A lamp which uses sodium in an excited state (and a small amount of neon and argon gas) to produce light. There are two varieties: low pressure sodium and high pressure sodium. 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).
Picture from www.t2.gstatic.com
High pressure sodium adds other elements such as mercury which improves colour rendering somewhat.
Picture from www.t2.gstatic.com
High pressure sodium adds other elements such as mercury which improves colour rendering somewhat.
