An important aspect of a luminaire's photometric effect in practice is the portion of the overall luminous flux of its light sources incident on the working plane. It is often referred to as effective luminous flux. Only this factor contributes to providing the required average illuminance on a working plane where a visual task is performed. The ratio of effective and overall luminous flux is referred to as utilization factor UF.
Luminaire classification by luminous flux and efficiency method
For luminaires with replaceable lamps, UF is composed of light output ratio LOR and utilance U, which depends on the luminous flux distribution of the luminaire as well as room geometry and reflectance of the room surfaces.
The utilization factor serves to determine the number of luminaires required to provide a specified average illuminance Em on the working plane of a room (see also chapter "Lighting design").
The efficiency formula is as follows:
In the formula,
Ē m |
is the maintained illuminance in lx, e.g. according to EN 12464-1 |
A |
is the floor space of the room in m2 |
n |
is the calculated number of luminaires |
z |
is the number of lamps per luminaire |
Φ |
is one lamp’s luminous flux in lm |
UF |
is the utilization factor in %. It encompasses the light output ratio as well as the photometrically effective properties of the room such as ceiling, wall and floor reflectance as well as room geometry. |
MF |
is the maintenance factor (see also chapter ) |
Historically, different methods for determining utilization factor or utilance have become established in Europe. They all require a classification of the luminaire’s luminous intensity distribution regarding its distribution of luminous flux to a manageable number of defined room segments. The contributions to the effective luminous flux are then rated and summed up per room segment. The classification according to DIN 5040 is illustrated in table.
Table 3.134: Classification of luminaires’ luminous flux distribution according to DIN 5040. The example of the luminous flux distribution with classification B41 of the depicted luminous intensity distribution curve is indicated in the table.
The various methods for determining the utilization factor are e.g.
the CIBSE method in England according to Technical Memorandum TM 5 (1980) "The calculation and use of utilisation factors",
the LiTG method in Germany according to publication no. 3.5 (1988) "Projektierung von Beleuchtungsanlagen nach dem Wirkungsgradverfahren" or DIN 5040 "Leuchten für Beleuchtungszwecke",
the NB documentation method in northern European countries, specified as computer program in the NBDOC report,
the standard NBN L 14-002 (1975) in Belgium, "Methoden ter voorafbepaling van verlichtingssterkten, luminanties en verblindingsindices bij kunstmatige verlichting in gesloten ruimten",
the standard NF (UTE) C71-121 (1995) in France, "Méthode simplifiée de prédétermination des éclairements dans les espaces clos et classification correspondante des luminaires", and
the international method according to CIE publication no. 40 (1978) "Calculations for interior lighting: Basic method” and no. 52 (1982) "Calculations for interior lighting: Applied method".
All efficiency methods represent room geometry using the room index , which is defined as follows:
With (see figure)
a |
being room length in m |
b |
being room width in m |
h |
being mounting height in m, calculated according to |
h |
= H − lp − e |
H |
room height in m |
lp |
suspension length of the luminaire from the ceiling in m |
e |
height of the rating level above the floor, e.g. in an office 0,75 m |
Figure 3.166: Calculation parameters for the room index
Room geometries with an identical room index can be regarded as equivalent regarding their photometric effect.
The determination of the contribution of the aforementioned partial luminous flux values (of the room segments) to the effective luminous flux is then made using factors documented in relevant tables depending on room index and room boundary surface reflectance.
The existing efficiency methods vary in terms of classification (segmentation) of luminous intensity distributions as well as the table entries for rating factors. The calculated utilization factors and thus the number of luminaires required for a given lighting task also noticeably vary (see also chapter "Design approximation using the efficiency method").
With the publication of European standard EN 13032-2 "Light and lighting - Measurement and presentation of photometric data of lamps and luminaires – Part 2: Presentation of data for indoor and outdoor work places", a uniform efficiency method was established for Europe, which also defines a harmonised determination for utilization factor UF. The harmonised European method according to EN 13032-2 resolves previous differences in determining the utilization factor and makes the number of luminaires determined for a certain illuminance using the efficiency formula and the expected illuminance due to installed lamps and luminaires comparable.
In the early 2000s, the utilization factor UF tables generated using the calculation method according to EN 13032-2
for the luminaire in question (luminous flux distribution),
for various room sizes (room indexes k), and
for ceiling, wall and floor reflectance
were published in the product documentation of many manufacturers. Today, this is no longer conventional or required since the manual application of the method is no longer relevant in practice. It has been implemented into many, freely available computer programs (see also chapter "Design approximation using the efficiency method").