Lighting Systems
Efficacy, illuminance, and losses
Lighting analysis rests on four numbers: efficacy (lumens per watt), illuminance (footcandles or lux; 1 fc ≈ 10.76 lux), lighting power density (LPD, W/ft²), and the lumen method. Delivered lumens per luminaire = lamp lumens × ballast/luminaire efficiency (ballast factor) × coefficient of utilization (CU) × light loss factor (LLF, combining lamp lumen depreciation, dirt, and ambient temperature effects). LED sources are typically rated to L70 (70% of initial lumens) rather than burnout.
Controls deliver savings that hardware upgrades cannot: occupancy/vacancy sensors, daylight dimming and switching, scheduling, bi-level and task tuning, and personal dimming. Retrofit paths include LED tube lamps on existing ballasts, ballast-bypass (Type B) tubes with line voltage, and full luminaire replacement; check driver compatibility, harmonic distortion, and dimming protocol (0–10 V, DALI).
Traps. Using raw lamp lumens as delivered light; forgetting ballast factor and LLF; assuming a 1:1 wattage reduction from an LED retrofit; ignoring the cooling-coil savings and heating penalty from reduced lighting wattage; confusing lamp life in hours with calendar years; and citing LPD without applying the correct ASHRAE 90.1 space-by-space value.
Worked example
A 40 ft × 30 ft room (1,200 ft²) needs 50 fc = 60,000 lumens. Each luminaire delivers 4,000 lm × 0.70 CU × 0.80 LLF = 2,240 lm. Luminaires required = 60,000 ÷ 2,240 = 27. At 30 W each, LPD = (27 × 30) ÷ 1,200 = 0.68 W/ft² - comfortably under a typical 1.0 W/ft² allowance for offices.