Atoms of low density Sodium (Na) gas are excited and emit radiation when they return to lower energy levels. The sparse line pattern (extremely
intense yellow and little else) characteristic of Sodium is "energy efficient" (little energy wasted in non-visible wavelengths) but leads to poor "color rendition" of objects
(the only color that can be reflected is yellow, so objects appear in various shades of yellow only). Complaints about the color have led many cities to replace low-pressure
Sodium bulbs with high-pressure ones which include other elements (e.g., Mercury) and produce a spectrum with more colors. Good for drivers, bad for astronomers
(more "pollution" of the spectrum).
Spectrum of Low Pressure Sodium Lamp