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LED-based Lighting: Light Bulbs and Basics

LED-based lighting is fast turning into 'the topic of the year.' There is a lot of talk about it but the information available is not easy to understand, and often not easy to find. There is a lot of interesting technical wizardry in this new technology, and I'll help explain it here. This article will explain how LED-based lighting works, what makes it different, and the series should be some help in deciding if you want to use it in your home.

What's Inside

This illustration gives you an idea of the construction of an LED made for lighting. Because they are emitting much greater power than a little LED made for an indicator (like the power indicator on a TV for example) these devices have to be constructed in a much different manner. The amount of power being handled within the device is approximately 800 times greater, so structures like the wires and the die (the emitting element) are made to handle the power. But the biggest difference is that these devices must have a heat sink to get rid of all the heat that's being generated within the device itself.



The 'heat sink' arrow in the above illustration is somewhat misleading. In the case of this manufacturer's package, the heat sink is actually the leads, (it is thermally coupled to the leads, not electrically) and the designer has to design his circuit board to take the heat coming from the leads and dissipate it away from the LED.

Heat sinks are our greatest design challenge right now, and deserve a little more discussion. Because there is about 800 times the power being dissipated within the emitter, we now have to handle about 800 times the heat. And all that heat is being generated within a tiny space. So the challenge is to take all that heat and move it away from the emitter in some manner that is both effecient and asthetically pleasing.

Another way to think of it: for an LED that has an output equivalent to a 150W light bulb, it generates about 25W of heat. Think of trying to hold a lit 25W bulb in your hand and it gives you an idea of the problem. If you could use the proper heat sink, you could have that 25W light bulb in your hand but it would feel only a little bit warm. That's the amount of heat sinking required for today's LEDs made for lighting applications.

How to Make Good Quality Light
LEDs have a skewed light output spectrum, with a lot of light in some areas of the visible spectrum but not much in others. So the best quality light is generated by using blue Indium-Gallium-Nitride die and using that output to activate yellow- and amber-colored phosphors to create a pleasing, white light. This allows for good control of the color and through process controls, consistent color from fixture to fixture. All high-quality bulbs currently use this method.

Some cheap methods of making light with LEDs involves taking a big cluster of inexpensive, hobbyist-grade indicator-type LEDs and putting them all together in some kind of form to create a usable light. You can look into these clusters and not see any of the amber or yellow phosphor in the LED emitters themselves. Quality of light from such fixtures is lower than inexpensive fluorescent tubes sold as "shop lights." Here's an example of this:




Part of the necessary infrastructure for an LED bulb is the driver circuitry. LED lighting modules (they are called this because they have more than just the die, but a structure to help dissipate the heat) operate at a different set of voltages and currents than simple indicator LEDs. This set of voltages and currents fluctuates somewhat with temperature, and the driver has to continuously optimize its output to keep the bulb lit at a consistent brightness.

By contrast, some inexpensive "LED bulbs" use an internal wiring method like that of the old series-wired Christmas light strings: if one quits, they all quit. There is also no compensation for line voltage variations, so when the fridge kicks on, you notice the light dims considerably.

But LEDs do have many advantages.



Why do you want to use an LED-based light bulb?
Let's answer with some general stuff:

They're way more efficient. LED bulbs use about half the power of a compact fluorescent for a given light output. And they use about 1/10 the power of an incandescent.

They last longer. Some of the specifications for the components that make up these LED light-producing devices point to lifetimes of up to 25 years, if the rest of the 'bulb' is designed properly.

The light is better-looking. Manufacturers have realized that if you're going to put up the bucks for one of their super-duper light bulbs, then it had better look like a real improvement over compact fluorescent. And typically, they do.

They're instant-on. No more waiting around for the light bulb to warm up (especially if the house is cold) so you can see what you're doing.

Use one anywhere you'd use a standard incandescent bulb; this includes places where you'd never be able to use a compact fluorescent: cold areas, outside, or high-vibration situations. Manufacturers claim they attract almost no bugs because they don't emit light in that part of the spectrum where bugs are attracted. And they don't put out the heat that attracts some types of insects.

No separate ballasts to replace or interference with electronics in the house.

They screw into existing fixtures. Manufacturers have been mindful of the design of the base of the light bulb so that they can be used with fancy fixtures that use the socket to hold a shade. Compact fluorescent bulbs often have the big heavy ballast at the bottom that won't let you screw in a 'bulb-looking' unit into one of these fixtures.

Gradual failure. No more dark rooms and smoking, stinking compact fluorescent ballasts. LED bulbs will gradually put out less and less light, until they fail to light altogether. But by that time, you will know that the bulb is going bad. And because the bulb doesn't have to make high voltage inside (unlike compact fluorescents), they will be safe when they fail.

No hazardous waste when they fail. LED bulbs contain no mercury or hazardous substances, and are non-pressurized, so if you drop one, you don't have to worry about all the glass or the mercury. Often they will survive the fall.

Okay, so what are the down sides?

They're lots more expensive. You'll pay about $25 (more or less) for a good-quality LED bulb, and about $35 for one that's dimmable. It appears that the dimmable variety are what's being brought to market first, so what's on some store shelves (as of this writing) are the more-expensive variety. The up side of this when you do the math is that it's a break-even or coming out somewhere ahead at the end of the lamp's service life. Some manufacturers claim that you'll be hundreds of dollars ahead by the time you factor in energy savings and bulb replacements.

They will have a heat sink designed as part of the bulb. The tiny parts (the actual LEDs) that emit all the light still generate a good amount of heat, and you have to get rid of that heat. Heat is the enemy of these devices, so if there's anything blocking the heat sink (the 'fins') then the unit will have a very short life. See the photo below, and note the fins under the yellow light-emitting areas. The manufacturer's done a good job of designing it with lots of heat sink ability, but at the same time kept it looking fairly conventional.

There will be cheap versions with short service lives and terrible-looking light output. This was the case with compact fluorescent bulbs, the same will happen now.

They look different. Here's a high-quality LED bulb currently being sold. It gives you the same amount of light as a 60W bulb, and good color. The shape is approximately the same as a standard light bulb, but if you're using it in a vintage fixture where the bulb shows, well...





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How to buy an LED-based light bulb
So you've seen what's coming into stores and you're wondering what's good, and what's worth owning. This should help you figure out if you're getting a good buy or not. There are three things you should look at when buying:
  • Light output
  • Color Rendering
  • Dimmability
Light Output Rule of Thumb

The correct way to measure light output is in Lumens. This is a weighted measurement that models the human eye's sensitivity to light. It also corresponds well to known incandescent values. For instance, a "standard" 75W incandescent bulb gives off about 1000 lumens of light. So since you know how much light to expect out of a 75W bulb, then you know how bright 1000 lumens is also. So for a 60W bulb, figure on about 800 lumens, and for a 1000W bulb, figure on 1500 lumens.

The easy number to remember is 1000 lumens = 75W incandescent.

So going on this number, you get these approximate figures:

25W = 200 lumens
40W = 500 lumens
60W = 800 lumens
75W = 1000 lumens
100W = 1500 lumens
150W = 2000 lumens (above 150W, most incandescent bulbs become more efficient due to the heat generated)
200W = 3500 lumens
300W = 5000 lumens

So now when you look at a replacement, you now have a pretty good guess as to how much light it's going to put out.

High-temperature bulbs like halogens are much more efficient because they use the generated heat to help them put out more light. Typically, the hotter a filament can be allowed to burn, the more efficient it will be at making light.

It's difficult to be really accurate when quoting figures about incandescent light output, because even ordinary light bulbs vary wildly in efficiency. Some are good at making light, some are good at making heat! Some of this is due to the materials used in making the filament, some is due to any interior or exterior coatings on the bulb to color the light, whether it's frosted or not, and if the frost is on the inside or the outside, for instance.

The correct term for efficiency in lighting is known as luminous efficacy, and is measured in lumens (output) per watt (used). You can think of it as sort of a miles-per-gallon figure for light. Here are some common efficacy ranges, courtesy of GE:
  • Thomas Edison's first lamp — 1.4 lm/W
  • Incandescent lamps — 10-40 lm/W (I used 13.3 in the example above)
  • Halogen incandescent lamps — 20-45 lm/W
  • Fluorescent lamps — 35-105 lm/W
  • Mercury lamps — 50-60 lm/W
  • Metal halide lamps — 60-120 lm/W
  • High-pressure sodium lamps — 60-140 lm/W
The values shown here for arc-type lighting (fluorescent and the industrial-type lamps below it) do not include the effect of the ballasts. When you take ballast losses into account, you lose an additional 10% to 20% of efficiency.


Color Rendering

There is a measure of color rendering called CRI, or Color Rendering Index. This is one of those "higher is better" measurements. Incandescent bulbs have a CRI of 100, or nearly perfect. Shop light fluorescent tubes typically have a rating of only 60, which means people look sickly, and it's almost impossible to match paint colors. That's why you have to take color samples outside to make sure they match well.

Dimmability

Some LED replacement bulbs can't be used with dimmers, just like some compact fluorescent bulbs can't be used with dimmers. Always check the box if you're going to be using one of these new-technology bulbs on a light dimmer. This same caution also applies if you're using something like a proximity sensor to switch them on and off, since the proximity sensor is an electronic switch and not a mechanical one.

Rumors of Incandescent's Death

Incandescent isn't dead yet. On the horizon (at the time of this writing) are new types of bulbs that combine capsular burning of the filament (like some halogens do) with standard incandescent envelopes; GE is showing prototypes of a combined-mode incandescent/compact fluorescent that uses the incandescent filament to give you instant light and at the same time preheats a highly efficient fluorescent tube, all encapsulated within an incandescent-size envelope.


Hope you found this helpful!

Articles to come:
Automotive-type Lighting and Lamps

Steamguy

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