Kevin Gauna, Project Engineer at the California Lighting Technology Center , breaks down the science behind solid-state light sources.
Residential Lighting: What are LEDs?
Kevin Guana: A light-emitting diode or an LED is a small semi-conductor chip often less than one millimeter square. Just as you would run a current through a filament in an incandescent light bulb, you can run current through a semi-conductor chip, and it emits light in the visible spectrum. So, you’ve got this very small chip and these little wires that go to the chip, what happens now is we make a lead frame package that supports the wires and holds the chip in place. This is the substrate of an LED. It’s typically a little piece of metal with a lead coming off of it and another piece of metal with a wire that connects to the other side of the LED. That whole thing gets encapsulated in an epoxy or clear silicon. That individual package is called an LED, and it’s essentially a miniature light bulb.
RL: How do they work?
KG: When you pass current through the LED, it doesn’t get hot like a filament in an incandescent light bulb gets hot. An LED produces light through a completely different physical process. It’s an electronic process essentially, and the details aren’t important here. The main thing to understand is that since the chip itself isn’t getting white hot, it’s simply producing light directly from the current that’s passing through it. The whole thing gets encapsulated into that epoxy, and there’s no air gap inside it. If you look at an LED, there’s usually a little dome or lens of some sort as part of that package, but the whole thing is solid. That’s where we get the term solid-state lighting.
Most of the time we cluster that individual LED with other similar LEDs on a circuit board, and that package is called an LED array. To power the array, we need an electronic driver. The electricity can come straight from your standard wall jack, and the electronic driver conditions the 120V AC electrical power and sends it to the LED array in a form that’s appropriate, usually DC. The whole thing — the LED array, the electronic driver and any reflectors or lenses that are added to provide a very specific light distribution — gets put into some sort of housing that brings everything together. You sell that as the luminaire: your table lamp or your downlight.