JP4421407B2 - Image reader - Google Patents

Description

  The present invention relates to an image reading apparatus capable of simultaneously reading the front and back of a document.

  In this type of apparatus, the front and back surfaces of the document are read by the first and second image reading units while the front and back surfaces of the document are illuminated by the first and second light sources, respectively.

  However, although most of the light from the first and second light sources is reflected by the front and back surfaces of the document, a small amount of the light from the first and second light sources is transmitted through the document. Accordingly, the first image reading unit not only reads the image on the surface by receiving the light irradiated from the first light source and reflected from the surface of the document, but also slightly transmits the document transmitted from the second light source. Is also received, and an image on the back surface is read even though the density is low. Similarly, the second image reading unit receives the light irradiated from the second light source and reflected from the front surface of the document, and not only reads the image on the back surface but also irradiates the first light source and transmits the document. A small amount of light is received, and the surface image is also read at a low density. As a result, a double image in which the front and back images overlap is read by both the first and second image reading units.

  For this reason, for example, in Patent Document 1, spectral sensitivity characteristics are made different between the front side reading unit and the back side reading unit that respectively read the front side and the back side of the original, and light transmitted through the original is transmitted by the front side reading unit and the back side reading unit. Techniques have been disclosed for preventing double image reading in a manner that does not sense.

In Patent Document 2, the front side and the back side of a document are respectively read, the image read on the back side is inverted to form a mirror image, the image read on the front side is compared with the mirror image, and the image is read on the front side. A technique is disclosed in which when the image matches the mirror image and the density of the mirror image is low, the mirror image is subtracted from the image read on the surface side, leaving only the surface image.
JP 2000-188668 A JP 2001-313830 A

  However, if the spectral sensitivity characteristics are made different between the front side reading unit and the back side reading unit that respectively read the front side and the back side of the document as in Patent Document 1, the spectral sensitivity range of each reading unit becomes narrower, so that the color image Could not be read.

  In Patent Document 2, mirror image formation, surface-side image and mirror image comparison, mirror image density determination, and mirror image subtraction must be performed. It became a burden.

  Therefore, the present invention has been made in view of the above-described conventional problems, and it is possible to prevent reading of a double image in which images on the front surface and the back surface are overlapped by a simple arithmetic processing, and also supports a color image. An object of the present invention is to provide an image reading apparatus capable of performing the above.

In order to solve the above problems, the present invention provides first and second light sources for illuminating the first and second surfaces of a document, respectively, and first and second image readings for reading the first and second surfaces of the document, respectively. In the image reading apparatus comprising the first and second light sources, the received light output of the first image reading unit when the first light source illuminates the white background portion of the first surface of the document and the first and second surfaces of the document by the first and second light sources. The first image reading when the first and second light sources illuminate the image areas of the first and second surfaces of the document using the ratio of the received light output of the first image reading unit when the white background portion of the document is illuminated The received light output of the unit is corrected.

Further, in the present invention, the light reception output of the first image reading unit when the white background portions of the first and second surfaces of the document are illuminated by the first and second light sources is set to Iso, and the first light source of the document is output by the first light source. The light receiving output of the first image reading unit when illuminating the white background portion of the surface is Iro, and the first and second light sources illuminate the image areas of the first and second surfaces of the document. If the light reception output is Is and the correction value of the light reception output Is of the first image reading unit is Irc,
Irc = Is · Iro / Iso (1)
It is.

On the other hand, according to the present invention, an image reading unit includes first and second light sources that illuminate the first and second surfaces of the document, respectively, and first and second image reading units that respectively read the first and second surfaces of the document. In the apparatus, the first and second surfaces of the document are illuminated by the first and second light sources, and the same position where the first and second surfaces of the document overlap each other is read by the first and second image reading units. When the light reception outputs of the first and second image reading units are Is1 and Is2, the coefficient relating to the light transmittance of the original is Ts, and the correction value of the light reception output of the first image reading unit is Ird.
Ird = (1−Is2 · α ^ (1 + α)) Is1 (2)
It is.

Or, approximately,
Ird = (1−Is2 · α) Is1 (3)
It is.

  In the present invention, sampling of data for shading correction is performed during illumination of either one of the first and second light sources.

  According to the present invention, the first and second light sources emit the first and second documents based on the light reception output of the first image reading unit when the first or second light source illuminates the first or second surface of the document. The light reception output of the first image reading unit when the second surface is illuminated is corrected. As a result, as the correction value of the light reception output of the first image reading unit when the first and second surfaces of the document are illuminated by the first and second light sources, the light reception of the first image reading unit accompanying the illumination of the second light source. An output component is suppressed. That is, the light reception output of the first image reading unit at the time of illumination by the first and second light sources includes a light reception output component obtained by receiving the light emitted from the first light source and reflected by the first surface of the document, The latter light receiving output component is suppressed although it includes the light receiving output component resulting from receiving the light emitted from the second light source and transmitted through the document. Further, it is not necessary to regulate the spectral characteristics of the first and second image reading units at all, and application to a color image is possible.

  For example, when the first light source illuminates the white background portion of the first surface of the document and the received light output of the first image reading unit and the first and second light sources illuminates the white background portion of the first and second surfaces of the document. Is used to correct the light reception output of the first image reading unit when the first and second light sources illuminate the image areas of the first and second surfaces of the document. . More specifically, the light reception output of the first image reading unit when the first and second light sources illuminate the white background portions of the first and second surfaces of the document is set to Iso, and the first light source outputs the first surface of the document. The light reception output of the first image reading unit when illuminating the white background of the image is Iro, and the light reception of the first image reading unit when the first and second light sources illuminate the image areas of the first and second surfaces of the document Assuming that the output is Is and the correction value of the light reception output Is of the first image reading unit is Irc, the correction value Irc of the light reception output Is of the first image reading unit is expressed by the following equation (1): Irc = Is · Iro / Iso Based on.

  According to the present invention, the first and second surfaces of the document are illuminated by the first and second light sources, and the first and second image readings are performed at the same position where the first and second surfaces of the document overlap each other. If the light reception outputs of the first and second image reading units at this time are Is1 and Is2, the light transmittance of the document is α, and the correction value of the light reception output of the first image reading unit is Ird, The correction value Ird of the light reception output Is1 of one image reading unit is obtained based on the equation (2) of Ird = (1−Is2 · α ^ (1 + α)) Is1. Alternatively, it is approximately calculated based on the formula (3) of Ird = (1−Is2 · α) Is1.

  Also here, as the correction value of the light reception output of the first image reading unit, a value obtained by suppressing the light reception output component of the first image reading unit accompanying the illumination of the second light source is obtained. In addition, it is possible to suppress the light reception output component that changes according to the image of the second surface. Therefore, only the image on the first side is read by the first image reading unit, the image on the second side is hardly read, and the double image is not read. Further, it is not necessary to regulate the spectral characteristics of the first and second image reading units at all, and application to a color image is possible.

  Furthermore, according to the present invention, since sampling of data for shading correction is performed when one of the first and second light sources is illuminated, stray light is received by the first or second image reading unit. Data error due to stray light can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a side view showing Embodiment 1 of the image reading apparatus of the present invention. The image reading apparatus according to this embodiment includes at least one of a lower housing 1 provided with a first reading unit 10, an upper housing 2 provided with a second reading unit 23, and first and second reading units 10 and 23. And a paper discharge tray 3 for receiving and accumulating the originals read by.

  The upper housing 2 is pivotally supported on one side of the lower housing 1 by a hinge (not shown) at the back, and is opened and closed by moving the front portion up and down. When the upper housing 2 is opened, the platen glass 11 of the lower housing 1 is opened.

  The first reading unit 10 of the lower housing 1 includes a platen glass 11, a first scanning unit 12, a second scanning unit 13, an imaging lens 14, and a CCD (Charge Coupled Device) 15. The first scanning unit 12 includes an exposure lamp (halogen lamp, xenon lamp, fluorescent lamp, etc.) 50 and a first reflecting mirror 51, and the direction from the first position P 1 to the second position P 2 along the platen glass 11. The document on the platen glass 11 is exposed by the exposure lamp 50 while moving at a constant speed V by a distance corresponding to the size of the document detected by a sensor (not shown), and the reflected light is first reflected. Reflected by the mirror 51 and guided to the second scanning unit 13, thereby scanning the image on the surface of the document in the sub-scanning direction. The second scanning unit 13 includes second and third reflecting mirrors 52 and 53, and follows the first scanning unit 12 while moving at a speed V / 2, while reflecting the reflected light of the document to the second and third. The light is reflected by the reflection mirrors 52 and 53 and guided to the lens 14. The lens 14 focuses the reflected light of the document on the CCD 15 and forms an image on the surface of the document on the CCD 15. The CCD 15 repeatedly scans the document image in the main scanning direction, and outputs an image signal of one main scanning line each time. The image signal is an analog signal, and the image signal is converted into digital image data by an image processing unit (not shown), and the image data is stored in an image memory (not shown).

The first and second scanning units 12 and 13 are provided with respective pulleys (not shown), wires (not shown) are bridged between these pulleys, and the wires are driven by a stepping motor, The first and second scanning units 12 and 13 are moved synchronously.
During standby, the first scanning unit 12 is positioned at a home position (not shown) between the first position P1 and the third position P3.

  Further, instead of the first and second scanning units 12 and 13, a reduction reading optical system or a unit of the same magnification reading optical system in which an exposure lamp, a lens, a CCD, and the like are integrated may be scanned at a scanning speed V. I do not care.

  The first reading unit 10 reads not only a stationary document but also an image on the surface of the document conveyed by the document conveying unit 70. In this case, the first scanning unit 12 of the first reading unit 10 is moved and positioned at the third position P3, and the second scanning unit 13 is positioned at a predetermined position. In this state, the document is pulled out from the document tray 22, and the document is conveyed to the paper discharge tray 3 through the upper side of the platen glass 11. During the conveyance, the exposure lamp 50 of the first scanning unit 12 irradiates the surface of the document through the platen glass 11, and the reflected light of the document is guided to the lens 14 by the respective reflection mirrors of the first and second traveling units. Then, the reflected light of the document is condensed on the CCD 15 by the lens 14, and an image of the surface of the document is formed on the CCD 15, thereby reading the image of the surface of the document.

  On the other hand, the second reading unit 23 of the upper housing 2 reads an image on the back side of the document when the document transport unit 70 transports the document. The second reading unit 23 is disposed above the platen glass 11, and is an exposure lamp (LED array, fluorescent lamp, etc.) 23a that irradiates the back side of the document, and a cell that collects the reflected light of the document for each pixel. A FOC lens array 23b and a contact image sensor (CIS) 23c that photoelectrically converts reflected light of the original received through the SELFOC lens array 23b and outputs an analog image signal are provided.

  The document transport unit 70 is a document tray 22 on which documents are placed, each document on the document tray 22 is sequentially pulled out and sent to the first document transport path 32 one by one. A first document conveyance path 32 provided with a conveyance roller 34, a second document conveyance path 35 provided with a belt conveyance unit 33 for conveying the document to the second reading unit 23 on the platen glass 11, and a document on the paper discharge tray 3. Each of the paper discharge rollers 40 is provided.

  About 100 originals can be placed on the original tray 22. An original sensor 43 including an optical sensor and an actuator is provided, and the original sensor 43 detects the original. The elevating plate 62 is pivotally supported by a support shaft 64, and the rotation of the output shaft of the elevating motor 61 is transmitted to the support shaft 64 through the gear unit to rotate the elevating plate 62. Raise and lower. The document tray 22 is pivotally supported by a support shaft 22a, and a rib 63 at the right end thereof is placed on the right end of the elevating plate 62. As the right end of the elevating plate 62 is raised and lowered, the right end of the document tray 22 is also raised and lowered.

  The document tray sensor 42 is a reflection type sensor or the like, and detects the standby position of the document tray 22. Based on the detection output of the document tray sensor 42, the elevation motor 61 is driven and controlled to move the document tray 22 to the standby position. The standby position of the document tray 22 is set in advance by a service person, an administrator, or the like, so that when the number of documents on the document tray 22 is the most frequently used, the document can be easily pulled out by the document calling unit 30. Set to Normally, since the most frequently used sheet is about one to several sheets, the highest position is set as the standby position of the document tray 22.

  The document drawing unit 30 includes a drawing roller 37 that pulls out a document from the document tray 22, a separation roller 36 that separates the document one by one and sends it to the first document transport path 32. The attracting roller 37 is pivotally supported on one end of the arm 31, and the other end of the arm 31 is pivotally supported by the rotating shaft of the separation roller 36. Further, a drive unit 39 made of a solenoid or the like is connected to the arm 31, and the arm 31 is rotated by the drive unit 39, and the height of the draw roller 37 is detected by the draw roller position sensor 41, while the draw roller 37 can be pressed against the document on the document tray 22. The separation roller 36 is provided in contact with a roller or a friction pad whose rotation operation is controlled by a torque limiter.

  In the standby state, the arm 31 is pulled up by an urging unit 38 such as a coil spring, and the drawing roller 37 is lifted to release the document tray 22 so that the document can be easily placed on the document tray 22. When a document is placed on the document tray 22, the drive unit 39 is driven and controlled, and the attracting roller 37 is pressed against the document on the document tray 22, so that the document can be pulled out by the attracting roller 37. Each document on the document tray 22 is sequentially pulled out by the drawing roller 37 and separated and conveyed one by one by the separation roller 36.

  The first document conveyance path 32 includes a curved conveyance guide, each conveyance roller 34, and a paper feed sensor 44 that detects the conveyance timing of the document. When receiving a document from the document tray 22, the first document transport path 32 transports the document through a transport guide and guides it to the second document transport path 35.

  The second document conveyance path 35 includes a belt conveyance unit 33 that spans an endless belt made of urethane or the like between a driving roller and a driven roller, and serves as a conveyance guide that guides the document between the belt conveyance unit 33 and the platen glass 11. Yes. A position where the first and second reading units 10 and 23 read the original is located at a position separated from the belt conveying unit 33 by a few ten millimeters downstream. The second document transport path 35 guides the document to each discharge roller 40 through the reading point. Each paper discharge roller 40 discharges a document to the paper discharge tray 3. The paper discharge sensor 45 detects whether or not the original is discharged to the paper discharge tray 3.

  In the image reading apparatus having such a configuration, the original is stopped and the first reading unit 10 reads the image of the original. In the still reading mode, the first and second reading units 10 and 23 convey the original while the original is being conveyed. Either a single-sided reading mode for reading an image or a double-sided reading mode for reading images on both sides of a document by the first and second reading units 10 and 23 while conveying the document can be selected and set.

  In the stationary reading mode, the upper housing 2 is opened, the document is placed on the platen glass 11, and then the upper housing 2 is closed. The first reading unit 10 moves the first scanning unit 12 from the home position to the first position P1 in response to an instruction from a reading control unit described later, and further moves the first scanning unit 12 from the first position P1 to the first position P1. The image is moved in the direction of the two-position P2 by a distance corresponding to the size of the document, and the surface of the document on the platen glass 11 is irradiated while the second scanning unit 13 follows this. Read by.

  In the single-sided reading mode, the document is placed on the document tray 22. In response to an instruction from a reading control unit to be described later, the first scanning unit 12 of the first reading unit 10 is moved from the home position to the third position P3 and positioned, and the second scanning unit 13 is positioned at a predetermined position. . Thereafter, the original is guided between the first and second reading units 10 and 23 while the original on the original tray 22 is pulled out and conveyed by the original conveying unit 70, and the original is further discharged to the discharge tray 3. Then, one of the images on both sides of the document read by the first and second reading units 10 and 23 is selected and output.

  Even in the duplex reading mode, the document is placed on the document tray 22. In response to an instruction from a reading control unit to be described later, the first scanning unit 12 of the first reading unit 10 is moved from the home position to the third position P3 and positioned, and the second scanning unit 13 is positioned at a predetermined position. . Thereafter, the original is guided between the first and second reading units 10 and 23 while the original on the original tray 22 is pulled out and conveyed by the original conveying unit 70, and the original is further discharged to the discharge tray 3. Then, both images on both sides of the document read by the first and second reading units 10 and 23 are output.

  By the way, in the double-sided reading mode, most of the light from the exposure lamp 50 of the first reading unit 10 and the exposure lamp 23a of the second reading unit 23 is reflected by the front and back surfaces of the original P, respectively. Passes through the original P. Therefore, the CCD 15 of the first reading unit 10 receives the light irradiated from the exposure lamp 50 and reflected from the surface of the document P, and not only reads the image on the surface but also irradiated from the exposure lamp 23a to the document P. Even a small amount of light transmitted through the light is received.

  Therefore, in this embodiment, for example, the light receiving output of the CCD 15 of the first reading unit 10 when the white background portion and the white background portion of the original P are illuminated by the exposure lamp 50 and the exposure lamp 23a is set to Iso. The light receiving output of the CCD 15 of the first reading unit 10 when the white surface portion of P is illuminated is Iro, and the first reading when the front image area and the back image area of the original P are illuminated by the exposure lamp 50 and the exposure lamp 23a. If the light reception output of the CCD 15 of the unit 10 is Is, and the correction value of the light reception output Is of the CCD 15 is Irc, the correction value Irc is obtained from the exposure lamp 23a by obtaining the correction value based on the following equation (1). The transmitted light component transmitted through P is suppressed. The correction value Irc of the light reception output of the CCD 15 accurately indicates the image density of the surface image area of the document P.

Irc = Is · Iro / Iso (1)
Here, the light reception output Iso of the CCD 15 of the first reading unit 10 is sampled by illuminating the front and back surfaces of the original P with the exposure lamp 50 and the exposure lamp 23a, for example, at the timing of reading the leading end portion of the original P. Similarly, the light reception output Iro of the CCD 15 of the first reading unit 10 is sampled by illuminating only the surface of the document P by the exposure lamp 50 at the timing of reading the leading end portion of the document P. The leading edge of the document P is read because the leading edge is white and is not affected by the image of the document P.

  As shown in FIG. 2, when the white surface portion of the original P is illuminated by the exposure lamp 50, the light r reflected by the white surface portion is received by the CCD 15 of the first reading unit 10, and the light receiving output Iro of the CCD 15 is obtained. It is done. When the white background portion of the original P is illuminated by the exposure lamp 23a, the light t transmitted therethrough is received by the CCD 15 of the first reading unit 10, and the light reception output Ito of the CCD 15 is obtained. The light receiving output Iso of the CCD 15 of the first reading unit 10 when the white background portion and the white background portion of the original P are illuminated by the exposure lamp 50 and the exposure lamp 23a are the light receiving output Iro of the reflected light r and the transmitted light t. It becomes equal to the sum of the light reception output Iro.

Iso = Iro + Ito (a1)
When the reflectance of the white background portion of the document P is W, the reflectance of the black pigment is b, the area ratio of the black background on the surface of the document P is S, and the surface image area of the document P is illuminated by the exposure lamp 50 If the light receiving output of the CCD 15 is Ir, the light receiving output Ir of the CCD 15 is
Ir≈W * (1−S) + b · S (a2)
And is inversely proportional to the area ratio S of the black background.

Further, if the light transmittance of the document P is α and the light reception output of the CCD 15 when the back surface of the document P is illuminated by the exposure lamp 23a is It, the light reception output It of the CCD 15 is
It≈α * (1-S) (a3)
It is also inversely proportional to the area ratio S of the black background.

If the light reception output of the CCD 15 when the front and back surfaces of the original P are illuminated by the exposure lamp 50 and the exposure lamp 23a is Is, the light reception output Is of the CCD 15 is
Is = Ir + It (a4)
It is also inversely proportional to the area ratio S of the black background.

  The graph of FIG. 3 shows characteristics of Is, Ir, and It that are inversely proportional to the area ratio of the black background of the surface image area of the document P.

  In the graph of FIG. 3, when both the front white background portion and the back white background portion of the original P are illuminated by the exposure lamp 50 and the exposure lamp 23a, the light reception output of the CCD 15 of the first reading unit 10 becomes Iso, and the white background of the original P When the portion is illuminated, the light reception output of the CCD 15 becomes Iro. As is apparent from the graph of FIG. 3, the ratio of Is, Ir, and It is always constant.

Accordingly, the light reception output Ir of the CCD 15 when the surface image area of the original P is illuminated is
Ir = Is · Iro / Iso (a5)
It becomes. By comparing this equation (a5) with the above equation (1), the correction value Irc of the light reception output of the CCD 15 received the light reception output It of the CCD 15 when the back surface of the original P is illuminated, that is, the light transmitted through the original P. It is clear that the light reception output It of the CCD 15 is not included.

  Therefore, if the correction value Irc of the light reception output of the CCD 15 is obtained based on the above equation (1), the transmitted light component irradiated from the exposure lamp 23a and transmitted through the original P is suppressed.

  Each table A, B, and C in FIG. 4 sets the light transmittance α of the original P to 0.1, 0.3, and 0.5, respectively, and the surface image of the original P for each of the light transmittance α. The area ratio of the black background of the region is changed stepwise, the light reception output Ir of the CCD 15 when the surface of the original P is illuminated, the image density, the light reception output It of the CCD 15 when the back side of the original P is illuminated, and the surface of the original P In addition, the light receiving output Is of the CCD 15 when the back surface is illuminated and the correction value Irc of the light receiving output Is of the CCD 15 are obtained and shown.

  In each of Tables A, B, and C, as is apparent from a comparison of the light reception output It of the transmitted light, the light reception output Is of the reflected light and the transmitted light, and the correction value Irc, the correction value Irc is calculated from the light reception output Is. The light receiving output It is suppressed.

  As described above, in this embodiment, the correction value Irc of the light reception output of the CCD 15 that accurately indicates the image density of the surface image area of the document P is obtained based on the above equation (1). For this reason, it is not necessary to regulate the spectral characteristics of the CCD 15 of the first reading unit 10 and the contact image sensor 23c of the second reading unit 23 at all, it can be applied to a color image, and complicated image processing is required. do not do.

  If the transmittance of the original P does not change greatly, Iro / Iso may be preset as a correction coefficient, and this correction coefficient may be stored in the memory of the image reading apparatus.

  In addition, the contact image sensor 23c of the second reading unit 23 receives light reflected from the surface of the original P by being irradiated from the exposure lamp 23a, and not only reads an image on the surface but also irradiated from the exposure lamp 50. Since a small amount of light transmitted through the document P is also received, the light reception of the contact image sensor 23c of the second reading unit 23 when the relationship between the front and back surfaces of the document P is reversed and the front and back surfaces of the document P are illuminated. It is also possible to correct the output.

  Furthermore, since sampling of data for shading correction is performed with the reference white plate illuminated when there is no document, it is performed when either the exposure lamp 50 or the exposure lamp 23a is illuminated. Thereby, it is not necessary to receive stray light by the CCD 15 of the first reading unit 10 or the contact image sensor 23c of the second reading unit 23, and data errors due to stray light can be suppressed.

  Next, as a second embodiment of the image reading apparatus of the present invention, another calculation process for correcting the light reception output of the CCD 15 of the first reading unit 10 and suppressing the transmitted light component transmitted through the document P will be described. .

  Note that the image reading apparatus of the present embodiment also has the same configuration as that of the apparatus of FIG. 1, and only the arithmetic processing for suppressing the transmitted light component is different.

  First, in the double-sided reading mode, as described above, although most of the light from the exposure lamp 50 of the first reading unit 10 and the exposure lamp 23a of the second reading unit 23 is reflected by the front and back surfaces of the document P, A small amount of the light passes through the original P. For this reason, the CCD 15 of the first reading unit 10 receives both the reflected light reflected by the original P and the transmitted light transmitted through the original P, and reads a double image in which the front and back images overlap.

  Therefore, in this embodiment, the front image area and the back image area of the document P are illuminated by the exposure lamp 50 and the exposure lamp 23a, and the CCD 15 of the first reading unit 10 sets the same position where the front and back surfaces of the document P overlap each other. And the light receiving output of the CCD 15 and the contact image sensor 23c at this time are Is1 and Is2, respectively, the correction value of the light receiving output Is1 of the CCD 15 is Irc, and the light transmission of the document P is performed. When the rate is α, the correction value Ic of the light reception output Is1 of the CCD 15 is obtained based on the following equation (2), and the transmitted light component irradiated from the exposure lamp 23a and transmitted through the original P is suppressed. The correction value Ird of the light reception output Is1 of the CCD 15 accurately indicates the image density of the front surface image area of the original P and suppresses the image density of the back surface image area. Therefore, the correction value Ird of the light reception output Is1 of the CCD 15 shows only the image on the front surface of the document P, and does not show the double image in which the images on the front surface and the back surface overlap.

Ird = (1−Is2 · α ^ (1 + α)) Is1 (2)
Here, the reflectance of the white background portion of the document P is W, the reflectance of the black pigment is b, the area ratio of the black background of the surface of the document P is S1, and the first image when the surface image area of the document P is illuminated is the first. If the light reception output of the CCD 15 of the reading unit 10 is Ir1, the light reception output Ir1 of the CCD 15 is
Ir1≈W * (1−S1) + b · S1 (b1)
It becomes.

If the light transmittance of the original P is α and the light reception output of the CCD 15 when the back surface of the original P is illuminated is It1, the light reception output It1 of the CCD 15 is
It1≈α * (1-S1) (1-S2) (b2)
It becomes.

Further, if the light receiving output of the CCD 15 when the front and back surfaces of the original P are illuminated is Is1, the light receiving output Is1 of the CCD 15 is:
Is1 = Ir1 + It1 (b3)
It becomes.

Similarly, if the area ratio of the black background on the back side of the document P is S2, and the light reception output of the contact image sensor 23c of the second reading unit 23 when the back image area of the document P is illuminated is Ir2, this contact image sensor 23c. The received light output Ir2 of
Ir2≈W * (1-S2) + b.S2 (b4)
It becomes.

If the light transmittance of the document P is α and the light reception output of the contact image sensor 23c when the surface of the document P is illuminated is It2, the light reception output It2 of the contact image sensor 23c is
It2≈α * (1-S2) (1-S1) (b5)
It becomes.

Furthermore, if the light reception output of the contact image sensor 23c when the front and back surfaces of the document P are illuminated is Is2, the light reception output Is2 of the contact image sensor 23c is
Is2 = Ir2 + It2 (b6)
It becomes.

  As apparent from the above formula (b3), the light receiving output Ir1 of the CCD 15 when the front image area of the document P is illuminated is the back surface of the document P from the light receiving output Is1 of the CCD 15 when the front and back surfaces of the document P are illuminated. Is obtained by subtracting the light reception output It1 of the transmitted light when the light is illuminated, and the same can be said for the correction value Ird of the above equation (2). The received light output of the transmitted light when the back surface of the document P is illuminated corresponds to any of the above formulas (b2) and (b5) and Is1 · Is2 · α ^ (1 + α) in the above formulas (2). It varies depending on the area ratio of the black background on the front and back surfaces of the document P, and reflects the image density on the front and back surfaces of the document P. Therefore, if the correction value Ird of the light reception output of the CCD 15 is obtained based on the above equation (2), the image density on the front surface of the document P can be accurately shown and the image density on the back surface can be suppressed.

  Each table D, E, and F in FIG. 5 sets the light transmittance α of the original P to 0.1, 0.3, and 0.5, respectively, and the surface image of the original P for each of the light transmittance α. The area ratio of the black background of the area is kept constant, and the area ratio of the black background of the back image area is changed stepwise to illuminate the front and back surfaces of the original P. The correction value Ird of the light reception output Is2 and the light reception output Is1 of the CCD 15 is obtained and shown.

  As is apparent from Tables D, E, and F, the correction value Ird of the light reception output Is1 of the CCD 15 can be accurately obtained regardless of the black area ratio of the back image area.

  In this way, in this embodiment, based on the above equation (2), the correction value Ird of the light reception output of the CCD 15 that accurately indicates the image density of the front surface image area of the original P and suppresses the back side image density is obtained. ing. For this reason, it is not necessary to regulate the spectral characteristics of the CCD 15 of the first reading unit 10 and the contact image sensor 23c of the second reading unit 23 at all, it can be applied to a color image, and complicated image processing is required. do not do.

  Note that the transmittance of the original P is, for example, by illuminating the back surface of the original P by the exposure lamp 23a at the timing of reading the leading end portion of the original P, sampling the light reception output of the CCD 15 of the first reading unit 10, and receiving this light. What is necessary is just to obtain | require based on an output.

  Further, the contact image sensor 23c of the second reading unit 23 also receives both the reflected light reflected by the original P and the transmitted light transmitted through the original P, and reads a double image in which the front and back images are overlapped. It is also possible to correct the light reception output of the contact image sensor 23c of the second reading unit 23 when the front and back surfaces of the document P are illuminated by reversing the relationship between the front and back surfaces of the document P.

  Furthermore, since sampling of data for shading correction is performed with the reference white plate illuminated when there is no document, it is performed when either the exposure lamp 50 or the exposure lamp 23a is illuminated. Thereby, data errors due to stray light can be suppressed.

  Further, the correction value Ird1 of the light reception output Is1 of the CCD 15 may be obtained using the following approximate expression (3) instead of the above expression (2).

Ird = (1−Is2 · α) Is1 (3)
Table G in FIG. 6 changes the light transmittance α of the document P stepwise, illuminates the front and back surfaces of the document P for each light transmittance α, and receives the light reception output Is1 of the CCD 15 and the contact image sensor 23c. , The correction value Ird based on the above formula (2), and the correction value Ird1 based on the above formula (3).

  As is apparent from Table G, if the light transmittance α of the original P is low, a correction value Ird1 based on the above equation (3) can be obtained with a small error.

It is a side view which shows Example 1 of the image reading apparatus of this invention. FIG. 2 is a diagram conceptually illustrating illumination on the front surface and the back surface of a document in the image reading apparatus of FIG. 1. 6 is a graph showing characteristics of Is, Ir, and It that are inversely proportional to the black area ratio of the surface image area of the document. A, B, and C are graphs obtained by obtaining Ir, It, Is, Irc, and the like by gradually changing the area ratio of the black background of the surface image area of the document for each light transmittance of the document. D, E, and F maintain the black area ratio of the front image area of the document constant and change the black area ratio of the back image area step by step for each light transmittance of the document. Is1, Is2 It is a chart which calculates | requires and shows Ird. G is a chart obtained by obtaining Is1, Is2, Ird, and Ird1 for each light transmittance of the document.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lower housing | casing 2 Upper housing | casing 3 Paper discharge tray 10 1st reading part 11 Platen glass 12 1st scanning unit 13 2nd scanning unit 15 CCD
22 Document tray 23 Second reading unit 23a Exposure lamp 23b Selfoc lens array 23c Contact image sensor 50 Exposure lamp

Claims (5)

An image reading apparatus including first and second light sources that illuminate first and second surfaces of a document, respectively, and first and second image reading units that respectively read first and second surfaces of the document.
The light receiving output of the first image reading unit when the first light source illuminates the white background portion of the first surface of the document and the first and second light sources when the white background portion of the first and second surfaces of the document are illuminated. Using the ratio with the light reception output of the one image reading unit, correcting the light reception output of the first image reading unit when the first and second light sources illuminate the image areas of the first and second surfaces of the document. A characteristic image reading apparatus. When the first and second light sources illuminate the white background portions of the first and second surfaces of the document, the light reception output of the first image reading unit is Iso, and the first light source illuminates the white background portion of the first surface of the document. The light reception output of the first image reading unit is Iro, the light reception output of the first image reading unit when the first and second light sources illuminate the image areas of the first and second surfaces is Is, If the correction value of the light reception output Is of one image reading unit is Irc,
Irc = Is · Iro / Iso (1)
The image reading apparatus according to claim 1, wherein: An image reading apparatus including first and second light sources that illuminate first and second surfaces of a document, respectively, and first and second image reading units that respectively read first and second surfaces of the document.
The first and second light sources illuminate the first and second surfaces of the document, and the first and second image reading units read the same overlapping positions of the first and second surfaces of the document. The light reception outputs of the first and second image reading units are Is1 and Is2, the light transmittance of the original is α, and the correction value of the light reception output Is1 of the first image reading unit is Ird.
Ird = (1−Is2 · α ^ (1 + α)) Is1 (2)
An image reading apparatus characterized by the above. An image reading apparatus including first and second light sources that illuminate first and second surfaces of a document, respectively, and first and second image reading units that respectively read first and second surfaces of the document.
The first and second light sources illuminate the first and second surfaces of the document, and the first and second image reading units read the same overlapping positions of the first and second surfaces of the document. The light reception outputs of the first and second image reading units are Is1 and Is2, the light transmittance of the original is α, and the correction value of the light reception output Is1 of the first image reading unit is Ird.
Ird = (1−Is2 · α) Is1 (3)
An image reading apparatus characterized by the above. 5. The image reading apparatus according to claim 1, wherein sampling of data for shading correction is performed during illumination of one of the first and second light sources.

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