Ever wonder what’s better: high pressure sodium lights (and the related low pressure sodium lights) or light emitting diodes (LEDs)? Well here’s a head-to-head comparison of the two followed by an in-depth discussion of each technology in turn.
High pressure sodium vapor (HPS) lights, similar to LPS lights, are a specific type of gas- discharge light (also known as a high intensity discharge, HID or arc light). The principal difference between low and high pressure sodium lights is the operating pressure inside the lamp. As indicated by the name, “high” pressure sodium vapor lights operate at a higher internal pressure. The arc tube is made of aluminum oxide and the sodium metal is combined with several other elements like mercury which counter-balances the yellow glow with some white to light blue emissions.
Low pressure sodium vapor (LPS) lights are a specific type of gas-discharge light (also known as a high intensity discharge, HID or arc light). The bulb principally contains solid sodium metal inside a borosilicate glass tube that vaporizes once the lamp is turned on. During start (while the sodium is still in solid form) the lamp emits a dim reddish/pink glow. Once the metal is vaporized the emissions become the characteristic bright yellow associated with sodium vapor lamps. The spectrum of visible emissions from an LPS light is actually very close together (589 and 589.6 nm, virtually monochromatic) resulting in the colors of illuminated objects being nearly indistinguishable.
Both low and high pressure sodium lights require ignition which is typically provided by a voltage pulse or a third electrode (an additional metal part) internal to the bulb. Starting is relatively simple with small tubes but can require significant voltage with larger lights. Sodium vapor lighting typically requires a “warm-up” period in order to evaporate the internal gas into plasma. Additionally, as the light heats up it requires additional voltage to operate which is balanced by a ballast (a magnetic or electric device designed to provide the light constant current). As sodium vapor lights age, more and more voltage is required to produce the same amount of light until eventually the voltage exceeds the fixed resistance provided by the ballast and the light goes out (fails). The lights become less efficient over time because they must use more and more voltage to produce the same lumen output as the light degrades. That said, HPS lights in particular maintain fairly good light output (roughly 80%) at their typical end-of-life (24,000 operating hours).
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Sodium vapor lighting has been around since the middle of the 20th century (in commercial production since the 1930s) and it generally represents a high efficiency way to provide lighting over a vast area. Sodium lights operate in a range where the human eye is very sensitive and so there is less power required to achieve the same lighting effect. For this reason they are very efficient. Additionally, despite their long warm-up period (5-10 minutes), low pressure sodium lamps will re-ignite immediately in the event of a power interruption. It is particularly useful for outdoor lighting where energy efficiency is at a premium (such as with municipalities lighting the streets or other common areas like parking lots.) LPS and HPS lights are much more efficient as well as longer lasting than incandescent bulbs, many fluorescent bulbs, and most high intensity discharge lamps in general. It is only recently with the advent of affordable and prevalent LED lighting that they are being consistently surpassed in terms of energy efficiency and lifespan.
Among the deficiencies in this lighting are the following:
Sodium vapor lights have the worst color rendering on the market. LPS lamps in particular are monochromatic which means that the objects illuminated by them appear shadowy black rather than the color you would see during daylight. HPS lamps are better but are still surpassed by virtually every other light on the market.
Sodium vapor lights require a brief warm-up period. Once the arc is ignited it melts and evaporates metal salts (sodium) internal to the device. The light doesn’t arrive at full power until the salts are fully evaporated into plasma (which can sometimes take up to 10 minutes). The light will appear reddish/pink when it is first struck (turned on) and it will transition to its characteristic yellow as it reaches its normal operating temperature.
Among the minor deficiencies in sodium vapor lighting are the following:
High pressure sodium lights contain a small amount of toxic mercury. The mercury inside the bulbs is a hazardous material that can present a waste disposal issue at the end of light’s life. Broken bulbs release a small amount of toxic mercury as a gas and the rest is contained in the glass itself.
Sodium vapor lights are omnidirectional. Omnidirectional lights produce light in 360 degrees. This is a large system inefficiency because at least half of the light needs to be reflected and redirected to the desired area being illuminated. The need for reflection and redirection of light means that the output is much less efficient for omnidirectional lights due to losses than it would be for the same light if it were directional by its nature. It also means that more accessory parts are required in the light fixture itself in order to reflect or focus the luminous output of the bulb (thus increasing unit costs).
Common applications for sodium vapor lighting includes street lighting and parking lots as well as for tunnel lighting where color rendering isn’t a major issue. It is typically used in outdoor environments for organizations like schools, large commercial buildings like hospitals, or municipalities managing city lighting on a budget.
LED stands for light emitting diode. A diode is an electrical device or component with two electrodes (an anode and a cathode) through which electricity flows - characteristically in only one direction (in through the anode and out through the cathode). Diodes are generally made from semi-conductive materials such as silicon or selenium - solid state substances that conduct electricity in some circumstances and not in others (e.g. at certain voltages, current levels, or light intensities). When current passes through the semiconductor material the device emits visible light. It is very much the opposite of a photovoltaic cell (a device that converts visible light into electrical current).
If you’re interested in the technical details of how an LED works you can read more about it here.
There are four major advantages to LED lighting:
LEDs have an extremely long lifespan relative to every other lighting technology. New LEDs can last 50,000 to 100,000 hours or more. The typical lifespan for a fluorescent bulb, by comparison, is 10-25% as long at best (roughly 10,000 hours).
LEDs are extremely energy efficient relative to every other commercially available lighting technology. There are several reasons for this to include the fact they waste very little energy in the form of infrared radiation (much different than most conventional lights to include fluorescent lights), and they emit light directionally (over 180 degrees versus 360 degrees which means there are far fewer losses from the need to redirect or reflect light).
Very high light quality.
Very low maintenance costs and hassle
In addition to the major advantages, LED lights also offer several smaller perks. These include the following:
Accessories: LEDs require far fewer accessory lamp parts.
Color: LEDs can be designed to generate the entire spectrum of visible light colors without having to use the traditional color filters required by traditional lighting solutions.
Directional: LEDs are naturally directional (they emit light for 180 degrees by default).
Size: LEDs can be much smaller than other lights.
Warm-Up: LEDs have faster switching (no warm-up or cool-down period).
Considering the upside you might think that LED lights are a no-brainer. While this is increasingly becoming the case, there are still a few tradeoffs that need to be made when you choose LED:
In particular, LED lights are relatively expensive. The up-front costs of an LED lighting project are typically greater than most of the alternatives. This is by far the biggest downside that needs to be considered. That said, the price of LEDs are rapidly decreasing and as they continue to be adopted en masse the price will continue to drop. (Don't give up if you received an expensive proposal to upgrade to LEDs. Our value engineering service can help.)
The first practical use of LEDs was in circuit boards for computers. Since then they have gradually expanded their applications to include traffic lights, lighted signs, and more recently, indoor and outdoor lighting. Much like fluorescent lights, modern LED lights are a wonderful solution for gymnasiums, warehouses, schools and commercial buildings. They are also adaptable for large public areas (which require powerful, efficient lights over a large area), road lighting (which offer significant color advantages over low and high pressure sodium lights), and parking lots. We also have a blog post with an interesting take on the history of street lighting in the United States.
The two different technologies are entirely different methods of producing light. Sodium vapor bulbs contain metals that are evaporated into inert gas within the glass casing while LEDs are a solid state technology. Both technologies are very efficient. The difference is that sodium vapor lights were the most efficient technology of the 1970s while LEDs are the modern day equivalent. Although sodium vapor lighting beats virtually every other technology in terms of energy efficiency (which is why it was chosen to illuminate the streets of so many cities), it loses out to LEDs. Both LEDs and sodium vapor lights emit electromagnetic radiation across a small portion of the visible light spectrum, however, LEDs waste much less energy producing waste heat and they also provide an incredibly better variety of high color rendering index options to the user (thus eliminating the monochromatic black appearance of objects illuminated by LPS and HPS bulbs).
Sodium lamps have the worst color rendering of any bulb. They produce a dark yellow glow which is generally a very low quality light. Additionally, there are serious waste disposal issues with sodium lamps. In particular, they have been known to start fires in the event that the lamp is broken and the sodium metal is exposed. The sodium can catch fire even in the event that the lamp is broken on the ground. For this reason it is safest to break sodium lights under water and then to subsequently dispose of the destroyed bulb. Lastly, HPS and LPS lights are monochromatic, so they can mess with your color vision if you look at them for an extended period of time.
Perhaps more importantly, in the last few years LED efficiency has surpassed that of even LPS and HPS lights and its efficiency improvements are progressing at a much more rapid rate. The largest selling point of LPS and HPS lights is the cheap selling price, the high energy efficiency (low operating costs), and the relatively long lifespan. LPS and HPS still retain these advantages over most conventional bulbs but they lose on all three counts to LEDs. In some areas (e.g. lifespan) they are drastically inferior to LEDs. The extremely low maintenance and replacement costs with LEDs is actually a major cost benefit over the long term. LED lifespan can be greater than 100,000 hours (more than four times that of LPS or HPS). Having to purchase one bulb versus three or four bulbs over the course of time is a significant selling point for LEDs. The bottom line is that having lost their traditional advantage of being the most energy efficient bulb on the market, there’s very little reason to use a sodium vapor light when LED lighting is available.
Correlated Color Temperature (Read more Here)
LEDs are available in a wide range of color temperatures that generally span from 2200K-6000K (ranging from “warm” yellow to light or “cool” blue).
Low and High Pressure Sodium lights are well-known for their warm yellow glow (CCT values around 2200K). Although High Pressure Sodium lamps emit visible light across a slightly more broad spectrum than Low Pressure Sodium lamps, they are still very limited. The downside is that there are very few options outside the narrow range to choose from. In other words, if you’re not looking for a warm deep yellow light, you’re going to have to use something besides Low or High Pressure Sodium to achieve it.
WINNER: LEDs
CRI
CRI for LED is highly dependent on the particular light in question. That said, a very broad spectrum of CRI values is available ranging generally from 65-95.
Low Pressure Sodium lamps are notorious for having the worst CRI values on the market. Typically they fall around 25 on a scale of 100 where 100 is the best possible. LPS lamps emit a monochromatic yellow light which very much inhibits color vision at night.
Color rendering for High Pressure Sodium lights is slightly improved (HPS lights emit a yellow to white light) but it is still much worse than other types of lamps.
WINNER: LEDs
Cycling (Turning On/Off)
LEDs are an ideal light for purposely turning on and off because they respond rather instantaneously (there is no warm up or cool down period). They produce steady light without flicker.
High pressure sodium bulbs may flicker and/or cycle on and off as the bulb reaches the end of its useful life. Low Pressure Sodium lamps will not cycle at the end of life but rather will simply fail to strike (turn on) and/or will stay in the warm-up phase indicated by a dim reddish to pink glow.
Both Low and High Pressure Sodium lights exhibit a short delay when turning on because they need to be ignited before they operate at steady state. Problems with the starter and/or improper matching of a starter and an HPS lamp may cause cycling even though the starter is otherwise working properly as the lamp is continually trying to light itself.
WINNER: LEDs
Dimming
LEDs are very easy to dim and options are available to use anywhere from 100% of the light to 0.5%. LED dimming functions by either lowering the forward current or modulating the pulse duration.
HID lights can be manually dimmed through the use of different electric or magnetic ballast but the process changes the voltage input to the light and can consequently alter the light characteristics. In some cases (particularly with older HID bulbs) dimming can cause the light to prematurely expire. Continuous dimming typically alters the light output from 100% to 30% for High Pressure Sodium lamps.
WINNER: LEDs
Directionality
LEDs emit light for 180 degrees. This is typically an advantage because light is usually desired over a target area (rather than all 360 degrees around the bulb). You can read more about the impact of directional lighting by learning about a measurement called “useful lumens” or “system efficiency.”
All High Intensity Discharge lights (of which HPS and LPS are examples) emit light omnidirectionally. This means they emit light for 360 degrees, requiring fixture housings or reflectors to direct a large portion of the emissions to the desired target area.
WINNER: LEDs
Efficiency
LEDs are very efficient relative to every lighting type on the market. Typical source efficiency ranges 37 and 120 lumens/watt. Where LEDs really shine, however, is in their system efficiency (the amount of light that actually reaches the target area after all losses are accounted for). Most values for LED system efficiency fall above 50 lumens/watt.
Low and High Pressure Sodium lights are the only light whose source efficiency compares to LEDs (values range between 50 and 160 lumens/watt for LPS and slightly less for HPS). They lose out to LEDs in many cases because their system efficiency is often much lower due to losses associated with omnidirectional light output and the need to redirect it to a desired area.
WINNER: -
Efficiency Droop
LED efficiency drops as current increases. Heat output also increases with additional current which decreases the lifetime of the device. The overall performance drop is relatively low over time with around 80% output being normal near the end of life.
High Pressure Sodium lights maintain their luminescence fairly well with 90% still available halfway through their lifespan (around 12,000 hours). HPS bulbs typically emit 80% of their original rated output at the end of life (around 24,000 hours).
WINNER: -
Viable Light Emissions
LEDs produce a very narrow spectrum of visible light without the losses to irrelevant radiation types (IR, UV) or heat associated with conventional lighting, meaning that most of the energy consumed by the light source is converted directly to visible light.
Low and High Pressure sodium lights produce a very narrow spectrum of light (particularly LPS lights). For this reason LPS lights are actually desirable as they minimize electromagnetic interference near facilities conducting astronomical observation.
WINNER: LEDs
Infrared & Ultraviolet
LED - None
HPS - None
WINNER: -
Heat Emissions
LEDs emit very little forward heat. The only real potential downside to this is when LEDs are used for outdoor lighting in wintery conditions. Snow falling on traditional lights like HID will melt when it comes into contact with the light. This is usually overcome with LEDs by covering the light with a visor or facing the light downward towards the ground.
Low and High Pressure Sodium lights emit heat that is absorbed by the ballast and/or lost to the environment. Roughly 15% of the emissions are lost due to energy dissipation and heat losses. In some circumstances heat emissions could be beneficial, however, it is a generally a bad thing to emit heat as it represents an energy inefficiency. The ultimate purpose of the device is to emit light, not heat.
WINNER: LEDs
Failure Characteristics
LEDs fail by dimming gradually over time. Because LED lights typically operate with multiple light emitters in a single luminaire the loss of one or two diodes does not mean failure of the entire luminaire..
Low and High Pressure Sodium lights can fail in a number of different ways. Generally they exhibit an end-of-life phenomenon known as cycling where the lamp goes on and off without human input prior to eventually failing entirely.
WINNER: LEDs
Foot Candles (Read more Here)
Foot candle is a measure that describes the amount of light reaching a specified surface area as opposed to the total amount of light coming from a source (luminous flux).
LEDs are very efficient relative to every lighting type on the market. Typical source efficiency ranges 37 and 120 lumens/watt. Where LEDs really shine, however, is in their system efficiency (the amount of light that actually reaches the target area after all losses are accounted for). Most values for LED system efficiency fall above 50 lumens/watt.
HIDs are very efficient compared to CFL and incandescent lights (120 lumens/watt source efficiency). They lose out to LEDs principally because their system efficiency is much lower (<30 lumens/watt) due to all of the losses associated with omnidirectional light output and the need to redirect it to a desired area. By far the most efficient HID variant is the Low Pressure Sodium lamp whose source efficiency can range from 60-190 lumens/watt.
WINNER: Foot Candle ratings are very application specific and vary case by case so it is difficult to say if LPS/HPS or LED would be better performing without the specifics of the particular situation.
Lifespan
LEDs last longer than any light source commercially available on the market. Lifespans are variable but typical values range from 25,000 hours to 200,000 hours or more before a lamp or fixture requires replacement.
High Pressure Sodium lights have excellent lifespan as well (although not as good as LED) which is why they have traditionally been used for outdoor street lighting in municipalities where energy efficiency is at a premium. Typical lifespan values for an HPS bulb are around 24,000 hours. According to American Electric Lighting, “HPS lamps still generate 90% of their initial light output at the midpoint of their life span. Lumen maintenance at the end of life still is excellent at around 80%.” LPS lights last slightly shorter periods of time (typically failing around 18,000 hours of operation).
WINNER: LEDs
Lifetime Costs
LED lighting has relatively high initial costs and low lifetime costs. The technology pays the investor back over time (the payback period). The major payback comes primarily from reduced maintenance costs over time (dependent on labor costs) and secondarily from energy efficiency improvements (dependent on electricity costs).
Low and High Pressure Sodium lights are very cheap to purchase as well as fairly cheap to maintain. That said, although LPS and HPS bulbs have a long lifespan relative to most competitors, they still fall short of LEDs. LPS or HPS lights will still likely need to be purchased several times and the associated labor costs will need to be paid in order to attain the equivalent lifespan of a single LED light.
WINNER: LEDs
Maintenance Costs
LED has virtually zero maintenance costs and the frequency with which bulbs have to be changed out is by far the best on the market.
Low and High Pressure Sodium bulbs are long lasting but will still require replacement several times in addition to the labor cost to monitor and replace aging or expired components all within the lifespan of a single LED.
WINNER: LEDs
Upfront Costs
LED light costs are high but variable depending on the specifications. The typical 100W-equivalent LED light costs somewhere between $10 and $20.
Low and High Pressure Sodium costs vary depending on the specific type of light. Generally they are cheap compared to LEDs ($5-$10 for a 100W incandescent-equivalent LPS or HPS bulb).
WINNER: Low & High Pressure Sodium
Shock Resistance
LEDs are solid state lights (SSLs) that are difficult to damage with physical shocks.
Low and High Pressure Sodium bulbs are fragile relative to LEDs. This is especially true of those bulbs in linear tubes. Like most legacy lamps HPS and LPS lamps operate by using a glass bulb.
WINNER: LEDs
Size
LEDs can be extremely small (less than 2mm in some cases) and they can be scaled to a much larger size. All in all this makes the applications in which LEDs can be used extremely diverse.
Low and High Pressure Sodium Lamps come in all shapes and sizes but are typically used for outdoor applications where size isn’t a major factor. Even so, they typically aren’t produced below roughly a centimeter in width and so they do not compare to the small size and robust build of a solid state light like LED.
WINNER: LEDs
Cold Tolerance
LEDs: Minus 40 Degrees Celsius (and they will turn on instantaneously).
Low and High Pressure Sodium: -40 Degrees Celsius.
WINNER: LEDs
Heat Tolerance
LEDs: 100 Degrees Celsius. LEDs are fine for all normal operating temperatures both indoors and outdoors. They do, however, show degraded performance at significantly high temperatures and they require significant heat sinking, especially when in proximity to other sensitive components.
Low and High Pressure Sodium: We couldn’t find any objective data on fluorescent bulb performance in high temperature situations. If you have any information please contact us.
WINNER: -
Warm-Up Time
LEDs have virtually no warm-up time. They reach maximum brightness near instantaneously.
Low and High Pressure Sodium lights require a warm-up time that varies depending on the light. It can take up to 10 minutes to get the LPS or HPS lamp up to its normal operating temperature.
WINNER: LEDs
Warranty
LEDs: Often 5-10 Years
Low and High Pressure Sodium: Typically 1-4 Years
WINNER: LEDs
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