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White Paper - LM-84 TM-28 Update May 2016

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Trevor Leighton

LM-84 and TM-28 aim to provide luminaire-level lifetime predictions, but they have flaws that make them unsuitable for high-performance LED roadway luminaires. LM-84's thermal testing method using cameras is impractical and alters the luminaire's thermal performance. TM-28's short testing duration and small sample sizes mean extrapolating lifetime predictions beyond 10,000 hours is not realistic. For roadway luminaires requiring 50,000 hour lifetimes, the standards would require testing 15 luminaires for over a year per product. Thermal testing per LM-80 and lumen maintenance projections using TM-21 remain the best options for comparing and assessing quality of high-powered LED roadway lighting

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Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 1 Date: 20-APR-2016 LM-84 / TM-28 Effectiveness for High Performance LED Roadway Lighting Luminaires Author: Trevor Leighton MILP MSLL Regional Technical Officer ¨C AMEA ¨C Schr¨¦der Group Objectives: For an LED roadway luminaire manufacturer, it is a significant challenge to clearly communicate the expected lifetime lumen depreciation for a system (luminaire) based on extant LM-80 / TM-21 standards at chip level. Whilst in-situ temperature tests to find the hottest solder point temperatures combined with appropriate LM-80 alpha and beta values fed into the TM-21 formulae offer the most available solution, they discuss mainly what is happening at chip level, not luminaire level. This is further complicated by delayed phosphor settlement on recent chip platforms leading to unrealistic lumen maintenance extrapolations. The promise of LM-84, combined with its partner TM-28, was to shift the focus to a luminaire level in determining what the effect of SSL in complete systems would have on the lamp lumen depreciation for the expected useful lifetime. This paper will explore how relevant, and ultimately how useful, these new standards are for investors in SSL roadway lighting in defining and comparing quality. 1. IES LM-84-14 1.1. Testing Regime ¨C Photometric over time The scope of LM-84 fundamentally rests on two principles; successive photometric tests over time and a methodology for thermal testing. Please note here that the release of LM-84 was considerably sooner than its counterpart TM-28 by more than a year or more. LM-84 states that the luminaire should be tested to the LM-79 standard and then removed to a rack to stay energised for an ¡°amount¡± of time, the amount not being specified. Furthermore the luminaire should periodically be returned for LM-79 tests, again at unspecified time intervals, apparently ad infinitum. (Reference: Section 6, Photometric and Thermal Measurement Procedure specifically part 6.1) Whilst no clear indications of time intervals are given, some parameters of lumen depreciation are mentioned in section 8.9.2 where L50 and L70 may be considered. For a typical high-power, 1W LED platform of 2mm2, the CREE XP-G2 chip, for example, L70 values exceed 400,000 hours. This would therefore involve LM-84 testing for more than 45 years, given the aforementioned flaws in TM-21.
Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 2 1.2. Testing Regime ¨C Use of thermal camera Annex A IEC 60598 and other non LED-luminaire specific testing regimes do not effectively address the issue of identifying the hottest solder point temperatures within an LED luminaire, or LED driver Tc point measurements for that matter. LM-84 suggests a methodology based on the use of a thermal camera to identify ¡°approximate¡± locations of ¡°worst case LED Ts¡± within the luminaire and based on that information, the more accurate thermocouples can be placed where necessary to reveal precise thermal data. Then the lenses and cover glass are replaced for that process to continue. Thermal cameras are not effective at penetrating glass covers and optical lenses, therefore this standard requires that both are removed prior to camera measurement. Removing both the cover glass and the lenses alters the thermal system of any luminaire and is therefore rendered redundant. This is neither logical nor scientific. Quality LED luminaire manufacturers design their luminaires so that the (heat) energy is dissipated as effectively as possible from the most challenging regions of the luminaire: 1. The centre of the luminaire 2. The centre of a collection of LEDs 3. The part of the luminaire which houses the LED driver Generally manufactures are held to considerable warranties, 10 years in many cases. The manufacturer will most likely be more vigilant on thermal issues than the investor.
Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 3 2. IES TM-28-14 2.1. Extrapolation Criteria ¨C ¡°Direct Method¡± TM-28 now clearly defines the time intervals that LM-84 did not. TM-28 also determines the sample size; most existing standards for a luminaire only require one test unit. TM-28 states that the nominal period of testing time is 6,000 hours. (Section 5.1 Procedures for Projection Method 1, Direct Extrapolation of LM-84 Data) Once this data is accumulated and put into the alpha and beta style LM-80 mode, a typical TM-21 formula is used. However, the sample size and extrapolation times are not compatible with roadway luminaires: Extract: TM-28-14, 5.1.5 Adjustment of results A sample size of 3 will allow for a lamp lumen depreciation to be estimated for 3 x 6,000 hours. It is generally accepted that 50,000 hours of useful life will be expected if not more.
Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 4 2.2. Extrapolation Criteria ¨C ¡°Combined Method¡± TM-28 allows a second method of predicting lumen maintenance by combining LM-80/TM-21 data with LM-84/TM-28. The sample size and multiplication factors being slightly different: Extract: TM-28-14, 5.2.7 Adjustment of results A sample size of 5 luminaires now only allows 1.5 x 6,000 hours = 9,000 hours. Not at all consistent with what the market requires of LED roadway luminaires. In order to reach a minimum market requirement of 50,000 hours, the manufacturer would have to test 15 luminaires for a period of 8,333 hours (1 year more-or-less) to apply the TM-28 (TM-21) formula. If third party tests were demanded, this would cost in the region of 60-70K USD without product cost or shipping to laboratories,
Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 5 3. Conclusions ? LM-84 Thermal testing using a thermal camera is neither useful nor practical ? LM-84/TM-28 do not address the specific concerns of LED roadway investors in terms of performance over useful lifetime within a reasonable technology cycle time frame ? The chip industry is currently evolving at a rate of, this year on last, +>20% light output per chip platform. If LM-84/TM-28 are invoked, by the time the luminaire manufacturer has completed the certification, they no longer have that technology within their luminaire (N.B. ¨C Manufacturers wait 6,000 to 10,000 hours of LM-80 before committing chip platforms to their products) The only obvious conclusion is that LM-84/TM-28 is more suited to products like retro-fit LED lamps where the technology/lumen output point is established and not required to push the performance boundaries of roadway and other demanding lighting applications like 2KW replacement floodlighting of a stadium for example. The conclusion of this paper is that, for performance LED roadway luminaires, a thermal test, LM-80 and TM-21 are the only current solutions for sensible comparison and quality assessment. 4. References IES LM-84-14 IES TM-28-14 IES LM-80-15 IES TM-21-11 IEC 60598

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White Paper - LM-84 TM-28 Update May 2016

  • 1. Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 1 Date: 20-APR-2016 LM-84 / TM-28 Effectiveness for High Performance LED Roadway Lighting Luminaires Author: Trevor Leighton MILP MSLL Regional Technical Officer ¨C AMEA ¨C Schr¨¦der Group Objectives: For an LED roadway luminaire manufacturer, it is a significant challenge to clearly communicate the expected lifetime lumen depreciation for a system (luminaire) based on extant LM-80 / TM-21 standards at chip level. Whilst in-situ temperature tests to find the hottest solder point temperatures combined with appropriate LM-80 alpha and beta values fed into the TM-21 formulae offer the most available solution, they discuss mainly what is happening at chip level, not luminaire level. This is further complicated by delayed phosphor settlement on recent chip platforms leading to unrealistic lumen maintenance extrapolations. The promise of LM-84, combined with its partner TM-28, was to shift the focus to a luminaire level in determining what the effect of SSL in complete systems would have on the lamp lumen depreciation for the expected useful lifetime. This paper will explore how relevant, and ultimately how useful, these new standards are for investors in SSL roadway lighting in defining and comparing quality. 1. IES LM-84-14 1.1. Testing Regime ¨C Photometric over time The scope of LM-84 fundamentally rests on two principles; successive photometric tests over time and a methodology for thermal testing. Please note here that the release of LM-84 was considerably sooner than its counterpart TM-28 by more than a year or more. LM-84 states that the luminaire should be tested to the LM-79 standard and then removed to a rack to stay energised for an ¡°amount¡± of time, the amount not being specified. Furthermore the luminaire should periodically be returned for LM-79 tests, again at unspecified time intervals, apparently ad infinitum. (Reference: Section 6, Photometric and Thermal Measurement Procedure specifically part 6.1) Whilst no clear indications of time intervals are given, some parameters of lumen depreciation are mentioned in section 8.9.2 where L50 and L70 may be considered. For a typical high-power, 1W LED platform of 2mm2, the CREE XP-G2 chip, for example, L70 values exceed 400,000 hours. This would therefore involve LM-84 testing for more than 45 years, given the aforementioned flaws in TM-21.
  • 2. Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 2 1.2. Testing Regime ¨C Use of thermal camera Annex A IEC 60598 and other non LED-luminaire specific testing regimes do not effectively address the issue of identifying the hottest solder point temperatures within an LED luminaire, or LED driver Tc point measurements for that matter. LM-84 suggests a methodology based on the use of a thermal camera to identify ¡°approximate¡± locations of ¡°worst case LED Ts¡± within the luminaire and based on that information, the more accurate thermocouples can be placed where necessary to reveal precise thermal data. Then the lenses and cover glass are replaced for that process to continue. Thermal cameras are not effective at penetrating glass covers and optical lenses, therefore this standard requires that both are removed prior to camera measurement. Removing both the cover glass and the lenses alters the thermal system of any luminaire and is therefore rendered redundant. This is neither logical nor scientific. Quality LED luminaire manufacturers design their luminaires so that the (heat) energy is dissipated as effectively as possible from the most challenging regions of the luminaire: 1. The centre of the luminaire 2. The centre of a collection of LEDs 3. The part of the luminaire which houses the LED driver Generally manufactures are held to considerable warranties, 10 years in many cases. The manufacturer will most likely be more vigilant on thermal issues than the investor.
  • 3. Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 3 2. IES TM-28-14 2.1. Extrapolation Criteria ¨C ¡°Direct Method¡± TM-28 now clearly defines the time intervals that LM-84 did not. TM-28 also determines the sample size; most existing standards for a luminaire only require one test unit. TM-28 states that the nominal period of testing time is 6,000 hours. (Section 5.1 Procedures for Projection Method 1, Direct Extrapolation of LM-84 Data) Once this data is accumulated and put into the alpha and beta style LM-80 mode, a typical TM-21 formula is used. However, the sample size and extrapolation times are not compatible with roadway luminaires: Extract: TM-28-14, 5.1.5 Adjustment of results A sample size of 3 will allow for a lamp lumen depreciation to be estimated for 3 x 6,000 hours. It is generally accepted that 50,000 hours of useful life will be expected if not more.
  • 4. Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 4 2.2. Extrapolation Criteria ¨C ¡°Combined Method¡± TM-28 allows a second method of predicting lumen maintenance by combining LM-80/TM-21 data with LM-84/TM-28. The sample size and multiplication factors being slightly different: Extract: TM-28-14, 5.2.7 Adjustment of results A sample size of 5 luminaires now only allows 1.5 x 6,000 hours = 9,000 hours. Not at all consistent with what the market requires of LED roadway luminaires. In order to reach a minimum market requirement of 50,000 hours, the manufacturer would have to test 15 luminaires for a period of 8,333 hours (1 year more-or-less) to apply the TM-28 (TM-21) formula. If third party tests were demanded, this would cost in the region of 60-70K USD without product cost or shipping to laboratories,
  • 5. Schr¨¦der AMEA Region Sidra Tower, Dubai, UAE Tel.: +971 4 420 40 23 Member of Schr¨¦der Group 5 3. Conclusions ? LM-84 Thermal testing using a thermal camera is neither useful nor practical ? LM-84/TM-28 do not address the specific concerns of LED roadway investors in terms of performance over useful lifetime within a reasonable technology cycle time frame ? The chip industry is currently evolving at a rate of, this year on last, +>20% light output per chip platform. If LM-84/TM-28 are invoked, by the time the luminaire manufacturer has completed the certification, they no longer have that technology within their luminaire (N.B. ¨C Manufacturers wait 6,000 to 10,000 hours of LM-80 before committing chip platforms to their products) The only obvious conclusion is that LM-84/TM-28 is more suited to products like retro-fit LED lamps where the technology/lumen output point is established and not required to push the performance boundaries of roadway and other demanding lighting applications like 2KW replacement floodlighting of a stadium for example. The conclusion of this paper is that, for performance LED roadway luminaires, a thermal test, LM-80 and TM-21 are the only current solutions for sensible comparison and quality assessment. 4. References IES LM-84-14 IES TM-28-14 IES LM-80-15 IES TM-21-11 IEC 60598