The inclusion of at least one transparent window is a simple, yet highly effective security feature, and is a standard feature in all Guardian® substrate designs. However, more than one window can be incorporated and they can be of any shape and size. A clear window is a feature which is readily identifiable, and enables the public to quickly ascertain the authenticity of a banknote.

This feature can range from total transparency to a high level of opacity. The image is divided into various levels of opacity by varying the number of opacifying layers – from no layers (clear window) to maximum opacity.
This method of design makes insertion of a piece of clear film into a paper copy much more difficult. Once again, this device uses the transparency of the substrate to make a simple, secure and aesthetic feature.

The WinTHRU® complex window is also an excellent medium for housing security features such as:

- Transitory Images
- Vignettes
- DOVD’s
- WinDOE® and
- Self-Verifying features

Half Window
The half window is a variation to the clear/complex window. The difference lies with the fact the window is opacified on one side of the note only, therefore one side acts as a glossy surface while the other side acts as a normal printing surface. The glossy surface is difficult to copy using photocopiers and scanners. The result acts as a carrier for other security features such as DOVD’s, OVI and reflective inks.

The WinDOE® (Diffractive Optical Element) is a holographic structure applied to the surface of the clear window. When collimated light such as a distant point light source passes through the WinDOE®, it is transformed by the WinDOE® structure into a recognisable pattern (image) by the process of diffraction. The user can view the image in two ways. By holding the WinDOE® up to the eye and looking directly at a distant point source the user will see the image appear in space between the note and the light source. The appearance of the image will depend on the light source used.

The WinDOE® allows the user to verify their banknote interactively using any commonly available point light source such as downlights, household light bulbs, car taillights, distant street lamps or the moon. The WinDOE® is virtually unique in that it works best in poor lighting conditions or complete darkness, for example at night, where most other security features cannot be used. This is because the dark background of such lighting conditions provides excellent contrast to the diffracted image seen against the point light source.

Safety Warning
The WinDOE® is visible in a range of lighting conditions, but dangerous sources of light such as the sun and laser lights should NEVER be used to view the WinDOE®.  The WinDOE® feature does not provide any protection against the harmful affects of these light sources.

G-switch® is a dynamic optical feature that changes colour when tilted under a light source. The bright and transient colour-switching effect is produced in the substrate layers. When viewed at different angles, it alternates between two contrasting colours, creating an optically reflective effect. The colour change is easily observed without special equipment or skill, making it easy for the public to recognise.

Guardian® polymer substrate can incorporate a spectrum of colours, which represents an innovative and effective security enhancement to aid in reducing the threat of counterfeiting. MultiCLR™ is available in many colour and design options and includes a choice ranging from a dual-coloured substrate – one side of the substrate is a different colour to the other side, or multi-coloured substrate – which uses different colour layering combinations producing 3, 4 or even 5 colour substrate. It also presents excellent design opportunities to produce multi-coloured half windows.

The ability of Guardian® substrate to accept a permanent emboss is an important feature which utilises a further element of the intaglio process to enhance the security of the banknote. This is achieved by leaving the engraved areas of the intaglio plate uninked to create an embossed design during the intaglio printing process.This is most effective when the uninked design is a 'transitory image' (bottom) embossed into the transparent window area, generating an image that is visible in both transmission and reflection.

This is a metallic pigment that is made up as ink and printed on top of the Guardian® substrate as a patch. This patch may then be used as the platform for other security features such as ICE® (Intaglio Contrast Effect) and TIED® (Transparent Intaglio Disappearing Effect).
The ICE® involves printing an intaglio image on top of the metallic patch using specially developed intaglio inks. It produces an optically variable colour effect where the colour of the intaglio print intensifies when viewed at a highly oblique angle to the surface of the note.
TIED® is also printed onto the reflective metallic gold or silver substrate. This combination creates a disappearing effect because the intaglio print is only visible at viewing angles where the background is in high reflection.
The metallic patch is enhanced by the smoothness of polymer and provides protection against reproduction methods, which utilise four-colour process methods, including colour laser copiers and ink jet printers. The feature is also significantly enhanced when it is used as a secure base for other more advanced features such as ICE® and TIED®.

A colour changing ink with a pearlescent sheen is used to print broad colour bands or images onto the substrate. When the note is viewed at different angles the colour and the texture of the iridescent feature will change. Iridescent features on notes are strong visual features, which are easy to recognise and to see the colour change.

The shadow image in Guardian® substrate is similar in effect to the watermark on a paper banknote. It is an excellent optically variable device that is not obvious in reflected light but visible when the note is held up to the light. It is produced by altering the opacity and sometimes the colour of the opacifying layers and is far more durable than a traditional watermark. The shadow image can be a portrait or text.

Threads have been one of the most recognisable overt security features for currency authentication. Threads can be introduced in the Guardian® substrate. These threads contain metallic, magnetic, phosphorescent and fluorescent pigments. As these threads are printed they can vary in their shape and size and include micro-text. In addition, they can be windowed. Printed security threads can be incorporated during the manufacture of Guardian® substrate. The method of producing Guardian® substrate allows for an added dimension in security threads. As well as being straight along the machine direction of the sheet, the thread can be a complex curved pattern, non-continuous or multi-directional. The reflective metallic, windowed or optically variable threads are particularly effective in preventing reproduction by colour photocopying or scanning.

This is an immediately recognizable design feature placed in the window which adds both an aesthetic and security dimension to the WinTHRU® complex window.  It creates another degree of difficulty for the casual counterfeiter who is already challenged with the task of emulating a window.  Complex line structures can be introduced into the design to make it difficult to create a window.

Additionally, the WinVU™ feature can be integrated with other complex window features, such as half windows and shadow images to make it more difficult, time-consuming and costly to try to counterfeit.    

The self-authenticating MicroSam® (Joh. Enschede) feature is composed of two elements: A microSam screener and a microSam image. The screener is printed in the clear window and the microSam image is printed elsewhere on the opacified substrate (note). They are both composed of a series of fine lines that cannot currently be photocopied; they are finer than the resolution of the eye. The functionality of the microSam® is achieved by superimposing the screener over the image and a hidden message or code appears to the viewer.

The MicroSam® feature provides many security benefits.  The ‘hidden image’ is difficult to reproduce using electrophotographic means and provides an anti-scan moire effect.  The decoding device, or screener is very difficult to reproduce, obviously it is especially difficult to reproduce on paper-based substrates.