JP3597586B2 - Image reading device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to an improvement in shading correction technology in an image reading device.
[0002]
[Prior art]
2. Description of the Related Art Image reading apparatuses are widely known as apparatuses for appropriately converting images to be read, including photographs, printed matter, various documents, and other three-dimensional objects, into electrical signals.
[0003]
<Overview of conventional image reading device>
FIG. 9 is a diagram showing main components of a conventional image reading apparatus. As shown in FIG. 1, in this apparatus, an object 2 to be read is illuminated by an illuminating means 1 such as a fluorescent lamp, and the image is formed on a sensor 5 through an optical system such as a mirror 3 and a lens 4 to thereby form the same image. To an electrical signal 10.
[0004]
Since an electric signal (hereinafter, referred to as a sensor output signal) 10 output from the sensor 5 is an analog signal, the signal is subjected to A / D conversion through a sample and hold circuit 11 and a gain controller 12 so as to be convenient for signal processing by digital technology. To a digital signal (hereinafter referred to as digital image data) 14A. Further, the digital image data 14A is passed through a shading correction circuit 15 using a technique known in Japanese Patent Publication No. 1-58909 or Japanese Patent Publication No. 61-56668, and the obtained digital image data 16 is stored in an image memory. 17 is stored. The A / D converter 13 is supplied with a reference voltage VFIXED fixed to a constant value from a voltage source 18.
[0005]
The digital image data 16 stored in the image memory 17 is supplied to a television monitor, a printer, and other various devices as it is or after performing appropriate signal processing as needed.
[0006]
<Image reading unit and reference density plate>
The sensor 5 is an aggregate of image pickup devices such as a CCD, and includes a one-dimensional array line sensor and a two-dimensional array area sensor. Normally, a line sensor is used, so that an image reading section 6 is formed by integrating the sensor 5, the illuminating means 1, the mirror 3 and the lens 4 together with the sensor 5, as shown in the figure. (Not shown), the sensor 5 is relatively moved in a direction perpendicular to the longitudinal direction (the arrangement direction of the imaging elements). This movement allows the sensor 5 to scan and read the entire image of the reading target 2 having a planar spread. Note that this relative movement direction is called a sub-scanning direction.
[0007]
When the sensor 5 is an area sensor, it may not be necessary to perform the above-described scan.
[0008]
A window 7 made of a transparent material such as glass for mounting the reading object 2 opposite to the image reading unit 6 and a plate having a uniform density such as all white as a reference for correcting shading distortion (Hereinafter referred to as a reference density plate) 8.
[0009]
The reference density plate 8 has a size corresponding to the field of view of the sensor 5 and is arranged so as to be able to face the mirror 3 at the start position in the sub-scanning direction.
[0010]
<Conventional shading correction technology>
The shading distortion is basically caused by uneven density of an image read by the sensor 5 due to uneven distribution of light amount by the illumination unit 1 on the reading target 2. A reference density plate 8 is used.
[0011]
The technique disclosed in Japanese Patent Publication No. 1-58909 is intended to correct not only uneven light amount of the illumination means 1 but also uneven light amount of the optical system and variation in sensitivity between the image sensors of the sensor 5. A reference white plate is used as the plate 8, the reference white plate is read by the sensor 5 before scanning of the reading object 2, and the digital image data 14 B for one line at that time is stored in the image memory 17 through the shading correction circuit 15. .
[0012]
Since the digital image data 14B after the A / D conversion obtained here is obtained by reading a reference white board, it should originally have a uniform signal level between pixels. Variations occur in the signal levels of the pixels. Therefore, the shading correction circuit 15 previously obtains a value proportional to the reciprocal of the signal level of each pixel from the digital image data 14B as a correction coefficient for each pixel, and relatively moves the image reading unit 6 to detect the object 2 to be read. When reading is performed, shading correction is performed by multiplying the signal level of each pixel by the corresponding correction coefficient for each digital image data 14A after A / D conversion of each line, and the result 16 is stored in the image memory 17. .
[0013]
The technique disclosed in JP-B-61-56668 is intended to correct the sensitivity variation between the elements of the sensor 5, and uses two types of reference density plates 8, for example, a reference white plate and a reference blackboard, and sets a read target 2. The reference white board is set prior to the scan, and the digital image data 14B for one line at that time is passed through the shading correction circuit 15 and stored in the image memory 17, and then replaced with the reference blackboard, and the one-line image at that time is replaced. Is also stored in the image memory 17 through the shading correction circuit 15.
[0014]
Thereafter, when reading the reading target 2, the shading correction circuit 15 uses the digital image data 14B and 14C and the regular data 14D and 14E obtained by separately reading the black and white reference density plates with a sensor having no sensitivity variation between elements. The digital image data 14A of each line is corrected for each pixel by a linear interpolation method or the like, and the result 16 is stored in the image memory 17.
[0015]
[Problems to be solved by the invention]
However, in any of the prior arts, since the shading correction is performed on the digital image signal 14A after the A / D conversion of the sensor output signal 10, reduction in the number of gradations due to shading distortion cannot be improved. As a result, the image quality deteriorated. The reason is as follows.
[0016]
If the sensor output signal 10 when reading the reference white plate has shading distortion as shown in FIG. 10, the reference voltage that determines the dynamic range of the A / D converter 13, in other words, the interval between the quantization levels, is equal to the above-mentioned value. As shown in FIG. 11, in the digital image data 14A after the A / D conversion and the digital image data 16 after the shading correction, as shown in FIG. There is. This means that in the corrected digital image data 16, the intervals between the quantization levels become coarse depending on the pixels, which leads to deterioration of the image quality.
[0017]
In FIG. 11, the A / D converter 13 is of an 8-bit type, and a maximum of 255 gradations can be obtained. However, in the portion where the level of the sensor output signal 10 is reduced, the gradation is reduced to 170 gradations.
[0018]
SUMMARY OF THE INVENTION It is an object of the present invention to provide an image reading apparatus capable of improving shading correction and reducing the number of gradations in view of the above-described related art.
[0019]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, and a 100 A voltage source that generates a voltage that reaches a signal level of 100% white as a reference voltage of the A / D converter when the reference density plate of the% white is read in a range where the output of the A / D converter is not saturated. The reference voltage of the A / D converter at the time of reading the object to be read is determined based on the output of the A / D converter at the time of reading the reference density plate. Control means for controlling the output of the / D converter to be maximum without variation,
The control means obtains, as shading distortion, a difference between the output of the A / D converter at the time of reading the reference density plate and the maximum value of the A / D converter. Superimposing a voltage on a voltage generated by the voltage source as a reference voltage of the A / D converter;
An image reading apparatus, wherein digital image data in which shading distortion has been corrected is output from the A / D converter.
[0020]
According to a second aspect of the present invention, there is provided a sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, and a sensor for reading a reference density plate by the sensor. A voltage source for generating a constant voltage as a reference voltage of the A / D converter, and a signal level of 100% white within a range where the output of the A / D converter is not saturated when reading the reference density plate. Means for controlling an exposure time, and a reference voltage of the A / D converter at the time of reading the object to be read, based on an output of the A / D converter at the time of reading the reference density plate. Detecting means for detecting shading distortion so as to maximize the output of the A / D converter without variation when reading is read, and a base of the A / D converter based on the detected shading distortion. And control means for controlling a voltage,
The control means obtains, as shading distortion, a difference between the output of the A / D converter at the time of reading the reference density plate and the maximum value of the A / D converter. Superimposing a voltage on a voltage generated by the voltage source as a reference voltage of the A / D converter;
The digital image data corrected for shading distortion is output from the A / D converter.
[0021]
A third aspect of the present invention is characterized in that a means for maintaining a constant reference voltage of the A / D converter while reading the reference density plate with the sensor is provided.
[0022]
[Action]
The dynamic range of the A / D converter is determined by the reference voltage. If the number of bits is the same, increasing the reference voltage increases the interval between the quantization levels. Conversely, decreasing the reference voltage decreases the interval between the quantization levels. Narrows.
[0023]
Therefore, according to the present invention, since the quantization level interval of the A / D converter changes in accordance with the shading distortion, the number of gradations does not decrease due to the shading distortion, and the shading correction is performed simultaneously with the A / D conversion. It can be performed. Therefore, there is no deterioration in image quality.
[0024]
【Example】
Hereinafter, the present invention will be described in detail with reference to the drawings together with embodiments thereof. In the drawings, FIG. 1 shows a configuration of a main part of an image reading apparatus according to a first embodiment of the present invention, and FIGS. 2 to 5 show waveform examples of respective parts for explaining the operation. FIG. 6 shows a second embodiment of the present invention. FIGS. 7 and 8 show control examples when a reference density plate having an arbitrary density other than white is used. In these figures, parts having the same functions as those of the conventional apparatus shown in FIG. 9 are denoted by the same reference numerals.
[0025]
<First embodiment>
An image reading apparatus according to the embodiment shown in FIGS. 1 to 5 will be described. In FIG. 1, an image reading apparatus according to the present embodiment includes an illumination unit 1 such as a fluorescent lamp, an optical system such as a mirror 3 and a lens 4, a sensor 5, a window 7 made of a transparent member, and a 100% white reference. A density plate (hereinafter referred to as a reference white plate) 8, a sample and hold circuit 11, a gain controller 12, an A / D converter 13, an image memory 17, a voltage source 19, a digital operation circuit 20, An A / A converter 21, a superimposing circuit 22, and a system control circuit 23 are provided.
[0026]
<Overview of control system>
Although details will be described later, in this embodiment, the sensor 5, the reference white plate 8, the A / D converter 13, and the voltage source 19 constitute a shading distortion detecting means 40.
[0027]
The voltage source 19, the digital operation circuit 20, the D / A converter 21, and the superimposing circuit 22 constitute reference voltage control means 50, and the output voltage V 1 of the voltage source 19 is applied to the output of the D / A converter The voltage V2 is superimposed by the superimposing circuit 22 to provide a reference voltage Vref, which is supplied to the A / D converter 13. (Vref = V1 + V2)
[0028]
The voltage source 19 is provided so that when the sensor 5 reads the reference white plate 8, the voltage source 19 reaches a signal level of 100% white as much as possible within a range where the output of the A / D converter 13 does not saturate when the sensor 5 reads the reference white plate 8. A constant voltage V1 required for the A / D converter 13 is generated so that the reference voltage can be supplied to the A / D converter 13 while maintaining a small reference voltage.
[0029]
In the reference voltage control means 50, the digital operation circuit 20 and the D / A converter 21 constitute a shading correction voltage generation means 51. As described later, the digital operation circuit 20 generates the shading correction value from the shading distortion detection data 24. (Digital amount) SDi is obtained, and this is converted by the D / A converter 21 into an analog voltage. That is, the output voltage V2 of the D / A converter 21 is the shading correction voltage.
[0030]
The system control circuit 23 controls the image memory 17 and the D / A converter 21 based on the mode information signal 25 in addition to the overall control of the apparatus, and distinguishes between a mode for reading the reference white board 8 and a mode for reading the object 2 to be read. Inform
[0031]
Based on the mode information signal 25 from the system control circuit 23, the D / A converter 21 makes the output voltage V2 of the reference white board 8 zero while reading it.
[0032]
Accordingly, while reading the reference white plate 8, the reference voltage of the A / D converter 13 is a constant voltage V1 generated by the voltage source 19. (Vref = V1 = constant)
[0033]
Based on the mode information signal 25 from the system control circuit 23, the image memory 17 separates the digital image data 14B when the reference whiteboard 8 is read from the digital image data 14 when the target 2 is read, and separates them. In the area.
[0034]
<Description of image reading unit>
The sensor 5, the sample and hold circuit 11 and the gain controller 12 have the same functions as those of the conventional device shown in FIG. 9, and the sensor 5 has an image of the reading object 2 or the reference white plate 8 illuminated by the illumination means 1 such as a fluorescent lamp. Is imaged through the optical systems 3 and 4. The sensor 5 converts the image formed thereon into an electric signal 10, passes the sensor output signal 10 through a sample-and-hold circuit 11, amplifies the signal with a gain controller 12, and supplies the amplified signal to an A / D converter 13.
[0035]
In the present embodiment, the entire image of the reading target 2 is scanned by moving the sensor 5 in the sub-scanning direction. Therefore, the illuminating means 1, the optical systems 3, 4 and the sensor 5 are integrally formed as an image reading section 6, and the image reading section 6 is moved in the sub scanning direction, and the main scanning direction is perpendicular to the sub scanning direction. The reference white plate 8 is disposed at the starting point in the sub-scanning direction, facing the image reading unit 6.
[0036]
<Description of shading distortion detection and reference voltage control>
The A / D converter 13 converts the sensor output signal 10 that has passed through the sample and hold circuit 11 and the gain controller 12 according to the reference voltage Vref (= V1 + V2 or = V1) given from the superimposing circuit 22 of the reference voltage control means 50. The image data is converted into digital image data 14 or 14B, and supplied to the image memory 17. The image memory 17 stores the digital image data 14 and 14B from the A / D converter 13 in a predetermined area.
[0037]
Here, in the mode of reading the reference white plate 8, the digital image data 14B obtained from the A / D converter 13 reflects the shading distortion as illustrated in FIG. 2 because the reference voltage is constant at Vref = V1. The digital image data 14B itself can be used as the shading distortion detection data 24.
[0038]
In the example of FIG. 2, the horizontal axis indicates n pixels P1 to Pn for one line in order, and the vertical axis indicates the signal level. The signal level at the ith pixel Pi is represented by Di. . Dmax is the maximum value of the digital image data after A / D conversion. In an 8-bit A / D converter, Dmax = 2 ⁸ -1 = 255.
[0039]
The digital arithmetic circuit 20 fetches the shading distortion detection data 24 from the image memory 17 and uses the signal level Di (i = 1 to n) of each pixel, the maximum value Dmax, and the constant voltage value V1 used as the reference voltage. , Equation (1) given by Equation (1) is digitally calculated to obtain a shading correction amount SDi for each pixel Pi as a digital value.
[0040]
(Equation 1)
SDi =-((Dmax-Di) / Dmax) V1 Equation (1)
[0041]
Further, in the mode in which the reading target 2 is read, the digital operation circuit 20 synchronizes the shading correction amount column SDi (i = 1 to n) line by line and in pixel order in synchronization with the operation of the A / D converter 13. It is provided to the D / A converter 21.
[0042]
The D / A converter 21 converts the shading correction amount sequence SDi (i = 1 to n) into an analog shading correction voltage V2 and supplies the analog shading correction voltage V2 to the superimposing circuit 22. Since the output voltage waveform of the D / A converter 21 is the same as SDi and V2 =-((Dmax-Di) / Dmax) V1 (i = 1 to n), it corresponds to the digital image data 14B of FIG. The result is as shown in FIG.
[0043]
The reference voltage Vref output from the superimposing circuit 22 is Vref = V1 + V2 = V1.times.Di / Dmax (i = 1 to n), which is, as shown in FIG. 4, between the effective pixels P1 to Pn. The waveform is completely similar to that of the waveform 2 and changes according to the shading distortion.
[0044]
As a result, if it is assumed that the reference white plate 8 is read using the reference voltage V1 + V2 = Vref, the digital image data obtained from the A / D converter 13 at that time is, as shown by a waveform 26 in FIG. There is no variance at all. (Di → Di × V1 / Vref = Di × V1 / (V1 × Di / Dmax) = Dmax)
[0045]
Therefore, when the waveform 26 in FIG. 5 is compared with the conventional waveform in FIG. 11, when the reading target 2 is read, the sensor output signal 10 is A / D converted and the number of gradations is not reduced. It can be clearly seen that shading correction is performed.
[0046]
<Second embodiment>
In the first embodiment, the reference voltage control means 50 is configured using the voltage source 19, the shading correction voltage generation means 51 (the digital operation circuit 20, the D / A converter 21) and the superimposition circuit 22. As shown in (1), it can be constituted by the digital operation circuit 31 and the D / A converter 21. In FIG. 6, portions having the same functions as those in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted. Also in this embodiment, a reference white plate is used as the reference density plate 8.
[0047]
In FIG. 6, the digital arithmetic circuit 31 has a register 32. The register 32 stores a voltage value V1 provided from the system control circuit 23 and suitable for reading the reference white board 8. In the mode for reading the reference whiteboard 8 in response to the mode information signal 25 from the system control circuit 23, the voltage value V1 in the register 32 is given to the D / A converter 21. As a result, a constant reference voltage V1 is supplied from the D / A converter 21 to the A / D converter 13.
[0048]
Next, the digital operation circuit 31 uses the digital image data 14B for the reference whiteboard 8 provided from the A / D converter 13 through the image memory 17 as the shading distortion detection data 24, and uses the voltage value V1 in the register 32 and each of the aforementioned values. From the signal level Di of the pixel Pi and the maximum value Dmax of the digital image data after A / D conversion, the equation (2) given by Expression 2 is digitally calculated for each pixel Pi. Thus, a digital reference voltage value sequence Vrefi that changes according to the shading distortion Di is obtained.
[0049]
(Equation 2)
Vrefi = V1 × Di / Dmax (i = 1 to n) Equation (2)
[0050]
Further, in the mode in which the digital object circuit 31 reads the object 2 to be read, the reference voltage value sequence Vrefi = V0 × Di / Dmax (in line with the order of pixels, in synchronization with the operation of the A / D converter 13). i = 1 to n) to the D / A converter 21. As a result, the D / A converter 21 converts the voltage into an analog reference voltage Vref and supplies the analog reference voltage Vref to the A / D converter 13.
[0051]
As a result, the reference voltage Vref changes in accordance with the shading distortion as in FIG. 4, and similarly to the first embodiment, the sensor output signal 10 is A / D converted and, at the same time, the shading correction is performed without reducing the number of gradations. It can be performed.
[0052]
In any of the first and second embodiments, the setting of the voltage V1 may be performed from the system control circuit 23 or may be performed inside the digital arithmetic circuits 20 and 31.
[0053]
<When using an arbitrary reference density plate other than the reference white plate>
In the first and second embodiments, a reference white plate is used as the reference density plate 8. However, by controlling the exposure time or controlling V1 as described below, it is possible to use a reference white plate other than white if the density is uniform. Can be used.
[0054]
<Exposure time control>
When the reference density plate 8 other than white is used, the voltage generated by the voltage source 19 is the value V1 corresponding to the reference white plate, so that the digital image data after A / D conversion when the reference density plate 8 is read is As shown in a waveform 27 in FIG. 7, the signal level is lower than the maximum value Dmax. This is because the exposure time is usually set in advance so that the maximum value of the signal level when the reference white plate is read becomes Dmax.
[0055]
Therefore, in the mode for reading the reference density plate 8, the exposure time is controlled so that the maximum value of the signal level reaches Dmax as shown by the waveform 14B in FIG.
[0056]
As described in the first or second embodiment, the shading correction amount SDi is obtained using the digital image data 14B for the reference density plate 8 after controlling the exposure time. Note that the shading distortion does not change at all even if the exposure time is changed, since the two waveforms 14B and 27 are similar as can be seen from FIG.
[0057]
Then, in the mode for reading the object to be read 2, the exposure time is returned to the original value, and the shading correction is performed by controlling the reference voltage Vref based on the shading correction amount SDi as described in the first or second embodiment. .
[0058]
<V1 control>
When the voltage generated by the voltage source 19 is changed from V1 to V1 'as shown in FIG. 8 instead of performing the exposure time control, the signal level of the digital image data 14B after the A / D conversion with respect to the reference density plate 8 is changed. The maximum value can be adjusted to be Dmax.
[0059]
Therefore, in the mode for reading the reference density plate 8, the voltage of the voltage source 19 is changed from V1 to V1 'so that the maximum value of the signal level becomes Dmax.
[0060]
Using the digital image data 14B for the reference density plate 8 after the control from V1 to V1 ', the shading correction amount SDi is obtained in accordance with the first or second embodiment.
[0061]
In the mode for reading the object to be read 2, the voltage V1 'is returned to the original voltage V1, and the reference voltage Vref is controlled based on the shading correction amount SDi as described in the first or second embodiment. Perform shading correction.
[0062]
However, the signal level Di in the equations (1) and (2) is read as the value when Dmax is set to V1 '.
[0063]
<Others>
In each of the embodiments described above, the sensor 5 is described as a line sensor. However, a two-dimensional array of area sensors may be used.
Further, the shading distortion is detected by the shading distortion detecting means 40 including the sensor 5, the reference density plate 8 and the A / D converter 13, but other than this, the light quantity distribution of at least the illumination means 1 such as a light quantity detector is detected. Anything that can be used may be used.
[0064]
【The invention's effect】
As described above with the embodiment, the image reading apparatus according to the present invention has the following effects.
[0065]
According to the present invention , since the reference voltage of the A / D converter is controlled according to the shading distortion, the shading distortion can be corrected at the same time as the A / D conversion without reducing the number of gradations.
[0066]
Further, according to the present invention, a shading correction is easily performed without lowering the number of gradations during A / D conversion by superimposing a correction voltage that changes according to shading distortion on a fixed reference voltage that is normally used. be able to.
[0067]
Further, according to the present invention, since shading distortion is detected by the sensor, the reference density plate, and the A / D converter, not only the light amount distribution of the illumination means but also the light amount unevenness of the optical system and the sensitivity variation between the sensor elements are included. Thus, the shading distortion can be detected.
[Brief description of the drawings]
FIG. 1 is a diagram showing a main configuration of an image reading apparatus according to a first embodiment of the present invention.
FIG. 2 is a diagram showing a waveform example of shading distortion detection data.
FIG. 3 is a diagram illustrating a waveform example of a shading correction voltage.
FIG. 4 is a diagram showing a waveform example of a reference voltage obtained by superimposing a shading correction voltage on a constant voltage.
FIG. 5 is a diagram showing a waveform example of digital image data after shading correction.
FIG. 6 is a diagram showing a main configuration of an image reading apparatus according to a second embodiment of the present invention.
FIG. 7 is a diagram illustrating a control example (exposure time control) when a reference density plate other than white is used.
FIG. 8 is a diagram showing another control example (V1 control) when a reference density plate other than white is used.
FIG. 9 is a diagram showing a main configuration of a conventional image reading apparatus.
FIG. 10 is a diagram showing a waveform example of a sensor output signal.
FIG. 11 is a diagram showing a decrease in the number of gradations in conventional shading correction.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Illumination means 2 Reading object 3 Mirror 4 Lens 5 Sensor 6 Image reading part 7 Window 8 Reference density plate (reference white plate)
Reference Signs List 10 sensor output signal 11 sample / hold circuit 12 gain controller 13 A / D converter 14 digital image data 14B corresponding to a reading target digital image data 17 corresponding to a reference density plate 17 image memory 19 voltage sources 20, 31 digital operation circuit 21 D / A converter 22 Superposition circuit 23 System control circuit 24 Shading distortion detection data 25 Mode information signal 32 Register 40 Shading distortion detection means 50 Reference voltage control means 51 Shading correction voltage generation means

Claims (3)

A sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, and the A / D converter when the sensor reads a 100% white reference density plate A voltage source for generating a reference voltage of the converter so as to reach a signal level of 100% white within a range where the output of the A / D converter is not saturated, and the A / D conversion at the time of reading the reference density plate The reference voltage of the A / D converter at the time of reading the object to be read is determined based on the output of the A / D converter so that the output of the A / D converter when the reference density plate is read is maximized without variation. And control means for controlling,
The control means obtains, as shading distortion, a difference between the output of the A / D converter at the time of reading the reference density plate and the maximum value of the A / D converter. Superimposing a voltage on a voltage generated by the voltage source as a reference voltage of the A / D converter;
An image reading apparatus, wherein digital image data in which shading distortion has been corrected is output from the A / D converter. A sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, and a reference for the A / D converter when the sensor reads a reference density plate A voltage source for generating a constant voltage as a voltage, and a means for controlling an exposure time so as to reach a signal level of 100% white in a range where the output of the A / D converter is not saturated when reading the reference density plate; Based on the output of the A / D converter at the time of reading the reference density plate, the reference voltage of the A / D converter at the time of reading the object to be read is the A / D when the reference density plate is read. Detecting means for detecting shading distortion so that the output of the D converter becomes maximum without variation, and control means for controlling a reference voltage of the A / D converter based on the detected shading distortion Equipped with,
The control means obtains, as shading distortion, a difference between the output of the A / D converter at the time of reading the reference density plate and the maximum value of the A / D converter. Superimposing a voltage on a voltage generated by the voltage source as a reference voltage of the A / D converter;
An image reading apparatus, wherein digital image data in which shading distortion has been corrected is output from the A / D converter. 3. The image reading apparatus according to claim 1, further comprising: means for maintaining a reference voltage of the A / D converter constant while the reference density plate is read by the sensor.

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