Apparatus and a method for testing the authenticity of documents

In an apparatus for examining the authenticity of a document, including at least a machine-readable authenticity feature in the form of an optical microstructure for diffracting at least one component of a beam of light rays having a central axis, an intensity distribution and a wavelength of a first predetermined value, and impinging on said microstructure along a predetermined direction, a light source for emitting said beam of light rays, the components of the light rays diffracted from said microstructure having intensity distribution defining an effective center of intensity distribution along one direction of diffraction, light-sensing means for sensing the light rays diffracted by said microstructure and generating electrical signals from the sensed light rays, and evaluating means for processing the electrical signals generated by said light sensing means, and for providing from information contained in the processed electrical signals a YES/NO decision as to whether the document is authentic or not,

• in combination,
• control means connected to said light source for the control thereof so that said light source switches the wavelength of said beam of light rays from said first predetermined value to a second predetermined value, and being free from disturbing the central axis and the intensity distribution of said beam;
• said light sensing means including at least an electro-optical position sensor having a detector surface, wherein the components of said beam of light rays diffracted from said microstructure and having said first predetermined wavelength value impinge on said detector surface at a first region defining said effective center of the intensity distribution at said first predetermined wavelength value, and wherein the components of said beam of light rays diffracted from said microstructure and having said second predetermined wavelength value impinge on said detector surface at a second region defining an effective center of the intensity distribution at said second predetermined wavelength value, said regions being displaced from one another by a certain displaced amount, said evaluating means providing said YES decision if said displaced amount lies within predetermined limits.

In a method of examining the authenticity of a document including at least a machine-readable authenticity feature in the form of an optical microstructure

• the steps comprising
• alternately illuminating said microstructure by first and second beams of light rays at a predetermined direction, each having components diffracted from the microstructure at an intensity distribution defining an effective "center line of the intensity distribution" along one direction of diffraction,
• sensing an angular difference between said directions of diffraction, and
• deciding that the document is authentic if said angular difference lies between predetermined limits.