A light source unit comprises a support for supporting a lamp unit, e.g., a gas discharge lamp or the like. The lamp unit has longitudinal and transversal fiduciary surfaces and the support has guide rails for guiding the lamp unit as it is inserted transversely with the longitudinal fiduciary surfaces sliding along a reference surface until transversal fiduciary surfaces engage one or more stops. The support may comprise a first part for supporting the lamp unit, a second part for supporting a 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 lamp unit incident upon the baffle, the arrangement being such that light from the lamp unit must pass through the apertures in both baffles to be incident upon the light guide.