Electrophotography – Supplemental electrophotographic process – Exposure or charging
Reexamination Certificate
1999-06-18
2001-03-20
Royer, William J. (Department: 2852)
Electrophotography
Supplemental electrophotographic process
Exposure or charging
C399S296000
Reexamination Certificate
active
06205306
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an electrophotographic apparatus employed in, for example, a copying machine, a printer, a facsimile, and a publishing system.
Among image forming apparatuses, a laser printer which employs an electrophotographic system has been known as a high speed, low noise printer.
Referring to
FIG. 12
, which depicts the general structure of a typical conventional laser beam printer, a photosensitive drum
1
(photosensitive member) is rotatively driven in the direction indicated by an arrow mark R
1
. As it is rotatively driven, the peripheral surface of the photosensitive drum
1
is charged by a charging device
2
(rotated in the direction indicated by an arrow mark R
2
), is exposed by an exposing means
3
, and is subjected to a development process carried out by the development roller
4
a
(rotated in the direction indicated by an arrow mark R
4
) of a developing device
4
. As a result, a toner image is formed on the peripheral surface of the photosensitive drum
1
. This toner image is transferred onto a piece of transfer medium P (for example, paper) by a transferring apparatus
5
(rotated in the direction indicated by an arrow mark R
5
). After being transferred onto the transfer medium P, the toner image is fixed to the surface of the transfer medium P by a fixing device
7
. After the toner image transfer onto the recording medium P, the toner (transfer residual toner) remaining on the peripheral surface of the photosensitive drum
1
is removed by a cleaning apparatus
6
, in order to prepare the photosensitive drum
1
for the following cycle of image formation process.
The aforementioned laser beam printer employs a binary system, a system based on whether or not a given spot on the peripheral surface of the photosensitive drum
1
is to be exposed to a laser or pictorial shapes. If a laser beam printer is intended for recording only an image such as a letter, it does not need to record in intermediary tone, and therefore, its structure can be simple. As is known, it is possible to reproduce intermediary tone with the use of a printer of a binary recording type, as long as it is used with an intermediary tone reproduction method, such as a dither method or a density pattern method, which reproduces intermediary tone on the basis of dot area ratio. However, a printer which employs a dither method, a density pattern method, or the like suffers from a problem that it can not print in high resolution.
Thus, an image forming apparatus based on a pulse width modulation system (PWM system) has been proposed. According to this PWM system, intermediary tone is reproduced by each picture element, making it possible to record in high resolution without reducing recording density. More specifically, a PWM system based image forming apparatus forms picture elements with intermediary tone by changing the length of time an exposure laser beam is turned on in response to image signals. Since it is capable of forming a high resolution image with excellent tone gradation, its superiority becomes more apparent when forming a full-color image. Elaborating further, according to the aforementioned PWM system, in order to reproduce intermediary tone, the area ratio can be changed for each dot created per picture element by a laser beam spot, making it possible to reproduce intermediary tone without reducing resolution.
However, in the case of a PWM system based image forming apparatus, as picture element density is increased, the size of each picture element becomes smaller relative to the diameter of a beam spot, which creates a problem in that intermediary tone can not be satisfactorily reproduced by changing the length of time an exposure beam is turned on.
In order to improve resolution while maintaining tone gradation, it is necessary to reduce the beam spot diameter. For example, when a laser based optical scanning system is employed, it is necessary to reduce the wave length of the laser beam, to increase the NA of the f-&thgr; lens, or to take the like measures. In order to employ these measures, an expensive laser must be used. Further, as a lens or a scanner is increased in size, mechanical accuracy must be improved to compensate for the reduction in focal depth. In other words, a PWM has a problem in that when it is employed, the increase in the apparatus size and cost cannot be avoided. Also in the case of a solid state scanner such as an LED array or a liquid crystal shutter array, there is the same problem: cost increase cannot be avoided because of the high prices of these scanners, the cost increase for the improvement in the accuracy with which the scanner must be mounted, and the cost increase for the electrical circuit for driving these scanners.
Recently, regardless of the problems described above, the demand for the increase in resolution and the level of tone gradation achievable by an electrophotographic system based image forming apparatus has been rapidly increasing.
In an attempt to accommodate such demand, Japanese Laid-Open Patent Application Nos. 169,454/1989 and 172,863/1989, for example, proposed the usage of a photosensitive drum characterized in that its sensitivity is low when the amount of exposure light is low, and increases as the amount of exposure light increases. With the use of such a photosensitive drum, in each exposure spot which displays a certain light intensity distribution pattern, the areas with low intensity are ignored so that the same effects as those obtained when the exposure spot diameter is reduced can be obtained. In this specification, a photosensitive member capable of producing such effects is called an induction type photosensitive member. The employment of an induction type photosensitive member as the photosensitive member for an image forming apparatus in which the photosensitive member is exposed to a scanning exposure spot with a light intensity distribution pattern, made it possible to achieve a resolution level higher than what was expected from the diameter of the exposure spot. When an electrostatic latent image formed on a photosensitive member with a high level of surface charge density was developed through the application of a high frequency development bias, a strong electric field was created on the peripheral surface of the photosensitive member due to the high level of charge density. As a result, not only relatively large image patterns such as lines or letters, but also image patterns constituted of a plurality of independent dots, such as the pattern in the half tone portions of a picture image, could be formed in an extremely high toner density.
However, in order to transfer by a transferring apparatus, the toner particles held fast to the photosensitive member by an extremely strong force, the toner particles on the photosensitive drum, which constitute the toner image, must be ripped away from the photosensitive member with the use of a strong transfer electric field. As a transfer electric field is strengthened, transfer efficiency increases. However, if the strength of a transfer electric field exceeds a certain level, electrical discharge or the like occurs which causes toner particles to aggregate, resulting in reduction in image quality. Normally, in the formation of an image of any pattern, the strength of the transfer electric field is set to strike an optimal balance between transfer efficiency and dot reproduction. However, when the aforementioned induction type photosensitive member is employed, a resultant toner image displays a high level of toner density and is excellent in terms of sharpness of contour while it is on the peripheral surface of the photosensitive drum, but while, or after, it is transferred onto a piece of transfer medium, it suffers from the problem that the toner particles scatter from the image, or the image fails to be satisfactorily transferred. In other words, a satisfactory image cannot be outputted.
SUMMARY OF THE INVENTION
The present invention was made in consideration of the above described issues,
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Royer William J.
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