JP2021047220A - Image formation apparatus - Google Patents

Abstract

To provide an image formation apparatus which can perform the highly accurate gradation correction while suppressing the consumption amount of a developer.SOLUTION: An image formation apparatus creates a test chart by forming a test image for gradation correction on a sheet. The test chart is read by an image reading device. The image formation apparatus generates a correction value used in the gradation correction from the reading result of the test chart by the image reading device. The test image is constituted by the combination of a gradation pattern group 911 (first gradation pattern) in a prescribed size at a higher density than a prescribed image density and a second gradation pattern group 912 (second gradation pattern) in a smaller size than the first gradation pattern group 911 at a lower density than the prescribed image density.SELECTED DRAWING: Figure 8

Description

The present invention relates to a gradation correction technique for an image forming apparatus.

In recent years, there have been an increasing number of cases in which color image forming devices such as color multifunction devices and color copiers are installed in offices and copy shops. The quality (image quality) of the image formed by the color image forming apparatus tends to be unstable due to aging and environmental changes. As one of the processes for stabilizing the image quality, a gradation correction process for correcting the gradation characteristics of the image forming apparatus is known. In the gradation correction process, a test image combining gradation patterns is formed on a sheet to create a test chart, the test chart is read by an image reading device attached to the image forming device, and gradation characteristics are obtained based on the reading result. It is done by correcting.

The image forming apparatus performs image forming processing in, for example, a plurality of different image processing modes. The image forming apparatus forms an image in an image processing mode according to the content of the output. The image processing mode includes, for example, an image forming process that involves a gamma characteristic correction (γ correction) process using a γ look-up table (LUT). The contribution rate of the image forming apparatus to the characteristic variation differs depending on the image processing mode. Therefore, it is necessary to execute the gradation correction process for each image processing mode. That is, it is necessary to create an optimum γLUT for each image processing mode by generating and reading a test chart for each image processing mode.

Conventionally, large-sized and expensive full-color image forming devices have been the mainstream. In recent years, with the development of technology, small and inexpensive full-color image forming devices have begun to spread. In particular, in SOHO (Small Office Home Office) and personal use, a small full-color image forming apparatus specialized for generating a small-sized output is becoming widespread. Even in a small and inexpensive full-color image forming apparatus, it is becoming indispensable to improve the image quality and stabilize the image. However, in order to perform the same gradation correction as in the conventional case for the characteristic fluctuation of the image forming apparatus, there is a peculiar problem due to the small size of the apparatus. For example, the size of a sheet capable of forming an image by a small image forming apparatus is smaller than the size of a sheet capable of forming an image by a large image forming apparatus. Therefore, in a small image forming apparatus, it is necessary to make the gradation pattern smaller than before or reduce the number of gradation patterns included in the test image to create a test chart.

Further, in a large image forming apparatus, test images for each of a plurality of image processing modes are formed on different sheets. However, in a small image forming apparatus, it is required to perform gradation correction without bothering the user as much as possible. That is, there is a desire to print a test image of each image processing mode on one sheet. In order to realize this, it is necessary to reduce the size of the gradation pattern constituting the test image. However, this has an adverse effect that the reading accuracy of the image reading device that reads the gradation pattern is lowered. This is because as the area of the gradation pattern becomes smaller, the influence of the surface of the sheet around the gradation pattern (so-called base) on the reading result of the gradation pattern becomes larger. Therefore, in order to reduce the size of the gradation pattern, a method for reducing the influence from the background is required. Patent Document 1 discloses a technique of adding a background pattern to the outer periphery of a test image to reduce the influence of the background when reading the test image and suppress a decrease in the accuracy of gradation correction.

Japanese Unexamined Patent Publication No. 2010-85744

When a background pattern is formed on the outer periphery of the test image (gradation pattern), the consumption of the developer increases and the running cost increases. Therefore, a main object of the present invention is to provide an image forming apparatus capable of performing highly accurate gradation correction while suppressing the consumption of a developer even if the size is small.

The image forming apparatus of the present invention is tested by forming a test image on the sheet by an image forming means for forming an image on the sheet, a reading means for reading the image formed on the sheet, and the image forming means. A control means for creating a chart and generating a correction value used for gradation correction from the reading result of the test chart by the reading means is provided, and the test image is formed in a different size according to an image density. It is characterized in that it is composed of a combination of a first gradation pattern and a second gradation pattern.

According to the present invention, it is possible to perform highly accurate gradation correction while suppressing the consumption of the developer.

The block diagram of an image forming apparatus and an image reading apparatus. Explanatory drawing of gamma correction. Diagram of the configuration of the image processing unit. A flowchart showing the gradation correction process. Explanatory drawing of a general test chart. Explanatory drawing of the reading area of a gradation pattern. (A) and (b) are relationship diagrams between the size of the gradation pattern and the detected luminance value. Explanatory drawing of the test image of this embodiment. Another illustration of the test image.

An embodiment of the present invention will be described with reference to the drawings.

(overall structure)
FIG. 1 is a configuration diagram of an image forming apparatus and an image reading apparatus of the present embodiment. The image forming apparatus 100 is a color image forming apparatus capable of forming a full-color image on the sheet 6. The image forming device 100 and the image reading device 101 form, for example, a color copier or a color multifunction device. The image forming apparatus 100 and the image reading apparatus 101 may be connected via a host computer.

The image reader 101 is sometimes called an image reader or an image scanner. The image reading device 101 includes a platen glass 102, a CCD (Charge-Coupled Device) sensor 105, and a reference white plate 106. The document G is placed on the platen glass 102. The document G placed on the platen glass 102 is irradiated with light by a light source. The CCD sensor 105 is a reading sensor, reads an image of the document G, and generates a reading signal representing the image as a reading result. Therefore, the CCD sensor 105 receives the reflected light from the document G via an optical system such as a lens. The CCD sensor 105 includes, for example, three rows of CCD line sensors corresponding to red, green, and blue. The CCD sensor 105 generates a read signal by photoelectrically converting the light reflected by the received document G. The CCD sensor 105 transmits the generated read signal to the image forming apparatus 100. The reference white plate 106 is arranged on the platen glass 102. The reference white plate 106 is used for determining the white level of the CCD sensor 105 and for shading correction in the thrust direction of the CCD sensor 105.

The image forming apparatus 100 includes an image processing unit 108 and a printer engine that forms an image on the sheet 6. The image processing unit 108 acquires a read signal from the CCD licensor 105 and performs predetermined image processing on the read signal to generate an image signal used in the image forming process. The image processing unit 108 performs image processing on the read signal in a plurality of image processing modes. The image processing unit 108 uses an image as an image signal (hereinafter, referred to as “test pattern signal”) of a test image for creating a test chart according to a predetermined image processing mode among a plurality of image processing modes related to gradation characteristics. Perform processing.

Since the image forming apparatus 100 of the present embodiment is a tandem type, the image forming units 111 and 112 corresponding to the developed colors (yellow (Y), magenta (M), cyan (C), black (K)), 113 and 114 are arranged in parallel. Each image forming unit 111, 112, 113, 114 includes an exposure device 2, a developing device 3, a photosensitive drum 4, a charging device 8, and a cleaner 9. Each image forming unit 111, 112, 113, 114 generates an image to be formed on the sheet 6 according to the image signal generated by the image processing unit 108. The image forming apparatus 100 includes an intermediate transfer body 5 and a fixing device 7 in order to form an image of each color generated by each of the image forming units 111, 112, 113, and 114 on the sheet 6.

The configuration of the image forming unit 111 will be described. The image forming units 112, 113, and 114 differ only in the colors of the images to be formed, and the same processing is performed with the same configuration as the image forming unit 111. Therefore, the description thereof will be omitted.

The photosensitive drum 4 is a drum-shaped photosensitive member having a photosensitive layer on its surface. The photosensitive drum 4 rotates in the direction of arrow A when forming an image. The charger 8 is, for example, a roller charger, and when a bias voltage is applied, the surface of the photosensitive drum 4 is uniformly charged with a negative electrode property. The exposure device 2 includes a laser driver, a laser light source, and a polygon mirror 1, and irradiates the charged surface of the photosensitive drum 4 with a laser beam modulated by an image signal generated by the image processing unit 108. The laser beam is reflected by the polygon mirror 1 and an optical system such as a lens and a mirror, and scans the surface of the photosensitive drum 4 in one direction according to the rotation of the polygon mirror 1. By scanning the laser beam and rotating the photosensitive drum 4, an electrostatic latent image corresponding to the image signal is formed on the surface of the photosensitive drum 4.

The developer 3 stores a developer (for example, toner) and develops an electrostatic latent image formed on the photosensitive drum 4 to form a developer image (toner image). This toner image is primarily transferred to the intermediate transfer body 5 which is pressure-contacted with the photosensitive drum 4. The toner remaining on the photosensitive drum 4 without being transferred to the intermediate transfer body 5 after the primary transfer is scraped off by the cleaner 9 and collected in a waste toner container (not shown). Such a process is performed for each image forming unit 111, 112, 113, 114.

The intermediate transfer body 5 is an endless belt member and rotates in the direction of arrow D. At the timing corresponding to the rotation of the intermediate transfer body 5, the toner image is sequentially superimposed and transferred from the image forming portion of each color to the intermediate transfer body 5. In the example of FIG. 1, the toner image is transferred to the intermediate transfer body 5 in the order of yellow, magenta, cyan, and black. As a result, a full-color toner image is formed on the intermediate transfer body 5.

The full-color toner image formed on the intermediate transfer body 5 is conveyed in the rotation direction of the intermediate transfer body 5 and secondarily transferred to the sheet 6. The sheet 6 on which the toner image is transferred is conveyed to the fixing device 7. The fixing device 7 performs a fixing process for fixing the transferred toner image on the sheet 6. The sheet 6 is sometimes called a recording material, paper, transfer material, transfer paper, or recording medium.

(Gradation correction)
Generally, the image forming apparatus 100 performs gamma correction. The gamma correction is a process of converting an image signal so that the density characteristic of the image forming apparatus 100 becomes an ideal characteristic. In the gamma correction, a gamma lookup table (hereinafter, referred to as “γLUT”) prepared in advance is used.

FIG. 2 is an explanatory diagram of gamma correction. The horizontal axis shows the brightness value (input signal) represented by the image signal used for image formation, and the vertical axis shows the actual brightness value (output signal) of the image formed on the sheet. The alternate long and short dash line 201 shows the original gradation characteristics to which the gamma correction is not applied. The broken line 202 indicates an ideal gradation characteristic (target gradation characteristic). The solid line 203 shows the characteristics of γLUT (correction value of gradation characteristics). By applying the correction value of γLUT (solid line 203) and performing the correction process, the original gradation characteristic (dashed line 201) is converted into the target gradation characteristic (broken line 202). The density characteristic of the image forming apparatus 100 fluctuates according to aging and the environment. Therefore, the γLUT needs to be updated according to the fluctuation of the concentration characteristic.

(Image processing unit)
FIG. 3 is a configuration explanatory view of the image processing unit 108. The image processing unit 108 is connected to the CCD sensor 105 of the image reader 101, the exposure device 2, the CPU 308 (Central Processing Unit), and the pattern generator 306. The operation unit 307 and the memory 309 are connected to the CPU 308.

The operation unit 307 is a user interface in which an input device and an output device are combined. The input device is a key button such as a numeric keypad or an input key, or a touch panel. The output device is a display or a speaker. The CPU 308 is a controller that controls the operations of the image forming apparatus 100 and the image reading apparatus 101 by executing a predetermined computer program. The CPU 308 receives instructions and data input from the operation unit 307 and executes processing. Further, the CPU 308 displays an instruction image to the user and a state image of the image forming apparatus 100 by the operation unit 307. The memory 309 provides a work area for processing by the CPU 308.

The image processing unit 108 includes an A / D (Analog / Digital) converter 302, a shading unit 303, a LOG (Laplacian Of Gaussian) conversion unit 304, and a γLUT unit 305 for image processing. The image processing unit 108 generates and outputs an image signal from the reading signal acquired from the image reading device 101.

The A / D conversion unit 302 converts the analog luminance signal included in the read signal acquired from the CCD sensor 105 into a digital luminance signal. The shading unit 303 performs shading correction on the digitally converted luminance signal. This is a process for correcting variations in sensitivity of a plurality of light receiving elements constituting the CCD sensor 105. The LOG conversion unit 304 LOG-converts the luminance signal after shading correction. The γLUT unit 305 performs gamma correction on the luminance signal after LOG conversion. The image signal including the gamma-corrected luminance signal is transmitted to the exposure device 2. The exposure device 2 modulates and outputs the laser beam according to this image signal.

The pattern generator 306 generates a test pattern signal, which is an image signal for forming a test image, when the test chart is created. The test image contains gradation patterns for four colors of cyan, magenta, yellow, and black. The pattern generator 306 is an example of a pattern generator that generates a test pattern signal for forming a test image including a plurality of gradation patterns having different gradations. The γLUT unit 305 also performs gamma correction on the test pattern signal.

(Gradation correction)
FIG. 4 is a flowchart showing the gradation correction process. When the CPU 308 receives an instruction to start the gradation correction process from the operation unit 307, the CPU 308 starts the gradation correction process.

When the gradation correction process is started, the CPU 308 creates a test chart using the pattern generator 306 (S1301). The CPU 308 instructs the pattern generator 306 to output a test pattern signal. The test pattern signal output from the pattern generator 306 is gamma-corrected by the γLUT unit 305. The gamma-corrected test pattern signal is transmitted from the image processing unit 108 to the exposure device 2. The exposure device 2 scans the photosensitive drum 4 with a laser beam modulated according to the test pattern signal. After that, a test image is printed on the sheet by the above-mentioned process, and a test chart is created.

When the test chart is created, the CPU 308 instructs the user to place the test chart on the platen glass 102. The CPU 308 displays, for example, an image or characters instructing the operation unit 307 to place the test chart on the platen glass 102. The user places the test chart on the platen glass 102 in response to the instruction, and then inputs the test chart reading instruction by the operation unit 307. When the CPU 308 receives the test chart reading instruction, the CPU 308 performs an image reading process by the image reading device 101 (S1302). As a result, the image processing unit 108 acquires a reading signal which is a reading result of the test image formed on the test chart from the image reading device 101.

The CPU 308 stores in the memory 309 the relationship between the image density of the test image obtained from the read signal acquired by the image processing unit 108 and the output level of the laser beam indicated by the test pattern signal (S1303). The CPU 308 calculates the correction value of the gradation characteristic from the image density of the test image stored in the memory 309 and the output level of the laser beam to generate the γLUT. The CPU 308 updates the γLUT by setting the generated γLUT in the γLUT unit 305 (S1304).

FIG. 5 is an explanatory diagram of a general test chart. As described above, the test chart is created by printing the test image 320 on the sheet 6. The test image 320 is composed of a combination of gradation patterns of each color of yellow, magenta, cyan, and black. The test image 320 of FIG. 5 is composed of a black gradation pattern group 321, a yellow gradation pattern group 322, a magenta gradation pattern group 323, and a cyan gradation pattern group 324.

The black gradation pattern group 321 includes a plurality of black gradation patterns K1 to K64 having different gradations (image densities). The yellow gradation pattern group 322 includes a plurality of yellow gradation patterns having different gradations (image densities). The magenta gradation pattern group 323 includes a plurality of magenta gradation patterns having different gradations (image densities). The cyan gradation pattern group 324 includes a plurality of cyan gradation patterns having different gradations (image densities). In the test image 320 of the present embodiment, the gradation pattern group of each color is composed of 64 gradation patterns. That is, a gradation pattern of 64 gradations is formed for each color.

In the black gradation pattern, the gradation pattern K1 has the highest density, the density gradually decreases, and the gradation pattern K64 has the lowest density. In the yellow gradation pattern, the gradation pattern Y1 has the highest density, the density gradually decreases, and the gradation pattern Y64 has the lowest density. In the magenta gradation pattern, the gradation pattern M1 has the highest density, the density gradually decreases, and the gradation pattern M64 has the lowest density. In the cyan gradation pattern, the gradation pattern C1 has the highest density, the density gradually decreases, and the gradation pattern C64 has the lowest density.

The density characteristics of each color are different for each image forming unit 111, 112, 113, 114. Therefore, the test image 320 includes a gradation pattern group for each color (for each color of the developing agent). In the processing of S1304, the output level of the laser beam indicated by the test pattern signal for each color used when forming the gradation pattern group of each color and the brightness detected from the reading result (reading signal) of the test chart. ΓLUT is generated from the relationship with the value (image density). γLUT is generated by correcting an existing γLUT.

(Relationship between gradation pattern size and brightness value)
FIG. 6 is an explanatory diagram of a reading area of a gradation pattern. The gradation pattern 701 is a square having a side length of X. When reading each gradation pattern of the test chart, the image reading device 101 reads not the entire surface of the gradation pattern but the reading area 702 which is a part of the gradation pattern. If the size of the gradation pattern 701 is sufficiently larger than the size of the reading area 702, the brightness value detected by reading the gradation pattern 701 is affected by the background 703 (the surface on which the image of the sheet 6 is formed). It becomes difficult to receive.

This is because the distance Y between the reading area 702 and the base 703 is sufficiently large, and the reading area 702 is sufficiently separated from the base 703. However, as the size of the gradation pattern 701 becomes smaller, the distance Y between the reading area 702 and the base 703 becomes shorter. Therefore, the influence of the base 703 on the luminance value detected by reading the gradation pattern 701 cannot be ignored. In particular, in the high density region, the brightness value detected from the read signal obtained by reading the gradation pattern 701 is higher than the original brightness value due to the reflected light from the base 703.

FIG. 7 is a relationship diagram between the size of the gradation pattern and the detected luminance value. FIG. 7A shows the case of a high-density gradation pattern. FIG. 7B shows the case of a low density gradation pattern. The solid line represents the detected brightness value, and the broken line represents the target value of the brightness value. The gradation pattern 701 is a square, and the size is represented by the length X of one side.

According to FIG. 7A, in the high-density gradation pattern, the luminance value detected when the length X of one side is 15 [mm] or more is close to the target value, and is detected when it is 10 [mm] or less. It can be seen that the obtained brightness value becomes higher than the target value due to the influence of the base 703. When the detected luminance value is higher than the target value, it means that the image density converted from the luminance value fluctuates to a value lower than the original image density. Therefore, when the correction value of the gradation characteristic is calculated using the luminance value of the detection result as shown in FIG. 7A, the image processing unit 108 executes the gradation correction in the direction of increasing the image density. It will be. From this, the size of the gradation pattern having a density higher than the predetermined image density has a side length X of 15 [mm] or more so that the detected luminance value is stable at a value close to the target value. Is preferable.

According to FIG. 7B, in the low density gradation pattern, the luminance value detected when the length X of one side is 10 [mm] or more is close to the target value, and is detected when it is 8 [mm] or less. It can be seen that the obtained brightness value becomes higher than the target value. That is, in a gradation pattern having a density lower than a predetermined image density, if the size of one side is 10 [mm] or more, the detected luminance value is less likely to be affected by the background 703, and accurate gradation correction can be performed. It will be possible. Therefore, the gradation pattern included in the test image 320 may be formed so that the gradation pattern having a density lower than the predetermined image density is smaller than the gradation pattern having a density higher than the predetermined image density. preferable.

(Test image of this embodiment)
FIG. 8 is an explanatory diagram of a test image of the present embodiment. For comparison with the test image of this embodiment, a general test image illustrated in FIG. 5 is also shown. Here, the black gradation pattern group 321 (gradation patterns K1 to K64) will be described, but the gradation pattern groups of other colors have the same shape.

Each gradation pattern K1 to K64 of the general black gradation pattern group 321 is a square having a side of 15 [mm] and the same size. On the other hand, the black gradation pattern group 910 of the test image of the present embodiment is composed of a combination of the first gradation pattern and the second gradation pattern formed in different sizes according to the image density. In the present embodiment, in the black gradation pattern group 910, the size of the second gradation pattern having a density lower than the predetermined image density is smaller than the size of the first gradation pattern having a density higher than the predetermined image density. It has become. The second gradation pattern constitutes the gradation pattern group 912, and the first gradation pattern constitutes the gradation pattern group 911.

Specifically, the high-density first gradation patterns K1 to K48 are formed of squares having a side of 15 [mm], and the low-density second gradation patterns K49 to K64 have a side of 10 [mm]. ] Is formed by a square. The pattern generator 306 generates a test pattern signal for generating a test image formed by combining gradation patterns of different sizes in this way. In this way, by reducing the size of the gradation pattern having an image density equal to or lower than the predetermined image density, the consumption of the developer can be suppressed to be lower than that of a general test image.

A predetermined image density as a reference for changing the size of the gradation pattern is determined based on the relationship between the size of the gradation pattern and the luminance value detected from the read signal as illustrated in FIG. 7. A gradation pattern for reducing the size is determined according to the degree to which the correction value of the gradation correction calculated from the detected luminance value affects the accuracy of the gradation correction.

By using a test image having the above gradation pattern group, it is possible to prevent the accuracy of the reading result (luminance value) from being lowered by the background and maintain the accuracy of gradation correction, while consuming the developer used for the test chart. The amount can be suppressed. Further, although the case where the gradation pattern is square has been described here, the shape of the gradation pattern may be rectangular.

(Other examples of test images)
FIG. 9 is another exemplary diagram of the test image of the present embodiment. In the test image of FIG. 9, each gradation pattern is circular. The gradation pattern group 1001 having a density higher than the predetermined image density is composed of a circular first gradation pattern having a diameter of 15 [mm], and the second gradation pattern group 1002 having a diameter lower than the predetermined image density is the diameter. It is composed of a circular second gradation pattern of 10 [mm]. The pattern generator 306 generates a test pattern signal for generating a test image formed by combining gradation patterns of different sizes in this way.

Even when the circular gradation pattern is used, the same effect as when the square gradation pattern of FIG. 8 is used can be obtained. That is, it is possible to suppress the consumption of the developer used in the test chart while maintaining the accuracy of gradation correction by preventing the accuracy of the reading result (luminance value) from being lowered by the background. Further, the circular gradation pattern can further suppress the consumption of the developer as compared with the square gradation pattern.

As described above, the image forming apparatus 100 of the present embodiment forms a test image used for gradation correction by a plurality of gradation patterns having different sizes. By optimizing the size of the gradation pattern according to the image density, the influence of the background on the reading result when reading the gradation pattern is suppressed. As a result, it is possible to prevent a decrease in the accuracy of gradation correction and suppress the consumption of the developer. Therefore, the image forming apparatus 100 can provide a stable image for a long period of time at low cost.

Claims (8)

An image forming means for forming an image on a sheet,
A reading means for reading the image formed on the sheet and
A control means for forming a test image on the sheet by the image forming means to create a test chart and generating a correction value used for gradation correction from the reading result of the reading means of the test chart is provided.
The test image is characterized in that it is composed of a combination of a first gradation pattern and a second gradation pattern formed in different sizes according to the image density.
Image forming device. Further provided with a pattern generation means for generating a test pattern signal for forming the test image,
The pattern generating means includes the first gradation pattern having a density higher than a predetermined image density and a predetermined size, and the first gradation pattern having a density lower than the predetermined image density and a size smaller than the first gradation pattern. It is characterized in that the test pattern signal for forming the test image composed of a combination with a two-gradation pattern is generated.
The image forming apparatus according to claim 1. The image forming means forms an image of a plurality of colors on the sheet.
The pattern generation means generates the test pattern signal for forming the test image in which the first gradation pattern and the second gradation pattern are combined for each of the plurality of colors.
The control means is characterized in that the image forming means forms the test image corresponding to the test pattern signal on the sheet to create the test chart.
The image forming apparatus according to claim 2. An image processing means for correcting an image signal representing an image formed by the image forming means based on the correction value is further provided.
The control means is characterized in that the image forming means forms an image based on the image signal corrected by the image processing means.
The image forming apparatus according to any one of claims 1 to 3. The first gradation pattern and the second gradation pattern are both square, and the length of one side of the second gradation pattern is shorter than the length of one side of the first gradation pattern. ,
The image forming apparatus according to any one of claims 1 to 4. The length of one side of the first gradation pattern is 15 [mm] or more, and the length of one side of the second gradation pattern is 8 [mm] or more.
The image forming apparatus according to claim 5. The first gradation pattern and the second gradation pattern are both circular, and the diameter of the second gradation pattern is shorter than the diameter of the first gradation pattern.
The image forming apparatus according to any one of claims 1 to 4. The diameter of the first gradation pattern is 15 [mm] or more, and the diameter of the second gradation pattern is 8 [mm] or more.
The image forming apparatus according to claim 7.

Images