Abstract: A security blank is formed with a light-redirecting spatial structure including a deterministic component establishing at least one authenticating feature, and a stochastic component superimposed on the deterministic component, and sufficiently obscuring the deterministic component so as largely to frustrate recognition of the function of the authenticating feature by intensity measurement of light reflected therefrom. Recognition of the function of the authenticating feature is made possible by a method and an apparatus measuring the degree of coherence of two narrow bundles of light rays obtained from light reflected from the spatial structure upon its illumination by light of a selected wavelength.

Abstract: A document is disclosed, which contains information invisible to the naked eye, but which information can be made visible by a simple method. Said information is represented by markings on the document, which markings comprise a first color at least partially opaque to at least part of the invisible light spectrum, and a second color which serves to conceal said first color in the visible spectrum, but which second color is itself transparent to said part of the invisible light spectrum.