LED in use - temperature?

So, everyone knows too hot the temp --> LED lifetime and performance will suffer.
But at what temperature might this happen?

Say the Luxeon LED is mounted on a sheet of aluminium;
running at spec, the alu gets warm on the touch, but not hot enough to cause more than feeling the heat (warmth).
Is this already 2 much?

How about beeing warmer, but still far fom burning skin?
/ubbthreads/images/graemlins/confused.gif

(Problem: weight and space restrictions)

@@"
Those Luxeon is hot!
Those are not just warm.
buring skin... well...When the Luxeon LED heating up, before it can burn your skin.It breaks.<--no joke /ubbthreads/images/graemlins/smile.gif

Please try to clear your question.

Check the current data sheets /ubbthreads/images/graemlins/icon3.gif
From what I still have in mind, anything above 20° C at the emitter will decrease both lifetime and performance (as compared to the specs). Anything else is either calculation (error laden) or measurement (difficult).
Calculation: you add up the termal resistances (emitter -> slug -> heat sink -> ambient medium), the temperature at the emitter is then (thermal resistance * power) + temperature of the ambient medium. Problem is, the thermal resistances are hard to get right - assume a factor of two as basic error; more, if you can't get good values for your specific heat sink.

In practice, this means if you can feel the heat sink getting warm, the emitter may already be too hot. Not a problem with flashlights with a comparatively short lifetime. Again, check the data sheets - the LED will not die instantly (as can be seen e.g. in the RAW), but performance will degrade over time and efficiency will suffer.

Bye
Markus