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Chartered Institute of Building Service Engineers
(CIBSE)
‘code for interior lighting design’
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The maintenance factor gives an estimate of how lighting
conditions will deteriorate through everyday use.
Dust and dirt
Aging of lamps
Cleaning of room surfaces
It should be noted that without detailed knowledge of the maintenance plan a maintenance factor of 0.8 must be used.
MF = RSMF x LMF x LLMF x LSF
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Where:
RSMF:
Room Surface Maintenance Factor
(dirt on the surfaces of the room)
LMF
Luminaire Maintenance Factor
(dirt on luminaire)
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Where:
LLMF:
Lamp Lumen Maintenance Factor
(dirt on the lamp)
LSF
Luminaire Survival Factor
(reduced light output due to lamps failing)
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UF = lumens received on working plane
lumen output of luminaires
Utilisation factor takes account of the loss of
light due to absorption on room surfaces
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Type of luminaire
Room index
Reflectance of room surfaces
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A luminaire with a concentrated light output
directed on the working plane will have a
higher UF than a luminaire with a dispersed
output
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This takes into account the length and
width of the given room, also the height of
the luminaires above the working plane
(Hm).
RI = L x W
(L + W) Hm
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Brighter colours with high reflectance result
in a higher UF.
Note: A high UF means fewer lamps are
needed (see formula) resulting in a more
energy efficient light design.
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Ceiling Walls
Floor
Room index
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A garage 20m long by 12m wide with a ceiling
height of 6m is required to be illuminated to a
level of 200 lx at a working plane height of 1 m.
The luminaires will be suspended 2m below the
ceiling height. The reflectance's are as follows:
Ceiling 0.5, Walls 0.5, Floor 0.2.
The utilisation factor can be calculated from the
previous table.
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Room index:
RI = L x W
(L + W) Hm Therefore:
= 20 x 12
(20 + 12) 3
= 240
96
= 2.5
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Therefore UF is ……..
C.W.F
RI
UF
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A garage 26m long by 16 m wide with a ceiling
height of 7m is required to be illuminated to a level
of 500 lx at a working plane height of 1.5 m. The
luminaires will be suspended 1.5m below the
ceiling height. The reflectance's are as follows:
Ceiling 0.3, Walls 0.5, Floor 0.2.
Calculate the utilisation factor from the previous
table.
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Room index:
RI = L x W
(L + W) Hm Therefore:
= 26 x 16
(26 + 16) 4
= 416
168
= 2.48 (2.5)
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UF
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This is the ratio of space between luminaires
to their installed height above the working
plane.
Manufactures will generally specify a
recommended SHR to ensure their products
best performance.
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Example
Room dimensions: Ceiling height = 4m
Working plane = 1m
(recessed luminaire)
Manufactures recommended SHR 2:1
Therefore: SHR = 2:1
= 6:3
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Calculate the number of luminaires required for a given room. Dimensions: L = 20m W = 40m H = 8m • Luminaire suspended 1.5m from ceiling • Working plane 1m Manufactures recommended SHR (1.5:1)
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Gap from working plane to luminaire = 5.5m
Therefore: SHR 1.5 : 1
8.25 : 5.5
Width = 40m = 4.8 luminaires
8.25
Length = 20m = 2.4 luminaires
8.25
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s o u rce I ca n d e la
flo w lu m e n s
i l l u m in a n ce E lu x
Fig . 1 .9
Luminous Intensity
Luminous Flux
Illuminance
Luminance
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Luminous Intensity
The measure of light from a source .i.e. a lamp. It
is measured in candelas (cd).
Symbol: I
Luminous Flux
It is the rate at which light flows from a source. It is
measured in lumens (lm).
Symbol: F
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Illuminance
The measure of the amount of light falling on a surface. It is
measured in lux (lx).
Symbol: E
Luminance
Is the measure of the light intensity leaving the surface which
has been illuminated by a given source. It is measured in
candelas per meters squared (cd / m²)
Symbol: L
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Typical recommended maintained Lux levels:
Corridors and stairs 100
Warehouses 100 - 200
Medium bench and machine work 500
Fine painting spraying and finishing 750
Printing inspection 1000
Proof reading / drawing offices 750
General offices (desk based) 500
General offices (screen based) 300
Supermarkets 750
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E = (F x n) x N x MF x UF
A
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Where:
E = Average Illuminance (lux)
F = Initial lamp lumen output (lm)
N = Number of luminaires
n = Number of lamps per luminaire
MF = Maintenance factor
UF = Utilisation factor
A = Area (m²)
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This method is most suitable for interior lighting design,
where a high proportion of light on the working plane is
reflected by internal surfaces.
The lumen method, sometimes called the luminous flux
method of calculation, is normally used to calculate the
average illuminance (Lux) on working planes, or to calculate
the number of luminaires required providing a specified
average illuminance in rooms.
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A factory floor, 30m x 20 m, is to be illuminated
with 2 x 58 W LPMV luminaires. Calculate the
number of luminaires needed to maintain an
average illuminance of 500 lux. The initial lamp
output is 5,200 lumens. Calculations are to be
carried out using a MF of 0.8 and a UF of 0.7
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E = (F x n) x N x MF x UF
A
Therefore:
N = E x A
(n x F) x MF x UF
N = 500 x (30 x 20)
(2 x 5,200) x 0.8 x 0.7
= 51.51 (52 luminaires)
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A hotel reception area, 18m x 11 m, is to be
illuminated with 40, 70 W, 2 pin pod MBI
luminaires. The initial lamp output is
6,600lm, the MF is 0.8 and the UF is 0.65.
Calculate the average illuminance in the
reception area. Also calculate the efficacy of
the lamp.
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E = (F x n) x N x MF x UF
A
= 6600 x 40 x 0.8 x 0.65
18 x 11
= 693.33 lux
Efficacy = 6600 / 70 = 94.3 lm/W