LEDs normally show a efficiency peak at relatively low drive currents. For older types of LEDs (up to Cree XP-G) I can find measured efficiencies in this forum, but not for XM-L. Can someone point me to the corresponding thread where I can find efficiency of XM-L in the range 10 to 350 mA? The diagrams in the original datasheet doesn't allow to read exact values at low currents.