A lamp, containing .ltoreq.100 ppm Hg, includes a burner chamber having an axis, the axis having a section .gtoreq.0.05 mm. The burner chamber has a recess oriented toward a light arc produced in the burning chamber to suppress arc curvature. The lamp has an arc-luminescence profile of luminescence (measured in cd/m.sup.2)/mm with an average slope over that section which is .gtoreq.|300,000,000| (cd/m.sup.2)/mm. Preferably, the profile maximum value is .gtoreq.90,000,000 cd/m.sup.2. In one embodiment, the arc-making device includes two electrodes disposed for producing the light arc across their two tips, the depth of the recess being sufficiently small so as not to extend between the two tips.