Excimers are formed in a high pressure gas by applying a potential between a first electrode (14, 214) and a counter electrode (25, 226) so as to impose an electric field within the gas, or by introducing high energy electrons into the gas using an electron beam. A phosphor for converting the wavelength of radiation emitted from the formed excimers is disposed within the gas and outside a region (62, 162) where the excimers are expected to be formed, so as to avoid degradation of the phosphor.