Image forming device

Abstract

To provide an image forming apparatus that can improve the invisibility of an invisible image.SOLUTION: An image forming apparatus has an image forming unit that forms an image on a recording medium and a control unit. The image forming unit forms, on the recording medium, an image obtained by superimposing a visible image formed by a visible material on an invisible image formed by an invisible material, and the invisible image is located closer to a recording medium side than the visible image. When a first adhesion amount is the amount of adhesion of the visible material and the invisible material per unit area of a predetermined image in a portion where the invisible image and the visible images are superimposed, and a second adhesion amount is an adhesion amount smaller than the first adhesion amount and is the amount of adhesion of the visible material and the invisible material per unit area of the predetermined image in the portion where the invisible image and the visible images are superimposed, in forming an image with the second adhesion amount, the control unit performs control of reducing the amount of the invisible material in the second adhesion amount compared with the amount of the invisible material in the first adhesion amount.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] In recent years, the copying performance of image forming devices has improved to the point where it has become difficult to distinguish between legitimate copies and illegally copied copies. Therefore, in order to prevent illegal copying and protect copyrights, a technology has been proposed for forming an invisible image, which is difficult to see with the naked eye, on a recording medium in addition to a visible image, which is visible with the naked eye.

[0003] For example, Patent Document 1 discloses a toner set having a color toner and an invisible toner containing a near-infrared absorbing material, and attempts to improve the visibility of the image quality of the color toner image and the reading accuracy of the invisible toner image. Patent Document 1 is characterized in that the 60 degree gloss of a solid image of the invisible toner is 30 or more, and is 10 or more higher than the 60 degree gloss of a solid image of the color toner. Incidentally, the color toner is also called a visible toner, the color toner image is also called a visible image, and the invisible toner image is also called an invisible image.

[0004] It is believed that by using the invisible toner and visible toner disclosed in Patent Document 1 to form an invisible image under a visible image, the invisibility of the invisible image, that is, the invisibility of the invisible image, can be ensured. Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the conventional technology, when the color of the visible toner is close to the visible wavelength component contained in the invisible toner, the invisible image is not masked by the visible image and is visible, so there is still a need to improve the invisibility of the invisible image.

[0006] SUMMARY OF THE PRESENT EMBODIMENTS In view of the above, an object of the present invention is to provide an image forming apparatus capable of improving the invisibility of an invisible image. [Means for solving the problem]

[0007] In order to solve the above problems, an image forming apparatus of the present invention is an image forming apparatus having an image forming section which forms an image on a recording medium, and a control section which controls the image forming section, wherein the image forming section forms an image on the recording medium by superimposing an invisible image made of an invisible material and a visible image made of a visible material on the invisible image in that order, the invisible image being positioned closer to the recording medium than the visible image, and when an adhesion amount of the visible material and the invisible material per unit area of ​​a predetermined image in a portion where the invisible image and the visible image are superimposed is defined as a first adhesion amount, and an adhesion amount of the visible material and the invisible material per unit area of ​​a predetermined image in a portion where the invisible image and the visible image are superimposed is defined as a second adhesion amount which is less than the first adhesion amount, the control section: changing a position where the invisible image is formed in accordance with information of the visible image; When forming an image with the second adhesion amount, the amount of the invisible material in the second adhesion amount is controlled to be smaller than the amount of the invisible material in the first adhesion amount. and performing control so as to make the size of the invisible image larger than when the invisible image is formed with the first adhesion amount. It is characterized by: Effect of the Invention

[0008] According to the present invention, it is possible to provide an image forming apparatus capable of improving the invisibility of an invisible image. [Brief description of the drawings]

[0009] [Figure 1] 1 is an overall schematic diagram showing an example of an image forming apparatus according to the present invention. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing an example of an image on a recording medium. [Diagram 3] 1 is a schematic diagram showing a main part of an example of an image forming apparatus according to the present invention; [Figure 4] 1A and 1B are schematic cross-sectional views showing other examples of images on a recording medium. [Diagram 5]1 is a diagram showing an example of the relationship between the adhesion amount of an invisible developer on a recording medium and invisibility. [Figure 6] 11 is a diagram showing an example of the relationship between the amount of invisible developer adhered to a recording medium and readability. FIG. [Figure 7] 11A and 11B are diagrams illustrating an example of control of the amount of invisible developer adhered to a recording medium. [Figure 8] 5A to 5D are plan schematic diagrams showing other examples of images on a recording medium. [Figure 9] 11 is a schematic cross-sectional view showing another example of an image on a recording medium. FIG. [Figure 10] FIG. 11 is an overall schematic view showing another example of an image forming apparatus according to the present invention. [Figure 11] FIG. 4 is a schematic view showing a main part of another example of an image forming apparatus according to the present invention. [Figure 12] FIG. 4 is a schematic view showing a main part of another example of an image forming apparatus according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The image forming apparatus according to the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following embodiment, and can be modified within the scope of what a person skilled in the art can imagine, such as other embodiments, additions, modifications, deletions, etc., and any aspect is included in the scope of the present invention as long as it achieves the functions and effects of the present invention.

[0011] The image forming apparatus of the present invention is an image forming apparatus having an image forming unit that forms an image on a recording medium and a control unit that controls the image forming unit, wherein the image forming unit forms an image on the recording medium by sequentially superimposing an invisible image made of an invisible material and a visible image made of a visible material on the invisible image, the invisible image being located closer to the recording medium than the visible image, and when a first adhesion amount is set as an adhesion amount of the visible material and the invisible material per unit area of ​​a specified image in a portion where the invisible image and the visible image are overlapped, and a second adhesion amount is set as an adhesion amount that is less than the first adhesion amount, the control unit, when forming an image with the second adhesion amount, performs control to make the amount of the invisible material in the second adhesion amount less than the amount of the invisible material in the first adhesion amount.

[0012] An embodiment of an image forming apparatus according to the present invention will be described. The image forming apparatus of this embodiment is an image forming apparatus that forms an image by an electrophotographic method, and includes an image carrier, a charging unit that charges the image carrier, an exposure unit that exposes the image carrier to form an electrostatic latent image, a development unit that visualizes the electrostatic latent image formed on the image carrier with a developer to form a visible image, and a transfer unit that transfers the visible image formed on the image carrier to an intermediate transfer body and transfers the visible image transferred to the intermediate transfer body to the recording medium. The image forming section includes the image carrier, the charging section, the exposure section, the developing section, and the transfer section. The developing section forms the visible image by using a visible developer as the visible material, and the visible image by using an invisible developer as the invisible material. When the visible image formed by the visible developer transferred onto the recording medium is designated as the visible image, and the visible image formed by the invisible developer transferred onto the recording medium is designated as the invisible image, the developing unit and the transfer unit perform the visualization and the transfer so as to overlay the invisible image and the visible image on the invisible image, in that order, on the recording medium. In addition, when the invisible image is formed with the second adhesion amount, the control unit controls the amount of the invisible developer on the intermediate transfer body to be less than when the invisible image is formed with the first adhesion amount.

[0013] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus (hereinafter also simply referred to as a printer) according to the present embodiment. The printer shown in this figure has an endless belt (intermediate transfer belt 11) as an intermediate transfer body.

[0014] Four image creating stations (also referred to as image forming units, image creating units, etc.) 10A, 10B, 10C, and 10D are arranged in parallel along the upper running edge of the intermediate transfer belt 11. The printer of this embodiment configures a tandem image creating section in which the image creating stations 10A, 10B, 10C, and 10D are arranged in parallel.

[0015] The imaging stations 10A, 10B, 10C, and 10D may have the same configuration, but may use different toner colors, etc. Therefore, in FIG. 1, only the imaging station furthest downstream in the transport direction of the intermediate transfer belt 11, i.e., imaging station 10D, is labeled.

[0016] In this embodiment, the imaging stations 10A, 10B, 10C, and 10D can be imaging stations using toner of each color, for example, yellow (Y), magenta (M), cyan (C), and black (K), respectively. Although not limited thereto, it is preferable to place the black imaging station at the most downstream position. The arrangement of these imaging stations can be changed as appropriate and is not limited to the above.

[0017] The image forming station 10D has a photoconductor drum 2d (image carrier), a charger 7d, a developing device 4d, a primary transfer roller 12d, a cleaning member 6d, etc. The image forming stations 10A, 10B, and 10C also have these members.

[0018] Hereinafter, when there is no need to distinguish between the image forming stations 10A, 10B, 10C, and 10D, they will simply be referred to as the image forming station 10. The same applies to the charger 7, the developer 4, the primary transfer roller 12, and the cleaning member 6.

[0019] As shown in the figure, the image forming station 10 is composed of a photosensitive drum 2 at the center, a charger 7, a developing device 4, a primary transfer roller 12, a cleaning member 6, etc., which are arranged around the photosensitive drum 2.

[0020] In this embodiment, the photosensitive drum 2 serving as an image carrier is a cylindrical photosensitive drum having a diameter of 30. The photosensitive drum 2 rotates at a peripheral speed of, for example, 50 to 200 mm / s.

[0021] A roller-shaped charger 7, which is a charging section, is pressed against the surface of the photosensitive drum 2 and rotates in response to the rotation of the photosensitive drum 2. A bias of DC or DC superimposed with AC is applied to the charger 7 from a high-voltage power source, thereby uniformly charging the photosensitive drum 2. The surface potential during charging is not particularly limited, but may be, for example, a surface potential of -500V.

[0022] The photoconductor drum 2 is exposed to image information by exposure light 3 from an exposure section, forming an electrostatic latent image. In this exposure process, for example, a laser beam scanner using a laser diode or an LED can be used. Although not particularly limited, the surface potential of the exposed section of the photoconductor drum 2 drops to, for example, -50 V by the exposure process.

[0023] The developing unit 4, which is a developing section, develops the electrostatic latent image formed on the photoconductor drum 2 into a visible image (also called a toner image, etc.) using a developer. This developing process is not particularly limited, but can be performed using a predetermined developing bias, such as -200 V, supplied from a high-voltage power supply.

[0024] In the development process in this embodiment, a one-component development method (also called a one-component contact development method) is mainly used, and one-component toner is stored in the developing device 4. Although not particularly limited, one-component toner having a negative charge polarity can be used. In addition, although one component is described in this embodiment, a two-component development method may also be used.

[0025] In this way, while the photosensitive drum 2 rotates in the direction of the arrow in the figure, the photosensitive drum 2 is charged by the charger 7, an electrostatic latent image is formed on the photosensitive drum 2 by the exposure light 3, and the electrostatic latent image is made visible by the developer 4.

[0026] Next, the visible image on the photosensitive drum 2 is transferred (primary transfer) to the intermediate transfer belt 11 using the primary transfer roller 12 as well. The intermediate transfer belt 11 is conveyed and rotated in the direction of the arrow in the figure by a belt conveying roller 35 and a secondary transfer opposing roller 13.

[0027] On the other hand, the recording medium in the paper feed cassette 15 is transported and supplied by a paper feed means 16 (paper feed roller) and a pair of transport rollers 17. The recording medium may also be supplied by a manual feed tray 60. When the recording medium passes between the secondary transfer opposing roller 13 and the secondary transfer roller 19, the visible image on the intermediate transfer belt 11 is transferred onto the recording medium (secondary transfer).

[0028] In this embodiment, the transfer section transfers the visible image formed on the image carrier to the intermediate transfer body, and then transfers the visible image transferred to the intermediate transfer body to a recording medium. The transfer section in this embodiment has a primary transfer roller 12, a secondary transfer roller 19, etc., and transfers the visible image formed on the image carrier to the intermediate transfer body, and then transfers it to a recording medium.

[0029] Next, the visible image on the recording medium is fixed by the fixing device 20. The recording medium that has passed through the fixing device 20 is discharged onto a paper discharge tray 30. In this manner, an image is formed.

[0030] If necessary, a belt cleaning blade 37 and an opposing member 38 for cleaning the intermediate transfer belt 11 may be provided. If necessary, the image forming apparatus may have a waste toner bottle 33. The waste toner bottle 33 collects toner that has not been transferred onto the photoconductor or transfer belt and has been removed by the cleaning blade.

[0031] 1 also shows a schematic diagram of a control unit 34. The control unit 34 is not particularly limited, but may be, for example, a CPU or the like, and controls the developing unit, the transfer unit, etc. For example, the amount of invisible developer per unit area in the invisible image is changed according to the amount of visible developer per unit area in the visible image on the recording medium.

[0032] Here, a supplementary explanation will be given regarding the imaging station. Although this can be changed as appropriate, when a full-color image is formed, in the primary transfer, the visible images are transferred onto the intermediate transfer belt 11 in the order of, for example, yellow, magenta, cyan, and black. In this case, when the secondary transfer is performed, the visible images are transferred onto the recording medium in the order of black, cyan, magenta, and yellow. In the illustrated example, the visible image visualized by the developer in the imaging station (imaging station 10D in this example) arranged at the most downstream side in the transport direction of the intermediate transfer belt 11 becomes the bottom layer on the recording medium.

[0033] The developing unit in this embodiment forms a visible image using a visible developer as a visible material, and a visible image using an invisible developer as an invisible material. For example, by using a primary transfer roller 12, both the visible image using the visible developer and the visible image using the invisible developer are transferred to an intermediate transfer belt 11. Next, by using a secondary transfer roller 19, both the visible image using the visible developer and the visible image using the invisible developer transferred to the intermediate transfer belt 11 are transferred to a recording medium.

[0034] In this embodiment, visualization and transfer are performed so that an image made with an invisible developer (also referred to as an invisible image) and an image made with a visible developer (also referred to as a visible image) are superimposed on the invisible image on the recording medium. To achieve this, for example, an image-making station that performs development with an invisible developer is disposed on the most downstream side of other image-making stations in the conveying direction of the intermediate transfer belt 11. For example, an image-making station that uses an invisible developer is provided downstream of the image-making station 10D. This allows an image made with an invisible developer and an image made with a visible developer to be superimposed on the recording medium in this order. The device configuration in this case is a configuration using five image-making stations.

[0035] To explain the configuration of this example again, the printer of this example has a plurality of imaging stations each having an image carrier, a charging unit, and a developing unit. An imaging station having a developing unit that visualizes an image using an invisible developer is disposed on the most downstream side of the intermediate transfer body in the transport direction.

[0036] In addition to the above, for example, the black imaging station 10D may be an imaging station that uses an invisible developer. In this case, a visible image is formed using yellow (Y), magenta (M), and cyan (C), and an invisible image is formed using invisible developers (K, Bk).

[0037] An example of this case will be described with reference to Fig. 2. Fig. 2 is a diagram showing a schematic example of an image on a recording medium. In the example shown here, an image is formed on a recording medium 45, in which an invisible image 500 and a visible image 600 are layered. The visible image 600 has a cyan image 601, a magenta image 602, and a yellow image 603. In this embodiment, the invisible image 501 is located at the bottom on the recording medium 45. Note that an example in which the black imaging station is an imaging station for an invisible developer will be described later with further examples as other embodiments.

[0038] Since the invisible image 500 is formed below the visible image 600, it may be expressed as embedding the invisible image 500 in the visible image 600. The invisible image 500 is formed below the visible image 600, and is an image that is not visible or is difficult to be visible.

[0039] Considering the arrangement of the invisible image and the visible image on the recording medium, it is preferable to arrange the image forming station using the invisible developer at the most downstream position in the transport direction of the intermediate transfer belt 11, but this is not limiting. For example, any one of the image forming stations 10A, 10B, and 10C in the example shown in Fig. 1 can be an image forming station using an invisible developer. However, care should be taken to ensure that the invisible image is the bottommost layer of the image on the recording medium.

[0040] By forming an image in which an invisible image and a visible image are layered in this order on a recording medium, additional information can be embedded in the image. For example, by embedding an invisible image that cannot be seen with the naked eye, it is possible to determine if an illegal copy has been made. Furthermore, in this embodiment, since the invisible image is the bottom layer on the recording medium, the invisibility of the invisible image can be increased. In other words, the invisible image can be made difficult to see with the naked eye.

[0041] In the developing unit in this embodiment, a one-component developing method is mainly used. Therefore, when the developer is referred to, it mainly refers to the toner. Therefore, when the developer is referred to, it means the visible toner, and when the developer is referred to, it means the invisible toner. In addition, although the one-component developing method is described in this embodiment, a two-component developing method may also be used.

[0042] In this embodiment, the difference between the visible developer and the invisible developer is, for example, the difference in the degree of absorption of visible light, the difference in the degree of absorption of infrared light, and the like.

[0043] In addition to the above, the visible developer and the invisible developer may differ in the materials, for example, the presence or absence of an infrared absorbing material, such that the invisible developer contains an infrared absorbing material, whereas the visible developer does not contain or substantially does not contain an infrared absorbing material.

[0044] The invisible developer may contain a coloring material and may have a color. The invisible developer may also absorb ultraviolet light. Visible and invisible developers can also be distinguished from each other by the degree of ultraviolet light absorption.

[0045] As described above, the configuration of the image forming station using the invisible developer can be appropriately selected. Another example will be described with reference to Fig. 3. Fig. 3 is a diagram similar to Fig. 1, but it shows only the main parts of the image forming apparatus of this embodiment in a schematic manner.

[0046] 3, the most downstream imaging station is the imaging station 10D, which is equipped with both a black developer and an invisible developer. This example makes it possible to use the black developer when it is not necessary to use the invisible developer.

[0047] In this example, the multiple image forming stations are image forming stations that use black developer, cyan developer, magenta developer, or yellow developer as the visible developer, respectively. The image forming station that uses black developer is disposed on the most downstream side in the transport direction, and the black developer and the invisible developer are exchangeable.

[0048] The term "replacement" can be modified as appropriate, and may refer to replacement on an image forming station basis, for example. Alternatively, a replaceable unit having an invisible developer and a replaceable unit having a black developer may be replaced. Alternatively, a container having a black developer (which may be called a toner bottle, etc.) and a container having an invisible developer may be separated, and a developing device for the black developer and a developing device for the invisible developer may be separated, and both the container and the developing device may be replaced. Alternatively, a single photoconductor drum 2d may have two developing devices, a black developer and a developer for the invisible developer, and the developing process may be performed by switching between them as appropriate.

[0049] In this way, by making it possible to replace the invisible developer and the black developer in the same image forming station, it is possible to prevent the device from becoming large.

[0050] Here, an example of the mechanism by which an invisible image is visually recognized will be described with reference to FIG. 4. FIG. 4(A) is a schematic diagram of a case where an invisible image 500 and a cyan image 601 are formed on a recording medium 45. The arrows in the figure typically indicate visible light. The formation of the invisible image 500 may cause a change in glossiness of the cyan image 601 (visible image 600). When the glossiness of the cyan image 601 changes, the reflected light of the cyan image 601 also changes, and it is considered that the position of the invisible image 500 will be recognized.

[0051] In such a case, in the conventional example, it has been proposed to reduce the change in the reflected light of the color image by reducing the glossiness of the color image such as the cyan image 601. This is intended to reduce the recognition of the position of the invisible image 500, that is, to increase the invisibility. However, as described below, when the invisible image 501 includes wavelengths in the visible wavelength range, it is difficult to increase the invisibility even if a method of reducing the glossiness of the color image is used.

[0052] 4(B) is a schematic diagram of an invisible image 501 and a yellow image 603 formed on a recording medium 45. This is an example of a case where the invisible image 501 contains a small amount of wavelengths in the visible wavelength range. When the invisible image 501 contains a material that reflects the visible wavelength range (e.g., a yellow coloring material), it is difficult to increase the invisibility of the invisible image 501 by measures such as adjusting the color image. In the case of such an invisible image 501, it is considered difficult to improve the visibility of the invisible image 501 even if the glossiness of the color image is reduced.

[0053] In these examples, improvements are required with respect to the invisibility of the invisible image, i.e., the invisibility of the invisible image to the naked eye. Therefore, the present inventor has conducted extensive research, including the above-mentioned content. For example, in the example shown in FIG. 4(B), one possible solution would be to reduce the amount of invisible developer for forming the invisible image and increase the amount of visible developer for forming the color image. However, if the amount of invisible developer is simply reduced, the invisibility of the invisible image will increase, but the readability of a mechanical reader will decrease. In this case, it is considered that it will be difficult to obtain advantages such as the prevention of illegal copying.

[0054] Another possible solution is to improve the invisibility of the invisible image by adjusting the color and density of the invisible developer. However, since the information of the visible image differs depending on the user, it is difficult to adjust the color and density of the invisible developer.

[0055] Therefore, the present inventors have further studied and found that good results can be obtained by controlling the amount of invisible developer for forming an invisible image according to the information of a visible image, and have arrived at the present invention. More specifically, the amount of invisible developer per unit area in the invisible image is changed according to the amount of visible developer per unit area in the visible image on the recording medium. This makes it possible to improve the invisibility of the invisible image. Furthermore, the image size of the invisible image (which may also be referred to as the size of the invisible image, the area of ​​the invisible image, etc.) is changed according to the amount of visible developer per unit area in the visible image on the recording medium. This makes it possible to ensure machine readability by, for example, increasing the image size when the amount of invisible developer is small.

[0056] In this embodiment, the method for changing the amount of invisible developer per unit area in the invisible image can be appropriately selected. When increasing or decreasing the amount of invisible developer per unit area, one example is a method of controlling the area of ​​dots formed per unit area. In this case, for example, the control unit controls by changing the area exposed by the writing unit of the device on the photoconductor. In addition, by changing the voltage applied to the developing roller, the amount of toner on the photoconductor can be changed to change the amount of invisible developer per unit area on the recording medium. By controlling in this way by the control unit 34, the amount of invisible developer on the intermediate transfer belt 11 or the photoconductor drum 2 is changed, and the amount of invisible developer on the recording medium is changed.

[0057] In addition, when referring to the amount of visible developer or the amount of invisible developer on a recording medium, unless otherwise specified, it refers to the amount of visible developer per unit area in a visible image formed on a recording medium or the amount of invisible developer per unit area in an invisible image formed on a recording medium. The amount of invisible developer per unit area in the invisible image is changed as will be described below.

[0058] Fig. 5 shows a diagram for explaining the relationship between the amount of invisible developer adhered to a recording medium and invisibility. The horizontal axis indicates the amount of invisible developer adhered to a recording medium, which is the amount of invisible developer per unit area. The vertical axis indicates the sensory rank of invisibility (also called invisibility rank).

[0059] The invisibility rank is a rank of 1 to 5 based on the degree of difficulty in seeing an invisible image by a visual sensory evaluation by an evaluator. The smaller the number, the easier it is to see the invisible image. The invisibility rank is preferably 3 or higher, more preferably 3.5 or higher, and even higher numbers are preferable. When the invisibility rank is 3 or higher, it can be said that it is difficult to recognize with the naked eye, and the confidentiality of the invisible image is ensured.

[0060] In assessing the invisibility rank, images were formed as follows. The image forming apparatus shown in Fig. 1 was used, and the imaging station had the configuration shown in Fig. 3. My Paper (manufactured by Ricoh Co., Ltd.) was used as the recording medium, and a 15 mm x 15 mm barcode image was formed as an invisible image on the recording medium. Development and transfer were performed so that the following visible images were formed on this invisible image, and evaluation images were formed. Three types of evaluation images were prepared according to the following visible images.

[0061] The following three visible images were taken. (2), (4), and (6) correspond to Figure 7 described below. (2) Cyan single color, adhesion amount on recording medium is 0.32 mg / cm 2 (4) Two colors, cyan and magenta, with a deposition weight of 0.70 mg / cm on the recording medium 2 (6) Three colors, cyan, yellow, and magenta, with a deposition weight of 1.10 mg / cm on the recording medium 2

[0062] The amount of adhesion on the recording medium is taken into consideration for each color image. For example, (6) indicates that the above numerical value is the total amount of visible developer in each visible image formed with cyan, yellow, and magenta developers. As can be seen from the units of the numerical value, the amount of adhesion indicates the amount per unit area. The amount of adhesion of the visible developer on the recording medium in the above is the amount of adhesion of the visible developer measured in the area where the invisible image is formed.

[0063] For each of the evaluation images having the above visible images, a barcode image was formed while changing the amount of invisible developer per unit area as shown on the horizontal axis in the figure. The amount of invisible developer was measured by measuring the amount of invisible developer in the area formed under the visible image.

[0064] The evaluation results are shown in the figure. As can be seen from the figure, the invisibility rank changes depending on the amount of visible developer in the visible image. When the amount of invisible developer attached to the recording medium is the same, the greater the amount of visible developer in the visible image, the higher the invisibility. For example, when the amount of invisible developer attached to the recording medium is 0.50 mg / cm 2 Looking at the area, the invisibility ranks for (2), (4), and (6) are 1, 2, and 4, respectively. This is because the amount of visible developer on the invisible image increases.

[0065] It can also be seen that the invisibility rank changes depending on the amount of invisible developer attached to the recording medium. The lower the amount of invisible developer attached to the recording medium, the higher the invisibility rank, and the higher the amount of invisible developer attached to the recording medium, the lower the invisibility rank. In other words, the more the amount of invisible developer, the easier it is to see the invisible image.

[0066] Thus, the greater the amount of visible developer per unit area on the recording medium, or the smaller the amount of invisible developer per unit area, the higher the invisibility rank. A high invisibility rank indicates that the invisible image is difficult to see, and indicates that the secrecy of the invisible image is ensured. Therefore, when the amount of visible developer per unit area is large, an image with sufficient secrecy can be obtained even if the amount of invisible developer is increased. On the other hand, when the amount of visible developer per unit area is small, a large amount of invisible developer does not ensure sufficient secrecy, so control to reduce the amount of invisible developer is required.

[0067] In the present embodiment, taking the above into consideration, when an image is formed with the second adhesion amount described below, the amount of the invisible material in the second adhesion amount is controlled to be less than the amount of the invisible material in the first adhesion amount described below. This can improve the invisibility of the invisible image. Furthermore, when the amount of invisible material per unit area and the amount of visible material per unit area in the overlapping portion of the invisible image and the visible image are measured, when an invisible image is formed with the second adhesion amount described below, the amount of invisible material may be controlled to be less than when an invisible image is formed with the first adhesion amount described below.

[0068] In this embodiment, the adhesion amount of the visible material and the invisible material per unit area of ​​the predetermined image in the overlapping portion of the invisible image and the visible image is defined as a first adhesion amount, and the adhesion amount of the visible material and the invisible material per unit area of ​​the predetermined image in the overlapping portion of the invisible image and the visible image that is less than the first adhesion amount is defined as a second adhesion amount.

[0069] Figure 6 shows a diagram for explaining the relationship between the amount of invisible developer adhered to the recording medium and the readability of the mechanical reader. As with Figure 5 above, the horizontal axis indicates the amount of invisible developer adhered to the recording medium, and the amount of invisible developer per unit area. The vertical axis indicates the distance (readable distance) between the reader and the recording medium when the invisible image can be read by the mechanical reader. The smaller the distance, the closer the reader and recording medium need to be, indicating lower readability. Here, readability was deemed OK when the readable distance was 68 mm or more.

[0070] To evaluate machine readability, images were formed as follows. 5, the above-mentioned image forming apparatus was used, and MyPaper (manufactured by Ricoh Co., Ltd.) was used. The invisible image formed on the recording medium was as follows. (a) 18mm x 18mm barcode image (b) 22mm x 22mm barcode image

[0071] On this invisible image, development and transfer were performed so that a cyan monochrome visible image was formed, and an evaluation image was formed. For the cyan monochrome visible image, the amount of adhesion on the recording medium was 0.32 mg / cm 2 The image is a solid cyan image.

[0072] As shown on the horizontal axis in the figure, in the evaluation image for mechanical readability, a barcode image was formed while changing the amount of invisible developer per unit area. Note that the mechanical reader can be selected appropriately as long as it can recognize the invisible image.

[0073] The evaluation results are shown in the figure. As can be seen from the figure, the readability changes depending on the amount of invisible developer attached to the recording medium. The readability increases as the amount of invisible developer attached to the recording medium increases, and the readability decreases as the amount of invisible developer attached to the recording medium decreases. In other words, the invisible image becomes easier to recognize by a mechanical reader when the amount of invisible developer per unit area increases.

[0074] Also, as can be seen from comparing (a) and (b) in the figure, the readability improves when the size of the invisible image, i.e. the barcode image, is increased. For example, at point A in the figure, when the image size is small (a), the readability is NG, but when the image size is large (b), the readability is OK.

[0075] For this reason, it is preferable to control the image size of the invisible image according to the amount of the visible developer. For example, when the amount of the visible developer is small, the amount of the invisible developer is correspondingly small, so it is preferable to increase the image size of the invisible image. This makes it possible to ensure machine readability even when the amount of the invisible developer is small.

[0076] Considering only FIG. 6, it may be thought that machine readability can be ensured by simply increasing the amount of invisible developer. However, considering FIG. 5 and FIG. 6 together, if the amount of invisible developer is simply increased, invisibility decreases and sufficient concealment is not ensured. Therefore, there are cases where the amount of invisible developer is reduced to ensure invisibility, and taking this into consideration, the amount of invisible developer is controlled according to the amount of visible developer in the visible image, while controlling the image size of the invisible image when the amount of invisible developer is small. This makes it possible to achieve both human invisibility and machine readability.

[0077] 7 is a diagram showing an example of control. As in the above, in the figure, the amount of visible developer adhesion indicates the amount of visible developer per unit area in the visible image on the recording medium, and the amount of invisible developer adhesion indicates the amount of invisible developer per unit area in the invisible image on the recording medium.

[0078] As shown in the figure, in this embodiment, the amount of invisible developer adhered is controlled according to the amount of visible developer adhered. For example, when the amount of visible developer adhered is small, the amount of invisible developer adhered is reduced to ensure the invisibility of the invisible image. Also, when the amount of invisible developer adhered is small, the machine readability is reduced, so the image size of the invisible image is increased. Thus, according to this embodiment, in addition to ensuring the invisibility of the invisible image, machine readability can be ensured.

[0079] The control unit in this embodiment can control, for example, the first adhesion amount, the second adhesion amount, and the third adhesion amount as follows. The amount of invisible developer per unit area in an invisible image formed under a certain visible image is defined as the first adhesion amount, and the amount of invisible developer when forming a visible image having a smaller amount of visible developer per unit than the certain visible image is defined as the second adhesion amount. When forming an invisible image with the second adhesion amount, control is performed to reduce the amount of invisible developer compared to when forming an invisible image with the first adhesion amount.

[0080] Also, the amount of invisible developer per unit area in an invisible image formed under a certain visible image may be defined as a first adhesion amount, and the amount of invisible developer when forming a visible image having a larger amount per unit of visible developer than the certain visible image may be defined as a third adhesion amount. When forming an invisible image with the third adhesion amount, it is preferable to control the amount of invisible developer to be larger than when forming an invisible image with the first adhesion amount.

[0081] As described above, when the amount of invisible material is reduced or increased, the control unit in this embodiment preferably controls to adjust the amount of toner on the image carrier and adjust the amount of toner attached to the intermediate transfer body (intermediate transfer belt), and this is also done in this embodiment. The transfer rate of toner from the intermediate transfer body to the recording medium may vary depending on the paper type and environment. Therefore, by controlling the amount of invisible developer as described above, the influence of variations due to the paper type and environment can be reduced. Note that the concealment (invisibility) is determined by the amount of visible developer and invisible developer attached to the recording medium, so the above explanation uses the amount of attachment on the recording medium.

[0082] In this embodiment, control may be performed to adjust the amount of invisible developer on the recording medium, but in this case, it should be noted that the accuracy of the amount of invisible developer may be inferior to the case of controlling the amount of invisible developer on the intermediate transfer body or the image carrier.

[0083] As described above, the control unit in this embodiment can change the image size of the invisible image, and can perform control as, for example, a first method, a second method, or a third method as described below. For example, a method of forming an image with an amount of invisible developer per unit area in an invisible image formed under a certain visible image and an image size of the invisible image is defined as the first method. In this case, when forming an invisible image with a smaller amount of invisible developer than the amount of invisible developer in the first method, it is preferable that the control unit uses the second method of forming an invisible image with an image size larger than the image size in the first method.

[0084] Furthermore, when forming an invisible image with a larger amount of invisible developer than that in the first method, the control unit preferably uses a third method for forming an invisible image with an image size smaller than that in the first method. In this manner, it is preferable to appropriately select and control the first method, the second method, and the third method.

[0085] In addition, in this embodiment, it is preferable to perform control according to the brightness of the visible image or the color difference between the visible image and the invisible image. For example, it is preferable to control the amount of adhesion of the invisible developer on the recording medium according to the brightness of the visible image or the color difference between the visible image and the invisible image. This provides advantages such as improving the readability of the machine while ensuring the invisibility of the invisible image. In addition, since the visibility of the invisible image formed under the visible image differs depending on the visible image, it is possible to more appropriately control the amount of adhesion of the invisible developer, and to further ensure the invisibility of the invisible image.

[0086] In this example, the control method is, for example, to reduce the amount of invisible developer adhered when the brightness of the visible image is high, and to increase the amount of invisible developer adhered when the brightness of the visible image is low. Also, when the color difference between the visible image and the invisible image is small, the amount of invisible developer adhered is reduced, and when the color difference between the visible image and the invisible image is large, the amount of invisible developer adhered is increased. In addition, it is preferable to control the image size of the invisible image to be changed accordingly.

[0087] As described above, the amount of the invisible developer adhered here refers to the amount of the invisible developer per unit area in the invisible image formed on the recording medium. Although not particularly limited, the color difference is calculated, for example, by the L*a*b* color space.

[0088] Next, a supplementary explanation of FIG. 7 will be given with reference to FIG. 8. As shown in FIG. 7, the amount of visible developer attached on the recording medium, i.e., the amount of visible developer per unit area, differs depending on the type of visible image. For example, the amount of visible developer increases in the order of (1) cyan hatching, (4) blue solid, and (6) black solid in FIG. 7. Since the amount of visible developer per unit area is small for (1) cyan hatching, the invisible image 500 formed below the visible image is easily visible. On the other hand, since the amount of visible developer per unit area is large for (6) black solid, the invisible image 500 formed below the visible image is difficult to see.

[0089] Therefore, when the amount of visible developer attached to the visible image is small, the amount of invisible developer is controlled to be reduced in order to ensure invisibility, as shown in Fig. 7. When the amount of invisible developer is small, the image size of the invisible image is increased in order to ensure machine readability.

[0090] Fig. 8 is an image diagram for explaining such control. Fig. 8(A) to (C) are schematic diagrams showing the image sizes when an invisible image is formed when a visible image is formed with (1) cyan hatching, (4) solid blue, and (6) solid black in Fig. 7.

[0091] Fig. 8(A) is a schematic diagram of a case where a cyan image 601 is formed as a visible image 600 on the invisible image 500. Fig. 8(B) is a schematic diagram of a case where a blue image 604 is formed as a visible image 600 on the invisible image 500. Fig. 8(C) is a schematic diagram of a case where a black image 605 is formed as a visible image 600 on the invisible image 500. 8(A) to 8(C) are schematic diagrams of the visible image and the invisible image as viewed from above, and the visible image 600 formed on the invisible image 500 is omitted from the illustration.

[0092] As shown in the figure, the image size of the invisible image 500 is changed depending on the type of the visible image 600. By controlling in this way, it is possible to achieve both invisibility of the invisible image and machine readability.

[0093] On the other hand, when changing the image size of the invisible image, it is assumed that the image size of the invisible image may become larger than the image size of the visible image. In the example shown in Fig. 8 (A) to (C), the image size of the invisible image 500 is smaller than the image size of the visible image 600, so that the invisible image 500 is prevented from being exposed to the surface, and the invisible image 500 is difficult to be visually recognized. However, when the image size of the invisible image 500 is larger than the image size of the visible image 600, there is a possibility that the invisible image 500 is exposed to the surface and visually recognized.

[0094] 8(D) is a schematic diagram showing a case where the image size of the invisible image 500 is larger than the image size of the visible image 600. Although it is difficult to express in the figure, the invisible image 500 is formed below the visible image 600. As shown in the figure, the area of ​​the invisible image 500 where the visible image 600 is not formed is exposed on the surface. In other words, it can be said that the invisible image 500 spans over a blank area (an area where the visible image 600 is not formed).

[0095] Therefore, in this embodiment, it is preferable to change the position where the invisible image is formed according to the information of the visible image. For example, the position where the invisible image is formed is changed so that the invisible image is formed below the visible image. This makes it possible to prevent the invisible image from being exposed on the surface of the recording medium, and reduces the possibility that the invisible image will be visible.

[0096] The position where the invisible image is formed can be changed as appropriate. For example, the invisible image in the image information is moved in pixel units using the information on the visible image and the image size of the invisible image to find a position where the invisible image is not exposed on the surface or where the area exposed on the surface is small. The position information of the invisible image thus found is used to form the invisible image on the recording medium.

[0097] In addition, the image forming apparatus of the present embodiment preferably has a presentation unit that shows the position information and image size of the invisible image to the user before the invisible image is formed on the recording medium. By using the presentation unit, the user can check the suitability of the invisible image before the image is formed. This makes it possible to take measures such as changing the position or image size of the invisible image when the position or image size of the invisible image is not appropriate, thereby reducing unnecessary printing.

[0098] The presentation unit can be appropriately selected, and may be, for example, a panel. Before the invisible image is formed, the position information where the invisible image is to be formed and the image size are displayed on the panel. Alternatively, the information may be displayed on a display unit provided separately from the image forming device.

[0099] As described in FIG. 4, when the invisible developer contains coloring material, it reflects wavelengths in the visible wavelength range, making the invisible image more visible. For example, when the invisible developer has a yellowish tinge (yellow color), the invisible image becomes more visible. Therefore, in this embodiment, it is preferable to control the amount of invisible developer adhering to the recording medium according to the brightness, glossiness, etc. of the invisible image. This makes it possible to ensure the invisibility of the invisible image even when the invisible developer contains coloring material. In addition, it is preferable to control the image size of the invisible image to be changed accordingly.

[0100] In the present invention, the invisible material can be appropriately selected. For example, the invisible material can be a transparent invisible material that contains a near-infrared absorbing material. In the above embodiment, an example in which a transparent (including substantially transparent) invisible developer is mainly used is described. The transparent invisible developer (toner) in the present invention refers to a toner that contains a sufficiently small amount (e.g., one-tenth or less) of colorants that exhibit coloring due to the action of light absorption or light scattering (e.g., color pigments, color dyes, black carbon particles, black magnetic powder, etc.) compared to black, yellow, cyan, and magenta toners, or does not contain any colorants at all. The transparent invisible developer in this embodiment has a slightly yellowish transparency depending on the type and amount of binder resin, wax, and external additives that constitute the toner, but can be said to be substantially transparent compared to colored toners such as magenta. In the following description, a transparent toner that contains an infrared absorbing material may be referred to as an IR toner.

[0101] In addition to the above, the invisible material may be a black invisible material that contains a near-infrared absorbing material. An example of such an invisible material is black toner (which may be referred to as black toner, etc.). Black toner contains a black coloring material, such as carbon black. Therefore, black toner is different from other color toners (yellow, magenta, cyan, etc.) in that it contains carbon black that absorbs infrared rays, and can be read by an infrared reader (scanner). By using this characteristic, it is possible to form an invisible image (black invisible image) that is usually invisible and can be read by an infrared reader.

[0102] As described in the above example, the black imaging station 10D having black toner can be an imaging station using an invisible developer. In other words, the black (K) image can be made invisible by forming an image by overlaying a visible image using yellow (Y), magenta (M) and cyan (C) on the black (K) image. That is, the black (K) image can be made an invisible image. In this case, the invisible material is black toner.

[0103] Another embodiment of the present invention will now be described in detail using an example in which black toner is used as the invisible material. When black toner is used as the invisible material, for example, an image as shown in Fig. 2 can be formed, and in the image shown in Fig. 2, the invisible image 500 is a black (K) image. In the following description, the black (K) invisible image 500 may be referred to as the invisible image 500(K). Also, the black (K) invisible image may be referred to as the invisible image (K).

[0104] In this embodiment, since the invisible image 500(K) is formed under the visible image 600, it may be expressed as embedding the invisible image 500(K) in the visible image 600. The invisible image 500(K) is formed under the visible image 600, and is an image that is not visible or is difficult to see. However, in this embodiment, unlike the invisible image 500 formed by the invisible developer (IR toner) in the above embodiment, the invisible image is black, so the proportion of the visible image 600 needs to be higher than in the above embodiment.

[0105] Considering the arrangement of the invisible image (K) and the visible image on the recording medium, it is preferable to arrange the black imaging station at the most downstream position in the transport direction of the intermediate transfer belt, but this is not limited to this. For example, any one of the imaging stations 10A, 10B, and 10C in the example shown in Figure 1 can be the black imaging station. However, care should be taken to ensure that the invisible image (K) is the bottom layer of the images on the recording medium.

[0106] In this embodiment, a method for increasing the proportion of the visible image 600 can be appropriately selected. For example, a method for laminating the visible image 600 can be used. 9 is a schematic diagram of an image formed by stacking visible images 600. As shown in the figure, an invisible image 500, a visible image 600, and a visible image 600' are formed on a recording medium 45 in this order.

[0107] An example of the image forming apparatus of this embodiment will be described. The image forming apparatus of this example is disposed downstream of the image forming unit in the recording medium transport direction and has a re-transporting means for re-transporting the recording medium to the image forming unit. The image forming apparatus of this example also has an image forming mode that executes a first step of forming an image including an invisible image on the recording medium by the image forming unit, a second step of re-transporting the surface of the recording medium on which the image including the invisible image has been formed to the image forming unit by the re-transporting means, and a third step of forming a visible image on the recording medium by the image forming unit. The image forming mode is executed, for example, in the order of the first step, the second step, and the third step.

[0108] The re-conveying unit may be, for example, a duplex unit. An example in which a duplex unit is used will be described below. When the image forming apparatus has a duplex unit, for example, there is a method in which the image is passed through the duplex unit multiple times and transfer and fixing are repeated multiple times.

[0109] The image forming apparatus of this embodiment having a double-sided unit will be described with reference to FIG. The image forming apparatus of this example is provided with an intermediate transfer belt 11, which is an intermediate transfer body serving as an image carrier stretched over a number of rollers, located approximately in the center of the apparatus main body 1. Four imaging units 10M, 10C, 10Y, and 10Bk are provided along the belt surface of the intermediate transfer belt 11 that faces vertically downward.

[0110] Each of the imaging units 10M, 10C, 10Y, and 10Bk has a photoconductor drum 2 as a latent image carrier. A charging device, a developing device, a cleaning device, and the like are arranged around each of the photoconductor drums 2. In addition, a primary transfer roller 12 as a primary transfer means is provided on the inside of an intermediate transfer belt 11 at a position facing each of the photoconductor drums 2.

[0111] Further, an optical writing device 14 is provided below each of the imaging units 10M, 10C, 10Y, and 10Bk. Although not shown, the optical writing device 14 has a polygon mirror, a mirror group, and the like, and irradiates the surface of the photoconductor drum 2 of each color imaging unit with optically modulated laser light.

[0112] Further, two-stage paper feed cassettes 15 are set in the lower part of the device main body 1, and paper feed means 16 corresponding to each paper feed cassette are provided. A pair of transport rollers 17 is provided to transport image recording sheets (hereinafter also referred to as "paper") such as transfer paper fed by the paper feed means 16. A pair of registration rollers 18 is provided above the pair of transport rollers 17 (downstream in the paper transport direction). A secondary transfer roller 19 functioning as a secondary transfer means is disposed above the pair of registration rollers 18, and the secondary transfer roller 19 is provided to face a secondary transfer opposing roller 13, which is one of the rollers that stretch the intermediate transfer belt 11.

[0113] A fixing device 20 is provided downstream of the secondary transfer roller 19 in the paper transport direction. A switching claw 21 for switching the paper transport direction is disposed above the fixing device 20. The switching claw 21 is switched by an actuator such as a solenoid (not shown).

[0114] A two-stage paper discharge tray 30 is configured on the top surface of the device main body 1, and a pair of paper discharge rollers 29 for discharging paper onto the lower stage 30a of the paper discharge tray is provided at the upper left in the figure of the fixing device 20. A pair of paper discharge rollers 31 for discharging paper onto the upper stage 30b of the paper discharge tray is provided above the pair of paper discharge rollers 29 so as to be rotatable in both forward and reverse directions.

[0115] A duplex unit 50 serving as a re-conveying means is provided on the side of the apparatus body 1. A part of a re-feed path 52 for re-feeding the inverted paper is formed in this duplex unit 50. This re-feed path 52 starts from a pair of paper discharge rollers 31 for discharging the paper to the upper stage of the paper discharge tray 30b, and ends at a point where it joins the paper conveying path just before the pair of registration rollers 18. Then, conveying roller pairs 26, 27, and 28 are arranged at positions that divide the re-feed path 52 in the duplex unit into approximately four equal parts.

[0116] The duplex unit 50 is also equipped with a manual paper feed device, and a manual feed tray 60 serving as paper stacking means for the manual paper feed device is provided on the side of the duplex unit 50 so as to be openable and closable with respect to the duplex unit 50, with a rotating shaft 61 as a fulcrum. However, in this example, it is not essential that the tray be openable and closable.

[0117] Paper is selectively fed from a paper feed cassette 15 or a manual feed tray 60, and is sent toward the secondary transfer section by a pair of registration rollers 18 in time with the toner image carried on the intermediate transfer belt 11. When the paper onto which the toner image has been transferred in the secondary transfer section passes through a fixing device 20, the toner image is fused and fixed to the paper by heat and pressure. The fixed transfer material is discharged by a pair of paper discharge rollers 29 onto a paper discharge tray 30 arranged on the top surface of the device main body 1.

[0118] When printing on both sides of a sheet of paper, the sheet of paper with a toner image fixed on one side of it is conveyed toward the pair of discharge rollers 31, which can rotate forward and backward, by appropriately switching the switching claw 21. At this time, the pair of discharge rollers 31 is rotated in the forward direction (clockwise in FIG. 10). When the rear end of the sheet of paper conveyed by the pair of discharge rollers 31 toward the upper stage of the discharge tray 30b is detected by the sensor 40, the pair of discharge rollers 31 is rotated in the reverse direction (counterclockwise in FIG. 10) to send the sheet of paper into the re-feed path 52. The sheet of paper is conveyed through the re-feed path 52 by the pairs of conveying rollers 26, 27, and 28, and is further sent to the pair of registration rollers 18.

[0119] In this way, the paper switched by the pair of ejection rollers 31, which can rotate forward and backward, is transported through the re-feed path 52 so that the paper is turned over (from the front side to the back side), and by repeating this operation once more, the paper is turned over again (from the back side back to the front side), and the front side on which the image was once formed is transported again to the image forming section. Then, a toner image is transferred from the intermediate transfer belt 11 to the front side and fixed by the fixing device 20, so that further images can be stacked. The paper is then discharged to the ejection tray 30.

[0120] Next, an example of the control will be described. In this example, first, a visible image (toner image) made with an invisible developer and a visible image made with a visible developer are transferred and fixed onto the recording medium, forming an invisible image 500(K) and a visible image 600. Next, the recording medium is passed through the double-sided unit twice, and then the recording medium is transported again to the image forming section. Next, a visible image made with a visible developer is transferred onto the visible image 600 and fixed, forming a visible image 600'. In this way, a further visible image 600' can be formed on the visible image 600, thereby increasing the proportion of visible images.

[0121] In this case, the visible image 600 and the visible image 600' may have different colors and densities. For example, the visible image 600 may be a magenta solid image, and the visible image 600' may be a cyan and yellow halftone image.

[0122] In this example, the invisible image 500(K) can be formed in the bottom layer on the recording medium regardless of the position of the black imaging station. On the other hand, in order to place the required visible image 600 in the shortest time, it is preferable to place the black imaging station at the most downstream position in the transport direction of the intermediate transfer belt 11. Note that in the case of a direct transfer type rather than an intermediate transfer type, it is preferable to place the black imaging station at the most upstream position.

[0123] The above configuration is not limited to the case where the black imaging station is an imaging station for an invisible developer, but may be an imaging station for an invisible developer such as IR toner. The number of layers of visible images is not particularly limited and can be selected appropriately. When increasing the number of layers of visible images, the number of times the recording medium is passed through the double-sided unit 50 can be increased, for example, to four, six, or eight times. By increasing the number of layers of visible images, the proportion of visible images can be increased.

[0124] In the above control example, the visible image 600 and the visible image 600' are fixed in separate processes, with the visible image 600 being formed the first time the recording medium passes through the fixing device 20, and the visible image 600' being formed the second time the recording medium passes through the fixing device 20. Fixing the toner onto the recording medium in multiple steps in this manner has the advantage that fixing can be performed without causing fixing defects even with a fixing device (fixing unit) with fixing performance equivalent to that of the conventional fixing device.

[0125] As described above, by passing the recording medium through the double-sided unit twice and re-transporting the side of the recording medium on which an image including an invisible image has been formed to the image forming section, the number of layers of visible images can be increased.

[0126] Next, another example of the control will be described. In this example, first, a visible image (toner image) made of an invisible developer is transferred to a recording medium and fixed, forming an invisible image 500(K). Next, the recording medium is passed through the double-sided unit twice. Next, a visible image made of a visible developer is transferred onto the invisible image 500(K) and fixed, forming a visible image 600. Next, the recording medium is passed through the double-sided unit twice. Next, a visible image made of a visible developer is transferred onto the visible image 600 and fixed, forming a visible image 600'. In this manner, a visible image 600' can be further formed on the visible image 600, thereby increasing the proportion of visible images.

[0127] As described above, in this embodiment as well, additional information can be embedded in an image by forming an image in which an invisible image (K) and a visible image are layered in this order on a recording medium. For example, by embedding an invisible image (K) that cannot be seen with the naked eye, it is possible to determine if an illegal copy has been made. Furthermore, in this embodiment, since the invisible image (K) is the bottom layer on the recording medium, the invisibility of the invisible image (K) can be increased. In other words, the invisible image (K) can be made difficult to see with the naked eye.

[0128] In addition, when using a method in which transfer and fixing are repeated multiple times using a double-sided unit, a similar effect can be obtained because a visible image can be formed on top of the invisible image (K) even if the invisible image (K) is not the bottom layer.

[0129] The above example is an example for explaining an image forming apparatus having four imaging stations, black (K), yellow (Y), magenta (M), and cyan (C), but the present embodiment is not limited to this. For example, the image forming apparatus may have five or more imaging stations having IR toner in addition to black (K), yellow (Y), magenta (M), and cyan (C). In the case of an image forming apparatus having five or more imaging stations having black (K), yellow (Y), magenta (M), cyan (C), and IR toner, two types of invisible images, an invisible image (K) and an invisible image (IR), can be formed. In particular, by making the wavelengths of infrared light absorbed by the IR toner and the black toner different, it is possible to print a printed matter with a very high security level, and it can be applied to printed matter that requires a high level of authenticity determination.

[0130] In this manner, in the present embodiment, two or more imaging stations can be used to form an invisible image. As described above, for example, the image forming unit can have a first imaging station having a first invisible material and a second imaging station having a second invisible material, the first invisible material including a near-infrared absorbing material and being transparent, and the second invisible material including a near-infrared absorbing material and being black.

[0131] As described above, according to the present embodiment, an image forming apparatus is provided that includes an image forming unit that forms an image on a recording medium, a control unit that controls the image forming unit, and a re-conveying unit that is disposed downstream of the image forming unit in the conveying direction of the recording medium and re-conveys the recording medium to the image forming unit, and the image forming unit is capable of forming an image on the recording medium in which an invisible image made of an invisible material and a visible image made of a visible material are superimposed on the invisible image. The image forming apparatus of the present embodiment has an image forming mode that executes a first step of forming an image including the invisible image on the recording medium by the image forming unit, a second step of re-conveying the surface of the recording medium on which the image including the invisible image has been formed to the image forming unit by the re-conveying unit, and a third step of forming the visible image on the recording medium by the image forming unit, and the image forming mode executes the first step, the second step, and the third step in this order.

[0132] Also, as described above, for example, the first step includes a step of transferring a visible image formed by a visible developer as the visible material to the recording medium and a step of fixing the visible image to the recording medium, and the third step includes a step of transferring a visible image formed by an invisible developer as the invisible material to the recording medium and a step of fixing the visible image to the recording medium.

[0133] Next, another embodiment of the image forming apparatus of the present invention will be described. In the above embodiment, the image forming apparatus that forms an image by electrophotography has been described, but the present invention can also be applied to an image forming apparatus that forms an image by inkjet. Therefore, the description of this embodiment will be omitted.

[0134] The image forming apparatus of this embodiment is an image forming apparatus that forms an image by an inkjet method, and has a first ejection section that ejects invisible ink as the invisible material onto the recording medium, and a second ejection section that ejects visible ink as the visible material onto the recording medium onto which the invisible material has been ejected. The image forming unit has the first ejection unit and the second ejection unit, and forms an image on the recording medium by superimposing, in order, the invisible image formed with the invisible ink and the visible image formed with the visible ink on the invisible image, and the invisible image is located closer to the recording medium than the visible image. When forming the invisible image with the second adhesion amount, the control unit controls the amount of invisible ink ejected by the first ejection unit to be less than when forming the invisible image with the first adhesion amount.

[0135] In this embodiment, the same configuration as in the above embodiment can be used. The visible developer in the above embodiment is the visible ink in this embodiment, and the invisible developer in the above embodiment is the invisible ink in this embodiment, so the above configuration can be applied. Therefore, the same matters as in the above embodiment will not be described.

[0136] An example of an image forming apparatus of this embodiment is shown in Fig. 11. In Fig. 11, a control unit 34, a first discharge unit 101, a second discharge unit 102, a first drying unit 103, a second drying unit 104, transport rollers 105a and 105b, etc. are shown. An arrow in the figure indicates the transport direction of a recording medium 45. Note that when the first discharge unit 101 and the second discharge unit 102 are described without distinction, they may be referred to as discharge units.

[0137] Furthermore, the discharge unit may be referred to as an imaging station, etc. However, in the case of a configuration in which a first invisible material and a second invisible material are used, the imaging station having the first invisible material is referred to as a first invisible imaging station, and the imaging station having the second invisible material is referred to as a second invisible imaging station to be distinguished from the imaging station having a visible material.

[0138] The first ejection unit 101 ejects invisible ink as an invisible material onto the recording medium 45. The second ejection unit 102 ejects visible ink as a visible material onto the recording medium 45. For example, inkjet heads can be used as the first ejection unit 101 and the second ejection unit 102. These may be of a line type or a serial type. The first ejection unit 101 and the second ejection unit 102 may be scanned separately, or may be scanned as an integrated unit using, for example, a carriage member or the like.

[0139] The first ejection unit 101 is disposed upstream of the second ejection unit 102 in the transport direction of the recording medium. This allows an invisible image formed with invisible ink and a visible image formed with visible ink to be layered in this order on the recording medium. In other words, a visible image can be formed on an invisible image.

[0140] In this embodiment, the differences between visible ink and invisible ink include, for example, the difference in the degree of visible light absorption, the difference in the degree of infrared light absorption, and the like.

[0141] In addition to the above, the visible ink and the invisible ink are different in the materials. For example, the presence or absence of an infrared absorbing material is included, and the invisible ink includes an infrared absorbing material, while the visible ink does not include an infrared absorbing material or does not substantially include an infrared absorbing material. An example of the infrared absorbing material is a near-infrared absorbing material, and an example of the near-infrared absorbing material is one described in Patent Document 1.

[0142] The invisible ink may contain a coloring material and may have a color. In this embodiment, as in the above embodiment, the invisible material may be a transparent ink (also called IR ink) that contains a near-infrared absorbing material and is transparent, or a black ink that contains a near-infrared absorbing material and is black in color.

[0143] In this embodiment, the ink ejected by the ejection unit is dried by a first drying unit 103 and a second drying unit 104. As the drying unit, a known drying unit can be used, such as a heater or a hot air generator.

[0144] However, the first drying unit 103 can be omitted. For example, after the first ejection unit 101 ejects ink, the second ejection unit 102 does not eject ink, and the ink is dried in the second drying unit 104. Next, the recording medium is transported again to the image forming unit by the re-transport means, and the ink is ejected and dried in the second ejection unit 102. In this manner, bleeding of the ink can be suppressed.

[0145] In this embodiment, similarly to the above embodiment, the control unit 34 controls the amount of invisible ink ejected by the first ejection unit to be less when forming an invisible image with the second adhesion amount than when forming an invisible image with the first adhesion amount, thereby improving the invisibility of the invisible image.

[0146] In this embodiment, similarly to the above embodiment, when an invisible image is formed with the second adhesion amount, the control unit 34 preferably controls the size of the invisible image to be larger than when an invisible image is formed with the first adhesion amount. This ensures the machine readability of the invisible image as described in the above embodiment, and makes it possible to achieve both invisibility and machine readability of the invisible image.

[0147] In this embodiment, when determining the amount of adhesion of the visible material and the invisible material per unit area of ​​a predetermined image in a portion where an invisible image and a visible image are overlapped, the amount is calculated, for example, as follows. Amount of ink deposited per unit area = Amount of ink consumed in image formation ÷ Image area

[0148] In this embodiment, too, it is preferable that the control unit changes the amount of invisible material per unit area in the invisible image on the recording medium according to the brightness of the visible image or the color difference between the visible image and the invisible image. For example, it is preferable to control the amount of invisible ink ejected onto the recording medium according to the brightness of the visible image or the color difference between the visible image and the invisible image. This provides advantages such as improved machine readability while ensuring the invisibility of the invisible image. In addition, since the visibility of the invisible image formed under the visible image differs depending on the visible image, it is possible to more appropriately control the amount of adhesion of the invisible developer, and to better ensure the invisibility of the invisible image.

[0149] Next, another example of this embodiment will be described with reference to FIG. In this embodiment, it is also preferable that the image forming apparatus has the above-mentioned image forming mode. The image forming apparatus of this example is disposed downstream of the image forming unit in the recording medium conveying direction and has a re-conveying means for re-conveying the recording medium to the image forming unit. The image forming apparatus of this example also has an image forming mode that executes a first step of forming an invisible image on the recording medium by the image forming unit, a second step of re-conveying the recording medium to the image forming unit by the re-conveying means, and a third step of forming a visible image on the recording medium by the image forming unit. The image forming mode also executes the first step, the second step, and the third step in that order, and when a further visible image is formed on the visible image, the second step and the third step are further repeatedly executed one or more times.

[0150] The illustrated example will be described. In this example, invisible ink is discharged onto the recording medium 45 by the first discharge unit 101 and dried by the first drying unit 103 or the second drying unit 104 to form an invisible image. Next, the recording medium 45 is re-conveyed to the image forming unit by a re-conveying means 700 having a conveying roller 710. Next, visible ink is discharged onto the recording medium 45 by the second discharge unit 102 and dried by the second drying unit 104 to form a visible image on the invisible image. As a result, an image in which a visible image is formed on an invisible image is obtained.

[0151] The recording medium 45 may be discharged at this point, or if a further visible image is to be formed on the visible image, the recording medium 45 is transported to the image forming section by the re-transport means 700. Next, visible ink is discharged onto the recording medium 45 by the second discharge section 102 and dried by the second drying section 104, forming a further visible image on the visible image. In this manner, as shown in Fig. 9, a further visible image 600' can be formed on the visible image 600, thereby increasing the proportion of visible images. In this embodiment, a dedicated re-transport unit is used as the re-transport means 700, but the re-transport means is not limited to this, and a method of passing the sheet through a general double-sided unit multiple times as shown in the other embodiments may be used. [Explanation of symbols]

[0152] 2 Photoconductor drum 3 Exposure light 4. Developing Machine 5 Primary transfer roller 6 Cleaning materials 7 Charger 10 Imaging Station 11 Intermediate transfer belt 19 Secondary transfer roller 45 Recording media 101 First discharge section 102 Second discharge section 103 First Drying Section 104 Second Drying Section 105 Transport roller 500, 501 Invisible Images 600 Visible Images 601 Cyan Images 602 Magenta Images 603 Yellow Images 604 Blue Images 605 Black Images [Prior art documents] [Patent documents]

[0153] [Patent Document 1] JP 2018-060169 A

Claims

1. an image forming unit that forms an image on a recording medium; a control unit for controlling the image forming unit, the image forming unit forms an image on the recording medium by superposing an invisible image formed of an invisible material and a visible image formed of a visible material on the invisible image in that order, the invisible image being positioned closer to the recording medium than the visible image; a first adhesion amount of the visible material and the invisible material per unit area of ​​a predetermined image in a portion where the invisible image and the visible image are superimposed; a second adhesion amount is an adhesion amount less than the first adhesion amount, and an adhesion amount of the visible material and the invisible material per unit area of ​​a predetermined image in a portion where the invisible image and the visible image are overlapped is defined as a second adhesion amount; The control unit changes the position where the invisible image is formed in accordance with information about the visible image, and when forming an image with the second adhesion amount, controls the amount of invisible material in the second adhesion amount to be less than the amount of invisible material in the first adhesion amount, and controls the size of the invisible image to be larger than when the invisible image is formed with the first adhesion amount.

2. An image carrier; a charging section for charging the image carrier; an exposure section for exposing the image carrier to light to form an electrostatic latent image; a developing section for visualizing the electrostatic latent image formed on the image carrier with a developer to form a visible image; a transfer unit that transfers a visible image formed on the image carrier to an intermediate transfer body, and transfers the visible image transferred to the intermediate transfer body onto the recording medium, the image forming unit includes the image carrier, the charging unit, the exposure unit, the developing unit, and the transfer unit, the developing unit forms the visible image by a visible developer as the visible material, and the visible image by an invisible developer as the invisible material; When a visible image formed by the visible developer transferred onto the recording medium is defined as the visible image, and a visible image formed by the invisible developer transferred onto the recording medium is defined as the invisible image, the developing unit and the transfer unit perform the visualization and the transfer on the recording medium so that the invisible image and the visible image are superimposed on the invisible image in order; 2. The image forming apparatus according to claim 1, wherein the control unit performs control so that, when the invisible image is formed with the second adhesion amount, the amount of the invisible developer on the intermediate transfer body is smaller than when the invisible image is formed with the first adhesion amount.

3. a plurality of image forming stations each including the image carrier, the charging unit, and the developing unit; 3. The image forming apparatus according to claim 2, further comprising an image forming station having a developing unit which performs the visualization using the invisible developer, the image forming station being disposed on the most downstream side in the transport direction of the intermediate transfer body.

4. the plurality of image forming stations are image forming stations that use a black developer, a cyan developer, a magenta developer, or a yellow developer as the visible developer, 4. The image forming apparatus according to claim 3, wherein the image forming station using the black developer is disposed on the most downstream side in the transport direction, and the black developer and the invisible developer are exchangeable.

5. a first ejection unit that ejects invisible ink as the invisible material onto the recording medium; a second ejection unit that ejects visible ink as the visible material onto the recording medium onto which the invisible material has been ejected, the image forming unit includes the first discharge unit and the second discharge unit, forming an image on the recording medium by superposing the invisible image formed by the invisible ink and the visible image formed by the visible ink on the invisible image in that order, the invisible image being positioned closer to the recording medium than the visible image; 2. The image forming apparatus according to claim 1, wherein the control unit controls the amount of invisible ink ejected by the first ejection unit to be less when the invisible image is formed with the second adhesion amount than when the invisible image is formed with the first adhesion amount.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that the control unit changes the amount of the invisible material per unit area in the invisible image on the recording medium in accordance with the brightness of the visible image or the color difference between the visible image and the invisible image.

7. The image forming apparatus of claim 6, characterized in that the control unit performs control to make the amount of invisible material in the second adhesion amount less than the amount of invisible material in the first adhesion amount when the brightness of the visible image is higher than a reference value or when the color difference between the visible image and the invisible image is smaller than a reference value.

8. 8. The image forming apparatus according to claim 1, further comprising a presentation unit that shows a positional information and an image size of the invisible image to a user before the invisible image is formed on the recording medium.

9. 9. The image forming apparatus according to claim 1, wherein the invisible material contains a near-infrared absorbing material and is transparent.

10. 9. The image forming apparatus according to claim 1, wherein the invisible material contains a near-infrared absorbing material and is black in color.

11. a re-transport unit that is disposed downstream of the image forming unit in a transport direction of the recording medium and re-transports the recording medium to the image forming unit; a first step of forming an image including the invisible image on the recording medium by the image forming unit; a second step of re-transporting the surface of the recording medium on which the image including the invisible image is formed to the image forming unit by the re-transporting means; a third step of forming the visible image on the recording medium by the image forming unit; and an image forming mode for performing 11. The image forming apparatus according to claim 1, wherein the image forming mode executes the first step, the second step, and the third step in this order.

12. the image forming section includes a first imaging station having a first invisible material and a second imaging station having a second invisible material; the first non-visible material comprises a near infrared absorbing material and is transparent; 9. The image forming apparatus according to claim 1, wherein the second invisible material contains a near-infrared absorbing material and is black in color.

13. a re-transport unit that is disposed downstream of the image forming unit in a transport direction of the recording medium and re-transports the recording medium to the image forming unit; a first step of forming an image including at least one of a first invisible image formed by the first invisible material and a second invisible image formed by the second invisible material on the recording medium by the image forming unit; a second step of re-transporting the surface of the recording medium on which the image including the invisible image is formed to the image forming unit by the re-transporting means; a third step of forming the visible image on the recording medium by the image forming unit; and an image forming mode for performing 13. The image forming apparatus according to claim 12, wherein the image forming mode is executed in the order of the first step, the second step, and the third step.

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