Optical Identification System

Abstract

A system and method of using the same, wherein the system comprises: an optical surface having a diffractive image generating structure disposed thereon, the diffractive image generating structure itself comprising a layer of reflective material incorporating a plurality of grooved diffractive elements each having a periodic wave surface profile, the periodic wave surface profiles each having a groove alignment direction; a source of incident electromagnetic radiation arranged to illuminate the diffractive elements at an angle of incidence substantially normal to the plane of the surface of the diffractive elements; means for polarising the radiation from the source, and means for polarising radiation reflected from the diffractive elements; wherein the diffractive elements are configured such that, in use, polarisation conversion of the incident radiation takes place, and wherein the diffractive elements are disposed in a two dimensional array of pixels to represent an image; and further wherein the means for polarising is arranged to pass incident radiation having a polarisation state of approximately 45° azimuth to the groove alignment direction, and is arranged to select a polarisation, using the means for polarising the radiation reflected from the diffractive elements, and to pass radiation of the selected polarisation to a detection point.

Claims

1. A system comprising: an optical surface having a diffractive image generating structure disposed thereon, the diffractive image generating structure itself comprising a layer of reflective material incorporating a plurality of grooved diffractive elements each having a periodic wave surface profile, the periodic wave surface profiles each having a groove alignment direction; a source of incident electromagnetic radiation arranged to illuminate the diffractive elements at an angle of incidence substantially normal to the plane of the surface of the diffractive elements; a polariser for polarising the radiation from the source, and a polariser for polarising radiation reflected from the diffractive elements; wherein the diffractive elements are configured such that, in use, polarisation conversion of the incident radiation takes place, and wherein the diffractive elements are disposed in a two dimensional array of pixels to represent an image; and further wherein the polarisers for polarising i-s are arranged to pass incident radiation having a polarisation state of approximately 45° azimuth to the groove alignment direction, and are arranged to select a polarisation, using the polariser for polarising the radiation reflected from the diffractive elements, and to pass radiation of the selected polarisation to a detection point.

2. A system as claimed in claim 1, wherein the selected polarisation is that which has been polarisation-converted by the diffractive elements,

3. A system as claimed in claim 1, wherein the polariser for polarising the radiation from the source comprises a linear polariser arranged to pass radiation from the source of radiation having a first polarisation state, and wherein the means for polarising the radiation reflected from the optical surface comprises a linear polariser arranged to pass the reflected radiation having a second polarisation state orthogonal to the first.

4. A system as claimed in claim 2, wherein the polariser for polarising the radiation from the source, and for polarising the reflected radiation, comprises a single linear polariser arranged to reflect light from the source orthogonally towards the optical surface, and to pass orthogonally polarised light reflected from the optical surface.

5. A system as claimed in claim 1, wherein the polariser for polarising the radiation from the source, and for polarising the reflected radiation, comprises a circular polariser.

6. A system as claimed in claim 1, wherein the selected polarisation is that which has not been polarisation-converted by the diffractive elements,

7. A system according to claim 1, wherein the periodic wave surface profiles have a common groove alignment direction and/or wherein the periodic wave surface profile of each diffractive element has a pitch G and a profile depth d, and wherein the pitch G is comparable to the wavelength λ of polarised electromagnetic radiation incident upon the layer of reflective material.

8-9. (canceled)

10. A system according to claim 1, wherein the surface profile is a rectangular, square or pulsed waveform having a mark to space ratio M, and preferably wherein for each respective surface profile at least one parameter thereof is chosen to provide a particular colour response, the at least one parameter being selected from a list comprising the pitch G, depth d, mark, mark to pitch ratio, mark to space ratio M, Fourier harmonic content of the surface profile cross-section, permittivity of the layer of reflective material and permittivity of any optional protective coating layer.

11. (canceled)

12. A system according to claim 1, wherein the plurality of diffractive elements each have at least two different surface profiles so as to provide at least two different colour responses, and preferably 3 different surface profiles.

13. (canceled)

14. A system according to claim 1, wherein at least one of the pixels is arranged to have at least part of its surface area devoid of a grating structure.

15. A system according to claim 1, wherein the two dimensional array of diffractive elements is arranged to represent an image with sub-pixel rendering.

16. A system according to claim 1, wherein the surface area of respective diffractive elements is varied to provide differences in the perceived respective polarisation conversion intensity.

17. A system according to claim 1, wherein the reflective material comprises a metal, preferably a metal or an alloy, and the -a metal is preferably selected from the group consisting of aluminium and silver.

18-20. (canceled)

21. A system according to claim 1, wherein the layer of reflective material is coated with a protective layer, and/or wherein the reflective layer is disposed on a substrate layer.

22. (canceled)

23. A system according to claim 1, wherein the source of electromagnetic radiation is at least one of the following: i) polychromatic; ii) visible light iii) ambient light.

24. (canceled)

25. A system according to claim 1, wherein at least part of the polariser for polarising the incident radiation comprises the illumination source being arranged to emit polarised radiation.

26. (canceled)

27. A system according to claim 1, wherein the optical surface comprises or is disposed on an article, and preferably wherein the article is selected from any one of a banknote, cheque, credit card, identity card, medical card, ticket, legal document, deed, label, casing or shrink-wrap.

28. (canceled)

29. A system as claimed in claim 1, wherein the system further includes a detector for detecting radiation reflected from the reflective layer.

30-32. (canceled)

33. A method comprising: (i) providing an optical surface having a diffractive image generating structure disposed thereon, the diffractive image generating structure itself comprising a layer of reflective material incorporating a plurality of grooved diffractive elements each having a periodic wave surface profile, the periodic wave surface profiles each having a groove alignment direction, wherein the diffractive elements are configured such that polarisation conversion of incident radiation takes place, and wherein the diffractive elements are disposed in a two dimensional array of pixels to represent an image; (ii) illuminating the diffractive elements with electromagnetic radiation, the radiation being directed onto the diffractive elements at an angle of incidence substantially normal to the plane of the surface of the diffractive elements and having a polarisation state of approximately 45° azimuth to the groove alignment direction; and (iii) passing the radiation reflected from the diffractive elements through polarising means for selecting a polarisation and then passing radiation of the selected polarisation to a detection point.

34. A method according to claim 33 further comprising comparing the appearance of an image generated using the reflected radiation received at the detection point in step (iii) with a reference image so as to determine whether or not an object is genuine or counterfeit.

35. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like reference numerals are used for like parts, and in which:

[0084] FIG. 1 shows the geometrical arrangement of components in an embodiment of the invention;

[0085] FIG. 2 shows modelled results of the R.sub.ps conversion spectrum for different values of the mark to pitch ratio and pitch for a grating with a rectangular profile;

[0086] FIG. 3 shows actual measured results for gratings made in aluminium and silver;

[0087] FIG. 4 shows an image of the Mona Lisa produced using an embodiment of the invention;

[0088] FIG. 5 shows an alternative embodiment of the invention adapted to use circular polarisation.

DETAILED DESCRIPTION

[0089] FIGS. 1a and 1b show schematically, in a profile view and a plan view respectively, a typical representation of how various components may be arranged in an embodiment of the invention. A grating (2) comprises a repeating pattern of grooves (3) comprising an array of regions, each one defining a pixel or sub-pixel of an image, and each being of a predetermined pitch and depth, as described herein, with the surface of the grating defining a plane. A plane of incidence (5) is defined, orthogonal to the plane of the grating. A polarising beamsplitter (10) is arranged to reflect light of a given polarisation (denoted “p”) from an illumination source (1) orthogonally onto the grating (2), wherein the polarisation state p is parallel to the plane (5) and at 45° to the alignment of the grooves (3) The alignment of the plane of incidence (5) in relation to the grooves (3), therefore defines an azimuthal angle (4) of 45°, or π/4 radians.

[0090] The illumination source (1) may provide linearly polarised light , of polarisation state p, or it may provide unpolarised light. In the latter case, the light from the illuminator having a polarisation state orthogonal to state p (i.e. in state s) will pass through the polariser and has no further function. Linear polariser (6) within the polarising beamsplitter (10) is used to reflect the light of state p towards the grating (2).

[0091] Light hitting the grating (3) will undergo a polarisation conversion, R.sub.ps, and reflected light will therefore be in the s polarisation state. This light passes up to the beam splitter (10) where it is able to pass through the polariser (6) due to the R.sub.ps conversion that has taken place, and on to an observer or detector (11).

[0092] To produce images having defined colours the pixels (or sub-pixels forming a given pixel) forming the image need to be adapted to produce the desired colour. In an embodiment of the invention this is done by suitable selection of the grating pitch, depth, and (for rectangular grating structures) mark/space ratio. These parameters may be devised by e.g. theoretical calculation, or by computer modelling, or an iterative trial-and-error approach, or by a combination of such methods.

[0093] Modelling of colours that may be produced by a given grating structure has been done. A finite-element method model was set up using Ansys Inc.'s HFSS program to simulate the spectral reflectances of grating profiles. Each spectrum was converted to the well-known CIE xyY coordinate system with the purpose of identifying a set of R.sub.ps RGB primary colours enclosing a broad gamut of chromaticities and efficient R.sub.ps conversion. A set of formulae was obtained to enable the conversion of CIE xyY coordinates of any colour to a set of R.sub.ps RGB relative intensities. By combining this conversion process with published conversion formulae relating CIE xyY to other standards, for example sRGB, the relative intensities of the pixels of a digital image recorded using that standard may be used to obtain an array of subpixel grating areas that perform R.sub.ps with accurate reproduction of the colours and the spatial distribution of the image.

[0094] FIG. 2 shows various modelled and measured results from a rectangular grating profiles formed in aluminium and silver.

[0095] FIG. 2a shows modelled results for aluminium of the R.sub.ps conversion of gratings having various mark to pitch ratios, each having a depth (peak to peak) of 45 nm and a pitch of 380 nm, encapsulated in a lossless transparent dielectric having a refractive index of 1.5. The wavelength-dependent permittivity of aluminium was specified in the model using the data of, A. D. Rakić, “Algorithm for the Determination of Intrinsic Optical Constants of Metal Films: Application to Aluminum,” Appl. Opt. 34, pp. 4755-4767, 1995.

[0096] Table 1 presents the results of further modelling, showing pitch and mark/pitch parameters used to obtain red, green and blue colours, with a fixed grating depth of 45 nm. x, y and Y are the resultant CIE colour space parameters.

TABLE-US-00001 TABLE I R.sub.ps RGB Pitch Mark/ Primary (nm) pitch x y Y Red 385 0.475 0.5968 0.3308 0.1074 Green 330 0.35 0.3327 0.5477 0.3197 Blue 275 0.3 0.2224 0.2047 0.1721 Grating dimensions and chromaticity data for selected R.sub.ps RGB primaries.

[0097] FIG. 2b shows modelled results of the R.sub.ps conversion for red, green and blue sub-pixel primaries based on the properties of aluminium. The modelling assumes the grating is being illuminated with linearly polarised broadband white light corresponding to the CIE standard illuminant E, with direction of illumination normal to the plane of the grating, and the groove alignment direction being at 45° to the plane of polarisation, e.g. using the setup shown in FIG. 1. Curve 40 shows the blue R.sub.ps conversion, curve 41 shows the green, while curve 42 shows the red.

[0098] FIG. 2c shows a modelled R.sub.ps spectrum of a white pixel, comprising a combination of three sub-pixels, each comprising a separate colour from the three colour primaries shown in FIG. 4a. The simulation includes an area weighting of the sub-pixels in order to reproduce the white point of the CIE standard illuminant E. The respective weightings applied in the model were N.sub.RED=1.1065, N.sub.GREEN=0.8817, N.sub.BLUE=1.0118.

[0099] FIG. 3a shows measured R.sub.ps reflectance v wavelength data taken from various aluminium gratings, with pitch values of 295 nm, 320 nm, 350 nm, 370 nm and 395 nm, for the curves peaking from left to right, and their mark/space ratios were 0.34, 0.33, 0.35, 0.37 and 0.39 respectively. The depths of the gratings was 45 nm.

[0100] FIG. 3b shows measured R.sub.ps reflectance v wavelength data from various silver gratings, with all dimensions the same as in FIG. 3a. It will be observed that the reflectance varies more widely for these gratings as compared to those made in aluminium, but this can be taken into account by weighting the areas of sub-pixels and non-R.sub.ps conversion regions, to achieve a more complete colour range.

[0101] These primaries can then be used to produce concealed images using R.sub.ps, pixels comprising three sub-pixels, each providing a different primary colour and having the corresponding grating design contained in adjacent rectangular areas. The relative intensity of each polarisation-converted primary colour within a specific pixel may be controlled by adjusting the area of the grating, with unused space being left as flat metal. The flat metal does not convert the polarisation and therefore appears black under the appropriate viewing conditions and does not contribute to the reflected spectrum.

[0102] The arrangement of subpixels may be used to reproduce colours as they would appear under a particular illuminant spectrum. The illuminant may be chosen according to a particular requirement. Conveniently, the CIE standard illuminant E may be chosen, which has a flat spectral power distribution across visible wavelengths, and a corresponding white point with CIE chromaticity values x=0.333 and y=0.333. In order to reproduce the white point, the areas of the individual primary colours may be weighted to take account of the reflectivities and chromaticities of the individual primary colours. Alternatively or as well, areas within a sub-pixel may be arranged to not have a grating structure formed thereon (e.g. by comprising of smooth metal), and so may be used to adjust the apparent brightness of the sub-pixel.

[0103] The grating design for each of the R.sub.ps primary colours was established by an iterative process. Firstly, the simulation of the electromagnetic response of a candidate grating design was performed to obtain its R.sub.ps spectrum, from which the CIE xyY coordinates were calculated. The available design parameters were then adjusted iteratively to alter the R.sub.ps spectrum through the plasmon behaviour, in order to optimise the xyY values for maximised colour saturation and reflectance magnitude. In this way, designs were obtained to provide R.sub.ps RGB primary colours enclosing a broad gamut of chromaticities and offering efficient R.sub.ps conversion.

[0104] An image has been produced using the technique describe herein to prove the principle. The image was a digital photograph in JPEG format of the Mona Lisa by Leonardo Da

[0105] Vinci. Analysis of the CIE coordinates of the image showed that its RGB values fitted the gamut of the sRGB standard and accordingly, the data were treated as sRGB. These pixel data were extracted from the file as a matrix of values, which were then converted to R.sub.ps RGB values, which in turn were used to generate a layout file in GDS II format defining a pixel array containing area-weighted gratings. The weightings were calculated to reproduce the colours of the image when illuminated by the CIE standard illuminant E, which corresponds to a flat spectral power distribution across visible wavelengths.

[0106] The layout was written into a 45 nm thick layer of polymethylmethacrylate (PMMA) resist on a silicon substrate, and developed and processed using standard techniques. The resulting metal surface was encapsulated by bonding a glass superstrate using Norland NOA65 epoxy, which has a refractive index of 1.52.

[0107] The R.sub.ps spectra of the fabricated test patches were measured using a polarising microscope, with the illumination and viewing paths containing linear polarisers set orthogonally to each other, and the grating vector of the sample orientated at the intermediate 45° angle. The microscope was fitted with a broadband optical source and a fibre-coupled optical spectrometer. The R.sub.ps image of the Mona Lisa sample was measured with the spectrometer arrangement replaced by a camera, and a black and white rendition of the resulting image is shown in FIG. 4. Of course, the original is in colour.

[0108] The grating profile used for each colour (i.e. sub-pixel) in the production of FIG. 4 was rectangular with a 45 nm peak to trough depth, and the grating was designed to work with an overcoat of refractive index 1.5. Three sub-pixels were used per pixel, each having the following respective characteristics:

[0109] Red sub-pixel: Pitch 385 nm, mark/pitch ratio 0.475 (i.e. width of grating peak as a fraction of the pitch)

[0110] Green sub-pixel: Pitch 330 nm, mark/pitch ratio 0.35

[0111] Blue sub-pixel: Pitch 275 nm, mark/pitch ratio 0.3

[0112] The values used therefore for the gratings were the same as those shown in Table 1.

[0113] FIG. 5 shows an alternative embodiment of the invention that uses circular polarisation, instead of the linear polarisation discussed in embodiments described above. In FIG. 5, electromagnetic radiation comprising ambient light is arranged to illuminate a diffraction grating surface (52) from a direction substantially normal thereto, via a circular polariser. The circular polariser comprises a linear polariser (53), followed by a 90° phase-retardation plate (54), arranged with its principal axes orientated at ±45° azimuth to that of the linear polariser, the combination of (53) and (54) acting as said circular polariser. This arrangement filters the incident light so as to transmit only circularly polarised light. The circular polariser may be configured so that the transmitted light is either left-hand circular or right-hand circular. Light that is reflected from the surface is filtered by a return pass through the circular polariser. On the return pass, the circular polariser only passes circularly polarised light of the same handedness as that transmitted on the forward pass, converting it to a linear polarisation in the process. The radiation from the source, having been circularly polarised, arrives at the diffraction grating surface (52) on the article under detection. The circular polarisation may be resolved into two orthogonal linear components of equal amplitude, orientated at +45° and -45° respectively to the grating azimuth, whereby one component lags the other in phase by 90° . Both linear components undergo polarisation conversion due to the grating, so that the phase relation with respect to the selected axes is reversed. Taken in combination with the mirror reversal on reflection, this process results in the preservation of the circular polarisation handedness: the reflected beam can then be transmitted back through the circular polariser, and viewed by an observer or optical detector. If polarisation conversion did not occur (i.e.) if the correctly-profiled grating was absent) then the reflected radiation would be rotating in a sense that would be opposed to that of the polariser, and transmission could not occur. The reflected radiation will therefore only produce a signal visible to an observer or detector if the surface exhibits specifically-tailored diffractive properties.

[0114] A modification to the embodiment shown in FIG. 5 may comprise a similar arrangement, but wherein a broadband source of light is provided as an illumination source. This takes away a reliance upon there being sufficient ambient light in any given situation.

[0115] The embodiment of FIG. 5 could be employed, for example, on a document or article, wherein the grating (52) is located on one part of the document, while the polariser elements (53, 54) are located on another part, and wherein the different parts could be brought into the configuration shown in FIG. 5, e.g. by bending or folding the document appropriately. Thus such an article provides a convenient means for checking its authenticity without requirement for further optical components, by ensuring for example that the resulting image matches an expected image, such as a similar, but traditionally printed image located close thereto.

[0116] A further degree of resolution can be obtained by arranging two detectors in parallel, one detecting polarisation converted reflections, the other detecting remaining reflections. A comparison of the two detected signals provides a higher resolution measurement of the polarisation converted radiation.

[0117] Aspects and embodiments of the invention extend to a method substantially as herein described, with reference to the accompanying drawings.

[0118] Aspects and embodiments of the invention have been described with specific reference to the production of images in the visible waveband. It will be understood that this is not intended to be limiting and that aspects and embodiments of the invention may be used more generally at other wavelengths of electromagnetic radiation. Moreover, aspects and embodiments of the invention have been described in relation to hidden images, covert and anti-counterfeiting applications. This is not intended to be limiting, and other applications will occur to the skilled person.