Several gas or metal-vapor lamps can also be used. When operated at low pressure and current, these lamps generally produce light in various spectral lines, with one or two lines being most predominant. Because these lines are very narrow, the lamps can be combined with narrow-bandwidth filters to isolate the strongest line. A helium-discharge lamp will produce a line at 587.6 nm (yellow), while a mercury-vapor lamp produces a line at 546.1 (yellowish green). Cadmium vapor produces a line at 643.8 nm (red), but low pressure sodium produces a line at 589.3 nm (yellow). Of all the lights, low pressure sodium is the only one that produces a single line, requiring no filter.