
Lumen Method The quantity of light reaching a certain surface is usually the main
consideration in designing a lighting system. This quantity of light is specified by illuminance measured in lux, and as this
level varies across the working plane, an average figure is
used. CIBSE Lighting Guides give values of illuminance that are suitable for various areas. The section 
Lighting Levels in these notes also gives illuminance
values. The lumen method is used to determine the number of lamps that should be
installed for a given area or room. Calculating for the Lumen
Method The method is a commonly used technique of lighting design, which is
valid, if the light fittings (luminaires) are to be mounted overhead in a regular
pattern. The luminous flux output (lumens)
of each lamp needs to be known as well as details of the luminaires
and the room surfaces. Usually the illuminance
is already specified e.g. office 500 lux, kitchen 300
lux, the designer chooses suitable luminaires and then wishes to know how many are required. The number of lamps is given
by the formula:
where, N = number
of lamps required. E = illuminance level required (lux) A = area
at working plane height (m^{2}) F = average
luminous flux from each lamp (lm) UF= utilisation
factor, an allowance for the light distribution of the luminaire
and the room surfaces. MF= maintenance
factor, an allowance for reduced light output because of deterioration and
dirt. Example
1 A production area in a factory measures 60 metres x
24 metres. Find the number of lamps required if each lamp has
a Lighting Design Lumen (LDL) output of 18,000
lumens. The illumination required for the factory area is 200 lux. Utilisation factor = 0.4 Lamp Maintenance Factor = 0.75
N = ( 200 lux x 60m x
24m ) / ( 18,000 lumens x 0.4 x 0.75 ) N = 53.33 N = 54 lamps. Spacing The aim of a good lighting design is to approach uniformity in illumination over the
working plane. Complete uniformity is impossible in practice, but
an acceptable standard is for the minimum
to be at least 70% of the maximum
illumination level. This means, for example, that for a room with an
illumination level of 500 lux, if this is taken as
the minimum level, then the maximum level in another part of the room will be
no higher than 714 lux as shown below. 500 / 0.7 = 714
lux Data in manufacturer's catalogues gives the maximum
ratio between the spacing (centre to
centre) of the fittings and their height
( to lamp centre) above the working plane (0.85
metres above f.f.l.)
Example 2 Using data in the previous example show the
lighting design layout below. The spacing to mounting height ratio is 3 : 2. The mounting height (H_{m})
= 4 metres. The spacing between lamps is calculated from from Spacing/H_{m} ratio
of 3 : 2. If the mounting height is 4 m then the maximum
spacing is: 3 / 2 = Spacing / 4 Spacing = 1.5 x 4
= 6 metres _{ } The number of rows of lamps is calculated by
dividing the width of the building (24 m) by the spacing: 24
/ 6 = 4 rows of lamps This can be shown below. Half the spacing is used for the ends of
rows.
The number of lamps in each row can be calculated
by dividing the total number of lamps found in example 1 by the number of rows. Total lamps 54 / 4 = 13.5 goes up to nearest whole number = 14 lamps in each row. The longitudinal spacing between lamps can be
calculated by dividing the length of the building by the number of lamps per
row. Length of building 60 m
/ 14 = 4.28 metres. There will be half the spacing at both ends = 4.28 / 2 = 2.14 metres This can be shown below.
The
total array of fittings can be seen below.
For more even spacing the layout should be
reconsidered. The spacing previously was 6 m between rows and
4.28 m between lamps. If 5 rows of 11 lamps were used then the spacing
would be: Spacing between rows = 24 / 5 = 4.8 metres Spacing between lamps = 60 / 11 = 5.45 metres Installed
Flux Sometimes it is
useful to know the total amount of light or flux, which has to be put into a space.
Installed flux (lm) = Number of
fittings (N) x Number of lamps per
fitting x L.D.L. output of each lamp (F) Example 3 A factory measuring 50m x 10m has a lighting scheme
consisting of 4 rows of 25 lighting fittings each housing 2No. 65Watt fluorescent lamps. (a) Find
the installed flux in total. (b) What
is the installed flux per m^{2} of floor area. The output of the lamps in the above example may be
found from catalogues. For a 65Watt fluorescent lamp the Lighting Design
Lumens (LDL) is 4400 lm. (a) Installed flux (lm) = N x no. lamps/fitting x F = 4 x 25 x 2 x 4400 = 880,000 lumens (b) The floor area = 50 x 10 = 500 m^{2}. Installed flux per m^{2} = 880,000 / 500 = 1760 lm/m^{2}. Example
4 A room measures 15m x 7m x 3.6m high and the design
illumination is 200 lux on the working plane (0.85
metres above the floor). The Utilisation factor is 0.5 and the Maintenance
factor is 0.8. If the LDL output of each fitting is 2720 lumens,
calculate; (a) the number of fittings required. (b) the fittings layout. (c) If
the spacing/mounting height ratio is 1 : 1 determine
whether the current design is acceptable.
(a)
Number of fittings. N = ( 200 x 15 x 7 ) /
( 2720 x 0.5 x 0.8 ) N = 19.3 N = 20 lamps (b)
Fittings layout For shallow fittings, the mounting height (H_{m}) may be taken as the distance form the
ceiling to the working plane. Therefore H_{m} = 3.6  0.85 H_{m} = 2.75 metres If
3
rows of 7 fittings are considered then the spacing
is;
(c) Spacing/ mounting height. Spacing / H_{m}
ratio: 2.33 / 2.75 = 0.847 Therefore
ratio is 0.85 : 1.0 2.14
/ 2.75 = 0.778 Therefore
ratio is 0.78
: 1.0 Example
5 A room, as shown below, has a design illumination
is 500 lux on the working plane (0.85 metres above
the floor). The Utilisation factor is 0.5 and the Maintenance
factor is 0.8. If the LDL output of each fitting is 2720 lumens,
calculate; (a) the number of fittings required. (b) the fittings layout. (c) If the spacing/mounting
height ratio is 1 : 1 determine whether the current
design is acceptable. (a) N = ( 500 x 10 x 12 ) / ( 2720 x 0.5 x 0.8 )
N = 55.15 N = 56 lamps.
(b) Spacing, say 8 lamps x 7 rows. Spacing along 12 m wall = 12
/ 8 = 1.50 m Spacing along 10 m wall = 10
/ 7 = 1.43 m (c) Mounting height = 3.0  0.85 = 2.15
m Desired Ratio =
1:1 Actual ratio = 1.5 / 2.15 = 0.69 Therefore ratio is 0.69 : 1.0 Actual ratio = 1.43 / 2.15 = 0.67 Therefore ratio is 0.67 : 1.0

Copyright © 2003 A. McKeegan MIET
All Rights Reserved