When Star Wars was first released George Lucas and Steven Spielberg went to Hawaii to relax and get away for a little while and they kicked around some ideas. Raiders of the Lost Ark was one of them. The story goes, Mr. Spielberg was interested in doing a globe hopping adventure like James Bond and Lucas told him he had a better idea along the same lines (Indy). Later, when they were casting the part of Indy, a lot of actors were considered (including Jeff Bridges and Tom Seleck) but upon seeing a rough cut of Empire, Steven Spielberg wanted to use Harrison Ford.
But I would argue that that the darker, more realistic tone of Raiders may have been influenced by Empire as a whole. Looking at Empire one sees darker lighting (compared to Star Wars: A New Hope) used to great effect.
Steven Spielberg, being an avid film student and practitioner may have recognized this. Looking at Raiders, Minority Report, and many of his better adventure films, you can see a tendency to use shadows and lighting in a noirish style...much like was used in The Empire Strikes Back.