Pinter said:Interestingly, at 5mm leds heat exits not only through the leads; if you hold the "dome" with your fingers, the Vf will start decreasing slowly. Within 10-20 seconds I saw -0,02 V change in Vf, while the current went up by 0,2mA (simple resistored setup from NiMH batteries).
With 250C/W through the leads, it's not surpising that a considerable percentage of the heat is being dissipated through the epoxy encapsulent. This will not hold true for power LEDs - heat dissipation through the dome/lens of a power LED will be negligable - the dome does not prodivide an appreciable thermal path.
FYI - when you held the dome, you were actually causing the LED to heat up, reducing the amount of heat dissipated by the epoxy. The Vf of LEDs drop when they heat up.