JP2000180720A - Lens for reading image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a compact image sensor so that a viewing angle for reading is made comparatively large and chromatic aberration and the other various kinds of aberration are satisfactorily suppressed by specifying the lens constitution of the respective lens groups of a lens for reading images. SOLUTION: This lens for reading images is constituted of a 1st and a 2nd positive and negative meniscus lenses L1 and L2, whose convex surfaces are made to face an object side, a 34th lens L34 obtained by joining a 3rd lens L3 whose both surfaces are convex and a 4th lens L whose both surfaces are concave, a 5th and a 6th negative and positive meniscus lenses L5 and L6, whose convex surfaces are made face an image surface side in turn from the object side. Then, when the refractive indexes of the respective lenses L1-L6 are defined as N1-N6 in this order, Abbe's numbers and the focal distances of the lenses L3 and L4 are respectively defined as ν3 and ν4 and (f3) and (f4) in turn, the distance between the apex of the lens surface of the lens L1 on the object side and that of the lens L4 on the image side is defined as S, the distance to an image surface from an object surface is defined as M, and expressions 1.77<(N1+N2+N5+N6)/4, 1.05<N4/N3, ν4/ν3<0.5, -1.15<f3/f4<-1.10 and 0.080<S/M<0.083 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, and more particularly to an F number 5.0 and a reading angle of view of about 46 DEG which are suitable for reading image information on a document surface by forming an image on a line sensor surface. The present invention relates to an image reading lens suitable for an image reading apparatus such as an image scanner, which has a high optical performance over the entire screen and is composed of six lenses in five groups as a whole and has a small size and a simple structure.

[0002]

2. Description of the Related Art Conventionally, an image scanner, for example, is widely known as an image reading apparatus for reading image information on a document surface using image reading means such as a line sensor (one-dimensional CCD).

[0003] In this apparatus, an image reading lens (imaging lens system) is generally used for projecting and forming image information on a document surface onto a line sensor surface for reading.
The image reading lens forms an image of a linear reading area on the document surface on the line sensor surface. In other words, a linear area obtained by back-projecting the linear light receiving portion of the line sensor onto the document surface is a reading area to be read by the image reading lens.

The specification required for this image reading lens is to form a linear reading area correctly on the line sensor surface. For example, an image reading lens used in an image scanner or the like has a high resolution constant over the entire screen from on-axis to off-axis, a small distortion, and a flat image surface satisfying these optical performances, and a deep depth of focus. Is required.

The length of the reading area in the longitudinal direction and the distance from the document surface to the line sensor across the image reading lens, that is, the so-called object-image distance, determines the maximum angle of view with respect to the image reading lens. As a matter of course, the imaging performance of the image reading lens is disadvantageous in performance as the angle of view is larger. Since the size of the original is determined by what is to be the original, the angle of view required for the optical system is generally determined by the distance between object images. That is, if the document size is the same, the angle of view becomes smaller as the distance between object images becomes longer.

[0006]

One of the important performance items to be considered in the apparatus is that the entire apparatus is small and the number of parts is small and the structure is simple. For example, the devices are often used side by side on a desk or the like, and thus consume space. In addition, if an object is placed on the device even when the device is not in use, there is a problem in that the device must be removed when using the device, and the space is eventually dead space. Therefore, miniaturization is desired as much as possible.

[0007] Items required for an image reading lens (imaging lens system) in order to realize these are as follows.

Conventionally, as a method of miniaturizing the apparatus without making the distance between the object and the image of the optical system too short, for example, two scanning mirrors for scanning are scanned at a speed ratio of 1: 2 to read an image. There is an image scanner using a 1: 2 scanning optical system that maintains a constant object-image distance while fixing a lens and a line sensor inside the apparatus. However, this requires complicated speed control of the scanning mirror,
There is also a problem that the structure is complicated and the device becomes expensive.

On the other hand, scanning is performed by integrally moving a scanning mirror, an image reading lens, and a line sensor.
The so-called integrated scanning optical system has an advantage that the scanning drive method is simpler than the above-described 1: 2 scanning optical system and the number of components is small. However, since all of the optical systems are mounted in the movable reading portion, the movable reading portion becomes large, and therefore, the apparatus requires a document placement window having at least the same area as the document to be read, and also requires scanning of the document. At the start end or the end of the area, the movable reading portion protrudes from the original area, so that the housing enclosing it must be large. As a result, there is no other way to reduce the size of the apparatus except for reducing the size of the movable reading portion that moves when scanning the original.

On the other hand, the optical system needs a predetermined optical path length before and after the image reading lens as shown in the conventional example. Attempting to shorten this reduces the size of the readable document. Or the line sensor (CC
With the pixel size of D), the original reading resolution becomes coarse.

To correct this, if the distance between the object and the image of the optical system is shortened, the movable portion for reading can be reduced in size, so that the entire apparatus becomes smaller. However, shortening the distance between the object and the image increases the angle of view. As a result, there is a problem that the design of the optical system is difficult. It is also difficult to manufacture the lens itself,
Attempts to cope with such problems by increasing the number of lenses, for example, complicate the entire apparatus and increase the cost.

According to the present invention, the reading angle of view is set to 4 by appropriately setting the lens configuration of each lens group of the image reading lens.
It is relatively large at 6 °, chromatic aberration and other aberrations are suppressed well, and the number of lenses is suppressed to six, and the distance between object images is shortened. For example, a compact image scanner despite being an integrated scanning optical system It is an object of the present invention to provide an image reading lens capable of realizing the above.

[0013]

According to the present invention, there is provided an image reading lens comprising: (1) a positive meniscus first lens L1 having a convex surface facing the object side in order from the object side, and a negative lens having a convex surface facing the object side. A second lens L2 having a meniscus shape, a 34th lens L34 obtained by bonding a third lens L3 having both lens surfaces convex and a fourth lens L4 having both lens surfaces concave, and a negative lens having a convex surface facing the image surface side It has six meniscus-shaped fifth lenses L5 and a positive meniscus-shaped sixth lens L6 with the convex surface facing the image side, and the first, second, third, fourth, fifth and fifth lenses The refractive indices of the glass of the sixth lenses L1, L2, L3, L4, L5, L6 are respectively N1, N2, N3, N4, N5, N6,
The Abbe numbers of the glasses of the third and fourth lenses L3 and L4 are respectively ν3 and ν4, and the focal lengths of the third and fourth lenses L3 and L4 are f3 and f4, respectively, and the object side of the first lens L1. 1.77 <(N1 + N2 + N5 + N6) / 4 where S is the distance from the vertex of the lens surface to the vertex of the lens surface on the image surface side of the fourth lens L4, and M is the distance from the object surface to the image surface. ‥ (1) 1.05 <N4 / N3 ‥‥‥ (2) ν4 / ν3 <0.5 ‥‥‥ (3) −1.15 <f3 / f4 <−1.10 ‥‥‥ (4) 0 .080 <S / M <0.083 (5)

[0014]

FIG. 1 is a sectional view of a lens according to a numerical example 1 of the present invention, which will be described later. FIGS. 2, 3, 4, 5, and 6 show numerical examples 1, 2, and 3, respectively. Imaging magnification β = −0.1889, F-number = 5.0, half angle of view ω = 4, 5
It is a some aberration figure at the time of 23 degrees. In each numerical example, the focal length f is standardized to 1.0.

In FIG. 1, P is an object surface (original surface), L1 is a positive meniscus first lens having a convex surface facing the object side, and L2 is a negative meniscus lens having a convex surface facing the object side. The second lens L34 is a 34th lens obtained by bonding a third lens L3 having both lens surfaces convex and a fourth lens L4 having both lens surfaces concave, and L5 is a negative meniscus having a convex surface facing the image surface side. A fifth lens L6 is a positive meniscus sixth lens having a convex surface facing the image surface side, and G is a cover glass. Although the stop is not shown, it may be disposed between the second lens L2 and the third lens L3, for example. The various aberration diagrams are lateral aberration (spherical aberration, curvature of field, distortion) and longitudinal aberration. The displayed angle of view is y '= 0 at the object height,
0.25, 0.37 and 0.52. The wavelength is e
Line, g line, C line, and F line are shown, and correspondence is as shown in the figure.

In this embodiment, as shown in FIG. 1, the image reading lens is composed of six lenses of five groups each having a predetermined shape, and the refractive index of each lens, third and fourth lenses L3 and L4.
By setting the Abbe number, the focal length, the object-image distance, and the like as in the conditional expressions (1) to (5), excellent optical performance was obtained at an F-number of 5.0 and a reading angle of view of about 46 °. I have an image reading lens.

Next, the technical meaning of each of the conditional expressions (1) to (5) will be described.

Conditional expression (1) is a condition for suppressing the curvature of field due to a large angle of view and maintaining the resolving power from the on-axis to the edge of the screen while allowing the spherical aberration to a certain degree on the under side. . If the condition (1) is not satisfied, the above effects cannot be obtained.

Conditional expressions (2), (3) and (4) are conditions for correcting chromatic aberration of the entire system. Third,
If each of the first and second lenses L1 and L2 and the fifth and sixth lenses L5 and L6 before and after the fourth lens L3 and L4 is to correct each aberration, the chromatic aberration will be excessively corrected. This is a necessary condition for reversely correcting this by the third and fourth lenses L3 and L4 and balancing each aberration as a whole system. If any one of the conditional expressions (2), (3) and (4) is not satisfied, the above-mentioned effects cannot be obtained, which is not good.

Conditional expression (5) defines the size of the entire optical system. By satisfying this conditional expression (5), the size of the entire imaging system from the object plane to the image plane is
The overall length of the lens is kept small, the space occupied in the apparatus is limited, and the movable portion for reading is easily configured. Conditional expression (5)
If the value is out of the range, the above effects cannot be obtained, which is not good.

In this embodiment, since the half field angle can be made as large as approximately 23 ° by appropriately setting the lens configuration of each lens group of the image reading lens as described above, the original document size is the same as the conventional one. By further setting the principal point interval to be negative while corresponding, a lens system in which the object-image distance is reduced with respect to the angle of view can be obtained. It should be noted that the negative principal point interval is the opposite of the case where the distance from the principal point position on the object side to the principal point position on the image plane is measured and the traveling direction of light is positive.

Next, numerical examples of the present invention will be described. In the numerical examples, 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 spacing from the object side, and Ni and νi are the i-th lens surfaces in order from the object side. The refractive index and Abbe number of glass.

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

[0024]

[Outside 1]

[0025]

[Outside 2]

[0026]

As described above, according to the present invention, by appropriately setting the lens configuration of each lens group of the image reading lens and satisfying each conditional expression, the reading angle of view is relatively large at 46 °, and the chromatic aberration is improved. In addition, for suppressing image aberrations, reducing the number of lenses to six, and reducing the distance between objects and images, for example, it is possible to realize a compact image scanner despite being an integrated scanning optical system. A lens can be achieved.

[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 diagram showing various aberrations of Numerical Embodiment 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.

FIG. 5 is a diagram showing various aberrations of Numerical Example 4 of the present invention.

FIG. 6 is a diagram showing various aberrations of Numerical Example 5 of the present invention.

[Explanation of symbols]

 L1 1st lens L2 2nd lens L3 3rd lens L4 4th lens L5 5th lens L6 6th lens L34 34th lens P Object surface (original surface) G Cover glass S Sagittal image surface M Meridional image surface

Claims (1)

[Claims] 1. A positive meniscus first lens L1 having a convex surface facing the object side, a negative meniscus second lens L2 having a convex surface facing the object side, and both lens surfaces having a convex surface. A third lens L3 and a fourth lens L having both lens surfaces concave
No. 4, a 34th lens L34, a negative meniscus fifth lens L5 with a convex surface facing the image surface side, and a positive meniscus sixth lens L6 with a convex surface facing the image surface side
, And the first, second, third, fourth, fifth and sixth lenses L1, L2, L3, L4, L5, L6 have refractive indices N1, N2, N3, N4, N
5, N6, the Abbe numbers of the glasses of the third and fourth lenses L3 and L4 are respectively ν3 and ν4, and the third and fourth lenses L
The focal lengths of the third lens L4 and the fourth lens L4 are respectively set in order from f3 and f4 to the vertex of the object side lens surface of the first lens L1.
1.77 <(N1 + N2 + N5 + N6) / 4 1.05 <N4 / N3 .nu.4 / .nu.3 <0. 5-1.15 <f3 / f4 <-1.10 0.080 <S / M <0.083 An image reading lens satisfying the following condition.