Patent ID: 8894098

Claim:
A luminescent security device, the device comprising: an ultraviolet (UV) responsive luminescent layer having a luminescent area comprising a mixture of two or more luminescent materials, each of which absorbs electromagnetic radiation only in the ultraviolet (UV) region of the electromagnetic spectrum, each of which, when stimulated by UV radiation, luminesces to produce luminescent radiation of a wavelength in the visible spectrum of the electromagnetic spectrum that is different from luminescent radiation produced by the other luminescent material(s) present in the mixture, the luminescent radiation emitted from the luminescent area at least including first and second wavelengths in the visible spectrum, the second wavelength being different from the first wavelength; and an optical interference structure distinct from the luminescent layer, and comprising a plurality of adjacent layers of electromagnetic radiation transmissive materials with different refractive indices that receive from the luminescent area the luminescent radiation of the first and second wavelengths in the visible spectrum, to control emission of the received luminescent radiation of at least one of the first and second wavelengths, such that the relative intensities of the luminescent radiation of the first and second wavelengths changes with an emission angle from the device, resulting in an angle-dependent colour-shift in the luminescent radiation from the luminescent area, when transmitted through the optical interference structure and emitted from the device; the optical interference structure including an area from within which emission of the luminescent radiation from the device occurs, the luminescent radiation of the first wavelength transmitted at a first angle of emission, and the luminescent radiation of the second wavelength transmitted at one or both of (1) the first angle of emission and (2) a second angle of emission different from the first angle of emission, said optical interference structure arranged to cause, from the optical interference structure area, the emissivity of the device for luminescent radiation of the first or second wavelengths to change relative to the emissivity of the device for luminescent radiation of the other of the first or second wavelengths with a change in emission angle of luminescent radiation from the device from the first emission angle to the second emission angle.