JP2685799B2 - Image forming device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image of a copying machine, a laser printer or the like which forms a halftone image by a laser beam driven by an image forming signal which is multi-valued pulse width modulated. Forming apparatus

(Prior art)

First, an image forming apparatus which is a premise of the present invention will be described.

FIG. 7 is a perspective view showing an example of a writing optical system using a semiconductor laser. 1 is a semiconductor laser, 2 is a polygon mirror, 3 is a photosensitive drum, 4 is an fθ lens, 5 is a condenser lens, 6 is a cylindrical lens, 7 is a mirror, 8
Is an optical detector for keeping the writing position constant.

In such a configuration, the beam emitted from the semiconductor laser 1 is converted into a parallel beam by the condenser lens 5, and the parallel beam is linearly focused on the polygon mirror 2 by the cylindrical lens 6. The beam reflected by the polygon mirror 2 is imaged on the photosensitive drum 3 by the fθ lens 4, and the beam scans the photosensitive drum 3 by the rotation of the polygon mirror 2.

FIG. 8 is a schematic sectional view for explaining the overall structure of the image forming apparatus including the writing optical system shown in FIG. In the figure, reference numeral 11 is a writing optical system unit in which the writing optical system shown in FIG. 7 is unitized. The beam emitting portion of the unit is provided with a dustproof glass 17, and the unit has a closed structure. ing. Reference numeral 12 is a photosensitive drum shown as the photosensitive drum 3 in FIG. 7, 13 is a charger, 14 is developing means, 15 is transfer paper, and 16 is cleaning means. The photosensitive drum 12 is rotated in a direction indicated by an arrow by a driving unit (not shown), and is charged by a charger 13. Thereafter, scanning exposure is performed by a laser beam from the writing optical system unit 11 to form a latent image. And visualized by the developing means 14,
The image is transferred onto the transfer paper 15 at the transfer point. The toner remaining on the photosensitive drum 12 is removed by the cleaning unit 16.

Next, a pulse width modulation method for modulating a laser beam will be described.

In this pulse width modulation method, when a gradation signal is input from an image processing unit, a laser beam is emitted from a semiconductor laser by a modulation signal having a pulse width corresponding to a prepared gradation. is there.

FIG. 9 is a block diagram of a configuration in which a 2-bit parallel signal line is assigned to a gradation signal and an output of 4 gradations is obtained by 2 bits. D1 to D4 are delay lines, G1 and G2 are AND gates, G3 and G4 are OR gates, S is a selector, D is a semiconductor laser (LD) drive circuit, and 1 is a semiconductor laser (LD).

The pulse width modulation method shown in FIG.
And the number of gradations (here, four types) are provided to obtain the logical product or logical sum of the pixel clock CLK delayed by the delay line for an arbitrary time, and the selector S outputs the pulse width corresponding to the gradation signal. Then, the semiconductor laser (LD) 1 is driven by applying this to the LD drive circuit D.

Figures 10 and 11 show the delay line, AND gate, and OR.
FIG. 10A is an explanatory diagram for obtaining an arbitrary pulse width output from the pixel clock CLK by the gate. FIG. 10A is a configuration diagram with a delay line and an AND gate, FIG. (a) is a configuration diagram of the delay line D and the OR gate, and (b) is an operation waveform diagram thereof.

In FIG. 10, a signal having a pulse width of (T-ΔT ₁ ) is obtained by inputting a delayed signal that has passed through the pixel clock CLK and the delay line D (delay time: ΔT ₁ ) to the AND gate G. Can be done. Therefore, by arbitrarily selecting ΔT ₁ , it is possible to obtain a signal having an arbitrary pulse width (T−ΔT ₁ ).

In FIG. 11, a signal having a pulse width of (T + ΔT ₂ ) can be obtained by inputting a delayed signal passing through the pixel clock CLK and the delay line D (delay time: ΔT ₂ ) to the OR gate G. . As in Figure 10, ΔT ₂
A signal having an arbitrary pulse width (T + ΔT ₂ ) can be obtained by arbitrarily selecting

The configuration shown in FIG. 10 (a) and FIG. 11 (a) is provided by the number corresponding to the number of gradations, these signals are input to the selector, and by the gradation signal instructed by the image processing unit, It is possible to obtain a signal output having a pulse width according to the gradation.

Although FIG. 9 shows an example in which four gradations are output, higher gradations can be output by increasing the circuit configuration and the number of bits of the gradation signal (for example, eight kinds of pulse widths). (8 gradations can be output by preparing a setting circuit for setting and a 3-bit gradation signal).

Next, the change with time of the photoreceptor will be described. As an example, a case where an OPC photosensitive member is used and a negative-positive developing method (a method of forming a toner image on an exposed portion) is adopted will be described.

FIG. 12 shows how the charging potential V ₀ on the photosensitive member changes as the number of image formations increases.
V ₀ decreases as the number of image formations increases. Also, the twelfth
The figure also shows changes over time in the post-exposure potentials V _T1 , V _T2 , and V _T3 when beam exposure is performed on the photoconductor with pulse widths T1, T2, and T3 (T1 <T2 <T3). As can be seen from FIG. 12, the changes in the post-exposure potentials V _T1 , V _T2 , and V _T3 are different from the changes in the charging potential V ₀ . That is, the amount of change in post-exposure potentials V _T1 and V _T2 when exposed with short pulse widths T1 and T2 is larger than the amount of change in charging potential V ₀ (however, the amount of change in V _T2 is greater than that in V _T1. Is big). Further, the post-exposure potential V _T3 when exposed with a long pulse width T3 is almost constant from a certain point. as a result,
The density of the image of the part exposed with each pulse width changes as shown in FIG. 13 when the initial number and the number of times of image formation are overlapped.

In the figure, a is a characteristic over time, and b is an initial characteristic. That is, this results in an increase in the image density and a deterioration in the gradation reproducibility of the image with the passage of time, resulting in a deterioration in the image quality.

In order to solve this problem, conventionally, there has been a method of detecting the number of times of image formation and the fatigue of the photoconductor and controlling the developing bias voltage according to the detected values.

However, with this method, the overall density (from the low density area to the high density area) can be changed uniformly, but the gradation cannot be changed.

A method is also known in which the laser beam output is changed to change the image density. In this system, for example, there is a proposal described in Japanese Patent Laid-Open No. 59-136754. This proposal forms a latent image pattern of a halftone portion A and a white background portion B on a photoconductor, measures the surface potentials of the halftone portion A and the white background portion B, and measures the measured values V _A , V _B and the target. Value V _A0 , V _B0
The difference between is within the tolerance C ₁ , C ₂ , ie |
It is determined whether or not V _A −V _A0 | ≦ C ₁ and | V _B −V _B0 | ≦ C ₂ .

As a result of the judgment, if the above relational expression is not satisfied, the charging current I ₁ of the charger and the driving current I ₂ of the semiconductor laser are to be controlled according to the following control expressions.

ΔI ₁ = α ₁ ΔV _A + α ₂ ΔV _B ΔI ₂ = β ₁ ΔV _A + β ₂ ΔV _B (α ₁ , α ₂ , β ₁ and β ₂ in the equation indicate the slope of the function) However, in this way Only by changing the output of the laser beam, the tone characteristics of the entire halftone could not be returned to the initial state, and the reproducibility of the halftone could not be reliably improved.

Further, a proposal to change the pulse width setting value of the semiconductor laser drive pulse is described in, for example, Japanese Patent Application Laid-Open No. 61-189567. This proposal measures whether the surface potential V of the photoconductor is measured and whether the difference between it and the target halftone potential V _H (photoconductor surface potential at which a predetermined halftone is obtained when developed) exceeds an allowable value ΔV. Is determined. When the allowable value ΔV is exceeded, the target pulse width T _H is calculated according to the following control formula, and a pulse width set value equal to or close to the target pulse width T _H is to be selected.

T _H = T ₃ + α (V−V _H ) (T ₃ in the formula is the pulse width of the reference modulation signal, α is a constant) However, even if the pulse width setting value is changed in this way, It was not possible to return the tone characteristics of the entire tone to the initial state, and it was not possible to reliably improve the reproducibility of the intermediate tone.

As a result of various experiments to obtain faithful reproducibility of halftones, the present inventors found that the pulse width per pixel (simply referred to as pulse width is abbreviated as "pulse width" simply by changing the output of the laser beam as described above. It was confirmed that the gradation characteristics returned to the initial state before the photoconductor was fatigued in the low density area where the pulse width was short, but the gradation characteristics were not sufficiently recovered in the high density area where the pulse width was long. Further, it has been found that, only by changing the set value of the pulse width, there is an effect of recovering the gradation characteristic in a specific density part, but the recovery of the gradation property is insufficient in other density parts. In more detail, ... When the output of the laser beam is reduced with reference to the low density portion having a short pulse width, the gradation characteristics are not sufficiently restored in the high density portion having a long pulse width.

. On the other hand, when the output of the laser beam is reduced with reference to the high density portion having a long pulse width, the image density is too low in the low density portion having a short pulse width.

. Further, if the pulse width is shortened with reference to the low density portion having a short pulse width, the gradation characteristics are not sufficiently restored in the high density portion having a long pulse width.

. On the other hand, it was clarified that when the pulse width is shortened with reference to the high density portion having a long pulse width, a peculiar phenomenon that the image density is excessively reduced in the low density portion having a short pulse width occurs.

〔Purpose〕

The present invention has been made based on such knowledge, and it is an object of the present invention to provide an image forming apparatus capable of obtaining faithful reproducibility of gradation characteristics in a wide halftone from a low density portion to a high density portion. To aim.

〔Constitution〕

In order to achieve this object, the present invention provides a multi-valued pulse width modulation of an image signal to generate an image forming signal, and irradiates a photoconductor with a laser beam driven corresponding to the image forming signal. The present invention is intended for an electrophotographic image forming apparatus that forms an electrostatic latent image on the photoconductor, visualizes the electrostatic latent image by developing means, and further transfers the developed image to a transfer paper. .

Then, a photoconductor fatigue detecting means such as a surface potential detector for detecting the fatigue of the photoconductor, a laser beam output changing means for changing the output of the laser beam, and a setting of the pulse width of the multilevel pulse width modulation And a pulse width changing means for changing the value.

When the photoconductor fatigue detection unit detects the photoconductor fatigue, the laser beam output changing unit changes the laser beam output so that the image density in the low density portion having a short pulse width becomes the same as the image density before the photoconductor fatigue. In addition to changing the output, the pulse width setting means is configured to change the pulse width setting value so that the pulse width in the high density portion having a long pulse width becomes shorter than that before the photoconductor fatigue. It is a thing.

When the output of the laser beam is changed (lowered) so that the image density of the low-density portion becomes the same as the image density before the photoconductor is exhausted, the gradation characteristics of the low-density portion are restored. However, in this case, the gradation characteristic in the high density portion does not match the initial gradation characteristic.

Therefore, in the high-density portion, by changing (decreasing) the laser beam output and further changing (shortening) the pulse width setting value, the gradation characteristic in the high-density portion can be returned to the initial gradation characteristic. it can.

Before describing the embodiments of the present invention, a method for detecting the degree of fatigue of a photoconductor will be described first.

FIG. 4 shows an example in which a surface potential detector 18 for detecting the surface potential of the photosensitive drum 12 is provided in the image forming apparatus. In the figure, the surface potential detector 18 is the photosensitive drum 12
It is provided between the upper beam irradiation point P ₁ and the charger 13,
The surface potential detector 18 can detect the charge potential V ₀ which, changes in charge potential V ₀ which as shown in FIG. 12, i.e. it is possible to know the degree of fatigue of the photoreceptor.

FIG. 5 is a diagram for explaining a method of detecting the degree of fatigue of the photoconductor different from that of FIG. In the figure, reference numeral 19 is a reflection density detector, which is arranged near the photosensitive drum 12 at a position between the development point P ₂ and the transfer point P ₃ . Then, this reflection density detector 19 allows the beam non-exposed area (background area).
It is possible to know the fatigue of the photoconductor by detecting the reflection density of.

 This point will be described in detail below.

The charging potential V _{0 of} the photoconductor decreases due to long-term use (due to fatigue of the photoconductor over time, film deviation of the photoconductor, etc.). As a result, when the developing bias is set to the same value as the initial value, when the charging potential V ₀ becomes a certain value or less,
Toner also adheres to the area not exposed by the beam (background area).

FIG. 6 shows the relationship between the potential of the photoconductor and the amount of toner adhering to the photoconductor (the reflection density detector 19 is
The amount of reflected light of the light emitted from the detector is detected, and when the toner adhesion amount increases, the reflected light amount decreases). That is, when the charging potential becomes almost equal to or less than the developing bias due to long-term use of the photoconductor,
The amount of toner attached on the photoconductor increases. Therefore, by detecting the toner adhesion amount (reflection density), it is possible to know the decrease in the charging potential, that is, the fatigue of the photoconductor.

FIG. 1 is a system diagram of the entire image forming apparatus for explaining an embodiment of the present invention.

Surface potential detector 18 and potential detection circuit 20 described in FIG.
Thus, the charging potential V ₀ is detected. Assuming that the charging potential drops to V ₀ ′ over time with respect to the initial charging potential V ₀ , the low-density portion (pulse width of the pulse width of ΔV ₀ (= V ₀ −V ₀ ′) The laser beam output changing circuit 21 lowers the laser beam output so that the image density in the short range) becomes the same as the initial image density (before the photoconductor fatigue).

The gradation characteristic of the image density at that time is shown in FIG.
Shown in b (solid line) is an initial characteristic.

As can be seen from FIG. 2, by decreasing the output of the laser beam, the density is lowered as a whole (from the low density portion to the high density portion). The gradation characteristics in the low density portion can be made almost the same as the initial gradation characteristics,
However, the gradation characteristics in the high density area do not match the initial gradation characteristics.

Therefore, the pulse width in the high density portion (where the pulse width is long) is made shorter than the initial value through the pulse width setting circuit 22, the selector S, and the LD drive circuit D, so that the gradation characteristics in the high density area also have the initial gradation characteristics. It can be made almost the same as the gradation characteristic.

As an example, the initial charging potential: −800V is −70 over time.
The table shows the changes in the output and pulse width of the semiconductor laser (LD) when the voltage drops to 0V.

However, the condition is: pixel density; 16 lines / mm photoconductor linear velocity; 90 mm / s effective scanning period ratio; 71.5% pixel clock; 10 MHz 1 pixel lighting width; 100 ms.

Here, a method for changing the output of the beam in accordance with the output from the detecting means for detecting the state of the background portion of the photoconductor will be supplementarily described below.

It should be noted that, even if the apparatus is common to those shown in FIG. 7 and FIG.

First, FIG. 14 shows an example of the configuration of a laser printer to which the present invention is applied. A laser beam emitted from the semiconductor laser 101 is collimated by a collimator lens 102, deflected by an optical scanning device 103 composed of a rotating polygon mirror, and imaged on the charged surface of a photoconductor 105 by an fθ lens 4. This imaging spot repeatedly moves in the main scanning direction indicated by the arrow X in accordance with the rotation of the optical scanning device 103, and at the same time, the photosensitive member 105 rotates to perform sub scanning. Photo detector 1
06 is provided outside the information writing area in the axial direction of the photoconductor 105 and detects the laser beam deflected by the optical scanning device 103 to generate a synchronization signal (line synchronization signal LSYNC). The signal processing circuit 107 applies an information signal (video data) to the semiconductor laser driving circuit 108, and the timing thereof is controlled by a synchronization signal from the photodetector 106.

On the other hand, the semiconductor laser drive circuit 108 is a signal processing circuit 107.
The semiconductor laser 101 is driven according to the information signal from the. Therefore, the laser beam modulated by the information signal is irradiated onto the photoconductor 105 to form an electrostatic latent image. This electrostatic latent image is developed by a developing device (not shown), and transferred to paper or the like by a transfer device (not shown).

Further, the laser beam emitted rearward from the semiconductor laser 101 is incident on the photodetector 109 as a photodetector, and the light intensity thereof is detected. A control circuit 110 as a control unit controls the semiconductor laser drive circuit 108 according to the output signal of the photodetector 109 to control the output light amount of the semiconductor laser 101 to be constant. A laser beam emitted rearward from the semiconductor laser 101 is made incident on the photodetector 109 to detect its light intensity. This method is advantageous because it does not lower the light intensity of the laser beam that can be actually used unlike the light intensity detection method of guiding a part of the laser beam emitted forward to the photodetector.

Next, a detailed configuration of the control circuit 110 is shown in the block diagram of FIG. First, when the timing signal T ₁ for starting the output control operation is input, the JK flip-flop 116
Is cleared and its output signal becomes L level, thereby permitting the counting operation of the up-down counter 117. The output signal of the comparator 112 as the comparing means is latched by the D flip-flop 113 by the clock signal from the oscillator 118, and the output signal of the D flip-flop 113 is added to the up-down counter 117 as a counting mode signal to control this counting mode. At the same time, the D flip-flop 114 is latched by the clock signal from the oscillator 118. The non-inverted output of the D flip-flop 113 and the inverted output of the D flip-flop 114 are input to the NOR circuit 115,
By the output signal of this NOR circuit 115, JK flip-flop 1
16 is set.

The output proportional to the intensity of the laser beam detected by the photodetector 109 and amplified by the amplifier 111 is compared with the reference voltage Vref by the comparator 112, and the comparator 11 is output according to the comparison result.
A high level or low level signal is output from 3.
For example, when the output of the comparator 112 is at a high level (that is, the optical output of the semiconductor laser 101 is higher than the reference voltage Vref), the timing signal T ₁ causes an up-down counter 117.
When the count operation of the
117 operates as a down counter by the high level output of the D flip-flop 113. The output of the up-down counter 117 is converted into an analog output by the digital-analog converter 119, and the current from the semiconductor laser drive circuit 108 to the semiconductor laser 101 changes according to the output. Therefore, in this case, the drive current of the semiconductor laser 101 decreases, and the output voltage of the amplifier 111 decreases. And comparator 1
When the output of 12 is inverted from the high level to the low level, the output of the D flip-flop 113 becomes the low level and the NOR circuit 115
Becomes high level, the JK flip-flop 116 is set, and the counting operation of the up-down counter 117 is prohibited.

On the other hand, when the output of the comparator 112 is at a low level (that is, the optical output of the semiconductor laser 101 is smaller than the reference voltage Vref), the timing signal T ₁ causes an up-down counter 117.
When the count operation of the
117 operates as an up counter by the low level output of the D flip-flop 113. The output of the up-down counter 117 is converted into an analog output by the digital-analog converter 119, and the current from the semiconductor laser drive circuit 108 to the semiconductor laser 101 changes according to the output. Therefore, in this case, the drive current of the semiconductor laser 101 increases and the output voltage of the amplifier 111 increases. And comparator 1
When the output of 12 is inverted from the low level to the high level, the output of the D flip-flop 113 becomes the high level. As a result, the up-down counter 117 operates as a down counter. At this time, the output of the NOR circuit 115 remains at the low level, the JK flip-flop 116 is not reset, and the up-down counter 117 remains enabled for counting operation. That is, the up-down counter 11
7, the optical output of the semiconductor laser 101 increases and the reference voltage Vref
When the voltage exceeds Vref, the counting operation is not prohibited. When the optical output of the semiconductor laser 101 decreases and exceeds the reference voltage Vref, the counting operation is prohibited. Therefore, the holding current of the semiconductor laser 101 is always constant.

Contrary to the above example, the up-down counter 117 does not prohibit the counting operation when the optical output of the semiconductor laser 101 decreases and exceeds the reference voltage Vref.
Even if the count operation is disabled when the optical output of 101 increases and exceeds the reference voltage Vref, the semiconductor laser 10
The holding current of 1 is always kept constant.

That is, the portion surrounded by a frame 120 in FIG. 15 corresponds to an edge detection circuit that detects a transition of the output from the comparator 112 and permits or prohibits the counting operation of the up-down counter 117. Amplifier 11 as described above
The optical output from the semiconductor laser 101 is controlled so that the output voltage of 1 becomes a constant value with the reference voltage Vref as a reference (the optical output is always held at a constant value).

Thus, for example, a counter and a D / A converter are used, and such output control means is disclosed in, for example, Japanese Patent Laid-Open No.
171863, JP 61-174786, JP 61-17
It is disclosed in Japanese Patent No. 4787 and the like.

Alternatively, a method of stabilizing the beam output by a method different from the above circuit configuration will be illustrated below.

As shown in FIG. 16, the same can be applied to the case where all are configured by analog circuits.

That is, the laser beam emitted from the semiconductor laser 121 is received by the photodetector 122 as the photodetection means, and its light intensity is detected.

The detection signal of the photodetector 122 is amplified by the amplifier 123, compared with the reference voltage Vref by the comparator 124 as a comparison means, and an error signal is output as the comparison result. The sample hold circuit 125 as a control means holds the error signal from the comparator 124 by the timing signal T ₁ instructing the power check, and outputs it to the laser drive circuit 126.
The laser drive circuit 126 increases or decreases the current value given to the semiconductor laser 121 in accordance with the signal from the sample hold circuit 125 to make the light intensity constant.

As described above, in the examples described in FIGS. 15 and 16, the output from the amplifiers (111 and 123) is
By controlling so as to match Vref, the set light beam intensity is made constant. Therefore, when Vref is changed, the light beam intensity controlled according to the value also changes.

In the present invention, according to the output from the detection means for detecting the state of the background portion of the photoreceptor, by adjusting the Vref, it is possible to correct the image change due to the change of the state of the background portion of the photoreceptor. It is possible.

For example, in brief, as shown in FIG.
Alternatively, a method of inputting a plurality of set voltages to one of them and selectively outputting one of them as Vref in accordance with a detection signal from a detection unit that detects the state of the background portion of the photoconductor.

Alternatively, the detection output from the detecting means is further input to the calculating means, and the selector 134 outputs the output signal after the arithmetic processing.
If the output of is selected, the light beam intensity can be corrected more finely in response to the state change of the photoconductor.

For example, in the example of FIG. 18 (a), the detection signal is converted into an A / D converter 1
Digital data is input at 36 and input to the selector 135. The selector data is a plurality of voltages prepared in advance by the voltage setting circuit 137, and one of them is selected according to the A / D output.
Two voltages are output as Vref.

In this case, the plurality of voltage outputs may be independently settable.

Alternatively, in the configuration illustrated in FIG. 18B, the detection signal is converted into digital data by the A / D converter 138. This output is input to, for example, storage means (ROM 139 or the like).
In the storage means 139, output data for input data,
The data determined according to the characteristics of the photoconductor is stored, and is addressed by the output from the A / D, and new data is output (data corresponding to the correction curve is output). This is further converted into an analog signal by the A / D converter 140. Use this output as Vref.

Although such a configuration is slightly complicated as compared with FIG. 17,
Since it is possible to flexibly respond to the characteristics of the photoconductor,
A more faithful image reproduction becomes possible.

Next, FIG. 3 shows a block diagram of a pulse width setting circuit for changing the set value of the pulse width. The difference from the block diagram shown in FIG. 9 is that a delay time changing circuit is provided in the middle of the circuit for obtaining the pulse widths T3 and T4. The pulse width T3, T4 can be changed by inputting the potential detection signal to the delay time changing circuits d3, d4 and changing the delay time of the delay lines D3, D4.

〔effect〕

As described above, according to the present invention, it is possible to prevent or reduce the change in gradation and density of the formed image due to sensitivity deterioration due to long-term use of the photoconductor.

[Brief description of the drawings]

FIG. 1 is a system diagram of the entire image forming apparatus according to an embodiment of the present invention, FIG. 2 is the same as the gradation characteristic diagram of the image density, and FIG. 3 is the same as the block diagram of the pulse width setting circuit. 5 and 5 are views showing different configurations for detecting the fatigue level of the photoconductor, FIG. 6 is a view showing the relationship between the potential of the photoconductor and the amount of toner, and FIG. 7 is a perspective view of the laser writing optical system. FIG. 8 is a schematic diagram of an image forming apparatus according to a conventional example, FIG. 9 is a block diagram showing an example of a pulse width modulation method of a laser beam, and FIGS. 10 and 11 are diagrams showing details thereof. (A) is a block diagram, (b)
Is a time chart, FIG. 12 is a diagram showing the deterioration characteristic of the surface potential of the photoconductor over time, FIG. 13 is a gradation characteristic diagram of the image density according to the conventional example, and FIG. 14 and the following are supplementary methods for changing the laser beam output. FIG. 14 is a diagram for explanation, FIG. 14 is a laser printer configuration diagram, FIG. 15 is a control circuit diagram thereof, FIG. 16 is a beam output stabilization block diagram, FIG. 17, FIG. 18 (a), and (b). )
FIG. 3 is a diagram showing each example of a method of selecting a reference value of beam intensity. 18 …… Surface potential detector, 19 …… Reflection density detector, 20 ……
Potential detection circuit, 21 …… Laser beam output change circuit, 22
...... Pulse width setting circuit, d3, d4 …… Delay time changing circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoko Fujioka 1-3-6 Nakamagome, Ota-ku, Tokyo Within Ricoh Co., Ltd. (72) Yoshio Kaneko 1-3-6 Nakamagome, Ota-ku, Tokyo (56) Reference JP 59-136754 (JP, A) JP 61-189567 (JP, A) JP 1-261669 (JP, A) JP 1-257868 (JP, A) JP-A-1-276166 (JP, A) JP-A-63-30252 (JP, A)

Claims (3)

(57) [Claims] 1. A multi-valued pulse width modulation of an image signal to generate an image forming signal, a laser beam driven corresponding to the image forming signal is irradiated onto the photosensitive member, and the photosensitive member is irradiated with the laser beam. In an electrophotographic image forming apparatus that forms an electrostatic latent image, visualizes the electrostatic latent image by developing means, and further transfers the developed image to a transfer paper, a photoconductor that detects fatigue of the photoconductor. Fatigue detecting means, laser beam output changing means for changing the output of the laser beam, pulse width changing means for changing the set value of the pulse width of the multi-valued pulse width modulation, and photosensitive body fatigue detecting means When the fatigue of the background portion of the body is detected, the laser beam output changing means changes the laser beam output so that the image density in the low density portion where the pulse width per pixel is short becomes the same as the image density before the fatigue of the photoreceptor. Control means for changing the output and changing the pulse width setting value by the pulse width changing means so that the pulse width in the high density portion where the pulse width per pixel is long is shorter than that before the photoconductor fatigue. An image forming apparatus characterized by the above. 2. The image forming apparatus according to claim 1, wherein the detection unit is a potential detection unit that detects a charging potential on the photoconductor. 3. The image forming apparatus according to claim 1, wherein the detecting means is a reflection density detecting means for detecting the density of the background portion on the photoconductor.