The old renderer could only apply a handful of lights to each pixel.

Every segment of a level now has its own list of the lights reaching it: the lamps contributing most to it, plus room for the lights that come and go, like shots, flares and explosions. A light only gets on the list of a segment it can be seen from, so light does not shine through walls.

Lamps shine as the rectangles they are and not as points. The light of ordinary lamps is baked into lightmaps; lamps that flicker, can be shot out or are switched by triggers stay live and really go dark.

Shots, flares, explosions and shields under fire light up their surroundings, and the explosions of Earthshakers, Mega Missiles and bosses have a light of their own that takes a few seconds to fade.

Level authors can also give segments an ambient light level of their own in DLE, instead of plastering them with weak lights just to get them bright enough.

All this needs lighting quality set to high or ultra high in the render options.