The lumen method is the best known of the flux design methods and has the great virtue of simplicity. The basic formula is usually expressed as follows:

(1)

where E = average illuminance

                N = number of lamps used in the installation

                L = lighting design lamp lumens (the product of initial

                                 lumens and lamp depreciation factor)

                MF = maintenance factor

                AL = factor to represent atmospheric absorption losses

                UF = utilisation factor of floodlights

                A = area to be lit by floodlights

4.1 Maintenance Factor

Maintenance factor allows the effect of loss of light due to dirt between cleaning intervals. For a 18-month cleaning cycle in a fairly clean environment, the maintenance factor is either 0.6 or 0.7, depending on the cleaning characteristics. It is much better to design for a maintenance factor of between 0.9 and 1.0 by choosing floodlights with good cleaning characteristics and accepting short cleaning cycles.

4.2 Atmospheric Loss

The atmospheric loss is caused by air-borne moisture and solid particles. The loss varies with:

(a) time of the day,

(b) season,

(c) location,

(d) mounting height, and

(e) length of throw.

In an urban football installation using four 30 m to 45 m floodlight towers, a typical loss of illuminance on a clear night is as much as 20 to 30 %.

Lumen calculations should be based on the performance on a clear night, as the effects of other weather conditions are too changeable to be taken into account.

4.3 Utilisation Factor