Image forming apparatus

Abstract

To provide an image forming apparatus that can check a hue with high accuracy.SOLUTION: An image forming apparatus 100 comprises: a printer 150 that forms an image on recording paper; a reader 160 that reads the image formed on the recording paper; and a controller 110 that forms an image in a specific color on the recording paper with a printer 150, reads the image in the specific color formed on the recording paper by the reader 160, and on the basis of, a result of reading of the image in the specific color performed by the reader 160, checks a hue of the image in the specific color. The controller 110 creates a color calibration chart for performing color calibration of the reader 160 with the printer 150, creates a color calibration matrix for calibrating a measurement result from the reader 160 on the basis of a result of reading of the color calibration chart performed by the reader 160, and checks the hue by comparing a color difference between a color value of the result of reading of the image in the specific color and a color value of the specific color obtained from the color calibration matrix with a hue check threshold value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus having a function for inspecting the color of an image printed on a printed material. [Background technology]

[0002] Corporate colors, used in company logos and design marks, are defined as elements for identifying a company. Therefore, printed materials containing corporate colors require strict color adjustment and management. In recent years, color inspection systems have been proposed that read the color of the printed material during printing and inspect the hue of the read color. Patent Document 1 discloses an image forming apparatus for color management. This image forming apparatus prints a measurement patch of a specific color designated by the user. The image forming apparatus performs color stabilization control based on the results of color measurement of the measurement patch using an image sensor. If the color measurement result is outside the acceptable range, the image forming apparatus notifies the user and performs color stabilization control again.

[0003] Image sensors used to measure the color of printed images output luminance values ​​(RGB data) for three colors: R (red), G (green), and B (blue) as the color measurement result. This RGB data is converted to L, a, and b data in the CIELab space. A color conversion table, which is a lookup table, is used to convert the RGB data to the CIELab space. The color conversion table does not register color conversion values ​​(Lab values) for all input values ​​(luminance values ​​of R, G, and B), but rather generally registers color conversion values ​​only for a number of grid points that are regularly arranged in the input color space. When performing color conversion using such a color conversion table, color conversion values ​​other than those at the grid points are obtained by interpolation calculations based on the color conversion values ​​registered at the grid points (Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2013-1049 [Patent Document 2] Japanese Patent Publication No. 2002-64719 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Color conversion using a color conversion table is performed by evenly converting the entire color gamut, which can be represented by the three colors R, G, and B, to Lab values. This method converts the entire color gamut to a level that does not cause any noticeable discrepancies. However, in color gamuts where the Lab value is easily changed for each color in RGB data, the conversion error becomes large, making high-precision color inspection difficult.

[0006] This invention has been made in view of the above problems, and its main objective is to provide an image forming apparatus capable of performing high-precision color inspection. [Means for solving the problem]

[0007] The image forming apparatus of the present invention comprises: an image forming means for forming an image on recording paper; a reading means for reading the image formed on the recording paper; a control means for forming an image of a specific color on the recording paper using the image forming means, reading the image of the specific color formed on the recording paper using the reading means, and inspecting the color of the image of the specific color based on the reading result of the reading means, wherein the control means creates a color calibration chart for color calibration of the reading means using the image forming means, generates conversion conditions for calibration of the measurement result of the reading means based on the result of the reading means reading the color calibration chart, and inspects the color by comparing the color difference between the color value of the reading result of the image of the specific color obtained by the conversion conditions and the color value of the specific color, and a predetermined threshold. [Effects of the Invention]

[0008] According to the present invention, highly accurate color inspection for specific colors becomes possible. [Brief explanation of the drawing]

[0009] [Figure 1] Diagram illustrating the configuration of the printing system. [Figure 2] Diagram showing the configuration of an image forming apparatus. [Figure 3] Diagram illustrating the leader's structure. [Figure 4] Diagram illustrating the configuration of the line sensor. [Figure 5] Diagram illustrating the configuration of the spectroscopic sensor unit. [Figure 6] A flowchart illustrating the printing process, including color inspection. [Figure 7] An example diagram of a color proofing chart. [Figure 8] A flowchart illustrating the color proofing process. [Figure 9] A diagram illustrating the calculation method for L*, a*, and b* of colors surrounding a specific color. [Figure 10] (a) and (b) are explanatory diagrams of the color conversion lookup table. [Modes for carrying out the invention]

[0010] The embodiments will be described in detail below with reference to the drawings. Note that the following embodiments are not intended to limit the invention as defined in the claims, and not all combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0011] (Printing system) Figure 1 is a diagram illustrating the configuration of a printing system including an image forming apparatus according to this embodiment. The printing system comprises an image forming apparatus 100 and a host computer 101. The image forming apparatus 100 and the host computer 101 are communicated with each other via a network 105. The network 105 consists of a communication line such as a LAN (Local Area Network), a WAN (Wide Area Network), or a public communication line. Note that multiple image forming apparatuses 100 and host computers 101 may be connected to the network 105.

[0012] The host computer 101 is, for example, a server device, and transmits print jobs to the image forming apparatus 100 via the network 105. The print job includes various information necessary for printing, such as image data, the type of paper used for printing, the number of copies to print, and instructions for double-sided or single-sided printing.

[0013] The image forming apparatus 100 comprises a controller 110, an operation panel 120, a paper feed unit 140, a printer 150, and a reader 160. The controller 110, operation panel 120, paper feed unit 140, printer 150, and reader 160 are connected to each other via a system bus 116 so as to be able to communicate with each other. The image forming apparatus 100 controls the operation of the printer 150 based on a print job acquired from the host computer 101 and forms an image on recording paper according to the image data.

[0014] The controller 110 controls the operation of each unit of the image forming apparatus 100. The controller 110 is an information processing device equipped with a ROM (Read Only Memory) 112, a RAM (Random Access Memory) 113, and a CPU (Central Processing Unit) 114. The controller 110 also includes a communication control unit 111 and storage 115. Each module is connected to each other via a system bus 116 so as to be able to communicate with each other.

[0015] The communication control unit 111 is a communication interface that communicates with the host computer 101 and other devices via the network 105. The storage 115 is a large-capacity storage device such as an HDD (Hard Disk Drive) or SSD (Solid State Drive). The storage 115 stores computer programs and various data used for image forming processing (printing processing). The CPU 114 controls the operation of the image forming apparatus 100 by executing computer programs stored in the ROM 112 and storage 115. The RAM 113 provides a work area for when the CPU 114 executes computer programs.

[0016] The control panel 120 is a user interface and includes an input interface and an output interface. The input interface includes, for example, operation buttons, a numeric keypad, a touch panel, etc. The output interface includes, for example, a display such as an LCD (Liquid Crystal Display), a speaker, etc. The user can input print jobs, commands, and print settings to the image forming apparatus 100 using the control panel 120. The control panel 120 displays the settings screen and the status of the image forming apparatus 100 on its display.

[0017] The paper feed unit 140 is equipped with multiple paper feed stages, described later, for storing recording paper. The paper feed unit 140 feeds paper from the paper feed stage that contains the type of recording paper specified in the print job. Each paper feed stage contains multiple sheets of recording paper (a stack of recording paper), and the paper is fed sequentially starting from the top sheet. The paper feed unit 140 transports the recording paper fed from the paper feed stages to the printer 150. Each paper feed stage may contain the same type of recording paper, or it may contain different types of recording paper.

[0018] The printer 150 generates a printed document by printing an image onto recording paper supplied from the paper feed unit 140 based on the image data included in the print job. The reader 160 is an image reading device that reads an image from the printed document generated by the printer 150 and transmits the reading result to the controller 110. The image read by the reader 160 is an image (detection image) used to adjust the image formation conditions when the printer 150 performs image formation. The controller 110 detects the image state, such as image quality, from the reading result of the detection image by the reader 160 and adjusts the image formation conditions based on the detected image state. In this embodiment, the image density is detected from the detection image and the image formation conditions are adjusted based on the detected image density.

[0019] (Image forming apparatus) Figure 2 is a diagram of the configuration of the image forming apparatus 100. The image forming apparatus 100 comprises, in order from the upstream side in the direction of transporting the recording paper, paper feeding stages 140a to 140e, a printer 150, a reader 160, and a finisher 190. Paper feeding stages 140a to 140e constitute the paper feeding section 140. The finisher 190 is a post-processing device that performs post-processing on printed materials from the printer 150. For example, the finisher 190 performs stapling and sorting on multiple printed materials.

[0020] The printer 150 comprises multiple image forming units, each forming an image of a different color. In this embodiment, the printer 150 comprises four image forming units to form an image of four colors: yellow (Y), magenta (M), cyan (C), and black (K). Each image forming unit differs only in the color of the image it forms, and operates similarly with a similar configuration.

[0021] One image forming unit comprises a photosensitive drum 153, a charger 220, an exposure unit 223, and a developer unit 152. The photosensitive drum 153 is a drum-shaped photoreceptor having a photosensitive layer on its surface, and is rotationally driven in the direction of arrow R1 by a motor (not shown). The charger 220 charges the surface (photosensitive layer) of the rotating photosensitive drum 153. The exposure unit 223 exposes the charged surface of the photosensitive drum 153 with laser light. The laser light scans the surface of the photosensitive drum 153 in the axial direction of the photosensitive drum 153. The direction in which the laser light scans the surface of the photosensitive drum 153 is the main scanning direction of the printer 150 (depth direction in Figure 2). As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 153. The developer unit 152 develops the electrostatic latent image using a developer (toner). As a result, an image (toner image) is formed on the surface of the photosensitive drum 153 in which the electrostatic latent image has been manifested.

[0022] The printer 150 includes an intermediate transfer belt 154 to which the toner images generated by each image forming unit are transferred. The intermediate transfer belt 154 is rotationally driven in the direction of arrow R2. The toner images of each color are transferred at timings corresponding to the rotation of the intermediate transfer belt 154. As a result, a full-color toner image is formed on the intermediate transfer belt 154 by superimposing the toner images of each color. The full-color toner image is transported by the rotation of the intermediate transfer belt 154 to a nip section formed by the intermediate transfer belt 154 and the transfer roller 221. The full-color toner image is then transferred to the recording paper by the nip section.

[0023] The recording paper is housed in the paper feed stages 140a, 140b, 140c, 140d, and 140e of the paper feed unit 140, and is fed according to the timing of image formation by each image forming unit. The paper feed stage from which the recording paper is fed is indicated by the print job. The recording paper is transported to the nip section at the timing when the full-color toner image is formed by the intermediate transfer belt 154 and the transfer roller 221. This transfers the toner image to a predetermined position on the recording paper. The direction of transport of the recording paper is the sub-scanning direction, which is perpendicular to the main scanning direction.

[0024] The printer 150 includes a first fuser 155 and a second fuser 156 that fix the toner image onto the recording paper by heating and pressurizing. The first fuser 155 includes a fuser roller with a built-in heater and a pressure belt for pressing the recording paper against the fuser roller. The fuser roller and pressure belt are driven by a motor (not shown) to grip and transport the recording paper. The second fuser 156 is positioned downstream of the first fuser in the direction of transport of the recording paper. The second fuser 156 is used to increase the gloss and ensure the fixation of the image on the recording paper that has passed through the first fuser 155. The second fuser 156 includes a fuser roller with a built-in heater and a pressure roller with a built-in heater. Depending on the type of recording paper, the second fuser 156 may not be used. In this case, the recording paper is not transported to the second fuser 156 but to the transport path 130. To this end, a flapper 131 is provided downstream of the first fuser 155 to guide the recording paper to either the transport path 130 or the second fuser 156.

[0025] Downstream of the second fuser 156, at the point where the transport paths 130 merge, a transport path 135 and an output path 139 are provided. For this purpose, a flapper 132 is provided at the point where the transport paths 130 merge downstream of the second fuser 156 to guide the recording paper to either the transport path 135 or the output path 139. The flapper 132 guides the recording paper with an image formed on the first side to the transport path 135, for example, in the duplex printing mode. The flapper 132 guides the recording paper with an image formed on the first side to the output path 139, for example, in the face-up output mode. The flapper 132 guides the recording paper with an image formed on the first side to the transport path 135, for example, in the face-down output mode.

[0026] The recording paper transported to the transport path 135 is transported to the reversal unit 136. After the transport operation of the recording paper transported to the reversal unit 136 is temporarily stopped, the transport direction is reversed by a switchback. From the reversal unit 136, the recording paper is guided by the flapper 133 to either the transport path 135 or the transport path 138. For example, in the double-sided printing mode, the flapper 133 guides the switched-back recording paper to the transport path 138 in order to print an image on the second side. The recording paper transported to the transport path 138 is transported toward the nip between the intermediate transfer belt 154 and the transfer roller 221. As a result, the front and back sides of the recording paper are reversed as it passes through the nip, and an image is formed on the second side. For example, in the face-down output mode, the flapper 133 guides the switched-back recording paper to the transport path 135. The recording paper, transported to the transport path 135 by the flapper 133, is then guided to the discharge path 139 by the flapper 134.

[0027] The recording paper with the image formed by the printer 150 is transported from the output path 139 to the reader 160. The reader 160 is an image reading device that reads the image density detection image printed on the recording paper along with the user image printed on the recording paper according to the print job. The recording paper transported from the printer 150 to the reader 160 is transported to the transport path 313 inside the reader 160. The reader 160 is equipped with a document detection sensor 311, a line sensor unit 312, and a spectroscopic sensor unit 315 in the transport path 313. A flow-reading glass 314 is placed between the line sensor unit 312 and the transport path 313. A white plate 316 is placed opposite the spectroscopic sensor unit 315 across the transport path 313. While the reader 160 transports the recording paper with the detection image printed by the printer 150 to the transport path 313, it measures the color using the line sensor unit 312 and the spectroscopic sensor unit 315.

[0028] The document detection sensor 311 is, for example, an optical sensor having a light-emitting element and a light-receiving element. The document detection sensor 311 detects the leading edge of the recording paper being transported along the transport path 313. The detection result of the leading edge of the recording paper by the document detection sensor 311 is transmitted to the controller 110. Based on the timing of the detection of the leading edge of the recording paper by the document detection sensor 311, the controller 110 starts the reading operation by the reader 160 (line sensor unit 312 and spectral sensor unit 315). The line sensor unit 312 is provided on the image-forming surface side of the recording paper in the transport path 313 to read the detection image of the recording paper while it is being transported. The spectral sensor unit 315 is driven in the main scanning direction and is provided on the image-forming surface side of the recording paper in the transport path 313 to measure the color of the image on the recording paper.

[0029] (Leader) Figure 3 is an explanatory diagram of the configuration of the reader 160. In addition to the line sensor unit 312, the spectral sensor unit 315, and the document detection sensor 311, the reader 160 includes an image memory 303 and a color detection processing unit 305. The operation of the line sensor unit 312, the spectral sensor unit 315, the image memory 303, the color detection processing unit 305, and the document detection sensor 311 is controlled by the CPU 114 of the controller 110.

[0030] The line sensor unit 312 includes a line sensor 301, a memory 300, and an AD converter 302. The line sensor 301 is, for example, a CIS (Contact Image Sensor). The memory 300 stores correction information such as the light intensity variation between pixels, the step difference between pixels, and the distance between pixels of the corresponding line sensor 301. The AD converter 302 acquires an analog signal, which is the reading result from the line sensor 301. The AD converter 302 converts the acquired analog signal into a digital signal and transmits it to the color detection processing unit 305. The digital signal is R (red), G (green), and B (blue) image data.

[0031] The spectral sensor unit 315 includes a spectral sensor 306, a memory 304, an AD converter 307, and a spectral sensor drive unit 308. The spectral sensor 306 is composed of, for example, a light source, a lens, a diffraction grating surface, and a light receiving unit. The spectral sensor 306 irradiates the object to be measured with light from the light source and spectrally separates the reflected light by wavelength using a diffraction grating. The spectral sensor 306 receives the spectrally separated light by wavelength with pixels separately provided for each wavelength in the light receiving unit and converts it into a voltage value for each wavelength using photoelectric conversion. The output value of the light for each wavelength converted into a voltage value is an analog signal. The AD converter 307 converts this analog signal into a digital signal and transmits it to the color detection processing unit 305 as spectral reflectance data. The memory 304 stores various correction information such as stray light data and dark current data from the spectral sensor 306. The spectral sensor drive unit 308 is a drive source that drives the spectral sensor unit 315 in the main scanning direction.

[0032] The color detection processing unit 305 calculates the average value (average brightness value) of the RGB brightness values ​​of the detection image portion from the RGB image data acquired from the line sensor unit 312, and sends the calculation result to the CPU 114. The color detection processing unit 305 is composed of semiconductor devices such as FPGA (Field-Programmable Gate Array) and ASIC (Application Specific Integrated Circuit). The image memory 303 stores the image data necessary for image processing in the CPU 114.

[0033] (Line sensor) Figure 4 is an explanatory diagram of the configuration of the line sensor 301. The line sensor 301 comprises light-emitting units 400a and 400b, light guides 402a and 402b, a lens array 403, and a group of sensor chips 401. The line sensor 301a is approximately a rectangular parallelepiped and reads images with its longitudinal direction as the primary scanning direction.

[0034] The light-emitting units 400a and 400b are light sources composed of, for example, white-emitting LEDs (Light Emitting Diodes). The light guide 402a has the light-emitting unit 400a positioned at its end and irradiates the light emitted from the light-emitting unit 400a toward the recording paper. The light guide 402b has the light-emitting unit 400b positioned at its end and irradiates the light emitted from the light-emitting unit 400b toward the recording paper. The light guides 402a and 402b are formed in a straight line in the main scanning direction. Therefore, the line sensor 301 irradiates light in a straight line in the main scanning direction. The main scanning direction of the line sensor unit 312 and the main scanning direction of the printer 150 are the same direction.

[0035] The lens array 403a is an optical system that guides the reflected light from the recording paper, which is irradiated from the light-emitting units 400a and 400b, to the sensor chip group 401a. The sensor chip group 401a is composed of multiple photoelectric conversion elements (sensor chips) arranged in a straight line in the main scanning direction. One sensor chip reads one pixel of the image. The multiple sensor chips in this embodiment are configured in three lines. One line is coated with an R (red) color filter, another line is coated with a G (green) color filter, and another line is coated with a B (blue) color filter. The light guided by the lens array 403a is imaged onto the light-receiving surface of each sensor chip in the sensor chip group 401a.

[0036] Light emitted from light-emitting units 400a and 400b diffuses inside light guides 402a and 402b and is emitted from curved sections, illuminating the entire area of ​​the recording paper in the main scanning direction. Light guides 402a and 402b are positioned with the lens array 403a in between in the sub-scanning direction, which is perpendicular to the main scanning direction. For this reason, the line sensor 301a has a double-sided illumination configuration that irradiates the lens array 403a (image reading line) with light from two directions in the sub-scanning direction. The sub-scanning direction of the line sensor unit 312a and the sub-scanning direction of the printer 150 are the same direction.

[0037] (Spectroscopic sensor unit) Figure 5 is an explanatory diagram of the configuration of the spectral sensor unit 315. The spectral sensor unit 315 is a roughly rectangular parallelepiped with its main scanning direction being the longitudinal direction. In Figure 5, the recording paper is transported in the sub-scanning direction from the back of the spectral sensor unit 315. The spectral sensor 306, memory 304, and AD converter 307 are configured as a single unit. The AD converter 307 is connected to the color detection processing unit 305 by wiring such as a flexible flat cable (not shown).

[0038] The spectroscopic sensor 306 is mounted on a rail 309 that extends from the spectroscopic sensor drive unit 308 in the main scanning direction. The spectroscopic sensor 306 moves along the rail 309 by the spectroscopic sensor drive unit 308. The spectroscopic sensor drive unit 308 has a built-in stepping motor and is controlled by instructions from the CPU 114. The spectroscopic sensor drive unit 308 can move the spectroscopic sensor 306 to a predetermined position in the main scanning direction with high precision.

[0039] A home position HP is provided outside the transport area where the spectral sensor unit 315 can read the recording paper. The white plate 316 is placed at the home position HP. The recording paper is transported one line at a time in the sub-scanning direction and stops at the timing of color measurement. An opening 310 is provided at the position corresponding to the document transport area of ​​the spectral sensor unit 315, and the spectral sensor 306 reads the recording paper through the opening 310.

[0040] The spectroscopic sensor 306 is positioned at its home position HP before color measurement begins. When the CPU 114 instructs the spectroscopic sensor 306 to start color measurement, it reads the white plate 316 and performs calibration such as adjusting the light source intensity and aligning the white reference. After calibration, the spectroscopic sensor 306 starts moving at a constant speed in the main scanning direction from its home position HP and starts measuring the color of one line triggered by the detection of a trigger patch. Once the color measurement of one line is complete, the spectroscopic sensor 306 returns to its home position HP. Subsequently, when the recording paper moves one line in the sub-scanning direction, the spectroscopic sensor 306 starts moving again in the main scanning direction and measures the color of one line. By repeatedly performing this process of moving the recording paper one line at a time and measuring one line with the spectroscopic sensor 306, the color of one sheet of recording paper is measured.

[0041] (Color inspection) Figure 6 is a flowchart representing the printing process, including color inspection. This process is initiated when the user inputs a color inspection instruction via the control panel 120 and then inputs a copy start instruction. The color inspection instruction includes the paper size, print mode, and number of copies P. MAX , the color value to be inspected (specific color: L 00*,a 00 *,b 00 *) Color and flavor inspection designated area (area X = X on the paper S ~X E , Y = Y S ~Y E ) includes color and flavor inspection threshold Cth, etc.

[0042] The CPU 114 obtains an instruction for color and flavor inspection from the operation panel 120, and performs mode setting (S600) by setting the information necessary for the printing job to each device based on the instruction and storing various parameters included in the instruction in the RAM 113. The CPU 114 waits for a copy start instruction from the operation panel 120 after mode setting (S601: N).

[0043] When the CPU 114 obtains a copy start instruction (S601: Y), it performs color calibration of the line sensor 301 according to the instruction content of the color and flavor inspection, and creates a color calibration matrix M of the line sensor 301 (S602). The color calibration matrix M is a conversion condition for converting L*, a*, b* converted from the reading result of the line sensor unit 312 into color values for color calibration. Details of the processing in S602 will be described later. The CPU 114 initializes the print count value P to "0" after color calibration (S603). The print count value P represents the number of sheets of recording paper on which an image is formed by the printer 150.

[0044] The CPU 114 performs printing processing of an image of a specific color under the conditions according to the instruction of the color and flavor inspection by the printer 150 to generate a printed matter (S604). The CPU 114 measures the color of the printed matter by the line sensor unit 312 (S605). The color measurement is performed on the color and flavor inspection designated area of the printed matter (area X = X on the paper S ~X E , Y = Y S ~Y E ). As a result of the color measurement of the printed matter, RGB image data is transmitted from the line sensor unit 312 to the color detection processing unit 305. The color detection processing unit 305 calculates the average luminance value (R A , R G , RB The result is derived and sent to CPU114.

[0045] CPU114 is a color conversion lookup table (LUT) that converts the luminance values ​​of each RGB color (RGB data) to L*, a*, and b*. IN It has a color conversion lookup table LUT. CPU114 has a color conversion lookup table LUT. IN Using the average brightness value of each color (R A ,R G ,R B ) to L a* ,a a* ,b a* Convert to a value. CPU114 uses the color calibration matrix M created by processing S602 to convert the average luminance value (R A ,R G ,R B ) from L a* ,a a* ,b a* From the conversion result to the value, the color value (L Pa* ,a Pa* ,b Pa* Derive the following:

[0046] CPU114 determines the color value (L) obtained from the color measurement results. Pa* ,a Pa* ,b Pa* ) and specific colors (L 00 *,a 00 *,b 00 The color difference ΔE00 with the color value of *) is derived (S606). The CPU 114 compares the derived color difference ΔE00 with the color inspection threshold Cth (S607). The result of the color inspection is determined based on the comparison result between the color difference ΔE00 and the color inspection threshold Cth.

[0047] If the color difference ΔE00 is less than or equal to the color inspection threshold Cth (S607:Y), the CPU 114 determines that the difference between the specific color of the image printed on the recording paper and the specified color to be inspected is small. In this case, the CPU 114 increments the print count value P by 1 because printing in the specified color has been performed correctly (S608). The CPU 114 then determines that the print count value P is equal to the number of printed pages P MAX It is determined whether or not the print count value P has been reached (S610).MAX If the print count value P has not been reached (S610:N), CPU114 will perform the processing from S604 onwards, and the number of printed pages P MAX Repeat until the print count value P is reached. MAX If this condition is reached (S610:Y), CPU114 terminates the printing process, including the color inspection process.

[0048] If the color difference ΔE00 is greater than the color inspection threshold Cth (S607:N), the CPU 114 determines that there is a large difference between the specific color of the image printed on the recording paper and the specified color to be inspected. In this case, the CPU 114 displays a warning on the control panel 120 because printing in the specified color has not been performed correctly (S609). The warning display indicates that the color inspection designated area is the specified color L 00 *,a 00 *,b 00 * Indicates that the color difference is far from the acceptable color difference (color inspection threshold Cth), and that the color inspection result is unsatisfactory. In addition to being displayed on the screen, the warning may also be audible from the speaker. After displaying the warning, CPU 114 terminates the printing process, including the color inspection process.

[0049] (Color proofing process) The color calibration process for S602 will now be explained. Figure 7 is an example of a color calibration chart used for the color calibration process of the line sensor unit 312. The color calibration chart 501 is created by printing 49 patch images 504 as detection images on a recording paper that is long in the sub-scanning direction. The patch images 504 are arranged in 7 rows and 7 columns in the main scanning direction and the sub-scanning direction. A margin 502 is provided at the left end of the color calibration chart 501 in the main scanning direction, a black trigger patch 503 is provided to the right of the margin 502, and the 49 patch images 504 are provided to the right of the trigger patch 503.

[0050] 504 patch images of 49 colors for color proofing are for specific color L 00 *,a 00 *,b 00 * and specific color L 00 *,a 00 *,b 00An image of the image density value corresponding to each of the L*, a*, and b* values of the peripheral color calculated as a value separated by a predetermined color difference from 00 *. a 00 *. b 00 * of the image density value. The image density value is set for each color of yellow (Y), magenta (M), cyan (C), and black (K). Here, this image density value is referred to as the "YMCK value". A method for selecting the YMCK value for the L*, a*, and b* values of the 49 patch images 504 for color calibration will be described later. Note that the formation position of the patch image 504 on the color calibration chart 501 is not limited to FIG. 7.

[0051] FIG. 8 is a flowchart showing the color calibration process. FIG. 9 is an explanatory diagram of a method for calculating the L*, a*, and b* values of the peripheral color of a specific color. FIG. 10 is an explanatory diagram of a color conversion look-up table LUT OUT for converting colors from L*, a*, and b* values to YMCK values.

[0052] The CPU 114 calculates the L*, a*, and b* values of the peripheral color from the specific color L 00 *. a 00 *. b 00 * (S800). To do so, the CPU 114 first obtains the specific color L 00 *. a 00 *. b 00 * and the color taste inspection threshold Cth from the RAM 113. The CPU 114 calculates the peripheral color separated by a predetermined color difference from the specific color L 00 *. a 00 *. b 00 *. The peripheral color is selected such that the range of the predetermined color difference straddles the color taste inspection threshold Cth (ΔEmin < Cth < ΔEmax).

[0053] For example, as shown in FIG. 9, 48 peripheral colors are selected. FIG. 9 illustrates the case where the color tone inspection threshold Cth is "5". The CPU 114 selects ΔE00 = 2, 4 which are smaller than ΔE00 = 5 and ΔE00 = 6, 8, 10, 12 which are larger than ΔE00 = 5 so as to straddle the predetermined color difference of ΔE00 = 5. The CPU 114 calculates the L*, a*, b* of the following 48 peripheral colors corresponding to the selected ΔE00.

[0054] · Peripheral colors 01 to peripheral color 08 with a color separation of color difference ΔE00 = 2 → L*, a*, b* = L 01 *, a 01 *, b 01 * ~ L 08 *, a 08 *, b 08 * · Peripheral colors 09 to peripheral color 16 with a color separation of color difference ΔE00 = 4 → L*, a*, b* = L 09 *, a 09 *, b 09 * ~ L 16 *, a 16 *, b 16 * · Peripheral colors 17 to peripheral color 24 with a color separation of color difference ΔE00 = 6 → L*, a*, b* = L 17 *, a 17 *, b 17 * ~ L 24 *, a 24 *, b 24 * · Peripheral colors 25 to peripheral color 32 with a color separation of color difference ΔE00 = 8 s → L*, a*, b* = L 25 *, a 25 *, b 25 * ~ L 32 *, a 32 *, b 32 * · Peripheral colors 33 to peripheral color 40 with a color separation of color difference ΔE00 = 10 → L*, a*, b* = L 33 *, a 33 *, b 33 * ~ L 40 *, a 40 *, b 40 * ·Peripheral colors 41 to 48 with a color separation of ΔE00 = 12 →L*, a*, b* = L 41 *, a 41 *, b 41 * ~ L 48 *, a 48 *, b 48 *

[0055] The CPU 114 calculates the patch color (Y, M, C, K), which is the color of the patch image used for the color calibration chart 501 (S801). The CPU 114 uses L 00 *, a 00 *, b 00 * ~ L 48 *, a 48 *, b 48 * to convert based on the color conversion look-up table LUT stored in the ROM 112 OUT . As a result, the YMCK values corresponding to each L*, a*, b* value are calculated (calculation of the patch color (L*, a*, b*)). According to FIG. 10, the color conversion look-up table LUT OUT for converting the L*, a*, b* values into the YMCK values, which are the printing parameters, will be described.

[0056] FIG. 10 shows the concept of the color conversion look-up table LUT OUT . FIG. 10(a) shows the three-dimensional color conversion look-up table LUT OUT in the input color space (Lab space). The color conversion look-up table LUT OUT has cubes arranged at equal intervals on the Lab space. Each vertex (grid point) of the cube represents a position (L*, a*, b* value) in the Lab space. The patch color (YMCK value) corresponding to the L*, a*, b* value of that position is assigned to the grid point.

[0057] For example, when the L*, a*, b* value to be converted is the L β *, a β *, b β * specified on the grid point, the color conversion look-up table LUT OUT uses the corresponding patch color (YMCK value), which is Y β , Mβ ,C β ,K β The following will be output.

[0058] Figure 10(b) illustrates the table interpolation method. The L*, a*, and b* values ​​to be color-converted are within the region enclosed by grid points 1 to 8. If the distances from grid points 1 to 8 are d1 to d8 respectively, the patch color (YMCK value) is calculated as follows, depending on the distance to each grid point.

[0059] Y=(Y1 / d1+Y2 / d2+…+Y8 / d8) / (1 / d1+1 / d2+…+1 / d8) M=(M1 / d1+M2 / d2+…+M8 / d8) / (1 / d1+1 / d2+…+1 / d8) C=(C1 / d1+C2 / d2+…+C8 / d8) / (1 / d1+1 / d2+…+1 / d8) K=(K1 / d1+K2 / d2+…+K8 / d8) / (1 / d1+1 / d2+…+1 / d8)

[0060] Note: Color conversion lookup table LUT OUT The values ​​are stored in ROM112, and the conversion calculation from L*, a*, b* values ​​to patch colors (YMCK values) is performed by CPU114.

[0061] The CPU 114 creates the color calibration chart 501 shown in Figure 7 using the printer 150 based on the patch color (YMCK value) calculated in the processing of S801 (S802). The CPU 114 measures the color of the created color calibration chart 501 using the line sensor 301 and the spectral sensor unit 315 (S803).

[0062] The line sensor 301 outputs the luminance values ​​(RGB data) of each color, which are the color measurement results, to the color detection processing unit 305. The color detection processing unit 305 calculates the average luminance value (R) of each RGB color in the measurement area from the RGB data acquired from the line sensor unit 312. A , G A B AThe CPU 114 calculates the LUT (Color Conversion Lookup Table), which converts the R, G, and B luminance values ​​to L*, a*, and b*. IN Using the average brightness value (R A ,G A ,B A The CPU 114 converts the color measurement results from the line sensor unit 312 into L*, a*, b* values. L_A00 *,a L_A00 *,b L_A00 *~L L_A48 *,a L_A48 *,b L_A48 Get *.

[0063] The spectral sensor 306 outputs spectral reflectance data of the color calibration chart 501, which is the color measurement result, to the color detection processing unit 305. The color detection processing unit 305 outputs 49 L*, a*, b* values ​​as spectral reflectance data, L S_A00 *,a S_A00 *,b S_A00 *~L S_A48 *,a S_A48 *,b S_A48 The color detection processing unit 305 obtains the * value. The color detection processing unit 305 calculates the L*, a*, and b* values ​​from the spectral reflectance data obtained from the spectral sensor unit 315. The color detection processing unit 305 outputs the calculated L*, a*, and b* values ​​to the CPU 114.

[0064] The 49 L*, a*, b* values ​​of spectral reflectance data acquired by the color detection processing unit 305 are ZA 00 ,Z B00 ,Z C00 ~Z A48 ,Z B48 ,Z C48 Let's assume that Z A00 ,Z B00 *,Z C00 ~Z A48 ,Z B48 *,Z C48 The L*, a*, b* values ​​obtained by measuring the color of the same patch image with the line sensor 301 are X A00 ,X B00 ,X C00 ~X A48 ,X B48 ,X C48 Let's assume that.

[0065] CPU 114 generates the color calibration matrix M for the line sensor 301 (S804). CPU 114 then generates the Z A00 ,Z B00 ,Z C00 ~Z A48 ,Z B48 ,Z C48 and X A00 ,X B00 ,X C00 ~X A48 ,X B48 ,X C48 Using this as training data, a color calibration matrix M for calibrating the measurement results of the line sensor 301 is calculated using the following formula. The color calibration matrix M is 3x10. The CPU 114 stores the calculated color calibration matrix M in RAM 113. In this way, the color calibration matrix M is obtained through the color calibration process.

[0066]

number

[0067] As described above, in this embodiment, the printer 150 creates a color calibration chart by printing a specific color and surrounding colors located at a predetermined color difference from the specific color as a detection image on recording paper. The color calibration chart is read by the reader 160. From the reading result of the color calibration chart by the reader 160, a color conversion table for the specific color from RGB to Lab is created. This improves the accuracy of the conversion from RGB to Lab for colors near the specific color, enabling the realization of a highly accurate color inspection system.

Claims

1. an image forming means for forming an image on a recording sheet; a reading means for reading an image formed on the recording paper; a control means for forming an image of a specific color on the recording paper by the image forming means, reading the image of the specific color formed on the recording paper by the reading means, and inspecting the color of the image of the specific color based on the reading result of the image of the specific color by the reading means, the control means creates a color calibration chart for color calibration of the reading means by the image forming means, generates conversion conditions for calibrating the measurement results of the reading means based on the results of the reading means reading the color calibration chart, and inspects the color by comparing the color difference between the color value of the reading result of the image of the specific color obtained under the conversion conditions and the color value of the specific color with a predetermined threshold. Image forming device.

2. the control means calculates peripheral colors that are separated from the specific color by a predetermined color difference, and the image forming means creates the color calibration chart including patch images of the specific color and patch images of the peripheral colors.

2. The image forming apparatus according to claim 1.

3. the control means calculates the surrounding color such that the range of the predetermined color difference from the specific color is a range that spans the threshold value.

3. The image forming apparatus according to claim 2.

4. The reading means a line sensor that reads the color calibration chart and outputs a luminance value as a reading result; a spectroscopic sensor that reads the color calibration chart and outputs a spectral reflectance as a reading result; a color detection processing means for converting the average value of the luminance values ​​output from the line sensor into L*, a*, b* values, and for converting the spectral reflectances output from the spectral sensor into L*, a*, b* values, the control means generates the conversion conditions based on L*, a*, b* values ​​converted from the average value of the luminance values ​​and L*, a*, b* values ​​converted from the spectral reflectances. The image forming apparatus according to any one of claims 1 to 3.

5. the control means generates the conversion conditions using the L*, a*, b* values ​​converted from the average luminance values ​​and the L*, a*, b* values ​​converted from the spectral reflectance as training data.

5. The image forming apparatus according to claim 4.

6. the control means generates a matrix that serves as the conversion conditions using the L*, a*, b* values ​​converted from the average luminance values ​​and the L*, a*, b* values ​​converted from the spectral reflectance as training data.

6. The image forming apparatus according to claim 4.

7. The method further comprises input means for specifying the specific color and the threshold value. The image forming apparatus according to any one of claims 1 to 6.

8. The control means compares the color difference between the color value of the image of the specific color and the color value of the specific color with a predetermined threshold, and if the color difference is equal to or smaller than the threshold, determines that the image has been printed correctly in the specified specific color, and if the color difference is greater than the threshold, determines that the image has not been printed correctly in the specified specific color.

8. The image forming apparatus according to claim 7.

9. The control means issues a warning through a predetermined output device when it determines that the image is not printed correctly in the specified specific color.

9. The image forming apparatus according to claim 8.

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