JP3275388B2 - Image forming method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method used in an electrophotographic copying machine or printer for forming an image using a laser beam emitted from a semiconductor laser, a gas laser or the like, and an apparatus therefor. It is.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus,
For example, there is one as shown in FIG. This image forming apparatus irradiates a laser beam LB emitted from a semiconductor laser 100 to a polygon mirror 102 via a collimator lens 101 and reflects the laser beam LB by the polygon mirror 102 rotating at a high speed. Scanning exposure is performed along the axial direction (main scanning direction) of the photosensitive drum 104. At this time, the image forming apparatus converges the laser beam LB scanned and exposed on the photosensitive drum 104 into a spot shape by the fθ lens 103, keeps the moving speed in the main scanning direction constant, and uniformly charges the surface. By performing scanning exposure on the photosensitive drum 104 thus formed, an electrostatic latent image corresponding to image information is formed on the surface of the photosensitive drum 104 to form an image.

[0003] At this time, in order to make it possible to reproduce the gradation in the above-mentioned image forming apparatus, generally, the exposure time of a laser beam scanned and exposed on the photosensitive drum 104 is modulated so that the laser beam per pixel is modulated. An area gradation method for changing the beam exposure area according to the density is used.

[0004]

However, the above-mentioned prior art has the following problems. That is, the image forming apparatus uses an area gradation method in which the exposure time of the laser beam is modulated to change the exposure area of the laser beam per pixel in accordance with the density in order to reproduce the gradation. I have. Therefore, when the exposure time of the laser beam is changed according to the density of the image, the energy density of the laser beam has a distribution approximated by a Gaussian distribution, so that only the width of the image according to the input coverage of the image density is used. However, the maximum value of the energy density of the laser beam irradiated on the photosensitive drum 104 also changes as shown in FIG. Therefore,
When the exposure time of the laser beam is changed to reproduce the gradation, the maximum value of the energy density of the laser beam changes accordingly. Not only does the exposure time become shorter, but also the maximum value of the energy density of the laser beam becomes lower, and the surface potential of the photosensitive drum 104 does not change to a predetermined potential.

In this state, the electrostatic latent image formed as described above is developed by a developing device having a difference in speed between the photosensitive drum and the developer carrier of the developing device, and the developer carrier has a high coverage. When the development is performed while moving from the portion to the low coverage portion, the following problems occur.

That is, as shown in FIG. 10, an image formed on the photosensitive drum 104 has a low coverage area (for example, a 100% density area) next to a high coverage area (high density area). For example, when an image having a density of 20% is continuous, as shown by a broken line in FIG. 11, a high coverage portion where the latent image potential is high, and the maximum value of the energy density of the laser beam is low and the latent image potential is low. Low low coverage areas will be adjacent. Therefore, when this electrostatic latent image is developed by a developing device having a difference in the speed between the photosensitive drum and the developer carrier of the developing device, when the developer carrier develops the high coverage area, A large amount of toner is consumed during development, and the remaining developer on the developer carrier includes:
Many residual charges of the opposite polarity to the toner are generated. Thereafter, when the low-coverage portion is developed by the developer-carrying member, the developer-carrying member having many residual charges having the opposite polarity to the charged polarity of the toner passes through the low-coverage portion.
A reverse electric field is generated between the developer carrier and the photosensitive drum, and the toner once developed returns to the developer carrier side, and a blank image as shown in FIG. There was a problem.

In order to prevent the occurrence of such image defects, the present applicant has optimized the beam diameter of the laser beam and the resistivity of the developer as disclosed in Japanese Patent Application No. 4-55981. It is proposed to make.

In the developing method according to this proposal, a magnetic brush of a developer in which an insulating toner and magnetic particles are mixed is formed on the surface of a developer carrier having a magnetic pole whose position is fixed,
The magnetic brush is brought into contact with or close to an electrostatic latent image holding member having an electrostatic latent image formed on its surface by uniformly charging and irradiating a laser beam based on a digital signal. A developing method of applying a bias voltage including an AC component between the carrier and the developer to adhere the insulating toner to the electrostatic latent image to visualize the insulating toner, wherein the insulating toner has an average particle diameter of 8 μm or less. Use things,
The frequency f (Hz) of the AC component of the bias voltage is in the range of 4000 ≦ f, and the magnetic particles have an electrical resistivity ρ (Ω · c
m) with respect to the diameter r (μm) of the laser beam in a range of r ≦ −12 × log (ρ) +196 (r ≦ 100).

However, in this case, if the change in the maximum value of the energy density due to the change in the exposure time of the laser beam is reduced by reducing the beam diameter of the laser beam, the beam diameter of the laser beam is reduced. A special optical system is required, which causes a new problem that the cost is greatly increased. Also, by lowering the resistivity of the developer, the developer carrier releases the residual charge of the developer when developing the high coverage area, and prevents a reverse electric field from being generated when developing the low coverage area thereafter. In such a case, since the resistance of the developer is low, there is a new problem that deterioration in gradation and leakage of a developing bias are caused. For this reason, there is a new problem that there is a limit in optimizing the beam diameter of the laser beam and the resistivity of the developer while preventing the occurrence of a blank image and achieving other image quality and cost.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art. It is an object of the present invention to prevent the occurrence of a blank image while maintaining the cost and the image quality. It is an object of the present invention to provide an image forming method and an image forming apparatus capable of obtaining the above.

[0011]

Means for Solving the Problems In order to solve the above problems, the present invention provides a method of forming an image by scanning and exposing a laser beam modulated according to an image signal on an electrostatic latent image carrier. The exposure time T of the laser beam scanned and exposed on the electrostatic latent image carrier is set as follows:
-When the beam scanning speed is S and the coverage is C,
Coverage of image density defined by the relationship T = LC / S
In the image forming method capable of reproducing gradation by modulating according to the edge C , when forming an image in a low coverage area, the intensity modulation of the laser beam is performed in addition to the exposure time modulation of the laser beam. At the same time, so that the intensity of the laser beam is larger than that at the time of no intensity modulation, and the rate of change of the maximum energy density value of the latent image after the intensity modulation with respect to the input coverage is continuously changed. Have been.

Further, an image forming apparatus according to the present invention provides
The laser beam modulated according to the image signal is scanned and exposed on the electrostatic latent image carrier to form an image, and the exposure time T of the laser beam scanned and exposed on the electrostatic latent image carrier is The width of one pixel is L, the scanning speed of the laser beam
Is S and the coverage is C, T = LC / S
In an image forming apparatus capable of reproducing gradation by modulating in accordance with the coverage C of the image density defined by the relationship, when forming an image in a low coverage area, the exposure time modulation of the laser beam is performed. In addition, the intensity of the laser beam is simultaneously modulated, so that the intensity of the laser beam becomes larger than at the time of no intensity modulation, and the rate of change of the maximum energy density value of the latent image after the intensity modulation with respect to the input coverage changes continuously. Control means for performing the control as described above.

When an image is formed by scanning and exposing a laser beam modulated in accordance with the image signal on an electrostatic latent image carrier, for example, a line screen is used as a screen.

[0014]

According to the present invention, when forming an image in a low coverage area, the intensity of the laser beam is simultaneously modulated in addition to the exposure time modulation of the laser beam, so that the intensity of the laser beam becomes larger than that at the time of no intensity modulation. And the rate of change of the maximum energy density value of the latent image after the intensity modulation with respect to the input coverage is configured to be continuously changed, so that an image in which a low coverage portion is continuous with a high coverage portion is formed. Is developed by a developing device having a difference in speed between the electrostatic latent image carrier and the developer carrier of the developing device, and the developer carrier is developed while moving from a high coverage area to a low coverage area. In this case, the image in the low-coverage area is simultaneously modulated by the laser beam intensity modulation in addition to the laser beam exposure time modulation, and the intensity of the laser beam is increased. Because There is set to be greater than the time of no intensity modulation, the latent image potential of the low coverage section,
It is possible to prevent a reduction in image quality due to a decrease in the density of the low-coverage area due to a reduction in the reverse electric field that occurs near the high-coverage area and occurs on the developer carrier after developing the high-coverage area.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiment.

FIG. 2 shows a laser scanning type image forming apparatus as an embodiment of the image forming apparatus according to the present invention.

In FIG. 1, reference numeral 1 denotes a semiconductor laser, and an excitation current modulated in a pulse form in accordance with an image signal for each pixel is supplied to the semiconductor laser 1 from a laser modulator 6. The laser beam LB emitted from the semiconductor laser 1 is applied to a collimator lens 2
Is applied to the polygon mirror 3 via the. The collimator lens 2 is for condensing and collimating a laser beam LB emitted from the semiconductor laser 1 so as to be diffused.

The laser beam LB applied to the polygon mirror 3 is rotated at a high speed.
And is scanned and exposed along the axial direction (main scanning direction) of the photosensitive drum 4. At this time, the laser beam LB is converged by the fθ lens 5 and has a constant moving speed in the main scanning direction, and is scanned and exposed on the photosensitive drum 4 having a uniformly charged surface.
On the surface of the photosensitive drum 4, an electrostatic latent image corresponding to the image information is formed. The electrostatic latent image formed on the photosensitive drum 4 is developed by a developing device 7 with toner as a color pigment, and is visualized. The toner image formed on the photosensitive drum 4 is configured to be transferred onto a transfer sheet and fixed on the transfer sheet by heat and pressure by a fixing device to form an image. .

At this time, the laser beam LB used in this embodiment has, for example, a wavelength of 780 nm, a beam diameter of 64 μm, and an image screen of 40 μm per inch.
The potential of the zero line screen and the photosensitive drum 4 is such that the non-image portion potential is -650 V and the entire exposure portion potential V _low is -20.
0 is set for each.

As shown in FIG. 3, a developing device 7 for developing an electrostatic latent image formed on the photosensitive drum 4 has a housing 9 containing a two-component developer 8 composed of a toner and a carrier. A cylindrical developer carrier 10 that transports the developer 8 by adsorbing and rotating the developer 8 on the surface;
A developer regulating member 11 that regulates the amount of the developer 8 attached to the surface of the developer carrier 10, and a screw auger that rotates and stirs and transports the developer 8 to supply the developer to the developer carrier 10. 12 are provided.

The developer carrier 10 has a built-in magnet roll 13 fixed so as not to rotate and a sleeve 14 rotatably supported around the magnet roll 13. The sleeve 14 is arranged so as to face the photosensitive drum 4 at a distance of 0.5 mm, and a portion where the surfaces are close to each other is a development area. The magnet roll 13 disposed inside has a plurality of magnetic poles, and a magnetic brush of magnetic particles is formed on the surface of the sleeve by a magnetic field formed between adjacent magnetic poles. It can be transported. The rotational speed of the sleeve 14 is set such that the peripheral speed ratio between the photosensitive drum 4 and the sleeve 14 is approximately 1: 2. That is,
The developing device 7 has a difference in speed between the photosensitive drum 4 and the developer carrier 10 of the developing device 7.
Move at a higher speed than the periphery of the photosensitive drum 4.

A DC superimposed AC voltage is applied to the developer carrier 10 from a DC power supply and an AC power supply as a developing bias voltage. Thus, the development potential contrast (the difference between the DC component voltage and the entire surface exposure potential of the photosensitive drum 4) is about 300 V, that is, the DC component is -500 V.

The AC component of the developing bias voltage has a peak-to-peak amplitude value of 1.5 KV, a frequency of 6 KHz, and a rectangular waveform.

In this embodiment, when the laser beam LB emitted from the semiconductor laser 1 is irradiated onto the photosensitive drum 4 to form an electrostatic latent image, gradation can be reproduced. In addition to controlling the laser beam area, by simultaneously performing intensity modulation according to the coverage value, it is configured to prevent the occurrence of a blank image due to an excessive potential difference between the high coverage area and the low coverage area Have been.

[0025] That is, the output characteristics of the semiconductor laser 1, as shown in FIG. 4, the excitation current I is no optical output is hardly increased until the threshold current I _th be increased, the excitation current I is the threshold current I When the _th or more, the light output P is rapidly increased, so as to linearly increase to a maximum value I _max of the excitation current I.

Further, as shown in FIG. 5, the laser beam LB emitted from the semiconductor laser 1 has a distribution whose energy density can be approximated by a Gaussian distribution centering on the maximum energy density value.

Furthermore, in order to make the gradation reproducible in the above-mentioned image forming apparatus, basically, the exposure time of the laser beam LB scanned and exposed on the photosensitive drum 5 is modulated to thereby make it possible to reproduce one pixel per pixel. An area gradation method for changing the exposure area of the laser beam LB according to the density is used.

The semiconductor laser 1 that emits the laser beam is supplied from the laser modulator 6 with an excitation current modulated in a pulse shape in accordance with an image signal for each pixel. The time modulation of the pulse wave of the excitation current I is performed according to the input coverage having a width of one pixel. That is, 1
Assuming that the width of the pixel is L, the scanning speed of the laser beam is S, and the input coverage is C, the lighting time T of the laser is T =
Time-modulated so as to be L · C / S.

On the other hand, the intensity modulation of the pulse wave of the excitation current I supplied to the semiconductor laser 1 is performed according to the following procedure.

That is, assuming that the intensity distribution of the laser beam LB emitted from the semiconductor laser 1 is a Gaussian distribution, the exposure energy distribution when the laser beam LB is time-modulated as described above In a central cross section parallel to the scanning direction, it is expressed by the following equation (1).

[0031]

(Equation 1)

Here, x is a position in the main scanning direction, a is a beam diameter, and m is an intensity modulation coefficient.

The change of the maximum energy density value with respect to the coverage is expressed by the following equation (2).

[0034]

(Equation 2)

The intensity modulation coefficient m is 1 when the beam intensity gives the potential V _low of the entire exposed area, and the intensity modulation ratio is represented by a relative value.

The diameter a of the laser beam is defined as follows. That is, as shown in FIG. 5, the energy density of the laser beam decreases as the distance from the center of the laser beam increases in the radial direction, and the energy density is 1 / e ²の of the maximum value (the value of the beam center x = 0). 13.5
%, That is, the value of a shown in FIG.

In this embodiment, the intensity modulation coefficient m _max at the time of maximum modulation is set to 1.365. This value is determined so that the maximum output of the laser beam and the gradation reproducibility are good. FIG. 6 shows changes in dE / dC with respect to coverage at various intensity modulation coefficients m.

Further, in this embodiment, the intensity modulation coefficient m for each coverage is about 20% of the coverage where white spot image defects are most conspicuous, and the difference between the maximum energy density value of the latent image during non-intensity modulation and that during intensity modulation is about 20%. In FIG. 7, it is set so as to move on a straight line connecting 5% of the input coverage, the point A of the maximum modulation intensity coefficient m and 50% of the coverage, and the point B of the intensity modulation coefficient 1 in FIG.

That is, the intensity modulation coefficient m for each coverage was determined by the following equation (3).

[0040]

(Equation 3)

FIG. 8 is a graph showing the value of the difference in the maximum energy of the latent image between the non-intensity modulation and the intensity modulation with respect to the coverage.

However, the intensity modulation start and end coverages can be freely set according to the situation of a blank image defect.

FIG. 9 is a block diagram showing a control circuit of the image forming apparatus.

In the figure, reference numeral 20 denotes an image signal.
Input to the converter 22. One of the image signals 20 converted into a digital signal by the A / D converter 22 is input to a modulation intensity calculation circuit 23, a predetermined modulation intensity is calculated, and the AND signal is input via a D / A converter 24. 26. On the other hand, the other one of the image signals 20 converted into a digital signal by the A / D converter 22 is input to a pulse width modulator 25, where a predetermined pulse width modulation is performed, and the signal is passed through a D / A converter 24. Output to the AND circuit 26. An image signal is input from the AND circuit 26 to the semiconductor laser 1 via the amplifier 27.

In the above configuration, the image forming apparatus according to this embodiment modulates a laser beam as follows. That is, in order to form an image in the image forming apparatus, as shown in FIG.
Is supplied from the laser modulating means 6 with an excitation current modulated in a pulse shape in accordance with an image signal of each pixel, and a laser beam LB emitted from the semiconductor laser 1 in accordance with the excitation current is supplied to the collimator lens 2 and the polygon. Scanning exposure is performed along the axial direction (main scanning direction) of the photosensitive drum 4 via the mirror 3 and the fθ lens 5. Then, on the surface of the photosensitive drum 4, an electrostatic latent image corresponding to the image information is formed, and the image is formed.

At this time, the excitation current supplied to the semiconductor laser 1 makes it possible to reproduce the gradation, so that the exposure time T of the laser beam LB scanned and exposed on the photosensitive drum 5 is as shown in FIG. As shown, the pulse width modulator 25 modulates the input coverage corresponding to one pixel width so that T = LC / S as described above.

By the way, in this embodiment, when reproducing the gradation, in addition to the modulation of the area gradation, that is, the exposure time, the exposure intensity of the laser beam LB is changed to the input coverage (only in the area of 5 to 50%). 9), as shown in FIG. 9, the intensity modulation operation circuit 23 modulates according to the expression (1) as described above.

Therefore, even in the case where the maximum value of the energy density of the laser beam becomes lower as the exposure time of the laser beam LB becomes shorter in the area of the input coverage where the image density is low, even if the energy value of the laser beam becomes shorter, The exposure intensity of the laser beam LB is modulated so that the exposure intensity of the laser beam LB is higher than usual in the low input coverage area as shown in FIG.

As a result, in the image area on the photosensitive drum 4, as shown by the solid line in FIG. 1, the potential of the image changes in accordance with the input coverage, and in the low input coverage area, the conventional image shown by the broken line in FIG. The change in the potential of the image area is larger than in the technology.

Therefore, as shown in FIG. 10, the image formed on the photosensitive drum 104 has a low coverage area (for example, a 100% density area) next to a high coverage area (eg, 100% density area). For example, when an image having a density of 20% is continuous, as shown by a solid line in FIG. 11, a high coverage portion where the latent image potential is high, and the maximum value of the energy density of the laser beam is low and the latent image potential is low. Low low coverage areas will be adjacent. However, in the low input coverage area, the change in the potential of the image area is larger than that in the related art shown by the broken line in FIG. Therefore, when this electrostatic latent image is developed by the developing device 7 having a difference in speed between the photosensitive drum 4 and the developer carrier 10 of the developing device 7, when the developer carrier 10 develops a high coverage area, Developer carrier 1
A large amount of toner having a polarity opposite to that of the toner is generated on the developer remaining on the developer carrier 10 in a large amount of toner on the developer carrier 0. Thereafter, when the low-coverage portion is developed by the developer-carrying member 10, the developer-carrying member 10 having many residual charges having a polarity opposite to the charge polarity of the toner passes through the low-coverage portion.

However, in the low input coverage area, the potential change of the image area is large, and the potential of the image area is close to the potential of the image area of the high input coverage area. The reverse electric field generated between the developer carrying member 10 and the photosensitive drum 4 is small even when the developer carrying member 10 in which the toner exists has passed through the low coverage area. Therefore, the toner once developed on the photoconductor drum 4 is unlikely to return to the developer carrier 10 due to a reverse electric field generated between the developer carrier 10 and the photoconductor drum 4.
It is possible to prevent the occurrence of a blank image on the image, and to prevent the image quality from deteriorating.

As described above, when forming the image of the low coverage area, the intensity modulation of the laser beam LB is simultaneously performed in addition to the exposure time modulation of the laser beam LB, and the intensity of the laser beam LB is changed from that at the time of no intensity modulation. An image in which a low coverage portion is continuous with a high coverage portion because the configuration is such that the rate of change with respect to the input coverage of the maximum energy density value of the latent image after the intensity modulation is continuously changed. Formation of the photosensitive drum 4
And developing by the developing device 7 having a difference in the speed of the developer carrier 10 of the developing device 7, and developing while moving the developer carrier 10 from the high coverage area to the low coverage area, The image of the low coverage area is set so that the intensity of the laser beam LB is simultaneously modulated in addition to the exposure time modulation of the laser beam LB, and the intensity of the laser beam LB is set to be larger than that at the time of no intensity modulation. The latent image potential of the coverage portion becomes close to the high coverage portion, and the developer carrier 1 after developing the high coverage portion
It is possible to prevent a reduction in image quality due to a decrease in the reverse electric field generated on 0 and a decrease in the density of the low coverage area.

In order to confirm the effects of the present invention, the present inventors prototyped an image forming apparatus as shown in FIG. 2 and formed an image by changing the resistivity of the magnetic particles of the developing device 7. The results of comparison between a conventional image forming apparatus that only modulates exposure time and white image defects are shown below.

The developer used in this case is a mixture of a toner having an average particle diameter of 7 μm and magnetic particles (ferrite carrier) having an average particle diameter of 50 μm, and the toner concentration is reduced to 7%. Set.

FIG. 12 shows that the resistivity of the magnetic particles of the developer is 10%.
^It shows the results of observing the conditions under which white spot image defects occur when the resistance is set to ¹¹ , 10 ¹³ , and 10 ¹⁵ Ω · m.

In this figure, the state of occurrence of white spots was evaluated for the following three grades. Grade 1: No white spots occurred Grade 2: White spots occurred but acceptable range Grade 3: White spots occurred and out of allowable range

As is apparent from FIG. 12, according to the image forming apparatus of the present embodiment, the allowable range of the resistivity of the magnetic particles in which a blank image defect occurs can be expanded.

[0058]

According to the present invention, there is provided an image forming method and an apparatus capable of obtaining a good image by preventing the occurrence of a blank image while maintaining the cost and the image quality. Can be provided.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between latent image potential and coverage in an embodiment of an image forming method according to the present invention.

FIG. 2 is a configuration perspective view showing a main part of the image forming apparatus.

FIG. 3 is a configuration diagram illustrating a developing device.

FIG. 4 is a graph showing a relationship between a light output of a semiconductor laser and an excitation current.

FIG. 5 is a graph showing the energy density of the light output of the semiconductor laser.

FIG. 6 is a graph showing a rate of change of a maximum energy density value of a latent image with respect to input coverage.

FIG. 7 is a graph showing a rate of change of a maximum energy density value of a latent image with respect to input coverage.

FIG. 8 is a graph showing a relationship between a difference between a maximum energy value of a latent image at the time of non-intensity modulation and that at the time of intensity modulation and coverage.

FIG. 9 is a block diagram showing a control circuit.

FIG. 10 is a schematic diagram illustrating an image forming state.

FIG. 11 is a graph showing a latent image potential.

FIG. 12 is a graph showing the relationship between the resistivity of magnetic particles and the grade of white spots.

FIG. 13 is a configuration perspective view showing a main part of a conventional image forming apparatus.

FIG. 14 is a graph showing exposure energy.

FIG. 15 is a schematic diagram illustrating a state in which a blank image has occurred.

[Explanation of symbols]

1 semiconductor laser, LB laser beam, 4 photosensitive drum, 6 laser modulation means.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B41J 2/44 B41J 2/52 G03G 15/00 H04N 1/23

Claims (2)

(57) [Claims] An image is formed by scanning and exposing a laser beam modulated according to an image signal onto an electrostatic latent image carrier, and a laser beam scanned and exposed on the electrostatic latent image carrier. Exposure time T, width of one pixel is L, laser
When the scanning speed of the camera is S and the coverage is C, T =
Image density coverage defined by the relationship L · C / S
In the image forming method capable of reproducing gradation by modulating according to C , when forming an image in a low coverage area, the intensity modulation of the laser beam is simultaneously performed in addition to the exposure time modulation of the laser beam. The intensity of the laser beam is controlled to be greater than that at the time of no intensity modulation, and the rate of change with respect to the input coverage of the maximum energy density value of the latent image after the intensity modulation is continuously changed. Image forming method. 2. An image is formed by scanning and exposing a laser beam modulated according to an image signal on an electrostatic latent image carrier, and a laser beam scanned and exposed on the electrostatic latent image carrier. Exposure time T, width of one pixel is L, laser
When the scanning speed of the camera is S and the coverage is C, T =
Image density coverage defined by the relationship L · C / S
In the image forming apparatus capable of reproducing gradation by performing modulation according to C , when forming an image in a low coverage area, the intensity modulation of the laser beam is simultaneously performed in addition to the exposure time modulation of the laser beam. Control means for controlling the intensity of the laser beam to be greater than that at the time of no intensity modulation, and controlling the rate of change of the maximum energy density value of the latent image after intensity modulation with respect to the input coverage to be continuously changed. An image forming apparatus comprising: