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A privacy display comprises a spatial light modulator and a compensated switchable liquid crystal retarder arranged between first and second polarizers arranged in series with the spatial light modulator. In a privacy mode of operation, on-axis light from the spatial light modulator is directed with

A privacy display comprises a spatial light modulator and a compensated switchable liquid crystal retarder arranged between first and second polarizers 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.

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1. A switchable display device comprising: a backlight arranged to output light;a transmissive spatial light modulator arranged to receive the light output from the backlight,a display polariser arranged on a side of the spatial light modulator;an additional polariser arranged on the same side of th

1. A switchable display device comprising: a backlight arranged to output light;a transmissive spatial light modulator arranged to receive the light output from the backlight,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; andplural retarders arranged between the additional polariser and the display polariser,wherein the plural retarders comprise:a switchable liquid crystal retarder comprising: a layer of liquid crystal material having a positive dielectric anisotropy and having a retardance for light of a wavelength of 550 nm in a range from 500 nm to 1000 nm; 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 the plane of the layer of liquid crystal material that is parallel or anti-parallel or orthogonal to an electric vector transmission direction of the display polariser; andat least one passive compensation retarder that comprises either:a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm; ora pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed and each having a retardance for light of a wavelength of 550 nm in a range from +300 nm to +800 nm. 2. A switchable display device according to claim 1, wherein the display polariser and the additional polariser have electric vector transmission directions that are parallel. 3. A switchable display device according to claim 1, wherein the layer of liquid crystal material has a retardance for light of a wavelength of 550 nm in a range from 600 nm to 850 nm. 4. A switchable display device according to claim 1, wherein the layer of liquid crystal material has a retardance for light of a wavelength of 550 nm in a range from 700 nm to 800 nm. 5. A switchable display device according to claim 1, wherein the at least one passive compensation retarder comprises a passive retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range in a range from −350 nm to −600 nm. 6. A switchable display device according to claim 1, wherein the at least one passive compensation retarder comprises a passive retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range in a range from −400 nm to −500 nm. 7. A switchable display device according to claim 1, wherein the at least one passive compensation retarder comprises a pair of retarders which have optical axes in the plane of the retarders that are crossed, each retarder of the pair of retarders having a retardance for light of a wavelength of 550 nm in a range from 350 nm to 650 nm. 8. A switchable display device according to claim 1, wherein the at least one passive compensation retarder comprises a pair of retarders which have optical axes in the plane of the retarders that are crossed, each retarder of the pair of retarders having a retardance for light of a wavelength of 550 nm in a range from in a range from 450 nm to 500 nm. 9. A switchable display device according to claim 1, wherein the backlight provides a luminance at polar angles to the 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. 10. A switchable display device according to claim 1, wherein the backlight provides a luminance at polar angles to the 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. 11. A switchable display device according to claim 1, wherein the backlight provides a luminance at polar angles to the 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. 12. A switchable display device according to claim 1, wherein the at least one passive compensation retarder comprises a pair of passive retarders have optical axes that are crossed and extend at 45° and at 135°, respectively, with respect to an electric vector transmission direction that is parallel to the electric vector transmission of the display polariser. 13. A switchable 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 in its undriven state. 14. A switchable display device according to claim 1, wherein the components of the pretilt direction of the pretilts of each of the surface alignment layers are anti-parallel to each other. 15. A switchable 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. 16. A switchable display device according to claim 15, wherein the electrodes are on opposite sides of the layer of liquid crystal material. 17. A switchable display device according to claim 15, further comprising a control system arranged to control the voltage applied across the electrodes of the at least one switchable liquid crystal retarder. 18. A switchable display device according claim 1, further comprising at least one further retarder and a further additional polariser, wherein the at least one further retarder is arranged between the first-mentioned additional polariser and the further additional polariser. 19. A switchable display device according to claim 1, wherein the backlight comprises: an array of light sources;a directional waveguide comprising:an input end extending in a lateral direction along the directional waveguide, the light sources being disposed along the input end and arranged to input input light into the waveguide; andopposed 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. A switchable display device according to claim 19, wherein the backlight further comprises a light turning film and the directional waveguide is a collimating waveguide. 21. A switchable 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 arranged to deflect input light guided through the directional waveguide to exit through the first guide surface. 22. A switchable display device according to claim 1, wherein the display polariser is an input display polariser arranged on the 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. 23. A switchable display device according to claim 22, wherein the additional polariser is a reflective polariser. 24. A switchable display device according to claim 22, wherein the display device further comprises an output polariser arranged on the output side of the spatial light modulator. 25. A switchable display device according to claim 1, wherein the display polariser is an output polariser arranged on the output side of the spatial light modulator. 26. A switchable display device according to claim 25, wherein the display device further comprises an input polariser arranged on the input side of the spatial light modulator.