Guides & Tips — 09 Aug 2022

Ever Wondered How to Authenticate a Banknote? Look for a Window!

The last decade has seen a massive emergence of windows in banknotes throughout the world. Transparent, translucent, with transforming or hologram images, and serving as filters, all these security elements are constantly under scrutiny by Regula’s experts. The team never stops following and analyzing existing and novel security elements in all kinds of identity and security documents, have registered various types of windows in the notes of European countries, Asian states, Australia, in the Middle East and Latin America. Nowadays, a window in a banknote is an established and reliable security element that is almost impossible to counterfeit. It’s eye-catching, easily recognizable and can be checked for authenticity both with and without special equipment.

A window in one hundred euro banknote
Close-up of the portrait window in one hundred euro banknote

A 100 euro banknote, 2019 year. It contains a so-called “portrait window”, which is unique and can’t be found in any other banknote. This is a see-through diffractive feature that has different visible effects when viewed from the front (sign of euro), back (number 100) and in transmitted light (a portrait of Europe).

Banknotes with windows may start as a clear sheet of plastic onto which several layers of ink are applied. Windows are made by leaving some sections of plastic free of ink. However, a polymer sheet that forms the core of a banknote may be covered with paper on both sides. In this case, a window is made in the outer layers of paper. It is cut out, which makes plastic visible. Paper notes can also contain windows, and they are usually generated either at a sheet-making stage or are formed by a laser and then filled with some transparent film (using liquid crystals or nanomaterials, for instance). 

After being created, windows can be left as they are, or they may be framed or covered with some images, vignettes, embossing, metallized coating, etc. Not only all these elements make windows look fancy and captivate viewers, but contribute to the security of banknotes. To authenticate a window and all the elements it might contain, a person needs to look at it directly, rotate a note to see the window at another angle, hold it up to the light, view at a different background (light or dark). Let’s have a closer look. 

A window in a Cambodian real
Close-up of the window with an embossed flower in the Cambodian real

A 15,000 Cambodian riel, 2019 year. A clear window in transmitted (on the left) and in oblique light with an embossed flower on a gray background (on the right).

A so-called Varifeye window in an Iraqi dinar
Close-up of the window in the Iraqi dinar with a changing image depending on a background

A 50,000 Iraqi dinar, 2015 year. A so-called Varifeye window, made of a transparent film in a paper banknote. Its close-up shows the image of a spiral minaret in transmitted light. The image changes in oblique light and on a dark background.

An ‘Optiks’ window in a rupee of the Seychelles
Close-up of the window in the rupee of the Seychelles with an image made with metallized ink

A 500 rupee of the Seychelles, 2016 year. A clear window ‘Optiks’ in transmitted light (on the left)  and its opaque version in oblique light (on the right). The image is made with metallized ink on transparent foil. 

Some windows serve as filters – they reveal hidden images, texts or codes printed on the same banknote. To witness this effect, one needs to fold a note so that the window is placed above the area where something is encoded. Like in a Brazilian real, for example: 

A filter window in a Brazilian real
The window serving as a filter shows the number 10 on the banknote

A 10 Brazilian real, 2000 series. When the red window is placed above the sail of the ship (by folding the banknote), the number 10 shows.

Another wow-effect is shown in a window that contains a diffractive optical element (DOE) – a so-called winDOE. It visualizes a rainbow hologram image when seen in transmitted light from a point light source (lamp, flashlight, etc.). To get the point, let’s examine a Romanian leu:

A so-called winDOE in a Romanian leu
The diffraction of light reveals the number 100 in the clear window of the Romanian leu note

A 100 Romanian leu, 2019 year. The diffraction of light reveals the number 100 in the clear window of the note.

However, not all windows are transparent. In some cases, only one side of a polymer substrate of a banknote is left visible, while the other side is opacified. This is known as a half window. It’s glossy where it shows, and the rear side of a note serves as a regular printing surface.  

A half window in a Malaysian ringgit
The close-up of the half window in the Malaysian ringgit

A 1 Malaysian ringgit, the 4th series. The half window in the upper left shows the image printed on the rear side of the note. 

Indeed, examining windows in banknotes can be rather amusing, and anyone can easily see all the tricks with their own eyes. Nevertheless, using special equipment can add to verification of windows and thus to authentication of banknotes. For example, magnification with special devices will show whether the framework of a window is cut correctly. It will also allow you to define a method of printing for a vignette. If a window is created by a laser, magnification will reveal that its edges are correspondingly burnt. So, even if malefactors somehow try to counterfeit a banknote with a window (which is really hard, for example Australia has not yet come across a single attempt of falsification of such banknotes), forensic devices will show it immediately. For instance, Regula’s latest compact device – Regula 1031 – allows for 24x magnification, and this significantly helps to thoroughly examine security features of any banknote as well as identity document. And a spectral comparator Regula 4308 and its polarizing filter can help verify the presence of liquid crystals in a film that forms a window in a note. 

All the knowledge and data on various security documents, accumulated by the experts of Regula for over 30 years of their work, have translated into profound Information Reference Systems that assist in the authentication process at a border control point, in a currency exchange office, etc. These systems provide databases of travel documents, banknotes, driver’s licenses and vehicle registration certificates with detailed description of their security features. As for the “Currency” Information Reference System, it contains data on 200 currencies that are arranged by country and production series. Overall the database counts over 7,300 banknotes and coins.  
To learn more about various security elements in banknotes, please check Regula’s extensive glossary, which describes them all.

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