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A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure.

Low pressure / LPS / SOX

LPS Lamps (Low Pressure Sodium), also known as SOX Lamps (Sodium OXide), consist of an outer vacuum envelope of glass coated with an infrared reflecting layer of indium tin oxide, a semiconductor material that allows the visible light wavelengths out and keeps the infrared (heat) back. It has an inner borosilicate 2 ply glass U shaped tube containing sodium metal and a small amount of neon and argon gas Penning mixture to start the gas discharge, so when the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright orange/yellow color as the sodium metal vaporizes. These lamps produce a virtually monochromatic light in the 589 nm wavelength. As a result, objects have no color rendition under a LPS light and are seen only by their reflection of the 589 nm light.

LPS lamps are the most efficient electrically-powered light source when measured for photopic lighting conditions. — up to 200 lm/W. As a result they are widely used for outdoor lighting such as street lights and security lighting where color rendition is viewed by many to be less important. LPS lamps are available with power ratings from 10 W up to 180 W.

High pressure / HPS

High pressure sodium (HPS) lamps are smaller and contain some other elements (for example, mercury), produce a dark pink glow when first struck, and produce a pinkish orange light when warmed up. These lamps produce continuous spectrum light (not monochromatic), hence colors of objects under them can be distinguished. This leads them to be used in areas where good color rendering is important, or desired (such as to identify the color of a fleeing suspect's car).

High pressure sodium lamps are quite efficient — about 100 lm/W, up to 150 lm/W, when measured for Photopic lighting conditions. They have been widely used for outdoor lighting such as streetlights and security lighting. Understanding the change in human colour vision sensitivity from Photopic to Mesopic and Scotopic is essential for proper planning when designing lighting for roads.

Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminum oxide (alumina). This construction led General Electric to use the tradename "Lucalox" for their line of high-pressure sodium lamps.

Theory of operation

The operation of a high-pressure sodium lamp is illustrated in the diagram on the right. An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor in which the arc is drawn. The temperature of the amalgam is determined to a great extent by lamp power, and this temperature in turn determines the sodium and mercury pressures throughout the arc tube. An increase in the metal pressures will cause a decrease in the electrical resistance of the lamp. For a given voltage, there are generally three modes of operation:

1. the lamp is extinguished and no current flows
2. the lamp is operating, with liquid amalgam in the tube
3. the lamp is operating with all amalgam evaporated

The first and last states are stable, because the lamp resistance is weakly related to the voltage, but the second state is unstable. Any anomalous increase in current will cause an increase in power, causing an increase in amalgam temperature, which will cause a decrease in resistance, which will cause a further increase in current. This will create a runaway effect, and the lamp will jump to the high-current state (#3). Since the lamp is not designed to handle this much power, this would result in catastrophic failure. Similarly, an anomalous drop in current will drive the lamp to extinction.

The lamp is powered by an AC voltage source in series with an inductive "ballast" in order to supply a nearly constant current to the lamp, rather than a constant voltage, thus assuring stable operation. The ballast is usually inductive rather than simply being resistive which minimizes resistive losses. Also, since the lamp effectively extinguishes at each zero-current point in the AC cycle, the inductive ballast assists in the reignition by providing a voltage spike at the zero-current point.

The light from the lamp consists of atomic emission lines of mercury and sodium, but is dominated by the sodium D-line emission. This line is extremely pressure (resonance) broadened and is also self-reversed due to absorption in the cooler outer layers of the arc, giving the lamp its improved color rendering characteristics. In addition, the red wing of the D-line emission is further pressure broadened by the Van der Waals forces from the mercury atoms in the arc.

Light pollution considerations

For placements where light pollution is of prime importance (for example an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out.

One consequence of widespread public lighting is that on cloudy nights, cities with enough public lighting are illuminated by light reflected off the clouds. As sodium vapor lights are often the source of urban illumination, this turns the sky a tinge of orange. If the sky is clear or hazy, the light will radiate over large distances, causing large enough cities to be recognizable by an orange glow when viewed from outside the city.

See also

History of street lighting in the United States

External Links

A UK site containing information about Low-Pressure Sodium (SOX) lamps

References

• de Groot, J.J.; J.A.J.M. van Vliet (1986). The High-Pressure Sodium Lamp, Antwerp: Kluwer Technische Bocken B.V.. ISBN 9020119028.
• Waymouth, John (1971). Electric Discharge Lamps, Cambridge, MA: The M.I.T. Press. ISBN 0262230488.

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