FI117834B - Quality testing of a paper surface - Google Patents

Description

117834

FIELD OF THE INVENTION This invention relates to the testing of paper surface quality. The invention 5 can at least be utilized to test the printability properties of paper.

2. Description of the Related Art

Printability is very important for paper mills and printers. However, characterizing the surface structure of paper is challenging. A prior art solution for measuring the surface quality of paper is provided if the plate is arranged on the surface of the paper. This plate contains holes through which I

the ground is blown towards the surface of the paper. When the force by which the plate is pressed against the paper is kept constant by adjusting the air blown towards the paper surface, the amount of air leaking between the paper surface and the surface of the plate depends on the roughness of the paper surface. It is thus possible to determine the surface quality of the paper by measuring the amount of air blown out of the holes in the sheet.

One problem with the prior art solution described above is the accuracy of the measurements. Pores with a size in the pm range cannot be taken into account in these measurements, although they are important in evaluating the printability of the paper.

SUMMARY OF THE INVENTION »· **" * The object of the present invention is to solve the weakness described above and

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to provide a solution that enables the quality of the paper surface to be measured * · .. ** in such a way that the properties of the paper can be better defined than in prior art solutions. These and other objects of the present invention are achieved by the method of independent claim 1 and the apparatus of independent claim 4.

: *** In the present invention, coherent backscatter utilization • · ·. ** ·. to test the surface properties of the paper. By measuring the intensity of coherent backscattering at different angles, it is possible to obtain measurement results with much better accuracy than before. The solution enables measurement results that take into account the actual particles or structure variations in the magnitude of the incident light wavelength. Particle sizes, average particle distances, or pores on the surface of the paper to be tested enhance reflection near the backscatter angle due to interfere, so that, for example, the porosity of a paper coating can be taken into account when determining surface quality. .

Preferred embodiments of the method and apparatus of the invention are set forth in dependent claims 2 to 3 and 5 to 10.

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Brief description of the figures

The present invention will now be described in more detail by way of example and with reference to the accompanying drawings, in which: Figure 1 is a flowchart illustrating a first preferred embodiment of the invention, Figure 2 illustrating a first preferred embodiment of the invention, Figure 3 illustrating coherent backscatter the apparatus, Fig. 5 is a block diagram illustrating another preferred embodiment of the invention, Figs. 6 and 7 illustrate measurement results obtained with the apparatus of Fig. 5, and Fig. 8 is a block diagram illustrating a third preferred embodiment of the invention.

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Description of Preferred Embodiments.

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The method of Figure 1 can advantageously be used to test the surface quality of a paper ** "* paper to determine the printability characteristics of that paper.

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In block A, a light beam is produced by a light source such as a laser. In block B, this light source is directed toward the surface of the paper. Alignment may be accomplished, for example, by a partially permeable mirror which directs at least a portion of the beam substantially perpendicularly to the surface of the paper. For m ·]. 30 to provide a beam of substantially constant intensity, it is possible, if necessary, to arrange the detector on one side of the partially transmissive mirror.

* This allows you to obtain a measurement of intensity, and use this result to adjust the light source to achieve a constant intensity.

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In block C, the coherent backscattering intensity is measured at different angles. This measurement 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 may be used to carry out the method described with reference to Figure 1.

The light beam 2 is produced by a light source 1, which is preferably a laser. The beam is directed toward the alignment means 3, which is preferably a partially transmissive mirror (beam splitter). This orientation means orientates at least a portion of the radius of part 4 substantially perpendicular to the surface of the paper 5. The second part 6 of the beam passes through the partially penetrating mirror to the detector 7. The detector measures the intensity of the beam 6. The measurement result is used to adjust the light source 1 to provide a constant intensity to the beam 2. The adjustment means may be integrated with the light source. It will be appreciated that the detector 7 is not necessary if the light source 1 can be adjusted in some other way to produce a light beam of substantially constant intensity.

The apparatus shown in Figure 2 also includes a measuring means 9 for measuring the intensity of the coherent back scatter 8 at different angles from the surface. The surface properties of the paper 5 have an effect on backscatter. This is utilized in the present invention by employing measuring means to provide a measurement result that shows coherent backscattering intensity at various angles, as described in connection with Figure 4. The measuring means 9 can be practically implemented as a camera comprising a CCD (Charge Coupled Device) element, whereby the analyzing means can be implemented in circuits, in a program or in a combination thereof.

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The measurement result obtained with the measuring means 9 is analyzed in the analysis interval) ···. they 10 for determining the surface properties of the paper 5. Analysis tools • · 10 can be implemented, for example, with a computer program and a PC (Personal Computer). 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 may be implemented as part of: a paper making machine that continuously measures and analyzes the quality of the paper surface produced. If the analysis shows that a predetermined quality level * is no longer achieved, an alarm is triggered.

• · · ···! 35 If the goal is to make measurements with a narrowband light beam, this can be accomplished by using a broadband light source along with a filter which only permits passage of a portion of the beam of the desired wavelength. Thus, the filter may be disposed between the light source 1 and the guide means 3 in Figure 2. Alternatively, the filter may be disposed between the guide means 3 and the measuring means 9 in Figure 2.

Figure 3 illustrates the coherent backscattering that occurs when, for example, the apparatus of Figure 2 is used. Figure 3 shows two incoming waves 4 'and 4 "substantially perpendicular to the surface of paper 5. The paths of these waves are illustrated in the figure. 11 that are on the paper.

In the figure, the backscattered waves 8 'and 8 "have the same path length and are in the same phase. The result is a structural interference that can be detected by measurements made with the apparatus of Figure 2.

Figure 4 illustrates the measurement results obtained with the apparatus of Figure 2. The peak of coherent backscatter is clearly visible among the other "nor-15 paint" background scattering marked in relative intensity 1 in Figure 4.

The shape and height of the visible peak give an indication of the surface properties of the paper. The angular width of the peak depends on the volume density of the scatterers (particles or pores). More specifically, the angle width is relative to the 20 light / average free path in the small particle medium. The average free passage for spherical particles is the volume density v, the size of the individual particles, and the extinction efficiency: ***: qe (extinction cross section divided by the geometric cross section of the particle - + * *.

. la) function, / = (4a) / (3v </ c). For non-absorbent scatterers, the half-width at the half-maximum of the coherent backscatter peak a is roughly a = 0.6 (1-g) / (k * l), where g is the single-scattering asymmetry parameter and k = 2π / λ is the wave number. [HI Thus, the peak width depends on the physical properties of the irregularities, for example, * · ** 'narrows as the average free path increases and with the increasing asymmetry parameter. For example, it is possible to define limits for peak angle width and peak intensity, and use these limits to determine whether or not the paper quality is good enough for printing. : there.

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Figure 5 is a block diagram illustrating another preferred embodiment of the invention. embodiment. The apparatus shown in Figure 5 is very similar to that shown in Figure 2. Thus, the embodiment of Figure 5 will be described mainly by "**: showing the differences between the two embodiments.

117834 s

In Figure 5, the first polarizer 12 is disposed in a beam path between the light source 1 and the orientation means 3. A second polarizer 13 is disposed on the path of the coherent backscatter 8 between the orientation means 3 and the measuring means 9. Otherwise, the apparatus of Figure 5 corresponds to that of Figure 5 2. The reason for the use of the first and second polarizers 12 and 13 is that practical experiments have shown that they allow for improved measurement accuracy.

Figures 6 and 7 illustrate exemplary measurement results obtained with the apparatus of Figure 5. In Figures 6 and 7, measurements are made on paper 10 in a wavelength of 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 backscatter peak is higher when co-polarization (co-) is used. polarization); in other words, when the first polarizer 12 and the second polarizer 13 are selected such 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 to that shown in Figure 8. Thus, the embodiment of Figure 8 will be described mainly by showing the differences between the two embodiments.

Figure 8 relates to multi-wavelength measurements. Multi wavelength measurements allow information to be obtained regarding the size distribution of the pores in the paper. Such a multi-wavelength measurement can be implemented in at least two alternative ways: · · · 1) Using the apparatus of Figures 2 or 5. In this case, many- * * *; Narrow-band light beams of different wavelengths are used in turn.

. ···. 25 Coherent backscatter is measured separately for each beam, and the measurement results obtained for the different beams are analyzed to determine surface quality.

2) Using the apparatus of Figure 8. In this case, the broadband ** source light source 1 is used to produce the broadband beam. The multi-filter means 14, such as a rotating filter plate 14 having different bandpass filters 30 in different sectors of the plate, is provided in front of the measuring means 9. Thus, the multi-filter * * * means 14 permits coherent backscattering at different wavelengths to the measuring means at different times. This enables measurement results to be obtained for different wavelengths separately using the same measuring tool 9. If the plate * ·. rotating at high speed, it is possible to obtain measurement results for different wavelengths within a short period of time, in practice almost simultaneously.

• ·: 117834 6 Thus, it is possible, for example, to obtain measurement results at different wavelengths on the same part of the paper surface moving in a paper machine.

In Fig. 8, polarizer 12 and polarizer 13 are shown in dashed lines to indicate that they may be in use or inactive in an embodiment 5 utilizing a rotating plate 14.

It is to be understood that the foregoing description and the accompanying figures are merely intended to illustrate the present invention. It will be apparent to those skilled in the art that the invention may also be modified and otherwise modified without departing from the scope of the invention.

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Claims (10)

A method of testing the surface quality of paper by which method: a light beam (A) is generated, said light beam being at least partially directed (B) substantially perpendicular to said paper surface, characterized in that the properties of paper produced of a papermaking machine is fed (C) and analyzed continuously by measuring at different angles the intensity of coherent backing radiation from a surface of said paper, and the quality of said paper surface is determined (D) by analyzing the coherent backing radiation at different angles. intensity, that is, the tip of the coherent backward radiation, such that the printing properties of the paper surface are determined on the basis of the angular width and intensity of the tip. 15 Method according to claim 1, characterized in that said method comprises: generating a plurality of light beams of different path lengths which are at least partially directed substantially perpendicular to the surface of the paper, measuring the intensity of coherent backside radiation from said paper surface at different angles for said plurality of angles. light rays, and determination of the surface quality of said paper by analysis of measurement quotas obtained for said plurality of light rays. A method according to claim 1, characterized in that said method comprises: • Generating a light beam with a broad band, said light beam being partially directed substantially. perpendicular to the surface of said paper, "measuring the intensity of coherent backward radiation at different angles" for a plurality of path lengths from said paper's surface, and determining the surface quality of said paper by analyzing the measurement citation. which has been obtained for said plurality of longitudinal beams. 4. Device for testing the surface quality of paper, which device ..... comprises: • * *. a light source (1) for generating a light beam (2), directional means (3) for directing at least a portion (4) of said generated beam substantially perpendicular to said light beam. paper surface (5), characterized in that for continuous measurement and analysis of paper produced by a papermaking machine, the apparatus comprises: 5 measuring means (7) for measuring the surface of the paper at different angles measuring the intensity (8) of coherent backsetting radiation, and analyzing means (10) for analyzing the intensity measured at different angles, i.e. the tip of the coherent backward radiation to determine the printing properties of said paper on the basis of the angular width and intensity of the tip. 5. Device according to claim 4, characterized in that said light source (1) is a laser. Device according to claim 4 or 5, characterized in that said directing means (3) contains a partially permeable mirror arranged to direct at least a portion of said light beam (2) to the surface (5) of said paper and to allow the coherent back-ether radiation (8). ) through passage to said measuring instrument. Device according to any one of claims 4 to 6, characterized in that said device comprises: a detector (7) arranged to measure the intensity of the light beam (2) generated by the light source (1), and adjust the light beam (1) in response to the measured intensity to obtain a constant intensity light beam. * Device according to any of claims 4-7, characterized in that said device comprises: ··· · · · a first polarizer (12) arranged on the path of the light beam (2) between ··· · The light source (1) and the directional means (3), and · ♦ a second polarizer (13) arranged on the coherent back-beam radiation. , ·. the path (8) between the directional means (3) and the measuring means (9). »T · Device according to any of claims 4 to 8, characterized in that said measuring means (9) comprise a CCD element. • · · '. * ·; 10. Device according to any of claims 4-9, characterized in that! said light source generates a wide band light beam (1), said device comprising a multi-filter device (14) having multiple filters, each filter being arranged to allow through-passage of coherent baking radiation (8) having a filter-specific path length, said measuring means (9) are arranged to measure separately for each path length the intensity of the coherent baking radiation (8) passing through said multi-filter device (14), and said analyzing means (10) are arranged to analyze the intensity measured at different angles for different path lengths to determine the quality of said paper. ? * ··> • · * · * · · · · 1 φ · ····· · · · · · · · · i- • · · 4 • · · • 1 • · • · · φ · · ·· ·· · · · · · · · ··· · · · · · · · · · · · · · · · · · · · · · · · · · · 1 1 ·