The Complete Guide to LED Street-Lights




The purpose of street lights is to illuminate the surroundings – including the road and adjacent areas in a manner that human beings using the area can adequately perceive the area and its surroundings.  Street lights are thus a vital instrument in ensuring safety from accidents and unsocial elements. Indeed in several studies street lights have been linked with feelings of safety and reduction in accidents.



Let's start off with the ideal street light, from there; we will continue on with everything you need to know about street light LED.


The Perception of Lights and the Ideal Street Light

 Light is perceived by the retina of the eye. The cone cells in the eye work in bright light and register color while the rod cells work in dim light and register the presence of light. The retina in turn is protected from excessive light by the pupil. The pupil alters it size in response to changing light levels. Thus, the pupil is wide open in dim light and becomes very small in bright light – thus controlling the amount of light entering the eye and protecting the retina from excessive light that may damage the light sensitive cells.

The pupilary muscles are the most relaxed in dim light and are most tightly contracted in bright light.  Thus these muscles fatigue quickly when there is excessive light.

For these reason–

  • Street lights should produce good quality light to ensure good contrast between objects and good color discrimination.
  • Street lights should provide constant illumination levels. Fluctuating lights or changing illumination levels will cause the pupilary muscles to tire quickly.

Several factors are instrumental in determining the quality of visual perception and the performance  of street light luminaries

  • Light output by the lamp
  • Light reflection from the road and other surfaces
  • Color rendering ability
  • Mesopic vision as determined by the S/ P ratio of light
  • Ratio between maximum lumens, average lumens and minimum lumens
  • Uniform distribution of light
  • Cut off angle of the fixture
  • Height of the mounting pole – Optimum pole height is important for the correct lighting density. As the height of the mounting pole increases the footprint of the light increases but light density decreases.

Luminosity of Street Lights

Luminosity is the light output from a lamp. A small lamp producing a lot of light results in greater brightness and consequent glare when looked at directly. Our LED street lighting systems on the other hand consist of multiple light producing points thus reducing glare.

Light and the angle above the Nadir


The nadir is represented by the lamp median perpendicular to the ground. IES classification of lights as ‘full cut off’, ‘semi cut off’ and ‘non cut off’ is based on the light dispersion 80 and 90 degrees above the nadir.

The table below shows the percentage light levels and their relation to classification of luminaries.

Classification of Luminaire Percent of Total light output at 90° from Nadir Percent of Total Candlepower at 80° from Nadir
Full Cutoff 0% <= 10%
Cutoff <= 2.5% <= 10%
Semi Cutoff <= 5% <= 20%
Non Cutoff No limitation No limitation

 

Here is the same information represented visually:

 

Clearly the full cut off lights direct more of their light downward. This means that light spillage in the sky can be minimized. The added advantage is that lamps with lower electricity usage can be used as the wastage is minimized.

Simply adding a light shield allows more light to be focused on the area of interest thus providing better illumination with fewer lumens and reducing the possibility of run ins with irate neighbors. LED lights do away with the need to add light shields. Better light angles and finer quality optics make for a great outdoor light.

Recently IES has highlighted the BUG rating system for lamps. This system is more comprehensive and rates light on three parameters -Backlight, Up-light and Glare. Since most municipalities still work with the earlier classification we will stick with it in this discussion.

What is the importance of full cut off lights?

 Full cut off lights minimize environmental light pollution and help save money that was being used to light up the sky.  Reduction of ambient light levels in the sky makes the skies ‘night friendly’ allowing astronomers to observe heavenly bodies and minimizes the impact of anthropogenic lighting on the circadian rhythm of nocturnal animals and plants.

 

This is a picture of San Jose taken at night. The leakage of light from street lights to illuminate the sky is clearly visible. This night sky light pollution is a major challenge for astronomers. Full cut off LED StreetLights installed properly could counter this problem.

The single most important reason for installing full city off light, however, is that in most areas around the world anything but a full cut off light may be illegal.

For example the lighting code of Douglas County, Colorado requires that all light fixtures for outdoor lighting (except those specified) must be full cut off as defined by the IES. The aim of street lighting is not just to provide a safe nighttime environment but also to protect the average Americans right to enjoy the beauty of a star studded night sky. In fact tilting full cut off lights can lead to the violation of the outdoor lighting code.

Light footprint of different types of Luminaries

Some of the basic types of luminaries according to IES’s classification and their typical position along roads is shown in the figure above. Thus while Type 5 are suitable for installation for a 360 degree illumination footprint, Type 1 is most suitable for installation along a road median.

The problem with measuring light output of lamps in Lumens

The method of calculating luminous output from Lamps assumes zero light reflection. In reality depending on the surface color different amounts of light may be reflected resulting in different brightness. If yellow colored light illuminates a green colored object for example all the light will be absorbed and the object will appear black in color. If the green object is placed on a black surface – it will not be visible at all!

In practice a green object – grass for example consists of several pigments – chlorophyll, carotenoids and xanthophylls etc and reflects a small amount of yellow light and appears grayish in color. This is one of the reasons why a very high luminous output is needed when LPS lamps are used for street light applications as these lamps produce only one color of the light spectrum. LED StreetLights on the other hand have a fuller spectrum. The color differences between objects are therefore highlighted, contributing to contrast and resulting in a perception of better visual acuity greater feeling of safety with far fewer Lumens. Then there is the concept of scotopic lumens explained elsewhere on this website which results in better performance of high CRI lights due to the response of the rod cells of the eye. The net result is that far fewer lumens are needed to illuminate an area.

The S/ P ratio is critical in the determination of pupil lumens. The S/ P ratio for HPS lamps is 0.62 while that for LED lamps is 1.9. Using the formula

Pupil Lumens = Photopic Lumens * [S/P] 0.78

1 photopic lumen produced by HPS lamp = 0.69 pupil lumens and

1 photopic lumen produced by LED lamp = 1.65 pupil lumens.  In other words, when it comes to vision each photopic lumen produced by an LED lamp is equivalent to 1.65/0.69 = 2.4 photopic lumens produced by a HPS lamp.  In Wisconsin 8040 lumen producing LED lamps replaced 19000 lumens producing HPS lamps in a school parking. Yet users of the parking facility felt that there was a distinct improvement in the lighting. The ratio 19000/ 8040 or 2.36 is very close to the theoretical ratio of 2.4 calculated above. Of course a part of the improvement was because the light from LED luminaries was better controlled and uniform.

In this picture of LED Street light trial on Lincoln Avenue the first three lights (left, right, left) are LEDs and the rest are Metal halide lamps. LED StreetLights are marked by low glare and uniform light distribution on the ground. They also consume half the energy of metal halide lamps. Further down the street, metal halide lamps show considerable glare and it is easy to understand why these lights may pose a hazard to drivers.

The Impact of LED StreetLights on Insects is Serious

  • There are multiple reasons why there is an interest in the impact of LED StreetLights on insects.
  • Studies in Europe have demonstrated that millions of insects are attracted to and perish due to street lights.
  • Insects like Shad flies are attracted to street lights at night particularly during the summer months. These insects can be an urban nuisance besides adding to the cost of cleaning up lamp fixtures and the street sidewalk to remove dead insects.
  • North Bay conducted an assessment of the extent to which Shad flies are attracted to LED and HPS lights and found that while the HPS lights were literally coated with shad flies LED lamp containing fixtures had very few flies.

Whatever your motivation – saving life, lowering cleaning costs, reducing insect swarming nuisance or purely aesthetic interests – LED Street Lights hold a lot of promise.

Several municipalities had undergone major retrofit programs around 2 decades back. Many of the lamps and fixtures installed then are nearing the end of their useful lives. It is very important that the next set of LED Street lighting technology be chosen with great care for a wrong choice can saddle you with energy wasting or maintenance intensive fixtures for the next 20- 30 years. 

When choosing the lighting technology such as LED Street-Lights it is also prudent to take future development in the field of lighting into consideration. Some of the most promising developments are

  • Intelligent lights – either with motion sensors or those capable of varying their output according to the ambient light levels to maintain uniform illumination. A large retail store has installed LED lights that run on 49 watts of electricity but power on to their full brightness of 149 watts when they sense activity. Similarly it is possible to develop control technologies that will allow sensors to match light output to ambient light levels – using small amounts of electricity to produce small amounts of light early in the evening and near daybreak.
  • Use of lights as information gathering points – about meteorological conditions, and traffic movement.
  • Use of solar panels integrated with light poles to ensure distributed generation of electricity and further reduce the environmental impact of lighting. 
  • Use of manual control of lights through power cables

San Jose is currently experimenting with these technologies. It has chosen LED technology for lighting because it is the most versatile light on the market today. After all, the human race has come so far because of its intelligence. Lighting should be no different.

If you have been searching for information on LED lights on the net you would have come across many confusing claims about the light output of LED devices. Some sites would have told you that LEDs are poor at producing light while the others would have claimed that they are among the most efficient lighting technologies. The reason behind this anomaly lies in the rapid strides made in LED lighting technology. In fact, efficiency of LED bulbs has gone up during the few months that it took us to collect data for this series of articles.

R Raghavan and N Narendran, of the Lighting Research Center summarized the improvements in LED lighting technology in a recent paper.
 

Lighting Technology Light Output (lumens) Efficiency (Lumens / Watt) Energy Use in KW per hour Savings
Fluorescent Tube (32 W) 2200 68 0.76 Base Case
LED 38 Watts (2002 vintage) 1200 32 0.91 -18%
LED 12 Watts (2010 vintage)
1200
100
0.28
+62%

The improvements in LED lighting technology and the low profile nature of LEDs have made this the most energy efficient choice for almost all lighting applications.

Light Distribution Requirements for Street Lights

The amount of light emitted by a streetlight beyond 75 degrees from the Nadir should be minimized. Light beyond this angle can cause direct glare to motorists. LED streetlights that have far better control over the light that they produce are ideal for reducing this glare. HPS lamps that are more frequently used have an angle of distribution beyond 150 degrees and can cause direct glare.
 

The picture above shows the distribution of light when wide-angle HPS lights are used. A considerable amount of light is wasted. Residents of houses near the road suffer from light trespass and must keep their curtains drawn to tackle excessive and unneeded illumination of their windows. The consequent light pollution makes life difficult for astronomers and nature lovers too.

The light from an LED outdoor light is better focused in the task area. People living in houses adjacent to the outdoor light need not worry about drawing their curtains as the light is incident on the task area and is not distributed in a very wide angle. Obviously when the light is not being used to illuminate the sky and adjacent houses, far fewer lumens are needed.

Uniformity of light distribution

Uniformity of light distribution by a street light can be measured after installation. Uniformity depends on optical properties of the light source, quality of the luminaries’ reflectors, overlapping areas between luminaries among others. Uniformity of light distribution can be determined by three ratios –

  • Average to minimum light intensity ratio
  • Maximum to minimum light intensity ratio and
  • Maximum to average light intensity ratio.


Of these, the first two are easier to understand and are therefore used more often.

The uniformity of light distribution is vital to reduce glare. Glare control is particularly important in street lighting applications. Glare causes the pupil to shrink thereby reducing the amount of light entering the eye. Squinting, blinking, using devices like a peak cap (to cut the brightest portion of the visual field), using sunglasses (To lower the luminance of the entire field of vision) are the commonest methods of dealing with glare. None of these is a suitable method for motorists and other road users at night. Similarly, disability glare caused by light emitted in an angle beyond 75 degrees to the nadir is usually due to light scattering within the eye thus reducing the contrast of the image formed on the retina.

A practical example of the impact of uniformity of light produced by LED Street-Lights

Thus, when LED Street Lights were installed in the parking facility of the Prairie School in Wisconsin, LED lights producing barely 8040 lumens replaced HPS lights producing mammoth 19,000 lumens. This was made possible by

  • Better light spread – The maximum: minimum light intensity ratio was 8:1 for Led lights while for HPS lamps it was 27.25: 1.
  • Better Scotopic performance – The mean light intensity level was 1.01 photopic lumens which is equivalent to roughly 1.7 pupil lumens or 2.45 photopic lumens produced by an HPS lamps.
  • Better optical control of light - All the light is directed downward. There is therefore no need to use reflectors to direct light downward. The consequent loss in light output due to limitations on reflector efficiency is avoided.

Both theory and practice demonstrate the benefits of LED Street-Lights. They also underline the importance of system thinking, smart technologies and reducing wastage. Instead of waging a war of ‘Lumens’ on darkness the current crop of managers and lighting designers are opting for precise lighting with LED technology.

Should you buy new or just retrofit?

Another viable option today is to retrofit your existing lights with one of our retrofit products that are a good Metal Halide Replacement (and HPS). There are many reasons why you may want to retrofit instead of replace. One of the reasons is some cities have legislation that would require a large capital investment in infrastructure (changing out of poles etc) if the lights were replaced, but retrofitting the existing fixture allows for them to be grandfathered in and thus saving you the extra cost that you weren't counting on. Retrofits can work with your existing lenses and optics distributions, just think of it as a simple bulb and ballast replacement.



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