Optical system and device using the same

Details Optical system and device using the same Optical system and device using the same

2002-07-24

2004-03-09

Dunn, Drew (Department: 2872)

Optical: systems and elements

Holographic system or element

Using a hologram as an optical element

C359S013000, C359S831000, C359S833000, C359S630000

Reexamination Certificate

active

06704128

ABSTRACT:

This application claims benefit of Japanese Patent application No. 2001-222961 filed in Japan Jul. 24, 2001, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to an optical system and devices using the same, and more particularly to an optical system such as a viewing or image pickup optical system that is used with an image display device or the like which may be mounted over the head or face of an observer or added to cellar phones or easy-to-carry information terminals.
For the purposes of allowing individuals to enjoy large-screen images, image display devices, especially head or face-mounted type image display devices are now under increasing development. There is also a growing demand for providing large-screen viewing of image-wise or character-wise data on cellar phones or portable information terminals.
For instance, U.S. Pat. No. 2,993,319 proposes a vehicle-mounted display device using a reflection hologram element having functions similar to those of a beam splitter capable of reflecting and diffracting only light having a specific range of angles of incidence and transmitting light having other angles of incidence. As proposed and shown in this patent publication, the angle selectivity of the reflection hologram element is used to achieve the beam splitter function of guiding light from a light source to an observer.
A viewing optical system comprising a combination of a reflection hologram element formed on a spherical substrate in the air with a reflection hologram element formed on a planar substrate in the air is proposed in U.S. Pat. No. 4,874,214. In this case, the reflection hologram formed on the planar substrate makes use of angle selectivity, thereby achieving the beam splitter function.
The aforesaid U.S. Pat. No. 2,993,319 refers only to means for achieving a hologram beam splitter using the angle selectivity of a monochromatic (a single band: wavelength band) corresponding to the green wavelength region. For instance, this patent publication does not pay any attention to the case where, for instance, a beam splitter harnessing the angle selectivity of a reflection hologram element is designed for light in several (e.g., red (R), green (G) and blue (B) or three-band light) bands.
That patent publication does not also say anything specific about the profile, etc. of optical powers for achieving the beam splitter function making use of the angle selectivity of a reflection hologram element.
The viewing optical system proposed in U.S. Pat. No. 4,874,214 comprises a hologram element having a spherical shape. It is here noted that a hologram element has two powers, i.e., optical power due to a geometrical shape and optical power due to its diffraction effect. For instance, two powers of a hologram element formed on a substrate member of spherical shape are now explained with reference to FIGS.
27
(
a
) and
27
(
b
). The hologram element has power due to a difference in density between interference fringes as represented by the pitch of a periodical structure within the hologram element as shown in FIG.
27
(
a
), and optical power due to its geometrical shape as shown in FIG.
27
(
b
). Here assume R is the radius of curvature of the hologram substrate. Then, the optical power &PHgr; of a conventional optical refracting lens, and a conventional reflector may be calculated from the following equations:
&PHgr;=(n−1)(1/R) for a refracting system
&PHgr;=2/R for a front surface mirror
&PHgr;=2n/R for a back-surface mirror
Here &PHgr; is the optical power due to the geometrical shape,
n is the refractive index of a medium, and
R is the radius of curvature of the hologram substrate.
It is thus understood that to obtain a certain quantity of optical power by the geometrical shape, the radius of curvature, R, of the back-surface mirror should be gentler than that of the front surface mirror by 1
.
To put it another way, if the interior of the reflection hologram element is filled up with a medium with a refractive index n, for instance, a glass or plastic medium as is the case with a back-surface mirror, it is then possible to obtain large optical power due to geometrical shape, even when the geometrical shape has a gentle radius of curvature, R.
Thus, if an arrangement ensuring to generate large optical power at such a gentle radius of curvature R is used for an optical system, it is then possible to reduce aberrations produced at this hologram surface.
In the viewing optical system of U.S. Pat. No. 4,874,214 wherein the spacing between the planar surface and the spherical surface is not filled up with a glass or plastic medium, however, the geometrical shape must be constructed with a smaller radius of curvature R so as to obtain the required quantity of optical power by the geometrical configuration having a spherical shape.
When the geometrical shape is constructed with a smaller radius of curvature R, however, it is difficult to display satisfactory images because of increases in the aberrations produced at this reflecting surface. For lack of any optical surface in an optical path between the image plane and the aforesaid curved surface, it is difficult to make satisfactory correction for distortion.
SUMMARY OF THE INVENTION
Having been accomplished with a view of solving such problems with the prior art as described above, the primary object of the present invention is to provide a viewing or image pickup optical system used with image display devices, which system can be used with high efficiency at a plurality of wavelengths, enables bright images to be observed with high color reproducibility, is easy to assemble, resist to impacts such as vibrations, light in weight and compact in size, and makes it possible to observe images well corrected for aberrations, and devices using the same.
According to the first aspect of the present invention, the aforesaid object is achieved by the provision of an optical system that is disposed between an image plane and an optical pupil and having generally positive power, wherein:
said optical system comprises a first prism having a refractive index of greater than 1, a second prism having a refracting index of greater than 1, and a volume hologram element disposed between said first prism and said second prism and cemented thereto, wherein:
said volume hologram element comprises a first volume hologram optimized in such a way as to effect Bragg diffraction at least at a first wavelength and a second volume hologram optimized in such a way as to effect Bragg diffraction at a second wavelength different from said first wavelength, and said volume hologram element is designed in such a way that diffraction efficiency thereof reaches a maximum at a first angle of incidence and a first angle of reflection and diffraction which vary with the position of said volume hologram element at least at said first wavelength and at a second angle of incidence and a second angle of reflection and diffraction which vary with the position of said volume hologram element at least at said second wavelength (it is here noted that the volume hologram element includes an element obtained by multi-exposing a single-layer volume hologram film to light including a plurality of wavelengths or a multilayer hologram element, and that the first and second wavelengths refer to two wavelengths in the RGB wavelength region or, for instance, two wavelengths R
1
and R
2
chosen out of the R wavelength region of the RGB wavelength region),
said first prism is located on said optical pupil side and said second prism is located on said image plane side,
said second prism has at least one reflecting surface formed at a surface thereof different from a surface thereof having said volume hologram element,
a light beam, which propagates from said optical pupil to said image plane in a forward or backward direction and includes at least a ray component of said first wavelength and at least a ray component of said second wavelength, passes through said volume

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