Author Topic: Why do backlight LEDs burn out and go blue?  (Read 19964 times)

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Online NiHaoMike

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Re: Why do backlight LEDs burn out and go blue?
« Reply #25 on: November 06, 2021, 07:52:05 pm »
Could that be that the TV uses kind of 'remote phosphor'? Some TV-Sets use blue LEDs with a bar contain the phosphor or nano-crystals (most of them contain cadmium!) to transform the blue light to the specific spectra required. If that piece fell off/moved that only the light from the LEDs enters the diffusor of the LCD?
Isn't cadmium restricted by RoHS?
Quote
I think RGB LEDs for background illumination is not commonly used anymore. From what I've heard too many problems with the uniformity of the used LEDs (brightness, aging, color...) and too expensive.
There are displays that use a RGB backlight and a monochrome LCD, I'm pretty sure there aren't any direct view TVs using that technology although it works on the same principle as DLP projectors. The one application I heard of for it was a tablet that could operate as a regular color tablet with the backlight on or as an eink like tablet with the backlight off. Probably the main problem is that it requires a very fast LCD to not flicker and the power use of such a fast LCD offsets the savings of not having color filters.
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Offline Twoflower

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Re: Why do backlight LEDs burn out and go blue?
« Reply #26 on: November 06, 2021, 08:16:08 pm »
Isn't cadmium restricted by RoHS?
Yes it is but I think there is/was an exception for the Cd-based quantum dots for TV usage (e.g. https://optics.org/news/7/6/8).

There are displays that use a RGB backlight and a monochrome LCD, I'm pretty sure there aren't any direct view TVs using that technology although it works on the same principle as DLP projectors. The one application I heard of for it was a tablet that could operate as a regular color tablet with the backlight on or as an eink like tablet with the backlight off. Probably the main problem is that it requires a very fast LCD to not flicker and the power use of such a fast LCD offsets the savings of not having color filters.
An interesting technique replacing the filters by time multiplexing. Sounds logical, as it has also an advance in the resolution as only one single pixel is used for the three colors. But probably nothing for me as I seem to sensitive to the rainbow effect on DLP projectors.

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.
 

Offline amyk

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Re: Why do backlight LEDs burn out and go blue?
« Reply #27 on: November 07, 2021, 01:39:47 am »
Wow. :o That is certainly a lot worse failure than I thought it would be. Even the various generic Chinese COBs I've seen have not failed like that, despite being heavily overdriven - usually the die or the bond wire goes first. Almost looks like they were really trying to save on the phosphor application.
 

Offline james_s

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Re: Why do backlight LEDs burn out and go blue?
« Reply #28 on: November 07, 2021, 03:06:22 am »
An easy way to tell if it's the backlight is to look around the back and sides of the panel, almost always there is a gap somewhere that light from the backlight will show through. If that leaking light is blue then the backlight is blue.
 
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Offline etiTopic starter

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Re: Why do backlight LEDs burn out and go blue?
« Reply #29 on: November 07, 2021, 04:42:44 am »
An easy way to tell if it's the backlight is to look around the back and sides of the panel, almost always there is a gap somewhere that light from the backlight will show through. If that leaking light is blue then the backlight is blue.

yep, and it is.
 

Offline newbrain

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Re: Why do backlight LEDs burn out and go blue?
« Reply #30 on: November 07, 2021, 09:02:15 am »
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.
Exactly, my Dell 2713H display is halfway there, as it uses blue and green LEDs, with a red phosphor.
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Offline Brumby

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Re: Why do backlight LEDs burn out and go blue?
« Reply #31 on: November 07, 2021, 11:52:16 am »
Actually one can see the LEDs glowing this bluish purple through the grilles in the back of the set.

I might suggest that this would have been a useful observation to have included in the original problem description.
 

Offline tooki

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Re: Why do backlight LEDs burn out and go blue?
« Reply #32 on: November 07, 2021, 07:58:12 pm »
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).
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. .


Actually it's nowhere near that blue in reality - more like a mid purple haze (makes sense as it's UV).
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.
 
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Offline etiTopic starter

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Re: Why do backlight LEDs burn out and go blue?
« Reply #33 on: November 08, 2021, 03:31:28 am »
Actually one can see the LEDs glowing this bluish purple through the grilles in the back of the set.

I might suggest that this would have been a useful observation to have included in the original problem description.

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.
« Last Edit: November 08, 2021, 03:35:41 am by eti »
 

Offline bw2341

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Re: Why do backlight LEDs burn out and go blue?
« Reply #34 on: November 08, 2021, 03:53:57 am »
Actually it's nowhere near that blue in reality - more like a mid purple haze (makes sense as it's UV).
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.
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.
« Last Edit: November 08, 2021, 03:55:58 am by bw2341 »
 

Offline james_s

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Re: Why do backlight LEDs burn out and go blue?
« Reply #35 on: November 08, 2021, 04:30:50 am »
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.

They look pretty much like ordinary blue to me, I've removed the remote phosphor panels from some old Philips LED bulbs that were intermittent and the exposed blue LEDs are a very bright saturated deep blue. It definitely looks blue rather than violet. It's kind of pedantic to debate anyway, I don't remember where the exact cutoff between blue and UV is defined but it's a continuous spectrum, just deeper and deeper violet that your eyes are less and less sensitive to. All sorts of stuff will fluoresce across much of that span.
« Last Edit: November 09, 2021, 08:20:03 pm by james_s »
 
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Offline gnavigator1007

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Re: Why do backlight LEDs burn out and go blue?
« Reply #36 on: November 08, 2021, 04:44:24 am »
We have a similar issue with many of the street light leds around town
 

Offline magic

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Re: Why do backlight LEDs burn out and go blue?
« Reply #37 on: November 08, 2021, 08:50:33 am »
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.
[...]
Your original post consisted exactly of a single "muh tv has gone blue" picture and a few links to people who happened to have the same symptom and zero mention of any diagnosis performed by you on your TV.

That sort of posts are the staple of Internet geniuses who did their homework by googling for 5 minutes and know exactly what the problem is and only need help with "that one final bit".

The response was totally predictable.
 
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Offline Ranayna

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Re: Why do backlight LEDs burn out and go blue?
« Reply #38 on: November 08, 2021, 10:20:41 am »
Uhh, that should go into the pet peeve thread as well :D

Post as much information about the issue you want help with as you can.
People cannot read minds, even here  :-//

  • Name the device or software your question is about. If possible the exact version.
Software and Hardware versions can differ greatly. With software, many known issues can be fixed with updates.
  • Quote any error messages you get. And quote them exactly as you get them. Pro-Tip: In Windows, on many (sadly not all) error messages, hit CRTL-C on the message box to get the contents into the clipboard, ready to paste. On recent Windows 10 (i think 20H1 and higher) hit Windows-Shift-S to start the snipping tool. Or hit ALT-PrintScreen to screenshot your current window.
The Error message details, if available, are important
  • List what you already have done or researched. If you followed any online guides, link them.
If you do not want the helpers to repeat the most basic troubleshooting steps, you have to list what you have done already. It's generally fine if you have not done anything yet, but tell us.


This should be included in *any* post seeking help. I have posted lists similar to this, or have seen them, in many many forums that allow help questions. This is, i my opinion, just basic decency, to not waste the time of the helpers, and your own as well.

I do not know if people are afraid to post such information to protect from potential identification, or if people think that no one knows the device anyway. Or, and i am afraid that this is actually the most likely, if people are just too lazy.[/list]
 
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Online Zero999

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Re: Why do backlight LEDs burn out and go blue?
« Reply #39 on: November 09, 2021, 01:29:17 pm »
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.

They look pretty much like ordinary blue to me, I've removed the remove phosphor panels from some old Philips LED bulbs that were intermittent and the exposed blue LEDs are a very bright saturated deep blue. It definitely looks blue rather than violet. It's kind of pedantic to debate anyway, I don't remember where the exact cutoff between blue and UV is defined but it's a continuous spectrum, just deeper and deeper violet that your eyes are less and less sensitive to. All sorts of stuff will fluoresce across much of that span.
I've seen the term Royal blue used on data sheets. It is indeed a slightly shorter wavelength, than ordinary blue used for illumination, but colour perception is subjective and LEDs differ between manufacturers.

Phosphors are often mixed to produce several different wavelengths to ensure a good colour rendering index. It's possible the green-yellow part of the phosphor has faded more than the red, giving a more purplish/violet colour.
 

Offline PlainName

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Re: Why do backlight LEDs burn out and go blue?
« Reply #40 on: November 09, 2021, 03:19:47 pm »
Quote
I do not know if people are afraid to post such information to protect from potential identification, or if people think that no one knows the device anyway. Or, and i am afraid that this is actually the most likely, if people are just too lazy.

No doubt a combination, but sometimes it's not knowing what info is relevant. In some other forums, people post 100K-worth of log output, 99.99% of which is of no use and just obscures the relevant part. I suppose in some ways it is akin to quoting - IMO a succinct quote of the section being addressed is far better than the full quote and then trying to work out which is The Bit being dealt with.

But we also see quite comprehensive descriptions which include fixes that can't be used for some reason (usually quite sensible) and yet at least a couple of responses will be to use those specific fixes. Sometimes you can't win.
 
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Offline PlainName

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Re: Why do backlight LEDs burn out and go blue?
« Reply #41 on: November 09, 2021, 03:23:47 pm »
And... it pains me to say so but eti did actually ask a generic question (why do LEDs burn out) and perhaps his only fault is to include a specific example. So with a quick reading (mea culpa) one might assume he is asking what's wrong with his TV, whereas in fact he is asking why backlight LEDs in general go like that.

The answer would seem to be that stick-on phosphor patch, which is something I didn't know about so I am grateful for the question coming up!
 

Offline Cerebus

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Re: Why do backlight LEDs burn out and go blue?
« Reply #42 on: November 10, 2021, 12:33:23 am »
Post as much information about the issue you want help with as you can.
People cannot read minds, even here  :-//

The problem is that some people are quite good and do all that, even to the point of meticulousness, but then there are certain individuals who just seem congenitally incapable of actually reading it and shoot off at some tangent.

For a very simple example, I just read a thread where someone says "I want to buy an upmarket-brand-name model XYZ scope; can anyone recommend an upmarket-brand-name dealer in my-area". The very next poster says "You should get a downmarket-brand model ABC!".  :palm: Do we really need to start making people sit a reading comprehension test before they're allowed to post?

I'm not saying that's what's happening here, in fact it's not, but it just goes to show that some people are so dumb, or determined to just go their way, that clearly and completely stating the problem/issue for discussion/question isn't a foolproof way of getting a sensible discussion/answer. I've noticed that quite a lot of the people who derail things in this way have self-congratulatory superlatives like 'guru', 'expert', 'master', 'wizard' and so on in their chosen user names. I think the implications are quite self-explanatory.
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Offline etiTopic starter

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Re: Why do backlight LEDs burn out and go blue?
« Reply #43 on: November 10, 2021, 01:58:22 am »
Post as much information about the issue you want help with as you can.
People cannot read minds, even here  :-//

The problem is that some people are quite good and do all that, even to the point of meticulousness, but then there are certain individuals who just seem congenitally incapable of actually reading it and shoot off at some tangent.

For a very simple example, I just read a thread where someone says "I want to buy an upmarket-brand-name model XYZ scope; can anyone recommend an upmarket-brand-name dealer in my-area". The very next poster says "You should get a downmarket-brand model ABC!".  :palm: Do we really need to start making people sit a reading comprehension test before they're allowed to post?

I'm not saying that's what's happening here, in fact it's not, but it just goes to show that some people are so dumb, or determined to just go their way, that clearly and completely stating the problem/issue for discussion/question isn't a foolproof way of getting a sensible discussion/answer. I've noticed that quite a lot of the people who derail things in this way have self-congratulatory superlatives like 'guru', 'expert', 'master', 'wizard' and so on in their chosen user names. I think the implications are quite self-explanatory.

I shall freely admit my propensity to rocket off at tangents, it’s a lateral thinking trait of mine 😁 for which I make no apologies whatsoever; it’s a function of how my mind works. I am sorry if I’ve confused anyone, especially the “experts” <bows with great humility> 😉😉

I agree regarding those using the cliched superlatives in their surnames, and will quote a phrase I once heard, regarding one naming oneself an “expert”:

“An ex is a has-been, and a spurt is a drip under pressure.”😛

I find humility wins every time, and I feel that maybe a great deal of personal insecurity leads some people to feel a need to establish their prowess by over-compensating, when, in fact, talent and gifting can speak for itself FAR louder than any superficial, somewhat boastful moniker.

 

Offline station240

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Re: Why do backlight LEDs burn out and go blue?
« Reply #44 on: November 10, 2021, 11:14:39 pm »
I beg to differ. Google "LG TV blue picture fault" and count the endless results that show engineers replacing the backlight strips that have gone blue.

Ah there is a difference between a service technician/engineer and engineers in general.
engineer: That's can't happen, (assumes customer is an idiot), spends ages on other random guesses before giving up.
technician: (takes one look at the set) Yup had one of those in last month, backlight LEDs have failed and gone blue.
Still you'd think engineers would know white LEDs are just blue ones with a phosphor layer added.

I've got a LED backlight and PSU here, after some reverse engineering I found how to turn the LEDs on without the control board.
But turns out if you drive them 100% (no PWM signal), they all turn blue after a few hours, but leave them off for a few days and they go back to white.
Obviously that can't happen if the phosphor layer falls off entirely. Bad bonding material perhaps ?
 

Offline james_s

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Re: Why do backlight LEDs burn out and go blue?
« Reply #45 on: November 11, 2021, 02:26:19 am »
Still you'd think engineers would know white LEDs are just blue ones with a phosphor layer added.

I'd assume most of them do. I had never in my life until this thread ever heard of the phosphor coming off of an LED. I've had cheap white LEDs phoshor fail such that it got really dim and gray but I've never seen it fall off. That seems like a serious design flaw, maybe class action lawsuit if it's a common problem. LED backlights should last *at least* 10 years of normal use in a TV.
 

Offline IanB

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Re: Why do backlight LEDs burn out and go blue?
« Reply #46 on: November 11, 2021, 02:37:38 am »
I'd assume most of them do. I had never in my life until this thread ever heard of the phosphor coming off of an LED. I've had cheap white LEDs phoshor fail such that it got really dim and gray but I've never seen it fall off. That seems like a serious design flaw, maybe class action lawsuit if it's a common problem. LED backlights should last *at least* 10 years of normal use in a TV.

Right. Best Buy is full of stunning looking LG OLED TVs costing ~$1000. I'd be seriously pissed if the phosphor fell off the backlight LEDs.
 

Offline james_s

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Re: Why do backlight LEDs burn out and go blue?
« Reply #47 on: November 11, 2021, 04:24:03 am »
If they're OLED they don't have a backlight, perhaps you meant QLED? Those are just LCD, with a deliberately deceptive name clearly meant to confuse them with superior OLED displays.
 

Offline IanB

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Re: Why do backlight LEDs burn out and go blue?
« Reply #48 on: November 11, 2021, 05:07:36 am »
If they're OLED they don't have a backlight, perhaps you meant QLED? Those are just LCD, with a deliberately deceptive name clearly meant to confuse them with superior OLED displays.

I didn't know that about OLED displays. If they don't have a backlight, that's good in a way, there is less to go wrong. But I suspect there is a downside. I'll have to research it.
 

Offline bdunham7

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Re: Why do backlight LEDs burn out and go blue?
« Reply #49 on: November 11, 2021, 05:40:20 am »
But I suspect there is a downside.

Oh yes.  Every OLED display in history (so far) has been a short-lived POS.  TV screens are a newer application, but since LG did so well with their backlights, I can't wait to see the screen burn on the OLED models in a few years.
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