A Low Pressure Sodium (LPS) lamp is a highly efficient type of gas discharge lamp known for producing a characteristic monochromatic yellow light.
These lamps operate by generating an electrical discharge through vaporized sodium metal at a relatively low partial pressure. This discharge produces light primarily at two closely spaced wavelengths in the yellow part of the spectrum, specifically at 589.0 and 589.6 nanometers.
At the core of an LPS lamp is a borosilicate glass gas discharge tube (arc tube). This tube is specifically designed to withstand the operating temperature and chemical properties of sodium vapor. Inside the arc tube, you will find solid sodium along with a small amount of starting gas. The starting gas is typically a small amount of neon and argon gas in a Penning mixture. This gas mixture allows the lamp to strike (start the discharge) at a lower voltage before the sodium vaporizes and takes over the light production. The shape of the discharge tube can vary, being either linear (sometimes referred to as an SLI lamp) or often U-shaped to provide a more compact light source.
Key Characteristics of LPS Lamps:
- High Efficacy: LPS lamps are among the most efficient light sources in terms of lumens per watt.
- Monochromatic Light: They emit light in a very narrow yellow spectrum, leading to poor color rendering (colors appear in shades of yellow, brown, or black).
- Warm-up Time: They require a few minutes to reach full brightness and color as the sodium metal vaporizes.
- Durability: Generally have a long operational lifespan.
Common Applications:
- Street lighting
- Parking lot lighting
- Security lighting
- Tunnel lighting
- Areas where light pollution reduction is important (due to their narrow spectrum, which can be filtered more easily)
While incredibly efficient, their poor color rendering has led to their decline in popularity in many applications where color perception is important, being replaced by high-pressure sodium, metal halide, or LED alternatives.
