Patent ID: 7833610

Claim:
A document security device comprising a document onto which information can be provided and having applied to the whole or part of a surface of said document one or more iridescent materials, the one or more iridescent materials each comprising a diffraction grating incorporating a plurality of sheets of a first substantially transparent material in parallel alignment and spaced apart by quantities of a second substantially transparent material, the second material having a refractive index significantly lower than the refractive index of the first material, the sheets being tilted at an angle θ to an axis X and the grating having a period d(X) in the direction of axis X, the angle θ and period d (X) being such that a second diffraction grating is provided along an axis Z, perpendicular to axis X, the second diffraction grating having a period d (Z) and consisting of twenty five periods or less and at least five periods whereby, the diffraction conditions of the second diffraction grating can be simultaneously fulfilled by waveforms of a range of wavelengths thereby causing iridescence such that information which may be provided on the document can only be viewed clearly from a viewing point substantially perpendicular to the surface of the document.