4 The Lumen Method 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 Utilisation factor is a calculated ratio of the lumens effectively lighting an area to the total available lumens from the lamp. Table of utilisation foctors are not usually published for floodlight projectors so it is a common practice to use the zonal flux diagram to determine the utisation factor of floodlight.