JP5743861B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a document reading unit that reads a document conveyed by a document feeding unit such as an auto document feeder (ADF).

  When reading a document transported by the document feeder, it is necessary to detect the leading edge of the transported document. For detecting the leading edge of the document, the document can be detected without providing a dedicated paper detection sensor. It has been proposed to use an image reading sensor for reading images (see, for example, Patent Documents 1 and 2).

  In the document reading device disclosed in Patent Document 1, the document leading edge is detected by detecting the shadow of the document leading edge conveyed to the reference roller using an image reading sensor. Further, in the scanner device disclosed in Patent Document 2, an identification pattern is formed in a portion opposite to a reading portion by an image reading sensor, and an original is generated when an image signal output by the image reading sensor changes from that showing the identification pattern. The tip of the is detected.

Japanese Patent Laid-Open No. 2000-174988 JP 2008-245209 A

  However, the prior art of Patent Document 1 requires an extra configuration (illumination) for forming a shadow at the leading edge of the document, and if the document is thin, the shadow is not clearly formed, and the document leading edge is not accurately formed. There was a problem that it could not be detected. Further, in the prior art of Patent Document 2, there is a problem in that show-through occurs although an extra configuration is not required only by forming an identification pattern at a portion facing the reading portion by the image reading sensor. . In other words, white is generally used for the part facing the reading part by the image reading sensor in order to prevent the show-through of the image, but when the identification pattern is arranged in the image reading range, the identification pattern is transparent. There was a problem that show-through occurred. In particular, when the original is thin and the background color of the original is white, the show-through occurs significantly.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the prior art and reliably detecting the leading edge of a document without causing show-through.

The image forming apparatus of the present invention conveys the document by the document feeding unit at a position facing the document reading window, the irradiating light to the original document reading on the window, before SL document through a color separation filter Is received by the line sensor, and the color component transmitted through the color separation filter of the document is read in synchronism with the conveying operation by the document feeding unit based on the image signal output from the line sensor. The line sensor includes a red line sensor that receives reflected light of the red component from the original in the reading line X via a color separation filter R that transmits the red component, and a color that transmits the green component. A green line sensor that receives reflected light of the green component from the original on the reading line Y through the separation filter G and transmits the blue component. Is composed of a blue line sensor for receiving the reflected light of the blue component from the document in the reading line Z through the separation filter B, the document reading across the conveyor path in which the document is conveyed by the document feeding unit It includes windows and arranged to face white plate, a front reached before SL document based on the image signal output change from the line sensor with the time arrival, and a document tip detecting means for detecting the original tip On the white plate, the color identification mark blocked by the color separation filter R on the reading line X and the color identification mark blocked by the color separation filter G on the reading line Y are read. An identification mark of a color blocked by the color separation filter B is formed on the line Z, and an image signal output from the line sensor is Only the latest two lines of image signals are overwritten and retained in the same memory area until the document leading edge detection unit starts detecting the document feeding operation by the document feeding section. The document leading edge detection means detects the document leading edge when the image signal output changes by a predetermined threshold or more by comparing the held image signals for the latest two lines. It is characterized by that .
Et al of the image forming apparatus of the present invention, the original tip detecting means, when a change of the image signal output from the line sensor is not detected for a predetermined preset time, so as to detect a paper jam error Anyway .
Et al of the image forming apparatus of the present invention, the original tip detecting means, by comparing the timing of the image signal output in the light receiving elements in the main scanning direction of two different points is changed, the inclination of the document You may make it detect.
Furthermore, the image forming apparatus of the present invention further includes a light guide path from the document placing portion of the document feeding portion to the light receiving element outside the document reading range of the line sensor, and the document leading edge detecting means is configured to read the document. The presence or absence of the document may be detected based on the image signal output of the light receiving element outside the range .

  According to the present invention, the identification mark for detecting the leading edge of the document formed on the reading line by the line sensor is set to a color that is blocked by the color separation filter, so that the document leading edge does not occur. There is an effect that can be reliably detected.

1 is a schematic cross-sectional view showing an internal configuration of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a schematic schematic cross-sectional view illustrating an internal configuration of a document reading unit and a document feeding unit illustrated in FIG. 1. FIG. 3 is a front view showing the configuration of the CCD shown in FIG. 2. It is a front view which shows the structure of the white board shown in FIG. FIG. 2 is a block diagram illustrating a schematic configuration of the image forming apparatus illustrated in FIG. 1. 6 is a flowchart for explaining a document leading edge detection operation in the embodiment of the image forming apparatus according to the present invention. It is a figure which shows the example of an image signal output of the red line sensor shown in FIG. It is a figure which shows the image signal output example of the red line sensor shown in FIG. 3, a green line sensor, and a blue line sensor.

Next, embodiments of the present invention will be specifically described with reference to the drawings.
An image forming apparatus 1 according to the present embodiment is a copying machine. Referring to FIG. 1, a document reading unit 2, a document feeding unit 3, a body unit 4, a stack tray 5, and an operation unit 6 are provided. I have. The document reading unit 2 is disposed on the top of the main body unit 4, and the document feeding unit 3 is disposed on the top of the document reading unit 2. The stack tray 5 is disposed on the recording paper discharge port 41 side of the main body unit 4, and the operation unit 6 is disposed on the front side of the image forming apparatus 1. The document feeding unit 3 is configured to be retractable, and the upper surface of the document reading unit 2 can be opened by opening the document feeding unit 3 upward.

  The operation unit 6 is provided with a liquid crystal display unit 61 and operation buttons 62. The user operates the operation unit 6 to input an instruction, thereby performing various settings of the image forming apparatus 1 and executing various functions such as image formation. The liquid crystal display unit 61 displays the state of the image forming apparatus 1, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. As the operation button 62, a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, etc., and used for setting various settings of the image forming apparatus 1 to a default state. A reset button, a numeric keypad and the like are provided.

  The document reading unit 2 includes a platen glass 21, a document reading slit 22, a shading plate 23 for determining a white reference, and a scanner unit 24 that optically reads an image of the document and outputs an image signal. . The platen glass 21 is a document table which is made of a transparent member such as glass and is disposed on the upper surface of the document reading unit 2. The document reading slit 22 is a slit-shaped document reading window formed of a transparent member such as glass and formed in a direction orthogonal to the document transport direction by the document feeding unit 3.

  Referring to FIG. 2, the scanner unit 24 includes a light source 241, a first mirror 242, a second mirror 243, a third mirror 244, a first carriage 245, a second carriage 246, and an imaging lens 247. And a CCD (Charge Coupled Device) 248 which is an image reading sensor for reading a document. The light source 241 and the first mirror 242 are supported by the first carriage 245, and the second mirror 243 and the third mirror 244 are supported by the second carriage 246. Referring to FIG. 3, the CCD 248 includes a red line sensor 248R, a green line sensor 248G, and a blue line sensor 248B, which are formed as light receiving element arrays for three lines corresponding to the three primary colors RGB. In the red line sensor 248R, a color separation filter R that transmits the red component is formed, and only the red component is selectively received. In the green line sensor 248G, a color separation filter G that transmits the green component is formed, and only the green component is selectively received. In the blue line sensor 248B, a color separation filter B that transmits the blue component is formed, and only the blue component is selectively received.

  In the original reading operation by the scanner unit 24, the light source 241 irradiates the original on the platen glass 21 or the original reading slit 22, and the CCD 248 receives reflected light from the original. The reflected light for three lines in the main scanning direction from the original is reflected in the order of the first mirror 242, the second mirror 243, and the third mirror 244, enters the imaging lens 247, and enters the imaging lens 247. The reflected light for the line is imaged on the red line sensor 248R, the green line sensor 248G, and the blue line sensor 248B of the CCD 248, respectively. The CCD 248 outputs image signals for three lines of RGB to the main body unit 4.

  The document feeding unit 3 is an ADF (Auto Document Feeder), and includes a document placing unit 31, a document discharge unit 32, and a document transport mechanism 33 as illustrated in FIG. 2. The originals placed on the original placement unit 31 are sequentially fed out one by one by the original conveyance mechanism 33, conveyed to a position facing the original reading slit 22, and then discharged to the original discharge unit 32.

  The document transport mechanism 33 includes a document feed roller 331, document transport rollers 332 and 333, a guide unit 334, a document discharge roller 335, and a white plate 336. The document feeding roller 331 sequentially feeds the documents set on the document placement unit 31 one by one, and the fed documents are conveyed to the position of the document reading slit 22 via the document conveying rollers 332 and 333. After being read at the position of the document reading slit 22, the document is discharged to the document discharge section 32 by the document discharge roller 335. The white plate 336 is attached to the surface of the guide portion 334 facing the document reading slit 22. In other words, the white plate 336 is disposed to face the document reading slit 22 across the transport path for transporting the document.

  Referring to FIG. 4A, a pair of blue identification marks 336B is formed on the reading line X by the red line sensor 248R, and the reflected light is green. A pair of red identification marks 336R is imaged on the line sensor 248G, and a pair of red identification marks 336R is formed on the read line Y by the green line sensor 248G, and a pair of red light is formed on the read line Z by the blue line sensor 248B. Green identification marks 336G are respectively formed. Identification marks 336B, 336R, and 336G are arranged near both ends of the document reading range, respectively. Here, as shown in FIG. 4B, the identification marks 336R, 336G, and 336B are arranged so that each of the reading lines X, In Y and Z, they are arranged at positions where the leading edge of the document crosses. Hereinafter, in the identification marks 336R, 336G, and 336B, the respective areas in the main scanning direction where B is formed are referred to as a right mark area and a left mark area. In addition, areas other than the right and left mark areas are white areas where the white plate 336 is exposed as it is.

  The identification mark (blue identification mark 336B) formed on the reading line X may be any color that is blocked by the color separation filter R formed on the red line sensor 248R, for example, a blue identification mark 336B. Instead, a green identification mark 336G may be formed. Further, the identification mark (red identification mark 336R) formed on the reading line Y may be any color that is blocked by the color separation filter G formed on the green line sensor 248G. For example, the red identification mark 336R Alternatively, a blue identification mark 336B may be formed. Further, the identification mark (green identification mark 336G) formed on the reading line Z may be any color that is blocked by the color separation filter B formed on the blue line sensor 248B, for example, a green identification mark 336G. Alternatively, a red identification mark 336R may be formed. Furthermore, in the present embodiment, the pair of identification marks 336R, 336G, and 336B is formed. However, when different document sizes are read, the pair of identification marks 336R, 336G, and 336B has each document size (B5 to A4). Formed in the vicinity of both ends.

  When reading of the document is instructed by the operation button 62 of the operation unit 6 in a state where the document is not placed on the document placement unit 31, the flat reading mode for reading the document placed on the platen glass 21 is set, and the platen A document placed on the glass 21 is read. In the flat reading mode, the first carriage 245 is moved to a position facing the platen glass 21, the original is read while moving the first carriage 245 and the second carriage 246 in the sub-scanning direction orthogonal to the main scanning direction, and the image signal Is output to the main unit 4.

  When reading of a document is instructed by the operation button 62 of the operation unit 6 with the document placed on the document placing unit 31, the document conveyed by the document feeding unit 3 is moved to the position of the document reading slit 22. The ADF reading mode is read, and the original conveyed by the original feeding unit 3 is read. In the ADF reading mode, the first carriage 245 is moved to a position facing the original reading slit 22, and the original is read via the original reading slit 22 in synchronization with the conveying operation by the original feeding unit 3, and the image signal is read from the main body. 4 is output.

  Referring to FIG. 1, the main body unit 4 includes a recording unit 7, and includes a paper feeding unit 42, a paper conveyance path 43, a conveyance roller 44, and a discharge roller 45. The paper feed unit 42 includes a plurality of paper feed cassettes 42a1 to 42an and paper feed rollers 42b1 to 42bn that feed recording sheets from the paper feed cassettes 42a1 to 42an one by one to the paper transport path 43. The paper feed rollers 42b1 to 42bn, the transport roller 44, and the discharge roller 45 function as a transport unit, and the recording paper is transported. The recording paper fed to the paper transport path 43 by the paper feed rollers 42 b 1 to 42 bn is transported to the recording unit 7 by the transport roller 44. Then, the recording paper on which recording is performed by the recording unit 7 is discharged to the stack tray 5 by the discharge roller 45.

  The recording unit 7 includes a photosensitive drum 71, an exposure unit 72, an image forming unit 73, a transfer unit 74, and a fixing unit 75. The exposure unit 72 is an optical unit including a laser device, a mirror, and the like, and outputs a laser beam based on image data to expose the photosensitive drum 71 to form an electrostatic latent image on the surface of the photosensitive drum 71. To do. The image forming unit 73 is a developing unit that develops the electrostatic latent image formed on the photosensitive drum 71 using toner, and forms a toner image based on the electrostatic latent image on the photosensitive drum 71. The transfer unit 74 transfers the toner image formed on the photosensitive drum 71 by the image forming unit 73 to a recording sheet. The fixing unit 75 heats the recording paper on which the toner image is transferred by the transfer unit 74 to fix the toner image on the recording paper.

  FIG. 5 is a block diagram illustrating a schematic configuration of the image forming apparatus 1. The document reading unit 2, the document feeding unit 3, the transport unit (feed rollers 42 b 1 to 42 bn, the transport roller 44, the discharge roller 45), the operation unit 6, and the recording unit 7 are connected to the control unit 8. The operation is controlled by 8. In addition, an image processing unit 9 and a storage unit 10 are connected to the control unit 8.

  The control unit 8 is an information processing unit such as a microcomputer including a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The ROM stores a control program for controlling the operation of the image forming apparatus 1. The control unit 8 reads out a control program stored in the ROM and develops the control program in the RAM, thereby controlling the entire apparatus according to predetermined instruction information input from the operation unit 6. The control unit 8 detects the leading end of the document based on the image signal from the document reading unit 2 and stores the subsequent image signal in the storage unit 10 as image data.

  The image processing unit 9 is means for performing predetermined image processing on the image data. For example, image improvement processing such as enlargement / reduction processing, gradation adjustment, and density adjustment is performed.

  The storage unit 10 is a storage unit such as a semiconductor memory or an HDD (Hard Disk Drive), and stores image data of a document read by the document reading unit 2 and various management information.

Next, the document leading edge detection operation in the image forming apparatus 1 of the present embodiment will be described in detail with reference to FIGS.
The document leading edge detection operation in the image forming apparatus 1 is an operation for detecting the reading start timing when the document conveyed by the document feeding unit 3 is read, and is executed in the ADF reading mode.

  Referring to FIG. 6, when reading of the document is instructed by the operation button 62 of the operation unit 6 (Step A <b> 1), the control unit 8 determines whether or not the document is placed on the document placement unit 31. (Step A2). If no document is placed on the document placement unit 31 in step A2, the control unit 8 sets the flat reading mode and ends the document leading edge detection operation without executing the subsequent steps. . In step A <b> 2, the control unit 8 determines whether a document is placed on the document placement unit 31 based on the image signal output from the CDD 248. As shown in FIG. 2, apart from the light guide A for reading the document, a light guide B extending from the document placing portion 31 to the CCD 248 is provided. The light guide path B extends from an opening (not shown) provided in the document placement portion 31 to the region 25 outside the document reading range of the CCD 248 (in this embodiment, the green line sensor 248G), and the light receiving element included in the region 25 Whether or not a document is placed on the document placement portion 31 is determined based on the output of the image signal.

  If a document is placed on the document placement unit 31 in step A2, the control unit 8 sets the ADF reading mode to start the conveyance of the document by the document feeding unit 3, and the document reading unit. Document reading by the second scanner unit 24 is started (step A3). FIG. 7A shows an image signal output of the red line sensor 248R in a state where the original does not reach the reading line X. In this case, the reflected light that does not include the red component from the blue identification mark 336B in the left and right mark regions is blocked by the color identification filter of the red line sensor 248R, and the red component is included in the white region. Since the reflected light is received by the red line sensor 248R, the left and right mark areas where the blue identification marks 336B are formed have lower image signal outputs than the other white areas. An image signal output of the green line sensor 248G when the document does not reach the reading line Y, and an image signal output of the blue line sensor 248B when the document does not reach the reading line Z, The signal waveform is substantially the same as in FIG. 7A, and the image signal output is lower in the left and right mark areas than in other areas formed in other white.

  Next, the control unit 8 passes the preset time T (step A4), changes in the image signal output in the left and right mark areas (step A5), and changes in the image signal output in areas other than the left and right mark areas (step A5). A6) is monitored. The time T set in advance in step A4 has elapsed without detecting any change in the image signal output in the left and right mark areas in step A5 and no change in the image signal output outside the left and right mark areas in step A6. If so, the control unit 8 detects a paper jam error (step A7). Next, the control unit 8 displays a notification screen informing the paper jam on the liquid crystal display unit 61 of the operation unit 6, notifies the user of a paper jam error (step A8), and ends the document leading edge detection operation. In the present embodiment, the control unit 8 overwrites and saves the image signal output from the scanner unit 24 of the document reading unit 2 as the image data in the same area of the RAM, and the latest two lines of images. It is configured to hold only the signal. Therefore, the memory area can be effectively used.

Before the time T set in advance in step A4 elapses, either a change in the image signal output in the left or right mark area in step A5 or a change in the image signal output in the area other than the left or right mark area is detected in step A6. If it has been done, the control unit 8 detects the leading edge of the document at the timing when the output of the image signal changes (step A9), and ends the document leading edge detection operation. Change of the image signal output at step A5 and step A6 is to compare the latest image signals of two lines is performed held, when the image signal output is changed over a preset threshold, the original The leading end portion is detected, and the image data is stored in the storage unit 10 from the line where the change in the image signal output is detected.

  FIG. 7B shows an image signal output of the red line sensor 248R in a state where a document of a background color including red components such as white and red has reached the reading line X. In this case, the blue identification mark 336B is blocked by the document whose background color includes the red component, and the reflected light of the red component is received by the red line sensor 248R in the left and right mark areas. Accordingly, when the original reaches the reading line X, the image signal output in the left and right mark areas becomes high in step A5, and a change in the image signal output is detected.

  FIG. 7C shows an image signal output of the red line sensor 248R in a state where a background color original that does not include a red component such as green or blue has reached the reading line X. In this case, the white region other than the blue identification mark 336B is blocked by the document whose background color does not include the red component, and the reflected light of the red component is not received by the red line sensor 248R except in the left and right mark regions. . Therefore, when the original reaches the reading line X, the output of the image signal in the areas other than the left and right mark areas becomes low in step A6, and a change in the image signal output is detected.

  In FIG. 8 (a), the background color is white, and the portion facing the left and right mark areas where the blue identification mark 336B is formed is colored in a color that does not contain a green or blue red component. This is an image signal output of the red line sensor 248R in a state where a special document reaches the reading line X. In this case, even if the blue identification mark 336B is blocked by the document in the left and right mark areas, the red component reflected light is not received by the red line sensor 248R. Therefore, even if the document reaches the reading line X, the image signal output in the left and right mark areas does not change, and the leading edge of the document cannot be detected. However, if the position facing the left and right mark areas of the document is green, when the document reaches the reading line Y, as shown in FIG. Becomes higher, a change in the image signal output is detected, and the leading edge of the document can be detected. If the portion facing the left and right mark areas of the document is blue, when the document reaches the reading line Z, as shown in FIG. Becomes higher, a change in the image signal output is detected, and the leading edge of the document can be detected. Therefore, in this embodiment, the change in the image signal output in step A5 and step A6 is not only in the image signal output from the red line sensor 248R but also in the image signal output from the green line sensor 248G and the blue line sensor 248B. Configured to detect.

  In this embodiment, the leading edge of the document is detected using a CCD 248 that is a line sensor. Therefore, by comparing the timing at which the image signal output changes at two different locations in the main scanning direction, it is possible to detect the inclination of the document and correct the image data. It is preferable that the two places where the timing at which the image signal output changes are compared are separated. Therefore, as shown in FIG. 7B, when a change in the image signal output of the left and right mark areas is detected in step A5, the left and right mark areas are respectively formed near both ends of the document reading range. It is preferable to compare the timings at which the image signal outputs of the light receiving elements change in the left and right mark areas. Further, as shown in FIG. 7B, when a change in the image signal output other than the left and right mark areas is detected in step A6, the light reception located between the both ends of the document reading range and the left and right mark areas, respectively. It is preferable to compare the timing at which the image signal output of the element changes.

  As described above, in the present embodiment, the original conveyed by the original feeding unit 3 is irradiated with light, and the reflected light from the original on the reading line X via the color separation filter R is reflected on the red line sensor 248R. The image forming apparatus 1 that reads the color component that passes through the color separation filter R of the document based on the image signal output from the red line sensor 248R, and that transports the document on the reading line X. The white plate 336 is provided with a color identification mark 336B disposed opposite to each other and blocked by the color separation filter R, and the red line sensor 248R before and when the document reaches the reading line X is provided. The leading edge of the document is detected based on the change in the image signal output. As a result, since the identification mark 336B formed on the reading line X by the red line sensor 248R for detecting the leading edge of the document is a color blocked by the color separation filter R, the document leading edge does not occur. The part can be detected reliably.

  Further, in the present embodiment, the green line sensor 248G that receives the reflected light of the green component from the original in the reading line Y through the color separation filter G that transmits the green component, and the color separation that transmits the blue component. A blue line sensor 248B that receives reflected light of a blue component from the original on the reading line Z through the filter B, and a color identification mark 336R that is blocked by the color separation filter G on the reading line Y on the white plate. And an identification mark 336G of a color blocked by the color separation filter B is formed on the reading line Z, respectively, and the document is based on changes in image signal output from the red line sensor 248R, the green line sensor 248G, and the blue line sensor 248B. It is comprised so that a front-end | tip part may be detected. Thus, a special manuscript whose background color is white and where the blue identification mark 336B is opposed to the left and right mark areas is colored in a color that does not include green or blue red components. Even in such a case, the leading edge of the document can be reliably detected.

  Note that the present invention is not limited to the above-described embodiments, and it is obvious that the embodiments can be appropriately changed within the scope of the technical idea of the present invention. In addition, the number, position, shape, and the like of the constituent members are not limited to the above-described embodiment, and can be set to a suitable number, position, shape, and the like in practicing the present invention. In each figure, the same numerals are given to the same component.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Original reading part 3 Original feeding part 4 Main body part 5 Stack tray 6 Operation part 7 Recording part 8 Control part 9 Image processing part 10 Storage part 21 Platen glass 22 Original reading slit 23 Shading plate 24 Scanner part 31 Original Placement section 32 Document discharge section 33 Document transport mechanism 41 Discharge port 42 Paper feed section 42a1-42an Paper feed cassette 42b1-42bn Paper feed roller 43 Paper transport path 44 Transport roller 45 Discharge roller 61 Liquid crystal display section 62 Operation button 71 Photoconductor Drum 72 Exposure unit 73 Image forming unit 74 Transfer unit 75 Fixing unit 80 Foreign matter 241 Light source 242 First mirror 243 Second mirror 244 Third mirror 245 First carriage 246 Second carriage 247 Imaging lens 248 CCD
248R Red line sensor 248G Green line sensor 248B Blue line sensor 331 Document feed roller 332, 333 Document transport roller 334 Guide section 335 Document discharge roller 336 White plate 336R, 336G, 336B Identification mark

Claims (4)

Conveying the document by the document feeding unit at a position facing the document reading window, the irradiating light to the original document reading on the window, receives reflected light from the pre-SL document by the line sensor through a color separation filter An image forming apparatus that reads a color component transmitted through the color separation filter of the document based on an image signal output from the line sensor in synchronization with a transport operation by the document feeding unit ,
The line sensor includes a red line sensor that receives reflected light of the red component from the original on the reading line X via a color separation filter R that transmits the red component, and a color separation filter G that transmits the green component. Blue that receives the reflected light of the blue component from the original in the reading line Z via the green line sensor that receives the reflected light of the green component from the original in the reading line Y and the color separation filter B that transmits the blue component. It consists of a line sensor and
A white plate disposed opposite to the original reading window across a conveyance path through which the original is conveyed by the original feeding unit ;
Based on the change in the image signal output from the line sensor of the previous arrival before SL document and upon reaching, comprising a document tip detecting means for detecting the original tip,
The white plate has a color identification mark blocked by the color separation filter R on the reading line X, and a color identification mark blocked by the color separation filter G on the reading line Y. Each having a color identification mark blocked by the color separation filter B,
The image signal output from the line sensor is started simultaneously with the start of the sheet feeding operation by the document feeder, and the latest two lines are stored in the same memory area until the document leading edge detection unit detects the document leading edge. Only the image signal of the minute is overwritten and retained
The document leading edge detection means detects the document leading edge when the image signal output changes by a predetermined threshold or more by comparing the latest two lines of image signals held. An image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the document leading edge detection unit detects a paper jam error when a change in the image signal output from the line sensor is not detected for a predetermined time. .   3. The document leading edge detection unit detects the inclination of the document by comparing timings at which the image signal output changes at two light receiving elements having different main scanning directions. The image forming apparatus described. A light guide path extending from the document placement portion of the document feeder to the light receiving element outside the document reading range of the line sensor;
4. The image forming apparatus according to claim 1, wherein the document leading edge detection unit detects the presence or absence of the document based on an image signal output of a light receiving element outside the document reading range. 5.

Images