Substrate features are an integral part of the material on which the bank notes are printed .... why not call them paper features? Well some bank notes, such as the Australian Dollar, are printed on plastic.! The choice of substrate is very important as it has to be a very durable material resistant to tearing, withstanding crumpling and stable to environmental effects such as humidity.
For each currency produced a corresponding paper is manufactured. Banknote paper is made from cotton pulp which gives it better durability than commercial papers and a very distinctive feel. Much of the time, it is the initial feel of a counterfeit that urges someone to have a closer look at what they are holding. If bank note paper is held under ultra violet light it is dull compared to commercial papers
The paper manufacturing process allows for a number of features to be created
The watermark is one of the most obvious security features of a paper banknote. When held up to the light an image can be seen in the paper, usually a portrait similar to that printed on the note. The image of the watermark is caused by different thicknesses of paper, with light areas of the watermark being a result of thinner paper. The highlighted effect of " ultra thin" paper is sometimes used as an added security effect in small specific areas within a watermark, e.g. A denomination may appear as a "highlighted" portion compared to the main bulk of the watermark.
A watermark is an excellent security feature. A counterfeiter is very unlikely to manufacture his own paper. Having said that there are some ways watermarks can be simulated although the effects are crude.
Threads are embedded within the paper fibre and can be completely invisible or have a star burst effect, where the thread appears to weave in and out of the paper when viewed from one side. However when held up to the light the thread will always appear as a solid line. Features can be built into the thread material e.g microprinting on a transparent plastic thread or adding materials so they fluoresce under ultraviolet light.
The thread is a difficult feature to counterfeit but some counterfeiters have been known to print a thin grey line or a thin line of varnish in the area of the thread.
While the paper is in the pulp stage various elements can be added that then become embedded in the paper in a random fashion. e.g Tiny fibres which fluoresce under ultra violet light or tiny iridescent foils known as planchettes. It is also possible to tint the paper.
A plastic substrate has been adopted by some countries as the chosen medium for their currency. Security features are built into the plastic is coating with an opaque white leaving a window of transparency and pseudo watermarks are produced by printing additional designs in opaque white. These features offer good protection from colour copier counterfeiting.