JP2000039748A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a device capable of securing desired gradation characteristics, that is, the gradation characteristics linear or near to an ideal reproduction curve by detecting the image density of a solid image or a dotted image and switching exposure in accordance with the respective images. SOLUTION: The image density of the solid image or the dotted image is detected. In the case of the solid image, developing bias and the surface potential of a photoreceptor are set based on the detected value, and in the case of the dotted image, the exposure that the image density of a standard dot is equal to that of a reference dot is set, and the exposure is switched in accordance with the respective images. In this image forming device 6, for example, an optical density sensor 12 detects the optical density of a toner image 33, and measures the optical density of the solid image, the dotted image and a thin line image. A photosensor or an optical fiber sensor is used as the sensor 12. Thus, the device where the stability of the image density of the solid image and that of the dotted image is improved and which can secure the good gradation characteristics is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine and a printer.

[0002]

2. Description of the Related Art In a conventional image forming apparatus, there is known an image forming apparatus which performs image density control for adjusting the density of an image formed on a sheet to a predetermined level before forming an image on the sheet. Image density control includes, for example, forming a reference toner image on a photoconductor, detecting the density of the formed reference toner image with a density sensor, and, based on the detection value of the density sensor, detecting the potential of the photoconductor, the amount of exposure, Also, the value of a parameter relating to the amount of toner adhesion such as a developing bias is controlled.

[0003] Japanese Patent Application Laid-Open Nos. 9-244391 and 9-244313 disclose an image forming apparatus for performing such image density control.
And JP-A-63-131152.

[0004]

However, in the conventional image forming apparatus, the image density of the solid image portion can be maintained at a predetermined level, but the gradation is adjusted by increasing or decreasing the print area ratio using high-definition halftone dots. If it is attempted to express, a shadow part (a halftone dot having a high halftone dot area ratio) will be destroyed, or a highlight portion (a halftone dot having a low halftone dot area ratio) will be insufficiently reproduced. There was a tendency that characteristics could not be obtained.

A first object of the present invention is to provide an image forming apparatus which eliminates the above-mentioned disadvantages and can secure desired gradation characteristics, that is, gradation characteristics close to a straight line or an ideal reproduction curve. is there.

A second object of the present invention is to provide an image forming apparatus capable of stably maintaining image density and gradation characteristics of a solid image portion.

[0007]

SUMMARY OF THE INVENTION In order to solve at least one of the above-mentioned objects, the present invention provides a photoconductor, a charging unit for charging the surface of the photoconductor, and the charged surface of the photoconductor. An exposure unit that forms an electrostatic latent image by exposing with a predetermined exposure amount; a developing unit that applies toner to the photoconductor to develop the image; and a detection unit that detects an image density of an image developed by the developing unit. A solid density control unit that determines a value of a developing bias applied to the developing unit and a surface potential of the photoconductor based on image density information of the solid image detected by the detecting unit; A memory unit for storing the predetermined image density information of the reference halftone dot, and the image density of the predetermined standard halftone dot area ratio equal to the reference halftone dot, the image density of the reference halftone dot The standard halftone dot In an exposure amount control unit for setting an exposure amount against, and the solid image and the halftone image, and an exposure amount varying unit which switches an exposure amount.

As described above, the image density of a solid image or a halftone image is detected. In the case of a solid image, the developing bias and the surface potential of the photosensitive member are set based on the detected values. Sets the image density of the standard halftone dot to an exposure amount that is equal to the image density of the reference halftone dot, and switches the exposure amount in accordance with those images to obtain the image density of the solid image and the image density of the halftone image. The image forming apparatus can improve the stability of the image forming apparatus and secure good gradation characteristics.

[0009]

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

[First Embodiment] FIG. 1 shows a first embodiment of the present invention.
1 is a schematic configuration diagram of an image forming apparatus according to an embodiment.
The image forming apparatus 6 includes marks such as a charger 2, an exposure device 3, a developing device 4, a transfer device 5, a static eliminator 43, a cleaner 7, and a fixing device 8, which are arranged around the photoreceptor 1. A photographing unit, a charger power supply 9 for charging, a developing bias power supply 10 for developing, a potential sensor 11, a charging control unit 14, an optical density sensor 12, and an optical density detecting unit 18.
An adhesion amount control unit 58 including a solid density control unit 56, a thin line / halftone dot control unit 57, an exposure control unit 19, etc., a process control unit 15, an image designation unit 16, a test image generation unit 17, , An image recognition unit 64, a signal switching unit 65, and the like.

As the charger 2, a scorotron charger having a discharge wire 41 for performing corona discharge and a control electrode 42 as shown in FIG. 6 is used to control the surface potential of the photosensitive member 1. The power supply 9 for the charger for supplying power to the discharge wire 41 and the control electrode 42 of the charger 2 can set a discharge current flowing through the discharge wire 41 and a control voltage applied to the control electrode 42.

The exposure device 3 comprises a scanning optical system and a laser (or LED array), and has an exposure amount varying means 13 for varying the exposure amount of the exposure light beam 35. Further, the recording spot diameter D of the exposure light beam 35 is set to be larger than the scanning line pitch P in order to overlap each other. It is set to 2.0 times, preferably about 1.4 times to about 1.7 times. The potential sensor 11 detects the surface potential of the photoconductor 1, and measures the charged potential V0 after uniform charging by the charger 2 and the residual potential Vr after exposure by the exposure device 3.

The developing device 4 includes a magnetic brush-like developer 31.
Is brought into contact with the electrostatic latent image on the photoreceptor 1, and a bias voltage is applied to the developing roll 23 from the developing bias power supply 10 to form a toner image 33. As shown in FIG. 7, the magnetic brush-shaped developer 31 is composed of a toner 20 and a magnetic carrier 21, and is attracted by the magnetic force of the magnet 30 in the developing roll 23 and held in a chain around the outer circumference of the developing roll 23. Is done. The toner 20 is charged to a polarity opposite to that of the magnetic carrier 21 by frictional charging with the magnetic carrier 21 and adheres to the surface of the magnetic carrier 21 by electrostatic force. The developing roll 23 is configured by disposing a magnet body 30 having a plurality of magnetic poles S, N, S... Inside a rotatable developing sleeve 29.

The developing device 4 includes a regulating plate 2 for regulating the transport amount of the developer 22 held on the developing roll 23 to a predetermined amount.
8 and a toner hopper 24 for supplementing consumed toner
, A supply roller 25, a supply roller driving unit 62, a stirring member 26 for mixing the supplied toner 20 and the developer 22 to improve triboelectric charging, and a mixing ratio of the toner 20 and the magnetic carrier 21 (toner And a magnetic sensor 27 for detecting the density. When the magnetic brush-shaped developer 31 is brought into rotational contact with the photoconductor 1, a bias voltage having the same polarity as that of the toner 20 is applied to the developing roll 23, and an electric field is applied to the charged toner 20 to expose the toner 20 to light. The toner image 33 was formed on the photoconductor 1 by moving to the body 1 side.

The developer 22 includes the toner 20 and the magnetic carrier 2
A two-component developer consisting of 1 was used. Magnetic carrier 21
The ferrite, magnetite, and iron powder carriers whose surfaces are coated with resin can be used. When the average particle size is 120 μm or more, the sweep tends to occur, and when the average particle size is 30 μm or less, the carrier tends to adhere to the photoreceptor. Is 40 to 11
0 μm was used. If the toner 20 is too small (average particle size of 5 μm or less), the image density tends to decrease, and if it is 12 μm or more, dot reproducibility tends to decrease. Therefore, the toner 20 having an average particle size of 6 μm to 11 μm was used. The toner density (Tc≡100 × toner weight / developer) is 1.5 (w
When the amount is less than 7% by weight, the image density is reduced and the carrier adheres to the photoreceptor. When the amount is 7% by weight or more, fog (background stain) and toner scattering occur. Was set to 2 to 4 (wt%).

The toner concentration is controlled by a magnetic sensor 27,
Supply roll drive unit 62 that rotates the supply roll 25
Is performed by the mixing ratio control unit 32. That is, the magnetic substance content (that is, toner concentration) is detected by the magnetic sensor 27, and when the toner concentration is lower than a predetermined value, the mixture ratio control unit 32 generates a supply signal to drive the rotation of the supply roller 25. Toner 20 in toner hopper 24
Is supplied to the stirring and mixing section in which the stirring member 26 is disposed in accordance with the rotation of the supply roll 25, thereby controlling the toner concentration to be substantially constant.

The optical density sensor 12 detects the optical density of the toner image 33, and measures the optical density of a solid image, a halftone dot image, and a fine line image. The optical density sensor 12 includes a photo sensor 44 (FIG. 8) and an optical fiber sensor 47 (FIG. 9).
Can be used. As shown in FIG. 8, the photo sensor 44 is convenient when an integrated type in which the light emitting element 45 and the light receiving element 46 are integrated is attached. As the light emitting element 45 of the photo sensor, a GaAs infrared light emitting diode can be used, and as the light receiving element 46, a Si photo transistor can be used.

As shown in FIG. 9, the optical fiber sensor 47 includes a first fiber bundle 48 composed of a plurality of optical fiber cores for guiding irradiation light and a second fiber bundle 49 composed of a plurality of optical fiber cores for guiding reflected light. A light-emitting element and a light-receiving element are provided at the rear end (not shown), similarly to the photo sensor. The optical fiber sensor 47 can reduce the light irradiation portion at the tip to 0.5 to 3 mmφ and has a rod shape, so that the installation space can be reduced and the gap can be easily set.
Also, even if the charging means with corona discharge is arranged in the vicinity, there is an advantage that noise due to electrical induction does not occur because the light irradiation part is made of glass or plastic.

The gap between the optical fiber sensor 47 and the photoreceptor 1 is at the position where the output is maximum (optical peak).
Is desirable. This is because the effects of gap fluctuation and setting variation can be reduced. Further, in the arrangement of the optical fibers, a plurality of optical fiber cores for guiding the irradiation light and a plurality of fiber cores for guiding the reflected light may be mixed at random, but as shown in FIG. It is more desirable to divide each of the optical fiber core wires for guiding the irradiation light. For example, although the optical peak of the random arrangement is 0.2 to 0.5 mm, if the optical peak is divided, it becomes 3 to 5 mm, so that a wider gap can be set and the output gap dependency becomes gentler.
As a result, in the latter case, the effect of alleviating the influence of gap fluctuation and setting variation is further increased.

The relationship between the image density and reflectance and the detection value of the optical density sensor will be described. Image density D (O.
D) is obtained by equation (1), where R is the reflectance.

D = −log ₁₀ R (1) Further, if the reflectance R is y0, y, respectively, the detection values of the optical density sensor for the mirror surface and the toner image can be approximated by the equation (2), The relationship between the density D (OD) and the detection value y of the optical density sensor is obtained by equation (3).

R = y / y0 (2) D = −log ₁₀ (y / y0) (3) In the image forming apparatus 6 of the present invention, a test is performed at the time of startup and periodically at the above-described configuration. Image quality (image density)
And the image forming conditions are set. Hereinafter, description will be made with reference to the flowchart of FIG.

First, the solid image density control steps from S1 to S4 in FIG. 3 will be described.

(1) S1, S2: Setting of solid image exposure amount Is, provisional setting of developing bias Vb and surface potential V0, and detection of background density Dp.

The exposure amount Is for the solid image is stored in the memory 55 in advance by the exposure control section 19 and the exposure amount varying means 13.
Automatically set to the entered value. The developing bias Vb is automatically and temporarily set to a previously input value by the solid density control unit 56.

The charging control unit 14 automatically and temporarily sets the surface potential V ₀ after uniform charging to a previously input value. As a method of temporarily setting the surface potential V ₀ , a method of setting the control voltage Vg applied to the control electrode 42 of the charger 2 to a predetermined value may be used, but the potential sensor 11 is set as follows.
The method of actually measuring the surface potential V ₀ using the method and temporarily setting the surface potential V ₀ based on the detected value is better than the control voltage Vg and the surface potential V ₀ depending on the environmental conditions and the surface state of the photoconductor. Is more preferable when the deviation from the initial correlation is large because the variation (difference) between the target value and the measured value of the surface potential V ₀ can be reduced. Surface potential V ₀ after uniform charging
Is detected by selecting a white image pattern from the test image generating unit 17, generating a white image signal as a print signal, and detecting the potential of the exposure light 35 with the exposure light beam 35 turned off. The control voltage Vg or the discharge current ic (or the control voltage Vg and the discharge current ic) is controlled so that the detection value of the potential sensor 11 becomes a predetermined target value.

In the above-described white image printing mode,
The optical density Dp of the background portion of the photoreceptor (the surface of the photoreceptor without toner) is detected by the optical density sensor 12 and the optical density detector 18 for measuring the output, and the detected value yp
Alternatively, the optical density Dp is stored in the solid density controller 56 and the thin line / halftone dot controller 57.

[0028] (2) S3: Detection and development bias Vb, modification of the surface potential V ₀ solid image density Ds. A solid image pattern is selected from the test image generator 17 and after uniformly charged by the charger 2, a solid image signal is generated as a print signal.
The solid image is exposed by the exposure device 3, and a bias voltage Vb is applied to the developing roll 23 to apply a toner image (solid image) 33.
To form The residual potential Vr after the exposure may be measured by the potential sensor 11 and stored in the charging control unit 14.
The image density (optical density; Ds) of the toner image 33 on which the solid image is formed is detected by the optical density sensor 12 and the optical density detector 18. The detection signal of the optical density sensor 12 is transmitted to the solid density control unit 56 in the optical density detection unit 18 based on the pattern information from the image designation unit 16. In the solid density control unit 56, the developing bias voltage Vb is controlled by the variable developing bias power supply 10 so that the image density detection value Ds becomes equal to the target value (reference value xs) of the solid image density. That is, the optical density sensor 1
The output value y of the optical density sensor 12 for the image of the reference density xs stored in the solid density control unit 56 in advance.
The developing bias voltage Vb is corrected so as to be equal to the output value ys.

The optical density sensor 12 for a solid image
It is more preferable that the reference output ys is periodically calibrated. This is because in the long term, the output of the optical density sensor 12 tends to change due to contamination of the sensor window, drift of the zero point, and the like. The calibration of the reference output of the optical density sensor 12 can be performed by the following method. The reference output value ys obtained by the calibration of the optical density sensor 12 is input to the solid density control unit 56 and stored.

A method using a calibration plate having a reference density value xs;
The calibration plate placed on the photoreceptor is measured by the optical density sensor 12, and the output value is used as a reference output ys.

A method of setting a reference output ys based on the output value yt of the optical density sensor 12 with respect to the saturation value of the image density; when the correction value is Δy, ys =
It can be obtained from yt + Δy. Here, the correction value Δ
y is the difference between the previously measured saturated concentration and the reference concentration (ys
-Yt). Alternatively, (D
The output value ys of the optical density sensor 12 for the image of the reference density xs may be obtained by assuming the yD curve of FIG. 13 based on (t, yt) and (Dp, yp) for the photoconductor background portion. As an approximate expression of the y-D curve, for example, approximation may be performed using Expression (4) with a and b being constants.

Y = a · (10 ^−D ) + b (4) The saturation value Dt of the image density is set such that the toner density is set higher than the center value of the allowable range, and the developing potential difference | Vb−Vr |
Was substituted by the image density when printing at a value 50 to 100 V larger than the development potential difference at the time of actual printing. By printing under the condition that the developing ability is increased in this manner, a substantially constant saturated image density Dt can be obtained.

Incidentally, instead of the yD curve of FIG. 13, the toner adhesion amount (M
/ A) t and (M / A) s are measured in advance, and y−M /
An A-curve may be assumed. That is, considering that the toner adhesion amount on the background of the photoreceptor is zero, and assuming a y-M / A curve based on (yp, 0) and [yt, (M / A) t], By giving M / A) s, the output value ys of the optical density sensor 12 with respect to the image of the reference density xs can be obtained.

Further, in this step, | V0−Vb |
Since There tends to increase becomes smaller than 150 to 200V fog (background fouling), if so, transmits a signal for instructing correction of the solid density control unit 56 V0 to the charge control unit 14, the surface potential V ₀ which To the control voltage Vg applied to the control electrode 42 or the discharge current ic flowing through the discharge wire 41 (or the control voltage Vg applied to the control electrode 42 and the The flowing discharge current ic) is controlled.

(3) S4: Storage of Corrected Values of Developing Bias Vb and Surface Potential V _{0 Under} the solid image forming conditions other than the solid image exposure amount Is determined in the above-mentioned steps (1) and (2). A certain surface potential V ₀ and a developing bias voltage Vb are determined.
₀ is stored in the charging control unit 14 and the developing bias voltage Vb is stored in the solid density control unit 56. Incidentally, the solid image exposure amount Is is stored in the memory 55.

Next, the dot image density control process from S5 to S12 in FIG. 3 will be described. (4) Detection of Reference Halftone Image Density Dr The reference halftone image in the present invention is a test pattern required only when determining image forming conditions of halftone dots, and is a plurality of halftone dots having different halftone dot area ratios. Refers to a halftone image created for actually measuring and obtaining the reference image density for. Therefore, it is desirable that the reference halftone dot can faithfully reproduce the set halftone dot area ratio.

On the other hand, the standard halftone dot image is formed at a resolution used for normal printing (abbreviated as a standard mode).
Refers to a halftone image used for printing a gradation image.

The inventors have found through experiments that the characteristics required as a reference halftone image can be obtained by forming a halftone dot having the same area ratio as that of the standard halftone image but having a coarser area than the standard halftone dot. Was. (The experimental results will be described later with reference to FIG. 4.) The reason for this will be described below. FIG. 15 is a schematic diagram showing the relationship between the size of a halftone pixel and the reproducibility of black and white halftone dots. The black enlarged portion 61 around the image shown in gray is an example in which the enlarged width is enlarged to δ with respect to the target black halftone pixel 59. For black halftone dots having the same halftone dot area ratio, increasing the number of black halftone pixels 59 decreases the number of black halftone pixels, and as a result, the entire perimeter of the black halftone pixel 59 decreases (in this example, 1 / 2), so the black enlarged portion 6
It can be seen that the area of No. 1 is reduced and the reproduction of the dot area ratio is improved. Similarly, also in the white halftone dot, when the white halftone pixel 60 is increased, the number of white halftone pixels is reduced, and as a result, the entire perimeter of the white halftone pixel 60 is reduced (in this example, 1/2).
It can be seen that the area of the black enlarged portion 61 is reduced, and the reproduction of the halftone dot area ratio is improved.

FIG. 16 shows an example of a standard halftone image and a reference halftone image when the halftone dot area ratios are different. For example, the reference halftone dot in the double mode has a double length of one side of a dot forming the halftone dot in the standard mode, and the reference halftone dot in the quadruple mode is
The length of one side of a dot forming a halftone dot is four times the length of the standard mode, and the resolution is 倍 times the standard resolution, respectively.
It is 1/4 times the standard resolution. From this example, it can be seen that in order to form a halftone dot, the pixel shape and arrangement (dispersion degree) of the halftone dot should be similar, and the resolution of the reference halftone dot should be lower than the standard resolution. As a method of forming a coarse halftone dot under the condition that the halftone dot area ratio of the standard halftone dot is equal to the resolution, the number of lines of the reference halftone dot (screen ruling: 25.4 / p)
Although it is conceivable to form a halftone dot with s (books / inch)) using a smaller number of lines than the standard number of lines, it is necessary to increase the number of halftone pixels. Image. Further, from the viewpoint of reducing the number of pixel memories and simplifying the circuit configuration, it is preferable that the pixels of the reference halftone dot be similar and larger than the pixels of the standard halftone dot. It is preferable to enlarge the size by a factor of 2, more preferably 2 to 4 times the standard halftone dot. In addition, if the number of halftone pixels entering the measurement spot of the optical density sensor 12 is reduced (less than four), the measurement variation becomes large. Therefore, as a more preferable reference halftone, the resolution of the reference halftone is set to the standard value. It is preferable that the resolution is 1/2 to 1/4 the resolution and the dots of the reference halftone dot are formed in the standard 2 to 4 times mode.

In order to form the above-described reference halftone image, a plurality of reference halftone image patterns having different halftone dot area ratios are selected from the test image generation unit 17, uniformly charged by the charger 2, and output as a print signal. A plurality of reference halftone image signals are generated, the reference halftone image is exposed by the exposure device 3, and a bias voltage Vb is applied to the developing roll 23 to form a toner image (reference halftone image) 33. Note that the exposure amount, surface potential V ₀ , and developing bias voltage Vb when forming the reference halftone dot use the same values as the solid image forming conditions, and therefore do not need to be changed.

Next, the image density Dr of the reference halftone dot is detected by the optical density sensor 12 and the optical density detecting section 18. The detection value yr of the optical density sensor 12 corresponding to the optical density Dr of the reference halftone dot is supplied to the optical density detection unit 18 along with a signal indicating the halftone dot area ratio based on the pattern information from the image designation unit 16 and the fine line / dot control. It is stored in the unit 57.

(5) S7-S10: Halftone image exposure amount Im
Tentative setting and detection of the standard halftone dot density Dm A standard halftone dot image pattern is selected from the test image generation unit 17, uniformly charged by the charger 2, and then a standard halftone dot image signal is generated as a print signal. The standard halftone image is exposed by 3 and the bias voltage Vb is applied by the developing device 4 to form a standard halftone image.

The provisional setting value of the exposure amount Im for the standard halftone dot image is a value in the range of 1/3 to 2/3 of the solid image exposure amount Is, which has been input to the memory 55 in advance, and The exposure amount is changed by the exposure amount varying means 13 from the solid image exposure amount Is to the temporary setting value of the halftone image exposure amount Im in accordance with a command from the exposure control unit 19 based on the signal relating to the pattern and the dot area ratio. The standard dot image signal corresponds to a standard resolution dot image pattern used for normal printing.

Next, the optical density of the standard halftone dot image is detected by the optical density sensor 12 and the optical density detector 18. The detection value ym of the optical density sensor 12 for the image density Dm of the standard halftone dot is sent to the fine line / halftone dot control unit 57 via the optical density detection unit 18.

The thin line / halftone dot control unit 57 compares the image density Dm of the standard halftone dot with the image density Dr of the reference halftone using the halftone dot area ratio as a parameter, and determines that Dm becomes equal to the value of Dr. The following processing is performed.

First, when Dm is equal to the value of Dr, the halftone exposure Im is stored in the memory 55 and set.

Next, when Dm is not equal to the value of Dr, a signal proportional to Dm-Dr is output by the thin line / halftone dot control unit 57.
Is transmitted to the exposure control unit 19, and based on the signal, a signal for correcting the exposure amount is sent from the exposure control unit 19 to the exposure amount varying unit 13, and the current for driving the laser emission is changed.
Correct the exposure. The correction of the halftone image exposure amount Im is performed until Dm becomes equal to the value of Dr. When the Dm becomes equal to the value of Dr, the halftone image exposure amount Im is stored and set in the memory 55. You.

(6) S11: Storage of the determined halftone image exposure amount Im By the above-described steps (4) and (5), the halftone image exposure amount Im
Is determined. The surface potential V ₀ and the developing bias voltage Vb, which are other conditions for halftone dot image formation, are the same and the same value as the solid image formation conditions, and are stored in the charging control unit 14 and the solid density control unit 56, respectively. . The halftone image exposure amount Im different from the solid image forming condition is stored in the memory 55 via the exposure control unit 19.

For the fine line image, the fine line image exposure amount If for obtaining the target fine line width can be determined by the same method as the halftone image. In this case, a plurality of vertical lines, horizontal lines, and oblique lines having an equal pitch are used as the standard thin line image, and an image in a 2 to 4 times mode thereof may be used as the reference thin line image (equal to the area ratio of the standard thin line image). .

As described above, actual printing is performed using the determined image forming conditions. That is, in FIG. 1, the test image signal 36 is turned off according to a command from the process control unit 15.
As shown in FIG. 2, the signal switching unit 65 switches the signal so that the print image signal 40 can be transmitted to the exposure device 3.
Perform at

Hereinafter, the operation of the image recognition unit 64 (image recognition image quality control) during actual printing will be described with reference to FIG. As shown in FIG. 2, the image recognition unit 64
Memory 37 for temporarily storing image data, and image determination unit 38
And a timing correction circuit 63.

In the image determination section 38, the image memory 3
7, the image area ratio is determined together with the determination of the image pattern (identification of the solid image and the halftone image), and the information is transmitted to the exposure control unit 19. The exposure control unit 19 sends a signal corresponding to the solid image and the halftone dot image to specify the exposure amount Is and the exposure amount Im, respectively. The driving current is changed to control the exposure amount of the exposure light beam 35.

The operation of the image forming apparatus of this embodiment will be described below. In this example, a halftone image formed in a 2 to 4 times mode in which halftone pixels are larger than standard halftone pixels is used as a reference halftone image.

FIG. 4 is an image density characteristic diagram of a gradation image in which the dot area ratio is changed under the same solid image forming conditions (surface potential V ₀ , bias voltage Vb, exposure amount Is). A dot area ratio of 100% corresponds to a solid image, and 0% corresponds to a white image. As the developing method, reversal development in which the charging polarity of the photoconductor 1 and the charging polarity of the toner 20 are the same was used.

The gradation curves A, B, and C are examples in which the size of the dots forming the halftone dot as parameters is changed to the standard, double mode, and quadruple mode, respectively. The solid image densities of the respective gradation curves have substantially the same value (about 1.3 (OD)).

In the case of the gradation curve A, the shadow part (the area having a high halftone dot area ratio) tends to be crushed, the curve becomes convex upward, and the photographic image tends to be dark when printed. It turns out. This is because, as described above, each halftone image forming the gradation curve A corresponds to the standard halftone image of FIG. 16 and the pixels of the halftone dot are small, so that the halftone dot is formed by exposing with the solid exposure Is. Then, it is considered that the influence of the black enlargement around the image becomes remarkable, and as a result, the black portion increases more than the target halftone dot area ratio (see FIG. 15), and the above tendency is exhibited.

On the other hand, in the case of the gradation curves B and C,
It has been found that the gradation curve is good, and when a photographic image is printed, the tendency of the photographic image to be dark can be avoided. This is because each halftone image forming the gradation curves B and C corresponds to the reference halftone image in FIG. 16 and the pixel of the halftone dot is larger, so that the influence of black enlargement around the image is suppressed. Even if halftone dots are formed by exposure with the solid exposure Is, a halftone dot area ratio substantially equal to the target halftone dot area ratio can be reproduced (see FIG. 15).
It is considered that the above tendency is exhibited. In other words,
It can be said that the exposure amount for forming the reference halftone image may be the solid exposure amount Is.

When the 2 × mode and 4 × mode halftone dots are used for actual printing, the halftone portion tends to be rough (grainy).

In the present invention, based on these findings, a reference halftone dot is created in a mode two to four times as large as the standard dot, and the reference halftone dot density Dr formed by exposure with the solid exposure Is is used as a reference.
The halftone dot exposure amount Im is controlled so that the halftone dot density Dm in the standard mode becomes equal to the reference halftone dot density. Therefore,
A standard halftone dot of a desired density is obtained for each halftone dot area ratio,
A target gradation curve can be obtained. As a result, a bright photographic image can be obtained.

FIG. 5 shows the relationship between the halftone dot image density and the output of the optical sensor. As the optical sensor, an optical fiber sensor 47 having a fiber bundle diameter of about 1.6 mm was used. It can be seen that when the standard resolution is 480 dots / inch, the data of the standard, double mode, and quadruple mode dot sizes are all represented by a single curve with respect to the dot image density. In other words, in the range from the standard to the 4 × mode, as the measurement variation of the optical density (reflectance) becomes excessive,
It can be seen that the halftone pixels are not coarse. As for the dot size, a quadruple mode or more may be used as long as the measurement spot (fiber bundle diameter) is sufficiently large and the measurement variation is not excessive. In the case of the integrated photosensor, the measurement spot was about 5 mm to 8 mm, which was larger than the optical fiber sensor 47 described above.

The number of reference halftone dots is as follows, where the number of gradation steps is j.
The first stage (zero dot area ratio) is a white image, and the last stage (jth stage; halftone dot area ratio is 100%) is a solid image. It is desirable to create individual pieces. Since the standard halftone dot compares the image density with the reference halftone dot having the same area ratio, the image densities Dmi of the plurality of standard halftone dots are compared.
(I = 1, 3,..., J−2; j is the number of gradation steps)
The exposure amount Imi (i = 1, 1) of each halftone dot is set to be equal to the reference halftone image density Dri (i = 1, 2,..., J-2).
2,..., J−2). Note that the number of reference halftone dots may be in the range of 1 to (j-3). In this case, the exposure amount is substituted or interpolated for standard halftone dots having an area ratio without some reference halftone dots. Just set it.

The above-described image forming apparatus is applied to an electrophotographic printer, and an organic photoconductor (OPC) is used as the photoconductor 1.
And a photoconductor 1 having a peripheral speed of 200 to 300 mm / sec
₀ = -550 to -700 V to form a solid electrostatic latent image having a contrast potential | V ₀ -Vr | of about 500 to 650 V, and the ratio of the peripheral speed of the developing sleeve 29 to the peripheral speed of the photosensitive member
(K = developing sleeve peripheral speed / photoconductor peripheral speed) is 1.5 to 2.0.
And -350 to -50 on the developing sleeve 29.
When a reversal development was performed by applying a development bias of 0 V,
In long-term printing, the solid image density (Ds) is maintained at 1.25 to 1.35 (OD) and several hundreds to several tens.
The fluctuation width of the density occurring in the period of 000 pages could be suppressed small. Therefore, there was no density unevenness, and the printing of clear solid images and characters with high contrast could be maintained.

Further, with respect to the gradation image in which the dot area ratio is changed, the curves B to C in FIG.
A gradation curve similar to that of was obtained, and the image quality of a bright and clear photographic image could be maintained. Further, the tendency that the shadow part (the halftone dot with a high halftone dot area ratio) is destroyed and the reproduction of the highlight part (the halftone dot with a low halftone dot area ratio) becomes insufficient can be alleviated.

[Second Embodiment] In the first embodiment described above, the halftone dot exposure amount Im is controlled. However, the pulse width control for changing the halftone pixel exposure time tm instead of the halftone dot exposure amount Im is performed. May be performed. In this case, an image is formed by exposing a solid image with a pixel exposure time ts and exposing a halftone image with a pixel exposure time tm (ts ≧ tm).

[Third Embodiment] A third embodiment is shown in FIG.
7 will be described.

The third embodiment is different from the first and second embodiments in that the potential sensor 11 detects latent image potentials Φi (i = 1, 2,..., J−2) of a plurality of reference halftone dots. )
Next, the latent image potential Vi (i = 1, 2,..., J) of the standard halftone dot
-2) is detected and the two are compared, and the exposure amount for the standard halftone dot (halftone exposure amount Im) is set such that the latent image potential Vi of the standard halftone dot becomes the same as the latent image potential Φi of the reference halftone dot. Is set. In this example, the potential sensor 1
The latent image potential detection value of 1 is provided with information such as the dot area ratio by the latent image potential detection unit 66 and sent to the fine line / dot control unit. Also, instead of adjusting and setting the dot exposure amount Im, a dot pixel exposure time tm may be set.

FIG. 12 is a latent image potential curve of a halftone dot when the dot size is changed to the standard, double mode and quadruple mode under the exposure amount Is. As examples of the latent image potential of the reference halftone dot, a double mode curve E and a quadruple mode curve F correspond.

In this embodiment, similar to the first and second embodiments, there is an effect that the reproducibility of the standard halftone dot can be improved. Since there is no need to form an image, there is the advantage of not consuming toner.

[Fourth Embodiment] FIG. 10 shows another embodiment of the present invention. In the first to third embodiments, only one optical density sensor 12 is provided. In the example,
A second optical density sensor 50 in addition to the optical density sensor 12;
To detect the toner image 33 on the photoreceptor 1 and the toner image 52 on the sheet 34, respectively, and an image density correction unit 68
, The solid or halftone dot image density is corrected based on the optical density / detection value of the toner image 33 on the photoconductor 1 and the optical density / detection value of the toner image on the paper. The detection and correction by the second optical density sensor 50 may be performed when the machine is started or after printing a predetermined page. The second optical density sensor 50 is desirably attached to a position facing the guide roll 51. At this position, the sheet 34 is in close contact with the guide roll 51 and the gap between the second optical density sensor 50 and the sheet 34 can be kept constant.

In this embodiment, even if the transfer efficiency changes due to a change in the environmental conditions or the type of paper, the second optical density sensor 50
Since the image density can be corrected based on the detection value of, the stability of the image density is improved compared to an example in which the toner image 52 on the sheet 34 is not detected.

The value detected by the first optical density sensor 12 and the value detected by the second optical density sensor 50 are compared with each other.
It may be determined whether the operation of the first optical density sensor 12 is normal or not, and if it is abnormal, an abnormal signal is issued.

[Fifth Embodiment] FIG. 11 shows another embodiment of the present invention, in which a continuous paper is used as the paper 54, and a solid or halftone image density is detected, and at least the developing bias is adjusted. In the mode, the toner image 33 is
The pressing mechanism 70 and the pressing drive 7
1, the paper 54 and the transfer device 5 are moved in the direction of arrow Z from the paper path 53 (dashed line) at the time of contact with the guide roll 51 as a fulcrum so that the photoconductor 1 and the paper 54 are in a non-contact state. A process control unit 1 configured to select either the test mode or a print mode for printing on paper.
5 is provided.

In such a configuration, the toner image 33 can be formed at an arbitrary position on the photosensitive member 1 in the test mode for detecting the solid or halftone image density and adjusting at least the developing bias. Is paper 5
4, the paper 54 is not consumed except in actual printing (print mode).

When a cut sheet is used as the sheet 54, if the sheet 54 is not conveyed to the transfer section in the test mode, and the transfer unit 5 is configured to be in a non-contact state with the photosensitive member 1. Often, even when cut paper is used, the same effect as in the case of continuous paper described above can be obtained.

[Sixth Embodiment] The sixth embodiment is shown in FIG.
8 will be described.

The sixth embodiment is different from the first and third embodiments in that the halftone image forming conditions are set in the first and third embodiments. The toner image or the electrostatic latent image is formed on the photoreceptor 1 and the reference value is obtained by detecting the toner image or the electrostatic latent image. However, in the sixth embodiment, the density calculating unit 67 is provided to provide the solid image density and the background portion. Yule-Niel which gives the image density when ideal halftone image reproduction is performed from the density etc.
The point of using sen's formula is to set a reference value for each halftone dot area ratio. Instead of adjusting and setting the dot exposure amount Im, the dot pixel exposure time tm may be adjusted and set. A method using the above-mentioned Yule-Nielsen equation will be described below.

In the Yule-Nielsen equation, n is a parameter that changes depending on the light diffusivity and the number of lines of the background and toner.
The dot image density is Dm (OD), and the dot area ratio is f × 100.
(%) Is given by equation (6).

Dm−Dp = −n · log ₁₀ {1−f · [1−10− ^{(Ds−Dp) / n} ]} (6) where Dp is the density of the background, and Ds is the solid density. Image density.

For example, the condition shown in FIG. 4 (Dp ≒ 0.1,
Ds ≒ 1.3) and n is assumed to be 2, and the relationship between the halftone dot image density and the halftone dot area ratio is calculated as f × 100 (%).
It turned out that it almost coincides with the gradation curve C of FIG.

Further, the following equation (7) was obtained by modifying equation (6).

[0081] In ^{10 (-Dm / n) = (} 1-f) · [10 ^{(-Dp / n)]} + f · [10 ^{(-Ds / n)]} ... (7) (7), 10 ^{(- Dm)} = (y / y ₀ ), 10
^{By substituting (−Dp)} = (yp / y0), 10 ^(−Ds) = (ys / y0) and transforming, equation (8) is obtained.

Y ^{(1 / n)} = (1−f) · [yp ^{(1 / n)} ] + f · [ys ^{(1 / n)} ] (8) In the equation (8), y is an ideal This is an output value of the optical density sensor 12 (approximate value based on the Yule-Nielsen equation) when the halftone dot reproduction is performed. If yp, ys, f, and n are given, the optical values of the solid image and the non-image portion are obtained. The output value y of the optical density sensor 12 at each halftone dot area ratio f with respect to the detection values ys and yp of the density sensor 12 can be obtained. As an example, FIG. 14 shows a calculated value of the optical density sensor output y obtained assuming n = 2 in the equation (8) and an actually measured value of the halftone image (standard mode and double magnification mode). It was confirmed that the measured value in the 2 × mode was closer to the calculated value (than in the standard mode), and that the measured value in the 4 × mode (not shown) substantially matched the calculated value. In this embodiment,
The output value y (calculated value in FIG. 14) of the optical density sensor 12 obtained from the equation (7) is used as a reference value yri (i = 1, 3,..., J−
2; j is the number of gradation steps), and the output values ymi (i = 1, 3,..., J−2) of the optical density sensor 12 for a plurality of standard halftone dots having the same halftone dot area ratio f are detected, In order to be equal to the reference value yri (i = 1, 2,..., J−2),
For each halftone dot area ratio f, the exposure amount Imi of each halftone dot (i = 1,
2,..., J-2) are set.

In order to determine the parameter n in advance, the influence of the change in the background portion and the gradation characteristic data at the standard halftone frequency are necessary. In this embodiment, the first to third embodiments are used. Similarly to the above, in addition to the effect that the reproducibility of the standard halftone dot can be improved, by previously determining the parameter n, the step of actually measuring the reference halftone image or the reference halftone latent image can be omitted. This has the effect of greatly reducing the time required.

[0084]

According to the image forming apparatus of the present invention, not only the solid image density but also the halftone (gradation) image density can be improved in stability.

Further, according to the image forming apparatus of the present invention, a sufficient solid image density is ensured, and a gray scale image composed of a plurality of halftone dots having different halftone dot area ratios can be shadowed from a highlight portion. This has the effect of improving the reproducibility of the image reaching the part.

Further, it is possible to prevent the photographic image from becoming dark.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a partial configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating a flowchart of an image quality control unit according to one embodiment of the present invention.

FIG. 4 is a diagram illustrating image density characteristics of a gradation image according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between an image density and an optical sensor output according to an embodiment of the present invention.

FIG. 6 is a diagram showing a schematic configuration of a charger of one embodiment of the present invention.

FIG. 7 is a cross-sectional view showing a part of the developing device according to one embodiment of the present invention.

FIG. 8 is a diagram showing a schematic configuration of a reflection type photosensor according to one embodiment of the present invention.

FIG. 9 is a diagram showing a schematic configuration of an optical fiber sensor according to one embodiment of the present invention.

FIG. 10 is a diagram illustrating a partial configuration of an image forming apparatus according to another embodiment of the present invention.

FIG. 11 is a diagram illustrating a partial configuration of an image forming apparatus according to another embodiment of the present invention.

FIG. 12 is a diagram showing surface potential characteristics of a halftone halftone latent image according to another embodiment of the present invention.

FIG. 13 is a diagram showing output characteristics of a reflection sensor for a solid image according to one embodiment of the present invention.

FIG. 14 is a diagram showing output characteristics of a reflection sensor for a gradation image according to another embodiment of the present invention.

FIG. 15 is an explanatory diagram of the reproducibility of a halftone image according to one embodiment of the present invention.

FIG. 16 is an explanatory diagram of a standard and reference halftone image according to one embodiment of the present invention.

FIG. 17 is a diagram illustrating a schematic configuration of an image forming apparatus according to another embodiment of the present invention.

FIG. 18 is a diagram illustrating a schematic configuration of an image forming apparatus according to another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photoreceptor, 2 ... Charging device, 3 ... Exposure device, 4 ... Developing device, 5 ... Transfer device, 6 ... Image forming device, 7 ... Cleaner, 8
... Fixing unit, 9 ... Power supply for charger, 10 ... Power supply for developing bias, 11 ... Potential sensor, 12 ... Optical density sensor, 13 ...
Exposure amount variable means, 14 ... Charge control unit, 15 ... Process control unit, 16 ... Image designation unit, 17 ... Test image generation unit, 1
8: Optical density detection unit, 19: Exposure control unit, 20: Toner, 21: Magnetic carrier, 22: Developer, 23: Development roll, 24: Toner hopper, 25: Supply roll, 26
... stirring member, 27 ... magnetic sensor, 28 ... regulating plate, 29 ...
Developing sleeve, 30: magnet body, 31: magnetic brush-like developer, 32: mixing ratio controller, 33, 52: toner image,
34 ... paper, 35 ... exposure light beam, 36 ... test image signal,
37 image memory, 38 image determination unit, 39 print signal, 40 print image signal, 41 discharge wire, 42 control electrode, 43 neutralizer, 44 photosensor, 45 light emitting element, 46 light receiving element , 47 ... Optical fiber sensor, 4
8 a first fiber bundle, 49 a second fiber bundle,
50: second optical density sensor, 51: guide roll, 5
3: paper path at contact, 54: paper at non-contact, 55:
Memory, 56: Solid density control unit, 57: Fine line / halftone dot control unit, 58: Adhesion amount control unit, 59: Black halftone pixel, 60 ...
White halftone pixel, 61: black enlarged portion around image, 62: replenishment roll drive unit, 63: timing correction circuit, 64: image recognition unit, 65: signal switching unit, 66: latent image potential detection unit, 67
... Density calculation section, 68 image density correction section, 69 second optical density detection section, 70 pressure contact mechanism, 71 pressure contact drive section.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shinya Kobayashi 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in Hitachi Research Laboratory, Hitachi Ltd. 2H027 DA10 DE02 DE07 EA01 EA02 EA05 EC03 EE08 2H076 DA07

Claims (17)

[Claims] A photoconductor, a charging unit that charges the surface of the photoconductor, an exposure unit that exposes the charged surface of the photoconductor with a predetermined exposure amount to form an electrostatic latent image, A developing unit that attaches and develops toner on the photoreceptor, a detecting unit that detects an image density of an image developed by the developing unit, and, based on image density information of a solid image detected by the detecting unit, A solid density control unit that determines a value of a developing bias applied to the developing unit and a surface potential of the photoconductor; and a memory unit that stores image density information of a predetermined reference halftone dot detected by the detection unit. An exposure control unit that sets an exposure amount for the standard halftone dot so that an image density of a predetermined standard halftone dot having the same halftone dot area ratio as the reference halftone dot becomes the same as the image density of the reference halftone dot; And the solid image and the halftone dot In the image, the image forming apparatus having an exposure amount varying unit which switches an exposure amount. 2. The image forming apparatus according to claim 1, wherein said reference halftone dot is formed by enlarging pixels of said standard halftone dot by an integral multiple. 3. The image forming apparatus according to claim 1, wherein the exposure amount changing unit changes an exposure amount by changing a light emission amount of the exposure unit. 4. The image forming apparatus according to claim 1, wherein the exposure amount changing unit changes the light emission time of the exposure unit to switch the exposure amount. 5. A photoconductor, a charging unit for charging the surface of the photoconductor, an exposure unit for exposing the charged surface of the photoconductor with a predetermined exposure amount to form an electrostatic latent image, A developing unit for applying toner to the photoreceptor to develop the image; a detecting unit for detecting the image density of the solid image; and applying the image to the developing unit based on the image density information of the solid image detected by the detecting unit. A solid density control unit that determines the values of the developing bias and the surface potential of the photoconductor, a potential detection unit that detects electrostatic latent image potentials of predetermined reference halftone dots and standard halftone dots, A memory unit for storing information on an electrostatic latent image potential, wherein an electrostatic latent image potential of a predetermined standard halftone dot having the same dot area ratio as the reference halftone dot is equal to an electrostatic latent image potential of the reference halftone dot; Exposure amount control for setting the exposure amount for the standard halftone dot so that And parts, between the solid image and the halftone image, the image forming apparatus having an exposure amount varying unit which switches an exposure amount. 6. The image forming apparatus according to claim 1, wherein a resolution of said reference halftone dot is lower than a resolution of said standard halftone dot. 7. The image forming apparatus according to claim 1, wherein the number of lines of the reference halftone dot is smaller than the number of lines of the standard halftone dot. 8. An image processing apparatus according to claim 1, wherein an exposure amount for said reference halftone dot is equal to an exposure amount for said solid image. 9. The image forming apparatus according to claim 1, wherein the image density of the solid image is obtained by applying an image density detection value of a toner image developed by depositing toner on the photosensitive member on a recording medium. An image forming apparatus having an image density correction unit for correcting an image density based on an image density detection value of a toner image transferred to a printer. 10. The image forming apparatus according to claim 1, wherein the image density of the solid image is detected without transferring the toner image formed on the photoreceptor onto a recording medium and at least developing the toner image. An image forming apparatus comprising: a test mode for adjusting a bias; and a print mode for printing on a recording medium, and a process control unit for selecting one of the test mode and the print mode. 11. A photoconductor, a charging unit that charges the surface of the photoconductor, an exposure unit that exposes the charged surface of the photoconductor with a predetermined exposure amount to form an electrostatic latent image, A developing unit that attaches and develops toner on the photoreceptor, a detecting unit that detects an image density of an image developed by the developing unit, and, based on image density information of a solid image detected by the detecting unit, A solid density control unit for determining a value of a developing bias applied to the developing unit and a surface potential of the photoconductor; and assuming an ideal reproduction halftone dot for each halftone dot area ratio as a predetermined reference halftone dot, An image density calculation unit for calculating the image density of a point; a memory unit for storing the calculated image density; and an image density detection value of the standard halftone dot having the same area ratio as the reference halftone dot, Will be equal to the calculated image density of Wherein the exposure amount control unit for setting an exposure amount for the standard dot, between the halftone image and the solid image, an image forming apparatus having an exposure amount varying unit which switches an exposure amount. 12. The image forming apparatus according to claim 11, wherein the image density calculation unit is configured to calculate a Yule-Nielse based on image densities of the solid image and the white image detected by the detection unit.
An image forming apparatus for calculating an image density of the reference halftone dot using an expression of n. 13. The image forming apparatus according to claim 11, wherein the exposure amount changing unit changes an exposure amount by changing a light emission amount of the exposure unit. 14. An image forming apparatus according to claim 11, wherein said exposure amount changing unit changes an exposure amount by changing a light emission time of said exposure unit. 15. The image forming apparatus according to claim 1, wherein a halftone dot of the halftone image has a plurality of halftone dot area ratios, and the exposure amount variable unit includes a halftone dot. An image forming apparatus that switches an exposure amount based on an area ratio. 16. An image forming apparatus according to claim 1, wherein said detecting section has an optical sensor. 17. The image forming apparatus according to claim 1, wherein said detection unit has an optical fiber sensor.