Low pressure sodium (LPS) lamps are the most efficient source of light
commercially available, up to 183 lumens per watt. However, they are used in very few
applications because of their poor color rendering characteristics. An LPS
lamp's light output is monochromatic yellow. What this means is that reds, blues
and other colors illuminated by an LPS light source all appear as tones of
gray. The primary use of LPS lamps is for street and highway lighting as well as
outdoor area and security lighting. The typical applications of LPS lighting
include parking lots and garages, automobile and train tunnels, and street
lighting. Indoor applications such as warehouses are practical where color is not
important.
Low pressure sodium lamps range in size from 35 watts to 180 watts. Ballasts designed specifically for LPS must be used. There is a limited range of either linear-lamp or U-tube fixtures available.
Strengths:
Most energy efficient light source commercially available, with an efficacy of
100 to 185 lumens per watt.
Lamps have average life in the 14,000 to 18,000 hour range and have excellent
lumen maintenance (very little reduction in lumen output over life of lamp).
Most lamps will restart immediately after interruption of power supply, but
require some time to come up to full brightness.
Provides superior uniformity of light distribution over all HID lamps.
Weaknesses:
These lamps have the poorest color rendering characteristics. In fact, it is
almost impossible to distinguish colors under an LPS lamp because light
produced by this source is monochromatic (a single color).
Most expensive lamps to install.
Run time to full light output is the longest (7 to 15 minutes).
Wattage will increase over the life of the lamp to maintain lumen output.
Requires special disposal considerations.
