WO2007072426A2

WO2007072426A2 - Method of manufacturing a polarization retardation film

Info

Publication number: WO2007072426A2
Application number: PCT/IB2006/054953
Authority: WO
Inventors: Mark T. Johnson; Pim T. Tuyls; Mireille A. Reijme; Bianca M. I. Van Der Zande; Johan Lub
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-12-23
Filing date: 2006-12-19
Publication date: 2007-06-28
Also published as: WO2007072426A3

Abstract

The present invention relates to a method of manufacturing a polarization retardation film, the method comprising the steps of coating a surface (112) with a polymerizable mesogenic composition comprising a polymerizable mesogenic compound (113) and an isomerizable compound, inducing isomerization of non-mesogenic isomers in the composition, creating an anisotropic and an isotropic region in the coating by inducing phase separation of the polymerizable mesogenic composition, and polymerizing the coating in order to obtain a polarization retardation film. The invention also relates to a polarization retardation film generated thereby for use in security applications. The invention further relates to a system for identification/authentication using a polarization retardation film of objects associated with a polarization retardation film according to the present invention.

Description

Method of manufacturing a polarization retardation film

The present invention relates to a method of manufacturing a polarization retardation film, a well as to the use of a polarization retardation film obtainable by means of the method in security applications. The invention further relates to a system for identification/authentication of an object, using such a polarization retardation film, and a system for enrolment of an object comprising such a polarization retardation film.

The loss of revenues due to counterfeited and pirated objects constitutes a serious economic threat to manufacturers and vendors alike. Products that are known to be susceptible to counterfeit practices range from watches, clothing, shoes, jewelry, CDs, DVDs, to software. There is no clear definition of objects that are susceptible to counterfeiting and piracy, but there is communality; they are desirable and, generally, there is either cost or effort involved in obtaining the authentic object. In response to counterfeiting and piracy, vendors have added labels and tags to their products to provide a proof of authenticity. However, as counterfeiters become more sophisticated, they may duplicate such means/markings. Therefore, better means/markings for proving authenticity are required.

Documents such as passports, credit cards and banknotes have equally been plagued by counterfeiting, and consequently have also been provided with a multitude of means/markings for proving authenticity. In spite of these means/markings, there remains a need for new means for proof of authenticity in order to deter counterfeiters. European Patent Application EP 1 336 874, entitled "Method of preparing an anisotropic polymer film on a substrate with a structured surface" presents a method of manufacturing an anisotropic polymer film with improved alignment on a substrate having a structured surface. EP 1 336 874 indicates that such an anisotropic film can be used as a false-proof security marking. However, provided a machine is available for producing such a film, this machine may be used for duplicating and/or reproducing the film.

European Patent Application EP 0 952 477, entitled "Wide viewing angle polarization plate and liquid crystal display" discloses a wide viewing angle polarization plate and method of manufacturing thereof, which involves the demixing of two poorly miscible materials, particularly materials which undergo phase separation during manufacture of a wide viewing angle polarization plate. A problem associated with the method of manufacturing PRFs disclosed in EP 0 952 477 is that the method does not provide subtle control of the phase-separation process, which is a desirable feature for a method of manufacturing PRFs for use in security applications. International Patent Application WO 01/29148 discloses a method of manufacturing a, possibly birefringent, polymer film or coating, which provides elements for protection against forgery and copying, wherein one of the materials in the film or coating is removed by using a selective solvent, and wherein the structure of the film obtained is the result of the demixing process of two molecularly mixed components forming a homogeneous phase during cross-linking. The method as presented in WO 01/29148 requires the use of a selective solvent to obtain a suitable film.

It is an object of the present invention to provide a method of manufacturing a polarization retardation film that alleviates the above problems at least in part.

According to the present invention, this object is realized by a method of manufacturing a polarization retardation film, the method comprising the following steps: coating a surface with a polymerizable mesogenic composition comprising a polymerizable mesogenic compound and an isomerizable compound comprising E-isomers so as to form a nematic host, the isomerazable compound being cross-linkable with the polymerizable mesogenic compound, inducing E-Z isomerization in the composition, creating an anisotropic and an isotropic region in the coating by inducing phase separation in the polymerizable mesogenic composition, and polymerizing the coating in order to obtain a polarization retardation film. A surface may be coated with a polymerizable mesogenic composition, for example, a composition comprising a polymerizable mesogenic compound and an isomerizable compound that is cross-linkable with the polymerizable mesogenic compound comprising E-isomers. The surface may be coated, for example, by spin-coating the composition onto the surface. Typically, the surface is the surface of an alignment layer. The molecules of the polymerizable mesogenic compound are aligned by the alignment layer, resulting in what is known as an aligned nematic host.

Subsequently, E-Z isomerization in the composition is induced, followed by the induction of phase separation in order to form an anisotropic region and an isotropic region in the nematic host. Phase separation, also known as de-mixing, typically occurs when the temperature of the nematic host is below, but close to the nematic-to-isotropic transition temperature of the polymerizable mesogenic composition.

During the phase-separation process, random isotropic nuclei will form in the nematic host. How and where such nuclei form is determined to a large extent by the molecular distribution of the polymerizable mesogenic compound in the nematic host. The process of nucleation is a physically random process that provides features in the form of randomly formed anisotropic and isotropic regions.

Phase separation is typically induced through temperature control. The temperature range wherein both nematic and isotropic regions co-exist in the nematic host is called the bi-phasic region. For compositions comprising primarily polymerizable mesogenic compounds, the bi-phasic region tends to be small, in the range of about 3⁰C to about 15⁰C.

However, as a result of the isomerization of non-mesogenic isomers in the composition, the order in the aligned nematic host is disturbed. The extent to which the order is affected depends on the amount of non-mesogenic compound present. This typically results in a lower birefringence of the nematic host, but it also causes a lower nematic-to- isotropic transition temperature and a broader bi-phasic region. As a result of this broader range, it will be easier to obtain a satisfactory distribution between anisotropic and isotropic regions.

By adding an isomerizable compound to the composition, and thereby to the coating, isomerization may be used to actively control the nematic-to-isotropic transition temperature and the bi-phasic region. During the E-Z isomerization, the calamatic E-isomers, which form a nematic phase when mixed with a liquid crystal, transform into the non- mesogenic Z-isomer which has a bent structure. As a result of the bent structure, the order in the nematic host will be reduced. When the coating is subsequently polymerized, the (nematic) order is frozen, and thereby the anisotropic and isotropic nuclei. Such random regions/region boundaries represent characteristic features of the resulting polarization retardation film, hereinafter abbreviated to PRF. These characteristic features may be used during identification and authentication, similarly as the ridge pattern of a human fingerprint is used for identifying/authenticating an individual.

To establish the characteristic features, the PRF may be placed between two crossed polarizers and irradiated. Alternatively, the PRF may be placed between a reflective surface and a single polarizer. As the feature sizes of the PRF may be of the order of magnitude of micrometers, polarization microscopy may be used to establish the region boundaries.

The regions in the PRF result from the molecular distribution of the polymerizable mesogenic compound in the composition. As this distribution is not explicitly controlled, it will be virtually impossible to duplicate a particular PRF by repeating the same process. As a result, the present method is a method of creating a PRF, but not a method of duplicating or reproducing the PRF. In a further embodiment, the number of anisotropic and/or isotropic regions is maximized in order to obtain more characterizing features in the resulting PRF. This is particularly useful for security applications wherein a high probability of uniqueness is relevant.

In one embodiment of the present invention, isomerization is induced in a uniform manner over the entire coating, and the nematic-to-isotropic transition temperature of the coating may be lowered and the bi-phasic region broadened. Using isomerization, the nematic-to-isotropic transition temperature may be lowered to or below room temperature. Consequently, phase separation may be induced by either controlling the coating temperature and/or by controlling the isomerization process. Such isomerization may be thermal isomerization or photo-isomerization.

By locally forming larger or smaller quantities of Z-isomers, polarization retardation may be varied locally, resulting in gray-scale levels or colors when viewed while using two crossed polarizers. Such local variations will make the PRF even more difficult to duplicate. By adding a photo-isomerizable compound to the composition, isomerization may be controlled by means of a controlled exposure of the coating. One or more focused light sources, or a programmable mask may be used to control photo-isomerization locally. The illumination pattern may also be a random illumination pattern such as an interference pattern resulting from exposing an irregular object to a UV laser.

When using UV light for photo-isomerization, it is important to note that certain polymerizable mesogenic compounds may polymerize on exposure to UV light under suitable atmospheric conditions. To prevent polymerization during isomerization, atmospheric conditions should be adapted so that little or no polymerization occurs during isomerization.

In a further embodiment, the polymerizable mesogenic compound itself is isomerizable. In order to align the polymerizable mesogenic compound, the surface on which the coating is applied may be provided with an alignment layer. The alignment layer helps to induce the alignment of the polymerizable mesogenic compound.

In its turn, the alignment layer may comprise a plurality of regions each with different alignment directions. Ideally, these alignment directions are random as well. As a result, the polarization retardation of a PRF manufactured accordingly would not only have a plurality of randomly formed isotropic and/or anisotropic regions, but, in addition, the optical axis of the anisotropic regions will vary over the PRF as a result of the underlying alignment layer. A further embodiment comprises an additional annealing step, during which the coating is heated to a pre-determined temperature, below the bi-phasic region, for a predetermined duration, in order to allow the polymerizable mesogenic compound to align with the alignment layer.

After phase separation has been induced to a satisfactory level, which will typically be satisfactory when a plurality of both anisotropic and isotropic regions have formed, but which may differ from application to application, the order in the nematic host is frozen by means of a polymerization step.

The polymerization step is preferably followed by a postbake step in order to obtain mechanically, optically, and chemically stable PRFs. A postbake step generally involves heating the film in an oven for a substantial period of time.

The surfaces used to create PRFs according to the present invention may include reflective and/or transparent surfaces. As a result, it is possible after polymerization of the PRF to use the PRF as is, in conjunction with the underlying surface.

Optionally to improve mechanical stability and provide physical protection, a PRF is preferably sandwiched between two substrates, for example, foil substrates.

Preferably, the thickness and mechanical properties of the two substrates are substantially the same. In this manner, the PRF is less prone to damage by bending of the substrates.

In a further beneficial embodiment, the PRF after polymerization is removed from the surface, and attached to, or embedded on/in a physical object, such as a banknote, a passport, or an optical data carrier.

A further object of the present invention is to provide a polarization retardation film manufactured by means of a method according to the present invention for use in security applications. Examples of such applications can be found in the preamble and may range from security markings on passports, credit cards, documents, and banknotes. In one embodiment, the PRF comprises a polymerized mesogenic compound, and a non-mesogenic compound. In a particular embodiment, the non-mesogenic compound is an isomerized compound.

A PRF according to the present invention provides an anisotropic and an isotropic region, but it preferably comprises at least one of: a plurality of anisotropic regions, and a plurality of isotropic regions with randomly formed region boundaries. These regions and region boundaries may be used as features being characteristic of the PRF. In a further advantageous embodiment, a PRF obtainable by means of a method according to the invention is used for identification and/or authentication of an associated object. In order to identify/authenticate a PRF, the characteristic features of the PRF need to be detected and enrolled during an enrolment phase. The enrolment data for the PRF, and typically those of other PRFs, are subsequently stored for later use during the actual identification/authentication.

During identification, the characteristic features of the PRF offered for identification are detected and subsequently matched with at least one, but typically many, enrolment data. The goal is to establish whether the PRF offered for identification is one of the enrolled PRFs. Once a sufficient match is found, the identity is established as the identity of the PRF associated with e.g. the best matching characteristic features. An example of a situation wherein a PRF is used for identification purposes is that in which a person carries a badge comprising a PRF. The PRF provides the characteristic features that are used to establish the person's identity.

In contrast, during authentication, also known as validation, characteristic features of the PRF offered for authentication are matched with one enrolment data. Which particular enrolment data is matched is typically determined by an alleged identity, such as a serial number or another (unique) identifier. An example of a situation wherein a PRF is used for authentication purposes is that in which a PRF is embedded in a passport.

During manufacture of the passport, the PRF of the passport is enrolled, and the PRF and passport are associated with the identity of the passport holder. During authentication of the passport, the characteristic features of the PRF on the passport are detected and matched with the enrolment data of the authentic passport holder. The passport is said to be authentic if the data matches. In summary, both authentication and identification involve matching of characteristic features. Identification involves matching one to many, and authentication involves a one-to-one match.

In order to enable identification/authentication of the PRF, the characteristic features of the PRF are typically enrolled during an enrolment phase, and the resulting data is referred to as enrolment data. To this end, the PRF may be placed between two crossed polarizers and irradiated in order to acquire/detect the regions and/or region boundaries of at least part of the PRF. Given the very small feature sizes that can be realized by using the present invention, the resulting PRFs may require polarization microscopy to establish the characteristic features.

Further selection is needed if every PRF is required to be uniquely identified/authenticated. As PRFs are based on a physically random process, the probability that two PRFs are identical is small to remote, but there is no guarantee for uniqueness. If uniqueness is required, a pre-selection may be made in order to obtain truly unique PRFs. The enrolment data may be used at a later stage during the actual identification/authentication. During identification/authentication, an acquisition procedure similarly as during enrolment may be followed. This procedure serves to obtain features of the regions and/or region boundaries of the PRF that is being identified/authenticated. Subsequently, this data may be compared with one or more enrolled data in order to establish identity/authenticity of the associated object.

The polarization retardation of a PRF is determined by:

1. the birefringence Δn of the material,

2. the direction of the optical axis of the PRF, and

3. the thickness d of the PRF.

Each of these factors may be used to provide additional characteristic features that may be used during authentication and/or identification. The birefringence of the material may be varied through manipulation of the internal order of the mesogenic compounds in the material. The birefringence may be controlled through the formation of non-mesogenic compounds as well as by means of the temperature at which the PRF is polymerized.

The optical axis of the PRF is determined by the type of material as well as the alignment of the material. Several methods for aligning polymerizable mesogenic compounds are known to those skilled in the art. Such methods include the use of alignment layers and the use of photo-orientation.

The third factor, the material thickness, can also be used to vary the polarization retardation. It is known that, when manufacturing isotropic acrylate films, there is mass transport in such films upon lithographic exposure during manufacture.

Mass transport is a phenomenon wherein mobile monomers diffuse in the material, in the above case under the influence of light. As a result of this phenomenon, thickness variations will appear on the surface of and near the isotropic parts of the acrylate film. This phenomenon is particularly evident when a patterned mask is used, wherein exposed and unexposed areas are in mutually close proximity.

In general, corrugated surfaces are undesirable for PRFs used in e.g. displays, see "Technologies Towards Patterned Optical Foils applied in Transflective LCDs" by B. van der Zande et al. published at the 11^th International Display Workshop, IDW'04, Niigata, Japan, herein incorporated by reference. This document teaches the skilled person that such surface corrugation is highly undesirable. However, this surface corrugation may be used advantageously in the present invention; variations in film thickness in anisotropic regions provide additional polarization retardation features.

As a result, the phenomenon of mass transport may be used in an advantageous manner to obtain thickness variations in the resulting PRF, and thereby variations in the polarization retarding behavior of the nematic regions in the PRF. Such thickness variations, when viewed by using polarization microscopy, will result in gray-scale values or colors.

Alternatively, the thickness variations themselves may be measured, e.g. by means of interferometry, to provide further different characteristic features of the PRF that may be combined in a multi-modal system for identification/authentication.

When, during phase separation, a coating is irradiated by means of a random irradiation pattern, e.g. using an interference pattern, or speckle pattern, this may induce mass transport and result in gray- scale levels (or color) in addition to the already random domains.

In a further advantageous embodiment, the PRF is further characterized in that it comprises a first checksum based at least in part on the characteristic features of the PRF itself. The first checksum is placed on the PRF during the manufacturing process.

First, a substrate with an alignment layer is coated with a composition comprising a polymerizable mesogenic compound and a photo-isomerizable compound. Subsequently, the surface is partitioned in two parts, namely a first area of the PRF surface, hereinafter referred to as the top area, wherein the plurality of polarization retarding regions with randomly formed region boundaries will be formed, and a second area of the PRF surface, hereinafter referred to as the bottom area, wherein the checksum will be encoded. Next, the nematic-to-iso tropic transition temperature of the top area will be lowered by UV exposure of the top area through photo-isomerization. As a result of this, phase separation can be induced for the top area, e.g. by increasing the temperature in such a way that phase separation is induced in the top area, but not in the bottom area.

When phase separation has been successfully induced, the top area may be fully polymerized by e.g. a masked exposure of the top area to a second UV source in a nitrogen atmosphere. Next, the characteristic features of the top area are detected and quantified, resulting in an enrolment estimate based on at least part of the light passing through the top area of the PRF. Note that the enrolment estimate does not need to include all features, but may comprise a subset of the features provided by the PRF.

In a further step, a cryptographic checksum may be formed on the basis of the enrolment estimate. Such a cryptographic checksum may preferably involve a cryptographic one-way function such as a hash or message digest function on a digital representation of the characteristic features.

Once the checksum has been established, the nematic-to-isotropic transition temperature of the bottom area will be lowered by UV exposure. Subsequently, the cryptographic checksum may be encoded on the lower part, using PRF patterning techniques. In one embodiment, this involves application of lithographic exposure of the bottom area of the coating in order to encode the checksum using UV light. Subsequently, the PRF is heated above the nematic-to-isotropic transition temperature of the bottom area, thereby rendering the UV-exposed parts isotropic. The checksum may be encoded by using human-readable or machine-readable symbols. The bottom area of the PRF may now be polymerized by e.g. a masked exposure of the bottom area or a complete flood exposure of the film to a second UV source under suitable atmospheric conditions.

In a further embodiment, the checksum is a Message Authentication Code

(MAC). In yet a further embodiment, the checksum is digitally signed by a trusted party. Using the MAC, or the signed checksum, it may be verified that: the MAC, or the signed checksum, is authentic and corresponds to a checksum which is derivable from an enrolment estimate obtained from the characteristic features of the PRF used for authentication/ identification. In order to further improve security of such a PRF, the feature size of the top area may be chosen to be smaller than that of the bottom area. Consequently, the equipment used to manufacture the PRF cannot be used to duplicate or replicate such PRFs, not even in the hands of a malicious party. A PRF as defined in claim 9 may be used in a system for identification and/or authentication of objects as defined in claim 15.

The irradiation means in the above system is used to irradiate the PRF. Although a single irradiation source may suffice, the use of one or more controllable irradiation means enables further differentiation, for example, by: - selective irradiation of parts of the PRF, irradiation using light at one or more particular polarization angles, irradiation using light at one or more particular wavelengths, and combinations thereof.

The irradiation produced by the irradiation means can be seen as a challenge presented to the PRF, while the light meanwhile passing through the PRF can be interpreted as a response to this challenge.

The first acquisition means is arranged to form an identification/authentication estimate of a first response from the PRF based on detected light from the PRF. The identification/authentication estimate comprises one or more features that characterize the PRF associated with the object under identification/authentication.

The second acquisition means is arranged to obtain an enrolment data. A single enrolment data is required in a system for authentication. Which particular enrolment data is to be used may be determined by using further parameters such as e.g. an object serial number. The enrolment data may be derived from an enrolment database using the serial number. Alternatively, a trusted party may provide such data, e.g. over a network. In a system for identification, multiple enrolment data are generally compared until a match is found and the object is identified, or no match can be established.

The matching means is arranged to match data based on the identification/authentication estimate with the enrolment data in order to establish the identity/authenticity of the PRF by determining if the data based on the identification/ authentication estimate matches the enrolment data within a pre-determined tolerance. In an alternative embodiment, the object itself, or an arbitrary party, may provide the enrolment data. In order to prevent fraud/tampering, the enrolment data in such embodiments is preferably signed, preferably using a digital cryptographic signature. By verifying the digital signature, the system can establish that the enrolment data is authentic and not tampered with.

The digital signature may be generated by using e.g. a secret key known only to trusted parties, or by using a private key from a private/public key pair of the trusted party. Only parties with access to these keys will then be able to generate correctly signed enrolment data. Consequently, parties with access to the secret key or access to the public key corresponding to the private key can decrypt the enrolment data, or verify that the enrolment data is authentic. In order to simplify key distribution and improve security, the use of private/public keys pairs is preferable to the use of a secret symmetric key. A PRF, and particularly those with small feature sizes, may comprise a large amount of characteristic features. When all of these features are included in the enrolment data of a PRF, the resulting enrolment databases may become unpractically large. To reduce the amount of data without sacrificing the characteristic features unnecessarily, the enrolment data may be cryptographically hashed in order to generate a checksum that is based on the enrolment estimate but is more concise.

During identification/authentication, the same procedure is repeated, this time using the identification/authentication estimate. As a result, the amount of data that needs to be stored in order to identify/authenticate objects can be substantially reduced. Moreover, the cryptographic hash function prevents malicious parties from access to the underlying enrolment estimate. Consequently, the use of a cryptographic hash as a checksum is advantageous for reasons of both efficiency and security.

In a further embodiment, the cryptographic hash is cryptographically signed by a trusted party. The signed cryptographic hash is small in comparison with the signed enrolment estimate. In a system for authentication, the signed cryptographic checksum is preferably placed on the object itself or even on the PRF itself, as mentioned hereinbefore, thereby effectively rendering an enrolment database superfluous.

These and other aspects of the invention will be further elucidated and described by way of example and with reference to the drawings, in which:

Fig. 1 is a schematic diagram of several stages of manufacturing a PRF according to the present invention.

Fig. 2 is a graph depicting five curves of birefringence over temperature of a composition comprising a photo-isomerizable compound exposed to ultraviolet light. Fig. 3 presents an exposure time series of polarization microscopy images of photo-isomerized PRFs.

Fig. 4 presents a thermal series of polarization microscopy images of photo- isomerized PRFs. Fig. 5 depicts three aspects of a PRF featuring thickness variations.

Fig. 6 is a schematic diagram of a system for authenticating/identifying an object comprising a PRF according to the present invention.

Fig. 7 is a schematic diagram of a system for enrolment and a system for authenticating an object comprising a PRF according to the present invention. Throughout the drawings, the same reference numerals refer to the same elements, or to elements that perform the same function.

Fig. 1 is a schematic diagram of several stages of manufacturing a PRF according to the present invention. The first stage 110 of the method depicts a substrate 111, provided with an alignment layer 112. The alignment layer 112 may be an intrinsic part of the substrate 111 that is provided with a structured surface as a result of embossing, interferography, photolithography, ion/electron beam etching, or cast curing. Alternatively, the alignment layer may be realized by introducing anisotropy into the alignment layer material by means of techniques such as rubbing the alignment layer with a cloth, stretching, or deforming the layer in a different way.

A composition is coated on top of the alignment layer 112, resulting in coating 113. Here, the composition comprises a polymerizable mesogenic compound as well as a photo-isomerizable compound that is cross-linkable with the polymerizable mesogenic compound. The composition is spin-coated onto the surface. It will be clear to those skilled in the art that there is a variety of alternatives to spin coating, all of which are within the scope of the present invention, such as gravure coating/printing, offset coating/printing, flexographic coating/printing, Meyer bar coating, and inkjet printing.

Subsequently, the substrate 111, alignment layer 112 and coating 113 may be jointly placed on a hot plate in order to anneal the coating, and give the polymerizable mesogenic compound comprised therein time to align with the underlying alignment layer. In the second stage 120, the coating 113 is mask-122 exposed to ultraviolet (UV) light 124 in air under atmospheric conditions that inhibit the polymerization of the polymerizable mesogenic compound under the influence of the exposure to UV light 124. The photo-isomerizable compound present in the coating will isomerize while forming non- mesogenic compounds. The non-mesogenic compounds disturb the order in the exposed areas 123. The exposed areas 123 have a lower nematic-to-isotropic transition temperature than the unexposed areas, and will generally also have a broader bi-phasic region. In the third stage 130, the temperature of the nematic host is raised until it is within the bi-phasic region of the exposed area 123. As a result, phase separation will be induced, which results in anisotropic and isotropic nuclei in the exposed area 123. The unexposed area 131 has a higher nematic-to-isotropic transition temperature and will remain anisotropic. Finally, in the fourth stage 140, the entire coating is flood-exposed to UV light

124 in atmospheric conditions that allow polymerization of the coating. As a result, the order will be frozen, which results in nematic and isotropic nuclei in the exposed areas 123 of the PRF.

Fig. 2 represents a graph depicting five curves of birefringence over temperature of a composition comprising a polymerizable mesogenic compound and a photo- isomerizable compound. The birefringence Δn can be seen as a function of the retardation R, and thickness d:

Δn = R/d.

The curves presented in Fig. 2 are based on a measurement of spin-coated films of approximately 1.7 μm thick comprising (4-(3-acryloyloxypropyloxy)benzoyloxy)-2- methylphenyl 4-(3-acryloyloxypropyloxy)benzoate and trans-4-(6- acryloyloxyhexyloxy)cyclohexyl 4-(6-acryloyloxyhexyloxy)cinnamate in an approximate 3:1 weight ratio that have been irradiated in air for various periods of time a at an intensity of approximately 5 W/cm2, using a Fairlight exposure system.

In this embodiment, the weight percentage of non-mesogenic compounds is increased in a controllable manner by exposure to UV light by adding a photo-isomerizable compound that is preferably cross-linkable with a composition of polymerizable monomers. The E-Z isomerization of the photo-isomerizable compound will lead to a decrease of the nematic-to-isotropic transition temperature due to the non-mesogenic character of the Z- isomer (bent structure) compared to that of the E-isomer (calamitic structure).

The order parameter at room temperature of the monomers will accordingly decrease due to the shift of the nematic-to-isotropic transition. Simultaneously, as the weight percentage of non-mesogenic compounds is increased by the UV exposure, a bi-phasic region at room temperature will evolve. The formation of the bi-phasic region is initiated by random nucleation of isotropic nuclei in a nematic host. Continuation of the E-Z isomerization process by UV exposure will result in a larger amount of non-mesogenic compounds, a lower isotropic nematic transition temperature and, consequently, larger isotropic domains created by coalescent growing isotropic droplets in the nematic host.

The curves present the birefringence as a function of the temperature for the above composition. Curve 200 represents the birefringence for the unexposed composition, curves 216, 264, 2128, and 2256 represent the curves after exposure times of approximately 16, 64, 128, and 256 seconds, respectively.

The bi-phasic regions are not indicated in Fig. 2. However, as an indication, the bi-phasic region for this particular composition without UV exposure (curve 200) ranges from approximately 7O⁰C to approximately 86⁰C. After an exposure of 32 seconds, the various stages of phase separation occur in the range of approximately 5O⁰C to approximately 75⁰C.

It will be clear from these curves that, with a progressing isomerization: the birefringence at a constant temperature decreases, the transition from anisotropic (birefringent) to isotropic (non-birefringent) becomes less steep, and - prolonged UV exposure may result in loss of birefringence.

Although Fig. 2 is representative of a composition using a 3:1 weight ratio, this example should not be interpreted as limiting the present invention. In fact, other weight ratios may be used, such as 1 : 1 and 7:1. The above is merely an example of the effects of the formation of Z-isomers in the coating. Fig. 3 presents an exposure time series of polarization microscopy images of photo-isomerized PRFs. The images are based on a composition comprising approximately 0.75 g of (4-(3-acryloyloxypropyloxy)benzoyloxy)-2-methylphenyl 4-(3- acryloyloxypropyloxy)benzoate , approximately 0.25 g of trans-4-(6- acryloyloxyhexyloxy)cyclohexyl 4-(6-acryloyloxyhexyloxy)cinnamate, approximately 0.01 g of Irgacure 651 ® (Ciba Geigy) and 0.01 g of RM522 (Merck) in approximately 2 g of xylene at approximately 7O⁰C.

This composition was spin-coated on a substrate with rubbed polyimide AL 1051 (available from JSR Co. (Tokyo, Japan)) as the alignment layer. The spin conditions of the Convac Spinner were approximately 60 s at 1500 rpm, yielding a birefringent film with a thickness of about 1.7 μm. The rubbed polyimide established nearly planar alignment of the LC monomers in their nematic phase with its director parallel to the rubbing direction. The formation of a mono-domain was facilitated by an annealing step on a hot plate at approximately 55⁰C for about 60 s. In order to induce phase separation by E-Z isomerization, the annealed film was exposed in air, using a Fairlight exposure machine without filter. The UV intensity was approximately 5 mW/cm² at 313 nm.

The PRFs in Fig. 3 are not cross-linked. Cross-linking of the PRF may be achieved by a flood exposure in nitrogen for about 5 minutes, using a Philips HPA lamp (4 mW/cm², 365 nm). The cross-linking will influence the exact pattern of isotropic and nematic regions and will depend on the UV spectrum and temperature.

The polarization microscopy images 316, 364, 3128 in Fig. 3 correspond to exposure times of 16, 64, and 128 seconds, respectively. The orientation of the alignment/director in the nematic domain (bright) is approximately 45 degrees with respect to the transmissive axes of the crossed polarizers. The dark domains are isotropic. The bright areas are photo-polymerized in the nematic state.

Fig. 4 presents a thermal series of polarization microscopy images of photo- isomerized PRFs that were prepared by means of a method according to a preferred embodiment. After isomerization, the PRF was subsequently heated so as to induce phase separation.

The composition was prepared by dissolving approximately 0.75 g of (4-(3- acryloyloxypropyloxy)benzoyloxy)-2-methylphenyl 4-(3-acryloyloxypropyloxy)benzoate, approximately 0.25 g of trans-4-(6-acryloyloxyhexyloxy)cyclohexyl 4-(6- acryloyloxyhexyloxy)cinnamate, approximately 0.01 g of Irgacure 651 ® (Ciba Geigy) and approximately 0.01 g of RM522 (Merck) in 2 g of xylene at approximately 7O⁰C.

This composition was spin-coated on a substrate with rubbed polyimide AL 1051 (JSR Co. (Tokyo, Japan)) as the alignment layer. The spin conditions of the Convac Spinner were approximately 60 s at 1500 rpm, yielding a birefringent film with a thickness of approximately 1.7 μm. The rubbed polyimide established nearly planar alignment of the LC monomers in their nematic phase with its director parallel to the rubbing direction. The formation of a mono-domain was facilitated by an annealing step on a hot plate at approximately 55⁰C for approximately 60 s.

In order to induce phase separation, the annealed film was incrementally heated in a Mettler Toledo FP5 hot stage. The temperature is indicated in Figure 4. The films in this picture are not cross-linked. Cross-linking of the film can be achieved by a flood exposure in nitrogen for 5 minutes, using a Philips HPA lamp (4 mW/cm², 365 nm). The cross-linking will influence the exact pattern of isotropic and nematic regions as well as the exact composition. The polarization microscopy images 475, 480 in Fig. 4 correspond to the temperature settings of approximately 75⁰C and approximately 8O⁰C, respectively, on the Mettler Toledo hot stage. The orientation of the alignment/director in the nematic domain (bright) is 45 degrees with respect to the transmissive axes of the crossed polarizers. The dark domains are isotropic. The bright areas are photo-polymerized in the nematic state. Fig. 5 depicts three aspects of a PRF featuring variations in thickness resulting from mass transport. A composition was created by dissolving approximately 0.75 g of (4-(3- acryloyloxypropyloxy)benzoyloxy)-2-methylphenyl 4-(3-acryloyloxypropyloxy)benzoate , approximately 0.25 g of trans-4-(6-acryloyloxyhexyloxy)cyclohexyl 4-(6- acryloyloxyhexyloxy)cinnamate, approximately 0.01 g of Irgacure 651 (Ciba Geigy) and approximately 0.01 g of RM522 (Merck) in 2 g of xylene at approximately 70 ⁰C.

This composition was spin-coated on a substrate with rubbed polyimide AL 1051 (JSR Co. (Tokyo, Japan)) as the alignment layer. The spin conditions of the Convac Spinner were 60 s at 1500 rpm, yielding a birefringent film with a thickness of about 1.7 μm. The rubbed polyimide established nearly planar alignment of the LC monomers in their nematic phase with its director parallel to the rubbing direction. The formation of a mono- domain was facilitated by an annealing step on a hot plate at approximately 55⁰C for about 60 s.

In order to induce phase separation by E-Z isomerization, the annealed film was exposed in air through a mask, using a Fairlight exposure machine for about 256 s without filter. The UV intensity was approximately 5 mW/cm2 at 313 nm. Cross-linking of the film was achieved by a flood exposure in nitrogen for approximately 5 minutes, using a Philips HPA lamp (4 mW/cm2, 365 nm). The mask feature size used in exposure was approximately 100 x 100 μm2.

Polarization microscopy image 520 shows an overview of the PRF, wherein the mixed nematic and isotropic regions are recognizable as the black/white spotted regions. As a result of the mass transport of mobile monomers from non-exposed areas to the exposed areas, a mass build-up occurs in the regions where phase separation occurs. This can be clearly seen from the height cross-section 530. The height cross-section shows a substantial height difference between exposed and unexposed regions, but also in the mixed nematic and isotropic areas, as indicated by the difference 540. The height difference 540 in the partially nematic-isotropic area is approximately 0.5 μm.

The three-dimensional representation 510 was obtained by using interferometry. Representation 530 clearly indicates the spiky and random nature of the thickness variations in the mixed nematic-isotropic region.

Fig. 6 is a schematic diagram of a system for identification/authentication of a document 605. The document 605 comprises a PRF 610 according to the present invention. Alignment marks 615 help localize the PRF 610 on the document 605. The document 605 further comprises a document serial number 665. The document 605 may be a passport, a banknote, or a digital data carrier.

The system 660 comprises an irradiation means 620 arranged to irradiate the PRF 610. In this particular embodiment, the irradiation means 620 comprises a lamp 604, a programmable polarizer 606, a bandpass filter 607, and a collimator lens 608. The system further comprises a first acquisition means 625 arranged to form an identification/authentication estimate 630 based on a first response 621, i.e. light incident from the PRF 610. The system may also comprise magnification means in order to expand the image from 610 before being detected by 625.

The system also comprises a second acquisition means 635 arranged to obtain an enrolment data 640, and a matching means 650 arranged to match data based on the identification/authentication estimate 630 with the enrolment data 640 in order to establish identity/authenticity of the object 605 by determining if the data based on the identification/authentication estimate 630 matches the enrolment data 640 within a predetermined tolerance.

The matching means may be arranged to establish whether the identity of the document 605 matches that associated with the enrolment data 640, or whether document 605 is authentic. If the system is used for identification, the identification/authentication estimate will be compared with enrolment data until either the enrolment data is exhausted and the identity cannot be established, or a match is found upon which the identity is established as that being associated with the enrolment data. The resulting decision 655 is output by the system and may be used e.g. for access control, or as an input for a larger multi-modal system for identification/authentication.

If the above system is used for authentication purposes, the system 660 will have to establish which enrolment data is to be retrieved from the enrolment database 645. To this end, the second acquisition means 635 also obtains the document serial number 665 from the document 605. The document serial number 665 is subsequently used to retrieve the enrolment data 640 corresponding to the document serial number 665 from the enrolment database 645. Although a serial number is used in this embodiment, other unique identifiers may be used in other embodiments. A PRF according to the present invention may comprise a vast amount of characteristic features. Although this is particularly helpful in creating a reliable system for identification/authentication of objects, it may also present an issue with respect to the storage of such features. The characteristic features are preferably replaced by a concise first checksum that is dependent on these features and yet preserves the differentiating capabilities of the features.

A particularly advantageous first checksum can be generated by using a oneway cryptographic hash function or a message digest function. Furthermore, if checksum uniqueness is an issue, collision- free cryptographic hash functions may be employed. The application of one-way functions has two advantages; enrolment data, and thereby the enrolment database can be greatly reduced in size and, furthermore, the use of a one-way hash prevents a malicious party with access to the first checksum from establishing the PRF features used to establish the first checksum. As a result, the use of such cryptographic oneway functions further complicates matters for forgers.

The size of the first checksum is largely dependent on the number of objects that need to be authenticated/identified, and on the fact whether or not uniqueness of the first checksum is required. The use of cryptographic checksums also affects matching. When using a first checksum generated by means of a cryptographic one-way function, it is generally not allowable to tolerate "errors" during matching; a precise match is required. Minor changes in the PRF features generally result in major changes in the first checksum. Consequently, the matching tolerance is zero, i.e. the enrolled first checksum must be identical to a second checksum established during identification/authentication.

In order to create a more robust system for identification/authentication, error- correcting codes may be applied to obtain more robust PRF features. WO2004/104899, "Authentication of a physical object", (Attorney Docket PFINL030552) herein incorporated by reference, discloses the use of helper data and delta-contracting functions for use in the authentication of a physical object.

The above application discloses how helper data can be generated during enrolment, and how it can be used during subsequent identification/authentication of physical objects. During identification/authentication, the helper data is combined with the characterizing features of the object. Then a delta-contracting function is applied, which takes the first estimate and helper data as inputs. The delta-contracting function compensates detection errors and noise and provides a more robust estimate of the characterizing features.

The authentication systems described so far rely on a database for storing enrolment data. In a practical system, this database may be centralized and/or distributed over various sites in order to reduce the cost of a system for authentication. However, a system for authentication would have to contact the database during authentication.

In order to avoid the need for multiple databases and simplify the system for authentication, enrolment data may be signed by a trusted party during enrolment. Signing the enrolment data typically involves applying a message digest function on the enrolment data and subsequently encrypting the resulting value with a secret key, or a private key from a private/public key pair. In a particularly advantageous embodiment, the generation of the first checksum and digital signing are combined. The first checksum is generated and subsequently encrypted by using a private key of a private/public key pair. Consequently, any party with the appropriate public key can verify validity, or in the latter case extract the correct first checksum.

In a system for authentication that uses signed checksums, the system for authentication will verify the digital signature, thereby establishing authenticity of the first checksum. Subsequently, the system will match the first checksum with the second checksum that is derived from the object being authenticated. Such signed checksums may be small and can thus be printed or stored on the object, or even on the PRF itself.

Fig. 7 depicts a document 605, provided with a PRF 610 according to the present invention, a system 705 for enrolment of an object comprising a PRF according to the present invention, and a system 795 for authentication of an object comprising a PRF according to the present invention. The systems 705 and 795 jointly allow the enrolment and authentication of objects provided with a PRF 610 according to the present invention, without the need for an enrolment database.

The document 605 further comprises a first checksum 730 and an accompanying digital signature 740. The first checksum 730 and the accompanying digital signature 740 are generated during enrolment of the object 605 by the system 705.

The enrolment process can be illustrated by using the schematic representation of system 705. System 705 comprises an irradiation means 620 for irradiating a second PRF, here PRF 610. The system establishes an enrolment estimate 715 based on a second response 721 resulting from light incident from the irradiated PRF 610. A third acquisition means 722 establishes the enrolment estimate 715. The enrolment estimate 715 serves as an input for a cryptographic hashing means 725 that produces the first checksum 730. The first checksum 730 serves as an input for a digital signing means 735 that produces the digital signature 740. Together with the digital signature 740, the first checksum 730 forms the enrolment data 745 that is stored on the document 605.

This data may be printed thereon in a machine-readable code, or may alternatively be stored in an alternative machine-readable format, e.g. on a magnetic stripe on the document 605. The enrolment data is preferably encoded on the PRF 610 itself.

When the document 605 is enrolled, it can be authenticated by using the system 795.

The system 795 comprises a further first irradiation means 620 arranged to irradiate a PRF embedded on a first object, here the document 605. The system 795 comprises a first acquisition means 625 arranged to form an identification/authentication estimate 630 based on a first response 621 in the form of light incident from the PRF. The system 795 further comprises a second acquisition means 635 arranged to obtain the previously enrolment data 745.

The enrolment data 745 is passed on to a matching means 650. The matching means 650 is arranged to generate a second checksum on the basis of the identification/authentication estimate using the same cryptographic hash function as employed during enrolment. In parallel, the matching means 650 can verify the digital signature comprised in the enrolment data 745 and thereby establish that the enrolment data, and therefore the first checksum, is authentic.

The first checksum and the second checksum can be matched by the matching means 650. Provided that the first checksum is authentic, and the first and second checksums are identical, the object is established as being authentic, resulting in a positive decision 655. If either condition is not satisfied, a negative decision is output.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The use of particular compounds and substances such as photo-initiators and/or surfactants should not be interpreted as limiting the scope of the claims to the use of such substances.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the indefinite article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method of manufacturing a polarization retardation film (610), the method comprising the following steps: coating a surface (112) with a polymerizable mesogenic composition comprising a polymerizable mesogenic compound (113) and an isomerizable compound comprising E-isomers so as to form a nematic host, the isomerizable compound being cross- linkable with the polymerizable mesogenic compound (113), inducing E-Z isomerization in the composition, creating an anisotropic and an isotropic region in the coating by inducing phase separation of the polymerizable mesogenic composition, and - polymerizing the coating in order to obtain a polarization retardation film

(610).

2. The method of claim 1 , characterized in that the step of creating comprises creating at least one of a plurality of anisotropic regions and a plurality of isotropic regions.

3. The method of claim 1, characterized in that the composition comprises an isomerizable polymerizable mesogenic compound.

4. The method of claim 1, characterized in that the temperature of the coating is conditioned so as to induce phase separation.

5. The method of claim 1, characterized in that the surface is a surface of an alignment layer (112).

6. The method of claim 5, characterized in that the alignment layer (112) comprises at least two alignment regions with different alignment directions.

7. The method of claim 1, characterized in that the method further comprises the following steps: deriving (722) characteristic features (715) from at least part of the polarization retardation film (610), generating (725) a first checksum (730) based at least in part on the characteristic features, and - encoding the first checksum (730) on the polarization retardation film (610).

8. The method of claim 7, characterized in that the generation of the first checksum (730) comprises digitally signing (735) the first checksum.

9. A polarization retardation film (610) obtainable by means of the method of claim 1.

10. Use of the polarization retardation film (610) of claim 9 in a security application.

11. The polarization retardation film (610) of claim 9, characterized in that it comprises at least one of a plurality of anisotropic regions and a plurality of isotropic regions.

12. The polarization retardation film (610) of claim 9, characterized in that features that are characteristic of the polarization retardation film are derivable from at least one of: the region boundaries of the anisotropic region and the isotropic region, thickness variations in the polarization retardation film (610), birefringence variations in the polarization retardation film (610), and - variations in the direction of the optical axis in the polarization retardation film (610).

13. The polarization retardation film (610) of claim 12, characterized in that the polarization retardation film (610) further comprises a first checksum (730) encoded on the polarization retardation film (610), the first checksum (730) being based at least in part on an enrolment estimate (715).

14. The polarization retardation film of claim 13, characterized in that the first checksum (730) is digitally signed.

15. A system for identification/authentication (795) of a first object (605) comprising a first polarization retardation film (610) according to claim 9, the system (795) comprising: - a first irradiation means (620) arranged to irradiate the polarization retardation film (610), a first acquisition means (625) arranged to form an identification/authentication estimate (630) of a first response from the polarization retardation film based on light detected from the polarization retardation film, - a second acquisition means (635) arranged to obtain an enrolment data

(640,745) associated with the first object (605), and a matching means (650) arranged to match data based on the identification/authentication estimate (630) with the enrolment data (640,745) in order to establish the identity/authenticity of the first object (605) by determining if the data based on the identification/authentication estimate (630) matches the enrolment data (640,745) within a pre-determined tolerance.

16. The system for identification/authentication (795) of claim 15, characterized in that the enrolment data (640,745) is based at least in part on an enrolment estimate (715) of a second response (721) from the polarization retardation film (610).

17. The system for identification/authentication (795) of claim 15, characterized in that the enrolment data (640,745) is a first checksum (730) based at least in part on characteristic features of the polarization retardation film (610), wherein the matching means (650) is further arranged to: form (650) a second checksum based on the identification/authentication estimate, and establish (650) the identity/authenticity of the polarization retardation film by verifying if the second checksum matches the first checksum (730) within a pre-determined tolerance.

18. The system for identification/authentication (795) of claim 17, characterized in that forming (650) the second checksum involves a cryptographic hash, and the predetermined tolerance is zero.

19. The system for identification/authentication (795) of claim 18, characterized in that the first checksum (730) is digitally signed, the signature (740) being based at least in part on the first checksum (730), wherein the matching means (650) is further arranged to verify the cryptographic signature (740).

20. A system for enrolment (705) of a first object (605) comprising a first polarization retardation film (610) according to claim 12, the system (705) comprising: a first irradiation means (620) arranged to irradiate the polarization retardation film (610), and a third acquisition means (722) arranged to establish an enrolment estimate (715) of a second response from the polarization retardation film based on light detected from the polarization retardation film (610).