GB2542463A - Methods and apparatus for forming non-diffractive light control structures in or on a surface of a polymer substrate - Google Patents

Abstract

A method of forming non-diffractive light control structures comprises: providing a polymer substrate 90 with a thermoplastic, curable coating 92; thermally embossing the light control structures into the curable coating while the coating is at a temperature at which it can be plastically deformed: and curing the deformed coating during or after the embossing process;wherein the temperature of the coating during the embossing process is such that the coating is plastically deformed while the substrate is at a lower temperature than that at which the substrate can be deformed (i.e. the coating has a softening temperature lower than the substrate). The substrate may comprise one of (bi-axially oriented) polypropylene, polyethylene, polycarbonate, polyvinyl chloride, nylon, acrylic, cyclic olefin polymer, cyclic olefin copolymer or any combination thereof. Various further methods and apparatus are disclosed for forming non-diffractive light control structures, manufacturing security documents comprising thermally embossing light control structures which may include refractive structures such as microprisms, an array of tetrahedral, square pyramids or corner-cube structures; or focusing elements such as microlenses. The apparatus and method may be used to create an optical effect such as moire magnification, integral image formation or lenticular effect such as to be used to define a security document such as a banknote, cheque, passport, identity card, certificate or stamp.

Description

METHODS AND APPARATUS FOR FORMING NON-DIFFRACTIVE LIGHT CONTROL STRUCTURES IN OR ON A SURFACE OF A POLYMER SUBSTRATE

The invention relates to methods and apparatus for forming non-diffractive light control structures such as focusing lenses in or on a surface of a polymer substrate, for example arrays of microlenses used in the construction of optical devices such as moire magnifiers and the like. The invention can be used for security devices for securing documents and articles of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents.

An early disclosure of the use of micro-optic based security devices is described in WO-A-94/27254. This document describes a security device formed by an array of spherical microlenses located above a corresponding array of microimages and typically being offset slightly from the array of microimages so as to generate a moire magnifier effect.

Another optical device is a so-called “lenticular” device formed by an array of elongate focusing lenses having widths of up to a few hundred microns located over a corresponding array of image strips. The optical effect is observed by viewing the combined device at different rotational angles so that different combinations of image strips are viewed at the different angles. Examples of known lenticular devices can be found in US-A-4892336, WO-A-03/052680, USA-4402150 and WO-A-2011/051668.

Further examples of optical devices incorporating arrays of microlenses are described in WO-A-2006/125224.

One of the reasons why these devices make good security devices is the difficulty of fabricating lens arrays, particularly microlens arrays. In most cases, in the prior art, the microlenses are formed on a security thread for incorporation in a paper substrate using a UV cast curing technique although other options have been disclosed such as extrusion embossing, injection moulding and the like. These techniques generally involve the provision of an additional material on the surface of the substrate which can be formed with the lenses.

With the gain in popularity of polymer-based security documents such as polymer banknotes, there is a need to be able to provide on such polymer substrates optical security devices based on lenses but it has been found that the conventional techniques used with narrow threads are not particularly successful and also complex and costly. Due to the larger surface area of a polymer banknote compared to a security thread it is important the mechanical properties of the lenses and the base polymer substrate are similar to create flexible durable products and this is typically not the case when using UV cast cure as the cast cure resins are typically not as flexible as the polymer substrate.

In accordance with a first aspect of the present invention, a method of manufacturing a security document comprising non-diffractive light control structures in a surface of a polymer substrate of the security document comprises thermally embossing non-diffractive light control forms into the surface of the substrate while the substrate is at a temperature at which the surface can be deformed; chilling the embossed substrate; and, before, during or after either of the steps of thermally embossing and chilling, coating at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer such that at least the non-diffractive light control structures are uncoated.

In accordance with a second aspect of the present invention, apparatus for manufacturing a security document comprising non-diffractive light control structures in a surface of a polymer substrate comprises: an embossing die carrying embossing forms corresponding to the light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss the non-diffractive light control structures into the surface; a heating system for heating the polymer substrate, in the vicinity of the embossing die, to a temperature at which the surface can be deformed; chilling apparatus to chill the embossed substrate; and a coating system to coat at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer such that at least the non-diffractive light control structures are uncoated.

With this aspect of the invention, we directly emboss the light control forms into the surface of the polymer substrate. Although direct embossing of holograms has been described in the past, it has been found that this was not successful with non-diffractive light control structures due to the shrinkage or warping of the polymer material following the embossing step. One way of attempting to overcome this would be to emboss light control structures larger than required so that following the shrinkage of the polymer, the resulting structures would have the desired shape and size. However, this is very difficult to achieve in practice.

We have recognized that the temperatures to which the polymer substrate needs to be raised to enable embossing to be carried out are relatively high so as on cooling the material relaxes and the required form of the embossed features is lost. As a result, this aspect of the invention overcomes this problem by not simply allowing the polymer to cool following embossing but rather positively to chill the substrate, typically following the embossing step.

We have found that by positively chilling the polymer, the relaxation of the polymer substrate is minimised thus leading to the formation of very accurately defined light control structures.

The chilling process could be carried out in a variety of ways, for example by spraying a cold fluid such as water onto the embossed polymer, passing the polymer through a chilled atmosphere or the like but in the preferred case, the chilling apparatus comprises at least one chilling roller which engages the embossed substrate. In some examples, a plurality of chilling rollers are provided in sequence. Each could be maintained at the same temperature or each chilling roller downstream of the first being maintained at a lower temperature than the preceding chilling roller. In the latter case, the chilling process is carried out in a sequence of sub-steps in which the substrate is chilled to successively lower temperatures.

The chilling process could be carried out after the substrate has been separated from the embossing die or while it remains in contact with the embossing die. Thus, in one example, the embossing process is carried out by passing the polymer substrate through a nip between an embossing die and an impression roller, wherein at least part of the chilling process is performed by the impression roller.

In some cases, the polymer substrate can be held under tension against the embossing die with sufficient force that the light control structures are embossed into it. In the preferred arrangement, however, the apparatus further comprises a pressure roller forming a nip with the embossing die through which the substrate is fed. This increases the pressure with which the polymer substrate is urged against the embossing die.

In order to raise the temperature of the polymer substrate to the required level during embossing, conveniently one or both of the embossing die and pressure roller, if provided, are heated whereby heat is transferred to the substrate.

The temperature to which the polymer substrate must be raised to enable it to be deformed will depend upon the material of the substrate. Typical polymer materials include polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof. In the case of BOPP, the required temperature is above 90°C but more preferably in the range 95-100°C. The temperature should not exceed that at which the polymer loses its mechanical stability through distortion/shrinkage. A temperature at which deformation is possible at a certain pressure can be ascertained using the Vicat softening point test using the ASTM D 1525 and ISO 306 standards. The Vicat softening point is taken as the temperature at which the specimen is penetrated to a depth of 1 mm by a flat-ended needle with a 1 mm2 circular or square cross-section. For polypropylene preferably a heating rate of 50°C/hr is used with a loading rate of 50N.

In accordance with a third aspect of the present invention, apparatus for forming non-diffractive light control structures in the surface of a polymer substrate comprises: an embossing die roller carrying embossing forms corresponding to the non-diffractive light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss non-diffractive light control structures into the surface; a pressure roller defining a gap with the embossing die roller through which the polymer substrate passes in use as the rollers rotate; and a heating system for heating the polymer substrate, in the vicinity of the embossing die roller, to a temperature at which the surface can be deformed, wherein the embossing forms are provided on radially outwardly projecting pedestals of the embossing die roller whereby each pedestal forms a respective nip with the pressure roller as the rollers rotate thereby causing non-diffractive light control structures to be embossed into the surface of the polymer substrate.

This third aspect provides another approach to solving the problem outlined above which is to increase the embossing pressure as compared to that experienced with a conventional embossing die roller without raised pedestals. Increasing the embossing pressure enables a reducing embossing temperature. That in turn means that the risk of shrinkage and warping after embossing is also lowered.

The increase in pressure is achieved by utilizing pedestal forms on the embossing die roller so that as each pedestal form engages the polymer material at the nip, there will be an increase in pressure compared with the pressure involved with a conventional embossing die roller having a generally constant radius.

Typically, the pedestals will project beyond the circumference of the embossing die roller by between 3 and 5pm. A further advantage of this aspect of the invention is that the modified embossing die roller with pedestals can be installed into a conventional embossing machine and provide much higher pressures at the nip without the need for adjusting the process conditions of the machine.

Typically, the embossing roller carries a plurality of radially outwardly projecting pedestals. Typically, each pedestal is provided with a plurality of embossing forms. In this way, each pedestal may emboss a region or sub-region of lenses or other non-diffractive light control structures. Preferably, each pedestal comprises a two-dimensional array of embossing forms that correspond to a two-dimensional array of embossed non-diffractive light control structures.

In accordance with a fourth aspect of the present invention, a method of forming non-diffractive light control structures comprises: providing a polymer substrate with a thermoplastic, curable coating; thermally embossing non-diffractive light control structures into the curable coating while the coating is at a temperature at which it can be plastically deformed; and curing the deformed coating during or after the embossing process wherein the temperature of the coating during the embossing process is such that the coating is plastically deformed while the substrate is at a lower temperature than that at which the substrate can be deformed.

This aspect of the invention provides yet a further solution to the problems outlined above. Although there have been previous attempts to provide focusing lens forms into a coating on a substrate, the problems outlined above concerning shrinkage and warping occur. However, by suitably choosing a coating that can be plastically deformed at a temperature that is lower than the melting temperature at which the substrate will deform reduces the risk of the substrate elastically deforming during the embossing stage and thereafter shrinking or warping.

Examples of suitable coatings are given later.

The method of curing will depend upon the nature of the coating but typically is one of UV, IR and thermal curing. It will be appreciated here that curing is a chemical process which involves the formation of cross-links between polymer chains to increase the hardness or toughness of the coating material.

The method includes the step of thermally embossing non-diffractive light control structures into the thermoplastic, curable coating while the coating is at a temperature at which it can be plastically deformed. For example, the curable coating is heated beyond its glass transition temperature at the point of embossing such that it may be plastically deformed. The curing may occur while the curable coating is in contact with any embossing structure, or, because it has been plastically deformed, it may be removed from the embossing structure before curing to harden and fix the embossed structure in its surface. It should be stressed that the curable coating is a thermoplastic that is preferably solid, dry and non-tacky before embossing to aid handling. This is in contrast to, for example, cast-cure processes, in which a material is in a liquid state prior to its curing.

While the coating is curable, no curing before the embossing process is necessary, instead, the coating may be provided as a solid thermoplastic with curable additives which are used to facilitate hardening of the coating after embossing. However, in some cases an additional partial pre-curing step may be desirable.

In accordance with a fifth aspect of the present invention, a method of forming non-diffractive light control structures in a first surface of a polymer substrate comprises: providing a (typically non-fibrous) polymer substrate, the polymer substrate having either a skin region of the substrate at the first surface of the substrate comprising a radiation absorbing material, or a radiation absorbing material affixed to the first surface of the polymer substrate; exposing the radiation absorbing material to radiation whereby the radiation absorbing material absorbs the radiation and heats up the first surface of the polymer substrate to a temperature at which the surface can be locally deformed; thermal embossing non-diffractive light control structures into the first surface of the substrate; and allowing the substrate to harden.

In this approach to solving the problems outlined above, the amount of the substrate which is caused to be deformable is restricted to a region near the surface thereby avoiding elastic and/or plastic deformation of the bulk of the polymer substrate. This is achieved by providing a radiation absorbing material on the surface of the polymer substrate to absorb radiation and thus heat up so as to heat the adjacent surface of the polymer substrate. A typical radiation absorbing material will comprise a polymer in which is provided a radiation absorbing additive. A typical example comprises: a) a radiation, e.g. infrared, absorber; b) a polymeric binder, a polymeric solution or a monomer formulation which can be converted to a polymer layer on the exposure of heat or UV or both. The polymeric binder layer usually consists of substantially linear chains specially those of low Tg (<100C). Monofunctional acrylic precursors are specially suited for this.

Typical Infrared absorbers can be:

An infrared dye,

An infrared organic pigment and

An inorganic infrared pigment A preferred infrared absorber is an infrared dye due to the narrower absorption spectrum

Examples of infrared dyes are: indolizine dyes, quinone dyes, azo dyes quinoid dyes, merocyanine dyes, cyanine dyes, squarylium dyes, croconium dyes, polymethine dyes, oxyindolizine dyes, polymethyl indoliums, indocyanine green, bis(aminoaryl)polymethine dyes, chalcogenopyryloarylidene dyes, metal thiolate complex dyes, bis(chalcogenopyrylo)polymethine dyes, pyrylium dyes, phthalocyanine dyes, naphthalocyanine dyes, metal complex IR dyes, and combinations thereof. and a preferred infrared dye is: 5-[2,5-bis[2-[1 -(1 -methylbutyl)benz[cd]indol-2(1 H)- ylidene]ethylidene]cyclopentylidene]-1-butyl-3-(2-methoxy-1-methylethyl)-2,4,6 (1 H,3H,5H)-pyrimidinetrione

Suitable inorganic infrared pigments include:

Transparent IR absorbers from NYACOL Nano Technologies- NYACOL® SN902

NYACOL® SN902W

NYACOL® SN902SD NYACOL® SN902 NYACOL® SN903

NYACOL® SN903W

NYACOL® SN903SD

NYACOL® SN903-PM

NYACOL® ITO-EG

Suitable inorganic infrared pigments can also be carbon blacks including:

Carbon Black MA8™ (MITSUBISHI CHEMICAL),

Special Black 250, Special Black 350, Special Black 550, Printex™ 25, Printex™ 35, Printex™ 55 and Printex™ 90 from DEGUSSA.

Regal™ 400R and Elftex™ 320 (CABOT Co.),

Typical concentrations of additive are 0.01 to 2.0 g/m2, more preferably in an amount of 0.1 to 1 g/m2.

Typically, radiation absorbing materials transparent to optical wavelengths are preferred. Preferably the radiation absorbing materials are transparent to at least some wavelength(s) in the visible spectrum, preferably transparent across the entire visible spectrum.

Typically, the radiation absorbing material is exposed to radiation just before an embossing nip while the rest of the polymer substrate will effectively remain cold.

In accordance with a sixth aspect of the present invention, a method of forming non-diffractive light control structures in or on the surface of a transparent or translucent polymer substrate comprises: (i) embossing, under predetermined conditions, non-diffractive light control structures into the surface of the substrate or into a material provided on the surface of the substrate; (ii) locating the substrate over one or more indicia such that the formed light control structures are aligned with the indicia; (iii) determining the resultant optical effect of viewing the or each indicia in conjunction with, for example through, the light control structures, (iv) generating a feedback signal based on the determined resultant optical effect; (v) using the feedback signal to change one or more of the predetermined conditions based on the feedback signal; and (vi) repeating steps (i)-(iii) such that if the determined resultant optical effect is not satisfactory the changed predetermined conditions improve the resultant optical effect.

In this aspect of the invention the non-diffractive light control structures are focussing elements.

In accordance with a seventh aspect of the present invention, apparatus for forming non-diffractive light control structures in or on a surface of a polymer substrate comprises: an embossing die carrying embossing forms corresponding to the non-diffractive light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss non-diffractive light control structures into the surface; a surface carrying one or more indicia over which the embossed substrate is positioned in use with the non-diffractive light control structures aligned with the or each indicia; a detector, such as a camera, for detecting the optical effect resulting from viewing the or each indicia in conjunction with, for example through, the light control structures; and a system for generating a feedback signal based on the detected optical effect and using the feedback signal to change the predetermined embossing conditions based on the feedback signal.

In accordance with an eighth aspect of the present invention, apparatus for forming non-diffractive light control structures in or on a surface of a polymer substrate comprises: an embossing die carrying embossing forms corresponding to the non-diffractive light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss non-diffractive light control structures into the surface; a system for forming indicia on the surface of the substrate opposite to the surface having the light control structures; a detector, such as a camera, for detecting the optical effect resulting from viewing the or each indicia in conjunction with, for example through, the light control structures; and a system for generating a feedback signal based on the detected optical effect and using the feedback signal to change the predetermined embossing conditions based on the feedback signal.

In a further solution to the problems outlined above, we provide a feedback mechanism which enables the result of the method to be determined and then the process conditions adjusted to compensate for any unsatisfactory aspects. This is done by observing the optical effect of viewing indicia in conjunction with, typically through, the light control structures. The indicia can be provided on the opposite surface of the substrate itself or the substrate can be located over indicia on another surface. Alternatively, the indicia may be provided on one or more rollers for conveying the substrate.

Typically, the non-diffractive light control structures are focussing elements, such as lenses. When focussing elements are used, determining the resultant optical effect is not satisfactory typically comprises determining whether or not the indicia are in focus. Other means of determining the resultant optical effect could comprise identifying other optical effects such as a moire magnified effect, integral image and the like.

Having determined that the optical effect is not satisfactory then one or more of the predetermined conditions can be changed and these predetermined conditions can include embossing temperature, embossing pressure, and/or the tension under which the substrate is held during the embossing process. A wide variety of non-diffractive light control structures are envisaged for use with this invention including a series of parallel linear microprisms with planar facets arranged to form a grooved surface, a ruled array of tetrahedra, an array of square pyramids, an array of corner- cube structures, an array of hexagonalfaced corner-cubes and a saw-tooth microprismatic array. The invention is principally concerned, however, with non-diffractive light control structures in the form of focusing elements such as lenses which may be concave or convex.

The light control structures that are embossed into the polymer substrate may be sufficient by themselves to achieve the desired light control but in some cases could be provided with reflection enhancing coatings such as metallisations, ceramic materials including zinc sulphide or the like, etc.

In the case of focusing lenses, these can have a variety of sizes but the methods and apparatus are particularly applicable to the formation of microlenses. The term “microlens” can be preferably defined as lenses having an active dimension such as a diameter or width in the range 5-250 microns. In some cases, lens diameters less than 5 microns are possbile. Examples of microlenses are one or more of spherical, aspherical, polygonal base, and elongate microlenses. Particularly preferred microlenses for use in optical devices such as moire magnifiers are spherical microlenses having a base diameter of less than 250pm, preferably less than 50pm.

In the case of moire magnifier optical devices and other similar devices, the thickness of the polymer substrate is determined by the focal length of the microlenses which in turn relates to their diameter. A typical thickness for the polymer substrate will be in the rangel0-500 microns and more preferably ΙΟΙ 00 microns even more preferably 50-100 microns and most preferably substantially 70pm.

Typical examples of polymer materials which can be used are polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

The polymer substrate may be opaque, particularly where the light control structures are reflective (in which case a reflective coating may be provided on the embossed forms) but preferably is transparent or translucent. This then enables indicia to be provided on the opposite surface of the substrate to the light control structures and the formation of security devices such as moire magnifiers, lenticular devices and the like.

As mentioned above, an important application of the polymer substrates provided with light control structures is in the field of security devices such as banknotes and the like. In these security applications, the methods may further comprise coating at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer while leaving at least the light control structures uncoated. This coating step may be carried out before or after the embossing process and indeed could be carried our partially before and partially after the embossing process. For example, one side of the polymer substrate could be coated before the embossing process and the other side after.

Examples of the provision of print receptive coating layers onto polymer substrates, particularly for the production of banknotes, are described in US-A-6495231, WO-A-83/00659 and EP-A-1054778.

In order to create optical devices from the polymer substrate in which light control structures have been embossed, the method may further comprise, where the polymer substrate is transparent or translucent, before or after the embossing process, providing on the side of the substrate opposite to the light control structures, and in alignment with the structures, indicia that co-operate with the light control structures to create an optical effect.

The indicia may be provided directly on the opposite side of the substrate or on a layer joined or coated on that side of the substrate.

The coatings on opposite sides could both be partial so as to define a full window including the light control structures and indicia or microimages, or could be full on the side of the indicia so as to define a half-window.

The method of providing the indicia can be chosen from a wide variety of known possibilities as for example described in WO-A-2011/107788, WO-A-2006/125224, WO-A-2014/070079 and WO-A-2008/000350.

The invention has many applications, particularly in the security industry, for producing security devices for use in securing documents and articles of value such as banknotes, cheques, passports, identity cards, certificates of authenticity, fiscal stamps and other secure documents. Further examples include use of a security thread or stripe, a passport laminate or a layer in a passport data page,

Some examples of methods and apparatus according to the invention will now be described with reference to the accompanying drawings, in which:-Figure 1 is a schematic diagram of a first example of embossing apparatus according to the invention;

Figure 2 illustrates examples of different ways to implement microimages;

Figure 3 is a schematic cross-section through part of a banknote with lenses made according to examples of the invention;

Figure 4 is a view similar to Figure 1 but of a second example of embossing apparatus;

Figure 5 is an enlarged view of part of an embossing die provided with pedestals according to an example of the invention;

Figures 6A and 6B are enlarged, schematic views of the embossing die shown in Figure 5 in conjunction with a polymer substrate at different stages;

Figures 7 A and 7B are views similar to Figures 6A and 6B but using a modified embossing die;

Figures 8A-8C are enlarged, schematic views of a further example of an embossing die and of a polymer film embossed with embossing dies having different lens forms;

Figure 9 illustrates schematically another example of a polymer substrate having a curable coating;

Figure 10 illustrates an example of a polymer substrate having a radiation absorbing coating; and

Figure 11 is a schematic block diagram of apparatus for detecting manufacturing errors and applying a feedback control to the embossing apparatus.

In all these examples, non-diffractive light control structures in the form of focusing lenses (both convex and concave) will be described. It should be understood, of course, that the techniques can also be used for non-diffractive light control structures of many other types including, for example, a series of parallel linear microprisms with planar facets arranged to form a grooved surface, a ruled array of tetrahedra, an array of square pyramids, an array of corner- cube structures, an array of hexagonal-faced corner-cubes and a sawtooth microprismatic array.

Figure 1 illustrates a first example of apparatus for forming focusing lenses in a polymer substrate 1. The substrate may comprise a clear plastic substrate preferably formed from a typically transparent, polymeric material such as polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic

Olefin Copolymer (COC), or any combination thereof but advantageously is made of at least one biaxially-oriented polymeric film, such as BOPP. The substrate can comprise a single layer or multiple layers.

The substrate 1 is fed in web form into a nip 2 defined between an embossing die roller 3 having a shore hardness of at least 95 on the A scale and a pressure or impression roller 4 (of high shore hardness). The embossing roller 3 carries embossing forms shown schematically at 5 about its circumference in a predetermined pattern and in the shape of the inverse of a desired lens form. Thus, if the lenses to be embossed into the web 1 are spherical then the embossed forms on the roller 3 will be concave.

The temperature TE of the embossing roller 3 for soft embossing will be in the range 150-180°C for nip pressures less than 5N/mm2 and typically as low as 1N/mm2 while the temperature T| of the impression roller 4 will be in the range 50-80°C. This then generates a temperature gradient between the two rollers 4,5 such that the web 1 is heated to an average temperature across its thickness of about 100°C at which the polymer material is deformable. Increasing the nip pressure to between 5-15N/mm2 would be considered medium pressure embossing and would enable TE to be reduced by 10-15°C. Increasing the nip pressure above 15N/mm2 would be considered high pressure embossing and would enable TE to be reduced by a further 10-15°C.

The rollers 4,5 rotate in opposite senses as shown as the web 1 passes through the nip 2 and the desired lens forms are imparted into the polymer.

It should also be noted that the web 1 is maintained under tension as it passes through the embossing apparatus shown in Figure 1 (by means not shown).

Immediately downstream of the nip 2 is provided chilling apparatus 10 which comprises in sequence a large diameter chill roller 12 and two smaller diameter chill rollers 13,14. The web 1, after embossing and passing through the nip 2, extends around the three chill rollers 12-14.

The chill rollers 12-14 are maintained at a low, chill temperature (Tc) so that the web 1 is rapidly cooled from the deforming temperature used at the nip 2. The effect of this is that the embossed structure does not have time to relax significantly or the degree of relaxation is controlled and thus the lens forms embossed into the web are maintained in their original, desired form.

The temperature Tc is preferably less than 50°C and most preferably less than 30°C.

It should also be noted that the embossed side of the web 1 does not contact the chill rollers 12 to avoid compression of the lenses.

In this and in the later examples, the lenses embossed into the polymer web 1 are preferably microlenses such as spherical, aspherical, polygonal base, and elongate, cylindrical microlenses. They will typically have active dimensions, for example diameters, of no more than 250pm and preferably less than 50pm.

The thickness of the polymer material 1 is typically in the range 10-500 microns and more preferably 10-100 microns, even more preferably 50-100 microns and most preferably substantially 70pm for use in a polymer banknote, and will usually correspond to the focal length of the lenses.

Although the preferred polymer material is BOPP, other suitable materials include polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

Following manufacture of the microlens bearing polymer, it can be processed in a number of different ways. Firstly, the finished polymer could be used in its own right to provide an array of lenses for a variety of applications including a verifying device such as described in WO-A-94/27254. In other applications, where the lens array comprises an array of microlenses, an array of microimages or other indicia can be provided in association with the microlenses, typically on the opposite side of the web (the web being translucent or transparent), the microimages cooperating with the microlenses to define an optical device such as a moire magnifier, integral imager, lenticular device or the like. The association of the microimages with the microlenses can be achieved in a variety of ways known in the art.

In one example, the microimages are printed onto the web. In other cases, the microimages can be provided on a separate layer which is then laminated with the web. Examples of high resolution printing techniques suitable for forming microimages are described in WO-A-2014/070079 and WO-A-2008/000350.

It would also be possible to provide some or all of the image elements as relief structures and examples of some of these are shown in Figures 2A-2J. In these Figures, ΊΜ’ indicates the parts of the relief generating an image while ‘ΝΓ indicates those parts which do not generate an image.

Figure 2A illustrates embossed or recessed image elements. Figure 2B illustrates debossed image elements. Figure 2C illustrates image elements in the form of grating structures while Figure 2D illustrates moth-eye or other fine pitch grating structures.

These structures can be combined. For example, Figure 2E illustrates image elements formed by gratings in recesses areas while Figure 2F illustrates gratings on debossed areas.

Figure 2G illustrates the use of a rough embossing.

Figure 2H illustrates the provision of print on an embossed area while Figure 21 illustrates “Aztec” shaped structures.

Figure 2J illustrates ink filled recesses. A very important application of an embossed web having an array of lenses on one side and an array of microimages or other indicia on the other side is in the formation of a security document such as a banknote. An example is shown schematically in Figure 3. The transparent or translucent polymer web 1 is shown in cross-section and has on one side a two-dimensional array of embossed, spherical microlenses 20 and on the opposite side a two-dimensional array of microimages 21 which are sufficiently aligned with the microlenses 20 to generate a moire magnified image when the device is viewed in transmission. The web 1 has been provided with an ink receptive coating 23 on the side with the lenses 20 and an ink receptive coating 24 on the side with the microimages 21. The coatings 23,24 define gaps 25,26 respectively which are aligned, contain the lenses and microimages 20,21 and thereby form a window.

The coatings 23,24 carry printed indicia (not shown) relating to the security document, in this case a banknote, such as its denomination and other security patterns.

The ink receptive layers or coatings are preferably opaque or opacifying layers of printed material on which indicia relevant to the security document can be provided. The layers of opacifying ink may comprise pigmented coatings comprising pigments such as titanium oxide dispersed within a binder or carrier or cross-linkable polymeric material. The opacifying layers may be printed using any conventional printing technique including offset, gravure, intaglio printing and the like.

Examples of polymeric-based security articles are described in WO 83/00659 and EP-A-1054778 which also describe how indicia receptive coatings can be provided.

The provision of the coatings 23,24 can be carried out downstream of the embossing apparatus shown in Figure 1 but as an alternative, one or both of the coatings 23,24 could be provided on the web 1 upstream of the embossing apparatus with the embossing apparatus being designed to provide the lens array only in the window 25. A second example of an embossing apparatus is shown in Figure 4. This is a modification of the apparatus shown in Figure 1 in which the embossing roller 3 is located between two impression rollers 4A,4B, with which it defines respective nips 2A,2B. The rollers 3,4A,4B are maintained at temperatures TE and Ti respectively as in Figure 1.

This configuration has the benefit of minimizing deflection of the embossing roller 3 and also allows the option of using a clamped embossing plate.

Thus, in this example, a separate embossing plate carrying the desired lens forms is provided on the embossing roller 3 and has an impression length equal to one half of the overall roller circumference.

On exiting the nip 2A between the rollers 3,4A the web 1 passes to first chilling apparatus 10A including a chill roller 30 around which the web is wrapped through an angle of about 180°, the roller 30 contacting the non-embossed side of the web where the objective is to cool the substrate below the softening temperature of the substrate such that no further relaxation of the embossed lens relief occurs.

The web is then guided around a number of guide rollers 32, which may be further chill rollers, around a further chill roller 33 before passing into a nip 2B between the rollers 3,4B.

The chill rollers 30,33 are water cooled to a temperature Tc of less than 50°C, preferably less than 30°C.

It is important that the web must travel between the first and second embossing nips 2A,2B an integer number of embossed panel lengths and arrive at the second nip 2B at the correct position to emboss those alternate sections not embossed by the passage of the web through the nip 2A. The left and right arrows in Figure 4 illustrate that the path length and tension is adjusted to ensure this requirement is met.

It is preferred that both impression rollers 4A,4B will either be coated with a very high shore hardness polymer or may even be steel. The higher the shore hardness, the less the roller surface compresses, which in turn results in a higher nip pressure for a given cylinder load.

As shown in Figure 4, after leaving the second nip 2B, the web passes into second chilling apparatus 10B comprising chill rollers 35,36 water cooled to a temperature Tc less than 50°C, preferably less than 30°C, from where the cooled web then passes on for further downstream processing as described in more detail with respect to the Figure 1 example.

Figure 5 illustrates the principle of an alternative embossing die 3A for using in place of the embossing die 3 in the previous embodiments. The Figure shows in a very schematic and enlarged form, part of the embossing die 3A having a main body or base substrate 50 on which are provided individual raised pedestals 52 on which embossing die forms 54 are provided. In this case, the die forms 54 define convex lens forms which will be used to emboss corresponding concave lens forms into the polymer substrate. The main difference between the embossing dies 3 and 3A is the insertion of the pedestal section 52 having a height H. The sagittal height (or depth) of the lens forms 54 is also shown at S referred to as “SAG height”.

The effect of this modified embodiment is shown in more detail with respect to different examples in Figures 6 and 7. In the Figure 6 example, the embossing die 3A is provided with pedestals having a small height H relative to the SAG height S of the lenses, movement of the polymer and die being from left to right.

The total area occupied by the lens array regions will be a small fraction of the total web area and more pertinently if an imaginary line is drawn across the nip width then the lens regions will occupy some fraction of that nip width. Suppose this value for a particular design is 25%, then it follows that if we elevate the relief then initially at least contact will occur only in the raised lens area thus increasing the nip width by a factor of 4. This scenario is shown in Figure 6, where the pedestal height H is much less than the SAG height S (> 30% but less than 90%). Initially only the lens sag profile contacts the substrate 1, leading to an elevated contact pressure (for a given roller load) to impress the lens profile into the substrate by using the minimum softening temperature.

Also as well as increasing the lens - substrate contact pressure, the amount of heat transferred into the substrate 1 (and therefore distortion and shrinkage) is reduced via direct conduction simply because the direct physical contact between substrate and tool is initially at least limited to the lens region.

As the lens profile pushes into the softened substrate displacing substrate material (Figure 6B), eventually a state is achieved wherein the displaced substrate material flows up to contact the embossed die / surface at least in those areas adjacent to the lens regions.

By increasing the pedestal height (H) further relative to the SAG height (S) (Figure 7), it follows that even after the full height of the lens array is pressed into the substrate (Figure 7B), the increased pedestal will prevent the substrate 1 making contact with the surface of the embossing die base substrate 70 in any regions other than those defined by the patterned lens arrays.

For an example of pedestal height, and for lens SAG height values in the range 9-15pm, then in the scenario of Figure 6 a pedestal height (H) of only 1-3pm would be sufficient to limit both pressure and heat transfer to the lens regions only, for most of the emboss depth

In contrast, for the scenario of Figure 7, a pedestal height (H) greater than 3pm would ensure that pressure and heat transfer contact was always limited to the lens regions for all of the emboss stroke or cycle.

If the pedestal height is too high then it will tend to leave a proud relief on the opposing side of the substrate and therefore pedestal height should ideally be limited to less than half the SAG height.

Figure 8A is a view similar to Figure 5 but showing part of an embossing die 3B with pedestals 82 on a base substrate 80 and carrying embossing lens forms 84 for embossing convex lenses into the polymer substrate. Compared with the previous example, the major difference is that the pedestal height H needed to prevent full contact with the base plate of the embossing die 3B throughout the majority of the embossing stroke / cycle should have a minimum value approaching that of the SAG height S (i.e. H > 0.75S).

Note if the value of H indeed slightly exceeds the value of S then little or no contact will occur between substrate and base substrate 80 throughout the embossing process and hence a higher contact emboss pressure is achieved and also reduced heat transfer into the substrate film in areas not being impressed with lenses.

Such a die can be used to reduce or eliminate the elevation of the lenses above the base substrate thus reducing any negative impact on other downstream, print processes.

Figure 8B shows the embossed film 1 scenario where H < S and thus the peaks of the embossed lenses 86 lie slightly above the base substrate of the polymer film 1.

Figure 8C shows the scenario H > S and the lenses 86 lie below the surface of the substrate 1. The benefit here in addition to print processes is that the recessed convex lenses should have improved wear and soil resistance.

Figure 9 illustrates another embodiment of polymer film 1A for use in the embodiments of Figures 1-4 or indeed with other conventional embossing apparatus. In this case, the polymer substrate 1A comprises a primary substrate layer 90 made of any of the materials previously mentioned in connection with the substrate 1, the preferred material being biaxially-oriented polypropylene, the primary layer 90 being coated with a layer 92 containing a cross-linking chemical group. The group that provides secondary curing preferably has 2 curing groups-one based on an - acrylic- which cures during the pre-polymerisation and is a part of network; and a second group which cures during the secondary curing reaction.

The cross-linking additive layer 92 is chosen so that prior to embossing the layer 92 has, or can be modified to have, a deformation temperature which is lower than that of the layer 90. For example, the layer 92 will be deformable at a lower temperature than layer 90.

Figure 9 shows this modified substrate 1A in conjunction with the embossing die 3A of Figure 7 although it could be used with any of the embossing dies previously described.

At the nip 2, the layer 92 is embossed by the embossing die so as to create, in this case, concave focusing lenses in the surface of the layer 92 (but not the layer 90). Continued movement of the substrate 1A in the direction of the arrow 94 brings the embossed substrate (although the embossing is not shown in Figure 9) to a curing position 94 where the layer 92 is subjected to curing radiation which activates the secondary curing group so as to cure and harden the layer 92. Depending upon the secondary group and the curing chemistry, the curing radiation could be UV, IR or heat.

The softening or embossing temperature of the layer 92 will be less than that of the layer 90 so, when the layer 90 is BOPP, the embossing temperature of the layer 92 will be less than 100°C, for example 60-90°C. Following the emboss process, and after the secondary cross-linking group has been activated the cross-linkage bond formation increases the hardness of the layer 92 to provide similar mechanical properties to layer 90.

In order to form the substrate 1A, the material to form the layer 92 may be partially pre-cured and then coated on the layer 90 so that the layer 92 is soft and can easily be deformed during the embossing stage. This pre-curing allows the coating to be applied on flat surfaces and yields a dry, tack-free surface with sufficient deformability after drying.

In alternative embodiments, the material used to form the layer 92 is not precured and instead is applied as a substantially solid layer, e.g. by co-extrusion with the polymer substrate, or by applying the layer 92 in a molten state and cooling to solidify. In these embodiments, the thermoplastic nature of the material ensures that, as it is heated in the thermal embossing process to a point at which it can be embossed, it is plastically deformable during the embossing process. Materials that are applied without any pre-curing that are nevertheless solid prior to embossing may provide a dry, non-tacky surface that facilitates handling.

Once the lenses have been formed, the coating 92 is capable of undergoing a second curing step leading to a hard coating with increased chemical and mechanical resistance as a result of further cross linking.

Acrylatic monomers with secondary functionalities such as Glycidyl, Oxetane and Hydroxy are the main examples of suitable materials for synthesising such polymer coatings in combination with normal monofunctional acrylate components. There are multiple other functionalities which provide the same mechanism. The most common examples of such precursors are:

Glycidyl methacrylate (GMA) or Trimethylolpropane oxetane acrylate (TMPO acrylate)

One or more of Hydrox functional acrylates in presence of a solvent such as Isopropanol, Butanol, MIBK, MEK, Ethyl acetate, Butyle Acetate, Gamma Butyrolactone or a mixture thereof; and a free radical based thermal- or photo- initiator and a Photoacid generator.

And-Hydroethyl Acrylate, Hydroxyethyl Methacrylate or Hydroxypropyl I Acrylate.

Some examples of monofunctional acrylate monomers are:

Sartomer SR256- 2-(2-ethoxyethoxy) ethyl acrylate (EOEOEA) SR285- Tetrahydrofurfuryl acrylate (THFA) SR217- Tertiobutyl cyclohexanol acrylate (TBCHA) SR339C- 2-phenoxyethyl acrylate (2-PEA) SR395- Isodecyl acrylate (IDA) SR440- Iso octyl acrylate (IOA) SR489- Tridecyl acrylate (TDA) SR495B- Polycaprolactone acrylate (CAPA) SR506D- Isobornyl acrylate (IBOA) SR531- Cyclic trimethylolpropane formal acrylate (CTFA)

Sigma-Aldrich-n-butyl acrylate Ethyl hexyl acrylate Tertiary butylacrylate Methyl methacrylate Acrylic acid

The same monomers are also available for multiple chemical suppliers/ Manufacturers, including Allnex, IGM, BASF, Bayer, Miwon, Doublebond etc.

Some examples of initiators are:

Either normal free radical based photo-initiators such as Irgacure 181, 651, 500, 1173, 2959, 819, 907

Or Free radical based thermal initiators such as AIBN (Azobisisobutyronitrile), 1,1'-Azobis (cyclohexanecarbonitrile), Benzoyl peroxide or tert-Butyl hydroperoxide Plus,

Photo-acid generator (PAG) type cationic cure initiators- such as Cyracure 6976 and Cyracure 6974 (triarylsulfonium hexafluoroantimonate salts).

Initiator concentration: 4-5% (w/w) of free radical type photo-initiators (eg, Irgacure 819) and 3% (w/w) of PAG type initiators- Cyracure 6974.

Example of Polymer Synthesis: A pre-polymer was first synthesised by reacting a mixture of 1 or more low Tg acrylate monomers (examples given below) and either-

Glycidyl methacrylate (GMA) or / and

Trimethylolpropane oxetane acrylate (TMPO acrylate) or/and

Hydroxyethyl acrylate in presence of a solvent such as Isopropanol, Butanol, MIBK, MEK, Ethyl acetate, Butyle Acetate, Gamma Butyrolactone or a mixture thereof. A free radical based thermal- or photo- initiator and a Photoacid generator. As it is known to a technician trained in the art that exposure of UV light (a Few minutes) or heat (100 to 150 degree C for 14 hrs) would be required for initiating the polymerisation.

This pre-polymer can be further chemically modified to enhance chemical reaction or cross linking density in particular, as required for modifying the hardness appropriate for the applications. This can be achieved by introducing additional expoy, oxetane or hydroxyl groups in the prepolymer - such as by adding 1-15% of bisphenol-A novalac glycidyl ether with 8 epoxy groups (commonly known as SU8) or similar amount of isocyanate terminated monomers/ oligomers or other similar high functionality Epoxy’s or other chemical reactants. This pre-polymer can also be further concentrated or diluted to required concentration appropriate for coating applications or additional additives can be added to it.

This pre-polymer was coated on the polymer substrate and dried to tack-free transparent coatings of thickness in the range of 0.5 to 100 microns. Lenses were thermally embossed onto this layer by heating the pre-polymer coated substrates to <100 C following by embossing using a hard lens tool.

After the formation of lenses by thermal embossing the layer was exposed to strong UV light (>1000mj/cm2) for 1 to 2 min and subsequently heated at 80 to 100 degree for 2 min. This led to production of further cross-links and hardening of lenses.

Figure 10 illustrates yet another embodiment of the invention. In this case, the substrate 1 of the previous example is replaced by the substrate 1B comprising a primary layer 100 formed of a polymeric plastics material of the type described above in connection with the substrate 1, the substrate 100 being coated with a radiation absorbing layer 102 adapted to absorb, typically infrared, radiation. As can be seen in Figure 10, upstream of the embossing die 3A, is located an irradiation station 104 where infrared radiation from a laser or the like is applied to the layer 102 where it is absorbed. The layer 100 is transparent to the selected infrared radiation wavelength so that only the layer 102 absorbs the infrared radiation and heats up. This heating causes the adjacent surface region of the layer 100 to be raised to a temperature at which it is deformable while the remainder of the layer 100 remains below that temperature or indeed is unaffected. Typically, the temperature to which the layer 102 is raised is in the range 150-180°C while the base layer 100 remains unheated or at least is not heated to a value approaching the point at which the base layer is deformable.

It is preferable if the layer 102 is transparent in the optical wavelength range.

As far as suitable materials for the layer 100 are concerned, PET is substantially transparent in the infrared spectrum and so wavelength selection for IR source and absorber is not critical and therefore medium wave IR (2-4pm) or long wave IR (4-1000pm) radiation is acceptable. BOPP has absorption bands at 3 and 6pm and therefore long-wave IR radiation above 6pm is preferred.

As far as the layer 102 is concerned, typical compositions are described in the introduction to this specification.

Figure 11 illustrates schematically apparatus for checking the accuracy of lenses embossed into the substrate 1 and can be used with any of the examples previously described. The embossing apparatus is shown at 110 and the embossed substrate 1 is fed from the embossing apparatus to a checking station 112 where it is brought over a fixed surface 114 carrying indicia 116. The indicia 116 correspond to indicia that would in a final product be provided, such as printed, on the underside of the substrate 1 in alignment with the lenses.

The superposed lenses and indicia 116 are then viewed in transmission (the surface 114 being provided on a transparent plate 118) and the resultant optical effect viewed by a camera 120. The received image is then analysed by a processor 122 to determine if it is in focus. If it is not, this indicates that one of the process parameters for controlling the embossing apparatus 110 needs to be adjusted and the processor 122 outputs a suitable control signal to affect that adjustment. The reason for the out of focus condition would typically be because the lenses are not provided at the correct pitch. Suitable parameters that can be adjusted include the tension of the substrate 1 in the embossing apparatus 110, the temperature of the embossing roller, the speeds of the rollers, embossing temperature and the like.

In a modification of this apparatus, the processor 122 could simply display the required adjustment on a monitor or the like and the adjustment would then be manually made by an operator.

In another modification, the indicia 116 could be provided, for example printed or laminated, on the underside of the substrate 1 thus avoiding the need for the plate 118.

In yet another modification, the indicia 116 could be provided as markings on one or more rollers used to convey the substrate 1. Typically in these embodiments, the camera 120 views the indicia through the lenses in reflection.

Claims (51)

1. A method of forming non-diffractive light control structures, the method comprising: a) providing a polymer substrate with a thermoplastic, curable coating; b) thermally embossing non-diffractive light control structures into the curable coating while the coating is at a temperature at which it can be plastically deformed; and c) curing the deformed coating during or after the embossing process wherein the temperature of the coating during the embossing process is such that the coating is plastically deformed while the substrate is at a lower temperature than that at which the substrate can be deformed.

2. A method according to claim 1, wherein the temperature of the coating during the embossing process is lower than the temperature at which the substrate can be deformed.

3. A method according to claim 1 or claim 2, wherein the polymer comprises one of polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

4. A method according to claim 3, wherein the substrate comprises BOPP, and wherein the coating is at a temperature of less than 100°C during the embossing process.

5. A method according to any of claims 1 to 4, wherein the step of providing a curable coating comprises partially pre-curing and coating the radiation curable material on to the polymer substrate.

6. A method according to any of claims 1 to 5, wherein the or each curing process comprises one of UV, IR and thermal curing.

7. A method according to any of claims 1 to 6, wherein at least before step b) the thermoplastic, curable coating is a solid, preferably a dry, non-tacky solid.

8. A method of manufacturing a security document comprising non-diffractive light control structures in a surface of a polymer substrate of the security document, the method comprising thermally embossing non-diffractive light control forms into the surface of the substrate while the substrate is at a temperature at which the surface can be deformed; chilling the embossed substrate; and, before, during or after either of the steps of thermally embossing and chilling, coating at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer such that at least the non-diffractive light control structures are uncoated.

9. A method according to claim 8, wherein the chilling process is carried out in a sequence of sub-steps in which the substrate is chilled to successively lower temperatures.

10. A method according to claim 8 or claim 9, wherein the embossing process is carried out by passing the polymer substrate through a nip between an embossing die and an impression roller, wherein at least part of the chilling process is performed by the impression roller.

11. A method according to any of claims 8 to 10, wherein the embossing process is carried out by feeding the substrate through a nip defined between an embossing die roller and a pressure roller, the chilling process being performed downstream of the nip.

12. Apparatus for manufacturing a security document comprising non-diffractive light control structures in a surface of a polymer substrate of the security document, the apparatus comprising: an embossing die carrying embossing forms corresponding to the light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss the non-diffractive light control structures into the surface; a heating system for heating the polymer substrate, in the vicinity of the embossing die, to a temperature at which the surface can be deformed; chilling apparatus to chill the embossed substrate; and a coating system to coat at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer such that at least the non-diffractive light control structures are uncoated.

13. Apparatus according to claim 12, wherein the chilling apparatus comprises at least one chilling roller which engages the embossed substrate.

14. Apparatus according to claim 12 or claim 13, wherein the chilling apparatus comprises a plurality of chilling rollers which engage the embossed substrate in sequence, each chilling roller being maintained at substantially the same temperature, preferably less than 50°C, most preferably less than 30°C.

15. Apparatus according to claim 14, wherein the chilling rollers are water cooled.

16. Apparatus according to any of claims 12 to 15, further comprising a pressure roller forming a nip with the embossing die through which the substrate is fed.

17. Apparatus according to any of claims 12 to 16, wherein the heating system is adapted to heat one or both of the embossing die and pressure roller, if provided, whereby heat is transferred to the substrate from one or both of the embossing die and pressure roller.

18. Apparatus for forming non-diffractive light control structures in a surface of a polymer substrate, the apparatus comprising: an embossing die roller carrying embossing forms corresponding to the non-diffractive light control structures and which, at an embossing location, engages the surface of the polymer in use to emboss non-diffractive light control structures into the surface; a pressure roller defining a gap with the embossing die roller through which the polymer substrate passes in use as the rollers rotate; and a heating system for heating the polymer substrate, in the vicinity of the embossing die roller, to a temperature at which the surface can be deformed, wherein the embossing forms are provided on radially outwardly projecting pedestals of the embossing die roller whereby each pedestal forms a respective nip with the pressure roller as the rollers rotate thereby causing non-diffractive light control structures to be embossed into the surface of the polymer substrate.

19. A method according to claim 18, wherein the embossing forms are shaped to emboss concave lenses into the substrate, and wherein the height of a pedestal is less than half the SAG height of the lens forms.

20. A method according to claim 18, wherein the embossing forms are shaped to emboss convex lenses into the substrate, and wherein the height of each pedestal is between 50-100% of the SAG height of the lens forms.

21. A method according to claim 20, wherein the height of each pedestal is greater than the SAG height of the lens forms.

22. A method of forming non-diffractive light control structures in a first surface of a polymer substrate, the method comprising: providing a polymer substrate, the polymer substrate having either a skin region of the substrate at the first surface of the substrate comprising a radiation absorbing material, or a radiation absorbing material affixed to the first surface of the polymer substrate; exposing the radiation absorbing material to radiation whereby the radiation absorbing material absorbs the radiation and heats up the first surface of the polymer substrate to a temperature at which the surface can be locally deformed; thermal embossing non-diffractive light control structures into the first surface of the substrate; and allowing the substrate to harden.

23. A method according to claim 22, wherein the radiation absorbing material is transparent to optical wavelengths.

24. A method according to claim 22 or claim 23, wherein the radiation absorbing material absorbs infrared radiation.

25. A method according to any of claims 22 to 24, wherein the radiation absorbing material is a polymer film, such as BOPP, provided with a radiation absorbing additive.

26. A method according to any of claims 22 to 25, wherein the non-diffractive light control forms comprise focusing lenses and wherein the thickness of the radiation absorbing material is equal to or exceeds the SAG height of the lens, preferably by at least a factor of 2.

27. A method of forming non-diffractive light control structures comprising focussing elements in or on a surface of a transparent or translucent polymer substrate, the method comprising: (i) embossing, under predetermined conditions, focussing elements into the surface of the substrate or into a material provided on the surface of the substrate; (ii) locating the substrate over one or more indicia such that the formed focussing elements are aligned with the indicia; (iii) determining the resultant optical effect of viewing the or each indicia, in conjunction with, for example through, the focussing elements; (iv) generating a feedback signal based on the determined resultant optical effect; (v) using the feedback signal to change one or more of the predetermined conditions based on the feedback signal; and (vi) repeating steps (i)-(iii) such that if the determined resultant optical effect is not satisfactory the changed predetermined conditions improve the resultant optical effect.

28. A method according to claim 27, wherein step (iv) comprises determining that some or all of the indicia are not seen in focus.

29. A method according to claim 27 or claim 28, wherein step (ii) comprises placing the or each indicia on the surface of the substrate opposite to the formed focussing elements before or after forming the light control structures.

30. A method according to claim 27 or claim 28, wherein step (ii) comprises locating the substrate over a surface on which the or each indicia is provided.

31. A method according to any of claims 27 to 30, wherein the predetermined conditions include embossing temperature, embossing pressure, and/or the tension under which the substrate is held during the embossing process.

32. Apparatus for forming non-diffractive light control structures comprising focussing elements in or on a surface of a polymer substrate, the apparatus comprising: an embossing die carrying embossing forms corresponding to the focussing elements and which, at an embossing location, engages the surface of the polymer in use to emboss focussing elements into the surface; a surface carrying one or more indicia over which the embossed substrate is positioned in use with the focussing elements aligned with the or each indicia; a detector, such as a camera, for detecting the optical effect resulting from viewing the or each indicia, in conjunction with, for example through, the focussing elements; and a system for generating a feedback signal based on the detected optical effect and using the feedback signal to change the predetermined embossing conditions based on the feedback signal.

33. Apparatus for forming non-diffractive light control structures comprising focussing elements in or on a surface of a polymer substrate, the apparatus comprising: an embossing die carrying embossing forms corresponding to the focussing elements and which, at an embossing location, engages the surface of the polymer in use to emboss focussing elements into the surface; a system for forming indicia on the surface of the substrate opposite to the surface having the light control structures; a detector, such as a camera, for detecting the optical effect resulting from viewing the or each indicia, in conjunction with, for example through, the focussing elements; and a system for generating a feedback signal based on the detected optical effect and using the feedback signal to change the predetermined embossing conditions based on the feedback signal.

34. Apparatus according to claim 32 or claim 33, wherein the system for determining if the detected optical effect is satisfactory is adapted to determine if the or each indicia is seen in focus.

35. Apparatus according to any of claims 32 to 34, wherein the predetermined embossing conditions include embossing temperature, embossing pressure and/or tension under which the substrate is held during the embossing process.

36. Apparatus or method according to any of the preceding claims, wherein the non-diffractive light control structures comprise refractive structures.

37. Apparatus or method according to claim 36, wherein the non-diffractive light control structures comprise one of a series of parallel linear microprisms with planar facets arranged to form a grooved surface, a ruled array of tetrahedra, an array of square pyramids, an array of corner- cube structures, an array of hexagonal-faced corner-cubes and a saw-tooth microprismatic array.

38. Apparatus or method according to any of claims 1 to 35, wherein the non-diffractive light control structures comprising focusing elements such as focusing lenses.

39. Apparatus or method according to claim 38, wherein the focusing lenses formed in the surface of the substrate are microlenses.

40. Apparatus or method according to claim 39, wherein the microlenses are one or more of spherical, aspherical, polygonal base, and elongate, cylindrical microlenses.

41. Apparatus or method according to claim 40, wherein the microlenses comprise spherical microlenses having a base diameter of less than 250pm, preferably less than 50pm.

42. Apparatus or method according to any of the preceding claims, wherein the polymer comprises one of polypropylene (PP) (most preferably bi-axially oriented PP (BOPP)), polyethylene terephthalate (PET), polyethylene (PE), polycarbonate (PC), polyvinyl chloride (PVC), nylon, acrylic, Cyclic Olefin Polymer (COP) or Cyclic Olefin Copolymer (COC), or any combination thereof.

43. Apparatus or method according to any of the preceding claims, wherein the polymer substrate is transparent or translucent.

44. Apparatus or method according to any of the preceding claims, wherein the polymer substrate has a thickness in the range 10-500 microns and more preferably 10-100 microns, even more preferably 50-100 microns and most preferably substantially 70pm, preferably substantially 70pm.

45. Apparatus or method according to any of the preceding claims, further comprising coating at least partially one or both surfaces of the substrate with a, preferably opaque, coating layer while leaving at least the non-diffractive light control structures uncoated.

46. Apparatus or method according to claim 45, wherein the coating step is carried out partially or completely after the embossing process.

47. Apparatus or method according to at least claim 43, further comprising, before or after the embossing process, providing on the side of the substrate opposite to the non-diffractive light control structures, and in alignment with the light control structures, indicia that co-operate with the focusing lenses to create an optical effect.

48. Apparatus or method according to claim 47, wherein the indicia are provided, for example printed, directly on the said side of the substrate or are provided on a layer joined to or coated on the said side of the substrate.

49. Apparatus or method according to claim 47 or claim 48, wherein the optical effect is one of moire magnification, integral image formation, and a lenticular effect.

50. Apparatus or method according to any of the preceding claims, further comprising providing indicia on the substrate or coating layer(s), if provided, to define a security document such as a banknote, cheque, passport, identity card, certificate, or stamp.

51. Apparatus or method according to any of claims 1 to 6 or 18 to 49, wherein the polymer substrate is used as one of a security thread or stripe, a passport laminate or layer in a passport data page.