The security of bank notes against counterfeiting can be improved by the incorporation of optically variable
devices (OVDs), which change their appearance when the note is viewed in different directions. Three types of
OVD; holograms, multiple diffraction gratings, and thin film interference filters will be compared in relation to the
special requirements unique to bank notes; security, visibility and durability.
The Bank of Canada has recently issued $50 bank notes bearing a five-layer thin film interference filter. The
tests which led to this particular form of OVD will be described, together with methods of deposition, quality
control, and application to sheets of bank notes.
"Optically variable devices for use on bank notes", Proc. SPIE 1210, Optical Security and Anticounterfeiting Systems, (1 April 1990); doi: 10.1117/12.17908; https://doi.org/10.1117/12.17908