SilverWook said:

I think the experienced veteran Cinematographers George worked with probably helped too. ;)

Not to take anything away from GL, he was just not good enough to make THX 1138 as well as it came out. He knows it (he knew it even with the Star Wars films) and lamented it in one of the many George Lucas / Star Wars documentaries.

Don Pedro Colley explains, in this LucasFan.com interview ...
http://www.lucasfan.com/thx1138/interv.html :

At first, it felt like we were doing another student film. However, when the final cast was assembled, we worked very hard at letting George know that we were all responsible .... Every day, before we shot a scene, the four of us would brainstorm to get to the subtextral content in the script , on film. We kept telling George this film will not work unless we are able to breath life into these characters. Unless an audience can connect with these life forms, who live in that real world, we would have nothing!! George wasn't too happy to have to deal with our ideas, but he came around.