Security printing
Security printing is the field of the printing industry that deals with the printing of items such as banknotes, cheques, passports, tamper-evident labels, security tapes, 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.[1] Security printing is most often done on security paper, but it can also occur on plastic materials.
Security features detectable with simple tools[edit]
Test pen[edit]
A counterfeit banknote detection pen can be used to quickly determine the starch in wood-based paper substrate. While genuine banknotes hardly change color at all, counterfeits turn black or blue immediately. This method, which is not very reliable – there is no color change on newsprint – is often used in the retail trade for reasons of cost and time.
Halo[edit]
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. A known implementation is Scrambled Indicia.[15]
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.
Latent images[edit]
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. With intaglio printing, a similar effect may be achieved for viewing the banknote from a slanted angle.
False-positive testing[edit]
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 manufactured 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 characteristics 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[edit]
Fluorescent dyes react with fluorescence 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. Phosphorescence may accompany fluorescence and shows an after-glow when the UV light is switched off.