Title: Apparatus with offset light source for forming images on photosensitive surface
Abstract: Disclosed is an image forming apparatus which provides an image of an excellent quality which has a less variation in resolution and suppressed linear irregularity. A rod lens array includes two rows of rod lenses stacked one on the other. An LED array is offset by a predetermined offset amount from a plane passing the median position between the first row of rod lenses and the second row of rod lenses. This structure can realize an LED printer head which reduces a variation in the resolution of the rod lens array, thereby suppressing linear irregularity, and can thus provide an image having an excellent quality.
Patent Number: 6,972,785 Issued on 12/06/2005 to Takagi, et al.
| Inventors:
|
Takagi; Tomitaka (Osaka, JP);
Iki; Koichiro (Osaka, JP)
|
| Assignee:
|
Nippon Sheet Glass Co., Ltd. (JP)
|
| Appl. No.:
|
327218 |
| Filed:
|
December 23, 2002 |
Foreign Application Priority Data
| Dec 28, 2001[JP] | 2001-401307 |
| Current U.S. Class: |
347/244; 347/258 |
| Intern'l Class: |
B41J 027/00 |
| Field of Search: |
347/238,241,244,256,258,130
349/95
359/652-654
385/119
399/201-203,212-215,220-221,197
|
References Cited [Referenced By]
U.S. Patent Documents
| 4447126 | May., 1984 | Heidrich et al.
| |
| 4947195 | Aug., 1990 | Flynn et al.
| |
| 6340982 | Jan., 2002 | Taira et al.
| |
| Foreign Patent Documents |
| 0 786 353 | Jul., 1997 | EP.
| |
| 09052385 | Feb., 1997 | JP.
| |
| 10-309826 | Nov., 1998 | JP.
| |
| 2002331705 | Nov., 2002 | JP.
| |
Primary Examiner: Pham; Hai
Attorney, Agent or Firm: Fish & Richardson P.C.
Claims
1. An image forming apparatus comprising:
an image forming surface;
a point light source array including a plurality of point light sources arranged
in a line; and
a lens array located between the image forming surface and the point light source
array and including first and second rows of gradient index rod lenses, the lens
array having a center line lying on a median plane between the first row of rod
lenses and the second row of rod lenses, the point light sources being offset by
a predetermined offset amount from a median position between the first row of rod
lenses and the second row of rod lenses, the point light sources being asymmetrically
offset with respect to an optical axis of the first row of rod lenses and an optical
axis of the second row of rod lenses, and the center line being substantially parallel
to a line connecting the point light source array to an image formed on the image
forming surface.
2. The image forming apparatus according to claim 1, wherein the point light
sources are light emitting diodes and the point light source array is a light emitting
diode array.
3. The image forming apparatus according to claim 1, wherein the predetermined
offset amount is in a range given by the equation: 0.5p×(X
0/d)≦offset
amount≦2.5p×(X
0/d) where p is a pitch of the point light sources,
X
0 is a radius of a visual field of each of the gradient index rod lenses
and d is a lens interval between the gradient index rod lenses.
4. An image forming apparatus comprising:
an image forming surface;
a point light source array including an array of point light sources which are
activated in accordance with an image signal; and
a lens array located between the image forming surface and the point light source
array for forming an image on the image forming surface according to light information
from the point light source array, the lens array including first and second rows
of gradient index rod lenses, the lens array having a center line lying on a plane
passing through a median position between an optical axis of the first row of rod
lenses and an optical axis of the second row of rod lenses, the array of the point
light sources being offset by a predetermined offset amount from said plane, the
array of the point light sources being asymmetrically offset with respect to the
optical axis of the first row of rod lenses and the optical axis of the second
row of rod lenses, and the center line being substantially parallel to a line connecting
the point light source array to the image formed on the image forming surface.
5. The image forming apparatus according to claim 4, wherein the predetermined
offset amount is in a range given by the equation:
0.5p×(X
0/d)≦offset amount≦2.5p×(X
0/d) where
p is a pitch of the point light sources, X
0 is a radius of a visual field
of each of the gradient index rod lenses and d is a lens interval between the gradient
index rod lenses.
6. A light emitting diode printer head for emitting light of an image to an image
forming surface, the printer head comprising:
a first row of gradient index rod lenses;
a second row of gradient index rod lenses stacked on the first row of gradient
index rod lenses; and
a plurality of light emitting diodes located to face the gradient index rod lenses
and being offset by 18 micrometers to 200 micrometers from a plane passing through
a median position between the first row of rod lenses and the second row of rod
lenses, the plurality of light emitting diodes being asymmetrically offset with
respect to an optical axis of the first row of rod lenses and an optical axis of
the second row of rod lenses, and said plane being substantially parallel to an
imaginary plane that passes through the light emitting diodes and the image formed
on the image forming surface.
7. A method for manufacturing an image forming apparatus for forming an image
on an image forming surface, the method comprising the steps of:
preparing a lens array including first and second rows of gradient index rod
lenses and a point light source array including an array of point light sources,
which are activated in accordance with an image signal, wherein the lens array
has a center line lying on a plane passing through a median position between an
optical axis of the first row of rod lenses and an optical axis of the second row
of rod lenses; and
arranging the array of point light sources to be offset by a predetermined offset
amount from said plane so that said center line is substantially parallel to a
line connecting the point light source array to the image formed on the image forming
surface, the array of point light sources being asymmetrically offset with respect
to the optical axis of the first row of rod lenses and the optical axis of the
second row of rod lenses.
8. The method according to claim 7, wherein the arranging step includes adjusting
the predetermined offset amount to be in a range given by the equation:
0.5p×(X
0/d)≦offset amount≦2.5p×(X
0/d) where
p is a pitch of the point light sources, X
0 is a radius of a visual field
of each of the gradient index rod lenses and d is a lens interval between the gradient
index rod lenses.
9. An image forming apparatus comprising:
an image forming surface;
a point light source array comprising a row of point light sources; and
a lens array located between the image forming surface and the point light source
array, the lens array comprising a first row of gradient index rod lenses and a
second row of gradient index rod lenses, the lens array having a center line, lying
on a median plane between the first row of rod lenses and the second row of rod
lenses, that is substantially parallel to a line connecting the point light source
array to an image formed on the image forming surface, the row of point light sources
being asymmetrically offset with respect to the first row of rod lenses and the
second row of rod lenses.
10. A method for manufacturing an image forming apparatus for forming an image
on an image forming surface, the method comprising the steps of:
preparing a lens array including a first row of gradient index rod lenses and
a second row of gradient index rod lenses, the lens array having a center line
lying on a plane passing through a median position between an optical axis of the
first row of rod lenses and an optical axis of the second row of rod lenses; and
arranging a row of point light sources to be asymmetrically offset with respect
to the optical axis of the first row of rod lenses and the optical axis of the
second row of rod lenses, where the center line is substantially parallel to a
line connecting the row of point light sources to the image on the image forming surface.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
Pursuant to 35 USC § 119, this application claims the benefit of Japan
Patent Application No. 2001-401307 filed Dec. 28, 2001.
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus, and, more particularly,
to an image forming apparatus, such as an light emitting diode (LED) printer head,
which forms an image on a photosensitive surface by forming the image of light
information from an LED array having a plurality of point light sources by a lens array.
A conventional LED printer head includes an LED array having a plurality of LEDs
and a lens array which forms an image on a photosensitive surface by forming the
image of light information irradiated from the LED array. The lens array comprises
two rows of a plurality of gradient index rod lenses. Each rod lens forms the image
of light information within a limited range. The lens array forms a total image
by overlapping images formed by the lenses.
As shown in FIG. 8, the conventional LED printer head had to adjust the positions
of the LED array and a lens array 110 in such a way that LEDs 100
would be positioned on the median plane C of the two rows of rod lenses. For example,
Japanese Laid-Open Patent Publication No. 10-309826 discloses an image forming
apparatus which is so designed as not to be easily influenced by the mounting errors
of the LED array and the lens array in order to eliminate the troublesome position adjustment.
If the resolution of the lens array in an LED printer head which forms an image
by causing a plurality of LEDs to emit light in various patterns differs at various
locations, i.e., if the resolution of the lens array has a large variation, a linear
irregularity occurs in the amount of light. The irregular amount of light results
in the formation of uneven point images on the image forming surface, which makes
the amount of toner adhered uneven, thereby resulting in uneven printing. A variation
in the amounts of lights from the LEDs can be adjusted by compensating for the
amount of light from each LED based on the light amount distribution of the surface
of an image that has been measured in advance. Because a variation in the resolution
of the lens array is a variation in a light amount profile (light amount distribution
of a point image), however, the variation cannot be corrected by changing the brightness
of the light sources. It is therefore difficult to compensate for a variation in
the resolution of the lens array.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image forming apparatus
which forms an image of an excellent quality which does not have linear irregularity.
To achieve the above object, the present invention provides an image forming
apparatus
having a point light source array including a plurality of point light sources
arranged in a line, and a lens array located to face the point light source array
and including first and second rows of gradient index rod lenses. The point light
sources are offset by a predetermined offset amount from the median position between
the first row of rod lenses and the second row of rod lenses.
A further perspective of the present invention is a light emitting diode printer
head having a first row of gradient index rod lenses, a second row of gradient
index rod lenses stacked on the first row of gradient index rod lenses, and a plurality
of light emitting diodes. The light emitting diodes is located to face the gradient
index rod lenses and is offset by 18 micrometers to 200 micrometers from the median
position between the first row of rod lenses and the second row of rod lenses.
A further perspective of the present invention is a method for manufacturing
an
image forming apparatus. The method includes preparing a lens array including first
and second rows of gradient index rod lenses and a point light source array including
an array of point light sources, which are activated in accordance with an image
signal, and arranging the array of point light sources to be offset by a predetermined
offset amount from the median position between an optical axis of the first row
of rod lenses and an optical axis of the second row of rod lenses.
Other aspects and advantages of the present invention will become apparent
from the following description, taken in conjunction with the accompanying drawings,
illustrating by way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention that are believed to be novel are set forth
with particularity in the appended claims. The invention, together with objects
and advantages thereof, may best be understood by reference to the following description
of the presently preferred embodiments together with the accompanying drawings
in which:
FIG. 1 is a side view showing the layout of an array of LEDs and a lens array
in an LED printer head according to the present invention;
FIG. 2 is a schematic side view of an LED printer head according to one embodiment
of the present invention;
FIG. 3 is a schematic diagram of an LED printer which uses the LED printer head
in FIG. 2;
FIG. 4 is a perspective view showing a lens array for the LED printer head in
FIG. 2;
FIG. 5 is a diagram showing how an image is formed by a lens array in FIG. 2;
FIGS. 6A and 6B are graphs showing the relationships between MTFσ and
an offset amount in case of a lens array having an overlapping degree m of 1.9;
FIGS. 7A and 7B are graphs showing the relationships between MTFσ and
the offset amount in case of a lens array having an overlapping degree m of 1.7; and
FIG. 8 is a side view showing the layout of the array of LEDs and the lens array
in the conventional LED printer head.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An LED printer
11 and an LED printer head
13 according to one embodiment
of the present invention will be described below referring to the accompanying drawings.
As shown in FIG. 3, the LED printer
11 includes a cylindrical photosensitive
drum
12, the LED printer head
13, a charging unit
14, a developing
unit
15, a transfer unit
16, a fixing unit
17, a neutralization
lamp
18, a cleaning unit
19, a sheet cassette
20 and a stacker
21. The peripheral surface of the photosensitive drum
12 is formed
of a material having a photoconductivity (photosensitive material), such as amorphous
silicon. The photosensitive drum
12 is rotated in accordance with the speed
of printing. The charging unit
14 evenly charges the photosensitive surface
of the photosensitive drum
12. The LED printer head
13 irradiates
light of a dot image to be printed on the photosensitive surface of the photosensitive
drum
12. This neutralizes charging at a portion where the light hits. The
developing unit
15 supplies toner to charged portions of the photosensitive
surface. The transfer unit
16 transfers the toner onto paper
22 supplied
from the sheet cassette
20. The fixing unit
17 heats up the paper
22 to fix the toner. The stacker
21 receives the image-printed paper
22. The neutralization lamp
18 neutralizes charging of the photosensitive
drum
12 after transfer. The cleaning unit
19 cleans the toner off
the photosensitive drum
12.
Referring to FIG. 2, the LED printer head
13 will be discussed below.
The LED printer head
13 includes an LED array
23 having a plurality
of LEDs (point light sources) which are activated in accordance with an image signal
and selectively emit light, and a rod lens array
24. The distance, L, between
the rod lens array
24 and the LED array
23 is equal to the distance
between the rod lens array
24 and the photosensitive surface of the photosensitive
drum
12.
The LED array
23 is a module including an LED array chip and an IC driver
chip both mounted on a substrate. In a case where the LED array
23 is for
1200 dpi (24 line pairs/mm), a plurality of LEDs are formed at a pitch of approximately
21.2 micrometers. The individual LEDs are turned on or off in accordance with an
image signal.
The rod lens array
24 forms an image comprised of a plurality of point
images on the photosensitive surface of the photosensitive drum
12 (the
image surface in FIG. 5) by forming the image of lights output from the LEDs (the
object surface in FIG. 5). Each rod lens forms the image of output light within
a limited range. The image of the rod lens array
24 is the images of plural
rod lenses
25 which are overlapped one on another. The symbols in FIG. 5
are such that Z is the length of the lens, L is a working distance or the distance
between the end face of the lens to the object surface or the image surface, TC
is a total conjugate length or Z+2 L, X
0 is the radius of the visual field
of each rod lens
25, d is the horizontal interval of the rod lenses
25
and θ is an output angle.
As shown in FIGS. 4 and 5, the rod lens array
24 has two frames
26
and a plurality of rod lenses
25 stacked zigzag in two rows between the
frames
26. The rod lenses
25 are of a gradient index type and have
different refractive indexes in the radial direction. In each row of rod lenses
25, the rod lenses
25 are laid out at a predetermined interval from
an adjoining rod lens
25. The gaps between the rod lenses
25 are
filled with a black silicone resin
27 to eliminate flare light. In FIG.
4, the LED array
23 is located to the right of the rod lens array
24
and the photosensitive drum
12 is located to the left.
The LED array
23 has a plurality of LEDs laid out in a line at a predetermined
pitch. The pitch is about 21.2 micrometers for the LED printer head
13 for
1200 dpi. In FIG. 2, a row of LEDs
23a is perpendicular to the surface
of the paper. The end face of the rod lens array
24 is laid out so as to
face the LEDs. That is, the optical axis (longitudinal axis) of each rod lens
25
is parallel to the sheet of FIG. 2, and the plural rod lenses
25 are laid
out on the left row and the right row in FIG. 2. The row of LEDs
23a
is offset by a predetermined offset amount Y from a plane C which passes the
median position between the optical axis of the left row of rod lenses
25
and the optical axis of the right row of rod lenses
25. Specifically, the
row of LEDs
23a is laid out eccentric to the right row of rod lenses
25. This can allow the LED printer head
13 to form an image of an
excellent quality free of linear irregularity.
The following will discuss the offset amount Y. It is preferable that the offset
amount Y should be set within a range defined by an equation 1 given below.
where p is the pitch of the LEDs, X
0 is the radius of the visual field
of each rod lens
25 and d is the lens interval between the rod lenses
25
in each row. The term "X
0/d" is called the overlapping degree that indicates
the degree of overlapping of images formed by the adjoining lenses and is a parameter
which represents the performance of the rod lens array.
In case of the LED printer head
13 for 1200 dpi, for example, the pitch
p is 21.2 micrometers (25400 micrometers/1200 dots). In case of using the rod lens
array
24 whose overlapping degree m is 1.7, therefore, the desirable offset
amount Y lies in a range of about 18 micrometers to about 90 micrometers. In case
of using the rod lens array
24 whose overlapping degree m is 1.9, the desirable
offset amount Y lies in a range of about 20 micrometers to about 100 micrometers.
In case of the LED printer head
13 for 600 dpi, for example, the pitch
p is 42.4 micrometers (25400 micrometers/600 dots). In case of using the rod lens
array
24 whose overlapping degree m is 1.7, therefore, the desirable offset
amount Y lies in a range of about 36 micrometers to about 180 micrometers. In case
of using the rod lens array
24 whose overlapping degree m is 1.9, the desirable
offset amount Y lies in a range of about 40 micrometers to about 200 micrometers.
This embodiment has the following advantages.
The row of LEDs
23a is offset by the predetermined offset amount
Y from the median plane C of the rod lens array
24. This reduces a variation
in the resolution of the rod lens array
24, thereby suppressing a variation
in point images on the image forming surface so that a variation in the amount
of toner adhered becomes smaller. It is therefore possible to realize an LED printer
head which has linear irregularity reduced to thereby ensure an excellent image quality.
The reduction in a variation in resolution will be discussed by referring to
FIGS. 6A,
6B,
7A and
7B. A variation in resolution is measured
by MTFσ. MTF (Modulation Transfer Function) is the index of the resolution
of a rod lens array and MTFσ is the standard deviation of the MTF of the
rod lens array. The smaller the MTFσ is, the less the linear irregularly becomes.
The horizontal scales in FIGS. 6A through 7B represent the offset amount Y and
the vertical scales represent MTFσ.
FIG. 6A shows the measuring results for the LED printer head
13 of 1200
dpi which uses the rod lens array
24 with an overlapping degree m of 1.9.
It is apparent that in a case where the offset amount is set to about 20 micrometers
to 100 micrometers, MTFσ becomes less than 3 and an excellent image quality
with linear irregularity reduced can be acquired. In other words, if the offset
amount Y is smaller than 20 micrometers, MTFσ exceeds 3 which is not desirable.
If the offset amount Y is greater than about 100 micrometers, MTFσ also exceeds
3 which is undesirable.
FIG. 6B shows the measuring results for the LED printer head
13 of 600
dpi which uses the rod lens array
24 with an overlapping degree m of 1.9.
It is apparent that in a case where the offset amount Y is set to about 40 micrometers
to 200 micrometers, MTFσ becomes less than 2 and an excellent image quality
with linear irregularity reduced can be acquired. In other words, if the offset
amount is smaller than 40 micrometers, MTFσ exceeds 2 which is not desirable.
In a case where MTFσ is originally small as in this example, the row of
LEDs
23a need not be offset. Setting the offset amount Y in a range
of approximately 40 micrometers to 200 micrometers can however make a variation
in resolution smaller, thereby reducing linear irregularity. This can ensure a
higher image quality.
FIG. 7A shows the measuring results for the LED printer head
13 of 1200
dpi which uses the rod lens array
24 with an overlapping degree m of 1.7.
It is apparent that in a case where the offset amount Y is set to approximately
18 micrometers to 90 micrometers, MTFσ becomes less than 4 and an excellent
image quality with linear irregularity reduced can be acquired. In other words,
if the offset amount is smaller than 18 micrometers, MTFσ exceeds 4 which
is not desirable. If the offset amount is greater than approximately 90 micrometers,
MTFσ also exceeds 4 which is undesirable.
FIG. 7B shows the measuring results for the LED printer head
13 of 600
dpi which uses the rod lens array
24 with an overlapping degree m of 1.7.
It is apparent that in a case where the offset amount is set to approximately 36
micrometers to 180 micrometers, MTFσ becomes less than 2 and an excellent
image quality with linear irregularity reduced can be acquired. In other words,
if the offset amount is smaller than 36 micrometers, MTFσ exceeds 2 which
is not desirable. In a case where MTFσ is originally small as in this example,
however, the row of LEDs
23a need not be offset. Setting the offset
amount Y in a range of approximately 36 micrometers to 180 micrometers can make
a variation in resolution smaller, thereby reducing linear irregularity. This can
ensure a higher image quality.
It is apparent from the results shown in FIGS. 6A and 7A that the effect of reducing
a variation in the resolution of the rod lens array
24 is significant in
case of an LED printer head which forms an image with a higher recording density.
The invention therefore demonstrates an outstanding advantage particularly in an
image forming apparatus whose recording density is high.
It should be apparent to those skilled in the art that the present invention
may
be embodied in many other specific forms without departing from the spirit or scope
of the invention.
Although the row of LEDs
23a is offset to the right to the
median plane C in FIG. 2, it may be offset to the left. As apparent from the results
given in FIGS. 6A through 7B, a variation in resolution is reduced regardless of
the direction of offset.
The present invention is also adaptable to a case where there are plural rows
of LEDs. In case of two rows of LEDs, for example, the two rows of LEDs are offset
from the median plane C by offset amounts Y
1 and Y
2, respectively.
In this case, the rows of LEDs are offset to the same side from the median plane C.
In case of two rows of LEDs, one row of LEDs may be offset to the right to the
median plane C and the other row of LEDs may be offset to the left by the same
offset amount Y.
The light source array is not limited to the LED array
23. The light source
array can take any form as long as it generates and kills light element by element
or it passes and blocks light from an external light source pixel by pixel. The
light source array is a light source, such as a light shutter array, which has
a plurality of point light sources that selectively emit light in accordance with
an image signal. The light shutter array includes a liquid crystal shutter array
which passes and blocks light from a discharge tube pixel by pixel.
The present invention may be adapted to an optical writing head which comprises
a liquid crystal shutter array and the rod lens array
24, instead of the
LED printer head
13 which comprises the LED array
23 and the rod
lens array
24. In this case, the printer is a liquid crystal shutter printer.
The present invention is not limited to an optical printer, such as the LED printer
11, it may be adapted to a copying machine and a complex machine equipped
with a printer capability, a copying capability and a facsimile capability.
The present examples and embodiments are to be considered as illustrative and
not restrictive and the invention is not to be limited to the details given herein,
but may be modified within the scope and equivalence of the appended claims.
*
