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 highly 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 than the low-pressure lamps, but still poorer
color rendering than other types of lamps. Low-pressure sodium lamps only give
monochromatic yellow light and so inhibit color vision at night.