LED Basics

How LEDs Work

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.

Red LEDs are based on aluminum gallium arsenide (AlGaAs). Blue LEDs are made from indium gallium nitride (InGaN) and green from aluminum gallium phosphide (AlGaP). "White" light is created by combining the light from red, green, and blue (RGB) LEDs or by coating a blue LED with yellow phosphor.

LED Basics - Terms

Solid-state lighting (SSL) technology uses semi-conducting materials to convert electricity into light. SSL is an umbrella term encompassing both light-emitting diodes (LEDs) and organic light emitting diodes (OLEDs).

Light-emitting diodes (LEDs) are based on inorganic (non-carbon based) materials. An LED is a semi-conducting device that produces light when an electrical current flows through it. LEDs were first developed in the 1960s but were used only in indicator applications until recently.

Organic light-emitting diodes (OLEDs) are based on organic (carbon based) materials. In contrast to LEDs, which are small point sources, OLEDs are made in sheets which provide a diffuse area light source. OLED technology is developing rapidly and is increasingly used in display applications such as cell phones and PDA screens. However, OLEDs are still some years away from becoming a practical general illumination source. Additional advancements are needed in light output, color, efficiency, cost, and lifetime.

General illumination is a term used to distinguish between lighting that illuminates tasks, spaces, or objects from lighting used in indicator or purely decorative applications. In most cases, general illumination is provided by white light sources, including incandescent, fluorescent, high-intensity discharge sources, and white LEDs. Lighting used for indication or decoration is often monochromatic, as in traffic lights, exit signs, vehicle brake lights, signage, and holiday lights.

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.

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).

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

RGB stands for red, green, and blue, the three primary colors of light. When the primaries are mixed, the resulting light appears white to the human eye. Mixing the light from red, green, and blue LEDs is one way to produce white light. The other approach is known as phosphor conversion [see below].

Phosphor conversion is a method used to generate white light with LEDs. A blue or near-ultraviolet LED is coated with a yellow or multichromatic phosphor, resulting in white light.

Common LED Types and Packages

LEDs come in two basic categories: Low power LEDs commonly come in 5 mm size, although they are also available in 3 mm and 8 mm sizes. These are fractional wattage devices, typically 0.1 watt, operate at low current (~20 milliamps) and low voltage (3.2 volts DC), and produce a small amount of light, perhaps 2 to 4 lumens. High power LEDs come in 1-3 watt packages. They are driven at much higher current, typically 350, 700, or 1000 mA, and—with current technology—can produce 40-80 lumens per 1-watt package. High power LEDs come in many different shapes and sizes. Some current products from the leading LED manufacturers are shown below.			Structure of a 5mm type LED. Source: Lumileds™ Structure of a high-brightness LED. Source: Lumileds™

Cree® XLamp 7090

Philips Lumileds Luxeon® K2 Emitter

Osram® OSTAR Lighting

Energy Efficiency of White LEDs The energy efficiency of LEDs is expected to rival the most efficient white light sources by 2010. But how energy efficient are LEDs right now? This section discusses various aspects of lighting energy efficiency and the rapidly evolving status of white LEDs. Click on the topics below for more information. Luminous Efficacy Comparing LEDs to Traditional Light Sources Application Efficiency

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 best white LEDs approach the efficacy of compact fluorescent lamps (CFLs). However, there are several important caveats, as explained below.

Color Quality
To date, LED luminous efficacy similar to that of CFLs has been achievable only with higher color temperature products, which produce a "cool" or bluish-toned light and relatively low color rendering index (CRI) in the 70s. LEDs with warmer color appearance and higher CRI are only marginally more efficacious than incandescent sources. However, this is changing rapidly, with new performance improvements being announced regularly by the industry. For more detail, see Color Quality.

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.

Thermal Effects
The luminous flux figures cited by LED manufacturers assume 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. Well-designed systems with adequate heat sinking will maintain Tj well below the manufacturer's rated maximum temperature (typically 125°C)

Comparing LEDs to Traditional Light Sources Energy efficiency proponents are accustomed to comparing light sources on the basis of luminous efficacy. To compare LED sources to CFLs, for example, the most basic analysis should compare lamp-ballast efficacy to LED+driver efficacy in lumens per watt. Data sheets for white LEDs from the leading manufacturers will generally provide "typical" luminous flux in lumens, test current (mA), forward voltage (V), and junction temperature (Tj), usually 25 degrees Celsius. To calculate lm/W, divide lumens by current times voltage. As an example, assume a device with typical flux of 45 lumens, operated at 350 mA and voltage of 3.42 V. The luminous efficacy of the LED source would be: 45 lumens/(.35 amps * 3.42 volts) = 38 lm/W To include typical driver losses, multiply this figure by 85%, resulting in 32 lm/W. For a rough comparison, the range of system (lamp and ballast or LED and driver) efficacies for traditional and LED sources are shown below. Light Source Typical System Efficacy Range in lm/w (varies depending on wattage and lamp type) Incandescent 10-18 Halogen Incandescent 15-20 Compact Fluorescent (CFL) 35-60 Linear Fluorescent (T8, T5) 50-100 Metal Halide 50-90 White LED 5000K 45-59* Warm LED 3300K 22-37*   *Current as of October 2006.

Because LED light output is sensitive to temperature, some manufacturers recommend de-rating luminous flux by 10% to account for thermal effects. In this example, accounting for this thermal factor would result in a system efficacy of approximately 29 lm/W. However, actual thermal performance depends on heat sink and fixture design, so this is only a rough approximation. Accurate measurement can only be accomplished at the luminaire level.

 

Application Efficiency

Luminous efficacy is an important indicator of energy efficiency, but it doesn't tell the whole story, particularly with regard to directional light sources.

Due to the directional nature of their light emission, LEDs potentially have higher application efficiency than other light sources in certain lighting applications. Fluorescent and standard "bulb" shaped incandescent lamps emit light in all directions. Much of the light produced by the lamp is lost within the fixture, reabsorbed by the lamp, or escapes from the fixture in a direction that is not useful for the intended application. For many fixture types, including recessed downlights and under-cabinet fixtures, it is not uncommon for 40-50% of the total light output of the lamp(s) to be lost before it exits the fixture.

Light emissions from LEDs are inherently directional, reducing the need for reflectors and diffusers that can trap light, so well-designed fixtures and systems using LEDs can potentially deliver light more efficiently to the intended location.

For example, several manufacturers have introduced LED systems for lighting refrigerated display cases in grocery stores. These products are currently based on white LEDs with lower luminous efficacy than the fluorescent lamps they are designed to replace. But because the system design takes advantage of the directional nature of LEDs and their especially good performance under low temperatures, they are demonstrating energy savings of 50% or more compared to standard fluorescent case lights.

 

Color Quality of White LEDs

Color quality is one of the key challenges facing the market introduction of light-emitting diodes (LEDs) as a general light source. This section reviews the basics regarding light and color, and summarizes the most important color issues related to white light LEDs. Click on the links below for more information.

 

Light and Color Basics

Light-emitting diodes (LEDs) differ from other light sources, such as incandescent and fluorescent lamps, in the way they generate white light. We are accustomed to lamps that emit white light. But what does that really mean? What appears to our eyes as "white" is actually a mix of different wavelengths in the visible portion of the electromagnetic spectrum. The diagram below illustrates visible light as one small portion of the overall electromagnetic spectrum. Electromagnetic radiation in wavelengths from about 380 to 770 nanometers is visible to the human eye.



Light sources such as incandescent, fluorescent, and high-intensity discharge lamps radiate across the visible spectrum, but with varying intensity in the different wavelengths. This is what determines both the color appearance of the light source and how well it renders colors of objects and people under the light source. The spectral power distribution (SPD) for a given light source shows the relative radiant power emitted by the light source at each wavelength in the visible spectrum. Incandescent sources have a continuous SPD, but relative power is low in the blue and green regions. The typically "warm" color appearance of incandescent lamps is due to the relatively high emissions in the orange and red regions of the spectrum. SPDs for fluorescent and HID sources are provided for comparison. These sources have "spikes" of relatively higher intensity at certain wavelengths, but still appear white to our eyes. Unlike incandescent, fluorescent and HID sources, LEDs are near-monochromatic light sources. An individual LED chip emits light in a specific wavelength. This is why LEDs are comparatively so efficient for colored light applications. In traffic lights, for example, LEDs are rapidly replacing the old incandescent + colored filter systems. Using colored filters or lenses is actually a very inefficient way to achieve colored light. For example, a red filter on an incandescent lamp can block 90 percent of the visible light from the lamp. Red LEDs provide the same amount of light for about one-tenth the power (12 watts compared to 120+ watts) and last many times longer. However, to be used as a general light source, "white" light is needed. LEDs are not inherently white light sources.

Incandescent Spectral Power Distribution (SPD). GE Lighting SPX35 Tri-phosphor fluorescent. GE Lighting. ConstantColor® Ceramic Metal Halide. GE Lighting.

 

 

Correlated Color Temperature

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) and refers to the appearance of a theoretical black body (visualize a chunk of metal) when it's heated to high temperatures. As the metal gets hotter, it turns red, orange, yellow, white, and finally blue. The CCT of a light source is the temperature (in K) at which the heated theoretical black body matches the color of the light source in question. Currently available white LEDs typically have higher CCTs, often 5000 K or higher, which is at the cooler/bluer end of the spectrum. The leading manufacturers also offer "warm white" LEDs, which more closely approximate incandescent light. However, there is a significant trade-off between color temperature and luminous efficacy: LEDs with warmer color appearance have about half the luminous efficacy of the cooler versions.

 

Color Rendering Index

Another important measure of color quality used by the lighting industry is the color rendering index (CRI). CRI indicates how well a light source renders colors compared to a reference source. The reference source for "warm" light sources (those with CCT less than 5000K) is a reference incandescent lamp. For higher CCT sources, the reference is (a specifically defined spectrum of) daylight. CRI comparisons across different light sources are meaningful only if the sources have similar CCT. CRI is calculated according to a test procedure established by the International Lighting Commission (Commission Internationale de l'Eclairage or CIE). It involves measuring the extent to which a series of eight standardized color samples differ in appearance when illuminated under a given light source, relative to the reference source appropriate for the specific color temperature.		1	2	3	4
		5	6	7	8
		Eight standard color samples used in the test-color method for measuring and specifying the color rendering properties of light sources. Adapted from IESNA Handbook. Reprinted courtesy of the Illuminating Engineering Society of North America.
The average "shift" in those eight color samples is reported as Ra or what is commonly called CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an "R9" score, which refers to how well the light source renders a saturated deep red color, relative to an incandescent reference source.

 

Lifetime of White LEDs

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. This section provides information on lumen depreciation and life measurement for LEDs compared to other light sources.

Lumen Depreciation

All types of electric light sources experience lumen depreciation, defined as the decrease in lumen output that occurs as a lamp is operated. The causes of lumen depreciation in incandescent lamps are depletion of the filament over time and the accumulation of evaporated tungsten particles on the bulb wall. This typically results in 10% to 15% depreciation compared to initial lumen output over the 1,000 hour life of an incandescent lamp.

In fluorescent lamps, the causes of lumen depreciation are photochemical degradation of the phosphor coating and the glass tube, and the accumulation of light-absorbing deposits within the lamp over time. Specific lamp lumen depreciation curves are provided by the lamp manufacturers. Current high quality fluorescent lamps using rare earth phosphors will lose only 5-10% of initial lumens at 20,000 hours of operation. Compact fluorescent lamps (CFLs) experience higher lumen depreciation compared to linear sources, but higher quality models generally lose no more than 20% of initial lumens over their 10,000 hour life.



Lumen depreciation in LEDs varies depending on package and system design. The primary cause of lumen depreciation is heat generated at the LED junction. LEDs do not emit heat as infrared radiation (IR) like other light sources, so the heat must be removed from the device by conduction or convection. If the LED system design has inadequate heat sinking or other means of removing the heat, the device temperature will rise, resulting in lower light output. Clouding of the epoxy encapsulant used to cover some LED chips also results in decreased lumens making it out of the device. Newer high-power LED devices use silicone as an encapsulant, which prevents this problem. LEDs continue to operate even after their light output has decreased to very low levels. This becomes the important factor in determining the effective useful life of the LED.

 

Defining LED Useful Life

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.

 

Measuring Light Source Life

We've all heard the small "pop" as an incandescent lamp fails. It's the sound of the tungsten filament finally breaking as the electric current hits it. This makes it easy to recognize the end of life for an incandescent light source. With fluorescent lamps, end of life may involve flickering or the lamp may simply not activate when the switch is turned on. With LEDs, outright failure of the device is less likely, although it can happen due to component failure. Instead, the LED's light output slowly declines over time.

The lifetimes of traditional light sources are rated through established test procedures. The life testing procedure for compact fluorescent lamps, for example, is published by the Illuminating Engineering Society (IES) as LM-65. It calls for a statistically valid sample of lamps to be tested at an ambient temperature of 25 degrees Celsius using an operating cycle of 3 hours ON and 20 minutes OFF. The point at which half the lamps in the sample have failed is the rated average life for that lamp. For 10,000 hour lamps, this process takes about 15 months.

How are LED lifetimes rated? 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.

How do the lifetime projections for LEDs compare to traditional light sources?

Light Source	Range of Typical Rated Life (hours)* (varies by specific lamp type)	Estimated Useful Life (L70)
Incandescent	750-2,000
Halogen incandescent	3,000-4,000
Compact fluorescent (CFL)	8,000-10,000
Metal halide	7,500-20,000
Linear fluorescent	20,000-30,000
High-Power White LED		35,000-50,000


*Source: lamp manufacturer data.

 

Thermal Management of White LEDs

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. This section reviews the role of heat in LED performance and methods for managing it.

 

Comparison of Power Conversion of White Light Sources

All light sources convert electric power into radiant energy and heat in various proportions. Incandescent lamps emit primarily infrared (IR), with a small amount of visible light. Fluorescent and metal halide sources convert a higher proportion of the energy into visible light, but also emit IR, ultraviolet (UV), and heat. LEDs generate little or no IR or UV, but convert only 15%-25% of the power into visible light; the remainder is converted to heat that must be conducted from the LED die to the underlying circuit board and heat sinks, housings, or luminaire frame elements. The table below shows the approximate proportions in which each watt of input power is converted to heat and radiant energy (including visible light) for various white light sources.

Power Conversion for "White" Light Sources
	Incandescent† (60W)	Fluorescent† (Typical linear CW)	Metal Halide‡	LED*
Visible Light	8%	21%	27%	15-25%
IR	73%	37%	17%	~0%
UV	0%	0%	19%	0%
Total Radiant Energy	81%	58%	63%	15-25%
Heat (Conduction + Convection)	19%	42%	37%	75-85%
Total	100%	100%	100%	100%

† IESNA Handbook ‡ Osram Sylvania

* Varies depending on LED efficacy. This range represents best currently available technology in color temperatures from warm to cool. DOE's SSL Multi-Year Program Plan (March 2006) calls for increasing extraction efficiency to more than 50% by 2012.

 

Why Does Thermal Management Matter?

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.

The light output of different colored LEDs responds differently to temperature changes, with amber and red the most sensitive, and blue the least. (See graph below.) These unique temperature response rates can result in noticeable color shifts in RGB-based white light systems if operating Tj differs from the design parameters. LED manufacturers test and sort (or "bin") their products for luminous flux and color based on a 15-20 millisecond power pulse, at a fixed Tj of 25°C (77°F). Under constant current operation at room temperatures and with engineered heat mitigation mechanisms, Tj is typically 60°C or greater. Therefore white LEDs will provide at least 10% less light than the manufacturer's rating, and the reduction in light output for products with inadequate thermal design can be significantly higher.

Continuous operation at elevated temperature dramatically accelerates lumen depreciation resulting in shortened useful life. The chart below shows the light output over time (experimental data to 10,000 hours and extrapolation beyond) for two identical LEDs driven at the same current but with an 11°C difference in Tj. Estimated useful life (defined as 70% of initial lumen output) decreased from ~37,000 hours to ~16,000 hours, a 57% reduction, with the 11°C temperature increase.

However, the industry continues to improve the durability of LEDs at higher operating temperatures. The Luxeon K2, for example, claims 70% lumen maintenance for 50,000 hours at drive currents up to 1000 mA and Tj at or below 120°C. (Luxeon K2 Emitter Datasheet DS51, dated 5/06)



What Determines Junction Temperature?

Three things affect the junction temperature of an LED: drive current, thermal path, and ambient temperature. In general, the higher the drive current, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life, and color. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings.

The typical high-flux LED system is comprised of an emitter, a metal-core printed circuit board (MCPCB), and some form of external heat sink. The emitter houses the die, optics, encapsulant, and heat sink slug (used to draw heat away from the die) and is soldered to the MCPCB. The MCPCB is a special form of circuit board with a dielectric layer (non-conductor of current) bonded to a metal substrate (usually aluminum). The MCPCB is then mechanically attached to an external heat sink which can be a dedicated device integrated into the design of the luminaire or, in some cases, the chassis of the luminaire itself. The size of the heat sink is dependent upon the amount of heat to be dissipated and the material's thermal properties.

Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. Successful products will use superior heat sink designs to dissipate heat, and minimize Tj. Keeping the Tj as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs.

 

Recessed Downlights

Recessed downlights are very common in both residential and commercial buildings. Is this a good application for LEDs? This section explores issues unique to this type of luminaire, and the potential for use of LEDs in downlights.

 

Comparison to Traditional Light Sources

Recessed downlights are the most common installed luminaire type in residential new construction. Downlights are used for general ambient lighting in kitchens, hallways, bathrooms, and other areas of the home. Downlights with small apertures and more directional lensing and baffling are also used for wall-washing and accent lighting. In commercial settings, a wide variety of downlight types, sizes, and finishes are used in lobbies, perimeter areas, hallways, and restrooms.

The light output of a recessed downlight is a function of the lumens produced by the lamp and the luminaire efficiency. Reflector-style lamps are specially shaped and coated to emit light in a defined cone, while "A" style incandescent lamps and CFLs emit light in all directions, leading to significant light loss within the luminaire. Downlights using non-reflector lamps are typically only 50% efficient, meaning about half the light produced by the lamp is wasted inside the fixture. LEDs are more directional, but can they provide enough light? For comparison, the table below shows typical light output and efficiency of residential-style fluorescent and incandescent recessed downlights and an LED downlight.

Examples of Recessed Downlight Performance Using Different Light Sources
		Fluorescent*		Incandescent*		LED**
		26W pin-based CFL	15W R-30 CFL Edison base	65W R-30	100W A-19	LED 15W Downlight
Lamp	Related lamp lumens	1800	750	755	1700	unknown
	Lamp wattage (nominal W)	26	15	65	100	9x1W LEDs
	Lamp efficacy (lm/W)	70	50	12	17	45
Luminaire	Luminaire effciency	50%	90%	90%	50%	unknown
	Delivered light output (lumens), initial	900	675	680	850	300
	Luminaire wattage (nominal W)	27	15	65	100	15
	Luminaire efficacy (lm/W)	33	45	10	9	20

 

*Based on photometric data for commonly available products. Actual product performance depends on reflectors, trims, lamp positioning, and other factors. Assumptions available from PNNL.
**Based on one commercially-available product tested. Other LED-based downlights may differ. Lamp efficacy for the LED product refers to the manufacturer listed "typical luminous flux" of the LEDs used. Luminous flux of the 9-LED array is not known.

Even though the 26W CFL is the most efficacious light source listed, the 15W reflector CFL provides higher luminaire efficacy, i.e., total lumens out of the fixture per watt consumed. The 15W LED downlight provides less than half the delivered light output of the 15W reflector CFL. As LED technology matures, this performance is expected to improve.

Potential for Use of LEDs in Downlights

Given the prevalence of downlights in both residential and commercial buildings, potential energy savings from high-performing, energy-efficient downlights would be significant. The high-temperature environment has plagued attempts to use CFLs in downlights, although recent developments in reflector CFLs are promising. Would LEDs do better?

The inherent directionality of LEDs is a potential advantage for their use in downlighting applications. If designed effectively, LED downlights could essentially eliminate luminaire light losses. LEDs also work with standard wall-mounted dimmers, unlike CFLs.

However, to approach the light output typically expected for downlights requires multiple LEDs to be grouped together. Clustering LEDs in the relatively small downlight package generates considerable heat. Actual light output depends on good thermal management in the fixture. If the heat is not adequately managed, LED device temperature will rise, light output will fall, and the useful life of the fixture will be disappointingly short. This concern is particularly important in residential insulated ceiling applications.

LED downlights available to date provide about half the delivered light output of downlights using 65W R incandescent or 15W reflector CFLs. However, as LED technology and product designs mature, LEDs are expected to compete favorably with traditional light sources in downlighting applications.

Advantages and Disadvantages of Recessed Downlight Lamping Options

Comparison of Recessed Downlight Lamping Options
	Advantages	Disadvantages
Incandescant Reflector	Dimmable High color quality Low lamp cost	High wattage Short life (2000 hrs) Heat increases cooling load
CFL Reflector	High efficacy Long life (6000-8000 hrs)	Few dimmable products More expensive than incandescent
CFL Pin-based	High efficacy Long life (10000 hrs)	Few dimmable products More expensive than incandescent Replacement lamps can be difficult to find
LED Downlight	Dimmable Potentially long life Lower wattage than incandescent Directional light source	Relatively low light output* Expensive to purchase* Very sensitive to high-temperature environment* Replacement lamps not available

 

*Listed disadvantages reflect current status of LED technology (Nov 2006). Expected technology improvements in coming years will mitigate and possibly eliminate these disadvantages.

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