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. Security printing is most often done on security paper, but it can also occur on plastic materials.
Features detectable by humans Secured documents, such as banknotes, use visible and tactile features to allow humans to verify their authenticity without tools. The European Central Bank (ECB) recommends feel, look, and tilt: First check the tactility of the banknote (including the substrate), then look at the optical design and finally the characteristics of certain optical features when tilting the banknote in relation to the incident light.
In general, the introduction of a new banknote series is accompanied by information campaigns describing the design and the security features. Several central banks also provide mobile apps explaining the characteristics by interactive methods and enrich them by animated effects. In general, they use the camera of a mobile device to explain the features of a presented banknote. As they do not support the direct verification of authenticity they also work with simple printouts or screen displays.
SwissBanknotes from the Swiss National Bank for the Swiss franc with animated effects MalawiKwacha from the Reserve Bank of Malawi for the Malawian kwacha with simulations of tilting and tactility as well as interactive effects by enhanced reality SARBCurrency from the South African Reserve Bank for the South African rand as an offline application explaining the security features by enhanced reality Lilangeni from the Central Bank of Eswatini for the Swazi lilangeni with simulations of tilting and tactility as well as interactive effects by enhanced reality
Substrate Paper The substrate of most banknotes is made of paper, almost always from cotton fibres for strength and durability; in some cases linen or specially coloured or forensic fibres are added to give the paper added individuality and protect against counterfeiting. Paper substrate may also include windows based on laser-cut holes covered by a security foil with holographic elements. All of this makes it difficult to reproduce using common counterfeiting techniques.
Polymer Some countries, including Canada, Nigeria, Romania, Mexico, Hong Kong, New Zealand, Israel, Singapore, Malaysia, United Kingdom, and Australia, produce polymer (plastic) banknotes, to improve longevity and to make counterfeiting more difficult. Polymer can include transparent windows, diffraction grating, and raised printing.
Format
Most currencies use different dimensions of length, width, or both for the different denominations, with smaller formats for the lower denominations and larger formats for the higher denominations, to hinder reuse of the substrate with embedded security features for counterfeiting higher denominations.
Blind and visually impaired people may also rely on the format for distinguishing between the denominations.
Visible security features Watermark True watermark
A true 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 paper makers to identify their product. For proofing the authenticity, the thinner part of the watermark will shine brighter with a light source in the background and darker with a dark background. The watermark is a proven anti-counterfeit technology because most counterfeits only simulate its appearance by using a printing pattern.
Simulated watermark
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. A similar effect can be achieved by iriodin varnish which creates reflections under certain viewing angles only and is transparent otherwise.
Watermarks are sometimes simulated on polymer currency by printing an according pattern, but with little anti-counterfeiting effect. For example, the Australian dollar has its coat of arms watermarked on all its plastic bills. A Diffractive Optical Element (DOE) within the transparent window can create a comparable effect but requires a laser beam for its verification.
See-through register See-through registers are based on complementary patterns on the obverse and reverse of the banknote and constitute a complete pattern under backlight conditions. Examples are the D of the Deutsche Mark (1989 series, BBk III) and the value number of the first series of euro banknotes (ES1). Counterfeiting is difficult because the printing registration requires an extremely high printing accuracy on both sides and minor deviations are easily detectable.
See-through window Polymer banknotes which are printed on a basically transparent substrate easily provide clear areas by sparing the white coating. This window may be overprinted by patterns. Initially this was the main human security feature for polymer banknotes which cannot use watermark or security threads. It attracted counterfeiting of large volumes when printing technology for polymer substrate became commonly available. Therefore new designs additionally laminate this window with an ultra-thin security foil, e.g., on the Frontier series of the Canadian dollar which was issued from 2011, and the Australian dollar (2nd series) issued from 2016.
A very similar security feature is achieved with banknotes on paper substrate. For this an area of up to 300 mm² is punched out and sealed with a partially transparent security foil. The ES2 series of euro banknotes is using this feature for the higher denominations (EUR 20 and above) and calls it portrait window. The European Central Bank (ECB) recommends to look at the banknote against the light – the window in the hologram becomes transparent and reveals a portrait of Europa on both sides of the note.
Micro-perforation Micro-perforation is used as Microperf in the Swiss franc and the Romanian leu. Very small holes are punched or laser-engraved into the substrate or a foil application without generating a crater. In backlight illumination, the holes form a pattern, e.g., the value numeral like in the SFR 20 (eighth series).
Geometric lathe work 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.
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 without either close inspection or the use of a magnifying glass. Cheques, for example, use microprint as the signature line.
Optically variable ink Optically Variable Ink (OVI) displays different colors depending on the angle at which it is viewed. It uses mica-based glitter. As an example, the euro banknotes use this feature as the emerald number on the ES2 series. The ECB recommends to "tilt the banknote". The shiny number in the bottom left corner displays an effect of the light that moves up and down. The number also changes colour from emerald green to deep blue. The EUR 100 and EUR 200 banknotes also show € symbols inside the number.
Coloured magnetizable inks are prepared by including chromatic pigments of high colour strength. The magnetic pigments’ strong inherent colour 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 coloured magnetizable inks due to their lower blackness.
Homogeneous 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.
When pearlescent pigments are viewed at different angles the angle of the light as it's perceived makes the colour appear to change as the magnetic fields within the particles shift direction.
Holograms
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.
Security threads Metal threads and foils, from simple iridescent features to foil colour 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-metallized 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 multicolour sewing thread made from cotton or synthetic fibers, mostly UV fluorescent, for the bookbinding of passport booklets. In recent designs the security thread was enhanced with other security features such as holograms or three-dimensional effects when tilted.
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 "windowed" 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.
Prismatic colouration