JP2013220606A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which can generate a latent image having a low level of visibility under visible light.SOLUTION: An image forming apparatus 1 forms a K monochromatic gray sample image on an intermediate transfer printing belt. The density of the formed image is detected by a density sensor 112. The image forming apparatus 1 determines the density of CMY so that the K monochromatic gray and the CMY mixed color gray may be isochromatic under visible light on the basis of a K-CMY correlation table stored in an HDD 103 and the result of detection by the density sensor 112. The K-CMY correlation table may be a table in which values are fixed or a table in which they are variable. The image forming apparatus 1 generates a latent image in which a monochromatic gray dot image is hidden in a mixed color gray dot image and forms an image by superimposing it on another image.

Description

  The present invention relates to an image forming apparatus that forms a latent image that is visually recognized under specific conditions.

  In recent years, the function of an image forming apparatus for copying an image by image reading and image formation has been improved, and the copying accuracy of the image has been improved, so that a document image such as a document can be copied easily and with high accuracy. In such a high-function image forming apparatus, for example, an illegal copy can be easily created by copying a document that should not be copied, and a highly accurate copy that cannot be distinguished from the original is obtained. be able to. Therefore, in order to prevent unauthorized copying of a document image, a technique for combining a copy prevention pattern or a watermark character with a document image that should be prohibited from being copied has been realized.

  In Patent Document 1, information identifying a person is printed on a base material with visible ink, and information obtained by encoding the person identification information is not absorbed in the visible light wavelength region but is absorbed only in the infrared light wavelength region. ID cards printed with external absorption inks have been proposed. For example, a face photograph image is printed as the person identification information, and a two-dimensional code is printed as the encoded information. This can prevent forgery of the ID card.

  In Patent Document 2, a plurality of first halftone dot regions and second halftone dot regions are arranged on a base material to form a visible image, and the first halftone dot region does not include an infrared absorption characteristic. The second halftone dot region is composed of a second halftone dot region composed of a color material including infrared absorption characteristics and a color material not including infrared absorption characteristics. There has been proposed a halftone dot printed matter having a 2b halftone dot area configured as described above and having a latent image formed by the second a halftone dot area.

JP 2000-309154 A Japanese Patent No. 4142262

  However, since the technique described in Patent Document 1 needs to print encoded information using an infrared absorbing ink that absorbs only the infrared light wavelength region without absorption in the visible light wavelength region, such infrared absorption. A special image forming apparatus for handling ink is required. Therefore, there is a problem that it is difficult to apply this technique to a general image forming apparatus.

  The technology described in Patent Document 2 is based on ink (or toner) of basic four colors of C (cyan), M (magenta), Y (yellow), and K (black) used in a general image forming apparatus. , K ink is a black pigment mainly composed of carbon black and exhibits absorption characteristics over the entire range from the ultraviolet region to the infrared region, whereas the process black with the CMY ink superimposed does not absorb infrared rays. is there. By configuring the 2a halftone dot area with K ink and the other areas with CMY ink process black, when the printed matter is observed with a special device such as an infrared camera, the latent potential formed with K ink Only the image can be visually recognized.

  As in the technique described in Patent Document 2, a latent image is used based on a difference in characteristics between two color materials, a first color material including at least one of CMY and a second color material of K. When forming an image, it is required that the latent image cannot be visually recognized under visible light. For this reason, it is important to optimize the combination of the first color materials and perform color control so as to be the same color as the second color material. However, conventional image forming apparatuses such as printers, copiers, and multi-function machines that employ an electrophotographic method, for example, have insufficient color control as described above, resulting in a color difference that can be recognized by humans. . For this reason, there is a case that the latent image can be visually recognized without using a special device such as an infrared camera, and there is a problem that the performance as the latent image is not sufficient.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an image forming apparatus capable of forming a latent image having low visibility under visible light. is there.

  An image forming apparatus according to the present invention includes an image carrier that supports an image formed of a plurality of color materials until the image is transferred to the medium, and includes an image forming unit that forms an image on the medium, and a plurality of infrared reflecting members. An image including a specific image of the single color gray in the basic image of the mixed color gray and the single color gray using a mixed color gray including the first color material and a single color gray including the second color material that absorbs infrared rays. An image processing apparatus that performs image processing for forming the image on the image carrier, the image carrier having a predetermined image forming unit that forms a predetermined image on the image carrier, and the image Density detecting means for detecting the density of an image formed on the carrier, and based on the detection result of the density detecting means, the color mixture so that the color mixture gray and the single color gray are substantially the same color under visible light; Gure Characterized in that it comprises a determination means for determining the density of each color of the first color material constituting the.

  The image forming apparatus according to the present invention further includes a storage unit that stores a correlation table of the density of each first color material in which the mixed color gray is substantially the same color as the density of the single color gray, and the image carrier. The predetermined image forming unit forms the monochrome gray image on the image carrier, and the determining unit is based on the density of the monochrome gray detected by the density detecting unit and the correlation table stored by the storage unit. Thus, the density of each color of the first color material constituting the mixed color gray is determined.

  The image forming apparatus according to the present invention also includes a gradation table in which an input gradation and an output gradation are associated with each color of the first color material, and gradation adjustment of an image using the gradation table. Adjusting means for performing correction, wherein the determining means corrects the gradation table so that the density of each color of the first color material becomes the determined density.

  In the image forming apparatus according to the present invention, the correlation table is a fixed value table.

  In the image forming apparatus according to the present invention, the correlation table is a variable value table, and a medium on which the mixed gray and / or monochromatic gray image is formed by the image forming unit is read by the reading unit. The image processing apparatus includes: a read image acquisition unit that acquires a read image that has been read; and a correction unit that corrects the correlation table based on the read image acquired by the read image acquisition unit.

  In addition, the image forming apparatus according to the present invention includes a halftone dot image generation unit that generates a halftone dot image using the mixed color gray and the single color gray as an image including the specific image in the basic image, and the halftone dot image generation And a superimposing means for superimposing the halftone dot image generated by the means on another image.

In the present invention, a mixed color gray including a plurality of first color materials (for example, CMY chromatic color materials) that reflect infrared light and a second color material (for example, K achromatic color material) that absorbs infrared light are used. An image including a specific image of a single color gray is formed in the basic image of the mixed color gray and the single color gray using the single color gray included. In addition, the density of the first color material constituting the mixed color gray is determined so that the mixed color gray and the single color gray are substantially the same color under visible light, and the density of the first color material is corrected to be the determined density. ·maintain.
The image forming apparatus forms an image by first forming an image on an image carrier using a plurality of color materials based on an input image, and transferring the image on the image carrier onto a medium such as paper. A means for detecting the density of the image formed on the image carrier is provided, a predetermined image for determining the density of the first color material is formed on the image carrier, and the density is detected. By determining the density of the first color material constituting the mixed color gray according to the detection result, the density can be determined without using a special device. As for the image formed by the image forming apparatus, a basic image is visually recognized under visible light, and a specific image is visually recognized under an infrared environment.

  In the present invention, an image in which a halftone of monochrome gray is drawn is formed on the image carrier, and the density of this image is detected. Further, since the density of each color of the first color material for making the mixed color gray substantially the same color as the single color gray of the second color material is determined according to the characteristics of the image forming means, this is measured in advance and stored as a correlation table. Keep it. From the density detection result of the image formed on the image carrier, the actual monochrome gray density can be checked, and the corresponding density of each color of the mixed color gray can be determined from the correlation table.

  In the present invention, when an image is formed, gradation processing is performed on the input image using a gradation table. When the density of each color of the mixed color gray is determined as described above, the gradation table is corrected so that the determined density is obtained. As a result, in the subsequent image formation, the density of the mixed color gray is appropriately adjusted by the image processing for gradation adjustment.

The correlation table used when determining the density of each color of mixed color gray may be a fixed value table or a variable value table. In the case of a fixed value correlation table, the characteristics of the image forming means are measured in advance in the manufacturing process of the image forming apparatus, and the correlation table is generated and stored in advance in the memory of the image forming apparatus.
In the case of a variable value correlation table, for example, a sample image on which a halftone of mixed color gray and / or single color gray is drawn is formed on a medium such as paper, and the formed image is read by reading means such as a scanner. The correlation table can be generated or corrected based on the read image.

  Further, in the present invention, a halftone image with mixed color gray and single color gray is generated. This halftone dot image is an image in which a specific image of single color gray is embedded in a basic image of substantially equal color mixture gray and single color gray. By superimposing the generated halftone image and another image, an image in which the security latent image is embedded can be generated by a simple method.

  According to the present invention, it is possible to easily determine the density of each color without using a special device by detecting the density of a predetermined image formed on the image carrier and determining the density of each color constituting the mixed color gray. Therefore, it is possible to provide an image forming apparatus with high convenience and versatility. Since the image forming apparatus can make the single color gray and the mixed color gray substantially the same color, it is possible to generate an image in which a latent image having low visibility under visible light is embedded.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a block diagram illustrating a configuration of an image forming apparatus. 6 is a flowchart illustrating a procedure of image forming processing by the image forming apparatus. It is a schematic diagram for demonstrating a latent image. It is a schematic diagram for demonstrating a latent image. It is a schematic diagram for demonstrating a latent image. It is a schematic diagram for demonstrating a latent image. 6 is a flowchart illustrating a procedure of density determination processing performed by the image forming apparatus according to the present embodiment. It is a schematic diagram which shows an example of a sample image. It is a schematic diagram which shows an example of a K-CMY correlation table.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof. FIG. 1 is a schematic diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 1 according to the present embodiment has multiple colors for a predetermined medium such as a sheet (recording paper) according to image data input from an external device such as a PC (Personal Computer) via a network. An apparatus for forming a monochrome image. The image forming apparatus 1 includes an exposure unit 11, developing devices 12a to 12d, photosensitive drums 13a to 13d, cleaner units 14a to 14d, chargers 15a to 15d, an intermediate transfer belt 17, a fixing unit 22, and the like. Yes.

  The image forming apparatus 1 according to the present embodiment adds 4 (black) to cyan (C), magenta (M), and yellow (Y), which are three subtractive primary colors obtained by color separation of a color image. Color image formation using colors can be performed. For this reason, the image forming apparatus 1 includes four developing devices 12a to 12d, photosensitive drums 13a to 13d, cleaner units 14a to 14d, and chargers 15a to 15d corresponding to the respective colors. An image station to be formed is configured. The four image stations are spaced apart by an appropriate length along the moving direction (sub-scanning direction) of the intermediate transfer belt 17.

  The photoconductive drums 13 a to 13 d have a cylindrical shape, and are arranged below the intermediate transfer belt 17 so that the axial direction thereof intersects the moving direction of the intermediate transfer belt 17. The chargers 15 a to 15 d are for charging the surfaces of the photosensitive drums 13 a to 13 d to a predetermined potential. In the illustrated example, a contact type roller charger is mounted on the image forming apparatus 1. However, a contact brush type charger or a charger type charger may be used.

  The exposure unit 11 is configured using a laser scanning unit including a laser irradiation unit and a reflection mirror. However, an EL (Electro Luminescence) or LED (Light Emitting Diode) writing head in which light emitting elements are arranged in an array may be used. The exposure unit 11 irradiates the photosensitive drums 13a to 13d with laser beams modulated by CMYK color image data. As a result, the charged photosensitive drums 13a to 13d are exposed according to the input image data, and electrostatic latent images are formed on the surface of the CMYK color image data. The developing units 12a to 12d store toners of CMYK colors, respectively supply the toner to the surfaces of the photosensitive drums 13a to 13d on which the electrostatic latent images are formed, and develop the electrostatic latent images into toner images. Image. The cleaning units 14a to 14d remove and collect the toner remaining on the surfaces of the photosensitive drums 13a to 13d after development and transfer of the toner image.

  The image forming apparatus 1 also includes intermediate transfer rollers 16a to 16d, an intermediate transfer belt 17, a driving roller 171, a driven roller 172, a cleaning unit 19, an intermediate transfer belt tension mechanism (not shown), and the like. The intermediate transfer rollers 16a to 16d, the driving roller 171, the driven roller 172, and the intermediate transfer belt tension mechanism are for rotating the intermediate transfer belt 17 in a predetermined direction (see an arrow in the drawing). The intermediate transfer belt 17 is disposed above the photosensitive drums 13 a to 13 d and is stretched between the driving roller 171 and the driven roller 172. The intermediate transfer rollers 16a to 16d are rotatably supported at positions facing the respective photosensitive drums 13a to 13d with the intermediate transfer belt 17 interposed therebetween. The intermediate transfer rollers 16a to 16d have a polarity (+) opposite to the toner charging polarity (-) in order to transfer the toner image carried on the surface of the photosensitive drums 13a to 13d onto the intermediate transfer belt 17. A transfer bias is applied. The intermediate transfer belt 17 is provided so as to be in contact with the photosensitive drums 13 a to 13 d, and the toner images of the respective colors formed on the photosensitive drums 13 a to 13 d are sequentially superimposed on the outer peripheral surface of the intermediate transfer belt 17. By transferring, a color toner image is formed on the outer peripheral surface of the intermediate transfer belt 17. The intermediate transfer belt 17 is an endless film having a thickness of about 1100 μm to 1150 μm.

  Transfer of the toner image from the photosensitive drums 13 a to 13 d to the intermediate transfer belt 17 is performed by intermediate transfer rollers 16 a to 16 d that are in contact with the inner peripheral surface of the intermediate transfer belt 11. A high voltage transfer bias for transferring the toner image is applied to the intermediate transfer rollers 16a to 16d. The intermediate transfer rollers 16a to 16d are configured by covering a metal (for example, stainless steel) shaft having a diameter of about 18 to 110 mm with a conductive elastic material (for example, EPDM, urethane foam, or the like). A uniform high voltage can be applied to the intermediate transfer belt 17 by this conductive elastic material. Instead of the intermediate transfer rollers 16a to 16d, a brush-like intermediate transfer member can be used.

  Each of the photosensitive drums 13 a to 13 d is visualized according to each color, and the electrostatic image is an input image by being laminated on the intermediate transfer belt 17. The images stacked on the intermediate transfer belt 17 are conveyed to a position facing the transfer roller 21 by the rotation of the intermediate transfer belt 17 and transferred onto a sheet. At this time, the transfer roller 21 presses the intermediate transfer belt 17 with a predetermined nip pressure, and the transfer roller 21 has a voltage for transferring the toner onto the sheet (having a polarity (+) opposite to the toner charging polarity (−)). High voltage) is applied. In order to maintain the nip pressure between the transfer roller 21 and the intermediate transfer belt 17 at a predetermined value, either the transfer roller 21 or the drive roller 171 is made of a hard material (metal or the like), and the other is an elastic roller or the like. The soft material (such as an elastic rubber roller or a foaming resin roller) can be used.

  Toner adhering to the intermediate transfer belt 17 due to contact with the photosensitive drums 13a to 13d or toner remaining on the intermediate transfer belt 17 without being transferred onto the sheet by the transfer roller 21 prevents color mixing in the next step. Therefore, it is removed and collected by the cleaning unit 19. The cleaning unit 19 has, for example, a cleaning blade as a cleaning member that contacts the intermediate transfer belt 17. The intermediate transfer belt 17 in contact with the cleaning member is supported by a driven roller 172 from the opposite side.

  The sheet on which the toner image is transferred is guided to the fixing unit 22 and passes between the heat roller 24 and the pressure roller 25 and is heated and pressed. As a result, the toner image transferred to the paper is fixed on the surface of the paper. The paper on which the toner image is fixed is discharged onto the paper discharge tray 29 by the paper supply roller 27c.

  The image forming apparatus 1 also includes a paper feed tray 20, a manual feed tray 26, a paper discharge tray 29, a paper transport path S, and the like. The paper feed tray 20 is a tray for storing paper used for image formation, and is provided at the lower portion of the image forming apparatus 1. The paper discharge tray 29 is a tray for placing paper on which image formation has been performed face down, and is provided on the upper portion of the image forming apparatus 1. The paper transport path S is for sending the paper stored in the paper feed tray 20 to the paper discharge tray 29 via the transfer roller 21 and the fixing unit 22, and the inside of the image forming apparatus 1 is set in a substantially vertical direction. Has been placed. In the paper transport path S, a transfer roller 21, a fixing unit 22, a registration roller 23, transport rollers 27a to 27h, pickup rollers 28a and 28b, and the like are arranged.

  The transport rollers 27a to 27h are small rollers for promoting and assisting the transport of paper, and a plurality of the transport rollers 27a to 27h are provided along the paper transport path S. The pickup roller 28 a is a pull-in roller that is provided at the end of the paper feed tray 20 and supplies the paper stored in the paper feed tray 20 to the paper transport path S one by one. The registration roller 23 has a function of temporarily holding the sheet conveyed through the sheet conveyance path S and conveying the sheet to the transfer roller 21 at a timing when the leading edge of the toner image on the intermediate transfer belt 17 is aligned with the leading edge of the sheet. ing.

  The fixing unit 22 includes a heat roller 24, a pressure roller 25, and the like. The heat roller 24 and the pressure roller 25 are configured to rotate with a sheet interposed therebetween. The temperature of the heat roller 24 is detected by a temperature detector (not shown), and the temperature of the heat roller 24 is controlled to a predetermined fixing temperature by a printer controller described later. The heat roller 24 heat-presses the paper together with the pressure roller 25 to melt, mix and press the multicolor toner images transferred onto the paper, and heat-fix the paper on the paper.

  In addition, the image forming apparatus 1 is provided with a paper feed tray 20 for storing a large number of sheets in advance. When the user forms an image on a small number of sheets, the image forming apparatus 1 opens and closes the paper feed tray 20. A manual feed tray 26 on which a small number of sheets are placed is provided so as not to be performed. The pickup roller 28a or 28b guides the paper from the paper feed tray 20 or the manual feed tray 26 to the paper transport path S one by one.

  When image formation is performed on only one side of the paper, the paper stored in the paper feed tray 20 is fed by the pickup roller 28a and is transported through the paper transport path S to the registration roller 23 by the transport roller 27a. The toner image is transferred to a sheet of paper by being conveyed to the transfer roller 21 at the timing of matching the leading edge of the toner image on the transfer belt 17. Thereafter, the sheet passes through the fixing unit 22 so that unfixed toner is melted and fixed by heat, and is discharged onto the sheet discharge tray 29 through the conveyance rollers 27b and 27c.

  The paper placed on the manual feed tray 26 is fed by the pickup roller 28b, and is transported through the paper transport path S to the registration roller 23 through the transport rollers 27f, 27e, and 27d. Thereafter, the same processing as that for paper fed from the paper feed tray 20 is performed, and the paper on which the image is formed is discharged to the paper discharge tray 29.

  When image formation is performed on both sides of a sheet, the rear end of the sheet that has passed through the fixing unit 22 after the image formation on one side has been completed by the above processing is stopped by the conveyance roller 27c, and the conveyance roller 27c rotates backward. Is guided to the reverse path provided with the transport rollers 27g and 27h, and then the back side image is formed through the registration roller 23 and discharged to the paper discharge tray 29.

  The four-color toners of CMYK used by the image forming apparatus 1 are manufactured with a general prescription. Color materials used for these toners are organic pigments having absorption and reflection characteristics in the visible light region. For example, a cyan pigment is a copper phthalocyanine pigment, a magenta pigment is a dimethylquinacridone pigment, a yellow pigment is a benzimidazolone pigment, a black pigment is a carbon black pigment, both of which are It is a common organic material that is commercially available. Black pigments have absorption characteristics not only in the visible light region but also in the infrared region. For this reason, in an image formed using four color toners of CMYK, a portion formed of K toner is visually recognized under infrared rays.

  FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus 1. The image forming apparatus 1 receives image data such as characters, graphics, or photographs input from an external device such as a PC 2 via a network in the form of a page description language, and uses it for image formation based on the received image data. It is a device such as a printer that generates data and forms an image. The PC 2 is connected to the scanner 3, acquires a read image obtained by the reading process of the scanner 3, and can give the read image to the image forming apparatus 1. The image forming apparatus 1 includes a printer controller 100, a print engine 110, an operation panel 120, and the like. The print engine 110 and the operation panel 120 are connected to the printer controller 100 and exchange data with the printer controller 100. The printer controller 100 is connected to an external device such as a PC 2 via a network.

  The printer controller 100 includes a central processing unit (CPU) 101, a random access memory (RAM) 102, a hard disk drive (HDD) 103, a network controller 104, a video controller 105, and the like. The CPU 101 performs control processing for each unit in the printer controller 100. The RAM 102 temporarily stores various types of information such as image data received from an external device, execution data generated based on the image data, and density correction information. The HDD 103 stores image data, execution data, characteristics of the print engine 110 (for example, characteristics such as density, gradation, or dot reproduction) data, and the like. The network controller 104 transmits / receives data to / from an external device such as the PC 2 via the network. The video controller 105 controls the operation of the print engine 110.

  The print engine 110 includes an image forming unit 111, a density sensor 112, a temperature / humidity sensor 113, and the like. The image forming unit 111 performs a process of forming an image on a sheet, and controls the operation of each mechanism shown in FIG. The density sensor 112 and the temperature / humidity sensor 113 are used when the image forming apparatus 1 is calibrated. The density sensor 112 detects the density of the image formed on the intermediate transfer belt 17 that is an image carrier, and notifies the CPU 11 of the printer controller 100 of the detection result. Although not shown in FIG. 1, the density sensor 112 is disposed, for example, in the vicinity of the drive roller 171 so as to face the outer peripheral surface of the intermediate transfer belt 17. The temperature / humidity sensor 113 measures the temperature and humidity inside and / or outside the housing of the image forming apparatus 1 and notifies the CPU 101 of the printer controller 100 of the measurement result. The operation panel 120 receives an operation related to a user instruction such as an instruction to form an image stored in the HDD 103 and notifies the printer controller 100 of the operation.

  FIG. 3 is a flowchart showing a procedure of image forming processing by the image forming apparatus 1. When the user issues an image formation instruction of the original on the PC 2, an image is input to the image forming apparatus 1 from a printer driver or the like of the PC 2. The image forming apparatus 1 receives image input from the network controller 104 by receiving image data from the PC 2 (step S1). The image data input at this time is data represented by three colors of R (red), G (green), and B (blue).

  Next, the image forming apparatus 1 performs a color conversion process for converting the input RGB image data into CMYK image data (step S2). In the color conversion process, the CPU 101 performs a color conversion operation based on the color table of the ICC profile using a color management module installed in the firmware of the printer controller 100.

  Next, the image forming apparatus 1 performs gradation correction processing on the CMYK image data obtained by the color conversion processing (step S3). In the gradation correction processing, for example, low contrast processing that emphasizes output gradation reproducibility is performed by the CPU 101 using a gradation correction table stored in advance in the HDD 103. Thereafter, the image forming apparatus 1 performs halftone adjustment processing (step S4). The halftone adjustment process is a process that is performed in order to always maintain a constant state of an image to be formed by correcting variations in each color due to machine difference variations, temporal changes, environmental changes, and the like of the print engine 110 and is stored in the HDD 103 in advance. The CPU 101 executes the halftone table. Since these processes are general processes, a detailed description of the processing procedure is omitted. However, for example, the correspondence between the input gradation and the output gradation for each color of CMYK is stored in the halftone table, and the process of step S3 is performed. Processing for converting the gradation value of the processed image data according to the intermediate table can be performed. The CMYK image data obtained by the above steps S1 to S4 is stored in the RAM 102.

  The image forming apparatus 1 according to the present embodiment has a function of generating an image in which a latent image is embedded in an original image such as a document created by a user and forming the generated image. Although details of the latent image will be described later, the latent image is input from the PC 2 together with an image such as a document or stored in the HDD 103 in advance. When the user gives an instruction to embed a latent image, the image forming apparatus 1 performs the same processing as steps S1 to S4 on the latent image. However, the gradation correction table used in the gradation correction process in step S3 and the halftone table used in the halftone adjustment process in step S4 may be those for a latent image. The image data of the CMYK latent image obtained in the processes of steps S1 to S4 is stored in the RAM 102.

  Next, the image forming apparatus 1 determines whether or not there is a latent image (step S5). If there is a latent image (S5: YES), the original image such as a document and the latent image are read from the RAM 102 and combined. Processing is performed (step S6). In the image compositing process, calculation processing of pixels of the original image and pixels of the latent image is performed at the designated position of the original image. As the arithmetic processing, it is preferable to multiply the pixel values of both pixels. After performing the image composition process, the image forming apparatus 1 forms the image by providing the image data after the composition to the image forming unit 111 of the print engine 110 (step S7), and ends the process. If there is no latent image (S5: NO), the image forming apparatus 1 provides the data stored in the RAM 102 to the image forming unit 111 without performing the composition process, and performs image formation (step S7). Exit.

  In the above description, the image forming apparatus 1 performs all the processes of steps S1 to S6 in the flowchart shown in FIG. 3, but the present invention is not limited to this. A part or all of the processes in steps S1 to S6 may be performed in advance on the PC 2, and processed image data may be input to the image forming apparatus 1 to form an image.

  4 to 7 are schematic diagrams for explaining the latent image. In each figure, an enlarged image of a part of the image shown on the upper side is shown on the lower side. The latent image handled by the image forming apparatus 1 according to the present embodiment is a halftone dot image represented by a set of gray halftone dots as shown in FIG. In addition, the shape, size, arrangement, and the like of the halftone dots shown in the figure are merely examples, and the present invention is not limited to this. In the illustrated example, rectangular halftone dots having the same vertical and horizontal sizes (number of pixels) are used. Each pixel constituting the halftone dot is gray (or black), but a K single-color gray pixel and a CMY mixed-color gray pixel are mixed. Black shall be included in gray.

  FIG. 5 is an image obtained by extracting a K single-color gray portion from the halftone image shown in FIG. The image shown in FIG. 5 is an image obtained when the image shown in FIG. 4 is viewed under infrared rays, and becomes a latent image. Each halftone dot may be partly a monochrome gray pixel, or all may be a monochrome gray pixel. By changing the ratio of monochrome gray pixels included in the halftone dots, it is possible to realize area gradation expression. In the example shown in the lower part of FIG. 5, the dark portion of the image is composed of halftone dots of six single-color gray pixels, and the light portion of the image is composed of halftone dots of one single-color gray pixel. 5 constitutes the image shown on the upper side. The shape, size, arrangement, and the like of the monochrome gray dot are not limited to those shown in FIG.

  FIG. 6 is an image obtained by extracting the CMY mixed color gray portion from the halftone image shown in FIG. Each pixel constituting the halftone dot is composed of CMY mixed in a mixing amount determined so as to be the same color as a monochromatic gray under visible light. The image shown in FIG. 6 corresponds to an image obtained by subtracting the image shown in FIG. 5 from the image shown in FIG. 4, but the infrared ray is reflected under the infrared ray, and the image cannot be visually recognized.

  FIG. 7 is a composite image obtained by superimposing the halftone dot image shown in FIG. 4 on an image such as a document created by the user, and is an image obtained by the process in step S6 of the flowchart shown in FIG. Any method may be used for superimposing the two images, but it is preferable to use a multiplication process in order to retain the pixel value of the gray dot. When the synthesized image shown in FIG. 7 is viewed under infrared rays, only K single gray is visually recognized and becomes the image shown in FIG. 5 (however, the original image on which the latent image is superimposed (image such as a document) is K (It is assumed that monochrome gray is not used, and the image is a CMY color or monochrome image).

  As described above, the image forming apparatus 1 according to the present embodiment generates an image in which a latent image that is visible only under infrared rays is superimposed on an image such as a document, and forms the generated image on a sheet and outputs it. be able to. The formed image is visually recognized as an image such as a document under visible light, and only the latent image is visible under infrared light. However, in order that the latent image is not visually recognized under visible light, it is necessary that the K single color gray and the CMY mixed color gray be the same color under visible light. Therefore, the image forming apparatus 1 needs to appropriately determine the density of each color of CMY so that the mixed-color gray and the single-color gray are the same color. Therefore, the image forming apparatus 1 has a function of determining the density of each color of CMY. This density determination may be performed based on a user instruction at the time of maintenance of the apparatus or at the time of toner replacement, for example, and the presence or absence of an environmental change is determined based on the detection result of the temperature / humidity sensor 113, for example. 1 may be performed automatically. Hereinafter, the density determination processing for each color of CMY constituting the mixed color gray by the image forming apparatus 1 will be described.

  FIG. 8 is a flowchart showing the procedure of density determination processing performed by the image forming apparatus 1 according to this embodiment. In order to determine the mixing amount, the image forming apparatus 1 generates a sample image for K monochrome gray, and forms the generated sample image on the intermediate transfer belt 17 (step S21). FIG. 9 is a schematic diagram illustrating an example of a sample image. The sample image is a predetermined image, for example, image data is stored in the HDD 103. The sample image is an image in which a plurality of halftones are drawn for K monochrome gray, and its layout, gradation, and the like are determined in advance. In the illustrated sample image, a plurality of rectangles are arranged in the moving direction of the intermediate transfer belt 17, and each rectangle is colored so that the density of K monochromatic gray gradually changes. The colors in each rectangle are expressed in halftones by changing the dot area ratio. The illustrated sample image is an example, and the present invention is not limited to this. The sample image preferably has the shape of a latent image superimposed on the final output image as shown in FIGS. 4 to 7. However, in this shape, the dot area ratio is low, and the density sensor 112 is sufficiently detected. For example, it is preferable to use another shape having high linearity with respect to the dot gain of the halftone dot shape to be superimposed, for example, or to have a shape with an increased dot density.

  The image forming apparatus 1 sequentially detects the density of the sample image formed on the intermediate transfer belt 17 by the density sensor 112 while moving the intermediate transfer belt 17 (step S22). The density sensor 112 includes, for example, a light emitting element and a light receiving element, and measures the signal intensity such as regular reflection or scattered reflection when the sample image formed on the intermediate transfer belt 17 emits light with the light receiving element. A configuration that converts the signal intensity into a density value can be used. The image forming apparatus 1 stores the detection result of the density sensor 112 in the RAM 102.

  Next, the image forming apparatus 1 performs K gradation correction processing (step S23). In the K tone correction process, a preset K tone (density) of the sample image is compared with the density value detected by the density sensor 112 to calculate an appropriate input value-output value correspondence. . The image forming apparatus 1 calculates the input value-output value correspondence, for example, for 16 points in the halftone of the K drawn in the sample image, and outputs the output value for the input value by calculation such as linear interpolation for the remaining points. Is calculated. The image forming apparatus 1 stores the calculated K input value-output value correspondence in the HDD 103 as a halftone table and uses it in the halftone adjustment process in step S4 of the flowchart of FIG.

  The density of each CMY color for making the CMY mixed color gray and the K single color gray equal is determined uniquely according to the output characteristics of the print engine 110. Therefore, the correspondence relationship between the K density and the CMY mixture amount is examined in advance and stored in the HDD 103 as a K-CMY correlation table. FIG. 10 is a schematic diagram showing an example of the K-CMY correlation table, and shows the correspondence between K and each color of CMY in a graph. The K-CMY correlation table may be a fixed table with a fixed value (invariable) or a variable table with a changeable value. When the K-CMY correlation table is a fixed table, for example, output characteristics are measured in the manufacturing process of the image forming apparatus 1 to create a K-CMY correlation table and store it in the HDD 103 in advance. The table created in this way can be used as an initial value when the K-CMY correlation table is variable.

  When the K-CMY correlation table is a variable table, the image forming apparatus 1 performs table correction processing using the scanner 3 or the like. In the table correction process, the image forming apparatus 1 forms, on a sheet, a sample image on which, for example, a K gray-tone halftone and a CMY mixed-gray gray halftone are drawn, and the scanned image is read by the scanner 3. Is obtained from PC2. The image forming apparatus 1 calculates a correspondence relationship between K and CMY in which the K single color gray and the CMY mixed color gray are substantially the same color based on the read image, and corrects the K-CMY correlation table to obtain this correspondence relationship. And stored in the HDD 103. If the image forming apparatus 1 has a scanner function, the sample image may be read by the scanner function of the image forming apparatus 1.

  The image forming apparatus 1 reads the K-CMY correlation table generated or corrected in advance and stored in the HDD 103 (step S24), and the density determination process is continued.

  The image forming apparatus 1 performs CMY gradation correction based on the K gradation corrected in step S23 and the read K-CMY correlation table (step S25). In the CMY tone correction, an appropriate input value-output value correspondence is calculated for each color of CMY so that the K tone already corrected has the correspondence set in the K-CMY correlation table. To do. For example, the image forming apparatus 1 can obtain the actual density output for one gradation of K from the halftone table created in step S23, and the density of each color of CMY corresponding to the density of K. Since it can be acquired from the K-CMY correlation table, the correspondence between the input value and the output value for each color of CMY may be calculated so that the density acquired in this way becomes an actual output. The image forming apparatus 1 calculates an input value-output value correspondence for, for example, each of 16 CMY halftones, and calculates an output value for the input value for the remaining points by an operation such as linear interpolation. The image forming apparatus 1 stores the calculated input value-output value correspondence of each color of CMY in the HDD 103 as a halftone table, and uses it in the halftone adjustment process in step S4 of the flowchart of FIG.

  By these processes, the density of each color of CMY of the mixed color with respect to the K monochrome gray is appropriately determined, and by performing the halftone adjustment process on the latent image using the obtained halftone table, The density of CMY is appropriately controlled so that the mixed color of CMY becomes the same color.

  The image forming apparatus 1 having the above configuration forms a K single-color gray sample image on the intermediate transfer belt 17 and detects the K single-color gray and the CMY mixed-color gray based on the result detected by the density sensor 112. By adopting a configuration in which the CMY density is determined so as to be equal in color under visible light, an appropriate density according to the characteristics of the print engine 110 can be determined without using a special device. As a result, an image including a latent image that is not visually recognized under visible light can be formed.

  In the present embodiment, the image forming apparatus 1 is configured not to include a scanner, but is not limited thereto, and may be configured to include a scanner. Further, the density detection is performed by forming a sample image on the intermediate transfer belt 17 as an image carrier. However, the present invention is not limited to this, and the density detection is performed by forming a sample image on an image carrier other than the intermediate transfer belt 17. The structure which performs this may be sufficient.

  In the above-described embodiment, it is assumed that the image forming apparatus 1 handles a color image or a grayscale image in which the gradation of each color is represented by 8 bits or the like, and the configuration of the sample image, the K-CMY correlation table, etc. In addition, the procedure of processing using these has been described. However, it may be configured to handle a binary image (monochrome image) whose gradation is represented by 1 bit. In this case, only the maximum density point for each color is generated and the halftone table is generated. Processing such as correction may be performed. The sample image is preferably formed by dithering a halftone image, and the halftone dot of the sample image is preferably the same shape as the halftone dot shown in FIG.

  The density determination process described above is performed when the gradation change of each color is large, such as when the operating environment or the surrounding environment of the image forming apparatus 1 changes, or when image formation of a specified number or more is performed. It is preferred to implement. The determination as to whether or not the state change of the image forming apparatus 1 has occurred includes, for example, determination as to whether the number of images formed exceeds a threshold value, determination based on the detection result of the temperature / humidity sensor 113, development conditions, process conditions, or the like. For example, when it is determined that a state change has occurred, the above-described density determination process may be performed. The density determination process may be performed automatically by the image forming apparatus 1 when it is determined that a state change has occurred, or may be performed by a user giving an execution instruction from the operation panel 120 or the like.

1 Image forming apparatus 2 PC
3 Scanner 11 Exposure unit 12a to 12d Developer 13a to 13d Photosensitive drum 14a to 14d Cleaner unit 15a to 15d Charger 16a to 16d Intermediate transfer roller 17 Intermediate transfer belt (image carrier)
DESCRIPTION OF SYMBOLS 20 Paper feed tray 21 Transfer roller 22 Fixing unit 23 Registration roller 24 Heat roller 25 Pressure roller 27a-27h Conveyance roller 28a, 28b Pickup roller 29 Paper discharge tray 100 Printer controller 101 CPU (Image processing means, determination means, adjustment means, Correction means, halftone image generation means, superimposition means)
102 RAM
103 HDD (storage means)
104 Network controller (read image acquisition means)
105 Video Controller 110 Print Engine 111 Image Forming Unit (Image Forming Unit, Image Carrier Predetermined Image Forming Unit)
112 Concentration sensor (concentration detection means)
113 Temperature / humidity sensor 120 Operation panel

Claims (6)

A mixed color gray having an image carrier that carries an image formed of a plurality of color materials until the image is transferred to the medium, and includes an image forming unit that forms an image on the medium and a plurality of first color materials that reflect infrared rays And an image including the specific image of the monochromatic gray in the basic image of the mixed gray and the monochromatic gray using the monochromatic gray including the second color material that absorbs infrared rays. An image forming apparatus comprising image processing means for performing image processing for
An image carrier predetermined image forming means for forming a predetermined image on the image carrier;
Density detecting means for detecting the density of the image formed on the image carrier;
Determination to determine the density of each color of the first color material constituting the mixed-color gray so that the mixed-color gray and the single-color gray become substantially equal colors under visible light based on the detection result of the density detection unit And an image forming apparatus. Storage means for storing a correlation table of the density of each first color material in which the mixed color gray is substantially the same color with respect to the density of the monochrome gray;
The image carrier predetermined image forming means forms the monochrome gray image on the image carrier,
The determining means determines the density of each color of the first color material constituting the mixed color gray based on the density of the single color gray detected by the density detecting means and the correlation table stored by the storage means. The image forming apparatus according to claim 1, wherein the image forming apparatus is configured as described above. A gradation table in which an input gradation and an output gradation are associated with each color of the first color material;
Adjusting means for adjusting the gradation of the image using the gradation table;
The image forming apparatus according to claim 2, wherein the determination unit corrects the gradation table so that the density of each color of the first color material becomes a determined density. The image forming apparatus according to claim 2, wherein the correlation table is a fixed value table. The correlation table is a variable value table;
A read image acquisition means for acquiring a read image obtained by reading the medium on which the mixed-tone gray and / or the single-tone gray image is formed by the image forming means;
The image forming apparatus according to claim 2, further comprising: a correction unit that corrects the correlation table based on the read image acquired by the read image acquisition unit. As an image including the specific image in the basic image, halftone dot image generation means for generating a halftone dot image by the mixed color gray and the single color gray;
The image forming apparatus according to claim 1, further comprising: a superimposing unit that superimposes the halftone image generated by the halftone image generating unit on another image.