Most banknotes are made of heavy paper, almost always from cotton fibres for strength and durability, in some cases linen or speciality coloured or forensic fibres are added to give the paper added individuality and protect against counterfeiting. Some countries, including Canada, Nigeria, Romania, Mexico, New Zealand, Israel, Singapore, Malaysia, United Kingdom and Australia, produce polymer (plastic) banknotes, to improve longevity and allow the inclusion of a small transparent window (a few millimeters in size) as a security feature that is difficult to reproduce using common counterfeiting techniques. In November 2011 Canada joined the list of countries using polymer currency as it began the introduction of a new banknote series.
A watermark is a recognizable image or pattern in paper that appears lighter or darker than surrounding paper when viewed with a light from behind the paper, due to paper density variations. 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; as well as their use in security printing, they have also been used by papermakers to identify their product. Watermarks can also be made on polymer currency, for example, Australia has its coat of arms watermarked on all its plastic bills.
Printed with white ink, simulated watermarks have a different reflectance than the base paper and can be seen at an angle. Because the ink is white, it cannot be photocopied or scanned.
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. Intaglio also allows for the creation of latent images which are only visible when the document is viewed at a very shallow angle.
A guilloché is an ornamental pattern formed of two or more curved bands that interlace to repeat a circular design. They are made with a geometric lathe.
This involves the use of extremely small text, and is most often used on currency and bank checks. The text is generally small enough to be indiscernible to the naked eye. Cheques, for example, use microprint as the signature line.
Color changing inks are made from mica.
Color changing inks colored magnetizable inks are prepared by including chromatic pigments of high color strength. The magnetic pigments’ strong inherent color generally reduces the spectrum of achievable shades. Generally, pigments should be used at high concentrations to ensure that sufficient magnetizable material is applied even in thin offset coats. Some magnetic pigment are best suited for colored magnetizable inks due to their lower blackness. Homogenous magnetization (no preferred orientation) is easily obtained on pigment made of spherical particles. Best results are achieved when remanence and coercive field strength are very low and the saturating magnetization is high.
Magnetic pigments are also used in ribbon inks for coding and reading (MICR = Magnetic Ink Character Recognition). The pigment is dispersed in a binder system (resin, solvent) or a wax compound and applied either by pressing or by hot melt to a carrier film (usually polyethylene).
When pearlescent pigments are viewed at different angles the angle of the light as it's perceived makes the color appear to change as the magnetic fields within the particles shift direction.
A hologram may be embedded either via hot-stamping foil, wherein an extremely thin layer of only a few micrometers of depth is bonded into the paper or a plastic substrate by means of a hot-melt adhesive (called a size coat) and heat from a metal die, or it may be directly embossed as holographic paper, or onto the laminate of a card itself. When incorporated with a custom design pattern or logo, hologram hot stamping foils become security foils that protect credit cards, passports, bank notes and value documents from counterfeiting. Holograms help in curtailing forging, and duplication of products hence are very essential for security purposes. Once stamped on a product, they cannot be removed or forged, enhancing the product at the same time. Also from a security perspective, if stamped, a hologram is a superior security device as it is virtually impossible to remove from its substrate.
Metal threads and foils, from simple iridescent features to foil color copying to foils with additional optically variable effects are often used.
There are two kinds of security threads. One is a thin aluminum coated and partly de-metalized polyester film thread with microprinting which is embedded in the security paper as banknote or passport paper. The other kind of security thread is the single or multicolor sewing thread made from cotton or synthetic fibers, mostly UV fluorescent, for the bookbinding of passport booklets.
On occasion, the banknote designers succumb to the Titanic effect (excess belief in the latest technology), and place too much faith in some particular trick. An example is the forgery of British banknotes in the 1990s. British banknotes in the 1990s featured a metal strip through the paper about 1 mm wide that comes to the paper surface every 8 mm. When examined in reflected light, it appears to have a dotted metallic line running across it, but when viewed through transmitted light, the metal strip is dark and solid.
Duplicating this was thought to be difficult, but a criminal gang was able to reproduce it quickly. They used a cheap hot-stamping process to lay down a metal strip on the surface of the paper, then printed a pattern of solid bars over it using white ink to leave the expected metal pattern visible. At their trial, they were found to have forged tens of millions of pounds’ worth of notes over a period of years.
Because of the speed with which they can be read by computer systems, magnetic ink character recognition is used extensively in banking, primarily for personal checks. The ink used in magnetic ink character recognition (MICR) technology is also used to greatly reduce errors in automated (or computerized) reading.
Some people believe that the magnetic ink was intended as a fraud prevention concept, yet the original intent was to have a non-optical technology so that writing on the cheque, like signatures, would not interfere with reading. The main magnetic fonts (E13-B and CMC7) are downloadable for a small fee and in addition magnetic toner is available for many printers. Some higher resolution toners have sufficient magnetic properties for magnetic reading to be successful without special toner.
Serial numbers help make legitimate documents easier to track and audit. To help detect forgeries serial numbers will normally have a Check digit to verify the serial number.
Another method of protection is to create trap numbers within the serial number range. For example, the system may automatically invalidate numbers which are in a range of 200-300 (e.g. 210, 205 would be invalid). The system may even take single random numbers within a block (e.g. ending in 51, 37, 48 within a 200-300 range would be invalid).
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.
Sometimes only the original document has value. An original signed cheque for example has value but a photocopy of it does not. An original prescription script can be filled but a photocopy of it should not be. Copy-evident technologies provide security to hard copy documents by helping distinguish between the original document and the copy.
The most common technology to help differentiate originals from copies is the void pantograph. Void pantographs are essentially invisible to the untrained, naked eye on an original but when scanned or copied the layout of lines, dots and dashes will reveal a word (frequently VOID and hence the name) or symbol that clearly allows the copy to be identified. This technology is available on both traditional presses (offset and flexographic) and on the newer digital platforms. The advantage of a digital press is that in a single pass through the printer a void pantograph with all the variable data can be printed on plain paper.
Copy-evident paper, sometimes marketed as ‘security paper’, is pre-printed void pantograph paper that was usually produced on an offset or flexographic press. The quality of the void pantograph is usually quite good because it was produced on a press with a very high resolution and when only a small number of originals are to be printed it can be a cost effective solution however the advent of the digital printer has rapidly eroded this benefit.
A second technology which complements and enhances the effectiveness of the void pantograph is the Verification Grid. This technology is visible on the original, usually as fine lines or symbols but when photocopied these lines and images disappear; the inverse reaction of the void pantograph. The most common examples of this technology are on the fine lines at the edge of a cheque which will disappear when copied or on a coupon when a symbol, such as a shopping cart, disappears when an unauthorized copy is made. Verification Grid is available for either traditional or digital presses.
Together the void pantograph and the Verification Grid complement each other because the reactions to copying are inverse, resulting in a higher degree of assurance that a hard copy document is an original.
The use of color can greatly assist the prevention of forgeries. By including a color on a document a color photocopier must be used in the attempt to make a copy however the use of these machines also tends to enhance the effectiveness of other technologies such as Void Pantographs and Verification Grids (see Copy-evident above).
By using two or more colors in the background and blending them together a prismatic effect can be created. This can be done on either a traditional or a digital press. When a document using this technique is attempted to be photocopied the scanning and re-creation by a color copier is inexact usually resulting in banding or blotching and thereby immediate recognition of the document as being a copy.
A frequent example of prismatic coloring is on checks where it is combined with other techniques such as the Void Pantograph to increase the difficulty of successful counterfeiting.
Carefully created images can be hidden in the background or in a picture on a document. These images cannot be seen without the help of an inexpensive lens of a specific line screening. When placed over the location of the image and rotated the image becomes visible. If the document is photocopied the Halo image is lost.
Halo can be printed on traditional or digital presses. The advantage of traditional presses is that multiple images can be overlaid in the same location and become visible in turn as the lens is rotated.
Halo is used as a technique to authenticate the originality of the document and may be used to verify critical information within the document. For example, the value of a coupon might be encoded as a Halo image that could be verified at the time of redemption or similarly the seat number on a sporting event ticket.
False-positive testing derives its name because the testing requires both a false and a positive reaction to authenticate a document. The most common instance is the widely available counterfeit detector marker seen in many banks and stores.
Counterfeit detector markers use a chemical interaction with the substrate, usually paper, of a document turning it a particular color. Usually a marker turns newsprint black and leaves currency or specially treated areas on a document clear or gold. The reaction and coloring varies depending upon the formulation. Banknotes, being a specially treated substrate, usually behave differently than standard newsprint or other paper and this difference is how counterfeits are detected by the markers.
False-positive testing can also be done on documents other than currencies as a means to test their authenticity. With the stroke of a marker a symbol, word or value can be revealed that will allow the user to quickly verify the document, such as a coupon. In more advanced applications the marker creates a barcode which can be scanned for verification or reference to other data within the document resulting in a higher degree of assurance of authenticity.
Photocopied documents will lack the special treating of the substrate so are easily detectable. False-positive testing generally is a one time test because once done the results remain visible so while useful as part of a coupon this technique is not suitable for ID badges for example.
Fluorescent dyes are dyes which fluoresce under ultraviolet light or other unusual lighting. These show up as words, patterns or pictures and may be visible or invisible under normal lighting. This feature is also incorporated into many banknotes and other documents - e.g. Northern Ireland NHS prescriptions show a picture of local '8th wonder' the Giant's Causeway in UV light. Some producers include multi-frequency fluorescence, such that different elements fluoresce under specific frequencies of light.
Banknotes are typically printed with fine alignment between the printing on each side of the note. This allows the note to be examined for this feature, and provides opportunities to unambiguously align other features of the note to the printing. Again, this is difficult to imitate accurately enough in most print shops.
With the advent of RFID, it is possible to insert extremely small RF-active devices into the printed product to enhance document security. This is most apparent in modern biometric passports, where an RFID chip mirrors the printed information.
Security ink with a normal "trigger" temperature of 88 °F (31 °C), which will either disappear or change colors when the ink is rubbed, usually by the fingertips.
Pressure-sensitive or hot stamped labels characterized with a normal (gray or colored) appearance. When viewed via a special filter (such as a polarizer) an additional, normally latent, image appears.