Sodium vapor lamp

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.

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Low pressure LPS/SOX

Low Pressure Sodium (LPS) Lamps, also known as Sodium oxide (SOX) Lamps, 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 two inner borosilicate glass U-pipes that hold solid sodium 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 yellow color as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging at a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colours of objects cannot easily be distinguished since they are seen almost entirely by their reflection of this narrow bandwidth yellow light.

LPS lamps are the most efficient electrically powered light source when measured for photopic lighting conditions—up to 200 lm/W.^[1] 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; however, length increases greatly with wattage creating problems for designers.

LPS lamps are more closely related to fluorescent than High Intensity Discharge lamps, since they have a low–pressure, low–intensity discharge source and a linear lamp shape. Also like fluorescents they do not exhibit a bright arc as do other HID lamps; rather they emit a softer luminous glow, resulting in less glare.

Another unique property of LPS lamps is that, unlike other lamp types, they do not decline in lumen output with age. As an example, Mercury Vapor HID lamps become very dull towards the end of their lives, to the point of being ineffective, whilst still drawing their full rated load of electricity. LPS lamps, however, do increase energy usage towards their end of life, which is usually rated around 18,000 hours for modern lamps.

High pressure HPS/SON

High pressure sodium (HPS) lamps are smaller and contain additional elements such as mercury, and produce a dark pink glow when first struck, and a pinkish orange light when warmed. Some bulbs also briefly produce a pure to bluish white light in between. This is probably from the mercury glowing before the sodium is completely warmed. The sodium D-line is the main source of light from the HPS lamp, and it is extremely pressure broadened by the high sodium pressures in the lamp; 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. Thus, its new model name SON is the variant for "Sun".

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 color 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.

Xenon at a low pressure is used as a "starter gas" in the HPS lamp. It has the lowest thermal conductivity and lowest ionization potential of all the non-radioactive noble gases. As a noble gas, it does not interfere with the chemical reactions occurring in the operating lamp. The low thermal conductivity minimizes thermal losses in the lamp while in the operating state, and the low ionization potential causes the breakdown voltage of the gas to be relatively low in the cold state, which allows the lamp to be easily started.

White SON

A variation of the high pressure sodium, the White SON, introduced in 1986, has a higher pressure than the typical HPS lamp, producing a color temperature of around 2700K, with a CRI of 85; greatly resembling the color of incandescent light.^[2] These are often indoors in cafes and restaurants to create a certain atmosphere. However, these lamps come at the cost of higher purchase cost, shorter life, and lower light efficiency.

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. The higher the lamp power, the higher will be the amalgam temperature. The higher the temperature of the amalgam, the higher will be the mercury and sodium vapor pressures in the lamp. An increase in these 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 actual lamps are 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. It is the second state which is the desired operating state of the lamp, because a slow loss of the amalgam over time from a reservoir will have less effect on the characteristics of the lamp than a fully evaporated amalgam. The result is an average lamp life in excess of 20,000 hours.

In practical use, 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. Sodium emits light on only one wavelength, and therefore 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.

End of life

At the end of life, high-pressure sodium lamps exhibit a phenomenon known as cycling. These lamps can be started at a relatively low voltage but as they heat up during operation, the internal gas pressure within the arc tube rises and more and more voltage is required to maintain the arc discharge. As a lamp gets older, the maintaining voltage for the arc eventually rises to exceed the voltage provided by the electrical ballast. As the lamp heats to this point, the arc fails and the lamp goes out. Eventually, with the arc extinguished, the lamp cools down again, the gas pressure in the arc tube is reduced, and the ballast can once again cause the arc to strike. The effect of this is that the lamp glows for a while and then goes out, repeatedly.

More sophisticated ballast designs detect cycling and give up attempting to start the lamp after a few cycles. If power is removed and reapplied, the ballast will make a new series of startup attempts.

LPS lamp failure does not result in cycling; rather, the lamp will simply not strike, and will maintain its dull red glow exhibited during the start up phase.

See also

• List of light sources
• History of street lighting in the United States
• Street light

References

1. ^ Why is lightning colored? (gas excitations). WebExhibits. Retrieved on 2007-09-24.
2. ^ Philips SDW-T High Pressure Sodium White SON. WebExhibits. Retrieved on 2007-09-24.

• de Groot, J.J.; J.A.J.M. van Vliet (1986). The High-Pressure Sodium Lamp. Antwerp: Kluwer Technische Bocken B.V.. ISBN 90-201-1902-8. 
• Waymouth, John (1971). Electric Discharge Lamps. Cambridge, MA: The M.I.T. Press. ISBN 0-262-23048-8.