Hi Bogus1,
Assuming that the NG is in regulation, the current delivered to the led is the same regardless to the Vf of the Luxeon bin.
ie, NG 500mA will deliver 500mA whether it is a J bin or K bin Lux.
Assuming that the flux is the same, brightness should be about the same at a certain current.
Power consumed (Watts) = Current (Ampere) x Volt, therefore the higher the Vf, the more power consumed.
To answer your question,
The K bin and the J bin will have the same output but the J bin will have a longer runtime assuming that the flux is the same and the NG is in full regulation.
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Also, using the J bin would probably result in an increase in converter efficiency because the battery voltage will be closer to Vf than the K bin, although I don't have a clue of how much difference this will make.
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Hope this clears you up.
Sorry for OT, just passing by /ubbthreads/images/graemlins/smile.gif
Takifugu