Abstract: An elongated discharge lamp which is particularly intended for use in photo-chemical reaction processes. The discharge tube of the lamp is supported at its ends by two end mountings, one of which carries the input terminals for the lamp. A conductor extending along the length of the discharge tube joins one of the terminals to the electrode at the remote end of the tube, and the tube and conductor are separately enclosed in respective transparent tubular envelopes sealed to the end mountings at their ends.A liquid-cooling jacket is provided, cooling liquid being circulated therethrough by means of a tube of transparent material supported in the end mountings.