Femtoprojector optical systems
A variety of femtoprojector optical systems are described. Each of them can be made small enough to fit in a contact lens using plastic injection molding, diamond turning, photolithography and etching, or other techniques. Most, but not all, of the systems include a solid cylindrical transparent substrate with a curved primary mirror formed on one end and a secondary mirror formed on the other end. Any of the designs may use light blocking, light-redirecting, absorbing coatings or other types of baffle structures as needed to reduce stray light.
1. A femtoprojector optical system for projecting an image produced by an image source to a retina of a user's eye, the femtoprojector optical system comprising:
a convex secondary mirror facing the image source, where image-forming rays from the image source are incident on and reflected by the secondary mirror;
a concave primary mirror facing the secondary mirror and further reflecting the image-forming rays from the secondary mirror, the secondary mirror and primary mirror cooperating to project the image from the image source onto the user's retina; and
a light baffle system that includes an obscuration, the obscuration blocking stray rays from the image source that are not reflected by the secondary mirror, where a ratio of (a) power in the stray rays that exit the femtoprojector optical system but do not form the projected image on the user's retina; to (b) power in the image-forming rays that exit the femtoprojector optical system and do form the projected image on the user's retina, does not exceed 1:2;
where the femtoprojector optical system is small enough to mount in a contact lens wearable by a human user;
where the femtoprojector optical system has a height h as measured from the image source to the secondary mirror of less than 2 mm and an aspect ratio of (height h):(width w) of not more than 2:1 where the width w is a width of the primary mirror.
2. The femtoprojector optical system of claim 1 where the light baffle system prevents any stray rays from being projected to an area that overlaps with the projected image.
3. The femtoprojector optical system of claim 1 where the obscuration blocks all axially-propagating rays from the image source that are not reflected by the secondary mirror.
4. The femtoprojector optical system of claim 1 where the obscuration is axially aligned with the secondary mirror.
5. The femtoprojector optical system of claim 4 where the obscuration is annular in shape, and an inner edge of the annular obscuration is adjacent to an outer edge of the secondary mirror.
6. The femtoprojector optical system of claim 4 where the obscuration is annular in shape, and a maximum lateral dimension a of the annular obscuration complies with
a≥b +FOV( h/n fp ),
where n fp is the index of refraction of material into which the image source emits light, h is a height of the femtoprojector optical system, b is a maximum lateral dimension of the image source, and FOV is a field of view occupied by the image produced by the image source.
7. The femtoprojector optical system of claim 1 where the obscuration is axially offset from the secondary mirror.
8. The femtoprojector optical system of claim 1 where the obscuration is annular in shape.
9. The femtoprojector optical system of claim 1 where the obscuration is segmented.
10. The femtoprojector optical system of claim 1 where the obscuration is sloped away from the image source.
11. The femtoprojector optical system of claim 1 where the obscuration reflects incident stray rays back to the image source.
12. The femtoprojector optical system of claim 1 where the light baffle system further includes a sidewall that is external to the primary mirror and blocks stray rays emitted from the image source at oblique angles.
13. The femtoprojector optical system of claim 12 where the obscuration is sloped to reflect stray rays towards an adjacent sidewall.
14. The femtoprojector optical system of claim 12 where the obscuration is sloped to reflect stray rays towards an opposite sidewall.
15. The femtoprojector optical system of claim 1 where a maximum lateral dimension of the secondary mirror is smaller than a maximum lateral dimension of the image source.
16. The femtoprojector optical system of claim 15 where a maximum lateral dimension of the obscuration is larger than the maximum lateral dimension of the image source.
17. The femtoprojector optical system of claim 1 where the femtoprojector optical system has a total length of not more than 1.1 mm.
18. The femtoprojector optical system of claim 1 where the femtoprojector optical system has an exit aperture with a maximum lateral dimension of not more than 1 mm.
19. The femtoprojector optical system of claim 1 where the aspect ratio of (height h):(width w) is not more than 1:1.