HADCO_LM79_LM80_Information_Brief

LM-79 and LM-80IntroductionLight emitting diodes (LEDs) are a relatively new and unique source for outdoor lighting. They are more reliant upon effective thermal management than any previous source, more of a directional source, and have to be designed and tested as an entire lighting system. Therefore, LEDs require new guidelines and practices for testing. There also needs to be a correlation between how LED manufacturers test their LEDs and how fixture manufacturers test their LED fixtures. In response, the Illuminating Engineering Society of North America or IESNA developed LM-79-08 and LM-80-08 for LED fixture and LED device testing.This Information Brief provides specifiers, engineers, lighting designers, commercial property owners, municipalities and utilities with an understanding of these documents. As an industry leader, Philips® remains at the forefront of LED technology and associated industry documentation. Philips Lumileds® applies LM-80-08 to their LUXEON LEDs, including the Rebel, and Philips HADCO® applies LM-79-08 to their outdoor LED lighting fixtures. Both companies remain active members of the IESNA in the development of methods and guidelines that help advance the use of LEDs in outdoor lighting.The IESNAThe IESNA is a 100+ year old lighting industry group with membership that includes manufacturers (both sources and fixtures), lighting designers and architects, utilities, and others affiliated with lighting such as consultants, government, researchers and educators.1 Both Philips and HADCO are active, contributing members of the IESNA.As a lighting organization the IESNA plays an important role in developing lighting documentation. Its chief publication is The Lighting Handbook which covers topics from light, lighting and vision to performance guidelines for indoor and outdoor lighting fixtures and applications. The IESNA also publishes numerous other lighting documents including: DG or Design GuidesLM or Lighting MeasurementsRP or Recommended PracticesTM or Technical Memorandumsall of which are utilized by lighting engineers, lighting professionals, lighting designers and LED and fixture manufacturers. Two LED LMs are LM-79-08 and LM-80-08; they will be covered in reverse order for reasons that will be evident after reading this IB.LEDs 101 – A RefresherBefore getting into the LMs, a refresher of LEDs is helpful. LEDs are solid state electronic devices that directly convert electricity into light with heat as a by-product. See Figure 1.Figure 1. Electrical model of an LED.Illustration courtesy of IESNA TM-16-05.The light is emitted forward while the heat occurs in the P/N junction where the work of converting electricity into light occurs. This junction temperature or T J heat must be conducted through the thermal pad and away from the LED. See Figure 2.Figure 2. Cross-section of a LUXEON ® Rebel LED. Illustration courtesy of Philips Lumileds Lighting Company.conducted heat (T J )In contrast, traditional sources rely upon secondary process to produce light such as heating a filament (incandescent), causing a gas to glow (fluorescent), or igniting a gas in an arc tube (HID or High Intensity Discharge). For a detailed discussion of LED light generation and the relationship between LED performance and heat, see HADCO’s Information Brief entitled LED Thermal Management.Since LEDs are solid state and directly convert electricity to light, their lumen maintenance is better than traditional sources. Lumen maintenance is a measure of how well a light source maintains its lumen output over time, typically shown as a percentage of initial light output. Typical LED lumen maintenance is 70% for 50,000+ hours – a marked improvement over traditional sources. As an example, compare LEDs to pulse start Metal Halide (p.s. MH) – see Figure 3 on the following page. The p.s. MH lamp has to be replaced four times within the same 50,000 hours, and the LED’s lumen maintenance remains above 70% while the p.s. MH lamp drops down to 50% lumen maintenance toward the end of its life.70% lumen maintenance means the LEDs will still produce 70% of their initial light output, or only 30% lumen degradation. According to the Lighting Research Center, this is “…close to the threshold for detecting gradual reductions in light output. Research shows also that reductions to 70% of initial light output are considered acceptable by the majority of occupants within a space.”2Figure 3. Lumen maintenance of LED versus pulse start Metal Halide.LM-80-08 for the LEDs themselvesLM-80-08 Approved Method: Measuring Lumen Maintenance of LED Light Sources was published by the IESNA Solid State Lighting (SSL) Subcommittee in the third quarter of 2008. Simply referred to as LM-80, this document covers lumen maintenance measurement for inorganic LED-based packages, arrays, and modules3; it does not cover any other aspect of LED performance. See the Appendix at the end of this IB for definitions of terms.One of the key reasons for the development of LM-80 is due to differences in measuring LED performance criteria. LED manufacturers typically measure LEDs in pulse mode operation with no heat sink. The pulse is very short – typically 10 or 20 milliseconds (that is, thousandths of a second) – which will not heat up the LED; therefore, no heat sink is required and T J can be assumed to be equal to ambient temperature T A (typically held constant at 25°C). This is useful for doing high yield LED measurements quickly. This also explains why LED manufacturer data sheets typically show LED performance for T J = 25°C.In contrast, LED fixture manufacturers measure LED performance in situ, which means while it is in their fixture. Under these conditions, the LED is operated in constant DC mode and there are typically numerous LEDs configured together often in close proximity to one another, elevating T J above 25°C. This elevated T J affects the photometric and colorimetric performance of the LEDs. In order to compare “apples to apples”, a new testing criteria needed to be developed – enter LM-80.LM-80 prescribes uniform test methods for LED manufacturers under controlled conditions for measuring LED lumen maintenance while controlling the LED’s T S or case temperature, the DC forward voltage and forward current to the LED. LM-80 requires 55°C, 85°C and one other T S chosen by the LED manufacturer. It also requires lumen maintenance data out to at least 6,000 hours of constant DC mode (not pulse mode) operation4.Philips Lumileds chose 110°C for the third T S for their Rebel LED and they have recorded data out to 10,000 hours which is the preferred duration in LM-80. Based upon LM-80 data, LED manufacturers then extrapolate lumen maintenance out to tens of thousands of hours; Philips Lumileds goes out to 50,000 hours and beyond. While LM-80 does not specify the extrapolation method, many LED manufacturers use more conservative exponential extrapolation due to the exponential behavior of LEDs and most electronic components. The SSL Subcommittee is working on TM-21 which will standardize this extrapolation method.The data resulting from LM-80 measurements are matrices of lumen maintenance values. LED fixture manufacturers use this data in combination with their UL in-situ thermal testing to predict the lumen maintenance of the LEDs when used in their fixtures and, subsequently, the lumen maintenance of the LED fixtures themselves. For example, if we measure 85°C T S at the hottest Rebel LED in one of our fixtures, then we look up that particular data set from Philips Lumileds to determine the LED fixture’s lumen maintenance based upon and correlated with the LED’s lumen maintenance at that same T S. Fixture manufacturers also use the data to predict LED color stability over time at the various T S temperatures.LM-79-08 for the LED fixtureLED lumen maintenance and color stability are only part of the puzzle; it is helpful to characterize the performance of LEDs in fixtures so that the entire system is considered. That’s where LM-79 comes in.LM-79-08 Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products was published by the IESNA Solid State Lighting (SSL) Subcommittee in the first quarter of 2008. LM-79 covers photometric and colorimetric performance as well as electrical power measurements for inorganic LED fixtures5. LM-79 prescribes uniform test methods for LED fixture manufacturers under controlled conditions using LED fixtures as they would be manufactured for production.Unlike traditional sources which are typically tested using relative photometry with test lamps and ballasts, LED fixtures are tested using absolute photometry with production LEDs and fixtures in the orientation in which it will be installed6 to ensure a more true test of LED performance when in situ. For a detailed discussion of absolute photometry, see HADCO’s Information Brief entitled Understanding Relative and Absolute Photometry. As previously discussed, LEDs operated in situ will perform differently due to the elevated T J which will be further impacted by fixture orientation and thermal conditions; if the LED array or module were removed from the fixture, its performance would change. This is precisely why absolute photometry is a must for LED fixtures.LM-79 testing is typically performed with either an integrating sphere for all photometric and colorimetric measurements or an integrating sphere in combination with a goniophotometer. The integrating sphere is recommended for colorimetric measurements; alternately, a gonio-spectroradiometer or gonio-colorimeter may be used.HADCO has a custom designed goniophotometer lab and equipment (again, see the Understanding Relative and Absolute Photometry IB for more details) where we test LED fixtures following LM-79 procedures. We also test key LED fixtures in independent DOE-approved labs – look for the .ies photometric files on our web site that indicate LM-79.ConclusionWith over 150 years of combined lighting and LED experience between them, Philips and HADCO understand LED technology, fixture manufacturing and LED and fixture testing. Our vertical integration enables us to better understand everything from the LED device itself to its proper use in fixtures. As active participants in the IESNA, both companies will continue to engage lighting organizations and other lighting professionals to ensure that we keep developing and manufacturing LED and other products with optimum performance that stand the test of time. To learn more about HADCO’s growing line of LED fixtures and our entire commercial and landscape product line, go to .Appendix – DefinitionsThe definitions in this Appendix are per ANSI / IESNA RP-16-05 Addendum a Nomenclature and Definitions for Illuminating Engineering, IES Nomenclature Committee, New York: Illuminating Engineering Society of North America, 2008.Please refer to the latest edition of this document for updates and revisions.LED die – A small block of semi-conducting material on which a given functional circuit is fabricated. {Author’s note: Also referred to as an LED chip; the light emitting part.}The following three definitions are for devices that do not include a power source and are not connected directly to the branch circuit.LED package – An assembly of one or more LED dies that contains wire bond connections, possibly with an optical element and thermal, mechanical, and electrical interfaces. {Author’s note: What we commonly refer to as the LED itself.}LED array – An assembly of LED packages on a printed circuit board or substrate, possibly with optical elements and additional thermal, mechanical, and electrical interfaces.LED module – A component part of an LED light source that includes one or more LEDs connected to the load side of LED power source or LED driver. Electrical, electronic, optical, and mechanical components may also be part of an LED module.Appendix – Definitions <continued>Power source – A transformer, power supply, battery, or other device capable of providing current, voltage, or power within its design limits. This device contains no additional control capabilities.Power supply – An electronic device capable of controlling current, voltage, or power within design limits.LED control circuitry – Electronic components located between the power source and the LED array designed to limit voltage and current, to dim, to switch, or otherwise control the electrical energy to the LED array. The circuitry does not include a power source.LED driver – A power source with integral LED control circuitry designed to meet the specific requirements of a LED lamp or a LED array.References1. IESNA web site , About Us page, 2009.2. Bullough, J. et al., ASSIST recommends… LED Life for General Lighting: Definition of Life, Volume 1, Issue 1. Troy, NY: Lighting Research Center, Rensselaer Polytechnic Institute, February 2005.3. Subcommittee on Solid State Lighting of the IESNA Testing Procedures Committee, Approved Method: Measuring Lumen Maintenance of LED Light Sources, LM-80-08, New York: Illuminating Engineering Society of North America, 2008.4. Ibid.5. Subcommittee on Solid State Lighting of the IESNA Testing Procedures Committee, Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products, LM-79-08, New York: Illuminating Engineering Society of North America, 2008.6. Ibid.November 2009, Rev. 2©2009 HADCO, a Philips group brand. All rights reserved. Product specifications are subject to change without notice. HADCO is a registered trademark of HADCO.。