Pupil-replicating lightguide with switchable out-coupling efficiency distribution and display based thereon

Abstract

A pupil-replicating lightguide includes a slab of transparent material for guiding image light in the slab, and an out-coupling structure supported by the slab for out-coupling portions of the image light from the slab. The portions are laterally offset from one another along a path of the image light in the slab. The out-coupling grating structure has a switchable distribution of out-coupling efficiency for redirecting the portions of out-coupled light to a desired location such as a current location of an eye of the viewer determined by an eye tracking system. The out-coupling grating structure may include a plurality of diffraction gratings having different local slant angles of grating fringes.

What is claimed is: 
     
         1 . A pupil-replicating lightguide for expanding image light, the pupil-replicating lightguide comprising:
 a slab of transparent material for guiding the image light therein by a series of internal reflections from opposed surfaces of the slab; and   an out-coupling grating structure supported by the slab for out-coupling portions of the image light from the slab, wherein the portions are laterally offset from one another along a path of the image light in the slab, and wherein the out-coupling grating structure has a switchable distribution of out-coupling efficiency.   
     
     
         2 . The pupil-replicating lightguide of  claim 1 , wherein the out-coupling grating structure comprises a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switchable between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. 
     
     
         3 . The pupil-replicating lightguide of  claim 2 , wherein the second threshold is at least ten times lower than the first threshold. 
     
     
         4 . The pupil-replicating lightguide of  claim 2 , wherein the plurality of switchable diffraction gratings comprises switchable gratings with spatially varying slant angle of the grating fringes along the path of the image light. 
     
     
         5 . The pupil-replicating lightguide of  claim 4 , wherein the switchable gratings are disposed in a stack configuration parallel to the opposed surfaces of the slab. 
     
     
         6 . The pupil-replicating lightguide of  claim 2 , wherein the plurality of diffraction gratings comprises a polarization volume hologram (PVH) grating. 
     
     
         7 . The pupil-replicating lightguide of  claim 2 , wherein the plurality of diffraction gratings comprises a tunable liquid crystal (LC) surface-relief grating. 
     
     
         8 . The pupil-replicating lightguide of  claim 1 , wherein the out-coupling grating structure comprises a fluidic grating. 
     
     
         9 . The pupil-replicating lightguide of  claim 1 , wherein the out-coupling grating structure comprises a grating having a wavelength-dependent refractive index contrast. 
     
     
         10 . A near-eye display comprising:
 a projector for providing image light; and   a pupil-replicating lightguide coupled to the projector and comprising:
 a slab of transparent material for guiding the image light therein by a series of internal reflections from opposed surfaces of the slab; and 
 an out-coupling grating structure supported by the slab for out-coupling portions of the image light from the slab, wherein the portions are laterally offset from one another along a path of the image light in the slab, wherein the out-coupling grating structure comprises a plurality of grating fringes having a switchable distribution of out-coupling efficiency. 
   
     
     
         11 . The near-eye display of  claim 10 , further comprising:
 an eye tracker for determining a position of a pupil of a user’s eye at an eyebox of the near-eye display; and   a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to:
 cause the eye tracker to determine the position of the pupil; and, 
 responsive to the determined position of the pupil, switch the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position. 
   
     
     
         12 . The near-eye display of  claim 10 , wherein the out-coupling grating structure comprises a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switchable between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. 
     
     
         13 . The near-eye display of  claim 12 , wherein the plurality of switchable diffraction gratings comprises switchable gratings with spatially varying slant angle of the grating fringes along the path of the image light. 
     
     
         14 . The near-eye display of  claim 12 , wherein the second threshold is at least ten times lower than the first threshold. 
     
     
         15 . The near-eye display of  claim 12 , wherein the plurality of diffraction gratings comprises at least one of: a polarization volume hologram (PVH) grating; a fluidic grating; or a grating having a wavelength-dependent refractive index contrast. 
     
     
         16 . The near-eye display of  claim 12 , further comprising:
 an eye tracker for determining a position of a pupil of the user’s eye at an eyebox of the near-eye display; and   a controller operably coupled to the projector, the eye tracker, and the pupil-replicating lightguide, and configured to cause the eye tracker to determine the position of the pupil; and, responsive to the determined position of the pupil, switch a diffraction grating of the plurality of diffraction gratings to the high-efficiency state.   
     
     
         17 . The near-eye display of  claim 16 , wherein the controller is further configured to switch the remaining diffraction gratings of the plurality of diffraction gratings to the low-efficiency state. 
     
     
         18 . A method for displaying an image, the method comprising:
 providing image light to a pupil-replicating lightguide comprising a slab of transparent material;   guiding the image light in the slab by a series of internal reflections from opposed surfaces of a slab of transparent material;   out-coupling portions of the image light from the slab by an out-coupling grating structure, wherein the portions are laterally offset from one another along a path of the image light in the slab; and   switching angular distribution of out-coupling efficiency of a plurality of grating fringes of the out-coupling grating structure.   
     
     
         19 . The method of  claim 18 , wherein the switching of the angular distribution of the out-coupling efficiency comprises switching a plurality of diffraction gratings having different local slant angles of grating fringes, wherein the diffraction gratings are switched between a high-efficiency state, in which a percentage of the image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of the image light out-coupled from the slab is below a second threshold lower than the first threshold. 
     
     
         20 . The method of  claim 18 , further comprising:
 using an eye tracker to determine a position of a pupil of a user’s eye at an eyebox of the near-eye display; and, responsive to the determined position of the pupil,   switching the angular distribution of diffraction efficiency to increase an amount of the image light illuminating the pupil at the determined position.