JP3126408B2 - Image processing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for processing a gradation image.

[0002]

2. Description of the Related Art Conventionally, a gradation pattern is formed on an image carrier such as a photoreceptor, and the density of the pattern is read and fed back to a toner supply amount and image forming conditions, thereby improving image stability. Techniques for causing this to occur are known.

[0003]

However, in the above-mentioned conventional example, a single machine uses a mode such as a character / line drawing mode or a photograph mode depending on the type of a document, for example, a screen angle of pulse width modulation or the like. When changing the tone reproduction method, the specific pattern on the image carrier has different reflection characteristics depending on the tone reproduction method, so if the mode changes, the same density conversion method can detect an accurate density. However, there is a problem that an accurate feedback system cannot be configured.

Further, the same problem arises because the reflection characteristics of a specific pattern on an image carrier such as a photoreceptor differ depending on the image forming material such as toner.

[0005] The present invention has been made in view of the problems of the related art, and has as its object to improve the accuracy of gradation control.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention is directed to a method of reproducing a predetermined pattern for a plurality of color components by using a tone reproduction method in which a pattern to be formed is different from each other. Forming means for forming a pattern on a medium, density detecting means for detecting the density of a pattern formed by the pattern forming means, and gradation control for performing tone control in accordance with the density detected by the density detecting means Means, the density detecting means comprises a light source and a light receiving element, measures reflected light of a pattern formed by the pattern forming means on the medium, and converts the obtained measurement amount into an optical density. At the time of conversion, the arithmetic processing for the conversion set for each of the color components is switched according to a tone reproduction method corresponding to the color component.

[0007]

【Example】

[Embodiment 1] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

In this embodiment, electrophotography will be described as an example. However, it goes without saying that the present invention can be applied to other types of image processing apparatuses such as an ink jet printer, a silver halide photographic printer, and a thermal transfer printer. .

FIG. 1 shows an embodiment of the present invention. In this embodiment, the image signal is converted into laser light by a laser driver and a laser light source (neither is shown),
The laser light is reflected by the polygon mirror 1 and the mirror 2 and is irradiated on the photosensitive drum 4. The photosensitive drum 4 on which the latent image has been formed by the scanning of the laser beam rotates in the direction of the arrow shown in the figure. Then, development for each color is performed by the rotary developing device 3. (FIG. 1 shows development with yellow toner.) On the other hand, the transfer paper 6 is wound around a transfer drum 5 to be Y (yellow), M (magenta),
The image is rotated once in the order of C (cyan) and Bk (black), and the image is rotated four times, and the transfer is completed.

When the transfer is completed, the transfer paper 6 separates from the transfer drum 5 and is fixed by the fixing roller pair 7 to complete a color image print.

Reference numeral 8 denotes an LED used as an irradiating means for emitting near-infrared light (main wavelength of about 960 nm), and 9 denotes a CCD sensor used to read a toner image formed on the photosensitive drum 4. .

FIG. 2 shows an image signal processing circuit for obtaining a gradation image according to this embodiment.

An image luminance signal is obtained by the CCD 21, and the luminance signal is converted into a digital luminance signal by the A / D conversion circuit 22.

In the obtained luminance signal, sensitivity variations of individual CCD elements are corrected by a shading circuit 23,
The corrected luminance signal is output, for example, by RA
It is converted by the LUT 25 composed of M and ROM.

FIG. 3 is a quaternary chart showing how the gradation is reproduced.

The first quadrant shows the reading characteristics for converting the original density into a density signal, the second quadrant shows the conversion characteristics of the LUT 25 for converting the density signal into a laser output signal, and the third quadrant shows the laser output signal. The printer characteristic of converting the image density into the output density is shown, and the fourth quadrant shows the total gradation characteristic of the image forming apparatus showing the relationship between the original density and the output density.

Since the number of gradations is processed by an 8-bit digital signal, there are 256 gradations.

In this image forming apparatus, in order to make the gradation characteristics of the IV quadrant linear, the nonlinearity of the printer characteristics of the III quadrant is corrected by the LUT 25 of the IV quadrant.

The contents of the table of the LUT 25 are described in C below.
It is generated and set according to the operation result by the PU.

The signal converted by the LUT 25 is converted into a signal corresponding to the dot width by a pulse width modulation circuit 26 and sent to a laser driver 27.

In this embodiment, for all colors, a tone reproduction means by pulse width modulation processing in which pixels are arranged in the sub-scanning direction is used.

Then, the photosensitive drum 4 is scanned by laser scanning.
A latent image having a gradation characteristic due to a change in dot area is formed on the top, and a gradation image is obtained through a process of development, transfer, and fixing.

The above-described image forming apparatus includes the photosensitive drum 4
A test pattern generator 29 to be output above is built in.

When the original image is composed of halftone dots and the tone reproduction method is the same for all colors, the image signal may cause interference and cause an image defect called moiré. .

As a countermeasure, a reproduction method (for example, a cycle or a phase of pulse width modulation) by a gradation reproduction means for each color.
The moire can be made inconspicuous by changing.

Further, if the mechanical vibrations during the sequential development of the four colors are different, a color shift occurs. This method has the effect of making the color shift less noticeable.

In this embodiment, by shifting the pulse appearance synchronization of the tone reproduction means by pulse width conversion, the arrangement angle of the constituent pixels (screen angle) is set to 0 ° for yellow, as shown in FIG. 45 ° for magenta, 0 for cyan
° and black were set at 26.5 °.

FIG. 16 is a block diagram showing the configuration of the pulse width modulation circuit of this embodiment.

The multicolor image signal A (for example, in this embodiment, a digital signal sent in a plane-sequential manner for each of the colors M, C, Y, and Bk) is converted into a D / A converter 2 for each color component.
01 is converted to an analog signal a. On the other hand, a reference signal b (a triangular wave in the embodiment) determined for each color component is generated under the control of the CPU 203 using a clock frequency-divided from the high-speed clock by the pulse pattern generator 202. Further, the reference signal b is input to the delay line 206 and is delayed. Day Ray Line 2
06 has eight types of delay patterns in the present embodiment, and one of them is selected by the selector 207.
It is.

Eight types of delay patterns are prepared in which a 1/4 pixel delay is set as one unit of the delay amount.

In this embodiment, a unit of pulse width modulation (PWM) as a reference is one unit of two pixels, and a delay of 1/4 pixel is a unit of delay amount. The eight types of delay patterns are patterns based on a PWM signal (not shown) delayed from 0/4 to 7/4 pixels.

The signals of Y, M, C, and Bk are previously stored in the above-mentioned 8
One of the types of delay patterns is associated with each other, and the selector 207 selects a delay pattern corresponding to each line of Y, M, C, and Bk. This selector 207 is connected to the CPU 203
Thus, the delay pattern information is read from the phase change pattern generator 204 for each line. In this embodiment, the pattern generator 204 is a RAM,
It contains delay pattern information to be selected for each line for each color. The counter 205 counts the horizontal synchronizing signal, and repeats 1 to 8 to count every line. This counter 20
5 sequentially performs the counting operation in accordance with the delay pattern written in the pattern generator 204 every time the counting operation is performed.
Selects a delay pattern and outputs it as a reference signal C. The unit of the triangular wave is one unit for two pixels (that is, the period of the triangular wave corresponds to two pixels), and the delay amount is ４.
One unit is a pixel. The analog signal a is compared with the reference signal c by the comparator 208,
The signal d is amplified by the amplifier 209 and output as a processed signal B.

The above operation is performed for each color component, and the M component is not delayed for odd lines, but is delayed for 4/4 pixels for even lines, so that an image is formed at a screen angle of 45 ° C, Y The components are imaged with no delay for all lines, ie at a screen angle of 0 °, Bk
The components have no delay for the (i + 1) th line (i = 0, 1, 2,...), A 2/4 pixel delay for the (i + 2) th line, and a 6/4 pixel delay for the i + 4th line. Image formation occurs at 5 °. In this way, each color is output as a multiplex image by the above-described full-color copying machine.

By setting the screen angles for forming the first and fourth images to be different from the screen angles of the second and third colors, color unevenness and the like can be eliminated.

On the other hand, the present inventor has found that, by adopting the pixel pattern as described above, the reflection characteristic of the toner on the photosensitive drum varies depending on the angle.

FIG. 12 shows a conversion table for converting a magenta reflected light amount signal into an optical density.

The solid line is a conversion table when the angle of arrangement of the constituent pixels is set to 45 °, and the broken line is a conversion table when the angle of the constituent pixels is set to 0 °.

Therefore, a conversion table for converting the reflected light amount signal into the optical density is prepared for each color according to the screen angle, so that even in such a case, the amount of toner on the photosensitive member can be detected with high accuracy. it can.

FIG. 13 shows an example of the black conversion table.

The solid line indicates the angle of arrangement of the constituent pixels is 26.5 °.
Is a conversion table when the angle of the constituent pixels is set to 0 °.

In this embodiment, the density conversion accuracy of all colors is improved by changing the shape of the conversion table in accordance with the angle of arrangement of the constituent pixels for each color. Improved accuracy can be achieved.

FIG. 4 shows a processing circuit for processing a signal from the sensor 9. The near-infrared light incident on the sensor 9 is converted into an electric signal by the sensor 9, and the electric signal is converted from an output voltage of 0 to 5 V into a digital signal of 0 to 255 level by the A / D conversion circuit 41. And a density conversion circuit 4
2 is converted to density.

The toners used in this embodiment are yellow, magenta, and cyan toners, each of which is formed by dispersing a color material of each color using a styrene copolymer resin as a binder.

The spectral characteristics of the yellow, magenta, and cyan toners are in this order as shown in FIGS.
(0 nm) of 80% or more. Further, in forming these color toner images, a two-component developing method which is advantageous in color purity and transparency is adopted.

On the other hand, in the present embodiment, a one-component magnetic toner, which has a proven track record in running cost reduction, is used as a black toner for monochrome copying. As shown in FIG. 8, reflection of near-infrared light (960 nm) is performed. The rate is about 10%. Black employs a one-component jumping development system, but may employ a two-component black toner.

The photosensitive drum 4 is an OPC drum, has a reflectance of near infrared light (960 nm) of about 40%, and may be an amorphous silicon drum or the like.

FIG. 9 shows the relationship between the output image density and the output of the sensor 9 when the density on the photosensitive drum 4 is changed stepwise according to the area gradation of each color. When the toner is not attached to the photosensitive drum 4, the output of the sensor 9 is 2.5
V, that is, 128 levels.

As can be seen from FIG. 9, the output of the sensor 9 for the yellow magenta and cyan color toners becomes larger than that of the photosensitive drum 4 alone as the area coverage increases.
On the other hand, as the area coverage of the black toner increases, the output of the sensor 9 becomes smaller than that of the photosensitive drum 4 alone.

Based on the difference in the reflection characteristics of the toner for each color, the output signal of the sensor 9 for each color shown in FIG.
Providing a table dedicated to each color for converting to a density signal makes it possible to accurately read the density signal for each color.

Before the copy operation is performed, a specific gradation pattern (16 levels, 3 levels,
2 levels, 48 levels, 64 levels, 80 levels, 96
Level, 112 level, 128 level, 144 level,
FIG. 1 shows the output of a 160-level 10-tone pattern.
As shown in FIG. 3, the image is formed continuously in the circumferential direction of the drum, measured at appropriate timing by the LED 8 and the sensor 9, and the gradation characteristics, that is, the printer characteristics in the third quadrant of FIG. It can be obtained accurately without being transferred to and fixed on a sheet.

LUT for correcting this printer characteristic
25 can be easily obtained from the printer characteristics obtained from the above measurement. That is, the LUT 25 can be obtained by reversing the input / output relationship of the printer characteristics.

It is generally known that various characteristics of an electrophotographic copying machine change depending on the number of copies used. This is because the photoreceptor causes light fatigue, or the surface layer of the photoreceptor is scraped by a cleaning blade that is in contact with the photoreceptor, thereby changing the photosensitivity characteristics or changing the tribo-imparting ability of the developer. Things.

Therefore, by performing the above operation periodically before the copying operation and sequentially creating the LUT 25, the optimum gradation reproduction characteristic can always be obtained.

As described above, according to the embodiment of the present invention, the gradation reproduction method for the input image data is formed on the medium on which the reference image is formed when forming the feedback system by using the same gradation reproduction method. Therefore, accurate gradation control corresponding to the gradation reproduction method can be performed.

Further, it is possible to detect the density of the reference image formed on the medium in accordance with the gradation reproduction method with high accuracy.

[Embodiment 3] In this embodiment, a dither method and a pulse width conversion method are combined as a tone reproduction method.

More specifically, Yellow is disclosed in Japanese Unexamined Patent Publication No.
By the method indicated by reference numeral 742, the gradation is reproduced by selectively switching the two dither matrix patterns shown in FIG. For magenta, the arrangement of the constituent pixels is 0 ° by the pulse width conversion, and for cyan, the arrangement of the constituent pixels is 45 ° by the pulse width conversion.
In ゜, black changes the arrangement of the constituent pixels to 2 by pulse width conversion.
A full color image was constructed at 2.5 °.

Although the above-described dither matrix is used in this embodiment, another type of dither matrix may be used, or a halftone processing method such as an error diffusion method may be used.

As described above, by employing different tone reproduction means for each color, it is possible to prevent the reproduced pixels of each color from interfering with each other and to suppress the moire phenomenon and the color shift.

FIG. 15 shows a conversion table for converting the amount of light reflected from the toner on the photosensitive drum of each color into an optical density.

In this embodiment, the density conversion accuracy of all colors is improved by changing the shape of the conversion table in accordance with the gradation reproducing means and the angle of arrangement of the constituent pixels for each color, and furthermore, the value is reduced. The accuracy of the used gradation control can be improved.

[0062]

According to the present invention, a pattern forming means for forming a predetermined pattern on a medium by using a tone reproduction method in which a plurality of color components are formed and corresponding to the color components, wherein the formed pattern is different, and the pattern forming means. An image processing apparatus comprising: a density detecting unit configured to detect the density of a pattern formed by the method; and a gradation control unit configured to perform gradation control in accordance with the density detected by the density detecting unit. And a light-receiving element, measuring reflected light of a pattern formed on the medium by the pattern forming means, and converting the obtained measurement amount into an optical density, the conversion being set for each of the color components. Are switched according to the tone reproduction method corresponding to the color component, so that when the reflection characteristics of the formed pattern are different, the reflected light can be accurately reflected. Can be converted from the signal into a density signal, it is possible to improve the accuracy of gradation control.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of the present invention.

FIG. 2 is a block diagram showing a processing circuit for processing an electric signal from a CCD.

FIG. 3 is a 4-limit chart showing gradation reproduction characteristics;

FIG. 4 is a block diagram showing a processing circuit for processing an electric signal from a sensor.

FIG. 5 is a diagram illustrating an example of spectral characteristics of a yellow toner.

FIG. 6 is a diagram illustrating an example of magenta toner spectral characteristics.

FIG. 7 is a diagram illustrating an example of cyan toner spectral characteristics.

FIG. 8 is a diagram illustrating an example of spectral characteristics of black (one-component magnetic) toner.

FIG. 9 is a diagram showing an example of a density signal level versus a sensor output.

FIG. 10 is a diagram showing a table for converting a reflected light amount signal of each color into a density signal.

FIG. 11 is a diagram showing angles of arrangement of constituent pixels.

FIG. 12 is a diagram showing a table for converting a magenta reflected light amount signal into a density signal.

FIG. 13 is a diagram showing a table for converting a black reflected light amount signal into a density signal.

FIG. 14 is a diagram showing a dither matrix pattern.

FIG. 15 is a diagram showing a table for converting a reflected light amount signal of each color into a density signal.

FIG. 16 illustrates a pulse width modulation circuit.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-64-41375 (JP, A) JP-A-1-102584 (JP, A) JP-A-1-276166 (JP, A) JP-A-2- 155771 (JP, A) JP-A-1-204742 (JP, A) JP-A-62-287774 (JP, A) JP-A-1-184136 (JP, A) JP-A-1-206368 (JP, A) JP-A-3-221764 (JP, A) JP-A-63-65461 (JP, A) JP-A-63-307954 (JP, A) JP-A-58-182372 (JP, A) (58) (Int.Cl. ⁷ , DB name) G03G 15/00 G03G 21/00 B41J 2/435 H04N 1/40 H04N 1/46

Claims (3)

(57) [Claims] 1. A pattern forming means for forming a predetermined pattern on a medium by using a tone reproduction method in which a plurality of color components are formed and corresponding to the color components and formed differently, and the pattern forming means An image processing apparatus comprising: a density detecting unit configured to detect a density of a pattern formed by the method; and a gradation control unit configured to perform gradation control according to the density detected by the density detecting unit. And a light-receiving element, measuring reflected light of a pattern formed on the medium by the pattern forming means, and converting the obtained measurement amount into an optical density, the conversion being set for each of the color components. The image processing apparatus according to claim 1, wherein the arithmetic processing is switched in accordance with a tone reproduction method corresponding to the color component. 2. An image processing apparatus according to claim 1, wherein said gradation control means controls gradation expression by dot forming means such as pulse width modulation and dither conversion. 3. The image processing apparatus according to claim 1, wherein the tone reproduction method is a screen angle forming process in pulse width modulation.