Illumination device

1. An illumination device comprising:

a light output unit having an output end that outputs illumination light;

a diffusion layer that is disposed in a circumferential direction centered on a predetermined axis and that receives the illumination light from the output end, guides the illumination light while diffusing the illumination light, and outputs the illumination light from a surface thereof; and

a reflective layer that is provided adjacent to a surface of the diffusion layer at an inner side in a radial direction and that reflects the illumination light outward in the radial direction,

wherein the diffusion layer comprises a light guide material that guides the illumination light and a diffusion material dispersed and supported in the light guide material, and

the diffusion layer is a substantially cylindrical member having a cutout formed by removing, in a longitudinal direction, a portion extending in the circumferential direction.

2. The illumination device according to claim 1 , wherein the output end is circular, oval, polygonal, or annular-sector-shaped.

3. The illumination device according to claim 2 , wherein the diffusion layer is a substantially cylindrical member having a cutout formed by removing, in a longitudinal direction, a portion extending in the circumferential direction.

4. The illumination device according to claim 3 , wherein

the diffusion layer is divided into a plurality of diffusion layers arranged in the circumferential direction centered on the predetermined axis, and

each diffusion layer is substantially horseshoe-shaped.

5. The illumination device according to claim 2 , comprising a plurality of the diffusion layers arranged in the circumferential direction,

wherein each diffusion layer is a pillar member having a flat side surface at the inner side in the radial direction, and

the reflective layer is provided on the flat side surface.

6. The illumination device according to claim 2 , wherein the diffusion layer is ring-shaped.

7. The illumination device according to claim 2 , wherein the density of the diffusion material is non-uniform in the diffusion layer.

8. The illumination device according to claim 2 , wherein the diffusion layer has an outer circumferential surface and a distal-end surface having a conical shape that is smoothly continuous with the outer circumferential surface and that becomes gradually narrower toward a distal end.

9. The illumination device according to claim 1 , wherein the diffusion layer is ring-shaped.

10. The illumination device according to claim 1 , wherein

the diffusion layer is divided into a plurality of diffusion layers arranged in the circumferential direction centered on the predetermined axis, and

each diffusion layer is substantially horseshoe-shaped.

11. The illumination device according to claim 1 , wherein the density of the diffusion material is non-uniform in the diffusion layer.

12. The illumination device according to claim 1 , wherein the diffusion layer has an outer circumferential surface and a distal-end surface having a conical shape that is smoothly continuous with the outer circumferential surface and that becomes gradually narrower toward a distal end.

13. An illumination device comprising:

a light output unit having an output end that outputs illumination light;

a diffusion layer that is disposed in a circumferential direction centered on a predetermined axis and that receives the illumination light from the output end, guides the illumination light while diffusing the illumination light, and outputs the illumination light from a surface thereof;

a reflective layer that is provided adjacent to a surface of the diffusion layer at an inner side in a radial direction and that reflects the illumination light outward in the radial direction; and

a plurality of the diffusion layers arranged in the circumferential direction,

wherein the diffusion layer comprises a light guide material that guides the illumination light and a diffusion material dispersed and supported in the light guide material,

each diffusion layer is a pillar member having a flat side surface at the inner side in the radial direction, and

the reflective layer is provided on the flat side surface.

14. The illumination device according to claim 13 , wherein the output end is circular, oval, polygonal, or annular-sector-shaped.

15. The illumination device according to claim 14 , wherein the diffusion layer is a substantially cylindrical member having a cutout formed by removing, in a longitudinal direction, a portion extending in the circumferential direction.

16. The illumination device according to claim 15 , wherein

the diffusion layer is divided into a plurality of diffusion layers arranged in the circumferential direction centered on the predetermined axis, and

each diffusion layer is substantially horseshoe-shaped.

17. The illumination device according to claim 14 , comprising a plurality of the diffusion layers arranged in the circumferential direction,

wherein each diffusion layer is a pillar member having a flat side surface at the inner side in the radial direction, and

the reflective layer is provided on the flat side surface.

18. The illumination device according to claim 14 , wherein the diffusion layer is ring-shaped.

19. The illumination device according to claim 14 , wherein the density of the diffusion material is non-uniform in the diffusion layer.

20. The illumination device according to claim 14 , wherein the diffusion layer has an outer circumferential surface and a distal-end surface having a conical shape that is smoothly continuous with the outer circumferential surface and that becomes gradually narrower toward a distal end.

21. The illumination device according to claim 13 , wherein the diffusion layer is ring-shaped.

22. The illumination device according to claim 13 , wherein the density of the diffusion material is non-uniform in the diffusion layer.

23. The illumination device according to claim 13 , wherein the diffusion layer has an outer circumferential surface and a distal-end surface having a conical shape that is smoothly continuous with the outer circumferential surface and that becomes gradually narrower toward a distal end.