LED Technology

1. Why LED ?

The only way to solve the problem of the increasing energy demand is to use lamps that meet the light requirements while using less power. This apparent paradox is solved nowadays by the new generation of high power LEDs. LEDs are the next revolution in light, since they provide superior light output with very low power consumption. This makes LED the most energy efficient light source.

The outstanding characteristics of LEDs include robustness, long functional life and a high luminous efficacy with high potential for further increase. White LED light is free of ultraviolet and infrared components and therefore has advantages from a conservation point of view. Daylight white LEDs offer higher efficiency, whereas warm white LEDs have better color rendition quality. LEDs are dimmable and offer high switching stability, making them ideally suited for lighting control and additive color mixing. Unlike other light sources, RGB LEDs do not produce any transmission losses through color filters. To exploit the luminous flux of the temperature sensitive LEDs to the full, good thermal management is indispensable.

One of the defining features of LEDs is that they emit light in a specific direction. Since directional lighting reduces the need for reflectors and diffusers that can trap light, well-designed LED fixtures can deliver light efficiently to the intended location. In contrast, fluorescent and "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 undercabinet fixtures, it is not uncommon for 40 to 50% of the total light output of fluorescent and incandescent lamps to be lost before it exits the fixture.

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LED chip -

source PHILIPS

2. How energy efficient are LEDs ?

Two aspects of energy efficiency are important to consider: the efficiency of the LED device itself (source efficacy); and how well the device and fixture work together in providing the necessary lighting (luminaire efficacy). How much electricity is used to provide the intended lighting service depends not only on the LED device, but on the lighting fixture design. Because they are sensitive to thermal and electrical conditions, LEDs must be carefully integrated into lighting fixtures. Poorly designed fixtures using even the best LEDs may be no more efficient than incandescent lighting. Conversely, a well-designed LED based refrigerated display case light that takes advantage of the directional nature of LEDs may use only about half the total watts of a linear fluorescent system to provide the necessary lighting, even though the LEDs have lower source efficacy than the linear fluorescent lamps.

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LED efficacy -

source DOE

3. What is an LED chip ?

LEDs are semiconductor diodes, electronic devices that permit current to flow in one direction. The diode is formed by bringing two doped semiconductors to form a PN junction, wherein the P side contains excess positive charge whereas the N side contains excess negative charge. When a forward voltage is applied to the PN junction electrons move from the N area toward the P area and hole move toward N area. Near the junction the electrons and holes combine releasing energy in the form of light when this happens.

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LED efficacy -

source DOE

4. What is an LED lighting system ?

An LED luminaire is an electromechanical system that includes in essential, light-emitting source, provisions for heat transfer, electrical control, optical conditioning, mechanical support and protection as well as aesthetic design elements. Because the LEDs themselves are expected to have long life, all other components must be equally long-lived or they will be the limiting factor of the system.

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Exploded view of an LED luminaire -

source CONTINENTAL LIGHTING

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LED luminaire components -

source PHILIPS

5. What is an LED driver?

An LED driver performs a function similar to a ballast for discharge lamps. Its primal function is to control the current flowing trough the LED. Latest generation of drivers will also monitor the temperature junction and will protect the LED in case this temperature goes beyond the safe zone for the LED.

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LED driver printed circuit - LED driver

6. How LEDs are affected by heat ?

High-power LEDs generate heat resulting from the resistance to the flow of current in the semiconductor. To ensure that the nominal luminous flux and functional life are attained, the identified maximum temperature of the PN junction must not ever be exceeded even in continuous operation. Therefore, thermal management plays a key role in the luminaire design.

The temperature in the junction will be determined by 3 factors:

  1. Ambient temperature: higher ambient temperatures leads to higher junction temperatures, which can increase the degradation rate of the LED junction element, causing the light output of and LED system to irreversibly decrease over the long term.
  2. Current trough the LED: the amount of heat generated is proportional to the current flowing trough the junction.
  3. Amount of heat sink material in and around the LED: although LED lighting systems do not produce significant amounts of radiated heat, LEDs still generate heat within the junction, which must be dissipated by convection and conduction. Any material that can conduct heat away from the LED can serve as heat sink, metals being the more widely used materials for such matter.
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Thermal image downlight -

source MARTINI

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Variation of efficacy with temperature junction -

source PHILIPS

7. Why do we require optics in LED lighting ?

Typically, an LED package uses an optically-clear material (encapsulant) to form a lens atop the LED die. In some cases this material forms the body of the whole device. This provides an optical path, a mechanical means to hold everything together, and protection for the wire bond to the die. As an optical element, the encapsulant should have a high index of refraction and good stability in the presence of humidity, high temperature, and high intensity light. Often additional collimation or tertiary optics is required to properly direct the light from LEDs or LED arrays.

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Optics narrow beam -

source MARTINI

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Collimating lens -

source ERCO

8. What is the life of LEDs ?

The true reliability and lifetime of LED lighting systems is currently unknown. Nowadays lumen maintenance values of LED devices are widely used as a proxy for the lifetime of an LED lighting system, which is misleading since light degradation or lumen maintenance is but one component of the reliability of a luminaire.

Whatever the stated lifetime of any lighting product, it is a statistic measure of the performance of a given design. For an individual LED package, lifetime has typically been considered to be the hours of operation at which the light output has fallen to 70% of its original value (L70). Lifetime is typically reported as the median time to failure of a population of diodes under normal conditions, called B50. In other words, after this period of time, half of units will fail due to low light output. While B50 represents a time interval, L70 is the lumen performance level defining a low-light failure. If a lifetime claim is made it is recommended to use L70 reference value for light output and reporting two time values as follows:

The lumen maintenance lifetime at which for xx% (e.g., 50%) of the product light output fall below 70% of the nominal initial value.

The electrical failure time for which yy% (e.g., 10%) of the population has experienced conventional lights-out failure.

The definition of lifetime does not include color shift. Depending upon the application, excessive color shift might also be deemed a failure.

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LM80 test results for lumen maintenance -

source OSRAM

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Useful life of HB LEDS at different operating temperatures -

source LRC