Optical: systems and elements – Compound lens system – With curved reflective imaging element
Reexamination Certificate
1999-07-19
2001-01-30
Sikder, Mohammad Y. (Department: 2872)
Optical: systems and elements
Compound lens system
With curved reflective imaging element
C359S365000, C359S366000, C359S726000, C359S267000
Reexamination Certificate
active
06181470
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical element, and more particularly, to an optical element suitable for use in an optical system of a video camera, a still video camera and a copying machine.
2. Description of the Related Art
Conventionally, various picture taking optical systems have been proposed in which reflecting surfaces such as concave mirrors and convex mirrors are utilized.
FIG. 1
schematically illustrates a main part of a so-called mirror optical system including one concave mirror and one convex mirror.
In the optical system of
FIG. 1
, light flux
134
from an object is reflected from concave mirror
131
, travels toward the object while being converged, is reflected from convex mirror
132
, and then forms an image on image surface
133
.
The mirror optical system is based on the construction of the so-called Cassegrainian reflecting telescope, and is intended to shorten the total length of the optical system by folding the optical path of a telephoto lens system composed of refracting lenses by the use of two opposite reflecting mirrors.
In addition, in an objective lens system constituting a telescope, a number of methods in addition to the Cassegrainian method have been known in which the total length of the optical system is shortened by the use of a plurality of reflecting mirrors, based on the principles described above.
Thus, a method for obtaining a compact optical system has been conventionally known in which the optical path is efficiently folded with use of reflecting mirrors in place of a lens optical system having a long total length.
However, in the mirror optical system of the Cassegrainian reflecting telescope and the like, there is a problem that the light from the object is partially eclipsed by the convex mirror
132
.
In order to solve the problem, a mirror optical system has been also proposed in which reflecting mirrors are decentered with respect to an optical axis in order to prevent the other parts of the optical system from shielding the passage region of the light flux
134
, that is to say, the principal ray of the light flux is separated from the optical axis
135
.
FIG. 2
schematically illustrates a main part of the mirror optical system disclosed in U.S. Pat. No. 3,674,334. In this mirror optical system, the center axis itself of the reflecting mirrors is decentered with respect to an optical axis to separate the principal ray of the light flux from the optical axis, thereby solving the problem of eclipse.
The mirror optical system of
FIG. 2
includes concave mirror
111
, convex mirror
113
and concave mirror
112
in the order of passage of the light flux. These mirrors are originally rotary symmetrical with respect to optical axis
114
, as shown by a chain double-dashed line. Only the upper portion of the concave mirror
111
with respect to optical axis
114
, only the lower portion of convex mirror
113
with respect to the optical axis, and only the lower portion of concave mirror
112
with respect to optical axis
114
are used to construct an optical system in which a principal ray of light flux from an object is separated from optical axis
114
, and the eclipse of light flux
115
is eliminated.
FIG. 3
schematically illustrates a main part of a mirror optical system disclosed in U.S. Pat. No. 5,063,586. In the mirror optical system of
FIG. 3
, a center axis of the reflecting mirrors itself is also decentered with respect to the optical axis to separate the principal ray of the light flux from the optical axis, thereby solving the problem of eclipse.
Referring to
FIG. 3
, when a vertical axis of object surface
121
is defined as optical axis
127
, center coordinates and center axes (axes connecting centers of reflecting surfaces and centers of curvatures thereof)
122
a
,
123
a
,
124
a
and
125
a
of the reflecting surfaces of convex mirror
122
, concave mirror
123
, convex mirror
124
and concave mirror
125
are decentered with respect to optical axis
127
. By suitably setting the amount of decentering and the radius of curvature of each surface, the eclipse of light flux
128
from an object due to reflecting mirrors is prevented, so that an object image is efficiently formed on an image surface
126
.
These reflecting-type picture taking optical systems contain many parts or components. Thus, in order to obtain a required optical performance, it is necessary that each of the optical parts are accurately assembled. More particularly, according to the picture taking optical system of a type in which reflecting mirrors are decentered with respect to the optical axis for the prevention of an eclipse of the light ray from the object, each of the reflecting mirrors must be disposed with different decentering amounts. As a result, structures for mounting reflecting mirrors thereto become complicated, and extremely precise mounting accuracy is required.
As one of the methods for solving the above problems, a method may be considered in which assembly error of the optical parts is avoided by combining mirror systems into one block.
Hitherto, as examples of such mirror systems, there have been optical prisms such as pentagonal roof prisms and Porro prisms which are used for viewfinder systems, and a color separation prism for separating the light flux from a picture taking lens is separated into three colors of red, green and blue to form object images based on each color of the light on the surface of each image pick-up device.
In these optical prisms, since a plurality of reflecting surfaces are integrated, the reflecting surfaces are placed accurately in relation to one another, so that positions of the reflecting surfaces need not be adjusted.
In these optical prisms, however, there is a problem in that a harmful ghost light associated with an irregular incident light incident from positions and angles other than those of an effective light ray is frequently generated.
A function of the pentagonal roof prism which is often used in a single-lens reflex camera as a typical example of the optical prisms will now be described with reference to FIG.
4
. Referring to
FIG. 4
, there are provided a picture taking lens
101
, a quick-return mirror
102
, a focal plane
103
, a condenser lens
104
, the pentagonal roof prism
105
, an eyepiece
106
, an observer's pupil
107
, an optical axis
108
and an image surface
109
.
A light flux from an object (not shown) is reflected upward of a camera from the quick-return mirror
102
after passing through the picture taking lens
101
so as to form an image on the focal plane
103
located at a position equivalent to the image surface
109
. A condenser lens
104
for forming an exit pupil of the picture taking lens
101
on the observer's pupil is disposed behind the focal plane
103
, and the pentagonal roof prism
105
for making an object image on the focal plane
103
into a correct image is disposed behind the condenser lens
104
.
The object light which has entered the incident surface
105
a
of the pentagonal roof prism
105
is subjected to a reversal of an object image from right to left, and then is emitted to the observer's side as the object light by the reflecting surface
105
c.
The object light emitted to the observer's side by the reflecting surface
105
c
passes through an emergent surface
105
d
of the pentagonal roof prism
105
, reaches the eyepiece
106
so as to be formed into a substantially parallel light by a refracting force of the eyepiece
106
, and then reaches the observer's pupil
107
so as to be observed.
In the pentagonal roof prism constructed as described above, a ghost light shown by an arrow in
FIG. 4
incident at an angle different from that of an effective light is reflected in the order of the roof surface
105
b
and reflecting surface
105
c
, is totally reflected from the incident surface
105
a
, and then is emitted from the lower portion of the emergent surface
105
d
to the observation side. Since the ghost light differs from the
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Sikd-er Mohammad Y.
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