Security printing is the field of the printing industry that deals with the printing of items such as banknotes, cheques, 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 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. These are by far the most common type of hologram - and in fact they are not holograms in any true sense of the words. The term "hologram" has taken on a secondary meaning due to the widespread use of a multilayer image on credit cards and driver licenses. This type of "hologram" consists of two or more images stacked in such a way that each is alternately visible depending upon the angle of perspective of the viewer. The technology here is similar to the technology used for the past 50 years to make red safety night reflectors for bicycles, trucks, and cars. These holograms (and therefore the artwork of these holograms) may be of two layers (i.e. with a background and a foreground) or three layers (with a background, a middle ground and a foreground). In the case of the two-layer holograms, the matter of the middle ground is usually superimposed over the matter of the background of the hologram. These holograms display a unique multilevel, multi-colour effect. These images have one or two levels of flat graphics “floating” above or at the surface of the hologram. The matter in the background appears to be under or behind the hologram, giving the illusion of depth.

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.

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.

Serial numbers

These types of hologram are created using highly sophisticated and very expensive electron-beam lithography systems. This kind of technology allows the creation of surface holograms with a resolution of up to 0.1 micrometres (254,000 dpi). This technique requires development of various algorithms for designing optical elements that shapes scattered radiation patterns. This type of hologram offers features like the viewing of four lasers at a single point, 2D/3D raster text, switch effects, 3D effects, concealed images, laser readable text and true colour images. The various kinds of features possible in security holograms are mentioned below:

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.

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.