Privacy display apparatus
A privacy display comprises a liquid crystal spatial light modulator, a switchable retarder and a passive compensation retarder arranged between a pair of polarisers. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without change of image contrast, whereas off-axis light has reduced contrast to reduce the visibility of the display to off-axis snoopers over a wide polar angular range. In a wide angle mode of operation, the retardance of the switchable retarder is adjusted so that off-axis contrast is substantially unmodified.
1. A display device comprising:
a spatial light modulator comprising a layer of liquid crystal material arranged to output light on an output side;
an input polariser arranged on an input side of the spatial light modulator;
an output polariser arranged on the output side of the spatial light modulator; and
plural retarders arranged between the input polariser and the output polariser on either the input side or the output side of spatial light modulator,
wherein
the plural retarders include at least one switchable liquid crystal retarder comprising a layer of liquid crystal material and electrodes arranged to apply a voltage for switching the layer of liquid crystal material, and at least one passive retarder, and
the plural retarders are arranged to introduce no phase shift to polarisation components of light passed by the input polariser along an axis along a normal to the plane of the retarder and are arranged to introduce a phase shift to polarisation components of light passed by the input polariser along an axis inclined to the axis along a normal to the plane of the retarder in a switchable mode of the at least one switchable liquid crystal retarder.
2. A display device according to claim 1 , wherein the switchable liquid crystal retarder comprises two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof and each arranged to provide homeotropic alignment in the adjacent liquid crystal material.
3. A display device according to claim 2 , wherein the liquid crystal material has a negative dielectric anisotropy.
4. A display device according to claim 2 , wherein the at least one passive retarder comprises a passive retarder having an optical axis perpendicular to the plane of the passive retarder and having and having a retardance for light of a wavelength of 550 nm in a range from −250 nm to −500 nm.
5. A display device according to claim 1 , wherein the switchable liquid crystal retarder comprises two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof and each arranged to provide homogenous alignment in the adjacent liquid crystal material.
6. A display device according to claim 5 , wherein the liquid crystal material has a positive dielectric anisotropy.
7. A display device according to claim 5 , wherein the at least one passive retarder comprises a passive retarder having an optical axis perpendicular to the plane of the passive retarder and having and having a retardance for light of a wavelength of 550 nm in a range from −400 nm to −200 nm.
8. A display device according to claim 5 , wherein the at least one passive retarder comprises a pair of passive retarders having optical axes parallel to the plane of the passive retarder that are crossed and having a retardance for light of a wavelength of 550 nm in a range from 250 nm to 450 nm.
9. A display device according to claim 5 , wherein
the spatial light modulator comprises a surface alignment layer disposed adjacent to the layer of liquid crystal material on the side closest to the plural retarders that is arranged to provide homogenous alignment in the adjacent liquid crystal material, there being a non-zero angle between the alignment direction of the surface alignment layer of the spatial light modulator and the alignment direction of the surface alignment layer of the switchable liquid crystal retarder on the side closest to the spatial light modulator, and
the display device further comprises a half waveplate arranged between the spatial light modulator and the switchable liquid crystal retarder and arranged to rotate a polarisation component of light incident thereon by said non-zero angle.
10. A display device according to claim 9 , wherein the at least one passive retarder comprises a passive retarder having an optical axis perpendicular to the plane of the passive retarder and having and having a retardance for light of a wavelength of 550 nm in a range from −400 nm to −200 nm.
11. A display device according to claim 1 , wherein the at least one passive retarder comprises a pair of passive retarders having optical axes parallel to the plane of the passive retarder that are crossed.
12. A display device according to claim 11 , wherein the pair of passive retarders have optical axes that extend at 45° and at 135°, respectively, with respect to an electric vector transmission direction of the input polariser or the output polariser.
13. A display device according to claim 1 , wherein the at least one passive retarder comprises a passive retarder having an optical axis perpendicular to the plane of the passive retarder.
14. A display device according to claim 1 , wherein the spatial light modulator is transmissive and the display device further comprises a backlight on the input side of the input polariser.
15. A display device according to claim 14 , wherein the backlight provides a luminance at lateral angles greater than 45 degrees, that is at most 20% of the luminance in a direction normal to the display surface.
16. A display device according to claim 14 , wherein the spatial light modulator is one of an in-plane switching device, a twisted nematic device or a fringe field switching device.
17. A display device according to claim 1 , further comprising
an additional polariser arranged on the input side of the input polariser or on the output side of the output polariser; and
at least one retarder arranged between the input polariser and the additional polariser in the case the additional polariser is arranged on the input side of the input polariser; or at least one retarder arranged between the output polariser and the additional polariser in the case the additional polariser is arranged on the output side of the output polariser.
18. A display device according to claim 17 wherein the at least one retarder comprises a switchable liquid crystal retarder comprising a layer of liquid crystal material.
19. A display device according to claim 18 further comprising at least one passive compensation retarder.
20. A display device according to claim 17 , wherein the additional polariser is on the output side of the output polariser and further comprising a reflective polariser arranged between the output polariser and the at least one retarder.