Abstract:
An image reading apparatus has an automatic document feeder and a flat bed, wherein a backing part which is read so as to ensure that a leading edge of an original is read is configured to have a white area and a gray area, either the white area or the gray area being read depending on a type of the original, and the original being read subsequently.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image reading apparatuses, methods of reading images, whiteness level correcting methods and whiteness level generating methods, and more particularly to an image reading apparatus and an image reading method for reading an image on an original, and to a whiteness level correcting method and a whiteness level generating method employed in reading an image on an original. 
     Recently, with less paper being used in offices, image inputting products (image scanners) have been used as inputting means for storing documents in optical disks or the like. As the volume of images increases, high-speed input systems are used more often. Hence, use of automatic paper feeding is being practiced more and more widely. 
     In the case where automatic paper feeding is used in image reading, a CCD (charge coupled device) starts a reading operation at a member (hereinafter, referred to as a backing part or a backing member) for supporting the backside of an original so that the leading edge of the original can be read. 
     Since a reading is started at the backing part, a ground color (reference color) of the original cannot be read properly. In some cases, an image reading apparatus may produce a darkened output. Another problem in a conventional image reading apparatus is that, when a large volume of input originals are processed, the backing part and the input original may undergo friction, resulting in a flaw extending in a longitudinal direction being created in he backing part. When the flaw is created, the output f the image reading apparatus often accompanies a white or black streak. 
     Thus, an arrangement, whereby it is ensured that the leading edge of an automatically fed original is read, is met by a difficulty in obtaining a high-quality image which is true to the original. 
     For this reason, there is required an image reading apparatus capable of producing a high-quality image true to the original that is fed automatically. 
     2. Prior Art 
     FIG. 1 shows a construction of a conventional image reading apparatus  91 . The image reading apparatus  91  has an electrooptical converter  92  for reading an image optically, an image processing unit  93  for processing a signal obtained in the electrooptical converter  92 , a feeding mechanism as for transporting an original  94 , a carriage driving mechanism  96  for moving the electrooptical converter  92 , and a controller  97  for controlling the image processing unit  93 , the feeding mechanism  95  and the carriage driving mechanism  96 . 
     The image reading apparatus  91  includes: a flat-bet (FB) function whereby the electrooptical converter  92  is moved by the carriage driving mechanism  96  to scan an original  94  so that the image thereon is read; and an automatic document feeder (ADF) function whereby the image is read while the original  94  is being moved by the feeding mechanism  95  and scanned. 
     As shown in FIG. 1, if the reading is executed in an FB mode, the controller  97  controls, upon power ON of the apparatus, the carriage driving mechanism  96  so that the electrooptical converter  92  is moved in a direction indicated by an arrow A 1  to reach a home position P 0  and stand by for an instruction signal. An original cover  98  is opened so that the original  94  is set in a platform  99  for an original. When an instruction to read an image arrives, a fluorescent lamp provided in the electrooptical converter  92  is turned on, whereupon the electrooptical converter  92  is further moved in the A 1  direction to reach a position P 1  so as to scan a reference plate  100  and read a reference whiteness level. Subsequently, the electrooptical converter  92  scans the original  94  so as to start reading the image thereon at a predetermined position P 2 . When the reading is completed, the electrooptical converter  92  is returned to the home position P 0  and stands by for a next instruction. In reading the original  94 , the reference whiteness level is used as an initial value, and other whiteness levels are successively made to follow the initial value. In this way, correction is made so that proper contrast is obtained. 
     If the reading is executed in an ADF mode, the controller  97  controls, upon power ON of the apparatus, the carriage driving mechanism  96  so that the electrooptical converter  92  is moved in the A 1  direction to reach a home position P 0  in the left half of the apparatus in FIG.  1  and stands by for an instruction signal. The original  94  is set in a shooter  101 . When an instruction to read an image arrives, the fluorescent lamp in the electrooptical converter  92  is turned on, and the electrooptical converter  92  is further moved in the A 1  direction to reach the position P 1  and read the reference whiteness level of the reference plate  100 . Subsequently, the electrooptical converter  92  is moved to an ADF reading position P 3  in the left half of the apparatus in FIG.  1  and stands by for an instruction. The feeding mechanism  95  is then driven so as to draw the originals  94  mounted in the shooter  101  one by one into the ADF. Reading is started from a predetermined position. The document  94  read is ejected to a stacker  102 . 
     The original  94  is guided by a backing part  103  so as to face directly opposite the electrooptical converter  92 . 
     FIG. 2 shows a conventional backing part  103 . In ADF reading, in order to ensure that a leading edge  94   a  of the original  94  is read, reading is started before the original  94  is transported to a reading position. The backing part  103  is read first, whereupon the original  94  is transported and read. Conventionally, the entire backing part  103  is painted in white in an assumption that the ground color of a commonly used original  94  is white. 
     The image processing unit  93  converts an analog electric signal from the electrooptical converter  92  into a digital signal on the basis of the reference whiteness level. In order to prevent data from being ruined by a level of the analog electric signal becoming higher in brightness level than the reference whiteness level, a conventional apparatus is equipped with a circuit whereby the electric signal is compared with the whiteness level required for conversion of the analog electric signal into the digital data. When it is determined that a difference between the analog electric signal and the whiteness level exceeds a predetermined level, the analog electric signal is made to indicate the whiteness level to make sure that the ground color of the original is white. Since image reading is started at the backing part  103  in the ADF, the color (white) of the backing part  103  is read first, whereupon other whiteness levels are made to follow. That is, a white-level following operation is started at the leading edge of the original. 
     It is to be noted that an original read by an image reading apparatus can be an original characterized by a contrast between the ground color of the original and a line drawing etc. on the original, or an original like a photograph whose image representation does not depend on the ground color. 
     The original which contains a line drawing etc. does not necessarily have the ground color of white. Hence, in order to obtain optimal contrast, a white-level following operation for determining the whiteness level should be carried out in accordance with a read signal. A reading mode wherein a reading is executed while whiteness levels are made to follow the ground color is referred to as a line art mode. 
     An image on an original such as a photograph is represented without the ground color being shown. An image true to the original cannot be obtained if a white-level following operation is conducted on an original such as a photograph. In this type of original, an image is read such that the reference whiteness level is determined by employing white as the fixed reference color. An image reading mode wherein the reference whiteness level is determined by employing white as the fixed reference color is referred to as a photograph mode. 
     Thus, the conventional image reading apparatus is constructed such that an image reading mode (line art mode or photograph mode) adapted for the type of original (a line drawing or a photograph) is chosen for image reading suitable for the original to be carried out. 
     It is to be noted that, irrespective of whether the line art mode or the photograph mode is employed, there is provided only one (white) backing part in the conventional image reading apparatus. In the line art mode wherein reading (white-level following operation) is started at the backing part in order to ensure that the leading edge of the original is read, the color (white) of the backing part is stored as the ground color of the original, and shading correction is conducted accordingly. There is a problem in that, when the original has a darkened color, a gap is created between the reference white level data and the data for the ground color, with the result that an accurate white-level following operation cannot be performed, and the image becomes darkened. 
     It is also to be noted that the white-level following operation of an original is conventionally conducted at the backing part or at a leading edge (which has a width of about 3 mm) of the original. A flaw in the backing part may cause the reflectance thereof to vary, with the result that different data may be output even when same reference plate is read. Consequently, reading correction is conducted even when data relating to the ground color vary depending on CCD pixels, with the result that a normal white-level following operation cannot be conducted, and a streak may be created in the image. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an image reading apparatus, an image reading method, a whiteness level correcting method and a whiteness level generating method capable of obtaining a high-quality image true to an original. 
     Another and more specific object of the present invention is to provide an image reading apparatus comprising: 
     an electrooptical converter which, provided opposite an original, scans the original and converts reflected light from the original that is illuminated into an electric signal; 
     a backing member which has differently colored areas arranged in a direction in which the electrooptical converter scans the original, and is provided opposite the electrooptical converter so as to support the back side of the original; 
     a driving mechanism for moving the electrooptical converter with respect to the backing member; and 
     controlling means for controlling the driving mechanism in correspondence with a type of the original, and thereby controlling a position of the electrooptical converter with respect to the backing member so that the electrooptical converter faces one of the colored areas of the backing member. 
     Preferably, the image reading apparatus further comprises: 
     converting means for converting an analog signal output by the electrooptical converter into digital data in accordance with a whiteness level indicating a maximum brightness; and 
     a whiteness level following circuit which causes a whiteness level supplied to the converting means to follow the analog electric signal output from the electrooptical converter. 
     According to the image reading apparatus of the present invention, in an image reading scheme whereby an original is read while a whiteness level is made to approximate the ground color of the original, and a backing part is read first in order to make sure that the leading edge of the original is read, the color of the backing member is selected to be similar to the ground color of the original. With this arrangement, a whiteness level following operation can be properly performed even when the operation is started in the backing member, thus ensuring that an image having an optimum contrast can be reproduced. 
     In another preferable embodiment, the image reading apparatus comprises: 
     a driving mechanism for altering a relative position of the electrooptical converter and the backing member; and 
     controlling means for causing the electrooptical converter to read the backing member so as to detect a variation of an electric signal along a direction in which the electrooptical converter lies, and for controlling, in response to an occurrence of a variation greater than a predetermined level, the driving mechanism so as to move a position of the electrooptical converter with respect to the backing member. According to this aspect of the present invention, it is possible to obtain image data not affected by a flaw or a stain in an original, by altering a position at which a reading is performed in case there is a flaw or a stain, the flaw or the stain being detected as a difference, exceeding a predetermined level, between data for one pixel and data for an adjacent pixel. Thus, variations in reading condition between pixels on a given line is canceled, thus preventing any streak from occurring in a read image. Consequently, a high-quality image can be obtained. 
     Still another object of the present invention is to provide an image reading method comprising the steps of: 
     reading the backing member by the electrooptical converter; 
     altering a relative position of the original and the electrooptical converter; and 
     reading the original. 
     According to the image reading method of the present invention, the colored area on the backing member corresponding to the ground color of the original is read first, whereupon the original is read. In this way, it is possible to carry out the whiteness level following operation smoothly, and a high-quality image can be obtained. 
     Yet another object of the present invention is to provide an image reading apparatus comprising: 
     an electrooptical converter for converting reflected light from an original that is illuminated into an electric signal; 
     converting means for converting an analog electric signal obtained in the electrooptical converter into digital data in accordance with a whiteness level signal indicating a maximum brightness, reading of the original being conducted while a relative position of the original and the electrooptical converter is being altered; 
     thickness detecting means for detecting a thickness of the original; and 
     whiteness level correcting means for correcting the whiteness level to become a predetermined value in correspondence with the thickness, detected by the detecting means, of the original. According to this aspect of the present invention, the whiteness level is corrected to have an optimum level depending on the thickness of the original. Therefore, it is possible to obtain a high-quality image, irrespective of the thickness of the original. 
     Still another aspect of the present invention is to provide a whiteness level correcting method, wherein a whiteness level is corrected to become lower in level as the thickness of the original increases. Since a relatively thick original is less detached from a reading window than a relatively thin original, the thick original has a larger light reflectance. The above described aspect of the present invention ensures that the whiteness level in the thick original is corrected to become smaller in level than in the thin original. Accordingly, it is possible to read images in a stable manner, irrespective of the thickness of the original. 
     Still another object of the present invention is to provide an image reading apparatus having a first reading function wherein an original is illuminated and its image is read while the original is moved and an electrooptical converter for converting reflected light into an electric signal is fixed, and a second reading function wherein the original is illuminated and its image is read while the original is fixed and the electrooptical converter is moved, 
     the image reading apparatus comprising whiteness level correcting means for correcting a whiteness level indicating a maximum brightness to have a predetermined value responsive to whether the first or second reading function is used. According to this aspect of the present invention, the whiteness level is corrected responsive to whether the first reading function or the second reading function is used. Accordingly, it is possible to read images in a stable manner, irrespective of the thickness of the original. 
     Still another object of the present invention is to provide a whiteness level correcting method, wherein a whiteness level is corrected to become greater in level when the first reading function is used in reading the original than when the second reading function is used. Since the first reading function, in which the original is moved, is characterized by a larger degree of detachment of the original from the reading window than the second reading function, in which the electrooptical converter is moved, the whiteness level in the first reading function is corrected to have a smaller level than that in the second reading function. In this way, a difference in brightness in images read in the first and second reading functions is reduced. Thus, it is possible to obtain read images in a stable manner, irrespective of the reading function used. 
     Still another object of the present invention is to provide an image reading apparatus comprising: 
     an electrooptical converter for converting reflected light from an original that is illuminated into an electric signal; 
     converting means for converting an analog electric signal obtained in the electrooptical converter into digital data in accordance with a whiteness level signal indicating a maximum brightness, reading of the original being conducted while a relative position of the original and the electrooptical converter is being altered; and 
     whiteness level generating means for causing the electrooptical converter to read a blank sheet of a same kind as used for the original and for generating a whiteness level commensurate with data obtained by reading the blank sheet. According to this aspect of the present invention, the whiteness level is determined on the basis of the value obtained by reading a blank sheet of the same kind used for the original. Thus, it is possible to use a value corresponding to the ground color of the original to determine the whiteness level. In this way, an optimum whiteness level can be generated, and a high-quality image can be achieved. 
     Still another object of the present invention is to provide a whiteness level generating method, wherein an average value of the data obtained by causing the electrooptical converter to read the blank sheet is set to be a whiteness level. According to this aspect of the present invention, an average value of image data obtained by reading a blank sheet is calculated, and the average value is used as the whiteness level. Thus, an optimum whiteness level can be generated, and a high-quality image can be obtained. 
     Still another object of the present invention is to provide an image reading apparatus having a first reading function wherein an original is illuminated and its image is read while the original is moved and an electrooptical converter for converting reflected light into an electric signal is fixed, and a second reading function wherein the original is illuminated and its image is read while the original is fixed and the electrooptical converter is moved, 
     the image reading apparatus comprising converting means for converting an analog electric signal output from the electrooptical converter into digital data commensurate with a blackness level indicating a minimum brightness; and 
     blackness level correcting means for correcting a blackness level to have a predetermined value corresponding to whether the first or second reading function is used. 
     Since the first reading function is characterized by a larger degree of detachment of the original from the reading window than the second reading function is moved, the blackness level in the first reading function is corrected to have a level different from that in the second reading function. In this way, it is possible to make sure that the blackness level is regular irrespective of the reading function. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a schematic construction of a conventional image reading apparatus; 
     FIG. 2 shows a construction of an essential part of a conventional image reading apparatus; 
     FIG. 3 is a schematic diagram showing the principle of the present invention; 
     FIG. 4 shows a schematic construction of an image reading apparatus according to a first embodiment of the present invention; 
     FIGS. 5A and 5B show a construction of a backing part according to the first embodiment; 
     FIG. 6 is a block diagram showing a construction of an image processing unit according to the first embodiment; 
     FIG. 7 is a flowchart which explains an operation of the first embodiment; 
     FIG. 8 is a graph which explains an operation of the first embodiment; 
     FIGS. 9A and 9B show a construction of a variation of the backing part according to the first embodiment; 
     FIG. 10 is a flowchart which explains an operation of the variation of the first embodiment; 
     FIG. 11 shows a schematic construction of an image reading apparatus according to a second embodiment of the present invention; 
     FIG. 12 is a block diagram showing a construction of the image reading apparatus according to the second embodiment; 
     FIG. 13 shows a construction of a correcting circuit of the image processing unit according to the second embodiment; 
     FIG. 14 shows a construction of a first variation of the correcting circuit according to the second embodiment; 
     FIG. 15 shows a construction of a second variation of the correcting circuit according to the second embodiment; 
     FIG. 16 is a flowchart which explains an operation of the second variation of the correcting circuit according to the second embodiment; 
     FIG. 17 explains an operation of the second variation of the correcting circuit according to the second embodiment; 
     FIG. 18 shows a schematic construction of a third embodiment of the present invention; 
     FIG. 19 is a block diagram showing a construction of an image processing unit according to the third embodiment; and 
     FIG. 20 shows a construction of a correcting circuit according to the third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows the principle of the first embodiment of the present invention. 
     An electrooptical converter  2  converts light reflected by an illuminated original  1  into an electric signal. 
     A backing member  3  has colored areas S 1 -S n  arranged side by side in a direction in which the electrooptical converter  2  is moved by a carriage driving mechanism  4 . The colored areas S 1 -S n  have different colors. The backing member  3  is provided opposite the electrooptical converter  2  when the ADF function is activated in reading the original  1  so as to support the back side of the original  1 . 
     The carriage driving mechanism  4  causes relative positions of the original  1  and the electrooptical converter  2  to vary. 
     When the ADF function is activated in reading the original  1 , a controller  5  controls the carriage driving mechanism  4  in a manner adapted for the original  1 . The controller  5  controls a position at which the electrooptical converter  2  faces the backing member  3  so as to control which color of the backing member  3  is opposite the electrooptical converter  2 . 
     FIG. 4 is a schematic diagram showing a construction of a first embodiment of the present invention. An image reading apparatus  11  of the first embodiment includes: an image detecting unit  13  for optically detecting an image on an original  12  and converting the optical signal into an electric signal; an image processing unit  14  for processing the electric signal from the image detecting unit  13 ; a white reference plate  15  which is read by the image detecting unit  13  and provides a reference whiteness level necessary for signal processing in the image processing unit  14 ; a carriage driving mechanism  16  for moving the image detecting unit  13  in a direction indicated by an arrow A in the FB mode; a feeding mechanism  19  for feeding the original  12  from a feeder  17  to a stacker  18  in the ADF mode; a controller  20  for controlling the operation of the whole apparatus; an operation part  21  for providing operation instructions; and a backing part  22  which is provided opposite the image detecting unit  13  when the original  12  is read in the ADF mode so as to support the back side of the original  12 , and ensures that the leading edge of the original  12  is read in the ADF mode. 
     The image detecting unit  13  includes: a light source  23  for illuminating the original  12  surface provided with an image; an optical system  24  which has light reflected from the original  12  introduced in a predetermined direction; and an electrooptical converter  25  which has light introduced from the optical system  24  and converts the light thus introduced into an electric signal. The image detecting unit  13  is designed to be moved by the carriage driving mechanism  16  in the A direction. 
     The light source  23  is formed, for example, of a fluorescent lamp and emits light whose intensity is uniform in a longitudinal direction (in a direction indicated by an arrow B of FIGS.  5 A and  5 B). The light emitted by the light source  23  is incident on the original  12  via a reading window  33 . The reading window  33  is formed as an opening of a shade  35  provided as wires built in layers within a glass plate  34 . The original  12  reflects light incident thereon in correspondence with the image thereon. The light reflected by the original  12  is fed to the optical system  24 . 
     The optical system  24  comprises mirrors and lenses lying in the longitudinal (line direction) direction and compresses the light reflected by the original  12  in the line direction and feeds the result to the electrooptical converter  25 . The electrooptical converter  25  is embodied, for example, by a line-type two-dimensional CCD which generates and outputs, line by line, a video signal corresponding to the image on the original. The electric signal obtained in the electrooptical converter  25  is fed to the image processing unit  14 . 
     FIGS. 5A and 5B show a construction of the backing part  22 . The backing part  22  comprises a white area  22   a  and a gray area  22   b  lying parallel in the longitudinal direction (the B direction). FIG. 5 a  shows a position of the image detecting unit  13  at which position the gray area  22   b  is selected, and FIG. 5B shows a position of the image detecting unit  13  at which position the white area  22   a  is selected. 
     FIG. 6 shows a construction of the image processing unit  14 . The image processing unit  14  comprises an A/D converter  26  for converting an electric signal from the image detecting unit  13  into digital data; and a whiteness level generating unit  27  for generating a whiteness level signal which controls the level of the signal converted by the A/D converter  26 . 
     The analog signal obtained in the image detecting unit  13  is fed to a terminal T 1 . The terminal T 1  is connected to the A/D converter  26  and the whiteness level generating unit  27 . 
     The A/D converter  26  converts the analog electric signal from the image detecting unit  13  into 8-bit digital data. The analog signal is converted into the 8-bit digital data on the basis of the whiteness level signal output from the whiteness level generating unit  27 . The whiteness level generating unit  27  includes: a whiteness level determining unit  28  which compares the video signal supplied from the terminal T 1  and the whiteness level signal generated in the whiteness level generating unit  27  and determines the whiteness level; a whiteness level controller  29  which generates reference whiteness level data in accordance with the digital data output from the whiteness level determining unit  28  and the A/D converter  26 , and causes the whiteness level to vary in accordance with the comparison result yielded by the whiteness level determining unit  28  so as to effect the white-level following operation; a whiteness level memory  30  for storing the whiteness level data generated by the whiteness level controller  29 ; and a whiteness level D/A converter  31  which converts the whiteness level data generated by the whiteness level controller  29  into an analog signal and generates the output whiteness level signal to be fed to the A/D converter  26  and the whiteness level determining unit  28 . 
     The whiteness level determining unit  28  is embodied, for example, by a comparator and compares the video signal supplied to the terminal T 1  and the output whiteness level signal. The whiteness level determining unit  28  supplies a high-level whiteness level follow signal to the whiteness level controller  29  when the video signal is greater in level than the output whiteness level signal, and supplies a low-level whiteness level follow signal when the video signal is smaller in level than the output whiteness level signal. 
     The whiteness level controller  29  is embodied by a CPU etc. The whiteness level controller  29  fixes the whiteness level when the image such as a photograph is read. When a line drawing or the like is read, the whiteness level controller  29  generates the follow whiteness level data in accordance with the whiteness level follow signal from the whiteness level determining unit  28 . The generated data is stored in the whiteness level memory  30 . The stored data is used as the whiteness level data for the next line. The whiteness level controller  29  intensifies the level of the whiteness level data by a predetermined number of steps when the whiteness level follow signal is at a high level. When the whiteness level follow signal is at a low level, the whiteness level controller  29  compares the image data obtained and the whiteness level data. In case a difference therebetween exceeds a predetermined value, the whiteness level data is lowered in level by a predetermined number of steps so that the whiteness level data is made to follow so as to suppress the video below the whiteness level. 
     The whiteness level memory  30  is embodied by a RAM or the like and stores the whiteness level data for each pixel in a line. 
     FIG. 7 explains an operation of the present invention. When an instruction to read image is issued, the controller  20  controls the carriage driving mechanism  16  so that the image detecting unit  13  is moved to a position opposite the white reference plate  15  to read the white reference plate  15  (S 1 - 1 ). The data read from the white reference plate  15  is supplied to the whiteness level controller  29  via the A/D converter  26 . The whiteness level controller  29  uses this data as the reference whiteness data and stores the same in the whiteness level memory  30  (S 1 - 2 ). 
     The controller  20  then determines whether or not a line drawing is to be read (S 1 - 3 ). Selection of a line drawing is made through the operation of the operation part  21  by a user. 
     If it is determined in S 1 - 3  that a line drawing is to be read, the controller  20  controls the carriage driving mechanism  16  so as to move the image detecting unit  13  to a position indicated by  2  in FIGS. 5A and 5B which position is opposite the gray area  22   b  of the backing part  22  (S 1 - 4 ). When it is determined in S 1 - 3  that an image other than a line drawing is to be read, which means that a photograph or the like that includes halftones is to be read, the controller  20  controls the carriage driving mechanism  16  so as to move the image detecting unit  13  to a position indicated by  1  in FIGS. 5A and 5B which position is opposite the white area  22   a  of the backing part  22  (S 1 - 5 ). 
     The controller  20  then controls the image detecting unit  13  so that a normal reading operation is started at the white area  22   a  or the gray area  22   b  of the backing part  22  (S 1 - 6 , S 1 - 7 ). The reference whiteness level data obtained by reading the white reference plate  15  is compared with the image data obtained by reading the white area  22   a  or the gray area  22   b  of the backing part  22 , by the whiteness level determining unit  28 . The gray area  22   b  is designed such that a difference between the image data obtained by reading the gray area  22   b  and the reference whiteness level data obtained by reading the white reference plate  15  exceeds a predetermined level. 
     The whiteness level determining unit  28  determines the data read from the white area  22   a  or the gray area  22   b  of the backing part  22  to be the whiteness level. The image data obtained by reading the white area  22   a  or the gray area  22   b  of the backing part  22  and determined to be the whiteness level is stored as the whiteness level data and used as the whiteness level data for the next line. 
     When the gray area  22   b  of the backing part  22  is read in the line art mode, the whiteness level generating unit  27  employs the color of gray obtained by reading the gray area  22   b  of the backing part  22  as the reference whiteness level. This whiteness level is made to follow the image data read thereafter. 
     Since the ground color of the original  12  is not exceeded in level by any color data, the white-level following operation is conducted such that, if the read image data is greater, that is, brighter than the reference whiteness level, it can be determined that the current image data indicates a color similar to the ground color of the original  12 . In this case, irrespective of the difference between the image data and the reference whiteness data, the read image data is determined to be the whiteness level data. 
     In case the read image data is slightly lower in level than the reference whiteness level, it is determined that the associated area of the original is temporarily darkened due to warp etc. of the original  12 . If, in this case, the whiteness data is made to follow the image data, the whiteness level becomes unstable. For the whiteness level to become more stable, the whiteness level is made to follow the image data when the difference between the image data and the reference whiteness level exceeds a predetermined level. 
     FIG. 8 explains an operation of the first embodiment of the present invention. In the figure, time t is assigned for the x-axis, and the image data is assigned for the y-axis. A time interval t 0 -t 1  indicates a period of time during which the image detecting unit  13  is reading the backing part  22 , and a time interval beyond t 1  indicates a period of time during which the image detecting unit  13  is reading the original  12 . 
     L 1  indicates image data obtained by reading the gray area  22   b  of the backing part  22 ; L 2  reference whiteness level data obtained by reading the white reference plate  15 ; and L 3  image data obtained by reading the ground color of the original  12 . 
     When the backing part  22  is read at the time t 0 , the reference whiteness level is made to follow the image data L 1  obtained by reading the gray area  22   b  of the backing part  22  because there is a difference greater than a predetermined value L 0  between the reference whiteness level data L 2  obtained by reading the white reference plate  15  and the image data obtained by reading the gray area  22   b  of the backing part  22 . 
     When the reading of the original  12  is started at the time t 1 , the reference whiteness level data L 1  is made to follow the image data L 3  indicating the ground color of the original  12 , irrespective of the aforementioned difference, because the reference whiteness level data L 1  obtained by reading the gray area  22   b  of the backing part  22  is lower in level than the image data L 3  indicating the ground color of the original  12 . 
     In this way, the reference whiteness level is made to follow the level of the ground color of the original  12 . 
     As has been described, by reading the gray area  22   b  of the backing part  22 , the reference whiteness level can be smaller than the level of the data indicating the ground color of the original  12 . It is then possible to make the reference whiteness level follow the ground color of the original  12 , irrespective of the difference between the ground color of the original  12  and the reference whiteness level. Thus, it is possible to read an image using the ground color of the original  12  as the reference whiteness level. As a result, proper contrast can be obtained in a line drawing etc. in which an image is represented against the background of the ground color of the original  12 . 
     The first embodiment will be summarized in the following. When an image reading is started at the backing part  22  in order to ensure that the leading edge of the original  12  is read, the reference whiteness level can be made to follow the ground color of the original  12  as described above, by designing the backing part  22  to have a gray area whose brightness level is lower than the ground color of the original  12 . Accordingly, the ground color of the original  12  can be used as the reference whiteness level, thus ensuring proper contrast is achieved in reading the image, and the quality of the read image is improved. 
     FIGS. 9A and 9B show a construction of a variation of first embodiment. A backing part  32  of this variation is constituted such that each of white areas  32   a  and gray areas  32   b  is formed to be relatively wide so that a plurality of reference whiteness level reading operations corresponding to a plurality of lines (white areas {circle around ( 1 )}-{circle around ( 3 )}, gray areas {circle around ( 4 )}-{circle around ( 6 )}) is possible. 
     Accordingly, the operation of the controller  20  and the whiteness level controller  29  is modified a little. 
     The whiteness level controller  29  is embodied by a CPU etc. The ground color of a photograph is usually white; that is, a photographic image is constituted with white as the reference color. Hence, an image true to a photograph can be obtained by fixing the whiteness level at the reference whiteness level. In the case of a line drawing etc., the image is represented by a contrast against the ground color. This contrast is properly reproduced by using the ground color of the line drawing etc., as the whiteness level. The whiteness level controller  29  generates the follow whiteness level data in accordance with the whiteness level follow signal from the whiteness level determining unit  28 , when a line drawing or the like is read. The generated data is stored in the whiteness level memory  30 . The stored data is used as the whiteness level data for the next line. The whiteness level controller  29  intensifies the level of the whiteness level data by a predetermined number of steps when the whiteness level follow signal is at a high level. The whiteness level data is made to follow so as to suppress the video signal below the whiteness level. 
     The whiteness level memory  30  is embodied by a RAM or the like and stores the whiteness level data for each pixel in a line. 
     FIG. 10 explains an operation of the variation. When an instruction to read an image is issued, the controller  20  controls the carriage driving mechanism  16  so that the image detecting unit  13  is moved to a position opposite the white reference plate  15  to read the white reference plate  15  (S 2 - 1 ). The data read from the white reference plate  15  is supplied to the whiteness level controller  29  via the A/D converter  26 . The whiteness level controller  29  uses this data as the reference whiteness data and stores the same in the whiteness level memory  30  (S 2 - 2 ). 
     The controller  20  then determines whether or not a line drawing is to be read (S 2 - 3 ). Selection of a line drawing is made through the operation of the operation part  21  by a user. 
     If it is determined in S 2 - 3  that a line drawing is to be read, the controller  20  controls the carriage driving mechanism  16  so as to move the image detecting unit  13  to a position indicated by  4  in FIGS. 9A and 9B which position is opposite the gray area  32 B of the backing part  32  (S 2 - 4 ). When it is determined in S 2 - 3  that an image other than a line drawing is read, the controller  20  controls the carriage driving mechanism  16  so as to move the image detecting unit  13  to a positon {circle around ( 1 )} in FIGS. 9A and 9B which position is opposite the white area  32   a  if the backing part  32  (S 2 - 5 ). 
     The controller  20  then controls the image detecting unit  13  to read the white area  32   a  or the gray area  32   b  of the backing part  32  (S 2 - 6 ). The read data is supplied to the whiteness level controller  29  and stored in the whiteness level memory  30  as the reference whiteness level data. 
     The whiteness level controller  29  is then controlled so as to check the read reference whiteness level data (S 2 - 7 ). Assuming that the image data for two adjacent pixels is V n  and V n+1 , respectively, (or assuming that the image data for each pixel is V n+1  when the basic data is set to be V n ), or assuming that the data for a predetermined pixel is V n  and the data for another pixel is V n+1 , the whiteness level controller  29  determines whether a ratio (V n+1 )/V n  between the V n  and V n+1  exceeds 0.98 (S 2 - 8 ). If it is determined in S 2 - 8  that (V n+1 )/V n &lt;0.98, it means that there is a stain or a flaw. The carriage  16  is then controlled so as to move the image detecting unit  13  by several lines so that the image detecting unit  13  can read the position {circle around ( 2 )} or the position {circle around ( 5 )} indicated in FIGS. 9A and 9B, instead of the position {circle around ( 1 )} or the position {circle around ( 4 )} (S 2 - 9 ). The reading is then started over. 
     When it is determined in S 2 - 8  that (V n+1 )/V n ≧0.98, it means that position of the backing part  32  which is read is in a normal condition. A normal reading operation of the original  21  is then conducted (S 2 - 10 ). In the above description, a determination about a stain or a flaw is made on the basis of the image data for two adjacent pixels, the determination may also be made on the basis of the image data for pixels not adjacent to each other. 
     In the above described variation of the first embodiment, the position of the image detecting unit is shifted when there is a flaw or a stain in the backing part  32  so that the unit is moved to a position opposite the backing part portion that is free from a flaw or stain. Hence, the reading operation is not affected by a flaw or a stain. While a determination of a presence of a flaw or a stain is given when the condition V n+1 /V n ≧0.98 holds, other numerical values may be used as long as the read image is not unfavorably affected. For example, the determination may be given when the condition V n+1 /V n ≧1.02 holds. It is to be noted that the condition V n+1 /V n ≧0.98 and the condition V n+1 /V n ≧1.02 may be used jointly to give the determination because the image data for a pixel may be seriously affected by a flaw etc. and exhibits a largely different value from the data for the neighboring pixel. 
     FIG. 11 shows a schematic construction of a second embodiment of the present invention. FIG. 12 is a block diagram of an essential part of the second embodiment. In the figures, those components that are the same as the components of FIGS. 4 and 6 are designated by the same reference numerals, and the description thereof is omitted. 
     In a conventional image reading apparatus, the image quality, such as contrast, obtained in the ADF mode and that obtained in the FB mode may differ. This difference is due to difference in light reflectance between an original processed in the ADF mode and an original processed in the FB mode. In the ADF mode, the original is transported so that it is impossible for the original to be firmly in contact  35  with a reading window, whereas, in the FB mode, the original is fixed by being firmly in contact with the reading window. In the second embodiment, this difference is overcome by referring to different whiteness levels in the FB mode and in the ADF mode so that the image can be read with the same quality. The second embodiment differs from the first embodiment in that an image processing unit  41  is used instead of the image processing unit  14 . The image processing unit  41  of the second embodiment includes the A/D converter  26  and a whiteness level generating unit  42 . The whiteness level generating unit has a correcting circuit  43  for correcting digital data supplied from the A/D converter  26  to the whiteness level controller  29 . 
     FIG. 13 shows a schematic construction of the correcting circuit  43 . The correcting circuit  43  includes a ROM  44  which stores corrected whiteness level data, and an adder  45  for adding the corrected whiteness level data from the ROM  44  to the whiteness level data from the A/D converter  26 . 
     The ROM  44  is connected to the operation part  21  and outputs the corrected whiteness level data of different levels in the ADF mode and in the FB mode, the difference being in accordance with a reading mode switching operation at the operation part  21 . 
     Accordingly, in the ADF mode, the ROM  44  outputs the corrected whiteness level data of a smaller level than in the FB mode. 
     The corrected whiteness level data output from the ROM  44  is supplied to the adder  45 . The data obtained by the reading of the white reference plate  15  by the image detecting unit  13  is supplied to the AD converter  26 . The reference whiteness level data output from the AD converter  26  is then supplied to the adder  45 . 
     The adder  45  adds the corrected whiteness level data from the ROM  44  to the reference whiteness level data, and supplies a sum to the whiteness level controller  29 . The whiteness level data output from the adder  45  is corrected to become the whiteness level data adapted to the reading condition. By setting the whiteness level according to the data output from the adder  45 , it is possible to set the whiteness level in the ADF mode lower than that in the FB mode. Consequently, substantially the same image quality can be obtained irrespective of the reading mode. 
     FIG. 14 shows a construction of a first variation of the correcting circuit of the second embodiment. In FIG. 14, those components that are the same as the components of FIG. 13 are designated by the same reference numerals, and the description thereof is omitted. 
     The first variation is constructed such that the whiteness level data is corrected according to the thickness of the original  12 . For example, this variation includes: a transparent-type photosensor  51  which, provided in a path through which the original  12  is transported, detects the thickness of the original  12 ; an amplifier  52  for amplifying an output signal of the photosensor  51 ; an A/D converter  53  for converting the output of the amplifier  52  into digital data; and a ROM  54  which, supplied with the digital data output from the A/D converter  53  as the address, supplies the corrected whiteness level data corresponding to the address to the adder  45 . 
     Since the transparency of one original differs from that of another, the digital data commensurate with the thickness of the original  12  is obtained by detecting the transparency of the original  12  by the transparent-type photosensor  51 , amplifying the detected signal and converting the same into digital data. By supplying the digital data commensurate with the thickness to the ROM  54  as the address, the corrected whiteness level data commensurate with the thickness can be retrieved from the ROM  54  and used in correcting the whiteness level. This variation is employed in the ADF reading mode. Since, in the ADF mode, the amount by which the original  12  is detached from the reading window varies depending on the thickness of the original  12 , the whiteness level correction in accordance with this amount is conducted by detecting the thickness of the original  12 . 
     FIG. 15 shows a schematic construction of a second variation of the correcting circuit of the second embodiment. The correcting circuit of the second variation is constructed such that reference whiteness level data is obtained by reading a blank sheet of the same kind as that used for the original  12 . 
     The correcting circuit of the second variation includes: a one-line average value circuit  61  for calculating an average value of the image data for one line on the basis of the image data obtained by reading the blank sheet; and a one-page average value circuit  62  for calculating an average value of the image data for one page on the basis of the average value for lines calculated by the one-line average value circuit  61 . 
     FIGS. 16 and 17 explains an operation of the second variation. Referring to FIG. 17,  63  indicates a blank sheet of the same kind as used for the original  12  to be read. Referring to FIG. 16, assuming that, pixels ( 1 ,  1 )-(N, N) (column, row) totaling a number N times N in the sheet  63  are read (S 3 - 1 ), the image data for the pixels ( 1 ,  1 )-(N, N) is supplied to the one-line average value circuit  61 , whereupon the image data values for the pixels are totaled, and a sum A 1  of the image data values for a first line is obtained. An average value M 1  for the first line is calculated by dividing A 1  by the number of pixels N in one line, M 1  being supplied to the one-page average value circuit  62  (S 3 - 2 ). 
     The one-line average circuit  61  obtains average values A 1 -AN for the respective lines. The values A 1 -AN are successively supplied to the one-page average circuit  62 . The one-page average value circuit  62  is supplied with the average values A 1 -AN for the respective lines by the one-line average value circuit  61 , and calculates a sum ANN of the average values A 1 -AN. ANN is divided by the number of lines N contained in one page so as to obtain an average value MN for one page (S 3 - 3 ). 
     The average value MN for one page obtained by the one-page average value circuit  62  is supplied to the whiteness level controller  29  as the reference whiteness level (S 3 - 4 ). 
     According to this variation, it is possible to obtain the whiteness level specific to the type of sheet on which the image to be read is provided. Accordingly, a whiteness level suitable for each original can be obtained so that a high-quality image is obtained. As in the above described variation, by adding the correcting circuit  43  in the whiteness level generating circuit  27  of the first embodiment, additional effects can be achieved on top of the effects derived from the first embodiment. Thus, the whiteness level control can be performed more accurately, and images with higher quality can be obtained. 
     FIG. 18 shows a schematic construction of a third embodiment of the present invention. FIG. 19 shows a construction of an image processing unit according to the third embodiment. In the figures, those components that are the same as the components of FIGS. 11 and 12 are designated by the same reference numerals, and the description thereof is omitted. 
     In the third embodiment, not only whiteness level correction but blackness level correction is performed. The third embodiment differs from the second embodiment in that the construction of the image processing unit is modified, and a black reference plate  72  is provided. An image processing  71  of the third embodiment includes: the A/D converter  26 ; the whiteness level generating unit  42 ; and a blackness level generating unit  73 . 
     The blackness level generating unit  73  has substantially the same construction as the whiteness level generating unit  42 . The blackness level generating unit  73  includes: a blackness level determining unit  74  for comparing the video signal read by the image detecting unit  13  and supplied via the terminal T 1  with the blackness level signal generated in the blackness level generating unit  73  so as to determine whether or not the video signal corresponds to the blackness level; a blackness level controller  75  for generating reference blackness level data in accordance with the digital data output from the blackness level determining unit  74  and the A/D converter  26 , and for causing the whiteness level to vary in correspondence with the comparison result supplied by the blackness level determining unit  74 , thereby effecting a following operation; a blackness level memory  76  for storing the blackness level data generated by the blackness level controller  75 ; a whiteness level D/A converter  77  for converting the blackness level data generated by the blackness level controller  75  into an analog signal, generating an output whiteness level signal, and supplying the output whiteness level signal to the A/D converter  26  and the blackness level determining unit  74 ; and a correcting circuit  78  for correcting the digital data supplied from the A/D converter  26  to the blackness level controller  75 . 
     The blackness level determining unit  74  is embodied, for example, by a converter, and compares the video signal supplied via the terminal T 1  with the output blackness signal. When it is determined that the video signal is smaller in level than the output blackness level signal, the blackness level determining unit  74  outputs a high-level blackness level follow signal and supplies the same to the blackness level controller  75 . When it is determined that the video signal is greater in level than the output blackness level signal, the blackness level determining unit  74  outputs a low-level blackness level follow signal and supplies the same to the blackness level controller  75 . 
     The blackness level controller  75  is embodied by a CPU etc. The blackness level controller  75  fixes the blackness level when the image such as a photograph is read. When a line drawing or the like is read, the blackness level controller  75  generates the follow blackness level data in accordance with the blackness level follow signal from the blackness level determining unit  74 . The generated data is stored in the blackness level memory  76 . The stored data is used as the blackness level data for the next line. The blackness level controller  75  lowers the level of the blackness level data by a predetermined number of steps when the blackness level follow signal is at a high level. The blackness level data is made to follow so as to ensure that the video signal is above the blackness level. 
     The blackness level memory  76  is embodied by a RAM or the like and stores the blackness level data for each pixel in a line. 
     FIG. 20 shows a construction of the correcting circuit  78 . The correcting circuit  78  includes: a ROM  79  for storing corrected data; and an adder  80  for adding the corrected data from the ROM  79  to the blackness level data from the A/D converter  26 . 
     The ROM  79  is connected to the operation part  21  and outputs the corrected data having different levels in the ADF mode and in the FB mode, the difference being in accordance with a reading mode switching operation at the operation part  21 . For example, the corrected data of a smaller level is output from the ROM  44  in the ADF mode than in the FB mode. 
     The corrected data output from the ROM  79  is supplied to the adder  80 . The data obtained by the reading of the black reference plate  72  by the image detecting unit  13  is supplied to the AD converter  26 . The reference whiteness level data output from the AD converter  26  is then supplied to the adder  80 . 
     The adder  80  adds the corrected whiteness level data from the ROM  79  to the reference blackness level data, and supplies a sum to the blackness level controller  75 . The blackness level data output from the adder  80  is corrected to become the blackness level data adapted to the reading condition. By setting the blackness level according to the data output from the adder  80 , substantially the same image quality can be obtained irrespective of the reading mode such as the ADF mode or the FB mode. 
     The present invention is not limited to the above described embodiments, and variations and modifications may be made without departing from the scope of the present invention.