Lumen

Lumen stacks several tracing methods: cheap screen-space rays first, then software tracing against the scene's mesh distance fields, and on capable hardware full ray tracing. The result is kept in a surface cache so indirect light need not be recomputed every frame. The point is that lighting follows along when the sun swings or a wall comes down — something baked maps never managed.

It replaces decades of workflow built around lightmap baking. The cost is noise and some latency in how light propagates, and the cheapest modes inherit screen space's blind spots. It launched with UE5 in 2021.