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Sodium-vapor lamp

A sodium vapor lamp is a gas discharge lamp that uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure. Because sodium vapor lamps cause less light pollution than mercury-vapor lamps, many cities that have large astronomical observatories employ them.[1]


  • 1 Low pressure sodium
  • 2 High pressure sodium
    • 2.1 "White" SON
    • 2.2 Theory of operation
  • 3 Light pollution considerations
  • 4 End of life
  • 5 See also
  • 6 Notes
  • 7 References

Low pressure sodium

An unlit 35W LPS/SOX lamp

A running 35W LPS/SOX lamp

Spectrum of a low-pressure sodium lamp. The intense yellow band is the atomic sodium D-line emission, comprising about 90% of the visible light emission for this lamp type.

Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium and a small amount of neon and argon gas Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp) [2] or U-shaped. 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 as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colors of illuminated objects are not easily distinguished because they are seen almost entirely by their reflection of this narrow bandwidth yellow light.

LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier types of LPS lamps had a detachable dewar jacket (SO lamps).[3] Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation.[4] Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps.[5]

LPS lamps are the most efficient electrically-powered light source when measured for photopic lighting conditions—up to 200 lm/W,[6] primarily because the output is light at a wavelength near the peak sensitivity of the human eye. As a result they are widely used for outdoor lighting such as street lights and security lighting where faithful color rendition is considered unimportant. LPS lamps are available with power ratings from 10 W up to 180 W; however, longer bulb lengths create design and engineering problems.

LPS lamps are more closely related to fluorescent than high intensity discharge lamps because 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. Unlike HID lamps, which can go out during a voltage dip, low pressure sodium lamps restrike to full brightness rapidly.

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, while continuing to consume full rated electrical use. LPS lamps, however, do increase energy usage slightly (about 10%) towards their end of life, which is generally around 18,000 hours for modern lamps.

High pressure sodium

High pressure sodium lamp Philips SON-T Master 600W

Same lamp in operation

Spectrum of high pressure sodium lamp. The yellow-red band on the left is the atomic sodium D-line emission; the turquoise line is a sodium line that is otherwise quite weak in a low pressure discharge, but become intense in a high pressure discharge. Most of the other green, blue and violet lines arise from mercury.

Diagram showing the spectral output of a typical high pressure sodium (HPS) lamp.

Office building illuminated by high pressure sodium lamps.

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; because of this broadening and the emissions from mercury, colors of objects under these lamps 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" (a name used primarily in Europe and the UK). HPS Lamps are favoured by indoor gardeners for general growing because of the wide colour-temperature spectrum produced and the relatively efficient cost of running the lights.

High pressure sodium lamps are quite efficient—about 100 lm/W—when measured for photopic lighting conditions. The higher powered versions of 600 W have an efficacy of even 150lm/W. 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 aluminium oxide. 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/SON lamp, producing a color temperature of around 2700 K, with a CRI of 85, greatly resembling the color of an incandescent light.[7] These are often used indoors in cafes and restaurants to create a particular atmosphere. However, these lamps suffer from higher purchase cost, shorter life, and lower light efficiency.

Theory of operation

Diagram of a high pressure sodium lamp.

An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor that is needed to draw an arc. 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 an increase in the electrical resistance of the lamp. As the temperature rises, the flow of current being maintained constant results in an increase in power until the nominal power is reached. 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). Because 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 that 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 to minimize resistive losses. Because 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 because of 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, such as an astronomical observatory parking lot, or a large city nearby an astronomical observatory, low pressure sodium is preferred. Such lamps emit light on just one dominant spectral line (with other far-weaker lines), and therefore is the easiest to filter out. One consequence of widespread public lighting is that on cloudy nights, cities with enough 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, which is caused by a loss of sodium in the arc. Sodium is a highly reactive element, and is easily lost by reacting with the arc tube made of aluminum oxide and the products are sodium oxide and aluminum:

6 Na + Al2O3 → 3 Na2O + 2 Al

As a result, 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 maximum voltage output 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, as the repeated high voltage ignitions needed to restart the arc reduces the lifetime of the ballast. 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 or will maintain its dull red glow exhibited during the start up phase.




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  2. ^ [1]
  3. ^ [2]
  4. ^ [3]
  5. ^ [4]
  6. ^ "Why is lightning colored? (gas excitations)". WebExhibits. Retrieved 2007-09-24.
  7. ^ "Philips SDW-T High Pressure Sodium White SON". WebExhibits. Retrieved 2007-09-24.