JPH09211908A - Image forming method and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve gradation property of a low resolution area, while minimizing power consumption and the deterioration of a photoreceptor, by using a photoreceptor provided with the high-γ property. SOLUTION: This device is provided with potential detecting means 8 or density detecting means 13 for detecting the latent image potential or the developing density corresponding to respective pulse width on the photoreceptor 1 formed by detection image forming means 7 and 12. Threshold pulse width deciding means 9 and 14 determine the threshold pulse width WL of exposing means 3 corresponding to the low potential side point inflection in a rapid potential attenuation part of the photoreceptor 1, based on the latent image potential or the developing density being detected. Then, at the time of the image forming cycle, pulse width deciding means 10 and 15 decide whether the pulse width of multi-level image data is smaller than the threshold pulse width WL, if it is judged smaller than the threshold pulse width WL, exposure strength controlling means 11 and 16 set the exposure strength larger than the normal exposure strength, and the latent image potential level in the low resolution area provided with the pulse width smaller than the threshold pulse width WL, is changed to the developing possible level.

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 such as a copying machine, an electrostatic recording apparatus, a facsimile, a transmission apparatus and a laser printer, which forms an image on a photosensitive member by using an electrophotographic method, and more particularly, The present invention relates to improvement of an image forming apparatus of a type using a photoconductor having a so-called High-γ characteristic.

[0002]

2. Description of the Related Art Electrophotographic technology is currently used as an image forming apparatus for plain paper copying machines, laser printers, facsimiles, etc., because of its features such as rapid image forming speed, dry development, and high recording density. It has been put to practical use. The electrophotographic process is composed of the basic processes of charging, exposing, developing, transferring, fixing and cleaning, and the photoconductor is an important part responsible for latent image formation by charging and exposing.

The characteristics required of the electrophotographic photoreceptor are chargeability, photoconductivity, and the like, which are the dominant factors for latent image formation in the electrophotographic process. Image formation in electrophotography is achieved by uniformly charging and selectively eliminating the charge on the photoreceptor surface by light irradiation.

Typical electrophotographic photoreceptors which have been put into practical use until now are roughly classified into amorphous chalcogenide materials containing amorphous selenium and its alloys, II-IV group inorganic compound materials such as zinc oxide and cadmium sulfide, and Organic photoconductor (OPC) such as resin dispersion system of molecules and low molecular weight organic compounds,
Amorphous silicon materials can be used. 1970
Until the 1960s, photoconductors were monopolized by inorganic materials, but in 197
With the advent of OPC from the first half of the year, electrophotographic photoconductors have reached a major turning point, and most of the conventional inorganic photoconductors are OP.
It started to move in the direction of replacing C. A characteristic of this OPC is that it is easy to design the spectral sensitivity, and it has become a new requirement with the advent of laser printers. That is, since the mainstream of the recording light source is a semiconductor laser, the photoconductor has excellent monochromatic light sensitivity of 780 mm.

Explain why this factor is important.
Conventionally, electrophotographic technology has been put into practical use only as a plain paper copying machine using an analog optical system as a light source. But 19
In the 1980s, this technology began to be actively applied as a computer output device. In addition to this, the digitization and colorization of plain paper has come to the forefront. Since these systems use digital optics,
The photoreceptor is required to have sufficient sensitivity corresponding to the light source used in this system. That is, it is important to have sufficient sensitivity to monochromatic light of a specific wavelength. In particular, a semiconductor laser is often used as a light source for digital use, and most of them are 700 to 9 that are inexpensive and excellent in mass productivity.
Photoconductors having a large sensitivity to monochromatic light having a peak in the range of 00 nm have been developed. As a result, OPC has come to be widely used as a photoconductor for an electrophotographic system using a digital optical system.

The OPC currently in the mainstream is a laminated OPC comprising a charge transport layer in which a charge transport material is dissolved in a resin in a high concentration and a charge generation layer in which a charge generating pigment is dispersed in a resin in a high concentration. is there. In this type of OPC, the photoconductive basic function of the photoreceptor is divided and independent, and the selection of materials is expanded, and as a result, the performance of the photoreceptor is dramatically improved. In particular, organic photoreceptors using phthalocyanine pigments such as metal-free phthalocyanine, copper phthalocyanine, titanyl phthalocyanine, magnesium phthalocyanine, and vanadyl phthalocyanine are known to be suitable for digital applications.

However, a major problem with this laminated OPC is that the charging characteristic is negative charging. Negatively charged OP
C has a problem that a large amount of ozone is generated from the system used. For this reason, positively charged OPCs are also described in, for example, “T. NakagaWa et al, Japan Hardcopy′88 l. Ozawa.
et al, Japan Hardcopy '88 etc. "
It was put to practical use as an OPC for ordinary copying machines in the latter half of the 1980s.

Further, studies on positively charged single layer type OPC have been made, and those having new characteristics are described in, for example, "S. Johnson et a.
l., IS & T ′s Seventh International congress on Adva
nces in Non-impact Printing Technologies, '91 S.
Tsuchiya et al., Ibit, '91 ”.
These are both photoconductors in which a pigment is dispersed in a resin without a charge transporting agent in the photosensitive layer. Therefore, the price is lower than that of the conventional laminated type in terms of layer formation. Further, the most characteristic of these is that they are positively charged. Furthermore,
Another major feature of these photoconductors is High-γ
The characteristics can be mentioned. The High-γ characteristic means that the slope of the linear potential decay portion in the potential decay curve of the photoconductor is large, and it is proposed that this characteristic is particularly advantageous for digital image formation.

Conventionally, the light energy distribution of spot light used for dot exposure is a Gaussian distribution with a skirt length. If dot exposure with this hem length energy distribution is applied to the photoconductor where the potential decay starts according to the amount of incident light to form an image, the hem length dot exposure will be reproduced as it is, and blurring will occur around the dots. Occurs, and a dot image with poor resolution is formed. Therefore,
For example, Japanese Patent Application Laid-Open No. 1-169454 proposes a so-called High-γ photoconductor that shows almost no potential attenuation during weak exposure and shows steep potential attenuation characteristics when the amount of light increases and reaches a certain amount of light. It is described in the above-mentioned publication that a sharp dot-shaped latent image is formed even if the dot exposure has a Gaussian distribution with a hem length.

Further, the phenomenon that the High-γ characteristic appears in the single layer type OPC has been known as an induction effect from before. As shown in FIG. 22, the induction effect is a phenomenon in which there is a time delay until the potential decays after the photoconductor is irradiated with light, and is a characteristic peculiar to the resin dispersion type photoconductor. This phenomenon does not contribute much to potential decay because generated carriers are trapped in traps at the beginning of exposure, but traps are filled up as the number of generated carriers increases thereafter, and carrier transport rapidly occurs and large potential decay occurs. It is presumed that this occurs. As a result, a high gamma value is exhibited. In the past, the induction effect was considered unsuitable when trying to obtain a linear photosensitivity, so there was a development to reduce the induction effect, but recently, as mentioned above, Induction effect (the point that the decay process of the surface potential when the photoreceptor is charged and exposed is gradually attenuated with the increase of the exposure amount, but shows a rapid potential decrease as the exposure amount is gradually increased). There has been a movement to actively use and form images by digital methods (binarization).

[0011]

Certainly, from the viewpoint of layer formation, a single layer type OP having a High-γ characteristic, which is most characterized by being a positively charged type which is low in price and produces a small amount of ozone.
When C is applied to the electrophotographic process, the conventional laminated OP
Compared with C, the exposed dots are sharply resolved. However, the photoreceptor having the High-γ characteristic is
Since latent image formation is performed with an on / off-like (switching) behavior, after the potential is rapidly attenuated by exposure,
A stable latent image is not formed on the photosensitive member unless the amount of light reaches near the low inflection point potential VL at the inflection point on the low potential side of the abrupt potential decay portion. Therefore, High-γ
If the gradation method in which the pulse width is modulated is adopted in the image forming apparatus using the photoconductor having the characteristics, if the light quantity modulated with the pulse width less than or equal to the predetermined threshold value WL does not cause the potential attenuation or becomes a sufficient quantity. There is a situation in which the latent image is not formed on the photoconductor without reaching the target. Therefore, in order to realize a high gradation, it is necessary that the potential attenuation reaches near the low inflection point potential VL even with the light amount modulated with the pulse width equal to or less than the threshold value WL. Therefore, set the total amount of beam light for exposure to a large value,
Although it is possible to realize high gradation, it is not preferable to raise the total beam light amount because if the overall beam light amount is set to a large value, power consumption increases and durability of the photoconductor also decreases.

The present invention has been made to solve the above technical problems, and when a high-γ characteristic photoconductor is applied to an electrophotographic process, power consumption and deterioration of the photoconductor are minimized. It is an object of the present invention to provide a high-gradation image forming method and an apparatus thereof, in which the gradation of a low resolution region is enhanced while suppressing the above.

[0013]

That is, the present invention provides:
As shown in FIG. 1, the uniformly charged surface potential VP is used to expose the photosensitive body 1 charged in the charging process by using the photosensitive body 1 having a potential attenuation characteristic that abruptly attenuates at a certain exposure amount A0. In the image forming method in which a latent image according to an image pattern is written by exposure in the process and the latent image is visualized in the developing process, a latent image is formed in the exposure process with a pulse width corresponding to multi-tone image data. Alternatively, a pulse width determination step of determining whether or not the threshold pulse width WL is a level capable of developing and forming in the development step, and the pulse width is determined to be the threshold pulse width WL or more in this pulse width determination step. Under the condition, under the high-resolution region exposure step in which the irradiation beam performs exposure with a normal exposure intensity, and the condition in which the pulse width is determined to be less than the threshold pulse width WL in the pulse width determination step, Irradiation And an exposure step for a low-resolution area in which the latent image potential level of the low-resolution area is set to be larger than the normal exposure intensity and less than the threshold pulse width WL) to a developable level. Is characterized by.

Further, as shown in FIG. 1, the apparatus invention embodying such a method invention is a photosensitive material having a potential attenuation characteristic that abruptly attenuates at a certain exposure amount A0 having a uniformly charged surface potential VP. An image in which a latent image corresponding to an image pattern is written on the photosensitive member 1 charged by the charging unit 2 by the exposure unit 3 by using the body 1, and the latent image is visualized by the developing unit 4. In the forming apparatus, the exposure unit 3 includes a pulse width changing unit 5 that modulates the pulse width of the irradiation beam according to the multi-tone image data, and an exposure intensity changing unit 6 that variably sets the exposure intensity of the irradiation beam. When a photoconductor characteristic inspection cycle different from a normal image forming cycle is provided, the exposure means 3 irradiates a plurality of beams having different pulse widths on the photoconductor characteristic inspection cycle to expose the photoconductor 1 with a beam having a plurality of pulse widths. by An inspection image forming means 7 for forming an image stepwise, and a potential detecting means 8 for detecting a latent image potential corresponding to each pulse width on the photoconductor 1 formed by the inspection image forming means 7, Of the latent image potentials detected by the potential detecting means 8, the exposing means 3 corresponding to the low inflection point potential VL at the inflection point on the low potential side of the abrupt potential decay portion of the photoconductor 1.
Threshold pulse width determining means 9 for obtaining the threshold pulse width WL of
And whether the pulse width of the multi-tone image data variably set by the pulse width varying means 5 in the image forming cycle is smaller than the threshold pulse width WL determined by the threshold pulse width determining means 9. And the pulse width determining means 10 determines the exposure intensity by the exposure intensity varying means 6 under the condition that the pulse width determining means 10 determines that the pulse width of the multi-tone image data is smaller than the threshold pulse width WL. An exposure intensity control means 11 is provided, which is set to be larger than that when the pulse width is WL or more and changes the latent image potential level in the low resolution area less than the threshold pulse width WL to a developable level. is there.

Further, as shown in FIG. 1, another apparatus invention embodying the method invention comprises the same constituent elements 1 to 6 (photoconductor 1 to exposure intensity varying means 6) as in the apparatus invention described above. In addition, during the photoconductor characteristic inspection cycle different from the normal image forming cycle, the pulse width varying means 5 irradiates the exposure means 3 with a plurality of beams having different pulse widths to expose the photoconductor 1 with a plurality of pulse widths. In addition to forming latent images by stepwise, the developing means 4 visualizes each latent image, and the inspection image forming means 12 and each of the photoreceptor 1 formed by the inspection image forming means 12. A density detecting means 13 for detecting density information of the developed image corresponding to the pulse width, and a threshold value corresponding to the density information of the lower limit level capable of forming a developed image based on the density information detected by the density detecting means 13. Threshold for finding pulse width WL The pulse width of the multi-gradation image data variably set by the value pulse width determination means 14 and the pulse width varying means 5 during the image forming cycle is larger than the threshold pulse width WL determined by the threshold pulse width determination means 14. A pulse width judging means 15 for judging whether or not it is small, and an exposure intensity varying means under the condition that the pulse width judging means 15 judges that the pulse width of the multi-tone image data is smaller than the threshold pulse width WL. And an exposure intensity control means 16 for changing the latent image potential level of the low resolution area of less than the threshold pulse width WL to a developable level. It is characterized by.

With such technical means, the photosensitive member 1
Any material having a potential attenuation characteristic that abruptly attenuates the potential at a certain exposure amount A0 is applicable, for example, a single-layer type organic positively charged photoreceptor including an X-type metal-free phthalocyanine and a binder resin. And so on. Also, the form of the photoreceptor 1 may be a drum shape or a belt shape. Further, the image forming apparatus of the present invention includes the charging unit 2,
It is only necessary that the toner image on the photoconductor 1 is visualized by the exposing means 3 and the developing means 4, and the toner image on the photoconductor 1 may be directly transferred to the recording medium. Alternatively, various types such as one in which the toner image on the photosensitive member 1 is primarily transferred to the intermediate transfer member and then secondarily transferred to the recording medium are included.

Further, the exposure means 3 is a pulse width varying means 5
The laser scanning device is not limited as long as it has the exposure intensity varying means 6 and can be appropriately selected. The exposure unit 3 may perform pulse width modulation and exposure intensity modulation in a specific exposure unit. For example, the exposure unit for pulse width modulation and the exposure unit for exposure intensity modulation may be functionally separated. You may do it. In particular, it is possible to use an arbitrary beam dot diameter by the exposure means 3,
From the viewpoint of maintaining a good latent image forming property in the low resolution area, the diameter of the beam dot irradiated on the photoconductor 1 is set to be smaller in the main scanning direction than in the sub scanning direction. Is preferred.

Further, the inspection image forming means 7 and 12 may be operated during a photoreceptor characteristic inspection cycle different from the normal image forming cycle. For example, when the power is turned on or every predetermined number of image forming cycles. You can do it to.
Further, with respect to the inspection image forming means 7 and 12, basically, a latent image of a plurality of steps may be formed in the vicinity of the latent image corresponding to the threshold pulse width WL, and the latent image is formed over the entire pulse width. Need not be created in stages.

Further, regarding the threshold pulse width determination means 9 and 14, for example, the detection data from the potential detection means 8 or the density detection means 13 and the lower limit level (corresponding to the low inflection point potential VL) at which a latent image can be formed. ) Or the reference data, which is the lower limit level at which a developed image can be formed, is compared, and the detection data that is as close as possible to the reference data that does not reach the reference data among the detection data is obtained, and the multi-gradation image data corresponding to this detection data is obtained. The threshold pulse width WL may be determined from the pulse width, or conversely, the detection data that exceeds the reference data and is as close as possible to the reference data in the detection data is obtained, and the multi-gradation image data corresponding to this detection data is obtained. It is possible to use an appropriate algorithm such as discriminating the threshold pulse width WL from the pulse width.

In the exposure intensity control means 11 and 16, basically, the exposure intensity is set to 2 so that the exposure intensity when the threshold pulse width WL is less than the exposure intensity when the threshold pulse width WL is greater than or equal to the threshold intensity. The exposure intensity when the pulse width is less than the threshold pulse width WL is switched to several stages, and the generated pulse width and the exposure intensity are made to correspond in inverse proportion to each other, and the gradation of the low resolution area is improved. It can be better.

Next, the operation of the above-mentioned technical means will be described by taking the apparatus invention as an example. First, an image forming apparatus including the components 1 to 11 will be described.
In FIG. 1, the inspection image forming means 7 irradiates a plurality of beams having different pulse widths from the exposure means 3 by the pulse width varying means 5 during a photoreceptor characteristic inspection cycle different from the normal image forming cycle, and the photoreceptor 1 A latent image is created in stages by beam exposure with a plurality of pulse widths. Then, the potential detecting means 8 detects the latent image potential corresponding to each pulse width on the photoconductor 1 created by the inspection image creating means 7.
Then, the threshold pulse width determination means 9 corresponds to the low inflection point potential VL at the low potential side inflection point of the abrupt potential decay portion of the photoconductor 1 among the latent image potentials detected by the potential detection means 8. The threshold pulse width WL of the exposure means 3 is obtained. In this state, when a normal image forming cycle is carried out, the pulse width judging means 10 determines that the pulse width of the multi-tone image data variably set by the pulse width varying means 5 is the threshold pulse width W.
Judge whether it is smaller than L.

When the pulse width judging means 10 judges that the pulse width W = W1 of the multi-gradation image data is equal to or larger than the threshold pulse width WL, the exposure intensity control means 11 keeps the normal exposure intensity as it is. To the exposure intensity varying means 6. Therefore, as shown in FIG. 2A, the exposure unit 3 performs pulse width modulation of the pulse width W1 with the normal exposure intensity P1 and the latent image potential level on the photoconductor 1 is a developable level. The image ZIM (W1) is formed. On the other hand, the pulse width determination means 10 causes the pulse width W = W2 of the multi-gradation image data.
Is less than the threshold pulse width WL, the exposure intensity control means 11 sets the exposure intensity higher than the normal exposure intensity, and the latent image potential level in the low resolution area less than the threshold pulse width WL is the developable level. Change to. Therefore, as shown in FIG. 2 (b), the exposure means 3 has a normal exposure intensity P1.
Pulse width modulation of the pulse width W2 is performed with a larger exposure intensity P2 to form a latent image ZIM (W2) on the photoconductor 1 whose latent image potential level is a developable level.

The image forming apparatus having the constituent elements 1 to 6 and 12 to 16 will be described. The inspection image forming means 12 includes the pulse width varying means 5 during the photoconductor characteristic inspection cycle different from the normal image forming cycle. The irradiation means 3 irradiates a plurality of beams having different pulse widths to form a latent image on the photoconductor 1 stepwise by beam exposure with a plurality of pulse widths, and each latent image can be developed by the developing means 4. Visualize.
Then, the density detecting means 13 detects density information of the developed image corresponding to each pulse width on the photoconductor 1 created by the inspection image creating means 12. Then, the threshold pulse width determination means 1
Reference numeral 4 obtains a threshold pulse width WL corresponding to the density information of the lower limit level at which a developed image can be formed, based on the density information detected by the density detecting means 13. In this state, when a normal image forming cycle is performed, the pulse width judging means 15 determines whether the pulse width of the multi-tone image data variably set by the pulse width varying means 5 is smaller than the threshold pulse width WL. To judge. Then, the exposure intensity control means 16
Under the situation where the pulse width determination means 15 determines that the pulse width of the multi-tone image data is greater than or equal to the threshold pulse width WL, the exposure intensity by the exposure intensity varying means 6 is maintained at the normal level, while the pulse width is maintained. Under the situation where the judgment means 15 judges that the pulse width of the multi-tone image data is smaller than the threshold pulse width WL, the exposure intensity is set to be larger than that when it is equal to or larger than the threshold pulse width WL, and the threshold pulse width WL is set. The latent image potential level in the low resolution area below is changed to a developable level. Therefore, even in this type, a high resolution region (W ≧
2), the latent images in the low resolution region (W <WL) are both shown in FIG.
As shown in (a) and (b), it is formed in a potential state of a developable level.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on the embodiments shown in the accompanying drawings. First Embodiment FIG. 3 shows a first embodiment of an image forming apparatus to which the present invention is applied. In the figure, reference numeral 21 is a single-layer organic photoconductor having a potential decay characteristic (High-γ characteristic) in which an element is rapidly attenuated at a certain exposure amount, 22 is a charging device for positively charging the photoconductor 21, and 23 Is an optical writing device (exposure device) that irradiates the charged photoconductor 21 with light to form an electrostatic latent image (in the present embodiment, a negative latent image for image exposure), such as a semiconductor laser and a polygon mirror. A laser light generator with a built-in is used. Further, reference numeral 24 is a developing device that visualizes the electrostatic latent image formed on the photoconductor 21 with toner having the same polarity charge as that of the photoconductor 21, and 25 is a toner image on the photoconductor 21. A transfer device such as a corotron that transfers the recording paper 26 to the recording paper 26; a paper peeling device 27 such as a corotron that peels the recording paper 26 electrostatically attracted to the photoconductor 21;
Reference numeral 8 is a fixing device for fixing an unfixed toner image on the recording paper 26, 29 is a cleaner for removing residues such as residual toner on the photoconductor 21, and 30 is a static eliminator for removing residual charges on the photoconductor 21. is there.

In the present embodiment, a single-layer organic photoconductor in which X-type non-metallic phthalocyanine is dispersed in a binder resin is used as the photoconductor 21. Specifically, X
A positively charged single-layer type photoconductor was prepared by using a type metal-free phthalocyanine (manufactured by Dainippon Ink and Chemicals, Inc.) and Fastgen Blue as a photosensitive material. The details of the photoreceptor have already been disclosed in JP-A-3-287171. Further, in the present embodiment, the photosensitive member was manufactured in a drum shape using an aluminum drum main body. In the single-layer type photoreceptor 21 configured as described above, since charges are mainly transferred by holes, the surface is positively charged before use.

The potential attenuation characteristics of this resin-dispersed single-layer type photosensitive member 21 are shown in FIGS. 4 (a) and 4 (b). The solid line in FIG. 4A shows the light attenuation characteristic of the resin-dispersed single-layer type photoconductor 21. When light is irradiated after charging, the light attenuation process due to the exposure amount of the surface potential is gradually gradual at first. Attenuation is shown, and when the light quantity is further increased, the surface potential is rapidly attenuated during the gentle attenuation. On the other hand, FIG.
The solid line indicates the dark decay characteristics of the resin-dispersed single-layer photoreceptor 21. The change over time after charging to a predetermined potential shows a gradual decay at first, followed by a rapid decay with the passage of time. The curve shows that the potential drops.

Here, the light and dark decay characteristics of the conventional laminated type photoreceptor (shown by dotted lines in FIGS. 4A and 4B) and the light and dark decay characteristics of the resin dispersion single layer type photoreceptor 21 used in the present embodiment. Compare with. Since the surface potential of the layered photoreceptor is negatively charged and the resin-dispersed single layer photoreceptor 21 is positively charged, the potential as an absolute value is used in the layered photoreceptor of FIG. . As can be seen from FIG. 4, the sensitivity characteristics of the conventional laminated-type photoconductor have a relatively entire sensitivity region response to irradiation light, whereas the single-layer photoconductor according to the present embodiment has a certain irradiation range. It is understood that the surface potential is gradually attenuated even after exposure up to the light amount, and that the surface potential is rapidly attenuated when the light amount increases at a certain irradiation light amount as a boundary.

As the photoconductor having the above-described light attenuation characteristic and dark attenuation characteristic, which are sensitivity characteristics in which an S-shaped curve has an inflection point and changes on and off, a single layer type in which zinc oxide is dispersed in a resin is used. It has been reported that the photoconductor or the organic photoconductor in which phthalocyanine is resin-dispersed, but the relaxation time that the photoconductor surface potential starts to decay at the time of exposure is long, and a large amount of charge required for charging is required, It was not suitable for practical use due to large changes in potential holding performance and sensitivity during repeated use. Therefore, in the present embodiment, a single-layer type photoconductor in which an X-type non-metallic phthalocyanine is dispersed in a resin is used as a type which overcomes these problems and has a sensitivity characteristic at a practical level.

FIG. 5 shows the characteristics of the semiconductor laser 231 (see FIG. 3) of the optical writing device 23 used in this embodiment (relationship between the laser drive current and the output light intensity). Further, FIG. 6 shows the relationship between the exposure amount, which is the laser output light amount, and the photosensitive member surface potential. In these, the exposure amount A
(Corresponding to the laser drive current A ′) indicates the standard use output level of the semiconductor laser 231, exposure amount B (corresponding to the laser drive current B ′) indicates the maximum output level, exposure amount C indicates the zero level, and The exposure amount D indicates the level at which the low inflection point potential VL can be obtained at the inflection point on the low potential side in the portion where the potential is rapidly attenuated on the photoreceptor. Therefore, if the exposure amount A is used without pulse width modulation, a latent image is formed on the photoconductor.
If, for example, the exposure amount of less than D is obtained by the pulse width modulation and the potential attenuation of the photoconductor is also less than the low inflection point potential VL, the latent image is not formed on the photoconductor. Become. Therefore, in the present embodiment, when the pulse width is less than the threshold pulse width WL, that is, when the exposure amount becomes D when the pulse width is modulated with the exposure amount of the standard use output level A, the exposure amount is Is set so that the output light amount of the semiconductor laser 231 is set to be larger than A so that the value exceeds D.

Next, the exposure control system used in this embodiment will be described with reference to FIGS. FIG. 7 is a block diagram of an exposure control system that drives and controls a semiconductor laser (LD). In the figure, reference numeral 40 is a digital data output device for outputting multi-gradation image data, and a DA converter 41 converts the digital data from the digital data output device 40 into an analog video signal. Further, reference numeral 42 is a triangular wave generator having a period of one pixel, and 43 is a comparison between the triangular wave from the triangular wave generator 42 and the analog video signal from the DA converter 41 to generate a modulation signal having a pulse width corresponding to the digital data. Reference numeral 44 is a laser (LD) drive circuit that drives the semiconductor laser 231 with a predetermined light amount based on the modulation signal from the comparison circuit 43, and 45 is a light amount switching circuit that switches the laser light amount set value of the laser drive circuit 44. . Furthermore,
Reference numeral 46 is a potential detection sensor that detects the latent image potential on the photoconductor 21, and reference numeral 47 is a switching light amount determination circuit that determines switching of the laser light amount setting value from the potential detected by the potential detection sensor 46.

FIG. 8 is an explanatory diagram showing the details of the switching light amount judgment circuit 47. In the figure, the switching light amount determination circuit 47 operates during a photoconductor characteristic inspection cycle (a cycle for periodically inspecting changes in the photoconductor characteristic) different from the normal image forming cycle, and stores an analog video signal. A first memory 471, a second memory 472 for storing a latent image potential formed on the photoconductor 21 corresponding to the analog video signal, and a low inflection point potential which is a potential required for latent image formation. Low inflection point potential memory 47 storing VL
3 is provided. The reference numeral 474 designates the latent image potential and the low inflection point potential memory 4 stored in the second memory 472.
The low inflection point potential VL of 73 is compared with the second memory 472.
Potential does not reach the low inflection point potential VL
Under a condition that is extremely close to, the potential and its address in the second memory 472 are sent to the first memory 471, and the analog video signal that generated the potential is sent to the first memory 471.
It is a comparator for taking it from inside and passing it to the light amount switching circuit 45. In the present embodiment, the photoconductor characteristic inspection cycle is performed when the power is turned on or after every predetermined number of image formations.

Next, the operation of the image forming apparatus according to this embodiment will be described. Photoconductor Characteristic Inspection Cycle In the present embodiment, the photoconductor characteristic inspection cycle is a cycle for correcting a situation in which the characteristics of the photoreceptor 21 fluctuate over time and the sensitivity at the time of conversion of the potential with respect to the beam exposure amount fluctuates. is there. First, the digital data output device 40 sequentially outputs digital data whose pulse width is varied in several steps (selected in the front and rear range of the digital data corresponding to the low inflection point potential VL under the standard condition), and the DA converter 41. A laser (LD) drive circuit 44 passes through the comparison circuit 43 to form an electrostatic latent image of several stages on the photoconductor 21 by the laser light. In this state, the latent image potential of each pulse width is detected by the potential detection sensor 46, and the analog video signal and the latent image potential on the photoconductor 21 generated thereby are switched. The first memory 471 of the light amount determination circuit 47. , The second memory 47
2 respectively. Then, the latent image potential stored in the second memory 472 is compared with the low inflection point potential VL in which the potential required for latent image formation is compared, and the low inflection point potential VL is first shown.
To the first memory 47 and its potential not reaching
1, the analog video signal corresponding to the potential is taken out from the first memory 471 and is passed to the light amount switching circuit 45 as a light amount switching threshold signal.

Image Forming Cycle In a normal image forming cycle, when a digital signal consisting of multi-gradation image data is output from the digital data output device 40 and converted into an analog video signal by the DA converter 41, this analog signal is output. The video signal is sent to the laser (LD) drive circuit 44 through the comparison circuit 43 and output as a predetermined pulse width modulation signal. At this time, when the analog video signal is larger than the light amount switching threshold signal, the light amount switching circuit 45 does not send the light amount switching signal to the laser drive circuit 44, and the laser light amount set value of the laser drive circuit 44 is set to the normal set value. However, when the analog video signal is equal to or less than the light amount switching threshold signal, the light amount switching circuit 45 sends a light amount switching signal to the laser drive circuit 44 to set the laser light amount set value of the laser drive circuit 44. The beam exposure amount is switched so that it can be developed in the low resolution area. In the present embodiment, the beam exposure amount that can be switched in the low resolution region may be constant, but the laser light amount set value (exposure intensity) in the case of the light amount switching threshold signal or less is switched in several steps to generate the generated pulse width and exposure. It is preferable that the intensity and the intensity are inversely proportional to each other to improve the gradation of the low resolution region.

In such an image forming process, as will be confirmed from the examples described later, not only the high resolution area but also the latent image in the low resolution area is surely visualized to form an image. Of high gradation was achieved.

Second Embodiment Although the basic structure of the image forming apparatus according to the present embodiment is substantially the same as that of the first embodiment, it has an exposure control system as shown in FIG. 9 different from the first embodiment. doing. In the figure, in the exposure control system, a density detection sensor 56 is attached in the vicinity of the surface of the photoconductor 21 in place of the potential detection sensor 46 used in the first embodiment, and the switching light amount judgment circuit 57 uses the density detection sensor 56. The switching of the laser light amount set value is determined based on the density detected in (1). Note that components similar to those of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and a detailed description thereof will be omitted.

FIG. 10 is an explanatory diagram showing the details of the switching light amount judgment circuit 57. In the figure, the switching light amount determination circuit 57 operates during a photoconductor characteristic inspection cycle different from the normal image forming cycle (similar to the first embodiment), and the first memory 5 for storing analog video signals.
71 and the photoconductor 21 corresponding to the analog video signal.
A second memory 572 that stores the development density of the latent image formed above, and a reference that stores the reference density (corresponding to the low inflection point potential VL of the first embodiment) necessary for visualizing the image. And a density memory 573. Further, reference numeral 574 compares the development density stored in the second memory 572 with the reference density of the reference density memory 573, and the development density in the second memory 572 does not reach the reference density and the reference density is unlimited. Under similar conditions, the development density in the second memory 572 and its address are sent to the first memory 571, and the analog video signal that has generated the development density is taken from the first memory 571 and the light amount switching circuit It is a comparator for passing to 45.

Therefore, in the present embodiment, the following photoreceptor characteristic inspection cycle and image forming cycle are performed. ● Photoconductor characteristic inspection cycle First, the digital data output device 40 sequentially outputs the digital data whose pulse width has been changed in several steps (selected in the front-back range of the digital data corresponding to the reference density under standard conditions), and performs DA conversion. An electrostatic latent image of several stages is formed on the photoconductor 21 by the laser light generated by the laser (LD) drive circuit 44 through the container 41 and the comparison circuit 43, and is developed by the developing device 24. In this state, the development density of each pulse width is detected by the density detection sensor 56, and the analog video signal and the development density on the photoconductor 21 generated thereby are switched. 2 memories 572 respectively. Then, the development density stored in the second memory 572 is compared with the reference density, and the development density which does not reach the reference density for the first time and its address are returned to the first memory 571, and the analog corresponding to the development density is obtained. The video signal is taken out from the first memory 571 and passed to the light amount switching circuit 45 as a light amount switching threshold signal.

Image Forming Cycle In a normal image forming cycle, when a digital signal consisting of multi-gradation image data is output from the digital data output device 40 and converted into an analog video signal by the DA converter 41, it is executed. Similar to the first embodiment, when the analog video signal is larger than the light amount switching threshold signal, the light amount switching circuit 45 does not send the light amount switching signal to the laser drive circuit 44, and the laser light amount set value of the laser drive circuit 44 is normally set. However, if the analog video signal is equal to or less than the light amount switching threshold signal, the light amount switching circuit 4
Reference numeral 5 sends a light amount switching signal to the laser drive circuit 44 to switch the laser light amount set value of the laser drive circuit 44 to switch the beam exposure amount to a level at which development is possible in the low resolution area.

Also in this embodiment, not only the high-resolution area but also the latent image in the low-resolution area can be surely visualized,
High gradation of the image was achieved.

[0040]

【Example】

Example 1 This example is applied to a pixel having a resolution of 400 spi, and the laser beam radius is set to 20 μm in the main scanning direction.
m, the sub-scanning direction is 30 μm, and the beam power is 0.4
mW, and the optical writing device 23 has a scanning speed of 1
ROS (Raster Output Scanner) of 000 m / s
Was used. Then, a simulation experiment was carried out on the change in gradation when the pulse width was modulated and the beam power was increased in the low resolution region. The PIDC (Photoinduced) of the high-γ photoconductor used in this embodiment is used.
11 shows the discharge curve), and FIG. 12 shows the relationship between the ROS lighting time per unit pixel and the beam power (exposure amount).

FIG. 13 shows a pulse width WL (WL = WL in this embodiment).
RO when the beam power is increased below 30%)
FIG. 14 shows the relationship between the ROS lighting time and the exposure amount when the S lighting time (pulse width modulation 0 to 100%) is performed in steps of 10%. FIG. 14 shows the ROS lighting time and the photoconductor under the same conditions as in FIG. The relationship with the above latent image potential distribution is shown. Note that the horizontal axis lengths (FS direction) in FIGS. 13 and 14 represent pixel width dimensions in the main scanning direction. In this example, the increase width of the beam power was set to 0.4 mW to 0.5 mW at 20% and 0.4 mW to 0.8 mW at 10%.

In the present embodiment, high resolution was judged to be good when the latent image potential width at the time of cutting at the developing potential (here, 300 V) was a constant interval for each modulation pulse width. According to FIG. 13 and FIG. 14, when the beam power is increased with the pulse width WL or less, the high resolution can be maintained even between 0 and 30%, and the gradation of the low resolution portion that is often included in the color image is reduced to the beam. Comparative Example 1 in which power is not modulated (FIGS. 15 and 1)
6)), a longer range can be obtained, and a stable halftone image can be obtained.

Comparative Example 1 A beam width was kept constant at 0.4 mW and pulse width modulation was carried out under the same conditions as in Example 1 except for the above conditions, and a simulation experiment was carried out for changes in gradation. FIG. 15 shows the ROS lighting time Cin (pulse width modulation 0 when the beam power is not changed within the pulse width WL (WL = 30% in this embodiment).
FIG. 16 shows the relationship between the ROS lighting time and the latent image potential distribution on the photoconductor under the same conditions as in FIG. Show the relationship. According to FIGS. 15 and 16, when the beam power is not changed, the ROS lighting time (modulation pulse width) C
It was confirmed that when in was less than 30%, the resolution was poor, and when it was less than 20%, a latent image was not formed on the photoreceptor. Therefore, in order to obtain the gradation of the low resolution portion, the pulse width is 20 to 30
% Area must be used in a very narrow range.

Comparative Example 2 In Example 1, the case where the beam diameter on the photoconductor in the main scanning direction is smaller than that in the sub scanning direction was described. Next, a similar simulation experiment was conducted with a laser beam radius of 30 μm in the main scanning direction and 30 μm in the sub-scanning direction, a beam power of 0.6 mW, and a ROS scanning speed of 1000 m / s. FIG. 17 shows a latent image potential distribution on the photoconductor when the ROS lighting time (pulse width modulation is 0 to 100%) is performed in 10% increments when the beam power is not changed within the pulse width WL. Further, in order to increase the beam power below the pulse width WL and enhance the gradation of the low resolution area, the beam power at the pulse width of 20% is 0.85 m.
FIG. 18 shows a diagram in which the potential distribution when the pulse width is increased to W and the potential distribution when the pulse width is 30% are combined.

As can be seen from FIGS. 17 and 18, when the beam power is increased below the pulse width WL, the potential distribution graph with a pulse width of 20% and the potential distribution graph with a pulse width of 30% overlap. Be understood. From this, if the beam diameter of the beam in the main scanning direction is equivalent to that of the beam system in the sub-scanning direction, even if the beam power is increased below the pulse width WL, the width when viewed near the developing potential also increases, It is understood that the effect of enhancing the gradation of the low resolution area may not occur. Therefore, when the beam diameter in the main scanning direction is smaller than that in the sub-scanning direction, latent images are formed in a low resolution area of a pulse width WL or less, and the effect of achieving high gradation can be effectively exerted. It is confirmed that it is possible.

Comparative Example 3 In this comparative example, a laminated photoconductor was used, the beam power was kept constant at 0.28 mW, and the pulse width was modulated under the same conditions as in Example 1 except that the gradation power was changed. A simulated experiment was conducted on the changes. FIG. 19 shows the PIDC of the laminated photoconductor used in this comparative example, and FIG. 20 shows the ROS lighting time Cin (pulse width modulation 0 to 0 when the beam power is not changed).
FIG. 21 shows the relationship between the ROS lighting time and the latent image potential distribution on the photoconductor under the same conditions as in FIG. 20, when the ROS lighting time and the exposure amount are measured in 10% increments. Indicates. According to this comparative example, H according to the present example
In a photoreceptor having a high-γ characteristic, R corresponding to FIG.
It is confirmed that the relationship between the OS lighting time and the latent image potential distribution on the photoconductor is obtained.

[0047]

As described above, according to the present invention, a Hi curve having a sensitivity curve in which the potential is switched is attenuated.
In the photoconductor having the gh-γ characteristic, the beam power of the light quantity modulated by the pulse width less than the threshold value of the potential attenuation level is set to be larger than the beam power of the light quantity modulated by the pulse width more than the threshold value. By doing so, since the latent image is formed in the low resolution area below the threshold,
By using the photoreceptor having the High-γ characteristic, it is possible to surely achieve not only the high resolution region but also the gradation property in the low resolution region. For this reason, when a high-γ characteristic photoconductor is applied to an electrophotographic process, it is possible to enhance the gradation of a low-resolution area and form a high-gradation image while minimizing power consumption and deterioration of the photoconductor. It can certainly be realized.

Further, in the present invention, the threshold value change of the potential attenuation level due to the time-dependent change of the photoconductor or the environment change is periodically obtained, and the beam power is increased in the low resolution region below the obtained threshold value. For example, it is possible to enhance the gradation of the low-resolution area and surely realize the high-gradation image formation without being affected by the temporal change of the photoconductor or the environmental change.

Further, in the present invention, if the diameter of the beam dot irradiated on the photoconductor is set smaller in the main scanning direction than in the sub scanning direction, the gradation of the low resolution region Sex can be surely enhanced.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of an image forming method and an apparatus thereof according to the present invention.

FIG. 2 is an explanatory diagram showing an image forming principle according to the present invention.

FIG. 3 is an explanatory diagram showing an outline of the image forming apparatus according to the first embodiment.

FIG. 4A is an explanatory diagram showing a light attenuation characteristic of the photosensitive drum used in the first embodiment, and FIG. 4B is a first embodiment.
FIG. 4 is an explanatory diagram showing dark attenuation characteristics of a photoconductor drum used in FIG.

FIG. 5 is a graph showing the output characteristics of the laser used in the first embodiment.

FIG. 6 is a graph showing a relationship between an exposure amount, which is a laser output light amount used in the first embodiment, and a photosensitive member surface potential.

FIG. 7 is a block diagram showing an exposure control system used in the first embodiment.

8 is a block diagram showing details of a switching light amount determination circuit of FIG.

FIG. 9 is a block diagram showing an exposure control system used in the second embodiment.

10 is a block diagram showing details of a switching light amount determination circuit in FIG.

FIG. 11 is a graph showing PIDC of the photoconductor used in Example 1.

FIG. 12 is a graph showing the relationship between ROS lighting time and exposure amount per unit pixel of the photoconductor used in Example 1.

FIG. 13 is a graph showing R when the ROS lighting time (pulse width modulation 0 to 100%) is increased by 10% when the beam power is increased below the pulse width WL in the first embodiment.
It is a graph figure which shows the relationship between OS lighting time and exposure amount.

FIG. 14 is a graph showing the relationship between the ROS lighting time and the latent image potential distribution on the photoconductor under the same conditions as in FIG.

FIG. 15 shows RO in Comparative Example 1 when the ROS lighting time (pulse width modulation 0 to 100%) is performed in 10% increments when the beam power is not changed below the pulse width WL.
It is a graph which shows the relationship between S lighting time and exposure amount.

FIG. 16 is a graph showing the relationship between the ROS lighting time and the latent image potential distribution on the photoconductor under the same conditions as in FIG.

FIG. 17 is a latent image on the photoconductor in Comparative Example 2 when the ROS lighting time (pulse width modulation is 0 to 100%) is performed in 10% increments when the beam power is not changed below the pulse width WL. It is a potential distribution.

FIG. 18 is a graph showing the potential distribution when the beam power is increased to 0.85 mW when the pulse width is 20% and the potential distribution when the pulse width is 30% in Comparative Example 2.

FIG. 19 is a diagram showing the PI of the laminated photoconductor used in Comparative Example 3.
It is a graph which shows DC.

FIG. 20 is a graph showing the relationship between the ROS lighting time and the exposure amount when the ROS lighting time (pulse width modulation 0 to 100%) is performed in 10% increments when the beam power is not changed according to Comparative Example 3. It is a figure.

FIG. 21 is a graph showing the relationship between the ROS lighting time and the latent image potential distribution on the photoconductor under the same conditions as in FIG.

FIG. 22 is a graph showing potential decay characteristics of a single-layer type photoconductor due to an induction effect.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photoconductor, 2 ... Charging means, 3 ... Exposure means, 4 ... Developing means, 5 ... Pulse width varying means, 6 ... Exposure intensity varying means,
7, 12 ... Image forming means for inspection, 8 ... Potential detecting means, 9,
14 ... Threshold pulse width determination means, 10, 15 ... Pulse width determination means, 11, 16 ... Exposure intensity control means, 13 ... Density detection means, 21 ... Photoconductor, 22 ... Charging device, 23 ... Optical writing device, 24 ... Developing device, 40 ... Digital data output device, 41 ... DA converter, 42 ... Triangular wave generator, 43 ...
Comparing circuit, 44 ... Laser (LD) drive circuit, 45 ... Light quantity switching circuit, 46 ... Potential detection sensor, 47, 57 ... Switching light quantity judging circuit, 56 ... Density detection sensor

Claims (4)

[Claims] 1. A photoconductor charged in a charging step by using a photoconductor (1) having a potential decay characteristic that abruptly decays at a certain exposure amount (A0) with a uniformly charged surface potential (VP). In contrast to (1), in the image forming method in which a latent image corresponding to an image pattern is written by exposure in the exposure step and the latent image is visualized in the developing step, a pulse width corresponding to multi-tone image data is A pulse width determining step of determining whether or not a threshold pulse width (WL) which is a level capable of forming a latent image in the exposure step or development forming in the developing step, and the pulse width in the pulse width determining step. Under the condition of being determined to be (WL) or more, the exposure process for the high resolution region in which the irradiation beam performs exposure with a normal exposure intensity, and the pulse width in the pulse width determination process is the threshold pulse width (WL ) Less than Under the above conditions, the irradiation beam is set to be larger than the normal exposure intensity, and the latent image potential level of the low resolution area less than the threshold pulse width (WL) becomes the developable level. An image forming method comprising: an exposure step. 2. A uniformly charged surface potential (VP) is charged by a charging means (2) by using a photoconductor (1) having a potential decay characteristic that abruptly decays at a certain exposure amount (A0). In the image forming apparatus, the latent image corresponding to the image pattern is written on the photoconductor (1) by the exposure unit (3) and the latent image is visualized by the developing unit (4). The means (3) includes pulse width varying means (5) for modulating the pulse width of the irradiation beam according to the multi-tone image data.
And an exposure intensity varying means (6) for variably setting the exposure intensity of the irradiation beam, and the pulse width varying means (5) exposes the exposure means (3) at the photoconductor characteristic inspection cycle different from the normal image forming cycle.
From a plurality of beams having different pulse widths from each other to form a latent image stepwise on the photoconductor (1) by beam exposure with a plurality of pulse widths, and an inspection image forming means (7). Potential detecting means (8) for detecting the latent image potential corresponding to each pulse width on the photoconductor (1) created by the means (7), and the latent image detected by this potential detecting means (8) A threshold pulse for obtaining the threshold pulse width (WL) of the exposing means (3) corresponding to the low inflection point potential (VL) at the inflection point on the low potential side of the sharp potential decay portion of the photoconductor (1) among the potentials. The pulse width of the multi-tone image data that is variably set by the pulse width varying means (5) during the image forming cycle and the threshold width determined by the threshold pulse width determining means (9). Width (W
Pulse width determining means (10) for determining whether or not the pulse width of the multi-gradation image data is smaller than the threshold pulse width (WL). Under such a situation, the exposure intensity by the exposure intensity varying means (6) is set to be larger than that when the threshold pulse width (WL) is equal to or more than the threshold pulse width (WL), and the latent image potential level in the low resolution area less than the threshold pulse width (WL) is set. An image forming apparatus comprising: an exposure intensity control unit (11) for changing to a developable level. 3. A uniformly charged surface potential (VP) is charged by a charging means (2) by using a photoconductor (1) having a potential attenuation characteristic which is rapidly attenuated at a certain exposure amount (A0). In the image forming apparatus, the latent image corresponding to the image pattern is written on the photoconductor (1) by the exposure unit (3) and the latent image is visualized by the developing unit (4). The means (3) includes pulse width varying means (5) for modulating the pulse width of the irradiation beam according to the multi-tone image data.
And an exposure intensity varying means (6) for variably setting the exposure intensity of the irradiation beam, and the pulse width varying means (5) exposes the exposure means (3) at the photoconductor characteristic inspection cycle different from the normal image forming cycle.
Are irradiated with a plurality of beams having different pulse widths to form a latent image stepwise on the photoconductor (1) by beam exposure with a plurality of pulse widths, and each latent image is visualized by the developing means (4). Inspection image forming means (12) for forming an image, and density detection for detecting density information of a developed image corresponding to each pulse width on the photoconductor (1) formed by the inspection image forming means (12) Means (13) and threshold pulse width determination for obtaining a threshold pulse width (WL) corresponding to the density information of the lower limit level capable of forming a developed image, based on the density information detected by the density detecting means (13) Means (14) and the pulse width of the multi-gradation image data that is variably set by the pulse width varying means (5) during the image forming cycle. W
Pulse width determining means (15) for determining whether the pulse width is smaller than L), and the pulse width determining means (15) determines that the pulse width of the multi-tone image data is smaller than the threshold pulse width (WL). Under such a situation, the exposure intensity by the exposure intensity varying means (6) is set to be larger than that when the threshold pulse width (WL) is equal to or more than the threshold pulse width (WL), and the latent image potential level in the low resolution area less than the threshold pulse width (WL) is set. An image forming apparatus comprising: an exposure intensity control unit (16) for changing to a developable level. 4. The exposure means (3) according to claim 2 or 3, wherein the diameter of the beam dot irradiated on the photoconductor (1) in the main scanning direction is the diameter of the beam dot in the sub scanning direction. An image forming apparatus characterized in that the size is set smaller than the size.