Security printing
Security printing is the field of the printing industry that deals with the printing of items such as banknotes, passports and identity cards. The main goal of security printing is to prevent forgery.
A number of technical methods are used in the security printing industry.
Special paper
Most banknotes are made of heavy paper, sometimes mixed with linen, cotton, or other textile fibres. Some countries including Mexico and Australia produce banknotes made from plastic, in order to improve wear and tear, and permit the inclusion of a small transparent window a few mm in size as a security feature that is very difficult to reproduce using common counterfeiting techniques.
Watermarks
A watermark is a recognizable image or pattern in paper that appears lighter when viewed by transmitted light (or darker when viewed by reflected light, atop a dark background). A watermark is made by impressing a water coated metal stamp or dandy roll onto the paper during manufacturing. Watermarks were first introduced in Bologna, Italy in 1282; they have been used by papermakers to identify their product, and also on postage stamps, currency, and other government documents to discourage counterfeiting.
Intaglio printing
Intaglio is a printing technique in which the image is incised into a surface. Normally, copper or zinc plates are used, and the incisions are created by etching or engraving the image, but one may also use mezzotint. In printing, the surface is covered in ink, and then rubbed vigorously with tarlatan cloth or newspaper to remove the ink from the surface, leaving it in the incisions. A damp piece of paper is placed on top, and the plate and paper are run through a printing press that, through pressure, transfers the ink to the paper.
The very sharp printing obtained from the intaglio process is hard to imitate by other means.
Microprinting
Multicolored inks
Holograms
Embedded strips
Magnetic ink
Serial numbers
Anti-copying marks
In the late twentieth century advances in computer and photocopy technology made it possible for people without sophisticated training to easily copy currency. In an attempt to prevent this, banks have sought to add filtering features to the software and hardware available to the public that senses features of currency, and then locks out the reproduction of any material with these marks. One known example of such a system is the EURion constellation.