Apparatus for checking the authenticity of banknotes

An apparatus for checking the authenticity of bank notes in unprotected areas, has a sensor assembly containing a light source which produces light of a first wavelength with which the bank notes to be checked are illuminated; whereupon one or more feature substances present in and/or on the bank notes produces light of a second wavelength which is detected by the sensor assembly. A control device evaluates data from the sensor assembly about the light of the second wavelength and compares them with reference data to determine the authenticity of the bank notes. The structure and operation of the sensor assembly and/or the control device permit conclusions to be drawn only with difficulty on the feature substance or substances contained in the bank notes.