JPH106557A - Image forming method and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reconcile gradation properties and the enhancement of the density of a character or a fine line by ensuring the enhancement of gradation expression part, the prevention of the generation of a false contour in a low image signal part and the sufficient density of a character or a fine line by linearizing medium contrast gradation properties in the formation of an image up to the high image signal part. SOLUTION: In the case of a printer mode, according to whether data is font data or not, an image signal 100 passes through a font data generation circuit 102 and a UCR processing circuit 103 to be stored in an image density data memory circuit 300 or passes through a UCR processing circuit 104 to be stored in the image density data memory circuit 300. In the case of a copier mode, a manuscript is read by a color scanner 200 to be converted by a density conversion circuit 202 to be stored in the image density data memory circuit 300 through a UCR processing circuit 203 and, according to whether a gradation image signal at every pixel is the max. value, laser 305 emits laser beam on a photosensitive member by a laser driver 304. Therefore, the medium contrast gradation properties of an image and the uniform high density display of a fine line or a character can be reconciled.

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 and apparatus for forming an electrostatic image on the surface of a photoreceptor, and is applied to electrophotography, electrostatic recording, facsimile, transmission devices, laser printers and the like.

[0002]

2. Description of the Related Art In a multicolor image forming technique using so-called electrophotographic technique, a charging means, an exposing means, and each color such as black, yellow, magenta and cyan are applied to a photoreceptor surface having a photoelectromotive force. Developing means, a primary transfer means consisting of a semiconductive belt, a cleaning means and a uniform light charging means are arranged, and the charged photoreceptor surface is exposed to light to first form a black electrostatic latent image, After a black toner image is developed on the photoreceptor surface, the toner image is transferred onto a primary transfer member, and untransferred toner remaining on the photoreceptor surface is cleaned, and the photoreceptor is uniformly irradiated with light. In the same manner, the toner images of yellow, magenta, and cyan are sequentially superimposed on the primary transfer member and transferred to form toner images of four colors, and the toner images are recorded on paper or the like. On the medium Batch performing transfer multicolor image forming method for forming a multi-color image is performed.

In this multicolor image forming method, as a means for forming a halftone, a lighting time of an exposure means is controlled by modulating a pulse width of a laser drive to form an electrostatic latent image on a photosensitive member surface. The method of forming an image by using the relationship between the image signal and the lighting time at this time,
As shown in FIG. 8, the gradation forms a straight line proportional to the lighting time. The relationship between the development contrast (development bias-image portion potential) and the density is a straight line proportional to the density as shown in FIG.

On the other hand, due to the nature of the sensitivity characteristic of the photoreceptor, the photoreceptor exhibits a sensitivity above a certain value with respect to an increase in the amount of exposure and then tends to gradually saturate. The curve becomes as shown, and this tendency becomes more conspicuous as the maximum light amount is increased and the VL (image portion potential) is reduced.

Since the relationship between the image signal, the density, the image portion potential and the like is as described above, the final relationship between the image signal and the density is a curve as shown in FIG. That is, these laser beams are exposed in the form of spots whose intensity distribution forms a Gaussian distribution, but since there is a threshold value for the potential difference to which the toner adheres, as the exposure intensity increases, the surrounding range below the threshold value becomes adjacent. Each spot overlaps and exceeds the threshold value, which is similar to a substantial increase in light quantity. For this reason, the curves of these curves are further strengthened and tend to plateau near the maximum value. As a result, the change in density is saturated from the halftone to the high density portion, and as described above, this phenomenon becomes more remarkable as the maximum light amount is increased, and the image has a clear gradation change in the halftone. Although the density is lost and the density is improved, the state is crushed from the half tone to the high density portion. Accordingly, in the method of forming a halftone by such exposure, the relationship between the image signal and the density forms a curve as shown in FIG. There have been problems such as the inability to express gradation and the occurrence of false contours in the low image signal portion.

For this reason, if the maximum light amount is reduced with an emphasis on the gradation, the density at the maximum light amount is reduced, and even when it is necessary to represent a high density such as a character or a thin line, FIG. As shown, even if the development contrast is increased, the line density does not increase, and the density cannot be increased. in this way,
It has been impossible to maintain the gradation of an image and to improve the density for clearly expressing characters and fine lines by these conventional exposure methods.

To solve these problems, Japanese Patent Application Laid-Open No. 2-52765 discloses a method of increasing the density of characters by determining whether or not an input signal is a character and controlling a developing bias. It has been shown. However, if only the developing bias is changed, there are problems such as the occurrence of so-called fogging and the adhesion of the carrier on the photoreceptor. Unless it was difficult. Japanese Patent Application Laid-Open No. 5-83500 discloses that the maximum light amount is controlled by an intensity modulation method by determining whether the region is a character region or a halftone region.
No. 501 describes a method in which similar regions are determined and the maximum light amount is changed by a pulse width modulation method. However, with respect to both of these methods, the specific determination of the character area or the halftone area can be made, for example, by successively differentiating image density data corresponding to continuous pixels, and the differential value exceeding a specific value. However, since the case where these change rates are large actually does not always indicate the presence or absence of a character area, the above problem has not been solved.

Japanese Patent Application Laid-Open No. Hei 5-183711 discloses an elliptical beam having a major axis in the main scanning direction.
Further, a method is described in which the maximum light amount is changed by a pulse width modulation method by determining whether the region is a halftone region or a character region. Japanese Patent Application Laid-Open No. 5-183712 similarly discloses an ellipse having a major axis in the main scanning direction. A method is described in which a beam is used to determine whether the area is a halftone area or a character area, and the maximum light amount is changed by an intensity modulation method. However, there is no specific description for determining whether the area is a halftone area or a character area, and none of them has solved these problems.

[0009]

SUMMARY OF THE INVENTION The present invention has been devised in order to solve such a problem, and the halftone gradation in image formation is made linear up to a high image signal portion. Improves the gradation expression in the high image signal area, prevents the occurrence of false contours in the low image signal area, and secures sufficient density of characters and thin lines to achieve gradation and density of characters and thin lines. The purpose is to achieve both improvements.

[0010]

According to the first and third aspects of the present invention, the above object is achieved by applying a pulse width modulation of a laser beam to a charged photoreceptor surface to respond to an image signal for each pixel. A method and an apparatus for forming an electrostatic latent image by performing exposure by laser irradiation of a pulse width that sets a maximum value in an area of a light amount with a gradation corresponding to an image signal, A signal less than the maximum value of the image signal representing the gradation of the pixel is exposed to light with pulse width modulation corresponding to the image signal value within the range of the maximum value or less, and the image signal representing the gradation of the pixel has the maximum value. Is exposed by a constant light amount larger than the maximum value by intensity modulation in addition to the light amount by the pulse width modulation.

According to a second or fourth aspect of the present invention, an electrostatic latent image is formed by exposing a charged photoreceptor surface by laser irradiation of an intensity corresponding to an image signal for each pixel by intensity modulation of a laser beam. A method and an apparatus for forming, wherein a maximum value is set in an area of a light amount having a gradation with a density corresponding to an image signal, and a signal less than the maximum value of an image signal representing a gradation of a pixel is set. Exposure is performed with a light amount by intensity modulation according to an image signal value in a range equal to or less than the maximum value. For a signal whose image signal representing the gradation of a pixel has a maximum value, a constant value larger than the maximum value is obtained by intensity modulation. Exposure is performed by light quantity.

According to the present invention, the characteristics of a curve represented by the relationship between an image signal and the potential of an image portion, and the gradation and discrimination characteristics to be expressed in a halftone portion and a character portion of these images, etc. The demands for the concentration of are different. That is, characters and fine lines need to be expressed with a uniform high density in order to emphasize discrimination from the surroundings, while the halftone part needs to be relatively distinguished in density. The highest density among the gradations only overlaps with the density of a character portion or the like.

Therefore, the portion which does not require the highest density is exposed within the range of light amount which can maintain the gradation, and when the highest density is required, that is, when the image signal has the maximum value, the gradation is not increased. Regardless, if you expose with a large amount of light,
The halftone portion can maintain a linear relationship with the image signal value, and an image is generated with high uniform density for characters, fine lines, and the like. Between these halftone parts and the maximum density part,
If the density difference is discontinuous, the set value of the maximum light amount for the halftone portion may be adjusted.

More specifically, according to the first and third aspects of the present invention, for an image signal lower than the maximum value of the image signal,
A maximum value for exposing a halftone image signal is set in a range in which the light amount of the exposure is linearly related to the density or in a range that can be approximated. The exposure light amount is adjusted by pulse modulation of a laser beam corresponding to the image signal. For a signal value less than the maximum value of the image signal, the light amount is adjusted by distributing the signal value with the light amount up to this maximum value. As a result, an image with gradation is formed. On the contrary,
When the image signal has the maximum value, in addition to the maximum light amount by the pulse modulation, intensity modulation is performed, and a sufficiently large light amount is given so that the light amount has an intensity equal to or higher than a certain value, and the density is increased. When the setting of the maximum light amount of the halftone is low, the density for the maximum value of the image signal is higher than the density of the halftone, and the density of the character, the thin line, and the portion having the maximum density is higher than the other areas. The density becomes too high and discontinuous,
In this case, set the maximum density for halftone high,
It may be adjusted so as to be continuous with a high density for characters, thin lines, and the like.

According to the second and fourth aspects of the present invention, the amount of laser light is adjusted by intensity modulation of an image signal, and the maximum value of the amount of light for halftone is set. Even if the value is less than the value, the halftone exposure light quantity is still distributed. When the image signal has the maximum value, the light amount is set to a large value which is equal to or larger than a certain value.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of an image forming apparatus for executing a multicolor image forming method of the present invention will be described below with reference to FIGS. FIG. 1 shows a schematic configuration of a multicolor image forming apparatus to which the present invention is applied. Reference numeral 1 denotes a photoconductor (latent image carrier, OP
C-IR), and an electrostatic latent image corresponding to image information is formed on the surface thereof by a well-known electrophotographic process with rotation in the direction of arrow A. That is, the photosensitive member 1 is
After being charged to a predetermined negative dark potential by a,
Exposure according to an image signal is performed by a light beam Bm emitted from a laser beam scanner (not shown). Further, developing units 5 to 8 corresponding to respective colors of black (BK), yellow (Y), magenta (M), and cyan (C) are arranged around the photoconductor 1. The formed electrostatic latent image is developed by any one of these developing devices to form a toner image T. Therefore, if the electrostatic latent image written on the photoconductor 1 corresponds to the yellow image information, the electrostatic latent image is developed by the developing device 6 containing the yellow (Y) toner, and 1, a yellow toner image is formed.

Reference numeral 2 denotes a belt-shaped intermediate transfer member arranged so as to be in contact with the surface of the photoreceptor 1, which is stretched over a plurality of rolls and rotates in the direction of arrow B. The toner image T formed on the photoconductor 1 is composed of the photoconductor 1 and the intermediate transfer body 2
Are transferred from the photoconductor 1 to the surface of the intermediate transfer body 2 at the primary transfer position where At this primary transfer position, a corona discharger 9 is provided on the back side of the intermediate transfer body 2,
By applying a voltage having a polarity opposite to the charging polarity of the toner, that is, a positive voltage to the corona discharger 9,
The upper unfixed toner image T is electrostatically attracted to the intermediate transfer body 2 and transferred.

On the other hand, untransferred toner remaining on the photoreceptor 1 after the primary transfer is collected into the cleaning device 16 by the toner removing member 16b. In order to prevent the toner collected in the cleaning device 16 from dropping from the inside of the cleaning device 16, a toner receiving member 16a is provided in contact with the photoconductor 1. And the erase lamp 1
7, the surface potential of the photoreceptor 1 becomes 0 V,
Prepare for the next charging step.

When forming a monochrome image, the intermediate transfer member 2
The toner image T primarily transferred to the recording medium 3 is immediately secondarily transferred to the recording medium 3. However, when a multi-color image is formed by superimposing toner images of a plurality of colors, formation of a toner image on the photoconductor 1 and The primary transfer process of this toner image is repeated for the number of colors. For example, when a multicolor image is formed by superimposing four color toner images, a black, yellow, magenta, and cyan toner image T is formed on the photoreceptor 1 for each rotation. Image T is sequentially transferred to intermediate transfer body 2
Is primary-transferred. On the other hand, the intermediate transfer member 2 holds the photosensitive member 1 while holding the first primary-transferred black toner image T.
The yellow, magenta, and cyan toner images T are transferred onto the intermediate transfer body 2 so as to be superimposed on the black toner image T for each rotation.

The toner image T primarily transferred to the intermediate transfer body 2 in this way is conveyed to a secondary transfer position facing the conveyance path of the recording medium 3 as the intermediate transfer body 2 rotates. . At the secondary transfer position, the transfer roll 10 is in contact with the intermediate transfer body 2, and the recording medium 3 unloaded from the tray 12 by the feed roller 11 at a predetermined timing is located between the transfer roll 10 and the intermediate transfer body 2. It is sandwiched.
On the back side of the intermediate transfer body 2 at the secondary transfer position, a conductive roll 4 serving as a counter electrode of the transfer roll 10 is provided. Is applied, the toner image T carried on the intermediate transfer body 2 is moved to the recording medium 3 at the secondary transfer position.
Is electrostatically attracted and transferred.

Then, the recording medium 3 onto which the toner image T has been transferred is peeled off from the intermediate transfer body 2 by the peeling claw 13 and sent to a fixing device (not shown) to perform a toner image fixing process. On the other hand, the untransferred toner is removed by the cleaner 15 from the intermediate transfer body 2 on which the secondary transfer of the toner image has been completed. Further, a blade-shaped cleaning blade 14 made of polyurethane rubber or the like is constantly in contact with the transfer roll 10 to remove toner attached to the transfer roll 10.

In such a configuration, the transfer roll 10, the peeling claw 13, and the cleaner 15 are disposed so as to be able to freely contact and separate from the intermediate transfer body 2, and when a color image is formed, the final color toner is formed. These members are in a position retracted from the intermediate transfer member 2 until the image is primarily transferred to the intermediate transfer member 2. By performing the above steps, a multicolor image is formed on the recording medium 3. Next, a specific latent image forming method of the present invention will be described in detail with reference to the accompanying drawings with reference to the following embodiments.

Embodiment 1 An electrostatic latent image forming method combining pulse width modulation and intensity modulation according to the present invention will be described with reference to FIGS. The printer mode and the copier mode have different means for converting image data. In the printer mode, as shown in FIG. 2, an input image signal 100 enters a data determination circuit 101 and receives font data. It is determined whether or not. In the case of font data, the font data generation circuit 102 performs data processing based on the font data,
The image signal enters the UCR processing circuit 103 for determining R, and the input signal of each color is optimized and stored in the image density data storage circuit 300 as image density data. If the data is not font data, it enters the UCR processing circuit 104 for determining the UCR, and the input signal of each color is optimized and stored in the image density data storage circuit 300 as image density data.

In the copying machine mode, an original is read by a color scanner 200, A / D conversion is performed by an A / D conversion circuit 201, and further converted as density data by a density conversion circuit 202 to determine a UCR. The signal enters the UCR processing circuit 203, and the input signals of each color are optimized and stored as image density data in the image density data storage circuit 300.

Next, these image density data storage circuits 3
The image data stored in 00 is read by the reading circuit 301 for each pixel. Then, the read data for each pixel is determined by the image determination circuit 302 as to whether or not the image data is the maximum value 255 of the image signal representing the gradation. When the image data is smaller than 255, as shown in FIG. 3, the maximum light amount is set by the maximum light amount determining means as the light amount A having gradation in accordance with the input signal. The triangular wave generating circuit 308 causes the pulse width modulation circuit 303 to emit a laser 305 on the photoconductor by the laser driver 304 for a lighting time according to the image data, thereby forming an electrostatic latent image. Therefore, an image having a gradation characteristic according to the input signal is formed on the photoconductor exposed by the image data.

When the image data has 255 gradations, that is, when the image signal representing the gradation is the maximum, all the lights are turned on by the pulse width modulation circuit 306, and the light amount at this time is the light amount A.
Next, the maximum light quantity is converted into the light quantity A + B by the intensity modulation circuit 307, and as shown in FIG. 4, the final light quantity becomes the maximum light quantity = A + B. A latent image is formed. Since the light amount at this time does not require gradation, and forms the maximum density, an image having a high-density and clear contour is formed at a place requiring high density such as characters and thin lines. Is obtained.

Therefore, in the image formed in this way, the halftone is expressed with sufficient gradation, and at the same time, the parts that need to be identified from the surroundings, such as characters and thin lines, are uniform. Expressed with high density and clear outline.

Embodiment 2 An image forming apparatus using an electrostatic latent image forming method using intensity modulation according to the present invention will be described with reference to FIGS. In FIG. 5, similarly to the first embodiment, in the case of the printer mode, the input image signal 400 enters a data determination circuit 401 and determines whether or not the input image signal is font data. In the case of font data, a font data generation circuit 402 performs data processing based on the font data, and enters a UCR processing circuit 403 for determining the UCR. Stored as data. If it is not font data, U to determine UCR
The image data enters the CR processing circuit 404, and the input signal of each color is optimized and stored in the image density data storage circuit 600 as image density data.

In the case of the copying machine mode, the color scanner 5
A / D conversion circuit 501 reads the original
UCR processing circuit 503 which performs / D conversion and converts it as density data by a density data circuit 502 to determine UCR.
Then, the input signal of each color is optimized and stored in the image density data storage circuit 600 as image density data. The image data stored in the image density data storage circuit 600 is read by the reading circuit 601 for each pixel. Then, the read data for each pixel is judged by the image discriminating circuit whether or not the image data is the maximum value 255 of the image signal representing the gradation.

When the image data is smaller than 255, FIG.
As shown in the figure, the maximum light amount that can hold the gradation according to the input signal of the image is defined as the light amount A, and the light amount of the halftone lower than the maximum light amount A is evenly distributed according to the input signal. As a result, a laser beam 605 is emitted on the photoreceptor to form an electrostatic latent image. Image data is the maximum value 2
In the case of 55, as shown in FIG. 7, the intensity modulation circuit 606 sets the maximum light quantity capable of sufficiently increasing the density to be higher than the gradation, as the light quantity A + B, and the laser 605 emits the laser 605 by the laser driver 604. A high electrostatic latent image is formed.

Therefore, also in this case, since the area of the halftone image signal is exposed with the maximum light amount or less capable of maintaining the gradation, it is expressed with the halftone gradation until the high density region. At the same time, in places where high contrast such as characters and fine lines or their outlines need to be clearly shown, a uniform high-density image can be obtained, and these two conditions can be compatible.

[0032]

As described above, according to the present invention, it is possible to form an image in which both the halftone gradation of an image and the uniform high-density display of fine lines and characters can be achieved. Become.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a multicolor image forming apparatus.

FIG. 2 is a block diagram of an image data processing unit using pulse width modulation and intensity modulation according to the present invention.

FIG. 3 is a graph showing a relationship (pulse width modulation) between an input signal and a lighting time according to the present invention.

FIG. 4 is a graph showing the relationship between the input signal and the lighting time (pulse width modulation + intensity modulation) of the present invention.

FIG. 5 is a block diagram of an image data processing unit using intensity modulation according to the present invention.

FIG. 6 is a graph showing a relationship between an input signal and a light amount (input signal <255) when using the intensity unevenness of the present invention.

FIG. 7 is a graph showing the relationship between an input signal and the amount of light when using intensity modulation according to the present invention (input signal = 255).

FIG. 8 is a graph showing a relationship between an image signal and a lighting time.

FIG. 9 is a graph showing the relationship between development contrast and density.

FIG. 10 is a graph showing a relationship between an image signal and an image portion potential.

FIG. 11 is a graph showing a relationship between an image signal and a density.

FIG. 12 is a graph showing the relationship between development contrast and line density.

[Explanation of symbols]

1 photoconductor, 1a charger, 2 primary intermediate transfer body,
3 recording medium, 4 conductive roll, 5 developer (black), 6 developer (yellow), 7 developer (magenta), 8 developer (cyan), 9 corona discharger, 10 transfer roll, 11 feed roll, 1
2 tray, 13 peeling claw, 14 cleaning blade, 15 cleaner, 16 cleaning device,
16a toner receiving member, 16b toner removing member,
17 erase lamp, 19 density reading means, B
m Laser light, T toner image.

Claims (4)

[Claims] 1. An image forming method for forming an electrostatic latent image by exposing a charged photoreceptor surface by laser irradiation with a pulse width corresponding to an image signal for each pixel by pulse width modulation of a laser beam, A maximum value is set in an area of a light amount having a gradation corresponding to an image signal, and an image in a range of less than the maximum value for a signal less than the maximum value of an image signal representing the gradation of a pixel. Exposure is performed with a light amount by pulse width modulation according to the signal value, and for a signal having an image signal representing the gradation of a pixel having a maximum value, in addition to the light amount by the pulse width modulation, intensity modulation is performed so as to be larger than the maximum value. An image forming method comprising performing exposure with a constant light amount. 2. An image forming method for forming an electrostatic latent image by exposing a charged photoreceptor surface by laser irradiation of intensity corresponding to an image signal of each pixel by intensity modulation of a laser beam. A maximum value is set in an area of a light amount having a gradation corresponding to the density, and for a signal less than the maximum value of the image signal representing the gradation of the pixel, an image signal value within a range of the maximum value or less is set. An image characterized by performing exposure with a constant light amount larger than the maximum value by intensity modulation with respect to a signal having a maximum value of an image signal representing the gradation of a pixel according to the intensity modulation. Forming method. 3. An image forming apparatus for forming an electrostatic latent image by exposing a photoreceptor by laser irradiation with a pulse width corresponding to an image signal for each pixel by a pulse width modulation means of a laser beam. The light pulse width modulation means includes an image discriminating circuit that determines whether or not the density is a maximum value set in a light quantity region having a gradation in accordance with the image signal. For a signal less than the maximum value, a pulse width modulation circuit that sets the amount of light by pulse width modulation according to the image signal value in a range equal to or less than the maximum value, and the image signal representing the gradation of the pixel is changed to a signal having the maximum value. On the other hand, an image forming apparatus provided with an intensity modulation circuit for adding a constant light amount larger than the maximum value in addition to the light amount by the pulse width modulation. 4. An image forming apparatus for forming an electrostatic latent image by exposing a charged body by laser irradiation of an intensity corresponding to an image signal for each pixel by a laser light intensity modulating means. The intensity modulating means includes an image discriminating circuit for judging whether or not the density is a maximum value set in an area of a light quantity with a gradation in accordance with the image signal, and the image signal representing the gradation of the pixel is less than the maximum value. For the signal of, the light amount corresponding to the image signal value is set within the range of the maximum value or less. An image forming apparatus provided with an intensity modulation circuit for setting a constant light amount.