Quick answer: By the lumen method, average illuminance E = (N × F × UF × MF) / A, where N is the number of lamps, F is lumens per lamp, UF the utilisation factor, MF the maintenance factor and A the floor area. Rearranged, the number of fittings needed is N = (E × A) / (F × UF × MF).
The lumen method
IS 3646 gives recommended lux levels by task and space (e.g. offices ~300–500 lux, corridors ~100 lux). The lumen method sizes the number of luminaires to reach that target on the working plane.
N = (E × A) / (F × UF × MF)
E = required lux A = area (m²) F = lumens/fitting
UF = utilisation factor (room index + reflectance)
MF = maintenance factor (0.7-0.8 typical)Worked example
Office 10 × 8 m (80 m²), target 400 lux, fittings 4000 lm, UF 0.6, MF 0.8: N = (400 × 80) / (4000 × 0.6 × 0.8) = 16.7 → 17 fittings.
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What lux level is recommended for an office as per IS 3646?
IS 3646 recommends around 300-500 lux for general office work, higher for detailed tasks like drawing offices. Corridors and circulation need about 100-150 lux.
What is the maintenance factor in lighting design?
The maintenance factor accounts for lumen depreciation and dirt on fittings over time, typically 0.7-0.8. It ensures the design still meets target lux at the end of the maintenance cycle.
What is the utilisation factor?
The utilisation factor is the fraction of lamp lumens reaching the working plane. It depends on the room index (proportions) and surface reflectances, usually 0.4-0.7.