GB2240947A - Authentification of documents with luminescent security features - Google Patents

Abstract

Document authentication apparatus is described for authenticating a document (2) having a security feature (3) of luminescent material. The apparatus comprises a lamp (8) for exciting the luminescent material to emit radiation; a liquid crystal shutter (10) through which a document to be authenticated can be viewed and which is switchable between transparent and opaque states; and control circuitry for controlling switching of the excitation means between on and off states and switching of the shutter between its transparent and opaque states, the apparatus having a mode of operation so that there is a first period in which the excitation means is in its on state to excite the luminescent material while the shutter is in its opaque state and a second period in which the excitation means is in its off state while the shutter is in its transparent state, whereby phosphorescent emission only, and not fluorescent emission, is visible during the second period. <IMAGE>

Description

DOCUMENT AUTHENTICATION This invention relates to document authentication.

Luminescent materials are widely used to provide security features on security documents such as passports, banknotes, credit cards, pension books, building society passbooks, etc. These materials generally are excited by longwave ultraviolet light, and emit in the visible region of the spectrum.

Although some of these security features, such as those on banknotes, are intended for machine authentication, many are designed for authentication visually at a transaction point, such as a bank counter or an immigration desk.

At such a transaction point the security document is placed under an ultraviolet lamp, and a teller/immigration officer authenticates the document by viewing the visible luminescence from the security feature. Usually the feature is printed with an invisible ink, so that it cannot be seen unless placed beneath an ultraviolet lamp.

These luminescent security features are now so widely used that a person wishing to produce counterfeit documents would be aware of their existence, and hence would attempt to replicate them. As many of these luminescent materials used as security features are themselves secure materials (in that the formulation of these materials will be safeguarded by the manufacturer of the security documents), a counterfeitor is unlikely to have access to the actual material used.

However the counterfeitor does not have to use the same material as the document manufacturer if the document is to be only visually authenticated; all that is required is that the material used on the counterfeit document emits light of a similar colour to that of the secure material.

The luminescent material used for these security features can be either fluorescent or phosphorescent.

With visual authentication by the known method described above, a phosphorescent material cannot be distinguished from a fluorescent material, providing the emission spectra (colour) are similar, and so a counterfeitor can use a fluorescent material where the authentic material was phosphorescent, and, vice-versa, could use a phosphorescent material to reproduce a fluorescent security feature.

For the purposes of the present text, a fluorescent material is considered as one whose emission decays in less than 10.8 seconds after excitation, whilst a phosphorescent material is considered as one whose emission decays in a period longer than 10 8 seconds after excitation.

For the purpose of this invention, a fluorescent material can therefore be considered as one whose emission is only present when the excitation lamp is switched on, whilst a phosphorescent material will still emit light after the lamp has been switched off i.e. it will have an "after glow".

It is an object of the present invention to provide an apparatus which can be used to distinguish between an instantaneous fluorescent feature, and a phosphorescent feature with an "after glow".

According to the present invention there is provided document authentication apparatus, for authenticating a document having therein or thereon a security feature of luminescent material, the apparatus comprising: excitation means for exciting the luminescent material to emit radiation; a shutter through which a document to be authenticated can be viewed and which is switchable between transparent and opaque states; and control circuitry for controlling switching of the excitation means between on and off states and switching of the shutter between its transparent and opaque states, the apparatus having a mode of operation so that there is a first period in which the excitation means is in its on state to excite the luminescent material while the shutter is in its opaque state and a second period in which the excitation means is in its off state while the shutter is in its transparent state, whereby phosphorescent emission only, and not fluorescent emission, is visible during the second period.

Such an apparatus can hence be used by an observer to make a judgement as to whether a luminescent security feature contains phosphorescent material and thereby authenticate the document. The apparatus is particularly applicable for authenticating documents containing fluorescent and phosphorescent materials, either one upon the other or one adjacent the other.

Preferably the shutter comprises liquid crystal material whose state can be altered between transparent and opaque by the application of an electric field across the liquid crystal material.

The excitation means is conveniently an ultraviolet lamp arranged between the document to be authenticated and the shutter.

The control circuitry preferably includes a timing device operable to produce two pulse trains in antiphase for controlling respectively the shutter and the excitation means to provide a plurality of said first and second periods alternating on a regular basis.

A delay circuit can be provided to reduce the mark/space ratio of the pulse train controlling the shutter whereby a delay is introduced between turning off the excitation means and switching the shutter to its transparent state and between switching the shutter to its opaque state and turning on the excitation means.

To enable both fluorescent and phosphorescent materials to be viewed together, the apparatus can be operable in a second mode to control both the shutter and the excitation means in phase whereby the shutter is in its transparent state while the excitation means is on.

To distinguish phosphorescent materials having a relatively long lifetime, the apparatus can have a further time delay circuit for operation in a third mode to increase the delay between turning off the excitation means and switching the shutter to its transparent state.

A switch can then be provided for selecting one of said modes of operation, which switch can be automatically controlled to sequentially select said different modes of operation.

The invention also contemplates a security document having provided therein or thereon a security feature comprising fluorescent and phosphorescent materials which have similar emission characteristics so that they cannot be distinguished under normal excitation conditions. The phosphorescent and fluorescent materials can be adjacent one another or can be provided in the form of a background of one of a fluorescent and phosphorescent material with security markings of the other of said fluorescent and phosphorescent material formed on said background.

Such a security document provides an additional safeguard against forgery yet the phosphorescent material of the security feature can be viewed alone using the apparatus of the present invention.

Potential forgers would not realise when viewing the document under a conventional excitation lamp that the document contained both fluorescent and phosphorescent material. They would see only a single emission colour from the security feature.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a diagram of the general arrangement of the apparatus; Figure 2 is a block diagram of control circuitry for the apparatus; Figure 3 is a timing diagram showing the control waveforms in the control circuitry of Figure 2; Figure 4 is a modified timing diagram for use in observing long lifetime phosphorescent materials; and Figures 5a and 5b illustrate documents with different security features.

Referring to Figure 1, apparatus for authenticating a document 2 containing a phosphorescent security feature 3 will be described. The document 2 to be authenticated is placed on a support 4 below a window 6. An excitation means in the form of a W lamp 8, for example a low pressure phosphor coated mercury lamp whose principle light output is at 365nm, is placed between the window 6 and a shutter 10. The shutter 10 has transparent and opaque states and will be described in more detail hereinafter. With the shutter 10 in its transparent state, an observer 11 can view the document 2 through the shutter 10 and window 6.

The shutter 10 comprises a layer of liquid crystal (LC) material 12 which is sandwiched between two glass plates 14,16, which are made conductive by the deposition of a transparent conductive layer. Copper electrodes are attached to the conductive layer so that an electrical field may be applied to the LC material. When an electrical field is applied to the shutter the LC material becomes transparent, and when this field is reversed the material becomes opaque. The construction of liquid crystal shutters of this type is known for example in the field of eye protection during welding and will not be described further herein. As an example, attention is drawn to U.S. Patent No. 4237557.

Liquid crystal shutters are particularly suitable for use in the present invention as they have a fast response time, a high transmissivity when in the transparent state and they can be constructed at a size and weight suitable for use in a portable authentication apparatus.

Control circuitry for the apparatus is shown in Figure 2.

The lamp 8 and the shutter 10 are controlled by respective drive circuits 18,20 connected to the lamp 8 and shutter 10 via connections 22,24; 26,28 respectively. As shown in Figure 2, the lamp drive circuit 18 is controlled by a clock 30 through a first delay Dl. The shutter drive circuit is controlled by the clock 30 optionally through the first delay Dl, a second delay D2, and a combination of the second delay D2 and a third delay D3. A three way switch 32 selects the delay applicable to the shutter drive circuit in a manner to be described in more detail hereinafter.

The lamp drive circuit 18 comprises a very high frequency (in comparison to the frequency of clock 30) oscillator which is used to control a transistor which switches current into the primary of a transformer. The transformer steps up the primary voltage to a value that causes the gases in the lamp to ionise and conduct, and so emit light.

In a similar way the LC shutter 10 is also driven by a high frequency (in comparison to that of clock 30) drive voltage. It is desirable to drive the LC shutter with a high frequency a.c. voltage, as opposed to a d.c. voltage, to avoid polarisation effects in the LC material and possible damage.

The control circuit of Figure 2 is used to operate the apparatus of Figure 1 so that the lamp 8 is modulated by an electronic signal 5L which is a pulse train arranged to switch the lamp on and off at a fixed frequency. The frequency of modulation of the lamp can be from 25Hz up to several kHz. This modulating signal SL is illustrated in the timing diagram of Figure 3. The LC shutter 10 is also modulated by a similar electronic signal s to that used to modulate the lamp, but in this case the signal is is in antiphase to the signal SL applied to the lamp. This is also illustrated in the timing diagram of Figure 3.

The timing is such that whilst the luminescent security feature on the document is excited by the lamp 8, the LC shutter 10 is opaque, and when the lamp 8 is switched OFF the LC shutter is transparent. Therefore if a document with a phosphorescent security feature is placed beneath the apparatus, the observer will see emission from the phosphorescent material because of the "after glow", but a fluorescent material placed beneath the apparatus will appear black because the emission lifetime is too short.

The apparatus hence enables phosphorescent material to be detected visually.

As a further feature of the invention the switch 32 enables the opening of the LC shutter 10 to be in phase with the ON period of the lamp i.e. the shutter is transparent when the lamp is ON, and opaque when the lamp is OFF.

In this mode of operation when the security document 2 is placed beneath the window 6 both phosphorescent and fluorescent features will be seen. In operation the user could switch between in phase and antiphase modulation of the LC shutter 10, and he would see fluorescent features in one mode which would disappear in the other mode, while phosphorescent features would be present in both modes.

The operation of the invention is described in more detail below with reference to the block diagram of Figure 2.

The clock 30 is an oscillator which produces two pulse trains l' 02 in antiphase which form the basic timing signals for both the lamp and LC shutter modulation. The modulating waveforms SL,Ss (Figure 3) are used to switch on and off the high frequency oscillators of the drive circuits 18,20, which in turn causes the lamp and shutter to be switched between their respective two states. The first and second delays Dl,D2 have the effect of altering the mark/space ratio of the signal Ss modulating the shutter 10 to delay its opening and advance its closure relative to switching of the lamp 8.This is advisable because both the lamp and the LC shutter have finite switching times, and if the shutter were "partially" open whilst the lamp was ON the observer would see some instantaneous fluorescent emission when the apparatus was in the mode of operation to observe phosphorescent emission only. A delay T is thus introduced both between the lamp turning off and the LC shutter opening, and between the LC shutter closing and the lamp switching on again.

This is accomplished in that each of the first and second delays D1, D2, comprises a monostable M1,M2 respectively connected to receive the relevant timing signal l' 2' respectively and an AND gate 34,36 respectively one input of which is the output of the monostable M1,M2 and the other input of which is the input to the monostable N' 2- The monostables M1,M2, are triggered by the leading edge of the respective timing signals 1' 162' and their outputs are shown as waveforms A,B in Figure 3.

The AND gates combine these signals A,B with the respective timing signals l' 2 to produce an alteration in the mark/space ratios of the modulating signals Ss, SL so that they are non-overlapping.

In the first position i of the switch 32, the apparatus is in a normal phosphorescence detection mode with the waveforms Ss, 5L as shown in Figure 3; in the second position ii of the switch 32, the drive circuits are operated in phase as described earlier for a phosphorescent/fluorescent detection mode and in the third position iii of the switch 32, a further refinement is possible to detect long lifetime phosphorescent materials. The delay D3 comprises a monostable M3 and an AND gate 38. The shutter modulating signal Ss is supplied to the monostable and to one input of the AND gate 38 and the output of the monostable is supplied to the other input of the AND gate.In switch position iii, the combination of delays D2 and D3 cause the opening of the shutter 10 to be delayed from the instant the lamp is switched off by a value T11 greater than T so that only phosphorescent materials with a longer lifetime (a more persistent "after glow") will be seen. This method of operation is illustrated by the timing diagram of Figure 4.

As a further refinement to the invention the insertion of a document under the apparatus could be detected automatically, by a photodetector or proximity detector, and the apparatus would then automatically be switched on.

Following automatic detection of the presence of a document 2 under the apparatus, a timing sequence could be initiated such that function of the switch 32 becomes automatic. Thus the LC shutter 10 would switch automatically at a low frequency ( e.g. 0.1 to 1Hz) from being in phase with the lamp 8 (position ii) to being out of phase with the lamp (position i), and the observer will see instantaneous emission followed immediately by delayed emission. If both fluorescent and phosphorescent materials are present on a security document, the user of the apparatus will see the fluorescent feature appear and disappear at this low frequency, while the phosphorescent feature will be seen constantly. The switching sequence could include the third mode, position iii of the switch 32, i.e. to detect phosphorescent materials having a long lifetime.

The apparatus would thus slowly switch between the instantaneous emission mode, the delayed emission mode and the long persistence mode. If on a security document, for example, a security features comprises both a fluorescent material and a short lifetime phosphorescent material, the observer using the apparatus in this three mode switching arrangement would see: 1. Both the phosphorescent and fluorescent features (in phase); 2. The phosphorescent feature only (anti-phase); and 3. Neither feature (delayed anti-phase).

If in producing a counterfeit document the forger did not use a phosphorescent material with a short lifetime, the feature would be observable in the final mode, and the document would not pass the authentication check.

Although it would be possible to incorporate photodetectors to detect delayed emission, and to produce an indication of this emission, it is an important advantage of this invention that the verification can be made by a human observer.

Another application of this apparatus is to detect a novel type of security feature, that is having a combination of phosphorescent and fluorescent materials in the same security feature. Figures 5a and 5b illustrate two exemplary security features.

If a fluorescent material were printed on a document, and subsequently overprinted with a phosphorescent material (the overprinting taking the form of alphanumeric characters or a motif for example), the overprinting would be invisible under a conventional ultraviolet lamp, providing the instantaneous emission spectra of the two materials were similar. However, by using the apparatus of this invention the fluorescent background would become invisible, and an observer would see the overprinted characters. An example of this type is shown in Figure 5b, with the fluorescent material indicated by shading and the phosphorescent material in black.

At present many building societies in the U.K. use their customer's signature in an invisible phosphorescent material as a safeguard. The customer is required to sign a phosphor coated paper placed in the passbook.

Consequently the pressure of signing transfers an invisible impression of the signature onto the passbook.

On presentation of the passbook for a withdrawal the customer's signature is compared with that in the passbook, the signature being observed using an ultraviolet lamp.

This safeguard is now so well known, that a person wishing to present a stolen passbook could purchase an ultraviolet lamp and thereby observe the signature, so that he could practice forging it.

If the phosphorescent signature were impressed onto a fluorescent background in the passbook, and the phosphorescent and fluorescent instantaneous emissions were similar in colour, the signature would be invisible under the conventional ultraviolet lamp presently used.

However, placed under the apparatus of the present invention the phosphorescent signature would stand out, because the background printing would become invisible.

Figure 5a shows a security feature in which the phosphorescent and fluorescent materials are printed adjacent one another. If the phosphorescent and fluorescent materials have similar emission spectra, they will be indistinguishable unless the document is viewed using the apparatus of the present invention.

Claims (14)

CLAIMS:

1. Document authentication apparatus, for authenticating a document having therein or thereon a security feature of luminescent material, the apparatus comprising: excitation means for exciting the luminescent material to emit radiation; a shutter through which a document to be authenticated can be viewed and which is switchable between transparent and opaque states; and control circuitry for controlling switching of the excitation means between on and off states and switching of the shutter between its transparent and opaque states, the apparatus having a mode of operation so that there is a first period in which the excitation means is in its on state to excite the luminescent material while the shutter is in its opaque state and a second period in which the excitation means is in its off state while the shutter is in its transparent state, whereby phosphorescent emission only, and not fluorescent emission, is visible during the second period.

2. Apparatus as claimed in claim 1 in which the shutter comprises liquid crystal material whose state can be altered between transparent and opaque by the application of an electric field across the liquid crystal material.

3. Apparatus as claimed in claim 1 or 2 in which the excitation means is an ultraviolet lamp arranged between the document to be authenticated and the shutter.

4. Apparatus as claimed in any preceding claim wherein the control circuitry includes a timing device operable to produce two pulse trains in antiphase for controlling respectively the shutter and the excitation means.

5. Apparatus as claimed in claim 4 which comprises a delay circuit for altering the mark/space ratios of the pulse train controlling the excitation means and the shutter whereby a delay is introduced between turning off the excitation means and switching the shutter to its transparent state and between switching the shutter to its opaque state and turning on the excitation means.

6. Apparatus as claimed in any preceding claim which is operable in a second mode to control both the shutter and the excitation means in phase whereby the shutter is in its transparent state while the excitation means is on, whereby fluorescent and phosphorescent material may be viewed together.

7. Apparatus as claimed in claim 5 or claims 5 and 6 comprising a further delay circuit for operation in a third mode to increase the delay between turning off the excitation means and switching the shutter to its transparent state, thereby to distinguish phosphorescent materials having a relatively long lifetime.

8. Apparatus as claimed in claim 6 or 7 comprising a switch for selecting one of said modes of operation.

9. Apparatus as claimed in claim 8 comprising means for automatically controlling said switch to sequentially select said different mode of operation.

10. Apparatus substantially as hereinbefore described with reference to or as shown in Figures 1 to 4 of the accompanying drawings.

11. A security document having provided therein or thereon a security feature comprising phosphorescent and fluorescent materials which have similar emission characteristics so that the security feature cannot be observed under normal excitation conditions.

12. A security document as claimed in claim 11 wherein the phosphorescent and fluorescent materials are adjacent one another in the security feature.

13. A security document as claimed in claim 11 wherein there is formed on a background of one of the phosphorescent and fluorescent materials security markings of the other of said phosphorescent and fluorescent materials.

14. A security document substantially as hereinbefore described with reference to or as shown in Figure 5a or 5b of the accompanying drawings.