Document having a security feature and method of determining the authenticity of the document

Claim 1

A security blank including a substrate having two major surfaces, comprising, in combination, a transparent thermoplastic layer having a predetermined refraction index as defined by light incident thereon, and being suitable for embossing thereon a machine-readable authenticity feature in the form of an optical microstructure, and a transparent protective layer having a prearranged refraction index for covering and protecting said microstructure, said prearranged refraction index of said transparent protective layer having a value different from said predetermined refraction index of said transparent thermoplastic layer, and wherein said microstructure includes means for generating a characteristic diffraction effect from the light incident thereonto, said characteristic diffraction effect being normally detectable by the unaided eye of an observer, a surface of the substrate facing said transparent thermoplastic layer including within a region of said microstructure means for producing diffuse reflection characteristics of a predetermined magnitude, the difference between said refraction indexes of said transparent layers being sufficiently small, and the magnitude of said diffuse reflection characteristics of the surface of the substrate facing said transparent thermoplastic layer being sufficiently large so that said characteristic diffraction effect of said microstructure is substantially not detectable by the unaided eye of an observer.