Low-Pressure Sodium Lamps
Low-pressure sodium lamps-producing up to 180 lumens per watt-have the highest efficacy of all commercially available lighting sources. Even though they emit a yellow light, a low-pressure sodium lamp shouldn't be confused with a standard high-pressure sodium lamp-a high-intensity discharge lamp. Low-pressure sodium lamps operate much like a fluorescent lamp and require a 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.
Figure 112: A low pressure sodium lamp
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.