Optical element and optical system having the same

Details Optical element and optical system having the same Optical element and optical system having the same

1999-09-24

2003-10-28

Shafer, Ricky D. (Department: 2872)

Optical: systems and elements

Mirror

Plural mirrors or reflecting surfaces

C359S859000, C359S861000, C359S727000, C359S729000, C359S731000

Reexamination Certificate

active

06637899

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical element and an optical system using the same and, more particularly, to an optical system for a video camera, still video camera viewfinder, or a copying machine using an optical element integrating a plurality of reflection surfaces with curvatures.
2. Related Background Art
Conventionally, a variety of optical systems using the reflection surfaces of concave mirrors or convex mirrors have been proposed.
For example, U.S. Pat. No. 4,775,217 or Japanese Patent Application Laid-Open No. 2-297516 discloses an optical prism whose optical block has a reflection surface with a curvature.
U.S. Pat. No. 4,775,217 is associated with the arrangement of the eyepiece of an observation optical system.
FIG. 11
shows this arrangement.
In the observation optical system shown in
FIG. 11
, display light
215
from an information display
211
is incident from an incident surface
218
, is reflected to the object side by a total reflection surface
212
, and reaches a concave surface
213
having a curvature.
The display light
215
that is output from the information display
211
as divergent light is converted into almost collimated light by the power of the concave surface
213
and enters a pupil
214
of the observer through the total reflection surface
212
, so the observer recognizes the image displayed on the information display
211
.
In this prior art, the object image can also be recognized simultaneously with observation of the displayed image.
Object light
216
is incident on a surface
217
nearly parallel to the total reflection surface
212
and reaches the concave surface
213
. For example, a semi-transparent film is deposited on the concave surface
213
. The object light
216
half-transmitted through the concave surface
213
passes through the total reflection surface
212
and enters the pupil
214
of the observer. Hence, the observer can observe the object light
216
and the display light
215
in a superposed state.
In a non-coaxial optical system, when asymmetrical aspherical surfaces are formed as constituent surfaces on the basis of the idea of a reference axis (to be described later), a compact observation optical system whose aberration is sufficiently corrected can be constructed. Japanese Patent Application Laid-Open No. 9-5650 discloses a method of designing the optical system. Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 disclose examples of design.
Such a non-coaxial optical system is called an off-axial optical system (An off-axial optical system is defined as an optical system including surfaces (off-axial optical surfaces) whose surface normals at the intersections between the surfaces and a reference axis are not present on the reference axis which is along a light beam passing through the image center and the pupil center. At this time, the reference axis bends).
In this off-axial optical system, generally, the constituent surfaces are non-coaxial, and the reflection surfaces do not generate an eclipse. For this reason, an optical system using reflection surfaces can be easily constructed. In addition, an integrated optical system can be easily constructed by integrally molding the constituent surfaces. With this method, the optical path can be relatively freely guided.
Hence, a compact reflection optical element with a high space efficiency and free shape can be formed.
However, in the integrally molded optical block of the off-axial optical system, when the number of reflection surfaces is increased for the purpose of, e.g., aberration correction of the optical block, influences of surface shape errors or surface distortion as manufacturing errors of the reflection surfaces accumulate. The error amount allowable in each reflection surface becomes smaller and stricter as the number of reflection surfaces increases. For this reason, the surface shape of each reflection surface must be accurately guaranteed.
An optical system with a small image size, which is disclosed in, e.g., Japanese Patent Application Laid-Open No. 8-292371 or 8-292372, has large curvatures, and the required accuracy against surface errors or surface distortion is high.
This also applies to the observation optical system disclosed in U.S. Pat. No. 4,775,217 or Japanese Patent Application Laid-Open No. 2-297516 when a compact high-performance optical system is constructed.
The characteristic features of the reflection optical elements disclosed in Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372 will be described next.
FIG. 12
shows an embodiment disclosed in Japanese Patent Application Laid-Open No. 8-292371. This optical system has an intermediate imaging plane N
1
and a pupil N
2
of the optical system. The intermediate imaging plane is formed near a second reflection surface R
4
counted from an incident surface R
2
along the optical path, i.e., one of reflection surfaces having curvatures.
The pupil is formed near a second reflection surface R
6
reversely counted from the exit surface along the optical path, i.e., one of the reflection surfaces having curvatures. If a first reflection surface R
3
having a curvature, which is counted from the incident surface R
2
along the optical path, has a convergence function, the intermediate imaging plane N
1
readily forms near the above-described reflection surface R
4
. If a final reflection surface R
7
having a curvature, which is counted from the incident surface along the optical path, has a convergence function, the pupil N
2
readily forms near the above-described reflection surface R
6
.
These surfaces are sensitive to distortion and spherical aberration, so the surface shapes must be accurately guaranteed.
To form an optical block having a plurality of reflection surfaces, molding using a mold is widely used because of the recent requirement for simplicity. When the mold is larger than the optical effective portion to some extent, the influence of the surface distortion near the reflection surfaces on the optical effective portion becomes small.
A large mold is also advantageous in guaranteeing the positional accuracy of each reflection surface. In a process using a synthetic resin, changes in dimensions due to shrinkage in the molding or the use environment must be taken into consideration because the thermal expansion coefficient of the synthetic resin is larger than that of an inorganic material by one order of magnitude. In association with the optical characteristics, not only the molding accuracy but also molding shrinkage and molecular orientation need to be taken into consideration.
Molding shrinkage influences the dimension accuracy of the entire molded body. Local shrinkage in cooling appears as residual distortion or deformation. Generally, when a molding material hardens in a mold, shrinking stress remains because the material cannot freely shrink. When a molded body formed from a soft material is released from a mold, such stress is released to warp the molded body. For a hard material such as polystyrene, polymethyl methacrylate, or polycarbonate, stress is not released, and a molded body maintains its shape with residual stress.
This stress is called internal stress. When the molded body comes into contact with, e.g., a solvent, a crack readily forms. The molded body may spontaneously break during use.
In consideration of this problem, Japanese Patent Application Laid-Open No. 8-122505 discloses an examination in which when a plurality of optical components are to be integrally formed as one optical member, contact surfaces at a joint portion are formed into appropriate shapes, and two surfaces adjacent to each other are smoothly joined at the boundary. This decreases residual stress on the optical member in the molding process to reduce manufacturing errors.
However, not all optical members can always be smoothly joined. When a design is made to smoothly join an optical member, the optical performance cannot be maintained.
FIG. 8
is a perspective view showing the surfa

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