JPH11277809A - Imaging system and exposure control method for electrophotographic process - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce gray level correctly and stably over the entire region of gray level. SOLUTION: In an imaging system for printing out a multilevel halftoning image data through electrophotographic process, intensity modulation exposure is performed for a pixel having gray level on the side where the quantity of exposure required for a set value Dth is low whereas pulse width modulation exposure is performed for a pixel having gray level on the side where the quantity of exposure required for the set value Dth is high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for printing out multi-tone image data by an electrophotographic process.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus such as a digital copying machine, a page printer, and a facsimile apparatus, which prints out image data, a negative latent image corresponding to an original image is formed on a photosensitive member, and the negative latent image is formed. The reversal development in which toner is attached to the charge erased portion is performed.

When printing a multi-tone image including a color image, in order to increase the number of reproducible tones,
Pseudo gradation control in a pixel matrix unit and multi-value density control in a pixel unit are used together. An error diffusion method is generally used as a pseudo gradation control method. As for the density control for each pixel, there is a method of setting the light emission amount of a light source in forming a latent image by pulse width modulation or intensity modulation.

[0004]

FIG. 14 is a schematic diagram of tone reproduction characteristics of pulse width modulation and intensity modulation. Pulse width modulation is excellent in linearity. That is, the density indicated by the image data is substantially proportional to the density actually reproduced. Further, it is easy to control the tone reproduction characteristic to be constant irrespective of the change of the operating environment, and the print quality can be stabilized.

However, in the pulse width modulation, when the pulse width is shorter than a fixed value depending on the rising characteristic of the light source,
The amount of light emitted from the light source is insufficient and the charge on the photoconductor is not erased. For this reason, there is a problem that the gradation of the highlight portion of the original image is not correctly reproduced.

On the other hand, according to the intensity modulation, the gradation can be reproduced even in the highlight portion. However, the tone reproduction characteristics are non-linear. In particular, when a laser light source is used, the light emission characteristics greatly change depending on environmental conditions such as temperature, so that it is difficult to stabilize the tone reproduction characteristics as compared with pulse width modulation. In color printing in which toner images of a plurality of colors are superimposed, it is advantageous that the gradation reproduction characteristics be linear in order to make the characteristics of each color uniform. Further, in the case where the speed of printing is increased by adopting a tandem format in which toner images of a plurality of colors are formed in parallel, since there are a plurality of light sources, it becomes more difficult to stabilize the tone reproduction characteristics.

SUMMARY OF THE INVENTION It is an object of the present invention to reproduce a gradation correctly and stably over the entire gradation range.

[0008]

In the present invention, switching between intensity modulation and pulse width modulation according to the gradation level is applied to the setting of the exposure amount for latent image formation. More specifically, in the case where the exposure amount is set only by pulse width modulation, the minimum gradation level at which exposure can be performed or the gradation level on the side where the pulse width is longer than that is set as a boundary, and exposure is performed from the boundary. Intensity modulation is applied to the gradation level on the smaller amount side, and pulse width modulation is applied to the other gradation levels.

According to a first aspect of the present invention, there is provided an image forming apparatus for printing and outputting multi-gradation image data by an electrophotographic process. The pixel having the value of (1) is exposed by intensity modulation, and the pixel having a value on the side where the required exposure amount is larger than the set value is exposed by pulse width modulation.

According to a second aspect of the present invention, there is provided an image forming apparatus for printing out multi-gradation image data by an image exposure type electrophotographic process for forming a negative latent image, wherein a gradation level value is set. A pixel having a value on the highlight side with respect to the value is subjected to exposure by intensity modulation, and a pixel having a value on the shadow side with respect to the set value is subjected to exposure by pulse width modulation.

According to a third aspect of the present invention, an exposure amount is set by intensity modulation for a pixel whose gradation level value is a value on the highlight side with respect to the set value, and is a value on the shadow side with respect to the set value. This is an exposure control method for an electrophotographic process in which an exposure amount is set for pixels by pulse width modulation.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A digital copying machine will be described as an application example of the present invention. First, a configuration relating to print output of image data will be briefly described, and then a configuration specific to the present invention will be described. [Overall Configuration] FIG. 1 is a view showing the overall configuration of a copying machine 1 according to the present invention.

The copying machine 1 is a digital color copying machine, and includes an automatic document feeder (ADF) 10, a reduced projection type image reader unit 20, and an electrophotographic printer unit 30.
It is composed of Further, the copying machine 1 includes an interface 27 for data communication with an external device, and outputs image data read by the image reader section 20 to the external device, and conversely, image data from the external device. The data can be sent to the printer unit 30 for printing.

The automatic document feeder 10 conveys the document set on the document set tray 11 to a reading position of the image reader section 20 and, after reading is completed, a discharge tray 13.
Discharge up. The document transport operation is performed according to an instruction from an operation panel (not shown), and the document is discharged by the image reader unit 20.
This is performed in response to the read end signal from. When a plurality of documents are set, these control signals are continuously generated, and a series of operations of document transport, reading, and document discharging are efficiently performed.

In the image reader section 20, the original positioned on the original platen glass 28 is irradiated from below with an exposure lamp 21, and the reflected light from the original is reflected by the mirror group 2.
2 and a CCD image sensor 24 through an imaging lens 23
Incident on. The scanner to which the exposure lamp 21 and the first mirror are attached is moved by the motor 29 in the sub-scanning direction at a speed V according to the magnification. Thus, the original can be scanned over the entire surface. As the scanner moves,
The second mirror and the third mirror move in the same direction at a speed V / 2, and keep the optical path length constant. The position of the scanner is controlled based on the calculated value of the amount of movement (the number of steps of the motor) from the time when the sensor 26 detects that the scanner has left the home position. The reading resolution of the image reader unit 20 is, for example, 40
0 dpi.

Light incident on the CCD image sensor 24 is photoelectrically converted for each of R, G, and B color components. The image processing circuit 25 performs analog signal processing, A / D conversion, and digital image processing on the photoelectric conversion signal of each color. From the image processing circuit 25 to the printer unit 30 or the interface
Image data corresponding to the document is sent to the face 27.

A white plate for shading correction is arranged outside the original reading area of the original platen glass 28, and reading of the white plate for generating shading correction data is performed prior to reading of the original. Will be

In the printer section 30, image data sent from the image reader section 20 is converted into cyan (C), magenta (M), yellow (Y), and black (K) by the data processing system of the print head 31. Is converted into image data (print data) for printing. The control unit of the print head 31 causes a semiconductor laser, which is an exposure light source, to emit light in accordance with print data of each color. The laser light is deflected in the main scanning direction by a polygon mirror, and a total of four imaging units 32c, 32m, 32
y, 32k.

Each of the imaging units 32c, 32m,
Elements required for the electrophotographic process are arranged inside the photoconductors 32y and 32k. In the figure, the photoconductor rotates clockwise, and each process of charging → exposure → development → transfer is continuously performed. The type of exposure is an image exposure type in which charges are erased from pixels other than the background portion of the original image to form a negative latent image, and the type of development is a reversal development type in which toner adheres to the non-charged area of the photoconductor. . Imaging units 32c, 32m, 32y, 3
2k is independently integrated for each color, and is configured to be detachable from the printer unit main body.

The toner image obtained by developing the latent image on the photoreceptor is transferred to a transfer charger 33c, 3 which is disposed at a position opposite to the photoreceptor with a sheet conveying belt 34 interposed therebetween.
By 3m, 33y, and 33k, the images are transferred onto the sheet in the order of C, M, Y, and K. However, in the case of a monochrome copy using only the black imaging unit 32k, the paper transport belt 34 has the other three color imaging units 32c, 32m, and 32y as indicated by the dashed lines in the drawing.
Placed away from the This is to prevent abrasion of the photoconductor.

On the downstream side of the imaging unit 32k in the paper transport path, a registration correction sensor 36, an AID
A C sensor 37, a cleaner 38, and a fixing roller pair 39 are arranged. The registration correction sensor 36 detects a color shift, and the detection signal is used for drawing position correction and distortion correction. The AIDC sensor 37 is a photo sensor that detects the density of the AIDC pattern. The AIDC pattern is a toner image for checking the reproduction state of the density, and is formed in a region of the paper transport belt 34 where there is no paper. The cleaner 38 erases the AIDC pattern.

Below the sheet transport belt 34, sheet cassettes 41a, 41b and 41c are mounted. Paper of a predetermined size is supplied to the transport path by a paper feed roller from the paper feed cassette selected as an alternative, and is sent to the transport belt 34 by a group of rollers. After the sensor 35 detects the fiducial mark on the paper transport belt 34, the paper transport belt 3
4 is started. Then, the sheet that has gone through the electrophotographic process including fixing is discharged onto a discharge tray 49. In the case of a double-sided copy, the sheet that has passed through the fixing roller pair 39 is turned over by switchback conveyance, sent to the screen unit 48, and fed again from the screen unit 48 to the sheet conveyance belt 34. [Signal Processing in Image Reader Unit] FIG. 2 is a block diagram of the image processing circuit 25 of the image reader unit 20.

The image processing circuit 25 includes an A / D conversion circuit 25
1, shading correction circuit 252, log conversion circuit 2
53, UCR / BP processing circuit 254, masking circuit 2
55, a density correction circuit 256, an MTF correction circuit 257, and an additional function circuit (not shown). These circuits are given predetermined instructions from a first CPU 201 which controls the image reader unit 20. Additional function circuits include a circuit for correcting chromatic aberration of the reading optical system, a circuit for realizing various kinds of image editing including enlargement / reduction,
And a circuit for preventing counterfeiting of banknotes and securities.

The A / D conversion circuit 251 performs offset and gain correction on the analog signal input from the CCD image sensor 24, and outputs the corrected signal to R, G,
The image data is converted into 8-bit (256 gradation) image data for each B color. The shading correction circuit 253 detects the uneven light distribution of the exposure lamp 21 and the CCD for the image data of each color.
Correction according to the variation in sensitivity between pixels of the image sensor 24 is performed. The log correction circuit 253 converts the image data representing the luminance into image data representing the density according to the relative luminosity factor of a human. The UCR / BP processing circuit 254 extracts a dark color component to be reproduced with black toner from the image data,
The data values of R, G, and B are corrected according to the extracted value. The masking circuit 255 generates C, C based on the three color image data.
Image data of four colors of M, Y, and K is generated. The density correction circuit 256 performs color correction by multiplying the C, M, and Y image data by a predetermined coefficient. Then, the MTF correction circuit 257
Perform processing for improving image quality such as smoothing. MT
The output of the F correction circuit 257 is sent to the printer unit 30 as print data.

Here, the filter processing in the MTF correction circuit 257 will be described. The filters to be applied are a general first-order differential filter and a Laplacian filter. The relationship between the input D and the output DD of the MTF correction circuit 257 is expressed by equation (1).

DD = D × [F (ΔV) × G (ΔD)] (1) ΔV in the expression is Laplacian data of lightness V obtained by conversion from the RGB color space to the VHC color space. (Δ
V) is a spatial frequency correction function shown in FIG. Further, ΔD is a primary differential value of the input data (density) from the density correction circuit 256, and G (ΔD) is an edge intensity output shown in FIG. In this example, when the edge strength G is equal to or greater than a predetermined threshold, the pixel of interest is regarded as an edge,
Activate the edge signal. This edge signal is sent to the printer unit 30 together with the image data, and is used for controlling halftone reproduction.

FIG. 3 is a block diagram of a main part of the MTF correction circuit 257, and FIG. 4 is a view showing an example of an output waveform of each part in FIG. The image data (D) from the density correction circuit 256 is 1
An edge emphasis signal G (ΔD) having a value corresponding to the differential value ΔD is output from the first table 2572 that receives the differential value ΔD obtained by differentiating by the next differential filter 2571. On the other hand, the lightness V is differentiated by the Laplacian filter 2574, and the obtained differential value ΔV is calculated in the second table 2
575. The table 2575 outputs a correction function F (ΔV) corresponding to the differential value ΔV. Image data (D), edge enhancement signal G (ΔD), and correction function F
(ΔV) is multiplied by a multiplier 2578. Also,
The differential value ΔD obtained by the primary differential filter 2571 is also input to the comparator 2579. The comparator 2579 calculates the differential value Δ
An edge signal is output when D is larger than a predetermined threshold.

When an annular original image as shown in FIG. 4 is read along a line crossing the original image, four edges are present. The transition of the edge intensity along the line is less sharp than that of the original image.

FIG. 5 is a diagram showing a correction function F (ΔV) of the interrogation frequency. The relationship between the differential value ΔV of the brightness V by the Laplacian filter 2574 and the correction function F (ΔV) is expressed by the following equation.

F (ΔV) = a · ΔV + b (ΔV> 0) (2) F (ΔV) = a ′ · ΔV + b ′ (ΔV <0) (2 ′) That is, the differential value ΔV is positive (image portion ) Or negative (base portion), the correction function F (that is, the image data value DD) is made positive or negative, so that the contrast between one side and the other side of the edge is increased.

FIG. 6 is a graph showing the relationship between the primary differential data (ΔD) and the edge intensity G (ΔD). A portion having a negative edge strength G corresponds to a halftone portion, and a positive portion corresponds to an edge. In the figure, three lines A1, A2, An
Is shown. An is n in the sub-scanning direction
The function Gn (ΔD) of the main scanning line is shown. When the first-order differential data ΔD is obtained for the n-th pixel in the sub-scanning direction, the edge intensity at that pixel is obtained from the function Gn (ΔD). Functions G1 (ΔD) to Gn (Δ
D) is stored in advance in the MTF correction circuit 257 as a table 2572. If this value is a negative value for each density, a smoothing filter according to that value is selected. An averaging filter or a median filter is used as the smoothing filter. The smoothing process may be performed by a moving average filter.

In FIG. 6, when the primary differential value (ΔD) becomes larger than the threshold value and the edge strength G becomes positive, the image data value DD represents the edge component of the image.
In this case, the edge signal is activated as described above. G (ΔV) increases as the first derivative (ΔD) increases. This component is usually dull with respect to the original image as shown in FIG. [Signal Processing in Printer Unit] The configuration of the printer unit 30 is composed of four imaging units 32c, 32m, and 32.
This is a “tandem format” in which a color image is formed using y and 32k simultaneously. To the printer unit 30, print data of four colors of C, M, Y, and K is simultaneously transferred from the image reader unit 20 during one scanning period of a document. Therefore, the signal processing in the printer unit 30 is basically a parallel processing of four colors.

In color copying, four color toner images must be transferred onto a sheet without being shifted, but the four imaging units 32c, 32m, 32y and 32k are arranged at equal intervals along the sheet transport direction. Have been. Therefore, in order to align the transfer positions on the paper, the process from exposure to transfer is performed at a timing shifted by a predetermined time for each color. Therefore, since the print data of four colors that are input simultaneously are used for exposure control at different timings, it is necessary to sequentially delay the print data of each color by a time determined by the arrangement interval of the photoconductors and the paper conveyance speed. Further, since four laser beams are deflected by one polygon mirror, the main scanning direction is reversed between the two colors (C, M) on the upstream side of the paper conveyance and the two colors (Y, K) on the downstream side. become. Further, a color shift occurs due to a shift of an exposure position in a main scanning direction, a distortion in a main scanning magnification, a bow distortion in a sub scanning direction, and a skew distortion when the axial direction of the photoconductor is not parallel to the deflection direction. For these reasons, the printer unit 30 incorporates a delay memory having a predetermined capacity and a correction circuit for preventing color misregistration. Further, the printer unit 30 includes a memory unit for storing an image on one side when copying on both sides.

FIG. 7 shows a signal processing system 300 of the printer unit 30.
FIG. The C, M, Y, and K image data D20 transferred from the image reader unit 20 is transmitted to a tone reproduction circuit 5.
Input to 0. The tone reproduction circuit 50 includes the printer unit 30.
In accordance with an instruction from the CPU 301 which controls the image data, the 8-bit image data D20 of each color is converted into 3-bit pseudo gradation data (for example,
56 gradations). However, for the edge, a simple ternarization process is performed instead of the multi-value error diffusion process to increase the sharpness. Switching of this process is in accordance with a tone reproduction attribute signal (LIMOS signal) from the image reader unit 20.

In an image distortion circuit (not shown),
K in resist detection test pattern with four colors superimposed
Based on the detection results of the shift amounts of C, M, and Y with respect to
The main-scanning magnification distortion of the C, M, and Y components and the skew distortion of the sub-scanning are corrected by density distribution type interpolation processing.
Then, the corrected data is replaced with solid black data as necessary based on the bill recognition result.

The image data of each color lowered in the tone reproduction circuit 50 is appropriately delayed by the delay memories 59a to 59f as described above, and the gamma correction / PWM control circuits 61 and 62 provided for each color. , 63, 64. In the delay, since the image data is reduced to 3 bits, the required memory capacity is smaller than in the case of 8 bits.
The gamma correction / PWM control circuits 61 to 64 correct the gradation distortion due to the gamma characteristic due to the electrophotographic process, and generate a PWM pulse having a peak value and a duty ratio according to the corrected data value. The photosensitive member 8 is generated by the generated PWM pulse.
On / off control of semiconductor lasers 71 to 74 as exposure light sources 1 to 84 is performed.

FIG. 8 is a block diagram of the tone reproduction circuit 50. Since the circuit configuration corresponding to each of the four colors C, M, Y, and K is the same, the circuit configuration for one color is shown here.

Image data D20 is input to the tone reproduction circuit 50 in synchronization with the pixel clock. The latched image data D20 is sent in common to the simple ternarization circuit 51 and the multi-level error diffusion processing circuit 52, and each circuit receives a predetermined process in parallel. Then, either the output of the simple ternarization circuit 51 or the output of the multi-level error diffusion processing circuit 52 is selected by the selector 55, and 1 is set as the image data D50.
It is output to the subsequent stage together with the bit tone reproduction attribute signal. The selection control signal of the selector 55 is an edge determination signal (corresponding to a tone reproduction attribute signal). That is, when the pixel of interest corresponds to the edge, the minimum value “0” of the input dynamic range “0-255” and the median value “12”
The output of the simple ternarization circuit 51 which takes either the value "8" or the maximum value "255" is selected. In other cases, the output of the multi-level error diffusion processing circuit 52 described below is selected. .
Note that the tone reproduction circuit 50 is provided with selectors 53 and 54 according to the effective image area signal. When scanning outside the effective image area having a margin around the original, the image data D20 and the edge discrimination are determined. Each signal is replaced with data of a fixed value indicating an invalid area.

The multi-valued error diffusion processing circuit 52 is a circuit for reducing the number of gradations by applying a well-known error diffusion method as a method of reproducing a pseudo gradation, and includes a lowering circuit 520 and an error detection table (lookup). Up table) 530, selector 55
0, an error diffusion matrix 560, a divider 570, and an adder 580. The outline of the function of the multi-level error diffusion processing circuit 52 is as follows.

The adder 580 outputs the image data D of the pixel of interest.
20 is added to the error allocated from the surrounding pixels.
The lowering circuit 520 outputs the output of the adder 580 and the CPU 30
1 is compared with the set threshold value, and gradation conversion from 256 levels to 8 levels is performed. The value of the error associated with this gradation conversion can be calculated in advance. The data set indicating the error corresponding to each of the 256 input gradation levels is stored in the error detection table 5.
30. Here, the actual error takes a positive or negative value, but for convenience of error diffusion processing, the error detection table 530 is used.
Is added or subtracted from the offset value corresponding to the maximum negative error. First, the offset value (= 32) is subtracted from the input value (Din).
An error in the gradation range corresponding to the threshold used in step 20 is obtained. Finally, the offset value is added. This series of calculations is performed by the CPU 301, and the calculation results are downloaded to the table memory as the error detection table 530.

The output of the adder 580 is input to an error detection table 530 to determine an error for the next pixel. An error corresponding to the input value is obtained by the error detection table 530.
Output from The selector 550 is provided to fix the error to a fixed value (32 in the example) outside the effective image area.

The error diffusion matrix 560 is a functional element that assigns weights to the peripheral pixels and distributes the errors of the gradation conversion. Here, the weights are assumed to be integers for simplicity, and the divider 570 performs an operation of dividing the output of the error diffusion matrix 560 by the sum of the weights. As described above, at the stage of setting the contents of the error detection table 530, the subtraction and the addition of the offset are performed so that the error is always positive, so that the calculation of the negative number in the error diffusion matrix 560 becomes unnecessary. This makes the circuit configuration simple and small,
Operation speeds up. In the error diffusion, it is necessary to calculate an error to be added before the image data D20 of the target pixel is input, and therefore, the processing time of the error diffusion defines the transfer speed of the image data. Therefore, the copy speed can be improved by increasing the speed of the error diffusion matrix 560.

FIG. 9 is a block diagram of the lowering circuit 520. The lowering circuit 520 includes threshold setting registers 521, 7
Comparators 522a-g and seven selectors 523a
To g. The CPU 301 loads seven threshold values th0 to th6 into the threshold value setting register 521. The comparators 522a to 522g commonly receive the image data D580 to which the error is distributed, and individually input corresponding ones of the thresholds th0 to th6 from the threshold setting register 521. The selectors 523a-523g
Image data D580 and threshold value t by comparators 522a-522g
One of two options (output gradation levels) is selected and output in accordance with the result of the magnitude determination of h0 to h6.

The set values of the threshold values th0 to th6 are 17,5 in order.
3,90,127,164,201,238
The dynamic range of the image data D580 is set so as to be approximately equal to seven. Table 1 shows the correspondence between the input gradation levels and the output gradation levels. Then, the input tone levels (32, 64,...) Near the center in each of the seven input tone ranges divided by the threshold values th0 to th6.
96, 128, 160, 192, and 224) are specific input gradation levels at which an error becomes zero during gradation conversion.

[0045]

[Table 1]

FIG. 10 is a block diagram of the gamma correction / PWM control circuit 61, and FIG. 11 is a waveform chart showing an example of the PWM control. Since the configurations of the gamma correction / PWM control circuits 61 to 64 corresponding to the respective colors of C, M, Y, and K are the same, here, the gamma correction / PWM control circuit 61 corresponding to C will be described as a representative example.

Image data D50 from the tone reproduction circuit 50
Is corrected by the γ correction table 601 and converted into 8-bit data. Then, the D / A converter 602
, And becomes analog density data S602, which is commonly input to the two comparators 604 and 605. One comparator 604 receives a triangular wave signal having a two-pixel cycle as a reference signal Sref1, and the other comparator 605 outputs 1
A triangular wave signal of a pixel cycle is input as a reference signal Sref2. Each of the comparators 604 and 605 converts a magnitude discrimination signal between the density data S602 and the reference signals Sref1 and Sref2 into PWM data (pulse width modulation data) D60.
4, and D605 to the selector 606. That is, the exposure pattern of each pixel is determined according to the value of the density data S602. The selector 606 selects one of the outputs of the comparator 604 and the comparator 605 in accordance with the tone reproduction attribute signal and sends it to the second selector 607. When the tone reproduction attribute signal is “H” indicating a continuous tone portion, PWM control of a two-dot cycle is performed to make the tone reproducibility smoother, and when “L” indicating an edge, it is suitable for sharpening. 1d
PWM control of the ot cycle is performed. The image quality is improved by automatically switching the modulation frequency. In addition, 2do
In the PWM control of the t-period, the phase of the reference signal Sref1 is shifted by a half period for each line in order to provide a screen angle of 45 degrees so as to improve the granularity of the image.

On the other hand, the image data D50 from the tone reproduction circuit 50 is also input to the intensity modulation circuit 610, and is converted into intensity modulation data D610 in which the signal level changes according to the density. The selector 607 selects one of the pulse width modulation data D606 from the selector 606 and the intensity modulation data D610 from the intensity modulation circuit 610 according to the instruction signal S607 from the CPU 301, and outputs it as an LD drive signal D61. The light emission of the semiconductor laser 71 is controlled using the LD drive signal D61. That is, the printer unit 30
In setting the exposure amount according to the gradation level, switching between pulse width modulation and intensity modulation, which is a feature of the present invention, is performed. [Exposure Control to which the Present Invention is Applied] In the copying machine 1 of the present embodiment, the tone reproduction characteristics of each color of C, M, Y, and K are linear to facilitate color adjustment, and four light sources (semiconductors) In order to keep the gradation reproduction characteristic constant irrespective of the characteristic fluctuation of the lasers 71 to 74), the exposure amount is basically set by pulse width modulation. Then, the exposure amount is set by intensity modulation only in the gradation level range in which the pulse width becomes too short to obtain the laser light amount required for exposure.

FIG. 12 is a graph showing an exposure control method. FIGS. 12 (A) and 12 (B) show substantially the same contents although the expression form is different. A pixel in which the value of the gradation level of the input image data is a value on the highlight side with respect to a preset threshold Dth, that is, from zero to the threshold Dth
The exposure time (pulse width) w for turning on the semiconductor lasers 71 to 74 is constant for the pixels within the gradation level range up to and the current supplied to the semiconductor lasers 71 to 74 is controlled so as to correspond to the gradation level. To increase or decrease the amount of laser light. That is, intensity modulation is performed. On the other hand, for a pixel whose gradation level value is on the shadow side with respect to the threshold value Dth, that is, for a pixel within the gradation level range exceeding the threshold value Dth, the laser light amount is fixed and Increase or decrease the pulse width w. That is, pulse width modulation is performed. However, on the circuit configuration, as described with reference to FIG. 10, the intensity modulation and the pulse width modulation are performed in parallel, and the semiconductor lasers 71 to 74 are selectively used by using one of the modulation data according to the gradation level. Drive.

Here, the threshold value Dth is set in the following manner. Print density 400 dpi, paper transport speed 120 m
When m / sec and the frequency (modulation frequency) of the reference signal Sref2 are 12 MHz, the maximum exposure time that can be assigned to one pixel is 83 msec. When the rise time when the semiconductor lasers 71 to 74 change from the off state to the light emitting state is about 5 msec, the gradation level corresponding to the rising point in the 256 gradation ranges is about 15.
4 [= 256 × (5/83)]. Therefore, the minimum value of the settable threshold value Dth is 16. In the present embodiment, the threshold Dth is set to 17 with some margin, and intensity modulation is applied to the pixels having the gradation levels of 1 to 17,
Apply pulse width modulation to 18 or more pixels. In accordance with this, the threshold value th0 is set to 1
7 is set (see FIG. 9).

FIG. 13 is a flowchart of an exposure control process executed by the CPU 301 of the printer unit 30. If the pixel density (gradation level) is equal to or smaller than the threshold value Dth, intensity modulation control for driving the semiconductor lasers 71 to 74 using the intensity modulation data D610 is performed (# 101, # 102). When the pixel density exceeds the threshold value Dth, pulse width modulation control for driving the semiconductor lasers 71 to 74 using the PWM data D604 and D05 is performed (# 101, # 10).
3).

In the above embodiment, the threshold value t for lowering the value
By rewriting the contents of h0-6 and the error detection table 530, the density reproduction characteristics can be flexibly adjusted. That is, the AID indicating the actual print output density
The density reproduction characteristic can be corrected based on the measurement result of the C pattern.

[0053]

According to the first to third aspects of the present invention,
The gradation can be correctly and stably reproduced over the entire gradation range.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of a copying machine according to the present invention.

FIG. 2 is a block diagram of an image processing circuit of the image reader unit.

FIG. 3 is a block diagram of a main part of the MTF correction circuit.

4 is a diagram illustrating an example of an output waveform of each unit in FIG.

FIG. 5 is a diagram showing a correction function of an interrogation frequency.

FIG. 6 is a graph showing a relationship between primary differential data and edge strength.

FIG. 7 is a schematic diagram of a signal processing system of the printer unit.

FIG. 8 is a block diagram of a tone reproduction circuit.

FIG. 9 is a block diagram of a low-value circuit;

FIG. 10 is a block diagram of a γ correction / PWM control circuit.

FIG. 11 is a waveform chart showing an example of PWM control.

FIG. 12 is a graph showing an exposure control method.

FIG. 13 is a flowchart of an exposure control process executed by a CPU of the printer unit.

FIG. 14 is a schematic diagram of tone reproduction characteristics of pulse width modulation and intensity modulation.

[Explanation of symbols]

 30 printer unit (image forming apparatus) Dth threshold value (set value) D20 image data 301 CPU (control means)

Claims (3)

[Claims] An image forming apparatus for printing out multi-gradation image data by an electrophotographic process, wherein a pixel whose gradation level value is a value on a side where a required exposure amount is smaller than a set value is used. Exposure by intensity modulation,
An image forming apparatus, wherein exposure is performed by pulse width modulation for a pixel whose required exposure amount is larger than the set value. 2. An image forming apparatus for printing out multi-tone image data by an image exposure type electrophotographic process for forming a negative latent image, wherein a value of a tone level is a value on a highlight side with respect to a set value. An image forming apparatus, comprising: performing exposure by intensity modulation on a pixel having a value of; and performing exposure by pulse width modulation on a pixel having a value on the shadow side with respect to the set value. 3. A pixel whose gradation level is a value on the highlight side with respect to a set value is set to an exposure amount by intensity modulation, and a pixel whose value is a shadow side with respect to the set value is set by pulse width modulation. An exposure control method for an electrophotographic process, comprising setting an exposure amount.