Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1997-04-09
2001-02-13
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S237000, C347S136000
Reexamination Certificate
active
06188425
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an exposure device and, in particular, to an exposure device using an optical shutter.
BACKGROUND OF THE INVENTION
FIG. 12
shows a configuration of an image forming device
700
with a conventional image exposure device.
Referring to
FIG. 12
, arranged in image forming device
700
are a photosensitive drum
701
, as an electrostatic latent image holder; a corona charger
702
, which charges a surface of photosensitive drum
701
around drum
701
in the direction in which drum
701
rotates; an exposure device
700
having an optical shutter array (also referred to as an optical shutter head)
703
which exposes an image by one line on the charged surface of photosensitive drum
701
and an optical shutter driver
704
which drives optical shutter array
703
, a developer
705
which develops an electrostatic latent image with a toner, a transfer/separation charger
706
which transfers a toner image obtained by the development onto a recording sheet, a cleaner
107
which removes residual toner on the photosensitive drum
701
, and a residual charge elimination lamp
108
which illuminates the photosensitive drum
701
and removes residual charge of the photosensitive drum
701
. A recording sheet
709
is transported by a transportation roller
710
between photosensitive drum
701
and transfer/separation charger
706
.
Optical shutter array
703
is a solid scanner component having a plurality of optical shutter elements arranged in the longitudinal direction along the rotation axis of photosensitive drum
701
.
FIGS. 13A and 13B
show arrangements of optical shutter elements of optical shutter array
703
shown in FIG.
12
.
Referring to
FIGS. 13A and 13B
, optical shutter elements of optical shutter array
703
are those formed of liquid crystal, PLZT (Pb Lanthanum-added Zirconate Titanate) or the like which are arranged in one line, as shown in
FIG. 13A
, or staggered in two lines, as shown in
FIG. 13B
, such that the number thereof corresponds to a width to be recorded on a recording sheet.
FIG. 14
is a cross sectional view of a configuration of a PLZT optical shutter head
900
as an example of optical shutter array
703
shown in FIG.
12
.
Referring to
FIG. 14
, PLZT optical shutter head
900
includes a light emitting portion
30
, a rod lens
31
which collects a linear pencil of light from light emitting portion
30
, an optical shutter portion
32
which selectively transmits light from rod lens
31
, and a rod lens array
33
which converges light transmitted from optical shutter portion
32
.
Light emitting portion
30
includes a halogen lamp
34
as a point source of light, and a fiber optic light guide
35
which converts a pencil of light from the point source of light to a linear pencil of light.
Optical shutter portion
32
is constituted by a polarizer
36
which selectively transmits only the light in a certain polarization direction of incident light having a random plane of polarization and an analyzer
38
which transmits only the light in the polarization direction angled by an angle of 90ø relative to the polarization direction of the light transmitted from polarizer
36
, with a PLZT optical shutter array
37
having electro-optic effect disposed therebetween, an optical shutter element of PLZT optical shutter array
37
having micropixels.
When a voltage is applied to an optical shutter element having micropixels and forming the PLZT optical shutter array
37
in the optical shutter portion
32
, a plane of polarized light, transmitted from polarizer
36
, is rotated and then passes through analyzer
38
. On the other hand, when a voltage is not applied to an optical shutter element, a plane of polarized light, transmitted from polarizer
36
, is unchanged and hence blocked by analyzer
38
. Light is selectively transmitted depending on whether or not voltage is applied to each of optical shutter elements forming PLZT optical shutter array
37
.
In an image forming device as described above, a driving voltage for an optical shutter element is preset. At the driving voltage, a period during which the optical shutter is opened is predetermined depending on the output pixel tone and thus it is operated.
However, as an accumulated drive period, i.e., an accumulation of periods during which an optical shutter is opened, is increased, durability of the optical shutter element is degraded and the quantity of light tends to be gradually decreased.
FIG. 6
shows how the amount of light emission of an optical shutter element changes relative to an accumulated optical shutter element drive period.
Referring to
FIG. 6
, in a conventional exposure device in which as the accumulated drive period is increased, the amount of light emission is decreased due to durability degradation of an optical shutter. When a predetermined period elapses, an accumulated drive period of an optical shutter element for a pixel A arranged in the longitudinal direction differs from that of an optical element for a pixel B arranged in the longitudinal direction, for example, depending on the image patterns which have been output and a drive period difference &Dgr;t is caused. That is, if there is a drive period difference of &Dgr;t between the optical shutter elements corresponding to pixels A and B, as shown in
FIG. 6
, there also is a difference in the amount of decreased light emission (the amount of degradation) and thus the difference &Dgr;E in the amount of light emission is caused.
Thus, since a plurality of optical shutter elements arranged in the longitudinal direction each have different accumulated drive periods, the reduced amount of light emission is different for each optical shutter element, thus causing unevenness in the quantity of light.
FIG. 15
is a state diagram illustrating a relationship between the period during which an optical shutter is opened (referred to as an optical shutter open period hereinafter) and the quantity of light outputted.
Referring to
FIG. 15
, in a conventional image forming device, which is assumed to be free from degradation and thus ideal, the optical shutter open period and the quantity of light are assumed to satisfy a linearly proportional relationship designated by the letter a and thus an optical shutter element open period corresponding to a tone is fixed independently of the drive period of each optical shutter element. In practice, however, the quantity of light is decreased due to degradation, as designated by the letter b, and thus it can be difficult to reproduce a pixel in a tone of interest.
If a voltage which drives an optical shutter element (referred to as a driving voltage hereinafter) is increased to compensate for the reduced quantity of light to achieve the state designated by the letter a, then the relation between the optical shutter open period and the quantity of light transmitted from the optical shutter will become nonlinear. If an optical shutter element is extremely degraded, deficiency in the quantity of light can be caused as the optical shutter open period reaches or exceeds T
1
, as designated by the letter c. If an optical shutter element is not so degraded, increase in the driving voltage can lead to saturation of the quantity of light, as designated by the letter d.
Thus, increasing the driving voltage to output a pixel in a multivalue tone cannot compensate for a change of the quantity of light due to durability degradation of an optical shutter element, since the optical shutter open period is preset for each pixel. Thus, a half tone or the like can not be reproduced successfully, and an image of high quality cannot be obtained.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an exposure device capable of reducing unevenness in a quantity of light due to durability degradation of each optical shutter element.
Another object of the present invention is to provide an exposure device capable of forming an image formed of pixels having a multivalue tone always in an appropriate state.
The above obje
Matsuura Kuniya
Sakamoto Kazuhiro
Le N.
Minolta Co. Ltd
Nguyen Lamson D.
Sidley & Austin
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