Abstract:
Described is a process for the production of a multi-layer body having a volume hologram with at least two different items of image information, wherein a photosensitive layer ( 46 ) of the multi-layer body is directly or with the interposition of a transparent optical medium ( 44   s   , 45 ) brought into contact with the front side of a master ( 44 ), in which interlaced regions with different asymmetrical surface structures or kinoform structures are shaped, which embody the at least two different items of image information. The photosensitive layer ( 46 ) and the master are exposed with a coherent light beam ( 47 ) whereby a volume hologram is formed in the photosensitive layer ( 46 ). Also described are a master for the production of the multi-layer body and a security element having said multi-layer body.

Description:
This application claims priority based on an International Application filed under the Patent Cooperation Treaty, PCT/EP2007/003091, filed on Apr. 5, 2007 and German Application No. DE 102006016139.4-51, filed on Apr. 6, 2006. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a process for the production of a multi-layer body having a volume hologram, a master for the production of the volume hologram and a security element having the multi-layer body. 
     Holograms are used as security elements for protecting security documents such as banknotes, identity cards or passports, security cards or the like to achieve a high level of forgery-proofness. For mass-produced items, surface holograms are frequently used, which on the one hand do not produce an optimum image impression and which on the other hand can be copied by copying the shape of the surface relief. 
     Volume holograms, also referred to as white light holograms or reflection holograms, are based on light diffraction at what are referred to as the Bragg planes of a transparent layer which has local differences in refractive index and they produce a brilliant image impression. They cannot be copied by shaping of a surface relief. 
     It will be noted however that simultaneously writing two different items of image information into a volume hologram gives rise to weak-light and/or blurred reproduction in the regions in which the two items of information are in overlapping relationship. That can be avoided if two separate holograms are arranged one over the other. A disadvantage in that respect however is on the one hand the increased thickness—volume holograms are of a thickness which is a multiple of the light wavelength used to produce them—and the high demands in terms of register accuracy of the volume hologram layers. 
     EP 1 187 728 B1 describes a process which provides that a transmission hologram and a reflection hologram are respectively recorded in a hologram layer and then the two hologram layers are laminated on to each other. 
     EP 1 217 469 A2 discloses a process of coating a surface relief hologram with a photosensitive layer and producing a volume hologram by means of the contact process. 
     EP 1 511 636 A1 describes a process in which a volume hologram is produced by an optical copying operation from a master with a surface relief, the surface relief being a hologram. 
     Such volume holograms are admittedly more forgery-proof than the master hologram, but they are optically no higher in quality than a surface hologram. 
     SUMMARY OF THE INVENTION 
     Now, the object of the invention is to provide a process, suitable for mass production, for the production of a volume hologram, which permits brilliant reproduction of two or more separate items of image information, as well as a master for the production of the volume hologram. 
     The object of the invention is attained by a process for the production of a multi-layer body having a volume hologram with at least two different items of image information, wherein it is provided that a photosensitive layer of the multi-layer body is brought into contact directly or with the interposition of a transparent optical medium with the front side of a master in which there are formed interlaced regions with at least two different surface structures which contain the at least two different items of image information, wherein one of the surface structures has an asymmetrical relief structure or a kinoform structure; the photosensitive layer and the master are exposed with a coherent light beam; and the volume hologram introduced into the photosensitive layer in that way is fixed by hardening of the photosensitive layer. 
     The object is further attained by a master for the production of a volume hologram having at least two different items of image information in the optical contact process, wherein it is provided that shaped in a master layer of the master are interlaced regions having at least two different surface structures which contain the at least two different items of image information, wherein one of the surface structures has an asymmetrical relief structure or a kinoform structure. 
     The object is further attained by a transparent layer with a non-homogeneous refractive index, in which the nodes of the Bragg planes of a volume hologram are formed by a refractive index variation, wherein the volume hologram contains at least two different items of image information as a holographic image of at least two interlaced different surface structures, wherein one of the surface structures has an asymmetrical relief structure or a kinoform structure. 
     The process according to the invention is distinguished in that the volume hologram is produced by an optical contact copy of a master in which interlaced regions with different asymmetrical surface structures or kinoform structures, which contain different items of image information, are formed. 
     The regions can be interlaced in different ways. They can involve for example interlaced raster grids, for example line grids. In that case the one region can reproduce for example an item of text information and the other region an item of image information. It can however also be provided that the one region affords an item of information and the other region forms the background surroundings from which the information stands out. The information can be for example a logo which appears light against a dark background in the one viewing position and dark against a light background in the other viewing position. It can therefore be provided that a change from a positive representation to a negative representation and vice-versa occurs when the volume hologram is tilted or moved. Furthermore the regions can be such that the one region forms the edge of the other region. Thus the one region can for example reproduce the border of an alphanumeric character and the other region can reproduce the alphanumeric character itself. 
     The asymmetrical surface structures are preferably periodic structures which deflect or diffract the incident light into preferred directions. Brilliant representations are produced in that way. Equally the surface structures can also be in the form of what are referred to as kinoform structures which are also known as zone lenses or Fresnel zone plates. These involve annular structures in which the zones differ in their transparency and/or in their optical path length. In the former case the light is diffracted at the annular gaps and intensified by constructive interference at focal points. In the latter case the light is deflected in a preferred direction by the differing phase shift at the rings. The kinoform structures, specifically for a wavelength, can highly efficiently diffract the coherent light into precisely defined angle regions. The kinoform structures are therefore also structures which concentrate the deflected light in a preferred direction and in that way produce brilliant representations. 
     The clean separation of the different items of image information by virtue of nested regions with different asymmetrical surface structures or kinoform structures in the master and holographic imaging of the master in a volume hologram by means of optical contact copying means that comparatively low demands are made on the reproduction procedure. The processes required for production of the master also impose comparatively low levels of demand. The master can be for example in the form of a film body having a thermoplastic or UV hardenable master layer in which the asymmetrical surface structures or kinoform structures are shaped. It is therefore possible to provide master copies which can be used just once for optical contact copying and which can be produced for example in a roll-to-roll process. The photosensitive material can be either liquid, viscous or solid. The viscosity of the photosensitive material can be increased by preliminary exposure with UV light. As tests have shown that operation is temperature-dependent. It can therefore be provided for example that the photosensitive material is processed at a temperature of 30° C. Following the optional preliminary UV exposure step which precedes exposure with the laser the material which is pre-treated with UV light is of the optimum viscosity for further processing. 
     The security element according to the invention is distinguished by an optimally produced volume hologram, the thickness of which is limited downwardly only by the optical laws relating to formation of volume holograms. The security element can therefore also be inserted into carriers which are subjected to a flexural loading during use as is the case for example with banknotes. Because the volume hologram is produced in a transparent material the unexpected optical effect of producing non-transparent reflecting images in a transparent window is particularly striking. 
     It can be provided that the at least one other surface structure has an asymmetrical relief structure or a symmetrical relief structure or a relief structure with a varying period or a random relief structure or a pseudo-random relief structure. 
     It can further be provided that the surface structures of the master which do not contain any image information are in the form of a motheye structure and/or in the form of a mirror and/or in the form of a matt structure and/or in the form of a scatter grating. 
     In that way the items of information are particularly clearly emphasised against the background. 
     An advantageous configuration provides that the at least two surface structures are in the form of asymmetrical surface structures. 
     It can be provided that the at least two asymmetrical surface structures are provided with a different relief structure and/or have a different k-vector. The asymmetrical relief structure can typically be a sawtooth-shaped structure in which the rising edge includes an acute angle of inclination with the surface normal of the surface and the falling edge extends in the surface normal, that is to say forms a vertical edge. The relief structure can therefore differ in terms of the angle of inclination. It can further differ in respect of the spacing of two successive raised portions. The surface structures can also differ from each other in their k-vectors, that is to say in the propagation direction of the diffracted light wave. 
     It can further be provided that a plurality of coherent light beams of differing wavelength and/or with a differing direction of incidence and/or polarisation are used. 
     As tests have clearly demonstrated the diffraction efficiency of the volume holograms with multiple beam exposure is lower than the diffraction efficiency of the volume holograms with single-beam exposure, in particular for fluid-like photosensitive material which has a high degree of mobility of its components. 
     It can further be provided that the coherent light beam passes through the photosensitive layer and is deflected at least at the asymmetrical surface structures and/or kinoform structures of the master, wherein no air layer is provided between the photosensitive layer and the master. In that way the coherent light beam which passes into the photosensitive layer forms the object wave and the coherent light beam issuing from the photosensitive layer and the coherent light beam diffracted or reflected by the asymmetrical surface structure or the kinoform structure of the master into the photosensitive layer forms the object wave which interferes in the photosensitive layer with the object wave and in that case at the interference nodes alters the refractive index of the photosensitive layer. 
     It can also be provided that the coherent light beam passes through the photosensitive layer and the master and is deflected at the rear side of the master, wherein an air layer is provided between the photosensitive layer and the master. 
     It can further be provided that the coherent light beam is divided into a first and a second beam portion and the first beam portion passes through the photosensitive layer and the second beam portion passes from the rear side of the master through the master. In that case the master can be formed with a transparent master layer without an additional reflection layer. 
     In an advantageous configuration it can be provided that the surface structures of the master are shaped in the surface, that is towards the master, of the photosensitive layer. In that way it is possible to provide an additional security feature because upon imitation both the surface structure and also the volume hologram have to be copied and association in accurate register relationship has to be implemented. 
     It can be provided that the master remains on the fixed photosensitive layer. 
     It can however also be provided that applied to the front side of the master is a transparent release layer, the refractive index of which is equal or approximately equal to the refractive index of the photosensitive layer and that then the photosensitive layer is applied to the release layer. To keep the optical influence of the release layer low, it is provided that the difference in refractive index between the photosensitive layer and the release layer is kept low or entirely avoided. 
     Frequently the quality of the photosensitive layer is such that there is no need to apply a release layer to the master. The release layer however can optionally be provided and can facilitate release of the master from the exposed photosensitive layer when it is intended that the master is released from the exposed photosensitive layer. 
     It can be provided that the photosensitive layer is used in a thickness of from 5 μm to 40 μm. The optimum thickness of the photosensitive layer is dependent inter alia on the material used and can be ascertained by tests. 
     It can further be provided that a photopolymer layer is used as the photosensitive layer. Photopolymers are resins which crosslink due to the action of energy-rich light, in particular UV light, and therefore polymerise. To produce volume holograms, special photopolymers are provided, the refractive index of which changes due to intense exposure such as for example OmniDex 706 from DuPont. 
     It can be provided that the photosensitive layer and the master are exposed by coherent light beams of differing wavelength and/or differing direction. In that way it can be provided that the items of image information stored in the volume hologram appear in different colours and/or are visible at different viewing angles. 
     It can advantageously be provided that the coherent light beam is afforded by a laser. 
     An advantageous configuration provides that the multi-layer body is produced in a roll-to-roll process by a procedure whereby the photosensitive layer is passed over a replication cylinder, on the peripheral surface of which is arranged the master, the photosensitive layer being illuminated with the coherent light beam. The roll-to-roll process is particularly advantageous for mass-production. As tests have shown the peripheral speed of the replication cylinder can be at least 5 m/min, it can be increased to at least 40 m/min. Because the multi-layer body is at rest with respect to the surface of the master, there are no losses of quality due to the rotating master. It can be provided that hardening of the photosensitive layer is effected when the multi-layer body is still in contact with the rotating master. At least the beginning of the hardening operation can be provided when the multi-layer body is still in contact with the rotating master. 
     It can be provided that the coherent light beam includes an acute angle with the surface normal of the replication cylinder. 
     An advantageous configuration provides that the coherent light beam includes an angle of 10° to 20° with the surface normal of the replication cylinder. An angle of 14° is particularly advantageous. 
     Further advantageous configurations of the invention are directed to the design of the master. 
     It can be provided that the at least one other surface structure has an asymmetrical relief structure or a symmetrical relief structure or a relief structure of a varying period or a random relief structure or a pseudo-random relief structure. 
     It can further be provided that the surface structures of the master which do not contain any image information are in the form of a motheye structure and/or in the form of a mirror and/or in the form of a matt structure and/or in the form of a scatter grating. 
     In a further advantageous configuration it is provided that the at least two surface structures are in the form of asymmetrical surface structures. 
     It can be provided that the k-vectors of the asymmetrical surface structures are turned through 180° relative to each other. Such an orientation of the surface structures is particularly advantageous because the different items of image information become visible by simply tilting the multi-layer body. With other positional associations, tilting and rotary movements have to be performed simultaneously or in succession to make all items of image information visible. That effect which is a hindrance in terms of simplest possible viewing can however be advantageous if for example, besides two items of image information intended for a human observer, a third item of image information is stored in the volume hologram, which is intended for an electronic reading device. That concealed image information can also be readable for example only under UV light or infrared light. 
     It can advantageously be provided that the asymmetrical surface structures are blaze gratings. Blaze gratings are distinguished by particular brightness because, on account of their asymmetrical configuration, they deflect as much light as possible into one of the two symmetrical diffraction orders, preferably into one of the two first orders. That is advantageous as, in the case of conventional gratings, the light is distributed to a larger spatial angle and the main component of the output is concealed in the zero order and thus remains unused. 
     It can preferably be provided that the blaze grating has a spatial frequency of 100 lines/mm to 150 lines/mm. The preferred blaze grating therefore has a grating line spacing of 10 μm to 6.7 μm. In especial cases however very large grating line spacings (&gt;10 μm) and also very small grating line spacings (&lt;1 μm) can also be preferred. Gratings with very great grating line spacings such as for example asymmetrical achromatic gratings with grating line spacings of over 10 μm can reflect all the incident light into one order. Equally, in particular with a non-normal angle of incidence of the exposure beam, it is possible to produce high-frequency gratings, in respect of which an order, for example the −1st order, negates almost all the energy of the diffracted field. The two situations are advantageous and there is no competition between the various ‘object waves’ (that competition potential reduces the diffraction efficiency of the volume hologram produced). 
     It can further be provided that the blaze grating involves a grating depth of 1 to 2 μm. 
     Blaze gratings involving the above-mentioned dimensions can be produced by hot embossing, for example by means of a heated embossing roller, or photomechanically by exposure of a UV hardenable lacquer. 
     It can be provided that the regions with the at least two items of image information are arranged in a raster grid with a grid width of 50 μm to 20 μm. Under particularly advantageous conditions, that is to say when considering high-contrast motifs, with good illumination, the limit of the resolution capability of the human eye is at 20 μm. The resolution capability can be worsened by a factor of 3 to 5 when low contrast is involved, with poor illumination. Grid widths of 20 μm to 50 μm can therefore no longer be resolved by the naked human eye so that it is not possible to perceive the rastering of the image information. 
     It can further be provided that the raster grid is a stripe raster grid. A stripe raster grid is particularly simple to implement. It is however also possible to provide other raster grids, in particular if more than two different items of image information are to be interlaced. The arrangement may involve for example a pixel raster grid, wherein the master can be produced by electron beam technology. The rastering effect ensures that the items of image information are also separated from each other in the volume hologram so that there are no losses in brightness and/or sharpness, due to superpositioning of items of image information in the volume hologram. 
     It can be provided that the master layer is formed from a transparent replication layer. 
     As already stated hereinbefore it can be provided that the coherent light beam incident on the photosensitive layer is reflected at the front side of the master or at the rear side thereof. 
     It can therefore be provided that a reflection layer is applied to the front side of the master or a reflection layer is applied to the rear side of the master. 
     It can further be provided that the reflection layer is in the form of a metallic layer. The metallic layer can be formed for example from a metal which is a good reflector such as aluminium, silver, gold or copper, or from a metal alloy. The metallic layer can be formed with a layer thickness of some nanometers. The layer thickness can also be so selected that it reflects in incident light and appears transparent in transmitted light. The preferred layer thickness can advantageously be determined by tests as the transparency, besides the layer thickness, depends inter alia on the material of the metallic layer and the aspect ratio of the surface structure. 
     If the reflection layer is provided on the front side of the master it can be provided that the master is formed from reflecting material, for example a metal which is a good reflector. The master can be for example in the form of a rotating master cylinder. 
     It can also be provided that the reflection layer is in the form of an optical separation layer. This can involve an inorganic dielectric such as for example ZnS. 
     It can further be provided that the reflection layer is in the form of an HRI layer. 
     It can also be provided that, instead of the reflection layer, there are provided a plurality of high-refraction dielectric layers which each involve a layer thickness of λ/2 or λ/4, wherein X denotes a light wavelength. 
     It can further be provided that use is made of the reflection at surfaces which interface with air or another low-refraction medium and in the case of a transparent master it is also possible in particular to dispense with the reflection layer on the rear side of the master. 
     The metallic layer and/or the dielectric layer and/or the thin film layer system and/or the liquid crystal layer and/or the print layer can either cover the entire surface of the photosensitive layer, they can be applied in register relationship with the images of the volume hologram as in the same way in relation to a KINEGRAM®, they can be applied in partial register relationship with the images of the volume hologram as in the same manner in relation to normal demetallisation or they can be partially applied in the form of a pattern which is not in register relationship with the images of the volume hologram. Especially if the surface relief is coated with metal or HRI material, it can acquire an optical function which supplements the volume hologram. 
     It can further be provided that the volume hologram is arranged in a window of a banknote or an ID card. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described hereinafter by way of example by means of a number of embodiments with reference to the accompanying drawings in which: 
         FIG. 1   a  shows a diagrammatic plan view of a master according to the invention for producing a volume hologram, 
         FIG. 1   b  shows diagrammatic detail views of  FIG. 1   a,    
         FIG. 2   a  shows a view illustrating the principle of the process according to the invention, 
         FIG. 2   b  shows a view illustrating the principle of the function of a volume hologram produced as shown in  FIG. 2   a,    
         FIGS. 3   a  to  3   i  show diagrammatic views of production steps for a first embodiment of the production process according to the invention, 
         FIGS. 4   a  to  4   h  show diagrammatic views of production steps for a second embodiment of the production process according to the invention, 
         FIGS. 5   a  to  5   g  show diagrammatic views of production steps for a third embodiment of the production process according to the invention, 
         FIGS. 6   a  and  6   b  show a diagrammatic view of a production apparatus for executing a fourth embodiment of the production process according to the invention, 
         FIGS. 7   a  to  7   d  show diagrammatic views of production steps for the fourth embodiment of the production process according to the invention in  FIG. 6 , and 
         FIGS. 8   a  and  8   b  show an example of use in the form of a security element in strip form. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a diagrammatic plan view on an enlarged scale of a master  1  for the production of a volume hologram. The master  1  can be in the form of a multi-layer body with at least one replication layer  1   r  which on the top side has image regions  2   a  and  2   b  formed from surface regions  3   a  and  3   b  which are in strip form and which are arranged in mutually parallel relationship. The strip-form surface regions  3   a  and  3   b  are each 50 μm in width and are arranged at a spacing of 50 μm from each other, wherein the intermediate space between the strip-form surface regions  3   a  is filled by the strip-form surface regions  2   b  and vice-versa. The strip-form surface regions  2   a  and  2   b  form interlaced line raster grids which are below the resolution capability of a naked human eye. Therefore the image regions  2   a  and  2   b  appear to the viewer as closed regions, in which respect in the embodiment shown in  FIG. 1  the image region  2   a  is a logo and the image region  2   b  are alphanumeric characters. 
     The surface regions  3   a  and  3   b  have relief structures in the manner of a blaze grating and are provided with a reflection layer  1   m . In the embodiment shown in  FIG. 1   a  the reflection layer  1   m  is a thin metallic layer. Blaze gratings are special diffraction gratings in optics. The grating elements are inclined through what is referred to as the blaze angle. That leads to a step-shaped asymmetrical surface relief structure with a rising front edge which is inclined at an acute angle relative to the surface normal and a steeply falling rear edge. As can be seen from  FIG. 1   b  the relief structures of the surface regions  3   a  and  3   b  involve identical relief structures which are arranged turned through 180° relative to each other (identified in  FIG. 1   b  with 0° azimuth and 180° azimuth). 
     Because of the relief structures of the surface regions  3   a  and  3   b , which are turned through 180° relative to each other, the image regions  2   a  and  2   b , upon tilting of the master, appear as clearly separated image regions which in that case light up brightly. 
       FIG. 2   a  now diagrammatically shows use of the master  1  of  FIGS. 1   a  and  1   b  for the production of a volume hologram which has the optical properties of the master  1 . 
     The reflection layer  1   m  arranged on the replication layer  1   r  is covered by a transparent cover layer  5  which can be a release layer which can later facilitate separation of a photopolymer layer  6  applied to the relief layer. In the illustrated embodiment the photopolymer layer  6  has a refractive index n=1.6. The photopolymer layer  6  is firstly non-crosslinked or only slightly crosslinked. Advantageously the cover layer  5  has the same refractive index or an approximately equal refractive index to the photopolymer layer  6  so that the cover layer  5  optically has no effect. 
     A laser beam  7   e  impinging on the photopolymer layer  6  for writing in a volume hologram is firstly refracted at the photopolymer layer  6  and then deflected at the reflection layer  1   m  by diffraction at the grating structure of the replication layer  1   r . In the diagrammatic view shown in  FIG. 2   a  the diffracted 1st-order beam is denoted by  7   g  while the zero order beam reflected at the illustrated edge is denoted by  7   a . Because the grating structure is a blaze grating the 1st-order beam is of the highest intensity. The first-order beam embodies the object wave which interferes with the reference wave embodied by the incident beam  7   e  and in that case triggers local polymerisation in the photopolymer layer  6 . As a consequence of polymerisation the refractive index of the photopolymer layer is changed. The changes in refractive index are localised in what are referred to as the Bragg planes which were first described in connection with the X-ray structural analysis of crystals. 
       FIG. 2   b  now shows a photopolymer layer  6   e  which is the exposed photopolymer layer  6  in  FIG. 2   a . The photopolymer layer  6   e  has a position-dependent refractive index n′=n+δ, whereby a three-dimensional refractive index pattern is stored in the photopolymer layer  6   e  in the form of a volume hologram, in which reconstruction of the interference pattern influenced by the surface structure of the replication layer  1   r  is permanently stored. 
     The photopolymer layer can be the photopolymer OmniDex 706 from DuPont, which has the specified property of a local change in refractive index due to exposure to light. Photopolymers are also known, which are in the form of a liquid substance and which for example polymerise due to the action of UV light and as a result harden. It can also be provided that the photopolymer is cast as a layer and is subjected to preliminary hardening by weak UV light action and/or is hardened after the formation of the volume hologram by the action of UV light or by a heat treatment. 
     In  FIG. 2   b  a light beam impinging on the photopolymer layer  6   e  for reconstruction of the grating images is identified by  8   e  and a beam which issues from the photopolymer layer  6   e  and which is diffracted at the volume hologram is denoted by  8   g . The beam  8   g  issuing from the photopolymer layer  6   e  corresponds in direction and strength of light to the diffracted beam  7   g  in  FIG. 2   a . To clearly illustrate the described events the nodes of one of the Bragg planes are diagrammatically represented by circles. 
     Accordingly the photopolymer layer  6   e  produced in accordance with the above-described process has an optical action which arouses the impression that there is a blaze grating having a reflecting relief structure. A volume hologram has a high level of safeguard against copying because production parameters such as the precise wavelength of the laser light and the precise exposure angles must be known to reproduce the volume hologram. Copying with polychromatic light is excluded from the outset. However copying with monochromatic light is also rendered considerably more difficult because, due to a series of causes, there is a drift of the wavelength which can be used for reconstruction relative to the original wavelength which was used when producing the volume hologram. One of the causes is shrinkage or upsetting of the Bragg gratings upon hardening of the photopolymer layer  6   e . Added to that is the fact that the drift does not occur homogeneously over the entire extent of the volume hologram and also varies in production. Such deformation phenomena can further be caused by sticking on the photopolymer layer with a hot melt adhesive or can be specifically and targetedly introduced in order for example to provide the volume hologram with personalised information. Because of the described variation in the drift between the wavelength which can be used for reconstruction of the volume hologram relative to the original wavelength and further the lack of homogeneity of the drift, the level of safeguard against forgery of the volume hologram according to the invention is very high. 
       FIGS. 3   a  to  3   i  now show the process steps for the production of a first embodiment of the multi-layer body according to the invention. 
       FIG. 3   a  is a diagrammatic view in section showing a replication layer  34  which can be formed from a thermoplastic material, in the top side of which are provided regions in which asymmetrical relief structures  30   a  and  30   b  in the form of blaze gratings are formed, the relief structures involving an arrangement turned through 180°, of the same configuration. The relief structures  30   a  and  30   b  in the embodiment shown in  FIGS. 3   a  to  3   i  have a grating width of 10 μm, that is to say a spatial frequency of 100 lines/mm, and a grating depth of 2 mm, with the total thickness of the replication layer  34  being 22 μm. The top side of the replication layer  34  further has regions with a relief structure  30   h  forming a background region for the regions with the relief structures  30   a  and  30   b . The relief structure  30   h  has a markedly higher depth-to-width ratio than the relief structures  30   a  and  30   b , the depth-to-width ratio of which, with the above-specified parameters, is 2/10=0.2. The relief structure  30   h  can for example have a depth-to-width ratio of 1 to 5. The relief structure  30   h  is not a blaze grating but a ‘motheye’ structure which absorbs the incident light and therefore appears dark to a viewer. It forms a neutral background for the images generated by the relief structures  30   a  and  30   b . The relief structure  30   h  can also be a reflective flat surface, a matt structure or a grating structure which diffracts the incident light. 
     The dimensionless depth-to-width ratio which is also called the aspect ratio is defined as the ratio of the depth of the ‘troughs’ to the spacing of two adjacent ‘peaks’ of a preferably periodic relief structure. 
       FIG. 3   b  now shows the replication layer  34  with a metallic layer  34   m  applied on the top side of the replication layer. The metallic layer  34   m  can be applied for example by sputtering. The metallic layer  34   m  can be of metals which are good reflectors such as aluminium, silver, gold or the like or it can be a metal alloy. 
       FIG. 3   c  now shows the replication layer  34  with the metallic layer  34   m  partially removed. The metallic layer is removed in the regions with the relief structure  30   h  which thus form non-reflecting regions. It will be noted however that it is also possible to dispense with demetallisation of the relief structure  30   h  because such a matt structure reflects little or not at all, because of the undirected scatter effect. 
       FIG. 3   d  now shows the replication layer  34  with the metallic layer  34   m  partially removed, with a release layer  35  being applied thereto. 
       FIG. 3   e  shows the layers indicated in  FIG. 3   d , with a photopolymer layer  36  applied to the release layer  35  in a thickness of 5 μm to 20 μm. The photopolymer layer  36  has the same refractive index as the release layer  35  so that no optical diffraction occurs at the interface between the photopolymer layer  36  and the release layer  35 . Depending on the consistency of the non-crosslinked photopolymer layer, after application the photopolymer layer  36  can be hardened to afford adequate stability in respect of shape, for the next processing step. For example the photopolymer layer  36  can be subjected to initial polymerisation by a preliminary exposure procedure. 
       FIG. 3   f  now shows exposure of the photopolymer layer  36  with laser light  37 . In that way, an interference pattern is produced in the photopolymer layer  36  by interference of the radiated laser light  37  (reference wave) with the laser light which is diffracted or reflected by the metallic layer  34   m  (object wave) and the photopolymer layer  36  is polymerised or further polymerised at the interference nodes. Consequently there the refractive index of the photopolymer layer is altered and a volume hologram is produced in the photopolymer layer  36  by a local change in refractive index. In the embodiment of  FIG. 3   f  the beams of the laser light  37  impinge perpendicularly on the photopolymer layer  36 . It can however also be provided that the beams of the laser light are directed on to the photopolymer layer  36  at an inclined angle, for example at an angle of 14° relative to the surface normal. 
       FIG. 3   g  shows the layer structure in  FIG. 3   f  with the photopolymer layer  36   e  which has been exposed and hardened by means of UV radiation and to which an adhesive layer  38  is applied. The photopolymer layer  36  can be applied to a substrate  39 , as shown in  FIG. 3   h , by means of the adhesive layer  38 . After application of the photopolymer layer  36   e  and release of the replication layer  34 , which is now no longer required, together with the release layer  35 , the relief structures  30   a ,  30   b  and  30   h  are unprotected and can therefore be worn away, damaged or soiled. Accordingly there is a protective layer  36   s  which, as shown in  FIG. 3   i , covers over the photopolymer layer  36   e  over its full area. The side of the protective layer  36   s , that is remote from the relief structures  30   a ,  30   b  and  30   h , forms a smooth surface. 
       FIGS. 4   a  to  4   h  now show the process steps for the production of a second embodiment of the multi-layer body according to the invention. 
       FIG. 4   a  shows a diagrammatic view in section illustrating a replication layer  44  which can be formed from a thermoplastic material and in the top side of which are provided regions in which are shaped asymmetrical relief structures  40   a  and  40   b  which are in the form of blaze gratings and which, being of the same configuration, are in an arrangement of being turned through 180°. The relief structures  40   a  and  40   b , in the embodiment shown in  FIGS. 4   a  to  4   h , involve the same parameters as in the embodiment illustrated hereinbefore in  FIGS. 3   a  to  3   i  (10 μm grating width, 2 μm grating depth, 22 μm total thickness for the replication layer  44 ). 
       FIG. 4   b  now shows the replication layer  44  with a metallic layer  44   m  applied to the top side of the replication layer. The metallic layer  44   m  can be applied for example by sputtering. The metallic layer  44   m  can involve metals which are good reflectors such as aluminium, silver, gold or the like, or a metal alloy. 
       FIG. 4   c  now shows the layer composite comprising the replication layer  44  and the metallic layer  44   m  with a protective lacquer layer  44   s  which is applied to the metallic layer  44   m  in a thickness of 1 μm to 3 μm and completely fills up the relief structures  40   a  and  40   b . The side of the protective lacquer layer  44   s , that is remote from the metallic layer  44   m , has a flat surface. 
     In  FIG. 4   d , applied to the protective lacquer layer  44   s  is a release layer  45  to which a photopolymer layer  46  is applied in  FIG. 4   e . Depending on the respective consistency of the non-crosslinked photopolymer layer, after application the photopolymer layer  46  can be subjected to preliminary hardening to afford adequate stability in respect of shape for the next processing step. For example the photopolymer layer  46  can be subjected to initial polymerisation by preliminary exposure. 
       FIG. 4   f  shows exposure of the photopolymer layer  46  with laser light  47 . In that way an interference pattern is produced in the photopolymer layer  46  by interference of the irradiated laser light  47  (reference wave) with the laser light reflected or diffracted by the metallic layer  44   m  (object wave) and the photopolymer layer  46  is polymerised or further polymerised at the interference nodes. Consequently the refractive index of the photopolymer layer is altered there. 
     In  FIG. 4   g  the photopolymer layer  46  is converted into an exposed and hardened photopolymer layer  46   e  and provided with an adhesive layer  48 . 
       FIG. 4   h  now shows the hardened photopolymer layer  46   e  applied to the carrier substrate  49  and fixed on the carrier substrate  49  by means of the adhesive layer  48 . The adhesive layer  48  can be a hot melt adhesive. The shrinkage effects, which are involved with the adhesive procedure, in respect of the developed photopolymer layer  46   e  can further increase the level of safeguard against forgery of the volume hologram stored in the photopolymer layer  46   e  because the Bragg planes of the volume hologram, that are deformed by the adhesive procedure, can at best be read out point-wise. 
       FIGS. 5   a  to  5   g  now show the process steps for the production of a third embodiment of the multi-layer body according to the invention. 
       FIG. 5   a  shows a replication layer  54  of PET with regions having relief structures  50   a  and  50   b  which, as in the above-described embodiments, differ from each other substantially by their arrangement of being turned through 180° and which involve blaze gratings. The relief structures  50   a  and  50   b  however can also be of different configurations and can differ from each other for example in grating spacing and/or grating depth and/or the angle of inclination of the grating flanks. In the  FIG. 5   a  embodiment the grating spacing of the relief structures  50   a  and  50   b  is 10 μm, the grating depth is 2 μm and the overall thickness of the replication layer  54  is less than 12 μm. 
     Applied to the top side of the replication layer  54 , that has the relief structures  50   a  and  50   b , is a metallic layer  54   m  which can be like the metallic layers in the above-described embodiments (see  FIGS. 3   b  and  4   b ). 
     In  FIG. 5   b , applied to the underside of the replication layer  54  is a release layer  55 , the refractive index of which is equal or approximately equal to that of the replication layer  54  so that no optical refraction occurs in the interference between the replication layer  54  and the release layer  55 . 
     Now in  FIG. 5   c  applied to the release layer  55  is a photopolymer layer  56  having the properties of the above-described photopolymer layers  36  (see  FIG. 3   e ) and  46  (see  FIG. 4   e ). 
       FIG. 5   d  shows the exposure of the photopolymer layer  56  by means of laser light  57  and  FIG. 5   e  shows the multi-layer body which is prepared for transfer on to a carrier substrate and in which an adhesive layer  58  is applied to an exposed and hardened photopolymer layer  56   e  produced from the photopolymer layer  56  in  FIG. 5   d.    
       FIG. 5   f  shows a variant of  FIG. 5   e  having a replication layer  54   e  produced from the replication layer  54  by removal of the relief structures  50   a  and  50   b  and the metallic layer  54   m.    
     Finally  FIG. 5   g  shows a carrier substrate  59  which can be a security document, with the hardened photopolymer layer  56   e  which is permanently fixed thereto by means of the adhesive layer  58 . 
       FIG. 6   a  is now a diagrammatic view showing a production apparatus  60  for the production of a fourth embodiment of the multi-layer body according to the invention. The production apparatus  60  is a production apparatus for what is referred to as the roll-to-roll process. A replication cylinder  61  is coated on its outside with a surface relief master  61   m . In the  FIG. 6   a  embodiment the replication cylinder  61  is of a diameter of 200 mm and rotates at a peripheral speed of 5 m/min. A maximum peripheral speed of up to 40 m/min can be provided. 
     As can be seen from  FIG. 6   b  the surface relief master  61   m  has a first blaze grating  63   a  with a grating period of 1 μm and of a grating depth of 300 nm and a second blaze grating  63   b  with a grating period of 0.78 μm and of a grating depth of 280 nm. The surface regions  63   h  of the surface relief master  61   m , which are not occupied by the first blaze grating  63   a  or the second blaze grating  63   b  have a surface relief with a matt structure which diffusely scatters incident light and therefore causes the optical impression of a ‘black mirror’. In this embodiment the surface relief master  61   m  is formed from a nickel-cobalt alloy. The surface of the surface relief master  61   m  is coated with a thin reflection layer  61   r  of a highly reflective metal, for example gold. 
     A transparent carrier film  65  is unwound from a supply roll  65   v , passed over the replication cylinder  61  and wound on to a take-up roller  65   a  again. In the  FIG. 7   a  embodiment a separation layer  65   t  is firstly applied to the carrier film  65  before the photopolymer layer  66  is printed thereon. The separation layer  65   t  can be provided to facilitate subsequent release of the carrier film  65  from the photopolymer layer. 
     The carrier film  65  extends around the replication cylinder  61  through 180° in the embodiments shown in  FIGS. 6   a  and  7   a, b . A viscous photopolymer layer  66  is applied by printing on the inside of the carrier film  65 , that faces towards the surface relief master  61   m , upstream of the replication cylinder  61 , by means of a rotating printing cylinder  64   a . It can be provided that a low-viscosity photopolymer layer is subjected to preliminary hardening in the printing operation or shortly thereafter by means of UV light so that the optimum viscosity for further processing is set. 
     Provided for exposure of the photopolymer layer  66  is a laser  67  which emits a laser beam  67   s  directed on to the surface relief master  61   m  at an angle of 14°. That angle can be optimised for example by tests. It is dependent inter alia on the flank inclination of the blaze gratings  63   a  and  63   b . Reflection and diffraction at the blaze gratings  63   a  and  63   b  (see  FIG. 6   b ) result in the formation in the photopolymer layer  66  of a volume hologram which is fixed by hardening of the photopolymer layer  66  by the UV light of a UV lamp  68  arranged downstream of the laser  67  (see  FIG. 7   b ). 
     Provided between the laser  67  and the replication cylinder  61  is a cylindrical lens  67   z  which focuses the laser beam  67   s  and directs it on to the surface of the surface relief master  61   m . It is however also possible to provide at that location a one-dimensional scanner or a two-dimensional mask, for example a liquid crystal modulator. This can therefore also involve a shutter or a modulator which switches the laser beam on and off, desirably in register relationship with the motif of the hologram. 
     The modulator can for example be in the form of an electro-optical or acousto-optical modulator. The laser  67  can be a monochromatic laser or a laser with a plurality of wavelengths or a plurality of monochromatic lasers. By way of example it can be provided that the master is illuminated with a red laser beam in one regions and illuminated with a green laser beam in other regions. In such a situation of use the complication and expenditure in terms of optical system and modulator are higher than when using a monochromatic laser. 
     Arranged downstream of the replication cylinder  61  and upstream of the take-up roller  65   a  is a rotating printing cylinder  64   b  which prints an adhesive layer  69  on to the side of the hardened photopolymer layer  66 , that is remote from the carrier film  65 . 
     The carrier film  65 , the hardened photopolymer layer  66  and the adhesive layer  69  now form a multi-layer body  70  which is then wound on to the take-up roll  65   a.    
       FIG. 7   c  shows a multi-layer body  70 ′ which is finished for further processing and which differs from the above-described multi-layer body  70  in that a separation layer  65   t  is arranged between the carrier film  65  and the photopolymer layer  66 . 
       FIG. 7   d  now shows the photopolymer layer  66  which has been pulled off the carrier film and which is applied to a carrier substrate  71  by means of the adhesive layer  69 . The carrier substrate  71  can be for example a banknote or an ID card, that is to say, a security document, the level of safeguard against forgery of which is considerably improved by the volume hologram which is shaped in the photopolymer layer  66 . 
       FIGS. 8   a  and  8   b  now show a strip-shaped security element  80  in two views on an enlarged scale (on the scale of 2:1), which are formed by tilting the security element  80  about a horizontal axis. The security element  80  has the following security features:
         a flip image  81  formed from a first image portion  81   t  which reproduces a text and a second image portion  81   k  which reproduces a series of crosses. The two image portions  81   t  and  81   k  are rastered in strip form and are interlaced (see in that respect hereinbefore also the embodiment of  FIG. 1   a ).   A logo  82  which in  FIG. 8   a  appears as a light logo  82   h  and in  FIG. 8   b  as a complementary dark logo  82   d . The light logo  82   h  and the dark logo  82   d  are rastered in strip form and interlaced, as described hereinbefore.       

     To achieve that effect it can also be provided that a first surface structure forms the logo  82  and a second surface structure forms the regions outside the logo. In the first viewing position shown in  FIG. 8   a , in this embodiment the logo  82  appears as a light logo  82   h  and the regions outside the logo  82  appear dark. In the second viewing position shown in  FIG. 8   b  the logo  82  appears as a dark logo  82   d  and the regions outside the logo  82  appear light. Rastering is therefore not necessary in this embodiment.
         A value identification  83  which in  FIG. 8   a  appears as an outline  83   u  and in  FIG. 8   b  as a solid image  83   v . The two image portions  83   u  and  83   v  of the value identification  83  are formed without rastering, the outline  83   u  surrounding the solid image  83   v.