The lighting industry is buzzing over the prospects for light-emitting diode (LED) technology for general lighting use. But when will this technology be ready for the mass market? While LED lighting is evolving rapidly, it’s still a young technology that will take time to mature.

Lighting for general use

When discussing LED lighting for general use, this nearly always refers to “white” LED and includes a wide variety of products. More and more companies, both new and established, are investing in LED research or acquiring other LED companies to expand their product lines. This presents some challenges for market providers because LED technology is changing so rapidly, with advancements occurring almost monthly. New vendors continue to make astounding claims of energy efficiency and long life, yet no established testing information is available. While draft standards exist for ENERGY STAR® qualification, including testing and performance, these will not be active until fall 2008. In the meantime, the U.S. Department of Energy’s Solid State Commercialization effort is encouraging demonstration studies and other research. Several fact sheets about LED technology can be found at its Web site. (See “For More Information” at the end of this article for links to specific resources.)

LED pros and cons

What’s so special about LED, anyway? The excitement stems from a great potential for energy savings (eventually) and a very long useful life. Other benefits include compact size, instant on/off, reliable operation in cold temperatures, the ability to dim, no mercury content (safe and easy disposal), and no emissions of infrared or ultraviolet light.

The downsides? LED is sensitive to heat, which can reduce light output and rated life. Variations in color can make LEDs difficult to match, and some LEDs may have poor color properties. Comparing products is also challenging because manufacturer data has not been standardized, and is sometimes unreliable. Testing procedures vary. In addition, LED is expensive compared to other technologies. And finally, efficacies vary among products and most are not yet ready for widespread use.

What is SSL?

SSL is an acronym for “solid state lighting,” which is what an LED is—a microchip that emits light. Different chemicals produce different colors of light, and specific color combinations can produce “white” light. Currently, LEDs can produce white light through a mixture of red, green and blue LEDs, or by coating a blue LED with a yellow phosphor.

Comparing light output

Measuring LED light output can seem complicated because standards for testing have only recently been finalized and unlike other light sources, the light output for LEDs must include system performance for the whole luminaire, not just the light source.

This is because, unlike other technologies that emit light in many directions, LEDs are directional in nature (think of a flashlight). In traditional light sources, up to 50 percent of the total light output may be lost inside a fixture. But the directional nature of LEDs means that light travels in a single direction. Instead of bouncing around inside a fixture, the light can exit the luminaire in a straight line. As a result, a direct comparison of lumens does not accurately represent the amount of light that will reach the task surface.

Measuring rated life

Heat is the enemy of electronics, and LED is no exception. Unlike other light sources, LEDs do not emit heat as infrared radiation (IR), so heat must be removed from the device by conduction or convection—for example, a heat sink or fan. If a fixture or light source is not designed to remove heat from the LED, this will result in drastically decreased light output and shorter rated life.

IESNA (Illuminating Engineering Society of North America) LM80 specifies standards for reporting estimated useful life, but is still in draft form. This is defined as the point at which the LED component has depreciated to 70 percent of its initial lumens and is therefore no longer providing the light output needed for the rated application. In other words, LED “end of life” is the point at which the product’s light output degrades to 70 percent of rated light output.

Evaluating LED products

When weighing the advantages and disadvantages of LED lighting, look for the following:

• Reliability and warranty. Look for products from well-established manufacturers or companies that have extensively researched LED products for general lighting purposes rather than new players in the market who may be importing products from overseas. Quality products will include a warranty.
• A good thermal management system (typically a heat sink or fan). This will help ensure better performance and a longer lifespan.
• Ask for photometric test data. This should reveal delivered light levels for a given application as well as the light distribution pattern for the fixture. A complete fixture layout showing projected light levels will help determine whether a specific fixture is appropriate for the customer’s site. Manufacturers should be eager to share this information. IESNA LM79 sets standards for testing photometry of LED luminaries.
• Verify the product’s rated life by reviewing test data documentation from the manufacturer. Is UL or other independent lab testing data available?
• Look for local demonstration sites where you can see the product in action.

Focus on Energy will continue to track this exciting technology and keep you up to date on the latest developments in LED lighting.

For more information

Visit the following links on the US Department of Energy's Solid State Lighting website to learn more about LED technology:
LED basics
Energy Efficiency of White LEDs
Color Quality of White LEDs
Lifetime of White LED
Thermal Management of White LEDs
ENERGY STAR Draft Specification