Luminous efficacy is the most commonly used measure of the energy efficiency of a light source. It is stated in lumens per watt (lm/W), indicating the amount of light a light source produces for each watt of electricity consumed. For white high-brightness LEDs, luminous efficacy published by LED manufacturers typically refers to the LED chip only, and doesn't include driver losses. (Source: DOE)

Correlated color temperature (CCT) is the measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow/gold/orange or more blue, in terms of the range of available shades of "white." CCT is given in kelvins (unit of absolute temperature). (Source: DOE)

Color rendering index (CRI) indicates how well a light source renders colors of people and objects, compared to a reference source. (Source: DOE)

How do building owners, facility managers, and lighting specifiers choose lighting products? Purchase price and operating costs (energy and maintenance) are usually the top concerns but a host of other aspects may come into play, depending on the application. Here are some unique LED characteristics:

Directional light emission – directing light where it is needed.

Size advantage – can be very compact and low-profile.

Breakage resistance – no breakable glass or filaments.

Cold temperature operation – performance improves in the cold.

Instant on – require no "warm up" time.

Rapid cycling capability – lifetime not affected by frequent switching.

Controllability – compatible with electronic controls to change light levels and color characteristics.

No IR or UV emissions - LEDs intended for lighting do not emit infrared or ultraviolet radiation.

(Source: DOE)

LEDs differ from traditional light sources in the way they produce light. In an incandescent lamp, a tungsten filament is heated by electric current until it glows or emits light. In a fluorescent lamp, an electric arc excites mercury atoms, which emit ultraviolet (UV) radiation. After striking the phosphor coating on the inside of glass tubes, the UV radiation is converted and emitted as visible light. An LED, in contrast, is a semiconductor diode. It consists of a chip of semiconducting material treated to create a structure called a p-n (positive-negative) junction. When connected to a power source, current flows from the p-side or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers (electrons and electron holes) flow into the junction from electrodes. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon (light). The specific wavelength or color emitted by the LED depends on the materials used to make the diode. (Source: DOE)

One of the main "selling points" of LEDs is their potentially very long life. Do they really last 50,000 hours or even 100,000 hours? It depends on LED quality, system design, operating environment, and other factors. (Source: DOE)

All types of electric light sources experience lumen depreciation, defined as the decrease in lumen output that occurs as a lamp is operated. (Source: DOE)

To provide an appropriate measure of useful life of an LED, a level of acceptable lumen depreciation must be chosen. At what point is the light level no longer meeting the needs of the application? The answer may differ depending on the application of the product. For a common application such as general lighting in an office environment, research has shown that the majority of occupants in a space will accept light level reductions of up to 30% with little notice, particularly if the reduction is gradual. Therefore a level of 70% of initial light level could be considered an appropriate threshold of useful life for general lighting. Based on this research, the Alliance for Solid State Illumination Systems and Technologies (ASSIST), a group led by the Lighting Research Center (LRC), recommends defining useful life as the point at which light output has declined to 70% of initial lumens (abbreviated as L70) for general lighting and 50% (L50) for LEDs used for decorative purposes. For some applications, a level higher than 70% may be required. (Source: DOE)

Life testing for LEDs is impractical due to the long expected lifetimes. Switching is not a determining factor in LED life, so there is no need for the on-off cycling used with other light sources. But even with 24/7 operation, testing an LED for 50,000 hours would take 5.7 years. Because the technology continues to develop and evolve so quickly, products would be obsolete by the time they finished life testing. A life testing procedure for LEDs is currently under development by the Illuminating Engineering Society of North America (IESNA). The proposed method is based on the idea of "useful life," i.e., the operating time in hours at which the device's light output has declined to a level deemed to no longer meet the needs of the application. For example, for general ambient lighting, the level might be set at 70% of initial lumens. Useful life would be stated as the average number of hours that the LED would operate before depreciating to 70% of initial lumens. The leading LED manufacturers have begun using the L70 language, stating that their white LEDs "are projected" to have lumen maintenance of greater than 70% on average after 50,000 hours when used in accordance with published guidelines. Electrical and thermal design of the LED system or fixture determine how long LEDs will last and how much light they will provide. Driving the LED at higher than rated current will increase relative light output but decrease useful life. Operating the LED at higher than design temperature will also decrease useful life significantly. (Source: DOE)

Based on how long a fixture is illuminated per day, here's what 50,000 hours works out to:

Hours of operation = years of life

24 hours a day = 5.7 years

18 hours per day = 7.6 years

12 hours per day = 11.4 years

8 hours per day = 17.1 years

LEDs won't burn your hand like some light sources, but they do produce heat. In fact, thermal management is arguably the most important aspect of successful LED system design. Excess heat directly affects both short-term and long-term LED performance. The short-term (reversible) effects are color shift and reduced light output while the long-term effect is accelerated lumen depreciation and thus shortened useful life. Continuous operation at elevated temperature dramatically accelerates lumen depreciation resulting in shortened useful life. (Source: DOE)

LED technology is rapidly becoming competitive with high-intensity discharge light sources for outdoor area lighting. Lighting of outdoor areas including streets, roadways, parking lots, and pedestrian areas is currently dominated by metal halide (MH) and high-pressure sodium (HPS) sources. These relatively energy-efficient light sources have been in use for many years and have well-understood performance characteristics. Recent advances in LED technology have resulted in a new option for outdoor area lighting, with several potential advantages over MH and HPS sources. Well-designed LED outdoor luminaires can provide the required surface illuminance using less energy and with improved uniformity, compared to HID sources. LED luminaires may also have significantly longer life (50,000 hours or more, compared to 15,000 to 35,000 hours) with better lumen maintenance. Other LED advantages include: they contain no mercury, lead, or other known disposal hazards; and they come on instantly without run-up time or restrike delay. Further, while MH and HPS technologies continue to improve incrementally, LED technology is improving very rapidly in terms of luminous efficacy, color quality, optical design, thermal management, and cost. (Source: DOE)

As a new technology, LED luminaires currently cost more to purchase than traditional fixtures lamped with commodity-grade HPS or MH light sources. The reduction in relamping cost and potential power savings with LEDs may reduce the overall lifecycle cost. Economic evaluation of LED outdoor luminaires is highly site-specific, depending on variables including electric demand (kW) and consumption (kWh) rates; labor costs, which may be bundled in a broader maintenance contract for the site; and other options available for the site. LED outdoor lighting demonstrations documented by DOE to date have shown estimated paybacks in as little as three years.

In some cases, LED technology may address new requirements that change the comparison to traditional sources. For example, some jurisdictions have implemented mandatory reductions in nighttime illumination. LED luminaires can be designed with control circuits that reduce the light output by half after curfew, without affecting the uniformity of light on the street or parking lot. Compare this to a design where a single, high-wattage HID luminaire is replaced with two lower-wattage luminaires on the same pole, so that half the fixtures can be extinguished at curfew without affecting the light distribution. (Source: DOE)