The Meaning of LED Life

Mark Kirkham, National Product Manager of Mark Herring Lighting (MHL) in Auckland, New Zealand believes that these LED-specific performance metrics are not well understood and often not applied fairly or correctly. “This may result in light levels falling below compliant levels part way through the expected application life of the luminaires,” he explains. Thus, luminaires will provide a shorter application life than expected.

Lighting standards prescribe minimum illuminance/luminance values that need to be maintained at all times during the application life of the lighting system. Mark elaborates that the depreciation in light output of the luminaires should be taken into account in the lighting design to ensure the light levels do not fall below compliant levels over the prescribed application life of the luminaires.

AS/NZS 1158 provides clear guidance in respect of determining maintenance factors for exterior lighting. These standards are currently undergoing an update in which LED technology are incorporated (refer to sidebar).

The current standard prescribes that overall maintenance factors (to be referred to as LLF in the updated standard), shall not exceed 0.7 for IP5X luminaires and 0.8 for IP6X luminaires. “Too often we see these absolute maximums of 0.7 or 0.8 used by default in the lighting design which suggests the designer may not have bothered to determine accurate and realistic MFs or is overly optimistic with their assessment,” Mark declares - stating that a similar method often applies to interior lighting designs.

Mark initially gained an interest in LEDs since their infancy and introduction to New Zealand in 2006. Before moving to MHL, Mark’s LED involvement was almost exclusively focused on exterior lighting solutions. He became one of the pioneers in New Zealand to push LED products into municipal (council) road lighting applications.

Being one of the first credible lighting suppliers in New Zealand to promote LEDs meant that Mark focused mostly on educating and proving that LEDs were up to meeting its very high performance and longevity claims. “There was also time invested in lobbying for changes of standards to incorporate LEDs,” he remarks. Mark quickly realised that not all LED products were created equal and those decision makers needed the knowledge and tools to be able to evaluate products fairly. “I saw this as a great opportunity to upskill on this topic and educate our specifiers and the end user,” Mark confirms.

As LED products are not created equal and can range in quality, performance and longevity, Mark emphasises that many decision-makers still do not realise or adequately assess these implications. “Too often we see users apply the same, unrealistically optimistic maintenance factors to all LED products,” he comments. Mark adds that the mortality of LEDs has never really been an issue and still isn’t. “Until recently, lumen depreciation was considered the attribute that determined the effective lifetime of LEDs,” he states. “With a huge (and growing) installed base of LEDs now operating and the performance of these being analysed, there is a developing belief that colour shift may in the future become the new determinant of life.”

Mark believes there was also a lack of knowledge in regards to LED life for interior applications, so he found himself inspired to educate on the topic further. “Some of my educational efforts were executed under the IES, which was very well received,” he comments. Mark presented to record crowds across New Zealand, which helped get the message through to some of the IES members to understand and apply the principles. He explains that unfortunately only a minority of decision makers correctly determine and apply accurate and realistic maintenance factors in their designs. “There is much more work still to be done to educate and encourage accurate and realistic maintenance factors to be determined and applied in the lighting design and specification process,” he reveals.

When it comes to what an investor or developer considers for an LED lifetime, Mark reveals it’s all about whole-of-life (WoL) costs (capital + operating + maintenance) over the lifetime of the lighting installation. “It is usually the case that better performing/longer life (albeit higher price) luminaires will often have a lower WoL cost than cheaper, poor performing/shorter life luminaires,” he informs. Mark says, unfortunately, a developer who will quickly sell or lease a building is usually only interested in the up-front capital cost (supply + installation = capital). “They are not usually responsible for, or concerned about the operating/maintenance and consequently utilise lower cost, less efficient, shorter life luminaires,” he reveals.

For building owner-operators, quality landlords and long-term tenants, they are more likely to invest more up front in higher quality, higher efficiency, longer life luminaires, resulting in lower WoL costs. Mark explains that by utilising higher quality and better performing lighting solutions, developers are likely to raise the spec of the lighting - and likely demand a higher lease rate.

LED chip quality, along with operating temperatures in the luminaires will determine the LED lifetime. LED life metrics are defined as Lxx and Byy. Mark explains that Lxx is the primary metric that defines lumen depreciation. The ‘xx’ denotes the % of initial light output after a nominated number of hours of operation. – L70 60,000hrs denotes the light output is predicted to depreciate to 70% of initial after 60,000hrs of operation. The Byy is a ‘statistical confidence’ metric which further defines the ‘L’ metric. The ‘yy’ denotes the proportion of lamps whose light output is expected to have depreciated to below the nominated ‘L’ value at the nominated number of hours.

Mark explains that luminaires with old technology will be re-lamped and cleaned multiple times during the application life; thus the output is fully restorable to new (or near new) performance during each maintenance (re-lamp and clean) cycle. Mark says for LED luminaires, it is generally assumed that LED boards/light engines will not need to be replaced during the application life and that maintenance (if any) is limited to cleaning.

The LED chip manufacturer tests lumen depreciation in accordance with IESNA LM-80. LM-80 prescribes a sample batch of chips are tested under controlled thermal conditions at a range of operating temperatures for a minimum of 6,000hrs. Mark elaborates that luminaire manufacturers then need to conduct In-Situ Temperature Measurement Tests (ISTMT) to determine the operating temperature of the LED chips installed in the luminaires while operating at the rated ambient temperature.

While LEDs have been around for over 50 years, they are a relatively new technology for general lighting applications. As LEDs have such a long expected life, there is insufficient test data upon which to determine long term depreciation accurately. As a consequence, to determine lumen depreciation beyond the test period in LM-80, the US EPA has developed their TM-21 calculator. This predicts the long term lumen depreciation by extrapolation based on the LM-80 LED chip test data and the operating temperature of the chips in the specific luminaire as determined by the ISTMT. Mark noted that while this TM-21 has been widely adopted, there are some concerns over the accuracy of the calculator along with the lifetime predictions generated from it.

Mark believes that it is also very likely that some manufacturers are inflating their claims, or in the absence of carrying out all of the necessary testings, may just be guessing LED lifetime. “I have always encouraged specifiers to challenge manufacturers to substantiate their performance claims by providing the necessary LM-79, LM-80, ISTMT and TM-21 test reports,” he remarks. “It’s surprising how many manufacturers struggle to do this - reputable LED luminaire manufacturers should be able to provide these.”

Mark expresses that it is important to question the lifetime, performance and reliability of LEDs given it will directly affect the application life of the luminaires, and thus the WoL cost of the lighting installation.

Mark believes lighting designers and specifiers still fail to understand the fundamental difference between old technology (halogen/fluorescent/HID) luminaires and LED luminaires. He emphasises that for LEDs; maintenance factors will vary widely depending on the chips used, operating current, the thermal design of the luminaires and the ambient temperature. “The use of unrealistically high maintenance factors for LED products will result in light levels falling below compliant levels part way through the expected application life, resulting in a shorter application life than expected,” he states.

Mark continues his efforts to educate the market that LED products are not all created equal and will vary in quality, performance and longevity. He says he will always provide the insight and knowledge to his customers to accurately assess the lighting performance over the expected service life of the installation so that they can make informed decisions in respect of the most appropriate products to use for each application.