Back in the 80’s when Light-Emitting Diode (LED) technology was introduced in emergency lighting the event was considered a technological break-through. Compared to the incandescent lamps, the LEDs needed only a fraction of the bulb electrical power and demonstrated an extremely low failure rate, translated to a Mean Time Between Failure (MTBF) of 200,000 hours or more. From this perspective, the LED-based exit signs would virtually eliminate the need and cost to replace failed lamps (re-lamping) as required periodically for the incandescent exits. Following the estimates of LED manufactures, most emergency lighting OEMs were proud to announce in their catalogues the extremely long life of the LEDs, in the range of 20 – 25 years.
These findings still hold today, with some necessary updates. The main question is: how do we define the life of an LED?
In the traditional approach, LED life is associated with catastrophic failure of the component: the LED was considered “failed” when it ceased emitting any light, like an incandescent bulb with the filament burned out.
How does the lamp life test work for incandescent lamps? A sample quantity of lamps is powered with nominal voltage at the same time, at room temperature. The lamp average life (Mean time between failures – MTBF) is the duration until half the lamps under test fail. For example, if we power-up a test sample of lamps rated “1000 hours”, approximately half of them will fail after 1000 hours. Obviously, some lamps will last longer (say, 1500 –2000 hours) while some others may fail very early in the test.
Based on this definition LED life can - indeed - be measured in the tens of years, but is this all we need? As LED applications gained popularity, a particular trend was observed and identified as critical for LED performance: the gradual loss of light output over time. In real life, LEDs don’t burn (fail instantly) like incandescent, filament lamps do; instead they slowly dim, to a point where the light level becomes insufficient for the application. Depending on several factors (technology ingredients, LED design, application design, environment) the light level could depreciate anywhere from 15% to more than 50% per year of continuous use. This trend is relatively easy to track on the Gallium-Aluminum-Arsenide (GaAlAs, or GaAs) red LEDs of early technology which are still used due to their low cost.
Life Degradation Phenomenon
In recent times, sustained R&D efforts in the optoelectronics industry have lead to a new development in LED manufacturing: the AlInGaP technology. Based on a compound of four elements, Aluminum, Indium, Gallium and Phosphorus, the AlInGaP LED has a better light efficacy, with the Lumen/Watt ratio 300% to 500% higher than the traditional GaAs LED. The maintained light output of the LED is also improved, due to semiconductor materials less degraded by aging and temperature.
Increasing market awareness has motivated the LED manufacturers to publish test results and statistical data related to the light degradation phenomenon. Among other data publicly available, an article by Agilent Technologies (Application Brief I-018) describes the results of a High-Temperature Operating Life (HTOL) test carried out on AlInGaP LEDs over a 16,000-hour time frame. Based on the test results, the authors predict that the LEDs exposed to 100,000 hours (11.4 years) of continuous use at an ambient temperature of +55ºC will exhibit an overall light output degradation of about 27%. This translates to an annual rate 10 times lower than the average light loss of GaAs LEDs.
Such test results and projections are very useful: they enable the application engineers to design LED signage equipment (exit signs, traffic lights, etc.) with built-in performance to meet the lighting standards for ten years and more without need for re-lamping.
LED Life: A New Definition
In parallel with the LED technology development, the specialists in the lighting industry have started to consider other definitions for LED life, more appropriate for LED usage in lighting applications. By comparison with traditional sources such as fluorescent and HID lamps, at least two parameters were considered critical: the functional time until the light output diminishes to a level noticeable by the human eye (e.g. 50% light loss), and the chromatic shift (noticeable changes from the original color) of the emitted light.
A notable definition of LED life was formulated by the Association for Solid-State Illumination Systems and Technologies (ASSIST). Founded by the Lighting Research Center of the Rensselaer Polytechnic Institute, Troy, NY and sponsored by LED manufacturers like Nichia, Gelcore, Osram Sylvania and Philips lighting, the association recommends the following in its publication in February 2005:
According to the authors, for general lighting applications 70% lumen maintenance (i.e. 30% light loss) is close to the threshold of human eye detection of reduction in light output and is considered acceptable by most viewers. For other applications – such as decorative lighting – reductions to 50% might still be acceptable. Both these criteria are extremely useful tools for lighting specialists and designers.
Other LED Colors: Amber, Yellow, Green, Blue
The amber (orange) and yellow LEDs can also be manufactured in the AlInGaP technology, with operating life for each in the same range as for the red: 100,000 hours or more.
Green and Blue LEDs are based on different technologies using a combination of Indium and Gallium Nitride (InGaN), or Aluminum, Indium and Gallium Nitride (AlInGaN). So far, their operating life is lower than for AlInGaP, in the range of 25,000 to 50,000 hours, when built in the 5mm package and powered with a current of 20 mA.
White LEDs: Why So Expensive?
An old friend once asked me: “Why are white LEDs so expensive? They should be cheaper than the red or green ones, since they don’t need color pigments in their plastic resin”. The reality is that the LED semiconductor dice does NOT emit white light, but a very narrow chromatic spectrum clustered around one wavelength. Regular LEDs are considered monochrome devices and – as described above - it takes different materials and technologies to generate the light in each particular color. However, some manufacturers add color pigments to the plastic resin covering the dice just to diffuse the beam and accentuate its color.
How, then, are the white LEDs made? There are basically two methods. One is to encapsulate together two or three LED dices with complementary colors (e.g. red-green-blue, or yellow-blue), which will synthesize a light beam that “looks” white.
The main drawbacks are the cost of assembly, the need to control the color mix (light balance), and the shift in color over time due to different operating lives of each component.
The popular manufacturing technique uses a blue LED dice covered with a thin coating of Phosphorus. Monochrome light energizes the Phosphorus atoms, which in turn emit secondary white light. The principle is somewhat similar to that employed in fluorescent lamps, where the lamp tubes are coated with a phosphor-based powder. By mixing the phosphorus with other ingredients the color temperature of the light can be modified from cool- to warm-white.
What is the life expectancy of white LEDs? For one, it is shorter than that for blue LEDs. The main reasons are the aging of the phosphorous coating and the color shift (yellowing) of the plastic resin, which acts as a barrier against white light emission. Microscopic cracks in the plastic resin due to mechanical or thermal stress also contribute to the gradual loss of light. As a result, LED life varies largely upon the type of packaging, the manufacturing technology and the application design and assembly. For LEDs with the traditional 5mm-diameter epoxy package, most manufacturers estimate LED life to be in the range of 5,000 to 10,000 hours. Ledtronics Inc., for example, shows results of in-house tests of 5mm white LEDs with an operating life around 8,000 hours.
Lumileds Lighting makes a comparison between regular 5mm LEDs and their series of hi-power Luxeon LEDs based on a special dice-packaging design. Their graph shows that, after 10,000 hours of continuous use the hi-power Luxeon LED maintains 90% of its initial output while a regular 5mm LED has dropped below 40% of the initial light.
Conclusions
As LEDs have become more popular in the lighting industry, particular attention is increasingly given to the definition of LED life. The initial approach based on catastrophic failure (MTBF) has been replaced by concepts more useful to LED technology and lighting applications, for example length of operating life until reaching a certain level (e.g. 50%) of light output depreciation. More thoughtful evaluations (the Alliance for Solid-State Illumination Systems and Technologies) also include limits for acceptable chromatic shifts during the LED functioning. Based on the above noted operating life criteria, the LEDs of AlInGaP technology (red, amber, yellow) last over 100,000 hours while other colors (green, blue, phosphor-based white) last anywhere from 6,000 to 50,000 hours, depending on the semiconductor and packaging technologies. Most LED manufacturers publish these data, allowing the application engineers and lighting designers to create lighting fixtures engineered to specific sustained performance criteria.
