Optical: systems and elements – Compound lens system – With curved reflective imaging element
Reexamination Certificate
1997-02-14
2003-04-15
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
With curved reflective imaging element
C359S407000, C359S630000, C359S728000
Reexamination Certificate
active
06549332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a reflecting optical system and an imaging apparatus using it and, more particularly, to a reflecting optical system suitably applicable to imaging optical systems, observing optical systems, and so on in video cameras and still video cameras for forming an object image on a predetermined surface, using an optical element having a plurality of reflecting surfaces.
2. Related Background Art
A variety of imaging or observing optical systems using a refracting optical system have been proposed heretofore. These optical systems are well corrected for spherical aberration, coma, curvature of field, and so on at a reference wavelength in the visible wavelength region and also corrected similarly for various aberrations at wavelengths other than the reference wavelength. In particular, since a refracting system has so-called dispersion that refractive indices of a material such as glass differ depending upon wavelengths, imaging performance is improved by correction for chromatic aberration occurring because of the dispersion characteristics (which is so called achromatization).
For example, in the case of an optical system using a refracting lens, it is theoretically impossible to effect achromatization as well as the imaging action with a single lens. Therefore, correction for chromatic aberration is done by a combination of plural lenses different in index and dispersion.
On the other hand, there have been a variety of proposals on photographing optical systems using reflecting surfaces such as a concave mirror or a convex mirror. Since the reflecting surfaces theoretically cause no chromatic aberration, such photographing optical systems are often applied to telescopes the imaging performance of which is very susceptible to chromatic aberration.
FIG. 17
is a schematic view of a so-called mirror optical system comprised of a concave mirror and a convex mirror. In the mirror optical system of the same drawing, an object beam
104
from an object is reflected by the concave mirror
101
then travels toward the object, is converged and then is reflected by the convex mirror
102
to form an image on the image plane
103
thereafter. Reference numeral
105
is a reference axis.
This mirror optical system has the basic configuration of the so-called Cassegrainian reflecting telescope, which reduces the total length of optical system by folding an optical path of a telephoto lens system with a long total lens length comprised of refracting lenses by two opposite reflecting mirrors and which avoids chromatic aberration specific to the telephoto lens by using the mirror optical system.
In this way, in the photographing lenses with longer total lens lengths, the reflecting mirrors are conventionally used instead of the lenses to fold the optical path efficiently, thereby obtaining the mirror optical system compact and free of chromatic aberration. In a so-called catoptric optical system using only the reflecting system, it is, however, difficult to correct all aberrations occurring at the reflecting mirrors by the limited number of surfaces or in a limited space.
There are thus examples of advantageously combining the reflecting system with the refracting system to increase degrees of freedom and thereby to correct aberrations as a total system.
FIG. 18
shows an example of a so-called catadioptric system as a combination of the reflecting system with the refracting system. In
FIG. 18
, an object beam
116
from an object is subject to refraction in refracting lenses
111
,
112
, thereafter is reflected by a concave mirror
113
, then travels toward the object as being converged, thereafter is reflected by a convex mirror
114
, and then forms an image on the image plane
115
. The refracting lens system is constructed so as to correct aberration occurring at the reflecting mirrors.
However, the refracting system is the combination of the convex lens
111
with the concave lens
112
in order to suppress chromatic aberration. Although the optical path is folded efficiently by only the reflecting system to achieve the compact arrangement, it has a disadvantage of an increase of size because it requires refracting lenses with a large aperture in fact.
In addition, because of an increase in the number of components, it was necessary to assemble the respective optical components with high accuracy in order to attain necessary optional performance. In particular, because high accuracy is required for the relative position between the reflecting mirrors or for the relative position between the reflecting mirrors and the refracting lenses, adjustment of position and angle of each reflecting mirror was essential.
Proposed as a method for solving this problem is a method for forming the mirror system in a block, thereby avoiding assembling errors of the optical components caused upon assembling, for example.
Conventional examples of such elements incorporating many reflecting surfaces in a block include optical prisms such as a pentagonal roof prism or a Porro prism used in a finder system or the like, for example.
Since these prisms include a plurality of reflecting surfaces integrally formed, they are formed with high accuracy for the relation of relative position among the reflecting surfaces, which obviates a need for positional adjustment between the reflecting surfaces. In many cases, however, the principal function of these prisms is to change the traveling direction of light rays so as to invert the image and the reflecting surfaces are often flat.
In contrast with the foregoing, there are known optical systems with the reflecting surfaces of prism having curvature.
FIG. 19
is a schematic drawing of the major part of the observing optical system disclosed in the specification of U.S. Pat. No. 4,775,217. This observing optical system is an optical system for observing a view in the external field and for observing a display image displayed on an information display as overlapping the view.
In this observing optical system, a display beam
125
emitted from the display image on the information display
121
is reflected by a surface
122
to travel toward the object and then to enter a halfmirror surface
123
being a concave surface. After being reflected by this halfmirror surface
123
, the display beam
125
is changed to a nearly parallel beam by refracting power of the concave surface
123
, then is refracted and transmitted by the surface
122
, and forms an enlarged, virtual image of the display image as entering the pupil
124
of an observer. Thus the observer can visually recognize the display image.
On the other hand, the object beam
126
from an object is incident to a surface
127
nearly parallel to the reflecting surface
122
to be refracted and then to reach the halfmirror surface
123
of concave surface. A semi-transparent film is evaporated over the concave surface
123
. Thus, part of the object beam
126
passes through the concave surface
123
, then is refracted and transmitted by the surface
122
, and thereafter enters the observer's pupil
124
. By this, the observer visually recognizes the display image overlapping the view of the external field.
FIG. 20
is a schematic drawing of the major part of the observing optical system disclosed in Japanese Patent Application Laid-open No. 2-297516. This observing optical system is also an optical system for observing the view in the external field and for observing the display image displayed on the information display as overlapping the external view.
In this observing optical system, the display beam
134
emitted from the information display
130
passes through a flat surface
137
forming the prism Pa to enter the prism Pa and then to be incident to a parabolic reflecting surface
131
. The display beam
134
is reflected by this reflecting surface
131
to become a converging beam to form an image on the focal plane
136
. The display beam
134
reflected by the reflecting surface
131
at this time is totally reflecte
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