WO2017001424A1

WO2017001424A1 - Multilayer light-diffracting structure

Info

Publication number: WO2017001424A1
Application number: PCT/EP2016/065049
Authority: WO
Inventors: Valentin Konstantinov MONOVSKI; Georgi Ivanov STOILOV; Mario Rumenov DOBREV; Andrii Ivanovskyi; Ihnat POHAN; Angel Georgiev BALDZHIEV; Evgeniy Anastasov LASKOV; Dimitar Antonov TONCHEV; Ivo Rumenov DELCHEV
Original assignee: Demax - Holograms Ad
Priority date: 2015-06-29
Filing date: 2016-06-28
Publication date: 2017-01-05
Also published as: WO2017000048A1

Abstract

The present invention provides a multilayer light-diffracting structure (10) comprising a transparent diffraction layer (5) and a light transforming layer (6) in contact with the diffraction layer (5). The diffraction layer (5) comprises superposed first and second diffraction gratings oriented at an angle (α) to each other. The first and second diffraction gratings each comprise elements (9) having a triangular cross-section in a depth-wise direction of the diffraction layer (5). A period (X) of the first diffraction grating and a period (Y) of the second diffraction grating are each in a range of from 0.2 to 0.8 micrometres. The diffraction layer (5) has a first refractive index n₁(λ) as a function of wavelength (λ) and the light transforming layer (6) has a second refractive index n₂(λ) as a function of wavelength (λ). The absolute value of the difference between the first refractive index n₁(λ) and the second refractive index n₂(λ) is greater than 0.5 for the same wavelength (λ). The multilayer light-diffracting structure (10) can appear black if viewed normal to its surface, can be manufactured by a process using stamping or pressing, and can be used as a security feature of an item of value, such as a bank card, banknote, passport or identity card, branded goods, ticket or legal document, for example.

Description

MULTILAYER LIGHT-DIFFRACTING STRUCTURE

The present invention concerns a multilayer light-diffracting structure, an optical security device comprising such a multilayer light-diffracting structure, an item of value, such as a bank card or banknote, comprising such an optical security device, and a method of manufacturing such a multilayer light-diffracting structure.

US4062628 describes a black-and-white light subtractive light filter, wherein neutral black is achieved when two superposed, crossed, sine-wave diffraction gratings embossed in plastic have different depths which are selected to provide a zero-order diffraction pattern which appears black to a viewer from a direction normal to the filter. By "black" is meant a transmittance of incident light by the filter of less than about 1 % and a chromaticity on a Commission Internationale de I'Eclairage (CIE) colour chart within 5% of that of white light. However, the filter described therein cannot be manufactured by a process using stamping or pressing.

US2012/0229368, US8147945 and US2008/0272883 all describe multilayer light-diffracting structures comprising diffraction layers with elements having a triangular cross-section in a depth-wise direction of the diffraction layer.

An object of the present invention is to provide a multilayer light-diffracting structure, which can appear black if viewed from a direction normal to its surface, which can be manufactured by a process using stamping or pressing, and which can be used as a security feature of an item of value, such as a bank card, banknote, passport or identity card, branded goods, ticket or legal document, for example.

Accordingly, in a first aspect, the present invention provides a multilayer light-diffracting structure comprising a transparent diffraction layer and a light transforming layer in contact with the diffraction layer. The diffraction layer comprises superposed first and second diffraction gratings oriented at an angle, a, to each other. The first and second diffraction gratings each comprise elements having a triangular cross-section in a depth-wise direction of the diffraction layer. A period, X, of the first diffraction grating and a period, Y, of the second diffraction grating are each in a range of from 0.2 to 0.8 micrometres. The diffraction layer has a first refractive index η-ι(λ) as a function of wavelength λ and the light transforming layer has a second refractive index η2(λ) as a function of wavelength λ. The absolute value of the difference between the first refractive index η-ι(λ) and the second refractive index η2(λ) is greater than 0.5 for the same wavelength λ. Since the elements of the first and second diffraction gratings have a triangular cross-section in a depth-wise direction of the diffraction layer, they can be manufactured by a process using stamping or pressing. Since the periods of both the first and second diffraction gratings are in a range of from 0.2 to 0.8 micrometres, they are suitable for diffracting incident light in the visible spectrum. The relationship between the refractive indices of the diffraction layer and the light transforming layer ensures Fresnel reflection from the interface between the diffraction layer and the light transforming layer.

The light transforming layer may reflect light or it may transmit light. Thus the multilayer light- diffracting structure can respectively operate in a light-transmitting mode or in a light-reflecting mode according to whether the light transforming layer reflects light or transmits light.

Preferably, the angle (a) is substantially equal to 90 degrees. If so, the multilayer light- diffracting structure would appear most, black, if viewed from a direction normal to its surface. For illustration see Figure 9.

For the diffraction gratings the above equation can be relied upon to define relation between wavelength (λ), order of diffraction (m), period (d), angle of illumination (Θ1 ) and angle of diffraction (Θ2). Zero diffraction order corresponds to simple reflection.

For the most grating, that are used for holography the period d lies in range from 2μηι to 0.8μηι. And the angle of diffraction of the first order is in range from 20 to 60 degree for common condition of illumination. For the scattered illumination zero-order and the first orders can be viewed for some range of illuminating and observing angles, but not only one value. For illustration see Figure 10.

For the gratings with period less than Ο.δμηι the light diffracts to very oblique angles and special conditions should be set up to see the first order of diffraction. The hologram should be tilted and the source of light should be preferably from the back side of the observer. In this case only zero order of the grating is easily observed. For illustration see Figure 1 1 . Thus, for common conditions of illumination and observation gratings with period above Ο.δμηη period produces rainbow coloured image in first order. For gratings with period in range from Ο.δμηη to 0.2μηι only zero-order of diffraction is visible.

The intensity of the zero order depends on relief height of the grating. The minimal value of reflection is 7% for optimal grating depth. We choose the threshold level of 10% for "black" colour to maintain possible production errors. For illustration see Figure 12.

In the context of the present invention, the term "black" is to be understood to mean a transmittance of incident light by the filter of less than about 10% and a chromaticity on a Commission Internationale de I'Eclairage (CIE) colour chart within 5% of that of white light.

However, the angle a may be less than 90 degrees, in which case, the multilayer light- diffracting structure will appear more grey as the angle a decreases. Preferably, the period, X, of the first diffraction grating is equal to the period, Y, of the second diffraction grating.

Preferably, at least one of the first and second diffraction gratings is quasiperiodic.

Preferably, the diffraction layer comprises a plurality of first areas, each of which comprises superposed first and second diffraction gratings oriented at a respective angle, a, to each other. If so, at least two of the first areas may be obliquely oriented with respect to each other.

Preferably, the diffraction layer further comprises at least one second area comprising chaotically scattered elements having a triangular cross-section in a depth-wise direction of the diffraction layer.

The light transforming layer may be reflective. If so, a depth, d_g, of each of the first and second diffraction gratings should preferably be in a range of from 0.1 to 0.5 micrometres. Alternatively, the light transforming layer may be transparent. If so, a depth, d_g, of each of the first and second diffraction gratings should preferably be in a range of from 0.2 to 1 .0 micrometres.

If the light transforming layer is reflective, a depth, dt, of the light transforming layer should preferably be within 15% of λ 2η2, more preferably within 10%, still more preferably within 5%, and most preferably, substantially equal to hi

If however, the light transforming layer is transparent, a depth, dt, of the light transforming layer should preferably be within 15% of λ / I ni - Π21 , more preferably within 10%, still more preferably within 5%, and most preferably, substantially equal to λ / | ni - n21 .

In one possible embodiment, the light transforming layer may comprise an aperture exposing at least a part of the diffraction layer to the environment, which may be air or a vacuum.

In a second aspect, the present invention also provides an optical security device comprising a multilayer light-diffracting structure according to the first aspect of the invention. The optical security device may be a security label, for example.

In a third aspect, the present invention also provides an item of value comprising an optical security device according to the second aspect of the invention.

In a fourth aspect, the present invention further provides a method of manufacturing a multilayer light-diffracting structure according to the first aspect of the invention, wherein the method comprises the following steps. Firstly, generating a mathematical description of the diffraction layer. Using the mathematical description to create a file for an electronic beam lithography (EBL) process. Exposing a plate coated with resist to an electronic beam directed by the file to create a pattern on the plate defined by the file. Developing the resist-coated plate. Transferring the pattern electrochemically from the plate to a metal matrix. Stamping the pattern from the metal matrix onto a transparent blank to create the diffraction layer, and coating the diffraction layer with the light transforming layer.

Further features and advantages of the present invention will become apparent from the following detailed description, which is given by way of example and in association with the accompanying drawings, in which: Figures

Fig. 1 is a schematic cross-sectional view of an embodiment of a multilayer light-diffracting structure;

Fig. 2 is a schematic plan view of a representative part of a first embodiment of a diffraction layer of a multilayer light-diffracting structure;

Fig. 3 is a schematic plan view of a representative part of a second embodiment of a diffraction layer of a multilayer light-diffracting structure;

Fig. 4 is a schematic cross-sectional view through the diffraction layer of Fig. 2;

Fig 5 is a schematic cross-sectional view through the diffraction layer of Fig. 3;

Fig. 6 is a schematic perspective view of a representative part of the diffraction layer of Fig. 2;

Fig. 7 is a schematic perspective view of a representative part of a third embodiment of a diffraction layer of a multilayer light-diffracting structure;

Fig. 8A is a plan view of an optical security device comprising a multilayer light-diffracting structure;

Fig. 8B is an oblique view from a first direction of the optical security device of Fig. 8A; and

Fig. 8C is an oblique view from a second direction of the optical security device of Fig. 8A.

Fig. 9 the multilayer light-diffracting structure appears most black, if viewed from a direction normal to its surface.

Fig. 10 the scattered illumination zero-order and the first orders can be viewed for some range of illuminating and observing angles, but not only one value.

Fig. 1 1 illustrates tilting of the hologram and the source of light should be preferably from the back side of the observer where only zero order of the grating is easily observed. Fig. 12 illustrates that the minimal value of reflection being 7% for optimal grating depth where the threshold level of 10% for "black" colour is selected to maintain possible production errors.

Detailed description of the invention

Referring firstly to Fig. 1 , there is shown an embodiment of a multilayer light-diffracting structure 10, comprising a support layer 1 , a separation layer 2, one or more transparent protective layers 3, an anti-reflection coating 4, a diffraction layer 5, a light transforming layer 6, a layer 7 of adhesive, and a removable protective sheet 8. The support layer 1 may, for example, be formed of polyethylene terephthalate (PET). The separation layer 2 may, for example, be formed of wax. The diffraction layer 5 is transparent, as is the layer 7 of adhesive. The protective sheet 8 is removable so that the multilayer light-diffracting structure 10 may be affixed via the layer 7 of adhesive to another object, such as an item of value, like a credit card or banknote.

The transparent diffraction layer 5 will be described in greater detail below in relation to Figs. 2 to 7.

The light transforming layer 6 is in contact with the diffraction layer 5 and may reflect light or it may transmit light. Thus the multilayer light-diffracting structure 10 can respectively operate in a light-transmitting mode or in a light-reflecting mode according to whether the light transforming layer 6 reflects or transmits light. For example, the light transforming layer 6 may be a reflective aluminium coating of the diffraction layer 5, in which case, the multilayer light- diffracting structure 10 will operate in a light-reflecting mode. In such a case, light illuminating the multilayer light-diffracting structure 10 from the direction A shown in Fig. 1 will be reflected by the light transforming layer 6 and the reflected light will be viewable from the same side A of the multilayer light-diffracting structure 10 as the illuminating light. Alternatively, the light transforming layer 6 may, for example, be a transparent polymer coating of the diffraction layer 5, in which case, the multilayer light-diffracting structure 10 will operate in a light-transmitting mode. In such a case, light illuminating the multilayer light-diffracting structure 10 from the direction B shown in Fig. 1 will be transmitted by the light transforming layer 6 and the transmitted light will be viewable from the opposite side A of the multilayer light-diffracting structure 10 to the illuminating light.

Thus when the protective sheet 8 is removed, the multilayer light-diffracting structure 10 may be affixed either to a transparent object or to an opaque object. The light transforming layer 6, as well as optionally the layer 7 of adhesive, may further comprise an aperture or window, so that when the protective sheet 8 is removed, at least a part of the diffraction layer 5 in the region of the aperture or window is exposed to the environment, such as air or a vacuum, in which case the environment can take the place of the light transforming layer 6 in that region.

The purpose of the light transforming layer 6 is to transform light passing through the diffraction layer 5 by Fresnel reflection from the interface between the diffraction layer 5 and the light transforming layer 6. Thus, if the diffraction layer 5 has a first refractive index η-ι(λ) as a function of wavelength λ and the light transforming layer 6 has a second refractive index η2(λ) as a function of wavelength λ, then the absolute value of the difference between the first refractive index η-ι(λ) and the second refractive index η2(λ) should be greater than 0.5 for the same wavelength λ.

When the light transforming layer 6 is reflective, so that the multilayer light-diffracting structure 10 operates in a light-reflecting mode, a depth dt of the light transforming layer 6 is preferably within 15% of hi 2n2, more preferably within 10%, still more preferably within 5%, and most preferably, substantially equal to λ/ 2n2.

When the light transforming layer 6 is transparent, so that the multilayer light-diffracting structure 10 operates in a light-transmitting mode, the depth dt of the light transforming layer 6 is preferably within 15% of λ / | ni - ri21 , more preferably within 10%, still more preferably within 5%, and most preferably, substantially equal to h i | ni - ri21 .

The diffraction layer 5 comprises superposed first and second diffraction gratings oriented at an angle, a, to each other. Preferably, the angle a is substantially equal to 90 degrees. Figs. 2 and 3 respectively show first and second embodiments of the diffraction layer 5 in which the angle a is substantially equal to 90 degrees, so that the first and second diffraction gratings are oriented at right angles to each other. However, in other possible embodiments, the angle a may be anything within a range from zero to 90 degrees.

In the first embodiment of Fig. 2, both the first and second diffraction gratings are periodic. The first grating has a period X and the second grating has a period Y. In this example, the period X of the first grating is substantially equal to the period Y of the second grating. However, in alternative embodiments, the respective periods X and Y of the first and second diffraction gratings may instead be different from each other. In the second embodiment of Fig. 3, both the first and second diffraction gratings are quasiperiodic. This means that the first grating has a sequence of periods Xi , X2, X3, X₄ up to X_n, which subsequently repeat the same sequence, and that the second grating also has a sequence of periods Yi , Y2, Y3, Y₄ up to Y_n, which also subsequently repeat the same sequence. In this example, the quasiperiodic sequence of the first grating is substantially equal to the quasiperiodic sequence of the second grating. However, in alternative embodiments, the respective quasiperiodic sequences of the first and second diffraction gratings may instead be different from each other.

In alternative embodiments not represented in Figs. 2 and 3, one of the first and second diffraction gratings may be periodic and the other of the first and second diffraction gratings may be quasiperiodic, instead. However, regardless of whether the first and second diffraction gratings are periodic or quasiperiodic and regardless of whether the respective periods or respective quasiperiodic sequences of the first and second diffraction gratings are the same as or different from each other, the values of X and Y should in all cases both lie within a range of from 0.2 to 0.8 micrometres.

Figs. 4 and 5 respectively show cross-sectional views through the diffraction layers of Figs. 2 and 3 taken along the lines C-C and D-D' respectively marked in Figs. 2 and 3. As may be seen in Figs. 4 and 5, the diffraction gratings each comprise elements 9 having a triangular cross-section in a depth-wise direction of the diffraction layer 5. In both Figs. 4 and 5, cross- sectional views of the first diffraction gratings can be seen. However, cross-sectional views taken at 90 degrees to each of the lines C-C and D-D' would also show the second diffraction gratings in each case to comprise elements 9 having a triangular cross-section as well. Thus both the first and second diffraction gratings each comprise elements 9 having a triangular cross-section in a depth-wise direction of the diffraction layer 5.

Since the first and second diffraction gratings are superposed on each other, in the embodiments illustrated in Figs. 2 and 3, the diffraction layer 5 in both cases comprises a plurality of elements 9 which are square-based (or tetragonal) pyramids arranged in two- dimensional arrays. In the embodiment illustrated in Figs. 2, the elements 9 form a regular two-dimensional array of tetragonal pyramids, as may be seen in the perspective view of Fig. 6.

However, in other possible embodiments, the diffraction layer 5 may instead comprise a plurality of elements 9 which are pyramids having bases with any other numbers of sides, such as triangular-based pyramids (also called tetrahedra), pentagonal-based pyramids and pyramids having bases with six or more sides. The diffraction layer 5 may also comprise a plurality of elements 9 which are instead cones, and therefore have bases which are either circles or ellipses. In all cases, however, the elements 9 have a triangular cross-section in a depth-wise direction of the diffraction layer 5. This allows the elements 9 to be formed by a stamping or pressing process. The elements 9 may be upright pyramids or upright cones which stand perpendicular to their bases, or they may be inclined to their bases at an oblique angle instead. They may be rotationally symmetric or rotationally asymmetric.

Fig. 7 shows a perspective view of a third possible embodiment, in which the diffraction layer 5 comprises a plurality of first areas 1 1 a, 1 1 b, 1 1 c and a plurality of second areas 12a, 12b, 12c, 12d. The first areas each comprise superposed first and second diffraction gratings oriented at a respective angle a to each other to constitute regular arrays of elements 9 in the manner of the diffraction layer 5 illustrated in Fig. 6. The first areas 1 1 a and 1 1 b are aligned with each other, whereas the first area 1 1 c is obliquely oriented with respect to the first areas 1 1 a and 1 1 b. The second areas 12a, 12b, 12c, 12d each comprise chaotically scattered elements 9 having a triangular cross-section in a depth-wise direction of the diffraction layer 5. As may be seen in Fig. 7, the second areas 12a, 12b, 12c, 12d can comprise a mixture of different elements 9, such as cones, tetrahedra, tetragonal pyramids and pentagonal pyramids.

If the light transforming layer 6 is reflective, so that the multilayer light-diffracting structure 10 can operate in a light-reflecting mode, the height of the elements 9, which defines the depth d_g of each of the first and second diffraction gratings, is preferably within a range of from 0.1 to 0.5 micrometres. If, on the other hand, the light transforming layer 6 is transparent, so that the multilayer light-diffracting structure 10 can operate in a light-transmitting mode, the height of the elements 9, which again defines the depth d_g of each of the first and second diffraction gratings, should preferably be twice as great, so that it is preferably within a range of from 0.2 to 1 .0 micrometres.

Fig. 8A is a plan view of an optical security device 13 comprising a multilayer light-diffracting structure such as described above. The optical security device 13 comprises two different areas which respectively define an object 14 and a background 15. For the sake of example only, in Fig. 8A, the object 14 represents a pair of musical notes. Different optical effects may be achieved by filling the object 14 and the background 15 in various different ways with different multilayer light-diffracting structures, such as those described above. Some representative examples of these different cases are listed in Table 1 , below. For example, either one or both of the object 14 and the background 15 may be filled with different multilayer light-diffracting structures. In Table 1 , for the sake of example only, the object 14 is supposed always to be filled with a first multilayer light-diffracting structure and the background 15 in different cases either to be filled or not to be filled by a second multilayer light-diffracting structure, which is different from the first multilayer light-diffracting structure of the object 14. Again for the sake of example only, the second multilayer light-diffracting structure of the background 15 is supposed to be aligned with the first multilayer light-diffracting structure of the object 14. Table 1

In all cases, when the optical security device 13 is viewed from a direction normal to its surface, the object 14 can appear black, whereas if the background 15 is not filled with a multilayer light-diffracting structure, the background 15 can appear neutral. However, if the background 15 is filled with a multilayer light-diffracting structure different from that of the object 14, the background 15 can appear a different shade, such as dark grey, for example.

Figs. 8B and 8C respectively show the appearance of the optical security device 13 if it is viewed firstly at an oblique angle from a direction perpendicular to the first diffraction grating (Fig. 8B), and secondly at an oblique angle from a direction perpendicular to the second diffraction grating (Fig. 8C). In other words, Figs. 8B and 8C represent the different appearances of the optical security device 13 if rotated by the angle a between the first and second diffraction gratings. In case no. 1 , where the first and second diffraction gratings are both periodic and have the same period as each other, the object 14 will appear the same colour from both different directions. The period X = Y of the two superposed diffraction gratings can be selected to define the colour. The background 15 will appear unchanged. In case no. 2, where the first and second diffraction gratings are both periodic but have different periods from each other, the object 14 will appear different respective colours from the two different directions.

Once again, the periods X≠ Y of the two superposed diffraction gratings can be selected to define the colours. The background 15 will again appear unchanged. In case no. 3, the object

14 will appear the same as in case no. 2 from the two different directions, but the background

15 will not remain unchanged. Instead, the background 15 will also appear different respective colours from the two different directions. Once again, the periods X≠ Y of the two superposed diffraction gratings can be selected to define the colours. For example, if the values of X and Y in the background 15 are chosen to be the opposite of the values of X and Y in the object 14, then the colours of the background 15 observed from the two different directions will also be the opposite of the colours of the object 14 observed from the two different directions.

In case no. 4, where the first and second diffraction gratings both have quasiperiodic sequences which are different from each other, the object 14 will appear grey from the two different directions. The background 15 will appear unchanged. In case no. 5, where the first diffraction grating has a quasiperiodic sequence, but the second diffraction grating is periodic, the object 14 will appear grey from one direction (the direction of the quasiperiodic grating), but coloured from the other direction (the direction of the periodic grating). The period of the periodic grating can again be chosen to define the colour observed. The background 15 will again appear unchanged. In case no. 6, the object 14 will appear the same as in case no. 5 from the two different directions, but the background 15 will not remain unchanged. Instead, the background 15 will appear grey from one direction (the direction of the quasiperiodic grating), but coloured from the other direction (the direction of the periodic grating). The period of the periodic grating can again be chosen to define the colour observed.

Thus if the direction of the quasiperiodic grating in the background 15 is aligned with the direction of the periodic grating in the object 14 and the direction of the periodic grating in the background 15 is aligned with the direction of the quasiperiodic grating in the object 14, the appearance of the background 15 from the two different directions will be the opposite of the appearance of the object 14 from the two different directions. In all cases, if a colour is observed at an oblique viewing angle, the observed colour changes through the colours of the rainbow as the viewing angle is increased or decreased. In all cases, if grey is observed at an oblique viewing angle, the observed shade of grey varies from black through grey to white as the viewing angle is increased or decreased. Thus various different combinations of greyscale and rainbow colours can be achieved by an appropriate choice of the variable parameters of the multilayer light-diffracting structure.

Such an optical security device as shown in Figs. 8A to 8C may be applied to an item of value, such as a bank card or banknote, passport or identity card, branded goods, ticket or legal document, for example, as a security and/or certification and/or authentication and/or identity feature.

A multilayer light-diffracting structure 10 may be manufactured according to the following process.

Firstly, a mathematical description of the diffraction layer 5 is generated. This mathematical description is used to create a file for an electronic beam lithography (EBL) process. A plate coated with resist is then exposed to an electronic beam directed by the file to create a pattern on the plate defined by the file. The plate may, for example, be formed of silicon or glass. The resist-coated plate is then developed according to whatever technique is recommended for it by a manufacturer of the resist-coated plate. The pattern is then transferred electrochemically from the plate to a metal matrix. The metal matrix may, for example, be formed of nickel. A blank transparent diffraction layer 5, already provided with an anti- reflection-coating 4, one or more transparent protective layers 3, a wax separation layer 2 and a support layer 1 , is then stamped with the pattern from the metal matrix, and the pattern now on the diffraction layer 5 is coated with a light transforming layer 6.

Finally, a layer 7 of adhesive is applied to the light transforming layer 6 and covered with a removable protective sheet 8. The multilayer light-diffracting structure 10 is then ready for use in an optical security device, such as a security label, for example.

Claims

A multilayer light-diffracting structure (10), comprising: a transparent diffraction layer (5) comprising superposed first and second diffraction gratings oriented at an angle (a) to each other; and a light transforming layer (6) in contact with the diffraction layer (5); wherein the first and second diffraction gratings each comprise elements (9) having a triangular cross-section in a depth-wise direction of the diffraction layer (5); a period (X) of the first diffraction grating and a period (Y) of the second diffraction grating each being in a range of from 0.2 to 0.8 micrometres; wherein the diffraction layer (5) has a first refractive index η-ι(λ) as a function of wavelength λ and the light transforming layer (6) has a second refractive index η2(λ) as a function of wavelength λ; and wherein the absolute value of the difference between the first refractive index η-ι(λ) and the second refractive index η2(λ) is greater than 0.5 for the same wavelength λ.

A multilayer light-diffracting structure according to claim 1 , wherein the angle (a) is substantially equal to 90 degrees.

A multilayer light-diffracting structure according to claim 1 or claim 2, wherein the period (X) of the first diffraction grating is equal to the period (Y) of the second diffraction grating.

A multilayer light-diffracting structure according to any one of the preceding claims, wherein at least one of the first and second diffraction gratings is quasiperiodic.

A multilayer light-diffracting structure according to any one of the preceding claims, wherein the diffraction layer (5) comprises a plurality of first areas (1 1 a, 1 1 b, 1 1 c), each of which comprises superposed first and second diffraction gratings oriented at a respective angle (a) to each other.

A multilayer light-diffracting structure according to claim 5, wherein at least two of the first areas (1 1 a, 1 1 b, 1 1 c) are obliquely oriented with respect to each other.

7. A multilayer light-diffracting structure according to any one of the preceding claims, wherein the diffraction layer (5) further comprises at least one second area (12a, 12b, 12c, 12d) comprising chaotically scattered elements (9) having a triangular cross-section in a depth-wise direction of the diffraction layer (5).

8. A multilayer light-diffracting structure according to any one of the preceding claims, wherein the light transforming layer (6) is reflective and a depth (d_g) of each of the first and second diffraction gratings is from 0.1 to 0.5 micrometres. 9. A multilayer light-diffracting structure according to any one of claims 1 to 7, wherein the light transforming layer (6) is transparent and a depth (d_g) of each of the first and second diffraction gratings is from 0.2 to 1 .0 micrometres.

10. A multilayer light-diffracting structure according to any one of claims 1 to 8, wherein the light transforming layer (6) is reflective and a depth (dt) of the light transforming layer (6) is within 15% of λ/ 2n2.

1 1 . A multilayer light-diffracting structure according to any one of claims 1 to 7 and 9, wherein the light transforming layer (6) is transparent and a depth (dt) of the light transforming layer (6) is within 15% of λ / | ni - ri21 .

12. A multilayer light-diffracting structure according to any one of the preceding claims, wherein the light transforming layer (6) comprises an aperture exposing at least a part of the diffraction layer (5) to the environment.

13. An optical security device (13) comprising a multilayer light-diffracting structure according to any one of the preceding claims.

14. An item of value comprising an optical security device (13) according to claim 13.

15. A method of manufacturing a multilayer light-diffracting structure (10) according to any one of claims 1 to 12, the method comprising: generating a mathematical description of the diffraction layer (5); using the mathematical description to create a file for an electronic beam lithography (EBL) process; exposing a plate coated with resist to an electronic beam directed by the file to create a pattern on the plate defined by the file; developing the resist-coated plate; transferring the pattern electrochemically from the plate to a metal matrix; stamping the pattern from the metal matrix onto a transparent blank to create the diffraction layer (5); and coating the diffraction layer (5) with the light transforming layer (6).