Low Pressure Sodium LPS lamps are more closely related to fluorescent than High Intensity Discharge lamps, since they have a low–pressure, low–intensity discharge source and a linear lamp shape. Also like fluorecents they do not exhibit a bright arc, rather they emit a softer luminous glow, resulting in less glare.

Close-up view of the electrical discharge inside a low-pressure sodium spectral lamp. This source has not yet reached its steady-state temperature resulting in a low sodium vapor pressure that still permits argon to be excited. This phenomenon can be seen from the argon purple emission in the axis of the discharge tube. Sodium emission is mainly located at the walls because sodium ions are driven toward the negatively-charged glass vessel.

Zeeman Effect Doublet Effect

Spectrum of high pressure sodium lamp. The yellow-red band on the left is the atomic sodium D-line emission, and the blue and green lines are sodium lines which are otherwise quite weak in a low pressure discharge, but become intense in a high pressure discharge.