JP2006255976A - Image forming device, and control method for printing head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device which can correct an unevenness in the exposure energy distribution while taking characteristics which are different depending on the difference in a conversion efficiency of each light-emitting element or an unevenness in circuit constants under consideration, and to provide a control method for a printing head. <P>SOLUTION: As shown in Fig.3, an image is formed by exposing a photo-conductor drum 12 by using an LED printing head (LPH) 16 for which a plurality of LEDs 46 are arranged into a line shape. Correction data 68 which is led out in advance from a correction resolution coefficient K which is peculiar for each light-emitting element and an exposure amount distribution when at least a plurality of LEDs 46 affecting each other of the LPH 16 are lit up is respectively stored in an EEPROM 66. In this case, the correction resolution coefficient K is led out in advance from a variation amount of characteristic values which vary in response to the increase/decrease of a pulse width for making respective LEDs 46 of the LPH 16 emit light. When a control that respective LEDs 46 of the LPH 16 are respectively lit up by the pulse width based on the image data is performed, the pulse width is corrected by using the characteristic values, the correction resolution coefficient K and the correction data 68. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using a print head in which a plurality of light-emitting elements are arranged in a line, and a method for controlling the print head.

  As a print head of an image forming apparatus such as a printer, a copying machine, or a facsimile, there is a print head in which a plurality of recording elements are arranged in a line. A typical example is an LED print head (LPH) that is used in an electrophotographic image forming apparatus and uses LEDs as recording elements.

  In general, an LED print head has an LED array in which a plurality of LED chips each having a large number of LEDs arranged in a line are arranged, and a plurality of rod lenses arranged to form an image of light output from the LEDs on the surface of the photoreceptor. The SELFOC lens. In the image forming apparatus, each LED of the LED print head is driven based on the image data, light is output toward the photoconductor, and the light output by the Selfoc lens is imaged on the surface of the photoconductor. Perform exposure based on the image data of the photoconductor, and move the photoconductor and the LED print head relative to each other (this movement direction is referred to as the “sub-scanning direction”) to move the exposure position and form an image on the photoconductor To do.

  In such an image forming apparatus using an LED print head, the variation in the output energy amount causes unevenness in the exposure energy distribution, which appears on the image as a streak in the sub-scanning direction, causing a reduction in image quality.

  Conventionally, in order to correct variations in the amount of output energy, for example, in Patent Document 1, in a line recording head, an image density is measured for each pixel and compared with a reference density to obtain correction data for each printing element. A technique has been proposed in which each recording element is driven based on corresponding correction data. More specifically, the drive data (pulse width) and gradation correction (TRC) are used for the correction data, and the actual correction is corrected only by the density signal for the corresponding recording element.

  Japanese Patent Laid-Open No. 2004-228688 measures the beam profile (output light amount distribution) of each light emitting element in an LED print head, and makes each light emission constant so that the light emission amount exceeding a predetermined threshold in the beam profile is constant. Techniques for controlling the output of the element have been proposed. In this technique, the amount of light emission is determined by excluding the skirt portion of the beam spot, so that the exposure energy at each light emission point is made uniform.

  In Patent Document 3, a plurality of light emitting elements are turned on except for adjacent light emitting elements, and the light emission intensity distribution (output light amount distribution) of each light emitting element turned on by a PD (Photo Diode) is measured. Based on the feature points, specifically, the displacement of the peak position, the change in the peak value, the change in the emission diameter, the emission diameter, the amount of light, and the emission area, the emission intensity of each light emitting element is calculated based on the feature points. Correction techniques have been proposed.

  Patent Document 4 proposes a technique for controlling the light energy (output light amount) of a target dot based on the peripheral dots for each target dot in consideration of points that affect each other (peripheral dots). . Specifically, the light energy is controlled according to the distance from the peripheral dots. At this time, the influence from the peripheral dots is held in advance as a matrix, and the influence from the peripheral dots is stored as image data. It is calculated from

By the way, in addition to the amount of output energy of each recording element, the cause of unevenness in the output exposure energy distribution in a print head in which a plurality of recording elements are arranged in a line is the spread of the output energy from each recording element and its position. And variations in characteristic values such as superimposition of output energy between adjacent pixels.
JP-A-2-36962 JP-A-11-342650 JP-A-11-227254 JP 2000-198233 A

  However, each of the techniques described in Patent Document 1 to Patent Document 3 controls the output (signal, drive current value, etc.) supplied to drive each recording element individually to a predetermined characteristic value. Since the exposure energy distribution is made uniform and unevenness is not taken into account in the exposure energy distribution due to superposition, there is a problem that the image quality is deteriorated when an image is formed at a high resolution.

  In addition, in the technique described in Patent Document 4, the influence from surrounding dots around the target dot is taken into account. However, as described above, variations in characteristic values such as the amount of output energy, and exposure due to the variations. No consideration is given to uneven energy distribution. In other words, this technology assumes that there is no variation in the profile of each light emitting element, and focuses on the size of the target dot on the recording paper for the shape of the composite profile that combines the light emission profiles without variation. Since the above-mentioned variation is unavoidable in a print head in which the recording elements are arranged in a line shape, a good image quality cannot be obtained even with this technique.

  The present invention has been made to solve the above-described problems, and is an image forming apparatus capable of correcting unevenness in exposure energy distribution in consideration of different characteristics depending on differences in conversion efficiency of each light emitting element and variations in circuit constants. It is another object of the present invention to provide a print head control method.

  In order to solve the above problems, the invention of claim 1 is an image forming apparatus that forms an image by exposing a photosensitive member using a print head in which a plurality of light emitting elements are arranged in a line. A parameter unique to each light-emitting element derived in advance from a variation amount of a characteristic value that varies according to an increase or decrease in the pulse width for causing each light-emitting element of the print head to emit light, and a plurality of the print heads that affect each other at least mutually Storage means for storing correction data derived in advance based on the exposure amount distribution when the light emitting elements are turned on, and each light emitting element of the print head is turned on with a pulse width based on the image data. Lighting control means for correcting the pulse width using the characteristic value, the parameter, and the correction data. .

  According to the first aspect of the present invention, the pulse width of each light emitting element based on correction data derived in advance based on the exposure amount distribution when at least the plurality of light emitting elements that affect each other of the print head are turned on. Therefore, it is possible to correct the unevenness of the exposure energy distribution including the influence of superposition by neighboring recording elements.

  Further, the correction data includes a parameter specific to each light emitting element derived in advance from a variation amount of a characteristic value that varies according to increase / decrease of the pulse width for causing each light emitting element to emit light. Since the width is corrected, the resolution of the correction value is adjusted for each light emitting element in consideration of different characteristics depending on the difference in conversion efficiency of each light emitting element and the variation in circuit constants, and the unevenness of the exposure energy distribution is corrected appropriately. Can do.

  Examples of the characteristic value include a peak position, a peak value, a light emission shape, a light amount, a light emission area, and the like.

  According to a second aspect of the present invention, the lighting control unit can correct the pulse width in units of a predetermined number of light emitting elements and share the parameters in units of the predetermined number of light emitting elements.

  Further, the characteristic value may be a light amount when the light emitting element of the print head is turned on or a normalized light amount ratio.

  The characteristic value may be an image density of an image formed using the print head.

  Furthermore, as described in claim 5, the parameter may be a polynomial coefficient describing a relationship between a pulse width and the characteristic value.

  According to a sixth aspect of the present invention, the lighting control unit is configured to be capable of executing a test mode in which an image showing a predetermined test pattern including a halftone image is formed by the print head, and executing the test mode. An acquisition unit that reads an image formed using the print head to acquire density information of the test pattern, and an update unit that updates and stores the correction data based on the density information acquired by the previous acquisition unit. , May be further provided.

  On the other hand, in order to solve the above problems, the invention described in claim 7 is a method for controlling a print head, which is used to form an image and exposes a photosensitive member by a plurality of light emitting elements arranged in a line. In addition, a parameter specific to each light emitting element is derived in advance from a variation amount of a characteristic value that varies according to an increase or decrease in the pulse width for causing each light emitting element of the print head to emit light, and the derived parameter is taken into consideration. Correction data is derived in advance based on an exposure distribution when at least a plurality of light-emitting elements that affect each other of the print head are turned on, and the derived parameters and correction data are stored in storage means, respectively. When controlling each light emitting element of the head to be lit with a pulse width based on the image data, the characteristic value, the parameter, and the previous It is characterized by correcting the pulse width by using the correction data.

  According to the seventh aspect of the invention, since it operates in the same manner as the first aspect of the invention, the unevenness of the exposure energy distribution is corrected in consideration of different characteristics depending on the difference in conversion efficiency of each light emitting element and the variation in circuit constants. can do.

  As described above, the present invention provides an image forming apparatus and a print head control method capable of correcting unevenness in exposure energy distribution in consideration of different characteristics depending on differences in conversion efficiency of each light emitting element and variations in circuit constants. It has an excellent effect of being able to.

  Next, an example of an embodiment according to the present invention will be described in detail with reference to the drawings.

[Overall configuration of image forming apparatus]
FIG. 1 shows the overall configuration of an image forming apparatus 10 to which the present invention is applied. As shown in FIG. 1, the image forming apparatus 10 includes a photosensitive drum 12 that rotates at a constant speed in the direction of arrow A. Note that the rotation direction (arrow A) of the photosensitive drum 12 corresponds to the sub-scanning direction.

  Around the photosensitive drum 12, along the rotation direction of the photosensitive drum 12, a charger 14, an LED print head (hereinafter referred to as "LPH") 16, a developing unit 18, a transfer roller 20, and a cleaner (not shown). ) And an erase lamp (not shown) are arranged in this order.

  That is, the surface of the photosensitive drum 12 is uniformly charged by the charger 14, and then a light beam is irradiated by the LPH 16 to form a latent image on the photosensitive drum 12. The LPH 16 is connected to a control device (not shown) and the like, and is controlled by the control device and the like to output a light beam based on the image data.

  The latent image formed on the photosensitive drum 12 is supplied with toner by the developing unit 18 and developed, and a toner image is formed on the photosensitive drum 12. The toner images on the photosensitive drum 12 are taken out one by one from the paper tray 24 by the transfer roller 20 and transferred to the paper 28 conveyed by the paper conveying belt 26. The toner remaining on the photosensitive drum 12 after the transfer is removed by a cleaner (not shown), neutralized by an erase lamp (not shown), charged by the charger 14 again, and the same processing is repeated.

  On the other hand, the paper 28 onto which the toner image has been transferred is conveyed to a fixing device 30 including a pressure roller 30A and a heating roller 30B, and subjected to a fixing process. As a result, the toner image is fixed and a desired image is formed on the paper 28. The paper 28 on which the image is formed is discharged out of the apparatus.

[Detailed configuration of LPH]
Next, the configuration of the LPH 16 will be described in detail. FIG. 2 shows a cross-sectional view of the LPH 16. As shown in FIG. 2, the LPH 16 includes an LED array 40, a printed circuit board 42 on which a circuit for supporting the LED array 40 and supplying various signals for controlling the driving of the LED array 40 is formed, and the LED array. And a SELFOC lens array 44 for forming an image of the light emitted from 40 on the photosensitive drum 12.

  The LED array 40 includes a plurality of SLED chips 48 in which a plurality of SLEDs (Self-Scanning LEDs) 46 are arranged in a line (see FIG. 3A). , SLED 46 corresponds to the number of pixels (dots) corresponding to the resolution, for example, up to A3 size (420 mm × 297 mm) paper, and about 7020 are provided when printing at 600 dpi in the main scanning direction.

  The SLED chips 48 are arranged in the main scanning direction with the arrangement direction of the respective LEDs 46 aligned with the main scanning direction, and the positions in the sub-scanning direction are alternately shifted (staggered arrangement: see FIG. 4), and are attached to the printed circuit board 42. It has been. As a result of the staggered arrangement as described above, even if the pitch between the SLEDs 46 is narrow for high resolution, overlap is provided between adjacent chips in the main scanning direction, and the SLED 46 is adjusted at a predetermined pitch by adjusting the overlap size. It can be arranged in the main scanning direction. Alternatively, the SLED chips 48 may be arranged in a line in the main scanning direction with the arrangement direction of the LEDs 46 aligned with the main scanning direction.

  The SELFOC lens array 44 includes a refractive index distribution type plastic rod lens arranged as an imaging lens corresponding to each pixel (dot) corresponding to the resolution, and a light beam emitted from each LED 46. Is imaged on the photosensitive drum 12.

[SLED circuit configuration]
Next, the circuit configuration of each SLED chip 48 will be described. In the present embodiment, a case where the self-scanning SLED chip 48 is used will be described as an example. However, the present invention may not be a self-scanning type. In FIG. 3, the number of SLED chips 48 is assumed to be four in order to simplify the description. Moreover, below, when distinguishing each SLED chip 48, the chip number of 1-4 is provided and demonstrated.

  As shown in FIG. 3A, each SLED chip 48 has a lighting control signal ΦI (1-4: chip number), transfer signals CK1, CK2, and start signal for each SLED chip 48 from the LPH driving unit 100. CKS is input.

  As shown in FIG. 3B, the SLED chip 48 is provided with a power supply line 120 and a GND (ground) line 122, and a predetermined voltage VDD (5 V) is supplied from a power supply device (not shown). The

  In FIG. 3B, in order to distinguish each SLED chip 48, the chip numbers 1 to 4 are shown in parentheses () at the end of the code, and this will be followed in the following description. Similarly, members provided for each SLED chip 48 and generated signals will be described by indicating the chip number in parentheses at the end of the reference numerals. In FIG. 3B, the configuration of each SLED chip 48 (1-4) is the same, so only the SLED chip 48 (1) is shown in detail, and the remaining SLED chips (2-4) are shown. Is omitted.

  The SLED chip 48 includes a thyristor 124 for each of the plurality of LEDs 46 arranged in the SLED chip 48. When the thyristor 124 is turned off when the trigger is set to the high level, the current Itr is changed to a point G. At the same time, a current Ib2 flows from the point G to the base of the transistor Q2 (Itr≈Ib2). Thereby, the transistor Q2 is turned on, and the collector current of the transistor Q2 flows. That is, the base current Ib1 of the transistor Q1 flows, and the transistor Q1 is also turned on.

  When the transistor Q1 is turned on, the collector current IC1 of the transistor Q1 flows, the voltage at the point P increases, and the current Itr stops flowing. However, since the collector current Ic1 of the transistor Q1 flows to the base of the transistor Q2 (current Ib2), the transistor Q2 is kept on.

  Thereby, even if the trigger becomes a low level, the transistors Q1 and Q2 maintain the on state. In this state, the voltage VDD is held, the LED can be turned on, and a predetermined light amount can be obtained by performing pulse width modulation.

  As shown in FIG. 3B, the anode side of each thyristor 124 is connected to the power supply line 120 and supplied with a predetermined voltage VDD. The first-stage thyristor 124 has a start signal CKS (voltage) as a trigger for lighting the LEDs 46 of the SLED chip 48 from the point G1 (numbers following the point G indicate the order of the plurality of LEDs 46) connected to the gate side. Is applied. Further, the point G (1 to 128) connected to the gate side of the thyristor 124 at each stage is connected in series via the diode 126. Further, the points G (1 to 128) of each stage are connected to the GND line 122 through the resistors 128, respectively. The GND line 122 maintains a predetermined voltage at the first stage and decreases by a predetermined potential as it goes to each stage.

  Further, the point G (1 to 128) is connected to the anode side of the LED 46, and the cathode side of the LED 46 is supplied with the lighting control signal ΦI (1 to 4: chip number) from the LPH driving unit 100. It is connected to the. When the lighting control signal ΦI is at a low level (L), the LED 46 is lit if the thyristor 124 whose gate is the point G (1 to 128) is ON.

  Further, the cathode side of the odd-numbered thyristor 124 is connected to the transfer signal CK1, and the cathode side of the even-numbered thyristor 124 is connected to the transfer signal CK2. According to the transfer signals CK1 and CK, the potential of the point G (1-128) is increased by a predetermined potential. That is, the potential at the point G reaches the predetermined potential at which the LED 46 can be turned on in order from the first point G1 to the subsequent stage, and the SLED chip 48 can be self-scanned.

  The LPH drive unit 100 generates a drive signal for driving the SLED chip 48 based on the screen-processed image data and Tag data, and outputs the drive signal to the SLED chip 48. The SLED chip 48 is driven based on this drive signal, that is, the LED 46 is turned on. More specifically, the LED 46 emits light that is pulse-modulated based on the screen-processed image data and intensity-modulated based on the low-density correction data or the high-density correction data selected based on the Tag data. Is output.

  Further, as shown in FIG. 3B, the LPH drive unit 100 includes a drive signal generation unit 140, three flip-flops 142A1 to A3, and three flip-flops 142B1 to B3. In addition, the LPH driving unit 100 includes, for each of the SLED chips 48 (1 to 4) provided in the LPH 16, a correction memory 144 (1 to 4) and a flip-flop 146A (1 to 4) as storage means of the present invention. 4) It has 146B (1-4) and the drive element part 148 (1-4) which bears the function as a correction | amendment means of this invention.

  The flip-flops 142A1 to A3, the flip-flops 142B1 to B3, the correction memories 144 (1 to 4), the flip-flops 146A (1 to 4), 146B (1 to 4), and the drive element units 148 (1 to 4) are driven. A signal generated by the drive signal generator 140 is connected to the signal generator 140.

  The drive signal generator 140 receives the line synchronization signal LS and the transfer clock SCLK from the timing signal generator (not shown).

  The drive signal generator 140 generates the transfer signals CK1, CK2 and the start signal CKS at a predetermined timing synchronized with the line synchronization signal LS and the transfer clock SCLK, and outputs them to each SLED chip 48 (1-4). Then, a lighting strobe signal STB indicating a lighting possible period of each LED 46 is generated and output to each driving element unit 148 (1 to 4).

  The drive signal generation unit 140 generates select signals SCK1, SCK2, and SCK3 at predetermined timings synchronized with the line synchronization signal LS and the transfer clock SCLK, and the select signal SCK1 is the clock terminal (flip-flops 142A1 and 142B1). CK), the select signal SCK2 is output to the clock terminals of the flip-flops 142A2 and 142B2, and the select signal SCK3 is output to the clock terminals of the flip-flops 142A3 and 142B3.

  In addition, the drive signal generation unit 140 generates the latch signal LCH at a predetermined timing synchronized with the line synchronization signal LS and the transfer clock SCLK, and the flip-flops 146A (1-4) and flip-flops 146B (1-4). To each output. Further, the drive signal generation unit 140 generates a 7-bit address signal ADL for designating an address, and outputs it to the correction memory 144 (1 to 4).

  The lighting data VDATA is branched and inputted to the input terminals (D) of the flip-flops 142A1 to A3. The output terminals (Q) of the flip-flops 142A1 to A3 are respectively connected to the input terminals (D) of the flip-flops 146A (1 to 3), and the input lighting data VDATA is input to the flip-flops 146A (1 to 3). Is output. The lighting data VDATA is input as it is to the input terminal (D) of the flip-flop 146A (4). The output terminals (Q) of the flip-flops 146A (1 to 4) are connected to the corresponding drive element units 148 (1 to 4).

  The flip-flops 142A1 to A3 output signals based on select signals SCK1, SCK2, and SCK3 input from the respective clock terminals, and the flip-flops 146A (1 to 4) output latch signals LCH input from the clock terminals. Input and output signals based on Accordingly, the lighting signals VD (1 to 4) of the corresponding SELD chips 48 (1 to 4) are input to the driving element units 148 (1 to 4) from the flip-flops 146A (1 to 4), respectively. It has become.

  On the other hand, TAG data is branched and input to the input terminals (D) of the flip-flops 142B1 to B3. The output terminals (Q) of the flip-flops 142B1 to B3 are connected to the correction memories 144 (1 to 3), respectively, and the inputted TAG data is output to the correction memories 144 (1 to 3). The TAG data is input as it is to the correction memory 144 (4).

  The correction memory 144 (1 to 4) stores low density correction data and high density correction data for each of the 128 LEDs 46 provided in the corresponding SLED chips 48 (1 to 4). For example, this storage is performed by reading from an EEPROM 66 (not shown, see FIG. 6) provided on the printed circuit board 42 and connected to the LPH driving unit 100 when the power is turned on (details will be described later). In the present embodiment, the storage location of each correction data in the correction memory 144 is designated by an 8-bit address, the upper 1 bit indicates low / high density, and the lower 7 bits indicate LED correction data.

  The correction memory 144 (1 to 4) is a flip-flop corresponding to each of the TAG data as the correction signal COR (1 to 4) stored as the correction signal COR (1 to 4) with the address specified as the upper 1 bit and the address signal ADL as the lower 7 bits. 146B (1-4). That is, it is specified by the address signal ADL which correction data corresponding to which LED 46 of the LEDs 46 provided in the SLED chip 48 is read out, and the correction data for low density or the correction data for high density is selected by the TAG data. Whether to read is specified. The outputs (Q) of the flip-flops 146B (1-4) are connected to the corresponding drive element units 148 (1-4).

  The flip-flops 142B1 to B3 input / output signals based on select signals SCK1, SCK2, and SCK3 input from the respective clock terminals, and the flip-flops 146B (1 to 4) are input from the respective clock terminals. A signal is input / output based on the latch signal LCH. Accordingly, the correction signals COR (1-4) of the corresponding SELD chips 48 (1-4) are read from the correction memories 144 (1-4) to the drive element units 148 (1-4), respectively. The signal is input from the flip-flop 146B (1 to 4).

  In each drive element unit 148 (1-4), as shown in FIG. 5, a 4-bit counter 150, a comparator 152, AND circuits 154A, 154B, 154C, 154D, and transistors (n-channel MOSFETs) 156, 158A 158B, 158C, 158D.

  In the 4-bit counter 150, a pulse modulation clock PWMCLK that divides the lighting-enabled period of each LED 46 indicated by the lighting strobe signal STB into 16 is input to the clock terminal (CLK), and a drive signal generator is provided to the clear terminal (CLR). The strobe signal STB from 140 is input. The 4-bit counter 150 counts the number of pulses of the input pulse modulation clock PWMCLK, outputs the count value CD, and resets the count value CD when the strobe signal STB is input.

  The output terminal (Q) of the 4-bit counter 150 is connected to the negative side input terminal of the comparator 152, and the comparator 152 receives the count value of the number of pulses of the pulse modulation clock PWMCLK. The lighting signal VD (1-4) input from the corresponding flip-flop 146A (1-4) to the drive element unit 148 is input to the plus side input terminal of the comparator 152.

  The comparator 152 compares the input count value with the lighting signal VD, and the comparison result is “1” when “count value CD ≦ VD” and “0” when “count value CD> VD”. Is output. The output of the comparator 152 branches to the AND circuits 154A to 154D and the gate side of the transistor 156 and is input to each of them.

  To the AND circuits 154A to 154D, the 4-bit correction signals COR (1 to 4) input from the corresponding flip-flops 146B (1 to 4) are branched and input to the bits (COR0 to COR3). The outputs of the AND circuits 154A to 154D are connected to the gate sides of the transistors 158A to 158D, and the AND circuits 154A to 154D output the output of the comparator 152 and each bit value (COR0 to COR3) of the correction signal COR. An AND operation is performed, and the result is input to the gate side of the transistors 158A-D.

  The sources of the transistors 156 and 158A to 158D are grounded, and the drain sides are connected in parallel via resistors R, RA, RB, RC, and RD. The potential at the connection point P of the resistors R, RA, RB, RC, and RD is supplied from the drive element unit 148 to the corresponding SLED chips 48 (1 to 4) as the lighting control signal ΦI (1 to 4). .

  Incidentally, low density correction data and high density correction data (hereinafter referred to as correction data) for correcting the amount of light emitted from the LED 46 stored in the correction memory 144 (1 to 4) are obtained in advance and described above. Stored in the EEPROM 66 provided on the printed circuit board 42.

  As shown in FIG. 6, the EEPROM 66 measures the light intensity distribution of the LPH 16 and generates correction data based on the light intensity distribution. It is configured to be connectable to each of correction data generation devices 72 (details will be described later) for correcting correction data of the EEPROM 66 based on the formed image density, and the correction data 68 can be written by the measuring device 70. The correction data 68 written in the EEPROM 66 is rewritten by the generation device 72.

  Each of the measurement device 70 and the correction data generation device 72 is configured as a device physically different from the image forming device 10 and is used by connecting with a predetermined cable or the like when necessary. The correction data generation device 72 may be built in the image forming apparatus 10.

  In this embodiment, as an example, before the LPH 16 is mounted on the image forming apparatus 10, the light intensity distribution of the LPH 16 is measured as an output characteristic by the measuring device 70, and the light intensity distribution is used as the first correction data. The correction data 68 based on the above (hereinafter referred to as correction data 68A) is written in the EEPROM 66. Further, before the factory shipment after assembly of the image forming apparatus 10, the correction data generating apparatus 72 and the image forming apparatus 10 are connected, and the correction data generating apparatus 72 actually uses the image forming apparatus as the second correction data. Correction data 68 (hereinafter referred to as correction data 68B) is calculated on the basis of the image density formed in Step 10, and the correction data 68A is read from the EEPROM 66 and is combined with the correction data 68B as third correction data. 68 (referred to as correction data 68C) is generated and written back to the EEPROM 66. The image forming apparatus 10 is shipped after the correction data is written back to the EEPROM 66 by the correction data generating apparatus 72.

[Detailed configuration of measuring device]
The measuring device 70 may be a conventionally known one, and in this embodiment, as an example, the exposure energy distribution of the print head is measured by the measuring device 70 shown in FIG.

  As shown in FIG. 7, the measuring apparatus 70 includes an LED array 40 in which a plurality of LEDs are arranged in a line in the arrow B direction (main scanning direction), and a selfoc lens array (SLA) 44. A sensor 80 for measuring the exposure energy distribution by the LPH 16 is provided. The LPH 16 is set at a predetermined position by a holder member (not shown).

  The sensor 80 includes a magnifying lens (× 10 magnification in this embodiment) 80B and a transmission diffusion plate 80C on the light receiving surface side of a line CCD 80A in which a plurality of CCDs (Charge Coupled Devices) are arranged in a line. It is configured. Further, the sensor 80 has the light receiving surface of the line CCD 80A facing the light output direction of the LPH 16, and the CCD array direction is orthogonal to the main scanning direction indicated by the arrow B, etc. It is installed on a sensor moving stage 82 that can move at high speed.

  That is, the sensor 80 receives light incident on each light receiving surface by each CCD while moving at a constant speed in the main scanning direction (LED arrangement direction), and outputs an electrical signal corresponding to the received light amount (this book). In the embodiment, the output energy distribution of the LPH 16 in a direction orthogonal to the LED arrangement direction (hereinafter referred to as “sub-scanning direction”) can be measured.

  Here, the transmission diffuser plate 80C is disposed at the focal position of the LPH, and the light from the LPH is uniformly diffused toward the magnifying lens 80B. Therefore, the sensor 80 is normalized in the LPH main scanning direction. The relative exposure energy distribution can be measured. Note that it is preferable to use a transmissive diffusion plate 80C that is closer and thinner than complete diffusion.

  The sensor 80 is connected to a personal computer (PC) 86 via a driver 84, and the personal computer 86 is also connected to the LPH 16 via a driver 84. The personal computer 86 is also connected to a drive unit (not shown) of the sensor moving stage 82.

  The personal computer 86 outputs lighting data to the LPH 16 via the driver 84, controls the lighting of each LED of the LPH 16, and outputs a moving stage control signal to a driving unit (not shown) of the sensor moving stage 82 to move the sensor. The drive of the stage 82 is controlled to move the sensor 80 at a constant speed in the main scanning direction, and a measurement timing signal is output to the sensor 80, and exposure energy measurement by the sensor 80 is controlled to be ON / OFF.

  The output of the sensor 80 is connected to the arithmetic processing unit 88. The measurement result by the line CCD 80A from the sensor 80, that is, an electric signal (8-bit data) corresponding to the amount of light received by each CCD is serially input to the arithmetic processing unit 88. Is done.

  By the way, in the so-called electrophotographic process in which the uniformly charged photosensitive drum 12 is exposed to form an electrostatic latent image and then developed as in the image forming apparatus 10, the exposure energy and the developed image are generally obtained. There is a relationship (development characteristics) as shown in FIG. 9A between the densities, and a desired image density can be obtained by adjusting exposure energy using these development characteristics. When this development characteristic is simplified, as shown in FIG. 9B, a linear area where the exposure energy amount and the image density are substantially proportional to each other, and an image density is produced even when exposure is performed on the lower exposure energy side of the linear area. There is no dead (not developed) dead zone and a saturated region where the image density does not increase even if the exposure amount on the high exposure energy side is increased from the linear region. That is, the image density has a correlation with the exposure energy in the linear region and is hardly affected by the exposure amount in the dead zone and the saturated region.

  Here, ideally, it is desirable to obtain correction data by converting the light profile into a density in consideration of the overlap in the main scanning direction and the sub-scanning direction in accordance with the actual lighting pattern for each density. However, it is not practical to calculate correction data for every lighting pattern in view of resources required for the calculation.

The following points are listed as characteristics that should be considered in practice.
(1) In high-resolution exposure (for example, 1200 × 2400 dpi × 2 bits), the exposure energy distribution due to the adjacent light emitting point (light emitting point adjacent to the target light emitting point) with respect to the exposure energy distribution of the light emitting point of interest (LED 46) is very large. The exposure energy distribution shape is greatly affected by the overlap of exposure energy at adjacent light emitting points. The overlap of exposure energy due to neighboring light emitting points other than adjacent light emitting points has a small contribution to the exposure energy distribution shape and greatly affects the average level of exposure energy.
(2) Since the light beam from the proximity emission point of LPH passes through the position in the vicinity of the pole within the SELFOC lens, there is little sudden change in the shape of the exposure energy distribution.
(3) Complete isolated points that do not have lighting around them cannot achieve stable resolution due to the limitations of development, and it is possible to develop stably only after multiple points of light emission overlap. The light emitting point is turned on. Therefore, it is necessary to consider the overlap of the exposure energy distribution between the adjacent light emitting points in the low density region and the high density region as well.

  In the same LED chip 48, the exposure energy between adjacent LEDs 46 is substantially the same. For example, in a 2on2off lighting pattern (LPH16 LEDs 46 are alternately turned on two by two), No. 1, 2, 5, There is almost no difference in the shape of the exposure energy distribution between when the LEDs 46 of No. 6, 9, 10,... Are turned on and when the LEDs 46 of No. 3, 4, 7, 8, 11, 12,.

  FIG. 10 shows the exposure energy distribution when the lighting pattern is 2 on 2 off. (A) shows the exposure energy distribution at the time of low density writing, and (b) shows the exposure energy distribution at the time of high density writing. As shown in the figure, the average level of exposure energy differs between low density writing and high density writing. Considering the difference in average level, it is practical to apply the above development characteristics after converting the scale for each density.

  Further, FIG. 11 schematically shows a cross section in the sub-scanning direction of the exposure energy distribution. As shown in FIG. 11A, when the influence of stray light is included in the cross section in the sub-scanning direction of the exposure energy distribution, addition is performed by shifting by a predetermined interval in the sub-scanning direction. And a cross section in the sub-scanning direction considering the influence of stray light as shown in (C) is obtained. However, as described above, the exposure energy amount that is equal to or greater than the threshold value and reaches the saturation value affects the image density, so the energy amount in the shaded area in FIG. 11D affects the image density.

  Due to the above characteristics, the light profile is measured with a lighting pattern for lighting adjacent light emitting points, such as 2 on 2 off and 4 on 4 off, and the development characteristics are scale-converted for each of the low density region and the high density region.

  Therefore, as shown in FIG. 8, the arithmetic processing unit 88 includes a memory 32, a sub-scanning direction overlay calculator 31, a memory 34, a comparator 36, and And an adder 38. In the following description, when the low concentration and the high concentration are distinguished from each other, the low concentration memory 34, the comparator 36, and the adder 38 have “A” at the end of their respective symbols. The memory 34, the comparator 36, and the adder 38 will be described with “B” added to the end of each code.

  In the sub-scanning direction overlay calculator 31, the measurement results of the CCDs of the line CCD 80A are added while being shifted by a predetermined interval in the sub-scanning direction. The predetermined interval can be set according to the writing resolution in the sub-scanning direction of the image forming apparatus.

  The memory 34A stores a predetermined low density threshold TH (low) set in advance, and the memory 34B stores a predetermined high density threshold TH (high) and image density set in advance. A high density saturation value SAT (high) corresponding to a saturated exposure amount is stored. The low density threshold and the high density threshold are values corresponding to the minimum exposure amount required to obtain the low density and high density image densities.

  Each of the comparators 36A and 36B receives the measurement result of each CCD input from the line CCD 80A. The comparator 36A compares the measurement result of each CCD with the low density threshold value stored in the memory 34A, and outputs data indicating the excess of the low density threshold value as the comparison result. Further, the comparator 36B compares the measurement result of each CCD with the high concentration threshold value and the high concentration saturation value stored in the memory 34B, and the comparison result shows the high concentration threshold value up to the high concentration saturation value. Data indicating the excess is output as data indicating the excess of the high concentration threshold.

  Adders 38A and 38B add the output data from comparators 36A and 36B, respectively. In other words, the adders 38A and 38B, of the measurement results of the CCDs of the line CCD 80A, exceed the threshold for low density or the threshold for high density that affects the image density (the part exceeding the saturation value for high density). Are added to the output data from the CCD for one line of the line CCD 80A, the addition result is stored in the memory 32, and the addition value is reset.

  As a result, for each measurement by the line CCD 80A, the memory 32 sequentially stores data corresponding to the addition (integration) value of the exposure energy amount exceeding the predetermined low density threshold and high density threshold for one line CCD. Then, when the measurement is performed in the main scanning direction of the print head, the profile of the data in the main scanning direction for the low density and the high density (hereinafter referred to as “light profile for low density”, “high density”). For each). That is, by extracting and adding only the exposure energy that affects the image density, a characteristic value correlated with the image density is obtained, and the image density is indirectly measured.

  The arithmetic processing unit 88 is connected to the personal computer 86 via the driver 84, and the low-density optical profile and the high-density optical profile stored in the memory 32 are respectively transferred. The personal computer 86 calculates correction data for low density based on the light profile for low density, and calculates correction data for high density based on the light profile for high density (details will be described later). Explained in the section).

[Detailed configuration of correction data generator]
Next, the configuration of the correction data generation device 72 will be described in detail. As shown in FIG. 6, the correction data generation device 72 includes a reading device 90, a density profile derivation circuit 92, a correction data calculation circuit 94, a data generation circuit 96 that generates data to be written to the EEPROM 66, and the correction data generation. When the apparatus 72 is connected to the image forming apparatus 10, a driver 98 for reading and writing data from and to the EEPROM 66 is provided.

  The reading device 90 may be a conventionally known device as long as it can read the density of the output image (test pattern) output from the image forming apparatus 10. For example, as shown in FIG. 16, the reading of the output image density is performed in such a manner that CCDs 352 are arranged in a row as reading elements below the platen glass 350 as shown in FIG. , And a test pattern is recorded by a reading device 358 including a carriage 356 that houses an optical system 354 that forms an image of reflected light from a document placed on the platen glass 350 and enters the CCD 350. The sheet 360 is placed at a predetermined position on the platen glass 350 determined by the positioning unit 362 so that the arrangement direction of the LED array 40 of the LPH 16 on which the test pattern is recorded is orthogonal to the pixel arrangement direction of the CCD 352. While moving the carriage 356 in a direction (indicated by an arrow V) perpendicular to the arrangement direction, the CCD 352 causes a test pattern. Can be applied shall be performed by reading a density of down.

  In the reading device 90, the reading result (density data) read by the reading device 90 is input to the density profile deriving circuit 92. Note that the reading device 90 is not necessarily built in the correction data generating device 72, and may be used by connecting with a predetermined cable or the like when necessary. For example, if the image forming apparatus 10 has a scanner function, the scanner function can be used as the reading device 90.

  The density profile deriving circuit 92 is also connected to the correction data calculation circuit 94, and based on the density data input from the reading device 90, the density profile generated on the image formed by the image forming apparatus 10 is derived and corrected. The data is output to the data arithmetic circuit 94.

  The correction data calculation circuit 94 is also connected to the data generation circuit 96, calculates correction data 68B based on the density profile input from the density profile derivation circuit 92, and outputs the result to the data generation circuit 96.

  The data generation circuit 96 is accessible to the EEPROM 66 via the driver 98, reads the correction data 68A stored in the EEPROM 66, and reads the correction data 68A read from the correction data calculation circuit 94. The correction data 68C is generated by combining with the data 68B, and the correction data 68C is written back to the EEPROM 66 via the driver 98.

[Action]
Next, the operation of the present embodiment will be described.

[Correction data calculation process]
First, correction data calculation processing by the personal computer 86 will be described. FIG. 12 shows a control routine for correction data calculation processing executed by the personal computer 86. When performing the correction data calculation process, the LPH 16 used for the image forming apparatus 10 is held in advance in the measuring apparatus 70 by a holder member (not shown) and set at a predetermined position.

  First, in step 200 of FIG. 12, the personal computer 86 outputs lighting data instructing lighting to the LPH 16 with a lighting pattern of 2 on 2 off. Accordingly, the LPH 16 receives the lighting data and turns on the LED 46 of the LPH 16 by 2 on 2 off.

  In the next step 202, the personal computer 86 outputs a movement stage signal to a drive unit (not shown) of the sensor movement stage 82 and causes the sensor movement stage 82 to move the sensor 80 at a constant speed in the main scanning direction. In the next step 204, a measurement timing signal is output to the sensor 80 to cause the sensor 80 to measure a cross section in the sub-scanning direction of the exposure energy distribution of the LPH 16.

  Then, until the movement of the sensor 80 in the main scanning direction is completed, the process proceeds from step 206 to step 208, the process returns from step 208 to step 204 every time a predetermined time elapses, and the measurement timing signal is output again. Thereby, until the movement of the sensor 80 in the main scanning direction is completed, the cross section in the sub-scanning direction of the exposure energy distribution of the LPH 16 is measured by the sensor 80 every predetermined time.

  In other words, in this embodiment, the personal computer 86 turns on the LED 46 of the LPH 16 with 2 on 2 off, and the sensor 80 moves at a constant speed in the main scanning direction, and the cross section in the sub scanning direction of the exposure energy distribution of the LPH 16 at a predetermined time period. As measured, a lighting signal, a moving stage control signal, and a measurement timing signal are output. The predetermined time period is determined so that the measurement result of the sensor 80 has a resolution of about several μm.

  At this time, the measurement result by the sensor 80, that is, each CCD output provided in the line CCD 80A of the sensor 80 is input to the arithmetic processing unit 88, and each CCD output for one line of the line CCD 80A is given a predetermined interval in the sub-scanning direction. Overlay calculation is performed by adding each shifted one by one, and among the measurement results after overlay calculation, the amount exceeding the low concentration threshold is added, and separately from the addition, the high concentration threshold is exceeded. (Excluding the portion exceeding the saturation value for high concentration) is added. That is, in the measurement result, an integral value obtained by rounding down the threshold value for low concentration and an integral value obtained by rounding down a portion below the threshold value for high concentration and a portion exceeding the saturation value for high concentration are calculated. This calculation result is stored in the memory 32.

  As a result, the sensor 80 is repeatedly measured at predetermined intervals while moving at a constant speed in the main scanning direction, the calculation processing unit 88 performs calculations, and the integrated values are sequentially stored in the memory 32. Finally (when the movement of the sensor 80 in the main scanning direction is completed), the exposure energy amount profile exceeding the low density threshold value in the main scanning direction profile (light profile for low density), and the exposure exceeding the high density threshold value. Each profile in the main scanning direction of the amount of energy (light profile for high density) is obtained.

  When the movement of the sensor 80 in the main scanning direction is completed, the process proceeds from step 206 to step 210. In step 210, the personal computer 86 reads the low-density and high-density optical profiles stored in the memory 32, and in the next step 212, the read low-density and high-density optical profiles are substantially omitted. The correction data for the low density and the correction data for the high density are respectively calculated so as to be flattened.

  Hereinafter, calculation of correction data will be described.

  FIG. 13 is a diagram showing characteristic values (≈light quantity) of the nth LED 46 (Ln in the drawing) and the n + 1th LED 46 (Ln + 1 in the drawing) when the pulse width of the lighting control signal ΦI is changed.

  As shown in the figure, the LED has a region (hereinafter referred to as an offset) that does not emit light even when a pulse is applied when the pulse width is increased from 0 and is increased. Different for each LED.

  Further, as shown in the figure, the amount of change in the characteristic value of the LED in accordance with the change in the pulse width of the lighting control signal ΦI (the slope of the characteristic of each LED in FIG. 13) is unique to each LED. Different for each. Note that Ln + 1 ′ indicated by a dotted line in the drawing indicates a characteristic value of Ln + 1 when only the offset is corrected. In order to adjust the amount of exposure energy, it is necessary to consider the variation in inclination for each LED, and it is possible to set a correction value for correcting the variation in inclination. Further, in FIG. 13, it is approximated by a straight line, but strictly speaking, it is slightly curved, and a coefficient determined by an error from the straight line is set as a correction resolution coefficient K.

  FIG. 14 shows an example of an optical profile acquired when the LPH is turned on with a 2 on 2 off pattern. The correction value is derived as an integer from 1 to 256 represented by 8 bits, and a peak (valley) is detected from the light profile obtained by the sensor 80 as shown in FIG. Is integrated, and the integrated value Sn is divided by the distance Ln from the valley to the valley to obtain the exposure energy density En between the valleys. The exposure energy density En thus obtained in each valley is used as a characteristic value of each light emitting point, and a correction value is set according to an error from the target value so that this characteristic value is matched with a predetermined target value. Then, planarization is performed by increasing or decreasing the exposure amount. By performing such flattening for each of the relatively low density and high density, a plurality of correction data corresponding to each density can be calculated.

  The predetermined target value may be an ideal characteristic value for obtaining a density (here, 50% density) according to the LPH lighting pattern (here, 2 on 2 off) at the time of measurement. Adjacent characteristic values may be used.

Considering the above, the set pulse width of each LED can be expressed by the following equation (1) using a preset reference pulse width.
Pulse width = K × reference pulse width × (1+ (correction value / 256)) + offset variation
... (1)
Here, in the equation (1), the pulse width is adjusted in the range of K × reference pulse width to twice K × reference pulse width. That is, the reference pulse width is set in consideration of various conditions, and if the pulse width becomes too short, the lighting of the LED becomes unstable. Therefore, it is preferable that the pulse width does not become less than K × reference pulse width. Considering that K takes a value smaller than 1, it is also necessary to secure a margin for the reference pulse width. The margin amount is a design item according to the performance of the device. Further, as shown in the equation (1), by deriving the correction resolution coefficient K for each light emitting element, the amount of change in the pulse width due to the change of 1 bit of the correction value is adjusted for each light emitting element. Will be. The offset variation indicates the offset variation of each LED, and needs to be measured and set in advance.

  First, in order to derive the correction resolution coefficient K for each LED using the above equation (1), it is assumed that the correction resolution coefficient K = 1 for all LEDs and the correction value = 128, A temporary correction value is derived. Thereafter, each LED is controlled using this temporary correction value, and the measurement by the measuring device 70 is performed again to derive a correction resolution coefficient K.

  Here, the derivation of the correction resolution coefficient K will be described in more detail.

First, Equation (1) is modified to define Equation (2).
Effective pulse width = pulse width-offset variation
= K × reference pulse width × (1+ (correction value / 256)) (2)
Here, the relationship between the effective pulse width and the light amount ratio is a linear expression passing through zero. In other words, the ratio between the light amount ratio and the effective pulse width is a constant. Therefore, if the arbitrary LED light quantity ratio measured by the measuring device 70 is 10% smaller than the desired light quantity ratio, the desired light quantity ratio should be obtained by increasing the effective pulse width by 10%.

Since K = 1 and correction value = 128 in the first measurement, the correction value Corr to be obtained is
1+ (Corr / 256) = 1.1 × (1+ (128/256))
Therefore, Corr = 166.

  However, as described above, since the relationship between the effective pulse width and the light amount ratio is not strictly a linear relationship, an error occurs. The error amount is measured by setting the correction value 166 and measuring again, and the correction resolution coefficient K is calculated from the ratio with the average value at that time.

  By deriving a correction value again using the obtained correction resolution coefficient K, a correction value considering the correction resolution coefficient is obtained, and the characteristic value of each LED becomes flat.

  If the LPH lighting pattern at the time of measurement is 2 on 2 off, the correction value and the correction resolution coefficient K become one value by the four LEDs 46 (however, if the LED is turned on for 2 on 2 and the process is performed twice, the two LEDs To obtain one correction value). Further, since there is no significant difference in the correction resolution coefficient in units of LED chips, the correction resolution coefficient K can be shared among the LEDs 46 provided in the same LED chip 48 (the correction resolution corresponding to each LED is thereby set). The capacity of the LUT for storing the coefficient K can be reduced).

  In this way, when the correction data capable of satisfactorily correcting unevenness is determined for each of the high density area and the low density area, the personal computer 86 stores the correction data in the EEPROM 66 of the LPH 16 in step 214 of FIG. Store and end the process.

  Thereafter, the LPH 16 is loaded into the image forming apparatus 10 and used for image formation by the image forming apparatus 10. That is, in the image forming apparatus 10, light corresponding to the image data is transmitted from the LPH 16 toward the photosensitive drum 12 whose surface is uniformly charged by the charger 14 while adjusting the light amount based on the determined correction data. Irradiate to form an electrostatic latent image on the photosensitive drum 12. The electrostatic latent image is developed by the developing device 18 and transferred onto the paper 28 by the transfer roller 20. In this way, the paper 28 onto which the toner image has been transferred is subjected to a fixing process by the fixing device 30 and then discharged out of the apparatus.

  In the above description, the plurality of SLEDs 68 are arranged in a staggered pattern with their positions shifted in the sub-scanning direction, and as shown in FIG. 15, the sub-scanning direction cross section (A) of the exposure energy distribution at the time of measurement and the image forming time Although the cross section (B) of the exposure energy distribution is different from that in the sub-scanning direction (B), as described above, the sub-scanning direction cross section of the measured exposure energy distribution is shifted and added in the sub-scanning direction, and the overlap in the sub-scanning direction is added. By obtaining a cross section in the sub-scanning direction of the exposure energy distribution that is taken into consideration, as shown in (C) and (D), an exposure energy distribution substantially equivalent to the exposure energy distribution during image formation can be obtained. In other words, the present invention can be applied even when a plurality of SLEDs 68 are arranged with their positions shifted in the sub-scanning direction by considering the overlap in the sub-scanning direction.

  Next, processing by the correction data generation device 72 will be described.

  The correction data generating device 72 is connected to the image forming apparatus 10, and the paper 28 printed with the test pattern printed by the image forming apparatus 10 is set on the reading device 90 of the correction data generating device 72 and recorded on the paper 28. Measure the density of the test pattern.

  In the correction data generation device 72, the correction data B is derived so as to flatten the density of the measured test pattern, using the correction resolution coefficient K previously derived by the measurement device 70 and stored in the EEPROM 66.

  Further, the correction data generation device 72 reads the correction data 68A stored from the EEPROM 66 and multiplies or adds the correction data 68B input to the correction data 68A to generate correction data 68C that combines the two. . That is, the correction data 68C is data obtained by merging the correction data 68A and the correction data 68B for correcting density unevenness that cannot be corrected by the correction data 68A.

  Finally, the correction data 68C is written in the EEPROM 66 as new correction data 68 (overwriting the correction data 68A), and the correction data setting is completed. Thereby, the setting of the correction data is completed.

  That is, since the image forming apparatus 10 is shipped with the correction data 68C stored in the EEPROM 66, the image forming apparatus 10 uses the correction data 68C to correct the amount of light when used by the user after shipment. Print the image as you go.

  The correction data generation device 72 is configured to be detachable from the image forming apparatus 10 and may be connected to adjust correction data before shipment or during maintenance, but the correction data generation device 72 is provided in the image forming apparatus 10. It may be built in so that it can cope with changes over time after shipment. Since the correction resolution coefficient K hardly changes with time, only the correction value is corrected.

  In this way, by performing the light amount correction using the correction data 68C obtained by merging the correction data 68A and the correction data 68B, the correction using the correction data 68A is performed after the correction using the correction data 68A. . In other words, the image forming apparatus 10 corrects the output unevenness of the high frequency component of the optical print head 16 with the correction data 68A component obtained based on the light intensity distribution of the optical print head 16 included in the correction data 68C. The output unevenness, that is, the output unevenness of the low frequency component, can be corrected with the correction data 68B component obtained based on the density distribution of the actual output result by the image forming apparatus 10 included in the correction data 68C. A high-quality image with little density unevenness can be obtained.

  As described above in detail, according to the present embodiment, an image is formed by exposing the photosensitive drum 12 using the LPH 16 in which a plurality of LEDs 46 are arranged in a line, and each LED 46 of the LPH 16 emits light. When a plurality of LEDs 46 that affect at least one of the correction resolution coefficient K inherent to each light emitting element and the LPH 16 that are derived in advance from the fluctuation amount of the characteristic value that fluctuates in accordance with the increase / decrease of the pulse width to be activated are lit. The correction data 68 derived in advance based on the exposure amount distribution is stored in the EEPROM 66, and when controlling each LED 46 of the LPH 16 to light with the pulse width based on the image data, the characteristic value and the correction Since the pulse width is corrected using the resolution coefficient K and the correction data 68, the conversion effect of each light emitting element is corrected. Unevenness of exposure energy distribution in consideration of the different characteristics due to variations in the difference and the circuit constants of can be corrected.

  In the present embodiment, the density unevenness is calculated by reading an image (test pattern) formed on the paper by the reading device 90, but the present invention is not limited to this. For example, as shown by a dotted line in FIG. 1, a sensor 200 that measures the latent image potential on the photosensitive drum 12 is attached in the image forming apparatus 10, and correction is performed so that the potential unevenness measured by the sensor 200 is reduced. The data 68B may be calculated, or a sensor 202 for measuring the toner amount (toner image density) on the photosensitive drum 12 is attached, and the correction data 68B is set so that the density unevenness measured by the sensor 202 is reduced. It may be calculated.

  In the present embodiment, the case where correction data 68C obtained by combining correction data 68A and correction data 68B is stored in EEPROM 66 has been described as an example, but the present invention is not limited to this. The correction data 68A and the correction data 68B are stored in the EEPROM 66, and when the correction data is transferred to the driver 64, the correction data 68A and the correction data 68B are combined to generate the correction data 68C and transferred to the driver 64. You may make it do. However, in this case, the image forming apparatus requires a means for combining the correction data 68A and the correction data 68B, and it is necessary to perform a combining process every time the correction data is transferred. Thus, it is preferable to store the correction data 68C in the EEPROM 66 in advance.

  In this embodiment, the case where correction data is set before shipment from the factory has been described as an example. However, the correction data may be set after shipment. For example, when the photosensitive drum 12 after installation of the apparatus is replaced, a test pattern is output, correction data 68B for correcting density unevenness after replacement is calculated, and the correction data 68C may be updated. .

  In this case, the correction data 68C before the replacement and the correction data 68B may be combined to generate new correction data 68C and written to the EEPROM 66. More preferably, the correction data calculated before the factory shipment is stored in the EEPROM 66. 68A may be left without being overwritten, and the correction data 68A may be combined with the correction data 68B calculated after the replacement to generate new correction data 68C and written to the EEPROM 66. This is because the correction data 68 </ b> C before replacement includes a component due to a shift between the light intensity measurement position of the optical print head 16 in the measuring device 70 and the position of the photosensitive drum 12 before replacement in the image forming apparatus. It is because it is.

  In the present embodiment, the case where the LPH lighting pattern at the time of measurement by the measuring device 70 is 2 on 2 off has been described. However, if half of the light sources are on, such as 4 on 4 off, and valleys are easily detected. Good. This lighting pattern can be appropriately set depending on the presence / absence of a diffusion plate, the degree of diffusion, and the like.

  Note that the configuration (see FIGS. 1 to 8 and 16) and the flow of processing (see FIG. 12) of the image forming apparatus 10 according to the present embodiment are merely examples, and needless to say, can be changed as appropriate.

1 shows an overall configuration of an image forming apparatus according to an embodiment. Sectional drawing of the LED print head which concerns on embodiment is shown. (A) is a figure which shows the input signal from a printed circuit board to SLED, (B) is a circuit diagram which shows the detailed structure of SLED and a LPH drive part. It is a figure which shows arrangement | positioning (staggered arrangement | positioning) of the LED chip which concerns on embodiment. It is a drive circuit diagram of LED single-piece | unit in SLED. It is a block diagram which shows the part which concerns on the setting of the correction data which concerns on embodiment, and the structure of a correction data generation apparatus. It is a glock figure showing the composition of the measuring device concerning an embodiment. It is a block diagram which shows the detailed structure of the arithmetic processing part which concerns on embodiment. (A) is a general development characteristic, (B) is a diagram showing development characteristics obtained by simplifying (A). The exposure energy distribution when the lighting pattern is 2 on 2 off is shown. A cross section in the sub-scanning direction of the exposure energy distribution is schematically shown. A control routine for correction data calculation processing is shown. It is a figure which shows the characteristic value (≒ light quantity) of LED when the pulse width of the lighting control signal ΦI is changed. An example of a light profile acquired when the LPH is turned on with a 2 on 2 off pattern is shown. (A) is a view showing a cross section in the sub-scanning direction of the exposure energy distribution at the time of measurement, (B) is a view showing a cross section in the sub-scanning direction of the exposure energy distribution at the time of image formation, and (C) and (D) are prints. It is a figure which shows the exposure energy distribution at the time of measurement and at the time, respectively. It is a figure which shows the structure of the correction data generation apparatus which concerns on embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 12 Photosensitive drum 14 Charging device 16 LED print head (print head)
DESCRIPTION OF SYMBOLS 18 Developing device 20 Transfer roller 24 Paper tray 26 Paper conveyance belt 28 Paper 30A Pressure roller 30B Heating roller 30 Fixing device 31 Sub-scanning direction overlap calculator 32 Memory 34 Memory 36 Comparator 38 Adder 40 LED array 42 Printed circuit board 44 Selfoc lens array 46 LED (light emitting element)
48 LED chip 64 Driver 66 EEPROM (memory means)
68 Correction data 70 Measuring device 72 Correction data generation device (acquisition means, update means)
80 sensor 80B magnifying lens 80C transmission diffuser plate 82 sensor moving stage 84 driver 86 personal computer 88 arithmetic processing unit 100 drive unit (lighting control means)

Claims (7)

An image forming apparatus that forms an image by exposing a photoreceptor using a print head in which a plurality of light emitting elements are arranged in a line,
The parameters specific to each light emitting element, which are derived in advance from the variation amount of the characteristic value that varies according to the increase / decrease of the pulse width for causing each light emitting element of the print head to emit light, and at least the print head influence each other. Storage means for storing correction data derived in advance based on an exposure amount distribution when a plurality of light emitting elements are turned on, and
A lighting control unit that corrects the pulse width using the characteristic value, the parameter, and the correction data when controlling each light emitting element of the print head to light with a pulse width based on image data,
An image forming apparatus. The lighting control unit corrects the pulse width in units of a predetermined number of light emitting elements,
The image forming apparatus according to claim 1, wherein the parameter is common to the predetermined number of light emitting element units.   The image forming apparatus according to claim 1, wherein the characteristic value is a light amount when the light emitting element of the print head is turned on or a normalized light amount ratio.   3. The image forming apparatus according to claim 1, wherein the characteristic value is an image density of an image formed using the print head.   The image forming apparatus according to claim 1, wherein the parameter is a polynomial coefficient describing a relationship between a pulse width and the characteristic value. The lighting control means is configured to be able to execute a test mode for forming an image showing a predetermined test pattern including a halftone image by the print head,
An acquisition means for reading the image formed using the print head by executing the test mode and acquiring density information of the test pattern;
Updating means for updating and storing the correction data based on the density information acquired by the previous acquisition means;
The image forming apparatus according to claim 1, further comprising: A method for controlling a print head that is used to form an image and exposes a photoconductor with a plurality of light emitting elements arranged in a line,
Deriving parameters specific to each light emitting element in advance from the amount of fluctuation of the characteristic value that varies according to the increase or decrease of the pulse width for causing each light emitting element of the print head to emit light,
In consideration of the derived parameters, correction data is derived in advance based on an exposure amount distribution when at least a plurality of light emitting elements that affect each other of the print head are turned on,
Store the derived parameters and correction data in the storage means,
The pulse width is corrected using the characteristic value, the parameter, and the correction data when controlling each light emitting element of the print head to be turned on with a pulse width based on image data. Head control method.

Images