Technical

CORNEA. A real Breakthrough in LED Technology
Eradicate the T5 Fluoro and the deadly MERCURY
Get rid of transformers
Mnemonic Codes
Importance of LSZH compliance
Risk-Free and Sustainable Lighting for Sporting Facilities

 

 

Light Emitting Diode (“LED”) lights are the same cool lights found on every computer and the new LED TVs. There is an industry consensus that these ‘Solid-State’ lights will continue to play an increasingly important role in energy reduction programs. These high-efficiency LEDs slash Energy bills by up to 90% because they use as little as one-tenth the energy of Incandescent or Halogen bulbs with the same luminosity. They produce warm light and last for up to 25-50 times longer. LEDs are the longest-life, maintenance-free, light source on the market. Their long-life means that fewer lamps need to be disposed-off in our Environment.

Halogen lamps are no energy savings bulbs! Using the same energy consumption as incandescent bulbs, due to their built-in reflector, halogen bulbs appear to be about two times brighter as compared to filament bulbs. Each halogen bulb needs a step-down transformer to convert the main voltage from 240 volts which causes 20% extra loss of power, year round. They contain the highly-toxic Fluorine.

To the contrary, LED lamps are Solid-State (contain no gas or filament), they are totally Flicker-FREE, UV-Free and Lead, Mercury and Argon FREE. They are disposable and 100% recyclable.

Halogens and CFLs also cause headaches as a result of the 50Hz AC power flicker. The justification is now obvious for replacing Halogens and CFLs with energy saving LED lamps on the basis of power consumption, energy efficiency, life span and service costs.

Originally used as indicator lights and novelty lighting, LED bulbs are now the standard in many applications such as 87% of traffic-lights worldwide, car brake lights, indicators and headlights. They have the same luminosity with halogen bulbs and they will soon overtake CFL (Fluoro) lamps as the energy-efficient lighting source of choice.

LEDs appear to have a higher upfront cost than CFLs (which contain Mercury). However, they become a far better investment within a relatively short term. The energy and product cost savings quickly add-up and the return on investment is only 5 months. Moreover, the on-going energy cost savings are simply staggering. The LED’s extremely long life span of 50,000 hours, means you continue to see energy savings for approx. 15-20 years without having to change bulbs. To the average house or commercial application this works out to savings of $4,000 per year.

The average car produces about 4,100kgs of CO2 emissions per year. Replacing just one 50W Halogen bulb for a 6W LED prevents 1,560kgs of carbon dioxide and Sulphur emissions over the LED’s life. So to put that in context, replacing just 3 Halogen lamps with LEDs, is equivalent to taking your car off the road for an entire year. Start saving the world today.

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Solid state lighting (SSL) is mainly driven by silicon based Light Emitting Diode (LED) technology, although some interest has been shown more recently in organic polymer based Organic Light Emitting Diodes (OLED).

LED devices work by exciting electrons at a junction between two differently tuned semiconductors. These electrons fall back to their ground state by emitting a photon of energy. The scale of the electron excitement is dependent on the tuning (or doping) of the semiconductors, resulting in the photons of light from any individual LED being emitted in a narrow colour band. Sometimes a photon is not successfully produced by this process and instead the energy goes into heating the semiconductor. Increasing the luminous efficiency of LED lighting therefore is predominantly about ensuring that a greater proportion of the energy results in emitted photons rather than heating of the semiconductor.

Heating of the semiconductor reduces the efficiency of the LED and can lead to long term damage if the lamp design does not include enough cooling. The standard testing procedure for LED energy efficiency involves lighting the lamp for a very short burst at a stable temperature of 20°C. This means that the efficiency of the LED lamp in operation is generally 10-15% less than the quoted figure, depending on the quality of the heat sink design. Lamps used in fixtures with poor air flow will have substantially lower performance and lifetime.

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This image clearly shows the influence of air flow in the lighting fixture on the temperature environment of an LED down light. The left side shows a cooler lamp environment provided by an unobstructed vertical convective airflow, the right shows the heating influence of an obstructed airflow. Higher operating temperatures reduce the efficiency, output and lifetime of the lamp.
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Commercial and industrial applications of LED lighting are growing and include indoor and outdoor commercial & industrial lighting, signage, architectural lighting and general indoor and outdoor lighting. The narrow colour band of LED lighting provides a distinct energy efficiency advantage over colour filtered light sources for signage and architectural lighting applications. However, successful implementation of LED technology for general lighting applications requires the use of LED designs that provide for full colour rendition by humans and the appearance of white light.

White light can be formed from narrow colour band LED by either including a number of red, green and blue chips in the lamp (corresponding to the red, green and blue absorbency peaks of the human vision system). Alternatively, white light can be formed by using a near ultraviolet LED covered in layers of different coloured phosphors similar to a fluorescent lamp. There are also hybrid techniques that combine a UV-phosphor system with specific coloured LEDs to boost colour in different parts of the spectrum. Currently, the tricolour technique is the most efficient way to deliver white light, but the colour rendition can be poor. The multi-layer coloured phosphor system gives the best CRI, but it is inherently less efficient due to losses in the phosphor layers.

The rated life of an LED light source is different from the lumen-maintenance life, and is an essential reliability value that is required by luminaire makers and end users.

Using IES test method TM-21-11, the SSL industry now has a standard method of obtaining long-term lumen-maintenance information for LED light sources. The method is made up of two steps. First, the LED light sources must be tested per LM-80. The new TM-21 method is then applied to the collected measurement data to make lumen-maintenance projections, including in-situ temperature calculations.

However, there is still one measure that is missing: the rated life of LED light sources. Rated life is an essential reliability property for LED integrators that design LED luminaires, providing luminaire users with warranty and usage information.

Rated life

The rated life of a lamp or light source is defined, per ANSI/IES RP-16, as “the life value assigned to a particular type lamp. This is commonly a statistically-determined estimate of median operational life.” The rated life in hours of an LED lamp or light source, specified by the manufacturer, applies under certain operational conditions and for defined failure criteria. The statistical measure for the rated life is designated Bp and is measured in hours, where p is a percentage.

For example, a B50 rated life of 1,000 hours means that 50% of the tested products have lasted 1,000 hours without failure. B50 is also known as the products’ rated average life. If a product has a B10 rated life of 1,000 hours, this means that only 10% of tested products failed within 1,000 hours, so the product should last much longer than a product with a B50 rated life of 1,000 hours.

Lumen-maintenance life

For LED light sources, LM-80 defines lumen-maintenance life as “the elapsed operating time at which the specified percentage of the lumen depreciation or lumen maintenance is reached, expressed in hours.” Different from rated life, the rated lumen-maintenance life is defined as “the elapsed operating time over which an LED light source will maintain the percentage (p) of its initial light output.”

Rated lumen-maintenance life is measured in hours with associated percentage of light output, noted as Lp. In other words, L70 of 30,000 hours means that the tested LEDs produce 70% of the initial light output at 30,000 hours. If an LED has L50 of 30,000 hours, its lumen output decays faster than one with L70 of 30,000 hours.

While Bp life is a statistical measure, Lp life is a defined durability measure. When testing for Bp life, a large statistically-meaningful sample size is required. When testing Lp life, there is no sample size requirement. However, when LM-80 test data is utilized to make lumen-maintenance projections (per TM-21), the sample size will affect the uncertainty of the projection. As a consequence, a smaller sample size will lead to shorter projected life in order to increase the statistical certainty.

Failure

For LED light sources, one can define failure as when the LED can no longer produce a certain percentage of the initial light-output value. For example, failure might be defined as when the light output of an LED reaches 70% or lower of the initial light output (including if the LED’s light output is zero). In other words, for a given period of time, if an LED produces insufficient light or no light, the LED is considered at failure.

Using this definition of failure criteria, the statistical measure can be combined with the defined durability measure. The combination of lumen-maintenance life (Lp) with statistically-measured failures (Bp) is the LED light source’s rated life, or BpLp value. For example, if an LED light source is claimed to have B50L70 of 30,000 hours, then 50% of tested samples should have a lumen-maintenance life of 30,000 hours.

Ideally, to obtain the rated life for LEDs, the statistical failure measurement can be integrated with lumen-maintenance measurements during the life test. One can use a large LED sample size, large enough to be statistically meaningful as when measuring traditional lamps, and then track and record the sample behaviors including light-output change and failures during the life test. When 50% of the tested samples reach a light output equal to 70% of initial lumens, including the samples that failed to produce light, then B50L70 (in hours) is obtained.

Obviously, as is the case with lumen maintenance, it is not practical to conduct real-life tests to get B50L70 values when such a value can be as long as 30,000 hours, or nearly three and a half years. The challenge is how to make a projection using the data obtained in a shorter testing period.

Projections

LED manufacturers have been conducting studies and establishing practices for reliable approaches to project the rated life for LEDs; in general, there are two approaches.

The first approach is to conduct LM-80 testing with a large sample size. The test data are collected for both light-output changes and failures. The data is then fitted into a mathematical model with a statistical-certainty band. In addition to the lumen-maintenance projection curve, the associated sample distribution bandwidth is also plotted. By analyzing the curve and bandwidth, an estimated B50L70 life is projected.

The second approach is to conduct the lumen-maintenance (LM-80) test separately from the accelerated-failure-modes test. Using TM-21, the lumen-maintenance projection can be established. The data collected in the accelerated-failure-modes test are modeled with a different mathematical expression. The rated life is then projected by mathematically combining both models.

There are some discussions in standardization organizations regarding development of a document or recommendation to address LEDs’ rated life. To help the LED lighting industry to properly use LED light-source information, it is necessary to clarify that rated life is not lumen-maintenance life.

Before the industry establishes a recommendation for a standard practice, LED integrators may need to request more testing and modeling information from the manufacturers in regards to the statistical failures of LED light sources.

 

Uni NewcastleT16 Building lighting efficiency v0.9 (download PDF official Government sheet) Contributed by Jianzhong Jiao.