Wednesday, April 28, 2010

You can’t melt an action figure with an LED

When I was a kid, my brother and I were playing with our action figures; G. I. Joes to be exact. We got distracted and turned our attention elsewhere for a while until we started smelling burning plastic. My mother’s keen nose helped her quickly sniff out the problem. My (not very bright) brother stuck his action figure in the lamp as a hiding place and the heat from the light bulb was melting poor Joe’s backside. Childhood tales like this will soon no longer be possible, not because G.I. Joe is no longer popular, but because of the heat source that melted his nether-regions. The goal of a “light bulb” is to produce light, not heat. Any heat production is an unwanted byproduct and represents conspicuous inefficiency. Australia has already banned the sale of incandescent light bulbs because of their inherent inefficiency and other countries, including the US, are following suit.

The two big alternatives to incandescent bulbs are compact fluorescent lamps (CFLs) and light emitting diodes (LEDs). You can read a lot online about the pros and cons of each. Since we are stuck with these technologies for the foreseeable future, we might as well understand how they work. As I’m a big fan of beating a dead horse, I’ll explain their operation in the same context as my last blog – through dance!

In my last blog, I talked about semiconductor physics and how they generally do not conduct electricity until they are given a stimulus. When stimulated, the electrons can move in the semiconductor but after the stimulus is removed the electrons transition back down to their natural non-conductive state. When electrons transition from “conductive” to “insulating” states, light is often emitted in the process. Now picture the semiconductor dancer from my previous blog. When stimulated with the spot light, she dances freely, but when the light turns off she falls to the floor as an insulator. In a dramatic move right before collapsing to the floor, she gives off a burst of light (radiative recombination to physics folk) – this is basically how a light emitting diode works. Taking this analogy way beyond its limit, her heritage determines the color of light that she emits. For example, a Latin dancer will produce a red light, a Russian dancer a blue light, an Asian dancer a green light. Translated from the stage back to semiconductor reality, this means that the color of light emitted from an LED depends on what materials are used (amaze your friends by saying, “an LED’s wavelength is determined by the material’s electronic band structure”). I often joke that LEDs come in any color you want as long as it’s red, amber, green or blue. These colors happen to correspond to semiconductor materials that are readily mass producible and illustrates the point that LEDs emit one color only. Next time you look at multi-color LED Christmas lights, you will notice the aforementioned LED colors because they are the least expensive to produce.

Anyway, since the light from LEDs is produced by electrons transitioning between states, there is very little waste energy, therefore they generally do not get “hot” like a light bulb and are more energy efficient. I know what you are thinking; because the light in an LED is produced through radiative recombination events it is inherently monochromatic, so how do you explain white LEDs? More on this later…..

1 comment:

  1. Thanks for doing such a great job explaining this topic! You make it very easy to understand using your analogies.

    I feel smarter already!