All of the inks used in bank note printing offer a certain amount of security because they are not commercially available and hence not available to would be counterfeiters.
The inks used have to have a very high performance and be very resistant to environmental conditions such as sunlight, heat, moisture, etc.It would be no good if we accidentally dropped a note in a puddle and all the ink fell off, or we casually left our money lying around in the sunshine and all the ink faded away !
Surprisingly the choice of colours in which a note is printed can provide quite a security element. Many commercial reproduction methods have problems telling some colours apart for instance colour copiers tend to reproduce lime green as yellow. Knowing the limitations of technology used in counterfeiting can enable the bank note printers to add protection in a cheap but effective way.
Materials which fluoresce under ultra violet light can be added to most inks. They can be incorporated into a visible design element or an invisible design element (ie printed as a transparent feature). When viewed under ultra violet light all is revealed. The following graphic shows what this note revealed under UV light. The 10 will have been printed as an invisible feature. Note also the fluorescent fibres in the paper.
Metallic inks produce a sheen effect when printed as compared to the matt effect seen with other inks. They are generally used in large areas of solid colour so that their effect is maximised. They offer good protection from colour copy counterfeiting.
Metameric inks work on the principle of metamerism ....two colours matching under one set of lighting conditions can appear and quite different under another set. The effect of such a feature can be seen below. Under normal viewing conditions nothing is apparent but when viewed under a red filter a numeral appears.
Magnetic inks enable areas of a note to be read by a magnetic detector. They are sometimes used for the letterpress component of the bank note, the serial numbers.
Optically variable inks or OVI contain tiny flakes of special film which changes colour as the viewing angle is varied. The result is an ink which has this same optical property, changing colour as the viewing angle is varied. They are very expensive inks and generally only used in small areas. An OVI feature is sometimes printed using the silk screen process
They do however offer excellent protection against all counterfeiting methods.
This effect is generally seen as a band or either solid or patterned design. It is applied by the silk screen printing process before any other printing is carried out. The iridescent or pearlised effect cannot be reproduced by a colour copier and therefore gives good protection from this threat.