A lamp which uses sodium in an excited state (and a small amount of sodium and neon and argon gas) to produce light. There are two varieties of such lamps: low pressure and high pressure. The low pressure sodium produces a very yellow quality of light which makes it very hard to discern colours (a problem for law enforcement as these lamps are frequently used outdoors because of great energy efficiency: about 40 - 50 lumens/watt).
High pressure sodium adds other elements such as mercury which improves colour rendering somewhat.
High pressure sodium adds other elements such as mercury which improves colour rendering somewhat.