I tried to light a woodland mansion by using the light transparency of half slabs (cutting a lot of holes vertically from the roof to the bottom floor and covering them with half slabs/carpets, so at least at daytime it's brighly lighted), and noticed a strange thing.

Principally the idea works, but after saving and reloading the game, it get's very dark. It seems that after reloading, the light passing through the slabs is treated like artificial light, and decreases with distance, not like sunlight.

You can easily reproduce this with a 4x4 tower, maybe 15 blocks high. Directly after building the tower, if you enter it and close the opening behind you, it's bright as daylight in there, but if you do the same after reloading the game, it's pitch black in there.

Maybe it is intended, that light behaves like artificial light after passing through a half slab and decreases, but if so, it should do that right from the begin.