The low pressure sodium lamp is regarded as the most efficient of all known lightsources. It has low running costs and long life time(about 18,000 hours). It is much used in street lights and is easily recognized by it's intense orange light. The fact that the light is monochromatic limits the use of this lightsource to applications where colour recognition isn't important

The LPS lamp consists of a arc tube made from sodium resistant glass. The tube is filled with a neon-argon mix and sodium. When the lamp is energized this gasmix starts ionizing. The heat from this procces vaporizes the sodium and starts to emit visible light. Outside the arc tube is another layer of glass. This is coated on the inside with heat reflective indium oxide, this keeps the heat inside the lamp so the sodium vaporizes faster. The LPS lamp often needs several minutes to reach the optimum operating temperature.
The low pressure sodium vapor lamp is one of the most common lamps used in outdoor lighting, and especially that of street lights. It has been used in Europe since the 1940s and was heavily marketed within the United States in the 1970s.

This lamp has a distinctive yellow orange color that is familiar to many. Its primary advantage is that is among the highest efficency light source and produces up to 180 lumens/watt and range from 18 to 180 watts. Furthermore, these lamps have a service life of up to 18,000 hours and thus require much less frequent changing (18,000 hours at 11 hours a day, 6 days a week would last for 5.2 years). The 18,000 hour figure makes the assumption that each 'on' cycle is at least 5 hours - these lamps do better with long periods of 'burning' and do not take well to being turned on and off rapidly.

Lamp                 Lumens/watt Average Lamp life
-------------------- ----------- ------------------
Incandescent          8 -  25     1000 -  2000
Mercury Vapor        13 -  48    12000 - 24000
Metal Halide         60 - 100    10000 - 15000
High Pressure Sodium 45 - 110    12000 - 24000
Fluorescent          60 - 600    10000 - 24000
Low Pressure Sodium  80 - 180    10000 - 18000

Often these lamps are used where maintaining the color of the environment is not important (as mentioned above street lights), and thus are not as appropriate for lighting buildings or artwork at night (such as monuments and flags). This light also works very well with surveillance cameras for two reasons: first, the color rendition on black and white cameras is not important, and secondly the peak wavelength of the light closely matches that of the peak sensitivity of most black and white video cameras.

The arc tube itself is made of ceramic known as polycrystalline alumina (also known as PCA). This material is necessary to use because the sodium in the tube would etch glass, while PCA does not react with sodium.

At times, mercury is added to the arc to strengthen the blue and green colors in the lamp from the predominately yellow orange of sodium.

The nature of the very narrow band of color emitted by a sodium vapor lamp is often used in photography. Within a darkroom, illumination must be provided by a safelight that does not expose the material (in this case, color blind printing paper).

When it comes to the topic of light pollution, the LPS lamp is especially useful. The light itself consists of two spectral lines at 589nm (about 95% of the output) and 586nm (about 5% of the output). To an astronomer it means that this light can be easily filtered out while the more 'white' lights of incandescent and similar lamps cover a large distribution of wavelengths.

The color of the low pressure sodium vapor lamp is approximately: red: 242; green: 202; blue: 0 (0xF2CA00). The 589nm wavelength of light is often used as the definition of the color yellow in light, however does not match exactly with the 255 255 0 color. The closest color familiar to those is that of 'gold' (255, 215, 0), however the x11 color 'gold2' is a better match (238 201 0).

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