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TECHNICAL FIELD OF THE INVENTION
This invention relates generally to electrophotographic printing with a
spatial light modulator (SLM), and more particularly to generating
grayscale images with data that represents time integration values and
that is selected for delivery to the SLM with a sliding window memory.
BACKGROUND OF THE INVENTION
Spatial light modulator ("SLM") technology has-found applications in many
fields, a significant one of which is electrophotographic printing. In
general, an SLM is an array of light-emitting, light-transmitting or
light-reflecting elements, which are individually addressable, usually
with electronic signals. Many SLMs are binary, having an addressing scheme
that switches its elements to either to an "on" or an "off" state to form
the image.
One type of SLM is a digital micro-mirror device (DMD), sometimes referred
to as a deformable mirror device. The DMD has an array of hundreds or
thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is
attached to one or more hinges mounted on support posts, and spaced by
means of an air gap over underlying addressing circuitry. The addressing
circuitry provides electrostatic forces, which cause each mirror to
selectively tilt. For printing applications, the DMD is addressed with
exposure data and in accordance with the data, light is selectively
reflected or not reflected from each mirror to the printer drum.
Existing electrophotographic printer technologies make use of an organic
photoconductive ("OPC") drum. Depending on the type of photoconductor
used, the drum is either charged or discharged to attract toner, with the
charging or discharging accomplished by reflecting light onto the drum. A
page is printed by exposing the drum, array-by-array. The drum rotates in
a direction known as the process direction, and overlapping arrays of data
are superposed on the drum to accumulate charge (or discharge) on the drum
by integration of several exposures. Thus, charge is integrated over time.
The toner is then transferred to paper.
U.S. Pat. No. 5,041,851, entitled "Spatial Light Modulation Printer and
Method of Operation", describes the use of a spatial light modulator for
exposing the OPC drum. That patent is herein incorporated by reference.
Ideally, the amount of toner that clings to any point on the drum is a
function of the level of charge (or discharge) on that point. In this
ideal case, grayscale printing (generating many shades of gray) could be
done simply by adjusting the charge or discharge of each point so as to
control the amount of toner at each point, with the desired gray scale
then being printed. However, with this approach, only a limited number of
shades of gray can be achieved.
Grayscales of the kind required for high resolution imaging are produced by
taking advantage of the ability of the human eye to integrate over an
area. For example, a mid-level gray dot will be perceived if smaller dots
of lighter and darker than mid-level gray are printed next to each other.
For example, if two light gray dots of 1/600 of an inch square and two
dark gray dots of 1/600 of an inch square are printed in a square, the eye
will integrate the four dots and perceive a mid-gray dot of 1/300 of an
inch square.
To assist in generating grayscale data, SLMs can be modulated in at least
two ways: intensity modulation and spatial modulation. These techniques
may be combined. These techniques are described in U.S. patent Ser. No.
08/038,398, entitled "Process and Architecture for Digital Micromirror
Printer", assigned to Texas Instruments Incorporated and incorporated
herein by reference. For each image to be exposed, the SLM generates a
series of microimages, which permit pixel-by-pixel variations in the
duration of exposure and the area exposed.
In both intensity modulation and spatial modulation, the SLM must be
addressed so that it exposes each pixel at the appropriate times. A goal
of SLM-based printing systems is to accomplish this addressing with a
minimum of processing and memory requirements.
SUMMARY OF THE INVENTION
The present invention is related to time integration for grayscale printing
with SLM-based printing systems. One aspect of the invention is a method
of compensating for elements of the spatial light modulator array that are
stuck in the ON position. A defect background value is determined and
added to all of the exposure values of all of the elements on the
modulator array, producing exposure data. The exposure data is stored in a
sliding window memory. A microimage is created on a drum by addressing
elements of a pixel array with the exposure data, the exposure data is
decremented, and the steps repeated until the entire image is formed.
Another aspect of the invention is the subtraction of a defect intensity
value from exposure values affected by the defective elements.
It is an advantage of the invention in that it allows fine control of
grayscale images with a spatial light modulator having defects without
increasing system cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the relationship between an OPC drum and a DMD array.
FIG. 2 illustrates an array of contone data to be printed on a page.
FIG. 3 is a graphical representation, in time, of the relationship between
lines on an OPC drum and a DMD array.
FIG. 4 is a row integration sliding window array according to the teachings
of the present invention.
FIG. 5 is a row integration gray scale lookup table according to the
teachings of the present invention.
FIG. 6 is a microimage array according to the teachings of the present
invention
FIG. 7 is a row integration sliding window array after decrementing and
sliding according to the teachings of the present invention.
FIG. 8 is a flow diagram for a sliding window approach according to the
teachings of the present invention.
FIG. 9 is a graphical representation, in time, of the relationship between
lines on an OPC drum and a DMD array.
FIG. 10 is a multiple intensities sliding window array according to the
teachings of the present invention.
FIG. 11 is a lookup table for gray scale using four light intensities
according to the teachings of the present invention.
FIG. 12 is a graphical representation, in time, of the relationship between
lines on an OPC drum and a DMD array.
FIG. 13 is a multiple phases and intensities sliding window array according
to the teachings of the present invention.
FIG. 14 is a gray scale lookup table with two phases and two intensities
according to the teachings of the present invention.
FIG. 15 is a graphical representation, in time, of the relationship between
lines on an OPC drum and a DMD array.
FIG. 16 is a gray scale lookup table for a two intensities, four phase
system according to the teachings of the present invention.
FIG. 17 is a pulse duration sliding window array according to the teachings
of the present invention.
FIG. 18 is a gray scale lookup table for a three phase pulse duration
modulation system according to the teachings of the present invention.
FIG. 19 is a gray scale lookup table for an anamorphic two duration pulse
duration modulation system according to the teachings of the present
invention.
FIG. 20 is an alternative embodiment gray scale lookup table for a two
intensity four phase system according to the teachings of the present
invention.
FIG. 21 is a block diagram of a printer according to the teachings of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
For purpose of example, the following description is in terms of printer
systems whose spatial light modulators (SLMs) are digital micro-mirror
devices (DMDs). However, the same concepts apply to printer systems that
use other types of pixel array imaging devices. For example, a printer
system having an array of liquid crystal elements instead of a DMD array
could used. In the case of a DMD, the image is generated by mirror
elements, but for pixel array devices in general, the terms "pixels" or
"elements" are often used. The pixels of the array usually have a
one-to-one correspondence with the pixels of the image, and the context of
the following description indicates where this is not the case.
FIG. 1 illustrates a printer exposure unit 10 having a DMD 12 and a memory
13, in accordance with the present invention. Throughout this description,
individual reflecting elements of DMD 12 are referred to as "mirrors" or
"mirror elements".
Various types of these mirror elements suitable for DMD 12 are described in
U.S. Pat. No. 4,662,746, entitled "Spatial Light Modulator and Method";
U.S. Pat. No. 4,956,619, entitled "Spatial Light Modulator"; U.S. Pat. No.
5,061,049, entitled "Spatial Light Modulator and Method"; U.S. Pat. No.
5,038,857, entitled "Multi-Level Deformable Mirror Device"; and U.S.
patent Ser. No. 08/171,303, entitled "Improved Multi-Level Digital
Micromirror Device." Each of these patents is assigned to Texas
Instruments Incorporated, and each is incorporated herein by reference.
In the example of FIG. 1, the mirror elements of DMD 12 are vertically
aligned. However, in other embodiments, each row of DMD 12 could be
staggered, such that the individual mirror elements are offset from row to
row. By offsetting the rows, advantage can be taken of spatial modulation
in the cross-process direction, as disclosed in U.S. patent application
Ser. No. 08/100,892, entitled "Method and Apparatus for Spatial Modulation
in the Cross-Process Direction," which is assigned to Texas Instruments
Incorporated, and incorporated herein by reference.
For purposes of providing a simple example, DMD 12 has only 4 rows of
mirror elements with 16 mirror elements per row. However, for practical
applications, DMD 12 may have many more rows and columns. A typical DMD 12
might have 1000 or more mirror elements per row. Light from a light source
14 is reflected by DMD 12 onto an OPC drum 16, in accordance with the "on"
or "off" state of each mirror element. This state is determined by data
delivered from an exposure data memory 13. As explained below, memory 13
delivers one bit of data for each mirror to be addressed during a single
line period.
The image is reflected and focused through an optics unit 18. As shown in
FIG. 1, light from DMD 12 falls on OPC drum 16, with each mirror providing
light for one pixel on the image. Only one line of pixels is explicitly
illustrated, it being understood that many lines of pixels are
simultaneously illuminated by DMD 12. Each pixel is either exposed or not,
and thereby either charged or discharged for toner attraction. Two typical
sizes for such pixels are 1/300 of an inch square and 1/600 of an inch
square, although other sizes are within the scope of the present
invention. The drum 16 will then rotate over the paper to,be printed and
the toner will be transferred from the drum 16 and fused to the paper, the
line of pixels printing a line on the paper.
Because drum 16 is rotating, light is received at a line of pixels on drum
16 from different rows of mirrors on DMD 12 at different times. The
desired amount of exposure time can be accomplished by pulsing light
source 14 or by leaving light source 14 on and switching the mirrors on or
off at the appropriate times. In one example, the "on" or "off" state of
the mirrors or the amplitude of light source 14 is updated once for every
line of drum movement (each line period). However, many other update
speeds may be used without departing from the scope of the invention.
As shown in FIG. 1, pixel 20 is exposed by a column of four mirrors 22-28
during four different time intervals as drum 16 rotates. Each time
interval is a "line period". Likewise, pixel 46 is exposed by mirrors
38-44. Pixel 21, which is in the second row of pixels, is exposed by
mirrors 22-28 but one line period later than pixel 20. Thus, each pixel of
drum 16 is exposed in accordance with data delivered to four different
mirrors in the same column of DMD 12 during four successive line periods.
U.S. patent application Ser. No. 08/038,398, entitled "Process and
Architecture for Digital Micromirror Printer", herein incorporated by
reference, describes methods of generating gray scales, but without the
sliding window implementation of the present invention. Generating a
particular shade of gray at a pixel on the image involves the following
steps. First, light is reflected onto the pixel (which may be divided into
smaller "phases") and accumulated through time integration. This light
discharges (or charges) the drum at the pixel and creates a
three-dimensional voltage profile. The relationship between the light
exposure and the voltage is non-linear, and the non-linear curve is called
the Photo-Induced Discharge curve ("PIDC"). A development potential is
applied to the drum and the toner particles are brought into contact. The
mass of toner attracted to the pixel has a non-linear relationship to the
voltage. The toner is then fused to the paper by applying heat. The mass
of toner fused determines the gray scale.
The relationship between the light exposure on the drum and the final gray
scale is not necessarily linear, and can be determined experimentally or
through modeling. The relationship can be expressed as a lookup table that
relates gray scale to the light exposure needed on each phase. Through the
following description, use of the term "grayscale" will refer to the
cumulative light exposure at a pixel, rather than the actual perceived
grayscale. Due to the non-linear relationship described above, it is
possible that two different exposure profiles may result in the same
perceived grayscale, and alternatively that two different profiles with
the same cumulative exposure may result in different perceived grayscales.
It should be understood that the mirrors shown in FIG. 1 are exemplary
only, and that other shaped mirrors may be used as well. In particular,
the mirrors shown in FIG. 1 are square, and are referred to as isomorphic
DMDs. Anamorphic DMDs may also be used. For example, rectangular DMDs
having half the height of those shown in array 12 may also be used.
Furthermore, the optics 18 may be used to make isomorphic mirrors
effectively anamorphic, and vice versa.
FIG. 2 illustrates a set of greyscale values, representing an image to be
printed. A number of lines are to be printed, designated as Line.sub.0
through Line.sub.L-1. Each row of values corresponds to a line, and
contains the greyscale data for each pixel on the line. In the example
shown, each line includes n pixels. For example, the first row of the
array of FIG. 2, Line.sub.0, contains data for pixel P.sub.0,0 through
P.sub.0,n-1. As explained below, these greyscale values are stored in
memory 13 and used as the basis for microimage data for delivery to DMD
12. The microimages are time-integrated to result in the desired grayscale
image.
Row Integration with a Single Light Intensity
FIG. 3 illustrates a method of successively exposing each pixel of the
image by light from mirror elements in the same column of DMD 12. For
example, a line of pixels will first be exposed by a first row of mirror
elements, then exposed by a second row of mirror elements, followed by
exposures from two succeeding rows of mirror elements. This accumulates
exposure on particular pixels, thus allowing for the printing of different
gray scales.
In the example of this description, each pixel is exposed by 4 mirrors.
More specifically, a particular pixel on image Line.sub.0 is exposed first
by mirror M.sub.3, then by mirror M.sub.2, then by mirror M.sub.1, then by
mirror M.sub.0. These exposures occur at times t=0 through t=3,
respectively. Although in reality, the exposures are in the same place,
for purposes of illustration in FIG. 3, the exposures are spread
horizontally. The mirrors M0-M3 might be any column of four mirrors on DMD
12, such as mirrors 22, 24, 26, and 28 of FIG. 1.
At the beginning of each new exposure period, the DMD 12 is addressed with
a new set of data. The data delivered to each mirror is binary in the
sense that it indicates whether that mirror should be ON or OFF during
that exposure period. The binary data (0 or 1 values) for all mirrors of
DMD to be addressed during a single exposure period is referred to herein
as "microimage" data. In the example of FIG. 3, four microimages, m.sub.0
-m.sub.3, are superposed to represent the final image.
Although FIG. 3 illustrates the complete exposure of only one pixel (on
Line 0), all other pixels are exposed in the same manner. Pixels in the
same column are exposed by the same mirror elements, but with other
microimage values. Pixels in different columns are exposed by different
mirror elements.
The microimages can be visualized as moving down the "page" represented by
pixels on drum 16. In the example shown, DMD 12 moves down the page one
line at a time. At each new position, DMD 12 receives new data and
generates a new microimage. At each new position, all but the top one of
the previously exposed lines of the image are part of the new microimage.
A new line of the image is also part of the new microimage.
The present invention involves the retrieval of data from memory 13 for
providing the microimage data to DMD 12. The memory 13 uses pointers to
simulate a sliding window, which at any given time contains the greyscale
data from which the current microimage is derived. The pointers are
reassigned so that the sliding window slides down the page. At each new
position of the sliding window, each pixel's greyscale value is updated to
account for the previous microimage exposure of that pixel's total
exposure.
FIG. 4 illustrates a sliding window array for a row integration system.
This array is stored in memory 13. As shown in FIG. 4, the sliding window
array has R rows, designated ROW.sub.0 through ROW.sub.R-1. The array
contains an exposure value for each pixel to be exposed. The number of
rows in the sliding window corresponds to the number of rows of the
microimages generated by DMD 12.
FIG. 5 illustrates a look-up table for obtaining the exposure values in the
sliding window array of FIG. 4. The lookup table maps a desired grayscale
into exposure values to be stored in the sliding window array. Each
exposure value is the number of exposures that a pixel is to receive.
Thus, the number of sliding window array rows must be at least as great as
the highest number of exposures in the lookup table. For example, the
lookup table of FIG. 5 has 9 as the highest exposure value, and thus at
least 9 sliding window array rows are needed.
In operation, the sliding window array is initialized with all zeros. The
exposure values for the first line of the image are then addressed as the
last row of the array. In this description, the array rows are identified
with pointers, so that no actual copying of data from one location from
another is required. Of course, the invention could also be implemented
with copying, such that data is written into appropriate locations in
memory.
FIG. 6 illustrates how microimage data is obtained from the exposure values
of FIG. 4. For the first microimage, all of the mirrors of DMD 12 are OFF
except the appropriate mirrors in the last row of DMD 12. This last row
will expose the first line of the image during the first exposure period.
The mirrors of DMD ROW.sub.R-1 will be ON or OFF, depending on the
information in ROW.sub.R-1 of the sliding window array. As illustrated, if
the exposure value for a mirror is .gtoreq.1, the mirror is ON. If the
exposure value is 0, the mirror is OFF.
FIG. 7 illustrates the sliding window array of FIG. 4, but after the first
exposure. The exposure values of ROW.sub.R-1 have been decremented to
reflect the first exposure. ROW.sub.R-1 has been readdressed as
ROW.sub.R-2. Furthermore, a new row of exposure values is addressed as
ROW.sub.R-1, for the second line of the image, which is Line 1 of FIG. 2.
The process of obtaining a microimage from the sliding window values, then
updating the sliding window continues until all the exposure values from
FIG. 2 have been used to generate microimages, and thus to generate the
page to be printed.
General Approach for Sliding Window Array
FIG. 8 illustrates a generalized process for using a sliding window array
of exposure values in accordance with the invention. It describes the
process described above in connection with FIGS. 3-7 for row integration,
as well as the processes described below for other types of greyscale
printing.
Step 80 is initializing the sliding window array with 0s. In step 82, the
sliding window array is addressed for the first microimage. In other
words, during the first exposure period, which begins at time t=0, row
ROW.sub.R-1 is loaded with exposure values. All other rows of the sliding
window array remain initialized at zero.
In step 84, a microimage is generated based on the exposure values in the
sliding window array. After generating the microimage, which includes
loading the microimage data into DMD 12 and exposing drum 16, in step 86,
the sliding window array is decremented. Decrementing is accomplished by
subtracting one unit from each exposure value in the sliding window array
that corresponds to a pixel that has been exposed.
After decrementing the sliding window array, exposure values will be
readdressed to the sliding window so that the sliding window corresponds
to the new position of drum 16. Thus, the process loops back to step 82.
Rows of exposure values receive addresses for preceding rows of the array,
such that each row receives exposure values from the next row down.
Expressed as a formula, ROW.sub.K =ROW.sub.K+1. The top row of exposure
values, which should by now by decremented to 0 drops out of the sliding
window. A new row of exposure values for the next line to be printed is
addressed as the last row of the sliding array. Thus, ROW.sub.R-1
=Line.sub.t. This process continues until all lines of the image have been
exposed.
Multiple Intensity System
FIG. 9 illustrates a multiple intensity approach to grayscale printing.
Like FIG. 3, FIG. 9 illustrates exposure of several pixels in one column
of an OPC drum. For purposes of illustration, the exposures are spread
horizontally, but are actually superposed. In the approach of FIG. 9, the
intensity of light source 14 of FIG. 1 is cycled through different
intensities. In particular, the representation of FIG. 9 illustrates four
different intensities, I.sub.1, I.sub.2, I.sub.4, I.sub.8, where the
second intensity is twice the first, the third intensity is twice the
second, and the fourth intensity is twice the third. It should be
understood that the particular intensities are exemplary only, and other
intensities may be used as well. With four intensity levels, and four
mirrors used to expose each pixel, 16 grayscale levels (0 through 15,
where 8+4+2+1=15) are available.
FIG. 10 illustrates the sliding window array for the multiple intensities
approach of FIG. 9. Each row of the sliding window array includes exposure
data for each intensity for each pixel on that row. Thus, each row is a
two-dimensional array in itself, having p-columns, where p is the number
of pixels in a row, and i rows, where i is the number of different
intensity levels available in the system.
The exposure data for each intensity corresponds to the number of exposures
that each pixel should receive at a particular intensity level. This
information is received from a lookup table such as that shown in FIG. 11.
The lookup table of FIG. 11 maps the desired grayscale from the data of
FIG. 2 into the number of exposures required at each intensity level. For
example, the sliding window array of FIG. 10 shows pixel P.sub.0 of
ROW.sub.R-1 having a grayscale level of 2, pixel P.sub.1 having a
grayscale value of 31, and pixel P.sub.p-1 having a grayscale value of 63.
As discussed above in connection with FIG. 8, for each exposure period, a
microimage is generated based on the sliding window array of FIG. 10,
followed by decrementing and sliding the sliding window array. In the
multiple intensities approach, decrementing is accomplished by subtracting
one from the exposure value for the appropriate intensity of each pixel
that has a value greater than zero. In the example of FIG. 10, after the
first exposure at intensity I.sub.1, the I.sub.1 data value for pixel
P.sub.0 of ROW.sub.R-1 will not be decremented, because it is zero. For
pixel P.sub.1 of the same row, the I.sub.1 data value is decremented from
the three to two, and for pixel P.sub.p-1, the data value I.sub.1 is
decremented from five to four.
Pulse Positioning
FIG. 12 illustrates a pulse position approach to grayscale printing. Like
FIGS. 3 and 9, FIG. 12 illustrates the relationship between a column of
pixels of several lines on drum 16 and mirrors in a column of DMD 12.
Pulse positioning is used to control where light from each of the mirrors
falls on the drum.
In the example of FIG. 12, the "on" time for every other microimage is
alternatively delayed or advanced by 1/4 pixel height. The mirror that
overlaps line L and line L+1 is counted as contributing to Line L. Thus, a
pixel on Line 0 is exposed by mirror M3 at time t=0 with intensity I1 at
phase P1, by mirror M2 at time t=1 with intensity I2 at phase P2, and so
on.
Thus, a particular pixel has two phases. These phases can be created by
controlling when the light is reflected onto the OPC drum. This control
can be performed by pulsing the light source at the desired times, or by
using the mirrors to switch light onto the drum only at the appropriate
times. Since the drum is always rotating, controlling the time at which
light is reflected controls the position at which the light falls on
logical pixels. Thus, the technique is referred to as pulse positioning.
Pulse positioning may be combined with changing of the light intensity,
although this is not required. By changing the light intensity, greater
numbers of different exposure profiles can be created, therefore allowing
for a greater number of gray scales to be printed.
FIG. 13 illustrates a sliding window array for a system using multiple
phases and intensities. Each row of this sliding window array includes
intensity data for each phase of the pixel. Thus, for a pixel having two
phases, and using two intensities, each element of the array will have
four exposure values. These four values are the number of exposures
required for a given pixel's first phase at the first and second intensity
levels, and that pixel's second phase at the first and second--intensity
levels.
FIG. 14 illustrates a lookup table that is used to map the greyscale of
FIG. 2 to the sliding window array of FIG. 13. As shown in FIG. 14, a
desired gray scale maps into a number of exposures for each phase at each
intensity. With mirrors that result in an exposure that is the height of
one line, the number of DMD rows equals the number of sliding window array
rows. This number equals the number of intensities times the maximum
number of exposures in the lookup table.
It should be understood that the number of intensity levels and the number
of phases shown in FIGS. 12, 13, and 14 is exemplary only, and more or
fewer intensities and phases may be used without departing from the
invention.
FIG. 15 illustrates another example of multiple intensities and multiple
phases. As shown in FIG. 15, anamorphic mirrors (or optics) are used, such
that light exposes areas that are not square. In the example of FIG. 15,
these areas are rectangular, and four phases correspond to each pixel.
With anamorphic optics as shown in FIG. 15, two phases are independently
exposed by two mirrors at the same time. Thus, for each microimage, data
for two phases in the sliding window array will be decremented.
With the greyscale method of FIG. 15, a sliding window array similar to
that shown in FIG. 13 is used, with the exception that each pixel has data
for four phases and two intensities rather than for two phases and two
intensities. FIG. 16 illustrates a lookup table that may be used to map a
desired grayscale level from the array of FIG. 2 to a sliding window
array.
The example of FIG. 15, with anamorphic optics, multiple intensities, and
overlapping phases, is--exemplary only. It should be understood that many
other combinations are possible. For example, anamorphic optics could be
used without overlapping phases and with multiple intensities. In a
situation where the anamorphic optics results in two mirrors per pixel,
and using two intensities, the sliding window array would look the same as
that shown in FIG. 13.
Pulse Duration
Another greyscale printing method is known as pulse duration modulation
("PDM"). With pulse duration modulation, the duration of a pulse is
controlled, either by pulsing the light or by switching the mirrors, such
that the exposure level is adjusted by controlling the length of time that
the light shines on the OPC drum.
FIG. 17 illustrates a sliding window array for pulse duration modulation.
The example of FIG. 17 has three different pulse durations, each having a
different delay. The graphical representation of the relationship between
the mirrors and the lines on the page is similar to that shown in FIG. 9,
with the intensity modulation being replaced by pulse duration modulation.
FIG. 18 illustrates a lookup table that is used to map the greyscale data
from the array of FIG. 2 into the sliding window array of FIG. 17. As
shown in FIG. 17, pixel P.sub.0 of ROW.sub.R-1 corresponds to a gray scale
of 15, pixel P corresponds to a gray scale of 30, and pixel P.sub.p-1
corresponds to a gray scale of 31. For an isomorphic system, exemplary
durations are 2.5%, 5%, and 10% for D.sub.1, D.sub.2, and D.sub.3,
respectively. These percent durations are expressed in terms of the
percentage of the line period, where the line period is the amount of time
for one row of pixels to rotate one row.
With pulse duration modulation, it is desirable in certain applications to
delay the shorter duration pulses, to align the centers of each area of
exposure. This is true because the drum 16 rotates while pixels are being
exposed, and thus the center of each exposure area depends on its
duration.
Pulse duration modulation may be used with both isomorphic and anamorphic
systems. FIG. 19 illustrates a lookup table that is used to map greyscale
data from the array of FIG. 2 into a sliding window array for an
anamorphic system (similar to that of FIG. 15) using pulse duration
modulation. Phases P.sub.1 and P.sub.3 are created by two adjacent mirrors
of a column, and phases P.sub.2 and P.sub.4 are created by two other
adjacent mirrors of the column. The appropriate duration times are shown
in FIG. 19 as 25% for phases P.sub.1 and P.sub.3 and 12.5% for phases
P.sub.2 and P.sub.4. Furthermore, phases P.sub.2 and P.sub.4 are delayed
6.25% to align their exposure centers with that of P.sub.1 and P.sub.3.
Alternative Lookup Table
As discussed above, the various lookup tables map the contone (gray scale)
data from the array of FIG. 2 to the appropriate sliding window array. The
data that is entered into the sliding window array corresponds to the
number of exposures for a particular pixel or phase at a particular
intensity. However, data other than this type of "number of exposure" data
can also be included in these lookup tables and sliding window arrays.
FIG. 20 illustrates a lookup table in which exposure information is
represented as a single number for all phases, rather than as separate
numbers for each intensity level. This single number represents the
desired exposure expressed in units of the lowest intensity for each
phase. For example, with a system using four intensity levels, 1, 2, 4,
and 8, times some base intensity level, a desired intensity level of 15
can be represented by one exposure at intensity level 8, one exposure at
intensity level 4, one exposure at intensity level 2, and one exposure at
intensity level 1, as in FIG. 11. With the lookup table of FIG. 20,
however, an exposure intensity of 15 would be entered as a single data
point of 15. This reduces the amount of memory required for the sliding
window array, since only these single data points will be entered for each
element in the array.
With such a lookup table, one decrementing strategy is as follows. First,
the sum of the intensities in the repeat cycles is computed. In the
example where the intensities cycle from 1 to 2 to 4 to 8, the sum is 15.
If the current desired exposure from a sliding window array is greater
than that sum, then the corresponding mirror is set to ON for the current
intensity level, and the sliding window array entry for that pixel is then
decremented by the current intensity. This continues until the desired
exposure of a particular pixel is less than the sum of the intensities in
the repeat cycle. Once this occurs, the binary representation of the
desired exposure is logically ANDed with the current intensity. If the
result is a one, then the corresponding mirror is switched on and the
appropriate entry in the sliding window is decremented.
Control System
FIG. 21 illustrates a block diagram of the circuitry that controls printer
10. As shown in FIG. 21, a processor 100 receives data representing the
image to be printed from data source 102 either directly or through memory
104. Source 102 may represent any device that can output data to be
printed, such as a personal computer. The memory 13 is used to store data
to be printed and to store other data, such as preprogrammed data for
later printing. A processor 100 includes instructions for controlling its
operation. Processor 100 is coupled to memory 13, light source 14, DMD 12,
and OPC drum and motor 16. Processor 100 is also coupled to printer paper
handling, and user I/O, and diagnostics block 106.
In operation, processor 100 processes the data to be printed by controlling
light source 14 and array 12. For example, processor 100 will determine
the gray scale that must be written for a particular pixel, and control
the particular mirrors of array 12 and light source 14 to accomplish that
intensity at the pixel.
The memory 13 includes sliding window arrays such as those discussed above.
Thus, processor 100 will control the DMD array 12 and light source 14
according to the sliding window array. After each exposure, processor 100
will decrement the sliding window array in memory 13 as discussed above.
In this manner, memory requirements are greatly reduced. In particular,
gray scale information can be maintained in as few as three memory blocks.
The first memory block will include the contone data to be printed on a
page, as represented by the array of FIG. 2. This data may be received
from block 102 of FIG. 21. The second memory block will include a lookup
table as discussed above. The third block will include the sliding window
array. Processor 100 will execute instructions to carry out the flow
diagram of FIG. 8, accessing these memory blocks from memory 13.
Defect Compensation
The invention provides for a simple method for-compensating defects in DMD
12. Three types of defects are black defects (where certain mirrors are
stuck in the OFF position), white defects (where certain mirrors are stuck
in the ON position), and non-uniformity (some mirrors resulting in greater
exposure than other mirrors).
Black defects are handled by simply not decrementing a pixel's exposure
value when the defective mirror is encountered. The microimage value for
that mirror is set to off.
White defects may be handled by creating a constant background to eliminate
their effect. First, the maximum exposure that will result from the white
defects in the column with the most number of white defects is determined.
This exposure level is then used as a background exposure level, which is
added to the exposure value for each pixel or pixel phase of the image.
During the print process, the background exposure is eliminated by
choosing an appropriate development threshold. With the addition of the
background, the effect of white defects may be subtracted during exposure
either by pre-subtraction or by decrementing. As an example of
pre-subtraction, if the background level is 8, and a pixel has a greyscale
value that maps to a desired exposure of 20, then this exposure is
incremented to 20+8=28. If one or more white defects affect this pixel
with intensity 4, then 4 is subtracted from the desired exposure to result
in a net exposure of 28-4=24, which is the value stored in the sliding
window array. Then, the sliding window method described above is used,
such that a pixel's exposure value in the sliding window array is not
decremented when the defective mirror is encountered. Alternatively,
instead of pre-subtraction, a pixel's exposure value in the sliding window
array is "force-decremented" when the defective pixel is encountered.
Another approach to white defect compensation is referred to herein as
"white balancing", where extra white defects are simulated in a separate
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