Home Up Introduction to the Lumen Method Lumen Method Calculations Summary


3. Summary of Procedures for Lumen Design Method

(a) Calculate the room index.

(b) Determine the effective reflectances of the ceiling cavity, walls and floor cavity.

(c) Determine the utilisation factor from the manufacturer's data sheet, using the room index and effective surface reflectances as found in (a) and (b) above.

(d) Determine the light loss factor.

(e) Inert the appropriate variables into the lumen method formula to obtain the number of luminaires required.

(f) Determine a suitable layout.

(g) Check that the geometric mean spacing to height ratio of the layout is within the SHR NOM range:

(h) Check that the proposed layout does not exceed the maximum spacing to height ratios (SHR MAX).

(i) Calculate the illuminance that will be achieved by the final layout and check against the standard.




Example 3

Design a lighting installation for a college seminar room so that the average illuminance is 500 lux on the horizontal working plane, using the data listed below. Suggest the layout and check appropriate spacing to mounting height.

        Room dimensions: 12 m long x 8 m wide x 3.2 m high

        Working plane at 0.7 m above floor

        Reflection factors: Ceiling 70 %

        Walls 50 %

        Working plane 20 %

        Light Loss factor: 0.779

        Luminaires: 1800 mm twin tube with opal diffuser

        Ceiling mounted

        Downward light output ratio 36 %

        SHR MAX 1.60 : 1

        SHR NOM 1.50 : 1

        Dimensions : 1800 mm long x 200 mm wide

        Lamps: 1800 mm 75 W plus white

        5800 average initial lumens per lamp

        2 lamps per luminaire



(a) Initial calculation

From manufacturer's photometric data sheet (Table 3), utilisation factor (UF) is 0.5336 by interpolation.

Therefore, the number of luminairs is 10.

Initial check on S/Hm ratio gives:

From the manufacture's photometric data, maximum S/Hm is 1.6 : 1. Therefore, it should be possible to use 10 luminaires.

(b) Proposed layout

A 5 x 2 array is proposed fro the lighting installation. (A 10 x 1 array is an alternative.)

(c) Checking the proposed layout

Since 2 x 1.8 m = 3.6 m < 8 m (width of room), the proposed layout will fit.

(Usually checking only the linear dimension of the fitting for space is enough as the other dimension (i.e. 200 mm in this case) is much smaller.)

For long axis,

For short axis,

Note that if the checks had worked out to be unsatisfactory, the number of luminaires should be reconsidered and the calculations on the illuminance should be repeated. For example, a 3x3 array for lower lux level or a 4x4 array for higher lux level.