LED Street Lighting Color Temperature Problem

Jan 25, 2016

How I discovered the push for LED street lighting could undo our outdoor lighting ordinance which restricts light to the night sky.

As many of you may have read in my last blog my wife and I completely retrofitted our home to LED lighting in a push to save even more energy and money over CFL's. This project was initiated when I begin to see LED lighting had arrived technologically and was very cost competitive with compact fluorescent lighting. It is also a superior lighting technology, producing more lumens per watt, it has a higher initial lumens, and lower lumens depreciation.

When I read an article stating our city of Las Cruces, NM had a project going to replace the city street lights with LED lighting I became concerned that they would choose the wrong color temperature, that the back scatter from blue weighted lighting would undo the intent of our outdoor lighting ordinance, which calls for all such lighting to be shielded. The daytime sky is blue because smaller wavelength blue light is more readily back scattered. There is a science term for this effect, it is called Rayleigh Scattering.

First of all I set up a short presentation with the astronomy club (Astronomical Society of Las Cruces) ASLC to explain the issue to them, and to get someone to attend a meeting with me, with the city engineer, and their sustainability officer. I purchased a 5000 K LED light bulb and a 2700 K LED light bulb for a demonstration. My presentation only took several minutes, but went very well. There were several questions and agreement with my mission. The club president offered to sign any letter we put together regarding this issue as a club position.

My research showed that the 4000 K LED street lights the city is proposing to use put out twice as much spectral distribution in the blue (less than 500 nano-meters) then the 3000 K LED lighting lighting we are proposing that they restrict their purchases too.

The city has done several demonstration projects but has not committed yet to retrofit the whole city which would be a long term project. I was not surprised to learn they had chosen 4000 K lighting which the lighting manufacturers promote for some reason.

Our meeting with the city is now set for Tuesday morning and I have printed graphs, IDA (International Dark Sky) material, and a graph of the spectral sensitivity of the human eye.

One of the major issues the lighting people may not understand is that lighting that emits a greater component of blue light may look more efficient, but it is not, because while the human eye is very sensitive to blue light, it is only sensitive to it a low light lighting levels. At high levels it washes the visual purple out of the eye, reduces dark adaptio as well as closing the pupil of the eye. In other words, the missing component in their calculations is the human eye itself.

We applaud the move to LED street lighting as it is more energy efficient then HPS lighting which has been used up until now. With a color temperature of 2200 K HPS lighting is not a concern for Rayleigh scattering. HPS Lighting does not effect dark adaption anywhere near as much as white lighting would.

Our prescription is simple really, it involves the city only installing 3000 K lighting which has half the spectral output distribution below 500 nano-meters. Our hook, is the Outdoor Lighting Ordinance which the city passed in the year 2000. It has a stated goal of keeping light out of the night sky. Although all such lighting is now full cut off shielding, this does not reduce back scatter due to blue lighting being reflected from the ground, or being back scattered due to emission near the horizontal plane. I think we have legal basis for asking them to restrict the color temperature of street lighting in this way.

We also have a safety issue in that blue weighted lighting creates more veiling glare. It is the very sensitivity of the human eye which causes this. I think we all have experienced this with oncoming cars using blue headlights, which can be very blinding. Good for the driver of the offending car, but bad for everyone else.

Wish me luck in our LED lighting meeting Tuesday morning. Our next steps will be to draft a letter outlining these issues, and our prescription to fix it. Such a letter will be addressed to the Mayor, City Council, and the Las Cruces Sun News. As the author of the current city outdoor lighting ordinance it ought to carry some weight. Having the ASLC sign the letter and supporting me is priceless of course. We need a definitive statement from the city as to why they are choosing the lighting they are now, and I do not think they can justify it. As we all know the facts don't always persuade power, but in this case they just might!

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