Eye tracking with switchable gratings
A pupil-replicating lightguide includes a slab of transparent material, a switchable out-coupling grating for out-coupling portions of image light to propagate towards an eyebox, and a switchable tracking grating for redirecting tracking light carrying an eye image towards an eye tracking camera. One of the two gratings may be turned ON while the other is turned OFF, in a time-sequential manner, allowing the combined use of the pupil-replicating lightguide for carrying image light and eye tracking light.
1. A near-eye display comprising:
an image projector for providing image light carrying an image in angular domain to an eye at an eyebox of the near-eye display;
an eye tracking camera for receiving tracking light carrying an image of the eye at the eyebox; and
a lightguide coupled to the image projector and the eye tracking camera, the lightguide comprising:
a slab of transparent material for propagating the image light and the tracking light therein;
a switchable out-coupling grating for out-coupling portions of the image light to propagate towards the eyebox; and
a switchable tracking grating for redirecting the tracking light towards the eye tracking camera;
wherein:
the switchable out-coupling grating is switchable between a high-efficiency state, in which an out-coupling efficiency of the switchable out-coupling grating is above a first threshold, and a low-efficiency state, in which the out-coupling efficiency of the switchable out-coupling grating below a second threshold; and
the switchable tracking grating is switchable between a high-efficiency state, in which an out-coupling efficiency of the switchable tracking grating is above a third threshold, and a low-efficiency state, in which the out-coupling efficiency of the switchable tracking grating is below a fourth threshold.
2. The near-eye display of claim 1 , wherein:
the slab is configured to transmit external light to the eyebox, providing a field of view of outside environment to the eye; and
both the switchable out-coupling grating and the switchable tracking grating are at least partially disposed in the field of view.
3. The near-eye display of claim 1 , wherein the first threshold is at least 10 times higher than the second threshold, and the third threshold is at least 10 times higher than the fourth threshold.
4. The near-eye display of claim 1 , further comprising a controller operably coupled to the switchable out-coupling grating and the switchable tracking grating and configured to:
switch the switchable out-coupling grating to the high-efficiency state and switch the switchable tracking grating to the low-efficiency state during a first time interval; and
switch the switchable out-coupling grating to the low-efficiency state and switch the switchable tracking grating to the high-efficiency state during a second, subsequent time interval.
5. The near-eye display of claim 4 , wherein the controller is operably coupled to the image projector and is configured to cause the image projector to display the image in angular domain during the first time interval.
6. The near-eye display of claim 4 , wherein the controller is operably coupled to the eye tracking camera and is configured to cause the eye tracking camera to obtain the image of the eye during the second time interval.
7. The near-eye display of claim 1 , wherein:
the lightguide is a pupil-replicating lightguide for propagating the image light and the tracking light by a series of internal reflections in the slab, wherein the tracking light is in-coupled into the slab by the switchable tracking grating, the pupil-replicating lightguide further comprising:
an in-coupling grating for in-coupling the image light provided by the image projector into the slab; and
an out-coupling tracking grating for out-coupling at least a portion of the tracking light from the slab and towards the eye tracking camera.
8. The near-eye display of claim 7 , wherein the in-coupling grating is switchable, and wherein the out-coupling tracking grating is switchable.
9. The near-eye display of claim 1 , wherein at least one of the switchable out-coupling grating or the switchable tracking grating comprises a switchable polarization volume hologram (PVH) grating.
10. The near-eye display of claim 1 , wherein at least one of the switchable out-coupling grating or the switchable tracking grating comprises a switchable Pancharatnam-Berry phase (PBP) liquid crystal (LC) grating.
11. The near-eye display of claim 1 , wherein at least one of the switchable out-coupling grating or the switchable tracking grating comprises a switchable liquid crystal (LC) surface relief grating.
12. The near-eye display of claim 1 , wherein at least one of the switchable out-coupling grating or the switchable tracking grating comprises a fluidic grating.
13. A method for displaying an image to an eye and tracking the eye, the method comprising:
providing image light carrying an image in angular domain;
propagating the image light in a slab of transparent material;
during a first time interval, switching an out-coupling grating to a high-efficiency state to out-couple portions of the image light towards an eyebox, and switching a tracking grating to a low-efficiency state; and
during a second, subsequent time interval, switching the tracking grating to a high-efficiency state to redirect tracking light carrying an image of the eye towards an eye tracking camera, and switching the out-coupling grating to a low-efficiency state.
14. The method of claim 13 , further comprising using an image projector to display the image in angular domain during the first time interval.
15. The method of claim 13 , wherein an out-coupling efficiency of the out-coupling grating is at least ten times higher in the high-efficiency state than in the low-efficiency state, and wherein an out-coupling efficiency of the tracking grating is at least ten times higher in the high-efficiency state than in the low-efficiency state.
16. A pupil-replicating lightguide comprising:
a slab of transparent material for propagating image light and tracking light, wherein in operation, the image light carries an image in angular domain to a user's eye at an eyebox, and the tracking light carries an image of the eye to an eye tracking camera;
a switchable out-coupling grating for out-coupling portions of the image light to propagate towards the eyebox; and
a switchable tracking grating for redirecting the tracking light towards the eye tracking camera;
wherein:
the switchable out-coupling grating is switchable between a high-efficiency state, in which an out-coupling efficiency of the switchable out-coupling grating is above a first threshold, and a low-efficiency state, in which the out-coupling efficiency of the switchable out-coupling grating below a second threshold; and
the switchable tracking grating is switchable between a high-efficiency state, in which an out-coupling efficiency of the switchable tracking grating is above a third threshold, and a low-efficiency state, in which an out-coupling efficiency of the switchable tracking grating is below a fourth threshold.
17. The pupil-replicating lightguide of claim 16 , wherein projections of the switchable out-coupling grating and the switchable tracking grating onto a clear aperture of the pupil-replicating lightguide overlap one another.
18. The pupil-replicating lightguide of claim 16 , wherein at least one of the switchable out-coupling grating or the switchable tracking grating comprises at least one of: a switchable polarization volume hologram (PVH) grating; a switchable Pancharatnam-Berry phase (PBP) liquid crystal (LC) grating; a switchable liquid crystal (LC) surface relief grating; or a fluidic grating.