The present invention relates to a method for controlling print quality in a web printing press by measuring at least the darkness of the colour surface of the printed product by aid of a web sensor, wherein on the moving paper web there is arranged a test spot and/or test patterns in the middle and/or aside the saleable printing, and the operation of the web sensor is synchronized with the said test spots while they move, along with the paper web, past the sensor, so that from the light source of the said web sensor there is focused a light beam onto the paper web, and the scatterings of the said light beam from the test surface are measured by employing a light detecto .

In the art or printing, it is necessary to measure the darkness of the product surface, as well as the reciprocal registration of the respective colours in order to achieve a product which has a high quality from the commercial point of view. Web densitometers have long been used for measuring colour darkness, as well as register measurement devices for measuring the reciprocal registration of different colours. The structure of web densitometers is explained for instance in the article "Design and construction of multichannel on-press densitometers for print quality monitoring" by Kimmo Simomaa and Tapio Lehtonen, published in Graphic Arts in Finland, 1982.

The standardization of print quality, for instance by employing closed-loop control, usually requires that the test spots indicating the relative shifting of the partial images printed using separate inks, i.e. cyan, magenta, yellow and black, are printed in the middle of the saleable printing, and that the web sensor is synchronized with the said test spots while they move past the sensor along with the paper web. A typical feature of the prior art web densitometers has been that they measure the optical reflection density of the test

spot within an area of roughly 9 square millimeters, which means that the light beam focused thereon has a diameter of roughly 3-4 mm. This practice originates from the manual desktop densitometers, where the measuring can be arranged to take place exactly at the desired spot in the printing. In the case of a web densitometer, the determination of the measuring spot is more problematic: the 3-4 mm light spot must fall on the printed test surface, such as the test spot, the length whereof - in the moving direction of the paper web - is in practice 6 mm at the most. In order to make a 3-4 mm spotlight to fall exactly on the test area, the measuring operation must be synchronized at the minimum accuracy of +/-1,5 mm. In reality the synchronization accuracy must be roughly +/- 0.5 mm,- if absolute certainty is required that the light detector of the web sensor does not see anything outside the 6 mm test area. Observation of the vicinity is a drawback, if the test area represents black printing and the vicinity is a light printing sheet or the like. Except that the 3-4 mm test area is a general practice, it is also indirectly recommended by the DIN standard 16536/16527: the large size of the test spot helps to filter the variation caused by irregularities in the paper and the printing.

By employing the method of the present invention, there is achieved a distinctive improvement compared to the measurement carried out by aid of the above described web densitometer. In order to realize this, the method of the invention is characterized by the features enlisted in the appended patent claims.

As for the advantages of the invention, the following is pointed out. The diameter, or the largest dimension of the light spot formed on the paper web by the light beam in the motional direction of the web, could be for instance 1-2 mm, in which case the synchronizing of the measurement with the 6 mm test area becomes easier, or, with careful synchronizing, a test pattern of even 4 mm can be used, which helps in selling the web sensors for customers in the printing trade. When employing web

densitometers, the diminishing of the test area is easily compensated by repeating the measurement on successive sheets or equivalent, and by calculating the measuring result proper as an average of the obtained partial measurements. In a microprocessor-based arrangement, there can also be carried out several separate measurements within the range of the same test area, if the edge readings are rejected. If the measuring area is reduced for instance to 1/10 of the original, the measurement is carried out on 10 successive sheets, or on 3 sheets, if for instance 3-4 measurements are performed on each sheet. Thus the good control interval, i.e. 20 sheets, can be maintained in the automated density control .

A small-size light spot can be realized by aid of a conventional lens arrangement, and the employed light source can be either a semiconductor lamp, a filament lamp or a Xenon flash tube, which are all known in the prior art. A new method for the projection of light onto paper is the use of cylindrical lenses. By aid of a cylinder lens which is installed in the transversal direction of the paper web, there can easily be realized the above mentioned short, small-sized light spot, which in this case means the duration of the illumination in the driving direction. In addition to this, the cylinder lens has the advantage that by employing one single light source, there can also be illuminated for instance the adjacent test area or test spot, which reduces the amount of required light sources. In the transversal direction of the web the light spot is limited for instance by using mechanical apertures.

The reduction of the light spot size used in the web densitometer measurement according to the invention makes it possible also to measure the registration of the colours with the same sensor. The data measured by the web sensor are processed for example simultaneously in two separate data processing units: there is provided specific equipment for measuring the density and register respectively. The measuring of the register is carried

out according to generally known principles.

The employed test pattern in the register measurement can naturally be of any possible type. The darkness of the colour surface could in fact be measured on the basis of the register test pattern, if the sensor of the present invention were utilized. The usefulness of the measuring arrangement here described works both ways: the accurate definition of the darkness on the basis of the density computation would make the definition of the edge position, i.e. the registration, more reliable. The greatest advantage gained by the twofold darkness and registration sensor is, however, the fact that the manufacturing costs are essentially reduced, and moreover the instalment of one common measuring sensor clamping bar in the printing press will be simple.

In the following the invention is explained in more detail with reference to the appended drawings, wherein figure 1 is an illustration of a web sensor applying the method of the present invention; figure 2 is an illustration of the same web sensor seen from above; figure 3 illustrates the comparison between a web sensor (A) employing a large light spot and measuring the darkness of the colour surface, and the web sensor (B) operated according to the method of the present invention; and figure 4 is an illustration of a measuring system according to the method of the present invention.

The web sensor of figure 1 comprises the light source 1, the optics connected thereto, which include for instance the condenser lens 2 and the focusing lens 3. On the paper web 4, or on some other respective material, there is arranged a test spot and/or test patterns 5 in the middle and/or aside the saleable printing. The operation of the web sensor is synchronized with the said test spots while they move past the sensor along with the paper web. The motional direction of the paper web 4 is upwards from the level of figure 1. The light detector or

detectors 6 are arranged at an angle with respect to the light beam directed towards the paper web, most advantageously so that the diffusely reflected, i.e. scattered light from the test pattern 5 can thereby be observed. In this case the light beam of the light source 1 is directed against the web surface at an angle of 45 , and the detector is arranged at an angle of 90 with respect to the web surface.

In this preferred embodiment, the focusing lens 3 is like in figures 1 and 2. The cylinder lens 3 is placed transversally with respect to the moving direction of the web, and the width of the light spot 7 is preferably 1-2 mm. In the transversal direction of the web, the light spots are limited for instance by aid of the mechanical aperture 3a. By means of this arrangement, for example 5 adjacent test spots can be illuminated (detectors 61, 62, 63, 64 and 65).

In figure 3A it is seen that in the prior art sensors for measuring the darkness of the colour surface, a large light spot 8 with respect to the test area 9 is employed, whereas the method of the present invention employs a small light spot 10.

The use of the small-size light spot enables the combination of two completely different measurements, i.e. the measurement of the darkness of the colour surface and the registration of different colours. Test areas are needed in both measurements, but they are different in nature, wherefore in conventional measurements both have their own respective marks, which consequently take up a lot of the printing space on the web. The registration measuring marks are relatively small, and usually there is only needed a right-angled triangle in order to measure the transversal and longitudinal registration of different colours. Thus the number of marks in four-colour printing is four. The location of the edges is the only feature of the marks to be measured. Normally these are observed by aid of a light source strongly focused by optics and by aid of a detector.

By means of the method of the present invention.

both of these aforementioned measuring procedures can be combined into one. One such measuring arrangement is illustrated in figure 4.

The said measuring arrangement comprises the web sensor 11, the tachometer 12, the synchronizing circuit 13, the fast A/D converter 14, the data processing unit 15 such as a microprocessor, the memory 16 and the interface units 17, 18 for instance for the user and additional data processing and/or process control activities respectively.

Both measurements are carried out by aid of the same web sensor 11, or by aid of two separate web sensors, which are connected for instance to the same fastening clamp or the like. While measuring the darkness of the colour surface, the measurement is repeated on successive printing sheets or equivalent, and the measuring result proper is calculated as an average of the partial measurements. The optical intensity of the scattered radiation, which is used in the measurement of the darkness of the colour surface, is utilized while defining the accurate location of the edges of the test surfaces, on the basis whereof the reciprocal registrations of the different colours are calculated, whereafter they are available for the information of the user and/or feedable into the adjusting system of the printing process.