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Terms

Lumen the SI unit o luminous fTe total amount o light emitted ba light source, without regard to ditionality, is given in lumens.

Luminous efcacy the total lumnous fux emitted by the light sourdivided by the lamp wattage; exprein lumens per watt (lm/W).

Luminaire efcacy the total lumfux emitted by the luminaire dividthe total power input to the luminaexpressed in lm/W.

Application efciency While this no standard denition o applicaeciency, we use the term here to dan important design consideration:the desired illuminance level and lig

quality or a given application shouachieved with the lowest practicable input. Light source directionality aintensity may result in higher applieciency even though luminous eis lower relative to other light sourc

Efciency or efcacy? Te termecacy normally is used wherethe input and output units dier.For example in lighting, we areconcerned with the amount o ligh(in lumens) produced by a certainamount o electricity (in watts). T

term eciency usually is dimensless. For example, lighting xture eciency is the ratio o the total lumeexiting the xture to the total lumeproduced by the light source. Ecis also used to discuss the broader ccept o using resources eciently.

Energy Efciency o White LEDsThe energy efciency o light-emitting diodes (LEDs) is expected to rival the most

efcient white light sources by 2010. But how energy efcient are LEDs right now?

This act sheet discusses various aspects o lighting energy efciency and the rapidly

evolving status o white LEDs.

Luminous Efcacy

Energy eciency o light sources is typically measured in lumens per watt (lm/W), meaningthe amount o light produced or each watt o electricity consumed. Tis is known asluminous ecacy. DOEs long-term research and development goal calls or white-lightLEDs producing 160 lm/W in cost-eective, market-ready systems by 2025. In the mean-time, how does the luminous ecacy o todays white LEDs compare to traditional lightsources? Currently, the most ecacious white LEDs can perorm similarly to fuorescentlamps. However, there are several important caveats, as explained below.

Color Quality

Te most ecacious LEDs have very high correlated color temperatures (CCs), oten

above 5000K, producing a cold bluish light. However, warm white LEDs (2600K to3500K) have improved signicantly, now approaching the ecacy o CFLs. In additionto warmer appearance, LED color rendering is also improving: leading warm white LEDsare now available with color rendering index (CRI) o 80, equivalent to CFLs.

Driver Losses

Fluorescent and high-intensity discharge (HID) light sources cannot unction without aballast, which provides a starting voltage and limits electrical current to the lamp. Similarly,LEDs require a power supply (commonly called a driver). Te power supply convertsline (AC) power to the appropriate DC voltage (typically between 2 and 4 volts DC orhigh-brightness LEDs) and current (generally 200-1000 milliamps, mA), and may alsoinclude supplementary electronics or dimming and/or color correction control.

Currently available LED drivers are typically about 85% ecient. So LED ecacy should

be discounted by 15% to account or the driver. For a rough comparison, the range oluminous ecacies or traditional and LED sources, including ballast and driver lossesas applicable, are shown below.

Light SourceTypical Luminous Efcacy Range in lm/W

(varies depending on wattage and lamp type)

Incandescent (no ballast) 10-18

Halogen (no ballast) 15-20

Compact fuorescent (CFL) (incl. ballast) 35-60

Linear fuorescent (incl. ballast) 50-100

Metal halide (incl. ballast) 50-90

Cool white LED >4000K (incl. driver) 60-92*

Warm white LED

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Research that Works

Application Efciency

Luminous ecacy is an important indicator oenergy eciency, but it doesnt tell the whole story,particularly with regard to directional light sources.

Due to the directional nature o their light emission,LEDs potentially have higher application eciencythan other light sources in certain lighting applica-tions. Fluorescent and standard bulb shapedincandescent lamps emit light in all directions.Much o the light produced by the lamp is lost

within the xture, reabsorbed by the lamp, orescapes rom the xture in a direction that is notuseul or the intended application. For manyxture types, including recessed downlights, troers,and under-cabinet xtures, it is not uncommon or40-50% o the total light output o the lamp(s) tobe lost beore it exits the xture.

LEDs emit light in a specic direction, reducingthe need or refectors and diusers that can traplight, so well-designed xtures, like the undercabinetlight shown below, can deliver light more ecientlyto the intended location.

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June 2009

Printed on 30% post-consumerrecycled paper.

Energy Efciency o White LEDs

A Strong Energy Portolio

or a Strong AmericaEnergy eciency and clean, renewaenergy will mean a stronger economa cleaner environment, and greaterenergy independence or America.Working with a wide array o state,

community, industry, and universitpartners, the U.S. Department oEnergys Oce o Energy Eciencyand Renewable Energy invests in adiverse portolio o energy technolo

For more inormation contact:EERE Inormation Center1-877-EERE-INF(1-877-337-3463)www.eere.energy.gov

Comparing LEDs to Traditional Light Sources

Energy eciency proponents are accustomed to comparing light sources on the basis oluminous ecacy. o compare LED sources to CFLs, or example, the most basic analysisshould compare lamp-ballast ecacy to LED+driver ecacy in lumens per watt. Datasheets or white LEDs rom the leading manuacturers will generally provide typicalluminous fux in lumens, test current (mA), orward voltage (V), and junction tempera-

ture (j), usually 25 degrees Celsius. o calculate lm/W, divide lumens by current timesvoltage. As an example, assume a device with typical fux o 45 lumens, operated at 350 mAand voltage o 3.42 V. Te luminous ecacy o the LED source would be:

45 lumens/(.35 amps 3.42 volts) = 38 lm/W

o include typical driver losses, multiply this gure by 85%, resulting in 32 lm/W.Because LED light output is sensitive to temperature, some manuacturers recommendde-rating luminous fux by 10% to account or thermal eects. In this example, account-ing or this thermal actor would result in a system ecacy o approximately 29 lm/W.However, actual thermal perormance depends on heat sink and xture design, so thisis only a very rough approximation. Accurate measurement can only be accomplished atthe luminaire level.

Te low-prole design of this undercabinet light takesadvantage of LED directionality to deliver light where itis needed. Available in 3W (shown), 6W, and 9W models. Photo credit: Finelite.

For Program Inormation

on the Web:www.ssl.energy.govDOE sponsors a comprehensiveprogram o SSL research,development, and commercializatio

For Program Inormation:Robert LingardPacic Northwest National LaboraPhone: (503) 417-7542E-mail: robert.lingard@pnl.gov

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