JP2002211035A - Apparatus and method for imazing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus and a method for imaging wherein variations of characteristics of a laser diode can be corrected by a simple constitution without requiring user's trouble. SOLUTION: The imaging apparatus is provided with a means 206 for converting bit map data to a laser ON/OFF signal, and the laser diode 212 for exposing an image carrier by a laser light based on the laser ON/OFF signal. A time of the laser ON/OFF signal is corrected on the basis of one or both of a threshold current A of the laser diode 212 and a laser diode-driving current B necessary for obtaining a predetermined light intensity at a surface of the image carrier. Variations of a laser emission time because of the characteristics of the laser diode can be reduced by correcting the laser ON/OFF signal, and therefore minute dots are reproduced stably irrespective of the variations of the laser diode. Vertical lines and horizontal lines are uniformed in the thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and an image forming method.

More specifically, the present invention relates to an image forming apparatus for forming an image by irradiating a laser beam emitted from a laser diode to an image carrier, for example, an electrophotographic image forming apparatus such as a laser beam printer or a copying machine. The present invention relates to a forming apparatus and an image forming method.

[0003]

2. Description of the Related Art There are various types of image forming apparatuses, such as an electrophotographic type, an ink jet type, and a thermal type, for forming an image based on an image signal sent from a host computer.

Among these image forming apparatuses, electrophotographic image forming apparatuses have been widely used in recent years because of their advantages such as high speed, high image quality and quietness. In particular, the laser exposure method is advantageous for high speed and high resolution, and can easily improve the gradation by modulating the laser emission time. This is an advantage of the laser exposure type electrophotographic image forming apparatus as compared with the other types of image forming apparatuses described above.

In order to form an image with such a laser exposure type electrophotographic image forming apparatus, an image processing circuit for developing image data sent from a host computer into bitmap data, and an image processing circuit based on the bitmap data. Therefore, an image signal generating means for generating a laser ON / OFF signal is required. And this laser ON / OFF
A signal is input to a laser drive circuit to drive a semiconductor laser diode to emit light.

As described above, the laser ON / OFF corresponding to the image
An electrostatic latent image is formed by irradiating the surface of the photoreceptor with laser light emitted based on the OFF signal, and is visualized by a developing unit. Then, the toner image is transferred onto transfer paper and fixed by a fixing device, so that a user can obtain a desired image.

[0007]

The light emission time of the above-mentioned laser diode is ideally the same laser ON / OF.
It is needless to say that it is desirable that the same time is always applied to the F signal. However, in reality, even if the laser ON / OFF signal for the same time is input to the laser drive circuit, not only does the rise characteristic (time constant) of the drive current of the laser diode drive circuit vary, but also the laser diode Drive threshold and IL (current-output light)
Since there is a variation in the characteristics, the response of the laser emission will be different. As a result, there is a problem that the output laser emission time is not constant.

Generally, laser ON / O per dot
The FF time depends on the resolution of the image forming apparatus, the process speed, the configuration of the exposure apparatus and the setting conditions (the number of faces of the polygon mirror and the number of rotations). When the configuration of the exposure apparatus is the same, the laser ON / OFF time per dot is
The higher the resolution and the faster the process speed, the shorter.

For example, in an image forming apparatus having a resolution of 600 dpi, the laser ON time per dot is 10
It is assumed that it is about 0 nsec. Now, considering this as a standard, the resolution is 2 at the same process speed.
If the resolution is doubled to 1200 dpi, the resolution in the sub-scanning direction is 2
It is necessary to double the number of rotations of the polygon mirror to double the number. At this time, the scanning time for one line on the photosensitive member is 1 /
It becomes 2. Therefore, the laser ON time per dot must be shortened to 25 nsec, which is 1/4 of the reference 100 nsec. Further, if the process speed is doubled while keeping the resolution at 600 dpi, the laser ON time per dot must be reduced to 50 nsec, which is 1/2.

That is, the fluctuation width of the laser emission time is not negligible because the laser ON time per dot is shortened as the resolution of the image forming apparatus is improved and the speed is increased.

[0011] More specifically, the following problems arise as the resolution increases. When forming a one-dot image, the size of the obtained dot differs due to these variations, and in the worst case, the dot cannot be reproduced. There may be a problem that a vertical line and a horizontal line of one dot line have different thicknesses. The horizontal line forms a latent image by being scanned while the laser is continuously turned on. On the other hand, the vertical lines are formed by forming latent images having a lighting width of one dot in a line in the vertical direction, so that the shorter the laser ON time is, the more the fluctuation is affected. That is, when a vertical line image is formed, the latent image is more affected by the variation in laser light than the horizontal line latent image.

As techniques for making the line widths of the vertical and horizontal lines uniform, for example, the techniques described in Japanese Patent Application Laid-Open Nos. 1-167863 and 10-193683 are known.

However, in the technique described in Japanese Patent Application Laid-Open No. 1-167863, it is necessary for an operator using the image forming apparatus to adjust the set value of the line width by himself. Further, since the characteristics of the laser diode are not taken into consideration, the operator cannot obtain a desired image without trial and error of the operator himself / herself, resulting in a very troublesome operation for the operator.

The technique described in Japanese Patent Application Laid-Open No. Hei 10-193683 detects image data of one horizontal dot line, and turns on and off the laser when forming one horizontal dot line.
Although the line width of the vertical line and the horizontal line is made uniform by FF modulation, this also does not take into account the characteristic variation of the laser diode.

Further, a multi-beam type image forming apparatus which is advantageous for speeding up and increasing resolution has a plurality of laser diodes. Therefore, if the characteristics of the individual laser diode drive circuits and the laser diode characteristics vary, there is also a problem that even if an attempt is made to form the same dot in the same image, the size of the dot differs depending on the exposed laser diode. That is, the multi-beam type image forming apparatus has a problem that it is more susceptible to the variation in the characteristics of the laser diode than the single beam type image forming apparatus.

Accordingly, an object of the present invention is to provide
An object of the present invention is to provide an image forming apparatus and an image forming method capable of correcting variations in characteristics of a laser diode with a simple configuration without requiring a user's effort.

[0017]

According to one aspect of the present invention, there is provided an image forming apparatus for forming an image by irradiating a laser beam emitted from a laser diode to an image carrier. A conversion unit that converts input image data into a laser on / off signal; a laser driving unit that drives the laser diode in response to the laser on / off signal; a threshold current of the laser diode; Correction means for correcting the on time of the laser on / off signal input to the laser driving means, based on one or both of the laser diode driving currents required to obtain a predetermined light intensity on the surface of the image carrier; It is provided with.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the correction means is configured to determine a ratio between the threshold current and the laser diode driving current.
3. The image forming apparatus according to claim 1, wherein the on-time of the laser on / off signal input to the laser driving unit is corrected. Storage means for storing either one or both.

According to a fourth aspect of the present invention, in the image forming apparatus according to the first aspect, in addition to the threshold current,
A detecting means for detecting one or both of the laser diode driving currents is provided.

According to a fifth aspect of the present invention, there is provided an image forming method for forming an image by irradiating a laser beam emitted from a laser diode to an image carrier, wherein the laser diode is controlled based on a laser on / off signal. Upon driving, the threshold current of the laser diode and, based on one or both of a laser diode driving current necessary to obtain a predetermined light intensity on the surface of the image carrier, input to the laser driving unit. This is for correcting the on-time of the laser on / off signal.

According to a sixth aspect of the present invention, in the image forming method according to the fifth aspect, the laser on / off signal input to the laser driving means based on a ratio between the threshold current and the laser diode driving current. Correct the on-time of.

According to a seventh aspect of the present invention, in the image forming method according to the fifth aspect, the threshold current,
Storing one or both of the laser diode drive currents.

According to an eighth aspect of the present invention, in the image forming method according to the fifth aspect, in addition to the threshold current,
A step of detecting one or both of the laser diode drive currents.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Embodiment 1 A first embodiment of the present invention will be described with reference to FIGS. 1 to 4 and FIG.

First, the configuration of the electrophotographic image forming apparatus will be described with reference to FIG. FIG. 8 is a schematic sectional view of the image forming apparatus main body.

The image forming apparatus 200 is roughly classified into an electrophotographic process, and includes a charging unit, an image carrier, an exposing unit, a developing unit, a transferring unit, a cleaning unit, and a fixing unit.

In FIG. 8, reference numeral 2 denotes a cylindrical photosensitive drum serving as a latent image carrier. After the surface of the photosensitive drum 2 is uniformly charged by the charging device 1, a latent image is formed by the exposure device 7. In the present embodiment, the charged potential of the photoconductor is charged to Vd = −700 V, and the potential of the solid black portion (laser exposed portion) becomes Vl = −100 V.

The latent image formed on the photosensitive drum 2 is visualized by supplying a developer by a developing device 3. A bias supply power supply (not shown) in which a DC bias is superimposed on an AC bias is connected between the photosensitive drum 2 and the developing sleeve, so that an appropriate developing bias is applied. In the present embodiment, the gap between the photosensitive drum and the developing sleeve is about 300 μm, and a DC voltage: −500 V, an AC voltage:
A rectangular wave Vpp of 1500 V and a frequency of 2000 Hz are applied. Thereby, the portion V1 is reversely developed.

The transfer material P is fed by a feed roller (not shown), and sent to the transfer device 5 in synchronization with the image on the photosensitive drum 2. The image on the photosensitive drum 2 visualized by the developer is transferred to the transfer material P by the transfer device 5. Then, the visible image of the developer transferred to the transfer material P is transported to the fixing device 6 together with the transfer material P, and is fixed by heat or pressure to become a recorded image.

On the other hand, the developer remaining on the photosensitive drum 2 without being transferred after the transfer is removed by a cleaning device having a blade. Thereafter, the surface of the photosensitive drum is charged again by the charging device 1, and the above steps are repeated. The printing speed is 10 ppm for A4.

The exposure device 7 comprises a laser drive section 210, a collimator (not shown), an Fθ lens, and a polygon mirror. A unit in which these are integrated is referred to as an optical unit.

FIG. 1 is a block diagram showing electrical components including the above-mentioned laser driving unit 210.

The image processing circuit 205 develops image data input from the host computer HOST into bitmap data. Based on this bitmap data,
Laser ON by laser ON / OFF signal generation unit 206
/ OFF signal is generated. And this laser ON / O
The laser drive circuit 211 is driven based on the FF signal to cause a laser diode (hereinafter, referred to as LD) to emit light.

The laser driving section 210 includes a laser driving circuit 2
11, a photodiode (hereinafter, referred to as PD) 214 built in the same package as the LD to receive the backlight of the LD 212 and the LD, and an optical output detection circuit 21
5, a laser light amount control circuit 216 and a current detection circuit 217 are included.

The operation of the laser driver 210 is as follows. The laser ON / OFF signal is transmitted to the laser drive circuit 21
1 to drive the LD 212. The LD 212 emits two laser beams. That is, one of the laser beams is a collimator (not shown), a polygon mirror,
Irradiation is performed on the surface of the photoconductor 2 via an Fθ lens or the like to form an electrostatic latent image. The other laser beam is applied to PD 214 built in the same package. PD2
14 is connected to a light output detection circuit 215 for monitoring the intensity of the laser light.

PD that receives laser light from LD 212
A current proportional to the laser light amount flows through 214. The light output detection circuit 215 has a conversion function of outputting a voltage proportional to the current, and the output voltage is applied to a laser light amount control circuit 216 for controlling a drive current of the LD 212. The current detection circuit 217 monitors a current flowing through the LD 212.

The laser light amount control circuit 216 compares the voltage output from the light output detection circuit 215 with a voltage value which has been adjusted in advance so that the laser power on the surface of the photosensitive member 2 becomes an appropriate value. And it has a function of controlling the LD drive current I so that these voltages match. With this laser light amount control circuit 216, the output voltage value from the light output circuit 215 always becomes a predetermined value. That is, the LD drive current I is adjusted so that the peak emission intensity of the LD 212 is always constant.
Is controlled. This is called auto power control (APC). The LD drive current for obtaining the predetermined light emission intensity is defined as Iop.

The LD drive current Iop is determined so that the amount of light on the surface of the photosensitive drum (photosensitive member) becomes a predetermined value. The LD drive current Iop depends on the beam emission angle of the LD and the transmittance of the optical lens. Have different laser powers for giving desired light energy to the laser beams. That is, Iop differs for each optical unit.

Further, the threshold current (hereinafter referred to as Ith) is not constant among individuals depending on the LD.

Next, the relationship between the laser ON / OFF time, the LD drive current, and the laser emission time will be described with reference to the image diagram of FIG.

1. First, the laser ON / OFF signal generator 2
A laser ON / OFF signal whose ON time is Tw as shown in FIG.

2. When the laser ON / OFF signal is turned on, the LD drive current gradually increases with a slope as shown in FIG.

3. When the LD drive current reaches the threshold current Ith, laser light is emitted as shown in FIG.

4. When the laser ON / OFF signal is turned off, the LD driving current decreases, and when the laser driving signal becomes smaller than the threshold current Ith, the laser is turned off.

5. At this time, the laser emission time TL is as shown in FIG.
It is defined as the half value width of the time-light output waveform shown in FIG.

However, as shown in FIG.
The rising time Tr from when the signal N is received to when it reaches Iop, and the delay time Tf from when Iop goes to 0.
Exists. These Tr and Tf are the laser drive circuit 211
Is determined by the time constant of

Therefore, when Tr, Tf and Iop are constant, the laser emission time TL can be extended by reducing the threshold current Ith.

Next, the relationship between the LD drive current Iop and the laser emission time TL will be described with reference to FIG. Here, the time constant and Ith of the laser drive circuit are equal, and I
A description will be given by taking as an example two optical units that differ only in op.

FIG. 3B shows the change over time of the laser drive current in two optical units having different Iops.
(C) is an image diagram showing light output waveforms of these optical units. In these drawings, the solid line shows the laser drive current and optical output waveform of the optical unit having a relatively large Iop. The broken line indicates the laser drive current and the optical output waveform of the optical unit having Iop smaller than the optical unit.

Further, Iop of the optical unit is changed to Iop.
, The value of 100% of the optical output power is represented as 100%, and the half value thereof is represented as 50%. For the optical unit,
Notation is similarly given.

Thus, the time constants Tr,
When Tf and threshold current Ith are equal, laser ON /
The time from receiving the OFF signal to reaching Ith is
It can be seen that the larger the Iop, the shorter the length. That is, the larger the Iop is, the longer the laser emission time TL can be.

Here, the laser characteristics I by the optical unit
The relationship between the variation of th and Iop and the emission time of one dot of the laser will be described in detail.

As shown in FIG.
By using LDs having different characteristics and using optical lenses having different transmission efficiencies, five optical units having different Ith and Iop were produced. Table 1 below
Are the Ith and LD of the LD used for these five optical units.
Iop and pulse width of 30 nsec for these LDs
Laser emission time TL when the laser ON signal is input
5 shows the results of the measurement.

[0055]

[Table 1]

Here, the laser emission time TL was measured according to the following procedure. First, an image signal having a pulse width of 30 nsec is input to a laser driving circuit, an LD is driven, and the irradiated laser beam is incident on a PD (photodiode). The PD (photodiode) is connected to an oscilloscope as a light detection circuit only during this measurement. Then, the time-light output waveform is monitored by the oscilloscope. The time during which 50% or more of the observed waveform peak value was output was defined as the laser emission time TL. In this case, the light output is a voltage value obtained by current-voltage conversion by the light output detection circuit.

From this, it was confirmed that the larger the Ith / Iop, the shorter the laser emission time TL.

Therefore, in this embodiment, this Ith
The ON time Tw of the one-dot laser is corrected based on / Iop.

Next, a process for correcting variations in the characteristics of these optical units and obtaining a stable image will be described in detail.

In the present embodiment, Ith and Iop of the LD are measured in advance when the image forming apparatus or the optical unit is manufactured.

1. Measurement of threshold current Ith An output terminal of the optical output detection circuit 215 is connected to a digital voltmeter (not shown). Then, the laser light amount control circuit 2
The LD drive current is gradually increased from 0 by changing the resistance in the resistor 16. When the voltage output Vp from the PD 214 changes abruptly, the current value monitored by the current detection circuit 217 is read. This is regarded as Ith.

This operation may be performed by the optical unit alone, or may be performed after the optical unit is incorporated in the image forming apparatus.

2. Measurement of LD Drive Current Iop Also, at the time of manufacturing, the optical unit was connected to the image forming apparatus 20.
After assembling into the main body, the resistance value of the laser light amount control circuit 216 is adjusted so that a desired laser power is obtained at the photosensitive member surface position. At this time, the current value monitored by the current detection circuit 217 is read, and this is defined as Iop.

When the image forming apparatus 200 is manufactured, the measured Ith and Iop are written and stored in a unit including the LD 212 and the laser drive circuit 211, that is, in the present embodiment, the storage unit 220 provided in the optical unit. This storage unit 220 is not particularly limited as long as it can be written and stored. In this embodiment mode, a ROM which is an electric element is used.

The image forming apparatus 200 manufactured as described above
Is then available for user use.

Before the user starts using the image forming apparatus, the reading unit 204 provided in the modulation circuit 202 reads the Ith and Iop from the ROM 220.
Is read out and sent to the determination unit 203.

The following formula is stored in the judgment unit 203.

[0068]

Tw = Tw0 + k × (Ith / Iop) (1) where Tw0 is a default value of the Idot laser ON time, and k is a constant.

That is, as Ith / Iop increases, the one-dot laser ON time Tw is lengthened.

In the present embodiment, the Tw thus determined is maintained as long as the image forming apparatus is used. The Tw correction process may be performed once when using the image forming apparatus.

In this embodiment, the laser is actually turned on.
Table 2 shows the relationship between the time and the number of continuous dots n. That is, Tw is applied to an image in which only one dot is turned on in the main scanning direction, and for an image formed by two or more continuous dots, only Tw is applied to one dot at the start of laser lighting. The reason is that when Tw correction is applied to a line formed by a large number of dots, the vertical line is prevented from becoming too thick. This relationship is optimized depending on the image forming apparatus actually used, and is not limited to this relationship.

[0072]

[Table 2]

An experiment was conducted to verify the effects of the present embodiment using the above-described five optical units having different characteristics using the image forming apparatus described above.

The common conditions of the used image forming apparatus are as follows. Resolution: 1200 dpi Process speed: 60 mm / sec Default value of Tw: 30 nsec Laser spot diameter: 70 μm in main scanning direction × 80 μm in sub-scanning direction Judgment formula: Tw = 30 + 8 × (Ith / Iop) Using the apparatus, an image was output using five optical units shown in Table 1, and the effect of the present embodiment was verified.

The following three types of image patterns were output. 1.1 Vertical line pattern of 2 dots and 2 spaces 2.1 Horizontal line pattern of 2 dots and 2 spaces 3.1 Single dot of 1 dot × 1 dot And 2. Was measured with a Macbeth densitometer, and it was determined that the smaller the difference, the more the vertical line could be reproduced as well as the horizontal line. Also, 3. Regarding (2), an enlarged photograph was taken and the dot reproducibility was sensually evaluated.

Next, the results of the verification experiment will be described.

Table 3 shows the density difference between the vertical line and the horizontal line for each unit.

Table 4 shows the results of the sensory evaluation of the reproducibility of a single dot by each unit. Here, ○ is good,
Δ indicates slightly inferior to ○, and X indicates an unacceptable level.

In each of these tables, results when the correction operation described in the present embodiment is not performed are also shown as comparative examples. Also, the laser emission time TL in Table 3
Was measured by the method described above.

[0080]

[Table 3]

As shown in Table 3, in this embodiment, Tw is corrected by the variation of Ith / Iop between units, so that the laser emission time TL of one dot is constant regardless of the optical unit. As a result, the density difference between the vertical line and the horizontal line of the one-dot two-space pattern was small, and it was confirmed that the image was very good.

On the other hand, in the comparative example, the density of the horizontal line pattern was substantially constant regardless of the unit. But I
Since Tw is not corrected by th / Iop, It
When h / Iop is large, TL becomes short. Accordingly, the units 1 and 2 having a small Ith / Iop had a sufficient density of the vertical line and the density difference between the vertical line and the horizontal line was small, and were good images. However, the units 3 to 5 had a large Ith / Iop and had a large Ith / Iop. Because of poor response, the latent image of the vertical line became shallow and the density of the vertical line became thin. As a result, comparing halftones formed with the same pattern,
It is not preferable because a density difference occurs between the vertical line and the horizontal line.

[0083]

[Table 4]

Further, from Table 4, in the present embodiment, all good single-dot images could be output regardless of the unit. However, in the comparative example, the units 1 and 2 were good images, but the units 3 to 5 could not reproduce single dots at a sufficiently acceptable level.

As described above, by controlling the ON time Tw of the one-dot laser in accordance with Ith / Iop, it is possible to secure a sufficient laser emission time especially in a single dot or one dot line.

Therefore, regardless of the variation of the optical unit, the density difference between the vertical line and the horizontal line formed in the same pattern is always small, and an image with good single dot reproduction can be provided.

Note that the determination formula for determining Tw is not limited to the above case, but it can be changed according to the characteristics of each image forming apparatus.

In this embodiment, the laser ON / OF
Although the laser ON time was corrected by the F signal generation unit 206, without using the laser ON / OFF signal generation unit,
The configuration of the laser drive circuit may be used. As an example, the laser ON time may be corrected to be longer by delaying the fall timing of the laser drive current as Ith / Iop is larger.

Embodiment 2 Next, a second embodiment of the present invention will be described.

The difference between the present embodiment and the first embodiment is that the measurement of Ith and Iop is performed each time the present image forming apparatus is used, not when the present image forming apparatus or optical unit is manufactured. Then, the one-dot laser ON time Tw is optimized based on these results.

A configuration different from that of the first embodiment will be described with reference to FIG.

The output of the current detection circuit 217 is input to the CPU 208 provided in the present image forming apparatus. CPU2
08 adjusts the voltage supplied to the laser drive circuit stepwise, monitors the current value detected by the current detection circuit 217 and the output voltage value from the optical output circuit 215 ′, and sends the result to the correction circuit 209. Take the role of sending.

Next, details of the operation (hereinafter referred to as a Tw correction process) in the present embodiment for correcting variations in Ith and Iop will be described.

First, image information is sent from the host computer HOST to the present image forming apparatus. Then, the image forming apparatus performs the following operation. 1. The drive current flowing through the LD 212 is gradually increased by gradually increasing the voltage applied to the LD 212 under the control of the CPU 208. At this time, the CPU 208
Monitors the current value output from the current detection circuit 217 and the output voltage value from the optical output detection circuit 215 '. 2. The output current value from the current detection circuit 217 when the output voltage value of the optical output detection circuit 215 ′ changes rapidly is regarded as the threshold current Ith. 3. Next, the light output detection circuit 21 is controlled by the CPU 208.
The output voltage value from 5 'is compared with a voltage value adjusted in advance at the time of manufacture, and the LD drive current is controlled so that they match. The output current value of the current detection circuit 217 at this time is defined as the LD drive current Iop. 4. The CPU 202 converts Ith and Iop into a correction circuit 209
Send to 5. Then, Tw based on Ith / Iop is determined by the following equation stored in the correction circuit 209.

[0095]

ITw = Tw0 + k × (Ith / Iop) (2) where Tw0 is a default value of 1-dot laser ON time, and k is an arbitrary constant.

That is, similar to the first embodiment, Ith /
The larger the value of Iop, the lower the response of the laser diode.

Thereafter, the bitmap data developed by the image processing circuit 205 based on the image information is
In the N / OFF signal generation unit 206, the corrected Tw
Is converted to a laser ON / OFF signal based on

These operations are performed before the electrostatic latent image is formed on the photosensitive member during the image forming process. After the Tw correction process is completed, the image forming apparatus performs the image forming process described above and performs the image forming process. Is output.

This Tw correction process is performed every time an image is formed. Then, Ith and Iop are updated every time an image is formed. As a result, an appropriate Tw can be always obtained every time an image is formed.

The present embodiment was applied to an image forming apparatus, and an experiment for verifying the effect of the present embodiment was performed under the same conditions as in the first embodiment.

As a result, similar to the first embodiment, the results show that the density of the vertical line and the horizontal line pattern formed by the same pattern is uniform and the reproduction of fine dots is excellent irrespective of variations in LD characteristics. I got it.

In addition, according to the present embodiment, since the change in the characteristics of the laser diode is measured every time the image forming apparatus is used, the change in the characteristics of the laser diode due to the use environment and deterioration in durability is taken into consideration. Laser driving can be performed. Therefore, it is possible to always provide a stable image regardless of the use environment and the number of times of image formation.

In the present embodiment, the laser ON / OF
Although the laser ON / OFF time is corrected by the F signal generation unit, the configuration of the laser drive circuit may be used without using the laser ON / OFF signal generation unit. As an example, I
The laser ON time may be corrected to be longer by delaying the fall timing of the laser drive current as th / Iop is larger.

Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIGS. The third embodiment is different from the first and second embodiments in that a plurality of L
This is an application to a so-called multi-beam type image forming apparatus using D.

FIG. 6 is a block diagram showing a configuration of the modulation circuit 207 and the laser driver 210 according to the third embodiment.

In the present embodiment, two LDs are used. Thus, the photosensitive member can be scanned by two lines in the sub-scanning direction at the same time, which is suitable for high-speed and high-resolution image forming apparatuses. The distance in the sub-scanning direction of the laser beams emitted from the two LDs on the photosensitive member is substantially determined by the resolution of the image forming apparatus. For example, if the resolution is 6
If it is 00 dpi, it is 42 μm, and if it is 1200 dpi, it is 21 μm. The PD 214 and the current detection circuit 21
7. There is one light output detection circuit 215 each.

FIG. 7 shows that the LD 212-2 is
FIG. 11 is an enlarged image diagram of a 1200 dpi resolution image obtained when Ith / Iop is larger than −1. In FIG. 7, the LD 212-1 has one signal every four pixels in the main scanning direction.
Form a pixel image. LD212-2 is an LD212-
An image is formed at the same cycle as that of the LD 212-1 while shifting the image formed in 1 by two pixels in the main scanning. One section of the grid pattern in the figure corresponds to one pixel of 1200 dpi. Black circles indicate the obtained toner images.

Ideally, these dots must all be formed in the same size. However,
As illustrated in FIG. 7, the dot image formed by the LD 212-2 is small and insufficient in size compared to the dot image formed by the LD 212-1. This phenomenon is caused by the fact that Ith / Iop of LD 212-2 is LD 212-1.
The same image laser ON / O
Even if the FF signal is input, the laser lighting time is LD212-2.
This is because the length is shorter.

Next, Ith and Iop of these two LDs
Of the present embodiment (hereinafter, Tw) for correcting the variation of
The details of the correction process will be described below.

First, when image data is sent from the host computer HOST to the image forming apparatus, the modulation circuit 20
7. The following operation is performed by the laser driver 210.

First, Ith1 and Iop1 are detected with respect to the LD 212-1 in the same manner as in the above-described second embodiment, and the 1-dot laser ON time Tw1 of the LD 212-1 is determined based on the result.

Next, the It 21 is similarly applied to the LD 212-2.
h2, Iop2, and based on the result, LD21
The 2-2 one-dot laser ON time Tw2 is determined.

Here, Ith1 and Iop1 are LD
The threshold current of 212-1 and the LD drive current are represented by Ith2, Ith
op2 indicates a threshold current and an LD drive current of the LD 212-1, respectively.

The image data sent from the host computer is converted into bitmap data by the image processing circuit 205. Then, the laser ON / OFF signal generation unit 20
In step 6, a laser ON / OFF signal is generated based on Tw1 and Tw2 determined previously, and each laser drive circuit 21
1-1 and 211-2.

As a result, the laser driving circuits 211-1 and 211-2 drive the LDs 212-1 and 212-2. Then, the photosensitive member is exposed to form a latent image, and an image is formed.

With the above operation, even in a multi-beam type image forming apparatus using a plurality of LDs, it is possible to correct the laser variation between the LDs and make the lighting time equal regardless of the LD. Therefore, the same latent image can be formed on the same image video signal regardless of the LD, and a uniform image can be obtained.

In the present embodiment, the laser ON / OF
Although the laser ON / OFF time is corrected by the F signal generation unit, the configuration of the laser drive circuit may be used without using the laser ON / OFF signal generation unit. As an example, I
The laser ON time may be corrected to be longer by delaying the fall timing of the laser drive current as th / Iop is larger.

[0118]

As described above, according to the present invention, there is provided an image forming apparatus and an image forming method capable of correcting variations in characteristics of a laser diode with a simple configuration without requiring a user's effort. Can be realized.

More specifically, according to the first, second, fifth, and sixth aspects of the present invention, variations in the laser emission time due to the characteristics of the laser diode can be reduced by correcting the laser ON / OFF signal. Therefore, it is possible to obtain an image forming apparatus and an image forming method in which the reproduction of minute dots is stable irrespective of the variation of the laser diode or the exposure apparatus, and the thickness of the vertical and horizontal lines is uniform.

According to the third and seventh aspects of the present invention, the laser ON / OFF signal can be corrected based on the information on the variation between the individual laser diodes or the exposure apparatuses, so that it is optimal for any image forming apparatus. The laser ON / OFF signal time can be determined. Thus,
It is possible to obtain an image forming apparatus and an image forming method in which the reproduction of minute dots is stable irrespective of the variation of the laser diode or the exposure apparatus and the thickness of the vertical line and the horizontal line is uniform.

Further, according to the present invention, stable minute dots can be always reproduced regardless of the characteristics of the laser diode or the exposure apparatus over time and changes in the use environment. An image forming apparatus and an image forming method in which the thickness of the vertical line and the horizontal line are uniform can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a laser driver and a modulation circuit according to a first embodiment.

FIG. 2 is an explanatory diagram showing a relationship among a laser ON / OFF signal, a laser drive current, and an optical output waveform.

FIG. 3 is an explanatory diagram showing a relationship among a laser ON / OFF signal, a laser drive current, and an optical output waveform.

FIG. 4 is a diagram showing IL characteristics of a laser diode.

FIG. 5 is a block diagram showing a laser driver and a modulation circuit according to a second embodiment.

FIG. 6 is a block diagram showing a laser driver and a modulation circuit according to a third embodiment.

FIG. 7 is an enlarged image diagram of an image obtained in a comparative example of the third embodiment.

FIG. 8 is a schematic sectional view of an image forming apparatus to which the present invention is applied.

DESCRIPTION OF SYMBOLS 1 Charging device 2 Photosensitive drum (photoreceptor) 3 Developing device 6 Fixing device 7 Exposure device 100 Process cartridge 200 Image forming device 210 Laser drive unit 211 Laser drive circuit 212 Laser diode (LD) 214 Photodiode (PD) ) 215, 215 'light output detection circuit 216 laser light amount control circuit 220 storage unit

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takeshi Konishi 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Katsuharu Shibata 3- 30-2 Shimomaruko, Ota-ku, Tokyo Canon F term in reference (reference) 2C362 AA03 AA31 AA61 BB28 BB37 CB59 CB78 2H076 AB05 AB12 DA11 DA17 DA19

Claims (8)

[Claims] 1. An image forming apparatus for forming an image by irradiating a laser beam emitted from a laser diode onto an image carrier, comprising: a conversion unit configured to convert input image data into a laser on / off signal; Laser driving means for driving the laser diode in response to a laser on / off signal; one of a threshold current of the laser diode; and a laser diode driving current required to obtain a predetermined light intensity on the surface of the image carrier. An image forming apparatus comprising: a correction unit configured to correct an on-time of the laser on / off signal input to the laser driving unit based on one or both of them. 2. The image forming apparatus according to claim 1, wherein the correction unit is configured to output the laser on / off signal input to the laser driving unit based on a ratio between the threshold current and the laser diode driving current. An image forming apparatus, wherein the on-time is corrected. 3. The image forming apparatus according to claim 1, further comprising storage means for storing one or both of the threshold current and the laser diode drive current. 2. The image forming apparatus according to 1. 4. The image forming apparatus according to claim 1, further comprising a detection unit for detecting one or both of the threshold current and the laser diode drive current. 2. The image forming apparatus according to 1. 5. An image forming method for forming an image by irradiating a laser beam emitted from a laser diode onto an image carrier, wherein the laser diode is driven when the laser diode is driven based on a laser on / off signal. And a laser diode driving current required to obtain a predetermined light intensity on the surface of the image carrier, or both of the threshold current and the on time of the laser on / off signal input to the laser driving unit. An image forming method comprising: 6. The image forming method according to claim 5, wherein an on-time of the laser on / off signal input to the laser driving unit is corrected based on a ratio between the threshold current and the laser diode driving current. An image forming method comprising: 7. The image forming method according to claim 5, further comprising the step of storing one or both of the threshold current and the laser diode driving current. 8. The image forming method according to claim 5, further comprising a step of detecting one or both of the threshold current and the laser diode drive current.