I'm fairly certain this is a known issue but I'm not very good at searching the various forums to find it so I figured I'd put it here again. I have a completely underground farm setup but the lighting on it is a nightmare. I can flood the place both inside and outside the actual structure and still get either dark plants during the day or, when I trim it down to lights that "should" be more than enough, i get black shadows across various areas. It is like the light from one side of a solid block bleeds through and radiates out the other side and it's a toss of a coin if it's the dark or light that's going to bleed. Shouldn't solid opaque blocks NOT convey light? I'm building over on Yahgiggle's server Rising Cities if you'd like to come visit the build and see first hand what I'm talking about. I'll see if I can get a screen shot attached.
So they attached. A quick description for each to point out what I was trying to show with it:
Daylight3: The shadows in the corner wall despite there being 2 lamps directly beside the black area. I know from experience that if I were to place a light on the other side of the side wall there that it would go away.
Daylight4: Take note of the line on the ceiling. If I were to fly up into the stone above, remember this is underground completely, I could place a light on the top of that reinforced concrete and the shadow line would go away.
Please feel free to have either Yahgiggle or myself add you to the area so you can play with the various lights in creative mode and see exactly what I'm talking about if you need to. I'm not a programmer so I have no clue how you do any of this and I'm thankful that you do it at all. If I can be of assistance in helping fix the physics of the lights, please let me know anything I can do.