The present invention relates to a device and a method for producing a diffractive microstructure on the surface layer of a substrate by embossing.

BACKGROUND OF THE INVENTION

Diffractive microstructures may be attached to products e.g. for the vis¬ ual effect produced by them, or for authenticating the product.

Diffractive microstructures may be produced e.g. by embossing the surface of a substrate coated with a suitable lacquer. In the embossing process, the coated substrate is pressed between an embossing mem¬ ber and a backing member. The surface of the embossing member has a microstructure that corresponds to the microstructure to be produced. During the embossing process, the backing member supports the sub- strate from the back side so that a sufficient pressure, the embossing pressure, may be exerted on the surface of the substrate for shaping the surface to correspond to the microstructure of the surface of the embossing member. For the processing of the surface of the substrate, it is advantageous to plasticize the surface by heating. The tem- perature of the surface of the substrate during the embossing process is herein called the embossing temperature.

US patent 4,913,858 discloses a method for producing a diffractive microstructure on the surface of a paper coated with a thermoplastic material. The coating is provided with the microstructure by means of a heated embossing roll.

In the arrangements of prior art, the diffractive microstructure is pro¬ duced in such a way that the shape and the dimensions ^' of the Tnicro- structured area of the embossing member correspond to the shape and the dimensions of the microstructured area to be produced.

SUMMARY OF THE INVENTION

The main object of the present invention is to make it possible to reduce the time delay between the designing and the production of the shape of the microstructured area. Another object of the present invention is to reduce the number of embossing members or semi¬ finished embossing members needed for producing microstructured areas of different shapes.

To attain these objects, the method and the embossing member according to the invention are primarily characterized in what will be presented in the characterizing parts of the appended independent claims. The dependent claims will present some preferred embodi- ments of the invention.

To achieve this object, the method and the device according to the invention are primarily characterized in that the width of the micro- structured area of the embossing member is in at least one direction greater than the width of the microstructured area to be produced on the surface layer of the substrate by said microstructured area, wherein said microstructured area of the embossing member is, in said at least one direction, wider than the width of said substrate.

According to the invention, at least a part of the microstructured area of said embossing member takes part in the production of the embossing pattern of the microstructured area to be produced on the surface layer of said substrate.

In an advantageous embodiment of the present invention, the shape of the produced microstructured area is defined by a bulge, which is for example cut from a sheet, and which is placed between the substrate and the backing member, the shape of the bulge corresponding to the produced ^" microstructαred area. When the ^" embossing ^" member is pressed against the surface layer of the substrate, the embossed embossing member exerts a higher embossing pressure on the surface

layer of the substrate at the location of bulge than at locations where there is no bulge. Thus, the diffractive microstructured area is substantially produced only at the location corresponding to said bulge.

In another embodiment of the present invention, the shape of the microstructured area to be produced is defined by heating the corre¬ sponding location on the surface layer of the substrate by e.g. a laser beam. The microstructured area is formed substantially only on the area which is plasticized by heating.

In yet another embodiment of the present invention, the shape of the produced microstructured area is defined by coating a corresponding location of the substrate with a coating, which is embossable at a pre¬ determined temperature. In a corresponding manner, the other locations of the substrate are coated with a coating that is not embossable at said temperature. The other locations may also be left uncoated.

The manufacturing of an embossing member is typically an expensive and time-consuming process. Typically, microstructured areas are pro¬ duced on the surface of the embossing member by optical and electro¬ chemical methods or by using electron beam litography. According to the present invention, when the shape of the microstructured area is changed, it is not necessary to perform all the working steps of e.g. electron beam litography from the beginning, but it is possible to utilize, for example, prefabricated semi-finished embossing members. In some embodiments, it is possible to produce microstructured areas, which have different shapes and dimensions by using a single embossing member 10 only. In this way, the time delay between the designing and the implementation of the shape of the microstructured area is reduced, which entails a significant cost saving. Especially, also small production batches may be produced according to the invention in a cost-effective way.

The invention and its fundamental properties as well as the advantages to be attained by means of the invention will become more evident for a

person skilled in the art from the claims and the following description, in which the invention will be described in more detail by means of a few selected examples.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows schematically an embossing member, a substrate and a backing member before embossing according to prior art,

Fig. 1 b shows schematically the embossing member, the substrate and the backing member during embossing according to prior art,

Fig. 1c shows schematically the embossing member and the sub¬ strate after the embossing of prior art,

Fig 2a shows schematically a substrate, a bulge positioned under the substrate, an embossing member, and a backing member before embossing,

Fig. 2b shows schematically the substrate, the bulge positioned under the substrate, the embossing member, and the backing member during the embossing,

Fig. 2c shows schematically the embossing member and the sub¬ strate after the embossing according to Fig. 2b,

Fig. 3 shows by way of example the substrate, the bulge positioned under the substrate, the embossing member, and the backing member after the embossing according to Fig. 2b,

Fig 4a ^" shows, ^" in principle, the ^~ diffraction ^~ efficiency as ^~ a fϋήctfon of the pattern height of the microstructure,

Fig. 4b shows the diffraction efficiency as a function of the height of the microstructure for a microstructure having a sinusoidal profile, and for a microstructure having a rectangular profile in an example case,

Fig. 5 shows, in principle, the height of the produced microstruc¬ ture as a function of the embossing pressure,

Fig. 6 shows, in principle, the height of the produced microstruc- ture as a function of the embossing temperature,

Fig. 7 shows schematically an embossing device, in which the produced microstructured areas are defined by bulges, which are between the substrate and the backing member,

Fig 8a shows schematically the heating of a selected area on the surface of the substrate by a heating device,

Fig. 8b shows schematically the embossing of the area defined by heating,

Fig. 8c shows schematically the substrate after the embossing according to Fig. 8b,

Fig. 9 shows schematically an embossing device in which the produced microstructured areas are defined by heating with a laser beam,

Fig. 10 shows schematically the defining of an area to be heated on the surface of the substrate by using a mask,

Fig. 11 shows schematically the defining of an area to be kept warm on the surface of the substrate by using a mask,

Fig. 12 shows schematically the defining of the areas on the sur¬ face of the substrate to be heated and to be kept cool by means of heating and cooling devices,

Fig 13a shows schematically the formation of coating zones on the surface of the substrate, which zones have different compositions,

Fig. 13b shows schematically the behaviour of different coating zones of the substrate during embossing,

Fig. 13c shows schematically a substrate, on which the produced microstructured area has been defined by means of the composition of the coating zones,

Fig 14a shows schematically the application of an embossable sur¬ face layer onto a non-embossable surface layer,

Fig. 14b shows schematically the embossing of the portion of the embossable surface layer,

Fig. 14c shows schematically the substrate, on which the produced microstructured area has been defined by means of the shape of the embossable surface layer,

Fig 15a shows schematically an embossing member, a film to be placed between the embossing member and a backing member, a substrate, and the backing member before embossing,

Fig. 15b shows schematically the embossing member, the film placed between the embossing member and the backing member, the substrate, and the backing member during the embossing, ^"

Fig. 15c shows schematically the substrate after the embossing according to Fig. 15b,

Fig 16a shows schematically a substrate having a large microstruc- ture, and smoothing members,

Fig. 16b shows schematically the substrate after the smoothing by the smoothing members,

Fig 17a shows schematically an embossing member, a backing support of the embossing member, a bulge between the embossing member and the backing support, a substrate, and a backing member before embossing,

Fig. 17b shows schematically the embossing member, the backing support for the embossing member, the bulge between the embossing member and the backing support, the substrate, and the backing member during the embossing,

Fig. 17c shows schematically the substrate and the embossing member after the embossing according to Fig. 17b,

Fig 18a shows schematically a cut embossing member, a backing support, a bulge between the embossing member and the backing support, a substrate, and a backing member before embossing,

Fig. 18b shows schematically the cut embossing member, the backing support, the bulge between the embossing member and the backing support, the substrate, and the backing member during the embossing,

Fig. 18c shows schematically the substrate and the cut embossing member after the embσssing according to Figr16b,

Fig 19a shows schematically an embossing member having a beveled edge at least during the embossing,

Fig. 19b shows schematically an embossing member having a rounded edge at least during the embossing,

Fig 20a shows schematically an embossing member having a sharp edge, a backing support of the embossing member, a bulge between the embossing member and the backing support, a substrate, and a backing member before embossing,

Fig. 20b shows schematically the embossing member having the sharp edge, the backing support for the embossing member, the bulge between the embossing member and the backing support, the substrate, and the backing member during the embossing,

Fig. 20c shows schematically the embossing member having the sharp edge, and the substrate after the embossing according to Fig. 20b, and

Fig. 21 shows schematically an embossing device in which the produced microstructured areas are defined by protruding embossing members having rounded/beveled edges.

DETAILED DESCRIPTION OF THE INVENTION

Figures 1a to 1c show the production of a microstructure according to prior art. Referring to Fig. 1a, a microstructured area 41 is produced on the surface layer 40 of a substrate 30 by pressing the substrate 30 between an embossing member 10 and a backing member 20 in such a way that the surface 40 of the substrate is shaped to correspond as closely as possible to the microstructured area 11 on the surface of the embossingi member! OT

With reference to Fig. 1 b, an embossing force EF is exerted on the embossing member 10. Furthermore, local or spatial differences of the embossing pressure exerted by the surface of the embossing member 10 on the surface 40 of the substrate 30 cause local material flow and/or a compression in the surface 40 of the substrate.

With reference to Fig. 1c, the width W2 of the microstructured area 41 produced on the surface layer 40 of the substrate 30 is equal to the width W1 of the microstructured area on the surface of the embossing member 10, provided that the width W3 of the substrate 30 is at least equal to the width W1 of the microstructured area on the surface of the embossing member 10. If the substrate 30 is narrower than W1 , the width W2 of the produced microstructured area is narrower than W1.

For the produced microstructure 41 , it is possible to determine a local pattern height r between the highest point and the lowest point of the surface, which height is determined in the direction perpendicular to the macroscopic surface. In a corresponding manner, a local pattern height s can be determined for the microstructured area 11 on the surface of the embossing member 10. The pattern height r of the produced micro- structured area 41 is smaller than or equal to the pattern height s of the embossing member.

Typically, the embossing member 10 is e.g. a shim made of a nickel- based material, and which shim is provided by optical and electrolytic methods with reliefs corresponding to the desired microstructure. A method for manufacturing a shim with reliefs suitable for use as the embossing member 10 is described, for example, in US patent 3,950,839. The embossing member may also be manufactured by methods of electron beam litography. If required, said embossed shim may be bent and welded to form a cylinder which is placed on a rotatable, wherein the substrate 30 and its surface layer 40 are com¬ pressed when the rolls rotate in such a way that a microstructure cor¬ responding to the ^" surface of the embδssincfmember 1O ^~ is formed on the surface layer 40. A method for bending and welding the cylinder is

disclosed, for example, in US patent 6,651 ,338. The backing member 20 may be, for example, a rotating roll coated with epoxy resin.

The substrate 30 may be, for example, paper, cardboard or plastic. The surface layer 40 of the substrate may constitute, for example, of a thermoplastic polymer, such as polyvinyl chloride or polycarbonate, whose viscosity is reduced at a high temperature. Examples of such materials are listed, for example, in US patent 4,913,858. The surface

40 of the substrate may also consist of a material that contains a fine- grained mineral, for example kaolin. The surface 40 of the substrate may also consist of UV curable lacquer. The microstructure may also be embossed on printing ink as disclosed in US patent 5,873,305.

The Poisson's ratio for the material of the surface layer 40 may be sub- stantially equal to 0.5, in which case the surface material is substan¬ tially not compressed during the embossing but the shaping takes place primarily as a result of material flow.

The substrate 30 and its surface 40 may consist of the same material. The embossed surface may be coated with a metal film to enhance the visual effect. The embossed surface may also be coated with a film having a high refractive index, for example with a zinc sulphide film, to enhance the visual effect. The embossed surface may be protected against wear and smudging by, for example, a transparent film.

The diffractive microstructure embossed on the surface layer 40 of the substrate consists of several periodically organized microscopic protru¬ sions recurring in at least one direction at intervals of the so-called grating constant d. The value of the grating constant and the orienta- tion of said protrusions may vary in different locations on the surface, wherein the desired diffractive effect or holographic pattern is obtained.

The intensity of diffracted light has a maximum at angles of illumination and at angles of viewing which fulfill the grating ^~ equation:

mλl d — sin θ^ + sin θ _f (1 )

where m is an integer indicating the order of diffraction and λ is the wavelength of light, d is the grating constant, θ, is the angle between the direction of incoming light and the normal of the surface, as determined clockwise from said normal of the surface, and θ _d is the angle between the direction of diffraction and the normal of the surface, as determined clockwise from said normal of the surface.

The surface layer 40 of the substrate may be provided with several zones, which have similar or different diffractive microstructures in order to create a desired colour effect, motion effect, two-dimensional pattern, pattern depending on the direction of viewing, animation, pattern providing a three-dimensional impression, or visually invisible microstructure. The substrate 30 or its surface layer 40 may also comprise patterns or symbols produced with a dye. These may be produced before, simultaneously with, or also after the embossing. The patterns provided with a dye and the produced microstructures may overlap in whole or in part.

The visual effect produced by the microstructured area 41 may also be transparent, wherein transparency means that there is at least one direction, in which the visual effect caused by the diffracted light is not detected. Thus, graphic characters produced on the substrate 30 or the surface layer 40 by printing methods can be viewed in said at least one direction without a disturbance caused by the diffractive effect. When the substrate 30 and the surface layer 40 are substantially transparent, it is possible to view an object behind the substrate 30, for example a product in a product package, in said at least one direction without the disturbance caused by said effect.

The profile of the microstructure may be, for example, sinusoidal, trian¬ gular or rectangular. For reasons of manufacturing aspects, the produced form of the profile on the surface of the embossing member may also be different from the sinusoidal , ^~ triaήgϋϊar ^" όr rectangular profile.

Figure 2a shows an embodiment of the present invention, in which a bulge 51 corresponding to the microstructured area 41 to be produced is placed between the substrate 30 and a backing member 20. The bulge 51 may be, for example, a shape cut from an aluminium sheet, plastic film, rubber sheet, or cardboard, that may be attached to the backing member 20, for example, by means of glue or two-sided adhesive tape. The thickness of the bulge 51 should be preferably at least equal to the compression of the substrate 30, the surface layer 40 of the substrate, the surface of the backing member 20, and the bulge 51 itself during the embossing. In this way, no significant embossing pressure p _E will be exerted on the areas on the surface layer 40 of the substrate 30 which do not have the bulge 51 underneath.

The thickness of the bulge may also be smaller than the compression of the substrate 30, the surface layer 40 of the substrate, the surface of the backing member 20, and the bulge 51 itself during the embossing. In that case, however, it is possible that a low microstmcture will be formed also at locations, which do not have the bulge underneath.

With reference to Fig. 2b, the surface layer 40 of the substrate 30 is processed to correspond to the microstmcture of the embossing mem¬ ber only on those areas, which have the bulge 51 underneath.

With reference to Fig. 2c, the width W2 of the microstructured area 41 produced on the surface layer 40 of the substrate 30 is smaller than the width W1 of the microstructured area 11 of the embossing member 10. The width W2 of the produced microstructured area 41 is substantially equal to the width of the bulge 51.

With reference to Fig. 3, the bulge 51 may be used to define a pattern produced on the surface layer 40 of the substrate 30, in this example case the letter "A". The shape and the dimensions of the produced shape may be different from the shape and the dimensions of the microstructured area 11 of trie embossing member 10.

The ratio between the intensity of light diffracted from the microstruc- ture 41 in a predetermined direction and the intensity of light impinging on the microstructure, that is, the diffraction efficiency, depends not only on the pattern height r of the microstructured area 41 but also on the form of the profile of the microstructure and on the refractive index of the surface 40 of the substrate.

Figure 4a shows the typical diffraction efficiency Eff of the microstruc¬ ture as a function of the pattern height r of the microstructure. The maximum diffraction efficiency is achieved, for example, at the pattern height value r _O pτ, which is typically slightly higher than the quarter of the wavelength λ of light, e.g. 0.26 times the wavelength λ of light. The achieved visual effect is thus as strong as possible. The diffraction effi¬ ciency may be optimized, for example, for the green colour having a wavelength of λ = 550 nm. If the pattern height r is substantially lower than r _O pτ, then also the diffraction efficiency is substantially lower than the maximum diffraction efficiency.

The diffraction efficiency may be calculated by means of diffraction theories described, for example, in Chapter 2 (by Jari Turunen) of the book Micro-Optics, Elements, Systems, and Applications (Taylor & Francis, Cornwall, 1997).

Figure 4b shows, by way of example, the diffraction efficiency Eff as a function of the pattern height r of the microstructure for a sinusoidal and binary microstructure. The curve S1 depicts the diffraction efficiency of a microstructure having a sinusoidal profile with respect to the diffraction order m = -1 , and the curve B1 illustrates the diffraction efficiency of a microstructure having a rectangular (binary) profile with respect to the diffraction order m = -1. In the situation of Fig. 2b, the calculated wavelength of light is 550 nm (green light), the angle of incidence of light is -30 degrees, and the refractive index of the surface layer 40 of the substrate is 1.5. The ratio d/λ between the grating constant d ^~ and the wavelehgth ^~ λ is 1 ^~ 5. ln lhe example case, ^~ the maximum diffraction efficiency for a microstructure having the sinusoidal profile is achieved at the pattern height value of 170 nm. In

the example case, the maximum diffraction efficiency having the rectangular profile is achieved at the pattern height value of 140 nm. In the example cases, the maximum diffraction efficiency for both micro- structures is 0.017, or 1.7 %. For the microstructure having the rectangular profile, also at least a second maximum 0,012 of the diffraction efficiency is detected at the pattern height 410 nm. In general, it is noted that when the diffraction efficiency has several maxima, high diffraction efficiency Eff may also be achieved at micro- protrusion heights that are significantly higher than the quarter of the wavelength.

Figure 5 shows the pattern height r of the microstructure as a function of the embossing pressure p _E . When the pressure p _E is increased, the pattern height r is also increased, until the pattern height s of the micro- structure of the surface of the embossing member 10 is approached. Consequently, the shape of the microstructured area 41 to be pro¬ duced may be defined by means of the distribution of the embossing pressure.

Figure 6 shows the pattern height r of the microstructure as a function of the embossing temperature T _E . When the embossing temperature T _E is increased, the pattern height r is also increased, until the pattern height s of the microstructure of the surface of the embossing member 10 is approached. Consequently, the shape of the microstructured area 41 to be produced may be defined by means of the distribution of the embossing temperature. However, there is a limiting temperature T _R not to be exceeded, because the surface layer 40 would then be dam¬ aged or adhered to the embossing member 10.

For high quality products, the aim is typically to produce the micro- structure in the desired areas only. However, it is possible that when microstructured areas 41 are produced, for example, according to the embodiment shown in Figs. 2a to 3, a low microstructure is also pro¬ duced on such areas ofthfe surface ^" layer 40 of the substrate 30 where the microstructure is not desired.

When determining the width W2 of the microstructured area 41 , the points which have a low microstructure on the surface layer 40 are not to be taken into consideration. Said low microstructure is defined so that when the pattern height r of the produced microstructure at a pre- determined location is lower than the quarter of the maximum value of the pattern height r of the microstructure produced on the surface layer 40 of the substrate 30, there is a low microstructure at said location.

The width means the distance between two parallel lines, which are tangential to the microstructured area 41 or the substrate 30. When determining the width W2, the considered microstructured area 41 is continuous, that is, undivided. The microstructured area 41 is continuous, if any two randomly selected points of the area 41 can be connected by a curve where the pattern height r at each point is at least a quarter of the maximum value of the pattern height r of the microstructure produced on the surface layer 40 of the substrate 30. Consequently, the pattern height r may vary within the continuous microstructured area 41. Further, if the pattern height r of the produced microstructure at any location of the surface layer 40 is lower than the quarter of the maximum value of the pattern height r of the micro- structure produced in said microstructured area 41 , then said location does not belong to said microstructured area 41.

With reference to Fig. 7, in an embodiment of the present invention, the embossing member 10 and the backing member 20 of the embossing device 1000 are rolls. The substrate 30 and its surface layer 40 move in the direction SZ and are pressed between the embossing roll 10 and the backing roll 20 when said rolls are rotated. The surface layer 40 of the substrate is pre-heated with a heater 120, and the surface of the embossing roll is heated with a heater 100. The heating of the embossing member 10 may be based, in whole or in part, on the use of heat transfer media, such as hot oil. The embossing device 1000 may also comprise inductive heaters 100 or auxiliary rolls heated by electricity or by a heat transfeF medium. The embossing roll Ϊ0 may comprise thermoelements and pressure sensors for monitoring the pressure and the temperature. The temperatures are monitored by

temperature sensors 121 , 101 which may be, for example, pyrometric sensors.

The embossing pressure exerted by the embossing member 10 and the backing member 20 on the surface 40 of the substrate is adjustable. The adjusting takes place, for example, by actuators 140 connected to the bearings 142 of the backing member 20, by which actuators the backing member 20 may be moved in the direction SZ. Said actuators 140 comprise, for example, one or more hydraulic or pneumatic cylinders 140. The actuators 140 may also operate on the electromechanical principle. Furthermore, the actuator 140 may be fully manual in such a way that, for example, no electric, hydraulic or pneu¬ matic auxiliary energy is needed for performing the control movement. The actuators 140 may be provided with sensors 141 for detecting the embossing force, i.e. for indirectly detecting also the embossing pres¬ sure. A control unit 400 controls the operation of the embossing device.

Bulges 41 attached to the backing roll 20 define the microstructured areas 41 to be produced. The bulges 51 may also be permanently engraved or etched on the surface of the backing member 20. More¬ over, the bulges 51 may be attached to the substrate 30. The bulges 51 may also consist of an additional coating layer between the sub¬ strate 30 and its surface layer 40. Furthermore, the bulges 51 may be formed such that the surface layer 40 of the substrate 30 is thicker in the desired areas than at locations where no microstructure is desired. The bulges 51 may also be formed such that the substrate 30 or its surface layer 40 is made thinner at locations where no microstructure is desired. The thinning may be performed, for example, by pressing, grinding or milling. The bulges 51 may also be attached to a film that is placed between the substrate 30 and the backing member 20. The bulges 51 and the film may also consist of the same material.

With reference to Fig. 8a, the produced microstructured area 41 may also be defined by ^~ heating specifically only ^" that area of the surface layer 40 of the substrate 30 where said microstructured area 41 is desired. The heating may be performed, for example, by means of a

device 200 emitting laser beams or heat radiation HR. The heating may also be performed, for example, by means of a hot flow of gas or liquid, or by means of a heating surface. The heating surface may be, for example, an aluminium body heated with a hot heat transfer medium, which body may be have, for example, a polytetrafluoro- ethylene (PTFE) coating to prevent adhesion.

The shape and the size of the heated area, in turn, may be defined, for example, by moving the laser beam or hot gas jet directed to the desired area. Furthermore, the heated area may be defined by means of a mask covering the areas not to be heated. The heated area may also be defined by the shape of the heating surface.

With reference to Fig. 8b, the embossing member is pressed against the surface layer 40 of the substrate 30, wherein only the heated area of the surface layer 40 is shaped. In the unheated areas, the local embossing pressure is not sufficient to process the surface layer 40. To form a microstructure 41 in the heated area, the substrate 30, and/or its surface layer 40 and/or the backing member 20 must be compressed such that the compression is at least equal to the height of the micro- structure 41.

With reference to Fig. 8c, the microstructure is formed substantially only on the microstructured area 41 corresponding to the heated area, wherein only a weak microstructure or no microstructure is formed on the other areas. The width W2 of the microstructured area 41 produced on the surface layer 40 of the substrate 30 is smaller than the width W1 of the microstructured area 11 of the embossing member 10. The width W2 of the produced microstructured area 41 is substantially equal to the width of the initially heated area.

Figure 9 shows an embossing device 1000, in which the shape of the microstructured area is defined by a heating laser beam LB. A laser 210 emits the laser beam LB to heatiher surface layer 40 of the sub- strate 30. A control unit 400 controls by means of a directing mechanism 204 a mirror 202, which directs the laser beam. The control

unit 400 also controls the power of the laser beam LB. The mirror 202 is turned substantially around the axis 201. The turning mirror 202 directs the laser beam LB to a second mirror 203 that is preferably as wide as the substrate 30 in the direction of the axis of the embossing roll 10. The second mirror 203 directs the laser beam LB to the surface layer 40 of the substrate 30. As the substrate 30 moves in a direction SX, the power of the laser beam LB scanning laterally along the surface layer 40 may be adjusted to heat areas having the desired shape and size. It is important that the temperature of the surface is within a predetermined range at the moment of embossing. Therefore, a scanning speed and/or laser beam power according to the situation is used. In particular, the absorption coefficient of the surface layer 40 must be taken into account. The optimal values are determined empirically or by calculations.

The mirror 202 may also rotate at a constant speed of rotation, wherein the directing mechanism 204 transmits the mirror position data to the control unit 400 to adjust the power of the laser beam LB.

The surface layer 40 begins to cool down immediately after heating. For this reason, it is advantageous to perform the heating as close to the embossing point, that is, the nip between the rolls 10, 20, as possible. It is thus advantageous that the second mirror 203 is also located as close to the nip between the rolls 10, 20 as possible. The velocity of the substrate 30 in the direction SX must be sufficiently high so that the surface layer 40 does not have time to cool down before the embossing. Said cooling may be partly compensated for by heating the surface layer 40 to a temperature which is higher than the optimal temperature at the moment of embossing.

The wavelength of the laser beam LB is selected so that it is absorbed to a sufficient extent in the surface layer 40 of the substrate 30. The laser 210 may be, for example, a diode laser or a carbon dioxide laser.

The embossing member 10 and the backing member 20 are advanta¬ geously rolls. The substrate 30 and its surface layer 40 move in the

direction SZ and are pressed between the embossing roll 10 and the backing roll 20 when said rolls are rotated. The surface layer 40 of the substrate is pre-heated with a heater 120, and the surface of the embossing roll is heated with a heater 100. In this case, the pre-heat- ing of the surface layer 40 of the substrate 30 makes it possible to reduce the required laser power.

The temperatures are monitored by temperature sensors 121 , 101 , which may be, for example, pyrometric sensors. Two hydraulic actua- tors 140 are used to exert, via a bearing 140, forces onto the ends of the shaft of the backing roll 20, in order to press the embossing roll against the substrate 30 in the direction SZ. Said forces, and also indirectly the embossing pressure are monitored by a sensor 141 connected to the actuator 140. A control unit 400 controls the operation of the embossing device.

The embodiment of Fig. 9 has particularly the advantage that the produced microstructured area 41 may be defined quickly by means of a computer program, wherein each produced microstructured area 41 may have a different shape, if desired.

With reference to Fig. 10, the heated area on the surface layer 40 of the substrate may also be defined by a mask 60, which mask 60 pre¬ vents the heating effect from being directed to those areas in which no microstructure is desired. The heating may be implemented, for example, by heat radiation HR or by hot gas jets. The mask 60 may be, for example, a sheet of a thermally insulating material. The mask 60 may also be a sheet that reflects heat radiation. The mask 60 may also have a considerable thermal capacity, wherein it slows down the heating of the surface layer 40. After the heating, the mask 60 may be cooled down and re-used.

With reference to Fig. 11 , the hot area of the surface may also be defined by cooling. ^" Before the situation of Fig. ^~ 11 , ^~ the ^" whoie surface layer 40 has been heated. The location corresponding to the micro-

structured area 41 to be produced is protected with a mask 60, and the unprotected areas are quickly cooled down with a cold gas stream.

With reference to Fig. 12, the location corresponding to the produced microstructured area 41 may be heated, for example, with a heating element 230 having a PTFE coating. The locations where no micro- structure is desired may be cooled with cooling elements 240. The elements 230, 240 may also be attached to a rotating roll.

In the situations of Figs. 8a to 12, it is important that the embossing can be performed fast enough after the formation of the hot area of the surface layer 40. The usable time interval is short, when the surface layer 40 is thin. Thus, the time constant for cooling of the surface layer is short, respectively.

With reference to Fig. 13a, the shape of the produced microstructured area 41 may also be defined by forming zones 40a, 40b on the surface of the substrate 30, which zones have different compositions. The chemical composition of the zones is selected so that with pre- determined values of the embossing pressure and embossing temperature, a microstructure is formed on the zone 40a but only a weak microstructure or no microstructure is formed on the zone 40b.

The different zones 40a, 40b of the surface layer may be produced, for example, by printing rolls 71 by known methods of printing technology. The embossable area 40a may consist of conventional printing ink. The non-embossable areas 40b may consist of, for example, UV cured lacquer.

In one embodiment, the substrate 30 is coated with an embossable coating 40a for only those portions, on which a microstructured area 41 is desired. Thus, the non-embossable zones 40b may consist of, for example, the original surface material of the substrate 30.

With reference to Fig. 13b, the surface of the non-embossable zones 40b, the substrate 30 and/or the backing member must be compressed

such that the compression is at least equal to the height r of the micro- structured area 40 formed during the embossing.

Figure 13c shows the substrate 30 as well as the zones 40a and 40b, where a microstructure has been formed on the embossable zone 40a only.

With reference to the Figs. 14a to 14c, the surface layer 40a consisting of an embossable material may be applied onto the surface layer 40b by, for example, a printing roll. The defining of the shape of the micro- structured area 41 is thus based firstly on the fact that the total thick¬ ness of the substrate 30 and its surface layers 40a, 40b is greater at said microstructured area 41 than in those areas where no microstruc¬ ture is desired, wherein the embossing pressure p _E is also higher in the microstructured area 41. Further, the surface layer 40a advantageously consists of a non-embossable material, wherein no microstructure will be formed on it.

With reference to Fig. 15a, the produced microstructured area 41 may also be defined by smooth films 62 which are placed between the embossing member 10 and the substrate 30 and which are used as masks. The films may consist of, for example, plastic, aluminium or paper. The microstructure of the embossing means may also be covered partly by a removable grease or lacquer, which may later be peeled, dissolved or washed off when the produced pattern is changed.

With reference to Fig. 15b, the film 62 prevents the formation of the microstructure in areas where no microstructure is desired. The surface of the surface layer 40, the substrate 30 and/or the backing member 20 must be compressed during the embossing such that the compression is at least equal to the total thickness of the thickness of the film 62 and the pattern height r of the produced microstructured area 41. The substrate 30 and the surface layer 40 are slightly bent at the perimeter of the zones 40a and 40b.

With reference to Fig. 15c, the micro-structured area 41 is substantially formed only on the area where there is no film 62.

With reference to Fig. 16a, at an intermediate stage the microstruc- tured area may comprise even the whole surface layer 40. Thus, the produced microstructured area 41 may be defined by smoothing the microstructure on those portions where no microstructure is desired.

This may be performed, for example, by heating the surface layer 40 with heating elements 250. In connection with the heating, the surface layer 40 may be pressed or wiped.

The smoothing may also be performed by coating the microstructure partly, for example, with lacquer or with printing ink.

Figure 16b shows the substrate 30 and its surface layer 40 after the smoothing. The microstructure is left only in the desired area, wherein the microstructured area 41 is formed.

With reference to Fig. 17a, the shape and the dimensions of the produced microstructured area 41 may also be defined by means of a bulge 53 between the embossing member 10 and a backing support 19 for the embossing member. Said bulge 53 may be a separate piece, or it may be attached to said embossing member 10 and/or to said backing support 19.

The bulge 53 may be, for example, a metal sheet, a plastic sheet or a cardboard that has been cut to its shape, for example, by computer- controlled waterjet cutting or laser cutting. The bulge 53 may also be, for example, a pattern formed of lacquer by methods of printing technology. It is advantageous if the bulge 53 can be easily detached, dissolved or washed off when the shape of the microstructured area 41 to be produced is changed.

The embossing member 10 is, ^" foT example, an embossed nickel shim having a thickness of 0.05 to 0.2 mm, wherein it is slightly flexible. With reference to Fig. 17b, the embossing pressure presses the embossing

member 10 against the backing support 19 and the bulge 53 during the embossing, wherein the portion of the embossing member 10 corre¬ sponding to said bulge 53 is set to protrude. To produce the micro- structured area 41 , a sufficiently high embossing pressure is transmit- ted to the surface layer 40 substantially only at the location corresponding to the bulge 53.

The thickness of the bulge 53 is advantageously at least equal to the compression of the substrate 30 and of its surface layer 40 during the embossing.

When the embossing member 10 is bent, locations having a bevel and/or a rounding 17 are formed. If there is no support behind said bevels and/or roundings, there is a risk that the embossing member 10 will break at the respective location. Such an unsupported point may be formed, for example, so that the edge of the bulge 53 is perpendicular to the embossing member 10. Consequently, the bulge 53 has advan¬ tageously beveled or rounded edges to avoid the breaking or cutting of the embossing member 10 during the embossing.

In the embodiment of Fig. 17b, the same embossing member 10 may be used for the production of several different microstructured areas 41.

With reference to Fig. 17c, the width W2 of the microstructured area 41 produced according to Fig. 17b is smaller than the width W1 of the microstructured area 11 of the embossing member 10.

The embossing member 10 compresses the substrate 30 and/or the surface layer 40 slightly during the embossing procedure. The local embossing pressure p _E depends on the local compression. If the embossing member 10 comprises bevelled and/or rounded points 17, then a zone where the pattern height r of the microstructure is changedT is formed at the e ^" dge of the pToduced microstructured area 41. Advantageously, the pattern height r is reduced to zero at a length

that is 0.1 to 1 times the total thickness of the substrate 30 and its surface layer 40.

With reference to Fig. 18a, the embossing member 10 may also be cut to correspond to the bulge 53. For example, a piece having the shape of the bulge 53 may be cut of a prefabricated embossed nickel shim, which piece may be, for example, glued or soldered to the bulge 53. In this way, a large number of microstructured areas 41 with different shapes may be manufactured by using a few stored types of nickel shims.

With reference to Fig. 18b, the microstructured area 41 is formed at the location corresponding to the bulge 53 during the embossing. In the embodiment of Fig. 18b, the outline of the produced microstructured area 41 is typically sharper than in the embodiment of Fig. 17b. However, a problem with the embodiment of Fig. 18b is, for example, that the cut embossing member 10 can be used later to only a limited extent for the production of microstructured areas 41 with different shapes.

To prevent the cutting of the substrate 30 and/or its surface layer 40, the edge of the embossing member 10 is provided with a bevel and/or a rounding 17, at least during the embossing. This is advantageously provided such that also the bulge 53 has a bevel and/or a rounding.

With reference to Fig. 18c, due to the bevels and/or the rounding of the embossing member 10, the width W2 of the microstructured area 41 is smaller than the width W1 of the microstructured area 11 of the embossing member 10.

The embossing member 10 compresses the substrate 30 and/or the surface layer 40 slightly during the embossing procedure. The local embossing pressure p _E depends on the local compression. If the edge of the embossing member 10 comprises a bevel and/or a rounding 17, then a zone where the pattern height r of the microstructure is changed is provided at the edge of the produced microstructured area 41.

Advantageously, the pattern height r is reduced to zero at a length that is 0.1 to 1 times the total thickness of the substrate 30 and its surface layer 40.

Figure 19a shows the dimensions V1 and V2 of the bevel 17 of the embossing member 10 at the moment of embossing. Figure 19b shows the dimensions V1 and V2 of the rounding 17 of the edge of the embossing member 10. At the moment of embossing, the dimension of the bevel/rounding 17 parallel to the surface of the produced micro- structured area 41 is V1. At the moment of embossing, the dimension of the bevel/rounding 17 perpendicular to the surface of the produced microstructured area 41 is V2. The dimensions V1 and V2 are advantageously larger than or equal to the quarter of the total thickness of the substrate 30 and its surface layer 40. The dimension V1 may be, for example, 0.05 mm, and the grating period d of the microstructure may be, for example, 0.001 mm, so that there is space for several tens or hundreds of microscopic protrusions in the area of the bevel or rounding.

With reference to Figs. 20a to 20c, the microstructured area 41 may be defined by means of the bulge 53 also in those cases in which the embossing member 10 does not comprise a bevel and/or a rounding 17. If there is no bevel or rounding 17 in the embossing member 10 during the embossing, there is still a risk that the substrate 30 and/or its surface layer 40 are cut. This weakens the strength of the substrate 30 and degrades the visual appearance of the final product.

With reference to Fig. 20c, it is noticed that if the bulge 53 does not comprise a bevel/rounding and the embossing member 10 is cut to the same size as said bulge 53, then the width W2 of the produced micro- structured area is equal to the width W1 of the microstructured area 11 of the embossing member 10. The embossing member 10 may be cut, for example, of a prefabricated nickel shim having a relief.

Figure 21 shows an embossing device 1000 according to the invention, in which the backing support 19 for the embossing members 10 is a

rotating roll. Bulges 53 between the embossing members 10 and the backing support define the shape of the microstructured areas 41 to be produced. To prevent cutting of the surface layer 40 of the substrate, the edges of the embossing members 10 are provided with bev- els/roundings 17.

The substrate 30 and its surface layer 40 move in the direction SZ and are pressed between the embossing roll 10 and the backing roll 20 when the rolls are rotated. The surface layer 40 of the substrate is pre- heated with a heater 120, and the surface of the embossing roll is heated with a heater 100.

The temperatures are monitored by temperature sensors 121 , 101 which may be, for example, pyrometric sensors. Two hydraulic actua- tors 140 are used to exert, via a bearing 140, forces on the ends of the shaft of the backing roll 20, which forces press the substrate 30 against the embossing members 10 in a direction SZ. Said forces and also indirectly the embossing pressure are monitored by sensors 141 in connection with the actuators 140. A control unit 400 controls the function of the embossing device.

One or more diffractive microstructured areas 41 may be produced, for example, on a product, a product brochure or a product package, for example to attract the interest of a consumer or to show authenticity. The shape of the produced microstructured area 41 may be, for example, a geometrical pattern, a letter, a number, or a random pat¬ tern.

The invention is not limited solely to the embodiments presented in the above description or in the drawings. The aim is to limit the invention only by the presentation of the scope of the appended claims.