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Solid-State Lighting – Using Light Emitting Diodes

Common LED Types and Packages

Structure of a 5mm type LED. Structure of a high-brightness LED.
Source: Lumileds™

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.

Cree® XLamp 7090
Philips Lumileds
Luxeon® K2 Emitter

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. See "Color Quality" section 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.
Typical Incandescent Spectral Power Distribution SPX35 Tri-phosphor fluorescent.

Correlated Color Temperature
Correlated color temperature (CCT) describes the relative color appearance of a white light source, indicating whether it appears more yellow/gold or more blue, in terms of the range of available shades of white.
CCT is given in Kelvin (SI unit of absolute temperature) and refers to the appearance of a theoretical black body heated to high temperatures. As the black body 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 black body matches the color of the light source in question.

Color Rendering Index
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.

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, on a scale of 0 to 100, compared to a reference light source of similar color temperature.
The test procedure established by the International Commission on Illumination (CIE) 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.
The average "shift" in those eight color samples is reported as Ra or CRI. In addition to the eight color samples used by convention, some lighting manufacturers report an "R9" score, which indicates how well the light source renders a saturated deep red color.

Making White Light with LEDs
White light can be achieved with LEDs in two main ways: 1) phosphor conversion, in which a blue or near-ultraviolet (UV) chip is coated with phosphor(s) to emit white light; and 2) RGB systems, in which light from multiple monochromatic LEDs (red, green, and blue) is mixed, resulting in white light.
The phosphor conversion approach is most commonly based on a blue LED. When combined with a yellow phosphor (usually cerium-doped yttrium aluminum garnet or YAG:Ce), the light will appear white to the human eye. Research continues to improve the efficiency and color quality of phosphor conversion.

The RGB approach produces white light by mixing the three primary colors - red, green, and blue. The color quality of the resulting light can be enhanced by the addition of amber to "fill in" the yellow region of the spectrum.
Comparison of White Light LED Technologies

Each approach to producing white light with LEDs (described above) has certain advantages and disadvantages. The key trade-offs are among color quality, light output, luminous efficacy, and cost. The technology is changing rapidly due to intensive private and publicly funded research and development efforts in the U.S., Europe, and Asia. The primary pros and cons of each approach at the current level of technology development are outlined below.

Advantages Disadvantages
Phosphor conversion • Most mature technology
• High-volume manufacturing processes
• Relatively high luminous flux
• Relatively high efficacy
• Comparatively lower cost
• High CCT (cool/blue appearance)
• Warmer CCT may be less available or more expensive
• May have color variability in beam RGB
• Color flexibility, both in multi-color displays and different shades of white
• Individual colored LEDs respond differently to drive current, operating temperature, dimming, and operating time
• Controls needed for color consistency add expense
• Often have low CRI score, in spite of good color rendering

Most currently available white LED products are based on the blue LED + phosphor approach. A recent product (see photo) is based on violet LEDs with proprietary phosphors emphasizing color quality and consistency over time. Phosphor-converted chips are produced in large volumes and in various packages (light engines, arrays, etc.) that are integrated into lighting fixtures. RGB systems are more often custom designed for use in architectural settings.

Typical Luminous Efficacy and Color Characteristics of Current White LEDs
How do currently available white LEDs compare to traditional light sources in terms of color characteristics and luminous efficacy? Standard incandescent A-lamps provide about 15 lumens per watt (lm/W), with CCT of around 2700 K and CRI close to 100. ENERGY STAR-qualified compact fluorescent lamps (CFLs) produce about 50 lm/W at 2700-3000 K with a CRI of at least 80. Typical efficacies of currently available LED devices from the leading manufacturers are 80-130 lm/W. Improvements are announced by the industry regularly.

Luminous Efficacy
Is the total luminous flux emitted by the light source divided by the lamp wattage. 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 6000K, 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 90% efficient. So LED efficacy should be discounted by 10% 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) 35-60
Linear fluorescent (incl. ballast) 50-100
Metal halide (incl. ballast) 50-90
Cool white LED 5000K (incl. driver) 47-80*
Warm white LED 3300K (incl. driver) 25-50*
* As of October 2007. Now in 2010 the technology can produce up to 140 lm/W

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.

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 x 3.42 volts) = 38 lm/W
To include typical driver losses, multiply this figure by 85%, resulting in 32 lm/W. 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 very 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, troffers, 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.
LEDs emit light in a specific direction, reducing the need for reflectors and diffusers that can trap light, so well-designed fixtures, can deliver light more efficiently to the intended location.

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
• Defining LED Useful Life
• Measuring Light Source Life


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) 6,000-10,000
Metal halide 7,500-20,000
Linear fluorescent 20,000-30,000
High-Power White LED 45,000-60,000
*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
• Why Does Thermal Management Matter?
• What Determines Junction Temperature?


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.

Using LEDs to Their Best Advantage
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.
• 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.

What makes LEDs different from other light sources?
LEDs are semiconductor devices, while incandescent, fluorescent, and high-intensity discharge (HID) lamps are all based on glass enclosures containing a filament or electrodes, with fill gases and coatings of various types.
LED lighting starts with a tiny chip (most commonly about 1 mm2) comprising layers of semi-conducting material. LED packages may contain just one chip or multiple chips, mounted on heat-conducting material and usually enclosed in a lens or encapsulant. The resulting device, typically around 7 to 9 mm on a side, can produce 30 to 150 lumens each, and can be used separately or in arrays. LED devices are mounted on a circuit board and attached to a lighting fixture, architectural structure, or even a "light bulb" package.