I saw this relating to Detroit, Michigan in the U.S. It has been mentioned in Ireland before as a very good alternative to energy saving sodium lamps, and als with less maintenance costs, who knows...

How many Ann Arbor city workers does it take to screw in a light bulb?

Soon, none.

Instead, they will be installing light-emitting diodes, or LEDs, to replace about 1,400 street lights.

The eco-friendly city about 30 miles west of Detroit says it will be the nation's first to convert all downtown street lights to LED technology, which uses less than half the energy of traditional bulbs and could save the community $100,000 a year.

"LEDs pay for themselves in four years," said Mayor John Hieftje, who announced the city's plans this week as it joined Raleigh, North Carolina, and Toronto in the LED City initiative, an industry-government group working to evaluate, deploy and promote LED lighting.

"They provide the same light, but they last 10 years. We had to replace the old ones every two years."

LEDs, small chips usually encased in a glass dome the size of a matchstick head, have been used in electronics for decades. They usually gave off red or green light, but a scientific breakthrough in the 1990s paved the way for LEDs that produce white light.

Lighting consumes 22 percent of the electricity produced in the United States, according to the U.S. Department of Energy, and widespread use of LED technology could cut consumption in half.

Hieftje said Ann Arbor's lighting conversion will reduce the city's production of carbon dioxide and gases that contribute to global warming in an amount equal to taking 400 cars off the road.

The two-year project is being funded by a $630,000 grant from the city's Downtown Development Authority.

Greg Merritt, director of corporate marketing at Durham, North Carolina-based Cree Inc., which is making the components inside Ann Arbor's new lights, acknowledged LEDs can be costly. But "as we improve the technology, the economics make sense for more and more applications," he said.

Good for energy saving, but bad for astronomy ?

Unlike low-pressure sodium (standard Orange street lights), these are broad white light. It won't be easy to filter against simple sodium lines (but the same as high-pressure sodium, i would expect).

The article sounds strange, though. While normal bulbs have a short-life
compared to LEDs, etc, i would have thought that (standard sodium) street lights would be longer. Or do they mean traffic lights?

The cheaper and more energy efficient electric lights become the more people will choose to use them with careless abandon.

Notice how many external household lights are left on all night because they use energy efficient bulbs whch cost little to run.

Peter.

LED's are getting adopted here in some sectors but not streetlighting. The main benefit to designers is that it makes it easy to target exactly where the light will go.

The ESB program is to replace lights with high pressure sodium lamps. The newer designs result in less light going upward but are still not FCO.

Filtering out sodium lines is only useful to a few astronomers, and is not much help to the wider astronomical community or the general public. There are health and wildlife aspects to using white light though. The ESB is trialling white light installations as a possible alternative option for the future.

2 years sounds a very short lifetime for street lights though. I'll find out what the lifespan of our various lights is from the ESB.

LEDs are by design fairly directional, white light is a problem for us such as light from Mercury vapor lights, its much harder to filter out.
For observers, if they are like Mercury lights, then light pollution filters will not work due to the high number of spectral lines, so we'll all be heading out to the country more often.
Am I right in saying that we're actually better off with low pressure sodium lamps, just with better cut off then is there at the moment.??

Am I right in saying that we're actually better off with low pressure sodium lamps, just with better cut off then is there at the moment.??

If you want to use filters then yes. They are expensive to maintain, pretty much impossible to get FCO, and are not much more efficient than high pressure Sodium. Most lighting is actually High Pressure Sodium which does have a blue component, but this tends not to cause as much a problem for skyglow as the blue does not scatter back down.

If anyone is interested a good experiment would be to take a spectrum of a light polluted sky where a survey has been done regarding the types of lights in the source path. ie take a spectrum in the direction of a town with mainly low pressure sodium and also in the direction of a town with mainly high pressure sodium. It would be interesting to see how much blue crops up in the spectrum.

Whats the Ideal solution (apart from no lights at all)

Whats the Ideal solution (apart from no lights at all)

It's a combination of things. Using no lights at all is appropriate in some cases - I can certainly think of places where having streetlights provides no benefit!

Where streetlights are needed then the type of light and control of the light needs to be looked at. As for the type, the ideal is the most efficient FCO. Some studies show that insets are drawn to blue light, and melatonin production in humans is stopped more by blue light. However its easier for humans to distinguish things using colour. So the high pressure sodium that is what is currently being installed is a good option - they provide just enough blue to give colour perception.

The problem of skyglow is not so much which light emits which spectra, its almost all down to the angle that the light leave the luminarire at. Dr. Chris Baddley (CfDS) recently presented a paper on this to the ILE in Dublin showing that due to scattering the luminaires that contribute most to skyglow are the ones that emit light within a few degrees of the horizon.

The next aspect to consider is the timing and strength of streetlights. On a winters evening it may be desirable to have all our streetlights on as people come home from work and to help them find their way home after a pint or two. If there is no nightclub in the town, the place will have nobody walking around after midnight or so. So do we need the streetlights on? Can they be turned down or off completely until people are begin to go to work? I don't think this technology is being used in Ireland yet.

Basically you can't go too far wrong advocating that whatever lights are used they should be FCO and should only be used when they are needed.