COUNTERFEITING AND A DEVICE FOR ITS IDENTIFICATION

The invention relates to security technologies, in particular, holographic protection systems and can be applied to the high level protection of products (important documents, securities and industrial products) from counterfeits.

The rainbow (embossed) holographic systems are known as a holographic method of protection that found a wide application for protection of documents, bank notes, excise marks, industrial products, etc. [1]. The recorded information is directly observed with the eye. Disadvantage of such a type of protective holograms is that it is possibe to copy holograms by optical methods, which allows to counterfeit them rather easily.

The proposed system of protection is based on a new physical principle - a polarization- holographic method and the use of polarization-sensitive materials.

The methods of holographic protection using the polarization of light are known [3,4,5,6]. These methods include rainbow holograms with a certain image, on which the polarizing liquid- crystalline film or markers are coated that reflect either the right-circularly polarized or left-circularly polarized light. An analyzing device determines the direction of rotation (right-hand or left-hand) of the reflected circularly polarized light, thereby determining the authenticity of the protective element. However, as follows from the description, coding according to the polarization state is only limited by two cases - the right-hand or left-hand circulation, and in this cases only polarizing properties of the liquid crystal layer coated on a conventional rainbow hologram are used.

In [7,8] two latent polarization images are used, formed by special cells that turn the plane of polarization at a certain angle. The authentication is done by the device, which contains a matrix of linear polarizers with the orientation of the axes of polarizers that coincides with the turn of the polarization plane of each cell. Thus, linearly polarized light with different azimuths of the polarization plane is used in the protection systems of such a type.

In [9] the so-called photoaddressable polymers based on azobenzene dyes incorporated into the side chains are used for the storage of confidential information in the form of latent polarization holograms and the information is recorded by orthogonally circularly polarized beams.

The inventions [10, 1 1] are known in which the diffraction of a light beam on the protective element in the form of holographic protective structure is used for authentication, and these elements also cause the polarization of the reflected light. Authentication of the protective elements is carried out by the determination of the rotation direction of circularly polarized light in the diffracted orders while diffracted beams pass through a circular polarizer [10], or the azimuth of the polarization plane of the diffracted beams is determined [1 1]. It follows from the description of the above mentioned inventions that the disadvantage of these methods is the fact that in these methods for information coding only the simplest form of the polarization state of light are used - linear and circular polarization.

In general, the polarization of light is elliptical and is characterized by four main parameters: ellipticity, the azimuth of the major axis, the direction of rotation of the polarization ellipse and also the degree of polarization. In particular, light is linearly polarized when the ellipticity equals to zero, and the light is circularly polarized when ellipticity equals to unit. It is evident, that there is an infinite set of polarization states, with different ellipticity, azimuth, the direction of rotation and the degree of polarization.

The proposed polarization-holographic protection system includes polarization-holographic protective elements, in which, for recording code, the polarization-sensitive materials obtained by a certain technology are used, and an identifying device that makes a complete in real time analysis of the polarization state of two beams diffracted on the element, and provides an possibility the level of protection to be significantly increased at the expense of the fact that:

1. The polarization-holographic protective element is recorded by two recording beams with a one specific combination of polarization state, which is preselected from a large set of possible combinations of polarization states (the number of combinations is of the order of 10 ¹⁸ ) For example, the polarization state of the first recording beam is: ellipticity is 0.235, the azimuth of the major axis is 28 degrees 32 minutes, the right-hand direction of rotation, the degree of polarization is 96%; the second recording beam: ellipticity is 0.085, the azimuth of the major axis is 85 degrees 15 minutes, the left-hand direction of rotation, the degree of polarization is 98%.

2. The reading laser beam, when incident on the polarization-holographic protective element, diffracts on it and the polarization state of the diffracted beams is the code for identification of the authenticity of the element.

3. For obtaining polarization-holographic protective elements polarization-sensitive materials are used which should have high sensitivity to actinic polarized light, high stability and resistance to environmental influences: sufficient thermostability up to 80 degrees Celsius, resistance to direct sunlight and moisture. For example, we can use the materials synthesized by us on the basis of azo dye Mordant Pure Yellow modified by adding supplementary ionizing functional groups and introduced into gelatin matrix stained by using special tannins, that are made so that they possess a specific and defined meanings of scalar s, and two vector reactions v _L , v _G on the action of polarized light and which can be changed by using different technological regimes. Besides, polarization- sensitive materials can be used such as: different combinations of azo dyes and optically isotropic polymer matrices; azobenzene-containing polymers: a side-chain azobenzene polyester, liquid- crystalline side-chain polymers, amorphous side-chain polymers; several types of polymer systems: comb-shaped liquid-crystalline co-polymers; azogels; azobenzene based methacrilic copolymers; liquid-crystalline side-chain group polymers; azobenzene polyimide; azobenzene peptides; azobenzene dendrimers; photoaddressable polymers; photorefractive azobenzene polymers; polymer photochromic cholesterics; silver-halide materials; alkali-halide crystals containing anisotropic color centers; bacteriorodopsin; materials on the bases of xanthene dyes, and also main-chain azopolymers which are being developed by us, in which azo dyes are introduced into the main chain of the macromolecule of polymer, as well as other polarization-sensitive materials that satisfy the mentioned conditions.

4. It is possible to obtain protective elements both of a transmission and reflective type, however elements of a reflective type are more multipurpose and convenient. The elements of a transmission type are made by coating a polarization-sensitive material on a transparent isotropic polymer film, and elements of a reflective type are made by coating a polarization-sensitive material on a polymer film with a reflecting layer or on the usual rainbow hologram in order to increase the level of its protection.

5. The identification of the authenticity of a protective element is carried out by its illumination with a reading nonactinic laser beam with a definite polarization state and by a complete analysis of the polarization states of two reflected or transmitted beams formed in the process of diffraction of a reading beam on a protective element.

6. Copying a protective polarization-holographic element by optical methods is impossible because such an effort will cause the destruction of the element.

7. Protective elements visually look absolutely homogeneous, polarization information recorded on them is not visually observed either by an eye or by any usual identifying device (ultraviolet and infrared radiation, magnetic and electrical fields, microscopes, etc.). The precise determination of the polarization state of light beams diffracted on a protective element even by means of traditional polarization optics is a very difficult task.

8. Authenticity of a protective element is carried out by means of a special identifying device created by us. The main analyzing detail of this device is a polarization-holographic element created by us that enables complete analysis of the polarization state of incident light to be carried out in real time (simultaneous determination of ellipticity, azimuth, the direction of rotation of a polarization ellipse and the degree of polarization). The device automatically compares the meanings of these parameters with the etalon that is stored in the memory of a comparison block of the device.

9. In an identifying device there is the possibility of changing the polarization state of the reading light beam. This additionally increases the level of protection because the polarization state of light beams diffracted on the same protective element depends on the polarization of the reading light. In addition, the possibility to change the polarization state of light in the device allows the criteria of the authenticity of the same protective element to be changed.

The principle of the operation of the polarization-holographic protection system is as follows:

1. As is known, the polarization-holographic method is a method of recording and reconstruction of optical information by using one of the basic characteristics of light - the state of its polarization [12].

The polarization-holographic elements are recorded by two coherent arbitrary elliptically polarized actinic light beams (with arbitrary ellipticity, the direction of rotation, azimuth and the degree of polarization). For example, Fig. 1 shows one of the possible schemes of the recording of the polarization holographic protective elements. As a source of actinic coherent light, for example, DSSP laser with a wavelength of 473 nm and a power of 200 mW is used. The laser beam while passing through the Wollaston prism is divided into two beams with orthogonal linear polarization. Then, these beams are re-united by means of mirrors in the plane, where the polarization-sensitive material is placed. Between mirrors and the polarization-sensitive material the quarter-wave phase plates with the capability of rotation are placed, in order to be able to change the polarization state of recording beams.

It will be noted that theoretically there is an infinite set of combinations of polarization states of recording beams (practically the number of combinations is of the order of 10 ¹⁸ ). It follows that it is possible to choose its own combination for each concrete system of protection and to record a polarization hologram by these beams. In case two beams with a simple wave front (plane or spherical) are used for recording, a polarization hologram represents a polarization-holographic diffraction grating. In a case when one of recording beams (an object beam) contains some image then we shall receive a polarization hologram.

For example, while turning quarter-wave plates at a certain angle it is possible to obtain a polarization state of one of the recording beams: ellipticity of 0.235, an azimuth of the major axis of 28 degrees 32 minutes, the right-handed direction of rotation, a degree of polarization of 96 %; and for the second beam: ellipticity of 0.085, an azimuth of the major axis of 85 degrees 15 minutes, the left-handed direction of rotation, a degree of polarization of 98 %. It is possible to obtain recording beams with any polarization state by changing an angle of rotation of quarter-wave plates and an azimuth of linear polarization of a laser beam.

For recording polarization-holographic protective elements polarization-sensitive materials are used in which appropriate anisotropy and gyrotropy of optical parameters are induced under the action of polarized light that depends on the properties of the medium (on the meanings of the scalar and two vector reactions and v _G specific for the given medium) [12]. These materials should have high sensitivity to actinic polarized light, high stability and resistance to environmental influences: sufficient thermostability up to 80 degrees Celsius, resistance to direct sunlight and moisture.

When a nonactinic reading light beam with a definite polarization state is incident on the polarization-holographic protective element, then in the process of reflection (or passing) two diffracted beams (+1 and -1) and one nondiffracted beam are formed. The polarization state of diffracted beams unambiguously corresponds to the concrete protective element.

It is possible to change the meanings of these reactions of the material by using different technologies of obtaining polarization sensitive materials. This further increases the level of protection, since we obtain a different polarization state of the beams diffracted on a protective element while using a pair of recording beams with the same polarization state but using a medium with different meanings of the reactions.

For example, if a protective element was recorded by recording beams with the above- mentioned polarization states and the meanings of the reactions of material satisfy the relations s = v _L and v _L = -v _G , only in this case the polarization state of +1 and -1 diffraction orders is equal to the polarization state of recording beams. But in case when the meanings of reactions do not satisfy these relations, the polarization state of +1 and -1 diffracted orders will not be equal to the polarization of recording beams and depend on the meanings of these reactions.

It will be noted that the code which determines the authenticity of the protective element is the polarization state of two beams diffracted on the protective element.

It is possible to obtain polarization-holographic elements both of transmission and reflection type. In case of more universal and convenient protective elements of a reflection type, polarization- sensitive material is coated on the reflecting mirror layer of a polymer film or on a rainbow hologram and then a polarization hologram is recorded on this material. On the opposite side of the film an adhesive layer is coated in order to glue protective elements on the protected object.

Unlike holographic security systems existing now, a significant advantage of the proposed method of code recording is that it is impossible to copy a protective element by optical methods, because they have high absorption on an actinic wavelength and in case of high power density of actinic expos ure light, that is necessary for optical copying, the irreversible destruction of the recorded information takes place.

It will be noted that one more advantage of this protection system is that protective elements visually look absolutely homogeneous and it is impossible to identify by eye and by any commonly used identifying devices what kind of polarization information is recorded on the given element. 2. The authenticity of polarization-holographic protective elements can be easily determined only by means of a special identifying device created by us.

When a nonactinic reading beam with definite polarization state is incident on a polarization- holographic protective element, it diffracts and as a result two diffracted beams (+1 and -1) and one nondiffracted zero beam are formed during reflection (passage).

The polarization state of diffracted beams, that represents a recorded code, uniquely corresponds to a definite protective element. The device automatically makes a complete analysis of the polarization state of beams (i.e. reading of a code) and for each type of a protective element compares the polarization state of diffracted beams (the code) with an etalon that is stored in the memory of a comparison block of the device.

For the analysis of the polarization state of diffracted beams a polarization-holographic element created by us is used in an identifying device that allows the polarization state of diffracted beams to be determined simultaneously and in real time: ellipticity, the direction of rotation and the azimuth of the major axis of polarization ellipse.

For example, in Fig. 2 the principal scheme of an identifying device for the determination of the authenticity of the protective elements of a reflective type is given. As a source of a reading nonactinic beam, for example, a diode laser with wavelength of 635 nm and power of 10 mW is used. On the propagation path of the beam a quarter-wave plate is placed, by the rotation of which it is possible to change the polarization state of a readout beam. In turn the change of the polarization of a reading beam results in a corresponding change of the polarization state of beams diffracted on a protective element. This circumstance allows an additional increase of the level of protection. And this enables the criterion of authenticity of the same protective element to be changed.

Two diffracted beams (+1 and -1), reflected from the protective element, get on the polarization-holographic analyzing elements that represent the set of polarization-holographic diffraction gratings recorded by a certain way. These elements form eight light beams and combinations of the intensities of these beams correspond to the code recorded on a protective element. These beams get on photodetectors, for example, photoresistors.

Photodetectors are connected with a measuring device that is made on the basis of, for example, analog dividers and operational amplifiers. An electronic signal that came out of the measuring block gets in the block of comparison that is made on the basis of, for example, comparators. A decoder performs the reading of the code and comparison of the polarization state of the beams diffracted on the element with an etalon, that is stored in the memory of the block of comparison, and by this, determines the authenticity of a protective element.

The polarization-holographic protective method essentially increases a level of protection of important documents, securities and industrial products (including existing rainbow holograms). References:

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United States Patent Application 20050219669, October 6, 2005. (WO/2005/093648, Intenational Application No.PCT/IN2004/000071).

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