Image pickup optical system

Details Image pickup optical system Image pickup optical system

2000-09-29

2002-10-15

Sikder, Mohammad (Department: 2872)

Optical: systems and elements

Prism

C359S837000, C359S883000, C359S834000

Reexamination Certificate

active

06466383

ABSTRACT:

This application claims benefit of Japanese Application No. Hei 11-281032 filed in Japan on Oct. 1, 1999, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates to an image pickup optical system and, more particularly, to an image pickup optical system including a reflective decentered image pickup optical element and a diffractive optical element.
Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 10-333040 is known as a conventional compact reflective decentered image pickup optical system capable of two-dimensional imaging. The known optical system has at least a rear optical unit on the image side of a pupil plane. When a light ray emanating from the center of an object and passing through the pupil center to reach the image center is defined as an axial principal ray, the rear optical unit has an optical system with at least three surfaces each decentered so that the whole surface is tilted with respect to the principal ray. The surfaces include a rotationally symmetric curved surface that has both reflecting and transmitting actions and a rotationally asymmetric curved surface that has a reflecting action and corrects rotationally asymmetric decentration aberrations caused by decentration.
Chromatic aberration produced in the conventional reflective decentered image pickup optical system cannot satisfactorily be corrected by the optical system alone. Consequently, the chromatic aberration causes the captured image to be degraded in image quality.
Conventionally, a coaxial optical system is corrected for chromatic aberration by combining a negative lens with a positive lens. With respect to a refractive positive lens, a negative lens can correct both chromatic aberration and curvature of field. With respect to a reflective positive lens, however, a negative lens tends to cause curvature of field to become unfavorably large, although it can correct chromatic aberration. The reason for this is that a reflective positive lens has positive curvature of field, whereas a refractive positive lens has negative curvature of field.
Meanwhile, a conventional reflective decentered ocular optical system has a relatively long focal length. Therefore, if an image is formed on a small-sized image pickup device, the image taking range becomes unfavorably narrow.
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 compact image pickup optical system satisfactorily corrected for chromatic aberration as well as decentration aberrations and capable of providing a clear image with minimal distortion even at a wide field angle.
To attain the above-described object, the present invention provides an image pickup optical system including an image pickup optical element and a diffractive optical element, which are decentered with respect to each other. The image pickup optical element has at least three optical surfaces adjacent to each other. At least one of the three optical surfaces is formed from a curved surface. At least two reflections take place between the optical surfaces.
The reasons for adopting the above-described arrangement in the present invention, together with the function thereof, will be described below.
The image pickup optical system according to the present invention is characterized by including a reflective decentered image pickup optical element and a diffractive optical element placed on the object or image side of the image pickup optical element.
A diffractive optical element has very strong negative dispersion (Abbe's number: −3.45) and is therefore capable of correcting chromatic aberration produced by a positive lens. Moreover, because the Petzval sum is zero, the diffractive optical element has no effect on curvature of field. Therefore, only chromatic aberration can be further corrected without increasing curvature of field by combining a diffractive optical element with the reflective decentered image pickup optical element, which is a reflective positive lens.
Accordingly, by incorporating a diffractive optical element into a reflective decentered image pickup optical system with a wide image taking range, it is possible to correct chromatic aberration without increasing curvature of field and to provide a clear image with minimal distortion even at a wide field angle.
It is preferable to satisfy the following condition:
−1
<F<
1  (1)
wherein F is the value of (the focal length of the image pickup optical system) divided by (the focal length of the diffractive optical element).
It should be noted that the focal length of a decentered optical system is defined as follows. As shown in
FIG. 9
, when the direction of decentration of the optical system is taken in the Y-axis direction, a light ray which is parallel to an axial principal ray
2
and which has a small height d in the YZ-plane is made to enter the optical system from the object side thereof. The sine of the angle that is formed between the two rays exiting from the optical system in the YZ-plane is denoted by NA′yi, and NA′yi/d is defined as the power Py in the Y-axis direction of the entire optical system. Similarly, a light ray which is parallel to the axial principal ray
2
and which has a small height d in the XZ-plane is made to enter the optical system from the object side thereof. The sine of the angle that is formed between the two rays exiting from the optical system in a plane perpendicularly intersecting the YZ-plane and containing the exiting axial principal ray is denoted by NA′xi, and NA′xi/d is defined as the power Px in the X-axis direction of the entire optical system. Furthermore, the reciprocals of the powers Px and Py are defined as the focal lengths Fx and Fy in the X- and Y-axis directions, respectively, of the entire optical system. In the present invention, the term (the focal length of the image pickup optical system) includes both the focal lengths Fx and Fy. In Examples (described later), however, the term (the focal length of the image pickup optical system) means the focal length Fy.
The condition (1) determines the aberration correction balance between the entire optical system and the diffractive optical element. If F is not smaller than the upper limit of the condition (1), i.e. 1, the amount of aberration corrected by the diffractive optical element becomes excessive. If F is not :larger than the lower limit, i.e. −1, the amount of aberration corrected by the diffractive optical element becomes deficient. In either case, the balance of aberration corrected by the diffractive optical element with respect to chromatic aberration produced in the optical system is destroyed, and it becomes impossible to attain favorable aberration correction.
It is even more desirable to satisfy the following condition:
 −0.1
<F<
0.1  (1-1)
It is still more desirable to satisfy the following condition:
0
<F<
0.1  (1-2)
It is practically preferable that the image pickup optical element in the image pickup optical system according to the present invention should be formed from a prism member in which the space defined by the at least three surfaces is filled with a :transparent medium having a refractive index larger than 1.
Preferably, at least one curved surface of the image pickup optical element is a rotationally asymmetric surface with no axis of rotational symmetry in the surface nor out of the surface. The rotationally asymmetric surface has a totally reflecting action or a reflecting action. When a light ray emanating from the center of an object and passing through the pupil center to reach the image center is defined as an axial principal ray, the rotationally asymmetric surface is tilted with respect to the axial principal ray. The rotationally asymmetric surface corrects rotationally asymmetric aberrations due to decentration by the rotationally asymmetric surface configuration. The diffractive optical element is pla

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