Security device

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 emit luminescent radiation of a wavelength in the visible spectrum of the electromagnetic spectrum that is different from that 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, the second wavelength being different to the first wavelength;
• and
• an optical interference structure distinct from the luminescent layer, and comprising 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 the intensity of emission of the received luminescent radiation with an angle of emission of the luminescent radiation from the device, based on the wavelength of said luminescent radiation, such that relative intensities of the luminescent radiation of the first and second wavelengths changes with emission angle, resulting in an angle-dependent colour shift in the luminescent radiation from the luminescent area, when transmitted by the optical interference structure and emitted from the device.

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 that is different from that produced by other luminescent materials present in the mixture, the luminescent radiation emitted from the luminescent area at least including first and second wavelengths, the second wavelength being different to the first wavelength;
• and
• an optical interference structure distinct from the luminescent layer, and comprising adjacent layers of electromagnetic radiation transmissive materials with different refractive indices, the optical interference structure including an optical interference structure area for receiving and controlling emission from the device of the luminescent radiation of at least one of the first and second wavelengths in the visible spectrum emitted from the luminescent area, the device being arranged to permit, from within the optical interference structure area, emission of the luminescent radiation of the first and second wavelengths from the device, wherein the optical interference structure is arranged to cause, from said optical interference structure area, the relative emissivity of the device for the luminescent radiation of the first and second wavelengths to change with at least one or both of (1) a change in emission angle of luminescent radiation from the device and (2) a change in the direction or angle of incidence of UV stimulating radiation on the device for stimulating the luminescent layer.