Optical stack for imaging directional backlights

Abstract

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. Retarder stack arrangements are provided to reduce the display visibility to snoopers located in polar viewing regions of the display while achieving minimal reduction of head-on luminance. Further visibility of light reflections from automotive windscreens may be reduced.

1 - 69 . (canceled) 
     
     
         70 . A display device comprising:
 a transmissive spatial light modulator;   a backlight comprising:
 two pluralities of light sources; 
 two waveguides disposed behind the transmissive spatial light modulator, each waveguide comprising:
 an input end extending in a lateral direction along a side of the waveguide; and 
 opposed first and second guide surfaces extending across the waveguide from the input end for guiding light input at the input end along the waveguide, wherein the waveguide is arranged to extract input light guided through the waveguide to exit through the first guide surface in a grazing direction, 
 wherein the input ends of the two waveguides are on opposite sides of the display device, and a respective plurality of light sources is disposed along the input end of each waveguide and arranged to input input light into the waveguide; and 
 
 a prismatic input layer arranged to receive the light extracted from the two waveguides and to direct the received light to be incident on the spatial light modulator, 
 wherein the backlight is arranged so that the light from the respective waveguides is directed to provide different viewing windows; 
   an input polariser arranged on the input side of the spatial light modulator between the backlight and the spatial light modulator;   an output polariser arranged on the output side of the spatial light modulator;   an additional polariser arranged on the input side of the input polariser between the input polariser and the backlight or on the output side of the output polariser; and   at least one retarder arranged between the at least one additional polariser and the input polariser in the case that the additional polariser is arranged on the input side of the input polariser or between the additional polariser and the output polariser in the case that the additional polariser is arranged on the output side of the input polariser.   
     
     
         71 . A display device according to  claim 70 , wherein the backlight is arranged so that the light from the respective waveguides is directed to provide viewing windows having different widths. 
     
     
         72 . A display device according to  claim 70 , wherein the backlight further comprises a rear scattering reflector arranged behind the two waveguides. 
     
     
         73 . A display device according to  claim 70 , wherein the backlight further comprises an intermediate diffuser layer arranged between the two waveguides. 
     
     
         74 . A display device according to  claim 70 , wherein at least one of the first and second guide surfaces of the two waveguides comprise microstructures. 
     
     
         75 . A display device according to  claim 70 , wherein the prismatic input layer comprises first input facets and second input facets, wherein the light extracted from one of the two waveguides is received by the first input facets and the light extracted from the other of the two waveguides is received by the second input facets. 
     
     
         76 . A display device according to  claim 70 , further comprising a control system arranged to control the light sources. 
     
     
         77 . A display device according to  claim 76 , wherein at least one retarder comprises a switchable liquid crystal retarder, and the control system is further arranged to control the applied voltage across the switchable liquid crystal retarder. 
     
     
         78 . A display device according to  claim 76 , wherein the control system is arranged to provide switching between
 in a first mode of operation, the plurality of light sources disposed along the input end of one of the two waveguides being operated; and a first applied voltage across the switchable liquid crystal retarder;   in a second mode of operation the light sources disposed along the input end of the other of the two waveguides being operated; and a second applied voltage across the switchable liquid crystal retarder that is different to the first applied voltage.   
     
     
         79 . A display device according to  claim 70 , wherein the additional polariser is arranged on the input side of the input polariser and said at least one retarder is arranged between the additional polariser and the input polariser. 
     
     
         80 . A display device according to  claim 79 , wherein the additional polariser is a reflective polariser. 
     
     
         81 . A display device according to  claim 70 , wherein the additional polariser has an electric vector transmission direction that is parallel to the electric vector transmission of the input polariser in the case that the additional polariser is arranged on the input side of the input polariser or is parallel to the electric vector transmission of the output polariser in the case that the additional polariser is arranged on the output side of the input polariser. 
     
     
         82 . A display device according to  claim 70 , wherein the additional polariser is arranged on the output side of the output polariser and said at least one retarder is arranged between the additional polariser and the output polariser. 
     
     
         83 . A display device according to  claim 70 , wherein the at least one retarder comprises at least one switchable liquid crystal retarder and at least one correcting passive retarder. 
     
     
         84 . A display device according to  claim 83 , wherein the at least one correcting passive retarder comprises a pair of retarders which have slow axes in the plane of the retarders that are crossed. 
     
     
         85 . A display device according to  claim 83 , wherein the at least one correcting passive retarder comprises a retarder having a slow axis perpendicular to the plane of the retarder. 
     
     
         86 . A display device according to  claim 83 , wherein the at least one correcting passive retarder comprises a retarder having a slow axis orientation with a component perpendicular to the plane of the retarder, and at least one component in the plane of the retarder. 
     
     
         87 . A display device according to  claim 83 , wherein the at least one switchable liquid crystal retarder has an optical thickness between 500 nm and 1000 nm, preferably between 700 nm and 900 nm, and most preferably between 775 nm and 825 nm. 
     
     
         88 . A display device according to  claim 87 , wherein the at least one correcting passive retarder has an optical thickness between 400 nm and 800 nm, preferably between 550 nm and 750 nm, and more preferably between 625 nm and 675 nm. 
     
     
         89 . A display device according to  claim 70 , further comprising at least one further additional polariser and at least one further correcting passive retarder and at least one further switchable liquid crystal retarder layer arranged between the at least one further additional polariser and the input polariser in the case that the further additional polariser is arranged on the input side of the input polariser or between the further additional polariser and the output polariser in the case that the further additional polariser is arranged on the output side of the input polariser.