A light source unit, for supplying light via a light guide to a remote location, comprises a support for supporting a light source and a light guide in a prescribed alignment relative to each other, the support comprising a first part for supporting the light source, a second part for supporting the light guide, and at least first and second light baffles between the first part and the second part, the first and second baffles each having an aperture for passing only a portion of light from the light source incident upon the baffle, the arrangement being such that light from the light source must pass through the apertures in both baffles to be incident upon the light guide. A mounting arrangement for replaceably mounting a gas discharge lamp or the like having longitudinal and transversal fiduciary surfaces comprises guide rails for guiding the lamp as it is inserted transversely with the longitudinal fiduciary surfaces sliding along a reference surface until transversal fiduciary surfaces engage one or more stops.