Purple Street Lights?

If the place where you live has been installing LED street lights to save energy, you may have noticed some of them are emitting purple light instead of white light. At least, people here in Saint Paul have noticed this and we wondered if we were losing our minds.

Daughter Number Three-Point-One recently pointed me to this article in Scientific American that explains it, and I'm so relieved to understand it, though it doesn't help with the annoying light color.

Essentially, LEDs are not really white lights: they're either three bulbs of red, green, and blue, which together are perceived as white, or just blue with a phosphor coating or laminate that absorbs the blue wavelengths, leaving behind the red and green (or yellow? it's confusing) to be emitted as white light. Using a single blue bulb is both cheaper and more energy-efficient, so you can see why cities would choose it.

The problem is that for some reason, the phosphor laminate sometimes peels off, leaving the blue (purplish) light to shine through. 

The reason for this failure is not known. The article lists possible causes:

• heat that builds up within the light fixture
  
• vibrations from passing vehicles
• gravity pulling the phosphor layer down
  

I don't know anywhere near enough about how the phosphor layer is attached to know if any of those reasons make sense. Heat seems like it would make the most sense, except that LEDs emit almost no heat. How sensitive is the phosphor to heat? That seems like a question someone would know the answer to.

Also, the number of purple lights out on the street is not that many. As one of the experts in the article said, “Outside of my university, we’ve had LED lighting on the street for ... years, and there’s [been] no problem.”

It just occurred to me to wonder if this problem happens in all kinds of climates, or if it's a cold-climate problem (since we know it happens in Minnesota). With a little searching, I see that purple lights are reported in lots of warm places too, but according to a story just published tonight in the Pioneer Press, St. Paul Public Works reports that more of them do fail as the weather gets colder. 

This is a technology mystery that I hope will be solved.