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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to an image forming apparatus using an
electrophotographic process and more particularly to an image forming
apparatus for exposing an image carrying member by an LED (light emitting
diode) array.
2. Description Relative to the Prior Art
Apparatus for selectively removing electric charge on an image-carrying
member using LEDs in electrophotographic copiers or printers are well
known.
In LED (light-emitting diode) electrophotographic copiers or printers,
several thousand LEDs are typically arranged in one or more rows for
recording on a suitable photoconductive web or drum. Driver circuitry are
provided for selectively activating the LEDs to emit light to record in
accordance with electronic data signals. In grey level LED printers such
as disclosed in PCT Publication WO 91/10311 and U.S. application Ser. No.
07/498,512, now U.S. Pat. No. 5,200,765 (the contents of both of which are
incorporated herein by this reference), the data signals may be multibit
digital signals for determining an exposure duration for recording each
pixel. As also noted in the aforementioned publication and application,
the LEDs are known to be non-uniform light emitters relative to each other
and correction is therefore desirable to overcome image degradation due to
non-uniformities between the LEDs. One form of correction is adjustment of
pulsewidth duration for recording each pixel so that any two LEDs on the
printhead, when each is enabled to record a pixel of a desired density,
will provide approximately identical densities, even though the light
output (intensity) from each is very different. While the known correction
methods may work well with printing of grey level halftone, it is not
sufficient to handle more stringent requirements of printing
continuous-tone images.
In correcting for non-uniformities a particular LED will have assigned
thereto a number, N, of exposure times for exposing a corresponding
number, N, of pixel sizes or density. As there are thousands of LEDs but
only 255 available exposure times for say an eight-bits per pixel system,
any correction thus represents for most LEDs an approximation. Thus, some
LEDs will be provided with better correction than others. In recording of
continuous tone images artifacts resulting from inadequate correction may
be visible in the images and appear as thin lines in the in-track
direction of recording. In addition to errors in LED non-uniformity
correction, there are similar artifacts caused by improper placement of
LEDs during assembly of the LEDs on the printhead. In assembly of the
printheads, arrays of LED chips are positioned in a row. A row of LEDs on
any one chip array will be uniformly spaced (pitched) apart at say 400
LEDs to the inch due to the accuracy of the manufacturing process of such
chips. However, LEDs on the ends of adjacent chip arrays may not be
positioned so as to be also spaced or pitched with this same spacing. It
is desirable and therefore an object of the invention that the
above-mentioned problems can be reduced.
SUMMARY OF THE INVENTION
These and other objects and advantages as will become apparent are realized
by an image forming apparatus comprising an electrophotoconductive imaging
member, a plurality of light-emitting units arranged in a row for
recording a corresponding first row of pixels on the member; first means
for energizing selected units to record individual pixels in said row, the
first means energizing a selected one of said units to record a respective
pixel in response to an exposure parameter determining signal and an
exposure clocking signal; second means for generating the exposure
parameter determining signal in response to both a signal having a factor
relating to a grey level of a first pixel and a signal relating to a
correction factor for correcting for non-uniformities of the one unit;
third means for generating a set of exposure clocking signals for timing
an exposure duration during recording of the first pixel; and fourth means
responsive to a change in recording line for changing both the correction
factor and a corresponding exposure clocking set for said unit for
recording a successive pixel by said one unit wherein the exposure
determining signals used for recording the successive pixel is determined
by changing of the correction factor even though the factor related to
grey level remains the same for said first and successive pixels.
In accordance with another aspect of the invention, there is provided an
apparatus for supplying corrected image data signals to a printer having a
plurality of recording units, each unit being energizable for recording a
pixel on a recording medium, the apparatus comprising a first memory means
for storing data representing exposure parameters for recording pixels of
plural grey levels by at least one of said units, said data for each grey
level and for said one unit including plural different selectable exposure
determining parameters; a second memory means including a plurality of
sets of exposure clocking data; and option selection means responsive to a
new recording line for selecting one of the signals representing an
exposure determining parameter and a corresponding one of the sets of
exposure clocking data.
In accordance with still another aspect of the invention, there is provided
an image forming method comprising the steps of generating data signals
representing an image, at least a portion of which image includes a flat
field; energizing selected light-emitting units to record a row of
individual pixels on a recording medium, selected ones of said units being
energized to record respective pixels in response to respective exposure
parameter determining signals and a set of exposure clocking signals;
generating the respective exposure parameter determining signals in
response to both signals having factors relating to grey levels of the
pixels and signals relating to respective correction factors for
correcting for respective non-uniformities of the units; generating a set
of exposure clocking signals for timing exposure durations; and in
response to changes in recording lines, changing a set of respective
correction factors for at least some of the units and a corresponding
exposure clocking set associated with said set of correction factors and
recording a successive row of pixel by said units to reduce artifacts in
the portion of the image including the flat field.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a copier/printer forming one embodiment of
apparatus of the invention;
FIG. 2 is a schematic in block diagram form of controls used in the
copier/printer apparatus of the invention; and
FIG. 3 is a schematic of a driver circuitry on a printhead forming a part
of the copier/printer apparatus of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The apparatus of the preferred embodiment will be described in accordance
with an electrophotographic recording medium. The invention, however, is
not limited to apparatus for creating images on such a medium, as other
media such as photographic film, thermal, etc. may also be used with the
invention. The invention is also useful in recording with ink jet,
electrographic, laser, etc.
Because electrophotographic reproduction apparatus are well known, the
present description will be directed in particular to elements forming
part of or cooperating more directly with the present invention. Apparatus
not specifically shown or described herein are selectable from those known
in the prior art.
With reference now to FIG. 1, an electrophotographic reproduction apparatus
10 includes a recording medium such as a photoconductive web 12 or other
photosensitive medium that is trained about plural transport rollers 14,
16, 18 and 20, thereby forming an endless or continuous web. In lieu of a
web, a drum recording medium may be used. Roller 20 is coupled to a driver
motor M in a conventional manner. Motor M is connected to a source of
potential when a switch (not shown) is closed by a logic and control unit
(LCU) 31. When the switch is closed, the roller 20 is driven by the motor
M and moves the web 12 in a clockwise direction as indicated by arrow A.
This movement causes successive image area of the web 12 to sequentially
pass a series of electrophotographic work stations of the reproduction
apparatus.
For the purposes of the instant exposure, several work stations are shown
along the web's path. These stations will be briefly described.
First, a charging station 300 is provided at which the photoconductive
surface 16 of the web 12 is sensitized by applying to such surface a
uniform electrostatic primary charge of a predetermined voltage. The
output of the charger 17 may be controlled by a grid connected to a
programmable power supply (not shown). The supply is in turn controlled by
the LCU 31 to adjust the voltage level Vo applied onto the surface 16 by
the charger 17.
At an exposure station 19 an electrostatic image is formed by modulating
the primary charge on an image area of the surface 16 with selective
energization of point-like radiation sources in accordance with signals
provided by a suitable data source. In accordance with one embodiment of
the invention, the information to be copied is formed on a multisheet
document supported as a stack in a tray 114 that forms part of a
recirculating feeder RF. In such a feeder the document sheets are fed
seriatim from the bottom of the stack and are scanned by an image reading
device 320 that includes lamps 321, gradient index lens array 122 and
photosensors such as conventional photodiodes arranged as in a charge
coupled device (CCD) array 123. The signals are processed by image scanner
processor and buffer 75 and stored in a bit map memory storage array 73 so
that the bit map now contains in digital signal format a bit mapped
representation of the visible information on the original document sheets
being copied. The point-like radiation sources are supported in a
printhead 100 to be described in more detail below.
A development station 38 includes developer which may consist of iron
carrier particles and electroscopic toner particles with an electrostatic
charge opposite to that of the latent electrostatic image. Developer is
brushed over the photoconductive surface 16 and toner particles adhere to
the latent electrostatic image to form a visible toner particle,
transferable image. The development station may be of the magnetic brush
type with one or two rollers. Preferably, the toner particles may have a
charge of the same polarity as that of the latent electrostatic image and
develop the image in accordance with known reversal development techniques
wherein the toner develops in areas discharged by energization of the
point-like radiation sources. As shown, plural development stations 41, 43
may be provided for reproducing the image with two colors.
The apparatus 10 also includes a transfer station 25 shown with a corona
transfer and detack chargers 61 at which the toner image on web 12 is
transferred to a copy sheet S; and a cleaning station 28, at which the
photoconductive surface 16 is cleaned of any residual toner particles
remaining after the toner images have been transferred. After the transfer
of the unfixed toner images to a copy sheet S, such sheet is transported
to a heated pressure roller fuser 67 where the image is fixed to the copy
sheet S and exited to a hopper 71 or finishing accessory 71'.
A copy sheet S is fed from a hopper supply 76 by driver roller 78, which
then urges the sheet to move forward onto the web 12 in alignment with a
toner image at the transfer station 25.
To coordinate operation of the various work stations 17, 19, 38, and 25
with movement of the image areas on the web 12 past these stations, the
web has a plurality of indicia such as perforations along one of its
edges. These perforations generally are spaced equidistantly along the
edge of the web 12. At a fixed location along the path of web movement,
there is provided suitable means 26 for sensing web perforations. This
sensing produces input signals into the LCU 31 which has a digital
computer, preferably a microprocessor. The microprocessor has a stored
program responsive to the input signals for sequentially actuating, then
deactuating the work stations as well as for controlling the operation of
many other machine functions. Additional encoding means may be provided as
known in the art for providing more precise timing signals for control of
the various functions of the apparatus 10.
Programming of a number of commercially available microprocessors is a
conventional skill well understood in the art. This disclosure is written
to enable a programmer having ordinary skill in the art to produce an
appropriate control program for the one or more microprocessors used in
this apparatus. The particular details of any such program would, of
course, depend on the architecture of the designated microprocessor.
The printhead 100 is provided with a multiplicity of energizable point-like
radiation sources, preferably light-emitting diodes (LEDs). Optical means
may be provided for focusing light from each of the LEDs onto the
photoconductive surface 12. The optical means, for example, preferably
comprises an array of optical fibers such as sold under the name SELFOC, a
trademark for a gradient index lens array sold by Nippon Sheet Glass,
Limited. Due to the focusing power of the optical means 29, a row of
emitters will be imaged on a respective transverse line on the recording
medium.
The printhead 100 further comprises a suitable support with a series of
chips mounted thereon. Each of the chips includes in this example, say, 96
or 128 LEDs arranged in a single row. Chips are also arranged end-to-end
in a row and where, for example, twenty-seven LED chips are so arranged,
the printhead will extend across the width of the web 12 and include 2592
or 3456 LEDs arranged in a single row. To each side of this row of LEDs
there are provided twenty-seven identical driver chips. Each of these
driver chips include circuitry for addressing the logic associated with
each of 48 or 64 LEDs to control whether or not an LED should be energized
as well as to determine the level of current to each of the LEDs
controlled by that driver chip 40. Two driver chips 40 are thus associated
with each chip of 96 or 128 LEDs. Each of the two driver chips will be
coupled for driving of alternate LEDs. Thus, one driver chip will drive
the odd numbered LEDs of the 96 or 128 LEDs and the other will drive the
even numbered LEDs of these 96 LEDs. The driver chips are electrically
connected in parallel to a plurality of lines 34-37 providing various
electrical control signals. These lines provide electrical energy for
operating the various logic devices and current drivers in accordance with
their voltage requirements. A series of lines provide clock signals and
other pulses for controlling the movement of data to the LEDs in
accordance with known techniques, see for example, U.S. Pat. No.
5,126,759, the contents of which are incorporated herein by reference.
With reference now being made to FIG. 2, a circuit 110 is provided to
generate an eight-bit corrected grey level image data signal for
transmission to the driver chips on printhead 100. In addition, the
circuit 110 generates non-linear exposure clock pulses, see
above-mentioned publication WO 91/10311, for controlling the duration of
exposures by the LEDs. Assume in this example that the printhead 100 is a
400 dpi 8-bits per pixel LED printhead that is used to record on a
recording medium such as the aforedescribed electrophotographic web 12. Of
course, the web may be replaced by an electrophotographic drum. Printhead
non-uniformity data is stored in an 8k.times.8 SRAM lookup table (LUT)
memory LUT 111. There is one LUT set 111 for the even LEDs and another LUT
set 111' for the odd LEDs. Four correction data lookup table memories 121
for the even and an additional four memories 121' for the odd use the
respective outputs (8-bits) of the LUTs 111 and 111' as an input pointer.
Each LUT 121, 121' may be a 64K.times.8-bit SRAM and may be subdivided
into four tables LUT 121A, B, C or D and LUT 121A', 121B', 121C' or 121D'.
LUTs 121, 121' contain the correction data for each grey level for their
respective LEDs. In addition, there are four non-linear exposure clock
LUTs 131 A, B, C or D each of which contains a 16K.times.4 data field for
the exposure clock. That is, each of the LUTs 131A, B, C or D when
selected provides the data for generating a different set of exposure
clock pulses. The non-linear exposure clock LUT 131 is triggered by an
exposure master clock 181 that can have a frequency that can be adjusted
from 20 Mhz to 40 Mhz for process control purposes. However, as adjusted
its function is to provide regular clock pulses at the particular
frequency to which it is adjusted. The selection of which tables in LUT
121, 121' and LUT 131 are used is controlled by exposure options LUT 141
that contains 256.times.2 bits of data. The data output from LUT 141
controls which of the tables (131 A, B, C or D) of the exposure clock LUT
131 and which correction data tables (121A, B, C or D) of LUT 121 that are
to be used in a particular data line for recording a row of pixels in the
main scan (crosstrack) direction. These lines may be triggered from the
position encoder 28 attached to the recording medium drive M.
In accordance with the invention, a random or more preferably changing
correction scheme for recording is desired. Thus, signals to enable a
changing selection of tables in LUTs 121, 121' and LUT 131 will be loaded
into LUT 141. These signals comprise 256, 2-bit signals that are of random
or preferably optimized order are arranged at successive addresses in the
LUT 141. The invention proposes that change of the non-linear clocking
signals and corresponding non-uniformity correction tables can be used to
reach a better uniformity in a flat-field exposure area so that
non-uniformities in the flat-field are minimized. Alternately, a random or
other number generator may be used for generating a signal that is used to
randomly or optimally change the particular tables selected in memories
121, 121' and 131. As the addresses in LUT 141 are successively addressed
by the LCU 31 for each recording line of pixels a 2-bit signal is output
to select one of the 4 tables of each of LUTs 131, 121 and 121'. An LED
counter 161 under control of the logic and control unit 31 is also
provided to identify the LED to which the current uncorrected pixel data
pertains. The count value output from counter 161 and input to LUTs 111,
111' may identify both an odd and even LED since there are the same number
of odd and even LEDs.
As noted above, the exposure clock LUT 131 comprises four different tables.
Assume LUT 131A is selected in response to a two-bit selection signal from
exposure options LUT 141 and which signal is stored in latch 171. Table
131A will include, for an eight-bit printhead, a string of at least 512
binary bits, i.e., 1's and 0's, that are output four bits at a time into
shift register 11. This outputting is done in response to an address
signal created by counter 101 that counts every fourth pulse from the
master clock 181. A divide-by-four device 191 is connected to clock 181
and provides a pulse, for each series of 4-clock pulses, to increment the
address output by counter 101. The bits are clocked out of the 4-bit shift
register 11 in response to clock pulses from master clock 181. The clocked
out signals from shift register 11 have rising edges that represent to the
printhead clock pulses that are non-uniformly spaced in accordance with
the original arrangement of 1's and 0's in memory 131A. The arrangement of
1 's and 0's in each of memories 131A, B, C or D will be different to
provide a different scheme of spacings between the generated rising edges
to which the clocking circuitry is synchronized. For further details of
such an exposure clock pulse generator, reference is made to U.S.
application Ser. No. 07/807,522 and to IBM Technical Disclosure Bulletin,
Vol. 12, No. 4, page 614 (September 1969), the contents of which are
incorporated herein by this reference. Other exposure clock pulses
generating circuits are known and also may be used such as disclosed in WO
91/10311, U.S. Pat. Nos. 5,111,217 and 5,025,322.
In operation, signals representing say 8-bits per pixel uncorrected image
data are provided over each of data lines 30, 30' (even, odd,
respectively). This data may originally have come from a scanner 320 or a
computer 87 and raster image processor 88. This data has been processed to
define pixel data of appropriate grey level for printing by the printhead
at a particular pixel location. Thus lines 30, 30' each comprises a means
for carrying grey level defining signals and the 8-bits per pixel
uncorrected image data are represented by said signals and are grey level
defining signals. In synchronization with this data, the LED counter 161
counts clock pulses from logic and control unit LCU 31. The signals output
from the counter 161 represents a count identifying two particular LEDs
(odd and even) that is associated with the respective uncorrected odd and
even pixel data. The count from counter 161 is input as an address to
respective LUTs 111, 111', each of which stores respective correction
factors as correction signals relative to the respective odd and even
LEDs. Typically, the data stored in such a LUT is empirically determined
by measuring the individual light outputs of each of the LEDs and
identifying LEDs having similar characteristics. The LEDs with such
similar characteristics are identified as a group and provided with a
correction factor that is stored in the respective LUTs 111, 111' and
addressable by the count from counter 161. The outputted correction
factors from LUTs 111, 111' are carried respectively by lines 120, 120'.
Lines 120, 120' are connected respectively to LUTs 120, 121 and comprise
means for carrying correction signals and the correction signals are used
in conjunction with the uncorrected image data to define an address in
exposure correction LUTs 121, 121'. LUTs 121, 121' each comprise four
table memories 121A, B, C and D and 121A', B', C' and D' that are adapted
to, in response to address inputs over lines 120, 120', provide on
respective lines 130, 130' an exposure time-related corrected image data
signal for recording a pixel of appropriate grey level to the respective
LED to which the uncorrected image data pertains. With reference now to
FIG. 3, the exposure-related corrected image data signals each comprises
an exposure parameter determining signal and are transmitted to the
printhead 100 over data bus 130 and latched by a data latch 234 on the
printhead associated with each LED. In order to latch the eight-bit
corrected data signals in registers 234, the data signals may be first
latched off of bus 130 into registers 235 in response to a token bit
signal that is shifted through a token bit register 250 in response to
token clock pulses. Thus, as a token bit shifts through the token bit
registers, a particular latch register 235 is enabled to latch the data
currently on the data bus 130 and associated with the particular LED 230.
In response to a subsequent latch enable (LEN) signal, data is shifted
from respective latches 235 to latches 234. Exposure of the LED is
controlled by comparing in a comparator 237 associated with each LED this
multibit corrected image data signal stored in respective latch 234 with
the output of a down-up counter 260 that is counting exposure clock pulses
shifted out from register 11. The exposure clock pulses shifted out from
register 11 comprise exposure clocking signals. The counter 260 is
initially set with a count of 255 and counts down to zero and then back up
to a count of 255. Thus, a pulsewidth modulated exposure duration is
generated for each LED selected to be energized during a pixel main line
recording period. A suitable current generated by driver current
generating circuits 232 is driven through each LED selected to be
energized to cause light outputs from the LEDs to be made for periods
determined by the corrected data signals and the exposure clock pulses. A
latch 236 ensures that current to an LED continues from the time them is a
match between inputs A and B of comparator 237 during a down-count phase
until there is no longer a match during an up-count phase of counter 260.
The exposure duration for each recorded pixel will depend upon the
corrected eight-bit image data signal and the exposure clocking scheme
selected from LUT 131. The density of the pixel will vary depending upon
the brightness characteristic of the LED and the exposure duration used
for recording that pixel.
Considering exposure correction LUTs 121 and 121', it will be noted that
them is also connected thereto as an address input a 2-bit signal from the
256 .times.2 bits SRAM exposure options LUT 141. In response to signals
from the LCU 31 for each pixel recording line, the output LUT of 141
either randomly changes or changed in a pattern that has been determined
to provide either an optimum or a suitable reduction to the production of
artifacts to thereby select one of the four correction data memories 121A,
B, C or D in LUT 121, one of the four correction data memories 121A', B',
C' or D' in LUT 121' and through latch 171, one of the four exposure clock
memories 131A, B, C or D. It is desirable, however, that the selection of
the particular exposure clock memory be associated with a particular
correction data memory as empirically determined. The correction data in
each of the correction memories 121A', B', C' and D' are different. A
particular combination of correction data and set of exposure clock pulses
will provide for a particular grey level for recording a pixel by a
particular LED. Therefore, the data in the LUTs 121, 121' and 131 are
determined so as to provide different combinations of exposure correction
and exposure clocking. Consider an example where there are only two
combinations of corrected exposure data and clock sets. One combination
might provide, for example, an exposure for all the pixels on the
cross-track line that is on average +5% of target while a different
combination might be -5% of target on average. By alternately shifting
combinations for each pixel recording line (also known as main scan
recording period) over say 20 of these recording lines for one combination
and 19 of these recording lines for the other combination the average
non-uniformities can be reduced to 5%/19=0.26%. Thus, with the possible
available combination and with optimized selection as determined
empirically (via the exposure option LUTs 141 selection of which
correction table and corresponding clock set to use for what line) for
exposure, one can reduce the two dimensional non-uniformity and push the
noise, i.e. artifacts, to a higher spatial frequency that is less visible
to observers. Thus, one can even build a much more robust line or dot
screen for the electrophotographic process simultaneously with grey level
non-uniformity correction.
Note, too, that on one line an exposure for a particular LED might be more
than 5% of optimum target even though its exposure is corrected but on the
next line this LED's corrected exposure might be right on target so that
through optimum selection of correction factors from line to line the
likelihood for the production of artifacts is minimized.
Although the invention is described herein with pulse width modulation of
exposure time, the invention is also applicable to adjusting of exposure
intensity for recording pixels of different sizes or densities. In
addition, in lieu of look-up tables a microprocessor or other rapid
calculator may be used to calculate data otherwise stored in the disclosed
tables.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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