JP2003244408A - Image reader - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image reader in a configuration in which the satisfactory results can be maintained in the image read accuracy, especially, of density nonuniformity or color slurring and further of color reproducibility of an original in the image reader having temporally different light attenuation characteristics for each pixel position in the main scanning direction. <P>SOLUTION: The device is provided with a white main scanning member 8 positioned between an original platen 2 and a sheet read position 3 and uniform all over the area of the main scanning direction and a white subscanning reference member 11 positioned on the line of the sheet read position 3 and provided outside the terminal of the original in the main scanning direction and when turning on a power source, the read data of the white main scanning member 8 are fetched for a prescribed time to be stored as initial data. When reading a sheet-like original, the attenuation rate for each pixel in a line sensor 4 is estimated from the data read by the white subscanning reference member 11 and the initial data and the gain of an output level is controlled for each pixel. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading device, and more particularly, to a configuration for ensuring the reading accuracy of image information.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a printing machine, an electrostatic latent image is formed on a latent image carrier according to the result of scanning an original image. An imaging optical element capable of photoelectric conversion (hereinafter referred to as CC
(Referred to as D). In this system, the reflected light from the original is imaged on the CCD and an output signal for each pixel of the CCD is obtained as image information. The image information obtained in the image forming apparatus is used to set the output form of the writing optical system at the time of optical writing on the photoconductor which is an electrostatic latent image body.

In the reading system using CCD,
Line scanning is performed in the main scanning direction corresponding to the width direction of the document, and the document transport direction corresponding to the direction perpendicular to the width direction is the sub-scanning direction. By moving the document in the sub-scanning direction, the entire surface of the document is scanned. Scanning is performed.

At the time of image reading, a document placed on the document table is scanned by a reading optical system capable of reciprocating in the sub-scanning direction, and the document is moved with the reading optical system fixed. There is a case where one of the formats to be scanned by the reading optical system is selected.

A common light source is used in both types, but depending on the configuration used for the light source, the light amount distribution in the main scanning direction may change, and a reading error in the main scanning direction may occur. In other words, when using a fluorescent lamp,
It is known that the amount of light changes according to environmental changes, lighting time, or cumulative time.

A change in the illuminance distribution due to a change in the amount of light from the light source may cause density unevenness when the image of the original is read. That is, the result that the reading condition of the original document differs for each original document is brought about. Therefore, conventionally, correction control called shading correction is executed in order to prevent a change in light intensity, in other words, a change in image density due to a change in illuminance distribution (for example, JP-A-001-94737). Japanese Patent Laid-Open No. 8-223415).

In the former one of the above publications, a white reference member is provided outside the original scanning area, and density data for this reference member is stored before the original is scanned. A structure is disclosed in which the unevenness of the light amount distribution of the device, the unevenness of the sensitivity of the CCD, and the output fluctuations from these data are corrected by comparing with the data to prevent the accuracy change of the image information of the original. In addition to this, when the reading optical system is fixed and the original is moved and read, another white reference member for moving the original intended for moving the original is read, and the read output signal is always used as a reference for constant reading. There is disclosed a configuration in which the gain in the drive circuit of the line sensor using the CCD is changed so that the output signal from the line sensor is corrected.

In the latter publication, the deterioration of the throughput due to the reading of the white reference member is performed every time the original scanning is started, and the correction in the main scanning direction due to the change in the light amount in the main scanning direction with the lighting time or the accumulated time is performed. In order to prevent a decrease in accuracy, the correction data for the light intensity change at the edge is calculated based on the light intensity at the center of the main scanning direction where the temperature changes drastically every time the document is read, and the light intensity change at the edge is corrected and the document is read. Regarding the light amount correction at time, a configuration is disclosed in which the data for light amount correction is acquired only by reading the white reference portion provided at the document reading position.

[0009]

In the structures disclosed in the above publications, the light amount distribution in the main scanning direction is made constant by performing the correction at the end portion in the main scanning direction where the light amount changes remarkably. It is possible to avoid a change in the read density of the.

However, in each of the configurations disclosed in these publications, the light quantity data is not always acquired for the white reference member provided outside the original scanning area, and the copy quantity is not changed every time the copy job starts or the original is read. Since each scan is temporary (instantaneous), no consideration is given to the light amount distribution in the main scanning direction based on the attenuation rate due to a temporal change. Therefore, when the gain in the line sensor drive circuit is adjusted so as to make the output of the line sensor constant based on the above data, it is possible to make the image density at the end portion in the main scanning direction constant, but At the central part in the main scanning direction, where the temporal attenuation rate is different from that at the edge, a deviation from the actual light amount occurs, causing density unevenness and color deviation when a color image is targeted, resulting in poor color reproducibility. There is a problem such as doing.

In view of the problems in the image reading apparatus having the conventional structure for stabilizing the image density, an object of the present invention is to read an image having a light attenuation characteristic that temporally differs for each pixel position in the main scanning direction. An object of the present invention is to provide an image reading apparatus having a configuration capable of maintaining a good result in image reading accuracy of an original, particularly density unevenness, color shift, and color reproducibility.

[0012]

The invention according to claim 1 is
The reflected light from a document illuminated by using a light source whose light amount distribution changes with time is imaged on a line sensor capable of photoelectric conversion through an imaging optical system including an illumination source and an imaging lens, whereby the document main In an image reading device for reading a line in the scanning direction, a document placed on a document placing table installed in advance is moved in the sub-scanning direction by moving the image forming optical system, and the front face of the document can be read. The image reading means of No. 1 and the image forming optical system are stopped, and the sheet original is conveyed in the sub-scanning direction toward a sheet reading position different from the original placing table, so that the front surface of the sheet original is moved. A second image reading unit for reading, a main scanning white member located between the document mounting table and the sheet reading position and uniform in the entire main scanning direction for reading, and the sheet reading unit. It is provided with a sub-scanning white reference member which is located on the line of the taking position and is provided on the outer side of the end portion in the main scanning direction of the document. When the sheet-like document is read by the second image reading means, the attenuation factor for each pixel in the line sensor is estimated from the read data by the sub-scanning white reference member and the initial data, and each image difference is obtained. Is equipped with a means for adjusting the gain of the output level.

The invention described in claim 2 is characterized in that, in addition to the invention described in claim 1, the initial data is read while thinning out within a predetermined time.

The invention according to claim 3 is the invention according to claim 1 or 2.
In addition to the described invention, the initial data is characterized in that it is read while thinning out the reading time within a predetermined time so as to be sequentially longer from the start of reading.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing an overall configuration of an image reading apparatus according to an embodiment of the present invention. In the figure, the image reading apparatus 1 is placed on a document placing table 2 provided on an upper part of an apparatus housing 1A. A second reading unit for reading a sheet-like original moving on a sheet-through glass 3 made of transparent glass arranged at a reading position for the sheet-like original separately from the first reading means A for reading the original and the original placing table 2. A means B and an image forming optical system C for forming an image by the line sensor 4 capable of photoelectrically converting the reflected light from the original when the first and second reading means A and B are used are provided.

The first reading means A comprises an original placing table 2 and a swingable original pressing member 5 which covers the original placing table 2 and presses the original placed, and the original placing table 2 is shown in FIG. The sub-scanning side scale member 6 which forms an edge placement reference portion in the sub-scanning direction (left and right direction in FIGS. 1 and 2) of the document to be placed, and the document which is located in one of the directions perpendicular to the sub-scanning side scale member 6. The main-scanning side scale member 7 which forms an edge mounting reference portion in the main-scanning direction (vertical direction in FIG. 2) is provided.

In the first reading means A, the document table 2
The document to be read is positioned by abutting the edges of the document against the respective scale members 6 and 7 in the main scanning direction and the sub-scanning direction provided at the edges of the document. The original document is pressed by the document holder 5 to be read in a state where light leakage is prevented.

A main scanning white reference plate 8 is arranged below the sub-scanning scale 6 on the document mounting table 2. The main-scanning white reference plate 8 is a member having a uniform white color in the main-scanning direction, and is used to read data that serves as a reference for determining a change in light amount during reading.

Although not described in detail, the second reading means B reads a document fed from an automatic document feeder (ADF) 9 capable of separating and feeding the stacked sheet-like documents one by one. An arrangement tray using a sheet through glass 3 provided separately from the document placing table 2 and arranged at a reading position of a sheet-like document, and an upper surface of a document retainer 5 located above the document placing table 2. 5A and. The sheet through glass 3 is positioned in the vicinity of the sub-scanning side scale 8 on the document mounting table 2, and a sheet document reading roller 10 that rotates while pressing a sheet document is disposed on the upper surface.

As shown in FIG. 2, the sub-scanning white reference plate 11 is arranged on the sheet through glass 3 at a position outside the image area in the main scanning direction. Sub-scanning white reference plate 11
Is used to read data as a reference for determining a change in the light amount at the time of reading, similarly to the main scanning white reference plate 8.

The image forming optical system C includes an image forming lens 12 which is provided in a stationary state, a line sensor 4 such as a photoelectrically convertible CCD, a first carriage 13 and a second carriage which are provided with an illumination source and a reflecting mirror. 14 and. Hereinafter, each carriage will be described as follows.
In FIG. 1, the first carriage 13 includes an illumination source 15 for a document and reflected light from the document on the second carriage 1.
The second carriage 14 is equipped with a first reflecting mirror 16 for guiding the reflected light from the first reflecting mirror 16 toward the imaging lens 12, and second and third reflecting mirrors 17, 18 are provided. Is equipped with.

As the illumination source 15 mounted on the first carriage 13, a cold cathode discharge tube (hereinafter, referred to as Xe lamp for convenience) whose light amount changes with time is used, and further direct light is directed toward the document. Light other than direct light is the illumination source 15
The original is illuminated in a state in which the insufficient amount of light to the original is prevented by being reflected by the illumination light reflection plate 19 arranged so as to face the original. The illumination light reflection plate 19 is a member in which a resin film is coated with a reflection film such as aluminum, and has a concave circumferential surface so that the light from the illumination source 15 can be efficiently condensed toward the document. It is attached to the support portion of the first carriage 13.

The first and second carriages 13 and 14 are shown in FIG.
Is set as the initial position, that is, the position where the first carriage 13 having the illumination source 15 faces the sheet through glass 3 is set as the initial position, and the speed ratio when moving is determined by the structure of the drive system to be described later. First using the principle of
Carriage 13 is half of second carriage 14 (1/2)
It is supposed to be the relationship of speed.

FIG. 3 shows the first and second carriages 13 and 14.
FIG. 1 is a perspective view showing the overall configuration of the first carriage 13, in which a first carriage 13 is composed of a frame that extends in the main scanning direction and has an open upper surface, and an Xe lamp used as an illumination source 15 is provided on the open surface side. A longitudinal direction is set in the main scanning direction, and the device is laterally bridged. An illumination reflection plate (not shown) and a first reflection mirror 16 (see FIG. 1) are arranged below the illumination reflection plate, which are not shown.

The second carriage 14 is composed of a frame having a U-shape in a plan view, and as shown in FIG. 4, an optical path is provided between end plates located at both longitudinal ends parallel to the main scanning direction. The longitudinal ends of the second and third reflecting mirrors 17 and 18 for reversing 180 degrees are attached. First
All of the third to third reflecting mirrors 16 to 18 have a reflecting surface fixed by being urged by a plate spring (not shown) made of stainless steel in the supporting portion. The reflecting surfaces of the second and third reflected lights 17 and 18 are positioned with the facing angle set to 90 degrees,
Image forming optical system C in which the light from the reflecting mirror 16 is inverted by 180 degrees
It guides the reflected light from the original toward.

The image forming optical system C includes a bracket 20 supported by a stationary portion of the image reading apparatus 1, and a lens unit 21 and an image forming lens 12 each having an image forming lens 12 housed on the upper surface of the bracket 20. CCD substrates 22 each having a line sensor 4 such as a CCD capable of photoelectric conversion located on the optical axis are attached and fixed.

The first and second carriages 13 and 14 are slidably provided in the sub-scanning direction by guide rails (not shown), and are reciprocated in the sub-scanning direction by a drive system having the configuration shown in FIG. Can move. That is, in FIG. 5, the drive system includes the first carriage belt 23 that drives the first carriage 13 and the second carriage belt 24 that drives the second carriage 14.

The first carriage belt 23 is mounted on a drive shaft 27 driven by a drive belt 26 which is arranged at one end in the sub-scanning direction and is stretched between the output shaft of a stepping motor 25 used as a drive source. The drive pulley 28 which is axially supported and the driven pulley 29 which is axially supported at the other end in the sub-scanning direction are wound around and each end is coupled to the sliding portion of the first carriage 13, and the first carriage 13 is stepped. The motor 25 is moved in a direction corresponding to the rotation direction.

The second carriage belt 24 includes a pulley 30 and a second pulley which are axially supported by an end plate of the second carriage 14 on which the second and third reflecting mirrors 17 and 18 are arranged.
The bracket 3 is wound around a pulley 31 pivotally supported by a U-shaped tip portion of the carriage 14 in a plan view, and an upper extension portion is attached to the first carriage 13 side.
2 is integrated with the first carriage 13 and a part of the lower extension part is connected to a bracket 33 provided on a non-moving part of the image reading apparatus 1.

Belts 23 and 24 for the first and second carriages
Among them, the second carriage belt 24 can be interlocked with the movement of the first carriage 13, and in the case of interlocking, a part of the lower extension part is coupled to the stationary bracket 33. Therefore, a known moving pulley structure is configured by the above, whereby the second carriage 14 can be moved to the first
The carriage 13 is moved in the same direction as the carriage 13 at a speed half that of the first carriage 13 and a half distance.

The first carriage belt 23 is provided with a mounting support portion of a stepping motor 25, which is a drive source, slidable with respect to a stationary portion of the image reading apparatus 1, and a biasing force of a spring (not shown). Is used to apply tension, and the second carriage belt 24
Is configured such that the shaft support portion of the pulley 31 is slidably provided and that tension is applied by utilizing the urging force of a spring (not shown).

In the above structure, the first reading means A,
In the image reading apparatus 1 including the second reading unit B and the image forming optical system C, the mounted original reading mode which is the reading mode of the original using the first reading unit A and the second reading unit B are used. The original sheet-shaped document reading mode can be executed.

In the placed original reading mode, the original placed on the original placing table 2 is scanned by moving the first and second carriages 13 and 14 provided in the imaging optical system C in the sub-scanning direction. By doing so, the original image is read. Therefore, in the imaging optical system C, the first carriage 13 and the second carriage 14 are moved at a speed ratio of 2: 1 (first carriage 13: second carriage 14) from the initial position,
The first reflecting mirror 16 and the second carriage 14 which irradiate the original with illumination light from an illumination source 15 mounted on the first carriage 13 and have the reflected light from the original on the first carriage 13.
The light is transmitted through the image forming lens 12 through the second and third reflecting mirrors 17 and 18 provided on the line sensor 4 to form an image on the line sensor 4. The line sensor 4 performs photoelectric conversion processing corresponding to the reflected light that has formed an image, and outputs the image information converted into an electric signal to an image formation control unit (not shown).

In the sheet-shaped document reading mode, the first document in the image-forming optical system C is synchronized with the document fed from the ADF 9 passing through the sheet-through glass 3 with the image-forming optical system C fixed at the initial position. Illumination light from the illumination source 15 provided in the carriage 13 is applied to the document, and the reflected light from the document is output as image information by the same processing as in the above-described placed document reading mode.

Next, the characteristic part of this embodiment will be described. In the present embodiment, it is known that the amount of light at the illumination source 15 is attenuated with time, and therefore, the illumination source 15
It is noted that the light amount attenuation at each position in the longitudinal direction is not constant. That is, FIG. 6 is a diagram showing output values when a uniform white portion is read, and as is clear from the figure, attenuation starts near the central portion where heat generation is large in the longitudinal direction of the illumination source 15, Attention is paid to the fact that the damping begins at a slight delay at the end.

The vertical direction in FIG. 6 indicates the amount of light, the horizontal direction indicates the respective positions in the longitudinal direction of the Xe lamp used as the illumination source 15, and the vertical central line P1 in the horizontal direction indicates the center of the image reading apparatus 1. , And an area between the vertical lines P2 indicates an image area.

The results indicated by the symbols L1 to L6 in FIG.
It shows a state in which it changes sequentially with time, and shows a case where the result indicated by the reference L1 is shorter than the result indicated by the reference L2. In addition, FIG. 7 shows the results shown in FIG.
It is the diagram displayed dimensionally. In FIG. 6, reference numerals L1 and L6
As can be seen from the results shown by and, after a certain period of time, the distribution form is the same as that of the light amount distribution immediately after lighting, and such a phenomenon progresses within a relatively short period of time. For this reason, the time corresponds to the leading edge of the document when reading an image. For example, when reading a color image, especially halftones where the density difference is subtle, the light intensity distribution is different and (Main scanning direction) Color unevenness or color misregistration easily occurs between the center and the end.

In the present embodiment, the problem caused by the difference in the light amount attenuation rate described above is targeted especially at the sheet-shaped document reading mode which is a reading mode in which the light amount changes with time when the illumination source 15 is continuously lit. As a result, it is supposed to be solved by the following method.

In the present embodiment, the attenuation rate is estimated by using the main scanning white reference plate 8 and the sub-scanning white reference plate 11 shown in FIG. 2 based on the read data obtained from each white reference plate. The output level gain of the line sensor 4 is adjusted so as to cancel out the light intensity distribution, and the light amount distribution in the main scanning direction is stabilized. Hereinafter, a specific description will be given. FIG. 9 is a block diagram showing the configuration of the control unit 34 used for suppressing the change in the density of the read image based on the change in the light amount distribution. In FIG. 9, the control unit 34 is
A microcomputer provided for controlling operation of each member according to selection of each mode in the image reading apparatus 1 is a main part, and is a member related to the present embodiment via an I / O interface (not shown). An input unit for an output signal from a certain line sensor 4, a sensor drive circuit 35, and an image forming processing unit 36 for controlling the writing mode to the latent image carrier by the output signal from the line sensor 4 are connected.

As shown in FIG. 10, the control section 34 moves the image forming optical system C from the initial position to the position of the main scanning white reference member 8 when the power of the image reading apparatus 1 is turned on (ST.
1) The reading operation is performed for a predetermined time (ST2).

In the present embodiment, after the illumination source 15 is turned on, the reading operation is performed for 30 seconds by the time management of the control unit 34, and the output data from the line sensor 4 is stored (stored).
Yes (ST3). In this case, the reading scan is performed for about 30 seconds when the number of pixels in the main scanning direction is about 7500 pixels. However, if the data for each pixel is obtained in such a time, the data amount becomes too large. 5 to 20 in the embodiment
Reading is performed by thinning out the lines. Then, as a thinning method, 0 second, 1 second, 2 seconds, 4
Second, 6 seconds, 9 seconds, 12 seconds, 16 seconds, 20 seconds, 25 seconds, 30 seconds, and so on. It reads finely and coarsely. By using such a reading procedure, the light amount attenuation of the Xe lamp used as the illumination source 15 decreases sharply at the beginning and gradually decreases gradually, so that the error in complementing the data at equal intervals becomes large. It is possible to prevent it. Note that FIG. 10 shows that the thinning process is performed in step ST2.

In addition, when the data of all pixels for one line is complemented, the amount of data becomes too large, which may increase the time required for the complementing process. The data is thinned out. In the present embodiment, the data obtained by such a procedure, so-called initial data, is stored (stored) as attenuation reference data.

When the reading scan for the main scanning white reference member 8 is completed (ST4), the imaging optical system C is returned to the initial position (ST5), and the reading of the sheet-like original fed from the ADF 9 is started. (ST6) Since the sheet-through glass 3 arranged at the initial position where the imaging optical system C returns is provided with the sub-scanning white reference member 11, the sub-scanning white reference member is read together with the reading of the sheet original. 11 is also read.

Since the sub-scanning white reference member 11 is provided outside the image forming area on the reading line for the sheet-like original, it corresponds to the data for the uniform white reference plate shown in FIG. , As shown in FIG. 8, it decays with time. In this embodiment, this data is captured as attenuation data.

In the present embodiment, the complementing process is performed using the attenuation reference data that has been read in and saved in advance, as shown in FIG.
The three-dimensional light amount data as shown in Fig. 3 is partially and sequentially restored along the attenuation reference data, and the data obtained at the position of the sub-scanning white reference plate on the attenuation reference data is compared with the attenuation data. It is judged which time axis line data in the result shown in FIG. 7 matches (ST7), the attenuation curve for each pixel on the obtained time axis line is calculated, and the gain (correction value) of the output data is calculated. Is calculated (ST8), and a correction process is performed on each pixel data so as to cancel the influence of light amount attenuation (ST9). The correction processing result in step ST9 is output to the drive circuit 35 of the line sensor 4. Such correction processing is performed for each scanning line or at regular intervals so that the original image is faithfully or optimally reproduced by making the attenuation rate constant over the entire surface of the image so that the image density does not change. Enable to output data.

When the attenuation data becomes smaller than the data of the sub-scanning white reference plate position on the attenuation reference data,
After that, it is determined that the light amount maintains substantially the same distribution state, and the attenuated light amount data for each pixel in the final line (in the present embodiment, this means the restored attenuated light amount data after 30 seconds, and in FIG. The curve on the right is equivalent), the gain (correction value) is calculated based on the attenuation data, and the correction process is performed on each pixel data to cancel the influence of the light amount attenuation. By such processing, the image density change due to the light amount change over the entire main scanning direction is eliminated, and especially in the case of a halftone image, color shift or color unevenness caused by the difference in light amount distribution between the central portion and the end portion is eliminated. Can be prevented.

[0047]

According to the first aspect of the invention, when the sheet-shaped original is read by the second reading means, the attenuation rate for each pixel is obtained by using the read data of the sub-scanning white reference member and the initial reference data. Since it is possible to adjust the gain of the output level for each pixel by estimating the above, even if an illumination source having a light amount attenuation characteristic that temporally differs for each pixel is used, there is a difference in the attenuation rate in the main scanning direction. Regardless, it is possible to stabilize the output level and make the image reading density constant.

According to the second and third aspects of the invention, the reading is performed in the thinned state for a predetermined time, and particularly in the third aspect of the invention, the thinning is performed so that the reading becomes longer sequentially from the start of reading. It is possible to acquire data for performing an appropriate gain adjustment of the output level in a state where the sampling load is reduced without an excessive amount of data within a predetermined time.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an overall configuration of an image reading device according to an embodiment of the present invention.

FIG. 2 is a plan view for explaining a configuration of a document placing table peripheral portion used in the image reading apparatus shown in FIG.

FIG. 3 is a perspective view for explaining a main configuration of first and second reading units used in the image reading apparatus shown in FIG.

FIG. 4 shows the first and second reading devices shown in FIG.
It is the figure shown from a different direction, and (A) is a top view,
(B) is a side view.

5 is a perspective view for explaining the configuration of a drive system used in the first and second image reading units shown in FIG.

FIG. 6 is a diagram showing a change over time in read data when a white reference plate is read by the image reading apparatus according to the embodiment of the present invention.

FIG. 7 is a three-dimensional diagram showing the change over time shown in FIG.

FIG. 8 is a diagram for explaining a decay state with time in an illumination source.

FIG. 9 is a block diagram illustrating a configuration of a control unit used in the image reading apparatus according to the embodiment of the present invention.

FIG. 10 is a flowchart for explaining the operation of the control unit shown in FIG.

[Explanation of symbols]

1 image reading device 2 document placing table 3 on which a document can be placed 3 sheet through glass on which a sheet-like document moves 4 image forming element 6 sub-scanning direction edge of document Main-scanning side scale 8 which forms an edge placement reference portion in the scanning direction 8 main-scanning white reference plate 9 automatic document feeder (ADF) 11 sub-scanning white reference plate 12 imaging lens

Continued front page    F term (reference) 5B047 BA01 BA02 BB02 BC05 BC09                       BC18 CB04 CB25 DA04 DC06                 5C072 AA01 BA08 CA02 DA02 DA04                       EA05 FB12 FB13 LA18 RA16                       UA02 UA05                 5C077 LL04 MM27 PP08 PP47 PQ12                       PQ20 PQ24 SS01 TT06

Claims (3)

[Claims] 1. An image of reflected light from a document illuminated by using a light source whose light amount distribution changes with time is formed on a line sensor capable of photoelectric conversion through an image forming optical system including an illumination source and an image forming lens. In the image reading apparatus for reading the lines in the main scanning direction of the document by scanning the document, the document placed on the document mounting table installed in advance is moved in the sub-scanning direction by moving the image forming optical system to scan the front face of the document. By transporting the sheet-shaped document in the sub-scanning direction toward a sheet reading position different from the first image reading unit which can read the image forming optical system and the image forming optical system. Second image reading means for reading the front surface of the sheet-like document, and a main scanning white member located between the document placing table and the sheet reading position and uniform in the entire main scanning direction for reading. It is provided with a sub-scanning white reference member which is located on the line of the above-mentioned sheet reading position and is provided outside the end portion of the document in the main scanning direction. Data is stored as data, and at the time of reading a sheet-like document by the second image reading means, the attenuation rate for each pixel in the line sensor is estimated from the read data by the sub-scanning white reference member and the initial data for each image. An image reading apparatus comprising means for adjusting a gain of an output level. 2. The image reading apparatus according to claim 1, wherein the initial data is read while thinning out within a predetermined time. 3. The image forming apparatus according to claim 1 or 2, wherein the initial data is read while thinning out the reading time so that the reading time sequentially becomes longer within a predetermined time from the start of reading. .