Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1999-09-13
2003-04-29
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
Reexamination Certificate
active
06556233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrophotographic apparatus and an electrophotographic method and, more particularly, to an electrophotographic apparatus provided with an optical scanner for scanning an area on a recording medium with a light beam deflected by a light reflection type deflector, such as laser beam printers, laser copiers, laser facsimile machines, and so on, and an electrophotographic method carried out by the apparatus.
2. Related Background Art
In recent years laser printers are drawing attention because of such characteristics thereof as good quality of image, quick printout, and so on. The performance of scanning the area on the recording medium while deflecting a laser beam in the laser printers considerably affects the performance of the image quality, print speed, etc. of the laser printers.
As illustrated in
FIG. 1
, an optical scanner is normally composed of a laser diode
100
, a rotary polygon mirror
102
, a light-source optical system
104
for guiding a laser beam emitted from the laser diode
100
to the rotary polygon mirror
102
, and a scanning optical system
108
for guiding the laser beam deflected by the rotary polygon mirror
102
to a recording medium
106
to scan it.
Relations between the rotary polygon mirror and the incident beam are generally classified under two types described below. The first type is an optical system of a type in which the width D of the incident light beam L is narrower than the width W of one reflecting facet
112
A of the rotary polygon mirror
112
and in which all the incident light beam L is guided to the scanning lens (which will be referred to hereinafter as an underfield type optical system or UFS), as illustrated in FIG.
2
.
The second type is an optical system of an overexposure type in which the width D of the incident light beam L is wider than the width W of one reflecting facet
112
A of the rotary polygon mirror
112
(which will be referred to hereinafter as an overfield type optical system or OFS), as illustrated in FIG.
3
.
Assuming that the same scanning lens is used, the overfield optical system is characterized by lower rotational speed of a motor etc., because the diameter of the rotary polygon mirror in the overfield optical system can be lower than that in the underfield optical system. However, because the incident beam is cut down in part to be directed toward the scanning lens, amounts of emerging light differ depending upon positions on the recording medium, which poses a problem of the locally different densities of image or the like.
To overcome the problem, U.S. Pat. No. 3,558,208 suggests that a light amount distribution of the light beam L is preliminarily shaped by a transmittance-distributed filter
126
before incidence of the incident beam to the rotary polygon mirror
112
, as illustrated in
FIG. 4
, so as to make light amounts of emerging rays toward the scanning lens
128
constant. This device, however, had the problems of the increased number of parts, increased cost, etc. because of the use of the special filter.
On the other hand, it is known that inconsistency of the light beam reflected by one reflecting facet
112
A of the rotary polygon mirror
112
can be reduced by increasing the width of the light beam L incident to the rotary polygon mirror
112
by the light-source optical system, as illustrated in FIG.
5
.
Further, image forming methods of the electrophotographic apparatus of the digital type are generally classified under two types described below, as to relations between image information and exposed part. The first type is an image exposure method for exposing image areas to light (hereinafter referred to as IAE) and the second type is a background exposure method for exposing nonimage areas (background areas) to light (hereinafter referred to as BAE).
BAE is the same image forming method as that used in the electrophotographic apparatus of the analog type and thus has the merit of permitting component sharing of development, cleaning, developer, etc. with the electrophotographic apparatus of the analog type.
On the other hand, IAE requires inversion development with a developer of opposite polarity in order to gain a normal image.
Both types are put in practical use, and in many cases either one of them is determined according to restrictions such as a photosensitive member, the developer, etc. used.
A variety of developing methods, including one-component development, two-component brush development, and so on, are devised or adopted according to the needs, such as monochrome or color, or the like, and it is understood in general that image reproduction characteristics in the two-component brush development are superior to those in the one-component development. However, they have their respective features.
The principal features of the developing methods are as follows; (a) the image characteristics of the BMT method and the FEED method (one component, electrically insulative, magnetic, contact), particularly, the image characteristics of the FEED method, are approximately equivalent to those of the two component brush development; (b) the touchdown method (one component, electrically insulative, nonmagnetic, contact) has the problem of fog due to contact development; (c) the jumping method (one component, electrically insulative, magnetic, noncontact) seldom suffers the problems of fog and scratches because of noncontact; (d) the projection method (one component, electrically insulative, nonmagnetic, noncontact) rarely experiences the problems of fog and scratches because of noncontact and permits application to color image formation because of the non-magnetic property; (e) the magnedynamic method (one component, electrically conductive, magnetic, contact) involves inductive charging by a latent-image electric field and brush development to permit development of either positive or negative latent image, but transfer is difficult; (f) the IMB method (two components, electrically insulative, nonmagnetic, contact) suffers storage of charge of opposite polarity after the development because of its electrically insulative carrier, and reproducibility is not good in solid areas but is good in fine lines; (g) the CMB method (two components, electrically conductive, nonmagnetic, contact) has good reproducibility of solid areas without storage of the opposite polarity charge after the development because of its electrically conductive carrier, but reproducibility is inferior in fine lines of low density; and so on.
In general, the development is a trade-off between fog and density and the latitude thereof considerably affects the developing performance. The latitude of BAE is wider than that of IAE.
On the other hand, the transfer-separation performance is greatly influenced by the transfer efficiency and the latitude of separation and retransfer, and the latitude of BAE is wider than that of IAE, because in IAE the potential at the nonimage areas (background areas) is higher than that at the image areas.
Since the potential of the photosensitive member is decreased at rush into the cleaning, a lot of developer is likely to attach to the photosensitive member in the cleaning section in the case of the IAE which is the developing method of lower-potential portions and, therefore, the latitude of BAE is also wider as to the cleaning than that of IAE.
As described above, BAE has the potential of easier design and, as a result, the potential of capability of supplying stable electrophotographic apparatus with wide latitude.
BAE, however, has a drawback of narrower latitude in image recording by scanning with the laser beam than IAE, as described below.
In the image recording technology by the laser beam scanning, it is generally known that the size, shape, power, etc. of the laser spot greatly affect the image quality and stability.
Particularly, in the laser beam printers based on electrophotography, a latent image is formed by exposing a uniform surface potential distribution obtained by charging
Ehara Toshiyuki
Kawada Masaya
Nakayama Yuji
Owaki Hironori
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Pham Hai
LandOfFree
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