I can't find any help for this online and it seems nobody else has ever had this issue. I'm using Gunmod Radiance Lighting 2.5 and some lots have this very choppy lighting on the walls/objects, whereas other lots won't have the problem at all and look good. Any ideas??

If you notice patches of unusually dark/bright ceiling in the middle of areas they shouldn't be (no walls separating the patches, adequate light sources, etc.), that's a bit more buggy. Possibly a Radiance issue, though I've never been quite sure.

I've had the ceiling issue with patchy ceilings a bit on and off. Not sure if it's related to the Radiance lighting mod, but I've used a couple different versions of the Radiance mods (Gunmod's and one of Dreadpirate's edits), and had it happen with both (can't remember if it happens with regular maxis light, though). I think the ceiling issue was a bit worse with Gunmod's version.

The ceiling issue sometimes seems to have some connection to the wall placement and room lighting (dark patches follow the wall placement, and if a room above is dark, the tiles tend to go dark too).

As for the walls, it's either a Radiance bug, or a game bug. I think I've had that wall issue for ages. The shadow gradient tends to be there to some extent, but gets either better or worse with different light and/or window placement.

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In your case, I think it's just how the light sources and the walls are put together. The light from one room doesn't light up another room, which explains why there's a dark patch close to a bright patch (the light is up against the wall on one side, making that patch bright, while the only light in the other room is up against the opposite wall, so the ceiling tile next to the bright one (separated by wall) would naturally be darker.

It does look strange as-is when you see the ceiling as a whiole, but you wouldn't notice it as much if the walls were up instead of cutaway.

Okay, thank you for the heads up then I thought there was something wrong with my shaders as a whole but the more I've tested it, it just seems to be a quirk with the lighting system in the game in general when there are multiple light sources, and Radiance system just makes the issue more noticeable.