General > General Technical Chat
Why do backlight LEDs burn out and go blue?
newbrain:
--- Quote from: Twoflower on November 06, 2021, 08:16:08 pm ---I remember some very expensive monitors had also RGB backlight with filters. They supposed to have an excellent color space in that time. But the phosphors for LEDs and filters on the LCDs got better to allow the huge color space of modern monitors/TVs.
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Exactly, my Dell 2713H display is halfway there, as it uses blue and green LEDs, with a red phosphor.
The coverage is 99% of Adobe RGB colour space, with excellent linearity due to the internal programmable LUT.
Brumby:
--- Quote from: eti on November 06, 2021, 01:18:53 am ---Actually one can see the LEDs glowing this bluish purple through the grilles in the back of the set.
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I might suggest that this would have been a useful observation to have included in the original problem description.
tooki:
--- Quote from: eti on November 06, 2021, 12:49:59 am ---There seems to be a lack of understanding with others in this thread about exactly how white LEDs manage to emit white light - it's UV light which strikes a phosphor and that causes the phosphor to emit white light (I forget the physics term).
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Yes but in this case it’s your understanding that is incorrect.
White LEDs are blue LEDs with yellow phosphors. While white LEDs based on UV theoretically exist, I’ve been unable to find even a single example of one, despite extensive searching. (UV LEDs require appreciably higher forward voltage than blue, and every single white LED I’ve looked at has had a Vf matching the blue LEDs of the same series, not UV.)
The exact spectral output of the yellow phosphors depends on the application: for general illumination a broad spectrum is desirable, whereas for LCD backlighting you’d ideally want just two sharp peaks (one green, one red) with wavelengths exactly matching the red and green filters on the respective subpixels.
You can easily test this by the way: but some white, blue, and UV LEDs. Alternately shine the blue and UV light onto the phosphor of the unlit white LED. You’ll find that it lights far brighter with the blue than with the UV, because it’s optimized for blue. .
--- Quote from: eti on November 06, 2021, 12:53:07 am ---
Actually it's nowhere near that blue in reality - more like a mid purple haze (makes sense as it's UV).
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But since they’re not UV at all, it’s actually because the yellow phosphors have degraded and thus the ratio of blue to red and green is out of whack.
eti:
--- Quote from: Brumby on November 07, 2021, 11:52:16 am ---
--- Quote from: eti on November 06, 2021, 01:18:53 am ---Actually one can see the LEDs glowing this bluish purple through the grilles in the back of the set.
--- End quote ---
I might suggest that this would have been a useful observation to have included in the original problem description.
--- End quote ---
One would think, would one not, that a thread asking "why", and not "if" the LEDs have lost their white output, would be enough to satisfy people that I've sussed pretty quickly that THE LEDS **HAVE** GONE OFF COLOUR. Instead of a thread full of pedantry and wise-acrey, we could've got to the (excruciatingly obvious) point that I've ascertained the state of the LEDs, and was enquiring from that known point onward. But then, that wouldn't allow people to brag and boast and belittle one another, to-ing and fro-ing, would it. No wonder poor Dave gets fed up of all the "have you tried X Y and BLAH?" on his YouTube comments - people are so eager to be right, they DO NOT PAY ATTENTION.
I should have included it maybe, but I'm pretty sharp, being an ex TV engineer, and I think my lateral thinking circuits are granular enough to discern whether it's just "Muh TV has gone blue!" like an average person, or "The LEDs are no longer white", as you'll see is the purpose of the thread.
I've even stated, a few times now, how simple it is to Google this fault and see how prevalent it is for LG TVs. A trivial Google for 5 mins can sum this up without explicit declarations from me.
Wow. Just amazing.
bw2341:
--- Quote from: tooki on November 07, 2021, 07:58:12 pm ---
--- Quote from: eti on November 06, 2021, 12:53:07 am ---Actually it's nowhere near that blue in reality - more like a mid purple haze (makes sense as it's UV).
--- End quote ---
But since they’re not UV at all, it’s actually because the yellow phosphors have degraded and thus the ratio of blue to red and green is out of whack.
--- End quote ---
https://www.digikey.ca/en/articles/royal-blue-leds-decoding-the-datasheet
It seems that remote phosphor products use royal blue LEDs which are shorter wavelength than regular blue LEDs. I haven't seen royal blue LEDs myself, but they apparently do look strange compared to regular blue.
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