Lumen Method Illuminance: The quantity of light reaching a certain surface. Its measured inlux,and as this level varies across theworking plane, an average figure is used.CIBSELighting Guides (Chartered Institute of Building Services Engineers)give values ofilluminancethat are suitable for various areas. The section - Lighting Levels in these notes also givesilluminancevalues.

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 MethodThe 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 theluminairesand the room surfaces.Usually theilluminanceis already specified e.g. office 500lux, kitchen 300lux, the designer chooses suitableluminairesand then wishes to know how many are required.

1- Number of LampsThenumber of lampsis given by the formula:LLFCU

where,N:number of lamps required.E:illuminancelevel required (lux or lm/ m2)A:area at working plane height (m2)F:average luminous flux from each lamp (lm) CU :Coefficient ofutilization utilisation factor, an allowance for the light distribution of theluminaire andthe room surfaces.LLF:light loss factor, an allowance for reduced light output because of deterioration and dirt.LLF= LLD x LDDExample 1A 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 of18,000 lumens.The illumination required for the factory area is200lux.Utilisation factor = 0.4Lamp Maintenance Factor = 0.75

N=(200luxx60mx24m)/(18,000 lumensx0.4x0.75)N=53.33N=54 lamps.SpacingThe aim of a good lighting design is to approachuniformityin illumination over the working plane.Complete uniformity is impossible in practice, but an acceptable standard is for theminimumto be at least 70% of themaximum illumination level.This means, for example, that for a room with an illumination level of 500lux, if this is taken as the minimum level, then the maximum level in another part of the room will be no higher than 714luxas shown below.500/0.7=714luxData in manufacturer's catalogues gives the maximum ratio between thespacing(centre to centre) of the fittings and theirheight( tolamp centre) above the working plane (0.85 metres abovef.f.l.)

Example 2

Using data in the previous example show the lighting design layout below.The spacing to mounting height ratio is3 :2.The mounting height (Hm) = 4 metres.The spacing between lamps is calculated fromfromSpacing/Hmratio of3 :2.If the mounting height is 4 m then the maximum spacing is:3 / 2=Spacing / 4Spacing=1.5x4=6 metresThe number of rows of lamps is calculated by dividing the width of the building (24 m) by the spacing:24 /6=4rows of lampsThis 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 lamps54/4=13.5goes 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 metresThis can be shown below.

The total array of fittings can be seen below.

For more even spacing the layout should be re-considered.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=4.8metresSpacing between lamps=60/11=5.45 metresInstalled FluxSometimesitis useful to know the total amount of light orflux, whichhas to be put into a space.Installed flux (lm)=Numberof fittings (N)x Number of lamps per fittingxL.D.L. output of each lamp (F)Example 3A factory measuring 50m x 10m has a lighting scheme consisting of 4 rows of 25 lighting fittings each housing 2No.65-Watt fluorescent lamps.(a)Find the installed flux in total.(b)What is the installed flux per m2of floorarea.

The output of the lamps in the above example may be found from catalogues. For a 65-Watt fluorescent lamp the Lighting Design Lumens (LDL) is 4400 lm.(a)Installed flux (lm)=Nxno. lamps/fitting x F =4x25x2x4400=880,000 lumens(b)The floor area=50x10=500m2.Installed flux per m2=880,000/500 =1760lm/m2.Example 4A room measures 15m x 7m x 3.6m high and the design illumination is 200luxon 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)thenumber of fittings required.(b)thefittings layout.(c)If the spacing/mounting height ratio is1 :1 determine whether the current design is acceptable.(a)Number of fittings.N=( 200x15x7 )/(2720x0.5x0.8 )N=19.3N=20 lamps(b) Fittings layoutFor shallow fittings, the mounting height (Hm) may be taken as the distance form the ceiling to the working plane.ThereforeHm=3.6-0.85Hm=2.75metresIf3rows of7fittings are considered then the spacing is;(c) Spacing/ mounting height.Spacing /Hmratio:2.33/2.75=0.847Therefore ratio is0.85 : 1.02.14/2.75=0.778Therefore ratio is0.78:1.0

Example 5A room, as shown below, has a design illumination is 500luxon 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)thenumber of fittings required.(b)thefittings layout.(c)If the spacing/mounting height ratio is1 :1 determine whether the current design is acceptable.(a)N=( 500x10x12)/( 2720x0.5x0.8 )N=55.15N=56 lamps.(b)Spacing,say8lampsx7rows.Spacing along 12 m wall=12 / 8= 1.50 mSpacing along 10 m wall=10 / 7= 1.43 m(c)Mounting height=3.0-0.85=2.15 mDesired Ratio=1:1Actual ratio=1.5/2.15=0.69Therefore ratio is0.69 : 1.0Actual ratio=1.43/2.15=0.67Therefore ratio is0.67 : 1.0