The lamp in the BIRNS CORONA is a commercially-available 1,000-watt high-pressure sodium vapor lamp. It consists of two envelopes: an inner, high purity alumina ceramic tube within an outer envelope of heat-resistant glass, which is fully evacuated.

The Arc Tube

The light-producing element in the lamp is a wall-stabilized arc discharge contained within a refractory envelope (arc tube) within the bulb. This inner envelope is made of a special ceramic, polycrystalline aluminum oxide. This material is extremely resistant to attack by sodium vapor and can withstand the required very high operating temperatures, yet has excellent transmission characteristics (typically over 90%) for visible light.

The electrodes in the arc tube are manufactured from tungsten and are coated with an emission mix of barium calcium tungstate or barium yttrium tungstate. The end caps of the arc tube are made of niobium (columbium) or ceramic. Polycrystalline alumina cannot be directly fused to the metal of the end caps so an intermediate seal is used on the arc tube. That seal is made of either solder glass or metal (or, sometimes, a ceramic plug), which is sufficiently impervious to the high-temperature sodium.

Arc Tube Contents

The arc tube contains small amounts of metallic elements. Although the primary radiating element in the arc is sodium, mercury is included as a buffer gas for color and voltage control, and small amounts of inert high purity xenon (or, in rare cases, argon plus neon) are used as a "starting gas". The resulting sodium/mercury amalgam has a mass of approximately 25 milligrams, and the sodium:mercury ratio is approximately 3:1.

Other Details

The support structure of the lamp uses nickel-plated steel, nickel, stainless steel and/or titanium. The base is typically brass, with lead-soldered connections to that base. The outer envelope is heat-resistant borosilicate glass which contains 5-10% lead. It prevents chemical attack of the arc tube metal parts, and maintains the arc temperature by isolating it from ambient temperature effects.