WO2013156664A1

WO2013156664A1 - A method and an apparatus for producing markings on a moving web

Info

Publication number: WO2013156664A1
Application number: PCT/FI2012/050393
Authority: WO
Inventors: Lauri KURKI; Petri Laakso; Teuvo LEPPÄNEN; Jussi Tenhunen; Pasi Virtanen
Original assignee: Upm-Kymmene Corporation
Priority date: 2012-04-20
Filing date: 2012-04-20
Publication date: 2013-10-24
Also published as: EP2838732A1; CN104245332B; CN104245332A; EP2838732A4

Abstract

A method for producing markings (MRK1 ) on a moving web (WEB1 ) comprises: - moving the web (WEB1 ) in a longitudinal direction (SX), - delivering laser light (LB2) to a moving aiming point (SP2) by using a first beam steering body (100) and a second beam steering body (200) such that the position (x(t),y(t)) of the aiming point (SP2) depends on the angular orientation (a) of the first body (100) and on the angular orientation (β) of the second body (200), - rotating the second body (200) from a first angular orientation (β1 ) to a second angular orientation (β2), the rotation from the first angular orientation (β1 ) to the second angular orientation (β2) defining a writing period (TA), wherein the rotation of the second body (200) during the writing period (TA) moves the aiming point (SP2) such that the average longitudinal velocity component (vx2) of the aiming point (SP2) is in the range of 50 to 150% of the velocity (v^ of the web (WEB1 ), - rotating the first body (100) such that the aiming point (SP2) crosses a longitudinal reference line (YREF) a plurality of times during the writing period (TA), and - controlling the intensity of the laser light (LB2) according to the angular orientation (a) of the first body (100) and according to the orientation (β) of the second body (200).

Description

A METHOD AND AN APPARATUS FOR PRODUCING MARKINGS ON A

MOVING WEB

FIELD OF THE INVENTION

The present invention relates to producing markings on a moving web.

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 a marking on a moving web. An object of the invention is to provide an apparatus for producing a marking on a moving web. An object of the invention is to provide a product comprising a marking produced by said apparatus.

According to a first aspect of the invention, there is provided a method according to claim 1 .

According to a second aspect of the invention, there is provided an apparatus according to claim 13. According to a third aspect of the invention, there is provided a computer program according to claim 25.

According to a fourth aspect of the invention, there is provided a computer program product according to claim 26. According to a fifth aspect of the invention, there is provided a product according to claim 27.

A marking may be produced on a moving web by a laser beam, which is directed to the web according to a predetermined scanning pattern, by using a first rotating beam steering body and a second rotating beam steering body. The intensity of the laser beam may be controlled based on the instantaneous position of the laser beam impinging on the web so as to locally alter the structure and/or chemical composition of the web at selected positions. For example, a plurality of holes may be formed on a paper web. The holes may together form at least a part of a marking.

Producing markings on a fast-moving web may be challenging due to a high peak intensity needed to form visible marks on the fast-moving web. A given portion of the moving web may spend only a very short time period in the vicinity of a marking apparatus, and the time period available for producing the marking may be short. A given portion of the moving web spends a very short time in the vicinity of a marking apparatus, and timing of operations needed to produce the marking may be critical.

A marking may be produced by steering the direction of the laser beam according to a two-dimensional scanning pattern, and modulating the intensity of the laser beam according to the instantaneous position of the laser spot. In particular, the scanning pattern may consist of a plurality of inclined tracks.

The marking laser beam may be deflected in a transverse direction by using a first beam steering body, which may be rotated to distribute laser light obtained from a single laser to a plurality of transverse positions. The marking laser beam may be deflected in the longitudinal direction by using a second beam steering body. The marking laser beam may be deflected such that a laser spot formed on the web follows the movement of the web. This may allow reducing the peak power of the laser, this may allow more precise positioning of the laser spot with respect to markings already produced on the web, and/or this may allow reducing the modulation frequency of the laser beam. The first body and/or the second body may comprise a plurality of light deflecting regions. In an embodiment, the first body and the second body may be polygons having reflective facets. When the first body comprises several light deflecting regions, this may allow shortening a useless time between two consecutive lateral sweeps. When the first body comprises several light deflecting regions, this may also allow transverse sweeping at a high frequency, without a need to use a high rotation speed of the first body. When the second body comprises several light deflecting regions, this may allow shortening a useless time between producing two consecutive markings on the moving web.

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 shows, in a three-dimensional view, a device for producing a marking on a moving web, shows, in a three-dimensional view, a beam steering body comprising several light deflecting regions, shows, in a three-dimensional view, a device for producing a marking on a moving web, wherein at least one beam steering body of the device comprises two or more light deflecting regions, shows, in a three-dimensional view, position of a laser spot with respect to a stationary reference point,

Fig. 2b shows, in a side view, angular orientation of the marking laser beam, shows, in an end view, angular orientation of the marking laser beam, shows, in a side view, changing the longitudinal position of a laser spot, shows angular orientations of light deflecting regions of the rotating first body at an instant of time, shows angular orientations of light deflecting regions of the rotating second body at an instant of time, shows, in top view, trajectories of the laser spot in a stationary coordinate system, shows, in top view, trajectories of the laser spot in a moving coordinate system, shows, in top view, markings consisting of a full array of dots formed on a web, shows, in top view, markings produced on a web, shows, in top view, trajectories of the laser spot in a stationary coordinate system, wherein the trajectories have been superposed on a still image of a dot pattern formed on the moving web, shows, by way of example, trajectory of a laser spot in a situation where rotation of the second body is stopped, and the rotation of the first body does not cause a longitudinal velocity component,

Fig. 6c shows, by way of example, trajectory of a laser spot in a situation where rotation of the second body is stopped, and the rotation of the first body causes a longitudinal velocity component, Fig. 6d shows a velocity diagram for a situation where the rotation of the first body does not cause a longitudinal velocity component,

Fig. 6e shows a velocity diagram for a situation where the rotation of the first body causes a longitudinal velocity component,

Fig. 6f shows, in top view, trajectories of the laser spot in the stationary coordinate system, wherein said trajectories are superposed on a still image of a dot pattern formed on the moving web,

Fig. 6g shows, in a top view, a first body having an inclined rotation axis,

Fig. 6h shows, in a three dimensional view, modifying the orientation of a trajectory by using a beam rotator,

Fig. 6i shows, in a three-dimensional view, a first body having inclined facets,

Fig. 7 shows, in a side view, a web processing apparatus comprising a marking device,

Fig. 8a shows units of a marking device,

Fig. 8b shows units of a marking device,

Fig. 9 shows, in a side view, an optical set-up of the marking device,

Fig. 10 shows, in a three dimensional view, an optical set-up of the marking device,

Fig. 1 1 shows, in a side view, a usable angular range for the marking laser beam,

Fig. 12a shows longitudinal position of the laser spot as a function of time in the moving coordinate system, shows, as a comparative example, longitudinal position of the laser spot as a function of time in the moving coordinate system, without using the second rotating body, shows transverse position of the laser spot as a function of time, shows longitudinal position of the laser spot as a function of time in the stationary frame, shows longitudinal position of the laser spot as a function of time in the moving frame, shows transverse position of the laser spot as a function of time, when using an additional direction modulator, shows modulating direction of a light beam by using a the first rotating body and an additional direction modulator, and shows a laser unit comprising a laser module and a controllable intensity modulator.

DETAILED DESCRIPTION Referring to Fig. 1 , an apparatus 500 may comprise a laser unit 400, a first beam steering body 100, and a second beam steering body 200. The apparatus 500 may be arranged to produce one or more markings MRK1 on a moving web WEB1 . The web WEB1 may be moved in the longitudinal direction SX at a velocity v-, . The laser unit 400 may provide laser light, which may be delivered by the beam steering bodies 100, 200 to form a marking laser beam LB2. The direction of the marking laser beam LB2 may define a laser spot SP2 on the moving web WEB1 .

The beam steering bodies 100, 200 may be rotatable. The first body 100 may comprise one or more light deflecting regions F1 a. The second body 200 may comprise one or more light deflecting regions F2a. The laser 400 may provide a primary beam LBO. The first beam steering body 100 may provide an intermediate beam LB1 by deflecting light of a primary beam LBO such that the direction of the intermediate beam LB1 depends on the angular orientation of the first beam steering body 100. The second beam steering body 200 may provide a (marking) laser beam LB2 by deflecting light of the intermediate beam LB1 such that the direction of the laser beam LB2 depends on the angular orientation of the second beam steering body 200. The laser beam LB2 may impinge on the web WEB1 at a spot SP2. The laser beam LB2 may be optionally focused to the spot SP2 by using (stationary) focusing optics 350 (Fig. 3a). The beam steering bodies 100, 200 may be arranged to deliver laser light LB2 to the spot SP2 such that the position of the spot SP2 depends on the angular orientation of the first body 100 and on the angular orientation of the second body 200. The first beam steering body 100 may be rotated to change the transverse position of the laser spot SP2 (in the direction SY). The second body 200 may be rotated to change the longitudinal position of the laser spot SP2 (in the direction SX). SX, SY and SZ denote orthogonal directions. When the second body 200 is rotated alone, without rotating the first body 100, the spot SP2 may move substantially in the longitudinal direction SX. Rotation of the second body 200 may be arranged to cause a longitudinal velocity component of the spot SP2 such that the spot SP2 may move in the same direction as the web WEB1 . A marking MRK1 may be formed during a time period, which may be called as a writing period. An average longitudinal velocity component of the spot SP2 during the writing period may be in the range of 50% to 150% of the velocity v₁ of the web WEB1 hus, the spot SP2 may "follow", "catch up" or even "overtake" the moving web WEB1 . When the first body 100 is rotated alone, without rotating the second body 200, the spot SP2 may move substantially in a transverse direction which is perpendicular to the longitudinal direction SX or inclined with respect to the longitudinal direction SX. Rotation of the first body 100 may be arranged to move the spot SP2 such that the spot SP2 crosses a reference line YREF. The reference line YREF may be a longitudinal reference line. The first body 100 may be arranged rotate such that the spot SP2 crosses the reference line YREF several times during the writing period. The bodies 100, 200 may be arranged to rotate simultaneously such that the laser spot SP2 moves along a plurality of inclined and adjacent tracks (See Figs. 4a and 6). The laser spot SP2 may be arranged to move according to a scanning pattern, which may comprise a plurality of adjacent (straight or curved) lines (see Fig. 4a). The scanning pattern may also be called as a sweeping pattern. The power of the laser beam LB2 may be varied according to the instantaneous position of the spot SP2 such that transformation of the web WEB1 takes place at the desired locations of the web WEB1 .

The intensity of laser light LB2 delivered by the beam-steering bodies 100, 200 to the spot SP2 may be controlled based on the instantaneous angular orientations of the bodies 100, 200 in order to form one or more markings MRK1 on a moving web WEB1 . A marking MRK1 may comprise e.g. graphical symbols, e.g. letters and/or numbers. A marking MRK1 may comprise e.g. the letters "ABC".

The device 500 may comprise a control unit CNT1 , which may be arranged to send a signal Si₀o for controlling rotation of the body 100. The control unit CNT1 may be arranged to send a signal S₂oo for controlling rotation of the body 200. The control unit CNT1 may determine the location of the spot SP2 e.g. based on the control signals S-ioo, S₂oo- The control unit CNT1 may determine the location of the spot SP2 e.g. based on position signals S_«, Sp obtained from positions sensors (See Figs.8a, 8b). The control unit CNT1 may be arranged to control the intensity delivered to the spot SP2 as a function of the location of the spot SP2. The control unit CNT1 may be arranged to send a signal S₄₀o for controlling the intensity of the light LB2 impinging on the spot SP2. The signals may S-ioo, S₂oo, S₄₀o may be communicated e.g. via electrical or optical cables CA1 , CA2, CA3.

The angular orientation of the first body 100 may be changed by a first actuator 120. The angular orientation of the second body 200 may be changed by a second actuator 220. The actuator 120 and/or 220 may be e.g. an electric motor, an electrodynamic actuator (e.g. based on a coil moving in a magnetic field, or based on a magnet moving in a varying magnetic field), an electrostatic actuator, in particular a MEMS actuator (Micro Electro Mechanical Systems), or a piezoelectric actuator.

The first actuator 120 may rotate the first body 100 about a first rotation axis AX1 . The second actuator 220 may rotate the second body 200 about a second rotation axis AX2. The angular orientation of the first body 100 may be changed at an angular velocity co-i , which may be constant or temporally varying. The rotational movement of the body 100 may be unidirectional or bidirectional. The body 100 may be arranged to rotate several complete revolutions or less than one revolution (i.e. less than 360°). The angular orientation of the second body 200 may be changed at an angular velocity co₂, which may be constant or temporally varying. The rotational movement of the body 200 may be unidirectional or bidirectional. The body 200 may be arranged to rotate several complete revolutions or less than one revolution (i.e. less than 360°).

Rotating the body 100 several complete revolutions at a substantially constant angular velocity coi may help to reduce vibrations. Thanks to rotating the first body at a substantially constant velocity, the amplitude of mechanical vibrations generated by the rotating first body may be smaller when compared with a situation where the beam would be steered in the transverse direction by a tilting mirror, which is accelerated and decelerated several times during producing a marking. The low vibrations may allow producing the markings improved precision. Rotating the body 100 several complete revolutions at a substantially constant angular velocity may provide a high number of transverse sweeps per unit time. Rotating the body 100 several complete revolutions at a substantially constant angular velocity may allow very accurate and/or reproducible timing of laser pulses with respect to the transverse position of the moving spot SP2.

Also the second body 200 may be rotated several complete revolutions at a substantially constant angular velocity co₂ e.g. in order to reduce vibrations and/or in order to provide accurate and/or reproducible timing of laser pulses with respect to the longitudinal position of the moving spot SP2_. Thanks to the rotation at substantially constant angular velocity, the instantaneous angular orientation of the bodies may be monitored with high precision even when using a very simple position sensor (e.g. an opto switch) to detect the instantaneous angular orientations of the bodies.

On the other hand, rotating the body 100 and/or 200 less than 360° at a temporally varying angular velocity may provide greater adaptability to produce different markings. Rotating the body 200 at a temporally varying angular velocity may allow producing three consecutive markings on the moving web WEB1 such that the distance L_BL between a first marking and a second marking is different from a distance between the second marking and a the third marking. Rotating the body 200 at a temporally varying angular velocity may allow producing consecutive markings on the moving web WEB1 such that the distance L_BL and/or L_SYNC can be rapidly adjusted (the distances L_BL and L_SYNC are shown e.g. in Fig. 5a).

The first body 100 may be rotated at a substantially constant angular velocity col to provide a high transverse sweeping rate, and the second body 200 may be rotated at a non-constant angular velocity co₂ in order to provide more freedom to select the longitudinal position of the marking MRK1 with respect to the moving web WEB1 .

The laser light LB2 may be delivered to the spot SP2 as short laser pulses so that the position of the spot SP2 is changed also when the intensity of light delivered to the spot SP2 is low or equal to zero. The spot SP2 may move according to the rotation of the bodies 100, 200 also when no laser light is delivered to the spot SP2. The spot SP2 may move according to the rotation of the bodies 100, 200 also between consecutive laser pulses. The spot SP2 may also be called as an aiming point. The apparatus 500 may be arranged to deliver laser light LB2 to the moving aiming point SP2 by using the beam steering arrangement comprising the first beam steering body 100 and the second beam steering body 200, wherein the position (x(t),y(t)) of the aiming point SP2 (see Fig. 2a) depends on the angular orientation a of the first body 100 and on the angular orientation β of the second body 200 (see Figs 3a and 3b). The spot SP2 may be moved according to a scanning pattern such that laser light is delivered (e.g. focused) to the moving laser spot SP2 (only) at those instants when the position of the laser spot SP2 coincides with the desired locations of the dots D1 of the marking MRK1 . For the majority of time, the spot SP2 may be moved according to the scanning pattern so that the laser light is not delivered to the spot SP2.

The web WEB1 may move at a velocity v-i . The velocity v may be e.g. in the range of 5 to 50 m/s. In particular, the velocity v may be in the range of 10 to 30 m/s.

The laser beam LB2 impinging on the moving web WEB1 may form a laser spot SP2 on the web WEB1 . The energy of the laser beam LB2 delivered to the spot SP2 may be at least partly absorbed in the web WEB1 , causing local and irreversible alteration of structure and/or chemical composition of the material of the web WEB1 . For example, a hole may be burned in the WEB1 . For example, the diameter of a hole formed in a moving paper web may be e.g. in the range of 0.05 mm to 1 mm. Material may be ablated away from the web WEB1 such that the thickness of the web WEB1 is locally decreased. Chemical composition of the web may be changed. In particular, the material of the web WEB1 may be locally carbonized (charred) so as provide brown or black color.

The web WEB1 may comprise e.g. paper and/or cardboard. The web WEB1 may comprise e.g. metal foil and/or polymer foil. The web WEB1 may comprise metal foil in addition to paper or cardboard. The web WEB1 may comprise polymer foil in addition to paper or cardboard. The web WEB1 may comprise metal foil and polymer foil in addition to paper or cardboard. The web WEB1 may optionally comprise material, which absorbs radiation at the wavelength of the laser beam LB2 (e.g. a dye). The laser 400 may be e.g. a carbon dioxide (C0₂) laser.

Referring to Fig. 1 b, the first rotating body 100 may comprise m beam deflecting regions F1 a, F1 b. The beam deflecting regions of the first body 100 may be regularly or irregularly spaced. In particular, the beam deflecting regions F1 a, F1 b may be positioned such that the rotating body 100 has m fold rotational symmetry with respect to a first axis AX1 of revolution. This may be advantageous e.g. when minimizing vibrations and/or the useless time between consecutive transverse sweeps. The number m may be e.g. in the range of 1 to 1000. The maximum transverse dimension W_A (Fig. 5a) of a marking MRK1 , which can be produced by the apparatus may depend on the number m-i . Smaller values of m may allow producing markings with a greater transverse dimension W_A. However, when the number m is very low (e.g. in the range of 1 to 4), the transverse dimension W_A may also be limited by the properties by other optical components of the apparatus 500, e.g. by an acceptance angle of (optional) focusing optics 350 (Fig. 3a) and/or by an acceptance angle of (optional) relay optics 320 (shown in Figs. 9 and 10). Increasing the value of m may allow producing small markings with a higher transverse duty cycle r\ This, in turn, may allow reducing the laser power and/or modulation frequency. The number m may be selected to be e.g. greater than or equal to 5 in order to avoid reduction of transverse duty cycle r\_v caused by the limited acceptance angle of the optics. The number m may be e.g. in the range of 5 to 1000. In particular, the number m may be e.g. in the range of 50 to 1000. Referring to Fig. 1 c, the second rotating body 200 may comprise m₂ beam deflecting regions F2a, F2b. The beam deflecting regions of the second body 200 may be regularly or irregularly spaced. In particular, the beam deflecting regions F2a, F2b may be positioned such that the rotating body 200 has m₂ - fold rotational symmetry with respect to a second axis AX2 of revolution. This may be advantageous e.g. when synchronizing production of markings with a web WEB1 , which will be later cut into a plurality of sheets having the same dimensions. The number m₂ may be e.g. in the range of 1 to 100. Advantageously, the number m₂ is in the range of 5 to 50. When the bodies 100, 200 are symmetrical, this may help to minimize mechanical vibration. The vibration may reduce the accuracy and/or operating life of the apparatus 500.

The term "deflection" may mean reflection, refraction and/or diffraction such that a (non-zero) change of direction takes place. The diffraction may be reflective diffraction in the order -3, -2, -1 , 0, 1 , 2 or 3 or transmissive diffraction in the order -3, -2, -1 , 1 , 2 or 3. Transmissive diffraction in the order 0 does not "deflect" light.

The beam deflecting regions F1 a, F1 b, F1 c, F2a, F2b, F2c may be e.g. reflective surfaces (mirrors), diffractive surfaces (diffractive gratings) or refractive surfaces (of prisms). The regions F1 a, F1 b, F1 c, F2a, F2b, F2c may also be called as "facets".

In particular, the body 100 may be a metallic or metal-coated polygon having m fold rotational symmetry about the rotation axis AX1 wherein the number m-ι may be e.g. in the range of 5 to 1000. In particular, the body 200 may be a metallic or metal-coated polygon having m₂-fold rotationally symmetry about the rotation axis AX2 wherein the number m₂ may be e.g. in the range of 5 to 50. The facets of the bodies 100, 200 may be polished and/or coated with a reflective coating. In particular, the bodies 100, 200 may comprise aluminum coated with gold or rhodium.

The use of a reflecting region may maximize a change in the direction of a deflected beam corresponding to change in the angular orientation of the reflecting region.

The regions F1 a, F2b, F2a, F2b may be reflective. This may maximize a change in the direction of the marking beam LB2 corresponding to the rotation of the body 100 or 200 by a given angular interval.

The normal of each region F1 a, F1 b of the first body 100 may be substantially perpendicular to the rotation axis AX1 . The normal of each region F2a, F2b of the second body 200 may be substantially perpendicular to the rotation axis AX2. This may also maximize change in the direction of the marking beam LB2 corresponding to the rotation of the body 100 or 200 by a given angular interval.

The regions F1 a, F1 b of the first body 100 may be arranged to provide a first deflected beam LB1 by deflecting light of an input beam LB0 such that the direction of the first deflected beam LB1 varies as the a function of the angular orientation a of the first body 100 when the first body 100 is rotated around the first rotation axis AX1 .

The regions F2a, F2b of the second body 200 may be arranged to provide a second deflected beam LB2 (i.e. the marking beam LB2) by deflecting light of the first deflected beam LB1 such that the direction of the second deflected beam LB1 varies the a function of the angular orientation β of the second body 200 when the second body 200 is rotated about the second rotation axis AX2.

Rotation of the first body 100 may change the orientation of the deflecting regions F1 a, F1 b of the body 100, in such a way that the spot SP2 repetitively moves in a transverse direction SY or ST. Rotation of the second body 200 may change the orientation of the deflecting regions F2a, F2b of the body 100, in such a way that the spot SP2 repetitively moves in a longitudinal direction SX.

In an embodiment, rotation of the first body 100 may change the orientation of the deflecting regions F1 a, F1 b of the body 100 such that the spot SP2 repetitively moves in an inclined direction.

Simultaneous rotation of the first body 100 and the second body 200 may cause the spot SP2 to move along several inclined and adjacent tracks TRAC1 , TRAC2 (Figs. 4a, 6). The tracks TRAC1 , TRAC2 may be straight or curved, depending on the optical configuration of the apparatus 500. The orientation of the tracks may be defined by a direction ST1 , which may deviate from the direction SY. An angle γΐ between the directions SY and ST1 may be e.g. in the range of 1 ° to 80°.

The web WEB1 may be arranged to move in the longitudinal direction SX during marking with the laser beam LB2. The plane of the web WEB1 may be parallel to a plane defined by the directions SX and SY. The operation of the apparatus 500 may be described in a stationary coordinate system (stationary frame), which may be fixed e.g. to the axes AX1 , AX2, and by using a movable coordinate system (moving frame), which is fixed to the moving web WEB1 . SX, SY and SZ denote orthogonal directions of the stationary frame. A position in the stationary frame may be defined e.g. by using coordinates x,y,z, respectively. SU, SY and SZ denote orthogonal directions of the moving frame. A position in the moving frame may be defined e.g. by using coordinates u,y,z, respectively. The direction SU is parallel to the direction SX. YREF denotes a reference line. The reference line YREF may be parallel to the longitudinal direction SX.

The orientation of a first facet F1 a of the first body 100 may be different from the orientation of a second facet F1 b of the first body 100. The orientation of a first facet F2a of the second body 200 may be different from the orientation of a second facet F2b of the second body 200. The first body 100 may be rotated several complete revolutions around the first rotation axis AX1 such that the facets F1 a, F1 b of the body 100 are simultaneously rotated at a substantially constant angular velocity co-i . The second body 200 may be rotated several complete revolutions around the second rotation axis AX2 such that the facets F2a, F2b of the body 200 are simultaneously rotated at a substantially constant angular velocity co₂. When using the substantially constant angular velocity coi and/or co₂, the first body and/or the second body may have considerable moment of inertia, which may facilitate maintaining a precisely controlled rotation speed and timing. It is not necessary to rapidly accelerate and decelerate the rotation speeds of the bodies when producing markings on a fast moving web.

Referring to Fig. 2a, the marking device 500 may provide a laser spot SP2, whose position may be defined by coordinates x, y in the stationary coordinate system. The laser beam LB2 may be approximated by a line, which is pivoted at a pivot point PP1 . The reference line N1 may represent a nominal (central) orientation of the laser beam LB2. The reference line N1 may intersect the web WEB1 at a point REF1 . x denotes the coordinate of the laser spot SP2 in the direction SX, and y denotes the coordinate of the laser spot SP2 in the direction SY. The direction of the laser beam LB2 may be defined by the two angles θ-ι , θ₂ shown in Figs. 2b and 2c. θι may denote an angle between the direction SY and the projection of the laser beam LB2 on a plane defined by the directions SX and SZ. θ₂ may denote an angle between the direction SY and the projection of the laser beam LB2 on a plane defined by the directions SY and SZ. The beam LB2 may also be considered to have a direction defined by the angular orientations of the bodies 1 00, 200 also when the power of laser light provided by a laser is zero. The beam LB2 may also be called as an aiming beam LB2.

Referring to Fig. 3a, the web may move at a velocity v-, . Thus, a dot D1 formed on the web WEB1 may propagate at a velocity v with respect to the stationary reference REF1 . The laser spot SP2 may be moved in the transverse direction SY by rotating the first body 1 00. Rotation of the first body 1 00 about the axis AX1 may cause a transverse velocity component v_y in the direction SY (see e.g. Fig. 6c). The laser spot SP2 may be moved in the longitudinal direction SX by rotating the second body 200. Rotation of the second body 200 about the axis AX2 may cause a longitudinal velocity component v^. In an embodiment, the laser spot SP2 may be moved in the longitudinal direction also by rotating the first body 1 00. Rotation of the first body 1 00 about the axis AX1 may cause a longitudinal velocity component v_x (see e.g. Figs. 6c and 6e). A total longitudinal velocity component v_x of the spot SP2 may be formed as the sum of the velocity components v_x , v_x2. The spot may move along an inclined trajectory at a velocity v_T, which is formed as the vector sum of the transverse velocity component v_y and the total longitudinal velocity component v_x.

The difference v-|-v_x2 may be called as the relative velocity V_REL- The relative velocity V_REL may represent a time-averaged longitudinal velocity of the spot SP2 with respect to the moving web WEB1 , when averaged during a single longitudinal sweep of the spot SP2. The movements of the spot SP2 may be expressed with respect to a predetermined point of the web WEB1 , in particular with respect to a predetermined dot D1 formed on the web WEB1 . The position of the laser spot SP2 may be defined by a time-dependent coordinate x(t). The position of the dot D1 may be defined by a time- dependent coordinate χροτ(ΐ). The light forming the laser beam LB2 may be focused in order to increase intensity at the spot SP2. The light may be focused e.g. by using focusing optics 350 positioned near the laser 400, after the second body 200, or at an intermediate location. However, the risk of damaging optical surfaces may be minimized when the focusing optics 350 is positioned after the second body 200. The focusing optics 350 may be e.g. a lens having a focal length f₂.

OC2 denotes the central axis of a deflected beam LB2 provided by the second body 200. OC1 denotes the central axis of a light beam LB1 , wherein the light beam LB1 may consist of light which has been deflected by the first body 100.

Referring to Fig. 3b, the angular orientation a of the first body 100 may be expressed e.g. by the angular orientation of a light deflecting region F1 a of the first body 100. A first region F1 a may have an angular orientation cci with respect to a reference (e.g. the direction SZ). A second region F1 b may have an angular orientation cc₂ with respect to the reference (e.g. the direction SZ). The laser light LBO may impinge on the region F1 a e.g. when the angular orientation a of the body 100 in the range cci to cc₂. The angular interval Δα-ι₂ of the region F1 a may be equal to the difference cci - cc₂. The angular interval Δα-ι₂ may represent the angular width of the region F1 a with respect to the axis AX1 . The interval Δα-ι₂ may also represent angular difference between the orientation of a first region F1 a of the first body 100 and the orientation of the second region F1 b of the first body 100.

In case of m-i-fold rotational symmetry, the interval Δα-ι₂ may be equal to 360°/m-| . However, in an embodiment, the regions of the body 100 and/or of the body 200 may be positioned at irregular angular intervals. In an embodiment, one or more of the regions may be removed or inactive (e.g. black).

The spot SP2 may make a single complete transverse sweep when the body 100 is rotated by the angle Δα-ι₂. When number of deflective regions F1 a, F1 b is equal to m-,, the spot SP2 may cross the longitudinal reference line YREF m-ι times during a single complete rotation of the first body 100

Referring to Fig. 3c, the angular orientation β of the second body 200 may be expressed e.g. by the angular orientation of a light deflecting region F2a of the second body 200. A first region F2a may have an angular orientation β with respect to a reference (e.g. the direction SZ). A second region F2b may have an angular orientation β₂ with respect to the reference (e.g. the direction SZ).

The laser light LB1 may impinge on the region F2a e.g. when the angular orientation β of the body 200 in the range βι to β₂. The angular interval Δβ ₂ of the region F1 a may be equal to the difference βι - β₂. The angular interval Δβ ₂ may represent the angular width of the region F2a with respect to the axis AX2. The interval Δβ ₂ may also represent angular difference between the orientation of a first region F2a of the second body 200 and the orientation of the second region F2b of the second body 200.

In case of m₂-fold rotational symmetry, the interval Δβ ₂ may be equal to 360°/m₂. However, in an embodiment, the regions of the body 200 may be positioned at irregular angular intervals. In an embodiment, one or more of the regions may be removed or inactive (e.g. black).

The spot SP2 may make a single longitudinal sweep when the body 100 is rotated by the angle Δβ ₂.

Fig. 4a shows a scanning pattern SCANPAT1 , which may be formed by rotating the first body 100 at the angular velocity co-i , and by rotating the second body at the angular velocity co₂. The pattern may comprise a plurality of adjacent lines TRAC1 , TRAC2. The direction of the beam LB2 may be changed such that the spot SP2 moves along the line TRAC1 , and then jumps to move along the adjacent line TRAC2. Fig. 4a shows the scanning pattern in the stationary coordinate system. YREF denotes a longitudinal reference line. The longitudinal reference line YREF may have the same transverse position as the reference position REF1 . The first line TRAC1 may represent points scanned when the beam LBO impinges on a (single) rotating facet F1 a of the body 100 (see Fig. 1 ). The second line TRAC2 may represent points scanned when the beam LBO impinges on an adjacent rotating facet F1 b (or F1 c) of the body 100 (The beam LBO is shown e.g. in Figs. 1 a-1 c).

The small open circles represent points where the laser 400 is arranged to deliver a laser pulse. Fig. 4b shows how the scanning pattern SCANPAT1 of Fig. 4a would appear in the moving coordinate system. Marking with the laser represents a mapping operation from the stationary coordinate system to the moving coordinate system. Sweeping along the line TRAC1 shown in Fig. 4a may provide markings at the line TRAC1 ' shown in Fig. 4b. Sweeping along the line TRAC2 shown in Fig. 4a may provide markings on the line TRAC2' shown in Fig. 4b. Sweeping along the tracks TRAC10, TRAC1 1 may provide the markings on the lines TRAC10', TRAC1 1 '.

The longitudinal velocity component of the spot SP2 caused by the second rotating body 200 may be lower than the velocity v-i . As shown in Fig. 4b the order of the lines TRAC2', TRAC1 ' may be reversed when compared with the order of lines TRAC1 , TRAC2 shown in Fig. 4a.

The longitudinal velocity component v_x2 may also be higher than the velocity v-i . In this case, the lines TRAC1 ', TRAC2' would have the same order as the lines TRAC1 , TRAC2 shown in Fig. 4a. Fig. 1 represents an example where the longitudinal velocity component is higher than the velocity v of the web WEB1 . The longitudinal velocity component v_x2 of the spot SP2 may be e.g. in the range of 50 to 150% of the velocity v-, . The longitudinal velocity component Vx2 of the spot SP2 may be called as the time-averaged longitudinal velocity component v_x2, which may be averaged during a single writing period T_A. In an embodiment, the longitudinal velocity component Vx2 of the spot SP2 may be e.g. in the range of 50 to 150% of the velocity v _; but different from the velocity v _; in order to produce a marking MRK1 which comprises dots D1 arranged in a two-dimensional array.

Using the longitudinal velocity component v^, which is smaller than the velocity v may be advantageous because it may be implemented by using a lower angular velocity co₂ of the second body 200.

In an embodiment, the longitudinal velocity component of the spot SP2 may be equal to the velocity v of the web WEB1 . If the longitudinal velocity component v_x2 of the spot SP2 is equal to the velocity v of the web WEB1 , this may allow producing a one dimensional marking MRK1 on the web WEB1 . In other words, the dots D1 of the marking would be arranged on a single line. The stationary reference point REF1 may draw a straight line NPATH in the moving coordinate system. A longitudinal position in the moving frame may be defined by a coordinate u with respect to a reference dot D1 , which moves together with the web WEB1 . When v_x2≠ v _; the longitudinal position u(t) of the laser spot SP2 may change as a function of time in the moving coordinate system fixed to the web WEB1 .

Several markings MRK0, MRK1 , MRK2 may be formed, which are separate from each other. Referring to Fig. 5a, the power of the laser beam LB2 may be modulated in order to form markings MRK1 , MRK2 comprising several dots D1 . Fig 5a shows the markings in the moving coordinate system (fixed to the web WEB1 ). A single dot D1 may have a longitudinal dimension DIM1 and a transverse dimension DIM2. The transverse dimension DIM2 is typically greater than or equal to the longitudinal dimension DIM1 . A desired value of the transverse dimension DIM2 may set by selected adjusting the duration of a pulse of the beam LB2. A short pulse may provide a short dimension DIM2. A long pulse may provide a long dimension DIM2. Increasing the number of dots D1 on a single line TRACT may require increasing the modulation frequency f₄₀o of the laser 400.

W_A denotes the maximum transverse dimension of a marking MRK1 . W_G denotes a transverse distance between adjacent dots. A desired value of W_G may be set by adjusting the time period between adjacent laser pulses and/or by adjusting the angular velocity coi of the first body 100 and/or by selecting the number of facets of the first body 100 and/or by adjusting the angular velocity co₂ of the second body 200 and/or by selecting the number of facets of the second body 200, and/or by selecting the focal length f₂ of the optics 350, according to the velocity v of the web WEB1 .

L_A denotes the maximum longitudinal distance between dots of a marking MRK1 . A desired value of L_A may be selected by adjusting the angular velocity of the second body 200, and/or by selecting the number of facets of the second body 200, and/or by selecting the focal length f₂ of the optics 350, according to the velocity v of the web WEB1 .

Ui may denote a longitudinal coordinate of the leading end of a first marking MRK1 , u₂ may denote a longitudinal coordinate of the trailing end of the first marking MRK1 , and u₃ may denote a longitudinal coordinate of the leading end of a first marking MRK2.

--SYNC may denote the distance between leading ends of the markings MRK1 (i.e.

). A desired value of L_SYNC may be selected by adjusting the angular velocity co₂ of the second body 200, and/or by selecting the number m₂ of facets of the second body 200, and/or by selecting the focal length f₂ of the optics 350, according to the velocity v of the web WEB1 . In an embodiment, the web WEB1 marked by the device 500 may be subsequently cut into separate sheets. The dimension L_SYNC may be matched with a longitudinal dimension of said sheets. In particular, the dimension L_SYNC may be matched with one of the standard sheet sizes 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 . Adjacent markings MRK1 , MRK2 may be separated by a blank space BLANK12, wherein it might be impossible to form marks on the blank space by using a single marking beam LB2. The blank space BLANK12 corresponds to a time period when the marking beam LB2 is available for producing the dots D1 . During said (blanking) time period the intermediate beam LB1 may only partly impinge on a facet of the body 200. The power of the laser beam LB1 may be switched off during said period e.g. in order to avoid damaging optical components and/or in order to avoid producing inferior or duplicate dots D1 . Also the properties of the focusing optics (acceptance angle) may have an effect on the length L_Bi_ of the blank space.

Referring to Fig. 5b, also dots D2, D3 having a greater transverse dimension may be formed in addition to shorter dots D1 . The dots D1 , D2, D3 may form graphical symbols, e.g. letters and/or numbers. A symbol may also be formed by using an inverted color scheme (i.e. the dots D1 , D2, D3 may cover more than 50% of the sites of a two-dimensional array).

Fig. 6a shows dots D1 a, D1 b arranged on sites of a two-dimensional array. The dots D1 a, D1 b are formed on the web WEB1 . The transverse positions of the dots may be defined by the transverse coordinates y-i , y₂, ... Fig. 6a also shows adjacent tracks TRAC1 , TRAC2 of the laser spot SP2 in the stationary coordinate system superposed on the instantaneous image of the moving web WEB1 . The image represents an instant where the laser spot SP2 is just forming the dot D1 b (i.e. the spot SP2 coincides with the dot D1 b).

The dots on the line TRACT were formed when the laser spot SP2 was swept along the (previous) track TRAC1 . The dot D1 a on the line TRACT was formed when the sweeping laser spot SP2 coincided with the point (xia,x-ia) on the track TRAC1 .

The sweeping laser spot SP2 has a transverse velocity component v_y and a longitudinal velocity component v_x. In this example, the longitudinal velocity component v_x is smaller than the velocity v of the web WEB1 . A second dot at the transverse position y₂ may be formed slightly later than the first dot at the transverse position y-, . The web WEB1 may be displaced by a small distance Δχ ₂ during a time period Δΐ₁₂ associated with moving the spot SP2 from the position y to the position y₂ along the track TRAC2. The displacement Δχ ₂ may be proportional to the relative velocity V_REL according to the equation Δχ ₂ = V_REL ·Δΐ ₂- The spot SP2 may be moved such that rotation of the first body 1 00 does not cause a longitudinal velocity component of the spot SP2. Consequently, the formed dots D1 may be located on a transverse line TRAC2', which is not perpendicular to the direction of movement of the web WEB1 . The formed dots D1 may be located on a transverse line TRAC2', which is not parallel to the direction SY.

The trajectories TRAC1 , TRAC2 may have an angle γ1 with respect to the direction SY. The angle γ1 may be e.g. in the range of 1 to 80°. The transverse lines TRAC1 ', TRAC2' may have an angle γ2 with respect to the direction SY (Fig. 5a). The formed dots may be arranged on sites of a slanted array. A marking MRK1 may comprise e.g. ten or more dots D1 arranged in a two-dimensional slanted array, the slant angle γ2 of the array being in the range of 1 0° to 45°.

REG5 may denote a usable portion of a trajectory TRAC1 where the laser beam LB0 impinges on (only) one facet of the first body 1 00 and where the focusing optics 350 (if used) is capable of delivering the beam LB2 to the surface of the web WEB1 . The full power of the laser beam may be delivered to the laser spot SP2 when the laser spot SP2 is in the usable portion REG5 of the trajectory TRAC1 .

The regions REG4 and REG6 of the trajectory TRAC1 may represent a situation where the initial laser beam LB0 simultaneously impinges on two adjacent facets F1 a, F1 b of the body 1 00 and/or an area between two adjacent facets. Thus, the full power cannot be delivered to the spot SP2 when the spot SP2 is in the region REG4 or REG6. Using a high laser power in the region REG4 or REG6 may involve a risk of damaging optical components. Furthermore, light of the beam LB0 may be split and directed into several different directions when the spot SP2 is in the region REG4 or REG6, so that ghost dots may be formed in inadvertent (i.e. wrong) positions of the web WEB1 .

W_T denotes full transverse dimension of the scanning tracks TRAC1 . The transverse dimension of the marking MRK1 may be equal to W_A. (Fig. 5a). The maximum transverse dimension of the usable portion REG5 may be W_A. The transverse dimension of the marking MRK1 may be smaller than or equal to the maximum transverse dimension W_A. The transverse sweeping may have a duty cycle r\_v, where r\_v = W_A/W_T. The duty cycle r\_v may be e.g. in the range of 10 to 90%.

Fig. 6b shows movement of the laser spot SP2 caused by the first body 100 in a hypothetical situation where the second body 200 is not rotating. When the first body 100 is rotating, the spot SP2 may repetitively move along the trajectory TRAC100 from a starting point POS1 to an end point POS2 at a transverse velocity v_y. A transverse sweeping cycle may comprise moving the spot SP2 from the starting point POS1 to the end point POS2 at the transverse velocity v_y. When the spot SP2 has reached the end point POS2, the spot SP2 may jump from the point POS2 to the point POS1 within a short period of time in order to start the next transverse sweeping cycle. In this case, the trajectory TRAC100 is perpendicular to the longitudinal direction SX, and rotation of the first body 100 does not cause longitudinal movement of the spot SP2. The spot SP2 may jump from the point POS2 to the point POS1 without a displacement in the longitudinal direction SX.

Fig. 6c shows movement of the laser spot SP2 caused by the first body 100 in a hypothetical situation where the second body 200 is not rotating. In this case, the trajectory TRAC100 is not perpendicular to the longitudinal direction SX, and rotation of the first body 100 may cause a longitudinal velocity component v_x of the spot SP2, in addition to the transverse velocity component v_y. A transition between two consecutive transverse sweeping cycles may be associated with a sudden longitudinal displacement when the spot SP2 jumps from the end point POS2 to the start point POS1 . The spot SP2 may repetitively return to the starting point POS1 several times during a single writing cycle. Thus, the time averaged value of the velocity component v_x of the spot SP2 may be substantially equal to zero. Figs. 6b and 6c may also be considered to show movement of the laser spot SP2 in a retarded coordinate system, which moves in the direction SX at the relative velocity V_REL-

Fig. 6d shows a velocity vector diagram for a situation where the second body 200 is rotating and where the rotation of the first body 1 00 does not cause longitudinal movement of the spot SP2. The spot SP2 may move along a trajectory TRAC1 at an inclined velocity v_T in the stationary coordinate system. Rotation of the first body 1 00 may cause a transverse velocity component v_y, and rotation of the second body 200 may cause a longitudinal velocity component v_x2. The velocity v_T may be formed as the vector sum of the velocity components v_x , Vx2 and v_y. The longitudinal velocity component v_x caused by rotation of the first body 1 00 may be zero, and the longitudinal velocity v_x of the spot SP2 may be entirely caused by rotation of the second body 200.

Fig. 6e shows a velocity vector diagram for a situation where the second body 200 is rotating and where the rotation of the first body 1 00 causes a first longitudinal velocity component v_x . Rotation of the second body 200 may cause a second longitudinal velocity component v^. The longitudinal velocity v_x of the spot SP2 may be formed as the sum of the components v^ and v_x . The velocity v_T may be formed as the vector sum of the velocity components v_x and v_y.

Fig. 6f shows adjacent tracks TRAC1 , TRAC2 of the laser spot SP2 in the stationary coordinate system superposed on the instantaneous image of the moving web WEB1 in a situation where rotation of the first body 1 00 causes a first longitudinal velocity component v_x and rotation of the second body 200 causes a second longitudinal velocity component v_x2. A marking MRK1 may be formed of a plurality of dots arranged on sites of a two-dimensional array. The resulting array of dots D1 may be substantially rectangular when the sum v^+v^ is selected to be substantially equal to the velocity v of the web WEB1 . The dots D1 may be arranged along the transverse lines TRAC1 ', TRAC2'. For certain applications, markings formed of dots arranged in sites of a rectangular array may be visually more pleasant than markings formed of dots arranged in sites of a slanted array.

Rotation of the first body 100 causes shifting of the primary beam LBO from a first facet to a second facet. The transition of the beam LBO from the first facet F1 a to the second facet F1 b may be associated with a sudden longitudinal and transverse displacement ("jump") of the spot SP2. The longitudinal displacement associated with the jump may be equal to the distance L_G between the adjacent lines TRAC1 ', TRAC2'.

Referring to Fig. 6g, the rotation axis AX1 of the first body 100 may be inclined with respect to the longitudinal direction SX such that rotation of the first body 100 about the axis AX1 causes a longitudinal velocity component v_x for the spot SP2, in addition to the transverse velocity component v_y. Consequently, the trajectory TRAC100 of the spot SP2 may be inclined with respect to the direction SY even in a situation where the longitudinal velocity component caused by the second body would be zero. The facets of the first body 100 may provide the intermediate beam LB1 by periodically deflecting light of the primary beam LBO, and a facet F2a of the second body 100 may provide the marking beam LB2 by deflecting light of the intermediate beam LB1 . The inclination angle between the direction SX and the axis AX1 may be e.g. in the range of 2 to 45°. The inclination angle φ1 of the trajectory TRAC100 may be equal to the inclination angle of the axis AX1 with respect to the longitudinal direction SX.

In an embodiment, an inclination angle between the rotation axis AX1 of the first body 100 and the vertical direction SZ may be e.g. in the range of 2 to 45°, in order to provide the inclined trajectory TRAC100 of the spot SP2 in a situation where the longitudinal velocity component caused by the second body would be zero.

The inclination angle between the direction SX and the axis AX1 may be e.g. in the range of 2 to 45° and/or the inclination angle between the axis AX1 and the vertical direction SZ may be e.g. in the range of 2 to 45°. When the inclined trajectory TRAC100 is provided by setting the axis AX1 to the inclined angular position (i.e. to the inclined orientation), the use of additional optical components may be minimized or avoided. Referring to Fig. 6h, rotation of the first body 100 about the axis AX1 may cause a longitudinal velocity component v_x when a (stationary) beam rotator 160 has been positioned between the first body 100 and the second body 200. In particular, rotation of the first body 100 may provide the longitudinal velocity component v_x even when the axis AX1 would be parallel to the longitudinal direction SX. The beam rotator 160 may be e.g. a Dove prism or a reflective beam-rotating set-up arranged to provide a beam-rotating functionality. The orientation of the beam rotator may be adjustable, but it may be held stationary during writing the marking MRK1 . The first body 100 may provide a first intermediate beam LB1 , which may be arranged to scan (i.e. sweep) along a trajectory TRAC101 , which is parallel to the direction SY. The beam rotator 160 may provide a second intermediate beam LBV by rotating the light of the first intermediate beam LB1 , wherein the second intermediate beam LBV may sweep along a trajectory TRAC10V, which deviates from the direction SY. The angle φ1 between the trajectory TRAC10V and the direction SY may be selected by setting a suitable orientation (e.g. 0.5·φ1 ) for the beam rotator 160. Light of the second intermediate beam LB1 ' may be subsequently deflected by the second body 200 in order to provide the marking beam LB2. Referring to Fig. 6i, rotation of the first body 100 about the axis AX1 may cause a longitudinal velocity component v_x also by using inclined reflective facets of the first body 100. The facet F1 a of the first body 100 may be inclined such that the normal of the facet F1 a is not perpendicular to the rotation axis AX1 , wherein the primary beam LB0 may impinge on the facet F1 a such that the centerline of the primary beam LB0 is not in a plane containing the axis AX1 .

Fig. 7 shows an apparatus 1000 comprising the marking device 500. The apparatus 1000 may be a web processing apparatus. The apparatus 1000 may be arranged e.g. to produce a paper web WEB1 or a cardboard web WEB1 . The apparatus 1000 may be arranged to process a paper web or a cardboard web. The apparatus 1000 may comprise e.g. a forming section, a press section, a drying section, a calender section, a coating section and/or a coating section. The apparatus 1000 may comprise one or more rolls 1010, 1020 to move the web WEB1 at a velocity v-, . The apparatus 1000 may comprise a processing unit 1 100 to process the web WEB1 . The processing unit 1 100 may be e.g. a heating unit, a coating unit, or a cutter. The WEB1 may be dry or wet when it is marked by the beam LB2.

Referring to Fig. 8a, the marking device 500 may comprise the first rotating body 100 and the second rotating body 200. The bodies 100, 200 may be rotated with motors 120, 220. The device 500 may comprise a control unit CNT1 arranged to control operation of the device 500. The device 500 may comprise a memory MEM1 which may store control data DATA1 for forming the desired markings. The control data DATA1 may e.g. specify the power of the laser beam LB2 as a function of the angular positions , β of the bodies 100, 200. The power of the beam LB2 may be modulated e.g. by a control signal S₄₀o communicated from the control unit CNT1 to the laser 400. The motors 120, 220 may be switched on and off by signals S-ioo, S₂oo- Optionally, the angular velocities of the bodies 100, 200 are adjustable, wherein the angular velocities may be set by the signals Si oo; S200- The motor 120 and/or 220 may be e.g. an electric motor. In particular, the motor 120 and/or 220 may be a brushless electric motor in order to provide accurate control of rotation speed and long operating life. In an embodiment, both bodies 100, 200 may also be rotated by the same motor, e.g. by using a gearbox. However, rotating the bodies with separate motors may provide additional freedom to select the dimensions of the markings in an operating situation.

The marking device 500 may optionally comprise one or more sensors to monitor the angular orientation of the first body 100 and to monitor the angular orientation of the second body 200. A first sensor may provide a first angular orientation signal S_«, which may specify the angular orientation a of the first body 100. A second sensor may provide a second angular orientation signal Sp, which may specify the angular orientation β of the second body 200. The sensors may be e.g. rotary encoders or opto switches. The orientation signal S_« may be e.g. a 1 -bit digital signal provided e.g. by an opto switch or by a Hall switch, which specifies when the first body 100 is at a predetermined angular position (e.g. at

). The angular velocity coi may be determined e.g. by measuring a time period corresponding to a complete rotation. The angular position a may be extrapolated based on the angular velocity coi and based on the time when the first body is at the predetermined angular position. The angular position β of the second body 200 may be determined based on the angular velocity co₂ and based on the time when the second body is at a predetermined angular position. However, the use of the feedback signals S_«, Sp is not necessary. For example, the motors may be synchronous electric motors, which rotate at a speed determined by the frequency of an electric current. The orientation of the bodies 100, 200 may also be determined based on the positions of dots in the markings formed by the device 500.

The device 500 may optionally comprise a velocity sensor VSENS1 to detect the velocity of the web WEB1 . The sensor VSENS1 may e.g. determine the speed v by monitoring rotation speed of a roll 1010 of the apparatus 1000 (Fig. 7). The angular velocities of the bodies 100, 200 may be set according to the detected velocity v-, . The velocity sensor VSENS1 may be omitted e.g. when the velocity v of the web WEB1 is constant and/or known by other means. For example, a control system of a paper processing machine may provide a signal which specifies the velocity of the web WEB1 . The velocity sensor VSENS1 might be omitted also when (random) variations in the longitudinal length of the markings MRK1 and/or distortions in the shape of the markings MRK1 may be tolerated.

Referring to Fig. 8b, the marking device 500 may further comprise one or more camera units CAM1 , CAM2 for monitoring the web WEB1 before and/or after marking with the laser beam LB2. Images captured by the camera units CAM1 , CAM2 may be analyzed by respective image analyzing units IAU1 , IAU2. The image analyzing units IAU1 , IAU2 may provide image analysis data ScAM-i , S_CAM2- An image captured by a camera CAM1 before marking with laser beam LB2 may be analyzed e.g. in order to determine the location of a preliminary marking. The control unit CNT1 may be arranged to control the location of the new laser marking MRK1 with respect to the location of the previous marking. Two or more images captured by the camera CAM1 and/or by the camera CAM2 may be used e.g. to determine the velocity of the web WEB1 , by determining the speed of a previous marking moving along the web WEB1 . Thus there is no need to use a separate velocity sensor VSENS1 . An image of a marking captured by a camera CAM2 after marking with the laser beam LB2 may be analyzed e.g. in order to provide feedback signal S_CAM2- For example, the control unit CNT1 may be arranged to adjust the angular speed coi and/or <¾ based on the feedback signal S_CAM2 determined by analyzing the image of the marking MRK1 . For example, the control unit CNT1 may be arranged to adjust the laser power based on the feedback signal S_CAM2- For example, the control unit CNT1 may be arranged to adjust the synchronization of the laser pulses with respect to the angular positions of the bodies 100, 200, based on the feedback signal S_CAM2-

The device 500 may further comprise a memory MEM2 for storing computer program code PROG1 , for executing the method of the invention.

The device 500 may further comprise an interface INTRF1 e.g. for receiving input signal S|_Ni from the apparatus 1000, e.g. from one or more sensors of the apparatus 1000 and/or from a control unit of the apparatus 1000. The input signal S|_Ni may be provided e.g. by process automation system. The interface INTRF1 may also be capable of receiving an input signal S|_N2 (directly or indirectly) from a user interface. The input signal S|_N2 may comprise e.g. the data DATA1 specifying the markings MRK1 produced on the web WEB1 by the device 500. The input signal S|_N2 may comprise data DATA1 for specifying positions the markings MRK1 , MRK2 with respect to each other. Operation of the marking device 500 may be started and/or stopped based on the input signal S_!N2- The interface INTRF1 may be arranged to send output data SOUT to the system 1000. The output data SOUT may e.g. comprise information about an operating temperature of the laser 400, information about the (monitored) power of the laser, and/or image analysis information obtained from the camera units CAM1 and/or CAM2. Operation of the marking device 500 may be switched off if the marking device 500 is not operating properly. Operation of the web processing apparatus 1000 may be stopped if the marking device 500 is not operating properly. Light of a light beam LB1 deflected by the facets of the first body 100 may be directly coupled to the facets of the second body 200 (as shown in Fig. 1 ), or by using optional further optical components. The marking device 500 may comprise an actuator 140 arranged to adjust the longitudinal velocity component caused by the first rotating body 100. The actuator may adjust e.g. the orientation of the axis AX1 or the orientation of a beam rotator 160 (Fig. 6h). The actuator 140 may be controlled by a signal

Fig. 9 shows a set-up where the direction of a light beam LB1 provided by a facet F1 a of a first polygon reflector 100 is changed by a reflector 315, and coupled via relay optics 320 to a facet F2a of a second polygon reflector 200. A primary laser beam LBO may propagate e.g. in the direction SY such that it impinges on the facet F1 a of the first polygon reflector 100. The relay optics 320 may provide a relayed beam LBV, which impinges on the facet F2a of the second polygon reflector 200.

The relay optics 320 may comprise e.g. two or more lenses 321 , 322. The distance between the lenses 321 , 322 may be e.g. substantially equal to the sum of the focal lengths of the lenses 321 , 322. The first lens 321 may provide a focused beam by focusing light of the intermediate beam LB1 , and the second lens 322 may provide a relayed beam LBV by collimating light of said focused beam.

The use of the relay optics 320 may e.g. help to increase ratio of the dimension W_A to the dimension W_T (Fig. 6a), in other words the relay optics 320 may increase the duty cycle r\_v of the transverse sweeping. A change of the direction of the beam LB1 may produce a change of the direction of the relayed beam LB1 '. The ratio of said changes may be called as the angular conversion ratio of the relay optics 320. The angular conversion ratio may be substantially equal to the ratio of the focal lengths of the lenses 321 , 322. The angular conversion ratio may be equal to one or it may be different from one. The transverse width W_A and/or W_T may be adjusted e.g. by selecting the angular conversion ratio of the relay optics 320. The transverse width W_A and/or W_T may be adjusted e.g. by selecting the number of light-reflecting regions F1 a, F1 b of the first body 100, by selecting the focal length f₂ of the focusing optics 350 and/ or by selecting the angular conversion ratio of the relay optics 320. In an embodiment, the relay optics may be implemented by using reflective optics, in particular by using paraboloidic reflective surfaces.

Fig. 10 shows, in a three-dimensional view, the optical set-up of a laser marking device 500. Fig 10 represents a view "below" the web WEB1 , i.e. the device 500 is on a first (e.g. upper) side of the web WEB1 and the viewer on the second (e.g. lower) side of the web WEB1 . In Fig. 10, a "visible" beam can be seen "through" the web (in reality, the beam may be e.g. an infrared light beam invisible to human eyes, and the web may be opaque at wavelengths visible to human eyes).

A reflector 310 may direct a primary beam LBO provided the laser 400 to a facet F1 a of a first rotating polygon reflector 100. The facet F1 a may provide a first deflected beam LB1 , which may be coupled via relay lenses 321 , 322 and a reflector 340 to a facet F2a of the second rotating polygon reflector 200. The marking laser beam LB2 may be provided by reflecting the light of the beam LB1 by the facet F2a, and by focusing the light by the lens 350. The polygon reflectors 100, 200 may be rotated with motors 120, 220.

Rotation of the first polygon 100 changes the orientation of the facet F1 a, which causes transverse sweeping of the beam LB2 (in the direction SY). Rotation of the second polygon 200 changes the orientation of the facet F2a, which causes longitudinal sweeping of the beam LB2 (in the direction SX).

Referring to Fig. 1 1 , a usable region REG2 of longitudinal sweeping may be shorter than a full sweeping length LFULL which could be provided by the rotating body 200. The end regions REG1 , REG3 may represent a situation where the intermediate beam LB1 simultaneously impinges on two adjacent facets of the body 200 and/or on an area between two adjacent facets. Producing a high quality dot D1 may be difficult or impossible when the spot SP2 is in the end region REG1 or REG3. The dimension L_REG2 may also be limited e.g. by the acceptance angle and/or aberrations of the optics 350. For example, the dimension L_REG2 may be shorter than the focal length f₂ of the optics 350. In particular, the dimension L_REG2 may be e.g. shorter than or equal to 50% of the focal length f₂.The longitudinal sweeping may have a longitudinal duty cycle η_χ. The longitudinal duty cycle η_χ is equal to LREG2/I-FULL- The longitudinal duty cycle η_χ may be e.g. in the range of 10 to 90%.

Fig. 12a shows the longitudinal position u of the laser spot SP2 during three consecutive marking cycles CO, C1 , C2. The position u is shown in the moving coordinate system of the web WEB1 . Writing of a first marking MRK1 may begin at the time t_u , and the writing may be stopped at the time t_u2. Writing of a second marking MRK2 may begin at the time t_u2. The longitudinal dimension of the first marking MRK1 may be equal to L_A. L_SYNC denotes the longitudinal distance between leading ends of the markings MRK1 , MRK2. L_Bi_ denotes the longitudinal length of a blank (i.e. empty) space between the markings MRK1 , MRK2.

The first marking MRK1 may be written during the time period T_A between the times t_ui and t_u2. The laser spot SP2 may jump to the starting position of the second marking MRK2 during the (blanking) time period T_Bi_ between the times t_u2 and t_u3.

The second rotating body 200 may be interpreted to perform a time warping operation, where the duration of the time period T_A available for writing the marking MRK1 may be increased by shortening the duration of the useless time period T_BL. T_t denotes the sum of the time periods T_A and T_BL.

Fig. 12b shows a comparative example of operation where the rotating second body 200 is replaced with a fixed mirror. In this comparative example, the markings must be written during writing cycles E0, E1 , E2. The writing cycle E1 starts at the time t_ui and ends at the time t_2iE-i . A marking having a length L_A should now be written between the positions U i and u₂ during a time period T_A>Ei , which is substantially shorter than the duration T_A shown in Fig. 12a. T_BL,EI represents a time period between writing two consecutive markings. The dots D1 forming a single marking MRK1 may need to be produced during the time period T_A. If the time period is shortened while keeping the number of the dots constant, the modulation frequency of the laser beam LB2 needs to be increased. Thus, a higher modulation frequency may be needed in the comparative example of Fig. 12b, when compared with the situation of Fig. 12a. The intensity of the laser beam LB2 impinging on the area of the marking MRK1 written according to Fig. 12a may be substantially lower than in the comparative example of Fig. 12b. This may allow operation by using a smaller and/or cheaper laser 400.

Referring back to Fig. 12a, the ratio L_A/L_SYNC may depend on the average longitudinal velocity component v_x2 of the laser spot SP2 and on the velocity

The average longitudinal velocity component v^ of the laser spot SP2 may be e.g. in the range of 50 to 150 % of the velocity v of the web WEB1 . The following equation may be derived to describe how the longitudinal dimension L_A of the marking MRK1 depends on the velocities.

Thus, if the relative velocity V_REL (=VI -V_X2) is e.g. 20% of the velocity vi of the web WEB1 , the longitudinal dimension L_A of the marking MRK1 would be smaller than 20% of the length LSYNC-

As the body 200 is rotated, the laser light LB1 provided by the first rotating body 100 impinges each light deflecting facet F2a, F2b of the second body 200, each in its turn. Each facet F2a may be replaced with a next facet F2b within a time period Τ_Δρΐ2- The time period Τ_Δρΐ2 corresponds to a frequency f₂₀o = 1 /Τ_Δρΐ2- The time period Τ_Δρΐ2 may be called e.g. as a facet time period of the second body, and the frequency f₂oo may be called e.g. as the facet frequency of the second body. The facet frequency f₂oo of the second body 200 may depend on the angular velocity co₂ of the second body 200 and on the number m₂ of the light-deflecting facets F2a, F2b of the second body 200 according to the following equation: m₂ · ω₂

^L200 (2)

2π

The intention may be to cut the web later into a plurality of sheets e.g. by using the unit 1 100 of the system 1000 (Fig. 7). For that purpose, the dimension LSYNC may be matched with the longitudinal dimension of said sheets. The operation of the apparatus 500 may be adjusted such that a dimension of said sheets is an integer (q-i) multiple of the dimension LSYNC- This may allow producing one or more markings on each sheet. The facet frequency f₂oo needed to implement the desired dimension LSYNC may be solved from: f 200 ^{= ~ '} (³)

LSYNC The corresponding angular velocity co₂ may be solved from the equation (4), by eliminating the frequency f₂oo from the equations (2) and (3): ω₂ = -^J— (4)

m2 ^{' L}SYNC For example, LSYNC may be selected to be 297 mm. This would allow producing a single marking on each sheet of A4 standard size by using each facet of the second body 200. For example, the number m₂ of facets may be selected to be equal to 8, and the velocity vi of the web may be e.g. equal to 10 m/s. The angular velocity co₂ matching with these conditions would be 26.44 s^"1 (from eq. (4)) and the corresponding rotation speed of the second body 200 would be 4.209 revolutions per second.

The longitudinal velocity component v_x2 of the laser spot SP2 (in the stationary coordinate system) may be approximated by the following equation (in case of reflection from a facet F2a whose normal is perpendicular to the rotation axis AX2): v_x2 = 2 - C02 - f₂ (5) The focal length f₂ may be selected to be equal to e.g. 150 mm. By using the equation (5) and the angular velocity co₂ = 26.44s^"1, the longitudinal velocity component v^ would be equal to 7.932 m/s. Thus, the relative velocity V_REL (=v-|-v_x2) would be equal to 2.068 m/s, which corresponds to 20.48 % of the velocity v of the web WEB1 .

The full longitudinal sweeping dimension L_FULL may be estimated by the following equation:

T ^LFULL - ^{27l ' f}2 ( (6^B))

m₂

By using the values f₂ = 150 mm and m₂ = 8, we get L_FULL = 120 mm As discussed above (see Fig. 1 1 ), high quality dots D1 may be produced (only) when the spot SP2 resides in the central region REG2 of the longitudinal sweeping. The longitudinal duty cycle η_χ may be e.g. 50%, and the corresponding length L_REG2 of the usable region REG2 may be e.g. 60 mm (=50% LFULL)- The moving spot SP2 may follow the movement of the web WEB1 by the longitudinal distance L_REG2- The average longitudinal velocity component of the laser spot SP2 during a single longitudinal sweep may be substantially equal to the velocity component v_x2 given by eq. (5). The time period T_A available for producing the (single) marking MRK1 may be equal to L_REG2/v_x2. By using the values L_REG2 = 60 mm and v_x2 = 7.932 m/s, we get T_A = 7.6-10^"3 s.

The maximum longitudinal dimension of the (single) marking MRK1 may be determined by the equation: L_A = T_A · VREL (7)

By using the values Τ_Α=7.6·10^"3 s and v_REL = 2.068 m/s, we get L_A = 16 mm. The first body 100 may be arranged to provide several transverse sweeps of the spot SP2 during a single longitudinal sweep provided by the second body 200. Fig. 12c shows transverse position of the laser spot SP2 during four consecutive transverse sweeps C1 1 , C12, C13, C14. The transverse sweep C1 1 may start at the time t_y-| . The positions y-i , y₂, y3, y4, ys may represent positions of five dots written during a first transverse sweep C1 1 . Writing of a dot at a location

, y₂, y₃, y₄, or y₅ may start at the time t_y , t_y2, t_y3, t_y4, or t_y5, respectively. The time t_yi may be e.g. the same as the time t_ui shown in Fig. 12a. Further dots may be written during the second sweep C12, which starts at t-121 and stops at t_{1 3} -

Writing of an individual marking MRK1 may comprise making n_k consecutive transverse sweeps, wherein the number n_k may be e.g. in the range of 2 to 100. In order to produce a single character, the number n_k may be e.g. greater than or equal to 5. The n_k transverse sweeps for producing a single marking MRK1 may be carried out during the time period T_A shown in Fig. 12a.

A single dot D1 written at the position y₂ may have a transverse dimension DIM2 (see also Fig. 5a). The transverse dimension DIM2 may correspond to the duration Δΐ_Ρι of laser pulse, according to the slope of the linear sweep curve C1 1 shown in Fig. 12c.

The substantially linear sweeping curves of Fig. 12c may be provided by a rotating body 100 without using other components to modulate the direction of the laser beam LB2 in the transverse direction. The locations y-i , y₂, ... and the number of dots may be freely selectable when using sweeping according to Fig. 12c . The transverse dimension DIM2 of each dot may be freely selectable.

As the body 100 is rotated, the laser light LB0 coming from the laser 400 consecutively impinges each light deflecting facet F1 a, F1 b of the first body 100, each in its turn. Each facet F1 a may interact with the primary beam LB0 during a time period Τ_{Δα 2}. The time period Τ_Δαΐ2 corresponds to a frequency f₁₀₀ = 1 /Τ_{Δα 2}. The time period Τ_Δαΐ2 may be called e.g. as a facet time period of the first body, and the frequency f-ioo may be called e.g. as the facet frequency of the first body 100.

The angular velocities co-i , co₂ and the number of facets m _; m₂ may be selected such that an integer number n_k of transverse sweeps is carried out during a single longitudinal sweep. The number n_k may be e.g. in the range of 2 to 100. The angular velocities co-i , co₂ and the number of facets m _; m₂ may be selected such that an integer number of transverse sweeps is carried out during an integer number of longitudinal sweeps. For example, 1 1 transverse sweeps may be used to produce the marking MRK1 shown in Fig. 4b. The number of transverse sweeps performed per unit time is equal to m co1 /(27t). The number of transverse sweeps performed per unit time may be called as the facet frequency f-ioo of the first body 100.

Due to the finite diameter of the beam LB1 , the duty cycle η_χ of the second body 100 is smaller than 1 , which means that the whole facet time period Τ_{Δβ 2} cannot be used for writing. The length of the usable time period may be e.g. 10 to 90% of the facet time period Τ_{Δβ 2}. The length of the usable time period may also be limited by the acceptance angle of the focusing optics 350.

For example, eleven transverse sweeps during a time T_A=7.6-10^"3 s may correspond to a facet frequency f-ioo = 1 147 Hz (=1 1 /T_A). If we select e.g. m = 200, the required rotation speed is 5.735 revolutions per second, corresponding to the angular velocity coi = 36.03 s^' The facet time period Τ_Δαΐ2 of the first body is now equal to 1 /f-ioo = 872 με. The transverse velocity component v_y of the spot SP2 may be calculated by the equation: v_y = 2 - co₁ - f₂ (8)

By using the values co-i = 36.03 s^' f₂=150 mm, we get v_y= 10.8 ms^"1. The equation (8) may contain the coefficient of 2 because reflection of light from a facet may cause doubling of the angular change. The transverse dimension W_T may be calculated by the equation: T = v_y - T_Aal2 (9) By using the values v_y= 10.8 ms^"1 and Τ_Δαΐ2 = 872 με, we get W_T = 9.4 mm. If we assume that the transverse duty cycle r\_v is equal to 50%, the maximum transverse dimension W_A of the marking MRK1 may now be equal to 4.7 mm, i.e. substantially equal to 5 mm. Thus, in this example, a plurality of markings MRK1 may be produced on a web WEB1 , which is moving at a longitudinal velocity v = 10 ms^"1. The web WEB1 may be later cut into pieces such that the size of each piece corresponds to the standard A4 sheet, wherein each piece comprises a single marking MRK1 . The size of each marking MRK1 may be e.g. 16 mm by 5 mm. The marking MRK1 may be formed on dots D1 arranged in e.g. eleven transverse columns. The markings MRK1 , MRK2 produced on the different pieces may be identical or different, depending on the modulation scheme of the laser beam LB2. Each transverse column of dots of a marking may be formed in a time period, which is equal to η_ν·Τ_Δαΐ2- By using r\_v = 50% and Τ_Δαΐ2 = 872 με, we get that each transverse column of dots may be formed in a time period of 436 με. If each transverse column of the marking MRK1 comprises e.g. seven dots D1 , the required modulation frequency f₄₀o of the laser beam LB2 may be e.g. greater than or equal to 7/436με = 16 kHz. The modulation frequency f₄₀o of the laser beam LB2 may also be called as the bit rate. The intensity of light LB2 impinging on the spot SP2 is modulated at the frequency f₄₀o-

Fig. 12d shows longitudinal position of the laser spot SP2 as a function of time in the stationary coordinate system. The curve portions C101 , C102, C103, ...C1 1 1 may represent e.g. eleven consecutive transverse sweeps of the spot SP2. Fig. 12d shows a situation where rotation of the first body causes a first longitudinal velocity component v_x-i . Rotation of the second body causes a second longitudinal velocity component v_x2. The total longitudinal velocity of the spot SP2 and the slopes of the individual curve portions C101 , C102 may be equal to the sum v_xi+v_x2. The slope of the envelope line may be equal to the (average) longitudinal velocity component Vx2. When the spot SP2 moves e.g. along the trajectory TRAC1 shown in Fig. 6a, the movement may correspond e.g. to the curve portion C101 shown in Fig. 12d. Movement along the trajectory TRAC2 may correspond to the curve portion C102, respectively.

Fig. 12e shows longitudinal position of the laser spot SP2 as a function of time in the moving coordinate system. Fig. 12e corresponds to the situation of Fig. 12d in the moving frame. The curve portions C201 , C202, C203, ...C21 1 may represent e.g. eleven consecutive transverse sweeps of the spot SP2. The sum v_x +v_x2. of the longitudinal velocity components may be substantially equal to the velocity v of the web WEB1 so that the longitudinal position of the spot SP2 in not changed when the spot SP2 is moving along an individual trajectory (e.g. TRAC1 in Fig. 6f). However, the longitudinal position of the spot SP2 may be changed when the spot SP2 jumps from a first trajectory TRAC1 to a second trajectory TRAC2. The longitudinal displacement may be equal to the longitudinal distance L_G between the dots formed on the web WEB1 . In an embodiment, the sloping curve portion C1 shown in Fig. 12a may consist of the portions C201 , C202, C203....C21 1 of Fig. 12e.

Fig. 12f shows transverse position of the laser spot SP2 during four consecutive transverse sweeps Q1 1 , Q12, Q13, Q14. This is an example of an embodiment where the transverse velocity v_y of the laser spot SP2 may be reduced in the vicinity of the locations y-i , y₂, y3, y4, ys-

The transverse dimension DIM2 of a dot produced on the location y₂ may correspond to the duration Δΐ_Ρ2 of laser pulse, according to the slope of the linear sweep curve C1 1 shown in Fig. 12c. The duration Δΐ_Ρ2 may be increased by reducing the transverse sweeping velocity at said one or more locations y-i , y₂, y₃, y₄, ys- This may allow reducing the modulation frequency of the laser 400 and/or this may allow reducing the power of the laser beam LB2. This sweeping mode (i.e. reducing transverse velocity in the vicinity of the locations of the dots) may be advantageous e.g. when writing separate dots D1 having a short transverse dimension DIM2. The "short" transverse dimension DIM2 may be e.g. in the range of 100% to 200% of the longitudinal dimension DIM1 of said dot D1 (Fig. 5a). The location of a dot D1 may advantageously match with one of the locations y-i , y₂, y3, y4, ys- The locations of dots D1 forming a symbol may match with one or more of the locations y , y₂, y₃, ΥΛ, ys-

The laser spot SP may jump from a first transverse position y to a second adjacent transverse position y₂. The transverse velocity of the laser spot SP2 may be reduced in the vicinity of the locations y-i , y₂, and the transverse velocity of the laser spot SP2 may be increased in a region between the locations yi and y₂, respectively. Writing of a dot at a location y _; y₂, y₃, y₄, or y₅ may start at the time t_yi , t_y2, t_y3, t_y4, or t_y5, respectively. The spot SP2 may have a reduced transverse velocity e.g. during a time period defined by times t_yi and t_si . The spot SP2 may have a reduced transverse velocity e.g. during a time period defined by times t_y2 and t_s2. The spot SP2 may have a reduced transverse velocity e.g. during a time period defined by times t_y5 and t_s5.

This may be implemented e.g. by using a further beam direction modulator, in addition to the first rotating body 100 and the second rotating body 200. Referring to Fig. 13, the further beam direction modulator 380 may be e.g. a movable reflector. The orientation of the reflector 380 may be rapidly changed by an actuator 390, which may be e.g. an electromagnetic actuator or a piezo-electric actuator. In particular, the direction modulator 380 may be a reflector arranged to change its tilt angle at a high frequency. The angular orientation of the reflector may be defined e.g. by an angular deviation Δφ/2 with respect to a reference direction REF380. This may cause a time- dependent angular deviation Δφ(ΐ) of the beam LB1 ' deflected by the direction modulator 380.

The first rotating body 100 may provide a first intermediate laser beam by reflecting light of a primary laser beam LB0. The first rotating body 100 may cause periodic variation ΔΘ1 (t) of the beam LB1 with respect to a reference direction REFDIR1 . The direction modulator 380 may provide a second intermediate beam LB1 ' by reflecting light of the first intermediate beam LB1 . Thus, the modulation caused by the direction modulator 380 may be combined with the periodic variation ΔΘ1 (ί) such that the direction of the second intermediate beam LB1 ' may be expressed as the sum ΔΘ1 (t) + Δφ(ί) The term ΔΘ1 (t) may be a substantially linear function of time in the vicinity of the transverse locations y-i , y₂, y3, y4, ys shown in Fig. 12d. The movements of the direction modulator 380 may be synchronized with the rotation of the first body 100 such that the temporally varying term Δφ(ΐ) reduces the transverse speed of the laser spot SP2 in the vicinity of the transverse locations y-i , y₂, y3, y4, ys- In particular, substantially sinusoidal mechanical vibration of the direction modulator 380 may be synchronized with the rotation of the first body 100.

The further beam direction modulator may be arranged to modulate the direction of light forming the marking laser beam LB2. The direction modulator may be positioned e.g. before the first rotating body 100, between the first rotating body 100 and the second rotating body 200, or after the second rotating body 200. For example, the reflector 315 shown in Fig. 9 may be a vibrating beam direction modulator, instead of being a stationary reflector. For example, the reflector 310 or the reflector 340 shown in Fig. 10 may be a vibrating beam direction modulator, instead of being a stationary reflector.

In an embodiment, the further beam direction modulator may also be a third rotating body comprising a plurality of beam-deflecting facets. In particular, the further beam direction modulator may be a rotating polygon reflector.

Referring to Fig. 14, the laser unit 400 of the marking device 500 may comprise an intensity modulating unit 420 arranged to modulate the intensity of the laser beam LB2, in order to control the timing of writing the dots of a desired marking MRK1 . The intensity needs to be rapidly changed according to the angular positions of the rotating bodies 100, 200 and according to desired marking MRK1 . The intensity modulating unit 420 may provide a primary beam LB0 by modulating the intensity of a beam LBC provided by a laser module 14. The beam LBC may be e.g. a continuous wave (CW) beam or it may be pulsed at a very high frequency, which does not need to be synchronized with the operation of the intensity modulating unit 420 (laser may be a free running pulse laser). The pulse frequency of the beam LBC may be e.g. greater than two times the controlled maximum modulation frequency needed to write the markings MRK1 .

The intensity modulating unit 420 may be controlled by a control signal S₄₀o obtained from a control unit (see Figs. 8a, 8b). The intensity modulating unit 420 may e.g. comprise an acousto-optic modulator, which may be arranged to transmit light or divert light to a beam dump, depending on the control signal S₄₀o-

The use of the external intensity modulating unit 420 is not necessary.

In an embodiment, the intensity may be modulated e.g. by controlling an electrical pumping current of the laser 400. The electrical pumping current may provide population inversion in a gas laser.

In an embodiment, the intensity may be modulated e.g. by a Q-switch in an optical cavity of the laser 400.

The maximum optical power of the beam LBO provided by the laser 400 may be e.g. smaller than 1000 W, advantageously smaller than 200 W. The laser 400 may be e.g. carbon dioxide laser providing laser light at a wavelength in the range of 9.3 μιτι to 10.7 μιη.

The laser 400 may be e.g. an optically pumped fiber laser. The fiber laser may comprise e.g. a doubly clad optical fiber doped with a rare earth metal (e.g. erbium, ytterbium, neodymium, dysprosium, praseodymium, or thulium). The laser 400 may be e.g. a Nd:YAG laser or a Yb:YAG laser. The wavelength of the laser may be converted by using one or more nonlinear crystals e.g. to provide second harmonic generation, third harmonic generation or fourth harmonic generation. The term "light" may also comprise light having a wavelength in the ultraviolet region (190 - 400 nm), in the visible region (400 - 780 nm) and/or in the infrared region (780 nm - 20 μιτι) of electromagnetic spectrum. The apparatus may be implemented by using refractive and/or reflective optics. The deflecting regions of the first body 100 and the second body 200 may be refractive, reflective and/or diffractive. In particular, when using a carbon dioxide laser, the apparatus 500 may comprise zinc selenide lenses and/or germanium lenses.

The rotation axis AX2 of the second body 200 may be substantially perpendicular to the direction (SX) of movement of the web WEB1 (e.g. within an angular range of 85° to 95°). The rotation axis AX1 of the first body 100 may be substantially perpendicular to the rotation axis AX2 (e.g. within an angular range of 85° to 95°. However, by using e.g. additional reflectors and/or beam rotators (e.g. a Dove prism), it might be possible to set the axis AX1 and or axis AX2 to nearly any orientation with respect to each other and with respect to the movement of the web. For the person skilled in the art, it will be clear that modifications and variations of the 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 for producing markings (MRK1 ) on a moving web (WEB1 ), the method comprising:

- moving the web (WEB1 ) in a longitudinal direction (SX),

- delivering laser light (LB2) to a moving aiming point (SP2) by using a first beam steering body (100) and a second beam steering body (200) such that the position ((x(t),y(t)) of the aiming point (SP2) depends on the angular orientation (a) of the first body (100) and on the angular orientation (β) of the second body (200),

- rotating the second body (200) from a first angular orientation (βι) to a second angular orientation (β₂), the rotation from the first angular orientation (β-ι) to the second angular orientation (β₂) defining a writing period (T_A), wherein the rotation of the second body (200) moves the aiming point (SP2) such that an average longitudinal velocity component (v^) of the aiming point (SP2) during the writing period (T_A) is in the range of 50% to 150% of the velocity (v^ of the web (WEB1 ),

- rotating the first body (100) such that the aiming point (SP2) crosses a reference line (YREF) several times during said writing period (T_A), and - controlling the intensity of the laser light (LB2) based on the angular orientation (a) of the first body (100) and according to the orientation (β) of the second body (200).

2. The method of claim 1 wherein the angular difference (Δβ ₂) between the second angular orientation (β₂) and the first angular orientation (βι) is smaller than or equal to 72°.

3. The method of claim 1 or 2 wherein rotation of the first body (100) causes an instantaneous longitudinal velocity component (v_x ) of the aiming point (SP2), in addition to the average longitudinal velocity component (v_x2) caused by rotation of the second body (100).

4. The method according to any of the claims 1 to 3 wherein the first body 100 is rotated about an axis (AX1 ), which is inclined with respect to the longitudinal direction (SX).

5. The method according to any of the claims 1 to 4 comprising forming a first deflected beam (LB1 ) by deflecting laser light (LBO) by the first body (100), forming a second deflected beam (LB2) by deflecting light of the first deflected beam (LB1 ) by the second body (200), and focusing light of the second deflected beam (LB2) to the aiming point (SP2).

6. The method according to any of the claims 1 to 5 wherein the first body (100) comprises two or more light deflecting regions (F1 a, F1 b) arranged to deflect laser light such that the aiming point (SP2) crosses the longitudinal reference line (YREF) five or more times during a complete revolution (360°) of the first body (100).

7. The method according to any of the claims 1 to 6 comprising locally altering the structure and/or chemical composition of the web (WEB1 ) by the laser light (LB2) delivered to the aiming point (SP2).

8. The method according to any of the claims 1 to 7 comprising forming a hole, which extends through the web (WEB1 ).

9. The method according to any of the claims 1 to 8 wherein the web (WEB1 ) comprises paper and/or cardboard.

10. The method according to any of the claims 1 to 9 wherein the velocity (v-i ) of the web (WEB1 ) is in the range of 5 to 50 m/s, advantageously in the range of 10 to 30 m/s.

1 1 . The method according to any of the claims 1 to 10 comprising processing the web (WEB1 ) after the structure and/or chemical composition of the web (WEB1 ) has been altered by the laser light (LB2).

12. The method according to any of the claims 1 to 1 1 comprising cutting the web (WEB1 ) into sheets, wherein the longitudinal size of the sheets is substantially matched with a longitudinal dimension (L_SYNC) defined by ends of two markings (MRK1 , MRK2) formed on the web (WEB1 ) by the laser light (LB2).

13. An apparatus (500,1000) for producing markings (MRK1 ) on a moving web (WEB1 ), the apparatus (500) comprising:

- a first beam steering body (100) and a second beam steering body (200) arranged to deliver laser light (LB2) to a moving aiming point (SP2) such that the position ((x(t),y(t)) of the aiming point (SP2) depends on the angular orientation (a) of the first body (100) and on the angular orientation (β) of the second body (200),

- a motor (120) arranged to rotate the first body (100) in order to change the transverse position (y(t)) of the aiming point (SP2), wherein the aiming point (SP2) is arranged to cross a reference line (YREF) several times during a complete revolution (360°) of the first body (100),

- a motor (220) arranged to rotate the second body (200) in order to change the longitudinal position (x(t)) of the aiming point (SP2), and

- a control unit (CNT1 ) arranged to control the intensity of the laser light (LB2) according to the orientation (a) of the first body (100) and according to the orientation (β) of the second body (100).

14. The apparatus (500) of claim 13 wherein the control unit (CNT1 ) is arranged to set the angular velocity (co₂) of the second body (200) according to a velocity value (v-,) such that an average longitudinal velocity component (v_x2) of the aiming point (SP2) is during a writing period (T_A) in the range of 50 to 150% of the velocity (v^ of the web (WEB1 ).

15. The apparatus (500) of claim 13 or 14 wherein the control unit (CNT1 ) is arranged to set the angular velocity (co-i) of the first body (100) such that the aiming point (SP2) crosses the longitudinal reference line (YREF) a plurality of times during said writing period (T_A).

16. The apparatus (500) according to any of the claims 13 to 15 comprising a velocity sensor (VSENS1 ) to detect the velocity (v-,) of the web (WEB1 ).

17. The apparatus (500) according to any of the claims 13 to 16 comprising an interface (INTRF1 ) to receive a velocity value (v-,) specifying the velocity (v of the web (WEB1 ).

18. The apparatus (500) according to any of the claims 13 to 17 wherein the aiming point (SP2) is arranged to cross the longitudinal reference line (YREF) five or more times during a full revolution (360°) of the first body (100).

19. The apparatus (500) according to any of the claims 13 to 18 wherein the first body (100) comprises a first light steering region (F1 a) and a second light steering region (F1 b), which are together arranged to be rotated about the rotation axis (AX1 ) of the first body (100), the orientation of the second light steering region (F1 b) being different from the orientation of the first light steering region (F1 a).

20. The apparatus (500) according to any of the claims 13 to 19 wherein the first body (100) is arranged to provide a first deflected beam (LB1 ) by deflecting laser light (LB2), and the second body (200) is arranged to provide a second deflected beam (LB2) by deflecting light of the first deflected beam (LB1 ).

21 . The apparatus (500) according to any of the claims 13 to 20 wherein the aiming point (SP2) is arranged to cross the longitudinal reference line (YREF) when the first region (F1 a) of the rotating first body (100) has been rotated to deflect the laser light (LB0), and the aiming point (SP2) is arranged to cross the longitudinal reference line (YREF) again when the second region (F1 b) of the rotating first body (100) has been rotated to deflect the laser light (LB0).

22. The apparatus (1000) according to any of the claims 13 to 21 comprising one or more rolls (1010) and/or belts to move the web (WEB1 ) in the longitudinal direction (SX).

23. The apparatus (1000) according to any of the claims 13 to 22 comprising one or more processing units (1 100) to process the web (WEB1 ) after the web (WEB1 ) has been marked by using the laser light (LB2).

24. The apparatus (1000) according to any of the claims 13 to 23 comprising a laser (400) to provide the laser light (LB0, LB2).

25. A computer program (PROG1 ), which when executed by a processor is for carrying out the method according to any of the claims 1 to 12.

26. A computer program product (MEM2) storing computer program code (PROG1 ), which when executed by a processor is for carrying out the method according to any of the claims 1 to 12.

27. A paper product (WEB1 ) comprising a marking (MRK1 ), the marking (MRK1 ) comprising ten or more dots (D1 ) formed by a laser beam (LB2) and arranged in a two-dimensional slanted array, the slant angle (γ2) of the array being in the range of 10° to 45°.

28. The paper product of claim 27 wherein the dots (D1 ) are holes extending through a paper sheet.

29. The paper product of claim 27 or 28 wherein the size of the paper product is selected from a group consisting of the standard sizes A5, A4, A3, A2, A1 , AO, ANSI A, ANSI B, ANSI C, ANSI D, and ANSI E, said standard sizes being determined in the standards ISO 216 and ANSI/ASME Y14.1 .