JP2010176011A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of maintaining a halftone image density during a continuous image output including a false continuous image output under a low humidity environment. <P>SOLUTION: The image forming apparatus is composed so that an image processing section 30 changes a gamma table used for relating an input image signal with the density of an output image according to the number of sheets of the image output during the continuous image output or during an intermittent image output within a constant period when the absolute amount of moisture in the environment is a prescribed value or below. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer employing an electrophotographic system.

  When using an image forming device, the environment in which the device is used and the storage environment for the paper as the recording material are different, so there is a difference between the moisture content of the atmosphere in the usage environment and the moisture absorption amount of the paper. A situation where the forming apparatus operates is conceivable. The situation in which the image forming apparatus operates in such a state can be considered even if the use environment and the paper storage environment are the same.

  When ordinary paper (neutral paper) generally used for copying is left in each environment from room temperature, the time required for one sheet to fully adapt to the environment is about one hour. . Even when the sheets loaded in the cassette and supplied into the image forming apparatus are allowed to adapt to the environment for a sufficiently long time, the moisture is absorbed and desorbed in a portion close to the surface in each environment.

  In particular, in the case of a low-humidity environment where the absolute moisture content in the surrounding environment of the image forming apparatus is low, if the continuous image output or the intermittent image output within a certain period of time is continuously performed for a certain time or more, The amount of water in the environment surrounding the device <the amount of water in the paper) may cause a problem in the image forming process. The intermittent image output within a certain period of time described above is continuously performed for a certain period or more is referred to as “pseudo continuous image output”.

  In the case of an electrophotographic image forming apparatus, the transfer means uses a transfer belt, a roller, or the like as a transfer member to transfer the toner image formed on the photoconductor to a sheet. This transfer member is in contact with the photoconductor. Alternatively, it is often used in a state where a certain amount of minute gap is maintained. The contact or non-contact transfer member allows the sheet to be brought into close contact with the photoreceptor in a certain amount of contact, whereby the toner image on the photoreceptor is efficiently transferred to the sheet.

  When continuous image output including pseudo continuous image output is performed, fluctuations in the latent image potential on the photoreceptor, which are considered to be due to the effects of peeling discharge during transfer, transfer voltage, and moisture content between the paper and the image forming apparatus, are considered. Occurs, and image density fluctuation occurs.

  Patent Document 1 discloses a technique for suppressing image density fluctuation due to continuous image output using paper in such a low-humidity environment. The image forming apparatus described in Patent Document 1 includes an environmental sensor, a correction unit I, and a correction unit II, and the correction unit II sets a development contrast that is set according to the absolute water content measured by the correction unit I. However, feedback control is performed with a correction amount commensurate with the development contrast fluctuation. The correction means II is a low humidity environment where the measured absolute moisture content is small, operates when paper is used as the recording material, and the image output is a continuous output. The correction means II calculates the potential decrease amount based on the relationship between the number of continuous or intermittent output sheets and the exposure part potential decrease amount of the photosensitive member within an arbitrarily set time in a low humidity environment obtained in advance. To relate to. Then, the correction unit II obtains a correction amount corresponding to the development contrast variation due to the potential decrease amount from the number of output sheets. Japanese Patent Application Laid-Open No. H10-228667 describes that the development contrast is controlled to be constant by correcting the exposure amount, or correcting the charging potential and the development potential, thereby suppressing image density fluctuation.

As described in Patent Document 1, by gradually changing the exposure amount during continuous image output in a low-humidity environment, a decrease in the exposure portion potential Vl is suppressed (Vl is kept constant), and the image density is increased. Variations can be suppressed. Further, by gradually switching between the charging potential and the developing potential during continuous image output in a low humidity environment, fluctuations in latent image contrast can be suppressed, and fluctuations in image density can be suppressed.
JP 2001-281940 A

  However, as described in Patent Document 1, there is a problem relating to fluctuations in image density that cannot be solved by gradually changing the exposure amount during continuous image output or by gradually switching between the charging potential and the developing potential. .

FIG. 17 shows the latent image contrast in each of the following cases during continuous image output. That is,
(1) First sheet (first image in continuous image formation),
(2) Nth sheet (no correction) (Nth image during continuous image formation),
(3) Nth sheet (exposure amount UP) (image obtained by increasing and correcting the Nth sheet exposure amount during continuous image formation)
(However, N> 1).

  According to the prior art, for example, the exposure amount is corrected (Nth sheet (exposure amount UP) in FIG. 17) in order to suppress image density variation (actually, image density decrease) during continuous image output. From FIG. 17, the latent image contrast of the maximum image density on the Nth sheet (exposure amount UP) (Dmax latent image contrast: right end on the horizontal axis) is equal to that of the first sheet, and the maximum image density (Dmax) is constant. You can see that it is preserved. On the other hand, from FIG. 17, the halftone latent image contrast (horizontal axis intermediate region) on the Nth sheet (exposure amount UP) is excessively corrected by increasing the exposure amount (first halftone latent image). You can see that it is above the contrast.

  FIG. 18 shows (1) N-th sheet (uncorrected) all gradation area color difference and (2) N-th sheet (exposure amount UP) when the first sheet is used as a color reference during continuous image output. It represents the tone range color difference. From FIG. 18, it can be seen that after the exposure amount is increased, the color difference in the vicinity of Dmax is improved as compared with the case without correction, whereas the color difference between halftones is deteriorated rather than without correction.

  As described above, it is difficult to suppress the halftone image density fluctuation caused by the latent image potential fluctuation during continuous image output by a conventional technique such as an increase in exposure amount.

  The halftone image density has the following characteristics. That is, (1) the human eye has a so-called halftone density range in which the optical density is 1.0 or less than the density fluctuation in the high density range such as the optical density 1.3, 1.4, 1.5. Sensitive to changes in the concentration of (2) Halftone image density fluctuation is important for gradation control in which gradation correction is performed based on a halftone density patch (test image). Therefore, it is important to suppress halftone image density fluctuations.

  As described above, it is necessary to take measures against density fluctuations in the middle tone among density fluctuations due to latent image contrast fluctuations on the photoconductor during continuous image output using paper in a low humidity environment.

  Therefore, an object of the present invention is to provide an image forming apparatus capable of maintaining a halftone image density during continuous image output including pseudo continuous image output in a low humidity environment.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a photosensitive member, a charging unit that charges the surface of the photosensitive member, an exposure unit that exposes the charged surface of the photosensitive member to form an electrostatic image, and an electrostatic image. Developing means for developing and forming a toner image, transfer means for transferring the toner image formed on the photoreceptor to a recording material, and processing the input image signal to form a toner image on the photoreceptor In the image forming apparatus having an image processing unit that generates a signal, the image processing unit is outputting a continuous image or outputting an intermittent image within a predetermined time when the absolute water content of the environment is equal to or less than a predetermined value. In addition, the image forming apparatus is characterized in that the gamma table used for relating the input image signal and the density of the output image is changed according to the number of output images.

  According to the present invention, it is possible to maintain a halftone image density during continuous image output including pseudo continuous image output in a low-humidity environment.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
1. 1 is a schematic diagram showing an embodiment of an image forming apparatus according to the present invention. In this embodiment, the image forming apparatus 100 includes a drum-type electrophotographic photosensitive member (photosensitive member), that is, a photosensitive drum 1 as an image carrier. The surface of the photosensitive drum 1 is uniformly charged to a predetermined potential by a charger 2 as a charging unit to which a charging bias is applied. Next, in this embodiment, image exposure is performed by an exposure device 3 as an exposure unit that irradiates laser, and an electrostatic latent image (electrostatic image) is formed on the surface of the photosensitive drum 1. Next, the electrostatic latent image formed on the photoreceptor is developed using a developer by a developing device 4 as a developing unit. As the developer, a one-component developer substantially composed of toner or a two-component developer containing toner and carrier is used. The developing device 4 includes a developing sleeve 4 a as a developer carrying member that carries the developer and supplies the developer to the developing unit facing the photosensitive drum 1. In this embodiment, during development, a developing bias is applied to the developing sleeve 4a, and toner is applied to the latent image on the photosensitive drum 1 by the action of an electric field formed between the developing sleeve 4a and the photosensitive drum 1 by the developing bias. To be visualized as a toner image (developed image).

  Here, it is important to stabilize the charging potential as much as possible during continuous image output. FIG. 4 shows a change in charging potential during continuous image output and an EV curve for the Nth sheet (N> 1) during continuous image output. The EV curve refers to a curve in which the horizontal axis plots the light amount and the vertical axis plots the exposure part potential. If the charging potential at the time of continuous image output is constant as shown in FIG. 4A, the N-th EV curve shape during continuous image output is uniquely determined as shown in FIG. 4B. On the other hand, if the charging potential at the time of continuous image output is unstable as shown in FIG. 4C, the Nth EV curve shape during continuous image output is not uniquely determined as shown in FIG. 4D. If the Nth EV curve during the continuous image output is not uniquely determined, it becomes difficult to obtain the reproducibility of the gamma characteristic change during the continuous image output, which is necessary for gamma correction described later. Therefore, it is desirable that the charging potential change as shown in FIG. 4A is ideal, and the charging potential during continuous image output is as stable as possible.

  As a technique for stabilizing the charging potential during continuous image output, for example, there is the following method proposed in Japanese Patent Application Laid-Open No. 7-110616. As a technique for always stabilizing a charging potential in a contact charging type charger, at least a first charging member as a charging member, and a second charging member positioned on the downstream side in the moving direction of the object to be charged with respect to the first charging member And prepare. Then, the charged body is charged to a substantially desired charging potential by the first charging member, and the non-uniformity of charging of the charged body made by the first charging member is corrected by the second charging member. In this embodiment, such a charging method can be adopted.

  The toner image formed on the photosensitive drum 1 is transferred to a recording material P conveyed to a position facing the photosensitive drum 1 from a recording material cassette (not shown) by a transfer charger 5 as a transfer unit. Next, the recording material P onto which the toner image has been transferred is conveyed from the photosensitive drum 1 to a fixing device 7 as a fixing unit, where the toner image is fixed by heating and pressing. Thereafter, the recording material P is discharged to the outside of the image forming apparatus 100, and an output image is obtained.

  The photosensitive drum 1, the charger 2, the exposure device 3, the developing device 4, the transfer charger 5, and the like constitute an image forming unit 10 as image forming means for forming an image on the recording material P.

2. Control Mode The image forming apparatus 100 includes a correction unit 20, and further includes an environment sensor 11, a time measurement unit (timer) 16, a number counter 17, and the like. Then, the development process condition is corrected and controlled by the correction unit 20 in accordance with the situation of the surrounding environment of the image forming apparatus 100 (environment correction).

  As shown in FIG. 1, the correcting means 20 includes two CPUs (first and second memories) as storage media storing a CPU as a calculation control means, a charging potential control means, a development potential control means, and an environment variation table. 13 and 14 etc. are provided.

  The contents of the correction control of the development process condition by the correction means 20 are that the toner charge amount Q / M used for development and the transfer parameters such as the transfer voltage change due to the influence of the environment. To change accordingly. The change in the relationship curve between the exposed portion potential Vl and the non-exposed portion potential Vd when the moisture amount (absolute moisture amount) increases is, for example, the direction of the bold arrow shown in FIG. FIG. 10 is a graph schematically showing the relationship between the non-exposed portion potential and the exposed portion potential in a low humidity environment. For example, when 1000 continuous images are output, the exposed portion potential Vl is greatly reduced from, for example, the point P on the solid line K to the point Q on the dotted line L in the drawing. The development contrast fluctuates (becomes larger) by the difference between the exposed portion potentials at points P and Q.

The necessary development contrast potential (development Vcont) in each environment is stored in the first memory 13 in the form of a table, and the correction unit 20 calculates the necessary development Vcont for the moisture amount. . Further, a necessary latent image contrast potential (latent image Vcont) is calculated. The necessary latent image contrast potential Vcont is a value obtained by adding a necessary back surface potential (Vback) as a countermeasure against background fog (a phenomenon in which unintended toner adheres to a non-image portion) to the necessary development Vcont.
Latent image Vcont = development Vcont + Vback

  Next, the correcting unit 20 calculates the non-exposed portion potential Vd and the exposed portion potential Vl necessary for obtaining the latent image Vcont obtained as described above on the photosensitive drum 1 by charging and exposure. The correcting means 20 is an environment table that is stored in advance in the second memory 14 and tabulates the relationship between the surface potential (non-exposed portion potential) Vd and the exposed portion potential Vl in each environment of the photosensitive drum 1. The CPU calculates the non-exposed portion potential Vd and the exposed portion potential Vl.

A value obtained by adding the calculated exposure portion potential Vl and the development Vcont obtained as described above is a potential applied to the development sleeve 4a of the developing device 4, that is, a development voltage (DC component of the development bias) Vdc. Become.
Development voltage Vdc = Vl + Development Vcont

  The setting of the developing voltage Vdc by the correcting means 20 is performed every time an image output job (a series of image outputs for one or a plurality of recording materials according to one image formation start command) is commanded. However, when image output is performed in an environment other than the low humidity environment, the setting value of the development voltage Vdc is not changed and is maintained at the value in that environment.

  The timer 16 operates after the end of one job and the drive of the image forming apparatus 100 is stopped, and measures the time until access is received from a continuous image output gamma correction unit described later. This time substantially corresponds to an elapsed time (time after stop) ΔT (seconds) until the next job is started. The timer 16 also has a role of transmitting this elapsed time to the continuous image output gamma correction unit.

  The sheet number counter 17 has a role of holding a sheet number history of continuous image output. Also, when an access is received from a continuous image output gamma correction unit, which will be described later, this number history is transmitted to the continuous image output gamma correction unit, and further according to a command from the continuous image output gamma correction unit. It has a role of changing the value of the number history.

  The environmental sensor 11 has a role of measuring the environmental temperature and humidity of the image forming apparatus 100 and calculating the environmental water content (absolute water content) based on the environmental data. Also, when an access is received from a continuous image output gamma correction unit in the image processing unit, which will be described later, this moisture amount is transmitted to the continuous image output gamma correction unit.

3. Image Processing Unit Next, the image processing unit will be described.

  FIG. 2 is a block diagram of the image processing unit 30 included in the image forming apparatus 100 of the present embodiment. Hereinafter, each element constituting the image processing unit 30 will be described.

  The CPU 701 that is an arithmetic control unit comprehensively controls each component of the image processing unit 30 using a RAM 703 that is a storage medium as a work memory based on a control program that is held in a ROM 702 that is a storage medium.

  A paper document read by an image reading device such as a scanner is converted into an electrical signal by a CCD sensor 704. The CCD sensor 704 is an RGB 3-line color sensor, and its electrical signal is input to the A / D converter 705 as R (red), G (green), and B (blue) image data.

  In the A / D conversion unit 705, after gain adjustment and offset adjustment, each color signal is converted into 8-bit digital image data. A shading correction unit 706 corrects the sensitivity variation of each pixel of the CCD sensor 704 and the variation of the light amount of the document illumination lamp for each color using the read signal of the reference white plate.

  The input gamma correction unit 707 is a one-dimensional lookup table (LUT) that performs correction so that the exposure amount and the luminance have a linear relationship with respect to each RGB input.

  The input direct mapping unit 708 is a three-dimensional LUT that converts an input RGB signal into an RGB signal in the device in order to unify the color space. The input direct mapping unit 708 is a part that converts the read color space determined by the spectral characteristics of the RGB filter of the CCD sensor 704 into a standard color space such as sRGB. The input direct mapping unit 708 can absorb various characteristics such as sensitivity characteristics of the CCD sensor 704 and spectral characteristics of the illumination lamp.

  In order to detect the background, a BE (background removal) sampling unit 709 discretely samples pixels in a designated rectangular area, creates a luminance histogram, and uses this for background removal during printing. .

  The background removal unit 710 performs non-linear transformation for removing the background part on the RGB image data read by the scanner based on the result of the BE sampling unit 709.

  Next, the output direct mapping unit 711 converts RGB image data into CMYK image data. In this conversion, RGB values are input to a lookup table, and a C (cyan) component is created from the sum of the output values. Similarly, each component of M (magenta), Y (yellow), and K (black) is also formed by a lookup table and its addition operation.

  The output gamma correction unit 712 performs density correction of the output image corresponding to each image forming apparatus 100. The output gamma correction unit 712 has a role of maintaining the linearity of output image data that differs for each image formation based on a CMYK one-dimensional lookup table stored in advance.

  The continuous image output gamma correction unit 714 will be described in detail later.

  The halftone processing unit 713 can alternatively apply different types of screening according to functions. In general, the copying operation uses an error diffusion screening that is less likely to cause moire, and the printing operation often uses a multi-value screen screening that uses a dither matrix in consideration of the reproducibility of characters and fine lines. . The former is a method in which the pixel of interest and its surrounding pixels are weighted with an error filter, and multilevel errors are distributed and corrected while maintaining the number of gradations. On the other hand, the latter is a method of expressing the halftone in a pseudo manner by setting the dither matrix threshold value to a multi-value. In this embodiment, the conversion is performed independently of CMYK, and switching between the low line number and the high line number is possible. .

4). Continuous Image Output Gamma Correction Unit The continuous image output gamma correction unit 714 is located after the output gamma correction unit 712 and before the halftone processing unit 713, and changes in halftone density during continuous image output in a low humidity environment. Perform correction and gamma correction.

  The continuous image output gamma correction unit 714 includes a reception unit 714a, a storage unit 714b, a calculation unit 714c, and a transmission unit 714d.

4-1. Reception Unit The reception unit 714a receives from the timer 16 an elapsed time ΔT from the end of the previous job to the present (when accessing the image forming apparatus). The receiving unit 714 a receives the moisture content in the image forming apparatus 100 from the environment sensor 11. Further, the receiving unit 714a receives from the sheet counter 17 the number N of sheets output by the image forming apparatus 100 during continuous image output at the present time (when accessing the image forming apparatus).

4-2. Storage Unit The storage unit 714b stores a correction coefficient curve, which will be described later, corresponding to each number of sheets at the time of continuous image output. Further, a plurality of correction coefficient curve groups stored corresponding to the respective numbers are stored in accordance with the moisture level (three levels in this embodiment) in the image forming apparatus 100 (see FIG. 5).

  Here, the correction coefficient curve will be described. FIG. 6 shows a correction in which the input image signal is multiplied by the horizontal axis and the input image signal corresponding to the Nth sheet at the time of continuous image output when the water content level in the image forming apparatus 100 is the water content level A. It represents a curve with the coefficients plotted on the vertical axis. The input image signal indicates an image signal that enters the continuous image output gamma correction unit 714. The curve shown in FIG. 6 corresponds to the Nth correction coefficient curve at the time of water content level A and continuous image output. The correction coefficient curve is input to the continuous image output gamma correction unit 714. The calculation parameter (for calculation unit 714c to be described later performs gamma correction on the image data corresponding to the Nth sheet during continuous image output ( Correction coefficient). The gamma correction is specifically an arithmetic correction for the input image signal.

4-3. Calculation Unit The calculation unit 714c first selects a moisture content level according to the moisture content in the image forming apparatus 100 received by the reception unit 714a (the moisture content level will be described later).

  Next, according to the timer value ΔT received by the receiving unit 714a, the image data that is currently in the continuous image output gamma correction unit 714 is output as the continuous image output as in the case 1, case 2, and case 3 below. Judge what number is the middle one.

Case 1: When ΔT is within 30 seconds When the last job before the next job is completed and the next job is started within 30 seconds after the driving of the image forming apparatus 100 is stopped (ΔT ≦ 30 seconds), it is determined that these jobs are continuous, and N is continuously counted. Specifically, it is determined that the image data currently entering the continuous image output gamma correction unit 714 is the (N + 1) th image during continuous image output.

  Note that the recovery time of the potential may be different depending on the type and material of the photosensitive drum 1, and therefore the threshold value (30 seconds) can be arbitrarily changed.

Case 2: When ΔT is 10 minutes or more When the last one job before the next job is finished and the next job is started after 10 minutes or more have passed after the drive of the image forming apparatus 100 is stopped ( ΔT> 10 minutes), the next job is determined to be discontinuous with respect to the previous job. Specifically, it is determined that the image data currently entering the continuous image output gamma correction unit 714 is the first image being output continuously.

  Here, the reason why the threshold value is determined to be 10 minutes is that, according to the result of Table 1, the lowered state of the exposed portion potential almost recovers after about 10 minutes. Table 1 shows the result of confirming the potential recoverability with the lapse of time of the exposed portion potential Vl in which the potential decreased in an environment with a water content of 0.89 g / kg.

  From Table 1, when the amount of decrease in the exposure part potential Vl is about 50 V, which is the maximum, it takes the longest time for the exposure part potential to fully recover, but the level has almost no effect in about 10 minutes. It turns out that it recovers. When the amount of decrease is less than that, the exposed portion potential returns almost completely in 10 minutes or less.

  Since the potential recovery time may vary depending on the type and material of the photosensitive drum 1, the threshold value (10 minutes) can be arbitrarily changed.

Case 3: When ΔT is from 30 seconds to 10 minutes After the last one job before the next job is completed and the driving of the image forming apparatus 100 is stopped, the next job is executed for a period longer than 30 seconds and shorter than 10 minutes. When the job is started (30 seconds <ΔT <10 minutes), the return correction by the function is performed. That is, in this case, the case 1 and the case 2 are in an intermediate state between continuous and discontinuous. In the case of this intermediate state, the decrease in the exposed portion potential Vl tends to recover. In this case, it is necessary not to determine that the image data currently entering the continuous image output gamma correction unit 714 is the (N + 1) th image, but to estimate the number of restored images by the amount of recovery and to determine by reducing that amount from N There is. As an example of the method for calculating the number of return sheets, an example in which the time ΔT is converted into the number of sheets in A4 horizontal size is as follows.

  FIG. 11 shows exposures when 1000 continuous images are output with the transfer charger 5 turned on, and when two continuous images are output 500 times intermittently and a total of 1000 pseudo continuous images are output. The fluctuation amount from the initial value of the partial potential Vl is shown. Note that two types of photosensitive drums 1, Lot A and B, were prepared, two types of Lot A and B were tested for continuous output, and one type of Lot B was tested for pseudo continuous image output. From FIG. 11, the potential drop of about 50 V occurs in the exposure portion potential Vl at the continuous output of 1000 sheets. In addition, a similar decrease in the potential of the exposed area occurs even when two continuous 1000 sheets of pseudo continuous images are output. That is, it can be seen that when the continuous image output is performed including the pseudo continuous image output in which the output is repeated at a short time interval, the similar exposure portion potential decrease occurs. The concept of continuous image output includes a case where a continuous state is continued in a pseudo manner even in an intermittent operation.

As in the experimental data of FIG. 11, assuming a case where the decrease in potential is almost saturated with 1000 consecutive sheets,
Returned number ΔS = {1000 (sheets) / 10 (minutes)} × {(ΔT (seconds) / 60)}
= 100 x (ΔT / 60) (sheets)
It becomes. As a result, it is determined that the image data currently entering the continuous image output gamma correction unit 714 is the N-ΔS + 1th sheet during continuous image output. Note that 1 is set when N−ΔS + 1 <1.

  Finally, the calculation unit 714c determines the input image signal as follows from the number of image data currently entered in the continuous image output gamma correction unit 714 and the moisture level determined as described above. Is transmitted to the halftone processing unit 713. For example, if it is the Nth sheet and the water content level A at the time of continuous image output, the input image signal is converted (corrected) into an image signal multiplied by the correction coefficient according to the correction coefficient curve of FIG. Transmit to the halftone processing unit 713.

4-4. Transmission Unit Simultaneously with the processing by the calculation unit 714, the transmission unit 714d transmits an instruction to the counter number 17 to change the counter value as follows in accordance with the determination of the calculation unit 714c. That is, in the case 1, an instruction is transmitted to change N to N + 1, in the case 2, N to 1, and in the case 3, N to N−ΔS + 1. The number counter 17 follows this counter value change command.

5). Next, the point that the storage unit 714b stores a plurality of correction coefficient curves will be described.

(1) Regarding points to be stored in accordance with the number of images at the time of continuous image output:
FIG. 7A shows an EV curve at the time of continuous image output. The EV curve refers to a curve in which the horizontal axis plots the light amount and the vertical axis plots the exposure part potential. FIG. 7B shows a change in gamma characteristics when a continuous image is output.

  As shown in FIGS. 7A and 7B, in a state where the charging potential during continuous image output is kept constant, the gamma characteristic tends to decrease in density with the number of sheets as the EV curve changes. In the state where the charging potential is kept constant during continuous image output, the above tendency has reproducibility. That is, the gamma characteristic at the Nth sheet of continuous image output can be predicted.

  In the case of a system in which the charging potential during continuous image output is not stable, the N-th gamma characteristic in the graph of FIG. 7B is not uniquely determined, and it is difficult to predict the N-th gamma characteristic. . For this reason, it is desirable that the charging potential be as stable as possible.

  Based on such a change tendency and reproducibility of the gamma characteristic, the correction coefficient curve is a coefficient (a graph in FIG. 6) so that the halftone density is appropriately kept constant according to the number of sheets at the time of continuous image output. It is necessary to store a plurality of correction coefficient curves in which the vertical axis is formed (see FIG. 5).

(2) Regarding points to be stored in a plurality of groups according to the moisture level:
FIG. 12 shows the relationship between the number of sheets continuously passed in a low-humidity environment and the amount of decrease in exposed portion potential obtained experimentally in a plurality of low-humidity environments with a moisture content of 7.6 g / kg or less. As can be seen from FIG. 12, the degree of decrease in the exposed portion potential is different even in a low humidity environment. Correspondingly, the degree of change in the gamma characteristic varies.

  Based on this phenomenon, in this example, the moisture content was 1 g / kg, 1 g to 2.9 g / kg, 2.9 g to 7.6 g / kg, 3% in a low humidity environment with a moisture content of 7.6 g / kg or less. The correction coefficient curve group is stored at the water content level of the rank. As a result, the difference in gamma characteristic change during continuous image output due to the difference in moisture content is absorbed (see FIG. 5).

6). Image Density Fluctuation FIG. 8 shows the density (gamma characteristic) of the gradation image on the first and Nth sheets when continuous image output is performed without the continuous image output gamma correction unit 714 functioning. is there. From FIG. 8, it can be seen that the density of the gamma characteristic is reduced over the entire signal range from the first to the Nth sheet during continuous image output.

  On the other hand, the N-th gamma characteristic after correction by the continuous image output gamma correction unit 714 is as shown in FIG. From FIG. 9, the gamma characteristic at the Nth sheet is corrected according to the correction coefficient curve for the Nth sheet, so that the halftone is approaching the gamma characteristic of the first sheet, that is, the density reduction of the halftone is suppressed. I understand that.

  As described above, the continuous image output gamma correction unit 714 performs correction on the input image signal in accordance with the moisture level and the number of images at the time of continuous image output, so that a halftone at the time of continuous image output in a low humidity environment is obtained. Image density fluctuation can be suppressed. In this embodiment, the relationship between the correction coefficient for correcting the input image signal and the input image signal (correction coefficient curve) created for each number of image outputs at the time of continuous image output according to the water content level is represented by a gamma table. Is stored in the storage unit 714b as information for changing the. At the time of continuous image output in a low humidity environment, a corresponding correction coefficient curve is selected from the storage unit 714b in accordance with the water content level and the number of output images. Then, each image input signal in the gamma table before correction is multiplied by a correction coefficient for the corresponding image input signal in the correction curve to relate the corrected image input signal and the density of the output image. Get the corrected gamma table.

7). Control Flow Next, the control in this embodiment will be described with reference to the flowchart of FIG.

  In step S1, when an output command is given to the image forming apparatus, first, in step S2, the amount of water is measured and calculated by the environment sensor 11.

  In order to obtain an appropriate image density Dmax value for the measured moisture content, the correction means 20 determines the necessity of environmental correction in step S3. When it is determined that environmental correction is necessary in the correction unit 20, the correction unit 20 performs correction control (environment correction) according to the control procedure described above, as indicated by a function A in FIG.

  Next, in step S4, the continuous image output gamma correction unit 714 determines the necessity of gamma correction during continuous image output. When the amount of moisture in the environment is> 7.6 g / kg, the continuous image output gamma correction unit 714 determines that gamma correction is unnecessary. That is, the image data received in the continuous image output gamma correction unit 714 is transmitted to the halftone processing unit 713 as it is without performing any correction / calculation.

  If it is determined in step S4 that gamma correction is necessary (when the amount of moisture in the environment is ≦ 7.6 g / kg), in step S5, the receiving unit 714a in the continuous image output gamma correction unit 714 The elapsed time ΔT from the stop and the history number N in the number counter 17 are read. In step S6, the calculation unit 714c in the continuous image output gamma correction unit 714 currently enters the continuous image output gamma correction unit 714 based on various information (ΔT, N) read by the reception unit 714a. It is determined which number of images in the continuous image output corresponds to the existing image.

  Next, in step S7, the calculation unit 714c selects a correction coefficient curve stored in the storage unit 714b from the number of sheets during continuous image formation and the moisture level determined as described above, and performs gamma correction. To do. At the same time, the transmission unit 714d transmits a signal for changing the history number N held by the number counter 17 in accordance with the number determined as described above to the number counter 17.

  Next, in step S8, the end of the job is determined. If a continuous image is being output, the process returns to step S5, and steps S5 and after are repeated. When the job is finished, it is in a standby state.

  By the correction by the continuous image output gamma correction unit 714 as described above, it is possible to suppress the halftone density fluctuation during the continuous image output in the low humidity environment.

  As described above, in the present exemplary embodiment, the image forming apparatus 100 includes the image processing unit 30 that processes the input image signal and generates a signal for forming a toner image on the photosensitive drum 1. Then, the image processing unit 30 changes the gamma table according to the number of output images during continuous image output or intermittent image output within a predetermined time when the absolute water content of the environment is equal to or less than a predetermined value. The gamma table is information used to relate the input image signal and the density of the output image. In this embodiment, the image forming apparatus 100 stores information for changing the gamma table according to the number of images output according to continuous image output or intermittent image output within a predetermined time. 714b. In particular, in the present embodiment, the storage unit 714b includes information for changing the gamma table in accordance with the number of images output by continuous image output or intermittent image output within a predetermined time, and a plurality of absolute values less than a predetermined value. It is stored corresponding to the amount of water. Then, during continuous image output or intermittent image output within a predetermined time, the image processing unit 30 uses the information stored in the storage unit 714b according to the absolute moisture content of the environment and the number of image output sheets, and uses a gamma table. To change.

  In this embodiment, the above information is a relationship (correction coefficient curve) between the input image signal for correcting the gamma table and the correction coefficient, which is obtained in advance and stored in the storage unit 714b. A gamma table is created and used (that is, changed and used as appropriate). However, the present invention is not limited to this, and the corrected gamma table itself is created in advance according to the number of image outputs and further according to the absolute moisture content of the environment and stored in the storage unit 714b. May be used (that is, used after being changed as appropriate).

  Further, in this embodiment, the number of images output in intermittent image output is such that the interval between two intermittently output images is less than the first time (30 seconds) within the predetermined time (10 minutes). When it is long and shorter than the second time (10 minutes) longer than the first time, it is as follows. That is, in this case, the number of image outputs is set to the number of image outputs obtained by subtracting the number of image outputs corresponding to the recovery of the decrease in the potential of the exposed portion of the photosensitive drum 1 during that time.

8). As described above, according to the present embodiment, the following control is performed for the potential drop of the exposed portion of the photosensitive drum 1 that occurs during continuous image output using paper as a recording material in a low humidity environment. That is, the continuous image output gamma correction unit 714 determines whether or not the image output is a continuous image output including the pseudo continuous image output from the output interval of the images. Based on the relationship between the number of continuous images output and the change in gamma characteristics obtained in advance, the halftone density reduction amount is related to the number of output sheets. Curve) and the image data is corrected. In this way, control is performed to keep the halftone density constant. As a result, during continuous image output using paper in a low-humidity environment, it is possible to satisfactorily control the halftone density to be constant and obtain an image in which halftone density fluctuation is suppressed.

  Further, according to the present embodiment, when the image output is in an intermediate state between the continuous output and the intermittent output, the amount of recovery of the decrease in the exposure portion potential at the time of the intermittent is related to the number of output and depends on the number of output. When the correction amount is obtained, the number of output sheets is corrected by an amount corresponding to the recovery of the potential decrease amount. Thereby, even if there is an intermediate output state during continuous output, the gamma characteristic can be controlled well.

Example 2
Next, another embodiment of the image forming apparatus according to the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Accordingly, elements having the same or equivalent functions and configurations as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the first embodiment, the case where gamma correction is performed in continuous image output in a low-humidity environment has been described. In the present embodiment, a case where gradation control is performed during continuous image output will be described with respect to the first embodiment.

1. Description of Image Forming Apparatus In this embodiment, the density of a toner patch that is a test image formed in the image forming apparatus 100 is detected for gradation control described later. Therefore, the image forming apparatus 100 according to the present exemplary embodiment includes a patch detection sensor 40 as an image density detection unit in addition to the configuration of the first exemplary embodiment.

  The patch detection sensor 40 has a mechanism for irradiating the toner patch with a light source such as an LED, receiving only the regular reflection light, or the regular reflection light and the diffuse reflection light, and calculating the density of the toner patch from the light amount. In this embodiment, as shown in FIG. 13, the patch detection sensor 40 is disposed at a position where the toner patch on the photosensitive drum 1 can be read.

2. Description of Image Processing Unit In this embodiment, the image processing unit 30 further includes an LUT correction unit 715 in addition to the configuration described in the first embodiment, as shown in FIG. The LUT correction unit 715 receives the patch density read by the patch detection sensor 40 for gradation control to be described later, creates a new lookup table based on the received patch density, and stores it in the output gamma correction unit 712. Responsible for updating the lookup table.

3. Explanation of Gradation Control In this embodiment, gradation control is performed at a constant timing in order to keep the gradation characteristics of the image forming apparatus 100 constant.

  In the gradation control, first, the image processing unit 30 transmits halftone density patch image data to the image forming unit 10, and the image forming unit 10 forms an image on the photosensitive drum 1 with toner based on the patch image data. It starts with that.

  Next, the patch detection sensor 40 reads the toner patch and calculates its density.

  Next, the image processing unit 30, more specifically the LUT correction unit 715, receives the toner density read by the patch detection sensor 40, and creates a new lookup table based on the received toner density.

  15 and 16 show an overview of the lookup table. The solid line in FIG. 15 plots the input image signal (horizontal axis) entering the halftone processing unit 713 and the toner density (toner density read by the patch detection sensor 40) obtained at that time (vertical axis). It is a curved line. In order to keep the gradation characteristic constant, it is necessary to make this curve a desired shape. The desired shape is called a target. In this embodiment, it is assumed that the target is linear as represented by a dotted line in FIG.

  In order for the gradation characteristics to conform to a curve represented by a dotted line in FIG. 15, the input image signal that enters the halftone processing unit 713 needs to be corrected in advance. The solid line in FIG. 16 is a curve in which the original input image signal is plotted on the horizontal axis and the corrected input image signal is plotted on the vertical axis, which is a lookup table.

  The LUT correction unit 715 plots a solid curve in FIG. 15 from the received toner density, and creates a solid line lookup table in FIG. 16 while collating with the target. Finally, the LUT correction unit 715 updates the lookup table stored in the output gamma correction unit 712 to the newly created lookup table, thereby completing the gradation control.

  Here, in this embodiment, the gradation control is performed at a constant timing, but this timing varies. Gradation control may be entered in the middle of continuous image output in a low humidity environment, gradation control may be entered after output and a brief pause, or gradation control is performed after 10 minutes or more have elapsed after the continuous image output is completed. May enter. However, the change to the image signal by the continuous image output gamma correction unit 714 is appropriately reflected in the input image signal input to the halftone processing unit 713 located in the subsequent stage. This is performed regardless of when the gradation control is entered and whether or not the continuous image output gamma correction unit 714 is functioning at that timing. Therefore, it is possible to appropriately create a lookup table during gradation control. That is, it is possible to achieve matching between the toner density and the input image signal input to the halftone processing unit.

  In the present embodiment, the main reason why the gradation control and the gamma correction control during continuous image output are simultaneously achieved is that the continuous image output gamma correction unit is located in the preceding stage of the halftone processing unit. If the continuous image output gamma correction unit is positioned after the halftone processing unit, the gradation control and the gamma correction control during continuous image output cannot be realized at the same time. The reason is as follows. That is, in this case, the image signal value that enters the halftone processing unit, which is referred to when creating the lookup table, is rewritten to an unexpected value by the continuous image output gamma correction unit located after the halftone processing unit. . For this reason, the patch density and the image signal value entering the halftone processing unit cannot be matched.

  In this embodiment, the gamma table that is changed according to the number of image outputs during the continuous image output or intermittent image output within a certain time by the image processing unit is the one before the halftone density gradation control processing. 3 is a gamma table for performing a correction operation on an image signal.

  Therefore, the gamma correction control during the continuous image output can be used in combination with the gradation control by performing it before the halftone process.

  As described above, according to the present embodiment, the same effects as those of the first embodiment can be obtained, and the gradation control based on the detection result of the patch detection sensor can be performed to improve the image gradation reproducibility.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. 1 is a schematic block diagram of an image processing unit in an embodiment of an image forming apparatus according to the present invention. It is a flowchart figure of the gamma correction control during continuous image formation according to this invention. It is a graph for demonstrating the relationship between the charging potential in the continuous image output, and the Nth sheet | seat EV curve shape. It is a graph for demonstrating the correction coefficient curve group which the memory | storage part in a continuous image output gamma correction part memorize | stores. It is a graph for demonstrating a correction coefficient curve. It is a graph which shows the change of EV curve and a gamma characteristic during continuous image output. It is a graph showing the N-th gamma characteristic and the first gamma characteristic before gamma correction during continuous image output. FIG. 6 is a graph showing the N-th gamma characteristic and the first gamma characteristic after gamma correction during continuous image output. It is a graph for demonstrating a mode that a charging potential exposure potential changes by the moisture content increase. It is a graph for demonstrating the relationship between a continuous output sheet number and exposure potential. It is a graph for demonstrating the exposure potential change difference by the moisture content difference. It is a schematic block diagram of the other Example of the image forming apparatus which concerns on this invention. FIG. 5 is a schematic block diagram of an image processing unit in another embodiment of the image forming apparatus according to the present invention. It is a graph for demonstrating the look-up table in gradation control. It is a graph for demonstrating the look-up table in gradation control. It is a graph for demonstrating a mode that the latent image contrast changes during the continuous image output. It is a graph showing the color difference from the first sheet in each case of the Nth sheet (without correction) and the Nth sheet (exposure amount UP) during continuous image output.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging device 3 Exposure device 4 Developing device 5 Transfer charging device 30 Image processing part 714 Continuous image output comma correction part

Claims (5)

A photosensitive member, a charging unit for charging the surface of the photosensitive member, an exposure unit for exposing the charged surface of the photosensitive member to form an electrostatic image, and developing the electrostatic image to form a toner image. A developing unit; a transfer unit that transfers a toner image formed on the photoconductor to a recording material; and an image processing unit that processes an input image signal to generate a signal for forming a toner image on the photoconductor. In an image forming apparatus having
When the absolute moisture content of the environment is equal to or less than a predetermined value, the image processing unit is configured to output the input image signal and the output image according to the number of output images during continuous image output or intermittent image output within a predetermined time. An image forming apparatus characterized by changing a gamma table used for relating density.   2. The image forming apparatus according to claim 1, further comprising storage means for storing information for changing the gamma table in accordance with the number of images output by continuous image output or intermittent image output within a predetermined time. .   In the storage means, information for changing the gamma table according to the number of images output by continuous image output or intermittent image output within a fixed time corresponds to a plurality of absolute moisture amounts below the predetermined value. The image processing unit stores the information stored in the storage unit according to the absolute moisture content of the environment and the number of image outputs during continuous image output or intermittent image output within a predetermined time. The image forming apparatus according to claim 2, wherein the gamma table is changed using an image.   The gamma table that the image processing unit changes according to the number of image outputs during continuous image output or during intermittent image output within a certain period of time is an image signal before performing halftone density gradation control processing. The image forming apparatus according to claim 1, wherein the image forming apparatus is a gamma table for performing a correction operation on the image forming apparatus.   The number of image outputs in the intermittent image output is a second time interval in which the interval between two intermittently output images is longer than the first time and longer than the first time within the predetermined time. 5. If the time is shorter than the time, the number of image outputs is calculated by subtracting the number of image outputs corresponding to the recovery of the decrease in potential of the exposed portion of the photosensitive member during that time. The image forming apparatus according to the item.