Image-forming device

Abstract

To perform conversion suitable for a type of calibration.SOLUTION: A test image for first calibration is formed by image-forming means, a first signal based on a light reception result of reflection light from the test image for first calibration outputted by detection means is amplified by amplification means, first density information is determined from the first amplified signal on the basis of a first determination condition, and gradation characteristics of an image that should be formed by the image-forming means is adjusted on the basis of the first density information. A test image for second calibration is formed by the image-forming means, a second signal based on a light reception result of reflection light from the test image for second calibration outputted by the detection means is amplified by the amplification means, second density information is determined from the second amplified signal on the basis of a second determination condition, and irregularities in the density of an image that should be formed by the image-forming means is adjusted on the basis of the second density information. The amplification factor of the second signal is higher than the amplification factor of the first signal.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] In an image forming apparatus, a reading device reads an original, and the sensor output (brightness signal) value output from the sensor of the reading device is converted into a density signal, and an image is formed on a sheet based on the density signal (Patent Document 1). Also, in an image forming apparatus in which toner of different particle sizes is replenished to a developing device, the result (measured value) of measurement of a measurement image for density control by a measuring unit is converted based on a conversion characteristic selected from a plurality of conversion characteristics according to the average particle size of the toner in the developing device (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-249503 [Patent Document 2] Japanese Patent Application Publication No. 2019-101302 Summary of the Invention [Problem to be solved by the invention]

[0004] A converter that converts an input signal into an output signal has a limit to the data size it can output. For example, a converter that can output an 8-bit output signal (such as a density signal) can only output the signal in 256 steps, from 0 to 255. This means that if the range of output signal values ​​is widened, the resolution decreases, and if the resolution is increased, the range of output signal values ​​that can be output narrows.

[0005] When correcting the gradation characteristics of an image to be formed by an image forming apparatus, the conversion unit must be able to acquire density signals from a measurement image over a wide range, from low to high density. Conventional image forming apparatuses always acquire density signals from measurement images under the same conditions, regardless of the density of the measurement image. Conventional image forming apparatuses use conditions that allow measurement of measurement images ranging from low to high density to detect gradation characteristics. However, measurement images used to detect density unevenness have a narrower density range than measurement images used to detect gradation characteristics (measurement images ranging from low to high density). To detect density unevenness, density signals should be acquired with high resolution within this narrow density range (specific density range). Therefore, conventional image forming apparatuses have been unable to detect density unevenness with high accuracy. Therefore, an object of the present invention is to perform conversion appropriate for the type of calibration. [Means for solving the problem]

[0006] The present invention is, for example, an image forming means for forming an image; a detecting means for receiving reflected light from a test image formed by the image forming means and outputting a signal based on the result of receiving the reflected light from the test image; an amplifier for amplifying the signal output from the detector; a control means for performing calibration to adjust the image quality of the image to be formed by the image forming means, The control means causing the image forming means to form a test image for first calibration, causing the amplifier means to amplify a first signal based on a result of receiving reflected light from the test image for first calibration output by the detector, determining first density information from the amplified first signal based on a first determination condition, and adjusting the gradation characteristics of the image to be formed by the image forming means based on the first density information; causing the image forming means to form a test image for second calibration, causing the amplifier means to amplify a second signal based on the result of receiving reflected light from the test image for second calibration output by the detection means, determining second density information from the amplified second signal based on a second determination condition, and adjusting density unevenness of the image to be formed by the image forming means based on the second density information; The image forming apparatus is characterized in that the amplification factor of the second signal is higher than the amplification factor of the first signal. [Effects of the Invention]

[0007] According to the present invention, it is possible to perform conversion suitable for the type of calibration. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an image forming system. [Figure 2] FIG. 2 is a diagram illustrating a control system and an image processing system. [Figure 3] FIG. 10 is a diagram showing the relationship between process colors and channels of a scanned image. [Figure 4] FIG. 10 is a diagram illustrating a brightness-density conversion table. [Figure 5] FIG. 10 is an enlarged view illustrating a brightness-density conversion table. [Figure 6] FIG. 2 is a diagram illustrating the functions of a CPU. [Figure 7] FIG. 10 is a diagram illustrating a menu screen. [Figure 8] FIG. 2 is a diagram illustrating a test pattern. [Figure 9] 10 is a flowchart showing a method for creating a tone correction table. [Figure 10] 10 is a flowchart showing a method for creating correction values ​​for correcting density unevenness. [Figure 11] FIG. 1 is a diagram illustrating an image forming system. [Figure 12] FIG. 2 is a diagram illustrating a control system and an image processing system. [Figure 13] FIG. 2 is a diagram illustrating the functions of a CPU. [Figure 14] FIG. 2 is a diagram illustrating a test pattern. [Figure 15] 10 is a flowchart showing a method for creating a tone correction table. [Figure 16] 10 is a flowchart showing a method for creating correction values ​​for correcting density unevenness. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention claimed. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0010] 1. Example 1 1-1. Image forming system 1, a copying machine 100 is an example of an image forming system. The copying machine 100 includes an image forming device (hereinafter referred to as a printer) 120, an image processing device 122, a reading device (hereinafter referred to as a reader) 130, and an operation unit 140.

[0011] 1-2.Printer The printer 120 is an electrophotographic image forming device that forms full-color images using yellow, magenta, cyan, and black (Y, M, C, K) toners. The image forming unit 123, called an image forming engine, forms a toner image on a sheet P. The printer control unit 121 receives raster image data through the image processing device 122 and controls the image forming unit 123 to form a toner image corresponding to the raster image data on the sheet P. In FIG. 1, various components that form the image forming unit 123 are given reference symbols. Each reference symbol is accompanied by the letters YMCK. When matters common to all four colors are described, the letters YMCK are omitted from the reference symbols.

[0012] The photosensitive drum 1 is an image carrier driven by a motor M1 shown in FIG. 14 and rotated. The charging roller 2 uniformly charges the surface of the rotating photosensitive drum 1. The charging roller 2 may be replaced with another charging member, such as a charging wire. The exposure device 3 exposes the surface of the photosensitive drum 1 to light based on an image signal supplied from the printer control unit 121 or the image processing device 122, forming an electrostatic latent image. The developing roller 4, called a developing sleeve, develops the electrostatic latent image using toner held in a toner container to form a toner image. The primary transfer roller 5 faces the photosensitive drum 1 via the intermediate transfer belt 6. The primary transfer roller 5 transfers the toner image from the photosensitive drum 1 to the intermediate transfer belt 6. The primary transfer roller 5 may be replaced with a transfer member called a transfer blade. The Y toner image, M toner image, C toner image, and K toner image are transferred sequentially to the intermediate transfer belt 6 to form a full-color image.

[0013] The sheet cassette 7 is a storage container that stores a plurality of sheets P. A feed roller 8 feeds the sheet P from the sheet cassette 7. A plurality of conveyance rollers 9 provided in the conveyance path convey the sheet P to the secondary transfer nip. The secondary transfer nip is formed when the intermediate transfer belt 6 and the secondary transfer roller 10 come into contact with each other.

[0014] A secondary transfer roller 10 transfers the toner image from the intermediate transfer belt 6 to the sheet P. A fixing device 11 has a heating film and a pressure roller. The sheet P passes through a fixing nip formed by the heating film and the pressure roller. As a result, the sheet P and the toner image are subjected to heat and pressure, and the toner image is fixed onto the sheet P. The sheet P is then discharged into a sheet tray 12.

[0015] 1-3. Leader The reader 130 includes a document table 132, a reading unit 133, and an image processing unit 136. The document table 132 is a flat, translucent glass plate. The reading unit 133 irradiates light from a light-emitting element 134, which is an illumination light source, onto the document 131 and receives reflected light from the document 131 with a light-receiving element 135. The light-emitting element 134 is, for example, a light-emitting diode. The light-receiving element 135 is, for example, a line sensor equipped with an RGB color filter. The light-receiving element 135 separates the reflected light from the document 131 into three RGB color components using the color filter and receives the light, and outputs a luminance signal as the light-receiving result (reading result) to the image processing unit 136. The reading unit 133 reads the document 131 while moving relative to the document 131 in the sub-scanning direction of the document 131. The document 131 may be transported by an automatic document feeder (ADF) while the reading unit 133 remains stationary. The image processing unit 136 performs predetermined image processing on the luminance signal output from the light receiving element 135 and outputs the signal to the printer 120. For example, red, green, and blue (RGB) raster image data is generated by the predetermined image processing.

[0016] The operation unit 140 has an input device that accepts instructions input by the user and a display device that displays information to the user. The input device includes hardware keys, switches, and touch sensors. The display device includes, for example, a liquid crystal display.

[0017] 1-4. Control systems and image processing systems 2 shows the control system and image processing system. The printer control unit 121 is a controller that controls the operation of the printer 120. The printer control unit 121 includes a central processing unit (CPU) 211 and a storage device (hereinafter referred to as memory) 212. The memory 212 includes a read-only memory (ROM), a random access memory (RAM), etc.

[0018] The printer control unit 121 is connected to the image forming unit 123, the operation unit 140, the communication circuit 213, etc. The printer control unit 121 is also connected to a drive source (e.g., motor M1) that operates the printer 120, a sheet sensor, an environmental sensor, a power supply, etc.

[0019] The CPU 211 executes a control program stored in the memory 212 and controls the image forming unit 123 and the like in accordance with the control program. The memory 212 can also store a brightness-density conversion table, which will be described later, and the like. The CPU 211 is connected to the reader 130, the image processing device 122, and an external device (host computer, not shown) via a communication circuit 213 by wire or wirelessly, and performs two-way communication.

[0020] The reader control unit 230 is a controller including a CPU 231 and a memory 232. The reader control unit 230 communicates with the printer control unit 121 and the image processing device 122 via a communication circuit 233. The CPU 231 executes a control program stored in the memory 232 and controls the reading unit 133 and the image processing unit 136 in accordance with the control program.

[0021] The image processing unit 136 includes an analog processing unit 234 and an AD conversion unit 235. AD is an abbreviation for analog / digital. The analog processing unit 234 adjusts the gain (e.g., amplification factor) of the electrical signal obtained by the light receiving element 135 according to the gain set by the printer control unit 121. The reading unit 133 is an example of a sensor that receives reflected light from a test image formed by the image forming unit 123 and outputs a signal based on the result of receiving the reflected light from the test image. The analog processing unit 234 is an example of an amplifier that amplifies the signal output from the sensor. The AD conversion unit 235 generates a digital signal by performing analog-to-digital conversion on the electrical signal output from the analog processing unit 234. The image processing unit 136 further performs predetermined image processing (e.g., shading correction) on the digital signal to generate a data string of a luminance signal. This results in two-dimensional RGB luminance data (hereinafter referred to as a scanned image). The CPU 231 outputs the scanned image to the printer 120 via the communication circuit 233.

[0022] The CPU 231 changes or selects the light intensity of the light emitting element 134 and the gain of the analog processing unit 234 depending on the type of document 131 to be read. For example, when a test pattern for gradation correction is read, the light intensity and gain are set so that a density greater than or equal to 0 and less than or equal to 2.3 is converted to a luminance greater than or equal to 0 and less than or equal to 255 without saturating. Also, when a test pattern for density unevenness correction is read, the light intensity and gain are set so that a density greater than or equal to 0.6 and less than or equal to 1.4 is converted to a luminance greater than or equal to 0 and less than or equal to 255 without saturating. Here, the density may be a reflection density value calculated using a spectral reflection densitometer with a weighting coefficient of ISOstatusA.

[0023] The image processing device 122 has an image processing control unit 240, a communication circuit 243, and an image processing unit 244. The image processing control unit 240 is a controller having a CPU 241 and a memory 242. The CPU 241 executes a control program stored in the memory 242 and controls the image processing unit 244 in accordance with the control program. The CPU 241 performs various image processing on a print job input from an external device via the communication circuit 243 and outputs 1-bit YMCK image signals to the corresponding exposure devices 3Y, 3M, 3C, and 3K. The print job includes instructions for the printer 120. The instructions are written, for example, in a page description language (PDL). In addition to the print content (original), the print job may also include various setting information such as the type of sheet P, single-sided / double-sided printing, image position adjustment, magnification, rotation, page layout, color processing, character / thin line processing, header / footer addition, etc. Such a print job is generated by the printer control unit 121 or an external device. In the latter case, the user uses driver software or the like to make various settings on a user interface on a client device connected to the printer 120 via a network, thereby generating a print job.

[0024] The image processing unit 244 extracts image data from the input print job (PDL data) via an interpreter and renderer. Furthermore, the image processing unit 244 rasterizes the image data for each surface of the sheet P using raster image data processing (RIP) to generate raster image data, which is a collection of pixels with a predetermined resolution and bit depth. The image processing unit 244 color-converts the raster image data to generate a YMCK image signal, and applies gradation correction and density unevenness correction to the image signal. Furthermore, the image processing unit 244 converts the resolution of the image signal to match the writing resolution of the exposure device 3, binarizes the image signal using halftoning, and outputs the binarized image signal to the exposure device 3.

[0025] 1-5. Brightness density conversion The printer control unit 121 refers to a brightness-density conversion table stored in memory 212 and converts brightness information (brightness signal) of the scanned image acquired by the reader 130 into density information (density signal). The brightness-density conversion table is a table that receives the brightness value of one of the RGB channels in the scanned image as input and outputs a YMCK density value.

[0026] Figure 3 shows the correspondence between YMCK and RGB. The K density value is obtained by inputting the luminance value of the G channel of the scanned image into a K luminance-density conversion table. The subject of luminance-density conversion is a scanned image obtained from an output printed in a single color of YMCK. Here, an output printed in a single color refers to a sheet P on which an image is formed using one of the color materials of YMCK.

[0027] The memory 212 stores a plurality of brightness-density conversion tables in advance. The CPU 211 selects a brightness-density conversion table from the plurality of brightness-density conversion tables according to the type of the document 131 to be read. For example, when a test pattern for gradation correction is read, a brightness-density conversion table for gradation correction is selected. When a test pattern for density unevenness correction is read, a brightness-density conversion table for density unevenness correction is selected. In the brightness-density conversion table for gradation correction, a density greater than or equal to 0 and less than or equal to 2.3 corresponds to a brightness greater than or equal to 0 and less than or equal to 255. In the brightness-density conversion table for density unevenness correction, a density greater than or equal to 0.6 and less than or equal to 1.4 corresponds to a brightness greater than or equal to 0 and less than or equal to 255.

[0028] The test pattern for gradation correction is further associated with the light amount and gain for gradation correction. That is, the brightness-density conversion table for gradation correction and the light amount and gain for gradation correction form one parameter set. Similarly, the test pattern for density unevenness correction is associated with the light amount and gain for density unevenness correction. That is, the brightness-density conversion table for density unevenness correction and the light amount and gain for density unevenness correction form one parameter set.

[0029] Fig. 4(A) shows brightness-density conversion tables 401Y, 401M, 401C, and 401K for gradation correction. Fig. 4(B) shows brightness-density conversion tables 402Y, 402M, 402C, and 402K for density unevenness correction. Fig. 5(A) is an enlarged view of Fig. 4(A). Fig. 5(B) is an enlarged view of Fig. 4(B).

[0030] The convertible density range of the brightness-density conversion tables 401Y, 401M, 401C, and 401K for gradation correction is wider than the convertible density range of the brightness-density conversion tables 402Y, 402M, 402C, and 402K for density unevenness correction. In other words, the convertible density range of the brightness-density conversion tables 402Y, 402M, 402C, and 402K for density unevenness correction is narrower than the convertible density range of the brightness-density conversion tables 401Y, 401M, 401C, and 401K for gradation correction.

[0031] The density resolution of the brightness-density conversion tables 401Y, 401M, 401C, and 401K for gradation correction is lower than the density resolution of the brightness-density conversion tables 402Y, 402M, 402C, and 402K for density unevenness correction. In other words, the density resolution of the brightness-density conversion tables 402Y, 402M, 402C, and 402K for density unevenness correction is higher than the density resolution of the brightness-density conversion tables 401Y, 401M, 401C, and 401K for gradation correction. Here, the smaller the change in density relative to a change in brightness of one level, the higher the density resolution. The larger the change in density relative to a change in brightness of one level, the lower the density resolution.

[0032] Here, a brightness-density conversion table 401 for gradation correction and a brightness-density conversion table 402 for density unevenness correction are illustrated as examples. However, a third brightness-density conversion table may be further added. For example, a brightness-density conversion table may be prepared for each sheet type (e.g., basis weight, thickness). A brightness-density conversion table may be prepared for each destination of the copier 100. Furthermore, when multiple readers 130 are connected to the printer 120, a brightness-density conversion table may be provided for each reader 130. In this case, the CPU 211 or the CPU 241 selects a brightness-density conversion table corresponding to each type of sheet P, destination, or reader 130.

[0033] 1-6.CPU and image processing unit 6 shows functions realized by the CPU 211 and functions realized by the CPU 241 of the image processing device 122. The functions of the CPU 211 described below may be realized by the CPU 241. In other words, a configuration may be adopted in which some or all of the functions shown in FIG.

[0034] The image processing unit 244 includes a color conversion unit 601, a gradation correction unit 602, a non-uniformity correction unit 603, and a binarization unit 604. The color conversion unit 601 converts the scanned image acquired from the original 131 or the raster image data generated from PDL data from RGB format to YMCK format. The gradation correction unit 602 converts the input YMCK image signal using a gradation correction table (γLUT) created by an LUT creation unit 615. LUT is an abbreviation for lookup table. The non-uniformity correction unit 603 corrects the image signal output from the gradation correction unit 602 using a correction value created to reduce density non-uniformity that occurs in the main scanning direction or the sub-scanning direction. The binarization unit 604 converts the 8-bit image signal output from the non-uniformity correction unit 603 into a 1-bit image signal by halftoning processing and outputs the image signal to the exposure device 3.

[0035] 1-6-1. Creating γLUT When a command for gradation correction is given via the operation unit 140, the pattern generation unit 610 outputs an image signal corresponding to the test pattern for gradation correction to the binarization unit 604. As a result, a test pattern (test image) for gradation correction is formed on the sheet P. The sheet P on which the test image is formed may also be called a test chart.

[0036] The setting unit 611 sets the amount of light and gain for tone correction in the reader 130. The reader 130 reads the test chart and generates a scanned image.

[0037] The selection unit 612 selects the first conversion unit 613. This corresponds to selecting the brightness-density conversion table 401 for gradation correction. The selection unit 612 transfers the scanned image acquired from the reader 130 to the first conversion unit 613. The first conversion unit 613 converts the brightness information of the scanned image into density information using the brightness-density conversion table 401 for gradation correction. As shown in FIG. 3, the brightness-density conversion table 401Y converts the brightness information of the B channel into Y density information. The brightness-density conversion table 401M converts the brightness information of the G channel into M density information. The brightness-density conversion table 401C converts the brightness information of the R channel into C density information. The brightness-density conversion table 401K converts the brightness information of the G channel into K density information. The LUT creation unit 615 creates a γLUT for converting the image signal so that the gradation characteristics of the image formed on the sheet P become ideal gradation characteristics. A γLUT is created for each of Y, M, C, and K. The LUT creation unit 615 sets the γLUTs for each of YMCK in the tone correction unit 602 .

[0038] 1-6-2. Creating correction values ​​for density unevenness correction When an instruction to correct density unevenness is given via the operation unit 140, the pattern generation unit 610 outputs an image signal corresponding to the test pattern for correcting density unevenness to the binarization unit 604. As a result, a test pattern (test image) for correcting density unevenness is formed on the sheet P.

[0039] The setting unit 611 sets the light intensity and gain for density unevenness correction in the reader 130. The reader 130 reads the test chart and generates a scanned image. In the present disclosure, a configuration is adopted in which both the light intensity and the gain are changed, but it is sufficient to adopt a configuration in which at least the gain is changed. In this configuration, the gain for gradation correction and the gain for density unevenness correction are different, and the light intensity for gradation correction and the light intensity for density unevenness correction are the same.

[0040] The selection unit 612 selects the second conversion unit 614. This corresponds to selecting the brightness-density conversion table 402 for density unevenness correction. The selection unit 612 transfers the scanned image acquired from the reader 130 to the second conversion unit 614. The second conversion unit 614 converts the brightness information of the scanned image into density information using the brightness-density conversion table 402 for density unevenness correction. As shown in FIG. 3, the brightness-density conversion table 402Y converts the brightness information of the B channel into Y density information. The brightness-density conversion table 402M converts the brightness information of the G channel into M density information. The brightness-density conversion table 402C converts the brightness information of the R channel into C density information. The brightness-density conversion table 402K converts the brightness information of the G channel into K density information. The correction value creation unit 616 creates correction values ​​that reduce density unevenness in the image formed on the sheet P. The correction values ​​are created for each position or block in the main scanning direction. The unevenness correction unit 603 corrects the image signal using a correction value corresponding to each pixel in the main scanning direction. In a second embodiment described later, a correction value for reducing density unevenness in the sub-scanning direction is generated.

[0041] 1-7. User Interface (UI) 7 shows a UI displayed on the display device of the operation unit 140. Button 701 is a button for instructing the copier 100 to make a copy of the original 131. Button 702 is a button for instructing the copier 100 to make a gradation correction. Button 703 is a button for instructing the copier 100 to make corrections to density unevenness in the main scanning direction. Button 704 is a button for instructing the copier 100 to make corrections to density unevenness in the sub-scanning direction. Correction of density unevenness in the sub-scanning direction will be described in detail in the second embodiment.

[0042] 1-8. Test Pattern 8A shows a test pattern 801 for tone correction. The test pattern 801 includes patch images with n different tones for each color of YMCK. Here, n is assumed to be 10, for example.

[0043] 8B shows a test pattern 802 for correcting density unevenness. The test pattern 802 includes patch images of a certain tone that extend in the main scanning direction for each of the colors YMCK.

[0044] 1-9. Flowchart 1-9-1. Tone Correction 9 shows a method for creating a γLUT executed by the CPU 211. As described above, the γLUT is a table for maintaining a constant relationship between the input image signal and the output density for each color of YMCK (hereinafter referred to as the gradation characteristics). The following steps may be executed by the CPU 241. In that case, the CPU 211 will be read as the CPU 241 in the following description.

[0045] In S901, the CPU 211 displays a menu screen on the display device of the operation unit 140. As shown in Fig. 7, the menu screen has a button 701 for instructing the execution of gradation correction. When the CPU 211 detects that the button 701 has been pressed by the user, the process proceeds from S901 to S902.

[0046] In S902, the CPU 211 (pattern generation unit 610) controls the image forming unit 123 to form the test pattern 801 on the sheet P. Specifically, the pattern generation unit 610 supplies an image signal that is the source of the test pattern 801 to the binarization unit 604. The binarization unit 604 binarizes the input image signal and supplies it to the exposure device 3. As a result, the test pattern 801 is formed on the sheet P. The sheet P (test chart) on which the test pattern 801 has been formed is discharged to the sheet tray 12.

[0047] In S903, the CPU 211 displays guidance on the display device of the operation unit 140. This guidance includes a message and an image instructing the user to place a test chart on the platen 132, the orientation of the test chart on the platen 132, etc. The guidance may also include a message or an image instructing the user to press a button to start reading the test chart. This button may be a button indicating that the user has completed placing the test chart on the platen 132.

[0048] In S904, the CPU 211 determines whether or not a read instruction has been input via the operation unit 140. If a read instruction has been input, the CPU 211 proceeds from S904 to S905.

[0049] In S905, the CPU 211 (setting unit 611) sets the amount of light and gain for tone correction in the reader 130.

[0050] In S906, the CPU 211 causes the reader 130 to read the test pattern 801. The reader 130 generates and transfers a scanned image of the test pattern 801. The printer control unit 121 acquires the scanned image via the communication circuit 213 and stores the scanned image in the memory 212.

[0051] In S907, the CPU 211 (selection unit 612) selects the brightness-density conversion table 401 for gradation correction (first conversion unit 613).

[0052] In S908, the CPU 211 (first conversion unit 613) converts the luminance information (luminance value) of the scanned image into density information (density value) using the luminance-density conversion table 401 for tone correction.

[0053] In S909, the CPU 211 (LUT creation unit 615) calculates density statistics. As shown in FIG. 8A, the test pattern 801 includes N patch images, each with a different gradation. Therefore, the LUT creation unit 615 acquires density values ​​at M positions for each patch image (one gradation), and calculates a statistical value (e.g., average value) from the M density values. This process is performed for each of YMCK. N statistical values ​​are obtained for each color.

[0054] In S910, the CPU 211 (LUT creation unit 615) generates a γLUT. For example, the LUT creation unit 615 generates a γLUT from N statistical values ​​for density, the input image signal used to create the test pattern 801, and a gradation target.

[0055] In S911, the CPU 211 (LUT creation unit 615) stores the γLUT for each color of YMCK in the memory 212. This enables the tone correction unit 602 to refer to the γLUT.

[0056] In S912, the CPU 211 notifies the user of the completion of the tone correction (γLUT creation process). For example, the CPU 211 may display a message on the display device of the operation unit 140 indicating that the tone correction has been completed.

[0057] 1-9-2. Correction of density unevenness in the main scanning direction Density unevenness correction in the main scanning direction is a process that corrects an image signal using a correction value to maintain uniform density in the main scanning direction for a given input image signal. Specifically, the image signal is corrected so that the density profile in the main scanning direction is uniform. Density unevenness correction requires high-resolution detection of densities within a predetermined range. For this reason, a set of light intensity, gain, and brightness-density conversion table 402 for density unevenness correction is used. In this embodiment, the patch image for each color of YMCK is divided into K blocks, and a density value is calculated for each block. K is, for example, 30.

[0058] 10 shows a method for creating correction values ​​executed by the CPU 211. The following steps may be executed by the CPU 241. In that case, the CPU 211 will be read as the CPU 241 in the following description.

[0059] In S1001, the CPU 211 displays a menu screen on the display device of the operation unit 140. As shown in Fig. 7, the menu screen has a button 703 for instructing the execution of density unevenness correction. When the CPU 211 detects that the button 703 has been pressed by the user, the process proceeds from S1001 to S1002.

[0060] In S1002, the CPU 211 (pattern generation unit 610) controls the image forming unit 123 to form the test pattern 802 on the sheet P. Specifically, the pattern generation unit 610 supplies an image signal that is the source of the test pattern 802 to the binarization unit 604. The binarization unit 604 binarizes the input image signal and supplies it to the exposure device 3. As a result, the test pattern 802 is formed on the sheet P. The sheet P (test chart) on which the test pattern 802 has been formed is discharged to the sheet tray 12.

[0061] In S1003, the CPU 211 displays guidance on the display device of the operation unit 140. This guidance includes a message and an image instructing the user to place a test chart on the platen 132 and the orientation of the test chart on the platen 132. The guidance may also include a message or an image instructing the user to press a button to start reading the test chart. This button may be a button indicating that the user has completed placing the test chart on the platen 132.

[0062] In S1004, the CPU 211 determines whether or not a read instruction has been input via the operation unit 140. When a read instruction has been input, the CPU 211 proceeds from S1004 to S1005.

[0063] In S1005, the CPU 211 (setting unit 611) sets the light amount and gain for density unevenness correction in the reader 130.

[0064] In S1006, the CPU 211 causes the reader 130 to read the test pattern 801. The reader 130 generates a scanned image of the test pattern 802 and transfers the scanned image to the printer control unit 121 using the communication circuit 213. The printer control unit 121 acquires the scanned image via the communication circuit 213 and stores the scanned image in the memory 212.

[0065] In S1007, the CPU 211 (selection unit 612) selects the brightness-density conversion table 402 (second conversion unit 614) for density unevenness correction.

[0066] In S1008, the CPU 211 (second conversion unit 614) converts the luminance information (luminance value) of the scanned image into density information (density value) using the luminance-density conversion table 402 for density correction.

[0067] In S1009, the CPU 211 (correction value creation unit 616) calculates density statistics. As shown in FIG. 8B, the test pattern 802 has patch images for each of Y, M, C, and K extending parallel to the main scanning direction. The correction value creation unit 616 divides each patch image into K small regions (blocks). For each block (main scanning position), the correction value creation unit 616 obtains density values ​​at J positions and calculates a statistical value (e.g., average value) from the J density values. This process is performed for each of Y, M, C, and K. K statistical values ​​are obtained for each color.

[0068] In S1010, the CPU 211 (correction value creation unit 616) calculates a correction value for the image signal for each block. For example, the correction value creation unit 616 calculates a correction value for the image signal for each block from K statistical values ​​for density and a gradation target. The correction value is calculated for each color of YMCK.

[0069] In S1011, the CPU 211 (correction value creation unit 616) saves the correction values ​​for each color of YMCK in the memory 212. This enables the unevenness correction unit 603 to refer to the correction values. The correction values ​​may be stored in the memory 212 for each pixel aligned in the main scanning direction.

[0070] In S1012, the CPU 211 notifies the user of the completion of the density unevenness correction (creation of correction values). For example, the CPU 211 may display a message on the display device of the operation unit 140 indicating that the density unevenness correction has been completed.

[0071] 1-10.Other According to the first embodiment, a brightness-density conversion table 401 for gradation correction and a brightness-density conversion table 402 for density unevenness correction are prepared. In the brightness-density conversion table 401 for gradation correction, a wide density range is prioritized over density resolution. In the brightness-density conversion table 402 for density unevenness correction, a high density resolution is prioritized over a wide density range. By selectively using the two brightness-density conversion tables, the CPU 211 can achieve both a wide density range and high density resolution.

[0072] As long as two brightness-density conversion tables are used appropriately, the first embodiment may be modified. For example, the bit depth of the scanned image, the bit depth of the brightness-density conversion table, the test pattern 801 for gradation correction, the test pattern 802 for density unevenness correction, the gradation target, the number of blocks in the main scanning direction, etc. may be changed. In the first embodiment, a correction value for correcting density unevenness in the main scanning direction is calculated using the test pattern 802 of a single gradation. However, correction values ​​may be calculated for each of multiple gradations. In this case, the test pattern 802 would have patch images of multiple gradations for each color. Furthermore, a brightness-density conversion table 402 corresponding to each gradation may be used.

[0073] The start conditions for tone correction and density unevenness correction do not need to be specified by a user. For example, the start condition may be that a component involved in image formation (e.g., a process cartridge or exposure device 3) has been replaced. The start condition may be that the cumulative number of printed sheets has exceeded a threshold number. The start condition may also be that a sensor has detected a predetermined state.

[0074] 2. Example 2 In the second embodiment, tone correction and density unevenness correction in the sub-scanning direction performed using a reading device connected in series to the image forming apparatus will be described.

[0075] FIG. 11 shows an image forming system 1100. FIG. 12 shows a control system and an image processing system. The image forming system 1100 has an image processing device 122, a printer 120, and a reading device (hereinafter referred to as a sensing unit) 1110. Most of the second embodiment is common to the first embodiment. Therefore, a description of the common parts will be omitted. The following description will mainly focus on the unique features of the second embodiment.

[0076] As shown in FIG. 11, the sensing unit 1110 has a reading unit 133. The reading unit 133 reads an image from the sheet P that is discharged from the printer 120 to the sensing unit 1110 and conveyed along the conveyance path by the conveyance rollers 19. As described above, the reading unit 133 has a light-emitting element 134, an imaging optical system 137, and a light-receiving element 135. The sheet P is discharged to the sheet tray 12 provided in the sensing unit 1110. The reader 130 of the first embodiment moves the reading unit 133 in the sub-scanning direction while causing the reading unit 133 to read the sheet P. The sensing unit 1110 causes the reading unit 133 to read the sheet P while keeping the reading unit 133 fixed. In either case, the reading unit 133 and the sheet P (original 131) move relatively in the sub-scanning direction.

[0077] In printer 120, photosensitive drums 1Y, 1M, 1C, and 1K have rotational phase sensors 21Y, 21M, 21C, and 21K, respectively. Rotational phase sensor 21 detects the rotational phase of photosensitive drum 1 and transmits the detection result to printer control unit 121. When printer control unit 121 receives a print job, it drives photosensitive drum 1 to rotate and adjusts the rotational phase so that the rotational phase of photosensitive drum 1 at the leading edge of the image is 0.

[0078] The rotational phase sensor 21 includes, for example, a photointerrupter attached to the printer 120 and a light-shielding plate attached to the cylinder of the photosensitive drum 1. The photointerrupter includes a light-emitting element and a light-receiving element. The light-shielding plate passes between the light-emitting element and the light-receiving element, blocking light from the light-emitting element toward the light-receiving element. Therefore, the rotational phase sensor 21 detects the rotational phase based on whether the photointerrupter is in a light-shielding state or a light-transmitting state. When the photointerrupter is in a light-shielding state, the rotational phase sensor 21 outputs 1. When the photointerrupter is in a light-transmitting state, the rotational phase sensor 21 outputs 0.

[0079] The rotational phase of the photosensitive drum 1 takes on values ​​between 0 and 2π. The printer control unit 121 determines that the rotational phase of the photosensitive drum 1 is 0 when the output of the rotational phase sensor 21 rises from 0 to 1. Every time the photosensitive drum 1 rotates 360 degrees, the rotational phase advances by 2π.

[0080] FIG. 13 shows the functions of the CPU 211 and the image processing device 122. FIG. 14 shows a test pattern 1400 for correcting density unevenness in the sub-scanning direction. The phase control unit 1311 determines the rotational phase of the photosensitive drum 1 based on the detection result of the rotational phase sensor 21. To align the leading edge of the image on the photosensitive drum 1 with the rotational phase of 0, the phase control unit 1311 monitors the rotational phase detected by the rotational phase sensor 21. The phase control unit 1311 controls the motor M1 so that the rotational phase detected by the rotational phase sensor 21 becomes 0. Furthermore, at the timing when the rotational phase becomes 0, the phase control unit 1311 instructs the pattern generation unit 610 to output an image signal that is the basis of the test pattern 1400. As a result, the leading edge of the test pattern 1400 in the sub-scanning direction coincides with the rotational phase of 0 (which may also be called the reference phase or home position). 14, a test pattern 1400 includes patch images of a constant gradation (for example, 50%) that extend parallel to the sub-scanning direction. The patch images are formed for each of the YMCK colors.

[0081] The setting unit 611 sets the light intensity and gain for correcting density unevenness in the sub-scanning direction in the reader control unit 230 of the sensing unit 1110. The light intensity and gain for correcting density unevenness in the sub-scanning direction are associated with a brightness-density conversion table 402 for correcting density unevenness in the sub-scanning direction and stored in the memory 212. The brightness-density conversion table 402 for correcting density unevenness in the main scanning direction may be different from the brightness-density conversion table 402 for correcting density unevenness in the sub-scanning direction. As in the first embodiment, the reader control unit 230 lights up the light-emitting element 134 at the set light intensity. In addition, the reader control unit 230 sets a gain in the analog processing unit 234 of the image processing unit 136. The image processing unit 136 generates a scanned image of the test pattern 1400 and transmits it to the printer control unit 121.

[0082] The CPU 211 (selection unit 612) selects the brightness-density conversion table 402 (second conversion unit 614) for correcting density unevenness in the sub-scanning direction. The second conversion unit 614 converts the brightness information of the scanned image of the test pattern 1400 into density information using the brightness-density conversion table 402 for correcting density unevenness in the sub-scanning direction. The correction value creation unit 616 creates a correction value for each rotation phase based on the density information and stores it in the memory 212. The unevenness correction unit 603 reads out the correction value corresponding to the rotation phase from the memory 212 and corrects the image signal.

[0083] 2-1. Gradation correction flowchart 15 shows a method for creating a γLUT executed by the CPU 211. The following steps may be executed by the CPU 241. In that case, the CPU 211 will be read as the CPU 241 in the following description.

[0084] In S1501, the CPU 211 displays a menu screen on the display device of the operation unit 140. As shown in Fig. 7, the menu screen has a button 701 for instructing the execution of gradation correction. When the CPU 211 detects that the button 701 has been pressed by the user, the process proceeds from S1501 to S1502.

[0085] In S1502, the CPU 211 (pattern generation unit 610) controls the image forming unit 123 to form the test pattern 801 on the sheet P. Specifically, the pattern generation unit 610 supplies an image signal that is the source of the test pattern 801 to the binarization unit 604. The binarization unit 604 binarizes the input image signal and supplies it to the exposure device 3. As a result, the test pattern 801 is formed on the sheet P. The sheet P (test chart) on which the test pattern 801 has been formed is discharged to the sheet tray 12.

[0086] In S1503, the CPU 211 (setting unit 611) sets the amount of light and gain for tone correction in the reader 130.

[0087] In S1504, the CPU 211 causes the sensing unit 1110 to read the test pattern 801. The sensing unit 1110 generates a scanned image of the test pattern 801 and transfers the scanned image to the printer control unit 121 using the communication circuit 213. The printer control unit 121 acquires the scanned image via the communication circuit 213 and stores the scanned image in the memory 212.

[0088] In S1505, the CPU 211 (selection unit 612) selects the brightness-density conversion table 401 for tone correction (first conversion unit 613).

[0089] In S1506, the CPU 211 (first conversion unit 613) converts the luminance information (luminance value) of the scanned image into density information (density value) using the luminance-density conversion table 401 for tone correction.

[0090] In S1507, the CPU 211 (LUT creation unit 615) calculates density statistics. As shown in FIG. 8A, the test pattern 801 includes N patch images, each with a different gradation. Therefore, the LUT creation unit 615 acquires density values ​​at M positions for each patch image (one gradation), and calculates a statistical value (e.g., average value) from the M density values. This process is performed for each of YMCK. N statistical values ​​are obtained for each color.

[0091] In S1508, the CPU 211 (LUT creation unit 615) creates a γLUT from N statistical values ​​for density, the input image signal used to create the test pattern 801, and the gradation target.

[0092] In S1509, the CPU 211 (LUT creation unit 615) stores the γLUT for each color of YMCK in the memory 212. This enables the tone correction unit 602 to refer to the γLUT.

[0093] In S1510, the CPU 211 notifies the user of the completion of the tone correction (γLUT creation process). For example, the CPU 211 may display a message on the display device of the operation unit 140 indicating that the tone correction has been completed.

[0094] 2-2. Correction of density unevenness in the sub-scanning direction Periodic density unevenness may occur in synchronization with the rotational phase of the photosensitive drum 1. Therefore, a correction value for the image signal is created for each rotational phase of the photosensitive drum 1. This makes the density profile uniform in the sub-scanning direction. Even when correcting density unevenness in the sub-scanning direction, it is necessary to detect a predetermined range of density with high resolution. Therefore, a set of light intensity, gain, and brightness-density conversion tables for density unevenness correction is used. In the second embodiment, the patch images of each color of YMCK in the test pattern 1400 are divided into L blocks (sub-scanning positions or rotational phases). L is, for example, 60.

[0095] 16 shows a method for creating correction values ​​executed by the CPU 211. The following steps may be executed by the CPU 241. In that case, the CPU 211 will be read as the CPU 241 in the following description.

[0096] In S1601, the CPU 211 displays a menu screen on the display device of the operation unit 140. As shown in Fig. 7, the menu screen has a button 703 for instructing the execution of density unevenness correction. When the CPU 211 detects that the button 703 has been pressed by the user, the process proceeds from S1601 to S1602.

[0097] In S1602, the CPU 211 (pattern generation unit 610) controls the image forming unit 123 to form the test pattern 1400 on the sheet P. Specifically, the pattern generation unit 610 supplies an image signal that is the source of the test pattern 1400 to the binarization unit 604. The binarization unit 604 binarizes the input image signal and supplies it to the exposure device 3. As a result, the test pattern 1400 is formed on the sheet P. The sheet P (test chart) on which the test pattern 1400 has been formed is discharged to the sensing unit 1110.

[0098] In S1603, the CPU 211 (setting unit 611) sets the light amount and gain for correcting density unevenness in the sub-scanning direction in the sensing unit 1110. In S1604, the CPU 211 causes the sensing unit 1110 to read the test pattern 1400. The sensing unit 1110 generates a scanned image of the test pattern 1400 and transfers the scanned image to the printer control unit 121 using the communication circuit 233. The printer control unit 121 acquires the scanned image via the communication circuit 213 and stores the scanned image in the memory 212.

[0099] In S1605, the CPU 211 (selection unit 612) selects the brightness-density conversion table 402 (second conversion unit 614) for density unevenness correction in the sub-scanning direction. In S1606, the CPU 211 (second conversion unit 614) converts the brightness information (brightness value) of the scanned image into density information (density value) using the brightness-density conversion table 402 for density correction.

[0100] In S1607, the CPU 211 (correction value creation unit 616) calculates density statistics. As shown in FIG. 14, the test pattern 1400 has patch images for each of Y, M, C, and K extending parallel to the sub-scanning direction. The correction value creation unit 616 divides each patch image into L small regions (blocks). For each block (main scanning position), the correction value creation unit 616 obtains density values ​​at J positions and calculates a statistical value (e.g., average value) from the J density values. This process is performed for each of Y, M, C, and K. L statistical values ​​are obtained for each color.

[0101] In S1608, the CPU 211 (correction value generating unit 616) calculates a correction value for the image signal for each block from the L density statistical values ​​and the gradation target. The correction value is calculated for each color of YMCK.

[0102] In S1609, the CPU 211 (correction value creation unit 616) associates the correction value for each color of YMCK with the block number, rotation phase, or sub-scanning position, and stores it in the memory 212. This enables the unevenness correction unit 603 to refer to the correction value based on the block number, rotation phase, or sub-scanning position. The correction value may be stored in the memory 212 for each pixel aligned in the sub-scanning direction.

[0103] In S1610, the CPU 211 notifies the user of the completion of the density unevenness correction (creation of correction values). For example, the CPU 211 may display a message on the display device of the operation unit 140 indicating that the density unevenness correction has been completed.

[0104] 2-3.Other The effects described in the first embodiment are also expected in the second embodiment. That is, the CPU 211 can achieve both a wide density range and high density resolution by selectively using two brightness-density conversion tables.

[0105] The CPU 211 may perform frequency analysis of the density profile detected from the test pattern 1400 by the sensing unit 1110. The CPU 211 may create a correction value that corrects only density unevenness at a specific frequency determined by the frequency analysis. Furthermore, the CPU 211 may perform density unevenness correction in the main scanning direction in addition to gradation correction and density unevenness correction in the sub-scanning direction.

[0106] 3. Other Examples The printer 120 and the image forming unit 123 are examples of image forming means that form an image on a sheet P using a coloring material (e.g., toner). The first conversion unit 613, the second conversion unit 614, and the brightness-density conversion tables 401 and 402 are examples of first and second conversion conditions. These convert brightness information acquired from a reading device (e.g., reader 130, sensing unit 1110) that irradiates light onto a sheet P on which an image has been formed by the printer 120 and receives light reflected from the sheet or image to generate brightness information into density information. The light-emitting element 134 functions as an irradiation unit that irradiates light onto a test image formed by the image forming unit 123. The light-receiving element 135 functions as an output unit that receives light reflected from the test image and outputs a signal based on the result of receiving the light reflected from the test image. The CPUs 211 and 241 function as control units. A first calibration may be performed to correct the gradation characteristics of an image to be formed by the image forming unit 123. In this case, the CPUs 211 and 241 may convert a first signal based on the result of receiving reflected light from a first test image formed by the image forming unit 123 under a first conversion condition and correct the gradation characteristics based on the converted first signal. A second calibration may be performed to correct density unevenness in a predetermined direction of the image to be formed by the image forming unit 123. In this case, the CPUs 211 and 241 convert a second signal based on the result of receiving reflected light from a second test image formed by the image forming unit 123 under a second conversion condition and correct the density unevenness based on the converted second signal. As illustrated in FIGS. 4A and 4B, the range of density information convertible from luminance information under the first conversion condition is wider than the range of density information convertible from luminance information under the second conversion condition. As illustrated in FIGS. 5A and 5B, the resolution of density information convertible from luminance information under the second conversion condition is higher than the resolution of density information convertible from luminance information under the first conversion condition. In this way, using multiple conversion conditions makes it possible to achieve both a wide density range and high density resolution.

[0107] Luminance-density conversion table 401 is an example of a first conversion table that converts luminance to density. Luminance-density conversion table 402 is an example of a second conversion table that converts luminance to density. As shown in Figures 4(A) and 4(B), the difference between the maximum and minimum densities in the second conversion table is smaller than the difference between the maximum and minimum densities in the first conversion table.

[0108] The LUT creation unit 615 is an example of a first creation unit that creates first image formation conditions (e.g., γLUT) to be applied to the image forming unit based on a first read result obtained by a reading device for a first test image (e.g., test pattern 801) formed on the sheet P. The correction value creation unit 616 is an example of a second creation unit that creates second image formation conditions (e.g., correction values) to be applied to the image forming unit based on a second read result obtained by a reading device for a second test image (e.g., test patterns 802, 1400) formed on the sheet P. The LUT creation unit 615 may convert luminance information about the first test image, which is the first read result, into density information using first conversion conditions (e.g., first conversion unit 613), and create first image formation conditions based on the density information. The correction value creation unit 616 may convert luminance information about the second test image, which is the second read result, into density information using second conversion conditions (e.g., second conversion unit 614), and create second image formation conditions based on the density information.

[0109] The first image forming conditions may include first image processing conditions (e.g., γLUT) for correcting the gradation characteristics of the image, and the second image forming conditions may include second image processing conditions (e.g., density correction values) for reducing density unevenness in the image.

[0110] The first image processing condition may include a gradation correction table (e.g., γLUT) that receives an image signal as input and outputs an image signal with corrected gradation characteristics. The second image processing condition may include a correction value for the density of each position or block where an image is formed on the sheet P.

[0111] As described in the second embodiment, the density correction value may include a correction value for reducing unevenness in density in a first direction (e.g., sub-scanning direction) parallel to the conveying direction P of the sheet P. As described in the first embodiment, the density correction value may include a correction value for reducing unevenness in density in a second direction (e.g., main scanning direction) perpendicular to the conveying direction P of the sheet.

[0112] The gradation correction unit 602 functions as a gradation correction unit that corrects the gradation characteristics of the image signal that is the source of the image using a gradation correction table (e.g., γLUT). The unevenness correction unit 603 functions as a density correction unit that corrects the image signal that is the source of the image using a density correction value.

[0113] The setting unit 611 functions as a setting unit that sets the light intensity of the light-emitting element 134 provided in the reader 130 and the gain of the light-receiving element 135 provided in the reader 130. As described in relation to S905 and S1503, the setting unit 611 sets a first light intensity and a first gain in the reader 130 when reading a first test image. As described in relation to S1005 and S1603, the setting unit 611 may be configured to set a second light intensity and a second gain in the reader 130 when reading a second test image.

[0114] The selection unit 612 functions as a selection unit that selects the first conversion condition or the second conversion condition. The selection unit 612 selects the first conversion condition when creating the first image forming condition to be applied to the image forming unit (e.g., image forming unit 123, image processing unit 244). The selection unit 612 selects the second conversion condition when creating the second image forming condition to be applied to the image forming unit (e.g., image forming unit 123, image processing unit 244). In this way, the multiple brightness-density conversion tables are used appropriately depending on the application.

[0115] The selection unit 612 may select a first conversion condition when a first adjustment mode (e.g., gradation correction) is designated from among a plurality of adjustment modes for adjusting the density of an image, and may select a second conversion condition when a second adjustment mode (e.g., density unevenness correction) is designated from among a plurality of adjustment modes for adjusting the density of an image.

[0116] The reading device is provided downstream of the image forming means in the sheet conveying direction, and may include an in-line image sensor (for example, sensing unit 1110, light receiving element 135) that reads an image from the conveyed sheet.

[0117] The reading device may include an offline image sensor (for example, a reader 130, a light receiving element 135) that reads an image from a sheet P that is ejected from the printer 120 and placed by a user.

[0118] The printer control unit 121, CPU 211, reader control unit 230, CPU 231, image processing control unit 240, and CPU 241 are examples of a control unit (controller) that performs calibration to adjust the image quality of an image to be formed by an image forming unit. The controller may cause the image forming unit 123 to form a test image for a first calibration, cause the analog processing unit 234 to amplify a first signal based on the result of receiving reflected light from the test image for the first calibration output by the reading unit 133, determine first density information from the amplified first signal based on a first determination condition, and adjust the gradation characteristics of the image to be formed by the image forming unit 123 based on the first density information. The controller may cause the image forming unit 123 to form a test image for a second calibration, cause the analog processing unit 234 to amplify a second signal based on the result of receiving reflected light from the test image for the second calibration output by the reading unit 133, determine second density information from the amplified second signal based on a second determination condition, and adjust density unevenness of the image to be formed by the image forming unit 123 based on the second density information. The amplification factor of the second signal is higher than the amplification factor of the first signal.

[0119] The reading unit 133 may have a light emitting element 134 that illuminates the test image. The amount of light with which the light emitting element 134 illuminates the test image for the first calibration may be different from the amount of light with which the light emitting element 134 illuminates the test image for the second calibration.

[0120] The range of densities that can be output as the first density information may be wider than the range of densities that can be output as the second density information.

[0121] The memory 212 stores various brightness-density conversion tables 401Y, 401M, 401C, 401K, 402Y, 402M, 402C, and 402K. The first determination condition may be a density conversion table (e.g., brightness-density conversion tables 401Y, 401M, 401C, and 401K) used to convert a digital signal with a predetermined number of bits into first density information in a first density range. The second determination condition may be a density conversion table (e.g., brightness-density conversion tables 402Y, 402M, 402C, and 402K) used to convert a digital signal with a predetermined number of bits into second density information in a second density range narrower than the first density range.

[0122] As shown in Fig. 8(A), the test image for the first calibration (e.g., test pattern 801) may have an image with multiple different gradations. As shown in Fig. 8(B), the test image for the second calibration (e.g., test patterns 802, 1400) may have a single gradation.

[0123] The conveying rollers 9 convey the sheet P to the conveying path. The image forming unit 123 forms an image on the sheet P conveyed by the conveying rollers 9. Here, the second calibration may be a calibration that adjusts density unevenness in a direction perpendicular to the conveying direction in which the conveying rollers 9 convey the sheet P. The second calibration may be a calibration that adjusts density unevenness in the conveying direction in which the conveying rollers 9 convey the sheet P.

[0124] The resolution of the signal value as the second concentration information may be higher than the resolution of the signal value as the first concentration information.

[0125] The image forming unit 123 may have an image processing unit 244 that converts the input image signal based on the gradation correction conditions. The image forming unit 123 forms an image based on the converted image signal. The controller (e.g., CPU 211, 241) may generate the gradation correction conditions based on the first density information in the first calibration.

[0126] The image forming unit 123 may have an image processing unit 244 that corrects the input image signal based on the correction value. The image forming unit 123 may form an image based on the corrected image signal. The controller (e.g., CPU 211, 241) may generate the correction value based on the second density information in the second calibration.

[0127] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0128] 120: printer, image forming unit 123, 130: reader, 613: first conversion unit, 614: second conversion unit

Claims

1. an image forming means for forming an image; a detecting means for receiving reflected light from a test image formed by the image forming means and outputting a signal based on the result of receiving the reflected light from the test image; an amplifier for amplifying the signal output from the detector; a control means for performing calibration to adjust the image quality of the image to be formed by the image forming means, The control means causing the image forming means to form a test image for first calibration, causing the amplifier means to amplify a first signal based on the result of receiving reflected light from the test image for first calibration output by the detection means, determining first density information from the amplified first signal based on a first determination condition, and adjusting the gradation characteristics of the image to be formed by the image forming means based on the first density information; causing the image forming means to form a test image for second calibration, causing the amplifier means to amplify a second signal based on the result of receiving reflected light from the test image for second calibration output by the detection means, determining second density information from the amplified second signal based on a second determination condition, and adjusting density unevenness of the image to be formed by the image forming means based on the second density information; The image forming apparatus according to claim 1, wherein the amplification factor of the second signal is higher than the amplification factor of the first signal.

2. the detecting means further comprises a light emitting means for illuminating the test image; 2. The image forming apparatus according to claim 1, wherein the amount of light with which said light emitting means illuminates said test image for the first calibration is different from the amount of light with which said light emitting means illuminates said test image for the second calibration.

3. 2. The image forming apparatus according to claim 1, wherein a density range that can be output as the first density information is wider than a density range that can be output as the second density information.

4. the first determination condition is a density conversion table used to convert a digital signal having a predetermined number of bits into the first density information in a first density range; 2. The image forming apparatus according to claim 1, wherein the second determination condition is a density conversion table used to convert the digital signal having the predetermined number of bits into the second density information in a second density range narrower than the first density range.

5. the test image for the first calibration includes an image having a plurality of different gray levels; 2. The image forming apparatus according to claim 1, wherein the test image for the second calibration is an image having a single tone.

6. The sheet conveying device further includes a conveying means for conveying the sheet to the conveying path, the image forming means forms the image on the sheet conveyed by the conveying means, 2. The image forming apparatus according to claim 1, wherein the second calibration is a calibration for adjusting density unevenness in a direction perpendicular to a conveying direction in which the conveying means conveys the sheet.

7. The sheet conveying device further includes a conveying means for conveying the sheet to the conveying path, the image forming means forms the image on the sheet conveyed by the conveying means, 2. The image forming apparatus according to claim 1, wherein the second calibration is a calibration for adjusting density unevenness in a conveying direction in which the conveying unit conveys the sheet.

8. 2. The image forming apparatus according to claim 1, wherein the resolution of the signal value as the second density information is higher than the resolution of the signal value as the first density information.

9. the image forming means has an image processing section that converts an input image signal based on a gradation correction condition, and forms the image based on the converted image signal; 2. The image forming apparatus according to claim 1, wherein the control unit generates the tone correction conditions based on the first density information in the first calibration.

10. the image forming means has an image processing section that corrects an input image signal based on a correction value, and forms the image based on the corrected image signal; 2. The image forming apparatus according to claim 1, wherein the control unit generates the correction value based on the second density information in the second calibration.

Images