JP3197017B2 - Digital imaging method - Google Patents

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 method in a digital copying machine, a digital printer and the like.

[0002]

2. Description of the Related Art Various electrophotographic image forming apparatuses such as a laser printer for reproducing an image by driving a laser unit based on image data converted into a digital value have been put into practical use. Various digital image forming methods for faithfully reproducing images have been proposed.

As this kind of digital image forming method, an area gradation method using a dither matrix or changing a laser pulse width (light emitting time) or light emitting intensity to change a laser light amount (= light emitting time × intensity) is used. A multi-valued laser exposure method (pulse width modulation method, intensity modulation method) or the like that expresses the gradation for one dot printed by printing is known (for example, JP-A-62-91077, JP-A-62-39972, JP-A-62-188562 and JP-A-61-22597), and a multi-valued dither method in which dither is combined with a pulse width modulation method or an intensity modulation method. Are known.

According to this type of gradation method, an image density having a gradation corresponding to the gradation of image data to be reproduced on a one-to-one basis should be able to be reproduced in principle. Various factors such as the photosensitive characteristics of the photoreceptor, the characteristics of the toner, and the use environment are intricately intertwined, and the original document density to be reproduced is not exactly proportional to the reproduced image density (hereinafter simply referred to as image density). As shown schematically in FIG. 5, a characteristic B deviated from a proportional characteristic A that should be originally obtained is shown. Such a characteristic is generally called a γ characteristic, and is a major factor in lowering the fidelity of a reproduced image particularly for a halftone original.

Therefore, in order to improve the fidelity of a reproduced image, conventionally, the read document density is converted using a predetermined gamma correction conversion table, and a digital image is formed based on the converted document density. Thus, so-called γ correction is performed so that the document density and the image density satisfy a linear relationship (characteristic A). As described above, normally, by performing the γ correction, an image can be faithfully reproduced according to the level of the document density.

On the other hand, another factor that affects the image density is that the amount of toner adhered to the photoconductor changes during development due to changes in the external environment such as temperature and humidity due to the characteristics of the photoconductor and toner. There is. In general,
In a high-temperature and high-humidity environment, the amount of adhered toner increases, the gradient of the γ characteristic at low and medium densities increases, and the reproduced image becomes darker. It is known that the inclination of the γ characteristic becomes small and the reproduced image becomes thin.

As described above, there is a problem that the density of a reproduced image changes due to a change in environment. In order to solve this problem and stabilize the image density, a general electrophotographic copying machine or printer has a maximum. A density control for controlling the image density to be constant is performed.

A method generally used as the density control will be described with reference to a block diagram of an image forming section including a photosensitive drum 41 and a developing roller 45r in FIG.

The photosensitive drum 41 is provided with a charging charger 43 having a discharge potential V _C facing the photosensitive drum 41. Grid grid voltage generating unit in the electric charger 43 (hereinafter, referred to as V _G generating unit.) 214 grid voltage V _G is applied by. The surface potential V Control of the surface potential V _O of the photosensitive drum 41 by the V _O sensor 44
Based on the detected value of the _{of O,} performed by adjusting the grid voltage V _G.

First, before laser exposure, a negative surface potential V is applied to the photosensitive drum 41 by the charging charger 43.
_O is also for the head phenomenon prevents the development bias voltage generating unit (hereinafter, referred to as V _B generating unit.) 21
5, a low potential negative developing bias voltage V _B (| V _O |> | V _B |) is applied to the developing roller 45r. At this time, the developing sleeve surface potential is also changed to the developing bias voltage V.
Equals _B.

[0011] potential of the photosensitive drum 41 is changed to the electrostatic latent image potential V _I during exposure to the maximum amount of light from the surface potential V _O decreases by the laser exposure. When the electrostatic latent image potential V _L is a potential lower than the developing bias voltage V _B, the photosensitive drum 4
1 adheres to the toner. The toner adhesion amount is larger as the difference of these developing bias voltage V _B and an electrostatic latent image potential V _L is large. Therefore, if the change of the surface potential V _O and the developing bias voltage V _B, the difference between the developing bias voltage V _B and an electrostatic latent image potential V _L is changed, it is possible to vary the amount of toner adhesion, controlled concentration can do.

This type of concentration control is performed by changing the potentials V _O and V _B manually or automatically to keep the maximum concentration constant.

In the automatic density control, first, a reference toner image serving as a reference for density control is formed on the surface of the photosensitive drum 41, and reflected from the reference toner image by an AIDC sensor 203 provided near the photosensitive drum 41. Detect light intensity. The detection value detected by the AIDC sensor 203 is input to the printer control unit 201,
In accordance with the comparison result between the detection value and the predetermined value from the AIDC sensor 203, the printer control unit 201 drives the V _G generating unit 214 and V _B generating unit 215.

The above operation is repeated until the toner adhesion amount reaches a predetermined value.

[0015]

However, when the density control is performed to keep the density of the reproduced image constant and the photosensitive drum surface potential V _O and the developing bias voltage V _B are changed as described above, the γ characteristic is reduced. Is greatly affected. An example of this is shown in FIG.

FIG. 7 is a graph showing the development characteristics NN in a reference environment and the development characteristics LL in a low temperature and low humidity environment. The solid line indicates the reference environment development characteristic NN, and the broken line indicates the low-temperature and low-humidity environment development characteristic LL. The reference potential setting in the reference _{environment, = (V O, V B} ) (- 700V,
-500 V). The maximum intensity of the laser used for exposure is 1.0 mW. While the potential setting, decreases when the environment changes to a low temperature and low humidity, and the developing characteristics NN varies in LL, the maximum concentration C1 represented by a point of intersection between the developing characteristic curve and the photosensitive member surface potential V _I to C2 I do. Therefore, if the environment becomes low temperature and low humidity when the density control is not performed, the density of the reproduced image becomes low.

Here, when the environment changes to low temperature and low humidity, each potential is set to (V
_{O, V B) = (-} 800V, change the settings -600 V). Then, the development characteristic LL in the low-temperature and low-humidity environment indicated by the broken line is shifted to the left and becomes the development characteristic LL ′ indicated by the one-dot chain line as indicated by the arrow in the figure. and intersection coincide in the surface potential V _I, compensation is performed for the maximum density level. Although not shown, when the environment changes to high temperature and high humidity, the amount of toner adhered increases and the image density increases in contrast to the case of LL. Therefore, in order to constantly compensate for the maximum density, for example, , in the case of high temperature and high humidity _{(V O, V B) =} (- 600V, -400
It is necessary to lower the potential setting as in V) and shift the characteristic curve to the right.

However, as is apparent from FIG. 7, the curve shape from the minimum density to the maximum density is significantly different between the reference environment development characteristic NN and the low temperature and low humidity environment development characteristic LL 'after the maximum density compensation. That is, if the image density is compensated by changing each of the potentials V _O and V _B , the γ characteristic will fluctuate.

FIG. 8 shows the respective γ characteristics after the maximum image density compensation in the low temperature and low humidity environment LL, the high temperature and high humidity environment HH, and the high temperature and high humidity environment SHH together with the γ characteristics of the reference environment NN.

As described above, when the density control is performed according to the use environment, the γ characteristic itself changes. For example, in the case of the intensity modulation method, as shown in FIG.
Conventionally, the laser emission intensity has been nonlinearly controlled in accordance with the γ characteristic by using a γ correction conversion table corresponding only to the γ characteristic of the reference environment NN. However, in the γ correction using a single γ correction conversion table, as shown in FIG.
In an environment other than the reference environment NN, correct gamma correction cannot be performed.

The present invention utilizes the characteristics of the photoreceptor,
An object of the present invention is to provide a digital image forming method capable of compensating for fluctuations in the γ characteristic generated as a result of the density control control and obtaining a reproduced image having a constant gradation reproducibility on a document. I have.

[0022]

In order to achieve the above object, a digital image forming method according to the present invention provides a gradation expression by changing the amount of light irradiated on a photoreceptor by an exposure means in accordance with image information. In the electrophotographic digital image forming method to be performed, at least one of a surface potential of the photoconductor before exposure by the exposure unit and a developing bias voltage previously applied to a developing device is set prior to image formation. In addition to controlling the density by changing the value, when the set values of the surface potential of the photoreceptor and the developing bias voltage are changed in a direction in which the absolute value is increased, the exposure of the exposing means is performed. By changing the exposure gain of the exposure means in the direction in which the intensity decreases, the gradation variation accompanying the change is compensated.

[0023]

In the electrophotographic digital image forming method according to the present invention, when the density control is performed by changing the surface potential of the photoreceptor before exposure or the developing bias voltage, the intensity or gain of the exposure means is also changed at the same time. Γ correction is performed.
Here, prior to image formation, the density control is performed by changing at least one of a surface potential of the photoconductor before exposure by the exposure unit and a development bias voltage previously applied to a developing device. When the set values of the surface potential of the photoreceptor and the developing bias voltage are changed in a direction in which the absolute value is increased, the exposure is performed in a direction in which the exposure intensity of the exposure unit is reduced. By changing the exposure gain of the means, the gradation fluctuation accompanying the change is compensated.

Specifically, regarding the characteristics of the photoreceptor used in the present invention, that is, when the maximum intensity of the laser is changed,
The fact that the γ characteristic fluctuates will be described. Here, the “maximum intensity” is the intensity of the exposure means such as a laser diode at the maximum density.

As shown in FIGS. 8 to 10, when the environment changes and the maximum image density is kept constant, the γ characteristic fluctuates. By the way, each γ characteristic shown in FIG. 8 is most characterized in that its rise is steep or gentle.
Therefore, in the example of FIG. 8, γ indicated by low temperature and low humidity LL
The characteristics have a more gradual rise, and the γ characteristics indicated by high-temperature and high-humidity HH have a more rapid rise,
If the rise of the γ characteristic shown by can be made even more steep, the γ characteristic shown by the reference environment NN can be approximated.

In FIG. 8, the γ characteristic fluctuates because the surface potential V _O of the photosensitive drum 41 and the developing bias voltage V _B are changed. 11 to 13 show that the γ characteristics fluctuate even when the intensity is changed.

FIG. 11 is a graph showing a change in surface potential when a charged photoreceptor is irradiated with a laser whose intensity is changed under a constant light emission time. If the surface potential drops to a certain extent, the saturation occurs, indicating that the image density does not change much.

On the other hand, FIG. 12 shows that the maximum intensity is 0.8 mW, 1.
FIG. 13 is a graph showing the surface potential of the photosensitive drum 41 when the laser intensity is changed from 0 to the maximum intensity of each of 0 mW, 1.1 mW, and 1.2 mW. 7 is a graph showing the relationship between the original density and the image density (γ characteristic) considered.

When the maximum intensity, that is, the maximum light amount is changed in this manner, it can be seen that the γ characteristic fluctuates in the same manner as when the potentials V _O and V _B are changed.

The present invention utilizes this phenomenon to suppress the fluctuation of the γ characteristic caused by changing the surface potential V _O before exposure of the photosensitive drum and the developing bias voltage V _B in order to keep the maximum density constant. Things.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital color copying machine according to an embodiment of the present invention will be described below with reference to the accompanying drawings.

(A) Configuration of Digital Color Copier FIG. 1 is a longitudinal sectional view showing the overall configuration of a digital color copier according to an embodiment of the present invention. The digital color copier includes an image reader unit 100 that reads a document image,
The main unit 200 reproduces an image read by the image reader unit 100.

In FIG. 1, a scanner 10 includes an exposure lamp 12 for irradiating a document, a rod lens array 13 for condensing light reflected from the document, and a contact type CCD for converting the condensed light into an electric signal. A color image sensor 14 is provided. The scanner 10 uses the motor 1 when reading a document.
1 to move in the direction of the arrow (sub-scanning direction) to scan the original placed on the platen 15. The image of the document surface irradiated by the exposure lamp 12 is photoelectrically converted by the image sensor 14. The multi-valued electrical signals of the three colors of R, G, and B obtained by the image sensor 14 are read by the read signal processing unit 20 into any of yellow (Y), magenta (M), cyan (C), and black (K). It is converted into such 3-bit gradation data. Next, the print head unit 31
Performs correction (γ-correction) according to the gradation characteristics of the photoconductor on input gradation data and dither processing as needed, and then converts the corrected image data into digital / analog data (hereinafter referred to as “digital / analog conversion”). , D / A conversion) to generate a laser diode drive signal, and drive the laser diode 221 with this drive signal.

The laser diode 2 corresponding to the gradation data
The laser beam generated from 21 is, as shown in FIG.
Photoreceptor drum 41 that is driven to rotate via reflection mirror 37
Is exposed. As a result, an image of the document is formed on the photoconductor of the photoconductor drum 41. The photosensitive drum 41 is irradiated by an eraser lamp 42 before being exposed for each copy, and is charged by a charging charger 43. When the photosensitive drum 41 is exposed in this uniformly charged state,
An electrostatic latent image is formed thereon. Only one of the yellow, magenta, cyan, and black toner developing units 45a to 45d is selected to develop the electrostatic latent image on the photosensitive drum 41. The developed image is transferred by the transfer charger 46 onto a copy paper wound around the transfer drum 51. The density of the toner image to be developed is optically detected by the AIDC sensor 203.

The above printing process is repeated for yellow, magenta, cyan and black. At this time, the scanner 10 repeats the scanning operation in synchronization with the operations of the photosensitive drum 41 and the transfer drum 51. Thereafter, the copy paper is separated from the transfer drum 51 by operating the separation claw 47, is fixed through the fixing device 48, and is discharged to the discharge tray 49. The copy paper is fed from a paper cassette 50, and the leading end thereof is chucked by a chucking mechanism 52 on the transfer drum 51, so that no positional deviation occurs during transfer.

FIGS. 2 and 3 are block diagrams of the entire digital color copying machine of the present embodiment.

As shown in FIG. 2, the image reader unit 10
0 is controlled by the image reader control unit 101. The image reader control unit 101 controls the exposure lamp 12 via a drive input / output device (hereinafter, drive I / O) 103 according to a position signal from a position detection switch 102 indicating the position of the document on the platen 15. The scan motor driver 105 is controlled via a drive I / O 103 and a parallel input / output device (hereinafter, referred to as a parallel I / O) 104. The scan motor 11 is driven by a scan motor driver 105.

On the other hand, the image reader control unit 101 is connected to the image control unit 106 via a bus. The image control unit 106 is connected to the CCD color image sensor 14 and the image signal processing unit 20 via a bus. The image signal from the image sensor 14 is input to an image signal processing unit 20, which will be described in detail later, and processed.

As shown in FIG. 3, the main body unit 200 includes a printer control unit 201 for controlling general copying operations and a print head control unit 202 for controlling print heads. The printer control unit 201 includes a V _O sensor 4 for detecting the surface potential V _O of the photosensitive drum 41 immediately before exposure.
4. A _VL sensor 60 for detecting the surface potential _VL of the photosensitive drum 41 immediately after exposure, and an AIDC sensor 2 for optically detecting the density of a toner image attached to the surface of the photosensitive drum 41
03, ATDC sensor 204 for detecting toner concentration in developing units 45a to 45d and temperature / humidity sensor 2
Analog signals from various sensors 05 are input. Various data are input to the printer control unit 201 via the parallel I / O 207 by key input to the operation unit key 206. The printer control unit 201 is connected to a control ROM 208 in which a control program is stored and a data ROM 209 in which various data are stored, and the printer control unit 201 determines the control based on the data in these ROMs as described later in detail. I do.

The printer control unit 201 is provided with each sensor 203
To 205, operation unit keys 206 and data ROM 209
Control the copy control unit 210 and the display panel 211 in accordance with the contents of the control ROM 208,
In addition, automatic by AIDC sensor 203, or
To perform the manual density compensation control by the input to the operation panel 206, and controls the V _G generating unit 214 and V _B generating unit 215 via the parallel I / O 212 and the drive I / O213. The printer control unit 201 sends the numerical data of the grid voltage V _G (surface potential V _O) and the developing bias voltage V _B to the print head controller 202.

The print head control unit 202 controls the control RO
It operates according to a control program stored in M216, is connected to the image signal processing unit 20 of the image reader unit 100 via an image data bus, and transmits an image signal received via the image data bus. Based on, γ
Data ROM 21 storing correction conversion table
7 is performed with reference to the contents of FIG. 7, and furthermore, when the multi-valued dither method is used as the gradation expression method, dither processing is performed, and drive I / O 218 and parallel I / O 219 are performed.
The laser diode driver 220 is controlled via the. The emission of the laser diode 221 is controlled by a laser diode driver 220.

The print head control unit 202 is connected to the printer control unit 201, the image signal processing unit 20, and the image reader control unit 101 via a bus and is synchronized with each other. In particular, in the present embodiment, the laser diode driver 220 operates based on numerical data of the grid voltage V _G (surface potential V _O ) and the developing bias voltage V _B sent from the printer control unit 201 as described later in detail. , The laser intensity of the laser diode 221 is changed.

(B) Image Signal Processing FIG. 4 is a block diagram for explaining the flow of processing of image signals from the CCD 14 to the print head control unit 202 via the image signal processing unit 20. Hereinafter, a read signal process for processing an output signal from the CCD color image sensor 14 and outputting gradation data will be described with reference to FIG.

In the image signal processing section 20, an image signal photoelectrically converted by the CCD color sensor 14 is converted into an R / G / B multi-value digital signal by an analog / digital conversion (hereinafter, A / D conversion) unit 21. Converted to image data. Each of the converted image data is subjected to predetermined shading correction by a shading correction circuit 22. Since the image data subjected to the shading correction is the reflection data of the original, the log conversion circuit 23
The image is converted into density data of an actual image by performing an og conversion. In addition, the under color remove / ink printing circuit 24
To remove the extra black color and generate true black data K from the R, G, B data. Then, the masking processing circuit 25 converts the data of the three colors R, G, and B into data of the three colors Y, M, and C. The density correction processing for multiplying the Y, M, and C data thus converted by a predetermined coefficient is performed by the density correction circuit 26, and the spatial frequency correction processing is performed by the spatial frequency correction circuit 27, and then output to the print head control unit 202. I do.

In the print head control unit 202,
The image signal processed by the image signal processing unit 20 is represented by γ
The conversion unit 28 performs γ conversion based on the conversion table for γ correction in the data ROM 217, and the dither processing unit 29 uses the dither threshold data in the data ROM 217 when the multi-valued dither method is adopted as the gradation expression. Processing is performed, and the processed digital image signal is output to the laser diode driver 220.

FIG. 25 shows a laser diode driver 22.
0 shows a block diagram. As shown in FIG. 25, the laser diode driver 220 includes a D / A converter 254, a driving amplifier 260, a gain switching unit 255, and a gain switching signal generation circuit 256.

The digital image signal output from the print head control unit 202 is converted into an analog voltage signal by the D / A converter 254 and then output to the laser diode 221 via the driving amplifier 260 and the gain switching unit 255. You. In the gain switching unit 255, the amplifier 2
The input terminal connected to the output terminal 60 is connected to the ground through a circuit in which eight resistors R1 to R8 each having a predetermined resistance value are connected in series. The input terminal of the gain switching unit 255 is connected to its output terminal via a switch SW1, and the connection point between the resistor R1 and the resistor R2 is connected to the switch SW1.
2 connected to its output terminal, and a resistor R2 and a resistor R3
Is connected to its output terminal via a switch SW3, and the connection point between the resistors R3 and R4 is connected to its output terminal via a switch SW4. The resistance R4 and the resistance R
5 is connected to its output terminal via a switch SW5, the connection point between the resistors R5 and R6 is connected to its output terminal via a switch SW6, and the connection point between the resistors R6 and R7 is connected to a switch SW7. Connected to its output end via
The connection point between the resistor R7 and the resistor R8 is connected to its output terminal via a switch SW8. Here, each of the switches SW1 to SW8 is selectively turned on / off according to a gain switching signal output from the gain switching signal generation circuit 256.
Therefore, if the switch SW1 is turned on, the attenuation of the gain switching unit 255 is minimum, that is, the amplification degree of the amplifier circuit including the amplifier 260 and the gain switching unit 255 is maximum. Also, if the switch SW8 is turned on, the attenuation of the gain switching unit 255 becomes maximum,
That is, the amplification degree of the amplifier circuit is minimized.

On the other hand, the data of the gain setting value to be set in the gain switching section 255 is input from the printer control section 201 to the gain switching signal generation circuit 256, and in response to this, the gain switching signal generation circuit 256 In order to selectively turn on one of the switches SW1 to SW8 so that the amplification degree of the amplifier circuit including the gain switching unit 255 becomes the data of the input gain setting value, each of the switches SW1 to SW8 is turned on. To output a gain switching signal. The gain setting value is a value determined according to a detection signal from various sensors or an input signal from the operation panel 206.

Therefore, the digital image signal sent from the image signal processing unit 20 via the print head control unit 202 and the parallel I / O 219 is converted into an analog voltage signal by the D / A converter Amplification is performed at an amplification degree according to the gain setting value input from the control unit 201. The amplified analog drive signal is output to the laser diode 221, thereby causing the laser diode 221 to emit light. In the present embodiment, as described in detail below, the amplifier 26 is determined according to environmental conditions and in accordance with a gain set value input from the printer control unit 201.
Since the amplification degree of the amplifier circuit composed of 0 and the gain switching unit 255 is changed, when the intensity modulation method is used, for example, the laser intensity of the laser diode 221 is changed in all gradation regions as shown in FIG. Let me. In the following, for convenience of description, the expression “change the maximum intensity” or “select the maximum intensity” is used, but in practice, as shown in FIG. The "maximum intensity" is selectively changed using the part 255, and the laser intensity is also changed in the intermediate density region.

In the embodiment of FIG. 25, the analog image signal is amplified according to the gain setting value. However, the present invention is not limited to this, and the light emission pulse of the laser diode 221 may be adjusted according to the gain setting value. The width may be changed, or the supply current value for causing the laser diode 221 to emit light may be changed.

(C) Gradation Expression Method A multi-valued laser exposure method and a multi-valued dither method, which are gradation expression methods used in the present invention, will be described.

(C-1) Multi-level laser exposure method (c-1-1) Intensity modulation method The intensity modulation method of the multi-level laser exposure method as a gradation expressing means will be described.

The intensity modulation method is a gradation expression method in which the density of one dot to be printed is changed stepwise, and the intensity of the laser for a constant light emission time is changed in several steps according to a multi-level signal from the image reader unit 100. And irradiates the photoconductor with a different amount of laser light at each stage.
The density of one dot can be multi-valued.

FIG. 14 shows that the intensity is 8 by the intensity modulation method.
5 is a graph schematically showing a potential of a cross section of a one-dot latent image by a laser that has been multivalued in stages. As shown in FIG. 14, the gradation expression by the intensity modulation method is realized by a change in the amount of adhered toner, that is, a change in image density because the latent image potential changes stepwise.

(C-1-2) Pulse width modulation method The pulse width modulation method of the multilevel laser exposure method as a gradation expressing means will be described.

The pulse width modulation method is a gradation expression method in which the area of one dot to be printed is changed stepwise, and the light emission time of a laser having a constant intensity is set in several steps according to a multi-level signal from an image reader. Since the photoconductor is irradiated with laser beams of different amounts depending on the respective stages, the printing area of one dot can be multivalued.

FIG. 15 is a graph schematically showing a latent image potential in a latent image section of one dot by a laser having an intensity of 1.0 mW and having a multi-valued light emission time in eight steps by a pulse width modulation method. As shown in FIG. 15, the gradation expression by the pulse width modulation method is realized by a change in the toner attachment area, that is, a change in the reproduced image area because the latent image area changes stepwise.

(C-2) Multi-valued dithering method A multi-valued dithering method for performing gradation expression by combining the above-described multi-valued laser exposure method (intensity modulation method or pulse width modulation method) with the dither method. explain.

This multi-valued dither method is, for example, (N × M)
Each of the dots in this block is multi-valued into (L) values, thereby expressing (N × M × L + 1) gradations. The above-mentioned pulse width modulation method or intensity modulation method is used as means for multi-leveling. Therefore, when the pulse width modulation method or the intensity modulation method is simply used, one dot of a printed image by laser exposure becomes one pixel, and when the multi-valued dither method is used, the printed image by laser exposure (N ×
M) A dot area is one pixel.

FIG. 16 is composed of (2 × 2) dots.
FIG. 3 is a diagram illustrating an example of a multi-valued dither in which one dot is multi-valued into eight values. This dither is (2 × 2 × 8 + 1) 33
The gradation can be expressed. In addition, regarding 1 to 32 indicating the threshold values of these multi-valued dithers, the gradation expression when the intensity modulation method is used for multi-valued conversion of one dot is not an area gradation but a density gradation, Because it is difficult to show, it is represented by a strip shape as shown in FIG. 16 for convenience.

(D) Control Flow FIGS. 17 to 24 show control flows executed by the printer control unit 201 of the digital color copying machine of the present embodiment according to the present invention. FIGS. 17 and 18 are flowcharts in the case of manually compensating for the density change due to the environmental change. FIGS. 19 to 24 show the density change due to the environmental change detected by the AIDC sensor 203 and automatically change the density. It is a flowchart in the case of performing compensation,
In any case, necessary gamma correction is performed along with the density control.

(D-1) Manual Density Control and γ Correction FIG. 17 shows a main routine of the digital color copying machine when the density compensation is performed manually.

First, initialization such as parameter initialization is performed (step S1 (hereinafter, "step" is omitted)), and an internal timer is started (S2). Then, after executing a routine (see FIG. 18) for manually performing density (ID) control by key input to the operation panel 206 (S3), a copy operation is started (S3).
4). When the internal timer expires (S5), the process returns to S2.

FIG. 18 is a flowchart of a manual density control routine (S3 in FIG. 17) of the present embodiment according to the present invention, wherein a certain maximum density is set according to the environment code selected by the user according to the usage environment. In addition to selecting the surface potential V _O and the developing bias voltage V _B necessary for obtaining the data, the numerical data of the surface potential V _O and the developing bias voltage V _B is sent to the print head control unit 202 to determine the maximum intensity of the laser used for exposure. It is characterized in that an appropriate gamma correction is performed by changing it. In this embodiment, the environment code has four levels, low temperature and low humidity LL, reference environment NN, high temperature and high humidity HH, and maximum temperature and high humidity SHH.

First, an environment code corresponding to the temperature and humidity is selected by key input to the operation panel 206 (S1).
1).

When the environment code is LL (YE in S12)
S) The surface potential of the photosensitive drum 41 is set at V
_O1 (−800 V in this embodiment) and V _B1 (−600 V in this embodiment) are selected as the developing bias voltage (S1).
3). When each of the potentials V _O1 and V _B1 is selected, the laser intensity P1 required to prevent the γ characteristic shown by LL in FIG.
(0.8 mW in this embodiment) is selected (S14). The γ characteristic due to the laser intensity P1 is originally indicated by LL in FIG. 13, and the γ characteristic in which the rising is sharp on the low density side shown by LL in FIG. 8 and the γ characteristic on the low density side shown in FIG. 8 is suppressed from fluctuating from the γ characteristic indicated by NN in FIG. Therefore,
9 according to the conversion table for γ correction corresponding to NN in FIG.
A linear target gradation characteristic is realized by the light emission characteristic of (1).

When the environment code is NN (YE in S15)
S), V _O2 (−700 V in this embodiment), which is a reference potential setting, is selected as the surface potential of the photosensitive drum 41, and V _B2 (−500 V in this embodiment) is selected as the developing bias voltage (S16). When each of the potentials V _O2 and V _B2 is selected, FIG.
Then, the laser intensity P2 (1.0 mW in this embodiment) that realizes the γ characteristic indicated by NN is selected (S17). The γ characteristic according to the laser intensity is indicated by NN in FIG. 13, and this γ characteristic is the same as the γ characteristic indicated by NN in FIG. Accordingly, a linear target gradation characteristic is realized by the light emission characteristics of FIG. 9 according to the conversion table for γ correction corresponding to NN of FIG.

When the environment code is HH (YE in S18)
S) The surface potential of the photosensitive drum 41 is set to V _O3 in order to compensate for the amount of toner adhering which is larger than in the reference environment.
(-600 V in this embodiment) and V _B3 (-400 V in this embodiment) are selected as the developing bias voltage (S1).
9). When each of the potentials V _O3 and V _B3 is selected, the laser characteristic P3 required to prevent the fluctuation from the γ characteristic indicated by NN by suppressing the γ characteristic indicated by HH in FIG.
(1.1 mW in this embodiment) is selected (S20). The γ characteristic due to the laser intensity P3 is originally indicated by HH in FIG. 13, and the rise on the low concentration side indicated by HH in FIG. 8 is more gradual than that of NN and is indicated by HH in FIG. Combined with the γ characteristic whose rise on the low concentration side is steeper than that of the NN, the fluctuation from the γ characteristic indicated by NN in FIG. 8 is suppressed. Accordingly, a linear target gradation characteristic is realized by the light emission characteristics of FIG. 9 according to the conversion table for γ correction corresponding to NN of FIG.

When the environment code is SHH (Y in S21)
ES) In order to compensate for the larger amount of toner adhered than in the standard environment, V _O4 (−500 V in the present embodiment) is used as the surface potential of the photosensitive drum 41 and V _B4 (−500 V in the present embodiment) is used as the developing bias voltage. In the example, -300V) is selected (S22). When each of the potentials V _O4 and V _B4 is selected, the characteristic originally becomes the γ characteristic shown by SHH in FIG.
The laser intensity P4 (1.2 mW in the present embodiment) necessary to keep the γ characteristic indicated by NN from changing is selected (S2).
3). The γ characteristic based on the laser intensity P4 is originally indicated by SHH in FIG. 13, and the γ characteristic whose rise on the low concentration side indicated by SHH in FIG.
In combination with the γ characteristic whose rise on the low concentration side indicated by SHH is steeper than that of HH, fluctuation from the γ characteristic indicated by NN in FIG. 8 is suppressed. Accordingly, a linear target gradation characteristic is realized by the light emission characteristics of FIG. 9 according to the conversion table for γ correction corresponding to NN of FIG.

If the environment code is not one of the above, the process returns to S11 to start over.

In the above description of the routine, since the intensity modulation method is used for gradation expression, the selected laser intensity indicates the maximum intensity, and when the pulse width modulation method is used, a constant intensity is used. Exposure is performed by the laser of the above-mentioned, so that the selected laser intensity is the constant intensity.

(D-2) AIDC concentration control and γ
FIG. 19 shows a main routine of the digital color copying machine in the case where the density compensation by the AIDC sensor 203 is performed.

First, initialization such as initialization of parameters is performed (S51), and an internal timer is started (S5).
2). Then, after executing a routine (see FIGS. 20 to 24) for automatically controlling the density (ID) by the AIDC sensor 203 (S53), the copying operation is started (S54). When the internal timer expires (S55), S
Return to 52.

FIGS. 20 and 21 show the AID according to the present invention.
20 is a flowchart of a first embodiment of an automatic density control routine (S53 in FIG. 19) by the C sensor 203, which detects potential changes due to environmental changes and selects potentials V _O and V _B for compensating for a reference density; It is characterized in that numerical data of the potentials V _O and V _B are sent to the print head control unit 202, and appropriate γ correction is performed by changing the maximum intensity of the laser used for exposure.

First, it is checked whether the copy start key is turned on. If the copy start key is turned on, that is, if the copy start key is on-edge (YES in S81), the AIDC flag F for performing automatic density control is set. It is set to 1 (S82), and V _O2 = −700 V and V _B2 = −500 V are used as the set potentials V _O and V _B for forming a reference toner image for density detection on the surface of the photosensitive drum 41, respectively. (S83) Further, the laser diode is caused to emit light with the maximum light amount so as to have the maximum density under the set potential, and a reference toner image is formed (S85).

If the copy start key is not on-edge (NO in S81), S82 of the previous routine is executed.
If the flag F is set to 1 (YE in S84)
S), a reference toner image is created based on the previously set potential (S85). If the copy start key is not on-edge (NO in S81), and if flag F is 0 (S
NO at 84), no automatic density control is performed.

The above-mentioned reference toner image is transferred to the AIDC sensor 20.
It is determined by the timer whether or not the detection position of the AIDC sensor 3 has been reached (YES in S86).
03 detects the density of the reference toner image and inputs the numerical value to the printer control unit 201 (S87). The toner image is A
If it has not yet reached the detection position of the IDC sensor 203 (NO in S86), the process returns.

In S88 to S97, it is determined whether or not the maximum density (detected density) detected in S87 is equal to the reference density. If the detected density is higher than the reference density, the reference pattern in the next loop of this flow is determined. In order to reduce the density, the set potential is lowered by one step, and if the detected density is lower than the reference density, an operation of increasing the set potential by one step is performed to increase the density of the reference pattern in the next loop of this flow. . Of course, if the detected concentration is equal to the reference concentration, the maximum concentration compensation based on the set potential is appropriately performed, and the setting of the potential ends, and γ is set based on the set potential.
Select the maximum intensity of the laser for proper correction.

When the density detected by the AIDC sensor 203 is
When it is higher than the reference density (YES in S88), when the set potentials are V _O2 and V _B2 , respectively (YE in S89).
S), the surface potential V _O3 (= -600V), the developing bias voltage V _B3 (= -400V) select (S90), YE each setting potential in V _O3, if a V _B3 (S91, respectively
S), V _O4 (= −500 V), V _B4 (= −300 V) are selected (S 92), and the routine returns.

When the density detected by the AIDC sensor 203 is
When the density is lower than the reference density (YES in S93), when the set potentials are V _O2 and V _B2 , respectively (YE in S94).
S), V _O1 (= −800 V) and V _B1 (= −600 V) are selected (S 95), and the routine returns.

The density detected by the AIDC sensor 203 is
It is not higher than the reference density (NO in S88), and if the detected density is not lower than the reference density (N in S93)
O), it is determined that the detected concentration is equal to the reference concentration, and in the laser intensity selection routine (see FIG. 22), the laser intensity according to each of the currently set potentials V _O and V _B is selected (S).
96), resets the flag F to 0 (S97), and returns.

Since the set potential is set to four levels in the flow of FIG. 21, the detected density is lower than the reference density even though each of the set potentials V _O4 and V _B4 that _minimizes the reference pattern density is selected. If it is still darker,
The set potentials V _O1 , V _{B1 for maximizing} the reference pattern density
If the detected density is still lower than the reference density even though is selected, the potential setting is terminated with V _O4 , V _B4 and V _O1 , V _B1 respectively, and the printing operation (S54) is started. However, the present invention is not limited to this flow, and if a problem arises with this flow, the potential may be set more finely.

FIG. 22 is a detailed flowchart of the laser intensity selection routine shown in S96 of FIG. 21, which will be described below. Since the laser intensity selected by the surface potential V _O of the photosensitive drum 41 and the developing bias voltage V _B in this flow is the same as that described in FIG. 18, the detailed description of the selected laser intensity is omitted. I do.

As the surface potential of the photosensitive drum 41, V
_O1 (= −800 V) and V _B1 (=
When −600V is selected (YE in S100)
S) The same as when it is determined that the environment is low temperature and low humidity LL, and the laser intensity P1 for γ correction is selected (S).
101).

As the surface potential of the photosensitive drum 41, V
_O2 (= -700 V) and V _B2 (=
When (−500V) is selected (YE in S102)
S), which is the same as when the environment is determined to be the reference environment NN, and selects the laser intensity P2 for γ correction (S103).

As the surface potential of the photosensitive drum 41, V
_O3 (= -600 V) and V _B3 (=
When (−400V) is selected (YE in S104)
S) The same as when it is determined that the environment is high temperature and high humidity HH, and the laser intensity P3 for γ correction is selected (S).
105).

The surface potential of the photosensitive drum 41 is V
_O4 (= -500 V) and V _B4 (=
When (−300 V) is selected (S100, S10
2, S104: NO), which is the same as the determination that the environment is the highest temperature and high humidity SHH, and selects the laser intensity P4 for γ correction (S106).

FIGS. 23 and 24 show the AIDC sensor 20.
3 (S5 in FIG. 19)
3) is a flowchart of the second embodiment of the present invention, in which only one conversion table for γ correction is prepared, and each set potential V _O , V _B for detecting a density change due to an environmental change and compensating for a reference density is selected. At the same time, numerical data of the set potentials V _O and V _B are sent to the print head control unit 202 to detect the density change due to the environmental change and select the potentials V _O and V _B for compensating for the reference density. Numerical data of each of the set potentials V _O and V _B is sent to the control unit 202, and the maximum γ of the laser used for exposure is changed to perform appropriate γ correction.

Note that S18 in this flowchart is
In steps 1 to S187, a reference pattern image is formed with a potential set to a predetermined value, and an environment code is selected based on the density of the reference pattern image. The content is exactly the same as S87, and the description is omitted. Further, the laser intensity selected by the surface potential V _O of the photosensitive drum 41 and the developing bias voltage V _B in this flow is the same as that described with reference to FIG. 18, so a detailed description of the selected laser intensity is omitted. I do.

Hereinafter, the remaining steps S188 to S200 will be described.

Subsequently, an environment code indicating the environment is selected based on the density detected by the AIDC sensor 203 and a preset reference density (S188). That is, if the detected concentration is lower than the reference concentration, the environment code becomes LL,
If the detected density is equal to the reference density, the environment code is NN. If the detected density is higher than the reference density, the environment density is HH. If the detected density is higher than the reference density, the environment code is SHH.

When the environment is determined to be low temperature and low humidity,
That is, when the environment code is LL (YE in S189)
S), the surface potential of the photosensitive drum 41 as V _O1 (= −8)
00 V) and V _B1 (= −600) as a developing bias voltage.
V) is selected (S190), and the laser intensity P1 for γ correction is selected (S191).

When the environment is determined to be the reference environment,
That is, when the environment code is NN (YE in S192)
S), the surface potential of the photosensitive drum 41 as V _O2 (= −7)
00V) and V _B2 (= −500) as the developing bias voltage.
V) is selected (S193), and the laser intensity P2 for γ correction is selected (S194).

When it is determined that the environment is high temperature and high humidity,
That is, when the environment code is HH (YE in S195)
S), the surface potential of the photoconductor drum 41 as V _O3 (= −6)
00V) and V _B3 (= −400) as the developing bias voltage.
V) is selected (S196), and a laser intensity P3 for γ correction is selected (S197).

When it is determined that the environment is at the highest temperature and humidity, that is, when the environment code is SHH (S189, S189)
192, S195: NO), photosensitive drum 4
V _O4 (= -500V) as one of the surface potential, V _B4 (= -300V) as the developing bias voltage is selected (S19
8) Then, the laser intensity P4 for γ correction is selected (S199).

When the selection of each of the set potentials V _O and V _B and the laser intensity for γ correction is completed, the flag F is reset to 0 to indicate that the automatic density control has been completed (S200). Ends the routine.

In the above description of the routine, since the intensity modulation method is used as the gradation expression, the selected laser intensity indicates the maximum intensity, and when the pulse width modulation method is used, the intensity is constant. Exposure is performed by the laser of the above-mentioned, so that the selected laser intensity is the constant intensity.

In the embodiment according to the present invention described above, when the concentration compensation due to the environmental change is automatically performed, the AIDC sensor 203 is not used to detect the environmental change.
Although the embodiment in which the change in the toner adhesion amount caused by the environmental change is detected has been described, the change in the environment is directly detected by using the temperature / humidity sensor 205 to perform the density compensation. Or AID
The concentration compensation may be performed by combining the C sensor 203 and the temperature / humidity sensor 205.

In the above embodiment according to the present invention, the environmental change is made into four stages. However, the present invention is not limited to this, and it is possible to increase it more.
Fine gradation compensation can be performed.

[0100]

As described in detail above, according to the present invention, there is provided an electrophotographic digital image forming method for performing gradation expression by changing the amount of light irradiated on a photoreceptor by exposure means in accordance with image information. Prior to image formation, the density control is performed by changing at least one of a surface potential of the photoreceptor before exposure by the exposure unit and a development bias voltage previously applied to a developing device. When the set values of the surface potential of the photoreceptor and the developing bias voltage are changed in a direction in which the absolute value increases, the exposure intensity of the exposure unit decreases in a direction in which the exposure intensity of the exposure unit decreases. By changing the exposure gain, the gradation variation accompanying the above change is compensated, so that gradation expression is performed by changing the laser light amount, for example, digital color copying. By applying the present invention to or printer,
Even if the density of the photoconductor is changed by changing the surface potential of the photoconductor before exposure or the developing bias voltage in response to environmental changes, it is possible to obtain a reproduced image that is faithful to the original without degrading the gradation reproducibility. There are advantages.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing an entire configuration of a digital color copying machine according to an embodiment of the present invention.

FIG. 2 is a block diagram of an image reader unit of the digital color copying machine shown in FIG.

FIG. 3 is a block diagram of a main body of the digital color copying machine shown in FIG.

FIG. 4 is a block diagram showing a process of image signal processing of the digital color copying machine shown in FIG.

FIG. 5 is a graph showing an example of a γ characteristic.

FIG. 6 is a block diagram of an image forming unit in the vicinity of a photoconductor drum for explaining image density adjustment by changing a photoconductor surface potential and a developing bias voltage.

FIG. 7 is a graph showing a difference in development characteristics between a reference temperature / humidity environment and a low temperature / low humidity environment.

FIG. 8 is a graph for comparing the respective γ characteristics after the image density is compensated for due to an environmental change with the γ characteristics in a reference environment.

9 is a graph showing emission characteristics of a laser whose intensity is nonlinearly controlled by a γ correction conversion table for linearly correcting γ characteristics in the reference environment NN of FIG. 8;

10 is a graph showing a result of performing γ correction on the four γ characteristics of FIG. 8 using a laser having a light emission characteristic of the reference environment NN of FIG. 9;

FIG. 11 is a graph showing a change in the surface potential of the photoconductor drum when a charged photoconductor is irradiated with a laser whose intensity is changed under a constant light emission time.

FIG. 12 shows that the maximum intensity is 0.8 mW, 1.0 mW, and 1.1 m, respectively, corresponding to the document level, with a constant light emission time.
6 is a graph showing the surface potential of the photoconductor drum when the laser intensity is changed from 0 to the maximum intensity of each of W and 1.2 mW.

FIG. 13 is a graph showing the relationship between original density and image density (γ characteristic) in FIG. 12 in consideration of development characteristics;
FIG. 9 is a graph showing grayscale characteristics according to respective laser intensities to make all four grayscale characteristics shown in FIG. 8 NN grayscale characteristics.

FIG. 14 is a graph schematically showing a potential of an electrostatic latent image of one dot on a photoconductor by a laser whose laser intensity is multileveled into eight levels by an intensity modulation method.

FIG. 15 shows an emission time of 8 using a pulse width modulation method.
4 is a graph schematically showing the potential of an electrostatic latent image of one dot on a photoconductor by a laser that has been multi-valued in stages.

FIG. 16 is a diagram showing an example of a dither pattern used in a multi-valued dithering method in which one dot is multi-valued into eight values and gradation is expressed by 2 × 2 dots.

FIG. 17 is a flowchart of a main control routine in a case where density control is performed manually in the digital color copying machine shown in FIG. 1;

18 is a flowchart of a manual density control routine of the digital color copying machine shown in FIG.

19 is a diagram illustrating a digital color copier A shown in FIG.
9 is a flowchart of a main control routine when performing density control automatically using an IDC sensor.

20 is a flowchart of a first portion of the first embodiment of the automatic density control routine of the digital color copying machine shown in FIG.

FIG. 21 is a flowchart of a second part of the first embodiment of the automatic density control routine of the digital color copying machine shown in FIG.

FIG. 22 is a flowchart of a laser intensity selection routine shown in FIG. 21.

23 is a flowchart of a first part of a second embodiment of the automatic density control routine of the digital color copying machine shown in FIG.

24 is a flowchart of a second part of the second embodiment of the automatic density control routine of the digital color copying machine shown in FIG.

25 is a block diagram of the laser diode driver shown in FIG.

FIG. 26 is a graph showing characteristics of the laser intensity of the laser diode shown in FIG. 3 with respect to the document density when the intensity modulation method is used in the present embodiment.

[Explanation of symbols]

20: image signal processing unit, 41: photosensitive drum, 44: V
_O sensor, 45r developing roller, 101 image reader controller, 106 image controller, 201 printer controller, 202 printer head controller, 203 AIDC
Sensor, 205 ... temperature and humidity sensor, 206 ... control panel, 214 ... V _G generating unit, 215 ... V _B generating unit, 208, 216 ... control ROM, 209,217 ... data ROM, 220 ... laser diode driver, 22
1 ... Laser diode.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H04N 1/036 G03G 15/04 120 1/23 103 (72) Inventor Takanobu Yamada 2-3-3 Azuchicho, Chuo-ku, Osaka-shi, Osaka No. 13 Osaka Kokusai Building Minolta Camera Co., Ltd. In-house (56) References JP-A-1-196347 (JP, A) JP-A-60-249166 (JP, A) JP-A-56-113160 (JP, A) Hei 1-235973 (JP, A) JP-A-59-148069 (JP, A)

Claims (1)

(57) [Claims] 1. An electrophotographic digital image forming method in which the amount of light irradiated on a photoreceptor by an exposure unit is changed in accordance with image information to perform gradation expression , prior to image formation, exposure by the exposure unit is performed. The surface potential of the photoconductor before, the density control is performed by changing at least one of a set value of a developing bias voltage previously applied to the developing device, and the surface potential of the photoconductor and the above Each of the developing bias voltage
When the set value is changed so that its absolute value increases
In the direction in which the exposure intensity of the exposure means decreases.
By changing the exposure gain of the light means,
A digital image forming method for compensating accompanying gradation fluctuation .