Lighting Systems
Metrics and terminology
Lumens measure light output, efficacy is lumens per watt, illuminance is foot-candles (1 fc = 10.76 lux), and lighting power density is W/ft2. Codes regulate LPD, not illuminance, and LPD must be computed from fixture input watts, meaning lamp watts plus ballast or driver losses, not lamp watts alone. Color metrics matter too: CRI for rendering, CCT in kelvin, and BUG ratings for exterior luminaires.
Technology and controls
LEDs dominate with efficacies well above fluorescent T8/T5 and HID sources, and they dim smoothly, which makes them natural partners for controls. Fluorescent systems still appear on exams: ballast factor modifies both lamp lumens and system watts, and lamp lumen depreciation combined with dirt depreciation yields the light loss factor used to select fixture quantities. Controls deliver savings in a predictable order: occupancy sensing, daylight-responsive dimming at perimeter glazing, scheduled sweep, and personal tuning.
Traps
Comparing lamp efficacy with fixture efficacy; omitting ballast or driver watts from LPD; treating foot-candles and lux as interchangeable; assuming occupancy sensors pay in continuously occupied spaces such as control rooms; and forgetting that 90.1 offers both space-by-space and whole-building LPD methods, which give different allowances.
Worked example
A 60-fixture office retrofit: existing fixtures draw 3 x 32 W lamps at 1.0 ballast factor plus 12 W of ballast loss, or 108 W each; new LED fixtures draw 38 W. Savings = 70 W x 60 = 4.2 kW, or 12,600 kWh at 3,000 hours per year, plus reduced cooling load.