Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
1999-09-28
2001-07-10
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S716000, C359S790000
Reexamination Certificate
active
06259570
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 10-299670 filed on Oct. 21, 1998, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image readout lens used for reducing or magnifying an image in an optical system of an image readout apparatus such as image scanner; and, in particular, to an image readout lens with a simple configuration composed of three lens elements.
2. Description of the Prior Art
In a color image readout optical system, employed in an image scanner or the like, of a type adapted to form a reduced or magnified image of an original onto an imaging device such as CCD, a three-element lens has conventionally been known as one which attains a smaller size and lower cost with a simple configuration.
Recently, as demands for this three-element lens to reduce its cost and weight have been increasing, an image readout lens using a plastic as its lens material has been known to fulfill these demands. Though a plastic lens is mainly advantageous in terms of its light weight and low cost as well as its easiness to be processed into an aspheric surface, favorable performances are hard to maintain due to its strong temperature dependence and large deviations of imaging positions upon changes in temperature.
Though conventional examples of a three-element image readout lens are disclosed in Japanese Unexamined Patent Publication Nos. 4-288512 and 4-296808, for instance, they are still insufficient in terms of resolution.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a high-resolution image readout lens which reduces its cost and weight by using a plastic material and also suppresses deviations in its imaging position which occur as a characteristic feature of the plastic material upon changes in temperature.
In one aspect, the image readout lens in accordance with the present invention comprises, successively from an object side, a first lens with a positive refracting power having a convex surface directed onto the object side, a second lens made of a biconcave lens, and a third lens with a positive refracting power having a convex surface directed onto an image side, at least one of the lenses being made of a plastic material, the image readout lens satisfying the following conditional expressions (1) and (2):
0.08
<D
6
/F<
0.2 (1)
0.2
<R
4
/F<
1.2 (2)
where
D
6
is the center thickness of the third lens;
R
4
is the radius of curvature of the image-side surface of the second lens; and
F is the focal length of the whole lens system.
Preferably, the second lens is made of a plastic material, and at least one surface thereof is formed as a divergent aspheric surface.
In another aspect, the image readout lens in accordance with the present invention comprises, successively from an object side, a first lens with a positive refracting power having a convex surface directed onto the object side, a second lens made of a biconcave lens, and a third lens with a positive refracting power having a convex surface directed onto an image side, the first and second lenses being made of a plastic material, the image readout lens satisfying the following conditional expressions (1) and (3) to (5):
0.08
<D
6
/F<
0.2 (1)
1.5
<|R
6
/F<
2.2 (3)
0.5
<R
2
/F<
1.55 (4)
0.5
<R
4
/F<
1.2 (5)
where
D
6
is the center thickness of the third lens;
R
2
is the radius of curvature of the image-side surface of the first lens;
R
4
is the radius of curvature of the image-side surface of the second lens;
R
6
is the radius of curvature of the object-side surface of the third lens; and
F is the focal-length of the whole lens system.
Preferably, at least one surface of the second lens is formed as a divergent aspheric surface.
Preferably, this image readout lens further satisfies the following conditional expression (6):
0.3
<F
1
×F
3
/F
2
<0.4 (6)
where
F
i
(i =1 to 3) is the focal length of the i-th lens; and
F is the focal length of the whole lens system.
It is more preferable that this image readout lens further satisfy the following conditional expressions (7) and (8):
−4.5
<F
h
/F<−
2.0 (7)
&ngr;
2
<33.0 (8)
where
F
h
is the composite focal length of the first and second lenses;
&ngr;
2
is the Abbe number of the second lens; and
F is the focal length of the whole lens system.
In still another aspect, the image readout lens in accordance with the present invention comprises, successively from an object side, a first lens with a positive refracting power having a convex surface directed onto the object side, a second lens made of a biconcave lens, and a third lens with a positive refracting power having a convex surface directed onto an image side, the second and third lenses being made of a plastic material, the image readout lens satisfying the following conditional expressions (1) and (9) to (11):
0.08
<D
6
/F<
0.2 (1)
0.3
<R
1
/F<
0.45 (9)
0.2
<R
5
/F<
0.4 (10)
0.2
<|R
7
|/F<
0.3 (11)
where
D
6
is the center thickness of the third lens;
R
1
is the radius of curvature of the object-side surface of the first lens;
R
5
is the radius of curvature of the image-side surface of the second lens;
R
7
is the radius of curvature of the image-side surface of the third lens; and
F is the focal length of the whole lens system.
Preferably, at least one surface of the second lens is formed as a divergent aspheric surface.
REFERENCES:
patent: 6111704 (2000-08-01), Noda et al.
patent: 4-288512 (1992-10-01), None
patent: 4-296808 (1992-10-01), None
Epps Georgia
Fuji Photo Optical Co., Ltd.
Snider Ronald R.
Snider & Associates
Spector David N.
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