Light in Kerbal Space Program as it really falls: from the sky, the ground and other worlds, from engines, floodlights and lit windows by night, with dark nights and long evening shadows. With TUFX installed the picture is also developed as a film camera would develop it: bright things glow instead of going blank white, and the camera's eye follows the brightness of the view.

Left: the game's own picture. Right: with the mod, at the same moment.

For Kerbal Space Program 1.12.x. Needs Keystone, a separate download (on SpaceDock): this mod does not load without it. The film look needs TUFX; everything else works without it.
Copy the NaturalLight folder from the download into your KSP's GameData, beside Keystone. It is on as installed.
In the Keystone window: the keystone button on the game's toolbar, or Alt-K (Option-K on a Mac). Each part can be turned down or off by itself. What each one does.
Windows 11 (Direct3D 11 and OpenGL) and Mac: tried. Linux: never run.
More: the long description (everything in full, more pictures, and what was tried on each system) and the rest of the wiki.
CC BY 4.0: use it, change it and pass it on as you like, as long as you credit Ishiaki and link back here.
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