Light Leaks
Light showing through walls and corners
Also known as: Light leaking · Light bleeding · Lumen light leak · Light leak through walls
Quick answer
Light leaks are bright lines or patches where light seems to pass through walls, floors and corners. In Unreal Engine they usually appear when walls are single planes or very thin, or when meshes meet with a gap, because global illumination cannot tell that the surface is solid. The standard fix is to give walls real thickness, close the gaps and overlap pieces at the seams.
How to fix it
Build rooms from thick, closed geometry instead of one-sided planes, make sure wall pieces overlap at corners and do not leave hairline gaps, and keep floors and ceilings as real volumes. If leaks remain in a Lumen scene, use the Lumen Scene view mode to check that the walls are represented, and avoid letting a tiny trim piece be the only thing that closes a seam. Baked lighting leaks for related reasons, often overlapping or low-resolution lightmap UVs.