Optical testing for genuineness of bank notes and similar paper bills

#The present invention concerns a method and a meals for optical testing of genuineness of bank notes, forms, cheques and similar paper bills. In particular the invention relates to recognizing paper notes on the basis of the particular optical signature of a genuine multilayer colour printed paper note, as opposed to the signature of a counterfeit note which hits been produced by means of a modern colour copying machine.

#Until to-day, "good" forgeries of bank notes have been made by offset printing. It is just recently that colour copying machines have been enabled to copy bank notes with a passable result. Such modern colour copying machines are now entering the market, and banks are presented with a possibly very large problem with good colour copies of bank notes, cheques etc.

#Previously a genuineness test of bank notes has usually been made by analysing embedded security parameters, like e.g. security thread and water mark. However, an automatic reading of e.g. a water mark is very difficult technically, ant also rather expensive. Thus, an authenticity test by means of these security parameters is adapted to a relatively small volume of test machines.

#In FIG. 1b appears a typical sensitivity curve for the colour detectors in the analyser of a colour copying machine. It should be noted that the three curves respectively have a bandwidth of about 100 nm. It should also be noted that the three curves overlap with each other. This is quite necessary for an analyser which must be able to reproduce all colours.

#FIG. 1d shows the spectra of typical print colours for a tree-coloured print process. It appears that "narrow" colour bands can be composed by using these dyes.

#FIG. 1a shows a sketch of a spectrum from a bank note to be checked for genuineness, with relative intensity as a function of wavelength. A certain point has been chosen on the bank note, in which point there exists a colour combination which is difficult to handle for the copying machine. As appears from the curve shape, said point comprises rather much blue and red in relation to green. In FIG. 1c there is shown an example of a choice of colour filters to be used in front of each respective one of the possibly three detectors to be used in accordance with the invention. The three filters have one respective pass band in each respective of the three colour ranges in question, and the width of the three bands is about the same, in the example about 35 nm. Thus, the three bands do not overlap at all, and the pass bands should be as rectangular as possible, as shown in the figure. A detector system comprising filters of this type will not encounter difficulties in distinguishing between a good colour copy and a genuine bank note.