JP2001109219A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color image forming device capable of controlling gradation without temporarily stopping image formation in the middle of consecutive printing. SOLUTION: As in the conventional manner, general density gradation control is performed when the power source is turned on, reflected light quantity data obtained by measuring an image pattern (half tone) obtained in the case of the general density/tone control is defined as initial data. A pattern is formed on a non-image area between the leading and the rear ends of a formed image during the consecutive printing. Since all the image can not be formed, two patterns printing 2-dot and 8-dot patterns among the dot matrix of 4×4 are formed between first and second transfer materials, 4-dot and 10-dot patterns between second and third transfer materials, and 6-dot and 12-dot patterns between third and fourth transfer materials by using yellow toner first. The initial data are corrected by new data obtained by measuring the patterns, the tone correction coefficient x=f-1 (y) for yellow is updated whenever it is necessary. The updating of the gradation correction coefficients for magenta, cyan and the black are similarly and repeatedly performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus such as an electrophotographic color printer or copier, and more particularly to a density control and a gradation control thereof.

[0002]

2. Description of the Related Art FIG. 7 shows an electrophotographic color image forming apparatus as an example of a conventional image forming apparatus. Hereinafter, description will be given with reference to the drawings.

The image forming apparatus has a drum-shaped electrophotographic photosensitive member as an image carrier, that is, a photosensitive drum 1. The photosensitive drum 1 is driven to rotate in a direction indicated by an arrow in FIG. In the process, the surface is uniformly charged by the primary charger (charging roller) 2. Next, the exposure device (laser scanner) 3 irradiates a laser beam according to a first color image pattern, for example, a yellow image pattern, to form a first color electrostatic latent image on the surface of the photosensitive drum 1.

[0004] The latent image formed on the photosensitive drum 1 is developed by a developing device 4y containing yellow toner facing the photosensitive drum 1 in advance as the photosensitive drum 1 rotates, and is visualized as a yellow toner image. You. Developing device 4
Y, 4m, 4c and 4k are supported by a rotary support (rotary drum) 5, and a predetermined developing device is rotated to a position facing the photosensitive drum 1 prior to development.

[0005] The toner image obtained by the development is transferred onto the surface of an intermediate transfer belt (belt-shaped intermediate transfer body) 6 rotating in the direction of the arrow at substantially the same speed as the photosensitive drum 1 on a primary transfer roller 7.
The image is transferred by the primary transfer bias applied to a (primary transfer). The transfer residual toner remaining on the photosensitive drum 1 is
It is removed by cleaning means 8 using a blade.

The above-described steps of charging, exposure, development and primary transfer are performed for each color of magenta, cyan and black following yellow, and a toner image of four colors of yellow, magenta, cyan and black is formed on the intermediate transfer belt 6. Are superimposed to obtain a color image.

The four color toner images superimposed on the intermediate transfer belt 6 are collectively collected by a pickup roller 9 of a conveying means.
Is transferred to the surface of the transfer material sent to the intermediate transfer belt 6 at a predetermined timing by the secondary transfer bias applied to the secondary transfer roller 7b (secondary transfer).

The transfer material onto which the four color toner images have been transferred is sent to a fixing device 11 by a conveyor belt 10, where heat and pressure are applied to fuse and fix the toner to the transfer material.
The result is a final full-color image.

Not only the image forming apparatus of the present embodiment but also an image forming apparatus for outputting a full-color image generally has a mechanism for adjusting the density of the output image. Particularly, in an image forming apparatus intended to output a high-quality color image, a mechanism for performing gradation correction (γ correction) is often added in order to obtain a desired color balance in all gradations.

In the density detection of the present embodiment, a toner image of a specific halftone pattern is formed on the photosensitive drum 1 by area gradation, and the amount of reflected light of the halftone pattern on the photosensitive drum 1 is determined by a light emitting element and a light receiving element. The measurement is performed by a reflected light amount sensor (density sensor) 14.
Since the image density is controlled by image forming conditions such as the charging potential of the photosensitive drum 1, the exposure potential after laser exposure, and the developing bias potential, one or a combination of these image forming conditions is changed stepwise to obtain a plurality of image densities. Is formed, and the amount of reflected light is measured by the reflected light amount sensor 14 to obtain image forming conditions which are estimated to obtain a desired constant density (reflected light amount).

The above-mentioned reflected light amount sensor 14 uses infrared light and is capable of estimating the amount of toner on the photosensitive drum 1 irrespective of the color of the toner. Although the amount of infrared light received by the reflected light amount sensor 14 is substantially proportional or inversely proportional to the amount of adhered toner (toner amount of the developed image), the amount of adhered toner and the density of the output image are not generally proportional. However, since the amount of adhered toner and the density of the output image have a one-to-one correlation, the density of the output image can be estimated from the measurement value of the reflected light amount sensor 14.

The density / gradation control of the image forming apparatus of this embodiment will be described in detail below.

In this embodiment, the surface potential of the photosensitive drum 1 is -6.
It is charged to be 00V, and the potential of the laser exposed portion is approximately -200V at normal temperature and normal humidity (23 ° C, 60% Rh).
It is assumed that the sensitivity of the photosensitive drum 1 and the exposure amount of the laser are adjusted so that As a detection image pattern, a halftone pattern for printing 9 dots as shown in FIG. 3 in a 4 × 4 dot matrix is used. As the developing bias, a DC voltage obtained by superimposing a rectangular wave (frequency 2000 Hz, peak-to-peak voltage (Vpp) 1600 V) on a DC voltage as shown in FIG. 8 is used. Control.

As shown in FIG. 9, a density control image pattern Q is formed on a portion of the intermediate transfer belt 6 where the reflected light amount sensor 14 is installed, as shown in FIG. Are printed at a proper interval of 30 mm square. Each image pattern is developed on the photosensitive drum 1 with a developing bias of a different DC component, and the reflected light amount is measured by the reflected light amount sensor 14 for each of the image patterns. In this example, the number of image patterns to be formed is set to 5 for each color, and the DC component Vdc of the developing bias is set to −30.
The voltage was changed from 0 V to -500 V in steps of 50 V.

FIG. 10 shows an example of the result of measuring the amount of reflected light. In this example, the target value (appropriate density value) of the reflection density of the halftone pattern is set to 1.0, and the subsequent image is developed under the developing condition (in this example, the DC voltage component of the developing bias) estimated to be closest to this. It is controlled to perform formation. For example, in the case where five reflection density data indicated by circles in FIG. 10 are obtained, the development condition that the reflection density becomes 1.0 is that the DC component Vdc is between -400 V and -450 V, and this section Assuming that the DC component and the reflection density are approximately proportional to each other, it is estimated that the reflection density becomes 1.0 when the DC component is internally divided from the reflection densities of -400 V and -450 V and is about -420 V. Is done. Therefore, the DC component Vdc of the developing bias is controlled to -420 V as the subsequent image forming conditions.

As in the above-described density control, the gradation control is performed on a portion of the intermediate transfer belt 6 where the reflected light amount sensor 14 is installed, as shown in FIG. A plurality of halftone pattern images (patch images) are printed at intervals. However, the image forming conditions are determined as described above (the DC component Vdc of the developing bias is -420 V), halftone patterns having different printing ratios are drawn, and the amount of reflected light is measured for each of them. In this example, a halftone pattern for printing 2, 4, 6, 8, 10, and 12 dots in the 4 × 4 dot matrix shown in FIG. 3 is used.

FIG. 11 shows an example of the measurement result of the reflection density. The measured reflection densities of the 2, 4, 6, 8, 10, and 12 dot halftone patterns are plotted with white circles in the figure. The straight line in the figure indicates that the reflection density of the 9-dot halftone pattern is 1.0, and that the printing ratio per dot (6.25% in this example) and the density are proportional (ideal). (A case where a large area gradation expression is established).

The printing ratio of the halftone pattern is x
, And the measured reflection density is represented by y, and a curve connecting white circles in the figure is converted into a spline function and represented by y = f (x). This inverse function x = f ^-1 (y) is calculated and stored in the control device. Thereafter, the gradation is approximated to the straight line in FIG. 11 by converting the halftone printing ratio x 'corresponding to the input density information y' and outputting the halftone printing ratio x 'based on the above inverse function and gradation correction function. Can be done.

Image forming conditions may be greatly affected by fluctuations in sensitivity of the photosensitive drum 1 (fluctuations due to temperature and humidity and fluctuations in durability), and variations in sensitivity and charging characteristics during the production of the photosensitive drum and toner. The density control and the gradation control may be affected to some extent by the above-described density control and gradation control, and a stable image formation can be performed.

The color image forming apparatus is used when the power is turned on, when returning from a sleep state for labor saving, when replacing the photosensitive drum 1 and the developing device 4 (4 m to 4 k), and when supplying toner. In many cases, there is a density / tone control sequence that automatically operates the density control and the tone control. In some cases, the density / gradation control sequence is operated after a fixed number of printed sheets or a fixed image forming time in consideration of durability fluctuation.

[0021]

When the above-described density / tone control sequence is operated, there is an inconvenience that image formation must be temporarily stopped, and the frequency / tone control sequence is frequently performed. Do not go to For this reason, measures such as not performing the density / gradation control sequence during continuous printing may be taken.

However, if the execution interval (interval) of the density / tone control sequence is too long, there is a problem in particular that the tone varies, and the tint of the halftone image may change slightly. For example, if the same photographic image is continuously printed on 100 or more sheets, the color tone of the first printed image and the last printed image may change. Further, the change in gradation is also affected by the printing ratio and the like, and the number of printed sheets / time in which the change in gradation falls within a certain range varies depending on the image to be formed. In a full-color image, since the gradations of the four colors fluctuate independently of each other, a gradation control sequence must be frequently used to maintain a delicate color tone.

As described above, conventionally, continuous printing without pause and maintenance of delicate color tone / gradation were in conflict with each other.

An object of the present invention is to provide a color image forming apparatus capable of eliminating a gradation control sequence for temporarily stopping image formation and performing gradation control even during continuous printing.

[0025]

The above object is achieved by a color image forming apparatus according to the present invention. In summary, the present invention provides a density detecting means for detecting the density of an image pattern for density detection formed in a non-image area during an image forming process, and performs density control and gradation control based on the detected density. A color image forming apparatus having a control unit, wherein the density detection by the density detection unit is performed a plurality of times for each color to perform data correction of density control and gradation control. It is.

According to the present invention, when image formation is performed continuously, density detection is performed by forming an image pattern for density detection in a non-image area sandwiched between the front end and the rear end of the image to be formed.
The density detection image pattern to be formed next and / or the image formation period is controlled based on the density data obtained by the density detection means.

According to the present invention, a plurality of image carriers and a belt-like conveying means for carrying a transfer material for transferring an image formed on the image carriers and sequentially conveying the transfer materials to the image carriers are provided. After the image pattern for density detection is formed on the image carrier, the image pattern is transferred to the belt-shaped conveyance means, and the density of the image pattern for density detection is detected on the belt-shaped conveyance means. Alternatively, the image forming apparatus further includes a plurality of image carriers, and a belt-shaped intermediate transfer body that temporarily transfers an image formed on the image carrier before transferring the image to a transfer material, and the image pattern for density detection is formed on the image carrier. Then, the image is transferred to the belt-shaped intermediate member, and the density of the density detection image pattern is detected on the belt intermediate member.

[0028]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a sectional view showing an embodiment of the color image forming apparatus of the present invention. The image forming process in the present image forming apparatus is basically the same as that of the conventional image forming apparatus shown in FIG. 7 except for a part thereof. Only the characteristic portions of the present invention will be described below.

Also in the present embodiment, when the power of the image forming apparatus is turned on, when the image forming apparatus returns from the sleep state for power saving, when the photosensitive drum 1 and the developing device 4 (4y to 4k) are replaced, and when toner is supplied. Performs one type of density control and gradation control as in the conventional example. The density control and the gradation control basically adopt the method shown in the conventional example. The reflected light amount data of each halftone image obtained at this time is hereinafter referred to as “initial data”. In this embodiment, the amount of reflected light of the halftone pattern transferred onto the intermediate transfer belt 6 is measured by the reflected light amount sensor 12 installed opposite the intermediate transfer belt 6.

The density detection and gradation control during continuous printing, which are features of the present invention, will be described.

In the present embodiment, as shown in FIG. 2, during continuous printing, the density detection image pattern Q is formed in a non-image area (hereinafter referred to as "between images") sandwiched between the front and rear ends of the image to be formed.
To form In the case of using the intermediate transfer belt as in the present embodiment, “peripheral length of intermediate transfer belt” − “length of transfer material” corresponds to the above “between images”. The reflected light amount sensor 12 measures the reflected light amount at the timing when the density detection image pattern Q formed “between images” on the intermediate transfer belt 6 passes below the sensor 12.

Since the "between images" area is narrow, it is not possible to form all of the image patterns. Therefore, the image pattern to be printed here is divided into several parts and distributed to “between images”.

Specifically, between the first and second sheets of the transfer material to be continuously printed, first, yellow toner is used and two of the 4.times.4 dot matrices shown in FIG.
Two halftone patterns for printing dots and eight dots, four dots between the second and third sheets, two halftone patterns for printing ten dots, and six halftone patterns between the third and fourth sheets Two halftone patterns for printing dots and 12 dots are formed.

The "initial data" of each halftone measured first is corrected with the "new data" of each halftone measured by the reflected light amount sensor 12, and a tone correction function x = f ^-1 (yellow) for yellow is measured. y) is updated as needed. The correction of the “initial data” is performed by giving appropriate weights to the “initial data” and the “new data” in consideration of avoiding a measurement error of the reflected light amount sensor 12 and a sudden change in color. May be.

The same tone correction function update as described above is performed for each of the colors magenta, cyan, and black, and when all are completed, the process returns to yellow and repeats. In the above, one cycle corresponds to printing of 12 sheets. When printing is completed in the middle of one cycle, if the above-described gradation control is started from the color at that time, gradation control can be performed almost equally for each color.

When the gradation control does not need to be performed so frequently, an interval period in which no image patch is formed (no gradation control is performed) is set. For example, "between images"
After printing 12 sheets to form an image patch on
For example, 40 sheets may be operated as one cycle without forming an image forming patch.

Naturally, the number, type, size and shape of the image patches to be formed "between the images"
However, the method is not limited as long as the reflected light amount can be detected with sufficient accuracy and the throughput (productivity) of image formation is not delayed. Further, the order of the colors of the image patches to be formed, how to insert the interval period, and the like are not limited.

Specifically, a 4 × 4 dot matrix shown in FIG. 3 is provided between a first transfer material and a second transfer material to be continuously printed using yellow toner. Three halftone patterns for printing two dots, six dots, and ten dots are formed, and the same image patch is changed to cyan for the second image and the third image for the third image, and the same image patch is changed to magenta for the third image. The 4 × 4 dots shown in FIG. 3 are formed between the fourth and fifth sheets by changing the color similarly between the fourth and fifth sheets, and after the ninth sheet with four intervals. Printing 4 dots, 8 dots, and 12 dots out of the matrix 3
One halftone pattern may be formed in the same manner as above in the order of yellow, cyan, magenta, and black, and one cycle may be completed at intervals of four sheets (one cycle of 16 sheets).

Embodiment 2 FIG. 4 is a sectional view showing a color image forming apparatus according to another embodiment of the present invention. In the present image forming apparatus, there is an electrostatic transfer belt 19 that runs while being suspended by an odd number of rollers. Inside and around the transfer belt 19, transfer chargers 20y, 20m, 20c, 20k, and a suction charger 2 are provided.
1, suction roller 23, inner charger 25, outer charger 2
6. A separation charger 27 is provided. Of these chargers,
Transfer roller 20 as transfer charger (20y to 20k)
Are arranged as a part of the image forming units by the number of image forming units.

The photosensitive drum 15y as an image carrier and the primary charger 16 are arranged so as to face the transfer belt 19 described above.
y, an LED exposure device 29y serving as an image signal applying means for an electrostatic latent image type, a developing device 18y for storing yellow toner,
There is an image forming unit P1 including a cleaning blade 30y as a cleaning unit, a waste toner container 31y, and a transfer roller 20y provided on the opposite side of the transfer belt 19 to face the photosensitive drum 15y. Similarly to the image forming section P1, the photosensitive drum 15, the primary charger 16, the LED exposing device 29, and the developing device 1 for storing magenta, cyan, and black toners, respectively.
8. There are image forming units P2, P3, and P4 each including the cleaning blade 30, the waste toner container 31, and the transfer roller 20 (in FIG.
subscripts c and k). These image forming units P1 to P4
Are arranged on a straight line with respect to the traveling direction of the transfer belt 19.

The attraction charger 21 inside the transfer belt 19 and the attraction roller 23 serving as the opposite pole thereof electrostatically attract the transfer material 8 supplied by the paper feed roller 22 onto the transfer belt 19 prior to image formation. I do. Each image forming unit P1 to P4
The timing is such that the yellow toner image, the magenta toner image, the cyan toner image, and the black toner image sequentially overlap predetermined positions on the transfer material 8 as the transfer material 8 electrostatically adsorbed on the transfer belt 19 travels. To form an image. The formed toner image of each color is transferred to the transfer roller 2.
The image is superimposed and transferred onto the transfer material 8 sequentially by the transfer bias applied to the transfer material 8.

After the transfer, the transfer material 8 is transferred to the separation charger 27.
Then, the toner image is separated from the transfer belt 19 by the separating device 28 and reaches the fixing device 17, where the toner image is melted and mixed by heating and pressurization to form a full-color image.
After being fixed and fixed to the transfer material 8, the sheet is discharged out of the apparatus. The transfer residual toner and paper dust remaining on the transfer belt 19 are removed by a cleaning brush 32 of a cleaning unit that is in contact with the transfer belt 19, and stored in a container 33. Further, the transfer belt 19 is charged inside and outside. Vessels 25, 2
The charge is removed by 6 so as to be ready for the next image forming operation.

The image forming apparatus of this embodiment is of a so-called tandem type, and has a higher throughput of full-color image formation than the one in which four colors are sequentially superimposed on an intermediate transfer member as in the first embodiment. It is characterized by being fast.

In this embodiment, as shown in FIG. 5, during continuous printing, a non-image area on the transfer belt 19 sandwiched between the rear end of the transfer material 18a and the front end of the next transfer material 18b, The image pattern Q for density detection is formed in “between”. The reflected light amount of the density detection image pattern (halftone pattern) transferred on the transfer belt 19 is, as shown in FIG.
Measured at 4. The installation positions are the four image forming units P1.
~ P4 downstream. The reflected light amount sensor 34 measures the reflected light amount at the timing when the density detection image pattern Q formed between the “images” passes under the sensor 34. The density control and the gradation control basically employ the method shown in the conventional example.

In the present embodiment, unlike the first embodiment, the length in the feed direction "between images" can be set relatively freely. However, unnecessary “between images” merely sets the throughput speed, which is a feature of the present embodiment, to a low value, and is usually set to be as short (narrow) as possible. Therefore, in the present embodiment, image patches required for gradation control are distributed one by one to “between images”. Also in this embodiment, it is assumed that the six halftone patterns of 2, 4, 6, 8, 10, and 12 dots shown in FIG. 3 are used. "Between images"
If there is not enough room to form one image patch, the density detection image pattern (patch image) is formed by widening "between images" at the expense of some throughput.

Hereinafter, the control procedure of this embodiment will be specifically described according to the control flow shown in FIG.

Step 1 (abbreviated as S1 in the figure; the same applies hereinafter): Also in this embodiment, when the power of the image forming apparatus is turned on, when the image forming apparatus returns from the sleep state for power saving, or when the photosensitive drum and the developing device are When replacing or refilling toner,
One type of density control and gradation control are performed. The reflected light amount data of each halftone obtained by this gradation control is hereinafter referred to as “R−x
i] (x is the measured color, y (yellow), m
(Magenta), c (cyan) or k (black), i is the number of halftone dots measured, 2, 4,
6, 8, 10, or 12. Hereinafter the same). A counter “C-xi” corresponding to each “R-xi” is set, and an initial value as shown in Table 1 is given.

[0049]

[Table 1] Step 2: It is determined whether or not all “C-xi” are “0”. If there is “0”, “C-xi” is set to a numerical value “2”.
If "3" is given and there is no "0" in step 3, all the numerical values of "C-xi" are decremented by 1, and the process proceeds to step 8.

Step 3: A color and halftone image pattern corresponding to “C-xi” is formed “between images”.
According to Table 1, 12 dots of yellow (print ratio 75
%) Is formed. If there is no room for forming one image pattern between “images”, the image pattern is formed by expanding “between images”.

Step 4: The amount of reflected light of the image pattern formed in step 3 is measured to obtain measurement data "N-xi". At this time, the measured value of N-y12 is "86h" (16
Base notation).

Step 5: “N−x” obtained in step 4
If the absolute value of the difference between “i” and “R-xi” is within the allowable error data “Δ-xi” as shown in Table 2, go to step 6.
Otherwise, go to step 7. Where R-y
Assuming that the value of No. 12 is “84h”, Δ−y in Table 2 is obtained.
Since the value of “12” is within the range of the allowable error data from “06h”, the process proceeds to step 6. Here, the printing ratio is 75%
It is determined that there was almost no change in the yellow halftone area.

[0053]

[Table 2] Step 6: Add the counter number "A-xi" as shown in Table 3 to "C-xi", where the counter number of Cy12 is "23" in step 2 and "72" of Ay12. In addition, it becomes "95".

[0054]

[Table 3] Step 7: After performing appropriate “weighting” from “N-xi” and “R-xi”, “R-xi” is corrected, and the yellow gradation correction function is updated. However, the update timing is performed after the formation of one yellow image is completed, and is not updated during image formation.

Step 8: When the image formation is completed, "R-xi" and "C-xi" are stored in the memory in the main body to prepare for the next image formation. Before the next image formation, if the photosensitive drum or developing unit is replaced, toner is replenished, the power is turned off, the power is turned off, or the main unit is reset. In step 1, otherwise, image formation is started from step 2.

In this embodiment, the counter “Cx
The image pattern is formed in ascending order of the initial value of “i”, and gradation correction is performed. However, as described above, since only one image pattern can be formed “between images”, one cycle of gradation correction is performed. Are 24 sheets.

To perform more accurate gradation correction, more frequent gradation correction is necessary. However, as described above, it is not possible to form an image pattern for gradation correction between all "images". However, this is not preferable in terms of toner consumption and maintenance inside the machine. If there is not enough room to form one image pattern “between images”, the throughput must be sacrificed, and the number of tone corrections must be reduced as much as possible.

Steps 5 and 6 of the present embodiment are provided in consideration of the above. When the measured image pattern is within a certain error range with respect to the data one cycle before,
The gradation is determined to be “stable”, and the gradation correction for the next cycle (or for several cycles) is omitted. In the above example, the gradation correction of the halftone pattern of 12 dots of yellow is omitted for three cycles.

The grayscale region which actually causes a problem such as a change in color is mainly a highlight portion. In general, the correlation between the printing ratio and the error is small, and the error tends to have no difference in the middle gradation area regardless of the printing ratio. For example, when a neutral gray color tone is created with three colors of yellow, magenta, and cyan, it is assumed that the error is about 3% regardless of the printing ratio, and only magenta has a higher printing ratio of 3% than the other two colors. think of. When the neutral gray is made with each printing ratio of 30%, and when the printing ratio is 20%
When the color tone (not the density) is compared with that when the printing is made in%, the magenta ratio becomes 10% higher than the other two colors when the printing ratio is 30%, but the magenta ratio is 20% when the printing ratio is 20%. Becomes 1.5% more color tone, giving a more reddish impression. Considering that human color sensation is more sensitive to differences in hue than lightness and saturation, tone management of highlights where color tone tends to be out of order should be more important than high density parts. Also, the other three colors should be given more importance than the gradation management of black which is irrelevant to the hue.

In this embodiment, “Δ-xi” and “A-xi” are provided in consideration of the above, and “Δ-xi” in Table 2 depends on the color and printing ratio. The reference, “A-xi” in Table 3, defines the number of cycles to be omitted. Table 2
The numerical value of “Δ-xi” is determined based on the sensitivity of the human hue and the error of the reflected light amount sensor. In the control flow of FIG. 6, from “Δ-xi” in Table 2 and “A-xi” in Table 3,
Strict criterion for gradation is centered on the halftone region with a printing ratio of 25% to stabilize the color tone of highlights, and the number of omitted cycles is increased in gradation correction for the region with a printing ratio of 50% or more. It reduces toner consumption and throughput delay. Note that “C-xi” in Table 1 is considered so that gradation correction of each color is performed almost equally per printed sheet even if gradation control of a high printing ratio is omitted as described above.

As a matter of course, the contents of “C-xi” in Table 1, “Δ-xi” in Table 2, and “A-xi” in Table 3 indicate that an image patch formed in a certain “between images” is unique. However, this is not a limitation as long as it is set to be determined as follows. The matrix data “C-xi”, “Δ-xi”, “A-
xi ”may be prepared in plural and used in any combination according to the purpose of the output image. For example, as in the present embodiment, for photographic and CG images whose purpose is to manage the hue centering on the highlight region, for graphs and DTP images whose purpose is to manage the hue / density in a slightly higher density region, and for black gradation For monochrome photographic images that lived expression,
Matrix data “C-xi” and “Δ-
xi "and" A-xi "may be created and selected.

The type and number of the density detection methods are not limited. For example, a plurality of the above-described reflected light amount sensors may be used to detect the reflected light amounts of a plurality of density detection image patterns at once. Further, by forming a plurality of density detection image patterns in the non-image area, gradation control can be performed even during continuous printing, and based on the obtained data, the density detection image pattern to be formed next or its image forming cycle Can be controlled, it is not against the gist of the present invention even if the control flow is not the control flow of FIG. Of course,
The method of density control / tone control is not limited to the method described above as long as it is determined based on a plurality of density detection image patterns.

[0063]

As described above, according to the present invention,
In a color image forming apparatus having means for detecting the density of a specific density detection image pattern formed in a non-image area during an image forming step, and means for performing density control and gradation control based on the detected density By forming an image pattern for density detection in a non-image area sandwiched between the leading edge and the trailing edge of an image to be formed, density detection is performed by dividing each color into a plurality of times, and data for density control and gradation control is performed. Since the correction is performed, the gradation control can be performed without suspending the image formation even when the image formation is performed continuously. Also, by controlling the density detection image pattern to be formed next and the image forming period based on the density data obtained by the density detection means,
The gradation control can be performed more efficiently.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a color image forming apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram illustrating a method of forming an image pattern for density detection under control in the embodiment of FIG. 1;

FIG. 3 is an explanatory diagram showing a density detection image pattern used in the control of the present invention.

FIG. 4 is a cross-sectional view illustrating a color image forming apparatus according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a method of forming an image pattern for density detection under control in the embodiment of FIG. 2;

FIG. 6 is a diagram showing a control flow in control in the embodiment of FIG. 2;

FIG. 7 is a sectional view showing a conventional color image forming apparatus.

FIG. 8 is an explanatory diagram showing a developing bias.

FIG. 9 is an explanatory diagram showing a method of forming a density detection image pattern by conventional control.

FIG. 10 is an explanatory diagram showing a density control method in the conventional control.

FIG. 11 is an explanatory diagram showing a gradation control method in the conventional control.

[Explanation of symbols]

 1, 15 photosensitive drum 4 18 developing device 6 intermediate transfer belt 12, 34 reflected light amount sensor 19 transfer belt

Claims (5)

[Claims] 1. A density detecting means for detecting the density of an image pattern for density detection formed in a non-image area during an image forming step, and a control means for performing density control and gradation control based on the detected density. Wherein the density detection by the density detecting means is performed a plurality of times for each color to perform data correction of density control and gradation control. 2. A color image according to claim 1, wherein, when image formation is performed continuously, a density detection image pattern is formed in a non-image area sandwiched between a front end and a rear end of an image to be formed, and density detection is performed. Forming equipment. 3. A density detection image pattern to be formed next and / or an image formation period thereof is controlled based on density data obtained by said density detection means.
Or 2 color image forming apparatus. 4. A density detecting device comprising: a plurality of image carriers; and a belt-like conveying means for carrying a transfer material for transferring an image formed on the image carriers, and sequentially conveying the images to the image carriers. 2. The image forming apparatus according to claim 1, wherein the image pattern is formed on the image carrier, and then transferred to the belt-shaped conveying unit, and the density of the density detecting image pattern is detected on the belt-shaped conveying unit.
4. The color image forming apparatus according to any one of Items 3 to 3. 5. An image forming apparatus comprising: a plurality of image carriers; and a belt-shaped intermediate transfer member for temporarily transferring an image formed on the image carrier to a transfer material before transferring the image pattern for density detection. The color according to any one of claims 1 to 3, wherein the color is transferred to the belt-shaped intermediate body after being formed on a carrier, and the density of the density detection image pattern is detected on the belt-shaped intermediate body. Image forming device.