Recording media such as Compact Disks (CD), CD ROMs and Digital Video Disks (DVD) are manufactured on a very large scale worldwide and are subject to counterfeiting.

To try to reduce this risk, manufacturers package authentic media in sealed wrappers and provide certificates and the like. Nevertheless, determined counterfeiters are able to produce counterfeit copies by utilising correct packaging or selling on replicated media to third parties for counterfeit packaging.

US-A-5974150 describes a special, coded label for use with goods, the label being impregnated with a randomly distributed arrangement of fluorescent dichroic fibres.

The arrangement of fibres can be detected and used to constitute an authentication code which could be stored in a database to allow the record medium to be subsequently authenticated. One problem with this approach is that it requires the use of a separate label and such labels could be stolen and used again with counterfeit record media.

Also, the fibres are poorly located in space making detection difficult.

US-A-5920628 describes a method of"fingerprinting" magnetic media by detecting remenant noise in a magnetic medium such as a computer program diskette or video tape and the like. This overcomes the problem of a separate label but in practice it is difficult to detect the remnant noise which is a very small ac magnetic variation, due to voids, inclusions crystal defects etc. which are intentionally designed out of most recording media.

US-A-5434917 describes the incorporation of randomly distributed ferrite particles in a plastic support of a credit card to constitute an authentication code. The disadvantage of this is that it requires modification of

the primary card substrate. Modification of the substrate can lead to a weakness in the flex properties which would be detrimental to the card's lifetime. Also the nature of the card format requires a single pass through the detection system hence the possibility of mis-reads.

US-A-4114032 describes the manufacture of documents such as bank notes, credit cards and the like in which fibres coated with a magnetic or magnetisable material are embedded in the primary substrate. This has similar disadvantages to US-A-5434917.

In accordance with a first aspect of the present invention, a recording medium comprises a substrate storing data which can be read; and particulate matter provided in a layer spun coated, printed or transferred on the substrate, the particulate matter defining an authentication code when read.

In accordance with a second aspect of the present invention, a method of manufacturing a recording medium comprises providing a substrate which can store data, and spin coating, printing or otherwise transferring a layer incorporating particulate matter on to the substrate, the particulate matter defining an authentication code.

We have realised that in the conventional manufacture of recording media such as CDs, CD ROMs and DVDs, it is common practice to include a spun coated protective layer on the surface of the substrate. Further, this layer can be utilised to incorporate particulate matter to define a random or pseudo-random code by taking advantage of the spin coating process. There is thus no need to modify the primary manufacturing process, which the particular matter can simply be added into the material of the spun coated layer.

As alternatives to spin coating, a layer may be printed (eg using an ink or pigment with a low concentration of particles) or transferred (eg hot foil) on to the substrate.

The nature of the particulate matter depends on the extent to which their properties can be detected. Thus, if the particulate containing layer is covered with an opaque layer then the particulate containing layer must be detectable outside the visible wavelength range, for example by virtue of its magnetic or acoustic properties.

However, if the particulate containing layer is normally visible then the particulate matter can present visible or machine detectable characteristics such as fluorescence, light scatter, reflectance, luminescence and the like.

The complexity of the coding can be determined by varying the concentration of particular matter. Typically 5 to 50 particles could be used but numbers outside this range are envisaged.

Preferably, the property of the particulate matter is detectable under second predetermined conditions different from the first predetermined conditions used to read data stored on the record medium. This then avoids the normal data reading process from being confused by the presence of particulate matter. Typically, the stored data is read under an infra-red laser while the particulate matter detectable within the visible or ultra-violet range, usually from the opposite side of the recording medium since the particulate containing layer is usually provided on the opposite side to the stored data.

It is not necessary to use all the particulate matter to define the authentication code but, in a relatively simple approach, the code could be defined by monitoring the presence/absence of particulate matter at various locations across the recording medium with reference to the centre or outside circumference of the disc.

In a more complex approach, typically where the recording medium is rotated under a detector, this detector may be fixed relative to the recording medium and determine a lateral code across the record medium as a whole.

The rotation of the recording medium is advantageous since it allows repetitive detection of the authentication code hence reducing the signal to noise ratio.

Alternatively, a more conventional helical reading method could be used in which the record medium is read in a helical scan producing a more complex/longer code.

Preferably, the authentication code is read following manufacture of the recording medium and the code or an encrypted form of the code is provided on a package in which the recording medium is provided. This provides a unique link between having the packaging and the recording medium preventing being used on another recording medium.

The invention is particularly suitable for recording media such as in CD's or DVD's but of course is equally applicable to other recording media.

Some examples of recording media and methods relating to this invention will now be described with reference to the accompanying drawings, in which :- Figures 1A and 1B are a schematic cross-section and plan respectively of a CD ROM manufactured according to an example of the invention; Figure 2A illustrates schematically an example of a reading device; Figure 2B illustrates a typical signal obtained from the reading device shown in Figure 2A; Figure 3 is a block diagram illustrating an authentication system; and Figure 4 is a schematic view of packaging for the CD ROM shown in Figure 1.

The CD ROM shown in Figure 1A comprises a polycarbonate substrate 1 having it upper surface 2 embossed with digital information, using pressure and temperature. This embossed surface is metallised and then a protective lacquer layer 3 is spin coated onto this metallised surface. The spin coating process is known and an example is described in more detail in US-A-5395649.

The spun coated lacquer is then printed with information

defining the content of the CD ROM and other information such as the name of the issuing company and the like.

The CD ROM manufacture and process described so far is conventional.

In one example of the invention, particles are supplied to the lacquer prior to the spin coating process so that these particles indicated at 4 are spun with the lacquer material onto the surface of the substrate 1 and become embedded in the lacquer coating 3 in random positions spaced around the CD ROM as can be seen in Figure 1B.

As explained above, these particles 4 can be designed to be detected in a number of different ways. In one example, the particles are magnetic, while in other examples they luminesce, for example fluoresce, or simply act as radiation scattering points.

The locations of the particles 4 on the CD ROM can be used to define an authentication code. Figure 2A illustrates an example of a radiation source/detector unit 5 positioned above the CD ROM indicated at 6. The radiation source may comprise a set of LEDs, a UV lamp or the like (not shown) depending upon the nature of the particles 4 while the detector comprises, for example, a linear CCD array. As the CD ROM 6 is rotated beneath the radiation source/detector unit 5, the particles 4 are excited by the impinging radiation so as to fluoresce. The fluorescence is detected by those CCDs located above the particles 4 as they pass underneath. This fluorescence is collected by each CCD during a single rotation of the CD ROM 6 and then the charges are downloaded in a conventional manner resulting in a signal of, for example, the type shown in Figure 2B.

Figure 2B includes a positive charge at each lateral position where a particle was detected. In this case, there are six pulses indicating the presence of six particles at different radial positions. The signal from the unit 5 is then fed to an A/D converter 7 where it is

digitised and the form of the signal including the spacing between pulses is determined by a microprocessor 8 and stored in a database 9.

In more sophisticated apparatus, the radiation source/detector unit tracks a spiral groove in the CD ROM so as to detect all of the particles and develop a more complex code.

Packaging is then provided for the CD ROM and, as shown in Figure 4, the package 10, as well as including conventional printing describing the contents of the CD ROM as shown at 11, is printed with a bar code 12 which is associated with the authentication code of the CD ROM to be packaged. The bar code 12 may be a simple representation of the authentication code or, more preferably, an encrypted version. Alternatively, the bar code 12 may provide an address in the database 9 from which the authentication code can be obtained.

The authentication code on the CD ROM can be used simply to authenticate the CD ROM if there is a suspicion that it has been counterfeited, for example by comparing this manually or automatically with the bar code read by a bar code reader (not shown). However, it would also be possible to use an authentication code to control the use of the CD ROM. Thus, for example, a CD reader 13 (Figure 3) such as a PLAYSTATIONTM, CD player or the like may be provided with a modem 14 so that a signal representing the code as read by the CD reader 13 can be sent to the microprocessor 15 via a telephone line 16 and the microprocessor 15 can check from the database 9 as to whether or not the code is an authentic code. If it is authentic, the microprocessor 15 can provide an indication of this to the CD reader 13 and which indication could be used to allow the CD reader to operate. If the code is not authentic, the appropriate signal will not be sent back to the CD reader which will remain inoperative with that CD ROM.

To achieve this, a conventional CD reader could be modified so that the head used to read data also includes a light source/detector unit similar to the unit 5.