Low-pressure sodium lamps are electric light sources that produce visible light through a discharge process involving low-pressure sodium vapor, typically operating at a pressure of just a few kilopascals. These lamps were first developed in 1930 and commercialized by Philips in 1932, marking the beginning of their widespread use. Over time, improvements have led to luminous efficiencies reaching up to 200 lumens per watt, making them one of the most efficient light sources available today.
The working principle of a low-pressure sodium lamp is based on the discharge of a mixture of sodium and rare gases, such as argon and helium, which act as Penning gases. At room temperature, sodium is solid, so it needs to be heated to vaporize. When the lamp is powered on, the Penning gas initially creates a red glow, generating heat that causes the sodium to evaporate. Due to its lower ionization and excitation potentials compared to argon and helium, the discharge quickly shifts to the sodium vapor, producing visible light. The emitted light is concentrated around two double-D lines at 589.0 nm and 589.6 nm, wavelengths very close to the peak sensitivity of the human eye (555 nm), contributing to the lamp’s high luminous efficiency.
In terms of structure, low-pressure sodium lamps come in two main shapes: straight-tube and U-shaped. Straight-tube models resemble fluorescent lamps, featuring a double-ended arc discharge tube filled with a sodium-argon-helium mixture. Each end has an electrode, and the discharge tube is sealed within a vacuum outer sleeve with a dual-pin base. U-shaped lamps use a thin, long discharge tube bent into a U-shape to reduce the overall size and minimize heat loss from the curved ends. The discharge tube is made from special sodium-resistant glass, and it is enclosed in a vacuum cylindrical outer bulb. A transparent, infrared-reflective coating—often composed of indium oxide—is applied to the inner surface of the bulb to reduce heat loss and enhance efficiency. A bracket inside the outer bulb helps support the discharge tube.
These lamps are available in various power ratings, including 18W, 26W, 35W, 36W, 55W, 66W, 90W, and 135W. Their typical lifespan is around 19,000 hours, and they exhibit minimal light decay over time. At the end of their life, they can still maintain about 80–85% of their initial luminous output, making them a reliable and energy-efficient lighting option for applications such as street lighting and industrial environments.
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