Device and method for generating a hologram

Abstract

Device (1) for generating a hologram, comprising: an optical arrangement (20), wherein the optical arrangement (20) has, along an optical axis, at least one laser radiation source (21), a diffractive optical element (22) and a master receptacle (23), in or on which a holographic recording material (8) to be exposed and a hologram master (9) can be arranged, and wherein the diffractive optical element (22) is designed to image, in a focal plane (24), laser radiation (5) emitted by the laser radiation source (21) and transmitted through the diffractive optical element (22) onto an incident radiation plane (17) in a beam path of the optical arrangement (20) and thereby to irradiate an exposure region (10) which corresponds to a region (25) to be exposed to produce the hologram.

Description

[0001] The invention relates to a device and a method for producing a hologram, in particular in contact copy.

[0002] Holograms are used as security elements to protect valuable or security documents. Holograms make it more difficult to counterfeit or imitate the valuable or security document, thereby increasing its security.

[0003] In order to be able to use holograms as security features in the mass production of valuable or security documents, holograms must be produced, in particular copied, in an efficient manner during the production of the valuable or security documents.

[0004] Copying is typically performed using the so-called contact copying process. For this purpose, a hologram master to be copied is produced in the form of a master hologram in which the holographic information is stored. A holographic recording material to be exposed, for example, a hologram film made of a photopolymer, is then applied flatly to the hologram master. If the hologram master is irradiated with appropriate laser radiation, the hologram stored in the hologram master is reconstructed and exposed, i.e., copied, into the hologram film. A corresponding device and method are described, for example, in EP 0 896 260 A2. Further devices and methods are known from DE 10 2015 220 123 A1 and DE 10 2012 215 540 A1.

[0005] In particular, to create the hologram, laser radiation is shaped into a linear profile using a Powell lens. The laser radiation thus shaped is then directed onto the holographic recording material and the hologram master using a lens. The linear laser radiation is guided ("scanned") step by step over the hologram master and the holographic recording material using a rotatable mirror of a scanner arranged in the beam path. To limit the area in which exposure takes place, an additional aperture is located in the beam path in front of the holographic recording material and the hologram master. In this way, for example, circular or square exposure areas can be created that limit exposure.

[0006] However, the described process is complex and slow, as it always requires step-by-step exposure of the area to be copied. Furthermore, the process is expensive and error-prone due to the large number of optical components.

[0007] The invention is based on the object of creating a device and a method for generating a hologram, in which the generation can be carried out in an improved manner.

[0008] The object is achieved according to the invention by a device having the features of patent claim 1 and a method having the features of patent claim 8. Advantageous embodiments of the invention emerge from the subclaims. Definitions

[0009] Valuable or security documents are those items that have at least one security feature. Examples of security documents include identity cards, passports, ID cards, access control badges, visas, tax stamps, tickets, driver's licenses, vehicle registration documents, valuable documents such as banknotes, checks, postage stamps, credit cards, any chip cards, and adhesive labels (e.g., for product security).

[0010] A diffractive optical element is an optical element that comprises an optical grating and that shapes the beam by utilizing diffraction effects on this optical grating. In particular, a diffractive optical element is a glass substrate with microstructures applied to its surface that form the optical grating. The microstructures cause individual partial beams to have different path lengths, resulting in phase modulations that can lead to interference patterns. Amplitude is also modulated through constructive and destructive superposition. By specifically forming the microstructures, the shape and intensity of laser radiation transmitted through the diffractive optical element can be changed and adjusted, particularly with respect to a focal plane.The optically effective microstructures can be created by modulating the refractive index and / or by modulating a height profile of the surface.

[0011] A holographic recording material is any material suitable for exposing, in particular, a volume hologram. In particular, a holographic recording material can be a hologram film. Photopolymer-based hologram films are particularly suitable. However, hologram films made from silver halide, especially bleached silver halide films, can also be used. Other materials include dichromate gelatin or photorefractive crystals. In the following, it is assumed that the holographic recording material, or a hologram master or master hologram, is smooth and flat. Minor deviations to eliminate imaging errors or similar are not affected by this.

[0012] A hologram master is a pattern from which a hologram is produced. Such a hologram master can, for example, be a master hologram, a mirror, or a grating. A master hologram is a hologram master designed as a hologram. For example, a ground glass screen serves as a hologram master in the prior art. The volume reflection hologram produced from this in contact copy represents a master hologram, which can be used as a hologram master in the contact copying process to produce individualized security elements. This master hologram then also serves as the hologram master for the individually produced security elements in the contact copying process, each of which has an individualized volume hologram that is spatially selected and copied from the master hologram in the contact copying process.

[0013] A spatial light modulator (SLM) enables two-dimensional, spatially resolved illumination or irradiation of a mostly flat object with modulated intensity. This can be, for example, a DMD (Digital Micro Mirror Device) chip, an LCD (Liquid Crystal Display) transmission display, or an LCoS (Liquid Crystal on Silicon) display. What they all have in common is that they consist of a large number of SLM pixels, with each SLM pixel being able to be activated or deactivated independently of other SLM pixels (intermediate stages are also possible). This allows patterns or images to be projected by appropriately controlling the SLM pixels. Due to the free controllability, different images or patterns can also be easily generated one after the other in chronological order, for example in the form of a passport photo.

[0014] A code or pattern is individualizing if it is unique for a person or object, or for a group of people or objects from a larger total. A code that is individualizing for a group of people within the total population of a country is, for example, the city in which they live. A code that is individualizing for a person is, for example, the number on their ID card or passport photo. A code that is individualizing for a group of banknotes within the total volume of banknotes is the denomination. The serial number is what is individualizing for a banknote. Examples of non-individualizing codes or patterns are coats of arms, seals, national emblems, etc.

[0015] A basic idea of ​​the invention is to simplify the optical structure of a device for generating a hologram. To this end, the Powell lens, the scanner, other lenses, and the aperture in the beam path are omitted. Instead, a diffractive optical element is used. The diffractive optical element is designed or selected such that an exposure region corresponds to the region to be exposed to generate the hologram. Corresponding here means that the region to be exposed corresponds in terms of shape, size, or area to the part to be generated, in particular the part to be copied. However, it can also be provided that not the entire hologram master is illuminated, but only the part or region by means of which a hologram is to be generated or copied.If, for example, a circular area is to be copied, the diffractive optical element is designed such that the laser radiation emitted in the focal plane of the diffractive optical element corresponds to this circular area. If, on the other hand, a square exposure area is to be exposed, for example because the hologram to be created or copied has a square shape, the diffractive optical element is designed or selected such that the laser radiation emitted or transmitted into the exposure area corresponds to this square area. In this way, the hologram to be created, in particular to be copied, can be created or copied over the entire area in a single exposure step.

[0016] In particular, a device for generating a hologram is provided, comprising an optical arrangement, wherein the optical arrangement has at least one laser radiation source, a diffractive optical element and a master holder along an optical axis, in or on which a holographic recording material to be exposed and a hologram master can be arranged, and wherein the diffractive optical element is designed to image, in a focal plane, laser radiation emitted by the laser radiation source and transmitted through the diffractive optical element onto an incident irradiation plane in a beam path of the optical arrangement and thereby to irradiate an exposure region which corresponds to a region to be exposed for generating the hologram.

[0017] Furthermore, a method for generating a hologram is provided, comprising the following steps: arranging a hologram master and a holographic recording material to be exposed in a master holder, generating laser radiation by means of a laser radiation source, irradiating the generated laser radiation onto a diffractive optical element, wherein the diffractive optical element has been provided or selected in such a way that, in a focal plane, the laser radiation transmitted through the diffractive optical element is imaged onto an irradiation plane in a beam path of the optical arrangement and as a result, an exposure region is irradiated which corresponds to a region to be exposed for generating the hologram.

[0018] The advantage of the device and method is that a Powell lens, a scanner, a lens, and an aperture for beam shaping are eliminated. This allows for a significantly simplified device design and a simplified method, especially since the adjustment of the optical arrangement can be made less complex. Overall, costs and effort can be saved. Furthermore, the smaller number of components in the optical arrangement also reduces the susceptibility to errors, making the device and method more robust.

[0019] The specific formation of the microstructures of the diffractive optical element is determined depending on the desired position of the focal plane of the diffractive optical element and the area to be exposed or the exposure area. For this purpose, a backward calculation is performed based on the shape and size of the area to be exposed in order to form the diffracting microstructures. In other words, a specific position and shape of the microstructures on the surface of the glass carrier of the diffractive optical element are determined through simulation in such a way that the desired image of the laser radiation transmitted through the diffractive optical element is obtained in the focal plane for the incident irradiation plane, and thus in particular for the exposure area. The diffractive optical element is then manufactured by forming the calculated microstructures on the surface of a glass carrier.For this purpose, the calculated microstructures are formed through several successive etching steps. One example of a manufacturer of diffractive optical elements is HOLO / OR Ltd. in Israel (http: / / www.holoor.co.il / ).

[0020] In the simplest case, the laser radiation transmitted by the diffractive optical element is directed directly onto the exposure area. The incident irradiation plane then coincides with the plane in which the hologram is to be generated. If optical components are arranged between the diffractive optical element, the incident irradiation plane may need to coincide with another plane, and the diffractive optical element must be designed or selected accordingly.

[0021] The master holder is designed, for example, as a holding element, mount or table for receiving and / or holding the holographic recording material and the hologram master.

[0022] In particular, it may be provided that the generated or copied hologram is used as a security feature in a valuable or security document.

[0023] Depending on the shape of the hologram to be created or copied, the area to be exposed can vary in terms of shape, size, and / or exposure intensity. In one embodiment, the diffractive optical element is designed such that a beam shape and / or an intensity profile of the transmitted laser radiation is tailored to the area to be exposed.

[0024] It can be provided that a hologram is to be reconstructed or generated in the holographic recording material at a specific angle. In one embodiment, it is therefore provided that the optical arrangement comprises a mirror, wherein the mirror is designed and arranged such that the laser radiation can be imaged onto the holographic recording material to be exposed at an angle with respect to a surface normal of the latter. If such an angle is, for example, 45°, the mirror is arranged accordingly in the beam path on the optical axis between the diffractive optical element and the master holder. The alignment is then carried out such that the transmitted laser radiation reflected by the mirror strikes the holographic recording material and the hologram master at an angle of 45°.

[0025] In particular, it can be provided that the diffractive optical element is designed, manufactured, or selected in such a way that any distortion or compression / stretching of the beam profile occurring due to exposure at an angle is taken into account. If, for example, a beam shape or an intensity profile were circular after transmission through the diffractive optical element, the exposed area would be elliptical after reflection from a mirror at an angle of 45° with respect to the surface normal of the holographic recording material or the hologram master. However, this can be taken into account by appropriate consideration when designing the diffractive optical element, so that exposure with a circular beam shape or circular intensity profile can take place despite the reflection.

[0026] It may be desirable to copy holograms into a holographic recording material at different angles. One embodiment therefore provides for the mirror to be adjustable in its position and / or for at least one axis of the mirror to be rotatable, so that the angle can be changed. This allows the device to be flexibly adapted to changing requirements, i.e., changing angles, thus reducing setup and downtime of the device.

[0027] The method may include positioning the mirror and adjusting the angle.

[0028] It may be desirable to individualize holograms, for example by providing the holograms with an individualized number or an individual image. In one embodiment, it is therefore provided that the optical arrangement comprises a spatial light modulator (SLM) arranged in the beam path, wherein the spatial light modulator is designed to individualize the hologram to be generated, wherein the incident irradiation plane coincides with a coupling plane of the spatial light modulator. The SLM is then arranged behind the diffractive optical element. The laser radiation modulated by the SLM is then imaged onto the exposure area, for example via imaging optics, and individualized by appropriate control of the SLM.

[0029] In particular, in one embodiment it is provided that the spatial light modulator is a liquid crystal on silicon modulator.

[0030] In one embodiment, the diffractive optical element is designed to have a transmittance of at least 99% and an efficiency of at least 97%. This is achieved, in particular, by using at least 32 etching stages during the production of the diffractive optical element, i.e., performing at least five etching steps. This allows for a significant increase in efficiency compared to devices known from the prior art. Hologram copying can thus be carried out more efficiently and cost-effectively.

[0031] In a further embodiment, the diffractive optical element is configured to image the laser radiation emitted by the laser radiation source and transmitted through the diffractive optical element onto at least one further irradiation plane in the beam path of the optical arrangement in the focal plane, thereby irradiating at least one further exposure region that corresponds to a further region to be exposed to generate another hologram. This makes it possible to expose multiple exposure regions separately but simultaneously, thus enabling, for example, multiple holograms to be generated, in particular copied, simultaneously for different valuable or security documents.This allows, for example, multiple valuable or security documents on a multiple-use sheet to be provided with a hologram in a single step, since all holograms can be created or copied simultaneously. This simplifies the process.

[0032] It can also be provided that several different holograms are generated, in particular copied, simultaneously for a single valuable or security document. The diffractive optical element is then designed to expose several corresponding exposure areas.

[0033] The advantages of the method are the same as those of the device.

[0034] The invention will be explained in more detail below using preferred embodiments with reference to the figures. Fig. 1 is a schematic representation of a prior art device for generating a hologram; Fig. 2 shows a schematic representation of another device for generating an individualized hologram from the prior art; Fig. 3 is a schematic representation of an embodiment of the device for generating a hologram; Fig. 4 a schematic representation of another embodiment of the device for generating a hologram; Fig. 5 a schematic representation of another embodiment of the device for generating an individualized hologram; Fig. 6 a schematic flow diagram of an embodiment of the method for generating a hologram.

[0035] In Fig. 1 shows a schematic representation of a device 1 for generating or copying a hologram in contact copy from the prior art. The device 1 comprises a scanner 2 with a rotatable mirror 3 and a Powell lens 4, which generates a line profile from a laser radiation 5 with a circular intensity profile. This line profile is imaged via the rotatable mirror 3 and a lens 6 as well as a further mirror 7 onto a holographic recording material 8 and a hologram master 9 arranged underneath and designed as a master hologram. The exposure area 10, which is to be copied into the holographic recording material 8 by means of the laser radiation 5, is limited in its extent by means of an aperture 11, wherein the aperture 11 is circular in the present example.a surface exposure of the exposure area 10 is now carried out sequentially by scanning using the rotatable mirror 3 of the scanner 2, so that the linear . Fig. the laser radiation 5 is moved along a scanning direction 12 across the holographic recording material 8 and the hologram master 9. As a result, the hologram for the exposure area 10 is reconstructed in the hologram master 9 and exposed into the holographic recording material 8.

[0036] In Fig. Figure 2 shows a schematic representation of another device 1 for generating an individualized hologram from the prior art. The same reference numerals in the figures denote the same features. In contrast to the device shown in Fig. The device 1 shown in Figure 1 comprises the Fig. 2, the device 1 additionally comprises a polarizing beam splitter 14, on the top of which a spatial light modulator 15 in the form of an LCoS is arranged. The laser radiation 5, which is locally modulated by a rotation of a polarization of the laser radiation 5 and subsequent filtering by the beam splitter 14 on the basis of individualization information (e.g., a grayscale image of a person), is guided onto the holographic recording material 8 and the hologram master 9 by means of an imaging optics 16. The surface exposure takes place as in the Fig. 1 by scanning using the rotatable mirror 3 of the scanner 2. This creates the individualized hologram in the holographic recording material 8.

[0037] In Fig. Figure 3 is a schematic representation of an embodiment of the device 1 according to the invention for generating a hologram, shown in particular in a contact copy. The device 1 comprises an optical arrangement 20, wherein the optical arrangement 20 comprises a laser radiation source 21, a diffractive optical element 22, and a master holder 23. The master holder 23 is designed, for example, as a holding element or mount for receiving and holding the holographic recording material 8 and the hologram master 9.

[0038] In the embodiment shown, the hologram master 9 is in particular a master hologram.

[0039] In contrast to the devices 1 in the prior art, the device 1 in the embodiment shown has no Powell lens, no scanner, no lens and no aperture.

[0040] The diffractive optical element 22 is designed such that, in a focal plane 24, laser radiation 5 emitted by the laser radiation source 21 and transmitted through the diffractive optical element 22 is imaged onto an incident plane 17 in a beam path of the optical arrangement 20, thereby irradiating an exposure region 10 that corresponds to a region 25 to be exposed to generate the hologram. In this embodiment, the incident plane 17 corresponds to the plane in which the holographic recording material 8 is arranged. The exposure region 10 irradiated by the beam shaping of the diffractive optical element 22 therefore corresponds exactly to the region 25 in which the hologram is to be copied from the hologram master 9 into the holographic recording material 8. In the illustrated embodiment, this is a circular area. The intensity profile is preferably homogeneous (“flat-top hat” profile).

[0041] In Fig. Figure 4 is a schematic representation of another embodiment of the device 1 for generating a hologram according to the invention, particularly shown in contact copy. The device 1 is based on the device shown in Fig. 3. The same reference numerals in the figures denote the same features. In addition, the optical arrangement 20 also includes a positionable and rotatable mirror 7, via which the laser radiation 5 transmitted through the diffractive optical element 20 is imaged onto the exposure area 10 at an angle 26 to the surface normal 27 of the holographic recording material 8 or the hologram master 9. This enables exposure or copying of the hologram at an angle 26.

[0042] Fig. 5 shows a schematic representation of another embodiment of the device 1 according to the invention for generating a hologram, in particular an individualized hologram. The same reference numerals in the figures denote the same features. In addition to the laser radiation source 21, the diffractive optical element 22, and the master holder 23, the optical arrangement 20 comprises, in the beam path, a polarizing beam splitter 14, on the top of which a spatial light modulator 15 in the form of an LCoS is arranged. The laser radiation 5, spatially modulated by the spatial light modulator 15, is directed onto the holographic recording material 8 and the hologram master 9 with the aid of imaging optics 16 of the optical arrangement 20.

[0043] In this embodiment, the incident irradiation plane 17 coincides with a coupling plane 18 of the spatial light modulator 15, i.e., the diffractive optical element 22 is designed such that the focal plane 24 of the diffractive optical element 22 is located in the coupling plane 18. The laser radiation 5 transmitted through the diffractive optical element 22 is therefore imaged onto the spatial light modulator 15 or its coupling plane 18.

[0044] The laser radiation 5, which is spatially modulated by the SLM 15 or LCoS and in particular provided with individualization information, is then imaged with the aid of the imaging optics 16 onto an exposure area 10 that corresponds to a region 25 to be exposed. The exposure area 10 irradiated by the beam shaping of the diffractive optical element 22 thus corresponds exactly to the region 25 in which the hologram is to be (individualized) generated or copied. In the illustrated embodiment, this is a square area. Preferably, the intensity profile of the laser radiation 5 in the focal plane 24 or in the exposure area 10 is homogeneous over the entire area, so that uniform exposure or copying is possible.

[0045] In Fig. Figure 6 shows a schematic flow diagram of the method according to the invention for generating a hologram.

[0046] In a first method step 100, a diffractive optical element 22 (cf. e.g. Fig. 3) is selected and provided. The diffractive optical element 22 corresponds to the laser radiation 5 used, in particular with regard to a wavelength of the laser radiation 5, and in particular to the hologram to be generated, in particular to be copied, or the hologram master 9. It can additionally be provided that the laser radiation 5 is expanded in front of the diffractive optical element 22 in order to thereby make a beam profile of the laser radiation 5 transmitted through the diffractive optical element 22 more homogeneous. The diffractive optical element 22 is designed such that during subsequent exposure precisely the area in which a hologram is to be generated or copied is irradiated. The selected and provided diffractive optical element 22 is arranged in a beam path of an optical arrangement 20 of the device 1.

[0047] In a method step 101, a hologram master 9 and a holographic recording material 8 to be exposed are arranged in a master holder 23. The master holder 23 is designed, for example, as a flat table on which the hologram master 9 and the holographic recording material 8 are placed and arranged one on top of the other in a predetermined position. It can also be provided that the holographic recording material 8 is or will be laminated onto the hologram master 9. Furthermore, it can also be provided that the holographic recording material 8 is pressed and / or sucked onto the hologram master 23 so that the two are arranged as closely as possible to one another and, in particular, so that there are no air pockets between them.

[0048] In the next method step 102, laser radiation 5 is generated by means of a laser radiation source 21. The wavelength of the laser radiation 5 is matched to the hologram to be generated or the hologram to be reconstructed on the hologram master 9.

[0049] In method step 103, the generated laser radiation is irradiated onto the diffractive optical element 22 and thereby the hologram is generated in the exposure area 10, in particular copied from the hologram master 9 into the holographic recording material 8. List of reference symbols 1 device 2 scanners 3 mirrors 4 Powell lens 5 Laser radiation 6 lens 7 mirrors 8 holographic recording material 9 Hologram Masters 10 Exposure range 11 aperture 12 Scanning direction 13 linear illustration 14 polarizing beam splitters 15 Spatial Light Modulator 16 Imaging optics 17 Beam plane 18 Coupling level 20 optical arrangement 21 Laser radiation source 22 Diffractive optical element 23 Master recording 24 focal plane 25 Area 26 angles 27 Surface normals 100-103 Procedural steps

Claims

[1] Device (1) for generating a hologram, comprising: an optical arrangement (20), wherein the optical arrangement (20) has, along an optical axis, at least one laser radiation source (21), a diffractive optical element (22) and a master receptacle (23), in or on which a holographic recording material (8) to be exposed and a hologram master (9) can be arranged, and wherein the diffractive optical element (22) is designed to image, in a focal plane (24), laser radiation (5) emitted by the laser radiation source (21) and transmitted through the diffractive optical element (22) onto an incident radiation plane (17) in a beam path of the optical arrangement (20) and thereby to irradiate an exposure region (10) which corresponds to a region (25) to be exposed to produce the hologram. [2] Device (1) according to claim 1, characterized bythat the diffractive optical element (22) is designed such that a beam shape and / or an intensity profile of the transmitted laser radiation (5) is matched to the area to be exposed (25). [3] Device (1) according to claim 1 or 2, characterized by that the optical arrangement (20) comprises a mirror (7), wherein the mirror (7) is designed and arranged such that the laser radiation (5) can be imaged onto the holographic recording material (8) to be exposed at an angle (26) with respect to a surface normal (27). [4] Device (1) according to claim 3, characterized by that the mirror (7) is adjustable in its position and / or at least one axis of the mirror (7) is rotatable so that the angle (26) can be changed. [5] Device (1) according to one of the preceding claims, characterized byin that the optical arrangement (20) comprises a spatial light modulator (15) arranged in the beam path, wherein the spatial light modulator (15) is designed to individualize the hologram to be generated, wherein the irradiation plane (17) coincides with a coupling plane (18) of the spatial light modulator (15). [6] Device (1) according to claim 5, characterized by that the spatial light modulator (15) is a liquid crystal on silicon modulator. [7] Device (1) according to one of the preceding claims, characterized by that the diffractive optical element (22) is designed to have a transmittance of at least 99% and an efficiency of at least 97%. [8] A method for producing a hologram, comprising the following steps: Arranging a hologram master (9) and a holographic recording material (8) to be exposed in a master holder (23), generating laser radiation (5) by means of a laser radiation source (21), irradiating the generated laser radiation (5) onto a diffractive optical element (22), wherein the diffractive optical element (22) has been provided or selected such that in a focal plane (24) the light reflected by the diffractive optical Element (22) transmitted laser radiation (5) is imaged onto an incident irradiation plane (17) in a beam path of an optical arrangement (20) and thereby an exposure area (10) is irradiated which corresponds to an area (25) to be exposed to produce the hologram. [9] Method according to claim 8, characterized bythat the diffractive optical element (22) has been provided or selected such that a beam shape and / or an intensity profile of the transmitted laser radiation (5) is matched to the area (25) to be exposed. [10] Method according to claim 8 or 9, characterized by that the transmitted laser radiation (5) is imaged onto the holographic recording material (8) to be exposed by means of a mirror (7) at an angle (26) with respect to a surface normal (27).

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