Lighting design: lumen method
The lumen method is used by the lighting designer to calculate the number of luminaires needed to realise the illuminance required in the room.
A description of lighting design based on this method is found in "Projektierung von Beleuchtungsanlagen nach dem Wirkungsgradverfahren" published by the German lighting society LiTG. The publication contains utilance values for many standard luminaires. The following formula is used for calculating the number of luminaires required:
Key:
n = number of luminaires
E = illuminance required
A = area or partial area of the room
z = number of lamps per luminaire
F = luminous flux of one lamp
hLB = light output ratio
hR = utilance
hB = coefficient of utilisation (hLB . hR)
WF = maintenance factor
Coefficient of utilisation tables
Utilance depends on the luminous flux distribution of the luminaire, the geometry of the room and the reflectance of room surfaces. The coefficient of utilisation (hB) is the product of light output ratio (hLB) and utilance (hR).
Useful to know: luminaire manufacturers maintain extensive lists showing the light output ratios of their products.
