LIGHTING DESIGN
Room Index
The absolute values of room
dimensions are not important; it is the relationship between area, parameter
and height of the source of light which matters. It is found convenient to
describe room proportions in term of room index.
Room Index, k = LW
Hm (L + W)
Where,
L = Length
W = Width
Hm = height of luminaries above the
working plane
Mounting Height (HM)
Mounting height usually is the
vertical distance between a luminaire and the working plane, but it is
sometimes the distance between the luminaire and the floor.
Utilization Factor (UF)
U.F is the local flux reaching
the working plane (or some other specified surface) divided by the total lamp
flux. The value of the ratio depends partly on the room size and reflectance.
Light Loss Factor (LLF)
The ratio of the illuminance
provide at some stated time to the initial illuminance i.e after 100 hours. LLF
is the product of the lamp lumen maintenance factor (LLMF), the luminaire
maintenance factor (LMF) and the room surface maintenance factor (LMF) and the
room surface maintenance factor (SRSMF)
Lumens and lumen-hours
It would strictly be better to
say “lumen output” or “total luminous flux” since the lumen measure flow rather
the cumulative quantity a lamp which fail after 1100 hours delivering an
average flux of 700 lumens has given a light output of 770,000 lumen-hours, but
rarely concerned with this and in ordinary way we speak of a “light output” of
700 lumens.
Illuminance
The illuminance of a surface is a
measure of the concentration of light falling on it. To express it in numbers
we need unit of light the lumen (lm). Thus we can say that the output of a 18W
PLC tube downlight is 1250 lumens or that the recommended illuminance for a
particular location is so many lumens per unit area.
Lux
Illuminance is measured in lumens
per square meter “lux”
Example for required Light Quantity
The store
measuring 18.5m x 8.4m with 3.85 lamp height is to be provided with an
illuminance consistence with good practice.
·
150 LUX requirement
·
Using 2 x 36W Mirror recessed light fitting (5400 lumens)
·
Assume reflectance of the ceiling is 0.5 and walls is 0.3
The first step is
to calculate the flux received on the room.
Flux received =
illuminance x area
=
150 x (18.5 x 8.4)
=
23,310 lumens
The flux which
must be installed is such that when multiplied by utilization factor and by the
light loss factor the resulting product is equal to the flux received.
Flux installed x UF x LLF = 23,310
In absence of
special information it is normal to assume the light loss factor to be 0.75. To
find the utilization factor we must first work out the room index
Room Index, k = LW
Hm (L + W)
= 18.5 x 8.4
3.85 (18.5 + 8.4)
=
1.5
Turning to the
table utilization factor giving values for this type luminaire (Appendix I),
for a room index 1.5 and ceiling and wall reflectances of 0.5 and 0.3 respectively
we find that the UF is 0.38.
Installed Flux = 23,310
0.38 x 0.75
=
81,894.74 lumens
Required quantity of light = 81,894.74
5,400
= 15.16 Nos.
Thus the number of Light fitting
needed is 16 Nos.
In practice, the
fast calculation can do as follows:-
Quantity of Light Fitting = Area x Lux
Lumens x LLF x UF
= (18.5 x 8.4) x 150
5400 x 0.75 x 0.38
=
15.145
=
16 Nos.
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