A light source for use in a projection system comprises a pair of lamps each effective to produce a substantially collimated beam. Part of the light emitted by each lamp is directed through the light originating part of the same, or another, lamp. A light combiner is arranged within the path of the light beams from each of the two lamps to provide a composite light beam of a cross section less than the sum of the cross section of the beams produced by each of the lamps.