CA1138521A - Method and apparatus for digitally controlling the formation of colour images - Google Patents

Abstract

METHOD AND APPARATUS FOR DIGITALLY CONTROLLING
THE FORMATION OF COLOUR IMAGES
Abstract of the Disclosure The colour images are generated by the point-wise over-printing of the individual printing colours by means of an ink jet printer. During printing of the second and sub-sequent colours the record medium is illuminated with white light and monitored by a two-dimensional opto-electronic device carrying colour filters. The spectral components measured are transformed into electrical signals used in each colour channel to control droplet generation, ink pressure and droplet deflection for correction in the direction of movement of the record medium, and to control the print head transport mechanism for correction in the direction orthogonal to the said direction.
The ink jet printer for performing this method com-prises at least three nozzles arranged in a common print head and a white-light source illuminating the image portions printed on the record medium. An opto-electronic scanner equipped with colour filters is mounted in scanning position. Its outputs are connected via a control unit to the control inputs of the droplet generators, of the ink supply and the deflection electronic of each individual colour channel as well as with the control inputs of the print head transport mechanism.

Description

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 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 ~9780~5 -3-~138521 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 S~978005 -4-11385~1 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 SZ9780~5 -5-11385~:1 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 SZ97~005 -6-1 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-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 S~978005 -8-113~S21 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 ~Z~78005 -9_ 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 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 11385~1 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 1~385~1 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 11385~1 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).

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method for digitally controlling the colour image formation by means of an ink jet printer using at least three differently coloured inks ejected from individual nozzles of a common print head mounted on a carrier dis-placeable across a record medium, the ink droplets from the said nozzles being superpositioned on said record medium to form half-tone picture elements of desired composite, i.e.
additive or subtractive primaries or any other colours, characterized in that the indicia printed on the record medium are illuminated with white light, that said indicia are monitored with a two-dimensional opto-electric scanning device having sections individually sensitive for a par-ticular one of the printing colours, that the illumination and/or the scanning are timed such that the scanning can occur only during the second or subsequent colour deposition operation, and that the spectral components sensed are transformed into electric signals controlling the frequency and/or phase relationship of the ink drop generators for the individual colours, the ink pressure, and the deflection electronic associated with the individual ink jet nozzles for correction of the drop deposition in the (y-) direction of movement of the record medium, and for controlling the drive mechanism of the print head assembly for correction of the drop deposition in the (x-) direction transversal to the movement of the record medium.

2. Apparatus for digitally controlling the colour image formation by means of an ink jet printer having at least three nozzles individually associated with different additive or subtractive primary colour inks and arranged in a common head assembly displaceably mounted in front of a record medium which is fixed on a rotatable drum, the ink droplets from said nozzles being superpositoned to form half-tone picture elements of desired composite, i.e.
additive or subtractive primary colours, comprising a source of white light for illuminating indicia printed on the record medium; optoelectronic scanning means having sections individually sensitive to a particular one of the printing colours for scanning said printed indicia and operable for scanning only for the deposition operation of a second or subsequent colour; output means from said scanning means connected to control means; said control means being opera-tively connected to control ink droplet generators, ink supplies, ink deflection means and print head drive means for correction of drop deposition to assure proper drop registration.

3. Apparatus according to claim 2, characterized in that the light source consists of a white light stroboscope and that the opto-electronic scanning means comprises a first charge-coupled device without colour filter and con-nected via a control unit to said stroboscope and a group of charge-coupled devices which are each covered with a dif-ferent colour filter and connected each to a tapped delay line shift register whose taps are in turn connected to the inputs of said control means.

4. Apparatus according to claim 2, characterized in that the light source consists of an incandescent lamp, and that the opto-electronic scanning means comprises a matrix of light-sensitive devices mounted on a substrate, the devices of each column of the matrix being connected to a first set of common lines, which lines are connected to a vertical scan generator, the devices of each row of the matrix being connected to a second set of common lines, which lines are connected to a horizontal scan generator, that all of the light-sensitive devices are covered by individual colour filter masks such that the distribution of the colours of the filters is uniform about the matrix and that the substrate is connected to said control means.