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Luminous Efficacy
Luminous Efficacy
Energy efficiency of light sources is typically measured in lumens per watt (lm/W), meaning the amount of light produced for each watt of electricity consumed by the light source. This is known as luminous efficacy. DOE's long-term research and development goal calls for white-light LEDs producing 160 lm/W in cost-effective, market-ready systems by 2025. In the meantime, how does the luminous efficacy of today's white LEDs compare to traditional light sources? Currently, the most efficacious white LEDs can perform similarly to fluorescent lamps. However, there are several important caveats, as explained below.
Color Quality
The most efficacious LEDs have very high correlated color temperatures (CCTs), often above 5000K, producing a "cold" bluish light. However, warm white LEDs (2600K to 3500K) have improved significantly, now approaching the efficacy of CFLs. In addition to warmer appearance, LED color rendering is also improving: leading warm white LEDs are now available with color rendering index (CRI) of 80, equivalent to CFLs.
Driver Losses

Fluorescent and high-intensity discharge (HID) light sources cannot function without a ballast, which provides a starting voltage and limits electrical current to the lamp. LEDs also require supplementary electronics, usually called drivers. The driver converts line power to the appropriate voltage (typically between 2 and 4 volts DC for high-brightness LEDs) and current (generally 200-1000 milliamps or mA), and may also include dimming and/or color correction controls.

Currently available LED drivers are typically about 85% efficient. So LED efficacy should be discounted by 15% to account for the driver. For a rough comparison, the range of luminous efficacies for traditional and LED sources, including ballast and driver losses as applicable, are shown below.

Light Source Typical Luminous Efficacy Range in lm/W
(varies depending on wattage and lamp type)
Incandescent (no ballast) 10-18
Halogen (no ballast) 15-20
Compact fluorescent (CFL)
(incl. ballast)
Linear fluorescent (incl. ballast) 50-100
Metal halide (incl. ballast) 50-90
Cool white LED 5000K (incl. driver) 47-64*
Warm white LED 3300K (incl. driver) 25-44*
* As of October 2007

Thermal Effects

The luminous flux figures cited by LED manufacturers are based on an LED junction temperature (Tj) of 25°C. LEDs are tested during manufacturing under conditions that differ from actual operation in a fixture or system. In general, luminous flux is measured under instantaneous operation (perhaps a 20 millisecond pulse) in open air. Tj will always be higher when operated under constant current in a fixture or system. LEDs in a well-designed luminaire with adequate heat sinking will produce 10%-15% less light than indicated by the "typical luminous flux" rating.


Efficiency or efficacy?
The term "efficacy" normally is used where the input and output units differ. For example in lighting, we are concerned with the amount of light (in lumens) produced by a certain amount of electricity (in watts). The term "efficiency" usually is dimensionless. For example, lighting fixture efficiency is characterized as a ratio of the total lumens exiting the fixture to the total lumens produced by the light source. "Efficiency" is also used to discuss the broader concept of using resources efficiently.

The SI unit of luminous flux. The total amount of light emitted by a light source, without regard to directionality, is given in lumens.

Luminous efficacy:
The total luminous flux emitted by the light source divided by the lamp wattage; expressed in lumens per watt (lm/W).

Luminaire efficacy:
The total luminous flux emitted by the luminaire divided by the total power input to the luminaire, expressed in lm/W.

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