JPH06178112A - Digital image forming device - Google Patents

Abstract

PURPOSE:To realize a right gradient by suppressing the fluctuation of a generating gamma characteristic. CONSTITUTION:A CPU 20 receives each image density detection value in plural gradation patterns from a density sensor and calculates the number of gradation in each density by defining the density of a black solid part as a maximum. Namely, the multi-valued level for the gradation level is calculated and the table of conversion data from the gradation level to the multi-valued level is stored in a ROM table 24. In a gamma conversion part 21, the gradation level is converted into the corresponded multi-valued level when multi-valued image signal is inputted by the table, a dither processing is performed for the multi- valued level in a dither processing part 22, a pulse width modulation is performed in a laser diode driver 26 based on the data after correction, and a laser diode 27 is driven.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image forming apparatus which is applied to a copying machine, a facsimile, a laser printer or the like and which can form gradation in an image to be formed.

[0002]

2. Description of the Related Art Conventionally, various electrophotographic image forming apparatuses such as laser printers have been put into practical use, which drive laser means based on image data converted into digital values to reproduce images. Various digital image forming apparatuses for faithfully reproducing so-called halftone images have been proposed.

As a digital image forming apparatus of this type,
By the area gradation method using a dither matrix, or by changing the laser pulse width (light emission time) or light emission intensity to change the laser light amount (= light emission time x intensity), the gradation for one dot to be printed can be changed. The multi-valued laser exposure method to express (pulse width modulation method, intensity modulation method) and the like are known. Furthermore, a multi-valued dither method in which a dither method and a pulse width modulation method or an intensity modulation method are combined is also known.

By the way, according to this kind of gradation method, in a digital copying machine, in principle, it should be possible to reproduce an image density having a gradation corresponding to a gradation of image data to be reproduced. However, in reality, various factors such as the photosensitive characteristics of the photoconductor, the characteristics of the toner, and the specification environment are intricately intertwined, and the original density to be reproduced and the reproduced image density are not exactly proportional. As schematically shown in FIG. 9, a characteristic B deviated from the proportional characteristic A that should be originally obtained is shown. Such a characteristic is generally called a [gamma] characteristic, and is a major factor that lowers the fidelity of a reproduced image particularly for a halftone original.

Therefore, in order to improve the fidelity of the reproduced image, conventionally, the density of the read original is converted using a predetermined conversion table for γ correction, and a digital image is formed based on the converted original density. Thus, so-called γ correction is performed. As described above, normally, by performing the γ correction, the image can be faithfully reproduced (see JP-A-3-271778 and JP-A-4-126462).

[0006]

However, since the image reproduction by the multilevel laser exposure method or the multilevel dither method uses an analog latent image, the gradation characteristic is large due to the diffusion of the latent image on the photoconductor. Therefore, if the characteristics of the photoconductor change, the gradation characteristics also change.

That is, γ based on a single conversion table
Since the correction works correctly only when the γ characteristic is constant to obtain a linear gradation characteristic, if the γ characteristic changes, it is not possible to perform an appropriate γ correction and the linear gradation characteristic is reproduced. It is not possible to obtain a faithful reproduced image.

When laser exposure is applied to a photoconductor charged with corona discharge, a considerable amount of laser light is reflected and scattered, so that a latent image is generated around the laser light irradiation portion, and further, a laser beam is generated. Due to the phenomenon in which latent images are averaged over time due to the diffusion of charges during charge transport due to the fact that the electric resistance in the non-irradiated area is not infinite, the diameter of the laser beam that is actually irradiated is An electrostatic latent image with a large outline and blurry is formed. This phenomenon is generally called latent image diffusion.

This latent image diffusion phenomenon is schematically shown in FIG. FIG. 10 (a) shows an example of the intensity distribution of the laser beam, and FIG. 10 (b) shows the potential distribution of the latent image formed on the photoconductor in the scanning direction. In an ideal state where latent image diffusion does not occur, a latent image along the laser intensity distribution is formed as shown by the broken line in FIG. 10 (b), but in reality, the broken line in FIG. 10 (b) is formed. As shown by, the aperture is larger than the ideal latent image indicated by the broken line, and the potential drop is smaller, so that the amount of toner adhering at the time of development is reduced, so that an overall blurred latent image is formed.

Percentage of influence of the above-mentioned latent image diffusion (hereinafter,
The diffusivity) and the developing characteristics (toner particle size, toner charge amount) change with the environment and time. Therefore, the γ characteristic also changes.

Conventionally, γ correction has been performed for the γ characteristic. For example, as shown in FIG. 11A, even if the γ characteristic changes (curves a and c), the γ correction is always performed by the conversion table for correction corresponding to the γ characteristic b. The emission characteristics of the laser based on this γ correction conversion table are shown in Fig. 11.
It is the same curve as B shown in (b). Figure 11 (c) is shown in Figure 11 (a).
For each of the characteristics shown in (1), the γ characteristics when the γ correction corresponding to the γ characteristics b are performed are shown.

As can be seen from the figure, if the constant γ correction is performed regardless of the change of the γ characteristic, the correct γ correction cannot be performed when the γ characteristic changes. If the diffusivity and the development characteristics change in this way, the γ characteristic changes, so the γ correction is not performed properly, the gradation characteristics of the image change, and a stable image cannot be obtained.

It is an object of the present invention to provide a digital image forming apparatus capable of suppressing the fluctuation of the γ characteristic that occurs and realizing a correct gradation.

[0014]

In order to achieve the above-mentioned object, the present invention provides a gradation correction means for performing gradation correction to desired gradation data in order to deal with various gradations, and a plurality of one pixels. In a digital image forming apparatus having a means for expressing gradation by combining it with a matrix, and a detection means for detecting the image density formed on the photoconductor, the detection value from the detection means is Based on the above, a control means for compensating the gradation variation is provided by creating a γ correction conversion table according to the gradation characteristic so as to compensate the gradation characteristic variation in image formation.

Further, in the above digital image forming apparatus, the light quantity of the laser for exposing the photoconductor is controlled based on the detection value from the detecting means, and the gradation characteristic is adjusted so as to compensate the fluctuation of the gradation characteristic in the image formation. It is characterized in that a control means for compensating for gradation variation is provided by creating a γ correction conversion table according to the above.

Further, the above-mentioned digital image forming apparatus is characterized by comprising control means for preparing a conversion table for γ correction according to a designated image processing method.

[0017]

According to the above means, even if the γ characteristic changes, the gradation characteristic is compensated by the image density, so that the gradation level and the image density can be made to have a linear relationship.

Even if the γ characteristic changes, the gradation level and the image density can be made to have a linear relationship by controlling the laser light amount according to the image density.

Further, by changing the γ characteristic in accordance with the designated image processing method such as highlight enhancement or shadow enhancement, it is possible to easily obtain a high quality image even if arbitrary image processing is performed.

[0020]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a schematic structural view of the internal mechanism of a laser printer which is an embodiment of the present invention. 1 is a drum-shaped photosensitive member, 2 is a charger, 3 is a toner developing device, 4 is a toner developing device. A density sensor, which is a detection unit that detects the density of the image developed in 3, is a transfer charger, and 6 is a cleaning unit.

Further, 7 is a paper feed cassette in which the transfer paper 8 is stored, 9 is a paper feed roller, 10 is a registration roller, 11 is an image fixing unit, 12 is a paper discharge roller, and 13 is a paper discharge tray.

Reference numeral 15 is an optical writing section for exposing the photosensitive member,
A rotary polygon mirror 17 for deflecting a laser beam emitted from a laser diode described later, a motor 18 for rotationally driving the rotary polygon mirror 17, a reflection mirror 19 and the like.

FIG. 1 is a block diagram showing the configuration of the control system of this embodiment. 20 is a CPU (central processing unit) which is a control means, 21 is a γ conversion section which is gradation correction means, and 22 is a floor. A dither processing unit that performs dither processing, which is a means of expressing a key, 2
Reference numerals 3, 24 and 25 are ROM tables in which various data tables corresponding to the respective units are stored, and 26 is a laser diode driver for driving the laser diode 27 provided in the optical writing unit 15 to emit light.

Next, the control of the above embodiment will be described.

The input multi-valued image signal (gradation level) is subjected to correction (γ correction) according to the gradation characteristics of the photosensitive member 1 by the γ conversion unit 21, and dither processing is performed as necessary. After the processing by the processing unit 22, the laser diode driver 26 generates a laser diode drive signal in which the corrected image data is pulse-width modulated, and the laser diode 27 is driven by this drive signal. The laser beam generated from the laser diode 27 corresponding to the gradation data exposes the photoconductor 1 via the rotary polygon mirror 17 to form an electrostatic latent image on the photoconductor 1.

The electrostatic latent image is developed by the toner developing device 3 and converted into a visualized toner image. The toner image is transferred by the transfer charger 5 onto the transfer paper 8 that has been conveyed along the route indicated by the alternate long and short dash line.

The toner and electric charge remaining on the photosensitive member 1 are
Each of the toners removed by the cleaning unit 6 is collected by the cleaning unit 6.

The transfer paper 8 onto which the toner image is transferred is fed by the paper feed roller 9 of the paper feed cassette 7, abuts against the registration roller 10 and is temporarily stopped, and then is aligned with the toner image on the photoconductor 1. The toner image is transferred to the position of the transfer charger 5 by the registration roller 10 at a timing.

The transfer paper 8 on which the toner image has been transferred is conveyed to the image fixing section 11, heated and pressure-contacted, fixed by the heat and pressure, and then conveyed to the discharge roller 12 and discharged onto the discharge tray 13. Discharged on top.

In this embodiment, for convenience of explanation, the matrix size is 2 × 2, the number of gradations is 64, and the number of pulse width modulations is 64. Therefore, originally 2 × 2 × 64 is 256
The gradation image density is represented, but the γ curve is not linear. Therefore, it is necessary to convert from 256 gradations to linear 64 gradation data.

In a 2 × 2 matrix, FIG.
Considering a pattern having nine gradations as shown in FIG. 4, the photoreceptor 1 is divided into nine areas, and the pattern of each gradation is exposed by N × N laser beams in each area (see FIG. (See a) and (b), and FIG. 4 (b) is an enlarged view of the broken line portion of FIG. 4 (a). Then, the image density in each area after development is detected by the density sensor 4. Note that the concentration at this time is an average value of potentials measured at a plurality of points in the region. Each density value is stored in the ROM table 24. CP
In U20, the multi-valued level is "255", that is, the density of the black solid portion is set as IDmax, the value is divided into 63, and the number of gradations at each density is calculated. As an example, the gradation level "3
Multi-value level of 1 ”is obtained, that is, gradation level of“ 3
It suffices to select a multi-valued level such that the image density at 1 ”is (IDmax × 31/63). The density at this time is ID31.
Then, the CPU 20 searches the ROM table 24 for the multi-valued density of the ID 31. In this case, it can be seen that it is between the multi-valued levels “95” and “127” (see FIG. 5). If the respective densities are ID1 and ID2, the multi-valued level of ID31 can be obtained from the following equation.

[0033]

## EQU00001 ## Multivalued level = 95 + (127-95) .times. (ID31-ID1) / (ID2-ID1) In this way, the CPU 20 calculates the multivalued level for each gradation level, and calculates from the gradation level. A table of multi-valued level conversion data is stored in the ROM table 24.

When a multi-valued image signal is input, the γ conversion unit 21 converts the gradation level into a corresponding multi-valued level, and the dither processing unit 22 dither-processes the multi-valued level. Data for laser diode driver 26
The pulse width modulation is performed at to drive the laser diode 27.

Therefore, even if the γ characteristic varies, the gradation level and the image density have a linear relationship by performing the above control (see FIG. 6).

Next, another control example will be described. As shown in FIG. 7, due to the characteristics of the photoconductor 1, the γ characteristic changes even if the laser intensity is changed. By changing the laser emission intensity corresponding to the maximum gradation level (hereinafter referred to as maximum emission intensity), it is possible to reduce the fluctuation of the γ characteristic.

The photoconductor is divided into m areas, and each area is subjected to laser exposure with a maximum emission intensity of m steps. That is,
In the same area, exposure is performed with the same intensity to form a black solid image. Then, the image density in each area after development is detected by the density sensor 4, and the image density data in each area is stored in the ROM table 23.

The CPU 20 stores in advance the data of the proper image density in the ROM table 23 and selects the maximum light emission intensity of the image density closest to the value. By performing the above-mentioned control at the selected maximum emission intensity, a linear relationship between the image density and the gradation level with less error can be obtained.

Both of the above controls are performed when the power is turned on, when the photoconductor 1 and the toner developing device 3 are replaced, or when printing or copying is started.

Still another control example will be described. By setting the γ curve as shown in Fig. 8, highlighting
It is possible to process shadow enhancement, highlight enhancement, shadow enhancement, halftone enhancement, and density linear even with the same image data.

When the density is linear with respect to the maximum image density IDmax, as described above, when the number of gradations is "64", the image density for each gradation level is 63 evenly with IDmax.
The divided values correspond. The conversion of the above γ curve is ID
It can be changed depending on how to divide max into 63.

IDma for each highlight / shadow enhancement, highlight enhancement, shadow enhancement, halftone enhancement, density linear
The divided data of x is stored in the ROM table 23. Therefore, when the user specifies an image processing method using an operation panel (not shown), the γ conversion control can be performed by the divided data of the image processing method specified by the CPU 20.

[0043]

As described above, the digital image forming apparatus of the present invention has the features of γ
Since gradation characteristics can be compensated for fluctuations in characteristics, the effects of gradation characteristics due to deterioration or replacement of the photoconductor, replacement of the developing device, environmental changes, etc. can be suppressed, and images with excellent gradation reproducibility Can be obtained.

According to the third aspect of the invention, it is possible to easily obtain a high quality image by arbitrary image processing.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a control system in an embodiment of a digital image forming apparatus of the present invention.

FIG. 2 is a schematic configuration diagram of an internal mechanism of a laser printer that is an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a gradation pattern.

FIG. 4 is an explanatory diagram of image density detection by a density sensor.

FIG. 5 is a relationship diagram between image density and multi-valued levels.

FIG. 6 is a relationship diagram between image density and gradation level.

FIG. 7 is a relationship diagram between image density and gradation level due to a change in laser intensity.

FIG. 8 is a diagram showing the relationship between image density and gradation level obtained by image processing.

FIG. 9 is an explanatory diagram of a γ characteristic.

FIG. 10 is an explanatory diagram of a latent image diffusion phenomenon.

FIG. 11 is an explanatory diagram of γ correction.

[Explanation of symbols]

1 ... Photosensitive member, 3 ... Toner developing unit, 4 ... Density sensor
(Detection means), 15 ... Optical writing unit, 17 ... Rotating polygon mirror, 20
... CPU (control means), 21 ... γ conversion section (gradation correction means),
22 ... Dither processing unit (means for expressing gradation), 23, 24,
25 ... ROM table, 26 ... Laser diode driver, 27 ... Laser diode.

Claims (3)

[Claims] 1. A gradation correction unit for correcting gradation to desired gradation data to cope with various gradations, and a unit for dividing one pixel into a plurality of regions and expressing the gradation by combining with a matrix. And a detection means for detecting the image density formed on the photoconductor, in a digital image forming apparatus, based on the detection value from the detection means, so as to compensate for variations in gradation characteristics in image formation. A digital image forming apparatus comprising: a control unit for compensating a gradation variation by creating a γ correction conversion table according to a gradation characteristic. 2. A gradation correction means for correcting gradation to desired gradation data in order to correspond to various gradations, and a means for dividing one pixel into a plurality of regions and expressing the gradation by combining with a matrix. In the digital image forming apparatus having a detection unit for detecting the image density formed on the photoconductor, based on the detection value from the detection unit, to control the light amount of the laser for exposing the photoconductor, A digital image forming apparatus comprising a control means for compensating for gradation variation by creating a γ-correction conversion table corresponding to the gradation characteristic so as to compensate for gradation characteristic variation in image formation. . 3. Gradation correction means for performing gradation correction to desired gradation data in order to correspond to various gradations, and means for expressing gradation by combining one pixel into a plurality of areas and combining with a matrix. And a digital image forming apparatus including a detection unit that detects the image density formed on the photoconductor, and a control unit that creates a γ correction conversion table according to the designated image processing method. Characteristic digital image forming apparatus.