Use of folded optics to reduce volume in a virtual-reality system
Folding optics are used in conjunction with a lens system to reduce a back focal length of a head-mounted display. The lens system provides optical power to focus light from an optical display. The folding optics reflects light to fold upon itself to reduce a distance between the lens system and the optical display.
1. A system comprising:
an optical display;
a lens system;
a partial reflector between the lens system and the optical display;
a first quarter-wave plate between the lens system and the partial reflector;
a first reflective polarizer between the lens system and the first quarter-wave plate, wherein a majority of focusing power of the system comes from the lens system;
a second reflective polarizer, wherein:
the partial reflector is between the first reflective polarizer and the second reflective polarizer;
the first reflective polarizer has a transmission axis in a first orientation;
the second reflective polarizer has a transmission axis in a second orientation; and
the first orientation is orthogonal to the second orientation; and
a second quarter-wave plate, wherein the second quarter-wave plate is between the partial reflector and the second reflective polarizer.
2. The system of claim 1 , wherein:
the first quarter-wave plate is on a flat surface; and
the first reflective polarizer is on a flat surface.
3. The system of claim 1 , wherein the partial reflector is on an aspherical surface.
4. The system of claim 1 , wherein the partial reflector is on a flat surface.
5. The system of claim 1 , wherein the partial reflector has a transmission of 50%, +/−5%.
6. The system of claim 1 , wherein the partial reflector, the first quarter-wave plate, and the first reflective polarizer provide an on-axis fold of light from the optical display.
7. The system of claim 1 , wherein the first reflective polarizer and the second reflective polarizer are equidistant from the partial reflector.
8. The system of claim 1 , wherein:
the first reflective polarizer transmits linearly-polarized light of a first orientation and reflects linearly-polarized light of a second orientation; and
the first orientation is orthogonal to the second orientation.
9. A system comprising:
an optical display;
a partial reflector;
a first quarter-wave plate, wherein the partial reflector is between the optical display and the first quarter-wave plate;
a first reflective polarizer, wherein the first quarter-wave plate is between the first reflective polarizer and the partial reflector;
a second quarter-wave plate, wherein the second quarter-wave plate is between the optical display and the partial reflector; and
a second reflective polarizer, wherein the second reflective polarizer is between the optical display and the second quarter-wave plate, wherein the first reflective polarizer and the second reflective polarizer are equidistant from the partial reflector.
10. The system of claim 9 , further comprising a lens system for focusing light from the optical display to an eye box.
11. The system of claim 9 , wherein:
the first quarter-wave plate has a fast axis in a first orientation;
the second quarter-wave plate has a fast axis in a second orientation; and
the first orientation is offset from the second orientation by 90 degrees, +/−5 degrees.
12. The system of claim 9 , wherein:
the first reflective polarizer has a transmission axis in a first orientation;
the second reflective polarizer has a transmission axis in a second orientation; and
the first orientation is offset from the second orientation by 90 degrees, +/−5 degrees.
13. The system of claim 9 , wherein:
the first quarter-wave plate is on a flat surface; and
the first reflective polarizer is on a flat surface.
14. The system of claim 9 , wherein the partial reflector has a transmission of 50%, +/−5%.
15. A method comprising:
emitting light from a display to a partial reflector;
transmitting a first portion of light from the partial reflector through a first quarter-wave plate to a first reflective polarizer;
reflecting the first portion of light at the first reflective polarizer back through the first quarter-wave plate to the partial reflector;
reflecting the first portion of light at the partial reflector back toward the first reflective polarizer;
passing the first portion of light through the first reflective polarizer to a lens system;
using the lens system to focus the first portion of light to an eye box;
reflecting a second portion of light from the partial reflector through a second quarter-wave plate to a second reflective polarizer, wherein the first reflective polarizer and the second reflective polarizer are equidistant from the partial reflector;
reflecting the second portion of light at the second reflective polarizer back through the second quarter-wave plate toward the partial reflector;
transmitting the second portion of light through the partial reflector, through the first quarter-wave plate, and to the first reflective polarizer;
passing the second portion of light through the first reflective polarizer to the lens system; and
using the lens system to focus the second portion of light to the eye box.
16. The method claim 15 , wherein:
the first reflective polarizer has a transmission axis in a first orientation;
the second reflective polarizer has a transmission axis in a second orientation; and
the first orientation is offset from the second orientation by 90 degrees, +/−5 degrees.
17. The method of claim 15 , wherein:
the first quarter-wave plate has a fast axis in a first orientation;
the second quarter-wave plate has a fast axis in a second orientation; and
the first orientation is offset from the second orientation by 90 degrees, +/−5 degrees.
18. The method of claim 15 , further comprising transmitting light from the display through the second quarter-wave plate before transmitting light from the display to the partial reflector.
19. The method of claim 15 , wherein:
the first quarter-wave plate is on a flat surface; and
the first reflective polarizer is on a flat surface.
20. The method of claim 15 , wherein the partial reflector is on a surface used to correct for field-curvature.