Well, a filament lamp works by heating up a (small) bit of metal to very high temperature, hot enough that it glows just through random emission of many wavelengths of light. The problem is that much of the emitted light is given off in the infrared. The loss terms are: conduction of heat by fill gas, conduction of heat by the leads to the filament, and invisible light given off as infrared.
In something like a Luxeon, light is produced by the combination of an electron and a 'hole'...really this is an electron dropping into a lower energy space that the electron can occupy, giving off as a photon the excess energy that it now has. The light produced is generally of a very narrow frequency range. With a visible light LED, very little infrared is produced. With most white LEDs, you make blue light with the semiconductor materials, and then you use a phosphor to make the white light.
Major loss terms are: direct resistance loss in the leads and substrate carrying the electrons to the semiconductor junction, inefficiency in the conversion of electrons to photons, inefficiency in the escape of photons from the junction to the surroundings, inefficiency in the phosphor converting the blue to white. All of these loss elements pretty much result in heat production, which needs to be dealt with.
Regards,
Jonathan
