Unless I'm mistaken, I believe the first time we see stormtroopers in Empire is when they swarm in behind the group on Cloud City after Darth Vader takes Han Solo's gun. This seems so late in the film to reintroduce such an iconic element of the first film. It probably seemed noticeable in the theatre in 1980 but not so much now in a world crawling with Star Wars images. Correct me if I'm wrong about this being their first appearance. I don't count the snowtroopers on Hoth.