LUMEN, why inside a closed room I can even see lighting instead that darkness?

[Bits360]

@7Sol

With Lumen you should not use a simple plane (even with 2-sided mat) or box with tiny width (less then ~10 cm) as a walls cos this will lead to light leaking

Left wall from cube with enough width:

Left wall from cube with tiny width:

Left wall from plane with 2-sided mat:

Correct.