EP2699425B1

EP2699425B1 - Security label (versions), method of forming a security label and method of product authentication (versions)

Info

Publication number: EP2699425B1
Application number: EP11723630.7A
Authority: EP
Inventors: Vadim A. SHEVKO; Ihar A. KRASHEUSKI
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-04-21
Filing date: 2011-04-21
Publication date: 2015-08-05
Anticipated expiration: 2031-04-21
Also published as: EP2699425A1; WO2012143746A1

Description

This invention relates to validation of security papers, documents, banknotes, artworks and brand products.
To combat fraud, various security features, that are hard to reproduce, are added to protect various type of products, e.g., watermarks, microprinting, and embedded to paper metallized stripes. Optical security elements, that are able to change the polarization of incident light, may be used as such security features, such as holograms, liquid crystal optical elements, and polymer layers with hidden images visible in polarized light only.
It is known a method of product authentication by forming a security label placed on the outer surface of protected product where a security label is comprised of a subsequently disposed transparent flexible substrate, a transparent thermoplastic lacquer layer, a metallized reflective layer and a transparent heat-sealable adhesive layer. A relief-phase hologram with visible and hidden laser images is made in a transparent thermoplastic lacquer layer by hot embossing method. The visible image appears in white light, while hidden laser image becomes visible when the hologram is illuminated with coherent laser beam. A polarized light-sensitive material is embedded into the security label; a polarizing encoding element is placed before the security label and then it is illuminated with blue-green light in order to record the hidden polarized image into the polarized light-sensitive layer; a polarizer is placed on the security label and illuminated with white light to reproduce the hidden polarized image; the polarized light-sensitive layer is situated on the outer surface of the transparent flexible substrate made of optically transparent material (see Patent # 2165360 of the Russian Federation, the invention description, Application # 95117875 of the Russian Federation, 1997.)
It is known an ID card authentication system comprising an information carrier substrate, a partially light reflecting layer superposed over the said substrate, a phase quarter wave plate placed over the said reflecting layer. A linear light polarizer is used for identification (see US Patent # 4659112, published on 21 April 1987 ).
It is known a security printing anti-counterfeiting method based on fact that two or more accurately positioned rasters are printed on some part of protected paper. There is placed a transparent lacquer layer or film over the mentioned rasters, and another raster is printed on the said layer accurately positioned to the first two rasters. Different images become visible on this part of the paper when viewed at a different orientation of the paper to the light source (see Patent # 2168416 of the Russian Federation published on 10 June 2001).
It is known a security label, a method of its production and a document protected with this label where the label contains an embossed transparent film covered at least partially with a metallic layer and the film has two zones at that. The first zone is made so that a rainbow effect is observed in the first light source orientation and disappears at a 90° angle. The second zone is covered with a dielectric layer instead of a metallic one and the dielectric appears in a specific color in the first mentioned light source orientation and changes the color at the turn on 90° (see Patent # 2236951 of the Russian Federation published on 20 February 2004).
There are known security labels containing hidden color polarized images made by photo method. (Application # 2008217907 of Japan published in 2008).
The closest prior art reference to the claimed security label is a security label comprising: a sequentially disposed metallic reflective layer, a transparent substrate, a transparent thermoplastic lacquer layer and having an element that changes optical properties of the substrate to form a hidden polarized image. The element is made on a face side of the substrate in the form of a relief image that locally changes the optical depth of the substrate (see UK Patent # 2318180 published on 17 February 1999 ).
A prior art reference to the claimed method of forming a security label is a method of producing a security label in which the security label is made of a sequentially disposed metallic reflective layer, a transparent substrate, a transparent thermoplastic lacquer layer and there is produced an element that changes optical properties of the substrate to form a hidden polarized image. The element is made on a face side of the substrate in the form of a relief image that locally changes the optical depth of the substrate (see UK Patent # 2318180 published on 17 February 1999 ).
Also a prior art reference to the claimed authenticity verification method of products is a method for ensuring authenticity of products in which a security label formed by the method described above is placed on the outside surface of the certified object and a hidden polarized image is visualized by observing it through a circular polarizer. (see UK Patent # 2318180 published on 17 February 1999 ).
The visible outlines of the polarized image, complicated realization and non technologic manufacturing procedures may be mentioned as disadvantages of such security label.
Document WO 2009/154506 discloses as security element having a reflective latent image on an optically anisotropic translucent substrate, printed on both sides.
The technical result of the invention is an implementation of a cost-effective and mass-producible security label with hidden polarized color images without any visible outlines of such polarized images.
The technical result of the invention includes method of forming a cost-effective and mass-producible security label with hidden polarized images without any visible outlines of such polarized images and method of the product authenticity verification with the assistance of such security label.
The claimed technical results of the claimed security label comprising: a sequentially disposed metallic reflective layer and an optically anisotropic transparent substrate, has been achieved at the expense of that the element changing the optical properties of the substrate is made in the form of mask that covers a face side of the substrate by the same metallic material that is used for the reflective layer, and the mask is performed using a solid fill printing method with blank spaces made in accordance with the hidden image graphics. For each section of such optically anisotropic transparent substrate under the blank spaces of the mask the total optical thickness L = d(n_e - n_o ), where

d - thickness of optically anisotropic substrate;
n_e - the index of refraction for the extraordinary light beam in the substrate;
n_o - the index of refraction for the ordinary light beam in the substrate,

The technical result in the method of forming a security label has been achieved by ensuring that for each section of such optically anisotropic transparent substrates with the blank spaces in their masks laying under the blank space in the top substrate mask the total optical thickness $L = \sum_{i = 1}^{N} d_{i} (n_{e i} - n_{o i}),$
where

d_i thickness of optically anisotropic i^th substrate;
i - from 1 to N depending on the number of the substrates mentioned above;
n_ei - the index of refraction for the extraordinary light beam in the i^th substrate;
n_oi - the index of refraction for the ordinary light beam in the i^th substrate,

Preferentially the mask should be performed using a solid fill printing with blank spaces made in accordance with the hidden image graphics.
The second technical result is achieved by forming at least one more sequentially disposed additional optically anisotropic substrate with an element changing the optical properties of the substrate and made in the form of the mentioned mask.
The metallic reflective layer on a reverse side of the film may be formed by solid fill printing with a metallic ink without any blanks.
Preferentially the security label should be applied to the outer surface of the protected product using an extra adhesive layer.
The mask on a face side of the substrate may be covered by at least one masking layer made preferentially of the same metallic material as the reflective layer and the mask; the density and/or thickness of the said layer should be enough to hide the mask outline without impeding the light transmission through the masking layer.
The security label may be formed directly during the printing process as a part of protected products like packing, labels, scotch tape, sealing tape based on optically anisotropic transparent film.
The optically anisotropic transparent substrate may be an oriented polymer film selected from the group that includes polyethilentereftalate, biaxially-oriented and axially oriented polypropylene film, or oriented lacquer layer made from the polymer group that includes polyvinyl alcohol (PVA), polystyrene, polycarbonate and ethylcellulose.
In the following, no limiting examples of an implementation of the claimed invention are illustrated by the drawings wherein:

Figs. 1 to 4 are schematic representation of the method of forming a structure of the security label, while Fig. 4 shows a final structure of the first version of the security label;
Fig. 5 shows a final structure of the second and the third versions of the security label and the scheme of forming a hidden polarized color image when the light passes through the label;
Fig. 6 is a schematic illustration of the light passing through the system "polarizer - optically anisotropic film - reflective layer,
Fig. 7 represents the orientation of the polarizer axis with reference to main directions of the optically anisotropic film;
Fig. 8 shows the spectral dependence of transmittance T of the system for different thickness values of the optically anisotropic film L = (n_e - n_o )d;
Figs. 9 to 10 are an optical scheme for observing a hidden polarized image.

The claimed security label is based upon the optically anisotropic substrate made of the film (1) (see Fig. 4) with a reverse side covered with the metallic reflective layer (2). As it is shown in Fig. 5 a reverse side of only one (lower) film 1 is covered with the metallic reflective layer (2). On a face side of each film (1) there is made an element, changing the optical properties of the substrate, in the form of mask (3) printed with the same metallic material as that of used for the reflective layer. On a face side of the upper film (1) over the mask (3) there is applied the masking layer (4) made of the same metallic material as for the reflective layer and the mask, and additional masking dispersive layer (5) is applied over the layer (4). A heat sealable adhesive layer (not shown on the drawings) may be applied on the metallic reflective layer (2). Such adhesive may be used to stick together the layers of the film (1) shown in Fig. 5.
For instance according to the invention the claimed method of producing a security label can be realized as follows (see Figs. 1 to 4).
An optically anisotropic transparent film (1) is used for printing. The film thickness is of no importance.
One of the following printing methods is used to form a hidden polarized image: offset printing; letterpress printing; flexographic printing; intaglio printing, etc.; the same metallic ink is used during all printing stages.

1. The metallic reflective layer (2) is formed by printing (using any printing method) a reverse side of the transparent film (1) by metallic ink (silver, gold, pearl...) (see Fig. 1). The ink layer thickness in this sample is 0,8 µ. If the thickness of the reflective background layer is increased, the thickness of all other layers should be proportionally increased as well.
2. The mask (3) is formed by printing (using any printing method) a face side of the transparent film (1) by the same metallic ink (silver, gold, pearl...) as for the reflective background layer with a printing plate (not shown on the drawings) having a required image or text (see Fig. 2). The ink thickness in this sample is at least 0,1 µ.
3.The masking reflective layer (4) (see Fig. 3) is made by printing (using any printing method) over the mask (3) formed during Stage 2 by the same metallic ink (silver, gold, pearl...) as for the reflective background layer (2). The density and/or thickness of the masking reflective layer (4) should be enough to hide the mask outline without impeding the light transmission through the masking layer. The thickness of the masking reflective layer (4) in this sample is at least 0,1 µ. Depending on the result the thickness and/or density of the layer (4) may be increased or reduced. If the hidden image is insufficiently masked and remains visible with the naked eye, then the thickness of the masking layer (4) should be increased.
4. The masking dispersive lacquer layer (5) (see Fig. 4) is formed by printing over the masking layer (4) made during Stage 3 with guilloche or other image (using any printing method). Lacquers containing at least 15% of pearl pigments (Iriodin®, Pearlets, Phoenix, etc) should be used. The lacquer thickness in this sample is at least 0,3 µ. Depending on the result the thickness of the masking dispersive lacquer layer (5) may be increased or reduced. If the hidden image is insufficiently masked and remains visible with the naked eye, then the thickness of the masking dispersive lacquer layer (5) should be increased.

As a result, the finished security label is obtained (see Fig. 4) without any outlines or relief visible on the surface.
When a multilayer substrate is required, a relevant number of substrate layers is made during Stage 2 (without any reflective layer). The prepared additional substrates are laid on the substrate implemented during Stages 1 and 2 and sticked together by optically transparent adhesive. Operations specified for Stages 3 and 4 are performed as described above.
So with the mentioned method the security label can be made (printed) directly on the protected product. Alternatively, the metallic reflective layer (2) may be covered by a heat-sealable adhesive (not shown on the drawings) to stick the security label onto a surface of the protected product.
The claimed method of producing a security label is intended to form the hidden images that become visible in polarized light. It can be used with any printing methods: offset printing; letterpress printing; flexographic printing; intaglio printing, etc.
The claimed first version of a security label (Fig. 4) is used for the product authenticity verification as follows (see Figs. 9 to 10).
The security label is placed on the outer surface of the protected product or is embedded into the same. If the claimed security label is illuminated with white light no images or relief are visible on its surface. The light passing through a polarizer (6) to the security label becomes polarized. The polarized light passes through the layers (5) and (4) and the sections of the mask (3) that are not covered with metallic ink, then goes into the transparent substrate (1) and is reflected from the reflective layer (2). The mask (3) acts as an element changing the optical properties of the substrate (1) in the sense that the sections of the mask (3) that reflect the polarized light without altering the same transfer this property to the substrate (1) because under these sections of the mask there appears no light in the substrate. Light beams reflected by the layer (2) re-pass through the substrate (1). Due to the optical anisotropy of the transparent substrate (1), the polarization of light is altered, however, the human eye does not detect any changes in the polarized light reflected by the mask and passed through the optically anisotropic substrate, and therefore the image remains invisible with the naked eye. The hidden polarized image generated by changes in polarization of light becomes visible if observed through a circular polarizer (6), because the polarized light, reflected by the mask without any changes in polarization, is absorbed by a circular polarizer (6), while the light, that has passed through the optically anisotropic substrate and changed its polarization state, passes through a circular polarizer (6) without any changes. A polarizer (6) can be put on the security label or, alternatively, the system comprising of polarized glasses and a source of polarized light can be used. When a circular polarizer (7) is used for observation, the claimed security label enables the visualization of a white image (7) on a dark-blue background that coincides with the image shown in the mask (3).
The same way can be used for authentication with assistance of the second and the third versions of the security label, however, in this case, the hidden image is colored.
Colored image is generated in such security labels due to chromatic polarization (coloration of initially white light) of the light that has passed through linear/circular polarizer (6) and optically anisotropic film (1) with reflective layer (2) (see Figs. 6 and 7), the optical axis of this film is perpendicular to the light traveling direction.
A polarizer lets light pass through if the electric field oscillations E ₀, are directed along the polarizer axis. Passing through the optically anisotropic film a light wave splits into two waves with mutually orthogonal electric field oscillations E _x and E _y. Both waves (ordinary and extraordinary) propagate in the optically anisotropic film with different velocities; therefore the phase difference Δϕ is generated between them. At the polarizer output the fields in both waves are directed along to the polarizer axis, A_lin - for the linear polarizer and A_cir - for the circular polarizer. Due to the phase quarter wave plate used in a circular polarizer, the effective circular polarizer axis A_cir at the polarizer output is orthogonal to II polarizer axis at the input.
As a result, the oscillations are summed up causing two-beam interference.
The transmittance of the considered system for a linear polarizer is: $T_{lin} = E^{2} / E_{0}^{2} = 1 - \sin^{2} 2 α \sin^{2} (Δ ϕ / 2),$
and for a circular polarizer $T_{cir} = \sin^{2} 2 α \sin^{2} (Δ ϕ / 2),$
where

E₀, E - electric field strengths of the light at the input and at the output from the anisotropic substrate
α - angle between the polarizer axis and one of the main directions of the anisotropic substrate,
Δϕ = 4πd(n_e - n_o )/λ - phase difference between oscillations,
d - thickness of transparent optically anisotropic substrate;
n _e refractive index for the extraordinary light beam in the substrate;
n _o - refractive index for the ordinary light beam in the substrate,
λ - light wave length in a vacuum.

At zero anisotropy of the index of refraction n_e - n_o = 0, or zero film thickness d (i.e., no film or no anisotropy) the transmittance factor T = 0 for the system equipped with a circular polarizer (no light transmittance), T = 1 for the system with a linear polarizer (complete light transmittance).
In the visible-light spectrum range the transmittance quickly oscillates at high values L, therefore, when the illumination is made with white light, the light transmitted will remain white for observers (see Fig. 8, graph for L=2.5 micron) ).
However, if L is less than one micron (see Fig. 8, graphs for L -0.5 micron and 0.2 micron), the spectral dependence is rather smooth with one or two maximal values in the visible-light spectrum. In this case the transmitted light is get colored. If the polarizer is rotated, then the color is constantly changing. When the polarizer axis is parallel to one of the main directions of the anisotropic film (α = 0 or 90°), then the light beams of only one type (ordinary or extraordinary) will pass through the film; no interference is observed, while coloration disappears.

Table 1 shows interferential colors for different values of optical thickness of the optically anisotropic film (or total optical thickness of a pack of optically anisotropic films) with the reflective layer.

Table 1.

Interferential colors
Optical thickness, nm	Interferential colors for the circular polarizer	Interferential colors for the linear polarizer
0	Black	White
158	Grayish blue	Yellowish-white
259	White	Light -red (bright)
306	Light yellow	Indian blue
505	Reddish orange	Bluish-green
536	Flame red	Light-green
551	Dark red	Yellowish-green
565	Purple	Lighter green
575	Violet	Greenish-yellow
589	Indian blue	Golden-yellow
664	Skyblue	Orange
728	Greenish blue	Brownish-orange
747	Green	Crimson
866	Greenish yellow	Violet
948	Orange	Dark-blue
1101	Dark violet-red	Green
1128	Light-greenish-violet	Yellowish--green
1151	Indian blue	Dirty-yellow
1258	Light-blue (greenish)	Flesh-coloured
1334	Aquamarine	Brownish-red
1376	Bright green	Violet

Therefore, the claimed versions of the security labels with hidden polarized images, including color images, as well as the methods of their production and verification, may be realized by proper selection of the optical thickness of the substrate or substrates (for multicolored hidden images), by forming a reflective layer, a mask or masks (for multicolored hidden images) with the same metallic material as for the reflective layer, and a masking dispersing layer used to hide the outlines of the polarized image on the label. Verification of the security label with hidden polarized images is made using a polarizer.

Claims

A security label with the hidden polarized multicolor image comprising: a sequentially disposed metallized reflective layer and at least two sequentially placed transparent optically anisotropic substrates with the masks, applied on a face side of each substrate as a solid fill printing with blank spaces laying in a contour of the hidden image; characterized in that each mask is made by the same material that is used for the reflective layer on a reverse side of the, substrate, and each section of optically anisotropic transparent substrates in the non-printed areas of their masks has the total optical thickness selected to ensure that the transmittance T of the correspondent section for the light going through a corresponding polarizing filter and the security label has a maximum on the preset wave length, corresponding to the definite color of the hidden image.
The security label of claim 1, characterized in that each mask is with blank spaces corresponding to the graphics of the hidden image and these blank spaces are made in such way that they may coincide or not coincide for adjacent transparent optically anisotropic substrates.
The security label of claim 1, characterized in that at least one masking layer is placed on a face side of the substrate over the mask and the density / thickness of this layer is chosen such that allows to hide contours of the mask but does not block light passing through the masking layer.
A security label with hidden polarized one color image comprising: a sequentially disposed metallized reflective layer and at least one optically anisotropic transparent substrate with a mask, applied on a face side of the substrate as a solid fill printing with blank spaces laying in a contour of the hidden image; characterized in that the mask is made by the same material that is used for the reflective layer on a reverse side of the substrate, and each section of optimally anisotropic transparent substrate in the non-printed areas of its mask has the optical thickness selected to ensure that the transmittance T of the correspondent section for the light going through a corresponding polarizing filter and the security label has a maximum on the preset wave length, corresponding to the definite color of the hidden image.
The security label of claim 4, characterized in that each mask is printed as a solid fill color with blank spaces corresponding to the graphics of the hidden image and these blank spaces are made in such way that they may coincide or not coincide for adjacent transparent optically anisotropic substrates.
The security label of claim 4, characterized in that at least one masking layer is placed on a face side of the substrate over the mask and the density / thickness of this layer is chosen such that allows to hide contours of the mask but does not block light passing through the masking layer.
A method of forming a security label with the hidden polarized color image wherein a sequentially disposed metallized reflective layer and at least one transparent optically anisotropic substrate are formed; and there is made a mask, applies on a face side of the substrate as a solid fill printing with blank spaces laying in a contour of the hidden image; characterized in that the mask is made by the same material that is used for the reflective layer on a reverse side of the substrate, and each section of optically anisotropic transparent substrate in the non-printed areas of its mask has the optical thickness selected to ensure that the transmittance T of the correspondent section for the light going through a corresponding polarizing filter and the security label has a Maximum on the preset wave length, corresponding to the definite color of the hidden image.
The method of forming a security label of claim 7, characterized in that there is formed at least one sequentially disposed additional transparent optically anisotropic substrate with the mask, applied on a face side of this substrate as a solid fill printing with blank spaces laying in a contour of the hidden..
The method of forming a security label of claim 7, characterized in that at least one masking layer is placed on a face side of the substrate over the mask and the density / thickness of this layer is chosen such that allows to hide contours of the mask but does not block light passing through the masking layer.
The method of forming a security label of claim 7, characterized in that the security label is formed directly during a printing process on protected products such as packaging, label, scotch tape, sealing tape based on the anisotropic transparent film.
A method of product authentication by forming a security label, placed on an outside surface of the protected product, and visualization of the hidden polarized image by observing it through a circular polarizer, characterized in that the security label of any of claims 1 to 6 is used.
A method of product authentication by forming a security label, placed on an outside surface of the protected product, and visualization of the hidden polarized image by observing it through a circular polarizer, characterized in that the security label is formed in accordance with the method of any of claims 7 to 11.