The evolutionary use of holograms on banknotes.
The First Banknote DOVIDs
However, the challenge had been made for a technological advance which would resist colour copying and in 1988 the banknote world responded in definite fashion. In that year, and at opposite ends of the Earth, two banknotes were issued that would signal a permanent change to banknote security. In Austria, the high value 5000 schilling note appeared with a patch hologram of Mozart and, in an even bigger departure from convention, Australia issued a commemorative A$10 note made of polymer (biaxially orientated polypropylene) displaying a computer generated image of Captain Cook in a transparent window.
Although Austria discontinued the use of holograms on its notes and Australia suffered numerous technical problems (17.5 million $10 notes were manufactured but the ones that went into circulation did not age well) the stage was set for a revolution in banknote security which would proliferate. It became clear that the features could not be photocopied, no matter how refined the copier. This is because a defining feature of OVDs (optically variable devices) is that their appearance in terms of colour and imagery is strongly angular dependent. This means that as the note/OVD is handled, rainbow colours flash in front of the viewer and, by comparison, a photocopy of the OVD appears lifeless.
Polymer Outlasts Catpix
It is this varying rainbow pattern that most distinguished the Captain Cook image from anything that had been seen previously on a banknote. The development of a polymer substrate was merely a means to an end, that end being the ability to accept a game changing optically variable anti-counterfeit device. Ironically, the polymeric substrate had greater staying power for Australia than the OVD. Securency has had worldwide success with this robust substrate which has been adopted by 30 countries worldwide but Australia discontinued the use of holograms after this experiment.
Strictly speaking, this isn't a hologram, because the diffractive fringes were totally computer generated by CSIRO without any recourse to the phenomenon of interference, but to a wider non-specialist audience, this is hair-splitting. The image had no depth or parallax in any direction. Occasionally, it seems to flip from a positive to a negative but it doesn't do that in a consistent way. Indeed, its main visual features are that it resembles a portrait of Captain Cook and its metallic surface glistens with iridescent colours when caught in the light. That is enough to dramatically distinguish it from a photocopy.
Although this note was not primarily intended for circulation, the idea of using such OVDs for circulation currency caught the attention of the financial community and the idea caught on. Although initially appearing on notes as a metallic patch, these OVDs were to appear as stripes and embedded window threads such that, over the next 20 years, nearly 100 issuing authorities were to adopt the technology and apply it to over 200 denominations
Enter the Euro
The launch of the Euro in 2002 proved that the technology had truly arrived. With stripes on the lower denominations and patches on the higher, more than 13 billion notes were issued that year.
The OVDs used on the Euro are much more complex than the Captain Cook. In addition to rainbow colours, these 'Kinegram[R]' and 'al-phagrams[TM]' (mastered by OVD Kinegram and Hologram Industries respectively) contained dynamic imagery channelled to show different graphic designs at different angles. Such complexity presents greater challenges in communication to specialist examiners and the general public. Yet in spite of the challenge of communicating the salient visual features of security holograms through leaflets, adverts, crib cards etc., the complexity of imagery has continued unabated.
In a study conducted recently in The Netherlands, it was found that the hologram is a highly recognizable public recognition feature second only to the watermark. The study, conducted by De Nederlansche Bank BV found that the watermark and the hologram were the easiest to teach to the uninitiated in terms of what they should look for. They were also able to retain the information provided for longer. Colour changing inks did not score so well on these tests.
Rock, Tilt or Rotate?
Whereas the Euro stripe requires the observer to tilt it backwards and forwards to see the denomination number morph into the Euro symbol, the Canadian ten dollar demonstrates horizontal parallax when rocked from side to side. Even more complex is the Estonian 100 Krone. Its OVD has no depth parallax but shows morphing as it is rotated through 90[degrees].
Although all these holograms are viewed by reflection, they are really transmission images rendered suitable for viewing on an opaque background (paper) by means of a reflective metallic coating. Before the metallic layer the surface relief image is transparent and capable of showing its rainbow coloured imagery if held up to the light which is allowed to pass through it. When the banknote is made of paper, this fact is academic but it was only a matter of time before these OVDs would be applied to a transparent polymer note, allowing them to be appreciated by holding it up to the light in the same way as a watermark. This technology known as WinDOE[R] (Diffractive Optical Element) is patented by Securency, the supplier of the polymeric substrate for banknotes. It appears on the 10,000 Ringit of Brunei.
Competing Technologies Emerge
Thus, the optical waters are now becoming muddied with the emergence of new effects that are not holographic and do not use diffractive effects. Of these, the most significant to date is Motion[R], the microlenticular system developed in the US by Nanoventions and acquired by Crane. This combines extremely fine print in register with micro lenses to produce effects with startling, and counter-intuitive animation. If the note is tilted from side to side, the image appears to move up and down and vice versa. The images are also in colour but not the varying rainbow colours typical of metallic holograms. This material is typically viewed by reflection and appears on notes as a windowed thread embedded into paper. It was first deployed on Swedish Kroner banknotes in 2006 and has subsequently been adopted by several countries. It is due to be released on US dollar bills on February 10 2011. The US Treasury has shown no appetite for adopting holographic technology and so this microlenticular technology can be seen as a major threat.
A Warning to Holography
This microlenticular technology sounds a warning bell to the holographic industry because it flaunts a phenomenon originally associated with holograms and yet notably absent from most of the holograms present on banknotes, namely depth. People are fascinated by small images on a two dimensional surface which yet reveal depth. The interest is especially acute when the depth can be appreciated in dim, diffuse lighting as is the case with Motion and microlenticulars in general.
De La Rue tried to redress this anomaly with its Depth Image[TM] Hologram adopted by the Clydesdale Bank in 2009. The feature is claimed to have visible depth, three-dimensionality, strong colour switching and contrast. The Clydesdale notes use holograms which have a depth of 8mm.
New Levels of Complexity
The commemorative 1000 Tenge note produced by Papierfabrik Lou-isenthal for Kazakhstan and launched in January this year takes optical complexity to a new level. Not only is there a holographic feature showing typical rainbow colours but there is a separate microlenticular patch viewed by transmission. This is Varifeye[R] which is intended to combine the best features of paper and polymer. For Varifeye, a hole is created in the paper substrate. It is not cut out, rather the deckle-edge window is created during the process of cylinder-mould web formation as the stock fibbers collect against the deckle, leading to the characteristic feather look. Then a clear stripe of film is laminated over it running from top to bottom of the note. The clear stripe contains the microlenticular image of a camel interchanging with the letter 'K' when tilted.
This feature, known as Magic, looks surprisingly like Motion and can be viewed by transmission through the window. There is also a metallised holographic image of the Astana Baiterek monument above the text OSCE (Organization for Security & Co-operation in Europe), interchanging with the date 2010 and these are viewed by reflection where it falls over the paper. (This window Varifeye technology was first used on the Bulgarian lev banknotes in 2005 thus becoming the world's first paper notes with see through window).
For polymeric substrates, the Bank of Australia has developed its Non-diffractive Switching Image (NSI). This appears like a dynamic watermark in the clear window of a polymer-based note. Being non-diffractive, the images are seen in varying shades of grey rather than rainbow colours and switching of the image elements occurs by rotation rather than tilting.
The Mexican 100 peso note has an ingenious feature which many think is holographic but in fact is transparent optically variable inks (they are usually opaque) printed on the clear window of a polymer note. This offers the viewer the option of inspecting the feature either by transmission or reflection. The inks change colour in both modes but the colours seen by transmission are the complementary colours of those seen by reflection.
Variations of Holo Technologies
The latest innovation in holographic technology which makes use of traditional (though modified) embossing technology, is the Asterium feature newly offered by Toppan Printing in Japan. By normal, direct lighting, this feature appears black, like the night sky, but when inclined at an extreme angle, the rainbow colours of an embossed hologram appear. It is like viewing a latent image. The key feature is the optical black which imparts a new aesthetic to documents and only reveals the colourful security feature 'on demand', so to speak.
In the last few months there have been several fundamental advances in the way holograms can now be produced for banknotes. In 2006 the Banque de Suisse commissioned Kurz in Germany to come up with a revolutionary security feature which went beyond what is being used for the Euro. The response is the use of photopolymer to record a volume holographic image for the banknote. Kurz's challenge has been to make the material thin enough to be useable on a banknote, especially given that the reason this is called a 'volume' hologram is that the interference fringes are recorded within the depth of the photo-sensitive material.
Similar developments are taking place in Japan with Dai Nippon Printing taking the lead. It is very interesting to compare the similarities and differences in approach between the two security providers.
3D or not 3D?
The imagery developed by Kurz' subsidiary OVD Kinegram for use in photopolymer does not make use of the full parallax capability of the volume hologram. Instead, they show similar animation to Kinegrams in monochromatic form. Part of the reason for this is that depth, in holographic images, requires good lighting (preferably a point source) and a smooth surface. The natural rugosity of paper does not provide this ideally smooth surface. Moreover, whereas metallised, holographic images are transferred to the paper by means of a transfer foil, the photopolymer, as developed in Germany, is currently attached to the paper as a thin laminate. This means that the photopolymer is protected from chemical attack and physical abrasion by a discrete layer of polyester but it also means that there is an appreciable increase in thickness in addition to the paper.
Dai Nippon favours the use of 3D, full parallax imagery which a volume medium is ideally suited for. To achieve this, the problem of the roughness of the paper surface has to be tackled and preliminary results look very encouraging. Furthermore, if the protection of a plastic layer is necessary for physical and chemical resistance, then a better way of incorporating it could be as a window thread during the paper making process. Here again, DNP have shown they can successfully do this--see page 3.
Some might argue that the image complexity as seen in the Kinegrams of the Euro currency, reached a zenith. Certainly, the eye and the brain are challenged with that level of complexity, as is the hand/eye co-ordination necessary required to make the visual judgement calls.
And yet, just as we might have thought complexity had reached it limits, the OVD Kinegram division of Leonhard Kurz unveiled Kinegram reColor [R] in 2008. This is a Kinegram designed for transparent windows in banknotes and displays different images depending on the side of the note from which it is observed. On the front the viewer sees a normal metalized reflective, diffractive image, while the reverse view shows a patterned coloured foil also displaying the diffractive features. The trick is performed using selective demetallisation to highlight different parts of the images.
Any trend towards simplification must be a move in the right direction. It seems that technological developments have run ahead of the abilities of artists and graphic designers to make good use of the media or of the public's ability to appreciate and evaluate them as security features.
After all, it's not as though holograms represent the only security feature on a banknote. They are one of many. The 1000 Tinge note for Kazakhstan has at least 16 features including one intended for the blind or partially sighted. Therefore, it isn't necessary to fill the hologram with every whistle and bell imaginable. There is a point at which the brain shuts off and says 'if it sparkles, it must be genuine'. We would do well to recall why the hologram was introduced at all: it provided a feature that could not be photocopied. Photopolymers offer the prospect of doing the same thing but without blinding and dazzling us in the process.