JP2000249913A - Lens for reading image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wide-angle lens whose aperture efficiency is 100%, where some aberration is completely compensated and whose half angle of view is >=22 deg. by providing a 1st lens which is a positive meniscus, a 2nd lens whose both surfaces are concave, a 3rd lens whose both surfaces are convex and a 4th lens which is a negative meniscus in order from an object side and satisfying a specified condition. SOLUTION: This lens is provided with four lenses, that is, the 1st lens G1 which is the positive meniscus, the 2nd lens G2 whose both surfaces are concave, the 3rd lens G3 whose both surfaces are convex and the 4th lens G4 which is the negative meniscus in order from the object side. When it is assumed that the refractive index of the material and the Abbe number of the 1st lens G1 are ND1 and νD1 the thickness of the 3rd lens G3 is D3, a distance between the 3rd and the 4th lenses G3 and G4 is t3, the focal distance of the 4th lens G4 is f4 and the focal distance of an entire system is (f), the conditions being 1.67<ND1, 50.0<νD1, 1.0<D3/t3<3.0 and -1.5<f4/f<-1.0 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading lens.
D) The present invention relates to an image reading lens suitable for use in an image reading apparatus such as an image scanner or a digital copying machine, which is projected in a reduced size on a plane and the line information is read by the line sensor.

[0002]

2. Description of the Related Art Conventionally, an image reading lens (imaging lens) such as an image scanner or a digital copier for reducing and projecting image information of an original onto a line sensor (CCD) surface and reading the same has an aperture efficiency of 100%. %, Various aberrations including distortion must be sufficiently corrected.

In recent years, in order to reduce the size of the main body of the image reading apparatus, it has been desired to widen the angle of view of the image reading lens and to reduce the distance between object images. From the viewpoint of cost, it is also required that the number of lenses be as small as possible. Further, in a color scanner, a color digital copying machine or the like, chromatic aberration must be corrected particularly well.

Normally, when the angle of view of an image reading lens is widened, the field curvature, astigmatism, distortion and chromatic aberration of magnification increase in accordance with the angle of view, and it is difficult to obtain a sufficient contrast at all angles of view. There is a problem.

Conventionally, as lens types suitable for use in such a wide field angle range, for example, a Tesser type, a Dopgon type, and a telephoto type are known.

[0006] The Tessar type image reading lens is compact and can correct various aberrations well, but the upper limit of the half angle of view is about 18 °, and it is extremely difficult to correct astigmatism at a wide angle of view beyond that. .

A Dopgon type image reading lens can be used with a half angle of view of about 20 ° as proposed in, for example, Japanese Patent Application Laid-Open No. 7-294812. However, there is a problem that it is difficult to suppress the occurrence of the like.

[0008] Telephoto type image reading lenses are
For example, Japanese Patent Publication No. 61-9607 and Japanese Patent Application Laid-Open No. Hei 9-101452 propose proposals for widening the angle of view from about 20 °, but any correction of distortion, axial aberration, chromatic aberration of magnification, etc. There is a problem that it is difficult.

[0009]

According to the present invention, an aperture efficiency of 100 is obtained by appropriately setting the lens configuration of each lens group of a four-photograph type telephoto image reading lens.
It is an object of the present invention to provide an image reading lens having a wide angle of view of 22 ° or more at a half angle of view in which various aberrations including distortion are sufficiently corrected.

[0010]

An image reading lens according to the present invention is provided.
(1-1) Menisca with the convex surface facing the object side in order from the object side
First lens G1 having a concave shape, and a second lens having concave surfaces on both lens surfaces.
Lens G2, a third lens G3 having both lens surfaces convex, an image forming surface
Of the negative meniscus fourth lens G4 with the convex surface facing the side
It has four lenses, and the refractive index of the material of the first lens G1
And Abbe number are each N_d1, Ν_d1, The lens of the third lens G3
D_Three Between the third lens G3 and the fourth lens G4.
Air spacing t _Three And the focal length of the fourth lens G4 is f_Four ,
When the focal length of the entire system is f, 1.67 <N_d1 , 50.0 <ν_d1 ‥‥‥ (1) 1.0 <D_Three / T_Three <3.0 ‥‥‥ (2) −1.5 <f_Four /F<−1.0‥‥‥(3).

In particular, (1-1-1) a stop is provided between the second lens G2 and the third lens G3 so that the aperture efficiency is 100%.
It is characterized by having become.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2, 3, and 4 show numerical examples 1, 2, 3, and 4 of an image reading lens according to the present invention.
4, FIG. 5, FIG. 6, FIG. 7, and FIG. 8 are various aberration diagrams of Numerical Examples 1, 2, 3, and 4 described later, respectively.

In FIGS. 1, 2, 3, and 4, C1 is a contact glass. LG is an image reading lens, and has four lenses G1 to G4 described later. C
2 is a cover glass. The contact glass C1 side is the object side (the side on which the read image is provided), G1 is a meniscus-shaped positive first lens having a convex surface facing the object side, G2 is a second lens having both lens surfaces concave, and S is an aperture. G3 is a third lens whose both lens surfaces are convex, and G4 is a meniscus-shaped negative fourth lens whose convex surface faces the imaging surface side (the side on which a read image is formed).

In this embodiment, as shown in FIGS. 1, 2, 3, and 4, the image reading lens LG is constituted by four groups of four telephoto type lenses having a predetermined shape, and the first lens G1 The refractive index and Abbe number of the material with respect to the d-line, the lens thickness of the third lens G3, the air gap between the third lens G3 and the fourth lens G4, the focal length of the fourth lens G4 with respect to the d-line, and the d-line of the entire system Is set as in the above-described conditional expressions (1) to (3), the aperture efficiency is 100%, and the half angle of view 22 ° in which various aberrations are sufficiently corrected.
An image reading lens having excellent optical performance in the above wide angle of view is obtained.

In this embodiment, the second lens G2 and the third lens G2
By providing the stop S between the lens G3 and the lens G3, vignetting of off-axis rays can be eliminated, and the aperture efficiency can be made 100%. Note that the object of the present invention can be almost achieved if the opening efficiency is not less than 100% but is 95% or more.

Next, the technical meaning of each of the conditional expressions (1) to (3) will be described.

Conditional expression (1) is for the d-line of the first lens G1.
Index of refraction N_d1And Abbe number ν _d1Regarding the provisions of
It is. By satisfying condition (1), the first lens
G1 has a stronger refractive power and is suitable for widening the angle of view.
In both cases, the dispersion of the refractive index with respect to wavelength is small,
The difference can be corrected well. Deviated from conditional expression (1)
This makes it difficult to correct axial chromatic aberration well.
Not so good.

Conditional expression (2) relates to the ratio between the lens thickness D ₃ of the third lens G3 and the air gap t ₃ between the third lens G3 and the fourth lens G4, and the aperture efficiency is 100%. However, it is intended to favorably correct various aberrations such as astigmatism at a wide angle of view and aberration of field curvature. Conditional expression (2)
When the value exceeds the lower limit, it becomes difficult to correct astigmatism, which is not good. When the value exceeds the upper limit of conditional expression (2), the size of the third lens G3 becomes too large, and the aperture efficiency decreases.

Conditional expression (3) relates to the ratio of the focal length f ₄ of the fourth lens G4 to the d-line and the focal length f of the entire system to the d-line, and reduces various aberrations such as distortion and coma. This is for good correction. If the lower limit of conditional expression (3) is exceeded, it becomes difficult to correct distortion, which is not good.
If the value exceeds the upper limit of conditional expression (3), it becomes difficult to correct coma aberration, which is not good.

The image reading lens of Numerical Example 1 has a necessary and sufficient F / No (F number) for an image reading apparatus, and as shown in FIG. 5, the various aberrations from on-axis to off-axis are sufficient. And high imaging performance is obtained.

In the image reading lens of Numerical Example 2, various aberrations are sufficiently corrected both on-axis and off-axis, even though the half angle of view reaches 26 °, and high imaging performance is obtained. I have.

The image reading lens of Numerical Example 3 is F / N
Although o = 4.6, which is bright and the half angle of view exceeds 22 °, various aberrations are sufficiently corrected, and high imaging performance is obtained.

Although the image reading lens of Numerical Example 4 has an extremely wide half angle of view of 29 °, various aberrations are sufficiently corrected as shown in FIG. Performance has been obtained.

As described above, various aberrations are satisfactorily corrected in each of the numerical examples. In particular, astigmatism, curvature of field, distortion, chromatic aberration of magnification, and the like, which are difficult to correct when the angle of view is widened. Has been corrected satisfactorily, and thus has a high contrast at all angles of view.

Next, numerical embodiments 1 to 4 of the present invention will be described. In each numerical example, Ri is the radius of curvature of the i-th lens surface in order from the object side, Di is the i-th lens thickness and air gap from the object side, and Ndi and νdi are the i-th lens in order from the object side. Are the refractive index and Abbe number of the glass.

Table 1 shows the relationship between the above-described conditional expressions and various numerical values in the numerical examples.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

[Table 4]

[0031]

[Table 5]

[0032]

According to the present invention, the aperture efficiency can be reduced by appropriately setting the lens configuration of each lens group of the four-photograph type image reading lens and satisfying the conditional expressions as described above. At 100%, it is possible to achieve an image reading lens with a wide angle of view of 22 ° or more at a half angle of view of 22 ° or more in which various aberrations including distortion are sufficiently corrected.

[Brief description of the drawings]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention.

FIG. 2 is a sectional view of a lens according to a numerical example 2 of the present invention.

FIG. 3 is a sectional view of a lens according to a numerical example 3 of the present invention.

FIG. 4 is a sectional view of a lens according to a numerical example 4 of the present invention.

FIG. 5 is a diagram showing various aberrations of Numerical Example 1 of the present invention.

FIG. 6 is a diagram showing various aberrations of Numerical Example 2 of the present invention.

FIG. 7 is a diagram showing various aberrations of Numerical Example 3 of the present invention.

FIG. 8 is a diagram showing various aberrations of Numerical Example 4 of the present invention.

[Explanation of symbols]

 LG image reading lens G1 first lens G2 second lens G3 third lens G4 fourth lens C1 contact glass C2 cover glass S aperture

Claims (2)

[Claims] 1. A convex surface is directed to the object side in order from the object side.
Meniscus positive first lens G1, both lens surfaces concave
Second lens G2, and a third lens G whose both lens surfaces are convex
3. A negative meniscus fourth lens with the convex surface facing the imaging surface
Lens G4, and the refractive index and Abbe number of the material of the first lens G1 are respectively
N_d1, Ν_d1And the lens thickness of the third lens G3 is D_Three The second
The air gap between the third lens G3 and the fourth lens G4 is t _Three ,
Let the focal length of the fourth lens G4 be f_Four , The focal length of the whole system
1.67 <N_d1 , 50.0 <ν_d1 1.0 <D_Three / T_Three <3.0-1.5 <f_Four Characterized by satisfying the following condition: /f<-1.0
Z. 2. The second lens G2 and the third lens G
3. The image reading lens according to claim 1, wherein an aperture is provided between the lens and the aperture and the aperture efficiency is 100%.