JP2014170119A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that performs gamma correction by which stable density reproducibility can be achieved.SOLUTION: When an input signal value having a gradation characteristic curve obtained by reading a gradation test pattern inclined larger than a predetermined threshold is detected within a predetermined range in the gradation characteristic curve, an image forming apparatus 100 corrects a gamma correction table corresponding to a signal value equal to or larger than the signal value having the gradation characteristic curve inclined larger than the predetermined threshold.

Description

  The present invention relates to an image forming apparatus.

  In recent years, cases in which image forming apparatuses such as color multifunction peripherals and color printers are installed in offices and copy shops are increasing.

  2. Description of the Related Art Conventionally, a color image forming apparatus is generally composed of four color toner developing devices of CMYK (cyan, magenta, yellow, and black). The color toner developing device sequentially faces the photosensitive drum and performs development with each color toner. The surface of the photosensitive drum is uniformly charged by a charger.

  Next, the first color electrostatic latent image is formed on the photosensitive drum by exposing and scanning the laser beam with an exposure device controlled on / off according to the image data of the first color (for example, magenta). It is developed and visualized by a magenta toner developer of the color.

  This visualized toner image is primarily transferred to the intermediate transfer member. This primary transfer is repeated in the same manner for the other toners (yellow, cyan, black), and is secondarily transferred collectively to the recording material 6 fed from the paper feed unit, followed by a fixing process by the fixing device 7. It is discharged outside the machine.

  In the image forming apparatus having such a configuration, the following method is known as a gradation correction method for adjusting image processing characteristics.

  In this method, one specific gradation test pattern is first output, and the density of the output gradation test pattern is read by the image reading means.

  The relationship between the laser output level of the image forming apparatus and the read density is stored in a memory, and a γ lookup table (hereinafter referred to as “γLUT”) in which the read density matches the ideal target table of the image forming apparatus. Create).

  With this gradation correction method, it is possible to stabilize the image quality in accordance with the characteristic fluctuation amount due to deterioration of parts mounted on the image forming apparatus and the fluctuation amount due to environmental conditions.

  Further, a circuit that forms a specific pattern in a non-image area during an image forming operation, reads the density of the pattern, and determines an image forming condition such as a γLUT for each image forming operation based on the read density value. There is also known a method for accurately correcting image characteristics that change every moment by changing the operation.

  The gradation characteristic is detected by the optical detection means as density information which is an image characteristic having a plurality of density information such as a gradation pattern.

  In recent years, there has been a demand for a reduction in running cost regardless of color or monochrome. For example, an image forming apparatus using an amorphous silicon photoconductor that has a lifetime nearly 100 times longer than that of a conventional organic photoconductor has been developed. ing.

  The amorphous silicon photoconductor forms a sharper latent image as compared with the conventional organic photoconductor, so that an image reproduction more faithful to the input image signal is possible (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 2003-195584

  However, the above-described conventional technology has the following problems.

  If the density of the highlight portion of the image forming apparatus is not or very thin when the tone correction pattern is output by the tone correction means, the highlight portion of the generated γ look-up table will rise sharply. That is, it means that the stability of the highlight portion is lowered.

  The density reproducibility from the highlight in the reflected image density to the halftone (about 0.1 to 0.8) region is due to the fluctuation of the environmental conditions, the usage time of the developer, the mechanical dimensional error of the photosensitive member and the developing device, and the vibration of the main body. It tends to fluctuate depending on the cause.

  In particular, there is a problem that it is difficult to obtain stable density reproducibility in a highlight portion. That is, the rising point of the density is likely to fluctuate.

  If the conventional technique is applied in a state where the highlight portion is not stable, the gamma correction result in which the extreme highlight portion rises sharply tends to be obtained. Furthermore, since the gradient of the density reproduction curve becomes steep, there is a problem that density stability is impaired.

  An object of the present invention is to provide an image forming apparatus that performs gamma correction capable of realizing stable density reproducibility.

  In order to achieve the above object, an image forming apparatus according to claim 1 uses a gamma correction table for correcting an input signal value indicating a pixel value indicated in image data to an output signal value used when forming on a recording medium. An image forming apparatus for forming an image, comprising: a reading unit that reads a gradation test pattern; and an input signal value within a predetermined range in a gradation characteristic curve obtained by reading by the reading unit. Detecting means for detecting a signal value with which the slope of the characteristic curve is larger than a predetermined threshold; and when the signal value larger than the predetermined threshold is detected by the detecting means, the predetermined value is detected. Correction means for correcting the gamma correction table corresponding to a signal value equal to or greater than a signal value greater than a threshold value.

  According to the present invention, it is possible to provide an image forming apparatus that performs gamma correction capable of realizing stable density reproducibility.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. It is a figure which shows schematic structure of the reader image process part in FIG. It is a figure which shows an example of the gradation test pattern produced | generated by PG in FIG. The characteristics of the prime gradation characteristic obtained by converting the image signal into the density signal, the ideal target gradation characteristic, and the characteristic of the γLUT generated to make the gradation characteristic before correction ideal. FIG. It is a flowchart which shows the procedure of the LUT production | generation process performed by CPU of FIG. It is a figure which shows an example of the instruction | indication screen displayed on the operation panel in FIG. FIG. 2 is a diagram illustrating a configuration for correcting a γLUT in the printer unit of FIG. 1. FIG. 2 is a diagram illustrating a relationship between an output of a photosensor and an output image density when the density of a patch pattern formed on the photosensitive drum in FIG. 1 is changed stepwise according to the area gradation of each color. It is a flowchart which shows the procedure of the target value setting process performed by CPU in FIG. It is a flowchart which shows the procedure of the steep correction process performed by CPU in FIG. It is a figure for demonstrating determination of whether it is a highlight point. It is a figure which shows the target for producing | generating γLUT. It is a figure which shows an example in the case of changing an input signal so that an output signal may not be changed in cumulative color difference linear.

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

  FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus 100 according to an embodiment of the present invention.

  In the reader unit 200 of FIG. 1, a document 101 placed on a platen glass 102 is irradiated by a light source 103, and an image of the document 101 (hereinafter referred to as “document image”) is connected to a CCD sensor 105 via an optical system 104. Imaged.

  The CCD sensor 105 generates red, green, and blue color component signals for each line sensor by a group of red, green, and blue CCD line sensors arranged in three rows. These reading optical system units convert the document image into an electric signal data string for each line by scanning in the direction of the arrow.

  Further, a contact member 107 that contacts the position of the document 101 to prevent the document 101 from being obliquely placed on the document table glass 102 and a white level of the CCD sensor 105 on the surface of the document table glass 102 are determined. A reference white plate 106 is disposed for shading the CCD sensor 105 in the thrust direction.

  The image signal output from the CCD sensor 105 is subjected to image processing by the reader image processing unit 108, then sent to the printer unit 300, and image processing is performed by the printer control unit 109.

  In the printer unit 300, the corona charger 8 applies a bias to uniformly charge the surface of the photosensitive drum 4 to a negative polarity.

  The laser driver included in the printer control unit 109 controls the laser light source 110 based on the image signal, and the laser light output from the laser light source 110 is reflected by the polygon mirror 1 and the mirror 2 and is uniformly charged. The drum 4 is irradiated.

  The photosensitive drum 4 on which a latent image is formed by scanning with laser light rotates in the direction of the arrow shown in the figure. The rotary developing unit 3 visualizes the electrostatic latent image, and the rotary developing unit 3 includes developing units 3M, 3C, 3Y, and 3K for magenta, cyan, yellow, and black.

  The four developing units 3M, 3C, 3Y, and 3K are detachably held by the rotary developing unit 3, and rotate around the rotation axis while being held by the rotary developing unit 3 during image formation. . For example, the sleeve of the developing device 3M is opposed to the photosensitive drum 4 with a minute interval (about 400 μm), and a visible image is formed on the photosensitive drum 4. When developing different color toner images, for example, a cyan toner image or a yellow toner image, similarly, after rotation driving, the development is stopped and stopped at a position facing the photosensitive drum 4.

  When the above developing operation is completed, the rotary developing device 3 is separated to a position where the sleeves of the four developing devices of the rotary developing device 3 do not come into contact with the photosensitive drum 4, and this position is set as the home position.

  By setting the home position in this way, the toner of the rotary developing device 3 is prevented from inadvertently adhering to the surface of the photosensitive drum 4 and other colors stored in the rotary developing device 3 can be prevented. It is possible to prevent toner from entering the developing device.

  The toner images formed on the photosensitive drum 4 based on the image signals of the respective colors are sequentially transferred to the intermediate transfer member 5, and in the case of full color, the four color toners are transferred to the intermediate transfer member 5 and then supplied. The recording material 6 fed from the paper unit is transferred to the recording material 6 at a time, and is discharged to the outside through a fixing process by the fixing device 7.

  The fixing device 7 heats and fixes the toner image of the recording material 6, and includes a fixing roller for applying heat to the recording material 6 and a pressure roller for pressing the recording material 6 against the fixing roller. Yes.

  The cleaning device 9 cleans the residual toner on the photosensitive drum 4 after the image visualized by the rotary developing device 3 is transferred to the recording material, and the waste toner is stored in a cleaning container and becomes full. It is exchanged at the stage.

  Further, an LED light source 10 and a photodiode 11 for detecting the amount of reflected light of the toner patch pattern formed on the photosensitive drum 4 are provided. The light from the LED light source 10 has a dominant wavelength at about 960 nm.

  FIG. 2 is a diagram showing a schematic configuration of the reader image processing unit 108 in FIG.

  In FIG. 2, the CPU 508 controls the entire reader image processing unit 108. The luminance signal of the original image read by the CCD sensor 105 is input to the A / D conversion unit 502 and converted into a digital signal. This digital luminance signal is sent to the shading unit 503, and the unevenness in the amount of light due to variations in the sensitivity of each element of the CCD sensor 105 is subjected to shading correction. This shading correction improves the measurement reproducibility of the CCD sensor 105.

  The luminance signal corrected by the shading unit 503 is further LOG converted by the LOG conversion unit 504. Subsequently, the LOG-converted signal is corrected by the γLUT 505 (gamma correction table) so that the ideal density characteristic of the image forming apparatus matches the output image density characteristic processed according to the γ characteristic. The The image signal corrected in this way is transmitted to the printer unit 300 to form an image. As described above, the image forming apparatus 100 according to the present embodiment uses the gamma correction table for correcting the input signal value indicating the pixel value indicated in the image data to the output signal value used when forming on the recording medium. Form.

  The operation panel 507 displays information to the user and is operated by the user. The memory 509 stores various programs and data. A PG 506 is a pattern generator that generates a gradation test pattern having 64 gradations for four colors of cyan, magenta, yellow, and black. FIG. 3 is a diagram illustrating an example of the gradation test pattern generated by the PG 506 in FIG.

  FIG. 4 is generated in order to obtain an ideal target gradation characteristic that is an original gradation characteristic obtained by converting an image signal into a density signal, an ideal target gradation characteristic, and a gradation characteristic before correction. It is a figure which shows the characteristic of (gamma) LUT.

  In FIG. 4, the characteristic of the γLUT for correcting the input signal value to the output signal value is such that the target gradation characteristic is line symmetric with respect to the original gradation characteristic.

  FIG. 5 is a flowchart showing a procedure of LUT generation processing executed by the CPU 508 of FIG.

  In FIG. 5, a gradation test pattern generated by PG 506 is formed on a recording material (step S201). Next, the screen shown in FIG. 6 is displayed. FIG. 6 is a diagram showing an example of an instruction screen displayed on operation panel 507 in FIG.

  According to this screen, the recording material on which the gradation test pattern shown in FIG. 3 is formed by the user or service person is placed on the platen glass 102, and the recording material on which the gradation test pattern is formed when the reading start button is pressed. Is read (step S202).

  Next, the read gradation test pattern is converted into a light amount signal by the CCD sensor 105, subjected to LOG conversion, and stored as read density data in the memory 509 (step S203).

  Then, a γLUT is generated from the relationship between the density and the laser output level indicated in the density data, and the relationship between the laser output level and the target characteristic (step S204), and this process is terminated with this γLUT505.

  FIG. 7 is a diagram illustrating a configuration for correcting the γLUT in the printer unit 300 of FIG.

  In FIG. 7, the photosensor 40 represents the LED light source 10 and the photodiode 11 described in FIG. 1 together. The photo sensor 40 detects only the regular reflection light from the photosensitive drum 4, and detects the density of the patch pattern formed on the photosensitive drum 4 by the configuration of FIG. . The photosensor 40 corresponds to a reading unit that reads a gradation test pattern described later.

  Near-infrared light from the photosensitive drum 4 incident on the photosensor 40 is converted into an electric signal by the photosensor 40, and the electric signal is converted to an output voltage of 0 to 5 V by the A / D conversion circuit 41 from 0 to 255 levels. Converted to a digital signal. Then, the density is converted into a density by the density conversion circuit 42 using the table 42 a and output to the CPU 708.

  FIG. 8 shows the relationship between the output of the photosensor 40 and the output image density when the density of the patch pattern formed on the photosensitive drum 4 in FIG. 1 is changed stepwise by the area gradation of each color. FIG.

  In FIG. 8, since the output of the photosensor 40 in a state where the toner is not attached to the photosensitive drum 4 is 5 V, it is set to the 255 level.

  As shown in FIG. 8, in any toner, the output from the photosensor 40 decreases as the area coverage by the toner increases and the image density increases.

  From these characteristics, by having a table 42a for converting the output from the photosensor 40 dedicated to each color into a density signal, it is possible to read the density signal with high accuracy for each color.

  FIG. 9 is a flowchart showing a procedure of target value setting processing executed by the CPU 708 in FIG.

  The process in FIG. 9 is a process for setting a target value in order to stabilize color reproducibility using the LUT generated by the LUT generation process in FIG.

  In FIG. 9, a patch pattern is formed on the photosensitive drum 4 and is read by the photosensor 40 (step S301). Next, with the configuration described with reference to FIG. 7, the density is detected (step S302), the value obtained by the above-described γLUT is set as the target value (step S303), and this process ends.

  5 is a flowchart showing a procedure of steep correction processing executed by a CPU 508 in FIGS. 10 and 2.

  In FIG. 10, the recording paper on which the patch pattern shown in FIG. 3 is formed is read, and the density is detected (step S101). Here, the correction using the γLUT is not performed.

  Next, the gradation characteristic is obtained from the detected density, and it is determined whether or not it is steep (step S102). This determination method will be described later. If the result of determination in step S102 is not steep (NO in step S102), the LUT generated in FIG.

  On the other hand, if the result of determination in step S102 is steep (YES in step S102), a highlight point is acquired (step S103), a γLUT corrected from the highlight point is generated (step S104), and the signal value is Conversion is performed (step S105), and this process ends.

  The discrimination process in step S102 described above will be described.

  FIG. 11 is a diagram for explaining determination of whether or not a highlight point.

  As shown in FIG. 11, the slope Fn of the gradation characteristic curve before correction is obtained at each laser output level n from the highlight density part to the medium density part that is sensitive to skin color and gradation. In the present embodiment, the highlight density portion to medium density portion are defined as input signal values 0 to 32.

  That it is steep indicates that the gradient Fn is large. Therefore, it is determined whether or not it is steep downward depending on whether or not Fn exceeds the threshold value X. This determination is performed at all laser output levels from the highlight portion to the medium concentration portion. It is known that the smaller the threshold value X, the sharper the generated γLUT. As described above, step S102 described above is performed within a predetermined range (0 (minimum signal value) to 32 (predetermined signal value)) in the gradation characteristic curve obtained by reading with the photosensor 40. This corresponds to detection means for detecting a signal value in which the gradient Fn of the gradation characteristic curve is larger than a predetermined threshold value X.

  Next, the process of generating a corrected γLUT in step S104 will be described.

  FIG. 12 is a diagram illustrating a target for generating a γLUT.

  In FIG. 12, the highlight point is n1, and a straight line connecting the input signal value n at the highlight point n1 to the end is reset as the target T1. Then, γLUT_n1 is generated so as to become this target. Thus, step S104 is a gamma correction table corresponding to a signal value greater than or equal to the predetermined threshold value X (Fn or greater) when a signal value greater than the predetermined threshold value X is detected. This corresponds to correction means for correcting.

  The reason for changing the target is that if the conventional target is used as it is, an unstable highlight region is used, and even if the inclination is reduced, it is difficult to improve the density instability.

  Next, the process of converting the signal value in step S105 will be described.

  This process is a process of converting a conventional target signal area to a new target signal area because the target has been changed. That is, the input signal value S is converted to S ′. In this case where the input / output signal is 10 bits, S ′ can be expressed by the following equation.

S '= (2046-S) / 1023 * S
This conversion formula can be applied when the target is linear. On the other hand, the case of a downward convex target called cumulative color difference linear used in printing can be dealt with by an appropriate conversion formula.

  FIG. 13 is a diagram illustrating an example of changing the input signal so as not to change the output signal in the accumulated color difference linear.

  FIG. 13 shows that the input signal value S is converted to S ′ at the target T1 while maintaining the output signal value Os.

  The γLUT generated in step S104 described above becomes a new γLUT 505.

  In the above-described embodiment, in the process of FIG. 5, a human or service person places a recording material on which the gradation test pattern is formed on the platen glass 102 and a human operation has occurred.

  Therefore, the gradation test pattern may be transferred to the intermediate transfer member 5 and read by the photosensor 40.

  That is, instead of forming the gradation pattern on the recording material executed in step S201 in FIG. 5, the gradation pattern is formed on the intermediate transfer body 5 without forming it on the recording material. In step S202, the gradation pattern is read from the intermediate transfer member 5 instead of reading from the recording material.

  The processing of steps S203 and 204 executed after step S202 is executed as it is, and the processing of FIG. 10 can also be applied as it is.

  As described above, in this embodiment, when the reading unit is the CCD sensor 105, the gradation test pattern is read from the recording medium on which the gradation test pattern is formed. On the other hand, when the reading means is the photo sensor 40, the gradation test pattern is read from the intermediate transfer body 5 on which the gradation test pattern is transferred from the image carrier.

  Further, in the above-described embodiment, the reflection type sensor is used. However, if a highly transmissive material is used for the intermediate transfer body 5, the transfer belt, etc., a configuration using the transmission type sensor can be naturally applied.

  If the process of FIG. 5 is not executed, a provisional γLUT is provisionally set at the shipping stage at the time of installation of the main body, and the corrected γLUT is obtained by executing the process of FIG. 10 during actual operation. Will be generated.

  In the embodiment described above, the gradation characteristics of the image forming apparatus are detected for each preset number of sheets (for example, 1000 sheets). In this embodiment, since the gradation characteristics are detected by reading the gradation pattern of the recording material, the entire image forming apparatus that takes into account the influence on the gradation characteristics due to transfer, fixing, recording material, etc. The correction reflecting the shape of the gradation characteristic can be performed.

  As an effect of the present embodiment, the density stability at the input signal 20 when the highlight point is 16 is conventionally about ΔD0.15 (measured using Xrite 504), which is about ΔD0.09. We were able to.

  According to the embodiment described above, correction according to the gradation characteristics of the image forming apparatus can be performed in real time regardless of the deterioration of the parts mounted on the image forming apparatus 100 or the environmental change.

5 Intermediate transfer body 100 Image forming apparatus 105 CCD sensor 108 Reader image processing unit 200 Reader unit 300 Printer unit 505 γLUT
506 PG
508 CPU
509 memory

Claims (3)

An image forming apparatus that forms an image using a gamma correction table that corrects an input signal value indicating a pixel value indicated in image data to an output signal value used when forming on a recording medium,
Reading means for reading a gradation test pattern;
Detecting means for detecting a signal value at which an inclination of the gradation characteristic curve is larger than a predetermined threshold value with an input signal value within a predetermined range in the gradation characteristic curve obtained by reading by the reading means; ,
Correction means for correcting the gamma correction table corresponding to a signal value greater than or equal to a signal value greater than the predetermined threshold when the detection means detects a signal value greater than the predetermined threshold. An image forming apparatus comprising:   2. The gradation test pattern is read from a recording medium on which the gradation test pattern is formed or a transfer body on which the gradation test pattern is transferred from an image carrier. Image forming apparatus.   3. The image forming apparatus according to claim 1, wherein the predetermined range is a signal value determined in advance from a minimum signal value.