An electric arc discharge lamp comprises a cathode (2), an apertured diaphragm (4) and a cup shaped apertured anode (6). The anode (6) is provided with a flared extension (7) and is mounted on leads passing through ceramic tubes (14) (FIG. 4). The cathode (2) is enclosed in a cup shaped member comprising a cylindrical tube (5) having one end closed by the diaphragm (4). The electrodes are enclosed in a sealed envelope (8) (FIG. 4) which is filled with deuterium. A synthetic quartz window (81) is provided in one end of the envelope to enable ultra-violet radiation from the arc struck between the cathode and anode to pass through.The cathode (2), the aperture (42) in the diaphragm (4) and the aperture (62) in the anode (6) are aligned and lie on the axis of symmetry of the envelope (8). The anode is an efficient heat radiator because of its large surface area and thus works at a lower than normal temperature allowing the use of stainless steel rather than the usual refractory metals.