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Lighting Technology Comparison

1. Street lighting today

2. Comparing LED luminaries with traditional lighting fixtures

3. Street lighting technology comparison




Today, street lighting commonly uses high-intensity discharge lamps, often HPS high pressure sodium lamps. Such lamps provide the greatest amount of photopic illumination for the least consumption of electricity. However when scotopic/photopic light calculations are used, it can been seen how inappropriate HPS lamps are for night lighting. White light sources have been shown to double driver peripheral vision and increase driver brake reaction time at least 25%. When S/P light calculations are used, HPS lamp performance needs to be reduced by a minimum value of 75%.

A study comparing metal halide and high-pressure sodium lamps showed that at equal photopic light levels, a street scene illuminated at night by a metal halide lighting system was reliably seen as brighter and safer than the same scene illuminated by a high pressure sodium system.


New street lighting technologies, such as induction or LED lights, emit a white light that provides high levels of scotopic lumens allowing street lights with lower wattages and lower photopic lumens to replace existing street lights. LED lights are far superior to and can easily replace traditional lamps with far higher light output. Two of the most important reasons for this are directional light produced by LEDs and scotopic advantage of LED street lights.

Formal specifications around Photopic/Scotopic adjustments for different types of light sources enables municipalities and street departments to test, implement and benefit from this new technologies. Read more about Photopic and scotopic vision.




When comparing LED luminaries with traditional lighting fixtures, you have to know about 6 important factors:


1)    Luminaire/ System efficiency

Source efficiency is based on the amount of light produced by a lamp at room temperature. It is different from system efficiency that refers to the amount of usable light delivered to the target area. There are several causes why raw lumens produced by a light source are far higher than the actual light delivered on a surface.

Losses due to trapped light - Trapped light and reflection inefficiency are the first source of lower light output from traditional lamps. As long as LEDs had not entered the scene all bulbs produced lights in a 360 degree sphere and the comparison was easy. LEDs changed all that with the directional nature of their light. In a traditional bulb (incandescent, metal halide, HPS etc.) a considerable portion of the light output is directed upwards. This light must then be reflected down. The efficacy of the reflector in turn is determined by the quality of finish, operating conditions and ambient temperature. The quality of the reflector and, therefore, the amount of light reflected degrades over time. The actual amount of light coming out of a fixture is, thus, considerably lower. An LED light by contrast ensures better light distribution with several sources of light. All the light is produced and directed downwards. There are no problems of reflector effciency, aging of reflector coating and consequent loss in light output.

Losses due to Cover and lenses – Lenses and glass covers are needed to direct light and to protect the bulb and reflectors from dust and other damage. Both LED light and traditional fixtures suffer from these losses.

Losses due to operating temperature – The lumen output of LEDs is measured at 25 degrees but the operating junction temperatures of LEDs can be high, resulting in a 10 % decrease in light output. For the same reason LEDs are an excellent choice for outdoor lighting in cold climatic conditions as the junction temperature is closer to the optimum operating temperature.

Ballast and LED driver losses – Ballasts reduce efficiency by 20%. LED drivers are more efficient and reduce performance by 10% or less.


HPS light

LED light

Light output in lumens / watt

101 lm/W

100 lm/W

Loss due to trapped light

Up to 40%


Available lumens

60 – 80 lm/W

100 lm/W

Loss due to cover, lenses



Available lumens

48 – 64 lm/W

80 lm/W

Loss due to operating temperature



Available lumens

43-57 lm/W

72 lm/W

Loss due to ballast, driver



Final system efficiency

35-45 lm/W

65 lm/W

Thus, traditional HPS bulbs are as efficient at producing light as LEDs. When one considers the impact of systemic deficiencies, LEDs stand miles ahead of the competition.


2) Scotopic superiority

Human eye responds better to shorter wavelengths of 510 and 550 nm. These are conspicuous by their absence in HPS lights. Each LED photopic Lumen is equivalent to 2.4 Scotopic lumens. When it comes to Scotopic equivalent lumens HPS lamps are no match for LED lights with richer light spectrum.
Several advantages of LED lighting systems are immediately visible. LED lights have significantly less glare, more uniform light distribution, fewer shadows, and improved visibility. The best part is that this improvement is obtained with fewer lumens and 40 % or more savings in electricity and maintenance costs.


3)    Lumen Maintenance

Lumen maintenance is another area that decision makers need to consider. The human eye can adapt well to up to 30% reduction in light levels. When light levels fall below the 30 % threshold reduction in light levels become evident and vision is compromised. Thus, lights need to be designed based on average lumens over the useful life of a light.

A comparison of the lumen maintenance data of different types of lights is shown in the graph above. A summary of the abbreviations, common name, and life till 70% of peak light output levels is:
        PSMH / Pulse Start metal halide / 12000 hours
        CMH / Ceramic Metal Halide / 16000 hours
        HPS / High Power Sodium / 24000 hours
        LED / Light Emitting Diode / 50000 hours and more.

A pulse start metal halide lamp with a peak light output of 10000 lumens will require that the fixtures be designed for 7000 lumens. This would require compromises in lamppost height, ground coverage and reduced spacing of light fixtures resulting in increased costs. A long life LED light on the other hand, with a similar peak output would, need fixture design for 9000 lumens.


4)    Uniformity of light distribution and light hotspots under traditional lights

On the photo you can see the advantage of uniform lumen distribution achieved by LED outdoor lights. The upward directed light in a traditional luminaire is reflected straight down, which causes a ‘hotspot’ to form right below the luminaire while areas further away from the bulb have poor light intensity.





Power consumption (watts)

97 (lam pand ballast power)


Average light level (foot candles)



Maximum light level (foot candles)



Minimum light level (foot candles)



Uniformity (ratio between maximum and minimum light levels)

6 : 1

2.7 : 1



5)    Color reproduction

Color discrimination is important in terms of safety to identify the colors of environment and to instill a feeling of safety outdoors. The comparative CRI of different lighting technologies are:



Low pressure sodium


High pressure sodium


Metal halide


Mercury vapour






LED light




Uniform lighting with LED parking area lights, Light hot spots under HPS lights can be seen in the image above. The excellent color reproduction with LED lights can be evaluated by observing the color of the grass. (Public domain image from– The Office of Energy Efficiency and Renewable Energy Photo credit - Beta Lighting)






Light technology

Life time



efficacy (lm/W)

Color temperature

CRI (color rendering index)

Ignition time

Dimming control

Cost of installation

Cost of operation

Other considerations

incadescent light

1.000 -5.000

11 - 15






Very high

very inefficient, short life time

mercury vapour light

12.000 - 24.000

20 - 50


15 - 55

2 – 6 min

Not possible



very inefficient, ultraviolet radiation, contains mercury

metal halide light

10.000 - 15.000

50 - 100



5 – 10 min

Possible but not practical



high maintenance UV radiation, contains mercury and lead, risk of bursting at the end of life

high pressure sodium light

12.000 - 24.000

35 - 130



2 – 6 min

Possible but not practical



low CRI with yellow light, contains mercury and lead

low pressure sodium light

10.000 – 18.000

80 - 180



2 – 5 min

Possible but not practical



low CRI with yellow light, contains mercury and lead

fluorescent light

10.000 - 20.000

50 - 100


70 - 90

2 – 5 min




UV radiation, contains mercury, prone to glass breaking, diffused non-directional light

compact fluorescent light

8.000 - 20.000

40 - 72



1 – 5 min

With special lamps



low life / burnout, dimmer in cold weather (failure to start), contains mercury

induction light

60.000 - 100.000

60 - 90




Not possible


Very Low

higher initial cost, limited directionality, contains lead, negatively affected by heat

LED light

50.000 - 100.000

60 - 150


85 - 90




Very low

relatively higher initial cost




Incandescent lamps

Incandescent Lamps are “standard” electric light bulbs that were introduced more than 125 years ago by Thomas Edison. They have the lowest initial cost, good color rendering and are notoriously inefficient. They typically have short life spans and use significantly more watts than CFLs and halogen lamps do to produce the same lumens, or light output. Incandescent technology produces light by heating up a metal filament enclosed within the lamp’s glass. More than ninety percent of the energy used by an incandescent light bulb escapes as heat, with less than 10% producing light. Their use is most common in areas prone to frequent theft or vandalism of light fixtures. In these locations a very high rate of replacement may make a case for use of these cheap light bulbs. Anywhere else they are too wasteful to make sense. After all, 5 % efficiency and a few hundred hours lifespan are difficult to consider when replacement with LED systems use 7 times less energy.





High Intensity Discharge (HID) Lamps include:

•    Mercury Vapor lamps (outdated and almost extinct)
•    Metal Halide lamps
•    High Pressure Sodium lamps (HPS)




Mercury Vapor Lamps

Mercury Vapor Lamps were introduced in 1948. It was deemed a major improvement over the incandescent light bulb, and shone much brighter than incandescent or fluorescent lights. Initially people disliked them because their bluish-green light. Other disadvantages are that a significant portion of their light output is ultraviolet, and they "depreciate"; that is, they get steadily dimmer and dimmer with age while using the same amount of energy.

Mercury lamps developed in the mid 1960s were coated with a special material made of phosphors inside the bulb to help correct the lack of orange/red light from mercury vapor lamps (increasing the color rendering index(CRI)). The UV light excites the phosphor, producing a more "white" light. These are known as "color corrected" lamps. Most go by the "DX" designation on the lamp and have a white appearance to the bulb. As of 2008, the sale of new mercury vapor streetlights and ballasts was banned in the United States by the Energy Policy Act of 2005, although the sale of new bulbs for existing fixtures do continue.





Metal Halide Lamps

In recent years, metal halide lamp (MH) streetlights have illuminated roadways, parking lots and also warehouses, schools, hospitals and office buildings. Unlike the old mercury lights, metal halide casts a true white light. It is not nearly as popular as its sodium counterparts, as it is newer and less efficient than sodium. MH lamps operate at high temperatures and pressures, emit UV light and need special fixtures to minimize risk of injury or accidental fire in the event of a so called ‘non passive failure’ – or when the lamp bursts at the end of the useful life. A small fire at the Harvard University greenhouse was started by one such lamp that was not properly contained. These cannot start up at full brightness as the gases in the lamp take time to heat up.

Additionally, every time the light is switched on a re-strike time of 5 to 10 minutes is needed before the lamp can be switched on. These lamps are thus not suited to situations when intelligent control systems are used to switch lights on and off. MH lamps suffer color shift as they age though this has been improving. Actual life expectancy is about 10,000 to 12,000 hours on average.The mercury and lead content of these lamps is also a serious issue. A single 1500 watt lamp may contain as much as 1000 mg of mercury. High cost and low life hours has kept them from becoming popular municipal lighting sources even though they have a much improved CRI around 85.




High Pressure Sodium (HPS) lamps

HPS lamps were introduced around 1970 and are one of the more popular street lighting options, the most efficient light source when compared to mercury vapor and metal halide lamps (on a ‘lumen/ watt’ scale). The disadvantage is that they produce narrow spectrum light mostly a sickly yellow in color. These lights have a very low color rendering Index and do not reproduce colors faithfully. These lights do not find favor with police departments as it is difficult to determine the color of clothes and vehicles of suspects from eye witness accounts in the event of a crime. Color-corrected sodium vapor lamps exist but are expensive. These "color corrected" HPS lamps have lower life and are less efficient.

There are two types of sodium vapor streetlights: high-pressure (HPS) and low-pressure (LPS). Of the two, HPS is the more-commonly used type.


Low Pressure Sodium lights are even more efficient than HPS, but produce only a single wavelength of yellow light, resulting in a Color Rendering Index of zero, meaning colors cannot be differentiated. LPS lamp tubes are also significantly longer with a less intense light output than HPS tubes, so they are suited for low mounting height applications, such as under bridge decks and inside tunnels, where the limited light control is less of a liability and the glare of an intense HPS lamp could be objectionable.

Another issue of HPS lights is that they contain 1 to 22 mg of mercury for a 100 watt bulb with an average of 16 mg per bulb. They also contain lead. Unsafe disposal of these bulbs can lead to significant exposure of human beings and wild life to mercury contaminated water and food. Issues with mercury contamination and customer preference for full spectrum light has been fuelling the replacement of these lights particularly in areas like self managed residential complexes where people can directly pay for the quality of light.




Fluorescent lamps

The fluorescent lamp first became common in the late 1930s. These lamps are a form of discharge lamp where a small current causes a gas in the tube to glow. The typical glow is strong in ultraviolet but weak in visible light. However, the glass envelope is coated in a mixture of phosphors that are excited by the ultraviolet light and emit visible light. Fluorescent lamps are much more efficient than incandescent lamps, but less efficient than High Pressure Sodium.

The major problems with standard fluorescent lamps for street lighting is that they are large, and produce a diffused non-directional light. They are also susceptible to low voltage failure, prone to breakage of glass parts and contain harmful mercury. Therefore the fixtures needed to be large, and could not be mounted more than 20–30 feet above the pavement if they were to produce an acceptable light level. Fluorescent lamps quickly fell out of favor for main street lighting, but remained very popular for parking lot and outside building illumination for roadside establishments.




Compact fluorescent lamp


Compact fluorescent lamps (CFL) have been used more frequently as time has improved the quality of these lamps. These lamps have been used on municipal walkways and street lighting though they are still rare at this time. Improvements in reliability still need to be made. Some issues with them are limited lumen output, high heat build upin the self contained ballast, low life/burnout due to frequent cycling (on/off) of the lamp, and the problem where most fluorescent sources become dimmer in cold weather (or fail to start at all).


They also contain harmful mercury. CFL efficiency is high and CRI is excellent around 85. CFL produces a color temperature around 3000 K with its light being "soft white" around that color temperature. Higher color temperatures are available.








Induction lights

Induction based fixtures are relatively new to the market. Induction lamps use radio frequency or microwaves to create induced electrical fields, which in turn excite gases to produce light. Induction lights have a rapid start-up and work at peak efficiency with minimal warm-up time, much like LED technology.


This technology has some advantages versus HPS technology in the areas of efficiency and life cycle, however, initial cost barriers and the rapidly evolving nature of LED technology have led to limited adoption of induction based roadway lighting systems. Another limitation of Induction Lighting is that it has limited directionality when compared to LEDs. The life of induction light is negatively affected by heat and they also contain lead.






LED lights

Light emitting diodes are rapidly developing in light output, color rendering, efficiency, and reliability.
Achieving good maintenance-free thermal management in an often hostile environment while keeping product competitive is the largest challenge, which only few manufacturers managed to achieve. This latest high quality LED technologies are already exceeding all other available technologies by all technical parameters.


According to its numerous advantages, even higher initial cost quickly pays for itself due to vastly reduced cost of electricity and maintenance. But to fully benefit from outstanding advantages it is important to educate and recognize the difference between low quality and latest state of the art LED technologies, since low quality LED alternatives have quickly spread all over the world.