Optical: systems and elements – Mirror – Plural mirrors or reflecting surfaces
Reexamination Certificate
1998-12-01
2003-09-09
Cherry, Euncha (Department: 2872)
Optical: systems and elements
Mirror
Plural mirrors or reflecting surfaces
C359S633000, C359S861000, C359S727000, C359S834000
Reexamination Certificate
active
06616287
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical element to be used in a video camera, still video camera, copying machine, and the like and, more particularly, to an optical element having a plurality of reflection surfaces with curvatures.
Conventionally, as a photographing optical system including a reflection surface, for example, a so-called mirror lens system is known, as shown in FIG.
29
.
Referring to
FIG. 29
, object light
174
is converged and reflected toward the object side by a concave mirror
171
, and is imaged on an image plane
173
. This mirror lens system is based on the arrangement of a so-called Cassegrain reflecting telescope, and aims at a small total lens length by folding the optical path of a telescopic lens system with a large total lens length using two opposing reflection mirrors.
In the objective lens system of a telescope as well, many systems for shortening the total optical length using a plurality of reflection mirrors are known in addition to the Cassegrain type. That is, the optical path is efficiently folded by using a reflection mirror in a lens system with a large total lens length, thus obtaining a compact optical system.
However, in general, in the Cassegrain reflecting telescope, some object light rays are eclipsed by a concave mirror
172
.
This problem arises from the fact that a chief ray
176
of the object light
174
is located on an optical axis
175
. In order to solve this problem, many mirror optical systems which separate the chief ray
176
of the object light
174
from the optical axis
175
by using a reflection mirror at a decentered position have been proposed.
As the methods of separating the chief ray of object light from the optical axis, a method using a portion of a reflection mirror which is rotation-symmetric to the optical axis, as disclosed in, e.g., U.S. Pat. Nos. 3,674,334, 4,737,021, and the like, and a method of decentering the central axis itself of the reflection mirror from the optical axis, as disclosed in U.S. Pat. Nos. 4,265,510, 5,063,586, and the like, are available.
FIG. 30
shows an example of U.S. Pat. No. 3,674,334 as an example of the method of using a portion of a rotation-symmetric reflection mirror.
Referring to
FIG. 30
, a concave mirror
181
, convex mirror
182
, and concave mirror
183
are originally rotation-symmetric to an optical axis
184
, as indicated by the two-dashed chain lines. However, since the concave mirror
181
uses only its portion above the optical axis
184
, the convex mirror
182
uses only its portion below the optical axis
184
, and the concave mirror
183
uses only its portion below the optical axis
184
, the chief ray of object light
185
can be separated from the optical axis
184
, and the object light
185
can be output without being eclipsed.
FIG. 31
shows an example of U.S. Pat. No. 5,063,586 as an example of the method of decentering the central axis itself of the reflection mirror from the optical axis.
Referring to
FIG. 31
, when an axis perpendicular to an object plane
191
is defined as an optical axis
197
, the central coordinates and central axes of the surfaces of a convex mirror
192
, concave mirror
193
, convex mirror
194
, and concave mirror
195
are decentered from the optical axis
197
, and object light
198
can be efficiently imaged on an image plane
196
without being eclipsed by the reflection mirrors by appropriately setting the decentering amounts and the radii of curvature of the respective surfaces.
In this way, when the reflection mirrors that construct the mirror optical system are decentered, object light can be prevented from being eclipsed. However, since the individual reflection mirrors must be set with different decentering amounts, a structure for attaching the respective reflection mirrors is complicated, and it is very hard to assure high attachment precision.
As one method of solving this problem, for example, when a mirror system is formed as one block, assembly errors of optical parts upon assembly can be avoided. Conventionally, as optical systems having a large number of reflection surfaces as one block, for example, optical prisms such as a pentagonal roof prism, Porro prism, and the like, which are used in camera finder systems, a color-separation prism for separating a light beam coming from a photographing lens into three, red, green, and blue light beams, and imaging object images based on the respective color light beams on the corresponding imaging element surfaces, and the like are known.
The function of a pentagonal roof prism popularly used in a single-lens reflex camera as an example of the optical prism will be explained below with reference to FIG.
32
.
Referring to
FIG. 32
, reference numeral
201
denotes a photographing lens;
202
, a quick return mirror;
203
, a focal plane;
204
, a condenser lens;
205
, a pentagonal roof prism;
206
, an eyepiece;
207
, the pupil of the observer;
208
, an optical axis; and
209
, an image plane.
Light rays coming from an object (not shown) are transmitted through the photographing lens
201
, are reflected upward in the camera by the quick return mirror
202
, and are imaged on the focal plane
203
located at a position equivalent to the image plane
209
.
Behind the focal plane
203
, the condenser lens
204
for imaging the exit pupil of the photographing lens
201
on the pupil
207
of the observer is placed. Behind the condenser lens
204
, the pentagonal roof prism
205
for converting an object image on the focal plane
203
into an erected image is placed.
An object image defined by object light that enters the pentagonal roof prism
205
via an entrance surface
205
a
is horizontally inverted by a roof surface
205
b
. The object light is then reflected by a reflection surface
205
c
toward the observer.
The object light reflected toward the observer side is transmitted through an exit surface
205
d
of the pentagonal roof prism
205
, and reaches the eyepiece
206
, which converts the object light into nearly collimated light by its refractive power. The nearly collimated light beam then reaches the pupil
207
of the observer, and the observer can observe the object image.
As a major problem of such optical prisms represented by the pentagonal roof prism, harmful ghost light is likely to be produced due to irregular incoming light into the prism from positions and angles other than those of effective light rays.
In the pentagonal roof prism with the above-mentioned structure, ghost light that enters the prism at an angle different from that of effective light rays, as indicated by the arrow in
FIG. 32
, is reflected in turn by the roof surface
205
b
and reflection surface
205
c
, is totally reflected by the entrance surface
205
a
, and then leaves the prism from the lower portion of the exit surface
205
d
toward the observer.
If such ghost light is produced, since its number of times of reflection is different from that of normal effective light rays, a vertically inverted image appears on the lower side of the observation frame.
In order to remove the ghost light, a light-shielding groove
200
is formed on the exit surface
205
d
of the pentagonal room prism
205
.
By painting the entire prism surface except for the entrance surface
205
a
and exit surface
205
d
in black, a reflection film deposited on the roof surface
205
b
and reflection surface
205
c
is protected from environmental changes in, e.g., temperature, humidity, and the like, and light rays coming from outside the prism are intercepted. Since such optical prism has a plurality of reflection surfaces that are integrally formed, the respective reflection surfaces have a very accurate relative positional relationship, and do not require any positional adjustment.
Note that the principal function of such prism is to invert an image by changing the direction the light rays travel, and the individual reflection surfaces are defined by planes.
By contrast, optical prisms, the reflection surfaces of which have curvatures, are disclos
Kumagai Fumiaki
Sekita Makoto
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