Projection optical system for coupling image light to a near-eye display
Technology is described for a projection optical system which optically couples image light from an image source to a near-eye display (NED) of a wearable near-eye display device. The projection optical system and the image source make up a projection light engine. Light from the image source is directed to a birdbath reflective optical element which is immersed in high index glass. The image light is reflected and collimated by the birdbath element and travels outside a housing of the projection light engine forming an external exit pupil, meaning the exit pupil is external to the projection light engine. A waveguide optically couples the image light of the external exit pupil. An example of a waveguide which can be used is a surface relief grating waveguide.
1. A projection optical system of a projection light engine for coupling image light from an image source of the projection light engine to a near-eye display comprising:
a support structure of a near-eye display device which may be worn by a user;
a high index glass region supported by the support structure;
a birdbath reflective optical element immersed in the high index glass region and positioned by the support structure for optically coupling image light from the image source; and
the birdbath reflective optical element collimating and reflecting the image light through the high index glass region to an external exit pupil which is external to the projection light engine.
2. The projection optical system of claim 1 further comprising:
one or more polarizers optically coupled in the high index glass region between the birdbath lens and the external exit pupil, the one or more polarizers polarizing the image light reflecting from the birdbath optical element.
3. The projection optical system of claim 2 further comprising:
a beam splitter positioned by the support structure in the high index glass region between the image source and the birdbath optical element for optically coupling the image light from the image source to the birdbath optical element.
4. The projection optical system of claim 3 wherein the one or more polarizers further comprises a quarter waveplate positioned by the support structure between the beam splitter and the birdbath optical element which quarter waveplate polarizes the image light from the image source travelling towards the birdbath optical element and polarizes the image light reflecting from the birdbath optical element.
5. The projection optical system of claim 3 further comprising the beam splitter is a polarizing beam splitter.
6. The projection optical system of claim 1 wherein the high index glass region includes flint glass.
7. The projection optical system of claim 1 wherein the high index glass region includes glass having an index of refraction of at least 1.65.
8. The projection optical system of claim 1 wherein the high index glass region includes glass having an index of refraction between 1.7 and 1.8.
9. The projection optical system of claim 1 wherein the external exit pupil is about five millimeters outside the projection light engine.
10. The projection optical system of claim 1 wherein the birdbath optical element has a radius of curvature of at least −38 mm.
11. A near-eye display device comprising:
a near-eye support structure;
a near-eye display (NED) supported by the near-eye support structure, the near-eye display including a waveguide;
a projection light engine, including an image source and a projection optical system, supported by the near-eye support structure and having an external exit pupil which is external to the projection light engine;
the projection optical system comprising a high index glass region in which a birdbath reflective optical element is immersed, the birdbath optical element collimates and reflects image light from the image source to the external exit pupil; and
the near-eye support structure positions the waveguide for optically coupling the image light of the external exit pupil.
12. The near-eye display device of claim 11 wherein the waveguide is a diffractive waveguide.
13. The near-eye display device of claim 11 wherein the waveguide is a surface relief grating waveguide.
14. The near-eye display device of claim 11 wherein the waveguide is multi-layered; and
wherein the near-eye support structure positions the waveguide for optically coupling the image light of the external exit pupil further comprises an input grating of the waveguide is positioned by the near-eye support structure to couple the image light of the external exit pupil within an inner layer of the multi-layered waveguide.
15. The near-eye display device of claim 14 wherein the inner layer is a central waveplate layer of the waveguide.
16. The near-eye display device of claim 11 wherein the near-eye support structure further comprises one or more housing structures for the projection light engine and the one or more housing structures arrange the image source and the projection optical system to fit within a twenty (20) mm height.
17. The near-eye display device of claim 11 wherein the image source comprises
a reflective microdisplay which generates image light by reflecting light from one or more light sources;
an illumination unit including the one or more light sources generating light; and
an illumination directing element which polarizes and directs light from the illumination unit to a reflective surface of the reflective microdisplay and passes through image light reflected from the reflective microdisplay; and
the illumination directing element is positioned within the high index glass region.
18. A method for coupling image light from an image source of a projection light engine to a near-eye display comprising:
optically directing image light from the image source along a first optical path in a high index glass region within a projection optical system housing to a birdbath reflective optical element in the high index glass region;
collimating and reflecting the image light by the birdbath reflective optical element along a second optical path extending through the high index glass region to an external exit pupil outside the projection optical system housing;
polarizing the image light in at least one of the first optical path and the second optical path; and
an input grating of a waveguide of the near-eye display optically coupling at least the image light of the exit pupil into the waveguide.
19. The method of claim 18 wherein polarizing the image light in at least one of the first optical path and the second optical path further comprises polarizing the image light in the second optical path by one or more polarizers including a quarter waveplate and a red rotation waveplate.
20. The method of claim 18 wherein optically directing image light from the image source along a first optical path in a high index glass region within a projection optical system housing to a birdbath reflective optical element in the high index glass region is performed by a beam splitter.