CA2310768A1 - An optical authenticity feature - Google Patents
An optical authenticity feature Download PDFInfo
- Publication number
- CA2310768A1 CA2310768A1 CA002310768A CA2310768A CA2310768A1 CA 2310768 A1 CA2310768 A1 CA 2310768A1 CA 002310768 A CA002310768 A CA 002310768A CA 2310768 A CA2310768 A CA 2310768A CA 2310768 A1 CA2310768 A1 CA 2310768A1
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- Prior art keywords
- security feature
- ray
- modulated
- phase
- accordance
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- Abandoned
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- 230000003287 optical effect Effects 0.000 title abstract description 9
- 238000000034 method Methods 0.000 claims abstract description 38
- 238000004519 manufacturing process Methods 0.000 claims description 6
- 230000009466 transformation Effects 0.000 claims description 2
- 238000001514 detection method Methods 0.000 claims 1
- 239000000969 carrier Substances 0.000 abstract description 4
- 239000011888 foil Substances 0.000 description 10
- 230000010363 phase shift Effects 0.000 description 6
- 230000002452 interceptive effect Effects 0.000 description 3
- 230000005540 biological transmission Effects 0.000 description 2
- 230000001066 destructive effect Effects 0.000 description 2
- 230000000694 effects Effects 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000000386 microscopy Methods 0.000 description 1
- 230000000149 penetrating effect Effects 0.000 description 1
- 238000000859 sublimation Methods 0.000 description 1
Classifications
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/0005—Adaptation of holography to specific applications
- G03H1/0011—Adaptation of holography to specific applications for security or authentication
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
- B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
- B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
- B42D25/30—Identification or security features, e.g. for preventing forgery
- B42D25/324—Reliefs
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B42—BOOKBINDING; ALBUMS; FILES; SPECIAL PRINTED MATTER
- B42D—BOOKS; BOOK COVERS; LOOSE LEAVES; PRINTED MATTER CHARACTERISED BY IDENTIFICATION OR SECURITY FEATURES; PRINTED MATTER OF SPECIAL FORMAT OR STYLE NOT OTHERWISE PROVIDED FOR; DEVICES FOR USE THEREWITH AND NOT OTHERWISE PROVIDED FOR; MOVABLE-STRIP WRITING OR READING APPARATUS
- B42D25/00—Information-bearing cards or sheet-like structures characterised by identification or security features; Manufacture thereof
- B42D25/30—Identification or security features, e.g. for preventing forgery
- B42D25/328—Diffraction gratings; Holograms
-
- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03H—HOLOGRAPHIC PROCESSES OR APPARATUS
- G03H1/00—Holographic processes or apparatus using light, infrared or ultraviolet waves for obtaining holograms or for obtaining an image from them; Details peculiar thereto
- G03H1/04—Processes or apparatus for producing holograms
- G03H1/0402—Recording geometries or arrangements
- G03H1/041—Optical element in the object space affecting the object beam, not otherwise provided for
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- G—PHYSICS
- G07—CHECKING-DEVICES
- G07D—HANDLING OF COINS OR VALUABLE PAPERS, e.g. TESTING, SORTING BY DENOMINATIONS, COUNTING, DISPENSING, CHANGING OR DEPOSITING
- G07D7/00—Testing specially adapted to determine the identity or genuineness of valuable papers or for segregating those which are unacceptable, e.g. banknotes that are alien to a currency
- G07D7/003—Testing specially adapted to determine the identity or genuineness of valuable papers or for segregating those which are unacceptable, e.g. banknotes that are alien to a currency using security elements
- G07D7/0032—Testing specially adapted to determine the identity or genuineness of valuable papers or for segregating those which are unacceptable, e.g. banknotes that are alien to a currency using security elements using holograms
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09F—DISPLAYING; ADVERTISING; SIGNS; LABELS OR NAME-PLATES; SEALS
- G09F3/00—Labels, tag tickets, or similar identification or indication means; Seals; Postage or like stamps
- G09F3/02—Forms or constructions
- G09F3/0291—Labels or tickets undergoing a change under particular conditions, e.g. heat, radiation, passage of time
- G09F3/0292—Labels or tickets undergoing a change under particular conditions, e.g. heat, radiation, passage of time tamper indicating labels
-
- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09F—DISPLAYING; ADVERTISING; SIGNS; LABELS OR NAME-PLATES; SEALS
- G09F3/00—Labels, tag tickets, or similar identification or indication means; Seals; Postage or like stamps
- G09F3/02—Forms or constructions
- G09F3/0291—Labels or tickets undergoing a change under particular conditions, e.g. heat, radiation, passage of time
- G09F3/0294—Labels or tickets undergoing a change under particular conditions, e.g. heat, radiation, passage of time where the change is not permanent, e.g. labels only readable under a special light, temperature indicating labels and the like
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Computer Security & Cryptography (AREA)
- Theoretical Computer Science (AREA)
- Holo Graphy (AREA)
- Credit Cards Or The Like (AREA)
- Diffracting Gratings Or Hologram Optical Elements (AREA)
Abstract
The invention relates to an optical authenticity feature for application onto any carriers, for example onto documents, and to a method for reading out the authenticity feature in accordance with the invention. With the aid of the security feature in accordance with the invention, a phase profile is modulated on a light ray incident to the security feature. The modulated phase profile can be transformed into amplitude information visible to the eye by means of, for example, the phase-contrast method, the schlieren method or an interferometer. The security feature can in particular comprise a hologram applied to the carrier, with a light ray incident to the hologram restoring an object ray having a modulated phase profile.
Description
An optical authenticity feature The invention relates to an optical authenticity feature for application onto any carriers, for example onto documents, I.D. cards, bank notes or plastic cards, and to a method for reading out the authenticity feature in accordance with the invention.
Documents, deeds, bank notes, LD. cards, plastic cards, etc. can be reproduced in exact detail and true to colour with the aid of modern high-resolution colour scanners and with the aid of colour laser printers or thermo-sublimation printers. The general availability of colour photocopiers means that it has also become substantially easier to manufacture high-quality forgeries.
A need therefore exists to make documents, I.D. cards, bank notes, securities, plastic cards, etc. secure against forgery by means of additionally applied safety features. By means of such security features, an effect can at least be achieved whereby the manufacture of a high-quality forgery is made much more expensive. Watermarks, silk threads, twisting line structures (Guilloche structures) and the use of special paper are known as such security features. The application of metallised embossed
Documents, deeds, bank notes, LD. cards, plastic cards, etc. can be reproduced in exact detail and true to colour with the aid of modern high-resolution colour scanners and with the aid of colour laser printers or thermo-sublimation printers. The general availability of colour photocopiers means that it has also become substantially easier to manufacture high-quality forgeries.
A need therefore exists to make documents, I.D. cards, bank notes, securities, plastic cards, etc. secure against forgery by means of additionally applied safety features. By means of such security features, an effect can at least be achieved whereby the manufacture of a high-quality forgery is made much more expensive. Watermarks, silk threads, twisting line structures (Guilloche structures) and the use of special paper are known as such security features. The application of metallised embossed
-2-holograms onto bank notes, credit cards and Eurocheque cards has also become generally widespread.
However, it would be desirable to be able to apply a security feature to the respective carrier which cannot at first be recognised as a security feature. Such a security feature, which is generally only recognisable with the use of complex technologies, would have the advantage of not being perceived as a security feature by the general public. Such a hidden security feature would not be taken into account in a number of forgeries.
It is therefore the object of the invention to provide a security feature which can be applied to any carriers such as documents, bank notes, LD. cards, plastic cards, etc., with the information contained in the security feature not being directly recognisable when looked at.
This object of the invention is solved by a security feature in accordance with claim 1 tir 14 by a method for reading out the security feature in accordance with the invention according to claims 10 and/or 15 and by a method for the manufacture of the security feature in accordance with the invention according to claim 18.
The security feature in accordance with the invention can be applied to any carriers, for example to a document, an LD. card, a bank note, a plastic card, etc. By means of the security feature, a phase profile is modulated on a light ray incident to the security feature.
Phase shifts are generated between the individual spatial regions of an incident wave-front by the security feature, with a phase profile being modulated on the wave-front.
Whereas the eye can directly recognise an amplitude profile modulated on a wave-front as a brightness profile, a wave-front on which a phase profile has been
However, it would be desirable to be able to apply a security feature to the respective carrier which cannot at first be recognised as a security feature. Such a security feature, which is generally only recognisable with the use of complex technologies, would have the advantage of not being perceived as a security feature by the general public. Such a hidden security feature would not be taken into account in a number of forgeries.
It is therefore the object of the invention to provide a security feature which can be applied to any carriers such as documents, bank notes, LD. cards, plastic cards, etc., with the information contained in the security feature not being directly recognisable when looked at.
This object of the invention is solved by a security feature in accordance with claim 1 tir 14 by a method for reading out the security feature in accordance with the invention according to claims 10 and/or 15 and by a method for the manufacture of the security feature in accordance with the invention according to claim 18.
The security feature in accordance with the invention can be applied to any carriers, for example to a document, an LD. card, a bank note, a plastic card, etc. By means of the security feature, a phase profile is modulated on a light ray incident to the security feature.
Phase shifts are generated between the individual spatial regions of an incident wave-front by the security feature, with a phase profile being modulated on the wave-front.
Whereas the eye can directly recognise an amplitude profile modulated on a wave-front as a brightness profile, a wave-front on which a phase profile has been
-3-modulated appears as a uniformly bright area. Whereas, therefore, an amplitude profile is directly recognisable, a phase profile is hidden information which can only be made visible by means of technical aids. A security feature which modulates a phase profile on a light ray therefore represents a hidden security feature, as the modulated phase profile is initially invisible. The security feature in accordance with the invention therefore remains unrecognised in many cases and can by just this feature meet its purpose of ensuring the authenticity of a document, an LD.
card, a bank note, a plastic card, etc. so much better.
With another advantageous aspect of the invention, a film is applied to the respective carrier, with the film having a varying film thickness and with a phase profile corresponding to the film thickness variation being able to be modulated on an incident light ray by the film.
If a wave-front passes through a transparent film having a film thickness variation, then the film thickness variation causes different optical path lengths through the film and thus phase modulation. After passing through the transparent film, a ray which has penetrated a region of high film thickness is dephased over a ray which has penetrated a region of lower film thickness.
Such transparent films can additionally be provided with a reflecting film so that the incident light ray first penetrates the transparent film, is subsequently reflected at the reflecting film and then passes through the transparent film a second time.
The phase modulation is modulated on here by the two-fold passage through the film thickness profile.
In accordance with another advantageous aspect of the invention, the security feature comprises a film applied to the respective carrier, with the film having a varying refractive index.
card, a bank note, a plastic card, etc. so much better.
With another advantageous aspect of the invention, a film is applied to the respective carrier, with the film having a varying film thickness and with a phase profile corresponding to the film thickness variation being able to be modulated on an incident light ray by the film.
If a wave-front passes through a transparent film having a film thickness variation, then the film thickness variation causes different optical path lengths through the film and thus phase modulation. After passing through the transparent film, a ray which has penetrated a region of high film thickness is dephased over a ray which has penetrated a region of lower film thickness.
Such transparent films can additionally be provided with a reflecting film so that the incident light ray first penetrates the transparent film, is subsequently reflected at the reflecting film and then passes through the transparent film a second time.
The phase modulation is modulated on here by the two-fold passage through the film thickness profile.
In accordance with another advantageous aspect of the invention, the security feature comprises a film applied to the respective carrier, with the film having a varying refractive index.
-4-A wave-front which passes through a transparent film having :a varying refractive index is thereby subjected to phase modulation. A ray which has passed through a region having a high refractive index is dephased over a ray which was directed through a region having a low refractive index.
Phase modulation is respectively generated by parts of a wave-front passing through different optical path lengths when passing through a film, with this occurring independently of whether the different optical path lengths are generated by a variation in refractive index or a variation in film thickness. However, a variation in refractive index has the advantage over a variation in film thickness that it is less easy to see on the film.
In accordance with another advantageous aspect of the invention, the security feature comprises a hologram applied to the respective carrier, with an object ray having a phase profile modulated thereon being able to be restored by a light ray incident to the hologram.
This solution has the advantage that the modulated information cannot be recognised without aids either on the hologram or on the object ray exiting the hologram.
The hologram per se already represents a security feature difficult to copy.
However, it would not be suspected that the hologram itself contains additional hidden information. But a security feature which is not actually recognised a such represents the best protection against forgery.
It is advantageous if the light ray incident to the hologram generates an image of a phase object. This is the case if the hologram contains the holographic record of a phase object which is restored by the incident light ray. A phase object is not
Phase modulation is respectively generated by parts of a wave-front passing through different optical path lengths when passing through a film, with this occurring independently of whether the different optical path lengths are generated by a variation in refractive index or a variation in film thickness. However, a variation in refractive index has the advantage over a variation in film thickness that it is less easy to see on the film.
In accordance with another advantageous aspect of the invention, the security feature comprises a hologram applied to the respective carrier, with an object ray having a phase profile modulated thereon being able to be restored by a light ray incident to the hologram.
This solution has the advantage that the modulated information cannot be recognised without aids either on the hologram or on the object ray exiting the hologram.
The hologram per se already represents a security feature difficult to copy.
However, it would not be suspected that the hologram itself contains additional hidden information. But a security feature which is not actually recognised a such represents the best protection against forgery.
It is advantageous if the light ray incident to the hologram generates an image of a phase object. This is the case if the hologram contains the holographic record of a phase object which is restored by the incident light ray. A phase object is not
- 5 -recognisable as such so that the image of the phase object is also only recognisable with aids.
The hologram is preferably manufactured by recording the interference image which is created by the interference of a reference ray and an object ray directed through a phase object. If the hologram manufactured in this way is subjected to the reference ray, the object ray can be restored. The object ray restored by the interference image again has the same phase modulation as the object ray used for the recording, as the complete information on the object ray recorded is contained in the interference Image.
To read out the security feature in accordance with the invention, it is exposed to a light ray, with a phase profile being modulated on the light ray by the security feature.
Subsequently, the modulated phase profile is transformed into an amplitude modulation. This is necessary to make visible the phase information which is invisible per se.
One method to generate amplitude modulation from phase modulation is the phase contrast method. In this method, a so-called phase-shifting annulus is introduced into a Fourier plane of the ray path. This phase-shifting annulus generates a phase shift between the light penetrating the annular region of the phase annulus and the remaining light. This leads to the phase information becoming visible as an amplitude image.
Another common method of making phase modulations visible is the so-called schlieren method. In this method, the zeroth order is filtered in a Fourier plane of the ray path. The lack of the zeroth order then leads to the creation of an amplitude image which reflects the phase information.
The hologram is preferably manufactured by recording the interference image which is created by the interference of a reference ray and an object ray directed through a phase object. If the hologram manufactured in this way is subjected to the reference ray, the object ray can be restored. The object ray restored by the interference image again has the same phase modulation as the object ray used for the recording, as the complete information on the object ray recorded is contained in the interference Image.
To read out the security feature in accordance with the invention, it is exposed to a light ray, with a phase profile being modulated on the light ray by the security feature.
Subsequently, the modulated phase profile is transformed into an amplitude modulation. This is necessary to make visible the phase information which is invisible per se.
One method to generate amplitude modulation from phase modulation is the phase contrast method. In this method, a so-called phase-shifting annulus is introduced into a Fourier plane of the ray path. This phase-shifting annulus generates a phase shift between the light penetrating the annular region of the phase annulus and the remaining light. This leads to the phase information becoming visible as an amplitude image.
Another common method of making phase modulations visible is the so-called schlieren method. In this method, the zeroth order is filtered in a Fourier plane of the ray path. The lack of the zeroth order then leads to the creation of an amplitude image which reflects the phase information.
-6-Another advantageous possibility of making the modulated phase profile visible is the use of an interferometer. Here, the phase-modulated ray is brought into interference with a non-modulated plane wave . Depending on the phase position of the two superimposed waves, constructive or destructive interference is created, which means that the phase modulation is transformed into a brightness distribution.
In accordance with another advantageous embodiment of the invention, the security feature comprises a hologram having the following properties being applied to a carrier, for example a document, an I.D. card, a bank note or a plastic card:
a first object ray having a modulated phase profile can be restored by a first light ray incident to the hologram at a first incidence angle. A second object ray having no phase modulation can be restored by a second light ray incident to the hologram at a second incidence angle. Amplitude modulation corresponding to the modulated phase profile of the first object ray is created from the interference between the first and the second object rays.
Such a hologram can be manufactured by the recording of two interference images.
The first interference image which has to be recorded is created by the interference of a first reference ray incident at a first incidence angle and a first object ray, with a phase profile being modulated on the first object ray. The second interference image is created by the interference of a second reference ray incident at a second incidence angle and a second object ray, with no phase profile being modulated on the second object ray.
The double-exposed hologram presented here provides a very reliable proof of authenticity. To make the stored information visible, the hologram must be exposed to two reference rays of the right wavelength under exactly the pre-defined incidence angles. Only then can the two object waves be restored and only with in-phase restoration of the two object waves does that interference arise which allows the _7_ modulated information to become visible. If only one of the two reference rays is incident, the interference image is not created.
The solution in accordance with the invention having two object rays is more or less an "integrated holographic interferometer". Here, a second, unmodulated ray interfering with the first object ray is also generated out of the hologram (which is the reason for the expression "integrated interferometer"). The first and the second object rays can both be stored in the hologram by the hologram being subjected to double exposure. If the hologram is exposed to a second reference ray incident at a second incidence angle, the second object ray is restored. By means of this solution, complicated optical reading apparatuses can therefore be dispensed with.
The double-exposed hologram can be read out by it being exposed to a first light ray at a first incidence angle and to a second light ray at a second incidence angle. In this way, the two object rays are restored and their interference can now be detected.
The interference image created is preferably captured on a screen. This is the simplest and cheapest solution.
As an alternative to this, the interference of the two object rays can also be captured by a camera. This has the advantage that the interference image can be evaluated better. It is in particular possible to evaluate the interference image further by means of an image processing system.
Further details and advantages of the invention are described below by means of several embodiments illustrated in the drawings, in which:
Fig. 1 shows the generation of a phase-modulated wave by means of a transparent foil with a varying film thickness;
_g-Fig. 2 shows the generation of a phase-modulated wave by means of a transparent foil which has a refractive index variation;
Fig. 3 shows an arrangement for the holographic recording of a phase-modulated object wave;
Fig.4 shows the restoration of a phase-modulated object wave for a reflection hologram manufactured in accordance with Fig. 3;
Fig. 5 shows the functional principle of the dark-field method, the schlieren method and the phase-contrast method;
Fig. 6 shows the superimposition of a plane wave on a phase-modulated wave in an interferometer;
Fig. 7A ~ shows the first exposure of a double-exposed hologram during which the object ray has no phase modulation;
Fig. 7B shows the second exposure of a hologram during which the object ray is directed through a phase object effecting phase modulation;
Fig. 8 shows the restoration of both the phase-modulated and the non-phase modulated object ray by means of two reference rays;
Fig. 9A shows the interference image of the phase-modulated and the non-phase modulated object ray in the way it can be captured on a screen inclined relative to the hologram plane;
Fig. 9B shows the interference image of the phase-modulated and the non-phase modulated object ray in the way it can be captured on a screen parallel to the hologram plane.
Fig. 1 shows how a plane wave 1 penetrates a transparent phase object 2, with information being modulated on the light wave 1 as phase modulation by the phase object 2.
Fig. 1 shows how phase modulation can be modulated on an incident wave front 1 by means of a transparent foil 2. For this purpose, the transparent foil 2 has regions of different film thickness. A light ray 3, which has penetrated a region of low film thickness, appears dephased over a light ray 4 which has penetrated a region of high film thickness. The phase shift here is proportional to the difference of the optical path:
0~ = 2~ n ~ Od In this way, information can be modulated on an incident ray as phase modulation by means of a foil 2.
In Fig. 1, the transparent foil 2 is penetrated in transmission by the wave front 1.
However, it is equally possible to provide the foil 2 additionally with a reflecting film and to expose it to a plane wave front so that the wave front passes through the film thickness profile twice. Accordingly, the double phase shift is produced with this reflection geometry.
In Fig. 2, a foil 6 is shown having regions of different refractive indices.
When the transparent foil 6 is penetrated by a wave front 5, then the refractive index profile is modulated on this wave front as a phase profile. A light ray 7, which has passed l~ -through a region of high refractive index, appears dephased over a light ray 8 which has gone through a region of low refractive index. With a constant film thickness of the transmitting film, that phase shift is produced which is proportional to the difference in refractive indices:
Ode _ ~ On ~ d Such a foil 6, whose refractive index varies, can also be provided with a reflecting film and be used in reflection geometry.
A phase profile can also be modulated on an incident light ray by means of a hologram. The arrangement for the exposure of such a hologram suitable as a security feature is shown in Fig. 3. First, an object wave is generated having the desired phase profile. To do this, a light wave 10 penetrates a phase object 11 which modulates the desired phase modulation on the wave. The phase-modulated object wave 12 is applied to the holographic film 9 from one side and the reference ray 13 is incident to the holographic film from the other side. The interference image created is recorded as a hologram. As the object and reference rays are incident to the holographic film from opposite sides, a reflection hologram is generated. A transmission hologram could be generated by the object and reference rays being incident to the holographic film from the same side.
In Fig. 4, it is shown how the original phase-modulated object wave can be restored by means of the hologram 9. To do this, the reflection hologram 9 is exposed to a reference ray 13 which restores the object ray 14. The restored object ray 14 has the same phase modulation as the object ray 12 used for the recording.
As the phase information modulated on an incident light ray by the security feature is not visible as such, this phase information has to be transformed into an amplitude image when the security feature is read out. Various techniques known, for instance, from microscopy exist for this purpose such as the dark-field method, the phase-contrast method or the schlieren method. The basic principle of the three techniques is illustrated in Fig. S. Here, an incident, phase-modulated wave 15 is imaged on a screen 17 by means of a lens or a lens element 16. If no manipulation were to be performed in the ray path, the screen 17 would appear to be illuminated uniformly brightly. To make the phase differences visible, a diaphragm is introduced into the ray path, namely at the Fourier plane.
In the dark-field method, a diaphragm 18 is used at whose centre a blackening 19 is present which filters the zeroth order of diffraction. The schlieren method, in which the zeroth order of diffraction is blocked out with the aid of a sharp tip extending into the ray path, functions in a similar way. In both cases, an amplitude image 20 of the modulated phase information can be seen on the screen 17.
In the phase-contrast method, a diaphragm 20 is introduced into the ray path, which diaphragm 20 contains a so-called phase-shifting annulus 21. Light transmitted through the phase-shifting annulus 21 is dephased over the other light. By means of the phase-shifting annulus, phase shift is introduced between the different orders of diffraction. The result is that the phase modulation of the incident light 15 is transformed into amplitude modulation which becomes visible on the screen 17.
In Fig. 6, an Michelson interferometer is shown with the aid of which phase modulation can also be made visible. With an interferometer, the transformation of phase modulation into amplitude modulation is performed by the object wave interfering with a reference wave. Depending on the relative phase position between the two waves, constructive or destructive interference is created.
With the interferometer shown in Fig. 6, the phase-modulated object ray 21 is applied to the semi-transparent mirror 22 which deflects part of the incident light downwards.
The non-deflected part of the object ray 21 is reflected by the stationary mirror 23 and again impacts the semi-transparent mirror 22. Again, a part of the object ray is reflected (ray 24) and projected onto the screen 26 by the lens or the lens element 25.
The semi-transparent mirror 22 is moreover exposed to the reference wave 27.
One part of the reference wave is reflected, the other part is transmitted, and this transmitted part of the reference wave is also imaged through the lens or the lens element 25 on the screen 26. Due to the superimposition of the two waves, an interference pattern is created on the screen 26. Depending on whether the screen 26 is tilted or not with respect to the minors 22 and 23, either the interference image 28 (for the case of a tilted screen) or the interference image 29 (for an optimum adjustment of the screen) is created. Instead of showing the interference image on a screen, it is also possible to record the interference image with a camera. This has the advantage that the interference image is then available in digital form and can be supplied to an image processing system.
In Figures 7A and 7B, a special kind of double exposure of a holograph is shown.
Figure 7A represents the first exposure of the holographic film 33. The first reference ray 34 is incident to the holographic film 33 at a first incidence angle. From the other side, the object ray 35 is applied to the holographic film 33 and interferes with the reference ray 34. The interference image is recorded as the first exposure on the holographic film 33.
With the selected taking geometry, the reference ray and the object ray are incident to the holographic film from opposite sides. A hologram taken in this way can be restored after its development by a light ray incident from the observer side;
it is therefore a reflection hologram.
After the holographic film 33 has been exposed in accordance with Figure 7A, it is exposed to another, second exposure in accordance with Figure 7B. The second exposure is performed using a reference ray at a second incidence angle differing from the first incidence angle. To obtain the object ray, a wave-front 37 passes through a phase object 38 and the object ray 39 having phase modulation is created. In this way, information, for example a text, an emblem or a logo, can be modulated on the object ray. The object wave 39 and the second reference ray 36 generate an interference image which is recorded on the holographic film 33 during the second exposure.
Figure 8 shows how the double-exposed hologram can be restored. If the developed hologram 33 is exposed to the second reference ray 36 at the second incidence angle, then the phase-modulated wave is restored. Phase modulations are, however, invisible to the eye.
To make the phase information visible, the hologram 33 has to be exposed at the same time to the first reference ray 34 at the first incidence angle and to the second reference ray 36 at the second incidence angle. The first reference ray 34 restores a light wave without phase modulation. The second reference ray 36 restores an object wave on which information has been modulated as phase modulation. Interference arises between these two restored wave-fronts and by means of this interference, the invisible phase modulation is transformed into a visible amplitude modulation.
This amplitude modulation becomes visible on the screen 40. Depending on whether the screen has a certain tilt relative to the interfering wave-fronts or not, either the screen image shown in Figure 9A or that shown in Figure 9B is created.
A visible screen image is only created if the phase-modulated object wave restored by the second reference ray has the non-interfered wave restored by the first reference ray superimposed on it. To this extent, the double-exposed hologram 33 acts as an "integrated holographic interferometer"; as with an interferometer, the interference between an interfered wave and a plane wave-front is made visible.
It is important for the use of the double-exposure method described as a security feature that the information can be obtained neither by the first reference ray 34 alone nor by the second reference ray 36 alone. Only when both reference rays are applied to the holographic film at the respectively proper incidence angles can the stored information be obtained.
In accordance with another advantageous embodiment of the invention, the security feature comprises a hologram having the following properties being applied to a carrier, for example a document, an I.D. card, a bank note or a plastic card:
a first object ray having a modulated phase profile can be restored by a first light ray incident to the hologram at a first incidence angle. A second object ray having no phase modulation can be restored by a second light ray incident to the hologram at a second incidence angle. Amplitude modulation corresponding to the modulated phase profile of the first object ray is created from the interference between the first and the second object rays.
Such a hologram can be manufactured by the recording of two interference images.
The first interference image which has to be recorded is created by the interference of a first reference ray incident at a first incidence angle and a first object ray, with a phase profile being modulated on the first object ray. The second interference image is created by the interference of a second reference ray incident at a second incidence angle and a second object ray, with no phase profile being modulated on the second object ray.
The double-exposed hologram presented here provides a very reliable proof of authenticity. To make the stored information visible, the hologram must be exposed to two reference rays of the right wavelength under exactly the pre-defined incidence angles. Only then can the two object waves be restored and only with in-phase restoration of the two object waves does that interference arise which allows the _7_ modulated information to become visible. If only one of the two reference rays is incident, the interference image is not created.
The solution in accordance with the invention having two object rays is more or less an "integrated holographic interferometer". Here, a second, unmodulated ray interfering with the first object ray is also generated out of the hologram (which is the reason for the expression "integrated interferometer"). The first and the second object rays can both be stored in the hologram by the hologram being subjected to double exposure. If the hologram is exposed to a second reference ray incident at a second incidence angle, the second object ray is restored. By means of this solution, complicated optical reading apparatuses can therefore be dispensed with.
The double-exposed hologram can be read out by it being exposed to a first light ray at a first incidence angle and to a second light ray at a second incidence angle. In this way, the two object rays are restored and their interference can now be detected.
The interference image created is preferably captured on a screen. This is the simplest and cheapest solution.
As an alternative to this, the interference of the two object rays can also be captured by a camera. This has the advantage that the interference image can be evaluated better. It is in particular possible to evaluate the interference image further by means of an image processing system.
Further details and advantages of the invention are described below by means of several embodiments illustrated in the drawings, in which:
Fig. 1 shows the generation of a phase-modulated wave by means of a transparent foil with a varying film thickness;
_g-Fig. 2 shows the generation of a phase-modulated wave by means of a transparent foil which has a refractive index variation;
Fig. 3 shows an arrangement for the holographic recording of a phase-modulated object wave;
Fig.4 shows the restoration of a phase-modulated object wave for a reflection hologram manufactured in accordance with Fig. 3;
Fig. 5 shows the functional principle of the dark-field method, the schlieren method and the phase-contrast method;
Fig. 6 shows the superimposition of a plane wave on a phase-modulated wave in an interferometer;
Fig. 7A ~ shows the first exposure of a double-exposed hologram during which the object ray has no phase modulation;
Fig. 7B shows the second exposure of a hologram during which the object ray is directed through a phase object effecting phase modulation;
Fig. 8 shows the restoration of both the phase-modulated and the non-phase modulated object ray by means of two reference rays;
Fig. 9A shows the interference image of the phase-modulated and the non-phase modulated object ray in the way it can be captured on a screen inclined relative to the hologram plane;
Fig. 9B shows the interference image of the phase-modulated and the non-phase modulated object ray in the way it can be captured on a screen parallel to the hologram plane.
Fig. 1 shows how a plane wave 1 penetrates a transparent phase object 2, with information being modulated on the light wave 1 as phase modulation by the phase object 2.
Fig. 1 shows how phase modulation can be modulated on an incident wave front 1 by means of a transparent foil 2. For this purpose, the transparent foil 2 has regions of different film thickness. A light ray 3, which has penetrated a region of low film thickness, appears dephased over a light ray 4 which has penetrated a region of high film thickness. The phase shift here is proportional to the difference of the optical path:
0~ = 2~ n ~ Od In this way, information can be modulated on an incident ray as phase modulation by means of a foil 2.
In Fig. 1, the transparent foil 2 is penetrated in transmission by the wave front 1.
However, it is equally possible to provide the foil 2 additionally with a reflecting film and to expose it to a plane wave front so that the wave front passes through the film thickness profile twice. Accordingly, the double phase shift is produced with this reflection geometry.
In Fig. 2, a foil 6 is shown having regions of different refractive indices.
When the transparent foil 6 is penetrated by a wave front 5, then the refractive index profile is modulated on this wave front as a phase profile. A light ray 7, which has passed l~ -through a region of high refractive index, appears dephased over a light ray 8 which has gone through a region of low refractive index. With a constant film thickness of the transmitting film, that phase shift is produced which is proportional to the difference in refractive indices:
Ode _ ~ On ~ d Such a foil 6, whose refractive index varies, can also be provided with a reflecting film and be used in reflection geometry.
A phase profile can also be modulated on an incident light ray by means of a hologram. The arrangement for the exposure of such a hologram suitable as a security feature is shown in Fig. 3. First, an object wave is generated having the desired phase profile. To do this, a light wave 10 penetrates a phase object 11 which modulates the desired phase modulation on the wave. The phase-modulated object wave 12 is applied to the holographic film 9 from one side and the reference ray 13 is incident to the holographic film from the other side. The interference image created is recorded as a hologram. As the object and reference rays are incident to the holographic film from opposite sides, a reflection hologram is generated. A transmission hologram could be generated by the object and reference rays being incident to the holographic film from the same side.
In Fig. 4, it is shown how the original phase-modulated object wave can be restored by means of the hologram 9. To do this, the reflection hologram 9 is exposed to a reference ray 13 which restores the object ray 14. The restored object ray 14 has the same phase modulation as the object ray 12 used for the recording.
As the phase information modulated on an incident light ray by the security feature is not visible as such, this phase information has to be transformed into an amplitude image when the security feature is read out. Various techniques known, for instance, from microscopy exist for this purpose such as the dark-field method, the phase-contrast method or the schlieren method. The basic principle of the three techniques is illustrated in Fig. S. Here, an incident, phase-modulated wave 15 is imaged on a screen 17 by means of a lens or a lens element 16. If no manipulation were to be performed in the ray path, the screen 17 would appear to be illuminated uniformly brightly. To make the phase differences visible, a diaphragm is introduced into the ray path, namely at the Fourier plane.
In the dark-field method, a diaphragm 18 is used at whose centre a blackening 19 is present which filters the zeroth order of diffraction. The schlieren method, in which the zeroth order of diffraction is blocked out with the aid of a sharp tip extending into the ray path, functions in a similar way. In both cases, an amplitude image 20 of the modulated phase information can be seen on the screen 17.
In the phase-contrast method, a diaphragm 20 is introduced into the ray path, which diaphragm 20 contains a so-called phase-shifting annulus 21. Light transmitted through the phase-shifting annulus 21 is dephased over the other light. By means of the phase-shifting annulus, phase shift is introduced between the different orders of diffraction. The result is that the phase modulation of the incident light 15 is transformed into amplitude modulation which becomes visible on the screen 17.
In Fig. 6, an Michelson interferometer is shown with the aid of which phase modulation can also be made visible. With an interferometer, the transformation of phase modulation into amplitude modulation is performed by the object wave interfering with a reference wave. Depending on the relative phase position between the two waves, constructive or destructive interference is created.
With the interferometer shown in Fig. 6, the phase-modulated object ray 21 is applied to the semi-transparent mirror 22 which deflects part of the incident light downwards.
The non-deflected part of the object ray 21 is reflected by the stationary mirror 23 and again impacts the semi-transparent mirror 22. Again, a part of the object ray is reflected (ray 24) and projected onto the screen 26 by the lens or the lens element 25.
The semi-transparent mirror 22 is moreover exposed to the reference wave 27.
One part of the reference wave is reflected, the other part is transmitted, and this transmitted part of the reference wave is also imaged through the lens or the lens element 25 on the screen 26. Due to the superimposition of the two waves, an interference pattern is created on the screen 26. Depending on whether the screen 26 is tilted or not with respect to the minors 22 and 23, either the interference image 28 (for the case of a tilted screen) or the interference image 29 (for an optimum adjustment of the screen) is created. Instead of showing the interference image on a screen, it is also possible to record the interference image with a camera. This has the advantage that the interference image is then available in digital form and can be supplied to an image processing system.
In Figures 7A and 7B, a special kind of double exposure of a holograph is shown.
Figure 7A represents the first exposure of the holographic film 33. The first reference ray 34 is incident to the holographic film 33 at a first incidence angle. From the other side, the object ray 35 is applied to the holographic film 33 and interferes with the reference ray 34. The interference image is recorded as the first exposure on the holographic film 33.
With the selected taking geometry, the reference ray and the object ray are incident to the holographic film from opposite sides. A hologram taken in this way can be restored after its development by a light ray incident from the observer side;
it is therefore a reflection hologram.
After the holographic film 33 has been exposed in accordance with Figure 7A, it is exposed to another, second exposure in accordance with Figure 7B. The second exposure is performed using a reference ray at a second incidence angle differing from the first incidence angle. To obtain the object ray, a wave-front 37 passes through a phase object 38 and the object ray 39 having phase modulation is created. In this way, information, for example a text, an emblem or a logo, can be modulated on the object ray. The object wave 39 and the second reference ray 36 generate an interference image which is recorded on the holographic film 33 during the second exposure.
Figure 8 shows how the double-exposed hologram can be restored. If the developed hologram 33 is exposed to the second reference ray 36 at the second incidence angle, then the phase-modulated wave is restored. Phase modulations are, however, invisible to the eye.
To make the phase information visible, the hologram 33 has to be exposed at the same time to the first reference ray 34 at the first incidence angle and to the second reference ray 36 at the second incidence angle. The first reference ray 34 restores a light wave without phase modulation. The second reference ray 36 restores an object wave on which information has been modulated as phase modulation. Interference arises between these two restored wave-fronts and by means of this interference, the invisible phase modulation is transformed into a visible amplitude modulation.
This amplitude modulation becomes visible on the screen 40. Depending on whether the screen has a certain tilt relative to the interfering wave-fronts or not, either the screen image shown in Figure 9A or that shown in Figure 9B is created.
A visible screen image is only created if the phase-modulated object wave restored by the second reference ray has the non-interfered wave restored by the first reference ray superimposed on it. To this extent, the double-exposed hologram 33 acts as an "integrated holographic interferometer"; as with an interferometer, the interference between an interfered wave and a plane wave-front is made visible.
It is important for the use of the double-exposure method described as a security feature that the information can be obtained neither by the first reference ray 34 alone nor by the second reference ray 36 alone. Only when both reference rays are applied to the holographic film at the respectively proper incidence angles can the stored information be obtained.
Claims (19)
1. A security feature which is applied to a carrier, for example a document, an I.D. card, a bank note or a plastic card, with a phase profile being able to be modulated by the security feature on a light ray incident to the security feature.
2. A security feature in accordance with claim 1, which comprises a film being applied to the carrier, with the film having a varying film thickness and with a phase profile corresponding to the film thickness variation being able to be modulated on an incident light ray by the film.
3. A security feature in accordance with either of claims 1 or 2, which comprises a film being applied to the carrier, with the film having a varying refractive index and with a phase profile corresponding to the refractive index variation being able to be modulated on an incident light ray by the film.
4. A security feature in accordance with claim 1, which comprises a hologram being applied to the carrier, with an object ray having a modulated phase profile thereon being able to be restored by a light ray incident to the hologram.
5. A security feature in accordance with claim 4, wherein the light ray incident to the hologram generates an image of a phase object.
6. A security feature in accordance with either of claims 4 or 5, wherein the hologram is made by recording the interference image created by the interference of a reference ray and an object ray directed through a phase object.
7. A security feature in accordance with any of claims 1 to 6, wherein the modulated phase profile can be shown by means of the phase contrast method, with the modulated phase profile being able to be transformed into amplitude modulation by means of the phase contrast method.
8. A security feature in accordance with any of claims 1 to 6, wherein the modulated phase profile can be shown by means of the schlieren method, with the modulated phase profile being able to be transformed into amplitude modulation by means of the schlieren method.
9. A security feature in accordance with any of claims 1 to 6, wherein the modulated phase profile can be shown by means of an interferometer, with the modulated phase profile being able to be transformed into amplitude modulation by means of the interferometer.
10. A method to read out a security feature, being a security feature in accordance with any of claims 1 to 6, which has the following steps:
a) Exposure of the security feature to a light ray, with the light ray having a phase profile modulated on it by the security feature;
b) Transformation of the modulated phase profile into amplitude modulation.
a) Exposure of the security feature to a light ray, with the light ray having a phase profile modulated on it by the security feature;
b) Transformation of the modulated phase profile into amplitude modulation.
11. A method for the reading out of a security feature in accordance with claim 10, wherein the modulated phase profile is transformed into amplitude modulation by means of the phase-contrast method.
12. A method for the reading out of a security feature in accordance with claim 10, wherein the modulated phase profile is transformed into amplitude modulation by means of the schlieren method.
13. A method for the reading out of a security feature in accordance with claim 10, wherein the modulated phase profile is transformed into amplitude modulation by means of an interferometer.
14. A security feature which comprises a hologram applied to a carrier, for example to a document, an I.D. card, a bank note or a plastic card, with a first object ray having a modulated phase profile being able to be restored by a first light ray incident to the hologram at a first incidence angle;
with a second object ray having no modulated phase profile being able to be restored by a second light ray incident to the hologram at a second incidence angle;
with amplitude modulation being created by means of the interference between the first and the second object rays which corresponds to the modulated phase profile of the first object ray.
with a second object ray having no modulated phase profile being able to be restored by a second light ray incident to the hologram at a second incidence angle;
with amplitude modulation being created by means of the interference between the first and the second object rays which corresponds to the modulated phase profile of the first object ray.
15. A method for the reading out of a security feature, being a security feature in accordance with claim 14, which possesses the following steps:
a) Exposure of the hologram to a first light ray at a first incidence angle, whereby a first object ray is created;
b) Exposure of the hologram to a second light ray at a second incidence angle, whereby a second object ray is created;
c) Detection of the interference image created by the interference of the two object rays.
a) Exposure of the hologram to a first light ray at a first incidence angle, whereby a first object ray is created;
b) Exposure of the hologram to a second light ray at a second incidence angle, whereby a second object ray is created;
c) Detection of the interference image created by the interference of the two object rays.
16. A method for the reading out of a security feature in accordance with claim 15, wherein the interference image created by the interference of the two object rays is captured on a screen.
17. A method for the reading out of a security feature in accordance with claim 15, wherein the interference image created by the interference of the two object rays is captured by a camera.
18. A method for the manufacture of a security feature, being a security feature in accordance with claim 14, which has the following steps:
a) Recording of a first interference image generated by a first reference ray incident at a first incidence angle and by a first object ray, with the first object ray having a phase profile modulated on it; and b) Recording of a second interference image generated by a second reference ray incident at a second incidence angle and by a second object ray, with the second object ray having no phase profile modulated on it.
a) Recording of a first interference image generated by a first reference ray incident at a first incidence angle and by a first object ray, with the first object ray having a phase profile modulated on it; and b) Recording of a second interference image generated by a second reference ray incident at a second incidence angle and by a second object ray, with the second object ray having no phase profile modulated on it.
19. A method for the manufacture of a security feature in accordance with claim 18, wherein the first object ray has a phase profile modulated on it by being directed through a phase object.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19926689.1 | 1999-06-11 | ||
| DE19926689A DE19926689A1 (en) | 1999-06-11 | 1999-06-11 | Optical authenticity feature |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2310768A1 true CA2310768A1 (en) | 2000-12-11 |
Family
ID=7910944
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002310768A Abandoned CA2310768A1 (en) | 1999-06-11 | 2000-06-06 | An optical authenticity feature |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP1060908A3 (en) |
| JP (1) | JP2001034722A (en) |
| CA (1) | CA2310768A1 (en) |
| DE (1) | DE19926689A1 (en) |
| ZA (1) | ZA200002890B (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB0300555D0 (en) * | 2003-01-10 | 2003-02-12 | Durand Technology Ltd | Improvements in or relating to security or authentication markings or the like |
| US20080084592A1 (en) * | 2006-10-09 | 2008-04-10 | General Electric Company | Molded Article Incorporating Volume Hologram |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU584035B2 (en) * | 1985-03-01 | 1989-05-11 | Kenneth John Hayden | Holographic security devices and systems |
| GB8812890D0 (en) * | 1988-05-31 | 1988-07-06 | De La Rue Co Plc | Security device & methods & apparatus for verification |
| US5886798A (en) * | 1995-08-21 | 1999-03-23 | Landis & Gyr Technology Innovation Ag | Information carriers with diffraction structures |
| ES2171747T3 (en) * | 1995-11-28 | 2002-09-16 | Ovd Kinegram Ag | OPTICAL INFORMATION SUPPORT. |
| DE29819954U1 (en) * | 1998-11-07 | 1999-03-04 | Basler GmbH, 22926 Ahrensburg | Device for the optical inspection of holograms |
-
1999
- 1999-06-11 DE DE19926689A patent/DE19926689A1/en not_active Ceased
-
2000
- 2000-05-26 EP EP00111391A patent/EP1060908A3/en not_active Withdrawn
- 2000-06-06 CA CA002310768A patent/CA2310768A1/en not_active Abandoned
- 2000-06-07 JP JP2000170667A patent/JP2001034722A/en active Pending
- 2000-06-08 ZA ZA200002890A patent/ZA200002890B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| DE19926689A1 (en) | 2000-12-14 |
| EP1060908A3 (en) | 2001-09-19 |
| JP2001034722A (en) | 2001-02-09 |
| ZA200002890B (en) | 2000-12-12 |
| EP1060908A2 (en) | 2000-12-20 |
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| EEER | Examination request | ||
| FZDE | Dead |