Note: Descriptions are shown in the official language in which they were submitted.
113~S21
1 Method and Apparatus for Digitally Controlling the Formation
of Colour Images
. . .
The invention relates to a method for digitally con-
trolling the colour image formation for raster printing and
an apparatus for performing that method. Raster printing in
the context of this invention shall mean that a finite
number of half-tone colour picture elements is printed, the
selection being made such that the human eye, owing to its
own resolution power and its prior experience in viewing
similar pictures, will tend to be deceived so as to virtu-
ally see a continuous-tone rather than a rastered, i.e.
"interrupted", colour picture.
Raster printing of black-and-white pictures is rather
simple technically, and printers for this purpose are well
known. Printing colours is however less easy since it is
impossible to provide for printing all the different colours
between which the human eye is able to distinguish. It has
been found that for many purposes it is entirely sufficient
to use seven colours including black (or even less) to re-
produce a colour picture with acceptable quality on a white
record medium, and this by actually printing only three
colours, as will be explained below.
It is long known in the printing art that the human eye
will be irritated by certain Moiré patterns which occur in
raster pictures if the rasters of the different colours are
not appropriately rotated with respect to the horizontal
direction. Accordingly, in present-day raster printing, the
rasters of the different colours are mutually rotated.
In "parallel" printing all points of the entire picture
in one particular colour are printed at the same time ~'in
parallel") and subsequentl~ all points or a second lor
further) colour are printed with the orientation of the
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1 raster of the second or further colour turned away from the
orientation of the raster of the first colour. This method
requires of course the preparation of as many printing forms
as colours (including black) are to be printed and as many
passes of the record medium through the printer. The exact
orientation of these printing forms does, however, not pose
any significant problem.
In "serial" printing, the picture elements are sequen-
tially composed by superpositioning the required colours in
the form of dots. These dots are placed in juxtaposition so
as to form printing lines, and an appropriately large number
of printing lines is positioned to form the entire picture.
In view of the necessity to prepare individual raster
screens for each colour to be printed, the parallel printing
process is inherently very slow. In contrast, the serial
printing process is much faster, though more difficult.
There are essentially two types of printers available to
perform the serial printing process. The first type is the
impact printer which uses a column or matrix of wires
arranged in a common print head and individually activatable
to be impacted against a ribbon which in turn is impacted
against the record medium to produce a dot thereon. The
ribbon is a multi-colour ribbon containing the required
number of printing colours and black. A suitable ribbon
lift mechanism is responsible for presenting that colour to
the wire matrix of the print head, which is to be printed or
over-printed. In view of the time requirPd by the mechan-
ical parts of the wire printer to move, particularly in view
of the hammer recovery time, these printers are still rather
slow. The other type of printer operates with liquid inks
in different colours which are ejected from a common head
assembly housing an individual nozzle for each colour to be
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1 printed, including black. It is this type of printer which
is fastest today, particularly in view of the very fast dry-
ing inks now available.
Because of its speed, this printer has best potential
for use in a colour image reproduction system. Such a
system can either be a colour copying machine or a colour
image facsimile transmission system, depen~ing upon the
distance between scanner and printer. Although the present
invention is applicable also to stand-alone ink jet printers,
it will be explained hereafter at the example of a colour
image reproduction system, viz. a colour copier.
It may be useful to briefly explain the conventional
colour image reproduction process. The original colour image
is first bandpass-filtered into its additive primaries green,
blue and red by measuring the colour "amplitude", i.e. inten-
sity for all picture elements in the x-y raster of the origi-
nal. Intensities below a certain threshold are neglected.
For the reconstruction of the image on a white record
medium, the subtractive primaries magenta (green-absorbant),
yellow (blue-absorbant) and cyan (red-absorbantl are used.
The half-tone patterns corresponding to these subtractive
primaries are to be exactly superimposed at the same x-y
positions as in the original, yet the raster of the colours
has to be rotated with respect to the horizontal. This
requirement can not be fulfilled with conventional analog
or digital colour image reproduction techniques using
rotated printing screens. Conventional colour rendition,
therefore, lacks fidelity and its proper correction requires
elaborate, difficult to implement colour balancing procedures
3~ A method for scanning coded colour rings is described
in Swiss Patent 495.Q17 in connection with a process for
identifying ampullae containing pharmaceutical products of
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1 various kinds. The ampullae carry rings of the colours red,
green, blue, cyan, magenta and yellow. The scanner uses
dichroic mirrors to split the light reflected by the rings
into three spectral zones (red, green, blue), and the colour
signal obtained from three photomultipliers, if surpassing a
given threshold, is digitized using a self-clocking system.
This patent does, of course, not describe a reconstruction
mechanism for the colour code scanned, i.e. no printer is
shown.
An ink jet printer using three nozzles for respectively
printing red, yellow and blue and arranged at mutual dis-
tances of 120 around a rotatable drum on which the record
medium is fixed, is disclosed in Swiss Patent 468,630 which
is directed to the design of deflection electrodes for the
deflection of droplets not used for printing into a gutter.
While this specification briefly mentions the applicability
of the invention to colour printing, no details are disclosed.
Swiss Patent 537.757 teaches an ink jet printer having a
print head with a plurality of parallel nozzles either in
matrix or linear arrangement. The patent is directed to the
design of a dual manifold chamber but briefly addresses the
registration of the droplets emanated from plural parallel
nozzles by delaying the activation of the nozzles such that
the droplets hit the record medium at the correct location.
Also mention is made of the possihility to use different
colours for printing, with the colours either mixing on the
record medium or being placed thereon in juxtaposition.
A serious problem encountered in serial printers using
ink jets with different colours is the local dilatation which
most record media undergo when hit by an ink droplet, and
which makes it very difficult tc precisely register subse-
quent ink dxoplets ~of the second and further colours) on the
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1 same spot. Since the contents of the images to be repro-
duced vary largely, there is no way of anticipating that
dilatation by appropriate mechanical compensation means.
Also, amount and main direction of the dilatation vary un-
predictably since they depend on the prevailing ambient con-
ditions and on manufacturing parameters, respectively.
None of the known prior art references addresses this
problem whose solution is, however, of major importance for
improvement of the print quality of colour printing with ink
jet printers. It is, therefore, a main object of the present
invention to provide a method for digital colour image repro-
duction which permits the automatical compensation of mis-
registrations of the second and subsequent colour droplets
with the first printed droplet intended to all be placed at
the same location Details of embodiments of the invention
will hereafter be described with reference to the accompany-
ing drawings in which:
Fig. 1 is a schematic block diagram of a colour image
reproduction system,
Fig. 2 is a block diagram of the circuitry for develop-
ing control signals for the ink j~t nozzles,
Fig 3 shows examples of properly adjusted print colours,
Figs. 4, 5 and 6 show misaligned print colours,
Fig. 7 shows another embodiment of a circuitry for
de~eloping control signals for the ink jet nozzles,
Fig. 8 shows a perforated drum for carrying the record
medium,
Figs. 9 and 10 show cross sections of the drum surface
with an overlaying dry and wet record medium, respectively.
Multiple colour printing with ink jet printers may use
either the "additive" or "subtractive" method depending on
which set of the pri~ary colours is used. For example, in
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1 Swiss Patent 468.630 mentioned above, use of the additive
primaries red, yellow and blue is proposed. The present
invention relates to the use of the subtractive primaries
magenta, yellow and cyan. As is well known, the superposi-
tion of magenta (which is green-absorbant) and yellow ~which
is blue-absorbant) yields red, the superposition of magenta
and cyan (red-absorbant) yields blue, the superposition of
yellow and cyan yields green, and the superposition of all
three of them yields black.
Accordinglv, by using magenta, yellow and cyan as
printing colours, it is possible to reconstruct the colour
images which contain - besides those printing colours - red,
blue, green and black, a total of seven colours. The super-
positioning of the printing colours must obviously be very
exact to prevent a colour shift or hue of an unwanted colour
from occurring.
It has long been observed that printing rasters when
oriented in the direction of reading ~or orthogonally to
this direction) tend to create Moire~ patterns, mostly of low
frequency, i.e. in an area where the human eye is most
sensitive~ Therefore, the printing industry uses different
angles of rotation for the individual colours so as to
increase the frequency of the Moire-patterns to make them
less visible to the human eye. A recent investigation by
the inventor has shown that digital colour images with
tilted rasters are very resistent to mechanical inaccuracies
and that a Moire-free, true-colour image of good overall
quality can even be obtained when the subtractive primaries
magenta, yellow and cyan are all arranged under the same 45
angle of orientation.
Fig. 1 shows a repreduction apparatus constructed from
conventional elements with which the invention to be des-
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1 cribed later can be used. An original 1 is illuminated by a
light source 2 and scanned by a scanner 3 which is position-
controlled by a control unit 4 so as to sequentially scan
the picture elements of original 1. The optical output of
scanner 3 is now bandpass-filtered into its additive prim-
aries green, blue and red by dispersive means such as a first
dichroic mirror 5 which extracts the green portion, and a
second dichroic mirror 6 which extracts the blue portion
from the scanner output.
The green and blue portions reflected by the dichroic
mirrors 5 and 6 as well as the red portion having passed
both mirrors 5 and 6, are amplified by photomultipliers 7,
8 and 9 individually assigned to these colours. The elec-
trical output signals of photomultipliers 7, 8 and 9 are
passed through threshold devices 10, 11 and 12, respectively.
These are commonly clocked by a clock 13 which is also con-
nected to position control unit 4.
The output slgnals from threshold devices 10, 11 and 12
are, thus, digitized continuous-tone values of the green,
blue and red contents of each of the picture elements of
original 1. For printing, these continuous-tone values of
the additive primaries are first transformed into their
equivalent half-tone representations. The digitized con-
tinuous-tone value is quantized into an appropriate number
of amplitude levels and these are compared with predetermined
threshold values to produce binary output signals indicating
whether at the scanned spot the particular colour surpasses
those thresholds. As mentioned earlier, in colour reproduc-
tion the raster of the picture element matrix has to be til-
ted with respect to the horizontal direction, lest an un-
wanted Moiré-pattern should be generated. The algorithm for
creating a 45 tilt, as in the present case, is simple and
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1 obvious to those skilled in the art. An approach to digital-
ly create any tilts was published by the inventor in IBM
Technical Disclosure Bulletin Vol. 20, No. 6, November 1977,
pp. 2423-2425.
The resultant binary output signals which are represen-
tative of the intensities of the additive primaries are used
to control their corresponding subtractive primaries, i.e.
the "green" signal controls the printing of magenta, the
"blue" signal ccntrols the printing of yellow, the "red"
signal controls the printing of cyan. This control is
effected via conventional print control units 14, 15 and 16
whose outputs are connected to a multiple colour ink jet
printer 17.
Printer 17 has all of the elements generally present in
any conventional ink jet printer such as ink supply means
and ink reservoir, means for maintaining the ink under
appropriate pressure, droplet generating means and deflec-
tion means Printer 17 has a separate nozzle 18, 19, 20 for
each colour to be printed and, perhaps, an additional nozzle
~1 for black to permit contrast enhancement.
These nozzles are aligned such that the droplets they
emit and which are intended for the same x, y picture ele-
ment to be reconstructed, hit the record medium 22 at
different times, the time intervals in between permitting
the inks to dry and record medium 22 to be advanced. Printer
17 and the transport mechanism (not shown) for record medium
22 receive control signals from position control unit 4 so
as to guarantee correspondence of the x,y printing position
with the x,y scanning position.
~0 Of course, this control can not cope with the devia-
tions in printing position created ~y the record medium
becoming wetted ~y the first ink printed which results in a
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1 minute dilatation and, hence, offset of the droplets of the
second (and third) ink from the true printing position, thus
creating a colour shift or hue of one or more colours.
The invention, therefore, proposes to monitor the actual
printing at the record medium and to develop an appropriate
correction signal to be used for amending the position of
printer 17 with respect to record medium 22 and to influence
the ejection times of the ink droplets of the second and
third inks.
Fig. 2 schematically shows a first embodiment of a
monitoring arrangement in accordance with the present inven-
tion. The record medium 22 is held on a suitable support
such as a rotatable drum 23, for example. Drum 23 is rotated
by conventional, controllable drive means, not shown, in the
direction of arrow 24. A portion 25 of the reconstructed
image is composed of a plurality of print dots which for
simplicity are represented as squares 26. Each of the print
dots is illuminated by a white-light flash from a strobo-
scope 27 and the light reflected by record medium 22 and
passing through the printing colours present on the par-
ticular print dot 26 is shed through a group of filters, viz.
magenta filter 28, yellow filter 29 and cyan filter 30, onto
two-dimensional light detection areas such as charge-coupled
device areas 31, 3~ and 33, for example. These transform
the detected spectral components into electrical signals.
A fourth light detection area 34 is provided for sens-
ing the average spatial position of the superimposed half-
tone patterns. Area 34 provides an output signal over a line
35 to a stroboscope control unit 36 which controls the flash
frequency to keep the half-tone patterns in the center of
the light detection areas 31 through 33.
The output signals from areas 31 through 33 are supplied
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1 to a delay network 37 consisting of tapped shift registers
38, 39 and 40 whose outputs are connected to a controller 41
which in turn provides control signals to the ink droplet
generator, i.e. the piezocrystal, for controlling the drop
frequency, to the pump for controlling the ink pressure, and
to the deflection complex, all for correction of the ink jet
in the y-direction indicated by arrow 42, and to the ink jet
print head transport mechanism, for correction in the x-
direction indicated by arrow 43.
Figs. 3 through 6 show examples of colour (mis)align-
ments and the correction strategy. In Fig. 3 the colours
are properly superpositioned and, although the areas covered
by the individual colours are partly of different size, no
correction is necessary. Fig. 4 shows a situation where two
colours are mutually offset, in Fig. 5 two colours are
properly aligned whereas the third colour is offset, and
Fig. 6 shows all three colours mutually offset. Of course,
in the cases of Figs. 4 through 6 correction is required.
In printing with the subtractive primaries yellow,
magenta and cyan, in that sequence, the following cases may
occur:
1. If one of the colours alone is to be printed, only
the filter 28, 29 or 30 (Fig. 2) associated with that colour
will pass light to the appertaining light detection device
31, 32 or 33. The intensity of the light in this case will
be a maximum. However, no correction signal is to be gen-
erated rom controller 41 since no misalignment of one
colour with respect to any other has occurred.
2. If two colours are to be printed, they may either
be aligned (Fig. 3) or misaligned (Fig. 4). In the former
case, no light will be passing through any of the filters 28
through 30 because one of the colours is not present and the
others combine to either red, green or blue for which all
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1 filters are absorbant.
In the case of misalignment of the two colours, their
associated filters will pass light of low intensity, and
the appertaining light detectors 31, 32 or 33 will emit out-
put signals from which correction signals are generated by
controller 41.
3. In case of printing of all three colours, three
situations may occur, namely a) all three colours are
aligned, b) two of the colours are aligned and a third one
is misaligned with respect to them (Fig. 5), or c) two
colours are misaligned with respect to the colour printed
first (Fig. 6), and with respect to one another.
(a) Proper alignment of all three colours yields black
and, hence, no output is generated.
(b) The two aligned colours combine to form a new
colour (red, or green, or blue) which is blocked by all
of the filters. Where all three colours overlap, blac~
is generated. Thus, only the filter of the misaligned
colour will produce a low output signal.
(c) With all three colours mutually misaligned, there
will be a black area not creating an output signal, and
there will exist three areas of new colours (red, green,
blue) which are blocked by the filters, and there will
be three low output signals generated by the printing
colours where they do not overlap.
In summary, there may either occur a high output signal
for one colour printed alone, or a low output signal created
by a misaligned colour. In order to be able to correct a
secondly and subsequently printed colour with respect to the
one printed first, stroboscope 27 is energized only during
the second and subsequent colour deposition sweeps. Accord-
ingly, the generation of said high output signal for the
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1 first printed colour is avoided.
Fig. 7 shows another embodiment of the deviation detec-
tion optics wherein stroboscope 27 is replaced by a continu-
ous white-light source 44 which may be an incandescent lamp.
The light reflected by record medium 22 and filtered by the
inks printed thereon is shed onto an area scanning device 45
consisting of an x-y matrix of light sensitive devices 46
which are covered with filter masks 47 of different colours
as shown in Fig. 7, where the masks can consist of thin
layers of colour film. The light sensitive devices 46 may,
for example, consist of charge-coupled capacitors which
store the photo-generated charges. The capacitors 46 in
each of the horizontal matrix rows are connected to a common
line 48, 49, 50 and 51, respectively, which lines are in turn
connected to a vertical (y) scan generator 52, while the
capacitors 46 in each vertical matrix column are connected
to a common line 53, 54, 55 and 56, respectively, which in
turn are connected to a horizontal (x) scan generator 57.
If pulses from vertical and horizontal scan generators
52 and 57 coincide in the proper sequence at an x, y matrix
point, the integrated photoelectric charge previously stored
at that point is transferred int~ the substrate 58 on which
the capacitors 46 are arranged, and a correspondent video
current is derived from that charge.
Other charge transfer arrangements are described in C.H.
S~quin, M.F. Tompsett, Charge Transfer Devices, Ad~ances in
Electronics and Electron Physics, Academic Press, New York
1975.
The function of the replaced stroboscope in this embodi-
ment is per~ormed by a pulsed exposure/read-out circuit 59
which is under the control of a clock 60 also controlling
scan generators 52 and 57. The exposure/read-out circuit 59
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1 provides output signals for droplet generation frequency
control, for ink pressure control, for droplet deflection
control, and for the control of the ink jet printer head
assembly transport mechanism on the basis of the electronic
sorting of the charge distribution for each of the second
and subsequent colours printed. As in the embodiment employ-
ing the stroboscope, the result of the scanning of the
colour first printed has to be suppressed which, in the
present embodiment, is done electronically.
A further means of minimizing the dilatation of record
medium 22 owing to the wetting thereof by the printing inks
is shown in Fig. 8. Drum 23 carrying record medium 22 has a
plurality of holes 61 in its cylindrical surface while its
axial surfaces are closed. When air is pumped out of drum
23 through duct 62, record medium 22 will be tightly kept
around it and any dilatation thereof will tend to be absorb-
ed by the additional paper length being sucked into holes 61
as shown in Fig. 9 (dry record medium) and Fig. 10 (ink-
wetted record medium).
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