DE102020210936A1 - Reading device for reading out holographically stored information, method for reading out holographically stored information, and reading system for reading out holographically stored information. - Google Patents

Abstract

The invention relates to a reading device (100) for reading information stored holographically in a Fourier hologram, the Fourier hologram being able to have local disturbances, e.g. scratches, and at the same time being deformed, e.g. curved. For this purpose, the read-out device (100) comprises a light source (15) for emitting a light beam, a detector (4) for detecting the light beam propagated along a beam path, a delimiting element (7) with a delimiting surface (7.1) for delimiting the beam path and a transmission surface (7.2) for transmitting the light beam propagating along the optical path, an optical active element (3) for transforming the light beam propagating along the optical path by means of Fourier transformation. Furthermore, a method for reading information stored holographically in a Fourier hologram (5) and a reading system (100) for reading information stored holographically in a Fourier hologram (5) is disclosed.

Description

The invention relates to a readout device for reading out holographically stored information, a method for reading out holographically stored information, and a readout system for reading out holographically stored information.

Holographically stored information is used in a large number of applications. Holographic information is often used in areas to prevent and detect counterfeiting of high-priced goods or banknotes. However, there are also applications in which consumer goods are provided with holographically stored information. In order to enable the holographically stored information to be read out, for example to determine the authenticity of a bank note, product or consumer good, special reading devices are required. This reading device reconstructs the holographically stored information. For example, security elements such as adhesive labels that include an optical security feature in the form of a hologram are also read by reading devices.

The information read out is usually a sequence of numbers or letters or combinations thereof. However, the information can also be a bar code or a two-dimensional code, with holographic structures being introduced into a suitable carrier using a suitable manufacturing process, which then form the hologram.

The above information is particularly suitable for machine processing. Information stored in holograms is typically read out by machine by illuminating the hologram with coherent, monochromatic light. The light is diffracted at the structures of the hologram and a holographic reconstruction is generated in the form of a wave field. This wave field then usually hits an optical detector and is detected by it.

Reading devices for reading holographically stored information are known from the prior art.

In general, the goal when storing information is to be able to read out this information under all application-specific circumstances that occur. In general, a reading system should be designed in such a way that the information can always be read reliably and correctly under all typically occurring circumstances, or at least with the highest possible recognition rate. Mistakes should be avoided. External influences can adversely affect the readout, so that the stored information cannot always be reliably read out with the readout devices known from the prior art. External influences can be local disturbances in individual areas of the hologram or caused by deformations of the hologram or its carrier.

Local disturbances of a hologram:

External influences can cause local disturbances at the points where the information is stored in a hologram. Local interference can be scratches, influences from fingerprints, etc. Parts of the stored information can then no longer be read or are completely lost. For example, scratches and other local disturbances in holographically stored information can lead to parts of the stored information being incorrectly recognized by a reading device. In order to still receive the correct, stored user data under such circumstances, the user data for the reading devices known from the prior art has been changed beforehand by an error correction encoding algorithm in such a way that the information read out may be partially incorrect and still indicate the correct user information permit. This method of error correction encoding and error correction decoding is common practice in most types of data storage. This is also used when storing holographically stored information.

Compared to other types of information storage, holography offers an additional advantage: local disturbances caused by fingerprints, in particular fingerprints of papillary lines, scratches, small amounts of dirt, dust particles, etc. typically have small local dimensions.

If the hologram is illuminated by a reading device for reading out the holographically stored information with a light beam which - compared to the local extent of the local disturbances - has a significantly larger diameter, the light is diffracted at the points of the hologram that are not disturbed, and the light is reflected or diffracted or scattered or absorbed or otherwise deflected in undesired directions at the locations affected by interference. If all of the light that leaves the hologram is directed onto a detector, then after an optical Fourier transformation, the result is that despite the local disturbances of the holo gram a reconstruction of the hologram. Under these conditions, however, the reconstruction of the hologram has a greatly reduced signal/noise ratio. The useful information contained in the holographic reconstruction is typically stored in a way encoded by an error correction algorithm, so that when the holographic reconstruction is decoded a reduced signal-to-noise ratio may result in individual errors, but these can be corrected by the error correction algorithm . In such cases, the useful information can be successfully read out.

If the hologram is illuminated by a reading device for reading out the holographically stored information with a light beam which - compared to the local extent of the local disturbances - is of the same order of magnitude, i.e. the hologram is only illuminated over a small area, or is even smaller, then statistically, in many cases the light beam hits an undisturbed area of the hologram. In such a case, a holographic reconstruction with a high, ie very good, signal-to-noise ratio is produced on the detector, and the information can be read out without errors. The disadvantage of this type of reading, however, is that if such a beam, whose local extent is in the same order of magnitude as the local disturbance or is even smaller, hits a disturbed or partially disturbed area of the hologram, there is no or almost no undisturbed area of the hologram contributes to an undisturbed holographic reconstruction. As a result, a holographic reconstruction with a low, ie very poor, signal-to-noise ratio is produced on the detector. In such cases, the error correction algorithm can also fail when decoding the information of the holographic reconstruction and the useful information can no longer be read out.

Thus, if a light beam is used which only illuminates a hologram over a small area, the small area having a local extent which is of the same order of magnitude as the local extents of local disturbances such as scratches or disturbances caused by fingerprints or the like. , or the light beam is even smaller, then the information cannot be reliably read out of a hologram with a high recognition rate as desired. Because in many cases correct information is read out, but in an undesirably high, relative number of cases, which comes close to the relative area occupancy of disturbances on the hologram, an attempt to read fails.

If no other disturbances negatively affect the system, it is therefore advantageous to illuminate a hologram on an area that is significantly larger than the local dimensions of local disturbances.

Deformations of the hologram:

This is different, however, in the case of another disruption that occurs very frequently in a typical application, in which the hologram has a different geometric shape when it is read out than it would have to have for error-free readout. In an arrangement consisting of a reading device and a hologram, the hologram represents an optical element, whereby the geometric shape of the hologram also influences the course of the light from the light source, via the hologram to the formation of the holographic reconstruction on the detector . If the diffractive, holographic structures of the hologram are designed in such a way that the desired diffractive effect of the hologram occurs when the hologram is flat and even, the holographically stored information can no longer be read in many cases when the hologram is at the time of reading has a different geometric shape. Other geometric shapes, ie deformations, are caused, for example, by bending or bulging or any other deformation of the hologram. In general, a deviation in the shape of the hologram at the time of reading in relation to the shape for which the hologram was produced has a negative effect on the quality of the holographic reconstruction on the detector. In most applications, the hologram is manufactured in such a way that the readout works best when the hologram is flat and planar at the time of readout. If it is then deformed at the time of reading, holograms can usually only be read with errors or not at all by reading devices. This is very disadvantageous and limits the possible uses of holograms. This applies in particular to Fourier holograms.

The reason for the sensitivity of Fourier holograms with regard to deformations such as bending, bulging or all other deformations of the hologram is that the optical Fourier transformation required for the reconstruction of the hologram has the property of light rays depending on their angle of incidence in an optical active element different locations, for example on a detector. If a Fourier hologram is deformed, no holographic reconstruction with sharp contours or even no holographic reconstruction at all is produced, whereby the information stored in the Fourier hologram can only be recovered with great effort or not at all. Even minor deformations, in particular slight curvatures of a Fourier hologram or unevenness in the surface of the hologram, mean that a Fourier hologram can no longer be read. Although the holographic carrier can be applied to dimensionally stable materials in order to prevent deformation of a Fourier hologram after the information has been introduced into a carrier, this does not guarantee that the Fourier hologram will not later be damaged by external influences, e.g. B. mechanically or thermally deformed. This is very disadvantageous and limits the possible uses of Fourier holograms.

This property is particularly disadvantageous when using Fourier holograms as a security feature or as a security element on packaging or products themselves. Holograms are increasingly being used for such applications. Product packaging fulfills, among other things, a protective function against mechanical damage to the product it contains. It can happen that the packaging is exposed to strong curvatures or deformations, as a result of which a Fourier hologram located on it can also be deformed, making it illegible. The shape of products or product packaging itself poses a problem if a Fourier hologram is to be applied directly to this(s), since products and product packaging, such as e.g. B. plastic bottles, coffee capsules, cream tubes or containers, etc., are often not flat, but curved or rounded or uneven. On such items, the use of Fourier holograms is severely limited. A fortiori it cannot be ensured that a Fourier hologram applied to packaging can be read if the packaging is not dimensionally stable. Examples of such packaging are tubular bag packaging, blister packaging, or the like. If Fourier holograms are applied to such packaging, a holographic reconstruction can usually no longer be reliably produced, which is why the information stored cannot be reliably recovered. For this reason, Fourier holograms are rarely used to store information on such packaging. With others

At best, it would be possible to read out deformed Fourier holograms with readout devices of the prior art using very complex readout devices that could determine the deformation and have devices that enable adaptive adaptation to the determined deformation.

In order to reduce the negative effects of warpage and/or deformation on the reading process, a light beam that illuminates a hologram over only a small area can be used. In the case of existing curvatures and deformations of the hologram, it applies that a small illuminated area enables a better reading result than is the case with a larger illumination of a deformed hologram. This inevitably leads to the disadvantages mentioned above if the hologram is locally disturbed, since information can no longer be read from the disturbed areas of the hologram.

In summary, it can be stated that if a hologram has a local disturbance, it is advantageous for a light beam to illuminate the largest possible area of the hologram. However, this is precisely counterproductive and particularly disadvantageous if the hologram is also deformed. In order to read out a deformed hologram, it is advantageous for a light beam to illuminate the smallest possible area of the hologram. Accordingly, it is hardly possible with the readout devices known from the prior art to read out holographically stored information from a Fourier hologram which has both a local disturbance and is deformed. In the known readout devices, only one of the problems is usually addressed. These readout devices fail either in the event of local disturbances or in the event of a hologram being deformed.

In the pamphlet EP 2 463 645 A1 describes a reading system using a reading beam with a very small diameter of only 0.4 mm to 0.8 mm. The pamphlet EP 2 463 645 A1 further describes that a second reading beam and a second detector are used due to local disturbances occurring in areas of the hologram, and the hologram is illuminated at two different locations in succession in order to increase the probability of a correct reading result. Due to the only small illuminated areas of the hologram, the reading system described is suitable for achieving a correct reading result in the event of any curvatures, but due to the small area of the hologram that is illuminated, it requires a second reading beam, since in some cases local interference has lateral extensions in the same order of magnitude as the illuminated area of the hologram, and reading out the stored information would therefore fail. These The solution is disadvantageous in that it is very complex and expensive since two reading beams are required.

From the pamphlet DE 10 2006 012 991 A1 describes a method and a device for optically reading out information from an information carrier by means of a card swipe system. Information elements in the form of holographic diffraction structures are stored on an information carrier. The information carrier according to the invention can be read out by means of a manual run through a reading device. A disadvantage of the device and the method described is that the information carrier on which the information is stored must be flat in order to be able to read out the information.

The invention is therefore based on the object of providing a readout device and a method with which information stored in a Fourier hologram can be read out. It should be possible to read out both when a Fourier hologram is deformed after a manufacturing process, z. B. is curved or has a different type of deformation as well as when a Fourier hologram has local disturbances.

In addition, it is an object of the invention to provide a method for reading information stored holographically in a Fourier hologram, with which reading is possible both when a Fourier hologram is deformed after a manufacturing process, e.g. B. is curved or has a different type of deformation as well as when a Fourier hologram has local disturbances.

Disclosure of Invention

The object of the present invention is achieved by the readout device according to claim 1. In addition, the object according to the invention is achieved by the method according to claim 12. Furthermore, the object according to the invention is achieved by a readout system according to claim 16. Advantageous developments result from the subclaims.

On the one hand, the invention is based on the knowledge that information can still be read from a Fourier hologram even if this has local disturbances. In the context of this description, local disturbances are to be understood as optical influences caused by fingerprints, in particular impressions of the papillary lines, scratches, small dirt particles, dust particles, paint particles, adhesive particles or the like. In particular, a local disturbance should be understood to mean an impairment of the hologram that affects a part of the hologram to the extent that the hologram can no longer be reliably read at this point. In contrast to two-dimensional graphic images, such as e.g. B. barcodes or photos, the property that the stored information is known to be stored distributed over the entire surface of a hologram.

Among other things, the invention is based on the finding that it is disadvantageous if the extent of the light beam with which the hologram is illuminated is small, and this is, for example, of the same order of magnitude as the local extent of the local disturbances, or even smaller . A holographic reconstruction with a small, ie very poor, signal-to-noise ratio is then produced on the detector. It would then not be possible to read out the information. The reading device according to the invention therefore provides for illuminating a partial area, preferably illuminating the entire Fourier hologram. This can improve the signal-to-noise ratio. Local disturbances are then less important.

In order to solve the problem that deformed Fourier holograms, which are illuminated with an extended light beam that is many times larger than typical local disturbances, can also be read out, the invention provides that the delimiting element along the Beam path propagating light beam limited by the boundary surface. The cross-sectional area of the light beam reflected and diffracted by the hologram is thus limited.

In this respect, the invention is based on the further knowledge that in the reading device according to the invention, the reading out of deformed or disturbed Fourier holograms can be improved by limiting the beam path using a limiting element. From a practical point of view, limiting the beam path appears to be disadvantageous, since as a result the light intensity with which the holographic reconstruction is generated on the detector only comprises a fraction of the light beams incident on the illumination surface. As a result, it is to be expected that the signal-to-noise ratio will be reduced and consequently detection of the holographic reconstruction will be made more difficult.

However, it has been found that it is extremely advantageous if the portion of the diffracted light beams that are Fourier transformed is limited, ie fewer light beams are used to generate the holographic reconstruction in that the transmission surface of the delimiting element is designed to be smaller in terms of surface area than the illumination surface. Because although the light intensity with which the holographic reconstruction is generated on the detector is reduced, the signal-to-noise ratio of the reconstruction on the Fourier hologram improves as a result of the limitation.

By limiting the light beams, it is achieved in the present reading device that only a few light beams, which are deflected at a similar angle by a deformed Fourier hologram, hit the optical active element and are Fourier transformed by it. Since the optical Fourier transformation has the property that angles of incidence in an optical active element result in a local displacement, the limiting element ensures that the reconstructions of the Fourier hologram superimposed on the detector have a small spatial offset relative to one another. Otherwise, the deflected light beams, which are imaged at different locations on a detector by the optical Fourier transformation, would result in a large number of reconstructions being superimposed, which are all arranged in a spatially offset manner in relation to one another. This would only produce a random light pattern on the detector, but not a reconstruction of the hologram. In a figurative sense, the image would then be blurred.

However, if a large number of the light beams are filtered out by the limiting element or better blocked or absorbed, it becomes possible to read out the information stored in a deformed Fourier hologram, since the contour sharpness of the reconstruction of the hologram produced on a detector is improved. The signal-to-noise ratio of the reconstruction is improved, i.e. greater. The light rays that are not intercepted by the delimitation element but pass through the transmission surface of the delimitation element still contain all the information necessary to recover the information stored in the hologram. This is due to the property of the Fourier hologram, which has already been described and is known to those skilled in the art, that the information stored in the hologram is stored in a spatially distributed manner over the entire hologram. As a result, useful information, which is part of the information stored in the hologram and is comprised by it, can also be recovered from a small number of light beams diffracted at the Fourier hologram. As already mentioned, the use of the delimitation element means that the intensity of the holographic reconstruction of the hologram imaged on the detector is lower compared to the case where no delimitation element is used. Detection is still possible. Any optical detector that can convert a local intensity distribution into electrical signals is suitable as a detector for detecting the light beam propagated along a beam path. It is preferably adapted to the wavelength range of the light source and is particularly sensitive in this wavelength range. The detector can be sensitive over a wide range of wavelengths. Preferably the detector is a pixel-based detector such as an active pixel sensor or a CCD sensor. These detectors are sensitive enough to detect even weak light signals and convert them into electrical signals.

The device according to the invention thus has the advantage that the information stored in a Fourier hologram can be read despite local disturbances, and it also has the advantage that reading is also possible when the Fourier hologram is deformed. Furthermore, the device has the advantage that, for a readout, no adjustments or changes need to be made to the readout device in order to adjust it to a local disturbance or a deformation of a Fourier hologram or to compensate for this. In other words, the reading device comes without moving elements, such as. B. a focusing device. From an application point of view, this offers decisive advantages and improves the reliability of the readout device, since the moving parts that are not required can neither wear out nor jam. The readout device according to the invention represents a considerable improvement in that adaptive adjustments are not required and the readout device also has no moving parts.

In order to achieve the previously described effect of readability of deformed Fourier holograms, it is preferable for the transmission area to be smaller than the illumination area. If the transmission surface is smaller than the illumination surface, only a portion of the diffracted light beams ever passes through the transmission surface and the signal-to-noise ratio of the reconstruction is improved.

Any light source that emits narrow-band coherent light beams is suitable as a light source for emitting a light beam. Light beams with these properties are particularly suitable for being diffracted on Fourier holograms in order to then be imaged on a detector. Such light sources can be lasers or laser diodes. The advantage of lasers or laser diodes is that they can provide light beams with a high intensity and the out passing light beam has the aforementioned properties. In this way, Fourier holograms can be illuminated particularly brightly. However, light-emitting diodes, ie LEDs, which can also emit in a narrow spectral range and can emit light beams with sufficient intensity, are also suitable as light sources. The spectrum of the light source is preferably in the visible wavelength range. However, the device according to the invention and the method according to the invention are expressly not limited to the visible wavelength range. In particular, the wavelength range of infrared light and the wavelength range of UV light are also preferred. The advantage of these wavelength ranges is that light rays are imperceptible to the human eye. Thus, people are not dazzled or otherwise irritated by the readout device.

The illuminated area of the Fourier hologram should be understood as meaning an arbitrarily shaped area that is illuminated by the light source. In particular, the illuminated surface can also step out of a plane, ie not lie completely in a plane, if it is curved, for example, has a spherical shape or is very uneven, for example “wrinkled”. In other words, the illuminated area can take any shape. This is the case, for example, when the Fourier hologram, ie at least the holographic carrier of the Fourier hologram, is or becomes deformed, for example wrinkled or deformed. The illuminated area may be formed on part of the illuminated Fourier hologram, preferably the entire Fourier hologram is illuminated by the light source.

In the context of this invention, diffracted light beams are understood to mean light beams which are diffracted at the structures written into the holographic carrier of the hologram. The effects of diffraction are known to diffract light rays below the zero order, the first orders, and higher orders. For purposes of this invention, the diffracted light rays emanate from the illuminated area of the hologram. The diffracted light rays have specific diffraction angles.

A delimiting element with a delimiting surface for delimiting the beam path and a transmission surface for transmitting the light beam propagating along the beam path should be understood to mean any element which delimits a beam path and can also transmit light beams. Limiting can in particular be understood to mean a reduction in a cross section of a light beam. The confinement element can be made of a metal, a plastic or any other opaque material. The boundary surface of a boundary element is designed to absorb or reflect part of the light beams in the beam path. The boundary surface can have a round, oval, square or rectangular shape. But any other shape is also possible. The transmission surface of a delimiting element is designed to transmit part of the light beams located in the beam path. In this case, the transmitted light beams are preferably not influenced by the transmission surface. It is particularly preferred that the transmission surface is formed in the boundary surface. It is also preferred that the boundary surface and the transmission surface lie in one plane. In particular, it is preferred that the intensity and/or direction of the light beams are not influenced by the transmission surface. The transmission surface can be formed by a recess or an opening in a material or body. For example, it can be a passage opening in a metal sheet or a plastic plate. The transmission surface can have a round, oval, square or rectangular shape. But any other shape is also possible.

A transmission surface is to be understood as that surface of a delimiting element which does not block or absorb the beam path of the diffracted light beams, but through which the diffracted and reflected light beams pass. For this purpose, the delimiting element is inserted in the beam path, being arranged along the beam path in front of the optical active element or being formed by the optical active element itself. In other words, the diffracted and reflected light first passes through the transmission surface of the delimiting element and then enters the optically active element.

An optical active element for transforming the light beam propagating along the beam path by means of a Fourier transformation is to be understood as an optical element which is designed in such a way that it carries out an optical two-dimensional Fourier transformation. The optical active element influences the beam path of the diffracted light beams. In particular, an active element can break, bend, reflect or transmit light rays. For this purpose, the active element has at least one active surface, which influences the beam path accordingly. It is particularly preferred that the active element is a lens, in particular a converging lens or a Fourier lens. This allows a particularly simple optical structure to be implemented with which a two-dimensional optical Fourier Trans formation can be carried out. The lens can also be a plano-concave lens or a lens system. The detector is arranged in such a way that the light-sensitive surface of the detector is in the Fourier plane of the lens, ie in that plane in which a focal point or focal point of the lens lies. The Fourier plane of the lens is also often called the focal plane. In this focal plane, the holographic reconstruction of the Fourier hologram is generated as a two-dimensional, locally distributed intensity curve, ie a two-dimensional pattern of light. Alternatively, an active element can be a mirror element, in particular a concave mirror, since a mirror element, such as a concave mirror, can also produce a two-dimensional Fourier transformation.

The active optical element can also consist of more than one optical element; in particular, an active optical element can be an arrangement of a plurality of lenses and/or mirror elements. However, it is essential that the optical active element can generate an optical Fourier transformation of the light beams.

An optical active element can also be a lens array, with the lens array comprising a plurality of lenses arranged next to one another. At least one of the lenses of a lens array transforms the light beam propagating along the beam path by means of a Fourier transformation in order to generate at least one holographic reconstruction of the Fourier hologram on the detector. Preferably, each of the lenses in the lens array is configured to perform an optical Fourier transform and generate a holographic reconstruction of the hologram on the detector. As a result, a number of optical Fourier transformations can be carried out and a number of holographic reconstructions of the hologram can be generated on the detector, which can be processed further by means of image processing or can be calculated. The lenses, in particular a plurality of converging lenses, in the lens array can be arranged next to one another in rows, in a matrix or in a honeycomb pattern. Also multiple lenses

A Fourier transformation should be understood to mean a two-dimensional optical Fourier transformation. The active surface of the optical active element can be designed in such a way that diffracted light beams are deflected in such a way that the holographic reconstruction is generated on the detector surface. This holographic reconstruction represents, for example, a two-dimensional data field, a bar code or digits and/or letters as a two-dimensional intensity distribution on the detector surface.

The readout device can advantageously be further developed in that the optical active element comprises an active surface and the transmission surface is smaller than the active surface. An effective surface is to be understood as that surface through which the diffracted light rays impinge on the optical effective element. The active surface is thus an optically active surface of the optical active element.

It may be preferred that more than one delimiting element is used. In particular, it can be advantageous that a first delimiting element is arranged before the optical active element along the optical path of the propagating light beam and a second delimiting element is arranged after the optical active element along the optical path of the propagating light beam. The aforementioned advantages in relation to the arrangement of the limiting element can then be combined in the readout device.

The readout device can advantageously be further developed in that the transmission area is less than 50% of the illumination area, the transmission area is preferably less than 20% of the illumination area, the transmission area is particularly preferably less than 10% of the illumination area, the transmission area is more particularly preferably less than 5% of the lighting area. The smaller the transmission surface is in relation to the illumination surface, the more pronounced is the effect of the angular selectivity produced by the limiting element when the Fourier hologram is deformed. The smaller the transmission surface is in relation to the illumination surface, the more the Fourier hologram can be deformed. A transmission area that is less than 5% of the area of the illumination area thus enables imaging of a Fourier transform of a particularly severely deformed Fourier hologram. A very strong deformation within the meaning of this invention is, for example, a curvature of a Fourier hologram with a radius of 15 centimeters to 30 centimeters. The curvature can be convex or concave. However, these values are by no means to be understood as limit values, but are chosen purely as examples.

The readout device can advantageously be further developed in that the delimiting element is a diaphragm or is formed by an aperture of the optical active element. A diaphragm is to be understood as an optical diaphragm. An optical stop is a device used to limit the cross-section of light beams in an optical system. Apertures have an area that transmits light rays and thus a trans forms mission surface, and an area that absorbs or reflects light rays and thus forms a boundary surface. The portion of the diffracted light rays can be limited by an optical diaphragm. An optical screen preferably consists of opaque material and absorbs the non-transmitted portion of the light beams in the area of the boundary surface. Alternatively, the delimiting element can also be determined by the aperture of an optical element. The aperture meant here, which describes an opening width of an optical element, should not be confused with a numerical aperture of a lens, which describes the ability of an optical element to focus light rays. The aperture of a lens describes the free opening or that diameter of an optical element through which light beams can transmit, which is to be understood as a transmission surface within the meaning of this invention. In this case, the optical active element thus comprises a delimiting element, with the transmission surface of the delimiting element being formed by the active surface of the optical active element. Those areas through which light beams cannot transmit are to be regarded as boundary surfaces within the meaning of this invention. The aperture of a lens is often determined by the edge of the lens or by the lens mount into which a lens is inserted. If the delimiting element is formed by the aperture of a lens, the delimiting element corresponds to the optical active element and the transmission surface essentially corresponds to the active surface. If the active optical element is a mirror element, the transmission surface should be understood to mean the mirrored surface and the boundary surface should be understood to mean non-reflecting areas of a mirror element and/or areas of a mirror mount. Using the aperture of a lens or a mirror element as a delimiting element therefore has the advantage that no separate delimiting element has to be provided, since the lens or the mirror element is both an optical active element and also acts as a delimiting element. This simplifies the mechanical and optical construction of the readout device according to the invention.

It is particularly preferred that the detector and the optical active element are included in a camera module. It is particularly advantageous if the detector is implemented by at least one CCD sensor or a CMOS sensor and the optical active element is implemented by at least one converging lens or a lens system of the camera module. It is further preferred that the delimiting element is implemented through the aperture of the converging lens or the lens system of the camera module. Alternatively, it can also be preferred that the camera module additionally includes a screen that implements the delimiting element.

The readout device can advantageously be further developed in that the readout device additionally comprises a housing with at least one housing wall and the at least one housing wall comprises a delimiting element, the delimiting surface of the delimiting element being formed by the at least one housing wall and the transmission surface of the delimiting element being formed by an opening in the at least a housing wall is formed. This is advantageous since a passage opening in a housing can act as a delimiting element. The opening of a passage opening thus corresponds to the transmission area of a delimiting element. The use of a passage opening as a delimiting element has the advantage that the delimiting element is formed from a housing that may be present anyway. This simplifies the mechanical and optical construction of the readout device according to the invention. In the context of this invention, a housing is to be understood as a closed or non-closed structure which has at least one surface which includes a passage opening. The passage opening can also have an optically transparent medium, so it does not have to be open. Possible transparent media can be a transparent or partially transparent plastic pane, a transparent or partially transparent glass or a transparent or partially transparent film. The use of an optically transparent or partially transparent medium can also prevent dust, moisture, particles or other foreign bodies from penetrating the housing and negatively influencing or preventing the reading of the information stored in a Fourier hologram.

Likewise, it can be preferred that the passage opening comprises the optical active element. As a result, on the one hand, the installation space of the readout device according to the invention can be reduced and, on the other hand, the same advantages can be achieved as through the use of an optically transparent medium. In addition, the constructive mechanical structure is simplified since the housing can be designed as a holding device for the optical active element. In this case, the transmission surface can be determined by the aperture of the optical active element or by the surface of the passage opening.

The readout device can advantageously be further developed in that the optical active element comprises a lens array and at least one first lens a first part of the light beam propagating along the beam path by means of a Fourier transform and a second lens transforms a second part of the light beam propagating along the beam path by means of a Fourier transform, and the detector is adapted to detect the first part and the second part of the light beam propagating along the beam path. Thus, multiple holographic reconstructions can be generated on the detector. This can have the advantage that these multiple holographic reconstructions can be offset against one another.

The readout device can advantageously be further developed in that the readout device additionally comprises an evaluation unit, the evaluation unit being designed to process detector signals of the detector. The detector signals can be processed by an evaluation unit and information stored holographically in the Fourier hologram can be read out and made available for further processing. An evaluation unit can preferably derive position markers, useful data, hologram parameters or other information from the holographic reconstruction and make them available for further processing.

The readout device can advantageously be further developed in that the evaluation unit comprises an image processing unit and/or a channel decoding unit and/or an error correction unit. For example, an image processing unit can localize position markers in the holographic reconstruction.

The holographically stored information can be read out more easily by means of such marker structures. A channel coding added during the generation of the two-dimensional data field can be decoded by means of a channel decoding unit. Error correction of the user data can be carried out by an error correction unit. An error correction code was used to generate the two-dimensional data field in order to provide the user data with an error correction. In other words, the user data was encoded using an error correction algorithm. For example, cyclic codes such as Reed-Solomon codes or BCH codes can be used in the error correction unit. However, other cyclic codes can also be used in the error correction unit F. Depending on the error correction code used, error correction can correct single or multiple errors.

According to a second aspect, the invention relates to a method for reading out information stored holographically in a Fourier hologram according to claim 13.

Regarding the advantages of this method, reference is made to the previous statements, which explain the advantages and properties of the readout device according to the invention.

The method can be further developed in that, in a further step, detector signals generated by the detector are processed by an evaluation unit. Regarding the advantages of this further development, reference is made to the previous explanations, which explain the advantages and properties of the readout device according to the invention.

The method can be further developed in that the processing includes image processing and/or channel decoding and/or error correction. Regarding the advantages of this further development, reference is made to the previous explanations, which explain the advantages and properties of the readout device according to the invention.

According to a third aspect, the invention relates to a readout system for reading out information stored holographically in a Fourier hologram according to claim 16.

• 1 (a) - 1 (b) zeigen schematisch das Auslesen von holographisch abgelegten Informationen mit einer Auslesevorrichtung gemäß dem Stand der Technik.
• 2 stellt schematisch die Erzeugung eines Fourier-Hologramms aus Nutzdaten und das Auslesen der Nutzdaten aus einer holographischen Rekonstruktion dar.
• 3 zeigt schematisch das Auslesen von holographisch abgelegten Informationen mit einer Auslesevorrichtung gemäß einer beispielhaften Ausführungsform der vorliegenden Erfindung.
• 4 (a) - 4 (b) zeigen schematisch das Auslesen von holographisch abgelegten Informationen mit einer Auslesevorrichtung gemäß einer beispielhaften Ausführungsform der vorliegenden Erfindung unter Berücksichtigung der nullten und ersten Beugungsordnung des von dem Fourier-Hologramm reflektierten und gebeugten Lichts.

Further details, features and advantages of the invention result from the figures and their descriptions of the figures. The figures only illustrate exemplary embodiments of the invention, which do not restrict the essential idea of the invention.

• 1 (a) - 1 (b) show schematically the reading of holographically stored information with a reading device according to the prior art.
• 2 shows schematically the generation of a Fourier hologram from user data and the reading of the user data from a holographic reconstruction.
• 3 shows schematically the reading out of holographically stored information with a reading device according to an exemplary embodiment of the present invention.
• 4 (a) - 4 (b) show schematically the reading of holographically stored information with a reading device according to an exemplary embodiment of the present invention, taking into account the zeroth and first order of diffraction of the light reflected and diffracted by the Fourier hologram.

In the figures, the same elements are always provided with the same reference symbols.

1 (a) and 1 (b) 12 schematically show the reading out of holographically stored information with a reading device 200 according to the prior art. In the 1 (a) and 1 (b) are shown to illustrate the problems of the prior art, the courses of the light rays according to geometric optics. The illustration relates to a section through the reading device 200. A Fourier hologram 5 arranged on the surface 1 of an object, in which information is stored holographically, is illuminated by the light source (not shown) with the incident light beams 2.1. The Fourier hologram 5 is, for example, a reflection hologram. The incident light beams are directed onto the Fourier hologram 5 by a beam splitter cube (not shown) (indicated by an arrow pointing upwards in the direction of the object). The area illuminated by the incident light beams 2.1 forms an illuminated partial area 1b of the hologram. The incident light beams 2.1 are reflected on the Fourier hologram 5 and diffracted and reflected on the holographic structures introduced into the holographic carrier.

The light beams 2.2 diffracted by the Fourier hologram are refracted by a lens 3, the lens plane of which is located at a distance 1a from the surface 1, and deflected onto the detector 4.

The distance between detector 4 and lens 3 is the detector surface distance 1d.

In 1 (a) a Fourier hologram 5 is arranged on a flat surface 1 . The Fourier hologram is undistorted and has the same shape it was intended to have at the time of manufacture - here flat and planar. The light beams 2.2 diffracted at the illuminated partial area of the Fourier hologram impinge on the effective area of the lens (cf. 1 (b) , effective area 1c). The lens generates the Fourier transform of the diffracted light beams 2.2. The detector 4 is arranged in such a way that the Fourier-transformed diffracted light beams produce a holographic reconstruction on the surface of the detector 4 . This is the case since the detector surface distance 1d corresponds to the focal length of the lens 3 and the surface of the detector 4 lies in the Fourier plane or focal plane of the lens 3 . This effect is known from Fourier optics and will not be explained further at this point. The Fourier transform is a two-dimensionally expanded pattern of light. This light pattern has locally varying light intensities. In this arrangement, the holographic information stored in the Fourier hologram can usually be read out without any problems.

In 1 (b) the surface 1 on which the Fourier hologram 5 is arranged was curved. The Fourier hologram is severely deformed as a result. Thus, at the time of reading, the hologram is no longer flat and planar and has a different shape than that intended for reading. The deformation shown here is only an example and is shown as a bulge for easier explanation. The deformation can also be spherical or any other shape that is not planar. Due to the deformation of the hologram, the diffracted light beams 2.2 emerge in different directions from the illuminated partial area of the Fourier hologram 1b. Due to the deformation, the reflected and diffracted light experiences a change in angle (cf. 1 (b) diverge of the light rays 2.2, 2.2.0). Due to the deformation of the surface 1, the diffracted light beams 2.2 no longer hit the lens plane of the lens 3 perpendicularly everywhere, but at different angles. Because the optical Fourier transformation means that light beams entering a lens are imaged at different locations in the Fourier plane (focal plane) depending on their angle of incidence, no holographic reconstruction of the hologram is generated on the surface of the detector. Although the surface of the detector 4 is still in the Fourier plane of the lens 3 and the lens continues to produce the Fourier transform of the diffracted light beams 2.2, the deformation of the hologram no longer produces a holographic reconstruction on the surface of the detector. The information stored in the hologram can therefore not be read out.

In 2 the sequence of how holographic structures 18 are calculated from useful data 6 and how useful data 6′ can be read out again from the holographic reconstruction 9 is shown schematically.

In order to generate a Fourier hologram 5, 18', useful data 6, e.g. B. binary data with an error correction code (error correction coding) in order to then carry out a channel coding (channel coding) of the data (both steps not in 2 shown). A two-dimensional data field 17 is then generated, which includes the user data 6 provided with error correction and channel coding. This two-dimensional data field 17 is transformed by means of a Fourier transformation (no optical Fourier transformation here) in order to calculate the structuring 18 of a hologram and to provide the structuring template. The Fourier transformation is a two-dimensional Fourier transformation. This structuring template produced in this way is then transferred into a holographic carrier by an embossing method or by a laser-lithographic writing method, and the written structuring 18' is provided. For this purpose, 18 structures are introduced into a holographic carrier according to the structuring template. These structures can be written as local refractive index or layer thickness variations or they can be absorption variations in the support. A Fourier hologram 5, 18' is then provided in which information is stored holographically.

In order to read out the information 6 stored in the Fourier hologram 5, 18', the Fourier hologram is preferably illuminated with narrow-band light, as a result of which the light is diffracted at the structures that have been introduced into the holographic carrier. The diffracted light is then Fourier transformed in order to generate a holographic reconstruction 9 of the two-dimensional data field 17 on a detector. In this case, a two-dimensional local intensity distribution is generated on the surface of the detector 4 in the pattern of the reconstruction 9 . The detector 4 converts this local intensity distribution into electrical signals and feeds them to a channel decoding unit K, which decodes the channel coding. The resulting decoded information is fed to an error decoding unit F, which corrects errors in the user data. The error decoding unit F then provides error-corrected user data.

In 3 shows the reading device 100 according to the invention for reading out information stored holographically in a Fourier hologram 5 according to an exemplary embodiment. The Fourier hologram 5 is on a flat surface 1, z. B. a cardboard box arranged. The Fourier hologram 5 is not deformed and has the same shape that was intended for reading during production, namely flat and level. The light source 15 is a narrow-band emitting LED and illuminates the Fourier hologram 5 almost over its entire surface, as a result of which the illuminated partial surface 1b of the Fourier hologram 5 is produced. The light beams emerging from the light source 15 are not shown for the sake of clarity; however, the light source illuminates the Fourier hologram 5 almost completely, ie over a large area. The light beams 2.1, 2.2 and 2.2.0 are shown parallel to one another for better understanding, but this does not mean that the incident light beams 2.1 actually run parallel. The incident light beams 2.1 are diffracted at the structures written in the Fourier hologram 5. Shown here are only diffracted light beams 2.2 that were diffracted below the zeroth order 2.2.0, that is, were reflected. The delimiting element 7 is inserted into the beam path 2.2, 2.2.0 and arranged in front of the optical active element 3, which is realized here by a lens. The delimiting element 7 is implemented here by an optical diaphragm. A large part of the light beams 2.2.0 diffracted at the structures inscribed in the Fourier hologram 5 are absorbed by the boundary surface 7.1. Only a fraction of the diffracted light beams 2.2 are transmitted through the transmission surface 7.2 of the diaphragm 7 and propagate further along the non-limited part of the light beam 2.4. The lens 3 performs an optical Fourier transformation and since the surface of the detector 4 is in the Fourier plane of the lens 3, ie the detector surface distance 1d corresponds to the focal length of the lens, a holographic reconstruction is generated on the surface of the detector 4.

The reconstruction 9 (cf. 4 ) of the information stored in the hologram is generated and the reconstruction 9 is imaged on the detector 4 with sharp contours. The reconstruction 9 is converted into data signals by means of the detector, which is, for example, an active pixel sensor (CMOS sensor) or a CCD image sensor. The data signals are fed to a channel decoding unit K, which decodes a channel coding of the information stored in the hologram. For example, certain arrangements of data points in a two-dimensional data field can be understood as channel coding. In order to correct any errors in the reconstruction 9, the output signals of the channel decoding unit K are subjected to an error correction in an error correction unit F.

As in 2 described, an error correction code is already "encoded" into the user data when a two-dimensional data field is generated, i.e. the user data is encoded by an error correction algorithm. This coding is used in the error correction unit F to detect any errors in the user data and, if there are errors, to correct them. In the error correction unit F, cyclic codes such as Reed-Solomon codes or BCH codes are used, for example. However, other cyclic codes can also be used in the error correction unit F. Depending on the error correction code used, error correction can be used to detect and correct one or more errors. The error correction unit F then provides the error-corrected user data for further processing in a system.

4 (a) and 4 (b) 12 show, in another representation, the reading out of holographically stored information with the reading device 100 according to the invention according to an exemplary embodiment. The figures schematically represent diffracted light beams 2.2.1, which emanate from three different areas A, B and C from the illuminated partial area of the Fourier hologram 1b. The incident light rays 2.1 are not shown for the sake of clarity, but they strike as in 3 written on the surface of the Fourier hologram 5. 4 (a) 12 shows various exemplary optical paths of diffracted light rays (2.2.0, 2.2.1', 2.2.1'') in the case that the surface 1 has no deformation and the Fourier hologram 5 is not deformed. the 4 (b) represents the reading in the case that the surface 1 of the object is convexly curved, ie deformed.

In 4 (a) beam paths of diffracted light beams 2.2.0, 2.2.1' and 2.2.1" are shown, which emanate from different locations A, B and C and which are diffracted on the Fourier hologram 5 in area A, in area B and in area C . At each of the areas, the incident light rays (not shown) are also diffracted at the inscribed structures of the hologram (5), whereby at each of the areas reflected and diffracted light of the first order 2.2.1' and 2.2.1" and higher orders (not shown) is reflected by the Fourier hologram 5.

Due to the fact that the effective surface 1c, here the optically effective surface of the optical active element 3, i.e. here the lens, is small in relation to the illuminated surface of the Fourier hologram 5, i.e. the illuminated partial surface of the Fourier hologram 1b, only a fraction the diffracted light beams 2.2.0 and 2.2.1' and 2.2.1'' emanating from the illuminated partial area of the Fourier hologram 1b, namely the light beams from the areas A, B and C are transmitted through the transmission area 7.1 of the delimiting element 7 and spread out further out in the now limited beam path. In the exemplary case shown, zero-order diffracted light beams 2.2.0 from area A, first-order light beams 2.2.1" from area B and first-order light beams 2.2.1' from area C reach the active element 3 (lens) and are Fourier-transformed by this A holographic reconstruction 9 is thereby generated on the detector 4, which is located in the Fourier plane of the optical active element 3. The diffracted light from the areas A, B and C are thus superimposed in the lens plane In this case, light beams 2.2.0 diffracted to the zero order from regions B and C do not reach the optical active element 3 (lens).

In 4 (b) is the in contrast to in 4 (a) shown surface 1, not flat, but deformed. In the 4 (b) Also shown are beam paths of light beams 2.2.0 and 2.2.1 diffracted at the Fourier hologram 5, which emerge from various exemplary deformed areas A', B' and C' from the illuminated partial area 1b of the Fourier hologram. At each of the deformed areas A', B' and C', the incident light rays (not shown) are diffracted at the inscribed structures of the hologram, whereby zero-order light diffracted by each of the deformed areas A', B' and C' produces 2.2. 0 as well as first-order diffracted light 2.2.1' and 2.2.1" and higher orders (not shown). For the sake of clarity, only the zeroth-order diffracted light beams 2.2.0 from area A' are part of the first-order diffracted light beams 2.2.1" from the area B' and a part of the first-order diffracted light beams 2.2.1" from the area C'. The deformed areas A', B' and C' essentially correspond to the areas A, B and C from 4 (a) , with the difference that they are deformed by the deformation of surface 1. Due to the deformation of the hologram, the diffracted light beams 2.2.0, 2.2.1′ and 2.2.1″ emerge from the illumination surface 1b at an angle. Because of the diaphragm 7, only a fraction of the diffracted light beams 2.2.0, 2.2.1″ are transmitted 'and 2.2.1" through the transmission surface 7.1 of the diaphragm 7 in order to spread further in the now limited beam path.

Due to the deformation of the surface 1, the diffracted light beams 2.2.0, 2.2.1′ and 2.2.1″ also no longer hit the lens plane of the optical active element 3 (lens) everywhere only diffracted light beams 2.2.0, 2.2.1' and 2.2.1" from a small area of the illuminated partial area of the Fourier hologram 1b onto the optical active element 3, as a result of which only these diffracted light beams are Fourier-transformed. Due to the fact that these diffracted light beams are incident on the optical active element 3 at a similar angle, the in 1 (b) described case that no holographic reconstruction occurs on the detector. Rather, due to the slight angle differences at which the diffracted light beams enter the optical active element 3, a holographic reconstruction 9 is produced on the surface of the detector 4 despite the deformed surface 1. Despite the deformation of the hologram, this has sufficiently sharp contours to enable the holographically stored information to be read out.

Reference List

1

surface of an object

1a

surface clearance

1b

illuminated partial area of the Fourier hologram

1c

effective area

1d

detector surface distance

2.1

incident light rays

2.2

diffracted rays of light

2.2.0, 2.2.0', 2.2.0"

zero-order diffracted light rays

2.2.1, 2.2.1', 2.2.1"

first-order diffracted light rays

2.4

unconfined part of the light beam

3

optical active element

4

detector

5

Fourier hologram

6

payload

6'

read user data

7

limiting element (aperture)

7.1

boundary surface

7.2

transmission surface

9

holographic reconstruction

15

light source

17

two-dimensional array

18

calculated structuring

18'

written structuring (Fourier hologram)

100

reading device

200

Reading device (state of the art)

A

Area A

B

Area B

C

Area C

A'

deformed area A

B'

deformed area B

C'

deformed area C

f

error correction unit

K

channel decoding unit

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 2463645 A1 [0019]
• DE 102006012991 A1 [0020]

Claims (16)

Reading device (100) for reading out information stored holographically in a Fourier hologram (5), comprising - a light source (15) for emitting a light beam, - a detector (4) for detecting the light beam (2.1, 2.2, 2.2.0, 2.2.1, 2.4) propagated along a beam path, - a delimiting element (7), with a delimiting surface (7.1) for delimiting the beam path and a transmission surface (7.2) for transmitting the light beam (2.1, 2.2, 2.2.0, 2.2.1, 2.4) propagating along the beam path, - an optical active element (3) for transforming the light beam propagating along the beam path by means of a Fourier transformation, wherein a Fourier hologram (5) can be arranged in the read-out device (100), wherein the optical active element (3) and the limiting element (7) are arranged along the beam path in such a way that when the light source (15) emits the light beam and the Fourier hologram (5) is arranged in the beam path of the read-out device: - the light beam (2.1) illuminates at least a partial area of the Fourier hologram (5), and the hologram (5) bends the light beam (2.2) on the illuminated area (1b), - the light beam (2.2, 2.2.0, 2.2.1, 2.4) propagating along the beam path is limited by the delimiting surface (7.1) and the non-limited part of the light beam (2.4) is transmitted through the transmission surface (7.2), - the optical active element (3) transforms the non-limited part of the light beam (2.4) propagating along the beam path (2.2, 2.2.0, 2.2.1, 2.4) by means of the Fourier transformation, and - The detector (4) detects the non-limited part of the light beam (2.4) transformed by the optical active element (3) and transformed by means of the Fourier transformation. Readout device (100) according to claim 1 , characterized in that the transmission surface (7.2) is smaller than the partial surface (1b) of the Fourier hologram (5) illuminated by the light beam. Readout device (100) according to claim 1 or 2 , characterized in that the optical active element (3) comprises an active surface (1c) and the transmission surface (7.2) is smaller than or equal to the active surface (1c). Readout device (100) according to one of the preceding claims, characterized in that the delimiting element (7) is arranged in front of the optical active element (3) along the beam path (2.1, 2.2, 2.2.0, 2.2.1, 2.4) of the propagating light beam or that the limiting element (7) is arranged after the optical effect element (3) along the optical path of the propagating light beam. Read-out device (100) according to one of the preceding claims, characterized in that the transmission surface (7.2) is less than 50% of the partial surface (1b) of the Fourier hologram (5) illuminated by the light beam, the transmission surface (7.2) is preferably less than 20% % of the partial surface of the Fourier hologram illuminated by the light beam, the transmission surface is particularly preferably less than 10% of the partial surface of the Fourier hologram illuminated by the light beam, the transmission surface is further particularly preferably less than 5% of the partial surface of the Fourier illuminated by the light beam -hologram is. Readout device (100) according to one of the preceding claims, characterized in that the optical active element (3) is a lens or a mirror element. Readout device (100) according to one of the preceding claims, characterized in that the delimiting element (7) is a screen or is formed by an aperture of the optical active element (3). Readout device (100) according to one of the preceding claims, characterized in that the readout device (100) additionally comprises a housing with at least one housing wall and the at least one housing wall comprises a delimiting element (7), the delimiting surface (7.1) of the delimiting element being defined by the at least one Housing wall is formed and the transmission surface (7.2) of the delimiting element (7) is formed by an opening in the at least one housing wall. Readout device (100) according to one of the preceding claims, characterized in that the optical active element (3) comprises a delimiting element (7), the transmission surface (7.2) of the delimiting element (7) passing through the active surface (1c) of the optical active element (3) is trained. Readout device (100) according to one of the preceding claims, characterized in that the readout device additionally comprises an evaluation unit, the evaluations unit is designed to process detector signals of the detector (4). Readout device (100) according to one of the preceding claims, characterized in that the evaluation unit comprises an image processing unit and/or a channel decoding unit (K) and/or an error correction unit (F). Method for reading out information stored holographically in a Fourier hologram (5), comprising the steps: a) emission of a light beam by means of a light source (15), b) diffraction of the light beam (2.1, 2.2, 2.2.0, 2.2.1) propagating along a beam path by means of a Fourier hologram (5), c) delimiting the beam path by means of a delimiting surface (7.1) of a delimiting element (7), d) transmitting the light beam propagating along the beam path by means of a transmission surface (7.2) of the delimiting element (7), e) Fourier transformation of the light beam propagating along the beam path by means of an optical active element (3), and f) detecting the light beam propagated along the beam path by means of a detector (4). procedure according to claim 12 , characterized in that the transmission surface (7.2) is less than 50% of the partial surface (1b) of the Fourier hologram (5) illuminated by the light beam, the transmission surface (7.2) is preferably less than 20% of the partial surface of the Fourier illuminated by the light beam hologram, the transmission surface is particularly preferably less than 10% of the partial surface of the Fourier hologram illuminated by the light beam, the transmission surface is further particularly preferably less than 5% of the partial surface of the Fourier hologram illuminated by the light beam. procedure according to claim 12 or 13 , In a further step, detector signals generated by means of the detector (4) are processed by means of an evaluation unit. procedure according to Claim 14 , wherein the processing comprises image processing and/or channel decoding and/or error correction. Read-out system for reading out information stored holographically in a Fourier hologram (5), the read-out system comprising a read-out device (100) according to one of Claims 1 until 11 and the Fourier hologram (5), wherein the Fourier hologram is arranged in the beam path of the read-out device (100), and wherein, when the light source (15) emits the light beam (2.1), the light beam covers a partial area (1b) of the Illuminated Fourier hologram (5), the Fourier hologram diffracts the light beam in the area of the illuminated partial area.

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