Security blank with enhanced authenticating features, and a method and an apparatus for determining the genuineness of the security blank

In an apparatus for determining the genuineness of a security blank, said security blank including at least one authenticating feature in the form of a spatial structure, said apparatus being adapted to be used with a light source illuminating said spatial structure at a selected wavelength of light, and including at least two light-sorting members forming an angle between each other, each selecting a narrow bundle of light rays from light scattered from said spatial structure, and redirecting said bundle of light rays, photoelectric means receiving the redirected bundle of light rays and generating an electrical signal therefrom, signal processing means connected to said photoelectric means receiving said electrical signal and determining therefrom a characteristic measurement value for said spatial structure, and decision means forming "yes" or "no" outputs in dependence of said characteristic measurement value,

• the improvement comprising
• an elongated deterministic component having a plurality of measurement distances, and an elongated stochastic component, said components being included in said spatial structure, said stochastic component scattering light impinging thereon in a diffuse manner, and having a correlation length not exceeding a predetermined fraction of one of said measurement distances,
• said signal processing means determining said characteristic measurement value by determining the degree of coherence of said narrow bundles of light rays.