The light emitting part of an LED unit feels cool because LEDs don't produce heat in the form of infrared radiation. It’s the infrared radiation that heats halogen bulbs and their surroundings, making them very hot to the touch.

However, heat is still produced in LEDs, but within the LED's electronics (junction heat) due to the inefficiency of the semiconductor processes that generates the light (same reason computers get hot and need cooling). The efficiency (optical power out/electrical power in) of LED units is roughly 5 - 40% (the best being closer to 40%), meaning that 60 - 95% of the actual input power is lost as heat.

The energy consumed by a typical incandescent bulb produces around 5% visible light. An average LED produces about 15% visible light. The rest is lost as heat in both cases. You just don’t notice it in most smaller LEDs, such as a typical 10W LED spot lamp like the Vision-X/Cree, because it’s only 8.5W of conducted heat. In a 55W incandescent bulb it’s 52W or radiated heat, which you will feel!

With high-power LEDs the semiconductor inefficiency becomes a huge problem, particularly because the heat has to be conducted through the body of LED. Radiated heat produced in a halogen bulb is much easier to dissipate. Without a decent heat sink (and fan) the internal temperature of the LED electronics rise, critically affecting performance. As the junction heat rises the lumen output will decrease and the wavelength of light will change for the worse too.

It’s worth noting that in the promotional material advertising HP LEDs all the figures quoted for lumun output will have been measured at a junction temperature of 25C (room temperature) on a test bench in a laboratory. In real world operation the junction heat will almost always by massively higher than this and the lumen output a lot lower.

Go and have a look at a new LED equipped R1200GS - take a look behind the headlight and see the size of the heat sink they've had to use...

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