Lighting Practice: Criteria, Calculations, Compliance, and Delivery
Lighting practice tests whether you can turn a client's needs into a documented, code-compliant, energy-conscious lighting installation. Expect to move between design criteria, calculation, standards, and project delivery - and to justify each decision with numbers rather than preference.
Establish criteria before product selection
- Take task illuminance from the applicable IES recommendation for the space type, not from habit or from a previous project.
- Put uniformity (max:min and avg:min), veiling reflections, and glare metrics (VCP, DGP, UGR) into the criteria before choosing luminaires.
- Specify color with CCT plus CRI (Ra) or TM-30 Rf/Rg; CCT alone says nothing about color fidelity.
- Egress and emergency illumination are code minimums that override aesthetic intent.
Make the numbers work
Lumen method: N = (E × A) / (lumens per luminaire × CU × LLF). Maintained illuminance requires a light-loss factor combining lamp lumen depreciation, luminaire dirt depreciation, and room-surface dirt depreciation. Energy: compare calculated LPD (W/ft²) with the allowance in the adopted ASHRAE 90.1 or IECC edition, using the correct space-by-space or whole-building compliance path.
Worked example
A 20 ft × 20 ft office (400 ft²) needs 50 fc maintained. Each luminaire has two 3,000 lm lamps, CU 0.75, LLF 0.80. N = (50 × 400) / (2 × 3,000 × 0.75 × 0.80) = 20,000 / 3,600 = 5.6, so specify 6 luminaires. Check power: 6 × 60 W = 360 W, or 0.90 W/ft². Confirm that figure against the current office allowance for the code edition in force before signing off.
Controls, commissioning, documentation
Occupancy sensing, daylight dimming, and scheduling must be functionally tested and commissioned, with setpoints and scenes recorded in the O&M package. Issue luminaire schedules, control diagrams, and calculation assumptions so the design can be verified. Post-occupancy measurement closes the loop: compare actual illuminance and LPD with design intent.
Traps
- Mixing lux and footcandles (1 fc ≈ 10.76 lux).
- Using initial lumens where maintained illuminance is specified.
- Reporting an average illuminance without checking uniformity.
- Citing an LPD without the correct code version or space classification.