This invention relates to a means of invisibly marking articles for identification and authentication purposes, particularly (but not exclusively) in the retail trade.

The counterfeiting and parallel trading of retail goods is a long standing problem in the retail industry: lost revenue, legal liability for claims and guarantees, and damage to goodwill can seriously affect a company.

The need for an effective means of combating counterfeiting has been the subject of considerable effort in the past. Perhaps the simplest and most common way of indicating the origin of goods is by the use of trade marks. Such marks, which are directed primarily at the customer, are obvious to the counterfeiter and readily lend themselves to copying. As a countermeasure some marks have become increasingly complex in design, sometimes incorporating sophisticated optical effects such as holograms, diffraction effects etc., but the determined counterfeiter has proved equal to the task of copying even the most sophisticated mark.

The use of infrared technology in the retail trade is also known. For example EP 444 331 describes the marking of an object with a code which can be scanned with infrared radiation. The code is applied using a printing ink which is colourless in the visible spectrum but absorbing in the infrared spectrum.

The above patent is primarily concerned with use on decorative packs, where a visible code would be undesirable, and with the prevention of counterfeiting of banknotes. The need for infrared scanning equipment renders the technique unsuitable for many of the circumstances which arise in the ongoing campaign against illegal trading: often it is necessary to enter premises and inspect the suspect goods and this is a potentially dangerous situation for the inspector concerned. The need for a means of covertly inspecting suspect goods is well recognised.

Under the present invention an identification mark, which is not recognisable as such under normal viewing conditions, is applied to articles and means of visually checking articles for the presence of such labels is provided. Unlike the invention described in the above referenced patent, the current invention forms a visible two dimensional image of the identification mark. The images produced can be viewed on a suitable television monitor or recorded using standard video recording equipment for future reference. Moreover, with the benefit of current technology, the imaging equipment used can be small enough to be used covertly.

According to this invention, a method of identifying articles comprises the steps of applying a label to articles and forming an image of said label with the aid of infrared imaging equipment, wherein the images so formed display features, which are not apparent on viewing the labels without the use of infrared equipment.

In a preferred embodiment the infrared imaging equipment comprises a television camera which is sensitive to radiation within the infrared region of the electromagnetic spectrum.

In a further preferred embodiment the television camera is of the silicon charge coupled device (CCD) type.

In a further preferred embodiment an optical filter is used to reduce the amount of visible light entering the television camera.

In a further preferred embodiment the optical filter comprises a combination of gelatin filters.

In a further preferred embodiment the optical filter comprises crossed polaroids.

In a further preferred embodiment the optical filter may be switched in and out of operation.

In a further preferred embodiment the optical filter comprises a liquid crystal cell.

In a further preferred embodiment the switching of the optical filter is synchronised to the camera frame period.

In a further preferred embodiment an artificial source of infrared radiation is used to illuminate the articles.

In a further preferred embodiment the source of infrared radiation comprises an infrared radiation emitting diode.

In a further preferred embodiment the source of infrared radiation comprises a tungsten lamp.

In a further preferred embodiment the output from the source of infrared radiation is not visible under normal viewing conditions.

In a further preferred embodiment the source of infrared radiation comprises tungsten lamp used in conjunction with an optical filter.

In addition to its use as an anticounterfeiting tool, the current invention provides a means for combating parallel trading. Information concerning the legitimate retail route for an article (eg date of manufacture, batch no., legitimate distributor or retail outlet etc.) may be secretly incorporated into a label on the article. The articles may then be examined in a retail outlet or warehouse in order to ascertain whether they have gone through a legitimate route from the factory to that outlet.

The invention will now be described, by way of example only, with reference to the following figures in which:-

Figure 1 is a schematic representation of an embodiment of the invention.

Figures 2a and 2b show, by example, cross sections of identification marks which may be used in this embodiment.

Figure 3 is a schematic cross-section of a liquid crystal cell for use as a switchable optical filter in this embodiment.

Referring to figure 1, articles or their packaging 1 are labelled with a distinctive identification mark 2. The appearance of this mark when viewed with the aid of suitable IR imaging equipment is discernibly different to that when viewed with the unaided eye under normal lighting conditions. The label may if required contain coded information about, for example, the origin of the goods, date of manufacture etc. That component of the mark which is visible under normal lighting conditions can be incorporated into the visible features of the article or package so that it would not be obvious that a mark is present.

There are several means of devising suitable marks, for example by use of materials which are substantially transparent in the visible spectrum but absorbing, or reflecting, in the infrared (for example a thin layer of copper sulphate) . The fundamental requirement is ttfat the authentication mark is not recognisable as such to the unassisted human eye but is recognisable if irradiated with infrared radiation and viewed with the aid of suitable equipment, - in this case a CCD TV camera. Other embodiments may include some of the optical effects currently seen in the visible spectrum (eg diffraction, hologramatic etc, see for example "Principles of Optics" by M Born and E Wolf, Pergamon Press).

Articles are checked using a Pulnix model TM6 silicon CCD TV camera 3 available from Pulnix house, Aviary court, Wade Road, Basingstoke, Hampshire, RG24 OLP, UK. The sensitivity of this device extends from the lower end of the visible region of the electromagnetic spectrum (wavelength of 0.4 micrometre) into the near infrared region (up to a wavelength of 1.1 micrometer) where it is limited by the bandgap of silicon. Other imaging devices can be used if their sensitivity extends into the infrared spectrum.

The lens 4 used with this camera was an 8.5mm fl.3 item available from RS components, (stock no 625-132) P0 Box 99, Corby, Northants., NN17 9RS, UK. Any standard CCTV lens, selected to give the required range, field of view etc., and which is transparent to infrared within the wavelength range being used would be acceptable.

An optical filter 5 may be placed in the path of radiation entering the camera (most conveniently over the lens) to filter out visible radiation. In this embodiment an electronically switchable filter derived from polaroids and liquid crystal material is used. Use of a switchable filter, in conjunction with a camera sensitive to both infrared and visible radiation allows the investigator to reference the infrared images to the more familiar visible image of the article under scrutiny.

The infrared radiation which is present in the ambient lighting may be supplemented by means such as a switchable tungsten lamp 6. Covert illumination may be achieved by including a filter 7 to block out the visible component from the output of the lamp 6. In this embodiment, filter 7 was constructed using three Wratten (TM) gelatin filters: Nos. 25 (red), 58 (green) and 47B (blue).

Referring to figure 2a, authentication mark 2 comprises two layers. In this example the first layer 8 comprises a material which is absorbing in the infrared region (for example copper sulphate) and is covered by the second layer 9 which is printed in ink which is not transparent to all radiation in the visible region but transparent to infrared radiation within the sensitivity range of the camera being used. Thus under normal viewing conditions only the top layer may be seen, but with the aid of the CCD TV camera the bottom layer, which may be patterned or coded, can be imaged.

Figure 2b shows, in cross-section, a second example of an identification mark comprising a plurality of elements arranged in a single layer. One or more element 10 is formed in ink which is coloured under normal (visible) lighting but is highly transmissive to infrared radiation. If the substrate 12 is highly reflective in the infrared (for example paper) then overall there is high reflectivity of infrared radiation.

One or more element 11 is formed in ink which is coloured under normal (visible) lighting and absorbing or reflecting to infrared. The mark may blend in with the other visible features of its environment under normal viewing conditions but appears quite different when imaged using the CCD TV camera.

Referring to figure 3. switchable optical filter 5. takes the form of a liquid crystal cell. The construction of such a device is well documented (for example in Appl. Phys. Lettr. l£, 1971. pl27.) and it's usefulness in the present invention depends on the fact that whether in the blocking or transmitting state with regard to visible radiation, it remains at least partially transparent to infrared radiation of suitable wavelength (radiation in the near IR is unaffected by the Polaroid filters used in such a device) .

Two glass plates 13. held apart by spacers 14, sandwich a five micrometre layer of twisted nematic liquid crystal material E7 15 available from Merck chemical co. (formerly BDH) . The glass plates 13 are clad on their outer sides by polarisers 16a and 16b arranged so that their respective axes of polarisation are mutually perpendicular, the directions of these axes being represented by →έ > (in the plane of the page) and ^^ (normal to the plane of the page) .

The inner surfaces of the glass plates 13 are clad with electrodes which take the form of thin layers of InSnO 17- Prior to assembly these surfaces are treated so as to effect alignment of the liquid crystal molecules. This could be achieved by, for example, unidirectional rubbing. The direction of rubbing on each plate is substantially parallel with the polarisation axis of the polariser associated with that plate.

Light entering the cell substantially along the direction indicated by the arrow 18 is polarised on passing through polariser 16a. As the light then passes through the liquid crystal material 15. in the absence of an electric field, the axis of polarisation is rotated by 90° thus allowing the light to exit the cell through polariser l6b.

The presence of an electric field, applied via electrodes 17, removes this 90° rotation thus allowing no light to pass through polariser 15b.