Abstract:
An image forming apparatus for forming a visible image on a transfer sheet according to a job, the image forming apparatus including: a plurality of sheet feed trays adapted to load plural kinds of transfer sheets; an image forming section for forming a visible image according to image data on a transfer sheet; a conveying section for conveying a transfer sheet from each of the plurality of sheet feed trays toward the image forming section; a display section for displaying various items; a color detection section for detecting a transfer sheet color of the transfer sheet being conveyed by the conveying section; and a control section for controlling a conveyance of the transfer sheet and the image formation, wherein the control section displays a transfer sheet color designated by the job and the transfer sheet color detected by the color detection section on the display section.

Description:
RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2007-216756 on Aug. 23, 2007 in Japan Patent Office, the entire content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus and an image forming system, for forming images on appropriate transfer sheets when executing image formation in a state where the transfer sheets include color sheets. 
     DESCRIPTION OF THE RELATED ART 
     There is an image forming apparatus which has a plurality of sheet feed trays, loads different transfer sheets on the respective sheet feed trays, and switches the sheet feed trays during execution of the image formation. In such an image forming apparatus, color sheets are used as a partition sheet or as a separator for each job. 
     Such an image forming apparatus holds tray information on which sheet feed tray what color of transfer sheets are loaded, selects transfer sheets according to the tray information and job information, thereby executes image formation. 
     Further, regarding such an image forming apparatus, in the Japanese Unexamined Patent Application Publications indicated below, various proposals are made on selection and detection of the color of transfer sheets. 
     According to the method described in Japanese Unexamined Patent Application Publication No. 2002-193465 (Page 1, FIG. 1), each of a plurality of sheet feed trays has a color detection section of color sheets which detects whether there is a sheet feed tray where transfer sheets of the designated color are set or not by job information including color designation of transfer sheets, and the image formation is executed if there is. 
     Japanese Unexamined Patent Application Publication No. 58-117567 (Page 1, FIG. 1) discloses that a color R, G, and B light emission section (light source) for irradiating light to transfer sheets and a light receiving section for detecting light which is reflected from the transfer sheets by irradiating from the light source are installed, and discloses a technique of detecting the color of transfer sheets through performing operations for received light output and displaying it on the operation display section. 
     Further, in Japanese Unexamined Patent Application Publication No. 7-187475 (Page 1, FIG. 1) discloses a method of detecting color sheets loaded on a sheet feed tray by the color sensor and sending accurately the color sheets on issuing a color sheet sending instruction even if the sheet feed tray loading the color sheets used as a job separator is mounted at any sheet feed position of the image forming apparatus. 
     However, in the Japanese unexamined patent application publications aforementioned, the color of the transfer sheet at the uppermost position of each sheet feed tray is detected, thus the color of the transfer sheets loaded on each sheet fed tray is judged, and a problem of judging the color of the transfer sheet arises when there are color sheets of different colors mixed in the sheet feed trays. 
     Further, in this case, when even one different color transfer sheet is mixed in the sheet feed trays, a copy of whole image forming output material subjected to mixture is handled as spoilage which cannot be shipped and a large waste is caused. 
     In the image forming apparatus and image forming system used for print on demand, various transfer sheets are used and the colors of transfer sheets are also various. Therefore, as transfer sheet information, not only the sheet kind, sheet size, and weight but also the transfer sheet color are necessary. 
     Further, not only in image formation on simple monochromatic transfer sheets but also in duplex image formation, different colors may be used on the front and rear, though there is a problem which cannot be coped with by the unexamined patent application publications aforementioned. 
     Further, when the image forming apparatus is equipped with multistage sheet feed trays, a countermeasure for a setting error of the sheet feed trays by an operator is required. 
     Further, when detecting the transfer sheet color on the sheet feed trays as described in the unexamined patent application publications aforementioned, it is necessary to arrange a sensor on each sheet feed tray, thus when multistage sheet feed trays are installed, a problem of an increase in cost arises. 
     And, in the methods of the unexamined patent application publications aforementioned, although an attempt is made to make the designation of the transfer sheet color by the job coincide with the transfer sheet for actual image formation, a method for automatically confirming whether the designation of the transfer sheet color by the job coincides with the transfer sheet color of the actual image forming output material or not is not established, thus a problem arises that the operator must confirm actually the actual image forming output material. 
     SUMMARY OF THE INVENTION 
     The present invention was developed to solve the aforementioned problems and is intended to provide an image forming apparatus and an image forming system for executing image formation using transfer sheets of a desired color designated by the job and automatically confirming whether the designation of the transfer sheet color by the job coincides with the transfer sheet color of the actual image forming output material or not. 
     One aspect of the present invention is an image forming apparatus for forming a visible image on a transfer sheet according to a job, the image forming apparatus comprising: a plurality of sheet feed trays adapted to load plural kinds of transfer sheets; an image forming section for forming a visible image according to image data on a transfer sheet; a conveying section for conveying the transfer sheet from the transfer sheet trays toward the image forming section; a display section for displaying various items; a color detection section for detecting a transfer sheet color of the transfer sheet being conveyed by the conveying section; and a control section for controlling a conveyance of the transfer sheet and an image formation, wherein the control section displays a transfer sheet color designated by the job and the transfer sheet color detected by the color detection section on the display section. 
     Another aspect of the present invention is an image forming system comprising: a display apparatus for displaying various items according to a control to form image by a control section; and a image forming apparatus for forming a visible image on a transfer sheet according to a job, wherein the image forming apparatus comprises: the control section for controlling to form image; a plurality of sheet feed trays adapted to load plural kinds of transfer sheets; an image forming section for forming a visible image according to image data on a transfer sheet; and a conveying section for conveying the transfer sheet from the transfer sheet trays toward the image forming section; and a color detection section for detecting a transfer sheet color of the transfer sheet when the transfer sheet is conveyed by the conveying section, wherein the control apparatus displays the transfer sheet color designated by the job and the transfer sheet color detected by the color detection section on the display section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the constitution of the image forming apparatus of the embodiment of the present invention. 
         FIG. 2  is a block diagram showing the constitution of the image forming system of the embodiment of the present invention. 
         FIG. 3  is a block diagram showing the sectional constitution of the image forming apparatus of the embodiment of the present invention. 
         FIG. 4  is a flow chart showing the operation (registering of transfer sheets) of the image forming apparatus of the embodiment of the present invention. 
         FIG. 5  is an illustration showing a display example on the display section of the image forming apparatus of the embodiment of the present invention. 
         FIG. 6  is another illustration showing a display example on the display section of the image forming apparatus of the embodiment of the present invention. 
         FIG. 7  is a flow chart showing the operation (execution of the job) of the image forming apparatus of the embodiment of the present invention. 
         FIG. 8  is still another illustration showing a display example on the display section of the image forming apparatus of the embodiment of the present invention. 
         FIG. 9  is a further illustration showing a display example on the display section of the image forming apparatus of the embodiment of the present invention. 
         FIG. 10  is a block diagram showing the constitution of the image forming apparatus of the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, the preferred embodiment (hereinafter, referred to as the embodiment) for executing the present invention will be explained in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing the constitution of a concrete example of the copying apparatus as an image forming apparatus  100  of the first embodiment of the present invention. 
     In  FIG. 1 , numeral  101  indicates a control section composed of a CPU as a control section for controlling the whole image forming apparatus  100 ,  102  an operation input section for inputting a preset operation of the image forming apparatus by a user or a key operator, and  103  a display section for displaying the apparatus condition to the user. Here, the operation input section and display section for performing individually the input and display of the operation are shown, though a constitution of an operation display section for performing the two may be used. Numeral  106  indicates an information memory section for holding various information regarding image formation such as tray information and transfer sheet information. 
     Numeral  110  indicates an interface for receiving image data from an external apparatus, which receives the image data when the image forming apparatus  100  operates as a printer. 
     Numeral  120  indicates a scanner section for optically scanning and reading image information of a document, thereby generating image data, which is composed of an image reading section for scanning an image of a document and generating image data and an automatic document feeding section (ADF) for automatically feeding the document to the image reading section. 
     Numeral  130  indicates an image processing section for performing a predetermined image process for the image data read and obtained by the scanner section  120 , which has an image memory  135  having a predetermined capacity for holding image data as required. 
     Numeral  140  indicates a print engine for forming an image (a toner image) according to the image data on a recording sheet and outputting it, including a sheet feed section for feeding desired sheets from the sheet feed tray to the image forming section, a conveying section for conveying the fed recording sheet at a predetermined speed, an image write section for generating a laser beam according to the image data, an image forming section for generating a toner image from a latent image according to the laser beam and transferring it to the recording sheet, and a color detection section for detecting the color of transfer sheets. 
       FIG. 2  is a block diagram showing the constitution of a concrete example of the print system as an image forming system  1000  of the first embodiment of the present invention using an image forming apparatus  100 ′, an operation input apparatus  102 ′, a display apparatus  103 ′, an information memory apparatus  106 ′, a scanner apparatus  120 ′, and a post-processing apparatus  200 ′. 
     In  FIG. 2 , to the same components as those shown in  FIG. 1 , the same numerals are assigned and to the similar components as those shown in  FIG. 1 , “′” is attached to the same numerals. In this case, as a print system, the image forming apparatus  100 ′ for executing image formation and print and the apparatuses for executing operation input and various displays are installed in separate rooms or installed separately from each other in the same room and are connected to each other via various signal lines and a network. Further, the image forming system may have the operation input section  102  and display section  103  as an image forming apparatus  100 ′ or may have the operation input section  102 ′ and display section  103 ′ at separate positions. 
     Further, the constitution of each component of the image forming system shown in  FIG. 2  is the same as that of the image forming apparatus shown in  FIG. 1 , so that in the embodiment indicated below, the image forming apparatus  100  will be explained as a concrete example. 
     Further,  FIG. 3  is a block diagram showing the constitution of the essential section regarding the image forming operation of the image forming apparatus  100 . Further, here, as an example, an electrophotographic monochromatic image forming apparatus is used as a concrete example. Further, in  FIG. 3 , the circumference of the part necessary for operation explanation of the characteristic part of this embodiment is shown mainly and the other already-known parts of the image forming apparatus are omitted. 
     Firstly, the constitution and operation of the image forming apparatus  100  will be explained by referring to  FIG. 3 . On sheet feed trays  150   a  to  150   d , transfer sheets are loaded as a subject to be conveyed and are discharged into a conveying section by sheet feed rollers (not drawn). 
     The discharged transfer sheet is conveyed at a predetermined fixed speed suited to image formation, and the timing thereof is adjusted by loop rollers  161   a , registration rollers  161   b , and transfer-preceding rollers  161   c , and a toner image is transferred thereto from an image carrier  170  such as a photosensitive drum or a photosensitive belt. 
     Further, in this case, the displacement of transfer sheet is detected by a displacement detection sensor  161   s  arranged in the perpendicular direction to the sheet of paper of  FIG. 3  which is the longitudinal direction. The displacement detection sensor  161   s  is composed of a color line sensor and composes the color detection section  161   s  of this embodiment. And, the color detection section  161   s  detects the transfer sheet color on the image forming surface of the transfer sheet being conveyed by a conveying section. Namely, even in double-side image formation, it detects the transfer sheet colors on the first image forming surface and second image forming surface. The displacement detection result and transfer sheet color detection result are transferred to the control section  101  shown in  FIG. 1 . 
     The transfer sheet with the toner image transferred to is conveyed on a conveying path  162  and the toner image is fixed on the transfer sheet by heating and pressurization at a fixing section  180 . The transfer sheet is subject, when necessary, to switch-back system reverse conveyance at a reverse conveying section  163 , and the front and rear are reversed, and the transfer sheet is ejected with the image forming surface directed downward. Further, at the reverse conveying section  163 , to prevent the reversed transfer sheet and the succeeding transfer sheet from a collision, the switch-back is executed at a higher speed than a predetermined conveying speed. 
     Further, when executing the double-side image formation, the transfer sheet fixed at the fixing section  180  is led to a circulating conveying section  164 , is reversed the front and rear by the switch-back system reverse conveyance at a circulating reverse conveying section  165 , and furthermore is led from a circulating conveying section  166  to the conveying section  161 , thus the image formation is executed on the rear. Further, also at the circulating reverse conveying section  165 , to prevent the reversed transfer sheet and the succeeding transfer sheet from a collision, the switch-back is executed at a higher speed than the predetermined conveying speed. 
     Further, for transfer sheets outputted normally and abnormal transfer sheets of a color difference, as an image forming apparatus  100 , the known distribution section may be installed or as a post-processing apparatus  200 , distribution may be executed as a part of the post processing. 
     The Embodiment of the Present Invention 
     Hereinafter, the operation of the image forming apparatus of this embodiment will be explained by referring to the flow charts shown in  FIGS. 4 and 7 . 
     Firstly, by referring to the flow chart shown in  FIG. 4 , registration of a transfer sheet prior to image formation will be explained. 
     Here, an operator operates the operation input section  102  of the image forming apparatus  100  and changes the image forming apparatus  100  from the ordinary image forming mode to the transfer sheet registration mode (Step S 401  shown in  FIG. 4 ). The transfer sheet registration mode is a mode for registering the kind, size, weight, and color of transfer sheets loaded on each sheet feed tray as tray information. 
     In the transfer sheet registration mode, the control section  101  circulates and conveys the transfer sheets loaded on each sheet feed tray without executing image formation in the same procedure as double-side image formation. At this time, at the color detection section  161   s , the color detection results for both sides of each transfer sheet are obtained (Step S 402  shown in  FIG. 4 ). Further, the color detection of the transfer sheets can be executed by the scanner section  120  instead of the color detection section  161   s . And, the transfer sheet color detection results are transferred to the control section  101 . 
     The control section  101  receiving the color detection results displays the transfer sheet selection screen as shown in  FIG. 5  on the display section  103 . 
     On the transfer sheet selection screen shown in  FIG. 5 , the items such as the transfer sheet name ( FIG. 5(   a )), sheet kind ( FIG. 5(   b )), size ( FIG. 5(   c )), weight ( FIG. 5(   d )), color ( FIG. 5(   e )), and color name ( FIG. 5(   f )) are displayed and each item is displayed for each transfer sheet. 
     Here, regarding the size ( FIG. 5(   c )) and color ( FIG. 5(   e )), from the detection results, the control section  101  judges and displays. 
     When the display section  103  permits color display, it executes color display for the color ( FIG. 5(   e )) of each transfer sheet. Further, when one side of each transfer sheet is composed of a plurality of colors, the plurality of colors are displayed clearly ( FIG. 5(   e   1 )). Further, when both sides of each transfer sheet are different in color, the colors of both sides are displayed clearly ( FIG. 5(   e   2 )). 
     Further, regarding the transfer sheet name ( FIG. 5(   a )), sheet kind ( FIG. 5(   b )), weight ( FIG. 5(   d ), and color name ( FIG. 5(   f )), the operator inputs them from the operation input section  102 . 
     Here, in this embodiment, the color names are displayed as shown in  FIG. 5(   f ). Further, the color name may be selected from the selection candidates (not shown) and in the case of two colors on one side or different colors on both sides, a new color name may be inputted. 
     Further, without using the transfer sheet color detected by the color detection section  161   s , the operator may select the transfer sheet color ( FIG. 5(   e )) from the color pallet selection candidates (not shown). When using transfer sheets which are similar to chroma and different in brightness, the operator can correct them in this way. When the setting of each item for the transfer sheets loaded on each sheet feed tray is completed in this way and the operator clicks the icon of “OK” ( FIG. 5(   g )), the control section  101  registers the transfer sheets and preserves the items aforementioned in the information memory section  106  as transfer sheet information or tray information (Step S 403  shown in  FIG. 4) . 
     Next, by referring to the flow chart shown in  FIG. 7 , the execution of image formation will be explained. 
     Here, firstly, the operator sets the job from the operation input section  102  (Step  5701  shown in  FIG. 7 ). 
     Here, the control section  101  displays the job confirmation screen as shown in  FIG. 6  on the display section  103 . On the job confirmation screen, regarding a plurality of pages, for each page No. ( FIG. 6(   a )), the sheet kind of each transfer sheet to be used ( FIG. 6(   b )), size ( FIG. 6(   c )), weight ( FIG. 6(   d )), and transfer sheet color ( FIG. 6(   e )) are listed. 
     Further, the control section  101 , in the detailed display mode, displays the image forming-image display screen for each page as shown in  FIG. 8  on the display section  103 . 
     In the detailed display mode screen shown in  FIG. 8 , for example, every two pages, the image forming-image display screen for each page ( FIGS. 8(   a ) and ( f )) and the sheet kind of the transfer sheet to be used on each page ( FIGS. 8(   b ) and ( g )), transfer sheet size ( FIGS. 8(   c ) and ( h )), transfer sheet weight ( FIGS. 8(   d ) and ( i )) and, transfer sheet color ( FIGS. 8(   e ) and ( j )) are displayed. 
     By referring to this display, the operator confirms the transfer sheets to be used for the job, if necessary, clicks the corresponding icon, and sets such as selection of transfer sheets of a different sheet kind, size, weight, and color (Step S 701  shown in  FIG. 7 ). 
     When the aforementioned job setting by the operator is completed and the Start button is pressed from the operation input section  102 , the control section  101  gives an instruction to the image forming section  140  to start image formation (Step S 702  shown in  FIG. 7 ). 
     During execution of the image formation, the detection results of the color of the transfer sheets detected by the color detection section  161   s  are transferred to the control section  101 . As shown in  FIG. 3 , when using the detection results of the color detection section  161   s  positioned on the upstream side of the image carrier  170  in the conveying direction, immediately before execution of image formation, the color of each transfer sheet is ascertained. 
     Here, the control section  101  displays the job execution confirmation screen as shown in  FIG. 9  on the display section  103  and displays the transfer sheet color detection results on the screen (Step S 703  shown in  FIG. 7 ). 
     In this case, the control section  101 , as a job execution confirmation display screen, displays the set job contents and the image formation of the job executed actually in correspondence with them, as shown in  FIG. 9 , in the comparison state of the left with the right on the screen. 
     Here, the control section  101  displays the job execution confirmation screen as shown in  FIG. 9  on the display  103 . On the set job content display screen ( FIG. 9 ), in one half of the screen, regarding a plurality of pages, for each page No. ( FIG. 9(   a )), the sheet kind of each transfer sheet to be used ( FIG. 9(   b )), size ( FIG. 9(   c )), weight ( FIG. 9(   d )), and transfer sheet color ( FIG. 9(   e )) are listed and for the job executed actually, in the other half of the screen, regarding a plurality of pages, the tray ( FIG. 9(   f )) to which each transfer sheet is outputted, sheet kind of each transfer sheet used ( FIG. 9(   g )), size ( FIG. 9(   h )), weight ( FIG. 9(   i )), and transfer sheet color ( FIG. 9(   j )) are listed. Further, when the detail button ( FIG. 9(   k )) for each page is clicked, more detailed information is displayed. 
     As a result, the image formation is executed using transfer sheets of a desired color designated by the job and whether the designation of the transfer sheet color by the job coincides with the transfer sheet color of the actual image forming output material or not can be confirmed easily by the operator. Further, in this case, the color display of the color itself and also the color name display are executed, so that similar colors different in density and coloring can be distinguished. 
     Further, on the job execution confirmation screen of the display section  103  instructed by the control section  101 , the information on the transfer sheet in execution of image formation at present and the preceding and subsequent transfer sheets thereof is displayed, though when the operator clicks the icon of “Return” or “Go” on the screen, he can confirm the preceding set contents and output contents or pre-confirm the preceding set contents. 
     Further, the control section  101  compares the color of transfer sheets designated by the job with the color of transfer sheets conveyed actually (Step S 704  shown in  FIG. 7 ), when the transfer sheet colors coincide with each other (YES at Step S 704  shown in  FIG. 7 ), controls so as to execute straightly the image formation (Step S 705  shown in  FIG. 7 ), and ejects the image-formed transfer sheets to a predetermined paper receiving tray (Step S 706  shown in  FIG. 7 ). 
     On the other hand, when the transfer sheet colors do not coincide with each other (NO at Step S 704  shown in  FIG. 7 ), the control section  101  controls so as to stop the execution of image formation (Step S 707  shown in  FIG. 7 ), ejects transfer sheets of a different color from the designated color to another predetermined paper receiving tray (Step S 708  shown in  FIG. 7 ), and displays the purport of an occurrence of an error of conveyance of transfer sheets of a color different from the designated color on the display section  103  (Step S 709  shown in  FIG. 7 ). In this case, the control section  101 , on the display section  103 , may display the items of the transfer sheets judged as an error in the blinking state or may use blinking display together with information by a warning sound. 
     As mentioned above, the transfer sheet color designated by the job is compared with the transfer sheet color detected by the color detection section and when inconsistency of colors occurs, the execution of image formation is stopped, so that an image forming output material using a transfer sheet of a color other than a color designated by the job will not be outputted. Further, by doing this, the overall transfer sheets which are not transfer sheets of a different color and are outputted normally until then can be prevented from coming to naught. Namely, from the place where the transfer sheet of a different color is conveyed, the image formation is restarted using normal transfer sheets, thus the waste of transfer sheets can be minimized. 
     Further, in this embodiment, in the case of double-side image formation, the respective colors of the front and rear of a transfer sheet which are designated by the job and the respective colors of the front and rear of a transfer sheet which are detected by the color detection section are displayed on the display section  103 . As a result, it is possible to execute image formation on both sides of a transfer sheet of a desired color designated by the job and confirm whether the designated colors on both sides of the transfer sheet by the job coincides with the colors on both sides of a transfer sheet of an actual image forming output material or not. 
     Further, in the aforementioned embodiment, it is not needed to arrange a sensor on each sheet feed tray of multistage sheet feed trays and the cost of the image forming apparatus can be reduced. 
     Further, in the embodiment aforementioned, the displacement detection section for detecting conveyance of transfer sheets in the displacement state is used also as a color detection section  161   s , so that there is no need to newly install an exclusive detection section and the conventionally existing image forming apparatus can be used as it is. 
     Further, the image forming apparatus, when registering transfer sheets, regarding the color of the transfer sheets, has a function for registering the color name, so that by giving an appropriate name beforehand, similar colors different in density and coloring which are hardly distinguished by the color display of the display section  103  can be distinguished. 
     Further, in the embodiment aforementioned, when registering transfer sheets, regarding the color of the transfer sheets loaded on each sheet feed tray, the detection results of the color detection section  161   s  are used and the input from the operation input section  102  are used, thus the transfer sheets are registered. By doing this, the transfer sheet color can be set meticulously and it is possible to select appropriately transfer sheets loaded on each sheet feed tray and form images in correspondence to the job. 
     The concrete example that the color detecting section  161   s  is arranged on the upstream side of the image carrier  170  and executing image formation is stopped before executing image formation is shown in the embodiment aforementioned. Further, even if the color detecting section  161   s  is arranged on the downstream side of the image carrier, immediately after execution of image formation before the concerned transfer sheet of a different color is ejected, the operation of the image forming apparatus can be stopped or the concerned transfer sheet of a different color can be ejected to a separate predetermined paper receiving tray, thus an image forming output material using a transfer sheet of a color other than a color designated by the job will not be outputted. By doing this, the overall transfer sheets at the same section which are not transfer sheets of a different color and are outputted normally until then can be prevented from coming to naught. Namely, from the place where the transfer sheet of a different color is conveyed, the image formation is restarted using normal transfer sheets, thus the waste of transfer sheets can be minimized to only one sheet. 
     Further, in the embodiment aforementioned, the concrete example that the color detecting section  161   s  is arranged on the upstream side of the image carrier  170  and is arranged between the registration rollers  161   b  and the transfer-preceding rollers  161   c  is indicated, though the present invention is not limited to this position. The position may be set between the position where the path of the transfer sheets from each sheet feed tray and the path the transfer sheets circulating for double-side image formation meet and the position before the distribution section for distributing ejection sheets to each paper receiving tray. 
     Further, to detect the case that both sides are different in color, for the rears of transfer sheets which are circulated and re-fed for front and double-side image formation of transfer sheets fed from each sheet feed tray, the color may be sequentially detected by the color detection section  161   s , though as shown in  FIG. 10 , using a color detection section  161   sa  for detecting the color of the front of each transfer sheet and a color detection section  161   sb  for detecting the color of the rear of each transfer sheet, the colors of both sides of each transfer sheet may be detected at the same time. 
     Further, the color detection sections  161   sa  and  161   sb  may be separate sensors, though they may be integrally formed in a shape that the intermediate parts thereof hold transfer sheets and permit them to pass through. 
     The colors of both sides of each transfer sheet are detected simultaneously as mentioned above, thus when the color of the front coincides with the set color, while the color of the rear is different from the set color, before executing the image formation of the front, it is possible to find a color difference, thereby stop the image forming operation. 
     Further, in the embodiment aforementioned, the color is not limited to the ordinary colors in the visible ray zone and may include fluorescence generated by irradiation of specific excitation light and may be detected and controlled. Further, in the embodiment aforementioned, as a color difference, the color may be detected and controlled so as to include existence of gloss.