Low Pressure Sodium Lamps
ANSWER
Low pressure sodium lamps have a U tube containing the gas and an outer heat reflecting glass envelope.
The light output from Sodium Vapour is in the yellow part of the visible spectrum. The efficacy is the highest of all lamp types but with very poor colour rendering (showing reds as browns). Low pressure sodium lamps are used mainly for exterior applications, road and security lighting. They are not suitable for repeated on/off operations as they take some time to reignite.
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