Many properties of inorganic phosphors can be used in printing inks, or dispersed in plastics, to frustrate the activities of forgers and counterfeiters.
Apart from the obvious applications in currency printing, there are many other fields in which they can play a part. These include: original works of art, limited edition prints, admission tickets to sporting events, validation certificates for aircraft spare parts, cheques, passports and credit cards.
At the least sophisticated level, the phosphors used fluoresce with a characteristic colour when exposed to the commonly available long wavelength (300-400 nm) ultraviolet. Many of these materials are generally available and can be used by forgers. A first step beyond simple fluorescence is to use phosphors that do not fluoresce under long UV but do so under short (254 nm) UV.
It is also possible to provide phosphors that fluoresce one colour under long and another colour under short UV. However, recognition equipment using short UV will be more costly than long UV sources.
Phosphors responding to long UV can be provided that, at first glance, look like the easily available materials; but with very simple equipment, can be readily distinguished from them. There are other luminescent properties that can be used to distinguish particular materials. One such would be an obvious change in colour with temperature.
Other phosphors with emissions in the UV and IR regions, and even with excitation in the IR region have also been introduced. These materials have great potential in security applications, especially in areas where high-speed machine reading is required.
Materials that can be excited with IR radiation to emit radiation of a higher energy in the IR or visible region of the spectrum are known as ‘Anti-Stokes’ or ‘up-conversion’ phosphors. To understand the significance of this it is necessary to appreciate the basic mechanism of fluorescence.
Fluorescence … a quantum phenomenon in which energy is absorbed by the phosphor in the form of photons of ultraviolet radiation and is emitted as photons of visible light…
The energy carried by a photon is inversely proportional to the wavelength, thus a photon of UV at 366 nm carries 3.4 electron volts (ev) while a visible light photon in the green at 520 nm carries 2.4 ev. In all normal fluorescence it follows that the exciting radiation must have a shorter wavelength than the emitted light. This observation is referred to as Stoke’s Law. However, Anti-Stokes phosphors have the ability to absorb two or three photons of long wavelength infrared light and combine their energies to emit a single photon of visible light. Phosphors in this class have characteristic excitation spectra in the infrared, often coinciding with the wavelengths emitted by IR LEDs or lasers.
Since stable Anti-Stokes Phosphors are not generally available and are difficult to manufacture, they are attractive candidates for security applications.
Phosphor Technology offers a range of inorganic phosphors for anti-counterfeiting and tagging applications. Please contact us directly to discuss your specific requirements in confidence.