LEDs may be little, but new high-brightness models are producing a considerable amount of light.

First used as status and indicator lamps, and more recently in under-shelf illumination, accent lighting, and directional marking applications, high-brightness LEDs have emerged within the last six years. But only recently have they been seriously looked upon as a feasible option in general purpose lighting applications. Before you recommend or install this type of lighting system, you should understand the basic technology upon which these devices are based.

Light-emitting diodes (LEDs) are solid-state devices that convert electric energy directly into light of a single color. Because they employ “cold” light generation technology, in which most of the energy is delivered in the visible spectrum, LEDs don't waste energy in the form of non-light producing heat. In comparison, most of the energy in an incandescent lamp is in the infrared (or non-visible) portion of the spectrum. As a result, both fluorescent and HID lamps produce a great deal of heat. In addition to producing cold light,

LEDs:

• Can be powered from a portable battery pack or even a solar array.

• Can be integrated into a control system.

• Are small in size and resistant to vibration and shock.

• Have a very fast “on-time” (60 nsec vs 10 msec for an incandescent lamp).

• Have good color resolution and present low, or no, shock hazard.

The centerpiece of a typical LED is a diode that is chip-mounted in a reflector cup and held in place by a mild steel lead frame connected to a pair of electrical wires. The entire arrangement is then encapsulated in epoxy. The diode chip is generally about 0.25 mm square. When current flows across the junction of two different materials, light is produced from within the solid crystal chip. The shape, or width, of the emitted light beam is determined by a variety of factors: the shape of the reflector cup, the size of the LED chip, the shape of the epoxy lens and the distance between the LED chip and the epoxy lens. The composition of the materials determines the wavelength and color of light. In addition to visible wavelengths, LEDs are also available in infrared wavelengths, from 830 nm to 940 nm.

The definition of “life” varies from industry to industry. The useful life for a semiconductor is defined as the calculated time for the light level to decline to 50% of its original value. For the lighting industry, the average life of a particular lamp type is the point where 50% of the lamps in a representative group have burned out. The life of an LED depends on its packaging configuration, drive current, and operating environment. A high ambient temperature greatly shortens an LED's life.

Additionally, LEDs now cover the entire light spectrum, including red, orange, yellow, green, blue, and white. Although colored light is useful for more creative installations, white light remains the holy grail of LED technology. Until a true white is possible, researchers have developed three ways to deliver it:

• Blend the beams. This technique involves mixing the light from multiple single-color devices. (Typically red, blue, and green.) Adjusting the beams' relative intensity yields the desired color.

• Provide a phosphor coating. When energized photons from a blue LED strike a phosphor coating, it will emit light as a mixture of wavelengths to produce a white color.

• Create a light sandwich. Blue light from one LED device elicits orange light from an adjacent layer of a different material. The complementary colors mix to produce white. Of the three methods, the phosphor approach appears to be the most promising technology.

Another shortcoming of early LED designs was light output, so researchers have been working on several methods for increasing lumens per watt. A new “doping” technique increases light output several times over compared to earlier generations of LEDs. Other methods under development include:

• Producing larger semiconductors.

• Passing larger currents with better heat extraction.

• Designing a different shape for the device.

• Improving light conversion efficiency.

• Packaging several LEDs within a single epoxy dome.

One family of LEDs may already be closer to improved light output. Devices with enlarged chips produce more light while maintaining proper heat and current management. These advances allow the units to generate 10 times to 20 times more light than standard indicator lights, making them a practical illumination source for lighting fixtures.

Before LEDs can enter the general illumination market, designers and advocates of the technology must overcome several problems, including the usual obstacles to mainstream market adoption: Industry-accepted standards must be developed and costs must be reduced. But more specific issues remain. Things like lumen-per-watt efficacy and color consistency must be improved, and reliability and lumen maintenance should be addressed. Nevertheless, LEDs are well on their way to becoming a viable lighting alternative.

Hot new LEDs that push back the night

By Josey Paul

J ust before midnight , an unbroken bank of clouds floats in low over the forest. Not a trace remains of the moon and stars. This night is beyond black. And all of you LED deniers are about to eat crow.

Yep, I remember you from the last time I wrote about LED lights – and all of your whimpering about how they aren’t bright enough to read by. (For a description of how Light Emitting Diodes work, click on http://electronics.howstuffworks.com/led.htm )

So here goes. A Meterman LM631 light meter on my desk will referee this contest. And right now, if I could see it, it would read 0.000 lux. That means no light. Lux is a unit of light intensity, which for this test means how many photons are bouncing off my reading desk from a light source exactly 1 meter away – that’s 39 inches for those of you not hip to the metrics of Old Europe.

First up is the villain: Mr. 75-watt incandescent bulb himself, the bane of the off-grid set and the ubiquitous handmaiden to guzzlers of fossil and nuclear power. This bulb is the standard – an energy-hogging, short-lived and fragile appliance that was invented at the dawn of the Electricity Age. It’s what most folks still use.

So here goes. I flip on Mr. Incandescent, squint at the Meterman and read 80 lux. Just to be fair here, lux is the intensity of light hitting my reading desk, not the total amount of light coming from the bulb. Lumens are the measure of total light. This GE bulb is throwing lumens all over the room, but most of them are useless for my reading. The photons spraying on my stove don’t help me read a book at my desk across the room. Flashlights focus their light, so they tend to be better at lux than lumens. Light bulbs are better at lumens then lux.

Be all that as it may, the 75-watt incandescent bulb is throwing 80 lux at my reading desk and using a porky 74.8 watts to do so. The math works out to 1.1 lux per watt. Remember that number.

Next up is a 26-watt compact florescent light. This light is a real energy saver and should be the equivalent of a 120-watt incandescent bulb. I flip the switch again, and after a few minutes to let it warm up, the ref pronounces a very respectable 130 lux. The light is using 23.2 watts. The energy-efficiency factor is 5.6 lux per watt. The bottom line is that CFLs give you a lot more light for a lot less power – at least compared to incandescent lights. But you already knew that.

Now the test that you LED deniers have been dreading. I screw a 12-volt, 36-LED light into the fixture, position it 39 inches from the light meter and flip the switch.

Voila! (That’s French for “would you like ketchup with your crow?”) French horns fill the night, and the ref proclaims 140 lux. Better still, the light is using just 3.9 watts. The energy-efficiency factor is a whopping 35.9 lux per watt. Way more reading light than that nasty incandescent. And way less electricity.

Both my 12- and 24-LED lights are even better in terms of efficiency. They put out 90 lux using just 1.3 watts. The energy-efficiency factor is 69.2.

Better still, the LEDs are 100,000 hour bulbs, compared to 10,000 hours for CFLs and typically 700 hours for incandescents. What’s not to love?

Here’s a chart of the test results:

Now some caveats. The LED’s 100,000-hour rating is misleading. LEDs that old tend to have lost half of their output. Many LEDs are shorter lived because they are over-driven to make them brighter. In the chart above, you’ll see that the 12-LED light is over-driven compared to the 24-LED light.

And most LEDs are task lights, they focus their lights on what you’re doing, whether it’s reading or sewing or sharpening a knife. They don’t do as well in lumen tests as they do in tests measured in units of lux. My Luxeon LED light (a new type of LED) is a flood light rather than a narrow-beam task light. It is about as efficient as the 26-watt CFL above in terms of lux per watt.

And LEDs are still expensive. My 36-LED light cost $160.

Also, many of these lights are poorly designed, either with weak, low-quality LEDs or simple circuits that can’t take the changing voltage loads of typical solar systems. My 36-LED, for example, is designed for 12 volts DC; but my solar-system’s voltage varies between 12.3 volts and 14.5 volts. The higher voltages burn out the LEDs prematurely. You’re usually safe with AC-powered LEDs that run off inverters or regular household circuits; but if you buy DC LEDs, you will probably need to hook a voltage controller to them.

For about $30 at Jameco, an electronic-parts retailer, I purchased a DC/DC converter that solves the voltage problem. I can feed the converter DC electricity between 9 volts and 18 volts, and it will give me an exact output of 12 volts DC. You can also buy simple voltage regulators in a transistor package for just a couple of bucks at Radio Shack.

And if you’re ambitious and handy, you can design and build your own LED lights, and even make advanced electronic circuits to avoid the voltage problem. See Al Getz’s story on LED-light designs .

The LED industry is young and has the shortcomings of youth, but it is here to stay. The AC-powered LED lights are incredibly efficient and durable, and DC LEDs are getting better and more affordable.

Friends of mine used to use a bare, 60-watt incandescent bulb for their porch light. While it lit up their stairs and front door just fine, it threw most of its light into space or, worse, into your eyes as you walked up to the door. And it chugged electricity shamelessly. I gave them a 50-LED AC light that cost about $15. It shines plenty of light down on their door and steps, but not anywhere else. It uses almost no electricity. And it will probably never need replacing.

LEDs are one of the keys to making us an energy-efficient society. ☼

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