Why is an LED a more efficient source of light than a filament lamp?

Answer 1

What happens here is that a filament emits all sorts of radiation - much of it as invisible infrared ("heat radiation"). On the other hand, a LED will convert a large percentage of its energy to light.

As to WHY this is so... Well, a filament is just a heated object, and those tend to radiate a variety of frequencies. For more details, do some reading on "black body radiation" (for example, in the Wikipedia). I am assuming, for simplicity, that the filament is a so-called "black body"; which I think is close enough for a basic analysis. The different frequencies (colors, or types of radiation) are basically caused by the fact that the atoms move at different speeds.

The LED, on the other hand, is based on electrons of a specific element being excited - i.e., lifted to a higher energy level. In this case, they will soon fall back to the basic energy level. In this case, the difference in energy levels should be the same for all atoms; thus, they all emit light at the same energy, i.e., frequency.

Answer 2

LEDs are more efficient because they convert more energy into light rather than heat. Filament lamps waste much of their energy as heat, resulting in lower efficiency. LEDs also have a longer lifespan and consume less power for the same amount of light output.