failing led floodlight killing grass

turbodog

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Have a cree floodlight that's failing... it's got a LARGE purple tint. Noticed a oval patch of grass right where it shines has more or less died... or is _very_ sick to say the least.

Anyone seen this? If it were a HID/etc with a cracked lens I could see this... but in an led?
 

chillinn

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Best guess, the purple tint may have UV in it, which with enough energy can penetrate the ground and illuminate the roots, killing them.

But as I have learned in another thread, this doesn't really matter because no one really cares, and it's really nitpicking. LED are perfect, and any health problems they may cause are nothing to be concerned about. But out of an abundance of caution, do not look into the light.
 

jtr1962

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I would look into another cause. LED grow lights for plants are actually a combination of deep red and royal blue. Together they make purple. And they're typically used at far greater intensities than a floodlight would be. So if anything purple should be helping the grass grow, not killing it.

I've never seen any type of vegetation killed off by any type of manmade light, be it LED or anything else. Not saying it couldn't possibly happen, but I'd love to know the mechanism. Obviously shade-loving plants can be killed off if they're exposed to full sunlight, but sunlight is way more intense than most artificial lighting.
 
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chillinn

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One must not assume perfect accuracy, and allow for fudge. Even though plants use UV light, enough UV light will burn and kill even a plant. UV light has a shorter wavelength than visible light. Purple and violet light have shorter wavelengths than other colors of light, and ultraviolet has even shorter waves than violet does; so ultraviolet is sort of a "more purpler-than-purple." Anyone care to guess what UV light looks like, even though we (usually) can't see it? Or if it can't be seen, but is definitely there, any guess as to what that might look like? If you guessed "purple," you guessed correctly. Generally speaking, LED are not lasers outputting a narrow and single band of light. Though the output is more narrow than incandescent light, it still is not a laser of a single wavelength of light. Also, purple has a wavelength of ~380nm, while red starts around 635nm and goes up to about 700nm. Only in the RGB model does purple come from mixing red and blue. Because purple and red are so distant from each other on the spectrum, there is no red light in purple light. If an LED is producing purple, we can rest assured that it is very possible to likely that it is also outputting a little blue and UV as well. If the grass is dead, and if the LED killed the grass, then how could a purplish LED kill grass? We can deny it is happening, or we can accept the report at face value. Once we eliminate the impossible, whatever remains, no matter how improbable, is the truth: UV killed the grass.
 
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bykfixer

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So let me get this straight;
The air, the water, the food and now the LED lights are killing us?

Actually, according to this:
plants need some sleep too. Not sleep like us, but do need some time to recoup in darkness.
 

chillinn

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So let me get this straight;
The air, the water, the food and now the LED lights are killing us?
It's not a big deal. We were dying anyway.

plants need some sleep too. Not sleep like us, but do need some time to recoup in darkness.

I overlooked this, and it is a fair point. Constant light alone with no darkness rest period, even without excessive UV, can cause green to brown, whither and die. Maybe I've put too much internal emphasis on it being an oval-shaped patch of dead grass, but if it was merely lack of a rest period of darkness, I would not expect a neat oval, but no shape, just more dead grass at the spot, less further from the spot, still less further away, but kind of random and patchy.
 
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Test it by shining it in 10 more spots and see what happens. Then repeat the test in your neighbor's yard
Then with the yellow spots showing on the neighbors yard, get ready for accusations of public urination on private property.

@turbodog : "I swear officer! I was only shining a light on the grass!"

Officer: "Sure buddy, everyone has different nicknames for urination, but it's still illegal in your neighbor's yard."
 

Olumin

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Has happend with a lot of street lightning aswell. From what I understand the phosphor coating somehow fails or wears off due to manufacturing defects, revealing the "true" colour of the LED, which for most is a blue/purple tint. However, so far as I know the light shouldn't contain any dangerous levels of UV. Regardless, the new spectrum it outputs might have harmful effects on the plants anyway. What was the original colour of the light? Also worth testing if the effect is repeatable, just to eliminate other factors.
 

chillinn

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However, so far as I know the light shouldn't contain any dangerous levels of UV.

uCerAcH_d.webp


This should be familiar by now, the LED color spectrum. Notice the far left side of the output includes violet and deeper into the UV. Any amount of UV can be harmful.

Regardless, the new spectrum it outputs might have harmful effects on the plants anyway.

Whatever is happening, it must explain the oval of dead grass, and maybe I've read too far into the description, but it sounds like a cookie-cut oval of dead grass with living grass outside the oval, so it speaks to something occurring which is deadly to grass within the highest emission area and not so deadly or not at all hugging up next to that. What other spectrum could be deadly for plants? If the light is merely preventing darkness cycles, why such a distinct shape of dead grass surrounded by living grass rather than patches of dead grass with areas of living grass between? Also, the fact that the light appears purple suggests the blue spike shifted slightly towards violet, so undoubtedly there is more UV emitted than before the phosphor faded or failed.
 
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jtr1962

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Until I see I picture I wouldn't make any assumptions about the pattern of dead grass. The most plausible explanation so far is that the light is preventing darkness cycles. Even so, I think then the floodlight must be quite a bit brighter than normal outdoor lighting. After all, cities have had LED streetlights now for upwards of a decade. I'm not noticing any dead grass in my neighborhood where the light spills on to lawns. Then again, it's much lower intensity than a floodlight.
 

chillinn

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The most plausible explanation so far is
Proof by assertion.

Again, if the grass was killed by lack of a rest cycle, it could not form an oval. Instead, there would be patches of dead grass on the outside of the oval, and less of some patches of grass that lived in the shadow of other blades on the inside of the oval, so, in fact, while it is a valid suggestion, because of the given evidence that rules it out entirely, it happens to be the least likely explanation. Flood lights do exactly that... flood. They do not produce an oval spot light without a collimated beam and a shroud, like some installed lighting. The smoking gun here is the purple tint. It gives it away.
 
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jtr1962

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uCerAcH_d.webp


This should be familiar by now, the LED color spectrum. Notice the far left side of the output includes violet and deeper into the UV. Any amount of UV can be harmful.
That spectrum looks like it was made with a sensor that wasn't zeroed properly. Note the the intensity is constant on the extreme red and purple ends. I know for a fact most LEDs have no output below about 410-420 nm, or above about 750 nm. High CRI LEDs might have some red all the way to 800 nm but there still isn't much above about 750 nm.

UV pumped LEDs of course will have some output well below 400 nm.
 

idleprocess

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15 years ago at a different work building there was a pond behind the office; I would occasionally feed the herons and egrets using a bag of bread. Birds would eagerly fly right on up to where the bread landed - only the birds weren't interested in the bread.

Wonder if something similar is afoot - the light is attracting or encouraging something else that's hurting the grass because I can't imagine an outdoor floodlamp putting out enough radiated watts to hurt grass. As opposed to the slightly concave mirror-glass panes on the west side of that same old work building that would burn lines into the grass in the summer (figuratively burn - the grass would be dead along the shifting paths of the beams May through September).
 

jtr1962

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Proof by assertion.

Again, if the grass was killed by lack of a rest cycle, it could not form an oval. Instead, there would patches of dead grass on the outside of the oval, and some small patches of grass that lived in the shadow of other blades on the inside of the oval, so, in fact, that is the least likely explanation. Flood lights do exactly that... flood. They do not produce an oval spot light.
How do you know the floodlight didn't have a cutoff? That could produce a distinct oval. Besides, you're making unwarranted assumptions about the area with dead grass. I'll wait until I see a picture before coming to any conclusions.

It could be that light isn't directly a factor in any of this at all. I'm thinking perhaps some kind of insect. Light above a certain intensity attracts it, and it destroys grass in the process. The end result could be a neat oval of dead grass. Where the light drops off under a certain intensity, the insects avoid.
 

jtr1962

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Wonder if something similar is afoot - the light is attracting or encouraging something else that's hurting the grass because I can't imagine an outdoor floodlamp putting out enough radiated watts to hurt grass.
I posted a similar hypothesis while you were posting. I think that's the most likely explanation. Light attracts insects, among other things, UV/purple much more so than white light. And the biggest destroyer of vegetation by far is insects.
 
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chillinn

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I know for a fact most LEDs have no output below about 410-420 nm, or above about 750 nm

More proof by assertion, as well as sweeping generalization.

How do you know the floodlight didn't have a cutoff? That could produce a distinct oval. Besides, you're making unwarranted assumptions about the area with dead grass.
Have a cree floodlight that's failing... it's got a LARGE purple tint. Noticed a oval patch of grass right where it shines has more or less died... or is _very_ sick to say the least.
This is how I know, aka comprehension, which admittedly I am poor at, but the description is reasonably short so I didn't get distracted.

Here is a pretty good example of what bykfixer described in mere suggestion covering the long odds that apparently is the hill jtr1962 is deciding to die on. Notice the lack of oval. Notice that there is no clear line delineating living from dead grass. Notice there are veins of live grass.

the birds weren't interested in the bread.

You were being hunted.
 
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jtr1962

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Here is a pretty good example of what bykfixer described in mere suggestion covering the long odds that apparently is the hill jtr1962 is deciding to die on. Notice the lack of oval. Notice that there is no clear line delineating living from dead grass. Notice there are veins of live grass.
What's the mechanism involved here, if light is the only cause? And why wouldn't other types of light sources have a similar effect, not just the LEDs you seem to want to blame for everything?

Your hypothesis that the LEDs are emitting UV is completely wrong:


Standard royal-blue pumped white LEDs simply don't emit UV. They won't when they're failing, either. The emission range of an LED is determined by the semiconductor material and type of doping during manufacture. There are some slight shifts due to age/temperature but a royal blue LED is never going to emit UV. Indeed, even LEDs designed to emit in the UV range don't do so all that well. Royal blue LEDs now have commercial wall-plug efficiencies of 75% or more. UV LEDs are barely at 10%.

The hill I'm choosing to die on is science and logic, not fitting things to some chosen narrative. If indeed there's some mechanism here where only the light source is at fault, I'll accept that. More likely, the light is attracting something else that is damaging the grass.
 
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