According to some sources (SheldonFanSite in particular) this shirt is showing vertical test bars. Personally, I'm a tad bit dubious of this claim but, hey, it's the best description I've seen so far. In either case, I've not yet found a shirt visually similar to this one online.
Featured on The Big Bang Theory episodes
S03 E10: "The Gorilla Experiment"
S04 E03: "The Zazzy Substitution"