Over at the BLF collaboration sheet, I read that XM-L’s were great for single celled lights because li-ions can supply more voltage until linear or buck drivers went out of regulation. This was due to the lower Vf of XM-L’s compared to XM-L2’s.
However, XM-L2’s are known to be approximately 15-20% brighter at a given current.
Connecting the dots, I concluded that the reason why XM-L2’s are brighter was because of the higher power input, as observed by the higher forward voltage.
Completely incorrect, unfortunately. XM-L2 is brighter because it’s newer technology that is more efficient. LED Luminous output is relative to Current and has very little to do with Vf.
Although XM-L2 is roughly 20% brighter at a given drive current, it is not 20% more efficient. The higher Vf eats into some of the efficiency gains.
Example:
XM-L 3.0A at 3.4V Gives 800 Lumens
XM-L2 3.0A at 3.6V Gives 960 Lumens
Hence, XM-L2 is 20% brighter at 3.0 Amps, but uses 5.8% more power because of the higher Vf.
Calculating further, XM-L2 is 88.88 Lumens per Watt while XM-L is 78.43 Lumens per Watt.
XM-L2 is roughly 13% more energy efficient.
I wouldn’t doubt if going to 2 bond wires vs. 3 did cause the Vf to go up, but I am more talking about what increased brightness as opposed to what increased Vf.
What can you see on top of the XM-L die that you can’t see on the XM-L2?
No, there isn’t. XP-G2 is a prime example. It has higher lumens per watt and a lower Vf than the original XP-G. Vf has correlations to current and to temperature.