Hi Guys,
Id like to know the most efficient single LED and at what current.
Id guess its a tear-up between the XML2 and MT-G2, and that both of these are most efficient at very low drive levels?
Lets limit this to a min of 10 lumens from a single emitter, it must also be visible white light. Id also be interested if this is still the most efficient at a few hundred lumens?
Thanks for any input
Just for reference, not going to be the most efficient: Nichia 219 at 236ma, LTC3454 buck/boost gets 87.2% driver efficiency. On a full li-Co battery, this driver is in the buck mode, and that type of driver should be the king in this situation. Some people will say a linear driver in direct drive is the most efficient. So there is the catch, efficiency varies over the battery voltage range.
There should be buck drivers out there that get over 90% efficiency, subscribed to see people’s findings on this 
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EDIT: Looks like I’m off topic here, somehow I read a driver being involved, oops!
Ah yes, introducing a driver complicates it a bit. I guess this isn’t easy due to the fact of different Vf’s for various LEDs
The drivers an interesting and relevant point
Edited my first post on the driver. As far as the led, it seems that the highest bin, angry blue tint is in the cards?
XM-L2: 300 lumen @ 700mA @ 2.85V = 1.995W = 150.38lm/W
MT-G2: 800 lumen @ 1100mA @ 5.7V = 6.27W = 127.59lm/W
At least that is the minimum per data sheet.
Cheers NC, I was just checking Matchs results, looks like CREE specs are conservative
Going by his graph, so rough figures;
He has a low reading of ~100 lumens at 2.75v and 200mA so 0.55w = approx 180lm/W
At 1A the output is ~450 with 3v across it so 3W, about 150lm/w
Shame there is no Vf’s data for his MT-G2 results 
I believe that there are smaller emitters with max output of around 10 lumens with significantly greater efficiency than XM-L/XP-G2s at those low levels.
Cheers CC, exactly what I was after
~370lms at 500mA and 5.6v, so about 2.8W and 132 lm/w
Sounds good RedForest, any idea? Made by CREE?
I’m not sure to be honest, CREE do make lower powered LEDs as well but don’t focus on them to the same extent as some other companies like Nichia.
Cree says their MKR can do 200 lm/w
Yeah, Nichia has some smaller LEDs that are in the range of ~190lm/W at single figure lumen output. (NHSW157BT, maybe NSSW757AT-V1?)
XML2 U3 will do 200lm/w up at 400ma. (if you can handle the tint) or the T6 is at 175lm/w here.
http://ledcalc.fonarevka.ru/cgi-bin/LCalc.cgi?led=XML2
I think it’s fortuitous to bring up driver efficiency; it’s the total package (power, driver, led) ‘system efficiency’ that matters. I would like to find more efficient drivers……