A sodium-vapor lamp is a gas-discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure. Low-pressure sodium lamps are the most efficient electrical light sources, but their yellow light restricts applications to outdoor lighting such as street lamps. High-pressure sodium lamps have a broader spectrum of light but poorer color rendering than other types.
Because sodium vapor lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them.
Read more about Sodium-vapor Lamp: Low-pressure Sodium, High-pressure Sodium, End of Life, ANSI HPS Ballast Codes
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