Seeing the light

A printing company has invented a security label for banknotes and share
certificates whose pattern can only be seen with a polarising viewer (GB 2 328
180). De la Rue of London created the label by adding a layer of plastic to a
reflective aluminised film. Light striking the plastic is refracted in two
slightly different directions depending on its polarisation, so removing or
melting parts of the plastic creates a coloured pattern when the label is viewed
through a polariser. The plastic is protected with a smooth, transparent
coating. The labels are easily checked with a cheap polariser, but are difficult
to counterfeit owing to the high cost of processing the birefringent
plastics.