Abstract:
An image reading device includes a reference member, a reading unit, a first reference value setting unit, a detecting unit, a second reference value setting unit, a determining unit, and a pixel value setting unit. The reading unit scans an image on a document to obtain image data and scans the reference member to obtain reference data. The first reference value setting unit sets a first reference value based on the reference data. The detecting unit detects a usage state of the reading unit. The second reference value setting unit sets a second reference value in accordance with the usage state of the reading unit. The determining unit determines whether or not the first reference value is in a predetermined condition The pixel value setting unit sets a pixel value. The pixel value setting unit sets the pixel value based on the image data and on the first reference value, if the determining unit determines that the first reference value is in the predetermined condition; and the pixel value setting unit sets the pixel value based on the image data and on the second reference value, if the determining unit determines that the first reference value is not in the predetermined condition.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2006-306416 filed Nov. 13, 2006. The entire content of the priority application is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an image reading device and, more particularly, to an image reading device capable of accurately reading an image formed on a document even when the image has become abnormal due to influence of outside light. 
       BACKGROUND 
       [0003]    There is known an image reading device that irradiates light on a document from a light source and reads reflection light reflected off the document using a plurality of light-receiving sensor such as a CCD to form an image. Japanese Patent Application Publication No. 11-112800 discloses an image reading device configured to scan a reference member for correcting light-receiving sensitivity of a light-receiving sensor before scanning a document, and to set scanned values as reference values for document reading operation. Then, if the reference values include abnormal value, the abnormal value is rewritten based on data adjacent to the abnormal value. 
       SUMMARY 
       [0004]    However, in the image reading device disclosed in Japanese Patent Application Publication No. 11-112800, the abnormal value is rewritten based on data adjacent to the abnormal value. Therefore, when the reference member is affected by outside light, values (reference values) that are scanned from the entire reference member become abnormal. Since the reference values are abnormal, an image formed on a document cannot be scanned accurately. 
         [0005]    In general, the reference member having physical properties in which scanned value is hard to change with time is used. However, scanned value is not always the same but is deteriorated with time. Further, the performance of light sources and light-receiving sensors that scans an image formed on a document is subject to deterioration with time. Therefore, an image formed on a document cannot be reliably scanned because of deterioration of the reference member, light source, and light-receiving sensor with time. 
         [0006]    In view of the foregoing, it is an object of the present invention to provide an image reading device capable of accurately scanning an image formed on a document in consideration of deterioration of the reference member even if there is influence of outside light, 
         [0007]    In order to attain the above and other objects, the invention provides an image reading device including a reference member, a reading unit, a first reference value setting unit, a detecting unit, a second reference value setting unit, a determining unit, and a pixel value setting unit. The reading unit scans an image on a document to obtain image data and scans the reference member to obtain reference data. The first reference value setting unit sets a first reference value based on the reference data. The detecting unit detects a usage state of the reading unit. The second reference value setting unit sets a second reference value in accordance with the usage state of the reading unit. The determining unit determines whether or not the first reference value is in a predetermined condition. The pixel value setting unit sets a pixel value. The pixel value setting unit sets the pixel value based on the image data and on the first reference value, if the determining unit determines that the first reference value is in the predetermined condition; and the pixel value setting unit sets the pixel value based on the image data and on the second reference value, if the determining unit determines that the first reference value is not in the predetermined condition. 
         [0008]    According to another aspect, the invention also provides an image reading method for an image reading device including a reference member, and a reading unit that scans an image on a document to obtain image data and that scans the reference member to obtain reference data. The image reading method includes: 
         [0009]    setting a first reference value based on the reference data; 
         [0010]    detecting a usage state of the reading unit; 
         [0011]    setting a second reference value in accordance with the usage state of the reading unit; 
         [0012]    determining whether or not the first reference value is in a predetermined condition, and 
         [0013]    setting a pixel value based on the image data and on the first reference valuer if the first reference value is in the predetermined condition; and setting a pixel value based on the image data and on the second reference value, if the first reference value is not in the predetermined condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view showing an outer structure of an MFP provided with an image reading device according to an embodiment of the present invention; 
           [0015]      FIG. 2  is a perspective view showing the outer structure of the MFP in  FIG. 1  in a state where a document cover is opened; 
           [0016]      FIG. 3  is a schematic cross-sectional view showing a structure of the scanner according to the embodiment; 
           [0017]      FIG. 4  is a block diagram showing an electrical configuration of the MFP according to the embodiment; 
           [0018]      FIG. 5  is an example of threshold values stored in a threshold value memory in  FIG. 4 ; 
           [0019]      FIG. 6  is an example of reference values stored in a time dependent reference value memory in  FIG. 4 ; and 
           [0020]      FIG. 7  is a flowchart illustrating steps in a process executed by the MFP when a document is scanned. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    An embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a perspective view showing an outer structure of an MFP  1  (Multi Function Peripheral or Multi Function Printer) provided with an image reading device according to the present invention. The MFP  1  has various functions such as a facsimile function, a printer function, a scanner function, and copy function. 
         [0022]    As shown in  FIG. 1 , the MFP  1  mainly includes a printer  2  disposed at the lower section of the MFP  1 , a scanner  3  disposed at the upper section of the MFP  1 , and an operation panel  4  disposed on the front side of the scanner  3 . The printer  2  is so-called an ink-jet type image recording device (ink-jet printer) configured to selectively eject ink droplets based on image data scanned by the scanner  3  or externally input image data to form an image on a recording sheet. 
         [0023]    An opening  5  is formed on the front side of the printer  2 . A sheet supply tray  14  and a sheet discharge tray  15  are entirely housed in the opening  5 . The sheet supply  14  and sheet discharge tray  15  are arranged in a two-tiered structure, with the sheet discharge tray  15  on the upper tier and sheet supply tray  14  on the lower tier. The sheet supply tray  14  is a rectangular case. The rectangular recording sheets, each of which has a rectangular shape, are stacked on the tray  14  in such a manner that the longitudinal direction of the rectangular recording sheets is parallel to the front-to-rear direction. 
         [0024]    Inside the printer  2 , a sheet feeding path curves from the back of the sheet supply tray  14  to the front side and is connected to the sheet discharge tray  15 . A recording sheet set in the sheet supply tray  14  is supplied to the sheet feeding path in a state that the short side of the recording sheet is a leading edge, then turned around, and guided to an image recording position provided in the sheet feeding path. Then, an image is recorded onto the recording sheet at the image recording position by the printer  2 . After recording the image, the recording sheet is discharged to the sheet discharge tray  15 . 
         [0025]    The operation panel  4  is provided to operate the printer  2  and scanner  3  and includes various operation keys  40  such as a document scanning button and an LCD  41 . A user can use the operation panel  4  to input a desired instruction. When a predetermined instruction is input to the MFP  1 , a CPU  21  (see  FIG. 4 ) starts controlling the operation of the MFP  1  based on the input information. The MFP  1  is configured to operate based on not only the instruction input from the operation panel  4  but also an instruction transmitted through a printer driver or scanner driver from a computer connected thereto. 
         [0026]    A connection panel  70  is provided above the opening  5  of the printer  2 . A USB terminal  71  is formed at the left side portion of the connection panel  70 . The USB terminal  71  is a connector terminal for connecting an external device and MFP  1  in a communicable manner through a USB cable. A slot portion  72  is located at the right side of the connection panel  70 . A plurality of card slots to which a memory card can be attached are formed in the slot portion  72 . Image data scanned by the scanner  3  can be stored in a memory card or image data stored in the memory card can be printed out by the printer  2 . 
         [0027]    The scanner  3  includes a document reading base  6  serving as a Flat Bed Scanner (FBS) and a document cover  8  provided with an Auto Document Feeder (ADF)  7 . The document cover a is attached to the document reading base  6  through hinges  11  (see  FIG. 2 ), so as to be capable of opening and closing on the document reading base  6  about the hinges  11 . The ADF  7  is configured to feed a document from a document tray  9  to a document discharge tray  10  through a document feeding path. 
         [0028]    As shown in  FIG. 2 , the top surface of the document reading base  6  is formed with an opening, and a platen glass  12  is fitted in the opening. An image reading unit  3   a  (see  FIG. 3 ) is accommodated in the document reading base  6  and is capable of reciprocating in the main scanning direction (X direction). The detail of the image reading unit  3   a  will be described later. 
         [0029]    When the scanner  3  is used as the FBS, a user opens the document cover  8 , places a document on the platen glass  12 , and closes the document cover  8  to fix the document. Upon input of a scanning operation start instruction, the image reading unit  3   a  scans the document along the rear surface of the platen glass  12  in the main scanning direction (X direction), whereby an image on the document is scanned. In general, the document scanning operation is performed with the document cover a closed. However, when a page of a thick book is scanned, the document cover B cannot be closed. If the image reading unit  3   a  (see  FIG. 3 ) is affected by outside light in the case where the document scanning operation is performed with the document cover  8  opened, a reference value for scanning the document cannot accurately be acquired in some cases. The detail of the reference value will be described later. 
         [0030]    When the document is automatically fed by the ADF  7  for the scanning operation, the document passes through a reading surface  13  on the document reading base  6  in the document feeding process of the ADF  7 , and an image on the document is scanned by the image reading unit  3   a  that is moved on the lower portion of the reading surface  13 . The scanning operation using the ADF  7  is performed with the document cover B closed relative to the document reading base  6 . 
         [0031]    Next, the structure and operation of the scanner  3  will be described with reference to  FIG. 3   FIG. 3  is a schematic cross-sectional view showing a structure of the scanner  3  provided in the MFP  1  as viewed from the front side. As shown in  FIG. 3 , the platen glass  12  is fitted in the opening portion of the document reading base  6 . The image reading unit  3   a  is provided on the lower portion of the platen glass  12 . 
         [0032]    The image reading unit  3   a  mainly includes a light source (LED)  3   b,  a light guide  3   c,  a light guide  3   d,  and a CCD  3   e.  The light source (LED)  3   b  emits a light. The light guide  3   c  focuses a light onto the scanned portion of a document. The light guide  3   d  focuses a reflection light from the document onto the CCD  3   e.  The CCD  3   e  outputs a voltage in accordance with the strength of a receiving light. The image reading unit  3   a  is capable of reciprocating in the main scanning direction (X direction). A white reference board  37  is disposed under the platen glass  12  and above the light guide  3   d.  The white reference board  37  is a thin-plate member having at least one side colored white. The white colored side of the white reference board  37  faces downward. 
         [0033]    Upon depression of the document scanning button which is one of the operation keys  40  after a document G is placed on the platen glass  12 , scanning operation for the document G is started. Before the document G is scanned, the white reference board  37  is scanned by the image reading unit  3   a.  More specifically, first, the LED  3   b  is turned on to emit a light. The direction of the light is changed by the light guide  3 c, whereby the light is irradiated onto the white reference board  37  as an irradiation light S. A part of the light reflected on the surface of the white reference board  37  is directed to the CCD  3   e  as a reflection light R. The reflection light R is focused by the light guide  3   d  onto the CCD  3   e  and a voltage value corresponding to the strength of the focused light is output from the CCD  3   e.  The CCD  3   e  includes a plurality of light-receiving elements sequentially aligned. One element corresponds to one pixel of one line of image data. Next, the LED  3   b  is turned off and, then, the white reference board  37  is scanned in the same manner as in the case where the LED  3   b  is turned on and a voltage value is output from the CCD  3   e.    
         [0034]    After scanning the white reference board  37  both in the case where the LED  3   b  is turned on and off, the scanning range (white reference value, black reference value) of the document G is set. When the image reading unit  3   a  has been fed to the document scanning start position, scanning operation for the document is started. As in the case of the scanning operation for the white reference board  37 , the image reading unit  3   a  irradiates the document G with the irradiation light S. The reflection light R reflected on the surface of the document G is focused onto the CCD  3   e  and a voltage value corresponding to the strength of the focused light is output from the CCD  3   e,  which is then input to an ASIC  3   f.  Every time the document G is scanned by one line, the image reading unit  3   a  is fed in the main scanning direction (X direction) by one line. By repeating the reading operation and feeding operation, the entire area of the document G is scanned. 
         [0035]    The ASIC  3   f  is an integrated circuit that converts voltage values input from the respective elements of the CCD  3   e  into 16-bit numeric data and perform various data corrections for the numeric data to output the corrected data as image data. That is, one reference value and one pixel value corresponding to one element of the CCD  3   e  is composed of 16-bit numeric value. The image data output from the ASIC  3   f  is stored into an image memory  23   c  of an RAM  23  (see  FIG. 4 ) through a bus line  25 , whereby the image on the document G is stored as image data. 
         [0036]      FIG. 4  is a block diagram showing an electrical configuration of the MFP  1 . The MFP  1  mainly includes the printer  2 , the scanner  3 , a CPU  21 , a ROM  22 , a RAM  23 , a flash memory  24 , a network control unit (NCU)  31 , a modem  32 , the operation keys  40 , the LCD  41 , the USB terminal  71 , the slot portion  72 , an amplifier  73 , and a speaker  74 , all of which are connected to each other through a bus line  25 . 
         [0037]    The CPU  21  controls the abovementioned components according to fixed values or program stored in the ROM  22 , RAM  23 , or flash memory  24 , functions provided in the MFP  1 , and various signals transmitted/received through the NCU  31 . The ROM  22  is an unrewritable memory for storing various control program  22   a  executed in the MFP  1 . The RAM  23  is a rewritable memory for storing various data. The RAM  23  includes various memories such as a white reference value memory  23   a,  a black reference value memory  23   b,  and an image memory  23   c.    
         [0038]    The white reference memory  23   a  stores a white reference value used for a correction of a difference between respective elements of the CCD  3   e  After the conversion of a voltage value output from the CCD  3   e  into a numeric value, the correction is performed on the numeric value. The value of the white reference value memory  23   a  is updated when a facsimile function, scanner function, or copy function is executed. 
         [0039]    similarly, the black reference memory  23   b  stores a black reference value used for correction of a difference between respective elements of the CCD  3   e.  After the conversion of a voltage value output from the CCD  3   e  into a numeric value, the correction is performed on the numeric value. The value of the black reference value memory  23   b  is updated when a facsimile function, scanner function, or copy function is executed. The image memory  23   c  stores image data of a document scanned by the scanner  3 . 
         [0040]    The flash memory  24  is a rewritable nonvolatile memory. Data stored in the flash memory  24  is retained even after the power-off time of the MFP  1 . The flash memory  24  includes a count memory  24   a,  and a threshold value memory  24   b,  a time dependent reference value memory  24   c.  The count memory  24   a  is a page counter in which the number of sheets, which is used in the printer  2  since the MFP  1  was produced, is stored. Every time one sheet is used in the printing operation,  1  is added to the counter value in the count memory  24   a.    
         [0041]    The threshold value memory  24   b  stores threshold values. The threshold values are used for determining whether the white reference value stored in the white reference value memory  23   a  or black reference value stored in the black reference value memory  23   b  is abnormal or not. The time dependent reference value memory  24   c  stores white and black reference values that are used as alternative values in the case where the white or black reference value, set before the scanning operation for a document, in the white or black reference value memory  23   a,    23   b  includes an abnormal value. In order to adapt to a reference value which is changed with temporal change, a plurality of white and black reference values corresponding to the number of sheets that have been used in the printer  2  are stored as alternative values in the time dependent reference value memory  24   c.    
         [0042]    The NCU  31  transmits a dial signal to a telephone network (not shown) and responds to a calling signal from the telephone network. The modem  32  modulates/demodulates image data through the NCU  31  and transmits resultant data to another facsimile machine (not shown), as well as transmits and receives various procedure signals for transmission control. The USE terminal  71  is a known circuit for transmitting and receiving data to/from a computer through a USB cable (not shown). The amplifier  73  drives the speaker  74  connected thereto to output a calling sound. 
         [0043]    Next, the details of the white reference value memory  23   a  and black reference value memory  23   b  will be described. The scanner  3  scans the white reference board  37  before scanning a target document to set white and black reference values. Since voltage values to be output vary even if the same light amount is input to respective elements of the CCD  3   e,  when the voltage values are simply converted into numeric data for replacement of the voltage values by pixel values, unevenness occurs in an image formed from the pixel value. In order to cope with this, white and black reference values are stored for each element of the CCD  3   e.  Based on the white and black reference values corresponding to respective elements of the CCD  3   e,  the output numeric values are replaced by pixel values. Thus, pixel values having a well-controlled density can be acquired without effects of differences among the light-receiving sensitivities of the elements. Therefore, the occurrence of image unevenness can be prevented and an image on a target document can be scanned accurately. 
         [0044]    The white reference value is numeric data based on voltage value output from respective elements of the CCD  3   e  in the case where the white reference board  37  is scanned with the LED  3   b  turned on. The white reference value is stored in the white reference value memory  23   a.  The black reference value is numeric data based on voltage value output from respective elements of the CCD  3   e  in the case where the white reference board  37  is scanned with the LED  3   b  turned off. The black reference value is stored in the black reference value memory  23   b.  When the white reference board  37  is scanned, various data corrections are not executed but the scanned values are directly converted into 16-bit numeric data respectively and stored in the white reference value memory  23   a  and black reference value memory  23   b.    
         [0045]    The pixel value set based on the white and black reference values will be described. After a target document is scanned by the scanner  3 , the ASIC  3   f  compares the numeric data corresponding to the voltage value output from each element of the CCD  3   e  with white and black reference values corresponding to each element. The numeric data output from each element is within the range to the white reference value (maximum value) from black reference value (minimum value). Depending on to which position in the range the numeric data corresponds, the corresponding pixel value is determined. More specifically, the pixel value is represented by 16 bits, and is determined by setting the white reference value to the maximum pixel value (65,536: decimal number) and black reference value to the minimum pixel value (0) and linearly setting the numeric data to the pixel value between 0 to 65,536. 
         [0046]    Next, as shown in  FIG. 5 , a white reference threshold value and a black reference threshold value stored in the threshold value memory  24   b  will be described.  FIG. 5  is an example of white and black reference threshold values which are 16-bit numeric data stored in the threshold value memory  24   b.  The white reference threshold value is used for determining whether the white reference value stored in the white reference value memory  23   a  is abnormal or not, and the value thereof is 35,000 (decimal number). Similarly, the black reference threshold value is used for determining whether the black reference value stored in the black reference value memory  23   b  is abnormal or not, and the value thereof is 500. 
         [0047]    If the average value of the white reference values of the respective elements stored in the white reference value memory  23   a  is smaller than the white reference threshold value, the white reference value is determined to be abnormal. If the average value of the black reference values of the respective elements stored in the black reference value memory  23   b  is greater than the black reference threshold value, the black reference value is determined to be abnormal. As for the cause of making the white reference value or black reference value abnormal, there can be considered that the scanner  3  is used with the document cover opened and, therefore, the CCD  3   e  or white reference plate  37  is affected by outside light. When the white reference value or black reference value is abnormal, a plurality of white and black reference values shown in  FIG. 6  are used as reference value for setting the pixel value. 
         [0048]    Next, with reference to  FIG. 6 , the plurality of white and black reference values stored in the time dependent reference value memory  24   c  will be described.  FIG. 6  shows an example of, the plurality of white and black reference values stored in the time dependent reference value memory  24   c.  The plurality of white and black reference values corresponding respectively to a plurality of values of the page counter is stored in the time dependent reference value memory  24   c.  The plurality of white and black reference values in the time dependent reference value memory  24   c  are used as alternative reference values in ASIC  3   f  for setting the pixel value when the white reference value stored in the white reference value memory  23   a  or black reference value stored in the black reference value memory  23   b  is abnormal. 
         [0049]    The white reference board  37 , CCD  3   e,  and LED  3   b  of the scanner  3  change in characteristics with time. Thus, if only one fixed alternative reference value is always used to set the pixel value when the white reference value or black reference value output form the CCD  3   e  becomes abnormal due to temporal change, an accurate image data cannot be obtained. Therefore, in order not to change the image data output from the scanner  3  with time when the reference value becomes abnormal due to influence of outside light, a plurality of white and black reference values in accordance with temporal change are previously stored. 
         [0050]    In general, the output voltage of the CCD  3   e  is decreased in accordance with degradation of the light-receiving sensitivity due to aging and amount of the irradiation light of the LED  3   b  is decreased due to aging. Thus, in order to make a white reference value corresponding to aging, the plurality of white reference values in the time dependent reference value memory  24   c  are gradually reduced in accordance with the value of the page counter. On the other hand, 0 is always set for the black reference value in the time dependent reference value memory  24   c  since the minimum value is always preferable. 
         [0051]    If the average value of the white reference values stored in the white reference value memory  23   a  or average value of the black reference values stored in the black reference value memory  23   b  is abnormal, the value of the page counter stored in the time count memory  24   a  is referred to and, one white reference value in the time dependent reference value memory  24   c  is stored in the white reference value memory  23   a  as new reference value based on the referred value of the page counter. Similarly, one black reference value in the time dependent reference value memory  24   c  is stored in the black reference value memory  23   b  as a new reference value based on the referred value of the page counter. Thus, even if the white and black reference values become abnormal due to influence of outside light, an image on a document can accurately be scanned without influence of the temporal change of the scanner  3 . 
         [0052]    At this time, the same white reference value is set for respective elements of the CCD  3   e  and stored in the white reference value memory  23   a.  Similarly, the same black reference value is set for respective elements of the CCD  3   e  and stored in the black reference value memory  23   b.  Accordingly, the same white reference value and same black reference value are used as reference value for setting the pixel value in the ASIC  3   f  in all the elements of the CCD  3   e.    
         [0053]    Next, processing executed by the CPU  21  of the MFP  1  will be described with reference to  FIG. 7   FIG. 7  is a flowchart showing processing executed upon depression of the document scanning button which is one of the operation keys  40 . 
         [0054]    First, the CPU  21  turns on the LED  3   b  of the scanner  3  in S 1  and instructs the image reading unit  3   a  to scan the white reference board  37  for setting the white reference value in S 2  The image reading unit Sa scans the white reference board  37  in a state where the LED  3   b  is turned on, and voltage values are output from the respective elements of the CCD  3   e.  Each of the voltage values is converted into 16-bit numeric data by ASIC  3   f  and stored in the white reference value memory  23   a  as a white reference value corresponding to each element. 
         [0055]    Upon detecting that the white reference values have been stored in the white reference value memory  23   a  by the ASIC  3   f,  the CPU  21  reads out the white reference values corresponding to the respective elements from the white reference value memory  23   a  and calculates the average value thereof. Then, in S 3  the CPU  21  determines whether the average value of the white reference values of the respective elements is normal or not. If the average value of the white reference values is smaller than the white reference threshold value stored in the threshold value memory  24   b,  the CPU  21  determines the white reference values are abnormal (S 3 : NO). However, if the average value of the white reference values is not smaller than the white reference threshold value stored in the threshold value memory  24   b,  the CPU determines that the white reference values are normal (S 3 : YES). 
         [0056]    If the average value of the white reference values is normal (S 3 : YES), the CPU  21  turns off the LED  3   b  in S 4  and instructs the image reading unit  3   a  to read the white reference board  37  for setting the black reference value in S 5 . The image reading unit  3   a  reads the white reference board  37  in a state where the LED  3   b  is turned off, and voltage values are output from the respective elements of the CCD  3   e.  Each of the voltage values is converted into 16-bit numeric data by ASIC  3   f  and stored in the black reference value memory  23   b  as a black reference value corresponding to each element. 
         [0057]    Upon detecting that the black reference values have been stored in the black reference value memory  23   b  by the ASIC  3   f,  the CPU  21  reads out the black reference values corresponding to the respective elements from the black reference value memory  23   b  and calculates the average value thereof. Then, in S 6  the CPU  21  determines whether the average value of the black reference values of the respective elements is normal or not. If the average value of the black reference values is greater than the black reference threshold value stored in the threshold value memory  24   b,  the CPU  21  determines the black reference values are abnormal (S 6 : NO). However, if the average value of the black reference values is not greater than the black reference threshold value stored in the threshold value memory  24   b,  the CPU  21  determines that the black reference values are normal (S 6 : YES) and advances to S 8 . 
         [0058]    On the other hand, if the CPU  21  determines that the white reference values are abnormal (S 3 : NO), or that the black reference values are abnormal (S 6 : NO), the CPU  21  refers the value of the page counter stored in the count memory  24   a.  In S 7  the CPU  21  reads out one white reference value and one black reference value from the time dependent reference value memory  24   c  based on the counter value of the page counter and stores the one white reference value and the one black reference value in the white reference value memory  23   a  and black reference value memory  23   b  as a new reference value. Then, the CPU  21  advances to S 8  and instructs the image reading unit  3   a  to scan a document, 
         [0059]    When the image reading unit  3   a  scans the document by one line, voltage values output from the respective elements of the CCD  3   e  are each converted into 16-bit numeric data by the ASIC  3   f.  The numeric data of the voltage value output from the respective elements of the CCD  3   e  is each within a range to the white reference value (maximum value) from the black reference value (minimum value), and the ASIC  3   a  determines each pixel value based on the relative position of the numeric data with respect to the range. After completion of the setting of the pixel values in all the elements of the CCD  3   e,  the next line of the document is scanned. Following the entire document has been scanned, image data is output from the ASIC  3   f.  The image data is then stored in the image memory  23   c  of the RAM  23  through the bus line  25   
         [0060]    In S 9  the CPU  21  determines whether the entire document has been scanned If determining that the entire document has been read (S 9 : YES), the CPU ends the processing. However, if determining that the entire document has not been scanned (S 9 : NO), the CPU  21  returns to S 8  and instructs the image reading unit  3   a  to scan the document. 
         [0061]    Although the present invention has been described with reference to the preferred embodiment, the present invention is not limited to the above embodiment and, evidently, a variety of modifications and changes may be made without departing from the scope of the present invention. 
         [0062]    For example, in the above embodiment, the number of sheets, which is used in the printer  2  since the MFP  1  was produced, is stored in the count memory  24   a.  However, a timer circuit may be provided in the MFP  1  so as to store a total operating time during which the image reading unit  3   a  (the scanner  3 ) has been operated since the MFP  1  was produced in the count memory  24   a  and store a plurality of reference times based on the operating time in the time dependent reference value memory  24   c.  In this case, if the white reference value stored in the white reference value memory  23   a  or black reference value stored in the black reference value memory  23   b  is abnormal, the white and black reference values are set in accordance with the total operating time in the count memory  24   a.  With this configuration, even if the white and black reference values become abnormal due to influence of outside light, an image on a document can accurately be scanned without influence of the temporal change of the scanner  3 . 
         [0063]    Further, in the above embodiment, the number of sheets, which is used in the printer  2  since the MFP  1  was produced, is stored in the count memory  24   a.  However, the number of documents that have been scanned by the scanner  3  may be stored in the count memory  24   a  and a plurality of reference values based on the number of documents that have been scanned may be stored in the time dependent reference value memory  24   c.  In this case, if the white reference value stored in the white reference value memory  23   a  or black reference value stored in the black reference value memory  23   b  is abnormal, the white and black reference values are set in accordance with the number of scanned documents in the count memory  24   a.  With this configuration, even if the white and black reference values become abnormal due to influence of outside light, an image on a document can accurately be scanned without influence of the temporal change of the scanner  3 . 
         [0064]    Further, in the above embodiment, the plurality of the white and black reference values is stored in the time dependent reference value memory  24   c.  If the white and black reference values that have been scanned by the scanner  3  are abnormal, one white reference value and one black reference value based on the value in the count memory  24   a  are read out from the plurality of the white and black reference values. However, calculation expression for calculating the reference value from the value in the count memory  24   a  may be stored in the flash memory  21 . Then, if the reference values that have been scanned by the scanner  3  are abnormal, the value in the count memory  24   a  may be assigned to the calculation expression so as to calculate the reference value. 
         [0065]    Further, in the above embodiment, if the white and black reference values scanned by the scanner  3  become abnormal due to influence of outside light, one white reference value and one black reference value based on the value in the count memory  24   a  is read out from the time dependent reference value memory  24   c  and the one white reference value and one black reference value are stored in the white reference value memory  23   a  and black reference value memory  23   b  respectively by the number corresponding to the number of elements. That is, the number, corresponding to the number of the elements, of the white and black reference value is stored. However, if the reference value is abnormal and pixel value is set by the ASIC  3   f,  one white reference value and one black reference value may be stored in the white reference value memory  23   a  and black reference value memory  23   b  respectively for only one element so as to allow the residual memory region to be used for other functions or programs. That is, only one white reference value and only one black reference value may be stored.