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Sodium vapor lamps are some of the most efficient lamps in the world. They have an efficiency of up to 190 lumens per watt compared to an incandescent street lamp which has between 15 and 19 lumens per watt.
These sodium vapor lamps come in two major groups:
- High-pressure sodium (HPS) lamps
- Low-pressure sodium lamps (LPS)
Compared to LPS lamps, high-pressure sodium lamps tend to have a longer bulb life, less lumen per watt efficiency, and most importantly, a higher color rendering index. Low-pressure sodium lights are considered to have the worst color rendering index of any lamp but are incredibly efficient.
Spectral Output
The spectral output of LPS lamps is incredibly focused at two wavelengths 589 nm and 589.6 nm. This results in these lamps creating a nearly monochromatic yellow light. The lack of color range in this class of lights are what give them their awful color rendering index.
This low color rendering index essentially means that it is very difficult to differentiate between the different colors of objects illuminated by this lamp. In other words, it is difficult to perceive colors in a space illuminated by these lamps because of their almost monochromatic yellow light.
The uniquely focused color of low-pressure sodium lamps although largely considered a flaw of LPS lamps is one of the reasons why some call for wider use of the lamps. This output is in the range of wavelengths that the human eye is most sensitive to causing LPS lamps to appear brighter to the human eye. Our sensitivity to the color produced by LPS lamps is part of what makes them so efficient and makes them a green option for lighting.
Another unique characteristic of these LPS lamps emitting light in only these wavelengths is that it makes the lamps contribute less to light pollution. Astronomers have an easier time removing the spectral data of these lights as an outlier because it is so specific to the wavelengths of 589.6 nm and 589 nm.
High-pressure sodium lamps on the other hand normally produce a wide spectral output due to their white light. This white light is obtained by using other materials in combination with sodium to balance out the yellow light produced by the sodium vapor.
Construction of LPS Lamps
Low-pressure sodium lamps require a number of parts each working in tandem for the lamp to produce light. There is an arc tube with neon gas that initially makes the lamp has a red color to it when it is powered by two electrodes at the ends of the tube. This neon serves to heat up the sodium metal in the lamp and vaporize the sodium into sodium vapor. Which then begins to emit its yellow light as it is excited by the electric current.
The process of the neon heating up the sodium metal into sodium vapor means that it takes anywhere from five to ten minutes for the lamp to fully turn on. Since the sodium vapor will not immediately return to its solid metal form the light will reignite after a loss of power as long as power is returned relatively quickly to the lamp.
The Low Pressure Sodium Lamp
Construction of HPS Lamps
High-pressure sodium lamps are a little bit more complicated in their construction as they require a vacuum but have a relatively similar design. In HPS lamps there is an arc tube made of aluminum oxide ceramic which is resistant to the corrosive effects of the sodium housed within. The arc tube is maintained at a higher pressure in high-pressure sodium lamps to increase their efficiency, however, they require a ballast that reduces their efficiency while producing arcs for the lamp.
Most HPS lamps contain an ignitor that causes a pulse start that will arc through the xenon gas inside of the lamp. The xenon gas will then begin emitting light in a sky blue color. The xenon heats up mercury until the mercury becomes mercury vapor and begins emitting its own blue light. This, in turn, heats up the sodium and causes the sodium to vaporize and emit its own yellow light.
Most HPS lamps have other materials mixed in with the sodium so that the overall light emitted by the lamp is white light. This color balance is in part achieved through the use of the mercury and xenon which start the lamp and both produce a blue light that works to balance out the yellow light of the sodium vapor. HPS lamps avoid the poor color rendering index (CRI) of LPS lamps in this way.
The arc tube which houses all of these materials is held in place by a metal frame that looks like the letter C. This metal frame also holds the getter in place which helps maintain the vacuum by absorbing unwanted gases that seep into the lamp over time.
Applications
As a result of the very poor color rendering index of LPS lamps, they are rarely used for indoor lighting and are best suited for outdoor lighting where their high efficiency allows them to illuminate large at minimal electrical cost.
LPS lamps are also used in security lighting as their high efficiency makes LPS lamps more cost-effective than incandescent lamps when left on throughout the night. LPS lamps are also often used in long tunnels as the yellow light has less of a detrimental effect on the eyes than white light has as it flashes past drivers.
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The more acceptable color rendering index of high-pressure sodium lamps means that they are used far more often than LPS lamps despite the fact that they are less efficient. High-pressure sodium lamps see use in a lot of the same sectors as LPS lamps since they are still more efficient than incandescent lamps for outdoor lighting.
Unlike LPS lamps they are often used in more private settings such as a yard because the color rendering index of HPS lamps is much better than that of LPS lamps. The efficiency of LPS and HPS lamps is their greatest strength and the main reason for their global usage.
Sources and Further Reading
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