LED street lights are gradually turning purple in Charlotte

Enzo

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I wonder who is the manufacturer. I converted about 14,000 to Acuity about 4-5 years ago. Very pleased with the performance.
 

Dave_H

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Twitter comments don't explain much.

Unlikely to be RGB I think, though theoretically possible.

Is it purple or actually violet, sometimes distinction is not made. Some early 5mm white LEDs had a violet or aqua tint at very low current which I think was colour of the underlying LED; so maybe the streetlight LEDs are more towards violet than blue and perhaps the phosphor is failing, so whatever underlying wavelengths passing through are more dominant.

If streetlights were RGB I'd be asking for a remote control to remotely change the tint, maybe even colour. Today would be a good day for green.

Dave
 

Dave_H

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Something I recall is white generated by blue LED plus reg+green phosphors instead
of yellow. Perhaps if something happened to the green, blue+red = purple; another
theory.

BTW anyone on here recall the old Philips "alien head" bulb which used "remote
phosphor" panels with blue LEDS? Could be opened up and radiated a hideous blue
light. I still have one.

Dave
 

PhotonWrangler

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Yes, I still have a few of those "alien head" LEDs also. I liked the idea of keeping the phosphor away from the LED surface. Mine are still going strong.

The more I think about this streetlight failure, the more I suspect that either the phosphor is burning away or it's separating from the LED die from a combination of heat and gravity. I would be interested in seeing a failure analysis.

**Update**

Just found the answer. It is an Acuity brand fixture and the phosphor is separating from the die. This has happened in several states that have lamps from a particular vintage.
 
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PhotonWrangler

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Saw a purple-ish fixture recently in the general DFW area and wondered if it was the same failure mode.
Good chance that it is.
We have some LCD TVs at work that failed in the same way. The screens started to turn a blotchy purple, then all purple. It turned out that the LED backlight strips were defective and is a known issue with that particular manufacturer and batch of backlights.
 

PhotonWrangler

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I did a quick search of the news for purple streetlights again and it looks like the problem is pretty widespread. I also ran across one conspiracy theory about them that I won't reapeat here because it's just too stupid. :ohgeez:
 
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JoakimFlorence

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Assuming that both the red and green phosphor are deteriorating at the same rate, the thing you have to remember is that if you increase the proportion of blue light, you also have to increase the proportion of green light to stay on the color temperature curve.

I am not exactly sure but I think it could very well be possible that even if you remove both red and green by the same percentage, the resulting light could still adopt a magenta color tint.

One additional minor contributing cause is that a portion of the blue (and bluish green) light in an LED comes from the phosphor, not the LED chip. This is only a small percentage of the total blue light (maybe 20%) but still could have a significant effect. This is especially true at lower color temperatures (2700-3000K). Once more of the blue light coming directly from the LED chip manages to get through the phosphor, it is going to result in a color shift in the blue light. A standard 450nm blue LED can appear very "purple"-tinted compared to the overall hue of the blue light in a white LED. Some LED streetlights may even be using 440-445nm blue chips (behind the white LED), which would make the situation even worse.
 
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