Abstract: To improve the color rendition index R.sub.a, and to lower the color tempture without decreasing the light yield and efficiency of the lamp, the discharge vessel (1, 12, 22) is surrounded by an envelope which comprises a first, or inner envelope element (4, 13, 21) and a second, or outer envelope element (5, 14, 20). The space between the first and second envelope elements, preferably, is evacuated, and the space between the first, inner envelope element and the discharge vessel may include an inert gas, such as nitrogen and/or a noble gas; if the inner envelope element and the discharge vessel are both made of sodium resistant materials, such as Al.sub.2 O.sub.3, the space between the inner envelope element and the discharge vessel may also include sodium of such quantity as to balance the vapor pressure within the discharge vessel and said space outside of the discharge vessel. The lamp can be made double-ended (FIGS. 1, 2) or single-ended (FIG. 3).