security printing

abstracted from wikipedia:

Security printing is the field of the printing industry that deals with the printing of items such as banknotes, passports, tamper-evident labels, product authentication, stock certificates, postage stamps and identity cards. The main goal of security printing is to prevent forgery, tampering, or counterfeiting. More recently many of the techniques used to protect these high value documents have become more available to commercial printers whether they are using the more traditional offset and flexographic presses or the newer digital platforms. Businesses are protecting their lesser value documents such as transcripts, coupons and prescription pads by incorporating some of the features listed below to ensure that they cannot be forged or that alteration of the data cannot occur undetected.
A number of technical methods are used in the security printing industry.

Special paper
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 Romania, Mexico, New Zealand, Israel, Singapore and Australia produce plastic banknotes, in order to improve wear and tear, and permit the inclusion of a small transparent window a few millimeters in size as a security feature that is very difficult to reproduce using common counterfeiting techniques. In November 2011 Canada joined the list of countries using polymer currency as it began the rollout of a new banknote series.

Watermarks
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.

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. Intaglio also allows for the creation of latent images which are only visible when the document is viewed at a very shallow angle.

Geometric lathe work
A guilloché is an ornamental pattern formed of two or more curved bands that interlace to repeat a circular design. They used to be made with a geometric lathe. Special guilloche design software like Torapp guilloche designer can make guilloche designing rather trivial.

Microprinting
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.

Optically Variable Color-changing inks
Color changing inks are inks containing pearlescent pigments that change color when viewed at a different angle. The color of the ink does not actually change, but the angle of the light to the viewer's eye changes and thus creates the change in color. A number of types are available, including green to purple, gold to green and green to lilac.

Holograms
A hologram may be embedded either via hot-stamping foil, wherein an extremely thin layer of only a few microns 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.

Security threads
There are two kinds of security threads. One is a thin aluminum coated and partly demetalized 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.

Magnetic ink
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
Serial numbers are not difficult to forge, but make legitimate documents easier to track and audit.

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.

Copy-evident
Sometimes only the original document has value. An original signed check 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.

Prismatic Coloration
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.

Halo
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
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
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.

Registration of features on both sides
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.

Electronic devices
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.

Thermochromatic ink
Security ink with a normal "trigger" temperature of 88 degrees F, which will either disappear or change colors when the ink is rubbed, usually by the fingertips.

Latent Images
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.