GB2398759A

GB2398759A - An identification system using a random array of micro-lenses

Info

Publication number: GB2398759A
Application number: GB0304223A
Authority: GB
Inventors: Mike Simmonds; John Marcus Rodenburg
Original assignee: Sheffield Hallam University
Current assignee: Sheffield Hallam University
Priority date: 2003-02-25
Filing date: 2003-02-25
Publication date: 2004-09-01
Also published as: GB0304223D0

Abstract

Concave craters or surface pits 201, taking the form of micro-lenses, are formed in a substrate 200 by a plasma etching method which uses a plasma etch resistant coating such as amorphous carbon. A transparent coating layer may then be applied to form a window above the array. A random array of such micro-lenses is essentially unreproducible. The array may be used as a unique counterfeit resisting identification marker. In response to illumination by a light source 2102 through optical fibre bundle 2101, the unique image pattern produced by reflection may be read through optical fibre bundle 2103 by CCD camera 2104 and colour image processing device 2105 when optical fibre bundle 2100 abuts the window.

Description

RANDOM MICRO-LENS ARRAY

Field of the Invention

The present invention relates to the field of micro lenses, and particularly although not exclusively, to a marking system using a random array of micro lenses.

Background to the Invention

In the field of optics, it is known to produce microscopic lenses and other microscopic optical components on a substrate material. In prior art micro optical component systems, in order to be useful, components must be reproducible and be capable of being produced to a pre-determined design and using technologies which admit of controllable re-production.

Prior art optical micro components generally fall into two categories. Firstly, there are component types including lenses, reflectors etc. which are formed at specific places on a substrate surface, and are created using an etching or other multiple stage process, such as a lithographic process, to create specific optical so components at specific positions on the substrate, and to control the shape, size and type of the optical micro components. When applied in the field of marking technology, such optical micro components can be used to embody codes.

However, because the process for making the micro components is controllable and well defined it is inherently reproducible, and therefore there is an opportunity for forging identification marks based upon such prior art optical micro components.

In the field of identification marking, and in particular optically read identification marks, there have been prior art attempts to generate unique markers, which are non-forgeable. Prior art attempts at creating unique marks include systems in which reflective particles are embedded in an epoxy layer, where light reflected from the reflective particles provides a pseudo - random I reflective light pattern.

Other prior art systems include systems of reflective and transmitting beads. Each of these prior art systems suffer from difficulty of reading identification marks in a re-producible way, without the need for expensive and complicated equipment. The prior art identification marks suffer from the basic problem of non-readability using digital reading equipment.

o Summarv of the Invention According to a first aspect of the present invention there is provided a substrate comprising a region formed as a plurality of lenses, wherein said plurality of lenses are positioned substantially randomly across said region.

Typically, individual said lenses may have a width dimension in the range 0.5 lam to 250 m.

Preferably, the substrate is provided with a coating layer extending across 2 o said region, said coating layer being substantially transparent to light.

Preferably, a thickness of said coating is selected such that a focal plane of light reflected from said plurality of lenses subsists outside said coating. I Individual said lenses may have a width dimension in the range 10 Am to m. Each said lens is capable of acting as a reflective lens, under conditions of external light illumination.

Suitably, the substrate comprises a crystalline material. The substrate may o comprises a metal. The substrate may comprise semi-conductor material.

The substrate may comprise a material selected from the set: a m - v compound; and a II - VI compound. The substrate may comprise a material selected from the set: silicon; indium phosphide; gallium arsenide; aluminium gallium arsenide; and sapphire.

The invention includes an identification mark comprising a substrate as described with reference to the first aspect.

According to a second aspect of the present invention, there is provided an identification mark comprising: a substrate material, said substrate material having a region formed as a plurality of concave craters, wherein said plurality of craters are positioned substantially randomly across said region.

According to a third aspect of the present invention, there is provided a substrate material comprising a surface region formed as a plurality for concave craters, wherein said plurality of craters are positioned substantially randomly across said surface region.

According to a fourth aspect of the present invention, there is provided a method of forming a plurality of concave craters in a substrate, said method compnslng: applying a plasma etch resistant coating to a surface of said substrate; and plasma etching through said plasma etch resistant coating and into a region of said substrate, such that said plurality of concave craters are formed in o said substrate region.

Preferably, said method comprises continuing plasma etching through said etch resistant coating until said coating is completely etched away.

The plasma etching may be continued into said substrate, until said region of said substrate subjected to said plasma etch is completely covered by said plurality of craters.

The region may be substantially circular, although regions of other shapes may be created. The region may be substantially cylindrical. However, in some o embodiments, the region is not circular, and in the general case may be symmetric, or asymmetric in shape in order to provided an easily recognizable shape for a reader device to determine an orientation in which to read the marking, by correctly recognizing and aligning with the region. In general, the shape of the region depends on the shape of the anode used to contain the plasma.

Preferably, said plasma etch resistant coating comprises an amorphous carbon coating. Suitably, said plasma etch resistant coating is applied to said substrate by physical vapor deposition.

Suitably, said craters have a width dimension in the range 0.5 to 250 m.

Said craters may be formed having a depth in the range 5 to 700 nanometers.

Said craters may be formed as concave surfaces having a radius of curvature in the range 4 to 80 m.

According to a fifth aspect of the present invention, there is provided a method of forming a region of a plurality of substantially randomly arranged lenses in a substrate material, said method comprising: 3 o applying an etch resistant coating to a surface of said substrate; etching through said etch resistant coating and into said substrate, such that said plurality of lenses are formed in said region of said substrate.

Preferably, said method comprises continuing etching of said etch resistant coating, until said etch resistant coating is completely removed.

According to a sixth aspect of the present invention, there is provided a method of applying an identification mark to a substrate, said method comprising: applying an etch resistant coating to a surface of said substrate; and etching through said etch resistant coating and into said substrate, such that a plurality of craters are formed in said substrate.

Said method may comprise applying a light transparent surface coating layer across said plurality of lenses formed in said substrate region.

According to a seventh aspect of the present invention, there is provided o an identification system comprising: an identification mark comprising a reflective region capable of forming a random array of light reflections; and a reader device comprising and an optical detector; wherein said reader device is capable of reading an optical image formed by said light reflected from said reflective region.

In one embodiment, said reader device comprises a plurality of optical fibres. In another embodiment, said reader device comprises an optical microscope. The reader device may comprise a charge coupled detector (CCD) array.

According to an eighth aspect of the present invention, there is provided a reading system for reading a micro-lens array device, said micro-lens array device capable of generating an image comprising a substantially randomly arranged plurality of contrasting lighter and darker regions, said reading system comprising: To illumination means adapted for illuminating said plurality of micro-lenses; optical guidance means for capturing an image formed by said plurality of micro-lenses; and detector means for detecting said captured image.

Said system may comprise image processing means for processing said captured image as digital data.

Said system may further comprise alignment means for aligning said optical guide with said plurality of micro-lenses.

Said system may further comprise scanning means for moving said optical guidance means to scan across said image.

According to a ninth aspect of the present invention, there is provided a method of reading an image formed by a plurality of micro-lenses, said method comprising: arranging an optical instrument to capture at least a portion of an image formed at a focal plane adjacent said plurality of micro-lenses; arranging an optical detector device to capture a light output of said optical instrument; and digitising an output signal of said detector device to provide a digital image data representing said image.

Brief Description of the Drawings

For a better understanding of the invention and to show how the same may To be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Fig. 1 illustrates schematically a region of micro-lenses according to a first specific implementation of the present invention; Fig.2 illustrates schematically in cut-away close up view, a substrate material having a region comprising a plurality of randomly arranged reflector lenses; Fig. 3 illustrates schematically in view from above, a photograph of a region comprising a plurality of reflective lenses formed on a silicon substrate material; Fig. 4 illustrates schematically in view from above, a photograph of a region comprising a plurality of reflective lenses formed on an iron substrate material; Fig. 5 illustrates schematically in cutaway view a random micro-lens device having a transparent window material according to a second specific implementation of the present invention; Fig. 6 illustrates schematically a first reading method and apparatus for reading an optic image formed by a random micro-lens device according to a third specific implementation of the present invention; Fig. 7 illustrates a photograph view of an array of micro-lenses formed in a silicon substrate material; Fig. 8 illustrates a photograph of an image formed in a focal plane of the random micro-lens array of fig. 7; Fig. 9 illustrates schematically the image of fig. 8 at reduced light intensity; Fig. 10 illustrates schematically a first stage of manufacture of a random lens array device according to a specific method of the present invention; Fig. 11 illustrates schematically a second stage of manufacture of a random lens array device; Fig. 12 illustrates schematically a third stage of manufacture of a random o lens array device; Fig. 13 illustrates schematically a fourth stage of manufacture of a random micro-lens array device; as Fig. 14 and 15 illustrates schematically further detail of the fourth stage of manufacture of the random micro-lens array device; Fig. 16 illustrates schematically a final stage of manufacture of a random micro-lens array device; Fig. 17 illustrates schematically a final stage of manufacture of a micro-lens array device comprising forming a transparent optical window layer; Fig. 18 illustrates schematically a plot of lens depth against lens width for a plurality of randomly formed micro lenses; Fig. 19 illustrates schematically a plot of radius of curvature of lens against width of lens for a plurality of randomly formed micro lenses; Fig. 20 illustrates schematically a second reader apparatus according to a fourth specific implementation of the present invention; Fig. 21 illustrates schematically a second reading system according to the fifth specific implementation of the present invention; Fig. 22 illustrates schematically a third reading system according to a sixth :5 specific implementation of the present invention; and Fig. 23 illustrates schematically a signal output of the third reading system of fig. 22.

Detailed Description of a Specific Mode for Carrvina Out the Invention There will now be described by way of example a specific mode contemplated by the inventors for carrying out the invention. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

Specific implementations according to the present invention provide a micro lens technology, which combines the attributes of a reproducible fabrication process for creating an array of micro lenses, with a substantially random pattern of lenses. Consequently, specific implementations provide a fabrication process resulting in an identification mark which has a reproducible technology for basic parameters including reproducible physical size of mark; reproducible depth of mark; reproducible focal length; and reproducible distance of focal plane from a substrate surface. However, the technology provides for a random element in an arrangement of micro lenses, with a benefit of making any particular array of micro lenses difficult or impossible to reproduce exactly, and also difficult or impossible to reproduce approximately. This makes the technology suitable for high security marking applications.

To Specific implementations according to the present invention provide for a reproducible fabrication technology for fabricating an identification mark, but with an information content of the mark being effectively un-reproducible from one mark to another, and therefore for all practical purposes, un-forgeable.

A micro-lens product, and underlying technology for fabricating a micro lens array product as described by specific implementations herein, may find wide application throughout a range of scientific, engineering and commercial uses.

However, the novel micro lens array technology described herein, may find particular advantageous use in the fields of marking technology and product 2 o identification.

In one implementation a mark comprising a lens array is created by etching onto the surface of an object. Although the mark itself is visible, typically having a diameter of the order of 0.5 to 6 mm, a microstructure within the mark is invisible to the naked human eye, and difficult to observe using an optical microscope.

The mark consists of a series of concave pits, each of which acts as a reflecting lens. Lenses may have width dimensions of the order 0.5 Em to 2501lm, and typically in the range 10 1lm to 100 m. When illuminated by light, the reflective lenses create a pattern of light resembling a series of point sources, which so appears like a random distribution of star like pinpoints of light. The pattern occurs with high contrast and high definition over a range of distances from the surface of the object, typically at around 0.1 mm above the surface of the object, and can be recorded by an appropriately designed detector and reading system.

Depending upon an angular divergence of an illuminating beam of a light source, the pinpoints of light may each comprise a focussed image of the light source. Because the pattern of reflected points are very definitely present or not present, a high definition high contrast image having light and dark regions is formed. This enables the pattern to be converted into a unique digital code in the form of a long number. If the mark is re-read, the code can be regenerated, and JO can be used to identify the object to which the mark is applied. The information encoded in the mark may be many times more unique than available with any

prior art marking technology.

Core Technolonv Referring to Fig. 1 herein, there is illustrated schematically in perspective view, a substrate material 100 having a region 101 comprising an array of micro lenses. The micro-lens region 101 may have a wide range of width dimension and shapes, and is not restricted to a circular shape, but could be square, 2 o rectangular or any other required shape. A surface area of the micro- lens region 101 is variable as a designable parameter, and typically may have an area in the range of 65OIlm2 to 25mm2. In principle there is no reason why the surface area of the region cannot be greater than 25mm2.

: Referring to Fig. 2 herein, there is illustrated schematically in close up cut away view, a section of the region 101. The region 101 comprises a substrate material 200, an upper and outer surface of which is formed into a plurality of crater like depressions 201, each having a concave bowl like surface which can act as a reflector lens. The plurality of craters are substantially randomly o arranged in two dimensions. Each crater has an outer dimension, that is a width across the crater, in the range 0.5 lam to 250 m.

Each crater has a depth, measured as a distance from a line extending between opposite edges of the crater, and a central point of the crater, in the range 5 to 700 nanometers. Each crater has a concave reflective mirror surface, having a radius of curvature in the range 4 to 80 m.

The substrate material can be amorphous, or can be a regular lattice structure. The substrate material preferably comprises a crystalline material having a regular atomic lattice structure. Examples of suitable substrate 0 materials include: semi-conductors, including silicon, -V compound semi- conductors, II - VI compound semi-conductors, for example InP; and rr-v quaternary compounds. Other examples of substrate material may include metals such as iron. Any material which is capable of being plasma etched or chemically etched may provide a suitable substrate material.

Referring to Fig. 3 herein, there is illustrated schematically a scanning electron microscope image of a cratered region of a crystalline silicon substrate.

A portion of region of dimension approximately 530m x 53OIlm is illustrated is Fig. 3. The region comprises a plurality of concave craters which have width dimensions within pre-defined limits, whose spatial distribution is random, such that the center positions of each lens is placed substantially randomly across the region. In particular, within a maximum width dimension of the order of approximately 250 Am, a maximum width of individual craters is substantially randomly selected. Further, within a pre-determined range of depth of crater, of z up to 30 Elm, the depth of individual craters is substantially random. However, for each individual crater, there is a relationship between depth of crater and outer width dimension of the crater within pre-determined limits.

In the general case, no two sections of the region will have identical crater 3 o patterns, but the crater pattern will continuously vary without repetition across the whole of the region.

Further, the region comprises a plurality of relatively larger craters, interspersed with a plurality of relatively smaller craters. The distribution, pattern and arrangement of the relatively larger craters is substantially random over the region.

Referring to Fig. 4 herein, there is illustrated schematically a second surface region formed on a second substrate material, in this case iron (Fe). The portion of region showing in Fig. 4 is of dimension approximately 530 Am x 530 Am and To shows crater features having width dimensions in the range approximately 5 to 50pm.

Referring to Fig. 5 herein, there is illustrated schematically in crosssection view a micro lens array device comprising a substrate material 500 having a region comprising an array of randomly arranged micro reflective lenses 501; and a transparent coating layer 501 covering the array of micro lenses. The coating layer 501 may comprise any suitable light-transparent material, for example an epoxy resin, a transparent polymer, or a transparent crystallized material. A depth d of the transparent layer 501 above an average depth of the plurality of o micro lenses is selected such that an upper surface of the transparent layer is at, or slightly within, an average focal distance (0 of each micro lens, so that when the device is illuminated from above, each micro lens forms an image of the light source in a focal plane 502 where the focal plane is formed at the surface of the transparent layer, or slightly beyond the surface of the transparent layer in free space. Since the plurality of micro lenses all have a radius of curvature within a narrow range, and a focal length within a narrow range of each other, the respective focal lengths of each micro lens lie within a narrow range of distances from the surface of the transparent coating 501. Consequently, a viewing apparatus placed at the focal plane views a high contrast image comprising light 3 o focused by each of the plurality of reflective micro lenses.

Referring to Fig. 6 herein, there is illustrated schematically a reading system for reading a micro lens array as described herein before. The reading system comprises a light source 600 for illuminating the micro lens array; an optical beam splitter device 601 for directing illumination onto the micro lens array, and for transmitting an image formed by the micro lens array; one or more collimating lenses 602, or other light guidance means, for guiding the illumination onto the micro lens array, and for transmitting an image formed at a focal plane 603 of the micro lens array; and a two dimensional detector device 604, for example a charge coupled detector (CCD) array device.

It will be appreciated that the reading system of Fig. 6 is one embodiment of a variety of possible embodiments for reading an image formed by the micro lens array.

Referring to Fig. 7 herein, there is illustrated a surface photograph taken from above of a micro lens array. As shown in Fig. 7, the micro lens array comprises a substantially random pattern of lenses of variable sizes. Within the array, some lenses have a relatively larger width, and some lenses have a relatively smaller width. The whole of the surface of the substrate is covered by go lenses. Each lens is bounded by an outer perimeter, comprising a ridge.

Substantially, the whole of the region of the substrate is occupied by lenses, and the lenses abut each other sharing common ridge boundaries.

An amount of light reflected by each lens depends upon the surface area of as the lens, which is related to its size measured by its largest width span across the lens or its reflective surface area.

Referring to Fig. 8 herein, there is shown an image formed by the lens array of fig. 7. The image comprises a plurality of sub-images, each subimage so produced by a corresponding respective reflective lens. At the image plane, the I individual sub-images are in focus, and distinct from each other providing a high contrast image. At the focal plane, each lens generates a reflected image of the source illumination. The intensity of the reflected image from each micro-lens is related to the surface area of the micro lens which creates the reflected image.

Consequently, as can be seen by comparing the image of Fig. 8 with the lens structure of Fig. 7, where a relatively large lens occurs, light captured over the whole area of that lens is focused to a single image, which results in a relatively high intensity bright image, surrounded by a relatively dark area. Consequently, a level of image contrast in the focal plane is related to the sizes of the micro lenses. The larger the micro-lens is, then the brighter the corresponding image is, for a constant intensity illumination source.

Referring to Fig. 9 herein, there is illustrated schematically, the image of Fig. 8, under conditions of reduced intensity illumination. As the intensity of illumination is reduced, some of the sub-images produced by the relatively smaller lenses become invisible to the naked eye, and the image adopts, an appearance to the naked eye of having fewer light spots. However in reality, to an optical detector, the image will appear the same as in Fig. 8, but at lower intensity. The image detector can have gain applied to it in order to amplify the image.

For each different random lens array, a corresponding respective unique image pattern is produced as shown with reference to Figs. 7 and 8 herein. The image produced by a particular random micro lens array is unique to that random micro lens array. Since each random micro lens array is itself unique, then each lens array produces a unique image at its focal plane.

Fabrication There will now be described a method of fabrication of a micro lens array according to a specific method of the present invention.

Referring to Fig. 10 herein, there is illustrated schematically in cut away side view, a substrate layer 1001 prior to formation of a micro lens array. Various substrate materials may be used, but in this specific example, for convenience of description, an example of a silicon substrate layer is described.

Referring to Fig. 11 herein, the silicon substrate 1000 is coated with an amorphous carbon layer by a known physical vapour deposition (PVD) process, to a thickness of around 5 to 50 nanometers (nm), although this thickness may be varied outside this range as required.

Referring to Fig. 12 herein, there is illustrated schematically processing of a silicon substrate 1200 having an amorphous carbon etch resistant coating 1201 to create a region 1202 of micro lenses. The silicon substrate is placed in a vacuum chamber, and a substantially cylindrical tubular anode 1203 is placed over the amorphous carbon layer 1201 covering the silicon substrate. Typically, the anode may have internal diameter of the order of 4mm, although as will be appreciated by the person skilled in the art, various anodes of various sizes may be provided. The internal dimensions of the anode define the surface area of the region 1202 which is subjected to plasma etching.

The plasma is formed within the anode 1203, which etches into the etch so resistance amorphous carbon layer 1201. Typical plasma conditions are created at a pressure of around 10-2 Torr to 1 Torr, with an anode voltage of the order of 600 volts. The silicon substrate 1200, acts as a cathode.

Referring to Fig. 13 herein, there is illustrated schematically the silicon substrate with amorphous carbon layer coating in the plasma etched region 1202, in cross section, after plasma etching has commenced. The plasma etches into the etch resistant amorphous coating, breaking through the coating to the silicon substrate in various places. The places at which the plasma etches through to the silicon substrate are not predictable. The plasma etches through the amorphous carbon etch resistant layer initially at points of the layer which are weaker than other points, or which have minor defects in the etch resistance layer, which makes that position slightly more vulnerable to etching than the remainder of the layer. The positions at which the plasma etch break through the etch resistant coating is effectively uncontrolled, and substantially random.

Referring to Fig. 14 herein, there is illustrated schematically in cross section view, the same region of substrate as shown in Fig. 13, but at a more advanced stage of etching, where larger portions of plasma etch resistant coating have been etch away, and the plasma etch has begun to etch into the silicon substrate at various positions. Through-holes in the plasma etch resistant layer which have been etch away. At these positions, substantially crater like etch regions are To formed in the silicon substrate.

Referring to Fig. 15 herein, there is illustrated schematically the silicon substrate at a further stage of plasma etch, where the plasma has etched through a greater area of the etch resistant amorphous carbon layer. A greater proportion ls of the region of the silicon substrate hasbeen etched by the plasma, and residual islands of 1500 plasma etch may remain.

Referring to Fig. 16 herein, there is illustrated schematically the region of plasma etched silicon substrate, at a point when the etch resistant amorphous go carbon coating has been removed, and the plasma is continuing to etch directly into the silicon substrate. At this point, further continued plasma etching will tend to reduce the number of individual micro lenses, since the silicon substrate will tend to etch at the same rate across the region, and tend towards a flat surface.

Therefore, plasma etching must be terminated soon after the last remnant of the Q etch resistant layer has disappeared, in order to avoid destruction of the micro lens structure.

Referring to Fig. 17 herein, there is illustrated schematically the substrate region comprising the micro lens structure, after formation of a transparent 3 o window layer 1700 on top of the array of micro lenses. The transparent window layer fills the craters, and provides a smooth substantially flat surface which protects the micro lens crater structure, and provides a surface onto which a reading device may be place, in order to view an image reflected from the micro lens array, providing a reference datum for viewing of the image.

It would be appreciated by those skilled in the art, that formation of the craters, including the radius of curvature of the craters and the depth of the craters, may be variable, depending upon the type of plasma etch process used, the type of substrate material, the type of etch resistant layer used, and the thickness of the etch resistant layer. Other variable parameters include the anode voltage of the plasma, and the pressure in the plasma etch chamber.

Referring to Fig, 18 herein, there is illustrated an experimental plot of width dimension in 1lm and depth in Em for individual lenses represented by individual data points, for three difference silicon substrate samples which were fabricated.

In each case, the depth of the crater, measured as the distance from the deepest point of the crater to a line drawn between perimeter ridges across the widest dimension of the crater, increases approximately linearly with the width dimension of the crater.

o Referring to Fig. 19 of the accompanying drawings, there is illustrated a plot of radius of curvature of the reflective lenses against maximum width dimension (marked as diameter) in m, for 3 samples having width in the range 0.3 to 19 Em. For each sample, the relationship between radius of curvature of lens and width of crater is linear, and the radius of curvature varies only within a relatively narrow range as the diameter or largest width dimension of the reflective lenses increases, within each individual sample.

Each individual sample has a different absolute range of radius of curvature and diameter, which depends upon the plasma etch conditions, including pressure applied and anode voltage, as well as other factors including the thickness of the plasma etch resistant layer.

Because the radius of curvature is relatively invariant for a wide range of sizes of lens, and because there is a linear relationship between width of lens and radius of curvature, the focal length of a plurality of lenses on the same substrate varies only slightly from lens to lens, thereby enabling the lenses to focus within a narrow range of distances from an average deepest point of crater, with the result that an image from a plurality of micro lenses can be formed within a narrow range of parallel focal planes above the surface of the micro lens array.

JO Reading Methods and Equipment Referring to Fig. 20, there is illustrated schematically in end view, a second reader device for reading a light pattern reflected from a micro lens array. The reader device comprises a bundle of optical fibers arranged parallel to each other in close contact with each other, the bundle of optical fibres cut along a common plane to provide a flat end.

Referring to Fig. 21, there is illustrated schematically the second reader device in use in a second reading system. The bundle of optical fibres 2100 is o separated into two branches, so that some of the optical flares are drawn off into a first branch 2101, for transmitting light from a light source 2102 to the random lens array; and a second branch of optical fibres 2103 is drawn off and used to transmit light from the micro lens array to a detector device 2104, such as a charge coupled detector (CCD) device. Light passes from the light source 2102 as down the first branch 2101 to generally illuminate the substrate region having a micro lens array. The fibre optic bundle 2100 abuts the transparent window layer of the micro lens array device, and is placed at a position immediately opposite the array of micro lenses, and as close as possible to the focal plane of the miro lens array, which subsists at the surface of the transparent window layer, or a short distance outside the surface of the window layer in free space. The fibre bundle captures the image reflected from the micro lens array, and transmits it to the CCD camera. The CCD camera converts the two dimensional image into digital data which is input into an image processing device 2105.

The output from the CCD array comprises digital data describing a plurality of positions in X, Y Cartesian coordinates, i.e. a pixilated image data, along with a quantised intensity information representing a level of intensity of light at a particular X, Y coordinate. The CCD device may also produce red, green, blue (R. G. B) or cyan, magenta, yellow, black (CMYK) data in a known video image format. Where the light source 2102 is a broad band wave length, i. e. multi colour light source, reflected light from the micro lens array may undergo filtering through the transparent layer, or by selective reflection of the substrate layer, which depends upon a reflectivity response of the underlying substrate layer material. Consequently, a two dimensional image data having monochrome or ROB intensity level data in pixilated format may be obtained from the CCD camera for input into image processing device 2105.

Image Processinn In various specific implementations according to the present invention, different forms of image processing may take place. In one implementation, image processing may comprise storing the digitized reflected image from the micro lens array, for example as a jpeg format, or in an other photographic two dimensional format. Identification of the micro lens array is achieved by visual comparison of images.

In another implementation, information may be abstracted from the digitised image in order to simplify the amount of data stored to characterize the random lens array. For example a digital electronic filter may be applied to the data, such as to ignore any light spots having an intensity below a pre-determined level, 3 o thereby just highlighting the high intensity light spots. Using this method, has the advantage of reducing the amount of data which needs to be stored for each random micro lens array to characterize that array, but also increases the probability that two different random micro lens arrays will give rise to the same or an indistinguishable filtered digital image data. For example two random micro lens arrays may have relatively large reflector lenses in approximately the same X, Y positions as each other, but the rest of the array may differ significantly. If the information describing light reflected from the remainder of the array is I effectively filtered out, by selecting only the high intensity light spots in the image produced, then two different random lens arrays may give similar digitally filtered images. The skilled person will appreciate that the level of filtering and the I amount of data to be stored to characterize a particular random lens device is a design variable parameter. The higher the level of filtering, which is applied to the digital image, i.e. the more information which is discarded from the image data, then the higher the probability of misidentification of a random lens array. ! However, by maintaining a high resolution image and a relatively high amount of digitalised image data for each image, each random lens array is uniquely identified, but the data storage requirement, and data processing requirement to I read that data is relatively higher.

Further, for a second reading system as described herein, the fibre bundle itself applies some pre-filtering of the image data because the fibre bundle has a limited resolution due to its structure. Typically, each fibre optic has a mono mode or multi mode core of dimension of the range 1 to 50 1lm surrounded by a glass cladding layer which may be a few hundred Am radius. Consequently, the core area of each fibre optic cable is comparable, that is, the same order of magnitude as to a width dimension of the micro-lenses. Therefore some pre filtering of image data may occur through light being selectively collected along individual fibres.

In the best mode, the diameter of the fibre optic bundle exceeds a width of the micro-lens region, to ensure that all of the micro-lens region is imaged by the 3 o CCTV camera. i Referring to Fig. 22 herein, there is shown a third reading system according to a sixth specific implementation of the present invention. The third reading system comprises a read head having a pair of optic fibres each capable of receiving light when the ends of those optical fibres are drawn across the focal plane above a micro-lens device. The third reading system comprises a first I optical fibre 2200 for transmitting light from a light source 2201 to a surface of the transparent window above a random micro-lens array; a second optical fibre 2202 for collecting reflected light from the lens array and transmitting the reflected light to a detector device 2203; a scanning mechanism 2204 (shown dotted) for To controlling the first and second fibres to scan in a path along a surface of the transparent window, in the focal plane of the micro-lens array and opposite the micro-lenses; a controller 2205 for controlling the scanning device; and a data processor 2206 for processing the output of the detector.

Operation of the third reading system is as follows. The scanning device is I placed such that the first and second optical fibres are adjacent and opposite the micro-lens array, and in the focal plane of the microlenses. Controller 2205 controls the scanning mechanism 2204, which in the best mode may comprise a piezo electric (pzt) crystal, to draw the first and second fibres in a linear path 2 o across the micro-lens array. An alignment routine may occur prior to scanning, to ensure that the fibres are drawn across a same portion of the random micro-lens array, when reading for a first or subsequent time.

As the second fibre optic cable 2202 traverses across the surface of the 2 5 transparent window, within the focal plane of the micro-lens array, the optical fibre experiences light and dark regions, having intensities which depend upon the amount of light reflected from an immediately underlying micro-lens. Illumination is supplied via the first fibre optic cable 2200, reflected by an underlying lens, and a proportion of the reflected light is captured by the second optic fibre and o detected by detector 2203. An output of the detector 2203 is digitised, and input i into data processor 2206.

The data processor 2206 may collect data comprising a plot of detector output against distance, to provide a characteristic of the random microlens array.

Referring to Fig. 23 herein, there is illustrated schematically a plot of intensity verses distance along the surface of a transparent window material, obtained at the detector of the third reading system of Fig. 22 herein. Regions of relatively lighter and darker reflectivity are shown, which will be different for each individual micro-lens array, and therefore capable of charaterising the micro-lens To array. The data shown in Fig. 23 may be stored as digital data, to characterize a particular random lens array device.

Unique Product Identifier Device The random micro-lens array as described herein above, may find utility in the field of device identification and marking. In a further specific implementation of the present invention, there is provided a digital data storage device, for example a hard disk drive, or a solid state digital data storage device, having an applied random micro-lens array for the purpose of uniquely identifying the data So storage device. In various embodiments, a random micro-lens array may be externally fabricated and bonded to a data storage device, using a strong adhesive, such that the random lens array forms an identifier for a particular item of hardware.

A reading system may be provided built into the hardware device, for providing a digital data output representing the image pattern produced by illumination of the random micro-lens array device. When digitalised, the image pattern forms a unique digital signature data which uniquely identifies an individual hardware item, and because the digital image data has its origin in the 3 o random pattern of micro-lenses, no two data storage devices will have the same digital signature data.

Random Optical Pattern Generator In the field of computer science, random digital number generators already exist in the prior art, however these random number generators are not truly random, but are pseudo - random, and allow for some level of repetition and predictability. It is a difficult design problem to design a truly random number generator. Within the field of optics, including telecommunications and optical computing, for various optic signal processing applications, there is a need for a random optical signal generator. Likewise, in the field of optical signal o processing, design of a truly random optical signal generator is difficult.

A random micro-lens array device as described herein above, may be used as a source of a random optical signal. Generation of a two dimensional or a one dimensional random optical signal may occur by scanning an image formed by a micro-lens array using a two dimensional or one dimensional detector (as appropriate) across the surface of a transparent window layer, in the focal plane of the random micro-lens array device. The device is therefore capable of generating: 20(1) a two dimensional static random optical signal in the form of an array of light and dark regions randomly arranged; (2) a dynamically changing random optical signal, in the form of a two dimensional image of light and dark spots, as a two-dimensional image detector is drawn across the surface of the micro-lens array; and (3) a one dimensional dynamic optical signal, where a one dimensional detector, e.g. single fibre optic cable, is drawn in a path across the random lens array device.

Since the random lenses occupy a finite surface region of the substrate, an infinitely continuous dynamic random optical signal cannot be obtained, since at some point the reading device will need to re-trace a path which it has previously traced. However, for a limited burst of data, a random optic data signal, of either two dimensional or one dimensional form can be obtained.

Advantanes Specific implementations according to the present invention may provide a unique identification system based upon a random array of micro-lenses which is re-producibly readable, but in which an information content provided by the array To of micro-lenses is random and unforgeable.

Specific implementations described herein above may have an advantage of providing a high definition, high contrast image in response to illumination.

Specific systems disclosed herein may have an advantage of providing ease of readability of an optically generated image, in the form which can be easily digitalised using low cost reading apparatus.

A random micro-lens array as disclosed herein may have an advantage of being practically un-forgeable, whilst at the same time producing a high definition o high resolution image signal.

A key benefit of the specific implementation as described herein above is that given a particular code, it is exceedingly difficult to replicate a surface pattern, which can generate the same code. Therefore, copying of a mark based on a random lens array as herein described is extremely difficult. The lens array surface acts as a unique identification medium which is very difficult to replicate or forge.

In various applications in computer science, data and/or file systems are so restricted to exist on specified individual hardware devices. Particularly high security data such as passport details, citizen residence details and banking information are commercially sensitive, and therefore must be guarded against forgery. Since the random micro-lens array is generally un-forgeable, and provides a unique image pattern in response to illumination with light, the random micro-lens array device as described herein before can be used as a unique identification marker.

Claims

Claims 1. A substrate comprising a region formed as a plurality of lenses,

wherein said plurality of lenses are positioned substantially randomly across said region.
2. The substrate as claimed in claim 1, wherein individual said lenses have a width dimension in the range 0.5 Am to 250 Am.
3. The substrate as claimed in claim 1 or 2, comprising: a coating layer extending across said region, said coating layer being substantially transparent to light.
4. The substrate as claimed in claim 3, comprising: said coating extends along said region, wherein a thickness of said coating is selected such that a focal plane of light reflected from said plurality of lenses 2 0 subsists outside said coating.
5. The substrate as claimed in any one of the preceding claims, wherein individual said lenses have a width dimension in the range 10 Am to 100 Elm.
6. The substrate as claimed in any one of the preceding claims, wherein each said lens acts as a reflective lens, under conditions of external light illumination.

so
7. The substrate as claimed in any one of the preceding claims, comprising a crystalline material.
8. The substrate as claimed in any one of the preceding claims, comprising a metal.
9. The substrate as claimed in any one of the preceding claims, comprising a semi-conductor material.
10. The substrate as claimed in any one of the preceding claims, comprising a material selected from the set: a III - V compound; and a II - VI compound.
11. The substrate as claimed in any one of claims 1 to 9, comprising a material selected from the set: silicon; indium phosphide; gallium arsenide; 2 0 aluminium gallium arsenide; and sapphire.
12. An identification mark comprising a substrate as claimed in any one of the preceding claims.
13. An identification mark comprising: a substrate material, said substrate material having a region formed as a plurality of concave craters, wherein said plurality of craters are positioned 3 o substantially randomly across said region.
14. A substrate material comprising a surface region formed as a plurality for concave craters, wherein said plurality of craters are positioned substantially randomly across said surface region.
15. A method of forming a plurality of concave craters in a substrate, said method comprising: applying a plasma etch resistant coating to a surface of said substrate; and plasma etching through said plasma etch resistant coating and into a region of said substrate, such that said plurality of concave craters are formed in said substrate region.
16. The method as claimed in claim 15, comprising continuing said process of plasma etching through said etch resistant coating until said coating is completely etched away.
17. The method as claimed in claim 15 or 16, comprising: continuing plasma etching into said substrate, until said region of said substrate subjected to said plasma etch is completely covered by said plurality of craters.
18. The method as claimed in any one of claims 15 to 17 wherein said region is substantially circular.
19. The method as claimed in any one of claims 15 to 18 wherein said region is substantially cylindrical.
20. The method as claimed in any one of claims 15 to 19, wherein said plasma etch resistant coating comprises an amorphous carbon coating.
21. The method as claimed in any one of claims 15 to 20, wherein said plasma etch resistant coating is applied to said substrate by physical vapour deposition.
22. The method as claimed in any one of claims 15 to 21, wherein said craters have a width dimension in the range 0.5 to 250 m.
23. The method as claimed in any one of claims 15 to 22, wherein said JO craters are formed having a depth in the range 5 to 700 nanometers.
24. The method as claimed in any one of claims 15 to 23, wherein said I craters are formed as concave surfaces having a radius of curvature in the range 4to80,um. '
25. A method of forming a region of a plurality of substantially randomly arranged lenses in a substrate material, said method comprising: applying an etch resistant coating to a surface of said substrate; etching through said etch resistant coating and into said substrate, such that said plurality of lenses are formed in said region of said substrate.
26. The method as claimed in claim 25, comprising: continuing etching of said etch resistant coating, until said etch resistant coating is completely removed.
27. A method of applying an identification mark to a substrate, said so method comprising: applying an etch resistant coating to a surface of said substrate; and etching through said etch resistant coating and into said substrate, such that a plurality of craters are formed in said substrate.
28. A method as claimed in claim 27, further comprising: applying a light transparent surface coating layer across said plurality of lenses formed in said substrate region.
29. An identification system comprising: an identification mark comprising a reflective region capable of forming a substantially random array of light reflections; and a reader device comprising and an optical detector; wherein said reader device is capable of reading an optical image formed by said light reflected from said reflective region.
30. The identification system as claimed in claim 29, wherein said reader device comprises a plurality of optical fibres.
31. The identification system as claimed in claim 29 or 30, wherein said reader device comprises an optical mircroscope.
32. The identification system as claimed in any one of claims 29 to 31, comprising a charge coupled detector (CCD) array.

o
33. A reading system for reading a micro-lens array device, said micro lens array device capable of generating an image comprising a substantially randomly arranged plurality of contrasting lighter and darker regions, said reading system comprising: illumination means adapted for illuminating said plurality of micro-lenses; optical guidance means for capturing an image formed by said plurality of micro-lenses; and detector means for detecting said captured image.
34. The system as claimed in claim 33, further comprising: image processing means for processing said captured image as digital data.
35. The system as claimed in claim 33 or 34, further comprising: alignment means for aligning said optical guide with said plurality of micro lenses.
36. The system as claimed in any one of claims 33 to 35, further comprising: scanning means for moving said optical guidance means to scan across said image.
37. A method of reading an image formed by a plurality of micro- lenses, said method comprising: so arranging an optical instrument to capture at least a portion of an image formed at a focal plane adjacent said plurality of micro-lenses; arranging an optical detector device to capture a light output of said optical instrument; and digitising an output signal of said detector device to provide a digital image data representing said image.