JPH04211282A - Digital image forming method - Google Patents

Abstract

PURPOSE:To constantly obtain a fixed gradation reproducibility in relation to an original by changing the intensity of an exposure means to compensate for fluctuation in gamma-characteristic. CONSTITUTION:A printer control unit controls a VG generating unit 214 and a VB generating unit 215 based on data from an AIDC sensor 203 for detecting the concentration of toner-image that adheres on the surface of a photosensitive drum, an ATDC sensor 204 for detecting the concentration of toner in a developing unit, and a temperature and humidity sensor 205. Further, the printer control unit 201 sends numerical data on grid voltage VG (surface electric potential) and developing bias voltage VB to a print head control unit 202. The print head control unit 202 performs gamma-correction by referring to the content of data ROM 217 in which a correcting conversion table is stored, and controls a laser diode driver 220 to control the light emission of a laser diode 221.

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 digital copying machines, digital printers, and the like.

0002

2. Description of the Related Art Various electrophotographic image forming apparatuses such as laser printers that reproduce images by driving laser means based on image data converted into digital values have been put to practical use. Various digital image forming methods have also been proposed for faithfully reproducing images.

[0003] This type of digital image forming method includes an area gradation method using a dither matrix, and a method in which the amount of laser light (=light emission time x intensity) is changed by changing the pulse width (light emission time) or light emission intensity of the laser. Multilevel laser exposure methods (pulse width modulation method, intensity modulation method), etc., which express the gradation for one printed dot by (See Japanese Patent Application Laid-open No. 62-39972, Japanese Patent Application Laid-Open No. 62-188562, and Japanese Patent Laid-Open No. 61-22597), and also multilevel dithering methods that combine dither with pulse width modulation method or intensity modulation method. Are known.

By the way, according to this type of gradation method, it should be possible in principle to reproduce an image density having a gradation level that corresponds one-to-one to the gradation degree of the image data to be reproduced, but in reality it is not possible to reproduce the image density. Due to a complex interplay of various factors such as the photosensitive characteristics of the photoconductor, the characteristics of the toner, and the usage environment, the density of the original to be reproduced and the density of the reproduced image (hereinafter simply referred to as image density) are not exactly proportional. , as shown schematically in FIG. 5, shows a characteristic B that deviates from the proportional characteristic A that should originally be obtained. Such characteristics are generally referred to as γ characteristics, and are a major factor in reducing the fidelity of reproduced images, especially for halftone originals.

[0005] Therefore, in order to improve the fidelity of a reproduced image, conventional methods have been to convert the density of the read original using a predetermined conversion table for γ correction, and to form a digital image based on the converted density of the original. Accordingly, so-called γ correction is performed so that the document density and the image density satisfy a linear relationship (characteristic A). In this way, by performing γ correction, images can be faithfully reproduced depending on the density of the original.

On the other hand, another factor that affects image density is the phenomenon that the amount of toner adhering to the photoreceptor changes during development due to changes in the external environment such as temperature and humidity due to the characteristics of the photoreceptor and toner. There is. In general,
In a high temperature and high humidity environment, the amount of toner adhesion increases, and the slope of the γ characteristic at low and medium densities increases, resulting in a darker reproduced image.In addition, in a low temperature and low humidity environment, the amount of toner adhesion decreases.
It is known that the slope of the γ characteristic at low and medium densities becomes smaller and the reproduced image becomes thinner.

As described above, there is a problem that the density of the reproduced image changes due to changes in the environment, and in order to solve this problem and stabilize the image density, general electrophotographic copying machines and printers have a maximum Density control is performed to keep the image density constant.

The method generally employed for 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 shown in FIG.

A charger 43 having a discharge potential VC is installed opposite to the photosensitive drum 41. A grid voltage VG is applied to the grid of the charger 43 by a grid voltage generation unit (hereinafter referred to as a VG generation unit) 214 . The surface potential VO of the photosensitive drum 41 is controlled by the surface potential V by the VO sensor 44.
This is done by adjusting the grid potential VG based on the detected value of O.

First, before laser exposure, a negative surface potential V is applied to the photosensitive drum 41 by the charger 43.
O is also used in a developing bias voltage generation unit (hereinafter referred to as VB generation unit) 21 to prevent the fogging phenomenon.
5, a low potential negative developing bias voltage VB (|VO|>|VB|) is applied to the developing roller 45r. At this time, the surface potential of the developing sleeve also becomes equal to the developing bias voltage VB.

The potential of the photosensitive drum 41 decreases due to the laser exposure, and changes from the surface potential VO to the electrostatic latent image potential VI at the time of exposure with the maximum amount of light. When the electrostatic latent image potential VL becomes lower than the developing bias voltage VB, toner adheres to the photoreceptor drum 41. The amount of toner attached increases as the difference between the developing bias voltage VB and the electrostatic latent image potential VL increases. Therefore, by changing the surface potential VO and the developing bias voltage VB, the difference between the developing bias voltage VB and the electrostatic latent image potential VL changes, so the amount of toner adhesion can be changed and the density can be controlled. .

This type of concentration control is performed at each potential VO.
, VB are changed manually or automatically to keep the maximum concentration constant.

In automatic density control, first a reference toner image is formed on the surface of the photoreceptor drum 41 as a reference for density control, and the AIDC sensor 203 installed near the photoreceptor drum 41 detects the reflection from the reference toner image. Detect the amount of light. The detection value detected by this AIDC sensor 203 is input to the printer control unit 201,
Depending on the comparison result between the detected value from the AIDC sensor 203 and a predetermined numerical value, the printer control section 201 drives the VG generation unit 214 and the VB generation unit 215.

The above operations are repeated until the amount of toner adhesion reaches a predetermined value.

[0015]

[Problems to be Solved by the Invention] However, when density control is performed to keep the density of the reproduced image constant as described above, and the photosensitive drum surface potential VO and developing bias voltage VB are changed, the γ characteristic becomes large. I am affected by it. An example of this is shown in FIG.

FIG. 7 is a graph showing the development characteristics NN in a standard environment and the development characteristics LL in a low temperature and low humidity environment. The solid line represents the standard environment development characteristics NN, and the broken line represents the low temperature, low humidity environment development characteristics LL. The reference potential setting in the reference environment is (VO, VB) = (-700V
, -500V). Further, the maximum intensity of the laser used for exposure is 1.0 mW. If the environment changes to low temperature and low humidity while maintaining this potential setting, the development characteristics NN changes to LL, and the maximum density represented by the intersection of the development characteristics curve and the photoreceptor surface potential VI decreases from C1 to C2. . Therefore, if density control is not performed and the environment becomes low temperature and humid, the density of the reproduced image will become thinner.

Here, in order to compensate for a constant maximum concentration when the environment changes to low temperature and low humidity, each potential must be adjusted to (VO
, VB) = (-800V, -600V). Then, the development characteristic LL in a low-temperature, low-humidity environment indicated by the broken line is shifted to the left as indicated by the arrow in the figure, and becomes the development characteristic LL' indicated by the dashed line. The points of intersection at the surface potential VI coincide, and compensation is made for the maximum concentration level. Although not shown in the figure, when the environment changes to high temperature and high humidity, the amount of toner adhesion increases and the image density becomes darker, contrary to the case of LL, so in order to compensate for the maximum density at a constant level, for example, , in the case of high temperature and high humidity, (VO, VB) = (-600V, -400V
), it is necessary to lower the potential setting and shift the characteristic curve to the right.

However, as is clear from FIG. 7, the shape of the curve from the lowest density to the highest density is significantly different between the standard environment development characteristic NN and the low temperature, low humidity environment development characteristic LL' after maximum density compensation. That is, if the image density is compensated for by changing each of the potentials VO and VB, the γ characteristics will vary.

FIG. 8 shows the γ characteristics after 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 density control is performed depending on the usage environment, the γ characteristic itself changes. For example, in the intensity modulation method, the reference environment NN of FIG. Conventionally, the laser emission intensity has been nonlinearly controlled according to the γ characteristics using a γ correction conversion table that supports only the γ characteristics. However, with γ correction using a single γ correction conversion table, correct γ correction cannot be performed in environments other than the reference environment NN, as shown in FIG.

[0021] The present invention utilizes the characteristics of the photoreceptor to
The purpose of the present invention is to provide a digital image forming method that can compensate for fluctuations in γ characteristics that occur as a result of the density control described above, and that can always obtain reproduced images with constant gradation reproducibility for originals. There is.

[0022]

[Means for Solving the Problems] In order to achieve the above object, the digital image forming method of the present invention changes the amount of light irradiated onto a photoreceptor by an exposure means according to image information to express gradation. In the electrophotographic digital image forming method carried out, density control is performed by changing at least one of the surface potential of the photoreceptor before exposure by the exposure means and a development bias voltage applied in advance to a developing device. The present invention is characterized in that the intensity of the exposure means is changed to compensate for gradation fluctuations caused by changes in the surface potential of the photoreceptor and the developing bias voltage.

[0023]

[Function] In the electrophotographic digital image forming method according to the present invention, when density control is performed by changing the surface potential of the photoreceptor before exposure or the developing bias voltage, the intensity of the exposure means is also changed at the same time to obtain an appropriate γ value. Make corrections.

Specifically, regarding the characteristics of the photoreceptor used in the present invention, that is, when the maximum intensity of the laser is changed,
The fluctuation of the γ characteristic will be explained. 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 changes. By the way, each of the γ characteristics shown in FIG. 8 is most characterized by whether the rise is steep or gradual. 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 gamma characteristics shown in high temperature and high humidity HH have a steeper rise, and the highest temperature and high humidity SHH
If it is possible to make the rise even steeper, the γ characteristic shown by can be made closer to the γ characteristic shown by the reference environment NN.

Furthermore, in FIG. 8, the reason why the γ characteristic fluctuates is because the surface potential VO of the photoreceptor drum 41 and the developing bias voltage VB are changed, but due to the characteristics of the photoreceptor, the intensity of the laser FIGS. 11 to 13 show that the γ characteristic changes even when the γ characteristic changes.

FIG. 11 is a graph showing changes in surface potential when a charged photoreceptor is irradiated with a laser beam of varying intensity under a constant emission time. , it is shown that when the surface potential drops to a certain level, it becomes saturated, and therefore the image density does not change much.

On the other hand, in FIG. 12, the light emission time is constant and the maximum intensity is 0.8 mW, 1.
13 is a graph showing the surface potential of the photoreceptor drum 41 when the laser intensity is changed from 0 to the respective maximum intensity as 0 mW, 1.1 mW, and 1.2 mW. Furthermore, FIG. 7 is a graph showing the relationship (γ characteristic) between the document density and image density taken into consideration.

It can be seen that if the maximum intensity, that is, the maximum amount of light is changed in this way, the γ characteristic changes in the same way as when each of the potentials VO and VB is changed.

The present invention utilizes this phenomenon to suppress fluctuations in the γ characteristics caused by changing the surface potential VO before exposure of the photosensitive drum and the developing bias voltage VB in order to keep the maximum density constant. be.

[0031]

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

(a) Structure of digital color copying machine FIG. 1 is a longitudinal sectional view showing the overall structure of a digital color copying machine according to an embodiment of the present invention. Digital color copier is
An image reader section 100 that reads a document image, and a main body section 2 that reproduces the image read by the image reader section 100.
It can be broadly divided into 00 and 00.

In FIG. 1, a scanner 10 includes an exposure lamp 12 that irradiates an original, a rod lens array 13 that collects reflected light from the original, and a contact type CCD that converts the collected light into an electrical signal. It is equipped with a color image sensor 14. The scanner 10 uses a motor 1 when reading a document.
1 to move in the direction of the arrow (sub-scanning direction) and scan the original placed on the platen 15. The image of the document surface illuminated by the exposure lamp 12 is photoelectrically converted by the image sensor 14 . The three-color multi-value electrical signals of R, G, and B obtained by the image sensor 14 are processed by the read signal processing unit 20 to determine which of yellow (Y), magenta (M), cyan (C), and black (K). It is converted into 3-bit gradation data. Next, the print head section 31
performs correction (γ correction) on the input gradation data according to the gradation characteristics of the photoreceptor and dither processing as necessary, and then converts the corrected image data into digital/analog conversion (hereinafter referred to as γ correction). , D/A conversion) to generate a laser diode drive signal, and the laser diode 221 is driven by this drive signal.

Laser diode 2 corresponds to the gradation data.
The laser beam generated from 21 is as shown in FIG.
A photosensitive drum 41 that is rotationally driven via a reflecting mirror 37
to expose. As a result, an image of the document is formed on the photoreceptor of the photoreceptor drum 41. The photosensitive drum 41 is irradiated with an eraser lamp 42 and charged with a charger 43 before being exposed for each copy. When exposed to light in this uniformly charged state, an electrostatic latent image is formed on the photosensitive drum 41. Only one of the yellow, magenta, cyan, and black toner developers 45a to 45d is selected to develop the electrostatic latent image on the photoreceptor drum 41. The developed image is transferred by a transfer charger 46 to copy paper wound around a transfer drum 51. Also, the density of the toner image to be developed is determined by the AIDC sensor 2.
03 is optically detected.

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, passed through the fixing device 48, fixed, and discharged onto the paper discharge tray 49. The copy paper is fed from a paper cassette 50, and its leading edge is chucked by a chucking mechanism 52 on a transfer drum 51 to prevent positional deviation during transfer.

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

As shown in FIG. 2, the image reader section 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 referred to as drive I/O) 103 based on a position signal from a position detection switch 102 indicating the position of the document on the platen 15. It also controls a scan motor driver 105 via a drive I/O 103 and a parallel input/output device (hereinafter referred to as parallel I/O) 104. Scan motor 11
is driven by a scan motor driver 105.

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

As shown in FIG. 3, the main body section 200 is provided with a printer control section 201 that controls general copying operations and a print head control section 202 that controls the print head. The printer control unit 201 includes a VO sensor 4 that detects the surface potential VO of the photoreceptor drum 41 immediately before exposure.
4. VL sensor 60 that detects the surface potential VL of the photoreceptor drum 41 immediately after exposure; AIDC sensor 2 that optically detects the density of the toner image attached to the surface of the photoreceptor drum 41;
03. ATDC sensor 204 and temperature/humidity sensor 2 that detect the toner concentration in the developing devices 45a to 45d
Analog signals from various sensors of 05 are input. Furthermore, various data are input to the printer control unit 201 via the parallel I/O 207 by key input to the operation unit keys 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 its control based on the data in these ROMs, as will be described in detail later. do.

The printer control unit 201 controls each sensor 203
~205, operation panel keys 206 and data ROM 209
control the copy control unit 210 and the display panel 211 according to the contents of the control ROM 208 using the data from the control ROM 208;
Further, automatically by the AIDC sensor 203, or
In order to perform manual density compensation control by inputting to the operation panel 206, the VG generation unit 214 and the VB generation unit 215 are controlled via the parallel I/O 212 and the drive I/O 213. Further, the printer control unit 201 sends numerical data of the grid voltage VG (surface potential VO) and the developing bias voltage VB to the print head control unit 202.

The print head control unit 202 controls the control RO
It operates according to a control program stored in the M216, and is connected to the image signal processing section 20 of the image reader section 100 via an image data bus, and processes image signals received via the image data bus. Based on γ
Data ROM 21 in which the correction conversion table is stored
7, perform γ correction, and further perform dither processing when using the multilevel dither method as the gradation expression method, and then
The laser diode driver 220 is controlled via. The light emission of the laser diode 221 is controlled by a laser diode driver 220.

Further, the print head control section 202 is connected to the printer control section 201, the image signal processing section 20, and the image reader control section 101 via a bus so that they are synchronized with each other. In particular, in this embodiment, based on the numerical data of the grid voltage VG (surface potential VO) and the developing bias voltage VB sent from the printer control unit 201, the laser diode driver 220 drives the laser diode as described in detail later. The laser intensity of 221 is changed.

(b) Image Signal Processing FIG. 4 is a block diagram for explaining the flow of image signal processing from the CCD 14 to the print head control section 202 via the image signal processing section 20. Hereinafter, read signal processing for processing the 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 unit 20, the image signal photoelectrically converted by the CCD color sensor 14 is converted into R, G, and B multi-value digital by an analog/digital converter (hereinafter referred to as A/D converter) 21. Converted to image data. Each of the converted image data is subjected to predetermined shading correction in a shading correction circuit 22. Since this shading-corrected image data is reflection data of the original, the log conversion circuit 23 converts it into a log
The image is converted into density data of an actual image by performing g conversion. Furthermore, in the under color removal/inking circuit 24,
Excess black coloring is removed and true black data K is generated from R, G, and B data. Then, in the masking processing circuit 25, the three color data of R, G, and B are
It is converted into three color data of M and C. The density correction circuit 26 performs a density correction process of multiplying the Y, M, and C data thus converted by a predetermined coefficient, and the spatial frequency correction process is performed by the spatial frequency correction circuit 27 before outputting the data to the print head control unit 202. do.

In the print head control section 202,
The image signal processed by the image signal processing unit 20 is
The conversion unit 28 performs γ conversion based on the γ correction conversion table in the data ROM 217, and when the multilevel dither method is adopted as the gradation expression, the dither processing unit 29 performs dithering using the dither threshold data in the data ROM 217. The processed digital image signal is output to the laser diode driver 220.

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

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

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, 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 comprising the gain switching section 255 becomes the data of the input gain setting value, each switch SW1 to SW8 is used. Outputs a gain switching signal to. Note that the gain setting value is a value determined according to detection signals from various sensors and input signals from the operation panel 206.

Therefore, the digital image signal sent from the image signal processing section 20 via the print head control section 202 and the parallel I/O 219 is converted into an analog voltage signal by the D/A converter 254, and then sent to the printer. The signal is amplified with an amplification degree according to a gain setting value input from the control section 201. The amplified analog drive signal is output to the laser diode 221, thereby causing the laser diode 221 to emit light. In this embodiment, as will be described in detail later, the amplifier 26
Since the amplification degree of the amplifier circuit consisting of 0 and the gain switching section 255 is changed, for example, when using the intensity modulation method, the laser intensity of the laser diode 221 is changed in all gradation regions as shown in FIG. I'm letting you do it. Furthermore, in the following, for convenience of explanation, the expressions "changing the maximum intensity" or "selecting the maximum intensity" will be used, but in reality, as shown in FIG. The "maximum intensity" is selectively changed using the section 255, and the laser intensity is also changed in the intermediate concentration region.

In the embodiment shown in FIG. 25, the analog image signal is amplified according to the gain setting value, but the present invention is not limited to this. 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 The multi-value laser exposure method and the multi-value dither method, which are the gradation expression methods used in the present invention, will be explained.

(c-1) Multilevel laser exposure method (c-1-
1) Intensity Modulation Method The intensity modulation method among the multilevel laser exposure methods as a gradation expressing means will be explained.

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 fixed light emission time is changed to several steps according to the multivalued signal from the image reader section 100. The photoreceptor is multivalued and the photoreceptor is irradiated with a different amount of laser light at each stage.
The density of one dot can be multivalued.

FIG. 14 shows how the intensity can be increased to 8 by the intensity modulation method.
3 is a graph schematically showing the potential of a cross section of a latent image of one dot produced by a stepwise multivalued laser. As shown in FIG. 14, gradation expression using the intensity modulation method is achieved by changing the amount of toner adhering, that is, changing the 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 explained.

The pulse width modulation method is a gradation expression method in which the area of one dot to be printed is changed in stages, and the emission time of a laser with a constant intensity is changed in several stages according to the multivalued signal from the image reader. The printing area of one dot can be multivalued because the photoreceptor is irradiated with a different amount of laser light depending on each stage.

FIG. 15 is a graph schematically showing the latent image potential in the cross section of a one-dot latent image produced by a laser with an intensity of 1.0 mW in which the light emission time is multivalued in eight stages using a pulse width modulation method. As shown in FIG. 15, the gradation expression by the pulse width modulation method is achieved by changing the toner adhesion area, that is, changing the reproduced image area, because the latent image area changes stepwise.

(c-2) Multi-value dither method Regarding the multi-value dither method that combines the above-mentioned multi-value laser exposure method (intensity modulation method or pulse width modulation method) and dither method to express gradation. explain.

[0059] This multilevel dithering method uses, for example, (N×M)
Dots are treated as one block, and each dot in this block is multivalued into (L) values, thereby expressing (N x M x L + 1) gradations, and each dot's The above-mentioned pulse width modulation method or intensity modulation method is used as a means for multi-leveling. Therefore, when simply using the pulse width modulation method or the intensity modulation method, one dot of the printed image by laser exposure becomes one pixel, and when using the multilevel dither method, the printed image (N × M
) The dot area is one pixel.

FIG. 16 is composed of (2×2) dots,
FIG. 7 is a diagram showing an example of multi-value dithering in which one dot is multi-valued into eight values. This dither is (2 x 2 x 8 + 1) 33
It is possible to express gradations. Regarding 1 to 32 indicating the threshold values of these multi-value dithering, when the intensity modulation method is used for multi-value conversion of one dot, the gradation expression is not area gradation but density gradation, Since it is difficult to illustrate, it is represented by a rectangular shape as shown in FIG. 16 for convenience.

(d) Control Flow FIGS. 17 to 24 show the control flow executed by the printer control section 201 of the digital color copying machine of this embodiment according to the present invention. 17 and 18 are flowcharts for manually compensating for concentration changes due to environmental changes, and FIGS. 19 to 24 are flowcharts for manually compensating for concentration changes due to environmental changes. It is a flowchart when performing compensation,
In either case, necessary γ correction is performed along with density control.

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

First, initial settings such as parameter initialization are performed (step S1 (hereinafter, "step" will be omitted)), and an internal timer is started (S2). and,
Routine for manually controlling concentration (ID) by key input to the operation panel 206 (see FIG. 18)
After executing (S3), the copy operation begins (S4). When the internal timer expires (S5), the process returns to S2.

FIG. 18 is a flowchart of the manual density control routine (S3 in FIG. 17) of this embodiment of the present invention, in which a constant maximum density is controlled in accordance with the environment code selected by the user according to the usage environment. At the same time, select the surface potential VO and developing bias voltage VB necessary to obtain the desired surface potential, and send the numerical data of the surface potential VO and developing bias voltage VB to the print head control unit 202, and change the maximum intensity of the laser used for exposure to obtain an appropriate value. It is characterized by performing a γ correction. In this embodiment, the environmental code has four levels, which are 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 temperature and humidity is selected by key input on the operation panel 206 (S11).
).

[0066] When the environment code is LL (YES in S12)
S), in order to compensate for the amount of toner adhering that is smaller than in the standard environment, V is set as the surface potential of the photosensitive drum 41.
Select O1 (-800V in this example) and VB1 (-600V in this example) as the developing bias voltage (
S13). When each potential VO1, VB1 is selected, originally the γ characteristic shown by LL in FIG. 8 is obtained, but this is suppressed and
Select the laser intensity P1 (0.8 mW in this example) required so as not to vary from the γ characteristic indicated by NN (S14
). The γ characteristic due to this laser intensity P1 is originally L in Fig. 13.
The γ characteristic with a steep rise on the low concentration side shown in LL in FIG. 8 and the γ characteristic with a gradual rise on the low concentration side shown in FIG. N to 8
Fluctuations from the γ characteristic indicated by N are suppressed. Therefore, a linear target gradation characteristic is realized by the light emission characteristic shown in FIG. 9 according to the γ correction conversion table according to the NN shown in FIG.

[0067] When the environment code is NN (YES in S15)
S), the reference potential setting VO2 (-700V in this embodiment) is selected as the surface potential of the photosensitive drum 41, and VB2 (-500V in this embodiment) is selected as the developing bias voltage (S16). When each potential VO2, VB2 is selected, the laser intensity P2 realizes the γ characteristic shown by NN in FIG.
(1.0 mW in this embodiment) is selected (S17). The γ characteristic depending on the laser intensity is shown by NN in FIG. 13, and this γ characteristic is the same as the γ characteristic shown by NN in FIG. Therefore, a linear target gradation characteristic is realized by the light emission characteristic shown in FIG. 9 according to the γ correction conversion table according to the NN shown in FIG.

[0068] When the environment code is HH (YES in S18)
S), in order to compensate for the amount of toner adhesion that is larger than that in the standard environment, VO is set as the surface potential of the photoreceptor drum 41.
3 (-600V in this example) and VB3 (-400V in this example) as the developing bias voltage (S
19). When each potential VO3, VB3 is selected, originally the γ characteristic shown by HH in FIG. 8 is obtained, but this is suppressed and N
Select the laser intensity P3 (1.1 mW in this example) required so as not to vary from the γ characteristic indicated by N (S20)
. The γ characteristic due to this laser intensity P3 was originally shown in Figure 13 as HH
The γ characteristics shown by HH in FIG. 8 have a more gradual rise on the low concentration side than NN, and the γ characteristics shown by HH in FIG. 13 have a steeper rise on the low concentration side than in NN. Together with the characteristics, fluctuations from the γ characteristics shown by NN in FIG. 8 are suppressed. Therefore, a linear target gradation characteristic is realized by the light emission characteristic shown in FIG. 9 according to the γ correction conversion table according to the NN shown in FIG.

[0069] When the environment code is SHH (Y in S21)
ES) In order to compensate for the increased amount of toner adhesion compared to the standard environment, VO4 (-500V in this embodiment) is set as the surface potential of the photoreceptor drum 41, and VB4 (in this embodiment) is set as the developing bias voltage. -300V) (S22). When each potential VO4, VB4 is selected,
Originally, the γ characteristic is shown as SHH in FIG. 8, but the laser intensity P4 (1.2 mW in this example) necessary to suppress this and not vary from the γ characteristic shown as NN is selected (
S23). The γ characteristic due to this laser intensity P4 was originally shown in Figure 1.
3 shows SHH, and the rise on the low concentration side shown by SHH in FIG. Coupled with the steep γ characteristics, fluctuations from the γ characteristics shown by NN in FIG. 8 are suppressed. Therefore, NN in FIG.
A linear target gradation characteristic is realized by the light emission characteristics shown in FIG. 9 according to the γ correction conversion table corresponding to the γ correction conversion table.

If the environment code is not one of the above, the process returns to S11 to try again.

In the above routine description, 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, the intensity is constant. Since the exposure is performed by a laser, the selected laser intensity is that constant intensity.

(d-2) AIDC concentration control and γ
Correction FIG. 19 shows the main routine of the digital color copying machine when density compensation is performed by the AIDC sensor 203.

First, initial settings such as parameter initialization are performed (S51), and an internal timer is started (S52).
). After executing a routine (see FIGS. 20 to 24) for automatically controlling the density (ID) using the AIDC sensor 203 (S53), a copying operation begins (S54). When the internal timer ends (S55), S5
Return to 2.

FIGS. 20 and 21 show the AID according to the present invention.
Automatic concentration control routine using C sensor 203 (
20 is a flowchart of the first embodiment of S53) in FIG. 19, in which the density change due to environmental change is detected and each potential VO, VB for compensating for the reference density is selected, and each potential VO, VB is Send the numerical data of
The feature is that 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, and if it 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. 1 (S82), and each set potential VO, VB for forming a reference toner image for density detection on the surface of the photoreceptor drum 41.
As VO2=-700V, VB2=-500 respectively
Using V (S83), the laser diode is caused to emit light with the maximum amount of light so as to achieve the maximum density under this set potential, and a reference toner image is formed (S85).

Furthermore, if the copy start key is not on edge (NO in S81), S82 of this previous routine
If flag F is set to 1 (YES in S84), then
), a reference toner image is created based on the previously set potential (S85). The copy start key is not on edge (
If the flag F is 0 (NO in S81), furthermore, if the flag F is 0 (S8
4 (NO), automatic concentration control is not performed.

The above reference toner image is the AIDC sensor 20.
The timer determines whether the detection position No. 3 has been reached, and if the detection position No. 3 has been reached (YES at S86), the AIDC sensor 2
03 detects the density of the reference toner image and inputs the value to the printer control unit 201 (S87). Toner image is A
If the detection position of the IDC sensor 203 has not yet been reached (NO in S86), the process directly 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 make the density thinner, the set potential is lowered by one step, and if the detected density is lighter than the reference density, the set potential is raised by one step in order to make the density of the reference pattern darker in the next loop of this flow. . Of course, if the detected concentration is equal to the reference concentration, the maximum concentration compensation is properly performed using the set potential, so the potential setting is completed and the maximum intensity of the laser is used to properly perform γ correction based on this set potential. Select.

[0079] The concentration detected by the AIDC sensor 203 is
If the density is higher than the standard concentration (YES in S88), and if each set potential is VO2 and VB2 (YES in S89),
ES), surface potential VO3 (=-600V), and development bias voltage VB3 (=-400V) are selected (S90), and if each set potential is VO3 and VB3, respectively (S9
1 (YES), VO4 (=-500V), VB4 (=-
300V) (S92) and return.

[0080] The concentration detected by the AIDC sensor 203 is
If the concentration is lower than the reference concentration (YES in S93), and if each set potential is VO2 and VB2 (YES in S94),
ES), VO1 (=-800V), VB1 (=-600
V) (S95) and return.

[0081] The concentration detected by the AIDC sensor 203 is
If the detected concentration is not greater than the reference concentration (NO in S88), and if the detected concentration is less than the reference concentration (NO in S93).
), the detected concentration is determined to be equal to the reference concentration, and in the laser intensity selection routine (see FIG. 22), the laser intensity is selected according to the currently set potentials VO and VB (S
96), resets the flag F to 0 (S97), and returns.

By the way, in the flow shown in FIG. 21, the set potentials are set in four stages, so even though the set potentials VO4 and VB4 that make the reference pattern density the thinnest are selected, the detected density is still further than the reference density. Each setting potential VO1 that makes the reference pattern density the darkest or the darkest
, VB1 are selected, but if the detected concentration is still lower than the reference concentration, VO4
, VB4 and VO1, the potential setting is finished with VB1 and the print operation (S54) begins, but
The present invention is not limited to this flow, and if this causes any inconvenience, it may be possible to set the potential more finely.

FIG. 22 is a detailed flowchart of the laser intensity selection routine shown in S96 of FIG. 21, and will be described below. Note that the laser intensity selected based on the surface potential VO of the photoreceptor drum 41 and the developing bias voltage VB in this flow is the same as that explained with reference to FIG. 18, so a detailed explanation of the selected laser intensity will be omitted.

VO1 as the surface potential of the photosensitive drum 41
(=-800V), development bias voltage VB1 (=
-600V) is selected (YES in S100)
, it is the same as determining that the environment is low temperature and low humidity LL, and the laser intensity P1 for γ correction is selected (S10
1).

As the surface potential of the photosensitive drum 41, VO2
(=-700V), development bias voltage VB2 (=
-500V) is selected (YES in S102)
, the environment is determined to be the reference environment NN, and the laser intensity P2 for γ correction is selected (S1
03).

As the surface potential of the photosensitive drum 41, VO3
(=-600V), development bias voltage VB3 (=
-400V) is selected (YES in S104)
, it is the same as determining that the environment is high temperature and high humidity HH, and the laser intensity P3 for γ correction is selected (S10
5).

As the surface potential of the photosensitive drum 41, VO4
(=-500V), VB4 (=
-300V) is selected, (S100, S102
, S104 (all NO), this is the same as determining that the environment is the highest temperature and high humidity SHH, and the laser intensity P4 for γ correction is selected (S106).

FIGS. 23 and 24 show the AIDC sensor 20
3 automatic concentration control routine (S5 in FIG. 19)
3) is a flowchart of the second embodiment, in which only one conversion table for γ correction is prepared, and each set potential VO, VB detects concentration changes due to environmental changes and compensates for the reference concentration.
At the same time, numerical data of each setting potential VO, VB is sent to the print head control unit 202, and each potential VO, V is selected to detect density changes due to environmental changes and compensate for the reference density.
B is selected, and numerical data of each set potential VO, VB is sent to the print head control unit 202, and the maximum intensity of the laser used for exposure is changed to perform appropriate γ correction.

[0089] In this flowchart, S18
In steps 1 to S187, a reference pattern image is formed with a potential set to a predetermined value, and an environmental code is selected based on the density of this reference pattern image. The content is exactly the same as S87, and the explanation will be omitted. Further, since the laser intensity selected based on the surface potential VO of the photosensitive drum 41 and the developing bias voltage VB in this flow is the same as that explained with reference to FIG. 18, a detailed explanation of the selected laser intensity will be omitted.

The remaining steps S188 to S200 will be explained below.

Next, an environmental code indicating the environment is selected based on the concentration detected by the AIDC sensor 203 and a preset reference concentration (S188). In other words, if the detected concentration is lower than the standard concentration, the environmental code becomes LL,
If the detected concentration is equal to the reference concentration, the environmental code is NN, if the detected concentration is higher than the reference concentration, the environmental concentration is HH, and if the detected concentration is even higher than the reference concentration, the environmental code is SHH.

[0092] When the environment is judged to be low temperature and low humidity,
That is, when the environment code is LL (YES in S189)
, the surface potential of the photoreceptor drum 41 is VO1 (=-80
0V), development bias voltage VB1 (=-600V)
) is selected (S190), and furthermore, the laser intensity P1 for γ correction is selected (S191).

[0093] When the environment is determined to be the reference environment,
That is, when the environment code is NN (YES in S192)
, VO2 (=-70
0V), development bias voltage VB2 (=-500V)
) is selected (S193), and furthermore, the laser intensity P2 for γ correction is selected (S194).

[0094] When it is determined that the environment is high temperature and high humidity,
That is, when the environment code is HH (YES in S195)
, the surface potential of the photoreceptor drum 41 is VO3 (=-60
0V), development bias voltage VB3 (=-400V)
) is selected (S196), and furthermore, the laser intensity P3 for γ correction is selected (S197).

[0095] When it is determined that the environment is at the highest temperature and humidity, that is, when the environment code is SHH (S189, S1
92, all NO in S195), photosensitive drum 41
VO4 (=-500V) is selected as the surface potential of S1, and VB4 (=-300V) is selected as the developing bias voltage.
98), and further, laser intensity P4 for γ correction is selected (S199).

When the selection of each set potential VO and VB and the laser intensity for γ correction is completed, the flag F is reset to 0 (S200) to indicate that the automatic concentration control has been completed, and the above routine is repeated. end.

In the above routine description, 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, the intensity is constant. Since the exposure is performed by a laser, the selected laser intensity is that constant intensity.

In the embodiments of the present invention described above, when automatically performing concentration compensation due to environmental changes, instead of using the AIDC sensor 203 to detect environmental changes,
Although an embodiment has been described in which this is done by detecting changes in the amount of toner adhesion caused by environmental changes, it is also possible to directly detect changes in the environment using the temperature/humidity sensor 205 and perform density compensation. Also, AIDC
The sensor 203 and the temperature/humidity sensor 205 may be combined to perform concentration compensation.

[0099] In the above embodiments of the present invention, the environmental change was made into four stages, but the present invention is not limited to this, and it is possible to have more stages; in this case,
Fine-grained gradation compensation can be performed.

[0100]

As described in detail above, the present invention provides an electrophotographic digital image forming method in which gradation is expressed by changing the amount of light irradiated onto a photoreceptor by an exposure means in accordance with image information. , density control is performed by changing at least one of the surface potential of the photoreceptor before exposure by the exposure means and the development bias voltage applied in advance to the developing device, and the surface of the photoreceptor is Since the intensity of the exposure means is changed to compensate for gradation fluctuations caused by changes in the potential and the developing bias voltage, it is possible to use, for example, a digital color copying machine or a printer that expresses gradations by changing the amount of laser light. By applying the present invention, even if density is compensated for by changing the surface potential of the photoreceptor before exposure or the developing bias voltage in response to environmental changes, the gradation reproducibility will not deteriorate and the image will remain faithful to the original. This has the advantage that a reproduced image can be obtained.

[Brief explanation of the drawing]

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

2 is a block diagram of an image reader section of the digital color copying machine shown in FIG. 1. FIG.

3 is a block diagram of the main body of the digital color copying machine shown in FIG. 1. FIG.

4 is a block diagram showing an image signal processing process of the digital color copying machine shown in FIG. 1. FIG.

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

FIG. 6 is a block diagram of an image forming section near the photoreceptor drum for explaining image density adjustment based on changes in photoreceptor surface potential and developing bias voltage.

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

FIG. 8 is a graph comparing each γ characteristic after compensation of image density due to environmental change with the γ characteristic in a reference environment.

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

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

FIG. 11 is a graph showing changes in the surface potential of a photoreceptor drum when a charged photoreceptor is irradiated with a laser beam of varying intensity while the emission time is constant.

[Figure 12] The maximum intensity is 0.8 mW, 1.0 mW, and 1.1, respectively, depending on the document level with a constant light emission time.
2 is a graph showing the surface potential of the photoreceptor drum when the laser intensity is changed from 0 to the respective maximum intensity at mW and 1.2 mW.

13 is a graph showing the relationship (γ characteristic) between original density and image density in consideration of the development characteristics in FIG. 12, and all four gradation characteristics shown in FIG. 8 are set to NN gradation characteristics. 3 is a graph showing gradation characteristics depending on each laser intensity.

[Figure 14] By using the intensity modulation method, the laser intensity can be increased to 8
3 is a graph schematically showing the potential of a one-dot electrostatic latent image on a photoreceptor produced by a stepwise multivalued laser;

FIG. 15 is a graph schematically showing the potential of a one-dot electrostatic latent image on a photoreceptor produced by a laser whose light emission time is multivalued in eight stages using a pulse width modulation method.

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

17 is a flowchart of a main control routine when performing manual density control of the digital color copying machine shown in FIG. 1. FIG.

18 is a flowchart of a manual density control routine of the digital color copying machine shown in FIG. 17. FIG.

19 is a flowchart of a main control routine when density control is automatically performed using the AIDC sensor of the digital color copying machine shown in FIG. 3; FIG.

20 is a flowchart of the first portion of the first embodiment of the automatic density control routine of the digital color copying machine shown in FIG. 19; FIG.

21 is a flowchart of the second portion of the first embodiment of the automatic density control routine of the digital color copying machine shown in FIG. 19; FIG.

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

23 is a flowchart of a first portion of a second embodiment of an automatic density control routine for the digital color copying machine shown in FIG. 19; FIG.

24 is a flowchart of the second portion of the second embodiment of the automatic density control routine for the digital color copying machine shown in FIG. 19; FIG.

25 is a block diagram of the laser diode driver shown in FIG. 4. FIG.

26 is a graph showing the 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 this example.

[Explanation of symbols]

20... Image signal processing section, 41... Photosensitive drum, 44... V
O sensor, 45r...Developing roller, 101...Image reader control unit, 106...Image control unit, 201...Printer control unit, 202...Printer head control unit, 203...AIDC
Sensor, 205... Temperature/humidity sensor, 206... Operation panel, 214... VG generation unit, 215... VB generation unit, 208, 216... Control ROM, 209, 217...
Data ROM, 220...Laser diode driver, 2
21...Laser diode.

Claims (1)

[Claims] 1. In an electrophotographic digital image forming method in which gradation is expressed by changing the amount of light irradiated onto a photoreceptor by an exposure means according to image information, the photoreceptor is The density is controlled by changing at least one of the surface potential and the developing bias voltage applied to the developing device in advance, and the gradation is controlled by changing the surface potential of the photoreceptor and the developing bias voltage. A digital image forming method characterized in that the intensity of the exposure means is varied to compensate for variations.