FIELD OF THE INVENTION
[0001] The present invention is related to a printing device such as a printing or copying
system employing print heads containing discharging elements, e.g. nozzles, for image-wise
forming dots of a marking substance on an image-receiving member, where the marking
substance is in fluid form when discharged. Examples of such printing devices are
inkjet printers and toner-jet printers. Hereinafter reference will be made to inkjet
printers.
BACKGROUND OF THE INVENTION
[0002] Print heads employed in inkjet printers and the like usually each contain a plurality
of nozzles arranged in (a) linear array(s) parallel to the propagation direction of
the image-receiving member or in other words the sub scanning direction. The nozzles
usually are placed substantially equidistant. The distance between two contiguous
nozzles defines the nozzle pitch. In operation, the nozzles are controlled to image-wise
discharge ink droplets on an image-receiving member such as to form columns of image
dots of ink in relation to the linear arrays such that the printing pitch equals the
nozzle pitch. In scanning inkjet printers, a matrix of image dots of ink, corresponding
to a part of an image is subsequently formed by scanning the print heads across the
image-receiving member, i.e. in the direction perpendicular to the propagation direction
of the image-receiving member or in other words the main scanning direction. After
a first matrix is completed the image-receiving member is displaced such as to enable
the forming of the next matrix. This process may be repeated till the complete image
is formed. An advantage of forming an image of image dots of ink on an image-receiving
member as here described is the high productivity as only a single printing stage
is employed. However, image quality may be improved by employing printing devices
enabling the use of multiple printing stages. In the prior art two main categories
of such printing devices can be distinguished, i.e. so-called "interlace systems"
and "multi-pass systems".
[0003] In an interlace system, as e.g. disclosed in US 4,198,642, the print head contains
N nozzles, which are arranged in (a) linear array(s) such that the nozzle pitch is
an integer multiple of the printing pitch. Multiple printing stages, or so-called
interlacing printing steps, are required to generate a complete image. According to
this disclosure, the print head and the image-receiving member are controlled such
that in I printing steps, I being defined here as the nozzle pitch divided by the
printing pitch, a complete image part is formed on the image-receiving member. After
each printing step, the image-receiving member is displaced over a distance of N times
the printing pitch. Such a system is of particular interest because it allows to achieve
a higher print resolution with a limited nozzle resolution.
[0004] In a "multi-pass system", the print head contains N nozzles, which are arranged in
(a) linear array(s). In operation, the print head is controlled such that only the
nozzles corresponding to selected pixels of the image to be reproduced are image-wise
activated. As a result an incomplete matrix of image dots is formed in a single printing
stage, i.e. a horizontal scanning pass across the image-receiving member in one direction.
Multiple passes are required to complete the matrix of image dots. In-between two
passes the image-receiving member may be displaced in the sub scanning direction.
[0005] Both "interlace systems" and "multi-pass systems" as well as combinations thereof
share the advantage of an improved image quality and the inherent disadvantage of
a lower productivity. Such systems are known to be of particular interest to overcome
or at least reduce the visibility of some banding artefacts, particularly regional
banding artefacts. Regional banding artefacts are caused by irregularities which can
be attributed to individual nozzles or small regional clusters of nozzles within the
array(s). Such irregularities may lead to regional variations in dot-size or dot positioning.
Examples of such irregularities are differences in nozzle shape or size, differences
in the shape or size of the ducts connecting the ink reservoirs with the respective
nozzles. These differences can occur in the manufacturing or may arise during use,
e.g. caused by contamination of the ink. The so-called print mask contains the information
about the number and sequence of printing stages and defines which nozzles need to
be activated, or in other words contains the information defining for each printing
stage which pixels will be rendered by which nozzles such that when all printing stages
are completed, all the pixels are rendered. Prior art print masks are usually configured
such as to minimise the influence of random regional variations in dot size and positioning.
A print mask is associated with a printing mode. Selecting a printing mode enables
the user to exchange image quality for productivity and vice versa dependent on his
requirements. By selecting a printing mode also the nozzles on the print head which
will be effectively used are determined as well as the displacement step in the sub
scanning direction after each printing stage.
[0006] However besides banding artefacts caused by the above-described regional variations
in dot-size or positioning, also very disturbing banding artefacts caused by so-called
systematic variations in dot-size can arise in "interlace systems" and "multi-pass
systems" as well as combinations thereof. Systematic dot-size variations are caused
by differences in size of dots formed by different groups of nozzles. For instance,
in a print head comprising two linear arrays of nozzles for the same colour, the first
group of nozzles may constitute the first array of nozzles while the second group
of nozzles constitutes the second array of nozzles. When due to a small shift in the
manufacturing process all nozzles of the first array are sized slightly different
from the nozzles of the second array, systematic variations in dot-size can arise
between droplets originating form nozzles of said first and second group. Another
example is a print head comprising a single linear array of nozzles for a particular
colour wherein the nozzles are controlled such that first the even nozzles within
the array, i.e. the first group of nozzles, are discharged and thereafter the uneven
nozzles within the array. Again this may lead to a systematic dot-size variation which
in case of a thermal or thermal-assisted inkjet printer may be caused by e.g. a small
temperature variation, or in case of a piezoelectrical inkjet printer may be caused
by e.g. mechanically induced cross-talk. A further example is an inkjet printer comprising
multiple print heads for a particular colour wherein the respective groups are constituted
by the respective arrays of the respective print heads. In such a configuration, again
e.g. small differences of nozzle sizes of nozzles groups each associated with a different
print head may lead to systematic dot- size variations.
OBJECTS OF THE INVENTION
[0007] It is an object of the invention to control the print heads of "interlace systems"
and "multi-pass systems" as well as combinations thereof such as to overcome or at
least reduce the visibility of systematic image dot-size variations while limiting
the influence on productivity.
[0008] It is another object of the invention to control the print head and the image-receiving
member displacement means such that in operation for a given print mask an optimal
number of nozzles is actually image-wise activated and an optimal displacement distance
in the sub scanning direction is determined which limits the visibility of banding
artefacts while maximising productivity.
SUMMARY OF THE INVENTION
[0009] In a first aspect of the invention, a printing device is disclosed comprising:
at least one print head for image-wise forming dots of a marking substance at a printing
pitch, P, on an image-receiving member in relation to a pattern of image pixels, said
print head comprising a plurality of N discharging elements being arranged in at least
one linear array, being spaced at a predetermined element pitch, and being composed
of at least a first group of discharging elements which, in operation, image-wise
form dots of a marking substance of a first size and a second group of discharging
elements which, in operation, image-wise form dots of a marking substance of a second
size, different from said first size, on said image-receiving member,
displacement means for displacing said image-receiving member in the sub scanning
direction;
selecting means for selecting a print mask defining a number of S printing stages
required to completely render said pattern of image pixels, S being an integer number
of at least 2;
control means for controlling said displacement means and for controlling said plurality
of N discharging elements;
characterised in that
in operation, on the basis of the difference between said first size and said second
size, said control means controls said displacement means such that said image receiving
member is displaced over a distance of M and controls said plurality of N discharging
elements such that an effective number, N
eff, of discharging elements, is image-wise activated, N
eff ≤ N. The printing device may further comprise scanning means for scanning the print
heads in the main scanning direction.
The image-receiving member may be an intermediate member or a medium. The intermediate
member may be an endless member, such as a belt or drum, which can be moved cyclically.
The medium can be in web or sheet form and may be composed of e.g. paper, cardboard,
label stock, plastic or textile.
[0010] Further according to the present invention, the respective groups of discharging
elements forming image dots of different sizes may be part of a single linear array
of discharge element of a single print head. The respective groups of discharging
elements forming image dots of different sizes may be part of multiple linear arrays
of discharging elements of a single print head, particularly the respective arrays
may constitute the respective groups. The respective groups of discharging elements
forming image dots of different sizes may be part of linear arrays of discharging
elements of multiple print heads. The latter configuration is of particular interest
when the multiple print heads form image dots of the same colour. In an embodiment
of the invention, the print heads have a width, i.e. the maximal distance between
discharge elements of a print head in the main scanning direction, equal to or larger
than the width, i.e. the dimension in the main scanning direction, of the image-receiving
member.
[0011] In another embodiment of the invention, the distance M and the effective number of
discharging elements N
eff are determined, on the basis of the number of available discharging elements N, by
combining at least the number of printing stages S, the number, q, of said groups
of discharging elements, the printing pitch and the element pitch. Also the defect
number, d, may be used to determine M and N
eff. The defect number, d, is defined as the number of subsequent printed image dots
in the sub scanning direction originating from the same group of discharging elements
when executing all the passes required to image-wise render all the pixels in the
main scanning direction. Particularly, in case of an "interlace system" a single scan
is executed in the main scanning direction, while in case of a "multi-pass system"
multiple scans are executed according to the print mask.
For instance, in case of a "multi-pass system", the distance M and the effective number
of discharging elements N
eff can be obtained by satisfying the following conditions:


and

wherein n is an integer greater than or equal to 1,
p is the ratio between the element pitch and the printing pitch. Alternatively,
in case of an "interlace system" or a combination of a "multi-pass system" and an
"interlace system", the distance M and the effective number of discharging elements
N
eff can be obtained by satisfying the following conditions:

and

wherein n is an integer number greater than or equal to 1,
p, the ratio between the element pitch and the printing pitch is an integer number
of at least 2,
f is a non-zero integer number defined as the minimal offset, expressed in number
of positions in the print mask, between two subsequent printing stages. For instance,
the print mask of fig. 2a defines a sequence 1,2,3,4,1,2,3,4,... therefore, f=±1.
A print mask defining a sequence 1, 4, 2, 5, 3, 1, 4, 2, 5, 3, ..., yields f=±2; a
print mask defining a sequence 1,4,3,2,1,4,3,2, ..., yields f=-1.
[0012] In another aspect of the invention, a method is disclosed for image-wise forming
dots of a marking substance at a printing pitch, P, on an image-receiving member in
relation to a pattern of image pixels with a printing device comprising at least one
print head, said print head comprising a plurality of N discharging elements being
arranged in at least one linear array, being spaced at a predetermined element pitch,
and being composed of at least a first group of discharging elements which, in operation,
image-wise form dots of a marking substance of a first size and a second group of
discharging elements which, in operation, image-wise form dots of a marking substance
of a second size, different from said first size, on said image-receiving member,
said method comprising the steps of:
selecting a print mask defining a number, S, and sequence of printing stages required
to completely render said pattern of image pixels, S being an integer number of at
least 2;
image-wise activating on the basis of said print mask at least a part of an effective
number, Neff, of discharging elements, Neff ≤N and
intermittently displacing on the basis of said print mask said image-receiving member
in the sub-scanning direction over a distance, M;
characterised in that
said distance, M, and said effective number, N
eff, of discharging elements are determined on the basis of the difference between said
first size and said second size.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
Figure 1 depicts an example of an inkjet printer.
Figure 2a depicts an example of a print mask.
Figure 2b depicts the image dots formed when activating the nozzles of a print head
having a single linear array of 15 nozzles once.
Figure 2c depicts a part of a matrix of ink dots formed in relation to a pattern of
image pixels using the same print head as used in figure 2b and the print mask of
figure 2a.
Figure 3a depicts the image dots formed when activating the nozzles, selected according
to an embodiment of the present invention, of the same print head as used in figure
2b once.
Figure 3b depicts a part of a matrix of ink dots formed in relation to a pattern of
image pixels using the print mask of figure 2a, the nozzle selection as indicated
in figure 2b and a displacement distance in the main scanning direction determined
according to an embodiment of the present invention.
Figure 4 depicts the image dots formed when activating the nozzles of a print head
having 99 nozzles arranged in two linear arrays once.
Figure 5 depicts the image dots formed when activating the nozzles of a print head
having 99 nozzles arranged in two linear arrays once.
Figure 6a depicts an example of a print mask.
Figure 6b schematically depicts parts of a matrix of ink dots formed in relation to
a pattern of image pixels using the print mask of figure 6a.
DETAILED DESCRIPTION OF THE INVENTION
[0014] In relation to the appended drawings, the present invention is described in detail
in the sequel. Several embodiments are disclosed. It is apparent however that a person
skilled in the art can imagine several other equivalent embodiments or other ways
of executing the present invention, the scope of the present invention being limited
only by the terms of the appended claims.
[0015] The printing device of fig.1 is an inkjet printer comprising a roller (1) for supporting
an image-receiving member (2) and moving it along four print heads (3), each of a
different process colour. The roller is rotatable about its axis as indicated by arrow
A. A scanning carriage (4) carries the four print heads and can be moved in reciprocation
in the main scanning direction, i.e. the direction indicated by the double arrow B,
parallel to the roller (1), such as to enable scanning of the image-receiving member
in the main scanning direction. The image-receiving member can be a medium in web
or in sheet form and may be composed of e.g. paper, cardboard, label stock, plastic
or textile. Alternately, the image-receiving member can also be an intermediate member,
endless or not. Examples of endless members, which can be moved cyclically, are a
belt or a drum. The carriage (4) is guided on rods (5) (6) and is driven by suitable
means (not shown). Each print head comprises a number of discharging elements (7)
arranged in a single linear array parallel to the sub scanning direction. Four discharging
elements per print head are depicted in the figure, however obviously in a practical
embodiment typically several hundreds of discharging elements are provided per print
head. Each discharge element is connected via an ink duct to an ink reservoir of the
corresponding colour. Each ink duct is provided with means for activating the ink
duct and an associated electrical drive circuit. For instance the ink duct may be
activated thermally and/or piezoelectrically. When the ink duct is activated an ink
drop is discharged form the discharge element in the direction of the roller (1) and
forms a dot of ink on the image-receiving member.
[0016] To enable printing firstly a digital image is to be formed. There are numerous ways
to generate a digital image. For instance, a digital image may be created by scanning
an original using a scanner. Digital still images may also be created by a camera
or a video camera. Besides digital images generated by a scanner or a camera, which
are usually in a bitmap format or a compressed bitmap format also artificially created,
e.g. by a computer program, digital images or documents may be offered to printing
device. The latter images can be in a vector format. The latter images can also be
in a structured format including but not limited to a page description language (PDL)
format and an extensible markup language (XML) format. Examples of a PDL format are
PDF (Adobe), PostScript (Adobe), and PCL (Hewlett-Packard). The image processing system
typically converts a digital image with known techniques into a series of bitmaps
in the process colours of the printing device. Each bitmap is a raster representation
of a separation image of a process colour specifying for each pixel ("picture element")
an image density value for said process colour. The image density value is typically
an 8-bit value which enables the use of 256 grey levels per process colour. These
bitmaps are converted into a printable format by means of a halftoning technique.
In case of binary halftoning, these 8-bit values are converted into a single-bit value
specifying for each pixel whether or not an image dot of ink of the associated process
colour is to be formed. The image processing system may be incorporated in a computer
which can be coupled by a network or any other interface to one or more printing devices.
The image processing system may also be part of the printing device.
By image-wise activating the ink ducts in relation to the pattern(s) of image pixels
an image composed of ink dots can be formed on the image-receiving member.
Comparative example 1
[0017] A printing device as depicted in Figure 1 is used to reproduce a digital image. Instead
of using the print heads each provided with four discharging elements as in the figure,
each print head is provided with 15 discharging elements, i.e. nozzles, arranged in
a single linear array. The nozzles are positioned equidistant at a resolution of 150
npi (nozzles per inch). This means that the nozzle pitch or element pitch, being the
distance between the centres of two adjacent nozzles is about 169.3 µm.
[0018] Suppose the user selects a particular printing mode enabling to reproduce a digital
image at a printing resolution of 600 dpi (dots per inch) in both directions, or in
other words, the printing pitch, i.e. the distance between the centres of two contiguous
dots of ink both in the main scanning direction and in the sub scanning direction,
is about 42.3 µm. The print mode is such that all the available nozzles are selected.
To enable rendering of an image with a resolution higher than the nozzle resolution,
the print mask associated with the selected printing mode as in figure 2a defines
an interlacing system. The print mask defines a sequence of four printing stages required
to completely render the raster of image pixels. The sequence is such that during
the first printing stage, labelled as 1 in fig. 2a, each selected nozzle of a print
head renders all the associated pixels in the main scanning direction. In other words,
each selected nozzle image-wise forms a complete line of image dots of ink in the
main scanning direction. In the sub scanning direction only every fourth pixel is
rendered during the first printing stage. After the first printing stage the image-receiving
member is displaced over a distance M, being an integer multiple of the printing pitch
which is about 42.3 µm, such that in the second printing stage, labelled as 2 in fig.2a,
pixel rows which are shifted one pixel with respect to the pixel rows rendered in
the first printing stage are rendered. In other words, M= [(4 x m) ± 1] x printing
pitch, m being an integer number. Again, in the second printing stage, each selected
nozzle image-wise forms a complete line of image dots of ink in the main scanning
direction while in the sub scanning direction only every fourth pixel is rendered
being shifted one pixel compared to the first printing stage. After the second printing
stage the image-receiving member is again displaced over the distance M, such that
in the third printing stage, labelled as 3 in fig.2a, pixel rows which are shifted
two pixels with respect to the pixel rows rendered in the first printing stage are
rendered. In the third printing stage, each selected nozzle image-wise forms a complete
line of image dots of ink in the main scanning direction while in the sub scanning
direction only every fourth pixel is rendered being shifted two pixels compared to
the first printing stage. After the third printing stage the image-receiving member
is again displaced over the distance M, such that in the fourth printing stage, labelled
as 4 in fig.2a, pixel rows, which are shifted three pixels with respect to the pixel
rows rendered in the first printing stage, are rendered. In the fourth printing stage,
each selected nozzle image-wise forms a complete line of image dots of ink in the
main scanning direction while in the sub scanning direction only every fourth pixel
is rendered being shifted three pixels compared to the first printing stage. After
completing this fourth printing stage, at least a part of the raster of image pixels
is completely rendered. By displacing the print head again over a distance M and repeating
the sequence of printing stages as described above, the complete raster of image pixels
can be rendered.
[0019] Further according to this comparative example, Fig.2c depicts a part of a matrix
of ink dots formed in relation to a pattern of image pixels using the printing device
of this comparative example and the print mask of Fig.2a. For instruction purposes,
only the dots generated by a single print head are shown and a full coverage image
is assumed. In practice however, it is clear that in the same way multi-colour images
can be formed by adequately timing both the driving of the respective print heads
and the image-wise activation of the associated nozzles. The nozzle pitch of about
169.3 µm is indicated by the arrow D2. The printing pitch in the main scanning direction
of 42.3 µm is indicated by the arrow D3, while the printing pitch in the sub scanning
direction of 42.3 µm is indicated by the arrow D1. The distance M over which the print
head is displaced after each printing stage is indicated by the arrow D4. M equals
15 times the printing pitch D1 and is chosen such as to minimise regional banding
artefacts. The part of the matrix displayed in fig.2c contains an arbitrary subset
of rows and columns of image dots formed by a single print head of a particular colour
in relation to the associated part of the raster of image pixels of said colour. In
the left column of the matrix, the nozzle number is indicated used to form the image
dots of the associated row. As also indicated in fig.2a, the dots formed during the
first printing stage are represented by a blank circle, while for each of the other
printing stages a representation with a specific fill pattern is chosen. As depicted
in fig.2b, the 15 nozzles of the print head form image dots of ink of a different
size on the image-receiving member. The image dots formed by the second group of nozzles,
i.e. the even nozzles are smaller than the image dots formed by the first group of
nozzles, i.e. the uneven nozzles. As a result of this group variation a systematic
banding artefact is clearly visible in the sub scanning direction in fig. 2c. The
banding artefact has a size of four times the print pitch.
Example 1
[0020] When observing a systematic banding artefact on the image-receiving member caused
by dot-size variation on a group level, as described in the comparative example 1,
according to the present invention, on the basis of the dot-size differences, for
a given printing mode and associated print mask, an effective number of discharging
elements N
eff, N
eff ≤ N, and an optimum displacement distance, M, in the sub scanning direction is determined.
Particularly, given that the print mask as depicted in fig.2a defines four printing
stages, S, and that the ratio, p, between the element pitch and the printing pitch
equals four, to at least reduce the visible effect of a banding artefact caused by
systematic dot-size variation, the following conditions should be met:

and

wherein n is an integer number greater than or equal to 1,

q is the number of groups of nozzles yielding image dots with different sizes;
according to this example q equals 2 as there are two groups forming image dots of
different size: the even nozzles and the uneven nozzles,
d, the defect number, equals 1 according to this example as subsequent printed
dots in the sub scanning direction, printed in a single scan in the main scanning
direction, are alternately formed by an even and an uneven nozzle.
By consequence: N
eff = (8 x n) + 4 ± 1 and M= N
eff x P.
Knowing that the print mode and the print head are such that maximal 15 nozzles can
be selected, the most productive mode yields N
eff = 13, M= 13 times the printing pitch. Therefore, in operation, the print head is
controlled such that only 13 nozzles can be image-wise activated. As depicted in fig.3a,
these 13 nozzles of the print head form image dots of ink of a different size on the
image-receiving member. In the printing mode according to this example, the nozzles
1 and 15 can no longer be activated.
[0021] Fig.3b depicts a part of a matrix of ink dots formed in relation to the same pattern
of image pixels as described in the comparative example 1. The same printing device
of this comparative example and the print mask of Fig.2a are used, but the print head
is controlled such that only thirteen nozzles, i.e. the nozzles 2 to 14, can be image-wise
activated. As can be observed in the figure, after each of the four printing stages,
the image-receiving member is displaced over a distance equal to 13 times the printing
pitch. The systematic banding artefact with a size of four times the print pitch,
as in fig. 2c, is less visible to the human eye due to the higher spatial frequency
of the artefact. The image quality is clearly improved with a limited effect on productivity
using the same print mask.
Example 2
[0022] A printing device as depicted in Figure 1 is used to reproduce a digital image. Instead
of using the print heads each provided with four discharging elements as in the figure,
each print head is provided with 99 discharging elements, i.e. nozzles, arranged in
two staggered linear arrays. The nozzles are positioned equidistant at a resolution
of 150 npi (nozzles per inch). This means that the nozzle pitch or element pitch,
being the distance D2 of fig.4 between the centres of two adjacent nozzles is about
169.3 µm. Analogous to example 1 and the comparative example 1, user selects a particular
printing mode enabling to reproduce a digital image at a printing resolution of 600
dpi (dots per inch) in both directions using the same print mask as depicted in fig.
2a and previously described. The print mask as depicted in fig.2a defines four printing
stages, S, and that the ratio, p, between the element pitch and the printing pitch
equals four. When all the nozzles of the print head are activated once, an image dot
pattern as indicated in Fig.4 is formed on the image-receiving member. The dot-size
of the image dots generated by the nozzles of the left array is different from the
dot-size of the image dots generated by the nozzles of the right array. As this dot-size
difference may result in a systematic banding artefact, according to the present invention
an optimal effective number of nozzles, N
eff, as well as an optimal image-receiving member displacement distance, M, is determined
such that the following conditions are satisfied:


wherein n is an integer number greater than or equal to 1,

q is the number of groups of nozzles yielding image dots with different sizes;
according to this example q equals 2 as there are two groups of nozzles forming image
dots of different size: the nozzles of the left array and the nozzles of the right
array,
d, the defect number, equals 1 according to this example as subsequent printed
dots in the sub scanning direction, printed in a single scan in the main scanning
direction, are alternately formed by a nozzle of the left array and a nozzle of the
right array.
By consequence: N
eff = (8 x n) + 4 ± 1 and M= N
eff x P.
[0023] Knowing that the print mode and the print head are such that maximal 99 nozzles can
be selected, the most productive mode which reduces the visible effect of the banding
artefact caused by the described systematic dot-size variation, yields N
eff = 99, M = 99 times the printing pitch.
Example 3
[0024] The same configuration is used as in example 2, except that when all the nozzles
of a print head are activated once, an image dot pattern as indicated in Fig.5 is
formed on the image-receiving member. The dot-size of the image dots generated by
the even nozzles within an array is different from the dot-size of the image dots
generated by the uneven nozzles within an array. As this dot-size difference may result
in a systematic banding artefact, according to the present invention an optimal effective
number of nozzles, N
eff, as well as an optimal image-receiving member displacement distance, M, is determined
such that the following conditions are satisfied:

and

wherein n is an integer number greater than or equal to 1,

q is the number of groups of nozzles yielding image dots with different sizes;
according to this example q equals 2 as there are two groups of nozzles forming image
dots of different size: the even nozzles of the respective arrays and the uneven nozzles
of the respective arrays,
d, the defect number, equals 2 according to this example as subsequent printed
dots in the sub scanning direction, printed in a single scan in the main scanning
direction, are alternately formed by even nozzles of the respective arrays and uneven
nozzles of the respective arrays.
By consequence: N
eff = (16 x n) + 8 ± 1 and M= N
eff x P.
Knowing that the print mode and the print head are such that maximal 99 nozzles can
be selected, the most productive mode which reduces the visible effect of the banding
artefact caused by the described systematic dot-size variation, yields N
eff = 89, M = 89 times the printing pitch.
Example 4
[0025] A printing device as depicted in Figure 1 is used to reproduce a digital image. Instead
of using the print heads each provided with four discharging elements as in the figure,
each print head is provided with 99 discharging elements, i.e. nozzles, arranged in
a single linear array. The nozzles are positioned equidistant at a resolution of 600
npi (nozzles per inch). A particular printing mode is selected by the user enabling
to reproduce a digital image at a printing resolution of 600 dpi (dots per inch) in
both directions using the print mask as depicted in fig. 6a. The print mask as depicted
in fig.6a defines a "multi-pass" system with two printing stages, S, as depicted in
fig. 6b. As the element pitch equals the printing pitch, p=1. Suppose the dot-size
of the image dots formed by the even nozzles of the array is different from the dot-size
of the image dots formed by the uneven dots of the array. Then, according an embodiment
of this invention, to avoid or at least limit the visible effect of the associated
systematic banding artefact, an effective number of nozzles is determined and controlled
such that only these nozzles are selectable an can be image-wise activated. Particularly,
N
eff and M are chosen such as to satisfy the following conditions:

and

wherein n is an integer number greater than or equal to 1,
q is the number of groups of nozzles yielding image dots with different sizes;
according to this example q equals 2 as there are two groups of nozzles forming image
dots of different size: the even nozzles of the array and the uneven nozzles of the
array,
d, the defect number, equals 1 according to this example as subsequent printed
dots in the sub scanning direction after two scans in the main scanning direction
are alternately formed by an even and an uneven nozzle.
By consequence: N
eff = (4 x n) + 2 and M= N
eff x P/2. Knowing that the print mode and the print head are such that maximal 99 nozzles
can be selected, the most productive mode which reduces the visible effect of the
banding artefact caused by the described systematic dot-size variation, yields N
eff = 98, M = 49 times the printing pitch.
1. A printing device comprising:
at least one print head for image-wise forming dots of a marking substance at a printing
pitch, P, on an image-receiving member in relation to a pattern of image pixels, said
print head comprising a plurality of N discharging elements being arranged in at least
one linear array, being spaced at a predetermined element pitch, and being composed
of at least a first group of discharging elements which, in operation, image-wise
form dots of a marking substance of a first size and a second group of discharging
elements which, in operation, image-wise form dots of a marking substance of a second
size, different from said first size, on said image-receiving member,
displacement means for displacing said image-receiving member in the sub scanning
direction;
selecting means for selecting a print mask defining a number of S printing stages
required to completely render said pattern of image pixels, S being an integer number
of at least 2;
control means for controlling said displacement means and for controlling said plurality
of N discharging elements;
characterised in that
in operation, to limit the visibility of systematic banding artefacts in the sub scanning
direction, on the basis of the difference between said first size and said second
size, said control means controls said displacement means such that said image receiving
member is displaced over a distance of M and selects an effective number, N
eff, of discharging elements, N
eff, of said plurality of N discharging elements for image-wise activation, N
eff ≤ N.
2. The printing device as recited in claim 1, wherein, on the basis of the number of
available discharging elements N, said distance M and said effective number of discharging
elements Neff are determined by combining at least said number of printing stages S, the number,
q, of said groups of discharging elements, the printing pitch and the element pitch.
3. The printing device as recited in claim 1 and 2, wherein the following conditions
are satisfied:


and

wherein n is an integer greater than or equal to 1,
p is the ratio between the element pitch and the printing pitch,
d, the defect number, is defined as the number of subsequent printed image dots
in the sub scanning direction originating from the same group of discharging elements
when executing all the passes required to image-wise render all the pixels in the
main scanning direction.
4. The printing device as recited in claim 3, further comprising scanning means for scanning
said print head in the main scanning direction.
5. The printing device as recited in claim 1 and 2, wherein the ratio between the element
pitch and the printing pitch is an integer number, p, of at least 2.
6. The printing device as recited in claim 5, wherein the following conditions are satisfied:

and

wherein n is an integer number greater than or equal to 1,
f is a non-zero integer number defined as the minimal offset, expressed in number
of positions in the print mask, between two subsequent printing stages,
d, the defect number, is defined as the number of subsequent printed image dots
in the sub scanning direction originating from the same group of discharging elements
when executing all the passes required to render all the pixels in the main scanning
direction.
7. The printing device as recited in claim 6, said print head has a width equal to or
larger than the width of the image-receiving member.
8. The printing device as recited in claim 1 and 2, wherein said print head comprises
a plurality of N discharging elements arranged in at least a first and a second linear
array.
9. The printing device as recited in claim 8, wherein said first linear array is composed
of said first group of discharging elements and said second linear array is composed
of said second group of discharging elements.
10. The printing device as recited in claim 1 and 2, comprising a first print head of
a colour and at least a second print head of said colour, which together comprise
a plurality of N discharging elements being arranged in at least one linear array
on said first print head and at least one linear array on said second print head.
11. The printing device as recited in claim 10, wherein the discharging elements of said
first print head form said first group and the discharging elements of said second
print head form said second group.
12. A method for image-wise forming dots of a marking substance at a printing pitch, P,
on an image-receiving member in relation to a pattern of image pixels with a printing
device comprising at least one print head, said print head comprising a plurality
of N discharging elements being arranged in at least one linear array, being spaced
at a predetermined element pitch, and being composed of at least a first group of
discharging elements which, in operation, image-wise form dots of a marking substance
of a first size and a second group of discharging elements which, in operation, image-wise
form dots of a marking substance of a second size, different from said first size,
on said image-receiving member, said method comprising the steps of:
selecting a print mask defining a number, S, and sequence of printing stages required
to completely render said pattern of image pixels, S being an integer number of at
least 2;
image-wise activating on the basis of said print mask at least a part of an effective
number of discharging elements, Neff, Neff ≤N; and
intermittently displacing on the basis of said print mask said image-receiving member
in the sub-scanning direction over a distance, M;
characterised in that
said distance, M, is determined and said effective number of discharging elements,
N
eff, is selected from said plurality of N discharging elements on the basis of the difference
between said first size and said second size in order to limit the visibility of systematic
banding artefacts in the sub scanning direction.
13. The method as recited in claim 12, wherein said distance M and said effective number
of discharging elements, Neff, are determined by combining at least said number of printing stages S, the number,
q, of said groups of discharging elements, the printing pitch and the element pitch.