JPH04255814A - Lens for image read - Google Patents

Abstract

PURPOSE:To obtain the image read lens which has a 5.6 F number and high optical performance over the whole picture plane of about 57 deg. in read view angle by properly setting the lens shapes, refracting power, etc., of three lenses. CONSTITUTION:This image read lens consists of a 1st positive meniscus lens G1 which has a convex surface on a document surface side, a 2nd negative biconcave lens G2, a stop SP, and a 3rd positive biconvex lens G3 in order from the document surface side. Then 1.2<(f1+f3)/f<1.4, 0.015<D2/f<0.026, 0.018<D4/f<0.024, 2.1<f1/f21<2.4, and 0.84<1f2/f31<0.9 hold, where fi is the focal length of an (i)th lens, (f) is the focal length of the whole system, and Di is the lens thickness or air interval of the (i)th lens.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to an image reading lens, which has an F number of 5.6 and a reading angle of view of about 57 degrees, which is suitable for reading image information on a document surface by focusing it on a line sensor surface, for example. The present invention relates to an image reading lens suitable for facsimiles, scanners, etc., which has high optical performance over the entire screen and is composed of three lenses in three groups as a whole.

0002

2. Description of the Related Art Conventionally, a device has been designed to read image information on a document surface by focusing the image information on the surface of a solid-state image sensor array (line sensor) at a predetermined reduction magnification using an image reading lens. Various devices such as image scanners and facsimile machines have been proposed. Image reading lenses used in such devices are required to have a small number of lenses, the entire lens system to be compact, and to have predetermined optical performance.

Conventionally, such an image reading lens has been proposed, for example, in Japanese Patent Laid-Open No. 64-21411.

[0004] According to this publication, the image reading lens is constructed of a so-called triplet type consisting of three lenses each having a predetermined shape: a positive lens, a negative lens, and a positive lens in order from the document surface side.

[0005]

[Problems to be Solved by the Invention] Generally, an image reading lens has an aperture efficiency close to 100% up to the periphery of the screen, has sufficiently small distortion, and has a high MTF value over the entire screen. It is required to be present. Furthermore, it is required that the number of lenses is small and that the entire lens system is simple.

However, when trying to satisfy the above-mentioned conditions by reducing the number of lenses while maintaining a predetermined F-number, various aberrations, especially halo and coma aberration, increase, and the problem occurs over the entire screen. It becomes difficult to maintain good optical performance.

The present invention uses a triplet lens type, and by appropriately setting the lens configuration of each lens, it is possible to correct various aberrations such as halo and coma in a well-balanced manner over the entire reading angle of view. The purpose of the present invention is to provide an image reading lens with optical performance, an F number of about 5.6, and an image reading angle of view of about 57 degrees.

[0008]

[Means for Solving the Problems] The image reading lens of the present invention includes, in order from the document surface side, a meniscus-shaped positive first lens with a convex surface facing the document surface side, and a negative second lens with both lens surfaces concave. , an aperture, and a positive third lens with both lens surfaces convex, and the focal length of the i-th lens is fi,
When the focal length of the entire system is f and the i-th lens thickness or air gap is Di, 1.2<(f1+f3)/f<1.4 (1
) 0.015<D2/f<0.026 (2
) 0.018<D4/f<0.024 (3
) 2.1<|f1/f2|<2.4 (4
) 0.84<|f2/f3|<0.9 (5
) is characterized by satisfying the following conditions.

[0009]

[Example] Fig. 1 is a cross-sectional view of a lens of Numerical Example 1, which will be described later, of the present invention, and Figs. 2, 3, and 4 show Numerical Examples 1, 2, and 3 when the imaging magnification β = -0.112. FIG.

In FIG. 1, P is the original surface, and G1 is the original surface P.
Meniscus-shaped positive first lens with a convex surface facing the side, G2
is a negative second lens with both concave lens surfaces, G3 is a positive third lens with both convex lens surfaces, and Q is a cover glass.

SP is a diaphragm and is arranged between the second group and the third group. In the aberration diagram, Y is the image height.

In this embodiment, the image reading lens is constructed of a triplet type consisting of three lenses of a predetermined shape as shown in FIG. 1, and the refractive power of each lens, the distance between the lenses, etc. By setting as in 5), F
An image reading lens with a number of 5.6 and good optical performance at a reading angle of view of about 57 degrees has been obtained.

Next, the technical meanings of the above-mentioned conditional expressions (1) to (5) will be explained.

Conditional expression (1) appropriately sets the sum of the focal lengths of the positive first lens and the positive third lens with respect to the focal length of the entire system, mainly keeping the Petzval sum at an appropriate value and adjusting the image plane. This is to improve flatness.

When the upper limit of conditional expression (1) is exceeded, the astigmatism difference increases, and when the lower limit is exceeded, the astigmatism difference and field curvature increase at the periphery of the screen, resulting in a sharp decline in optical performance. .

Conditional expression (2) appropriately sets the air distance between the first lens and the second lens, and conditional expression (3) appropriately sets the air distance between the second lens and the third lens. This is to maintain a well-balanced optical performance.

When the upper limit of conditional expression (2) is exceeded, the astigmatism difference increases and the coma flare due to the downward beam of the off-axis beam also increases. Moreover, when the lower limit is exceeded, coma flare due to the upward beam of off-axis light at the periphery of the screen increases, resulting in a sharp decline in optical performance. Furthermore, the aperture efficiency also decreases, which is not good.

If the upper limit of conditional expression (3) is exceeded, astigmatism will be insufficiently corrected, and distortion at the periphery of the screen will also increase. On the other hand, when the lower limit is exceeded, astigmatism becomes overcorrected.

Conditional expressions (4) and (5) are the first, second, and third
Regarding the refractive power of the lens, this is to maintain the optical performance of the entire screen in a well-balanced manner by appropriately distributing the paraxial refractive power of the entire lens system when the entire lens system is composed of three lenses.

If either the upper limit or the lower limit of conditional expressions (4) and (5) is exceeded, it becomes difficult to maintain good optical performance of the entire screen.

Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and νi are the curvature radius of the i-th lens from the object side, respectively. These are the refractive index (e-line) and Abbe number of the glass.

Table 1 shows the relationship between each of the above-mentioned conditional expressions and the numerical values in the numerical examples. Numerical Example 1 f=20, f1=21.06, f2=-9.1
1, f3=10.36, F=5.6 Magnification β=
-0.112 ω=28.5 ° Aperture = Behind 4th surface 0.48mm Surface number R
D N
ν 1 5
．． 852 2.01 1.
83480 47.72 2
8.174 0.40
3 -26.228 0.9
4 1.59270
35.3 4 5.564
0.37 5 10.050
3.16 1.83480
42.72 6 -22.
395 Numerical Example 2 f=20, f1=19.72, f2=-9.0
9, f3=10.65, F=5.6 Magnification β=
-0.112 ω=28.5 ° Aperture = Behind 4th surface 0.47mm Surface number R
D N
ν 1 6
．． 081 2.08 1.
83480 42.72 2
9.055 0.42
3 -27.253 1.0
6 1.59270
35.3 4 5.615
0.45 5 9.997
2.84 1.77249
49.6 6 -20.
571 Numerical Example 3 f=20, f1=20.92, f2=-9.0
6, f3=10.59, F=5.6 Magnification β=
−0.112 ω=28.5°
Aperture = Behind 4th surface 0.39mm Surface number
R D
N ν 1
5.734 2.18
1.77249 49.6 2
8.543 0.31
3 -25.685 0
．． 96 1.59270
35.3 4 5.290
0.41 5 9.443
2.77 1.77249
49.6 6 -1
9.038

[0023]

[Table 1]

[0024]

According to the present invention, as described above, by appropriately setting the lens shape, refractive power, etc. of the three lenses, F.
It is possible to achieve an image reading lens suitable for facsimiles and image scanners that has high optical performance over the entire screen with a number of 5.6 and a reading angle of view of about 57 degrees.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a lens according to Numerical Example 1 of the present invention.

FIG. 2 is a diagram showing various aberrations of Numerical Example 1 of the present invention.

FIG. 3 is a diagram showing various aberrations of Numerical Example 2 of the present invention.

FIG. 4 is a diagram showing various aberrations of Numerical Example 3 of the present invention.

[Explanation of symbols]

G1 1st lens G2 2nd lens G3 3rd lens SP Aperture S Sagittal image plane M Meridional image plane S. C Sine condition

Claims (1)

[Claims] Claim 1: In order from the document surface side, a meniscus-shaped positive first lens with a convex surface facing the document surface side, a negative second lens with both concave lens surfaces, an aperture, and a positive lens with both lens surfaces convex. It has three lenses as a third lens, and when the focal length of the i-th lens is fi, the focal length of the entire system is f, and the thickness or air gap of the i-th lens is Di, 1.2<(f1+f3)/ f<1.4 0.015<D2/f<0.026 0.018<D4/f<0.024 2.1<|f1/f2|<2.4 0.84<|f2/f3|< An image reading lens characterized by satisfying the following condition: 0.9.