finger heating via optical energy while testing led?

converted a few cheap 12v halogen desklamps to 2x k2 tffc.
used the metal reflector as the heatsink.
doing the digital temperature probe test(finger) i noticed the led getting quite hot.but the heatsink was barely warm.figured it odd that the dome felt hotter than the heatsink.
so i let it run a bit and turned it off.touching it right as i shut it off the dome feels cool.
so is the intense point source of the die causing heating of my finger?

snakebite said:

converted a few cheap 12v halogen desklamps to 2x k2 tffc.
used the metal reflector as the heatsink.
doing the digital temperature probe test(finger) i noticed the led getting quite hot.but the heatsink was barely warm.figured it odd that the dome felt hotter than the heatsink.
so i let it run a bit and turned it off.touching it right as i shut it off the dome feels cool.
so is the intense point source of the die causing heating of my finger?

Click to expand...

Yes, yes it is

I learned that the hard way on my first quad rebel putting out over 1 watt optical in a relatively small area. Yes it burned ... as it would black tape if put over it.

Semiman

An XML up to 4W of light from 4mm^2 surface.

Thats 1W/mm^2. Sunlight at noon has about 1.2kW/m^2 (=0.12W/cm^2=0.0012W/mm^2).

Even if you consider the source enlargement due to the dome, its still 100 times more concentrated than sunlight -> Hot enough to burn.

The 4 watts is the "electrical" power, not "optical" power. The optical power is less than the electrical power but it is still more than the sun.

Actually I'd say IMSabbel is correct with the optical power, an XML driven at 4 Amps would be sucking 12+Watts of electrical power, say 30% efficiency, 4Watts of optical power sounds reasonable.
I know some here have been known to drive them at 6Amps+, so would be well over 4Watts of light in those cases.

I noticed this with XP-Gs run at medium-high levels.

Despite having similar lumens output, I never noticed it as an issue when handling lit earlier LEDs like the Seoul P4, presumably because of the larger dome keeping the finger rather further from the die.

While doing some hand-aligning of an XPG in a reflector a while ago, I put down the illuminated XPG/heatsink assembly, where it fell face-down on the fabric arm of an old chair for a few seconds.
When I picked it up it, had created a nice little dent in the fabric by melting the protruding fibres away towards the backing material. The LED dome was unharmed - I guess it had been able to melt stuff away from it before coming into contact with anything. That was only at something approaching half max power.