Monochromatic light is light with a very narrow wave-length bandwidth. One way to generate monochromatic light is to excite gases or vapours of certain elements to luminescence inside lamps. Hg vapour lamps, for example, produce light radiation with 546.1 nm wavelength, He tubes 587.6 nm, and Na lamps 589.3 nm. Monochromatic light is needed, among other things, for inspecting the flatness of seal faces (interference).