Low-pressure sodium lamps are specialized light sources that operate by creating a discharge in low-pressure sodium vapor. The lamp is enclosed in a special anti-sodium glass tube, which allows the emission of two distinct sodium spectral lines at 5890A and 5896A. These monochromatic yellow emissions make them ideal for use in optical instruments such as polarimeters, refractometers, and other precision devices that require a single wavelength of light. The lamp consists of a discharge tube filled with inert gas and metallic sodium, housed inside a high-vacuum outer bulb. A support frame secures the discharge tube within the bulb, and the electrodes are positioned at both ends. The anti-sodium sleeve, shaped like a U, is made of an outer layer of silicate glass and an inner layer of anti-sodium bismuth glass, ensuring durability and stability during operation.
The compact design of the U-shaped discharge tube contributes to a smaller overall size of the lamp, reducing the heat-dissipating surface area. This results in faster heating to the optimal operating temperature of 260°C, improving light collection efficiency. Low-pressure sodium lamps are widely used in various lighting applications, including roads, tunnels, warehouses, and docks across different climate zones. They offer high light output and strong penetration, making them particularly effective in foggy or low-visibility conditions. With a typical lifespan of around 19,000 hours, these lamps experience minimal light decay, maintaining 80–85% of their initial luminous flux at the end of life.
However, due to the long discharge tube, sodium migration at the ignition points can affect performance and longevity. To mitigate this, the lamp must be installed in a specific orientation, typically allowing a horizontal deviation of up to ±20%. For lower power versions, a slightly larger angle may be acceptable. While the yellow monochromatic light produced by low-pressure sodium lamps has limited color rendering, it is well-suited for areas where high illumination is required but color accuracy is not critical, such as highways, bridges, urban plazas, and industrial sites. Their energy-efficient nature makes them especially valuable in regions with power constraints, offering significant energy savings compared to traditional lighting technologies.
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