WO2005106436A1

WO2005106436A1 - Paper surface quality testing

Info

Publication number: WO2005106436A1
Application number: PCT/FI2005/050134
Authority: WO
Inventors: Kari Saarinen; Karri Muinonen
Original assignee: Abb Research Ltd
Priority date: 2004-04-28
Filing date: 2005-04-27
Publication date: 2005-11-10
Also published as: FI20040600A; FI20040600A0; WO2005106436A8; FI117834B

Abstract

The invention relates to an apparatus for testing the surface quality of paper. The apparatus comprises: a light source (1) for generating a light beam (2), directing means (3) at least a part (4) of said generated beam substantially perpendicularly towards the surface of said paper (5), measuring means (7) for measuring the intensity of coherent backscattering (8) from the surface of said paper at different angles, and analyzing means (10) for analyzing the measured intensity at different angles in order to determine the surface quality of said paper.

Description

PAPER SURFACE QUALITY TESTING

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates to the testing of the surface quality of paper. The invention can be utilized at least for testing the printability properties of paper.

2. Description of the Prior Art The printability properties are very important for papermakers and printing plants. The characterization of the surface structure of paper is, however, challenging. Previously there is known a solution for measuring the surface quality of paper, where a plate is arranged on the surface of the paper.

This plate includes holes through which air is blown towards the paper surface.

When the force pressing the plate against the paper is kept constant by controlling the air blown towards paper surface, the amount of air which leaks out between the paper surface and the surface of the plate depends on the surface roughness of the paper. Thus it is possible to determine the surface quality of the paper by measuring the amount of air which is blown out through the holes in the plate. A problem with the prior art solution described above is the accuracy of the measurements. Pores whose size is in the μm range cannot be taken into account in these measurements, though they are important when evaluating the printability properties of the paper.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawback and to provide a solution which makes it possible to measure the surface quality of paper in such a way that the properties of the paper can be determined better than in the prior art solutions. These and other objects of the present invention are achieved with the method of independent claim 1 and the apparatus of independent claim 4. In the present invention, coherent backscattering is utilized in order to test the surface properties of paper. By measuring the intensity of the coherent backscattering at different angles, it is possible to obtain measurement results with a much better accuracy than before. The solution makes it possible to obtain measurement results taking into account actual particles or structural variations in the order of the wavelength of incident light. The size of the particles, the average distance of the particles, or pores in the tested paper surface strengthen the reflection close to the backscattering angle due to interference. Thus, for instance, pores of paper coating can be taken into account when the surface quality is determined. Therefore, it is also possible to determine the printability properties of paper better than before. Preferred embodiments of the method and apparatus of the invention are disclosed in the dependent claims 2 to 3 and 5 to 10.

BRIEF DESCRIPTION OF DRAWINGS In the following, the present invention will be described in more detail by way of example and with reference to the attached drawings, in which Figure 1 is a flow chart of a first preferred embodiment of the invention. Figure 2 is a block diagram illustrating a first preferred embodiment of the invention. Figure 3 illustrates coherent backscattering. Figure 4 illustrates the measurement results obtained with the apparatus of Figure 2. Figure 5 is a block diagram illustrating a second preferred embodiment of the invention. Figures 6 and 7 illustrate measurement results obtained with the apparatus of Figure 5. Figure 8 is a block diagram illustrating a third preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The method according to Figure 1 can preferably be used to test the surface quality of paper in order to determine the printability properties of the paper in question. In block A, a light beam is generated with a light source, such as a laser. In block B, this light source is directed towards the surface of the paper. The directing can be accomplished for instance with a semi-transparent mirror, which directs at least a part of the beam substantially perpendicularly towards the surface of the paper. In order to obtain a beam with a substantially constant intensity, it is possible, if necessary, to arrange a detector on the other side of the semi-transparent mirror. This makes it possible to obtain a measurement result describing the intensity, and to use this result in order to adjust the light source to obtain constant intensity. In block C, the intensity of the coherent backscattering is measured at different angles. This measurement result is analyzed in block D to determine the surface quality. Figure 2 is a block diagram illustrating a first preferred embodiment of the invention. The apparatus shown in Figure 2 can be used to implement the method which is described with reference to Figure 1. A light beam 2 is generated with a light source 1 , which preferably is a laser. The beam is directed towards a directing means 3, which preferably is a semi-transparent mirror (beam-splitter). This directing means directs at least a part 4 of the beam substantially perpendicularly towards the surface of the paper 5. Another part 6 of the beam passes through the semi-transparent mirror to a detector 7. The detector measures the intensity of the beam 6. The measurement result is used to adjust the light source 1 in order to obtain constant intensity for the beam 2. The adjustment means can be integrated into the light source. It should be observed that the detector 7 is not necessary in case the light source 1 can be controlled in some other way to generate a beam with substantially constant intensity. The apparatus shown in Figure 2 also includes a measuring means 9 for measuring the intensity of coherent backscattering 8 from the surface at different angles. The surface properties of the paper 5 have an impact on the backscattering. This is utilized in the present invention such that the measuring means is used to provide a measurement result which indicates the intensity of the coherent backscattering at different angles, as explained in connection with Figure 4. The measuring means 9 may in practice be implemented as a camera comprising a CCD (Charge Coupled Device) element, in which case the analyzing means can be implemented with circuitry, software or a combination of these. The measurement result obtained with the measuring means 9 is analyzed by analyzing means 10 in order to determine the surface properties of the paper 5. The analyzing means 10 can be implementedwith a computer program and a PC (Personal Computer), for instance. Alternatively, it is also possible that the measuring means and the analyzing means are integrated into one physical component. The apparatus shown in Figure 2 can be implemented such that it is a part of a paper manufacturing machine which continuously measures and analyzes the surface quality of the produced paper. If the analysis indicates that a predetermined quality level is no longer achieved, then an alarm is initiated. In case the object is to carry out measurements with a narrow-band light beam, then this can be implemented by using a wide-band light source in combination with a filter, which allows only the part of the beam with the desired wavelength to pass. Thus the filter can be located between the light source 1 and the directing means 3 in Figure 2. Alternatively the filter can located between the directing means 3 and the measuring means 9 in Figure 2. Figure 3 illustrates coherent backscattering occurring when, for instance, the apparatus of Figure 2 is used. Figure 3 shows two incident waves 4' and 4" which are directed substantially perpendicularly towards the surface of the paper 5. The paths of these waves are illustrated in the figure. These paths depend on the irregularities, such as pores 11 , which are present in the paper. In the figure, the backscattered waves 8^f and 8" have the same path lengths and they are in the same phase. The result is constructive interference which can be detected in the measurements carried out with the apparatus of Figure 2. Figure 4 illustrates the measurement results obtained with the apparatus of Figure 2. The coherent backscattering peak is clearly visible from the "normal" background scattering which has been indicated with a relative intensity of 1 in Figure 4. The shape and the height of the visible peak gives an indication about the surface properties of the paper. The angular width of the peak depends on the volume density of the scatterers (particles or pores). In more detail, the angular width is related to the mean free path of light / in the medium of small particles. The mean free path is a function of the volume density v, the size of the individual particles a, and the extinction efficiency q_β (extinction cross-section divided by the geometrical cross-section of the particle), / = (4a)/(3vq_c), for spherical particles. For non-absorbing scatterers, the half-width at half-maximum of the coherent backscattering peak a is roughly a = 0.6 - g)/(k *l) , where g is the single-scattering asymmetry parameter and k = 2π/λ s the wave number. Thus the width of the peak depends on the physical characteristics of the irregularities, e.g., it becomes narrower with increasing mean free path and with increasing asymmetry parameter. It is possible, for instance, to define limits for the angular width of the peak and for the intensity of the peak, and to use these limits in order to determine whether or not the quality of paper is good enough for printing purposes. Figure 5 is a block diagram illustrating a second preferred embodiment of the Invention. The apparatus shown in Figure 5 is very similar to the one shown in Figure 2. Thus, the embodiment of Figure 5 will mainly be explained by pointing out the differences between these two embodiments. In Figure 5, a first polarizer 12 has been arranged on the travel path of the beam between the light source 1 and the directing means 3. A second polarizer 13 has been arranged on the travel path of the coherent backscattering 8, between the directing means 3 and the measuring means 9. Otherwise, the apparatus of Figure 5 corresponds to the one shown in Figure 2. The reason for using the first and second polarizer 12 and 13 is that practical tests have shown that they make it possible to improve the accuracy of the measurements. Figures 6 and 7 illustrate example measurement results obtained with the apparatus of Figure 5. In Figures 6 and 7 the measurements were carried out for paper in such a manner that the wavelength was 0.49 μm in Figure 6 and 0.66 μm in Figure 7. The results of both figures confirm that the relative intensity of the coherent backscattering peak is higher when co- polarization is in use; in other words, when the first polarizer 12 and second polarizer 13 are selected in such a manner that the polarization is the same. Figure 8 is a block diagram illustrating a third preferred embodiment of the invention. The apparatus shown in Figure 8 is very similar as the one shown in Figure 8. Thus, the embodiment of Figure 8 will mainly be explained by pointing out the differences between these two embodiments. Figure 8 relates to multi-wavelength measurements. Multi- wavelength measurements make it possible to obtain information about the size distributions on the pores in the paper. Such multi-wavelength can be implemented in at least two alterative ways: 1 ) By using the apparatuses of Figures 2 or 5. In this case several narrow-band light beams with different wavelengths are used in turns. The coherent backscatter is measured for each beam separately, and the measurement results obtained for the different beams are analyzed in order to determine the surface quality. 2) By using the apparatus of Figure 8. In this case a wide-band light source 1 is used to create a wide-band beam. A multi-filter means 14, such as rotating filter disk 14 with different band pass filters in different sectors of the disk, Is arranged in front of the measuring means 9. Thus the multi-filter means 14 will allow coherent backscattering light of different wavelengths to pass to the measuring means at different moments of time. This makes it possible to obtain measurement results for different wavelengths separately with the same measuring means 9. If the disk is rotated with a high speed, then it is possible to obtain measurement results for different wavelengths within a short period of time, in practice almost simultaneously. Thus, it is possible to obtain measurement results witj different wavelengths from the same part of a paper surface of a paper which moves within a papermaking machine, for instance. In Figure 8 the polarizer 12 and polarizer 13 have been indicated by dotted lines in order to show that they may or may not be used in the embodiment utilizing the rotating disk 14. It is to be understood that the above description and the accompanying figures are only intended to illustrate the present invention. It will be obvious to those skilled in the art that the invention can also be varied and modified in other ways without departing from the scope of the invention.

Claims

CLAIMS:

1. A method for testing the surface quality of paper, characterized in that said method comprises: generating (A) a light beam which is at least partly directed (B) substantially perpendicularly towards the surface of said paper, measuring (C) the intensity of coherent backscattering from the surface of said paper at different angles, and determining (D) the surface quality of said paper by analyzing the measured intensity of the coherent backscattering at different angles.

2. The method of claim ^ characterized in that said method comprises: generating a plurality of light beams with different wavelengths, which are at least partly directed substantially perpendicularly towards the surface of said paper, measuring the intensity of coherent backscattering from the surface of said paper at different angles for said plurality of light beams, and determining the surface quality of said paper by analyzing the measurement results obtained for said plurality of light beams.

3. The method of claim ^ characterized in that said method comprises: generating a wide-band light beam which is at least partly directed substantially perpendicularly towards the surface of said paper, measuring the intensity of coherent backscatteπng from the surface of said paper at different angles for a plurality of wave-lengths, and determining the surface quality of said paper by analyzing the measurement results obtained for said plurality of wave-lengths beams.

4. An apparatus for testing the surface quality of paper, characterized in that said apparatus comprises: a light source (1 ) for generating a light beam (2), directing means (3) for directing at least a part (4) of said generated beam substantially perpendicularly towards the surface of said paper (5), measuring means (7) for measuring the intensity of coherent backscatteπng (8) from the surface of said paper at different angles, and analyzing means (10) for analyzing the measured intensity at different angles in order to determine the surface quality of said paper.

5. The apparatus according to claim 4, characterized in that said light source (1) is a laser.

6. The apparatus according to claim 4 or 5, c h a r a c t e r i z e d in that said directing means (3) include a semitransparent mirror arranged to direct at least a part of said light beam (2) towards the surface (5) of said paper and to allow the coherent backscattering (8) to pass through to reach said measuring means (9).

7. The apparatus according to any one of claims 4 to 6, characterized in that said apparatus comprises: a detector (7) arranged to measure the intensity of the light beam (2) generated by the light source (1), and adjustment means for adjusting the light source (1) in response to the measured intensity in order to obtain a light beam with constant intensity.

8. The apparatus according to any one of claims 4 to 7, characterized in that said apparatus comprises: a first polarizer (12) arranged on the travel path of the beam (2) between the light source (1) and the directing means (3), and a second polarizer (13) arranged on the travel path of the coherent backscatter (8) between the directing means (3) and the measuring means (9).

9. The apparatus according to any one of claims 4 to 8, characterized in that said measuring means (9) include a CCD element.

10. The apparatus according to any one of claims 4 to 9, characterized in that said light source produces a wide-band (1) light beam, said apparatus includes a multi-filter means (14) with a plurality of filters, each filter being adapted to allow a filter specific wavelength of the coherent backscattering (8) to pass through, said measuring means (9) is arranged to measure the intensity of coherent backscattering (8) passing through said multi-filter means (14) separately for each wavelength, and said analyzing means (10) is arranged to analyze the the measured intensity at different angles for different wavelengths in order to determine the surface quality of said paper.