the input end and arranged to input input light into the waveguide; and
opposed first and second guide surfaces extending across the directional waveguide from the input end for guiding light input at the input end along the waveguide, the waveguide being arranged to deflect input light guided through the directional waveguide to exit through the first guide surface.
20. The display device according to claim 19 , wherein the backlight further comprises a light turning film and the directional waveguide is a collimating waveguide.
21. The display device according to claim 20 , wherein the collimating waveguide comprises
(i) a plurality of elongate lenticular elements; and
(ii) a plurality of inclined light extraction features,
wherein the plurality of elongate lenticular elements and the plurality of inclined light extraction features are oriented to deflect input light guided through the directional waveguide to exit through the first guide surface.
22. The display device according to claim 19 , wherein the directional waveguide is an imaging waveguide arranged to image the light sources in the lateral direction so that the output light from the light sources is directed into respective optical windows in output directions that are distributed in dependence on the input positions of the light sources.
23. The display device according to claim 22 , wherein the imaging waveguide comprises a reflective end for reflecting the input light back along the imaging waveguide, wherein the second guide surface is arranged to deflect the reflected input light through the first guide surface as output light, the second guide surface comprises light extraction features and intermediate regions between the light extraction features, the light extraction features being oriented to deflect the reflected input light through the first guide surface as output light and the intermediate regions being arranged to direct light through the waveguide without extracting it;
and the reflective end has positive optical power in the lateral direction extending between sides of the waveguide that extend between the first and second guide surfaces.
24. The display device according to claim 18 , wherein the display polariser is an input display polariser arranged on an input side of the spatial light modulator between the backlight and the spatial light modulator, and the additional polariser is arranged between the input display polariser and the backlight.
25. The display device according to claim 24 , wherein the additional polariser is a reflective polariser.
26. The display device according to claim 24 , wherein the display device further comprises an output polariser arranged on an output side of the spatial light modulator.
27. The display device according to claim 18 , wherein the backlight provides a luminance at polar angles to a normal to the spatial light modulator greater than 45 degrees that is at most 33% of the luminance along the normal to the spatial light modulator.
28. The display device according to claim 27 , wherein the backlight provides a luminance at polar angles to a normal to the spatial light modulator greater than 45 degrees that is at most 20% of the luminance along the normal to the spatial light modulator.
29. The display device according to claim 28 , wherein the backlight provides a luminance at polar angles to a normal to the spatial light modulator greater than 45 degrees that is at most 10% of the luminance along the normal to the spatial light modulator.
30. The display device according to claim 1 , wherein the display polariser is an output polariser arranged on an output side of the spatial light modulator.
31. The display device according to claim 30 , wherein the display device further comprises an input polariser arranged on an input side of the spatial light modulator.
32. The display device according to claim 1 , wherein the spatial light modulator comprises an emissive spatial light modulator arranged to output light and the display polariser is an output display polariser arranged on an output side of the emissive spatial light modulator.
33. The display device according to claim 1 , wherein the layer of liquid crystal material has a retardance for light of a wavelength of 550nm in a range from 600nm to 850nm.
34. The display device according to claim 30 , wherein the layer of liquid crystal material has a retardance for light of a wavelength of 550nm in a range from 700nm to 800nm.
35. The display device according to claim 1 , wherein the at least one passive compensation retarder has a retardance for light of a wavelength of 550nm in a range from −350nm to −600nm.
36. The display device according to claim 35 , wherein the at least one passive compensation retarder has a retardance for light of a wavelength of 550nm in a range from −400nm to −500nm.