JP7412942B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a copying machine, a multifunction device, and a printer.

In recent years, the market for on-demand image forming devices has been expanding. For example, electrophotographic image forming apparatuses are becoming more popular in the offset printing market. Furthermore, there are inkjet image forming apparatuses that have successfully developed a wide range of markets due to their large format, low initial cost, and ultra-high speed. However, expanding the market is not easy, and the image quality (hereinafter referred to as "image quality") of the previous image forming apparatus that has supported the market must be maintained.

Image quality includes gradation, graininess, in-plane uniformity, character quality, color reproducibility (including color stability), and the like. As an image control method for adjusting image quality, a method for adjusting gradation is known. The gradation is adjusted, for example, by forming a test image for gradation correction on an image carrier used for image formation, and based on the image density measured from this test image.

When an image forming apparatus is used for a long period of time, the image density measured from the image on the image carrier may not match the image density of the image actually formed on the sheet. In this case, a method is known in which a test image for gradation correction is formed on a sheet and image forming conditions are corrected based on the image density of the test image on the sheet (Patent Document 1). The position where the test image is formed on the image carrier can be a non-image area other than the area where the image desired by the user is formed on the image carrier (Patent Document 2). This eliminates the need to form a test image on a separate sheet of paper from the image desired by the user, making it possible to avoid waste paper.

Japanese Patent Application Publication No. 2000-238341 JP2014-107648A

When adjusting the gradation as described above, a test image is formed on the sheet. The gradation varies depending on the type (characteristics) of the sheet. Therefore, in order to adjust the gradation for various types of sheets, it is necessary to form test images for each type of sheet and adjust the gradation. This leads to an increase in the amount of developer used for adjusting gradation. The developer should originally be used to produce printed matter desired by the user, and it is preferable that the amount of developer used is as small as possible for stable operation of the image forming apparatus. For this reason, it is desirable to make the test image as small as possible.

However, some sheets on which images are formed have low smoothness (smoothness) and have uneven surfaces, such as embossed paper and embossed paper. A sheet with such surface properties has a property that an image is not easily transferred from an image carrier. For example, if the surface of the sheet has irregularities of several tens [μm] or more in height, the image carrier and the sheet may not be able to come into contact with each other in the recessed portions of the sheet, and gaps may be formed. This gap prevents the image from being transferred to the sheet properly. This may occur even in a configuration in which transfer is performed by applying a bias voltage while pressing the sheet with a transfer member made of, for example, a conductive rubber roller. If image transfer is not performed normally, image quality deterioration such as image dropout or density unevenness occurs. In addition, recycled paper may be used as a sheet on which images are formed. Recycled paper has a large amount of impurities, which causes large resistance unevenness, and poor transferability, which causes large unevenness within the image. For these reasons, depending on the type of sheet, if the test image is made small, the image quality of the test image itself deteriorates, making it impossible to adjust the image quality such as gradation with high precision.

In view of the above problems, the main object of the present invention is to provide an image forming apparatus capable of highly accurate image quality adjustment while suppressing the amount of developer used.

The image forming apparatus of the present invention includes an image forming means for forming an image on a sheet, a reading means for reading the image formed on the sheet, and a test image formed on the sheet by the image forming means, and the reading means for forming a test image on the sheet. control means for controlling the density of the image formed by the image forming means based on the reading result of the test image by the means, the control means controlling information regarding the smoothness of the sheet on which the test image is formed; and when the test image is formed on a sheet with the smoothness lower than the predetermined value, the image forming means is caused to form a test image with a first number of gradations, and the smoothness is lower than the predetermined value. In the case where the test image is formed on a sheet having a high height, the image forming means is configured to form a test image with a second number of gradations smaller than the first number of gradations .

According to the present invention, highly accurate image quality adjustment is possible while suppressing the amount of developer used.

FIG. 1 is a configuration diagram of an image forming apparatus. FIG. 2 is an explanatory diagram of the configuration of a printer control unit. An explanatory diagram of gamma correction. Flowchart showing gradation correction processing. An illustrative diagram of a guide image. FIG. 4 is an exemplary diagram of Bekk smoothness for each type of sheet. An explanatory diagram of variations in image density. FIG. 3 is an explanatory diagram of the relationship between the type (characteristics) of a sheet and the size of a patch image. (a) and (b) are exemplary diagrams of test images. A flowchart showing test image selection processing. FIG. 3 is an exemplary diagram of a selection image of a paper feed cassette. FIG. 1 is a configuration diagram of an image forming apparatus that reads a test chart inline. An illustrative diagram of a test image.

Embodiments of the present invention will be described with reference to the drawings. However, the relative arrangement of the constituent elements, mathematical formulas, numerical values, etc. described in this embodiment are not intended to limit the scope of the present invention only thereto, unless there is a specific description.

(overall structure)
FIG. 1 is a configuration diagram of an image forming apparatus. The image forming apparatus of this embodiment includes a reader section A and a printer section B. The reader section A is an image reading device that reads a document image. The printer section B prints, for example, an image corresponding to the original image read by the reader section A on a sheet 6 such as paper.

(Leader part)
The reader section A includes a document glass 102 on which a document 101 is placed, a light source 103 that irradiates light onto the document 101 on the document glass 102, an optical system 104, a light receiving section 105, and a reader image processing section 108. An abutment member 107 and a reference white plate 106 are arranged on the document table glass 102 . The abutting member 107 is used to place the original 101 in the correct position. The reference white plate 106 is used to determine the white level of the light receiving section 105 and to correct shading.

A light source 103 illuminates the original 101 placed on the original platen glass 102 . The light receiving unit 105 receives the light emitted from the light source 103 reflected by the original 101 via the optical system 104 . The light receiving unit 105 generates color component signals, which are electrical signals representing each color of R (red), G (green), and B (blue), based on the received reflected light and transmits them to the reader image processing unit 108. Such a light receiving section 105 includes, for example, a CCD (Charge Coupled Device) sensor as a light receiving element. For example, the light receiving unit 105 includes CCD line sensors arranged in three rows corresponding to each color of R, G, and B, and detects each color of R, G, and B based on the reflected light received by each CCD line sensor. Generate component signals. The light source 103, the optical system 104, and the light receiving section 105 are an integrated reading unit, and are movable in the left and right directions in the figure. The CCD line sensor of the light receiving section 105 includes a plurality of CCD sensors arranged in the depth direction in FIG. To this end, the reading unit sequentially reads the entire document 101 line by line by moving with the depth direction in FIG. 1 as one line, and generates color component signals for each line. The depth direction in FIG. 1 is the main scanning direction of the reader section A.

The reader image processing unit 108 performs image processing on the color component signals of each color and generates image data representing the image of the document 101. The reader image processing unit 108 sends the generated image data to the printer unit B.

(Printer section)
The printer section B includes a photosensitive drum 4 as an image carrier, a charger 8, a developer 3, a cleaner 9, an intermediate transfer member 5, a fixing device 7, and a laser light source 110 in order to form an image on a sheet 6 such as paper. , a polygon mirror 1, a mirror 2, and a printer control unit 109. A surface potential sensor 12 and a photosensor 40 are provided around the photosensitive drum 4 .

The photosensitive drum 4 is a drum-shaped photosensitive member, and rotates clockwise in the figure during image formation. The charger 8 is, for example, a corona charger, and uniformly charges the surface of the photosensitive drum 4 by applying a bias voltage to the rotating photosensitive drum 4. The laser light source 110 forms an electrostatic latent image on the charged surface of the photosensitive drum 4 under the control of the printer control unit 109 . The printer control unit 109 acquires image data from the reader image processing unit 108 of the reader unit A, and controls blinking of the laser light emitted from the laser light source 110 based on this image data. Note that, when image data is acquired from an external device such as a personal computer, the printer control unit 109 controls blinking of the laser light emitted from the laser light source 110 based on the acquired image data. Laser light emitted from the laser light source 110 illuminates the uniformly charged photosensitive drum 4 via the polygon mirror 1 and the mirror 2. The laser beam scans the photosensitive drum 4 by changing the deflection angle by rotating the polygon mirror 1 . The laser beam scans the surface of the photosensitive drum 4 with the main scanning direction being a direction perpendicular to the rotational direction of the photosensitive drum 4 (the depth direction in FIG. 1). By scanning the rotating photosensitive drum 4 with a laser beam, an electrostatic latent image for one page is formed on the surface of the photosensitive drum 4 based on image data.

The developing device 3 develops the electrostatic latent image formed on the photosensitive drum 4 with a developer (toner) to form a developer image (toner image). The developer 3 is a rotary developer and includes a magenta developer 3M, a cyan developer 3C, a yellow developer 3Y, and a black developer 3K. The four developing devices 3M, 3C, 3Y, and 3K are detachably held within the developing device 3, and when forming an image, the four developing devices 3M, 3C, 3Y, and 3K are rotated around the rotation axis of the developing device 3 while being held in the developing device 3. Rotate. For example, when developing with magenta developer (toner), the developing device 3M stops at a position facing the photosensitive drum 4. The sleeve in the developing device 3M faces the photosensitive drum 4 at a very small distance (for example, about 400 [μm]), and attaches toner to the electrostatic latent image on the photosensitive drum 4 to form a toner image. When forming a chromatic toner image such as cyan or yellow, the developing device 3 rotates and the developing devices 3C and 3Y stop at positions facing the photosensitive drum 4 to perform development. When forming a black toner image, the developing device 3 rotates and the developing device 3K stops at a position facing the photosensitive drum 4 to perform development.

When the above-described developing operation is completed, the developing devices 3 are separated to a position where the respective sleeves in the developing devices 3Y, 3M, 3C, and 3K do not come into contact with the photosensitive drums 4. The separated position becomes the home position of the developing device 3. By setting the home position of the developing device 3, toner in the developing device 3 is prevented from inadvertently adhering to the surface of the photosensitive drum 4. Furthermore, the toner stored in the developing device 3 is prevented from mixing with toner of other colors between the developing devices 3Y, 3M, 3C, and 3K.

The toner image formed on the photosensitive drum 4 is transferred to the intermediate transfer body 5. When forming a full-color image, four-color toner images are sequentially transferred onto the intermediate transfer member 5 in a superimposed manner. As a result, a full-color toner image is formed on the intermediate transfer member 5. The toner image transferred to the intermediate transfer member 5 is transferred all at once to a sheet 6 fed from a paper feeding unit (not shown). The sheet 6 onto which the toner image has been transferred undergoes a toner image fixing process by a fixing device 7. The fixing device 7 heats and fixes the toner image on the sheet 6. The fixing device 7 includes a fixing roller 7a for heating the sheet 6, and a pressure roller 7b for pressing the sheet 6 against the fixing roller 7a. The sheet 6 with the toner image fixed thereon is discharged outside the machine as a printed matter.

Note that toner remaining on the photosensitive drum 4 after transfer is removed by a cleaner 9. The removed toner is accumulated in a cleaning container (not shown). The cleaning container is replaced when it is full of toner. After the remaining toner is removed by the cleaner 9, the photosensitive drum 4 can be used for the next image forming process.

The surface potential sensor 12 is provided around the photosensitive drum 4 between a position where laser light is irradiated by the laser light source 110 and the developing device 3 . The surface potential sensor 12 detects the potential on the surface of the photosensitive drum 4.

The photosensor 40 is provided around the photosensitive drum 4 and between the developing device 3 and the intermediate transfer body 5. The photosensor 40 has a light source 10 and a photodiode 11. The light source 10 irradiates the surface of the photosensitive drum 4 on which the toner image is formed with near-infrared light having a dominant wavelength of about 960 [nm]. The photodiode 11 receives the light emitted from the light source 10 and reflected by the surface of the photosensitive drum 4 . Thereby, the photosensor 40 can measure the amount of light reflected from the toner image for image quality adjustment (hereinafter referred to as "test image") formed on the photosensitive drum 4.

Although the details will be described later, the test image is composed of a combination of a plurality of patch images. In the image forming apparatus of this embodiment, the design of the test image used for image quality adjustment is determined according to the characteristics of the sheet. The design of the test image is determined by the size, layout, number, etc. of patch images. Depending on the design, the amount of developer (toner) used when generating the test image differs. In the following description, gradation adjustment processing will be described as an example of image quality adjustment. The test image for gradation correction includes a gradation pattern for each color, which is composed of a plurality of patch images with different image densities.

(Printer control unit)
FIG. 2 is an explanatory diagram of the configuration of the printer control unit 109. The printer control unit 109 is connected to the reader image processing unit 108 of the reader unit A, the laser light source 110, the CPU (Central Processing Unit) 509, and the pattern generator 507. The CPU 509 is connected to an operation unit 508 and a memory 510.

The operation unit 508 is a user interface that is a combination of an input device and an output device. 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. A CPU 509 is a controller that controls the overall operation of the image forming apparatus by executing a predetermined computer program. The CPU 509 receives instructions and data input from the operation unit 508 and executes processing. Further, the CPU 509 displays an instruction image for the user and a status image of the image forming apparatus 100 using the operation unit 508 . Memory 510 provides a work area for processing by CPU 509.

The printer control unit 109 includes an A/D (Analog/Digital) conversion unit 502, a shading unit 503, a LOG (Laplacian Of Gaussian) conversion unit 504, and a γLUT unit 505 for image processing. The printer control unit 109 performs predetermined image processing on the image data acquired from the reader unit A, and controls light emission of the laser light source 110 according to the processed image data.

The A/D conversion unit 502 converts an analog luminance signal included in the image data obtained from the reader image processing unit 108 into a digital luminance signal. The shading unit 503 performs shading correction on the digitally converted luminance signal. This is a process for correcting variations in sensitivity of the plurality of light receiving elements constituting the light receiving section 105. The shading correction improves the measurement reproducibility of the light receiving section 105. The LOG conversion unit 504 performs LOG conversion on the luminance signal after the shading correction. Through LOG conversion, each of the R, G, and B luminance signals is converted into each of C (cyan), M (magenta), Y (yellow), and K (black) density signals. The γLUT unit 505 performs gamma correction on each of the C, M, Y, and K density signals generated by LOG conversion. The γLUT unit 505 performs gamma correction so that the ideal density characteristics of the printer unit B match the output image density characteristics processed according to the γ characteristics. The image data processed in this manner is transmitted to the laser light source 110 and used to control the blinking of the laser light.

A pattern generator 507 generates a test pattern signal that is image data for forming a test image. The test image includes gradation patterns for four colors: cyan, magenta, yellow, and black. The gradation pattern is composed of a plurality of patch images each having a different density (gradation). The pattern generator 507 of this embodiment is an example of a pattern generation unit that generates test pattern signals for forming test images of a plurality of different designs.

(gradation correction)
Gamma correction performed by the γLUT unit 505 is a process of converting image data so that the density characteristics of the image forming apparatus become ideal characteristics. In gamma correction, a gamma lookup table (hereinafter referred to as "γLUT") prepared in advance is used.

FIG. 3 is an explanatory diagram of gamma correction. The horizontal axis indicates the image density (input signal) represented by the image data used for image formation, and the vertical axis indicates the actual image density (output signal) of the image formed on the sheet. A dashed-dotted line 201 indicates the original gradation characteristics to which no gamma correction has been applied. A broken line 202 indicates an ideal gradation characteristic (target gradation characteristic). A solid line 203 indicates the characteristics of the γLUT (correction value of gradation characteristics). By performing correction processing by applying the correction value of the γLUT (solid line 203), the original gradation characteristic (dotted chain line 201) is converted to the target gradation characteristic (dashed line 202). Note that the density characteristics of the image forming apparatus 100 vary depending on aging and environment. Therefore, the γLUT needs to be updated in accordance with changes in concentration characteristics. The correction value of γLUT is an example of an adjustment value used for adjusting image quality.

FIG. 4 is a flowchart showing the tone correction process. In this process, the γLUT is set in the γLUT section 505. When the CPU 509 receives an instruction to start the tone correction process from the operation unit 508, the CPU 509 starts the tone correction process.

When starting the gradation correction process, the CPU 509 creates a test chart using the pattern generator 507 (S201). The CPU 509 instructs the pattern generator 507 to output a test pattern signal. The test pattern signal output from the pattern generator 507 is subjected to gamma correction by the γLUT section 505. The gamma-corrected test pattern signal is sent to laser light source 110. The laser light source 110 scans the photosensitive drum 4 with laser light modulated according to this test pattern signal. Thereafter, the test image is printed on the sheet 6 through the above-described process, and the test chart is completed. The test image printed on the test chart includes gradation patterns of each color. The gradation pattern of this embodiment is composed of rectangular patch images of different image densities for each color.
In this embodiment, a plurality of test images used for creating a test chart are prepared. The CPU 509 causes the pattern generator 507 to generate a test pattern signal of a test image according to the characteristics of the sheet 6 used for the test chart. Details of the test chart creation process will be described later.

When the test chart is created, the CPU 509 instructs the user to place the test chart on the document table glass 102. The CPU 509 displays, for example, on the operation unit 508 an image or text instructing to place the test chart on the document table glass 102. FIG. 5 is an exemplary diagram of such a guide image. The guide image provides guidance on how to place the test chart on the document table glass 102, in which direction, and so on. After placing the test chart on the document table glass 102 according to the guide image, the user inputs an instruction to read the test chart using the operation unit 508. The reading instruction is input by pressing the "Start reading" button on the guide image. When the CPU 509 receives the test chart reading instruction, the CPU 509 performs image reading processing using the reader unit A (S202). As a result, the printer control unit 109 obtains image data from the reader unit A, which is the result of reading the test image formed on the test chart. The image data is LOG-converted by the printer control unit 109 and input to the CPU 509 as density signals for each color.

The CPU 509 stores in the memory 510 the relationship between the density signal (image density) of each color acquired by the printer control unit 109 and the output level of the laser light instructed by the test pattern signal (S203). The CPU 509 calculates a correction value for the gradation characteristics from the image density of the test image stored in the memory 510 and the output level of the laser beam, and generates a γLUT. The CPU 509 updates the γLUT by setting the generated γLUT in the γLUT section 505 (S204).

(Variations in sheet properties and image density)
The image density detected from the test chart varies depending on the characteristics of the sheet used for the test chart. Examples of sheet properties that affect variations in image density include basis weight, thickness, unevenness, and smoothness. The basis weight is the mass of the sheet per 1 m ² and is expressed in s/m ² . Thickness is the distance between metal surfaces when a constant load is applied to the sheet between two smooth metal surfaces. Formation unevenness is two-dimensional structural unevenness of the sheet.

The smoothness of the sheet has the greatest influence on the variation in image density. Smoothness is the most important sheet property that governs image quality in laser printers for office use and on-demand machines for commercial printing. Smoothness is expressed in terms of the speed at which air leaks out of the gap created when a very smooth metal surface and the sheet surface come into contact, and the time (in seconds) it takes for a certain amount of air to leak out of the gap. A typical index is Beck smoothness. In this embodiment, Beck smoothness will be explained as a characteristic value of the sheet 6. The memory 510 stores in advance the smoothness of each type of sheet.

In this embodiment, as described above, a plurality of test images used for creating a test chart are prepared. Each test image has a different design. The test image used in the test chart is determined according to Bekk smoothness, which is a sheet characteristic. Note that the test image may be determined according to other sheet characteristics such as basis weight and texture. Here, a case will be described in which the sizes of patch images included in the test images are different. The size of the patch image is determined according to Beck smoothness.

FIG. 6 illustrates Bekk smoothness for each type of sheet. FIG. 6 shows the Beck smoothness of high-quality paper, embossed paper, and coated paper. The higher the Bekk smoothness, the smoother the surface of the sheet.

If variations in image density occur within a patch image, the image density cannot be detected accurately, making it impossible to perform highly accurate gradation correction. For this reason, in this embodiment, the size of a patch image formed on a sheet having a Bekk smoothness lower than a predetermined value (threshold value) is 10 [mm] x 10 [mm]. A sheet with a Beck smoothness higher than a predetermined value has smaller variations in image density than a sheet with a Beck smoothness lower than a predetermined value. Therefore, the size of a patch image formed on a sheet having a Bekk smoothness higher than a predetermined value can be made smaller than 10 [mm] x 10 [mm]. From this, it is possible to make the sizes of the patch images included in the plurality of test images different from each other.

FIG. 7 is an explanatory diagram of variations in image density detected from a three-tone gradation pattern. Here, a three-tone black gradation pattern of highlight (pixel data value 16), halftone (pixel data value 90), and solid (pixel data value 255) is printed on A4-sized high-quality paper. An example in which is printed will be explained. The size of the patch image of each gradation pattern is 10 [mm] x 10 [mm]. Image density detection is performed at intervals of 10 [mm]. In the example of FIG. 7, the standard deviation is 0.008 on the highlight side, 0.007 on the halftone side, and 0.006 on the solid side. All of these are acceptable as detection variations in image density used for gradation correction.

The variation in image density on the sheet depends on the unevenness of the surface of the sheet and the size of the unevenness. If there is not enough toner to cover the irregularities on the surface of the sheet during image formation, the surface of the sheet will be exposed from the toner and image unevenness will likely occur. This image unevenness causes variations in image density.

Embossed paper has surface irregularities with a width of about 1 [mm] to 5 [mm]. Therefore, the size of the patch image formed on the embossed paper is 10 [mm] x 10 [mm], which is larger than the width of the unevenness. A highly smooth sheet has surface irregularities with a width of 0.1 [mm] to 1 [mm]. Therefore, the size of the patch image formed on the highly smooth sheet can be set to 6 [mm] x 6 [mm], which is larger than the width of the unevenness. Naturally, the size of the patch image is not limited to these values.

In this embodiment, the smoothness that makes it difficult for variations in image density to become apparent is set to 60 (smoothness of high-quality paper). In other words, the predetermined value for distinguishing between high and low smoothness is set to "60". A patch image formed on a sheet with a smoothness of 60 or more is formed in a smaller size than a patch image formed on a sheet with a smoothness of less than 60. In this embodiment, the size of a patch image formed on a sheet with a smoothness of 60 or more is 6 [mm] x 6 [mm], and the size of a patch image formed on a sheet with a smoothness of less than 60 is 10 [mm]×10[mm].

FIG. 8 is an explanatory diagram of the relationship between sheet type (characteristics) and patch image size. For high-quality paper or coated paper whose smoothness is higher than a predetermined value, the patch image size is smaller than for other types of sheets whose smoothness is lower than a predetermined value. By reducing the size of the patch image, the amount of toner used during gradation correction can be reduced.

FIG. 9 is an exemplary diagram of a test image. FIG. 9A illustrates a test image in which the patch image size is large and is formed on a sheet whose smoothness is lower than a predetermined value. FIG. 9A illustrates a test image in which the patch image size is small and is formed on a sheet with a smoothness higher than a predetermined value. In this way, two patterns are prepared for the test image of this embodiment. Note that more patterns may be prepared. That is, test images may be prepared with three or more patch image sizes depending on the sheet characteristics.

The test images in FIGS. 9A and 9B include a gradation pattern composed of 16 gradation patch images for each color. The values of image data used to form each patch image are 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256. 256 represents a solid image. In addition to this, images with image data values of 0 can be detected.

When reducing the size of patch images, test images may be formed by reducing the margins between patch images. In this case, the light emitting time of the reader unit A (light source 103) for reading the test image is shortened, leading to a longer lifespan of the light source 103. The layout of patch images may be adjusted as appropriate depending on the characteristics of the image forming apparatus and the size of the sheet used.

The sheet characteristics (smoothness for each type of sheet) are registered in advance in the memory 510 by the user using the operation unit 508. Furthermore, the size and layout pattern of patch images may also be stored in the memory 510 in advance. The CPU 509 causes the pattern generator 507 to generate a test pattern signal of a test image according to the characteristics of the sheet used for the test chart. The CPU 509 may cause the pattern generator 507 to generate a test pattern signal with a layout according to the size of the patch image.

FIG. 10 is a flowchart illustrating test image selection processing. This process is performed during the process of S201 in FIG.

When creating a test chart, the CPU 509 acquires information representing the type of sheet used for the test chart (S1). Information indicating the type of sheet is input by the user using the operation unit 508 to select from which paper feed cassette the sheet is to be fed, for example, when there are multiple paper feed cassettes containing sheets. The types of sheets to be accommodated in the paper feed cassette are set at the time of accommodation and are stored in the memory 510. FIG. 11 is an exemplary diagram of a selection image of a paper feed cassette. The user selects a paper feed cassette from the selection image using the operation unit 508. The CPU 509 acquires from the memory 510 information representing the type of sheet accommodated in the selected paper feed cassette. Note that when there is one paper feed cassette, the types of sheets to be accommodated in the paper feed cassette are set in advance.

The CPU 509 checks whether the smoothness of the sheet is a predetermined value, 60 or more in this embodiment, based on the acquired information representing the type of sheet (S2). The smoothness is registered in the memory 510 in advance for each type of sheet that can be used in the image forming apparatus. The CPU 509 performs this process by referring to the registered smoothness, confirming the smoothness of the sheet used for the test chart, and comparing the confirmed smoothness with a predetermined value (60).

If the smoothness is equal to or greater than a predetermined value, that is, equal to or greater than 60 (S2: Y), the CPU 509 determines that the size of the patch image of the test image to be printed on the sheet may be small. In this case, the CPU 509 causes the pattern generator 507 to generate a test pattern signal of a test image including gradation patterns of each color configured by a rectangular patch image with a size of 6 [mm] x 6 [mm]. The CPU 509 creates a test chart by printing a test image according to this test pattern signal on a sheet (S3).

If the smoothness is less than a predetermined value, that is, less than 60 (S2:N), the CPU 509 determines that the size of the patch image of the test image to be printed on the sheet must be large. In this case, the CPU 509 causes the pattern generator 507 to generate a test pattern signal of a test image including gradation patterns of each color configured by a rectangular patch image with a size of 10 [mm] x 10 [mm]. The CPU 509 creates a test chart by printing a test image corresponding to this test pattern signal on a sheet (S4).

The test chart created as described above is read by the reader unit A in the process of S202 in FIG. The CPU 509 generates a γLUT based on the reading result of the test chart by the reader unit A, and sets it in the γLUT unit 505. As described above, tone correction processing is performed using a test image formed with an optimal design depending on the type of sheet.

By reducing the size of the patch image, the amount of toner used during gradation correction is suppressed. For example, when using a rectangular patch image with a size of 6 [mm] x 6 [mm], the amount of toner used is approximately less than when using a rectangular patch image with a size of 10 [mm] x 10 [mm]. It will be reduced by 1/3.

(Inline method)
In this embodiment, a test image is printed on a sheet and gradation correction is performed. The gradation correction performed in this embodiment can be performed off-line, in which the user manually reads the test chart with the reader unit A, or in-line, in which the test chart is automatically read without the user's intervention. FIG. 12 is a configuration diagram of an image forming apparatus that reads a test chart inline. This image forming apparatus has a configuration in which a reading device 50 is added to the image forming apparatus shown in FIG. The configuration of the reading device 50 will be explained.

The reading device 50 is provided at a destination where printed matter (sheets) are discharged from the printer section B. The reading device 50 includes a first conveyance section 51 , a second conveyance section 52 , a first reading sensor 53 , and a second reading sensor 54 . The reading device 50 is configured such that when images are formed on both sides of the sheet 6 by the printer section B, the first reading sensor 53 and the second reading sensor 54 can simultaneously read the images on both sides of the sheet 6. ing. The result of reading the test image by the reading device 50 is processed by the printer control unit 109.

The first reading sensor 53 reads an image printed on one surface (lower surface) of the sheet 6 conveyed by the first conveyance section 51 . The first reading sensor 53 is, for example, an optical sensor such as a line scanner that can perform image reading processing at a relatively high speed. The first reading sensor 53 reads the image formed on the sheet 6 and sends image data of each color of R, G, and B to the printer control unit 109 as the reading result. The second reading sensor 54 reads the image printed on the other surface (upper surface) of the sheet 6 transported by the second transport section 52 . The second reading sensor 54 has the same configuration as the first reading sensor 53, and sends image data of each color of R, G, and B to the printer control unit 109 as a reading result. The processing of the printer control unit 109 is the same as when using the reader unit A.

By using the reading device 50 as described above, in-line gradation correction becomes possible. That is, an image density detection section such as the reading device 50 is provided in the image forming apparatus and detects the gradation pattern of the test image inline. A test image is additionally printed on the sheet 6 in a printable margin area outside the image to be printed according to a print job instruction by the user. This test image is read and gradation correction is performed. If the smoothness of the sheet 6 is higher than a predetermined value, a test image composed of a gradation pattern including small-sized patch images is used. In the sheet 6 whose smoothness is higher than a predetermined value, not only the variation in the image density detection result but also the variation in the color detection result is reduced, so that the size of the patch image for color detection can also be reduced.

(Other examples of test images)
For the test image, in addition to reducing the size of the patch image according to the characteristics of the sheet, the number of gradations of the gradation pattern may be reduced to reduce the amount of toner used. As explained with reference to FIG. 7, a sheet with good surface properties (high smoothness) makes it difficult for image unevenness to become apparent, so that the error in detecting image density due to unevenness within a patch image is small. Therefore, even if the number of patch images included in a gradation pattern for performing gradation correction is reduced and the number of gradations is reduced, the accuracy of gradation correction is less affected. Since the number of patch images used for gradation correction is reduced, the amount of developer (toner) used can be reduced.

FIG. 13 is an exemplary diagram of a test image including a gradation pattern with a reduced number of patch images. In a test image printed on a sheet such as embossed paper whose smoothness is lower than a predetermined value, one gradation pattern is composed of 16 patch images (FIG. 9(a)). A test image printed on a sheet with a smoothness higher than a predetermined value, such as high-quality paper, has one gradation pattern composed of 8 patch images (FIG. 13). In this example, since the number of patch images is halved compared to the test image in FIG. 9A, the amount of toner (developer amount) consumed is also suppressed to approximately half.

As described above, the image forming apparatus of this embodiment is capable of printing a plurality of test images with different designs. The image forming apparatus generates adjustment values for adjusting image quality using a test image with an optimal design depending on the characteristics of the sheet. The test image is composed of a combination of multiple patch images. The design of the test image is determined by the size of the patch image, the layout of the patch image, the number of patch images used in the test image, and the like. For this purpose, the pattern generator 507 can generate test pattern signals for a plurality of test images having different patch image sizes, patch image layouts, and the number of patch images used in the test images. Depending on the design, the test images use different amounts of developer. By using such a test image, it is possible to perform image quality adjustment with high precision and reduce the amount of developer used for image quality adjustment.

Claims (4)

an image forming means for forming an image on the sheet;
reading means for reading the image formed on the sheet;
control means for causing the image forming means to form a test image on the sheet and controlling the density of the image formed by the image forming means based on the reading result of the test image by the reading means;
The control means includes:
obtaining information regarding the smoothness of the sheet on which the test image is formed;
when the test image is formed on a sheet having a smoothness lower than a predetermined value, causing the image forming means to form a test image with a first number of gradations;
When the test image is formed on a sheet having a smoothness higher than the predetermined value, causing the image forming means to form a test image with a second number of gradations smaller than the first number of gradations. characterized by
Image forming device. The image forming means has a conversion means for converting image data based on conversion conditions,
The control means controls the density of the image formed by the image forming means by generating the conversion condition based on the reading result of the test image by the reading means .
The image forming apparatus according to claim 1. The reading means is a reader that includes a document table on which a document is placed and a reading unit that reads the document placed on the document table, and outputs image data regarding the document to the image forming device. characterized by a certain
The image forming apparatus according to claim 1 or 2. The image forming means has a fixing means for fixing the image on the sheet,
The reading means is provided downstream of the fixing means in the conveying direction in which the sheet is conveyed ,
The image forming apparatus according to claim 1 or 2 .

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