JP3610214B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus that performs exposure scanning on an image carrier based on input image information to form an image on a recording medium.
[0002]
[Prior art]
Conventionally, in an electrophotographic image forming apparatus, when the usage status or usage environment of a photoconductive drum as an image carrier changes, the characteristics of the photoconductive drum change and a significant change occurs in the output image. well known. For example, when the temperature of the photosensitive drum rises, the sensitivity of the photosensitive member improves and the output density changes. Further, as the photosensitive drum is used, the thickness of the photosensitive member gradually decreases, which causes a decrease in charging performance and a deterioration in sensitivity, resulting in a change in output density.
[0003]
Furthermore, as the film thickness of the photoconductor decreases, the characteristics of the light scattered on the photoconductor drum change, which changes the reproducibility of the minute dots, and changes in the charging performance and sensitivity of the photoconductor drum. An unexplainable level of change in output density characteristics occurs.
[0004]
In addition, it is known that changes in the use environment of the apparatus affect not only the photosensitive drum but also the developing apparatus. That is, it is known that when the humidity of the usage environment changes, the charging performance of the developer in the developing device changes, and as a result, a significant change occurs in the output density characteristics.
[0005]
In order to solve these problems, various measures have been used conventionally.
[0006]
For example, regarding the change in the film thickness of the photosensitive drum, a detection unit that detects the film thickness of the photosensitive drum is provided, and the voltage supplied to the charging device is changed according to the detection signal, thereby charging the photosensitive drum. Methods have been proposed to compensate for performance changes.
[0007]
Similarly, there is also known a method in which a change in sensitivity of the photosensitive drum is corrected by changing an applied voltage of a lamp that illuminates a document in accordance with a detection signal for detecting a film thickness.
[0008]
Furthermore, regarding changes in the temperature and humidity of the usage environment, the temperature around the photosensitive drum is detected, and the voltage supplied to the charging device is changed in accordance with the temperature and humidity, thereby changing the performance of the photosensitive drum due to the temperature. A correction method has been proposed.
[0009]
Also known is a method of changing an image signal so as to maintain gradation characteristics in accordance with a detection signal obtained by detecting the film thickness of the photosensitive drum.
[0010]
By adopting such a method, it is possible to stabilize the output density characteristics against changes in sensitivity characteristics and charging performance due to film thickness fluctuations of a medium such as a photosensitive drum.
[0011]
On the other hand, recent electrophotographic printers and copiers have a mode that prioritizes character quality called character mode and character photo mode, and a mode that prioritizes gradation images such as photographs called photo mode. There are many cases. In this case, the image processing method is often changed between the character mode and the photo mode.
[0012]
For example, in the case of a 600 dpi printer, the character mode uses a binarization process of 600 dpi in order to reproduce the gradation characteristics while maintaining the resolving power. In the photo mode, the gradation characteristics and the smoothness of the image are obtained. In some cases, a 200 dpi PWM method may be used to achieve this.
[0013]
As described above, when a plurality of signal generating means are provided, the influence of the sensitivity characteristic due to the film thickness variation of the medium such as the photosensitive drum or the change in the charging performance on the gradation characteristic is often different in each signal generating means.
[0014]
For example, FIG. 17 and FIG. 18 show the output density characteristic results in the case of a printer using 600 dpi binarization processing as the character mode and 200 dpi PWM method as the photo mode.
[0015]
FIG. 17 shows the change in output density characteristics when the binarization process of 600 dpi is used when the film thickness of the photoconductor is changed to 30 μm, 20 μm, 16 μm, and 13 μm, and FIG. The change in the output density characteristic when the 200 dpi PWM method is used when the body film thickness is changed to 30 μm, 20 μm, 16 μm, and 13 μm is shown. This data shows through data that is not subjected to γ conversion. 17 and 18, the horizontal axis represents video data, and the vertical axis represents the output density of a conventional image forming apparatus.
[0016]
As described above, in the case of the binarization processing of 600 dpi shown in FIG. 17 and the case of the PWM method of 200 dpi shown in FIG. 18, the sensitivity characteristics and charging performance due to the film thickness variation of the medium such as the photosensitive drum are obtained. It can be seen that the influence of the change on the gradation characteristics is different.
[0017]
Even in such an image forming apparatus having a plurality of signal generating means, a method for optimizing the image signal conversion method and maintaining the gradation characteristics can be used in accordance with the detection signal for detecting the film thickness. .
[0018]
When such a method is taken, the image signal conversion method is usually not continuously optimized in accordance with the detection signal for detecting the film thickness due to the cost of the image forming apparatus. A technique of switching the image signal conversion method before and after the threshold value of the detection signal for detecting the film thickness is used.
[0019]
[Problems to be solved by the invention]
However, in the conventional image forming apparatus having the plurality of signal generating units as described above, when the same threshold value of the detection signal for detecting the film thickness is used in the plurality of signal generating units, as described above. Depending on the signal generation means, the sensitivity characteristics due to film thickness variation and the influence of the change in charging performance on the gradation characteristics are different, so depending on the signal generation means, the difference in the gradation characteristics before and after the threshold becomes large or just before the threshold. There is a problem in that there is a possibility that problems such as generation of pseudo contours may occur.
[0020]
The present invention has been made to solve the above-described problems, and detects a value corresponding to the film thickness of an image carrier, and based on a comparison between a detection result and a predetermined value, a density characteristic of an image signal. When determining the conversion characteristics of the conversion means for converting the image, the predetermined value for comparison is made different depending on the type of image forming mode including the photo mode and the character mode, so that it does not depend on the type of image forming mode. An object of the present invention is to provide an image forming apparatus capable of appropriately performing density gradation correction in accordance with the photoreceptor film thickness.
[0021]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided an image carrier that carries an image to be formed on a recording medium, conversion means for performing a process of converting density characteristics on an input image signal, and the image carrier. Detection means for detecting a value corresponding to the film thickness of the body, control means for comparing a detection result of the detection means with a predetermined value, and determining a conversion characteristic of the conversion means based on the comparison result; and the conversion means Processing means for processing the image signal converted in accordance with an image forming mode including a photo mode and a character mode, and exposure scanning on the image carrier based on the image signal processed by the processing means. An image forming unit that forms an image on a recording medium, and the control unit varies the predetermined value according to a type of the image forming mode.
[0022]
A second invention according to the present invention is characterized in that the conversion means has a plurality of conversion tables having different γ characteristics, and the control means selects a conversion table based on the comparison result.
[0023]
A third invention according to the present invention is characterized in that the conversion means can execute a plurality of primary conversion processes having different characteristics on an image signal, and determines the primary conversion process based on the comparison result. .
[0024]
According to a fourth aspect of the present invention, the image carrier is a photosensitive member (photosensitive drum 6 shown in FIG. 1).
[0025]
According to a fifth aspect of the present invention, the detecting means measures a current flowing on the image carrier when a voltage is applied to the image carrier, thereby determining a value corresponding to the film thickness of the image carrier. It is characterized by detecting.
[0026]
According to a sixth aspect of the present invention, there is further provided an operation means for selecting the image forming mode.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
[First Embodiment]
FIG. 1 is a schematic cross-sectional view illustrating the configuration of an image forming apparatus showing a first embodiment of the present invention.
[0028]
In the figure, reference numeral 2 denotes a document illumination lamp which illuminates the image information on the document 1 with light. A CCD image sensor (hereinafter referred to as a CCD) 3 receives reflected light from the document 1 and converts it into an image signal. An image signal processing circuit 4 performs predetermined image processing on the image signal output from the CCD 3 and outputs it as a drive signal for driving the laser driver 5. The laser driver 5 performs image exposure according to the image information on the document 1 on the photosensitive drum 6 that is uniformly charged by the drive signal output from the image signal processing circuit 4.
[0029]
A charging member 7 is disposed in contact with the photosensitive drum 6 and is applied with a predetermined voltage from the high voltage power source 11 to uniformly charge the photosensitive drum 6 before exposure. A developing unit 8 performs a developing process of developing the electrostatic latent image on the photosensitive drum 6 formed by the laser driver 5 with toner.
[0030]
A transfer device 9 performs a transfer process in which the toner image on the photosensitive drum 6 formed through the development process is transferred to the transfer paper P fed in synchronization. Reference numeral 10 denotes a fixing unit that fixes the toner image on the transfer paper P conveyed by the conveyance belt on the transfer paper P through a transfer process. A cleaner 12 removes residual toner on the photosensitive drum 6 after transfer.
[0031]
Reference numeral 13 denotes a detection unit that detects the amount of current flowing through the photosensitive drum 6 when a constant voltage is applied to the photosensitive drum 6 by the high-voltage power supply 11 through the charging member 7, and determines the thickness of the photosensitive drum 6. The corresponding detection signal is output. Note that the CPU 25 shown in FIG. 2, which will be described later, corrects the output density characteristic due to the change in the photoreceptor film thickness based on the detection signal output from the detection unit.
[0032]
Hereinafter, the operation of each unit will be described.
[0033]
First, the photosensitive drum 6 is uniformly charged by applying a predetermined voltage to the charging member 7 from the high voltage power source 11. The light illuminated by the document illumination lamp 2 is guided to the CCD 3, and the image information on the document 1 is converted into an image signal. This image signal is processed by the image signal processing circuit 4 and output as a drive signal for driving the laser driver 5. As a result, image exposure corresponding to the image information on the document 1 is performed on the uniformly charged photosensitive drum 6.
[0034]
Next, a developing process for developing with the toner of the developing device 8 is performed on the photosensitive drum 6 on which the electrostatic latent image is formed, and the toner image on the photosensitive drum 6 formed through the developing process is synchronized. The transfer device P is transferred to the transfer paper P that has been taken and fed by the transfer device 9, and the transfer paper P that has undergone this transfer process is transported to the fixing device 10 by the transport belt, and the toner on the transfer paper P is fixed to the transfer paper P. It is fixed by the device 10. Further, the cleaner removes residual toner on the photosensitive drum 6 after fixing.
[0035]
Further, by detecting the amount of current flowing through the photosensitive drum 6 when a constant voltage is applied to the photosensitive drum 6 by the high-voltage power source 11 through the charging member 7, a detection signal corresponding to the photosensitive member film thickness of the photosensitive drum 6. The output density characteristic due to the change in the photoreceptor film thickness is corrected according to the detection signal.
[0036]
FIG. 2 is a diagram showing a configuration of the detection unit 13 for detecting a value corresponding to the photosensitive film thickness of the photosensitive drum 6 shown in FIG. 1, and the same components as those in FIG. It is attached.
[0037]
In the figure, reference numeral 21 denotes an AC voltage source, which superimposes a predetermined voltage V0 on the voltage VPPO while the photosensitive drum 6 is rotating. An analog-digital converter (A / D converter) 24 converts the voltage across the detection resistor 23 corresponding to the current i into a detection signal TD that is a 6-bit digital signal. The detection signal TD is a signal corresponding to the thickness of the photosensitive member, which serves as a measure for measuring the newness of the drum.
[0038]
Reference numeral 25 denotes a CPU that performs overall control of the entire image forming apparatus based on a program stored in the ROM 26. Further, the CPU 25 classifies the photoconductor film thickness according to a predetermined reference based on the detection signal TD output from the detection unit 13, and stores a γ conversion table corresponding to a plurality of γ conversion characteristics γ1 to γ4 shown in FIG. One of them is selected, and control is performed to switch the conversion method of the output (density) characteristics of the image signal.
[0039]
Here, the CPU 25 determines that the greater the value of the detection signal TD, the newer the photoreceptor drum 6 is, and the greater the thickness of the photoreceptor drum, and the smaller the value of the detection signal TD, the thinner the photoreceptor drum thickness due to deterioration of the photoreceptor drum 6. Then, a control signal for selecting the γ conversion characteristic is output.
[0040]
FIG. 3 is a block diagram for explaining the configuration of the image signal processing circuit 4 shown in FIG. 1, and the same components as those in FIGS. 1 and 2 are denoted by the same reference numerals.
[0041]
In the figure, reference numeral 31 denotes an analog processing unit that performs gain adjustment and shading correction on an analog signal from the CCD sensor 3. Reference numeral 32 denotes an analog / digital conversion unit (A / D conversion unit) that converts an analog signal output from the analog processing unit 31 into an 8-bit digital signal.
[0042]
A logarithmic conversion unit 33 converts the digital signal output from the A / D conversion unit 32 into a density signal. Reference numeral 34 denotes a γ conversion unit that performs any γ conversion of γ conversion characteristics γ1 to γ4 on the density signal output from the logarithmic conversion unit 33 to convert the output (density) characteristics of the image signal. Reference numeral 35 denotes a PWM circuit as a signal transmission means, which converts the concentration signal output from the γ conversion section into a pulse width signal for driving the laser 5. Reference numeral 36 denotes a binarization processing circuit as a signal transmission means, which binarizes the density signal output from the γ conversion unit. An operation unit 50 includes a display screen and various setting keys, and can perform various settings including an image forming mode (photo mode, character mode, etc.).
[0043]
The PWM circuit 35 and the binarization processing circuit 36 are selected and controlled by the CPU 25 based on the image forming mode (photo mode, character mode) set by the operation unit 50.
[0044]
Each unit of the image signal processing circuit 4 including the γ conversion unit 34 is controlled by the CPU 25. The CPU 25 receives a value corresponding to the film thickness from the A / D converter 24 and performs control according to this value. A printer control unit 40 includes a charging voltage control circuit 41 that controls a charging voltage by the high-voltage power supply 11, a developing bias control circuit 42 that controls the developing bias of the developing device 8, and a laser light amount adjusting circuit that adjusts the laser light amount of the laser 5. 43. A fixing temperature control circuit 44 for controlling the fixing temperature of the fixing device 10 is included.
[0045]
FIG. 4 is a block diagram for explaining the configuration of the γ converter 34 shown in FIG. 3, and the same components as those in FIG. 3 are denoted by the same reference numerals.
[0046]
In the figure, reference numerals 401 to 404 denote γ conversion tables, which perform γ conversion using the γ conversion tables 401 to 404 respectively corresponding to γ conversion characteristics γ1 to γ4 shown in FIG. .
[0047]
The γ conversion unit 34 is a table conversion circuit that selects (γ conversion table switching) one γ conversion table among the γ conversion tables 401 to 404 based on a selection signal output from the CPU 25.
[0048]
The γ conversion unit 34 as the table conversion circuit can be realized, for example, by providing the ROM with input / output characteristics (γ conversion tables 401 to 404) of γ conversion characteristics γ1 to γ4. In the case of a RAM, the CPU 25 may create a γ conversion table corresponding to the film thickness by setting table data of characteristics to be selected in one set of RAM.
[0049]
FIG. 5 is a characteristic diagram showing the γ conversion characteristics γ1 to γ4 of the γ conversion tables 401 to 404 shown in FIG. 4, and (a) to (d) correspond to the γ conversion characteristics γ1 to γ4. The horizontal axis represents input data (data before γ conversion) to the γ conversion unit 34, and the vertical axis represents output data (data after γ conversion) of the γ conversion unit 34.
[0050]
As shown in FIGS. 4A to 4D, the slope of the curve increases as the γ conversion characteristics change from γ1 to γ4.
[0051]
In other words, the γ conversion characteristic corresponds to the conversion characteristic at the time of γ conversion to a photoconductor having a thin film thickness of a deteriorated photoconductor as it changes from γ1 to γ4 ((a) to (d)).
[0052]
Hereinafter, a method for selecting the γ conversion characteristics by the CPU 25 will be described.
[0053]
First, predetermined threshold values (switching points) A, B, and C (for example, A>B> C) are individually set in advance for each signal generation unit (PWM circuit 35 and binarization processing circuit 36 shown in FIG. 3). It is assumed that it is set and stored in the ROM 26.
[0054]
The CPU 25 selects the γ conversion characteristic γ1 when the predetermined reference for classifying the film thickness of the photoconductor, for example, the detection signal TD is “TD> A (for example, 20 μm)”, and “A ≧ TD> B” is satisfied. In this case, the γ conversion characteristic γ2 is selected. If “B ≧ TD> C”, the γ conversion characteristic γ3 is selected, and if “C ≧ TD”, the γ conversion characteristic γ4 is selected.
[0055]
Hereinafter, with reference to FIGS. 6 and 7, correction (conversion) of the output density characteristic of the image signal in accordance with the photoconductor film thickness of the photoconductor drum 6 will be described.
[0056]
6 and 7 are characteristic diagrams showing respective output density characteristics when the photosensitive drum film thickness of the photosensitive drum 6 shown in FIG. 1 is 30 μm, 20 μm, and 13 μm, and FIG. FIG. 7 corresponds to after the correction of the output density characteristic by the γ converter 34.
[0057]
6 and 7, the horizontal axis represents the document density, and the vertical axis represents the output density of the image forming apparatus of the present invention. This output density characteristic corresponds to a case where a binarization method of 600 dpi is used as the signal generating means.
[0058]
The output density characteristics shown in FIG. 6 change remarkably when the photosensitive film thickness of the photosensitive drum 6 changes.
[0059]
The output density shown in FIG. 7 has a constant output density characteristic regardless of the film thickness (30 μm, 20 μm, 13 μm) of the photosensitive drum 6.
[0060]
The corrected output density characteristics shown in FIG. 7 are selected based on a threshold value (switching point) set to be optimal in the 600 dpi binarization method (γ conversion table). This corresponds to the case where the output density characteristic is corrected by the γ converter 34.
[0061]
Hereinafter, with reference to FIGS. 8 to 10, optimization for each signal generation means for correcting the output density characteristic of the image signal will be described.
[0062]
8 and 9 are characteristic diagrams showing output density characteristics immediately before (for example, detected film thickness> threshold) and immediately after (for example, detected film thickness ≦ threshold) before switching the γ conversion table, and FIG. 8 shows 600 dpi. 9 corresponds to the 200 dpi PWM method (for example, character mode).
[0063]
The output density characteristic correction in FIGS. 8 and 9 is based on a threshold value (switching point, for example, the film thickness is “20 μm”) set to be optimal in the 600 dpi binarization method shown in FIG. This corresponds to the case where the γ conversion table is switched.
[0064]
8 and 9, the horizontal axis indicates the document density, and the vertical axis indicates the output density of the image forming apparatus of the present invention.
[0065]
As shown in FIGS. 8 and 9, compared with the output by the binarization method of 600 dpi shown in FIG. 8, the output by the PWM method of 200 dpi shown in FIG. The difference in output density characteristics is large.
[0066]
Further, the output by the 200 dpi PWM method shown in FIG. 9 has a large deviation from the ideal output density characteristic in the low density region in the output density characteristic immediately before switching the γ conversion table. The potential is great.
[0067]
FIG. 10 is a characteristic diagram showing output density characteristics immediately before (for example, detected film thickness> threshold) and immediately after (for example, detected film thickness ≦ threshold) before switching the γ conversion table, and a 200 dpi PWM method (for example, (Character mode).
[0068]
Note that the output density characteristic correction in FIG. 10 corresponds to the case where the γ conversion table is switched based on a threshold value (switching point, for example, the film thickness is “24 μm”) set to be optimal in the 200 dpi PWM method. To do.
[0069]
As shown in the figure, by setting the film thickness as the switching point to “24 μm”, the difference between the output density characteristics immediately before and after the switching of the γ conversion table indicates that the film thickness as the switching point is “20 μm”. Is substantially equivalent to the 600 dpi binarization method (shown in FIG. 8) when the γ conversion table is switched.
[0070]
Further, the deviation from the ideal output density characteristic in the low density region is small, and the occurrence of a pseudo contour can be prevented.
[0071]
As described above, the image forming apparatus shown in the embodiment uses, for example, a γ conversion table switching point when image formation is performed by the binarization processing circuit 36 that generates an image signal by a 600 dpi binarization method (for example, a photographic mode). For example, when the image forming is performed by the PWM circuit 35 that generates the image signal by the PWM method (character mode) of “20 μm” and 200 dpi, the switching point of the γ conversion table is, for example, “24 μm”, and the signal generating means (binarization processing circuit 36) , PWM circuit 35), and the CPU 25 is configured to optimize the switching point of the γ conversion table.
[0072]
As described above, by optimizing the switching point of the γ conversion table according to the signal generating means, the difference in the change in the output density characteristic before and after the γ conversion table switching (film thickness before and after the threshold) is reduced. In addition, it is possible to prevent the occurrence of defective images such as pseudo contours.
[0073]
[Second Embodiment]
In the first embodiment, the γ conversion characteristic (γ conversion table) is switched in accordance with the photosensitive film thickness of the photosensitive drum 6 to correct the output density characteristic of the image signal (the output (density) characteristic of the image signal). Although the configuration for performing non-linear conversion has been described, the primary conversion is switched in accordance with the film thickness of the photosensitive drum 6 to correct the output density characteristic of the image signal (linear conversion of the output (density) characteristic of the image signal). You may comprise. The embodiment will be described below.
[0074]
FIG. 11 is a block diagram illustrating the configuration of the image signal processing circuit 4 according to the second embodiment of the present invention. The same components as those in FIG. 3 are denoted by the same reference numerals.
[0075]
In the figure, reference numeral 1301 denotes a γ conversion unit that performs γ conversion on the density signal output from the logarithmic conversion unit 33. A primary conversion unit 1302 performs primary conversion on the density signal output from the γ number conversion unit 1301 to convert the output (density) characteristics of the image signal.
[0076]
FIG. 12 is a block diagram for explaining the configuration of the primary conversion unit 1302 shown in FIG. 11, and the same components as those in FIG. 3 are denoted by the same reference numerals.
[0077]
In the figure, L1 to L4 are primary conversion units, each performing predetermined primary conversion on the input density signal.
[0078]
The primary conversion unit 1302 selects one primary conversion unit among the primary conversion units L1 to L4 based on a selection signal output from the CPU 25.
[0079]
First, the density signal primary conversion Ln (where n is an integer of 1 to 4) will be described.
[0080]
Doutn is a density signal after the primary conversion Ln, Din is a density signal before the primary conversion, and An and Bn are coefficients, and the primary conversion Ln of the density signal is “Doutn = an × (Din + bn)”. However, n shall be an integer of 1-4.
[0081]
The primary conversion Ln includes the case where “an = 1” and “bn = 0”, that is, the case where the primary conversion is not performed.
[0082]
The primary conversion unit 1302 can be realized, for example, by providing the ROM with primary conversion coefficients a1 to a4 and b1 to b4. In the case of a RAM, the CPU 25 may set a coefficient data to be selected in a set of RAMs to realize a primary conversion unit corresponding to the film thickness.
[0083]
Hereinafter, a method for selecting the primary conversions L1 to L4 by the CPU 25 will be described.
[0084]
First, predetermined threshold values (switching points) A, B, and C (for example, A>B> C) are individually set in advance for each signal generation unit (PWM circuit 35 and binarization processing circuit 36 shown in FIG. 11). It is assumed that it is set and stored in the ROM 26.
[0085]
The CPU 25 selects the primary conversion L1 when the predetermined standard for classifying the film thickness of the photosensitive member, for example, the detection signal TD is “TD> A (for example, 20 μm)”, and when “A ≧ TD> B” is satisfied. The primary conversion characteristic L3 is selected. When “B ≧ TD> C”, the primary conversion characteristic L3 is selected, and when “C ≧ TD”, the primary conversion characteristic L4 is selected.
[0086]
Hereinafter, the correction (primary conversion) of the output density characteristic of the image signal in accordance with the photoconductor film thickness of the photoconductor drum 6 will be described with reference to FIG.
[0087]
FIG. 13 is a characteristic diagram showing the output density characteristics after the primary conversion when the photoconductor film thickness of the photoconductor drum 6 shown in FIG. 1 is 30 μm, 20 μm, and 13 μm.
[0088]
In the figure, the horizontal axis represents the document density, and the vertical axis represents the output density of the image forming apparatus of the present invention. This output density characteristic corresponds to a case where a binarization method of 600 dpi is used as the signal generating means.
[0089]
As shown in the figure, almost constant output density characteristics are obtained regardless of the thickness of the photoreceptor.
[0090]
Note that the output density characteristic after the primary conversion shown in FIG. 13 selects (switches) the primary conversions L1 to L4 based on a threshold (switching point) set to be optimal in the binarization method of 600 dpi. This corresponds to the case where the output density characteristic is corrected by the primary conversion unit 1302.
[0091]
Hereinafter, with reference to FIGS. 14 to 16, optimization for each signal generation means for correcting the output density characteristic of the image signal will be described.
[0092]
14 and 15 are characteristic diagrams showing output density characteristics immediately before the primary conversion switching (for example, detected film thickness> threshold) and immediately after (for example, detected film thickness ≦ threshold), and FIG. 14 is 600 dpi 2. FIG. 15 corresponds to a 200 dpi PWM method (for example, a character mode).
[0093]
The output density characteristic correction in FIGS. 14 and 15 is based on a threshold value (switching point, for example, the film thickness is “20 μm”) set to be optimal in the 600 dpi binarization method shown in FIG. This corresponds to the case where the γ conversion table is switched.
[0094]
14 and 15, the horizontal axis indicates the document density, and the vertical axis indicates the output density of the image forming apparatus of the present invention.
[0095]
As shown in FIGS. 14 and 15, the output by the 200 dpi PWM method shown in FIG. 15 is the output immediately before and after the primary conversion switching, as compared with the output by the binarization method of 600 dpi shown in FIG. Difference in density characteristics is large.
[0096]
Further, the output by the 200 dpi PWM method shown in FIG. 15 has a large deviation from the ideal output density characteristic in the low density region in the output density characteristic immediately before the primary conversion switching. In this case, a pseudo contour can be generated. The nature is great.
[0097]
FIG. 16 is a characteristic diagram showing output density characteristics immediately before the primary conversion switching (for example, detected film thickness> threshold) and immediately after (for example, detected film thickness ≦ threshold), and is a 200 dpi PWM method (for example, character Mode).
[0098]
Note that the output density characteristic correction in FIG. 16 corresponds to a case where primary conversion switching is performed based on a threshold value (switching point, for example, the film thickness is “24 μm”) set to be optimal in the 200 dpi PWM method. .
[0099]
As shown in the figure, by setting the film thickness as the switching point to “24 μm”, the difference between the output density characteristics immediately before and after the primary conversion switching is set to “20 μm” as the switching point. This is almost equivalent to the 600 dpi binarization method (shown in FIG. 14) when the γ conversion table is switched.
[0100]
Further, the deviation from the ideal output density characteristic in the low density region is small, and the occurrence of a pseudo contour can be prevented.
[0101]
As described above, the image forming apparatus according to the embodiment uses, for example, a primary conversion switching point when image formation is performed by the binarization processing circuit 36 that generates an image signal by a 600 dpi binarization method (for example, photo mode). When the image forming is performed by the PWM circuit 35 that generates the image signal by the PWM method (character mode) of 20 μm ”and 200 dpi, the γ conversion table switching point is set to“ 24 μm ”, for example, and the signal generating means (binarization processing circuit 36, Each is set according to the PWM circuit 35), and the CPU 25 is configured to optimize the primary conversion switching point.
[0102]
From the above, by optimizing the primary conversion switching point according to the signal generation means, the difference in the change of the output density characteristic before and after the primary conversion switching (film thickness before and after the threshold) can be reduced, Furthermore, it is possible to prevent the occurrence of defective images such as pseudo contours.
[0103]
Note that the conversion of the output (density) characteristics of the image signal may be a conversion method other than γ conversion and primary conversion.
[0104]
As described above, a storage medium storing software program codes for realizing the functions of the above-described embodiments is supplied to the system or apparatus, and the computer (or CPU or MPU) of the system or apparatus stores the storage medium in the storage medium. It goes without saying that the object of the present invention can also be achieved by reading and executing the program code.
[0105]
In this case, the program code itself read from the storage medium realizes the novel function of the present invention, and the storage medium storing the program code constitutes the present invention.
[0106]
As a storage medium for supplying the program code, for example, a floppy disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a CD-R, a magnetic tape, a nonvolatile memory card, a ROM, an EEPROM, or the like is used. it can.
[0107]
Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) or the like running on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.
[0108]
Further, after the program code read from the storage medium is written to a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the case where the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing.
[0109]
Further, the present invention may be applied to a system composed of a plurality of devices or an apparatus composed of a single device. Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or apparatus. In this case, by reading the storage medium storing the program represented by the software for achieving the present invention into the system or apparatus, the system or apparatus can enjoy the effects of the present invention. .
[0110]
Furthermore, by downloading and reading a program represented by software for achieving the present invention from a database on a network by a communication program, the system or apparatus can enjoy the effects of the present invention. .
[0111]
【The invention's effect】
As described above, according to the present invention, the conversion unit that detects the value according to the film thickness of the image carrier and converts the density characteristic of the image signal based on the comparison between the detection result and the predetermined value. When determining the conversion characteristics, the predetermined value for comparison is made different depending on the type of image forming mode including the photo mode and the character mode. There is an effect that the density gradation correction can be appropriately performed.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view illustrating the configuration of an image forming apparatus showing a first embodiment of the invention.
FIG. 2 is a diagram illustrating a configuration of a detection unit for detecting a value corresponding to the photosensitive film thickness of the photosensitive drum illustrated in FIG.
3 is a block diagram illustrating a configuration of an image signal processing circuit shown in FIG. 1. FIG.
4 is a block diagram illustrating a configuration of a γ conversion unit illustrated in FIG. 3. FIG.
FIG. 5 is a characteristic diagram showing γ conversion characteristics of the γ conversion unit shown in FIG. 4;
FIG. 6 is a characteristic diagram showing output density characteristics before γ conversion when the photoconductor film thickness of the photoconductor drum shown in FIG. 1 is 30 μm, 20 μm, and 13 μm.
FIG. 7 is a characteristic diagram showing output density characteristics after γ conversion when the photosensitive film thickness of the photosensitive drum shown in FIG. 1 is 30 μm, 20 μm, and 13 μm.
FIG. 8 is a characteristic diagram showing output density characteristics according to a 600 dpi binarization method immediately before and immediately after switching of a γ conversion table.
FIG. 9 is a characteristic diagram showing an output density characteristic according to a 200 dpi PWM method immediately before and immediately after switching a γ conversion table.
FIG. 10 is a characteristic diagram showing an output density characteristic according to a 200 dpi PWM method immediately before and after switching of a γ conversion table.
FIG. 11 is a block diagram illustrating a configuration of an image signal processing circuit according to a second embodiment of the present invention.
12 is a block diagram illustrating a configuration of a primary conversion unit illustrated in FIG. 11. FIG.
13 is a characteristic diagram showing output density characteristics after primary conversion in the case where the photoconductor film thickness of the photoconductor drum shown in FIG. 1 is 30 μm, 20 μm, and 13 μm.
FIG. 14 is a characteristic diagram showing an output density characteristic according to a binarization method of 600 dpi immediately before and after switching of the primary conversion.
FIG. 15 is a characteristic diagram showing output density characteristics according to the 200 dpi PWM method immediately before and immediately after the primary conversion switching.
FIG. 16 is a characteristic diagram showing an output density characteristic by a 200 dpi PWM method immediately before and immediately after primary conversion switching.
FIG. 17 is a characteristic diagram showing respective output density characteristics according to a binarization method of 600 dpi when the photosensitive member film thickness of the photosensitive drum is 30 μm, 20 μm, 16 μm, and 13 μm.
FIG. 18 is a characteristic diagram showing output density characteristics according to the 200 dpi PWM method when the photoconductor film thickness of the photoconductor drum is 30 μm, 20 μm, 16 μm, and 13 μm.
[Explanation of symbols]
6 Photosensitive drum
13 Detector
25 CPU
26 ROM
34 γ converter
35 PWM circuit
36 Binarization processing circuit
401-404 γ conversion unit table

Claims (6)

An image carrier for carrying an image to be formed on a recording medium;
Conversion means for performing a process of converting the density characteristic for an image signal input,
Detecting means for detecting a value corresponding to the film thickness of the image carrier;
A control means for comparing a detection result of the detection means with a predetermined value, and determining a conversion characteristic of the conversion means based on the comparison result ;
Processing means for processing the image signal converted by the conversion means in accordance with an image forming mode including a photo mode and a character mode;
Image forming means for exposing and scanning the image carrier based on the image signal processed by the processing means to form an image on a recording medium ;
The image forming apparatus is characterized in that the control unit varies the predetermined value according to a type of the image forming mode . The image forming apparatus according to claim 1, wherein the conversion unit includes a plurality of conversion tables having different γ characteristics, and the control unit selects a conversion table based on the comparison result . The image forming apparatus according to claim 1, wherein the conversion unit is capable of executing a plurality of primary conversion processes having different characteristics with respect to an image signal, and determines the primary conversion process based on the comparison result . The image forming apparatus according to claim 1, wherein the image carrier is a photoconductor. The detection means detects a value corresponding to a film thickness of the image carrier by measuring a current flowing on the image carrier when a voltage is applied to the image carrier. The image forming apparatus according to 1. 2. The image forming apparatus according to claim 1 , further comprising operation means for selecting the image forming mode .

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