This mod by Mishikedman is a complete overhaul of the lighting system within The Witcher 3, and while that may not seem like a huge deal, the results are incredibly impressive, although it's not the first mod of it's kind to do so.
While similar to the older Wiedzmin Lighting Mod, this particular mod promises more enhanced effects, and aims to make the game look great in all scenes and locations, rather than just cherry picking for the purposes of screenshots.
Difference here is that this mod does not cut corners and look good in one location but bad in another, or look fantastic in gameplay but bad in cutscenes, this is not a mod for screenshots but for playing the game and it looks consistent and good in all situations game can put you in.
And the results are admittedly impressive. As you can see from the screenshots below, the difference is noticeable and an improvement.
Lighting mods are hard to get right, mostly because it is very easy to ruin contrast and cause dark areas to be too black or bright areas to wash out. This mod seems to strike a very good balance.
Should you be interested in downloading the mod, it is available to download here.