JP3265674B2 - Density reproduction adjustment device for extreme highlights in image forming apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus in which a photosensitive member is scanned with a light beam modulated based on multi-tone digital image data and a formed latent image is developed. Without affecting the reproducibility of the high density area,
The present invention relates to an image forming apparatus capable of adjusting the density reproducibility of a very highlight portion.

[0002]

2. Description of the Related Art In an image forming apparatus which irradiates a photosensitive member with a light beam modulated based on multi-tone digital image data and develops a formed latent image, a smooth intermediate density portion can be reproduced. Therefore, in order to not only reproduce the high density part clearly by skipping the background part of the original, but also reproduce the original with photographic gradation, it is necessary to reproduce the stable density below the intermediate density part. Is a mandatory requirement. In particular, the sensitivity of the human eye is less than the middle density part (density,
(Approximately 1.2 or less), and the lower the concentration, the more conspicuous it becomes. Therefore, the stable reproducibility of the concentration below the intermediate concentration is required to be higher than that of the conventional high concentration. The stable reproducibility of the extremely highlighted portion (image area ratio of about 15% or less), which has hardly been a problem in the image forming apparatus, is particularly important for the digital image forming apparatus.

One of the methods for forming an image with multi-gradation digital image data is to convert multi-gradation digital image data into an analog signal, compare it with a reference pattern having a predetermined period such as a triangular wave, and apply a pulse width. A modulation method for obtaining a modulated binary signal is known. In this method, the output pulse width is determined by the magnitude relationship between the analog signal and the reference pattern. The magnitude relation greatly affects the gradation of the reproduced image. In particular, the setting of the relationship between the black level / white level of the analog signal and the upper and lower peak levels of the reference pattern depends on the resolution of the gradation of the extreme highlight and high density parts and the reproduction gradation of the entire image. Affect.

In order to adjust the pulse width characteristics of the binarized image signal, Japanese Patent Application Laid-Open No. 62-181575 discloses a black level by adjusting a full scale range of an analog signal and a bias of a triangular wave reference pattern. , And the pulse width of the white level are set. Further, in Japanese Patent Application Laid-Open No. 63-177153, in order to use the linear portion of the characteristic curve of the image data as widely as possible, the power at which the white level (00H) curve starts to become linear and the power of the image data A light energy measuring device is provided in the optical path so that the power immediately before deviating from the linear portion of the black level (FFH) curve, and a binarization circuit including a triangular wave generator, a gain, and an offset circuit is manually operated. A technique is disclosed in which the volume of a gain and an offset are adjusted to adjust the bias of the reference pattern and the light amounts of the black level and the white level. In Japanese Patent Application Laid-Open No. 62-284578, in order to obtain a stable image even when the sensitivity of the photoconductor changes, the photoconductor is exposed in a predetermined pattern, the charging voltage is detected, and the potential is detected. Or, the amount of light is adjusted.

On the other hand, in order to correct the gradation of the entire image forming apparatus, Japanese Patent Application Laid-Open No. 63-208368 discloses a method in which a plurality of gradation pattern densities from a white level to a black level copied on paper are detected by a light receiving element. A technique is disclosed in which a gamma correction characteristic curve is obtained by obtaining a signal level at which the density becomes equal, and a gamma correction of the entire image forming apparatus is performed by using a memory storing the gamma correction value.

[0006]

By the way, the reproduction start level of the density fluctuates depending on the setting of the white level side.
The setting on the white level side is an important matter because the sensitivity of the human eye is more sensitive to the fluctuation of the extreme highlight area than the high density area. Therefore, the pulse width between the white level and the black level may be set (Japanese Patent Application Laid-Open No. 62-181575), or the bias of the reference pattern, the black level, and the light level of the white level may be set (particularly). 63-177
153), as generally known, the photosensitivity of the photoreceptor fluctuates depending on the use time or between solids.
In addition to the problem that the potential on the photoconductor changes, the actual density reproducibility is not stable even if the above setting is performed.
Due to the difference in light transmission rate due to contamination in the optical path between the devices, the amount of light reaching the photoreceptor actually changes, and similarly, there is a problem that the density reproducibility is not stable.

Further, adjustment is made so that a stable image can be obtained irrespective of fluctuations in the potential of the photosensitive member (Japanese Patent Application Laid-Open No. Sho 62-2).
No. 84578), in electrophotographic development, the reproducibility of the extreme highlight portion varies, especially due to environmental conditions, changes in the developer due to use time, and mechanical dimensional errors of the developing device. Therefore, there is a problem that stable extreme highlight reproducibility cannot be obtained.

FIG. 26 shows an example of the above-mentioned problem, in which the medium / high-density portion is controlled and adjusted, and changes in the DRS (gap distance between the photosensitive member and the developing roll of the developing device), temperature, and humidity. FIG. 9 is a diagram showing a density fluctuation state of a correspondingly changed extreme highlight portion (note that a characteristic curve up to a middle / high density portion is omitted), specifically, a middle / high density portion; The characteristic curve of the image area ratio versus the density of the extreme highlight portion when the density of the image area ratio of 70% is constant at 1.35 is such that the larger the DRS and the lower the temperature and humidity, the more the extreme highlight portion. The density tends to increase. For example, when the image area ratio is 8%, the DRS is 0.375 mm in a low temperature and low humidity state.
There is a difference between 0.12 and 0.22 in image density between the devices (shown by a dashed line) and 0.50 mm (shown by a solid line),
On the other hand, in a high temperature and high humidity state, the concentration is 0.00 (dashed line).
And 0.02 (solid line). From this example, it can be seen that in the state where the density of the middle / high density part is adjusted, the extreme highlight part independently fluctuates based on the fluctuation of environmental conditions and the like. In other words, in order to secure the reproducibility of the gradation of the entire image, when correcting the density of the extreme highlight portion which is easily affected by environmental conditions, the density of the middle / high density portion is not affected. It is important to make correction possible independently.

A technique for detecting a large number of gradation pattern densities and performing gamma correction to correct the gradation of the entire image forming apparatus (Japanese Patent Application Laid-Open No. 63-208368) aims at gamma correction of the entire image forming apparatus. However, it is not intended to stably adjust only the density of the extreme highlight portion, which tends to fluctuate in response to changes in environmental conditions and the like, while adjusting the middle / high density portion.

Therefore, to adjust the reproducibility of the extreme highlight, which tends to fluctuate independently of the reproducibility of the medium / high density portion which has been conventionally controlled and adjusted by various methods, it is necessary to adjust the reproducibility of the medium / high density portion. It is necessary to develop an adjustment method capable of stably adjusting the density of only the extremely highlighted portion without changing the reproducibility.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to correct the density of a very-high-light portion with the maximum density of a photosensitive member or the light potential of a halftone density. Without changing the difference from the development bias potential,
The charging potential, light beam quantity, and developing bias potential are set so that the difference between the dark potential of the lowest density and the developing bias potential changes, so that the density reproducibility of the medium / high density portion is not affected at all. Another object of the present invention is to provide a density adjusting device for an extreme highlight portion in an image forming apparatus which can independently adjust the density of the extreme highlight portion.

Another object of the present invention is to detect the difference between the patch image density of the extreme highlight copied on the paper and the density of the white background portion of the paper, thereby obtaining the variation in the CCD of each color, the exposure lamp, and the solid-state difference. Another object of the present invention is to provide an apparatus for adjusting the density of an extreme highlight portion in an image forming apparatus which detects the density of an extreme highlight patch image without receiving shading errors.

[0013]

[0014]

[0015]

[0016]

In order to achieve the above-mentioned object, the present invention converts an original image photoelectrically read by image reading means into multi-gradation digital image data.
An image forming apparatus scans a photosensitive member charged by a charging unit with a light beam formed based on multi-tone digital image data and develops a latent image formed by a developing unit to form an image. Patch signal generating means for generating a patch signal corresponding to the maximum density portion or a patch signal corresponding to the density of the image data extreme highlight portion; and a charged and unexposed latent image patch (dark potential patch) on the photosensitive member. Patch generating means for generating a latent image patch (bright-potential patch) by charging and exposing with predetermined gradation digital data, and charging amount varying means for varying the charging potential of the charging means;
A light beam light amount correcting means for correcting the light amount of the light beam; and a developing bias variable means for correcting the developing bias voltage of the developing means. In controlling the potential of the photosensitive member at the maximum density, the dark potential patch is set to the target dark potential VHS. Potential, a light potential patch for obtaining the target light potential VLS, a light beam quantity LDS for obtaining the target light potential VLS, and a development baisic potential VB obtained by subtracting a preset fog prevention potential difference VC from the target dark potential VHS. In correcting the density of the extreme highlight portion, a correction value α obtained from the deviation between the density of the patch image on the paper developed with the latent image patch corresponding to the density of the extreme highlight portion detected by the image reading means and the reference value
Are added to the target dark potential VHS and the anti-fogging potential difference VC to obtain the corrected target dark potential VHS '= VHS + α and the corrected anti-fogging potential difference VC' = VC + α. Correction target dark potential V
The charging potential VGS 'for forming the HS' and the light beam quantity L for forming the target bright potential VLS with the bright potential patch.
DS ′ and the development bias potential VB = VH obtained by subtracting the correction fog prevention potential difference VC ′ from the correction target dark potential VHS ′.
It is characterized by comprising control means for calculating S'-VC ', and setting the charge amount variable means, the light beam light quantity variable means, and the developing bias variable means.

Further, the present invention is characterized in that the control means detects the patch image density from the difference between the patch image density on the paper detected by the image reading means and the white background density of the paper. I do.

Further, according to the present invention, instead of detecting the density of the patch image by the image reading means, the developed image of the patch latent image formed on the photosensitive member is irradiated with light, and the reflected light indicating the density is detected. A sensor is provided, and the control means sets a correction value obtained from a deviation between a detection output of the optical sensor and a reference value as a correction amount of the correction target dark potential and the correction fog prevention potential difference.

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

[0026]

[0027]

According to the present invention, the bias of the reference pattern is adjusted so that the difference between the reference image and the patch image density on the paper obtained by developing the latent image patch formed by the patch signal corresponding to the density of the extreme highlight portion is eliminated. Alternatively, the gain is adjusted to correct the density of the extreme highlight portion due to environmental conditions and dirt in the optical path without affecting the reproducibility of the middle / high density portion.

Further, according to the present invention, the error in reading the patch image density is corrected by detecting the difference between the image density of the patch image on the paper and the density of the white background portion of the paper by the image reading means.

Further, according to the present invention, the density of the extreme highlight portion is corrected using gamma correction data which reduces the difference between the image density of the patch image of the extreme highlight portion and the reference value.

Further, the present invention provides a charged potential and a light beam which change the difference between the dark potential and the developing bias potential at the minimum density without changing the difference between the bright potential and the developing bias potential of the predetermined gradation digital data of the photoreceptor. The amount of light and the developing bais potential are set, and the density of the extreme highlight portion is corrected.

Further, according to the present invention, the gap distance between the photosensitive member and the developing roll is adjusted by the distance adjusting means based on the correction value obtained based on the difference between the image density of the patch image in the extreme highlight area and the reference value. I do.

[0032]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is an overall configuration diagram of a first embodiment in which the present invention is applied to a color copying machine, and FIG. 2 is a detailed block diagram of the color copying machine shown in FIG. As shown in FIG. 1, the configuration of the color copier is roughly classified into a scanner unit 100 for reading an original, an image processing unit 200 for processing the read image data, and a laser driven in accordance with the processed image data to drive the photosensitive member. The ROS optical unit 300 irradiates a light beam, and the image forming unit 400 forms an image. Note that reference numeral 101 in the drawing denotes a document table.

In FIG. 2, in the scanner section 100,
The original is irradiated by an exposure lamp 102, and the reflected light is read by a known reduction optical system or a CCD sensor 103 such as a contact image scanner. The reduction optical CCD sensor will be described with reference to FIG. 5. A red CCD sensor 103a, a green CCD sensor 103b, and a blue CCD sensor 103c each having a large number of photoelectric conversion elements arranged in parallel are provided with a CCD sensor video signal amplifier. 103
d, A / D converter 103e, odd-numbered
An Odd / Even signal combining unit 103f for serially combining detection signals from the even-numbered photoelectric conversion elements and a shading correction LSI connected to the correction data storage memory 103h are provided. In consideration of the case where the CCD sensors 103a to 103c are not equally spaced, the green CCD sensor 103b has a blue CCD as a reference position.
A read position shift correction memory 103j for the sensor 103c is provided, and a read position shift correction memory 103k for the blue CCD sensor 103c is provided for the red CCD sensor 103a. The original image read by the reduction optical system CCD sensors 103a to 103c in a manner described later is used as the amplifier 10 shown in FIG.
After being amplified to an appropriate level in step 4, the data is converted into 8-bit digital image data by the A / D converter 105. Then, after performing shading correction and gap correction by the shading correction unit 106 and the gap correction unit 107, the data is converted from reflectance data to density data by the density converter 108 and sent to the image processing unit 200.

In the image processing section 200, the color conversion section 201 performs basic image processing as a color copying machine, that is, color signal conversion, black reproduction (UCR), MTF processing, and the like. The image data is converted into image data of four colors of black. Next, gamma correction is performed by the gamma correction unit 202, correction of each color gradation is performed according to the gradation of the image forming unit 400, and the data is converted into analog data by the D / A converter 203. In the comparator 206, the triangular wave generator 2
For example, the standard analog triangular wave shown by a solid line in FIG. 7 is compared with the input analog image data, and a portion where the analog image data is larger than the triangular wave is set to “0”, that is, The portion where the laser is off and the analog image data is small is “1”, that is, binarized image data for turning on the laser is formed and sent to the ROS optical section 300. Note that the waveforms indicated by the two-dot chain line and the one-dot chain line in FIG. 2 shows a process of forming pulse width modulated binary image data.

Further, as shown in FIG. 2, the image processing unit 200 includes a patch signal generator 206 for generating image data having an area ratio of 8% corresponding to the extreme highlight portion, analog image data, and an area ratio of 8%. And a selector 207 for selecting one of the% image data and inputting it to the comparator 205. The selector 207 selects analog image data at the time of normal copying, and at the time of performing adjustment of the extreme highlight portion according to the present invention, receives a patch creation instruction signal from an arithmetic unit 411 of the image forming unit 400 described later, and generates a patch signal. 8% image data from the unit 206 is selected and sent to the comparator 205.

The ROS optical unit 300 includes an image forming unit 40
The laser driving circuit 301 is provided with a laser light amount changing device 306 which is controlled by an instruction signal of the arithmetic unit 411 for changing the laser light amount, and a laser driving circuit 301.
01 controls on / off of the laser 302 based on the binarized data sent from the comparator 205. The laser light is deflected by the polygon mirror 303 and passes through the fθ lens 304 and the reflection mirror 305 to form the image forming unit 400.
Of the photosensitive member 401.

In the image forming section 400, a scorotron type charging device 402, a rotary developing device 403, a transfer device 406, a cleaner device 404,
Static elimination lamp 405, electrometer 4 for measuring potential on photoconductor
09 and an optical sensor 410 for measuring the reflected light of the patch developed image on the photoconductor. Further, a toner dispensing device 415 for supplying toner to the developing device of each color of the rotary developing device 403, a fixing device 408, a sheet conveying device 417, and a computing device 41 for controlling image formation.
1. There is provided a charge amount varying device 413 which is controlled by the arithmetic device 411 and changes the charge amount of the photosensitive member 401 by the charging device 402.

Then, an image is formed according to a well-known xerographic process. That is, the rotating photoconductor 4
01 is uniformly negatively charged by the charging device 402, and first a latent image of the first color is formed by the laser beam. The portion of the latent image written by the laser light with the negatively charged black toner in the first color black developing device of the rotary developing device 403 is developed, and the developed image is transported from the paper tray 407a by the paper transport device 407. The image is transferred to a sheet (not shown) wound around the transfer drum 406b by the transfer corotron 406a. The image remaining on the photoreceptor 401 without being transferred is removed by the cleaner device 404, the photoreceptor 401 is discharged by the discharging lamp 405, and is uniformly negatively charged again by the charging device 402, and the second color yellow image is formed. Formation follows. In this way, the developed images of the four colors up to the third color magenta and the fourth color cyan are transferred to the transfer drum 406.
When the sheet is sequentially transferred to the sheet on the sheet b, the sheet is separated from the transfer drum 406b by the separation corotron 406c and fixed by the fixing device 408 to form a color copy.
A neutralizing corotron 4 is provided around the transfer drum 406b.
06d is provided to remove excess charge on the paper after transfer of each color or after peeling of the paper and on the film of the transfer drum 406b.

Next, the method of adjusting the density reproduction of the extreme highlight portion according to the first embodiment will be described with reference to the adjustment flowchart shown in FIG. First, the patch signal generator 206 generates an extreme highlight patch signal of the area ratio 8% image data according to an instruction from the arithmetic unit 411, causes the selector 207 to select this patch signal, and sends it to the comparator 205. Thereafter, according to the process of the color copier described above, FIG.
As shown in FIG. 7, black 407c, yellow 407d, and magenta 407 are placed on paper 407b fed in the direction of the arrow.
The extreme highlight patch image of the image data having an area ratio of 8% for each color of e and cyan 407f is developed to create a copy (step S1).

Next, the created copy is sent to the scanner unit 10.
5, the red, green, and blue CCD sensors 103a through 103a are scanned in the direction of the arrow as shown in FIG.
03c, the specific point 4 of the cyan, magenta, and yellow highlight patch images 407c to 407f corresponding to the complementary colors
Read the reflected light at 07h ... The black patch 407c uses a signal read by the green CCD sensor 103b. When reading the reflected light indicating the development density of the extreme highlight patch image, each color C
CD sensors 103a to 103c, exposure lamp 102
In order to compensate for variations due to variations in image quality, variation between solids, and shading correction errors, the development density of specific points 407g... On the white background portion of the copy paper 407b of FIG. Sensor 103
Then, the arithmetic unit 411 obtains a subtraction output value of the reference white background reflected light from the reflected light of each patch image (step S2).

Next, the obtained subtraction output value of each color is compared with a target value previously set in the arithmetic unit 411 (step S).
3), an operation plate 5 shown in FIG.
In the liquid crystal display unit 501 of 00, the volume adjustment direction of the triangular wave adjuster 209 is displayed to the right or left for colors having a deviation from the target value, and OK is displayed for colors within the target value ( Step S4). If all colors are OK,
The adjustment is completed (step S5). Otherwise, the triangular wave adjuster 209 is set in accordance with the display contents of the liquid crystal display unit 501.
When the volume for changing the bias amount provided for each color is manually turned clockwise, a positive bias is applied to the triangular wave (two-dot chain line), and the pulse width output from the comparator 205 is widened. Therefore, the laser light amount for the image signal of the same 8% area ratio becomes large, and as a result, the density to be developed is increased, and when it is turned to the left, a negative bias is applied (dotted line), and the development density is reduced. The thinning is adjusted (step S6). In this way, the triangular wave adjuster 209 adjusts the bias amount for the triangular wave for each color until all the colors are OK, and adjusts the density reproducibility of the extreme highlight portion of each color.

In FIG. 7, when the bias amount for the triangular wave is adjusted, the entire pulse width, that is, the laser light amount changes from the white level to the black level, but the bias amount necessary for adjusting the extreme highlight portion density is changed. The actual adjustment amount is very small, and the fluctuation of the density in the middle / high density part is very small. This is shown by the variation curve of the photosensitive area potential from the image area ratio to 0% potential in FIG. 8. The extreme highlight characteristic curve a and the bias of the triangular wave are shifted by 2% in the image area ratio. The potential in the area ratio 8% image data with the characteristic curve b in the case is from 100 V to 75 V, that is,
While the voltage shifts by 25 V, the potential difference in the vicinity of the image data having a 30% image area ratio close to the middle density portion is very small, almost 5%.
V shift. As described above, even if the extreme highlight portion is adjusted by controlling the bias of the triangular wave, the extreme highlight portion has a larger pulse width, that is, a change amount of the photoconductor potential with respect to a change in the light amount, but the adjusted portion has a small amount. It can be seen that the / high density portion hardly shifts. Since the human eye is dull on the high density side as compared with the extreme highlight side, the change in density on the medium / high density side is not noticeable in the normal adjustment range of the extreme highlight reproducibility.

In the present embodiment, the adjustment of the triangular wave adjuster 209 is manually performed with a volume, but the remote adjustment means is provided, and the detection output read by the CCD sensor 103 and the target value set in the arithmetic unit 411 are adjusted. The deviation signal may be used as a control signal, and the volume of the triangular wave adjuster may be remotely controlled and adjusted so that the deviation is eliminated. Further, although the amount of bias to the triangular wave is varied by the triangular wave adjuster 209, the same effect can be obtained by varying the amplitude of the triangular wave by adjusting the gain. Further, in the present embodiment, a single triangular wave adjuster 2 is used for the triangular wave from one triangular wave generator 208.
Although an example having 09 is shown, it is also possible to provide a triangular wave generator and a triangular wave adjuster for each color. Further, in an image forming apparatus that reproduces an image having a different resolution depending on the type of a document such as a character or a photograph, the adjustment can be performed for each resolution by the above-described method.

FIG. 9 is a flow chart of the density reproduction adjustment of the extreme highlight portion according to a modification of the first embodiment of the present invention. In this example, as shown in FIG.
An extreme highlight patch developed image P of the image data having an area ratio of 8% is formed thereon (step S21), and the developed image P is irradiated with light from the light emitting diode 401a provided in the optical sensor 410, and the reflected light is reflected by the photodiode. Light is received at 401b, and reflected light of the patch developed image P is detected (step S22).
The optical sensor 410 is provided downstream of the rotary developing device 403 around the photoconductor 401 in FIG. 2, and detects a medium / high-density development patch of the photoconductor and performs toner dispense control, which is conventionally performed. We use what is used to do it together. In this way, the reflected light of each color patch is input to the arithmetic unit 411 and compared with the target value preset in the arithmetic unit 411 (step S23), and the comparison result is displayed on the liquid crystal display unit 501 to adjust the triangular wave. Vessel 209
Is adjusted in the same manner as in the previous example (step S
24). The advantage of this example is that it is not necessary to place the created copy on the platen 101 of the scanner unit 100.
It can be adjusted automatically even during copying.

Next, FIG. 11 is a detailed block diagram of a color copying machine to which the second embodiment of the present invention is applied in which the density adjustment of the extreme highlight portion is performed by gamma correction. The reproduction adjustment of only the density of the extreme highlight portion is performed without affecting the density fluctuation of the density portion. Further, the extreme highlight adjustment method shown in this embodiment uses not only a pulse width modulation method using a reference pattern such as a triangular wave for converting image data into digital, but also a known dither modulation method and an intensity modulation method. An example in which the pulse width modulation method is applied to an image forming apparatus will be described. 11 is different from the one having the configuration shown in FIG. 2 in that the triangular wave adjuster 209 is not provided and the density of the extreme highlight portion described later is corrected for gradation by gamma correction. A plurality of correction tables 210a to 210
c, and a gamma correction changing unit 210 including a selector 207a that outputs one gamma correction data from a plurality of correction tables 210a to 210c in accordance with a selection instruction of the arithmetic unit 411. In addition,
A known developing bias variable device 412 controlled by an instruction of the arithmetic unit 411 based on the detection amount of the electrometer 409 is provided, and the toner dispense control device 415 can be controlled based on the detection amount of the optical sensor 410. Since the other components are the same, the repetition is omitted.

Here, the gamma correction changing means 210 provided in the image processing section 200 includes a gamma characteristic correction curve c for normal use shown in FIG. 13 and a gamma characteristic correction curve for high density correction in the extreme highlight portion. The selector 207a selects one of the correction tables 210a, 210b, and 210c in which the curve d and the gamma characteristic correction curve e for low density correction are converted into data and stored based on a selection instruction of the arithmetic unit 411. And output it to the gamma correction unit 2.
Set to 02 to change the gamma correction content. Then, the data whose gradation has been corrected by the gamma correction table selected by the selector 207 is input to the D / A converter 203, converted into analog data by the D / A converter 203, and converted by the comparator 205 into a triangular wave generator. The signal is pulse-width-modulated in comparison with a signal of a predetermined period sent from the signal 208 and is converted into binary image data.

The extreme highlight reproduction adjustment method according to the second embodiment of the present invention will be described with reference to the adjustment flowchart shown in FIG. In this embodiment, the middle / high-density portion is already controlled by the electrometer 409 and the optical sensor 410 as conventionally performed in an electrophotographic image forming apparatus. Shall be.

In response to an instruction from the arithmetic unit 411, a patch signal generator 206 generates an extreme highlight patch signal of 8% area ratio image data, and the selector 207 selects this patch signal and sends it to the comparator 205. Thereafter, by the above-described xerographic process of the color copier, FIG.
Then, a copy in which the extreme highlight patch images 407c to 407f of the image data having the area ratio of 8% of each color are developed on the sheet 407b shown in FIG.

The copy made in step S31 is placed on the platen 101 of the scanner unit 100, and as described in the first embodiment, each color CCD sensor 103 shown in FIG.
a to 103c, which are cyan, magenta,
Yellow highlight patch images 407c to 407f
The reflected light at the specific point 407h. In order to compensate for variations in the color CCD sensors 103a to 103c and variations in the exposure lamp 102, errors in shading correction, and the like, specific points 407g... On a white background portion of the paper 407b in FIG. The difference between each reflected light of each of the patches 407c to 407f and the white background partial reflected light is obtained as a subtraction output value by the arithmetic unit 411 (step S32). Next, the subtraction output value is compared with a target value set in advance in the arithmetic device 411 (step S33).
An instruction to select the optimum correction data from the plurality of correction tables 210a to 210c so that the difference from the target value is reduced is sent to the selector 2 of the gamma correction changing unit 210.
07a (step S34).

Here, the gamma correction data will be described. FIG. 13 shows the input image data area ratio versus the output image data, and three types of typical data of the gamma correction table.
That is, FIG. 3 shows the standard correction data c, the high density correction data d, and the low density correction data e which are normally used.
The number of correction tables may be three or more. FIG. 14 shows the gradation f at the standard time when gradation correction is not performed with the middle / high density portion being adjusted to a constant value, as an input image density versus an output image density. FIG. 15 shows the target of this color copier. The linear gradation g is shown as an input image density versus an output image density.
Then, when the gradation property f at the standard time in FIG. 14 is corrected by the standard correction data c in FIG. 13, the target gradation property g in FIG. 15 can be obtained. In addition, the density correction data d and e for the extreme highlight portion need not be provided over the entire gradation, but only for the extreme highlight portion. Therefore, the amount of correction data can be reduced.

Therefore, when the difference output of the extreme highlight portion read in step S33 is higher than the target value, in step S34 the high density correction data d of FIG. 13 is selected and the gamma correction is performed via the selector 207a. If the setting is made in the section 202, a final gradation property substantially equal to the target gradation property g in FIG. 15 can be obtained. If the difference output of the extreme highlight portion is lower than the target value, step S34
Then, the low-density correction data e shown in FIG. 13 is selected, and the final gradation is substantially equal to the target gradation g shown in FIG. After selecting the optimum correction data, the operation from step S31 may be repeated and checked again if necessary.

FIG. 16 is a flowchart for adjusting the extreme highlight portion according to a modification of the second embodiment of the present invention. In this example, an extreme highlight patch of image data having an area ratio of 8% for each color on the photosensitive member 401 is shown. Is developed and created (step S41),
As shown in FIG. 10, this extreme highlight patch developed image P
Is irradiated with light from the optical sensor 410, the reflected light is detected by the optical sensor, and is input to the arithmetic unit 411 (step S
42). As described above, the optical sensor 410 is used to detect the middle / high density development patch on the photoconductor and control the toner dispense control unit 415, which is generally performed conventionally. Something is used together. next,
The arithmetic unit 411 compares the read reflected light of each color patch with a target value (step S43), and based on the comparison result,
The gamma correction data for reducing the difference from the target value set in the arithmetic device 411 is selected (step S44). Subsequent steps are the same as in the above-described second embodiment. The advantage of this example is that the created copy is transferred to the platen 10 of the scanner unit 100.
Since it is not necessary to place the image on the photoreceptor 1, if an extremely-highlight patch developed image is periodically formed on the photoconductor 401, the image can be automatically adjusted even during copying.

FIG. 17 shows a detailed block diagram of a color copying machine to which the third embodiment of the present invention is applied. The image data has an area ratio of 100% corresponding to the maximum density, and corresponds to an extreme highlight. Patch signal generator 206 for generating image data having an area ratio of 8%, analog image data, and an area ratio of 1
A selector 207 is provided for selecting one of the 00% and 8% image data and sending the selected image data to the comparator 205. The selector 207 selects analog image data at the time of normal copying, and selects the analog image data at the time of adjustment according to the present embodiment. A patch creation instruction is issued by the arithmetic unit 411 of the image forming unit 400 described later, and 100% area ratio image data or 8% area ratio image data from the patch signal generator 206 is selected and sent to the comparator 205. , And binarized by the comparator 205. The rest of the configuration is the same as that shown in FIG. Further, as described in the description of the second embodiment of the present invention, the third embodiment is also applicable to a pulse width modulation method using a triangular wave, a known dither modulation method, and a halftone conversion method of an intensity modulation method. However, in this example, an example in which a pulse width modulation method is applied will be described. Further, the image processing unit 200 does not affect the middle / high density portion by using the photoconductor potential control flow in a state where the control of the middle / high density portion is already performed as in the second embodiment. Then, reproduction adjustment of only the extremely highlighted portion is performed.

FIG. 18 is a flow chart of the control of the potential of the photosensitive member of the color copying machine. Although the photoconductor potential control of the maximum density is performed in accordance with this flow, it is needless to say that the present invention is not limited to this, and may be performed during copying according to the sensitivity characteristics of the photoconductor to be used. The calculation device 411 stores in advance the target dark potential VHS, the target bright potential VLS, and the fog prevention potential difference VC obtained by subtracting the development baisic potential VB from the target dark potential VHS. First, the grid voltage VGS of the scorotron type charging device 402 is changed to the charging amount variable device 41.
3, dark potentials VH1 and VH when VG1 and VG2 are set
2 is detected by the electrometer 409 (step S51), and the arithmetic unit 411 calculates a grid voltage VGS for obtaining the target dark potential VHS based on the relationship shown in FIG.
G1 + [(VG2-VG1) (VHS-VH1)] /
It is calculated from (VH2−VH1) (step S52).
Next, the grid voltage VGS calculated in step S52 is applied to the charging device 402 to charge the photoconductor 401 (step S53).

Then, in accordance with an instruction from the arithmetic unit 411, the selector 207 of the image processing unit 200 selects the 100% image data of the maximum density from the patch generator 206, sends it to the comparator 205, and compares the binary data with the triangular wave. The ROS data to the laser drive circuit 301 of the ROS optical unit 300,
The laser light amount variable device 306 is a laser light amount LD1, L
A latent image patch of 100% image data in D2 is created, and the exposure unit potentials VL1 and VL2 are detected by the electrometer 410. Next, based on the relationship shown in FIG.
The laser light amount LDS for obtaining the target light potential VLS is LDS
= LD2-[(LD2-LD1) (VLS-VL2)]
/ (VL1-VL2) (Step S5
4). Next, the fog prevention potential difference V is calculated from the target dark potential VHS.
C is subtracted to obtain the developing bias potential VB (step S
55), the grid voltage VGS, the laser light amount LDS, and the development vise potential VB are adjusted by the charge amount variable device 413, the laser light amount variable device 306, and the developing bias combustible device 4.
12, and the adjustment at the maximum density is completed (step S56).

Next, after the above-described adjustment is completed, a new dark potential VHS 'for correcting the density of the extreme highlight portion and a new dark potential VHS' based on the adjustment flowchart in the third embodiment of the present invention shown in FIG. The operation for determining the anti-fogging potential difference VC 'will be described.

In accordance with an instruction from the arithmetic unit 411, a patch signal generator 206 generates an extreme highlight patch signal of 8% area ratio image data, and the selector 207 selects this patch signal and sends it to the comparator 205. Thereafter, as shown in FIG. 4, a copy in which the extreme highlight patch images 407c to 407f of the image data having the area ratio of 8% of each color are developed on the paper 407b by the above-described xerographic process of the color copying machine. (Step S61). Next, FIG.
The specific point 407g of a white background portion where the toner of the copy in FIG.
, And the patch images 407c to 407f
The reflected light of the specific point 407h... And the reflected light of the white background part are detected (step S62). Next, a difference output between each color patch detected by the arithmetic unit 411 and the white background portion is obtained, and the difference output is compared with a set target value (step S63), and a correction amount α proportional to a subtraction value from the target value is calculated. (S6
4). The correction dark potential at the extreme highlight density obtained by adding the correction amount α to the target dark potential VHS described in the flow of the photoconductor potential control in FIG. 18, and the correction obtained by adding the correction amount α to the fog prevention potential difference VC After obtaining the fog prevention potential difference, VC ′ = VC + α (step S6)
5) Then, the flow of the photoconductor potential control shown in FIG. 18 is executed again (step S66).

The photoconductor potential control flow in this case will be described. First, the arithmetic unit 4 is operated in the same manner as described above.
11, the correction target dark potential VHS ', the fog prevention potential difference VC', and the same target bright potential VLS as the previous time are set. Next, the dark potentials VH1 and VH2 of the photoreceptor 401 charged with the grid potentials VG1 and VG2 are detected, a grid voltage VGS 'for obtaining the corrected target dark potential VHS' is calculated, and the photosensitive body 401 is charged with the grid voltage VGS '. Photoconductor 40
1, 100% exposed by laser light amount LD1, LD2
Detecting the maximum density patch potentials VL1 and VL2 of the area ratio;
Then, the laser light amount LDS 'for obtaining the target light potential VLS
Is calculated. Further, VC 'is subtracted from the corrected VHS', and the developing bias potential VB, that is, VB = VHS'-
VC ′ = (VHS + α) − (VC + α) is obtained. Then, the arithmetic unit 411 supplies the grid voltage VGS 'for obtaining the corrected dark potential VHS', the laser light amount LDS 'for obtaining the target light potential VLS, and the developing bias potential VB of the difference between the corrected dark potential VHS' and the fog prevention potential difference VC '. After setting, copy is executed.

Next, referring to FIG. 21, the dark highlight VHS, the light potential VLS, and the developing bias potential VB set in the flow of FIG. 18, and the extreme highlight density obtained by adding the correction amount α in the flow of FIG. Dark potential VHS ′ based on
The reason why the density of the extreme highlight portion can be changed without changing the middle / high density portion from the fog prevention potential difference VC 'will be described. As can be seen from FIG. 21, since the bright potential VLS of the maximum density and the developing bias potential VB are constant before and after the correction of the extreme highlight portion, the difference potential Vd = VB− The VLS is constant before and after the extreme highlight adjustment, so that the density on the high density side does not change. However, the density on the extreme highlight side is changed to the dark potential VHS 'obtained by adding the correction amount α to the dark potential, and the fog prevention potential difference VC is similarly obtained by adding the correction amount α to V
C ', but the developing bias potential VB is VB
= VHS'-VC '= (VHS + .alpha.)-(VC + .alpha.) And remains constant before and after the correction. Therefore, the dark potential VHS' and the anti-fogging potential difference VC 'remain almost unchanged on the high density side. It can be seen that by changing, only the density on the extreme highlight side can be adjusted independently. That is, without changing the difference Vd between the bright potential VLS corresponding to the maximum density and the developing bias potential VB, the grid potential, the laser light amount, and the developing amount are changed so that only the difference between the dark potential of the lowest density and the developing bias potential changes. By setting the bias potential, the density reproducibility of the extreme highlight portion is adjusted without changing the density reproducibility of the middle / high density portion.

More specifically, the image density is generally determined by the difference between the potential of the image area and the developing bias potential VB from highlight to high density.
The highlight part is the dark potential corresponding to the lowest density (white background),
It also changes depending on the difference between the developing bias potentials, that is, the fog prevention potential difference. FIG. 22 is a diagram showing the image area ratio vs. density variation when the fog prevention potential difference VC of 150 V shown by the solid line is changed to the fog prevention potential difference VC of 170 V shown by the dotted line (the difference between the potential of the image portion and the bias potential). Is constant)
It can be seen that even if the fog prevention potential difference VC is changed, the middle / high density side hardly changes. As described above, when correcting the extreme highlight density of the image data having the area ratio of 8% for each color,
By setting so that only the difference between the dark potential corresponding to the lowest density and the developing bias changes without changing the difference between the bright potential corresponding to the maximum density and the developing bias, the middle / high density portion is not affected. Can correct the extreme highlight patch density.

In this embodiment, the bright potential corresponding to the maximum density is used, and the density correction of the extreme highlight is performed after the photoconductor potential control flow is performed. For example, the image area ratio is 60%.
It is of course possible to correct the density of the extreme highlight portion after performing the photoconductor potential control flow using the bright potential patch corresponding to the halftone image data. In this case, the target dark potential, the target halftone potential VMS, and the fog prevention potential difference obtained by subtracting the developing bias potential from the target dark potential are stored in the arithmetic unit 411 in advance. Thereafter, the photoconductor potential control flow shown in FIG. 18 is executed to calculate the grid voltage for obtaining the target dark potential, and then, the image area ratio 60% image data and the binary image data compared with the triangular wave The photosensitive member is exposed based on the above, and a halftone potential is detected. Then, the amount of laser light for obtaining the target halftone potential is calculated, the fog prevention potential difference is subtracted from the target dark potential to determine the developing bias potential, and the grid voltage VG for obtaining the target dark potential as described above.
S, a laser light amount LDS for obtaining a target halftone potential VMS,
It is necessary to set the developing bias potential VB.

FIG. 23 is a flow chart of an adjustment method according to a modification of the third embodiment of the present invention. In this example, an extremely high-light patch of image data having an area ratio of 8% for each color on a photosensitive member 401 is shown. The exposed and developed extreme highlight patch developed image P is formed on the photoreceptor 401 as shown in FIG. 10 (step S71), and the reflected light is detected by the optical sensor 410 and input to the arithmetic unit 411 (step S71). Step S7
2) The reflected light of each color patch is compared with a target value (step S73). As a result of the comparison, the arithmetic unit 411 calculates a correction amount α in proportion to the difference from the target value (step S74), and sets the correction amount α in the target dark potential VHS and the fog prevention potential difference VC, respectively. And VHS '=
After calculating VHS + α and VC ′ = VC + α, the flow of the photoconductor potential control shown in FIG. 18 is executed again as described above. The advantage of this modification is that it is not necessary to place the created copy on the platen 101 of the scanner unit 100, and if a highlight patch is periodically created on the photoconductor, it can be automatically adjusted even during copying. is there.

FIG. 24 is a flow chart showing the adjustment in the fourth embodiment of the present invention for adjusting the density on the extreme highlight side by changing the gap distance between the photosensitive member and the developing roll. In this embodiment, as shown in FIG. 25, the developing devices of the respective colors of the rotary developing device 403 shown in FIG.
The distance adjustment rod 403d moves forward and backward with respect to the immovable reference plate 403c by the correction distance amount β of the developing roll 403b in the developing device 403a of each color provided in the rotary developing device 403 opposed to the photosensitive member 401 with respect to the photosensitive member 401. To change the gap distance. The remaining configuration is the same as that of the third embodiment shown in FIG.

The flow will be described below. As shown in FIG. 4, the extreme highlight patch images 407c to 407f of the image data having an area ratio of 8% for each color are developed on paper 407b to create a copy (step S81). Each of the CCD sensors 103a to 103a of the reduction optical system shown in FIG.
03c, the reflected light from the specific point 407g... Of the white background portion and the specific points 407h... Of the respective color patches 407c to 407f is detected and input to the arithmetic unit 411 (step S82). The difference output of each reflected light is compared with the set target value (step S83), and a correction amount β obtained by converting the difference output into the facing distance is obtained and displayed on the liquid crystal display unit 501 shown in FIG. 8 (step S84). .
Then, the operator recognizes the displayed correction amount β, and moves the distance adjustment rod 4 with respect to the reference plate 403c that is stationary by the correction amount β.
03d is moved forward and backward to change the gap distance (step S85). In this way, the distance between the photoconductor 401 and the developing roll is adjusted for each color, and the density adjustment of the extreme highlight portion is performed. At that time, the density on the medium / high density side also changes slightly, but the change in the highlight part is larger and the human eye is dull on the high density side compared to the highlight side, so normal extreme highlight reproduction In the range of adjusting the sex, the change in concentration on the middle / high concentration side is not bothersome.

In the first to fourth embodiments, when an image forming apparatus having an intermediate transfer member or a transfer film is used, the extreme highlight patch developed on the photosensitive member is Therefore, it is a matter of course that the measurement may be performed by an optical sensor after transfer to an intermediate transfer member or a transfer film.
Further, the above-described first to fourth embodiments can be applied not only to a color copying machine but also to a monochrome multi-tone digital image forming apparatus.

[0066]

As described above, according to the present invention, the dark potential of the lowest density and the developing bias are maintained without changing the maximum density of the photoreceptor or the difference between the bright potential of the halftone density and the developing bias potential. The charging control potential for forming the correction target dark potential, the light beam light amount for forming the target bright potential, and the developing bias potential are set so that the difference between the potential and the potential is changed. / It is possible to independently adjust the reproducibility of only the density of the extreme highlight portion without affecting the reproducibility of the high density portion.

Further, according to the present invention, the image reading means detects the patch image density of the latent image patch of the extreme highlight portion copied on the sheet and the density of the white background portion of the sheet, and determines the difference between the two densities. , Each color C
It is possible to perform reproduction adjustment of the extreme highlight portion without receiving variations in CDs and exposure lamps, individual differences, and shading errors.

[0068]

[0069]

[0070]

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a first embodiment in which the present invention is applied to a color copying machine.

FIG. 2 is a detailed block diagram of the color copying machine shown in FIG.

FIG. 3 is a flowchart of density adjustment of a very highlight portion according to the first embodiment of the present invention.

FIG. 4 is a copy in which an extreme highlight patch image of each color is formed on a sheet in the first embodiment.

FIG. 5 is a block diagram of a CCD sensor of a reduction optical system used in the first embodiment.

FIG. 6 shows an operation display board provided on the color copying machine for displaying whether or not to adjust a triangular wave bias and an adjustment direction.

FIG. 7 is a waveform diagram showing how to adjust the bias of the triangular wave reference pattern to obtain laser drive signals having different pulse widths.

FIG. 8 shows an extreme highlight variation curve represented by an image area ratio versus a potential difference from 0% potential when a triangular wave bias adjustment corresponding to 2% in image area ratio is performed.

FIG. 9 is an adjustment flowchart of an extreme highlight section according to a modification of the first embodiment.

FIG. 10 is a diagram illustrating an optical sensor that detects a developed image density of a latent image of an extreme highlight patch formed on a photoconductor.

FIG. 11 is a detailed block diagram of a color copying machine to which a second embodiment of the present invention has been applied.

FIG. 12 is a flowchart of density adjustment of a very highlight portion according to the second embodiment.

FIG. 13 is a diagram illustrating a gamma correction curve represented by an input data area ratio versus an output image data area ratio.

FIG. 14 is a diagram showing gradation characteristics at a standard time, which is represented by an input image density versus an output image density.

FIG. 15 is a diagram showing a target gradation characteristic represented by a relationship between an input image density and an output image density.

FIG. 16 is a flowchart of density adjustment of an extreme highlight portion according to a modification of the second embodiment of the present invention.

FIG. 17 is a detailed block diagram of a color copying machine to which a third embodiment of the present invention has been applied.

FIG. 18 is a flowchart of photoconductor potential control for performing density adjustment of a high density portion in the third embodiment.

FIG. 19 is a flowchart for calculating a dark potential and an anti-fogging potential difference of an extremely highlighted portion according to the third embodiment of the present invention.

20A is a view for explaining a relationship of calculating a grid potential for charging a photosensitive member with a target dark potential in the flowchart shown in FIG. 18, and FIG. 20B is a view showing a laser light amount LDS for generating a target bright potential on the photosensitive member. FIG. 6 is a diagram for explaining a relationship for calculating.

FIG. 21 is a diagram showing a new dark potential V obtained by adding a bright potential VLS, a developing bias potential VB, a dark potential VHS, and a fog prevention potential difference VC, and a correction amount α obtained at the time of adjusting the extreme highlight portion.
FIG. 7 is a diagram showing a relationship between HS ′ and a fog prevention potential difference VC ′.

FIG. 22 is a diagram showing a state of change between the middle / high density part and the density of the extreme highlight based on the fluctuation of the fog prevention potential difference in relation to the image area ratio body density.

FIG. 23 is an adjustment flowchart for executing density adjustment of an extreme highlight portion according to a modification of the third embodiment of the present invention.

FIG. 24 is an adjustment flowchart for executing the density adjustment of the extreme highlight portion according to the fourth embodiment of the present invention.

FIG. 25 is a schematic view of an adjusting device used in the fourth embodiment of the present invention, which adjusts the gap distance between the photosensitive member and the developing roll of the rotary developing device to adjust the density of the extreme highlight portion. .

FIG. 26 is an explanatory diagram showing a characteristic curve of a pole highlight portion that fluctuates with a change in DRS and temperature and humidity in a relationship between image area ratio and density.

[Explanation of symbols]

Reference Signs List 100 scanner unit, 101 platen, 102 illumination lamp, 106 shading correction unit, 107 gap correction unit, 108 density conversion unit, 200 image processing unit, 201 color conversion unit, 202 gamma correction unit, 205
Comparator, 210 gamma correction changing means, 206 a patch signal generator for generating a patch signal, 207 selector,
207a selector for outputting gamma correction data, 2
08 Triangular wave generator, 209 Triangular wave adjuster, 210
Gamma correction changing means, 300 ROS optical section, 301
Laser driving circuit, 302 laser, 303 polygon mirror, 305 reflecting mirror, 306 laser light amount variable device, 400 image forming unit, 401 photoconductor, 402
Charging device, 403 Rotary developing device 406 Transfer device, 409 Electrometer, 410 Optical sensor, 411 arithmetic device, 413 Variable charge amount device.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Satoshi Tomita 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. (56) References JP-A-63-14173 (JP, A) JP-A-2-150862 ( JP, A) JP-A-3-12671 (JP, A) JP-A-3-129365 (JP, A) JP-A-4-337749 (JP, A) JP-A-3-271764 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) G03G 15/00 G03G 15/02 G03G 15/04 G03G 15/06 G03G 21/00 H04N 1/23-1/31

Claims (3)

(57) [Claims] 1. An original image read photoelectrically by an image reading means is converted into multi-gradation digital image data.
An image forming apparatus for forming an image by scanning a photosensitive member charged by a charging unit with a light beam formed based on the multi-tone digital image data and developing the latent image formed by a developing unit to form an image. Patch signal generating means for generating a patch signal corresponding to the data maximum density portion or a patch signal corresponding to the density of the image data extreme highlight portion; and a charged non-exposed latent image patch (dark potential patches), and charged and the patch generation means for generating a latent image patch and the exposure (light potential patch) at a predetermined gradation digital data, charge amount varying means for varying the charging potential of the charging unit, the light beam light beam light amount correction means for correcting the amount of light, and provided with a developing bias varying means for correcting the developing Baaisu potential of said developing means, the photosensitive member at the maximum concentration Upon position control, charge potential for the dark potential patch obtaining the target dark potential VHS VGS
When asked the bright and <br/> light beam quantity LDS for potential patches obtaining the target light potential VLS, the developing Baaisu potential VB of subtracting antifogging potential difference VC set in advance from the target dark potential VHS, pole In correcting the density of the highlight portion, a correction value α obtained from a deviation between a patch image density on a sheet on which a latent image patch corresponding to the density of the extreme highlight portion detected by the image reading unit is developed and a reference value is calculated. The target dark potential VHS,
And the correction target dark potential VHS ′ = VHS + α and the correction fog prevention potential difference V
C ′ = VC + α is obtained, and when the potential control of the photosensitive member is performed again, the charging potential V for the dark potential patch to form the correction target dark potential VHS ′ is obtained.
GS ' , the light beam amount LDS ' for the light potential patch to form the target light potential VLS, and the developing bias potential VB obtained by subtracting the correction fog prevention potential difference VC 'from the correction target dark potential VHS'. = seeking VHS'-VC ',
The charge amount varying means, the light beam light quantity varying means,
And a control unit for setting the developing bias variable unit. The density reproduction adjusting unit of the extreme highlight portion in the image forming apparatus. 2. The apparatus according to claim 1, wherein the control unit detects the patch image density from a difference between a patch image density on a sheet detected by the image reading unit and a white background density of the sheet. Claim 1
A density reproduction adjustment device for an extremely highlighted portion in the image forming apparatus described in the above. 3. An optical sensor for irradiating a developed image of a patch latent image formed on the photosensitive member with light and detecting reflected light indicating the density, instead of detecting the density of the patch image by the image reading means. Wherein the control means sets a correction value obtained from a deviation between a detection output of the optical sensor and a reference value as a correction amount of the correction target dark potential and the correction fog prevention potential difference. 2. An apparatus for adjusting the density of an extremely highlighted portion in the image forming apparatus according to 1 .