JP2959031B2 - 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. As a digital image forming method of this kind, an area gradation method using a dither matrix or changing the laser pulse width (light emission time) or light emission intensity to change the laser light amount (= light emission time × intensity) is used. A multi-valued laser exposure method (pulse width modulation method, intensity modulation method) for expressing the gradation for one dot to be printed is known (for example, JP-A-62-91077, JP-A-62-91077). No. 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 developed image particularly to 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.

[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 laser beam diameter and the like. There was a problem that the characteristics also changed. 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 characteristic is only considered to be 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 effect of the laser beam diameter on the γ characteristic will be described below with reference to an example. The laser beam diameter may be changed by exchanging a print head assembly having a laser diode as its main component, and the change in the laser beam diameter changes the γ characteristic as described below. 5 and 6 are diagrams for explaining the relationship between the beam diameter and the electrostatic latent image on the photoconductor. FIGS. 5A and 5B show the beam diameter (radius in the main scanning direction of a region having a value of 1 / e ² or more with respect to its maximum value in the Gaussian distribution of laser energy in this specification). A laser diode referred to as a B type having a radius of 54 μm to 75 μm (indicated by the radius in the sub-scanning direction) and
Each type has a strength of 1.
This is a three-dimensional visualization of the intensity distribution of the laser beam emitted at mW. FIGS. 6 (a) and 6 (b) show the emission times of 50 × 1/8 and 50 × 2 using the B type and S type laser diodes, respectively, with an emission intensity of 1.0 mW. / 8,…, 50 × 7 / 8,50 × 8/8 (msec) Scanning direction at each stage of electrostatic latent image on photoreceptor formed using pulse width modulation method that multi-values into 8 steps 4 is a graph numerically showing a potential distribution at the point. FIG. 7A shows the γ characteristics obtained by irradiating the same photoconductor with the B and S type laser beams by the pulse width modulation method. From this figure, B type and S
It is clear that the γ characteristic changes depending on the difference in beam diameter from the type. As shown in FIGS. 6 (a) and 6 (b), the change in the γ characteristic is such that the latent image formed by the B type laser beam and the S type laser beam is formed by a laser beam having the same intensity (1.0 mW). Despite this, the rise of the γ characteristic becomes steeper as the laser beam diameter is smaller, and the larger the laser beam diameter,
The rise of the characteristic becomes gentle. If the laser beam diameter is changed, the γ characteristic fluctuates. Therefore, since the same correction as before the change is performed, the appropriate γ correction is not performed, and the gradation characteristic of the reproduced image changes. And a stable reproduced image cannot be obtained. That is, for example, γ due to the S type laser beam diameter
Using the gamma correction conversion table corresponding to the characteristics, the seventh
When the emission characteristic of the laser is nonlinearly controlled like the emission characteristic of FIG. 7B, and the linear gradation characteristic indicated by S in FIG. If the type is changed to the B type, the rise of the γ characteristic becomes gentle (FIG. 7 (a)). Therefore, the non-linear control of the laser emission based on the conversion table for γ correction (FIG. 7 (b) )) Does not work effectively,
The gradation characteristic is not linearized (characteristic curve B in FIG. 7 (c)), and the rise remains gradual. As described above, in the digital image forming method that emphasizes the reproducibility of the gradation, the change of the laser beam diameter is a serious problem that cannot be ignored because the change in the gradation characteristic. Furthermore, this also affects the reproducibility of fine lines, especially in the intensity modulation method. 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]

According to the present invention, when a beam diameter is changed due to conversion of a print head assembly including a laser diode as a main component, as shown in FIG. In the opposite direction, well γ
If the characteristic is changed, the fact that the γ characteristic does not change as a result is used. According to a first aspect of the present invention, in an electrophotographic digital image forming method in which a laser light amount is changed in multiple stages to perform gradation expression, replacement of a laser generating means for generating a laser beam is detected, and based on the detection result, the photosensitive device is exposed. The method is characterized in that the diameter of the laser beam incident on the body is determined, and the gradation expression is performed by changing the laser emission intensity for the image signal having the maximum density according to the laser beam diameter. According to a second aspect of the present invention, in an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, replacement of a laser generating means for generating a laser beam is detected, and the detection result is obtained. The diameter of the laser beam incident on the photoconductor is determined based on the laser beam diameter, and the amount of laser light is nonlinearly controlled using a conversion table for γ correction corresponding to the laser beam diameter. Further, a third invention is an electrophotographic digital image forming method in which a laser light amount is changed in multiple stages to perform gradation expression, and detects the exchange of a laser generating means for generating a laser beam, and outputs the detection result. A laser beam diameter incident on the photosensitive member is determined based on the laser beam diameter, and a threshold value of dither is changed according to the laser beam diameter.

[Action]

One of the electrophotographic digital image forming methods according to the present invention performs appropriate γ correction by changing the laser exposure intensity based on the value of the laser beam diameter. 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 laser beam diameter. In still another example of the electrophotographic digital image forming method according to the present invention, another dither pattern is selected based on a value of a laser beam diameter and an appropriate gamma correction is performed.

【Example】

Hereinafter, embodiments 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 photoreceptor drama 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 to a copy sheet wound around a transfer drum 51 by a transfer charger 46. 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 a paper cassette 50, and the leading end thereof is caught by a catching 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 _O for detecting the photosensitive drum 41 surface potential V _O before laser exposure
A sensor 44, a _VL sensor 60 for detecting the surface potential _VL of the photosensitive drum 41 after laser exposure, an AIDC sensor 203 for optically detecting the density of a toner image attached to the surface of the photosensitive drum 41, and developing units 45a to Analog signals from various sensors such as an ATDC sensor 204 and a density / humidity sensor 205 for detecting the toner density within 45d are input. 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. Printer control unit 201
Is connected to a control ROM 208 storing a control program and a data ROM 209 storing various data.
The printer control unit 201 determines the control based on the data of M. 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 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 with reference to the contents of the data ROM 217 stored in the conversion table for gamma correction, and control the laser diode driver 202 via the drive I / O 218 and the parallel I / O 219. ing. Laser diode 2
The emission of 21 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. In particular, in the present invention, the optimum γ correction is performed based on the laser beam diameter data sent from the printer control unit 201. (B) Image Signal Processing FIG. 3A 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 shading-corrected image data is the reflection data of the original,
The log conversion is performed by the g conversion circuit 23 to convert the data into density data of an 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. The converted Y, M, and C data are multiplied by a predetermined coefficient to perform a density correction process in a density correction circuit 26, and a spatial frequency correction process is performed by a spatial frequency correction circuit 127. Output. In the print head control unit 202, the image signal processing 2
The image signal processed by 0 is subjected to γ conversion by the γ conversion unit 28 based on the γ correction conversion table in the data ROM 217, and is output to the laser diode driver 220. The laser diode driver 220 controls the light emission intensity of the laser diode 221 according to the image signal. (C) Control Flow i. First Embodiment Main Routine FIGS. 9 to 11 relate to a first embodiment of the present invention, and illustrate a printer of a digital color copying machine using laser intensity control for gradation characteristic compensation. 5 is a control flow of the control unit 201. Hereinafter, these flows will be described. FIG. 9 shows a main routine of the digital color copying machine. First, initialization such as parameter initialization is performed (S1), and an internal timer is started (S2). Next, a beam code is input (S4). Thereafter, the routine enters a laser diode intensity determination routine (S5). This intensity determination routine depends on the beam diameter.
Laser emission intensity corresponding to the image signal of the maximum density (hereinafter, referred to as
The maximum laser intensity is changed to suppress the fluctuation of the γ characteristic, which will be described in detail with reference to FIG.
When the laser intensity is determined in S5, a copy operation is performed (S5
6) Wait for the end of the internal timer (S7) and end one cycle of the main routine. Note that, here, the fact that the γ characteristics fluctuate even when the laser intensity is changed due to the characteristics of the photoconductor is shown in FIG.
(B) and (c) show. FIG. 8 (a) 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. This indicates that the saturation occurs when the surface potential drops to a certain extent, and therefore the image density does not change much. On the other hand, FIG. 8B shows that the light emission time is constant and the maximum values are 0.8 mW, 1.0 mW, 1.1 mW, and 1.2 m respectively corresponding to the document level.
FIG. 8F is a graph showing the photoconductor surface potential when the laser intensity is changed from 0 to each maximum value as W, and FIG. 8F is a document based on FIG. 9 is a graph showing the relationship between the density and the image density (γ characteristic). If the maximum light quantity is changed in this way, it is understood that the γ characteristic fluctuates in the same manner as when V _O and V _B are changed. The present invention utilizes this phenomenon to suppress a change in gradation characteristics in image formation caused by a change in laser beam diameter, and to obtain a reproduced image faithful to a document. i.-1 Beam code input routine The beam code input routine is to input a code assigned according to the diameter of the photoreceptor incident beam of several kinds of laser diodes selected in advance, and to input the beam of the currently equipped laser diode. It is for judging the diameter. An example of the beam code input will be described with reference to the flowchart shown in FIG. The beam code C is determined in advance according to the beam diameter of the type of the laser diode (for example, the above-described B type or S type). In this embodiment, the beam code C corresponds to the following five types of beam diameters: minimum beam diameter: beam code C = 1, small beam diameter: beam code C = 2, normal beam diameter: beam code C = 3 Large beam diameter: beam code C = 4 Maximum beam diameter: beam code C = 5, so that the smaller the beam code C, the sharper the γ characteristic rises. First, it is checked whether the laser diode has been replaced (S11). When laser diode replacement is detected (YE
S), the beam code C stored so far is reset (S12), and the beam code C is newly input (S13). The laser diode has not been replaced (S
If NO at 11), and if the beam code C has not been input and stored (YES at S14, for example, during assembly), the beam code C is input (S13). If the laser diode has not been replaced and the beam code C has been stored, the process directly returns. For the replacement of the laser diode and the input of the beam code accompanying the laser diode, a photo interrupter type sensor is provided in the print head unit in which the laser diode is housed, and a code information mark of the beam code C is previously attached to the printer head unit. The sensor is used to monitor the attachment of the printer head unit, and the beam code C is read simultaneously with the attachment of the print head unit by a mark reader capable of reading the beam code information in the attached state. So that Therefore, forgetting to input the beam code C and input error can be prevented, and the user is not conscious of the input operation of the beam code C. The input of the beam diameter may be manually input from the operation panel. i.-2 Laser Diode Intensity Determination Routine FIG. 11 shows a routine for determining a laser intensity suitable for correcting a change in the γ characteristic resulting from the influence of the beam code C on the photoconductor. This embodiment utilizes an intensity modulation method, and the laser intensity here means the maximum laser intensity. In this embodiment, since there are five types of beam codes C, the optimum laser intensity for the five types also has five levels. As described above, the smaller the value of the beam code C, the sharper the γ characteristic rises. First, when the beam code C is 1 (YES in S41), since the rise of the γ characteristic is the steepest from the characteristics of the photoreceptor in FIG. To offset, the laser intensity which makes the γ characteristic the slowest,
That is, selecting P ₁ is the lowest intensity of the laser intensity of P ₁ to P _5, which is prepared in advance (S42). As described above, the optimum laser intensity that does not affect the γ characteristics of the photoconductor may be determined experimentally in advance according to the beam code C. Similarly, when the beam code C is 2 (YES in S43),
Select laser intensity P ₂ (S44), when the beam code C is 3 select (YES in S45), the optimum laser intensity P ₃ (S4
6), YES in (S47 when the beam code C 4), to select the optimum laser intensity P ₄ (S48). When the beam code C is 5, (S41, S43,..., S45,.
7; NO), from the characteristics of the photoreceptor shown in FIG.
Since the rise of the γ characteristic is the slowest,
In order to offset this gradual characteristic, the laser intensity that makes the γ characteristic the steepest, that is, P _{1 to} P ₅ prepared in advance
Selecting P ₅ the highest intensity of the laser intensity (S4
9). By performing the laser exposure of the selected intensity in this manner, the fluctuation of the γ characteristic is suppressed, so that the optimal γ correction is performed. FIG. 16 shows a circuit diagram in the case where the setting of the selected laser diode intensity is manually performed, and will be described below. The intensity control is determined by the blinking of LED1 and LED2 provided on the operation panel. In the illustrated circuit, a voltage V is connected in series via a variable resistor R to a laser diode controller 220a provided in a laser diode driver 220, and the laser input to the laser diode controller 220a by the variable resistor R. The value of the diode drive voltage V _LD is adjusted. V _LD is input to each of the comparators CMP1 and CMP2. On the other hand, each variable power source V _R the comparator CMP1 and CMP2 via the resistor R ₁ and _{R 2 (V R -ΔV),} (V R +
ΔV) is input. Voltage of the variable power source V _R is, it is possible to know the value precisely by scale such as dial or meter (not shown). Accordingly, if determined that the equal as compared with values of the V _LD of the known V _R of the value of V _LD can be adjusted. The outputs of comparators CMP1 and CMP2 are LE
Input to D driver 1 and LED driver 2, LED1 and LE
It controls the blinking of D2. In the above circuit, if V _LD is less than (V _R -ΔV), only LED1 is lit, if V _LD is greater than (V _R + ΔV), only LED 2 is lit, and V _LD is (V _R -ΔV). if between) and (V _R + ΔV), LED1 and LED2 are turned off together, V _LD will be adjusted as to match the V _R. Here, the smaller the [Delta] V, accuracy of the matching between the V _LD and V _R is increased. As described above, if both LED1 and LED2 on the operation panel are turned off, the adjustment of the laser output is completed,
An image can be formed with preferable γ characteristics. In the above-described embodiment, the case where the laser emission control by the intensity modulation method is performed is described. However, in the pulse width modulation method in which the laser hardness at the time of light emission is fixed and the light emission time is changed according to the image signal, By controlling the laser intensity at that time according to the diffusion rate and the laser beam diameter, it is possible to obtain a preferable γ characteristic as in the above-described embodiment. Further, the above-described embodiment can be applied to a multi-valued dither method in which an intensity modulation method or a pulse width modulation method is combined with dither. ii. Second Embodiment Main Routine FIG. 12 and FIG. 13 relate to a second embodiment of the present invention. As a gradation characteristic compensation, a digital color copying machine in which a conversion table for γ correction is switched in accordance with a beam diameter. Printer control unit
It is a control flow of 201. Hereinafter, these flows will be described. FIG. 12 shows a main routine of the digital color copying machine. First, initialization such as parameter initialization is performed (S101), and an internal timer is started (S102). Next, a beam code is input (S104). Thereafter, the control enters a gamma correction conversion table determination routine (S105). This conversion table determination routine is for switching the conversion table for γ correction according to the beam diameter to suppress the fluctuation of the γ characteristic, and will be described in detail with reference to FIG. When the laser intensity is determined in S105, a copy operation is performed (S106), and after the internal timer has ended (S107), one cycle of the main routine ends. The beam code input (S104) of the main routine is the same as that of FIG. 10 shown in the first embodiment, and the description is omitted. ii.-1 Gamma Correction Conversion Table Determination Routine FIG. 13 shows a routine for determining a gamma correction conversion table suitable for correcting fluctuations in gamma characteristics resulting from the photoconductor being affected by the beam code C. Show. In the present embodiment, since there are five types of beam codes C, there are also five types of .gamma. Note that the beam code C has a small value and the γ characteristic rises sharply as described above. First, when the beam code C is 1 (YES in S141),
From the characteristics of the photoreceptor shown in FIG.
Since the rise of the γ characteristic is the steepest, in order to offset the steep characteristic, a γ conversion table for making the γ characteristic the gentlest, that is, all the tables 1 to 5 prepared in advance Among them, the conversion table 1 with the characteristic that makes the laser emission characteristic the slowest rise is selected (S142). As described above, each conversion table for performing the γ correction by nonlinearly controlling the laser emission of the γ characteristic of the photoconductor according to the beam code C may be experimentally obtained in advance. Similarly, when the beam code C is 2 (YE in S143)
S), the optimum conversion table 3 is selected (S144), and when the beam code C is 3 (YES in S145), the optimum conversion table 3 is selected (S146), and when the beam code C is 4, (S146). (S147: YES), and selects the optimal conversion table 4 (S14).
8). When the beam code C is 5, (S141, S143,..., S145,
.., NO in S147), since the rise of the γ characteristic is the slowest from the characteristics of the photoreceptor in FIG. 7 (a), the rise is the steepest in order to offset this gentle characteristic. The conversion table 5 for nonlinearly controlling the laser emission is selected (S149). In this way, switching to the selected γ correction conversion table controls the laser emission to perform appropriate γ correction. This embodiment can be applied to an apparatus using various systems such as an intensity modulation system, a pulse width modulation system, a dither method, and a multi-valued dither method. iii. Third Embodiment Main Routine FIG. 3 (b) and FIG. 14 and FIG.
In this embodiment, a multi-valued dither method is used for gradation expression, and in this embodiment, the threshold value of dither is changed according to the beam diameter as gradation characteristic compensation. FIG. 3B shows the flow of processing of an image signal in this embodiment.
In the present embodiment, the signal output from the γ conversion unit 28 is subjected to dither processing in the ROM 217 by the dither processing unit 29 based on dither threshold data, and the laser diode driver
Output to 220. 14 and 15 are control flows of the printer control unit 201 of the digital color copying machine. Hereinafter, these flows will be described. FIG. 14 shows a main routine of the digital color copying machine. First, initialization such as parameter initialization is performed (S201), and an internal timer is started (S202). Next, a beam code is input (S204). Thereafter, a dither determination routine is entered (S205).
This dither determination routine is for performing an appropriate gamma correction in accordance with a fluctuating gamma characteristic by changing a dither threshold pattern according to a beam diameter when a multi-valued dither method is used for gradation expression. Yes, and will be described in detail in FIG. When the laser intensity is determined in S205, a copy operation is performed (S206), and after the internal timer has ended (S207), one cycle of the main routine ends. The beam code input (S204) of the main routine is the same as that of FIG. 10 shown in the first embodiment, and therefore, the description is omitted. iii.-1 Dither Determination Routine FIG. 15 shows a routine for determining a dither suitable for correcting a change in the γ characteristic resulting from the influence of the photoreceptor by the beam code C. In this embodiment, since there are five types of beam codes C, five types of optimal dithers are stored. As an example of this dither pattern,
17 (a), (b), (c), and (d) show dither 1, dither 2, dither 3, and dither 4, respectively. As described above, the smaller the value of the beam code C, the sharper the γ characteristic rises. First, when the beam code is C1 (YES in S241), the rise of the γ characteristic is the steepest from the characteristics of the photoreceptor in FIG. 7A described above. In the case of the type, even if the γ correction of the nonlinear control of the laser emission is performed by the predetermined γ correction conversion table for linearizing the γ characteristic by the S type, it cannot be completely corrected,
The fact that it remains nonlinear after the correction is the same as that shown in FIG. 7 (c). Therefore, in the present embodiment, even if the nonlinear control of the laser emission is performed only by the predetermined conversion table for the γ correction, the dither having the input / output characteristics that can further correct the linearity and maintain the nonlinearity. 1
(S242). As described above, the γ characteristics as a result of performing the laser emission control using the predetermined γ correction conversion table for all the beam codes C are experimentally obtained in advance, and therefore, finally, each γ characteristic is linearly determined. There is also a need for a dither threshold pattern that can be converted. Similarly, when the beam code C is 2 (YE at S243)
S), dither 2 is selected (S244). If beam code C is 3 (YES in S245), the optimal dither 3 is selected (S244).
246), when the beam code C is 4 (YES in S247), the optimum dither 4 is selected (S248). When the beam code C is 5, (S241, S243,..., S245,
.., NO in S247), since the rise of the γ characteristic becomes the slowest from the characteristics of the photoreceptor in FIG. 7A, for example, the S type γ characteristic in FIG. Is not completely corrected only by the γ correction of the non-linear control of the laser emission by the predetermined γ correction conversion table (FIG. 7 (b)), and the nonlinearity remains even after the correction. This is the same as that shown by the characteristic curve B in FIG. Therefore, in the present embodiment, even if the nonlinear control of the laser emission is performed only by the predetermined conversion table for γ correction, the input / output characteristics that can further correct the characteristics that remain non-linear and can be linearized. Switch to dither 5 (S242). In this way, switching to the selected dither, laser emission is controlled, and appropriate gamma correction is performed. In the above embodiment of the present invention, the beam code C 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, since the gradation characteristic is compensated for in response to the change in the laser beam diameter, the state of the electrostatic latent image formed corresponding to the laser output is reduced. It is possible to accurately cope with the problem, suppress the influence on the gradation characteristics due to the change in the diffusion rate due to the change in the beam diameter, and obtain a reproduced image that is faithful to a document whose gradation reproducibility is selected.

[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 second embodiment of the block diagram showing the configuration of the image signal processing unit. FIG. 4 is a diagram showing an example of the γ characteristic. FIG. 5 (a) shows the beam of a B-type laser diode visually rendered three-dimensional at an output of 1.0 mW. FIG. 5 (b) visually shows the beam of the S-type laser diode as three-dimensional at an output of 1.0 mW. FIG. 6A is a graph showing a longitudinal section of an electrostatic latent image by a laser beam in which the emission time of a B-type laser diode having an output of 1.0 mW is multileveled in eight stages. FIG. 6B is a graph showing a longitudinal section of an electrostatic latent image by a laser beam in which the emission time of an S-type laser diode having an output of 1.0 mW is multileveled in eight stages. FIG. 7A is a graph showing that the γ characteristics change depending on the laser beam diameter. FIG. 7 (b) is a diagram showing the emission characteristics of the laser whose intensity is nonlinearly controlled by the γ correction conversion table for linearly correcting the γ characteristic S shown in FIG. 7 (a). FIG. 7 (c) is a diagram showing the result of performing γ correction on the γ characteristics of S and B of FIG. 7 (a) using the laser having the light emission characteristics of FIG. 7 (b). FIG. 8A 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. 8 (b) shows a case where the laser light intensity is changed from 0 to each of the maximum values, with the maximum values being 0.8 mW, 1.0 mW, 1.1 mW and 1.2 mW, respectively, corresponding to the document level, with the light emission time constant. 4 is a graph showing the photoconductor surface potential of FIG. FIG. 8C is a graph showing the relationship between the original density and the image density (γ characteristic) in consideration of the development characteristics from FIG. 8B. FIG. 9 is a main control flowchart of the digital color copying machine according to the first embodiment of the present invention. FIG. 10 is a flowchart of a laser beam code input routine of the digital color copying machine according to the embodiment of the present invention. FIG. 11 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. 12 is a main control flowchart of the digital color copying machine according to the second embodiment of the present invention. FIG. 13 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. 14 is a main control flowchart of the digital color copying machine according to the third embodiment of the present invention. FIG. 15 is a flowchart of a dither determination routine of the digital color copying machine according to the third embodiment of the present invention. FIG. 16 is a circuit diagram of one embodiment for manual laser diode intensity control. FIGS. 17 (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 _O sensor 60 V _L sensor 101 image reader control unit 106 image control unit 201 printer control unit 202 print head control Unit 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

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-208368 (JP, A) JP-A-1-120577 (JP, A) JP-A-3-271778 (JP, A) JP-A 64-64 8046 (JP, A) JP-A-3-252269 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B41J 2/44 B41J 2/52 H04N 1/036 G03G 15/04

Claims (3)

(57) [Claims] In an electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, replacement of a laser generating means for generating a laser beam is detected, and a photosensitive member is detected based on the detection result. A digital image forming method comprising: determining the diameter of a laser beam to be incident on the image signal; and changing the laser emission intensity with respect to the image signal having the maximum density according to the laser beam diameter to perform the gradation expression. 2. An electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, wherein replacement of a laser generating means for generating a laser beam is detected, and a photosensitive member is detected based on the detection result. A digital image forming method comprising: determining a diameter of a laser beam incident on a laser beam; and nonlinearly controlling a laser light amount using a conversion table for γ correction corresponding to the laser beam diameter. 3. An electrophotographic digital image forming method in which gradation is expressed by changing a laser light amount in multiple stages, wherein replacement of a laser generating means for generating a laser beam is detected, and a photosensitive member is detected based on the detection result. A digital image forming method comprising: determining a diameter of a laser beam incident on a laser beam; and changing a dither threshold value according to the laser beam diameter.