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The light sources generally used for the polymerization of UV curable inks and paints in industrial processes are very different with respect to the method used to pump energy into the radiating medium, which in all cases is represented bya gas at very high temperature.
Pulsed Xenon lamps are actually not used in the field of polymerization of inks and coatings, but mostly, in reprography and therefore are not taken into consideration in this paper. The microwave power lamps can be included in the general category of high pressure mercury lamps, whose radiating mechanics is described below, with the distinction that the microwave energy is used to heat directly the plasma within the quartz tube. The high intensity electric field of the microwave initiates the discharge and brings the mercury vapour up to the temperature where UV radiation occurs.
The most widely used industrial UV sources are, however, the high intensity discharge medium pressure mercury lamps. it is useful to mention the , .basic principles of operation of this device; The As light is produced by an eiectric discharge occurring .between two oxide coated electrodes through
mercury vapour and argon at high pressure (>1 bar) within a fused silica (quartz) tube. At start, a
glow discharge occurs between the electrodes, sustained by argon gas. Collisions between
accelerated electrons and argon atoms can bring some of these into a metastabie' state. In turn
mercury atoms 'are ionized colliding with 'metastabie argon. An avalanche is therefore
triggered even if the 'distance between electrodes is very high. in this way the construction
of long lamps at relatively low operating voltage is possible. The gas is then heated by the electric
power input given by the current flowing times the voltage applied. At steady state the kinetic energy, of colliding particles is high enough to keep in the mean a large number of mercury atoms in an excited state, with their outermost electrons displaced on higher energy orbitals (Picture 1).