Patent Publication Number: US-8121501-B2

Title: Image forming apparatus with color detection of a reference toner image and sheet

Description:
CROSS-NOTING PARAGRAPH 
     This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-184788 filed in JAPAN on Jul. 13, 2007, the entire contents of which are hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an electrophotographic image forming apparatus, for example, such as a copying machine, and more specifically relates to an image forming apparatus which performs adjustment to maintain quality of an image formed on an image carrier or a transfer sheet carrier using a patch image. 
     BACKGROUND OF THE INVENTION 
     An usual electrophotographic image forming apparatuses are susceptible to an usage state of each process portion, surrounding environmental situations and the like, due to the principle of operation that image formation is performed using static electricity and therefore, they have a problem that a toner density of a formed image is also liable to change and as a result, image quality is liable to deteriorate. Against these circumstances, in Japanese Laid-Open Patent Publication No. 2003-15369, an electrophotographic image forming apparatus is developed, in which an adjustment step of adjusting control conditions (charging voltage, exposure amount, developing bias, transfer bias and the like) in each portion of an image forming portion (process control) is provided and the adjustment step is carried out in accordance with predetermined conditions, and thereby image quality is adjusted so as to be able to obtain excellent image quality at all times. 
     In the above-mentioned image quality adjustment, a toner patch for a test is formed on an image carrier such as a photoreceptor, an intermediate transfer body or a transfer body at a predetermined timing other than forming images, and the above control conditions are adjusted based on the value of an image (toner) density of the toner patch measured by a density sensor. 
     In a process control mode, a toner patch serving as a density reference is formed on an image carrier (including an intermediate transfer body such as a photoreceptor, an intermediate transfer drum or an intermediate transfer belt, or a transfer body such as a transfer drum or a transfer belt), a density of the toner patch (toner adhesion amount) is detected by an image sensor that is an optical detection means, and based on the detected density, feedback is applied to parameters (image forming conditions), such as charging voltage, exposure intensity, developing bias potential, transfer voltage and amount of toner supply (in the case of two component development) which affect the image density. 
     On the other hand, Japanese Laid-Open Patent Publication No. 10-272806 discloses a technology that in a color image forming apparatus, a color sensor for detecting a sheet color is provided near a sheet supply port, and at the time of performing color printing, color tones of a sheet to be printed are detected by the color sensor, so that optimization of more precise color adjustment is applied depending on the color of the sheet to be printed. 
     However, in a color image forming apparatus provided with a function of detecting a toner patch image by a toner density sensor to perform image quality adjustment, a color sensor for detecting the sheet color needs to be further provided on a sheet transport path in order to add a function of applying optimization of color adjustment depending on a sheet color, but this inhibits miniaturization of the apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a smaller image forming apparatus with smaller number of parts which is achieved by making a common color sensor detect a sheet color and a reference toner image not to further provide a color sensor for detecting the sheet color on a sheet transport path. 
     Another object of the present invention is to provide an image forming apparatus including an image carrier on which a toner image depending on input image data is formed, a transfer sheet carrier for forming a transfer sheet transport path and transferring the toner image formed on the image carrier onto a sheet being transported, a color sensor disposed in a vicinity of the transfer sheet transport path, a control portion for controlling an image forming operation, and an image adjustment function which performs an image adjustment using a reference toner image prior to printing when a color print mode is selected, wherein, a separation driving mechanism for separating the transfer sheet carrier from the image carrier and then contacting them and a sheet re-feeding transport path for feeding the sheet which has passed through the transfer sheet transport path again to the transfer sheet transport path are further included, and the control portion performs a series of control at the time of the image adjustment, to form the reference toner image on a surface of the transfer sheet carrier, to detect the reference toner image by the color sensor, to adjust developing conditions based on color information of the detected reference toner image, then, to transport the sheet on the transfer sheet transport path in a state where the transfer sheet carrier is separated from the image carrier by the separation driving mechanism, to detect colors of the sheet by the color sensor, and to correct a color tone of the input image data based on the detected color of the sheet. 
     Another object of the present invention is to provide the image forming apparatus, wherein the sheet re-feeding transport path is the path reversing the side of the sheet. 
     Another object of the present invention is to provide the image forming apparatus, wherein the control portion controls to pass the sheet transported at the time of the image adjustment through the reversing transport path twice in a state where the transfer sheet transport path is separated from the image carrier, and to transfer the toner image on the same surface as that of the sheet color has been detected. 
     Another object of the present invention is to provide an image forming apparatus including an image carrier on which a toner image depending on input image data is formed, a transfer sheet carrier for forming a transfer sheet transport path and transferring the toner image formed on the image carrier onto a sheet being transported, a color sensor disposed in a vicinity of the transfer sheet transport path, a control portion for controlling an image forming operation, and a function which performs an image adjustment using a reference toner image prior to a start of printing when a color print mode is selected, wherein the control portion controls at the time of the image adjustment, to form the reference toner image on the sheet transported to the transfer sheet transport path, to detect a color of the sheet and the reference toner image by the color sensor, and to adjust developing conditions based on the detected color of the sheet and color information of the reference toner image. 
     Another object of the present invention is to provide the image forming apparatus, wherein the control portion controls to discharge the sheet on which the reference toner image has been formed at the time of the image adjustment into a sheet discharge tray different from a sheet discharge tray generally used in printing. 
     Another object of the present invention is to provide the image forming apparatus, wherein the image adjustment is carried out at the time of the first sheet feeding after a sheet cassette has been changed. 
     Another object of the present invention is to provide the image forming apparatus, wherein the image adjustment is carried out at the time of the first sheet feeding after a sheet supplying has been performed. 
     Another object of the present invention is to provide the image forming apparatus, wherein when a sheet is fed from a specific sheet tray, the image adjustment is carried out for each sheet feeding. 
     Another object of the present invention is to provide the image forming apparatus further including an operation input portion and a storage device for storing a variety of information such as color correction information based on sheet color information detected by the color sensor, wherein when sheet type information indicating a relationship between a sheet feed source of a sheet and a type of the sheet is inputted from the operation input portion, the sheet type information is stored in the storage device, and the control portion carries out the image adjustment based on the sheet type information and the color correction information which are stored in the storage portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing the structure of a digital color copying machine as an image forming apparatus to which the present invention is applied; 
         FIG. 2  is a block diagram of an image processing portion of the color digital copying machine; 
         FIG. 3  is a block diagram showing the main structure of a control portion in the image forming apparatus according to the present invention; 
         FIGS. 4A and 4B  are diagrams illustrating operation of an image forming apparatus according to the first embodiment of the present invention; 
         FIGS. 5A and 5B  are diagrams illustrating operation of an image forming apparatus according to the second embodiment of the present invention; 
         FIG. 6  is a flowchart of processing in a basic embodiment in the first and second embodiments of the present invention; 
         FIG. 7A  is a flowchart of processing in the third embodiment of the present invention; 
         FIG. 7B  is a flowchart subsequent to the flowchart shown in  FIG. 7A ; 
         FIG. 8  is a flowchart of processing of changing development conditions in the image forming apparatus of the present invention; 
         FIG. 9  is a flowchart of processing in the fourth embodiment of the present invention; 
         FIG. 10  is a sectional view schematically showing the structure of an image forming apparatus in the fifth embodiment of the present invention; and 
         FIG. 11  is a flowchart of processing in the fifth embodiment of the present invention. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
       FIG. 1  is a sectional view schematically showing the structure of a digital color copying machine as an image forming apparatus to which the present invention is applied, and the digital color copying machine is provided with an image reading portion  110 , a platen  111 , a reversing automatic document feeder  112 , a sheet feed portion  210 , an image forming portion  220 , and a sheet discharge portion  240 . 
     The platen  111  is the glass plate on which originals for forming images are placed and is disposed on the upper side of a main body of the digital color copying machine. The reversing automatic document feeder (RADF)  112  transports documents to a predetermined position on the platen  111 . The reversing automatic document feeder  112  is installed to be opened and shut like a cover for the platen  111  so as to come to the predetermined position facing the upper side of the platen  111 . 
     The reversing automatic document feeder  112  transports a document first so that one side of the document faces the image reading portion  110  at a predetermined position of the platen  111 , and after reading the image on this side is finished, the document is transported such that the other side faces the image reading portion  110  at a predetermined position of the platen  111 . That is, the reversing automatic document feeder  112  transports the document so that an image or images on only one side or both sides of the document can be read, and after image reading for one document is completed, the document is discharged and the operation of reading and transporting is carried out on the following document. The transportation and the reversing operations are controlled according to an overall operation of the digital color copying machine. 
     The image reading portion  110  is disposed below the platen  111  to read an image in a document placed on the platen  111 . The document is transported onto the platen  111  by the reversing automatic document feeder  112  or directly placed on the platen  111 . The image reading portion  110  includes a document scanning body which moves to and fro in parallel along a lower side of the platen  111 , an optical lens  116 , and a CCD line sensor  17 . 
     The document scanning body  115  is composed of a first scanning unit  113  and a second scanning unit  114 . The first scanning unit  113  is provided with an exposure lamp for exposing the surface of an image in a document, and a first mirror M 1  for changing the course of a reflection image from the document in a predetermined direction. The first scanning unit  113  moves to and fro in parallel at a predetermined scanning speed with a fixed distance maintained from the lower surface of the platen  111 . The second scanning unit  114  is provided with a second mirror M 2  and a third mirror M 3  for further changing the course of the reflection image from the document, that has been deflected by the first mirror M 1  of the first scanning unit  113 , in a predetermined direction (the direction in which the optical lens  116  is disposed). The second scanning unit  114  moves to and fro in parallel with a fixed speed relative to the first scanning unit  113 . 
     The optical lens  116  reduces the reflection image from the document, that has been deflected by the third mirror M 3  of the second scanning unit  114 , and causes the reduced image to be formed on a light receiving surface on the COD line sensor  117 . The CCD line sensor  117 , which is an optical transducer, performs optical conversion to the light image formed by the optical lens  116  one after another to output electric signals The CCD line sensor  117  is preferably a so-called three-line color CCD which can read a monochrome image or a color image and output line data decomposed into each color component of R (red), G (green) and B (blue). The original image information converted into electric signals by the CCD line sensor  117  is further transferred to an image processing portion  41  shown in  FIG. 2  and given the predetermined image data processing. 
     The sheet feed portion  210  is composed of a sheet tray  211 , a sheet feed roller  212 , a resist roller  213  and the like, and separates sheets (recording mediums) P which are stacked and stored in the sheet tray  211  one by one and supplies them to the image forming portion  220 . The sheets P which have been separated and supplied one by one are transported to the image forming portion  220  with a timing being controlled by a pair of resist rollers  213  disposed between the sheet tray  211  and the image forming portion  220 . In a so-called double-sided image formation mode in which images are formed on both sides of the sheet P, the sheet P is firstly transported to the image forming portion  220  and an image is formed on one side of the sheet P, subsequently the sheet P is reversed and transported again to the image forming portion  220 , followed by forming an image on the other side. 
     In the above digital color copying machine, a cut sheet is used as the sheet P. When the sheet P is sent from the sheet tray  211  and supplied to a guide portion of a sheet feeding transport path of the sheet feed portion  210 , the end of the sheet P is detected by a sensor (not shown). Then, based on a detection signal outputted from the sensor, the sheet P is temporarily stopped by the pair of resist rollers  213 , and transported to the image forming portion  220  depending on a timing of the image formation. Note that, the sheet feeding can be also performed from a manual tray  214 . 
     The image forming portion  220  forms an image on the sheet P based on the image information of the document to which the image data processing is performed by the image processing portion  41  (refer to  FIG. 2 ). The image forming portion  220  is provided with a transfer sheet transport belt mechanism  221 , a laser writing unit  227 , an image forming station  228 , and a fixing device  229 . 
     The transfer sheet transport belt mechanism  221  is provided with a drive roller  231 , a driven roller  232 , and a transfer sheet transport belt (transfer sheet carrier)  230  which is tensioned between the drive roller  231  and the driven roller  232  so as to extend almost in parallel. The transfer sheet transport belt  230  is frictionally driven in the direction indicated by an arrow Z according to the rotation of the drive roller  231 . The sheet P fed through the sheet feed portion  210  is sent onto the transfer sheet transport belt  230  at the timing of forming images, and transported to the image forming station  228  in a state of being held by the transfer sheet transport belt  230  by electrostatic absorbing. 
     The image forming station  228  reproduces a toner image corresponding to image information of a document on the sheet P. The image forming station  228  is composed of a first image forming station  228   a  corresponding to a black component, a second image forming station  228   b  corresponding to a cyan component, a third image forming station  228   c  corresponding to a magenta component, and a fourth image forming station  228   d  corresponding to a yellow component. Each of the image forming stations  228   a  to  228   d  is disposed above the transfer sheet transport belt  230  so as to be close to the transfer sheet transport belt  230  in order from the upstream side of the sheet transport path. 
     The image forming stations  228   a  to  228  are substantially identical to one another in construction, hand respectively include photoreceptor drums  222   a  to  222   d  each of which is rotatively driven in the direction indicated by an arrow F. Around each of the photoreceptor drums  222   a  to  222   d , charging devices  223   a  to  223   d  for electrifying each of the photoreceptor drums uniformly, developing devices  224   a  to  224   d  for developing electrostatic latent images respectively formed on the photoreceptor drums, transfer rollers  225   a  to  225   d  which function as transfer electrodes for transferring toner images, which have been developed on the photoreceptor drums, onto the sheets P, and cleaning devices  226   a  to  226   d  for removing the remaining toner on the photoreceptor drums, are disposed in this order along a rotating direction of the photoreceptor drums. 
     The laser writing unit  227  writes electrostatic latent images on the photoreceptor drum  222  based on image information. The laser writing unit  227  is composed of a first laser writing unit  227   a  corresponding to a black component, a second laser writing unit  227   b  corresponding to a cyan component, a third laser writing unit  227   c  corresponding to a magenta component, and a fourth laser writing unit  227   d  corresponding to a yellow component. 
     The laser writing units  227   a  to  227   d  are provided above each of the photoreceptor drums  222   a  to  222   d , and are substantially identical to one another in construction. Each of the laser writing units  227   a  to  227   d  is composed of semiconductor laser elements (not shown) for emitting dot light modulated according to image data, polygon mirrors (deflecting devices)  240   a  to  240   d  for changing a direction of a laser beam from the semiconductor laser element in a main scanning direction, fθ lenses  241   a  to  241   d , mirrors  242   a  to  242   d , and mirrors  243   a  to  243   d  for forming an image of a laser beam, which is deflected by the polygon mirrors  240   a  to  240   d , on the surfaces of the photoreceptor drums  222   a  to  222   d , and the like. 
     A pixel signal corresponding to a black component image of a color document image is inputted to the laser writing unit  227   a , a pixel signal corresponding to a cyan component image of a color document image is inputted to the laser writing unit  227   b , a pixel signal corresponding to a magenta component image of a color document image is inputted to the laser writing unit  227   c , and a pixel signal corresponding to a yellow component image of a color document image is inputted to the laser writing unit  227   d . Each of the laser writing units  227   a  to  227   d  emits a laser beam corresponding to an inputted pixel signal, and forms an image of the laser beam on the surfaces of the photoreceptor drums  222   a  to  222   d . Then, the laser beam is scanned and exposed so that optical writing on the photoreceptor drums  222   a  to  222   d  is performed. 
     Thus, electrostatic latent images corresponding to image information of a document subjected to the color conversion are formed on each of the photoreceptor drums  222   a  to  222   d . Moreover, the developing device  224   a  stores black toner, the developing device  224   b  stores cyan toner, the developing device  224   c  stores magenta toner, and the developing device  224   d  stores yellow toner, respectively, and the electrostatic latent images on the photoreceptor drums  222   a  to  222   d  are developed by the toners of each color. Thereby, the image information of the document subjected to the color conversion in the image forming portion  220  is reproduced as toner images of each color. 
     The sheet P is transported to each of the image forming stations  228   a  to  228   d  in order by the transfer sheet transport belt  230 , and toner images developed on the photoreceptor drums  222   a  to  222   d  are transferred to the sheet P. That is, the toner images of each color are formed in each of the image forming stations  228   a  to  228   d , and are superimposed one after another on the sheet P which is electrostatically absorbed and transported on the transfer sheet transport belt  230 . When the image transfer by all of the image forming stations  228   a  to  228   d  is completed, the sheet P is released off the transfer sheet transport belt  230  and transported to the fixing device  229 . 
     The fixing device  229  fixes a toner image transferred and formed on the sheet P onto the sheet P. The fixing device  229  is disposed in a downstream side of the transfer sheet transport belt mechanism  221  in the sheet transport path, and is provided with a pair of fixing rollers made of a heat roller  229   a  and a pressure roller  229   b , a release member and the like. The sheet P is transported through a nip portion between the fixing rollers so that the toner image is formed on the sheet P, and released off the fixing rollers by the release member to be transported to a sheet discharge portion  240 . 
     The sheet discharge portion  240  discharges the sheet P on which image formation is completed to outside of the digital color copying machine. The sheet discharge portion  240  is provided with a transport direction switching gate  241 , a discharge roller  242 , and a sheet discharge tray  243 . In a one-side image formation mode, as described above, an image is formed on one side of the sheet P, and thereafter, the sheet P transported from the fixing device  229  is transported through the transport direction switching gate  241  and ejected by the discharge roller  242  on the sheet discharge tray  243 , which is mounted on an outer wall of the digital color copying machine. 
     The transport direction switching gate  241  switches a transport path of the sheet P on which the toner image has been fixed, and selectively switches a track for discharging the sheet P to the sheet discharge tray  243  and a sheet re-feeding transport track for feeding the sheet P again to the image forming portion  220 . In a double sided image formation mode, an image is formed on one side of the sheet P as described above, and thereafter a transport direction of the sheet P is switched by the transport direction switching gate  241  so that the sheet P can be transported to the image forming portion  220  again, and the sheet P is reversed by a switchback transport path  250 , to be supplied again to the image forming portion  220  through a sheet re-feeding transport path  260  which forms the sheet re-feeding transport route. 
     Here, in the above description, the laser writing unit  227  ( 227   a  to  227   d ) scans a laser beam for exposure so as to perform an optical writing on the photoreceptor drum  222  ( 222   a  to  222   d ). However, in the present apparatus, as a means for performing optical writing on the photoreceptor drum  222  ( 222   a  to  222   d ), a solid-state scanning writing optical system (LED head) which includes a light emitting diode array, an image-forming lens array and the like is also applicable, instead of the laser writing unit  227  ( 227   a  to  227   d ). The LED is smaller than the laser writing unit in size, and has no movable part which causes sound. Therefore, the LED head can be preferably used in an image forming apparatus such as a tandem system digital color copying machine, which requires a plurality of optical writing units. 
     A color sensor  15 , which is a feature of the present invention, for detecting both of a reference toner image formed on the transfer transport belt  230  and a color of a sheet to be transported with one sensor is disposed above the transfer sheet transport path in front of the fixing device  229  in the present embodiment. 
       FIG. 2  is a block structural diagram of an image processing portion provided in the color digital copying machine, and the image processing portion is composed of an image data input portion  40 , an image processing portion  41 , an image data storage portion  42 , an image data output portion  43 , a central processing unit (CPU)  44 , an image editing portion  45 , and external interface portions  46  and  47 . 
     The image data input portion  40  includes a three-line color CCD  40   a  capable of reading a monochrome document or a color document image and outputting line data decomposed into RGB color components, a shading correction circuit  40   b  for correcting a line image level of line data which is read in the color CCD  40   a , a line adjusting portion  40   c  such as a line buffer for correcting a shift of image line data which is read in the three-line color CCD  40   a , a sensor color correction portion  40   d  for correcting color data of line data of each color that is outputted from the three-line color CCD  40   a , an MTF correction portion  40   e  for making correction to emphasize a change in a signal of each pixel, a γ correction portion  40   f  for making visibility correction by correcting contrast of an image. 
     The image processing portion  41  includes a monochrome data generating portion  41   a  (monochrome document) for generating monochrome data from an RGB signal which is a color image signal inputted from the image data input portion  40 , an input processing portion  41   b  for converting an RGB signal into a YMC signal corresponding to each storage portion in a storage device and performing a clock converting operation, an area segmentation portion  41   c  for classifying input image data into characters, a halftone picture, and a photographic-picture, a black generating portion  41   d  for removing a base color in response to a YMC signal outputted from the input processing portion  41   b  so as to generate a black color, a color correction circuit  41   e  for adjusting each color of a color image signal based on each color conversion table, a zoom processing circuit  41   f  for zooming in and out image information which is inputted based on a set magnification, a spatial filter  41   g , a halftone processing portion  41   h  for displaying gradation such as multilevel error dispersion and multilevel dither, and the like. 
     Image data of each color that is subjected to halftone processing is temporarily stored in an image data storage portion  42 . A hard disc device  42   a  in the image data storage portion  42  includes four hard disks (rotatable memory mediums), which successively receive image data of 8 bits and 4 colors (32 bits) outputted from the image processing portion  41 , convert 32-bit data into 8-bit image data of four colors while temporarily storing the data in the buffer, and store and control the image data for each color. Moreover, since each of the image forming stations is disposed at different positions, an RAM  42   b  as a delay buffer memory is caused to temporarily store image data of each color so as to transmit the image data to each of the laser scanner units at different timings, causing no displacement of color. The image data storage portion  42  further includes an image synthesizing memory for synthesizing a plurality of images, and the like. 
     The image data output portion  43  includes a laser control unit  43   a  for carrying out pulse width modulation based on image data of each color from the halftone processing portion  41   h , and laser scanner units of each color  43   b ,  43   c ,  43   d , and  43   e  for performing laser recording based on a pulse width modulated signal corresponding to an image signal of each color that is outputted from the laser control unit  43   a.    
     The central processing unit (CPU)  44  controls the image data input portion  40 , the image processing portion  41 , the image data storage portion  42 , the image data output portion  43 , the image editing portion  45  (described later), and the external interface portions  46  and  47 , based on a predetermined sequence. The image editing portion  45  performs a predetermined image edit on image data, which has been transmitted through the image data input portion  40 , the image processing portion  41 , or the interfaces (described later) and has been temporarily stored in the image data storage portion  42 , and the edit of the image data is carried out by using an image synthesizing memory of the image data storage portion  42 . 
     The interface  46  is communication interface means for receiving image data from an external image input processing apparatus (a communication portable terminal, a digital camera, a digital video camera and the like) which is provided in addition to the color digital copying machine. Note that, image data, which has been inputted from the interface  46 , is also temporarily inputted to the image processing portion  41  for color space correction and the like, and the data level of the image data is converted to a data level applicable to the image editing portion  45  in the color digital copying machine, and then, the image data is stored and controlled in the hard disc device  42   a.    
     The interface  47  is a printer interface for inputting image data generated by a personal computer, or a monochrome or color FAX interface for receiving image data from a FAX machine. Image data inputted from the interface  47  which is already composed of a CMYK signal, is temporarily subjected to the halftone processing portion  41   h , and stored and controlled in the four hard disks of the hard disc device  42   a  in the image data storage portion  42 . 
       FIG. 3  is a block diagram showing the main structure of a control portion in the image forming apparatus according to the present invention. While  FIG. 1  shows the structure of the image forming apparatus,  FIG. 3  shows a control system, and therefore components which are not shown in  FIG. 1  are included in  FIG. 3 , on the other hand, some components shown in  FIG. 1  are omitted in  FIG. 3 . 
     The control portion shown in  FIG. 3  is mainly composed of a main CPU  10 , and is provided with a ROM  11  which stores a control program for controlling a whole of the present apparatus and image forming operation, a memory  12  for storing an adjustment value or a set value of an automatic document feeder, and color information detected by a color sensor  15 , a sensor  13  for detecting a sheet position in a sheet feed portion and each transport path, a sheet tray sensor  14  for detecting presence of a sheet in a sheet feed tray and change of a cassette, a color sensor  15  (described later) which is a feature portion of the present invention, a sheet feeding system driving mechanism  16  including a motor for driving a sheet feed roller and a resist roller, an image forming portion driving mechanism  17  including a motor for driving the drive roller in a transfer sheet transport belt mechanism  221  and a solenoid for driving a separation mechanism (not shown) for contacting and separating the transfer sheet transport belt  230  with and from the photoreceptor drum  222 , a sheet discharge system driving mechanism  18  including a motor for driving the discharge roller  242  and the like, a transporting system driving mechanism  19  including a motor for driving a transport roller which transports sheets, and a solenoid for driving the transport direction switching gate  241  which switches transport directions, a reversing transport system driving mechanism  20  including a solenoid for switching the transport direction switching gate  251  in the switchback transport path  250  and a motor for driving a transport roller in a reversing (non-reversing) transport path (the sheet re-feeding transport path  260 ), and an input operating portion  21  which is composed of an operation panel provided with a display device and input key etc., and inputs setting of copy modes such as a color print mode and a variety of operations. 
     First Embodiment 
     Operation performed by the control portion (CPU)  10  based on a control program stored in the ROM  11  in an image forming apparatus in the first embodiment of the present invention will hereinafter be described. 
     When a document is set on a color copying apparatus and a user executes a print job with a color print mode in the input operating portion  21 , then firstly, prior to printing, a reference toner image is formed on the transfer sheet transport belt  230  through the photoreceptor drum  222  and the reference toner image is detected by the color sensor  15 , so that adjustment of developing conditions is carried out based on color information of the reference toner image. The reference toner image formed on the transfer sheet transport belt  230  passes below the color sensor  15 , and thereafter cleaned by cleaning means (not shown). 
     Now, referring to  FIGS. 4A and 4B , image adjustment by color tone correction of input image data based on a sheet color, which is to be carried out after the above-mentioned image adjustment by adjusting the developing conditions, will be described. The present embodiment shows an example in a case where a reversing transport path (switchback transport path) is used. In  FIG. 4A , when the above-mentioned image adjustment by adjusting the developing conditions is completed, the sheet P is fed from the sheet tray  211  (T 1 ), transported in a state where the transfer sheet transport belt  230  is separated from the photoreceptor drum  222  ( 222   a ,  222   b ,  222   c , and  222   d ) (T 2 ), and a color of the sheet P is detected by the color sensor  15  which has detected the above-mentioned reference toner image (T 3 ). Subsequently, in  FIG. 4B , the sheet P the sheet color of which has been detected is guided to the switchback transport path  250  with a guide by transport direction switching gates  241  and  251  (T 4 ), and thereafter the transport direction switching gate  251  is switched to a side of the sheet re-feeding transport path  260  and the sheet P in the switchback transport path  250  is fed to the sheet re-feeding transport path  260  by a reverse rotation of the transport roller  252  (T 5 ), subsequently the sheet P is transported in a state of being reversed by the transfer sheet transport belt  230  which is being in contact with the photoreceptor drum  222  and a document image read by the image reading portion  110  is formed on the sheet P (T 6 ). After image formation on the sheet P, the sheet P on which the image is fixed by the fixing device  229  is discharged to the sheet discharge tray  243  (T 7 ). 
     The transfer rollers  225   a  to  225   d  are movable up and down by a contact/separation mechanism (not shown) and the transfer sheet transport belt  230  contacts with and separates from the photoreceptor drum  222  ( 222   a  to  222   d ) by moving the transfer rollers  225   a  to  225   d  up and down with a separation driving mechanism (not shown). 
     In the present embodiment, since the transfer sheet transport belt  230  separates from the photoreceptor drum  222  at the time of detecting a sheet color, the sheet P is not soiled. 
     When a reversing transport path is used, although an image is to be formed on the side different from the side which a sheet color has been detected, this method is applicable when both sides of a sheet have a same sheet color, and the present invention can be carried out using an existing reversing sheet transport path. 
     Second Embodiment 
       FIGS. 5A and 5B  show an example in the case of an image forming apparatus provided with a non-reversing transport path, as the second embodiment of the present invention. In the case of the image forming apparatus provided with the non-reversing transport path, different from the case where a reversing transport path is used, an image is not formed on the side of the sheet different from the side which a sheet color has been detected and the image can be formed on the side which the sheet color has been detected, thus making it possible to carry out an accurate color correction even when both sides of the sheet have different sheet colors. 
     In  FIG. 5A , when image adjustment by adjusting developing conditions is completed, similar to the case of the above-mentioned first embodiment, the sheet P is fed from the sheet tray  211  (U 1 ), transported in a state where the transfer sheet transport belt  230  is separated from the photoreceptor drum  222  ( 222   a  to  222   d ) (U 2 ), and a sheet color is detected by the color sensor  15  which has detected a reference toner image (U 3 ). Subsequently, in  FIG. 5B , the sheet P whose sheet color has been detected is guided to the sheet re-feeding transport path  260  with a guide by transport direction switching gates  241  and  261  to be fed again (U 4 ), transported to the transfer sheet transport belt  230  which is being in contact with the photoreceptor drum  222  ( 222   a  to  222   d ), then an image to be printed on the sheet P is formed (US) After image formation on the sheet P, the sheet P on which the image is fixed by the fixing device  229  ( 229   a  and  229   b ) is discharged to the sheet discharge tray  243  (U 6 ). 
     Next, the operation in the above-mentioned embodiment will be described using a flowchart. 
       FIG. 6  is a flowchart of basic processing in image adjustment based on a sheet color, and the image adjustment starts when image adjustment by a reference toner image is completed, as described above. 
     In  FIG. 6 , firstly, the transfer sheet transport belt  230  is separated from the photoreceptor drum  222  (step S 1 ). Then, a piece of sheet P is picked up from the sheet tray  211  (step S 2 ). Subsequently, the sheet P is transported and a sheet color is detected by the color sensor  15  (step S 3 ). Further, depending on the sheet color, image processing (color conversion) is applied to print data to reduce an influence of the sheet color (step S 4 ). 
     Here, image processing of the present invention, which is carried out depending on the detected sheet color, will be described below. For example, when color component values of a sheet color, for example, a Y component value of α, an M component value of β, and a C component value of γ are detected by the color sensor, the image processing can be carried out so that the values of α, β, and γ are reduced from image data YMC values recorded in an image memory, respectively. 
     Specifically, for example, a following corresponding relation is satisfied between a value of yellow (X 1 ) in print data and a value of a yellow component (X 2 ) when the sheet color is detected by the color sensor; {X 1 =f(X 2 )}, where f denotes a function. For example, there is a following relation; X 1 =α·X 2 . In the relation of “X1=α·X2”, when a value in detecting the sheet color (yellow component) is X 2 , “α·X2” is subtracted from the print data (yellow) to thereby reduce the yellow component of the sheet color from the print data, so that correction can be performed. 
     After going back to the flowchart shown in  FIG. 6  again to perform the above-mentioned image processing, the transfer sheet transport belt  230  is made in contact with the photoreceptor drum  222  ( 222   a  to  222   d ) (step S 5 ) and the sheet P is reversed by the switchback transport path  250  to be fed again (step S 6 ). Subsequently, image formation is performed on the sheet P (step S 7 ), and the sheet P is discharged to the sheet discharge tray  243  (step S 8 ). Above steps are repeated as far as there is a next print page (step S 9 ). 
     These are basic processing and may be carried out in the case of sheet feeding from a specific tray. For example, the processing may be carried out in the case of sheet feeding from a manual tray  214 . Since various kinds of sheets are placed on the manual tray  214 , the processing is always carried out at the time of sheet feeding so that color correction can be performed properly. 
     Third Embodiment 
       FIGS. 7A and 7B  are flowcharts of an embodiment in which a sheet color of only a first fed sheet is detected and the same image processing is performed based on the sheet color of the first sheet in subsequent image formation. 
     The present embodiment is suitable for a case where a type of sheets in the sheet tray is uniform and there is no need of detecting a sheet color for each sheet feeding. 
     Steps S 1  to S 9  are the same processing as that in the flowchart shown in  FIG. 6 , and description thereof will be omitted. 
     In the flowcharts, the image formation for the first sheet is performed at steps before step S 9 , subsequent image formation is performed at steps after step S 9 , and in image forming processing after step S 9 , the image processing is performed based on the sheet color of the first sheet. 
     At step S 10 , when it is detected that there is some sheets in the sheet tray  211  (step S 10 /Yes), the same processing as that of steps S 2 , S 4 , and S 7  to S 10  is performed (steps S 11  to S 16 ), and when it is detected that there is some sheets in the sheet tray  211  at step S 16  (step S 16 /Yes), the processing goes back to step S 11 . 
     When there is no sheet, the processing proceeds to step S 17  and waits until sheet supply is achieved. When sheet supply is achieved (step S 17 /Yes), the processing goes back to first step S 1  and a sheet color is detected. Thus, the image processing can be performed appropriately even if the color of the supplied sheet is different from that of the previous sheet. 
     In the above description, although the processing is performed at the time of sheet supply, the same processing can be also performed in the case where sheet feeding is conducted from a different sheet tray by switching to the different sheet tray when there is no sheet. 
     Note that, when a non-reversing transport path is used, step S 6  shown in  FIGS. 6 and 7A  can be realized by feeding sheets not using with the reversing transport path but using the non-reversing transport path. 
       FIG. 8  is a flowchart of processing of changing development conditions by a reference toner, which is performed before image adjustment based on a sheet color, in which when a user selects a color print mode and the processing starts. First, a reference toner image (toner patch) is generated on the transfer sheet transport belt  230  (step S 21 ), and a color of the reference toner image is detected by the color sensor  15  (step S 22 ). Whether or not the detected color of the reference toner image falls within a range of a predetermined reference value is determined (step S 23 ) When it is out of the reference value, process conditions such as charging voltage and developing bias electrode are changed (step S 24 ), and the processing is repeated until it falls within the range. 
     Fourth Embodiment 
       FIG. 9  is a processing flow in a fourth embodiment, in which when a reversing transport path is used, a sheet is reversed twice so that a side of a sheet which a sheet color has been detected can be the same side of the sheet on which a print image is to be formed. 
     Steps S 31  to S 34  are the same as steps S 1  to S 4  shown in  FIG. 6 . At step S 35 , the sheet is reversed so that the side of the sheet which the sheet color has been detected can be the printing side and fed again, and transported to the switchback transport path  250 , and thereafter the sheet is reversed and fed again at step S 36 , and the transfer sheet transport belt  230  is made in contact with the photoreceptor drum (the image carrier)  222  to perform image formation of a print image on the side of the sheet which the sheet color has been detected (step S 38 ), and the sheet is discharged (step S 39 ). Above steps are repeated as far as there is a next print page (step S 40 ). 
     Fifth Embodiment 
     Now, a fifth embodiment of the present invention will be described referring to  FIGS. 10 and 11 . In the present embodiment, a reference toner image is formed on a sheet to perform image adjustment processing, and as shown in  FIG. 10 , a transport direction switching gate  244  and another sheet discharge tray  245  are provided between the sheet discharge roller  242  and the sheet discharge tray  243 , and a sheet on which a reference toner image for image adjustment has been formed is discharged to the second sheet discharge tray  245  so as not to be mixed with a sheet on which a normal print has been performed. 
       FIG. 11  is a processing flow of the present embodiment, in which when a user executes a print job in a color print mode from the input operating portion  21 , the present processing starts prior to printing. First, a piece of sheet is picked up from a sheet feed cassette (step S 41 ), and a toner patch as a reference toner image is formed on the sheet transported to the transfer sheet transport path (step S 42 ). Subsequently, a sheet color and color information of the toner patch formed on the sheet are detected by the color sensor  15  (step S 43 ), and development conditions are adjusted based on the detected sheet color and the color information of the toner patch (step S 44 ). The transport direction switching gate  244  is switched to a side of the second sheet discharge tray  245  at a predetermined timing, and the sheet whose sheet color and toner patch have been detected is discharged into the second sheet discharge tray (step S 45 ), followed by completing the processing for the image adjustment. 
     Sixth Embodiment 
     Next, an embodiment that a relation between a sheet feed source (sheet feed tray) and a type of a sheet is inputted from an operation panel so that image adjustment can be performed effectively, will be described. 
     On the input operating portion  21  (refer to  FIG. 3 ) such as an operation panel, an input screen (not shown) for inputting sheet type information which is a relation between a sheet feed source and a type of a sheet is displayed, and the sheet type information inputted from the input screen is stored in the memory  12 . For example, sheet type information of “1” is stored in association with the sheet tray  211 , and sheet type information of “2” is stored in association with the manual tray  214 . 
     Information showing types of sheets is stored in the sheet type information, for example, a name of a sheet such as “a recycled sheet” and “a color sheet Y” is stored. 
     Further, color correction information related to sheet type information is stored separately in the memory  12  so that same color correction is performed when the sheet type is set. For example, as a sheet type, color correction information of “α·X2” is recorded in association with “a recycled sheet”. A value obtained by actually measuring a recycled sheet by the color sensor  15  is recorded and stored for X 2 . 
     By setting in this way, for example, when a user sets sheet type information of “a recycled sheet” for the manual tray and the manual tray is specified as a sheet feed source, an image forming apparatus reads color correction information corresponding to “a recycled sheet” and adopts it as a correction value. With such a structure, once a correction value is measured, color correction information thereof can be used thereafter, and color correction can be performed appropriately in printing from any sheet feed source. 
     According to the present invention, a color sensor for detecting a reference toner image on a transfer sheet carrier is also capable of detecting a sheet color and there is no need of disposing another color sensor for detecting the sheet color in a sheet transport path, and therefore, it is possible to prevent the number of components from being increased and reduce a size of an apparatus.