Fresnel lens assembly having varying facet angle

1. An apparatus comprising:

a lens assembly comprising a lens body having a first surface defining Fresnel prisms; and

wherein a Fresnel prism of the Fresnel prisms has a first facet angle when viewed in a first cross-section and has a second facet angle when viewed in a second cross-section parallel to the first cross-section, the first facet angle different than the second facet angle.

2. The apparatus of claim 1 , the Fresnel prism has equal facet heights in the first and second cross-sections.

3. The apparatus of claim 1 , wherein Fresnel prisms adjacent the Fresnel prism have approximately equal facet heights as the Fresnel prism.

4. The apparatus of claim 1 , wherein pitches between adjacent pairs of Fresnel prisms including the Fresnel prism are approximately equal.

5. The apparatus of claim 1 , wherein the Fresnel prism extends in a loop.

6. The apparatus of claim 5 , wherein the Fresnel prism extends in a circle.

7. The apparatus of claim 5 , wherein the Fresnel prism extends in an ellipse.

8. The apparatus of claim 1 , wherein the lens body is curved in each orthogonal pair of cross-sections intersecting at a location within the lens body.

9. The apparatus of claim 1 , wherein the lens body has a first fitted radius of curvature in the first cross-section and a second fitted radius of curvature in the second cross-section, the first fitted radius smaller than the second fitted radius.

10. The apparatus of claim 9 , wherein a radius ratio of the first fitted radius to the second fitted radius is not greater than 0.1.

11. The apparatus of claim 10 , wherein the radius ratio is not greater than 0.05.

12. The apparatus of claim 1 , wherein the lens body is anamorphic.

13. The apparatus of claim 1 , wherein the lens body further comprises second Fresnel prisms defined by a second surface opposite the first surface.

14. The apparatus of claim 13 , wherein a second Fresnel prism of the second Fresnel prisms has a third facet angle when viewed in a third cross-section and has a fourth facet angle when viewed in a fourth cross-section parallel to the third cross-section, the third facet angle different than the fourth facet angle.

15. The apparatus of claim 1 , wherein the Fresnel prism has a linear variable facet angle.

16. The apparatus of claim 1 , further comprising:

a head mounted display device comprising:

a display panel; and

the lens assembly mounted so that an optical axis of the lens assembly intersects the display panel.

17. The apparatus of claim 16 , wherein the first surface defining the Fresnel prisms faces the display panel.

18. The apparatus of claim 16 , further comprising a second lens assembly wherein the second lens assembly has a structure that is a reflection of the lens assembly.

19. The apparatus of claim 16 , wherein the lens assembly further comprises a second lens body having a second surface defining Fresnel prisms.

20. The apparatus of claim 19 , wherein the lens body and the second lens body provide the optical axis, and wherein the Fresnel prisms of the lens body are elliptical Fresnel prisms having a first axis and the Fresnel prisms of the second lens body are elliptical Fresnel prisms having a second axis.