JP4461913B2 - Reading lens - Google Patents

Description

  The present invention relates to a reading lens that is used in an image reading apparatus such as a copying machine, an image scanner, or a facsimile, and forms an image of light corresponding to image information of a document on a reading unit such as a CCD (Charge Coupled Device).

  Conventionally, a reading lens used in an image reading apparatus such as a copying machine is often made of a plurality of glasses. In order to achieve good aberration correction with a brightness of about F5, about six lenses are used. I needed a lens. However, when such a number of glass lenses is used, there is a limit to the cost reduction as a reading lens.

  Therefore, in recent years, some of the lenses that make up the reading lens are replaced with glass from plastic, and the surface is made aspherical to reduce the number of lenses, thereby reducing the cost of the reading lens. Has been made.

  However, plastic lenses have a larger coefficient of linear expansion than glass lenses and a large change in refractive index due to changes in temperature, so when the environmental temperature changes, the focal length and curvature of field change, which degrades performance. Will cause. Therefore, when a plastic lens is used as a part of the reading lens, it is necessary to compensate for a change in image plane movement and a field curvature due to a temperature change.

Therefore, in Patent Document 1, for example, at least one of a plurality of lenses constituting the reading lens is constituted by an anamorphic lens made of resin, and the reading lens is designed to satisfy a predetermined conditional expression. . For example, in Patent Document 2, three plastic lenses are used as a part of the reading lens, and the reading lens is designed to satisfy a predetermined conditional expression.
JP 2002-314767 A JP-A-9-43508

  By the way, when a light ray parallel to the optical axis is incident on the lens, even if the power of the lens (reciprocal of the focal length) is almost equal, the amount of the light beam bent by the position where the light ray strikes (the height of the light ray) Will change. In other words, when the refractive index of a lens changes due to a temperature change, even if the combined power of the two lenses is small (the combined focal length is long), the influence on the image plane movement changes if the height of the light beam is different. Therefore, in order to sufficiently reduce the movement of the image plane due to temperature changes, light rays that enter the plastic lens (particularly light rays that define FNo and are focused on the axis (hereinafter referred to as on-axis) It is necessary to design the reading lens in consideration of the height (referred to as FNo ray).

  However, since the reading lenses of Patent Documents 1 and 2 are not designed in consideration of the height of the axial FNo ray passing through the plastic lens, there arises a problem that the reduction of the image plane movement is insufficient. .

  The present invention has been made to solve the above-mentioned problems, and its purpose is to design a lens in consideration of the height of the axial FNo ray while reducing the cost by using a plastic lens. Accordingly, an object of the present invention is to provide a reading lens that can sufficiently reduce image plane movement due to temperature change.

The reading lens of the present invention is a reading lens including a front group, a diaphragm, and a rear group in order from the object side , and the front group is a positive lens (L1) made of a plastic material in order from the object side. And a negative lens (L2) made of a plastic material, and the rear group in order from the object side is a positive lens (L3) made of a glass material and a negative lens (L4 made of a plastic material) And the following conditional expressions (1) and (2) are satisfied. That is,
600 <| f × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <900 (1)
However,
f: focal length of the entire system,
f1: Focal length of L1, H1: Incident height of axial FNo ray to L1,
f2: Focal length of L2, H2: Incident height of axial FNo ray to L2,
f4: Focal length of L4, H4: Incident height of axial FNo ray on L4,
800 mm <f12 <1200 mm (2)
However,
f12: Composite focal length of L1 and L2,
It is. The unit of each parameter is [mm].

  According to the present invention, conditional expression (1) is set in consideration of the heights H1, H2 and H4 of axial FNo rays passing through the lens (L1), the lens (L2) and the lens (L4) which are plastic lenses. ing. In addition, conditional expression (2) is set for the combined focal length f12 of the lens (L1) and the lens (L2).

  Here, if the lower limit of conditional expression (1) is not reached, the amount of image plane movement in the minus direction (the direction in which the image plane approaches the lens) increases. On the other hand, if the upper limit of conditional expression (1) is exceeded, the amount of image plane movement in the plus direction (the direction in which the image plane moves away from the lens) increases. If the lower limit of conditional expression (2) is not reached, the amount of image plane movement in the positive direction increases, and if the upper limit of conditional expression (2) is exceeded, the amount of image plane movement in the negative direction increases.

  Therefore, by designing the reading lens so as to satisfy the conditional expressions (1) and (2), it is possible to reduce the amount of movement of the image plane when the environmental temperature changes, while reducing the cost of the lens by using a plurality of plastic lenses. It is possible to suppress the deterioration of the reading lens performance due to the environmental temperature change.

Here, the reading lens of the present invention may be further designed to satisfy the following conditional expression (3). That is,
| F × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <800 (3)
It is.

The reading lens of the present invention may be further designed to satisfy the following conditional expression (4). That is,
900 mm <f12 (4)
It is.

  If conditional expressions (3) and (4) are satisfied, the amount of image plane movement in the plus direction can be further reduced. Thereby, a material with a small linear expansion coefficient can be used as the lens lens frame, and the dimensional accuracy can be improved and the merit that the stability of quality is high can be obtained.

  According to the present invention, the reading lens is designed so as to satisfy the predetermined conditional expression in consideration of the height of the axial FNo ray passing through the plastic lens. The image plane movement amount can be reduced, and the performance deterioration of the reading lens due to the environmental temperature change can be suppressed.

  An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing a schematic configuration of a reading lens according to the present embodiment. The reading lens is provided in an optical path between a platen glass (1) on which an object (for example, a document) is arranged and a CCD cover glass (2) arranged in front of a CCD as a reading unit serving as an image plane. It has been. This reading lens includes a front group (F), a diaphragm (A), and a rear group (R) in order from the object side. The front group (F) is composed of two lenses in order from the object side: a positive lens (L1) made of a plastic material and a negative lens (L2) made of a plastic material. The rear group (R) is composed of, in order from the object side , a positive lens (L3) made of a glass material and a negative lens (L4) made of a plastic material.

In the present embodiment, the reading lens is designed to satisfy the following conditional expressions (1) and (2). That is,
600 <| f × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <900 (1)
However,
f: focal length of the entire system,
f1: Focal length of L1, H1: Incident height of axial FNo ray to L1,
f2: Focal length of L2, H2: Incident height of axial FNo ray to L2,
f4: Focal length of L4, H4: Incident height of axial FNo ray on L4,
800 mm <f12 <1200 mm (2)
However,
f12: Composite focal length of L1 and L2,
It is.

  Here, if the lens (L1) and the lens (L2) are arranged adjacent to each other, the difference between H1 and H2 becomes small. By making this point and f12 longer, it is possible to cancel each other the amount of image plane movement generated by each lens. In addition, by disposing the lens (L4) away from the aperture (A) and lowering H4, the generated image plane movement can be kept small.

In view of such points, in the present embodiment, conditional expression (1) is defined in consideration of H1, H2, and H4, and conditional expression (1) and conditional expression (2) for f12 are satisfied. The reading lens is designed to do this. Actually, in the conditional expression (1), if the value of | f × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | It is known that the amount of image plane movement in the direction of approaching the lens increases and exceeds the upper limit, and the amount of image plane movement in the plus direction (the direction in which the image plane moves away from the lens) increases. In conditional expression (2), it is known that when f12 falls below the lower limit, the image plane movement amount in the positive direction increases, and when it exceeds the upper limit, the image plane movement amount in the negative direction increases.

  Therefore, by designing the reading lens so as to satisfy the conditional expressions (1) and (2), it is possible to reduce the cost of the reading lens by using a plurality of plastic lenses as in this embodiment. The amount of image plane movement at the time of the environmental temperature change can be made sufficiently small, and the performance deterioration of the reading lens due to the environmental temperature change can be suppressed.

  In conditional expression (1), H3 is not taken into account, but this is because the lens (L3) is a glass lens, and glass is less susceptible to deterioration in characteristics such as image plane movement due to temperature changes than plastic. This is because there is little need to consider.

Here, in order to further reduce the moving amount of the image plane in the plus direction, it is preferable to satisfy the following conditional expression (3) or (4). That is,
| F × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <800 (3)
900 mm <f12 (4)
It is. In this case, a material having a small linear expansion coefficient can be used as the lens lens frame, and the effects of improving the dimensional accuracy and improving the quality stability can be obtained.

  Hereinafter, a detailed configuration of the reading lens according to the present embodiment will be described as a first example.

Example 1
Table 1 is an explanatory view showing the construction data of the reading lens in the present embodiment, and FIG. 2 is an explanatory view showing aberration diagrams in the present embodiment. In the astigmatism diagram, X indicates astigmatism on the sagittal image surface, and Y indicates astigmatism on the meridional image surface. The subscripts 1, 2, and 3 of X and Y indicate the results at wavelengths of 656.28 nm, 587.56 nm, and 435.84 nm, respectively.

なお、表１において、ｎｄは屈折率を示し、νｄはｄ線に対するアッベ数を示している。また、面番号ｎ（ｎ＝１〜１４）は、それぞれ、物体側から数えて第ｎ番目の面を指している。また、面番号に＊印を付しているものは、その面が軸対称の非球面であることを示している。軸対称非球面の定義式は、次の数１式で表される。

In Table 1, nd represents the refractive index, and νd represents the Abbe number with respect to the d line. Further, the surface numbers n (n = 1 to 14) respectively indicate the nth surface counted from the object side. A surface number with an asterisk (*) indicates that the surface is an axisymmetric aspherical surface. The defining formula of the axisymmetric aspherical surface is expressed by the following equation (1).

ただし、
Ｙ：光軸からの高さのＹ座標
Ｚ：光軸からの高さのＺ座標
ｃ：面頂点での曲率
ｋ：コーニック係数
Ａ_i：非球面係数
である。ここで、第３面、第５面および第６面における非球面係数は、それぞれ、表２、表３および表４に示す通りである。なお、Ｅ−ｎ＝×１０^-nとする。

However,
Y: Y coordinate of height from optical axis Z: Z coordinate of height from optical axis c: Curvature at surface vertex k: Conic coefficient A _i : Aspheric coefficient. Here, the aspheric coefficients on the third surface, the fifth surface, and the sixth surface are as shown in Table 2, Table 3, and Table 4, respectively. Note that E−n = × 10 ⁻ⁿ .

また、自由曲面の定義式は、次の数２式で表される。

Moreover, the definition formula of the free-form surface is expressed by the following formula 2.

ただし、
Ｙ：光軸からの高さのＹ座標
Ｚ：光軸からの高さのＺ座標
ｈ：ｈ²＝Ｙ²＋Ｚ²
ｃ：面頂点での曲率
ｋ：コーニック係数
Ａ_ij：非球面係数
である。ここで、第１０面および第１１面における自由曲面係数は、それぞれ、表５および表６に示す通りである。なお、Ｅ−ｎ＝×１０^-nとする。

However,
Y: Y coordinate of the height from the optical axis Z: Z coordinate of the height from the optical axis h: h ² = Y ² + Z ²
c: curvature at the surface vertex k: conic coefficient A _ij : aspherical coefficient. Here, the free-form surface coefficients on the tenth surface and the eleventh surface are as shown in Table 5 and Table 6, respectively. Note that E−n = × 10 ⁻ⁿ .

  In this embodiment, f1 = 52.6 [mm], f2 = −48.5 [mm], f3 = 53.8 [mm], f4 = −105.0 [mm], f12 = 999.2 [Mm], f = 75.27 [mm]. Also, FNo = 4.74, m (reading magnification) = 0.203, H1 = 7.70 [mm], H2 = 7.02 [mm], H3 = 7.07 [mm], H4 = 3.49 [Mm].

As a result of designing as above,
| F × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | ≈ | −655 | = 655
This result satisfies the above-described conditional expression (1), and further satisfies the conditional expression (3). F12 = 999.2 [mm] , which satisfies the above-described conditional expression (2) and further satisfies the conditional expression (4). Therefore, it can be said that the reading lens of this embodiment can sufficiently suppress the deterioration of the reading lens performance due to the environmental temperature change.

1 is a cross-sectional view illustrating a schematic configuration of a reading lens according to an embodiment of the present invention. It is explanatory drawing which shows the aberrational figure in one Example of this invention.

Explanation of symbols

L1 lens
L2 lens
L3 lens
L4 Lens A Aperture F Front group R Rear group

Claims (3)

In order from the object side, a reading lens having a front group, a diaphragm, and a rear group,
The front group is from the object side,
A positive lens (L1) made of plastic material,
It consists of two pieces with a negative lens (L2) made of plastic material,
The rear group is in order from the object side.
A positive lens (L3) made of glass material,
It consists of two pieces with a negative lens (L4) made of plastic material,
And a reading lens satisfying the following conditional expressions (1) and (2):
600 <| f × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <900 (1)
However,
f: focal length of the entire system,
f1: Focal length of L1, H1: Incident height of axial FNo ray to L1,
f2: Focal length of L2, H2: Incident height of axial FNo ray to L2,
f4: Focal length of L4, H4: Incident height of axial FNo ray on L4,
800 mm <f12 <1200 mm (2)
However,
f12: Composite focal length of L1 and L2,
It is. The reading lens according to claim 1, further satisfying the following conditional expression (3):
| F × f1 / (H1) ² + f × f2 / (H2) ² + f × f4 / (H4) ² | <800 (3)
It is. Further, the following conditional expression (4) is satisfied: The reading lens according to claim 1 or 2,
900 mm <f12 (4)
It is.

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