Suprisingly the choice of colors in which a note is printed can provide quite a security element. Many commerical reproduction methods have problems telling some colors apart for instance color copiers tend to reproduce lime green as yellow. Knowing the limitations of technolgy 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 color so that their effect is maximised. They offer good protection from color copy counterfeiting.

Metameric inks work on the principle of metamerism ....two colors matching under one set of lighting conditions can appear 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 color as the veiwing angle is varied. The result is an ink which has this same optical property, changing color as the viewing angle is varried. 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 color copier and therefore gives good protection from this threat.

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