WO2010010229A1

WO2010010229A1 - Imaging arrangement

Info

Publication number: WO2010010229A1
Application number: PCT/FI2009/050569
Authority: WO
Inventors: Juha Parviainen; Ate Korkalainen; Markku MÄNTYLÄ; Heimo Ihalainen; Toni Kuparinen; Hans Nyström; Bo Sundqvist; Markku Markkanen
Original assignee: Lappeenrannan Teknillinen Yliopisto
Priority date: 2008-07-23
Filing date: 2009-06-25
Publication date: 2010-01-28
Also published as: FI20085741A0; FI20085741A; EP2313738A1; FI122448B

Abstract

The invention relates to an arrangement for imaging the topography of the surface of a moving web in illumination that is implemented with such incoherent radiation that has at least a first wavelength component and a second wavelength component, arranged to be received with wavelength-component-specific cameras at the end of a wavelength-component-specific branch of an optical path leading to each camera, which optical path has sequentially in the travel direction of said radiation after the source of radiation said point of the surface to be imaged, a wavelength-selective film for directing said first wavelength component to a first wavelength-specific camera by reflection from said film along a first wavelength-component-specific branch of the optical path and for directing said second wavelength component to a second wavelength-specific camera through said film along a second wavelength-component-specific branch of the optical path, in order to store each image taken by a camera on a storage medium along with the pulsing of the lighting. The invention also relates to a method and a system as well as their use in a printing machine and/or a paper machine for monitoring the composition and/or structure of a paper web.

Description

IMAGING ARRANGEMENT

The invention generally relates to imaging details of a moving film, but specifically to an arrangement for imaging the topography of a specific surface of a moving web as described in the preamble of the independent claim relating to the imaging arrangement. The invention also relates to a method for imaging the topography of a specific surface of a moving web. The invention also relates to a system for imaging the topography of a specific surface of a moving web. The invention also relates to a printing machine comprising an arrangement according to the invention. The invention also relates to a paper machine comprising an arrangement according to the invention. In addition, the invention relates to the use of an arrangement according to the invention for monitoring the composition and/or structure of a web.

Background

Industrial manufacture is nearly always mass production, which is characterized in large quantities of product. For example, the industrial manufacture of a web sets several requirements, in which case in malfunction situations the amount of wasted material depends on the length of the reaction time for stopping erroneous productions and/or for eliminating the error, but also on the production rate.

For example, paper web has conventionally been monitored by a human by visually monitoring the quality and composition of the web. Later, different devices for revealing deviations in the web quality have been developed and therefore the problem in the quantity of erroneous mass production in industrial scale has been attempted to be limited. Deviations in the web properties may be significant, for example, when the web, for example paper, is moved in high speed. Thus, in printing, for creating print the quality and properties of the paper are visible in the print quality, but also in the actual paper manufacturing phase when the paper web conveys paper in the process. Thus, deviations in the composition and/or structure may also change the print, or through insufficient strength due to structural issues it may cause uncontrollable breaking of the paper. These aspects create a prob- lematic situation through stoppages and/or broken devices or their parts, unless caught early enough. Thus, a long delay from the impulse of a detected deviation and the production shutdown taking place in response to the impulse may cause an error production phase of the length of the delay, and therefore, at least mate- rial losses in the form of non-current product.

Patent publication 6,757,065 limits prior art known as such in imaging a moving surface by taking sequential images in order to determine the topography of a surface when using one camera and two light sources. Thus, when imaging a moving surface by means of alternating lighting, radiation from different light sources creates two different images of sequential points. In the publication, the images are, however, from different points of the surface due to its movement in relation to the camera and the alternating illumination of the light sources. In addition, shutter times may become unadvantageous in relation to the resolution of the created im- age.

Microscopic imaging creates significant demands for the greatest resolution of imaging, on one hand through the required depth of field, and on the other hand through diffraction limit. By using a small aperture in the objective (large aperture number), the depth of field can be increased, but at the same time the diffraction caused by the aperture increases problematically. Diffraction relates to the spreading of a point when light travels through the aperture. The magnitude of diffraction depends on the size of the aperture, the focal length of the lens and the wavelength of light. The individual pixels of the camera cells being used are in the mag- nitude of micrometers, and therefore when a point spreads past a pixel size due to diffraction, the imaging resolution suffers. Therefore, the optics must be optimized so that the smallest possible aperture is used, in which case the image is not diffraction-limited, which causes problems in the selection of light source for creating sufficient illumination efficiency. At the same time it is necessary to take into ac- count sufficient light exposure on the camera cell, because when the diameter of the aperture is halved, the luminosity through the aperture drops to a quarter of that of the non-halved aperture. Even though the above problems as such may into some extent be known to a person skilled in the art, the applicant has no knowledge prior to the priority date of the application of such publications, where a solution for arranging sufficient illumination in relation to stopping movement would be discussed more in detail.

Short summary

By means of the invention it is possible to solve prior art problems, including what is described above, especially in order to create sufficiently short but intensive il- lumination to create a still image of a specific point of a moving web simultaneously or substantially simultaneously by imaging on two wavelengths.

The arrangement according to the invention is characterized in what is disclosed in the characterizing part of the independent claim relating to the arrangement. The system according to the invention is characterized in what is disclosed in the characterizing part of the independent claim relating to the system. The method according to the invention is characterized in what is disclosed in the characterizing part of the independent claim relating to the method. The printing machine according to the invention is characterized in what is disclosed in the characterizing part of the independent claim relating to the printing machine. The paper machine according to the invention is characterized in what is disclosed in the characterizing part of the independent claim relating to the paper machine. Other embodiments of the invention are presented in the dependent claims. The embodiments of the invention can be combined as suitable.

By utilizing a pair of cameras in imaging according to the embodiments of the invention, the thus created images can be focused separately to cameras of the pair of cameras, in which case the effect of chromatic aberration is eliminated, but at the same the use of expensive achromatic optics can be avoided. Since according to the embodiments of the invention it is possible to use monochromatic, but at the same time incoherent illumination with sufficient efficiency but in short pulses, advantages over solutions based on broadband light are reached, in which case cross-coupling in a wavelength-specific component of a camera in another camera can be minimized. In an arrangement according to an embodiment of the invention it is possible to use a fixed focal length and a small enough aperture in order to reach depth of field, especially when using an incoherent laser source of an arrangement according to an embodiment of the invention, in which case it is possi- ble to reach illumination efficiencies that are sufficient for even a small aperture, even if the pulse were very short, for example below 250 ns according to an embodiment, but below 50 ns according to another embodiment.

The arrangement according to an embodiment of the invention comprises, for im- aging the topography of a specific point of the surface of a moving web in a illumination that is implemented with such incoherent radiation that has at least a first wavelength component and another wavelength component, arranged to be received with wavelength-component-specific cameras at the end of a wavelength- component-specific branch of an optical path leading to each camera, which opti- cal path has sequentially in the travel direction of said radiation after the source of radiation said point of the surface to be imaged, a wavelength-selective film for directing said first wavelength component to a first wavelength-specific camera by using said film to a first optical event along a first wavelength-component-specific branch of the optical path, and for directing said second wavelength component to a second wavelength-specific camera by utilizing said film to a second optical even along a second wavelength-component-specific branch of the optical path, in order to store each image taken by a camera on a storage medium along with the pulsing of the illumination. According to an embodiment of the invention the incoherence of illumination is arranged by collectively mixing the coherences of sev- eral coherent monochromatic light sources.

In an arrangement according to an embodiment of the invention a set of wavelength-specific laser radiation sources is arranged in order to create said first wavelength component and said second wavelength component with their own set of wavelength-specific laser beam sources, whose phasing and coherence- affecting variable are mixed in order to reach incoherence for each wavelength component. Thus it is possible to arrange several camera pairs to take images wavelength-specifically either from the same point or different points of the web. An arrangement according to an embodiment of the invention comprises radiation sources of said radiation arranged in order to create such set of wavelength components where there are at least two or more wavelength components, in which case said first and second wavelength component are certain ones in said set of wavelength components.

In an arrangement according to an embodiment of the invention, by using laser radiation whose coherence is mixed, it is possible to control the pulsing of the illumi- nation / select pulse properties quite freely on the basis of imaging resolution and/or web rate. According to an embodiment of the invention, the laser radiation sources in each set send radiation that is of the same wavelength, but when examining the radiation caused by their combined effect, its coherence is mixed, even if the individual laser sources were coherent as such.

Thus, in an arrangement according to an embodiment of the invention the frequency of the pulsing is below 100 MHz for a set of laser radiation sources for producing a first wavelength component of radiation of illumination. In an arrangement according to an embodiment of the invention the frequency of the puls- ing is below 1 MHz for a set of laser radiation sources for producing a first wavelength component. In an arrangement according to an embodiment of the invention the frequency of the pulsing is below 0.1 MHz for a set of laser radiation sources for producing a first wavelength component. In an arrangement according to an embodiment of the invention the frequency of the pulsing is below 0.01 MHz for a set of laser radiation sources for producing a first wavelength component.

In an arrangement according to an embodiment of the invention the frequency of pulsing of a laser radiation source belonging to a certain set of laser radiation sources is on at least one of the following frequency areas: 0,001 MHz- 0,03MHz; 0,08MHz- 0,1 MHz; 0,3MHz -0,8MHz; 1 MHz-3MHz; 8MHz-13 MHz; 15MHz- 50MHz; 55MHz-350MHz and one such a frequency area, where the lower limit of the frequency area is different from that of the highest of said limits of frequency areas, but the upper limit is higher than the lower limit, in order to produce another wavelength component, in which case the frequency of the pulsing and the resolution of details of the images may be varied and thus optimized, if necessary, according to the web.

An arrangement according to an embodiment of the invention comprises pulsing means arranged for setting the pulsing frequency of said second wavelength radiation as equal to a from zero deviating integer multiple of said first wavelength radiation.

An arrangement according to an embodiment of the invention comprises phasing means for setting a common frequency of pulsing for radiation of a first wavelength component and for radiation of a second wavelength component to a specific mutual phase by means of a phase difference and/or for affecting the mutual coherence of a set of radiation sources. According to an embodiment of the invention, the phase difference mentioned in the arrangement is set, in practice, to zero.

The phase difference mentioned in the arrangement according to an embodiment of the invention is arranged to correspond to the relaxation time of fluorescence and/or fosforence of a substance remaining in the web during its manufacture, in which case said first wavelength component is selected for tuning said fluorescence and/or fosforence.

In an arrangement according to an embodiment of the invention the duration of the pulse in a state corresponding to illumination on the second wavelength compo- nent is shorter that the duration of the pulse in a state to the illumination on the first wavelength component.

In an arrangement according to an embodiment of the invention the pulsing of radiation associated with a first wavelength is arranged on a different frequency than the pulsing of radiation associated with a second wavelength. In an arrangement according to an embodiment the wavelength-specific camera is arranged for storing an image being stored on a wavelength that in practice spikes only to the environment of a wavelength.

In an arrangement according to an embodiment of the invention the power of each laser radiation source belonging to a set of laser radiation sources arranged for arranging lighting is on at least one of the following power areas: 1 mW-300 mW, 800 mW-1W, 1W-3W, 8W-13W, 10W-55W, 100 W-IkW and one such a power area, where the lower limit of the power area is different from the highest of the limit of said areas, but the upper limit is higher than said lower limit. Thus, it is possible to optimize the lighting that is suitable for properties of the web, but also the shutter time for creating a specific desired resolution for the images.

According to an embodiment of the invention a method for imaging the topography of a specific point of the surface of a moving web in illumination implemented by means of incoherent radiation, in which case said incoherent radiation comprises at least a first wavelength component and a second wavelength component, comprises at least the following phases:

- incoherent illumination is directed along an optical path to a specific point of the surface of the moving web to be imaged,

- said illumination is pulsed with pulsing means for taking an instantaneous still image with each camera arrange at the end of an optical path,

- said illumination is directed from said specific point of the surface to an wavelength-selective film, which is on said optical path, o for creating illumination for distributing a first wavelength component of the used radiation via a first optical event to a first branch of the optical path to a first camera at the end of it, o for creating illumination for distributing a second wavelength component of the used radiation via a second optical event to a second branch of said optical path to a second camera at the end of it,

- an image is stored with said first camera as a first image on a first storage medium, and - an image is stored with said second camera as a second image on a second storage medium.

In a method according to an embodiment of the invention an image is taken along with the frequency of the pulsing of the lighting first with at least a first or a second camera, in which case the next image is located to a part of a specific lighting pulse within a specific delay, which is shorter than said lighting pulse.

In a method according to an embodiment of the invention the image taken with a camera is handled as a pixeled matrix to be added onto a cumulative image, to the value before taking the image of each variable of an elementary unit of which matrix is directed an arithmetic calculation with the value of the variable of the image taken of said elementary unit for forming a new cumulative image. In a method according to an embodiment of the invention the value of the variable of the elemen- tary unit of a cumulative image is standardized by means of a variable that depends on the number of images combined into to cumulative image. In the method according to an embodiment of the invention the value of the variable of the elementary unit is changed on the basis of values of a variable of neighboring elementary units in a specific direction.

A system according to an embodiment of the invention for imaging the topography of a specific point of the surface of a moving web comprises at least two arrangements according to an embodiment of the invention arranged for imaging the topography of the web surface from at least two points of the web.

A conveyor according to an embodiment of the invention for conveying the web comprises an arrangement according to an embodiment of the invention. Thus, the arrangement is arranged at such point of the web conveying track, which precedes the critical point of the track and/or after such a point. According to an embodiment of the invention the conveyor also comprises the track itself for conveying the web.

A printing machine according to an embodiment of the invention comprises an arrangement according to an embodiment of the invention. A paper machine accord- ing to an embodiment of the invention comprises an arrangement according to an embodiment of the invention.

The arrangement according to an embodiment of the invention can be utilized for monitoring the structural evenness of the web, but also additionally or alternatively for monitoring the evenness of the composition of the web.

The arrangement and method according to an embodiment of the invention are intended for imaging a specific point of the topography of a surface a rapidly moving web.

Drawings

The embodiments of the invention are described in the following with reference to figures as part of the description, which figures disclose examples of embodiments of the invention without limiting the invention as such. The same references in different figures refer to the same things, unless separately otherwise disclosed in connection with the figure in question. Thus, the parts and/or phases referred to with a reference do not necessarily need to be exactly the same. Since the nature of the images is to illustrate examples, the geometric figures and/or symbols displayed in them are not necessary scaled in relation to each other, unless otherwise specifically pointed out.

Fig. 1 illustrates an arrangement according to an embodiment of the inven- tion for imaging a moving surface with two cameras,

Fig. 2 illustrates a paper machine according to an embodiment of the invention,

Fig. 3 illustrates a printing machine according to an embodiment of the invention, Fig. 4 illustrates a web manufacturing unit according to an embodiment of the invention, and

Fig. 5 illustrates a method according to an embodiment of the invention for imaging a moving surface with two cameras, Detailed description of the invention

Figure 1 shows an example of an arrangement according to the invention, wherein the light source 101 may comprise several, even hundreds of coherent light sources in order to mix the coherence and to avoid the speckle phenomenon. The point of the surface to be imaged may be defined by a template and/or a lens 103, the form of which template and/or lens does not necessarily have to be limited to the one shown in figure 1. From the point being imaged light is transferred to a di- chromatic film 104, which film distributes the illumination wavelength-specifically along a route corresponding to the wavelength of the light source to a branch of an optical path and to a camera 102 located at the end of it. Even though figure 1 shows such an optical path where the wavelength-specific branch is both at the beginning and the end of the path, according to an embodiment of the invention the light sources may be located in a mixed manner. In addition it is noted that even though only one pair of light sources 101 has been drawn for illustration purposes, according to an embodiment of the invention there may be several light sources 101. The light source 101 is according to an embodiment of the invention monochromatic, but it is arranged as incoherent. This is implemented, for exam- pie, by utilizing a set of coherent light sources, whose coherence is intentionally mixed in order to prevent the speckle phenomenon, but still for creating a sufficient illumination efficiency for short pulses for taking a still image. According to an embodiment of the invention the light source 101 comprises a laser light source, which is pulsed.

The arrangement according to an embodiment of the invention can be implemented, for example, as a measuring device, but also as a module-like solution. Thus, the module may comprise other parts as well, which, as can be directly seen from prior art, may comprises parts relating to web processing, for example a con- veyor. According to an embodiment of the invention, said module (100) can be utilized in the printing machine as a part of it, in which case the printing machine is a printing machine (200) according to an embodiment of the invention (Figure 2). According to an embodiment of the invention, said module can alternatively be util- ized in a paper machine as a part of it, in which case the paper machine is a paper machine (300) according to an embodiment of the invention (Figure 3). According to an embodiment of the invention, said module can alternatively be utilized in a web manufacturing unit as a part of it, in which case the web manufacturing unit is a web manufacturing unit (400) according to an embodiment of the invention (Figure 4).

The web can, according to the following embodiments of the invention, be pulp, paper, textile, plastic film, expanded metal, composite material, thin metal film, wood veneer, or some combination of mutually compatible materials of the above. In addition, according to an alternate, extreme embodiment of the invention the web can be considered as the surface of a rigid body. According to such an embodiment of the invention said surface is a sawn surface, a planed surface, a milled surface, a forged surface, a rolled surface, an etched surface. Said rigid body can be, for its suitable parts or entirely, of wood, metal, ceramics, semiconductor, plastic or a substance combining the above.

Figure 5 illustrates the method according to an embodiment of the invention, for example, in an arrangement according to figure 1. The method for imaging the to- pography of a specific point of a moving web surface illumination implemented by means of incoherent radiation, which comprises at least a first wavelength component λ±ε and a second wavelength component λ, comprises the following phases:

- directing 501 the incoherent illumination along an optical path OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b to a specific point of the moving web sur- face to be imaged,

- said illumination is pulsed 501 with pulsing means for taking an instantaneous still image with each camera 102 arranged at the end of an optical path,

- said illumination is directed 502 from said specific point of the surface to a wavelength-selective λ, λ±ε film, which is on said optical path OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b, o for creating illumination for distributing 503 a first wavelength component λ+ε of the used radiation via a first optical event, such as reflec- tion, to a first branch OP.4a of the optical path to a first camera 102 at the end of it, o for creating illumination for distributing 503 a second wavelength component λ of the used radiation via a second optical event to a second branch OP.4b of said optical path to a second camera 102 at the end of it,

- an image is stored 504 with said first camera as a first image on a first storage medium, and

- an image is stored 504 with said second camera as another image on another storage medium.

Said phases can be partly overlapping in a manner that is reasonable from the point of view of directing, pulsing and other practical implementation of illumination. According to an embodiment of the invention, in the method an image is tak- en along with the frequency of the pulsing of the illumination with at least a first or a second camera. According to an embodiment of the invention it is therefore possible to create an image that has resolution on the microscopic level with an arrangement of one or more camera pairs.

According to an embodiment of the invention the image taken with a camera is handled 505 as a pixeled matrix to be added onto a cumulative image, to the value before imaging of each variable of an elementary unit of which matrix is directed an arithmetic calculation with the value of the variable of the image taken of said elementary unit for forming a new cumulative image. Thus, the image can also be processed in other ways, for example for detecting aspects affecting the feedback. Thus, an advantage is reached in that the controller can be presented with an image where deviations are shown better, in which case it is easier for the observer to stop erroneous production. According to an embodiment of the invention it is also possible to search certain aspects from the image for forming a feedback sig- nal, for stopping erroneous productions as early as possible, but in an embodiment for forming such a feedback signal, by means of which it is possible to control the web manufacture for changing the structure and/or composition of the web. Thus, in the method it is possible to process the image or its part in such a manner that a cumulative image is formed, in which case the value of the variable of its elementary unit is normed by means of a variable depending on the number of images combined in the cumulative image. According to an embodiment of the in- vention it is also possible to form an image, where the value of the variable of the elementary unit is changed 506 on the basis of values of a variable of neighboring elementary units in a specific direction.

Example 1. The arrangement 100 in the example is arranged as a device for microscopically imaging the surface of a moving target. In the example the surface 105 of the web is diffusively reflecting material. The surface is illuminated by a light source 101 , OP.1a, OP.1 b with oblique plane-wave-like light. In the example stopping the rapid movement is performed by a short and high-intensity light pulse, in which case special cameras are not necessarily needed.

As mentioned, in the embodiment example of the invention the coherence problem is solved by using in the device, for example, a diode laser structure, which may comprise even hundreds of individual lasers, which are directed in slightly different phases, in which case the coherence is intentionally mixed and therefore the speckle phenomenon is prevented. The energy of individual lasers can be directed to the same light fiber, in which case a point-form light source is created for forming an oblique photo of the surface. With the thus created incoherent light the speckle phenomenon is not a problem in image formation. In the device imaged in the example, commercial pulsed diode lasers by Cavitar Oy were used, with wavelengths of 690 nm and 804 nm. The images created by different wavelengths were directed to different cameras via a dichromatic mirror. The minimum pulse length was 42 ns. According to an embodiment of the invention it is possible to use the wavelengths of both the visible and the IR -area. By selecting ε in a different man- ner than above (where the deviation was 114 nm), according to each implementation variant of the invention it is possible to vary the illumination within visible light, but according to another embodiment variant at least one of the wavelengths is selected outside visible light for implementing illumination. Thus, the wavelengths used for an individual pair of cameras can both be in an embodiment in the IR area, while in a second embodiment in the UV area, but in a third embodiment the first on the IR area and a second in the UV area.

Example 2.

The moving web is textile, where deviations in the fiber structure are examined. An arrangement according to an embodiment of the invention is used for evaluating the roughness of the surface. As soon as the surface has been formed, images are taken by an arrangement according to the invention. By a second arrangement according to the invention images are taken of the surface when the coloring agent or other composite of the surface has been brought into contact with the surface. Said arrangements form a system according to an embodiment of the invention as an example, the use of which is not to be seen as limited solely to a textile web and/or two arrangements in a system.

Example 3.

The moving web is paper, whose printing results are to be evaluated before the paper is printed. Thus, with the imaging according to an embodiment of the invention are taken images, on the basis of which the suitability of the fine structure of the paper for a specific printing is determined. According to an embodiment example of the invention the arrangement according to an embodiment of the invention is a part of a printing machine. According to an embodiment of the invention a printing machine comprises an arrangement according to an embodiment of the invention.

Example 4.

The moving web is manufactured by a web manufacturing unit and with the arrangement according to an embodiment of the invention the web is imaged, information relating to its structure and/or its composition, for example the distribution of filler, adhesive and/or coloring agent on its surface. Example 5.

The moving web is monitored in a processing phase related to its manufacture. In addition, the web may be monitored while it is discharged for use, for example when taken off a roll, when used and/or when moved to another roll. The monitor- ing is performed with equipment according to an embodiment of the invention. Thus, the apparatus comprises two incoherent light sources with different wavelengths (λ, λ±ε). The light source pair and camera pair are on the same level. According to an embodiment of the invention the apparatus may also comprise a second corresponding light source pair, however without limiting the selection of the wavelength of the light sources of the pair itself as such, nor the number of the light source pairs. In an embodiment, which comprises a first light source pair and a second light source pair, the use is affected by how many branches of optical path are arranged on the optical path after a dichromatic mirror to lead to a camera. Thus, the response of said mirror for penetrating wavelength of radiation has an effect on the more individual placement of cameras and light sources.

When the topography of the web is examined as defined by a camera pair, for calculating the gradient of the surface being measured it is possible to use a photometric stereo known as such (R. Woodhamin (1980), Photometric Method for De- termining surface Orientation from Multiple Images, Optical engineering vol. 19, no. 1 , pp. 139-144 suitable parts). Hanson and Johansson (P. Hansson, P. Jo- hannson (2000), Topography and reflectance analysis of paper surface using a photometric stereo method, Optical engineering , Vol. 39, no.9, pp 2555-2561 ) disclose formulas for a 2-light-photometric stereo:

_{( 1} ) ^df ₌ ^{l 1}I - ¹I dx tan(σ) I₁ + 1₂ wherein /i is intensity to a first camera and I₂ is intensity to a second camera and σ is the angle of incidence of light in relation to the normal of the surface of the web for both light sources. Thus, the topography of the surface can be calculated by in- tegrating the gradient filed by the Hanson and Johansson method by using integration on Fourier level combined with a Wiener filter. The disclosed example does not as such limit the known calculation methods to be used in connection with embodiments of the invention.

Example 6 In the setting of the apparatus according to example 5, four light sources are used, but alternatively differing from example 5 only one camera. Thus, a first light source pair (which comprises a first and a second light source) and a camera are on the same first level and a second light source pair (which comprises a third and fourth light source) and a camera are on the same level so that said first and sec- ond level are perpendicular to each other. The optical axis of the camera is in the example parallel to the transversal of the levels and at the same time also parallel to the normal of the surface of the web. According to an embodiment of the invention the transversal forms an angle differing from a right angle in relation to the normal of the surface of the web.

According to an embodiment of the invention in example 6 the surface is illuminated alternately by four light sources, but very rapidly for creating still-like images and therefore in practice from the same point of the surface. The intensity scattered from the surface is measured by a camera, in which case four images of the surface are created, which are illustrated measured as intensities /W₄ from each point of the image, by using the corresponding light source.

For calculating the gradient of the measured surface is used the calculation according to example 5 as is suitable. For four-light-photometric stereo, where light sources are therefore every 90°, as seen from the direction of the normal of the surface of the web towards the web. The x and y gradients of the surface can thus be calculated point by point:

₍2₎ ^df - ² W₂ dx tan(σ) /, + 1₂ + /₃ + I₄

(3) £ = . ^{2 7}^ dy tan(σ) I₁ +I₂ +I₃ + 1₄ The topography of the surface is calculated by integrating the gradient field either by means of Hanson and Johansson's method by utilizing integration on Fourier level combined with a Wiener-filter and after that by combining the gradient fields with symmetrical weights (T. Kuparinen, V. Kyrki, J.Mielikainen and P. Toivanen (2007): On surface reconstruction from gradient fields. In Proc. of IEEE ICIP pages Il 545-548, September 16-19.), or by Wiener filtering the gradient fields and then integrating the filtered gradient fields by a method disclosed by Frankot and Chel- lappa (R. Frankot, R. Chellappa (1988): A method for enforcing integralibility in shape from shading algorithms, IEEE Trans. Pattern Anal. Machine Intell., vol. 10, no. 4 pp 435-446).

Example 7.

According to an embodiment of the invention the apparatus is according to example 6, but differing from example 6 in that the images are taken by the cameras of the moving web simultaneously or substantially simultaneously. According to a variant embodiment of the invention the images of a first pair of cameras are taken simultaneously by the pair of cameras, but at different times than the images of a second camera pair, which are also taken simultaneously by the cameras of the camera pair.

Example of a set of other embodiments In an embodiment of illumination created by leds, the length of the pulse is not a limiting aspect, but the energy received may be insufficient for illuminating the camera cell with a short pulse, especially if the surface of the web absorbs light heavily. In an embodiment utilizing stroboscopes, however, enough energy as such may be received, but the minimum pulse frequency of stroboscopes is limited to one microsecond. The embodiments using lasers as such have the disadvantage of coherence of laser light, which causes a speckle phenomenon. Speckle ruins the image received of the surface. Thus, despite the efficiency known as such and the rapidness as a light source, the laser alone is not advantageous as such, especially in the case of a rapidly moving surface.

According to an embodiment of the invention the light from the light sources is distributed to optical branches to be used in taking images. The distribution takes place optically through a first and a second optical event. According to an embo- diment of the invention said first optical event is one of the following: reflection, penetration, absorption, blocking. According to an embodiment of the invention said second optical event is one of the following: reflection, penetration, absorption, blocking. During blocking the light of a light source is prevented from entering a corresponding camera. According to an embodiment of the invention the blocking can be implemented by keying the light source, in which case, for example, an incoherent set of laser radiation sources is arranged on a specific wavelength to not illuminate for the time period of the blocking. By keying the light source by controlling it electronically it is therefore possible to reach higher rates than by means of mechanical slot etc. corresponding systems by breaking the light beam, and in relation to those implemented by means of rotating aperture and slot plates, mechanical vibrations can therefore be decreased for their part. The block can be used also for creating phasing according to an embodiment, for example in relation to afterglow and/or detecting it.

Said optical events follow, for example, on the surface of a dichromatic mirror, except for practical loss, the principle of conservation of energy for each illumination so that the sum of energy of radiation entering a certain point of radiation flux is the same as the energy that exits it through penetration, reflection and/or absorp- tion.

According to an embodiment of the invention, only one camera is simultaneously used for imaging. According to an embodiment of the invention the wavelengths of radiation of light sources are selected to correspond, for example, to the red (R), green (G) and blue (B) of visible light, in which case the RGB-sensitive pixels in the cell of the camera see the corresponding light of wavelength. Thus, by reading the same cell, but by using only certain pixels, it is possible to collect simultaneously at least three different images from the same camera. The fourth light source may function, for example, on the infrared area.

According to yet another embodiment of the invention the second camera of a camera pair is an RGB camera, which is arranged to take three images, but the first camera is arranged to operate, for example, on infrared area. The invention is described by examples above. It is, however, obvious that these examples are meant for understanding the invention and that the scope of protection of the invention may vary within the scope of the following claims.

Claims

Claims:

1. An arrangement for imaging the topography of a specific point of a surface of a moving web in such illumination, which is implemented by such incoherent radiation that has at least a first wavelength component (λ±ε) and a second wavelength component (λ), characterized in that in the arrangement

- said first wavelength component (λ±ε) and said second wavelength component (λ) are arranged to be received by a wavelength-specific camera (102) at the end of a wavelength-component-specific (OP.4b, OP4a) branch of an optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) leading to each camera, which optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) comprises said point of a surface (105) to be imaged after a source (101 ) of radiation in the travel direction of said radiation, and that the arrangement comprises - a wavelength-selective film (104) o for directing said first wavelength component (λ±ε) to a first wavelength-specific camera (102) by utilizing said film (104) for a first optical event along a first wavelength-component-specific branch (OP.4a) of an optical path and o for directing said second wavelength component (λ) to a second wavelength-specific camera (102) by utilizing said film (104) for a second optical event along a first wavelength-component-specific branch (OP.4b) of an optical path,, o for storing each image taken by means of a camera (102) on a sto- rage means along with the pulsing of the illumination.

2. The arrangement according to claim 1 , characterized by a set of wavelength-specific laser radiation sources arranged in order to create said first wavelength component and said second wavelength component with their own set of wavelength-specific laser radiation sources, whose phasing and coherence- affecting variable are mixed in order to reach incoherence for each wavelength component.

3. The arrangement according to claim 2, wherein said radiation sources of radiation are arranged in order to create such a set of wavelength components in which set there are at least two or more wavelength components, in which case said first and second wavelength component are certain ones in said set of wave- length components.

4. The arrangement according to claim 3, wherein the frequency of the pulsing is below 10 MHz for a set of laser radiation sources for producing a first wavelength component.

5. The arrangement according to claim 4, wherein the frequency of the pulsing is below 1 MHz for a set of laser radiation sources for producing a first wavelength component.

6. The arrangement according to claim 5, wherein the frequency of the pulsing is below 0.1 MHz for a set of laser radiation sources for producing a first wavelength component.

7. The arrangement according to claim 6, wherein the frequency of the puls- ing is below 0.01 MHz for a set of laser radiation sources for producing a first wavelength component.

8. The arrangement according to any of the preceding claims, wherein the frequency of pulsing of a laser radiation source belonging to a certain set of laser radiation sources is on at least one of the following frequency areas:

0,001 MHz- 0,03MHz; 0,08MHz- 0,1 MHz; 0,3MHz -0,8MHz; 1 MHz-3MHz; 8MHz-13 MHz; 15MHz-50MHz; 55MHz-150MHz and a such frequency area where the lower limit of the frequency area is different from the highest of said frequency area limits, but the upper limit is higher than said lower limit, for producing a second wave- length component.

9. The arrangement according claim 8, characterized by pulsing means arranged for setting the pulsing frequency of said second wavelength radiation as equal to and integer multiple of said first wavelength radiation which differs from zero.

10. The arrangement according to claim 1 , characterized by phasing means for setting the mutual frequency of pulsing for radiation of a first wavelength component and radiation of a second wavelength component to a specific mutual phase by means of phase difference.

11. The arrangement according to claim 10, wherein said phase difference is set in practice to zero.

12. The arrangement according to claim 10, wherein said phase difference is arranged to correspond to the relaxation time of the fluorescence and/or fosfor- ence of a substance remaining in the web during its manufacture, in which case said first wavelength component is selected for tuning said fluorescence and/or fosforence.

13. The arrangement according to claim 13, wherein the duration of the pulse in a state corresponding to the illumination on the second wavelength component is shorter that the duration of the pulse in a state corresponding to the illumination in the first wavelength component.

14. The arrangement according to any of the preceding claims, wherein the pulsing of radiation associated with a first wavelength is arranged on a different frequency than the pulsing of radiation associated with a second wavelength.

15. The arrangement according to any of the preceding claims, wherein the wavelength-specific camera is arranged to the environment of only one wavelength for storing an image stored in practice by spiking wavelength.

16. The arrangement according to any of the preceding claims, wherein the effect of each laser radiation source belonging to a set of laser radiation sources arranged for arranging illumination is on at least one of the following power areas: 1 mW-300 mW, 800 mW-1W, 1W-3W, 8W-13W, 10W-55W, 100 W-IkW and one such a power area, where the lower limit of the power area is different from the highest of the limits of said areas, but the upper limit is higher than said lower limit.

17. The arrangement according to any of the preceding claims, wherein the arrangement is arranged for imaging the topography of a specific point of the surface of a rapidly moving web.

18. A method for imaging the topography of a specific point of the surface of a moving web in illumination implemented by means of incoherent radiation, characterized in that said incoherent radiation comprises at least a first wavelength component (λ+ε) and a second wavelength component (λ), in which case the method comprises at least the following phases:

- directing (501 ) the incoherent illumination along an optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) to a specific point of the moving web surface to be imaged,

- pulsing (501 ) said illumination with pulsing means for taking an instantaneous still image with each camera (102) arranged at the end of an optical path, - directing (502) said illumination from said specific point of the surface (105) to a wavelength-selective (λ, λ±ε) film (104), which is on said optical path (OP.1a, OP.1b, OP.2, OP.3, OP.4a, OP4b), o for creating illumination for distributing (503) a first wavelength component (λ±ε) of the used radiation via a first optical event to a first branch (OP.4a) of the optical path to a first camera (102) at the end of it, o for creating illumination for distributing (503) a second wavelength component (λ) of the used radiation via a second optical event to a second branch (OP.4b) of said optical path to a second camera (102) at the end of it,

- storing (504) an image with said first camera as a first image on a first storage medium, and - storing (504) an image with said second camera as another image on another storage medium.

19. The method according to claim 18, wherein an image is taken along with the frequency of the pulsing of the illumination with at least a first or a second camera.

20. The method according to claim 18, wherein the image taken with a camera is handled (505) as a pixeled matrix to be added onto a cumulative image, to the value before imaging of each variable of an elementary unit of which matrix is directed an arithmetic calculation with the value of the variable of the image taken of said elementary unit for forming a new cumulative image.

21. The method according to claim 20, wherein the value of the variable of the elementary unit of a cumulative image is standardized by means of a variable that depends on the number of images combined into the cumulative image.

22. The method according to claim 21 , wherein the value of the variable of the elementary unit is changed (506) on the basis of values of a variable of neighbor- ing elementary units in a specific direction.

23. The method according to any of the preceding claims, wherein the topography of a specific point of the surface of a rapidly moving web is imaged.

24. A system for imaging a specific point of the surface of a moving web, characterized in that

- the system comprises at least two arrangements (100) for imaging the topography of the surface (105) of a moving web in at least two points in such illumination, which is implemented in at least two points to be imaged with such incoherent radiation that comprises at least a first wavelength component (λ±ε) and a second wavelength component (λ),

- which said at least first wavelength component and second wavelength component are in each at least two points to be imaged arranged to be re- ceived by a wavelength-component-specific camera (102) at the end of a wavelength-component-specific (OP.4b, OP4a) branch of an optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) leading to each camera, which optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) comprises in the travel direction of said radiation after the source (101 ) of radiation said one point of surface (105) to be imaged of said at least two points to be imaged, and that the system comprises - a wavelength-selective film (104) for each at least two points to be imaged o for directing said first wavelength component (λ±ε) to a first wavelength- specific camera (102) by utilizing said film (104) for a first optical event along a first wavelength-component-specific branch (OP.4a) of an optical path and o for directing said second wavelength component (λ) to a second wavelength-specific camera (102) by utilizing said film (104) for a second opt- ical event along a first wavelength-component-specific branch (OP.4b) of an optical path, o for storing an image taken by means of a first and a second wavelength- component-specific camera (102) forming a camera pair corresponding to each at least two points to be imaged, on a storage means along with the pulsing of the illumination.

25. The system according to claim 24, wherein the system is arranged for imaging the topography of a specific point of the surface of a rapidly moving web.

26. A track for conveying a web, characterized in that the track comprises an arrangement for imaging the topography of a specific point of a surface of a moving web in such illumination, which is implemented by such incoherent radiation that has at least a first wavelength component (λ±ε) and a second wavelength component (λ), in which arrangement - said first wavelength component (λ+ε) and said second wavelength component (λ) are arranged to be received by a wavelength-specific camera (102) at the end of a wavelength-component-specific (OP.4b, OP4a) branch of an optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) leading to each camera, which optical path (OP.1a, OP.1 b, OP.2, OP.3, OP.4a, OP4b) comprises said point of a surface (105) to be imaged after a source (101 ) of radiation in the travel direction of said radiation, and that the arrangement comprises - a wavelength-selective film (104) o for directing said first wavelength component (λ±ε) to a first wavelength-specific camera (102) by utilizing said film (104) for a first optical event along a first wavelength-component-specific branch (OP.4a) of an optical path and o for directing said second wavelength component (λ) to a second wavelength-specific camera (102) by utilizing said film (104) for a second optical event along a second wavelength-component- specific branch (OP.4b) of an optical path o for storing each image taken by means of a camera (102) on a sto- rage means along with the pulsing of the illumination.

27. The track according to claim 26, characterized in that the track is arranged in a paper machine (300).

28. The track according to claim 26, characterized in that the track is arranged in a printing machine (200).

29. The track according to claim 26, characterized in that the track is arranged in a web manufacturing unit (400).

30. The use of the arrangement according to claim 1 in monitoring the structural evenness of the web.

31. The use of the arrangement according to claim 1 in monitoring the even- ness of composition of the web.