JP2014060631A - Image reading device, image forming apparatus, and black level correction method - Google Patents

Abstract

Image quality can be improved even when the temperature change with time varies from pixel to pixel.
A light source that illuminates a document, a light source control unit that alternately turns on and off the light source, and a reflected light from the document are read for each pixel, and an image signal of the document is generated for each pixel. A photoelectric conversion / signal processing unit 240. The photoelectric conversion / signal processing unit 240 has, as an image signal of the original, an image level of the original read during the light source lighting period and a black level read during the light source off period. A black level correction unit that generates a corrected image level for each pixel.
[Selection] Figure 2

Description

  The present invention relates to an image reading apparatus, an image forming apparatus, and a black level correction method.

  There is known an image reading apparatus that reads an image of a document by irradiating the document with a light source such as an LED (Light Emitting Diode) and converting reflected light into an image signal with an image sensor. In such an image reading apparatus, for example, a complementary metal oxide semiconductor (CMOS) image sensor is used as the image sensor.

  When a CMOS image sensor is used as the image sensor, one of the factors that degrade the read image is fixed pattern noise that is generated due to pixel-to-pixel variations. When fixed pattern noise occurs, vertical stripes appear in the read image. In addition, as one of the factors of variation for each pixel, for example, there is variation in dark current noise generated in a photodiode that converts light constituting each pixel into electric charge. Dark current noise is a current generated when light is not exposed.

  Black shading correction is known as a technique for correcting pixel-to-pixel variations such as dark current noise. In black shading correction, the image is read with the light source turned off, the read value is held as a black level for each pixel, and the black level is calculated from the image level when the image is read with the light source turned on. This is a technique for correcting by subtraction.

  However, in black shading correction, it is necessary to turn off the light source before reading the image in order to generate the black level, so when reading images continuously, the fixed pattern noise changes due to temperature changes, etc. If this happens, black shading correction cannot be performed normally.

  For this reason, for example, Patent Document 1 discloses a technique for correcting the value of the black level of another pixel based on the fluctuation of the reading level of the light-shielding pixel of the image sensor.

  However, in the conventional technology as described above, when the temperature change over time is not uniform for each pixel, the black shading correction cannot be performed normally, and image degradation occurs.

  The present invention has been made in view of the above circumstances, and an image reading apparatus, an image forming apparatus, and a black level correction capable of improving image quality even when a temperature change with time varies from pixel to pixel. It aims to provide a method.

  In order to solve the above-described problems and achieve the object, an image reading apparatus according to an aspect of the present invention includes a light source that illuminates a document, a light source control unit that causes the light source to alternately turn on and off, An image signal generation unit that reads reflected light from the document for each pixel and generates an image signal of the document for each pixel, and the image signal generation unit uses the light source as an image signal of the document. A black level correction unit configured to generate, for each pixel, a corrected image level that is a difference between the image level of the original read during the lighting period and the black level read during the light-off period of the light source;

  An image forming apparatus according to another aspect of the present invention includes the image reading apparatus.

  Further, a black level correction method according to another aspect of the present invention includes a light source control step that alternately turns on and off a light source that illuminates a document, and a reading step that reads reflected light from the document for each pixel. And a black level correction step for generating, for each pixel, a corrected image level that is a difference between the image level of the original read during the light source lighting period and the black level read during the light source off period.

  According to the present invention, there is an effect that the image quality can be improved even when the temperature change with time varies from pixel to pixel.

FIG. 1 is a diagram illustrating an example of a schematic configuration of the image reading apparatus according to the first embodiment. FIG. 2 is a block diagram illustrating an example of a functional configuration of the image reading apparatus according to the first embodiment. FIG. 3 is a block diagram illustrating an example of a circuit configuration of the photoelectric conversion / signal processing unit according to the first embodiment. FIG. 4 is a block diagram illustrating an example of a functional configuration of the black correction unit according to the first embodiment. FIG. 5 is an explanatory diagram illustrating an example of black correction by the black correction unit of the first embodiment. FIG. 6 is a timing chart illustrating an example of processing timings of the light source control unit and the photoelectric conversion / signal processing unit according to the first embodiment. FIG. 7 is a block diagram illustrating an example of a functional configuration of the image reading apparatus according to the second embodiment. FIG. 8 is a block diagram illustrating an example of a circuit configuration of the photoelectric conversion / signal processing unit of the second embodiment. FIG. 9 is a block diagram illustrating an example of a functional configuration of the black correction unit according to the second embodiment. FIG. 10 is an explanatory diagram illustrating an example of black correction performed by the black correction unit according to the second embodiment. FIG. 11 is a timing chart illustrating an example of processing timings of the light source control unit and the photoelectric conversion / signal processing unit according to the second embodiment. FIG. 12 is a flowchart illustrating an example of gain calculation processing by adjustment of the gain adjustment unit of the second embodiment. FIG. 13 is a block diagram illustrating an example of a functional configuration of the black correction unit according to the first modification. FIG. 14 is a diagram illustrating an example of a schematic configuration of an image forming apparatus including the image reading apparatuses according to the above-described embodiments and modifications.

  Hereinafter, embodiments of an image reading apparatus, an image forming apparatus, and a black level correction method according to the present invention will be described in detail with reference to the accompanying drawings. Note that the image reading apparatus of each of the following embodiments can be realized by, for example, a scanner device, and may be mounted on the image forming apparatus or may be used alone. The image forming apparatus can be realized by, for example, a printing apparatus, a copier, a multifunction peripheral (MFP), a facsimile apparatus, and the like. The multifunction peripheral has at least two functions among a copying function, a printing function, a scanner function, and a facsimile function.

(First embodiment)
FIG. 1 is a diagram illustrating an example of a schematic configuration of the image reading apparatus according to the first embodiment. As shown in FIG. 1, the image reading apparatus 100 includes a contact glass 101, a reference white plate 110, a reading window 111, a first carriage 106, a second carriage 107, a lens unit 108, and a CMOS image sensor 109 ( An example of an image signal generation unit).

  The contact glass 101 has translucency and is disposed on a document placing portion of the image reading apparatus 100.

  The reference white plate 110 is used for correcting various distortions caused by the optical system included in the image reading apparatus 100. The reference white plate 110 is on the end of the contact glass 101 (in the example shown in FIG. 1, the left end as viewed from the front). (Above).

  The reading window 111 is a reading position when an image of a document is read using the sheet scanning function, and is arranged on the side surface side of the contact glass 101 (the left side surface when viewed from the front in the example shown in FIG. 1). ing. The reading window 111 is also translucent.

  In the example shown in FIG. 1, an ADF (Auto Document Feeder) 400 is mounted on the upper part of the image reading apparatus 100. The ADF 400 is connected to the image reading apparatus 100 via a hinge (not shown) so that it can be opened and closed with respect to the contact glass 101. The ADF 400 includes a document tray 401 and a feeding roller 402.

  A document tray 401 is a document placement table on which a document bundle composed of a plurality of documents is placed. A feeding roller 402 separates documents one by one from a bundle of documents placed on the document tray 401 and automatically feeds the documents toward a reading window 111 for sheet-through reading.

  The first carriage 106 includes a light source 102 that irradiates a document placed on the contact glass 101 and a first mirror 103 that reflects light reflected from the document.

  The second carriage 107 includes a second mirror 104 that reflects the reflected light from the first mirror 103 and a third mirror 105 that reflects the reflected light from the second mirror 104.

  Here, in the scan mode in which the image surface of the document is scanned and the image of the document is read, the first carriage 106 and the second carriage 107 are moved in the direction indicated by the arrow A by a drive unit such as a stepping motor (not shown). The document is scanned in the sub-scanning direction. However, in order to keep the optical path length from the contact glass 101 to the CMOS image sensor 109 constant, the second carriage 107 moves at a speed half that of the first carriage 106.

  As a result, the image surface, which is the lower surface of the document placed on the contact glass 101, is illuminated (exposed) by the light source 102, and the first carriage 106 and the second carriage 107 receive the reflected light from the document (image surface). Guide to the lens unit 108.

  In the sheet-through mode in which an original is automatically fed by the ADF 400 and an image of the original is read, the first carriage 106 and the second carriage 107 move to the lower side of the reading window 111. Thereafter, the document placed on the document tray 401 is automatically fed in the arrow B direction (sub-scanning direction) by the feeding roller 402, and the image surface of the document is scanned (scanned) at the position of the reading window 111. The document whose image has been read is discharged to a discharge port (not shown).

  As a result, the image surface which is the lower surface of the automatically fed document is illuminated by the light source 102, and the first carriage 106 and the second carriage 107 guide reflected light from the document (image surface) to the lens unit 108.

  The lens unit 108 converges the reflected light guided by the third mirror 105 and irradiates the CMOS image sensor 109.

  The CMOS image sensor 109 photoelectrically converts the reflected light emitted from the lens unit 108, converts the analog signal after the photoelectric conversion into a digital signal, and outputs the digital signal to an image processing unit (not shown). Thereby, digital image data is obtained. The image processing unit performs image processing such as shading correction on the digital signal, variation correction in the main scanning direction such as sensitivity variation of the CMOS image sensor 109 and uneven light distribution of the optical system, and outputs the result.

  Note that the CMOS image sensor 109 photoelectrically converts the reflected light from the reference white plate 110 by illumination of the light source 102 before reading the document in both the scan mode and the sheet-through mode, and outputs the analog after the photoelectric conversion. The signal is converted into a digital signal and output to the image processing unit. The output digital signal is used for shading correction performed in the image processing unit.

  In addition, when the ADF 400 includes a conveyance belt, the ADF 400 automatically feeds the document placed on the document tray 401 to the reading position on the contact glass 101 even in the scan mode. An image of the document can be read.

  FIG. 2 is a block diagram illustrating an example of a functional configuration of the image reading apparatus 100 according to the first embodiment. As shown in FIG. 2, the image reading apparatus 100 includes a control unit 210, a light source control unit 220, and a CMOS image sensor 109.

  The control unit 210 controls the image reading apparatus 100 in an integrated manner, and includes a CPU (Central Processing Unit), a ROM (Read Only Memory) storing a program executed by the CPU, and a RAM functioning as a work memory. (Random Access Memory).

  Specifically, the control unit 210 controls the light source control unit 220 and the CMOS image sensor 109, outputs a lighting on signal for lighting the light source 102 to the light source control unit 220, and outputs a CMOS image to the CMOS image sensor 109. A control signal for controlling the sensor 109 is output.

  The light source control unit 220 controls lighting of the light source 102 and can be realized by, for example, an IC (Integrated Circuit). Specifically, when the lighting on signal from the control unit 210 is asserted, the light source control unit 220 turns on the light source 102 according to the lighting timing signal from the CMOS image sensor 109.

  In the present embodiment, the light source control unit 220 sequentially switches on and off the light source 102 within the line for each line while the document is being read. In the present embodiment, the lighting time and the lighting time of the light source 102 in one line are the same time.

  The CMOS image sensor 109 includes a timing generation unit 230, a photoelectric conversion / signal processing unit 240, and an interface unit 290.

  The timing generation unit 230 generates timing signals for the light source control unit 220, the photoelectric conversion / signal processing unit 240, and the interface unit 290. Specifically, the timing generation unit 230 generates a lighting timing signal based on a control signal from the control unit 210, outputs the lighting timing signal to the light source control unit 220, generates a drive timing signal, and generates a photoelectric conversion / signal processing unit. The output timing signal is generated and output to the interface unit 290.

  The photoelectric conversion / signal processing unit 240 photoelectrically converts the reflected light from the document into charges in accordance with the drive timing signal from the timing generation unit 230, performs black correction on the signal based on the converted charges, and outputs the signal to the interface unit 290. To do.

  The interface unit 290 outputs the image signal output from the photoelectric conversion / signal processing unit 240 to an image processing unit (not shown) according to the output timing signal from the timing generation unit 230.

  FIG. 3 is a block diagram illustrating an example of a circuit configuration of the photoelectric conversion / signal processing unit 240 according to the first embodiment. As illustrated in FIG. 3, the photoelectric conversion / signal processing unit 240 includes PD 241-1 to PD 241-n (n is a natural number), C 242-1 to C 242-n, Tr_s 243-1 to Tr_s 243-n, and Tr_r 244. -1 to Tr_r244-n and black correction units 245-1 to 245-n. Note that n is the number of pixels constituting image data for one line.

  In the following description, when it is not necessary to distinguish the PDs 241-1 to PD241-n, they may be simply referred to as PD241, and when it is not necessary to distinguish the C242-1 to C242-n, C_242 may be referred to as Tr_s243-1 to Tr_s243-n, and may be simply referred to as Tr_s243. In some cases, it may be referred to as Tr_r244, and when it is not necessary to distinguish each of the black correction units 245-1 to 245-n, they may be simply referred to as black correction units 245.

  PDs 241-1 to PD241-n are photodiodes for n pixels constituting image data for one line, and are arranged in the main scanning direction. The PD 241 photoelectrically converts the reflected light from the document into electric charge and accumulates it.

  C242 is a capacitor, and the electric charge accumulated by the PD 241 is accumulated.

  Tr_s243 is a switch circuit. When the switch is turned on, the charge stored in the PD 241 is transferred to the C242.

  Tr_r244 is a switch circuit, and when the switch is turned on, the charge stored in C242 is reset to a certain level.

  The black correction unit 245 performs black correction on a signal based on the electric charge after photoelectric conversion, and outputs the signal to the interface unit 290.

  FIG. 4 is a block diagram illustrating an example of a functional configuration of the black correction unit 245 of the first embodiment, and FIG. 5 is an explanatory diagram illustrating an example of black correction by the black correction unit 245 of the first embodiment. As shown in FIG. 4, the black correction unit 245 includes an image level holding unit 246, a black level holding unit 247, a subtractor 248, and a corrected image level holding unit 249.

  The image level holding unit 246 holds the signal level of the signal read during the lighting period of the light source 102. Specifically, the image level holding unit 246 has a signal level of a signal based on a charge after photoelectric conversion output from the PD 241 in a state where the light source 102 is turned on, that is, a charge corresponding to reflected light and dark current. Hold the image level.

  The black level holding unit 247 holds the signal level of the signal read while the light source 102 is turned off. Specifically, the black level holding unit 247 is a signal level of a signal based on the electric charge after photoelectric conversion output from the PD 241, that is, the electric charge corresponding to the dark current, in a state where the light source 102 is turned off. Hold the level.

  The charge corresponding to the dark current is proportional to the accumulation time in the PD 241. However, in this embodiment, since the light-on time and the light-off time of the light source 102 are the same time, as shown in FIG. The signal level of the dark current occupying the level and the black level are the same level.

  The subtracter 248 subtracts the black level held in the black level holding unit 247 from the image level held in the image level holding unit 246. Thereby, a corrected image level obtained by correcting (removing) the black level from the image level is obtained.

  The corrected image level holding unit 249 holds the result of subtraction by the subtracter 248, that is, the corrected image level obtained by subtracting the black level from the image level, and outputs it to the interface unit 290.

  FIG. 6 is a timing chart illustrating an example of processing timings of the light source control unit 220 and the photoelectric conversion / signal processing unit 240 according to the first embodiment. Note that the processing timing of each process shown in FIG. 6 is controlled by a drive timing signal from the timing generator 230.

  The light source control unit 220 sequentially switches on and off the light source 102 once in a line while the original is being read. In the example shown in FIG. 6, as described above, the lighting time and the lighting time of the light source 102 in one line are the same time.

  When the light source 102 is turned on, charges corresponding to the reflected light and dark current from the original are accumulated in the PD 241, and when the light source 102 is turned off, charges corresponding to the dark current are accumulated in the PD 241. Is done.

  The Tr_r 244 resets the charge stored in the C242 when the light source 102 is turned on or off.

  When the charge stored in C242 is reset by Tr_r244, Tr_s243 transfers the charge stored in the PD 241 to C242 immediately before the light source 102 is switched on and off.

  As a result, when the light source 102 is turned on, a charge corresponding to the dark current accumulated in the PD 241 when the light source 102 is turned off immediately before is accumulated in C242. When the light source 102 is turned off, C242 contains Charges corresponding to the reflected light and dark current from the original accumulated in the PD 241 when the light source 102 is turned on immediately before are accumulated.

  The image level holding unit 246 holds an image level which is a signal level of a signal based on the light corresponding to the reflected light and dark current from the original stored in the C242. Note that the image level held by the image level holding unit 246 is updated once per line. In the example illustrated in FIG. 6, the image level update timing is when the light source 102 is turned off.

  The black level holding unit 247 holds a black level that is a signal level of a signal based on charges according to the dark current accumulated in C242. The black level held by the black level holding unit 247 is also updated once per line. In the example shown in FIG. 6, the black level update timing is when the light source 102 is turned on.

  The corrected image level holding unit 249 holds a corrected image level obtained by subtracting the black level from the image level. The corrected image level held by the corrected image level holding unit 249 is also updated once per line.

  As described above, according to the first embodiment, for each pixel, the black level is updated for each line, and black correction is performed using the updated black level. Even when dark current variation occurs for each pixel, which is not uniform, the dark current variation can be corrected correctly, the occurrence of vertical stripes in the image can be avoided, and the image quality can be improved.

(Second Embodiment)
In the second embodiment, an example of performing gain adjustment will be described. In the following, differences from the first embodiment will be mainly described, and components having the same functions as those in the first embodiment will be given the same names and symbols as those in the first embodiment, and the description thereof will be made. Omitted.

  FIG. 7 is a block diagram illustrating an example of a functional configuration of the image reading apparatus 1000 according to the second embodiment. As shown in FIG. 7, the image reading apparatus 1000 according to the second embodiment is different from the first embodiment in a light source control unit 1220 and a CMOS image sensor 1109.

  Also in the second embodiment, the light source control unit 1220 sequentially switches on and off the light source 102 within the line for each line while the document is being read. However, in this embodiment, the lighting time of the light source 102 in one line is longer than the extinguishing time. This is because when reading is performed at a constant speed, the longer the lighting time is, the more charge is accumulated in the PD 241 and the noise characteristics are improved.

  The CMOS image sensor 1109 is further provided with a gain adjustment unit 1291, and the photoelectric conversion / signal processing unit 1240 is different from the first embodiment.

  A gain adjustment unit 1291 (an example of a coefficient adjustment unit) performs gain adjustment. Specifically, the gain adjustment unit 1291 performs gain adjustment on the photoelectric conversion / signal processing unit 1240 based on the image signal output from the interface unit 290. Details of the gain adjustment will be described later.

  FIG. 8 is a block diagram illustrating an example of a circuit configuration of the photoelectric conversion / signal processing unit 1240 according to the second embodiment. As illustrated in FIG. 8, the photoelectric conversion / signal processing unit 1240 includes an APGA (Analog Programmable Gain Amplifier) 1251-1 to APGA 1251-n, an ADC (Analog to Digital Converter) 1252-1 to ADC 1252-n, and a CDS (Correlated Double Sampling) 1253-1 to CDS1253-n and the contents of the black correction units 1245-1 to 1245-n are different from those of the first embodiment.

  In the following description, when APGA 1251-1 to APGA 1251-n need not be distinguished from each other, they may be simply referred to as APGA 1251, and when it is not necessary to distinguish between ADC 1252-1 to ADC 1252-n, In some cases, it may be referred to as ADC 1252, and when it is not necessary to distinguish each of CDS 1253-1 to CDS 1253-n, they may be simply referred to as CDS 1253.

  The APGA 1251 amplifies a signal based on the electric charge after photoelectric conversion. The APGA 1251 can arbitrarily set the gain by a gain adjustment signal, but sets the gain so that the dynamic range of the ADC 1252 is widened.

  The ADC 1252 A / D converts the analog signal amplified by the APGA 1251 into a digital signal.

  The CDS 1253 calculates the difference between the value of the signal after A / D conversion by the ADC 1252 and the value of the signal after the ADC 1252 performs A / D conversion of the signal when the charge after photoelectric conversion is reset. The noise due to resetting is removed and output to the black correction unit 1245.

  FIG. 9 is a block diagram illustrating an example of a functional configuration of the black correction unit 1245 of the second embodiment, and FIG. 10 is an explanatory diagram illustrating an example of black correction by the black correction unit 1245 of the second embodiment. As shown in FIG. 9, the black correction unit 1245 is different from the first embodiment in that the black correction unit 1245 further includes a DPGA (Digital Programmable Gain Amplifier) 1254 and a subtractor 1248.

  In this embodiment, since the lighting time of the light source 102 is longer than the extinguishing time, the dark current signal level in the image level is higher than the black level (level) as shown in FIG. It has become.

  The DPGA 1254 amplifies the black level held in the black level holding unit 247 by multiplying the set gain (an example of a coefficient) and outputs the result to the subtracter 1248. The DPGA 1254 can arbitrarily set the gain by the gain adjustment signal, but as shown in FIG. 10, the black level is amplified so as to be the same level as the dark current signal level in the image level.

  The subtractor 1248 subtracts the black level amplified by the DPGA 1254 from the image level held in the image level holding unit 246.

  FIG. 11 is a timing chart illustrating an example of processing timings of the light source control unit 1220 and the photoelectric conversion / signal processing unit 1240 according to the second embodiment. Note that the processing timing of each process shown in FIG. 11 is controlled by a drive timing signal from the timing generator 230.

  The light source control unit 220 sequentially switches on and off the light source 102 once in a line while the original is being read. In the example shown in FIG. 11, as described above, the lighting time of the light source 102 in one line is longer than the lighting time.

  When the light source 102 is turned on, charges corresponding to the reflected light and dark current from the original are accumulated in the PD 241, and when the light source 102 is turned off, charges corresponding to the dark current are accumulated in the PD 241. Is done.

  The Tr_r 244 resets the charge stored in the C242 when the light source 102 is turned on or off.

  When the charge stored in C242 is reset by Tr_r244, Tr_s243 transfers the charge stored in the PD 241 to C242 immediately before the light source 102 is switched on and off.

  As a result, when the light source 102 is turned on, the charge corresponding to the dark current accumulated in the PD 241 when the immediately preceding light source 102 is turned off is accumulated in C242, and the CDS 1253 performs A / D of the signal based on the charge. The difference between the value after conversion and the value after A / D conversion of the signal when the charge after photoelectric conversion is reset is held. On the other hand, when the light source 102 is turned off, the charge corresponding to the reflected light and dark current from the document accumulated in the PD 241 when the light source 102 is turned on immediately before is stored in C242, and the CDS 1253 is based on the charge. The difference between the value after A / D conversion of the signal and the value after A / D conversion of the signal when the charge after photoelectric conversion is reset is held.

  The image level holding unit 246 holds the image level output from the CDS 1253. Note that the image level held by the image level holding unit 246 is updated once per line. In the example shown in FIG. 11, the update timing of the image level is when the light source 102 is turned off.

  The black level holding unit 247 holds the black level output from the CDS 1253. The black level held by the black level holding unit 247 is also updated once per line. In the example shown in FIG. 11, the black level update timing is when the light source 102 is turned on.

  The corrected image level holding unit 249 holds a corrected image level obtained by subtracting the amplified black level from the image level. The corrected image level held by the corrected image level holding unit 249 is also updated once per line.

  Here, gain adjustment by the gain adjustment unit 1291 will be described. Based on the image signal output from the interface unit 290, the gain adjustment unit 1291 performs gain adjustment on the DPGA 1254 so that the dark current signal level and the black level in the image level are the same level.

  The gain adjustment unit 1291 may calculate the gain according to the driving condition, or may calculate the gain by adjustment.

  When calculating the gain according to the driving conditions, the gain adjusting unit 1291 obtains the gain from the ratio between the lighting time and the lighting time of the light source 102. As described above, since the electric charge according to the dark current is proportional to the accumulation time in the PD 241, the gain adjustment unit 1291 sets the gain of the DPGA 1254 to the ratio of the lighting time to the turn-off time, so that the image level and the black level are changed. The dark currents can be matched. That is, the gain adjustment unit 1291 obtains the gain of the DPGA 1254 based on the lighting time / light-out time, and adjusts the gain of the DPGA 1254 to the obtained value.

  FIG. 12 is a flowchart illustrating an example of gain calculation processing by adjustment of the gain adjustment unit 1291 of the second embodiment. The gain calculation process shown in FIG. 12 is performed when the image reading apparatus 100 is turned on, so that a gain corresponding to the state of the image reading apparatus 100 can be set.

  First, when the lighting on signal from the control unit 210 is negated, the light source control unit 1220 turns off the light source 102 (step S301).

  Subsequently, the gain adjustment unit 1291 changes the gain of the DPGA 1254 (step S303).

  Subsequently, the gain adjustment unit 1291 makes a difference between the black level for adjustment, which is the black level when the light source 102 is turned off during the lighting period and the black level after amplification when the light source 102 is turned off during the lighting period. It is confirmed whether the absolute value of is less than or equal to a threshold value (step S305).

  When the absolute value of the difference is equal to or smaller than the threshold (Yes in Step S305), the gain adjustment unit 1291 sets the gain of the DPGA 1254 to the gain changed in Step S303 (Step S307), and ends the process.

  On the other hand, if the absolute value of the difference exceeds the threshold value (No in step S305), if the gain adjustment unit 1291 has not tried all the gains that can be changed (No in step S309), the process returns to step S303. If all gains that can be changed by the gain adjustment unit 1291 have been tried (Yes in step S309), the gain adjustment unit 1291 sets the gain of the DPGA 1254 to the gain having the smallest absolute value of the difference (step S311). The process ends.

  As described above, also in the second embodiment, the same effects as in the first embodiment are obtained. In particular, according to the second embodiment, by performing gain adjustment, the lighting time of the light source 102 in one line can be made longer than the extinguishing time, so that noise characteristics can be improved.

(Modification)
In addition, this invention is not limited to the said embodiment, A various deformation | transformation is possible.

(Modification 1)
In Modified Example 1, a modified example of the black correction unit of the second embodiment will be described. In the second embodiment, since the turn-off time of the light source 102 is short, dark current noise is also reduced, and it is easy to be affected by random noise or sudden noise. For this reason, in the second embodiment, stable black correction cannot be performed if the black level is updated. Therefore, in the second embodiment, the difference between the current black level and the new black level is attenuated, and smoothing is performed by adding the attenuated difference value to the current black level and updating the black level. .

  FIG. 13 is a block diagram illustrating an example of a functional configuration of the black correction unit 2245 according to the first modification. As shown in FIG. 13, the black correction unit 2245 is further provided with a subtracter 2255, an attenuator 2256, and an adder 2257, and a black level holding unit 2247 is different from the first embodiment.

  The subtracter 2255 calculates the difference between the current black level held in the black level holding unit 2247 and the input new black level.

  The attenuator 2256 attenuates the difference value obtained by the subtracter 2255. Note that the attenuation rate of the attenuator 2256 is set to a value that can follow changes in dark current due to temperature changes.

  The adder 2257 adds the difference value attenuated by the attenuator 2256 and the current black level held in the black level holding unit 2247.

  The black level holding unit 2247 holds the (updated) black level obtained by the adder 2257.

  In this way, the black level can be smoothed while reducing the influence of noise.

(Modification 2)
In the above embodiment, the description has been made assuming that the CMOS image sensor 109 is a line sensor that reads image data for one line. However, the RGB image sensor has a PD prepared for each color of RGB and can output RGB. The present invention can also be applied to a line sensor for three lines. In this case, the black correction unit described above may be provided for each RGB.

(Image forming device)
FIG. 14 is a diagram illustrating an example of a schematic configuration of an image forming apparatus 300 including the image reading apparatuses according to the above-described embodiments and modifications. In the example shown in FIG. 14, the image forming apparatus 300 is a copying machine, but is not limited to this.

  As shown in FIG. 14, the image forming apparatus 300 is provided with an ADF 400 on the upper part of the contact glass 101 on which the document is placed, and a hinge or the like (not shown) is provided so that the ADF 400 can be opened and closed with respect to the contact glass 101. Are connected through.

  The ADF 400 includes a document tray 401 as a document placement table on which a document bundle composed of a plurality of documents can be placed. In addition, when a print key on an operation unit (not shown) is pressed, the originals are separated one by one from the original bundle placed on the original tray 401 with the image surface facing upward, and automatically fed to the reading window 111 or the contact glass. Separation / feeding means including a feeding roller 402 and a conveying belt 403 that are conveyed toward 101 are also provided.

  The document fed by the feeding roller 402 or the conveyance belt 403 is read out on the upper surface of the ADF 400 by the conveyance belt 403 and the discharge roller 404 after image reading is performed.

  Here, the operation of the controller (not shown) and the ADF 400 when the document is conveyed to the reading position of the contact glass 101 by the ADF 400 will be described.

  The feed motor of the ADF 400 is driven by an output signal from the controller. When the feed start signal generated by pressing the print key on the operation unit is input, the controller corrects the feed motor.・ Reverse drive. When the feeding motor is driven to rotate forward, the feeding roller 402 rotates clockwise to automatically feed the document located at the uppermost position from the document bundle and transport it toward the reading window 111 or the contact glass 101. . When the leading edge of the document is detected by the document set detection sensor 405, the controller drives the feed motor in reverse based on the output signal from the document set detection sensor 405. This prevents subsequent documents from entering and prevents separation.

  When the document set detection sensor 405 detects the trailing edge of the document, the controller also counts a rotation pulse of a conveyance belt motor (not shown) from this detection point, and when the rotation pulse reaches a predetermined value, the conveyance belt 403. Is stopped and the conveying belt 403 is stopped, whereby the document is stopped at the reading position on the contact glass 101. Further, when the trailing edge of the document is detected by the document set detection sensor 405, the feeding motor is driven again, and the subsequent document is separated and automatically fed as described above. Then, the sheet is conveyed toward the contact glass 101, and when the pulse of the feeding motor from the time when the document is detected by the document set detection sensor 405 reaches a predetermined pulse, the feeding motor is stopped and the next document is detected. To wait first.

  When the document stops at the reading position on the contact glass 101, the image of the document is read. When this image reading is completed, a signal indicating that is input to the controller, so that the controller drives the conveyance belt motor in the normal direction by this signal, and the document is discharged from the contact glass 101 by the conveyance belt 403 to the discharge roller. Unload to 404.

  As described above, the original bundle placed on the original tray 401 in the ADF 400 with the image side of the original facing upward is automatically fed from the uppermost original by pressing the print key. For example, the reading position on the contact glass 101 is read. It is conveyed to.

  The original that has been conveyed to the reading position and stopped is discharged from the discharge port by the conveying belt 403 or the like after the image is read. Further, when it is detected that there is a next document on the document tray 401, the next document is automatically fed and conveyed onto the contact glass 101 like the previous document.

  The transfer sheets (paper sheets) stacked on the first tray 301, the second tray 302, and the third tray 303, which are sheet feeding trays, are a first sheet feeding unit 311, a second sheet feeding unit 312, and a third sheet feeding unit, respectively. The sheet is fed by 313 and conveyed by the vertical conveyance unit 314 to a position where it abuts on a drum-shaped photosensitive member (photosensitive drum) 315 as an image carrier. Actually, any one of the trays 301 to 303 is selected, and the transfer paper is fed therefrom. Also, a recording medium other than transfer paper can be used.

  On the other hand, the image data read by the image reading apparatus 100 is written on the surface of the photoreceptor 315 charged in advance by a charging unit (not shown) by a laser beam from a writing unit 350 in the printer as an image forming unit (there is When the surface is exposed), the portion passes through the developing unit 327, and a toner image is formed there. The developing unit 327 that performs the image formation, the charging unit, and the like constitute image forming means.

  The transfer sheet fed from the selected paper feed tray is transferred by the transfer belt 316 at the same speed as the rotation of the photoconductor 315, and the toner image on the photoconductor 315 is transferred. Further, the toner image is fixed by the fixing unit 317, and is discharged by a paper discharge unit 318 to a paper discharge tray 319 outside the apparatus.

  At this time, for example, in order to reverse the transfer paper on which the toner image is formed on one side in order to discharge face-down (the image surface faces downward in order to align the transfer paper in the page order), the transfer paper is discharged. The paper is conveyed to the double-sided paper conveyance path 320 by the unit 318, switched reverse by the reversing unit 321, and then discharged to the paper discharge tray 319 through the reverse paper conveyance path 322.

  When images are formed on both sides of the transfer paper, the transfer paper on which the image is formed on one side is conveyed to the double-sided input conveyance path 320 by the paper discharge unit 318 and is switched back by the reverse unit 321. After that, it is sent to the duplex conveying unit 323.

  The transfer paper sent to the double-sided conveyance unit 323 is fed again from the double-sided conveyance unit 323 to transfer the toner image formed on the photoconductor 315 again, and is again applied to the photoconductor 315 by the vertical conveyance unit 314. It is transported to the contact position. Then, after the toner image is transferred to the other surface, the toner image is fixed by the fixing unit 317 and discharged to the paper discharge tray 319 by the paper discharge unit 318.

  The photoconductor 315, the conveyance belt 316, the fixing unit 317, the paper discharge unit 318, and the development unit 327 are driven by a main motor (not shown), and the driving force of the main motor is transmitted to each of the paper feed units 311 to 313 by a paper feed clutch. Driven. The vertical conveyance unit 314 is driven by the driving force of its main motor being transmitted via an intermediate clutch.

  The writing unit 350 includes a laser output unit 351, an imaging lens 352, and a mirror 353. Inside the laser output unit 351, a laser diode that is a laser light source and a rotating polygon mirror (polygon mirror) that scans the laser light. Or it has a vibrating mirror. The laser light emitted from the laser output unit 351 is deflected by a polygon mirror or a vibrating mirror, passes through an imaging lens 352, is folded by a mirror 353, and is focused on the surface of the photoreceptor 315.

100, 1000 Image reading device 101 Contact glass 102 Light source 103 First mirror 104 Second mirror 105 Third mirror 106 First carriage 107 Second carriage 108 Lens unit 109, 1109 CMOS image sensor 110 Reference white plate 111 Reading window 210 Control unit 220 Light source controller 230 Timing generator 240, 1240 Photoelectric conversion / signal processor 241-1 to 241-n (241) PD
242-1 to 242-n (242) C
243-1 to 243-n (243) Tr_s
244-1 to 244n (244) Tr_r
245-1 to 245-n (245), 1245, 2245 Black correction unit 246 Image level holding unit 247, 2247 Black level holding unit 248, 1248 Subtractor 249 Correction image level holding unit 290 Interface unit 400 ADF
401 Document tray 402 Feed roller 1251-1 to 1251-n (1251) APGA
1252-1 to 1252-n (1252) ADC
1253-1 to 1253-n (1253) CDS
1254-1-1254-n (1254) DPGA
1291 Gain adjustment unit 2255 Subtractor 2256 Attenuator 2257 Adder

JP 2011-97528 A

Claims (9)

A light source that illuminates the document;
A light source controller that alternately turns on and off the light source; and
An image signal generation unit that reads reflected light from the document for each pixel and generates an image signal of the document for each pixel;
With
The image signal generation unit, as the image signal of the original, a corrected image level that is a difference between the image level of the original read during the light source lighting period and the black level read during the light source off period, An image reading apparatus having a black level correction unit generated for each pixel.   The image reading apparatus according to claim 1, wherein the black level correction unit multiplies the black level by a coefficient.   The image reading apparatus according to claim 2, wherein the coefficient is determined by a ratio between the lighting period and the extinguishing period.   The image reading apparatus according to claim 2, wherein the black level correction unit generates the black level by smoothing and updating the previous black level.   The image reading apparatus according to claim 2, wherein the image data generation unit further includes a coefficient adjustment unit that adjusts the coefficient when the power is turned on and the document is not read.   The image reading apparatus according to claim 5, wherein the coefficient adjustment unit adjusts the coefficient so that a value of an adjustment black level and the black level when the light source is turned off during the lighting period and the same period are matched. .   The image reading apparatus according to claim 1, wherein the image signal generation unit is a CMOS (Complementary Metal Oxide Semiconductor) image sensor.   An image forming apparatus comprising the image reading apparatus according to claim 1. A light source control step for alternately turning on and off the light source that illuminates the document;
A reading step of reading reflected light from the document for each pixel;
A black level correction step for generating, for each pixel, a corrected image level that is a difference between the image level of the document read during the light source on period and the black level read during the light source off period;
Including black level correction method.