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LED Frequently Asked Questions

LED Frequently Asked Questions
LED technology continues to develop rapidly as a general
light source. As more LED lighting products are introduced
on the market, what do retailers, energy efficiency advocates,
and consumers need to know to make informed
buying decisions?
Are LEDs ready for general lighting?
The number of white-light LED products available on
the market continues to grow, including a wide range of
replacement lamps, as well as integrated light fixtures,
such as portable desk/task lights, under-cabinet lights,
recessed downlights, track heads, and outdoor fixtures for
street and area lighting. Some of these products perform
very well, but the quality and energy efficiency of LED
products still varies widely, for several reasons:
1. LED technology continues to evolve very quickly.
Performance and pricing of LED packages/devices
are dynamic but both are steadily improving.
2. Lighting manufacturers face a learning curve in applying
LEDs. Because they are sensitive to thermal and
electrical conditions, LEDs must be carefully integrated
into lighting products. Manufacturers vary in their ability
to do this effectively.
3. Price pressures can affect the quality of components
used in LED products, particularly replacement lamps
targeted to the general consumer.
Courtesy of Osram Opto Semiconductor.
LED light sources used in a residential application.
SSL – solid-state lighting; umbrella term for semiconductors
used to convert electricity into light.
LED – light-emitting diode.
CCT – correlated color temperature; a measure of the color
appearance of a white light source. CCT is measured on the
Kelvin absolute temperature scale. White lighting products
are most commonly available from 2700K (warm white) to
5000K (cool white).
CRI – color rendering index; a measure of how a light
source renders colors of objects, compared to a “perfect”
reference light source. CRI is given as a number from 0 to
100, with 100 being equivalent to the reference source.
Lumen Maintenance – the percentage of initial light output
produced by a light source at some percentage of rated
useful life (usually 100% for LED and 40% for source types
characterized by sudden failure). An LED package used in lighting products
Courtesy of Philips Lumileds
Are LEDs energy-efficient?
The best white LED products meet or exceed the efficiency
of fluorescent and high-intensity discharge (HID) light
sources. However, many LED products currently available
in consumer market channels are only marginally
more efficient than incandescent lamps, and many suffer
from very low light output relative to incandescent lamps
and CFLs.
For several categories of luminaires (complete lighting
fixtures), LED products are now widely available and
meet or exceed the performance of conventional light
sources. For example, nearly 500 LED recessed downlights
are now listed by DOE’s Lighting Facts program
(www.lightingfacts.com), which requires verification of each
product’s light output, efficacy, and color characteristics.
More than half of those downlights exceed the initial
output and efficacy requirements of the ENERGY STAR®
program, indicating they may perform at least as well as
fluorescent downlights.

How long do LEDs last?
Unlike other light sources, LEDs usually don’t suddenly “burn out;” instead, they gradually fade in brightness over time. LED useful life is generally based on the number of operating hours until the LED is emitting 70% of its initial light output. Good quality white LEDs in well-designed fixtures are expected to have a rated useful life on the order of 30,000 to 50,000 hours. A typical incandescent lamp lasts about 1,000 hours; a comparable CFL lasts 8,000 to 10,000 hours, and some linear fluorescent
lamp-ballast system can last more than 40,000 hours. LED light output and useful life are strongly affected by temperature.
LEDs must be “heat sinked” (placed in direct contact with materials that can conduct heat away from the LED) and driven at an appropriate input current.
Do LEDs provide high quality lighting?
Color appearance and color rendering are important aspects of lighting quality. Until recently, most white LEDs had very high CCTs, often above 5000 Kelvin. High CCT light sources appear “cool” or bluish-white. While very high CCT LEDs are still common, products with neutral and warm-white LEDs are now readily available. They are less efficient than cool white LEDs, but have improved significantly, and the efficacy gap between cool and warm LEDs is narrowing. Whereas warm-white (2700 to 3000K) is appropriate for most indoor residential applications, neutral-white (3500 to 4000K) is more common in commercial settings.
The CRI measures the ability of light sources to render colors, compared to incandescent and daylight reference sources. The CRI has been found to be an unreliable predictor
of color preference of LED lighting products. A new metric called the Color Quality Scale (CQS) is under development, but in the meantime, color rendering of LED products should be evaluated in person and in the intended application if possible.
Are LEDs cost-effective?
Costs of LED lighting products vary widely. Good quality LED products currently carry a significant cost premium compared to standard lighting technologies. However, costs are declining rapidly. Recent industry roadmapping indicates
prices for warm white LED packages have declined by half, from $36 to $18 per thousand lumens (kilolumens, klm) from 2009 to 2010. Prices are expected to continue to decline significantly to approximately $2/klm by 2015. It is important to compare total lamp replacement, electricity,
and maintenance costs over the expected life of the LED product.
What other LED features
might be important?
Depending on the application, other unique LED characteristics
may merit consideration:
• Directional light
• Low profile / compact size
• Breakage and vibration resistance
• Improved performance in cold temperatures
• Life unaffected by rapid cycling
• Instant on / no warm up time
• No IR or UV emissions

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