A security device for authenticating bank notes, documents and other items, comprises a luminescent material for producing luminescent radiation of first and second wavelengths. The security device includes an optically variable structure for controlling emission of luminescent radiation of at least one of the first and second wavelengths from the security device, the security device being arranged to permit, from an area of the optically variable structure, emission of luminescence of the first and second wavelengths from the security device. The optically variable structure causes the relative emissivity of the security device for luminescent radiation of the first and second wavelengths to change with a change m emission angle, so that the security device produces an angle-dependent colour shift m the emitted luminescent radiation. The optically variable structure may comprise an optical interference stack that controls transmission of luminescent radiation therethrough in response to the wavelength of luminescent radiation.