JP2002236328A - Document reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a document reader which can avoid the increase of cost and assembling man-hours due to adoption of a sagittal stopper and realize further improvement of the read image quality. SOLUTION: The document reader is provided with: an irradiation means which irradiates the document with light; an imaging lens which forms an image of light reflected from the document and which is designed so that the resolution of the subscanning direction within the document reading wavelength band in the viewing angle in the vicinity of the document edge is lower than the resolution which shows the minimum value within document reading wavelength bands and with the viewing angle inside the vicinity of the document edge; a light shielding means which shields part of the reflected light images so that the pupil diameter of the subscanning direction in the reflected light image from the vicinity of the document edge becomes smaller; and a photoelectric conversion means which converts into an electric signal the light which is partly shielded with the light shielding means and is imaged with the lens. The light shielding means is disposed outside the imaging lens and is constituted so that it is movable while maintaining a fixed distance from the imaging lens.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a document reading apparatus for optically reading a document image, and more particularly to a document reading apparatus suitable for a color document reading apparatus and a color copier equipped with the color document reading apparatus. It concerns the device.

[0002]

2. Description of the Related Art Conventionally, as a document reading apparatus of this type,
The one configured as shown below is generally used.
As shown in FIG. 23, this document reading device illuminates an image of a document (not shown) placed on a platen glass 101 with a light source 102 and a reflector 103, and reflects a reflected light image from the document on a full-rate mirror. An imaging lens 107 via the 104 and the half-rate mirrors 105 and 106
Thus, a reduced image is formed on an image reading element 108 composed of a CCD (Charge Coupled Device) sensor or the like, and an image of a document is read by the CCD sensor 108 serving as the image reading element. The optical system of the document reading apparatus described above, if the optical path folded by the full-rate mirror 104 and the half-rate mirrors 105 and 106 is linearly represented along the optical axis, as shown in FIG. Can be expressed as a reduction optical system that forms an image on the CCD sensor 108 in the image space by the imaging lens 107. Note that FIG.
Reference numeral 110 in FIG. 3 denotes a platen cover for holding down a document.

[0003] In recent years, with the diversification of documents handled in offices and the like, such document reading apparatuses have been required to have higher resolution and higher image quality.
In addition, in addition to the above, a document reading device used in a copying machine is required to be capable of high-speed reading, wide width (for A3 size), and handling of document floating at a binding margin of a book.
Capturing various requirements of these document reading devices physically and optically, high Nyquist frequency (quantization limit frequency) and high MTF (Modulation Transfer Function)
on) Characteristics, high sensitivity of the sensor, wide angle of view, and deep depth of focus are required. In order to design a manuscript reading device that satisfies these requirements, after determining the characteristics of the sensor and the conjugate length of the manuscript reading device, the specifications of the imaging optical system include the MTF height and focus. Depth.

Here, as is well known to those skilled in the art, the MTF is a spatial frequency spectrum of the intensity distribution of an object (subject) O (μ, ν) and a spatial frequency spectrum of the intensity distribution of an image is I (Μ, ν), I (μ, ν) = O
The relationship of (μ, ν) H (μ, ν) holds, and H (μ, ν)
ν) is generally a complex number whose absolute value is MTF (Modulati
on Transfer Function) and the phase is PTF (Phas
e Transfer Function) (see RIKEN). Therefore, a high value of the MTF means that an image of a document as an object (subject) is formed on the CCD sensor 108 which receives light as an image at a high transmission rate over a wide spatial frequency without retrieving. Means you can do it.

Further, in the case of a color copying machine, a color scanner, or the like, in addition to the above-mentioned conditions, chromatic aberration of the imaging lens also becomes a problem. The chromatic aberration includes axial chromatic aberration, which is a shift in the optical axis direction, and chromatic aberration of magnification, which is a shift in the longitudinal direction of the CCD sensor, and has different adverse effects on the evaluation of the imaging optical system. Specifically, in the case of a single lens,
As shown in FIG. 25, the focus of each color of B (blue), G (green), and R (red) in the imaging optical system is
When G (green) light is used as a reference, B having a short wavelength
The light of (blue) is shifted to the near side, and the light of long wavelength (R) is shifted to the far side in the defocus direction. Even if correction is performed by combining a plurality of lenses, the axial chromatic aberration slightly remains, and as shown in FIG. 26, the peaks of the MTFs of B (blue), G (green), and R (red) are defocused. Since the color shifts in the direction, it is necessary to balance each color, and as a result, the MTF value must be adjusted to a relatively low color. As a result, the MTF is degraded and the image quality is degraded. On the other hand, the chromatic aberration of magnification indicates that the imaging position of each color is C
As shown in FIG. 27, the light received by the pixels of the CCD sensor corresponding to the document causes a difference in the amount of received light depending on the color, and as a result, the output of the CCD sensor differs depending on the color. Causes color misregistration.

The mechanism of the above-mentioned MTF characteristic is very complicated because, in addition to chromatic aberration, monochromatic aberrations degrade the chromatic aberration. The evaluation items for the MTF include the height of the MTF on the CCD sensor surface,
There is an allowable range (depth of focus) when the document floats, and a color balance (ΔMTF) at each angle of view. Here, the color balance (ΔMTF) at each angle of view is the difference between the MTF value of the color with the largest MTF value and the minimum color of the MTF value among the BGRs at each angle of view.

[0007] Among the evaluation items of the MTF, an increase in ΔMTF and an increase in chromatic aberration of magnification have different physical (optical) behaviors, but have similar results from the viewpoint of output from the CCD sensor. That is, the signal values to be output from the three color pixels located at the same position along the longitudinal direction of the CCD sensor change depending on the color,
As a result, there occurs a phenomenon that the color information obtained by combining them changes from the original value. The change in the color information means that the color reproducibility of the output image of the color copying machine with respect to the original is deteriorated. In the case of a color copying machine, information of a document read in three colors of B (blue), G (green), and R (red) by a document reading device is converted into Y (yellow), M ( There is a step of converting to magenta), C (sian), and K (black). If the read original BGR image data is shifted by an unspecified amount, the color shift is corrected when converting to YMCK. It is difficult to do.

[0008] Another drawback inherent to magnification chromatic aberration is the coloring of edges in the same color area. When copying and outputting an image image whose contour is often unclear, such as a photograph, slight color shift is often not bothersome. However, in the case of a document composed of a combination of characters and figures such as a business document, it is very conspicuous that only the edges of the characters and figures in the main scanning direction have different colors due to chromatic aberration of magnification, which gives a bad impression to the user. Will be given. When there is magnification chromatic aberration, for example, as shown in FIG.
When the original is read by the pixel of interest of the CD sensor, the original color of the original is K (black), so that the output values of B (blue), G (green), and R (red) are all equal and almost close to zero. Although the output value of the target pixel of the CCD sensor should be a value, the output value of B (blue) is the highest due to chromatic aberration of magnification, and the output value of G (green) is an intermediate value. Red) has the lowest output value. Therefore, the K value read by the target pixel of the CCD sensor is
The edge portion of the (black) image is read as a color such as blue mixed with green, for example.

On the other hand, in the case of the color balance (ΔMTF) at each angle of view, the MTF value at each angle of view is the difference between the MTF values of the maximum color and the minimum color. Despite the differences, only the edges are not different colors. In this respect, it can be said that the chromatic aberration of magnification has a greater adverse effect on the image quality of the original read by the original reading apparatus than the ΔMTF.

Furthermore, in the case of a digital color copying machine, image processing is generally made different depending on whether the original is a text or an image such as a photograph. If the original is text, it is easier to read if the contrast between the text and the background is clear. Therefore, in the case of a text original, image processing for emphasizing the edge portion of the text is performed. On the other hand, in the case of an image document, in contrast to the above-described text document, a smooth color change gives a better visual impression. Therefore, in the case of an image original, image processing is performed such that the color gradually changes. As described above, in order to change the manner of image processing depending on whether the original is a text or an image, a T / I (text / image) separation processing function is often incorporated in an image processing unit of a copying machine.

When the read color is black or gray, it is better to express the image with one K (black) toner than to express black or gray by superposing Y, M and C toners. Gives a beautiful output. Therefore, many color copiers have a black determination function in addition to the T / I separation function. At this time, when the black character is separated from the image information, if the output from the CCD sensor deviates from black due to the aberration, a serious problem occurs. When the T / I separation processing error and the black determination error occur simultaneously, the black character portion of the document is colored and output with blur, giving the user an impression that the output is far from the document.

Therefore, in an image processing apparatus used in combination with a conventional document reading apparatus, a color c ^* determined to be K (black) in the L ^* a ^* b ^* color space in the CIE color system ^. By setting the threshold level of (= √ (a ^{* 2} + b ^{* 2} )) to a relatively high value, L ^*
The fact is that the color of a slightly wide area in the color space of a ^* b ^* is determined to be black. Therefore, CIE
In the color system, despite being originally a color other than black,
The area reproduced as the original color has become narrow, and color reproducibility has to be reduced.

In particular, in recent years, with the development and spread of personal computers and presentation material creation tools, full-color originals including high-definition character information have begun to overflow, and the frequency of copying such originals has been increasing. Therefore, in the above-described conventional document reading apparatus, it is presumed that problems caused by chromatic aberration and ΔMTF in the imaging optical system frequently occur.

Therefore, as a measure for solving the above-mentioned various problems, in addition to image processing, in designing the imaging lens of the original reading apparatus, the material and lens parameters of the lens are set so as to minimize the aberration. It is common to make decisions and balance properties. According to the study of the present inventors, in order to perform the above-described T / I separation processing and black determination processing satisfactorily, color shift of pixels due to chromatic aberration of magnification is suppressed to within 0.1 pixel, and ΔMTF is reduced. About 20%
Simulation results indicate that it is necessary to keep within. However, when attempting to correct chromatic aberration of magnification when designing a lens, the curvature of field becomes rather large in order to match the imaging magnification for each color of BGR, and the MTF deteriorates. For this reason, even if anomalous dispersion glass effective for measures against chromatic aberration is used, there is a limit in improving the characteristics of a single lens. In addition, when anomalous dispersion glass is used, not only is the cost three to five times higher than ordinary glass materials, but also as a harmful substance, because the refractive index and Abbe number of a plurality of wavelengths are adjusted. Lead and arsenic, which are symmetrical in reduction, must be used, which is not preferable from the viewpoint of safety.

In general, when trying to reduce aberrations at the stage of designing a lens, the aperture is reduced or the angle of view is reduced. Due to space constraints, it cannot be changed much. Therefore, a method of reducing aberration by reducing the aperture of the imaging lens (darkening the lens) is often adopted. However, due to the S / N ratio of the CCD sensor, if the lens is darkened while maintaining high-speed and high-resolution reading, the amount of light from the lamp that illuminates the original must be increased in order to secure the required amount of light. Instead, power consumption is increased.

Normally, the electric power that can be taken from an office outlet is 1.5 kVA. However, in the case of an electrophotographic copying machine, since heat is used for fixing a toner image, a heater portion that generates this heat is used. Requires very large power. Even in the current 400 dpi reading, the power of the color electrophotographic copying machine is a value of just 1.5 kVA. For this reason, it is desirable to reduce the electric power allocated to the document reading portion as much as possible.

Further, when the resolution is increased to 600 dpi while maintaining the copying speed, the area of one sensor pixel is reduced to about 2/3 (400/600), and the relative sensor light receiving amount is less than half. become ⁽² 2/3 ² = 4/9).
In addition, since the amount of data increases, the video rate at which the pixel output of the CCD sensor is transferred also increases, and the power also tends to increase. Therefore, it is not currently possible to darken the lens and increase the power of the lamp. It is possible. Rather, the lens must be brighter than in the prior art to compensate for less than half the amount of light received.

On the other hand, since the cost of the memory has recently decreased, the read information is temporarily stored using the memory, and is asynchronous with the copy output side (so-called printer side), so that the read speed is reduced. A method has been considered in which the reading speed is slowed down, or full-color reading is performed in one scan, thereby reducing the speed of one reading.

However, if the read speed is reduced by temporarily storing the read information using a memory and making the read information asynchronous with the copy output side (so-called printer side), a black-and-white copy is performed in a color copying machine. The time taken tends to be slow because of the adoption of the same sequence as for color, but even when making a black and white copy in a color copier, it is necessary to achieve a copy speed comparable to that of a black and white dedicated machine. Therefore, ideally, it is desired that the reading speed of the document reading device be high.

There are other reasons for designing the lens as bright as possible. In a black-and-white copying machine, recently, a xenon lamp with low power consumption has been used instead of a halogen lamp conventionally used as a lamp for illuminating a document. Naturally, a full-color copying machine must be designed in consideration of the use of a xenon lamp in order to follow up from the viewpoint of energy saving. However, at present, there is an upper limit to the amount of light that can be output from a xenon lamp, so that a dark lens can only read data with a poor SN ratio.

As a countermeasure against the chromatic aberration of magnification, a technique of electrically correcting the center of gravity of the output of the CCD sensor is also known. However, this causes deterioration of the MTF as an electric signal, which affects the character reproduction. Will be given. In this case, even if the resolution is increased, the image quality will not be improved, and the catalog specification will increase from 400 dpi to 600 dpi, so that it looks as if the image quality is high. .

Further, correcting an image forming characteristic that cannot be covered by a lens with an electric circuit or another optical element surely leads to an increase in cost unless the cost of the lens is reduced by using these elements.

Therefore, as a technique for improving the optical system of the document reading apparatus without increasing power consumption due to an increase in the light amount of a lamp for illuminating the document, a lens characterized by high imaging characteristics is used. Many patents have been filed,
Some of them are already licensed. For example, Japanese Patent No. 2729039 discloses that 4 groups 6
Double Gaussian type lens with F number 4.5
Is disclosed.

However, in a normal design method, F
While ensuring the brightness of 4.5, the color shift of the pixel due to the chromatic aberration of magnification is suppressed to within 0.1 pixel, and the ΔMTF is 20 or less.
% Has already reached the design limit, and the technology disclosed in Japanese Patent No. 2729039 also disclose
This cannot be achieved.

In order to solve this problem, the present inventor disclosed in Japanese Patent Application Laid-Open No. H11-191830 designed the chromatic aberration of magnification to be less than half that of the conventional art. MTF in the sub-scanning direction of the document edge
A technique has been disclosed in which the MTF in the sub-scanning direction, which is degraded due to this concentration, is improved by using a light blocking member (sagittal stopper) that narrows the lens pupil in the sub-scanning direction. .

[0026]

However, the prior art described above has the following problems. That is, in the case of the technique disclosed in Japanese Patent Application Laid-Open No. H11-191830, a light-blocking member (sagittal stopper) that narrows the lens pupil in the sub-scanning direction is used. It is disposed as a separate member on the document side. Therefore, when adjusting the optical system at the time of assembling, in addition to adjusting the position of the lens and the CCD sensor, it is also necessary to separately adjust the position of the sagittal stopper (brightness adjustment). There was a problem that it increased. Further, in the case of the above-mentioned document reading apparatus, the increase in cost due to the provision of the sagittal stopper as a separate member cannot be ignored.

Further, in the case of the technique disclosed in Japanese Patent Application Laid-Open No. H11-191830, the lens parameters are set to specific values as described in claims 6 and 7. Although the reading image quality is greatly improved, the resolution is increased to 400 to realize a higher resolution.
When the resolution is increased from dpi to 600 dpi, the lens does not completely correct various aberrations, and from this point, there is a problem that it takes time for optical adjustment.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to avoid an increase in the number of assembling steps and an increase in cost by using a sagittal stopper. Can be
It is another object of the present invention to provide a document reading apparatus capable of further improving the read image quality.

[0029]

According to a first aspect of the present invention, there is provided an irradiating means for irradiating a document with light, and a member for forming a reflected light image from the document, wherein the member is provided near an edge of the document. An imaging designed such that the resolution in the sub-scanning direction of the document reading wavelength range at the angle of view has a lower resolution characteristic than the resolution indicating the lowest value of the document reading wavelength range at the angle of view inside the vicinity of the document edge. A lens, a light-shielding unit that shields a part of the reflected light image so that a pupil diameter in the sub-scanning direction in the reflected light image from the vicinity of the document end is reduced, and a part of which is shielded by the light-shielding unit and the lens And a photoelectric conversion unit for converting the light imaged in the step (c) into an electric signal, wherein the light shielding unit is disposed outside the imaging lens and maintains a constant distance from the imaging lens. So that it can be moved A document reading apparatus characterized by constituting the.

According to a second aspect of the present invention, there is provided an irradiation means for irradiating an original with light, and a member for forming a reflected light image from the original, the original reading wavelength at an angle of view near an end of the original. An imaging lens designed so that the resolution in the sub-scanning direction of the area is lower than the resolution indicating the lowest value of the original reading wavelength range at an angle of view inside the vicinity of the original end; A light-shielding member that shields a part of the reflected light image so that a pupil diameter in the sub-scanning direction in the reflected light image from the vicinity is reduced, and light that is partially shielded by the light-shielding unit and imaged by the lens. And a photoelectric conversion unit for converting the light into an electric signal, comprising a holding member for holding the imaging lens, wherein the light shielding member is integrally formed with the holding member. It is a document reading device.

Further, according to a third aspect of the present invention, in the document reading apparatus according to the first or second aspect, the light shielding member is provided outside the exit side of the imaging lens. is there.

According to a fourth aspect of the present invention, the distance between the light blocking member and the imaging lens is set as short as possible. It is a reading device.

According to a fifth aspect of the present invention, the image forming lens has a rotationally symmetric system with respect to the optical axis. It is a reading device.

Still further, the invention described in claim 6 is as follows.
Irradiating means for irradiating the original with light, and a member for forming a reflected light image from the original, wherein the resolution in the sub-scanning direction of the original reading wavelength band at an angle of view near the original end is greater than that near the original end. An imaging lens designed to have a resolution characteristic lower than the resolution indicating the lowest value of the original reading wavelength range at the inner angle of view, and a pupil diameter in a sub-scanning direction in a reflected light image from near the original end. A document comprising: a light-shielding member that shields a part of the reflected light image so as to reduce the size; and a photoelectric conversion unit that partially converts the light imaged by the lens into an electric signal, the light being partially shielded by the light-shielding unit. In the reading device, the imaging lens is a Gauss-type lens obtained by combining four or more lenses, and has a refractive index and dispersion at d-line,
An original reading apparatus includes a material having the following three characteristics. 1. Refractive index 1.658 Dispersion 50.9 2. Refractive index 1.639 Dispersion 55.5 3. Refractive index 1.648 Dispersion 33.8

Further, according to a seventh aspect of the present invention, in the case where the imaging lens is a Gaussian type lens in which six lenses in four groups are combined, and the lenses are numbered sequentially from the object side, The first characteristic, refractive index 1.658 dispersion
The lens having the characteristic of 50.9 is replaced with the first lens and the sixth lens.
The lens has a second characteristic, a refractive index of 1.639 and a dispersion of 5.
The lenses having the characteristic of 5.5 are referred to as a second lens and a fifth lens, and the third characteristic, the refractive index of 1.648 and the dispersion of 33.8
7. The document reading apparatus according to claim 6, wherein a lens having the following characteristics is used as the third lens.

According to the invention described in claim 8, in the imaging lens, ri is a radius of curvature of the i-th surface, di is a distance between the i-th surface and the (i + 1) surface, and ni is d of the i-th lens. The refractive index at the line, νi, is the Abbe number at the d-line of the i-th lens. r6 -16.27 d6 0.90 n4 1.603 ν4 38.0 r7 -220.00 d7 6.72 n5 1.639 ν5 55.5 r8 -22.87 d8 0.51 r9 -172.4 d9 4.92 n6 1.658 ν6 50.9 r10 -38.80 It is a document reading device.

[0037]

The sagittal stopper (light shielding member) basically has the effect of improving the MTF characteristics in the main scanning direction and the MTF characteristics in the sub-scanning direction with almost no effect on the chromatic aberration of magnification. Further, since the sagittal stopper limits the spread of the light beam only in the sub-scanning direction, there is a merit that the loss of the light amount is smaller than that of a normal circular stop.

Even if the specifications can be satisfied by the characteristic value of the lens alone, this method can reduce the cost of the glass material used for the lens and reduce the total cost.

Further, a sagittal stopper (light shielding member)
Is provided outside the exit side of the imaging lens, the light flux reduced and imaged by the imaging lens is focused in the sub-scanning direction by the sagittal stopper, so that the sagittal stopper itself can be downsized. .

Further, the distance between the sagittal stopper and the imaging lens is set as short as possible. Preferably, no space is provided between the sagittal stopper and the lens barrel supporting the imaging lens. Accordingly, it is possible to reliably prevent stray light that has entered from the gap between the imaging lens and the sagittal stopper from being reflected on the exit side surface of the imaging lens and adversely affecting the optical system.

Further, by rotating the lens with respect to the optical axis, it is possible to assemble the lens in such a manner that the influence of tolerance such as eccentricity or surface inclination hardly appears on MTF and other performances.

[0042]

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

Embodiment 1 FIG. 1 is a schematic diagram showing an original reading apparatus according to Embodiment 1 of the present invention.

As shown in FIG. 1, the original reading apparatus
An original reading apparatus main body 1 having an open upper surface is provided, and a platen glass 2 for mounting an original (not shown) is attached to an upper opening of the original reading apparatus main body 1. A document (not shown) is placed on the platen glass 2 at a predetermined position with an image to be read facing downward. A platen cover 3 for pressing a document onto a platen glass 2 is provided at an upper portion of the document reading device main body 1 so as to be freely opened and closed.

A lamp 4 as an illuminating means for illuminating an image of an original (not shown) placed on the platen glass 2 and a light radiated from the lamp 4 are provided inside the original reading apparatus main body 1. A reflector 5 for efficiently condensing the document surface, a full-rate mirror 6 for horizontally reflecting the reflected light image from the document, and two half-rate mirrors for reflecting the reflected light image from the full-rate mirror 6 in a folded manner. 7, 8 and these two half-rate mirrors 7, 8
Imaging lens 9 for reducing and imaging the reflected light image from the camera, a sagittal stopper 11 as a light shielding means, and a CCD (Cha) for converting the light image formed by the imaging lens 9 into an electric signal.
rge Coupled Device) A line image sensor 12 as a photoelectric conversion unit including an image sensor and the like is provided. As the imaging lens 9, for example, a double Gauss type lens is used. In order to read a color image as the CCD image sensor 12, the color image is converted into B (blue), G (green),
A color CCD image sensor that reads three colors (red) is used.

The lamp 4 and the reflector 5 and the full-rate mirror 6 are mounted on a full-rate carriage (not shown), and the full-rate carriage is configured to move at a predetermined speed V along the sub-scanning direction. I have. The two half-rate mirrors 7 and 8 are
It is mounted on a half-rate carriage (not shown),
The half-rate carriage is also configured to move at half the speed of the full-rate carriage 1/2 V in the sub-scanning direction. As a result, the original placed on the platen glass 2 has a lamp 4, a reflector 5 and a full rate mirror 6 moving at a predetermined speed,
By using the two half-rate mirrors 7 and 8, the entire image is scanned and exposed on the CCD image sensor 12 via the imaging lens 9 without changing the optical path length.

In the document reading apparatus, as shown in FIG. 1, an image of a document placed on the platen glass 2 is illuminated by light emitted from a lamp 4 and collected by a reflector 5, The reflected light image from the original is reduced and formed on the CCD image sensor 12 by the imaging lens 9 via the full rate mirror 6 and the half rate mirrors 7 and 8. At this time, a part of the reflected light image from the document, which is reduced and formed on the CCD image sensor 12 by the image forming lens 9,
1 is shaded. At this time, the CCD image sensor 12
Performs photoelectric conversion in accordance with the intensity of incident light for each pixel, thereby obtaining an image signal (RG
B signal).

The optical system of the above-described document reading apparatus can be configured such that the optical path turned back by the full-rate mirror 6 and the half-rate mirrors 7 and 8 is linearly represented along the optical axis ax.
As shown in FIG. 2, the image of the document 13 in the object space is
The imaging lens 9 can be expressed as a reduction optical system that forms an image on the CCD image sensor 12 in the image space.

By the way, in addition to demands for higher resolution and higher image quality, the original reading apparatus configured as described above has
High-speed reading, wide width (A3 size compatible), and handling of document floating at the binding margin of the book are required. However, if these requirements are physically and optically captured, a high Nyquist frequency (a limit frequency of quantization) ), High MTF (ModulationTr
ansfer Function) characteristics, high sensitivity of sensor, wide angle of view,
A deep depth of focus will be required. Therefore, in order to design a document reading device that satisfies these requirements,
After determining the characteristics of the CCD image sensor 12 and the conjugate length of the document reading device, the specifications of the imaging optical system include the height of the MTF and the depth of focus.

Next, basic characteristics of the imaging lens 9 which is a main component for determining the MTF and the depth of focus of the above-mentioned imaging optical system will be described with reference to FIG.

First, in the image forming lens 9, when forming an image at a position distant from the optical axis ax, that is, in the vicinity of the document edge, a line in the tangential direction (tangential direction) of a concentric circle centered on the optical axis ax. The pair information and the line pair information in the radial direction of the concentric circle (radial direction) have different MTF (Modulation Transfer Function) characteristics indicating the resolution of the lens. When such a lens 9 is used in a document reading apparatus using the line image sensor 12, the resolution (MTF) in the tangential direction (MTF) is reduced.
And the resolution in the radial direction corresponds to the resolution of the line image sensor 12 in the sub-scanning direction (transverse direction of the sensor). Due to the characteristics of the imaging lens 9, the resolution in the radial direction tends to be lower than the resolution in the tangential direction. Therefore, in the line image sensor 12, the resolution in the sub-scanning direction of the image at a position distant from the optical axis ax often decreases.

Light emitted from an object point off the optical axis is transmitted to the imaging lens 9.
, The light beam that has passed through the meridional surface of the entrance pupil does not deviate from the meridional surface even after exiting the lens 9 due to the axial symmetry of the imaging lens 9. Therefore, the resolution of the light passing through the meridional surface in the sub-scanning direction does not decrease.

From this, when the light beam emitted from the imaging lens 9 is narrowed down in the sagittal direction orthogonal to the meridional surface, the geometrical aberration is reduced and the imaging state is improved. In addition, by limiting the spread of the imaging light beam in the sagittal direction, the light beam in the image space becomes narrower, so that the spread of the point image at a position deviating from the ideal image plane becomes smaller, and the image formation state in the radial direction Of focus can be increased.

Therefore, in the first embodiment, as shown in FIG. 1, the reflected light from the vicinity of the end of the original is reduced so that the exit pupil diameter in the sub-scanning direction after exiting the imaging lens 9 is reduced. Is provided with a sagittal stopper 11 for blocking a light beam in the sagittal direction of the imaging lens 9 corresponding to the sub-scanning direction.

Further, in the document reading apparatus according to the first embodiment, the sagittal stopper 11 as a light shielding means is provided.
Is arranged outside the imaging lens 9 and is configured to be movable while maintaining a certain distance from the imaging lens 9. In FIG. 1, the sagittal stopper 11 is arranged between the imaging lens 9 and the sensor (photoelectric conversion means) 12 in the optical path of the reflected light from the document. The sagittal stopper 11 may be arranged before the lens 9.

More specifically, in the document reading apparatus according to the first embodiment, as shown in FIG. 1, the outside of the exit side of the imaging lens 9, that is, the connection between the imaging lens 9 and the CCD image sensor 12. In between, a sagittal stopper 11 as a light shielding means is provided. As shown in FIGS. 4 to 6, the sagittal stopper 11 is a black rectangular thin plate made of metal or the like, and a rectangular opening 11a is formed at the center thereof. The sagittal stopper 11 is arranged such that the center of the opening 11a coincides with the optical axis ax of the reflected light from the document, the long side of the opening 11a coincides with the main scanning direction, and the short side coincides with the sub-scanning direction. I have. That is, the long side of the opening 11a is arranged in parallel with the meridional surface of the reflected light from the original to the imaging lens 9, and the short side is arranged in parallel with the sagittal surface. The meridional plane is a plane that connects an off-axis object point and the optical axis in the optical system, and the sagittal plane is a plane perpendicular to the meridional plane. FIG. 7 shows the shape of the lens pupil when the sensor is viewed from the front of the lens when a conventional light shielding plate is employed.

As shown in FIG. 8, the sagittal stopper 11 is fixed to the lens plate 10 on which the imaging lens 9 is mounted at a predetermined distance from the imaging lens 9 by means of screws or the like. Installed. The lens plate 10 has an optical axis direction, a direction orthogonal to the optical axis direction, and an inclination angle with respect to the optical axis direction on a guide rail or the like (not shown) of the document reading apparatus main body 1 via a plurality of screw holes 15a to 15c. Adjustably mounted. Therefore, since the sagittal stopper 11 is attached to the lens plate 10 to which the imaging lens 9 is attached, the lens plate 10 is used to adjust the position of the imaging lens 9 when adjusting the original reading device.
Is moved so as to be able to move while maintaining a certain distance from the imaging lens 9.

As shown in FIG. 6, the short side of the opening 11a of the sagittal stopper 11 is set smaller than the range of the emitted light beam E limited by the exit pupil of the imaging lens 9 itself.
Thus, the exit pupil diameter of the imaging lens 9 in the sub-scanning direction near the document end is reduced by the sagittal stopper 11. That is, a light beam from an object point OP1 via the imaging lens 9 to a position above or below the opening 11a is blocked by the sagittal stopper 11 and does not enter the CCD image sensor 12. As a result, the exit pupil diameter of the imaging lens 9 is reduced by the sagittal stopper 11 only in the sagittal direction. The light blocking effect of the sagittal stopper 11 is uniform throughout the longitudinal direction (main scanning direction) because the opening 11a has a rectangular shape. That is, the light from the object point OP2 on the optical axis ax is partially blocked by the sagittal stopper 11,
Only the light that has passed through the opening 11a forms an image at the image point IP2,
Light from the object point OP1 distant from the optical axis ax is also partially blocked by the sagittal stopper 11, and only light that has passed through the opening 11a forms an image at the image point IP1. In FIG. 6A, the hatched portions indicate the portions of the light beam cut by the upper and lower portions of the sagittal stopper 11.

As described above, the sagittal stopper 11 cuts a light beam that is far from the meridional surface and affects the resolution in the sub-scanning direction. Therefore, the resolution in the sub-scanning direction at the end in the main scanning direction (near the document end) is reduced. The image quality is improved to improve the imaging performance of the lens 9. In addition, since the light amount is cut only in one direction (sagittal direction), a light amount loss for improving the imaging performance of the lens 9 can be reduced.

Incidentally, in this case, since the light beam in the sub-scanning direction is uniformly converged in the entire area in the main scanning direction, the light amount also decreases at a constant rate. At this time, the sagittal stopper 11 is most effective in improving the MTF of the angle of view at the end when the light amount in the sub-scanning direction is cut by about 20 to 25%, particularly about 25%. Correspondingly, the opening width of the opening 11a of the sagittal stopper 11 is also set. In other words, the sagittal stopper 11 improves the resolution in the sub-scanning direction by cutting light rays in the sagittal direction away from the meridional surface, but if the sagittal stopper 11 cuts light rays in the sagittal direction too much. , The optics darken,
The amount of light incident on the CCD image sensor 12 decreases. In order to suppress the decrease in the amount of incident light of the CCD image sensor 12, it is conceivable to set the aperture diameter in the lens wider by that amount. However, if the aperture diameter in the lens is set wider, aberration in the main scanning direction is reduced. The impact is greater,
Not preferred. Therefore, considering both the improvement of the resolution in the sub-scanning direction by using the sagittal stopper 11 and the reduction of the aberration in the main scanning direction, it is empirically that the sagittal stopper 11 exhibits the most effect. It is known that the amount of light in the sub-scanning direction is cut by about 20 to 25%, particularly about 25%.

For this reason, in consideration of the fact that the amount of light in the sub-scanning direction is reduced by about 25% by using the sagittal stopper 11, the brightness of about F4.5 can be obtained without increasing the illumination power and the imaging optical system. To secure the entire system,
It is necessary to design the lens as a single unit at about F4. This is because the required light quantity is proportional to the square of the F value ratio. If the current illumination power is assumed to be 100 W and a lens having a brightness of about F4 is adopted, 100 W × (4/4.
5) ² ÷ 0.75 ≒ 105 W, and the increase in illumination power is very small.

Regarding the improvement of the resolution (MTF) in the sub-scanning direction by the sagittal stopper 11, it is known that the effect increases as the angle of view in the main scanning direction increases. This is due to the fact that the entrance pupil of the imaging lens 9 is generally near the edge of the lens at the angle of view near the document edge, and the light beam spread in the sagittal direction increases aberration in the sub-scanning direction. On the other hand, in the design stage of the imaging lens 9, conventionally, the resolution at the angle of view near the document edge is increased at the expense of the resolution (MTF) at the angle of view inside the vicinity of the document edge, thereby improving the overall balance. I am taking it.

On the other hand, in the case of this embodiment,
Rather than controlling the resolution in a well-balanced manner with the imaging lens alone as in the prior art, the resolution in the sub-scanning direction at the angle of view at the end is intentionally reduced at the stage of parameter design that determines the characteristics of the imaging lens 9. To be designed. Regarding lens design, since optical simulation software is commercially available from related manufacturers, the weight of the angle of view at the end in the radial direction, that is, the original reading at the angle of view near the original end, is calculated by the automatic design program of the optical simulation software. By intentionally setting the resolution of the wavelength range low,
An image forming lens 9 having a resolution characteristic in the sub-scanning direction of the original reading wavelength range at an angle of view near the original end lower than the lowest resolution of the original reading wavelength range at an angle of view inside the vicinity of the original end is adopted.

More specifically, in each of the wavelength ranges of blue, green and red, which are generally in the visible light range, the spatial frequency (lines / mm)
Is different in each wavelength range. In particular, for good character recognition, 5
An important factor is the quality of the MTF at a spatial frequency in the vicinity of this / mm. On the other hand, in this embodiment,
The sub-scanning direction MTF of the original reading wavelength band at a spatial frequency near 5 lines / mm at the angle of view near the original end is smaller than the lowest MTF of the original reading wavelength region at the angle of view inside the vicinity of the original end on the best image plane. An imaging lens 9 having a low resolution characteristic is employed. As a result, the degree of freedom in lens parameter design is increased, so that axial chromatic aberration and lateral chromatic aberration, as well as field curvature, astigmatism, distortion, and the like can be suppressed as compared with those in the related art.

However, if the optical system of the original reading apparatus is constituted by using such an imaging lens 9, the MT in the sub-scanning direction in the vicinity of the end of the original in the main scanning direction is naturally understood.
F deteriorates. However, the resolution in the sub-scanning direction in the document reading wavelength range at the angle of view near the document end is intentionally set low by taking into account in advance the increase in the resolution (MTF in the sub-scanning direction) by the sagittal stopper 11 described above. By doing so, it is possible to offset the reduction in resolution due to the parameter design as described above. That is, in the document reading apparatus according to the present invention, the imaging lens 9 is designed in a well-balanced manner as in the related art, and the light shielding plate (sagittal stopper) is not used in an auxiliary manner. The sagittal stopper 11 is positioned as an essential element in constructing the system, and the characteristics of the imaging lens 9 are determined on the assumption that the sagittal stopper 11 is used, thereby optimizing the characteristics of the entire imaging optical system.

As compared with the conventional case where the MTF is balanced by a single imaging lens as in the related art, the degree of freedom in lens design is increased. The properties of the MTF at the corner can be improved. Here, the MTF in the sub-scanning direction in the visible light region
An example of the frequency characteristics is shown in the graphs of FIGS. The solid line curve in the graph shows the MTF characteristic at the end angle of view, and the broken line curve shows the MTF characteristic at the inner angle of view. In the simulation performed this time, for each of the blue, green, and red wavelength ranges, In each case, a characteristic curve as shown in the figure was shown.

That is, the MTF frequency characteristic of the lens without the sagittal stopper shown in FIG.
At a spatial frequency of 5 lines / mm, the MTF at the end angle of view has lower characteristics than the MTF at the inner angle of view. In contrast, the MTF frequency characteristic of the lens with the sagittal stopper shown in FIG.
The MTF at a spatial frequency of up to 15 lines / mm is lifted by the light-shielding effect of the sagittal stopper 11, and good MTF characteristics are uniformly obtained at both angles of view.

In this case, the MTF characteristic at the angle of view inside the vicinity of the document edge is set to a higher level than before without sacrificing it at the lens design stage. The MTF at the angle of view near the document edge is raised to the same level as the MTF at the inner angle of view by the light blocking effect of the sagittal stopper 11.
Therefore, as the characteristics of the entire image forming optical system, the best state is achieved by reducing the aberration as compared with the conventional case.

In setting the characteristics of the imaging lens 9, the difference between the resolution in the sub-scanning direction at the angle of view near the document edge and the lowest resolution at the angle of view inside the angle of view is calculated as follows.
If it is set to correspond to the increase in the resolution by the sagittal stopper 11, the MTF difference at a predetermined frequency without the sagittal stopper shown in FIG. Book / mm
The difference between the nearby MTFs (ΔMTF) becomes almost 0 (zero) in the state with the sagittal stopper shown in FIG. 9B. As a result, the character information written on the document can be suitably read, and the variation in the MTF of the entire image forming optical system can be suppressed, so that the deterioration of the read image quality due to this can be prevented.

The image reading wavelength ranges are blue, green,
Not only in the red visible light wavelength range, but also in the wider ultraviolet,
It may include five wavelength ranges to which infrared light is added. Then, the resolution of the imaging lens 9 satisfying the condition that the resolution in the sub-scanning direction of the image reading wavelength range at the angle of view near the document edge is lower than the minimum resolution of the image reading wavelength range at the angle of view inside the vicinity of the document edge. Characteristics include ultraviolet, red,
At least one of each of the green, blue, and infrared wavelength ranges
What satisfies the above conditions in one or more wavelength ranges may be used.

With the above configuration, the original reading apparatus according to this embodiment can avoid an increase in the number of assembling steps and an increase in cost due to the use of the sagittal stopper as described below, and can also improve the image quality of reading. It is possible to further improve.

That is, in the document reading apparatus according to this embodiment, as shown in FIG. 1, the image of the document placed on the platen glass 2 is illuminated by the lamp 4 and the reflector 5 while the document is read from the document. Is reflected on the CCD image sensor 12 via the full-rate mirror 6 and the half-rate mirrors 7 and 8 by the imaging lens 9 while the full-rate mirror 6 and the half-rate mirrors 7 and 8 are By moving the original in the scanning direction, the entire image of the original is scanned and exposed on the CCD image sensor 12, and the original image is converted into an electric signal by the CCD image sensor 12 and read.

At this time, as shown in FIG. 2, the image of the original is converted by the imaging lens 9 into the CCD image sensor 12.
Although the image is reduced and formed on the top, the CCD of the imaging lens 9
Since the sagittal stopper 11 is provided on the image sensor 12 side, the image forming characteristics on the CCD image sensor 12 are determined by various aberrations of the image forming lens 9 and the light blocking characteristics of the sagittal stopper.

By the way, in this embodiment, the MTF in the sub-scanning direction in the original reading wavelength range at the angle of view near the original end is described.
However, the imaging lens 9 having a resolution characteristic lower than the lowest MTF in the document reading wavelength range at the angle of view inside the vicinity of the document edge on the best image plane is intentionally adopted. As a result, the degree of freedom in lens parameter design is increased, so that axial chromatic aberration and lateral chromatic aberration, as well as field curvature, astigmatism, distortion, and the like can be suppressed as compared with those in the related art. As a result, the MTF characteristics at the angle of view inside the vicinity of the document edge can be set to a higher level than before without sacrificing the characteristics at the lens design stage.

The MTF at the angle of view near the document edge
Can be raised to the same level as the MTF at the inner angle of view due to the light blocking effect of the sagittal stopper 11. Therefore, as the characteristics of the entire image forming optical system, the aberration is reduced as compared with the related art, the best state can be achieved, and the read image quality can be greatly improved.

In this embodiment, as shown in FIG. 8, the sagittal stopper 11 is mounted on the lens plate 10 on which the imaging lens 9 is mounted, at a predetermined distance from the imaging lens 9, and It is attached by means such as a stop. Therefore, when adjusting the position of the imaging lens 9 and adjusting the focus when adjusting the original reading apparatus, when the lens plate 10 is moved, the sagittal stopper 11 is kept at a fixed distance from the imaging lens 9. You can move. Therefore, it is not necessary to move the imaging lens 9 and the sagittal stopper 11 separately and adjust the positions when adjusting the document reading apparatus. Therefore, the use of the sagittal stopper increases the number of assembling steps and the number of assembling steps. An increase in manufacturing cost can be avoided.

Further, in this embodiment, the sagittal stopper 11 is provided outside the exit side of the imaging lens 9 so that the light beam reduced and imaged by the imaging lens 9 is sub-scanned by the sagittal stopper 11. Since the stop is performed in the direction, the sagittal stopper 11 itself can be reduced in size.

Second Embodiment FIG. 10 shows a second embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals and described. And a holding member for holding the imaging lens, and the light blocking member is formed integrally with the holding member.

In the second embodiment, the distance between the light blocking member and the imaging lens is set as short as possible.

FIG. 10A shows the lens assay 20 (lens 9 and lens holding member 10) of the second embodiment viewed from above, and FIG. 10B shows the state viewed from the side. . Further, FIG. 11A shows a state in which the lens holding member 10 is viewed from above, and FIG. 11B shows a state in which the lens holding member 10 is viewed in the optical axis direction.

Lens plate 1 as lens holding member
0 is formed in an L-shape, and the vertical plate portion 10a of the lens plate 10 functions as a sagittal stopper 11, and the vertical plate portion 10a is provided with an opening 11a and pressed against the sagittal stopper 11. It is desirable to fix the imaging lens 9. As a result, the distance between the sagittal stopper 11 and the imaging lens 9 is set as short as possible.

As described above, the distance between the sagittal stopper 11 and the imaging lens 9 is set as short as possible, and if possible, no gap is provided between the sagittal stopper and the lens barrel supporting the imaging lens. With this configuration, it is possible to reliably prevent stray light incident from the gap between the imaging lens and the sagittal stopper from being reflected on the exit side surface of the imaging lens and adversely affecting the optical system.

The other configuration and operation are the same as those of the first embodiment, and the description is omitted.

Third Embodiment FIG. 12 shows a third embodiment of the present invention. The same parts as those in the first embodiment are denoted by the same reference numerals, and will be described. The imaging lens is formed of a Gaussian type lens in which four or more groups of lenses are combined, and the refractive index and dispersion at d-line include a material having the following three characteristics. 1. Refractive index 1.658 Dispersion 50.9 2. Refractive index 1.639 Dispersion 55.5 3. Refractive index 1.648 Dispersion 33.8

Further, in the third embodiment, the imaging lens is configured such that ri is the radius of curvature of the i-th surface and di is the i-th surface.
+1) The spacing between planes, ni is the refractive index of the i-th lens at the d-line, and νi is the Abbe number of the i-th lens at the d-line. 55.5 r4 70.70 d4 2.85 n3 1.648 ν3 33.8 r5 16.96 d5 21.77 r6 -16.27 d6 0.90 n4 1.603 ν4 38.0 r7 -220.00 d7 6.72 n5 1.639 ν5 55.5 r8 -22.87 d8 0.51 r9 -172.4 d9 4.92 n6 1.658 ν6 50.9 r10 -38 It is configured so that

In the document reading apparatus shown in FIG. 1, the most important component among the components of the optical system is the imaging lens 9. When designing the imaging lens 9, as described in the first embodiment, the sub-scanning direction MTF of the original reading wavelength band at the angle of view near the original end is set to be more inward than the original end near the original image plane. By intentionally adopting an imaging lens 9 having a resolution characteristic lower than the lowest MTF in the document reading wavelength range at the angle of view,
Since the degree of freedom in lens parameter design is increased, axial chromatic aberration and chromatic aberration of magnification, as well as field curvature, astigmatism, distortion, and the like can be suppressed as compared with the related art.

However, in addition to this, the imaging lens 9
It is also an important factor in designing that to secure the brightness without making the diameter of the imaging lens 9 too large.

In this regard, the prior art discloses lenses having various characteristics used for copiers, scanners and the like. Further, as a lens characterized by particularly high imaging characteristics, for example, Japanese Patent Application Laid-Open No. Hei 9-113802 discloses a Gauss type having four groups and six elements, and an F value (No.) of 4.
Five reading lenses are disclosed. However, the reading lens disclosed in this publication has water resistance, blue burn resistance,
There are drawbacks in that it is very difficult to handle in terms of latent scratch resistance and the like, and the cost of the lens alone is high.

On the other hand, by effectively utilizing the sagittal stopper described in the first embodiment, chromatic aberration of magnification and M
Regarding those that simultaneously improve the TF characteristics, for example, F4.
In order to secure a brightness of about 5, it is necessary to increase the lens diameter and decrease the F value of the lens alone (brighter) in order to compensate for the amount of light blocked by the sagittal stopper. Then, there is a concern about an increase in size and cost due to an increase in the lens diameter. However, when considering the lens configuration in combination with the sagittal stopper,
In some cases, it is possible to avoid the above-mentioned difficulties in handling the lens, and increase in size and cost due to enlargement of the lens diameter while securing the brightness of the optical system to F4.5. Hereinafter, a specific example will be described.

First, FIG. 12 shows the overall configuration of the imaging lens according to the third embodiment. As shown in the figure, the first lens unit, the second lens unit, the third lens unit, and the fourth lens unit are arranged in order from the object side to the image side (from left to right in FIG. 12). . The first lens 31 forming the first group is a positive meniscus lens having a convex surface facing the object side (left side in the figure). The second group includes a second lens 32 and a third lens.
It is composed of a cemented lens in which a lens 33 is bonded. Among them, the second lens 32 is a positive meniscus lens having a convex surface facing the object side, and the third lens 33 is a negative meniscus lens having a convex surface facing the object side.

The third group includes the fourth lens 34 and the fifth lens 3
5 is formed of a cemented lens. The fourth lens 34 is a negative meniscus lens having a concave surface facing the object side, and the fifth lens 35 is a positive meniscus lens having a concave surface facing the object side. The sixth that constitutes the fourth group
The lens 36 is a positive meniscus lens having a concave surface facing the object side. A stop 37 is disposed between the third lens 33 (second group) and the fourth lens 34 (third group), and the first and second groups are located in front of the stop 37. , And the third and fourth groups constitute the subsequent lens group. As a result, the entire lens is a Gaussian type lens having four groups and six elements.

Here, specific characteristic values of the lens (Gauss type having four groups and six elements) employed in the third embodiment will be described. First, ri is the radius of curvature of the i-th surface, di is the distance between the i-th surface and the (i + 1) surface, ni is the refractive index of the i-th lens at the d-line, and νi is the Abbe number of the i-th lens at the d-line. A lens that satisfies the following numerical conditions is employed.

R1 32.40 d1 5.82 n1 1.658 ν1 50.9 r2 80.80 d2 0.15 r3 25.11 d3 5.55 n2 1.639 ν2 55.5 r4 70.70 d4 2.85 n3 1.648 ν3 33.8 r5 16.96 d5 21.77 r6 -16.27 d6 0.90 n4 1.72 ν4 38.0 r7. 1.639 ν5 55.5 r8 -22.87 d8 0.51 r9 -172.4 d9 4.92 n6 1.658 ν6 50.9 r10 -38.80

The sagittal stopper 11 combined with the imaging lens 9 includes the first lens of the sixth lens 36.
On the surface 0, for example, an opening 11 of 9.6 mm in the sub-scanning direction
A rectangular type having a is adopted. The sagittal stopper 11 may be arranged anywhere between the original and the line sensor 12, but the opening width of the sagittal stopper 11 is
It is necessary to recalculate appropriately from the ratio of the light shielding to the lens pupil.

Incidentally, the shape of the sagittal stopper 11 employed here and the light shielding state with respect to the lens pupil are shown in FIG.
(B). Here, as to how much light quantity decreases, assuming that the light quantity distribution in the pupil is uniform,
From the ratio of the area shielded by the sagittal stopper 11 to the area of the entire pupil, the brightness of the entire optical system is ensured to be equivalent to F4.5 from the obtained result. For example, if the F-number of the lens is set to 4.0, the brightness of the entire optical system is set to F0.5 by setting the aperture width so that the amount of decrease in the amount of light due to light blocking of the sagittal stopper 11 becomes equivalent to F0.5.
It is assumed to be 4.5.

Next, the lens diameter will be described with reference to FIG. In FIG. 13, the intersection of the optical axis and the object plane is the origin (0, 0), the object image height is Yob, the radius of curvature of the first lens surface is r, the distance between the object and the first lens surface is Z1, the distance from the object to the lens is Z1. At the maximum ray height incident on the lens
If the coordinates of the intersection with the surface are p (z2, h), the thickness of the lens barrel is x, the length of the lens barrel protruding from the first surface of the lens toward the subject is m, and the lens diameter is φ, z2 = R-√
(R ² −h ² ) + Z1 tan θ = (Yob−h) / z2x
= Φ / 2−h− (z2−Z1 + m) tan θ, φ / 2 = x + h + (z2−Z1 + m) tanθ =
x + Yob- (Z1-m) (Yob-h) / z2.

Here, Yob is sufficiently large with respect to x,
Since Z1 is also sufficiently large with respect to m, x = m = 0, Z1 /
Assuming that z2 = k, φ / 2 = Yob (1-k) + kh, which is a simple ratio calculation. From this, in order to reduce φ, k → 1 and h → 0, that is, the radius of curvature of the first surface of the lens should be increased and the lens length should be reduced.

In consideration of this point, it is possible to reduce the diameter of the lens using a Gauss-type lens having a long focal length of the front group, that is, a configuration of four or more groups in which the curvature of the first surface is easily reduced. Will lead to a further increase in the effect of The combination of the sagittal stopper 11 and the one having the best balance between characteristics and cost is a Gauss type lens having four groups and six elements.

Normally, a Gauss type lens having four groups and six elements has a concave surface with a small radius of curvature on both sides of the stop, thereby correcting (small) the Petzval sum. Here, the Petzval sum is a sum of parameters that provide necessary conditions for correcting the curvature of the image plane in a coaxial spherical optical system. If the sum is equal to 0 (zero), This makes it possible to correct the curvature of the image plane. Therefore, it has been difficult to correct the aberration in the sagittal direction at a large angle of view, and it has been said that the use of a glass material having a high refractive index facilitates the correction of the aberration. In fact, in the automatic lens design, if the refractive index range of the optical glass used as the lens material is allowed to be high and used as a variable, the refractive indices of the first, fifth, and sixth lenses particularly change to higher ones. There is a tendency.

However, this tendency is attributable to the lens automatic design algorithm that simultaneously improves aberrations in the main scanning direction (meridional direction) and the sub-scanning direction (sagittal direction) with a single lens and reduces the so-called error function. This is a design method suitable for a camera lens that needs to read an area image at the same time. On the other hand, even when the above-described design means is applied to a configuration in which RGB information is read separately and one-dimensionally by a line sensor, such as a color image reading apparatus, the performance of the imaging optical system is not necessarily improved. .

That is, in a color image reading apparatus using a line sensor, RGB light having the same intensity is separately formed rather than having a low error function when forming white light or having a high MTF. The uniformity of the MTF, the chromatic aberration of magnification, and the uniformity with respect to each angle of view when performed are more important because of the necessity of synthesizing color information in a later process.
If the MTF at the Nyquist frequency is too high, it also causes moire.

Therefore, in the third embodiment, the sagittal flare correction load on the lens alone is reduced by introducing the sagittal stopper 11. The sagittal stopper 11 is an optimal means for compensating for the disadvantages of the design of the Gaussian type lens and at the same time maximizing the advantages thereof.

In general, as the refractive index of the glass material increases, the dispersion also increases. Therefore, considering the color correction of the imaging system,
Each of the first, second, fifth, and sixth lenses has a refractive index of 1.603 to 1.658 and an Abbe number of 50 at d-line.
It can be said that the use of the optical glass described above is preferable in the image forming optical system according to the third embodiment in terms of the balance between the color correction and the MTF characteristics, and furthermore, the cost. That is, in the imaging optical system according to the third embodiment,
Even if the lens has the highest refractive index, it is about 1.658 and the Abbe number is 50 or more, so that the cost of the optical glass constituting the lens can be reduced.

FIG. 14 is a diagram showing various aberrations when the imaging lens according to the third embodiment and a sagittal stopper are combined.

FIG. 14A shows the spherical aberration, and FIG.
14B shows astigmatism, and FIG. 14C shows distortion, respectively. As can be seen from these figures, various aberrations are suppressed to very small values, and the main scanning direction and the sub-scanning direction are suppressed. It can be seen that magnification chromatic aberration and axial chromatic aberration are small in the scanning direction, and the read image quality is greatly improved.

15 to 17 show simulation results of MTF versus frequency characteristics of R (red), G (green), and B (blue) of the lens having the above-mentioned numerical configuration (without sagittal stopper). Note that FIG.
16 shows the MTF versus frequency characteristic of G, and FIG. 17 shows the MTF versus frequency characteristic of B. In each of the characteristic diagrams, a line is a characteristic curve in the main scanning direction on the optical axis, a line is a characteristic curve in the sub-scanning direction, and a line is a characteristic curve in the main scanning direction at an angle of view of -13.67 °. The characteristic curve line indicates the characteristic curve in the sub-scanning direction, the line indicates the characteristic curve in the main scanning direction at an angle of view of -19.07 °, and the line indicates the characteristic curve in the sub-scanning direction.

In FIGS. 15 to 17, especially R, B
In the MTF vs. frequency characteristics of FIG.
07 °) in the sub-scanning direction is low. By intentionally keeping the characteristics of this portion low, the degree of freedom of the parameters in the lens design is increased, and efforts are being made to improve the characteristics of MTF and chromatic aberration of magnification in the main scanning direction. Here, the CCD sensor of this embodiment uses pixels whose one side in the longitudinal direction is 9.33 μm.

18 to 20 show simulation results of MTF versus frequency characteristics in a state where the lens having the above-described numerical configuration and the sagittal stopper are combined (with the sagittal stopper). As is clear from the figure,
It can be seen that the low MTF in the sub-scanning direction of the lens alone, as seen in FIGS. 15 to 17, has been completely corrected by the resolution improving effect of the sagittal stopper.

FIG. 21 shows the angle-of-view characteristics of the MTF at a spatial frequency of 7.87 line pairs / mm on the document surface. In FIG. 21, the horizontal axis represents the object height, which is a determinant of the angle of view, and the vertical axis represents the MTF. As shown in the figure, for the three colors of B, G, and R, the MTFs are very well aligned regardless of the change in the object height (the size of the angle of view). In this case, the difference between the highest resolution and the lowest resolution of the reflected light in the sub-scanning direction, that is, ΔMTF is within 20%. As a result, unevenness in the reading resolution is reduced, and high image quality can be achieved. If the ΔMTF is within 20%, a sufficiently high image quality can be achieved.

FIG. 22 shows the chromatic aberration of magnification versus angle of view characteristics.
Shown in In FIG. 22, the angle-of-view characteristic of the lens according to the third embodiment is indicated by a solid line, and the angle-of-view characteristic of the conventional lens is indicated by a broken line. As shown in the drawing, the chromatic aberration of magnification exceeds 0.933 μm in the conventional lens, but the chromatic aberration of magnification falls within 0.933 μm in the lens according to the third embodiment. That is, this indicates that the lens is designed so that the difference between the center of gravity of the image at the wavelength of G color and the center of gravity of the image at the wavelength of another reading color is 10% or less of the pixel. This reduces errors when performing achromatic determination based on image data read by the CCD sensor.

As described above, the combination of the lens having the characteristics according to the present embodiment and the sagittal stopper allows the chromatic aberration of magnification to be within 1 μm and the MTF difference (ΔMT
This means that an imaging optical system with less F) is realized. Thereby, in the image processing using the CCD sensor output,
It is possible to increase the black character determination threshold level (c ^* in the CIEL ^* a ^* b ^* color space) to 2/3 or less of the conventional value, expand the color reproducible range, and reduce the occurrence rate of black character erroneous determination. It becomes possible. Also, even when an energy-saving xenon lamp is used for the reading optical system of a color copier,
By using the memory, it is possible to maintain the same process speed as when using a halogen lamp.

In the above lens structure, the first, second, fifth, and sixth lenses each use optical glass having a refractive index of 1.603 to 1.658 at d-line and an Abbe number of 50 or more. This is extremely advantageous both in cost and handling.

Further, the radii of curvature of the first and sixth lenses (r
(1, r10) is set to ± 30 mm or more to reduce the curvature, and the sagittal stopper is used to improve the deterioration of the aberration with the sagittal stopper, thereby improving the imaging performance of the entire optical system, thereby improving the characteristics and reducing the lens diameter. Can be simultaneously realized.
Therefore, it is possible to substantially avoid an increase in size by canceling out the reduction in the lens diameter and the expansion in the lens diameter when the F value of the lens alone is reduced (F4.5 → F4.0). it can.

In addition, by using optical glass of the same material (glass material) for each of the first, second, fifth, and sixth lenses, cost reduction can be expected due to the common use of the glass material.

The other configuration and operation are the same as those of the first embodiment, and the description is omitted.

As described above, according to the first to third embodiments, in the color original reading optical system, brightness equivalent to F4.5 is ensured, chromatic aberration of magnification is corrected, and pixel shift in each color is reduced by 10%. (0.93 μm) and, in addition, by introducing a sagittal stopper and designing parameters corresponding to it, 7.81 lp / m
ΔMTF at a frequency of mm could be suppressed to within 20%.

As a result, the black character determination threshold level (CIEL ^* a) in the image processing after the CCD sensor is performed.
c ^* ) in the ^* b ^* color space to a value of 2/3 or less of the conventional value,
The color reproducible area is expanded, and erroneous black character determination does not occur.

Further, if a memory is used even if a xenon lamp is used for the reading optical system of a color copying machine, the process speed can be made the same as the conventional one.

Further, by dispersing the effect of improving aberration to the lens and the sagittal stopper, the design of the entire image forming optical system becomes easy. As a result, the range of choices of usable glass materials is expanded, and it is possible to use low-cost, easy-to-handle glass materials that do not contain lead and arsenic, which are symmetrical for reduction as harmful substances, and that new optical systems can be used. The number of man-hours required for the design was also reduced.

[0120]

As described above, according to the present invention, an increase in the number of assembling steps and an increase in cost due to the use of the sagittal stopper can be avoided.
A document reading apparatus capable of further improving the read image quality can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a document reading apparatus according to Embodiment 1 of the present invention.

FIG. 2 is a development explanatory view showing an optical system of the document reading apparatus according to the first embodiment of the present invention.

FIG. 3 is an explanatory perspective view showing an image forming optical system of the original reading apparatus according to the first embodiment of the present invention;

FIG. 4 is an explanatory perspective view showing a main part of an image forming optical system of the document reading apparatus according to the first embodiment of the present invention;

FIG. 5 is a front view showing a relationship between an imaging lens and a sagittal stopper.

FIG. 6 is an explanatory perspective view showing an imaging optical system of the original reading apparatus according to the first embodiment of the present invention;

FIG. 7 is a front view showing a relationship between a conventional imaging lens and a sagittal stopper.

FIGS. 8A and 8B are a plan view and a side view showing a relationship between an imaging lens and a sagittal stopper.

FIGS. 9A and 9B are graphs respectively showing a relationship between a spatial frequency and an MTF when a sagittal stopper is not used and when a sagittal stopper is used.

FIGS. 10A and 10B are a plan view and a side view showing a relationship between an imaging lens and a sagittal stopper in a document reading apparatus according to Embodiment 2 of the present invention.

FIGS. 11A and 11B are a plan view and a side view showing a lens holding member in a document reading apparatus according to Embodiment 2 of the present invention.

FIG. 12 is a sectional configuration diagram showing an imaging lens in a document reading apparatus according to Embodiment 3 of the present invention.

FIG. 13 is an explanatory diagram showing the aperture of the imaging lens.

FIG. 14 is a graph showing various aberrations of the optical system of the document reading apparatus according to Embodiment 3 of the present invention.

FIG. 15 is a diagram showing M of R (red) in a lens alone;
It is a figure showing TF vs. frequency characteristics.

FIG. 16 is a diagram showing MTF (Green) MTF versus frequency characteristics of a single lens.

FIG. 17 is a diagram showing the M of B (blue) with a single lens.
It is a figure showing TF vs. frequency characteristics.

FIG. 18 is a diagram showing MTF versus frequency characteristics of R (red) in a state where a lens and a sagittal stopper are combined.

FIG. 19 is a diagram showing a G (green) MTF versus frequency characteristic in a state where a lens and a sagittal stopper are combined.

FIG. 20 is a diagram showing MTF versus frequency characteristics of B (blue) in a state where a lens and a sagittal stopper are combined.

FIG. 21 is a diagram illustrating MTF versus view angle characteristics when the frequency on the document surface is 7.87 line pairs / mm.

FIG. 22 is a diagram showing the angle-of-view characteristic of chromatic aberration of magnification.

FIG. 23 is a schematic configuration diagram showing a conventional document reading apparatus.

FIG. 24 is a development explanatory view showing an optical system of a conventional document reading apparatus.

FIG. 25 is an explanatory diagram showing axial chromatic aberration.

FIG. 26 is an explanatory diagram showing axial chromatic aberration.

FIG. 27 is an explanatory diagram showing chromatic aberration of magnification.

[Explanation of symbols]

1: Document reading apparatus main body, 2: Platen glass, 3: Platen cover, 4: Lamp (irradiation means), 5: Reflector, 6: Full rate mirror, 7, 8: Half rate mirror, 9: Imaging lens, 10: Lens holding member, 11: sagittal stopper (light shielding member), 12: CCD image sensor (photoelectric conversion means).

Claims (8)

[Claims] An irradiating means for irradiating an original with light; and a member for forming a reflected light image from the original, wherein a resolution in a sub-scanning direction of an original reading wavelength range at an angle of view near an end of the original is: An imaging lens designed to have a resolution characteristic lower than the resolution indicating the lowest value in the document reading wavelength range at an angle of view inside the vicinity of the document edge, and a sub-scan in a reflected light image from near the document edge Light blocking means for blocking a part of the reflected light image so as to reduce the pupil diameter in the direction, and photoelectric conversion means for converting light partially blocked by the light blocking means and imaged by the lens into an electric signal. Wherein the light-shielding means is arranged outside the imaging lens and is configured to be movable while maintaining a certain distance from the imaging lens. Document reading device. 2. An irradiating means for irradiating a document with light, and a member for forming a reflected light image from the document, wherein a resolution in a sub-scanning direction of a document reading wavelength range at an angle of view near a document edge is: An imaging lens designed to have a resolution characteristic lower than the resolution indicating the lowest value in the original reading wavelength range at an angle of view inside the vicinity of the original end, and sub-scanning in a reflected light image from the vicinity of the original end A light-shielding member that shields a part of the reflected light image so that a pupil diameter in the direction becomes small; And a holding member for holding the imaging lens, wherein the light blocking member is formed integrally with the holding member. 3. The document reading apparatus according to claim 1, wherein the light blocking member is provided outside the emission side of the imaging lens. 4. The distance between the light blocking member and the imaging lens is
4. The document reading apparatus according to claim 1, wherein the document reading apparatus is set as short as possible. 5. The imaging lens according to claim 1, wherein the imaging lens forms a rotationally symmetric system with respect to an optical axis.
The document reading device according to any one of the above. 6. An irradiation means for irradiating a document with light, and a member for forming a reflected light image from the document, wherein a resolution in a document scanning wavelength range in a sub-scanning direction at an angle of view near a document edge is: An imaging lens designed to have a resolution characteristic lower than the resolution indicating the lowest value in the document reading wavelength range at an angle of view inside the vicinity of the document edge, and a sub-scan in a reflected light image from near the document edge A light-shielding member that shields a part of the reflected light image so that a pupil diameter in the direction becomes small; Wherein the imaging lens comprises a Gaussian type lens in which four or more lenses are combined, and the refractive index and dispersion at d-line include a material having the following three characteristics. Characterized by Document reading device. 1. Refractive index 1.658 Dispersion 50.9 2. Refractive index 1.639 Dispersion 55.5 3. Refractive index 1.648 Dispersion 33.8 7. The imaging lens is a Gaussian type lens in which six lenses in four groups are combined, and when the lenses are numbered in order from the object side, the first characteristic is a refractive index of 1.658 dispersion. The lenses having the characteristic of 50.9 are referred to as a first lens and a sixth lens, and the second characteristic, the refractive index
1.639 Dispersion 55.5
The third characteristic is a refractive index of 1.
7. The document reading apparatus according to claim 6, wherein a lens having a characteristic of 648 dispersion and 33.8 is used as the third lens. 8. The imaging lens, wherein ri is the radius of curvature of the ith surface, di is the distance between the ith surface and the (i + 1) surface, and ni is the ith surface.
The refractive index at the d-line of the lens, νi, is defined as the Abbe number at the d-line of the ith lens. 16.96 d5 21.77 r6 -16.27 d6 0.90 n4 1.603 ν4 38.0 r7 -220.00 d7 6.72 n5 1.639 ν5 55.5 r8 -22.87 d8 0.51 r9 -172.4 d9 4.92 n6 1.658 ν6 50.9 r10 -38.80 7. The document reading device according to 7.