Low Pressure Sodium Systems
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.
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.
Low Pressure Sodium lamps will start at ambient temperature as low as -40°F and will warm up to full wattage and light output at any temperature.
- 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.
- 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.
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.
Aside from its excellent energy efficiency, this source has another important advantage. It has been used a great deal in areas that are sensitive to light pollution. Arizona and southern California have used Low Pressure Sodium lamps extensively because the monochromatic light doesn't interfere with astronomical observation. Low Pressure Sodium light also provides good fog penetration.
Technology types (resource)
A Low Pressure Sodium lamp contains a U-shaped arc tube constructed of sodium-resistant lime borate glass inside an evacuated clear outer envelope. This glass envelope has an internal coating of heat-retaining indium oxide, which contributes to the extremely high efficacy of the LPS lamp. Inside the arc tube there is a mixture of neon and argon gases, which are used to start the lamp, together with pure sodium metal.
The arc tube surface features a series of dimples that serve as reservoirs to assure even distribution of the sodium. This promotes maximum lumen maintenance throughout the life of the lamp. The arc discharge takes place between triple-coil tungsten electrodes mounted at each end of the arc tube near the base of the lamp. The electrodes are coated with a special emission material.
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