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Performance Testing of LED Lighting Products – Part 2

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Thursday 1st March, 2018

Dr Gareth Jones, Chief Executive of Lux Tsi, the internationally accredited test laboratory, continues his article outlining the testing regime applied to new lighting products.  This externally verified methodology is key to building confidence in the rapidly emerging range of new LED luminaires coming to the market.  Part 1 dealt with Photometric testing which is obviously vital for understanding that a new luminaire is up to the job that it is specified for.  Here, in Part 2, Dr Jones outlines the other parameters key to product performance and compliance.

Electrical Performance

To assess the performance of a lighting product requires specialised electrical equipment in order to provide a stabilised power source with low harmonic distortion or ripple current for either DC or AC electrical supply.  In addition along equipment is required to properly measure the voltage, current and the phases of these to measure the power factor value and provide a true power measurement, which requires the simultaneous separate measurement of the current and voltage with time.   We use the Yokogawa power measurement systems for such testing. yokogawa Thus,  both the electrical power provided to the product and consumed by the product is well quantified.

Sometimes, it is necessary to measure the effect of the driver (control gear) separated from that of the LED luminaire and again care should be taken to ensure that proper measurements are taken in line with International standards.

Thermal Performance

The performance of LED lighting products is very dependent on the thermal management of the LED package and therefore the LED chip junction temperature.   LEDs are small semiconductor diodes and their efficiency varies with temperature.   The hotter the diode, the lower the efficiency and also the shorter the lifespan both of the LED and the packaging materials used to protect the LED and turn the blue light from the LED chip into the white light (using a material known as a phosphor).

Thus, the measurements of LED lighting products need to be performed at a stable and consistent temperature to assess the product performance.   This temperature is achieved by testing of the LED product at an ambient temperature of 25C and with the product in its operating orientation, free of drafts, as specified in the standard LM79.    The in-situ temperature measurement test (ISTMT) of the LED is achieved in a consistent manner that ensures that the junction (or case) temperatures of different products can be compared in a reliable manner.

Lumen Maintenance (applying the LM80 standard)

Lumen maintenance measures the changes in the light output of lighting products over a long period of time which is indicative of how the product will perform in its intended use.    The key standard for lumen maintenance measurements of LED lighting products is IES LM-80-08.  This is an International standard which was written and approved by the Illuminating Engineering Society (IES) of North America in response to concerns that the lumen maintenance parameters of LED modules and packages needed to be measured in a controlled and standardised manner.  It provides a standard means of measuring the lifetime performance of LED sources at well specified measurement conditions, utilising best practise for the measurement of the photometric performance of the LED light sources.

There is no standard approved yet in Europe which details the way in which to measure the lumen maintenance of LED lighting products and therefore LM80 is considered good practise for manufacturers' in-house and third party testing laboratories.

LM-80 provides guidance for measuring lumen maintenance of LED packages, arrays and modules.  It does not cover measurement of lamps and luminaires and a related standard for these is being developed and should be available in 2013.   However, the principles of measurement are a good approach for the measurement of lamps and luminaires.

The standard imposes the following requirements:

  • Steady state testing for at least 6000 hours at 3 different junction (case) temperatures for a sensible sample of devices - for LED packages at least 10 per temperature and current are sensible but the manufacturer can choose to put more through testing;
  • Regular assessment of the performance (lumen output) over the test period when measured at 25C ambient

LM80 ensures the lifetime of the LED is tested at component level early in the supply chain to avoid quality issues arising when incorporated into final lamps or luminaires.

It specifies 3 realistic temperatures for the testing – 2 are fixed (55C and 85C) and one can be determined by the manufacturer.  The standard refers to LM79 for the basis of the testing of the photometric output of the sources - thus ensuring good practise for the luminous flux assessment.

It is important the lamp and luminaire manufacturers check that LM-80 data is available from their LED supplier to ensure that lumen maintenance characteristics of the LED have been well quantified, thus ensuring that the LED output when in the final product will perform in a reliable manner with quantifiable estimates of the lumen maintenance of the product.

Photobiological Assessment (implementing EN 62471 standard)

The requirement for safe, reliable and effective products in the European Market places certain requirements on product manufacturers and importers to ensure compliance to product based standards for CE mark purposes.

One of the essential requirements of lighting products is the need to measure and assess the products for any photobiological hazards.  Photobiological means the influence of the light (UV, visible and IR) emitted on the skin or eyes of those using, making or installing the product.

Lighting products emit visible light, obviously, but can also emit ultraviolet and infrared light, which is not visible. It is important to assess whether products in normal use or during maintenance are safe with respect to the influence of the light on the eye or the skin. These are known as photobiological effects. Obviously, when people are using lighting for many hours a day then it is important to understand that there are no hazards.photobio

Lighting manufacturers must ensure that their products present no known hazards. The standard IEC/EN 62471 describes how to assess the products against these hazards and provide categories of risk to the user with advice on the labelling and instructions for such product categories.

The use of LEDs in lighting products has raised the issue of photobiological testing because LEDs can be configured in many more ways than traditional lighting and the sources have a large blue light content that could be damaging to the eye.   In addition, the installation and use of such products into domestic, commercial or other environments is required to be performed in a safe manner.

The application of the IEC/EN 62471 standard is highly complex and requires specialised equipment, and even when the measurement has been made the interpretation of the results against the specific hazard bands is complex.

So with claims, counter claims and occasionally some rather confused messages about the performance and efficiency of LEDs, independent testing is a valuable means of validating performance and demonstrating to lighting specifiers the true capabilities of a luminaire.  Building trust in the technology will be vital as energy efficiency climbs our collective agenda.  If all the lights are LEDs perhaps they are a little less likely to go out.

Read Part 1

Contacts

LUX-TSI is an independent internationally accredited laboratory that can support lighting companies and manufacturers with all of the above tests, please contact info@lux-tsi.com (www.lux-tsi.com) for more information. This post was written by guest author Dr. Gareth Jones

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