Which SMT white LED for CCFL backlight replacement?

[davorin]

EHLO (o;

I have some cheap 12" IBM TFT screens acquired form eBay where the CCFL have had their lives (o;

Did anyone some SMT LED replacement for those tiny CCFL tubes?
What should be the minimum mCandela to look for?

[Buriedcode]

Candela is a pretty poor indication of light output as it generally given for a certain angle.  This means LED's advertised with a high number generally have a narrow viewing angle - not good for backlighting.

CCFL's are still used because they provide a nice even light, which can easily couple to the lightguide (the polycarbonate/acrylic sheet that provides a planar light source).  In order to replace it with LED's you would need a thin strip of them, spaced close together, butted up against the light guide.  As these are "point" sources, you will have what are called "hot spots" where each LED is.  LED backlights usually cover the first few millimeters of the lightguide with reflective tape to both hide the hotspots, and to prevent light escaping.

All in all, I do not think you will achieve satsfactory results when compared to the original CCFL backlight.  However, if it is just experimenting, learning, or tinkering then by all means give it a go!  Perhaps you could buy a strip of white LED's (a 12V strip to make it easier) with 144 LED's per metre.  Cut this to length, power it with 12V and see how bright it is. 

Say each LED is spaced 100/144 = 7mm apart.  And you have a 12" 4:3 monitor - horizontal 24.4cm.  244 /7 = 34 LEDs.  Each strip of 3 LED's is powered by 12V @ 30mA.  34/3 = 11 groups.  So total power consumption = 11 * 12v * 0.03 = 3.96W.   So, if the original screen had a 4-5W ccfl tube, then this arrangement would provide similar light output (LEDs aren't more efficient than CCFL, but comparable).

Google "edge lit display" as this shows how LED backlights work.

[Brumby]

My first reaction was also about LED hotspots.  You are going to need quite a few of them spaced very closely together to minimise this with the existing diffuser - or try an alternate diffuser option.

I wouldn't look at this as anything other than an experiment from the outset - but if you come up with a workable solution, then all power to you!

I don't see this as impossible - but it could be a challenge.

[davorin]

Actually I finally went with CCFL after I tried the white SMT LEDs I ordered (o;

Wasn't aware that Digikey sells CCFL tubes...thought you only get them now from Aliexpress or whatever strange sources (o;

Still not as bright as current monitors...but I guess older TFTs weren't that bright....

[Gyro]

You can find a bunch of replacement LED strips on ebay these days, complete with dimming driver. The led strips are indeed narrow enough to replace 2mm CCFL tubes (up to 335mm anyway). You can also cut them to length too (in units of 3 LEDs). I'm not sure about Candela rating but they are certainly very bright.

e.g. https://www.ebay.co.uk/sch/i.html?_odkw=led+backlight+ccfl+12+13&LH_PrefLoc=2&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xled+backlight+ccfl+update.TRS0&_nkw=led+backlight+ccfl+update&_sacat=0

Cheap enough to have a play. Just make sure that the seller packs them in a rigid package though - I've already had one refund on some that were just packed in a plastic bag and arrived very curved and intermittent, so I don't actually have experience of them in a panel yet! 

You'd probably want either 255 or 275mm length.


P.S. I think they all use DF6113 driver ICs... https://people.xiph.org/~xiphmont/thinkpad/DF6113.pdf

[Buriedcode]

Actually I finally went with CCFL after I tried the white SMT LEDs I ordered (o;

Wasn't aware that Digikey sells CCFL tubes...thought you only get them now from Aliexpress or whatever strange sources (o;

Still not as bright as current monitors...but I guess older TFTs weren't that bright....

And older TFT's didn't have a great contrast ratio (modern glass can achieve 1000:1, comparable with OLED). So even if you did increase the brightness of the backlight, it would brighten the "black" as well as the white.

One thing I have done for monochrome STN displays - because they are old school and I have a thing for them - was to use PLCC2 LED's.  These are small enough so that they are no wider than the lightguide = so almost all the light is coupled into it, have >120 degree emission, and they can easily be soldered close together in series of 3-5 for efficiency.  I have since replaced these with SK6812, but they did provide quite a bright backlight, certainly comparable - if not brighter - than the original CCFL.   

An example (not mine):  http://www.turbokeu.com/mycomputer/myglcd.htm

[davorin]

So far I am satisfied with the displays....primary goal was to have a small display with DVI-D input to be driven directly from a MAX10 FPGA....
But I might consider ordering some of those LED backlight strips mentioned by Gyro.....

I guess I am just too old to solder small SMT devices (o;
Back then I used to repair TVs and radios with vacuum tubes (o;

[xiphmont]

Yes, I make them, though I have an order backlog right now:
https://people.xiph.org/~xiphmont/thinkpad/led-kit.shtml
The site says 'X series', but I also make them for T and W.

I use Nichia component-RGB leds for the retrofits; standard (read: cheap) general purpose white LEDs don't really work well for the task.  The output peak in the yellow region both kills the color saturation, and gets lousy efficiency as the LCD color filters mostly block it.

[xiphmont]

P.S. I think they all use DF6113 driver ICs... https://people.xiph.org/~xiphmont/thinkpad/DF6113.pdf

There are several different driver chips in use.  You probably want to avoid the ones based on the DF6113; that's a boost controller being used in a weird lobotomized buck-mode in most of the Chinese drivers.  The regulation precision is terrible and they flicker under load due to input voltage changes.

Personally, I use the TPS92510.

[james_s]

Those LED retrofit strips do work, my friend installed one in the display of an old Toshiba Libretto palmtop PC and it worked great.

TFT comparable to OLED? That's news to me, I've seen some pretty nice modern TFT displays but the contrast ratio is pathetic compared to OLED, nothing even comes close. OLED can manage nearly infinite contrast ratio, LCD can't touch it.

[Alex Eisenhut]

For LED backlights they use these weird inverted lens thingies that diffuse the light sideways. They don't have that in CCFL screens.

[Zero999]

Cheap LEDs typically have a poorer colour rendering index, compared to CCFLs, so it will work but the colours might not look so good.

I bought a cheap HD, TFT, LED backlit monitor a couple of years ago and was disappointed with the colour rendering. The reds just seem duller, than on a similar monitor, with a CCFL backlight. The problem is the mismatch between the filters inside the display and the spectrum of the LEDs. I imagine it would be even worse if the CCFL backlight in a display not designed for LEDs were replaced with a cheap LED backlight.

No doubt decent LEDs, which have a good phosphor or ideally red LEDs, in addition to the usual phosphor converted blue units, would be much better.

[james_s]

A lot of CCFL backlights have lousy color rendering too, that shouldn't be the case on a good quality computer monitor but I've seen lots of LCD panels that were mediocre at best. In the case of my friend's Libretto, the Chinese LED retrofit resulted in noticeably better color and an overall brighter screen but the CCFL it replaced was very tired. Whatever the case it looks great, but YMMV.

[xiphmont]

Cheap LEDs typically have a poorer colour rendering index, compared to CCFLs, so it will work but the colours might not look so good.
[...]
No doubt decent LEDs, which have a good phosphor or ideally red LEDs, in addition to the usual phosphor converted blue units, would be much better.

The problem isn't color rendering index.  The problem is that most white LEDs produce white via blue+yellow.  LCDs pass red, green and blue.

You can get LEDs that make white via blue pumped red&green phosphor rather than blue pumped yellow phosphor. I use these in my kits (and you can buy them from Nichia too.  They will sell single reels).  Past that point, I use a decent spectrometer to match the whitepoint via the right LED color bin.  The end result is better color than the original CCF and about 3x the light output per input watt.

[edit: and to be clear, although I sell retrofit kits, it's just a hobby for the Thinkpad community.  I don't make a profit on the kits, I only cover the parts costs]

[james_s]

Cheap LEDs typically have a poorer colour rendering index, compared to CCFLs, so it will work but the colours might not look so good.
[...]
No doubt decent LEDs, which have a good phosphor or ideally red LEDs, in addition to the usual phosphor converted blue units, would be much better.

The problem isn't color rendering index.  The problem is that most white LEDs produce white via blue+yellow.  LCDs pass red, green and blue.

That's typically quantified as color rendering index. The primary factor affecting the CRI of LEDs is the amount and wavelength of red emission. Getting a true deep red with decent efficiency is challenging, hence the 90+ CRI LED and fluorescent sources are lower efficiency than the 80+ stuff.

[NiHaoMike]

Maybe find LED backlit TVs and monitors that were discarded because of a broken/faulty panel and salvage the backlights out of them?

[xiphmont]

That's typically quantified as color rendering index. The primary factor affecting the CRI of LEDs is the amount and wavelength of red emission. Getting a true deep red with decent efficiency is challenging, hence the 90+ CRI LED and fluorescent sources are lower efficiency than the 80+ stuff.

CRI is assuming broadband spectrum, you're talking about a tristimulus display.  CRI simply doesn't apply.  It's like mixing watts and oranges.

In a tristimulus display, a high CRI emitter is almost as bad as a cheap white LED.  The main emission peak is still in the yellow region, it just adds some more red between 630 and 660nm (and usually some cyan to fill in the hole between the ~465nm pump and the yellow phosphor).  You want emission peaks aligned exactly with the transmission peaks of the LCD color filters, usually as close as possible 650nm, 550nm and 450nm.  Any emission outside those wavelengths reduces saturation, color accuracy, and overall efficiency.

There are LEDs made specifically to do this.

[Zero999]

Getting a true deep red with decent efficiency is challenging

It's not that challenging. The most efficient method is to use separate red LEDs, but then care needs to be taken to ensure good mixing of the light.

That's typically quantified as color rendering index. The primary factor affecting the CRI of LEDs is the amount and wavelength of red emission. Getting a true deep red with decent efficiency is challenging, hence the 90+ CRI LED and fluorescent sources are lower efficiency than the 80+ stuff.

CRI is assuming broadband spectrum, you're talking about a tristimulus display.  CRI simply doesn't apply.  It's like mixing watts and oranges.

In a tristimulus display, a high CRI emitter is almost as bad as a cheap white LED.  The main emission peak is still in the yellow region, it just adds some more red between 630 and 660nm (and usually some cyan to fill in the hole between the ~465nm pump and the yellow phosphor).  You want emission peaks aligned exactly with the transmission peaks of the LCD color filters, usually as close as possible 650nm, 550nm and 450nm.  Any emission outside those wavelengths reduces saturation, color accuracy, and overall efficiency.

There are LEDs made specifically to do this.

You're right. I shouldn't have mentioned CRI, but I did talk about the mismatch in LED spectrum and the filters inside the LCD, which is the real issue with swapping a CCFL with a cheap LED.

[james_s]

It's not that challenging. The most efficient method is to use separate red LEDs, but then care needs to be taken to ensure good mixing of the light.

I was referring more to discharge lamp sources but it applies to LEDs too. The most efficient red emitters are more orange-red than true blood red. You can get 660nm deep red LEDs but they are not nearly as bright, partly due to lower efficiency and partly due to the eye's lower sensitivity. The red phosphor used in the very first color TVs and some high-CRI fluorescent lamps emits a nice deep red but it's not nearly as efficient as the orangish-red rare earth phosphor used later. You still need that deep red to get 90+ CRI out of a fluorescent lamp though, and the 90+ stuff is always lower efficiency than 80+. That's true of every source I'm aware of, LED, fluorescent, HID.

[Zero999]

It's not that challenging. The most efficient method is to use separate red LEDs, but then care needs to be taken to ensure good mixing of the light.

I was referring more to discharge lamp sources but it applies to LEDs too. The most efficient red emitters are more orange-red than true blood red. You can get 660nm deep red LEDs but they are not nearly as bright, partly due to lower efficiency and partly due to the eye's lower sensitivity. The red phosphor used in the very first color TVs and some high-CRI fluorescent lamps emits a nice deep red but it's not nearly as efficient as the orangish-red rare earth phosphor used later. You still need that deep red to get 90+ CRI out of a fluorescent lamp though, and the 90+ stuff is always lower efficiency than 80+. That's true of every source I'm aware of, LED, fluorescent, HID.

You're probably right that deep red LEDs are less efficient, radiometrically speaking, than orange-red LEDs, but modern LEDs aren't that bad. According to the data sheet, the LUXEON Rebel deep red (650nm to 670nm) LEDs are typically 49% efficient, which is only slightly worse than their royal blue LEDs (440nm to 460nm), at 50% efficient. Unfortunately it's not easy to compare them with the red-orange LEDs, because their output is specified in lumens, rather than watts. In any case, it's much better to use separate red LEDs, rather than a phosphor, which would waste much more power.
https://www.luxeonstar.com/assets/downloads/DS68.pdf

I think the optimum solution would be RGB LEDs, rather than a phosphor converted blue LED. Even if green and pure blue LED are slightly radiometrically less efficient, than the royal blue LEDs, it will probably still yield a higher efficiency and colour rendering, than a royal blue LED + phosphor.