A High Frequency light source (11) has a central body (12) of fused quartz, with a central void (14), filled with a fill (16) in the void of material excitable by High Frequency energy to form a light emitting plasma. An inner sleeve (17) of perforate metal shim extends along the length of the central body to within 2.5mm of its void end to provide a launching gap (18). The sleeve has a transverse end portion (19) extending across the other, inner end of the central body. An outer cylinder of fused quartz (20) with an internal bore (21) such as to be a sliding fit with the inner sleeve, itself a sliding fit on the central body. An outer sleeve (22) of perforate metal, enclosing the outer cylinder and having an end portion (23) extending across the flush, void ends of the quartz body and cylinder (12,20). The outer sleeve has a skirt (25) extending past the flush other ends of the quartz elements over an aluminium carrier (26), where it is clamped, holding the quartz elements against the carrier. Thus the sleeve forms with, with its end (23) and the carrier (26), a Faraday cage around the quartz and the plasma void (14). An antenna (27) insulated from the carrier extends from it into a bore (28) in the quartz cylinder (20) for introducing HF radiation into the coaxial wave guide formed by the inner and outer sleeves (17,22).