GB2064113A

GB2064113A - Monitoring Colour of Printed Web

Info

Publication number: GB2064113A
Application number: GB8038182A
Authority: GB
Original assignee: Licentia Patent Verwaltungs GmbH
Current assignee: Licentia Patent Verwaltungs GmbH
Priority date: 1979-11-28
Filing date: 1980-11-28
Publication date: 1981-06-10
Also published as: DE2947791C2; GB2064113B; DE2947791A1

Abstract

A device for monitoring the colour of a web of material 1 in a printing apparatus comprises a row of optical conductors 5 arranged across the material 1 which are used to illuminate the material with light from a source 8 and to return light reflected from the material 1 to a detector 9. The illuminated portion of material 1 may be a special colour control strip 3 or part of the printed image. The light may return by the same optical conductor 5, in which case an optical divider (not shown) is provided at end remote from material 1. Alternatively the light may return by an adjacent optical conductor, in which case there is used an optical switching arrangement (Fig. 3 not shown) or a multiplexing arrangement (Fig. 5 not shown). The colour may be compared with a desired colour strip (29) and the information processed and displayed by a processor (28). The processor may also automatically control the printing apparatus. <IMAGE>

Description

SPECIFICATION A Device for Monitoring the Colour of Materials in Sheet or Web Form In offset printing machines telemetering is usually carried out by setting the colour measuring device on the ink container. A clearance exists between the colour measuring device and the inking roller through which the ink passes to the inking roller from the inking box.

The measuring device is flexible and is brought close to the roller to varying degrees over width of the web 6f material which is to be printed. The clearance is specially set with the aid of special setting devices over regions with an extent of 3 to 10 cm. A continuous transistion takes place between the regions because of the continuous but flexible measuring device (The Polygraph 4, 1979, pp.3l 4-317).

In the case of presetting colour regions the colour metering is dependent on location and only takes place by means of setting the clearance width depending on the location. In addition, the fact that the adjustment in one region can also affect adjacent regions, e.g. by distorting or bending a common suspension element, must be taken into account. In addition the adjustment may affect all other regions.

In more recent printing machines presetting of colour regions is undertaken before each new contact pressure and this is then corrected manually during printing until a satisfactory result is achieved. In the case of continuous printing further corrections may be necessary due to fluctuations in temperature, changes in moisture, differences in the paper etc.

The result of printing is checked not only visually but also densitometrically. In the case of offset machines printing sheets of paper or other material, individual printed and dried sheets of paper are removed for this purpose and measured on a test console which is located outside the machine (German Offenlegungsshcrift No. 27 28 738).

The density of the colour is measured by means of a colour control strip arranged transverse to the direction of movement of the sheet, said strip being located at the edge of the printed sheet and containing the four colours yellow, magenta (red), cyan (blue) and black in full colour or tone and in several raster shades in each colour region. The sheet is tensioned on the test console and its colour control strip is measured with the aid of one or more densitometers travelling over the colour control strip, the density or values of the colour being passed to an electronic evaluation device. With the aid of the evaluation device, a comparision is made between these colour values and the desired values recorded in the evaluation device. This comparison is used to control the colour metering (German Offenlegungsschrift No. 27 27 426).

Despite the partial automation which has already been achieved by printing machines, the time required to set up these machines needs to be shortened still further, the operation of these machines needs to be simplified and the quality of printing needs to be improved and, above all, paper wastage is to be reduced. Since a sheet of paper removed from the machine is tested on the test desk during the printing process, which operates at a high speed (operational speed of the machines to be printed of between approximately 1 m/s and 10 m/s) a relatively large period of time elapses between colour metering at the input and completion of the colour examination process carried out on the sheet which has been removed so that in some circumstances there can be large quantities of paper wasted.

It is an object of the present invention to achieve a device for monitoring the colours of a colour control strip provided on the printing material or of the printed image directly, which reduces the time between setting the colour metering at the input and completion of the examination process of the stationary colour control strip or printed image considerably. The present invention also seeks to provide a device which supplies objective measured values regarding colour saturation, printing contrast and raster shade values for all printing regions and colours and provides the possibility of fully automatic colour metering.

In accordance with a first aspect of the present invention there is provided a device for monitoring the colour of a material in a sheet or web form, comprising a plurality of optical conductors which are postioned transversely of and spaced from the material which are arranged to illuminate the surface of the material, to receive the light returned from the surface of the material and to transmit it for processing.

In accordance with a further aspect of the present invention there is provided a method of monitoring the colour of a moving material in sheet or web form employing a plurality of optical conductors arranged above the material and extending transversely to the direction of motion of the material, illuminating the surface of the material by means of the optical conductors, the optical conductors being arranged to receive light returned from the surface of the material, and subsequently processing the information contained in the returned light.

Preffered embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows a first embodiment of a colour monitoring device according to the present invention; Figure 2 shows a light sensor system opposite a web of material and having optical conductors; Figure 3 shows a colour monitoring device with optical switching of the optical conductors and having light detector; Figure 4 shows a diagram associating the reflected light with the colour areas, with reference to the device shown in Figs. 3 and 5; and Figure 5 shows a colour monitoring device which simultaneously illuminates all of the colour areas with the optical conductors and switches the light detectors associated with these optical conductors.

Figure 1 shows a portion only of a material 1 e.g. paper, which Is to be printed on, the material being moved in the direction of the arrow by a gulde roller 2 of a printing machine.

For example the material may be a rolled up printing material having a conventional width. The material has colour control strips 3 at certain spacings (for example 1 m) and these strips may have a length of up to 150 cm and a width of up to, say, 1 cm. The colour control strip has colour areas 4; the dimensions of each colour area are, say 1 cm in the direction of movement of the material and, say 0.5 cm transverse to the direction of movement.

A number of groups of flexible optical conductive bundles 5 Is distributed over a length of the colour control strip 3, the number of flexible bundles corresponding to a number of colour areas 4. The optical conductors 5 are used to illuminate the surface of the printing material 1 and to receive and pass on the radiation returned by the surface of the printing material 1, preferably to a light sensor system comprising light detectors 9 located outside the printing machine.Each group of optical conductors 5 comprises a plurality of individual fibres, for example 100, and at one end runs out into two group ends 6, 7. The end surface of the end 6 is light input E of the light of a light source 8, preferably arranged outside the printing machine, while the end surface of the end 7 is the light output A of the retumed light.

The end surfaces 10 of the optical conductor groups 5 with their statistically mixed individual fibres are arranged without any further optical means at a small distance (a few mm) above the material 1 which passes.

In the case of a colour control strip 3 having twenty colour areas for example, twenty optical conductor groups 5 are also provided above the track 1 while light is coupled into the input E of all of the groups 5 and the radiation which has been sent back from the surface of the material 1 arrives at the output A of all of the groups 5.

The ends 11 of all of the groups 5 near the material track 1 can be combined simply to form a sensor unit which is very small in width. The sensor unit can, because of its small space requirement and flexibility, be arranged without difficulty in fairly inaccessible areas requiring a small space within the region of the track of the printing machine.

The colour areas 4 are illuminated with a certain illumination intensity by means of the optical conductors groups 5 and the brilliance of the light which has been passed back from the surface of the moving material is measured with the aid of the light detectors 9.

Since the colour areas 4 of the colour control strip 3 generally are rectangular it Is advisable to use groups 5 having a rectangular cross-section which is slightly smaller than that of the colour area.

The flexible groups 5 may easily have a considerable length so that the light source 8 and the photodetectors 9 can be arranged outside the printing machine: the same is true also of the processor associated with the light detectors and mentioned below, which further processes the measured signals, stores them and displays them and influencies the printing machine in accordance with the measured data (Figs. 3, 5).

If there is a colour strip 3 having 200 colour areas and a width b of 1 cm and 200 colour areas n and the speed of movement v of the material is 5 m/s, then for the optical conductor group detector system there is measured time of T=b/v.n=1 .10-2/5.2.1 021 1 0-5s This measurement time is sufficient for analog/digital convertion of the measured data from the light detectors and for storing the data in a processor.

SI--PIN diodes may be used as the light detectors.

The optical conductor groups 5 do not necessarily have to split into two ends 6, 7; normally-formed groups (not shown) can be used too, their two ends being formed like the end 11 of the group 5. In such an arrangement a radiation divider which is transmissive is arranged at a certain angle in front of the end associated with the light source 8 so that the light from the light source 8 is coupled into the group 5 via this radiation divider, and emerges at the end associated with the surface of the moving material 1, the returned radiation being received by this end and passed to the other end where the said radiation reaches the radiation divider which defiects this radiation, for example by 90 , and the radiation is then absorbed by a light detector associated with the radiation divider.

The design and construction of the sensor system in relation to the web of material 1 is shown in Fig. 2.

The optical conductor groups 5 arranged in adjacent rows are clamped between two profiled rails or bass 40, 41 and are retained by these ralls at a standard spacing. The bars form the lateral termination of a half cylinder 42 and together therewith provide a stable measuring bar which can be mounted on the printing machine. A slit 43 through which compressed air is supplied is provided between the semi-cylinder 42 and the profiled bar 41, the said compressed air keeping the ends of the optical conductors free of dust and dirt.

In Fig. 3, a device for monitoring colour is shown in which light is coupled into the optical conductor groups successively in terms of time for example and thus the colour areas of the colour control strip are illuminated successively In time too; the light beams passed back by the colour areas and also occuring successively in terms of time are supplied to a single light detector only.

The printing machine is indicated only schematically by DM and only one guide roll 2 is shown. The spatial separation of the printing machine DM and of the electronic measuring unit ME coupled to the inputs and outputs of the optical conductors 5 is indicated by the broken lines S.

The groups 5 are retained on a measuring bar or track 22 in front of the material web 1. The measuring bar may have a graduated straight edge and may be used to accurately position the groups 5. Illumination takes place with the aid of a light source 8 which is similar to daylight, for example xenon lamp. The divided ends 6, 7 of the groups 5 are each circularly arranged on an optical switch comprising two rotating beam splitters 23, 24 and each having a slit 25. If the beam splitters 23, 24 rotate synchronously, then only one group 5 is illuminated at a time and this in turn iiluminates the associated colour area.

Switching takes place in very rapid sequence successively from one colour area to the next.

The colour area of the colour strip 3 illuminated by the associated optical conductor end 11 passes back the incoming light in accordance with its colour values; this light is received by the same group 5 and passed to the beam splitter 24. The group ends 7 are arranged circularly in front of this beam splitter. The light passing out in parallel in the first approximation is focused on to a common detector 27 with the aid of a convergent lens 26 or a concave mirror and is converted thereby into corresponding voltage values which are converted in a processor 28 into digitial values and further processed.

The beam splitter 23 leaves the light path to only one mixed optical conductor group 5. One half of the fibres of each group 5 are guided to the actual colour control strip 3 and the other half to a desired colour strip 29 which is stationary. The colour areas of the two strips 3, 29, which correspond to each other, are illuminated at the same time. The beam splitter 24 determines which of the two beams, which have been passed back and which carry the signal information, reaches the detector 27.

The detector voltage varies as shown in Fig. 4.

With the aid of the beam splitter 23 thecolour areas 1' to 8' may be illuminated successively via the optical conductor group by colour regions n.

Each colour area may have an edge length of 0.5 cm and each region n may have a length 4 cm.

Over a material web width of 100 cm for example 25 regions are to be measured and 8.25=200 measurements are to be carried out. In detail, the following measurements for example may be carried out one after the other in terms of time: 1. The photoconductor group associated with the colour area 1' leads to a black full shade area and passes the weak returned radiation to the detector 27.

2. The photoconductor group associated with the colour area 2' leads to the black area of the raster shade (or grey shade area) and passes the fairly strong returned radiation to the detector 27.

3. The yellow full colour area 3' is illuminated and the returned radiation reaches the detector 27.

4. The yellow raster shade area 4' is illuminated and the returned radiation reaches the detector 27.

In the other colour areas and regions the procedure is similar. In the arrangement of Fig. 4, the white unprinted paper is measured not in a colour area provided for the purpose, but rather after passing the colour control strip 3 and is checked-individually by each sensor.

If the photoconductor groups are arranged on a circle, then the radiating surface may be represented optically on a detector having a relatively small area. In this way individual detector may be coupled generally to all optical conductors. Because of the large dimensions in practice a parabolic mirror is preferably used instead of an imaging lens.

Instead of the mechanical beam splitter 23, 24 a solid state system of similar form may be used in which electro-optical solid state switches are arranged in front of the inputs E and outputs A of the photoconductor groups 5, these solid state switches being controlled in the necessary manner.

In order to provide independence of time changes of the light source 8 and detector 27 there is no absolute setting of the colour values but rather a comparison is made with the desired colour strip 29. The actual colour strip 3 and the desired colour control strip 29 are measured successively and the measured values are stored.

In order to process them further the measured voltages arising as a function of time at the detector 27 and associated with the colour areas are digitalized by an analog/digital converter, which is not shown in greater detail, and stored in the processor 28.elf the actual colour control strip 3 is printed in the printing machine and the desired colour strip 29 are measured alternately and the measurement voltage values are stored in the processor 28 than all of the colour values of interest can be derived from this. The alternating measurement of the web of material 1 and the desired colour strip 29 primarily leads to the measurement voltage graphs shown in Fig. 4.

By forming of the difference between the desired and actual recording (Fig. 4) area by area, the quantitative difference between the desired values in each case and the actual values is obtained. The desired, actual and difference values may be displayed optionally on an image screeen of an image device of the processor 28.

In the simplest case the printer himself may control the colour metering of the printing machine or the values may act via the processor 28 on adjusting elements associated with the colour metering devices.

If one is not satisfied with relative measurement as compared to a desired colour strip, then the printing contrast and the surface or area coverage may be calculated from the colour density values at each colour area. These magnitudes may also be displayed on the image screen of a video device once the processor 28 has performed the corresponding computations with the measurement data. Fig. 5 shows a device for monitoring colour in which light is coupled at the same time to all of the optical conductor groups. These groups illuminate all of the colour areas 4 of the colour areas 4 of the colour control strip 3 at the same time while a light detector is associated with each optical conductor group, and these are activated successively.

The desired and actual colour strips 29, 3 are illuminated with the aid of the group 5 over their entire length in continuous operation. Each output A is terminated by its own light detector 13. All of the detectors are connected to a common amplifier 31 in a sequence controlled by the processor 28 with the aid of a multiplexer 30.

The respective measurement information is stored subsequently in the processor 28 by means of an analog/digital converter (not shown).

The multiplexer 30 is programmable in conjunction with the processor 28 such that the actual colour measurement, the actual white measurement and the desired colour measurement take place successively, see Fig. 4, and the measured values are stored accordingly in the processor 28.

Due to the plurality of optical conductor groups and to the different spacings which may be present between the sensors and the colour strips and the plurality of detectors, the individual detector units may have a different sensitivity.

These differences can be eliminated from the test result by relating all of the measured values to their respective white values. The measurement may then take place for example in the following steps: 1. Desired colour measurement USF in accordance with Fig. 4.

2. Displacement of the desired colour strip until the white of the paper is present under all of the sensors.

3. Desired white measurement Usw in accordance with Fig. 4.

4. Forming a quotient USF,/USWn for each sensor field; the quotients represent the standardized desired colour values for the subsequent measurements.

5. Actual colour measurement UIF in accordance with Fig. 4.

6. Actual white measurement U,w in accordance with Fig. 4.

7. Forming the quotient UlFIUIWn for each sensor field: the quotients represent the standardized actual colour values of the measurement.

8. Forming the difference between the standardized desired colour values and the standardized actual colour values.

As a result of standardizing on the white values the measured result is made largely independent of: the illumination intensity, the possible breakages in individual glass fibres in the photoconductor group, difference in spacing of the sensors from the web of paper, diode sensitivity, changes in amplification etc.

Programming can be carried out with the desired colour strip via the measurement bar itself before beginning printing if the additional expenditure on the comparison measurement bar is to be avoided.

The system of light detectors and optical conductor groups provides the possibility of dispensing with the colour control strip 3. In this case the printed image itself is scanned by using colour filters in coloumns in the direction of movement of the paper over the width of the image and comparing with corresponding desired values.

The advantage achieved by the above-desired embodiments consist in that there is direct measurement at the moving printing material, and therefore renders unnecessary the checking of a sheet of material removed from the machine at a special testing station requiring an operator. The device can be used in printing machines in which the colour control strip is arranged, for example, between two successive printed images or sheets. The colour control strip may have a length of approximately 100 to 1 50 cm and approximately 200 colour areas which are measured semi-simultaneously with a corresponding number of sensors. The light source and light detectors associated with the optical conductors may be arranged at almost any desired location and therefore outside the machine.In addition, a colour control strip can be dispensed with and the colour image may be scanned itself in sections in the direction of movement.

In the devices of Figs. 3 and 5 a desired colour strip 29 need not necessarily be used. Rather, digital desired values associated with the respective colour areas of the colour control strip 3 may be recorded in the processor and compared to the digital values derived from measurement of the colour area.

The desired values of a colour control strip or of a complete printed image may be obtained for example by the fact that a desired colour strip or a desired colour image is illuminated with the aid of the optical conductor and sensor system before the actual printing process begins and is scanned electricaHy by light, the voltage values achieved being registered as desired digital values in the storage device of the processor 28.

The sensor system is not only suitable for testing materials in motion but also may be used advantageously instead of the densitometers which were previously used for stationary materials and which travel across the colour control strip.

In the simplest case, the voltage may be measured displayed with the aid of a digital storage oscillograph and the printer may look at the measured values, or values derived therefrom, in analog form on the screen.

The optical conductor and sensor system may also be used for measuring moisture, register control etc. The system can also be used for all materials in motion such as any desired printed webs of paper, piastics material etc. which are to be checked for colour errors.

Claims

1. A device for monitoring the colour of a material in sheet or web form, comprising a plurality of optical conductors which are positioned transversely of and spaced from the material, and which are arranged to illuminate the surface of the material, to receive the light returned from the surface of the material and to transmit it for processing.

2. A device according to claim 1 wherein the surface of the material has a plurality of colour test locations, each location being illuminated by an optical conductor and returning light to the same optical conductor which transmits it via a radiation divider to light detecting means.

3. A device according to claim 1 wherein the surface of the material has a plurality of test locations, each location being illuminated by an optical conductor and returning light to an adjacent optical conductor which transmits it to light detecting means.

4. A device according to any preceding claim which is arranged to monitor the colour of moving material.

5. A device according to claim 4 wherein the optical conductors extend transversely to the direction of motion of the material.

6. A device according to any preceding claim wherein the optical conductors transmit light to a respective plurality of light detectors which pass signals for processing via a multiplexing arrangement.

7. A device according to any of claims 1 to 5 wherein the optical conductors are arranged to be activated successively by means of an optical switching arrangement and that the light returned from the surface of the material is supplied to a single light detector by means of the optical switching arrangement.

8. A device according to any preceding claim comprising a plurality of desired colour locations each corresponding to said colour test location the desired colour locations being illuminated by optical conductors and the light returned therefrom being received by optical conductors for the purpose of measurement.

9. A device according to claim 8 wherein the material and the colour test locations are arranged to move and the desired colour locations are stationary.

10. A device according to any of claims 2 to 9 wherein the colour test locations are constituted by colour control strips on the material.

11. A device according to any of claims 2 to 9 wherein printed images on the material are themselves used to control the colour of the printing.

12. A device according to claim 10 wherein the material and the images are moving and the images are scanned in columns in the direction of their movement.

13. A device according to claim 11 or 12 wherein the printed images are checked and measured for colour with the aid of stationary desired colour image which is illuminated in a similar manner as the printed images and the light from this desired colour image is passed for purpose of measurement.

14. A device according to any preceding claim which is arranged to control the colour of the printed material of a printing machine.

1 5. A device according to any preceding claim wherein the information from the light detecting means is supplied to a processor having storage, computer and image reproduction devices.

1 6. A device according to claim 1 5 in which digitally converted information from the light detecting means is stored and made visible for the purpose of evaluation on a screen of the image reproduction device as a desired, actual and difference recording of the measured values.

1 7. A device according to claim 14 and claim 1 5 or 1 6 wherein digitally converted information from the light detecting means is used to control the printing machine.

18. A device according to any of claims 1 5 to 1 7 wherein the computer of the processor calculates the printing contrast and the surface coverage from the measured values for each colour location, these being made visible on a screen of the image reproduction device after feeding the information from the light detectors to the computer and/or is used to control the printing machine.

1 9. A device according to claim 14 and claim 1 7 or 1 8 wherein the calculated values of the printing contrast and the surface coverage are used to control the printing machine.

20. A device according to any of claims 1 5 to 1 9 wherein desired colour values are compared in the processor as digital words with digital words derived from the colour test locations, i.e. the colour control strip or the printed image.

21. A device according to any preceding claim wherein the material is moving and the ends of the optical conductors adjacent the material are arranged in a parallel disposition on a measurement track which is mounted above the material and transverse to the direction of movement thereof.

22. A device according to claim 21 wherein the measurement track has a channel for the supply of a compressed air flow to the end surfaces of the optical conductors.

23. A device according to any preceding claim wherein the optical conductors are glass fibres.

24. A method of monitoring the colour of a moving material in sheet or web form employing a plurality of optical conductors arranged above the material and extending transversely to the direction of motion of the material, illuminating the surface of the material by means of the optical conductors, the optical conductors being arranged to receive light returned from the surface of the material, and subsequently processing the information contained in the returned light.

25. A device for monitoring colour substantially as herein described with reference to the accompanying drawings.

26. A method of monitoring colour substantially as herein described with reference to the accompanying drawings.