Design interior lighting using the IES zonal cavity method.
Electrical designers, lighting reps, and facility teams use a lumen method lighting calculation to estimate fixture count, average illuminance, spacing, and room cavity ratio from lumens, coefficient of utilization, light loss factor, and area. It is useful for early layouts where a quick maintained foot-candle check is needed before photometric software.
Average illuminance equals the number of fixtures times fixture lumens times coefficient of utilization times light loss factor, divided by the illuminated area.
Coefficient of utilization estimates how much lamp or fixture light reaches the work plane. It depends on fixture photometry, room proportions, mounting height, and surface reflectances.
Light loss factor accounts for lumen depreciation, dirt accumulation, driver performance, and other maintenance conditions. It converts initial output into a maintained design estimate.
It is best for average illuminance estimates. Final design often needs point-by-point calculations, uniformity checks, glare review, emergency lighting, controls, and energy code compliance.