Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-10-15
2002-12-17
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
Reexamination Certificate
active
06496210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital electrophotographic imaging apparatus in which a latent electrostatic image is formed by exposing a photoreceptor by using an optical beam such as a laser beam and the image is visualized by a toner.
2. Description of the Background Art
Table 1 represents relation of resolution per inch (DPI: Dot Per Inch) of a laser beam printer, dot pitch Dp corresponding to the resolution and average beam diameters employed. It can be seen from the table that resolution of the optical system cannot follow the resolution of the printer, as the resolution increases.
TABLE 1
Dot Pitch
Beam Diameter
Resolution
Dp
Ds
Ds/Dp
300 DPI
84 &mgr;m
100~110
&mgr;m
1.2~1.3
600 DPI
42 &mgr;m
75~85
&mgr;m
1.8~2.0
1200 DPI
21 &mgr;m
55~65
&mgr;m
2.6~3.1
FIG. 2
is a schematic diagram of a laser beam scanning optics, in which a light beam
45
emitted from a laser light source
44
passes through a collimator lens
46
and a cylindrical lens
47
, reflected by a polygon mirror
41
and forms an image on a photoreceptor drum
2
. As polygon mirror
41
rotates, the light beam scans the photoreceptor drum
2
.
FIG. 3
is an enlarged view of the image forming portion. The expansion of the image forming beam (beam diameter) Ds with no aberration is given by the following equation, where &lgr; represents the wavelength of the laser beam, f represents focal length of the lens and D represents pupil diameter of the laser beam.
Ds=
2&lgr;/(&pgr;·
NA
)=4&lgr;·
f
/(&pgr;·
D
)
The focal length f of the laser scanning optics must be as large as necessary for scanning an A
3
object. Therefore, in order to make smaller the beam diameter Ds, it is necessary to shorten the wavelength &lgr; or to enlarge the pupil diameter D. In order to use a light source having shorter wavelength, the design of the optics must be changed and the photoreceptor material must be reviewed in accordance with the wavelength. When the pupil diameter D is enlarged, the optics becomes larger, and therefore, it is difficult to positively make smaller the beam diameter Ds in practice.
FIGS. 1A
to
1
C represent relations between the beam diameter Ds and the dot pitch Dp. When the beam diameter Ds and the dot pitch Dp are equal to each other, the ratio of the width W
1
of 1 line with respect to the width W
2
for 2 lines is 1:2. When the beam diameter Ds becomes larger than the dot pitch Dp, the relation of 1:2 is lost gradually.
Therefore, as can be seen from Table 1, in a current laser beam scanning apparatus, when the resolution is 600 DPI or higher, the ratio between the beam diameter Ds and the dot pitch Dp attains to 2 or higher. Therefore, it becomes difficult to ensure the ratio of the width W
1
of 1 line with respect to the width W
2
of 2 lines of 1:2.
SUMMARY OF THE INVENTION
The present invention was made in view of the foregoing and its object is to provide an electrophotographic imaging apparatus in which a toner image obtained by development can be fixed at a desired ratio (the ratio of the width of 1 line with respect to the width of 2 lines is 1:2) even when the beam diameter is larger than the dot pitch.
The above described object can be attained by an electrophotographic imaging apparatus employing a digital electrophotographic imaging process in which a latent electrostatistic image is written discretely by exposure modulation means on a charged photoreceptor and the image is visualized by development, characterized in that an unsaturated region of photo-induced discharge characteristic or gamma characteristics of development of the photoreceptor is used when an image of 1 dot or a 1-dot line, that is an image of which lines consist of single dots to be formed, and a saturated region of photo-induced discharge characteristics or gamma characteristics of development is used when an image of 2 dots or at least 2-dots line is to be formed.
Though
FIGS. 1A
to
1
C schematically show the relation between the beam diameter Ds and the line width W, in an actual digital electrophotographic imaging process, exposure energy in accordance with image information is emitted from an exposure apparatus
4
to a photoreceptor
2
which has been uniformly charged by a charger
3
, charges at the irradiated portion are lost and a latent image profile is formed, as shown in FIG.
4
. Thereafter, charged toner
10
is fixed on the portion where the charges have been lost, by developing means
11
, so that a development profile as a toner image is formed. More specifically, an image pattern including images of 1 line and 2 lines is converted through an exposure profile and a latent image profile to a development profile.
The exposure profile depends on power and size of an exposure beam as well as on exposure time. The process in which the exposure profile is determined will be described with reference to laser scanning as an example. Energy distribution I (x, y) per unit time when a Gaussian laser beam, which has laser power P and beam radius of wx, wy, is static is given by the equation (2).
I
(
x
,
y
)
=
2
P
π
·
wx
·
wy
Exp
(
-
2
x
2
wx
2
-
2
y
2
wy
2
)
(
2
)
Exposure energy profile Ev (x, y) when pulse width is &Dgr;t and scanning rate is v is given by the equation (3).
Ev
(
x
,
y
)
=
∫
0
Δ
t
I
(
x
-
v
·
t
,
y
)
ⅆ
t
=
2
P
π
·
wx
·
wy
Exp
(
-
2
y
2
wy
2
)
∫
0
Δ
t
Exp
(
-
2
(
x
-
v
·
t
)
2
wx
2
)
ⅆ
t
(
3
)
In digital exposure, when we represent exposure coordinates as (x
i
, y
j
), x
i
and y
j
assume discrete values dependent on the resolution. For example, when it is 1200 DPI, the dot pitch Dp is about 20 &mgr;m, and therefore, x
1
, x
2
, . . . will be x
1
, x
2
, . . . =0, 20, 40, 60, . . . Assume that a function G (x
i
, y
j
) representing the state of exposure of the coordinates (x
i
, y
j
) is defined by equation (4).
G
(
x
i
,
y
i
)
=
{
1
laser ON
0
laser OFF
(
4
)
Then, a total exposure profile En (x, y) is given by the following equation (5).
E
n
(
x
,
y
)
=
∑
i
∑
j
Ev
(
x
-
x
i
,
y
-
y
j
)
·
G
(
x
i
,
y
j
)
(
5
)
FIG. 6
represents the image profile transfer function of
FIG. 5
plotted in detail as a graph. Specifically, it is a chart plotting the process in which an exposure profile corresponding to image information of 1 line and 2 lines having the pixel density of 1200 DPI is converted to a latent image profile in accordance with photo-induced discharge characteristics of photoreceptor
2
, and further converted to a development profile in accordance with the gamma characteristics of development.
The exposure beam has a power of 0.2 mW and beam diameter of 60 &mgr;m. Exposure time per dot is 17.76 nsec, and scanning rate is 1191.9 m/sec. Initial potential voltage after charging of photoreceptor
2
is −600V, and half-decay exposure energy is 0.1 &mgr;J/cm
2
. More specifically, the characteristic is such that when irradiated with optical energy density of 0.1 &mgr;J/cm
2
, photoreceptor potential falls to −300V, that is, ½ of the initial potential voltage after charging.
In this example, the peak values of exposure energy distribution of 1 line and 2 lines both attain to the saturation values of photo-induced discharge characteristics of the photoreceptor, and the latent image profile and the development profile are both truncated profiles. Therefore, it is impossible to attain the line width ratio W
1
and W
2
of 1 line and 2 lines, that is, W1:W
2
to 1:2. Further, the line width of 1 line attains to 45 &mgr;m when the adhesion amount is 0.4 mg/cm
2
(amount of adhesion corresponding to one layer of toner having average grain diameter of 7 &mgr;m), which is larger than the target value (about 25 &mgr;m) of the width of 1 line with 1200 DPI.
FIG. 7
shows an example in which a latent image is fo
Azuma Nobuyuki
Iwamatsu Tadashi
Mutou Yoshinori
Toyoshima Tetsuro
Conlin David C.
Edwards & Angell LLP
Pendegrass Joan
Roos Richard J.
Sharp Kabushiki Kaisha
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