Optical stack for privacy display

Patent metadata

Field	Value
Publication number	US-10955715-B2
Application number	US-201916448235-A
Country	US
Kind code	B2
Filing date	Jun 21, 2019
Priority date	Jun 29, 2018
Publication date	Mar 23, 2021
Grant date	Mar 23, 2021

Abstract

A switchable privacy display apparatus comprises a polarised output spatial light modulator, and an additional polariser. A reflective polariser, switchable liquid crystal polar control retarder, passive polar control retarders and air gap are arranged between the display output polariser and additional polariser. The passive retarders are arranged to provide no phase difference to polarised light from the spatial light modulator for on-axis light; and simultaneously provide a non-zero phase difference for polarised light in off-axis directions. The polar control retarders are further arranged to achieve low reflectivity for light propagating through the air gap. A switchable privacy display that can be conveniently assembled at low cost can be provided with high contrast images for display users while maintaining high visual security level for off-axis snoopers.

First claim

The invention claimed is:

1. A display device comprising:
a spatial light modulator;
a display polariser arranged on an output side of the spatial light modulator, the display polariser being a linear polariser;
an additional polariser arranged on an output side of the display polariser, the additional polariser being a linear polariser; and
plural polar control retarders arranged between the additional polariser and the display polariser, wherein an air gap divides the plural polar control retarders into at least one air gap input retarder between the display polariser and the air gap and at least one air gap output retarder between the air gap and the additional polariser,
the at least one air gap input retarder being arranged to convert linearly polarised light passed by the display polariser into circularly polarised light, and the plural polar control retarders together being capable of simultaneously introducing no net, relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis along a normal to a plane of the plural polar control retarders and introducing a net, relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis inclined to a normal to the plane of the plural polar control retarders,
wherein the plural polar control retarders include at least one passive retarder,
wherein the at least one passive retarder comprises two passive retarders which have optical axes in a plane of the two passive retarders that are crossed,
wherein the two passive retarders are both included in one of the at least one air gap output retarder or the at least one air gap input retarder,
wherein the two passive retarders introduce a net, relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis along a normal to the plane of the plural polar control retarders, and the at least one passive retarder comprises a further passive retarder included in the other of the at least one air gap output retarder or the at least one air gap input retarder, the further passive retarder introducing a relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis along a normal to the plane of the plural polar control retarders that is equal and opposite to said net, relative phase shift introduced by the at least one passive retarder, and
wherein one of the two passive retarders and the further passive retarder have optical axes in a plane of the said one of the two passive retarders and the further passive retarder that are crossed, and the other of the two passive retarders has a retardance for light of a wavelength of 550 nm that is equal to a retardance of said one of the two passive retarders minus a retardance of the further passive retarder.

2. A display device according to claim 1 , wherein the display polariser and the additional polariser have electric vector transmission directions that are parallel.

3. A display device according to claim 1 , wherein
the display polariser and the additional polariser have electric vector transmission directions that are not parallel, and
the display device further comprises a rotator retarder arranged between the additional polariser and the display polariser, the rotator retarder being arranged to rotate a polarisation direction of polarised light incident thereon between the electric vector transmission directions of the display polariser and the additional polariser.

4. A display device according to claim 3 , wherein the spatial light modulator includes a layer of twisted nematic liquid crystal material.

5. A display device according to claim 1 , wherein a reflective polariser is arranged between the display polariser and additional polariser, the reflective polariser being a linear polariser.

6. A display device according to claim 5 , wherein the display polariser and the reflective polariser have electric vector transmission directions that are parallel.

7. A display device according to claim 1 , wherein a diffusing surface structure is provided on at least one of the at least one air gap input retarder and the at least one air gap output retarder.

8. A display device according to 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.

9. A 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.

10. A display device according to claim 9 , 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, preferably at most 20% of the luminance along the normal to the spatial light modulator, and most preferably at most 10% of the luminance along the normal to the spatial light modulator.

11. A display device according to claim 9 , 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.

12. A display device according to claim 11 , wherein the backlight further comprises a light turning film and the directional waveguide is a collimating waveguide.

13. A display device according to claim 12 , 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.

14. A display device according to claim 11 , 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.

15. A display device according to claim 14 , 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.

16. A

Assignees

• Reald Spark Llc

Inventors

• Woodgate Graham J
• Harrold Jonathan

Classifications

• G02F1/133638

  Waveplates, i.e. plates with a retardation value of lambda/n · CPC title

• G02F1/137 Primary

  characterised by the electro-optical or magneto-optical effect, e.g. field-induced phase transition, orientation effect, guest-host interaction or dynamic scattering · CPC title

• G02F1/13725

  based on guest-host interaction (G02F1/13762, G02F1/13737, take precedence) · CPC title

• G02F1/1323

  Arrangements for providing a switchable viewing angle · CPC title

• G02F1/133531

  characterised by the arrangement of polariser or analyser axes · CPC title