Multilayer foil with embossed diffractive structure, method of its production, and its use for optically securing documents

Abstract

The present invention relates to a multilayer foil (18), which comprises a base (1) as carrier, and at least layers (3) to (8) and optionally layer (2), which are different from one another, inter alia at least one embossing layer (3), which is obtainable from at least one thermoplastic material and comprises at least one embossed diffractive structure (4), which bears at least one reflective layer (5), at least one reinforcing layer (6), at least one leveling layer (7), which is present at least in portion on the at least one reinforcing layer (6), and at least one adhesive layer (8), which is present at least in portion on the at least one leveling layer (7), a method for its preparation, a use of the multilayer foil as an origin of a diffractive device (38), a method for optically securing a document (9), which makes use of the diffractive device originating from the multilayer foil, and to a document optically secured by means of the diffractive device.

Description

Multilayer foil with embossed diffractive structure, method of its production, and its use for optically securing documents The present invention relates to a multilayer foilwith at least one embossed diffractive structure, a method of its production,its use as an origin of a diffractive device, in particular of a holographic device, for optically securing documents, a method for optically securing a document, which makes use of the diffractive device originating from the multilayer foil, and to an optically secured document as such, which bears the diffractive device on at least one of its surfaces. Background of the invention Optical security elements are frequently used to prevent copying and misuse of documents including in particular plastic documents, e.g., for securing documents such credit cards, money cards, identity cards, passports, etc., but also for securing paper-based documents such as banknotes. The optical security elements can, e.g., be incorporated into a multilayer holographic foil, which is then stamped onto the suitable document, or can be integrated in a laminate that is heat sealed to the surface of a suitable document. Usually, the security elements used contain diffractive structures such as relief structures as optical security elements, which in turn are often equipped with at least one layer made of at least one reflective material, in order to create special optical structures on the surfaces of the documents to be secured, in particular based on visual optical effects created by diffraction phenomena.  Multilayer holographic foils are, e.g., known from GB 1 568 563 and US 7,192,641 B2. GB 1 568 563 relates to a multilayer foil comprising a carrier film as base, an embossing layer bearing a three-dimensional pattern such as a diffraction microrelief, optionally a release layer positioned in between to facilitate the separation of the embossing layer from the base, optionally a reflective layer, and an adhesive layer. US 7,192,641 B2 also discloses a multilayer foil for use as an embossing foil for transferring labels to documents of value. Said multilayer foil includes a carrier film as base, a label layer such as an embossing layer with a formed diffraction microrelief, optionally a release layer positioned in between to facilitate the separation of the label layer from the base, optionally a reflective layer, and an adhesion promoter layer applied before an adhesive layer, that partially covers the surface of the foil and forms the shape of a holographic device when applied to the document. A disadvantage of the multilayer foils according to GB 1 568 563 and US 7,192,641 B2 is that these foils cannot be used to apply a holographic device to a porous, rough document surface such as paper pages of passports, identity cards or banknotes, as it requires the use of a comparably thick layer of the adhesive, which creates, however, unwanted fringe foil residues at the boundaries of the stamp during application at the edges of the adhesive layer, where the embossing layer is torn up. Another disadvantage of the multilayer foils according to GB 1 568 563 and US 7,192,641 B2 is that when using a comparably thick adhesive layer and applying the foil to a comparably rough surface such as a paper surface, the diffraction pattern can be significantly damaged due to thermal and mechanical effects and the holographic image can become faded and unclear as a result thereof, e.g., may lose brightness and may become pale and matte. When using a thinner adhesive layer and applying this foil to a rough surface, it often only sticks to protruding parts of the roughness and fails to create sufficient overall adhesion. US 5,820,971 relates to a security document such as a bank note or identity card with a multilayer security element comprising least two plastic layers such as UV lacquers with a relief structure such as holographic structure being embedded in between. One plastic layer is applied directly to the document and the relief structure is embossed on said layer, which is activated before or during the relief embossing. Then, a reflective layer and the second plastic layer are applied on top. A disadvantage of the technology underlying US 5,820,971 is that each document such as a passport or banknote must participate in the technological process of forming a holographic security device. This, however, is in most cases technologically complex or even impossible for economic reasons. For example, vacuum deposition is commonly used to form reflective layers, which, however, it is unsuitable for be used for paper documents. US 6,302,989 B1 discloses a method for producing a multilayer foil for transferring optically variable elements to objects to be protected. The multilayer foil is prepared providing a carrier substrate, applying at least one embossable layer on said carrier substrate with the contour shape of said element to be transferred, forming a diffraction structure in said embossable layer in the form of a relief, and forming a reflecting layer on top of said diffraction structure. The applied layers have the contour of an applied holographic device. A disadvantage of the method according to US 6,302,989 B1 is that when layers are successively applied, their contours are often misaligned. This allows imitations to be made with widely available printing technologies. Misalignment between individual layers also decreases contour resolution and limits the design of the final holographic device. Thus, there is a need to provide multilayer foils having an embossed diffractive structure, in particular a holographic structure, which do not exhibit the disadvantages of the multilayer foils of the prior art. In particular, there is a need to provide multilayer foils having an embossed diffractive structure, in particular a holographic structure, which allow to be used to be adherently applied with sharp edges to surfaces of suitable documents, which have a surface with a comparably high porosity and / or which have a comparably rough surface, in particular to surfaces of paper documents, e.g., banknotes, in an economically advantageous, technologically non-complex, and easy to perform manner, wherein the contours of the layers of the multilayer foils after having been transferred to the documents to be secured are not misaligned, the diffraction pattern of the embossed diffractive structure does not suffer any damage including any optical damage during and after the transfer, the embossed structure after transfer still exhibits an excellent image resolution, and wherein the shape of the multilayer foil to be used as holographic device is of arbitrary complexity, and cannot be made, e.g., by cutting by a die or by stamping, and that said shape can be pre-formed before application to the document to be protected takes place. Problem It has been therefore an objective underlying the present invention to provide multilayer foils having an embossed diffractive structure, in particular a holographic structure, which allow to be used to be adherently applied with sharp edges to surfaces of suitable documents, which have a surface with a comparably high porosity and / or which have a comparably rough surface, in particular to surfaces of paper documents, e.g., banknotes, in an economically advantageous, technologically non-complex, and easy to perform manner, wherein the contours of the layers of the multilayer foils after having been transferred to the documents to be secured are not misaligned, the diffraction pattern of the embossed diffractive structure does not suffer any damage including any optical damage during and after the transfer, the embossed structure after transfer still exhibits an excellent image resolution, and wherein the shape of the multilayer foil to be used as holographic device is of arbitrary complexity, and cannot be made, e.g., by cutting by a die or by stamping, and that said shape can be pre-formed before application to the document to be protected takes place. Solution This objective has been solved by the subject-matter of the claims of the present application as well as by the preferred embodiments thereof disclosed in this specification, i.e., by the subject-matter described herein. A first subject-matter of the present invention is a multilayer foil (18), which comprises a base (1) as carrier, and at least layers (3) to (8) and optionally layer (2), which are different from one another, namely optionally at least one separation layer (2), which is present on at least a portion of at least one surface of the base (1), at least one embossing layer (3), which is present on at least a portion of at least one surface of the base (1), or which is present on at least a portion of the separation layer (2), if the separation layer (2) is present, wherein the embossing layer (3) is obtainable from at least one thermoplastic material and comprises at least one embossed diffractive structure (4), wherein at least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5),  at least one reinforcing layer (6), which is present at least on the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or which is present on the at least one reflecting layer (5), if the entire embossing layer (3) as such bears the at least one reflective layer (5), at least one leveling layer (7), which is present at least in portion on the at least one reinforcing layer (6), and at least one adhesive layer (8), which is present at least in portion on the at least one leveling layer (7). A further subject-matter of the present invention is method for preparation of the multilayer foil (18) according to the invention, which comprises a base (1), characterized in that it comprises at least steps 1), 3), 5), 6), 8) to 10), one of steps 4a) and 4b), one of steps 7a) and 7b), and optionally at least one of steps 2) and 11), namely 1) providing a base (1) as carrier, and2) optionally applying at least one material suitable to form a separation layer (2) at least to a portion of at least one surface of the base (1) to form the at least one separation layer (2), and3) applying at least one thermoplastic material at least to a portion of at least one surface of the base (1), or at least to a portion of the separation layer (2), if the separation layer (2) is present, which is suitable to form the embossing layer (3), and4a) embossing at least one diffractive structure into the at least one thermoplastic material to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4), and applying at least one reflective material at least to the at least one embossed diffractive structure (4) of the embossing layer (3) and optionally to the entire surface of the embossing layer (3) including its at least one embossed diffractive structure (4), to form the at least one reflective layer (5), or4b) applying at least one reflective material to the entire surface of the at least one thermoplastic material to form the at least one reflective layer (5), and embossing at least one diffractive structure into the reflective layer (5) present, to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), which bears the at least one reflective layer (5), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4), and5) applying at least one material suitable to form the at least one reinforcing layer (6) at least to the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or to the at least one reflecting layer (5), if the entire embossing layer (3) as such including its at least one embossed diffractive structure (4) bears the at least one reflective layer (5),6) applying at least one material suitable to form the at least one leveling layer (7) at least to a portion of the reinforcing layer (6), 7a) applying at least one material suitable to form the at least one adhesive layer (8) at least to a portion of the leveling layer (7), or7b) applying at least one material suitable to form the at least one adhesive layer (8) to only a portion of the leveling layer (7) and further applying at least one material suitable to form at least one further adhesive layer (8’) to both a portion of the leveling layer (7) not covered by the adhesive layer (8) and to a portion of the adhesive layer (8), wherein the materials suitable to form layers (8) and (8’) are identical to or different from one another, but wherein the material suitable to form the further adhesive layer (8’), in contrast to the material suitable to form the adhesive layer (8), is colored and / or is detectable and visible by UV light, preferably since the material suitable to form the further adhesive layer (8’) contains at least one dye and / or at least one constituent, which is detectable and visible by UV light,8) treating the multilayer foil obtained after step 7a) or step 7b) with an aqueous alkaline solution or composition, which preferably has a pH value in a range of from 10.0 to 14.0,9) washing the multilayer foil treated according to step 8) with water and / or mechanically treating the multilayer foil according to step 8),10) drying the multilayer foil obtained after step 9), and11) optionally irradiating the multilayer foil obtained after step 10) with radiation, preferably UV radiation, in order to cure any of the layers of the multilayer foil, which have been obtained from making use of any constituents, which are radiation curable (crosslinkable by means of radiation), preferably UV curable (crosslinkable by means of UV radiation) constituents, in particular layers 3) and / or 6). A further subject-matter of the present invention is a use of the multilayer foil (18) according to the invention as an origin of a diffractive device (38), in particular of a holographic device, preferably for optically securing suitable documents (9) and / or for preventing copying and / or misuse of suitable documents (9), more preferably of documents made at least partially of at least one kind of plastic and / or paper, even more preferably of documents selected from the group of passports, identity cards, money cards including credit cards, tax stamps, bank cheques, vignettes, tickets and paper-based documents such as banknotes, wherein said diffractive device (38) is obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated. A further subject-matter of the present invention is a method for optically securing a document (9), characterized in that it comprises at least a step A) or a step B), namely A) contacting the multilayer foil (18) according to the invention with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) and / or, if present, the at least one adhesive layer (8’), faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil (18) against said surface of the document (9), more preferably by means of a stamp (10), even more preferably by means of a hot stamp (10), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated, orB) contacting the multilayer foil (18) according to the invention with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) and / or, if present, the at least one adhesive layer (8’), faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil (18) against said surface of the document (9), more preferably by means of a roll (11), even more preferably by means of a hot roll (11), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated.  A further subject-matter of the present invention is an optically secured document (9) obtainable by the inventive method of optically securing a document and bearing the diffractive device (38) at least in portion on one of its surfaces. A further subject-matter of the present invention is a document (9) optically secured by means of a diffractive device (38), characterized in that the document (9) comprises at least in portion on one of its surfaces a diffractive device (38), comprising at least layers (88), (77), (6), (5), and (3), namely, at least one adhesive layer (88), which is present at least in portion on one of the surfaces of document (9), and which originates from adhesive layer (8) and optionally present adhesive layer (8’) of the multilayer foil (18) according to the invention, at least one layer (77), which is present at least in portion on the at least one adhesive layer (88), and which originates from layer (7) of the multilayer foil (18) according to the invention, at least one reinforcing layer (6), which is present at least in portion on the at least one layer (77), and at least one embossing layer (3), which is obtainable from at least one thermoplastic material, and which comprises at least one embossed diffractive structure (4), wherein at least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5),  wherein said at least one embossed diffractive structure (4), which bears at least one reflective layer (5), is embedded between layers (3) and (6). It has been surprisingly found that the inventive multilayer foil (18) having an embossed diffractive structure, in particular a holographic structure, allows to be adherently applied to surfaces of suitable documents (9), which are to be optically secured, which have a surface with a comparably high porosity and / or which have a comparably rough surface, in particular to surfaces of paper documents, e.g., banknotes. The term “adherently applied” in this context means that the diffractive device (38) obtainable from the multilayer foil (18) exhibits an excellent and strong adhesion to the surfaces of the documents (9) onto which the multilayer foil (18) is applied, despite, e.g., the comparably high porosity of their surfaces and / or despite their comparably high surface roughness. Moreover, it has been found that the inventive multilayer foil (18) can be used as an origin of a diffractive device (38) such as of a holographic device to optically secure suitable documents (9) such as credit cards, money cards, identity cards, passports, and paper-based documents such as banknotes. It has been found that the contours of the layers originating from the multilayer foil (18), after having been transferred as diffractive, in particular as holographic, device (38) to the documents (9) to be secured, are not misaligned. Further, it has been found that the embossed structure (4) after transfer still exhibits an excellent image resolution. In particular, it has been found that the embossed structure (4) can be transferred to the surfaces of the documents (9) to be secured with intact sharp edges of said structure. In addition, it has been found that the diffraction pattern of the embossed diffractive structure (4) does not suffer any damage including any optical damage such as a decrease in brightness and / or occurrence of paleness and / or of undesired matting effects during or after said transfer, e.g., due to any undesired thermal and / or mechanical effects arising during the transfer.  Further, it has been surprisingly found that the inventive multilayer foil (18) can be produced in an economically advantageous, technologically non-complex, and easy to perform manner. It has been found that the shape of the multilayer foil (18) to be used as holographic device (38) is of arbitrary complexity, and cannot be made, e.g., by cutting by a die or by stamping, and that said shape can be pre-formed before application to the document (9) to be protected takes place. Detailed description of the invention The invention is exemplarily described in a number of figures.  Fig. 1 illustrates an exemplary embodiment of the multilayer foil (18), which comprises a base (1) as carrier, according to the present invention with the aid of reference signs (1) to (10), (18), (38), (S2) and (S11). For the multilayer foil (18) including the base (1) and the optionally present separation layer (2) reference sign (18) is used. For the part of the multilayer foil (18) from which the diffractive device (38) is formed on the document to be optically secured, reference sign (38) is used. The multilayer foil (18) shown in Fig. 1 comprises a base (1) (also referred to as “carrier” (1) hereinafter), which is, e.g., made of at least one suitable polymeric material. Onto part of the surface of the base (1) a separation layer (2) (also referred to as “release layer” (2) hereinafter) is present, which is optional according to the present invention, but has been realized in the exemplary embodiment according to Fig. 1. Onto the surface of the separation layer (2) an embossing layer (3) is present, which is, e.g., obtainable from at least one suitable thermoplastic material such as a UV curable (crosslinkable by means of UV radiation) thermoplastic material. Layer (3) contains at least one embossed diffractive structure (4). Layer (3) including the embossed diffractive structure (4) further contains a reflective layer (5), which has, e.g., been applied by a vacuum deposition method and which covers the embossed diffractive structure (4). Onto the reflective layer (5) a reinforcing layer (6), which is, e.g., made of at least one UV curable (crosslinkable by means of UV radiation) material, is present in order to provide sufficient mechanical strength to the embossed diffractive structure (4) and helps to preserve the optical properties thereof such as brightness and clarity of the original image. Onto the reinforcing layer (6) a leveling layer (7), which is, e.g., made of at least one suitable polymeric thermoplastic material such as an (meth)acrylate and / or polyurethane material, is present. Layer (7) allows the reinforcing layer (6) to remain as smooth as possible and, further, allows the adhesive layer (8), which is present onto the leveling layer (7) as outermost layer, to take the shape of the roughness of the surface of the document (9) after the multilayer foil (38) has been transferred to the surface of document (9). The adhesive layer (8) covers at least an area (S11), which corresponds to the entire surface of leveling layer (7) of the exemplary multilayer foil (18) as shown in Fig. 1, and is in particular made of a material, which is chemically resistant to alkaline solutions and provides for a thermally activated adhesion. The aforementioned area (S11) defines the outline of the diffractive device (38) to be generated from the multilayer foil (18), when it is at least partially to be applied to the surface of a document (9) to be optically protected. The part of the surface of document (9) to be protected corresponds to aforementioned area (S11) of the multilayer foil (18), which, as already mentioned hereinbefore, defines the outline of the diffractive device (38). The remaining parts (S2) of document (9) not to be protected are also indicated in Fig. 1. Also shown in Fig. 1 is a stamp (10), which has a surface larger in size than the boundary size of area (S11) of the multilayer foil (18), to be used to press the multilayer foil (18) against document (9) in order to transfer the diffractive device (38) comprising the embossed diffractive structure (4) to document (9). “P” in Fig. 1 means pressure, and ”t°C” in Fig.1 means temperature. Both abbreviations illustrate the application process of applying the foil (18) to document (9) in order to provide the document (9) with the diffractive device (38), e.g. according to step A) as defined hereinbefore and hereinafter. Fig. 2 illustrates another exemplary embodiment of the multilayer foil according to the present invention with the aid of reference signs (1) to (9), (11), (18), (8’), (38), (S1), (S1’), (S2) and (S11). The multilayer foil (18) as shown in Fig. 2 corresponds to the multilayer foil (18) as shown in Fig. 1 as far as base (1) and layers (2), (3), and (5) to (7) as well as in the embossed diffractive structure (4) are concerned. The same applies to the diffractive device (38), for which reference sign (38) is used. However, onto the leveling layer (7) an adhesive layer (8) is present, which covers only part of the surface of leveling layer (7) of the exemplary multilayer foil (18), namely a first area (S1). The remaining part of the surface of leveling layer (7) is covered by a further adhesive layer (8’), which may be made of the same material as adhesive layer (8), but, in contrast to adhesive layer (8), is colored, e.g., by making use of at least one dye being present therein, and / or is UV visible, e.g., by making use of at least one UV visible ink being present therein. The further adhesive layer (8’) not only covers the remaining part of the surface of leveling layer (7), but also part of the adhesive layer (8). The overall area covered by adhesive layer (8’) is referred to as area (S1’) in Fig. 2. An area (S11), as in case of the embodiment of the multilayer foil (18) of Fig. 1, defines the outline of the diffractive device (38) to be generated from the multilayer foil (18), when it is at least partially to be applied to the surface of a document (9) to be optically protected. Said area (S11) is the sum of areas (S1) and (S1’) minus the area, where areas (S1) and (S1’) overlap. The part of the surface of document (9) to be protected corresponds to aforementioned area (S11) of the multilayer foil (18). The remaining parts (S2) of document (9) not to be protected are also indicated in Fig. 2. Also shown in Fig. 2 is a roll (11), which has a width larger than the width of the boundary size of area (S11) of the multilayer foil (18), to be used to press the multilayer foil (18) against document (9) in order to transfer the diffractive device (38) comprising the embossed diffractive structure (4) to document (9). “P” in Fig. 2 means pressure, and ”t°C” in Fig. 2 means temperature. Both abbreviations illustrate the application process of applying the foil (18) to document (9) in order to provide the document (9) with the diffractive device (38), e.g. according to step B) as defined hereinbefore and hereinafter.  Fig. 3 illustrates an exemplary embodiment of the diffractive device (38) according to Fig. 1 or Fig. 2, which has been used as embossed diffractive device and has been adhered to part of the surface of document (9) to form an optically secured document, e.g., by making use of the stamp (10) as shown in Fig. 1 or by making use of the roll (11) as shown in Fig. 2, and from which base (1) and separation layer (2) have already been removed afterwards. The diffractive device (38) shown in Fig. 3 originates from the multilayer foil (18) and comprises layers (3), (5), (6), (77) and (88) and includes the embossed diffractive structure (4). Leveling layer (77) originates from the leveling layer (7) shown in Fig. 1 and Fig. 2. Layer (77) has taken the shape of the roughness on the surface of the document (9) after the diffractive device (38) had been transferred to the surface of document (9). The adhesive layer (88) originates from the adhesive layer (8) shown in Fig. 1 or from adhesive layers (8) and (8’) together as shown in Fig. 2. Adhesive layer (88) as well has taken the shape of the roughness on the surface of the document (9) after the transfer has taken place. Area (S11) indicated in Fig. 3 shows the outline of the diffractive device (38), which has been used as embossed diffractive device with the contour of said device. Indicated are again also the remaining parts (S2) of document (9), which have not been protected.  Fig. 4 illustrates a birds-eye view of a passport as an exemplary document (9) containing on part of its surface the diffractive device (38) as illustrated in Fig. 3, which has been obtained from applying the multilayer foil (18) and making use of the diffractive device (38) shown in Fig. 2. Only reference signs (38), (9), (S1), (S1’) and (S2) have been indicated in Fig. 4.  Multilayer foil The multilayer foil (18) according to the present invention comprises the base (1) as carrier, and further comprises at least layers (3) to (8) and optionally layer (2), which are different from one another. The multilayer foil may optionally comprise one or more further layers, e.g., one or more layers, which are already present on the base (1) used for preparing the multilayer foil, but preferably the multilayer foil does not comprise one or more of such further layers. Preferably, the multilayer foil (18) represents an origin of diffractive device (38) and is suitable to transfer said diffractive device (38) due to the presence of the at least one embossed diffractive structure, which preferably is a holographic structure, to the surface of suitable documents (9) for optically securing documents. The term “origin” in this context means that the diffractive device (38) originates from the multilayer foil (18).  The term “comprising” in the sense of the present invention, in connection with the inventive multilayer foil, preferably has the meaning of “consisting of”.  Base (1)The multilayer foil (18) according to the present invention comprises a base (1). The base (1) of the multilayer foil (18) represents a and serves as a carrier.  Preferably, the at least one polymer used for generating the base (1) is selected from polymers, which have a sufficient mechanical strength and temperature resistance to withstand an embossing procedure, in particular as defined in step 4a) or 4b) of the inventive method for preparing the multilayer foil according to the present invention, in which the embossed diffractive structure (4) is generated in the embossing layer (3), and / or to withstand the procedure, according to which the multilayer foil is used as a diffractive device, in particular as a holographic device, to optically secure documents, in particular as defined in step A) and B) of the inventive method for optically securing a document according to the present invention. Preferably, said polymer is different from the thermoplastic material from which the embossing layer (3) is obtainable. The term "polymer" is known to the person skilled in the art. The term "polymer" includes both homopolymers and copolymers. Mixtures of homopolymers and / or copolymers can be used as well. Polymers can be used together, e.g., as blends, with one or more additives, e.g., functional additives, and / or one or more fillers. Suitable polymers for generating the base (1) are poly(meth)acrylates, polyesters including polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), hybrid polyesters including polyethylene terephthalate modified with furandicarboxylic acid (PET / F), polyethylene terephthalate modified with glycol (PET / G), and polycarbonates, polyvinylidene fluoride, polyvinylidene chloride, polyvinyl chloride, polyamides, polyimides, polyethers, polyurethanes, polyolefins including polyethylene, polypropylene, and polystyrene.  Preferably, the base (1) is made of at least one polymer selected from the group consisting of poly(meth)acrylates, polyesters including polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyethylene furanoate (PEF), hybrid polyesters including polyethylene terephthalate modified with furandicarboxylic acid (PET / F), polyethylene terephthalate modified with glycol (PET / G), and polycarbonates, and mixtures thereof, polyvinylidene fluoride, polyvinylidene chloride, polyvinyl chloride, polyamides, polyimides, polyethers, polyurethanes polyolefins including polyethylene, polypropylene and polystyrene, and mixtures thereof, more preferably of at least one polymer selected from the group consisting of polyesters including polyethylene terephthalate, polybutylene terephthalate, polycarbonates, and mixtures thereof, polyvinyl chlorides, polyamides, polyimides, and polyolefins including polyethylene, polypropylene and polystyrene. Most preferred are polyesters including polyethylene terephthalate, polybutylene terephthalates, polycarbonates, and mixtures thereof. Preferably, the base (1) has a uniform thickness, more preferably over its entire size. Preferably, the base (1) has a flat geometry such as in case of a foil or sheet, and, particularly, is not of any curved shape. More preferably, the base (1) has a flat geometry. Particularly preferred, the base (1) is a foil. The thickness of the base (1) is preferably in a range of from 5 µm to 300 µm, more preferably in a range of from 5 µm to 250 µm, even more preferably of from 10 µm to 150 µm.  Optionally present separation layer (2)The multilayer foil (18) according to the invention optionally comprises at least one separation layer (2), which is present on at least a portion of at least one surface of the base (1), preferably on at least a portion of precisely one surface of the base (1). The separation layer (2) functions as release layer and may also be referred to as release layer in this regard. The optionally present separation layer (2) can be made of any material suitable for producing said layer. Preferably, the material used for preparing the optionally present separation layer (2) is able to regulate, if needed, the adhesion between the base (1) and the embossing layer (3). Adhesion should be good enough when embossing a diffraction structure (4) such as a microrelief, in particular hologram, into the thermoplastic material from which the embossing layer (3) is obtainable during its preparation, in particular as defined in step 4a) or 4b) of the inventive method for preparing the multilayer foil (18) according to the present invention, in order to prevent any separation of the embossing layer (3) from the base (1). Further, when using the multilayer foil (18) as an origin of a diffractive device (38), in particular of a holographic device, to optically secure documents such as a document (9), in particular as defined in steps A) and B) of the inventive method for optically securing a document according to the present invention, the material used for preparing the optionally present separation layer (2) is able to ensure, if needed, that a separation of the base (1) from the embossing layer (3) is possible in order to remove the base (1) once the multilayer foil (18) has been contacted with the document (9) to be optically secured and the diffractive device (3) has been transferred to the document to be optically secured. Preferably, the material used for preparing the optionally present separation layer (2) is selected from waxes, silicones and mixtures thereof. More preferably, at least one wax is used for preparation of the separation layer (2) if present. The thickness of the separation layer (2) is preferably in a range of from 0.01 µm to 1.0 µm, if present. Depending on the materials used for preparing the base (1), e.g., on the at least one polymer used for its preparation, and for preparing the embossing layer (3), i.e., on the at least one thermoplastic material from which layer (3) is obtainable, there may be no need for a release layer (2), as the adhesion between these aforementioned materials is already sufficient and adequate, which is why the presence of the release layer (2) is only optional. In this case, the embossing layer (3) is applied directly to the base (1) and directly present on at least a portion of at least one of its surfaces. Embossing layer (3) with embossed diffractive structure (4)At least one embossing layer (3) is present on at least a portion of at least one, preferably of precisely one, surface of the base (1), or is present on at least a portion of the separation layer (2), if the separation layer (2) is present. Preferably, the at least one embossing layer (3) is present on at least one, preferably on precisely one, entire surface of the base (1), or is present on the entire separation layer (2), if the separation layer (2) is present. The embossing layer (3) is obtainable from at least one preferably polymeric and / or prepolymeric thermoplastic material, which preferably is a radiation curable material, more preferably is a UV radiation curable material, and comprises at least one embossed diffractive structure (4). At least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears a reflective layer (5).  The term “radiation curable” in the sense of the present invention means “crosslinkable by means of radiation” and the term “UV radiation curable” in the sense of the present invention means “crosslinkable by means of UV radiation”. The thickness of the embossing layer (3) is preferably in a range of from 0.5 µm to 10.0 µm. In order to generate the embossing layer (3) at least one thermoplastic material, preferably at least one polymeric and / or prepolymeric thermoplastic material, which preferably in each case is a radiation curable material, more preferably in each case is a UV radiation curable material, is preferably applied to at least a portion of at least one, preferably of precisely one, surface of the base (1), or is preferably applied to at least a portion of the separation layer (2), if the separation layer (2) is present. Then, the at least one embossed diffractive structure (4) is generated by a suitable embossing technique. Suitable embossing techniques and standard devices used to carry out the embossing are known to a person skilled in the art. Preferably, an embossing die or matrix bearing on part of at least one of its surfaces at least one structure, said structure being the negative image of the at least one embossed diffractive structure (4), is contacted with at least one surface, preferably with precisely one surface, of the at least one thermoplastic material of the layer, which represents, after embossing, the embossing layer (3). More preferably, a preferably heated die or matrix is pressed or stamped into at least one surface of said thermoplastic material to incorporate a negative image of the structure of the die or matrix into the at least one surface of the thermoplastic material. The die or matrix, preferably die, is preferably part of an embossing tool. The die or matrix can, e.g., be made of at least one kind of metal and / or metal alloy. An exemplary die is a shim made of at least one metal and / or alloy thereof. Preferred is nickel, but other metals can be used as well such as Cu, Cr, steel, etc. As already outlined hereinbefore, the embossing layer (3) is preferably obtainable from at least one thermoplastic material that is radiation curable. The term “curable by radiation” or "radiation curable" in this context preferably refers to a material that undergoes radical or cationic polymerization triggered by photons or electrons by which a three-dimensional crosslinked molecular network is generated. Radiation examples being electron beam or / and UV / VIS light in the wavelength range of λ=100 nm to 500 nm, preferably of λ=200 nm to 400 nm and more preferably λ=250 nm to 400 nm and / or electronic radiation in the range from 150 keV to 300 keV and more preferably with a radiation dose of at least 80 mJ / cm2, preferably 80 mJ / cm2 to 3000 mJ / cm2. Radiation curing employed with particular preference is UV radiation. The embossing layer (3) preferably is obtainable from at least one suitable reactive prepolymer (oligomer or monomer) or at least one reactive polymer, or a mixture thereof, to be used as thermoplastic material. Optionally, at least one non-reactive polymer may be additionally present in each case. The term “reactive” in this context preferably refers to the presence of specific groups in the prepolymer and / or polymer including a) unsaturated C=C groups (such as acrylate, methacrylate, vinyl including styrenyl, and / or allyl) being capable of leading to polymerization by a radical mechanism; b) cyclic ethers groups (such a oxiranes and / or oxetanes), cyclic formals groups, vinyl including styrene and hydroxyl groups capable of participating in cationic polymerization. In order to obtain a solid-state embossing layer (3) suitable for embossing, it is preferable that the embossing varnish is composed primarily of medium and high molecular weight and to a lesser extent of low molecular weight radiation curable polymers and / or prepolymers. The molecular backbone of the radiation curable prepolymers and / or polymers can be, but not necessarily, selected from different chemical classes such as: urethane, epoxy, polyester, polyether, polycarbonate, melamine, (meth)acrylic etc. The reactive prepolymers and / or polymers may contain, but not necessarily, polar groups such as: hydroxyl, carboxyl, carbonyl, ester, ether, amino, amide, phosphonate, phosphate, sulfo, sulfate etc. The embossing varnish can be both solvent-based (organic dispersion or organic solution) and water-based (dispersion and / or aqueous solution).  A suitable embossing composition can be used for generating the embossing layer (3), which composition preferably contains the at least one suitable reactive prepolymer (oligomer or monomer) or the at least one reactive polymer, or the mixture thereof. Besides prepolymer / s and / or polymer / s, the embossing composition used for preparing embossing layer (3) may include, but not necessarily: a) photoinitiators (type I, type II) suitable for initiating radical polymerization or / and photocatalysts suitable for initiating cationic polymerization; and / or b) other ingredients typical for formulating radiation-curing varnishes such as: stabilizers, photosensitizers, oxygen scavengers, amine synergists, defoamers, adhesion promoters, flatting agents, slip agents, wetting agents and / or c) hydrophilizing agents, which can be low molecular weight, medium molecular weight or / and high molecular weight water-soluble or water-swellable compounds and may contain, but not necessarily, the aforementioned reactive groups in their molecules. Before radiation curing, the embossing layer (3) is thermoplastic and highly sensitive to external influences such as temperature, mechanical stress, solvents, water and / or aqueous solutions with pH>7. After embossing, further in an appropriate step such as step 11) of the inventive method for preparing the inventive multilayer foil (18), the embossing layer (3) can be cured (cross-linked) using, e.g., UV / VIS or electron beam, whereby a polymer network is formed, which increases the heat resistance, chemical resistance and dimensional stability of the embossed layer (3). This makes it possible to better preserve the at least one embossed diffractive structure (4) such as a diffraction microrelief at the time of subsequent contacting the multilayer foil (18) with and transferring the diffractive device (38) to the document (9) to be optically secured. The embossed diffractive structure (4) is able to modify light by diffraction, which leads to generation of different optical effects such as two- or three-dimensional (2D or 3D) images and / or kinematic and / or color changing effects and / or brightness effects. This is particular due to presence of structural elements within the embossed diffractive structure (4), which preferably form said structure (4). Examples of such structural elements are gratings, which form a surface reliefs structure. Preferably, the embossed diffractive structure (4) is a holographic structure. A number of effects are possible in a holographic structure, e.g., 2D holograms with structural and / or color changes, 3D holograms with images, holograms with kinematic effects, etc. Preferably, the at least one embossed diffractive structure (4) is an embossed diffractive relief structure, in particular a microrelief structure. “Relief structure” in the sense of the present invention preferably means that at least both depressions and elevations (protrusions) are present as structural elements forming the relief structure, which preferably lead to the formation of gratings in the embossed structure (4). Microrelief structure” in the sense of the present invention preferably means that the depth of the depressions and the heights of any elevations present in the relief structure does not exceed 1000 nm, preferably does not exceed 500 nm. Preferably, the at least one embossed diffractive structure (4) comprises depressions and protrusions with a depth in a range of from 0.1 µm to 2.0 µm and / or distances between adjacent depressions and / or adjacent protrusions being in a range of from 0.2 µm to 100 µm. Reflective layer (5)The embossing layer (3) comprises at least one embossed diffractive structure (4). At least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5).  The term “reflective” in the sense of the present invention in connection with the reflective layer (5) is understood by a person skilled in the art. It preferably means that layer (5) is able to reflect the light of the visual spectral band from 380 nm to 750 nm. The reflective layer (5) is preferably present at least within all depressions of the at least one embossed diffractive structure (4), more preferably also on any further structural elements present including any elevations (protrusions).  Preferably, the reflective layer (5) has a layer thickness in a range of from 20 nm to 200 nm, more preferably of from 30 nm to 100 nm. Layer thickness is determined by spectral interferometry.  The reflective layer (5) can be opaque or transparent and preferably is obtainable from application of at least one preferably inorganic reflective material, more preferably being selected from the group consisting of metals, preferably metals such as Al, Ag, Cu, Cr, and / or Ni, and other inorganic materials such as zinc sulfide (ZnS), zinc oxide (ZnO) and / or titanium dioxide (TiO2). Said application can be performed by conventional techniques and standard devices known to a person skilled in the art. Preferred are vapor deposition techniques and sputtering techniques, more preferably vapor deposition. In case the reflective layer (5) is transparent it preferably is a layer having a high refractive index (HRI) and is preferably obtainable from application of at least one material having also a high refractive index (HRI material). The term “high” in this context preferably means that the refractive index of the reflective layer (5) is at least higher than the refractive index of the thermoplastic material of embossing layer (3) and of the reinforcing layer (6), since these layers are positioned adjacently to the reflective layer (5), more preferably higher than the refractive index of any of the materials of the other layers of the multilayer foil, still more preferably is >1.50, even more preferably is >1.80. For example, reflective layers formed from ZnS or TiO2 have refractive indices of about 2.40 to 2.50. Preferably, at least one preferably inorganic reflective material for generating the reflective layer (5) is applied at least to the at least one embossed diffractive structure (4), preferably by using the vacuum evaporation method. As mentioned hereinbefore, it is also possible to apply said material to the entire embossing layer (3) and not only to the area of said layer comprising the at least one embossed diffractive structure (4).It is also possible to apply at least one preferably inorganic reflective material for generating the reflective layer (5) onto the embossing layer (3) prior to embossing, i.e., prior to generating the at least one embossed diffractive structure (4) within layer (3), and to perform the embossing in order to generate the at least one embossed diffractive structure (4) only afterwards, i.e., after the reflective layer (5) has already been formed. The latter variant has the advantage that the already present reflective layer (5) may serve as release layer for the improving the release of the embossing die or matrix such as a metal shim from the at least one embossed diffractive structure (4) after embossing has been performed. It is also possible in this case to apply an additional reflective layer (5) of the same or different material after having generated the at least one embossed diffractive structure (4). Reinforcing layer (6)The at least one reinforcing layer (6) is present at least on the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or is present on the at least one reflecting layer (5), if the entire embossing layer (3) as such bears the at least one reflective layer (5).  Preferably, the at least one reinforcing layer (6) is applied over the at least one reflective layer (5) to provide sufficient mechanical strength to the at least one embossed diffractive structure (4) such as at least one diffraction microrelief. When applying the multilayer foil according to the invention to comparably rough surfaces of documents (9) to be secured, the reinforcing layer (6) preferably prevents mechanical and / or chemical actions on the at least one embossed diffractive structure (4) and thus preserves particularly the optical properties thereof such as brightness and clarity of the original image.  The thickness of the reinforcing layer (6) is preferably in a range of from 0.5 µm to 10.0 µm. Preferably, the reinforcing layer (6) is obtainable from at least one preferably polymeric and / or prepolymeric thermoplastic material, which preferably is a radiation curable, more preferably a UV-radiation curable material. The term “curable by radiation” or "radiation curable" in this context has already been defined hereinbefore in connection with the thermoplastic material from which the embossing layer (3) is obtainable. Radiation curing employed with particular preference also in connection with reinforcing layer (6) is UV radiation. The at least one thermoplastic material suitable for obtaining the reinforcing layer (6) can be identical to or different from the at least one thermoplastic material suitable for obtaining the embossing layer (3). Preferably, however, it is different therefrom.  The reinforcing layer (6) preferably is obtainable from at least one suitable reactive prepolymer (oligomer or monomer) or at least one reactive polymer, or a mixture thereof, to be used as thermoplastic material. Optionally, at least one non-reactive polymer may be additionally present in each case. The term “reactive” in this context has already been defined hereinbefore in connection with the thermoplastic material from which the embossing layer (3) is obtainable. In order to obtain a solid-state reinforcing layer (6), it is preferable that the reinforcing varnish is composed primarily of medium and high molecular weight and to a lesser extent of low molecular weight radiation curable polymers and / or prepolymers. The molecular backbone of the radiation curable prepolymers and / or polymers can be, but not necessarily, selected from different chemical classes such as: urethane, epoxy, polyester, polyether, polycarbonate, melamine, (meth)acrylic etc. The reactive prepolymers and / or polymers may contain, but not necessarily, polar groups such as: hydroxyl, carboxyl, carbonyl, ester, ether, amino, amide, phosphonate, phosphate, sulfo, sulfate etc. The embossing varnish can be both solvent-based (organic dispersion or organic solution) and water-based (dispersion and / or aqueous solution).  A suitable reinforcing composition can be used for generating the reinforcing layer (6), which composition preferably contains the at least one suitable reactive prepolymer (oligomer or monomer) or the at least one reactive polymer, or the mixture thereof. Besides prepolymer / s and / or polymer / s, the reinforcing composition used for preparing reinforcing layer (6) may include, but not necessarily: a) photoinitiators (type I, type II) suitable for initiating radical polymerization or / and photocatalysts suitable for initiating cationic polymerization; and / or b) other ingredients typical for formulating radiation-curing varnishes such as: stabilizers, photosensitizers, oxygen scavengers, amine synergists, defoamers, adhesion promoters, flatting agents, slip agents, wetting agents and / or c) hydrophilizing agents, which can be low molecular weight, medium molecular weight or / and high molecular weight water-soluble or water-swellable compounds and may contain, but not necessarily, the aforementioned reactive groups in their molecules. Before radiation curing, the reinforcing layer (6) is thermoplastic and highly sensitive to external influences such as temperature, mechanical stress, solvents, water and / or aqueous solutions with pH>7. In an appropriate step such as step 11) of the inventive method for preparing the inventive multilayer foil (18), the reinforcing layer (6) can be cured (cross-linked) using, e.g., UV / VIS or electron beam, whereby a polymer network is formed, which increases the heat resistance, chemical resistance and dimensional stability of the reinforcing layer (6). This makes it possible to better preserve the at least one embossed diffractive structure (4) such as a diffraction microrelief at the time of subsequent contacting the multilayer foil (18) with and transferring the diffractive device (38) to the document (9) to be optically secured. Leveling layer (7)The at least one leveling layer (7) is present at least in portion on the at least one reinforcing layer (6). The at least one leveling layer (7) is applied at least in portion, preferably entirely, over the at least one reinforcing layer (6). Preferably, the at least one leveling layer (7) is present on the entire surface of the reinforcing layer (6). Preferably, the leveling layer (7) is obtainable from and more preferably made of at least one thermoplastic material, more preferably of at least one polymeric thermoplastic material. Preferably, the at least one thermoplastic material of the leveling layer (7) is different from the at least one thermoplastic materials used for preparation of the embossing layer (3) and the reinforcing layer (6). Preferably, the at least one thermoplastic material of the leveling layer (7) is not obtained by radiation curing, in particular not by UV radiation curing.  Preferably, the at least one thermoplastic material of the leveling layer (7) is selected from polymers selected from the group consisting of poly(meth)acrylates, polyurethanes, polyvinyl acetate, poly(ethylene-vinyl acetate), ethylene-(meth)acrylic acid co-polymers, rosin and rosin derivatives, ketone and aldehyde resins, polyamides, polyesters, hydrocarbon resins and mixtures thereof, wherein the term polymer includes both homopolymers and copolymers. An example of a vinyl copolymer is a poly(vinyl acetate-co-crotonic acid) copolymer. The polymers and resins may contain, but not necessarily, polar groups such as: hydroxyl, carboxyl, carbonyl, ester, ether, amino, amide, phosphonate, phosphate, sulfo, sulfate etc. In the formulation of the leveling layer (7), hydrophilizing agents may be involved, but not necessarily, which can be low molecular weight, medium molecular weight or / and high molecular weight water-soluble or water-swellable compounds. The formulation of the leveling layer (7) is preferably selected such that in alkaline aqueous solutions and / or certain temperatures of these solutions, the leveling layer (7) lowers its mechanical strength. Preferably, the at least one thermoplastic material of the leveling layer (7) is softened by the temperature applied and used during the application of the multilayer foil to the document (9) to be optically secured and allows the reinforcing layer (6) to remain as smooth as possible and allows the at least one adhesive layer (8) to take the shape of the roughness of the surface of the document (9).  The thickness of the leveling layer (7) is preferably in a range of from 1.0 µm to 10.0 µm. The leveling layer (7) is preferably thick enough to compensate for the roughness of the surface of document (9), to which the multilayer foil (18) is contacted with and to which the diffractive device (38) is applied.  The thickness of the leveling layer (7) should be thick enough to compensate the roughness of the surface of document (9) to be optically secured and thin enough to not weaken the overall structure of the multilayer foil and to prevent any cohesive separation from the document (9). Adhesive layer (8) and optionally present further adhesive layer (8’)The at least one adhesive layer (8) is present at least in portion on the at least one leveling layer (7). Preferably, adhesive layer (8) covers at least an area (S11) of the leveling layer. The at least one adhesive layer (8) is applied over the at least one leveling layer (7). Preferably, the at least one adhesive layer (8) is present on the entire surface of the leveling layer (7), in which case area (S11) corresponds to the entire surface of the leveling layer (7). Preferably, the at least one adhesive layer (8) is a mask adhesive layer, more preferably a temperature activated adhesion layer. It is preferably used to protect certain areas such as a surface area corresponding to area (S11) of all layers of the multilayer foil in subsequent technological operations such as in the method of producing of the multilayer foil (18) according to the invention.  Preferably, the adhesive layer (8) comprises, more preferably consists of, at least one adhesive material that is chemically resistant to alkalinity sources such as alkaline solutions. As outlined hereinbefore, the at least one adhesive material further preferably is a temperature activated adhesive material. More preferably, at least one preferably polymeric adhesive material is selected from the group consisting of poly(meth)acrylate, polyurethane, polyurea, polyester, polyvinyl acetate, poly(ethylene-vinyl acetate), ethylene-(meth)acrylic acid co-polymers, rosin and rosin derivatives, hydrocarbon resins, ketone and aldehyde resins, and mixtures thereof, wherein the polymers can be homopolymers and / or copolymers. The adhesive layer (8) preferably is applied by printing, e.g., by using one of the known printing technologies known by a person skilled in the art, such as by rotogravure.  The thickness of the adhesive layer (8) is preferably in a range of from 0.5 µm to 5.0 µm.  The adhesive layer (8) after having been applied preferably defines an area such as area (S11) with the outline of the diffractive device (38) such as the holographic device. Preferably, the adhesive layer (8) represents the outermost layer of the multilayer foil (18). The adhesive layer (8) forms the final shape of the diffractive device (38) after its application onto the document (9) to be optically secured. The shape can be complex enough to make it difficult to repeat with alternative common die-cutting or stamping technologies and this can be used for some applications. However, in order to avoid the use of die or stamps for the possible imitations of the diffractive device (38), the multilayer foil (18) preferably comprises at least one further adhesive layer (8’) besides the adhesive layer (8), which is different therefrom, and contains, in contrast to adhesive layer (8), at least one dye and / or at least one UV light detectable constituent, which makes it colored and / or UV detectable. In particular, the further adhesive layer (8’), in contrast to the material suitable to form the adhesive layer (8), contains at least one dye and / or at least one UV light detectable constituent. Attempting to imitate such a contour with the die or stamp would show a misalignment between the contour of the die or stamp and the contour of the adhesive layer (8'). When the diffractive device (38) is applied to a document (9) to be secured optically, adhesive layers (8) and (8') preferably together form an adhesive layer (88) and together define an area (S11) with the contour of the device (38). Hence, a combined adhesive layer resulting from both layers (8) and (8’) in a diffractive device (38) used for optically securing documents (9) is referred to as adhesive layer (88). Preferably, when at least one adhesive layer (8’) is present, the at least one adhesive layer (8) is only applied in portion on the at least one leveling layer (7). In this case, the adhesive layer (8) preferably only covers a part of the surface, namely an area (S1), of the leveling layer (7), but not its entire surface. The remaining part of the surface of the leveling layer (7) is then preferably covered by the at least one further adhesive layer (8’). Preferably, the at least one further adhesive layer (8’) not only covers the remaining part of the surface of the leveling layer (7), but also the part of the adhesive layer (8). The overall area covered by adhesive layer (8’) is referred to as area (S1’). Aforementioned area (S11), which provides the outline of the diffractive device (38) such as the holographic device, in this case preferably is the sum of areas (S1) and (S1’) minus the area, where areas (S1) and (S1’) overlap. Preferably, the at least one further adhesive layer (8’) is a mask adhesive layer, more preferably a temperature activated adhesion layer. Preferably, the adhesive layer (8’) comprises, more preferably consists of, at least one adhesive material that is chemically resistant to alkaline solutions. As outlined hereinbefore, the at least one adhesive material further preferably is a temperature activated adhesive material. More preferably, at least one preferably polymeric adhesive material is selected from the group consisting of poly(meth)acrylate, polyurethane, polyurea, polyester, polyvinyl acetate, poly(ethylene-vinyl acetate), ethylene-(meth)acrylic acid co-polymers, rosin and rosin derivatives, hydrocarbon resins, ketone and aldehyde resins, and mixtures thereof, which is comprised in adhesive layer (8’). The adhesive layer (8’) preferably is applied by printing, e.g., by using one of the known printing technologies known by a person skilled in the art, such as by rotogravure.  The thickness of the adhesive layer (8’) is preferably in a range of from 0.5 µm to 5.0 µm.  The adhesive material of adhesive layer (8’) may be identical to or different from the adhesive material of adhesive layer (8). Preferably, the adhesive material of adhesive layer (8’) is identical to the adhesive material of adhesive layer (8) except that it further comprises at least one dye being present therein, which is able to color the adhesive layer (8’), in particular to color it differently than adhesive layer (8), and / or comprises at least one constituent, which is detectable and visible by UV light upon illumination such as at least one UV visible ink. Preferably, the adhesive layer (8’) represents the outermost layer of the multilayer foil if it is present. Method of preparation of the multilayer foil A further subject-matter of the present invention is method for preparation of the multilayer foil (18) according to the invention. The method comprises at least steps 1),3), 5), 6), 8) to 10), one of steps 4a) and 4b), one of steps 7a) and 7b), and optionally at least one of steps 2) and 11). The term “comprising” in the sense of the present invention, in connection with the inventive method, preferably has the meaning of “consisting of”.  All preferred embodiments described hereinbefore in connection with the inventive multilayer foil are also preferred embodiments in relation to the aforementioned method for preparation of the multilayer foil according to the invention. Step 1)In a base (1) as carrier is provided. Optional step 2)At least one material suitable to form a separation layer (2) is optionally applied at least to a portion of at least one surface of the base (1) to form the at least one separation layer (2). Step 3)At least one thermoplastic material, is applied in step 3) at least to a portion of at least one surface of the base (1), or at least to a portion of the separation layer (2), if the separation layer (2) is present, said material being suitable to form the embossing layer (3). Steps4a) and 4b)At least one diffractive structure is embossed into the at least one thermoplastic material in step 4a) to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4), and applying at least one reflective material at least to the at least one embossed diffractive structure (4) of the embossing layer (3) and optionally to the entire surface of the embossing layer (3) including its at least one embossed diffractive structure (4), to form the at least one reflective layer (5). Alternatively, according to step 4b) at least one reflective material is applied to the entire surface of the at least one thermoplastic material to form the at least one reflective layer (5), and embossing at least one diffractive structure into the reflective layer (5) present, to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), which bears the at least one reflective layer (5), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4). Step 5)At least one material suitable to form the at least one reinforcing layer (6) is applied in step 5) at least to the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or to the at least one reflecting layer (5), if the entire embossing layer (3) as such including its at least one embossed diffractive structure (4) bears the at least one reflective layer (5). Step 6)At least one material suitable to form the at least one leveling layer (7) is applied in step 6) at least to a portion of the reinforcing layer (6).  Steps7a) and 7b)At least one material suitable to form the at least one adhesive layer (8) is applied in step 7a) at least to a portion of the leveling layer (7). Alternatively, at least one material suitable to form the at least one adhesive layer (8) is applied to only a portion of the leveling layer (7) and, further, at least one material suitable to form at least one further adhesive layer (8’) is applied to both a portion of the leveling layer (7) not covered by the adhesive layer (8) and to a portion of the adhesive layer (8) in step 7b), wherein the materials suitable to form layers (8) and (8’) are identical to or different from one another, but wherein the material suitable to form the further adhesive layer (8’), in contrast to the material suitable to form the adhesive layer (8), is colored and / or is detectable and visible by UV light, preferably since the material suitable to form the further adhesive layer (8’) contains at least one dye and / or at least one constituent, which is detectable and visible by UV light. Step 8)An aqueous alkaline solution or composition, which preferably has a pH value in a range of from 10.0 to 14.0, is used for treating the multilayer foil obtained after step 7a) or after step 7b) in step 8). Preferably, the aqueous alkaline solution or composition used in step 8) has a temperature in a range of from 25 to 85 °C. Preferably, step 8) is performed for 5 to 60 seconds. Preferably, treating in step 8) comprises dipping of the multilayer foil into the alkaline solution or composition. Other techniques such as brushing or spraying are, however, also possible. Preferably, the aqueous alkaline solution or composition comprises at least one suitable alkali and / or earth alkali salt, more preferably at least one hydroxide and / or carbonate as alkaline agent. Examples are NaOH, KOH, NH3, Na2CO3, and / or K22CO3. Other suitable and preferably organic alkali and / or earth alkali salts can be used as well such as amines and / or sodium acetate. Preferably, by treatment according to step 8), the material of the layers (2), (3), (5), (6), (7) from the areas (S2) shown, e.g., in Figs. 1 to 4, that are not protected by adhesive layer (8) or, if present adhesive layer (8’), are removed. Step 9)The multilayer foil treated according to step 8) is washed with water and / or mechanically treated in step 9). Preferably, washing in step 9) is performed by making use of a water jet. Preferably, the mechanical treatment of step 9) is performed by applying a roller or brush onto the foil. Preferably, both washing and the mechanical treatment are performed in step 9), either simultaneously or one after another, preferably simultaneously. Preferably, by washing and / or the mechanical treatment according to step 9), the material of the layers (2), (3), (5), (6), (7) from the areas (S2) shown, e.g., in Figs. 1 to 4, that are not protected by adhesive layer (8) or, if present adhesive layer (8’), are removed. Step 10)The multilayer foil subjected step 9) is dried after step 9). Preferably, drying according to step 10) is performed by blowing with warm air. Optional step 11)Optionally, the multilayer foil obtained after step 10) is irradiated with radiation, preferably UV radiation, in order to cure any of the layers of the multilayer foil, which have been obtained from making use of any constituents, which are radiation curable, preferably UV curable constituents, in particular layers 3) and / or 6). Preferably, step 11) is performed, in particular to cure at least layers (3) and (6) by radiation such as UV irradiation. Use of the multilayer foil as an origin of a diffractive device  A further subject-matter of the present invention is use of the multilayer foil (18) according to the invention as an origin of a diffractive device (38), in particular of a holographic device, preferably for optically securing suitable documents (9) and / or for preventing copying and / or misuse of suitable documents (9), more preferably of documents made at least partially of at least one kind of plastic and / or paper, even more preferably of documents selected from the group of passports, identity cards, money cards including credit cards, tax stamps, bank cheques, vignettes, tickets and paper-based documents such as banknotes, wherein said diffractive device (38) is obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated. All preferred embodiments described hereinbefore in connection with the inventive multilayer foil and its method for preparation are also preferred embodiments in relation to the aforementioned inventive use. Method of optically securing a document A further subject-matter of the present invention is a method for optically securing a document (9), characterized in that it comprises at least step A) or step B). All preferred embodiments described hereinbefore in connection with the inventive multilayer foil and its method for preparation and the inventive use are also preferred embodiments in relation to the aforementioned method of optically securing a document. Step A) and step B)The multilayer foil (18) according to the invention is contacted in step A) with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) and / or, if present, the at least one adhesive layer (8’), faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil (18) against said surface of the document (9), more preferably by means of a stamp (10), even more preferably by means of a hot stamp (10), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated. The multilayer foil (18) according to the invention is contacted in step B) with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) and / or, if present, the at least one adhesive layer (8’), faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil (18) against said surface of the document (9), more preferably by means of a roll (11), even more preferably by means of a hot roll (11), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated.  In case of step A) the stamp (10) preferably heats and presses the multilayer foil (18) to the surface of the document (9) activating the adhesive layer (8) and, if present, the adhesive layer (8’), reforming them to the adhesive layer (88) and reforming the leveling layer (7) to the leveling layer (77). This is, e.g., shown in Fig. 1 (before use of stamp (10)) and Fig. 3 (after use of stamp (10)). In case of step B) the roll (11) preferably gradually passes over the multilayer foil (18) resulting in heating and pressing it to the surface of the document (9) activating the adhesive layer (8) and, if present, the adhesive layer (8’), reforming them to the adhesive layer (88) and reforming the leveling layer (7) to the leveling layer (77). This is, e.g., shown in Fig. 2 (before use of roll (11)) and Fig. 3 (after use of roll (11)). In step A) or B) a diffractive device (38) originating from the multilayer foil (18) is basically transferred to a document (9) and, hence, as optical security element. Preferably, in step A) the size of the surface of the stamp (10) is larger than the boundary size of area (S11) of the multilayer foil, which outlines the diffractive device (38) as described hereinbefore. Preferably, in step B) the width of the roll (11) is larger than the width of the area (S11) of the multilayer foil, which outlines the diffractive device (38) as described hereinbefore. This ensures that no breakage of the layers is occurred at the edges of the stamp (10) or roll (11) that would lead to forming unwanted fringe foil residues. It has been found that at the edges of the stamp (10) or of the roll (11) or the adhesive layer (88) there is no undesired tearing observed therefore no unwanted fringe residues are formed. It has been found that the comparably thick leveling (7) layer originally present in the multilayer foil compensates any surface irregularities of the document (9) and that the adhesive layer (88) may penetrates deep into the substrate of the document (9). The result is a strong bond between the surface of the document (9) and the diffractive device (38). Since the at least one embossed diffractive structure (4) is embedded between layers (3) and (6), its image, e.g., its holographic image, does not lose its brightness and clarity, in particular since as the materials used are resistant to mechanical and chemical actions. Base (1) optionally bearing the at least one separating layer (2) is preferably separated from the remaining part of the multilayer foil (18) adhered to the document (9) during performance of step A) or B), which leads to a document (9) bearing on one its surfaces the diffractive device (38), which originates from multilayer foil (18). Document obtainable from the method of optically securing a document A further subject-matter of the present invention is an optically secured document (9) obtainable by the inventive method of optically securing a document. The optically secured document (9) bears at least in portion on one of the surfaces of document (9) the diffractive device (38). All preferred embodiments described hereinbefore in connection with the inventive multilayer foil and its method for preparation and the inventive use as well as the inventive method for optically securing a document are also preferred embodiments in relation to the aforementioned method of optically securing a document. Optically secured document A further subject-matter of the present invention is a document (9) optically secured by means of a diffractive device (38), characterized in that the document (9) comprises at least in portion on one of its surfaces the diffractive device (38), comprising at least layers (88), (77), (6), (5), and (3), namely, at least one adhesive layer (88), which is present at least in portion on one of the surfaces of document (9), and which originates from adhesive layer (8) and optionally present adhesive layer (8’) of the multilayer foil (18) according to the invention, at least one layer (77), which is present at least in portion on the at least one adhesive layer (88), and which originates from layer (7) of the multilayer foil (18) according to the invention, at least one reinforcing layer (6), which is present at least in portion on the at least one layer (77), and at least one embossing layer (3), which is made of at least one thermoplastic material, and which comprises at least one embossed diffractive structure (4), wherein at least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5),  wherein said at least one embossed diffractive structure (4), which bears at least one reflective layer (5), is embedded between layers (3) and (6). Preferably, the optically secured document is obtained by the inventive method of optically securing a document (9) when performing step A) or step B).EXAMPLES The following examples further illustrate the invention but are not to be construed as limiting its scope.  1. Manufacture of a multilayer foil according to the invention 1.1Example 1A base (1) made of a PET-film with a thickness of 19 µm was provided. A separation layer (2) with a thickness of 0.05 µm made of polyethylene wax was coated over one entire surface of the base (1) by means of rotogravure technique. A medium molecular weight thermoplastic UV curable (crosslinkable by means of UV radiation) (meth)acrylate polymer mixture was coated over the entire separation layer (2) by means of rotogravure technique to form the embossing layer (3) with a thickness of 2 µm. Then, the relief of a diffractive structure (4) comprising depressions and protrusions was negatively copied to the surface of the embossing layer (3) from a nickel shim by means of the hot embossing technique. The surface of the embossing layer (3) bearing the diffractive structure (4) was then coated entirely with a ZnS material with a thickness of 50 nm to form the reflective layer (5) by means of vacuum vapor deposition method. The same UV curable (crosslinkable by means of UV radiation) polymer mixture used for forming the embossing layer (3) was coated over the entire reflective layer (5) by means of rotogravure technique to form the reinforcing layer (6) with a thickness of 5 µm. A poly(vinyl acetate-co-crotonic acid) was coated over the entire surface of the reinforcing layer (6) by means of rotogravure technique to form the leveling layer (7) with a thickness of 8 µm. A layer of polyurethane with a thickness of 1.0 µm was printed partially on the surface of the levelling layer (7) to form the adhesive layer (8) in the areas (S11). The multilayer foil obtained in this manner was then treated with an aqueous solution of NaOH having a pH value of 12, and a temperature of 50 °C for 30 seconds. The layers (3), (6), (7) in the areas (S2) that were not protected by the adhesive layer (8) were subjected to swelling and lost their mechanical strength. The multilayer foil was then washed by water having a temperature of 20 °C assisted by roller brushing and the material of the layers (3), (5), (6), (7) was removed in this manner from the unprotected areas (S2). The multilayer foil obtained was then subjected to drying by hot air. The multilayer foil was then exposed to a medium pressure mercury lamp to cure the embossing layer (3) and reinforcing layer (6) with UV radiation about 500 mJ / cm2. After exposure to the medium pressure mercury lamp the multilayer foil (18) had remaining layers (3), (5), (6), (7) only in the areas (S11) that had been protected by the adhesive layer (8). The multilayer foil (18) obtained in this manner was ready to use. 1.2Example 2The preparation of the multilayer foil of this example was identical to the preparation of the multilayer foil according to example 1 except that (i) the surface of the embossing layer (3) was first coated entirely with the ZnS material with a thickness of 50 nm to form the reflective layer (5) by means of vacuum vapor deposition method prior to performing the embossing, and only afterwards the relief of the diffractive structure (4) comprising depressions and protrusions was negatively copied to the surface of the embossing layer (3) through the reflective layer (5) from the nickel shim by means of the hot embossing technique, and except that (ii) the layer of polyurethane with a thickness of 1.0 µm was printed only partially on the surface of the levelling layer (7) to form the adhesive layer (8) within the areas (S11), and that a further colored adhesive layer (8’) with a thickness of 1.0 µm, made of the same material as adhesive layer (8) and colored by a dye in red color was printed partially on the relevant area (S11) of the surface of the levelling layer (7) and the adhesive layer (8). After exposure to the medium pressure mercury lamp the multilayer foil (18) had remaining layers (3), (5), (6), (7) only in the areas (S11) that had been protected by the colored adhesive layer (8’). The multilayer foil (18) obtained in this manner was ready to use. 2. Use of the multilayer foil as an origin of a diffractive device to obtain an optically secured document The multilayer foil (18) according to example 1 was used for optically securing a document by attaching a diffractive device (38) originating from the foil (18) to the surface of the document (9) to be secured by means of the adhesive layer (88) of the device (38). For this, the multilayer foil (18) obtained in example 1 was contacted with the surface of a passport document (9) to be optically secured, such that the adhesive layer (8) of the multilayer foil (18) faced the surface of the said document (9). The multilayer foil (18) was pressed against said surface of the document (9) by means of a hot stamp (10) to form an optically secured document (9) bearing on one of its surfaces the diffractive device (38) according to the invention. The temperature of the stamp (10) was about 140 °C and the pressure used was about 4 bar. The size of the stamp (10) was larger with 1 mm each side than the boundary size of the diffractive device (38) to be applied to the document (9). Base (1) bearing the release layer (2) was separated from the remaining part of the diffractive device (38) originating from the foil (18) of example 1 and adhered to the document (9), which led to a document (9) bearing on one its surfaces a diffractive device (38). At the edges of the stamp (10) or the adhesive layer (88) of the device (38) there was no undesired tearing observed and therefore no unwanted fringe residues were formed. The comparably thick leveling (7) layer originally present in the multilayer foil (18) of example 1 compensated any surface irregularities of the document (9) and the adhesive layer (88) of the device (38) was able to penetrate deep into the substrate of the document (9). The result was a strong bond between the surface of the document (9) and the diffractive device (38). Since the embossed diffractive structure (4) was embedded between layers (3) and (6), which were cured by UV radiation, the holographic image of the diffractive device (38) did not lose its brightness and clarity, in particular as the materials used were resistant to mechanical and chemical actions.   

Claims

1. A multilayer foil (18), which comprises a base (1) as carrier, and at least layers (3) to (8) and optionally layer (2), which are different from one another, namely optionally at least one separation layer (2), which is present on at least a portion of at least one surface of the base (1), at least one embossing layer (3), which is present on at least a portion of at least one surface of the base (1), or which is present on at least a portion of the separation layer (2), if the separation layer (2) is present,wherein the embossing layer (3) is obtainable from at least one thermoplastic material and comprises at least one embossed diffractive structure (4), wherein at least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5),  at least one reinforcing layer (6), which is present at least on the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or which is present on the at least one reflecting layer (5), if the entire embossing layer (3) as such bears the at least one reflective layer (5), at least one leveling layer (7), which is present at least in portion on the at least one reinforcing layer (6), and at least one adhesive layer (8), which is present at least in portion on the at least one leveling layer (7). 2. The multilayer foil according to claim 1, characterized in that the at least one embossing layer (3) is obtainable from at least one thermoplastic material, which is a radiation curable (crosslinkable by means of radiation) material, preferably is a UV radiation curable (crosslinkable by means of UV radiation) material, wherein the at least one thermoplastic material preferably is selected from reactive prepolymers, reactive polymers, and mixtures thereof, wherein at least one non-reactive polymer may be optionally additionally used.

3. The multilayer foil according to one claim 1 or 2, characterized in that the at least one embossed diffractive structure (4) is a relief structure, preferably a microrelief structure, which preferably comprises depressions and protrusions with a depth in a range of from 0.1 µm to 2.0 µm and / or comprises distances between adjacent depressions and / or adjacent protrusions being in a range of from 0.2 µm to 100 µm.

4. The multilayer foil according to one or more of the preceding claims, characterized in that the thicknesses of the at least one embossing layer (3) and of the at least one reinforcing layer (6) are, independently of one another, in a range of from 0.5 µm to 10.0 µm.

5. The multilayer foil according to one or more of the preceding claims, characterized in that the at least one reinforcing layer (6) is obtainable from at least one thermoplastic material, which is a radiation curable (crosslinkable by means of radiation) material, preferably is a UV radiation curable (crosslinkable by means of UV radiation) material, wherein the at least one thermoplastic material preferably is selected from reactive prepolymers, reactive polymers, and mixtures thereof, wherein at least one non-reactive polymer may be optionally additionally used.

6. The multilayer foil according to one or more of the preceding claims, characterized in that the reflective layer (5) is obtainable from application of at least one preferably inorganic reflective material being preferably selected from the group consisting of metals, preferably of metals such as Al, Ag, Cu, Cr, and / or Ni, zinc sulfide, zinc oxide, and titanium dioxide, and mixtures thereof, and / or in that the reflective layer (5) has a layer thickness in a range of from 20 nm to 200 nm, preferably of from 30 nm to 100 nm.

7. The multilayer foil according to one or more of the preceding claims, characterized in that the leveling layer (7) is obtainable from at least one thermoplastic material, preferably from at least one polymer selected from the group consisting of poly(meth)acrylates, polyurethanes, polyvinyl acetate, poly(ethylene-vinyl acetate), ethylene-(meth)acrylic acid co-polymers, rosin and rosin derivatives, ketone and aldehyde resins, polyamides, polyesters, hydrocarbon resins and mixtures thereof, wherein the term polymer includes both homopolymers and copolymers, wherein the term polymer includes both homopolymers and copolymers, and / or in that the thickness of the leveling layer (7) is in a range of from 1.0 µm to 10.0 µm, but preferably exceeds the layer thickness of the at least one reinforcing layer (6).

8. The multilayer foil according to one or more of the preceding claims, characterized in that the at least one adhesive layer (8) is a mask adhesive layer, preferably a temperature activated adhesion layer, which preferably has a layer thickness in a range of from 0.5 µm to 5.0 µm, wherein the at least one adhesive layer (8) is preferably obtainable from at least one adhesive material that is chemically resistant to alkalinity sources such as alkaline solutions, and preferably is obtainable from at least one adhesive material selected from the group consisting of poly(meth)acrylate, polyurethane, polyurea, polyester, polyvinyl acetate, poly(ethylene-vinyl acetate), ethylene-(meth)acrylic acid co-polymers, rosin and rosin derivatives, hydrocarbon resins, ketone and aldehyde resins, and mixtures thereof, wherein the polymers can be homopolymers and / or copolymers.

9. The multilayer foil according to one or more of the preceding claims, characterized in at least one further adhesive layer (8’) is present in portion on the at least one leveling layer (7) besides the at least one adhesive layer (8) also present, wherein the further adhesive layer (8’) is different from adhesive layer (8) in that it further contains, in contrast to adhesive layer (8), at least one of at least one dye and at least one constituent, which is detectable and visible by UV light upon illumination being present in the adhesive layer (8’).

10. A use of the multilayer foil (18) according to one or more of the preceding claims as an origin of a diffractive device (38), in particular of a holographic device, preferably for optically securing suitable documents (9) and / or for preventing copying and / or misuse of suitable documents (9), more preferably of documents made at least partially of at least one kind of plastic and / or paper, even more preferably of documents selected from the group of passports, identity cards, money cards including credit cards, tax stamps, bank cheques, vignettes, tickets and paper-based documents such as banknotes, wherein said diffractive device (38) is obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated.

11. A method for preparation of the multilayer foil (18) according to one or more of claims 1 to 9, characterized in that it comprises at least steps 1), 3), 5), 6), 8) to 10), one of steps 4a) and 4b), one of steps 7a) and 7b), and optionally at least one of steps 2) and 11), namely 1) providing a base (1) as carrier, and2) optionally applying at least one material suitable to form a separation layer (2) at least to a portion of at least one surface of the base (1) to form the at least one separation layer (2), and3) applying at least one thermoplastic material, at least to a portion of at least one surface of the base (1), or at least to a portion of the separation layer (2), if the separation layer (2) is present, which is suitable to form the embossing layer (3), and4a) embossing at least one diffractive structure into the at least one thermoplastic material to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4), and applying at least one reflective material at least to the at least one embossed diffractive structure (4) of the embossing layer (3) and optionally to the entire surface of the embossing layer (3) including its at least one embossed diffractive structure (4), to form the at least one reflective layer (5), or4b) applying at least one reflective material to the entire surface of the at least one thermoplastic material to form the at least one reflective layer (5), and embossing at least one diffractive structure into the reflective layer (5) present, to form the at least one embossing layer (3) comprising the at least one embossed diffractive structure (4), which bears the at least one reflective layer (5), by making use of an embossing die or matrix bearing at least one structure, which is the negative image of the embossed diffractive structure (4), and5) applying at least one material suitable to form the at least one reinforcing layer (6) at least to the portion of the embossing layer (3), which comprises the at least one embossed diffractive structure (4), which in turn bears the at least one reflective layer (5), or to the at least one reflecting layer (5), if the entire embossing layer (3) as such including its at least one embossed diffractive structure (4) bears the at least one reflective layer (5),6) applying at least one material suitable to form the at least one leveling layer (7) at least to a portion of the reinforcing layer (6), 7a) applying at least one material suitable to form the at least one adhesive layer (8) at least to a portion of the leveling layer (7), or7b) applying at least one material suitable to form the at least one adhesive layer (8) to only a portion of the leveling layer (7) and further applying at least one material suitable to form at least one further adhesive layer (8’) to both a portion of the leveling layer (7) not covered by the adhesive layer (8) and to a portion of the adhesive layer (8), wherein the materials suitable to form layers (8) and (8’) are identical to or different from one another, but wherein the material suitable to form the further adhesive layer (8’), in contrast to the material suitable to form the adhesive layer (8), is colored and / or is detectable and visible by UV light, preferably since the material suitable to form the further adhesive layer (8’) contains at least one dye and / or at least one constituent, which is detectable and visible by UV light,8) treating the multilayer foil obtained after step 7a) or step 7b) with an aqueous alkaline solution or composition, which preferably has a pH value in a range of from 10.0 to 14.0,9) washing the multilayer foil treated according to step 8) with water and / or mechanically treating the multilayer foil according to step 8),10) drying the multilayer foil obtained after step 9), and11) optionally irradiating the multilayer foil obtained after step 10) with radiation, preferably UV radiation, in order to cure any of the layers of the multilayer foil, which have been obtained from making use of any constituents, which are radiation curable (crosslinkable by means of radiation), preferably UV curable (crosslinkable by means of UV radiation) constituents, in particular layers 3) and / or 6).

12. A method for optically securing a document, characterized in that it comprises at least step A) or step B), namely A) contacting the multilayer foil (18) according to one or more of claims 1 to 9 with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) or, if present, the at least one adhesive layer (8’) as defined in claim 9, faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil against said surface of the document (9), more preferably by means of a stamp (10), even more preferably by means of a hot stamp (10), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated, orB) contacting the multilayer foil (18) according to one or more of claims 1 to 9 with at least one document (9) to be optically secured, such that the at least one adhesive layer (8) or, if present, the at least one adhesive layer (8’), faces the surface of the document (9), which is to be optically secured, preferably by pressing the multilayer foil (18) against said surface of the document (9), more preferably by means of a roll (11), even more preferably by means of a hot roll (11), to form an optically secured document (9) bearing on one of its surfaces a diffractive device (38), said device being obtainable from the multilayer foil (18), from which base (1) and the optionally present separation layer (2) have been separated.

13. The method according to claim 12, characterized in that a stamp (10) is used in step A), wherein the size of the surface of said stamp (10) is larger than the boundary size of an area (S11) of the diffractive device (38), and / or in that a roll (11) is used in step B), wherein the width of the roll (11) is larger than the width of the area (S11) of the diffractive device (38).

14. An optically secured document (9) obtainable by the method according to claim 12 or 13 and bearing the diffractive device (38) at least in portion on one of its surfaces.

15. A document (9) optically secured by means of a diffractive device (38), characterized in that the document (9) comprises at least in portion on one of its surfaces a diffractive device (38), comprising at least layers (88), (77), (6), (5), and (3), namely, at least one adhesive layer (88), which is present at least in portion on one of the surfaces of document (9), and which originates from adhesive layer (8) and optionally present adhesive layer (8’) of the multilayer foil (18) according to one or more of claims 1 to 9, at least one layer (77), which is present at least in portion on the at least one adhesive layer (88), and which originates from layer (7) of the multilayer foil (18) according to one or more of claims 1 to 9, at least one reinforcing layer (6), which is present at least in portion on the at least one layer (77), and at least one embossing layer (3), which is obtainable from at least one thermoplastic material, and which comprises at least one embossed diffractive structure (4), wherein at least the at least one embossed diffractive structure (4) of the embossing layer (3), and optionally the entire embossing layer (3) including its at least one embossed diffractive structure (4), bears at least one reflective layer (5),  wherein said at least one embossed diffractive structure (4), which bears at least one reflective layer (5), is embedded between layers (3) and (6). 

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