Enabling eye tracking in pancake lens optics
Ghost images can interfere with eye tracking in a system that uses folded optics, such as is an artificial-reality display. Optical elements, such as waveplates and/or polarizers, can be used to attenuate or eliminate light causing the ghost images.
1. A system comprising:
a lens arrangement for focusing light from a display to an eye of a user, the lens arrangement comprising:
a first mirror; and
a second mirror;
a camera;
a reflector, wherein the reflector is configured to direct a portion of light to the camera after the portion of light passes through the lens arrangement;
a first waveplate, wherein the first waveplate is between the lens arrangement and the reflector;
a second waveplate, wherein the second waveplate is between the reflector and the camera; and
a polarizer, wherein the polarizer is between the second waveplate and the camera.
2. The system of claim 1 , further comprising a third waveplate, wherein the third waveplate is between the polarizer and the camera.
3. The system of claim 2 , wherein a fast axis of the third waveplate is at 45 degrees to a transmission axis of the polarizer.
4. The system of claim 1 , wherein:
the first mirror is a partial reflector; and
the second mirror is a reflective polarizer.
5. The system of claim 4 , wherein the reflective polarizer is a linear polarizer.
6. The system of claim 1 , wherein:
the lens arrangement further comprises an optical retarder;
the optical retarder is a quarter-wave plate; and
the optical retarder is between the first mirror and the second mirror.
7. The system of claim 1 , wherein:
the camera is configured to capture images of the eye of the user; and
the system further comprises a controller configured to track a position of the eye of the user based on images of the eye captured by the camera.
8. The system of claim 1 , wherein the polarizer is an absorptive polarizer.
9. The system of claim 1 , wherein the first mirror is on a curved surface for focusing light from the display to the eye of the user.
10. The system of claim 1 , wherein the portion of light is from a reflection from the eye of the user.
11. A method comprising:
directing a portion of light to a camera, using a reflector, after the portion of light passes through a lens arrangement and a first waveplate, wherein the lens arrangement comprises a first mirror and a second mirror;
passing the portion of light through the first waveplate, after directing the portion of light with the reflector;
passing the portion of light through a second waveplate, wherein the second waveplate is between the reflector and the camera;
passing the portion of light through a polarizer, wherein the polarizer is between the second waveplate and the camera; and
detecting the portion of light with the camera, after the portion of light passes through the polarizer.
12. The method of claim 11 , further comprising passing the portion of light through a third waveplate, after passing the portion of light through the second waveplate and before detecting the portion of light with the camera.
13. The method of claim 11 , wherein:
the first mirror is a partial reflector;
the second mirror is a reflective polarizer; and
the reflective polarizer is a linear polarizer.
14. The method of claim 11 , wherein:
the lens arrangement further comprises an optical retarder; and
the optical retarder is between the first mirror and the second mirror.
15. The method of claim 11 , wherein the portion of light is from a reflection from an eye of a user.
16. The method of claim 11 , further comprising tracking a position of an eye based on detecting the portion of light with the camera.