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OLEDs and such



 
 
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  #21  
Old May 30th 13, 01:18 PM posted to uk.tech.digital-tv
R. Mark Clayton
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Default OLEDs and such


"Stephen" wrote in message
...

"R. Mark Clayton" wrote in message
...

That's easy - LCD just get switched, whereas LED's have to be driven.


Hmm. Although I thought the point of the transisitors was to allow more
drive to each pixel in the LCD. And I thought the brightness/darkness of
an LCD pixel was controlled by an analogue signal.


Not quite sure, but they are stable in the time scale of frame refresh, so
you address each one and tell it what level until the next frame or even
(e-ink) when you want to change it again.

Emissive LED's require a drive current. In a small display they can be
multiplexed, but in a large one the problem below emerges unless you have a
transistor for each one.


(I don't think they can be switched because the timing would be too
critical. Each pixel is only addressed for about a 10 nanoseconds per
frame in and HDTV, and you'd have to control the edge timing to within a
256th of that, or 1/25th of a nanosecond. (Equivalent to handling
frequencies of up to 25 GHz in the display, which sounds unlikely to me.)



  #22  
Old May 30th 13, 03:09 PM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Default OLEDs and such

In article , Stephen
wrote:

"Steve Thackery" wrote in message
...
R. Mark Clayton wrote:

about 2mm is the smallest LED you can get and you need three for each
pixel, so 1920X1080 display would be ~6MX3M


Ah, so we can be sure that is the limit? It's because we can't make
ordinary LEDs small enough for a domestic TV screen?



I don't know of any reason why you can't make an LED as small as you
like, the same as any other diode or transistor - it's a mystery to me
too. And if you did make a normal size TV from conventional LEDs, you
would only need a tiny output from each one. I estimate about 1
microAmp to each one, instead of the typical 20 milliAmps or so.


Again. my unreliable memory may be misleading me. But IIRC many 'vertical'
semiconductor devices suffer from problems with their surfaces/edges not
being active and causing parasitic problems. For a given layer thickness of
such 'not working correctly' surface material the fraction that fouls up
performance gets bigger when the device is made smaller.

In conventional LEDs this would, I think be an area/linear problem.

And you'd also have the amount of light output fall with the area and
current, with the 'bad surface' surround not contributing.

You also presumably need a minimum length to allow the required junction
profiles, etc. So if made too narrow the structure may become to easily
damaged. And if very narrow, the electron waveforms become affected so the
system changes behaviour anyway!

No idea at what size these factors may place the practical limits, though.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
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Audio Misc http://www.audiomisc.co.uk/index.html

  #23  
Old May 30th 13, 07:46 PM posted to uk.tech.digital-tv
Graham.[_2_]
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Default OLEDs and such

On Thu, 30 May 2013 10:54:43 +0100, "Rick" wrote:



"Jim Lesurf" wrote in message
...
In article , Stephen

wrote:

"Steve Thackery" wrote in message
...

Colleagues,

The ongoing progress of OLED displays led me to wonder something: does
anyone know why organic LEDs are considered suitable for making TV and
computer displays, but ordinary LEDs are not?



As far as I know LCD displays have a transisitor in the corner of every
pixel. Why can't these be ordinary LEDs instead?


My vague and unreliable recollection is that it may be because the OLEDs
are more flexible and easier to stick to something like a glass surface in
a thin layer. LEDs tend to be more rigid crystalline and damaged by small
defects. So thermal and flexure effects can cause failure. However I'm not
sure of this as I'm trying to recall articles on the OLEDs I read years
ago!



One supposes that this is also a factor which might have to be taken into
consideration by manufacturers, I mean, think of all the ensuing lawsuits if
it turns out to be true.
http://news.in.msn.com/international/article.aspx?cp-documentid=253019725



The lack of substance and writing style makes me very doubtful.
The first question I would ask is how does it compare to looking at
"stuff" illuminated by sunlight?

Anyhow, Blueband is margarine, init?


--
Graham.

%Profound_observation%
  #24  
Old May 30th 13, 09:07 PM posted to uk.tech.digital-tv
David Woolley[_2_]
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Default OLEDs and such

Stephen wrote:


I don't know of any reason why you can't make an LED as small as you like,
the same as any other diode or transistor - it's a mystery to me too. And if


What I suspect is difficult is radically changing the chemical profile
of the wafer on those scales to get the different colours.

you did make a normal size TV from conventional LEDs, you would only need a
tiny output from each one. I estimate about 1 microAmp to each one, instead
of the typical 20 milliAmps or so.


  #25  
Old May 31st 13, 12:45 AM posted to uk.tech.digital-tv
Stephen
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Default OLEDs and such


"David Woolley" wrote in message
...
Stephen wrote:


I don't know of any reason why you can't make an LED as small as you
like, the same as any other diode or transistor - it's a mystery to me
too. And if


What I suspect is difficult is radically changing the chemical profile of
the wafer on those scales to get the different colours.


Maybe they could use all white LEDs with a standard LCD colour filter in
front.


you did make a normal size TV from conventional LEDs, you would only need
a tiny output from each one. I estimate about 1 microAmp to each one,
instead of the typical 20 milliAmps or so.



  #26  
Old May 31st 13, 04:29 AM posted to uk.tech.digital-tv
Bill Wright[_2_]
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Default OLEDs and such

Stephen wrote:
"David Woolley" wrote in message
...
Stephen wrote:

I don't know of any reason why you can't make an LED as small as you
like, the same as any other diode or transistor - it's a mystery to me
too. And if

What I suspect is difficult is radically changing the chemical profile of
the wafer on those scales to get the different colours.


Maybe they could use all white LEDs with a standard LCD colour filter in
front.


But how is the 'white' constructed?

Bill
  #27  
Old May 31st 13, 08:17 AM posted to uk.tech.digital-tv
Richard Tobin
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Posts: 1,378
Default OLEDs and such

In article ,
Bill Wright wrote:

Maybe they could use all white LEDs with a standard LCD colour filter in
front.


But how is the 'white' constructed?


White LEDs either have multiple colour LEDs (e.g. RGB), or have
phosphors like fluorescent lights. It would seem pointless to use
multicolour LEDs for this - just use the colours directly. Using LEDs
with phosphors ought to work, just as LCDs can be illuminated by
fluorescent tubes.

-- Richard
 




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