Prism optical system, and image display apparatus and imaging apparatus incorporating the same

1. A prism optical system comprising:

a prism in which a space formed by at least two optical surfaces mutually decentered with respect to an axial chief ray of an incident light beam is filled up with a medium having a refractive index of greater than 1, wherein:

at least two surfaces out of said optical surfaces are rotationally asymmetric surfaces, five internal reflections take place inside said prism, and there is an intermediate image formed inside said prism, which image is in turn formed outside said prism, and

one surface out of a surface on which light is incident and a surface from which light exits is capable of at least one transmission and at least two internal reflections, and a transmitting area having said transmission and a reflecting area having said internal reflections at least partly overlap.

2. The prism optical system as recited in claim 1 , wherein at least one internal reflection off at least one surface of said entrance or exit surface is total reflection.

3. The prism optical system as recited in claim 1 , wherein a surface in opposition to said entrance or exit surface is defined by a single surface.

4. The prism optical system as recited in claim 1 , wherein light exiting out from an image display surface for displaying an image enters said prism optical system, and thereafter exits out from said prism optical system, forming an exit pupil to be projected onto the eyeball of an viewer, and such that in order along an optical path taken by light from incidence of said light on said prism optical system until said light exits out from said prism optical system, said light enters a first surface that is an entrance or transmitting surface, and is reflected at a second surface that is an internal reflection surface in opposition to said first surface, then again reflected at said first surface that is an internal reflection surface, then again reflected at said second surface that is an internal reflection surface, then again reflected at said first surface that is an internal reflection surface, and then again reflected at said second surface that is an internal reflection surface, exiting out from said prism optical system through said first surface.

5. The prism optical system as recited in claim 1 , wherein an angle α that an axial chief ray incident on said prism optical system forms with an axial chief ray exiting out from said prism optical system satisfies a condition: 0°≦α≦60°.

6. The prism optical system as recited in claim 1 , wherein the entrance surface and the exit surface are the same surface.

7. The prism optical system as recited in claim 1 , wherein the entrance surface is different from the exit surface.

8. The prism optical system as recited in claim 1 , wherein a surface in opposition to the surface on which light is incident or the surface from which light exits out is partially cut away.

9. An image display apparatus including the prism optical system as recited in claim 1 , wherein when said image display device is located in opposition to said first surface of said prism optical system and the eyeball of a viewer is located in opposition to a transmitting area of said first surface, an enlarged virtual image is presented to said viewer.

10. The imaging display apparatus as recited in claim 9 , wherein an exit pupil of said prism optical system is formed near an exit window of the first surface or between the first surface and the eyeball of the viewer.

11. An imaging apparatus including the prism optical system as recited in claim 1 , wherein an imaging device is positioned in opposition to said first surface, and an aperture stop is positioned in front of and near a transmitting area of said first surface to take images in the outside world.