Abstract:
A reference image signal level curve V 1  and a predetermined coefficient are provisionally stored in a ROM of a main controller. The predetermined coefficient provides a threshold level curve by multiplying by the curve V 1 . A white color image signal level curve V 2  is formed upon reading a white reference surface by optical sensors. The white color image signal level curve V 2  is compared with the reference image signal curve V 1 . If at least a part of the curve V 2  is not coincident with at least a part of the curve V 1 , the curve V 1  is multiplied by ΔV percent, and this percentage is stored in a RAM. These are performed repeatedly until a part of the curve V 2  is coincident with a part of the curve V 1  as a result of increase in cumulative ΔV percent. Thus, a correction coefficient K is determined based on the cumulative ΔV percent to provide a corrected image signal level curve. The corrected image signal level curve is compared with the threshold level curve to obtain a binary image data.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an image reading device that adjusts for weakening of scanner elements over time. The present invention also relates to a method for formulating a level of image signals, and to an electronic board which performs the image signal level formulating method and is provided with the image reading device. 
   There has been known an image reading device for use in an electronic whiteboard. The image reading device includes a scanner and a comparator. The scanner includes a plurality of sensor elements aligned in a single row for optically reading the image on the board. The optical sensors pick up an image and output an image signal accordingly. The comparator compares the image signal from the sensor elements with a predetermined threshold value to produce binary data of the image signal. That is, the comparator judges a pixel to be white or black based on the threshold value. 
   There is variance in the sensitivity of the optical sensors of the scanner. To adjust for this variance, a reference image signal level curve V 1  and a threshold level curve V 0  shown in  FIG. 1(   a ) are prepared before shipping the image reading device from the factory, and prestored in a memory unit of the image reading device as the characteristic of the image reading device. 
   That is, at the factory the scanner of the image reading device is used to scan a predetermined white color reference member. The obtained image signal is corrected to match its peak value with a maximum value Vmax, which is the maximum value established for the readable range of the image reading device. The corrected image signal is stored in the memory unit as the reference image signal level curve V 1 . Then, the threshold level curve V 0 , which is used to convert a signal of a read out image into binary image data as described above, is determined based on the image signal level curve V 1 . That is, the image signal level curve V 1  is multiplied times a predetermined coefficient to produce the threshold level curve V 0 . The threshold level curve V 0  is also stored in the memory unit. Later, when the image reading device is actually used to read images, the actual image signal level curve is compared with the threshold level curve V 0  to produce an output of binary image data. 
   However, the scanner of this conventional image reading device can degrade over time and/or for other reasons, so that the scanner outputs an image signal level smaller than the image signal level measured at the factory. However, because the weaker image signal is compared with the same threshold level curve V 0  to produce binary data, more pixels will be judged as being black, resulting in an overall darkening of images printed out based on the binary data. 
   Therefore, adjustments must be made before the scanner is used to correctly read images, in order to increase the image signal level as appropriate for the threshold level curve V 0 . For the adjustment, the scanner is used to scan a white reference surface disposed at the image reading position of the device, to read an image signal level curve V 2  shown in  FIG. 1(   b ). Then, the image signal level is adjusted so that a maximum value V 2 max of the image signal level curve V 2  matches the above-described maximum value Vmax, to produce a corrected image level curve V 2   corrected  as shown in  FIG. 1(   c ). After the adjustment of the image signal level, the image reading device is actually used to read images, and the actual image signal level curve is compared with the already stored threshold level curve V 0  to produce an output of binary image data. 
   However, the white reference surface disposed at the image reading position of the scanner can be stained or soiled. This is particularly common with electronic whiteboard that are written on using a felt-tipped marker, because the ink from the felt-tipped marker is often not completely erased from the white reference surface. In this case, the stained area will affect the image signal level curve. As shown in  FIG. 1(   b ) the entire shape of the image signal level curve V 2  will be different from the entire shape of the reference image signal level curve V 1 . (In  FIG. 1(   b ), a curve V 1 ′ is an imaginary curve provided that no stain portion is provided in the whiteboard but the signal level are entirely weakened as a result of degradation.) That is, the signal level are lowered at the stained area of the whiteboard. 
   With this state, if the curve V 2  is modified so that the maximum value V 2 max of the image signal level curve V 2  matches the above-described maximum value Vmax as shown in  FIG. 1(   c ), a broken line curve V 3  in  FIG. 1(   c ) which corresponds to the broken line curve V 1 ′ exceeds the maximum readable range Vmax. 
   In the factory before shipping, assuming that there is a black color area in a whiteboard, and a black dot is shown as an image signal level. This black dot is located below the threshold level curve V 0  as shown in  FIG. 1(   a ), so that the determination falls within a black color range. On the other hand, after the above-described adjustment, the black dot is positioned above the threshold level curve V 0  as shown in  FIG. 1(   c ) due to excessive adjustment of the image signal level, so that the determination falls within a white color range. In summary, due to the excessive adjustment, the read image data may produce an entirely white or pale image in comparison with the actual image drawn on the whiteboard. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image reading device capable of properly correcting the image signal level curve even when a portion of the white reference member disposed at the image reading position is stained. 
   Another object of the present invention is to provide an electronic board provided with the image reading device capable of properly correcting the image signal level curve even when a portion of the white reference member disposed at the image reading position is stained. 
   Still another object of the invention is to provide a method for formulating a level of image signals capable of providing a shape of the reference image signal level curve even after degradation of the image sensor elements and irrespective of a stained area in a reference white surface. 
   These and other objects of the present invention will be attained by an image reading device including an image reading unit, a characteristic storage unit, a correction coefficient calculator and a correction output unit. The image reading unit includes a plurality of optical reading sensors aligned in a row. The image reading unit outputs an image signal based on an image read by the plurality of optical reading sensors. The characteristic storage unit stores the characteristic of the image reading unit in the form of a reference image signal level curve and one of a predetermined coefficient and a threshold level curve. The reference image signal level curve is obtained by correcting an image signal curve outputted by the reading unit when the optical reading sensors reads an image of a predetermined white reference member at a factory so that a peak value of the image signal curve matches a predetermined maximum readable range. The threshold level curve is obtained by multiplying the reference image signal level curve by the predetermined coefficient. The correction coefficient calculator determines a correction coefficient which provides a part of the present image signal level curve being matched with a part of the reference signal level curve to produce a corrected image signal level curve. The correction output unit produces a binary output signal of the corrected image signal level curve by comparing the corrected image signal level curve with either the threshold level curve stored in the characteristic storage unit or a threshold level curve obtained by multiplying the reference image signal level curve by the predetermined coefficient. 
   The correction coefficient calculator includes reading means, comparing means, and determining means. The reading means reads an image from a white reference surface provided at a reading position to obtain a present image signal level curve before image data is actually retrieved using the reading unit. The comparing means compares the present image signal level curve with the reference image signal level curve stored in the characteristic storage unit. The determining means determines a correction coefficient required to match at least the portion of the present image signal level curve with the portion of the reference signal level curve. 
   The white reference surface positioned at the reading position of the image reading device may be partially soiled, so that the image signal level curve actually outputted differs from the reference image signal level curve already stored at the factory. In such a situation, a correction coefficient is determined that is required to match at least a portion of the actual image signal level curve with the reference image signal level curve. Therefore, a correction coefficient is determined for matching the image signal level obtained at non-soiled portions with the reference image signal level curve. In other words, because a portion of the actual image signal level curve, the portion corresponding to the soiled portion, has a shape different from the reference image signal level curve, such portion does not match the reference image signal level curve. An actual signal level curve is obtained by multiplying with the correction coefficient. The actual signal level curve becomes the image signal level curve with a shape that corresponds to the reference image signal level curve, regardless of the presence or absence of dirty areas on the white reference surface. 
   In another aspect of the invention, there is provided an electronic board including a white board on which an image is drawn, the image reading unit, the characteristic storage unit, the correction coefficient calculator, the correction output unit, and printing means that prints an image on an image recording medium based on the binary output signal. 
   In still another aspect of the invention, there is provided a method for formulating a level of image signal including the steps of provisionally storing a characteristic of a image reading unit in the form of a reference image signal level curve and one of a predetermined coefficient and a threshold level curve, the reference image signal level curve being obtained by correcting an image signal curve outputted by the reading unit when the reading unit reads an image of a predetermined white reference member at a factory so that a peak value of the image signal curve matches a predetermined maximum readable range, and the threshold level curve being obtained by multiplying the reference image signal level curve by the predetermined coefficient, determining a correction coefficient providing a part of a present image signal level curve being matched with a part of the reference signal level curve to produce a corrected image signal level curve, and generating a binary output signal of the corrected image signal level curve by comparing the corrected image signal level curve with either the threshold level curve stored in the characteristic storage unit or a threshold level curve obtained by multiplying the reference image signal level curve by the predetermined coefficient. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1(   a ) is a graph schematically showing a reference image signal level curve V 1  and a threshold level curve V 0 , which serve as the characteristic of a conventional image reading device; 
       FIG. 1(   b ) is a graph schematically showing an image signal level curve V 2  obtained by reading the image of a white reference member where a stain area partly exists for adjusting output of attenuated sensor elements of the conventional image reading device; 
       FIG. 1(   c ) is a graph schematically showing an excessively corrected image level curve V 2   corrected  resulting from a stain on the white reference member for adjusting output of attenuated sensor elements of the conventional image reading device; 
       FIG. 2  is a block diagram showing an electronic board provided with an image reading device according to an embodiment of the present invention; 
       FIG. 3  is a flowchart representing a correction coefficient determination routine performed in the image reading device shown in  FIG. 2 ; 
       FIG. 4(   a ) is a graph schematically showing a reference image signal level curve V 1  and a threshold level curve V 0 , which serve as the characteristic of the image reading device shown in  FIG. 2 ; 
       FIG. 4(   b ) is a graph schematically showing an image signal level curve V 2  obtained by reading the image of a white reference member, for adjusting output of attenuated retrieval elements of the image reading device shown in  FIG. 2 ; 
       FIG. 4(   c ) is a graph schematically showing the image signal level curve V 2  of  FIG. 4(   b ) after being corrected according to the routine represented by the flowchart of  FIG. 3 ; and 
       FIG. 5  is a flowchart representing a copy routine performed in the image reading device shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An electronic board according to an embodiment of the present invention will be described with reference to  FIGS. 2 to 5 . As shown in  FIG. 2 , the electronic board  1  includes a whiteboard  3 , a thermal printer  5 , and a main controller  7 . The main controller  7  controls the whiteboard  3  and the thermal printer  5  in a manner to be described below. 
   The whiteboard  3  includes a sheet member  11  on which figures and characters are drawn, upper and lower rollers (not shown) for supporting the sheet member  11 , a sheet member feed motor  15  for feeding the sheet member  11  upward and downward, and a contact image sensor (CIS)  13  for reading figure and character images drawn on the sheet member  11  and outputting image data signals accordingly. The sheet member  11  is in the form of an endless web with confronting front and rear sides. The thermal printer  5  includes a thermal head  21  with a plurality of heat generating elements, and a sheet feed motor  23  for feeding the heat-sensitive sheet. The main controller  7  includes a central processing unit (CPU)  31 , a read only memory (ROM)  33 , and a random access memory (RAM)  35 . 
   The CIS  13  includes a plurality of light-receiving elements aligned in the widthwise direction of the sheet member  11 , red LEDs and green LEDs. The CIS  13  can read figures and characters drawn on the sheet  11  in either black or red marker. That is, the CIS  13  reads black figures and characters by illuminating the red LEDs and reads black and red figures and characters by illuminating the green LEDs. 
   The heat-sensitive sheet set in the thermal printer  5  is coated on its surface with two heat-sensitive ink layers. The first ink layer turns red at a first temperature, and the second ink layer turns black at a second temperature lower than the first temperature. The plurality of heat generating elements of the thermal head  21  are aligned in the widthwise direction of the heat-sensitive sheet. These heat generating elements are controlled to be driven upon low level strobe signal from the CPU  31 . Image data retrieved by the CIS  13  and stored in the RAM  35  is outputted in one line basis at a time from the RAM  35 . Based on the one line image data, the thermal head  21  is controlled by the strobe signal from the CPU  31 . That is, the heat geneating elements are heated based on the outputted image data to the first temperature to turn the first ink layer red or to the second temperature to turn the second ink layer black. 
   The CPU  31  controls overall operation of the electronic board  1 . The CPU  31  follows a predetermined control program to read image data from the sheet member  11  using the CIS  13  and store the retrieved image data in the RAM  35  while driving the sheet member feed motor  15  to move the sheet member  11  in a vertical direction. The CPU  31  also controls driving mode of the thermal head  21  and the sheet feed motor  23  based on the image data stored in the RAM  35 , to reproduce on the heat-sensitive sheet the image that was drawn on the sheet member  11  in black or red marker. The ROM  33  stores the control program executed by the CPU  31  and also stores various types of control data. The RAM  35  stores image data retrieved by the CIS  13  and also functions as a work area for control processes of the CPU  31 . 
   The ROM  33  stores the “characteristic values” of the electronic board  1  in the form of “a reference image signal level curve V 1 ” and “a predetermined coefficient” for obtaining a threshold curve V 0 . The characteristic is a reference image signal level curve V 1  obtained by correcting an image signal outputted by the CIS  13  when it reads a predetermined white reference member. The image signal is corrected so that its peak value matches a maximum value Vmax of the readable range defined in the image reading device. Then, the threshold level curve V 0  can be provided by multiplying the curve V 1  by the predetermined coefficient. When the electronic board  1  actually reads an image, the actual image signal level curve is compared with the threshold level curve V 0 , to produce an output of binary image data. 
   Next, a correction coefficient determination routine executed in the electronic board  1  for determining a correction coefficient will be described. The light receiving elements of the CIS  13  may degrade over time, so that the level of their output signal drops. Because of this, there is a need to compute a correction coefficient K for correcting the image signal V outputted from the CIS  13  when image data is actually retrieved. 
     FIG. 3  is a flowchart representing processes performed in the correction coefficient determination routine. First, an actual white image signal level curve V 2  is retrieved in S 10  using a surface of the sheet member  11  as a white reference surface where no image is drawn. Next, in S 20  the actual white image signal level curve V 2  is compared with the reference image signal level curve V 1  stored in the ROM  33 . Then, whether or not the two curves V 1 , V 2  match each other is judged in S 30 . If not (S 30 :NO), then in S 40  the actual white image signal level curve V 2  is increased by a small amount ΔV (%), and in S 50  a correction coefficient K is calculated based on the value of ΔV and stored in the RAM  35 . That is, the correction coefficient K is first set to 1, and then increased in S 50  to a value greater than 1 using the formula K ΔV/100. 
   In S 60 , the present white image signal level curve V 2  is again compared with the reference image signal level curve V 1 . Then it is judged whether or not at least a portion of the curves V 1  and V 2  match. If it is judged that there are no matching portions between curves V 1  and V 2  (S 70 :NO), then the routine returns to S 40 , whereupon the present white image signal level curve V 2  is again increased by the small amount ΔV(%) in S 40 , and the correction coefficient K is calculated in S 50 . 
   The processes of S 40  to S 70  are repeated until it is judged that at least a portion of the curves V 1  and V 2  match. It should be noted that the two curves V 1  and V 2  “match” implies that a part of the curve V 2  overlaps with a part of the curve V 1  within the range of plus minus ΔV 1 (where ΔV 1  is a predetermined small amount) as a result of the successive change of the curve V 2  by the step S 50 . When it is judged that a portion of the curves V 1  and V 2  match (S 70 :YES), then the correction coefficient K stored in the RAM  35  at this time is set in S 80  as the coefficient to be multiplied by the output signal from the CIS  13  when an image is actually read. 
   Next, a correction coefficient determination routine will be described while referring to  FIGS. 4(   a ) to  4 ( c ). The ROM  33  stores therein, as the characteristic values of the electronic board  1 , a reference signal level curve V 1  and a predetermined coefficient. The reference signal level curve V 1  is obtained by scanning a white reference member at the factory to obtain an image signal, and increasing the image signal until the peak value of the image signal matches the maximum readable value Vmax of the scanning range of the image reading device  1 , whereupon the reference signal level curve V 1  shown in  FIG. 4(   a ) is set. The predetermined coefficient is used to obtain a threshold level curve V 0  shown in  FIG. 4(   a ) by multiplying the reference signal level curve V 1  by the predetermined coefficient. The threshold level curve V 0  is used to produce binary image data of an actual image signal level curve obtained when the electronic whiteboard  1  is actually used to read an image, that is, the threshold level curve V 0  is compared with the actual image signal level curve to obtain the binary image data. 
   The correction coefficient determination routine is performed when the light receiving elements of the CIS  13  degrade over time, so that the output signal from the light receiving elements weakens. An image signal level curve V 2  can be obtained, if the light receiving elements are degraded over time and if certain areas of the sheet member  11  are stained when the image of the sheet member  11  at the read position of the CIS  13  is read to perform the correction coefficient determination routine. The correction coefficient determination routine determines the correction coefficient K by increasing such an image signal level curve V 2  by increments of the small amount ΔV(%) until finally, as shown in  FIG. 4(   c ), the image signal level curve V 2  partially matches the reference image signal level curve V 1 . 
   The correction coefficient determination routine enables proper determination of a correction coefficient that will reliably correct for weakening of the output signal from the light receiving elements of the CIS  13 . Even if the sheet member  11  is partially stained as shown in  FIG. 4(   b ) when its image is retrieved as a white reference, the correction coefficient can be properly determined without being affected by this stain. 
   When the correction coefficient K is determined in this way for the image signal level in the electronic board  1 , then a copy routine is performed as shown in the flowchart of  FIG. 5 . First in S 110  the CIS  13  reads the image, which was drawn on the surface of the sheet member  11  using a felt-tipped marker, while the sheet member feed motor  15  is driven to slowly move the sheet member  11  upward at a predetermined speed. In S 120 , the image signal level curve V 2  that corresponds to the retrieved image data is multiplied by the correction coefficient K, and the resultant corrected image signal level curve K×V 2  is stored in the RAM  35 . In S 130 , sheet feed motor  23  of the thermal printer  5  is driven to set a heat-sensitive sheet at the printing position of the thermal printer  5 . 
   In S 140 , the corrected image signal level curve K×V 2  is compared with the threshold level curve V 0 , and in S 150 , a control signal for controlling the thermal printer  5  is produced. The control signal is produced by designating pixels in the corrected image signal level curve K×V 2  that are equal to or less than the threshold level curve V 0  as pixels that are to be colored. In S 160 , the control signal is outputted to the thermal printer  5  with a strobe signal. 
   In S 170 , it is judged whether or not a single page&#39;s worth of image data has been printed. If a single page&#39;s worth of image data has not been printed (S 170 :NO), then in S 180  the sheet feed motor  23  is driven to feed the heat-sensitive sheet a single line&#39;s distance. The routine then returns to S 140 , whereupon the image signal level curve K×V 2  of the next line is compared with the threshold level curve V 0 . A control signal for the next line is prepared in S 150 , and outputted to the thermal printer  5  along with the strobe signal in S 160 . 
   These processes are repeated until it is judged that a signal page&#39;s worth of image data has been printed (S 170 :YES), whereupon in S 190  the sheet-feed motor  23  is driven to discharge the heat-sensitive sheet from the thermal printer  5 . This ends the copy routine. 
   In this way, the degradation of the CIS  13  over time can be suitably corrected. Also, the correction coefficient used during this correction process can be properly calculated, even if the surface of the sheet member  11 , which serves as the white reference member, is partially soiled. That is, the influence of the soiled portion can be removed during the correction process. Therefore, proper correction can always be performed. The image drawn on the sheet member  11  can be faithfully reproduced, without portions appearing blanched, because of the binary processing based on the corrected data. 
   While the invention has been described in detail and with reference to specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention. For example, in the above described embodiment, a predetermined coefficient for obtaining the threshold level curve V 0  is stored in the ROM  33 . However, the entire threshold level curve V 0  can be stored instead. Also, the depicted embodiment pertains to an electronic board. However, the present invention can be applied to a facsimile machine or other devices instead.