JP2014153473A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of calculating a height of toner without requiring an optical sensor for height detection.SOLUTION: An image forming apparatus includes: a scanner unit 2 that acquires parameters of patch images formed on a recording material, including brightness, color saturation, hue, density, and dot area ratio; a control unit 401 that selects, from the parameters acquired by the scanner unit 2, two parameters according to the type of a toner used for forming the patch images on the recording material; and the control unit 401 that detects a height of the patch images on the basis of the two parameters selected by the control unit 401.

Description

  In the present invention, when an image is formed on a sheet material such as paper using a color material such as toner, the height of the color material laminated on the sheet material is detected without using an optical system sensor. The present invention relates to a technique for controlling the height of a color material to an optimum value.

  In an electrophotographic image forming apparatus, an electrostatic latent image is formed on the surface of a photosensitive member when the photosensitive member is exposed based on a result of reading an original image after the photosensitive member is charged by a charger. . The electrostatic latent image is developed with, for example, toner having an average particle diameter of about 5 to 8 [μm] to become a toner image. The toner image is transferred to a sheet material and fixed by a heat fixing device.

  In this type of image forming apparatus, when the density of the toner image is increased, the amount of toner stacked per unit area is increased. Therefore, a high density image has a higher toner height than a low density image. The toner height is referred to as “toner height” in this specification. If the toner is fixed on the sheet material in a state where the toner height exceeds a predetermined value, the image quality is deteriorated due to the deterioration of the quality of characters and lines and the scattering of dots. In order to solve such a problem, it is necessary to measure the toner height with high accuracy and to make the measured toner height less than a predetermined value.

  In the image forming apparatus described in Patent Document 1, a sensor called a laser displacement meter is used to detect the toner height based on the result of detecting the imaging position of the laser beam. Then, the toner bias is controlled to be less than a predetermined value by adjusting the developing bias value or the exposure time when forming the latent image in accordance with the detected toner height.

JP 09-233235 A

In the case where a dedicated sensor is provided for detecting the toner height, the dedicated sensor requires an optical system part using a laser, so that the cost of the apparatus is increased. In addition, in order to detect the toner height of about 5 to 10 [μm], there is a problem on the installation side that not only the sensor is attached with high accuracy but also a mechanism for suppressing the vibration of the sensor is required.
Further, it is necessary to correct irregularities on the surface of the image carrier such as a photosensitive member or an intermediate transfer member on which toner is carried. Such correction is referred to as “background correction”. However, in order to perform background correction, data obtained by measuring the unevenness of the surface of the image carrier when no toner is placed is necessary.
Such a problem occurs not only in toner but also in image forming apparatuses that perform image formation using ink or other pigment-containing color materials.

  An object of the present invention is to provide an image forming apparatus capable of detecting the toner height of an image without newly providing a dedicated sensor for detecting the height of a color material such as toner. And

  According to a first aspect of the present invention, there is provided an image forming apparatus that acquires parameters of brightness, saturation, hue, density, and dot area ratio of an image formed on a sheet, and the acquisition unit acquires the parameters. Among the parameters, a selection unit that selects two parameters according to the type of the color material that forms the image on the sheet, and the image based on the two parameters selected by the selection unit. And a detecting means for detecting the height.

  According to the present invention, the color material height can be detected without newly providing a sensor for detecting the color material height.

1 is a cross-sectional view of an image forming apparatus according to a first embodiment. FIG. 2 is a schematic configuration diagram of a scanner unit. The figure which shows the example of an operation part. 1 is a functional block diagram of an image forming apparatus according to a first embodiment. The functional block diagram of an input image processing part. The functional block diagram of an output image processing part. 6 is a flowchart of toner height correction processing in the first embodiment. The figure which shows the example of a test pattern. Graph of spectral reflectance difference of cyan. (A) is a graph showing the relationship between chromaticity coordinates and brightness, and (b) is a graph showing the relationship between saturation and toner height. The figure which shows a * b * of L * a * b * chromaticity coordinate. Graph of spectral reflectance difference of magenta. Graph of the spectral reflectance difference of yellow. (A) is explanatory drawing explaining the relationship between the thickness (width) of the line expressing a halftone, a density | concentration, and toner height, (b) is explanatory drawing which shows the state of the sheet | seat material at that time. The figure which shows the example of the content of a brightness correlation table. The figure which shows the example of the content of a saturation correlation table. 6 is a graph showing a correlation between hue and toner height at a specified saturation. The graph which shows the relationship between saturation and hue angle. A chart showing a relationship between development contrast, saturation, hue, and toner height. The figure which showed the numerical example in the case of performing linear interpolation in order to derive | lead-out the optimal value of development contrast. The graph for demonstrating the optimal value of development contrast. 9 is a flowchart of toner height correction processing according to the second embodiment. FIG. 4 is a diagram illustrating a relationship between toner fixing properties and hue.

Hereinafter, embodiments of the present invention will be described.
[First Embodiment]
FIG. 1 is a cross-sectional view of the image forming apparatus according to the first embodiment. The image forming apparatus includes a printer unit 1 that forms an image and a scanner unit 2 that optically reads an image. This image forming apparatus is a so-called multi-function machine having various image processing functions such as copying and printing, and also a communication function such as a facsimile. In this image forming apparatus, various color materials such as toner and ink can be used for image formation. In this embodiment, an example in which toner is used as a color material will be described.

<Printer section>
The printer unit 1 has functions of a laser exposure unit 3, an image forming unit 4, a fixing unit 5, and a paper feeding / conveying unit 6. The laser exposure unit 3 applies laser light for each color modulated according to the density of an image read by the scanner unit 2 or an image transferred from an external device to a photosensitive drum 41 that is an example of a photoconductor. Irradiate toward. The rotary polygon mirror (polygon mirror) 31 rotates at a predetermined angular velocity, and scans the modulated laser beam in a direction orthogonal to the rotation direction of the photosensitive drum 41.

  In addition to the photosensitive drum 41, the image forming unit 4 includes a charger 42, a transfer drum 43, and a developing unit 44. The charger 42 charges the photosensitive drum 41 that is rotationally driven. When the laser beam reaches the charged photosensitive drum 41, an electrostatic latent image is formed on the photosensitive drum 41. The developing unit 44 develops the toner by attaching toner to the electrostatic latent image to generate a toner image. The toner image is transferred to the sheet material by the transfer roller 43.

  In the case of color printing, the developing unit 44 uses four color toners. That is, the sheet material is wound around a predetermined position of the transfer drum 43 and is rotated four times. During this time, the photosensitive drum 41 is irradiated with laser light for each color, and development with toner of each color is repeatedly executed. As a result, a sheet material on which toner images of four colors are transferred is obtained. This sheet material is conveyed from the transfer drum 43 to the fixing unit 5. The image forming unit 4 collects toner remaining on the photosensitive drum 41 without being transferred.

  The fixing unit 5 incorporates a heat source such as a halogen heater, and the toner image transferred to the sheet material by the image forming unit 4 is melted and fixed by heat and pressure. The sheet material on which the toner image is fixed by the fixing unit 5 is discharged to the paper discharge tray 53 and stacked.

  The sheet feeding / conveying unit 6 feeds the sheet material one by one from the sheet material storage 51 containing the sheet material, and conveys the sheet material to the image forming unit 4. When the sheet material is placed on the manual feed tray 52, the paper feed / conveyance unit 6 feeds the sheet material on the manual feed tray 52 one by one and conveys it toward the image forming unit 4.

The toner of each color component has the following inorganic or organic pigment as a main component.
Magenta: Quinacridone Cyan: Phthalocyanine blue Yellow: Disazo yellow Black: Carbon black

The following can be used for the color material using an organic pigment.
Rhodamine lake, methyl violet lake, quinoline yellow lake, malachite green lake, alizarin lake, carmine 6B, lakelet C.
Disazo Yellow, Lakelet 4R, Chromophthalo Yellow 3G, Chromophthal Scarlet RN, Nickel Azo Yellow, Pendidazolone Azo, Permanent Orange HL.
Phthalocyanine blue, phthalocyanine green, flavanthrone yellow, thioindigo bordeaux, perinone red, dioxadone violet, quinacridone red.
Naphthol Yellow S, Pigrant Green B, Lummogen Yellow, Signal Red, Alkaline Blue, Aniline Black.
Monoazo yellow, disazo yellow, carmine, quinacridone, rhodamine, copper phthalocyanine.

<Scanner part>
FIG. 2 is a schematic configuration diagram of the scanner unit 2 shown in FIG. The scanner unit 2 starts a scan operation in response to the input of a scan start signal. Then, the original is illuminated and the original image is optically read, and the image data obtained thereby is RGB digital signals (R: red, B: blue, G: green. Hereinafter, referred to as “RGB data”). .). Here, the RGB data corresponds to a luminance signal. The scan start signal is output, for example, when the user operates the operation unit of the image forming apparatus.
An example of this operation unit is shown in FIG. The operation unit 300 includes a touch panel unit that also serves as a display device and a key input unit. The operation unit 300 allows a user to select one of a plurality of functions realized in the image forming apparatus or receives an operation instruction input to the image forming apparatus.

  Returning to FIG. 2, the scanner unit 2 includes a document table 201 on which the document D is placed, a first mirror unit 202, a second mirror unit 203, a home position sensor 204, and a document illumination lamp 205. The first mirror 206, the second mirror 207, the third mirror 208, the lens 209, the scanner motor 210, and the CCD sensor 2112 are configured.

  When the scanning operation is started, the scanner unit 2 returns the positions of the first mirror unit 202 and the second mirror unit 203 to the home position once detected by the home position sensor 204. The document illumination lamp 205 irradiates the document D on the document table 201 with light. This light is reflected by the document D and reaches the lens 209 via the first mirror 206 in the first mirror unit 202 and the second mirror 207 and the third mirror 208 in the second mirror unit 203. . The reflected light that reaches the lens 209 is imaged by the CCD sensor 211. The CCD sensor 211 generates the aforementioned RGB data from the formed reflected light.

<Functions of image forming apparatus>
The function of the image forming apparatus of this embodiment will be described in detail. FIG. 4 is a functional block diagram of the image forming apparatus.
The control unit 401 is a kind of computer, and controls the operation of each component of the image forming apparatus by executing a predetermined program. Thus, various image processing functions such as copying and printing, communication functions, and the like are realized.

The image forming apparatus also has functions of an input image processing unit 402, a FAX (facsimile) unit 403, and a NIC (Network Interface Card) unit 404. Further, as other interfaces, there are also functions of a dedicated I / F (Interface) unit 405 and a USB (Universal Serial Bus) I / F unit 406.
The input image processing unit 402 converts RGB data output from the CCD sensor 211 of the scanner unit 2 shown in FIG. 2 into parameters such as density, brightness, saturation, and hue (corner).
The FAX unit 403 transmits and receives image data using a telephone line. The NIC unit 404 transmits and receives image data corresponding to RGB data using a network. The dedicated I / F unit 405 transmits and receives image data and the like with an external device. The USB I / F unit 406 is used to exchange image data and the like with a USB device typified by a USB memory (a kind of removable media).

  The image forming apparatus also has functions of a compression / decompression unit 407, a post-processing unit 410, an output image processing unit 412, and a RIP (Raster Image Processor) 413.

  The compression / decompression unit 407 compresses the image data and stores it in the document management unit 408, or conversely compresses and stores the image data stored in the document management unit 408 for decompression. When image data is transmitted and received between an image forming apparatus and an external device via a network, compressed data such as JPEG, JBIG, and ZIP may be used. The compression / decompression unit 407 compresses image data into compressed data, and decompresses (decompresses) compressed data received from an external device into image data. The post-processing unit 410 performs finishing processing such as punching processing and stapling processing on the sheet material discharged from the printer unit 1.

  The output image processing unit 412 performs various image processing on the image data so that the printer unit 1 can actually form an image based on the image data. The RIP unit 413 converts image data in a PDL (Page Description Language) format input via the NIC unit 404 into image data in a bitmap format, for example.

  The image forming apparatus includes a document management unit 408 and a resource management unit 409 for data recording. The document management unit 408 stores various image data input via the input image processing unit 402, the FAX unit 403, the NIC unit 404, the dedicated I / F unit 405, and the USB I / F unit 406 in a storage device. . This storage device is a storage device such as a hard disk capable of storing a plurality of image data.

The resource management unit 409 uses reference data representing the correlation between parameters such as toner density, lightness, saturation, and hue (corner) and the toner height (the stacking height of the toner as the color material, the same applies hereinafter). Stored for each type of toner (each color material).
In the resource management unit 409, for example, reference data representing the correlation between the hue and the toner height at a predetermined saturation is held for a color material whose combination of saturation and hue correlates with the toner height.
The resource management unit 409 includes, for example, reference data representing a correlation between saturation and toner height at a predetermined halftone area ratio for a color material whose combination of halftone dot area ratio and saturation has a correlation with toner height. Is held. Alternatively, the resource management unit 409 may include, for example, reference data representing a correlation between brightness and toner height at a predetermined halftone dot area ratio for a color material in which a combination of halftone dot area ratio and brightness is correlated with toner height. Is held. The “halftone dot area ratio” is the ratio of the area of the toner image to which the toner is adhered within a predetermined area when the toner image is formed on the sheet material. That is, in the image forming apparatus that expresses the image density by area gradation, the halftone dot area ratio of the image with the maximum density is 100 [%]. Each reference data is held in a table format. These tables are called “correlation tables”.

The resource management unit 409 also holds information such as image formation conditions and gradation correction data described later.
The correlation table and the like held in the resource management unit 409 are necessary for determining conditions for forming an image, that is, image forming conditions.

  It should be noted that the correlation table and the like for the above parameters are desirably held for every color used in accordance with a test pattern to be described later, but it is not always necessary to do so. Only the toner having a correlation with the toner height that is known in advance may be held.

  In the image forming apparatus of the present embodiment, when calculating and correcting the toner height, a test pattern with various toner adhesion densities is used, and various parameter detection is performed, whereby the toner height in each pattern is detected. Is calculated. When the test pattern is read by the scanner unit 2, the input image processing unit 402 detects parameters such as the density, brightness, saturation, hue (corner) of toner attached to the test pattern. Then, the output image processing unit 412 calculates the toner height by comparing these parameters with the reference data.

  The control unit 401 refers to the target toner height (hereinafter referred to as “target height”) based on the image forming conditions when the test pattern is formed and the toner height calculated by the output image processing unit 412. ) Is determined. The image forming condition for achieving the target height is a condition for forming an image in which the toner height is less than a predetermined value.

In this embodiment, in order to form an image having a target height, for example, Vcont (hereinafter referred to as “development contrast”), which is a difference between the development potential Vdc on the photosensitive drum 41 and the exposure portion potential Vl, is referred to for each color. To control. The development potential Vdc is a surface potential of a region where the toner on the photosensitive drum 41 is not attached. The exposed portion potential Vl is a surface potential of a region on the photosensitive drum where a toner corresponding to a reference density, for example, a density of 1.6 is attached. “Density 1.6” can be measured, for example, with a spectral densitometer 504 manufactured by X-rite.
It is generally difficult to change the distribution of the development potential Vdc on the photosensitive drum 41. On the other hand, the exposure potential Vl on the photosensitive drum 41 can be easily changed by changing the laser power of the laser exposure unit 3 in the plane. Therefore, in this embodiment, the toner height correction is facilitated by changing the development contrast Vcont by the exposure potential V1, specifically, the laser power of the laser exposure unit 3.
Hereinafter, image processing during normal image formation and toner height correction in the input image processing unit 402 and the output image processing unit 412 will be described.

<Description of Input Image Processing Unit 402>
FIG. 5 is a functional block diagram of the input image processing unit 402. The input image processing unit 402 receives RGB data read by the CCD sensor 211 and input via the A / D conversion unit 502. The A / D conversion unit 502 performs gain adjustment and offset adjustment of RGB data, and converts the data into 8-bit data for each color.
The input image processing unit 402 includes a shading correction unit 503, a filter processing unit 504, an RGB → CMYK color conversion unit 505, an RGB → L * a * b * color conversion unit 506, an RGB → density color conversion unit 507, and L * / a. * / B * / Density composition unit 508 functions.

A shading correction unit 503 corrects variations in sensitivity of each pixel of the CCD sensor 211, variations in the amount of light of the document illumination lamp 205, and the like using a read signal of a reference white plate (also referred to as a shading plate) (not shown).
When the shading correction unit 503 forms an image based on the image of the original D read by the scanner unit 2 (hereinafter referred to as “normal image formation”), the shading correction RGB data is transmitted to the filter processing unit 504. To do. On the other hand, when the toner height is corrected, the RGB data subjected to the shading correction is transmitted to each of the RGB → L * a * b * color converter 506 and the RGB → density converter 507.

The filter processing unit 504 performs RGB data filtering processing during normal image formation, and transmits the filtered data to the RGB → CMYK color conversion unit 505. The filter process is a process for making an image captured from the CCD sensor 211 appear sharper.
The RGB → CMYK color conversion unit 505 converts the RGB data received from the filter processing unit 504 into CMYK data using, for example, a known multidimensional direct mapping method. The CMYK data is transmitted to the output image processing unit 412 (FIG. 4) under the control of the control unit 401.

When the RGB data is input to the RGB → L * a * b * color conversion unit 506, the RGB → L * a * b * color conversion unit 506 converts the RGB data into each parameter of L * a * b *. . Each parameter of L * a * b * is data representing lightness (L) and complementary color dimensions (a, b). Then, each parameter of L * a * b * after conversion is transmitted to L * / a * / b * / Density composition unit 508.
On the other hand, the RGB → density conversion unit 507 calculates a density value (Density data) from the RGB data and transmits the density value to the L * / a * / b * / Density composition unit 508. The L * / a * / b * / Density synthesizer 508 associates each parameter of the received L * a * b * with the density value (Density data) and associates it with the output image processor via the controller 401. 412.

<Output image processing unit>
FIG. 6 is a functional block diagram of the output image processing unit 412. The output image processing unit 412 includes a gradation correction unit 701, an HT (Half Tone) processing unit 702, a hue / saturation conversion unit 703, a toner height calculation unit 704, and an output condition determination unit 705.

  The gradation correction unit 701 receives the CMYK data converted by the RGB → CMYK conversion unit 505 during normal image formation. The tone correction unit 701 corrects the signal values for each color component of the cyan, magenta, yellow, and black components of the CMYK data using the tone correction data.

  The HT processing unit 702 executes pseudo halftone processing. Specifically, for example, a screen image of 134 [lpi] (Line Per Inch), 166 [lpi] or the like is formed using a known multi-value dither method. Since the density gradation of the printer engine differs depending on the screen, gradation correction is executed for each dither in this embodiment.

  On the other hand, when performing toner height correction, each parameter of L * a * b * converted by the input image processing unit 402 is input to the hue / saturation conversion unit 703. The hue / saturation conversion unit 703 converts each parameter of L * a * b * into L * CH data and sends it to the toner height calculation unit 704. The L * CH data is data representing saturation (C) and hue (H). This conversion is performed by the following equation.

Density data converted by the input image processing unit 402 is also transmitted to the toner height calculation unit 704.
The toner height calculation unit 704 calculates the toner height for each test pattern using the L * CH data and Density data, and transmits the result to the output condition determination unit 705.
The output condition determination unit 705 analyzes the toner height for each test pattern and determines an image forming condition for making the toner height less than a predetermined value for each color component.

<Toner height correction>
In the present embodiment, as described above, the control unit 401 controls the development contrast Vcont in order to perform the toner height correction process. However, when the development contrast Vcont is controlled, the maximum density of the image formed by the printer unit 1 changes. Therefore, in the present embodiment, control for forming an image with the maximum density is executed after the control of the development contrast Vcont.

Hereinafter, the procedure of the toner height correction process according to the present embodiment will be described with reference to FIG.
The control unit 401 includes an image forming mode in which an image is formed based on an image read by the scanner unit 2 and image data received via the NIC unit 404, the dedicated I / F unit 405, and the USB I / F 406, and a toner height. A toner height correction mode for correcting the height.
The toner height correction process is performed in response to an input of a toner height correction execution instruction from the operation unit 300. When this execution instruction is accepted (S101: Yes), the control unit 401 shifts to the toner height correction mode. In the toner height correction mode, the control unit 401 executes potential control (S102), prints a test pattern (first image) for calculating the toner height on the recommended paper, and then outputs it from the paper discharge tray 53. (S103). The recommended paper is a sheet material in which smoothness, whiteness, and basis weight are defined in advance.

The recommended paper on which the test pattern is printed is conveyed to the scanner unit 2 by the user.
An example of the test pattern is shown in FIG. Twenty-five test patterns are formed on the recommended paper for each of cyan, magenta, yellow, and black, with different development contrast Vcont and halftone dot area ratio.

In this embodiment, the halftone dot area ratio is set to five types of 30 [%], 32 [%], 34 [%], 36 [%], and 38 [%]. Thereby, the toner height in the halftone of each color can be calculated.
The development contrast Vcont is determined by the charging bias of the charger 42 and the laser power of the laser exposure unit 3 as described above. In this embodiment, for example, the development contrast Vcont is set to the following four types in addition to the standard Vcont determined in S102.
Vcont-10 [%], which is 10% smaller than the standard Vcont,
Vcont-20 [%], which is 20 [%] smaller than the standard Vcont,
Vcont-30 [%] 30 [%] smaller than the standard Vcont,
Vcont-40 [%] 40 [%] smaller than the standard Vcont.
As a result, various test patterns having different adhesion density and density of toner for each color can be formed and printed. The image forming conditions when the test pattern is formed are stored.

Returning to FIG. 7, the control unit 401 optically reads a test pattern in response to an input of a read instruction execution signal from the operation screen of the operation unit 300 (FIG. 3) by the user (S <b> 104). Then, parameters of brightness, saturation, hue, and density are detected from the image data thus obtained (S105).
Next, the control unit 401 compares the detection result of S105 with reference data indicating the correlation between the detected toner height and the detected parameter in the resource management unit 409 in advance, and calculates the toner height ( S106). Details of the processing in S106 will be described later.
The control unit 401 determines the development contrast Vcont based on the toner height calculated in S106 (S107). In step S107, the control unit 401 determines the development contrast Vcont at which the toner height becomes the target height, and determines the exposure conditions including the laser power so that the determined development contrast Vcont is obtained. Then, the exposure condition is stored in the resource management unit 409 (S108).

  In this embodiment, since the development contrast Vcont is changed in order to make the toner height less than a predetermined value, an image having a desired density may not be formed. Therefore, after calculating the toner height, the control unit 401 performs automatic gradation correction and updates existing gradation correction data in order to adjust the maximum density and gradation (S109). In addition, since a well-known technique can be used about automatic gradation correction | amendment, description here is abbreviate | omitted. The control unit 401 stores the updated tone correction data in the resource management unit 409 as part of the image forming conditions (S110), and returns to S101.

On the other hand, when a toner height correction execution signal is not input from the operation unit 300 (S101: No), the control unit 401 inputs a scan start signal from the operation unit 300 or input image data from the NIC unit 404. (S111: No). In S111, when a scan start signal is input or when image data is received via the NIC unit, the control unit 401 shifts to an image forming mode (S111: Yes). The control unit 401 refers to image forming conditions (including gradation correction data) held in the resource management unit 409 (S112), and an image based on an image read by the scanner unit 2 according to the image forming conditions, Alternatively, an image based on the image data received via the NIC unit 406 is formed (S113). Note that the image based on the image read by the scanner unit 2 and the image data received via the NIC unit 406 corresponds to the second image.
When the formation of all the images is completed (S114: Yes), the process is finished. If all the images have not been formed, the process returns to S111.

<Toner height calculation>
Here, an outline of the toner height calculation process will be described. In the present exemplary embodiment, parameters such as lightness (L), saturation (C), hue (H), and density (Density) of the toner image are detected and compared with reference data prepared in advance, thereby adjusting the toner height. calculate. However, depending on the type of toner (color, contained pigment, etc.), there is a case where the toner height cannot be calculated with high accuracy using only one parameter among the parameters of brightness, saturation, hue, and density. Therefore, the control unit 401 calculates the toner height by comparing at least two parameters with the reference table for the toner (for color material) according to the type of toner (color, pigment).

FIG. 9 shows the result of detecting the spectral reflectance of a cyan toner image having a different toner height. The dot area ratio of the toner image is 100% (so-called solid portion).
As shown in FIG. 9, cyan has a characteristic that absorption of light having a wavelength near 480 [nm] increases as the toner height increases. Therefore, the reflected light from the cyan toner image changes its hue from cyan to blue as the toner height increases. However, when the density of the cyan toner image is detected by irradiating light having a wavelength in the vicinity of 600 to 650 [nm], the reflectance of the cyan toner image does not change due to the toner height. That is, the toner height of the cyan toner image cannot be calculated with high accuracy using only the density parameter.

Brightness is an index of brightness that takes into account a wide range of spectral characteristics. For this reason, it is more susceptible to toner height than density. However, it has been found through experiments by the present inventors that the amount of change in lightness is small with respect to the toner height to be calculated, resulting in insufficient resolution.
Further, it is impossible to distinguish whether the thickness of the line expressing the halftone has changed or the toner height has changed in both density and lightness. For this reason, it is difficult to detect the toner height with high accuracy only by density or brightness. The relationship between the thickness of the line expressing the halftone and the toner height will be described later.

  It is also difficult to calculate the toner height with high accuracy only from the saturation. FIG. 10A is a graph showing the relationship between chromaticity coordinates and brightness of a plurality of toner images having different toner heights. The horizontal axis is “b” of chromaticity coordinates L * a * b *, and the vertical axis is lightness “L”. The chromaticity is a color value, and is a quantification of the nature of the color stimulus determined by the chromaticity coordinates. The chromaticity coordinates (color value coordinates) include color systems such as “Lab”, “LCH”, “xyz”, “RGB”, and the like. As described above, “L” represents “lightness”, and “a” and “b” represent “complementary color dimensions”. “C” indicates “saturation” and “H” indicates “hue”. In this graph, the relationship between the toner height is toner height 4> toner height 3> toner height 2> toner height 1.

  As the toner height increases, the saturation increases, but when it exceeds a certain value (toner height 3), the saturation decreases. FIG. 10B shows this in a graph of saturation and toner height. That is, even if a target (target) is set for the saturation C and the toner height is calculated by this, there is an inflection point (extreme value), so the toner height can be accurately determined based only on the saturation. Cannot be detected.

It is also impossible to detect the toner height with high accuracy only by the hue. FIG. 11 is a graph of a * b * of L * a * b * chromaticity coordinates. When the halftone saturation is compared between the gray level (density) from the paper white point (W) to the cyan solid part (C), the hue increases from cyan to blue when the toner height increases. It will change. Based on the spectral reflectance characteristics of FIG. 9, it has been described that the chromaticity changes from cyan to blue when the height is different, but it can be seen that this phenomenon occurs in a halftone.
As shown in FIG. 11, the height and the degree of hue change are different in each gradation (density) region. Therefore, the toner height cannot be detected with high accuracy only by the hue.

  The above relationships are summarized in Table 1. Table 1 shows that uniaxial quantitative evaluation values represented by density, brightness, saturation, and hue are not enough to calculate the cyan toner height.

  However, the hue can be converted into toner height by detecting the change under certain conditions combined with other parameters. Specifically, the hue (its change) is detected with one of the two parameters fixed to the specified value, such as “hue at specified saturation” or “hue at specified brightness”. This can be converted into toner height.

  As described above, when calculating the toner height of the cyan test pattern, it is only necessary to detect a change in hue at the specified saturation in the cyan toner image. However, when a pigment type is employed in which the variation amount of the density becomes sufficiently large according to the change in the toner height, it is desirable to adopt a configuration that detects a change in hue at a specified density.

FIG. 12 shows the result of detecting the spectral reflectance of magenta toner images having different toner heights. The dot area ratio of the toner image is 100 [%] (so-called solid portion).
As shown in FIG. 12, magenta increases the absorption of the blue component when the toner height is high, but the reflectance of the red component is relatively maintained. That is, as the toner height of the magenta toner image increases, the hue changes from magenta to red. Similarly to cyan, the reflectance of the magenta toner image when irradiated with light having a wavelength near 530 [nm] hardly changes due to the toner height. Therefore, the toner height of the magenta toner image cannot be detected with high accuracy only by the density. However, the same phenomenon as cyan occurs in saturation. That is, the hue can be converted into the toner height by detecting a change under a certain condition combined with other parameters. For this reason, when calculating the magenta toner height, for example, it may be configured to detect the hue at the specified saturation (change thereof) or the hue at the specified brightness (change thereof).

FIG. 13 shows the result of detecting the spectral reflectance of yellow toner images having different toner heights. The dot area ratio of the toner image is 100 [%] (so-called solid portion).
As shown in FIG. 13, since the spectral reflectance of yellow is absorbed only in the blue region, the hue does not change even if the toner height changes. That is, the toner height can be calculated by detecting a decrease in reflectance only in the blue region.
The yellow toner height may be detected by, for example, the blue filter density. On the other hand, since the brightness of yellow does not differ from the brightness of the sheet material, it is difficult to detect the height of the yellow toner image formed on the sheet material based only on the brightness.

  The factors that determine the color include not only pigments but also resins, charge control agents, and their dispersibility, but the same tendency is observed when the pigment content (also referred to as dilution rate) is increased. Has been confirmed by their experiments. That is, the pigment factor is dominant. Therefore, there is a correlation between the toner height and the pigment content. Therefore, if the pigment content and the spectral reflectance are known, it is possible to specify a colorimetric method (density / lightness / hue / saturation, 1 axis / 2 axes) effective for calculating the toner height.

The black toner image with a flat spectral reflectance does not change the hue even if the toner height changes. The black toner image has a characteristic that the absorption of visible light increases as the toner height increases. Therefore, similarly to yellow, the toner height can be calculated based on the density. However, unlike yellow, since a dynamic range of lightness can be secured, the toner height may be detected based on the lightness.
However, the uniaxial evaluation can be calculated only when the dot area ratio is 100 [%]. Even if the relationship between the toner height and the density is derived and the prescribed density is determined, it cannot be determined whether the area is increasing or the toner height is increasing. Therefore, even in yellow and black, the toner height of a halftone toner image cannot be calculated based only on the density.

FIG. 14A is an explanatory diagram for explaining the relationship between the thickness (width) of a line expressing a halftone, the density, and the toner height, and FIG. 14B shows the state of the sheet material at that time. It is explanatory drawing. As shown in these drawings, even if the specified density is the same, if the line width is large, the toner height is relatively low, and if the line width is thin, the toner height is high. Therefore, the toner height can be calculated by comparing with reference data representing the correlation (converted) between the dot area ratio and the density at the specified height. That is, for yellow and black, the toner height can be calculated based on the dot area ratio and density.
In the present embodiment, a density of 50 [%] in a test pattern is set with reference to a density of 50 [%] with respect to the difference between the maximum density and the minimum density in one test pattern, and the density is 50. The ratio of the area to the region that is less than [%] is obtained. Hereinafter, this ratio is referred to as “patch dot area ratio”.
By selecting an exposure condition in which the patch dot area ratio is a specified percentage and the density of the test pattern is a specified density, the toner height of yellow or black can be kept constant.

  Table 2 summarizes the quantification axes of the toner heights for cyan, magenta, yellow, and black. Thus, the toner height can be calculated by combining parameters according to the type (color, pigment) of the toner.

<Calculation of toner height and exposure conditions>
The details of the processing of the toner height calculation in S106 and the exposure condition calculation in S107 in the flowchart of FIG. 7 will be specifically described with respect to the portion relating to the cyan toner height. Since the method for detecting the toner height of the cyan and magenta test patterns is the same, the same applies to the toner height of the magenta test pattern.
In the toner height calculation in S106, image data generated by reading 25 cyan test patterns is converted from RGB data to L * a * b * and LCH in the input image processing unit 402. Also, the RGB data is converted into Density (density) and halftone dot area ratio. That is, the input image processing unit 402 acquires parameters detected from all test patterns.

According to Table 2, in the cyan toner image, the parameters having a correlation with the toner height were lightness and hue, and saturation and hue.
FIG. 15 is an example of the content of reference data representing the correlation between the hue at the specified brightness “55” and the average toner height (μm) at that time for cyan. In the illustrated example, the reference data is expressed as a brightness correlation table. FIG. 16 is a content example of reference data representing the correlation between the hue at the specified saturation “40” and the average toner height (μm) at that time for cyan. In the illustrated example, the reference data is expressed as a saturation correlation table. The average toner height is data measured in advance by a micrometer or the like. The content of the reference data in FIG. 16 is represented in a graph as shown in FIG.
As is clear from these figures, the cyan toner height can be calculated by comparing the hue at the specified brightness or the hue at the specified saturation with the brightness correlation table or the saturation correlation table. Recognize.

Next, an example of a method for calculating the optimum value of the development contrast Vcont when the cyan target height is 7 [μm] will be described.
FIG. 18 shows the relationship of development contrast Vcont when the saturation, hue, and halftone dot area ratio are 30 [%], 32 [%], 34 [%], 36 [%], and 38 [%]. The development contrast in the graph of FIG. 18 is the above-mentioned standard Vcont, Vcont-10 [%], Vcont-20 [%], Vcont-30 [%], and Vcont-40 [%]. FIG. 19 is a chart showing the relationship between the development contrast Vcont and the hue when the saturation is the specified saturation “40”. In FIG. 19, the toner height corresponding to each hue is derived from the correlation table of FIG. The hue when the target height is 7 [μm] is “245” (hue angle 245 degrees) with reference to FIGS. 15 and 16.

The control unit 401 forms the development contrast when a test pattern having a toner height (h1) closest to the target height and the target height is low among the toner heights for each development contrast shown in FIG. Is identified. In the example of FIG. 19, Vcont-30 [%] is specified. The development contrast at this time is Vcont1.
Next, the control unit 401 specifies the development contrast when a test pattern having a toner height (h2) that is higher than the target height and closest to the target height is formed. In the example of FIG. 19, Vcont-20 [%] is specified. The development contrast at this time is Vcont2. FIG. 20 shows these relationships. Thereafter, Vcont1 and Vcont2 are linearly interpolated to create a graph as shown in FIG.
From FIG. 21, it is possible to calculate the optimum development contrast Vcont that can form a cyan toner image with a target height of less than 7 μm. In this example, the optimum development contrast Vcont is about “190”.
Note that the hue when the target height is 7 [μm] is “245” (hue angle 245 degrees) according to the correlation tables of FIGS. 15 and 16.

  Since magenta is the same as cyan and has a different prescribed value (saturation: 40, hue angle 350 degrees), description thereof is omitted.

When calculating the toner height of the yellow test pattern, a graph of halftone dot area ratio and saturation is created. Then, the saturation at the specified halftone dot area ratio is detected for each development contrast, and is compared with reference data indicating the correspondence between the saturation and the toner height in the yellow test pattern with the specified halftone dot area ratio. In this way, the toner height for each development contrast is calculated.
Thereafter, binarization is executed with the difference between the maximum density and the minimum density in the test pattern as a threshold value of 50 [%], and the ratio of white and black is obtained. The halftone dot area ratio, which is the ratio, is replaced with the horizontal axis of FIG. 18 and the saturation is replaced with the vertical axis, and the development contrast that gives the yellow prescribed value <halftone dot area ratio: 50 [%], saturation: 55> calculate. Next, an optimum development contrast Vcont that can form a yellow toner image having a target height is calculated by the above-described linear interpolation.

When detecting the toner height of the black test pattern, a graph of the dot area ratio and the brightness is created. Then, the lightness when the specified halftone dot area ratio is obtained is calculated for each development contrast, and is compared with reference data indicating the correspondence between the lightness and the toner height in the black test pattern of the specified halftone dot area ratio. In this way, the toner height for each Vcont is calculated.
Thereafter, as in the case of yellow, binarization is executed with a threshold of 50 [%] for the difference between the maximum density and the minimum density in the test pattern, and the ratio of white and black is obtained. The halftone dot area ratio, which is the ratio, is replaced with the horizontal axis of FIG. 18 and the saturation is replaced with the vertical axis, and the development contrast is calculated so that the black specified value example <dot area ratio: 50 [%], lightness: 60> To do. Next, an optimum development contrast Vcont that can form a black toner image having a target height is calculated by the above-described linear interpolation.

  According to the image forming apparatus of the present embodiment, the toner height is calculated based on the brightness, saturation, hue, halftone dot area ratio, and the like, so it is necessary to provide a new sensor for detecting the toner height. Absent. Further, since the image forming conditions including the development contrast Vcont are changed so that the calculated toner height becomes the target height, it is possible to prevent deterioration of character line quality and graininess.

  In the present embodiment, the toner height is optimized by using a method of changing the exposure intensity (development contrast) as the image forming condition. In addition, the image forming conditions can be changed by various methods such as a method of changing the exposure spot diameter (Japanese Patent Laid-Open No. 2008-181080) and a method of changing the developing bias condition (Japanese Patent Laid-Open No. 2009-139664). can do.

  In the present embodiment, the toner height correction technique using the scanner unit 2 has been described. For example, a color sensor after fixing, which is becoming mainstream in POD printers, may be used as the scanner unit (Japanese Patent Laid-Open No. 2006-243276). At that time, the color sensor after fixing may be converted into saturation and hue and converted into toner height by using an RGB color sensor, a spectrophotometer, or an L * a * b * colorimeter. .

[Second Embodiment]
In the first embodiment, the control unit 401 uses the recommended paper as the sheet material on which the test pattern is formed in the toner height correction process. This is because the saturation and hue are affected by the whiteness and unevenness of the sheet material. In the second embodiment, in consideration of usability, a sheet material capable of forming a test pattern by a user can be selected from previously registered sheet materials. In this embodiment, in FIG. 5, the reading result of the scanner unit 2 is corrected according to the type of sheet material on which the test pattern is formed, and converted into saturation, hue, halftone dot area rate, and brightness.

FIG. 22 is a flowchart of toner height correction in the second embodiment. Since the processes other than S201 are the same as the processes of FIG. 7 described in the first embodiment, the corresponding processes are denoted by the same reference numerals and description thereof is omitted.
In the second embodiment, the control unit 401 accepts input of media information by the user before reading the test pattern (S201). That is, in S201, the control unit 401 displays a plurality of media information registered in advance on the screen of the operation unit 300. When the user selects desired media information, the control unit 401 sets the RGB → L * a * b * color conversion unit 506 and the RGB → density color conversion unit 507 according to the type of the selected sheet material. Switch the color conversion table.

Thereafter, the control unit 401 refers to data representing the correlation between hue and toner height at a predetermined saturation for cyan and magenta, and calculates the toner height of each test pattern. Further, the toner height of each test pattern is calculated based on data representing the correlation between the saturation and the toner height at a predetermined halftone dot area ratio with respect to the yellow test pattern. Similarly, the toner height of each test pattern is calculated based on the data representing the correlation between the brightness and the toner height at a predetermined halftone dot area ratio with respect to the black test pattern.
The control unit 401 derives, for each color component, an optimum development contrast Vcont that becomes the target height from the toner height of the test pattern of each color component calculated in this way.

  That is, in the second embodiment, the reference data for determining the toner height is switched according to the media information selected by the user from the media information registered in advance. Therefore, it is possible to accurately calculate the toner height of the test pattern formed in the registered media information, and to derive the optimum development contrast for the target height for each color component.

[Third Embodiment]
In the third embodiment, when a cyan and magenta test pattern is formed, the fixing property is lowered so that the hue is likely to change when the toner is fixed to the sheet material according to the heating condition. The fixability is controlled by the fixing speed by changing the heating conditions, but can also be controlled by the temperature control temperature or the applied pressure.

FIG. 23 shows the relationship between fixability and hue. As shown in FIG. 23, the toner melting characteristic varies depending on the fixing property. Generally, when the melting degree of the toner is lowered, the fixability is also lowered. A typical value of the toner melting characteristic is a gloss (gloss) characteristic. When the degree of melting of the toner is low, it adheres to the sheet material with the toner particles remaining as shown in FIG. When the degree of melting is high, the toner particles become smooth and the amount of specular reflection increases. If the degree of melting is high, the specular reflection characteristic is detected, so that the gloss is also high.
The density is determined by the spectral absorption characteristics of the color material and the irregular reflection component on the toner surface, and the density value changes as the gloss changes. For example, the density increases as the gloss increases even with the same amount of applied toner. In order to secure the dynamic range of density and achieve high accuracy, it is better to improve the fixability.

  In the case of black, the toner height is calculated using the lightness. However, the lightness is lowered when the irregular reflection light is lowered, as in the case of the density. Therefore, in order to secure a dynamic range of brightness and achieve high accuracy, it is preferable to improve the fixing property. In yellow, the toner height is calculated using the saturation, but the value only changes depending on the degree of absorption at a specific wavelength. Similar to the density, the saturation decreases as the diffusely reflected light decreases. Therefore, in order to secure a dynamic range of saturation and to achieve high accuracy, it is preferable to improve the fixability.

  On the other hand, with cyan and magenta, the toner height is calculated using the hue, but if the fixability is low, the transmittance of the toner layer is lowered and the reflected light from the background is reduced. Further, since the color material is present in the sheet material in a condensed state, the color material dispersion degree is low, and the turbid component of the color material appears as a hue change as it is.

  As described above, in the first and second embodiments, the change in hue is used to determine the toner height. Therefore, it is desirable to output a test pattern under fixing conditions that tend to cause a change in hue. Therefore, in the cyan and magenta test patterns, the fixing speed is increased so as to decrease the fixing property.

  Cyan phthalocyanine blue tends to change on the blue side, and magenta quinacridone tends to change on the red side. The saturation of cyan and magenta whose fixing property has been lowered tends to decrease as described in yellow. As described above, since the toner heights of cyan and magenta are calculated based on the change in hue, there is no problem even if the cyan and magenta test chart is output in a mode in which the fixing property is lowered.

  As described above, in the third embodiment, it is possible to calculate the toner height with higher accuracy by reducing the fixing property so that the hue easily changes when the cyan and magenta test patterns are output.

Claims (6)

Acquisition means for acquiring parameters of brightness, saturation, hue, density, and dot area ratio of an image formed on the sheet;
Of the parameters acquired by the acquisition means, a selection means for selecting two parameters according to the type of color material that formed the image on the sheet;
An image forming apparatus comprising: a detection unit configured to detect the height of the image based on the two parameters selected by the selection unit. The acquisition means includes
Reading means for acquiring a luminance signal of an image formed on the sheet by reading the sheet;
The image forming apparatus according to claim 1, further comprising a calculation unit that calculates brightness, saturation, hue, and density based on the luminance signal read by the reading unit. In accordance with predetermined image forming conditions, image forming means for forming an image by laminating a color material on the surface of the sheet material,
Reading means for outputting image data by optically reading the first image formed by the color material;
Parameter detection means for detecting a parameter correlated with the color material height in the sheet material from the image data, among the brightness, saturation, hue, density, and color material density parameters of the color material;
Resource management means for holding, for each color material, reference data representing a correlation between the parameter and the height of the color material from the surface of the sheet material;
A height calculation means for calculating a color material height of the color material by comparing the parameter detected by the parameter detection means with reference data about the color material held in the resource management means;
The image forming condition is changed based on the reference data so that the difference between the calculated color material height and the target color material height is small, and the first image is sent to the image forming means according to the changed image forming condition. And a control means for forming a different second image.
Image forming apparatus. The image forming unit is configured to perform image formation in accordance with the image forming condition including a fixing condition for changing the fixing property of the color material to the sheet material;
The control means changes the fixing condition.
The image forming apparatus according to claim 3. The image forming means fixes the color material to the sheet material according to a heating condition for heating the color material,
The control means changes the heating condition,
The image forming apparatus according to claim 4. The image forming conditions include a development contrast that is a difference between a development potential that is a surface potential of a region where the coloring material is not attached to a photosensitive member that is sensitive to light and an exposure portion potential that determines the intensity of the light,
The control means controls the exposure portion potential to change the development contrast, thereby bringing the color material height close to the target color material height,
The image forming apparatus according to claim 3.