WO2023047145A1

WO2023047145A1 - Method, computer program product and computer readable medium for creating a hologram

Info

Publication number: WO2023047145A1
Application number: PCT/HU2022/050068
Authority: WO
Inventors: Imre LAKATOS; Zsolt ERTL; Zsolt Papp; Tamás KORNIS
Original assignee: Lakatos Imre
Priority date: 2021-09-23
Filing date: 2022-09-23
Publication date: 2023-03-30

Abstract

A method for creating a hologram having an array of hogels (100), by a holographic imaging system having an aperture (120), on a holographic substrate having a predefined hologram area (140), wherein - the method comprises a sequence of exposure steps to sequentially expose each hogel (100) of the array of hogels (100) by the holographic imaging system via projecting light onto the holographic substrate through the aperture (120), wherein the aperture (120) has a projection area on the holographic substrate in each exposure step, The method is characterized in that - each hogel (100) within the hologram area (140) has a hogel surface area that is smaller than the projection area of the aperture (120), and - in each exposure step the aperture (120) of the holographic imaging system is positioned over the hologram area (140) by creating an overlap between the projection area of the aperture (120) and an unexposed portion of the hologram area (140), wherein the overlap corresponds to one hogel (100) of the array of hogels (100). The invention is furthermore a computer program product and a computer readable medium carrying out the above method.

Description

METHOD, COMPUTER PROGRAM PRODUCT AND COMPUTER READABLE MEDIUM FOR CREATING A HOLOGRAM

TECHNICAL FIELD

The invention relates to a method for creating a hologram with improved resolution. The invention also relates to a computer program product and a computer readable medium implementing the method.

BACKGROUND ART

The use of holograms, especially digital holograms are more and more common, because digital holograms can be purely computer-generated, i.e., a holographic image can be generated for example by digitally computing a holographic interference pattern and printing it onto a mask, a film or any suitable substrate for subsequent illumination by a suitable coherent light source. It has the advantage that an object that one wants to show do not have to possess any physical reality at all (completely synthetic hologram), or digital images of an actual object can also be transformed into a digital hologram.

The digital holograms comprise so called hogels, i.e., holographic elements (holographic pixels). Usually, an array of hogels form a complete image of a holographic recording. Contrary to two-dimensional (2D) pixels, hogels contain information about direction and intensity of light rays from many perspectives. The size and arrangement of the hogels can affect the resolution and quality of the digital holograms. Several methods are known in the art for generating a digital hologram.

Document KR 101901966 B1 discloses a system for digital holographic stereogram, wherein an n x m sized array of hogels is created, just like pixels of a two- dimensional image. According to the document the pixels are re-arranged in a manner that individual hogels are generated for each R, G and B pixels. During the hogel generating process the hogels are generated individually, without any overlap.

EP 1 785 987 B1 discloses a holographic storage medium, where data are stored as data pages having plurality of pixels within a pixel array. The document aims for increasing the storage capacity within a recording medium, i.e., to maximize the amount of data that can be stored on a given recording medium, or to minimize the size of the recording medium for a given quantity of information. In order to achieve the above aims, a first type of data is stored in an inner part of the data pages, and a second type of data is stored in an outer part of the data pages, wherein the second type of data is stored within a pixel size equal to or less than the pixel size of the first type of data.

US 2003/0128324 A1 discloses an optical encoder or decoder including an array of addressable elements with varying sized pixels. The size of the pixels can be changed by including different numbers of addressable elements into the pixels. The document describes that an optical signal created with and detected by arrays of addressable elements, such as a spatial light modulator (SLM) or a photo-detector array, typically suffers from distortions, e.g., images created by SLM suffer from non- uniform quality over its extent, i.e., the center of the image may be of higher quality than the edge. A primary source of distortions can be a poor optical component, a mechanical misalignment, or a Gaussian distribution of the light beam. In order to improve the image quality or detection, the document teaches that the pattern of the addressable elements of the array can be modified or grouped together into different sized pixels. The disadvantage of this solution is that a non-uniform image resolution is created by the different sized pixels. It is also to be noted that the intensity or the signal quality is the greatest near the centre of the array, where smaller physical pixels are arranged and these include a larger amount of information. It is also known that forming smaller pixels by applying a smaller aperture increases the diffraction effect and leads to pixels that are more difficult to read. The larger physical pixels that are arranged or addressed farther apart carry less information, and are less susceptible to image quality problems. As a result, not all hogels contain all the information of the image.

US 2008/0316894 A1 discloses a holographic recording/reproducing apparatus having aperture of variable size. The document shows examples wherein one or four pixels of an SLM is used as an information unit for recording information. When more pixels are used for storing the information, the reproducing reliability of the holographic recording medium can be improved. Depending on the number of pixels in an information unit, the size of the aperture is preferably changed, e.g., by electrical or mechanical methods. Changing the size of the aperture can increase the complexity of the system and also increase the number of failure modes.

CN 106646905 B discloses a holographic display panel, a holographic display device and a holographic display method by the help of which clear pixel edge and reduced crosstalk between neighbouring pixels can be achieved. The pixel sizes are limited by the Fraunhofer diffraction angle, i.e., due to the diffraction on the aperture of the imaging device, pixel sizes cannot be reduced below a certain limit. To reduce the adverse effect of the diffraction, a phase-plate is used to control the angle of diffraction. According to Fig. 4 of the document, a plurality of pixels can be grouped into pixel-groups, wherein each pixel of the pixel-group has the same content. By storing the same information over a plurality of pixels the recording medium is not effectively used.

In view of the known approaches, there is a need for a method for creating a hologram having an increased information storage capability with the same holographic medium and also an increased resolution.

DESCRIPTION OF THE INVENTION

The primary object of the invention is to provide a method for creating holograms with an increased resolution, which is free of the disadvantages of prior art approaches to the greatest possible extent.

The object of the invention is to provide a method for generating a hologram with increased resolution without modifying the optical settings of a holographic imaging system, i.e., without modifying or changing the optical elements and/or aperture of the holographic imaging system, and/or using additional imaging elements. Accordingly, the object of the invention is to provide a reliable method that is less prone to mechanical or optical failures.

A further object of the invention is to increase the information storage capabilities of a holographic medium, i.e., to maximize the amount of data that can be stored on a given recording medium, and/or to minimize the size of the recording medium for a given quantity of information by using the same holographic imaging system. Furthermore, the object of the invention is to provide a non-transitory computer program product for implementing the steps of the method according to the invention on one or more computers and a non-transitory computer readable medium comprising instructions for carrying out the steps of the method on one or more computers.

The objects of the invention can be achieved by the method according to claim 1 . The objects of the invention can be further achieved by the computer program product according to claim 9, and by the computer readable medium according to claim 10. Preferred embodiments of the invention are defined in the dependent claims.

The main advantage of the method according to the invention compared to prior art approaches comes from the fact that it can generate smaller-sized hogels with the same aperture, for example hogels having a size of a fraction of the aperture. By creating smaller sized hogels, more information can be stored within a holographic medium, or alternatively, smaller sized mediums can be used to store the same amount of information. This allows using the created holograms on smaller objects. Furthermore, the size of the holograms can be reduced until a level, wherein the holograms can be hardly seen by a human eye, thus the hologram can be used as a security feature of an object.

The smaller sized hogels also result in a better resolution, i.e. , the quality of the holographic image can be enhanced by the method according to the invention.

It has been recognized that each segment of a typical substrate of a hologram can only be exposed once, i.e., further exposures over the same segment are ineffective. This allows shifting the aperture of the holographic imaging device by smaller steps than the size of the aperture, thereby creating smaller-sized or differently shaped hogels.

A further advantage of the method according to the invention is that uniform hogel- sizes can be generated throughout the whole hologram that enhances userexperience, i.e., image distortions can be reduced, and the hologram will have a uniform resolution. The method according to the invention can be used for creating any digital holograms, i.e. , also colour holograms.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of example with reference to the following drawings, where

Fig. 1 is an illustration of an arrangement of hogels and a usual method for forming the hogels of a hologram, wherein the hogel size equals to the size of the aperture of the hologram imaging system,

Fig. 2 is an illustration of arrangements of hogels of a hologram according to the method of the invention, wherein the hogel size is smaller than the size of the aperture of the hologram imaging system, and

Fig. 3A - Fig. 3C are illustrations of preferred steps of the method according to the invention.

MODES FOR CARRYING OUT THE INVENTION

The invention relates to a method for creating a hologram with improved resolution. Detailed examples of the method according to the invention are described below in connection with the figures.

In general, the method according to the invention is a method for creating a hologram having an array of hogels 100 on a holographic substrate, wherein the holographic substrate has a predefined hologram area 140. The hologram is preferably created within the boundaries of the hologram area 140. In some embodiments of the method, see for example Fig. 2, holographic information can also be written on the holographic substrate outside of the hologram area 140 that however does not affect the quality and readability of the hologram created by the method according to the invention.

The hologram is created by a holographic imaging system having an aperture 120, and the method according to the invention comprises a sequence of exposure steps to sequentially expose each hogel 100 of the array of hogels 100 by the holographic imaging system via projecting light onto the holographic substrate through the aperture 120, wherein the aperture 120 has a projection area on the holographic substrate in each exposure step. Depending on the distance between the aperture 120 and the holographic substrate, the projection area, i.e., the area covered by light through the aperture 120 during an exposure step, can be different. Larger distance between the aperture 120 and the holographic substrate results in a larger projection area, and smaller distance results in a smaller projection area, because of the diverging light that exits the aperture 120. The light of the holographic imaging system is preferably originating from a uni-colour laser, a multi-colour laser or a set of lasers. The lasers can have separate light emitting units, or can be integrated into one unit. The multi-colour laser is preferably an RGB laser, and the set of lasers preferably have red, green, and blue lasers. Preferably, the aperture 120 has a predetermined aperture size and a predetermined aperture shape. Preferably, the predetermined aperture shape is a square, a rectangle, a triangle, or a hexagon.

The method according to the invention is characterized in that each hogel 100 within the hologram area 140 has a hogel surface area (i.e., a hogel size) that is smaller than the projection area of the aperture 120, i.e., by the method according to the invention smaller hogels 100 can be created than by traditional methods. In case of traditional methods, the size of each hogels (i.e., the hogel surface area) are determined by the projection area of the aperture 120. Contrary to this traditional approach, the method according to the invention allows for creation of smaller sized hogels 100, e.g., the hogel surface area of the hogels 100 is a fraction of the projection area of the aperture 120, preferably, the hogel surface area is half or one- fourth or one-nineth of the projection area of the aperture 120. The smaller sized hogels 100 result in a hologram having an increased resolution.

In each exposure step, the aperture 120 of the holographic imaging system is positioned over the hologram area 140 by creating an overlap between the projection area of the aperture 120 and an unexposed portion of the hologram area 140, i.e., a portion of the hologram area 140 that has not yet been exposed and thus no hogels 100 are created yet. The overlap between the projection area of the aperture 120 and an unexposed portion of the hologram area 140 corresponds to one hogel 100, i.e., one yet unexposed hogel 100 of the array of hogels 100, and this particular hogel 100 will be exposed in the respective exposure step. Preferably, the method according to the invention comprises a positioning step before each exposure step, and the positioning step comprises shifting and/or rotating the aperture 120 of the holographic imaging system.

Preferably, the hogel surface area is equal for each hogel 100 of the array of hogels 100, i.e., the hologram created by the method according to the invention has a uniform resolution and uniform information density.

Fig. 1 illustrates a classical method for generating a hologram by a holographic imaging device. The hologram builds up from holographic elements or holographic pixels (hogels 100) that are preferably arranged in an array. Each hogel 100 is created by the holographic imaging device by directing a coherent light, such as a laser beam, onto a suitable substrate. Suitable substrates include photosensitive materials, such as photopolymers, silver halide and/or dichromated gelatine (DCG). Some photosensitive materials are rewritable, i.e., the information stored in the material can be erased by a separate process, and after erasing the information, the material can be exposed to light again to store different holographic information. Rewritable materials include photorefractive (ferroelectric) liquid crystals, bacteriorhodopsin, amorphous chalcogenide, polarization sensitive materials (e.g., organic and inorganic materials, such as azo dyes), or photorefractive materials (e.g., lithium niobate, barium titanate or gallium arsenide). Rewritable materials can also be used as a holographic substrate for creating a hologram by the method according to the invention.

As a light projected to a substrate has a cylindrical intensity distribution, usually an aperture 120 is applied to form the beam and its projection to a desired shape, which is typically a rectangular shape. The aperture 120 is preferably formed in a frame 110.

According to Fig. 1 , the size of the hogels 100 corresponds to a projection area of the aperture 120, i.e., the hogels 100 have a shape and a size corresponding to the shape and the size of the projection area of the aperture 120. According to the example shown in Fig. 1 , the aperture 120 has a square shape with both sides of the aperture 120 having the same length (aperture-width), and the size and dimensions hogels 100 correspond to the respective size and dimensions of the aperture 120, i.e. , the aperture-width and the width of the hogels 100 (hogel-width 130) are approximately equal (considering a slight widening of the projected light, i.e., that the projection area of the aperture 120 is slightly larger than the aperture 120 itself).

Fig 1. shows an example, wherein the hogels 100 are arranged closely to each other, i.e., without any gaps between adjacent hogels 100. In other exemplary cases, there is a gap between adjacent hogels 100, wherein the gap is usually smaller than the hogel-width 130. In cases according to Fig. 1 , the hogels 100 are created one-by-one, preferably by moving the aperture 120 into positions corresponding to individual hogels 100. The positions of each hogel 100 can be visited randomly by the aperture 120 until every hogel 100 is created, or the hogels 100 can be created in a more arranged manner, i.e., creating a row or a column of hogels 100 as also indicated in Fig. 1.

According to Fig. 1 , the hogels 100 are created by moving the aperture 120 in a zigzag like manner, i.e., the aperture 120 is shifted along a first direction 150 of the substrate, and along said first direction 150 the hogels 100 are formed one-by-one creating a first row or first column of hogels 100. Upon reaching the last hogel 100 to be formed along the first direction 150, the aperture 120 is shifted along a second direction 160 and a second row or second column of hogels 100 are formed one after each other. According to Fig. 1 , the second row or second column of hogels 100 are created by shifting the aperture 120 along a counter-direction 170 of the first direction 150. Alternatively, after shifting the aperture 120 to the second row or second column, the aperture 120 can return next to the first hogel 100, and the second row or second column of hogels 100 can be created by moving the aperture 120 along the first direction 150. The above method is continued until every hogel 100 of the hologram is created.

It is to be noted, that during the method according to Fig. 1 , each hogel-position is written only once, and for this reason, the sequence of writing the individual hogels 100 does not matter.

Fig. 2 shows a preferred implementation of the method according to the invention, i.e., a method for creating a hogel-array within a hologram area 140 by the same holographic imaging device as, for example, in Fig. 1 , wherein the size of the hogels 100 is smaller than the size of the aperture 120, e.g., the hogel-width 130 is smaller than the aperture-width. It is known that once a portion of a photopolymer substrate has been exposed (i.e., a hogel 100 is written) and the photopolymerization process is finished, further irradiation on the same portion has no effect. Based on this information it has been recognized that instead of shifting the aperture 120 by a full aperture-width (or by a full aperture-with and the width of the gap between the adjacent hogels 100), the hogel-size can be reduced with the same settings of the same holographic imaging device.

In a preferred embodiment of implementing the method of the invention according to Fig. 2, the hogels 100 and the aperture 120 both have a square shape, and the hogel-width 130 equals to half of the aperture-width (i.e., the width of the projection area of the aperture 120), thus the hogel-size is one-fourth of the size of the projection area of the aperture 120. In other preferred embodiments of the method according to the invention, the hogel-width 130 can be any width smaller than the aperture-width, but preferably not less than 0.2 mm or 0.3 mm in order to avoid disturbing diffraction effects.

Fig. 2 shows a preferred sequence of forming the array of hogels 100, indicated by consecutive numbers. A preferred way of creating a first line (a first row or a first column) of hogels 100 is shown in more detail in Fig. 3A. The hologram is created in a substrate made of a photosensitive material that does not change its properties once it has been exposed with a suitable light source, i.e., further irradiations have no effect on the substrate. The hologram is preferably created in a hologram area 140, the boundaries of which are indicated by bold lines in Fig. 2. The holographic substrate can have the same size as the hologram area 140, or the hologram area 140 can be a region of the holographic substrate. In cases, wherein the holographic substrate has a larger area than the hologram area 140, holographic information can also be written outside of the boundaries of the hologram area 140. Such information written outside of the boundaries of the hologram area 140 is considered redundant information and it does not affect the readability or quality of the hologram created by the method. According to the method of the invention, the aperture 120 is positioned above a holographic substrate in a way that an overlap between the projection area of the aperture 120 and the hologram area 140 corresponds to one hogel 100. As it can be in Fig. 2, a first hogel 100 is created in a corner of the hologram area 140, and as the aperture 120 extends over the boundaries of the hologram area 140, which means that redundant information is written (encoded) into the holographic substrate outside of the hologram area 140. Such an extension, however, does not reduce the quality or readability of the hologram. Once the first hogel 100 is formed, the aperture 120 is preferably shifted along a first direction 150 by a hogel-width 130 and a second hogel 100 is formed, i.e. , written into the holographic substrate. As it can be seen in Fig. 2, the information encoded into the second hogel 100 also extends over the boundary of the hologram area 140, furthermore, all the information encoded into the first line of hogels 100 extend over the hologram area 140, because the aperture 120 is shifted in a way that the aperture 120 overlaps only a one-hogel sized unexposed portion of the hologram area 140.

Fig. 3A shows creating a hogel 100 (the 5th hogel 100) of the hologram, wherein the aperture 120 is moved into a position that covers an already exposed portion of the holographic substrate, see an exposed area 100a (the 4th hogel 100), and an unexposed area 100b (an area that has not yet been irradiated, exposed). In Fig. 3A, unexposed area 100b extends over the hologram area 140 (not indicated in Fig. 3A). Although the light illuminating the aperture 120 in the exposure step shown in Fig. 3A fills the whole aperture 120, i.e., creates a projection area larger than a size of a single hogel 100, the information will only be encoded into the unexposed area 100b. The information content of exposed area 100a will not be changed due to the further exposure. The method according to the invention is continued until the last hogel 100 (Nth hogel 100) of the first line (the first row or column) is reached and exposed.

According to the sequence of hogels 100 indicated in Fig. 2, a second line of hogels 100 is formed in a counter-direction 170 of the first direction 150, the implementation of which is shown in more detail in Fig. 3B. In order to create a second line (a second row or column) of hogels 100, the aperture 120 is shifted by one hogel-width 130 in the first direction 150 and by one step (in this particular example, by one hogel-width 130) in the second direction 160. In this position of the aperture 120, the projection area of the aperture 120 overlaps with the Nth hogel 100 being an exposed area 100a of the substrate and an unexposed area 100b that extends over the boundaries of the hologram area 140. From the unexposed area 100b only a one- hogel sized portion lays within the boundaries of the hologram area 140, thus the (N+1 )th hogel 100 will have the same size as any other hogel 100 of the hologram.

The aperture 120 is moved step-by-step in a way that in each step the projection area of the aperture 120 only covers a one-hogel sized area of the hologram area 140, thus in each step only one hogel 100 of the array is created. Fig. 3B further indicates the position of the aperture 120 and the frame 110 to create a last hogel 100 of the second line, i.e., the (2N)th hogel 100.

Fig. 3C depicts a preferred way of creating a (2N+1 )th hogel 100, i.e., a first hogel 100 of a third line (a third row or column) of the hologram, which is similar to creating the first hogel 100 of the second line of the hologram. As indicated in Fig. 3C, to create the first hogel 100 of the third line of the hologram, the aperture 120 is shifted along the second direction 160 and simultaneously it is moved by one hogel-width 130 along the counter-direction 170 of the first direction 150. In this position, the projection area of the aperture 120 covers an already exposed area 100a (i.e., the area of the (2N)th hogel 100), and an unexposed area 100b. According to the position of the aperture 120, only a one-hogel sized area of the unexposed area 100b is within the boundaries of the hologram area 140, thus by exposing the area covered by the projection area of the aperture 120, only a one-hogel sized area (i.e., the area of the (2N+1 )th hogel 100) is created within the boundaries of the hologram area 140. The third line of hogels 100 is created by moving the aperture 120 along the first direction 150.

The sequence of exposure steps can also be different from the example described above. For example, instead of taking the exposure steps of the hogels 100 in a zigzag pattern (i.e., moving along a first direction 150 followed by a step in a second direction 160 and moving along a counter-direction 170 of the first direction 150), the second line (second row or the second column) of hogels can be started from next to the first hogel 100, i.e., the second line (second row or column) of hogels 100 can be exposed along the first direction. Alternatively, the aperture 120 can be moved along the boundary of the hologram area 140, i.e., forming the outermost hogels 100 first, then the inner hogels 100 in a spiral-like motion. In every step of the method according to the invention it is to be ensured that the overlap between the projection area of the aperture 120 and the unexposed area 100b of the hologram area 140 corresponds to a size of one hogel 100 only.

The method according to the invention is capable of creating holograms having different hogel sizes and shapes, wherein the shapes can include a triangle (e.g., a right-triangle, an equilateral triangle), a rectangle, a square, or a hexagon, etc. Triangle shaped hogels 100 can be created for example by a triangle shaped aperture 120, and hexagon shaped hogels 100 can preferably be created by a hexagon shaped aperture 120, i.e., a triangle or a hexagon shaped aperture 120 of a given size can be used to create triangle or hexagon shaped hogels 100 having smaller dimensions than that of the projection area of the aperture 120.

Alternatively, hogels 100 having a shape of a right triangle can also be created by square or rectangle shaped apertures 120, in which case the aperture 120 is to be rotated when moved to make exposure in a new hogel-position.

The hologram created by the method according to the invention can be a digital hologram that can be based on virtual data, i.e., a virtual design of an existing on a non-existing object, on digital images, photographs, or an analogous hologram (i.e., a copy of the analogous hologram). Preferably, the hologram created by the invention is a colour-hologram, i.e., the holographic imaging system uses a multi- coulour laser such as an RGB laser producing red, green and blue coloured light to create each hogel 100, or alternatively, a set of lasers can be used to encode multiple colour information into the holographic substrate. More preferably, the hologram created by the invention can have a viewing angle in the range of 10 to 120 degrees depending on the optical properties of the holographic imaging system, especially a lens of the holographic imaging system, wherein the lens has a Fourier transforming property. Furthermore, the hologram created by the method according to the invention preferably can be channelled in a rendering phase (preferably using up to 8 channels for depicting different scenes within different viewing angle ranges of the same hologram), and/or the hologram can be a parallax or a full-parallax hologram depending on the amount of images recorded from different viewing angles.

The hologram created by the method according to the invention can have various applications, a preferred application is to use the hologram on objects or as parts of an object, such as a phone or a phone case. The object can be made of any material such as glass, silicone or other plastic material, or a combination of any of these materials with a metal, rubber, leather or wood. In order to protect the hologram from environmental impacts and to ensure a long-term readability of the hologram, the substrate of the hologram can be covered by a protective layer, and/or it can be embedded in one or more layers. The protective layer covering the substrate of the hologram (i.e., a cover layer) can be a polycarbonate layer, a polyethylene terephthalate (PET) layer, a polyethylene (PE) layer, a polypropylene (PP) layer, or other optically clear polymer. To attach the substrate of the hologram to a surface, preferably, an adhesive layer is to be applied. Depending on the viewing direction of the hologram, preferably an optically clear adhesive is to be used, especially, if the hologram is to be viewed from the direction of the adhesive layer, e.g., in cases when the substrate of the hologram is to be attached on a transparent object.

When the hologram created by the method of the invention is used on a phone case, it is preferably attached to the inner side of the phone case so that the hologram is sandwiched between the phone and the phone case. It preferably further helps to prevent any damages of the substrate and the cover layer of the hologram during use. As already mentioned above, the hologram (preferably in a form of a holographic film) can be attached to the inner side of the phone case via an adhesive layer, preferably an optically clear adhesive layer allowing undisturbed reconstruction and view of the hologram, or the hologram can be placed between the case and the phone like a very thin card in which case the hologram is easily replaceable. If the outer shell of the phone has a colour or an image, then preferably an additional layer is arranged on the backside of the phone (i.e., under the hologram to increase the diffraction. The additional layer can be a decoration layer, preferably made of paper, metallized paper, plastic, leather, wood, fabric, metal or any combination of these. Most preferably, the additional layer is a black film, that helps to enhance the view of the hologram. When the hologram is illuminated by a light source or even sunshine, the light diffracts on the holographic layer, and the holographic image is reconstructed. The holographic image is usually reconstructed in front of the holographic layer, creating an illusion of an image extending from the holographic layer.

The hologram created by the method according to the invention can be combined with different printing techniques, including techniques known in the art such as offset printing, screen printing, digital printing, UV printing, litography, inkjet printing, laser printing, etc. The hologram when applied as a holographic layer can also be combined with different materials such as paper, plastic, metallized paper, leather, wood, metal, textile, etc.

The invention, furthermore, relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out an embodiment of the method according to the invention.

The computer program product may be executable by one or more computers, and/or the computer program product can be a non-transitory computer program product.

The invention also relates to a computer readable medium comprising instructions which, when executed by a computer, cause the computer to carry out an embodiment of the method according to the invention.

The computer readable medium may be a single one or comprise more separate pieces, and/or the computer readable medium can be a non-transitory computer readable medium.

The invention is, of course, not limited to the preferred embodiments described in detail above, but further variants, modifications and developments are possible within the scope of protection determined by the claims. Furthermore, all embodiments that can be defined by any arbitrary dependent claim combination belong to the invention. LIST OF REFERENCE SIGNS

100 hogel

100a exposed area 100b unexposed area

110 frame

120 aperture

130 hogel-width

140 hologram area 150 first direction

160 second direction

170 counter-direction

Claims

1. A method for creating a hologram having an array of hogels (100), by a holographic imaging system having an aperture (120), on a holographic substrate having a predefined hologram area (140), wherein

- the method comprises a sequence of exposure steps to sequentially expose each hogel (100) of the array of hogels (100) by the holographic imaging system via projecting light onto the holographic substrate through the aperture (120), wherein the aperture (120) has a projection area on the holographic substrate in each exposure step, characterized in that

- each hogel (100) within the hologram area (140) has a hogel surface area that is smaller than the projection area of the aperture (120), and

- in each exposure step the aperture (120) of the holographic imaging system is positioned over the hologram area (140) by creating an overlap between the projection area of the aperture (120) and an unexposed portion of the hologram area (140), wherein the overlap corresponds to one hogel (100) of the array of hogels (100).

2. The method according to claim 1 , characterized by comprising a positioning step before each exposure step, and the positioning step comprises shifting and/or rotating the aperture (120) of the holographic imaging system.

3. The method according to claim 1 or claim 2, characterized in that the aperture (120) has a predetermined aperture size and a predetermined aperture shape.

4. The method according to claim 3, characterized in that the predetermined aperture shape is a square, a rectangle, a triangle, or a hexagon.

5. The method according to any one of claims 1 to 3, characterized in that the hogel surface area is a fraction of the projection area of the aperture (120), preferably, the hogel surface area is half or one-fourth or one-nineth of the projection area of the aperture (120).

6. The method according to any one of claims 1 to 3, characterized in that the hogel surface area is equal for each hogel (100) of the array of hogels (100).

7. The method according to any one of claims 1 to 5, characterized in that the light of the holographic imaging system is originating from a uni-colour laser or a multi-colour laser or a set of lasers.

8. The method according to any one of claims 1 to 5, characterized in that the holographic substrate is made of a photosensitive material, such as photopolymers, silver halide, dichromated gelatine (DCG), or rewritable materials, wherein rewritable materials include liquid crystals, bacteriorhodopsin, amorphous chalcogenide, polarization sensitive materials, or photorefractive materials.

9. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method of any of claims 1 -8.

10. A computer readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method of any of claims 1 -8.