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How long will it take before we see energy-efficient, cost-competitive, white-light products on the market?
DOE's SSL R&D plan spans 20 years (2000-2020), and includes three components: Core Technology Research, Product Development, and Commercialization Support activities. The good news is that tremendous progress is being made, faster than originally anticipated. Researchers have already improved the efficacy of white LEDs to approximately 50 lumens per watt, almost four times more efficient than incandescent sources. Costs are still high, but continue to drop significantly, from approximately $250/kilo-lumen in 2004 to around $50/kilo-lumen in 2006 (based on manufacturer estimates for volume purchase). For comparison, conventional light sources (incandescent, fluorescent) cost around $1/kilo-lumen.

There already appear to be a lot of white-light LED products available now… what should I look for?
Some of the LED products available today are marketed as “energy-efficient,” but actually have very low light output compared to typical light sources. The combination of high price and low light output may actually make them a poor replacement for current technology. It is important to compare new LED products to the most efficient conventional technology (such as fluorescent, incandescent, or metal halide) that could be used in your specific application. The following checklist will help you determine if an LED product is right for your application:

	Ask how many watts the product consumes and how many lumens of light it produces. Lumens per watt (lpw) is the commonly used measure of how efficiently a light source is converting electricity into useable light. For comparison, incandescent lamps typically produce 12-15 lumens per watt of electric power. Compact fluorescent lamps (CFLs) produce at least 50 lumens per watt. Currently available high-brightness LEDs can produce about 30-35 lumens per watt. In task lighting applications, LEDs may be able to provide enough light on the task, even though the total lumens are less than comparable incandescent or fluorescent sources. This is because LEDs emit light in a less diffuse pattern than conventional light sources. In contrast, standard incandescent bulbs and fluorescent lamps emit light in all directions, and much of the light output is absorbed inside the fixture or escapes in an unintended direction.
	Evaluate the cost. For comparison, a 75-watt incandescent light bulb typically produces about 1,000 lumens and costs less than $1. The problem is, it only lasts about 1,000 hours and only converts about 5% of the electricity it consumes into light (the rest is wasted as heat). A comparable CFL is 5 times more efficient, lasts 10,000 hours, and costs less than $5. So the conventional light sources cost around $1 per thousand lumens. Today's white LED products cost more than $50 per thousand lumens. But that's only part of the story. If you have lights that are on most of the time, or in a hard-to-reach area, LEDs could save significant maintenance and energy costs.
	Assess the need for unique LED features. In some applications, the extra durability that LEDs can provide is worth a higher purchase price. Outdoor pathway and step lighting is an example of a sensible application for today's white LEDs. They provide a small amount of light right where it's needed, and can be powered by solar cells, eliminating the need for running wire outdoors. They are also good for applications where vibration typically leads to early failure of conventional light sources. Being a solid-state device, LEDs are highly resistant to damage caused by vibration.
	Check the color of white LEDs. If you've ever tried to match white paint, you know there are actually many shades of white. There is a similar issue with lighting. White light varies from “warm” or more yellow/gold in appearance, to “cool” or more blue. Today's white LEDs are typically “cooler” and bluer, even compared to fluorescent sources. Further, there is a trade-off between efficiency and color. The “warmer” colored white LEDs provide less light per watt of electricity consumed, compared to the “cooler” white LEDs.
	Get a sample of the product if possible, so you can evaluate it yourself. Because of the current lack of standardized testing procedures and product specifications among LED product manufacturers, there is not yet a reliable way to compare product performance based on information provided by the manufacturer. The best way to assess a product you are considering is to ask for a sample.

Do LEDs really last 50,000 hours? 100,000 hours?
There is no simple answer to this question. Considering that 100,000 hours is more than 11 years of 24/7 operation, it's difficult to do life testing on these products! However, SSL researchers have developed ways to estimate LED life, based on shorter test periods. Unlike other light sources, LEDs don't typically “burn out;” they simply get dimmer over time. Although there is not yet an official industry standard defining “life” of an LED, the leading manufacturers report it as the point at which light output has reached 70% of initial light output. Using that definition, the best white LEDs have been found to have a useful life of around 35,000 hours (that's four years of continuous operation). LED lifetime is strongly dependent on appropriate electrical and thermal design of the fixture or system. While LEDs do not emit heat in the form of infrared radiation (IR), they do generate conductive heat that must be managed in order for the LED to maintain expected light output. Reputable LED manufacturers provide detailed electrical and thermal design specifications. When considering an LED-based fixture or system, it may be worthwhile to ask about the type of LEDs used and what provisions the product manufacturer has made for heat management.

Source: U.S. Department of Energy