Lighting mods have a few components, and how these interact is important to the overall feel of the mod.
Before I go on, by lighting mods I mean ELFX, RLO, ELE, and Relighting Skyrim. Other mods such as Darker Dungeons for ENB also fall in this category, although I haven’t specifically looked at this one. Revamped Interior/Exterior fog does not fall in this category.
For examples of what these look like, please see Gopher’s awesome video here: https://www.youtube.com/watch?v=YceSkJgy2gw or the carefully crafted comparison screenshots by MangaClub here: https://staticdelivery.nexusmods.com/mods/110/images/36067-0-1432843158.jpg
Lighting in Skyrim is accomplished through several means. The first is simple: Lightbulb objects emit light. How much light they emit is controlled by the specific object. A lightbulb for a candle doesn’t emit as much as one for a fireplace, which doesn’t emit as much as a bonfire.
All of the above lighting mods tweak both the lit radius and the color of the various lightbulb types in game.
This is part of what makes RLO so dark – it decreases the radius of all the lights by a lot!
The second is lighting template, and light in the cells itself. This controls the ambient light and color in a cell. (Now, people complain a lot about ambient light… but you know what it is like when the power goes out and you have to navigate by candlelight alone! It’s a nightmare! Ambient light is essential for good and non-frustrating gameplay. And if realism is more important.. well aren’t we lucky to have choices via a healthy modding scene!)
The final is imagespace modifier, which also controls color in the scene, and the overall “feel” of the scene. It’s basically control for Bethesda’s built in lighting shaders.
The extreme example of imagespace modifiers is Night Eye, or maybe the drunk effect in RND, or when the screen goes black for a cutscene. But it’s also a core part of what makes an inn look warmer and cozier than a dungeon, even though they’re using the same fireplace and similar lighting templates.
Besides the maligned ambient lighting, there is another major issues with Bethesda lighting. The first is the infamous shadowcaster limit. This limit is hardcoded in the engine; not even people really good at fixing that kind of thing have been able to fix it yet.
To understand this limit, understand that lightbulbs come in two forms, those that cast shadows and those that don’t. Most small lights don’t cast shadows. This saves a ton of performance and also prevents running up against that limit. Many large lights do, but not all. And that’s about the limit of my understanding there.
If there are more than four lightbulbs attempting to control shadows on a single mesh, the shadowcasting calculations will turn off on the additional lightbulbs. This leads the infamous flickering where sometimes an area will be light and sometimes not based on the number of lights trying to cast on it.
JawZ attempted to fix this with his JIT scripts, turning off extra lightbulbs in a way that’s mean to be unobtrusive. It works pretty well.
ELFX actually fixes this by editing meshes to be smaller, so that for example, instead of the entire bannered mare being one mesh, it’s actually multiple smaller meshes. This leads to more compatibility issues, but when it works it works pretty well.
RLO does not have a built in fix for this and instead tries to play nice with the limit by keeping a smaller number of shadowcasting lights.
There are two more minor issues. The first is misplaced lightbulbs. In order to keep a whole room lit while staying within the shadowcaster limit, Bethesda’s designers would do things like place lightbulbs at the very top of the room, instead of actually in the fireplace, or one lightbulb halfway between two braziers, or place a lightbulb every third candle instead of at each one. This leads to pretty obviously unimmersive and bad-looking lighting.
The final minor issue is shadowstriping, which seems to be an issue with how shadows are rendered. I’ve actually never had this happen to me so I can’t guarantee how it’s fixed, but either moving light sources with a lighting mod or changing some settings in the ini file seems to work for most people.
So, lighting mods attempt to do many things. They reduce the amount of ambient light by changing lighting templates. They improve the character of the game and the specific locations by altering the imagespaces to be more unique – warmer inns and darker dungeons. They move lightbulbs to more logical locations and in many cases increase the number of lightbulbs (especially to make up for the lack of ambient lighting).
RLO and ELFX do all of these things. Relighting Skyrim edits lightbulb location and cells colors, but not lighting templates or imagespaces. ELE focuses on imagespaces and overall feel.
Which one looks best is really a matter of taste, and because of how imagespaces interact so strongly with ENB, a matter of ENB or other post-processing.