There is described an ultraviolet light source comprising an ultraviolet bulb, a pulsed microwave energy source for exciting said ultraviolet bulb and an enclosure for enclosing the ultraviolet lamp, the enclosure comprising an optically transparent waveguide. The optically transparent waveguide wholly surrounds the bulb. The ultraviolet light source is particularly suitable for use in the sterilisation of substances; the promotion of photochemical reactions; and the promotion of molecular dissociation in liquids.