JP2936630B2 - Digital imaging method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a digital image forming method in a digital copying machine, a digital printer, and the like.

[Prior art]

Various electrophotographic image forming apparatuses, such as laser printers, which drive laser means based on image data converted into digital values and reproduce images, have been put to practical use, and faithfully reproduce so-called halftone images such as photographs. Various digital image forming methods have been proposed. Examples of this type of digital image forming method include an area gradation method using a dither matrix and changing the laser light amount (= light emission time × intensity) by changing the laser pulse width (light emission time) or light emission intensity. 1 to be printed
A multi-level laser exposure method (pulse width modulation method, intensity modulation method) and the like for expressing gradation for dots is known (for example, JP-A-62-91077, JP-A-62-39972, JP-A-62-188562 and JP-A-61-22597), and a multi-valued dither method in which dither is combined with a pulse width modulation method or an intensity modulation method are also 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 corresponding to the gradation of the image data to be reproduced on a one-to-one basis. Various factors such as the photosensitive characteristics of the toner, the characteristics of the toner, and the use environment are complicatedly intertwined, and the density of the original to be reproduced is not exactly proportional to the reproduced image density (hereinafter simply referred to as image density). As shown schematically in the drawing, a characteristic B deviating from the originally obtained proportional characteristic A is shown. Such a characteristic is generally called a γ characteristic, and is a major factor in lowering the fidelity of a reproduced image particularly for a halftone original. Therefore, in order to improve the fidelity of a reproduced image, conventionally, the read original density is converted using a predetermined conversion table for γ correction, and a digital image is formed based on the converted original density. So-called γ correction is performed so that the density and the image density satisfy a linear relationship (characteristic A). As described above, normally, by performing the γ correction, an image can be faithfully reproduced according to the level of the document density. On the other hand, another factor that affects the image density is a phenomenon in which the amount of toner adhered to the photoconductor changes during development due to changes in the external environment such as temperature and humidity due to the characteristics of the photoconductor and toner. Generally, in a high-temperature and high-humidity environment, the amount of adhered toner increases and the γ characteristic rises, and the reproduced image becomes darker. Is known to be thinner. As described above, there is a problem that the density of a reproduced image changes due to a change in the environment. In order to solve this problem and stabilize the image density, a general electrophotographic copying machine or printer requires a maximum image density. Density control for constant control is performed. The method generally used as the density control described above is described with reference to the photosensitive drum 41 and the developing roller shown in FIG.
This will be described with reference to a schematic diagram of an image forming unit including 45r. The photosensitive drum 41, a charger discharge potential V _C 43
Are installed facing each other. A grid voltage V _G is applied to the grid of the charger 43 by the grid voltage generation unit 214.
Is applied. The control of the potential V _{0 on} the surface of the photoconductor drum 41 is based on the detected value of the potential V ₀ by the V ₀ sensor 44,
Performed by adjusting the grid voltage V _G. First, before the laser exposure, a negative surface potential V ₀ is applied to the photosensitive drum 41 by the charging charger 43, and a low potential negative is applied to the developing roller 45r by the developing bias generating unit 215 to prevent the fogging phenomenon. Development bias V _B (|
V ₀ |> | V _B |). That is, the developing sleeve surface potential is also V _B. The potential of the photosensitive drum 41 is reduced by the laser exposure, and the electrostatic latent image potential V _I at the time of exposure with the maximum light amount from the surface potential V ₀
Transition to. When the electrostatic latent image potential V _L is a potential lower than the developing bias V _B, the toner adheres to the photosensitive drum 41.
The larger the difference between _VB and _VL , the larger the toner adhesion amount. Therefore, if changing the developing bias V _B, the difference between V _B and V _L is changed, it is possible to vary the amount of adhered toner,
The concentration can be controlled. This type of concentration control is performed in such a manner that the maximum concentration is made constant by manually or automatically changing V ₀ and V _B. In the automatic density control, first, a reference toner image serving as a reference for density control is formed on the surface of the photoconductor drum 41, and the amount of reflected light from the reference toner image is detected by an AIDC sensor 203 provided near the photoconductor drum 41. . this
The detection value detected by the AIDC sensor 203 is input to the printer control unit 201, and the printer control unit 201 is controlled based on a comparison result between the detection value from the AIDC sensor 203 and a predetermined numerical value.
Drives the V _G generating unit 214 and V _B generating unit 215. The above operation is repeated until the toner adhesion amount reaches a predetermined value. At this time, in order to prevent the carrier from adhering to the photoreceptor in the two-component developer of the background portion and fog of the image, conventionally, the density was controlled while keeping (V ₀ −V _B ) constant.

[Problems to be solved by the invention]

However, in order to a constant concentration of the reproduced image as described above, when the concentration control by changing the (V ₀ -V _B) while the maintaining constant the photosensitive drum surface potential V ₀ which and the developing bias V _B The γ characteristics are greatly affected. An example of this is shown in FIG. FIG. 6 shows the development characteristics NN in the reference environment and the development characteristics LL in the low-temperature and low-humidity environment. The solid line is the reference environment development characteristic NN, and the dashed line is the low temperature and low humidity environment development characteristic.
LL is shown. As the reference potential setting in the reference environment, (V ₀ , V _B ) = (− 700 V, −500 V). The maximum intensity of the laser used for exposure is 1.0 mW. If the environment changes to low temperature and low humidity with this potential setting, the development characteristics NN
There was variation in LL, the maximum density represented by the intersection of the developing characteristic curve and the photosensitive member surface potential V _I is reduced from C1 to C2. Therefore, if the environment becomes low temperature and low humidity when the density control is not performed, the density of the reproduced image becomes low. Here, when the environment changes to low temperature and low humidity, in order to keep the maximum concentration constant, (V ₀ , V _B ) = (− 800 V, −60)
Change the setting to 0V). Then, the development characteristic LL in the low-temperature and low-humidity environment indicated by the dashed line shifts to the left to become the development characteristic LL ′ indicated by the one-dot chain line, as indicated by the arrow in the figure. The intersections at _I coincide, and compensation is made for the maximum density level. Although not shown, when the environment changes to high temperature and high humidity, contrary to the case of LL, the amount of adhered toner increases and the image density increases.
In order to compensate the maximum concentration constantly, for example, in the case of high temperature and high humidity, the characteristic setting is shifted to the right by lowering the potential setting such as (V ₀ , V _B ) = (− 600 V, −400 V). Need to be done. However, as is apparent from FIG. 6, the curve shape from the minimum density to the maximum density is significantly different between the reference environment development characteristic NN and the low-temperature and low-humidity environment development characteristic LL 'after the maximum density compensation. That is, if the image density is compensated by changing V ₀ and V _B , the γ characteristic will change. Γ after maximum image density compensation in low temperature and low humidity environment LL, high temperature and high humidity environment HH and further high temperature and high humidity environment SHH
FIG. 7A shows the characteristics together with the γ characteristics of the reference environment NN. As described above, when the concentration control is performed according to the usage environment, the γ characteristic itself changes, for example,
In the intensity modulation method, the emission intensity of the laser is conventionally adjusted according to the γ characteristic by using a conversion table for γ correction corresponding to only the γ characteristic of the reference environment NN shown in FIG. 7 (a) as shown in FIG. 7 (b). Had nonlinear control. However, in the γ correction using a single γ correction conversion table, as shown in FIG. 7C, correct γ correction cannot be performed in an environment other than the reference environment NN. Therefore, unlike the conventional, it is performed while the concentration control and for fog-preventing contamination of the image background (V _B -V _I) constant tone characteristic will change, obtaining a faithful reproduction image In addition, the change in the gradation characteristic was unstable, and could not correspond to the user's personal preference for the reproduced image. The present invention makes use of the characteristics of the photoreceptor to compensate for fluctuations in the γ characteristics that occur as a result of the above-described density control, thereby producing a reproduced image having a constant gradation reproducibility for the original document. It is an object to provide a digital imaging method that can be obtained. Further, according to the present invention, the tone characteristics can be corrected even when the density control is performed, and a reproduced image having the desired tone characteristics can always be obtained in accordance with the preference of each individual, the type of the document, and the like. It is intended to provide a digital imaging method.

[Means for Solving the Problems]

Therefore, in order to achieve the above object, the present invention provides an electrophotographic digital image forming method for performing gradation expression by changing the amount of light irradiated by an exposure unit according to image information. The surface potential of the photosensitive member before exposure and the developing bias potential previously applied to the developing device are changed to control the density. The density control is performed while maintaining a constant ratio between a difference between an electrostatic latent image potential and a difference between the developing bias and the electrostatic latent image potential. Further, a value of a ratio of a difference between the photoconductor surface potential and the electrostatic latent image potential at the time of the maximum light amount irradiation by the exposure unit and a difference between the developing bias and the electrostatic latent image potential can be selected. It is characterized by having done.

[Action]

In the electrophotographic digital image forming method according to the present invention, the difference between the surface potential of the photoreceptor before exposure and the electrostatic latent image potential at the time of irradiation of the maximum amount of light, and the difference between the developing bias and the potential of the electrostatic latent image. The gradation characteristic can be changed by making the ratio selectable, and the fluctuation of the gradation characteristic is suppressed by keeping this ratio constant during density control.

【Example】

Hereinafter, a digital color copying machine according to an embodiment of the present invention will be described with reference to the accompanying drawings. (A) Configuration of Digital Color Copier FIG. 1 is a longitudinal sectional view showing the overall configuration of a digital color copier according to an embodiment of the present invention. The digital color copier includes an image reader unit 100 that reads an original image,
Main unit 20 that reproduces the image read by the image reader
It is roughly divided into 0. In FIG. 1, a scanner 10 includes an exposure lamp 12 for irradiating a document, a rod lens array 13 for collecting light reflected from the document, and a contact type CCD color image for converting the collected light into an electric signal. The 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. The multi-valued electrical signals of the three colors R, G, and B obtained by the image sensor 14 are read by a read signal processing unit 20 into any of yellow (Y), magenta (M), cyan (C), and black (K). It is converted into such 3-bit gradation data. Next, the print head unit 31 performs correction (γ correction) according to the gradation characteristics of the photoconductor and dither processing as needed on the input gradation data, and then outputs the corrected image data. D / A
The laser diode 221 is driven by this conversion to generate a laser diode drive signal. The laser beam generated from the laser diode 221 corresponding to the grayscale data exposes the rotatably driven photosensitive drum 41 via the reflecting mirror 37 as shown in FIG. As a result, an image of the document is formed on the photosensitive drum of the photosensitive drum 41. The photosensitive drum 41 is irradiated by an eraser lamp 42 before being exposed for each copy, and is charged by a charger 43. When exposure is performed in this uniformly charged state, an electrostatic latent image is formed on the photosensitive drum 41.
Yellow, magenta, cyan, and black toner developing units
Only one of 45a to 45d is selected, and the electrostatic latent image on the photosensitive drum 41 is developed. The developed image is transferred by the transfer charger 46 onto a copy paper wound around the transfer drum 51. Further, the toner image density to be developed is
It is optically detected by the AIDC sensor 203. The above printing process is repeated for yellow, magenta, cyan and black. At this time, the scanner 10 repeats the scanning operation in synchronization with the operations of the photosensitive drum 41 and the transfer drum 51. Thereafter, the copy paper is separated from the transfer drum 51 by operating the separation claw 47, is fixed through the fixing device 48, and is discharged to the discharge tray 49. The copy paper is fed from 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 digital color copying machine according to the present invention. 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 and a position signal from the position detection switch 102 indicating the position of the original on the exposure lamp 12 via the drive I / O 103.
Controls the drive I / O 103 and parallel I / O 104
The scan motor driver 105 is controlled via the. 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 an image signal processing unit 20 described later and processed. The main body unit 200 includes a printer control unit 201 that controls general copying operations and a print head control unit 202 that controls print heads. In the printer control unit 201,
V ₀ sensor for detecting the surface potential V ₀ of the photosensitive drum 41 before exposure
44, a _VL sensor 60 for detecting the surface potential _VL after exposure of the photosensitive drum 41, an AIDC sensor 203 for optically detecting the density of the toner image attached to the surface of the photosensitive drum 41, and developing units 45a to 45d
Analog signals are input from various sensors such as an ATDC sensor 204 and a temperature / humidity sensor 205 for detecting the toner concentration in the inside. Various data is input to the printer control unit 201 via the parallel I / O 207 by key input to the operation unit key 206. The printer control unit 201 is connected to a control ROM 208 in which a control program is stored and a data ROM 209 in which various data are stored, and the printer control unit 201 determines the control based on the data in these ROMs. The printer control unit 201 controls the control R based on the data from the sensors 203 to 205, the operation unit keys 206, and the data ROM 209.
According to the contents of OM208, the copy control unit 210 and the display panel 211
Controlling the door further automatically by AIDC sensors 203, or, for performing the manual density compensation control by the input to the operation panel 206, a parallel I / O 212 and drive via an I / O213 V _G generated for the high voltage unit 214 and V _B
The generation high-voltage unit 215 is controlled. The printer control unit 201 sends the numerical data of V _G (V ₀₎ and V _B to the print head controller 202. The print head control unit 202 operates according to a control program stored in the control ROM 216, and is connected to the image signal processing unit 20 of the image reader unit 100 via an image data bus. Based on the incoming image signal, perform gamma correction by referring to the contents of the data ROM 217 stored in the conversion table for gamma correction, and further use dither processing when using a multi-valued dither method as a gradation expression method. Drive I / O 218 and parallel I / O
The laser diode driver 220 is controlled via O219. The emission of the laser diode 221 is controlled by a laser diode driver 220. Further, the print head control unit 202
01, image signal processing unit 20 and image reader control unit 101
Are connected by a bus and synchronized with each other. (B) Image Signal Processing FIG. 3 is a diagram for explaining the flow of processing of image signals from the CCD 14 to the print head control unit 202 via the image signal processing unit 20. 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 202, the image signal photoelectrically converted by the CCD color sensor 14 is converted into R, G, B signals by the A / D converter 21.
Is converted into multi-valued digital image data. Each of the converted image data is subjected to predetermined shading correction by a shading correction circuit 22. Since the image data subjected to the shading correction is the reflection data of the original, the log data is converted by the log conversion circuit 23 into the density data of the actual image. Further, the undercolor remove / black printing circuit 24 removes the extra black color and generates true black data K from the R, G, B data. Then, the masking processing circuit 25 converts the data of the three colors R, G, B into data of the three colors Y, M, C. A density correction process for multiplying the converted Y, M, and C data by a predetermined coefficient is performed by the density correction circuit 26, and a spatial frequency correction process is performed by the spatial frequency correction circuit 27, which is then output to the print head controller 202. I do. In the print head control unit 202, the image signal processed by the image signal processing unit 20 is subjected to γ conversion by the γ conversion unit 28 based on the conversion table for γ correction in the data ROM 217, and multi-valued dithering is performed as gradation expression. When the method is adopted, the dither processing unit 29 performs dither processing based on the dither threshold data in the data ROM 217, and outputs the result to the laser diode driver 220. (C) Gradation Expression Method A multi-valued laser exposure method and a multi-valued dither method, which are gradation expression methods used in the present invention, will be described. i. Multi-level laser exposure method i.-1 Intensity modulation method The intensity modulation method of the multi-level laser exposure method as a gradation expressing means will be described. The intensity modulation method is a gradation expression method in which the density of one dot to be printed is changed stepwise, and the intensity of the laser for a constant light emission time is divided into several steps according to a multilevel signal from an image reader (multilevel ), And a different amount of laser is applied to the photoconductor at each stage, so that the density of one dot can be multi-valued. FIG. 8 schematically shows a cross section of a latent image of one dot by a laser in which the intensity is multileveled into eight levels by an intensity modulation method by the potential. As shown in the figure, the gradation expression by the intensity modulation method is realized by a change in the amount of adhered toner, that is, a change in image density because the latent image potential changes stepwise. i.-2 Pulse width modulation method The pulse width modulation method of the multilevel laser exposure method as a gradation expressing means will be described. The pulse width modulation method is a gradation expression method in which the area of one dot to be printed is changed stepwise, and the light emission time of a laser having a constant intensity is divided into several steps according to a multilevel signal from an image reader. ), And the photoreceptor is irradiated with a different amount of laser light at each stage, so that the printing area of one dot can be multivalued. FIG. 9 schematically shows a cross section of a latent image of one dot by a laser having an intensity of 1.0 mW in which the light emission time is multi-leveled into eight levels by a pulse width modulation method by the potential. As shown in the figure, the gradation expression by the pulse width modulation method is realized by a change in the toner attachment area, that is, a change in the reproduced image area because the latent image area changes stepwise. ii. Multi-valued dither method A multi-valued dither method for performing gradation expression by combining the above-described multi-valued laser exposure method (intensity modulation method or pulse width modulation method) and the dither method will be described. In the multi-valued dither method, for example, (N × M) dots are treated as one block, and each dot in this block is multi-valued to (L) values.
It expresses (N × M × L + 1) gradations, and uses the above-described pulse width modulation method or intensity modulation method as means for multi-leveling each dot. Therefore, when the pulse width modulation method or the intensity modulation method is simply used, one dot of the printed image by laser exposure becomes one pixel, and when the multi-valued dither method is used, the printed image by laser exposure (N × M) A dot area is one pixel. FIG. 10 shows an example of a multi-valued dither composed of (2 × 2) dots, in which one dot is multi-valued into eight values. This dither can represent 33 gradations of (2 × 2 × 8 + 1). The threshold values of these multi-valued dithers are 1 to
Regarding 32, the gradation expression when the intensity modulation method is used for multi-valued formation of one dot is not an area gradation but a density gradation,
Because it is difficult to show, it is represented by a strip shape as shown in FIG. 10 for convenience. (D) Photoconductor Characteristics and γ Characteristics The characteristics of the photoconductor used in the present invention will be described below. FIG. 7D shows an example of a characteristic curve of the photoconductor representing the relationship between the photoconductor surface potential and the exposure laser light amount. The characteristic curve of the photosensitive member surface potential V ₀ before laser exposure
The plots are plotted for four types of LL, NN, HH, and SHH, which are V ₀₁ , V ₀₂ , V _03, and V _04. Even if V ₀ changes from the figure, the surface potential of the photoreceptor is changed by laser exposure, respectively. it can be seen that descends at approximately the same proportion between the previous highest potential and the lowest potential V _I. As shown in FIG. 7 (d), it can be regarded as substantially the same for the photosensitive member surface potential V _I when the laser emission by the maximum intensity does not change much even by changing the V _0. From the characteristics of the photoreceptor, V _B2 of the NN is set arbitrarily, and (V _B2 −V _I ) / (V ₀₂ −V _I ) = (V _B1 −V _I ) / (V _0I −V _I ) = the _{(V B3 -V I) / (} V 03 -V I) = (V B4 -V I) / (V 04 -V I) become such _{V B, LL, NN, HH} , in SHH, individually selected As shown in the figure, the points at which the photoconductor surface potentials are V _B1 , V _B2 , V _B3 , and V _B4 in each curve are aligned in a straight line in parallel with the vertical axis (photoconductor surface potential). . Therefore,
The minimum laser light emission level P _SH to be developed by the toner becomes almost the same when each V _{0 is} set. Further, FIG. 7 (e) shows the difference between the developing bias V _B and the photosensitive member surface potential V _L and (V _B -V _L) the relationship between the laser light quantity. In Figure 7 (e), the laser light intensity of each of the curves are normalized by the value of (V _B -V _L) at the maximum (MAX), 4 single curves substantially overlap. The toner adhesion amount is (V _B −
V _L ), all γ characteristics will be consistent. The characteristics of the photosensitive member as described _{above, (V B -V I) /} (V 0 -V I) = β is satisfied the relationship (constant), even if the V ₀ is changed, gamma
Characteristics can always be constant. On the other hand, if the value β of (V _B −V _I ) / (V ₀ −V _I ) to be kept constant can be selected, the gradation characteristic can be changed.
A reproduced image according to the user's preference can be obtained. This is shown below. FIG. 7 (f) is a diagram showing that the γ characteristic changes according to the change in β when the change in the light amount of the laser is made linear, and is plotted according to the set values in the table below. FIG. 7 (f) shows that as the β value increases, the rise of the γ characteristic becomes steeper. Also, FIG. 7 (g) shows that β = shown in FIG. 7 (f).
The respective gradation characteristics when the γ correction is performed based on the γ characteristic at 0.667 are shown. From this figure, as the β value increases, the rise in the low-density portion becomes sharper, and this gradation characteristic generally becomes an image called low-key in printing or the like, and the reproducibility of a halftone image such as a photograph is improved. Get better. On the other hand, if the β value is small, the rise of the low density portion becomes dull, and the reproducibility of the low density portion of the document is deteriorated.
For example, an image such as a character or a thin line with a clear contrast has excellent reproducibility. As described above, if the β value can be changed, reproducibility according to the image tone of the document can be selected. Further, in the above example of the potential setting of V _B constant -
As 500V, by changing only the value of V _0, it is configuring the β value. This makes it possible to change the γ characteristic while keeping the maximum image density constant. (E) Control Flow FIGS. 11 to 14 show a control flow executed by the printer control unit 201 of the digital color copying machine according to the present invention. FIGS. 11 and 12 show the case where the concentration change due to the environmental change is manually compensated, and FIGS. 13 and 14 show the concentration change due to the environmental change using the AIDC sensor 203.
, And the density is automatically compensated. In any case, the necessary γ correction is performed along with the density control. i. Manual Density Control and γ Correction FIG. 11 shows the main routine of a digital color copying machine when performing density compensation manually. First, initialization such as parameter initialization is performed (S
1), start the internal timer (S2). Then, the density (I
D) After executing the control routine (see FIG. 12) (S3), the copying operation starts (S4). When the internal timer expires (S5), the process returns to S2. FIG. 12 shows a manual density control routine (S3 in FIG. 11) according to the present invention, which determines an environment code and a gradation characteristic selected by a user according to changes in temperature and humidity indicating the use environment (V _B −V _I ) / (V ₀ −V _I ) V ₀ and V _B that are necessary to obtain a constant maximum density corresponding to the value β and that do not change the γ characteristic are selected. . In this embodiment, the environment code has four levels, each of which has a low temperature and a low humidity.
LL, standard environment NN, high temperature and high humidity HH, and maximum temperature and high humidity SHH. Further, in this embodiment, V _B for each environment, ie, L
V _B1 = −567 V, V _B2 = −50 for each of L, NN, HH, SHH
0 V, V _B3 = −433 V, V _B4 = −367 V are stored in advance. Further, as a characteristic of the photosensitive member in the following description, V _I
Is assumed to be constant even if V ₀ changes, and V _I = −
100V. First, an environment code corresponding to temperature and humidity and β for determining a gradation characteristic are selected by key input to the operation panel 206 (S11). When the environment code is LL (YES in S12), V _B1 (−567V in this embodiment) is selected as the developing bias in order to compensate for the amount of toner adhesion that is smaller than in the reference environment, and for example, Assuming that 0.667 is selected as the β value,
Based on these V _B1 and β, (V _B −V _I ) / (V ₀ −
V _I ) = V ₀₁ that satisfies β is selected. From (V _B1 −V _I ) / (V ₀₁ −V _I ) = β (−567 + 100) / (V ₀₁ + 100) = 0.667, V ₀₁ = −800 V is selected and set according to these selected values. V _G generating unit 214 to the V _B generating unit 215
Is actually controlled (S14), and since the γ characteristic does not change from that shown by NN in FIG. 7 (a), for example,
A linear target gradation characteristic is realized by the generation characteristic in FIG. 7B according to the conversion table for γ correction corresponding to the NN in FIG. 7A. When the environment code is NN (YES in S15), V _B2 (−500 in this embodiment) is set as the developing bias which is the reference environment setting potential.
V) and, for example, if 0.667 is selected as the β value, based on these V _B2 and β, (V _B −V _I ) /
_{(V 0 -V I) = V} 02 satisfying β is selected. From (V _B2 −V _I ) / (V ₀₂ −V _I ) = β (−500 + 100) / (V ₀₂ +100) = 0.667, V ₀₂ = −700 V is selected and set according to these selected values. V _G generating unit 214 to the V _B generating unit 215
Are actually controlled (S17). When V ₀₂ and V _B2 are selected, the γ characteristic indicated by NN in FIG. 7A is obtained. Therefore, the γ characteristic according to the γ correction conversion table corresponding to NN in FIG.
A linear target gradation characteristic is realized by the light emission characteristic of FIG. When the environment code is HH (YES in S18), V _B3 (−433 V in the present embodiment) is selected as a developing bias in order to compensate for an increased amount of adhered toner compared to the reference environment.
For example, assuming that 0.667 is selected as the β value, (V _B −V _I ) / (V ₀ −V _I ) = based on these V _B3 and β.
V ₀₃ to meet the β is selected. From (V _B3 −V _I ) / (V ₀₃ −V _I ) = β (−433 + 100) / (V ₀₃ + 100) = 0.667, V ₀₃ = −600 V is selected and set according to these selected values. V _G generating unit 214 to the V _B generating unit 215
Is actually controlled (S20), and the γ characteristic does not change from that shown by NN in FIG. 7 (a), for example.
A linear target gradation characteristic is realized by the light emission characteristics of FIG. 7B according to the conversion table for γ correction corresponding to the NN of FIG. 7A. When the environment code is SHH (YES in S21), V _B4 (−367V in this embodiment) is selected as the developing bias in order to compensate for a larger amount of toner adhesion than in the standard environment. Assuming that 0.667 is selected as the β value, based on these V _B4 and β, (V _B −V _I ) − (V ₀
−V _I ) = V ₀₄ that satisfies β is selected. (V _B4 −V _I ) / (V ₀₄ −V _I ) = β (−367 + 100) / (V ₀₄ +100) = 0.667 From the equation, V ₀₄ = −500 V is selected and set according to these selected values. V _G generating unit 214 to the V _B generating unit 215
Is actually controlled (S23), and the γ characteristic does not fluctuate, for example, from that shown by NN in FIG. 7 (a).
A linear target gradation characteristic is realized by the generation characteristic in FIG. 7B according to the conversion table for γ correction corresponding to the NN in FIG. 7A. If the environment code is not one of the above, the process returns to S11 to start over. In the above embodiment, the tone characteristic is linearly corrected in all environments because the β values are all set to 0.667, but as described above, by changing the β value, the tone characteristic of the reproduced image is changed. Can be changed in various ways to obtain a reproduced image according to the user's preference, and to obtain a reproduced image with gradation characteristics according to each original such as a halftone original such as a photograph or a character original with high contrast. It becomes possible. ii. AIDC Density Control and γ Correction FIG. 13 shows a main routine of the digital color copying machine when the AIDC sensor 203 performs density compensation. First, initialization such as parameter initialization is performed (S5
1), start the internal timer (S52). And AI
After executing a routine (see FIG. 14) for automatically controlling the concentration (ID) by the DC sensor 203 (S53),
The copying operation starts (S54). When the internal timer expires (S
55) Return to S52. FIG. 14 shows an automatic density control routine (S53 in FIG. 13) by the AIDC sensor 203 according to the present invention, which is necessary for detecting a density change due to an environmental change and compensating for a certain maximum density, and a γ characteristic. V ₀ , V _B without fluctuation
Is determined by the value β of (V _B −V _I ) / (V ₀ −V _I ) that determines the gradation characteristic.
To choose from. In this embodiment, there are four levels of environmental codes, which are low-temperature low-humidity LL, reference environment NN, high-temperature high-humidity HH, and maximum temperature high-humidity SHH, respectively. Furthermore V _B for each environment in the present embodiment is assumed to be pre-memory as in the case of manual. Further, as in the case of the manual, in the following description, it is assumed that the relationship of V _I = −100 V is maintained even if V ₀ changes as a characteristic of the photoconductor. First, a value β of (V _B −V _I ) / (V ₀ −V _I ) for determining the gradation characteristic (β = 0.667 in the present embodiment) is input (S6).
1). Next, as a potential setting for forming a reference toner image for density detection on the surface of the photosensitive drum 41, a reference environment NN
V _B2 = −500 V, β = 0.667 and V _I = −
From 100 V, it derives the V ₀₂ = -700 V, and controls the V ₆ generating units 214 and V _B generating unit 215 (S62), forming a reference toner image (S63). Then, the AIDC sensor 203 detects the specular reflection light and the scatter reflection light of the reference toner image, obtains a difference between the two detection signals, and compares the difference with a predetermined numerical value to obtain a detection density (S64). The detected concentration is compared with the reference concentration. In the maximum temperature and high humidity environment SHH, that is, when the detected density is considerably higher than the reference density (YES in S66), the density is considerably lowered to match the reference density.
V _B4 with selecting (-367V in this embodiment), the photosensitive drum 41 surface potential V ₀₄ to select the -500V derives from the values of β and V _I, to be set in accordance with these selected values Back to V _G generating unit 214 and is the V _B generating unit 215 is actually controlled (S68), re-create the reference toner image (S63) to. In the high-temperature and high-humidity environment HH, that is, when the detected density is slightly higher than the reference density (YES in S69), in order to lower the density to match the reference density, the developing bias is set to V _B3 (−433 V in this embodiment). ) as well as select, select -600V photosensitive drum 41 surface potential V ₀₃ was derived from the value of β and V _I, V _G generating unit 214 and V _B to be set in accordance with these selected values The generation unit 215 is actually controlled (S71), and the process returns to the reference toner image creation (S63). When a low-temperature low-humidity environment LL, i.e., if the detected concentration is lower than the reference concentration (S65, S66, S69 all NO), to match the reference density by increasing the concentration, the V _B1 (present embodiment as a development bias - 567V) and the photosensitive drum
41 surface potential V ₀₁ to select the -800V derives from the values of β and V _I, V _G to be set in accordance with these selected values
It is a generating unit 214 and the V _B generating unit 215 is actually controlled (S73), returns to create reference toner image (S63). If the detected density is equal to the reference density (S65), the process returns because the potential setting has been completed. In the above embodiment, the tone characteristic is linearly corrected in all environments because the β values are all set to 0.667, but as described above, by changing the β value, the tone characteristic of the reproduced image is changed. Can be changed in various ways to obtain a reproduced image according to the user's preference, and to obtain a reproduced image with gradation characteristics according to each original such as a halftone original such as a photograph or a character original with high contrast. It becomes possible. In the description of the flow of V _0, V _B set control Figure 12 and Figure 14 when Iii.V _I varies, the photosensitive member surface potential V ₀ before the exposure is also vary, during exposure by the maximum amount of light If the potential V _I has been described the control of the V ₀ and V _B in the case that can be regarded as unchanged, depending on the setting of the maximum light amount characteristics and laser photoconductor, a change in the V _I due to the change in V ₀ is not negligible There is also. Therefore, S1 in FIG.
4, S17, S20, S23 and a of FIG. 14 S62, S68, S71, V ₀ of the S73, V _B
The setting subroutine is shown in FIG. 15 and will be described below. First, select a V ₀ corresponding to the environment by manual or AIDC sensor 203 (S100). Then, by turning on the main charger by V _G generating unit 214 charges the photosensitive member to V ₀ (S101), performing laser exposure of sufficient quantity to potential drop to V _I (S102), the photosensitive member surface Established in
The V _L sensor 60 is measured V _I (S103), calculates the developing bias V _B from V ₀ and (V ₀ = -700V) and entered the beta (= 0.667) in this and the previously stored reference environment NN Is performed (S104). Then, it is controlled to the developing bias V _B obtained by the V _B generating unit 215 (S105). In the above-described embodiment of the present invention, the environmental change is made into four stages. However, the present invention is not limited to this, and it is possible to increase the number, and fine gradation compensation can be performed. .

【effect】

By performing gradation expression by changing the laser light amount, for example, if the present invention is applied to a digital color copying machine or a printer,
It is possible to select a desired gradation characteristic for the reproduced image or a gradation characteristic suitable for the reproduced image.Furthermore, in response to environmental changes, the surface potential of the photoconductor before exposure and the developing bias are changed to compensate for the density. Even if the reproduction is performed, a desired reproduced image can always be obtained without deteriorating the gradation characteristics of the selected reproduced image.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing the entire configuration of a digital color copying machine. FIG. 2 is an overall block diagram of the digital color copying machine. FIG. 3 is a block diagram showing a process of image signal processing of the digital color copying machine. FIG. 4 is a diagram showing an example of the γ characteristic. FIG. 5 is a diagram for explaining image density adjustment by changing the photoconductor surface potential and the developing bias. FIG. 6 is a diagram showing a difference in development characteristics between a reference temperature / humidity environment and a low temperature / low humidity environment. FIG. 7A is a diagram for comparing the respective γ characteristics after the image density is compensated for due to the environmental change with the γ characteristics in the reference environment. FIG. 7 (b) shows the γ in the reference environment NN of FIG. 7 (a).
FIG. 7 is a diagram illustrating light emission characteristics of a laser whose intensity is nonlinearly controlled by a conversion table for γ correction for linearly correcting characteristics. FIG. 7 (c) is a diagram showing the result of performing γ correction on the four γ characteristics of FIG. 7 (a) using a laser having the emission characteristics of the reference environment NN of FIG. 7 (b). is there. FIG. 7 (d) shows the relationship between the photoconductor surface potential and the amount of exposure laser light, and is a characteristic curve of the photoconductor in which the maximum electrostatic latent image potential becomes substantially constant regardless of the initial photoconductor surface potential. FIG. 7E shows a potential drop characteristic (toner adhesion amount) from the developing bias when the laser light amount is changed from the minimum to the maximum with respect to the photosensitive member having the characteristic of FIG. 7D.
FIG. FIG. 7 (f) is a diagram showing that the γ characteristic changes according to the change in β when the change in the light amount of the laser is linear. FIG. 7 (g) is a diagram showing the respective gradation characteristics when the γ correction is performed based on the γ characteristics when β = 0.667 shown in FIG. 7 (f). FIG. 8 is a longitudinal sectional view schematically showing an electrostatic latent image of one dot on a photoreceptor by a laser whose intensity has been multileveled into eight levels by an intensity modulation method, by using its potential. FIG. 9 is a longitudinal sectional view schematically showing an electrostatic latent image of one dot on a photoconductor by a laser whose emission time is multileveled into eight levels by a pulse width modulation method, with the potential thereof. FIG. 10 is an example of a dither pattern used for a multi-valued dither method in which one dot is multi-valued into eight values and gradation is expressed by 2 × 2 dots. FIG. 11 is a flowchart of a main control routine in a case where the density control is manually performed in the digital color copying machine. FIG. 12 is a flowchart of a manual density control routine of the digital color copying machine shown in FIG. FIG. 13 is a flowchart of a main control routine when the density control is automatically performed using the AIDC sensor of the digital color copying machine. FIG. 14 is a flowchart of an automatic density control routine of the digital color copying machine shown in FIG. FIG. 15, the photosensitive member which the value of V _I have not stable characteristics, to a constant _{(V B -V I) / (} V 0 -V I),
₅ is a flowchart for setting V ₀ and V _B. 20 ...... image signal processing unit 41 ...... photosensitive drum 44 ...... V ₀ sensor 45r ...... developing roller 60 ...... V _L sensor 101 ...... image reader control unit 106 ...... image control unit 201 ...... printer control unit 202 … Printer head controller 203… AIDC sensor 205… Temperature / humidity sensor 206… Operation panel 214… V _G generation unit 215… V _B generation unit 208,216… Control ROM 209,217… Data ROM 220… Laser diode driver 221 …… Laser diode

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI G03G 15/06 101 B41J 3/00 D H04N 1/29 (56) References JP-A-2-212864 (JP, A) 57-76563 (JP, A) JP-A-1-234862 (JP, A) JP-A-1-250971 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03G 15 / 00 303 G03G 21/00 370-540 G03G 21/14

Claims (2)

(57) [Claims] 1. An electrophotographic digital image forming method in which a light amount irradiated by an exposure unit is changed in accordance with image information to perform gradation expression, wherein: a photoconductor surface potential before exposure by the exposure unit;
By changing the developing bias potential previously applied to the developing device to control the density, the difference between the surface potential of the photoconductor and the potential of the electrostatic latent image at the time of the maximum light amount irradiation by the exposure unit, A digital image forming method, wherein the density control is performed while maintaining a constant ratio between the developing bias and the difference between the electrostatic latent image potential. 2. An electrophotographic digital image forming method in which the amount of light irradiated by an exposure unit is changed according to image information to express a gradation, wherein the surface potential of the photoconductor and the maximum amount of light irradiation by the exposure unit are used. 2. The digital image forming method according to claim 1, wherein a value of a ratio of a difference between the electrostatic latent image potential and a difference between the developing bias and the electrostatic latent image potential is selectable.