High-Pressure Sodium Discharge Arc Lamps
Schmidt has studied high-pressure discharges through the vapors of alkali metals, sodium, potassium, rubidium and cesium, and discovered that sodium provides the highest efficacy in a light source with a good color rendition. The high-pressure sodium discharge is enclosed in an arc-tube envelope of high-temperature, alkali-vapor-resisting, high-density, polycrystalline alumina. Nelson and Rigden have since made similar studies and have substantiated these findings. Operation of the high-pressure sodium discharge differs from that of the high-pressure mercury-metallic-iodide discharge in that the discharge is wall stabilized with high-volume loading, the sodium pressure is higher by a factor of several hundred, and it is primarily the sodium atoms that are excited. The iodide lamp operates constricted and, convection determined, the sodium pressure is ordinarily a few torr; it uses mercury as well as metal iodides, and atoms of all the metals and mercury are excited. The low-pressure sodium lamp in comparison has high electron temperature and low gas temperature with low-volume loading. The sodium pressure in the arc is several microns compared to 200 mm (200 torr) in the high-pressure sodium lamp.