Luminaire efficiency factor system for
general lighting
Axel Stockmar
LCI Light Consult International
D-29227 Celle
KEY WO R D S
general lighting, luminaire efficiency, utilisation factor,
European utilisation factor method, European luminaire
efficiency labelling system.
A B S T RA C T
Lighting requirements concern the criteria illuminance
level, luminance distribution, glare limitation, modelling,
colour appearance, and colour rendition. Besides these
photometric quality criteria special attention is paid to the
costs for acquisition, installation, maintenance, and operation of the illumination. Here it is of particular interest to
what extent the total costs could be reduced by utilising
lamps with higher efficacy, ballast with lower losses, and
luminaires with higher light output ratios and more appropriate luminous intensity distributions. For lamps and
ballasts appropriate classification and labelling systems
have been developed and are in use throughout Europe.
For luminaires there are a number of proposals for energydependent classification and/or labelling procedures, but
for various reasons non of the methods has been developed
towards a usable system. As luminaires are being designed
to fulfil particular needs, it seems to be impossible to create
a system which would cover all luminaires. However for luminaires to be used for general lighting of e.g. offices,
stores, workshops, sports facilities etc. a luminaire efficiency factor system could be developed based on an utilisation
factors. As the utilisation factor is in proportion to the light
output ratio and is dependent on the luminaire flux distribution, the luminaire layout, and the reflection properties
of a space, it is necessary to define appropriate standard
conditions. Concerning the luminous flux distribution
there is a need for more than one category, i.e. direct/semidirect, direct/indirect, and semi-indirect/indirect. To keep
the system simple there should be only a few standard
rooms, but with two luminaire arrangements for ceiling
mounted and suspended installations. The reflectances of
ceiling and walls should be suitable also for luminaires
with considerable upward flux components. Based on a
comprehensive European utilisation factor method a luminaire efficiency factor labelling system for general lighting
will be discussed.
INTRODUCTION
Lighting requirements concern the criteria illuminance
level, luminance distribution, glare limitation, modelling,
colour appearance, and colour rendition. Besides these
photometric quality criteria special attention is paid to the
costs of acquisition, installation, maintenance, and operation of the illumination. Here it is of particular interest to
what extent the total costs could be reduced by utilising
lamps with higher efficacy, ballasts with löwer losses, and
luminaires with higher light output ratios and/or more appropriate luminous intensity distributions. For lamps and
ballasts appropriate classification and labelling systems
have been developed and are in use throughout Europe.
For luminaires a number of proposals have been emerged
for energy-dependent classification or labelling procedures, but none of the ideas has been developed towards an
approved system. The need for such a system has been
stressed by the European Commission, which has initialised activities to create a mandated European standard. As
luminaires are being designed to fulfil particular needs, it
seems to be impossible to create a system, which would
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cover all types of luminaires at the same time, such as
floodlights, wall washers, and/or road lighting lanterns.
However, for luminaires to be used for general lighting of
e.g. offices, stores, workshops, sports facilities etc. a luminaire efficiency factor system could be developed based on
utilisation factors. At the moment there are several utilisation factor methods being used in Europe showing remarkable differences. A European approach towards a
luminaire efficiency factor labelling system requires as a
first step a harmonised European utilisation factor method. Lamps, ballasts, and luminaires are usually regarded as
independent components having different measures for
energy efficiency classifications. But in many cases the
combined effect of lamp, ballast, and luminaire type matter, expressed in e.g. system efficacy. If this is specified in
addition, it would help to take into account incompatibilities, ambient temperature influences etc.
FE A S I B I L I TY S T U D I E S A N D P R O P O S A L S
The ratio of the achievable illumination level, expressed
in terms of an average illuminance on a reference surface
(usually the utilisation plane), to the necessary electric
power depends on the selected lamps, ballasts, and luminaires as well as on the reflectances of all interior surfaces.
For lamps and ballasts there are appropriate measures, efficacy and ballast-lamp circuit power respectively, which
serve as a basis for energy efficiency labelling systems.
However, for luminaires the obvious measure, the light
output ratio, is not a suitable quantity, as there exists no relationship to the achievable illumination level. In an early
study [1] it was concluded that for general lighting in interiors only a system based on utilisation factors could describe photometric properties while taking into account
the necessary electric energy, as the utilisation factor is in
proportion to the light output ratio and is dependent on
the luminaire flux distribution for given standard conditions. Furthermore it was proposed to consider at the same
time the glare limitation, as for some highly effective luminaires the glare could be exceedingly high. When this study
has been made public [2] it was felt that the system should
be developed further to cover more than one standard
room, but without glare considerations. This was regarded
not simple enough for a labelling system. In a different
proposal [3] the light output ratio was considered as the
suitable measure to describe the energy efficiency of luminaires. In building regulations where energy efficiency is
expressed in terms of luminaire lumens per circuit watt the
light output ratio plays an important role, but it will not
describe the lighting performance of the installation. The
more recent proposal [4] for the representation of luminaire efficiency factors based on utilisation factors for three
standard rooms with room indices k=1.00, k=2.00, and
k=5.00 seems to be a logic step towards a luminaire efficiency labelling systems. Unfortunately the
envisaged conversion of the room indices into approximate floor plan area sizes is misleading and valid only for
ceiling mounted luminaires. The room index depends not
only on length and width of the interior but also on the
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mounting height of the luminaires, i.e. the distance between luminaire plane and reference plane. For a given suspension ratio SR, the ratio of suspension height to
mounting plus suspension height, a room index of for example k=5,00 for a suspended luminaire arrangement corresponds with a room index of only 3.5 to 4.0 for a ceiling
mounted luminaire arrangement (figure 1) in the same interior, which makes it very difficult to compare different
lighting systems. In other words, the floor area leading to
the same room index differs by a factor of up to two dependent on the suspension ratio (figure 2). A further problem arises from the fact that there are several utilisation
factor methods being used in Europe showing considerable differences.
U T I L I S A T I O N FA C TO R M E T H O D S B E I N G U S E D I N
EUROPE
Throughout Europe there are at the moment several
methods being used for the calculation of utilisation factors In particular these are the British as described in CIBSE Technical Memoranda No 5 [5], the German as
published in LiTG Publication 3.5 [6], the French as given
in the French Norm NF C71-121 [7], the Nordic as part of
the NB-Documentation of luminaires [8], and the CIEMethod as specified in Publications No 40 [9] and No 52
[10]. In all utilisation factor methods considered it is assumed that the interiors are empty rectangular parallelepipeds. The working plane is assumed to be materialised by
a surface, which will be counted as one of the room surfaces - with reflectance of the floor cavity. All room surfaces
(ceiling, walls, and working plane) reflect uniformly and
according to Lambert's law. The luminaires are arranged in
regular patterns in the luminaire plane at a specified distance above the working plane. The indirect component of
the utilisation factors is calculated using the common flux
transfer theory. Tabulated transfer factors show virtually
no differences whether they have been evaluated for rooms
with a square plan or with a rectangular plan (length to
width equal to 1.6 to 1). Differences experienced between
the different utilisation factor methods are caused by the
procedures applied to the calculation of the utilization factors for direct illumination. The important influencing
factors are the luminaire layout - described best by the
spacing-to-height ratio and the proximity - and the way in
which the direct flux onto the working plane is calculated
– zonal multipliers, stripe multipliers, point-by-point calculation etc.
For the standard room with room index k=2.00 the luminaire layouts used in the different utilisation factor
methods are shown in figure 3. Similar differences in luminaire arrangements are found for all room indices resulting
in considerable differences between the utilisation factors
obtained. An example is given in figure 4 – for the proposed reflectance combination. Following the proposal for
representation of lighting efficiency factors [4] but using
the room index instead of the plan area size as parameter
leads in consequence to different results applying the different utilisation factor methods (figure 5). The discrepancies are larger for smaller room indices, but also smaller
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discrepancies for larger room indices could have quite
some commercial impact.
RE Q U I R E M E N T S FO R A L U M I N A I R E EF F I C I E N C Y
FA C TO R L A B E L L I N G SYS TE M
Utilisation factors for a set of standard rooms seem to be
an adequate measure to form the basis for a luminaire efficiency factor labelling system. From the presented results it
is obvious that there is still a need for a common European
utilisation factor method. A harmonised method should
reflect as far as possible current practice. Comparisons
have shown that utilisation factors calculated according to
the CIE method are somewhere in the middle of the bandwidth of the utilisation factors obtained using the other
methods (figure 4) [11]. Therefore a
European utilisation factor method should apply the
CIE method for the calculation of the utilisation factors for
direct illumination. To take advantage of more pronounced luminous intensity distributions the method
should allow for luminaire arrangements with different
spacing-to-height ratios, preferably between 1.00 and 2.00
in steps of 0.25. For the luminaire efficiency factor system
it is proposed to fix the spacing-to-height ratio at 1.00, otherwise it has to be stated every time. In addition to ceiling
mounted systems the method should be extended to enable the calculation of utilisation factors for suspended arrangements. This would make it possible to compare
different systems regarding luminaire efficiencies. In this
case a suspension ratio has to be standardised; a realistic
suspension ratio would be _. The standard sets of reflectance combinations should include the one proposed for
the representation of luminaire efficiency factors, i.e. 0.70
for ceiling, 0.50 for walls, and 0.20 for the floor cavity, and
also a set with higher reflectances for lighting systems with
considerable indirect components [12].
CO N C L U S I O N S
Provided a comprehensive European utilisation factor
method could be established, calculated utilisation factors
for well-defined standard conditions could be used for a
luminaire efficiency factor labelling systems. It seems to be
necessary to distinguish between ceiling mounted and suspended luminaires to make useful comparisons possible.
The borderline between ceiling mounted and suspended
luminaires could be set using a downward flux fraction of
0.90. Furthermore, for indirect lighting systems with
downward flux fractions smaller than 0.10 it is probably
more realistic to use higher than proposed standard reflectances for ceilings and walls. In any case the mounting
mode should be stated in the luminaire efficiency factor label and/or the conversion from room index to floor plan
area size should reflect the standard suspension ratio. In
addition it could be useful to complement the luminaire
efficiency factor label with information concerning the luminaire lumens per circuit watt – here called the system efficacy – which is needed to check conformity with some
building regulations as mentioned in an earlier proposal
[3]. This could help to identify energy efficient luminaires,
which can use only less efficient lamps or ballasts (figure
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6). The only problem remaining is the fact that very efficient luminaires could cause considerable glare which will
not be notified. Therefore the glare limitation has to be
evaluated separately or a glare dependent modifying factor
would have to be introduced.
B I B L I O G RA P HY
[1] Stockmar, A.Evaluation of a Luminaire Specific Index
for the Description of Photometric Properties of Interior Luminaires Taking into Account the Efficient Use
of Energy, Study on behalf of CELMA, 1994.
[2] Stockmar, A.Proposal for a Luminaire Specific Index
for the Description of Photometric Properties of Interior Luminaires Taking into Account the Efficient Use
of Energy, Proceedings RIGHT LIGHT 4, Volume 2, p.
41-45, 1997.
[3] Hampton, B.Conservation of Fuel and Power: Regulations for Energy Efficiency to Protect the Environment
– ‘An Industry Proposal’, CIBSE National Lighting
Conference Lancaster,1998.
[4] CELMA Proposal for luminaire efficiency factor presentation, CEN TC 169 document N 418, 2001.
[5] CIBSE Technical Memoranda 5, The calculation and
use of utilisation factors, 1980.
[6] LiTG Publikation 3.5, Projektierung von Beleuchtungsanlagen nach dem Wirkungsgradverfahren,
1988.
[7] NF C71-121, Méthode simplifiée de prédétermination
des éclairement dans les espaces clos et classification
correspondante des luminaires, UTE, 1993.
[8] NBDOC, A computer program for NB-documentation of luminaires, LTLI Notat 248, 1987.
[9] CIE Publication No 40, Calculations for interior Lighting, Basic method, 1978.
[10] CIE Publication No 52, Calculations for interior lighting, Applied method, 1982.
[11]Stockmar, A.Comparison of Utilisation Factor Methods, CEN TC 169 / WG2 document N184, 2001.
[12] Stockmar, A.Proposal for a European Utilisation Factor Method, Proceedings ILUMINAT 2001, paper no.
4, p. 1-4.
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Figure 1: Room Index Dependence for Different Suspension Ratios
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Figure 2: Floor Area for same Room Index / Different Suspension Ratios
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Figure 3: Different Standard Luminaire Arrangements
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Figure 4: Comparison of Utilisation Factor Methods
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Figure 5: Comparison of Luminaire Efficiency Factors
Figure 6: Luminaire Efficiency Factors
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