IES Frequently Asked Questions
Q: What is an LED?
A: LED is short for Light Emitting Diode. Like other solid state devices (switches, diodes, triodes, amplifiers, etc.) they are made in a way that is similar to a computer chip. The light output is very specific to the chemistry used in the layers, and can be designed or tuned to be any number of colors. For instance, a high efficiency red LED (635nm) would be made with Gallium Arsenic Phosphide while a 555nm green would use Gallium Phosphide layers (no arsenic).
Q: What are the basic advantages of LED’s that everyone is getting so excited about?
A: Space Saving Vibration resistant Low temperature resistant, Long Lasting, Doesn't burn out (fades out over time instead) Low power consumption, Little heat to light ratio.
Q: Can you change the colors After the LED is made?
Q: What colors are available?
A: LED's Come in a multitude of colors. In this case, color really means the frequency at which the light is emitted from the LED. Humans can percieve a visible spectrum that starts in the red (700nm+) and goes down to blue (less than 400nm). Some colors are common such as red, green, and blue, but commercially you can buy LED’s that range anywhere from 940nm all the way down to 280nm.
Q: How is an LED different than an incandescent bulb?
A: An incandescent bulb has a tungsten filament that power (current) passes through. An LED is a solid state device. Due to resistance Tungsten filaments (incandescent bulbs) get really, really hot. So hot in fact that they glow, or incandesces. This is the light you and I see and recognize from incandescent bulbs. As it continuously heats and cools the filament actually starts to evaporate (sublimate) and becomes thinner over time. Eventually the filament will get so hot that it will melt, creating an “open” circuit. This is the standard, “oh dang, my light just burned out.” An LED is a solid state device. Light diminishes, but there’s no burning out quickly like an incandescent.
Q: Where do LED’s work well?
A: For white light, or general lighting. LED’s work well in task lighting, parking garages, landscape lighting, cove lighting, architectural lighting, highlighting, and generally most places where a lot of “light throw” is not needed. They’re great for applications where UV light is bad, like museums and art galleries. Colored LED’s work very well in outdoor architectural lighting, and any application where colors, or color changing is desired. Note that the color changing is typically done with multiple different colored LED’s clustered together and controlled by a computer of some sort.
Q: Where do LED’s not work well?
A: The Holy Grail of lighting is office space (commercial) and street lighting. Office spaces may be lit with white LED’s at present, but the immediate economic benefit is yet to be realized when compared to other classic lighting technologies. Street lighting has been a real quest, and today there are a couple of companies that can do it well. What scares the towns, cities, and municipalities is the initial cost of purchasing the LED lighting. The economic benefits are there however.
Q: What is a heat sink, and why is it important?
A: Typically a heat sink is some material that is good at conducting heat. If you stick a silver spoon into a hot cup of tea, the handle of the spoon will get hot quickly….all the way to the end. That is a good heat sink. Copper is more typical, and aluminum is also commonly used to conduct heat away from a heat source. The goal of the designer is to push the LED to put out as much light as possible (meaning push lots of electricity through it) which will generate a wee bit of heat. That little bit of heat is in a very small volume, and it is important to draw the heat away from the LED itself. That’s why there’s a heat sink on almost every LED. Certainly on all high power LED’s!
Q: If LED’s need a heat sink, why do people say they give off no heat?
A: LED's do give off heat. But significantly less compared to traditional (incandescent and flourescent) bulbs. Let’s look at the incandescent lamp one more time. They operate by making the filament so hot that it glows, or incandesces. Basically its 92% heat, and 8% light. Not a real good return for your energy. Fluorescents are much better. Both the linear and twister lamps are about 30% heat, and 70% light. Pretty darned good improvement over incandescent, wouldn’t you say? Sure enough, moving over to the LED, we have roughly 92% light, and 8% heat. That isn’t much heat at all, and unfortunately, there’s some folks out there making claims of “no heat” but this is really not the truth. This small percentage of heat is clearly a vast improvement, but because this small amount of heat is in such a tiny volume it has to be addressed. By that I mean it must be removed from the LED die, and that’s why there’s a heat sink. Most of the manufacturers will tell you a lot about how to best manage the heat, given some technical parameters. If you’re buying a complete LED lighting system, all this is already done for you. The big thing to watch out for is that the manufacturer has to do it right. Otherwise, you’re going to be sorely disappointed!
Q: Why isn’t there a replacement LED lamp for my standard at-home 90 watt incandescent lamp?
A: The future of LED lighting will see an LED lamp that "looks, feels, and illuminates" like the classic incandescent bulb. While we're not quite there, it’s not far off. Past that, its going to be a quest to optimize the power consumption to light output and then to minimize the cost so consumers start buying them In large volume. First, you have to recall that the incandescent lamp gives off light is all directions, and the LED is very directional. So the first issue is that you’ll need a number of LED’s to realize the same “omnidirectional,” or all direction, light output. Because you’re asking for a direct replacement, it means that the fixture you’re putting the lamp into is designed for the optical characteristics of that omnidirectional lamp. That’s one of the reasons its more common to see a whole LED luminaire. A Luminaire is both the fixture AND the lamp, when combined. Right away the cost is up there, and for most folks, they’re simply not ready to spend the money. I heard a guy complaining about having to buy two 90 watt light bulbs ($0.78) when he only needed one. Do you think this is the same guy who will spend $100.00 on an LED luminaire? I don’t think so! The good news is that there are a lot of applications that simply make sense, and are such good economic paybacks that the initial cost of ownership is a no-brainer. We’re seeing this more now in the industrial and commercial arena, where there is a higher level of awareness towards the total cost of ownership over a period of time.
Q: How are LED’s made?
A: LED’s are made in a way that is similar to a computer chip. A flat wafer of very pure gallium arsenide, or sapphire is the foundation. In a super hot oven, under extremely high vacuum, certain gases are introduced, and they deposit onto the base foundation, or substrate. Different layers are “grown” by the introduction of different gases or chemicals during each period of time. By controlling the thickness of the layers, and by changing the chemistry involved, different color emissions will be realized. When the wafer is cooled down, it is removed, and cut into tiny squares, or “die” and it is these die that are packaged, wire bonded (so they can be connected electrically to the outside circuit), and ultimately have other lenses and optics items added. Don’t try this at home….its a wee bit more complicated and requires some equipment that you probably don’t have in your garage!
Q: How long do LED’s last?
A: Up to 50,000 hours! That's 34 years if its used four hours per day. Because we live in a world of standards and specifications, the LED world had to come up with a specification. It was decided that the lifetime of an LED would be defined as the point at which the LED die only produced 70% of its original light output. Most die manufacturers have figured this to be about 50,000 hours. This “light depreciation” (technically called Lumen Depreciation) is common in fluorescents, metal halide, mercury vapor, and other lamps, so it’s a measure that we’re all accustomed to. Taking this thought one step further, if you run the LED for 100,000 hours, it will likely only be at 50% of its original light output. But that’s not the whole story. The LED is a part of a system, and the driver (some say ballast, transformer, electronics, etc.) has a life too. The real question is “what is the life of the system?” Most of the luminaire manufacturers are having a hard time coming up with a reasonable answer, because it all boils down to how long is the warranty! Look for a warranty on the system, and not just a claim on the life of the die.
Q: Do LED’s reduce Greenhouse emissions?
A: While a good question, it is poorly phrased. LED’s by themselves don’t reduce greenhouse emissions. By reducing your electrical use while still realizing the lighting effect you want does result in an indirect reduction. Its all about reducing the amount of electricity used for a given task, over a period of time. The easy answer is that for electricity that comes from a coal-based power generating system, every Kilowatt saved will result in NOT sending 996 grams of CO2 up the stack. For example, in 2001 there were 107 million homes in the United States. In total, they consumed about 1,140 billion KwH. (Roughly 10,644 KwH per house per year). If each house were to reduce their energy consumption by 5% (533 KwH), each house would reduce their greenhouse emission by 531Kg. (That assumes all our power comes from coal fired generators, but you get the idea. It really does add up quickly)!