JP5998567B2 - Density detector and image forming apparatus - Google Patents

Description

  The present invention relates to a density detection apparatus and an image forming apparatus.

  Conventionally, in an image forming apparatus, in order to correct an image density, a technique for forming a density detection image on an image carrier and detecting the density of the density detection image is used.

  Patent Document 1 discloses a toner image forming apparatus that forms a toner image for measuring toner density in an image forming apparatus that forms a toner image by developing an electrostatic latent image formed by exposing a charged photoreceptor. A toner that measures a toner density, a reflection member having a constant reflectance with respect to light, a light-emitting unit that can control the amount of light, and a light-receiving unit that receives reflected light from the reflection member or the toner image A density sensor, a first light amount determining means for determining a first light emission amount of the light emitting unit so that the light receiving unit has a predetermined output value by reflected light received from the reflection member, and the first light emission amount Second light amount determining means for determining different second light emission amounts of the light emitting units, and an output of the light receiving unit of the toner density sensor by reflected light received from the reflection member when the first and second light emission amounts are used. Value and the above The toner measured by the first light emission amount based on the relationship with the standard output value of the light receiving unit of the standard sensor obtained in advance by the reflected light received from the reflection member when the first and second light emission amounts are used. Measurement density correction means for correcting the output value of the light receiving unit with respect to density so as to approach the standard output value, and the second light quantity determination means corresponds to a plurality of color toners smaller than the first light emission quantity. An image forming apparatus with a toner density measuring function is disclosed in which the respective light emission amounts are determined, and the measured density correcting means corrects the measured value for each of a plurality of colors of toner.

  In Patent Document 2, an image forming system for forming a toner image on an image carrier, and light emitted from a light emitting element are irradiated with the density of a control toner image formed on the image carrier by the image forming system. A density detection sensor that detects the reflected light from the output value obtained by receiving the reflected light with a light receiving element, and a control unit that controls the imaging conditions of the imaging system according to the detection result obtained by the density detection sensor. In the image forming apparatus, the density detection sensor includes a box-shaped support body that houses and installs the light-emitting element and one light-receiving element, and a part of light emitted from the light-emitting element inside the support body The sensor is provided with a reference reflecting surface that reflects the light and the light is received by one of the light receiving elements, and inside the support, the light emitted from the light emitting element is directed toward the irradiation object within the support. A light exiting optical path for guiding the light to emit light, a light entrance optical path for allowing a part of the reflected light reflected from the irradiation object to enter the support and guiding it to the light receiving element, and a light emitting element An image forming system comprising: a first optical path for guiding a part of light to a reference reflecting surface; and a second optical path for guiding light reflected by the reference reflecting surface to the light receiving element. An apparatus is disclosed.

Japanese Patent No. 3331294 Japanese Patent No. 3661446

  The object of the present invention is to correct the reflected light amount from the density detection image detected by the light amount detection unit even when the output characteristic of the light amount detection unit deviates from the output characteristic of the standard detector, thereby obtaining an accurate image density. It is an object of the present invention to provide a density detection apparatus and an image forming apparatus that can acquire the above.

In order to achieve the above object, the invention of claim 1 is characterized in that an image forming means for forming a plurality of density detection images having different gradations on an image carrier and a reference plate while stepwise changing measurement conditions. Measuring means for measuring at least one of the reflected light amount and the reflected light amount of the image holding body, and measuring the reflected light amounts of the plurality of density detection images formed on the image holding body under predetermined measurement conditions; Storage means for storing a plurality of reflected light amounts measured while changing measurement conditions in stages by the measurement means as a plurality of reference values corresponding to a plurality of gradations of the density detection image; A density detection device having density acquisition means for acquiring the detection density of the density detection image to be measured based on the amount of reflected light of the detection image and a reference value corresponding to the gradation of the density detection image .

  According to a second aspect of the present invention, when the reflected light amount of the reference plate and the reflected light amount of the image holding member are measured by the measuring unit, the reflected light amount from the reference plate is stored in the storage unit as a plurality of first reference values. The density detection apparatus according to claim 1, wherein the amount of reflected light from the image carrier is stored in the storage unit as a plurality of second reference values.

  The invention according to claim 3 is the concentration detection apparatus according to claim 1 or 2, wherein the measurement condition includes at least one of an irradiation light amount and a light receiving sensitivity.

  According to a fourth aspect of the present invention, there is provided the density detection device according to any one of the first to third aspects and a plurality of detection densities corresponding to the plurality of density detection images acquired by the density detection device. And an image forming apparatus having correction means for correcting the output image density based on the image forming apparatus.

  According to a fifth aspect of the present invention, the image forming means for forming a plurality of density detection images having different gradations on the image carrier, the amount of reflected light from the reference plate while measuring the measurement conditions continuously, Measuring means for measuring the amount of reflected light of each of the plurality of density detection images formed on the image carrier under defined measurement conditions, the detection density of the density detection image by the standard detector, and the output value of the standard detector And storage means for storing in advance the relationship between the light emission amount of the standard detector and the reference plate output value of the standard detector, and the relationship between the light emission amount of the standard detector and the reference plate output value of the standard detector On the basis of the reference detector output value of the standard detector corresponding to the amount of reflected light of the density detection image to be measured, and the detected density of the density detection image by the standard detector and the output value of the standard detector, Obtained standard based on the relationship of The detection density of the density detection image by the standard detector corresponding to the reference plate output value of the output unit is acquired, and the acquired detection density by the standard detector is set according to the gradation of the density detection image to be measured. And a concentration acquisition unit that acquires a detection concentration.

  According to a sixth aspect of the present invention, the image forming means for forming a plurality of density detection images having different gradations on the image holding member, and measuring the reflected light amount of the image holding member while continuously changing the measurement conditions, Measuring means for measuring the amount of reflected light of the density detection image formed on the image carrier under a predetermined measurement condition, and the detected density of the density detection image by the standard detector and the output value of the standard detector Storage means for storing in advance the relationship between the light emission amount of the standard detector and the image carrier output value of the standard detector, and the relationship between the light emission amount of the standard detector and the image carrier output value of the standard detector To obtain the image carrier output value of the standard detector corresponding to the amount of reflected light of the density detection image to be measured, and the detected density of the density detection image by the standard detector and the output value of the standard detector Based on the relationship between The detection density of the density detection image by the standard detector corresponding to the output value of the image carrier of the measuring device is acquired, and the acquired detection density by the standard detector is in accordance with the gradation of the density detection image to be measured And a concentration acquisition unit that acquires a detection concentration.

  The invention according to claim 7 is obtained by the density detection apparatus according to claim 5 or 6, the relationship between the gradation of the density detection image and the target density of the density detection image, and the density detection apparatus. The image forming apparatus includes a correction unit that corrects the output image density based on a plurality of detected densities corresponding to the plurality of density detection images.

The invention according to claim 8 is an image forming means for forming a plurality of density detection images having different gradations on the image carrier, and the amount of reflected light from the reference plate and the amount of reflected light from the image carrier under predetermined measurement conditions. Measuring means for measuring and measuring the reflected light amount of the plurality of density detection images formed on the image carrier under predetermined measurement conditions, and the reflected light amount from the reference plate measured by the measuring means Is stored as a first reference value, and the first storage means for storing the amount of reflected light from the image carrier measured by the measurement means as a second reference value; the first reference value and the standard detector A second storage means for storing in advance a relationship between the reference plate output value and a relationship between the second reference value and the image carrier output value of the standard detector; and a density detection image to be measured. The amount of reflected light is closer to the first reference value than the second reference value In this case, the amount of reflected light of the measurement target of the density detection image, the first reference value, and based on the reference plate output value of the standard detector obtains the detected concentration of the measurement target density detection image, When the reflected light amount of the density detection image to be measured is closer to the second reference value than the first reference value, the reflected light amount of the density detection image to be measured, the second reference value, and the image of the standard detector It is a density | concentration detection apparatus which has a density | concentration acquisition means which acquires the detection density | concentration of the density | concentration detection image of a measuring object based on a holding body output value .

  The invention according to claim 9 is a correction for correcting the output image density based on the density detection apparatus according to claim 8 and a plurality of detection densities corresponding to the plurality of density detection images acquired by the density detection apparatus. And an image forming apparatus.

  According to the present invention, even when the output characteristics of the light quantity detector deviate from the output characteristics of the standard detector, the reflected light quantity from the density detection image detected by the light quantity detector is corrected to obtain an accurate image density. Can be obtained.

1 is a schematic configuration diagram illustrating an example of a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus illustrated in FIG. 1. It is sectional drawing along the optical axis which shows an example of a structure of a light quantity detection part. (A) And (B) is a schematic diagram which shows an example of the image for a density | concentration detection formed on an image holding body. (A) is a graph which shows the relationship between the gradation value of the image for density detection, and the acquired image density. (B) is a graph showing the relationship between the input gradation value and the output gradation value. It is a figure which shows the ideal result of the conventional density correction process. (A) is a graph which shows the output characteristic of a light quantity detection part and a standard detector, (B) is a graph which shows the example from which the density | concentration detection result of a light quantity detection part and a standard detector corresponds. It is a figure which shows the problem of the conventional density correction process. (A) is a graph which shows the output characteristic of a light quantity detection part and a standard detector, (B) is a graph which shows the example from which the density | concentration detection result of a light quantity detection part and a standard detector does not correspond. It is a figure which shows the result of the density correction process which concerns on the 1st Embodiment of this invention. (A) is a graph which shows the output characteristic of a light quantity detection part and a standard detector, (B) is a graph which shows the relationship of the density | concentration detection result of a light quantity detection part and a standard detector. It is a flowchart which shows the procedure of the density correction process which concerns on the 1st Embodiment of this invention. (A) And (B) is a schematic diagram which shows the flow of the density correction process which concerns on the 1st Embodiment of this invention. It is a graph explaining the procedure of the density | concentration correction process which concerns on the 2nd Embodiment of this invention. (A) And (B) is a schematic diagram which shows the flow of the density correction process which concerns on the 2nd Embodiment of this invention. It is a figure which shows the result of the density correction process which concerns on the 3rd Embodiment of this invention. (A) is a graph which shows the output characteristic of a light quantity detection part and a standard detector, (B) is a graph which shows the relationship of the density | concentration detection result of a light quantity detection part and a standard detector. It is a schematic diagram which shows the flow of the density correction process which concerns on the 3rd Embodiment of this invention. It is a flowchart which shows the procedure of the density | concentration detection process which concerns on the 3rd Embodiment of this invention. It is a graph explaining the setting method of the emitted light amount which concerns on the 1st Embodiment of this invention. It is a graph which shows the application object of the density correction process which concerns on the 1st Embodiment of this invention.

<Image forming apparatus>
(Configuration of image forming apparatus)
First, an example of the configuration of the image forming apparatus will be described.
The image forming apparatus is an electrophotographic image forming apparatus that forms an image on a sheet using an electrophotographic developer containing toner. In the present embodiment, a so-called tandem type intermediate transfer type image forming apparatus will be described. The image forming apparatus may be an image forming apparatus that forms a density detection image on an image carrier, acquires the density of the density detection image, and corrects the image density. It is not limited.

  FIG. 1 is a schematic configuration diagram showing an example of the configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus shown in FIG. As shown in FIGS. 1 and 2, the image forming apparatus according to the present embodiment includes an operation display unit 10, an image reading unit 20, an image forming unit 30, a paper supply unit 40, a paper discharge unit 50, and a light amount detection unit 60. , An image processing unit 70, a communication unit 80, a storage unit 90, and a control unit 100. Each of the image forming unit 30, the paper supply unit 40, and the paper discharge unit 50 is arranged in the order of the paper supply unit 40, the image formation unit 30, and the paper discharge unit 50 along the paper conveyance path illustrated by a dotted line. ing.

  In the present embodiment, the image carrier is an intermediate transfer belt 36 described later. The light quantity detection unit 60 is disposed around the intermediate transfer belt 36 constituting the image forming unit 30 so as to face the intermediate transfer belt 36. The light quantity detection unit 60 is disposed on the downstream side in the moving direction of the intermediate transfer belt 36 with respect to the image forming unit 32 constituting the image forming unit 30, and is formed on the intermediate transfer belt 36 by the image forming unit 30. The amount of reflected light of the density detection image is measured.

  The control unit 100 is configured as a computer that controls the entire apparatus and performs various calculations. That is, the control unit 100 includes a CPU (Central Processing Unit) 100A, a ROM (Read Only Memory) 100B storing various programs, a RAM (Random Access Memory) 100C used as a work area when executing the programs, A nonvolatile memory 100D for storing various information and an input / output interface (I / O) 100E are provided. Each of CPU 100A, ROM 100B, RAM 100C, nonvolatile memory 100D, and I / O 100E is connected via a bus 100F.

  The operation display unit 10, the image reading unit 20, the image forming unit 30, the paper supply unit 40, the paper discharge unit 50, the light amount detection unit 60, the image processing unit 70, the communication unit 80, and the storage unit 90 are each configured as a control unit 100. Connected to the I / O 100E. The control unit 100 includes the operation display unit 10, the image reading unit 20, the image forming unit 30, the paper supply unit 40, the paper discharge unit 50, the light amount detection unit 60, the image processing unit 70, the communication unit 80, and the storage unit 90. To control.

  Further, the control unit 100 acquires a detection result output as a detection signal from the light amount detection unit 60. Note that the image forming apparatus has a plurality of transport rollers 46. The plurality of transport rollers 46 are arranged along the paper transport path illustrated by dotted lines. The plurality of transport rollers 46 are driven by a driving mechanism (not shown) and transport the paper according to the image forming operation.

  The operation display unit 10 includes various buttons such as a start button and a numeric keypad, and a touch panel for displaying various screens such as a warning screen and a setting screen. With the above configuration, the operation display unit 10 accepts user operations and displays various types of information to the user. The image reading unit 20 includes an image reading device that optically reads an image formed on a sheet, such as a CCD image sensor, and a scanning mechanism for scanning the sheet. With the above configuration, the image reading unit 20 reads an image on a document sheet placed on the image reading unit 20 and generates image information.

  The image forming unit 30 forms an image on a sheet by electrophotography. The image forming unit 30 includes an image forming unit 32K that forms a K toner image, an image forming unit 32C that forms a C toner image, an image forming unit 32M that forms an M toner image, and a Y toner. An image forming unit 32Y for forming an image is provided. The image forming unit 30 also includes an intermediate transfer belt 36 wound around a plurality of rollers 34 so as to move in the direction of arrow B, and a secondary transfer device 38 that collectively transfers the toner images on the intermediate transfer belt 36 onto a sheet. , And a fixing device 39 for fixing the secondary transferred toner image.

  Each of the image forming units 32K, 32C, 32M, and 32Y is arranged in the order of Y, M, C, and K colors on the intermediate transfer belt 36 when the intermediate transfer belt 36 moves in the arrow B direction. The toner images are arranged in the order shown so as to form a toner image. Hereinafter, when it is not necessary to distinguish each color, they are collectively referred to as an image forming unit 32. The image forming unit 32 includes a photosensitive drum, a charging device, an exposure device, a developing device, a transfer device, a cleaning device, and the like. The photosensitive drum is configured to rotate in the direction of the arrow.

  The intermediate transfer belt 36 is wound around a driving roller 34A, a back support roller 34B, a tension applying roller 34C, and a driven roller 34D. When it is not necessary to distinguish between these rollers, they are collectively referred to as a plurality of rollers 34. The plurality of rollers 34 are driven by a drive mechanism (not shown). When the drive roller 34A is driven to rotate by the drive mechanism, the intermediate transfer belt 36 moves in the arrow B direction at a predetermined speed. Further, a predetermined tension is applied to the intermediate transfer belt 36 by the tension applying roller 34C being moved outward by the drive mechanism.

Specifically, the image forming unit 30 forms an image according to the following procedure.
The toner image of K color is transferred onto the intermediate transfer belt 36 by the image forming unit 32K. In the image forming unit 32K, the photosensitive drum is charged by a charging device. The exposure apparatus exposes the charged photosensitive drum with light corresponding to the K color image. As a result, an electrostatic latent image corresponding to the K-color image is formed on the photosensitive drum. The developing device develops the electrostatic latent image formed on the photosensitive drum with K color toner. The transfer device transfers the K-color toner image formed on the photosensitive drum onto the intermediate transfer belt 36.

  Similarly, the C-color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32C. Further, the M color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32M. Further, the Y-color toner image is transferred onto the intermediate transfer belt 36 by the image forming unit 32Y. On the intermediate transfer belt 36, toner images of K color, C color, M color, and Y color are superimposed to form an “overlapped toner image”. The secondary transfer device 38 collectively transfers the “superimposed toner image” on the intermediate transfer belt 36 onto a sheet. The fixing device 39 fixes the “superimposed toner image” that is collectively transferred onto the sheet by heating or heating.

  The paper supply unit 40 includes a paper storage unit 42 that stores paper, a supply mechanism that supplies paper from the paper storage unit 42 to the image forming unit 30, and the like. The supply mechanism includes a take-out roller 44 that takes out the paper from the paper storage unit 42, a transport roller 46, and the like. A plurality of paper storage sections 42 are provided according to the type and size of the paper. The paper supply unit 40 takes out the paper from one of the paper storage units 42 and supplies it to the image forming unit 30. The paper discharge unit 50 includes a discharge unit 54 that discharges paper, a discharge mechanism that discharges the paper onto the discharge unit 54, and the like.

  The light amount detection unit 60 is an optical sensor that irradiates the detection object with detection light and detects the amount of reflected light reflected from the detection object. The detection signal output from the light amount detection unit 60 represents the amount of reflected light from the object to be detected. Hereinafter, the output from the light quantity detection unit 60 may be simply referred to as “sensor output”. The object to be detected is the intermediate transfer belt 36 on which the density detection image is not formed, or the density detection image formed on the intermediate transfer belt 36 (see FIG. 4). The configuration of the light quantity detection unit 60 will be described in detail later. The density correction and density detection image will be described later.

The image processing unit 70 performs predetermined image processing on the image information of the acquired electronic document. The predetermined image processing includes color conversion processing for converting image data in RGB format into image data in YMCK format, gradation correction, and the like. In this embodiment, as will be described later, the control parameter of the image processing unit 70 is changed, such as changing the toner amount for each gradation by adjusting the relationship between the input gradation value and the output gradation value of the image data, it may be corrected output image density based on the image density D _p. Note that the image processing may be executed by the control unit 100.

  The communication unit 80 is an interface for communicating with an external device via a wired or wireless communication line. The communication unit 80 acquires print parameters including print attributes such as pages and the number of copies, together with a print instruction and image information of an electronic document, from an external device. The storage unit 90 includes a storage device such as a hard disk. The storage unit 90 stores various data such as log data, control programs, and the like.

In the present embodiment, a case where a control program for “density correction processing” to be described later is stored in the storage unit 90 in advance will be described. The control program stored in advance is read and executed by the CPU 100A. The control program may be stored in another storage device such as the ROM 100B. In the present embodiment, the light emission amount of the light emitting element 62 of the light amount detection unit 60 for acquiring various reflected light amounts is stored in advance in the storage unit 90 as a “light amount detection condition”. The various reflected light amounts include the reflected light amount V _ref from the reference plate, the reflected light amount V _clean from the image holding member, the reflected light amount V _p from the density detection amount image, and the like.

(Configuration of light intensity detector)
Here, the configuration of the light amount detection unit will be described.
FIG. 3 is a cross-sectional view along the optical axis showing an example of the configuration of the light quantity detection unit. In the present embodiment, a case will be described in which the light amount detection unit 60 is an optical sensor that detects the amount of specularly reflected light and the amount of diffusely reflected light. However, the optical sensor may be an optical sensor corresponding to the density detection image, and is not limited to the configuration shown in FIG. For example, for a K-color density detection image, a regular reflection type optical sensor that detects the amount of regular reflection light may be used. In addition, for the density detection amount image of each color of YMC, a diffuse reflection type optical sensor that detects the amount of diffuse reflection light may be used.

  As shown in FIG. 3, the light quantity detection unit 60 includes a light emitting element 62 that emits detection light that irradiates the detected object, a light receiving element 63 that receives specularly reflected light from the detected object, and diffuse reflection from the detected object. It includes a light receiving element 64 that receives light, a window portion 66 that transmits detection light and reflected light, and a reference plate 67. Each of the light emitting element 62, the light receiving element 63, and the light receiving element 64 is supported by a support member (not shown) and housed in the housing 61. The light receiving element 63 is disposed at a position where the regular reflection light of the detection light is received. The light receiving element 64 is disposed at a position where the diffuse reflected light of the detection light is received.

  The light emitting element 62 is driven to be lit by a drive circuit (not shown) in accordance with a control signal from the control unit 100. The light receiving elements 63 and 64 are connected to the control unit 100 via an A / D converter (not shown), and output a detection signal that has been digitally converted to the control unit 100. The term “sensor output” means a voltage value of an analog signal output from the optical sensor. The light emitting element 62, the light receiving element 63, and the light receiving element 64 are housed in the housing 61 in a state of being formed on the substrate 65 together with a drive circuit for driving them.

  The casing 61 has a rectangular parallelepiped outer shape, and a surface facing the intermediate transfer belt 36 is a facing surface 61A. The facing surface 61A is provided with an opening through which detection light is emitted and reflected light is incident. The window portion 66 is fitted into the opening of the facing surface 61A so that the surface of the window portion 66 has substantially the same height as the facing surface 61A. The housing 61 includes a light guide path 62A that guides detection light from the light emitting element 62 to the window 66, a light guide path 63A that guides specular reflection light from the window 66 to the light receiving element 63, and diffuse reflected light. A light guide path 64 </ b> A for guiding light from the window portion 66 to the light receiving element 64.

As will be described later, the reference plate 67 is a plate-like member prepared for obtaining a “reference value V _ref ” for correcting the detected reflected light amount. The reference plate 67 is disposed between the intermediate transfer belt 36 and the light amount detector 60 so as to be movable in the direction of arrow C. The reference plate 67 is driven by a driving mechanism (not shown) and is inserted between the intermediate transfer belt 36 and the light amount detection unit 60 or retracted from between the intermediate transfer belt 36 and the light amount detection unit 60. In the present embodiment, the reference plate 67 also serves as a shutter that blocks light from entering the light amount detector 60. For this reason, the reference plate 67 is configured as a light-shielding plate-like member, and the facing surface facing the light amount detection unit 60 is a white surface.

  Next, the operation of the light amount detection unit 60 will be described. Here, an operation for detecting the amount of reflected light from the density detection image G will be described. Accordingly, the reference plate 67 is retracted from between the intermediate transfer belt 36 and the light amount detection unit 60. The detection light emitted from the light emitting element 62 propagates through the light guide path 62 </ b> A, passes through the window 66, and is applied to the density detection image G on the intermediate transfer belt 36. The specularly reflected light that is specularly reflected by the density detection image G passes through the window 66, propagates through the light guide path 63 </ b> A, and is received by the light receiving element 63. A part of the diffusely reflected light diffusely reflected by the density detection image G is transmitted through the window 66, propagates through the light guide path 64 </ b> A, and is received by the light receiving element 64.

  Further, the amount of reflected light from the intermediate transfer belt 36 is detected by the same operation as that for detecting the amount of reflected light from the density detection image G. Further, when detecting the amount of light reflected from the reference plate 67, the reference plate 67 is inserted between the intermediate transfer belt 36 and the light amount detector 60. In this case, the detection light emitted from the light emitting element 62 is applied to the white surface of the reference plate 67. A part of the diffusely reflected light diffusely reflected by the reference plate 67 is received by the light receiving element 64.

  In a third embodiment to be described later, detection light emitted from the light emitting element 62 is applied to the reference plate 67, and regular reflection light regularly reflected by the reference plate 67 is received by the light receiving element 63. In this case, the reference plate 67 is disposed at substantially the same position as the intermediate transfer belt 36.

<Density detection image>
Next, the density detection image will be described.
4A and 4B are schematic views showing an example of a density detection image formed on the image carrier. As shown in FIGS. 4A and 4B, the density detection image group G has a plurality of density detection images P (hereinafter referred to as “patch images P”). Each of the plurality of patch images P is a toner image formed by one specific color (for example, K color). The plurality of patch images P are arranged in a one-dimensional manner on the intermediate transfer belt 36 along the moving direction (arrow B direction) of the intermediate transfer belt 36. That is, an image group in which a plurality of patch images P are arranged is a density detection image group G.

  One patch image P is an image formed at a predetermined area ratio in a predetermined area. In the example shown in FIGS. 4A and 4B, the plurality of patch images P have different “area ratios”. The “area ratio” of the patch image P is represented by a toner coverage per unit area, for example, “60%”. In the present embodiment, since the area ratios of the plurality of patch images P change stepwise, the area ratio of the patch images is hereinafter referred to as “gradation” or “gradation value”. That is, the plurality of patch images P are arranged so that the “gradation” increases or decreases along the arrangement direction.

In the example shown in FIG. 4A, the density detection image group G includes four Y-color patch images _P1-4 , four M-color patch images _P5-8 , and four C-color patches. It has patch images _P9-12 . The twelve patch images P _{1 to 12} are arranged in the order of Y color, M color, and C color from the left side to the right side in the drawing. For each YMC color, the gradation decreases from left to right in the drawing. In the example shown in FIG. 4B, the density detection image group G includes four K-color patch images P1 to _P4 . The four patch images P1 to _P4 have gradations decreasing from the left side to the right side in the drawing.

  When the intermediate transfer belt 36 moves in the arrow B direction, the light amount detection unit 60 detects the amount of reflected light from the density detection image group G formed on the intermediate transfer belt 36. The amount of reflected light is detected in order for a plurality of patch images P. The density detection image group G is an example, and different images may be used. For example, the plurality of patch images P may be formed so that the plan view is not rectangular but the plan view is circular.

<Density correction processing>
(Outline of density correction processing)
The image forming apparatus forms a density detection image on the image carrier, acquires the density of the density detection image, and corrects the image density at the time of image formation based on the acquired density. Is running. FIG. 5A is a graph showing the relationship between the gradation value of the density detection image and the acquired image density. FIG. 5B is a graph showing the relationship between the input gradation value and the output gradation value.

As shown in FIG. 5A, a plurality of density detection images are formed with different gradation values (patch gradation values), and the image density D _p (patch density) of each density detection image is acquired. In this example, four kinds of image density D _p according to the gradation values is shown with black circles as "patch point". As will be described later, the image density D _p is a value obtained by detecting the reflected light amount V _p from each density detection image and correcting the detected reflected light amount V _p based on the reference value. _{Let the} acquired image density _Dp be the “current density”. The “current density” is compared with the “target density” corresponding to the gradation value, and the amount of deviation of the acquired image density D _{p from} the target density is obtained.

Then, based on the amount of deviation from the target density of the obtained image density D _p, the density of an image formed on a sheet (output image density) is to approach the target density, and change the image forming condition Correct the output image density. As means for correcting the output image density, there are means for changing the control parameter of each part of the image forming unit 30 and means for changing the control parameter of the image processing unit 70. The former includes means for changing the toner supply amount by adjusting the toner density in the developing unit, changing the toner amount on the image carrier by adjusting the charge amount on the photosensitive member, the exposure amount, and the developing bias of the developing roll. There is means to do. As the latter, there is means for changing the toner amount for each gradation by adjusting the relationship between the input gradation value and the output gradation value of the image data.

As shown in FIG. 5B, when adjusting the relationship between the input tone value (C _in ) and the output tone value (C _out ) of the image data, the output image density becomes the target density. , C _in and C _out are changed. For example, when the relationship between C _in and C _out is given by a look-up table (LUT), for each input tone value (C _in ), the corresponding output tone value (C _out ) is the target density. The LUT is changed so that Note that the input tone value (C _in ), that is, each tone value of a plurality of density detection images, may be changed each time “density correction processing” is executed.

(Correction of reflected light amount)
The amount of reflected light detected by the light amount detector 60 varies depending on various causes such as individual differences of the optical sensors, the mounting state of the optical sensors, contamination of the optical path of the optical sensor, temperature characteristics of the optical sensor, and the like. In general, the change in the reflected light amount V _p from the density detection image due to these causes is corrected using the reflected light amount V _ref from the reference plate or the reflected light amount V _clean from the image holding member as a reference value. Conventionally, each of the reflected light amount V _p , the reflected light amount V _ref, and the reflected light amount V _clean is measured with the light emission amount of the light emitting element 62 being constant.

Usually, YMC for each color density detection image is measured reflected light amount V _p by the diffuse reflection light, the correction amount of the reflected light V _ref by diffuse reflection light from the reference plate as a "reference value" is performed. As the density of the density detection image for each color of YMC increases, the sensor output by diffuse reflected light increases. For the K-color density detection image, the amount of reflected light V _p due to specularly reflected light is measured, and correction is performed using the amount of reflected light V _clean from the image carrier as a “reference value”. As the density of the K color density detection image increases, the sensor output due to the specularly reflected light decreases.

  Conventionally, the output characteristics of the optical sensors constituting the light quantity detection unit 60 are adjusted to be the same as the output characteristics of a standard detector (hereinafter referred to as “standard sensor”) at the time of shipment. Hereinafter, an example of the adjustment method will be described. Three types of external reference plates are prepared: a first reference plate, a second reference plate, and a third reference plate. The amount of diffuse reflected light from the first reference plate is acquired as the first sensor output, and the amount of diffuse reflected light from the second reference plate is acquired as the second sensor output. In addition, the amount of reflected regular light from the third reference plate is acquired as the third sensor output.

  The reflected light amount of the diffusely reflected light from the first reference plate by the standard sensor is acquired in advance as the first standard sensor output. Further, the reflected light amount of the diffusely reflected light from the second reference plate by the standard sensor is acquired in advance as the second standard sensor output. Furthermore, the amount of reflected regular light from the third reference plate by the standard sensor is acquired in advance as the third standard sensor output.

  Here, the light emission amount of the light emitting element 62 is adjusted so that the first sensor output by the diffusely reflected light from the first reference plate becomes the first standard sensor output. Further, the light receiving sensitivity of the light receiving element 64 is adjusted so that the second sensor output by the diffusely reflected light from the second reference plate becomes the second standard sensor output. Further, the light receiving sensitivity of the light receiving element 63 is adjusted so that the third sensor output by the regular reflection light from the third reference plate becomes the third standard sensor output. Thereby, the output characteristic of the light quantity detection unit 60 is adjusted to be the same as the output characteristic of the standard sensor when using either diffuse reflection light or regular reflection light.

  Note that the above adjustment method is an example, and other adjustment methods may be used. For example, the light receiving sensitivity can be adjusted by narrowing the optical path. Further, a reference plate attached to the optical sensor may be used instead of the first reference plate (external). Further, when the light receiving element is one type of sensor, the light receiving sensitivity of the light receiving element may be adjusted.

  However, even if the output characteristics of the light quantity detection unit 60 are adjusted at the time of shipment, the output characteristics of the light quantity detection unit 60 are different from the output characteristics of the standard sensor during use due to various causes such as the temperature characteristics of the optical sensor. There is a case. Further, if the output characteristics of the light quantity detector 60 are adjusted at the time of shipment, adjustment costs are required, and it is necessary to secure a yield.

For this reason, when the output characteristics of the photosensors constituting the light quantity detection unit 60 have changed significantly from the output characteristics of the standard sensor, the reflected light quantity V _p from the density detection image using the reference value described above. be corrected, no accurate image density D _p is obtained. Hereinafter, specific description will be given with reference to the drawings. FIGS. 6A and 6B are diagrams showing the results of matching the sensor output sensitivities by the conventional density correction processing. On the other hand, FIGS. 7A and 7B are diagrams showing problems in the conventional density correction processing. In this example, correction is performed on the patch image P of each color of YMC with the reflected light amount V _ref from the reference plate as the “reference value”. Since the image density D _p is a value obtained by correcting, in the following, the image density D _p may be referred to as "density detection result".

As shown in FIG. 6A, the density of the density detection image changes from A → B → C → D (A <B <C <D), and in response to this, the sensor output of the light quantity detector 60 is V. _p4 → _{V p3} → changes _{V p2} → _{V p1.} In the case of the patch image P for each color of YMC, when the density (that is, the toner amount) of the density detection image increases, the amount of diffuse reflected light increases. Hereinafter, the sensor outputs V _{p1 to} V _p4 are collectively referred to as “sensor output V _pn ”. The output sensitivity curve (illustrated by a dotted line) of the light quantity detector 60 does not match the output sensitivity curve (illustrated by a solid line) of the standard sensor.

However, the output sensitivity of the light quantity detection unit 60 has a relationship that changes at a similar ratio to the output sensitivity of the standard sensor regardless of the density of the density detection image. Therefore, as shown in FIG. 6B, if the density detection result is (V _pn / V _ref ) obtained by dividing the sensor output V _pn by the reference value V _ref , the density detection result by the light amount detection unit 60 is the standard. It agrees with the density detection result by the sensor. That is, an accurate image density _Dp is obtained.

On the other hand, in the example shown in FIG. 7A, the output sensitivity curve (shown by a dotted line) of the light quantity detection unit 60 does not match the output sensitivity curve (shown by a solid line) of the standard sensor, and the light quantity detection unit The output sensitivity of 60 has a relationship that changes at a different ratio depending on the density of the density detection image with respect to the output sensitivity of the standard sensor. In this case, as shown in FIG. _7B , even if (V _pn / V _ref ) is the density detection result, the light intensity detector 60 detects the density as the value of the sensor output V _pn becomes farther from the reference value V _ref. The result does not coincide with the density detection result by the standard sensor. That is, when a change in density of the density detection image, accurate image density D _p is not obtained.

In the present embodiment, the reflected light amount V _p from the density detection image acquired by the light quantity detection unit 60 is corrected using a plurality of “reference values” corresponding to the gradation of the density detection image. As a result, even when the output characteristics of the light quantity detector 60 are significantly changed from the output characteristics of the standard sensor, an accurate image density _Dp can be obtained regardless of the gradation of the density detection image.

<First Embodiment>
(Outline of density correction processing)
Next, density correction processing according to the first embodiment of the present invention will be described.
FIGS. 8A and 8B are diagrams showing the results of density correction processing according to the first embodiment of the present invention. FIG. 8A is a graph showing the output characteristics of the light quantity detector and the standard detector, and FIG. 8B is a graph showing the relationship between the density detection results of the light quantity detector and the standard detector.

As shown in FIG. 8A, in the density correction processing according to the first embodiment, a plurality of reference values V _{ref1 to} V _refn according to the density of the density detection image are used. _{Each of the} plurality of reference values V _{ref1 to} V _refn is acquired by irradiating the reference plate 67 of the light amount detection unit 60 with detection light having a different light amount from the light emitting element 62. The amount of light emitted from the light emitting element 62 is switched to a predetermined amount of light so that a reflected light amount V _refn similar to the reflected light amount V _pn from the density detection image of the corresponding density can be obtained. The setting of the light emission amount of the light emitting element 62 will be described later.

In this example, the density of the density detection image changes from A → B → C → D, and accordingly, the sensor output of the light quantity detection unit 60, that is, the reflected light quantity from the density detection image is V _p4 → V _p3. → V _p2 → V _p1 Thus, "reference value V _ref1" amount of reflected light equal to that of the reflected light amount V _p1, "reference value V _ref2" of the reflected light quantity substantially equal to that of the reflected light amount V _p2, the amount of reflected light V _p3 and comparable amount of reflected light " reference value V _ref3 ", each of" reference value V _ref4 "reflected light amount V _p4 the same level of the reflected light amount is obtained.

As shown in FIG. 8B, if the density detection result is (V _pn / V _refn ) _obtained by dividing the sensor output V _pn by the corresponding reference value V _refn , the density detection result by the light quantity detection unit 60 is the standard. It almost coincides with the density detection result by the sensor. That is, an accurate image density D _p is acquired regardless of the deviation of the output characteristic of the light amount detection unit 60 from the standard value. In addition, since the density detection result (V _pn / V _refn ) substantially matches the density detection result by the standard sensor, adjustment according to the output characteristics of the standard sensor at the time of shipment becomes unnecessary, and the adjustment cost can be reduced. it can.

Note that the value of the density detection result (V _pn / V _refn ) is “1” if the detected image density D _p matches the target density. Therefore, the deviation amount {(V _pn / V _refn ) −1} from “1” may be set as the deviation amount from the target density of the detected image density D _p . Based on the deviation amount {(V _pn / V _refn ) −1}, the output image density is corrected so as to approach the target density.

In this example, the reflected light amounts V _p4 , V _p3 , V _p2 , and V _p1 are detected according to the densities A, B, C, and D of the density detection image, and the density detection results (V _p4 / V _ref4 ), ( _{V p3 / V ref3), (} V p2 / V ref2), it is acquired _{(V p1} / _{V ref1).} Further, the deviation amount _{{(V p4 / V ref4)} -1}, {(V p3 / V ref3) -1}, {(V p2 / V ref2) -1}, {(V p1 / V ref1) -1 } Is acquired.

(How to set the amount of emitted light)
Next, a method for setting the light emission amount of the light emitting element 62 of the light amount detection unit 60 will be described.
FIG. 16 is a graph illustrating a method for setting the amount of emitted light according to the first embodiment of the present invention. This graph shows the relationship between the input tone value C _in and the toner amount per unit area (upper right), the relationship between the toner amount per unit area and the sensor output of the standard sensor (upper left), and the light emission amount of the standard sensor. This represents the relationship with the sensor output (lower left). Here, the “toner amount per unit area” represents the toner amount per unit area on the image carrier.

For convenience, the upper right is referred to as “first quadrant”, the upper left as “second quadrant”, and the lower left as “third quadrant”. The unit of the input gradation value C _in is “%”, the unit of toner amount per unit area is “g / m ² ”, the unit of sensor output is “V (volt)”, and the unit of emitted light quantity is “mA”. (Milliamperes) ".

  Further, as described above, the relationship between the toner amount per unit area and the sensor output of the standard sensor is “standard sensor output characteristics”, and the relationship between the toner amount per unit area and the sensor output of the light amount detector 60 is “Output characteristics of the light quantity detector 60”. The “relation between the amount of light emitted from the standard sensor and the sensor output” is referred to as “light quantity characteristic of the standard sensor”. When the optical sensor is the light amount detection unit 60, it is similarly referred to as “light amount characteristic of the light amount detection unit 60”.

As shown in the “first quadrant”, the amount of toner per unit area (that is, the target amount corresponding to each of the four types of input gradation values C _inA , C _inB , C _inC , and C _inD ) , Density) are four density detection images of A, B, C, and D. Further, as shown in the “second quadrant”, the sensor outputs V _pstd4 , V _pstd3 , V _pstd2 , V _{pstd1 of the} standard sensor corresponding to each of the four density detection images of density A, B, C, D. Is acquired. The sensor output of the standard sensor represents the amount of reflected reflected light measured by the standard sensor with a predetermined amount of emitted light.

As shown in the “third quadrant”, when the light emission amount of the detection light is switched to A ₄ → A ₃ → A ₂ → A ₁ and irradiated to the reference plate of the standard sensor, the light emission amount A ₄ , A ₃ , A 1 ₂ , sensor outputs V _refstd4 , V _refstd3 , V _refstd2 , and V _{refstd1 of} the standard sensor are acquired corresponding to each of ₂ and A ₁ . Here, sensor output V _refstd4 and sensor output V _pstd4 , sensor output V _refstd3 and sensor output V _pstd3 , sensor output V _refstd2 and sensor output V _pstd2 , and sensor output V _refstd1 and sensor output V _pstd1 match. The light emission amounts A ₄ , A ₃ , A ₂ , A ₁ are set.

The output characteristic of the light quantity detection unit 60 is adjusted to match the output characteristic of the standard sensor. The reflected light amounts V _p4 , V _p3 , V _p2 , and V _p1 obtained by the light amount detection unit 60 corresponding to the respective density detection images of the densities A, B, C, and D are the densities A, B, and C, respectively. , D corresponding to each of the sensor outputs V _pstd4 , V _pstd3 , V _pstd2 , V _pstd1 obtained by the standard sensor corresponding to each of the density detection images. Similarly, the reflected light _amounts (reference values) V _ref4 , V _ref3 , V _ref2 , and V _ref1 from the reference plate 67 obtained by the light amount detection unit 60 are the sensor outputs V _refstd4 , V _refstd3 , and V _refstd2 of the standard sensor. , V _refstd1 .

In addition, the light quantity characteristic of the light quantity detection unit 60 is adjusted to match the light quantity characteristic of the standard sensor. Therefore, when the light emission amount of the light emitting element 62 of the light amount detection unit 60 is switched from A ₄ → A ₃ → A ₂ → A ₁ to irradiate the reference plate 67, each of the densities A, B, C, and D of the density detection image. Corresponding to the reference values V _ref4 , V _ref3 , V _ref2 , and V _ref1 . In other words, the light emission amount _An is set so that the reflected light amount V _refn is approximately the same as the reflected light amount V _pn from the density detection image having the corresponding density.

For example, the toner amount per unit area is 1 g / m ² , 2 g / m ² , 3 g / m ² corresponding to each of four types of input gradation values C _in 20%, 40%, 60%, and 80%. Four density detection images of 4 g / m ² are formed. Corresponding to each of the four density detection images, if the light emission amount of the standard sensor is 800 mA, sensor outputs 0.5 V, 1.2 V, 1.7 V, and 2.0 V of the standard sensor are acquired. In order to obtain the sensor output 0.5V, 1.2V, 1.7V, 2.0V of the standard sensor when the reference plate is used, the light emission quantity of the standard sensor should be 200 mA, 400 mA, 600 mA, 800 mA. .

Similarly, the light emission amount of the light emitting element 62 of the light amount detection unit 60 may be switched to, for example, 200 mA, 400 mA, 600 mA, and 800 mA. In this embodiment, a plurality of light emitting quantity A _n that is set as described above, is stored in advance as "the light amount detection condition" in the storage unit 90.

(Concentration detection result)
Here, the significance of acquiring the density detection result (V _pn / V _refn ) will be described.
FIG. 17 is a graph showing an application target of the density correction processing according to the first embodiment of the present invention. In the light quantity detection unit 60, the sensor output “V _px ” is acquired from the density correction image corresponding to the input gradation value C _inD . For example, the sensor output of the light amount detection unit 60 is 2.2 V with respect to the input gradation value C _inD 80%. When this phenomenon occurs, there are two types, “Case A” indicated by a white circle (◯) and a one-dot chain line, and “Case B” indicated by a black circle (●) and a solid line. Note that there are cases that fall between “case A” and “case B”.

In case A, the output characteristics of the light quantity detection unit 60 are the same as the output characteristics of the standard sensor, and the toner amount per unit area is “D” in the sensor output “V _px ” represented by a white circle (◯) → It is shifted to “X”. For example, the sensor output increases to 2.2 V as a result of the toner amount per unit area increasing to 4.6 g / m ² . In Case B, the output characteristics of the light quantity detection unit 60 are deviated from the output characteristics of the standard sensor. In the sensor output “V _px ” represented by a black circle (●), the toner amount per unit area is “D”. It remains. For example, the amount of toner per unit area remains 4.2 g / m ² and only the sensor output increases from 2.0 V to 2.2 V.

  In case A, since the toner amount per unit area is deviated, it is necessary to correct the output image density by changing the image forming conditions so that the current density approaches the target density. On the other hand, in case B, since the toner amount per unit area is not deviated, it is not necessary to correct the output image density.

In case B, the amount of reflected light from the reference plate 67 obtained by the light amount detector 60 is also shifted. Accordingly, in consideration of fluctuations in the sensor output due to the deviation of the output characteristics of the light amount detector 60, the ratio (V _pn) between the reflected light amount V _pn from the density detection image and the reference value V _refn that is the reflected light amount from the reference plate. / V _refn ) is acquired as the density detection result. If the density detection result is close to “1”, it corresponds to “Case B”, and it is determined that there is no need to correct the output image density. Then, the deviation amount {(V _pn / V _ref ) −1} from “1” is set as the deviation amount from the target density of the current density.

Conventionally, the density detection result is acquired using one reference value V _ref for the reflected light amount V _pn . For example, when the amount of reflected light V _pn from four density detection images is 0.5 V, 1.2 V, 1.7 V, and 2.0 V, the reference value V _ref is only 2.0 V. In this case, the density detection result (2.0 V / 2.0 V) represents an appropriate amount of toner, but the 2.0 V area and the 0.5 V area do not always change at the same ratio. The density detection result (0.5 V / 2.0 V) obtained by normalizing the 0.5 V area with the 2.0 V area does not always represent an appropriate toner amount.

According to the present embodiment, the reference value V _refn close to the value of the reflected light amount V _pn is set for each of the reflected light _amounts V _pn from the density detection images having different gradations, and (V _pn / V _refn ) is acquired as the density detection result, so that an appropriate density detection result (V _pn / V _refn ) is acquired regardless of the density of the density detection image, and the deviation amount of the current density from the target density is more accurately determined. Be grasped.

(Density correction procedure)
Next, a specific procedure of the density correction process will be described.
FIG. 9 is a flowchart showing the procedure of density correction processing according to the first embodiment of the present invention. The “density correction process” is executed by the CPU 100A of the control unit 100. FIGS. 10A and 10B are schematic diagrams showing the flow of density correction processing according to the first embodiment of the present invention.

  The image forming apparatus starts density correction processing when a predetermined condition is satisfied. A normal image forming operation is not performed during the density correction process. In this embodiment, the number of image formations is counted, and density correction processing is started when the number of image formations exceeds the limit number. The conditions for starting the density correction process may be other conditions. For example, the density correction process may be started when a predetermined period has elapsed. Further, the density correction process may be started when the image forming apparatus is turned on, or the density correction process may be started after each member of the image forming unit is replaced.

First, in step 100, the image forming unit 30 is instructed to form the Y, M, and C density detection image groups G illustrated in FIG. 4A in the region B of the intermediate transfer belt 36. To do. The image forming unit 30 forms a density detection image group G on the intermediate transfer belt 36. The density detection image group G is a YMC three-color image group in which _N patch images P _{1 to} P _N are arranged one-dimensionally.

Next, in step 102, the image forming unit 30 is instructed to form the K density detection image group G illustrated in FIG. 4B in the region D of the intermediate transfer belt 36. The image forming unit 30 forms a density detection image group G on the intermediate transfer belt 36. The density detection image group G is a K-color image group in which M (four in FIG. 4) patch images P _{1 to} P _M are arranged one-dimensionally.

  Next, in step 104, the light amount detection unit 60 is instructed to insert the reference plate 67 between the intermediate transfer belt 36 and the light amount detection unit 60. In the light amount detection unit 60, the reference plate 67 is driven by a drive mechanism (not shown) and is inserted between the intermediate transfer belt 36 and the light amount detection unit 60.

Next, in step 106, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission quantity of the light emitting element 62 for obtaining the reflected light quantity V _ref from the reference plate is obtained. In the present embodiment, the light emission amount of the light emitting element 62 and the switching timing of the light emission amount for acquiring the plurality of reference values V _{ref1 to} V _refN are acquired.

Next, in step 108, the light amount detector 60 measures the amount of diffuse reflected light from the reference plate 67 in the region A of the intermediate transfer belt 36 to obtain a plurality of reference values V _{ref1 to} V _refN. To instruct. In the light amount detection unit 60, the light emitting element 62 is driven to be turned on at the switching timing of the light emission amount and the light emission amount according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The light receiving element 64 receives diffusely reflected light from the reference plate 67. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 110, a plurality of reference values V _{ref1 to} V _refn are acquired.

Next, in step 112, the light amount detection unit 60 is instructed to retract the reference plate 67. In the light amount detection unit 60, the reference plate 67 is driven by a drive mechanism (not shown) and is retracted from between the intermediate transfer belt 36 and the light amount detection unit 60. Next, in step 114, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission quantity of the light emitting element 62 for obtaining the reflected light quantity V _p from the density detection amount image of the YMC three colors is obtained. .

Next, in step 116, the amount of diffusely reflected light from the _N patch images P _{1 to} P _{N of} the YMC three-color density detection image group G formed in the region B of the intermediate transfer belt 36 is measured. Instruct the light amount detection unit 60. In the light quantity detection unit 60, the light emitting element 62 is driven to turn on with the light emission quantity of the light emitting element 62 according to the “light quantity detection condition”, and the density detection image group G is irradiated with the detection light. The light receiving element 64 sequentially receives diffuse reflected light from the _N patch images P _{1 to} P _N. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 118, the reflected light _amounts V _{p1 to} V _pN from the _N patch images P _{1 to} P _N of the YMC three-color image group G are acquired.

Next, in step 120, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission quantity of the light emitting element 62 for obtaining the reflected light quantity V _clean from the image holding body is obtained. In the present embodiment, the light emission amount of the light emitting element 62 and the switching timing of the light emission amount for acquiring the plurality of reference values V _{clean1 to} V _cleanM are acquired.

Next, in step 122, in the region C of the intermediate transfer belt 36, the amount of specularly reflected light from the intermediate transfer belt 36 that is an image carrier is measured to obtain a plurality of reference values V _{clean1 to} V _cleanM. Then, the light amount detection unit 60 is instructed. In the light amount detection unit 60, the light emitting element 62 is driven to be turned on at the switching timing of the light emission amount and the light emission amount according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The regular reflection light from the intermediate transfer belt 36 is received by the light receiving element 63. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 124, a plurality of reference values V _{clean1 to} V _cleanM are acquired.

Next, in step 126, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the emitted light quantity of the light emitting element 62 for obtaining the reflected light quantity V _p from the density detection amount image of K color is obtained. .

Next, in step 128, the amount of specularly reflected light from the _M patch images P _{1 to} P _{M of} the K color density detection image group G formed in the region D of the intermediate transfer belt 36 is measured. Instruct the light amount detection unit 60. In the light amount detection unit 60, the light emitting element 62 is driven to turn on with the light emission amount according to the “light amount detection condition”, and the detection light is irradiated to the density detection image group G. The regular light reflected from the _M patch images P _{1 to} P _M is sequentially received by the light receiving element 63. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 130, the reflected light amounts V _{p1 to} V _pM from the N patch images P _{1 to} P _M of the K color image group G are acquired.

Next, in step 132, the image densities D _{p1 to} D _pN of the _N patch images P _{1 to} P _N of the YMC three-color image group G are acquired according to the following equation (1).

D _pn = (V _pn / V _refn ) × K _std equation (1)

In the above equation (1), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is a normalization coefficient for converting the division result into an integer (0 to 255, 0 to 1023, etc.).

D _pn : patch image density V _pn : patch image reflected light amount (diffuse reflected light amount)
V _ref : Amount of reflected light from the reference plate corresponding to the patch image (diffuse reflected light amount)

Next, in step 134, the image densities D _{p1 to} D _pM of the _M patch images P _{1 to} P _M of the K color image group G are acquired according to the following equation (2).

D _pn = (V _pn / V _cleann ) × K _std equation (2)

In the above equation (2), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is the normalization coefficient.

D _pn : patch image density V _pn : patch image reflected light amount (regular reflected light amount)
V _cleann : Reflected light quantity (regular reflected light quantity) of the image carrier corresponding to the patch image

Next, in step 136, density correction processing corresponding to the acquired image density _Dpn is executed, and the routine is terminated. For example, in the density correction process using the YMC three-color image group G, the output image density is determined using the deviation amount {(V _pn / V _refn ) −1} as the deviation amount from the target density of the detected image density D _pn. Is corrected.

<Second Embodiment>
(Outline of density correction processing)
Next, density correction processing according to the second embodiment of the present invention will be described.
In the first embodiment, by acquiring the density detection result (V _pn / V _refn ), the accurate image density D _pn is obtained regardless of the deviation of the output characteristic of the light amount detection unit 60 from the output characteristic of the standard sensor. Was acquired. However, with this method, if the toner amount of the density correction image on the image carrier is significantly deviated, there is a possibility that an accurate image density D _pn may not be acquired.

For example, when the toner amount per unit area relative to the input gradation value C _{in 20%} becomes 1 g / m ^2, when the toner amount per unit area became 2 g / m ² by characteristic variation of the image forming apparatus It is. In this case, the density detection result (1.2 V / 0.5 V) does not represent an appropriate toner amount.

Therefore, in the density correction processing according to the second embodiment, the output characteristics and light quantity characteristics of the standard sensor are acquired in advance, and the light quantity of the light quantity detector 60 is continuously changed by changing the light emission quantity of the light emitting element 62. Get characteristics. By acquiring the light quantity characteristics of the light quantity detector 60, the sensor output of the light quantity detector 60 can be converted into the sensor output of the standard sensor. By obtaining the density detection result in terms of the sensor output of the standard sensor, the influence of the deviation from the output characteristics of the standard sensor is eliminated, and an accurate image density D _pn is obtained from the density detection result. This will be specifically described below.

,
FIG. 11 is a graph illustrating the procedure of density correction processing according to the second embodiment of the present invention. As in the graph shown in FIG. 16, this graph shows the relationship between the input tone value C _in and the toner amount per unit area (upper right: first quadrant), and the relationship between the toner amount per unit area and the sensor output ( Upper left: second quadrant) and the relationship between the amount of emitted light and the sensor output (lower left: third quadrant). The solid line represents the output characteristics of the photosensors constituting the light amount detection unit 60. An alternate long and short dash line represents an output characteristic or the like of the standard sensor.

As shown in the “first quadrant”, when the toner amount per unit area is not shifted, one point corresponding to each of the four types of input gradation values C _inA , C _inB , C _inC , and C _inD As indicated by the chain line, four density detection images having toner amounts per unit area (that is, target densities) of A, B, C, and D are formed. However, the toner amount per unit area deviates due to various causes. As a result, as indicated by a solid line in the “first quadrant”, the target densities A, B, C, and D are different for each of the four types of input gradation values C _inA , C _inB , C _inC , and C _inD. Four density detection images are formed in terms of density.

Further, as indicated by the one-dot chain line in the “second quadrant”, the sensor outputs V _pstd4 , V _pstd3 , V _{pstd2 of the} standard sensor corresponding to each of the four density detection images of density A, B, C, D. , V _pstd1 is acquired in advance. The sensor output of the standard sensor represents the amount of reflected reflected light measured by the standard sensor with a predetermined amount of emitted light. The “relation between the toner amount per unit area and the sensor output of the standard sensor (standard sensor output characteristics)” indicated by the alternate long and short dash line in the “second quadrant” is acquired in advance and stored in the storage unit 90 or the like. .

The output characteristics of the light amount detector 60 deviate from the output characteristics of the standard sensor due to various causes. As a result, as indicated by the solid line in the “second quadrant”, the sensor outputs V _p4 , V _p3 , and so on of the light quantity detection unit 60 correspond to each of the four density detection images of density A, B, C, D. Each of V _p2 and V _p1 is acquired. Each of the sensor outputs V _p4 , V _p3 , V _p2 , and V _p1 of the light amount detection unit 60 is associated with input gradation values C _inA , C _inB , C _inC , and C _inD . However, the sensor outputs V _pstd4 , V _pstd3 , V _pstd2 , and V _pstd1 of the standard sensor have different values.

Further, as indicated by a one-dot chain line in the “third quadrant”, the sensor output V of the standard sensor corresponds to each of the light emission amounts A ₄ , A ₃ , A ₂ , A ₁ of the detection light irradiated on the reference plate. _refstd4 , _Vrefstd3 , _Vrefstd2 , and _Vrefstd1 are acquired. The “relationship between the light emission quantity and the sensor output of the standard sensor (light quantity characteristic of the standard sensor)” indicated by the one-dot chain line in the “third quadrant” is acquired in advance and stored in the storage unit 90 or the like. As described above, the output characteristics of the light amount detector 60 deviate from the output characteristics of the standard sensor due to various causes. As a result, as indicated by a solid line in the “third quadrant”, the light quantity characteristic of the light quantity detection unit 60 deviates from the light quantity characteristic of the standard sensor in accordance with the fluctuation of the output characteristic.

In the present embodiment, the sensor output of the light amount detector 60 corresponding to the input tone value _CinA is “V _p4 ”. The reference value V _ref4 corresponding to the input tone value C _inA in the light amount detection unit 60 is equal to the sensor output “V _p4 ”. The reference value “V _ref4 ” of the light amount detection unit 60 is converted into the sensor output “V _refstd4R ” of the standard sensor obtained with the same light emission amount (third quadrant). Here, the influence of the deviation from the output characteristics of the standard sensor is eliminated.

The sensor output “V _refstd4R ” of the standard sensor for the reference plate is equal to the sensor output “V _pstd4R ” of the standard sensor for the density correction image (second quadrant). Therefore, the sensor output of the standard sensor is a converted value of the sensor output of the light amount detector 60 "V _{p4" "V} pstd4R _{(V refstd4R)",} the difference between the sensor output of the standard sensor "V _Pstd4" is a predetermined, Obtained as the density detection result (V _pstd4R -V _pstd4 ). Note that the sensor output of the standard sensor is converted value _"V pstd4R (V refstd4R)", the ratio of the predetermined sensor output of the standard sensor "V _Pstd4", even as the concentration detection result _(V pstd4R / V pstd4) Good.

For example, the toner amount per unit area is 1 g / m ² , 2 g / m ² , 3 g / m ² corresponding to each of four types of input gradation values C _in 20%, 40%, 60%, and 80%. Four density detection images of 4 g / m ² are formed. Corresponding to each of the four density detection images, if the light emission amount of the standard sensor is 800 mA, sensor outputs 0.5 V, 1.2 V, 1.7 V, and 2.0 V of the standard sensor are acquired.

The sensor output of the light amount detector 60 at the input gradation value C _in 20% is 1.2 V (V _p4 = V _ref4 ), and the difference between the light amount characteristic of the light amount detector 60 and the light amount characteristic of the standard sensor is 0. Suppose that it is 2V. The sensor output 1.2V of the light quantity detector 60 is converted into a sensor output 1.0V (V _pstd4R = V _refstd4R ) of the standard sensor. Therefore, if the sensor output of the standard sensor preset for the input gradation value C _in 20% is 0.5 V (V _pstd4 ), the difference (V _pstd4R −V _pstd4 ) of 0.5 V is the density detection. As a result.

In this case, the toner amount “A ′” per unit area corresponding to the sensor output 1.0 V (V _pstd4R ) of the standard sensor is 1.5 g / m ² and the unit corresponding to the input gradation value C _in 20%. When the toner amount “A” per area is 1.0 g / m ² , density correction processing is performed so as to reduce the toner amount per unit area by 0.5 g / m ² .

As described above, in the density correction processing according to the second embodiment, the sensor output of the light amount detection unit 60 is converted into the sensor output of the standard sensor, and the sensor output of the standard sensor that is the converted value and the predetermined standard sensor By making the difference or ratio of the sensor output from the sensor output “density detection result”, the influence of the deviation from the output characteristics of the standard sensor (that is, output sensitivity difference) is eliminated, and the accurate image density D _{pn is obtained} from the density detection result. Is acquired.

Incidentally, the value of the density detection results _(V pstdnR -V _pstdn), represents the amount of deviation from the target density of the image density D _pn, the detected image density D _pn is the if equal to the target concentration "0" Become. The image density D _pn is represented by {V _pn − (V _refn −V _refstdn )} when the sensor output of the light amount detection unit 60 is “V _pn ”.

(Density correction procedure)
Next, a specific procedure of the density correction process will be described.
FIGS. 12A and 12B are schematic diagrams showing the flow of density correction processing according to the second embodiment of the present invention. As shown in FIGS. 12A and 12B, the emitted light amount of the light emitting element 62 of the light amount detecting unit 60 is continuously changed to obtain “the emitted light amount and the reflected light amount V _ref (sensor output) from the reference plate”. ”And“ Relation between the amount of emitted light and the amount of reflected light V _clean (sensor output) from the image holding member ”and obtaining the image density D _pn using the density detection result (V _pstdnR −V _pstdn ) and the like. The procedure of the density correction process has many parts that overlap with the procedure of the first embodiment, so the flowchart is omitted and only the differences from the flowchart shown in FIG. 9 will be described.

The light emission amount and the light emission amount of the light emitting element 62 for obtaining “the relationship between the light emission amount and the reflected light amount V _ref from the reference plate” and “the relationship between the light emission amount and the reflected light amount V _clean from the image holding member”. Each of the switching timing, the output characteristic of the standard sensor, and the light quantity characteristic of the standard sensor will be described as being acquired in advance and stored in the storage unit 90 or the like.

In step 106, the light quantity detection condition stored in advance in the storage unit 90 is read, and the light emission quantity of the light emitting element 62 for obtaining the reflected light quantity V _ref from the reference plate is obtained. In the present embodiment, the light emission amount of the light emitting element 62 and the switching timing of the light emission amount for obtaining the “relation between the light emission amount and the reflected light amount V _ref from the reference plate” are obtained. Next, in step 108, the amount of diffusely reflected light from the reference plate 67 is measured in the region A of the intermediate transfer belt 36, and "the relationship between the amount of emitted light and the amount of reflected light _Vref from the reference plate" is obtained. In this way, the light amount detection unit 60 is instructed. In the next step 110, “the relationship between the light emission amount and the reflected light amount V _ref from the reference plate” is acquired.

In step 120, the light amount detection condition stored in advance in the storage unit 90 is read, and the light emission amount of the light emitting element 62 for acquiring the reflected light amount V _clean from the image holding body is acquired. In the present embodiment, the light emission quantity of the light emitting element 62 and the switching timing of the light emission quantity for obtaining the “relation between the light emission quantity and the reflected light quantity V _clean from the image holding member” are obtained. Next, at step 122, in the area C of the intermediate transfer belt 36, by measuring the specular reflection light amount of light from the intermediate transfer belt 36 as an image carrier, "reflected light amount V _clean from the light emission amount and the image carrier The light quantity detection unit 60 is instructed to acquire the “relationship with”. In the next step 124, “the relationship between the light emission amount and the reflected light amount V _clean from the image holding member” is acquired.

Further, in step 132, for each of the _N patch images P _{1 to} P _N of the YMC three-color image group G, the diffuse reflected light amount V acquired with reference to the output characteristics of the standard sensor and the light quantity characteristics of the standard sensor is obtained. Based on _{p1 to VpN} , a density detection result (V _pstdnR -V _pstdn ) is acquired.

Next, in step 134, for each of the _M patch images P _{1 to} P _M of the K-color image group G, the diffuse reflected light amount acquired with reference to the output characteristics of the standard sensor and the light quantity characteristics of the standard sensor is obtained. A density detection result (V _pstdnR −V _pstdn ) is acquired based on V _{p1 to} V _pM .

Next, in step 136, density correction processing is executed according to the acquired density detection result (V _pstdnR −V _pstdn ), and the routine is terminated.

<Third Embodiment>
(Outline of density correction processing)
Next, density correction processing according to the third embodiment of the present invention will be described.
FIG. 13 is a diagram showing the result of density correction processing according to the third embodiment of the present invention. FIG. 13A is a graph showing the output characteristics of the light quantity detector and the standard detector, and FIG. 13B is a graph showing the relationship between the density detection results of the light quantity detector and the standard detector.

As shown in FIG. 13 (A), in this embodiment, depending on the density of the density detection image, a combination of a reference plate output V _Refstd standard sensors corresponding to the reference value V _ref and the reference value V _ref, Any one of a combination of the reference value V _clean and the image carrier output V _cleanstd of the standard sensor corresponding to the reference value V _clean is used. The relationship between the reference value V _ref and the reference plate output V _refstd of the standard sensor, and the relationship between the reference value V _clean and the image carrier output V _cleanstd of the standard sensor are acquired in advance and stored in the storage unit 90 or the like. It will be explained as a thing.

The reference value V _ref is acquired by irradiating the reference plate 67 of the light amount detector 60 with detection light from the light emitting element 62. The reference value V _clean is acquired by irradiating detection light from the light emitting element 62 to the intermediate transfer belt 36 that is an image carrier. By setting the light emission amount of the light emitting element 62 as a predetermined light amount (that is, without changing the light amount), both the reference value V _ref and the reference value V _clean are acquired by inserting or retracting the reference plate 67.

The amount of reflected light when the diffused reflected light from the reference plate 67 is received by the light receiving element 64 is “reference value V _ref1 ”, and the amount of reflected light when the regular reflected light from the reference plate 67 is received by the light receiving element 63 is “reference”. Value V _ref2 ”. The amount of reflected light when the diffused reflected light from the intermediate transfer belt 36 is received by the light receiving element 64 is “reference value V _clean1 ”, and the amount of reflected light when the regular reflected light from the intermediate transfer belt 36 is received by the light receiving element 63 is set. _{Set to} “reference value V _clean2 ”.

In this example, the density of the density detection image changes from A → B → C → D, and accordingly, the sensor output of the light quantity detection unit 60, that is, the reflected light quantity from the density detection image is V _p4 → V _p3. → V _p2 → V _p1 The reference value V _ref and the reference value V _clean are acquired according to the type of density detection image. For the density detection image of each color of YMC, “reference value V _ref1 ” and “reference value V _clean1 ” that are the amount of diffuse reflected light are acquired. Further, for the K-color density detection image, “reference value V _ref2 ” and “reference value V _clean2 ” that are reflected light amounts of the regular reflection light are acquired.

As shown in FIG. 13B, a reference value closer to the sensor output V _pn is selected from the reference value V _ref and the reference value V _clean , and the selected reference value and this reference value are associated. Correction is performed using the output value of the standard sensor. Alternatively, it may be set in advance which one of the reference value V _ref and the reference value V _clean is used according to the density of the density correction image.

When the reference value V _ref is selected, the sensor output V _pn is multiplied by (V _ref / V _refstd ) _obtained by dividing the reference value V _ref by the reference plate output V _refstd of the standard sensor, and {(V _ref / V _refstd ) × V _pn } is the density detection result. When the reference value V _clean is selected, the sensor output V _pn is multiplied by (V _clean / V _cleanstd ) _obtained by dividing the reference value V _clean by the image carrier output V _cleanstd of the standard sensor, and {(V _clean / V _cleanstd ) × V _pn } is taken as the density detection result.

If {(V _ref / V _refstd ) × V _pn } or {(V _clean / V _cleanstd ) × V _pn } is a density detection result, the density detection result by the light quantity detector 60 is the same as the density detection result by the standard sensor. It becomes almost coincident. That is, an accurate image density D _p is acquired regardless of the deviation of the output characteristic of the light amount detection unit 60 from the standard value. In this case, the amount of deviation of the detected image density D _{pn from} the target density is obtained by comparing the density detection result with the density target value indicated by the solid line and obtaining the “difference” or “ratio” between them. Then, density correction processing is performed.

In this example, the reflected light amounts V _p4 , V _p3 , V _p2 , and V _p1 are detected according to the densities A, B, C, and D of the density detection image. For the amount of reflected light V _p4 , {(V _clean / V _cleanstd ) × V _p4 } is taken as the density detection result. For each of the reflected light _amounts V _p3 , V _p2 , and V _p1 , { (V _ref / V _refstd ) × V _p3 }, { (V _ref / V _refstd ) × V _p2 }, { (V _ref / V _refstd ) × V _p1 } is the density detection result.

(Density correction procedure)
Next, a specific procedure of the density correction process will be described.
FIG. 14 is a schematic diagram showing the flow of density correction processing according to the third embodiment of the present invention. FIG. 15 is a flowchart showing the procedure of density correction processing according to the third embodiment of the present invention. The “density correction process” is executed by the CPU 100A of the control unit 100.

  First, in step 200, the image forming unit 30 is instructed to form the Y, M, and C density detection image groups G illustrated in FIG. 4A in the region F of the intermediate transfer belt 36. To do. The image forming unit 30 forms a density detection image group G on the intermediate transfer belt 36.

  Next, in step 202, the image forming unit 30 is instructed to form the K density detection image group G illustrated in FIG. 4B in the region G of the intermediate transfer belt 36. The image forming unit 30 forms a density detection image group G on the intermediate transfer belt 36.

  Next, in step 204, the light amount detection unit 60 is instructed to insert the reference plate 67 between the intermediate transfer belt 36 and the light amount detection unit 60. In the light amount detection unit 60, the reference plate 67 is driven by a drive mechanism (not shown) and is inserted between the intermediate transfer belt 36 and the light amount detection unit 60.

Next, in step 206, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the emitted light quantity of the light emitting element 62 for obtaining the reflected light quantity V _ref from the reference plate is obtained. In the present embodiment, the light emission amount of the light emitting element 62 that is predetermined for acquiring the reference value V _ref1 and the reference value V _ref2 is acquired.

Next, at step 208, the light quantity of the diffusely reflected light from the reference plate 67 is measured to obtain the reference value _Vref1, and the light quantity of the regular reflected light from the reference plate 67 is measured to obtain the reference value _Vref2 . In this way, the light amount detection unit 60 is instructed. In the light amount detection unit 60, the light emitting element 62 is turned on with the light emission amount according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The light receiving element 64 receives diffusely reflected light from the reference plate 67. The regular reflection light from the reference plate 67 is received by the light receiving element 63. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Therefore, in the next step 210, the reference value V _ref1 and the reference value V _ref2 are acquired.

  Next, in step 212, the light amount detection unit 60 is instructed to retract the reference plate 67. In the light amount detection unit 60, the reference plate 67 is driven by a drive mechanism (not shown) and is retracted from between the intermediate transfer belt 36 and the light amount detection unit 60.

Next, in step 214, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission quantity of the light emitting element 62 for obtaining the reflected light quantity V _clean from the intermediate transfer belt 36 is obtained. In the present embodiment, the light emission amount of the light emitting element 62 that is predetermined for acquiring the reference value V _clean1 and the reference value V _clean2 is acquired.

Next, in step 216, in the region E where the density detection image of the intermediate transfer belt 36 is not formed, the amount of diffuse reflected light from the intermediate transfer belt 36 is measured to obtain the reference value V _clean1 and the intermediate transfer. The light amount detection unit 60 is instructed to measure the light amount of the regular reflection light from the belt 36 and obtain the reference value V _clean2 . In the light amount detection unit 60, the light emitting element 62 is turned on with the light emission amount according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The light receiving element 64 receives diffusely reflected light from the intermediate transfer belt 36. The regular reflection light from the intermediate transfer belt 36 is received by the light receiving element 63. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Therefore, in the next step 218, the reference value V _clean1 and the reference value V _clean2 are acquired.

Next, in step 220, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission of the predetermined light emitting element 62 is obtained in order to obtain the reflected light quantity V _p from the density detection amount image of the YMC three colors. Get the amount of light.

Next, in step 222, the amount of diffusely reflected light from the _N patch images P _{1 to} P _{N of} the YMC three-color density detection image group G formed in the region F of the intermediate transfer belt 36 is measured. Instruct the light amount detection unit 60. In the light amount detection unit 60, the light emitting element 62 is driven to turn on with the light emission amount of the light emitting element 62 according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The light receiving element 64 sequentially receives diffuse reflected light from the _N patch images P _{1 to} P _N. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 224, the reflected light _amounts V _{p1 to} V _pN from the _N patch images P _{1 to} P _N of the YMC three-color image group G are acquired.

Next, in step 226, the “light quantity detection condition” stored in advance in the storage unit 90 is read, and the light emission of the predetermined light emitting element 62 is obtained in order to obtain the reflected light quantity V _p from the density detection amount image of K color. Get the amount of light.

Next, in step 228, the amount of specularly reflected light from the _M patch images P _{1 to} P _{M of} the K color density detection image group G formed in the region G of the intermediate transfer belt 36 is measured. Instruct the light amount detection unit 60. In the light amount detection unit 60, the light emitting element 62 is driven to turn on with the light emission amount according to the “light amount detection condition”, and the reference light 67 is irradiated with the detection light. The regular light reflected from the _M patch images P _{1 to} P _M is sequentially received by the light receiving element 63. The light amount detection unit 60 outputs a detection signal indicating the reflected light amount to the control unit 100. Accordingly, in the next step 130, the reflected light amounts V _{p1 to} V _pM from the N patch images P _{1 to} P _M of the K color image group G are acquired.

Next, in step 230, a first density acquisition process for acquiring the image densities D _{p1 to} D _pN of the _N patch images P _{1 to} P _N of the YMC three-color image group G is executed. Next, in step 232, performing a second concentration acquiring process for acquiring an image density _D p1 _{to D pM} of the M patch image _P 1 to P _M of the image group G of K color. Next, in step 234, density correction processing corresponding to the acquired image density _Dpn is executed, and the routine is terminated.

(First concentration acquisition process)
Next, the “first density acquisition process” for the YMC three-color density detection image will be described. At a first concentration acquisition process, when the reflected light amount V _pn is closer to the reference value V _ref1, based on the relationship between the reference value V _ref and the reference plate output V _Refstd standard sensors, corresponding to the reference value V _ref1 V _refstd1 is acquired, and the image density D _pn of the patch image P _n of the reflected light amount V _pn is acquired according to the following equation (5).

D _pn = {(V _ref1 / V _refstd1 ) × V _pn } × K _std equation (5)

In the above equation (5), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is the normalization coefficient described above.

D _pn : patch image density V _pn : patch image reflected light amount (diffuse reflected light amount)
V _ref1 : _Light amount reflected from the reference plate (diffuse reflected light amount)
V _refstd1 : Standard sensor reference plate output (diffuse reflected light amount)

On the other hand, if the amount of reflected light V _pn is closer to the reference value V _Clean1, based on the relationship between the reference value V _clean and the image carrier output V _Cleanstd standard sensors, obtains the V _Cleanstd1 corresponding to the reference value V _Clean1 , according to the following equation (6), and acquires the image density _{D pn} of the patch image _{P n} of the reflected light amount _{V pn.}

D _pn = {(V _clean1 / V _cleanstd1 ) × V _pn } × K _std equation (6)

In the above equation (6), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is the normalization coefficient described above.

D _pn : patch image density V _pn : patch image reflected light amount (diffuse reflected light amount)
V _clean1 : Reflected light amount of the image carrier (diffuse reflected light amount)
V _cleanstd1 : Standard sensor image carrier output (diffuse reflected light amount)

(Second concentration acquisition process)
Next, the “second density acquisition process” for the K color density detection image will be described. At a second concentration acquiring process, when the reflected light amount V _pn is closer to the reference value V _ref2 based on the relationship between the reference value V _ref and the reference plate output V _Refstd standard sensors, corresponding to the reference value V _ref2 V _refstd2 is acquired, and the image density D _pn of the patch image P _n of the reflected light amount V _pn is acquired according to the following equation (7).

D _pn = {(V _ref2 / V _refstd2 ) × V _pn } × K _std equation (7)

In the above equation (7), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is the normalization coefficient described above.

D _pn : patch image density V _pn : patch image reflected light amount (regular reflected light amount)
V _ref2 : _Amount of reflected light from the reference plate ( amount of regular reflected light)
V _refstd2 : Standard sensor reference plate output (regular reflection light quantity)

On the other hand, if the amount of reflected light V _pn is closer to the reference value V _Clean2, based on the relationship between the reference value V _clean and the image carrier output V _Cleanstd standard sensors, obtains the V _Cleanstd2 corresponding to the reference value V _Clean2 , according to the following equation (8), to obtain an image density _{D pn} of patch images _{P n} of the reflected light amount _{V pn.}

D _pn = {(V _clean2 / V _cleanstd2 ) × V _pn } × K _std equation (8)

In the above equation (8), each parameter of the _nth patch image _Pn is defined as follows. “K _std ” is the normalization coefficient described above.

D _pn : patch image density V _pn : patch image reflected light amount (regular reflected light amount)
V _clean2 : Reflected light amount of image carrier (regular reflected light amount)
V _cleanstd2 : Standard sensor image carrier output (regular reflection light quantity)

<Modification>
In the first and second embodiments described above, the example in which the light emission amount of the light emitting element is changed to obtain a plurality of reference values has been described, but the temperature dependency of the light emission amount of the light emitting element is large. When the light emission amount of the light emitting element is an error factor with the output characteristic of the light amount detection unit at the time of density correction image detection, the light receiving sensitivity of the light receiving element may be changed by changing the electrical amplification factor, The light receiving sensitivity of the light receiving element may be changed by narrowing the optical path. That is, it is only necessary that the output characteristics of the optical sensor at the time of density correction image detection be close.

In the first and second embodiments described above, the K color density detection image is corrected using the amount of reflected light from the image carrier as the “reference value V _clean ”. Since the reference value V _clean varies depending on the surface condition or the like, the reflected light amount is measured for an area larger than the density detection image, and the representative value (for example, average value) of the measured reflected light amount is set as the “reference value V _clean ”. Also good. In addition to the “reference value V _clean ”, the representative value of the measured reflected light amount may be used for the reflected light amount of the reference plate, the reflected light amount of the image holding member, and the reflected light amount of the density detection image. For example, when 20 measured values are collected and an average value excluding the maximum and minimum values is used as the representative value, a stable measurement result is obtained.

In addition, correction is performed with the amount of reflected light from the image carrier as “reference value V _clean ”, but there is a possibility that the amount of reflected light is larger than the amount of reflected light “reference value V _ref ” of the reference plate. "reference value V _clean" reflections used to adjust the specular reflection mirror light receiving element based on a "reference value V _ref" of the specular reflection to be constant, only when the specular reflection output taken "reference value V _clean" It is also possible to devise such as adjusting the amount of emitted light so that is constant.

  Further, the reference plate may be disposed inside the housing of the light amount detection unit. However, as exemplified in the above embodiment, the configuration of the external reference plate disposed outside the housing of the light amount detection unit is less affected by the contamination of the detection window.

  In the first and second embodiments described above, examples have been described in which correction is performed by setting different reference values for each of the density detection images. However, a plurality of density detection images having similar sensor output values are used. For this, a common reference value may be used.

  In the above embodiment, the example in which the reference value is acquired immediately before detecting the reflected light amount of the density detection image has been described. However, in order to shorten the time, the reference value acquired at another timing is used. Also good.

Further, in the above embodiment, the description of the expression is narrowed down to an important part. For example, the sensor dark current V _dark (the sensor output of the light receiving elements 63 and 64 in a state where the light emitting element 62 does not emit light is used to detect the density Is subtracted from the numerator and denominator to obtain D _pn = (V _pn −V _dark ) / (V _refn −V _dark ) × K _std as equation (1). Including changes in formulas within the scope of the subject matter of this project, such as use.

  Further, the configuration described in each of the above embodiments is an example, and it goes without saying that the configuration may be changed without departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 10 Operation display part 20 Image reading part 30 Image forming part 32 Image forming unit 34 Roller 34A Drive roller 34D Follower roller 34C Tension applying roller 34B Back support roller 36 Intermediate transfer belt 38 Secondary transfer apparatus 39 Fixing apparatus 40 Paper supply part 50 Paper Discharge unit 60 Light amount detection unit 61 Case 61A Opposing surface 62 Light emitting element 62A Light guide path 63 Light receiving element 63A Light guide path 64 Light receiving element 64A Light guide path 65 Substrate 66 Window part 67 Reference plate 70 Image processing part 80 Communication part 90 Storage part 100 Control Part G Density detection image group P Density detection image (patch image)

Claims (9)

Image forming means for forming a plurality of density detection images having different gradations on the image carrier;
Measure at least one of the reflected light amount of the reference plate and the reflected light amount of the image holding body while changing the measurement conditions in stages, and for detecting the plurality of densities formed on the image holding body under predetermined measurement conditions Measuring means for measuring the amount of reflected light of each image;
Storage means for storing a plurality of reflected light amounts measured while changing the measurement conditions step by step as a plurality of reference values according to a plurality of gradations of the density detection image;
A density acquisition means for acquiring a detection density of the density detection image of the measurement target based on the amount of reflected light of the density detection image of the measurement target and a reference value corresponding to the gradation of the density detection image;
Concentration detection device having When the reflected light amount of the reference plate and the reflected light amount of the image carrier are measured by the measuring means,
The amount of reflected light from the reference plate is stored in the storage unit as a plurality of first reference values, and the amount of reflected light from the image carrier is stored in the storage unit as a plurality of second reference values.
The concentration detection apparatus according to claim 1.   The concentration detection apparatus according to claim 1, wherein the measurement condition includes at least one of a light emission amount and a light reception sensitivity. The concentration detection apparatus according to any one of claims 1 to 3,
Correction means for correcting the output image density based on a plurality of detected densities corresponding to the plurality of density detection images acquired by the density detection device;
An image forming apparatus. Image forming means for forming a plurality of density detection images having different gradations on the image carrier;
Measuring means for measuring the reflected light amount of the reference plate while continuously changing the measurement conditions, and for measuring the reflected light amounts of the plurality of density detection images formed on the image carrier under predetermined measurement conditions. When,
Storage means for storing in advance the relationship between the detection density of the image for density detection by the standard detector and the output value of the standard detector, and the relationship between the light emission amount of the standard detector and the reference plate output value of the standard detector,
Based on the relationship between the light emission amount of the standard detector and the reference plate output value of the standard detector, the reference plate output value of the standard detector corresponding to the reflected light amount of the density detection image to be measured is obtained, Based on the relationship between the detected density of the density detection image by the standard detector and the output value of the standard detector, the density detection image of the standard detector corresponding to the acquired reference plate output value of the standard detector is obtained. Density acquisition means for acquiring a detection density and acquiring the acquired detection density by the standard detector as a detection density according to the gradation of the density detection image to be measured;
Concentration detection device having Image forming means for forming a plurality of density detection images having different gradations on the image carrier;
Measuring means for measuring the amount of reflected light of the image carrier while continuously changing the measurement conditions, and measuring the amount of reflected light of the plurality of density detection images formed on the image carrier under predetermined measurement conditions When,
Storage means for storing in advance the relationship between the detection density of the image for density detection by the standard detector and the output value of the standard detector, and the relationship between the light emission amount of the standard detector and the image carrier output value of the standard detector,
Based on the relationship between the light emission amount of the standard detector and the image carrier output value of the standard detector, the image carrier output value of the standard detector corresponding to the reflected light amount of the density detection image to be measured is acquired. Based on the relationship between the detected density of the density detection image by the standard detector and the output value of the standard detector, the density detection by the standard detector corresponding to the acquired image carrier output value of the standard detector A density acquisition means for acquiring a detection density of the image for detection, and acquiring the acquired detection density by the standard detector as a detection density according to the gradation of the density detection image to be measured;
Concentration detection device having The concentration detection apparatus according to claim 5 or 6,
Based on the relationship between the gradation of the density detection image and the target density of the density detection image, and a plurality of detection densities corresponding to the plurality of density detection images acquired by the density detection device, the output image density is determined. Correction means for correcting;
An image forming apparatus. Image forming means for forming a plurality of density detection images having different gradations on the image carrier;
The amount of reflected light from the reference plate and the amount of reflected light from the image carrier are measured under predetermined measurement conditions, and the amount of reflected light from the plurality of density detection images formed on the image carrier is measured under predetermined measurement conditions. Measuring means for measuring;
The amount of reflected light from the reference plate measured by the measuring means is stored as a first reference value, and the amount of reflected light from the image carrier measured by the measuring means is stored as a second reference value. Storage means;
Second storage means for storing in advance the relationship between the first reference value and the reference plate output value of the standard detector, and for storing in advance the relationship between the second reference value and the image carrier output value of the standard detector. When,
When the reflected light amount of the density detection image to be measured is closer to the first reference value than the second reference value, the reflected light amount of the density detection image to be measured, the first reference value, and the reference of the standard detector Based on the plate output value, the detection density of the density detection image to be measured is acquired,
When the reflected light amount of the density detection image to be measured is closer to the second reference value than the first reference value, the reflected light amount of the density detection image to be measured, the second reference value, and the image of the standard detector Density acquisition means for acquiring the detected density of the density detection image to be measured based on the holding body output value ;
Concentration detection device having A concentration detection apparatus according to claim 8;
Correction means for correcting the output image density based on a plurality of detected densities corresponding to the plurality of density detection images acquired by the density detection device;
An image forming apparatus.