WO2014033356A1

WO2014033356A1 - Security-marked web

Info

Publication number: WO2014033356A1
Application number: PCT/FI2012/050842
Authority: WO
Inventors: Pasi Virtanen; Teuvo LEPPÄNEN; Petteri Lannes
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-08-30
Filing date: 2012-08-30
Publication date: 2014-03-06
Also published as: EP2890846B1; CN104884707B; CN104884707A; EP2890846A4; EP2890846A1

Abstract

A method of producing markings on a web (WEB1) comprises: - forming a first base portion (DOT0) in a primary web (WEB0) by locally altering at least one optical property of the primary web (WEB0) with a first laser beam (LB1), and - forming a covered web (WEB1) by covering and/or impregnating the primary web (WEB0) with an additive (AD1) after the first base portion (DOT0) has been formed, wherein the primary web (WEB0) comprises cellulose fibers, the covered web (WEB1) comprises an optically detectable first altered portion (DOT1), the first altered portion (DOT1) comprises the first base portion (DOT0) and an amount of additive (AD1) bound to the first base portion (DOT0), and the composition of the additive (AD1) has been selected such that exposing the covered web (WEB1) to a second laser beam (LB2) causes an optically detectable alteration of the additive (AD1) contained in the covered web (WEB1) in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity (IMIN,1) needed to cause optically detectable alteration of the primary web (WEB0) contained in the covered web (WEB1).

Description

SECURITY-MARKED WEB

FIELD OF THE INVENTION

The present invention relates to producing a paper or cardboard web, which comprises one or more security markings.

SUMMARY

It is known that a paper document may comprise a watermark in order to improve visual appearance of the document or in order to make counterfeiting of the document more difficult.

SUMMARY

An object of the invention is to provide a method for producing markings on a paper or cardboard web. An object of the invention is to provide paper or cardboard web, which comprises one or more markings.

According to a first aspect of the invention, there is provided a method for producing paper or cardboard web according to claim 1 .

According to a second aspect of the invention, there is provided a paper or cardboard web according to claim 27.

A layer of a primary web may be marked with a laser beam. The layer and the marking may be subsequently covered and/or impregnated with a protective additive in order to provide a covered web, which comprises a security marking. The covered web may have a capability of providing an optical indication if someone has modified the security marking, or attempted to modify the security marking at a later stage. The security marking may comprise one or more covered altered portions. Each covered altered portion may be formed by locally altering the structure and/or chemical composition of a primary web by a laser beam, and subsequently covering and/or impregnating the altered portions and the primary web with the additive. The altered portions of the primary web may be called as the base portions.

The primary web contained in the covered web may have been treated with the additive such that a laser beam cannot interact with the primary web without passing through the additive. Thus, the additive may act as a guard so that the primary web cannot be marked with a laser through the additive without causing a noticeable telltale marking on the additive. The composition of the additive may be selected such that exposure to the laser beam causes an optically detectable alteration of the additive. The composition of the additive may be selected such that the spectral properties of the additive are changed when the additive is exposed to an intense laser beam. The additive may e.g. change color and/or it may lose its fluorescent properties.

The covered web may comprise a first base portion, which was formed by a first laser beam before covering and/or impregnating with the additive. Thus, the additive bound to the first base portion has not been altered due to exposure to the first laser beam.

The primary web contained in the covered web may be buried beneath the additive such that the buried primary web can be altered only by using a second laser beam or by bringing a heated stamp into contact with the covered web.

However, if someone tries to produce a second (fake) base portion by directing the second laser beam to the covered web, the second laser beam inevitably interacts also with the additive. Thus, an attempt to produce further markings at a later stage is likely to cause an optically detectable alteration of the additive. When the produced web comprises portions having altered additive, this may be an indication that additional markings have been produced after the primary web has been covered and/or impregnated with the additive. The produced web may be monitored optically e.g. by an eye or by an image sensor in order to detect whether the web comprises portions having altered additive.

Even if the parameters of the second laser beam would be selected such that the second base portion would have the same optical properties as the first base portion, the additive superposed on second base portion would be optically different from additive superposed on the first base portion, because the additive superposed on second base portion has been exposed to laser radiation and the additive superposed on the first base portion has not been exposed to laser radiation.

For example, the additive may be arranged to lose its fluorescent properties when exposed to an ultraviolet laser beam. The additive may be arranged to lose its fluorescent properties due to heating caused by absorption of a laser beam. The additive may be arranged to change its color due to heating caused by absorption of a laser beam. The additive may be arranged to exhibit yellowing or photo-bleaching when exposed to a laser beam.

For example, a written document or a package for a product may comprise the covered web, which comprises the security marking. The covered web may be cut into sheets e.g. to form a ticket or a label for a medicament. The security marking may indicate e.g. a trade name, a name of a person, a date stamp, or a page number. By applying suitable covering layers (e.g. starch sizing and/or pigment coating comprising the additive) on top of the primary web, the altered base portion made in the primary web can be effectively "sealed". Thus modification of the base portion and/or producing a further base portion may become very difficult or impossible. Falsifying the security marking may be difficult or impossible, because the optical alteration of the additive may indicate whether the marking has been modified after the covering and/or impregnating with the additive. Producing further markings without causing optical alteration of the additive may be difficult or impossible without removing the previous additive and covering the primary web again with a new layer of the additive. The security-marked web may be easily mass-produced at low costs e.g. by using a modified paper machine, which has been equipped with a laser marking device. In an embodiment, it is not necessary to use an additional security printer or a security converter after producing the web by the modified paper machine.

In an embodiment, the covered web may optionally comprise further (i.e. factory-made) markings formed by a laser beam after covering and/or impregnating the primary web with the additive. The further markings may make counterfeiting of a document even more difficult.

If a counterfeiting person wants to produce a falsified marking, which is optically identical to an original marking, he should typically have knowledge about the laser parameters, which were used when producing the original marking. The laser parameters may be e.g. intensity, pulse duration, and wavelength. The laser parameters may be kept secret so that a person intending to counterfeit a document would need to make experimental tests to find the suitable laser parameters. If the counterfeiting person has access to only one piece of the paper, i.e. to the document, which he desires to falsify, he cannot typically use said document for making the experimental tests.

The composition of the additive may be selected such that intense heating of the additive changes at least one optical property of the additive. If someone tries to alter the buried primary web by bringing a hot stamp into contact with the covered web, this is likely to alter the superficial additive layer. Thus, the altered additive may indicate that a second (fake) base portion has been produced after the primary web was covered and/or impregnated with the additive.

Marking with a laser beam may allow higher spatial resolution, i.e. marking with the laser beam may provide a narrower base portion than which would be possible by using a hot stamping tool to cause conductive heating. Bringing a hot stamp into contact with the covered web is likely to produce a marking, which has slightly blurred boundaries. The authenticity of a marking comprising the covered altered portions may be checked e.g. by comparing the marking with a reference marking. For example, the manufacturer, a customer, or a public authority may have access to a piece of a web, which comprises the reference marking. The authenticity of the marking may also be checked e.g. based on information retrieved from a database. The information may e.g. specify the locations of the altered portions with respect to each other. The manufacturer, a customer, or a public authority may have access to the information e.g. via the Internet.

In an embodiment, the covered web may comprise a combined marking, which comprises one or more first altered portions covered and/or impregnated with the additive, and one or more second altered portions formed by a laser after applying the additive. Said combined marking may be called e.g. as a hybrid marking. For example, the markings MRK1 , MRK2 shown in Fig. 1 b may together form a hybrid marking. The hybrid marking may be formed of several altered portions produced in different phases of manufacturing a paper web. Falsification of the hybrid marking may be very difficult, because exact copying or modifying the hybrid marking may require knowledge of a high number of process parameters. The altered hybrid marking may represent e.g. a graphical symbol. In particular, the hybrid marking may represent e.g. an alphanumeric code. The first altered portions may have a predetermined size and/or position with respect to size and/or position of the second altered portions. The optical properties of the first altered portions are different from the optical properties of the second altered portions. The first altered portions and the second altered portions may together form the graphical symbol. The first altered portions of the hybrid marking may represent the graphical symbol, and also the second altered portions of the hybrid marking may represent substantially the same the graphical symbol. In an embodiment, the visual appearance of the first altered portions may be different from the visual appearance of the second altered portions in order to facilitate inspection of the hybrid marking with the naked eye when the web is illuminated with visible light and/or with ultraviolet light. In an embodiment, the difference between the optical properties of the first altered portions and the optical properties of the second altered portions may be detectable only by using special optical equipment (e.g. by using a microscope or a spectral analyzer).

Checking the authenticity of the hybrid marking may comprise checking whether the positions of the altered portions of the hybrid marking substantially match with the positions specified by a reference marking.

In an embodiment, the positions of the first altered portions with respect to the positions of the second altered portions may be determined by an algorithm. The size and/or position of the second altered portions with respect to the size and/or position of the first altered portions may be determined by using an algorithm, which is based on the use of a (secret) password. Thus, is someone attempts to modify the hybrid marking by forming additional second altered portions, it is highly probable that the positions of the additional second altered portions do not match with the positions given by the algorithm. Checking the authenticity of the hybrid marking may comprise checking whether the positions of the first and the second altered portions match with the positions given by the algorithm. In order to further improve the security level, counterfeiting of the markings may be made even more difficult by applying two or more covering layers on the primary web. One or more first altered portions may be formed before covering and/or impregnating with a first additive. One or more second altered portions may be formed after covering and/or impregnating with a first additive but before covering and/or impregnating with a second additive (the first additive and the second additive may have the same chemical composition or different chemical composition).

In order to further improve the security level, a combined marking may comprise one or more first altered portions formed before applying a first additive, one or more second altered portions formed after applying the first additive, and one or more third altered portions formed after applying a second additive. The second altered portions may be formed before applying the second additive. The compositions of the first additive and the second additive may be selected such that at least one optical property of the additives is changed when exposed to the intense laser beam. Thus, the first additive may serve as guard for the underlying layers below the first additive, and also the second additive may serve as a guard for the layer of the first additive and the other underlying layers. In an embodiment, the covered web may comprise three or more superposed additive layers, wherein each additive layer may serve as a guard for markings implemented in the underlying layers.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 a shows, in a top view, a web comprising a first marking,

Fig. 1 b shows, in a top view, a web comprising a modified marking, shows, in a three-dimensional view, inspecting a web by the naked eye, shows, in a three-dimensional view, inspecting a web by an optical sensor, shows, in a cross-sectional side view, forming a first base portion by altering the structure and/or composition of a primary web with a laser beam, shows, in a cross-sectional side view, a web comprising a covered altered portion, shows, in a cross-sectional side view, a web comprising different types of covered altered portions, shows, in a cross-sectional side view, producing a second base portion by directing a second laser beam to the covered web, shows, by way of example, intensity ranges for suppressing fluorescence, shows, by way of example, intensity ranges for suppressing fluorescence, shows, in a three-dimensional view, fluorescence from a web illuminated by ultraviolet light, shows, in a cross-sectional side view, fluorescence from a web illuminated by ultraviolet light, shows, by way of example, fluorescence spectrum of a reference portion, shows, by way of example, fluorescence spectrum of a covered altered portion, shows, by way of example, fluorescence spectrum of a second altered portion, shows, by way of example, intensity ranges for altering color, shows, in a cross-sectional side view, light reflected from a web, shows, by way of example, spectrum of light reflected from a reference portion, shows, by way of example, spectrum of light reflected from a covered altered portion, shows, by way of example, spectrum of light reflected from a second altered portion, shows intensity ranges for altering color, shows, in a cross-sectional side view, fluorescence light from a web and light reflected from the web, shows intensity ranges for suppressing fluorescence, and an intensity range for altering color, shows, in a cross-sectional side view, fluorescence from a web and light reflected from the web, shows, in a cross-sectional side view, producing a hole in the primary web by a laser beam, shows, in a cross-sectional side view, forming a covered web by covering and/or impregnating the primary web with an additive, shows, in a cross-sectional side view, a covered web comprising a hole filled with an additive, shows, in a cross-sectional side view, producing a hole in the covered web, shows, in a cross-sectional side view, light reflected from the web of Fig. 14a, shows, by way of example, the contrast between an altered portion and a reference portion, shows steps for producing a paper or cardboard web, shows an apparatus comprising a laser marking device and a unit for supplying the additive, shows, in a cross-sectional side view, a covered web comprising altered portions, Fig. 18b shows, in a cross-sectional side view, a covered web comprising altered portions, Fig. 18c shows, in a cross-sectional side view, a covered web comprising altered portions,

Fig. 19 shows a laser device for producing markings, Fig. 20 shows a laser device for producing markings,

Fig. 21 shows providing several individually controllable laser beams, Fig. 22 shows a laser device for producing markings, Fig. 23a shows, in a three dimensional view, a covered web comprising a a plurality of identical markings,

Fig. 23b shows, in a three dimensional view, a covered web comprising several different markings,

Fig. 23c shows, in a three dimensional view, a covered web where the area treated with the additive encloses an altered portion, Fig. 23d shows, in a three dimensional view, a covered web comprising a hybrid marking,

Fig. 23e shows, in a three dimensional view, a covered web comprising a hybrid marking, and Fig. 23e shows, in a three dimensional view, a covered web comprising a hybrid marking.

DETAILED DESCRIPTION Referring to Fig. 1 a, a covered web WEB1 may comprise a first marking MRK1 . The first marking MRK1 may comprise one or more covered altered portions DOT1 . The first marking MRK1 may be formed of one or more covered altered portions DOT1 . An item ITE1 may comprise a piece of the covered web WEB1 . The ITE1 may comprise the marking MRK1 .

The covered altered portions DOT1 may be produced by altering the structure and/or composition of a primary web WEBO by a laser beam, and by subsequently covering and/or impregnating the primary web WEBO with a protective additive AD1 (See Figs. 3a and 3b). In other words, the covered web WEB1 may be formed by providing an additive AD1 such that the primary web WEBO and the first base portion DOT0 are impregnated with the additive AD1 and/or such that the primary web WEBO and the first base portion DOTO are covered with a layer, which comprises the additive AD1 . Each covered altered portion DOT1 may be formed by locally altering the structure and/or chemical composition of a primary web WEBO by a laser beam LB1 , and subsequently covering and/or impregnating the primary web WEBO with the additive AD1 such that the altered portion DOTO is covered and/or impregnated with the additive AD1 .

The additive AD1 may be applied such that the altered portion DOTO of the primary web WEBO and an (unaltered) area surrounding the altered portion DOTO are covered and/or impregnated with the additive AD1 . The additive AD1 may be applied such that the altered portion DOTO of the primary web WEBO and an (unaltered) area surrounding the altered portion DOTO are substantially simultaneously covered and/or impregnated with the additive AD1 . In an embodiment, substantially the whole surface of the primary web WEBO may be covered and/or impregnated with the additive AD1 . The primary web WEBO may be directly in contact with the additive AD1 , but this is not necessary. One or more intermediate layers may be located between the primary web WEBO and the layer comprising the additive AD1 . The covered altered portion DOT1 comprises an optically detectable base portion DOTO formed by a laser beam, and an amount of an additive AD1 bound to the base portion DOTO. The additive AD1 may be directly bound to the base portion DOTO, or the additive AD1 may be bound to the base portion DOT0 by one or more intermediate layers. The additive AD1 may be transparent or translucent in the visible regime and/or in the ultraviolet regime of optical spectrum so that the optically altered base portion can be optically detected also after covering and/or impregnating with the additive AD1 .

Impregnating the primary web WEBO with the additive AD1 may convert the surface of the primary web into a material layer, which comprises the additive. Removal of the additive may be very difficult or impossible when the primary web has been impregnated with the additive AD1 .

The covered web WEB1 and/or the item ITE1 may comprise one or more covered altered portions DOT1 . A first marking MRK1 may comprise one or more covered altered portions DOT1 . One or more covered altered portions DOT1 may together constitute a first marking MRK1 . A first marking MRK1 may consist of one or more covered altered portions DOT1 . The first marking MRK1 may also be called e.g. as an original marking, a genuine marking, a covered marking, a covered marking, or a sealed marking.

SX, SY, and SZ denote orthogonal directions. The direction SZ is shown e.g. in Fig. 2a.

The method of producing markings on a web WEB1 may comprise:

- forming a first base portion DOT0 in a primary web WEBO by locally altering at least one optical property of the primary web WEBO with a first laser beam LB1 , and

- forming a covered web WEB1 by providing an additive AD1 such that the primary web WEBO and the first base portion DOT0 are impregnated with the additive AD1 and/or such that the primary web WEBO and the first base portion DOTO are covered with a layer, which comprises the additive AD1 , wherein the primary web WEBO comprises cellulose fibers, the covered web WEB1 comprises an optically detectable first altered portion DOT1 , the first altered portion DOT1 comprises the first base portion DOTO and an amount of the additive AD1 bound to the first base portion DOTO, and the composition of the additive AD1 has been selected such that exposing the covered web WEB1 to a second laser beam LB2 causes an optically detectable alteration of the additive AD1 contained in the covered web WEB1 in a situation where the intensity of the second laser beam LB2 is equal to a minimum threshold intensity I _MIN,I needed to cause optically detectable alteration of the primary web WEBO contained in the covered web WEB1 . Referring to Fig. 1 b, the covered web WEB1 may comprise a second marking MRK2. The second marking MRK2 may comprise one or more second altered portions DOT2, which have been formed by a laser beam after the primary web WEBO has been covered and/or impregnated with the protective additive AD1 . The composition of the additive AD1 may be selected such that exposure to an intense laser beam changes at least one optical property of the additive AD1 . Thus, the presence of an altered additive may be an indication that the covered web has been exposed to intense laser radiation after covering and/or impregnating with the additive AD1 . Altered additive AD1 may be utilized for detecting the presence of the second altered portion DOT2 which has been added or modified after covering and/or impregnating with the additive AD1 . In an embodiment, the presence of a second altered portion DOT2 may be an indication that a genuine first marking MRK1 has been modified at a later stage. The presence of a secondary altered portion DOT2 may be an indication of a falsified marking.

The covered altered portion DOT1 of the first marking MRK1 may be implemented such that it is difficult or impossible to produce a second altered portion DOT2, which has the same spectral properties as the first covered altered portion DOT1 . For example, the portions DOT1 and DOT2 may have different color. For example, the portions DOT1 and DOT2 may emit different fluorescence spectra when illuminated with the same ultraviolet light.

The altered portions DOT1 , DOT2 are optically different from the surrounding area REF1 . In other words, the covered altered portions are optically detectable. The presence of an altered portion DOT1 , DOT2 may be optically detected by comparing the optical properties of the altered portion DOT1 , DOT2 with the optical properties of the reference portion REF1 . The reference portion REF1 may surround an altered portion DOT1 and/or DOT2. The reference portion REF1 may surround the first marking MRK1 and/or the second marking MRK2. The distance between the altered portion DOT1 and the reference portion REF1 may be smaller than the dimension of the altered portion DOT1 in the direction SX or SY. In particular, the distance between the altered portion DOT1 and the reference portion REF1 may be substantially equal to zero. The reference portion REF1 may mean the intact area of the web WEB1 which does not comprise any altered portions and which does not comprise any printed portions. The markings MRK1 , MRK2, TXT1 may cover e.g. less than 50% of the total (one-sided) surface area of the web WEB1 . The surface area of the reference portion REF1 may be e.g. greater than 50% of the total (one-sided) surface area of the web WEB1 . In an embodiment, the markings MRK1 , MRK2, TXT1 may cover less than 20% of the total (one-sided) surface area of the web WEB1 . The surface area of the reference portion REF1 may be e.g. greater than 80% of the total (onesided) surface area of the web WEB1 , respectively.

A portion of the covered web WEB1 may be classified to be a covered altered portion DOT1 or a second altered portion DOT2 based on optical properties. In particular, a portion may be classified to be a covered altered portion DOT1 or a second altered portion DOT2 based on the spectral properties. The color and/or fluorescence spectrum of the second altered portion DOT2 may be different from the color and/or fluorescence spectrum of the covered altered portion DOT1 . The color and/or fluorescence spectrum of the second altered portion DOT2 produced by a second laser beam LB2 may be different from the color and/or fluorescence spectrum of the covered altered portion DOT1 produced by a first laser beam LB1 . The color and/or fluorescence spectrum of the second altered portion DOT2 produced by a second laser beam LB2 may be different from the color and/or fluorescence spectrum of the covered altered portion DOT1 produced by a first laser beam LB1 even in a situation where the first laser beam LB1 and the second laser beam LB2 have the same intensity, pulse duration, and wavelength λ₀. The second altered portion DOT2 may reflect, transmit and/or scatter light in a different manner than the covered altered portion DOT1 .

The first marking MRK1 may be a security marking, wherein it may be difficult or impossible to produce a second marking MRK2 on the web WEB1 at a later stage such that the optical properties of the second marking MRK2 are substantially identical to the optical properties of the first marking MRK1 . A first marking MRK1 , a second marking MRK2 and/or a hybrid marking HMRK1 formed as a combination of the markings MRK1 , MRK2 may be detected by monitoring optical properties of the altered portions DOT1 , DOT2. (Examples of hybrid markings are shown e.g. in Figs. 1 b and 23d). The monitoring may comprise comparing the spectrum of light emitted and/or reflected from an altered portion DOT1 , DOT2 with the spectrum of light emitted and/or reflected from a reference portion REF1 . The monitoring may comprise comparing the spectrum of light emitted and/or reflected from a first portion DOT1 with the spectrum of light emitted and/or reflected from a second portion DOT2. The monitoring may comprise comparing the spectrum of light emitted and/or reflected from a portion DOT1 , DOT2 with a reference spectrum stored in a database. The database may be accessed e.g. via the Internet.

The primary web WEBO may comprise cellulose fibers. In particular, the primary web WEBO may be a paper web or a cardboard web, which comprises cellulose fibers. The cellulose fibers may easily absorb various additives. The cellulose fibers may be charred (carbonized) by heating with a laser beam such that black, grey or brown color is formed. The material of the cellulose fibers may be easily ablated by a laser beam.

The primary web WEBO may be uncoated before altering the structure by the laser beam LB1 . For example, uncoated paper or cardboard web may be used as the primary web WEBO. The primary web WEBO may also be coated, i.e. it may have been coated with one or more coating layers before altering the structure by the laser beam LB1 . The primary web WEBO may also be called e.g. as the substrate layer.

The cellulose fibers of the primary web WEBO may be natural cellulose fibers. The primary web WEBO may comprise natural cellulose fibers. The cellulose fibers of the primary web WEBO may be natural vegetable fibers. The natural vegetable fibers may be selected e.g. from a group consisting of wood fibers, cotton fibers, linen fibers, flax fibers (i.e. linen fibers), hemp fibers, sisal fibers, jute fibers, kenaf fibers, bamboo fibers and coconut fibers. In particular, the cellulose fibers may be selected e.g. from a group consisting of wood fibers, cotton fibers and linen fibers. These fibers are traditionally utilized in when making paper and/or cardboard. The wood fibers may be e.g. pine fibers, spruce fibers, and/or eucalyptus fibers.

The covered web WEB1 may comprise cellulose fibers. The covered web WEB1 may comprise paper and/or cardboard.

An individual altered portion DOT1 , DOT2 may have e.g. a substantially circular or linear shape, or it may have e.g. the shape of a letter or a number. A first marking MRK1 may comprise a covered altered portion DOT1 but not the second altered portion DOT2. The second marking MRK2 may comprise the second altered portion DOT2 but not the covered altered portion DOT1 . One or more altered portions DOT1 , DOT2 may form e.g. alphanumeric symbols (i.e. letters and/or numbers), whose height may be e.g. in the range of 1 mm to 20 mm.

The marking MRK1 and/or MRK2 may indicate e.g. a trade name, a name of a person, a date stamp, or a page number.

The web WEB1 may optionally comprise text and/or graphics INF1 , which has been printed on the covered web WEB1 or on the primary web WEB0. (the text may be e.g. a marking "TXT1 "). The graphics and/or text may be printed by using conventional printing means (e.g. flexography, offset printing, inkjet printing). An item ITE1 may be formed from the covered web WEB1 . An item ITE1 obtained by cutting a piece of the web covered WEB1 may comprise one or more covered altered portions DOT1 . The item ITE1 may comprise one or more second altered portions DOT2. The item ITE1 may e.g. a paper sheet or a cardboard sheet. The size of the sheet may be e.g. A5, A4, A3, A2, A1 , AO, ANSI A, ANSI B, ANSI C, ANSI D, and ANSI E, as determined in the standards ISO 216 and ANSI/ASME Y14.1 . The item ITE1 may optionally comprise text and/or graphics INF1 . The item ITE1 may be e.g. a receipt, a label, a certificate of warranty, a diploma, a written agreement, or a product package. The item ITE1 may be optionally lined with an adhesive. The item ITE1 may be e.g. a label. In particular, the item ITE1 may be a label for a medicament. Referring to Fig. 2a, an optical difference between an altered portion DOT1 and a reference portion REF1 may be detected e.g. by illuminating the covered web WEB1 . The illuminating light may be e.g. visible light VISO and/or ultraviolet light UVO. The optical properties of the portions DOT1 , DOT2 may be monitored by a human eye E1 when the web WEB1 is illuminated with visible light VISO and/or with ultraviolet light UVO. The portion DOT1 and/or DOT2 may be detectable by an unaided eye E1 , i.e. by a naked eye E1 . Alternatively, an optical device may be used to facilitate detection of the portions DOT1 and/or DOT2, when viewed by a human eye E1 . The optical device may be e.g. a microscope or an optical narrowband filter positioned between the covered web WEB1 and the eye E1 . The illuminating light VISO and/or UVO may be obtained from a light source 1210 and/or 1220. The light source 1210, 1220 may be e.g. a light-emitting diode, gas discharge lamp (in particular a fluorescent tube), or a tungsten halogen lamp. The same light source may provide visible light VISO and ultraviolet light UVO.

Referring to Fig. 2b, the optical properties of the portions DOT1 , DOT2 may be monitored by using an optical sensor CAM1 when the web WEB1 is illuminated with visible light VISO and/or with ultraviolet light UVO. The optical sensor CAM1 may be e.g. an image sensor. For example, the digital camera of a mobile phone may be used as the image sensor. The optical sensor CAM1 may be spectrally selective so as to measure e.g. reflectance spectrum and/or a fluorescence spectrum. The optical sensor CAM1 may be arranged to detect visible light, ultraviolet light, and/or infrared light.

Referring to Fig. 3a, the mechanical structure and/or chemical composition of a primary web WEBO may be locally altered by a laser beam LB1 to form a first base portion DOT0. The laser beam LB1 may e.g. locally change the color of the primary web WEBO, deactivate fluorescence from the primary web WEBO, and/or change a light-scattering property of the primary web WEBO. The laser beam LB1 may be used to ablate material away from the primary web WEBO. The primary web WEBO may comprise a fluorescent substance, e.g. stilbene, coumarin or pyrazoline. The fluorescent substance of the primary web WEBO may comprise e.g. a fluorescent trans-isomer, wherein the fluorescent trans-isomer may be converted into a non-fluorescent cis-isomer by exposing the fluorescent substance to ultraviolet light.

The additive AD1 may comprise a fluorescent substance, e.g. stilbene, coumarin or pyrazoline. The fluorescent substance of the additive AD1 may comprise a fluorescent trans-isomer, wherein the fluorescent trans-isomer may be converted into a non-fluorescent cis-isomer by exposing the fluorescent substance to ultraviolet light. The marking laser beam LB1 may be obtained from a laser 500. The laser 500 may be e.g. an excimer laser, a C02-laser or a fiber laser. The peak wavelength λ₀ of the laser beam LB1 may be e.g. in the ultraviolet range (e.g. in the range of 180 to 380 nm), in the visible range (e.g. in the range of 400 to 760 nm), or in the infrared range (e.g. in the range of 800 nm to 1 1 μιτι). The laser beam LB1 may have certain (predetermined) laser parameters. The laser parameters may include e.g. wavelength λ₀, intensity (when measured at the surface of the primary web WEBO), and pulse duration.

The base portion DOT0 may be formed in the uncovered primary web WEBO by using the laser beam at a wavelength λ₀. Interaction of laser radiation with the additive AD1 may have an effect on the minimum intensity level needed to alter the optical properties of the web after the primary web WEBO has been covered and/or impregnated with the additive AD1 . The covered web WEB1 may have a minimum threshold intensity Ι ΜΙΝ,-Ι , which is needed to cause optically detectable alteration of the primary web WEBO by a laser beam at the wavelength λ₀, after the primary web WEBO has been covered and/or impregnated with the additive AD1 . The minimum threshold intensity I_MIN,I is shown e.g. in Figs. 6a, 6b, 9a, 1 1 a and 1 2a. The energy of a laser beam may be converted into heat by absorption of radiation in the primary web and/or by absorption of radiation in the additive. The additive AD1 may be selected such that it has a high absorbance for laser radiation. In particular, the additive AD1 may be selected such that it has a high absorbance in the ultraviolet regime and/or in the infrared regime. The additive AD1 may be selected such that it has a high absorbance in the range of 180 nm to 380 nm and/or in the range of 800 nm to 1 1 μιτι. The additive AD1 may comprise e.g titanium dioxide and/or an organic dye.

Referring to Fig. 3b, a covered altered portion DOT1 may be formed by covering and/or impregnating the primary web WEB0 with an additive AD1 such that the base portion DOTO is also covered and/or impregnated with the additive AD1 . The covered altered portion DOT1 comprises the base portion DOTO and an amount of the additive AD1 bound to the base portion DOTO. Said amount of the additive AD1 is located above the base portion DOTO or impregnated within the base portion (DOTO). Said amount of the additive AD1 may be directly or indirectly bound to the base portion DOTO. In other words, the additive AD1 may be directly in contact with the base portion DOTO or the covered altered portion DOT1 may comprise one or more intermediate coating layes located between the base portion DOTO and said amount of the additive AD1 .

The additive AD1 of the covered altered portion DOT1 has not been exposed to the laser beam LB1 . Thus, the visible portion of the additive AD1 of the covered altered portion DOT1 may have substantially the same optical properties as the visible portion of the additive AD1 of the reference portion REF1 .

The additive AD1 may be applied substantially to the whole upper and/or lower surface of the primary web WEB0. The additive AD1 may be applied e.g. by spraying, curtain-coating, dip-coating or brushing.

The additive AD1 may comprise e.g. an optical brightener. If the primary web WEB0 comprises a first optical brightener, the additive AD1 may comprise a second different optical brightener. The brightener may be e.g. stilbene, coumarin or pyrazoline. The thickness d₀ of the base portion DOTO may be smaller than the thickness d_Wo of the primary web WEBO. In other words, the base portion DOTO may be implemented so that it does not extend through the primary web WEBO. The bottom side of the base portion DOTO may be covered by a material layer, which has not been altered due to exposure to a laser beam. The bottom side of the covered altered portion may be protected e.g. by an unaltered layer of the primary web and/or by a layer of the additive AD1 on the bottom side. If the base portion DOTO extends fully through the primary web WEBO (i.e. when do = d_Wo), it may be advantageous to cover and/or impregnate both sides of the primary web WEBO with an additive AD1 . Otherwise a fake base portion might be produced by directing the second laser beam LB2 to the reverse side (bottom side) of covered web WEB1 , without causing alteration of the additive. Both sides may be covered with the same additive AD1 or with different additives AD1 , AD2. The composition of the second additive AD2 may also be selected such that the second additive AD2 is altered when exposed to the second laser beam LB2. Fig. 4 shows covered altered portions DOT1 having different optical properties.

For example, a primary web WEBO may comprise a fluorescent substance. The fluorescence from the primary web WEBO may be suppressed by exposing the fluorescent substance to an intense laser beam LB1 . The exposed portion may be subsequently covered and/or impregnated with a UV-transmitting additive AD1 to form a covered altered portion DOT1 . In this case, the covered altered portion DOT1 may appear darker than the reference REF1 when the covered web WEB1 is illuminated with ultraviolet light UV0 and observed in the visible range of wavelengths (e.g. by the naked eye E1 ).

The color of a primary web WEBO may be locally altered by exposing the primary web WEBO to an intense laser beam LB1 . In particular, the base portion DOTO may be carbonized so that it has a black, grey or brown color. The base portion formed by the intense laser beam may be subsequently covered and/or impregnated with the additive AD1 to form a covered altered portion DOT1 . In this case, the covered altered portion DOT1 may appear darker than the reference portion REF1 when the covered web WEB1 is illuminated with visible light VISO and observed in the visible range of wavelengths (e.g. by the naked eye E1 ).

An intense laser beam LB1 may be arranged to ablate material away from the primary web WEBO and/or to oxidize the material of the primary web WEBO such that a hole is formed. The base portion DOT0 formed by the laser beam may comprise a hole. The primary web WEBO may be subsequently covered with the additive AD1 such that the additive fills the hole. The covered altered portion may be a hole filled with the additive AD1 . The covered altered portion DOT1 may reflect and/or scatter illuminating light in a different way than the reference portion REF1 .

Yet, the covered altered portion DOT1 may be optically detectable based on its light-scattering properties. The gloss value of the covered altered portion DOT1 may be substantially different from the gloss value of the reference portion REF1 . The gloss values may be measured e.g. by a method defined in the standard "TAPPI T480". The portion DOT1 may have a glossy visual appearance, and the reference portion REF1 may have a matte visual appearance. The portion DOT1 may have a matte visual appearance, and the reference portion REF1 may have a glossy visual appearance. The different gloss value may be provided e.g. by processing the primary web WEBO with the laser beam LB1 so that the grain structure of the primary web WEBO is changed, or so that small bubbles are formed in the primary web WEBO, before covering and/or impregnating the primary web WEBO with the additive AD1 . Fig. 5 illustrates producing a further marking (e.g. a marking MRK2) on the web WEB1 after covering and/or impregnating with the additive AD1 . The further marking comprises an altered portion DOT2.

A laser beam LB2 may be arranged to impinge on the top surface of the covered web WEB1 , and the primary web WEBO contained in the covered web WEB1 may be covered and/or impregnated with the additive AD1 such that the laser beam LB2 cannot interact with the top layer of the primary web WEBO without interacting with the additive AD1 . The primary web WEBO contained in the covered web WEB1 may be covered and/or impregnated with the additive AD1 such that the laser beam LB2 cannot interact with the top layer of the primary web WEBO without passing throuch a layer, which contains the additive AD1 .

The first covered altered portion DOT1 may comprise additive AD1 , which has not been exposed to laser beam. An attempt to form the second altered portion DOT2 through the additive AD1 by the laser beam LB2 will inevitably expose also the additive AD1 to the laser beam LB2. Thus, the altered portion DOT2 will comprise additive exposed to the laser light LB2. The composition of the additive AD1 may be selected such that an attempt to form the altered portion DOT2 by the laser beam LB2 causes an optically detectable transformation of the additive AD1 contained in the top layer of the portion DOT2. When exposed to the intense laser beam, the additive AD1 may e.g. change color, or the additive AD1 may lose its fluorescent properties. For example, the composition of the additive AD1 may be selected such that exposing the additive AD1 to the second laser beam LB2 locally reduces the fluorescence yield of the additive AD1 contained in the covered web WEB1 , in a situation where the intensity of the second laser beam LB2 is equal to a minimum threshold intensity I _MIN,I needed to cause optically detectable alteration of the primary web WEBO contained in the covered web WEB1 .

The "fluorescence yield" is defined as the ratio of the number of photons emitted to the number of photons absorbed, wherein the wavelengths of the emitted photons are longer than the wavelength of the absorbed photons. The fluorescence yield may also be called as the fluorescence efficiency. The expressions "deactivating fluorescence" and "suppressing fluorescence" mean that the fluorescence yield is reduced.

When exposed to the intense laser beam, the light-scattering properties of the additive AD1 may be changed. For example, a glossy layer of the additive AD1 may be locally changed to have a matte appearance. The composition of the additive AD1 may be selected such that the additive AD1 covering the primary web WEBO initially has a matte appearance, wherein the portion of the additive AD1 exposed to the laser beam LB2 has a glossy appearance. The composition of the additive AD1 may be selected such that the additive AD1 covering the primary web WEBO initially has a glossy appearance, wherein the portion of the additive AD1 exposed to the laser beam LB2 has a matte appearance. The composition of the additive AD1 may be selected such that exposure to the laser beam LB2 locally changes the gloss value of the additive layer AD1 covering the altered portion DOT1 primary web WEBO.

The composition of the additive AD1 is advantageously selected such that at least one optical property of the second altered area DOT2 is different from a corresponding optical property of the covered altered area DOT1 . The composition of the additive AD1 may be selected such that it is difficult or impossible to produce a second altered portion DOT2 optically similar to the covered altered portion DOT1 by delivering a laser beam LB2 to the web WEB1 . The composition of the additive AD1 may be selected such that it is difficult or impossible to produce a second altered portion DOT2 optically similar to the covered altered portion DOT1 , without removing the previous additive AD1 . The composition of the additive AD1 may be selected such that it is difficult or impossible to produce a second altered portion DOT2 optically similar to the covered altered portion DOT1 , without applying a new layer of the additive AD1 to the WEB1 .

Figs. 6a and 6b show, by way of example, intensity ranges for suppressing fluorescence.

Fig. 6a relates to a situation where the primary web WEBO contains a first fluorescent substance, and the additive AD1 contains a second fluorescent substance, which has a different fluorescence spectrum than the first substance. A first base portion DOT0 may be produced by exposing the primary web WEBO to a laser beam LB1 before applying the additive AD1 , so that the fluorescence from the base portion DOTO is partly or completely deactivated. When producing the first marking MRK1 , the intensity of the laser beam LB1 may be kept between the values I_MIN,O and ΙΜΑΧ,Ο- ΙΜΙΝ,Ο may represent the minimum threshold intensity sufficient for deactivating the fluorescence of the bare primary web WEBO, which has not yet been covered with the additive AD1 . I _MAX,O may represent the maximum threshold intensity which does not yet alter the visible color of the primary web WEBO. In particular, I _MAX,O may represent the maximum threshold intensity which does not yet cause carbonizing of the first base portion DOTO. The bar BO represents the parameter range from I_MIN,O to Ι ΜΑΧ,Ο- I LBI may denote the intensity of the first marking laser beam LB1 . A second base portion DOTO' may be formed by directing a second laser beam LB2 to the covered web WEB1 . The additive AD1 may absorb and/or scatter laser light such that the minimum threshold intensity I _MIN,I sufficient for deactivating the fluorescence of the buried primary web WEBO may be higher than in case of the uncovered primary web. I_MAX,I may represent the maximum threshold intensity which does not yet cause a charring of the second base portion DOTO' when the laser beam LB2 is directed to the covered web WEB1 (i.e. when directed to the primary web through the additive AD1 ). The range from I _MIN,I to I _MAX,I has been marked by the bar B1 in Fig. 6a. The bar B1 represents the parameter range from I _MIN,I to Ι ΜΑΧ,Ι - l|_B2 may denote the intensity of the second laser beam LB2.

The bar B2 represents the suitable intensity range for deactivating the fluorescence of the additive AD1 . I_MIN,2 may represent the minimum threshold intensity sufficient for deactivating the fluorescence of the additive AD1 .

The composition of the additive AD1 may be selected such that the minimum level I _MIN,I for deactivating the fluorescence of the primary web WEBO through the additive AD1 is substantially higher than the minimum level I_MIN,2, which causes deactivation of fluorescence from the additive AD1 . Consequently, it may be impossible to deactivate the fluorescence of the primary web WEBO through the additive AD1 without simultaneously deactivating fluorescence from the additive AD1 . An attempt to alter the properties of the primary web through the additive AD1 may be impossible without simultaneously altering the additive AD1 . A local change in the fluorescence spectrum of the additive AD1 may be interpreted to be an indication that someone has deactivated or attempted to deactivate the fluorescence of the primary web WEB1 after the additive AD1 has been applied.

Thus, the second altered portion DOT2 can be detected simply by monitoring the fluorescent properties of the top layer. The covered altered portion DOT1 may appear to be brighter than the second altered portion DOT2 when illuminated with ultraviolet light UVO. The additive bound to the second altered portion DOT2 may have a reduced fluorescence yield when compared to the intact additive AD1 .

The laser beams LB1 , LB2 may have the same wavelength λ₀ and pulse duration.

The parameter shown in the vertical axis of Fig. 6a may also be pulse duration instead of the intensity. The expression "deactivation of fluorescence" means that the fluorescence yield is decreased. The expression "suppressing fluorescence" means that the fluorescence yield is decreased. Fig 6b shows shows a situation where a base portion may be produced by directing a first laser beam LB1 to a bare (uncovered) primary web WEBO at a first intensity level l_LB2, and an optically similar base portion may be produced by directing a second laser beam LB2 to a covered web WEB1 at the same intensity level l_LB2- Also in this case, the intensity level l_LB2 may be sufficient to deactivate the fluorescence from the additive AD1 . The laser beams LB1 , LB2 for producing the covered altered portion DOT1 and the second altered portion DOT2 may have the same wavelength λ₀ and pulse duration. Figs. 7a and 7b show fluorescence light emitted from a covered web WEB1 . The first covered altered portion DOT1 of the first marking MRK1 has been formed by covering and/or impregnating the primary web WEBO with the additive AD1 after it has been exposed to a first laser beam LB1 . The second altered portion DOT2 of the second marking MRK2 has been formed by exposing the covered web WEB1 to the second laser beam LB2 after the additive AD1 has been applied. The portions DOT, DOT2, REF1 may emit fluorescence light F1 , F2 when the covered web WEBO is illuminated with ultraviolet light UVO. The first altered portion DOT1 may emit light F1 , the second altered portion DOT2 may emit light F2, and the reference portion REF1 may emit fluorescence light F0.

Referring to Fig. 7b, the primary web of the reference portion REF1 may emit light FOa, and the additive layer bound to the reference portion REF1 may emit light FOb. The components FOa and FOb may together form the light F0. The base portion of the covered altered portion DOT1 may emit light F1 a, and the additive AD1 bound to the covered altered portion DOT1 may emit light F1 b. The base portion DOT0' of the second altered portion DOT2 may emit light F2a, and the additive of the second altered portion DOT2 may emit light F2b. The additive of the second altered portion DOT2 may have reduced fluorescence yield, because it has been exposed to the intense laser light LB2.

The primary web WEBO may contain a first fluorescent substance, and the additive AD1 may contain a second fluorescent substance such that the (initial) fluorescence spectrum of the additive AD1 is substantially different from the fluorescence spectrum of the primary web WEBO. When the reference portion REF1 is illuminated by ultraviolet light UVO, the primary web WEBO may emit (i.e. fluoresce) light having a first visible spectrum FOa, and the additive AD1 may emit (i.e. fluoresce) light having a second visible spectrum FOb. An observer E1 and/or an optical detector CAM1 (Fig. 1 ) may now detect that the reference portion REF1 emits light, whose spectrum FO is a combination of the spectra FOa and FOb.

The web WEB1 may be illuminated by the ultraviolet light UVO such that the irradiation (W/m²) by visible light VISO is less than 10% irradiation by the ultraviolet light UVO, advantageously less than 1 %, so as to enhance the contrast cause by spatial differences in the fluorescent properties. The web WEB1 may be illuminated by using a substantially uniform spatial irradiance. The eye E1 of an observer and/or an optical detector CAM1 (Fig. 1 ) may now detect that the covered portion DOT1 emits light, whose spectrum F1 is a combination of the spectra F1 a and F1 b. The covered altered portion DOT1 may comprise the base portion DOT0 whose fluorescence was deactivated by the laser beam LB1 , and the top layer, which contains the fluorescent additive AD1 . The first marking MRK1 may now be detected because the spectrum F1 of light emitted from the covered altered portion DOT1 is substantially different from the spectrum FO of light emitted from the reference portion REF1 . As the fluorescence from the base portion DOT0 was deactivated, the contribution of the component F1 a to the combined spectrum F1 may be small or negligible.

The additive AD1 was applied after the fluorescence of the underlying portion DOT0 was deactivated. The spectrum F1 b of light emitted by the top layer of the covered portion DOT1 may be substantially identical to the spectrum of light emitted by the top layer of the reference portion REF1 . Both top layers may contain active fluorescent material and may have substantially identical fluorescent properties.

The composition of the additive AD1 may be selected such that an attempt to locally deactivate the fluorescence of the primary web WEBO by delivering a laser beam LB2 through the additive AD1 will also partially or completely deactivate the fluorescence from the additive AD1 . Thus, the second altered portion DOT2 produced after applying the additive AD1 may emit light whose spectrum F2 is different from the spectrum F1 of light emitted from the covered portion DOT1 . Thus, the second (e.g. fake) marking MRK2 may be distinguished from the first (e.g. genuine) marking MRK1 . Fig. 8a shows the fluorescence spectrum FOa of the fluorescent primary web WEBO, and the fluorescence spectrum FOb of a fluorescent additive AD1 . The spectrum FO of fluorescence light from the reference portion REF1 may be formed as a combination of the spectrum FOa and the spectrum FOb. The reference portion REF1 may emit light having the combined spectrum FO, when illuminated by ultraviolet light UVO. The spectrum FOa may be different from the spectrum FOb, or the spectrum FOa may be similar the spectrum FOb

Fig. 8b shows the fluorescence spectrum F1 a of a base portion DOTO after exposure to the laser beam LB1 . The fluorescence yield of the base portion DOTO may be e.g. smaller than 50% of the initial fluorescence yield of the primary web WEBO. A covered altered portion DOT1 may comprise the base portion DOTO and the additive AD1 bound to the base portion. The spectrum F1 of fluorescence light from the covered altered portion DOT1 may be formed as a combination of the spectrum F1 a and the spectrum F1 b, when the covered web WEB1 is illuminated by ultraviolet light UVO. F1 b denotes the spectrum of light emitted from the additive AD1 of the covered altered portion DOT1 . The fluorescence yield of the additive AD1 of the covered altered portion DOT1 may be substantially equal to the fluorescence yield of the additive AD1 of the reference portion REF1 . The spectrum F1 b may be substantially identical to the fluorescence spectrum FOb shown in Fig. 8a.

The fluorescence from the primary web WEBO may be partly or completely deactivated by the laser beam LB1 so that the spectrum F1 a is substantially different from the fluorescence spectrum FOa shown in Fig. 8a, when the illuminating light UVO of Fig. 8b has same intensity and spectral composition as the illuminating light UVO of Fig. 8a.

In particular, the covered altered portion DOT1 may appear to be darker than the reference portion REF1 when the covered web WEB1 is illuminated by ultraviolet light UVO.

The covered altered portion DOT1 may be a part of a covert marking MRK1 , which becomes visible only when illuminated by ultraviolet light UVO.

The fluorescence spectrum F1 of the covered altered portion DOT1 may correspond to a different (visible) color than the fluorescence spectrum FO of the reference portion REF1 . Thus, the first marking MRK1 may be detected by the naked eye E1 when the covered web WEB1 is illuminated by the ultraviolet light UVO. Alternatively, the fluorescence spectrum F1 of the covered altered portion DOT1 may be only slightly different from the fluorescence spectrum F0 of the reference portion REF1 such that it is difficult or impossible to detect the covered altered portion DOT1 by using unaided eye E1 even when the covered web WEB1 is illuminated by the ultraviolet light UVO. For example less than 10%, or even less than 1 % of the power of the fluorescence spectra F0 and F1 may be in the visible range of wavelengths (e.g. in the range of 400 to 760 nm). In that case, the covered altered portion DOT1 may be detected e.g. by an image sensor, which is capable of capturing images at two different (ultraviolet) wavelengths.

Fig. 8c shows the fluorescence spectrum F2 of a second altered portion DOT2. Exposure to the intense laser beam LB2 may have deactivated the fluorescence from the primary web and also from the additive. Thus, fluorescence from the second marking MRK2 may be partially or completely suppressed, wherein the top layer of the first marking MRK1 may be fluorescent. The visual difference between the covered altered portion DOT1 and the second altered portion DOT2 may be detected e.g. by using the naked eye E1 when the covered web WEB1 is illuminated by the ultraviolet light UVO.

Alternatively, the optical difference between the covered altered portion DOT1 and the second altered portion DOT2 may be invisible to naked eye E1 . The optical difference may be detected e.g. using an image sensor, which is capable of capturing images at two different (ultraviolet) wavelengths.

Fig. 9a to 10c relate to a situation where exposure to an intense laser beam may change the color of the primary web WEB0 and the color of the additive AD1 .

Referring to Fig. 9a, the primary web WEB0 may contain a substance, which is arranged to change color when exposed to the marking laser beam LB1 , and the additive AD1 may contain a substance, which is arranged to change color when exposed to the laser beam LB2. A first base portion DOT0 may be produced by exposing the primary web WEB0 to the laser beam LB1 before applying the additive AD1 , so that the color of the base portion DOTO is altered. When producing the first marking MRK1 , the intensity of the laser beam LB1 may be kept above the value I_MIN,O- The bar BO represents the intensity range for producing the first base portion DOTO.

A second base portion DOTO' may be formed by directing a second laser beam LB2 to the covered web WEB1 . The additive AD1 may absorb and/or scatter laser light such that the minimum threshold intensity I_MIN,I needed to alter the color of the buried primary web WEBO may be higher than in case of the uncovered primary web WEBO. The bar B1 may represent the intensity range for producing the second base portion DOTO'. I_LB2 may denote the intensity of the second laser beam LB2.

The bar B2 represents the suitable intensity range for altering the color of the additive AD1 . I _MIN,2 may represent the minimum threshold intensity sufficient for altering the color of the additive AD1 .

The composition of the additive AD1 may be selected such that the minimum level I_MIN,I for altering the color of the primary web WEBO through the additive AD1 is substantially higher than the minimum level I_MIN,2, which causes altering the color of the additive AD1 . Consequently, it may be impossible to change the color of the primary web WEBO by the laser beam LB2 through the additive AD1 without simultaneously changing the color of the additive AD1 .

Thus, the second altered portion DOT2 can be detected simply by monitoring the color of the top layer. The altered portions of the first marking MRK1 may appear to be have a different color than the altered portions of the second marking MRK2, when illuminated by visible light VISO, which does not contain ultraviolet light UVO.

The laser beams LB1 , LB2 may be in the same predetermined wavelength range. The laser beams LB1 , LB2 may have the same wavelength λ₀. In particular, the laser beams LB1 , LB2 may have the same wavelength λ₀ and pulse duration. The parameter shown in the vertical axis of Fig. 9a may also be pulse duration instead of the intensity. Referring to Fig. 9b, the primary web WEBO of the reference portion REF1 may have a color COa, and the additive layer of the reference portion REF1 may have a color COb. The color CO of the reference portion REF1 may be formed as a combination of the colors COa and COb, when the covered web WEB1 is illuminated by visible light VISO. The base portion DOT0 of the covered altered portion DOT1 may have a color C1 a, and the additive AD1 of the covered altered portion DOT1 may have a color C1 b. The color C1 of the covered altered portion DOT1 may be formed as a combination of the colors C1 a and C1 b. The base portion DOT0' of the second altered portion DOT2 may have a color C2a, and the additive of the second altered portion DOT2 may have a color C2b. The additive of the second altered portion DOT2 may have a dark color because it has been exposed to the intense laser beam LB2. The color C2 of the second altered portion DOT2 may be formed as a combination of the colors C2a and C2b.

Fig. 10a shows spectrum CO of light reflected from a reference portion REF1 when the web WEB1 is illuminated by visible light VISO. COa denotes the spectrum of light reflected from the primary web WEBO and COb denotes the spectrum of light reflected from the additive AD1 . The spectrum CO may be a combination of the spectrum COa and the spectrum COb. The spectrum CO may correspond to a substantially neutral color. In particular, the spectrum CO may create the sensation of a white color or grey color when viewed by the eye E1 .

The same symbol (e.g. CO, C1 or C2) is herein used for the reflected light, for the spectrum, and for the corresponding color.

Fig. 10b shows spectrum C1 of light reflected from a covered altered portion DOT1 when it is illuminated by visible light VISO. The base portion DOTO may have e.g. a dark color. The spectrum C1 a of light reflected from the base portion DOTO may correspond e.g. to a black color or to a brown color. The spectrum C1 b of light reflected from the additive AD1 (top layer) may correspond e.g. to white color, to grey color, or to "transparent" color. The spectrum C1 may be a combination of the spectrum COa and the spectrum COb. The covered altered portion DOT1 may have a lower reflectance than the reference portion REF1 , and the visual color C1 of the covered altered portion DOT1 may appear to be darker than the visual color CO of the reference portion REF1 . Thus, the presence of the covered altered portion DOT1 may be detected by the naked eye E1 when the web WEB1 is illuminated by visible light VISO.

Fig. 1 0c shows spectrum C2 of light reflected from a second altered portion DOT2 when it is illuminated by visible light VISO. In this case the color of the additive AD1 has been changed so that the second altered portion DOT2 appears to be darker than the covered altered portion DOT1 , when the covered web WEB1 is illuminated by visible light VISO.

The difference between the spectrum C1 and the spectrum CO may also be so small, that it is difficult or impossible to detect by unaided eye E1 . The difference between the spectrum C1 and the spectrum CO may be detected e.g. by a color-sensitive image sensor CAM1 . The difference between the spectrum C2 and the spectrum C2 may also be so small, that it is difficult or impossible to detect by unaided eye E1 . The difference between the spectrum C2 and the spectrum C1 may be detected e.g. by a color-sensitive image sensor CAM1 .

Figs. 1 1 a and 1 1 b relate to a situation where exposure to an intense laser beam may change the color of the primary web WEB0 and deactivate the fluorescence of the additive AD1 .

Referring to Fig. 1 1 a, the primary web WEB0 may contain a substance, which is arranged to change color when exposed to the first laser beam LB1 , and the additive AD1 may contain a fluorescent substance, which is deactivated when exposed to the second laser beam LB2. A first base portion DOT0 may be produced by exposing the primary web WEBO to a laser beam LB1 before applying the additive AD1 , so that the color of the base portion DOTO is altered. When producing the first covered portion DOT1 , the intensity of the laser beam LB1 may be kept above the value ΙΜΙΝ,Ο- The bar BO represents the intensity range for producing the first base portion DOTO. A second base portion DOTO' may be formed by directing a second laser beam LB2 to the covered web WEB1 . The additive AD1 may absorb and/or scatter laser light such that the minimum threshold intensity I _MIN,I sufficient for altering the color of the buried primary web WEBO is higher than in case of the uncovered primary web WEBO. The bar B1 may represent the intensity range for producing the second base portion DOTO'. I _LB2 may denote the intensity of the second laser beam LB1 .

The bar B2 represents the suitable intensity range for deactivating the fluorescence from the additive AD1 . I _MIN,2 may represent the minimum threshold intensity sufficient for deactivating the fluorescence from the additive AD1 .

The composition of the additive AD1 may be selected such that the minimum level I_MIN,I for altering the color of the primary web WEBO through the additive AD1 is substantially higher than the minimum level I MIN,2, which causes deactivation of the fluorescence from the additive AD1 . Consequently, it may be impossible to change the color of the primary web WEBO through the additive AD1 without simultaneously deactivating the fluorescence.

Thus, the second altered portion DOT2 can be detected by monitoring the fluorescent properties of the top layer. The covered altered portions DOT1 of the first marking MRK1 may appear to be fluorescent, whereas the altered portions of the second marking MRK2 may exhibit reduced fluorescence.

The laser beam LB1 , LB2 may have the same wavelength λ₀ and pulse duration. The parameter shown in the vertical axis of Fig. 1 1 a may also be pulse duration instead of the intensity. Referring to Fig. 1 1 b, the primary web of the reference portion REF1 may have a color COa, and the additive layer of the reference portion REF1 may have a color COb. The color CO of the reference portion REF1 may be formed as a combination of the colors COa and COb, when the covered web WEB1 is illuminated by visible light VIS0. When illuminated by ultraviolet light UV0, the primary web of the reference portion REF1 may emit light FOa, and the additive layer of the reference portion REF1 may emit light FOb. The components FOa and FOb may together emit light FO. The base portion DOT0 of the covered altered portion DOT1 may have a color C1 a. The additive of the covered altered portion DOT1 may emit light F1 b. The base portion DOT0' of the second altered portion DOT2 may have a color C2a. The additive of the second altered portion DOT2 may emit light F2b. The additive of the second altered portion DOT2 may have reduced fluorescence yield, because it has been exposed to intense laser light.

A laser beam LB2 directed through the additive AD1 may deactivate the fluorescence of the additive AD1 . Thus, the deactivated fluorescence may be interpreted to be an indication of an attempt to alter the color of the primary web WEBO through the additive AD1 . Thus, the second altered portion DOT2 may appear darker than the covered altered portion DOT1 , when illuminated with by the UV light UVO.

The covered altered portion DOT1 may have a color C1 , which appears to be darker than the color CO of the reference portion REF1 when illuminated by visible light VISO. In this case, the primary web WEBO may be initially fluorescent, but it does not need to be. If the primary web WEBO is initially fluorescent, exposure to the laser beam LB1 may deactivate the fluorescence. In that case, the covered altered portion DOT1 may appear darker than the reference portion REF1 when illuminated by UV light UVO. If the primary web WEBO is initially non-fluorescent, the covered altered portion DOT1 may appear as bright (or as dark) as the reference portion REF1 when illuminated by UV light UVO, because both portions may comprise a substantially similar fluorescent top layer. The presence of the covered altered portion DOT1 may be detected based on the color and based on the fluorescent properties. The second altered portion DOT2 may be distinguished from the covered altered portion DOT1 based on the difference in the fluorescent properties. The covered web WEB1 may be illuminated by the visible light VISO and by the ultraviolet light UVO. The web WEB1 may be simultaneously illuminated with the lights VISO and UVO. The WEB1 may be illuminated with the lights VISO and UVO in an alternating manner such that during a first period of time the web WEB1 is illuminated with light UVO, which does not contain visible wavelengths, and during a second period of time the WEB1 is illuminated with light VISO, which does not contain ultraviolet wavelengths.

Figs. 1 2a and 12b relate to a situation where the primary web WEBO contains a fluorescent substance, and the composition of the additive AD1 has been selected such that the color of the additive AD1 is changed when exposed to an intense laser beam.

Referring to Fig. 1 2a, the primary web WEBO may lose its fluorescent properties when exposed to the first laser beam LB1 , and the additive AD1 may be arranged to change color when exposed to the second laser beam LB2. In partricular, the composition of the additive AD1 may be selected such that it has a high absorbance for UV and/or IR radiation. UV means ulraviolet and I R means infrared.

A first base portion DOT0 may be produced by exposing the primary web WEBO to a laser beam LB1 before applying the additive AD1 , so that the fluorescence is deactivated. When producing the covered altered portion DOT1 , the intensity of the laser beam LB1 may be kept above the value ΙΜΙΝ,Ο- The bar BO represents the intensity range for producing the first base portion DOTO. A second base portion DOTO' may be formed by directing a second laser beam LB2 to the covered web WEB1 . The additive AD1 may absorb and/or scatter laser light such that the minimum threshold intensity IMIN,I sufficient for deactivating fluorescence of the buried primary web WEBO may be higher than in case of the uncovered primary web. The bar B1 may represent the intensity range for producing the second base portion DOTO'. li_B2 may denote the intensity of the second laser beam LB2.

The bar B2 represents the suitable intensity range for changing the color of the additive AD1 . I_MIN,2 may represent the minimum threshold intensity sufficient for changing the color of the additive AD1 . The composition of the additive AD1 may be selected such that the minimum level I_MIN,I for deactivating the fluorescence of the primary web WEBO through the additive AD1 is substantially higher than the minimum level I_MIN,2, which changes the color of the additive AD1 . Consequently, it may be impossible to deactivate the fluorescence of the primary web WEBO through the additive AD1 without simultaneously changing the color of the additive AD1 . Thus, the second altered portion DOT2 can be detected by monitoring the color of the top layer. The altered portions of the first marking MRK1 may appear to have brighter color than the altered portions of the second marking MRK2. The laser beam LB1 , LB2 may have the same wavelength λ₀ and pulse duration. The parameter shown in the vertical axis of Fig. 12a may also be pulse duration instead of the intensity.

Referring to Fig. 12b, the primary web WEBO of the reference portion REF1 may have a color COa, and the additive layer AD1 of the reference portion REF1 may have a color COb. The color CO of the reference portion REF1 may be formed as a combination of the colors COa and COb, when the covered web WEB1 is illuminated by visible light VISO. When illuminated by ultraviolet light UVO, the primary web WEBO of the reference portion REF1 may emit light FOa, and the additive layer AD1 of the reference portion REF1 may emit light FOb. The components FOa and FOb may together emit light FO. The base portion DOTO of the covered altered portion DOT1 may emit fluorescence light F1 a. The additive AD1 of the covered altered portion DOT1 may have a color C1 b. The base portion DOTO' of the second altered portion DOT2 may emit fluorescence light F2a. The additive of the second altered portion DOT2 may have a color C2b. The additive of the second altered portion DOT2 may have a dark color, because it has been exposed to intense laser light LB2. The composition of the additive AD1 may be selected such that an attempt to deactivate the fluorescence of the primary web WEBO through the additive AD1 alters the color of the additive AD1 . Thus, an attempt to deactivate the fluorescence of the primary web WEBO through the additive AD1 may be revealed by the change of color of the additive AD1 .

The additive AD1 may be fluorescent but it does not need to be. Referring to Fig. 13a, the laser beam LB1 may be arranged to ablate and/or burn a hole in the primary web WEBO. Referring to Fig. 13b, the primary web WEBO may be covered and/or impregnated with an additive AD1 , after the base portion DOT0 has been formed. The additive AD1 may be delivered e.g. by a nozzle 620. Adhesion of the additive AD1 to the primary web WEBO may be optionally improved e.g. by using a heater 630 and/or compression rolls 641 ,642. The primary web WEBO may be moved in the direction SX at a velocity v-i .

In case of a hole, the additive may advantageously fill the hole, which was formed by using the laser beam LB1 . The resulting web WEB1 is shown in Fig. 13c.

Referring to Fig. 14a, the composition of the additive AD1 may be selected such that an attempt to form a further hole in the primary web WEBO through the additive AD1 by the laser beam LB2 will form a hole also in the additive layer AD1 of the web WEB1 . The (fake) second altered portion DOT2 may be a hollow hole, and the (genuine) covered altered portion DOT1 may be filled with the additive AD1 . Thus, the second altered portion DOT2 may have different light-scattering properties and/or different light-transmitting properties than the covered altered portion DOT1 . Thus, the (fake) altered portion DOT2 may be distinguished from the covered altered portion by visual inspection.

In particular, the composition of the additive AD1 may be selected such that it has a high absorbance in the UV and/or IR region. Thus, the energy of a UV or IR laser beam may be absorbed in the additive layer AD1 so that an attempt to form a new hole underneath the additive layer AD1 may also cause optically detectable alteration of the additive layer AD1 .

It may also be noticed that the laser beam LB1 may be arranged to produce a very narrow hole in the primary web WEBO. The hole may be so narrow that it may be difficult or impossible to produce a second hole having the same diameter by mechanically sticking a needle through the covered web WEB1 .

Referring to Fig. 14b, the color C2 of the second altered portion DOT2 may be different from the color C1 of the covered altered portion DOT1 . The fluorescence spectrum F2 of the altered portion DOT2 may be different from the fluorescence spectrum F1 of the covered altered portion DOT1 . The light- scattering property of the altered portion DOT2 may be different from the light-scattering property of the covered altered portion DOT1 . The light- reflecting property of the altered portion DOT2 may be different from the light- reflecting property of the covered altered portion DOT1 . The transmittance of light through the altered portion DOT2 may be different from the transmittance of light through the covered altered portion DOT1 . Referring to Fig. 15, the altered portions may be distinguished from the reference portion based on spatial variations of brightness, when the web is illuminated with light, which has spatially uniform intensity distribution. An altered portion may appear darker (or brighter) than the background. The web WEB1 may have e.g. a first altered portion DOT1 a having an altered color, and a second altered portion DOT1 b having an altered fluorescence. An altered portion DOT1 a, DOT1 b may appear darker (or brighter) than the reference portion REF1 when inspected by the naked eye E1 . An altered portion DOT1 a, DOT1 b may appear darker (or brighter) than the reference portion REF1 when inspected by an image sensor CAM1 . An altered portion DOT1 a, DOT1 b may appear darker (or brighter) than the reference portion REF1 , depending on the spectral properties of illuminating light, depending on the spectral properties of the altered portion DOT1 a, DOT1 b, and depending on the spectral properties of the image sensor CAM1 or eye E1 . When inspecting the web WEB1 by an optical sensor CAM1 , the predetermined wavelength range may e.g. be in the visible range (e.g. from 400 nm to 760 nm) or in the ultraviolet range (e.g. from 180 nm to 380 nm). The predetermined wavelength range of an optical sensor CAM1 may be defined by one or more optical filters. An altered portion DOT1 , DOT1 a, DOT1 b may be detected by measuring the radiance of the altered portion, and by comparing the radiance value of the altered portion with the radiance value of the reference portion REF1 and/or with a reference value stored in a memory.

An altered portion DOT1 , DOT1 a, DOT1 b may be detected by comparing the luminance of the altered portion with the luminance of the reference portion REF1 and/or with a reference value stored in a memory. The uppermost curve CR1 of Fig. 15 shows, by way of example, luminance l_vis,i at different locations of the web WEB1 when the web WEB1 is illuminated by visible white light VISO, which does not contain ultraviolet light UVO. The illuminating light may have spatially uniform intensity distribution. The luminance L is the luminous intensity reflected (or emitted) from a unit area. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. The luminance L takes into account the spectral sensitivity of the eye. The unit of the luminance may be cd/m² (candela per unit area). The "visible white light" means the visible portion of blackbody emission spectrum when the blackbody temperature is in the range of 3000 K to 6500K. When the illuminating light does not contain ultraviolet light, this means that the illuminating light does not contain any spectral components whose wavelength is shorter than 400 nm.

The curve CR1 may also be interpreted to represent a weighted average of spectral reflectance of the web WEB1 at different locations, wherein the spectral reflectance is weighted by the spectral sensitivity of the eye and averaged over the range of visible wavelengths from 400 nm to 760 nm. The reference portion REF1 may have a luminance value L_REF_,I -

The altered portion DOT1 a may be e.g. carbonized such that it appears to be black when illuminated by the visible light VISO, and when it is viewed by the eye E1 . The first altered portion DOT1 a may have a luminance value L _a> . For reliable detection by the naked eye, there should be a minimum depth of spatial modulation of luminance. The depth of spatial modulation of the luminance may be called as the "visual contrast". A contrast relevant for inspection by the naked eye may be called as the "visual contrast". A portion may be considered to be "visible to the naked eye" when the visual contrast is higher than or equal to 2%. A portion may be considered to be "substantially invisible to the naked eye" when the visual contrast is smaller than 2%.

The visual contrast of the portion DOT1 a is equal to (L_REF,i -Lia,i ) LREF,i , with respect to the reference portion REF1 . The visual contrast of the portion DOT1 a may be e.g. higher than 5%, in order to facilitate easy detection by the naked eye E1 . l_ib,i denotes the luminance of the portion DOT1 b. The visual contrast of the covert portion DOT1 is equal to (L_REF,i -Lib,i ) LREF,i , with respect to the reference portion REF1 . The visual contrast (L_REF,i -Lib,i )/L_REF,i of the portion DOT1 b may be e.g. smaller or equal to 2% such that it is difficult or impossible to detect the presence of the portion DOT1 b by the naked eye E1 when the web WEB1 is illuminated by visible white light VISO, which does not contain ultraviolet light UV0.

The visual contrast (L_REF,i -Lib,i )/L_REF,i of the portion DOT1 b may be e.g. smaller or equal to 0.5% such that it is nearly impossible to detect the presence of the portion DOT1 b by the naked eye when the web WEB1 is illuminated by visible white light VISO, which does not contain ultraviolet light UV0.

In case of a very low visual contrast, the portion DOT1 b may still be detectable e.g. by using an image sensor CAM1 . The second curve CR2 shows, by way of example, luminance L_Vis,2 at different locations of the web WEB1 when the web WEB1 is illuminated by ultraviolet light UVO, which does not contain visible light. Also in this case the luminance L_Vis,2 indicates how much luminous power will be detected by an eye E1 looking at the surface from a particular angle of view. The primary web WEBO may be initially fluorescing, wherein the fluorescence from the portions DOT1 a and DOT1 b may be suppressed, as a consequence of exposure to the laser beam. Thus, the portions DOT1 a, DOT1 b may appear darker than the reference portion REF1 , when illuminated by ultraviolet light UVO and inspected by the naked eye E1 .

LREF,2 denotes the luminance of the reference portion REF1 . L _A>2 denotes the luminance of the portion DOT1 b. L _b,2 denotes the luminance of the portion DOT1 b. In this case, the visual contrast of the portion DOT1 a is equal to (l-REF,2-l-ia,2)/l-REF,2- The visual contrast of the portion DOT1 b is equal to (LREF,2-Lib,2) I-REF,2- When illuminating with the ultraviolet light UVO, the visual contrast of the portion DOT1 b may be substantially greater than 2% even if the visual contrast of the covert portion DOT1 would be less than 2% when illuminated with visible light VISO. Fluorescence from underlying layers of the web WEB1 may cause residual radiance L_REsi - Fluorescence light emitted from the surrounding portions and scattered from the portions DOT1 a, DOT1 b may also cause residual radiance L_REsi- The third curve CR3 shows, by way of example, ultraviolet radiance Ruv at different locations of the web WEB1 when the web WEB1 is illuminated by ultraviolet light UVO, which does not contain visible light. The ultraviolet radiance Ruv is equal to the integral of spectral radiance over a range of ultraviolet wavelengths. Spatial variations of the radiance Ruv may be detected by an optical sensor CAM1 , which is sensitive to ultraviolet wavelengths. Spatial variations of the radiance Ruv cannot be detected by the naked eye E1 . The curve CR3 may also represent spatial variations of ultraviolet reflectance of the web WEB1 . R _a>3 denotes the ultraviolet radiance of the portion DOT1 a. R-i _b,3 denotes the ultraviolet radiance of the portion DOT1 b. RREF,3 denotes the ultraviolet radiance of the reference portion REF1 . In this case, the ultraviolet contrast of the portion DOT1 a is equal to (RREF,3-R-ia,3) RREF,3- The ultraviolet contrast of the portion DOT1 b is equal to (RREF,3-Rib,3) RREF,3- The ultraviolet contrast cannot be detected by the naked eye. The web WEB1 may be monitored by an optical sensor CAM1 , which detects ultraviolet light. The (carbonized) portion DOT1 a may have a low reflectance for ultraviolet light such that the portion DOT1 a appears to be darker than the reference portion REF1 when viewed by a UV-sensitive optical sensor CAM1 . Fluorescence from the portion DOT1 b has been suppressed by altering the primary web WEBO with the laser beam. Exposure to the laser beam may change the chemical structure of the fluorescent substance so that the substance may still absorb ultraviolet radiation, even if the fluorescent property would be deactivated. The ultraviolet radiance R-i_B,3 of the portion DOT1 b may be lower than the ultraviolet radiance RREF_,3 of the reference portion REF1 .

The laser beam LB1 may decompose fluorescent material and/or ablate fluorescent material from the primary web WEBO such that absorbance in the UV regime is locally reduced. In this case, the altered portion DOT1 b may appear to be brighter than the reference portion REF1 , when viewed by a UV-sensitive optical sensor CAM1 . Thus, the ultraviolet radiance L _B,3 of the second portion DOT1 b may also be higher than the ultraviolet radiance L_REF_,3 of the reference portion REF1 .

In an embodiment, the laser beam may convert the first fluorescent substance to a second substance, which has a different fluorescence spectrum than the first substance. In an embodiment, the laser beam may convert a first fluorescent substance to a second substantially non-fluorescent substance.

In an embodiment, the visual contrast may be increased or maximized by selecting the wavelength range of the illuminating light VIS0. In an embodiment, a portion DOT1 a may be substantially invisible when illuminated by white visible light, but said covert portion DOT1 a may be visible when illuminated by visible light having narrow spectral band. The contrast may be e.g. less than 2% when illuminated by white light (e.g. sunlight, tungsten halogen lamp), wherein the contrast may be higher than 2% when illuminated by light having narrow spectral band (e.g. by using visible laser light, or light from a blue, red, green or yellow light-emitting diode). In an embodiment, the portion DOT1 b may be implemented such that it is difficult or impossible to detect by monitoring only visible light, wherein the presence of the portion DOT1 b may be detected by using an optical sensor CAM1 , which is arranged to detect ultraviolet light reflected and/or fluoresced from the web WEB1 .

A fluorescent material may absorb optical energy at a shorter wavelength, and emit a part of the optical energy at a longer wavelength. In an embodiment, the reference portion REF1 may be fluorescing such that it absorbs visible light, and emits infrared light. In this case, the presence of the portion DOT1 b may be detected by using an optical sensor CAM1 , which is sensitive to infrared light.

Referring to Fig. 16, production of a paper or cardboard web may typically comprise one or more of the following steps:

- compressing, where wet cellulose fiber web is compressed between rolls in order to remove water,

- drying, where water is removed from the web by heating,

- calandering, where the surface of the web is smoothed by compressing between rolls,

- sizing, where one or more sizing agents are added to the web e.g. in order to improve the mechanical strength of the web,

- coating, where the web is coated with one or more fillers e.g. in order to produce a smooth surface, to modify optical reflectance of the web, and/or to facilitate subsequent printing on the web. The marking with a laser beam may be carried out e.g. in one or more of the positions POS12, POS1 1 , POS1 , POS2, POS 3, POS4.

A base portion DOT0 may be formed e.g. between drying and calandering (position POS1 1 ). A covered altered portion DOT1 may be subsequently formed by covering and/or impregnating the base portion DOT0 with the additive AD1 .

A base portion DOT0 may be formed e.g. between calandering and adding a sizing (position POS1 ). A covered altered portion DOT1 may be subsequently formed by covering and/or impregnating the base portion DOT0 with the additive AD1 .

A base portion DOT0 may be formed e.g. between adding a sizing and adding a filler (position POS2). A covered altered portion DOT1 may be subsequently formed by covering and/or impregnating the base portion DOT0 with the additive AD1 .

The additive AD1 may be e.g. a sizing agent or a filler. The sizing agent may comprise e.g. starch, resin, and/or glue. The filler may comprise e.g. calcium carbonate or china clay. The filler may be suspended in a binder of cooked starch and styrene-butadiene latex.

The composition of the primary web WEBO may be selected such that color and/or fluorescent properties of the primary web WEBO are altered when the primary web WEBO is locally heated to a temperature which is higher than or equal to a first threshold temperature T_THRI -

The composition of the additive AD1 may be selected such that color and/or fluorescent properties of the additive AD1 are altered when the additive AD1 is locally heated to a temperature which is higher than or equal to a second threshold temperature T_THR2-

The composition of the primary web WEBO and/or the composition of the additive AD1 may be selected such that the first threshold temperature T_THRI is higher than the second threshold temperature T_THR2, in order to prevent falsification by conductive heating.

Referring to Fig. 17, an apparatus 1000 for processing a paper web or a cardboard web may comprise at least one marking unit 500 and at least one unit 600 arranged to supply the additive AD1 . The unit 600 may be a coating unit. The laser marking unit 500 may be arranged to provide a laser beam LB1 , which may impinge on the primary web WEB1 at a laser spot SP1 . The coating unit 600 may be arranged form a covered web WEB1 by covering and/or impregnating the primary web WEB0 with the additive AD1 . In particular, the coating unit 600 may be arranged cover the primary web WEB1 with a layer of sizing, with a layer of pre-coating and/or with a layer of top-coating. The apparatus 1000 may optionally comprise e.g. rolls 1010, 1020 e.g. for moving the web and/or for compressing the web. The apparatus may be a paper machine. The apparatus may optionally comprise a cutting unit arranged to cut the covered web WEB1 into a plurality of pieces.

Referring to Figs. 18a-18c, different types of markings may be produced on different layers of a paper or cardboard web WEB1 .

Different lasers may be used. For example, an infrared laser (IR laser) may be used to produce an overt marking comprising an altered portion, which has a dark color when illuminated by visible light VIS0. The infrared laser may be used to blacken a layer of the web WEB0 or WEB1 .

An infrared laser (IR laser) may be used to produce an overt marking comprising an altered portion, which comprises a hole or engraving in the paper structure. An ultraviolet laser (UV laser) may be used to cause de-activation of a fluorescent chemical comprised in the primary web or in the additive. The ultraviolet laser may be used to produce a covert marking, which may be substantially invisible to the naked eye E1 when the web WEB1 is illuminated by visible light VIS0, which does not contain ultraviolet light UV0, and the marking may be visible to the naked eye E1 when illuminated with ultraviolet light UV0. Fig. 18a shows a web WEB1 formed by covering a primary web WEBO with a layer of additive AD1 . One or both sides of the primary web WEBO may be covered with a layer of the additive AD1 . The primary web WEBO may be fluorescing. The additive AD1 may be arranged to change color and/or to change its fluorescence response when exposed to a laser beam LB1 .

The web WEB1 may comprise covered altered portions DOT1 a, DOT1 b, DOT1 c, and bare altered portions DOT2a, DOT2b, DOT2c. DOT2d, DOT2e. A carbonized (black) portion and the superposed additive layer AD1 may (together) form a covered altered portion DOT1 a. A non-fluorescent base portion and the superposed additive layer AD1 may form a covered altered portion DOT1 b. A hole filled with the additive AD1 may form a covered altered portion DOT1 c.

The color of the additive layer AD1 may be locally altered to form a bare altered portion DOT2a or DOT2d. The fluorescence of the additive layer AD1 may be locally deactivated to form a bare altered portion DOT2b or DOT2e. A hole extending through the web WEB1 may form a bare altered portion DOT2C.

The term "bare" may mean that the outermost surface of the portion is not covered by an intact layer of the additive AD1 . The term "bare" may mean that at least a part of the light of the portion is emitted and/or reflected from an altered additive layer AD1

Fig. 18b shows a web WEB1 formed by covering and/or impregnating a primary web WEBO with a first additive AD1 , and subsequently covering the first additive AD1 with a second additive AD2. One or both sides of the web WEB1 may be covered. The upper side of the web WEB1 may be covered with the additive AD1 , and also the lower side of the web WEB1 may be covered with the additive AD1 or AD2.

The web WEB1 may comprise covered altered portions DOT1 d, DOT1 e, DOT1 f, DOT1 g, DOT1 h, DOT1 i, DOT1j, and/or DOT1 k. The web WEB1 may comprise bare altered portions DOT2f, DOT2h, DOT2i. A hole filled with the first additive AD1 and subsequently covered with the second additive AD2 may form a covered altered portion DOT1 d. A hole may be formed after the covering and/or impregnating with the first additive AD1 , and the hole may be subsequently filled with a second additive AD2 in order to form a covered altered portion DOT1 e.

A hole may be filled with the first additive AD1 . The first additive AD1 may be locally altered by the laser beam LB1 . The altered portion may be covered with the second additive AD2 in order to form a covered altered portion DOT1 f.

A carbonized (black) portion and the superposed two additive layers AD1 , AD2 may (together) form a covered altered portion DOT1 h.

The color of the first additive AD1 may be locally altered by the laser beam LB1 , and the altered portion of the first additive AD1 may be subsequently covered with the second additive AD2 in order to form a covered altered portion DOT1 i.

The fluorescence of the primary web WEBO may be locally deactivated, and the deactivated portion may be covered with two additive layers AD1 , AD2 to form a covered altered portion DOT1j.

The fluorescence of the first additive AD1 may be locally deactivated, and the deactivated portion may be covered with the second additive AD2 to form a covered altered portion DOT1 k. The color of the superficial layer AD2 may be locally altered to form a bare altered portion DOT2h. The fluorescence of the superficial layer AD2 may be locally deactivated to form a bare altered portion DOT2h.

Referring to Fig. 18c, the web of Fig. 18b may be covered with a third layer of additive AD3. One or both sides of the web may be covered with the third additive AD3. The bare portions DOT2f, DOT2h, DOT2i shown in Fig. 18b may be covered by covering them with the third additive AD3.

Referring to Fig. 19, a laser marking unit 500 may be arranged to provide one or more laser beams LB1 for locally altering the primary web WEBO and/or locally altering the covered web WEB1 . The laser marking unit 500 may comprise e.g. one or more beam deflecting optics 100, 200 arranged to direct a laser beam LB1 to a laser spot SP1 , which is arranged to move with respect to the primary web WEBO (or the web WEB1 ). The intensity of light impinging on the primary web WEBO may be controlled according to the position of the spot SP1 in order to produce the desired altered portions DOT0 (or DOT2).

The web WEBO may be moved in the longitudinal direction SX at a velocity v-i , and a first beam deflector 100 may be arranged to move the laser spot SP1 in a transverse direction SY in order to provide a two-dimensional marking MRK1 and/or MRK2, which has a desired size and shape. The laser spot SP1 may be moved such that it crosses a longitudinal line YREF several times during writing a marking MRK1 and/or MRK2. The altered portions DOT0 of desired size, shape and position may be produced by controlling the intensity of the beam LB1 as a function of the position of the spot SP1 .

An optional second beam deflector 200 may be arranged to periodically move the laser spot SP1 in the longitudinal direction SX. The use of the second beam deflector 200 may allow producing the marking MRK1 and/or MRK2 at an increased velocity v of the web WEBO. The velocity v may be e.g. in the range of 5 to 50 m/s.

The optional second beam deflector 200 may be arranged to move the laser spot SP1 in the longitudinal direction SX. The use of the second beam deflector 200 may allow producing the marking MRK1 and/or MRK2 on a stationary (non-moving) web.

The laser marking unit 500 may comprise e.g. a first rotatable mirror 100 which can be rotated about an axis AX1 by an actuator 120 at an angular speeds co-i . The laser marking unit 500 may comprise e.g. a second rotatable mirror 200 which can be rotated about an axis AX2 by an actuator 220 at an angular speed co₂. The mirror 100 and/or 200 may comprise one or more reflecting facets F1 a, F1 b. The mirror 100 and/or 200 may be a rotating polygon mirror.

The laser module 400 may provide a primary beam LB0. The first rotatable mirror 100 may provide an intermediate beam LB0' by reflecting light of the primary beam LB0. The second rotatable mirror 200 may provide a marking beam LB1 by reflecting light of the intermediate beam LB0'. The light of the marking beam LB1 may be focused to the primary web WEB0 or to the covered web WEB1 by focusing optics, e.g. by a lens.

The laser marking unit 500 may comprise a control unit CNT1 to control the intensity of the laser beam LB1 based on the position of the laser spot SP1 . The control unit CNT1 may be arranged to provide control signals S-ioo, S₂oo, S₄₀o to the laser module 400 and to the actuators 120,220. The signals S-ioo, S200, S₄₀o may be delivered e.g. via cables CA1 , CA2, CA3.

The laser marking apparatus 500 may be arranged to produce markings MRK1 , MRK2, which comprise a dot-matrix pattern. In other words, the apparatus 500 may be arranged to produce a MRK1 and/or MRK2, which comprises a plurality of dots DOT0, DOT2 arranged in a two-dimensional array. The marking MRK1 may be produced by covering and/or impregnating the primary web WEB0 with the additive AD1 after the dots DOT0 have been formed by the laser beam LB1 .

Referring to Fig. 20, a laser marking apparatus 500 may comprise a plurality of individually controllable lasers 400a, 400b, 400c, 400d, 400e. The laser marking apparatus 500 may be arranged to produce markings MRK1 , MRK2, which comprise a dot-matrix pattern. The number N of individually controllable lasers may be e.g. in the range of 4 to 10. A higher number of lasers may provide a marking, which is visually more pleasant. A lower number of lasers may be less expensive. Each laser 400a, 400b, 400c, 400d, 400e may provide a marking laser beam LB 1 a, LB1 b, LB1 c, LB1 d, LB1 e. The intensity of each laser beam LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be individually controllable so as to produce a plurality of different dot-matrix patterns. The intensity of each laser beam LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be controlled individually based on time and/or based on the position of a moving reference point fixed to the primary web WEBO. A marking MRK1 produced by the laser marking apparatus 500 may consist of e.g. 5 x 5 dots or 7 x 5 dots. The marking MRK1 , MRK2 may be e.g. an alphanumeric symbol selected from the group consisting of the latin alphabets from A to Z and the arabic numerals from 0 to 9. The apparatus 500 may also produce dot-matrix markings MRK2 on the covered web WEB1 .

In an embodiment, the laser marking apparatus 500 may be arranged to produce a marking MRK1 on a moving primary web WEBO, which moves at a velocity v1 . In an embodiment, the laser marking apparatus 500 does not need to comprise any moving parts. In an embodiment, the laser marking apparatus 500 may be arranged to produce a marking MRK2 on a moving covered web WEB1 , which moves at a velocity v1 .

Each laser beam may be focused to the primary web WEBO by common focusing optics 350. Alternatively, each laser may have its own focusing optics, i.e. a first focusing optics may be used for focusing a first beam LB1 a, and a second focusing optics may be used for focusing a second beam LB1 b. The apparatus 500 may simultaneously provide a plurality of laser spots SP1 a, SP1 b, SP1 c, SP1 d, SP1 e such that the intensity of each laser spot is individually controllable.

Referring to Fig. 21 , the laser marking apparatus 500 of Fig. 20 or Fig. 22 may also comprise a beam-splitting unit, which is arranged provide a plurality of laser beams LB1 a, LB1 b, LB1 c, LB1 d, LB1 e by distributing light of a primary laser beam LBC. The intensity of each beam LB2a, LB2b, LB2c, LB2d, LB2e may be rapidly modulated e.g. by using a high-speed modulator 420a, 420b, 420c, 420d, 420e. The primary laser beam LBC may be provided by a single laser 400. The laser beam LBC may be a substantially continuous-wave beam. Alternatively, the laser beam LBC may be pulsed in synchronization with producing the altered portions. Alternatively, the laser beam LBC may be pulsed at a frequency, which is substantially higher than the maximum modulation frequency of the modulators 420a, 420b, 420c, 420d, 420e.

Also in this case, the intensity of each laser beam LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be individually controllable so as to produce the desired dot-matrix patterns. The control unit CNT1 may be arranged to control the intensity of the laser beams.

The beam-splitting unit may comprise a plurality of beam splitters 415a, 415b, 415c, 415d. The power division ratios of the splitters 415a, 415b, 415c, 415d may be selected so that each beam LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may have equal maximum intensity. The power division ratio of the first splitter 415a may be e.g. 20%/80%. The power division ratio of the second splitter 415b may be e.g. 25%/75%. The power division ratio of the third splitter 415b may be e.g. 33%/67%. The power division ratio of the fourth splitter 415b may be e.g. 50%/50%. The component 415e may be a reflector, which reflects 100% of the power into the beam LB1 e.

An intensity modulator 420a, 420b, 420c, 420d, 420e may be e.g. an acousto-optic modulator or a MEMS modulator (i.e. a modulator based on a Micro Electro Mechanical System). Each laser beam may be focused to the primary web WEB0 by common focusing optics 350. Alternatively, each beam may have its own focusing optics, i.e. a first focusing optics may be used for focusing a first beam LB1 a, and a second focusing optics may be used for focusing a second beam LB1 b.

The power of the laser 400 may be substantially increased and/or the price of the laser 400 may be substantially decreased if there is no need to rapidly modulate the power of the primary beam LBC provided by the laser 400.

Referring to Fig. 22, the maximum modulation frequency f_MoD of the marking laser beams LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be substantially reduced when the marking laser beams are arranged to move in the same direction as the primary web WEB0. Each marking laser beam may provide a moving laser spot on the primary web WEBO. The longitudinal velocity of the laser spots may be slightly different from the velocity v1 of the primary web WEBO so as to allow writing of a two-dimensional marking MRK1 (or MRK2). The laser spots may be arranged to move e.g. by using one or move rotating beam deflecting facets F2a. A facet F2a may provide the marking beams LB1 a, LB1 b, LB1 c, LB1 d, LB1 e by deflecting light of intermediate beams LBOa, LBOb, LBOc, LBOd, LBOe. In particular, the beam deflecting facet F2a may be reflective surface of a rotating mirror 200. The facet F2a may be rotated by an actuator 220, which may be e.g. an electric motor. All beams LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be moved simultaneously by the same facet F2a or by using several facets. The beams LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be focused by a common focusing optics 350 to form the laser spots. The apparatus 500 may simultaneously provide a plurality of laser spots SP1 a, SP1 b, SP1 c, SP1 d, SP1 e such that the intensity of each spot is individually controllable. The intensity of each beam LB1 a, LB1 b, LB1 c, LB1 d, LB1 e may be individually controlled. Each laser 400a, 400b, 400c, 400d, 400e may be individually controllable. Control signals to for controlling the intensity of the beams may be provided by a control unit CNT1 . A signal for controlling the rotation of the facet F2a may be provided by a control unit CNT1 . The control unit CNT1 may receive a position signal from the actuator 220.

The set-up of Fig. 21 may be used also in the laser marking apparatus 500 of Fig. 22, i.e. a common laser 410 and a plurality of intensity modulators 420a, 420b, 420c, 420d, 420e may be arranged to provide the individually controllable beams LBOa, LBOb, LBOc, LBOd, LBOe instead of using several individually controllable lasers. In an embodiment, graphics and/or text may be subsequently printed on the covered web WEB1 by using a conventional printer. The graphics/text may be known already when the covered web WEB1 is produced. The covered web WEB1 may be produced such that the covered web WEB1 contains a first marking MRK1 , which in turn comprises one or more covered altered portions DOT1 . The first marking MRK1 may be comprise e.g. a fingerprint of the graphics/text, wherein later modification of the graphics/text in a counterfeiting purpose can be detected by using the fingerprint. The first marking MRK1 may be comprise e.g. a checksum representing the graphics/text Referring to Fig. 23a, a covered web WEB1 may comprise a plurality of identical markings MRK1 , e.g. an alphanumeric code "SN98765432". The marking MRK1 may indicate e.g. a serial number of a product, and the covered web WEB1 may be used to provide a plurality of paper documents related to said product (e.g. a contract or a warranty certificate). The marking MRK1 may specify e.g. a manufacturing batch, and the covered web WEB1 may be used to provide documentation related to said manufacturing batch.

In an embodiment, substantially the whole surface of the primary web WEBO may have been covered and or impregnated with the additive AD1 .

The additive AD1 may also be applied e.g. as a stripe so that the covered web WEB1 comprises a treated portion PR1 covered and/or impregnated with the additive AD1 after forming one or more base portions DOT0. Thus, the covered web WEB1 may also comprise an uncovered portion PRO which has not been covered with the additive AD1 and which has not been impregnated with the additive AD1 . The width of the treated portion PR1 (in the direction SY) may be smaller than the total width of the covered web WEB1 (in the direction SY). This may reduce the consumption of the additive AD1 . For example, less than 40% of the top surface of the primary web WEBO may be coated and/or impregnated with the additive AD1 . For example, the area of the treated portion PR1 may be in the range of 1 % to 40% of the area of the top surface of the primary web WEBO. The additive AD1 may be applied on a moving web so that the treated portion PR1 is aligned in the direction of propagation of the web WEB1 . The additive may be applied e.g. by curtain-coating, spraying or brushing.

The additive AD1 may be applied e.g. such that the width of the band-shaped treated portion PR1 of each sheet is e.g. in the range of 2 mm to 50 mm. When coating the whole surface, or when applying the additive AD1 as a continuous band, the coating unit 600 used for applying the additive AD1 does not need to be controlled at a high speed. Referring to Fig. 23b, a covered web WEB1 may comprise different markings MRK1 , e.g. alphanumeric codes "31 OCT 2012 P1 " and "31 OCT 2012 P2". The markings may specify e.g. a date and/or a page number. The web WEB1 may be subsequently cut e.g. along a line CUT1 to form separate sheets. Referring to Fig. 23c, the shape of the treated portion PR1 covered and/or impregnated with the additive AD1 may also substantially correspond to the shape of the altered portion DOT1 . The treated portion PR1 may enclose the altered portion DOT1 such that the treated portion PR1 extends over the base portion DOT0. The treated portion PR1 may also cover an unaltered portion, which surrounds to the base portion DOT0.

For example, the covered altered portion DOT1 may have the shape of the letter "A", and the treated portion PR1 may have a shape which substantially resembles the shape of the letter "A".

The additive AD1 may be applied to a moving primary web WEBO e.g. by using an array of nozzles, wherein the flow of additive through each nozzle may be individually controlled by using fast-acting valves. Referring to Fig. 23d, the covered web WEB1 may comprise a hybrid marking HMRK1 , which comprises one or more covered altered portions DOT1 and one or more second altered portions DOT2. The covered altered portions DOT1 may be produced by forming base portions DOT0 in the primary web WEBO by locally altering at least one optical property of the primary web WEBO with a laser beam LB1 , and by covering and/or impregnating the base portions DOT0 with the additive AD1 . The second altered portions DOT2 may be formed by a laser beam after said covering and/or impregnating. The covered altered portions DOT1 may be formed e.g. by charring the primary web WEBO, and the second altered portions DOT2 may be formed e.g. by deactivating the fluorescence from the additive AD1 . Thus, a subsequent attempt to modify the covered altered portions DOT1 will cause additional deactivation of the fluorescence from the additive AD1 . The covered altered portions DOT1 may be formed e.g. by deactivating the fluorescence from the primary web WEBO, and the second altered portions DOT2 may be formed e.g. by changing the color of the additive AD1 . Thus, a subsequent attempt to modify the covered altered portions DOT1 will cause additional change of color of the additive AD1 .

In an embodiment, the covered altered portions DOT1 may form a first marking MRK1 , and the second altered portions DOT2 may form a second marking MRK2. The shape of the second marking MRK2 may be substantially identical to the shape of the first marking MRK1 , wherein the second marking MRK2 may be displaced with respect to the first marking MRK1 so as to allow comparison of the second marking MRK2 with the first marking MRK1 . A subsequent attempt to modify the first marking MRK1 by a laser beam will also modify the second marking MRK2 so that the shape of the modified second marking MRK2 will be different from the shape of the modified first marking MRK1 . Thus, the modification may be easily detected by comparing the second marking MRK2 with the first marking MRK1 . Both marking MRK1 , MRK2 may form e.g. the same alphanumeric code, e.g. a code "SEC001 ". The hybrid marking HMRK1 may be formed by covering and/or impregnating susbtantially the whole surface of the web WEB1 with the additive AD1 .

Referring to Fig. 23e, the additive AD1 may also be applied as a stripe. The covered web WEB1 may comprise an uncovered portion in order to reduce consumption of the additive AD1 . The additive AD1 may be applied to the moving primary web as a substantially continuous band.

Referring to Fig. 23f, the treated portion PR1 does not need to extend continuously in the longitudinal direction SX. The covered web WEB1 may be formed e.g. so that the flow of the additive AD1 applied to the primary web is switched on and off. This may further reduce consumption of the additive AD1 .

In an embodiment, the composition of the primary web WEBO and the composition of the additive AD1 may be selected such that a covered altered portion DOT1 may be modified at a later stage without altering the additive AD1 , but only when using a laser beam whose wavelength is in a predetermined wavelength range. The composition of the additive AD1 may be selected such that the wavelengths of most common laser types are outside said predetermined wavelength range. The composition of the additive AD1 may be selected such that said predetermined wavelength range does not comprise any of the following wavelengths: 193 nm (excimer laser), 337 nm (nitrogen laser), 1064 nm (Nd:YAG), 10.6 μιτι (carbon dioxide laser). Thus, an attempt to falsify the marking by using e.g. a C02 laser is likely to cause an optically detectable alteration of the additive.

For the person skilled in the art, it will be clear that modifications and variations of the products, methods, and devices according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

1 . A method of producing markings on a web (WEB1 ), the method comprising:

- forming a first base portion (DOT0) in a primary web (WEBO) by locally altering at least one optical property of the primary web (WEBO) with a first laser beam (LB1 ), and

- forming a covered web (WEB1 ) by covering and/or impregnating the primary web (WEBO) with an additive (AD1 ) after the first base portion (DOTO) has been formed,

wherein the primary web (WEBO) comprises cellulose fibers, the covered web (WEB1 ) comprises an optically detectable first altered portion (DOT1 ), the first altered portion (DOT1 ) comprises the first base portion (DOTO) and an amount of additive (AD1 ) bound to the first base portion (DOTO), and the composition of the additive (AD1 ) has been selected such that exposing the covered web (WEB1 ) to a second laser beam (LB2) causes an optically detectable alteration of the additive (AD1 ) contained in the covered web (WEB1 ) in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity (I _MIN,I) needed to cause optically detectable alteration of the primary web (WEBO) contained in the covered web (WEB1 ).

2. The method of claim 1 wherein the primary web (WEBO) comprises a fluorescent substance, and the fluorescence yield of the first altered portion (DOT1 ) is smaller than the fluorescence yield of a reference portion (REF1 ) of the covered web (WEB1 ).

3. The method of claim 1 or 2 wherein a contrast between the first altered portion (DOT1 ) and a reference portion (REF1 ) is not detectable by a naked eye (E1 ) when the covered web (WEB1 ) is illuminated by visible light (VISO) which does not contain ultraviolet light, and wherein the contrast between the first altered portion (DOT1 ) and the reference portion (REF1 ) is detectable by the naked eye (E1 ) when the covered web (WEB1 ) is illuminated by ultraviolet light (VISO).

4. The method according to any of the claims 1 to 3 wherein primary web (W EBO) comprises a fluorescent substance selected from a group consisting of pyrazoline, coumarin and stilbene.

5. The method according to any of the claims 1 to 4 wherein additive (AD1 ) comprises a fluorescent substance.

6. The method of claim 5 wherein the composition of the additive (AD1 ) has been selected such that exposing the additive (AD1 ) to the second laser beam (LB2) locally reduces the fluorescence yield of the additive (AD1 ) contained in the covered web (W EB1 ) , in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity (I _MIN,I ) needed to cause optically detectable alteration of the primary web (W EBO) contained in the covered web (W EB1 ) .

7. The method according to any of the claims 1 to 6 wherein the additive (AD1 ) comprises a fluorescent substance selected from a group consisting of pyrazoline, coumarin and stilbene.

8. The method of claim 1 wherein the color (C1 ) of the first altered portion (DOT1 ) is darker than the color (CO) of a reference portion ( REF1 ) of the covered web (W EB1 ) , the additive (AD1 ) is fluorescent, and the composition of the additive (AD1 ) has been selected such that exposing the additive (AD1 ) to the second laser beam (LB2) locally reduces the fluorescence yield of the additive (AD1 ) contained in the covered web (W EB1 ), in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity (I _MIN,I) needed to cause optically detectable alteration of the primary web (W EBO) contained in the covered web (W EB1 ).

9. The method of claim 1 wherein the color (C1 ) of the first altered portion (DOT1 ) is darker than the color (CO) of a reference portion ( REF1 ) of the covered web (W EB 1 ), and the additive (AD1 ) is arranged to change color when exposed to the second laser beam (LB2).

10. The method according to any of the claims 1 to 9 wherein the optical absorbance of the additive (AD1 ) at the wavelength (λ₀) of the first laser beam (LB1 ) is higher than the optical absorbance of the primary web (WEBO) at the wavelength (λ₀).

1 1 . The method according to any of the claims 1 to 9 wherein the wavelength (λ₀) of the first laser beam (LB1 ) is in the range of range of 1 80 nm to 380 nm.

12. The method of claim 1 1 wherein the wavelength of the second laser beam (LB2) is in the range of range of 1 80 nm to 380 nm.

13. The method according to any of the claims 1 to 9 wherein the wavelength (λ₀) of the first laser beam (LB1 ) is in the range of 800 nm to 1 1 μιτι.

14. The method of claim 13 wherein the wavelength (λ₀) of the second laser beam (LB2) is in the range of 800 nm to 1 1 μιτι.

15. The method according to any of the claims 1 to 14 wherein the wavelength of the second laser beam (LB2) is equal to the wavelength (λ₀) of the first laser beam (LB1 ).

16. The method according to any of the claims 1 to 15 wherein the thickness (do) of the base portion (DOT0) is smaller than the thickness (D_Wo) of the primary web (WEBO).

1 7. The method of claim 1 wherein the first altered portion (DOT1 ) comprises a hole filled with the additive (AD1 ), and the additive (AD1 ) is transparent or translucent.

18. The method according to any of the claims 1 to 17 wherein the composition of the additive (AD1 ) has been selected such that exposing the additive (AD1 ) to the second laser beam (LB2) locally changes the gloss value of the covered web (WEB1 ), in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity (I _MIN,I ) needed to cause optically detectable alteration of the primary web (WEBO) contained in the covered web (WEB1 ).

19. The method according to any of the claims 1 to 18 wherein the upper side of the primary web (WEBO) is covered and/or impregnated with an additive (AD1 ), and also the lower side of the primary web (WEBO) is covered and/or impregnated with an additive (AD1 , AD2).

20. The method according to any of the claims 1 to 19 further comprising producing a second altered portion (DOT2) on the covered web (WEB1 ) by a laser beam (LB2) after the primary web (WEBO) has been covered and/or impregnated with the additive (AD1 ).

21 . The method of claim 20 wherein the second altered portion (DOT2) has a predetermined position with respect to the position of the first altered portion (DOT1 ).

22. The method of claim 20 or 21 wherein the second altered portion (DOT2) has a predetermined size with respect to the size of the first altered portion (DOT1 ).

23. The method according to any of the claims 20 to 22 comprising forming an alphanumeric marking (MRK1 , MRK2), which comprises the first altered portion (DOT1 ) and the second altered portion (DOT2).

24. The method according to any of the claims 1 to 23 wherein the velocity (v1 ) of the primary web (WEBO) during the exposure to the first laser beam (LB1 ) is in the range of 5 to 50 m/s.

25. The method according to any of the claims 1 to 25 wherein substantially the whole top surface of the primary web (WEB1 ) is covered and/or impregnated with the additive (AD1 ).

26. The method according to any of the claims 1 to 25 wherein less than 40% of area of the top surface of the primary web (WEB1 ) is covered and/or impregnated with the additive (AD1 ).

27. A covered web (WEB1 ), comprising a primary web (WEBO) covered and/or impregnated with an additive (AD1 ), the covered web (WEB1 ) further comprising an optically detectable first altered portion (DOT1 ) obtained by forming a first base portion (DOT0) in the primary web (WEBO) by using a first laser beam (LB1 ) before the primary web (WEBO) is covered and/or impregnated with the additive (AD1 ),

wherein the primary web (WEBO) comprises cellulose fibers, and the additive (AD1 ) has been selected such that exposing the web (WEB1 ) to a second laser beam (LB2) causes an optically detectable alteration of the additive (AD1 ) in a situation where the intensity of the second laser beam (LB2) is equal to a minimum threshold intensity needed to cause optically detectable alteration of the primary web (WEBO) contained in the covered web (WEB1 ).

28. The web (WEB1 ) of claim 27 wherein the fluorescence yield of the first altered portion (DOT1 ) is smaller than the fluorescence yield of a reference portion (REF1 ) of the web (WEB1 ).