GB2403798A

GB2403798A - Evaluating the quality of a hologram

Info

Publication number: GB2403798A
Application number: GB0315609A
Authority: GB
Inventors: David Roy Winterbottom; Ben Bowmaker
Original assignee: Ver Tec Security Systems Ltd
Current assignee: Ver Tec Security Systems Ltd
Priority date: 2003-07-03
Filing date: 2003-07-03
Publication date: 2005-01-12
Anticipated expiration: 2023-07-03
Also published as: GB0315609D0; GB2403798B

Abstract

A metallised hologram or diffractive optical variable device (DOVD) is applied to e.g. a banknote for security reasons. The hologram quality can be checked during production of the banknote by forming an image of the metallization pattern of the hologram using camera 440. To produce this image, the hologram is illuminated from behind using diffuse lighting from light source 408. The metallization pattern is then compared to a reference pattern to determine the quality of the metallization of the hologram. In addition, a second light source 424 may illuminate the hologram from above, producing the diffractive holographic image. This diffractive image is captured by a second camera 444. Information from the diffractive image may be used in association with information from the metallization image to give a further indication of the quality of the hologram. The apparatus shown is suitable for use in a banknote production line, where a sheet 402 of notes with metallised holograms is passed through the apparatus. The apparatus may also be used to test holograms on a tape prior to application to the banknotes.

Description

1 2403798 Apparatus and methods for device quality evaluation This

invention generally relates to apparatus and methods for evaluating diffractive optical variable devices (DOVDs) such as holograms, for example for quality control of a bank note production process.

It is known to use a diffractive optical variable device (DOVD) such as a hologram as an anti-counterfeiting device on packaging, labelling, and paper or plastic-based security documents such as passports, visas, identity cards, driving licenses, government bonds, bank notes and the like. Thus a DOVD may be provided on a substrate and applied to the document, labelling, or packaging to make counterfeiting impossible or uneconomic. Broadly speaking a DOVD has a plurality of light diffracting features which create a variable image as lighting or viewing conditions change, to hinder reproduction with a photocopier or scanner. A DOVD is generally capable of creating a plurality of graphic elements, often in three dimensions, which may change according to the angle of observation andlor illumination. In this way kinetic effects such as the appearance and disappearance of graphic elements, animations and image transformations may be created (sometimes termed a Kinegram - trademark). An image may also vary in contrast and/or brightness with viewing angle, typically converting from a positive to a negative image (sometimes termed a Pixelgram - trademark).

In this specification references to light and to optics are to be interpreted as including ultraviolet and infrared light and optics as well as visible light and optics. Thus a DOVD may incorporate 'hidden' elements, for example only apparent when viewed in the infrared.

A DOVD such as that described above typically comprises a surface relief structure and when embossed may be referred to as an embossed hologram. Embossed holograms can be mass produced as described further below. Generally the surface relief structure is metallised, for example by covering the structure with a thin layer of aluminium. A master surface relief structure for mass producing an embossed DOVD may be created by conventional holography or by directly writing a fringe structure, for example using an electron beam. The abbreviation DOVD (diffractive optically variable image device) is sometimes used in preference to the term 'hologram' since the former is more strictly correct. However in this specification 'hologram' is employed in its commonly used looser meaning to include directly written structures and embossed structures as well as structures created by the interference of light recorded on lightsensitive film. We are particularly concerned with holograms (DOVDs) which form an image on or in close proximity to the hologram plane.

The diffracting structures of DOVDs used for anti-counterfeiting purposes generally create 'holograms' which are positionally associated with the DOVD so that a particular graphic element is associated with a particular portion of the DOVD. More than one graphic element may be associated with the same portion of the DOVD, and a reconstructed graphic element need not relate to an object which has had any real existence.

For additional security it is known to pattern the metallisation of a DOVD, generally by partially demetallising the DOVD 'foil' either by laser ablation or chemically, for example using a resist and etch process. In this way fixed, or in some instances variable, information may be securely included with the hologram. Generally bank notes use a fixed demetallisation pattern for each note denomination, at a resolution which is visible to the unaided eye. However higher resolution demetallisation patterns may also be employed, for example to provide a semi-transparent film or halftones.

A typical process for producing a demetallised embossed hologram and some of the properties of security DOVDs will next be outlined.

Referring to figure 1 a hologram 100 typically comprising a light or electron sensitive resist 102 on a glass substrate 104 has a coating of silver and then nickel electrode deposited to form a stamper plate 106. The initial hologram 100 may comprise a second generation hologram recorded by illuminating a photoresist with a reference beam and a reconstructed image from a master hologram so that a lateral position in the second generation hologram is associated with a lateral position in an image it creates.

Alternatively a hologram 100 may be directly written using electron beam lithography, for example to create an optically variable image such as a Kinegram (trademark) or Pixelgram (trademark).

The stamper plate 106 is then wrapped around a rotating drum 108 to emboss the diffraction structure onto a polyester film 110 moving in the direction of arrow 112.

This embossing process is performed carefully as the diffracting structures of the hologram have a typical spatial frequency of around 1000 lines per millimetre to be reproduced in film 110. The embossed film 110' is then coated with aluminium 114, typically by sputtering, to form a tape 116 (shown in plan view) comprising a plurality of metallised holograms or DOVDs 118, which can be cut into a length and stored on a reel. Typically 150mm web is slit into four or five and tape 116 has a width of 3 or 4 cm and may accommodate more than one hologram 118 across its width; sometimes the web may be lm across or more. The metallised surface of tape 116 may be provided with an adhesive to allow the holograms to be used for labelling or for hot stamping.

The holograms 118 may be selectively demetallised by means of a photo-or printed resist process using a caustic alkali such as sodium hydroxide to remove the aluminium.

This leaves demetallised tape 116' comprising the clear polyester substrate carrying demetallised holograms 120. An enlargement of one such demetallised hologram is shown together with one set of diffraction images, in this example a plurality of '5's.

Typically a registration mark 124 such as a 5 mm square of (diffractive) metallisation is provided for alignment purposes.

A dernetallised hologram (without the registration mark) may then be attached to an article such as bank note 126 by hot stamping. Thus the aluminium metallisation may be provided with a hot melt adhesive and polyester film 110 may have two layers separated by a release layer such as wax. The film can then act as a carrier for the holograms which may be embossed into some other material such as resin.

Alternatively holograms 120 may be cut from tape 116' prior to application to an article.

There is a need for technology to facilitate quality control of demetallised holograms at a variety of stages. For example it is desirable to be able to check the quality of holograms on tape 116' as they are produced (typically the demetallisation is a continuous process applied to a moving tape), as well as prior to and following application to a security document such as a bank note. Generally bank note production takes place at a different location to that at which the holograms 120 are produced and it is therefore desirable to be able to check the quality of incoming reels of holographic tape and the quality of applied holograms both before and after printing where, as in some cases, print overlies a hologram. It is further desirable to be able to perform forensic' checks and to monitor the quality and validity of bank notes and other security documents in circulation, or on withdrawal from circulation.

It is helpful briefly to outline a typical bank note production process. The substrate on which bank notes are printed is generally paper although some countries such as Australia and Singapore employ plastic. Following an initial offset printing stage optically variable ink (OVI) and then optically variable devices (OVDs) are then applied and checked by eye. Intaglio printing is then applied and the bank notes are then stored for back end processing such as numbering and cutting. In a typical bank note printing process sheets of notes are printed at high speed, for example 10,000 sheets per hour with eight columns of notes (in a direction perpendicular to a direction of motion of a sheet) per sheet. This implies a production rate, in this example, of 22 notes per second per column or approximately 40 milliseconds to check each note, allowing for a margin of error. With a 650 mm sheet travailing at 1.8 metres per second approximately one second is available to make a decision on whether to accept or reject a sheet prior to the sheet reaching a storage stack. The back end storage is secure and with manual checking, which is slow, a considerable volume of poor quality notes can build up in the storage before errors are detected, and secure disposal of a large batch of rejected notes can be very expensive and difficult. From the foregoing discussion it can be seen that it is desirable to be able to inspect the quality of holograms in real time at a rate of approximately 25 per second, with a maximum latency of preferably less than one second.

It is helpful to next review some of the properties of a typical diffractive optically variable device. Figures 2a and 2c show two views of a DOVD 200 which in figure 2a shows a first image, in this case a note denomination ('5') and in figure 2c a second image, in this case an image of Britannia symbolised by the letter B. In this example selection of the first or the second image is performed by rotation of the DOVD 200 about a vertical axis 202 which, for a second generation hologram in effect corresponds to looking through a master hologram 204 in one of two directions 206a, b (figure 2d).

As can be seen in figure 2a different portions of the first image are associated with different lateral positions on the DOVD 200 as different (lateral) portions of the virtual object are recorded in different positions in the plane of the hologram.

Figure 2b shows, in effect, a cross-sectional view of the hologram showing the physical plane of the hologram 200 which corresponds to the virtual object plane of the hologram. Both the '5' of figure 2a and the 'B' of figure 2c are in the plane of the hologram but, as illustrated in figure 2b, additional images may be present in a plane 208 in front of the plane of the hologram (which here is also termed the focal plane) and in a plane 210 behind the plane ofthe hologram. In practice it is rare for additional images to be provided in front of the plane of the hologram but one or more images may be provided behind the hologram, for example a fan or line pattern 212. Typically such an image is 1 or 2 mm behind the plane of the hologram and when viewed at any one angle generally appears in a different colour from that of an image in the plane of the hologram. As with a conventional view of an object such an image, in the background, may be obscured by an image in the foreground. A background (or foreground) image may be removed by filtering.

Referring again to figure 2a rotation of the DOVD about axis 214, perpendicular to axis 202, changes the colour of the first image, for example from blue to red. Under broadband illumination a so-called 'vertical channel' image may appear at extreme angles. As the DOVD 200 is rotated away from a perpendicular viewing direction the grating spacing of the diffracting structures effectively decreases and thus an image which is present in the infrared may appear in the visible at large angles of rotation thus giving rise to a flick-in effect. Similarly an unwanted foreground or background image may be removed by tilting the DOVD until the unwanted image effectively moves into the ultraviolet. It will further be appreciated that when DOVD200is viewed under narrowband rather than broadband illumination an image such as the '5' of figure 2a or the 'B' of figure 2c will only appear at an angle matched to the wavelength of the illumination. Thus, in effect, the wavelength of illuminating light may be used to select a viewing angle or, in a related way, to select a recorded DOVD diffraction image.

DOVDs such as that illustrated in figure 2 may suffer from a number of defects and many of these are related to the quality of the metallisation of the DOVD film or foil, particularly in a DOVD which has been subject to a demetallisation procedure. Thus, for example, the thickness and opacity of the metallisation and the integrity of demetallisation detail - for example the fuzziness of edges - can vary. The relative positions of the diffractively generated images of the DOVD and the metallisation/demetallisation pattern can also vary, as can be appreciated from figure 2a.

Where a DOVDis applied to a security document the relative position of the demetallisation pattem, in effect the position of the applied hologram, can also vary with respect to printing applied to the document either before or after the hologram.

Background prior art relating to the authentication of optically diffracting structures can be found in US4,544,266 and WO92/01975. A bank note fitness inspection system is described in 'Optical inspection techniques for security instrumentation', RL van Renesse, Proceedings SPE Volume 2659,pl59-167 but this system is explicitly stated to be not fit for inspection of highly reflective features such as OVDs. There is therefore a need for apparatus and methods for evaluating pattemed, metallised diffractive optically variable devices (DOVDs).

According to a first aspect of the present invention there is therefore provided apparatus for evaluating a metallised diffractive optically variable security device (DOVD), the apparatus comprising a first light source configured to illuminate said DOVD; a first image capture device configured to capture an image of a metallisation pattern of said DOVD using light transmitted through said DOVD and to provide metallisation pattern image data; and a data processor coupled to said first image capture device to receive said captured image data, configured to evaluate said patterned DOVD by comparing said image data with data for a metallisation reference image, said metallisation reference image comprising a reference version of at least a portion of said metallisation pattern.

It has been recognised that capturing an image of a DOVD metallisation pattern by backlighting the metallisation and then comparing this with a reference or ideal pattern image facilitates the evaluation of a number of parameters of a metallised DOVD, in particular a metallised embossed DOVD. For example dimensional changes, skew and other distortions may be detected in a relatively straightforward manner, and it has also been found that the metallisation opacity variation is related to hologram degradation.

Thus the data processor is preferably configured to evaluate at least one of metallisation thickness, metallisation uniformity, and metallisation distortion responsive to the image comparison. However in another aspect of the invention the image comparison is replaced by a direct evaluation of the metallisation pattern, for example by evaluating or determining a measure of the sharpness of edges within the captured image of the metallisation pattern.

In embodiments the metallisation may be examined at a plurality of wavelengths, for example using broadband back illumination. A plurality of image capture devices may then be employed to capture a plurality of images at the different wavelengths, for example by filtering sensors of the devices, preferably concurrently to reduce differential positioning effects which can occur with serial image capture from a fast moving production line. A light level sensor may also be employed to compensate for light absorption by a substrate, such as a paper or plastic substrate, carrying the DOVD.

Preferably the apparatus also includes means for determining when the DOVDis in focus for the image capture device and/or means for controlling the image capture device focus.

The apparatus may further include means for compensating for light absorption from any printing or other patterning of a substrate on which the DOVD is mounted, for example by matching to a known printing pattern and then correcting the captured metallisation pattern image by subtracting an estimated absorption.

In a preferred embodiment the data processor further includes means for storing a result of the evaluation of the DOVD, preferably a quantitative measure of the DOVD, with a time and/or date stamp or in a time series data store to facilitate trend analysis. This is particularly useful in a production line where gradually reducing quality of a DOVD or security document may be monitored and corrective action taken before the quality falls below a permitted threshold level.

In an embodiment adapted for installation in-line in a manufacturing process the apparatus preferably also provides an evaluation output signal such as a reject signal, to indicate in substantially real time when a DOVD falls below a quality threshold. This signal may then be used to halt or divert the output of a production line. In such an embodiment the plastic or paper substrate to which the metallised DOVDs are applied will generally be moving rapidly on a system of belts, rollers and/or supports, in which case the apparatus is preferably configured to be installed around such an existing arrangement, preferably within an enclosure to reduce or substantially exclude ambient light.

The data processor may comprise a personal computer or workstation, for example with one or more DSP (digital signal processing) cards to facilitate real-time operation. The image processing functions may be pipelined to take advantage of any latency which may be available in providing an evaluation signal output whilst maintaining a necessary degree of throughput. Thus in the example described above a throughput of image captures/evaluations per second is needed but up to one second is potentially available at least for the image evaluation. In some embodiments where particularly rapid processing is needed data processing functions may be provided in hardware, for example using an FPGA (field programmable gate array) rather than in software, or a combination of dedicated hardware and software may be employed.

It has been found that a particularly useful parameter for evaluation equality of a patterned metallised DOVDis a determination of the relative positions of the metallisation pattern and an image, here referred to as a diffraction image, generated by diffraction of light by the DOVD. Thus a particularly preferred embodiment includes a second light source configured to illuminate the viewing side of the DOVD to generate such a diffraction image, and the data processor is configured to determine the position of one of the metallisation pattern and the diffraction image with respect to the other to generate evaluation data such as an accept/reject signal. The diffraction image may be captured by the same image capture device used to capture an image of the metallisation pattern, for example by controlling the first and second light source to illuminate the DOVD at different times and, if necessary, moving the image capture device according to which light source is illuminated. However such a technique is relatively slow and cumbersome although cheaper to implement and it is therefore strongly preferable in the context of apparatus for monitoring a production line that two image capture devices are employed, one to capture the image of the metallisation pattern and a second to capture one or more diffraction images generated by the DOVD. Employing two image capture devices such as two digital cameras allows the metallisation pattern image and diffraction image to be captured substantially simultaneously, which is important when determining the relative position of these two images in the context of a production line in which the DOVDs are moving relatively quickly. Furthermore the use of two image capture devices and two illumination sources facilitates the positioning of these devices and, optionally, filtering of one or more of the first and second light sources and the first and second image capture devices, to provide some optical pre-processing of the captured signals. Thus, for example, the second light source and second image capture device may be positioned to select a desired diffraction image generated by the DOVD thus reducing or eliminating the need for more expensive and time consuming digital signal processing. Some techniques for selecting one or more DOVD diffraction images have already been described with reference to figure 2 above, and it will be recognized that selection of an image may be made by illumination/viewing angle andlor illumination/viewing wavelength (including waveband) selection.

In such an embodiment the data processor may determine a position of the metallisation with respect to a diffraction image by determining positions of the metallisation pattern (or a portion thereof) and a captured diffraction image (or a portion thereof) by comparing the metallisation and diffraction images with reference images, then comparing the two positions to determine a relative position of the two images. It will be recognised that employing two separate image capture devices the relative positions of which can be calibrated beforehand considerably simplifies this procedure.

In preferred arrangements the second light source is configured to provide a substantially collimated beam of light, in effect to appear as a distant point source, to facilitate reproduction of just a desired one or a few diffraction images of the DOVD.

Preferably the direction of this collimated light is offset from a direction used by the first light source and image capture device to capture an image of the metallisation pattern in transmission. This reduces the effect of specular reflection from a metallised DOVD into the metallisation image capture device (although some residual scatter may still be present due to effects such as surface roughness of the substrate or carrier of a DOVD). The second light source may further be configured to illuminate the DOVD at such an angle as to select a single recorded image (in the plane of the DOVD) that is, for example, to substantially select between the images shown in figures 2a and 2c above, although without necessarily removing, say, background images such as the illustrated fanning line pattern, which may be addressed by additional filtering. To reduce mutual interference between the metallisation pattern image capture system and the diffractive image capture system filtering may be employed, for example capturing one image at a first wavelength or waveband and another image at a second wavelength or waveband. For example the metallisation pattern image capture system may be filtered to reduce the illumination at a particular wavelength and the diffraction image capture system may be filtered, say at the camera, so that the camera has an increased relative sensitivity at this wavelength. As previously mentioned the metallisation pattern image and the diffraction image are captured substantially simultaneously and in embodiments this may be accomplished by controlling the timing of the illumination of the DOVD from the first and second light sources. Thus, in a preferably low ambient light level environment, the first and second light sources may be controlled to illuminate the DOVD substantially simultaneously, although for ease of processing this may be implemented by separate hardware rather than as an additional control process of the data processor.

In embodiments of the apparatus with a second image capture device for capturing the diffraction image this second image capture device will generally be offset at an angle to a perpendicular direction to the plane of the DOVD. Capturing an image at an angle in this way can introduce a perspective distortion which can affect the evaluation of the DOVD, for example by affecting position determination by comparison with a reference image. A perspective correction may be made in software using a standard library procedure but this imposes an additional burden on the data processor where, in a system for a production line, any additional burden is undesirable, making real time operation harder. In a preferred embodiment of such an apparatus, therefore, a perspective-correcting lens is employed for the second image capture device.

Depending upon the distances and angles involved the perspective distortion may only be a second order effect and therefore the perspective correction need not be complete although the better the correction the potentially more accurate the DOVD evaluation.

Such a perspective correction may be made using an off-axis lens intended to maintain a captured image of substantially constant size despite changes in viewing angle.

Alternatively an image capture device moving in an arc with its centre at the DOVD can provide a suitable image, particularly when a camera with a relatively small aperture or high f number such as fit and above is employed. In this latter case a known viewing angle can be employed to perform a digital perspective correction of the captured image in either hardware or software.

In a related aspect the invention provides apparatus for evaluating the quality of an embossed hologram by demetallisation detection, the apparatus comprising means to illuminate said hologram from behind; means to capture an image of a demetallisation pattern of said hologram; and means to evaluate a quality of said hologram using said captured demetallisation image.

The invention further provides apparatus for evaluating a metallised diffractive optically variable security device (DOVD), said DOVD having a viewing side for viewing a variable image generatable by said DOVD and a back side, the apparatus comprising a first light source configured to illuminate said back side of said DOVD; a second light source configured to illuminate said viewing side of said DOVD to generate a DOVD diffraction image; a first image capture device configured to capture an image of a metallisation pattern of said DOVD using light transmitted through said DOVD from said first light source to provide metallisation pattern image data; a second image capture device configured to capture said DOVD diffraction image; and a data processor coupled to said first image capture device to receive said captured metallisation pattern image data, and to said second image capture device to receive said captured DOVD diffraction image data, and to determine a relative position of said metallisation pattern and said diffraction image from said captured image data to generate evaluation data for said DOVD.

According to another aspect of the present invention there is provided apparatus for evaluating a metallised security device, the apparatus comprising a first light source configured to illuminate said metallised security device; a first image capture device configured to capture an image of a metallisation pattern of said metallised security device using light transmitted through said metallised security device and to provide metallisation pattern image data; and a data processor coupled to said first image capture device to receive said captured image data, configured to evaluate said patterned metallised security device by comparing said image data with data for a metallisation reference image, said metallisation reference image comprising a reference version of at least a portion of said metallisation pattern.

In another aspect the invention provides a data processing system for determining the quality of a metallised diffractive optically variable security device (DOVD), the metallisation of said DOVD being patterned, the system comprising an image input for receiving data for an image of said metallisation pattern of said DOVD; data memory for storing a metallisation reference image for said metallisation pattern; program memory storing processor control code; and a processor coupled to said first image input, to said data memory, and to said program memory for loading and implementing said processor control code, said code comprisingcode for controlling said processor to input a metallisation pattern image of said DOVD; retrieve said metallisation reference image from said data memory; match said metallisation pattern image to said metallisation reference image; determine quality data for said DOVD responsive to said matching.

In a preferred embodiment the system further includes an image input for receiving data for a diffractively generated image formed by the DOVD, although in embodiments a single image capture card may be employed with switchable first and second inputs for capturing images from first and second digital cameras. These cameras may themselves have captured the metallisation pattern and diffractively generated images of the DOVD substantially simultaneously, for example using strobed illumination. Preferably the data processing system code further comprises code for comparing data for a diffractively generated image formed by the DOVD with a reference image to generate position data for use in determining a relative position of the diffractively generated image and the metallisation pattern. This position data may be used directly or indirectly to provide quality data for the DOVD. Where an additional image such as a print image is also present, for example on a substrate carrying the DOVD, this additional image may also be located by matching with a reference image and then subtracted from an image such as the metallisation pattern image and/or the diffractively generated image to provide a correction prior to determining the DOVD quality data.

Alternatively the relative positions of the metallisation pattern and additional image may themselves be used to generate the quality data, with or without reference to the diffractively generated image. Where the additional image is used to correct the metallisation image this correction is preferably performed prior to a position determination of the metallisation pattern, although in other embodiments the correction may simply be applied, for example, as a correction to a determination of metallisation opacity or uniformity. Preferably the quality data is stored as a time series in data memory such as a non-volatile data store to facilitate on-line or off-line data analysis.

For example the code may further comprise code to determine a trend in the quality data over time and then to extrapolate this trend to determine whether or when a DOVD quality is predicted to fall outside a tolerance threshold.

In a first related aspect the invention provides a method for determining the quality of a metallised diffractive optically variable security device (DOVD), the metallisation of said DOVD being patterned, the method comprising inputting a metallisation pattern image of said DOVD; retrieving a reference image of said metallisation pattern image from a data store; matching said metallisation pattern image to said metallisation reference image; determining quality data for said DOVD responsive to said matching.

In another related aspect the invention provides a method of evaluating a demetallised embossed DOVD, the method comprising comparing a position of a demetallisation pattern of said DOVD with a position of a diffractively generated image from said DOVD; and determining a quality measure for said DOVD responsive to said comparing.

Preferably the method employs a pair of image capture devices to substantially simultaneously capture the diffractively generated image and an image of the DOVD metallisation pattern. Preferably the method includes capturing these two images from fixed relative positions as for a fixed diffractively generated image capture position the captured image is substantially fixed with respect to the physical position of the embossed DOVD.

In a further related aspect the invention provides a method of determining a quality measure for a demetallised embossed DOVD, the method comprising matching a pattern of said demetallisation with an ideal pattern of demetallisation for said DOVD; and determining a quality measure for said DOVD responsive to said matching.

The invention further provides a method of monitoring a bank note production process, bank notes produced by said process each bearing an embossed, demetallised DOVD, the method comprising capturing an image of a DOVD demetallisation pattern of a said bank note; comparing said demetallisation pattern image with a reference demetallisation pattern image; and providing a control signal for said production process responsive to said pattern comparing.

Preferably the method is performed in substantially real time, optionally using a pipelined process for image capture and image processing so that one image is captured whilst a previously captured image is processed. In a preferred embodiment the control signal comprises a reject signal to control the production line, for example to redirect a note or sheet of notes or to mark a note, sheet or batch as rejected.

In a still further aspect the invention provides a method of monitoring a bank note production process, bank notes produced by said process each bearing an embossed, demetallised DOVD, the method comprising capturing a holographic image generated by a said DOVD; capturing an image of a DOVD demetallisation pattern of a said bank note; comparing a relative position of said holographic image and said demetallisation pattern image; and providing a control signal for said production process responsive to said image position comparing.

The invention further provides a data processor configured to operate in accordance with the above described methods. The data processor may comprise a suitably programmed computer system, or hardware such as an FPGA or ASIC (application specific integrated circuit), or a combination of the two.

In a still further aspect the invention provides the above described data processing system processor control code, and in a further aspect processor control code to implement the above described methods. Such processor control code may be provided on a data carrier such as a disk, CD- or DVD-ROM, programmed memory such as read- only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. The processor control code may comprise code in a conventional programming language such as C, or microcode, or code for setting up or controlling an ASIC or FPGA, or code for a hardware description language such as Verilog (trademark), VHDL (very high speed integrated circuit hardware description language), or SystemC. As the skilled person will appreciate such code (and/or data) may be distributed between a plurality of coupled components in communication with one another, for example on a network.

These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows an example of an embossed demetallised diffractive optically variable image device (DOVD) fabrication process; Figures 2a to 2d show, respectively, a first image of a DOVD, image planes of a DOVD, a second image of a DOVD, and an illustration of an origin of the diffractively generated images of figures 2a and 2c; Figures 3a and 3b show, respectively, a bank note production line incorporating DOVD quality inspection apparatus, and a plan view of a bank note sheet transport arrangement; Figure 4 shows DOVD evaluation of apparatus according to an embodiment of the present invention; Figures Sa to Sc show, respectively, a schematic illustration of a plan view of the apparatus of figure 4 and DOVD diffraction images obtained therefrom, a shift lens arrangement for the DOVD camera of figure 4, and a stylised example of a DOVD metallisation pattern image obtainable using the apparatus of figure 4; Figures 6a and 6b show first and second examples of DOVD registration mark sensing apparatus; Figure 7 shows a block diagram of a data and control processor and associated interface and control systems for the apparatus of figure 4; Figure 8 shows timing waveforms for the data processing and control apparatus of figure 7; Figures 9a and 9b show a flow diagram illustrating operation of the data and control processor of figure 7; and Figure 10 illustrates a procedure and programme code for analysing DOVD evaluation data.

Referring to figure 3a, this shows a schematic illustration of a portion of a bank note production line 300 incorporating DOVD quality control apparatus 302 housed within a light restricting enclosure. A sheet of bank notes 304 on the production line passes through the QC apparatus 302, for example via light restricting baffles, and then progresses on to secure storage and back end processing facilities for further printing, numbering, note cutting and the like. As illustrated the sheet of bank notes comprises two columns of notes across the width of the sheet and the housing of apparatus 302 preferably incorporates one set of DOVD evaluation apparatus per column. There is approximately 1/25 of a second between each note in a column available for capturing one or more images of a DOVD for evaluation. Figure 3b shows a plan view of a portion of the production line illustrating one example of a transport arrangement for a sheet of notes comprising, in this case, sets of pulleys 306a, b linked by belts 308a, b supporting the sheet of notes. An arrangement for inspecting DOVDs on a tape prior to application to a substrate is similar to that shown in figure 3a but on a smaller scale as the width of a reel of DOVD tape is typically only a few centimetres.

The DOVD evaluation measurements performed by apparatus 302 may include an evaluation of DOVD metallisation thickness, metallisation uniformity and integrity of metallisation detail by inspection, for example, at a single wavelength or over a range of wavelengths, or at a plurality of separate wavelengths. It is further desirable to make a detemmination of the relative position of the demetallisation pattern and one or more diffractive images fommed by the DOVD; optionally the demetallisation pattern position may be inferred from a registration mark associated with the DOVD.DOVDs may be evaluated on a tape substrate, for example following production of a tape or on receipt at a bank note production facility, and it is also preferable to evaluate DOVD quality after the DOVD has been applied to a substrate such as a bank note. In this latter case there is generally no specific registration mark for detemmining the relative position of a demetallisation pattern and DOVD diffractive image making such evaluation more difficult. Furthemmore since on some substrates such as bank notes printing may be applied either under or over the DOVD the evaluation process may be further complicated, and an additional evaluation parameter, that is a detemmination of the relative position of the DOVD demetallisation pattern and the printing may also be desirable.

Referring next to figure 4, this shows one embodiment of DOVD evaluation apparatus suitable for in-line installation in a bank note production line for evaluating a DOVD after application to a bank note; similar apparatus may be employed for example for evaluating DOVDs on tape. A moving substrate 402 such as a sheet of bank notes is carried on rollers 404 or a belt drive, optionally with additional guides 406 and/or air pressure to ensure that the surface of substrate 402 between rollers 404 remains substantially flat to within less than lmm (optionally guide wheels may be provided in the gaps between the notes; less preferably the substrate may be flattened over a glass bed. A DOVD on substrate 402 is illuminated from beneath, that is from a back or non- viewing side of the DOVD by underside illumination apparatus 408 providing diffuse back illumination. Apparatus 408 comprises an illumination source 410 such as a Tungsten lamp, Xenon strobe or laser followed by an optical system 412 which controls the area over which the light is directed and, optionally, provides filtering. Light from optical system 412 impinges upon a diffusing plate 414, for example of ground glass, and thence on the underside of substrate 404. To control the underside illumination apparatus 408 is preferably provided with an ambient light restricting enclosure 416. A control signal 418 controls the timing ofthe illumination, for example by determining when illumination source 410 is on and off end optionally allows selection of a filter within optical system 412, for example by means of a filter wheel, to select one or a range of wavelengths for back illumination of substrate 402. Typically the back illumination wavelength or waveband is in the visible or near infrared, in particular for a polyester film substrate which attenuates strongly in the ultraviolet. Preferably underside 420 and top 422 light level sensors such as photocells are included to provide corresponding reference illumination level signals from below and above substrate 402 respectively.

Illumination apparatus 408 illuminates substrate 402 from beneath to provide a shadow demetallisation pattern image when the substrate is viewed from above, for example by a camera. A second illumination source 424, with a control input 425 for controlling the timing of the illumination it provides, is provided above substrate 402 to illuminate the DOVD for capturing a diffractively generated image of the DOVD.To generate a diffractive image of good quality illumination source 424 preferably provides a substantially collimated output light beam 426 which facilitates recreation of only selected images recorded in the DOVD. In particular use of collimated illumination reduces blurring of background images such as image 212 of figure 2. Illumination source 424 is preferably a broadband illumination source although a narrow band source such as a light emitting diode or laser, or filtering may optionally be employed. As previously described with reference to figure 2 use of a broadband source allows selection of a colour in which to view a DOVD diffraction image by selecting a viewing angle, and filtering at a viewing camera may then be employed to reduce interference from underside illumination apparatus 408. To further reduce crosstalk the underside illumination may be filtered, for example to provide blue underside illumination when viewing an orange/red DOVD diffraction image. In embodiments of the apparatus the lateral position, height and angle of illumination of illumination source 424 with respect to substrate 402 may be varied, for example to facilitate setting up the apparatus, although once installed in a production line the illumination position and angle will generally be fixed.

The top or viewing side DOVD illumination source 424 may also be employed to provide illumination for detecting a DOVD registration mark, for example by means of a small diagonal mirror 428 and a registration detector 430 to detect specular reflection from a registration mark and provide an output signal 432 for controlling image capture timing by determining when a DOVD to be inspected is at a desired position within the apparatus 400. Registration detector 430 may be configured remotely using line 432 by a data processing system coupled to the apparatus. Where a registration mark is not available a trigger signal 434 may be derived from a roller 404 or from, for example, a stepper motor drive coupled to the roller. The apparatus may also incorporate a z- position sensor 436 either to provide a z-position output 438 or a z- signal to indicate when the substrate 402 is outside a permitted height tolerance. The z-position sensor 436 may comprise a radar ranging device or a 'Barnes Wallace' type device where the coincidence of two light spots from beams of light angled towards one another indicates that substrate 402 is at the correct height for camera focus.

A shadow image of the dametallisation pattan of a DOVD is captured by a demetallisation camera 440 with a data and control link 442 to output image data and to receive camera configuration data and an image capture trigger signal. Data link 442 may comprise, for example, a fast parallel bus such as CamLink or Firewire (trademark). Preferably demetallisation camera 440 has a viewing direction substantially perpendicular to the plane of substrate 402 for reduced distortion. The distance of camera 440 from substrate 402 may be varied according to the parameters of the apparatus such as illumination level, size of image to be captured, vertical motion of substrate 402 (a larger distance providing a better depth of focus) and the like. In one embodiment demetallisation camera 440 is positioned approximately 150mm from substrate 402 and has a wide angle Super Angulon (available from Schneider Optics, Inc. of NY, USA) wide angle lens. Where illumination source 424 has a limited range of wavelengths demetallisation camera 440 may be filtered to reduce the camera's sensitivity at these wavelengths. Optionally more than one demetallisation camera, or a camera with more than one sensor (such as a three CCD colour camera) may be employed to capture demetallisation pattern images simultaneously at a plurality of different wavelengths, for improved characterization of the demetallisation by inspection of the metallisation pattern at these different wavelengths.

Since the diffraction image or images generated by a DOVD are generally in the plane of the DOVD and therefore in substantially the same plane as the metallisation pattern image a single camera may be employed to capture images of both the metallisation pattern and a diffractively generated DOVD image, for example by alternately illuminating substrate 402 from beneath and from above. This is a relatively slow procedure and is therefore preferably restricted to 'off-line' use, for example for validation at a point of sale, rather than for in-line production use. More particularly, however, it is important to capture an image of the demetallisation pattern substantially simultaneously with a diffractively generated image of the DOVD where substrate 402 is moving if an accurate comparison of the relative positions of these two images is to be established. It is therefore strongly preferable that an additional digital camera 444 is employed to capture a diffractively generated DOVD image. Like the demetallisation camera the DOVD camera 444 has a data and control link 446 for triggering image capture and for providing captured image data to a data processor, as well as for configuring the camera. As illustrated, and described further later, camera 444 is preferably moveable with respect to substrate 402 to select a lateral diffractive image component from diffractive images recorded in the DOVD, although again for production line monitoring camera 444 will generally be fixed at a selected position and angle. More than one DOVD camera may be employed to simultaneously capture more than one DOVD diffractive image, for example to simultaneously capture images as shown in figures 2a and 2c for more complete and/or more rapid DOVD characterization.

Cameras 440 and 444 are preferably integrating cameras, that is cameras which provide an output which sums or integrates photons reaching the camera to facilitate simultaneous metallisation pattern and diffractive image capture by controlling the timing of illumination from sources 408, 424. Thus in one embodiment cameras 440, 444 comprise CCD cameras with a progressive scan (to avoid motion effects on interlaced frames) and an asynchronous trigger capability for single image capture. The sensitivity or f number of the camera depends upon the level of illumination and the exposure time needed to freeze the image, which is in turn dependent upon the camera resolution. Suitable parameters may be determined according to the circumstances by routine experiment. In one embodiment cameras 440 and 444 comprise monochromatic cameras with a resolution of 640 x 480 pixels and an exposure time of 1/31 500 seconds, in particular Model No CV-Al lE available from JAI of Copenhagen, Denmark. During the exposure interval a sheet of bank notes travelling at 1.8 m per second moves 0.057 mm and assuming this corresponds to a pixel dimension this provides an image capture area of approximately 27 x 36 mm with a metallisation pattern resolution of approximately 0.1 mm. To operate at this speed requires a relatively high light level and Xenon strobe lighting or high intensity LED (light emitting diode) strobe systems such as the Edmunds Optics type X54-844 may be employed. With such lighting and image capture arrangements it is relatively straightforward to highlight a desired diffractive DOVD image by selecting illumination and camera angles and, if necessary, filtering.

In a less preferred variant of the system the demetallisation pattern may be captured using a camera behind the note and using the illumination from the (strobe) lamps which are being used to get the diffractive image to also provide illumination for the demetallisation. This is because it is strongly preferable to capture the position of the demet and the diffractive images at the same time and in some configurations it may be difficult to separate out all the spectral information on all images if the nominally black image of the demetallisation is being captured from the illuminated side. Use of LED strobed illumination helps with the separation of spectral information.

Refernug next to figure Sa this shows a schematic plan view of the positions of cameras 440, 444 of figure 4 in relation to substrate 402. A DOVD 500 is shown on substrate 402 and a second DOVDis concealed beneath demetallisation camera 440. In one embodiment DOVD camera 444 is moveable on a portion of a surface of a sphere approximately centred on the DOVD under the demetallisation camera 440 so that by moving in a first arc 502 one or more lateral DOVD diffraction images may be selected and by moving in a second arc 504 perpendicular to the first arc a colour of the selected lateral image may be varied (bearing in mind that the in-plane and background portions of the selected lateral image generally have different colours at any one angle). Thus by selecting a viewing angle of DOVD camera 444 and, optionally, by selecting a filter for the camera (for example using a filter wheel) a desired image may be substantially isolated from other diffractive images recorded in a DOVD, simplifying later processing. As can be seen from the plan view in general both cameras may be mounted a similar distance away from substrate 402 without significant mechanical interference between the two.

Figure 5b shows an alternative arrangement for implementing a moveable DOVD camera 444 in which the camera is shown in three different lateral positions 444a, b, c with respect to a DOVD 500. the embodiment illustrated in figure 5b camera 444 and the sensor therein moves substantially parallel to the plane of substrate 402 but the camera is provided with a 'shift' lens 445 which moves relative to the camera body as the lateral position of camera 444 changes. In this way perspective distortion is reduced by keeping the sensor plane parallel to the DOVD reducing the need for digital perspective correction processing. To achieve this lens 445 projects a large image area, substantially larger than the sensor of camera 444, so that as the lens is moved in a direction parallel to the sensor plane the sensor still receives a useful fraction of the DOVD diffraction image. An example of such a shift or perspective correction lens is the Super Angulon. In still other embodiments of the system one or both of cameras 440, 444 may employ a telecentric lens.

Figure 5c shows an example of a shadow or outline demetallisation pattern image 500' such as may be captured by demetallisation camera 440. Where the DOVD is carried on a printed substrate the printing is generally also visible (depending upon the colour of the ink and any filtering applied) and may or may not be visible through the metallisation, depending upon the thickness and uniformity of the metallisation.

Figures 6a and 6b show examples of registration mark sensing apparatus. In figure 6a a laser 600 directs a beam of light 602 at a substrate 604 bearing a reflective registration mark and light reflected from the mark is detected by a detector 606 which provides a trigger output 608. Where the registration mark comprises a diffraction pattern the angles of incidence and reflection of the light are selected so that reflection at the chosen angle is obtained at the wavelength at which the laser 600 operates. An arrangement such as that shown in figure 6a may also be used to identify an edge of a DOVD where such an edge which diffracts in a useable direction is available.

Figure 6b shows a second example of a registration mark sensor 610 comprising a fibreoptic cable 612 with inner and outer portions, an illumination arrangement 614 directing light into the inner portion of the cable, light reflected from a registration mark on substrate 604 being collected by an outer portion of cable 612 and directed by a mirror 616 into a detector 618. A controller 620 receives an output from detector 618 and controls illumination 614 and has a control and data interface 622 for configuring the sensor and for providing a registration mark sense output signal. Such a device is available from Omron Corporation of Japan, and can be configured to detect both leading and trailing edges of a registration mark, averaging these to provide a more accurate indication of the mark's position.

Figure 7 shows a block diagram of a data processing system suitable for use with the apparatus of figure 4 for DOVD evaluation. As illustrated the processing system uses a programmed data and control processor but in other arrangements part or all of the data processing may be implemented on FPGAs (field programmable gate rays) or on ASICs for increased speed.

The data processing system 700 comprises timing control hardware 702 and a data and control processor 704 comprising a processor 704a, data memory 704b, programme memory 704c, and a time series data store 704d. Although in the illustrated embodiment these all form part of data and control processor 704 the skilled person will recognise that these components may comprise a distributed system, for example on a network. The processor 704 is coupled to user interface devices 706 such as a pointing device, for example a mouse, a screen, keyboard and optionally a printer. Processor 704 also has one or more image capture cards 708 for interfacing to cameras 440, 444, for example a two channel image capture card, or a pair of synchronized single channel image capture cards such as National Instruments type 1407, or a multi-channel image capture card such as the National Instruments type 1409. Processor 704 further has a plurality of analogue and digital interfaces 710 such as analogue-to- digital, digital-to- analogue and digital interface cards. Processor 704 may include one or more additional processors (not shown) specifically adapted for image processing. Data memory 704b comprises non-volatile memory such as Flash RAM, or ROM, or a hard disk storing a reference or 'golden' metallisation image, a reference print image, and a reference DOVD diffraction image or images. Programme memory 704c also comprises nonvolatile memory and stores metallisation pattern image capture code, DOVD diffractive image capture code, illumination level input and compensation code, print matching and image correction code, DOVD diffractive image matching and position determination code, system control code for configuration, illumination control and optionally camera positioning, database interface code for interfacing to time series data store 704d, user interface code for providing a user interface with user interface devices 706, and operating system code. Some or all of the data in data memory 704b andlor the code in programme memory 704c may be provided on a removable storage medium 712. Time series data store 704d may be implemented on a separate machine to processor 704a and again preferably comprises non-volatile memory and includes data which may be provided on a removable storage medium. Time series data store 704d stores data such as metallisation uniformity data, metallisation average opacity data,metallisation edge sharpness data, metallisation distortion data, overall metallisation (match) quality data, and relative metallisation position data with respect to a diffractive DOVD image and/or substrate printing, with time andlor date information. This historical evaluation data may be used to determine trends within and between processed batches and to predict and address problems before they arise.

Timing and control hardware 702 has a timing controlled configuration input from data and control processor 704 but preferably operates substantially independently of this processor to reduce the processing burden, providing a trigger output signal 716 to the processor. Thus timing control hardware 702 has a trigger input 718, for example from a registration sensor or line 434 of figure 4 and provides trigger/control outputs 720, 722, 724, 726 for the front illumination, back illumination, demetallisation camera and DOVD camera respectively. Image capture device 708 has two-way interfaces 708a, b to the demetallisation camera and to the DOVD camera respectively for configuring these cameras and capturing image data from the cameras. Alternatively as the control signal to the cameras specified above is via RS232 it may come from the processor 704 rather than the capture card 708. The analogue and digital interfaces 710 provide inputs 728, 730 from the top 422 and underside 420 illumination level sensors and provides an underside illumination control output 732 for controlling illumination apparatus 408, for example for controlling adjustable filtering and/or illumination intensity. Interfaces 710 also provide a DOVD reject signal output 734 for, in a production line version, signalling to production line control equipment that a DOVD, bank note or batch is to be rejected. Interfaces 710 also provide interfaces for other control signals 736, for example for sensing Reposition and for sensing/controlling DOVD camera position where a DOVD camera or cameras is under machine control.

Figure 8 shows examples of timing waveforms for the data processing system of figure 7. Broadly speaking the timing of all these waveforms is controlled by a leading edge 802 of a trigger input waveform 800 on trigger input line 718 of timing control hardware 702. Thus following this trigger input control signals 802, 804 are provided on outputs 724, 726 for the demetallisation camera and DOVD camera respectively to begin integrating received light to form an image. The integration periods of these cameras are generally determined by an internal camera clock in accordance with the configuration of the camera. Shortly afterwards outputs 720, 722 provide demetallisation illumination 806 and DO'ID illumination 808 control signals to corresponding illumination sources 408, 424 to control simultaneous illumination of the DOVD from beneath for demetallisation pattern image capture and from above for diffractive DOVD image capture. A trigger output 810 on line 716 is also provided to data and control processor 704 and this processor then reads the level of demetallisation illumination using inputs 728 and/or 730 once it has allowed a short time to elapse for the illumination to be fumed on. The trigger output signal 810 is also used by processor 704 to begin processing of a previously captured image and in turn provides a GUI (graphical user interface) and database strobe 814 indicating that previously processed image data is valid for output to the GUI and/or data store 704d.

Figure 9 shows a flow diagram illustrating the operation of the data processing system 700 of figure 7. The image processing functions of figure 9 may be implemented using standard library routines such as the IMAQ Vision suite of routines available from National Instruments; National Instruments also provide corresponding libraries for implementation of the image processing functions in hardware. The functions illustrated in the flow chart of figure 9 need not all be perfommed when evaluating every DOVD and in some implementations it is preferable to perform a basic evaluation without, for example, relative diffractive image-metallisation pattern determination, performing more complex evaluation procedures on selected DOVDs.

Referring to figure 9 at step S900 processor 704 captures a metallisation pattern image using a card 708 and stores this in working memory. The subsequent steps which are marked with a double asterisk are preferably performed when evaluating a DOVD on a printed substrate but need not be performed when there is no printing. Thus at step S904 processor 704 loads a reference print image and matches this to the metallisation pattern image, where this includes print which has not been optically removed, and determines the position of the print. Then, at step S906, one or more areas of the metallisation pattern image without metallisation are selected and the average density of the print is determined from the intensity of pixels imaging printed portions of the substrate. Then the reference print image is weighted using this average print density (S908) and subtracted from the metallisation pattern image to correct for the presence of the print. Then at step S910 (or immediately following step S900 where no print compensation is applied) processor 704 loads the reference metallisation image from data memory 704b, matches this to the metallisation pattern image captured by the metallisation camera, and determines the metallisation position. The matching process also provides a score indicating the goodness of match and this score can be used as a direct measure of the quality of demetallisation, providing a demetallisation quality data output 911. The relative position of the metallisation pattern and print may then be determined (S918) using the print position data provided by step S904, providing DOVD positioning quality data 919. At step S912 a diffractive DOVD image from the DOVD camera is captured using card 708 and stored in working memory. Then processor 704 loads a reference DOVD image from data memory 704b, matches this to the captured DOVD diffractive image, and determines the DOVD diffractive image position. Steps S912, S914 may be performed in parallel with the metallisation pattern image capture and processing, or sequentially, depending upon the configuration of processor 704. It will be recognized that the above described pattern and shape matching techniques need not be applied to a complete image of DOVD metallisation or to a complete diffractive DOVD image but may be applied to partial images or to parts of a captured image or images.

At step S916 the processor uses the deme.allisation position data from step S910 and the DOVD diffractive image data from step S914 to determine the relative position of the DOVD diffractive image and the metallisation pattern, to provide relative position data for storage and output. Optionally more complex processing may be employed, for example by matching three local demetallisation and diffractive image portions a measure of relative distortion (rotation and skew) may be obtained. Prior to determining the relative positions of demetallisation pattern and of a diffractive image a calibration procedure may be performed to determine any offset between the captured images from the two cameras which may need to be subtracted. In some embodiments, for example where colour DOVD diffractive images are captured, images at a plurality of wavelengths or having different colours may be compared with the metallisation pattern image position to provide more DOVD characterization data. The DOVD diffractive image data from step S914 may also be used as a further measure of DOVD quality. For example a poor match score may indicate that a DOVD has been applied to a relatively rough substrate causing a loss in diffractive image quality.

The data obtained at step S910 is sufficient for a basic DOVD metallisation pattern evaluation, and that from step S916 for a useful evaluation of DOVD diffractive image performance. However further processing may be applied to the captured metallisation pattern image and thus in embodiments at step S920 one or more areas of demetallisation are selected and an average pixel intensity in each of these areas is determined. Optionally metallisation pattern image edge sharpness may also be determined in one or more of these areas or for the entire demetallisation pattern image to provide further demetallisation quality data. Then, at step S922, processor 704 reads the demetallisation illumination level, preferably from above the substrate, and adjusts the pixel levels determined in step S920 to correct for absorption effects within the DOVD substrate, for example by siting sensor 422 over a relatively clear substrate portion. Then at step S924 the metallisation thickness or opacity is determined for each selected region and an average metallisation thickness is preferably also determined, providing metallisation thickness and uniformity data for storage and further processing.

At step S926 some or all of the DOVD evaluation data, in particular the demetallisation quality data, the relative demetallisation and diffractive DOVD image position data, the DOVD positioning quality data, the metallisation thickness and uniformity data, and other demetallisation quality data may be compared with stored tolerances and dependent upon the result a reject signal 927 provided to the production process, preferably with an associated stored log of the evaluation data outside the permitted limits. Also at step S928 some or all of the aforementioned DOVD evaluation data may be output to the user and/or stored in a time series database 704d.

In embodiments some of the operations of the apparatus of figure 4 and the associated processing system of figure 7 may be pipelined so that, for example, one pipeline step comprises the capture of metallisation and diffractive DOVD images by cameras 440, 444, a second pipeline step comprises transfer of these images to data and control processor 704 and subsequent processing to match the captured images with the stored reference images, a third pipeline step comprising further evaluation of the captured metallisation pattern image and a comparison of relative image positions, and a fourth pipeline step comprising storage of the DOVD evaluation data resulting from the previous steps in the time series database 704d and/or output of the evaluation data to a user. As previously mentioned, depending upon the speed of a production line up to one second may be available for the complete pipeline although only a few tens of milliseconds may be available for each stage.

Figure 10 shows one example of further data processing and analysis performed on DOVD evaluation data held in time series data store 704. Thus this data is read by time series/trend analysis code 1000 under control of GUI code (which is also useable to provide system configuration data), one output of the trend analysis comprising a graph 1002 of quality data against time. Trend analysis code 1000 is preferably configured to extrapolate a trend such as trend 1004 to predict DOVD quality/evaluation data 1006 and to compare the prediction against a quality limit 1008. Such a prediction and comparison may be performed by quality evaluation trend extrapolation and warning signal generation code 1010, without the need for any explicit construction of a graph such as graph 1002, to provide a quality warning signal 1012. Such code may be employed by an operator to control a production process or may be implemented for automatic control of a production process or for operator warning of parameters which are likely to fall outside a permitted limit at some point in the future. The apparatus 400 of figure 4 may be adapted to facilitate such a process. For example it is observed experimentally that with an embossed DOVD the blue tends to fall in quality before the red portions of recorded diffractive images and thus the DOVD camera 444, or another dedicated camera, may be arranged to give weight to blue-end spectrum diffractive image portions, for example by appropriate angle selection andlor filtering, to facilitate subsequent analysis and early detection of potential problems.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: 1. Apparatus for evaluating a metallised diffractive optically

variable security device (DOVD), the apparatus comprising: a first light source configured to illuminate said DOVD; a first image capture device configured to capture an image of a metallisation pattern of said DOVD using light transmitted through said DOVD and to provide metallisation pattern image data; and a data processor coupled to said first image capture device to receive said captured image data, configured to evaluate said patterned DOVD by comparing said image data with data for a metallisation reference image, said metallisation reference image comprising a reference version of at least a portion of said metallisation pattern.
2. Apparatus as claimed in any claim 1 wherein said DOVD has a viewing side for viewing a variable image generatable by said DOVD and a back side; wherein said apparatus is configured to generate a DOVD diffraction image; and wherein said data processor is configured to receive data for said diffraction image, and to determine a relative position of said metallisation pattern and said diffraction image to generate evaluation data for said DOVD.
3. Apparatus as claimed in claim 2 wherein said data processor is configured to determine a metallisation position of said metallisation pattern from said metallisation reference image comparing, to determine a diffraction image position of said diffraction image by comparing said diffraction image data with data for a DOVD reference image, said DOVD reference image comprising a reference version of at least a portion of said diffraction image, and to determine said relative position from said metallisation and diffraction image positions.
4. Apparatus as claimed in claim 2 or 3 further comprising a second image capture device configured to capture said diffraction image; and wherein said data processor is coupled to said second image capture device to receive said diffraction image data.
5. Apparatus as claimed in claim 2, 3 or 4 wherein said first light source is configured to illuminate said back side of said DOVD, the apparatus further comprising: a second light source configured to illuminate said viewing side of said DOVD to generate said DOVD diffraction image.
6. Apparatus as claimed in claim 5 wherein said second light source is configured to provide a substantially collimated beam of light in a direction offset from a line joining said first light source and first image capture device.
7. Apparatus as claimed in claim 5 or 6 wherein said second light source is configured to illuminate said DOVD at an angle to select in one plane of said DOVD substantially a single diffraction image of said DOVD.
8. Apparatus as claimed in any one of claims 2 to 7 configured to capture said metallisation pattern image and said diffraction image substantially simultaneously.
9. Apparatus as claimed in any one of claims 4 to 8 wherein one or both of said first light source and said second image capture device are filtered to reduce interference to said captured diffraction image from said first light source.
10. Apparatus as claimed in any one of claims 4 to 8 wherein said first light source is filtered to attenuate the first light source at a first wavelength, and wherein said second image capture device is configured such that said captured diffracted image is captured in a colour approximating a colour of said first wavelength.
11. Apparatus as claimed in any one of claims 4 to 10 wherein said second image capture device has an associated perspective-correcting lens system.
12. Apparatus as claimed in claim 11 wherein said second image capture device is moveable in conjunction with said lens to view one or more diffraction images generated by said DOVD over a range of angles whilst maintaining said perspective correction.
13. Apparatus as claimed in any preceding claim wherein said data processor is configured to evaluate at least one of a metallisation thickness, a metallisation uniformity, and a metallisation distortion responsive to a result of said metallisation reference image comparing.
14. Apparatus as claimed in any preceding claim further comprising means to capture two images of said metallisation pattern at two different wavelengths, and wherein said data processor is configured to receive data for both said images and to determine evaluation data for said DOVD responsive to data for both said images.
15. Apparatus as claimed in any preceding claim wherein said data processor is further configured to evaluate said metallisation pattern image by determining the sharpness of at least a portion of said image.
16. Apparatus as claimed in any preceding claim further comprising means to store time series result data from said evaluation in a data store, whereby a DOVD evaluation trend may be determined.
17. Apparatus as claimed in any preceding claim wherein said DOVDis carried on a substrate also bearing an additional pattern, and wherein said data processor is further configured to correct said metallisation pattern image for said additional pattern prior to comparing with said metallisation reference image.
18. Apparatus as claimed in any preceding claim wherein said DOVD comprises an embossed demetallised DOVD.
19. Apparatus for monitoring a bank note production line comprising apparatus as claimed in any preceding claim, wherein said DOVDis carried on a said bank note, the apparatus further having an evaluation output to provide an evaluation signal for controlling said production line responsive to said evaluating.
20. Apparatus as claimed in any one of claims 1 to 19 for monitoring a production process, configured for pipelined, real-time capturing and processing of said image data for said evaluating, in particular comprising a plurality of said first light sources and first image capture devices for parallel evaluation of a plurality of said DOVDs.
21. Apparatus for evaluating the quality of an embossed hologram by demetallisation detection, the apparatus comprising: means to illuminate said hologram from behind; means to capture an image of a demetallisation pattern of said hologram; and means to evaluate a quality of said hologram using said captured demetallisation image.
22. Apparatus for evaluating a metallised diffractive optically variable security device (DOVD), said DOVD having a viewing side for viewing a variable image generatable by said DOVD and a back side, the apparatus comprising: a first light source configured to illuminate said back side of said DOVD; a second light source configured to illuminate said viewing side of said DOVD to generate a DOVD diffraction image; a first image capture device configured to capture an image of a metallisation pattern of said DOVD using light transmitted through said DOVD from said first light source to provide metallisation pattern image data; a second image capture device configured to capture said DOVD diffraction image; and a data processor coupled to said first image capture device to receive said captured metallisation pattern image data, and to said second image capture device to receive said captured DOVD diffraction image data, and to determine a relative position of said metallisation pattern and said diffraction image from said captured image data to generate evaluation data for said DOVD.
23. A data processing system for determining the quality of a metallised diffractive optically variable security device (DOVD), the metallisation of said DOVD being patterned, the system comprising: an image input for receiving data for an image of said metallisation pattern of said DOVD; data memory for storing a metallisation reference image for said metallisation pattern; program memory storing processor control code; and a processor coupled to said first image input, to said data memory, and to said program memory for loading and implementing said processor control code, said code comprising code for controlling said processor to: input a metallisation pattern image of said DOVD; retrieve said metallisation reference image from said data memory; match said metallisation pattern image to said metallisation reference image; determine quality data for said DOVD responsive to said matching.
24. A data processing system as claimed in claim 23 further comprising an image input for receiving data for a diffractively generated image formed by said DOVD, wherein said data memory is further configured to store a reference diffractively generated image for said DOVD, wherein said matching of said metallisation pattern image generates position data for said metallisation pattern, and wherein said code further comprises code to: input a diffractively generated image formed by said DOVD; retrieve said reference diffractively generated image for said DOVD from said data memory; match said diffractively generated image to said reference diffractively generated image to generate position data for said diffractively generated image; compare said position data for said metallisation data image and said position data for said diffractively generated image; and wherein said quality data is determined responsive to said comparison.
25. A data processing system as claimed in claim 23 or 24 wherein said metallisation pattern image includes an additional image, wherein said data memory is further configured to store a reference additional image, and wherein said code further comprises code to: locate said additional image in said metallisation pattern image using said reference additional image; and correct said metallisation image for said additional image responsive to said location prior to said metallisation image matching.
26. A data processing system as claimed in any one of claims 23 to 25 wherein said code further comprises code to store said quality data in data memory such that a time series of said quality data is retrievable.
27. Processor control code as claimed in any one of claims 23 to 26.
28. A carrier carrying the processor control code of claim 27.
29. A method for determining the quality of a metallised diffractive optically variable security device (DOVD), the metallisation of said DOVD being patterned, the method comprising: inputting a metallisation pattern image of said DOVD; retrieving a reference image of said metallisation pattern image from a data store; matching said metallisation pattern image to said metallisation reference image; determining quality data for said DOVD responsive to said matching.
30. A method as claimed in claim 29 wherein said matching of said metallisation pattern image generates position data for said metallisation pattem, and further comprlsmg: inputting a diffractively generated image fommed by said DOVD; retrieving a reference diffractively generated image for said DOVD from said data store; matching said diffractively generated image to said reference diffractively generated image to generate position data for said diffractively generated image; and wherein said determining of said quality data comprises comparing said position data for said metallisation data image and said position data for said diffractively generated image.
31. A method of evaluating a demetallised embossed DOVD, the method compnsmg: comparing a position of a demetallisation pattern of said DOVD with a position of a diffractively generated image from said DOVD; and determining a quality measure for said DOVD responsive to said comparing.
32. A method of determining a quality measure for a demetallised embossed DOVD, the method comprising: matching a pattern of said demetallisation with an ideal pattern of demetallisation for said DOVD; and determining a quality measure for said DOVD responsive to said matching.
33. A method of monitoring a bark note production process, bank notes produced by said process each bearing an embossed, demetallised DOVD, the method compnsmg: capturing an image of a DOVD demetallisation pattern of a said bank note; comparing said demetallisation pattern image with a reference demetallisation pattern image; and providing a control signal for said production process responsive to said pattern comparing.
34. A method of monitoring a bank note production process, bank notes produced by said process each bearing an embossed, demetallised DOVD, the method compnsmg: capturing a holographic image generated by a said DOVD; capturing an image of a DOVD demetallisation pattern of a said bank note; comparing a relative position of said holographic image and said demetallisation pattern image; and providing a control signal for said production process responsive to said image position comparing.
35. A data processor configured to operate in accordance with the method of any one of claims 29 to 34.
36. Processor control code, in particular on a data carrier, configured to implement the method of any one of claims 29 to 34.
37. Apparatus for evaluating a metallised security device, the apparatus comprising: a first light source configured to illuminate said metallised security device; a first image capture device configured to capture an image of a metallisation pattern of said metallised security device using light transmitted through said metallised security device and to provide metallisation pattern image data; and a data processor coupled to said first image capture device to receive said captured image data, configured to evaluate said patterned metallised security device by comparing said image data with data for a metallisation reference image, said metallisation reference image comprising a reference version of at least a portion of said metallisation pattern.