Mechanical Engineering Wiki / Guilloches
Guilloches - complicated line patterns for greater safety
In short, a guilloche is a special line pattern: several lines overlap each other, run parallel, intersect and thus create a highly complex pattern. Viewed from the outside, guilloches often have an appealing ornamental look, which is why they are often used in art. In connection with securities and chip card security, however, they are a relevant security feature that is largely classified as forgery-proof.
History and development of guilloches
The origin and history of guilloches can no longer be clearly clarified from today's perspective. There are four possible theories on the development of guilloches and their naming. The most common theory is that the guilloches were named after their inventor, the Frenchman Guillot. Regardless of the history of their origin, however, one thing is certain: guilloches were used many years ago as a forgery-proof feature on various documents. Banknotes, securities, identity papers, certificates and passports in particular were printed with the elaborate line patterns, which often take the form of asymmetrically closed ellipses or differently shaped circular paths. In earlier times, a so-called guilloché machine was used for this purpose, in which the patterns were created using engraved printing plates. This alone made it difficult to forge the complex line ornaments without further ado.
Use and creation of guilloches today
Guilloches are still used today as a security-relevant element on chip cards or documents. Depending on the area of application, however, the guilloches are no longer produced using engraved printing plates, but are calculated on the computer using specific databases and software programs. These programs use various line patterns to produce a guilloche. A distinction can be made between parallel lines, known as thread guilloche, wave guilloche and ornamental guilloche. All three elements are combined with each other. Today, there are specific guidelines for the production of guilloches. For example, in its 1991 printing guidelines, the German stock exchange stipulates that guilloches should be designed either as positive guilloches or as negative guilloches. With positive guilloches, colored lines are applied to an unprinted surface, whereas with negative guilloches, the surface is printed and the lines run unprinted through these surfaces. A distinction is also made between single-color and multi-colored guilloches.
High-precision line geometry as counterfeit protection
In order for guilloches to be classified as forgery-proof, it is important that the individual guilloches, i.e. line patterns, match each other. The perfect accuracy with which the individual lines intersect is ultimately what makes guilloches so attractive as a security feature: This accuracy cannot normally be reproduced, as the lines often become frayed, thickened or blurred at the latest at the intersections. In addition, two- or multi-colored guilloches are often developed using a combination of basic and interfering colors, which makes it difficult for even the most modern reading devices to produce a 100% accurate reproduction. This is prevented not least by the fact that guilloches do not allow either an end or a starting point to be identified. Exact recalculation is therefore not straightforward, even for modern software programs. A classic feature that can be used to identify manipulated chip cards are broken guilloches or guilloches with blurred, frayed or unevenly thickened lines. Even if the basic patterns of the guilloches are asymmetrical, they ultimately appear to be perfectly regular, which is why conspicuous features in the course of counterfeiting are immediately obvious. Nevertheless, guilloches are nowadays combined with other security features on chip cards, for example, in order to ensure greater protection.
