JPH01128028A - Original reading lens - Google Patents

Abstract

PURPOSE:To satisfactorily correct aberration even in case of <=0.1 low image pickup magnifications by constituting a triplet lens system consisting of three groups of three lenses and forming the image-side lens face of a double concave lens of a second group into an aspherical face and specifying its conic coefficient. CONSTITUTION:In the triplet lens system consisting of three groups of three lenses, first-third groups are arranged in order from the object side to the image side. The first group consists of a convex meniscus lens 10 whose convex is directed to the object side, and the second group consists of a double concave lens 12, and the third group consists of a double convex lens 14, and the image- side lens face of the double concave lens 12 of the second group is an aspherical face. A conic coefficient K of this aspherical face is -0.09<K<-0.002. An original placing glass 16 on which an original is placed and a cover glass 18 for the light receiving face of a solid-state image pickup element are provided. Thus, a satisfactory performance is obtained even in case of <=0.1 low image pickup magnifications, and especially, aberrations are satisfactorily corrected from the center to the peripheral part of an image.

Description

[Detailed description of the invention] (Technical field) The present invention relates to a document reading lens.

(Prior art) A document in which a document to be read is illuminated in a slit shape and scanned with illumination, and 'IHK' in the illuminated portion of the document is imaged onto the light receiving surface of a solid-state image sensor 1, such as a COD, using a document reading lens to read the document. The reading method is well known in connection with digital copying machines, facsimile machines, and the like.

The document reading lens used in such document reading method includes:
The following performance is required.

That is, first of all, it is necessary to have high resolution and high contrast. For example, the arrangement pitch of the light receiving elements is 14.
When using a line COD of μm and 2048 bits,
The imaging performance of the reading lens is a spatial frequency of 35.7 lines/
Resolving power of m m or higher is required, and the CCD output S/
It is desired that the contrast (MTF) be high in order to increase the N ratio.

Second, it needs to be bright. This is to speed up document reading.

Thirdly, it is necessary that the aperture efficiency be high.

That is, in general, the amount of light at the periphery of the image plane at the half angle of view θ decreases with respect to the amount of light at the center according to the cosine fourth power law. For this reason, light amount correction is performed in order to obtain a uniform light amount on the image plane, and this is because correction becomes easier if the aperture efficiency of the lens is close to 100X.

Fourth, it is necessary that distortion be as small as possible. When reading a manuscript, the most important thing is to read the manuscript faithfully.
This is because if a reading lens with large distortion is used, a large reading bit position error will occur at the edge of the document. for example,
Using the above 2048-bit CCD, distortion is 0.5
If reading is performed with a lens of 10%, a reading bit position error of about 10 bits will occur at the edge of the document.

Further, it is desirable that the document reading lens be small, compact, and inexpensive.

Conventionally, a triplet-type document reading lens disclosed in Japanese Patent Application Laid-Open No. 134720/1983 has been known as a document reading lens that can be realized in a small size and at a low cost.

However, since all lens surfaces of this document reading lens are spherical, there are limits to aberration correction, especially when the imaging magnification is 0.
When the magnification is as low as 1 or less, comatic aberration, which determines the quality of the image, cannot be sufficiently corrected.

(Purpose) The present invention was made in view of the above-mentioned circumstances, and
It is an object of the present invention to provide a good document reading lens that can obtain good imaging performance even at a low magnification of 0.1 times or less.

(composition) The present invention will be explained below.

The document reading lens of the present invention is a triplet type having three lenses in three groups in which the first to third groups are arranged in this order from the object side to the image side.

As shown in FIG. 1, the first group is a convex meniscus lens 10 with its convex surface facing the object side (left side in the figure), the second group is a biconcave lens 12, and the third group is a biconvex lens 14. , second
The image side lens surface of the biconcave lens 12 of the group is an aspherical surface.

The conic constant of the aspherical surface is in the range -0,09<K<-0,002. In FIG. 1, reference numeral 16 indicates an original glass on which an original is placed, and reference numeral 18 indicates a cover glass on the light-receiving surface of the solid-state image carrier.

In the present invention, as described above, the image side lens surface, that is, the fourth surface, of the biconcave lens 12 of the second group is made an aspherical surface, and its conic coefficient is used as a parameter for aberration correction, thereby achieving a low magnification of 0.1 or less. However, each aberration can be corrected well.

In particular, the F/No is 5.56 and the width is 297mm.
When a document of 1 is reduced and imaged at a reduction magnification of about 0.088, it is possible to satisfactorily correct various aberrations up to a half angle of view of about 22 degrees and increase the aperture efficiency to about 95X.

To explain the above range in which the conic constant is satisfied, flare due to comatic aberration occurs more greatly as the degree of convergence of the outer shell ray with respect to the principal ray is poor. Therefore, in order to correct the degree of convergence of the outer shell ray when the ray is incident and to reduce the above flare component, the fourth
The surface is aspherical. The conic constant that defines the aspheric surface in this case is −1<K, which forms an ellipsoid that is rotationally symmetrical about the major axis.
<O range. At this time -0,09<K<
-0,002 time.

Flare caused by comatic aberration can be efficiently corrected.

Therefore, if the above range is exceeded, flare will increase even on an elliptical surface that is rotationally symmetrical about the long axis, and coma aberration will worsen.

(Example) Three specific examples are listed below. In each embodiment, rol and r02 are the radius of curvature of the object side and image side of the original glass 16, r1 to r6 are the radii of curvature of the lens surface of the original reading lens sequentially from the object side, and r7. r8 is the radius of curvature of each surface of the cover glass, dol is the thickness of the original glass 16, dol is the air gap between the original glass 16 and the first surface (aspherical surface), dl2d5 are the radius of curvature of each surface of the cover glass The lens surface spacing, d6, is the air gap between the sixth surface and the cover glass 18, d7 is the thickness of the cover glass 18, n01. ν0
1 is the refractive index and Atsube number of the original glass 16, n4.
ν4 is the refractive index and Atsube number of the cover glass 18, n
l"n3.Sil~S3 are the refractive index and Abbe number of the first to third groups, f is the composite focal length of the entire system, F/No is the brightness of the lens, ω is the half angle of view, and m is the reduction Y is the object height, and K is the conic constant.

Example 1 f = 30. F/No=5.56. ω=22”,
m =0.088. Y=148.5r01 00
dol 3 nol 1.51680 ν016
4.2r02 ood02367.672 rl 9.672 dl 4.028 n
l 1.80420 v 146.5r2 15
．． 054 d2 0.491r3 -22.689
d3 0.84 n21.69895 y2
30.1r4 9.942 d4 0.731r
5 19.014 d5 2.8gI n31.
80420? 346.5r6 -17.042
d6 26.968r7 oo d7
0.7 n41.5168'o V464
．． 2 (f3 o.

K=-0,083075 Example 2 f=30. F/No=5.56. c, +=22”
, m =0.088. Y=148.5ro1'
Odol 3 n011.51680 vol 6
4.2r02 ω d02357.692r1
9.938 dl 4.261 nl 1.804
20 v'l 46.5r2 15.634 d2
0.49r3 -22.151 d3 0.84
n21.69895 y230.1r4 10.2
64 d4 0.774r5 19.922 d5
2.82I n31.80420 v, 346.
5r6 -16.784 d626.865r7
oo d7 0.7 n41.51680
y464.2r8o.

K=-0,042915 Example 3 f=30. F/No =5.56. ω=22',
m=0.088. Y=148.5r01 00
dol 3 n011.51680 V 016
4.2r02 oo d02367.626r1
10.071 dl 4.382 nl 1.8
0420 ν146.5r2 15.888 d2
0.491r3 -21.842 d3 0.84
n21.69895 v230.1r4 10.
466 d4 0.8 r5 20.463 d5 2.765 n31.
80420 v346.5r6 -16.63 d
626.81r7 00 d7 0.7 n
41.51680 v464.2 (8o.

K = -0,003121 FIG. 2 shows various aberration diagrams regarding the above-mentioned Example 1. Third
In the figure, various aberration diagrams related to the above-mentioned Example 2 are shown.
The figure shows various aberration diagrams related to the third embodiment. In these aberration diagrams, SA and SC represent spherical aberration and sine conditions, the broken line represents the sine condition, DS and DM represent astigmatism, the solid line represents radial, and the broken line represents tangential.

DIST indicates distortion aberration, and COM^ indicates coma aberration. Also, ■ is a wavelength of 587.56 nm, and ■ is a wavelength of 656.28 nm.
m and ■ indicate that the wavelength is 486.13 cm.

(Effects) As described above, according to the present invention, it is possible to provide a novel M document reading lens.Since this document reading lens has a triplet type configuration of 3 elements in 3 groups, it can be made smaller and more compact, and is therefore inexpensive. This can be achieved with However, since the fourth surface is aspherical, various aberrations are well corrected, and the performance required for an original reading lens is well satisfied. In particular, in the above embodiments, as is clear from each aberration diagram, aberrations are well corrected from the center of the image to the image periphery, and coma aberration, which affects the quality of contrast, is also well corrected. Therefore, each example has F/N.

It is a document reading lens that is bright at 5.56 mm, satisfactorily corrects various aberrations up to a half angle of view of 22 degrees, maintains a high aperture efficiency of about 95%, and has high performance from the center of both sides to the periphery of the screen.

It goes without saying that the document reading lens of each embodiment can be effectively used not only at reduction ratios of o and n88, but also at reduction ratios in the vicinity thereof.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the lens configuration of the present invention, and FIGS. 2 to 4 are aberration diagrams.

Claims (1)

[Claims] First, second, and third groups are arranged in this order from the object side toward the image side, and the first group is a convex meniscus lens with a convex surface facing the object side;
The second group is a biconcave lens, and the third group is a biconvex lens, and the image side lens surface of the biconcave lens in the second group is an aspherical surface.
A triplet-type document reading lens comprising a group of three elements, and characterized in that the conic constant K of the aspherical surface is within the range of -0.09<K<-0.002.