JP2906557B2 - 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-valued laser exposure method (pulse width modulation method, intensity modulation method) for expressing the gradation with respect to 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 document density is converted using a predetermined conversion table for γ correction, and a digital image is formed based on the converted document density. So-called gamma correction is performed so that the high density and the image density satisfy a linear relationship (characteristic A). Normally, by performing the γ correction, an image can be faithfully reproduced according to the level of the document density.

[Problems to be solved by the invention]

However, since the image reproduction by the multilevel laser exposure method or the multilevel dither method is based on an analog latent image, the gradation characteristics greatly depend on the diffusion of the latent image on the photoconductor, and when the characteristics of the photoconductor change. In addition, there is a problem that the gradation characteristics also change. That is, γ correction based on a single conversion table is γ correction
When the γ characteristic fluctuates, an appropriate γ characteristic is obtained because the function operates properly only when the characteristic is constant and a linear gradation characteristic is obtained.
Since the correction cannot be performed and the linear gradation characteristics cannot be reproduced, a faithful reproduced image cannot be obtained. The latent image diffusion phenomenon and the effect of the photoconductor on the γ characteristic will be described below with reference to examples. The effect of the latent image diffusion phenomenon of the photoconductor on the γ characteristics will be described below. As is well known, the photoconductor is formed by depositing or applying an insulating photoconductive material to a thickness of several tens of μm on a conductive substrate. When laser exposure is performed on a photoreceptor charged by corona discharge in a dark place, the exposed insulative photoconductive material reduces its electrical resistance, and the retained charge is transported to a conductive substrate and grounded . As a result, the exposed portions are electrically neutralized, and an electrostatic latent image is formed. By the way, since laser light is reflected and scattered by the insulating photoconductive material and the conductive substrate at the time of laser exposure, a latent image is also generated around the laser irradiation part, and the electric resistance in the laser non-irradiation part is infinite. Due to the phenomenon that the latent image is averaged over time due to the diffusion of charge during charge transport due to the small size, the electrostatic latent image with a larger outline and blurry than the actually irradiated laser beam Will be formed. This phenomenon is generally called latent image diffusion. This latent image diffusion phenomenon is schematically shown in FIGS. 5 (a) and 5 (b). FIG. 5A shows an example of the intensity distribution of a certain laser beam, and FIG. 5B shows the potential distribution in the scanning direction of a latent image formed on the photoconductor. Fifth
In an ideal state where the latent image diffusion phenomenon does not occur due to the laser beam having the intensity distribution of FIG. 5A, the latent image along the intensity distribution of the laser beam as shown by the broken line in FIG. However, the latent image diffusion phenomenon is actually unavoidable, and as shown by the solid line in FIG. 5B, the aperture becomes larger than the ideal latent image indicated by the broken line, and Since the falling potential decreases and the toner adhesion density decreases, a blurred latent image is formed as a whole. Since this latent image diffusion can be considered to be substantially the same as the laser beam diameter changing and becoming larger than the actual diameter, the latent image diffusion ratio (diffusion rate α) is defined, for example, by the following equation. can do. α = Wx ′ / Wx (= Wy ′ / Wy) where Wx and Wy are the beam diameters of the actually irradiated laser beam in the main scanning direction and the sub-scanning direction, respectively, and Wx ′ and Wy ′ are the laser beams. The main scanning direction, obtained from the potential distribution of the electrostatic latent image formed by the beam,
This is the laser beam diameter in the sub-scanning direction. Diffusion rate α
Is not constant due to variations in the manufacturing of the photoconductor, deterioration of the photoconductor itself, and the like, and the gradation characteristic deteriorates with time. FIG. 6A shows an example of a change in the γ characteristic due to a change in the spreading factor α when the intensity modulation method is used.
Five levels of 1.1, 1.3, 1.5, 1.7, and 1.9 are shown. As shown in the figure, it can be seen that the rise of the γ characteristic tends to become slower as the diffusion rate α increases. Conventionally, γ correction corresponding to a change in the diffusion rate α has not been performed, and γ correction has been performed on γ characteristics at a specific diffusion rate α. For example, as shown in FIG. 6 (a), even when the diffusion rate α changes variously, the γ correction is always performed by the correction conversion table corresponding to the γ characteristic when α = 1.3. The emission characteristic of the laser according to the conversion table for γ correction is the same curve of α = 1.3 in FIG. 6B. FIG. 6C shows γ characteristics when γ correction corresponding to γ characteristics when α = 1.3 is performed for each α shown in FIG. 6A. As can be seen from the figure, if the constant γ correction is always performed irrespective of the change in the diffusion rate α, correct γ correction cannot be performed on a photoconductor having characteristics other than α = 1.3. If the diffusion rate α changes as described above, the γ characteristic fluctuates, so that the γ correction is not properly performed, the gradation characteristic of the reproduced image changes, and a stable reproduced image cannot be obtained.
Further, the diffusivity α is not constant due to variations in the manufacture of the photoconductor, deterioration of the photoconductor itself, and the like, and the gradation characteristics change with time. As described above, in the digital image forming method that emphasizes the reproducibility of gradation, the latent image diffusion phenomenon on the surface of the photoreceptor changes the gradation characteristics, and is a serious problem that cannot be ignored. The present invention has been made in view of such a problem, and in an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple steps, a change in γ characteristics that occurs is suppressed and a correct gradation is obtained. It is an object of the present invention to provide a digital image forming method capable of realizing the above.

[Means for Solving the Problems]

In the present invention, as shown in FIG. 6 (a), if the γ characteristic is successfully changed in the direction opposite to the direction in which the γ characteristic fluctuates with the change in the diffusion rate α, the result is that γ The fact that characteristics do not fluctuate is used. The first invention shows the spread degree of the latent image potential with respect to the actual beam diameter of a laser beam incident on a photoreceptor in an electrophotographic digital image forming method in which a book light is expressed by changing a laser light amount in multiple stages. Determine the spread rate,
The gradation expression is performed by changing a laser emission intensity for an image signal having a maximum density according to the diffusion rate. According to a second aspect of the present invention, in an electrophotographic digital image forming method in which a laser light amount is changed in multiple steps to perform gradation expression, the spread of a latent image potential with respect to an actual beam diameter of a laser beam incident on a photosensitive member. It is characterized in that the diffusion rate indicating the degree is determined, and the amount of laser light is nonlinearly controlled using a conversion table for γ correction corresponding to the diffusion rate. According to a third aspect of the present invention, there is provided an electrophotographic digital image forming method in which a laser light amount is changed in multiple stages to perform gradation expression, and the spread of a latent image potential with respect to an actual beam diameter of a laser beam incident on a photosensitive member. The spread rate indicating the degree is determined, and the threshold value of dither is changed according to the spread rate. According to a fourth aspect of the present invention, in a digital image forming method of an electrophotographic system that performs gradation expression by changing a laser light amount in multiple steps, the diffusivity is determined by detecting a photoconductor surface potential of a laser exposure degree. It is characterized in that a diffusion rate is calculated from a detected value to make a determination. According to a fifth aspect of the present invention, there is provided an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, wherein the diffusion rate is input by an input unit. According to a sixth aspect of the invention, in the digital image forming method of the electrophotographic system in which gradation is expressed by changing a laser light amount in multiple steps, the input of the diffusion rate is performed when the photosensitive member is replaced.

[Action]

In one of the electrophotographic digital image forming methods according to the present invention, appropriate gamma correction is performed by changing the laser exposure intensity based on the value of the diffusivity of the photoconductor. In another electrophotographic digital image forming method according to the present invention, an appropriate gamma correction is performed by switching to another gamma correction conversion table based on the value of the diffusion rate of the photoconductor. In still another example of the electrophotographic digital image forming method according to the present invention, another dither pattern is selected based on the value of the diffusivity of the photoconductor, and an appropriate gamma correction is performed. In still another electrophotographic digital image forming method according to the present invention, an appropriate gamma correction is performed based on the detected value of the diffusion coefficient of the photoconductor.

【Example】

 Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
And will be described in detail. (A) Configuration of digital color copying machine FIG. 1 is a digital color copying machine according to an embodiment of the present invention.
FIG. 2 is a longitudinal sectional view showing the entire configuration of the machine. Digital color compound
The copying machine includes an image reader unit 100 that reads a document 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 is an exposure device for irradiating a document.
Lamp 12 and rod lens for collecting the reflected light from the original
Array 13 and adhesion to convert collected light into electrical signals
It has a CCD color image sensor 14 of a type. Scya
The scanner 10 is driven by a motor 11 at the time of reading a document.
Moves in the direction of the mark (sub-scan direction) and is placed on the platen 15
The scanned document is scanned. Document illuminated by exposure lamp 12
The image of the surface is photoelectrically converted by the image sensor 14. Image
R, G, B multi-valued electrical signals obtained by the image sensor 14
The signals are read by the read signal processing unit 20 for yellow (Y)
Any of the following colors: (M), cyan (C) and black (K)
The data is converted into 3-bit gradation data. Then print
The head unit 31 receives the photosensitive data for the input gradation data.
Correction according to body tone characteristics (γ correction) and if necessary
After performing dither processing, the corrected image data is converted to D / A.
To generate a laser diode drive signal,
The laser diode 221 is driven by the signal. Generated from laser diode 221 corresponding to gradation data
The reflected laser beam, as shown in FIG.
Then, the photosensitive drum 41 driven to rotate is exposed. This
As a result, an image of the original on the photoconductor of the photoconductor drum 41 is formed.
Is done. The photoreceptor drum 41 receives exposure for each copy
It is irradiated by the eraser lamp 42 before
More charged. Exposure in this uniformly charged state
When received, 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 photoconductor
The electrostatic latent image on the ram 41 is developed. The developed image is transferred
The copy wound around the transfer drum 51 by the charger 46
Transcribed on paper. Further, the toner image density to be developed is
It is optically detected by the AIDC sensor 203. The above printing process includes yellow, magenta, cyan, and blue.
It is repeated for the rack. At this time, the photoconductor
In synchronization with the operation of the drum 41 and the transfer drum 51, the scanner 10
Repeat the scanning operation. After that, operate the separation claw 47
The copy paper is separated from the transfer drum 51 by
The sheet is fixed through the receiving device 48 and is discharged to the discharge tray 49.
You. The copy paper is fed by the paper cassette 50 and
The chucking mechanism 52 on the photo drum 51 causes
To prevent misalignment during transfer.
ing. FIG. 2 shows an overall block diagram of a digital color copying machine according to the present invention.
FIG. The image reader unit 100 is controlled by the image reader control unit 101.
Is controlled. The image reader control unit 101 includes a platen 1
5 Position from the position detection switch 102 indicating the position of the original on
Signal and 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. Scya
Motor 11 is driven by the scan motor driver 105.
It is. On the other hand, the image reader control unit 101
It is connected by a bus. The image control unit 106 controls the CCD color
A bus with each of the image sensor 14 and the image signal processing unit 20
Are connected to each other. Image from image sensor 14
The signal is input to an image signal processing unit 20 described later and processed.
Is managed. The main body 200 includes a printer that controls general copying operations.
A print head for controlling the print head with the control unit 201
And a controller 202. In the printer control unit 201,
Surface potential V of photosensitive drum 41 before laser exposure₀V to detect₀
Sensor 44, surface potential V of photosensitive drum 41 after laser exposure_LTo
V to detect_LThe sensor 60 adheres to the surface of the photosensitive drum 41
AIDC sensor 203 that optically detects the density of the toner image
ATDC sensor for detecting toner concentration in imagers 45a to 45d
From the sensors 204 and the temperature / humidity sensor 205.
The analog signal is input. In addition, the key to the operation
Printer input via the parallel I / O 207
Various data are input to the control unit 201. Printer control unit 201
The control ROM 208 that stores the control program and various
A data ROM 209 storing data is connected to
The printer control unit 201 performs the control based on the ROM data.
decide. The printer control unit 201 includes sensors 203 to 205 and an operation unit key.
Control 206 according to the data from the
According to the contents of OM208, the copy control unit 210 and the display panel 211
Control by the AIDC sensor 203,
In other words, manual density compensation by input to operation panel 206
Parallel I / O 212 and driver
V via Eve I / O213_GHigh-pressure unit 214 and V for generation_B
The generation high-voltage unit 215 is controlled. The print head control unit 202 is stored in the control ROM 216.
It operates according to the control program
Image signal processing unit 20 of image reader unit 100 and image data bus
Connected via the image data bus
The conversion table for gamma correction is stored based on the image signal.
Perform gamma correction with reference to the contents of the
Laser die via live I / O 218 and parallel I / O 219
It controls the Aether driver 220. Laser diode 2
21 emits light by laser diode driver 220
Is controlled. 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. Especially the present invention
Then, the diffusion rate α sent from the printer control unit 201 is
Optimum gamma correction is performed based on data
You. (B) Image signal processing FIG. 3 (a) shows the image signal processing from the CCD 14 through the image signal processing unit 20.
Of image signal processing reaching the printhead control unit 202
It is a figure for explaining this. Referring to this, the CCD camera
Process the output signal from the color image sensor 14 to
Read signal processing for outputting data will be described. In the image signal processing unit 202, the CCD color sensor 14
Therefore, the image signals subjected to photoelectric conversion are converted into R, G, B signals by the A / D converter 21.
Is converted into multi-valued digital image data. This conversion
Each of the image data obtained by the shading correction circuit 22
Predetermined shading correction is performed. This shading
Since the corrected image data is the reflection data of the original,
The log conversion is performed by the log conversion circuit 23 to obtain the actual image density.
Is converted to degree data. In addition, under color remover
If you remove excess black color with the black ink printing circuit 24,
First, true black data K is generated from the R, G, B data.
Then, in the masking processing circuit 25, the data of the three colors R, G, B
The data is converted into three color data of Y, M, C. Thus strange
Density correction processing for multiplying the converted Y, M, C data by a predetermined coefficient.
The spatial frequency correction process is performed
After performing the inter-frequency correction circuit 27, the print head
Output to the control unit 202. In the print head control unit 202, image signal processing is performed.
The image signal processed by the unit 20 is
Γ conversion based on γ correction conversion table in data ROM 217
And outputs the result to the laser diode driver 220. Les
Laser driver 220 according to the image signal
The light emission intensity of the diode 221 is controlled. (C) Control Flow i. First Embodiment Main Routine FIGS. 7 to 9 relate to the first embodiment of the present invention,
Digital Color Using Laser Intensity Control for Tonal Compensation
6 is a control flow of the printer control unit 201 of the copying machine. Less than
Next, these flows will be described. FIG. 7 shows the main routine of the digital color copying machine.
Show. First, make initial settings such as parameter initialization.
(S1), the internal timer is started (S2). Next, expand
The dispersion rate α is determined (S3). Then, enter the laser diode intensity determination routine.
(S5). This intensity determination routine is based on the diffusion rate α
Laser emission intensity corresponding to the image signal of the maximum density (hereinafter, referred to as
The maximum laser intensity), and change the pulse
When using the width modulation method, change the specified laser intensity.
To suppress the fluctuation of the γ characteristic.
This will be described in detail with reference to the drawings. When the laser intensity is determined in S5,
Perform copy operation (S6), wait for internal timer to expire
(S7) One cycle of the main routine ends. Here, due to the characteristics of the photoconductor, the laser intensity changes.
6D shows that the γ characteristic fluctuates even if the
(E) and (f) show. FIG. 6D shows the light emission time for the charged photoconductor.
When irradiating a laser whose intensity is changed under a constant
It is a graph showing the change of the surface potential, the laser intensity changes
Even if the surface potential drops to some extent,
This indicates that the image density does not change much.
doing. On the other hand, FIG.
0.8mW, 1.0mW, 1.1mW, 1.2m respectively corresponding to
As W, change the laser intensity from 0 to its maximum value.
FIG. 9 is a graph showing the photoconductor surface potential when
FIG. 6 (f) shows the development characteristics from FIG. 6 (e).
Graph showing the relationship between the original document density and the image density (γ characteristic).
It is. By changing the maximum light amount in this way, V_O, V_BChanged
It can be seen that the γ characteristic fluctuates in the same manner as when the γ characteristic is changed. The present invention takes advantage of this phenomenon to change the diffusivity α.
The fluctuation of the gradation characteristics in the image formation caused by
Thus, a reproduced image faithful to the original is obtained. i.-1 Spreading Factor Determination Routine When the Spreading Factor is Obtained by Inputting An example of the spreading factor α input is a flowchart shown in FIG.
This will be explained with reference to FIG. In this flow, when the photoconductor is replaced
Or when the spreading factor is not entered for some reason
Enter α. In addition, the diffusion rate α
It is assumed that it is measured in a place. In this embodiment, the diffusion rate α is in the range of the numerical value.
Is conveniently coded by
Α ≦ 1.2: spreading factor code P = 1 1.2 <α ≦ 1.4: spreading factor code P = 2 1.4 <α ≦ 1.6: spreading factor code P = 3 1.6 <α ≦ 1.8: spreading factor code P = 4 1.8 <α ≦ 2.0: Diffusivity code P = 5 Accordingly, the smaller the diffusivity code P, the smaller the latent image
The degree of diffusion is small, and the sharp rise
Will have the property. First, it is checked whether the photoconductor has been replaced (S1).
6). This photoconductor replacement is detected as follows, for example.
You. As shown in the top view of FIG.
Is an insertion slot opened in the front side plate PF of the photoreceptor copier body
Until the end of the assembly 410 abuts on the back plate PR.
Inserted and mounted, just sandwiched between both plates PF and PR
It will be dressed. The front side plate RF has a photo-interrupter type photoconductor.
A lock switch 230 is attached, and the photoconductor assembly is
The photoconductor lock is fully inserted into the yellowtail 410.
The light emitting unit and the light receiving unit (
(Not shown in the figure).
Is provided. Therefore, photoconductor assembly 410 is removed for replacement.
When the light shielding unit 230a is
The light from the light-emitting part is detected by the light-receiving part, and is also shown in FIG.
As shown, the detection signal is sent to the control unit 201, and the control unit 201
Checks that photoconductor assembly 410 has been removed
Wear. Return to the description of the flow again. Photoconductor replacement detected
And (YES), the spreading factor code P stored so far is
It is reset (S17), and the spreading factor code P is newly input.
(S18). The photoconductor has not been replaced (N in S16
O), if the spreading factor code P is not entered and stored
If (YES in S19, for example, during assembly), the spreading factor code P is input.
(S18). If the photoconductor is not replaced and the diffusion rate
If the mode P is stored, the process returns. In the input of the spreading factor code P in S18, FIG.
As shown in FIG.
The code information mark 231a of
Monitor the installation of the assembly 410 with the photoconductor lock switch 230
So that the above code information can be read
Possible photoconductor mark reader 231 for photoconductor mounting
Of the spreading factor code P at the same time
I do. Therefore, if the user forgets to input the spreading factor code P or makes an input error,
User can input the spreading factor α code.
There is no consciousness. This diffusion rate can be manually entered from the operation panel.
You may make it perform by input. When the Diffusivity is Obtained by Detection The flow of determining the diffusivity shown in FIG.
To enter the diffusion rate α measured at the factory, etc. during manufacturing
However, here, as shown in FIG.
The case where the copying machine itself detects itself will be described. As described in the definition of the diffusion rate,
When detecting the laser beam diameter and the latent image aperture,
Measurement, but as a practical matter, the
Since it is difficult to measure the diameter directly, the electrostatic latent
A predetermined pattern of the image is formed, and the surface potential after laser exposure is
V to measure_LUsing the sensor 60 (see Fig. 1),
Initial surface potential V₀To V_LThe potential drop of the latent image to the
The diffusion rate α from the degree of potential drop
I'm trying. If the potential drop is large, the diffusion rate α is small
If the potential drop is small, the diffusion rate α is large. An example of calculation of the diffusion rate α is shown in a flowchart in FIG.
This will be explained by a chart. The calculation of the diffusion rate α is performed according to the following procedure.
You. First, in a dark place, the photoconductor is evenly distributed by corona discharge.
A surface potential on the surface. Then, this photosensitive member N area
(1,2,…, N), and the γ characteristics of each area
Within the concentration range showing a straight line, the intensity (W (1), W (2),.
(N)) The laser beam with different
Laser exposure is performed. In other words, the same strength in the same area
Laser exposure. And an electrostatic latent image for each area
The potentials (V (1), V (2),..., V (N)) are obtained. What
Note that the potential at this time is measured at multiple locations within the region.
It is the average value of the applied potentials. On the other hand, if there is a known diffusion rate α1
With another photoreceptor having a known diffusivity α2
Each of the photoconductors has a laser beam
Laser exposure at the same intensity as
The electrostatic latent image potential with respect to the laser intensity is measured in advance, and the value is
Stored as data. That is, the photoreceptor having the diffusion rate α1
Latent image potential V1 with respect to laser intensity W (1)
(1), V1 (2) for W (2), ..., W (N)
V1 (N) and the laser intensity on the photoreceptor with a diffusivity α2
To the latent image potential V2 (1) and W (2) with respect to the degree W (1).
V2 (2), ..., V2 (N) for W (N)
Data. These data and the photosensitivity for determining the diffusion rate α
Latent image potential V with respect to laser intensity W (1) in the body
(1), V (2) for W (2), ..., W (N)
V (N), the diffusivity α is determined in proportion to each region.
Can be That is, from the calculation formula V (n) = V1 (n) + {V2 (n) -V1 (n)}｝ (α-α1) / (α2-α1) ｛n = 1,2,. For all the regions, the diffusion rate α is obtained, and
Is the final value of α. Hereinafter, the flowchart of FIG. 8B will be described. First, the operation key to start the copy operation is pressed
Check (S20), and if pressed (YES), charge
Turn on the charger 43, eraser 42 and motor 11
(S21), preparing for exposure. And the duplication described above.
A flag indicating the number of the image part in several areas
Resetting the log N to 0 (S22), and
Loop (S23-S)
Enter 27). In this loop, after increasing N by 1,
(S23) Laser exposure is performed on the Nth image portion.
(S24), the electrostatic latent image potential V (n) of the image portion is V
_LMeasured at multiple locations with sensor 60 and averaged to determine
Is set (S25). And for exposure by the next laser output
The laser is turned off once in preparation (S26), and N is the last value
If it does not reach A (NO in S27), return to S23, and N is the last
The loop is repeated until the value A is reached (YES in S27). Le
After exiting the loop, the diffusion rate α averages the values for each region.
(S28), according to the value, the spreading factor code P = 1 when α ≦ 1.2, the spreading factor code P = 2 when 1.2 <α ≦ 1.4, and the spreading factor when 1.4 <α ≦ 1.6. Rate code P = 3, spreading factor code P = 4 when 1.6 <α ≦ 1.8, spreading factor code P = 5 when 1.8 <α ≦ 2.0 (S29-S38). By the way, in the present embodiment, the diffusion rate α
If not in the range, that is, if the diffusion rate α is greater than 2.0
If the photoreceptor deteriorates and exceeds the usage limit,
To output a signal prompting replacement (S3
9) For the convenience of the user. In the present embodiment, the measurement of the diffusivity is performed as described above.
It takes place at the start, but this does not limit the invention,
For example, it may be performed when the power is turned on or when the photosensitive member is replaced. i.-2 Laser Diode Strength Determination Routine FIG.
To compensate for the change in γ characteristics resulting from the photoconductor being affected
4 shows a routine for determining a laser intensity suitable for performing the above.
In this embodiment, the intensity modulation method is used.
The laser intensity means the maximum laser intensity. In the present embodiment, the spreading factor code P is set in advance in five ways.
Appropriate for each spreading factor code P
The laser intensity also has five levels. The smaller the value of the spreading factor code P, the sharper the γ characteristic.
It is as described above that the user stands up. First, when the spreading factor code P is 1 (YES in S41),
From the characteristics of the photoreceptor shown in FIG.
Since the rise is steep, this steep characteristic
To offset, the laser intensity which makes the γ characteristic the slowest,
That is, the previously prepared P₁~ P_FiveLaser intensity
P which is the lowest strength₁Is selected (S42). In this way, for all values of the spreading code P,
Each laser intensity that does not affect the gamma characteristics of the
It may be required experimentally. Similarly, when the spreading factor code p is 2 (YES in S43),
Laser intensity P_TwoIs selected (S44) and the spreading factor code P is 3
When (YES at S45), the optimal laser intensity P_ThreeSelect (S4
6), when the spreading factor code P is 4 (YES in S47),
Laser intensity P_FourIs selected (S48). When the spreading factor code P is 5, (N in S41, S43, S45, S47
O) From the characteristics of the photoreceptor shown in FIG.
Since the rise of the γ characteristic becomes gentle,
To offset the slow characteristics, make the γ characteristics the steepest
Laser intensity, that is, P₁~ P_FiveNo
P which is the highest strength_FiveIs selected (S49). In this way, to perform laser exposure of the selected intensity
As a result, fluctuations in γ characteristics are suppressed, so that appropriate γ complementarity is
Done. FIG. 15 shows the selected laser diode intensity.
A circuit diagram for setting manually is shown below.
explain. Intensity control of LED1 and LED2 provided on the operation panel
Judge by flicker. The circuit shown is installed in the laser diode driver 220.
Voltage to the laser diode controller 220a
V is connected in series via a variable resistor R.
Enter the laser diode controller 220a by the variable resistance R
Laser diode drive voltage V_LDThe value of is adjusted
You. Also, V_LDIs that of comparator CMP1 and CMP2
Each has been entered. On the other hand, the comparators CMP1 and CMP2_RBut
Resistance R₁And R_TwoThrough each (V_R−ΔV), (V_R+
ΔV) is input. This variable power supply V_R
Is measured by a dial, meter, or other scale (not shown).
It is possible to know the value accurately. Therefore, the known V_RValue and V_LDCompared to the value of
If it can be determined that_LDCan be adjusted. The outputs of comparators CMP1 and CMP2 are LED
Input to Driver 1 and LED Driver 2, LED1 and LED
It controls the flicker of 2. In the above circuit, V_LDIs (V_R-ΔV)
If only LED1 lights up, V_LDIs (V_R+ ΔV)
Only LED2 lights up, and V_LDIs (V_R−ΔV) and
(V_R+ ΔV), LED1 and LED2 are both turned off.
Lit, V_LDIs V_RBe adjusted to match
become. Note that the smaller ΔV here, the more V
_LDAnd V_RThe accuracy of the match increases. As described above, LED1 and LED2 on the operation panel
If both are turned off, the laser output adjustment is completed,
An image can be formed with preferable γ characteristics. In the embodiment described above, the intensity modulation method is used.
The case where laser emission control was performed is shown,
And the emission time according to the image signal.
In the variable pulse width modulation method,
By controlling the laser intensity according to the diffusivity,
It is possible to obtain favorable γ characteristics as in the above-described embodiment.
You. In the above-described embodiment, the intensity modulation method or the pulse width modulation method is used.
Applicable to multi-valued dither method combining tone method and dither
can do. ii. Second Embodiment Main Routine FIGS. 10 and 11 relate to a second embodiment of the present invention,
The conversion table for gamma correction
Printer control unit 20 of digital color copier that switches over
1 is a control flow. The following describes these flows.
Do. Fig. 10 shows the main route of the digital color copier.
You. First, initialization such as initialization of parameters is performed (S1
01), the internal timer is started (S102). Next, expand
The scattering rate α is determined (S103). After that, the routine enters a γ correction conversion table determination routine.
(S105). This conversion table decision rule is based on the diffusion rate α.
Therefore, the conversion table for γ correction is switched to change the γ characteristic.
This is described in detail in FIG.
You. When the laser intensity is determined in S105, a copy operation is performed.
(S106), wait for the internal timer to expire (S107),
One cycle of the routine ends. Regarding the determination of the diffusion rate of the main routine (S103)
8 (a) and 8 (b) shown in the first embodiment
The description is omitted because it is the same. ii.-1 Gamma correction conversion table determination routine In FIG. 11, the photoconductor is affected by the diffusion coefficient P.
Gamma compensation suitable for compensating for variations in gamma characteristics resulting from
4 shows a routine for determining a primary conversion table. This embodiment
Then, since the spreading factor code P is set in five ways,
Five types of conversion tables optimal for each spreading factor code P
Becomes The smaller the value of the spreading factor code P, the sharper the γ characteristic.
It is as described above that the user stands up. First, when the spreading factor code P is 1 (YES in S141),
From the characteristics of the photoreceptor shown in FIG.
Since the rise of the γ characteristic is the steepest,
To offset the steep characteristic, γ that makes the γ characteristic the slowest
Conversion table, that is, table prepared in advance
Out of 1 to 5, the emission characteristics of the laser are sharpest
The conversion table 1 of the characteristic to be performed is selected (S142). As described above, the gamma characteristic of the photoconductor is determined by the value of the diffusion coefficient P.
To perform gamma correction by controlling the laser emission nonlinearly.
Each conversion table may be obtained experimentally in advance. Similarly, when the spreading factor code P is 2 (YE in S143)
S), selects the optimal conversion table 2 (S144), and sets the spreading factor
If the code P is 3 (YES in S145), the optimal conversion
Is selected (S146), and when the spreading factor code P is 4,
(YES in S147), the most suitable conversion table 4 is selected (S14).
8). When the spreading factor code P is 5, (S141, S143, S145, S147
NO), from the characteristics of the photoreceptor shown in FIG.
This is also the case when the γ characteristic rises or becomes gentle.
Rising and steepest to offset the gradual characteristics of
Conversion table 5 for nonlinearly controlling laser emission so that
Is selected (S149). Switching to the selected gamma correction conversion table
Thus, the laser emission is controlled to perform appropriate gamma correction. In this embodiment, the intensity modulation method, the pulse width modulation method,
The system uses various methods such as the multi-valued dithering method and the multi-valued dithering method.
Can be applied. iii. Third Embodiment Main Routine FIG. 3 (b) and FIG. 12 and FIG.
In this embodiment, the gradation expression is
Using the multi-valued dither method, as the gradation characteristic compensation,
The threshold is changed according to the spreading factor. Fig. 3 (b) shows the actual
4 shows a flow of processing of an image signal in the embodiment. Real truth
In the embodiment, the signal output from the γ conversion unit 28 is
The dither threshold value in the ROM 217 by the dither processing unit 29.
Dithered and applied to laser diode driver 220
Is output. Figures 12 and 13 show digital color copiers.
5 is a control flow of the printer control unit 201. Below, this
The flow will be described below. Figure 12 shows the main routine of the digital color copier.
Show. First, make initial settings such as parameter initialization.
(S201), the internal timer is started (S202). Next
Next, the spreading factor α is determined (S203). Thereafter, a dither determination routine is entered (S205).
This dither determination routine uses a multi-valued dither method for gradation expression.
, The dither threshold is determined by the diffusion rate α.
Change the value pattern to suit the changing γ characteristics
This is for performing a gamma correction, and is described in detail in FIG.
I do. When the laser intensity is determined in S205, the copy operation is
(S206) and wait for the internal timer to expire (S207).
One cycle of the in-routine is completed. Determination of the diffusion rate of the main routine (S203) and
The input of the frame code (S204) is described in the first embodiment.
8 (a) and 8 (b) shown in FIG.
Is omitted. iii.-1 Dither determination routine In FIG. 17, the photoconductor is affected by the diffusion coefficient P.
Suitable for compensating for variations in gamma characteristics resulting from
2 shows a routine for determining the user. In this embodiment, the diffusion coefficient
Since the code P is set in five ways, each spreading factor
There are also five types of optimal dither for the code P. This dither
As an example of a turn, FIGS. 16 (a), (b), (c),
(D) shows dither 1, dither 2, dither 3, and dither, respectively.
FIG. The smaller the value of the spreading factor code P, the sharper the γ characteristic.
It is as described above that the user stands up. First, when the spreading factor code P is 1 (YES in S241),
From the characteristics of the photoreceptor shown in FIG.
Is sharp, for example, P = 2
That is, a predetermined γ that linearizes the γ characteristic when α = 1.3
Γ of nonlinear control of laser emission by correction conversion table
Correction alone cannot completely compensate, and even after correction
This is the same as shown in FIG. 6 (c).
You. Therefore, in the present embodiment, a predetermined conversion for γ correction is performed.
Performing nonlinear control of laser emission using only the table
Can be further corrected to linearize
Switch to dither 1 which has input / output characteristics (S2
42). As described above, the laser based on the predetermined gamma correction conversion table is used.
Γ as a result of performing the light emission control on the diffusion coefficient code P
The characteristics are determined experimentally in advance, and therefore,
Dither threshold pattern that can linearize
Is also required. Similarly, when the spreading factor code P is 2 (YE at S243)
S), dither 2 is selected (S244), and the spreading factor code p is 3
In the case of (YES in S245), the optimum dither 3 is selected (S24
6), when the spreading factor code P is 4 (YES in S247),
The appropriate dither 4 is selected (S248). When the spreading factor code P is 5, (S241, S243, S245, S247
NO), from the characteristics of the photoreceptor shown in FIG.
Is also the case when the γ characteristic rises or becomes gradual.
For example, linearly change the γ characteristic when P = 2, that is, when α = 1.3.
Of the laser emission by the predetermined conversion table for gamma correction
The gamma correction of linear control alone cannot completely correct
The fact that it remains nonlinear after that is shown in FIG. 6 (c).
It is the same as Therefore, in this embodiment, the predetermined
Non-linear control of laser emission using only γ-correction conversion table
Even after control, the characteristics that remain non-linear
Cut to dither 5 with input / output characteristics that can be shaped
(S242). In this way, switching to the selected dither,
, And appropriate γ correction is performed. In the above embodiment of the present invention, the spreading factor code P is set to 5
But the invention is not limited to this,
The more it is, the more precise compensation is possible.

【effect】

In the electrophotographic digital image forming apparatus according to the present invention, the gradation characteristic is compensated for in accordance with the diffusion rate of the photoconductor. By suppressing the influence on the characteristics, it is possible to obtain a reproduced image that is faithful to a document having excellent gradation reproducibility.

[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. 3A is a first example of a block diagram showing a configuration of the image signal processing unit. FIG. 3B is a block diagram showing a second embodiment of the configuration of the image signal processing unit. FIG. 4 is a diagram showing an example of the γ characteristic. FIG. 5A is a schematic diagram showing a laser intensity distribution. FIG. 5 (b) is a schematic view showing a latent image formed by exposing the photosensitive member with the laser shown in FIG. 5 (a) in comparison with an ideal latent image. FIG. 6 (a) is a diagram showing that the γ characteristic changes depending on the difference in the diffusivity. FIG. 6 (b) shows the emission characteristics of the laser whose intensity is nonlinearly controlled by the γ correction conversion table for linearly correcting the γ characteristics at the values of the respective diffusion rates shown in FIG. 6 (a). FIG. FIG. 6 (c) shows γ of the four γ characteristics of FIG. 6 (a) using a laser having an emission characteristic of α = 1.3 in FIG. 6 (b).
It is a figure showing the result of having performed amendment. FIG. 6 (d) is a graph showing a change in surface potential when a charged photoreceptor is irradiated with a laser whose intensity is changed under a constant light emission time. FIG. 6 (e) shows the case where the laser light intensity is changed from 0 to each of the maximum values, with the maximum values being 0.8mW, 1.0mW, 1.1mW, and 1.2mW, respectively, corresponding to the document level, with the light emission time constant. 4 is a graph showing the photoconductor surface potential of FIG. FIG. 6 (f) is a graph showing the relationship between the document density and the image density (γ characteristic) in consideration of the development characteristics from FIG. 6 (e). FIG. 6 (g) is a diagram showing the input / output characteristics of the dither which can linearly correct the four gradation characteristics shown in FIG. 6 (c). FIG. 7 is a main control flowchart of the digital color copying machine according to the first embodiment of the present invention. FIG. 8A shows a diffusion rate determination routine of the digital color copying machine according to the embodiment of the present invention, in which the diffusion rate is determined by inputting. FIG. 8 (b) shows a diffusion rate determination routine of the digital color copying machine according to the embodiment of the present invention, in which the diffusion rate is determined by detection. FIG. 9 is a flowchart of a laser diode intensity determination routine of the digital color copying machine according to the first embodiment of the present invention. FIG. 10 is a main control flowchart of the digital color copying machine according to the second embodiment of the present invention. FIG. 11 is a flowchart of a conversion table determination routine of the digital color copying machine according to the second embodiment of the present invention. FIG. 12 is a main control flowchart of the digital color copying machine according to the third embodiment of the present invention. FIG. 13 is a flowchart of a dither determination routine of the digital color copying machine according to the third embodiment of the present invention. FIG. 14 is a top view showing a state where the photoreceptor assembly is sandwiched between the front side plate and the back side plate of the photoreceptor copier main body. FIG. 15 is a circuit diagram of one embodiment for manual laser diode intensity control. FIGS. 16 (a), (b), (c) and (d) each show 1
This is an example of a dither threshold pattern used in a multi-valued dithering method in which a dot is multi-valued into eight values and gradation is expressed by 2 × 2 dots. 20 ...... image signal processing unit 41 ...... photosensitive drum 44 ...... V ₀ sensor 60 ...... V _L sensor 101 ...... image reader control unit 106 ...... image control unit 201 ...... printer control unit 202 ...... printhead controller Section 203 AIDC sensor 206 Operation panel 208, 216 Control ROM 209, 217 Data ROM 220 Laser diode driver 220a Laser diode controller 221 Laser diode 230 Photoconductor lock switch 231 Photoconductor Mark reader 231a …… Code information mark 410 …… Photoconductor assembly

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-208368 (JP, A) JP-A-1-120577 (JP, A) JP-A-64-8046 (JP, A) JP-A-3- 271779 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B41J 2/44

Claims (6)

(57) [Claims] 1. An electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple steps, showing a spread degree of a latent image potential with respect to an actual beam diameter of a laser beam incident on a photosensitive member. A digital image forming method, wherein a diffusion rate is determined, and the gradation expression is performed by changing a laser emission intensity for an image signal having a maximum density according to the diffusion rate. 2. The degree of spread of a latent image potential with respect to an actual beam diameter of a laser beam incident on a photosensitive member in an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages. A digital image forming method, wherein a diffusion rate is determined, and a laser light amount is nonlinearly controlled using a conversion table for γ correction according to the diffusion rate. 3. The degree of spread of a latent image potential with respect to an actual beam diameter of a laser beam incident on a photosensitive member in an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple steps. A digital image forming method, comprising: determining a diffusion rate; and changing a dither threshold according to the diffusion rate. 4. An electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, wherein the diffusion rate is determined by detecting a surface potential of a photoreceptor at a laser exposure degree. The digital image forming method according to any one of claims 1 to 3, wherein the determination is made by calculating a diffusion rate from the value. 5. An electrophotographic digital image forming method in which gradation expression is performed by changing a laser light amount in multiple stages, wherein the diffusivity is input by an input means. Item 4. The digital image forming method according to any one of Items 3 to 8. 6. An electrophotographic digital image forming method in which gradation is expressed by changing the amount of laser light in multiple stages, wherein the input of the diffusivity is performed when the photosensitive member is replaced. The digital image forming method as described above.