Abstract:
The invention relates to an encrypted synthetic hologram formed from the Fourier transformation of an image ( 40 ) and consisting of a matrix of elementary cells. Half of the elementary cells, with a 10% margin, selected according to a motif ( 52 ), are dephased in relation to the elementary cells of a hologram directly produced by the Fourier transformation of the image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a National Stage of PCT International Application Serial Number PCT/FR2011/052054, filed Sep. 8, 2011, which claims priority under 35 U.S.C. §119 of French Patent Application Serial Number 09/50183, filed Sep. 8, 2010, the disclosures of which are incorporated by reference herein. 
         [0002]    1. Field of the Invention 
         [0003]    The present invention relates to synthetic holograms. More specifically, the present invention relates to a method for concealing a synthetic hologram in a binary image. 
         [0004]    2. Discussion of Prior Art 
         [0005]    In many fields, especially in the luxury goods industry (for example, perfumery, jewelry or leather goods), or in the field of drugs, fighting against the copy of branded products is an everyday concern. Several methods are currently used to attempt to guarantee the authenticity of branded products. The simplest is to reproduce or to affix a brand logo on the products. However a fraudster can easily reproduce a logo. 
         [0006]    Other marking methods, which are more difficult to detect and to copy, are known. One of them comprises placing a transparent identification chip, invisible for the naked eye, on each of the products of a batch, a hologram being formed on the transparent chip. The hologram may be obtained by calculating the Fourier transform of an image representing, for example, the brand logo. The origin of the products is thus guaranteed by the presence or the absence of the hologram. 
         [0007]      FIG. 1  illustrates a method for forming a synthetic hologram, for example, a coded-aperture hologram. It is started from an initial image  10  (IMAGE), for example, a logo or a brand, after which the Fourier transform of this image is calculated at a step  12  (TF). The calculation of this Fourier transform enables to obtain an amplitude image  14  (A) and a phase image  16  (φ) of the Fourier transform. A synthetic hologram is then formed (step  18 , HOLOGRAM) from the Fourier transform amplitude image A and from Fourier transform phase image φ. 
         [0008]    As an example a coded-aperture synthetic hologram is formed as follows. The images of amplitude A and of phase φ are divided into a predefined number of elementary cells. The hologram is also formed of a same number of elementary cells. 
         [0009]      FIG. 2  illustrates an example of a synthetic coded-aperture hologram  20 . In each elementary cell  22  of the hologram is formed an aperture  24 . Apertures  24  are aligned, in each line, along a first direction y of the hologram. Surface area  24  of the apertures in each of the elementary cells of the hologram corresponds to the amplitude of the associated elementary cell of amplitude image A. The shifting of apertures  24  along a second direction x of the hologram corresponds to the phase of the associated elementary cell of phase image φ. 
         [0010]    Practically, a synthetic coded-aperture hologram may be formed on a thin glass plate having a thin opaque layer, for example, made of platinum oxide, deposited thereon. Portions of the platinum oxide are then etched to form transparent or opaque regions. In the case of the hologram of  FIG. 2 , the etched portions are the portions located outside of the contours of apertures  24 . 
         [0011]    Other types of synthetic holograms are known, and are especially described in the publication entitled “Computer generated holograms: an historical review”, by G. Tricoles, Applied Optics 1981, vol. 26 no 20, pp. 4351-4360. Especially, synthetic holograms in which each of the elementary cells comprises several apertures having their respective sizes coding the amplitude and the phase of the Fourier transform of the corresponding elementary cell, holograms having each cell representing an interferogram portion having its width and its positioning coding the amplitude and the phase of the Fourier transform, or again holograms which directly code the phase by thickness changes of the glass plate on which the hologram is formed. 
         [0012]    It is also known to combine synthetic holograms with binary images, that is, images with two color levels. 
         [0013]      FIG. 3  illustrates such a combination. 
         [0014]      FIG. 3  shows a visible binary image  30  comprising a repetition of word “Graphisme”. On a portion of binary image  30  is integrated a synthetic hologram  32  having its contour delimited by dotted lines. 
         [0015]    To integrate a hologram in a binary image without losing information, the elementary cells of the hologram superimposed to the dark portions of the binary image are inverted and phase-shifted by a phase shift close to π according to a known technique, for example disclosed in unpublished French patent application FR 09/56913 of the applicant, filed on Oct. 5, 2009 (B9741). 
         [0016]    An inversion of an elementary cell comprises, for example, in the case of the coded-aperture synthetic hologram of  FIG. 2 , inverting the clear and opaque portions of this cell. An elementary cell with a different shading than the initial cell is thus formed. A phase shift close to π is then applied to the central pattern of the concerned cells. In known fashion, an elementary cell and an inverted and phase-shifted elementary cell provide identical diffraction effects. 
         [0017]    A problem with the use of simple synthetic holograms is that such holograms are visible. Indeed, due to the calculation of the Fourier transform at step  12 , a synthetic hologram is formed of a central portion more strongly marked (darker) than the rest of the hologram (see  FIG. 3 ). Thus, a fraudster may spot the hologram and use an optical system capable of calculating the inverse Fourier transform of the hologram and thus obtain the image used to form this hologram. Once this image has been obtained, this person can easily recreate and copy the hologram. 
         [0018]    Thus, there is a need for a method for forming integrated synthetic holograms in a visible image and with a location that cannot be detected. 
       SUMMARY 
       [0019]    An object of an embodiment of the present invention is to provide a method of marking with a synthetic hologram in a visible image, avoiding for the hologram to be perceptible. 
         [0020]    Another object of an embodiment of the present invention is to provide a method of marking with a synthetic hologram which cannot be directly read. 
         [0021]    Thus, an embodiment of the present invention provides a synthetic hologram comprising first elementary cells having first apertures defined in each of them, integrated in a first portion of a binary pattern, the binary pattern comprising at least one second portion having second elementary cells comprising second apertures defined therein, the second apertures having an average size equal, to within 5%, to the average size of the first apertures, and a random phase shift. 
         [0022]    According to an embodiment of the present invention, the binary pattern comprises a third portion in which is integrated a second synthetic hologram, phase-shifted by π, to within 10%, with respect to the hologram integrated in the first portion. 
         [0023]    According to an embodiment of the present invention, the binary pattern further comprises one or several additional portions in which is integrated the synthetic hologram integrated in the first portion and one or several additional portions in which is integrated the second synthetic hologram. 
         [0024]    According to an embodiment of the present invention, the synthetic hologram is of coded-aperture type. 
         [0025]    An embodiment of the present invention further provides a method for concealing a synthetic hologram, formed from an initial image, in a binary pattern having dimensions greater than that of said hologram, comprising the steps of: (a) calculating a scrambled Fourier transform of the initial image to obtain a scrambled amplitude image and phase image; (b) forming a synthetic hologram from the scrambled amplitude image and phase image; (c) combining the synthetic hologram with a first portion of the binary pattern; (d) defining second apertures having an average size equal, to within 5%, to the average size of the first apertures; and (e) combining the second apertures with a second portion of the binary pattern. 
         [0026]    According to an embodiment of the present invention, the second apertures are combined with the entire surface of the binary pattern which is not combined with the synthetic hologram. 
         [0027]    According to an embodiment of the present invention, the second apertures have a random phase shift. 
         [0028]    According to an embodiment of the present invention, the method further comprises, before step (d), a step of combining a second synthetic hologram with a third portion of the binary pattern, the second synthetic hologram being obtained from the synthetic hologram combined in the first portion by a π phase shift, to within 10%. 
         [0029]    According to an embodiment of the present invention, the method further comprises, before step (d), a step of combining the synthetic hologram combined in the first portion with one or several additional portions of the binary pattern and combining the second synthetic hologram with one or several additional portions of the binary pattern. 
         [0030]    According to an embodiment of the present invention, the combination of the hologram(s) with the binary pattern is performed by inserting apertures directly into the light portions of the binary pattern and by inserting apertures of the hologram inverted and phase-shifted by π, to within 10%, into the dark portions of the binary pattern. 
         [0031]    An embodiment of the present invention further provides a method for reading a hologram such as defined hereabove, comprising a step of masking the third portion of the binary pattern. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0033]      FIG. 1 , previously described, is a flowchart of a method for forming a synthetic hologram; 
           [0034]      FIG. 2 , previously described, illustrates an example of a coded-aperture synthetic hologram; 
           [0035]      FIG. 3 , previously described, illustrates a synthetic hologram superimposed to a visible binary pattern; 
       
    
    
       [0036]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         [0037]      FIG. 1 , previously described, is a flowchart of a method for forming a synthetic hologram; 
         [0038]      FIG. 2 , previously described, illustrates an example of a coded-aperture synthetic hologram; 
         [0039]      FIG. 3 , previously described, illustrates a synthetic hologram superimposed to a visible binary pattern; 
         [0040]      FIG. 4  is a flowchart of a method for scrambling a synthetic hologram according to an embodiment of the present invention; 
         [0041]      FIG. 5  is a curve of the amplitude of the Fourier transform of an image used to form a hologram, along a plane crossing the center of the amplitude image; 
         [0042]      FIGS. 6A ,  6 B,  6 C, and  6 D respectively illustrate the amplitude of a first modified Fourier transform of an image used to form a hologram along a plane crossing the center of the amplitude image, an amplitude image obtained by the application of this modified Fourier transform, a phase image obtained by application of this modified Fourier transform and an image obtained by direct reading of a hologram formed from the amplitude and phase images; 
         [0043]      FIGS. 7A ,  7 B,  7 C, and  7 D respectively illustrate the amplitude of a second modified Fourier transform of an image used to form a hologram along a plane crossing the center of the amplitude image, an amplitude image obtained by the application of this modified Fourier transform, a phase image obtained by application of this modified Fourier transform, and an image obtained by direct reading of a hologram formed from the amplitude and phase images; 
         [0044]      FIG. 8  illustrates a stacking of a visible binary pattern and of a scrambled hologram according to an embodiment of the present invention; 
         [0045]      FIGS. 9A and 9B  are enlarged views illustrating a processing applied to a structure such as that in  FIG. 7 ; and 
         [0046]      FIG. 10  illustrates the result obtained in the case of the integration of two scrambled holograms in a real image. 
         [0047]    For clarity, the various drawings are not to scale. 
         [0048]      FIG. 4  is a flowchart of a method for scrambling a synthetic hologram according to an embodiment of the present invention; 
         [0049]      FIG. 5  is a curve of the amplitude of the Fourier transform of an image used to form a hologram, along a plane crossing the center of the amplitude image; 
         [0050]      FIGS. 6A ,  6 B,  6 C, and  6 D respectively illustrate the amplitude of a first modified Fourier transform of an image used to form a hologram along a plane crossing the center of the amplitude image, an amplitude image obtained by the application of this modified Fourier transform, a phase image obtained by application of this modified Fourier transform and an image obtained by direct reading of a hologram formed from the amplitude and phase images; 
         [0051]      FIGS. 7A ,  7 B,  7 C, and  7 D respectively illustrate the amplitude of a second modified Fourier transform of an image used to form a hologram along a plane crossing the center of the amplitude image, an amplitude image obtained by the application of this modified Fourier transform, a phase image obtained by application of this modified Fourier transform, and an image obtained by direct reading of a hologram formed from the amplitude and phase images; 
         [0052]      FIG. 8  illustrates a stacking of a visible binary pattern and of a scrambled hologram according to an embodiment of the present invention; 
         [0053]      FIGS. 9A and 9B  are enlarged views illustrating a processing applied to a structure such as that in  FIG. 7 ; and 
         [0054]      FIG. 10  illustrates the result obtained in the case of the integration of two scrambled holograms in a real image. 
         [0055]    For clarity, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0056]    To avoid for one or several holograms formed on a chip, having a binary pattern formed thereon, to be directly visible, it is provided to scramble this or these hologram(s). 
         [0057]      FIG. 4  is a flowchart of such a method, the different steps of the method being detailed hereafter in the following description. 
         [0058]    At a step  40 , it is started from an image which is desired to be turned into a hologram. At a step  42 , a scrambled Fourier transform of the image (B_TF) is calculated, which provides, at a step  44 , a scrambled amplitude image (B_A) and, at a step  46 , a scrambled phase image (B_φ) of the Fourier transform. 
         [0059]    Based on scrambled amplitude image B_A and phase image B_φ of the Fourier transform, a synthetic hologram is formed at a step  48  (B_HOL), for example, a coded synthetic hologram such as the hologram of  FIG. 2 . 
         [0060]    Then, at a step  50  (F_CALC), a number of characteris tics of the hologram obtained at step  48  associated with the visible aspect of this hologram is calculated, to be able, at a step  52  (DEF_BACK), to define a contour region of the hologram. Finally, at a step  54  (HOL_INT), a hologram having its contour defined by the region formed at step  52  is integrated in the visible binary image, the hologram and the contour having a total size equal to that of the binary image. 
         [0061]    The steps discussed in relation with the flowchart of  FIG. 4  are detailed hereafter. Especially, step  42  is specified in relation with  FIGS. 5 to 8  and steps  52  and  54  are specified in relation with  FIGS. 9A and 9B . 
         [0062]      FIG. 5  is a curve of the amplitude of the Fourier transform of an image used to form a hologram, along a plane crossing the center of the amplitude image. The Fourier transform of the image has a very high peak at its center, which results in having the hologram directly formed from this Fourier transform mainly shaded at its center. Indeed, with a constant sampling to form the hologram from the Fourier transform, only the center of the hologram corresponds to a significant amplitude, and thus to a heavier shading at the hologram level. 
         [0063]    To avoid the shading effect at the center of the hologram, it is provided to scramble the Fourier transform before forming the hologram (step  42 ). To achieve this, several techniques may be used, and especially scrambling techniques disclosed in patent application U.S. Pat. No. 4,013,338. Any other known scrambling technique may also be used. Methods for clipping the Fourier transform may also be used, as described hereafter. 
         [0064]      FIGS. 6A ,  6 B,  6 C, and  6 D illustrate a first case where the Fourier transform is clipped and respectively show the amplitude of a clipped Fourier transform, an amplitude image obtained by application of this clipped Fourier transform to an initial image, a phase image obtained by the application of this clipped Fourier transform to the initial image, and the image obtained by direct reading of a hologram formed from the amplitude and phase images of  FIGS. 6B and 6C . The use of a clipped Fourier transform is relatively easy since it is sufficient, in order to obtain it, to limit the value of the central peak of the transform, as well as part of the secondary peaks which surround it 
         [0065]    Thus, a hologram formed from this Fourier transform has a central peak which is less marked than in the case of the Fourier transform of  FIG. 5  (see  FIG. 6B ). However, although the central peak is attenuated, it is not totally concealed in the hologram and the hologram remains visible for an ill-intentioned person. 
         [0066]    Further, the image reconstructed on reading of a hologram formed from a clipped Fourier transform is readable (in this case, a bidimensional matrix, “DataMatrix”), as illustrated in  FIG. 6D , but comprises a central portion of low quality, the reading quality decreasing with the application of a significant clipping. 
         [0067]    The Fourier transform thresholding technique thus enables to slightly conceal the hologram, which may however remain visible if the thresholding is not sufficient. If the thresholding is increased, the hologram is more difficult to read. 
         [0068]    Thus, a scrambling such as described hereafter will be preferred over a thresholding, although a thresholding may be used at step  42 . 
         [0069]      FIGS. 7A ,  7 B,  7 C, and  7 D respectively illustrate the amplitude of a scrambled Fourier transform of an image used to form a hologram, an amplitude image obtained by the application of this scrambled Fourier transform to an initial image, a phase image obtained by application of this scrambled Fourier transform to an initial image, and an image obtained by direct reading of a hologram formed from the scrambled amplitude and phase images. 
         [0070]    In the example of  FIGS. 7A to 7D , the scrambling used is a phase scrambling which results in distributing the amplitude of the Fourier transform over the entire image, with no loss of information. As an example, this phase scrambling may be a scrambling such as the scrambling provided in patent application U.S. Pat. No. 4,013,338, or any known scrambling type, for example, a random phase scrambling. 
         [0071]    The application of a phase scrambling of the Fourier transform enables to distribute the amplitude of the Fourier transform over the entire image, and thus to obtain a uniform amplitude image ( FIG. 7B ). 
         [0072]    Advantageously, the hologram obtained from the amplitude image of  FIG. 7B  and from the phase image of  FIG. 7C  is uniform across its entire surface. Further, as illustrated in  FIG. 7D , the image obtained by direct reading of the hologram, with an adapted optical device, is of very good quality (data matrix). This is due to the fact that the application of the scrambling implies no loss of information in the hologram. 
         [0073]      FIG. 8  illustrates a stacking of a visible binary pattern and of a scrambled hologram according to an embodiment of the present invention. 
         [0074]    In  FIG. 8 , a scrambled hologram  64  is integrated in a visible image  60  comprising a number of binary patterns  62  (word “graphisme”). As previously described, to integrate the hologram in the binary image, said hologram is modified at the level of the dark portions of the binary image. For example, in the case of coded-aperture synthetic holograms, the cells at the level of the dark portions of the visible image are inverted (negative cells) and phase-shifted by π (to within 10%). 
         [0075]    Hologram  64  integrated in image  60  is, in the case of  FIG. 8 , visible since it forms a shaded area in the non-shaded background of patterns  62 . 
         [0076]    To conceal hologram  64  in image  60 , it is provided to form a “decoy” area in image  60 , all along the contour of hologram  64 . 
         [0077]      FIGS. 9A and 9B  illustrate this principle.  FIG. 9A  shows an enlargement of hologram  64  of  FIG. 8 , at a border of this hologram, the hologram contour being materialized by a dotted line. 
         [0078]      FIG. 9B  illustrates the enlargement of  FIG. 9A  after having formed a decoy area  66  all around the contour of hologram  64 . The decoy area is formed of elementary cells of same size as the elementary cells of hologram  64 , each elementary cell of the decoy area comprising one or several apertures having a shape similar to that of the apertures of the elementary cells of hologram  64 . 
         [0079]    The forming of apertures in the decoy area enables to obtain a contour of hologram  64  having a shading level identical to that of the hologram. Thus, the hologram cannot be distinguished from the decoy area. 
         [0080]    To obtain such an aspect, the average, minimum, and maximum sizes of the apertures formed in hologram  64  may be determined, after which apertures may be defined in decoy area  66  having a size ranging between the minimum size and the maximum size, the apertures of decoy area  66  altogether having an average size equal or very close, to within 5%, to the average size of the apertures of hologram  64 . 
         [0081]    The decoy area may also be formed by separately harmonizing the areas formed in the dark portions of the visible image and the areas formed in the light portions of the visible image. To achieve this, the average, minimum, and maximum sizes of the apertures formed in the elementary cells of the dark areas of the visible image at the hologram level are defined, and random apertures are defined at the level of the dark portions of the decoy area corresponding to these characteristics. The same operation is then carried out between the light regions of the visible image at the hologram level and the light regions of the visible binary image of the decoy area. 
         [0082]    It should be noted that any method enabling to provide for the aspect in the dark and light areas of the decoy area to be the same as in the corresponding dark and light areas of the hologram. Especially, the apertures in the decoy area may also be larger than the largest apertures at the hologram level or smaller than the smallest apertures at the hologram level, as long as the average size of the apertures in the decoy area is equal or close, within a 5% limit, to the average size of the aperture in the hologram area. 
         [0083]    The decoy area must generally have an aspect similar to the aspect of the scrambled hologram. Other techniques than those discussed herein may also be used to achieve this object 
         [0084]    The apertures formed in the decoy area are randomly phase-shifted so that the reading of the hologram is not disturbed by their presence. Indeed, if they are randomly generated, with no coherence, the signal that they diffract adds to the signal of the main grating diffraction orders, but not in holographic reconstruction orders. Thus, the reading is not disturbed by the presence of the random apertures of the decoy area. 
         [0085]      FIG. 10  illustrates the result obtained in the case of the integration of scrambled holograms in a real image. 
         [0086]      FIG. 10  shows real binary image  60  of  FIG. 8 , comprising dark regions  62 , where a hologram  64 A is integrated. 
         [0087]    The forming of the decoy area around hologram area  64 A enables to conceal the hologram in a background having a same texture. 
         [0088]    According to an alternative embodiment shown in  FIG. 10 , it may also be provided to form two or several synthetic holograms  64 A and  64 B in a same visible image of large size, the two synthetic holograms being phase-shifted with respect to each other by π, or by a phase shift close to π, to within 10%. This phase shift comprises phase-shifting each elementary cell of hologram  64 B by π with respect to each corresponding elementary cell of hologram MA. 
         [0089]    This enables to avoid for an ill-intentioned person knowing the existence of a hologram in image  60  to be able to trace back the image which has been used to form it Indeed, if a person attempting to fraud illuminates image  60  comprising the two phase-shifted holograms  64 A and  64 B with a device capable of reading a hologram, the beams originating from holograms  64 A and  64 B destructively interfere and do not enable to obtain the initial image used to form the hologram. 
         [0090]    Thus, even if this person knows that a hologram is concealed in image  60 , he cannot trace back the image used to form the hologram. 
         [0091]    To properly read the hologram and avoid the occurrence of destructive interferences, it is sufficient to mask one of the two holograms  64 A and  64 B. This reading is relatively easy when the location where the holograms are formed is at least approximately known. 
         [0092]    It may also be provided to form more than two holograms  64 A and/or  64 B in image  60 . This enables, once a first hologram or a first group of identical holograms have been masked, to ease the reading of the unmasked holograms. Indeed, when a single hologram is unmasked, the reading of this hologram is optimized if the reading beam aims at this hologram. When several identical holograms are not masked, the reading may be carried out without specifically aiming at a hologram. Indeed, in this case, the reading beam intercepts several hologram portions in phase, which enables to read the image with a good quality without requiring precisely aiming at the hologram. 
         [0093]    It should be noted that, in addition to the above-described hologram scrambling steps, it may also be provided, before step  50  of the flowchart of  FIG. 4 , to perform an equalization of the amplitude of the Fourier transform of the initial image. Such an equalization may be achieved by any known method, and enables to obtain a scrambled amplitude image with a very smooth visual aspect. 
         [0094]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the concealing method disclosed herein applies to coded-aperture synthetic holograms, but also to any type of known synthetic hologram. 
         [0095]    Further, the holograms provided herein may for example be formed by the structuring of an opaque layer formed on a support or substrate. As an example, the opaque layer may be made of a metal layer, for example, aluminum or chromium, formed by deposition. It may also be made of other materials. The structuring of the opaque layer may be performed by a lithographic-type step. 
         [0096]    The support or substrate will preferably be transparent, for example, made of glass, sapphire, or quartz. It may also be opaque in the visible range if the reflectivity contrast with the metal is sufficient A combination of an aluminum layer deposited on the silicon substrate for example ensures this contrast 
         [0097]    Advantageously, the current writing resolution with this first lithography method is smaller than one micrometer, which is compatible with a pitch of the hologram cells ranging between 1 and 10 μm (which is here advantageous for a coded-aperture synthetic hologram such as shown in  FIGS. 9A and 9B ). 
         [0098]    The hologram may also be formed on the substrate by a modification of its index, for example, by photosensitive effect or by modification of the thickness of a transparent layer formed at the surface of the substrate (it is then spoken of a phase or kinoform hologram). 
         [0099]    In this case, generally, a lithography of a resin layer formed on the substrate is performed. After development of the insolated resin, the device surface has two thickness levels, that of the remaining resin and that of the substrate. This topography is transferred to the substrate by etching thereof, the resin being used as a mask. The height differences typically are on the order of the wavelength, that is, of a few hundreds of nanometers. This process is repeated several times with different patterns to eventually form a complex thickness structuring. Substrate thickness differences form phase differences on an incident beam, which enables to modify the phase thereof by following the desired holographic function. 
         [0100]    Holographic materials, for example, holographic resins having an index capable of being modified proportionally to an insolation level, may also be used. Such materials are known by those skilled in the art. 
         [0101]    In practice, to take advantage of the visual aspect of the hologram and align the read key thereof, for example, in the case described in relation with  FIGS. 11A and 11B , the first solution for forming an etched metal layer on a support or substrate may be preferred.