JP4876413B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus that reduces color unevenness in an output image.

  Conventionally, in an image forming apparatus that prints out based on a digital image signal, color reproduction faithful to the color information of the input image signal is performed, so that the input image signal is an image corresponding to the gradation characteristics of the image forming apparatus. A so-called density conversion process for converting the signal is performed. In general, a lookup table is widely used for this density conversion processing. The look-up table is a table in which a relationship (input / output characteristics) between an input image signal and an output image signal is stored in advance in a ROM or RAM. Thereby, the color fluctuation of the output image with respect to the input image signal is reduced.

  In particular, image forming units for forming toner images of each color of Y (yellow), M (magenta), C (cyan), and K (black) are arranged in series, and each unit is formed on the intermediate transfer member. A color image forming apparatus that forms a color image by transferring (multiple transfer) a toner image formed on the recording medium through an intermediate transfer member or directly and sequentially superimposing it on the recording medium is nonlinear in input / output characteristics. Is very high, and color variation (color unevenness) is likely to occur.

  In order to reduce such color unevenness, an image processing apparatus that converts an image signal using a multidimensional direct look-up table (DLUT) has been proposed (see, for example, Patent Document 1).

  Here, the cause of the occurrence of nonlinearity and color unevenness due to multiple transfer will be briefly described. The toner image transferred from the photosensitive member to the intermediate transfer member (or the paper in the case of a device that transfers to the paper by primary transfer without using the intermediate transfer member) by the image forming unit on the upstream side in the process direction is formed on the downstream side. When passing through the transfer portion of the unit, a phenomenon that is attached to the photoreceptor of the downstream image forming unit and peeled off from the intermediate transfer member, so-called retransfer, may occur. This is the main cause of color unevenness. Furthermore, the occurrence of retransfer is caused by the color of the output image, that is, the primary color (color by any one type of toner of YMCK), secondary color (color by two types of toner such as RGB), or tertiary color (gray or gray). The color varies depending on the color of the three types of YMC toner such as black.

  FIG. 8 shows an image forming unit for forming a Y toner image, an image forming unit for forming an M toner image, an image forming unit for forming a C toner image, and an image forming unit for forming a K toner image from the upstream side to the downstream side in the process direction. FIG. 6 is an explanatory diagram for explaining retransfer that occurs when a single-color image (primary color) of Y is formed in an image forming apparatus in which units are arranged in series. In FIG. 8, the intermediate transfer belt 22 and the photosensitive members 10y, 10m, 10c, 10k, developing units 16y, 16m, 16c, 16k, and primary transfer units 18y, 18m that constitute the image forming units. , 18c, 18k.

  As shown in FIG. 8, the Y toner image Ty formed on the photoconductor 10y by the Y image forming unit is transferred to the intermediate transfer belt 22 (when the transfer medium to be primarily transferred is a recording medium). However, here, a case where the transfer object to be primarily transferred is an intermediate transfer member will be described as an example) and then conveyed to the M image forming unit side on the downstream side. Retransfer occurs in which a part of the toner image Ty adheres to the surface of the M photoconductor 10m and is peeled off. Retransfer may also occur when passing through the subsequent C, K photoconductors 10c, 10k. Therefore, the final Y toner image Ty has a shape different from that formed first by the Y image forming unit by retransfer.

  FIG. 9 shows retransfer that occurs when an R image (secondary color) is formed from a toner image Yy with 60% Y coverage and a toner image Tm with 60% M coverage in the same image forming apparatus of FIG. It is explanatory drawing explaining these. In this case, the Y toner image Ty is formed in the Y image forming unit as in the case of FIG. 8, but in the subsequent M image forming unit, the M toner is formed on the Y toner image Ty. Since the image Tm is formed, retransfer of the Y toner image Ty to the M photoconductor 10m does not occur (or is small). In the subsequent primary transfer of C and K, retransfer of the M toner image Tm occurs, but no retransfer occurs for the Y toner image Ty because the M toner image Tm is formed in the upper layer (or less). ).

As described above, in the primary color and the secondary color, even if the Y toner image Ty is similarly formed in the Y image forming unit, the Y toner image that is finally output differs depending on the retransfer state. It becomes a shape. This is the main cause of nonlinearity due to multiple transfer.
JP 2002-135610 A

  However, retransfer does not occur in the same way in all YMCK image forming units. For example, as shown in FIG. 10, the toner image Tk formed by the image forming unit (K image forming unit in FIG. 10) provided at the last stage of the image forming units arranged in series will be described later. Since there is no subsequent image forming unit, retransfer of the toner image Tk does not occur.

  However, in the conventional technique, in any case, a multi-dimensional DLUT using all image signals as input in consideration of retransfer generation from the primary color to the tertiary color is used for density conversion processing. Necessary, resulting in an increase in the cost of the device.

  The present invention has been made to solve the above problems, and an object thereof is to provide an image forming apparatus capable of reducing color unevenness with a small memory capacity.

In order to achieve the above object, the image forming apparatus of the present invention exposes the surface of the photoreceptor based on an image signal to form a latent image, and then develops the toner using the toner, thereby developing CMYK color toners on the photoreceptor surface. Four image forming units that form an image are arranged along the process direction, and output by sequentially superimposing and transferring toner images of different colors formed on the surface of the photoreceptor of each image forming unit onto the transfer target Among image forming means for forming an image, an image forming unit located on the most downstream side in the process direction, an image forming unit for forming a K toner image, and an image forming unit for forming a CMY toner image The remaining image forming units except for the first image forming unit that forms a toner image are the downstream image forming units on which the toner image transferred to the transfer target is transferred. Since the image forming unit forms a toner image that generates retransfer, which is a phenomenon that adheres to the photosensitive member of the first image , the remaining image forming unit needs to consider the effect of the retransfer. When the image forming unit excluding the first image forming unit among the four image forming units is the second image forming unit that does not need to consider the influence of the retransfer, Among a plurality of image signals provided corresponding to one image forming unit and forming a toner image superimposed on another toner image when the output image is formed, other than the corresponding first image forming unit A toner image formed by the corresponding first image forming unit is formed in consideration of an image signal formed by another image forming unit. In order to correct the image signal to be corrected, each of the plurality of image signals is input, and color unevenness due to the retransfer and color unevenness due to a predetermined factor other than the retransfer are determined so as not to occur in the output image. The image signal supplied to the first image forming unit is corrected using the first conversion table that outputs the image signal that has been output, and is provided corresponding to the second image forming unit. An image signal that forms a toner image formed by the corresponding second image forming unit is corrected without considering an image signal formed by another image forming unit other than the second image forming unit. The color unevenness due to a predetermined factor other than the re-transfer is input to the image signal supplied to the corresponding second image forming unit as the output image. Image processing means for correcting an image signal to be supplied to the corresponding second image forming unit using a second conversion table that outputs an image signal determined so as not to occur. Yes.

In order to suppress the color unevenness of the output image, an image forming unit located on the most downstream side in the process direction, an image forming unit that forms a K toner image, and an image forming unit that forms a CMY toner image The remaining image forming units except for the first image forming unit that forms a toner image are retransfers in which the toner image transferred to the transfer target adheres to the photoreceptor of the downstream image forming unit. since there is an image forming unit for forming a toner image occurs, the remaining image forming units as a first image forming unit is necessary to consider the influence of the retransfer, among the four image forming units The image forming units other than the first image forming unit do not need to consider the influence of the retransfer. The first image forming unit needs to correct the image signal in consideration of color variations caused by factors other than retransfer such as the gradation characteristics of the image forming apparatus and color variations caused by retransfer. There is. That is, when forming an output image, it is necessary to correct the image signal in consideration of each image signal that forms a toner image to be superimposed with another toner image. On the other hand, for the second image forming unit, the image signal may be corrected in consideration of only the color variation due to factors other than retransfer.

Accordingly, the present invention is provided corresponding to the second image forming unit, and does not take into account image signals formed by other image forming units other than the corresponding second image forming unit. In order to correct an image signal for forming a toner image formed by the second image forming unit, an image signal supplied to the corresponding second image forming unit is input and color unevenness due to a predetermined factor other than retransfer is caused. Since the image signal supplied to the corresponding second image forming unit is corrected using the second conversion table that outputs the image signal determined so as not to occur in the output image, the image of another color is corrected. Since the signal is not taken into consideration, the memory capacity can be reduced. Other than the corresponding first image forming unit among a plurality of image signals provided corresponding to the first image forming unit and forming a toner image superimposed on another toner image when forming an output image. Each of the plurality of image signals is corrected so that an image signal forming a toner image formed by the corresponding first image forming unit is corrected in consideration of an image signal formed by another image forming unit. First image formation using a first conversion table as an input and outputting an image signal determined so that color unevenness due to retransfer and color unevenness due to predetermined factors other than retransfer do not occur in the output image Since the image signal supplied to the unit is corrected, color unevenness considering retransfer can be suppressed.

Note that when a toner image is formed by a plurality of image forming units and sequentially transferred onto the transfer target, the toner image transferred to the transfer target at the end is transferred next to the toner image. Since there is no image forming unit for forming a toner image, retransfer does not occur (see, for example, FIG. 10). Therefore, the image forming unit that forms the toner image that is finally transferred to the transfer medium can be used as the second image forming unit.

  The degree of retransfer is also affected by the lower layer of the toner layer in contact with the photoreceptor. In an experiment conducted by the inventors of the present invention, a toner image directly transferred to a transfer target and a toner image transferred onto another toner image previously transferred to the transfer target are not attached. The result was that retransfer was less likely to occur in the former due to the difference in the wearing force. Hereinafter, the experiment conducted by the inventors and the result thereof will be briefly described.

  The inventors form an output image having a higher density in the main scanning direction by an image forming apparatus in which image forming units of each color are arranged in series in the order shown in FIG. 10, and each color toner image constituting the output image The concentration of was measured. 11A is a graph showing the measurement result of the density of the Y color toner image, FIG. 11B is a graph showing the measurement result of the M color toner image, and FIG. 11C is the C color measurement result. 6 is a graph showing measurement results of a toner image. In each graph, the horizontal axis indicates the position in the main scanning direction, and the vertical axis indicates the density.

  In FIG. 11A, the value indicated by Y indicates the density of the Y color toner image when the output image is Y color (that is, the primary color of Y). The value indicated by YinG indicates the density of the Y color toner image that constitutes the G color output image when the output image is green G color (ie, Y color + C secondary color). The value indicated by YinR indicates the density of the Y color toner image constituting the output image of R color when the output image is red R color (that is, secondary color of Y color + M color). Further, the value indicated by YinPB indicates the density of the Y color toner image constituting the output image of the PB color when the output image is the process black PB (that is, the tertiary color of Y color + M color + C color). Yes.

  Similarly, the value indicated by M in FIG. 11B is the density of the M color toner image when the output image is the primary color M, and the value indicated by MinR is the value when the output image is the secondary color R. The density of the M color toner image, the value indicated by MinB is the density of the M color toner image when the output image is the secondary color B, and the measured value indicated by MinPB is the M color when the output image is the tertiary color PB The density of the toner image is shown.

  Similarly, the value indicated by C in FIG. 11C is the density of the C color toner image when the output image is the primary color C, and the value indicated by CinB is the value when the output image is the secondary color B. The density of the C toner image, the value indicated by CinG is the density of the C color toner image when the output image is the secondary color G, and the measured value indicated by CinPB is the C color when the output image is the tertiary color PB The density of the toner image is shown.

  FIG. 12 shows the state of the Y, M, and C color toner images formed by the image forming apparatus when the output image is primary, secondary, and tertiary colors. Is a table showing how many times it passes through another image forming unit after being transferred to the intermediate transfer member).

  In this table, Δ indicates the number of passes through another image forming unit in a state where the toner image is directly transferred onto the intermediate transfer member, and × indicates the top of the toner image previously transferred to the transfer member. The number of passes through another image forming unit in a state where the toner image has been transferred to is shown.

  For example, as shown in FIG. 8, when the output image is a Y single color image, a Y color toner image is formed by the Y color image forming unit and directly transferred to the intermediate transfer member, followed by M, C, The three K image forming units pass through the primary transfer unit (indicated by ΔΔΔ in FIG. 8). As shown in FIG. 9, when the output image is a Y-color toner image when the output color is the secondary color R, a Y-color toner image is first formed and directly transferred directly to the intermediate transfer member, followed by M-color image formation. Since the M toner image is transferred to the upper layer of the Y toner image by the unit, no retransfer occurs in the lower Y toner image.

  When the output image is the secondary color G, the Y color image forming unit forms a Y color toner image, which is directly transferred directly to the intermediate transfer member, and the M color following the Y color image forming unit. Passes through the primary transfer unit of the image forming unit. Since the C toner image is transferred to the upper layer of the Y color toner image by the subsequent C color image forming unit, no retransfer occurs in the subsequent K color image forming unit, and the retransfer is applied to the Y color toner image. It occurs only when it passes through the M color image forming unit.

  Here, looking at the measurement result of FIG. 11A, with respect to the Y color toner, there is no variation in density in any of the primary color, secondary color, and tertiary color, and color unevenness due to retransfer almost occurs. You can see that it is not.

  Further, the M color toner image when the output image is the primary color M passes through the subsequent C and K image forming units (ΔΔ) after the M color toner image is directly transferred to the intermediate transfer member. It is shown). Further, for the R color toner image when the output image is the secondary color M, the M color toner image is transferred onto the Y color toner image that has been directly transferred to the intermediate transfer body, and the subsequent Pass through the C and K image forming units (indicated by xx).

  Here, in the measurement result of FIG. 11B, even if the image forming unit passes through another image forming unit the same number of times, retransfer hardly occurs when the lower layer is an intermediate transfer member, and retransfer is not performed at all. It can be seen that there is almost no difference from the concentration when it does not occur (MinB, MinPB). This is presumably because the adhesive force between the toner layer and the intermediate transfer member is higher than the adhesive force between the toner layer and the toner layer.

  Similarly, in the case of a C color toner image, the measurement result in FIG. 11C shows that the C color toner image when the output image is the primary color C is directly applied to the intermediate transfer member. Because the image is transferred, retransfer by the subsequent K-color image forming unit hardly occurs. However, when the output image is the secondary color G or B, or the tertiary color PB, the toner image of the other color is intermediately transferred first. Since the image is transferred onto the body and then transferred, the density is lowered by retransfer by the subsequent K-color image forming unit.

From this, it can be said that the retransfer is small when the lower layer is an intermediate transfer member, and retransfer hardly occurs. In addition, even if a slight amount of retransfer occurs, it can be corrected to a level where there is no problem in use by optimizing the parameters such as the transfer current of the primary transfer device, so that the formed toner image is always transferred directly to the transfer target. The image forming unit, that is, the image forming unit that forms the toner image first transferred to the transfer target can be the second image forming unit.

In addition, even if a toner image of another color is superimposed on black (K color), the color of the image becomes black. Therefore, it is usually rare that the toner image of K color and the toner image of another color are superimposed. Therefore, the K color toner image is always transferred directly to the transfer medium as the primary color, so the image forming unit that forms the black toner image can be the second image forming unit.

  Further, the first conversion table and the second conversion table can further receive information related to the formation position of the output image.

  The information related to the formation position of the output image may be information related to the position in the main scanning direction, information related to the position in the sub-scanning direction, or information related to the two-dimensional position, that is, main scanning / Information regarding both sub-scanning positions may be used. As a result, color variation due to in-plane uniformity within the image forming surface can be prevented, and density conversion accuracy can be greatly improved.

  As described above, according to the present invention, there is an excellent effect that color unevenness can be reduced with a small memory capacity.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram showing a first embodiment of an image forming apparatus of the present invention. The image forming apparatus includes an image input unit 1, an image processing unit 2, and an image forming unit 3.

  The image input unit 1 captures color image data by inputting color image data from an external device or by reading a color document with the image reading unit 1a (see FIG. 2). Then, an image signal of each color of Y (yellow), M (magenta), C (cyan), and K (black) and a recording position coordinate signal x on a sheet on which an image corresponding to the image signal is formed are output.

  The recording position coordinate signal x on the sheet indicates the recording position coordinates in the main scanning direction during exposure by the ROS 14 in the image forming unit 3. Since this coincides with the position coordinate of the image signal, the recording position coordinate signal x is input from the image input unit 1 to the image processing unit 2 here.

  The image processing unit 2 uses an image signal (hereinafter referred to as an image formation signal) for forming an image signal of each color of YMCK input from the image input unit 1 with the image forming unit 3 so that color unevenness does not occur in the output image. And then transferred to the image forming unit 3. In the present embodiment, the recording position coordinate signal x indicating the recording position coordinate in the main scanning direction is also input from the image input unit 1, and the YMCK image signals of the YMCK colors are also converted using the recording position coordinate signal x. Convert to formation signal. As a result, it is possible to correct the color variation caused by the in-plane uniformity within the image forming surface, and reduce the color unevenness of the final output image. Details of the configuration and operation of the image processing unit 2 will be described later.

  The image forming unit 3 forms an image on a sheet according to the image forming signal transferred from the image processing unit 2. FIG. 2 is a schematic configuration diagram illustrating a configuration example of the image forming unit 3. The image forming unit 3 includes a Y color image forming unit 33Y that forms a Y color toner image, an M color image forming unit 33M that forms an M color toner image, and a C color image forming unit 33C that forms a C color toner image. , K color image forming units 33K for forming K toner images are arranged in a series direction from the upstream side to the downstream side in the process direction.

  Each image forming unit includes rotating photoreceptors 10y, 10m, 10c, and 10k, chargers 12y, 12m, 12c, and 12k that charge the surface of each photoreceptor, and an image processing unit that treats each charged photoreceptor surface. ROS (laser output units) 14y, 14m, 14c, and 14k that form an electrostatic latent image on each photosensitive member by exposure with exposure light modulated based on the image forming signal of each color transferred from 2 and each color Developers 16y and 16m each having a developing roll for carrying toner and applying a developing bias to the developing roll to develop an electrostatic latent image on each photoreceptor with each color toner to form a toner image on the photoreceptor. 16c, 16k, primary transfer devices 18y, 18m, 18c, 18k for transferring the color toner images on the photosensitive member to the intermediate transfer belt 22, and a cleaner for cleaning the surface of each photosensitive member. 0y, and includes 20 m, 20c, and 20k, discharger 21y, 21m, 21c, and 21k.

  Further, the image forming section 3 further includes a secondary transfer device 24 that transfers the toner image on the intermediate transfer belt 22 to the recording paper, a fixing device 26 that fixes the toner image transferred to the recording paper, and the recording paper. The paper tray 28 is configured to be stored.

  Here, an image forming operation in the image forming apparatus shown in the configuration diagram will be briefly described. The driving of the image forming apparatus is started by a signal from the outside, and the surface of the photoconductors 10y, 10m, 10c, and 10k is applied to the chargers 12y, 12m, 12c, and 12k by applying a charging bias from a power source to a predetermined charging potential. Charges up. When the surfaces of the photoreceptors 10y, 10m, 10c, and 10k charged by the chargers 12y, 12m, 12c, and 12k reach the positions of the color developing devices 16y, 16m, 16c, and 16k, the color developing devices 16y, 16m, 16c, and A developing bias is applied to the predetermined developing potential from the power source at 16k as well.

  On the other hand, image signals Yi, Mi, Ci, Ki corresponding to the colors Y, M, C, K and the recording position coordinate signal x are transferred from the image input unit 1 to the image processing unit 2. In the image processing unit 2, the image forming signals Yo, Mo, Co, Ko are input to the ROSs 14y, 14m, 14c, 14k from the input image signals Yi, Mi, Ci, Ki of each color and the recording position coordinate signal x. The laser beam (exposure light) is modulated. The modulated laser beam is applied to the surfaces of the photoreceptors 10y, 10m, 10c, and 10k that are uniformly charged by the chargers 12y, 12m, 12c, and 12k. When the surface of each photoconductor is raster-irradiated with a laser beam, an electrostatic latent image corresponding to the image forming signal is formed on each photoconductor. Subsequently, the electrostatic latent images on the photoconductors are developed with toner by the color developing devices 16y, 16m, 16c, and 16k, and the color toner images are formed on the photoconductors. The toner images of the respective colors formed on the respective photoconductors are sequentially superimposed on the intermediate transfer belt 22 by the primary transfer units 18y, 18m, 18c, and 18k (here, in the order of Y, M, C, and K colors). Transcribed. After the transfer of the toner image to the intermediate transfer belt 22 is completed, the toner 20y, 20m, 20c, and 20k are cleaned of the adhering matter such as residual toner on the surface, and the static eliminators 21y, 21m, 21c, Residual charges are removed by 21k.

  Next, the toner image on the intermediate transfer belt 22 is transferred onto the recording paper sent from the paper tray 28 by the secondary transfer device 24 and then transferred onto the recording paper by the fixing device 26. The image is fixed and a desired image is obtained.

  As illustrated in FIG. 3, the image processing unit 2 includes a first conversion table 30 and a second conversion table 70 k. In the present embodiment, density conversion (correction) is performed using the first conversion table 30 for the YMC color image forming units 33Y, 33M, and 33C in which retransfer may occur according to the color of the output image. ) Is supplied to the intermediate transfer belt 22 and the K image forming unit 33K in which the retransfer of the toner image that is primarily transferred to the photosensitive member again does not always occur is performed by the second conversion table 70k. A density-converted (corrected) image forming signal is supplied.

  The second conversion table 70k is a look-up table in which a correspondence table between the input signal (Ki, x) and the image forming signal Ko is held. The image processing unit 2 uses the second conversion table 70k to input the second conversion table 70k. The processed image signal Ki is converted into an image forming signal Ko (two inputs and one output). The second conversion table 70k generates an image signal that does not cause color unevenness in the output image without considering other colors transferred to the intermediate transfer belt 22. With this second conversion table 70k, the density of the image represented by the input image signal is appropriately reproduced on the output image according to the color reproducibility such as the gradation characteristics of the image forming apparatus. It can be converted into an image forming signal. Even if the in-plane uniformity of the image forming apparatus is low, the image signal can be converted in accordance with the recording position coordinate signal x, so that color unevenness can be suppressed.

  In the present embodiment, the Y color image forming unit 33Y, the M color image forming unit 33M, the C color image forming unit 33C, and the K color image forming unit 33K are arranged in series from the upstream side to the downstream side in the process direction. Has been. Therefore, retransfer does not occur in the K color image forming unit that finally transfers the toner image to the intermediate transfer belt 22 (that is, located on the most downstream side). (See FIG. 10). Therefore, in the present embodiment, the K color image forming unit 33K is converted by the second conversion table 70k in consideration of color fluctuations caused by factors other than retransfer (that is, the toner images of other colors are not considered). The image forming signal (converted in step 1) is supplied.

  The first conversion table 30 is a multi-dimensional lookup table in which a correspondence table of input signals (Yi, Mi, Ci, x) and image formation signals for each color of Y, M, and C is held. 2 converts the input image signals Yi, Mi, and Ci into image forming signals Yo, Mo, and Co using the first conversion table 30 (four inputs and three outputs).

  In the present embodiment, even when the multiple transfer characteristics are nonlinear (when there is retransfer), density conversion is performed so that color unevenness does not occur. Therefore, even at the same recording position, the intermediate transfer belt Density conversion is performed according to the image signals of the respective colors superimposed on each other. Accordingly, it is possible to correct not only in-plane uniformity and gradation characteristics, but also color variations caused by multiple transfer characteristics.

  In the present embodiment, the image forming units 33Y, 33M, and 33C other than the K-color image forming unit 33K located on the most downstream side are regarded as image forming units that generate retransfer, and the image forming signals converted by the first conversion table are used. Are supplied to these image forming units 33Y, 33M, and 33C. Note that, even if the other color toner image is superimposed on the K color, the color of the image will be black. Therefore, the K color toner image and the other color toner image are usually rarely superimposed. Therefore, it is not necessary to consider the K color toner image for retransfer, and the input signal of the first conversion table 30 does not include the image signal Ki.

  The second conversion table 70k can be obtained as follows. First, the image forming unit 3 prints out patches for an arbitrary value of the image signal Ki on a sheet at intervals corresponding to a predetermined number of divisions in the main scanning direction, and the output patch density is determined by a colorimeter (not shown). taking measurement. A correspondence table of the density measurement value, the original image signal, and the recording position coordinate x in the main scanning direction is created. Based on this correspondence table, a correspondence table between the input signal (Ki, x) and the image forming signal Ko that appropriately reproduces the density of the image represented by the input image signal on the output image is created. And stored in a ROM or the like.

  Similarly to the second conversion table 70k, the first conversion table 30 also prints out patches for any value of the image signals Yi, Mi, and Ci on the paper at intervals according to the number of divisions, and sets the density of the output patches. taking measurement. Then, a correspondence table is created in the same manner as described above, and finally a correspondence table between the input signals (Yi, Mi, Ci, x) and the image forming signals Yo, Mo, Co is created and stored in the ROM or the like.

  As described above, with respect to the image forming units other than the image forming unit in which retransfer does not occur, the density conversion is performed using the first conversion table 30 using the YMC image signals and the recording position coordinate signal x as input signals. For the image forming unit in which retransfer does not occur, such as the most downstream image forming unit 33K, density conversion is performed using the second conversion table 70k using the K color image signal and the recording position coordinate signal x as input signals. Therefore, the memory capacity of the lookup table necessary for the density conversion process can be reduced.

  For example, an image signal input from the image input unit 1 is an 8-bit (256 gradation) image signal, and the number of divisions of the recording position coordinate signal x in the main scanning direction input from the image input unit 1 is 16 (17 (Lattice points), the total memory capacity required for the density conversion processing in this embodiment can be calculated as follows.

First, the memory capacity of the first conversion table 30 is
256 (Y) x 256 (M) x 256 (C) x 17 (x) x 3 (output)
= 855, 638, 016 bits = 106, 954, 752 bytes.

On the other hand, the memory capacity of the second conversion table 70k is
Since 256 (K) × 17 (x) = 4352 bits = 544 bytes, the total memory capacity of the conversion table in this embodiment is
106,954,752 + 544 = 106,955,296 bytes (1)
It becomes.

Here, the memory capacity of the conversion table (DLUT) 40 used in the conventional density conversion process is illustrated as a comparative example. In the prior art, as shown in FIG. 4, YMCK image signals Yi, Mi, Ci, Ki and recording position coordinate signal x are input and YMCK image signals Yo, Mo, Co, Ko are output. The memory capacity of the DLUT 40 required for the density conversion process is
256 (Y) x 256 (M) x 256 (C) x 256 (K) x 17 (x) x 4 (output)
= 292,057,776,128 bits = 36,507,222,016 bytes (2)
It becomes.

  As is clear from the calculation results (1) and (2), the memory capacity is reduced as compared with the conventional case.

[Second Embodiment]
The adhesion force differs between when the toner image is transferred directly to the intermediate transfer belt 22 and when transferred onto another toner image. Usually, in the former case, adhesive force is large and retransfer hardly occurs.

  Accordingly, an image forming unit in which the formed toner image is always transferred directly to the intermediate transfer body belt 22, that is, an image forming unit that forms a toner image first transferred to the intermediate transfer body belt 22 (positioned at the most upstream in the process direction). The image forming unit) can be an image forming unit that does not generate retransfer. In the present embodiment, the image forming signal supplied to the Y color image forming unit 33Y positioned at the most upstream is also generated using the same conversion table as the second conversion table described in the above embodiment.

  Note that the overall configuration of the image forming apparatus in the present embodiment is the same as that shown in FIGS.

  As shown in FIG. 5, the image processing unit 2 according to the present embodiment includes a first conversion table 50 and two second conversion tables 70y and 70k. In the present embodiment, for the MC color image forming units 33M and 33C in which retransfer may occur according to the color of the output image, the image forming signal whose density has been converted using the first conversion table 50 is used. Are supplied to the Y and K image forming units 33Y and 33K for which retransfer does not always occur, by supplying the image forming signals whose density has been converted by the second conversion tables 70y and 70k.

  The second conversion table 70y is a lookup table in which a correspondence table between the input signal (Yi, x) and the image forming signal Yo is held, and the operation thereof is the second conversion table 70k shown in the first embodiment. Since it is the same as that of FIG.

  The first conversion table 50 is a multi-dimensional lookup table in which a correspondence table of input signals (Yi, Mi, Ci, x) and image formation signals for each color of M and C is held. The first conversion table 50 is used to generate image forming signals Mo and Co. In this embodiment, there are four inputs and two outputs.

  In the present embodiment, as in the first embodiment, the image signal input from the image input unit 1 is an 8-bit (256 gradation) image signal, and the main scanning direction input from the image input unit 1 is the same. If the number of divisions of the recording position coordinate signal x is 16 (17 grid points), the total memory capacity necessary for the density conversion process can be calculated as follows.

First, the memory capacity of the first conversion table 50 is
256 (Y) x 256 (M) x 256 (C) x 17 (x) x 2 (output)
= 570, 425, 344 bits
= 71,303,168 bytes.

On the other hand, the memory capacities of the second conversion tables 70y and 70k are respectively
Since 256 × 17 = 4352 bits = 544 bytes, the total memory capacity of the conversion table in this embodiment is
71,303,168 + 544 + 544 = 71,304,256 bytes (3)
It becomes.

  Therefore, as is apparent from the calculation results (1) and (2) shown in the first embodiment, the memory capacity is further reduced in this embodiment.

[Third Embodiment]
Even if the other color toner image is superimposed on the K color, the color of the image becomes black. Therefore, the K color toner image and the other color toner image are usually rarely superimposed. Therefore, the K color toner image is always transferred directly to the intermediate transfer belt 22 as the primary color. Accordingly, an image forming unit that forms a black toner image can be regarded as an image forming unit that does not always generate retransfer.

  In the present embodiment, the K-color image forming units 33K are arranged at positions other than the most upstream and downstream positions in the process direction. Here, as shown in FIG. 6, the Y color image forming unit 33Y, the M color image forming unit 33M, the K color image forming unit 33K, and the C color image forming unit 33C are arranged in series in this order from upstream to downstream in the process direction. Array.

  With this arrangement, retransfer is considered for the Y color image forming unit 33Y arranged at the most upstream position, the C color image forming unit 33C arranged at the most downstream position, and the K image forming unit 33K. The density conversion process can be performed without the need.

  As shown in FIG. 7, the image processing unit 2 according to the present embodiment includes a first conversion table 60 and three second conversion tables 70y, 70k, and 70c. In the present embodiment, an image forming signal whose density has been converted using the first conversion table 50 is supplied to the M color image forming unit 33M in which retransfer may occur depending on the color of the output image. The Y, K, and C image forming units 33Y, 33K, and 33C in which retransfer does not always occur are supplied with the image forming signals whose density has been converted by the second conversion tables 70y, 70k, and 70c.

  The second conversion table 70c is a lookup table in which a correspondence table between the input signal (Ci, x) and the image forming signal Co is held, and the operation thereof is the second conversion table 70k shown in the first embodiment. Since it is the same as the second conversion table 70c shown in the second embodiment, a description thereof will be omitted.

  The first conversion table 60 is a multi-dimensional lookup table in which a correspondence table of input signals (Yi, Mi, Ci, x) and M-color image formation signals Mo is held. An image forming signal Mo is generated using one conversion table 60. In this embodiment, there are four inputs and one output.

  In the present embodiment, as in the first embodiment, the image signal input from the image input unit 1 is an 8-bit (256 gradation) image signal, and the main scanning direction input from the image input unit 1 is the same. If the number of divisions of the recording position coordinate signal x is 16 (17 grid points), the total memory capacity necessary for the density conversion process can be calculated as follows.

First, the memory capacity of the first conversion table 60 is
256 (Y) x 256 (M) x 256 (C) x 17 (x)
= 285, 212, 672 bits = 35, 651, 584 bytes, and the memory capacities of the second conversion tables 70y, 70k, 70c are respectively
Since 256 × 17 = 4352 bits = 544 bytes, the total memory capacity of the conversion table in this embodiment is
35,651,584 + 544 + 544 + 544 = 35,652,128 bytes
... (4)
It becomes.

  Therefore, as is clear from the calculation results (1), (2), and (3) shown in the first and second embodiments, the memory capacity is further reduced in this embodiment.

  In the above embodiment, an example of inputting the recording position coordinate signal in the main scanning direction has been described. However, in addition to the recording position coordinate signal in the main scanning direction, the recording coordinate signal in the sub scanning direction on the photosensitive member is input. You may make it do. For example, a photoconductor encoder may be provided on the photoconductor, and a rotation angle signal output from the encoder may be input to the first and first conversion tables.

  In the above embodiment, the image forming apparatus that performs primary transfer from the photoreceptor to the intermediate transfer belt and secondary transfer from the intermediate transfer belt to the paper has been described as an example. For example, an image forming apparatus that directly transfers from a photosensitive member to a recording sheet may be used.

1 is a block diagram showing a first embodiment of an image forming apparatus of the present invention. It is a schematic block diagram which shows one structural example of an image formation part. It is the figure which showed the input / output state of the 2nd conversion table with which the image processing part of 1st Embodiment was equipped, and the 1st conversion table. It is the figure which showed the input / output state of the conversion table (DLUT) used by the prior art. It is the figure which showed the input / output state of the 2nd conversion table with which the image processing part of 2nd Embodiment was equipped, and the 1st conversion table. FIG. 10 is a diagram illustrating an arrangement of image forming units according to a third embodiment. It is the figure which showed the input / output state of the 2nd conversion table with which the image processing part of 3rd Embodiment was equipped, and the 1st conversion table. An image forming unit for forming a Y toner image, an image forming unit for forming an M toner image, an image forming unit for forming a C toner image, and an image forming unit for forming a K toner image are arranged in series from the upstream side to the downstream side. FIG. 6 is an explanatory diagram for explaining retransfer that occurs when a single-color image (primary color) of Y is formed in the image forming apparatus. Description of retransfer that occurs when an R image (secondary color) is formed from a toner image Yy with a Y coverage of 60% and a toner image Tm with a M coverage of 60% in the same image forming apparatus of FIG. FIG. FIG. 6 is an explanatory diagram for explaining that no retransfer occurs in a toner image formed by an image forming unit provided at the last stage of image forming units arranged in series (the toner image formed and transferred last). An output image having a high density in the main scanning direction was formed by an image forming apparatus in which image forming units of each color were arranged in series in the order shown in FIG. 10, and the density of each toner image constituting the output image was measured. (A) is a graph showing the measurement result of the density of the Y color toner image, (B) is a graph showing the measurement result of the M color toner image, C) is a graph showing the measurement result of the C toner image. In the image forming apparatus in which the image forming units of each color are arranged in series in the order shown in FIG. 10, what kind of toner images of each color Y, M, and C can be obtained when the output image is primary, secondary, and tertiary colors. 7 is a table showing how many times the image is transferred to a transfer target (in this case, an intermediate transfer member) and passed through another image forming unit after transfer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image input part 2 Image processing part 3 Image formation part 10y, 10m, 10c, 10k Photoconductor 12y, 12m, 12c, 12k Charger 14y, 14m, 14c, 14k ROS
16y, 16m, 16c, 16k Developing devices 18y, 18m, 18c, 18k Primary transfer device 22 Intermediate transfer belt 24 Secondary transfer device 26 Fixing devices 30, 50, 60 First conversion table 33Y Y color image forming unit 33M M color Image forming unit 33C C color image forming unit 33K K color image forming units 70y, 70k, 70c Second conversion table

Claims (2)

Four image forming units are arranged along the process direction to form C, M, Y, and K toner images on the photosensitive member surface by exposing the surface of the photosensitive member based on an image signal to form a latent image and then developing with toner. Image forming means for forming an output image by sequentially superimposing and transferring toner images of different colors formed on the surface of the photoreceptor of each image forming unit on a transfer target;
An image forming unit that forms the first toner image among an image forming unit that is located on the most downstream side in the process direction, an image forming unit that forms a K color toner image, and an image forming unit that forms a CMY color toner image. The image forming unit other than the forming unit forms an image that forms a toner image in which retransfer occurs, which is a phenomenon in which the toner image transferred to the transfer target adheres to the photoreceptor of the downstream image forming unit. Since it is a forming unit , the remaining image forming unit is a first image forming unit that needs to consider the influence of the retransfer, and the first image forming unit is excluded from the four image forming units. The first image forming unit is the second image forming unit that does not need to consider the influence of the retransfer. Other than the corresponding first image forming unit among a plurality of image signals provided corresponding to the image forming unit and forming a toner image superimposed on another toner image when forming the output image Each of the plurality of image signals is input so that the image signal forming the toner image formed by the corresponding first image forming unit is corrected in consideration of the image signal formed by the image forming unit. The first conversion table that outputs an image signal determined so that color unevenness due to the retransfer and color unevenness due to a predetermined factor other than the retransfer do not occur in the output image is used. The image signal supplied to the image forming unit is corrected and provided corresponding to the second image forming unit, and the corresponding second image forming unit. The corresponding second signal is corrected so that the image signal forming the toner image formed by the corresponding second image forming unit is corrected without considering the image signal formed by the other image forming unit outside. A second conversion table that receives an image signal supplied to the image forming unit as an input and outputs an image signal determined so that color unevenness due to a predetermined factor other than the retransfer does not occur in the output image; Image processing means for correcting an image signal supplied to the corresponding second image forming unit;
An image forming apparatus including:   The image forming apparatus according to claim 1, wherein the first conversion table and the second conversion table further receive information regarding a formation position of the output image.

Images