Back in the old days (when I was just a kid) the lights in our homes, appliances, flashlights, cars, etc.) were primarily incandescent lamps. When enough electric current is high enough the wire filament gets so hot that it gives off light. Unfortunately, it takes a lot of power just to get the filament hot enough to start glowing, and because of this incandescent lamps tend to be relatively inefficient. Most of these lamps use no more than 10% of their power to create light; the remaining 90+% of power is lost as heat.

Light-Emitting Diodes (LEDs) are far more efficient at converting electrical power to light, so they are rapidly replacing incandescent bulbs in most applications. However, LEDs are a bit trickier to use, since they can be permanently damaged if they get too hot. And because these light-emitting devices are diodes, they are sensitive to polarity. Let’s look at a typical LED datasheet to get a better idea of these issues

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