JP2003250028A - Digital image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a digital image forming device configured to have excellent operability and minimize a time required for automatically detecting an original size to the utmost. <P>SOLUTION: The digital image forming device configured such that a scanner unit scans an original placed on a platen, an image sensor receives a light reflected in the original, and write processing is applied to an image storage medium on the basis of image data after analog/digital conversion, is characterized in that an exposure lamp is lighted by using as a trigger a detection signal of a first detection means for detecting a midway of a closing operation of a platen cover placed in an opening/closing enabled way, move the scanner unit, the image sensor reads lights by a plurality of lines of the original reflected in the original to detect the original size in the main scanning direction and the original size in the subscanning direction through an output of a photosensor. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image forming apparatus such as a copying machine or a printer, and more particularly to a digital image forming apparatus having an improved structure for detecting the size of a document placed on a platen.

[0002]

2. Description of the Related Art Conventionally, a scanner unit including an exposure lamp scans and exposes an original placed on a platen which is an original placing table, and the reflected exposure light from the original is line C.
After the image is formed on the CD, photoelectric conversion is performed, and after analog processing, A
After D / D conversion and appropriate image processing such as shading correction and γ correction, image writing using a laser light source is performed on an image carrier made of a photoconductor according to the image data, and a transfer material is formed by an electrophotographic method. The digital image forming apparatus configured to obtain the toner image thereon is configured to automatically detect the size of the original document and automatically select and feed the transfer material having a size suitable for the original size.

The structure of the automatic document size detection is as follows.
For example, the width size of the document (document size in the length direction of the exposure lamp, that is, the document size in the main scanning direction) is determined from the output information from the line CCD, and the document length size is determined from the output information from the separately provided photosensor. A configuration has been proposed for detecting (the document size in the scanning direction by the exposure lamp, that is, the sub-scanning direction).

Such an automatic document size detecting technique improves the operability of the image forming apparatus and is a useful technique. On the other hand, on the other hand, the document is placed on the platen,
After the document is covered with the platen cover, it is provided on the operation panel, for example, since the exposure lamp is turned on by pressing the copy button, the operation feels a little annoying, and at the same time, it takes a little time. Even so, it gave the user a feeling that it took too long before the original copy was made.

Therefore, in order to eliminate the troublesomeness in the operation and to waste more time, it is conceivable that the exposure lamp is turned on during the operation of closing the platen cover. In this case, the exposure lamp is used for exposure. If the corners of the document serving as the area are broken, the output information from the line CCD that picks up the reflected light from the document becomes impossible size information, and as a result, there is a problem that the image formation control is hindered. Was.

Further, the form of the original is, for example, graph paper, or there is a black line or the like in the design drawing in the area exposed by the exposure lamp, and reading by the line CCD is just a ruled line or a black line. If it is a portion, it cannot be determined by the control means whether or not it is an image, and there is a concern that the image formation control may be hindered as described above.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and its main object is to have a good operability and to reduce the time spent for automatic document size detection as much as possible. It is to provide a digital image forming apparatus.

Another object of the present invention is to provide a digital image forming apparatus having a structure capable of accurately detecting the size of an original without being substantially affected by the corner of the original or the image of the original such as graph paper. Especially.

[0009]

The objects of the present invention can be achieved by the following constitutional requirements.

(1) A document placed on the platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the document is received by an image sensor, and based on image data after A / D conversion. In the digital image forming apparatus configured to perform writing processing on the image carrier after being charged, the exposure lamp is triggered by the detection signal of the first detection unit that detects the closing operation of the platen cover that can be opened and closed. While the light is turned on, the scanner unit is moved, the reflected exposure light for a plurality of lines of the document is read by the image sensor to detect the document size in the main scanning direction, and the document size in the sub-scanning direction is detected by the photo sensor. A digital image forming apparatus characterized in that it is detected through an output.

(2) The digital image forming apparatus as described in (1) above, wherein the lighting time of the exposure lamp at the time of detecting the document size is variable.

(3) After the document size detection operation,
The digital image forming apparatus according to (1), wherein the original size detection operation in response to the detection signal from the first detection unit again within a certain period of time is prohibited.

(4) A document placed on the platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the document is received by an image sensor, and based on image data after A / D conversion. In the digital image forming apparatus configured to perform writing processing on the charged image carrier, an automatic document feeder that can be opened and closed and a platen cover that can be opened and closed independently of the automatic document feeder. The exposure lamp is turned on by using the detection signal of the first detection unit that detects the closing operation of the platen cover as a trigger, and the scanner unit is moved in a predetermined direction in the sub-scanning direction to scan the original document. The plurality of lines are read by the image sensor, and the completion of the subsequent closing operation of the automatic document feeder is detected by the second detecting means. The exposure lamp is turned on again by using the detection signal of the detection means as a trigger, and the scanner unit is moved in a direction opposite to the moving direction to read a plurality of lines of the document by the image sensor, and the image sensor. A digital image forming apparatus characterized in that the document size in the main scanning direction is determined from the two pieces of information read by, and the document size in the sub scanning direction is detected via a photo sensor.

(5) The digital image forming apparatus as described in (4) above, wherein the lighting time of the exposure lamp when the document size is detected is variable.

(6) A document placed on the platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the document is received by an image sensor, and based on image data after A / D conversion. In a digital image forming apparatus configured to perform a writing process with a laser beam on an image carrier after charging, an automatic density setting function is provided, and reflected exposure light from a document at the time of scanning for automatic density setting is reflected by the document. The light is received by the image sensor, the amount of laser light is determined by calculation by the calculation means, and the document size in the main scanning direction is detected from the light reception information by the image sensor.
A digital image forming apparatus characterized by the above.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION A digital image forming apparatus according to an embodiment of the present invention will be described below with reference to FIG.

In FIG. 1, reference numeral 1 denotes an entire digital image forming apparatus (hereinafter, simply referred to as an image forming apparatus or apparatus). The image forming apparatus 1 is an automatic document feeder (commonly called A).
DF) A, a document image reading section B for reading an image of a document conveyed by the automatic document conveying device A,
An image control board C for processing the read original image, a writing device D for performing a writing process on an image carrier (described later) based on the image data subjected to the image processing, a toner image is formed on the image carrier, Further, it has an image forming section E by an electrophotographic system for transferring the toner image onto a transfer material, and a storage section F for storing a transfer material P made of paper such as plain paper.

The automatic document feeder A is attached to the upper part of the apparatus so as to be openable and closable, and can be opened and closed with the back side of the figure as a fulcrum.

The automatic document feeder A includes a document stacking table 1
00, a roller group including the roller R1, and a document transport processing unit 110 including a switching unit (no reference symbol) for appropriately switching the document moving path.

Reference numeral 120 denotes a platen cover for covering the area on the platen G. The platen cover 120 is formed by integrally attaching a rubber plate to a resin frame, and like the automatic document feeder A, is independent with the back side of the drawing as a fulcrum. Can be opened and closed.

Since the automatic document feeder A and the platen cover 120 are in a state in which the former overlaps the latter, in the opening operation, first the automatic document feeder A is opened and then the platen cover 120 is opened. , The closing operation is performed in the reverse order.

The upper part of the platen cover 120 functions as a paper discharge tray for documents processed by the automatic document feeder A.

The automatic document feeder A and the platen cover 120 may be integrally opened and closed as in the conventional apparatus.

In the case where both are integrally formed, this structure (structure) is included in the concept of a platen cover in view of the relationship with the present invention.

That is, as will be understood from the following description, one invention relating to the present application is not affected by the existence of the automatic document feeder, but the function of the platen cover,
In other words, it is indispensable to cover the document placed on the platen and to provide a means capable of applying a predetermined pressing force, and the function of the automatic document feeder is not required. On the contrary, the structure itself obviously has a function as a platen cover.

Further, as described above, the automatic document feeder A
The platen cover 120 and the platen cover 120 are configured separately from each other.
This is to meet the demand for multi-functionality by using the automatic document feeder. At the same time, in a mode in which an original is placed on the platen and an image is formed, the automatic original feeder having a large weight is used for placing the original on the platen. This is to reduce the burden on the user and improve the operability by adopting a mode in which only the lightweight platen cover is operated rather than operating each time.

That is, most of the originals placed on the platen are sheets, and the originals can be satisfactorily pressed onto the platen by the mass of the platen cover.

If the original placed on the platen is thick like a book, the central part of the spread may float more than necessary due to the weight of the platen cover alone. With the automatic document feeder also closed, its mass can be used as a pressing means, and the operability can be further improved.

The document image reading section B is provided below the automatic document feeder A and the platen G, and reciprocates in the left-right direction (sub-scanning direction) in the figure while maintaining the optical path length. A first mirror unit 200 capable of scanning a document placed on the platen G, a second mirror unit 210, and the first mirror unit 200.
Exposure lamp L which is an exposure means for a document provided integrally with
A moving optical system (which is referred to as a scanner unit) 250 including an image forming lens (hereinafter, simply referred to as a lens) 2
Image sensor 2 consisting of 20 and line CCD
30 etc. are provided.

When the original function of the automatic document feeder A is used and an image is read from a document fed from the document stacking table 100, the exposure lamp is passed through a slit provided below the roller R1. L is capable of exposing a document, and reflected exposure light from the document is made incident on a mirror on the first mirror unit 200 and finally imaged on the image sensor 230. Each mirror unit Are kept in a fixed position.

The image control board C performs analog / digital (A / D) conversion of the analog signal of the document photoelectrically converted by the image sensor 230 and then performs image processing such as shading correction and γ correction. Image data is prepared by using the writing device D, and finally, dot exposure (writing) for image formation is performed via the writing device D.

The writing device D comprises a semiconductor laser 300 as a light source for writing, a rotating polygon mirror 310 as a light deflecting means, and the like.

In the image forming section E, an image carrier (hereinafter, simply referred to as a photoconductor) 400 including a photoconductor drum, a charging electrode 410, a magnetic brush type developing device 420, a transfer electrode 430, a separation electrode 440, and a cleaning. Means 450 and the like are additionally provided, and the writing device D is added to these to form an image forming means.

The storage section F has a two-stage paper feed tray 50.
0 and 550, and sheet feeding means 510 and 555 provided for each of these sheet feeding trays are provided.

Reference numeral S is a movable plate provided in the paper feed tray, and its tip portion is raised in accordance with the decrease of the transfer material P, and the regulating means for bringing the position of the uppermost transfer material P to a predetermined position. Function as one element of.

In the moving direction of the transfer material P, a plurality of conveying roller pairs (no reference numeral) for forming a conveying path for the transfer material P are provided on the downstream side of the paper feed rollers 510 and 555.

R10 shown at a position on the front side of the transfer electrode 430 is a registration roller, and a roller pair shown downstream of the separation electrode 440 is a fixing roller H constituting a fixing device. At least, an unfixed toner image after transfer is formed. The roller on the side opposite to the surface of the transfer material P having a heat source has a heating source such as a halogen lamp.

Further, Z is a cleaning means for the fixing roller H, and has a cleaning web provided as a winding element as a main element.

Reference numeral 600 denotes a path switching member provided downstream of the fixing roller. When an image is formed on both surfaces of the transfer material P, the path switching member 600 is fixedly held at the position of a broken line and a toner image is formed on the first surface. The transfer material P conveyed by
Send to 10.

The transfer material P carried into the carrying path 610 is carried downward, is stopped in a state where its rear end is nipped, and then enters the carrying path 620 by a pair of carrying rollers R20 rotated in the opposite direction. Then, the image is formed on the second surface by being conveyed toward the registration roller R10.

Reference numeral 650 is a paper discharge roller for discharging the transfer material P, on which the image formation has been completed, onto the paper discharge tray T.

An image forming process using the image forming apparatus having the above-mentioned structure is roughly as follows. The description will be given with some control added.

When the copy button on the operation plate is pressed after the document is placed on the platen cover 120 via the opening and closing of the platen cover 120, the photoconductor 400 starts rotating counterclockwise as indicated by the arrow, and The exposure lamp L that constitutes the scanner unit is turned on, and the first mirror unit 200 and the second mirror unit 210 start moving forward.

The reflected exposure light from the original is the first and the second.
Mirror units 200, 210 and lens 220
Then, the image is formed on the image sensor 230 and read.

The image information read by the original image reading section B, in other words, the original image information read by the image sensor 230 is processed by the image processing means,
It is converted into data (image data) and temporarily stored in the memory provided on the image control board C.

The image data is called according to the image forming process, the semiconductor laser 300 in the writing device D is drive-controlled according to the image data, and the rotary polygon mirror 3 is driven.
Writing (exposure) is performed on the photoconductor 400 via 10.

Prior to the writing, the photoconductor 40
The surface of 0 is given a predetermined potential by the corona discharge action of the charging electrode 410, but the writing light reduces the potential of the exposed portion according to the exposure amount, and as a result, the electrostatic latent image according to the image data. Are formed on the photoconductor 400.

The electrostatic latent image is reversely developed by the developing device 420 to form a visible image (toner image).

On the other hand, before the front end of the toner image on the photoconductor 400 reaches the transfer area, for example, the paper feed tray 500.
The transfer material P inside is fed and conveyed, reaches the registration roller R10, and is in a state in which the front end is regulated.

The transfer material P is conveyed toward the transfer area by the registration roller R10 which is synchronously controlled so as to overlap with the toner image area on the photoconductor 400.

In the transfer area, the photoreceptor 400
The upper toner image is transferred onto the transfer material P by the bias of the transfer electrode 430, and then the transfer material P is transferred to the separation electrode 20.
Is separated from the photoconductor 400.

The separated transfer material P is pressed and heated by a pair of fixing rollers H, and as a result, the toner image is fixed on the transfer material P and then discharged through a discharge roller 650. The paper is discharged onto the tray T.

Although not shown in FIG. 1, the image forming apparatus 1 is closed or closed by being turned on or off by a driving action from the platen cover during the closing operation of the platen cover 120. The first detecting means for detecting an operation in progress and the closing operation of the automatic document feeder A by being turned on or off by the action of the automatic document feeder A immediately before the closing operation of the automatic document feeder A. Second to detect the end
Detection means are provided.

Next, a detailed description of the present invention will be given with reference to FIGS. FIG. 2 is a schematic diagram for explaining the configuration of the platen cover and a detection unit that detects the closing operation of the platen cover. FIG. 3 is a detection unit that detects the closing operation of the automatic document feeder and the automatic document feeder. FIG. 3 is a schematic diagram for explaining the configuration of FIG. 1, and the configuration of FIG. 1 will be described below with occasional reference.

In FIG. 2, a first detecting means 122 for detecting the closing operation of the platen cover is provided at an appropriate position on the fulcrum side of the platen cover which can be opened and closed in the arrow direction with the left side of the drawing as the fulcrum. There is.

Specifically, the first detecting means 122 is biased so that a part thereof protrudes from the upper surface of the apparatus main body in the area occupied by the platen cover 120, and
An actuator 124 provided to be movable up and down, and a structure 125 made up of a light receiving element and a light emitting element provided in a lower movement range of the actuator are provided. Due to a change in output of the light receiving element when the actuator 124 blocks an optical path, , It is detected that the platen cover 120 is in the process of closing.

Then, as described above, the first detecting means 12
When the detection signal from 2 is obtained, in the present embodiment, the exposure lamp L is turned on by using the detection signal as a trigger, and the first and second mirror units 200,
210, that is, the scanner unit 250 is moved by a predetermined distance in the sub-operation direction, the reflected exposure light based on the scanning of a plurality of lines on the original 800 is read by the image sensor 230, and the original size in the main scanning direction is detected. Configured.

In other words, with the platen cover not fully closed, the width of the document is detected by detecting the exposure light reflected by the white background area of the document 800 in the main scanning direction.

In the present embodiment, when the platen cover 120 is opened up to about 40 degrees with respect to the platen surface, the platen cover 120 can maintain its position even if the operator releases his hand, and the platen cover 120 is closed. In operation, the actuator 124 is brought into contact with the actuator 124 in a state of about 30 degrees, and the actuator 124 is pressed in the subsequent closing operation.

In the case of copying an original placed on the platen, the exposure lamp L is set to a predetermined position (standby position) within the original area (meaning the area where the platen is).
Then, the exposure lamp is turned on based on the detection signal of the first detection means 122, and at the same time, the exposure lamp is moved in the sub-scanning direction, that is, toward one end of the platen which is the exposure start position for image formation. Then, a plurality of lines of the document are scanned.

In FIG. 3, reference numeral 130 denotes a second detecting means,
It has a function of detecting completion of the closing operation of the automatic document feeder A.

The platen cover 12 described with reference to FIG.
In a mode in which the automatic document feeder A is closed and the document is copied after the closing operation of 0 as shown in FIG. 3, the automatic document feeder A is driven (pressed) immediately before the end of the closing operation. The exposure lamp L is turned on again by using the detection signal of the second detection means 130 as a trigger, and the exposure lamp L is moved in the sub-scanning direction opposite to the above-described direction, so that a plurality of lines, which are the same area on the original document, are moved. To scan.

The basic structure of the second detecting means 130 is the same as that of the first detecting means 122.

The above-described configuration detects a black character portion (indicating a character portion, an image portion, etc.) of the original document while the embodiment of FIG. 2 detects the white background portion of the original document as described above. Therefore, both detection methods enable accurate document size (width size) detection.

The detection of the black portion means that the lower surface of the platen cover 120 or the document pressing surface of the automatic document feeder A which performs the same function as the platen cover is generally white. In such a case, It is assumed that it is difficult to distinguish the white background portion of the document from the pressing surface of the document, but the above-described configuration according to the present invention can eliminate such fears.

The size of the original in the sub-scanning direction is, for example, as shown in Japanese Patent No. 3168666, the original size (length size) is obtained by providing a photosensor at a position corresponding to the longitudinal direction of each original size. Can be detected.

Further, the document is positioned on the platen by the center reference (a method of positioning by aligning the center position of the abutting plate of the document with the center position of the abutted document).
However, one-sided reference (a method in which a corner portion of a document is pressed against an abutting plate in the XY directions which constitutes a predetermined corner portion of the platen to position the original portion) may be used.

As described above, the information via the first detecting means 122 or the image sensor 230 based on the first detecting means 122 and the second detecting means 130 and the photo sensor for detecting the length size of the original document are stored. The size of the original and the direction of the original are determined based on the information received, and a transfer material of an appropriate size can be automatically selected based on the information on the original size.

As described above, the output information from the image sensor, the photo sensor, etc. is actually shown in FIG.
The document size is specified by the calculation means (for example, one subroutine in multitask) provided in the image control board C.
The present invention does not have a feature in the calculation means, and since the document size detection is based only on the output information of the image sensor or the like, the width of the document is simply measured by the image sensor or the photo sensor. Described that size or length size is detected.

After detecting the document size through the above steps, the first mirror unit 200 including the exposure lamp L is detected.
And returning the second mirror unit 210 to the intended position for image formation (when only the platen cover is used as the document pressing member, the second mirror unit 210 is moved from the standby position in the sub-scanning direction for image forming exposure). In the case of the form described above, the returning operation can be eliminated), and the image forming is prepared.

That is, the return operation is performed, for example, by operating the copy button, and after returning the related means such as the exposure lamp L to the desired position, the operation exposure for the image forming process is subsequently executed.

The scanning exposure (prescan) distance for detecting the document size has been described as a plurality of lines of the document, but this can be appropriately determined, and the document is abutted even in the scanning area. It is not necessary to include the end position.

The scanning by the exposure lamp L as described above can detect the area with and without the line as the detection range even if the original is in the form of graph paper, so that the reading by the image sensor is not affected and the accurate size detection is performed. It can be carried out.

In order to efficiently perform the image formation process including the above-described prescan and the actual image formation process, or the image formation per unit time, the prescan distance is preferably as short as possible. In that form, the distance is about 15 to 20 mm, and the time is about 400.
˜500 milliseconds.

Note that this set time (which substantially corresponds to the moving distance, but there is also a mode in which lighting is continued even while the exposure lamp is stopped at a predetermined position) is rewritably stored in a non-volatile memory. It is extremely convenient to store some time or select some time.

For example, if the exposure lamp is turned on and then extinguished (called a sky shot) during an extremely short time in which the platen cover does not settle at a predetermined position, some users feel uncomfortable or, on the contrary, feel too long. The user who feels prefers to make it as short as possible, but making it rewritable can mitigate such a problem.

It is also conceivable that the platen cover once detected may be opened and closed again due to some force majeure during the operation of the platen cover, and the second detection may be performed. In order to prevent this, in this embodiment, the detection of the platen cover is detected again within a predetermined time after the exposure lamp is turned on in response to the detection signal of the first detection means and the operation of detecting the document size is started. Even if the means outputs, the detection signal is not accepted as a result.

Specifically, the timer is started in response to the detection signal of the first detecting means, and it is calculated whether or not the second detection signal is generated within a predetermined time. In some cases, the signal is monitored by a subroutine so as to judge that the signal is invalid.

The predetermined time is set to about 500 milliseconds in the present embodiment, but a suitable time can also be determined through various experiments.

Further, the first detecting means and the second detecting means are not limited to the above-described aspect, and are required without being driven by the closing operation of the platen cover and the automatic document feeder. It is possible to adopt a configuration for detecting those modes that

For example, a photo sensor is provided on the main body of the apparatus capable of detecting the opening / closing operation of the platen cover, or the closing operation is detected by using the rotating member on the platen cover side at the fulcrum. The detecting means may be disposed in the automatic document feeder, and a light emitting element and a light receiving element are provided on the automatic document feeder and the apparatus main body side, and the end of the closing operation of the automatic document feeder is detected from the received light intensity. It can be achieved by configuring as follows.

For example, in the case of an original with a color background,
The density of the background area is detected by pre-scanning using the image sensor described above, and the power of the light source consisting of a semiconductor laser is changed to increase or decrease the amount of light. In the image forming apparatus having a so-called automatic density setting (EE) function capable of performing the above), the size of the document in the main scanning direction is used by using the collected data at the time of scanning the document for the automatic density setting. Detection can be performed.

[0083]

EFFECTS OF THE INVENTION Even though it has a simple structure, it is possible to accurately detect the size of a document in a short time, and the operability is good.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a schematic configuration of a digital image forming apparatus.

FIG. 2 is a schematic diagram for explaining a configuration of a platen cover and a detection unit that detects a closing operation of the platen cover.

FIG. 3 is a schematic diagram for explaining a configuration of an automatic document feeder and a detection unit that detects the end of the closing operation of the automatic document feeder.

[Explanation of symbols]

1 Digital image forming device 120 platen cover 122 first detecting means 130 Second detection means 230 image sensor 300 semiconductor laser 400 image carrier (photoreceptor) A Automatic document feeder B Original image reading section C image control board D writing device E Image forming section F storage G Platen

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H04N 1/10 1/107 (72) Inventor Katsunori Takahashi 2970 Ishikawa-cho, Hachioji-shi, Tokyo Konica Stock Company ( 72) Inventor Watanabe Masayuki 2970 Ishikawa-cho, Hachioji, Tokyo Konica Stock Company F-term (reference) 2H012 CA07 2H076 AB05 AB12 AB22 BA08 BA24 BA47 BA52 BA87 BA95 BB04 BB06 DA05 5B047 AA01 BA02 BB02 BC12 CB04 CB05 CB05 CB05 CB05 AA01 BA05 EA05 LA02 LA08 RA01 UA12 XA01

Claims (6)

[Claims] 1. An original placed on a platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the original is received by an image sensor, and based on image data after A / D conversion. In a digital image forming apparatus configured to perform writing processing on an image carrier after charging, the exposure lamp is turned on by a detection signal of a first detection unit that detects a closing operation of a platen cover that can be opened and closed. At the same time, the scanner unit is moved, the reflected exposure light for a plurality of lines of the document is read by the image sensor to detect the document size in the main scanning direction, and the document size in the sub-scanning direction is output by the photo sensor. The digital image forming apparatus is characterized in that the image is detected through the. 2. The digital image forming apparatus according to claim 1, wherein a lighting time of the exposure lamp when a document size is detected is variable. 3. The document size detecting operation according to the second detection signal from the first detecting means within a certain time after the document size detecting operation is prohibited. Digital image forming device. 4. An original placed on a platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the original is received by an image sensor, and based on image data after A / D conversion. A digital image forming apparatus configured to perform writing processing on the charged image carrier, an automatic document feeder that can be opened and closed, and a platen cover that can be opened and closed independently of the automatic document feeder. The exposure lamp is turned on by using a detection signal of a first detection unit that detects a closing operation of the platen cover as a trigger, and the scanner unit is moved in a predetermined direction in the sub-scanning direction to move the plurality of documents. The line sensor is read by the image sensor, and the completion of the subsequent closing operation of the automatic document feeder is detected by the second detecting means. The exposure lamp is turned on again by using the detection signal of the output means as a trigger, and the scanner unit is moved in a direction opposite to the moving direction to read a plurality of lines of the document by the image sensor. A digital image forming apparatus characterized in that the document size in the main scanning direction is determined from the two pieces of information read by, and the document size in the sub scanning direction is detected via a photo sensor. 5. The digital image forming apparatus according to claim 4, wherein the lighting time of the exposure lamp when the document size is detected is variable. 6. A document placed on a platen is scanned and exposed by a scanner unit including an exposure lamp, reflected exposure light from the document is received by an image sensor, and based on image data after A / D conversion, In a digital image forming apparatus configured to perform writing processing with a laser beam on an image carrier after charging, an automatic density setting function is provided, and the reflected exposure light from an original at the time of scanning for automatic density setting is used as the image. A digital image forming apparatus, characterized in that a sensor receives light, a calculation means calculates the laser light amount, and a document size in the main scanning direction is detected based on light reception information from the image sensor.