WO1997006502A1

WO1997006502A1 - Optical image authenticator

Info

Publication number: WO1997006502A1
Application number: PCT/AU1996/000498
Authority: WO
Inventors: Peter Samuel Atherton; Charles E. Chandler; Robert J. Potter
Original assignee: Mikoh Technology Limited
Priority date: 1995-08-07
Filing date: 1996-08-07
Publication date: 1997-02-20
Also published as: EP0843860A4; AUPN462995A0; EP0843860A1; JPH11510283A; CA2226321A1

Abstract

Optical Image Authenticator and method where a light intensity pixel array is detected and compared with a reference pixel array on a pixel by pixel basis to determine if each pixel is either good or bad. Authentication is done on the basis of the number of good/bad pixel's or their ratio. Pixel's are assigned a grey scale value or a binary value. Values are allocated with respect to different threshold levels. The optical memory can be a pixilated diffraction grating or 'pixelgram'. Detection is via a CCD sensor and a laser pulses every frame. Detected pattern starting point position is matched with the respective positions in the authentic pattern. The authentic pattern is stored in a variety of forms which have been rotated or linearly translated. For use in the field of securities and credit cards.

Description

OPTICAL IMAGE AUTHENTICATOR

Field of the Invention

This invention generally relates to optical memory technology, and applications of such technology to the authentication of cards (such as credit cards, debit cards, access cards and the like), documents and products of various types. The invention particularly relates to methods and apparatus for detecting or reading data from a diffraction surface (i.e. an optical memory), and further to authenticating that data in order to authenticate the diffraction surface.

Background of the Invention

The fraudulent copying and counterfeiting of cards, documents and products is a substantial and growing problem due to the ease with which many of the existing anti- counterfeiting devices (such as holograms) can themselves be copied or counterfeited. Other related problems, including repackaging and parallel importation, are also resulting in substantial revenue losses to brand owners and manufacturers. Various security devices are currently employed to protect against counterfeiting and the abovementioned related problems. These existing devices include optical devices such as holograms, pixelgrams, kinegrams, gyrograms and optically refractive surfaces based on the inclusion of patterns of glass microspheres. Reference is made to the document having International Publication No. WO 94/06097 (PCT/AU93/00455) in the name of the present applicant. This document summarises a large number of prior art documents relevant to the holographic patterns and kinegrams. International Publication No. WO 94/06097 also describes an optical memory medium formed as a diffraction surface, and illustrates in Fig. 2b thereof an intensity pattern obtainable by illumination of the diffraction surface by a single collimated monochromatic incident optical beam. Furthermore, in Figs. 3a and 3b thereof, a system for reading and authenticating the diffraction surface is disclosed.

Summary of the Invention The present invention is directed to overcoming or at least substantially ameliorating one or more of the disadvantages of the first mentioned prior art documents. Furthermore, the present invention is directed to improvements in data detection and authentication over the arrangements disclosed in noted International Publication No. WO 94/06097. Therefore, in one aspect, the invention discloses apparatus for authenticating a detected light intensity pattem divided into an array of pixels, the apparatus comprising memory means for storing an authentic pattem in the format of said pixel array; and data processing means for receiving a said detected pattem and for assigning on a pixel- by-pixel basis one of a number of digital values to each pixel in the detected pattern, and further for comparing each detected pixel with the corresponding pixel of said stored authentic pattem to determine whether the pixel is good or bad, and yet further for authenticating said detected pattem or rejecting said detected pattem on the basis of the number of good pixels and/or the number of bad pixels.

Preferably, said pixels have assigned the binary value of 0 or 1, and said data processing means discriminates on a pixel-by-pixel basis which pixels of said detected pattem are light and which are dark on the basis of the binary value, and further determines whether a pixel is good if it is light or dark as expected from the corresponding authentic pixel, otherwise it is determined to be bad.

In a preferred form the data processing means can authenticate a detected pattem on the basis of the ratio of good to bad pixels.

Advantageously, the data processing means further provides for comparing a said detected pattem with one of a number of a threshold values, the threshold values discriminating the detected pixels into said binary values. Preferably, the invention further comprises optical reading means for reading a diffraction pattem to detect a said light intensity pattem. The optical reading means can comprise a light source for illuminating said diffraction pattem to cause generation of a retum light pattem, said retum light pattem being detected as light intensity by charge coupled device (CCD) or CMOS sensor means, for example.

Further preferably, said optical reading means reads a diffraction pattern periodically to detect said light intensity pattern in frames. The light source can be a laser beam source that is pulsed once within a frame. The CCD sensors respond to a said return light pattem over time, said response time being less than the period of a frame.

Yet further, said memory means can store said authentic pattern in a plurality of forms, each form being angularly rotated with respect to any other, and further wherein one or more of said forms of the authentic pattem are compared with the detected pattem so that the detected pattem can be authenticated against any one said forms of the authentic pattem.

The invention further discloses a method for authenticating a detected light intensity pattem divided into an array of pixels, the method comprising the steps of: storing an authentic pattem in the format of said pixel array; receiving a said detected pattem; assigning on a pixel-by-pixel basis one of a number of digital values to each pixel in the detected pattem; comparing each detected pixel with the corresponding pixel of said stored authentic pattem to determine whether the pixel is good or bad; and authenticating said detected pattem or rejecting said detected pattem on the basis of the number of good pixels and/or the number of bad pixels.

Preferably, the pixels have assigned the binary value of 0 or 1, and said step of assigning discriminates, on a pixel-by-pixel basis, which pixels of said detected pattem are light and which are dark on the basis of the binary value, and the step of comparing determines whether a pixel is good if it is light or dark as expected from the corresponding authentic pixel, otherwise it is determined to be bad.

Brief Description of the Drawings An embodiment of the invention now will be described with reference to the accompanying drawings, in which:

Fig. 1 is a top view of a diffraction surface;

Fig. 2 shows a diffracted beam intensity pattern produced by the surface of Fig. 1; Fig. 3 shows, as a cross-sectional view, a diffraction surface reader;

Fig. 4 is a schematic block diagram of the CCD sensor of the reader; Fig. 5 is a schematic block diagram of the reader and authentication apparatus; Fig. 6 is a schematic block diagram summarising the authentication process; and Figs. 7 and 8 are schematic block diagrams showing the authentication process in detail.

Description of Preferred Embodiments

Fig. 1 is an illustration of a diffraction surface 10 as viewed from above the surface. The surface 10 includes a mesh pattem of etched regions 15 enclosed by ridges 11 and 13, with the mesh pattem varying across the surface 10. The diffraction surface 10 is arranged to produce a number of retum diffracted beams when illuminated by monochromatic coherent light, each of which retum beams produces a specified intensity pattem that can be machine-recognised in order to authenticate the diffraction surface 10.

Fig. 2 illustrates a type of diffracted beam intensity pattem which may be produced by a surface of the type shown in Fig. 1 when illuminated by a single, collimated, approximately monochromatic incident optical beam. In Fig. 2, white indicates the maximum diffracted optical intensity, black indicates zero optical intensity, and different degrees of shading represent different intermediate levels of optical intensity in the diffracted beam intensity pattem projected onto an intercepting surface. The central spot of Fig. 2 represents specular reflection of the incident optical beam, illustrating that in this example the diffracted beam forms a pair of symmetrical images around the specular reflection beam. It should be appreciated that the diffraction surface 10 can be designed to produce a specified combination of angular size and angular position of the diffracted images in order to suit machine reading of the diffracted images. It should also be appreciated that part or all of the diffracted beam optical intensity pattem may be utilised to be machine read for the purposes of authentication of a device bearing the diffraction surface 10. The diffraction surface 10 thus constitutes an optical memory.

Fig. 3 shows, as a cross-sectional view, a reading device 20 suitable for reading a diffraction surface of the type shown in Fig. 1. The reader has a light shield 22, the ends of which butt against a medium 16 carrying on one side thereof a diffraction surface 10 generally of the type shown in Fig. 1. The diffraction surface 10 can occupy only a part or the whole of the substrate medium 16.

The reader 20 further has a source of near monochromatic light, in the form of a laser diode 24 carried by a focusing sleeve 26, in turn set in and secured to an outer shell 28 by set screws 30. The laser light output from the device 24 passes through a lens system 32, following which it is in the form of a beam of approximately coherent light. The beam then is incident on the diffraction surface 10 in a direction normal to the surface. The incident light source results in the production of a conjugate pair of retum diffracted beams 34, 34'. Within the outer shell 28 are arranged sites for optical detector arrays 36, 38 coincident with the path of the diffracted beams.

The lens system is preferably adjusted so as to produce the sharpest diffracted images at the sites 36 and 38. This will result in the output beam from the lens system 32 being slightly converging. In the embodiment shown in Fig. 3, only the single sensor array 38 is provided. The sensor array 38 is constituted by a charge coupled device (CCD) array positioned so that the diffracted beam 34 is incident upon approximately 60% of the total active pixel area of the CCD array.

Hence in this embodiment the CCD sensor array 38 detects only part of the total diffracted intensity pattem produced by the diffraction surface 10. The diffraction surface 10 and CCD array 38 will usually be configured so as to enable the CCD array 38 to detect one complete component of the zeroth order diffracted image produced by the surface 10 (with the remaining components of the zeroth order image being mirror images of the detected component). In operation of the reading head, the laser diode light source 24 is pulsed once per frame with an On-time' which produces retum signals at approximately 50% of the CCD sensor full well capacity. CCD devices function by accumulating charge over time in response to incident light. Thus the incident light must be pulsed for a discrete and uniform instant of time, and the sensor output appropriately 'framed' in consideration of the minimum time taken for the sensors to accumulate the charge due to the light pulse duration.

In one embodiment, the CCD sensor 38 is a 192 by 165 pixel format sensor. The pixels typically are of a size 8.5 μm by 10.0 μm. The CCD sensor in this embodiment utilises only 4 wires for complete operation, and the clock input signal to the sensor may preferably be TTL compatible.

The relative orientation of the collimating lens 32 and the diffraction surface 10 is important so that the retum beams 34, 34' occur at the location of the detector arrays 36, 38. Correct focusing can be obtained by placing adjusting the location of the collimating lens 32 to produce the optimum diffracted image at the detector array 38. The diffractive surface 10 can be made up of a repetitive pattem of basic cell units, so that each cell unit produces a unique diffracted intensity image. In some instances the optical properties of the laser diode 24 and collimating lens 32 will result in an elliptical light pattem incident upon the diffractive surface 10. Configuring of the optical components in some cases will need to take into account the lengths and orientations of the major and minor axes of said elliptical pattern, in particular to ensure that one or other of said axes coincides with a particular direction on the optical surface 10 and that the length of one or other of said axes is comparable with the characteristic dimension of the individual cell units of the diffraction surface 10. Most usually, this involves configuring the optical components to ensure that the minor axis of said ellipse is parallel to a specified direction in the optical surface 10 and that the length of said minor axis is comparable with said characteristic dimension, thereby ensuring that cells from no more than one row or column of the basic cell structure of said diffractive surface 10 are fully illuminated at any one time. Configuring said optical components to meet such requirements may in some instances involve placing an aperture in the path of the optical beam incident on the optical surface 10.

Fig. 4 shows a schematic diagram of the CCD sensor 38, that includes an array driver section 42 driving the pixel array 40. Further, there is a serial driver section 44 driving a multiplexing stage 46. The output from the multiplexer 46 is provided to an output amplifying section 48 and in turn to the output terminal 50. Input power supply terminal 52 receives a power supply for the clocking and drive electronics, together with providing power for the CCD array 40 and output amplifier section 48. A clock input line 54 controls integration time, parallel array shifting and serial register pixel readout. A ground reference connection point 56 also is provided. Thus the output from terminal 50 represents an amplified form of serial register pixel data updated at the frame rate. For each frame, the output data for each pixel represents the received light intensity.

Fig. 5 shows a schematic block diagram of reading and authentication apparatus in accordance with a preferred embodiment. A low voltage (6 V) supply and high voltage supply are provided. The 6 V supply is regulated by the low voltage regulator 60 to a 5 V level. This voltage level supports all logic type devices and the pulsed laser driver circuit 62. The 18 V supply is regulated by the high voltage regulator 64 to a 15.5 V supply, that provides power for all analogue circuits, including the CCD array 40 and the output amplifier 48, and is regulated down once again to + 10 V DC for the video comparator 74.

Overall timing is derived from the master clock 66, which in an embodiment is a 10.0 MHz TTL oscillator, fed to a timing generator circuit 68, such as an Altera EPS-448 timing generator chip. The timing generator circuit 68 thus has responsibility for frame timing, which is chosen to be twice the mains frequency (i.e. 120 Hz in the United States) to reduce the effects of line noise and stray light from sources such as fluorescent sources. In countries where the mains frequency is 50 Hz, the frame rate would be set at 100 Hz. The pixel readout from the CCD sensor 40 is at one half of the master oscillator frequency, i.e. at 5 MHz. The laser diode 24 is fired once per frame with an 'on time' in one embodiment of approximately 2 μs. The laser source remains idle for the remainder of the frame time, and such a low duty cycle is possible due to the sensitivity of the CCD sensors 40 and high reflectance of the diffraction surface 10. For diffraction surfaces 10 with lower diffractive efficiencies - for example, "clear" diffractive surfaces 10 - the laser diode 24 may need to have a longer on-time.

The laser drive circuit 62 consists of a charging circuit, a firing circuit, and preferably may also include a failsafe circuit feature to prevent damage to the laser diode 24 in the event of a circuit failure. The drive circuit is logic 1 to fire the laser diode 24 and logic zero in the off state.

A frame timing of 120 Hz gives a period of 8.333 ms. For a 2 μs on-time, this represents a duty cycle of 0.024%. Thus there is very little power dissipation by the laser diode, virtually eliminating the need for laser diode heat sinking. The noise filter 70 filters the regulated supply from high frequency noise that otherwise would affect the output noise floor of the CCD array 40.

The amplified output from the CCD sensor 40 passes to a video buffer offset amplifier 72 to provide a DC offset so that the output signal is suitable for passing to the video comparator 74. Also provided to the video comparator 74 is a DC input voltage that sets the threshold between black (dark) and white (light) pixels. The threshold level is previously determined by minimising false white pixels due to noise within the circuit and matching the number of illuminated pixels to a pre-stored digital image located in the pattem read-only memory 78. The video comparator 74 digitises the thresholded retum pattem to a 1-bit digital data stream at the pixel rate. Thus the video comparator 74 determines, for each frame, definitively which pixels of the retum diffraction intensity pattem are black or white. The thresholded retum pattem then is passed to the first one of two LSI devices 80, 82 where the authentication processing takes place.

Fig. 6 shows a typical pictorial representation of analogue output image from the CCD sensor 40 and subsequent thresholded analogue image as performed by the video comparator 74. The frame comparator 100 and the pixel accumulators 102, 104 represent a number of the components shown in Fig. 5, and particularly the LSI devices 80, 82 and the pixel sealers 86, 88.

From the pre-stored image stored in the pattem memory 78, each pixel is classified as "expected light" or "expected dark" . There then follows a comparison with the thresholded retum pattem, and pixels which are light where dark backgrounds are expected are classed as "bad"; pixels which are light where expected light are classed as "good". The converse comparison can in some preferred embodiments also be performed, with pixels which are dark where dark is expected being classed as "good", and which are dark where light is expected being classed as "bad". This testing is performed by the pixel accumulator 102. The total number of "good" and "bad" pixels are stored in two counters in the second pixel accumulator 104. At the end of each frame, the total pixel counts in each of the good and bad groups are subtracted, and further by the application of minimum difference rules, the determination of an authentic or non-authentic return intensity image per frame is determined. Fig. 7 is a schematic block diagram further detailing the authentication process. The thresholded data (VID) from the video comparator 74 is input to a two- cycle delay latch 110 to be latched and delayed by two pixel cycles. The pattem ROM 78 is addressed by a 16 bit generator 112, being incremented at the pixel rate via the CLK input. The address generator 112 is cleared once per frame by the CLR* signal from the timing generator 68 and delay line 84.

Thus the digital data from the pattem ROM 78 is input to the three-cycle delay latch 114, thereby being latched and delayed by three pixel cycles to match the difference in propagation times from the CCD sensor 40 and the pattem ROM 78. The latched delay signal then is provided to a processing unit 116, that determines the good/bad pixel count and contains the steering logic. Each pixel from the pattem ROM 78 therefore is matched in time with the corresponding pixel from the CCD sensor 40 to steer the detected pixels in terms of those pixels which are light and expected to be light and those pixels which are light but expected to be dark. Pixels that are expected to be light and are determined to be so are fed to a rate multiplier (sealer) 118 as "good" pixels. The sealer 118 can be programmed via ADIV bits 0-3 of the selector 86 to feed through one clock cycle for each pixel received in this category down to one clock cycle for each 15 pixels received. The scaling of expected good pixels relative to bad pixels provides user selectivity in the authentication process. In that case, for minimal security applications where low to moderate ratios of good to bad pixels would be allowable for the purposes of authentication, a direct count of good and bad pixels can be made without the use of the sealer circuits. Conversely, for high security applications, only one-of-eight good pixels might, for example, be passed for the purposes of authentication, thus requiring a ratio of at least eight good to every one bad pixel counted.

Fig. 7 also shows a "bad pixel" sealer 120. In that case, there is scaling of both good and bad pixel paths. The scaling of pixels which are light when expected to be dark, via BDIV bits 0-3 of the selector 88, could be necessary under conditions of low security or where ambient lighting or high speed reader applications may require a reduction in bad pixel count sensitivity. Thus flexibility is included by the inclusion of both sealers, since this allows a wide range of good to bad pixel ratio count in the authentication process. The outputs CKA and CKB from the LSI device 80 thus are classified as good and bad pixel count respectively, and either or both categories have been scaled depending on the level of security desired. Both the good pixel count and bad pixel count thus are passed to the second LSI device 82, as shown in particular detail in Fig. 8. Two diagnostic LED indicators 90, 92 also are provided by way of indication of good pixel count and bad pixel count.

The good pixel count and bad pixel count are separately passed to 16-bit counters 130, 132. Each counter's 12 lower significant bits are routed to a 12-bit subtracter circuit 134 that maintains a difference count at the pixel rate. In the simplest form, total counts in the good pixel counter which equal or exceed the bad pixel total count will result in a non-borrow condition in the subtracter 134. The 12-bit subtracted values and borrow bit are passed to a logic unit 136, where the minimum difference between good and bad pixel counts are combined with the subtracter borrow output to set a niinimum difference requirement. Further logic can be incoφorated to limit the absolute number of bad pixels in the bad pixel (CKB) count, ensuring that non-authentic diffraction patterns having a random remm pattem outside the expected count ranges are rejected. Additional levels of logic can be added to accommodate unique good to bad pixel count situations.

The output from the logic circuit 136 is a valid signal (AGEB) that is passed to the LED indicator 94 shown in Fig. 5. It should be noted that authentication of a number of different remm patterns can be achieved by electronically selecting different EPROM chips, each storing a different valid pattem, and comparing the remm pattem with each stored pattem to determine whether the retum pattem matches any of the stored patterns. The embodiments described above exemplify the case where a diffraction surface is at a known and repeatable position with respect to the optical reading system, for example such as is the case where a "swipe-slot" credit card configuration is used. In other instances the reader might be hand-held, and thus can have a varying positional distance from the diffraction surface, and furthermore, a differing rotational orientation. Two methods of compensating for such a simation are described as follows.

Rotation of the reading apparams relative to the diffractive surface will result in rotation of the retum diffracted images about the axis of specular reflection of the incident light beam. For such rotated images reaching the CCD sensor 38, the retum pattem will have more pixels light which are expected to be dark than in the non- rotated situation. Thus there could be rejection of an otherwise valid diffraction surface.

As has been described, a one-bit serial string of data is stored in the pattem ROM 78, which is usually an EPROM device, and is used for comparison against the return beam pattern. Most common EPROM devices are stmcmred for 8-bit bytes, hence there are effectively seven unused channels which are output along the single pattem channel being sampled at the pixel rate. By using six of the remaining seven channels in the EPROM, it is possible to provide for three images rotated on either side of the "central" pattem also to be loaded into the same EPROM. Thus a total of seven images can be stored, each one being a relative rotation with respect to the others. The degree of rotation for each step can be determined by considering the field of view of the CCD sensor and the largest rotational angle at which the pattern would still strike the CCD sensor, with the total rotational angle being divided into three segments on either side of the original image. Thus a comparison can be made with each one of the stored image pattems in sequence against a detected image pattern for the process of authentication, having the property of greater rotational freedom. The process basically is one of electronic de-rotation of the retum images. The effective sample rate of the pattem data can be maintained by adding circuitry to allow the parallel (i.e. simultaneous) processing of the seven channels. Altematively, a shift register could be clocked once per frame to select the next data bit pattem from the pattem EPROM for comparison with the detected retum pattem (the serial technique). Using the (serial) shift register l-of-7 technique would result in the authentication process increasing in time from a minimum of 8.333 ms up to a maximum 58.3 ms if the last pattem in the authentication loop was the match for the actual retum image. One the other hand, the parallel reading and comparison of the 7 data streams would not suffer any such time penalty, but would involve considerable additional circuitry and a commensurate increase in power consumption. The use of "wider" EPROM devices (with more bits per word, such as 16 bit devices), with more unused channels, could further improve the ability to authenticate rotated retum images using either the serial or parallel technique referred to above. Thus in general the serial comparison of the actual retum images against electronically rotated pattem data results in a time penalty, while the parallel comparison of the retum image with electronically rotated pattem data does not result in a time penalty but does involve increased electronic complexity and increased power consumption.

As follows, the use of EPROM devices having higher number of bit bytes could further improve the ability to authenticate rotated images. The above technique deals with the electronic de-rotation of images. As noted, a similar situation arises where the retum patterns are likely to shift linearly in a particular direction (which will here be arbitrarily termed the vertical direction) - for example due to a relative motion between the diffraction surface and machine reading apparams. A technique similar to the rotationally displaced authentic images can be employed. Again, a plurality of authentic images are stored, each having a differing relative linear translation (in one or two directions).

Remm patterns generally can include dots, lines and even circles. Thus, in a second technique, by selecting patterns which have a singular region of return signal at the top vertical extreme (i.e. one horizontal tangent point) of the image, then by implementing a "floating point" pattern start location, the absolute vertical position of the retum image can be easily detected, and thus the degree of vertical offset resolved. The first illuminated pixel would reset the pattem EPROM address generator, hence the stored pattem would begin with the first pixel of the first line in which the pattem information was present for comparison with the retum image.

In this way, less EPROM pattem data would be required in the authentication process, since only illuminated lines would be mapped. Restarting of the address generator (by a thresholded reset of the address generator) would start the comparison of the stored pattern data to the digitised return pattem. There would be no time penalty associated with this process, and data matching of the return data and the EPROM pattem would be handled in the same manner as the full frame authentication process.

The technique for compensating for linear movement of the remm image can be extended to two dimensions, thereby allowing a "floating point" location of the remm image, and hence the start point for the image authentication, in both the horizontal and vertical directions (i.e. the X and Y directions in a Cartesian coordinate system).

The retum pattem data described above utilises only one threshold in advance of a comparison with the stored pattem - i.e. each pixel in the remm pattem is converted into a simple binary "light/dark" signal. High security methods can incorporate grey scale comparison between stored patterns and the return patterns in which each pixel in the stored and retum patterns is assigned one of a number of digital values rather than a simple binary 1 (light) or 0 (dark) value. Moving from one-bit to multi-bit data may be achieved through the addition of either (i) more comparators to detect each of the values (levels), or a flash type analogue to digital converter. Data stored in the EPROM pattem array would be matched to the various digitised levels for comparison with the multi-bit digitised retum patterns using a technique similar to that described herein for single bit comparison.

In the reading head illustrated in Fig. 3 the output beam from the laser diode 24 and lens 32 is perpendicular to the diffraction surface 10. It is to be appreciated that the output beam may instead be incident on the diffraction surface 10 at a different angle, with the CCD sensor 38 positioned appropriately to accept the return diffracted beam 34.

In the embodiment described herein the output of the authenticator is used to activate an LED to indicate whether or not the return pattem from the diffraction surface is authentic. It should also be appreciated that the output of the authenticator could instead be used to activate or trigger another device or process, thereby acting as an optoelectronic security "key" for said device or process.

Numerous alterations and modifications, apparent to one skilled in the art, can be made without departing from the basic inventive concept. All such alterations and modifications are to be considered within the scope of the present invention.

Claims

CLAIMS:

1. Apparatus for authenticating a detected light intensity pattem divided into an array of pixels, the apparatus comprising memory means for storing an authentic pattem in the format of said pixel array; and data processing means for receiving a said detected pattem and for assigning on a pixel-by-pixel basis one of a number of digital values to each pixel in the detected pattern, and further for comparing each detected pixel with the corresponding pixel of said stored authentic pattem to determine whether the pixel is good or bad, and yet further for authenticating said detected pattem or rejecting said detected pattem on the basis of the number of good pixels and/or the number of bad pixels.

2. Apparams as claimed in claim 1, wherein said digital values are assigned in a grey scale representation.

3. Apparams as claimed in claim 1, wherein said pixels have assigned the binary value of 0 or 1, and said data processing means discriminates on a pixel-by-pixel basis which pixels of said detected pattem are light and which are dark on the basis of the binary value, and further determines whether a pixel is good if it is light or dark as expected from the corresponding authentic pixel, otherwise it is determined to be bad.

4. Apparams as claimed in claim 3, wherein the data processing means can authenticate a detected pattem on the basis of the ratio of good to bad pixels.

5. Apparams as claimed in claim 4, wherein the data processing means further provides for comparing a said detected pattem with one of a number of threshold values, the threshold values discriminating the detected pixels into said binary values.

6. Apparatus as claimed in any one of the preceding claims, further comprising optical reading means for reading a diffraction pattem to detect a said light intensity pattem.

7. Apparams as claimed in claim 6, wherein the optical reading means can comprise a light source for illuminating said diffraction surface to cause generation of a return light pattem, said return light pattem being detected as light intensity by charge coupled device (CCD) sensor means.

8. Apparams as claimed in claim 7, wherein said optical reading means reads a diffraction pattem periodically to detect said light intensity pattern in frames.

9. Apparatus as claimed in claim 8, wherein the light source can be a laser beam source that is pulsed once within a frame.

10. Apparams as claimed in claim 9, wherein the CCD sensors respond to a said remm light pattem over time, said response time being less than the period of a frame.

11. Apparams as claimed in any one of the preceding claims, wherein said data processing means is further operable to detect the position of a first detected pixel in a detected pattem and match that pixel position with the respective position in said authentic pattem as an assignment starting point.

12. Apparatus as claimed in claims 1 to 10, wherein said memory means stores said authentic pattem in a plurality of forms, all of which forms being rotated or linearly translated with respect to any other, and further wherein one or more of said forms of the authentic pattem are compared with the detected pattem so that the detected pattem can be authenticated against any one said forms of the authentic pattem.

13. A method for authenticating a detected light intensity pattem divided into an array of pixels, the method comprising the steps of: storing an authentic pattem in the format of said pixel array; receiving a said detected pattem; assigning on a pixel-by-pixel basis one of a number of digital values to each pixel in the detected pattem; comparing each detected pixel with the corresponding pixel of said stored authentic pattem to determine whether the pixel is good or bad; and authenticating said detected pattem or rejecting said detected pattern on the basis of the number of good pixels and/or the number of bad pixels.

14. A method as claimed in claim 14, whereby said digital values of said assigning step are in a grey scale representation.

15. A method as claimed in claim 13, whereby the pixels have assigned the binary value of 0 or 1, and said step of assigmng discriminates, on a pixel-by-pixel basis, which pixels of said detected pattem are light and which are dark on the basis of the binary value, and the step of comparing determines whether a pixel is good if it is light or dark as expected from the corresponding authentic pixel, otherwise it is determined to be bad.

16. A method as claimed in claim 15, whereby said step of authenticating comprises the steps of forming a ratio of good pixels and bad pixels and comparing said ratio against a predeteπ-iining number which delineates authentic from unauthentic.

17. A method as claimed in claim 16, whereby the step of assigning comprises the steps of comparing a detected pattem with one of a number of threshold values, and discriminating each detected pixel into said binary values on the basis of the comparison.

18. A method as claimed in any one of claims 13 to 17, comprising the further steps of detecting a first detected pixel in a detected pattem and matching that pixel position with the respective position in said authentic pattern as a starting point for the pixel-by-pixel step of assigning.

19. A method as claimed in any one of claims 13 to 18, whereby the step of storing stores said authentic pattem in a plurality of forms, all of which being rotated or linearly translated with respect to any other, and said step of comparing compares one or more of said forms of the authentic pattem with the detected pattem so that the detected pattem can be authenticated against any one of said forms of the authentic pattem.