JP4820195B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a printer, and a facsimile machine, and more particularly to an image forming apparatus that forms an electrostatic latent image on an image carrier by irradiating a laser beam.

Conventionally, an electrostatic latent image is formed by irradiating the surface of a charged photosensitive drum with laser light, and a toner image formed by developing the electrostatic latent image with toner is transferred and fixed on a recording sheet. Thus, an electrophotographic image forming apparatus that forms an image is known. In such an image forming apparatus, a technique is known in which pseudo gradation and high resolution are realized by using dots smaller than normal size dots (hereinafter referred to as sub-dots). In this case, since the beam diameter of the semiconductor laser provided in the exposure apparatus and the characteristics of the photosensitive drum affect the image quality, in the above exposure apparatus, the pulse width of the drive current corresponding to the sub-dot emission time is shortened. By doing so, the beam diameter when stationary is apparently reduced. Here, for example, in the case of a photosensitive drum made of amorphous silicon, the dark decay after exposure is fast and the charge holding ability is weak, so it is difficult to maintain an electrostatic latent image of sharp small-sized dots. Techniques have been proposed such as intentionally overshooting the laser power of the laser light to stabilize the beam diameter and maintain good image quality (see, for example, Patent Document 1).
JP 2002-303814 A

  However, in the method of Patent Document 1, it is difficult to quantify the overshoot of the laser beam, and the overshoot may reach a kink-free region that causes the semiconductor laser to be destroyed. Conversely, if the overshoot is kept within the rating, the formation of subdots may become unstable. For this reason, it is difficult to stabilize and improve dot reproducibility when forming small-sized dots.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of stabilizing and improving dot reproducibility when forming small-sized dots. .

According to a first aspect of the present invention, there is provided light source means for outputting a laser beam for forming an electrostatic latent image on an image carrier,
Pulse width calculating means for calculating the pulse width of the driving signal of the light source means according to the size of each dot constituting the image;
The light emission intensity corresponding to the pulse width calculated by the pulse width calculation means is calculated based on the light emission intensity when the light emission intensity does not change with the passage of time , which is a predetermined value. The emission intensity that determines the emission intensity corresponding to the calculated pulse width by setting the emission intensity to increase with the passage of time from the specified value at a predetermined increase rate to a value that increases as the value becomes shorter A determination means;
Pulse width modulation means for modulating the pulse width of the drive signal to the pulse width calculated by the pulse width calculation means;
Emission intensity increasing means for increasing the emission intensity of the laser beam at an increase rate determined by the emission intensity determining means,
The light source means is an image forming apparatus that outputs a laser beam to the image carrier with a pulse width modulated by the pulse width modulation means and a light emission intensity increased by the light emission intensity increasing means.

According to this configuration, the pulse width of the drive signal of the light source unit is calculated by the pulse width calculation unit in accordance with the size of each dot constituting the image, and the emission intensity determination unit emits light corresponding to the calculated pulse width. The intensity is determined based on the emission intensity when the emission intensity is not changed with the passage of time, and the emission intensity of the specified value is increased by a predetermined increase rate as the calculated pulse width becomes shorter. The emission intensity increases with time. Then, based on the calculated pulse width, the pulse width of the drive signal is modulated by the pulse width modulation means, and the light emission intensity increasing means increases the light emission intensity of the laser beam at the determined increase rate. A laser beam for forming an electrostatic latent image on the image carrier is output from the light source means using the light emission intensity thus increased.

  Therefore, an accurate electrostatic latent image corresponding to each dot constituting the image can be reliably formed with a laser beam having an appropriate emission intensity, and particularly when a small-sized dot is formed, the emission intensity of the laser beam is reduced. Since the light emission intensity is adjusted appropriately according to the pulse width of the small size dot, the dot reproducibility when forming the small size dot can be stabilized and improved.

  Further, in this configuration, when the pulse width is short, the increase rate of the emission intensity is increased to maintain the image density more stably and constant.

The invention according to claim 2 is the image forming apparatus according to claim 1, wherein a light emission intensity table in which the pulse width is associated with the increase rate of the light emission intensity according to the pulse width is provided. A light emission intensity table storage means for storing in advance;
The light emission intensity determining means refers to a light emission intensity table stored in the light emission intensity table storage means, and determines an increase rate of the light emission intensity corresponding to the pulse width calculated by the pulse width calculation means. It is.

  In this configuration, the light emission intensity determining means refers to the light emission intensity table in which the pulse width and the increase rate of the light emission intensity according to the pulse width are associated with each other, so that the light emission intensity corresponding to the calculated pulse width can be easily obtained. Can be determined.

The invention according to claim 3 is the image forming apparatus according to claim 1 or 2, wherein the increase rate of the emission intensity corresponding to the pulse width calculated by the pulse width calculation means is calculated. A function storage means for storing a function to be obtained;
The emission intensity determining means calculates an increase rate of the emission intensity corresponding to the pulse width calculated by the pulse width calculating means, using a function stored in the function storage means.

  In this configuration, the light emission intensity determination means can easily determine the light emission intensity corresponding to the calculated pulse width by using a function for obtaining the light emission intensity corresponding to the pulse width.

  According to the first aspect of the present invention, an accurate electrostatic latent image corresponding to each dot constituting an image can be reliably formed with a laser beam having an appropriate emission intensity, and in particular, a small-sized dot is formed. In this case, since the emission intensity of the laser beam can be adjusted to an appropriate emission intensity according to the pulse width when forming the small-sized dot, the dot reproducibility of the small-sized dot can be stabilized and improved. it can.

  According to the first aspect of the present invention, when the pulse width is short, the increase rate of the emission intensity is increased to increase the peak value of the emission intensity, so that the constant image density can be maintained more stably. can do. In this case, since the laser beam is output based on the emission intensity increased using the peak value of the emission intensity according to the pulse width without using the overshoot, the emission according to the size of each dot The strength can be adjusted accurately.

  According to the second aspect of the present invention, the emission intensity determining means can easily determine the emission intensity corresponding to the calculated pulse width by referring to the emission intensity table.

  According to the invention described in claim 3, the emission intensity determining means easily determines the emission intensity corresponding to the calculated pulse width by using a function for obtaining the emission intensity corresponding to the pulse width. Can do.

  Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing an internal configuration of an image forming apparatus according to an embodiment of the present invention. In this figure, an image forming apparatus constitutes a digital color copying machine 1 (hereinafter referred to as a copying machine 1). An image reading unit 12 for reading a document image and an image reading unit 12 read the image. The main body 14 for transferring the original image to the recording paper (transfer paper), and the recording paper discharge section 16 for discharging the recording paper on which the original image is transferred by the main body 14 are configured.

  The image reading unit 12 is disposed above the main body unit 14, and includes a first contact glass 18 that forms a reading region for a stationary document and a first contact glass that forms a reading region for an automatically fed document. Two contact glasses 20, an automatic document feeder 22 that feeds stacked documents one by one toward the second contact glass 20, and a scanner 24 that reads a document image composed of a CCD or the like. ing. That is, the image reading unit 12 reads an image of a document placed on the first contact glass 18 by the scanner 24, and is sent out by the automatic document feeding unit 22 and passes over the second contact glass 20. Scan the original image to be scanned. The image data read by the scanner 24 is stored in an image memory provided in a control unit (not shown).

  The main body 14 includes a paper feed cassette 30 disposed in the lower part of the housing 28, an image forming unit 32 disposed in the center of the housing 28, and an image forming unit 32 in the housing 28. The recording paper transport unit 34 disposed below, the fixing unit 36 disposed on the downstream side of the recording paper transport unit 34 in the housing 28, and the recording paper fed from the paper feed cassette 30 are image forming units. A first transport path 38 that feeds the recording paper to the side 32 and a second transport path 40 that discharges the recording paper that has passed through the fixing unit 36 to the recording paper discharge section 16 are provided.

  The paper feed cassette 30 is configured such that the recording paper P accumulated therein is fed out one by one by the paper feed roller 42 to the first transport path 38 side.

  The image forming unit 32 is for transferring the original image read by the image reading unit 12 to a recording sheet, and is rotated in the illustrated counterclockwise direction (arrow R1 direction) to form a toner image on the peripheral surface. A photosensitive drum 44, an intermediate transfer drum 46 which is rotationally driven in the clockwise direction (in the direction of arrow R2) shown in the figure, and a toner image formed on the photosensitive drum 44 is transferred to the peripheral surface, and an intermediate transfer drum 46 And a transfer belt 48 for transferring the toner image transferred to the intermediate transfer drum 46 to the recording paper conveyed from the first conveyance path 38.

  A bias voltage having a positive potential is supplied to the transfer belt 48 so that the toner image transferred to the intermediate transfer drum 46 can be easily transferred to the recording paper side. In this way, since a positive bias voltage is supplied to the transfer belt 48, the recording paper is charged with a positive charge when a toner image is transferred.

  Further, on the outer periphery of the photosensitive drum 44, a charging unit 52 that faces the peripheral surface of the photosensitive drum 44, an exposure unit 54 that irradiates laser light in a downstream region of the charging unit 52 of the photosensitive drum 44, and exposure A developing unit 56 facing the circumferential surface of the photosensitive drum 44 on the downstream side of the unit 54 and a cleaning unit 58 facing the circumferential surface of the photosensitive drum 44 on the downstream side of the developing unit 56 are disposed. The developing unit 56 forms a cyan toner image, a first developing unit 60 that forms a yellow toner image on the peripheral surface of the photosensitive drum 44, a second developing unit 62 that forms a magenta toner image, and the like. A third developing unit 64 and a fourth developing unit 66 for forming a black toner image are sequentially arranged from the upstream side to the downstream side.

  Further, on the outer periphery of the intermediate transfer drum 46, a cleaning unit 68 facing the peripheral surface of the intermediate transfer drum 46 is disposed on the downstream side of the transfer belt 48. Further, the transfer belt 48 is stretched between a driving roller 76 and a driven roller 78, and transfers the recording paper received from the first transport path 38 in the direction of arrow R3 and delivers it to the recording paper transport unit 34. .

  The recording paper transport unit 34 transports the recording paper received from the transfer belt 48 to the fixing unit 36, and is configured by a mesh-shaped transport belt 84 being bridged between a driving roller 80 and a driven roller 82. It is. A vacuum suction section (not shown) is provided on the back side of the mesh-like transport belt 84, and the recording paper is transported in the direction indicated by the arrow R4 in a vacuum-sucked state and delivered to the fixing unit 36. is there.

  The fixing unit 36 performs fixing processing by heating the recording paper on which the toner image is transferred. The fixing unit 86 includes a heater, and a pressure roller 88 disposed in pressure contact with the fixing roller 86. And.

  The copying machine 1 configured as described above operates as follows. That is, a document is placed on the first contact glass 18 or a document is placed on the automatic document feeder 22 and a start button (not shown) is operated to capture a document image by the scanner 24. And stored in an unillustrated image memory. When the document image capturing is completed, the image data is read from the image memory and transmitted to the image forming unit 32.

  In the photosensitive drum 44 of the image forming unit 32, an electrostatic region is formed on the surface of the charging unit 52 by rotating the photosensitive drum 44, and this electrostatic region is exposed by the laser light from the exposure unit 54. Then, an electrostatic latent image based on the image data transmitted from the image reading unit 12 is formed, and then a yellow toner image is formed by the first developing unit 60, for example. The yellow toner image is transferred to an intermediate transfer drum 46 that rotates in synchronization with the photosensitive drum 44. Similarly, toner images of each color are sequentially formed on the photosensitive drum 44 by the second to fourth developing units 62 to 66, and the toner images are transferred to the intermediate transfer drum 46 in an overlapping manner.

  The toner image transferred onto the intermediate transfer drum 46 is transferred to the recording paper conveyed on the transfer belt 48 in synchronization with the intermediate transfer drum 46, whereby a toner image is formed on the recording paper. The transfer to the recording paper is executed by drawing a toner image on the intermediate transfer drum 46 to the recording paper side by supplying a bias voltage having a positive potential from the back surface side of the transfer belt 48. At this time, the recording paper is charged with a positive charge by the bias voltage supplied to the transfer belt 48.

  The recording paper onto which the toner image has been transferred is transferred to the recording paper transport unit 34 by the transfer belt 48. The recording paper is charged when passing below the conductor 70. As a result of being attracted to 70 and discharged, the recording paper can be easily peeled off from the intermediate transfer drum 46, while the delivery to the recording paper transport unit 34 is performed smoothly. The recording paper delivered to the recording paper transport unit 34 is transported to the fixing unit 36 in a vacuum-sucked state, and is sandwiched between the fixing roller 86 and the pressure roller 88 while being heated by the fixing roller 86, and downstream. The paper is transported and discharged to the recording paper discharge unit 16 through the second transport path 40.

  Note that the toner remaining on the surface of the photosensitive drum 44 after the toner image is transferred to the intermediate transfer drum 46 is removed by the cleaning unit 58. Similarly, the toner remaining on the surface of the intermediate transfer drum 46 having the toner image transferred to the transfer belt 48 is removed by the cleaning unit 68.

  FIG. 2 is a block diagram showing a schematic configuration of the copying machine 1. The copying machine 1 includes an exposure unit 54, a development unit 56, a charging unit 52, a CPU (Central Processing Unit) 100, a ROM (Read Only Memory) 540, a RAM (Random Access Memory) 120, a frame buffer 130, and a light emission control unit 140. The drum motor 150 and an I / F (interface) unit 160 are provided.

  The I / F unit 160 is connected to an external device such as a PC (personal computer) and receives image data transmitted by the external device. The ROM 110 stores an operation program for the entire copying machine 1. The RAM 120 stores image data received by the I / F unit 160.

  The CPU 100 controls the copying machine 1 as a whole, reads the image data stored in the RAM 120, and generates bitmap data from the read image data. The frame buffer 130 stores bitmap data generated by the CPU 100 in units of pages.

  The light emission control unit 140 controls the exposure unit 54, and in accordance with the size of the dots (normal size dots and a plurality of sub dots smaller than the normal size) constituting the bitmap data, the exposure unit 54. The pulse width of the drive signal for forming each dot is calculated, the light emission intensity is determined according to the pulse width, and the pulse width and light emission intensity of each dot are output to the exposure unit. The drum motor 150 rotationally drives the photosensitive drum 44 based on the control of the CPU 100.

  FIG. 3 is a diagram showing a configuration of the light emission control unit 140 shown in FIG. The light emission control unit 140 illustrated in FIG. 3 includes a raster buffer 141, a smoothing instruction unit 142, a pulse width calculation unit 143, a pulse width data table storage unit 144, a light emission intensity determination unit 145, and a light emission intensity LUT (lookup table) storage unit 146. And an output determination unit 147.

  The raster buffer 141 stores bitmap data read from the frame buffer 130 by a raster scan method. The bitmap data stored in the frame buffer 130 is sequentially read from the upper left end portion of the page to the lower right end portion in the horizontal line direction in the horizontal line direction at a constant timing and written to the raster buffer 141.

  The smoothing instructing unit 142 instructs smoothing processing for smoothing the edge region by adding or deleting sub-dots smaller than the unit dot in the step portion of the unit dot in the edge region of the bitmap data. In the smoothing process, for example, a process of adding a sub-dot having a size of 1/4 of the unit dot to the stepped part of the unit dot in the edge area of the bitmap data is performed.

  The pulse width calculation unit 143 calculates the pulse width according to the gradation value for each pixel of the bitmap data. Note that the pulse width is obtained by referring to a pulse width data table in which the gradation value and the pulse width stored in the pulse width data table storage unit 144 are associated in advance. The pulse width data table storage unit 144 stores a pulse width data table in which gradation values of each pixel, for example, 0 to 255 and pulse widths corresponding to the gradation values are associated with each other.

  In addition, when the pixel of the bitmap data is a sub-dot added or deleted by the smoothing process, the pulse width calculation unit 143 further increases the pulse width calculated according to the gradation value of the pixel of the bitmap data. The pulse width corresponding to the size of the subdot is calculated. That is, for example, when forming a sub-dot having a size that is ¼ of a unit dot, the pulse width calculation unit 143 multiplies the pulse width calculated according to the gradation value by ¼ to obtain a sub-dot. The pulse width corresponding to the size of is calculated.

  The light emission intensity determination unit 145 refers to the light emission intensity LUT stored in the light emission intensity LUT storage unit 146, and the light emission intensity value (specified value) used when forming a normal size dot and the normal dot The increase rate of the laser beam corresponding to each pulse width calculated by the pulse width calculation unit 143, which is used when forming smaller subdots, is determined, and the emission intensity is determined therefrom.

  The light emission intensity LUT storage unit 146 stores the specified value, each pulse width calculated by the pulse width calculation unit 143, and each light emission intensity increase rate associated with each pulse width. Is stored. The light emission intensity LUT stores an increase rate that increases as the pulse width becomes shorter in association with the pulse width. This is to maintain a constant image density by increasing the increase rate of the emission intensity even when the pulse width is shortened (details will be described later).

  The output determination unit 147 determines pulse width data and light emission intensity data of each pixel of the bitmap data, and outputs the data to the exposure unit 54. The pulse width data includes data related to the pulse width and data related to the pulse position.

  In the present embodiment, the pulse width calculation unit 143 corresponds to an example of a pulse width calculation unit, the emission intensity determination unit 145 corresponds to an example of an emission intensity determination unit, and the emission intensity LUT storage unit 146 corresponds to an emission intensity table. This corresponds to an example of storage means.

  FIG. 4 is a diagram showing a configuration of the exposure unit 54 shown in FIGS. 2 and 3. The exposure unit 54 shown in FIG. 4 includes a pulse width modulation unit 541, a light emission intensity changing unit 542, and a semiconductor laser 543.

  The pulse width modulation unit 541 modulates the pulse width of the laser light output from the semiconductor laser 543. The pulse width modulation unit 541 outputs a drive signal for driving the semiconductor laser 543 on / off with a pulse width based on the pulse width data output from the light emission control unit 140.

  The emission intensity changing unit 542 changes the emission intensity of the laser beam based on the prescribed value of the emission intensity and the emission intensity increase rate determined by the emission intensity determining unit 145, and outputs a drive signal to the semiconductor laser 543. When the light emission intensity changing unit 542 outputs a drive signal for forming a dot having a normal size, the light emission intensity changing unit 542 outputs the drive signal having the pulse width modulated by the pulse width modulation unit 541 to the light emission intensity having the specified value. The level is stopped and output to the semiconductor laser 543. On the other hand, when outputting a drive signal for forming subdots, the light emission intensity changing unit 542 generates a drive signal having a pulse width modulated by the pulse width modulation unit 541 from the above specified value for each pulse width. The pulse width is increased over the entire pulse width at the determined emission intensity increase rate, and a drive signal for driving the semiconductor laser 543 with such emission intensity is output to the semiconductor laser 543. A specific example of this increase rate will be described later with reference to FIG.

  The light emission intensity change unit 542 includes a light emission intensity increase rate instruction unit 5421, a light emission intensity control voltage generation unit 5422, and an operational amplifier 5423. The emission intensity increase rate instruction unit 5421 outputs a control signal based on the emission intensity increase rate sent from the emission control unit 140 to the emission intensity control voltage generation unit 5422. The light emission intensity control voltage generation unit 5422 includes a D / A converter or the like, converts the control signal output from the light emission intensity increase rate instruction unit 5421 into an analog signal, and outputs the analog signal to the operational amplifier 5423. The operational amplifier 5423 outputs a drive signal obtained by amplifying the output from the light emission intensity control voltage generation unit 5422 to the semiconductor laser 543.

  The semiconductor laser 543 is a visible red semiconductor laser having a wavelength of 670 nm, for example, and outputs a laser beam having an emission intensity corresponding to the drive signal.

  In the present embodiment, the semiconductor laser 543 corresponds to an example of a light source unit, the pulse width modulation unit 541 corresponds to an example of a pulse width modulation unit, and the light emission intensity changing unit 542 corresponds to an example of a light emission intensity increasing unit. To do.

  Next, the operation of the copying machine 1 shown in FIGS. FIG. 5 is a flowchart for explaining the operation of the copying machine 1 shown in FIGS.

  First, the CPU 100 determines whether image data to be printed is input from a PC on the network via the I / F unit 160 (S1). If the CPU 100 determines that no image data is input (NO in S1), the CPU 100 waits until image data is input.

  On the other hand, when the CPU 100 determines that image data has been input (YES in S1), the CPU 100 generates bitmap data from the input image data (S2). Next, the CPU 100 stores the bitmap data in the frame buffer 130 (S3).

  Next, the light emission control unit 140 reads the bitmap data stored in the frame buffer 130 by the raster scan method and stores it in the raster buffer 141 (S4). Then, the pulse width calculation unit 143 reads the gradation value for each pixel of the bitmap data from the raster buffer 141, refers to the pulse width data table stored in the pulse width data table storage unit 144, and stores the bitmap data The pulse width of the drive signal is calculated according to the gradation value for each pixel (S5).

  Subsequently, the pulse width calculation unit 143 determines whether there is a smoothing instruction in the bitmap data read from the raster buffer 141 (S6). If it is determined that there is a smoothing instruction (YES in S6), the pulse width calculation unit 143 calculates a pulse width corresponding to the size of the subdot (S7). The smoothing instruction includes information regarding the size of the subdot. At this time, for example, when the size of the sub dot is 1 / n times the size of the unit dot, the pulse width calculation unit 143 may multiply the pulse width calculated according to the gradation value by 1 / n. The pulse width corresponding to the size of the subdot is calculated.

  In this embodiment, the pulse width corresponding to the size of the sub dot is calculated by multiplying the pulse width calculated according to the gradation value by the ratio of the sub dot to the unit dot. The invention is not particularly limited to this. For example, a correction table in which the size of the sub dot is associated with the correction rate of the pulse width set in advance according to the size of the sub dot is stored, and the pulse width calculation unit 143 refers to the correction table. Alternatively, the pulse width may be calculated by selecting a correction ratio of the pulse width corresponding to the size of the subdot and multiplying the selected correction ratio by the pulse width calculated according to the gradation value.

  Next, the light emission intensity determination unit 145 refers to the light emission intensity LUT stored in the light emission intensity LUT storage unit 146, and determines the light emission intensity increase rate of the light emission intensity corresponding to the pulse width calculated by the pulse width calculation unit 143. Determine (S8). As described above, the light emission intensity LUT is stored in association with the pulse width and the current increase rate of the light emission intensity corresponding to the pulse width. For example, assuming that the emission intensity LUT has a 1-dot pulse width of 79 nsec and sub-dot pulse widths of 10 nsec, 20 nsec, 40 nsec, and 60 nsec, for each of these pulse widths, a drive current of a specified value is set. The current increase rate from (mA / nsec) is stored. The light emission intensity determination unit 145 reads and determines the light emission intensity increase rate of the light emission intensity corresponding to the calculated pulse width from the light emission intensity LUT.

  When the emission intensity increase rate corresponding to the pulse width is determined, in S9, the output determination unit 147 determines and determines the pulse width data, the prescribed value of the emission intensity and the increase rate data to be output to the exposure unit 54. Pulse width data, emission intensity peak value, and current increase rate data are output to the exposure unit 54 (S9). If it is determined that there is no smoothing instruction (NO in S6), the output determination unit 147 defines the data of the pulse width as calculated by the pulse width calculation unit 143 according to the gradation value and the emission intensity. The value data is output to the exposure unit 54 (S9).

  The pulse width modulation unit 541 modulates the pulse width based on the pulse width data output from the output determination unit 147, and changes the drive signal for driving the semiconductor laser 543 on / off (S10).

  The light emission intensity changing unit 542 changes the drive signal whose pulse width is modulated by the pulse width modulation unit 541 so that the semiconductor laser 543 is turned on at the specified value and the light emission intensity increase rate (S11). If there is no smoothing instruction (NO in S6) and no data indicating the emission intensity increase rate is output from the output determination unit 147 (the increase rate may be output as 0), the emission intensity is increased. Output to the exposure unit 54 (S11).

  The semiconductor laser 543 outputs a laser beam corresponding to the drive signal output from the emission intensity changing unit 542, and starts exposure (S12).

  With reference to FIG. 6, the light emission control of the semiconductor laser 543 by the pulse width modulation unit 541 and the light emission intensity changing unit 542 at the time of forming the sub dots will be described. In the following, a case where the pulse width of a normal dot is 79 nsec will be described as an example.

  The pulse width modulation unit 541 generates a drive signal for the semiconductor laser 543 with the pulse widths of the sub-dots having different sizes calculated by the pulse width calculation unit 143 (for example, 10 nsec, 20 nsec, 40 nsec, 60 nsec, etc.). The light emission intensity changing unit 542 adjusts the drive signal based on the specified value and the light emission intensity increase rate output from the output determining unit 147. For example, as shown in FIG. 6, the light emission intensity changing unit 542 raises the drive current to the specified value, and when the drive current reaches the specified value, the pulse width is increased by the light emission intensity increase rate from the output determining unit 147. The drive current is increased over the entire area. Note that, under the conditions of the present embodiment, for the light emission with a pulse width of 60 nsec or more, a current increase rate of 0 mA / msec is sufficient.

  By controlling the increase in drive current in this way, the problems seen when controlling the formation of the beam diameter by intentionally overshooting the laser power of the laser light to be emitted, namely (1) Difficult to obtain different emission values depending on dot size (difficulty in quantifying laser light overshoot), (2) risk of overshoot reaching the kink-free region leading to semiconductor laser destruction Can be eliminated. Therefore, according to the copying machine 1 of the present embodiment, it is possible to form an electrostatic latent image of each dot constituting an image with a laser beam having an appropriate emission intensity while solving the above-described problems. When forming dots of size, the laser beam emission intensity can be changed to the emission intensity corresponding to the pulse width of the small dot, so that the dot reproducibility when forming small dots is stabilized and improved. Can be made.

  Furthermore, a light emission intensity table in which the pulse width and the drive current increase rate of the light emission intensity corresponding to the pulse width are associated with each other is stored in advance, and the light emission corresponding to the calculated pulse width is referred to this light emission intensity table. Since the driving current increase rate of intensity is selected, the emission intensity corresponding to the calculated pulse width can be easily determined.

  Next, an experiment on the relationship between the pulse width of the subdot and the dot reproducibility will be described. FIG. 7 is a diagram showing experimental results of dot reproducibility when printing is performed with the emission intensity corrected according to the pulse width of the subdots and when printing is performed without correcting the emission intensity.

  In FIG. 7, it is assumed that the pulse width of a dot having a normal size (hereinafter simply referred to as 1 dot) is set to 79 nsec, as in FIG. In this case, good dot reproducibility can be obtained without increasing the drive current.

  Here, for example, if the pulse width is 10nsec, which is about 1/8 of the pulse width of 1 dot, good dot reproducibility can be obtained by printing subdots without correcting the emission intensity. I could not. However, when the emission intensity increase rate was set to 5 mA / nsec and the emission intensity was corrected and sub-dots were printed, good dot reproducibility could be obtained.

  In addition, if the pulse width is 20nsec, which is about 2/8 of the pulse width of one dot, if the subdot is printed without correcting the emission intensity, the subdot is not corrected without correcting the emission intensity. When printing, good dot reproducibility could not be obtained. However, when the emission intensity increase rate was set to 3 mA / nsec, the emission intensity was corrected, and sub-dots were printed, good dot reproducibility could be obtained.

  In addition, when the pulse width is 40nsec, which is approximately 4/8 of the pulse width of one dot, if the sub-dots are printed without correcting the light emission intensity, it is confirmed that the dot reproducibility is improved although it is insufficient. It was done. Moreover, when the emission intensity increase rate was set to 1 mA / nsec, the emission intensity was corrected, and sub-dots were printed, good dot reproducibility could be obtained.

  Furthermore, when a pulse width of 60 nsec, which is approximately 6/8 of the pulse width of one dot, was used, good dot reproducibility could be obtained without increasing the drive current.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. For example, in the above embodiment, the light emission intensity determination unit 145 refers to the light emission intensity LUT stored in the light emission intensity LUT storage unit 146 and increases the light emission intensity corresponding to the pulse width calculated by the pulse width calculation unit 143. Although the rate is selected, the present invention is not particularly limited to this. For example, a function storage unit is provided instead of the light emission intensity LUT storage unit 146, and the light emission intensity determination unit 145 indicates a drive current increase rate of the light emission intensity corresponding to the pulse width calculated by the pulse width calculation unit 143. It may be calculated using a function stored in the.

  In this case, since a function for obtaining the drive current increase rate of the light emission intensity corresponding to the pulse width is used, the light emission intensity corresponding to the calculated pulse width can be easily determined.

  Further, the emission intensity LUT of the emission intensity LUT storage unit 146 includes an emission intensity peak value corresponding to the sensitivity characteristic of the photosensitive drum 44 and an increasing rate for increasing the emission intensity toward the peak value. You may make it memorize | store and match | combine for every width | variety. The sensitivity characteristic represents the degree to which the light-irradiated portion of the surface of the photosensitive drum 44 becomes conductive and loses the surface charge. The light emission intensity LUT storage unit 146 stores the light emission intensity LUT in which the peak value and the increase rate are stored as the light emission intensity according to the sensitivity characteristic of the photosensitive drum 44. In this case, since the light emission intensity determination unit 145 determines the light emission intensity peak value and the increase rate according to the sensitivity characteristic of the photosensitive drum 44, a desired sensitivity characteristic can be obtained when a laser beam is output with a modulated pulse width. The emission intensity can be determined. That is, the light emission intensity can be increased at different rates depending on the sensitivity characteristics of the photosensitive drum 44 over the entire pulse width.

  Furthermore, in the present embodiment, the light emission intensity LUT of the light emission intensity LUT storage unit 146 is replaced with the light emission intensity peak value corresponding to the sensitivity characteristic of the photoconductor drum 44, according to the dark attenuation characteristic of the photoconductor drum 44. The peak value and the increase rate of the emission intensity may be stored in association with each pulse width, and the emission intensity determination unit 145 may determine based on these values. The dark decay characteristic represents the charge holding ability in the dark place on the surface of the photosensitive drum 44. In this case, the light emission intensity determination unit 145 determines the peak value and the increase rate of the light emission intensity according to the rate of decrease of the surface potential due to the dark decay characteristics after exposure of the photosensitive drum 44, so that the modulated pulse width is used. When the laser beam is output, the emission intensity corresponding to the dark attenuation characteristic unique to the photosensitive drum can be determined.

  As described above, when the light emission intensity determination unit 145 determines the light emission intensity based on the peak value and the increase rate of the light emission intensity in accordance with the sensitivity characteristic or the dark decay characteristic of the photosensitive drum 44, the series of processes illustrated in FIG. In the processing, the processing of S8 to S11 is performed using the peak value and the increase rate that the light emission intensity determination unit 145 reads from the light emission intensity LUT.

  In the above embodiment, when the smoothing process is performed in FIG. 5, the sub-dot drive signals shown in S7 and S8 are changed. However, the present invention is applied only when the smoothing process is performed. The present invention is not intended to be limited to changing the drive signal of the sub-dots. For example, the present invention can also be applied to a quality improvement process of a color image formed by superimposing toner images formed by respective color dots. is there.

1 is a diagram illustrating a mechanical configuration of a printer as an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating a schematic configuration of a printer that is an example of an image forming apparatus according to the present invention. It is a figure which shows the structure of the light emission control part shown in FIG. It is a figure which shows the structure of the exposure part shown in FIG.2 and FIG.3. 5 is a flowchart for explaining the operation of the printer shown in FIGS. It is a figure for demonstrating the relationship between emitted light intensity and a pulse width. It is a figure which shows the experimental result about the dot reproducibility in the case where it prints by changing the pulse width of a subdot, and correct | amends light emission intensity, and when it prints without correcting light emission intensity.

DESCRIPTION OF SYMBOLS 1 Copier 16 Recording paper discharge part 54 Exposure part 140 Light emission control part 142 Smoothing instruction | indication part 143 Pulse width calculating part 144 Pulse width data table memory | storage part 145 Light emission intensity determination part 146 Light emission intensity LUT memory | storage part 541 Pulse width modulation part 542 Light emission intensity Change unit 543 Semiconductor laser

Claims (3)

Light source means for outputting a laser beam for forming an electrostatic latent image on the image carrier;
Pulse width calculating means for calculating the pulse width of the driving signal of the light source means according to the size of each dot constituting the image;
The light emission intensity corresponding to the pulse width calculated by the pulse width calculation means is calculated based on the light emission intensity when the light emission intensity does not change with the passage of time , which is a predetermined value. The emission intensity that determines the emission intensity corresponding to the calculated pulse width by setting the emission intensity to increase with the passage of time from the specified value at a predetermined increase rate to a value that increases as the value becomes shorter A determination means;
Pulse width modulation means for modulating the pulse width of the drive signal to the pulse width calculated by the pulse width calculation means;
Emission intensity increasing means for increasing the emission intensity of the laser beam at an increase rate determined by the emission intensity determining means,
The light source means is an image forming apparatus for outputting a laser beam to the image carrier with a pulse width modulated by the pulse width modulation means and a light emission intensity increased by the light emission intensity increasing means. A light emission intensity table storing means for preliminarily storing a light emission intensity table in which the pulse width and the increase rate of the light emission intensity according to the pulse width are associated;
The emission intensity determination means refers to an emission intensity table stored in the emission intensity table storage means, and determines an increase rate of the emission intensity corresponding to the pulse width calculated by the pulse width calculation means. Item 2. The image forming apparatus according to Item 1. Function storage means for storing a function for obtaining an increase rate of the emission intensity corresponding to the pulse width calculated by the pulse width calculation means;
The light emission intensity determination means calculates the increase rate of the light emission intensity corresponding to the pulse width calculated by the pulse width calculation means by using a function stored in the function storage means. Item 3. The image forming apparatus according to Item 2.

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