Critical lighting: when a mark on the wall isn’t a defect

Critical lighting is light raking sharply across a surface, exaggerating any imperfection. It’s not how walls and ceilings are normally viewed, so imperfections that only show up under critical lighting are not defects. Knowing this saves you arguing over a “fault” that no test would ever call a fault.

What critical lighting actually is

It’s all about the angle the light hits the surface. When light comes in almost flat, every wave and trowel mark throws a shadow.

• Light at a ≤15° angle to the surface.
• Everyday examples: low-angle morning or evening sun raking across a wall, or a torch held flat against plasterboard.
• By contrast, walls and ceilings are designed to be viewed under normal overhead or diffuse lighting, from ≥1.2m away.

What this means on the job

Once you know a mark only appears under critical lighting, you know it’s not a defect — but you still need to handle sign-off and client questions the right way.

• Don’t sign off plasterboard finish in critical lighting — the finish will fail any test under that lighting.
• If a client complains about a wave or trowel mark in low-angle sun, point them to MBIE Guide §10 — it’s not a defect.

Get ahead of known problem spots

Some areas are always going to cop critical lighting — think a long single-pane window facing low sun. Spec for it up front rather than fighting it later.

• For known critical-lighting areas, spec a Level 5 finish.
• Pair it with light-coloured matt paint to soften what the raking light picks up.