Low-pressure sodium lamps—producing up to 180 lumens per watt—have the highest efficacy of all commercially available lighting sources. A low-pressure sodium lamp is shown in the image below.
Even though they emit a yellow light, a low-pressure sodium lamp should NOT be confused with a standard high-pressure sodium lamp, which is a high-intensity discharge lamp.
Low-pressure sodium lamps operate much like fluorescent lamps and require ballast. The lamps are also physically large—about 4-feet long for the 180-watt size—so light distribution from fixtures is less controllable. There is a brief warm-up period for the lamp to reach full brightness.
With a CRI of 0, low-pressure sodium lamps are used where color rendition is not important but energy efficiency is. They're commonly used for outdoor, roadway, parking lot, and pathway lighting. Low-pressure sodium lamps are preferred around astronomical observatories because the yellow light can be filtered out of the random light surrounding the telescope.