Both systems have high first costs that are somewhat offset by reduced maintenance and related costs. Both currently have similar efficiencies, although the LEDs are continually improving on that measure. The induction systems have been in use for many years with proven results, while LED systems are new and much of their performance is “projected.”
The high pressure sodium (HPS) lighting system you currently use has been, and continues to be, the street lamp of choice in many communities because of their efficacy and life span. The yellowish-pinkish light they cast is adequate for general lighting and security. For a variety of reasons many cities are opting to retrofit their street lighting with white light. In addition to the LEDs and induction lamps you are considering, metal halide and fluorescent lamps also provide white light.
One of your first decisions concerning new lighting systems will be whether you need the new products to utilize the existing pole infrastructure. That will determine fixture type and mounting height to control the light distribution, and you need to know if the new source will be capable of providing the desired light level. Both induction and LED products should be seen as luminaire packages (lamp, driver/ballast/starter, fixture), not retrofit components to place into existing fixtures. For this reason the efficacy of the fixture, not just lumens per watt of the light source, is what is important to proper light distribution. Also consider whether the new technology is compatible with your current or planned control system.
Because of the difficulties in comparing two very different light sources with standard measurements, test installations are strongly recommended so color and intensity can be viewed in the real world where they will matter. White light, as you know, can have many shades, from yellow to blue.
Information on the U.S. Department of Energy’s Solid State Lighting (SSL) website offers good information about measuring and describing the light performance of LEDs and some comparative information as well. Look at the information in the “Using LEDs” section.
The induction lamp comes in several color temperatures, from almost warm to cool, with a CRI of 80. For a better understanding of LED color and measurements, see the publication Color Rendering Index and LEDs.
With a high first cost but an actual useful life of approximately 70,000 hours, induction lamps are more of a solution to a maintenance problem than an improvement in efficiency (at 50-60 lumens per watt). They are especially good in tunnels, on bridges, and other places where the long life reduces the risks and costs involved with changing out spent lamps. Induction lamps do very well in cold weather, and so do LEDs. Be sure with either technology that your operating conditions fall within their tolerances.
White LEDs are starting to be used in low-level street lighting. LED lights do not currently have the power to act as high-mast general lighting, but are being installed as high as 35 feet high in some GATEWAY demonstration projects, which you can read about at the “Solid-State Lighting GATEWAY Demonstration Results” website. LEDs do carry a high first cost, but promise an extended lamp life and around 40% energy savings over existing technology. Current efficacy is about the same as fluorescent lamps with the very cool, bluish colors having the best performance, but these numbers change rapidly as the technology develops.
The useful life of LED systems is still a matter of much debate, as they do suffer from lamp depreciation, fading away rather than abruptly failing. Lamp life in the range of 35,000 to 50,000 hours of useful life is being suggested rather than the 100,000 or 200,000 hours that some manufacturers initially claimed. Standards have been and continue to be developed for LEDs and you can keep up with them at the “Standards Development for Solid-State Lighting” website. Read more about lamp life and see comparisons to other common sources in the publication Lifetime of White LEDs (PDF file). Note that linear fluorescent lamp life is coming very close to projections for LEDs, and induction lamps (though not listed) at 60,000 hours or more may be longer lasting.
Plenty of bright white light is not the only factor involved with street lighting. A growing number of cities are responding to the request of the International Dark Sky Association to preserve our view of the stars in the night sky. Besides the light source, the Dark Sky group seeks to reduce light trespass and light pollution, or sky glow. Light trespass is the light that falls onto another’s property, or shines through their windows, and can be addressed by fixture placement and design, or additional shielding. Light pollution can be partially addressed by fixtures that do not allow light above a particular angle. Both full- and semi-cutoff fixtures reduce how much light is directed upwards, but a percentage of light will be reflected off the ground surface and contribute to sky glow, often visible for miles away above a city. LEDs have the advantage of being highly directional so good design can aim light only in the areas it is desired. Other light sources often produce light that is bounced off the back of a fixture before exiting it, while some light never escapes the fixture.
When looking at the cost of a lighting system, the first costs of both purchase and installation must be considered, as well as its care and maintenance (the costs of which, over time, tend to be much higher). Don’t forget to include disposal costs of spent lamps. Also be aware that the presence of mercury in a lamp may be affected by state laws. LEDs do not contain mercury.
As you know, there is more to a cost-effective street lighting system than just energy-efficient lamps—labor for maintenance, the ability to retrofit existing infrastructure versus a new design to accommodate different technology and usage needs, controls, complying with recommended lighting levels, and applicable codes all must be considered. Below are some additional resources on this topic I hope will be interesting to you.
The Lighting Research Center has two older (2003) publications that provide excellent information about the issues encompassed in street lighting systems, although they are not up-to-date with the induction or LED/SSL technology, and mercury vapor lamps are considered obsolete. Access them through the Lighting Research Center website.
- NYSERDA How-to Guide to Effective Energy-Efficient Street Lighting for Municipal Elected/Appointed Officials – 32 page booklet
- NYSERDA How-to Guide to Effective Energy-Efficient Street Lighting for Planners and Engineers
You can watch for details about the soon-to-be-activated U.S. Department of Energy Municipal Solid-State Street Lighting Consortium. The consortium is being set up specifically to help the many municipalities hoping to upgrade their street lighting to LEDs as a result of stimulus funding.
