Optical device

1. An optical device comprising:

a plate-shaped Fresnel lens portion made of a first optical material, the Fresnel lens portion comprising a Fresnel-shaped portion on one side of Fresnel lens portion in a thickness direction of the Fresnel lens portion, the Fresnel-shaped portion having a plurality of grooves, and each of the grooves has a reflecting surface; and

a partially reflective mirror layer provided on a surface of the Fresnel-shaped portion,

wherein the plurality of grooves of the Fresnel-shaped portion is configured such that an angle of the reflecting surface varies continuously along each of the grooves depending on a position on a circumference along the groove, so that a surface configuration of the Fresnel-shaped portion has optically freeform surface characteristics.

2. The optical device according to claim 1 , wherein the Fresnel lens portion is configured to reflect display light emitted from a display unit that enters the Fresnel lens portion from the Fresnel-shaped portion on the one side such that the light is projected toward an eye point, and to transmit, in the thickness direction, external light entering the Fresnel lens portion from another side opposite the Fresnel-shaped portion on the one side.

3. The optical device according to claim 1 , wherein each of the grooves has a non-uniform depth.

4. The optical device according to claim 1 , wherein each of the grooves of the Fresnel-shaped portion is configured have a ring shape such that at least a distance from a central portion of the Fresnel-shaped portion to the circumference along the groove varies continuously depending on the position on the circumference.

5. The optical device according to claim 4 , wherein each of the grooves of the Fresnel-shaped portion have an elliptical shape.

6. The optical device according to claim 1 , further comprising:

a second optical material as a transparent filler that fills projections and recesses of a surface of the partially reflective mirror layer to form a flat surface; and

a third optical material that protects an outer surface of the second optical material,

wherein a refractive index of the first optical material, a refractive index of the second optical material, and a refractive index of the third optical material are substantially the same.

7. The optical device according to claim 1 , wherein a configuration of the plurality of grooves of the Fresnel-shaped portion is designed based on parameters including three-dimensional coordinates corresponding to respective locations on the Fresnel lens portion, a normal vector, and a Fresnel angle.

8. The optical device according to claim 1 , wherein the surface of the Fresnel-shaped portion has a concave central portion surrounded by the plurality of grooves.

9. An optical device configured to transmit light to an eye point of a user, the optical device comprising:

a Fresnel lens portion made of a first optical material, the Fresnel lens portion having a Fresnel-shaped portion on one side thereof in a thickness direction, the Fresnel-shaped portion having a surface with a central portion and a plurality of non-uniform depth grooves so to have optically freeform surface characteristics, a central axis being defined by the central portion in the thickness direction of the Fresnel-shaped portion; and

a partially reflective mirror layer provided on the surface of the Fresnel-shaped portion,

wherein each of the plurality of grooves has a reflecting surface facing inward at an angle toward the central axis, and

wherein each of the grooves of the Fresnel-shaped portion is configured have a ring shape such that at least a distance from the central portion of the Fresnel-shaped portion to a circumference along the groove varies continuously depending on a position on the circumference.

10. The optical device according to claim 9 , further comprising:

a display unit configured to emit display light toward the Fresnel-shaped portion on the one side of the Fresnel lens portion,

wherein the one side of the Fresnel lens portion is configured to thereby redirect the display light to project toward the eye point of the user and external light enters the Fresnel lens portion from another side thereof opposite the Fresnel-shaped portion on the one side.

11. The optical device according to claim 9 , wherein the central portion of the surface of the Fresnel-shaped portion is concave.