THE MAINTENANCE FACTOR FOR LED LIGHTING
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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
► Fluorescent lighting
► LED lighting
► Conclusion
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.
LLMF:
(1)
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:
LSF:
(2)
LMF:
(3)
MF = LLMF x LSF x LMF x RSMF (CIE97)
Whereby:
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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
RSMF:
(4)
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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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► Maintenance Factor
► Fluorescent lighting
► LED lighting
► Conclusion
Fluorescent lighting
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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
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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► Maintenance factor
► Fluorescent lighting
► LED lighting
► Conclusion
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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
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 + ( [Rth j-sp] x [VF] x [IF] )
TSP :
[Rth j-sp] :
VF:
IF :
solder-point temperature
thermal resistance between the LED junction and the solder
point of the LED.
forward voltage
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).
Figure 4: LLMF curve of U7 luminaire with Cree XPG LEDs , Ta=25°C
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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.
Short circuit LED in series connection
• current remains the same
• no accelerated aging other LED’s
1 Defect
1 Defect
Short circuit LED in parallel strings
• No more balanced current
• accelerated aging other LED’s (runaway)

LMF for LED lighting
Luminaire's maintenance factor: can be determined in the same way
as for fluorescent lighting.
The value of LMF that we use in annex 1 is 95%. This value is based
on the available reports (CIE97,TNO2004-GGI-R027) in combination
with ETAP’s experience and measurements.
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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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THE MAINTENANCE FACTOR FOR LED LIGHTING
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► Maintenance factor
► Fluorescent lighting
► LED lighting
► Conclusion
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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Terminology
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
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.
THE MAINTENANCE FACTOR FOR LED LIGHTING
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Annexe 1: Maintenance factor of ETAP LED products (status February 2013)
For the latest version, please check www.etaplighting.com
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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.
February 2013
© 2013, ETAP NV
ETAP NV
Antwerpsesteenweg 130 ● B-2390 Malle ● Tel. +32 (0)3 310 02 11 ● Fax +32 (0)3 311 61 42
e-mail: info.be@etaplighting.com ● www.etaplighting.com
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THE MAINTENANCE FACTOR FOR LED LIGHTING