Optically variable devices have become a ubiquitous part of everyday life, with holographic, opalescent and colour-shifting features adorning a multitude of products. Of these, colour-shifting inks and films have been particularly successful. These devices provide the user with a clear and easily taught mode of observation, with a first colour viewable at near-normal incidence, and a second colour seen at more oblique angles. Because of this observed colour change, such features cannot be scanned and reproduced by traditional printers, thereby providing a barrier to casual counterfeiting. In this article, André Arsenault reviews a technology for the creation of colour-shift patterns in a single printing pass.

It is clear that optical variability (OV) is no longer restricted to only high-security documents. From effect pigments for vehicles to holographic packaging to colour-shifting wrappings and toys, OV effects can be increasingly found almost everywhere you look, as aesthetic and eye-catching effects on a multitude of products. There is no question, however, that colour-shift devices and materials used in high-security products have much higher performance, with brighter, unique colours and special effects – such as polarisation, colour ‘travel’ and dynamic movement – not found in widespread commercial products. 

Graphics and fine patterns
The uniqueness and security of colour-shifting devices can be further improved by introducing graphics or fine patterns. Colour-shifting foils, laminates and threads can be patterned by overprinting or overlamination of a printed or demetallised graphic. Colour-shifting inks, on the other hand, can be directly printed onto any substrate, typically using screen-printing. While the resolution in screen-printing is not extremely high, the resulting print can be harmoniously integrated with the base design of the document or substrate using registration. 

Colour-shift patterning: a route to increased security
In order to bring colour-shift patterning to the next level, it would be highly attractive to be able to pattern separate, multiple colours onto the same device. To be truly effective, these different colours should also be patterned in high-resolution, and with excellent registry. Such multicolour lateral colour-shift patterning is quite limited in film-based materials, although some features which are currently being developed, may allow for this effect. On the other hand, colour-shifting inks can enable multicolour patterning by using registered screen-printing of multiple inks, although achieving high-resolution and registry can be quite difficult and expensive. It is also needless to say that for both approaches small custom runs are not economical, and personalisation and serialisation is virtually impossible. 

OpalPrint: colour-shift printing on demand
The Canadian company Opalux has recently developed a technology called OpalPrint, arising out of its platform of technologies based on active photonic colour. Opalprint allows for the creation of registered, full-colour, personalised/serialised colour-shift patterns in a single printing pass, with zero consumables. OpalPrint is based upon a unique thermally reactive photonic colour film, illustrated schematically in figure 1. As can be seen at the top of the image, OpalPrint films consist of an array of organised voids in a specially tuned polymer matrix. The even spacing of layers of organised voids generates a periodic difference in refractive index, which leads to coherent Bragg diffraction of visible light1. When this material is exposed to heat above a preset threshold, the polymer matrix will begin to sag and melt homogeneously. This sagging and melting causes the layers of voids to shrink closer together, and the decrease in spacing results in a blue shift in the colour reflected from the material.

figure 1. Schematic illustration of how OpalPrint works

 

OpalPrint is continuously adjustable: the more heat the material absorbs, the greater the shift in reflected wavelength, as can be seen in the bottom part of figure 1. For instance, such a film may initially reflect red light, by virtue of the initial spacing between void layers. By inputting a small amount of heat, this spacing may decrease only slightly and result in an orange-coloured film. Progressively increasing the heat input will shift the colour to yellow, green and then blue. The colour may even be ‘erased’ to the human eye by shifting the colour into the invisible UV range. If one can accurately control the heat input into an OpalPrint film, a continuum of multiple colours spanning the entire rainbow can be created. Furthermore, each of these colours is optically variable and will show its own unique start colour, end colours, and colour-travel range

Thermal printers
Opalux has been able to optimise the thermal tunability for use with widespread thermal printers. This is done by inputting a grayscale image into the thermal printer, after which it is ‘inscribed’ into the OpalPrint substrate via the thermal print head. Each grey level of the printer corresponds with a specific amount of heat input into the substrate. The grey level will translate to a specific heat input for each printed pixel in the OpalPrint film, and consequently to a specific and highly reproducible colour. 

figure 2.

Advantages
Since colours are produced by varying the nanostructure of the material itself, the thermal printing process requires no ink, toner or other consumables. Because thermal printing is a digital patterning method, colour-shift prints can be generated ‘on-demand’, with high-resolution (up to 600 dpi). And as the printing is performed in a single pass, multicolour graphics can be created with perfect registry. The resulting prints are highly durable, and will withstand the high heat and pressure of common lamination, transfer, and other integration processes.
To illustrate its strong potential in security and identity documents, the films can be easily integrated into laminates for use in passports, incorporating customised optically variable features such as registry and serial data (passport number), personal data (the holder’s portrait or signature) and distinctive logos, as illustrated in figure 2. The complexity in the optically variable patterns generated with this material is truly unprecedented, as can be seen in the details and richness of printed features, in particular the colour-shifting portrait of the holder. It is also readily apparent that while providing striking reflected colours (bottom images), OpalPrint also shows distinctive transmission colours (top images) and an excellent clarity, allowing for obstruction-free viewing of the data page. By harnessing the full power of high-resolution digital printing, features such as hidden or full-colour images, microtext, secure patterns (i.e. guilloches), logos, barcodes and many others can be generated.

figure 3

Films are manufactured using a high-precision roll-to-roll process, resulting in spools of material with extremely high brightness (~80% reflectivity for each colour), as seen in figure 3. These materials can also be easily customised to selected brightness, colours, and colour-shifting range chosen from the visible, ultraviolet, and infrared spectral ranges. Opalux has developed multiple integration routes targeted to different applications in the field of documents and products security. The active film can be converted into threads, clear and opaque laminates, foils for hot-stamping, and adhesive labels, and can thus be integrated onto paper and plastic substrates of all types. 

Outlook
In summary, OpalPrint provides a breakthrough technology platform for countless secure products and applications. The heat from either standard or customised printers can be used to provide the necessary activating conditions, multiple colours of colour-shift in a single pass, with zero consumables and in perfect registry. It readily supports various customisation options such as colour patterns, colour ranges, programmable information and data, as well as fine graphics. It can make use of existing thermal printing equipment, or be upgraded to work with specific, customised printers that enable secure data and materials interfaces. It can facilitate the unprecedented ability to provide personalised, serialised colour-shift features for passports, ID cards and driving licences, as well as high-resolution and customised colour-shift graphics on security labels, banknotes, and countless other security products. 

Reference
1 Lopez, C. Materials aspects of photonic crystals. Article in Advanced Materials, Vol 15, Iss 20, pp 1679-1704).