Lighting design is the intersection of science and human experience — a warehouse needs 100 lux to find a box, but an operating room needs 1,000+ lux to save a life. The key metric is illuminance, measured in lux (lm/m²) or footcandles (lm/ft², where 1 fc = 10.76 lux). The IES Lighting Handbook publishes task-based illuminance recommendations: 300-500 lux for general office work, 500-1,000 lux for detailed mechanical assembly, 50-100 lux for warehouses and corridors, and 30-50 lux for parking areas. These values ensure visual acuity, safety, and productivity appropriate to each environment.

The lumen method is the standard approach for calculating the number of luminaires: N = (E × A) / (Φ × CU × MF), where E is target illuminance, A is area, Φ is lumens per fixture, CU is coefficient of utilization, and MF is maintenance factor. The CU accounts for room geometry (quantified as Room Cavity Ratio = 5h(L+W)/(L×W) where h is cavity height) and surface reflectances (ceiling 70-80%, walls 50-70%, floor 20-30%). The MF accounts for lamp depreciation (lumens decline over time) and dirt accumulation on fixtures — typically 0.70-0.80 for clean environments, 0.50-0.65 for industrial spaces.

NEC Article 220, Part III requires minimum lighting load calculations based on volt-amperes per square foot for different occupancy types: 3.5 VA/ft² for offices, 1.5 VA/ft² for warehouses, 3 VA/ft² for retail, and 3 VA/ft² for schools. These values determine minimum branch circuit capacity regardless of the actual luminaires installed — a building designed with highly efficient LED fixtures must still provide branch circuit capacity per NEC Table 220.12. ASHRAE 90.1 imposes Lighting Power Density (LPD) maximums in the opposite direction: 0.82 W/ft² for offices, 0.63 W/ft² for retail (2022 edition).

LED lighting has fundamentally changed the calculation landscape. LED efficacy (100-200 lm/W) is 2-3× that of fluorescent (60-100 lm/W) and 5-10× incandescent (10-17 lm/W). This means fewer fixtures, smaller branch circuits, and lower energy costs. However, LED quality varies enormously: Color Rendering Index (CRI) ranges from 70 (minimum acceptable) to 95+ (gallery/retail quality), Color Temperature (CCT) from 2700K (warm) to 6500K (daylight), and L70 rated life from 25,000 to 100,000+ hours. Power factor of LED drivers is critical — cheap LED fixtures may have PF as low as 0.5, affecting circuit loading.

Emergency lighting per NEC 700 and NFPA 101 requires illumination of egress paths at minimum 1 footcandle (10.76 lux) average with a minimum-to-maximum uniformity ratio not exceeding 40:1. Emergency lighting must activate within 10 seconds of normal power failure and provide illumination for at least 90 minutes. Battery-backed emergency fixtures, unit equipment, or generator-backed systems satisfy this requirement. Exit signs must be visible from 100 feet per NFPA 101 and illuminated at 5 footcandles (per NEC 600.5) for externally illuminated or per UL 924 for self-luminous types.

Lighting controls and dimming systems are increasingly required by energy codes. ASHRAE 90.1 mandates occupancy/vacancy sensors in most commercial spaces, daylight-responsive controls within 15 feet of fenestration, and the ability to reduce lighting power by at least 50% in each space. 0-10V dimming is the most common analog control protocol; DALI (Digital Addressable Lighting Interface) provides digital control with individual fixture addressing. Dimming LED fixtures requires compatible drivers — not all LED drivers are dimmable, and mismatched driver/dimmer combinations cause visible flicker at frequencies that can trigger headaches or seizure in sensitive individuals.