LIGHTGUIDES WITH COLOR- AND TIME-SEQUENTIAL GRATINGS
A lightguide for conveying image light to an eyebox of a display device includes a tunable grating, e.g. an out-coupling grating for out-coupling the image light from the lightguide. The tunable grating may be tuned or switched to effectively diffract light of a color channel of a color-sequential display. In augmented reality display systems, a lightguide combiner element may include a switchable grating to out-couple the image light only during short time intervals and to not out-couple the image light in between these time intervals. Effectively, the out-coupling grating is present only a portion of overall operation time of the display, which improves the transparency of the combiner element to the outside light and reduces rainbow effects.
1 . A pupil-replicating lightguide for expanding image light carrying an image in angular domain, 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 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 has a diffraction efficiency switchable between a high-efficiency state, in which a percentage of image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of image light out-coupled from the slab is below a second threshold lower than the first threshold.
2 . The pupil-replicating lightguide of claim 1 , wherein the out-coupling grating comprises a polarization volume hologram (PVH) grating.
3 . The pupil-replicating lightguide of claim 1 , wherein the out-coupling grating comprises a tunable Pancharatnam-Berry phase (PBP) liquid crystal (LC) grating.
4 . The pupil-replicating lightguide of claim 1 , wherein the out-coupling grating comprises a tunable liquid crystal (LC) surface-relief grating.
5 . The pupil-replicating lightguide of claim 1 , wherein the out-coupling grating comprises a fluidic surface-relief grating.
6 . The pupil-replicating lightguide of claim 1 , further comprising an acoustic actuator coupled to the slab, wherein the out-coupling grating is formed by an acoustic wave generated by the acoustic actuator.
7 . The pupil-replicating lightguide of claim 1 , wherein the second threshold is at least ten times lower than the first threshold.
8 . The pupil-replicating lightguide of claim 1 , wherein the out-coupling grating has less rainbow effects in the low-efficiency state than in the high-efficiency state.
9 . A near-eye display comprising:
a light engine for providing image light carrying an image in angular domain;
a pupil-replicating lightguide coupled to the light engine 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 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 has a diffraction efficiency switchable between a high-efficiency state, in which a percentage of image light out-coupled from the slab is above a first threshold, and a low-efficiency state, in which a percentage of image light out-coupled from the slab is below a second threshold lower than the first threshold; and
a controller operably coupled to the light engine and the pupil-replicating lightguide and configured to:
during a first time interval, cause the light engine to provide the image light;
during the first time interval, switch the out-coupling grating to the high-efficiency state; and
during a subsequent second time interval, switch the out-coupling grating to the low-efficiency state.
10 . The near-eye display of claim 9 , wherein the pupil-replicating lightguide is configured to transmit outside visible light therethrough during the first and second time intervals.
11 . The near-eye display of claim 9 , wherein the controller is configured to cause the light engine to provide no image light during the second time interval.
12 . The near-eye display of claim 9 , wherein the second threshold is at least ten times lower than the first threshold.
13 . The near-eye display of claim 9 , wherein the second time interval is at least two times larger than the first time interval.
14 . The near-eye display of claim 9 , wherein the light engine comprises a display panel coupled to a projector lens for converting an image in linear domain displayed by the display panel into the image in angular domain.
15 . The near-eye display of claim 9 , wherein the light engine comprises a beam scanner for rastering the image in angular domain.
16 . A method for augmenting a view of outside environment with a generated image, the method comprising:
coupling image light carrying the generated image to a slab of transparent material, wherein the outside environment is observable through the slab;
guiding the image light in the slab by a series of internal reflections from opposed surfaces of the slab;
during a first time interval, switching an out-coupling grating supported by the slab to a high-efficiency state, in which the out-coupling grating out-couples laterally offset portions of the image light from the slab, wherein a percentage of the image light out-coupled from the slab is above a first threshold; and
during a second time interval, switching the out-coupling grating to a low-efficiency state, in which a percentage of image light out-coupled from the slab is below a second threshold lower than the first threshold.
17 . The method of claim 16 , wherein the image light is coupled to the slab during the first time interval and not during the second time interval.
18 . The method of claim 16 , wherein the out-coupling grating has less rainbow effects in the low-efficiency state than in the high-efficiency state.
19 . The method of claim 16 , wherein at least one of: the second threshold is at least ten times lower than the first threshold; or the second time interval is at least nine times larger than the first time interval.
20 . The method of claim 16 , further comprising switching the out-coupling grating to the high efficiency state during a third time interval following the second time interval, and switching the out-coupling grating to the low efficiency state during a fourth time interval following the third time interval.