Exit pupil expander with total internal reflection between wedge surfaces
An exit pupil expander (EPE) has entrance and exit pupils, a back surface adjacent to the entrance pupil, and an opposed front surface. In one embodiment the EPE is geometrically configured such that light defining a center wavelength that enters at the entrance pupil perpendicular to the back surface experiences angularly varying total internal reflection between the front and back surfaces such that the light exiting the optical channel perpendicular to the exit pupil is at a wavelength shifted from the center wavelength. In another embodiment a first distance at the entrance pupil between the front and back surfaces is different from a second distance at the exit pupil between the front and back surfaces. The EPE may be deployed in a head-wearable imaging device (e.g., virtual or augmented reality) where the entrance pupil in-couples light from a micro display and the exit pupil out-couples light from the EPE.
1 . A method of operating an optical channel comprising:
directing light through an entrance pupil of an optical channel body, wherein an exit pupil enables the light to exit the optical channel body, wherein the optical channel body is geometrically configured such that a front surface and a back surface form a continuous wedge defining an angle α that quantifies an amount of non-parallelism between the front surface and the back surface and between a plane of the entrance pupil and a plane of the exit pupil, a first distance at the entrance pupil between the front surface and the back surface is greater than a second distance at the exit pupil between the front surface and the back surface, and wherein light defining a center wavelength that enters the optical channel body at the entrance pupil perpendicular to the back surface experiences angularly varying total internal reflection between the front and back surfaces such that the light that exits the optical channel body perpendicular to the exit pupil is at a wavelength shifted from the center wavelength.
2 . The method of claim 1 , wherein the front and back surfaces define at least first and second discrete geometric segments of the optical channel body, wherein the front and back surfaces are parallel in the first discrete geometric segment and non-parallel in at least the second discrete geometric segment.
3 . The method of claim 1 , wherein the light defining the center wavelength is expanded by the angularly varying total internal reflection such that:
a first portion of the expanded light that exits the optical channel body perpendicular to the exit pupil is at a wavelength shifted from the center wavelength; and
a second portion of the expanded light that exits the optical channel body non-perpendicular to the exit pupil is at the center wavelength.
4 . The method of claim 3 , wherein:
the second portion of the expanded light that exits the optical channel body non-perpendicular to the exit pupil exhibits a greater intensity than the first portion of the expanded light that exits the optical channel body perpendicular to the exit pupil.
5 . The method of claim 1 , wherein the front surface is adjacent to the exit pupil and the optical channel body is a see-through exit pupil expander further comprising:
a compensating wedge disposed adjacent to a portion of the back surface opposite the exit pupil, said compensating wedge transparent to incident environmental light and geometrically configured to offset angular variance the optical channel body imposes on incident environmental light that passes into the optical channel body via the compensating wedge.
6 . The method of claim 5 , wherein the compensating wedge has a surface that is at the angle α relative to the plane of the entrance pupil.
7 . The method of claim 5 , comprising a wedge overlay, wherein:
the said front and back surfaces are front and back surfaces of the optical channel body;
the wedge overlay defines non-parallel front and back overlay surfaces;
the wedge overlay is disposed adjacent to the front surface of the optical channel body opposite the entrance pupil; and
at least some of the angularly varying total internal reflection between the front and back surfaces are between the front surface of the wedge overlay and the back surface of the optical channel body.
8 . The method of claim 7 , wherein:
disposition of a distal end of the wedge overlay relative to the entrance pupil filters incident light passing through the entrance pupil such that only wavelengths above or below a threshold experience the angularly varying total internal reflection while remaining wavelengths experience total internal reflection that is not angularly varying.
9 . A method of operating an optical channel comprising:
directing light through an entrance pupil of an optical channel body, wherein the optical channel body is configured such that front and back surfaces of the optical channel form a continuous wedge defining an angle α that quantifies an amount of non-parallelism between the front surface and the back surface and between a plane of the entrance pupil and a plane of the exit pupil, light that enters the optical channel body at the entrance pupil experiences total internal reflection between the front and back surfaces, and a first distance at the entrance pupil between the front surface and the back surface is greater than a second distance at the exit pupil between the front surface and the back surface, wherein the front surface is adjacent to the exit pupil and the optical channel body is a see-through exit pupil expander further comprising:
a compensating wedge disposed adjacent to a portion of the back surface opposite the exit pupil, said compensating wedge transparent to incident environmental light and geometrically configured to offset exit pupil expansion that the optical channel body imposes on incident environmental light that passes into the optical channel body via the compensating wedge.
10 . The method of claim 9 , wherein the front and back surfaces define at least first and second discrete geometric segments of the optical channel body, wherein the front and back surfaces are parallel in the first discrete geometric segment and non-parallel in at least the second discrete geometric segment.
11 . The method of claim 9 comprising a wedge overlay, wherein:
the front and back surfaces are front and back surfaces of the optical channel body;
the wedge overlay defines non-parallel front and back overlay surfaces;
the wedge overlay is disposed adjacent to the front surface of the optical channel body opposite the entrance pupil such that the first distance is between the back surface of the optical channel body and the front overlay surface; and
at least some of the light entering the optical channel body via the entrance pupil is reflected from the front overlay surface back into the optical channel body towards the back surface of the optical channel body.
12 . The method of claim 11 , wherein:
disposition of a distal end of the wedge overlay relative to the entrance pupil filters incident light passing through the entrance pupil such that only wavelengths above or below a threshold are reflected from the front overlay surface into the optical channel body while remaining wavelengths reflected from the front overlay surface are not.
13 . A method of operating an optical channel comprising a wedge overlay, the method comprising:
directing light through an entrance pupil of an optical channel body, wherein the optical channel body is configured such that front and back surfaces of the optical channel form a continuous wedge defining an angle α that quantifies an amount of non-parallelism between the front surface and the back surface and between a plane of the entrance pupil and a plane of the exit pupil, light that enters the optical channel body at the entrance pupil experiences total internal reflection between the front and back surfaces, and a first distance at the entrance pupil between the front surface and the back surface is greater than a second distance at the exit pupil between the front surface and the back surface, wherein:
the front and back surfaces are front and back surfaces of the optical channel body;
the wedge overlay defines non-parallel front and back overlay surfaces;
the wedge overlay is disposed adjacent to the front surface of the optical channel body opposite the entrance pupil such that the first distance is between the back surface of the optical channel body and the front overlay surface; and
at least some of the light entering the optical channel body via the entrance pupil is reflected from the front overlay surface back into the optical channel body towards the back surface of the optical channel body.
14 . The method of claim 13 , wherein the front and back surfaces define at least first and second discrete geometric segments of the optical channel body, wherein the front and back surfaces are parallel in the first discrete geometric segment and non-parallel in at least the second discrete geometric segment.
15 . The method of claim 13 , wherein:
disposition of a distal end of the wedge overlay relative to the entrance pupil filters incident light passing through the entrance pupil such that only wavelengths above or below a threshold are reflected from the front overlay surface into the optical channel body while remaining wavelengths reflected from the front overlay surface are not.