In this article, Malcolm Knight discusses the history, technology and status of the banknote feature termed Optiks. The journey has not been straightforward and represents a case study in how development programmes can become diverted by changed circumstances and constraints to results and outputs somewhat different to those originally envisaged.

figure 1

The origins of Optiks may be traced all the way back to the 1970s when, based on work undertaken by the National Research Council of Canada, a patent specification was published proposing the use of thin film interference filters for the protection of security documents such as banknotes and credit cards1. Subsequently, the Bank of England proposed a specific embodiment of such devices whereby a thin film dichroic filter element, i.e. a security thread, was incorporated within a sheet material such that the thin film element was visible through a pair of superposed windows present in the sheet material on each side of the element2.

figure 2

The intention was that when the dichroic filter was viewed in reflected light, it would appear to have a specific colour (determined by the detailed construction of the filter), but when it was viewed in transmitted light it appeared as a second colour which was complementary to the reflected colour. The Bank utilised precision vacuum sputtering equipment to manufacture a five layer all-dielectric material suitable for conversion to a security thread conforming to this concept. The resulting product, known as Thin Film Thread (or TFT for short), had a brilliant green colour when viewed in reflection and a complementary magenta colour when viewed in transmission (see figures 1 and 2).

figure 3

In order to achieve the desired effect, it was necessary that the TFT was incorporated into the paper sheet such that the thread was, in places, fully exposed on both sides; such a region became known as an ‘Aperture’ (see figure 3) with the thread exposed in the central region, but supported by the paper on the edges. This requirement presented a major challenge for the papermaker, Portals Limited. At that time, all banknote security threads were fully embedded within the thickness of the paper substrate and thus enclosed in fibre on both sides along their full length. There was no known papermaking process to enable exposure of a thread in an aperture on an industrial scale. The challenge was further complicated by economic constraints. The vacuum sputtering process needed to create the TFT with sufficient precision on the layer thickness was relatively slow and thus costly. Therefore, in order to produce industrial quantities of film material at an acceptable cost, the thread prepared from the film had to be limited to a width of 1.0mm. The objective was therefore to find a means of exposing the thread on both sides to a width of nominally 0.5mm while retaining support from the paper on the edge regions of width nominally 0.25mm on each side – and all of this being undertaken for multiple notes across a 2.8m wide paper machine while running at a commercially acceptable speed! Various ideas were trialled but the challenge could not be met on an industrial basis.

As an interim measure while trials on the Aperture programme continued, the Bank of England proposed a revised banknote structure whereby a security thread within the note would be exposed in discrete locations on one side of the paper only; such a region became known as a ‘window’. Since such a structure had a layer of fibre all along the back of the security thread, it was not compatible with the TFT interference filter; the thread structure selected for use in windowed form was to be vacuum metallised aluminium, similar to that widely used in fully embedded threads at the time, which with its mirror-like surface could not be replicated using photocopiers and similar equipment. 

Windowed thread concept
This revised objective led to a different research programme at Portals Limited. Many ideas were generated followed by a succession of trials on both a new papermaking process to create the thread exposure in windows and, once the essential papermaking principles were developed, means of retaining the thread in the paper to provide normal levels of banknote durability in circulation. The full details of this programme are outside the scope of the present article, but ultimately the project was successful and led to a new papermaking process3. The result was the world’s first ‘windowed thread’ banknote: the Bank of England series D GBP 20 (see figure 4), which was introduced into circulation on 15 November 1984. This was followed by the introduction of the windowed thread into the series D GBP 10 and GBP 50 banknotes.
In the mid-1980s, another thread was converted to windowed form, i.e. the magnetic metallic thread. In 1990, the negative demetallised thread termed Cleartext®4  was also introduced in windowed form and became very successful, with adoption in the currencies of many countries.

figure 4

Thus, although initially conceived as an interim measure pending resolution of the aperture problem, the windowed thread concept and manufacturing process proved to be highly successful and development work on security threads was mainly directed at additional variants. In 1992, an optically active windowed security thread comprising a three layer metal/dielectric structure was introduced, making use of the original principles of optical interference filters to provide security to banknotes5. Thereafter, a wide range of windowed thread structures providing security at the public, teller and covert machine readable levels has become available from several suppliers6. Nearly 30 years after its introduction, the windowed thread concept continues to contribute to banknote security in currencies around the world, and the limit on thread width has now been extended from the original 1.0mm to 6.0mm.

The development of Optiks 
While much attention was paid to the development of windowed thread variants, the original objective of creating a banknote structure where the thread could be exposed on both sides simultaneously was not forgotten and at intervals, further thought and experimentation was given to the aperture challenge. The breakthrough was made in 1998 when a new papermaking process was introduced in conjunction with a wholly new construction for security threads. The original concept of a 1.0mm wide interference filter was replaced by a step-change in thread width to 18mm, a different thread substrate and a structure comprising complex designs in demetallised vacuum deposited aluminium.

New papermaking process
The principles of the new papermaking process involve modifications to the cylinder mould paper machine, which is widely used to make most of the world’s banknotes. The basis of papermaking on such machines is the drainage of aqueous paper stock through a fine wire mesh (the ‘mould cover’) placed around a rotating cylinder located within a vat. Fibres from the paper stock are deposited onto the surface of this mould cover and progressively build up in thickness until that part of the rotating cylinder emerges from the vat. The wet paper fibres are then peeled off the rotating cylinder, drained of water, pressed and dried to form paper. 

For the modified process, a technique was developed whereby sections of the mould cover of a defined shape, for example an oval, are blocked off or ‘blinded’, such that no fibre can be deposited in those regions. The superwide security thread is then inserted into the paper machine in such a way that it overlies the blinded regions; since there is no fibre present between the blinded region and the adjacent surface of the thread, the thread is exposed in these regions on that side of the note in the final paper.

The thread itself, at 18mm, is very wide compared to the average length of the paper fibres (~ 1mm) and is impervious to the paper stock. Once the thread has been introduced into the paper machine and made contact with the rotating cylinder, no paper stock can drain through the thread and no fibre can be deposited over the back of it. Therefore, in the finished paper the main portion of the back of the thread is free of fibre and exposed for the full length of the thread. A small amount of fibre is deposited over the back of the thread at the edges and this contributes to bonding or ‘keying’ the thread into the finished paper7

The resultant paper thus incorporates a superwide security thread which, on one side, is fully exposed along the central regions for the full height of the banknote and on the other side is exposed in a controlled manner in a specific region of defined shape, for example, an oval; in this defined region, the thread is thus free of paper and exposed on both sides – the aperture. Either side of the banknote may be deemed front or back of the note design according to the preference of the issuing authority. The banknote structure resulting from this novel process was given the trade name ‘Optiks8.

The security thread to be used in conjunction with this new paper manufacturing process had to be very different to all previous threads, embedded and windowed. In the first instance, a new thread substrate – polypropylene – was adopted, instead of the polyester used for embedded and windowed threads, to provide the flexibility and elastic properties needed for such a superwide thread. This was then metallised with vacuum deposited aluminium and selectively demetallised in a specific pattern characteristic of the banknote in question; the pattern is designed to give a good balance between reflective and transparent regions which make use of the two-sided exposure in the aperture. A complex sequence of protective coatings and adhesives is then applied to enable the superwide thread to bond to paper on the front and the back edges, while enabling print-receptive regions which will take and hold banknote inks during the subsequent printing processes.

figure 5

The new technology was first introduced to the banknote industry at the Currency Conference in Rome in 2004. A dedicated housenote was designed around the theme of Sir Isaac Newton to illustrate the combination of properties available from the Optiks approach. Figure 5 shows this housenote and, to the right of the intaglio portrait, the exposed oval region in the paper (the Optiks aperture), revealing the security thread with a demetallised design representing the famous apple linked to Newton’s discovery of the theory of gravity. 


figure 6

Figure 6 comprises close-up views of the two sides of the security thread viewed in reflected light, on the left illustrating the thread exposed in the oval opening in the paper and on the right showing exposure of the thread for the full height of the note.

figure 7

In figure 7, the banknote is viewed firstly in combined reflected and transmitted light, when the opaque metallic content of the thread is seen in silhouette in the aperture region, and then in transmitted light when the full width of the thread becomes apparent and the demetallised pattern is perceptible for the full height of the banknote. The demetallised regions are then seen to be transparent and in the aperture enable perception of material behind the Optiks note. 
Holding a banknote up to a source of transmitted light is an instinctive response in many countries whenever members of the general public wish to authenticate a banknote. Thus the check on the Optiks feature is readily achieved alongside the check of the watermark.

Optiks aspects
It is important to note that the Optikssecurity thread is, like fully embedded and windowed threads, an integral part of the banknote. The paper is formed around the thread; it is not applied in a subsequent operation. This gives rise to a number of properties which contribute to the overall security of the banknote:

• The watermark, embedded thread and other traditional paper-borne security features, including the characteristic feel and crackle of a cotton-based banknote, can all be retained.
• The banknote is fully compatible with normal printing processes, including intaglio with the full range of tactility.
• The region around the exposed hole in the paper where the mould cover was blinded has a fibrous ‘furry’ edge, sometimes referred to as a ‘deckle edge’. The presence or absence of such fibre pattern helps distinguish between a genuine Optiks™ banknote and a counterfeit produced by cutting or punching a hole in paper and then overlaying a plastic strip.

figure 8

The above aspects of Optiks are aimed at authentication by the general public, i.e. they make it a level 1 feature. Optiks can also provide security for bank tellers and other professional handlers of cash (level 2a) by the addition of luminescent material in selected regions of the security thread. Normally, this is applied as a strip running along the two edge regions; when the banknote is viewed under UV light placed 
over the coated side, the luminescent regions are seen as continuous strips, but when the banknote is viewed with the light source behind it, only the demetallised regions have the luminescent glow observable through the clear regions (see figure 8).

The Optiks technology thus retains all of the security opportunities available from a paper-based banknote 
while also containing an aperture with distinctive patterns surrounded by clear, transparent regions. In that respect, it can be regarded as providing many of the security benefits of both a paper- and a polymer-based banknote, although as outlined above, its origins do not lie with this objective. Since its introduction, Optiks has been adopted for the currencies of ten nations; the most recent example is the GIP 100 issued by HM Government of Gibraltar (see figure 9).

figure 9

Recent developments
Like its windowed thread predecessor, Optiks represents a technology platform rather than a single product. The underlying technology will lead to a succession of product variants providing additional security benefits supplementing the core properties. 
A recent development along these lines, termed InSight(see figure 10) makes use of interaction between a specific fine demetallised pattern on the thread and a related design formed from complementary coloured translucent inks applied in tight register on both sides of the aperture during the banknote printing process 8.

figure 10

Here, the demetallised pattern on the Optiks thread comprises an intricate fine line pattern masking a characteristic image, such as a lighthouse. During the normal banknote printing process, the aperture region is overprinted on each side with a different specific pattern, for example a light bulb, using two complementary colours such that on one side the light bulb is printed in, for example, silver ink with a blue background, and on the other side in a blue ink with a silver background. When the note is viewed in reflected light, only a light bulb is seen – in silver or blue according to which side is uppermost.
However, when the note is viewed using transmitted light, both the silver and blue inks appear translucent and together merge to form a uniform colour such that the image of the light bulb disappears altogether. However, the vacuum deposited metal on the underlying Optiks thread is opaque and thus is readily seen in silhouette – in this case in the form of a lighthouse . This change in appearance represents an easy effect to describe in a public education campaign but is a significant additional challenge to counterfeiters.
In recognition of the outcome of the complex development programme and novel combination of properties available from Optiks and InSight, in April 2013 the technology was granted the UK’s Queen’s Award for Enterprise: Innovation.

In many cases, development programmes start with a tight project definition and the primary challenge is to deliver the defined end result in the shortest possible time and at the lowest possible cost. However, on occasion, experimental work can lead to different or additional opportunities to that originally intended. 

In the case of the windowed thread manufacturing process, which was initially seen as an interim measure, the spin-off technology can itself assume considerable significance and, in this specific case, has led to the development of a succession of products offering a range of properties. 

Furthermore, modifying the constraints of a project may lead to a different end result to the original objective, but one which offers its own opportunities. While the origins of Optiks lie in an attempt to expose a 1mm wide strip in an aperture, the end result – a new papermaking process enabling an 18mm wide security thread fully incorporated within the body of the paper – represents a technology platform in its own right with scope for a new family of security devices reliant on the availability of an aperture within a paper-based banknote. 


1 Patent US 3858977.
2 Patent application GB 1552853 A.
3 Patent EP 0059056 B1.
4 Cleartext® is a registered trademark of De La Rue International Limited.
5 Knight, M.R.M. Looking Back and Looking Forward – A Papermaker’s View. Currency Automation Conference Presentations, 3-6 May 1992, San Francisco.
6 See for example InfoSecura Number 54, November 2012.
7 Patent EP 1141480 B1.
8 Optiks and InSight are trademarks of De La Rue International Limited.