Why Is Money So Complicated to Print? Features Nobody Is Meant to Copy

Security features serve three audiences that need different things. The public must be able to check a note in seconds without equipment, which requires features that are obvious, memorable and hard to imitate convincingly, and there is little point in a feature nobody knows about. Cash handling machines must authenticate notes mechanically at speed, which requires features detectable by sensors, frequently invisible to a person. And central banks and forensic examiners must be able to determine authenticity definitively and to identify the source of a forgery, which requires features that are covert and not published. A note therefore carries overt, machine-readable and covert features together, and the first group is the one that matters for ordinary use.

The public ones are designed to be checked by feel, by tilting and by holding to light:

• •Raised print produced by intaglio printing, which can be felt with a fingertip
• •A watermark formed in the substrate itself, visible when held up
• •A security thread embedded in the material, appearing continuous against the light
• •Holographic or colour-shifting elements that change with the viewing angle
• •Microprinting readable only with magnification
• •See-through registration, where parts printed on each side align to form a shape

The substrate is a security feature before anything is printed on it. Traditional banknote paper is made from cotton and linen rather than wood pulp, which gives it a distinctive feel, different acoustic properties when snapped, and durability under repeated folding, and it is unavailable commercially. Polymer notes, introduced in Australia from 1988 and adopted widely since, use a plastic film that lasts several times longer, resists soiling and moisture, and permits genuinely transparent windows, which are extremely difficult to imitate and are the most conspicuous feature of such notes. Both materials incorporate fibres, planchettes and chemical markers during manufacture. Controlling the supply of the substrate is itself a security measure and is one reason production is concentrated in very few facilities.

The design process balances requirements that pull against each other and takes years. Security features must be prominent enough to be checked and must not crowd the design into illegibility. Denominations must be distinguishable by people with impaired vision, which is addressed with size differences, contrasting colours and tactile marks, and the arrangements differ between countries with some doing it far better than others. The portrait or image must be recognisable enough that an unfamiliar face reads as wrong, which is why faces are used, since people detect errors in faces unusually well. Durability matters for cost, since a note lasting twice as long halves production. And the subjects depicted are a public and political choice that frequently generates argument, which is the part of the process the public actually sees.

The threat is more varied than the image of a master craftsman suggests. The overwhelming majority of counterfeits are produced on ordinary digital printers and are poor, relying on being passed quickly in low light to people not looking, which is why the simplest checks catch most of them. A small number of operations produce high-quality work using offset presses and specialist materials, and those are treated as a national security matter rather than ordinary crime. Colour copiers and image editing software include a recognition system that refuses to reproduce banknotes, triggered by a pattern of small circles present on notes worldwide. Raising the value of a genuine low-denomination note by altering it is a separate technique, as is producing notes of a denomination that does not exist.

Features serve the public, cash machines and forensic examiners, and the public ones must be checkable in seconds by feel, by tilt and against the light. Raised intaglio print, a watermark in the substrate and an embedded thread are the standard checks. The substrate is itself a feature, with cotton and linen paper or polymer film unavailable commercially, and most counterfeits are poor digital prints passed quickly.