Lighting Design The Lumen Method Explained

When Designing and calculating lighting design for internal areas, the quickest system to use is the lumen method. I have used this method for many years and the installed results were always accurate to the designed and required lighting level.





In design it is important to keep the balance between the minimum requirements as called up by relevant standards and by local authorities and balance it against the cost of installed and running the lighting for the development, if you over design the client wont thank you for an non green design and if you under design you could have serious problems with dark areas, or dim lighting.

By following this guide you will over come most of this pit falls and reach the required balance.

Step One.
Take the measurement of the room you are designing for in Meters and find the area. Then you must determine the working plane, if the room is to be an office then this will be at desk height (0.75m) from the floor, if the room is to be an office then this will be zero.

Take the height from the floor to the ceiling and take the height of the working plane from it.

Example: you have a room with the following.
length 23.8m
Width 14.5m
gives and area of 345.1m

the room is a function room so we take the working plane as 0.75m
The mounting height is 3m (floor to ceiling) - 0.75m gives 2.25m

use the following formula to calculate the room index.

Room index (RI) = A/h(a+b) 345.1/2.25(23.8+14.5) = 4.0

Step 2.
When the lights are installed at first they will give out 100% of there illumination but over time dust and grime will reduce this value so we must make allowances for that now. If the maintenance on the lighting is regular then a high value can be attributed, if the maintenance factor is unknown at design stage then a MF of 0.85 is acceptable.


Step 3.
When designing lighting systems you have to select exact fittings with known values to work to, So select a lighting catalogue that contains detailed data on the fitting and lamp types available and not catalogues that a focused towards architects and interior designers as these are generally just full of pretty pictures, I would recommend using Thorn or Philips are they have a good range of medium to high quality fittings. I would also recommend visiting there web sites to check for updates to there selection or for further technical information.

For our calculation example above the Coefficient of Utilisation is 0.46, which was obtained from the manufactuers catalogue with the use of the RI and the reflectances. For office areas or hotels expect a high reflectance values which are based on the colour of the ceilings walls and floor. Areas such as stores or back of house areas would be expected to have lower relectances values and will have an effect on the coefficient of utilisation fugure you get.

Step 4.
Note the number of lamps (tubes, bulbs) in the selected fitting and the luminour flux per tube, this figure will come from the manufacturers catalogue. Also note the wattage per lamp.

In the case of our example from above the following values resulted:

Luminour Flux per tube 20000

No of Tubes per Fitting 1

Watts per Tube 250

Step 5.
It is important to know for the application the required lux levels to be achieved in the room, for calculations in the UK we refer to the British Standard for minimum levels. For our example the require illumination value was 250.

Step 6.

Riv = Required Illumination Value
A = Area
MF = Maintenance Factor
CU = Coefficient of Utilisation
L = Luminous Flux per tube
T = No of Tubes

Fixtures Required = RIV x A
MF x CU x L x T

250 x 345.1 =
1 x 0.46 x 20000 x 1

86275 = 11.03
7820

11.03 is rounded off to 11 No. fixtures required to meet the lux levels in this room with this type of fitting and this type of lamp installed.

For asthehics this figure was increase to 12 No fittings with 4 rows of 3 No fittings installed.
We use the following Formula to determine the lux output achieved for 12 No fittings installed.

where F = No of Fitting Installed.

Average Illuminance =

F x CU x MF x L x T
A

12 x 0.46 x 0.85 x 20000 x 1 = 271.9 Lux

345.1

There are lighting software packages available that can calculate far more complex rooms and areas which I will discuss in a latter issue.

If you have any questions or queries please leave a comment.

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