JPS6057868A - Image density controlling method - Google Patents

Abstract

PURPOSE:To maintain proper image density accurately with high precision by detecting the density of an image close to a final image in a polychromatic image forming device, and feeding it back and controlling the image density. CONSTITUTION:A solid image 37 for density measurement transferred to an intermediate transfer sheet 18 when reaching an image density detector 21 is irradiated by a light source 21a, and its transmitted light is converted photoelectrically by a photodetecting element 21b. A control circuit 42 compares this density measured value with a proper density value; when the density measured value of the solid image 37 in every color is nearly equal to a corresponding proper density value, developing biases of developing devices 10-13 for respective colors are held as they are and the applied voltage to a primary charger 8 is also held as it is to enter the next process of actual printing. If some density measured value of the image 37 is not equal to its proper density value, the process of controlling the developing bias to the corresponding developing device among 10-13 through the control circuit or varying the applied voltage to the primary charger is repeated until the measured value enters the permissible range of the proper density value.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density control method for image forming apparatuses such as color printers and color copiers that produce multicolor images using an electrophotographic method. The present invention relates to an image density control method for forming an image by performing density adjustment.

Conventionally, in a method of forming an image in a color printer, a color copying machine, or the like, first, images corresponding to respective color components obtained by color-separating an original image are formed on a photoreceptor by exposing the photoreceptor to light. For example, in the case of a color copier,
The yellow component of the original image is extracted by passing the original image through a blue filter, and a latent image of the yellow component is electrostatically formed on the photoreceptor. This latent image of the yellow component is visualized by a yellow developer using a developing device, and this developed powder image of the yellow component is placed on the transfer drum or a transfer material mechanically or electrostatically wound around the drum. transcribed. This transfer step is performed multiple times on the same transfer portion for each color component number. In other words, the transfer process is performed three times for yellow, magenta, and cyan, or four times in addition to black. Note that when the toner image is transferred to the transfer drum, the toner image on the transfer drum is transferred onto the transfer material at one time. In this way, the transfer material after the transfer process is separated from the transfer drum, and the transferred developer powder image is fixed on the transfer material in the fixing section.

In the case of a color printer, when reading an image of a document, the document image is separated into images of various components via each color separation filter and a solid-state image sensor and then read, and the image signals of the various components thus read are Temporarily stored in buffer memory. This accumulated image signal is given to a laser light source, and a modulated laser beam emitted from this laser light source exposes and scans the photoreceptor, thereby forming an electrostatic latent image on the photoreceptor according to each various component. . The subsequent steps are the same as those described above, and their explanation will be omitted.

A conventional image density control method used in such a process will be considered. In general, it is sufficient to detect the image density for adjusting the image density by measuring the density of the final image using a method such as optical means, but since there are various forms of actual images, the density must be regularly measured. It is difficult to do so. Therefore, a predetermined image (for example, a solid image with a constant density) is formed on a photoreceptor using a developer of each color, and the density is measured on the photoreceptor before cleaning, and the image is formed according to this density information. It is preferable to control the concentration of However, in an image forming apparatus that uses several types of developers with different colors, when the color of the photoreceptor itself is the same as the color of the developer, for example, when the photoreceptor is made of CdS and the yellow developer is used, the photoreceptor Due to the influence of the color of the developer itself, it may be difficult to detect developers of the same color among the developers.

For this reason, in actual image forming apparatuses such as color printers and color copiers, it is necessary to control the formation of electrostatic latent images around the photoreceptor (controlling the formation of electrostatic latent images at a constant amount of light so that the standard electrostatic latent image characteristics can be changed). The image density is controlled by simultaneously controlling the voltage applied to the charger to form an electrostatic latent image and controlling the amount of exposure to the photoreceptor and keeping the amount of developer in the developing device at a constant level. There is.

According to this image density control method, it is possible to obtain the effect of adjusting the image a degree as described in Section 7. However, when the measurement target for image density adjustment is There is a problem in that image density measurement lacks accuracy and simplicity, such that a person has to judge the image density by looking at the image.

The present invention has been made in view of the above points and to eliminate the above drawbacks, and forms an electrostatic latent image by exposing a photoreceptor to light for each color according to image color information of a document separated into various components. Then, this electrostatic latent image is developed with a developing device having a developer of a predetermined color to form a developed powder image, and this developed powder image is transferred to a transfer section and separated into at least two colors. A multicolor image forming method in which a transfer unit holds multiple images of the corresponding number of colors, transfers these images onto a transfer material at once, and then colors the multiple powder images on the transfer material using a fixing device. In this step, a predetermined image for image density detection is transferred to a transfer section,
It is an object of the present invention to provide an image density control method in which the density of this image is detected by an image density detector using optical means, and image density forming conditions are determined and controlled based on the detection results.

Embodiment of the Image Density Control Method According to the Present Invention FIG. 1 is a schematic cross-sectional view of a color printer to which the image density control method according to the present invention is applied. In the figure, reference numeral 1 denotes a semiconductor laser as a laser light source, from which a laser beam indicated by a dashed dot line is emitted which is modulated according to image color information of an original document separated into each color. Reference numeral 3 denotes a rotating polygon mirror as a deflector, which is driven by a motor 2 to repeatedly scan a high-speed rotating laser beam. Reference numeral 4 denotes an imaging lens which, for example, has an f-θ characteristic and forms an image of the laser beam reflected by the rotating polygon mirror 3 onto the photosensitive drum 7 via a mirror 5 for changing the optical path and a transparent glass 6 for dust protection. let

Reference numeral 7 denotes an electrophotographic photoreceptor drum as a photoreceptor, and around it are a primary charger 8 for primary charging, a potential sensor 9 for detecting surface potential, and a developer for developing with yellow developer (hereinafter referred to as Y developer). 10. A developing device for developing with magenta color developer (hereinafter referred to as M developing device) 11. A developing device for developing with cyan color developer (hereinafter referred to as C developing device for short).

) 12, a developing device for developing with a black developer (hereinafter referred to as a BK developing device) 13, a cleaning device 14, and a pre-exposure source 15 for preventing history of electrostatic latent images are arranged in this order. . Note that the photosensitive drum 7 rotates in the direction of the arrow shown in the figure with the shaft 7a as the rotation center, and the Y, M, C, and BK developing units 10 to 13 are configured so that each developer comes into contact with the photosensitive drum 7 only during development. has been done.

A transfer drum 19 is in contact with the surface of the photosensitive drum 7 between the BK developing device 13 and the cleaning device 14, and has cylindrical flanges 16 on both sides, and these flanges 16 are connected by a connecting member 17. A cylindrical intermediate transfer sheet 18 made of a transparent plastic sheet, such as a polyester resin sheet, whose side in contact with the photosensitive drum 7 has been subjected to conductive treatment is provided on these surfaces. Further, this transfer drum 19 rotates about a shaft 19a as a rotation center in the direction of the arrow shown in the figure.
The rotations of the photosensitive drum 7 and the photosensitive drum 7 are synchronized by transmitting a driving force through a gear (not shown) or the like. A transfer charger 22 for transferring the developing powder image formed on the photosensitive drum 19 onto the intermediate transfer sheet 18 on the transfer drum 19 side is provided on the inner side of the transfer drum 19 where the transfer drum 19 and the photosensitive drum 7 are in contact with each other. is located. This transfer charger 2
At a position slightly shifted from 2 in the rotational direction of the transfer drum 19,
The light source 21a and the light receiving element 21 are connected via the intermediate transfer sheet 18.
b are arranged at a distance from each other, and this configuration constitutes an image density detector 21 that detects the density of the developing powder image.

Furthermore, according to the rotational direction of the transfer drum 19, transfer chargers 24a and 24b are provided for transferring the image transferred onto the transfer drum 19 onto a transfer material 30, and a separation device is provided for separating the transfer material 30 from the transfer drum 19. Charger 25, cleaning device 26 having a blade 26a, fur 27
The cleaning devices 27 having a cleaning device 27a are arranged around the transfer drum 19 in this order. 30 is the aforementioned transcription shrine,
The register rollers 23a and 2 are stored in the cassettes 29a and 29b, and are fed by the supply rollers 31a and 31b.
3b side, and is supplied to the transfer drum 19 side with timing taken by the rollers 23a and 23b. 3
2 is a conveyor belt, 33 is a fixing device, and 34 is a paper discharge tray.

3(a) is an explanatory diagram for determining the developing bias, FIG. 4 is a time chart for explaining FIG. 2, and FIGS. 5 and 6 are for determining the developing bias. FIG. 3 is an explanatory diagram showing the relationship between the amount of developer and the amount of light.

FIG. 7 is a partially broken simplified diagram showing a solid image for density measurement being measured by an image density detector, 37a, 37b, 37c,
37d is a solid image (stationary image) for density measurement transferred onto the intermediate transfer sheet 18 of the transfer drum, and yellow, magenta, cyan, and black images are arranged in the circumferential direction. FIG. 8 is an overall schematic block diagram of the image density control device.
The control circuit 42 inputs the outputs of the potential sensor 9 and the light receiving element through amplifiers 41 and 40, respectively, and controls the voltage of the primary charging system 8a of the circuit including the wide charger 8, and the Y developer 10.
The developing bias of each of the M developing device 11, C developing device 12, and BK developing device 13 is controlled. In addition, in the case of this example,
Since a microprocessor is used as the control circuit 42, on its input side there is an analog-to-digital converter and on its output side it is digital.

Next, an embodiment of the image centrality control method according to the present invention will be explained with reference to FIGS. 1 to 3(a) and FIGS. 4 to 8.

In FIGS. 1 and 2, first, the photosensitive drum 7 is mounted on the shaft 7a.
The transfer drum 19 rotates in the counterclockwise direction of the arrow in the figure with the center of rotation at , and in synchronization with this, the transfer drum 19 rotates in the clockwise direction of the arrow in the figure with the center of rotation at the support 19a. As the photosensitive drum 7 rotates, the pre-exposure source 15 turns on, and the surface charge on the photosensitive drum 7 is erased. Next, the photosensitive drum 7 is charged to a certain polarity by a primary charger 81- to which a predetermined voltage is applied (negative polarity in this embodiment).

The surface of the charged photosensitive drum 7 is not exposed as it is, and the potential sensor 9 measures the surface potential of the dark area. Here, if the surface potential is Vp, the process moves to the next step, but if the surface potential Vl) cannot be obtained, this measured value is fed back again to the primary charger 8 so that the surface potential VD can be obtained.
The above steps are repeated by changing the voltage used to obtain the surface potential vD. 101° Next, the modulated laser beam from the semiconductor laser 1 is applied to the rotating polygon mirror 3 which is being rotated at high speed by the motor 2, and the imaging lens 4. A mirror 5 and a transparent film expose and scan the surface of a photosensitive drum 7 via a lath 6.

The surface potential (I) of the bright part of the photosensitive drum 7 exposed from the second direction is 102 degrees as measured by the potential sensor 9. From the surface potential VD of this dark part and the surface potential V4 of the bright part , C, BK each developing device 10 to 13) is determined (103o).
This will be explained in detail using . The surface potential vLt of the bright area of the photosensitive drum 7 measured as described above versus Y.

The potential vB obtained by subtracting a predetermined voltage VB' of any one of the M, C, and BK developing devices 10 to 13 becomes the developing voltage of this developing device.

Since this predetermined voltage I3' is different for each of the Y, M, C, and BK developing devices 10 to 13, the developing bias VU is naturally different for each developing device. At this time,
Since the developer used in the developing device is charged to a certain polarity (negative polarity in this embodiment), the developing agent (r thicker than IAS V11) is applied to the bright area surface side of the photosensitive drum 7 having a surface potential vL. The developer is attracted and adheres to the surface of the dark area.Since the potential is lower than the development bias VI3 on the dark area surface side, the developer is repelled by the negative polarity of the developer and does not adhere to this surface.

According to the above explanation, the larger the predetermined voltage Vg, the more
That is, the greater the potential difference between the surface potential VL of the bright area and the developing bias V11, the stronger the electric field acts between the photosensitive drum surface and the developing device, and the amount of developer attached to the bright area of the photosensitive drum 7 increases. . This means that the lower the developing bias Vll is set, the higher the image density becomes. In addition, by changing the predetermined voltage VB' and adjusting the voltage of the charger 8, the surface potential Vll of the dark region is changed, thereby relatively changing the surface potential VL of the bright region. The image density can be changed with the same effect as changing the developing bias V13.

Next, the surface potential of the dark area of the photosensitive drum 7 is set to Vl), and the surface potential of the bright area is measured to Y, M, and C.

The following steps will be described with reference to FIGS. 1, 2, and 4 to 8. - The photosensitive drum 7, which has been charged so that the surface potential becomes VD by the charger 8, is measured by the laser beam from the semiconductor laser 1 through the polygon mirror 3, the imaging lens 4, the mirror 5, and the transparent glass 6, and the image density is measured. Four fixed solid images for specific purposes are placed at regular intervals (at least Y, M, C).

Exposure scanning is performed so that the BK developing units 10 to 13 are formed at a distance greater than the distance between them. The surface potential of the photosensitive drum 1 exposed in this scan (second exposure) is vL, and among the exposed and scanned areas, the area that is scanned and exposed first is converted into yellow developing powder by the Y developing device 10. The area where the image (yellow solid image) is formed and then scanned is exposed to M developer 1.
1, a magenta developing powder image (magenta solid image) is formed.
A black developed powder image (black solid image) is formed by 11'lJ, '-/T'l bone 4, 1 rotation of the r-down drum 7, Rows are carried out in order at intervals, as shown in Fig.
, M, and BK are abbreviations for yellow, magenta, cyan, and black, respectively. At this time, when each developing device develops, the developing bias determined as described above is applied.

The solid images for image density measurement formed on the photosensitive drum I are sequentially transferred to the transfer drum 19 by the transfer charger 22.
This transferred portion is shown in FIG. 7, where 37a,
37b, 37c, and 37d are yellow, magenta, cyan, and black solid images, respectively. Immediately after these solid images are transferred to the transfer drum 19, they are sequentially transferred to the image density detector 2.
1, the amount of transmitted light is measured and compared with the standard appropriate value to determine whether the image density is appropriate.106 At that time, the method of operating the detector is to activate the detector and set it to the brim II position. Possible methods include obtaining input sequentially or activating the detector in accordance with the laser exposure period.

This will be further explained in detail with reference to FIGS. 5 to 8. The image density detector 21 shown in FIGS. 1 and 7 receives the amount of transmitted light from a light source 21a with a light receiving element 21b (for example, a photoelectric conversion element), and determines the amount of transmitted light based on the amount of photoelectric conversion. A detection method is used. The reason why it is possible to detect the density of a solid image for image density measurement formed on the intermediate transfer sheet 18 by measuring the amount of transmitted light is as described below.

In Figure 5, the left side is the intermediate transfer sheet! - The case where there is no developer on 18 is shown. At this time, the amount of irradiated light is I. If the amount of transmitted light during binding is shifted from 11, Io and 11 are almost equal to I. 1. It is. However, if there is developer on the intermediate transfer sheet 18 and a solid image 37 is formed as shown on the right side of FIG.
7 is one of the yellow, magenta, cyan, and black solid images 37a to 37d shown in FIG. ), as shown in the figure, the irradiated light is absorbed, reflected and scattered by the developer of the solid image 37 on the intermediate transfer sheet 18.
The transmitted light amount I is the irradiated light amount I. (II<IO). The reason why the irradiated light is scattered is because the surface of the developer layer and the parts of the intermediate transfer sort 18 and the solid image 37 that are in contact with the developer are not smooth and have very complicated shapes. .

FIG. 6 shows the relationship between the amount of developer per unit area on the intermediate transfer sheet 18 and the ratio between the amount of transmitted light and the amount of irradiated light. When the amount of developer on the intermediate transfer sheet is small (very rare), the amount of transmitted light is 11 and the amount of irradiated light is I. However, as the amount of developer increases, the amount of transmitted light 11 gradually decreases, and when a certain placement plate is reached, there is almost no amount of transmitted light.

In other words, the amount of developer per unit area on the intermediate transfer sheet, that is, the density of the solid image for measuring the density of the intermediate transfer sheet is the amount of transmitted light in the area where the developer is on the intermediate transfer sheet. By detecting this, it becomes possible to determine its concentration.

This a degree measurement and judgment will be further explained in detail using FIG. 8. When the solid image 37 for density measurement transferred onto the intermediate transfer sheet 18 comes to the position of the image density detector 21, this solid image 37 is irradiated by the light source 21a,
This transmitted light is received by the light receiving element 21b and photoelectrically converted. Since this photoelectric conversion amount becomes smaller as the density of the solid image 37 becomes higher, the control circuit 42 inputs this photoelectrically converted density measurement value as a digital value via an amplifier. Further, the control circuit 42 compares this density measurement value with an appropriate density value to make a judgment, and here, the solid images 37a to 3 of each of yellow, magenta, cyan, and black shown in FIG.
If all of the measured density values of 7d substantially match their respective appropriate density values, the control circuit 42 selects Y, M, and C.

BK, the developing bias of each of the developing devices 10 to 13 is maintained at the state of '>, and the primary charging system 8a including the -order distortion electric device 8 is controlled, that is, the voltage applied to the primary charging device 8 is maintained as it is. \ state and move on to the next actual printing process (YES in 106).

If the density measurement value of the solid image 37 for density measurement does not substantially match the appropriate density value, that is, if it is outside the allowable range of the appropriate density value, one of the Y, M, C, and BK developing units 10 to 13 The developing bias of the developing device corresponding to the measured solid image 37 is controlled by the control circuit 42 as described above, or
The voltage applied to the primary charger 8 of the −th order distortion electric system 8a is changed (NO in 106). As described above with reference to FIG. 2, this process is repeated until the measured density value falls within the allowable range of appropriate density values. That is, when only the developing bias is controlled again, the developing bias of each developing device is determined 103, then a solid image for density measurement is produced 104, and then this solid image is transferred to the transfer drum 105. Next, the density of the solid 7% image is detected and judged whether it is appropriate or not (106), and this process loop is repeated until the appropriate density is achieved.

Note that when the -forming charger 8 is controlled, the setting 101 of the surface potential Vl of the dark part of the photosensitive drum 7 and the measurement 102 of the surface potential VL of the bright part of the photosensitive drum 7 are used to determine the developing bias of each developing device 103, as described above. A loop is formed until the appropriate concentration is reached.

In FIGS. 1 and 7, after a solid image is transferred to the transfer drum 19 by the transfer charger 22, the photosensitive drum 7
The remaining toner adhering to the surface of the solid images 37a to 37d transferred to the transfer drum 19 is cleaned by the cleaning device 14, and the density of the solid images 37a to 37d transferred to the transfer drum 19 is detected by the image density detector 21. (-The blade 26a of the cleaning device 26 comes into contact and cleans the developer attached to this surface, and then the fur 27a of the cleaning device 27 rotates at high speed,
The cleaning device 27 swings and the fur 27a comes into contact with the transfer drum 19, and the intermediate transfer sheet 18
Clean the developer puddle that has formed at the rear edge of the upper image. After cleaning the developer reservoir, the fur 27a is separated from the transfer drum 19 again. These cleaning devices 26.2
7, the residual developer on the transfer drum 19 is cleaned, and the transfer drum 19 is prepared for the next transfer process.

Conditions for image formation are set in the manner described above.
Move on to the next actual printing process. This step is almost the same as the step of forming the solid image for density measurement, so it will be briefly explained. In Figure 1, the photosensitive drum 7 is the pre-exposure source 1.
5, is then charged by a primary charger 8, and is scanned and exposed by a laser beam modulated according to the image from the semiconductor laser 1. The electrostatic latent image formed by this scanning exposure is developed by the Y developing device 10 into a yellow developed powder image, and this powder image is transferred to the transfer drum 19 side by the transfer charger 22. On the other hand, after this transfer is completed, the surface of the photosensitive drum 7 is cleaned by a cleaning device 14. The above process is repeated three times for the M developing device 11, C developing device 12, and BK developing device 13, and a multiplexed image of four colors is transferred onto the intermediate transfer sheet 18 of the transfer drum 19. There is. On the other hand, cassette)
29a to the transfer drum 1 by rotation of the supply roller 31a.
The transfer material 30 supplied to the 9 side is transferred to the register roller 23a,
The transfer charger 24a is timed by the transfer charger 23b.
, 24b is sent to the transfer drum 19 side, and the four-color powder image on the intermediate transfer sheet 18 is transferred to the transfer charger 24.
This transfer material 3 fed by the operation of a and 24b
Transferred to 0. After transcription M'-old 15, M tlc
'IF Hsun IJ fan, mlt; IKri REQ Q
l; t-P isolated from 1hC'EV lamb 19;
The transfer material 30 is conveyed by a conveyor belt 32, and after the transferred four-color powder image is fixed by a fixing device 33 so as to develop color, the transfer material 30 is sent to a paper discharge tray 34.

After the powder image has been transferred onto the transfer material 30 by the transfer chargers 24a and 24b, the transfer drum 19 is cleaned again by the cleaning devices 26 and 27 as described above to prepare for the next transfer. Actual printing is performed as shown in (101 in FIG. 2). In the above embodiment, solid images for density measurement were formed in four colors, but the present invention is not limited to this, but a solid image for density measurement in at least one color is formed, and the density measurement value of this solid image is used as a reference. Density control for image formation may be performed as described above.

Further, in the above embodiment, an example was shown in which the image density was detected prior to actual printing. Solid images 37a to 37d for image density detection are formed and transferred outside the range (portion other than the width in FIG. 9).
If it is possible to form and transfer image density, it becomes possible to always detect the image density during actual printing, which is useful for controlling the image density during the next actual printing. As shown in the figure, solid images 37a to 37a for density measurement on the intermediate transfer sheet 18
An example of detecting the image density of 37d using a reflection method will be described. 21a is a light source, 2Tb is a light receiving element, and light receiving element 2
The light source 1b is sensitive to the reflected light of the developer used for the solid images 37a to 37d, and the light source 21a strongly emits light at a wavelength of a portion with a high reflectance among the spectral reflection characteristics of each developer. The riveted image 37a is used to operate the light source 2+a and the light-receiving element 21b to determine whether there is developer on the intermediate transfer sheet 18 or not from the output of the light-receiving element 21b.
Detect r3 degree of ~37d.

For example, in FIG. 10, the density of a cyan solid image 37c is being detected, and a portion of the irradiated light from the light source 21a is reflected to the light receiving element 21b with an intensity corresponding to the image density. incident. Here, the higher the density of the solid image 37c, the stronger the degree of light reflected by it, and the higher the output of the light receiving element 21b. The image density may be controlled using the output from the light receiving element 21b in the same manner as in the above embodiment. Note that since the black solid image 37d has low light reflectivity, it is difficult to detect the density thereof, and this portion may be excluded in this embodiment.

In each of the above embodiments, when determining the image forming conditions by increasing the image density of the solid image for image a degree measurement (this is done by increasing the laser beam irradiation density), An example was shown. Therefore, the reproduction of the density of the image of the actual print formed in each of these embodiments is necessarily based on the high density area, and is reproduced according to the change in the irradiation density of the Lede beam onto the photosensitive drum. However, when performing this kind of image density adjustment, the density may only be reproduced within a certain range due to environmental fluctuations such as temperature and humidity. For example, if the width of the laser beam irradiation density is narrow, This means that the change in image density occurs rapidly at low a degrees.

One way to deal with this phenomenon is to change the irradiation density of the laser beam onto the photosensitive drum when setting the electrostatic latent image forming conditions (when setting and detecting the surface potential of the dark and bright areas of the photosensitive drum). Then, the surface potential of each part is measured to determine the voltage applied to the primary charger and the developing bias, a steady image for density measurement with different density is formed depending on these conditions, and the density is detected to determine the image forming conditions. FIG. 3(b) shows the setting of the electrostatic latent image forming conditions at that time.

VLI and VL2 are the respective surface potentials of the bright areas when the irradiation density of the laser beam on the photosensitive drum is changed. Voltage VBI' + VB2' respectively surface potential VLl
, shows the difference between VL2 color development bias and V13. It is not necessary to actually correct the image a degree by changing the difference y between the developing bias vB and the surface potential vD of the dark area from the result of density measurement. Figure 3(b) Te is vI, l,
The irradiation density of the laser beam was changed in two steps, V and 2, but the image density can be controlled by changing the irradiation density of the laser beam in multiple steps. It is possible to control the image density.

In each of the above embodiments, an example of a color printer has been described, but electrical processing is performed before exposing the photosensitive drum. Before copying the original, place the test chart on the original platen, or place it on the original platen at a different location from where the original is placed, as shown in Figure 9. Just make a copy.

As described above in detail about the image density control method of the present invention, in a multicolor image forming apparatus such as a color printer or a color copier, the density of an image close to the final image is detected and fed back to control the image density. (7), the proper image density can be maintained with high precision and accuracy. Also, since there is no need to print out part of the image and detect the image density, unnecessary printouts are avoided. Yx<
In addition, the waiting time for images to be printed out is minimized, and the burden on the operator is greatly reduced compared to conventional systems.

[Brief explanation of the drawing]

Fig. 1 is a schematic overall sectional view of a color printer to which the present invention is applied, Fig. 2 is a flowchart of an embodiment of the image density control method of the present invention, and Fig. 3 fa) and fb) are times when determining the developing bias. 4 is a time chart for explaining FIG. 2, and FIGS. 5 and 6 are diagrams showing the relationship between the amount of developer on the intermediate transfer sheet and the amount of light. FIG. 7 is an explanatory diagram showing density measurement using an image a-degree detector, FIG. 8 is an overall schematic block diagram of the image density control device, and FIGS. It is an explanatory view of each example of. 1... Semiconductor laser, 7... Photosensitive drum 8...-
Charger 9...Potential sensor 10...Yellow developer 11...Magenta developer 12...Cyan developer 13...Black developer 15...Pre-exposure source 18.
...Intermediate transfer sheet 19...Transfer drum 21...
Image density detector 21a...Light source 21b...Light receiving element 22...Transfer charger 23a, 23b...Register roller 24...Transfer charger 30...Transfer shrine 33
... Fixing device 37 ... Solid image 37a (for a degree measurement) ... Yellow solid image 37b (for density measurement) ...
・Magenta solid moving image 37c (for density measurement)...(
Cyan solid image 37d (for density measurement)...Black solid image 40.41 (for density measurement)...Amplifier 42...Control circuit 8a...-Nth distortion electric system patent applicant Canon Corporation Figure 6 Figure 7

Claims (3)

[Claims] (1) A photoreceptor is exposed to light according to the color-separated image information of the original to form an electrostatic latent image, and this electrostatic latent image is developed with a developing device containing a developer of a predetermined color to form an image. form,
This image is transferred onto a transparent member of an image holding/transfer section, and the above steps are carried out in rows for the number of color separations. In a multi-color image forming method in which an image is formed by transferring images in one go, a predetermined stationary image for image density detection for each developer is generated under predetermined electrostatic latent image formation conditions and development bias. A small Yx is formed on the photoreceptor by at least one side, and then transferred to the image retention/transfer section 11Gt, and the image density of the steady image is determined by an image density detector disposed near the transparent member of the image retention/transfer section. , and this concentration detection i txr += i-, P / (1, N f i
Place the jar 7. An image density control method characterized by controlling the density of an image formed in a next step by adjusting f-M-, m a & L. (2) the image density detector is arranged such that a pair of a light source and a light receiving element face each other and are separated from each other with the transparent member interposed therebetween;
2. The image density control method according to claim 1, wherein the image density is measured by the light receiving element receiving light when the light from the light source passes through the stationary image on the transparent member. (3) The image density detector has a light source and a light receiving element arranged near one side of the transparent member, and detects the light when the light from the light source is reflected by the steady image on the transparent member. The image density control method according to claim 1, wherein the image density is measured by the light receiving element receiving light.