DIFFRACTIVE DEVICE HAVING EMBEDDED LIGHT SOURCE MECHANISM

Abstract

An optical device comprising an at least substantially transparent substrate having a first side comprising a source layer having an arrangement of source elements and a second side comprising an Optically Variable Device (OVD) layer having a corresponding arrangement of diffractive elements, wherein each source element is configured to, upon illumination of the first side, provide an embedded light source for an associated diffraction element, and wherein the diffractive elements are configured for producing an optical effect observable when the diffractive elements are viewed by a viewer, such as the naked eye, upon illumination by the source elements.

Claims

1. An optical device comprising an at least substantially transparent substrate having a first side comprising a source layer having an arrangement of source elements and a second side comprising an Optically Variable Device (OVD) layer having a corresponding arrangement of diffractive elements, wherein each source element is configured to, upon illumination of the first side by an external light source, provide an embedded light source being a light source which provides light for an associated diffraction element substantially independent of the external light source, and wherein the diffractive elements are configured for producing an optical effect observable when the diffractive elements are viewed by a viewer, such as the naked eye, upon illumination by the source elements, wherein each diffraction element is configured in accordance with the shape of its associated source element.

2. An optical device as claimed in claim 1, wherein either or both of: a) the source elements define images which are varied between the source elements; and b) the surface relief of the diffractive elements is varied between the diffractive elements, such that the observed image appears to change in magnification and/or move and/or change form as the angle of view is changed.

3. An optical device as claimed in claim 2, wherein only the diffractive elements are varied.

4. An optical device as claimed in claim 2, wherein only the source elements are varied.

5. An optical device as claimed in claim 1, wherein each source element defines a source image and wherein each diffractive element defines a diffractive focussing element, preferably a circular or cylindrical zone plate type diffractive element, configured such as to provide a magnified and/or displaced projection of the source image of the associated source element.

6. An optical device as claimed in claim 1, wherein the substrate comprises a characteristic thickness, and wherein the surface relief of each diffractive element is determined in part by the characteristic thickness.

7. An optical device as claimed in claim 1, wherein each diffractive element is uniquely associated with one source element.

8. An optical device as claimed in claim 1, wherein each source element has at least one linear dimension less than a spacing between the source element and its associated diffractive element, preferably approximately half the spacing.

9. A document, preferably a security document, comprising the optical device of claim 1.

10. A document as claimed in claim 9, wherein the document comprises a transparent document substrate, a region of which corresponding to the same substrate as the optical device, preferably wherein the document also comprises opacifying layers on each side of the document substrate, each absent in overlapping regions thereby defining a window in which the optical device is located.

11. A document as claimed in claim 9, wherein the optical device is formed separately to the document and affixed to the document in a window region, wherein the window is either a transparent portion of the document or corresponds to a removed portion of the document.

12. A method of manufacturing the optical device of claim 1, including the steps of: preparing a shim having an inverse profile to a required OVD layer profile; determining a printing pattern corresponding to a required source layer; applying to a surface of a transparent substrate a radiation curable ink; embossing the radiation curable ink with the shim, and curing the radiation curable ink, thereby forming the diffraction layer; and printing onto an opposing surface of the substrate the printing pattern, preferably in register with the surface profile of the diffraction layer.

13. A method as claimed in claim 12, wherein the step of embossing and the step of printing are performed substantially simultaneously.

14. A method as claimed in claim 12, wherein the transparent substrate includes opacifying layers located on each surface, the opacifying layers absent in the region of the radiation curable ink thereby defining a window comprising the optical device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given by way of illustration only and the invention is not limited by this illustration. In the drawings:

[0046] FIGS. 1a and 1b show documents having optical devices according to different embodiments of the invention;

[0047] FIG. 2 shows a simplified representation of an optical device according to the present invention;

[0048] FIG. 3 shows a source layer and a OVD layer according to an embodiment;

[0049] FIG. 4 shows the interaction between a light source, a grating structure, and an eye;

[0050] FIG. 5 shows a source element configured as a slit and a corresponding grating of a diffraction element;

[0051] FIG. 6 shows an external light source illuminating a source element, and a diffraction element configured to provide a magnified equivalent to the image of the source element; and

[0052] FIG. 7 shows a method for manufacturing an optical device.

DESCRIPTION OF PREFERRED EMBODIMENT

[0053] FIGS. 1a and 1b each show a document 2 having an optical device 4 according to embodiments of the invention. The optical device 4 comprises a transparent (or at least substantially transparent) substrate 8. The document 2 also comprises a substrate (herein, document substrate 9). In the embodiment of FIG. 1a, the two substrates 8, 9 are the same, that is, the optical device 4 and the document 2 share the same substrate 8, 9. In the embodiment of FIG. 1b, the document substrate 9 is different to the substrate 8 of the optical device 4.

[0054] In each case, the document 2 includes first and second opacifying layers 7a, 7b. The opacifying layers 7a, 7b act to reduce or eliminate the transparency of the document 2 in the regions in which the layers 7a, 7b are present. In the embodiments shown, both opacifying layers 7a, 7b are not present in the area of the optical device 4, thereby causing the optical device 4 to be located within a window region of the document 2.

[0055] It is also possible for the document 2 to be inherently opaque (or substantially opaque), for example where the document substrate 9 is paper or a paper composite material. In this case, the opacifying layers 7a, 7b are not necessarily required. The optical device 4 in this case is still located in a window region of the document 2, which can be achieved using knowing methods such as forming the optical device 4 as a foil, and applying the foil to a cut-out area of the opaque document substrate 9.

[0056] The optical device 4 typically provides a security function, that is, the optical device 4 acts to decrease the susceptibility of the document 2 to counterfeiting. The optical device 4 can be referred to as a security device or security token when used for this purpose. A document 2 requiring protection to counterfeiting is often referred to as a security document.

[0057] FIGS. 1a and 1b also show further security features 6 (6a in FIG. 1a, 6b in FIG. 1b), which can assist in reducing the susceptibility of the document 2 to counterfeiting in combination with the optical device 4. In FIG. 1a, the further security feature 6a is implemented in a window region of the document 2, whereas in FIG. 1b the further security feature 6b is implemented in an opaque (i.e. non-window) region of the document 2. The illustrated arrangements are simply examples, and generally the document 2 can include one or more security features 6, each implemented in a window, half-window, or opaque region of the document 2. Example further security features 6 include: optically variable devices such as diffractive optical elements, Kinograms, microlens based features, holograms, etc; watermark images; fine print; etc.

[0058] As shown in FIGS. 1a and 1b, and in more detail in FIG. 2, the optical device 4 generally includes a substrate 8 having on a first side 16a a source layer 10 and on a second side 16b an OVD layer 12 opposite the source layer 10.

[0059] FIG. 3 shows the source layer 10 and OVD layer 12 in further detail. The source layer 10 comprises an arrangement of source elements 18. The source elements 18 typically correspond to a pixelated printed source pattern, that is, they are created by selectively printing onto areas of the source layer 10 and each source element 18 constitutes a pixel of the source pattern. The arrangement can be as shown; that is, a regular square array. According to an implementation, the arrangement is selected such that the source elements 18 are arranged in any repeating manner, for example by arranging according to one of the five 2-dimensional Bravais lattices. In an alternative implementation, the arrangement of source elements 10 is not required to be repeating.

[0060] Each source element 18 of the source layer 10 defines an image which is defined by a transparent portion and an opaque portion. Typically, the opaque portion defines at least a border of the image, such that the entire transparent portion of the source element 10 is within the border. The source elements 10 are typically created using a printing process, such as rotogravure, silkscreen, intaglio, etc., where ink is only applied in the opaque portions. In this way, the source elements 18 define transparent images.

[0061] FIG. 3 also shows a specific example of a source element 18, being source element 18a which has an image in the form of a transparent line or slit surrounded by an opaque printed border.

[0062] In an embodiment, each source element 18 is identical. Therefore, the arrangement of source elements 18 constitutes an arrangement of identical printed source pixels. In another embodiment, not shown, the source layer 10 comprises different source elements 18, that is, the source layer 10 includes at least two different images. Having different source elements allows for a change in depiction as the viewing position is changed.

[0063] It is understood that the images defined by the source elements 18 can be selected from very simple concepts, for example a line or dot image, or more complicated concepts, such as characters, symbols, or depictions.

[0064] An external light source 30 is located such as to illuminate the source layer 10. The external light source 30 is of arbitrary shape, for example point source, fluorescent tube, uniform cloudy sky, etc. Furthermore, the external light source can illuminate the source layer 10 from an arbitrary angle or direction.

[0065] Each source element 18 transmits the light incident from the light source only through the non-opaque regions of the source element 18. The overall effect is that each source element 18 acts as an embedded light source with a predefined shape corresponding to the image of the source element, for example the slit shown in FIG. 3.

[0066] The substrate 8 is transparent, allowing the light incident onto each source element 18 to propagate from the first side 14a of the substrate 8 to the second side 14b. The substrate 8 acts as a spacer for the source layer 10 and the OVD layer 12. Typically, the substrate 8 is sourced from a bulk material, and will have a characteristic thickness. For example, biaxially oriented polypropylene material used in polymer banknotes typically has a thickness between 70 and 100 m.

[0067] Still in reference to FIG. 3, the OVD layer 12 includes an arrangement of diffractive elements 26. The diffractive elements 26 typically correspond to a pixelated OVD microstructure. In essence, each diffractive element 18 can constitute a pixel of a larger diffractive OVD structure. The diffractive elements 26 are configured for viewing by a viewer 20, typically the naked eye.

[0068] Each diffractive element 26 is associated with a source element 18. Typically, each diffractive element 26 is uniquely associated with a source element 18 and vice versa (as shown in FIG. 3), in that each diffractive element 26 is illuminated by its associated source element 18. Alternatives are however envisaged, for example, each source element 18 may be uniquely associated with a fixed number (greater than one) of diffractive elements 26, or each diffractive element 26 may be uniquely associated with a fixed number (greater than one) of source elements 18. For example, a source element 18 may be arranged to provide an artificial light source for four diffractive elements 26, or one diffractive element 26 may be configured to interact with four separate artificial light sources, each corresponding to a different source element 18.

[0069] As the source elements 18 provide an embedded light source with a consistent shape that is independent, or at least relatively independent, of the external light source 30, it is possible to design each diffractive element 26 in accordance with the particular image of the associated source element 18. Each diffractive element 26 has a surface relief configured for producing an optically variable image when the device is observed by the naked eye; the image being optically variable in that it varies in form and/or brightness with changing angle of view of the device. Optionally, the surface relief of each diffractive element 26 will be configured specifically for that diffractive element 26, though there may ultimately be diffractive elements 26 having the same surface relief.

[0070] In general, it can be preferred that the linear dimensions of the source elements 18 are less than the spacing between source elements 18 and diffractive elements 26. Typically, the source elements 18 will have linear dimensions roughly half the spacing between the source elements 18 and the diffractive elements 26. For example, when utilised as a security device on a banknote, the spacing between the source elements 18 and the diffractive elements 26 is approximately 70 microns. In this example, each source element 18 has two linear dimensions of 30 microns.

[0071] Reference is made to R. A. Lee, Generalized curvilinear diffraction gratings I. Image diffraction patterns, OPTICA ACTA, 1983, vol. 30, no. 3, 267-289 (herein referred to as GCDG1), which describes a general theory for curvilinear diffraction gratings illuminated by an arbitrarily extended diffuse light source. Each source element 18 is effectively an arbitrarily extended diffuse light source within the context of GCDG1.

[0072] Referring now to FIG. 4, the grating function for a particular diffractive element 26 is given by W(x, y), and the grating grooves of the diffractive element 26 are defined by the indicial equation of the form W(x, y)=n, where n is the groove index number (i.e., n=1, 2, 3, . . . ). The figure shows a generalised relationship between source element 18 (that is, the light source), diffraction element 26, and the viewer 20. As described in R. A. Lee, Generalised Curvilinear Diffraction Gratings II, OPTICA ACTA 1983, vol. 30, no. 3, 291-303 (herein referred to as GCDG2), W(x, y) can also be regarded as the contour map of an abstract phase surface transferred to a planar light wave as it passes through or is diffracted from the grating groove pattern W(x, y).

[0073] The geometric optics diffraction grating ray equations for the above situation are given by:

[00001]$\begin{array}{cc}{p}_1+w_1+Q_{01}=\frac{x}{G}-.Math..Math.h.Math.\frac{W}{x}& \left(1\right)\\ p_1+w_1+Q_{01}=\frac{y}{G}-.Math..Math.h.Math.\frac{W}{y}& \left(2\right)\end{array}$

where (Q.sub.01, Q.sub.02) are the coordinates of the centre of the light source coordinate system (i.e., the centre of the associated source element 18), located at a distance R.sub.s from the centre of the grating, as shown in FIG. 4. The coordinates (w.sub.1, w.sub.2) are the coordinates of a particular point on the light source, while (p.sub.1, p.sub.2) are the coordinates of an eye (or other viewer) observation point located a distance R.sub.0 from the centre of the grating, also as shown in FIG. 4. The parameter G is defined by G.sup.1=R.sub.0.sup.1+R.sub.s.sup.1, while h is the diffraction order number and is the wavelength of the incident light.

[0074] In GCDG1 and GCDG2 it was shown that the observed fringe pattern (that is, the set of (x, y) points in the grating plane that are observed to diffract light to the eye at a particular angle of view) can be described by an equation of the form:

S.sub.(w1,w2))(w.sub.1+Q.sub.01,w.sub.2+Q.sub.02)=0(3)

which defines the angular shape of the embedded light source in terms of individual points within the light source represented by (w.sub.1, w.sub.2) which in turn are defined with respect to the centre point of the light source defined by (Q.sub.01, Q.sub.02). The observed or perceived illuminated points on the grating are calculated by substituting the grating ray equations of equations (1) and (2) into equation (3).

[0075] Consider the example of a generalised diffraction grating observed at an angle normal to the plane of the grating and illuminated by an extended incoherent polychromatic source in the form of a very thin slit illuminated by a polychromatic external light source such as shown in FIG. 3, oriented in a direction parallel to the x-axis (as defined in FIG. 4) of the grating. Applying equation (2) in this situation results in the expression:

[00002]$\begin{array}{cc}{p}_2+Q_{02}=\frac{y}{G}-.Math..Math.h.Math.\frac{W}{y}& \left(4\right)\end{array}$

where w.sub.2=0 as the slit can be approximated by an infinitely thin line. The coordinate w.sub.1 does not enter into the calculation because the slit is also approximated by a line of infinite length so that equation (1) applied equally to all points in the x direction. Q.sub.02 defines the angle of the source with respect to the y direction and h is the diffraction order number and takes values of h=1, 2, 3, etc, although usually only the first, and possibly second, orders need to be included in the calculation for those gratings whose brightness or diffraction efficiency drops off rapidly with increasing order number.

[0076] For the particular case of a zone plate type OVD where W=A(x.sup.2+y.sup.2), with A being a constant, equation (4) gives

[00003]$p_2+Q_{02}=y\left(\frac{1}{G}-2.Math..Math..Math.\mathrm{hA}\right).Math..Math.\mathrm{or}.Math..Math.y=\left(p_2+Q_{02}\right)/\left(\frac{1}{G}-2.Math..Math..Math.\mathrm{hA}\right),$

which describes a series of straight lines (one for each value of h) parallel to the source line.

[0077] Referring now to FIG. 5, a detailed view of the interaction between a single source element 18 and diffractive element 26 is shown. Here, the source element 18 is in the form of the printed slit of FIG. 3. Shown spaced apart and opposite the source element 18 is the diffractive element 26, being a series of straight lines parallel to the printed slit (source line) of source element 18.

[0078] A particular embodiment is shown in FIG. 6. Here, the diffractive elements 26 are configured as diffractive lenses, that is, they act in a similar manner to a concave or convex lens. When coupled to a source element 18 defining an arbitrary shape (in this case, a star), the viewer 20 perceives the same shape (that is, a star) when viewing the diffraction element 26.

[0079] For a diffractive element 26 configured as a diffractive lens, the grating function can be of the form W(x, y)=A(x.sup.2+y.sup.2)+Bx+Cy, where A, B, and C are constants with A defining the focussing characteristic and B and C defining off-axis focal points of the diffractive element 26. For example, if B and B are both zero then the diffractive element 26 would be of a circular type diffractive lens as shown in FIG. 6. Equations (5) and (6) are obtained by substituting this expression for the grating function into equations (1) and (2):

[00004]$\begin{array}{cc}{p}_1+w_1+Q_{01}=x\left(\frac{1}{G}-2.Math.A.Math..Math..Math..Math.h\right)+B& \left(5\right)\\ p_1+w_2+Q_{02}=y\left(\frac{1}{G}-2.Math.A.Math..Math..Math..Math.h\right)+C& \left(6\right)\end{array}$

and substituting these into equation (3) provides:

S.sub.(w1,w2)(x(G.sup.12Ah)+Bp.sub.1,y(G.sup.12Ah)+Cp.sub.2)=0(7)

where the calculation is applied at every (w.sub.1, w.sub.2) point within the embedded light source. Note that the source equation which was originally a function of (w.sub.1, w.sub.2) (see equation (3)) is now a function of (x, y) with a linear relation between points (w.sub.1, w.sub.2) and (x, y).

[0080] The result shows that a diffractive lens array where each diffractive element 26 is described by a grating function of the form W(x, y)=A(x.sup.2+y.sup.2)+Bx+Cy produces an observed diffraction fringe pattern having the same shape as the image defined by its associated source element 18. The only difference being that the diffractive fringe pattern is a magnified and/or displaced version of the image (magnified according to the parameter A and displaced according to the parameters B and C).

[0081] The degree of magnification can be calculated by considering two points (w.sub.1, w.sub.2) and (w.sub.1, w.sub.2) on a source element 18 and observed through the corresponding diffractive element 26 at point (p.sub.1, p.sub.2). Substituting into equations (5) and (6) gives the observed image points occurring at (x, y) and (x, y). The degree of magnification can then be found by:

[00005]$\begin{array}{cc}M=\frac{\left(x^{}-x\right)}{\left(w_1^{}-w_1\right)}=\frac{\left(y^{}-y\right)}{\left(w_2^{}-w_2\right)}=\frac{G}{\left(1-2.Math.A.Math..Math..Math..Math.\mathrm{hG}\right)}& \left(8\right)\end{array}$

[0082] As G.sup.1=R.sub.0.sup.1+R.sub.s.sup.1 and that the observed distance R.sub.o is much larger than the thickness of the document substrate R.sub.s, it is possible to simplify the magnification to:

[00006]$\begin{array}{cc}M=\frac{1}{\left(R_s-2.Math.A.Math..Math..Math..Math.h\right)}& \left(9\right)\end{array}$

[0083] This relationship allows for suitable selection of the lens focus parameter A depending on the desired magnification for a particular substrate thickness, wavelength, and image characteristics required.

[0084] The optical device 4 disclosed herein can be manufactured according to the method shown in FIG. 7. A transparent substrate is provided, such as a biaxially oriented polypropylene substrate, and a radiation curable ink is applied through a printing process to one side of the substrate, at RCI step 100. The radiation curable ink is then embossed with a shim and cured at embossing step 101. The shim has a surface profile the opposite to the intended surface profile of the OVD layer 12.

[0085] Printing step 102 is undertaken simultaneously with, before, or after, embossing step 101. Printing step 102 corresponds to the creation of the source layer 10 through printing of an opaque (or substantially opaque) ink to the opposite side of the substrate, the opaque ink absent in areas defining the source images. Typically, it is necessary to ensure registration between the diffraction elements 26 and the source elements 24, which may be achieved using known methods.

[0086] As will be clear, the method of FIG. 7 requires that a shim has previously been created, for example utilising known e-beam techniques, and that a suitable printing pattern is been formulated for creating the source elements 24. FIG. 7 shows the optional shim preparation step 103. Typically, the design of both the shim and the printing pattern is assisted by a computer. Usually, the intended source images 18 are determined as well as the intended projected image and projection direction. From this, the required grating profile for each diffraction element 26 can be determined utilising computational methods implementing the relationships described herein.

[0087] Further modifications and improvements may be incorporated without departing from the scope of the invention.