Abstract:
A document deciding apparatus includes: a light source; a light-quantity detection unit detecting a reference light quantity based on irradiation of the light source, and respective light quantities of plural colors based on reflection from or transmission through a document; a decision unit deciding whether the document is colorless or colored, in accordance with a predetermined criterion on the basis of the light quantities of plural colors detected by the light-quantity detection unit; and a control unit performing a control so as to alter the criterion of the decision unit or a value corresponding to the light quantities detected by the light-quantity detection unit, on the basis of a change of the reference light quantity detected by the light-quantity detection unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from the prior Japanese Patent Application No. 2005-333835, filed on Nov. 18, 2005; the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to, for example, a document deciding apparatus, a document reading apparatus, an image forming apparatus and a document deciding method which decide whether a document is colorless or colored. 
     BACKGROUND 
     An image processing apparatus identifies whether a document is a color one or a black-and-white one, in such a way that a light of a lamp is projected onto the document and that reflected light from the document is read by a color CCD. 
     SUMMARY 
     According to an aspect of the invention, a document deciding apparatus includes: a light source; a light-quantity detection unit detecting a reference light quantity based on irradiation of the light source, and respective light quantities of plural colors based on reflection from or transmission through a document; a decision unit deciding whether the document is colorless or colored, in accordance with a predetermined criterion on the basis of the light quantities of plural colors detected by the light-quantity detection unit; and a control unit performing a control so as to alter the criterion of the decision unit or a value corresponding to the light quantities detected by the light-quantity detection unit, on the basis of a change of the reference light quantity detected by the light-quantity detection unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which: 
         FIG. 1  is a side view showing an outline of an image forming apparatus according to an embodiment; 
         FIG. 2  is a side view showing details of a document reading apparatus according to the embodiment; 
         FIG. 3  is a graph showing light quantity characteristics of a light source; 
         FIG. 4  is a block diagram showing outlines of a processing unit which processes RGB electric signals outputted by a photoelectric transducer, and the surroundings thereof; 
         FIGS. 5A and 5B  are graphs exemplifying auto gain adjustments which an analog correction portion performs, wherein  FIG. 5A  shows a gain before the light quantity degradation or the like of a light source occurs, while  FIG. 5B  shows a gain after the light quantity degradation or the like of the light source has occurred; 
         FIG. 6  is a block diagram showing a configuration of an ACS portion; 
         FIG. 7  is a graph showing pixel decision threshold values which are used in cases where a pixel decision portion decides whether a pixel is colorless or colored; 
         FIGS. 8A and 8B  show decided results of individual pixels as are inputted from the pixel decision portion to a block decision portion, with respect to an identical block in an identical document in a case where the analog correction portion changes a gain value, wherein  FIG. 8A  shows the decided results before an occurrence of the light quantity degradation or the like of the light source, while  FIG. 8B  shows decided results after the occurrence of the light quantity degradation or the like of the light source; 
         FIGS. 9A and 9B  are model diagrams exemplifying number-of-colored-pixels decision threshold values which are used in cases where a block decision portion decides whether each block is colorless or colored, wherein  FIG. 9A  shows the number-of-colored-pixels decision threshold value before the occurrence of the light quantity degradation or the like of the light source, while  FIG. 9B  shows the number-of-colored-pixels decision threshold value after the occurrence of the light quantity degradation or the like of the light source; 
         FIGS. 10A and 10B  show decided results of individual blocks as are inputted from the block decision portion to a document decision portion, with respect to the identical document in the case where the analog correction portion changes the gain value, wherein  FIG. 10A  shows the decided results before the occurrence of the light quantity degradation or the like of the light source, while  FIG. 10B  shows the decided results after the occurrence of the light quantity degradation or the like of the light source; 
         FIGS. 11A and 11B  are model diagrams exemplifying document decision threshold values which are used in cases where the document decision portion decides whether the document is colorless or colored, wherein  FIG. 11A  shows the document decision threshold value before the occurrence of the light quantity degradation or the like of the light source, while  FIG. 11B  shows the document decision threshold value after the occurrence of the light quantity degradation or the like of the light source; 
         FIG. 12  is a document top view showing an ACS decision region relative to the document, in the case of varying the region (ACS decision region) in which pixels to be decided by the ACS portion lie; 
         FIG. 13  is a flow chart showing a first example (S 10 ) of an operation in which the document reading apparatus decides whether the document is colorless or colored; 
         FIG. 14  is a flow chart showing a second example (S 20 ) of the operation in which the document reading apparatus decides whether the document is colorless or colored; 
         FIG. 15  is a flow chart showing a third example (S 30 ) of the operation in which the document reading apparatus decides whether the document is colorless or colored; 
         FIG. 16  is a table showing a result obtained in such a way that the number of pixels to be decided as colored ones due to noise is decreased using a filter coefficient “0”; and 
         FIG. 17  is an ACS-decision-parameter setting region diagram showing a fourth example of the operation in which the document reading apparatus decides whether the document is colorless or colored. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described in conjunction with the drawings. 
     Shown in  FIG. 1  is an outline of an image forming apparatus  10 . The image forming apparatus  10  is for color use by example, and it includes an image formation section  12  and a document reading apparatus  14 . The image formation section  12  is of, for example, xerography scheme, it includes a paper feed tray  16  in which sheets are stacked, and it forms an image on the sheet which has been fed from the paper feed tray  16  to a sheet conveyance path  20 . 
     More specifically, the image formation section  12  includes a photosensitive drum  22 , a charger  24  which uniformly charges the photosensitive drum  22 , an exposure device  26  which forms a latent image on the photosensitive drum  22  uniformly charged by the charger  24 , a development device  28  which visualizes with toners, the latent image on the photosensitive drum  22  as has been formed by the exposure device  26 , a primary transfer device  32  which transfers the toner images formed by the development device  28 , onto an intermediate transfer belt  30 , and a photosensitive-drum cleaner  34  which removes the toners remaining on the photosensitive drum  22 . The exposure device  26  is of, for example, laser scan scheme, and it changes the image of a document read by the photoelectric transducer  86  of the document reading apparatus  14  as will be stated later, into the ON/OFF signals of a laser and outputs these ON/OFF signals. The development device  28  is of, for example, rotary type, in which development units  36   a ,  36   b ,  36   c  and  36   d  of four colors Y (yellow), M (magenta), C (cyan) and K (black) are circumferentially arranged, and in which they are respectively rotated so as to confront the photosensitive drum  22  when the images of the corresponding colors are developed. The primary transfer device  32  is constructed of, for example, a corotron, and the toner images of the four colors are transferred onto the intermediate transfer belt  30  by the transfer device  32 . The toner images transferred on the intermediate transfer belt  30  is transferred onto the sheet by a secondary transfer device  38 . The sheet bearing the toner images is delivered to a fixation device  40 , and the toner images are fixed onto the sheet by the fixation device  40 . The sheet on which the toner images have been fixed is ejected onto an ejection tray  42 . The toners remaining on the intermediate transfer belt  30  is scraped off by an intermediate-transfer-belt cleaner  44 . 
     Besides, when an operation mode (ACS mode: Auto Color Selection mode) in which the image forming apparatus  10  decides whether the document is colorless or colored is selected through a user interface not shown, the image formation section  12  forms, for example, a black-and-white image or the color image on the sheet in accordance with a result decided by the document reading apparatus  14  as to whether the document is colorless or colored. 
     Registration rolls  46  are arranged along the sheet conveyance path  20 . The registration rolls  46  are controlled so as to temporarily stop the fed sheet, and to feed the sheet to the secondary transfer device  38  in synchronism with a timing at which the toner images are formed on the intermediate transfer belt  30 . 
     The document reading apparatus  14  includes an automatic document feed device  48 , and an optical system  50  which optically reads the document. This document reading apparatus  14  has the function of roughly reading the document fed by the automatic document feed device  48 , and the function of reading the document placed on a platen glass plate  52 . 
     Shown in  FIG. 2  is the details of the document reading apparatus  14 . 
     The automatic document feed device  48  includes a platen  54  on which a large number of documents are placed, a document conveyance path  56  along which the document is conveyed, and an ejection bed  58  onto which the document having had its image read is ejected. The document conveyance path  56  includes a main conveyance portion  60  and a reversal portion  62 . The main conveyance portion  60  is formed in the shape of letter U, and it is provided with a pickup roll  64 , feed rolls  66 , preregistration rolls  68 , registration rolls  70 , out-rolls  72  and ejection rolls  74  which constitute a conveyance device. In feeding the document, the pickup roll  64  descends and picks up the documents placed on the platen  54 . The feed rolls  66  separate the documents delivered from the pickup roll  64 , and feed only the uppermost document. The preregistration rolls  68  temporarily stop the document delivered from the feed rolls  66 , and form a loop so as to correct oblique traveling. The registration rolls  70  temporarily stop the document delivered from the preregistration rolls  68 , so as to establish a read timing. In case of ejecting the document, the ejection rolls  74  are rotated forward, and the document is ejected onto the ejection bed  58  through the out-rolls  72  as well as the ejection rolls  74 . 
     The reversal portion  62  has one end connected to the main conveyance portion  60  between the out-rolls  72  and the ejection rolls  74 , while it has the other end connected to the main conveyance portion  60  by the preregistration rolls  68 . A reversing gate  76  is disposed near one end of the reversal portion  62 . In case of reversing the document, the ejection rolls  74  are rotated reversely at a stage at which the rear end of the document has arrived near these ejection rolls  74 , and the reversing gate  76  is located down, thereby to guide the document to the reversal portion  62 . The reversing gate  76  may be opened by the thrust of the document and closed by its own weight, or an actuator for opening and closing the reversing gate  76  may well be disposed. 
     A conveyance document reading position  78  is set between the registration rolls  70  and the out-rolls  72 , and the optical system  50  reads the document fed by the automatic document feed device  48 , at the conveyance document reading position  78 . 
     The optical system  50  includes a full-rate carriage  80 , a half-rate carriage  82 , a lens  84  and the photoelectric transducer  86 . The full-rate carriage  80  includes a light source  88  and a first mirror  90 . With a scan direction being the subscan direction of the document (rightward from the left as viewed in  FIG. 2 ), the full-rate carriage  80  moves over a full stroke in the scan direction within the document reading apparatus  14 . 
     The light source  88  is a lamp of, for example, halogen or xenon as extends in the main scan direction of the document. As shown in  FIG. 3 , the light source  88  has the characteristics that the light quantity of this light source is smaller at each end part thereof than at the middle part thereof in the main scan direction, and that the light quantity decreases entirely due to secular degradation. The half-rate carriage  82  includes a second mirror  92  and a third mirror  94 , and it moves over a half stroke in the subscan direction within the document reading apparatus  14 . 
     The light source  88  projects light onto the document which is placed on the platen glass plate  52  disposed over the moving ranges of the full-rate carriage  80  and the half-rate carriage  82 , or the document which passes through the conveyance document reading position  78 . In this regard, the lens  84  accepts the reflected light of the projected light through the first mirror  90 , second mirror  92  and third mirror  94  so as to focus the reflected light. 
     The photoelectric transducer  86  is, for example, a 3-line color CCD which receives the reflected light at the focused position of this reflected light as based on the lens  84 . The 3-line color CCD is configured of photodiodes which are respectively provided with filters of, for example, R (red), G (green) and B (blue) (primary color filters), and from which analog electric signals corresponding to the light quantities of the respective colors R, G and B in pixel units are outputted to a processing unit  100  to be explained later. 
     Besides, a reference white plate  98  which reflects the light projected by the light source  88  is disposed in the vicinity of the conveyance document reading position  78 . The reflective surface of the reference white plate  98  for reflecting the light is tinged with a reference white color. The photoelectric transducer  86  can receive the reflected light of the light which the light source  88  has projected onto the reference white plate  98  (reference light quantity light) through the first mirror  90 , second mirror  92 , third mirror  94  and lens  84 . 
     In a case where the document reading apparatus  14  reads the document placed on the platen glass plate  52 , the full-rate carriage  80  and the half-rate carriage  82  are respectively moved in the scan direction while the light source  88  is projecting the light toward the document, whereby the photoelectric transducer  86  successively receives the rays of the reflected light corresponding to the whole surface of the document. 
     Besides, the photoelectric transducer  86  receives the reflected light of the light which the light source  88  has projected onto the reference white plate  98  at a predetermined timing. 
       FIG. 4  is a block diagram showing the outlines of the processing unit  100  which processes the RGB electric signals outputted by the photoelectric transducer  86 , and the surroundings thereof. 
     As shown in  FIG. 4 , the processing unit  100  includes an analog correction portion  102 , a shading correction portion  104 , an image processing portion  106  and an ACS (Auto Color Selection) portion  108 , and it is controlled by an image read control unit  110  which includes a CPU. 
     The analog correction portion  102  includes an auto gain control portion (AGC portion), and it performs processes such as auto gain adjustments and auto offset adjustments, for the respective analog electric signals inputted from the photoelectric transducer  86 . Further, the analog correction portion  102  subjects the resulting analog signals to A/D conversions so as to output adjusted digital RGB signals to the shading correction portion  104 . Besides, the analog correction portion  102  makes the gain adjustments in accordance with the light quantities of the reflected light based on the reference white plate  98 , immediately after the turn-ON of a power source or after a prescribed time period since the turn-ON of the power source. The gain adjustments are made for the individual colors of the electric signals so that the signal levels of the respective analog electric signals inputted from the photoelectric transducer  86  which has received the reflected light based on the reference white plate  98  may become predetermined values. The adjusted gain values are outputted to the image read control unit  110 . 
       FIGS. 5A and 5B  are graphs exemplifying the auto gain adjustments which the analog correction portion  102  performs, wherein  FIG. 5A  shows a gain before the light quantity degradation or the like of the light source  88  occurs, while  FIG. 5B  shows a gain after the light quantity degradation or the like of the light source  88  has occurred. 
     As seen from  FIGS. 5A and 5B , the analog correction portion  102  is set so that each signal level outputted by the photoelectric transducer  86  may be amplified, for example, double before the light quantity degradation or the like of the light source  88  occurs. Besides, the analog correction portion  102  amplifies each signal level outputted by the photoelectric transducer  86  to, for example, the quadruple so as to be brought to a predetermined AGC target value, after the light quantity degradation or the like of the light source  88  has occurred. Here, even when the signal level outputted by the photoelectric transducer  86  has lowered due to the occurrence of the light quantity degradation or the like of the light source  88 , substantially the same noise component as before the occurrence of the light quantity degradation or the like of the light source  88  is contained in the signal outputted by the photoelectric transducer  86 , and hence, the analog correction portion  102  amplifies also the noise component of the RGB signal to the quadruple. 
     The shading correction portion  104  performs shading corrections for the RGB signals inputted from the analog correction portion  102 , and outputs the resulting signals to the image processing portion  106 . The image processing portion  106  performs a process in which the RGB signals inputted from the shading correction portion  104  are converted into image data of, for example, L*a*b* color representation system, an arithmetic process to be explained later, etc., and it outputs the resulting signals to the ACS portion  108 . 
     The ACS portion  108  accepts the image data of the L*a*b* color representation system from the image processing portion  106  and analyzes the image data by a process as explained later, thereby to decide whether the document is colorless or colored. The result of the decision is outputted to the image read control unit  110 . 
     Besides, the image read control unit  110  controls a sensor  112 , an automatic-document-feed-device control unit  114 , a light-source control unit  116 , a scan control unit  118 , a ROM  120 , a RAM  122  and an NVM (Non-Volatile Memory)  124 . 
     The sensor  112  is configured of a group of sensors which include, for example, a sensor for sensing the existence or nonexistence of the document. The automatic-document-feed-device control unit  114  controls the automatic document feed device  48 . The light-source control unit  116  controls the light source  88 . The scan control unit  118  controls a motor  126 , thereby to control the movements of the full-rate carriage  80  and the half-rate carriage  82  in the subscan direction of the document (the scan direction of the carriages). The ROM  120  stores therein programs for operating the CPU which is included in the image read control unit  110 . The RAM  122  stores therein the gain values accepted from the analog correction portion  102  by the image read control unit  110 , predetermined set values, image data, etc. The NVM  124  stores therein, for example, the initial values of the gain values of the analog correction portion  102 , gain threshold values “0”-“3” to be explained later, gain threshold value ratios “0” and “1”, ASC decision parameters “0”-“4” and filter coefficients “0”-“2”. 
     Next, the ACS portion  108  will be detailed. 
     Shown in  FIG. 6  is the configuration of the ACS portion  108 . As shown in  FIG. 6 , the ACS portion  108  is configured of a pixel decision portion  130 , a block decision portion  132  and a document decision portion  134 . 
     The pixel decision portion  130  accepts the image data of the L*a*b* color representation system from the image processing portion  106  every pixel, it decides whether the pixel is colorless or colored, in accordance with the pixel decision threshold value inputted from the image read control unit  110 , and it outputs the result of the decision to the block decision portion  132  as a decision flag every pixel. 
       FIG. 7  is a graph showing pixel decision threshold values which are used in the cases where the pixel decision portion  130  decides whether the pixel is colorless or colored. 
     The pixel decision portion  130  decides each pixel as being colorless, in a case where the chromaticity of the pixel lies within that region (black-and-white decision region) of an a*b* plane which is enclosed with the pixel decision threshold value, and it decides each pixel as being colored, in a case where the chromaticity lies outside the region which is enclosed with the pixel decision threshold value. Besides, when the gain value which the analog correction portion  102  changes becomes large in the case of the occurrence of the light quantity degradation or the like in the light source  88 , or the like, the pixel decision portion  130  acquires from the image read control unit  110 , the pixel decision threshold value which spreads the black-and-white decision region within the a*b* plane. Thus, unless a value indicating a chromaticity is larger than before the occurrence of the light quantity degradation or the like in the light source  88 , the pixel is prevented from being decided as a colored one. 
     The block decision portion  132  ( FIG. 6 ) accepts the decision flag from the pixel decision portion  130  every pixel, it counts the number of colored pixels which are contained in each of N×M blocks (where letters N and M denote integral numbers of pixels as predetermined) for dividing the document (or an ACS decision region to be explained later), and it decides whether the block is colorless or colored, in accordance with a number-of-colored-pixels decision threshold value inputted from the image read control unit  110 . The result of the decision is outputted to the document decision portion  134  as a decision flag every block. 
       FIGS. 8A and 8B  show the decided results of individual pixels as are inputted from the pixel decision portion  130  to the block decision portion  132 , with respect to the identical block in the identical document in the case where the analog correction portion  102  changes the gain value. Here,  FIG. 8A  shows the decided results before the occurrence of the light quantity degradation or the like of the light source  88 , while  FIG. 8B  shows the decided results after the occurrence of the light quantity degradation or the like of the light source  88 . 
     As shown in  FIGS. 8A and 8B , when the analog correction portion  102  enlarges the gain value on account of the light quantity degradation or the like of the light source  88 , the decided results in which the number of the colored pixels increases are inputted from the pixel decision portion  130  to the block decision portion  132  with respect to the identical block in the identical document. 
       FIGS. 9A and 9B  are model diagrams exemplifying the number-of-colored-pixels decision threshold values which are used in cases where the block decision portion  132  decides whether each block is colorless or colored. Here,  FIG. 9A  shows the number-of-colored-pixels decision threshold value before the occurrence of the light quantity degradation or the like of the light source  88 , while  FIG. 9B  shows the number-of-colored-pixels decision threshold value after the occurrence of the light quantity degradation or the like of the light source  88 . Regarding the block of, for example, 12 pixels×12 pixels, the block decision portion  132  acquires from the image read control unit  110 , the number-of-colored-pixels decision threshold value which is enlarged (from 48 pixels to 72 pixels by way of example) in accordance with the gain value that the analog correction portion  102  changes in the case of the occurrence of the light quantity degradation or the like in the light source  88 , or the like. Thus, unless the number of the colored pixels within the block is larger than before the occurrence of the light quantity degradation or the like in the light source  88 , the block is prevented from being decided as a colored one. 
     The document decision portion  134  ( FIG. 6 ) accepts the decision flag from the block decision portion  132  every pixel, it counts the number of colored blocks which are contained in the document (or the ACS decision region to be explained later), and it decides whether the document is colorless or colored, in accordance with a document decision threshold value inputted from the image read control unit  110 . The result of the decision is outputted to the image read control unit  110 . 
       FIGS. 10A and 10B  show the decided results of individual blocks as are inputted from the block decision portion  132  to the document decision portion  134 , with respect to the identical document in the case where the analog correction portion  102  changes the gain value. Here,  FIG. 10A  shows the decided results before the occurrence of the light quantity degradation or the like of the light source  88 , while  FIG. 10B  shows the decided results after the occurrence of the light quantity degradation or the like of the light source  88 . 
     As shown in  FIGS. 10A and 10B , when the analog correction portion  102  enlarges the gain value on account of the light quantity degradation or the like of the light source  88 , the decided results in which the number of the colored blocks increases are inputted from the block decision portion  132  to the document decision portion  134  with respect to the identical document. 
       FIGS. 11A and 11B  are model diagrams exemplifying the document decision threshold values which are used in cases where the document decision portion  134  decides whether the document is colorless or colored. Here,  FIG. 11A  shows the document decision threshold value before the occurrence of the light quantity degradation or the like of the light source  88 , while  FIG. 11B  shows the document decision threshold value after the occurrence of the light quantity degradation or the like of the light source  88 . Regarding the document of, for example, 216 blocks, the document decision portion  134  acquires from the image read control unit  110 , the document decision threshold value which is enlarged (from 24 blocks to 32 blocks by way of example) in accordance with the gain value that the analog correction portion  102  changes in the case of the occurrence of the light quantity degradation or the like in the light source  88 , or the like. Thus, unless the number of the colored pixels is larger than before the occurrence of the light quantity degradation or the like in the light source  88 , the document is prevented from being decided as a colored one. 
       FIG. 12  is a document top view showing the ACS decision region relative to the document, in the case of varying the region (ACS decision region) in which pixels to be decided by the ACS portion  108  lie. 
     The light source  88  exhibits the smaller light quantities at the end parts in the main scan direction, as compared with the light quantity at the middle part in the main scan direction (refer to  FIG. 3 ). Therefore, the ACS portion  108  may well variably set the width of the ACS decision region relative to the document so as to narrow inward in the main scan direction as shown in  FIG. 12 , in the case where the analog correction portion  102  has enlarged the gain value. 
     Next, there will be described an operation in which the document reading apparatus  14  decides whether the document is colorless or colored. 
       FIG. 13  is a flow chart showing a first example (S 10 ) of the operation in which the document reading apparatus  14  decides whether the document is colorless or colored. 
     As shown in  FIG. 13 , at a step  100  (S 100 ), the image read control unit  110  decides whether or not the ACS mode has been selected by a user through the user interface not shown. Subject to the decision that the ACS mode has been selected, the routine proceeds to a step S 102 , and subject to the decision that the ACS mode has not been selected, the routine is ended. 
     At the step  102  (S 102 ), the image read control unit  110  decides whether or not the gain value adjusted by the analog correction portion  102  is less than the gain threshold value “0”. In a case where the gain value is less than the gain threshold value “0”, the routine proceeds to a step S 104 , and in a case where the gain value is, at least, the gain threshold value “0”, the routine proceeds to a step S 106 . Here, the gain threshold value “0” is a threshold value which is previously set in order to decide whether or not the ACS portion  108  ought to decide the document by using the ACS decision parameter “0” that is the combination of, for example, the pixel decision threshold value, the number-of-colored-pixels decision threshold value, the document decision threshold value and the ACS decision region. 
     At the step  104  (S 104 ), the image read control unit  110  sets the ACS decision parameter “0” for the ACS portion  108 . 
     At the step  106  (S 106 ), the image read control unit  110  decides whether or not the gain value adjusted by the analog correction portion  102  is less than the gain threshold value “1”. In a case where the gain value is less than the gain threshold value “1”, the routine proceeds to a step S 108 , and in a case where the gain value is, at least, the gain threshold value “1”, the routine proceeds to a step S 110 . Here, the gain threshold value “1” is a threshold value which is previously set in order to decide whether or not the ACS portion  108  ought to decide the document by using the ACS decision parameter “1” that is the combination of, for example, the pixel decision threshold value, the number-of-colored-pixels decision threshold value, the document decision threshold value and the ACS decision region. 
     Incidentally, the ACS decision parameter “1” is altered so as to bring the pixel decision threshold value onto a high chromaticity side, to bring the number-of-colored-pixels decision threshold value onto an increase side, to bring the document decision threshold value onto an increase side and to make the ACS decision region small in area, with respect to the ACS decision parameter “0”, and it makes the document difficult to be decided as a colored one. 
     At the step  108  (S 108 ), the image read control unit  110  sets the ACS decision parameter “1” for the ACS portion  108 . 
     At the step  110  (S 110 ), the image read control unit  110  sets the ACS decision parameter “2” for the ACS portion  108 . Incidentally, the ACS decision parameter “2” makes the document difficult to be decided as a colored one, more than the ACS decision parameter “1”. 
     At a step  112  (S 112 ), the ACS portion  108  decides whether the document is colorless or colored, by using the set one of the ACS decision parameters “0”-“2”. 
       FIG. 14  is a flow chart showing a second example (S 20 ) of the operation in which the document reading apparatus  14  decides whether the document is colorless or colored. 
     As shown in  FIG. 14 , at a step  200  (S 200 ), the image read control unit  110  decides whether or not the ACS mode has been selected by the user through the user interface not shown. Subject to the decision that the ACS mode has been selected, the routine proceeds to a step S 202 , and subject to the decision that the ACS mode has not been selected, the routine is ended. 
     At the step  202  (S 202 ), the image read control unit  110  reads the initial value of the gain value of the analog correction portion  102  out of the NVM  124 , and it stores the initial value in, for example, the RAM  122 . 
     At a step  204  (S 204 ), the image read control unit  110  decides whether or not the ratio of a present gain value to the initial value of the gain value of the analog correction portion  102  is less than the gain threshold value ratio “0”. In a case where the ratio is less than the gain threshold value ratio “0”, the routine proceeds to a step S 206 , and in a case where the ratio is, at least, the gain threshold value ratio “0”, the routine proceeds to a step S 208 . Here, the gain threshold value ratio “0” is a predetermined threshold value which is previously set in order to decide whether or not the ACS portion  108  ought to decide the document by using the ACS decision parameter “0”. 
     At the step  206  (S 206 ), the image read control unit  110  sets the ACS decision parameter “0” for the ACS portion  108 . 
     At the step  208  (S 208 ), the image read control unit  110  decides whether or not the ratio of the present gain value to the initial value of the gain value of the analog correction portion  102  is less than the gain threshold value ratio “1”. In a case where the ratio is less than the gain threshold value ratio “1”, the routine proceeds to a step S 210 , and in a case where the ratio is, at least, the gain threshold value ratio “1”, the routine proceeds to a step S 212 . Here, the gain threshold value ratio “1” is a predetermined threshold value which is previously set in order to decide whether or not the ACS portion  108  ought to decide the document by using the ACS decision parameter “1”. 
     At the step  210  (S 210 ), the image read control unit  110  sets the ACS decision parameter “1” for the ACS portion  108 . 
     At the step  212  (S 212 ), the image read control unit  110  sets the ACS decision parameter “2” for the ACS portion  108 . 
     At a step  214  (S 214 ), the ACS portion  108  decides whether the document is colorless or colored, by using the set one of the ACS decision parameters “0”-“2”. 
       FIG. 15  is a flow chart showing a third example (S 30 ) of the operation in which the document reading apparatus  14  decides whether the document is colorless or colored. 
     As shown in  FIG. 15 , at a step  300  (S 300 ), the image read control unit  110  decides whether or not the ACS mode has been selected by the user through the user interface not shown. Subject to the decision that the ACS mode has been selected, the routine proceeds to a step S 302 , and subject to the decision that the ACS mode has not been selected, the routine is ended. 
     At the step  302  (S 302 ), the image read control unit  110  decides whether or not the gain value adjusted by the analog correction portion  102  is less than the gain threshold value “2”. In a case where the gain value is less than the gain threshold value “2”, the routine proceeds to a step S 304 , and in a case where the gain value is, at least, the gain threshold value “2”, the routine proceeds to a step S 306 . Here, the gain threshold value “2” is a threshold value which is previously set in order to decide whether or not the image processing portion  106  ought to smooth RGB signals indicating an image, by using the filter coefficient “0”. 
     The filter coefficient “0” is a coefficient for decreasing the number of pixels to be decided as colored ones due to noise, in such a way that, as shown in  FIG. 16  by way of example, the RGB signals are respectively smoothed by 1×3 filtering (the pixel value “44” of the signal B, for example, is brought to 12×0.25+44×0.5+14×0.25≈28). 
     At the step  304  (S 304 ), the image read control unit  110  sets the filter coefficient “0” for the image processing portion  106 . 
     At the step  306  (S 306 ), the image read control unit  110  decides whether or not the gain value adjusted by the analog correction portion  102  is less than the gain threshold value “3”. In a case where the gain value is less than the gain threshold value “3”, the routine proceeds to a step S 308 , and in a case where the gain value is, at least, the gain threshold value “3”, the routine proceeds to a step S 310 . Here, the gain threshold value “3” is a threshold value which is previously set in order to decide whether or not the image processing portion  106  ought to smooth the RGB signals indicating the image, by using the filter coefficient “1”. Incidentally, the filter coefficient “1” is a coefficient which smoothes the RGB signals more than the filter coefficient “0”. 
     At the step  308  (S 308 ), the image read control unit  110  sets the filter coefficient “1” for the image processing portion  106 . 
     At the step  310  (S 310 ), the image read control unit  110  sets the filter coefficient “2” for the image processing portion  106 . Incidentally, the filter coefficient “2” is a coefficient which smoothes the RGB signals more than the filter coefficient “1”. 
     At a step  312  (S 312 ), the ACS portion  108  decides whether the document is colorless or colored, by using the RGB signals smoothed by the image processing portion  106 . 
       FIG. 17  is an ACS-decision-parameter setting region diagram showing a fourth example of the operation in which the document reading apparatus  14  decides whether the document is colorless or colored. 
     As shown in  FIG. 17 , the document reading apparatus  14  may well be made capable of setting the ACS decision parameters for the respective positions (X 1 -X 8 ) of the document in the main scan direction thereof, in accordance with a light quantity distribution corresponding to the respective positions (X 1 -X 8 ) of the document in the main scan direction thereof. Incidentally, the ACS decision parameters “1”-“4” are set in an ascending order so that, when the document is colored, the ACS decision parameter “4” may be the most difficult of the decision. 
     That is, the ACS decision parameters are permitted to be set for the respective positions of the document in the main scan direction thereof, whereby the ACS decision parameters may well be set irrespective of the gain value of the analog correction portion  102  and in consideration of the light quantity distribution, a stain on the optical path of the light source  88 , or the like. 
     Besides, the document reading apparatus  14  may have a plurality of ACS decision parameters or filter coefficients set beforehand, or it may well calculate a plurality of ACS decision parameters or filter coefficients by arithmetic operations employing a reference ACS decision parameter or filter coefficient. 
     Further, the respective ACS decision parameters may be ones which differ from one another in any of the pixel decision threshold value, the number-of-colored-pixels decision threshold value, the document decision threshold value and the ACS decision region, or they may well be ones which differ from one another in all of the above values and region. 
     Besides, the embodiments have been described by exemplifying the case where the photoelectric transducer  86  receives the reflected light from the document, but the document reading apparatus  14  may well be configured so that the photoelectric transducer  86  may receive transmitted light through the document. 
     Further, the photoelectric transducer  86  may be a CCD provided with complementary color filters, or the like, or it may well be a CCD provided with a dichroic filter of red and black, or the like. 
     According to the above-embodiments, even in a case where a difference occurs in the light quantity of a light source illuminating a document, erroneous decisions on whether the document is a color one or a colorless one can be mitigated. 
     The foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.