Lighting Design
What this domain tests
Lighting questions test the layers of light, lumen-based calculation (not wattage), coordination of switching with the reflected ceiling plan, IP zones in wet areas, and energy reasoning.
Layers and quantities
- Layers: ambient/general, task, accent, decorative, plus architectural cove and wall-wash.
- Illuminance targets (maintained lux): living general 100–150, reading task 300–500; kitchen worktop 300–500; desk 500; bathroom mirror 300–500; stairs and corridors 100–150.
- Lumen method: N = (E × A) ÷ (Φ × CU × MF), where E is target lux, A area, Φ lumens per luminaire, CU utilisation factor 0.6–0.8, MF maintenance factor about 0.8.
- CCT: 2,700–3,000 K warm residential; 4,000 K neutral work areas; 6,500 K daylight. CRI ≥ 80 domestic, ≥ 90 where colour matching or textiles are judged.
- Glare: UGR ≤ 19 in offices; deep-baffle recessed downlights; match beam angle to ceiling height.
Energy and control
- Compare lumens, not watts: LED efficacy is roughly 80–150 lm/W; L70 lifetime defines useful life.
- Dimming protocol must match the driver (TRIAC mains dimming, 0-10 V, DALI); mismatches cause flicker or failure.
- Daylight: glazing orientation, blinds or overhangs for glare control; separate switching near windows.
Wet zones (HD 60364 / IEC)
- Zone 0 (inside bath or shower tray): IPX7.
- Zone 1 (above the bath to 2.25 m): IPX4, IPX5 in public showers.
- Zone 2 (extending 0.60 m sideways from zone 1): IPX4. Shaver sockets must have an isolating transformer.
Worked example
A living room is 4.50 × 3.60 m (16.2 m²) with a 2.70 m ceiling and a 150 lux general target. Required flux = 150 × 16.2 = 2,430 lm; divide by CU 0.7 × MF 0.8 = 0.56, giving about 4,340 lm at the source. With four 1,100 lm (10 W LED) downlights, spacing follows roughly 1.2 × mounting height; measured to a 0.75 m working plane the mounting height is about 1.95 m, so 2.3 m centres, with the outer row about one third of ceiling height (0.9 m) off the walls. Add a 300 lux task lamp at the reading chair on a separate, dimmable circuit.
Traps that cost marks
- Sizing by watts ("a 100 W lamp") instead of lumens and lux.
- No separate switching or dimming for the layers, so nothing can be controlled.
- Ignoring ceiling height when choosing beam angles.
- Wrong IP rating in bathrooms or above showers.
- Confusing CRI with CCT, or specifying 6,500 K in a warm residential scheme.