Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2000-02-17
2002-01-08
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S631000, C359S633000, C359S637000
Reexamination Certificate
active
06337776
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to prism optical systems including a reflecting surface that is decentered and has a power, for example, a prism optical system for use in an image-forming optical system, a finder optical system, etc. used in cameras, video cameras and so forth.
Recently, there have been proposed optical systems designed to be compact in size by giving a power to a reflecting surface and folding an optical path in the direction of the optical axis. In such optical systems, a prism or a mirror is mainly used as a member having a reflecting surface with a power. An optical system having a prism and an optical system having a mirror are largely different in characteristics from each other although these optical systems are the same in terms of the structure using a reflecting surface.
When a curvature (radius r of curvature) is given to a reflecting surface of a prism and to a reflecting surface of a mirror, the power of each of the reflecting surfaces is given by the paraxial power calculating equation as follows. The power of the reflecting surface of the prism is −2n/r in a case where the prism is filled therein with a medium having a refractive index n larger than 1, whereas the power of the reflecting surface of the mirror is −2/r. Thus, even when these reflecting surfaces have the same curvature, the powers are different from each other. Accordingly, the curvature required for the prism is 1
of the curvature required for the mirror to obtain the same power. Therefore, the prism produces a smaller amount of aberration at the reflecting surface than in the case of the mirror. Thus, the prism is more favorable than the mirror in terms of performance. Moreover, the prism has two refracting surfaces, i.e. an entrance refracting surface and an exit refracting surface, in addition to a reflecting surface as a single member. Therefore, the prism is advantageous from the viewpoint of aberration correction in comparison to the mirror, which has only a reflecting surface as a single member. Furthermore, because the prism is filled with a medium having a refractive index larger than 1, it is possible to obtain a longer optical path length than in the case of the mirror, which is placed in the air. Accordingly, it is relatively easy with the prism to provide the required reflecting surface even when the focal length is short. In general, reflecting surfaces require a high degree of accuracy for assembly because decentration errors of reflecting surfaces cause the performance to be degraded to a considerable extent in comparison to refracting surfaces. In a case where an optical system is constructed by arranging a plurality of reflecting surfaces, the prism is more advantageous than the mirror because the prism enables a plurality of reflecting surfaces to be integrated into one unit so as to fix the relative positions and is therefore capable of preventing performance degradation due to assembling. Thus, the prism is superior to the mirror in many respects.
Meanwhile, when a surface with a power is placed at a tilt to the optical axis, rotationally asymmetric aberrations are produced. For example, if a rotationally asymmetric distortion occurs, a square object may become trapezoidal undesirably. Such rotationally asymmetric aberrations (hereinafter referred to as “decentration aberrations”) are impossible to correct by a rotationally symmetric surface in theory. For this reason, rotationally asymmetric curved surfaces, e.g. anamorphic surfaces, are used in conventional prism optical systems.
Such prism optical systems include the disclosure of Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as “JP(A)”] 8-313829. JP(A) 8-313829 discloses an ocular optical system comprising a prism in which there are two reflections, and a first transmitting surface and a second reflecting surface, as counted from the pupil side, are formed from the identical surface. In this optical system, all reflecting surfaces are rotationally asymmetric anamorphic surfaces.
Among the conventional prism optical systems using rotationally asymmetric curved surfaces, prism optical systems in which there are four reflections, in particular, are disclosed in JP(A) 8-292372, 9-73043, 9-197336 and 10-161018.
JP(A) 8-292372 discloses a zoom optical system in which a second reflecting surface and a first transmitting surface, as counted from the object side, are formed from the identical surface, and a third reflecting surface and a second transmitting surface, as counted from the object side, are formed from the identical surface. A first reflecting surface and a fourth reflecting surface are formed independently of the other transmitting surfaces and reflecting surfaces. The first and fourth reflecting surfaces are rotationally asymmetric surfaces, but the second and third reflecting surfaces are plane surfaces, which have no power. The zoom optical system is arranged to form an image once in the prism in order to relay the image.
Example 9 of JP(A) 9-73043 is an ocular optical system formed from a prism in which a first reflecting surface and a third reflecting surfaces, as counted from the pupil side, are formed from the identical surface with a second transmitting surface, and two other reflecting surfaces, i.e. second and fourth reflecting surfaces, are formed independently of the other transmitting surfaces and reflecting surfaces. The first, second and third reflecting surfaces are rotationally asymmetric anamorphic surfaces.
Example 3 of JP(A) 9-197336 is an ocular optical system formed from a prism in which a second reflecting surface and a fourth reflecting surface, as counted from the pupil side, are formed from the identical surface with a first transmitting surface, and a third reflecting surface is formed from the identical surface with a second transmitting surface. Only one reflecting surface, i.e. a first reflecting surface, is formed independently of the other transmitting surfaces and reflecting surfaces. All the reflecting surfaces are rotationally asymmetric anamorphic surfaces.
Example 21 of JP(A) 10-161018 is an optical system formed from a prism in which a second reflecting surface is formed from the identical surface with a first transmitting surface, and a third reflecting surface is formed from the identical surface with a second transmitting surface. Two other reflecting surfaces, i.e. first and fourth reflecting surfaces are formed independently of the other transmitting surfaces and reflecting surfaces. However, no numerical example is shown, and no detailed arrangement is mentioned.
These prior art prism optical systems suffer, however, from various problems as stated below.
In JP(A) 8-313829, because the prism optical system has only two reflecting surfaces, there is a limit in achieving high performance even if the prism reflecting surfaces are formed into rotationally asymmetric surfaces. Therefore, if the aperture becomes large or the field angle becomes large, the optical system may fail to fulfill the required high performance.
Accordingly, it is conceivable to increase the number of reflections so that aberration correction can be made satisfactorily even in the above case. However, high performance cannot always be attained in the prior art prism optical system even if the number of reflections is increased.
In JP(A) 8-292372, there are four reflections. However, there are only two reflecting surfaces having a power. The other two reflecting surfaces are formed from plane surfaces, which have no aberration correcting effects. Accordingly, JP(A) 8-292372 is not substantially different in performance from a prism in which there are two reflections. Moreover, because an image is formed once in the optical path, the powers of the surfaces need to be increased. This results in an increase in the amount of aberration produced. It is difficult to fulfill the required performance satisfactorily unless a large number of reflecting surfaces are used. In addition, because the image is re
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
Spyrou Cassandra
Treas Jared
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