Observation optical system using volume hologram

Details Observation optical system using volume hologram Observation optical system using volume hologram

2001-03-16

2003-11-18

Juba, John (Department: 2872)

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

C359S016000, C359S643000, C359S630000, C359S631000

Reexamination Certificate

active

06650448

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an image observation optical system. To be specific, it relates to an image observation optical system used in an image display apparatus that can be held on the head or face of an observer and that can be attached to a cellular phone or a portable intelligent terminal.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a large image. Also, in accordance with recent popularization of cellular phone and portable intelligent terminal, requirements for large view of graphics or text data on these apparatuses have grown.
As conventional image observation optical systems used in image display apparatuses, there are those as disclosed in Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 7-333551 and Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 8-234137. In such an optical system, a prism whose surface having reflecting action has a rotationally asymmetric shape, such as anamorphic, toric or free curved shape, is employed, to compensate image distortion, curvature of field, and astigmatism.
However, if the image display apparatus is designed to achieve high image definition and wide field angle using such a prism, chromatic aberration of magnification by the prism itself becomes large, to cause the problem of imaging performance degradation.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an image observation optical system which is made so compact as to be applicable, as an image display apparatus, to a cellular phone or a portable intelligent terminal, and which can achieve high image definition and wide field angle while controlling chromatic aberration of magnification to be small.
An image observation optical system according to the present invention comprises an image display element and an eyepiece optical system which introduces, without forming an intermediate image, an image formed by the image display element into the center position of an eye of an observer so as to allow the observer to observe the image as a virtual image. The eyepiece optical system is configured to bend the optical axis using reflecting surfaces for compact-sizing of the optical system, with the optical axis lying in a plane, with respect to which the optical system is symmetrically formed, and to have an optical element that has an entrance surface, a plurality of reflecting surfaces and an exit surface, wherein at least one of the reflecting surfaces is provided with a volume hologram (HOE).
Here, the refractive index of the optical element is preferably greater than 1.7. It is because an element that exerts the same power with higher refractive index produces smaller amount of aberrations.
Also, according to the present invention, it is preferred that a prism with a positive refracting power and a HOE are provided and that, upon an image point F0 at the midpoint, and, of image points at the end points under a condition without the HOE, an image point Fb showing the larger chromatic aberration of magnification and an image point Fa showing the smaller chromatic aberration of magnification being defined on a segment that is an intersection formed by the plane of symmetry and an image display surface, the following conditions (1) and (2) are simultaneously satisfied:
−1<&phgr;
y
(
HOE, Fa
)/&phgr;
y
(Total)<2  (1)
−1<&phgr;
y
(
HOE, Fb
)/&phgr;
y
(Total)<1  (2)
where &phgr;y(HOE, Fa) is a y-direction power of the HOE at the image position Fa, &phgr;y(HOE, Fb) is a y-direction power of the HOE at the image position Fb, and &phgr;y(Total) is a y-direction power of the entire system.
Also, according to the present invention, it is preferred that, in a prism optical system having at least two reflecting surfaces, the HOE has one or two plane of symmetry of power, and the plane of symmetry of power coincides with a plane of symmetry of shape of a base on which the HOE is provided.
According to the present invention, an optical system whose optical axis is bent using reflecting surfaces is configured as an eyepiece optical system, to achieve compact-sizing.
A prism with a positive refracting power is used as this optical system.
As stated above, if high image definition and wide field angle is attempted using a prism, the problem of large chromatic aberration of magnification should occur. The present applicants have thought of making a chromatic aberration of magnification derived from a prism canceled by endowing a diffraction element an effect of a chromatic aberration of magnification with inverse tendency. First, the applicants examined a case where compensation of a chromatic aberration of magnification derived from a prism is made using a relief hologram (DOE) as the diffraction element.
A diffraction element is provided, as scarcely having a thickness, on a transparent base. As shown in
FIG. 33
, for example, its power distribution varies by position on the surface thereof. Since a chromatic aberration of magnification varies according to the variation of power of the diffraction element, if a diffraction element having a power distribution that causes a chromatic aberration of magnification having an inverse tendency to the chromatic aberration of magnification derived from the prism is provided on a predetermined surface of the prism, the chromatic aberration of magnification derived from the prism can be compensated without size increase of the optical system. Also, if the diffraction element is formed integral with the prism, disorder of alignment regarding the prism and the diffraction element does not occur.
Regarding the hologram element, there are two types; i.e. a relief hologram (DOE) and a volume hologram (HOE). The DOE has the property of low selectivity regarding incident angle and low selectivity regarding wavelength. Thus, the DOE diffracts rays with a particular wavelength incident thereon at a particular angle and images them as desired order rays, while diffracting, at a low diffraction efficiency, other rays with different wavelengths incident thereon at different angles and imaging them as undesired order rays. In contrast, the HOE has the property of high selectivity with respect to incident angle and high selectivity with respect to wavelength. Thus, the HOE exclusively diffracts rays with a particular wavelength incident thereon at a particular angle, while transmitting the remaining rays as 0
th
order rays so that undesired order rays should hardly be imaged.
If mass-productivity is considered, it is efficient to fabricate the DOE by lathing. In this case, the DOE is necessarily shaped rotationally symmetric. Therefore, it cannot compensate a chromatic aberration of magnification derived from a prism having a rotationally asymmetric surface such as a free curved surface as shown in FIG.
34
. Also, if the relief hologram is used, undesired order rays (0
th
order rays, 2
nd
order rays) are generated, in addition to the desired order rays, to degrade the image and obstruct observation.
Then, the present applicants examined a case where compensation of a chromatic aberration of magnification derived from a prism is made using a reflection-type HOE instead of the DOE. As shown in
FIG. 35A
, since the HOE can be constructed to have a rotationally asymmetric power distribution, it can be configured to exert an effect to cancel a chromatic aberration of magnification derived from a prism that has a free curved surface. Also, in general, if a HOE is applied to a reflecting surface, 0
th
order rays alone are generated as undesired order rays. Moreover, if the HOE is disposed on a surface that is directed opposite to the eye of the observer, as shown in
FIG. 1
, 0
th
order rays, which are generated only by a small amount, are effused in directions deviated from the eye of the observer, and thus do not obstruct

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