Cree's XLamp/XM-L High-Bay reference design document is a facinating thing to read... Indeed; the statement that I found most profound therein was: "The XLamp XM-L LED operates at up to 10 watts of electrical power, depending on the drive current, and requires a heat sink to dissipate this thermal load. About 40% of that energy is converted to radiant flux and the rest to heat..." (ref: here - under Thermal Requirements)
Whoa! Can it be that simple? Bear with me, this really got me thinking..
Imho, they're probably alluding to a T6 bin emitter w/o lens running at 700 mA and 3.337 Volts, but that is *still* amazing to me because too many people get hung-up trying to convert luminous flux (Lumens) into radiant flux (Watts) using the widely misunderstood 683 Lu/W value from Wikipedia..
Keep-it-simple: 40% of 700 mA * 3.337 Volts is 0.93436 Watts of radiant power emitting from said LED, give or take a little..
At this point I thought; don't get too excited; Flashlight-Wiki states here that that the XM-L (T6) emits between 280 & 300 Lumens when driven at 700 mA; So, we're nowhere near the maximum output yet. Let's just do some math:
290 Lu / 0.93436 W ==> 310 "XM-L Cool White" Lumens per emitted Watt!
Let's consider that number in comparison to Wikipedia's vaulted 683 Lu/W for pure 555 nm "green" light. Countless people have said that our eyes are most sensitive to green light, but I have nearly gone blind reading stuff that never addresses Lu/W for XM-L lights or any white LED that I would recognize in a flashlight... But wait; we just calculated that ourselves! And, for CPF technical purists; Cree documents XM-L's relative spectral power distribution here. Further, if the spectral power distribution between led's is fairly similar, then my 310 Lu/W value shouldn't vary much between LED's either.
Of course I don't know the accuracy or details behind the 40% efficiency that someone slipped into one of Cree's application notes, but I don't think I've ever seen it so concisely published before and I hope that people reading this will embrace an approximately 310 Lumens-per-Watt value for XM-L emitters. The main thing to note is that it's only 45% of Wikipedia's 683 value for pure green.
Last, but not least, I assume that radiant efficiency is proportional to luminous efficacy and both obviously degrade as power levels increase from 700 mA. For example, the radiant efficiency of said XM-L (T6) emitter drops to 28% at 3A corresponding with its efficacy drop from 124 to 94 Lu/W - i.e. 72% of said emitter's power has to be dissipated as heat at 3A - Btw, didn't MikeAusC already try to demonstrate 75% dissipation for XM-L here?
==> Does anybody see any glaring flaws in my logic or care to post refinements upon the 310 Lu/W value that I derived for "XM-L Cool White"?
Looking forward to your posts, William