JP2007158454A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an image forming apparatus for forming a color image using toner of only three colors of cyan (C), magenta (M) and black (K) in which identification information can be added. <P>SOLUTION: When a color image is formed, an image making unit of black (K) color system reproduces pixels 81-85 at the upper left corner of partial regions 71-75 representing apparatus series number "14402" as identification information in the block region 70 of 40 longitudinal pixels and 20 lateral pixels with black color. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms a color image.

In the field of color image forming apparatuses, copying machines and printers that form color images using toners of at least four colors of C (cyan), M (magenta), Y (yellow), and K (black) have been developed. It is actively done.
In such a copying machine, for example, image forming units for each color of C, M, Y, and K are arranged along the transfer belt, and each color toner image formed by each image forming unit is placed on the intermediate transfer belt. There is a so-called tandem type in which the toner images are sequentially superimposed and transferred, and the transferred toner images are collectively transferred onto a recording sheet. In the tandem system, color images can be formed with a single sheet passing, and therefore, the copying process can be speeded up.

By the way, in order to cope with counterfeiting of banknotes or the like, for example, a copying machine that forms a color image has identification information such as a device-specific number for tracking which device the color image is formed at the time of image formation. In some cases, a function of adding toner on a recording sheet using Y-color toner is mounted. The reason why the Y color toner is used is that it is difficult for the Y color to be recognized by human eyes compared to other colors, and it is difficult to perceive deterioration in image quality due to the addition of identification information.
Japanese Patent No. 2614369

In general, a copying machine or the like is often installed in a limited space such as in a company office, and it is desired to make it as small as possible. However, in the configuration in which the four color image forming units are arranged as in the above tandem method, for example, the size can be reduced only to the extent that a gap between adjacent image forming units is reduced.
Accordingly, the present inventor has created an image forming apparatus that forms a color image using only C, M, and K color toners in order to achieve a significant reduction in size. This image forming apparatus is not equipped with an image forming unit for Y color, so that the apparatus can be reduced in size and the cost can be reduced.

However, since the created image forming apparatus is not equipped with a Y-color image forming unit, there is a problem in that an image indicating Y-color identification information cannot be added as in the conventional case.
Further, regarding the problem that Y-color identification information cannot be added, Y-color toner is consumed in an image forming apparatus that forms a color image using the conventional toners for C, M, Y, and K colors. The same can occur when the image forming unit does not remain.

  The present invention has been made in view of the above problems, and provides an image forming apparatus capable of adding an image indicating identification information regardless of Y color in an image forming apparatus for forming a color image. The purpose is that.

  In order to achieve the above object, an image forming apparatus according to the present invention forms a color image by superimposing and transferring magenta, cyan and black toner images on a sheet, and at the time of forming a color image. A specific pattern image is formed on the sheet using a black toner different from the color image.

  As a result, when forming a color image with cyan, magenta and black toner images, it is possible to form a certain pattern image using black toner as identification information. If the pattern image is formed in black, for example, even if the pattern image overlaps the cyan toner image, the shade of the cyan color itself remains substantially unchanged and the brightness changes only in units of pixels. It is possible to prevent the image quality from being deteriorated due to a change in hue when the image is formed in magenta.

The pattern image includes pixels reproduced with black toner and pixels reproduced with white, and an interval between two pixels reproduced with the black toner is equal to or more than one pixel. And
This makes it difficult for the pattern image to be seen by the human eye and prevents image quality deterioration due to the formation of the pattern image.

Furthermore, the interval is in the range of not less than 3 pixels and not more than 95 pixels.
In this way, each dot of the reproduced black pixel is difficult to be seen by human eyes, and when the formed pattern image is detected by a detection means such as a photoelectric sensor, the interval between the black pixels is set. It can be prevented that a part of the pattern image protrudes from the detection region due to being too far away and cannot be detected.

The pattern image includes pixels reproduced with black toner and pixels reproduced with white toner, and the concealment rate indicating the ratio of pixels reproduced with black toner per unit area is 0. .01 [%] to 10 [%].
This makes it difficult for the pattern image to be seen by the human eye and prevents image quality deterioration due to the formation of the pattern image.

In addition, a detection unit that detects the formed pattern image and outputs a value for indexing the concealment rate is provided. From the output data from the detection unit, it is determined whether the concealment rate is within a predetermined range. When it is determined that it is not within the predetermined range, the pattern image forming conditions are changed so that the concealment rate falls within the predetermined range.
In this way, even when the concealment rate is out of the predetermined range, it is possible to make adjustments so that the concealment rate falls within the predetermined range, and further prevention of image quality degradation can be achieved.

Further, the pattern image is formed at a predetermined position on the sheet regardless of the position where the color image is formed on the sheet.
In this way, the pattern image forming position can be determined in advance, and the control for forming the pattern image can be simplified.
Further, the present invention is characterized in that the black color toner image and the pattern image are formed on the sheet before the other color toner images.

In this way, the color reproducibility can be further improved.
Further, the magenta color toner has a hue with a yellow chromaticity point on the chromaticity diagram, compared to a full-color magenta toner used for forming a color image with four colors of magenta, cyan, yellow, and black. Than the cyan toner for full color used in the case of forming a color image with four colors of magenta, cyan, yellow, and black. The toner is characterized in that the chromaticity point on the chromaticity diagram is a toner shifted in a direction approaching a yellow hue.

In this way, the reproducibility of the yellow color of the image formed on the sheet can be further improved.
In addition, magenta, cyan, and black color developing units other than yellow are provided, and the magenta color toner image is developed by the magenta color developing unit. The black toner image and the pattern image are developed by a cyan developing unit, and are developed by a black developing unit.

In this way, it is only necessary to provide three developing units, and the size can be greatly reduced as compared with an apparatus including a yellow color.
The color image is configured to be capable of forming an image including a yellow color toner image, and further includes a determination unit that determines whether or not the yellow color toner image can be formed. Only when it is determined that the toner image cannot be formed, the pattern image is formed.

  In this way, for example, even when the yellow toner is not consumed, it is possible to form a color image with the other three colors and to add identification information.

Hereinafter, an example in which the embodiment of the image forming apparatus according to the present invention is applied to a tandem color digital printer (hereinafter simply referred to as “printer”) will be described.
(First embodiment)
FIG. 1 is a diagram illustrating the overall configuration of the printer 1.
As shown in the figure, the printer 1 includes an image forming unit 2M, 2C, 2K corresponding to each of magenta (M), cyan (C), and black (K), and an intermediate transfer that rotates in the direction of arrow A. An intermediate transfer unit 10 including a belt 11, a feeding unit 30, a fixing unit 40, and a control unit 100 are provided, connected to a network, here a LAN, from an external terminal device (not shown). When a print instruction is received, image formation is executed based on the instruction. Hereinafter, the reproduction colors of magenta, cyan, and black are represented as M, C, and K, and the M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.

  The image forming units 2M to 2K face the intermediate transfer belt 11 at a predetermined interval from the upstream side in the rotational direction (hereinafter simply referred to as “upstream side”) to the downstream side in the rotational direction (hereinafter simply referred to as “downstream side”). Are arranged in series, and are photosensitive drums 3M, 3C, and 3K as image carriers, chargers 4M, 4C, and 4K disposed around the photosensitive drums, exposure units 5M, 5C, and 5K, and M colors. A developing device 6M containing a developer containing a system toner, a developing device 6C containing a developer containing a C color toner, a developing device 6K containing a developer containing a K color toner, Primary transfer rollers 7M, 7C, and 7K facing the photosensitive drums 3M to 3K across the intermediate transfer belt 11 and cleaners 8M, 8C, and 8K are provided.

  The intermediate transfer unit 10 includes a drive roller 12, a driven roller 13, a tension roller 14, an intermediate transfer belt 11 stretched around these rollers, a drive motor (not shown) that drives the drive roller 12 to rotate, A cleaner 15 for cleaning the residual toner on the transfer belt 11 and a photoelectric sensor 16 for detecting the density of a pattern image (described later) with K-color (black) toner as identification information formed on the intermediate transfer belt 11. Etc.

The feeding unit 30 includes a paper feed cassette 31 that stores paper S as a sheet, a feed roller 32 that feeds the paper S in the paper feed cassette 31 one by one, and a transport roller pair 33 that transports the fed paper S. And a timing roller pair 34 for taking the timing of feeding the sheet S to the secondary transfer position 21, a secondary transfer roller 35, and the like.
Upon receiving a print instruction from an external terminal device, the control unit 100 receives the transmitted image signal, converts it into a digital image signal for the M, C, K color system, and exposes 5M to 5K. A drive signal for driving is generated. The exposure unit 5M receives a drive signal from the control unit 100, emits a laser beam for forming an image for the M color system, and performs exposure scanning on the photosensitive drum 3M in the main scanning direction. Similarly, the exposure units 5C and 5K emit laser beams for image formation for cyan and black colors, and expose and scan the photosensitive drums 3C and 3K.

The photosensitive drums 3M to 3K are cleaned by the cleaners 8M to 8K before the exposure, and the surface of the photosensitive drums 3M to 3K is uniformly discharged by the chargers 4M to 4K. When being exposed to the laser beam in the state of being charged uniformly in this way, electrostatic latent images are formed on the surfaces of the photosensitive drums 3M to 3K.
The electrostatic latent images are developed by the developing devices 6M to 6K for the respective colors, whereby toner images as M, C, and K color developer images are formed on the surfaces of the photosensitive drums 3M to 3K. Transfer is sequentially performed on the rotating intermediate transfer belt 11 by the electrostatic force generated by the voltage applied to the primary transfer rollers 7M, 7C, and 7K disposed on the back side of the intermediate transfer belt 11 at the primary transfer positions 9M, 9C, and 9K. It will be done.

At this time, the image forming operation for each color is executed while shifting the timing from the upstream side toward the downstream side so that the toner image is primarily transferred to the same position on the intermediate transfer belt 11.
The color toner images superimposed on the intermediate transfer belt 11 are moved to the secondary transfer position 21 by the rotation of the intermediate transfer belt 11.

  On the other hand, the sheet S is fed from the feeding unit 30 via the timing roller pair 34 in accordance with the movement timing of the intermediate transfer belt 11, and the sheet S is connected to the rotating intermediate transfer belt 11. The toner image on the intermediate transfer belt 11 is transferred onto the sheet S by the electrostatic force generated by the voltage applied to the secondary transfer roller 35 at the secondary transfer position 21 while being sandwiched between the secondary transfer rollers 35 and conveyed. Secondary transferred.

The sheet S that has passed through the secondary transfer position 21 is conveyed to the fixing unit 40, where the toner image is heated and pressed to be fixed on the sheet S, and then onto the discharge tray 41 via the discharge roller pair 36. Discharged.
As a result, a color image is formed by toner images of three colors of M, C, and K. In the case of monochrome, for example, K color image formation, only the image forming unit 2K is used, and the above-described color image is formed. A K color image is formed on the sheet through a process such as transfer.

In this embodiment, the yellow (Y) color is not used, and therefore the chromaticity points of the M and C color toners are changed from those of the conventional configuration using the Y color.
FIG. 2 is a chromaticity diagram showing chromaticity points of M and C color toners in the L ^* a ^* b ^* color system. Here, for the M and C colors, the white circle plots indicate the chromaticity points of the toner of the conventional configuration (a configuration in which a color image is formed using the Y, M, and C colors), and the square marks are the present implementations. The chromaticity point of the toner in the configuration of the form (configuration of forming a color image without using Y color) is shown.

As shown in the figure, for example, regarding the M color toner, the toner of this embodiment has a chromaticity point at a position shifted in the red (R) direction as compared with the conventional toner, and the C color toner. Looking at the toner, it can be seen that the toner of this embodiment has a chromaticity point at a position shifted in the green (G) direction as compared with the conventional toner.
Here, the red direction means the hue direction of the R (red) color resulting from the mixture of the M color and the Y color. Having a chromaticity point at a position shifted in the red direction means that the hue of the M color is the color of the R color. It means getting close to hue.

Similarly, the green direction means the hue direction of G (green) color by the mixed color of C color and Y color, and having a chromaticity point at a position shifted in the green direction means that the hue of C color is changed to that of G color. It means getting close to hue.
In this way, as the M and C color toners, toners whose chromaticity points are shifted in the direction of R and G colors as compared with the conventional configuration are used. Therefore, R color (R = M + Y) and G color (G = C + Y) color reproducibility can be ensured to some extent even without Y-color toner, and reproducibility of frequently used colors such as R color and G color can be improved.

The shift amount and the shift direction of the chromaticity points of the M and C color toners can be obtained in advance by experiments or the like, and are not limited to those described above. Color reproduction can be achieved by using M, C color toners with chromaticity points shifted in a direction having a directional component approaching the hue of Y color on the chromaticity diagram with respect to the chromaticity points of the toner in the conventional configuration. This is because the performance can be improved.
For example, when Japan Color is used as a reference, the chromaticity point of the M-color toner is set to R color (a ^* : 68.5, b ^* : 48.0) under the observation condition of the 2-degree visual field D50. Alternatively, the chromaticity point of the C-color toner may be set around or very close to the G color (a ^* : −73.5, b ^* : 25.0).

Furthermore, even when another color standard such as SWOP (Specifications for Web Offset Publications) or Euro Color is used, the chromaticity point of the M and C color toners approaches the hue of R, G color or Y color. By doing so, the same effect can be obtained.
In addition, as a manufacturing method of the above-described toner, a method in which a coloring material having the above chromaticity point is contained in a binder resin or the like in a conventional toner manufacturing method is used.

FIG. 3 is a diagram illustrating a configuration of the control unit 100.
As shown in the figure, the control unit 100 includes, as main components, a CPU 101, a communication interface (I / F) unit 102, an image processing unit 103, an identification information adding unit 104, an image memory 105, An LD drive unit 106, a ROM 107, a RAM 108, an identification information storage unit 109, and an exposure amount storage unit 110 are provided so that data can be exchanged via a bus 111.

The communication I / F unit 102 is an interface for connecting to a network such as a LAN such as a LAN card or a LAN board.
The image processing unit 103 performs a known correction process such as shading on the image signal received through the communication I / F unit 102, for example, red (R), green (G), and blue (B) data. Thereafter, each pixel is converted into a digital image signal (image data) for reproduction colors of M, C, and K. This conversion is performed by the MCK color conversion unit 1031 using an image processing method such as UCR (under color removal), BP (black generation), or γ correction.

Specifically, conversion formulas and conversion tables for converting R, G, B color image signals used in the conventional configuration into Y, M, C, K color image data are represented by C, This can be realized by changing to the case where only M and K color toners are used.
More specifically, for example, the coefficient relating to Y color data in the conversion formula or the like is set to zero, and the color reproducibility is optimized in consideration of the chromaticity points of M and C color toners. In addition, the values of the coefficients related to the data for the M, C, and K color systems are obtained from experiments and the like.

Note that the present invention is not limited to the method of changing the coefficient value, and various methods such as a method of newly obtaining a conversion formula so as to suit a configuration for forming a color image using only M and C color toners can be employed. . The obtained data such as a conversion formula is stored in the ROM 107 in advance, is read out during the image processing, and is used for conversion processing to image data.
The image data for M, C, and K colors is stored in the image memory 105 for each pixel. The image data for the K color is added to the image memory 105 after the identification information is added by the identification information adding unit 104. Stored in This identification information adding process will be described later.

Under the control of the CPU 101, the LD driving unit 106 reads M, C, and K color image data for each scanning line from the image memory 105, and drives the exposure units 5M to 5K.
The identification information storage unit 109 is composed of a nonvolatile memory, and stores data indicating a manufacturing number as one piece of identification information. Here, it is assumed that data indicating a numerical value “14402”, for example, is stored as the manufacturing number. Since the manufacturing number is for identifying the device, a different number is assigned to each device at the time of manufacturing and is stored in the identification information storage unit 108.

  The exposure amount storage unit 110 is composed of a nonvolatile memory, and stores intensity data indicating the intensity of the laser beam during exposure by the exposure unit 5K. The LD driving unit 106 reads the intensity data stored in the exposure amount storage unit 110 and instructs the exposure unit 5K to emit the laser beam with the intensity data. The value of the intensity data is updated by an exposure amount control process described later.

The ROM 107 stores programs related to the image forming units 2M to 2K, the image forming operation in the intermediate transfer unit 10 and the sheet S feeding operation by the feeding unit 30, the identification information addition process, the program related to the exposure amount control process, and the like. .
The RAM 108 serves as a work area when the CPU 101 executes a program.
The CPU 101 reads out a necessary program from the ROM 107 and controls the image formation, feeding operation, etc. in a unified manner while taking a timing to execute a smooth printing operation. Further, an exposure amount control process is executed.

Further, the identification information adding unit 104 is instructed to execute an identification information adding process, and a manufacturing number, an output date and time, and secret information as identification information are added to the K-color image data.
Here, the output date and time refers to the date and time when the print is output, and the secret information may be arbitrary information such as a device name. The date and time is measured by a timer (not shown) in the control unit 100. The secret information may be stored in the ROM 107 in advance, or may be configured to be set and input by a user or the like from an operation panel or the like (not shown).

FIG. 4 is a schematic diagram showing what information is added as identification information to which position on the printed paper. In the figure, only the pattern image as identification information is shown, and the original formed image is omitted.
As shown in the figure, a pattern image 61 indicating the production number is displayed in the rectangular area 51, a pattern image 62 indicating the output date and time is displayed in the area 52, and a pattern image 63 indicating the secret information is displayed in the area 53. A plurality of each is formed at a predetermined interval in the main scanning and sub-scanning directions.

FIG. 5 is an enlarged view of one pattern image 61.
In the figure, one rectangle corresponds to one pixel, and a block area 70 composed of 20 horizontal pixels × 40 vertical pixels (a total of 800 pixels) is shown, and five pixels 81 to 85 shown in black are The pattern image 61 is composed of 795 pixels shown in white. Hereinafter, black pixels are referred to as black pixels.

The horizontal axis (X axis: corresponding to the main scanning direction) is used to indicate the number of digits (1 to 5) of the serial number, and the vertical axis (Y axis: corresponding to the sub scanning direction) is used to indicate the numerical values (0 to 9). .
The number of digits and the numerical value are specified in units of the partial area, with 16 pixel groups each consisting of 4 horizontal pixels × 4 vertical pixels as one partial area (area distinguished by a thick line, 50 in total).
Specifically, since the black pixel 81 belongs to the partial area 71 indicating the numerical value “1” of the “1” digit indicated by the horizontal axis and the vertical axis, the first digit of the serial number is set to “1”. Become. Similarly, since the black pixels 82 and 83 belong to the partial areas 72 and 73 indicating the numerical value “4” in the “2” and “3” digits, the second and third digits are “4”. Since the black pixels 84 and 85 belong to the partial areas 74 and 75 indicating the numerical values “0” and “2” in the “4” and “5” digits, the “0” and “2” are in the fourth and fifth digits. "become. From this, it can be seen that the pattern image 61 indicates the production number “14402”.

The reason why the pixels located at the upper left corner of each partial area are used as the black pixels 81 to 85 representing the number of digits and the numerical value is to allow the black pixels to be spaced apart by three pixels in the main scanning direction.
That is, if the black pixels are adjacent or the interval is as narrow as about one pixel, for example, when the reproducibility deteriorates due to the sensitivity variation of the photosensitive drum, the two black pixels are reproduced as if they were stuck together. This is because it may be difficult to recognize the number. The minimum value of the pixel interval is the same in the sub-scanning direction. However, if there is an interval corresponding to three pixels in the main scanning direction, the interval in the sub-scanning direction may be narrower than three pixels. It is desirable to leave an interval corresponding to three pixels in any direction, but when the degree of reproducibility is small, for example, a configuration may be adopted in which black pixels are adjacent to each other or only one pixel is left. .

The maximum value of the distance between the black pixel 81 and the black pixel 85 corresponding to both ends is desirably up to 95 pixels.
This is related to the pixel resolution and the detection area of the photoelectric sensor 16. That is, in the exposure amount control process, a pattern image 61 is formed on the intermediate transfer belt 11 as described later, and the formed pattern image 61 is detected by the photoelectric sensor 16. This is because if the interval is excessively widened, the black pixel at one end may protrude from the detection area and be undetectable.

Specifically, for example, if the width of the detection region on the intermediate transfer belt 11 by the photoelectric sensor 16 in the main scanning direction is 4 [mm] and the pixel resolution [dpi: dot per inch] is 600 [dpi], 4 [ mm] is about 95 pixels. The interval between the black pixels at both ends is determined according to the detection area of the photoelectric sensor 16 and the resolution of the apparatus.
Further, it is desirable to set the concealment rate P indicating the ratio of black pixels per unit area to 10% or less. Here, the concealment rate P can be expressed by the following (Formula 1).

P = (number of black pixels) × 100 / (number of pixels in a rectangular area surrounding the partial area to which the black pixel belongs) (Expression 1)
In the case of the example of FIG. 5, the number of black pixels is “5”, the partial area to which the black pixel belongs is “partial areas 71 to 75”, and the rectangular area surrounding the partial areas 71 to 75 is “digit number × 5 numerical values”. The number of pixels in the rectangular area ”is“ 400 (= 25 × 16) ”, so P = (5 × 100) / (400) = 1.26 [ %〕become. In order to simplify the process, for example, the denominator value may be fixed, specifically “800” or the like.

The concealment rate P can be said to indicate the ratio of the black area per unit area of the white portion on the paper (sheet) from the above formula (1). This means that the area of the portion increases, and the pattern 61 is easily felt by human eyes.
By the way, in the field of image forming apparatuses, toner particles adhering to the background (original non-image forming area) of paper is called “covering”, and the density is expressed as “fogging density”. .

The fog density D can be usually obtained by measuring the reflectance R of a portion where fog occurs using an optical reflection densitometer. The reflectance R is a common logarithmic value and is expressed by the following (Equation 2).
D = log ₁₀ (1 / R) (Formula 2)
Empirically, the fog density D increases as the value increases, and the density increases due to adhered toner, and the human eye begins to see an image when the value of D exceeds 0.2, that is, it becomes easier to feel. I know.

FIG. 6 is a diagram showing the relationship between the fog density D and the concealment rate P. As shown in FIG.
As shown in the graph of the figure, the value of the concealment rate P corresponding to the fog density D of 0.2 is 10 [%], so that the value of P is 10 [%] or less. It can be seen that if the image 61 is formed, it is difficult to be felt by human eyes. In the example of FIG. 5, it is 1.25 [%] as described above, and is in a range where it is difficult to feel.

  Note that the minimum value of the concealment rate P is desirably 0.01 [%] or more. This is due to the following reason. That is, according to the graph of FIG. 6, it can be seen that the smaller the concealment rate P, the smaller the fog density D becomes, and the more difficult it is to be felt by human eyes. However, decreasing the concealment rate P means that the interval between the black pixels is widened. Therefore, if the concealment rate P is decreased, the length from the black pixel 81 at one end to the black pixel 85 at the other end is increased accordingly. It will be long. This is because the detection area by the photoelectric sensor 16 is usually about several millimeters, and if the pixel interval becomes too long, the detection area may protrude and pixels that cannot be detected may appear. It has been confirmed from experiments and the like that such a lack of pixel detection does not occur if it is set to 0.01 [%] or more.

  In the above description, the pattern image 61 has been described. Similarly, for the pattern images 62 and 63, a pattern determined to indicate the output date and time by arranging a plurality of black pixels is added to the rectangular areas 52 and 53. The The formation position of each of the pattern images 61 to 63 within one page of the paper is determined by the coordinate position of the XY coordinate system, and the data is stored in the ROM 107 in advance.

FIG. 7 is a flowchart showing the contents of identification information addition processing. This process is executed by the identification information adding unit 104.
As shown in the figure, when image data for K color is received from the image processing unit 103 (step S11), the image data is temporarily stored in the RAM 108 (step S12).
The image data is data indicating any one of 0 to 255 when the gradation value is set for each pixel, for example, 256 gradations. Here, the gradation value becomes higher (closer to black) as the value becomes smaller.

And identification information is acquired (step S13). Specifically, for the manufacturing number, data stored in the identification information storage unit 109 is read. For the date and time, a value measured by the timer is acquired. As for the secret information, data stored in the ROM 107 as the information is read out.
Next, a pixel for representing the identification information is specified (step S14). This specific method will be described using an example in which the identification information is the production number “14402”.

(A) The coordinate position data indicating the formation position of the pattern image 61 stored in the ROM 107 is read.
(B) A block area 70 (FIG. 5) corresponding to the read coordinate position is specified for each pattern image 61 among all the pixels of the K color image data developed in the storage area of the RAM 108.

(C) Of the 50 partial areas included in the specified block area 70, which of the five partial areas corresponds to the numerical value “14402” from the first digit to the fifth digit of the read manufacturing number To determine whether or not In the example of FIG.
(D) The pixels located at the upper left corner of each of the five specified partial areas, that is, the pixels 81 to 85 in the example of FIG. 5, are specified as the pixels for representing the manufacturing number. Pixels can be specified in the same way for other output dates and times.

  Subsequently, the gradation value of the specified pixel is changed to a value indicating the maximum density value (step S15). For example, in the case of the serial number, the gradation values of the five specified pixels (black pixels 81 to 85) in the block area 70 are set to “0”. As a result, when the gradation value of all the pixels in the block area 70 of the K color image data originally represents “255”, that is, represents white, in the K color image formation at the time of print output, As the pattern image 61 (FIG. 4), only the five pixels 81 to 85 in the block area 70 turn black, and the remaining 795 pixels are reproduced as they are in white. When the original gradation value is “0”, it is set to “0” as it is.

For other output dates and times, the gradation value of the pixel is changed in the same manner. As a result, K-color image data including identification information is generated.
The K color image data including the identification information is stored in a predetermined storage area of the image memory 105 (step S16), and the identification information addition process is terminated.
As a result, when printing a color image, pattern images 61 to 63 showing identification information different from the input image are formed on the paper with the K-color toner (FIG. 4).

  As described above, since the pattern images 61 to 63 are formed at predetermined positions on the paper, for example, black pixels may overlap an image composed of only M colors, but the overlapping portion is as small as one pixel. Because it only needs a range, it is hardly noticeable (not noticed) by human eyes. Further, in the overlapping portion, M color + K color is obtained, and it is only necessary to reduce the brightness of the M color to some extent in units of pixels. If the configuration is such that a pattern image is formed with C-color toner, the color becomes M + C color. The M color does not change to a bluish color.

FIG. 8 is a flowchart showing the contents of the exposure control process.
This process is performed in order to keep the concealment rate P substantially constant. That is, it is desirable that the concealment rate P is constant at 1.25 [%], but in practice, the development characteristics and the like are improved due to the local sensitivity variation of the photosensitive drum and the toner charge amount distribution. It varies locally, and the diameter of one dot of a black pixel is often larger or smaller than the original one pixel. When the size of the black pixel varies, the concealment rate P is also affected. For example, when the concealment rate P increases, the pattern image 61 is easily perceived by human eyes. This is because it may be difficult to identify.

This process is performed at a predetermined timing, for example, every time the apparatus is turned on, every time a predetermined time elapses, every time the cumulative number of printing operations exceeds a predetermined number, or every time an error (such as a paper jam) is cleared. This is executed each time a member involved in image formation such as a photosensitive drum is replaced.
As shown in the figure, the intermediate transfer belt 11 is rotated and a predetermined number of pattern images 61 are formed as test patterns on the intermediate transfer belt 11 using the image forming unit 2K (step S21). At that time, as the exposure amount, the intensity data currently stored in the exposure amount storage unit 110 is read, and exposure with the K-color laser beam is executed with the intensity. The image forming units 2M and 2C are kept stopped.

  The formed pattern image 61 is detected by the photoelectric sensor 16 (step S22). Specifically, the distance t from the primary transfer position 9K on the intermediate transfer belt 11 to the detection position of the photoelectric sensor 16 is L, the transport speed of the intermediate transfer belt 11 is V, and the time t ( = L / V) is obtained in advance. Then, timing is started from when the pattern image 61 is primarily transferred onto the intermediate transfer belt 11, and when the elapsed time reaches t, a signal from the photoelectric sensor 16 is received.

Then, the concealment rate P is estimated from the received signal (step S23).
Specifically, first, data of a graph showing the correspondence between the output voltage V of the photoelectric sensor 16 and the concealment rate P shown in FIG. And the value of the concealment rate P corresponding to the output voltage value V of the photoelectric sensor 16 is calculated | required from the said graph, and the calculated | required value of P is made into an estimated value. Referring to the graph in the figure, when the output voltage of the photoelectric sensor 16 is V1, the concealment rate P corresponds to the target 5 [%], and when the output voltage is V2, P is the upper limit of 10 [%]. It turns out that it corresponds to.

The reason why the output voltage V and the concealment rate P are in an inversely proportional relationship is as follows.
That is, the photoelectric sensor 16 is of a reflective type including a light emitting element such as an LED and a light receiving element such as a photodiode that receives reflected light from the pattern image 61 of light emitted from the light emitting element. .
Here, the concealment rate P can be said to indicate the ratio of the black portion per unit area of the white portion. When the concealment rate P increases, the black portion increases and the light received by the light receiving element of the photoelectric sensor 16 is reflected. This is because when the light is decreased and the output voltage V is decreased and the concealment rate P is decreased, the black portion is decreased, the reflected light is increased and the output voltage V is increased.

  Note that the graph shown in FIG. 9 can be obtained in advance from experiments or the like, and the data is stored in the ROM 107. In this sense, the value of the output voltage V can be said to be a value for indicating the concealment rate. The value is not limited to the output voltage, and may be, for example, the value of the output current or the amount of reflected light as long as the concealment rate can be obtained from the relationship with the concealment rate. May be. Further, when the concealment rate is directly measured, the measured value can be used.

It is determined whether or not the value of the concealment rate P estimated in step S23 is within a predetermined range (step S24). Here, the predetermined range is a range of 0.01 [%] or more and 10 [%] or less.
If it is determined that the concealment rate P is within the predetermined range (“YES” in step S24), the process is terminated as it is.
On the other hand, when it is determined that the concealment rate P is not within the predetermined range, that is, P <0.01 or 10 <P (“NO” in step S24), processing for changing the exposure amount is performed (step S25). Here, the value of intensity data stored in the exposure amount storage unit 110 is changed.

  For example, in the case of P <0.01, it is considered that the diameter of one dot formed as a black pixel is smaller than one pixel due to sensitivity variation of the photosensitive drum 3K. A value obtained by adding a predetermined value to the value of the current intensity data is overwritten and saved so that the intensity of the laser beam to be used becomes larger, and the process returns to step S21. For example, when the charging characteristics of the photosensitive drum and the toner are negative (minus) and the developing method is a reversal developing method, the value of the intensity data is rewritten (updated) so that the intensity of the laser beam becomes larger. )

  As a result, when the pattern image 61 is formed for the second time, the intensity of the laser beam is higher than that for the first time. Since the laser light has a Gaussian type, when the beam intensity is increased, not only the original black pixel region on the photosensitive drum but also the potential of the peripheral portion thereof is attenuated to some extent. When the potential is attenuated, the toner easily moves from the developing unit to the photosensitive drum in the developing process, so that the black toner tends to adhere to the peripheral portion of one pixel, and the diameter of the black pixel becomes longer. Thus, the concealment rate P can be increased.

In the case of 10 <P, the reverse of P <0.01, that is, the value obtained by subtracting a predetermined value from the current intensity data value is overwritten and saved so that the intensity of the laser beam is lowered, and the process goes to step S21 Return. When the pattern image 61 is formed for the second time, the intensity of the laser beam is lower than that for the first time, the diameter of the black pixel can be reduced, and the concealment rate P can be lowered.
Until the concealment rate P falls within the predetermined range, the processes of steps S21 to S25 are repeatedly executed. The method for increasing or decreasing the diameter of the black pixel is not limited to changing the intensity of the laser beam as the pattern image formation condition. For example, the charging potential of the photosensitive drum or the developing bias applied to the developing device is used. A configuration in which the voltage or the like is variable may be employed.

If the value of P does not fall within the predetermined range even if the number of repetitions exceeds a predetermined number, for example, 10 times, the processing is stopped and a message or the like is displayed on a display means such as an operation panel (not shown). May be displayed.
By controlling in this way, it is possible to form a pattern image in an appropriate state even if the sensitivity variation of the photosensitive drum occurs.

(Second Embodiment)
In the above embodiment, the configuration example of the printer including the image forming units 2M to 2K for the three colors M, C, and K excluding the Y color has been described. However, in this embodiment, the configuration of four colors including Y is included. This point is different. The following description will focus on the parts different from the configuration in the first embodiment, and the description of the same parts will be omitted and the same reference numerals will be given.

FIG. 10 is a diagram illustrating the configuration of the printer 201 according to the present embodiment.
As shown in the figure, the printer 201 includes an image forming unit 2Y for Y color in addition to the image forming units 2M to 2K.
The image forming unit 2Y includes a photosensitive drum 3Y as an image carrier, a charger 4Y disposed around the photosensitive drum 3Y, an exposure unit 5Y, and a developer containing a developer containing Y-color toner. 6Y, a primary transfer roller 7Y, and a cleaner 8Y. The Y-, M-, C-, and K-color toners used here correspond to toners in a conventional color reproduction configuration (four-color configuration) using four colors (having the same chromaticity point as the four-color configuration). Equivalent to toner).

When forming a color image, the Y image forming unit 2Y is interlocked with the other image forming units 2M and the like, and Y, M, C, and K color toner images are primarily transferred onto the intermediate transfer belt 11. The toner images are collectively transferred (secondary transfer) onto the paper S at the secondary transfer position 21. This operation itself is the same as that of a conventional four-color printer.
The developing device 6Y is provided with a remaining toner detection sensor 210Y (see FIG. 11) for detecting the remaining amount of toner contained therein. Similarly, the developing devices 6M to 6K are provided with toner remaining amount detection sensors 210M to 210K. As the toner remaining amount detection sensors 210Y to 210K, known liquid level sensors, photoelectric sensors, or the like can be used. Here, the specific description is omitted.

Detection signals from the remaining toner detection sensors 210 </ b> Y to 210 </ b> K are sent to the control unit 202. The control unit 202 can detect whether or not toner remains in the developing devices 6Y to 6K based on detection signals from the remaining toner detection sensors 210Y to 210K.
FIG. 11 is a block diagram illustrating a configuration of the control unit 202.
As shown in the figure, the control unit 202 includes 203 as an image processing unit, 204 as an LD driving unit, and further includes an identification information adding unit 205.

The image processing unit 203 includes a YMCK conversion unit 211 and an MCK conversion unit 212.
The YMCK conversion unit 211 converts R, G, and B color data into image data for Y, M, C, and K reproduction colors. This conversion method is basically a conventional four-color configuration. The same method as that of the apparatus can be used.
On the other hand, the MCK conversion unit 212 corresponds to the MCK conversion unit 1031 in the first embodiment, but here, as the M, C color toner, the toner corresponding to the conventional four-color configuration is used. The conversion formulas, coefficients, and the like are determined in advance by experiments or the like so as to be appropriate when a color image is formed using the toner.

The identification information adding unit 205 adds identification information to the Y-color image data, and can basically use one having the same function as the conventional one. As a result, a pattern image is formed using a Y-color toner image.
FIG. 12 is a flowchart showing the contents of conversion processing in the image processing unit 203.
As shown in the figure, before forming a color image, it is determined whether or not a Y-color toner image can be formed, here, whether or not there is Y-color toner (step S31). This determination is made by referring to a detection signal from the toner remaining amount detection sensor 210Y. It is assumed that other color toners are accommodated in the developing devices.

When it is determined that a Y-color toner image cannot be formed, that is, no Y-color toner remains in the developing device 6Y (“NO” in step S31), the MCK conversion unit 212 is selected as the conversion processing unit to be used. (Step S32).
Then, the R, G, B color data as the input data is sent to the selected conversion unit, here the MCK conversion unit 212 (step S33). In the MCK conversion unit 212, the R, G, B color data is converted into M, C, K color image data (step S34), the converted data is output (step S35), and the process ends. .

As a result, the image data of M, C, and K colors are sent to the image memory 105, and the K color is sent to the image memory 105 via the identification information adding unit 104 (solid line in FIG. 11). As a result, a K-color pattern image is formed as identification information. This point is the same as in the first embodiment.
The M, C, and K color image data stored in the image memory 105 is read by the LD driving unit 204. The LD drive unit 204 generates drive signals for the exposure units 5M, 5C, and 5K based on the read image data, and drives the exposure units 5M, 5C, and 5K. Thereby, a color image of the three colors is formed.

On the other hand, if it is determined that Y toner remains (“YES” in step S31), the YMCK conversion unit 211 is selected as the conversion processing unit to be used (step S36), and the process proceeds to step S33.
As a result, the R, G, B color data is sent to the YMCK conversion unit 211 (step S33). In the YMCK conversion unit 211, the R, G, B color data is Y, M, C, K color image data. (Step S34) and then output (step S35).

In this case, the Y color image data is sent to the image memory 105 via the identification information adding unit 205, and the K color image data is sent to the image memory 105 without going through the identification information adding unit 104. Is taken (see the dashed line in FIG. 11). As a result, a Y-color pattern image is formed as identification information.
The LD drive unit 204 reads Y, M, C, and K color image data stored in the image memory 105, and generates drive signals for the exposure units 5Y, 5M, 5C, and 5K based on the read image data. The exposure units 5Y to 5K are driven. Thereby, a color image of four colors is formed.

As described above, in this embodiment, four color images including Y color can be formed. When there is no Y color toner, M, C, and K color toners are used. Since a color image is formed and an image pattern as identification information is formed in K color, there is no case where identification information cannot be added because there is no Y color toner as in the prior art.
This means that it is possible to form a color image without Y-color toner. For example, if a chart such as a graph can be expressed in color even if it cannot be accurately reproduced in a company office, etc. This is very convenient for users who think it is good.

In the above description, whether or not the Y color toner image can be formed is determined based on the presence or absence of the Y color toner, but is not limited thereto. For example, it may be determined whether the image forming unit 2Y cannot be operated due to a failure or the like, or the replacement time has been reached.
Further, for example, a configuration may be adopted in which the user is notified that toner other than Y color is used. Specifically, it is possible to take a method of displaying a message to that effect or outputting sound before image formation. In this case, it is more convenient for the user if the image formation is started when the image formation permission is received from the user.

  Note that the present invention is not limited to the image forming apparatus, and may be a method for forming the pattern image. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.

  Further, the program according to the present invention does not have to include all modules for causing the computer to execute the processing described above. For example, a separate information process such as a communication program or a program included in an operating system (OS) is performed. You may make it make a computer perform each process of this invention using the various general purpose programs which can be installed in an apparatus. Accordingly, it is not always necessary to record all the modules on the recording medium of the present invention, and it is not always necessary to transmit all the modules. Further, there are cases where predetermined processing can be executed using dedicated hardware.

(Modification)
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, the manufacturing number is represented by the formation positions of five black pixels. However, if a specific pattern image is formed with black toner, the number of pixels, the formation positions, etc. Of course, the way of expression is not limited to this. For example, a method of expressing characters or numerical values themselves with black pixels or forming them at arbitrary positions on the sheet may be used.

  (2) In the first embodiment, the M and C color toners in the Y color direction on the chromaticity diagram are compared with the conventional toner using the four colors M, C, Y, and K. Although the shifted one is used, it is not limited to this. For example, M and C color toners having the same chromaticity point as in the past may be used. In this case, the reproducibility of the Y color cannot be denied to some extent, but it is convenient for a user who thinks that a chart such as a graph can be expressed in color in a company office as described above.

Furthermore, depending on the adjustment of the chromaticity point of the toner, it is also conceivable to use a toner having a chromaticity point equivalent to the conventional one that is not shifted for either the M or C color. Specifically, a color image is formed by using a combination of a shifted M color toner and an unshifted C color toner.
(3) In the above embodiment, the photoelectric sensor 16 is disposed at a position facing the intermediate transfer belt 11 and the pattern image formed on the intermediate transfer belt 11 is detected. However, if the pattern image can be detected, the photoelectric sensor The position of 16 is not limited to that position. For example, the photoelectric sensor 16 may be disposed at a position close to the surface of the photosensitive drum 3K, and a pattern image formed on the photosensitive drum 3K may be detected.

(4) The image forming apparatus in the first embodiment is not limited to a printer, as long as it is an apparatus that forms a color image using only three image forming units for M, C, and K colors. It can be applied to a machine, an MFP (Multiple Function Peripheral), a facsimile machine, and the like.
Further, the tandem method is not limited to the above, and for example, a recording sheet is conveyed on a transfer belt, and M, M, and M formed on the photosensitive drums 3M to 3K are conveyed on the conveyed sheet. The present invention can also be applied to a configuration in which C and K color toner images are sequentially transferred.

  Further, the image forming means is not limited to the tandem system, and includes, for example, an image forming means for sequentially forming toner images of respective colors on the image carrier by switching three developing devices for M, C, and K colors. A so-called transfer drum system that forms a color image by multiple transfer of a toner image formed on the image carrier onto a sheet conveyed by a transfer material carrier such as a transfer drum, or the image carrier A so-called intermediate transfer member that forms a color image by transferring the toner image formed on the transfer member such as an intermediate transfer belt in a multiple transfer, and further transferring the multiple transferred toner images onto a sheet at once. It can also be applied to an image forming apparatus such as a method. The same applies to the second embodiment, not limited to the tandem method.

  (5) In the first embodiment, the image forming unit 2K (photosensitive drum 3K) is positioned on the most downstream side in the transport direction of the transport belt 11, and M, C, and K colors on the paper S are displayed. When the toner images are overlaid, the K color is in the lowermost layer, and the M and C color toner images are positioned thereon. This is because when the K color is positioned at the lowermost layer, the M and C color toners positioned thereon can be further melted in the fixing unit 40, thereby improving the color and further improving the color reproducibility. Because it can.

  This is not limited to the tandem system, but can be applied to a transfer drum system and the like, and the position of the photosensitive drum for each color, the switching order of the developing units, and the like may be determined so that the K color is positioned in the lowest layer. Depending on the development characteristics, fixing characteristics, etc., there may be a case where the difference in reproducibility hardly changes regardless of the position of the image forming unit 2K. In that sense, the image forming unit 2K is positioned on the most downstream side. It is not limited to. It is possible to arrange the image forming units 2M to 2K at an optimum position from experiments or the like. The same applies to the second embodiment.

  Further, the above embodiment and the above modification examples may be combined.

  The present invention can be widely applied to an image forming apparatus having a function of adding a specific pattern image to a color image.

1 is a diagram illustrating an overall configuration of a printer 1 according to a first embodiment. FIG. 5 is a chromaticity diagram showing chromaticity points of M and C color toners in an L ^* a ^* b ^* color system. 2 is a diagram illustrating a configuration of a control unit 100 of the printer 1. FIG. It is a schematic diagram showing which information is added as identification information at which position on the printed paper. It is the figure which expanded and showed one pattern image 61 as identification information. It is a figure which shows the relationship between the fog density | concentration D and the concealment rate P. FIG. It is a flowchart which shows the content of the addition process of identification information. It is a flowchart which shows the content of exposure amount control processing. It is a figure which shows the correspondence of the output voltage V of the photoelectric sensor 16, and the concealment rate P. FIG. 2 is a diagram illustrating a configuration of a printer 201 according to a second embodiment. FIG. 2 is a block diagram illustrating a configuration of a control unit 202 of a printer 201. FIG. 6 is a flowchart showing the contents of conversion processing in the image processing unit 203 of the control unit 202.

Explanation of symbols

1,201 Printer 2Y, 2M, 2C, 2K Image forming unit 6Y, 6M, 6C, 6K Developer 61-63 Image pattern 81-85 Black pixel 100, 202 Control unit 104 Identification information addition unit 1031 MCK conversion unit

Claims (10)

  A magenta, cyan, and black toner image is transferred onto a sheet to form a color image. When a color image is formed, a specific pattern using a black toner that is separate from the color image An image forming apparatus for forming an image on a sheet. The pattern image is composed of pixels reproduced with black toner and pixels reproduced with white,
The image forming apparatus according to claim 1, wherein an interval between two pixels reproduced with the black toner is equal to or more than one pixel.   The image forming apparatus according to claim 2, wherein the interval is in a range not less than 3 pixels and not more than 95 pixels. The pattern image is composed of pixels reproduced with black toner and pixels reproduced with white,
4. The concealment rate indicating a ratio of pixels reproduced by the black toner per unit area is 0.01% or more and 10% or less. 5. 2. The image forming apparatus according to item 1. Detecting means for detecting the formed pattern image and outputting a value for indicating the concealment rate;
It is determined from the output data from the detection means whether or not the concealment rate is within a predetermined range, and when it is determined that the concealment rate is not within the predetermined range, the pattern image is set so that the concealment rate falls within the predetermined range. The image forming apparatus according to claim 4, wherein the forming condition is changed.   The image forming apparatus according to claim 1, wherein the pattern image is formed at a predetermined position on the sheet regardless of a formation position of the color image on the sheet.   The black color toner image and the pattern image are formed on the sheet prior to the other color toner images. Image forming apparatus. The magenta color toner has a chromaticity point closer to the yellow hue than the full-color magenta toner used when forming a color image with four colors of magenta, cyan, yellow, and black. Directionally shifted toner and / or
The cyan toner has a chromaticity point closer to the yellow hue than the full-color cyan toner used when forming a color image with four colors of magenta, cyan, yellow, and black. The image forming apparatus according to claim 1, wherein the toner is a toner shifted in a direction. Has development sections for magenta, cyan, and black except yellow
The magenta color toner image is developed by the magenta color developing unit,
The cyan toner image is developed by the cyan developing unit,
9. The image forming apparatus according to claim 1, wherein the black color toner image and the pattern image are developed by a black color developing unit. The color image is configured to be capable of forming an image including a yellow toner image, and includes a determination unit that determines whether or not the yellow toner image can be formed.
9. The image formation according to claim 1, wherein the formation of the pattern image is executed only when it is determined by the determination means that a yellow toner image cannot be formed. apparatus.

Images