This is for those who have always thought: "Doom lights are the way they are and there is nothing you can do about them!" because you can. And you know it: Better lights means better atmosphere if done well.
Shots one and two demonstrate the first light enhancement that is already available since Boom and Hexen introduced the TranslucentLine special. But although this trick is already well known and has already been used in a few wads, most of the active projects don't use it at all. The idea behind this is adding a color gradient (from black to full color/brightness) and placing this at the correct offsets as middle texture with the TranslucentLine special activatet (additionally arg1 = 1 making it additive for a better effect).
Shots three and four demonstrate a very new light effect that is done via ZDooms DECORATE feature and was developped by Paul "NMN" exclusively for Knee-Deep in ZDoom. Here we are using circular sprites instead of gradients that use the Additive Translucency effect for new actors and decorations where the sprites themselves are just circle gradients (from black to full color/brightness). With DECORATE you can adjust their size (scale) and their brightness (translucency amount)
I think it is still a matter of taste if you want to use these new light techniques or not but in my eyes they are a real enhancement and therefore part of Knee-Deep in ZDoom.
Summary: "If you like em and if they fit into your project, use them, cause they are simple but convincing!"