JP3021524B2 - Digital image forming equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a digital image forming apparatus (digital printer, digital copying machine, etc.) having a reversal development type electrophotographic process.

(Prior Art) In a digital image forming apparatus having a reversal developing electrophotographic process, in order to obtain an optimum reproduced image corresponding to variations in the characteristics of photoconductors and toners, variations in originals, and usage environments, Control and gradation correction are performed.

Density of an image formed can be adjusted by controlling the bias potential V _B of the developing device and the grid potential V _G of the electric charger. Grid potential V _G is the surface potential V _O of the photosensitive member produced by the main charger determines, therefore, it affects the decay potential V _I of the electrostatic latent image formed on the photosensitive member by the exposure.
The bias potential V _B is development potential | affect adhesion amount of toner carried on the electrostatic latent image from the developing device surface through | V _B -V _I.

Therefore, in this kind of digital image forming apparatus, a density sensor for measuring the reflected light of the reference toner image of the intermediate density obtained by irradiating the photosensitive body with a predetermined amount under predetermined grid potential V _G and the bias potential V _B the provided to detect the toner adhesion amount of the reference toner image, an automatic density control for automatically controlling the grid potential V _G and the bias potential V _B so that the maximum density of an image corresponding to the detected value becomes constant Has been done.

(Problems to be Solved by the Invention) There are other points to be considered when automatically controlling the maximum density by changing the grid potential and the bias potential.

In digital image forming apparatuses, especially in full-color digital image forming apparatuses, fogging is a serious problem. The head, because basically dependent on the grid potential V _G and the bias potential V _B, in order to prevent the head may be those potentials to properly control. However, fog removal must not affect automatic density control.

Therefore, in another application, the applicant can set the fog removal level by the user, and in response to this, change the grid potential of the charging charger and the bias potential of the developing device to perform digital density control and fog removal. An image forming apparatus was proposed.

Further, in the case of a halftone image, it is necessary to consider the influence on gradation correction (so-called γ correction) due to changes in the grid potential and the bias potential. In general, the reading density of a document image to be reproduced and the density of an image reproduced by copying are not proportional to the photosensitive characteristics of the photoconductor, the characteristics of the toner, and the use environment (humidity, temperature, etc.). Tone correction (γ correction) for correcting and outputting the read density data so as to increase the fidelity is required.

By the way, in the detection of the toner adhesion amount of a reference toner image of a predetermined intermediate density as a basis of density control by a density sensor, a laser beam of a predetermined reference intensity is applied under a predetermined reference grid potential and a reference bias potential. An image is formed. The reference grid potential and the bias potential are also set so that fog can be removed. At this time, the exposure is performed at a light emission level converted corresponding to a predetermined reference halftone density by the gradation correction characteristic. However, in such a correction of the light emission level, the development potential, which is the difference between the surface potential and the bias potential determined by the grid potential, changes, so that the toner adhesion amount is not constant, and accurate density information can be obtained. Absent. Therefore, unless the conditions for forming the reference toner image are further controlled so as to have the same gradation, the accuracy of the automatic density control cannot be improved.

An object of the present invention is to provide a digital image forming apparatus capable of controlling a grid potential and a bias potential to control a density and accurately detecting a reference density as a reference for the density control in a reversal developing electrophotographic process. It is to provide.

(Means for Solving the Problems) A digital image forming apparatus according to the present invention is a digital image forming apparatus that modulates the amount of laser light based on image data, performs exposure, and forms an image by a reversal developing electrophotographic process. An image forming apparatus, wherein a reference grid potential of a charging charger and a reference bias potential of a developing unit are respectively set to predetermined values, and a density detection for detecting a toner density when exposing and developing with a laser beam having a reference intensity. Means, and a density control means for setting a grid potential of the charging charger and a bias potential of the developing device so that the maximum density becomes constant in accordance with the toner density detected by the density detection means. At the time of density detection by the density detecting means, a reference potential setting for setting a reference grid potential and a reference bias potential so as to remove fog. Setting means,
When the density is detected by the density detecting means, the reference intensity of the laser beam is set so that the developing potential at the exposure position is constant under the reference grid potential and the reference bias potential set by the reference potential setting means. And reference intensity setting means for setting the reference intensity. Preferably, the density detecting means sets the reference grid potential in response to fog removal while keeping the reference bias potential constant.

(Function) In the density detection by the density detecting means, the reference grid potential and the reference bias potential are selected and set so as to remove fog, and the selected values are also set in the toner image formation of the reference density formed at the time of the density detection. Accordingly, the reference intensity of the laser beam can be set so that the developing potential of the density detection pattern becomes constant.

Hereinafter, embodiments of the present invention will be described in the following order with reference to the accompanying drawings.

(A) Configuration of a digital color copier (b) Image signal processing (c) Integration of automatic density control and fog removal in a reversal developing electrophotographic process (d) Integration with gradation correction <d-1> Gradation correction <D-2> Integration of fog removal and gradation correction <d-3> Broken line approximation <d-4> Gradation correction using final correction addition corresponding to fog removal level <d-5> Gradation by β code Selection of correction data (e) Semiconductor laser power control <e-1> Temperature fluctuation and maximum light amount correction <e-2> Fog removal and gradation control (f) Printer control flow (g) Automatic fog removal (h) Pulse Tone Correction in Width Modulation Method The present invention particularly relates to the sections (c), (d) and (e) (particularly <e-2>).

(A) Configuration of Digital Color Copier FIG. 1 is a 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 for reading a document image and a copying unit 200 for reproducing an image read by the image reader unit.
It can be broadly divided into

In the image reader unit 100, the 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 color for converting the condensed light into an electric signal. An image sensor 14 is provided. The scanner 10 is driven by the motor 11 at the time of reading a document, moves in the direction of the arrow (sub-scanning direction), and scans the document placed on the platen 15. The image on the document surface irradiated by the exposure lamp 12 is photoelectrically converted by the image sensor 14. R, G, B 3 obtained by the image sensor 14
The multi-valued electrical signal of the color is converted by the read signal processing unit 20 into gradation data of four colors of 8 bits each of yellow (Y), magenta (M), cyan (C), and black (K). Are stored in the synchronization buffer memory 30.

Next, in the copying section 200, the print head section 31
Performs gradation correction (γ correction) on the input gradation data according to the gradation characteristics in the image forming process, and then D / A converts the corrected image data to generate a laser diode drive signal. Then, the semiconductor laser is caused to emit light by this drive signal (see FIG. 4).

The laser beam generated from the print head unit 31 in accordance with the gradation data exposes the photoreceptor drum 41, which is rotationally driven, via the reflecting mirror 37. The photoreceptor drum 41 is irradiated with an eraser lamp 42 before receiving exposure for each copy, and is uniformly charged by a charger 43. When exposure is performed in this state, an electrostatic latent image of the document is formed on the photosensitive drum 41. Only one of the cyan, magenta, yellow, and black toner developing units 45a to 45d is selected to develop the electrostatic latent image on the photosensitive drum 41.
The developed toner image is transferred by a transfer charger 46 onto a copy paper wound around a transfer drum 51. Also,
The toner adhesion amount of the reference toner image developed by exposing a predetermined area on the photoreceptor with a predetermined amount of light is optically detected by the AIDC sensor 210. That is, light is obliquely incident on the reference toner image, and reflected light from the reference toner image is detected. The toner adhesion amount is determined by the intensity of light reflected from the toner image.

The printing process includes yellow (Y), magenta (M),
This process is repeated for four colors of cyan (C) and black (K). At this time, the photosensitive drum 41 and the transfer drum
The scanner 10 repeats the scanning operation in synchronization with the operation of 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 the paper cassette 50, and the leading end thereof is chucked by the chucking mechanism 52 on the transfer drum 51, so that no displacement occurs during transfer.

FIG. 2 is an overall block diagram of a control system of the digital color copying machine according to the embodiment.

The image reader unit 100 is controlled by the image reader control unit 101. The image reader control unit 101 includes a platen 1
5 controls the exposure lamp 12 via a drive I / O 103 in accordance with a position signal from a position detection switch 102 indicating the position of the original on the scanner 5, and scan motor driver 105 via a drive I / O 103 and a parallel I / O 104. Control. 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 by a bus. The image control unit 106 is connected to each of 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 the image signal processing unit 20 and processed.

The copying unit 200 includes a printer control unit 201 that controls general copying operations.

A printer control unit 201 having a CPU is connected to a control ROM 202 storing a control program and a data ROM 203 storing various data (such as γ correction data). The printer control unit 201 controls the printing operation based on the data in the ROM.

The printer control unit 201 determines the surface potential V _O of the photosensitive drum 41.
V _O sensor 44 for detecting, for detecting a toner adhesion amount of the reference toner image adheres to the surface of the photosensitive drum 41 optically AIDC
Sensor 210, ATDC sensor 211 for detecting toner concentration in developing units 45a to 45d, temperature sensor 212, and humidity sensor
Analog signals from various sensors 213 are input.

Further, the printer control unit 201 has a fog input switch (2 bits) 21 for setting a fog removal level.
4 and a color balance switch (4 bits each) 216 for setting the color balance level of each color, and a photoconductor lot switch (3 bits) indicating lot dependency of photoconductor characteristics
218 are connected via I / Os 215, 217, and 219, respectively.
The fog input value (four levels) is set by a serviceman or a user with a DIP switch in this embodiment, but may be input from the operation panel 221 via the parallel I / O 222. Various data is input to the printer control unit 201 via the parallel I / O 222 by key input on the operation panel 221.

The printer control unit 201 includes sensors 44, 210 to 213, an operation panel 221, input switches 214, 216, 218, and a data R.
The data from the OM 203 controls the copy control unit 231 and the display panel 232 in accordance with the contents of the control ROM 202.
To perform manual concentration control by inputting to 21,
Parallel I / O241 and drive I / O242 for V _G generating generates the grid potential V _G of the electric charger 43 through the high voltage unit 243 and a developing unit V _B generating high voltage for generating a developing bias potential V _B of 45a~45d The unit 244 is controlled.

The printer control unit 201 is also connected to the image signal processing unit 20 of the image reader unit 100 via an image data bus,
Based on the image density signal coming via the image data bus, the semiconductor laser driver 263 is controlled via the drive I / O 261 and the parallel I / O 262 with reference to the contents of the data ROM 203 storing the γ correction table. ing. The light emission of the semiconductor laser 264 is driven by the semiconductor laser driver 263. The gradation expression is performed by modulating the emission intensity of the semiconductor laser 264. (In the modification, a pulse width modulation method for modulating the light emission time is used ((h)
Section).

(B) Image Signal Processing FIG. 3 is a diagram for explaining the flow of processing of image signals from the CCD color image sensor 14 to the printer control unit 201 via the image signal processing unit 20. With reference to this, read signal processing for processing the output signal from the CCD color image sensor 14 and outputting gradation data will be described.

In the image signal processing unit 20, the image signal photoelectrically converted by the CCD color image sensor 14 is converted by the A / D converter 2
At 1, the data is converted into R, G, B multi-valued digital image data. Each of the converted image data is subjected to shading correction by the shading correction circuit 22. Since the image data subjected to the shading correction is the reflected light data of the original, the log data is converted by the log conversion circuit 23 into density data of an actual image. Further, the under color removal and black printing circuit 24 removes unnecessary black color development and generates true black data K from R, G, B data.
Then, the masking processing circuit 25 converts the data of the three colors R, G, B into data of the three colors Y, M, C. The density correction processing for multiplying the Y, M, and C data thus converted by a predetermined coefficient is performed in the density correction circuit 26, and the spatial frequency correction processing is performed in the spatial frequency correction circuit 27. The density data is output to the printer control unit 201 as density data of ~ 255.

FIG. 4 is a block diagram of image data processing in the printer control unit 201. Here, the image data (8 bits per color) from the image signal processing unit 20 is input to a first-in first-out memory (hereinafter referred to as a FIFO memory) 252 via an interface unit 251.
The FIFO memory 252 is a line buffer memory capable of storing gradation data of a predetermined number of lines in the main scanning direction, and absorbs a difference in operation clock frequency between the image reader unit 100 and the copying unit 200. It is provided in order to.
Next, the data in the FIFO memory 252 is input to the γ correction unit 253. As will be described later, the γ correction data in the data ROM 203 is sent from the printer control unit 201 to the γ correction unit 253, and the γ correction unit 253 corrects the input data (ID) and changes the output level to D.
Send to / A conversion unit 254. The analog voltage converted from the output level (digital value) by the D / A conversion unit 254 is then input to the gain switching unit 255 in accordance with the gain setting value from the printer control unit 201. strength after being amplified by switching the switch SW ₁ to SW ₈ (corresponding to different power P ₁ ~P _8), is sent to the semiconductor laser driver 263 via the drive I / O261, the value of the semiconductor laser 264 by (Or a pulse width of that value in a modified example). The printer control unit 201 sends a clock signal to the semiconductor laser driver 263 via the parallel I / O 262.

(C) the integrated reversal development system electrophotographic process of automatic density control with the fog removal in the reversal development system electrophotographic process, the concentration is controlled by the grid potential V _G and the bias potential V _B.

FIG. 5 shows the charging charger 43 around the photosensitive drum 41.
And the arrangement of a developing unit (for example, 45r) are schematically shown. Here, the photosensitive drum 41, a charger discharge potential V _C
43 are installed facing each other. The grid of the main charger 43 negative grid potential V _G is applied by the grid potential generating unit 243. Surface potential of photoreceptor drum 41
V _O is measured by Vo sensor 44, and based on the measured value, it is controlled by adjusting the grid voltage V _G.

First, before the laser exposure, a negative surface potential V _O is applied to the photosensitive drum 41 by the charging charger 43, and a low potential negative bias voltage V _B ( | V _B |> | V _O |). That is, the developing sleeve surface potential is V _B.

Potential of the irradiated position on the photosensitive drum 41 is shifted to decay potential V _I of the electrostatic latent image from the surface potential V _O decreases by the laser exposure. When decay potential V _I is a potential lower than the developing bias voltage V _B, the toner that has been carried to the sleeve surface of the developing device 45r (negatively charged) adheres to the photosensitive drum 41. The difference between V _O and V _B may not be too large or too small.
The larger the developing voltage ΔV = | V _B −V _I |, the larger the toner adhesion amount. On the other hand, decay potential V _I will vary as the surface potential V _O may be the same exposure amount changes. Therefore, while maintaining the difference between V _O and V _B within a certain range, for example, while a substantially constant difference, if the change of the surface potential V _O and the developing bias potential V _B, the difference between V _B and V _I , The amount of toner adhesion can be changed, and the density can be controlled.

On the other hand, the amount of toner adhering to an image
Detected by sensor 210. That is, the photosensitive drum 41
A reference toner image serving as a reference for density control of the toner image is formed.Specifically reflected light and scattered reflected light of the reference toner image are detected by an AIDC sensor 210 provided near the photosensitive drum 41. The toner is input to the control unit 201, where the toner adhesion amount is obtained from the difference between the two detection signals. Therefore, it is possible to perform automatic density control to maintain V _O corresponding to the detected value, the toner adhesion amount of the maximum density level be changed to V _G constant. For example, the attenuation characteristics of the toner charge amount change due to changes in the environment such as photoconductor sensitivity and relative humidity. However, the maximum density can be automatically kept constant by changing V _O and V _B. Therefore, in the present embodiment to correspond to one grid potential V _G to one bias voltage _{V B, (V B, V} G)
Is changed in accordance with the density detection level (LBA) of 0 to 28 corresponding to the detection value of the AIDC sensor 210.

Table 1 shows an example of data of a set of (V _B , V _G ) set in this way. (Note, V _B, V _G but is negative, in the table indicated by the absolute value for simplicity.) Detection value of AIDC sensor 210, the level of 0 to 28 shown in the leftmost column on the basis of its size is made to correspond to, 20 from V _B is 100V corresponding to each level
It changes by V and becomes 660V at the maximum. V _G is kept at 100V greater than V _B and therefore varies from 200V to 760V.
(The amount of change in (V _G , V _B ) may be determined according to the accuracy of the control.) FIG. 6 is a schematic graph showing the change in potential at each level in the photoconductor. Here, V _G , V _O , and V _B are finally given subscripts of the corresponding levels. Further, curve shows the distribution of potential at the time of irradiation with the one dot at the maximum amount of light corresponding to the density level 255 in each of the V _G, V _I denotes the decay potential.

Further, in a digital image forming apparatus, particularly in a full-color image forming apparatus, removal of fog is a serious problem.

Therefore, in this embodiment, when (V _B , V _G ) is changed in accordance with the detection value of the AIDC sensor 210, the user further determines that the grid potential V _G is appropriate by looking at the reproduced image. Four levels of fog removal levels (LBK) 0 to 3 can be set by the fog input switch (2 bits) 214. That is, as shown in FIG. 7 for the same bias potential when the input level (OD) = 5, the bias potential V corresponds to the detection level (LBA) of the AIDC sensor 210.
And determines the _B5, the grid potential also correspond to the head removal level (LBK) 0 to 3 set by the user is varied in four steps V _G50 ~V _G53. As shown in Table 1, the grid potential corresponding to the same _{V B (V GM, M =} 0~
Change △ V _G of V _G between levels 3) is as small as 20V at low V _B, the high V _B 30 V, are sequentially increased to 40V. △ V _G is V _G
, The influence on the automatic density control and the gradation is small.

Since the digital image forming apparatus according to the present embodiment is a full-color copying machine, the color balance must be adjusted. Therefore, for each color of cyan, magenta, yellow, and black, a 4-bit (ie, +7 to -7) positive / negative color balance bias level (LBC) is set by the color balance switch 216 at the time of factory shipment or by a service person or a user. You. Based on the color balance bias level of each color, the maximum value of the image density corresponding to the image signal level 255 is increased or decreased to adjust the color balance. This allows the color balance switch 216 to set the color balance bias level (LBC) even in areas where color adjustment is not possible due to the toner density control in the developing unit by the AITC sensor 211, or when the user wants to adjust the color as he or she likes. it can. Therefore, it is necessary to consider the relationship between the color balance and the above-described automatic density control and fog removal.

When changing the grid potential V _G and the bias voltage V _B for the present the embodiment, after corresponding to the detected value of the AIDC sensors 210 as discussed remove fog and automatic density control, the response to this color Tone correction is performed every time. If this gradation correction is ideally performed, the color balance should have been automatically realized in principle without special consideration. However, the color balance is greatly affected by the gradation of each color, and the accuracy of the color balance may be insufficient in the above-described gradation correction due to the change in the grid potential and the bias potential. So, the user can switch the color balance
The color balance can be set using the 216 (see FIG. 22).

This color balance can be performed by controlling the grid potential and the bias potential. However, it is easier to perform processing by associating the grid potential and the bias potential for automatic density control and fogging removal. One stage of the balance bias level (LBC) was set to be the same as one stage of the concentration detection level (LBA) by the AIDC sensor 210. Therefore, it is possible to adjust the color balance by changing the grid potential V _G and the bias potential V _B at the correction detection level obtained by adding the LBA and LBC. Since this has little effect on automatic density control and gradation correction, color balance adjustment can be realized together with automatic density control and gradation correction.

Note that the characteristics of the AIDC sensor 210 are also subject to noise due to sensor mounting, sensitivity changes, dirt, and the like. In addition, the characteristics of the photoconductor also change due to environmental changes. Therefore, by performing the color balance adjustment together with these adjustments, a highly accurate color balance can be obtained.

Even in the case of a non-color copying machine, fine adjustment of the detection level (LBA) can be generally performed by associating the above-mentioned color balance bias level (LBC) with a bias level representing a characteristic change or an environmental change of various components.

Thus, each color (V _G, V _B) value will be determined by the final following data.

Based on the detection level (LBA) 0 to 28 Color by balancing switch balance bias level (LBC) -7~ + 7 head input switch according to the head input level (LBK) 0 to 3 of these data by the AIDC sensors, the bias potential V _B values are selected by the correction detection level LBXN (= LBA + LBC), the value of the grid voltage V _G, the correction detection level LBXN and (head removed even considered to) is selected by the LBK (Fig. 25 S206~S208 reference ).

(D) Integration with Tone Correction <d-1> Tone Correction By the way, in copying a halftone image, tone characteristics must be considered. In general, various factors such as the photosensitive characteristics of the photoreceptor, the characteristics of the toner, and the use environment are intertwined, and the reading density level (hereinafter also referred to as input level) (OD) of the document to be reproduced and the light emission intensity level of the laser beam (therefore, reproduction) The image density level (ID) thus obtained is not exactly proportional, but shows a characteristic B deviating from the originally obtained proportional characteristic A, as schematically shown in the upper right of FIG. (It does not pass through the origin because of fogging or the like.) Such a characteristic is generally called a γ characteristic (gradation characteristic), and particularly, the fidelity of a reproduced image printed on a halftone original is reduced. This is a major factor. Therefore, the output power (also referred to as laser energy) P of the semiconductor laser 264 is corrected by the γ correction unit 253.
The output characteristic is controlled in advance like the exposure correction characteristic at the lower right of FIG. 8 to realize the proportional characteristic A. This is called gradation correction (so-called γ correction). That is, the output power is increased at a low gradation and the output power is decreased at a high gradation, and the density of the reproduced image is made proportional to the gradation.

Incidentally, as shown in photoreceptor characteristics of Figure 8 the lower left, decay potential V _I of the photosensitive member corresponding to the output power of the semiconductor laser varies nonlinearly. Further, the toner is deposited in the V _I <V _B, as shown in the development characteristics of Figure 8 the upper left, the toner adhesion amount also changes nonlinearly.

FIG. 9 shows the grid potential V _G under the conditions shown in Table 2, that is, while maintaining the bias potential V _B at −591 V.
7 is a graph of various characteristics in a case where is changed in six steps in a range from -700V to -919V.

Further, FIG. 10, under the conditions shown in Table 3, i.e., in the range of -362V bias potential V _B from -637V,
Shows a graph of various characteristics when changed to 8 stage grid potential V _G in the range from -835V to -565V.

Thus, when changing the bias potential V _B and grid potential V _G can control various characteristics.

Returning to FIG. 8, the explanation will be continued. Now, when the photoconductor is exposed to the power Px of the semiconductor laser 264 at the level of Lx,
The potential of the photoreceptor drops to Vx, so that the developing potential △ Vx becomes | V
_B −Vx |. At this time, the toner amount IDx adheres to the photoreceptor by the toner development.

In the automatic density control, the AIDC sensor 210 outputs a detection value V _AIDC (analog value) corresponding to the toner adhesion amount IDx to the printer control unit 201 (this output characteristic is also non-linear). The printer control unit 201 determines the detection level LBA (0 to 28) In response to this, the maximum density corresponding to the detected level determined to maintain a constant (V _G, V _B) the value of Table 1 Select according to the data.

Integration with <d-2> Head removal and gradation control, however, (V _G, V _B) Fig. 9 is only keeps simply maximum density by changing the value constant, the gradation characteristic as shown in FIG. 10 Changes, and the fidelity of halftone image reproduction deteriorates. Now the eighth
When the gradation characteristic as shown in FIG curve B, and increasing the V _G, the toner adhesion amount is increased even with the same power Px, gradation characteristic varies as B1.

For example, as shown in the eleventh diagram side, when the damping potential of the photosensitive member corresponding to the semiconductor laser of output power (laser energy) were changed as → to changes in grid potential V _G, the As shown in the lower part of FIG. 11, the development voltage | V _B -V _I | related to the toner adhesion amount changes as shown by →. Effect of the gradation according to a change in the V _G is greater in the particular low density side. Therefore, when high-precision halftone reproduction is desired, especially in color copying, when controlling the maximum density and fog removal, the nonlinearity originally existing in toner development is improved (normal γ correction), and automatic grid potential V _G for density control, keeping the gradation characteristics when changing the bias potential V _B constant, and grid potential V _G to the head removed, even when changing the bias potential V _B An integrated tone correction must be performed so as to keep the tone characteristics constant.

In the present embodiment the input switch 214 user fog sets the head input level in 4 steps (LBK), varying the V _G correspondingly.

Now, as shown schematically in FIG. 12, the same bias potential V _B of the input level fog the grid potential V _G under 0-3
Changes in response to, | V _G -V _B | when the sequentially changing, the rise of the output image density ID is gradually slows, the gradation curve is changed. (In this figure, the output ID is normalized by the maximum value and the minimum value.) Therefore, the output ID greatly changes even for the same input level (OD). Accordingly, if the output level ID is corrected and output to the semiconductor laser 264 as shown by curves 0 to 3 in FIG. 13, a target curve (that is, an output proportional to the input) is obtained. In the present embodiment, as shown in the right column of Table 1, a plurality of γ correction tables representing the gradation correction data are set as (V _B , V
Corresponding to _G) value is stored in advance in the data ROM203 to, gamma correction unit 253, (V _G, V _B) to select the gamma correction table than the set value (see FIG. 26), the tone correction Gamma correction is performed with reference to the data (see FIG. 26). In this example, the bias potential V _B is changed in 29 steps, since changing the V _G in four stages in the V _B, 29 × 4 = 116 pieces of γ correction table is stored in the data ROM 203.

Tables 4 (a) and (b) show examples of the γ correction table. An input level of 0 to 255 is an output level of 0 to 1023 (10
Bit). The gamma correction table (a) is
In the figure, the γ correction table (b) corresponds to that in FIG. Here, the reason why the 8-bit data is converted to the 10-bit data is to prevent the conversion accuracy from deteriorating in the gradation correction calculation as described later.

<D-3> Broken Line Approximation To convert the read density data OD (8 bits) 0 to 255 of the image into output data (0 to 255) in the γ correction,
Conventionally, output data for each of the input values 0 to 255 is stored in the data ROM 203 (this is referred to as a γ correction table), and data conversion is performed with reference to the data in the data ROM 203. Therefore, 256 data must be stored in the data ROM 203. Moreover, in this embodiment, it must be carried out γ correction using different γ correction table for many (V _G, V _B). Therefore, there is a problem that a very large memory capacity is required for the gradation correction data.

There is also a method in which the γ correction is performed with emphasis on only a portion where the γ characteristic changes linearly. However, this method is not preferable because reproducibility is low particularly at low density and fog is removed.

Therefore, in this embodiment, a polygonal line approximation is adopted as shown in FIGS. 14 (a) and 14 (b). Here, the broken line in FIG. 14 (a) represents raw data. 14 (a) and 14 (b), (a) shows a case where the input level itself is approximated by a broken line (variable section), and (b) shows a case where the width of each section is made constant. This shows a case where a broken line approximation is performed. It is desirable that the tone characteristics be approximated by dividing into at least three sections of a low density part, an intermediate density part and a high density part which change most rapidly from the shape. Therefore, a polygonal line approximation is made at least at two or more intermediate points. Thus, in this embodiment, data obtained by approximating the input levels 0 to 255 with ten broken lines is stored in the data ROM 203. As a result, the memory capacity is about 1/10
Can be reduced.

Eleven contact points for dividing input levels 0 to 255 into ten sections were selected with emphasis on reproducibility of low-density parts. Specifically, as shown in Table 5, the section at low concentration is increased,
(0,4,8,16,32,64,128,160,192,224,255) points were selected. Table 5 shows 10 sections (N = 1) determined by these points.
The inclination a (N) and the intercept b (N) of each polygonal line in 〜10) are shown. Tables 6 (a) and 6 (b) also show examples in which the same 10 sections are broken-line approximations. Here, Tables 6 (a) and 6 (b) respectively correspond to those in FIG.

Therefore, the γ correction unit 253 stores data corresponding to the detection value of the AIDC sensor 210, the color balance input value, and the fog input value.
The γ correction table in the ROM 203 is selected (see FIG. 26).
When receiving the input level value X, the coefficient a corresponding to the section corresponding to the value is obtained from the γ correction table.
(N), b (N) are read, and the operation a (N) · X + b
(N) is performed, and the result Y is output. For example, the fifth
When the γ correction table in the table is used, if the input level = 50, the section N = 5, and a (5) = 5 and b (5) = 24
Is read out, and the conversion level Y is obtained from the calculation of a (5) × 50 + b (5).

In this polygonal line approximation, if an 8-bit operation is performed, the conversion efficiency of the gradation characteristic is substantially reduced to 1 or less at the maximum slope of the gradation characteristic. When the maximum slope γ = 4 as in the example at the upper right of FIG. 8, the conversion efficiency is X = 25 to 12
In the range of 5, it drops to about 25%. Therefore, while the input level X is 8 bits, the gamma correction operation is performed with 10 bits, which is 2 bits larger than the number of input bits. Therefore, even if the maximum gradient γ is 4, the gradation number conversion efficiency is not substantially reduced by the calculation, and an ideal gradation characteristic (γ = 1) can be obtained. In other words, if the ratio of the number of output gradations to the number of input gradations is substantially equal to or greater than the value of the maximum gradient in the gradation characteristics, the output value is also corresponding to one step change of the input level X. Since it always changes, the number of gradations does not decrease in the γ correction operation.

In the above, the tone correction curve is approximated by a polygonal line, but another approximation that can reduce the number of tone correction data may be used. In this case, data representing an approximate expression may be stored in the same manner as the polygonal line approximation.

<D-4> Gradation correction using final correction addition corresponding to fog removal level As a modified embodiment for reducing the capacity of gradation correction data stored in data ROM 203, gradation correction according to fog removal level (LBK) A method of correcting data may be adopted.

Since the change of the grid potential V _G by fog removal level (LBK) is smaller than the size of the grid potential itself, even the (V _B, V _G) is gradation correction due to a change in V _G in a substantially identical This is a good approximation. Therefore, it is a good approximation to process the change of the gradation correction accompanying the fog removal by the addition correction, and it also reduces the memory capacity of the gradation correction data.

Therefore, for automatic density control and tone control corresponding thereto, a pair of bias potential V _B and grid potential V _G (V _B,
V _G ) is stored for each gradation correction table. further,
An addition correction table is stored for each of the fog input levels (LBK) 0, 1, 2, and 3 set by the fog input switch 214. However, if the fog removal level is 0, no correction is made. Then, by adding the output values of both tables for the same reading density (OD), a finally corrected conversion level can be output. Therefore, (V _B , V
_{When G} ) is changed in 29 steps, only 29 + 3 = 32 γ correction tables need to be stored, and the memory capacity is reduced to about 1/4 as compared with the case of using 29 × 4 = 116 γ correction tables. Can be reduced.

Α1, α2, and α3 in Table 7 show examples of addition correction tables when the fog removal level (LBK) is 1, 2, and 3, respectively. Now, assuming that the table A0 in Table 8 is the γ correction table selected by (V _B , V _G ), the γ correction data of the γ correction table is added to the addition correction tables α1, α2, α
The results corrected by adding in Table 3 are shown in Tables A1 and A, respectively.
The conversion level Y value indicated by 2, A3 is obtained.

If it is desired to further reduce the memory capacity of the gradation correction data, the above-described polygonal line approximation may be used. in this case,
(V _B, V _G) to the γ correction table selected in stores the data of the polygonal line approximation, the actual tone correction operation be performed final correction operation on the converted level data from γ correction table Good.

<D-5> Selection of gradation correction data by β code By the way, in the method described above, there are four levels of V _G (for fog removal) and 29 levels of V _B (for density control). It is necessary to prepare x29 = 116 gamma correction tables. Therefore, in another modified embodiment, the number of γ correction data is reduced as follows.

The gradation characteristics for many combinations of (V _B , V _G ) as described above are often similar to each other. V _B, the more those similar as the difference V _G is reduced. Therefore, if almost similar ones among many gradation characteristics can be classified by appropriate parameters, it is possible to reduce the number of γ correction tables by assigning parameters to each set of (V _B , V _G ) in advance. It is considered possible.

The gradation characteristics are calculated from the bias potential V _B , the surface potential V _O, and the attenuation potential V _i at the maximum light quantity when the laser output level is 255. β = (V _B −V _I ) / (V _O −V _I ) Is largely determined by That is, β is appropriate as the above parameter. Therefore, in advance, FIG. 9, the decay potential V _I at correspondingly surface voltage V _O and a maximum amount of light in various characteristics change as shown in FIG. 10 was measured in each selectable (V _G, V _B) And β
= (V _B -V _I) previously determining the / (V _O -V _I) (see Table 9). Table 9 shows the obtained results and the corresponding β codes (0 to 51). The β code is a code determined by dividing the β value (0.420 to 0.939) by 0.010 as shown in Table 10. Then, the optimum γ correction data is stored for each β code. As a result, the number of γ correction tables may be 51, and the memory capacity is reduced to about 1/2 compared to the above example. If the pitch of the section of the β value is further increased, the memory capacity of the γ correction data can be further reduced.

In actual correction operation, beta code is obtained from the table by V _G and V _BM selected, the beta gamma correction table corresponding to the code is read from the data ROM 203, the output data for light emission using the same Is output.

When the above-described polygonal line approximation is used for the γ correction table, the memory capacity of the γ correction data can be further reduced.

(E) Semiconductor Laser Power Control <e-1> Temperature Fluctuation and Maximum Light Amount Correction Some organic photoconductors have sensitivity characteristics (decay curve of surface potential) with respect to the light emission level of the semiconductor laser 264 that change with temperature. . When gradation is expressed by modulating the intensity of the semiconductor laser 264 using such a photoconductor,
The fidelity of tone reproduction changes depending on the temperature. Therefore, in this embodiment, as described below, the maximum light amount in the intensity modulation method is controlled according to the surface temperature of the photoconductor.

FIG. 15 shows the sensitivity characteristics of the photoreceptor at 10 ° C. (low temperature) and 25 ° C. (normal temperature) (humidity is 55% RH in the following description). The surface potential at 10 ° C is lower than the surface potential at 25 ° C. For example, half the amount of light the surface potential to 1/2, as shown by a broken line, while requiring laser energy 0.5μJ / cm ² at 25 ° C., at 10 ℃ 0.62μJ / cm ²
Laser energy and sensitivity is reduced by about 20%.

FIG. 16 shows the decay of the surface potential when the emission energy of the semiconductor laser 264 is changed at 10 ° C. and 25 ° C. here,
The maximum emission level is 25 when the emission energy is 1.6 μJ / cm ² at 10 ° C. and 1.0 μJ / cm ² at 25 ° C.
At 5, the attenuation of the surface potential is almost the same. But,
Conversely, in the halftone portion, the attenuation is large, and the shift due to the temperature change at each level is large as a whole.

However, when the emission energy at 10 ° C. is 1.35 μJ / cm ² , the surface potential at level 255 is slightly larger than when the emission energy at 25 ° C. is 1.0 μJ / cm ² , The entire light emission level (output level) is almost the same. That is, the sensitivity characteristics are substantially similar.

FIG. 17 shows the gradation characteristics on the upper side and the emission level conversion characteristics on the lower side. Emission energy at a maximum level of 255 at 10 ° C
Assuming 1.35 μJ / cm ² , the normalized gradation characteristics are almost the same as shown above, compared to the case where the emission energy at 25 ° C. is 1.0 μJ / cm ² at the maximum level of 255. As shown on the lower side, the light emission level conversion characteristics for making the gradation characteristics linear with respect to the level are also substantially the same.

Therefore, when copying at 10 ° C., it is understood that almost the same gradation characteristics can be obtained by increasing the emission energy at the level 255 to 1.35 μJ / cm ² .

Similarly, in other temperatures may be to control the maximum amount of light at a level 255 such that the attenuation curve of the surface potential V _I to the emission level in the same. Thus even if the temperature changes developing voltage | V _B -V _I | no level change is made tone characteristic substantially the same, the developing characteristics are stabilized.

Specifically, the temperature range is classified in advance in accordance with, for example, the luminous energies at which the maximum amounts of light at a plurality of levels as shown in Table 11 are generated. Then, in response to the temperature detected by the temperature sensor 212, the photoconductor temperature code LPC to which the temperature belongs is determined and stored (see the flow in FIG. 21).

On the other hand, since the sensitivity characteristics vary depending on the lot of the photoconductor, a 3-bit photoconductor lot code (LLOT) is assigned to the photoconductor drum 41 in advance. This code corresponds to a maximum emission energy difference of 0.1 mW so that it can be added to the photoconductor temperature code (LPC). Therefore, the sum of the two codes LLOT and LPC is defined as the power code LPOW (Table 11,
(See the flow in FIG. 23). However, the code LPOW is set to 7 when the sum exceeds 7, but the number of bits is increased to 8
As described above, the maximum power may be further increased. When the power code LPOW determined in this way is sent as a level setting value to the gain switching signal generating circuit 256 of the gain switching section 255 (see FIG. 4), only the switch corresponding to the level setting value becomes conductive, and the gain is reduced. Is set. In this way, the gain switching section 255 controls the maximum light amount in accordance with the fluctuation of the surface temperature, and can make the sensitivity characteristics of the photoconductor substantially the same. Therefore, the gradation characteristics are stabilized with respect to the temperature change.

Further, after controlling the maximum light amount of the semiconductor laser 264 in this way (S6 in FIG. 20), the density control by the AIDC sensor 210 is performed (S7 in FIG. 20). Therefore, also in the detection by the AIDC sensor 210, the surface potential in the exposure area of the reference toner image becomes constant, and only the change in the development characteristics can be detected, and the automatic density control with high accuracy can be performed.

Furthermore, even when controlling the grid potential V _G and the bias potential V _B to the tone correction by the detection signal by the AIDC sensors 210, the decay curve of the photoreceptor is substantially the same, the halftone area even change temperature Can be accurately reproduced.

<E-2> Fogging removal and gradation control The user can also set the reference value of (V _G , V _B ) in the preparation of the reference toner image at a predetermined intermediate gradation for detecting the amount of toner adhesion using the AIDC sensor. Set fog removal level (LBK)
Is changed correspondingly. As already described, so as not substantially alter the overall tone characteristic for head removal (V _G, V _B) tone correction conversion of emission levels while maintaining the maximum amount of light by changing the constant is made. However, the development potential,
Thus the detection value of the AIDC sensors 210, each (V _G, V _B) during the image formation of the reference toner image in this level of gradation correction accuracy will vary by value, not the exact amount of toner adhesion information is available. Therefore, if the fog removal level fluctuates, gradation correction cannot be performed accurately.

Therefore, separately from this gradation correction, the light emission amount of the semiconductor laser 264 is changed in accordance with the fog removal level set by the fog input switch 214 when forming a reference toner image for detecting the amount of adhered toner by the AIDC sensor 210. . That is, with respect to (V _B , V _G ) for forming a reference toner image for detecting the amount of toner adhered by the AIDC sensor 210, the bias potential V _B is kept constant but corresponds to the fog removal level (LBK). varying the grid potential V _G in four stages Te. At this time, when the semiconductor reference output power of the laser 264 for imaging the reference toner image is the same, as shown in FIG. 18, decay potential V _I of the photosensitive member varies. So even after changing the grid potential V _G corresponds to the head removal level (LBK), as indicated by broken lines in FIG. 19, as decay potential V _I on the photosensitive member is constant, i.e., the development voltage | V _B
The output power of the semiconductor laser 264 is changed so that −V _I | becomes constant. For example, as shown in FIG. 19, when the decay potential V _I in response to changes in the grid potential is changing,
_Assuming that the output level at the minimum grid potential V _G0 is 100,
Grid potential V _G1, V _G2, the output level at V _G3 120,140,165
, The developing voltage ΔV = | V _B −V _I | can be kept constant. Therefore, in this embodiment, the output level of the laser corresponding to each grid potential is stored in the data ROM2.
03 is stored in, when creating a reference toner image, performs exposure at the output level corresponding to the grid potential V _G value at that time. As a result, the toner adhesion amount of the reference toner image can be kept constant, so that the density can be controlled based on an accurate detection value even if the fog removal level varies.

(F) Flow of Print Control Hereinafter, a flow of print operation control in the printer control unit 201 will be described.

FIG. 24 shows a main flow of the printer control unit 201. First, an initial setting is performed (Step S1, hereinafter, "Step" is omitted). Next, it waits for a key input of a print switch on the operation panel 221 (S2, S3). When there is a key input of the print switch (YES in S3), a process of inputting data of each sensor is performed (S4, see FIG. 21), and a switch input process (S5, see FIG. 22) is performed. power setting processing (S6, see FIG. 23), AIDC measurement process (S7, see FIG. 24), V _G, V _B code selection processing (S8, see FIG. 25), gamma
The correction table selection process (S9, see FIG. 26) is performed sequentially,
The copying operation is performed (S10), and it is determined whether the copying operation has been completed (S11). If not completed, the process returns to S8, and if completed, the process returns to S2.

FIG. 21 shows a flow of the sensor input process (S4). First, the temperature of the photoconductor detected by the temperature sensor 212 is input, and a 3-bit photoconductor temperature code LP according to Table 11 is input.
It is stored as C (S31). Further, data detected by other sensors is stored (S32), and the process returns.

FIG. 22 shows a flow of the switch input processing (S5).
First, P / C lot switch (3-bit DIP switch)
The P / C lot switch code is input and stored in the photoconductor lot code LLOT (S51). This code may be input by reading a mark such as a bar code attached to the photosensitive drum 41. Next, the color balance switches 21 for each color
The color balance switch codes of cyan (C), magenta (M), yellow (Y), and black (K) are input from 6 and stored as LBCC, LBCM, LBCY, and LBCK, respectively (S52 to S55). Next, fog level switch codes of cyan (C), magenta (M), yellow (Y), and black (K) are input from the fog input switches 215 of the respective colors, and stored as LBKC, LBKM, LBKY, and LBKK, respectively. (S56-S5
9). Furthermore, other switch inputs are stored (S60),
To return.

FIG. 23 shows a flow of the semiconductor laser power setting processing (S6). First, LLOT (photoconductor lot switch code,
S51) and LPC (photoconductor temperature code, see S31) are added and stored as a 3-bit added power code (LPOW) (S71). Next, it is determined whether or not the power code LPOW is 7 (maximum value) or more (S72). If the power code LPOW is 7 or more, the power code LPOW is set to 7 (S73). Next, the gain of the gain switching unit 255 (FIG. 4) is switched based on the LPOW code (S
74), return.

FIG. 24 shows a flow of AIDC measurement (S7). First, to set the standard value as the bias potential V _B (S101). next,
The cyan developing device 45a is set (S102).

Then, LBKC, LBKM, LBKY, among LBKK, after setting the V _G value according to those corresponding to the developing color at that time (S103),
The photosensitive drum 41 is rotated (S104), and the charger 43 is charged.
Is activated (S105), and the eraser lamp 42 is turned on in the data ROM 203.
More (corresponding to V _G) that the read out is turned at a power level (S106), to operate the measurement developing device 45 which is set (S1
07), a detection pattern for AIDC measurement is formed on the photoconductor (S108), and the measured AIDC value is stored in AM (S109).

As explained in <e-2>, upon detection pattern imaging (S108), the output power of the semiconductor laser 264 is changed corresponding to the set value (V _G, V _B).

Next, the developing device is determined. If the cyan developing device 45a is set (YES in S111), the data of AM is stored in the LBAC (S112), and the image formation is stopped (S113). Then, the magenta developing device 45b is set (S114), the process returns to S103, and the measurement is continued.

If the magenta developer 45b is set (YE in S121)
S), the data of AM is stored in LBAM (S122), and the image formation is stopped (S123). Then, the yellow developing device 45c is set (S124), the process returns to S103, and the measurement is continued.

If the yellow developing unit 45c is set (YE in S131)
S), the data of AM is stored in the LBAY (S132), and the image formation is stopped (S133). Then, the black developing device 45d is set (S134), the process returns to S103, and the measurement is continued.

If the black developing unit 45d is set (S111, S12
1 and S131: NO), store AM data in LBAK (S1
41), stop imaging (S142), and return.

FIG. 25 shows V _B, V _G code selection processing flow (S8). First, it is determined whether or not scan switching is performed (S201).
If not, the process returns.

If the switched scan is a cyan scan (YES in S202), the AIDC measurement value LBAC is stored in the detection level LBA (S203), the color balance input value LBCC is stored in the color balance bias level LBC (S204), and fogging is performed. Input value L
BKC is stored in the fog removal level LBK (S205). Then, the process proceeds to S206.

Similarly, if the switched scan is a magenta scan (YES in S211), the AIDC measurement value LBAM is set to the detection level L.
The color balance input value LBKM is stored in the color balance bias level LBC (S213), and the fog input value LBKM is stored in the fog removal level LBK (S21).
Four). Then, the process proceeds to S206.

Similarly, if the switched scan is a yellow scan (YES in S221), the AIDC measurement value LBAY is set to the detection level L.
The color balance input value LBCY is stored in the color balance bias level LBC (S223), and the fog input value LBKY is stored in the fog removal level LBK (S22).
Four). Then, the process proceeds to S206.

Similarly, if the switched scan is black (S2
02, S211, S221: NO), AIDC measurement value LBAK is stored in detection level LBA (S231), and color balance input value LBCK
Is stored in the color balance bias level LBC (S23
2) The fog input value LBKK is stored in the fog removal level LBK (S233). Then, the process proceeds to S206.

Next, the detection level LBA and the color balance bias level LBC set as described above are added and stored in the correction detection level LBXN (S206). Correction detection level LBXN corresponds to the value of the bias potential V _B. Next, the fog input level LBK is further added to the correction detection level LBXN and stored in the level LBXM (S207). In this flow, the fog removal level LB
Since the difference between V _G by K is equal to the difference between V _G due to the difference of the correction detection level LBXN, the level LBXM the sum of both,
Corresponds to the grid potential V _G. Then, the bias potential VG corresponding to the correction detection level LBXN is set, and
Set the grid potential V _G corresponding to LBXM (S208), the process returns. (Note that when varying the difference between the two V _G as Table 1, (LBXN, it is sufficient to correspond to V _G matrix of LBK).) Thus removing fogging and color balance set by the user V _G, is V _B is selected considering (see <d-2> section).

FIG. 26 shows a flow of the gamma correction table selection processing (S9). First, it is determined whether or not scan switching is performed (S25).
1) For scan switching, set γ from the set values of V _G and V _B
Select a correction table (S252) and return.

Incidentally, <d-5> as described in Section in β code (V _G, V _B) in the modified embodiment of selecting selects the γ correction table from β codes corresponding to the (V _G, V _B) .

Further, <d-4> In the case of using the additive correction table as described in section with selecting the γ correction table corresponding to the grid potential V _B, the addition correction table corresponding to the head removal level LBK Select

The γ correction table thus selected is read out in the copy operation processing (S10) and used to convert the input level (OD) into the output level.

(G) Automatic Fog Removal In the embodiment described above, the fog removal level is set in four steps by the fog input switch 214 after the user views the reproduced image. However, if the fog removal level can be automatically set, the usability of the user is improved. Therefore, in a modified embodiment described below, the fogging amount of each of the developing units 45a to 45d is detected by using the AIDC sensor 210, and four fog removal levels (LBK) of 0 to 3 are automatically set. Grid potential for fog removal
V _G is selected.

In order to detect the fogging amount, the AIDC sensor 210 detects the amount of toner adhering to the reference toner image and simultaneously sets the AI
In a region other than the reference region for DC level detection, exposure is performed with a predetermined weak light emission amount (for example, the minimum light emission amount of a semiconductor laser) to form a second reference toner image.
_{Find the} 210 detected value V _AIDC . For example, as shown in FIG. 27, the detection value V _AIDC changes according to the amount of fogging on the photoconductor. Therefore, as shown in the figure, if the detected value of the toner adhesion amount is divided into, for example, four sections 0 to 3 and the fog removal levels 0 to 3 correspond to each section, the detection value V for each color is obtained. _The fog removal level LBK can be automatically set from _AIDC .

As a result, the AIDC sensor 210 can be effectively used for two purposes, the fog can be automatically removed, and the gradation characteristics can be kept constant, so that a high-quality image can be stably reproduced.
In addition, since the life of the developer, the photoreceptor, and the like can be systematically extended for removing fog, the running cost of the copying machine can be reduced.

In this modified embodiment, the fog removal level (LB
K) is automatically set, so the control system (see Fig. 2)
, The fog input switch 214 is unnecessary. In response to this, switch input processing S5 of the printer control unit 201 is performed.
In FIG. 28, the processing (S56-S59) relating to the fog input switch in FIG. 22 becomes unnecessary. AIDC
In the flow of the measurement process S7, as shown in FIG. 29,
A process for automatically measuring the fog value is added. Other aspects of printer control remain unchanged. Hereinafter, the changed flow will be described.

FIG. 28 shows a flow of the switch input process (S5).
First, P / C lot switch (3-bit DIP switch) 218
A photoconductor lot switch code indicating lot dependence of photoconductor characteristics is input and stored in LLOT (S301). This code may be input by reading a mark such as a bar code attached to the photosensitive drum 41. Next, cyan (C), magenta (M), yellow (Y), and black (K) color balance switch codes are input from the color balance switches 216 for each color, and LSCC, LBCM, and LB, respectively.
It is stored as CY, LBCK (S302-S305). Further, other switch inputs are stored (S306), and the process returns.

FIG. 29 shows the flow of AIDC measurement (S7). First, to set the standard value as the bias potential V _B (S321). next,
The cyan developing device 45a is set (S322).

Then, LBKC, LBKM, LBKY, after setting the V _G value according to those corresponding to the development at that time among the LBKN (S323), rotates the photosensitive drum 41 (S324), actuates the electric charger 43 ( S325), the eraser lamp 42 corresponding to the read (V _G value than the data ROM 203) are turned on at a power level (S326), operates the set measurement for the developing device (S32
7) An image of a detection pattern (for fog measurement) is formed on the photoconductor (S328). The fog value of the detection pattern thus formed is measured by the AIDC sensor 210 (S32
9) The fog level code is selected from the measured values and stored in the LBK (S330). Next, V is set for AIDC measurement based on this LBK value.
_G, sets the V _B (S331). Next, a detection pattern for AIDC measurement is formed (S332), and the AIDC measurement value is stored in AM (S3).
33).

Next, the developing device is determined. If it is determined that the cyan developing device 45a is set (YES in S351), the data of AM is stored in LBAC, the data of LBK is stored in LBKC, and (S35)
2) Stop image formation (S353). Then, the magenta developing device 45b is set (S354), the process returns to S323, and the measurement is continued.

If it is determined that the magenta developing device 45b is set (YES in S361), the data of AM is stored in LBAM, the data of LBK is stored in LBKM (S362), and the image formation is stopped (S36).
3). Then, the yellow developing unit 45c is set (S36
4) Return to S323 and continue measurement.

If it is determined that the yellow developing device 45c is set (YES in S371), the data of AM is stored in LBAY, the data of LBK is stored in LBKY (S372), and the image formation is stopped (S37).
3). Then, set the black developing device 45d (S37
4) Return to S323 and continue measurement.

If it is determined that the black developing device 45d is set (NO in S351, S361, S371), the AM data is LBAK
Then, the data of the LBK is stored in the LBKK (S381), the image formation is stopped (S382), and the process returns.

(H) Gradation correction in pulse width modulation method It is necessary to perform gradation correction also in gradation expression in pulse width modulation method. FIG. 30 is a diagram comparing gradation characteristics in each gradation expression method. It can be seen that those which are not multi-valued only by the dither method (4 × 4) have basically high linearity of gradation characteristics. However, although the pulse width modulation method (two-dot period) is not as nonlinear as the light intensity modulation method described above, it is necessary to perform tone correction for the nonlinearity.

FIG. 31 shows gradation characteristics in pulse width modulation (2 dot cycle-400 DPI). Here, the surface potential V _O is fixed (−
While keeping to 700V), -600V bias potential V _B, -50
0V and -400V. That is, β = (V _B −
(V _I ) / (V _O −V _I ) is changed to 0.833, 0.667, and 0.500. Developing voltage of the minimum dot unit by the change (| V _B -V _O |) is changed, thereby gradation characteristics changes at a low density part, shifting the overall shape.

32 Figure (a), (b), (c) is a model diagram of an electrostatic latent image when the bias voltage V _B changes. The solid line shows the change in the electrostatic latent image (decay voltage) when the pulse width changes. In contrast, the bias potential V _B of Figure 31,
, Change as well. Therefore, when the pulse width is small, that is, on the minimum dot side, the adhesion of toner is prevented. For example, when the pulse width is small and attenuates from the surface voltage V _O (−700 V) as indicated by a one-point line, toner adheres in (a), but does not adhere in (b) and (c). Thus it can be seen that it is removed fogging by a change in the bias potential V _B. Therefore, a change in gradation characteristics at a low density as shown in FIG. 31 is caused.

Therefore, even in the pulse width modulation method, the grid potential
By changing the bias potential V _B while maintaining the V _G constant, can be performed simultaneously fog removing Doing automatic density control.

Similarly, it can be performed similarly head removed even by adjusting the V _G while maintaining the V _B constant. More generally, it allows the automatic density control and fog removal by changing the combination of the grid potential V _G and the bias voltage V _B, the same as in the light intensity modulation method.

FIG. 33 shows a tone correction curve corresponding to the tone characteristics of FIG. Therefore, the gamma correction table corresponding to this curve may be stored in the data ROM 203. As with the light intensity modulation method, each (V _G, V _B) by preparing a γ correction table for each set of, while performing the automatic density control and fog removing also the tone correction performed. It is useful to reduce the memory capacity if the gradation correction curve is approximated by broken lines.

The parameter _{β (= (V B -V I} ) / (V O -V it)) is determined in advance a beta code corresponding to, to determine the beta code corresponding to (V _G, V _B), the beta The gamma correction table that is optimally determined may be selected. Thereby, the γ correction data is reduced, and the memory capacity can be reduced.

(Effect of the Invention) In the automatic density control in the digital image forming apparatus, the toner density can be detected more accurately by using the reference grid potential, the reference bias potential and the reference intensity of the laser beam in consideration of fog removal. As a result, the density is more accurately controlled during image formation.

[Brief description of the drawings]

FIG. 1 is a sectional view showing the overall configuration of a digital color copying machine according to an embodiment of the present invention. FIG. 2 is a block diagram of a control system of the digital color copying machine. FIG. 3 is a block diagram of an image signal processing unit. FIG. 4 is a block diagram of an image data processing system of the printer control unit. FIG. 5 is a diagram schematically showing the arrangement around the photosensitive drum. FIG. 6 is a diagram schematically showing a potential change of the photoconductor corresponding to the detection level of the AIDC sensor. FIG. 7 is a diagram schematically showing a potential change of the photoconductor when V _G is changed while V _B is kept constant. FIG. 8 is a diagram of various characteristics including gradation characteristics. 9 is a diagram with respect to a change in the V _G by keeping the V _B constant. FIG. 10 is a diagram when both changing the V _B and V _G. FIG. 11 is a diagram showing a decay potential and a development potential with respect to luminescence energy. FIG. 12 is a diagram of a change in the γ characteristic. FIG. 13 is a diagram of a γ correction curve. FIGS. 14 (a) and (b) are graphs of a broken line approximation of the γ correction curve. FIG. 15 is a graph showing the temperature change of the potential on the photoconductor with respect to the output power. FIG. 16 is a graph showing a change in potential on the photoconductor with respect to a light emission level. FIG. 17 is a graph showing gradation correction and exposure correction characteristics with respect to the light emission level. FIG. 18 is a graph showing a change in the potential of the photoconductor when the grid potential is changed. FIG. 19 is a graph of the potential of the photoconductor when the grid potential changes. FIG. 20 is a main flowchart of the printer control unit. FIG. 21 is a flowchart of a sensor input process. FIG. 22 is a flowchart of a switch input process. FIG. 23 is a flowchart of a semiconductor laser power setting process. FIG. 24 is a flowchart of AIDC measurement. FIG. 25 is, V _B, is a flowchart of V _G code selection process. FIG. 26 is a flowchart of the gamma correction table selection process. FIG. 27 is a graph showing the setting of the fog removal level. FIG. 28 is a flowchart of a switch input process in the modified embodiment. FIG. 29 is a flowchart of AIDC measurement in a modified embodiment. FIG. 30 is a diagram of reading characteristics and output characteristics. FIG. 31 is a diagram of gradation comparison in various gradation expressions. FIGS. 32 (a), (b), and (c) are model diagrams of an electrostatic latent image in the pulse width modulation method. FIG. 33 is a diagram of gradation characteristics in the pulse width modulation method. 12 Exposure lamp, 20 Image signal processor 31, Print head 41 Photosensitive drum 43 Charger 45a, 45b, 45c, 45d Developing device 201 Printer control Unit, 203 Data ROM, 210 AIDC sensor, 212 Temperature sensor, 213 Humidity sensor, 214 Fog input switch, 218 Photoconductor lot switch, 243 V _G generation unit, 244 ...... V _B generating unit, 253 ...... gamma correction unit, 255 ...... gain switcher, 264 ...... semiconductor laser.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H04N 1/29 (56) References JP-A-1-196347 (JP, A) JP-A-2-27366 (JP, A) JP-A-1-259378 (JP, A) JP-A-1-207767 (JP, A) JP-A-62-223763 (JP, A) JP-A-60-260067 (JP, A) JP-A-1-235973 (JP JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/00 303 G03G 21/00 370-540 G03G 13/04-13/056 G03G 15/04-15/056

Claims (2)

(57) [Claims] 1. A digital image forming apparatus which modulates the amount of a laser beam based on image data and performs exposure to form an image by a reversal development type electrophotographic process, comprising: a reference grid potential of a charger; A density detector for detecting a toner density when the exposure and development are performed with a laser beam having a reference intensity after setting a reference bias potential of the developing device to a predetermined value; and a toner density detected by the density detector. Density control means for setting the grid potential of the charger and the bias potential of the developing device so that the maximum density becomes constant in accordance with the density. Further, when the density is detected by the density detection means, fog is removed. A reference potential setting means for setting a reference grid potential and a reference bias potential; and A reference intensity setting unit that sets a reference intensity of the laser beam so that a development potential at an exposure position is constant under the reference grid potential and the reference bias potential set by the reference potential setting unit. A digital image forming apparatus characterized by the above-mentioned. 2. A digital image forming apparatus according to claim 1, wherein said density detecting means sets a reference grid potential corresponding to fog removal while keeping a reference bias potential constant. Digital image forming apparatus.