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An extremely compact prism optical element, image observation apparatus and image display apparatus which are capable of providing an observation image that is clear and has minimal aberration and minimal distortion even at a wide field angle. Light rays emitted from an image display device ( 7 ) en

An extremely compact prism optical element, image observation apparatus and image display apparatus which are capable of providing an observation image that is clear and has minimal aberration and minimal distortion even at a wide field angle. Light rays emitted from an image display device ( 7 ) enter an ocular optical system ( 12 ) through a fourth surface ( 6 ) and are totally reflected toward an observer's pupil ( 1 ) by a third surface ( 5 ). The reflected light rays are reflected by a first surface ( 3 ) disposed immediately in front of the observer's pupil ( 1 ) and then reflected toward the observer's pupil ( 1 ) by a second surface ( 4 ). The reflected light rays pass through the first surface ( 3 ) and are projected into an observer's eyeball ( 15 ) with the observer's iris position as an exit pupil ( 1 ). When an external-scene image is observed, light rays from an object point in the external scene enter the ocular optical system ( 12 ) through the third surface ( 5 ), pass through the first surface ( 3 ) and are projected into the observer's eyeball ( 15 ). Assuming that the angle of internal reflection of an arbitrary light ray at the third surface ( 5 ) is θ r3 , the ocular optical system ( 12 ) satisfies the condition of sin −1 (1/n d )≦θ r3 ≦60°, where n d is the refractive index for the spectral d-line of the medium of the ocular optical system ( 12 ).

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1. An image observation apparatus comprising image forming means and an ocular optical system having an action by which an image formed by said image forming means is led to an eyeball of an observer,wherein said ocular optical system includes at least a prism member,said prism member having at leas

1. An image observation apparatus comprising image forming means and an ocular optical system having an action by which an image formed by said image forming means is led to an eyeball of an observer,wherein said ocular optical system includes at least a prism member,said prism member having at least four optical surfaces having a transmitting or reflecting optical action, wherein a space surrounded by said at least four optical surfaces is filled with a single medium having a refractive index (n) larger than 1 (n>1),said at least four optical surfaces including a first surface having both a transmitting action and a reflecting action and disposed on a side of said prism member that is closer to said observer's eyeball; a second surface having a reflecting action and disposed to face said first surface across said medium, said second surface being at least decentered or tilted with respect to an observer's visual axis; a third surface having a reflecting action and disposed to face said first surface across said medium at a position substantially adjacent to said second surface; and a fourth surface disposed such that one end thereof is substantially adjacent to said first surface, and the other end thereof is substantially close to said third surface,wherein at least said third surface has a totally reflecting action, and said first surface, said single medium and said third surface are arranged to have an external-scene observation action by which an external scene can be observed through said first surface, said single medium and said third surface, andwherein the first surface is identical in surface shape with the third surface. 2. An image observation apparatus according to claim 1, wherein said image forming means is an image display device having an image forming surface disposed to face said fourth surface, and said second surface is formed from a curved surface. 3. A head-mounted image display apparatus comprising:the image observation apparatus of claim 2; anda retaining member that retains both said image display device and said ocular optical system in front of the observer's eyeball,wherein said prism member is arranged such that a bundle of light rays emitted from said image display device enters said prism member through said fourth surface, and the light rays are reflected successively by said third surface, said first surface and said second surface so as to exit from said first surface. 4. An image observation apparatus according to any one of claims 1 to 3, wherein said external-scene observation action is formed such that a composite power of said first and third surfaces at at least one region of each of them is approximately zero. 5. An image observation apparatus according to any one of claims 1 to 3, wherein said first surface and said third surface are formed from curved surfaces, respectively. 6. An image observation apparatus according to any one of claims 1 to 3, wherein said first surface and said third surface are formed from spherical surfaces, respectively. 7. An image observation apparatus according to any one of claims 1 to 3, wherein said first surface and said third surface are planar shaped surfaces. 8. An image observation apparatus according to any one of claims 1 to 3, which satisfies the following condition: t1 ≦0.5 (1/millimeter)  (3)where ø t1 is a composite power of said first and third surfaces at respective arbitrary regions thereof. 9. An image observation apparatus according to claim 1, wherein said prism member is fixed at a same position regardless of whether the observer views the image formed by said image forming means or an image of the external scene. 10. An image observation apparatus according to claim 9, wherein the image formed by said image forming means and the image of the external scene can be observed in respective partial regions through said first surface and said third surface. 11. An image observation apparatus according to a ny one of claims 1 to 3, wherein said prism member has switching means that causes observation modes to change between observation of the image formed by said image forming means and observation of an image of the external scene, said switching means having a function of moving said prism member. 12. An image observing apparatus according to any one of claim 11, wherein said switching means moves said prism member such that an optical path extending from said prism member to said observer's eyeball to observe the image formed by said image forming means is approximately coincident with an optical path extending from said prism member to said observer's eyeball to observe the image of the external scene. 13. An image observation apparatus according to claim 11, wherein said switching means causes said prism member to move along a plane containing an optical path of an axial principal ray. 14. An image observation apparatus according to claim 11, wherein said switching means causes said prism member to move in a direction perpendicular to the observer's visual axis. 15. An image observation apparatus according to claim 11, wherein said switching means causes said prism member to rotate. 16. An image observation apparatus according to claim 1, further comprising positioning means for positioning said image forming means and said ocular optical system with respect to an observer's head. 17. An image observation apparatus according to claim 1, further comprising support means for supporting at least a pair of said image observation apparatuses at a predetermined spacing. 18. An image observation apparatus comprising image forming means and an ocular optical system having an action by which an image formed by said image forming means is led to an eyeball of an observer,wherein said ocular optical system includes at least a prism member,said prism member having at least four optical surfaces having a transmitting or reflecting optical action, wherein a space surrounded by said at least four optical surfaces is filled with a single medium having a refractive index (n) larger than 1 (n>1),said at least four optical surfaces including a first surface having both a transmitting action and a reflecting action and disposed on a side of said prism member that is closer to said observer's eyeball; a second surface having a reflecting action and disposed to face said first surface across said medium, said second surface being at least decentered or tilted with respect to an observer's visual axis; a third surface having a reflecting action and disposed to face said first surface across said medium at a position substantially adjacent to said second surface; and a fourth surface disposed such that one end thereof is substantially adjacent to said first surface, and the other end thereof is substantially close to said third surface,wherein at least said third surface has a totally reflecting action, and said first surface, said single medium and said third surface are arranged to have an external-scene observation action by which an external scene can be observed through said first surface, said single medium and said third surface, andwherein the surface shape of the first surface, and the third surface is determined such that the external scene observed via the first surface and the third surface is the same as the external scene observed other than via the first surface and the third surfaceswherein the composite power of the power of said first surface plus the power of said third surface is substantially equal to zero.