Author Topic: EEVblog #915 - Dumpster Dive LCD TV Salvage  (Read 5361 times)

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EEVblog #915 - Dumpster Dive LCD TV Salvage
« on: August 26, 2016, 10:51:04 am »
Turning a dumpster dive 50" LG LCD TV into a light panel
Fun with a spectrometer, and light meter measurements.
Also looking at the diffusion and prism layers of typical LCD TV's.

3M Brightness Enhancement Films: http://bit.ly/2blggIR

 

Offline Kleinstein

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #1 on: August 26, 2016, 11:25:31 am »
LCD backlight does not need a good CRI. For a large color range and best efficiency they even want 3 peaks, just where the color filters of the LCD panel are. So no wonder to see the spectrum drop all the way to zero. On thing I am wandering is why there is so little red light. So maybe the TV was nor the best in showing bright red pictures.
 

Offline FloFo

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #2 on: August 26, 2016, 11:51:29 am »
I expected the LEDs not to work without the main electronics PCB due to the fact that the brightnes of the TV can be controlled (which would be handeled by the electronics). Most likely one of the lines from the main elecronics board to the PSU will contain a PWM signal for this (and maybe for synchronisation with the picture refresh to blank out the LEDs during pixel switching). So it should be possible to replace the main electronics board with a simple pwm circuit.

regarding the spectrum: maybe they choose LEDs with high blue intensity due to the spectral properties of the diffusors and the transmission properties of the LC panel/polarisation filters.
 

Offline Brumby

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #3 on: August 26, 2016, 12:09:01 pm »
I recently converted an old tablet to a lightbox for an artist friend.  I should have done some photos.....
 

Offline rs20

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #4 on: August 26, 2016, 09:30:13 pm »
To be very pedantic, the "CRI is poor" --> "you can see the imperfect white in the screen" conflation is a bit of a red herring -- although the huge blue peak contributes to both a blue cast in the light and a poor CRI, those are totally different and independent consequences.

For example, you could tune ( :-/O) three red, green and blue lasers to produce a mixture that looked like perfect white to the human eye. It's only when you use that light to illuminate some fruit* that you can suddenly tell that something is terribly wrong. Hence the three lasers would combine to make a very pure white, but with a very poor CRI.

On the flip side, a cool white incandescent (if such a thing even exists) would have a noticeable blue tinge, but fruit* would look right -- it'd have a pretty decent CRI. So the warmth/coolness/pinkishness of a light source is almost completely orthogonal to its CRI, hence the existence of the CRI measurement.

* "Fruit" used here as a placeholder for "things that our brains are deeply familiar with the normal daylight response for."

LCD backlight does not need a good CRI. For a large color range and best efficiency they even want 3 peaks, just where the color filters of the LCD panel are. So no wonder to see the spectrum drop all the way to zero. On thing I am wandering is why there is so little red light. So maybe the TV was nor the best in showing bright red pictures.

Indeed -- the absolute optimal spectrum for a domestic light source is a CRI=100, broad, daylight-like spectrum. Whereas the optimal spectrum for an LCD display is three sharp peaks (which this newfangled quantum dot TV stuff is ostensibly working towards) in order to maximise the gamut of the display.

Regarding the weak red response; maybe the LCD panel had a warm/reddish cast when it was told to turn fully white, so that the entire system has a fairly neutral look?
 

Offline larrybl

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #5 on: August 27, 2016, 02:36:50 am »
I am having fun with Street Light LED bulbs. The driver fails and they toss them. So I grab the up. I added LEDS to one of my Garden tractors and it looks like car head lights at night now. I did need to add a 10 ohm 10W ballast to each side to keep them from over heating.
 

Offline larrybl

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Re: EEVblog #915 - Dumpster Dive LCD TV Salvage
« Reply #6 on: August 27, 2016, 01:32:03 pm »
Where I live it is almost a necessity, at least into late dusk. The stock 1156 bulbs (Car tail light bulbs) leave a lot to be desired. I came across the LED's at work and thought I would experiment. These have a Lenz that focused the light outward, unlike Dave's.
 


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