THE MAINTENANCE FACTOR FOR LED LIGHTING

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WHITE PAPER

INTRODUCTION

During the lifetime of a lighting installation there is a decline of light on the work surface due to pollution and ageing of the installation and room surfaces. In order to factor in this decline, the so-called maintenance factor (also referred to as depreciation factor) was created.

The values usually applied for fluorescent lighting are based on independent studies, such as CIE97 and TNO (TNO 2004-GGI-R027). These values are mainly independent of the useful service life of a lighting installation (i.e., the period over which an installation is effectively used).

For LED lighting there are few studies for the determination of the correct maintenance factor and therefore no market consensus. As a result the same maintenance factor is often used as that of fluorescent lighting, which over time can have a negative impact on your LED installation since the useful service life does play a crucial role here.

In this paper we will point out the difference between maintenance factor for fluorescent and LED lighting. The maintenance factor for fluorescent lighting will first be discussed, and subsequently the dissimilarities with LED lighting will be emphasised.

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Maintenance factor

The EN12464-1 standard determines the required luminous intensity. This is the minimum light level to be safeguarded, independently from the number of the installation's burning hours and service life. The light quantity on the work surface will decrease during this service life, which can be the result of declining luminous flux of the light sources, defective light sources and pollution of luminaires or spaces. A ‘surplus' must therefore be provided for calculations of the lighting design and in order to calculate the latter, the maintenance factor was introduced. (CIE 97).

This maintenance factor (MF) is a reduction factor, which is combined with the initial luminous intensity (Ei) in the dimensioning of the installation in order to

achieve the required luminous intensity (Em):

Em= Ei x MF (CIE 97)

E.g.: 500lx = 625lx x 80%

In order to maintain the required luminous intensity of 500lx, an initial luminous intensity of 625lx must be installed.

EN12464-1 states that the following must be taken into account in the determination of the

maintenance factor:

 Decrease in the lamp's luminous flux (1)  Frequency of lamp defects without immediate

replacement (2)

 Decline in luminaires' output due to pollution (3)

 Space pollution (4)

These four parameters are included in the maintenance factor definition in the form of four multiplication factors:

MF = LLMF x LSF x LMF x RSMF (CIE97) Whereby:

LLMF: Lamp Lumen Maintenance Factor

LSF: Lamp Survival Factor (the lamp's service life without immediate replacement )

LMF: Luminaire's Maintenance Factor

RSMF: Room Surface Maintenance Factor

LLMF: (1) LSF: (2) LMF: (3) RSMF: (4)

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Fluorescent lighting

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LED lighting

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Conclusion

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Documenting the maintenance factor

EN12464-1 prescribes that those carrying out the lighting study, must reference the assumptions (regarding both the reduced lamp luminance and pollution of luminaires and space) made to use a specific maintenance factor. These assumptions must be included in the study.

After all there is a risk that incorrect comparisons are made in calculations between manufacturers due to a difference in assumptions with respect to the maintenance factor.

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Independent studies (such as CIE97 and TNO2004-GGI-R027) in combination with many years of experience are used to determine and calculate the maintenance factor for fluorescent lighting.

 LLMF for fluorescent lighting

LLMF is the relative light output during the service life of a burning lamp compared to its initial light output. This factor is provided by the lamp’s manufacturer and is specified in function of the burning hours of the lamp .

The LLMF in a lighting study depends on the replacement cycle of the lamps; the LLMF that is used is the value until replacement of the lamp. As a result the total maintenance factor (MF) will depend on the

replacement cycle of the lamps and not the useful service life of the luminaire.

A typical value used for the LLMF of fluorescent lighting is 90%.

Figure 1: sample LLMF graphic for fluorescent lighting

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Maintenance Factor

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LED lighting

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 LSF for fluorescent lighting

LSF is the likelihood that the lamps continue to burn for a specific period of time. In the report TNO2004-GGI-R027 a distinction is made between two different lamp replacement plans:

o Group replacement of the lamps: all lamps are replaced at the same time after a defined average reduced lamp luminance o Individual replacement combined with group replacement: if a

lamp is defective before the group replacement, it is replaced immediately

In the event of an individual replacement combined with a group replacement LSF=1 can be taken into account

 LMF for fluorescent lighting

LMF is the relative light output of a luminaire due to dirt deposit on the lamp and on (or in) the luminaire over a specific period. Depending on the application and maintenance frequency, a value is applied. Both the CIE97 and TNO report provide sample values in table form.

 RSMF for fluorescent lighting

RSMF is the relative proportion of the initial inter-reflected component of illuminance from the installation after a certain period due to dirt on room surfaces.

In addition to the maintenance interval and the area of application, the reflection values for the various surfaces also play a role in determining the RSMF. In order to do so, both the CIE97 and the TNO report provide sample values in table form.

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LED lighting

For LED lighting the determination and underpinning of the maintenance factor requires more thought. The determination of the pollution degree for the luminaire (LMF) and for the space (RSMF) is analogous to that of fluorescent lighting, but especially the reduced light source luminance (LLMF) requires a specific approach. Given the long service life of LEDs the majority of LED luminaires are used without one lamp, or in this case LED replacement taking place during the luminaire's useful service life. Contrary to fluorescent lighting the LLMF, and consequently also the maintenance factor, depends on the useful service life of the luminaire . For the maintenance factor in light calculations with LED luminaires, it is therefore necessary to indicate which useful service life was used. This parameter is of critical importance in the determination of the initial light level and the number of installed luminaires, in order to be able to guarantee the necessary light level at the end of the useful service life. Calculation with a useful service life of 20,000 hours will therefore provide a different result than a calculation with a useful service life of 50,000 hours. Contrary to fluorescent lighting a single standard maintenance factor cannot be used for LED lighting, regardless of the installation's service life.

Figure 2: LLMF for LED lighting compared to fluorescent lighting

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LED lighting

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 LLMF for LED lighting

As to LED lighting the light source can no longer be considered independently from the luminaire. The LED luminaire's manufacturer chooses the brand and type of LED.

Performance, service life and maintenance of the luminous flux (LLMF) are determined by a combination of choice of LEDs (manufacturer and type Low/Medium/High Power) and design of the LED luminaire. The combination of these two parameters is unique for each luminaire.

1. Choice of LEDs (package design & materials)

In order to publish valid values for the service life and LLMF of LED luminaires (see annex 1), ETAP exclusively works with manufacturers who are able to provide the necessary data that allows determining the service life and lumen retention of the used LEDs in a specific

luminaire.

The requested data must be drawn up with the help of two globally accepted measuring methods: the LM80 and the TM21.

2. Thermal design

Heat has a negative impact on the operation of LEDs. Good thermal management is required in order not to allow the junction temperature (Tj) around the LEDs to rise.

High temperatures have a negative impact on the performance of the LEDs and associated luminaires and can cause colour shifts.

ETAP becomes low junction temperature by making use of very low power to control the LEDs, combined with advanced thermal design.

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The junction temperature is not measured directly, but can be determent by following equation;

TJ = TSP + ( [Rthj-sp] x [VF] x [IF] ) TSP : solder-point temperature

[Rth j-sp] : thermal resistance between the LED junction and the solder

point of the LED. VF: forward voltage

IF : forward current

Figure 3: LED component

The specific junction temperature, together with the LM-80/TM-21 result in a representative curve for the LLMF for each type of luminaire (curve according to Weibull distribution).

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 LSF for LED lighting

If the effective failure rate of LEDs during their useful service life is less than 0.5% and failures are uncorrelated, LSF =1 can be considered.

The influence of a defective LED depends on the electrical design. If the LEDs are connected in series and we assume that a defective LED results in a short circuit (which seems to be the case in practice for LEDs in 99% of cases) a defective LED will have no impact on the operation of the remaining LEDs. For other scenarios (e.g., several parallel strings), however, this no longer applies.

The most critical component in an LED luminaire is the driver. In most cases account is taken of a spot replacement for the driver, so that it does not affect the LSF.

 LMF for LED lighting

1 Defect

Short circuit LED in series connection • current remains the same

• no accelerated aging other LED’s

Short circuit LED in parallel strings • No more balanced current

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 RSMF for LED lighting

Area maintenance factor: can be determined in the same way as for fluorescent lighting.

The values of RSMF that we use in annex 1 are different for office or industry application;

• Office RSMF=94% ; Reflectances of 70/50/20 in a clean environment and cleaning interval of 3 year.

• Industry RSMF=89% ; Reflectances of 50/30/10 in a normal environment and cleaning interval of 3 year.

These values are the same for fluorescent or LED lighting and are based on the available reports (CIE97, TNO2004-GGI-R027) in combination with ETAP’s experience.

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Conclusion

 Contrary to fluorescent lighting a standard maintenance factor cannot be used for LED lighting.

 With LED lighting the light source can no longer be considered independently from the luminaire. Performance, service life and the reduced light source luminance (LLMF) of the luminaire are determined by a combination of choice of LEDs and the thermal design of the LED luminaire. The combination of these two parameters is unique for each luminaire.

 For a correct comparison between different types of LED luminaires in a light study, you need to calculate with their specific maintenance factor. The difference in MF is mainly due to their proper LLMF, since the other parameters (soiling of luminaire and room) remain the same for different types of luminaires in one project.

 The expected useful lifetime is a critical parameter to determine the MF and consequently needs to be included in light studies with LED luminaire.

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LED lighting

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Conclusion

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Terminology

 EN12464-1: European lighting application standard. The original standard was drawn up by Work Group 2 of Technical Committee (TC) 169 of the European Committee for Standardisation (CEN).

 CIE 97; Guide on the maintenance of indoor electric lighting systems  LM-80-08: IES (illuminating Engineering Society) approved method for

measuring the lumen retention of LED light sources with representative measurements every 1,000 hours and this over a minimum period of 6,000 hours. It addresses the measurement of lumen maintenance testing for LED light sources including LED packages, arrays and modules only. It does not provide guidance or recommendations regarding prediction estimations or extrapolations for lumen maintenance beyond the limits of the lumen maintenances determined from actual measurements.

 TM-21-11: This IES document recommends a method for projecting the lumen maintenance of LED light sources from the data obtained by the procedures found in IES document LM-80-08 Approved Method for Measuring Lumen Maintenance of LED Light Sources.

 TNO report: 2004-GGI-R027, 'Determination of the Maintenance Factor for EN12464'. Report drawn up by TNO, Building and Construction Research, at the request of ETAP, Philips, Osram and Zumtobel.

 Junction temperature: the highest temperature of the actual semiconductor in an electronic device. The temperature of the PN junction in a LED.

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This document has been composed by ETAP with greatest care. However, the data in this publication are without any obligation and can change in pursuance of technical evolution. ETAP accepts no liability for damage, of any kind, that results from using this document.

ETAP NV

Antwerpsesteenweg 130 ● B-2390 Malle ● Tel. +32 (0)3 310 02 11 ● Fax +32 (0)3 311 61 42 e-mail: [email protected]●www.etaplighting.com