JP2023085826A - Image forming apparatus - Google Patents

Abstract

To prevent or alleviate a reduction in the life of a photoconductor drum by not uniformly changing the frequency of electrification AC voltage to avoid moire.SOLUTION: An image forming apparatus 10 has: an electrifying roller 3 that electrifies a photoconductor drum 2; an electrification high voltage power supply 30 that applies, to the electrifying roller 3, a voltage obtained by superimposing DC voltage and AC voltage; an image processing unit 9 that generates an output image signal for image formation from image data; an image diagnosis unit 700 that performs diagnosis about the density of an image of the output image signal generated by the image processing unit 9; and a control unit 400 that, based on a result of the diagnosis conducted by the image diagnosis unit 700, changes the frequency of the AC voltage applied to the electrifying roller 3 by the electrification high voltage power supply 30.SELECTED DRAWING: Figure 3

Description

The present invention relates to an electrophotographic image forming apparatus applied to copiers, laser printers, facsimiles, printing apparatuses, multifunction machines, and the like.

Conventionally, the frequency of the AC voltage applied to the contact charging roller is generally set according to the image forming process speed so that the potential pitch formed on the photosensitive drum is difficult to see. At this time, it is preferable that the absolute value of the set frequency be as small as possible. The reason for this is that when the absolute value of the frequency is increased, the followability of the current deteriorates, so the voltage needs to be increased, and the deterioration of the photosensitive drum is accelerated according to the voltage increase.

Japanese Unexamined Patent Application Publication No. 2002-100003 discloses an image forming apparatus that forms a full-color image by superimposing toner images of halftone images of multiple colors formed by a screen pattern. In the image forming apparatus of Patent Document 1, the screen pitch of the image written on the photoreceptor is identified, and the frequency of the AC voltage is automatically set so as to form uneven charging that does not interfere with the screen pattern. In the image forming apparatus of Patent Document 1, when interference (hereinafter referred to as "moire") occurs between the screen pattern and the pitch of the potential difference formed on the photoreceptor due to the frequency of the AC charging voltage, The frequency of the alternating voltage is selected according to the screen pattern.

JP-A-2004-85735

However, in Japanese Unexamined Patent Application Publication No. 2004-100000, since the lowest possible frequency is usually set as a default, simply changing the frequency according to only screen pattern information increases the frequency, accelerating the deterioration of the photosensitive drum. There is a problem to do.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of preventing or reducing the shortening of the life of a photosensitive drum by not uniformly changing the frequency of the charging AC voltage to avoid moire. be.

An image forming apparatus according to the present invention comprises a photoreceptor, a charging member for charging the photoreceptor, a charging high-voltage power supply for applying a voltage obtained by superimposing a DC voltage and an AC voltage to the charging member, and image data. an image processing unit for generating an output image signal for image formation; an image diagnosis unit for diagnosing image density of the output image signal generated by the image processing unit; and a control section for changing the frequency of the AC voltage applied from the charging high-voltage power supply to the charging member.

According to the present invention, shortening of the life of the photosensitive drum can be prevented or reduced by not uniformly changing the frequency of the charging AC voltage in order to avoid moire.

1 is a schematic diagram of an image forming apparatus according to Embodiment 1 of the present invention; FIG. 2 is a schematic diagram showing the configuration of an image forming section of the image forming apparatus according to Embodiment 1 of the present invention; FIG. 1 is a block diagram showing the configuration of an image forming apparatus according to Embodiment 1 of the present invention; FIG. 2 is a block diagram showing the configuration of an input image processing section of the image forming apparatus according to Embodiment 1 of the present invention; FIG. FIG. 3 is a diagram showing a screen pattern employed by the image forming apparatus according to Embodiment 1 of the present invention; FIG. 5 is a diagram showing changes in density of an output image of the image forming apparatus according to Embodiment 1 of the present invention; It is a schematic diagram explaining the generation principle of moire. FIG. 4 is a schematic diagram illustrating parameters related to moire; FIG. 5 is a flowchart showing moiré avoidance mode processing performed by the image forming apparatus according to Embodiment 1 of the present invention; 4 is a diagram showing the relationship between moire intensity and spatial frequency in moire avoidance mode processing performed by the image forming apparatus according to Embodiment 1 of the present invention; FIG. FIG. 5 is a diagram showing the relationship between moiré intensity and brightness in moiré avoidance mode processing performed by the image forming apparatus according to Embodiment 1 of the present invention; FIG. 5 is a diagram showing the relationship between lightness and image signal value in moire avoidance mode processing performed by the image forming apparatus according to Embodiment 1 of the present invention; FIG. 4 is a diagram showing an example of an output image used by an image diagnosis section of the image forming apparatus according to Embodiment 1 of the present invention; FIG. 9 is a flowchart showing moire avoidance mode processing performed by the image forming apparatus according to Embodiment 2 of the present invention; FIG. 9 is a diagram showing the relationship between moiré intensity and brightness for each relative humidity in moiré avoidance mode processing performed by the image forming apparatus according to Embodiment 2 of the present invention; FIG. 9 is a diagram showing the relationship between relative humidity and moiré intensity in moiré avoidance mode processing performed by the image forming apparatus according to Embodiment 2 of the present invention; FIG. 5 is a diagram showing test results of the image forming apparatuses according to Embodiments 1 and 2 of the present invention;

Hereinafter, embodiments will be described in detail with reference to the drawings.

(Embodiment 1)
<Configuration of Image Forming Apparatus>
A configuration of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail with reference to FIGS. 1 and 2. FIG.

The image forming apparatus 10 includes image forming sections 1a, 1b, 1c and 1d, an intermediate transfer belt 8, an image processing section 9, a drive roller 11, a secondary transfer counter roller 12, a tension roller 13, a secondary and a transfer roller 15 . The image forming apparatus 10 also includes a belt cleaning device 16, a fixing device 17, a charging high-voltage power supply 30, a control section 400, an environment sensor 500, a storage section 600, and an image diagnosis section 700. there is The image forming apparatus 10 here is an in-line (4-drum system) color image forming apparatus.

The image forming units 1a, 1b, 1c and 1d include photosensitive drums 2a, 2b, 2c and 2d; charging rollers 3a, 3b, 3c and 3d; developing devices 4a, 4b, 4c and 4d; 5b, 5c and 5d. The image forming sections 1a, 1b, 1c and 1d also include drum cleaning devices 6a, 6b, 6c and 6d, and exposure devices 7a, 7b, 7c and 7d. Each of the image forming sections 1a, 1b, 1c, and 1d is an image forming unit arranged in a line with a constant interval.

The image forming section 1a forms a yellow image. The image forming section 1b forms a magenta image. The image forming section 1c forms a cyan image. The image forming section 1d forms a black image.

In addition, when the subscripts a, b, c and d are described in the reference numerals, they indicate the respective members or devices of yellow, magenta, cyan and black colors, and are simply the reference numerals without description of the subscripts. In some cases, each member or device is shown collectively.

The photosensitive drum 2 is a negatively charged organic photosensitive member having a photosensitive layer on a drum substrate such as aluminum (not shown). The photosensitive drum 2 is rotationally driven at a predetermined surface speed in the conveying direction (hereinafter referred to as "process speed") by a driving device (not shown). A charging roller 3 , a developing device 4 , a primary transfer roller 5 and a drum cleaning device 6 are arranged around the photosensitive drum 2 . The photosensitive drum 2 is contact-charged to a predetermined polarity and potential by the charging roller 3 .

The charging roller 3 as a charging member is made of conductive rubber, and both end portions of a metal core (not shown) are rotatably held by bearing members while maintaining a constant distance between the shafts. The charging roller 3 is biased toward the photosensitive drum 2 by a pressing spring, and performs contact charging by pressing the surface of the photosensitive drum 2 with a predetermined pressing force. The charging roller 3 rotates following the rotation of the photosensitive drum 2 . A charging bias voltage of predetermined conditions is applied to the core metal of the charging roller 3 by a charging high voltage power supply 30 .

The developing devices 4a, 4b, 4c and 4d contain yellow toner, cyan toner, magenta toner and black toner, respectively. The developing device 4 develops the electrostatic latent image formed on the surface of the photosensitive drum 2 to form a toner image on the surface of the photosensitive drum 2 .

The primary transfer roller 5 transfers the toner image formed on the photosensitive drum 2 to the intermediate transfer belt 8 by applying a primary transfer voltage.

The drum cleaning device 6 cleans toner remaining on the surface of the photosensitive drum 2 .

The exposure device 7 is installed above the photosensitive drum 2 in FIGS. The exposure device 7 is electrically connected to the image processing section 9 and forms an electrostatic latent image on the charged surface of the photosensitive drum 2 according to the output image signal transmitted from the image processing section 9 . .

The intermediate transfer belt 8 is a rotatable endless intermediate transfer member provided at a position facing the image forming section 1 . The intermediate transfer belt 8 is stretched by a drive roller 11 , a secondary transfer counter roller 12 and a tension roller 13 .

The image processing unit 9 outputs to the exposure device 7 and the image diagnosis unit 700 an output image signal obtained by performing predetermined processing on the input unprocessed image signal. The details of the configuration of the image processing unit 9 will be described later.

The drive roller 11 is connected to a motor (not shown), and is driven by the motor to rotate the intermediate transfer belt 8 in the direction of the arrow shown in FIG. 1 (counterclockwise direction in FIG. 1).

The secondary transfer opposing roller 12 is in contact with the secondary transfer roller 15 via the intermediate transfer belt 8 to form a secondary transfer portion.

The tension roller 13 stretches the intermediate transfer belt 8 together with the driving roller 11 and the secondary transfer counter roller 12 .

The secondary transfer roller 15 presses and nips the transfer material together with the secondary transfer counter roller 12 to secondarily transfer the toner image primarily transferred to the intermediate transfer belt 8 onto the transfer material.

The belt cleaning device 16 is provided outside the intermediate transfer belt 8 to remove and collect transfer residual toner remaining on the surface of the intermediate transfer belt 8 .

The fixing device 17 is provided on the downstream side in the conveying direction of the secondary transfer portion where the secondary transfer opposing roller 12 and the secondary transfer roller 15 are in contact with each other. The fixing device 17 includes a fixing roller 17a and a pressure roller 17b. By heating and pinching the transfer material between the fixing roller 17a and the pressure roller 17b, the toner image secondarily transferred to the transfer material is formed. A heat pressure treatment is performed to fix the image onto the transfer material.

The charging high-voltage power supply 30 applies an oscillating voltage obtained by superimposing the DC voltage generated by the DC power supply 31 and the AC voltage generated by the AC power supply 32 to the charging roller 3 as a charging bias voltage under the control of the control unit 400 .

The control unit 400 determines the frequency of the DC voltage, the frequency of the AC voltage, and the peak-to-peak voltage of the AC voltage of the charging bias voltage to be applied to the core metal of the charging roller 3 . The control unit 400 controls the charging high-voltage power supply 30 so as to apply the charging bias voltage having the determined frequency and peak-to-peak voltage. Control unit 400 executes moire avoidance mode processing. By executing moire avoidance mode processing, the control unit 400 causes the charging high-voltage power supply 30 to move the core metal of the charging roller 3 from the charging high-voltage power supply 30 during image formation when the diagnosis result input from the image diagnosis unit 700 indicates that the image is an image in which moire is more likely to be visually recognized. Moire is avoided by changing the frequency of the AC voltage applied to the . Note that the moire avoidance mode processing will be described later.

The environment sensor 500 detects the temperature and humidity outside the image forming apparatus 10 and outputs an electrical signal corresponding to the detected temperature and humidity to the control unit 400 .

The storage unit 600 stores various data.

The image diagnosis unit 700 diagnoses the density of the image based on the image information such as the density, luminance, and area of the output image of the output image signal input from the image processing unit 9, and outputs the diagnosis result to the control unit 400. do.

<Configuration of image processing unit>
The configuration of the image processing section 9 of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail with reference to FIG.

The image processing section 9 includes a selector 100 , a scanner section 110 , a rendering section 120 , an input image processing section 200 and an output image processing section 300 .

The selector 100 is connected to the scanner section 110 when the image forming apparatus 10 performs copying, and outputs an unprocessed image signal input from the scanner section 110 to the input image processing section 200 . The selector 100 connects to the rendering unit 120 when the image forming apparatus 10 receives a page description language (PDL) output from a terminal on the network, and converts an unprocessed image signal input from the rendering unit 120 into an input image. Output to the processing unit 200 .

The scanner unit 110 converts the image data of the document to be copied into an unprocessed image signal and outputs the converted unprocessed image signal to the selector 100 .

The rendering unit 120 converts PDL image data received from a terminal on the network into an unprocessed image signal, and outputs the converted unprocessed image signal to the selector 100 .

The input image processing unit 200 performs predetermined processing on the unprocessed image signal input from the selector 100 and outputs the processed image signal to the output image processing unit 300 as an input image signal. The details of the configuration of the input image processing unit 200 will be described later.

The output image processing unit 300 performs predetermined processing on the input image signal input from the input image processing unit 200 and outputs the processed image signal to the exposure device 7 and the image diagnosis unit 700 as an output image signal. The details of the configuration of the output image processing unit 300 will be described later.

<Configuration of Input Image Processing Unit>
A configuration of the input image processing unit 200 of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail with reference to FIG.

The input image processing unit 200 includes a sub-scanning color shift correction unit 201, a main scanning color shift correction unit 202, an image area determination unit 203, a filter processing unit 204, a histogram unit 205, an input color correction unit 206, It has

The sub-scanning color shift correction unit 201 corrects the color shift in the sub-scanning direction of the input image of the unprocessed image signal input from the selector 100 and outputs it to the main scanning color shift correction unit 202 .

The main scanning color shift correction unit 202 corrects the color shift in the main scanning direction of the input image of the signal input from the sub scanning color shift correction unit 201 and outputs the corrected signal to the image area determination unit 203 .

The image area determination unit 203 identifies the image type in the input image of the signal input from the main scanning color misregistration correction unit 202 . The image area determination unit 203 identifies pixels constituting each image type such as a photograph portion or a character portion, a chromatic portion or an achromatic portion in the input image, and generates attribute flag data indicating the identified image type. is generated pixel by pixel. The image area determination unit 203 corrects the pixel density based on the identified image type, and outputs a signal with the corrected pixel density to the filter processing unit 204 .

The filter processing unit 204 arbitrarily corrects the spatial frequency of the input image of the signal input from the image area determination unit 203 and outputs the result to the histogram unit 205 .

A histogram unit 205 performs a process of sampling the histogram of the image data in the input image of the signal input from the filter processing unit 204, and determines whether the input image is color or monochrome. The histogram unit 205 outputs a signal containing the discrimination result to the input color correction unit 206 .

The input color correction unit 206 performs correction processing regarding the color of the input image of the signal input from the histogram unit 205 based on the determination result included in the signal input from the histogram unit 205 . For example, the input color correction unit 206 converts the color space of the input image into an arbitrary color space in the correction process. The input color correction unit 206 outputs the corrected signal to the output image processing unit 300 as an input image signal.

<Configuration of Output Image Processing Unit>
A configuration of the output image processing unit 300 of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail with reference to FIG.

The output image processing unit 300 includes a background removal unit 301 , a monochrome generation unit 302 , an output color correction unit 303 , a filter processing unit 304 , a gamma correction unit 305 and a halftone processing unit 306 .

The background removal unit 301 removes unnecessary background fog as processing for removing the background color of the input image of the input image signal input from the input image processing unit 200 . The background color removal unit 301 performs background color removal using a one-dimensional LUT (lookup table) or the like, for example, as processing for removing the background color of the input image. The background removal unit 301 outputs the signal subjected to the background removal processing to the monochrome generation unit 302 .

A monochrome generation unit 302 converts the color image data included in the signal input from the background removal unit 301 into monochrome image data. A monochrome generation unit 302 multiplies color image data such as RGB data by an arbitrary constant to convert the data into a monochrome gray signal when printing as a monochrome image. The monochrome generation unit 302 outputs the signal converted into monochrome image data to the output color correction unit 303 .

The output color correction unit 303 performs color correction on the signal input from the monochrome generation unit 302 by, for example, performing processing such as direct mapping in accordance with the characteristics of the printer unit that outputs the image data. The output color correction unit 303 outputs the color-corrected signal to the filter processing unit 304 .

The filter processing unit 304 arbitrarily corrects the spatial frequency of the image data of the signal input from the output color correction unit 303 and outputs the corrected image data to the gamma correction unit 305 .

A gamma correction unit 305 performs gamma correction on the signal input from the filter processing unit 304 in accordance with the characteristics of the output printer unit. For example, the gamma correction unit 305 uses a one-dimensional LUT to perform color balance adjustment for adjusting the density balance of each color of CMYK, or density adjustment for adjusting the overall density increase or decrease. The gamma correction unit 305 outputs the gamma-corrected signal to the halftone processing unit 306 .

A halftone processing unit 306 performs arbitrary halftone processing on the signal input from the gamma correction unit 305 in accordance with the number of gradations of an output printer unit (not shown). For example, the halftone processing unit 306 performs arbitrary screen processing such as binarization or 32-value conversion or error diffusion processing as halftone processing. The halftone processing unit 306 outputs the halftone processed signal to the exposure device 7 and the image diagnosis unit 700 as an output image signal for image formation.

<Operation of Image Forming Apparatus>
The operation of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail.

When a start signal for starting an image forming operation is issued from the control unit 400, transfer materials (recording media) contained in a cassette (not shown) are fed one by one from the cassette and conveyed to registration rollers (not shown). be done. At this time, the registration rollers are stopped and the leading edge of the transfer material is waiting just before the secondary transfer portion.

Further, the photosensitive drum 2 of the image forming section 1 starts rotating at a predetermined process speed.

The photosensitive drum 2 is uniformly charged by the charging roller 3 . Specifically, the photosensitive drum 2 is supplied with a DC voltage of −700 V, a frequency of 2.6 kHz, and a peak-to-peak voltage Vpp of 1.6 kV during image formation in a room temperature environment of 23° C. and a relative humidity of 50%. An oscillating voltage obtained by superimposing a sinusoidal AC voltage is applied from the charging roller 3 . As a result, the surface of the photosensitive drum 2 is uniformly charged to −700 V (dark potential Vd), which is the same DC voltage applied to the charging roller 3 .

Here, the average potential of the photosensitive drum 2 converges to a potential corresponding to the applied DC voltage by applying an oscillating voltage obtained by superimposing a DC voltage and an AC voltage from the charging roller 3 . Further, a periodic potential difference occurs on the photosensitive drum 2 according to the period of the AC voltage.

When the process speed of the photosensitive drum 2 is v (mm/s) and the frequency of the AC voltage is f (Hz), the charging pitch T (mm) is the interval between the potential height differences formed on the photosensitive drum 2. is expressed by the formula (1).

T=v/f (1)

(1), the charging pitch T increases as the value of the frequency f decreases. As the electrification pitch T becomes larger, the difference in density of the printed matter due to the difference in potential level becomes more visible. It has been found that this effect becomes more pronounced when the charging pitch T exceeds 0.14 mm.

Further, according to the formula (1), the charging pitch T becomes smaller as the value of the frequency f becomes larger. As the charging pitch T becomes smaller, it becomes more difficult to visually recognize the difference in density of the printed matter due to the difference in potential level. However, in this case, since the followability of the current flowing through the charging roller 3 deteriorates as the frequency f increases, it is necessary to increase the voltage value of the charging bias voltage, which accelerates the deterioration of the photosensitive drum 2. tend to

Therefore, in order to suppress deterioration of the photosensitive drum 2, it is preferable to set the frequency f to a value as small as possible within the range where the charging pitch T does not exceed 0.14 mm.

Therefore, in this embodiment, when the process speed in the conveying direction of the photosensitive drum 2 is 360 mm/sec, the standard frequency f of the AC voltage is set to 2.6 kHz. The charging pitch T at this time is T=v/f=0.138 (mm) according to the equation (1).

The exposure device 7 scans and exposes the photosensitive drum 2 with laser light according to an output image signal from the image processing unit 9 to form an electrostatic latent image. The charge amount and the exposure amount of the photosensitive drum 2 are adjusted so that the potential after being charged by the charging roller 3 is -650V and the potential after being exposed by the exposure device 7 is -200V. Also, the developing bias is -500V. The process speed is 360 mm/sec, the image forming width, which is the length in the direction perpendicular to the conveying direction, is 300 mm, the toner charge amount is about -30 μC/g, and the toner amount on the photosensitive drum 2 in the solid image portion is about 0.00 mm. It is set to be 4 mg/cm ² .

As for the order of image formation, first, in order to form a yellow image, a yellow toner is caused to adhere to the electrostatic latent image formed on the photosensitive drum 2a by the developing device 4a to visualize it as a toner image. . The yellow toner image on the photosensitive drum 2 a is primarily transferred onto the rotating intermediate transfer belt 8 .

As the intermediate transfer belt 8 rotates, the area on the intermediate transfer belt 8 onto which the yellow toner image has been transferred moves toward the image forming portion 1b. Then, the magenta toner image formed on the photosensitive drum 2b of the image forming portion 1b is superimposed on the yellow toner image on the intermediate transfer belt 8 and transferred. Subsequently, the cyan and black toner images formed by the photosensitive drums 2c and 2d are successively superimposed on the yellow and magenta toner images superimposed and transferred on the intermediate transfer belt 8 in the same manner to form a full-color toner image. It is formed on the intermediate transfer belt 8 .

A registration roller (not shown) conveys the transfer material to the secondary transfer portion at the timing when the leading edge of the full-color toner image formed on the intermediate transfer belt 8 is moved to the secondary transfer portion.

The full-color toner image formed on the intermediate transfer belt 8 is collectively secondary-transferred onto the transfer material by the secondary-transfer roller 15 to which a secondary-transfer voltage (a voltage opposite to that of the toner (positive polarity)) is applied. be transcribed.

The transfer material on which the full-color toner image is formed is conveyed to the fixing device 17 .

The full-color toner image secondarily transferred onto the transfer material conveyed to the fixing device 17 is heated and pressurized at the fixing nip portion formed by the fixing roller 17a and the pressure roller 17b, and heat is applied to the surface of the transfer material. be established.

The transfer material on which the full-color toner image is thermally fixed is discharged to the outside.

<About Moire>
Moire associated with the operation of the image forming apparatus 10 according to Embodiment 1 of the present invention will be described in detail with reference to FIGS. 6 to 8. FIG.

Moire occurs due to interference between the screen pattern and the charging pitch T (mm), which is the difference in potential caused by the AC voltage applied to the charging section. Moire is an unintended pitch-like density unevenness that occurs on an output image and leads to deterioration in image quality, so it is desired that it is not visually recognized as much as possible.

FIG. 6 shows screen patterns for cyan, magenta, yellow and black. The ruling of the screen is the distance between parallel straight lines on which the centroids of the dots of the screen are aligned, expressed in units of dpi (dots per inch). The angle of the screen is the angle between the main scanning direction of exposure and the straight line on which the centroids of the dots of the screen are aligned.

For example, on a magenta screen, the number of lines is 185 dpi and the angle is 67°. Also, as indicated by the solid line and the dashed line in FIG. 6, when the straight lines on which the centers of gravity of the dots are aligned are calculated by two methods, the smaller angle is taken. Therefore, in the case of FIG. 6, the angle of the solid line is treated as the angle of the screen.

As shown in FIG. 7(a), when an AC voltage is applied to the charging roller 3, the charging potential rises and falls in a wavy pattern at intervals of the charging pitch T (mm) along the sub-scanning direction. are the same potential. The zones with low charging potential are L1, L2, L3 and L4. Here, when a screen pattern is drawn, interference occurs between the periodicity of the screen pattern and the periodicity of high and low charging potential generated by applying an AC voltage, and moire appears on the transfer material. end up

The direction of the moire generated in this way is the direction connecting the closest ones of the arrays of the portion having a density higher than that of the neighborhood. Therefore, as shown in FIG. 7(b), the direction of moire is the point where adjacent bands L1, L2, L3, and L4 with low charging potentials intersect with the adjacent screen patterns (in FIG. 7(b) The part indicated by the circle) is connected. In addition, the space|interval of the direction of an adjacent moiré is called moiré pitch P. FIG.

Here, the screen angle θ [rad] is the angle formed by the direction of the screen pattern (hereinafter referred to as “screen direction”) and the main scanning direction. The angle formed by the moire direction and the main scanning direction is assumed to be a moire angle θ2 [rad] (−π/2<θ<π/2). The relationship between the charging pitch T, the screen pitch L, and the screen angle θ, and the moiré angle θ2 and moiré pitch P calculated therefrom will be described with reference to FIG. However, since the screen angle θ and the moiré angle θ2 have a periodicity of π and are bilaterally symmetrical about 0 as an axis, the respective ranges are −π/2<θ<π/2 and −π/2<θ2 <π/2 can be considered.

is .pi./2, the moiré angle |.theta.2| is .pi./2, and the moire pitch P is the screen pitch L or the charging pitch T. Therefore, consideration of this case is omitted.

First, in FIG. 8, low charging potential bands are repeated along the sub-scanning direction at a period of charging pitch T, and two adjacent low charging potential bands are denoted by L11 and L12, respectively. Two adjacent lines in the moire direction forming interference fringes are M1 and M2.

Here, let A and B be the points where L11 intersects M1 and M2, respectively, and let D and E be the points where L12 intersects M1 and M2, respectively. Also, let G be the intersection of the perpendicular drawn from point B to L12 and L12.

When the screen angle .theta.=0, the moire angle _.theta.2 and the moire pitch P are given by the following formulas (2) and (3).

θ ₂ =0 (2)

P=|(1/T)-(1/L)| ^-1 (3)

Further, when the screen angle θ is 0<|θ|<π/2 in the above, the moire angle _θ2 and the moire pitch P can be calculated based on the following calculation method.

In this calculation method, first, the length of the line segment EG is obtained. The lengths of the line segments DE and DG can be obtained from the scan pitch Z, the screen pitch L, and the screen angle θ as shown in FIG. 8 using the following equations (4) and (5).

DE=L/sin θ (4)

DG=T/tan θ (5)

Since the line segment EG is the difference between the line segment DE and the line segment DG, the following formula (6) is obtained from the formulas (4) and (5).

EG=DE−DG=(L/sin θ)−(T/tan θ) (6)

As a result, the lengths of the two sides (line segments BG and EG) sandwiching the right angle in the right-angled triangle BEG can be specified, so the moire angle _θ2 can be obtained by the following equation (7).

θ ₂ = tan ⁻¹ (T/((L/sin θ)−(T/tan θ)))
= tan ^-1 (T cos θ/(L-T cos θ)) (7)

Also, since the moire angle _θ2 has been specified, the moire pitch P can be obtained as in the following equation (8).

P=|DE sin θ ₂ |=|(L/sin θ)×sin θ ₂ | (8)

<Moire avoidance mode processing>
Moire avoidance mode processing executed by the image forming apparatus 10 according to the first embodiment of the present invention will be described in detail with reference to FIGS. 9 to 13. FIG.

The frequency of the AC voltage to be changed in the moiré avoidance mode and the image conditions to which the moiré avoidance mode is applied will be described in detail below.

First, mention is made of the frequency of the AC voltage that is changed during the moire avoidance mode.

When the process speed of the photosensitive drum 2 is 360 mm/sec, the control unit 400 sets the frequency f1 of the AC voltage to 2.9 kHz for avoiding moiré. The value of the charging pitch T at this time is as follows from the formula (1).

T=v/f1=0.124 (mm)

In this way, in the moire avoidance mode, the control unit 400 performs control to reduce the visibility of moire by narrowing the charging pitch T of 0.138 mm in the standard mode.

Next, image conditions for applying the moire avoidance mode will be described in detail.

In the present embodiment, the visibility of moire is quantified. Specifically, first, the brightness component is read from the printed image by a scanner, and Fourier transform is performed to obtain the amplitude of fluctuation of the brightness component. Then, the product of the obtained amplitude of the fluctuation of the brightness component and the visual spatial frequency characteristic was defined as the moiré intensity. The visibility of moire can be quantified by this moire intensity.

For example, a method of quantifying one density level in a single magenta color under conditions of screen ruling of 185 dpi, screen angle of 67°, and process speed of 360 mm/sec will be described as an example.

The electrophotographic copier used in the image evaluation was a full-color copier imagePRESS800 manufactured by Canon Inc. It is a machine for A3 landscape output, and the image resolution is 1200 dpi. Using this machine, an image of constant density was printed on plain paper in a 50 mm square area. An EPSON ES-2200 was used as a scanner, and images were read at a resolution of 800 dpi.

In order to convert the scanned image data into the L*a*b* space and analyze the quantified brightness for each frequency, a two-dimensional FFT (Fast Fourier Transform) is performed on the L* component image, and the spatial A Fourier transform image with a frequency resolution of about 0.06 cycle/mm was obtained.

A radial frequency method was used to make the obtained Fourier transform image one-dimensional and evaluate it. This method does not obtain the power spectrum in a specific direction, but is a method of defining a radial frequency centered on the origin of the two-dimensional power spectrum and averaging the entire circumference, obtaining an average power spectrum in all directions. be able to. A power spectrum obtained in this way is called a radially averaged power spectrum (hereinafter referred to as "RAPS"). RAPS can quantify the brightness amplitude.

However, due to the characteristics of human vision, there are spatial frequency bands that are easy to perceive and spatial frequency bands that are difficult to perceive, so it is necessary to weight variations in the brightness component in consideration of the characteristics of human vision. In the present embodiment, a VisualTransfer Function (hereinafter referred to as "VTF") is used in order to weight variations in lightness components in consideration of human visual characteristics. As the VTF, although there are several reports, the VTF of brightness variation used by Dooley et al. was used. The VTF function used is Equation (9).

VTF(u)=5.05e- ^{(0.138πlu/180)} (1-e- ^{(0.1πlu/180)} ) (9)
where u is the spatial frequency
l is the observation distance

In this embodiment, the observation distance l is set to 300 mm.

FIG. 10 shows the one-dimensional power spectrum multiplied by the VTF(u) function. The moiré pitch P can be obtained from the screen pitch L, the screen angle θ, and the charging pitch T, as described above. As shown in FIG. 10, a peak can be seen in the spatial frequency of the moire. The value of this peak is the moire intensity of the image obtained this time.

These operations were performed multiple times while changing the image signal value. The lightness L ^* changes with changing the image signal value.

FIG. 11 shows the relationship between lightness L ^* and moire intensity obtained using the above machine. Two charging frequencies, 2.6 kHz used in the standard mode and 2.9 kHz used in the moire avoidance mode, were used for measurement.

The intensity of moire and the visibility of moire are in good agreement. Therefore, as shown in FIG. 11, when the moire intensity was 2e-8 or less, the moire pattern was barely visible, and when the moire intensity was greater than 2e-8, the moire pattern was more visible as the moire intensity increased.

At the charging frequency of 2.6 kHz used in standard mode, moire intensity can exceed 2e-8 when the lightness L ^* is approximately 80 or higher. On the other hand, at the charging frequency of 2.9 kHz used in the moire avoidance mode, the moire intensity never exceeded 2e-8 regardless of the lightness L ^* .

Therefore, in the present embodiment, the brightness L ^* =80 is used as the threshold for determining whether or not to implement the moire avoidance mode.

FIG. 12 shows the relationship between the image signal value obtained using the above machine and the lightness L* with respect to the image output by the image forming apparatus 10. In FIG.

The image signal value is a signal value in 256 stages from 0 to 255, and is designed so that the larger the signal value, the larger the dot area formed by the screen. That is, the higher the image signal value, the higher the density of the output image to be formed. As shown in FIG. 12, the higher the image signal value, the lower the lightness L ^* . Accordingly, the image signal value is a value corresponding to the density of the output image.

As shown in FIG. 12, in this embodiment, it has been found that the image signal value is approximately 112 or less when the lightness L ^* is 80 or more. Therefore, the threshold value of lightness L ^* =80 can be rephrased as 112 or less as the threshold value Sth of the image signal value. Therefore, the threshold Sth of the image signal value as the first threshold is set to 112. FIG.

Note that the threshold value Sth of the image signal value is not limited to 112, and may be increased or decreased depending on which of the moire visibility and the life of the photosensitive drum 2 is prioritized. Further, the threshold value Sth of the image signal value may be set to a different value for each paper size or type, or may be set to a different value for each station color.

Next, moire avoidance mode processing will be described in more detail with reference to FIG.

The moire avoidance mode process shown in FIG. 9 is started when control unit 400 receives job data.

First, the control unit 400 receives an image formation start signal (S1).

Next, the control unit 400 determines whether or not the moire avoidance mode is selected (S2). For example, the control unit 400 determines whether or not a signal indicating that the moire avoidance mode has been selected has been input from the operation unit (not shown) by the user operating the operation unit, thereby determining whether the moire avoidance mode has been selected. determine whether

When the control unit 400 determines that the moire avoidance mode is selected (S2: Yes), the image diagnosis unit 700 converts the output image signal of each color processed by the image processing unit 9 under the control of the control unit 400 to receive. Then, the image diagnosis unit 700 performs image diagnosis based on the received output image signal (S3) and outputs the diagnosis result to the control unit 400. FIG.

For example, when the image shown in FIG. 13 is composed of magenta and cyan, the image diagnosis unit 700 receives a magenta output image signal and a cyan output image signal. Then, the image diagnosis unit 700 detects the density distribution for each color image and extracts the image area. Here, the image area is an area that can be clearly distinguished from a background area with a density of 0, and is an area in which images with a density greater than 0 are continuous.

The image diagnosis unit 700 extracts image regions Ic1 to Ic3 from the cyan image shown in FIG. ˜Sc3 are calculated. Here, the average image signal value is a value obtained by averaging the image signal values in the calculated region areas Ac1 to Ac3. The image diagnosis unit 700 outputs the calculated region areas Ac1 to Ac3 and average image signal values Sc1 to Sc3 to the control unit 400 as diagnosis results.

Next, the control unit 400 determines whether or not the image requires the moire avoidance mode based on the area areas Ac1 to Ac3 and the average image signal values Sc1 to Sc3 input from the image diagnosis unit 700 as diagnosis results. (S4). Specifically, the control unit 400 performs moire avoidance based on the results of comparison between the region areas Ac1 to Ac3 and the threshold Ath as the second threshold, and the results of comparison between the average image signal values Sc1 to Sc3 and the threshold Sth. It is determined whether or not the image requires a mode. As a result, it is possible to determine whether or not the image requires the moire avoidance mode using the average image signal values Sc1 to Sc3.

When the control unit 400 determines that the image requires the moire avoidance mode (S4: Yes), the control unit 400 controls the charging high-voltage power supply 30 to set the charging frequency for moire avoidance (S5). The frequency for moiré avoidance is exemplified here as 2.9 kHz.

For example, the image diagnosis unit 700 determines whether there is an Icn that satisfies Scn<Sth (integer of 1≦n≦3 in the case of FIG. 13) (Icn less than the threshold). When Icn satisfying Scn<Sth exists, the image diagnosis unit 700 selects the largest region area among the Icn satisfying Scn<Sth, and sets the selected region area to Acmax. When Acmax [cm ² ]>Ath [cm ² ] (S4: Yes), the image diagnosis unit 700 determines that the image requires the moiré avoidance mode, and the charging high-voltage power supply 30 performs charging for moiré avoidance. Control to set the frequency.

For example, the control unit 400 sets the visibility of moire to a threshold Ath=20, and selects Ic1 as having the largest region area among Icn that satisfies Scn<30. Since the area Ac1=25 of the selected Ic1 is larger than Ath=20, the control unit 400 controls the charging high-voltage power supply 30 to set the charging frequency for avoiding moire.

Next, the control unit 400 starts an image forming operation using an AC voltage having a frequency for avoiding moiré (S6), and ends the process. For example, since the area Ac1=25 of the selected Ic1 exceeds Ath=20, the control unit 400 sets the AC voltage of cyan to the frequency for avoiding moiré and starts the image forming operation.

As described above, by changing the frequency of the AC voltage to the frequency for avoiding moiré when the moiré avoiding mode is selected, moiré can be avoided as necessary.

On the other hand, if the moire avoidance mode is not selected in step S2 (S2: No), the control unit 400 controls the charging high-voltage power supply 30 to set the default charging frequency (S7). The process of S6 is performed. The default charging frequency is exemplified here as 2.6 kHz.

Further, when the control unit 400 determines in step S4 that the image does not require the moire avoidance mode (S4: No), it then performs the processing in step S7.

Specifically, if there is no Icn that satisfies Scn<Sth, or if the largest region area Acmax among Icn that satisfies Scn<Sth is equal to or less than the threshold Ath (Acmax is equal to or less than the threshold), It is determined that the image does not require the moire avoidance mode.

For example, the control unit 400 sets the visibility of moire to Ath=20, and selects Im1 as having the largest region area among Imns that satisfy Smn<Sth=112. Since the region area Am1=8 of the selected Im1 is Ath=20 or less, the control unit 400 determines that the image does not require the moire avoidance mode. As a result, the control unit 400 sets the magenta AC voltage to the default frequency and starts the image forming operation. As described above, since the frequency of the AC voltage is not changed for an image for which the moire avoidance mode is unnecessary because the area area is equal to or smaller than the threshold value Ath, shortening of the life of the photosensitive drum 2 can be prevented or reduced.

In this embodiment, the image diagnosis section 700 diagnoses the image density of the output image signal generated by the image processing section 9 . Also, the control unit 400 changes the frequency of the AC voltage applied to the charging roller 3 from the charging high voltage power supply 30 based on the diagnosis result of the image diagnosis unit 700 . Accordingly, shortening of the life of the photosensitive drum 2 can be prevented or reduced by not uniformly changing the frequency of the charging AC voltage in order to avoid moire.

Note that in the present embodiment, the threshold for the area area is set to 20 cm ² or more because moire is easily visible, but the threshold for the area area is not limited to 20 cm 2 and can be set to a value other than 20 cm ² . In addition, a different area threshold may be set for each size or type of paper, or a different threshold for area area may be set for each station color.

(Embodiment 2)
The configuration of the image forming apparatus according to Embodiment 2 of the present invention is the same as that shown in FIGS. 1 to 5, so description thereof will be omitted.

<Moire avoidance mode processing>
Moire avoidance mode processing executed by the image forming apparatus according to the second embodiment of the present invention will be described in detail with reference to FIGS. 14 to 16. FIG.

Here, the visibility level of moire fluctuates depending on the relative humidity during image printing. In a high-humidity environment, the toner charge amount (tribo) tends to be small, and the density fluctuation with respect to the potential fluctuation of the photosensitive drum 2 tends to be large. Therefore, in a high-humidity environment, even if the potential pitch of the photosensitive drum 2 due to the charging AC voltage is the same, the density of the portion of the screen where halftone dots are present fluctuates greatly, so moire tends to be visible. On the other hand, in a low-humidity environment, the characteristics are reversed, and moire tends to be less visible.

FIG. 15 shows the relationship between lightness L ^* and moire intensity for each relative humidity (5%, 60%, 80%) obtained using the above machine. In addition, FIG. 15 was measured with the default charging frequency of 2.6 kHz. From FIG. 15, the moire intensity increases as the humidity increases. As a result, the life of the photosensitive drum 2 can be prevented from being shortened by performing the operation in the moire avoidance mode only when the humidity is high.

In this way, it is desirable to operate the moire avoidance mode only when the humidity is equal to or higher than a preset threshold value that increases as the moire intensity becomes visible.

Next, moiré avoidance mode processing will be described in more detail with reference to FIG. 14 .

The moire avoidance mode process shown in FIG. 9 is started when the control unit 400 receives the job data of the first image.

First, the control unit 400 receives an image formation start signal (S11).

Next, the control unit 400 determines whether or not the relative humidity is equal to or greater than a preset threshold α based on the measurement result input from the environment sensor 500 (S12). The threshold α is exemplified here as 30%.

When the relative humidity is equal to or higher than the threshold value α (S12: Yes), the control unit 400 determines that moire is visible at the default charging frequency, and determines whether the moire avoidance mode is selected ( S13).

When the control unit 400 determines that the moire avoidance mode is selected (S13: Yes), the image diagnosis unit 700 converts the output image signal of each color processed by the image processing unit 9 under the control of the control unit 400. receive. Then, the image diagnosis unit 700 performs image diagnosis based on the received output image signal (S14) and outputs the diagnosis result to the control unit 400. FIG.

Next, the control unit 400 determines whether or not the image requires the moire avoidance mode based on the diagnosis result input from the image diagnosis unit 700 (S15).

When the control unit 400 determines that the image requires the moire avoidance mode (S15: Yes), the control unit 400 controls the charging high-voltage power supply 30 to set the charging frequency for moire avoidance (S16).

Next, the control unit 400 starts the image forming operation with the frequency for avoiding moiré (S17), and ends the process.

On the other hand, in step S12, when the relative humidity is less than the threshold α (S12: No), the control unit 400 determines that moire is not visually recognized even at the default charging frequency. Then, the control unit 400 controls the charging high-voltage power supply 30 to set the default charging frequency (S18), and then performs the process of step S17.

If the moire avoidance mode is not selected in step S13 (S13: No), the control unit 400 then performs the process of step S18.

Further, when the control unit 400 determines in step S15 that the image does not require the moire avoidance mode (S15: No), the process of step S18 is subsequently performed.

Here, FIG. 16 shows a plot of the relationship between the relative humidity and the moire intensity by averaging the brightness L ^* =80 to 90 where the moire intensity is relatively high from the graph shown in FIG. It can be seen from FIG. 16 that the relative humidity and the moire intensity increase approximately linearly. Further, from the approximate straight line shown in FIG. 16, it is clear that when the relative humidity is 30% or less, even the default charging frequency does not exceed 2e-8, which is the moiré intensity threshold at which moiré is visually recognized. be.

<Effect of Execution of Moire Avoidance Mode Processing>
The effect of executing the moire avoidance mode process executed by the image forming apparatuses according to the first and second embodiments of the present invention will be described in detail with reference to FIG.

In FIG. 17, in addition to Embodiments 1 and 2, two Comparative Examples 1 and 2 were used in order to confirm the effect of improving moiré in the high image quality mode and the life of the photosensitive drum 2. was used to compare the visibility of moiré and the life of the drum in the durability test. The term "drum life" as used herein means the number of printed sheets at which image defects occur due to abrasion of the surface layer of the drum during an endurance test.

Here, the electrophotographic copier used in the image evaluation was a full-color copier ImagePRESS800 manufactured by Canon Inc. (the magenta station was used as the station).

In a machine for A3 landscape output, the recording media speed is 360 mm/sec and the image resolution is 1200 dpi. A screen ruling of 190 dots was selected. The charging frequency was fixed at 2.52 kHz in Comparative Example 1 and fixed at 2.9 kHz in Comparative Example 2. As the test environment, three environments with different humidity (temperature 23° C. relative humidity 5%, temperature 23% relative humidity 50%, temperature 23° C. relative humidity 80%) were used.

As the conditions for the durability test, the following images 1 to 3 are used, and the image is printed in the following order: 100 prints of image 1 → 100 prints of image 2 → 100 prints of image 3 → 100 prints of image 1 → ... Images 1 to 3 were repeated in order to print 100 sheets each.

As image 1, an image having a density of 1.5 was prepared on an A4 sheet with the entire surface excluding margins (vertical: 30 mm, horizontal: 30 mm). This corresponds to about 255 as an image signal value and about 50 as lightness L ^* measured by ES-2200.

As the image 2, an image having a density of 0.8 was prepared on an A4 sheet with the entire surface excluding margins (vertical: 30 mm, horizontal: 30 mm). This corresponds to about 150 as an image signal value and about 70 as lightness L ^* measured by ES-2200.

As the image 3, an A4 sheet was prepared with a density of 0.3 over the entire surface excluding margins (vertical: 30 mm, horizontal: 30 mm). This corresponds to about 80 as an image signal value and about 85 as lightness L ^* measured by ES-2200.

The evaluation rank for moire was determined as follows.

Rank A: Moire is not visually recognized in all images 1 to 3.

Rank B: Some of images 1 to 3 have visible moire.

The evaluation rank of the results of the durability test was determined as follows.

Rank A: Drum life is 500,000 sheets or more Rank B: Drum life is less than 500,000 sheets

In the test results shown in FIG. 17, Comparative Example 1 was rated as rank B because moiré was observed in environments with humidity of 50% and humidity of 80%. Further, in Comparative Example 2, the life of the drum under the environment of 5% humidity and 50% humidity is less than 500,000 sheets, which is rank B judgment.

In the first embodiment, the rank of moiré is determined as A in any environment, and the occurrence of moiré can be prevented. Further, in the first embodiment, compared with the comparative example 2, the life of the drum was improved in the environment of temperature 23° C. and humidity 50%, which is considered to be the most frequently used environment, and the rank A was determined. ing. On the other hand, in the first embodiment, the drum life is less than 500,000 sheets and rank B is determined only in an environment with a humidity of 5% where the surface layer is more likely to be scraped.

As for the second embodiment, the moire and drum life ranks were A judgments in all three environments. As a result, in the second embodiment, it is possible to prevent the occurrence of moire regardless of the humidity and prevent the life of the photosensitive drum 2 from being shortened.

(Other embodiments)
In Embodiments 1 and 2 above, the process speed of the photosensitive drum 2 is only 360 mm/sec, but the process speed is not limited to 360 mm/sec. In this case, the slow process speed is served when forming relatively high quality images. The reasons for the low speed are to stably convey coated paper or thick paper in order to form high-quality images, to prevent uneven toner melting by keeping the fixing temperature high, and to achieve uniform gloss. This is to ensure reliability.

In view of the above, it is desirable to positively avoid moire when forming images at a low process speed. Therefore, the image forming apparatus 10 may change and adjust the frequency of the AC voltage when the moire avoidance mode is performed according to the process speed.

Specifically, the control unit 400 pays attention to the charging pitch T calculated from the process speed and the charging frequency. is set to be narrower than the charging pitch T of .

For example, the image forming apparatus 10 has a standard mode (hereinafter referred to as "high-speed mode") in which images are formed at a process speed v of 360 mm/sec, and a low-speed mode in which images are formed at a process speed v' of 180 mm/sec. .

Further, the image forming apparatus 10 sets the frequency f or f1 of the AC voltage to 2.6 kHz or 2.9 kHz, respectively, and sets the charging pitch T or T1 to 0 when forming images in the standard mode or the moiré avoidance mode in the high-speed mode. .138 mm or 0.124 mm. Further, the image forming apparatus 10 sets the frequency f2 or f3 of the AC voltage to 1.45 kHz or 1.6 kHz, respectively, when forming an image in the standard mode or the moire avoidance mode in the low speed mode.

The values of the charging pitch T2 or T3 at this time are as follows from the equation (1).

T2=v'/f2=0.138 (mm)
T3=v'/f3=0.113 (mm)

Accordingly, the charging pitch T2 in the standard mode of the low speed mode is equivalent to the charging pitch T=0.138 (mm) in the standard mode of the high speed mode. Also, the charging pitch T3 in the low-speed moire avoidance mode is set narrower than the charging pitch T1=0.124 (mm) in the high-speed moire avoidance mode. By setting in this way, in the low-speed mode in which high-quality and high-quality images are often formed, it is possible to form high-quality images in which degradation in image quality is difficult to see even from the viewpoint of moire.

According to the present embodiment, the charging frequency is changed when it is determined that there is an area where moiré is likely to be visually recognized on the output image due to the pitch of the height difference of the drum potential due to the charging AC voltage and the pattern of the screen. to avoid moire. Further, according to the present embodiment, when it is determined that the moire is difficult to see on the output image, electrical damage to the photosensitive drum 2 can be suppressed by not changing the charging frequency. This makes it possible to prevent or reduce the shortening of the life of the photosensitive drum 2 compared to the case of uniformly changing the frequency to avoid moire.

It goes without saying that the present invention is not limited to the above-described embodiments, and can be modified in various ways without departing from the gist of the present invention.

1 Image Forming Section 2 Photosensitive Drum 3 Charging Roller 4 Developing Device 5 Primary Transfer Roller 7 Exposure Device 9 Image Processing Section 10 Image Forming Device 17 Fixing Device 30 Charging High Voltage Power Supply 31 DC Power Supply 32 AC Power Supply 200 Input Image Processing Section 300 Output Image Processing Unit 400 Control Unit 500 Environment Sensor 600 Storage Unit 700 Image Diagnosis Unit

Claims (6)

a photoreceptor;
a charging member that charges the photoreceptor;
a charging high-voltage power supply that applies a voltage obtained by superimposing a DC voltage and an AC voltage to the charging member;
an image processing unit that generates an output image signal for image formation from image data;
an image diagnosis unit that diagnoses image density of the output image signal generated by the image processing unit;
a control unit that changes the frequency of the AC voltage applied to the charging member from the charging high-voltage power supply based on the diagnosis result of the image diagnosis unit;
An image forming apparatus comprising: The control unit
changing the frequency based on a comparison result between an image signal value of the output image signal corresponding to the concentration as the diagnostic result and a first threshold;
2. The image forming apparatus according to claim 1, wherein: The control unit
when the area of the image area where the image signal value is less than the first threshold is greater than the second threshold, the image signal value is greater than or equal to the first threshold or the area is less than or equal to the second threshold; higher than the frequency set for
3. The image forming apparatus according to claim 2, wherein: The control unit
The frequency is changed when a moire avoidance mode for avoiding interference between a screen pattern and a screen pattern during image formation is selected. do,
4. The image forming apparatus according to any one of claims 1 to 3, characterized by: having an environment sensor that measures the temperature and humidity outside the image forming apparatus;
The control unit
changing the frequency based on measurements in the environmental sensor;
5. The image forming apparatus according to any one of claims 1 to 4, characterized by: The control unit
changing the frequency according to process speed;
6. The image forming apparatus according to any one of claims 1 to 5, characterized by:

Images