embodiment of fig3 alternatively the cylindrical second housing having a flat end may be used instead as illustrated in the embodiments shown in fig1 and 13 below the embodiment of fig1 is similar to fig1 except that a single vertical circuit board is provided in the embodiments of fig1 and 13 the discharge tube 10 is folded to define a series of four u shapes arranged in the form of a square in this embodiment it is more appropriate for the second housing 23 to be cuboid with a square cross section in order to make the maximum use of the space defined within the folded discharge tube as in the previous embodiments the second housing 23 is provided with a reflective coating the electronic control circuitry may be provided on vertical fig1 or horizontal fig1 circuit boards it will be understood that there are a number of other possible ways of implementing the invention for example a suitably sized cylindrical second housing could be used even when the discharge tube is arranged in a rectangular manner and similarly a suitably sized cuboid second housing could be used in the space defined by a spiral discharge tube or circumferentially arranged discharge tube sections the particular examples shown in the embodiments are preferred however as they provide maximum space efficiency housings of a triangular or polygonal cross section may also be useful the reflective coating on the second housing may be a smooth coating or it may comprise a plurality of individual small reflective elements the reflective coating may be applied either only to the sides of the second housing or to the.