Optical: systems and elements – Prism – With reflecting surface
Reexamination Certificate
2004-05-17
2004-12-07
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Prism
With reflecting surface
C359S631000, C359S633000, C359S637000, C362S299000, C362S327000, C362S331000, C396S384000, C396S386000, C353S038000, C353S081000, C348S333090, C348S333100, C348S341000
Reexamination Certificate
active
06829113
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to image-forming optical systems. More particularly, the present invention relates to a decentered optical system with a reflecting surface having a power for use in optical apparatus using a small-sized image pickup device, e.g. video cameras, digital still cameras, film scanners, and endoscopes.
Recently, with the achievement of small-sized image pickup devices, image-forming optical systems for use in video cameras, digital still cameras, film scanners, endoscopes, etc. have also been demanded to be reduced in size and weight and also in cost.
In the general rotationally symmetric coaxial optical systems, however, optical elements are arranged in the direction of the optical axis. Therefore, there is a limit to the reduction in thickness of the optical systems. At the same time, the number of lens elements unavoidably increases because it is necessary to correct chromatic aberration produced by a rotationally symmetric refracting lens used in the optical systems. Therefore, it is difficult to reduce the cost in the present state of the art. Under these circumstances, there have recently been proposed optical systems designed to be compact in size by giving a power to a reflecting surface, which produces no chromatic aberration, and folding an optical path in the optical axis direction.
Japanese Patent Application Unexamined Publication (KOKAI) Number [hereinafter referred to as “JP(A)”] 7-333505 proposes to reduce the thickness of an optical system by giving a power to a decentered reflecting surface and thus folding an optical path. In an example thereof, however, the number of constituent optical members is as large as five, and actual optical performance is unclear. No mention is made of the configuration of the reflecting surface.
JP(A) 8-292371, 9-5650 and 9-90229 each disclose an optical system in which an optical path is folded by a single prism or a plurality of mirrors integrated into a single block, and an image is relayed in the optical system to form a final image. In these conventional examples, however, the number of reflections increases because the image is relayed. Accordingly, surface accuracy errors and decentration accuracy errors are transferred while being added up. Consequently, the accuracy required for each surface becomes tight, causing the cost to increase unfavorably. The relay of the image also causes the overall volumetric capacity of the optical system to increase unfavorably.
JP(A) 9-222563 discloses an example of an optical system that uses a plurality of prisms. However, because the optical system is arranged to relay an image, the cost increases and the optical system becomes large in size unfavorably for the same reasons as stated above.
JP(A) 9-211331 discloses an example of an optical system in which an optical path is folded by using a single prism to achieve a reduction in size of the optical system. However, the optical system is not satisfactorily corrected for aberrations.
JP(A) 8-292368, 8-292372, 9-222561, 9-258105 and 9-258106 all disclose examples of zoom lens systems. In these examples, however, the number of reflections is undesirably large because an image is relayed in a prism. Therefore, surface accuracy errors and decentration accuracy errors of reflecting surfaces are transferred while being added up, unfavorably. At the same time, the overall size of the optical system unavoidably increases, unfavorably.
JP(A) 10-20196 discloses an example of a two-unit zoom lens system having a positive front unit and a negative rear unit, in which the positive front unit comprises a prism of negative power placed on the object side of a stop and a prism of positive power placed on the image side of the stop. JP(A) 10-20196 also discloses an example in which the positive front unit, which comprises a prism of negative power and a prism of positive power, is divided into two to form a three-unit zoom lens system having a negative unit, a positive unit and a negative unit. However, the prisms used in these examples each have two transmitting surfaces and two reflecting surfaces, which are all independent surfaces. Therefore, a relatively wide space must be ensured for the prisms. In addition, the image plane is large in size in conformity to the Leica size film format. Accordingly, the prisms themselves become unavoidably large in size. Furthermore, because the disclosed zoom lens systems are not telecentric on the image side, it is difficult to apply them to image pickup devices such as CCDs. In either of the examples of zoom lens systems, zooming is performed by moving the prisms. Accordingly, the decentration accuracy required for the reflecting surfaces becomes tight in order to maintain the required performance over the entire zooming range, resulting in an increase in the cost.
When a general refracting optical system is used to obtain a desired refracting power, chromatic aberration occurs at an interface surface thereof according to chromatic dispersion characteristics of an optical element. To correct the chromatic aberration and also correct other ray aberrations, the refracting optical system needs a large number of constituent elements, causing the cost to increase. In addition, because the optical path extends straight along the optical axis, the entire optical system undesirably lengthens in the direction of the optical axis, resulting in an unfavorably large-sized image pickup apparatus.
In decentered optical systems such as those described above in regard to the prior art, an imaged figure or the like is undesirably distorted and the correct shape cannot be reproduced unless the formed image is favorably corrected for aberrations, particularly rotationally asymmetric distortion.
Furthermore, in a case where a reflecting surface is used in a decentered optical system, the sensitivity to decentration errors of the reflecting surface is twice as high as that in the case of a refracting surface, and as the number of reflections increases, decentration errors that are transferred while being added up increase correspondingly. Consequently, manufacturing accuracy and assembly accuracy, e.g. surface accuracy and decentration accuracy, required for reflecting surfaces become even more strict.
SUMMARY OF THE INVENTION
In view of the above-described problems with the prior art, an object of the present invention is to provide a high-performance and low-cost image-forming optical system having a reduced number of constituent optical elements.
Another object of the present invention is to provide a high-performance image-forming optical system that is made compact and thin by folding an optical path using reflecting surfaces arranged to minimize the number of reflections.
The image-forming optical system according to the present invention provided to attain the above-described objects is an image-forming optical system having a positive refracting power as a whole for forming an object image. The image-forming optical system has a prism member formed from a medium having a refractive index (n) larger than 1 (n>1). The prism member has a first entrance surface through which a light beam from an object enters the prism member. The prism member further has a first reflecting surface, a second reflecting surface, a third reflecting surface and a fourth reflecting surface, which reflect the light beam in the prism member. Further, the prism member has a first exit surface through which the light beam exits from the prism member. An optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the first entrance surface and the first reflecting surface, and the optical path connecting the second reflecting surface and the third reflecting surface intersects an optical path connecting the fourth reflecting surface and the first exit surface. At least either one of the first reflecting surface and the second reflecting surface has a curved surface configuration that gives a power to a light beam. The curved s
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