How does light work in this game?

[LostManAbroad]

The reason I am asking this is that I have my main base setup at Durin's Forge. The plinth in the middle where the Cave Troll's throne was is now my farm as it has natural light shining in.

The problem is that I also wanted to grow deep mushrooms so I tried to build a dark room. On the side of the chamber I built a square stone room with no windows and just one door at the front. I filled it with farming plots and filled them with deep mushrooms. However, it still says that there is too much light so I built another structure of granite walls completely encasing the first structure. Now there should be no way for light to get in. It should be pitch black. But is it? NO.

Still too much light. How is that possible?

[Smitey MacSmiterton]

You have to find an area that registers as Dark before any modifications.

Destroy light sources, put out your torch, and walk around the area. I find the area on either side of the stairs to be fairly dark, as well as the base of the stairs

[LostManAbroad]

In the end this was not possible. There is too much light in the chamber regardless of what walls and ceilings you build to try and block it. Instead I tried a different tactic. I just setup a farm at the bottom of the Mithril mine shaft. Down there even if you fill the room with torches, it still considers it to be dark.

[Athos42]

I think that walls you build don't really influence if it is dark or light for plant growth. If you build in an area which is filled with daylight, it will always be too bright for mushrooms to grow. On the other hand, if you build in an area which is dark and place a Afhan Lamp you create a radius in which it is bright enough for plants to grow. The other lamps or torches don't create any light which would influence plant growth in any way.

In Durin's Forge, try to place some mushrooms up there near the entrance to the Desolation, it does not look to bright up there.

[LostManAbroad]

It is better the way I have it set up. It would be actually much farther to walk all the way back to the entrance of Durin's forge than to walk to the Map Stone. I just think it is sad that the developers did not program the light mechanics better. Making a solid box room out of Adamant or Granite should make for a completely blacked out room.

[Chaoslink]

Basically, an area is either light or dark based on the environment itself. Lights you place don't matter at all and walls won't affect it. I think that room in particular is entirely lit as you've found out.