JP5594154B2 - Reading lens, image reading apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to a reading lens, an image reading apparatus, and an image forming apparatus.

  An image reading apparatus used in an image forming apparatus such as a facsimile machine or a digital copying machine reduces image information to be read from a document with a reading lens, and forms an image on a solid-state image pickup device such as a CCD, thereby converting the image information into a signal. To do. In order to read the image information of the document in color, for example, a so-called three-line CCD in which light receiving elements having red, green and blue filters are arranged in three rows on one chip is used, and a reduced image of the document is formed on the light receiving surface. There is an optical system that forms a color image and separates it into three primary colors to convert color image information into a signal.

In general, a reading lens used in such an optical system is required to have a high contrast in a high spatial frequency region on the image plane and to have an aperture efficiency close to 100% up to the periphery of the angle of view.
Furthermore, in order to read a color original satisfactorily, it is necessary to match the image forming positions of red, green and blue colors on the light receiving surface in the optical axis direction, and chromatic aberration of each color must be corrected extremely well. . Further, in order to realize a reduction in the size of the optical system, it is necessary to “shorten the conjugate length of image formation of the reading lens”, and a lens having a wide angle of view is required.

Various reading lenses have been proposed.
Examples of wide angle of view include those described in Patent Documents 1 and 2.
These reading lenses described in Patent Documents 1 and 2 have a five-group five-element configuration and a relatively small number of elements, and have achieved a half angle of view of about 30 degrees, but both use an anamorphic lens, so However, this is not always easy, and the manufacturing cost of the anamorphic lens becomes high, and the reading lens tends to be expensive.

  Further, examples of the five-group five-element lens using an axisymmetric aspheric surface include those described in Patent Documents 3 and 4, and those described in Patent Documents 3 and 4 have a half angle of view of about 21.5 degrees. However, it is difficult to realize downsizing because the conjugate length is long.

  The applicant has also proposed various reading lenses having five groups and five elements having negative, positive, negative, positive, and positive structures (Patent Documents 5 to 8).

  In the present invention, the number of elements in the five groups is relatively small and the number of elements is relatively small, and “axial chromatic aberration” is well corrected in a wide wavelength range from the C line (656.27 nm) to the g line (435.83 nm). Angle of view: Wide angle of 38 degrees, field curvature is well corrected, F number: Large aperture of about 4.2, aperture efficiency is close to 100% to the periphery, and various aberrations are also well corrected, It is an object to realize a small and low-cost reading lens having high contrast in a high spatial frequency region, and an image reading apparatus and an image forming apparatus using the reading lens.

  As shown in FIG. 1, the reading lens of the present invention forms a reduced image of the original on the original placement surface (the left side of the contact glass shown in FIG. 1) on the light receiving surface of the image sensor. The first group having positive power, the second group having negative power, the third group having positive power, and the positive power sequentially from the object side (left side in FIG. 1) to the image side. A fourth group having a negative power and a fifth group having a negative power, each having a stop between the second group and the third group, and each of the first group to the fifth group is a single lens. 5 groups and 5 groups.

The first lens constituting the first group is a positive meniscus lens having a convex surface facing the object side.
The second lens constituting the second group is a negative lens.
The third lens constituting the third group is a positive lens.
The fourth lens constituting the fourth group is a positive lens.
The fifth lens constituting the fifth group is a biconcave lens.

  An aspherical surface is employed for at least the fifth lens.

Focal length of first group (first lens) with respect to e-line: f1, total focal length with respect to e-line of entire system: f, refractive index of first, third, and fourth lens materials that are positive lenses with respect to d-line Average: n- convex, average refractive index of the second and fifth lens materials that are negative lenses with respect to the d-line: n-concave, average of Abbe number with respect to the d-line of the positive lens material: ν-convex, the negative lens Average Abbe number for d-line of material: ν concave, condition:
(1) 1.1 <f1 / f <3.8
(2) −0.035 <n convex−n concave <−0.03
(3) 15.0 <ν convex−ν concave <15.3
Satisfied.
The reading lens according to claim 1, wherein the second lens is “a negative meniscus lens or a biconcave lens arranged with the convex surface facing the object side”, and the third lens is “a positive meniscus arranged with the convex surface facing the object side”. The lens or the biconvex lens ”and the fourth lens are preferably“ a positive meniscus lens or a biconvex lens disposed with the convex surface facing the object side ”(Claim 2).

In the reading lens described in claim 1 or 2, it is preferable that the fifth lens is made of plastic and has a “strip shape that is long in the main scanning direction of a line sensor that is an image sensor” (claim 3).
The “main scanning direction” is a direction in which minute photoelectric conversion elements (light receiving elements) are arranged in a line in the line sensor.

The image reading apparatus according to the present invention is an “image reading apparatus that reads a reduced image of a document on a document placement surface by forming an image on a light receiving surface of an image sensor with a reading lens”, and includes an illumination system and an imaging lens. And a line sensor (claim 4).
The “illumination system” illuminates the document on the document placement surface.
The “imaging lens” is an imaging lens for reducing and imaging the reflected light of the original illuminated by the illumination system, and the reading lens according to any one of claims 1 to 3 is used.

The “line sensor” photoelectrically converts the reduced document image formed by the reading lens.
The line sensor can have a one-line configuration or a three-line configuration.

  According to a fourth aspect of the present invention, there is provided an image reading apparatus having color separation means in the image forming optical path of the reading lens and reading document information in full color.

  An LED can be suitably used as the “illumination light source” in the image reading apparatus according to claim 4 or 5 (claim 6).

  An image forming apparatus according to the present invention comprises the image reading apparatus according to any one of claims 4 to 6 (claim 7).

  Supplementing the explanation, the condition (1) in claim 1 defines the power of the first group relative to the power of the entire system. If the upper limit is exceeded, the power of the first group becomes relatively weak, The total length of the lens system is large, which tends to increase costs. If the lower limit is exceeded, the power of the first group becomes relatively strong, and a large “negative spherical aberration” tends to occur, making correction in other groups difficult.

Condition (2) defines the “refractive index range” of the materials of the positive lens (first, third and fourth lenses) and the negative lens (second and fifth lenses) constituting the reading lens. When the upper limit is exceeded, the Petzval sum becomes small, and the image surface tends to fall to the positive side and the field curvature tends to increase. Beyond the lower limit, the Petzval sum increases, the image plane falls to the negative side, and the astigmatic difference increases.
Outside the range of the condition (2), “good imaging performance over the entire screen” cannot be obtained.

  Condition (3) is a condition for satisfactorily correcting “axial chromatic aberration”. If the upper limit is exceeded, the axial chromatic aberration will be overcorrected, and “on the shorter wavelength side than the dominant wavelength, the axial chromatic aberration will be larger on the positive side”. If the lower limit is exceeded, the on-axis chromatic aberration will be undercorrected, and “on the shorter wavelength side than the dominant wavelength, the on-axis chromatic aberration will become larger on the negative side”.

  As shown in the examples to be described later, in the power distribution of “positive / negative / positive / positive / negative” of the first to fifth lenses, by satisfying the above conditions (1) to (3), it is made compact. Low cost and good performance can be realized.

  By defining the lens shapes of the second lens, the third lens, and the fourth lens as in claim 2, it is possible to correct the aberration more satisfactorily while keeping the reading lens compact.

  As described in claim 3, the fifth lens is made of plastic and has a strip shape which is long in the main scanning direction, and has the following significance.

  Since the reading lens is used at “reduction magnification”, the arrangement position of the fifth lens is “near the imaging plane”. For this reason, the fifth lens is required to have a length approximately the same as the size of the line sensor in the longitudinal direction. For this reason, if the fifth lens is configured as a normal “circular lens”, the outer shape of the fifth lens increases, leading to an increase in the size of the image reading device, the lens cost of the fifth lens, and the reading lens / image reading. It is easy to increase the cost of the device.

  According to a third aspect of the present invention, the fifth lens is formed of an inexpensive plastic material, and the outer shape thereof is a “strip shape that is long in the main scanning direction”.

  Further, as described in claim 6, by using the LED as the “illumination light source”, the power consumption in the image reading apparatus can be reduced.

As described above, according to the present invention, a novel reading lens can be provided.
This reading lens has five positive, negative, positive, positive, and negative lens configurations. As shown in an example described later, the reading lens has a wide wavelength range from C line (656.27 nm) to g line (435.83 nm). "Chromatic aberration on the axis" is corrected well, half field angle: a wide field angle of about 38 degrees, field curvature is well corrected, F number: 4.2, large aperture, aperture efficiency up to 100 %, The aberration is also corrected well, and a good performance with a high contrast in a high spatial frequency region can be realized.

For this purpose, an image reading apparatus using a reading lens is compact and can read an image satisfactorily. An image forming apparatus using such an image reading apparatus can be made compact, and a good image can be formed.
According to the eighth aspect of the present invention, since an image reading apparatus using the reading lens is provided, an image is formed on the basis of good read image quality, so that a high-quality image forming apparatus can be obtained at low cost. is there.

It is a figure for demonstrating the lens structure of a reading lens. 1 is a diagram illustrating a lens configuration of Example 1. FIG. FIG. 6 is an aberration diagram for Example 1. 6 is a diagram illustrating a lens configuration of Example 2. FIG. FIG. 6 is an aberration diagram for Example 2. 6 is a diagram illustrating a lens configuration of Example 3. FIG. FIG. 6 is an aberration diagram for Example 3. It is a figure for demonstrating the shape of a 5th lens. It is a figure for demonstrating one Embodiment of an image reading apparatus. It is a figure for demonstrating one Embodiment of an image forming apparatus.

Embodiments of the invention will be described below.
2, 4 and 6 show three examples of embodiments of the reading lens. These embodiments specifically correspond to reading lenses of Examples 1 to 3 described later. In order to avoid complications, the symbols are shared in these drawings.

  2, 4, and 6, reference numeral CG indicates a contact glass on which an original is placed, and reference numeral F indicates a transparent parallel plate. The transparent parallel flat plate F represents various filters, a cover glass of an image sensor, etc. as one transparent parallel flat plate optically equivalent to these.

  The left side of these figures is the “object side of the reading lens”, that is, the document side, and the right side is the image side.

  As the reading lens, the first lens L1, the second lens L2, the third lens L3, the fourth lens L4, and the fifth lens L5 are arranged in this order from the object side, and the second lens group L2 and the third lens group L3 are arranged. A diaphragm S is arranged between the two.

The first lens L1 constituting the first group is a positive meniscus lens having a convex surface facing the object side.
The second lens L2 constituting the second group is a negative lens, the third lens L3 constituting the third group is a positive lens, and the fourth lens L4 constituting the fourth group is a positive lens. The fifth lens L5 constituting the fifth group is a biconcave lens.

  In the embodiment shown in FIG. 2 (corresponding to Example 1 described later), the second lens L2 is “a negative meniscus lens disposed with a convex surface facing the object side”, and the third lens L3 is a “biconvex lens”. The fourth lens L4 is “a positive meniscus lens disposed with a convex surface facing the object side”.

  In the embodiment shown in FIG. 4 (corresponding to Example 2 described later), the second lens L2 is “a biconcave lens, the third lens L3 is a“ biconvex lens ”, and the fourth lens L4 is“ a convex surface on the object side ”. “Positive meniscus lens”.

  In the embodiment shown in FIG. 6 (corresponding to Example 3 described later), the second lens L2 is “a biconcave lens, and the third lens L3 is“ a positive meniscus lens disposed with a convex surface facing the object side ”. The fourth lens L4 is a “biconvex lens”.

Next, one embodiment of an image reading apparatus using these reading lenses is shown in FIG.
In FIG. 8, a document 2 having an image to be read is placed flat on a “document placement surface” on the contact glass 1.

  The image reading unit 7 drawn at the bottom of the contact glass 1 has a box shape, and an illumination system 3, a reading lens 5, and a line sensor 6 are arranged in a predetermined positional relationship, and the object side connection of the reading lens 5 is performed. Folding mirrors M1 to M5 constituting the image optical path are arranged in a predetermined positional relationship.

  The illumination system 3 includes a tube lamp (xenon lamp, halogen lamp, etc.) that is long in a direction perpendicular to the drawing and a reflector, and illuminates the “slit-like portion that is long in the direction perpendicular to the drawing” of the document 2.

  Reflected light from the illuminated portion of the document 2 is sequentially reflected by the folding mirrors M1 to M5, and a reduced image of the document image is formed on the imaging surface of the line sensor 6 as the imaging element by the reading lens 5. As the reading lens 5, the one described in claims 1 to 3, specifically, any one of Examples 1 to 3 is used.

  The image reading unit 7 is moved in the direction of the arrow (to the right in the drawing) by a driving unit (not shown), and is displaced at a constant speed to a “position indicated by a broken line” to read the entire document information.

  The line sensor 6 that is an “imaging device” has “photoelectric conversion elements (6A, 6B, 6C) having red (R), green (G), and blue (B) filters as color separation means“ 3 on one chip. This is a “3-line CCD” arranged in a row, and converts the document image into an image signal as the document 2 is illuminated and scanned.

  In this way, reading of the document 2 is executed, and the color image of the document 2 is read by color separation into three primary colors of red, green, and blue.

  This image reading apparatus is an “apparatus that reads an image in full color”, and is provided with “color separation means (red, green, and blue filters provided in the three-line CCD) provided in the imaging optical path of the reading lens 5. Is included.

  In “color separation”, separately from the above, a color separation prism or filter is selectively inserted between the reading lens and the line sensor to separate the colors into R (red), G (green), and B (blue). This method may be performed by the method or the “method of illuminating the original by sequentially turning on the R, G, and B light sources (in this case, a line sensor having a single-row configuration”) may be used.

  It is also possible to read document information as “monochrome image information” without disposing a color separation element in the imaging optical path. In this embodiment, an example in which five folding mirrors M1 to M5 are used is shown, but the number of folding mirrors is not limited to this, and may be four or less, or six or more.

  In addition, although a long tube lamp such as a xenon lamp or a halogen lamp is used as the light source of the illumination system 3, LEDs (a plurality of LEDs are arrayed and used in a direction orthogonal to the drawing of FIG. 1). LED is excellent in energy saving because of low power consumption.When reading an original as a color image, a white LED is used, but when reading an original as a monochrome image, a “monochromatic LED” is used. Can be used.

  In the embodiment of FIG. 8, the fifth lens L5 used for the reading lens 5 has an outer shape of “a strip shape long in the main scanning direction” as shown in FIG. The left diagram of FIG. 9 is a perspective view, and the right diagram is a diagram showing a shape viewed from the optical axis direction.

An embodiment of the image forming apparatus will be described with reference to FIG.
The image forming apparatus includes an image reading apparatus 200 positioned at the upper part of the apparatus and an “image forming unit” positioned below the image reading apparatus 200. The portion of the image reading apparatus 200 is the same as that described with reference to FIG. 8, and the same reference numerals as those in FIG.

  The image signal output from the three-line line sensor (imaging means) 6 of the image reading apparatus 200 is sent to the image processing unit 120 and processed by the image processing unit 120 to generate a “writing signal (yellow, magenta, cyan, Signal for writing each color of black) ”.

  The image forming unit includes a photoconductive photosensitive member 110 formed in a cylindrical shape as a “latent image carrier”, and a charging roller 111 as a charging unit, a revolver type developing device 113, and a transfer belt around the photosensitive member 110. 114 and a cleaning device 115 are provided. As the charging means, a “corona charger” can be used instead of the charging roller 111.

  An optical scanning device 117 that receives a writing signal from the signal processing unit 120 and writes on the photosensitive member 110 by optical scanning performs optical scanning of the photosensitive member 110 between the charging roller 111 and the developing device 113. Yes.

  Reference numeral 116 denotes a fixing device, reference numeral 118 denotes a cassette, reference numeral 119 denotes a registration roller pair, reference numeral 122 denotes a paper feed roller, reference numeral 121 denotes a tray, and reference numeral S denotes a transfer sheet as a “recording medium”.

  When image formation is performed, the photoconductive photosensitive member 110 is rotated at a constant speed in the clockwise direction, the surface thereof is uniformly charged by the charging roller 111, and is subjected to exposure by optical writing of the laser beam of the optical scanning device 117. An electrostatic latent image is formed. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.

  “Image writing” is performed in the order of a yellow image, a magenta image, a cyan image, and a black image in accordance with the rotation of the photoconductor 110, and the formed electrostatic latent image is stored in each developing unit Y ( Development with yellow toner), M (development with magenta toner), C (development with cyan toner), K (development with black toner) are sequentially reversed and visualized as a positive image. The respective color toner images are sequentially transferred onto the transfer belt 114 by the transfer voltage application roller 114A, and the respective color toner images are superimposed on the transfer belt 114 to form a color image.

  The cassette 118 storing the transfer paper S is detachable from the main body of the image forming apparatus. When the cassette 118 is mounted as shown in the drawing, the uppermost sheet of the stored transfer paper S is fed by the paper feed roller 122. The leading edge of the fed transfer sheet S is caught by the registration roller pair 119.

  The registration roller pair 119 feeds the transfer sheet S to the transfer unit at the timing when the “color image by toner” on the transfer belt 114 moves to the transfer position. The transferred transfer paper S is superimposed on the color image at the transfer portion, and the color image is electrostatically transferred by the action of the transfer roller 114B. The transfer roller 114B presses the transfer sheet S against the color image during transfer.

  The transfer sheet S on which the color image has been transferred is sent to the fixing device 116, where the color image is fixed, passes through a conveyance path by a guide means (not shown), and is discharged onto the tray 121 by a pair of paper discharge rollers (not shown). The Each time each color toner image is transferred, the surface of the photoreceptor 110 is cleaned by the cleaning device 115 to remove residual toner, paper dust, and the like.

Three specific examples of the reading lens will be described below.
The meanings of symbols in each embodiment are as follows.

f: Composite focal length of e-line in the entire system FNo: F number m: Reduction ratio ω: Half angle of view (degrees)
Y: object height ri (i = 1 to 11): radius of curvature of the i-th surface (including the stop surface) counted from the object side di (i = 1 to 9): i-th surface counted from the object side interval
nj (j = 1 to 5): Refractive index of the material of the jth lens counted from the object side
vj (j = 1 to 5): Abbe number of the material of the j-th lens counted from the object side rc1: radius of curvature on the object side of the contact glass rc2: radius of curvature on the image side of the contact glass rc3: object on a transparent parallel plate Radius of curvature rc4: radius of curvature of the image side of the transparent parallel plate dc1: thickness of the contact glass
dc3: Thickness of transparent parallel plate nc1: Refractive index of material of transparent parallel plate nc3: Refractive index of material of contact glass vc1: Abbe number of material of contact glass vc3: Abbe number of material of transparent parallel plate nd: d line Refractive index for ne: Refractive index for e-line
f1: focal length with respect to e-line of the first lens
n-convex: average nd of lenses having positive refractive power n-concave: average nd of lenses having negative refractive power ν-convex: average nd of lenses having positive refractive power ν-concave: negative refractive power Average of nd of lens having these symbols are as shown in FIG. In FIG. 1, the refractive index n is indicated by the capital letter “N”.

  The aspherical surface is the distance from the tangent plane at the aspherical vertex of the aspherical surface at the height Y from the optical axis: X, the height from the optical axis: Y, the paraxial radius of curvature: R, the conic constant: K, the aspherical coefficient : Represented by the following well-known formula using A4, A6, A8, and A10.

X = (1 / R) × Y ² / [1 + √ {(1− (1 + K) × (Y / R) ² )}]
+ A4 × Y ⁴ + A6 × Y ⁶ + A8 × Y ⁸ + A10 × Y ¹⁰
"Example 1"
Example 1 has the lens configuration shown in FIG.
f = 19.305, F = 4.2, m = 0.11012, Y = 152.4, ω = 38.3 ° Table 1 shows the data of Example 1.

"Aspherical surface"
The aspherical data is shown in Table 2.

In the above aspherical notation, for example, “5.85626E-12” means “5.85626 × 10 ⁻¹² ”. The same applies to the following other embodiments.

Aberration diagrams relating to Example 1 are shown in FIG.
In the aberration diagrams, “e” is e-line (546.07 nm), “g” is g-line (436.83 nm), “c” is c-line (656.27 nm), “F” is F-line (486.13 nm) ). Further, in the spherical aberration diagram, “broken line indicates sine condition”, and in astigmatism diagram, “solid line indicates sagittal ray and dotted line indicates meridional ray”. The same applies to the aberration diagrams regarding the other examples.

"Example 2"
Example 2 has the lens configuration shown in FIG.
f = 19.275, F = 4.2, m = 0.11012, Y = 152.4, ω = 38.3 °
The data of Example 2 is shown in Table 3.

"Aspherical surface"
Table 4 shows the aspherical data.

  FIG. 5 shows aberration diagrams related to Example 2.

"Example 3"
Example 3 has the lens configuration shown in FIG.
f = 19.456, F = 4.2, m = 0.11012, Y = 152.4, ω = 38.1 °
The data of Example 3 is shown in Table 5.

"Aspherical surface"
Table 6 shows the aspherical data.

  FIG. 7 shows aberration diagrams related to Example 3.

"Parameter values for conditional expressions"
Table 7 shows the parameter values of the conditions (1) to (3) in Examples 1 to 3.

  In each example, “axial chromatic aberration” is well corrected in a wide wavelength range from the C line (656.27 nm) to the g line (435.83 nm), and the half field angle is about 38 degrees. Good field curvature is corrected, F number is 4.2, large aperture, aperture efficiency is close to 100% to the periphery, and aberrations are also well corrected and high contrast in high spatial frequency range. Has been realized.

L1 first lens
L2 second lens
L3 3rd lens
L4 4th lens
L5 5th lens
S Aperture

Japanese Patent No. 3862446 JP 2002-244033 A Japanese Patent No. 3939908 Japanese Patent No. 4496231 Japanese Patent Laid-Open No. 4-191809 JP-A-4-191810 Japanese Patent Laid-Open No. 4-181909 JP-A-4-184409

Claims (7)

A reading lens for forming a reduced image of a document on a document placement surface on a light receiving surface of an image sensor, a first group having a positive power sequentially from the object side to the image side, a negative power A second group having a positive power, a third group having a positive power, a fourth group having a positive power, and a fifth group having a negative power, and a diaphragm between the second group and the third group. The first group to the fifth group each have a five-group five-element configuration including one lens.
The first lens constituting the first group is a positive meniscus lens having a convex surface facing the object side,
The second lens constituting the second group is a negative lens,
The third lens constituting the third group is a positive lens,
The fourth lens constituting the fourth group is a positive lens,
The fifth lens constituting the fifth group is a biconcave lens,
Having an aspheric surface in at least the fifth lens;
Focal length for the first group e-line: f1, total focal length for the entire system e-line: f, average refractive index of the positive, first, third, and fourth lens materials for the d-line: n-convex , the average of the refractive index for the first 2, d line of the material of the fifth lens is a touch lens: n concave, the average Abbe number to the d-line of the material of the positive lens: [nu convex, the material of the negative lens d Average Abbe number for line: ν concave, condition:
(1) 1.1 <f1 / f <3.8
(2) −0.035 <n convex−n concave <−0.03
(3) 15.0 <ν convex−ν concave <15.3
A reading lens characterized by satisfying The reading lens according to claim 1.
The second lens is a negative meniscus lens or biconcave lens arranged with the convex surface facing the object side, the third lens is a positive meniscus lens or biconvex lens arranged with the convex surface facing the object side, and the fourth lens is an object A reading lens, which is a positive meniscus lens or a biconvex lens disposed with a convex surface facing the side. The reading lens according to claim 1 or 2,
A reading lens, wherein the fifth lens is made of plastic and has a strip shape that is long in a main scanning direction of a line sensor that is an image sensor. An image reading apparatus that reads a reduced image of a document on a document placement surface by forming an image on a light receiving surface of an image sensor with a reading lens,
An illumination system that illuminates the original on the original placement surface, an imaging lens that reduces and forms an image of the reflected light of the original illuminated by the illumination system, and a reduced original image formed by the imaging lens is photoelectrically converted. And a line sensor
An image reading apparatus using the reading lens according to any one of claims 1 to 3 as the imaging lens. The image reading apparatus according to claim 4.
An image reading apparatus having color separation means in an imaging optical path of a reading lens and reading document information in full color. The image reading apparatus according to claim 4 or 5,
An image reading apparatus characterized in that a light source of an illumination system is an LED.   An image forming apparatus comprising the image reading apparatus according to any one of claims 4 to 6.

Images