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.
This article is part one of a two part series that is classified at "the complete guide to LED street lights" for those who are interested in the topic, are roadway agency employees, or even those on a local level looking to gain insight on street lighting and how LED technology can greatly improve roadway safety and eye appeal.
Let's start off with the ideal street light, from there; we will continue on with everything you need to know about street light LEDs.
Already read part one or want to skim part two ahead of time? Click here to go to part two of this guide.
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. 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
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 street lights 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 street lights 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 street lights 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 Street Lights on Insects is Serious...
- There are multiple reasons why there is an interest in the impact of LED street lights 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.
Should you buy new or just retrofit?
Another viable option today is to retrofit your existing lights with one of our LED retrofit kits 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 Kits can work with your existing lenses and optics distributions, just think of it as a simple bulb and ballast replacement.
Read part two of this series by clicking here.