The invention concerns a method and a system for control- ing whether the colours printed on a medium in a printing process are placed correctly with respect to each other, i.e. whether they are in register.

In the prior art the medium is printed with a control print in the margin which is subsequently cut away. The control print can e.g. be a cross and may be provided by printing all the colours of the print on the medium so that they form a coinciding and substantially colourless cross. The register of the colours is controlled by in- specting the control print manually, where the operator can observe a shadow on the cross in case of misregister, the shadow of the colour depending upon the displaced colour. This technique has the drawback that the displace¬ ment is observed by random sampling, so that quite large amounts of print may be out of register before the damage is discovered. These prints must then be rejected if they do not satisfy the quality requirements.

It is known from the US Patent Specification 4 534 288 to check control marks on a web, to detect misregistration of a printing unit and to obtain correction signals to bring the printing unit back into registration. The control marks comprise a structure of partially overlapping lines. The control marks are controlled with measuring equipment detecting the relationship between a print area and a non- print area. The detection must be made for each of the colours with respect to black. It is a serious drawback of this art that paper is used for separate control marks for each colour to control registration with black. Further, it takes a long time to perform the many control measure¬ ments. This problem increases with the number of colours.

Another US Patent Specification 4 546 700 discloses a corresponding system, but with a different control mark, where the colours are printed individually with a black margin, following which the relationship between the areas of the coloured field and a complementary white field within the margin is used for controlling the printing unit. Since, here too, the registration of the printing units is controlled individually with respect to the black margin, the above-mentioned drawbacks are also present here.

The object of the invention is to provide a method and a system to detect whether the colours in a print are placed correctly with respect to each other. Further, the system may advantageously be designed such that it is possible, on the basis of the detected colour of the control print, to generate control signals capable of correcting the printing device causing the misregistration, which can be done without interruption of the printing process.

This object is achieved in that a characteristic pattern coinciding for the colours is printed on the medium, said pattern leaving a plurality of evenly distributed, un- coloured fields, and detecting the colours of the fields which are uncoloured when correctly positioned.

Thus, performance of simultaneous control of the printing devices for all the colours incorporated in the print re¬ sults in a considerably reduction of the number of control marks necessary to obtain the same results as in the prior art. This capacity can then be used for increasing the frequency of measurements, so that misregistration is de¬ tected more rapidly and thus also corrected more rapidly. Further, the amount of paper used for printing the control marks is reduced.

In the preferred embodiment, at least two and preferably four identical base patterns preferably with black colour are printed on the medium. These base patterns are then printed with the characteristic patterns coinciding for the colours, which can be formed in pairs by complementary parts of the base pattern. Since the pattern coinciding for the colours is printed on the base pattern and is con¬ tained in it, registration will make it appear as if it was only the base pattern that was printed on the medium. When one of the printing devices is displaced, the colour detectable on the usually uncoloured background in the base patterns will depend upon the direction in which the printing device is displaced. The order of the printing of the colours is of secondary importance.

Thus, on the basis of at least two base patterns it will be possible to provide information on which of the colours has been displaced, the direction in which it has been displaced and how much it has been displaced. This infor- mation can then be converted to control signals, making it possible to correct such a misregistration of a printing device immediately after the error has occurred. It will thus be possible to avoid having to reject part of the production because the quality requirements cannot be satisfied.

The invention will be explained more fully below with re¬ ference to the drawing, in which

fig. 1 schematically shows a preferred embodiment of a printing system with colour displacement control according to the invention,

fig. 2 shows the colour coordinate system used in the colour control equipment in the preferred embodiment,

fig. 3 shows the preferred position of the control fields according to the invention,

fig. 4 shows an enlarged section of the preferred embodi¬ ment of the patterns used in the detection of colour dis¬ placement according to the invention,

fig. 5 shows enlarged details of the patterns shown in fig. 4, and

fig. 6 shows enlarged sections of alternative embodiments of the characteristic patterns coinciding for the colours according to the invention.

Fig. 1 shows a printing unit 10 with associated colour control equipment 20. A printing medium, which is a web 5 of paper in this case, is moved past the printing machine 10, where its printing devices 11-19 apply colour to the paper web. The printing machine contains one printing de- vice per colour and typically has a total of 2-9 devices. These printing devices 11-19 apply colour to the paper web 5 to form the actual colour image, and in addition also a plurality of control prints which will be described later. The colour of the control prints is measured with the colour control equipment 20 comprising a light source 22 and a colour detector 24. The colour control equipment may e.g. be of the type stated in the Danish Patent Appli¬ cation 5875/87. The colour control equipment 20 generates an output signal in response to the measured colour, which is added through a connection 26 to a register control system 30 capable of adjusting the individual printing devices upon adjustment of the print and during printing, so that the print can be moved in the longitudinal and transverse direction of the paper web, i.e. be x-y con- trolled. This control can take place through the signal path 32.

The colour control equipment 20 typically has three sen¬ sors, which are sensitive in the green, the red and the blue spectral range respectively. The intensity measured by the sensors in the red and the green spectral bands is normalized with the sum of the measured intensities in the three spectral bands. This provides two figures between 0 and 1 which can be related to a so-called CIE colour co¬ ordination system which is shown in fig. 2. The relative intensity measured with the red sensor is plotted on the x axis of the system of coordinates, while the relative in¬ tensity measured with the green sensor is plotted on the y axis of the system of coordinates. The pure spectral co¬ lours will be plotted on a substantially horseshoe-shaped curve. The system of coordinates will have a colourless point E with the coordinates (0.33, 0.33). The blue spec¬ tral colour will be represented in the blue point A in the horseshoe, while B is the green point, C the yellow point and D the red point. Blends between the colours will be represented on connecting lines between the respective colour points, where e.g. blends between red and blue will be represented on the connecting line between A and D, which applies e.g. for the colours violet, lilac and ma¬ genta. If the colour measuring region consists of a blend of grey, i.e. uncoloured regions, and yellow, the measure- ment result will be on the connecting line between E and C.

Fig. 6 shows some patterns with which it is possible to detect colour displacement, but it is not possible to de- tect in which direction the colour has been displaced.

When these patterns are used, the control device 30 must either cause the printing unit 10 to stop automatically, or sound an alarm signal which can tell the operator that at least one of the colours is out of register. The patterns in question leaving an evenly distributed number of uncoloured fields can e.g. comprise a first plurality

of parallel lines, with an optionally corresponding number of intersecting lines, which appears from fig. 6a. Another applicable pattern may consist of parallel lines as shown in fig. 6t, which, however, has the limitation that dis- placement longitudinally along the lines cannot readily be detected. Alternatively, the design may consist of dots as appears from fig. 6c. A pattern with concentric circles where the radius and line thickness of the individual cir¬ cles are adapted so as to form a substantially evenly dis- tributed uncoloured background face as shown in fig. 6d. Finally, the pattern may also consist of screen fields, which, as shown in fig. 6e, consist of four-sided fields with a line in parallel with the diagonal, where the area below the line is provided with colour, while the area above the line is uncoloured. A figure in % can indicate how great a portion of the entire field is provided with colour. As already mentioned, these patterns can only be used for detecting whether a colour has been displaced.

Fig. 3 shows how the control fields are placed in the pre¬ ferred embodiment. Forwardly there is a plurality of pre¬ ferably four-sided control fields 41, which are fields with a coverage of 100%. Then follows a plurality of con¬ trol fields 42 where the screen coverage corresponds to 50%. The number of control fields 41 as well as 42 corres¬ ponds to the number of colours or devices. With the con¬ trol fields 41 it is possible to place the measured colours in the colour system coordinates shown in fig. 2. It is moreover possible to identify which colour origi- nates from the individual devices. The control fields 42 may be used for controlling the point extension and there¬ by e.g. controlling the viscosity of the inks which can be changed by filling new ink. Then follows the control prints 43 to 46, which may likewise be composed as screen fields, but may advantageously have a chessboard chequered black base print. A pattern characteristic of the colours

is then printed on top of this, said pattern being sub¬ stantially formed by base pattern parts complementary in pairs. Fig. 4 shows an enlarged section of these patterns where the patterns characteristic of the colours, in case of correct positioning of the colours, are within one half of the black checks of the chessboard pattern divided by the diagonal. The checks indicated by 50 will be without ink and can assume the colour the background material, e.g. paper, has. The triangular fields 52 and 51 will be dark colourless areas, both of which have received dark colourless ink, preferably black. The regions 51 are more¬ over printed with the characteristic pattern coinciding for the colours, which will appear colourless owing to the blend of several colours and will be difficult to distin- guish from the areas 52. It appears from figs. 4a-4d that the area 51 in the individual coloured checks is turned 90° with respect to the intersection of the diagonal for the individual control fields. In case of colour displace¬ ment the areas of the checks 50 forming part of the con- trol prints 43-46 and provided with colour will vary in fixed mutual relationship in response to the direction in which the colour is displaced. Measurement of the control areas 43-46 with the colour control equipment gives a set of coordinates making it possible to determine, on the basis of the colour system of coordinates used, which colour has been displaced, in which direction and how far.

With correct positioning, the control fields 43-46 will all have a colour corresponding to the one shown with E in the colour system of coordinates (fig. 2), which corres¬ ponds to the colourless point. The position of the four colours, used as an example, can be determined with the colour measurement of the fields 41 and will show that magenta, cyan, yellow and black are present at the points M, S, C and about E, respectively. If the yellow colour is displaced, it can be observed in part of the otherwise

uncoloured field 50 and can contribute to the colour measurement since the resulting measured colour will be on the connecting line between E and C, where the distance from E depends upon how far the colour is displaced.

In fig. 5 the colour prints 43-46 are additionally en¬ larged so that only a black field from the chessboard chequered background pattern is visible. There will still be a large fields formed by the areas 52 and 51' . In case of colour displacement, a colourless adjacent field will be coloured in an area 55 which depends upon the displaced colour and the size of the displacement. As will be seen, the size of the area 55 in the four control fields 43-46 depends upon the direction in which the colour control takes place, and on how much the colour is displaced. On the basis of the coordinates calculated for each of the control fields 43-46 in the colour system of coordinates, it will be possible to determine unambiguously which co¬ lour has been displaced, in which direction it has been displaced, and how far it has been displaced. This deter¬ mination is made in an associated calculating member of the control device 30, so that this is capable of correcting the position of the individual devices 11-19. It will be relatively simple to generate the mentioned control signals on the basis of simple algorithms once the colour coordinate set has been calculated for the four control fields 43-46.

To determine which colour has been displaced, how far it has been displaced and in which direction, the preferred embodiment has been explained with reference to four con¬ trol prints 43-46 which give the greatest accuracy. If it is merely desired to detect that a colour has been dis¬ placed, while the direction and distance are of no impor- tance, a simple control field 43-46 will be sufficient. It is not necessary currently to measure all the fields in

the control fields 41 and 42 if the colour measuring equipment cannot keep up with the production rate. A measurement may e.g. be performed on a single field of those forming part of the control fields 41 and 42 each time the control fields 41 and 42 pass by. The colour of the control fields 43-46 may advantageously be measured each time these pass by. The information from these fields 41 and 42 comprises firstly a correlation between which colour the individual devices contain, then information on whether these colours have been applied correctly to the medium, i.e. control of the colour amounts and control of correct viscosity. The greatest sensitivity to colour displacement is obtained by colour measurements with a relationship between the printed pattern and the back- ground of 50%.