a gas-discharge light source in which emission in the optical range takes place during electrical discharge in sodium vapor.
A low-pressure sodium-vapor lamp is a tube of sodium-resistant glass filled with sodium vapor and a mixture of inert gases, with electrodes sealed into the ends of the tube. The gas pressure in the tube is 1.3–2.0 kilonewtons per sq m (kN/m2), or 10–15 millimeters of mercury (mm Hg). The power of sodium-vapor lamps is 45–200 watts (W), the service life is 5,000–7,000 hr, and the luminous efficiency is 100–170 lumens per watt (lm/W). Because of their pure yellow light, sodium-vapor lamps are not suitable for general lighting; they are used abroad for lighting intercity highways and for decorative illumination.
The discharge tubes of high-pressure sodium-vapor lamps are made from transparent polycrystalline A12O3, which is resistant to electrical discharge in sodium vapor up to temperatures in excess of 1200°C. After the removal of air, measured quantities of sodium, mercury, and an inert gas are introduced into the discharge tube at a pressure of 2.6–6.5 kN/m2 (20–50 mm Hg). The power of such lamps is 125–1,000 W; the luminous efficiency, 100–140 lm/W; and the service life, 10,000–20,000 hr. Such sodium-vapor lamps, which give a pleasant golden-white light, are used for exterior and interior lighting. All sodium-vapor lamps are connected to power-supply systems through start-control devices. To achieve maximum yield of the resonance radiation of sodium, the discharge tubes of sodium-vapor lamps are heated by placing them in evacuated glass envelopes.
G. N. ROKHLIN