A hand-held unit that produces light in the visible spectrum within a specific waveband which initiates the polymerization process by activating a photo-initiator contained within resin composite. The light waveband lies within the range of 400–500 nanometres (nm) at the blue end of the light spectrum. Ultra-violet light is no longer used because of safety issues. Lights with light emitting diodes (LEDs) that emit light at a waveband of 470nm may also be used. Plasma arc curing lights generate a high-voltage pulse and a reduced curing time, although this may be insufficient to cure all types of resin composite.