For a closer look at how the Hawk-Eye works, launch the photo gallery by clicking 'View Photos' at left.

Its accuracy may be based on a complex computer-generated algorithm, but the Hawk-Eye Tennis Officiating System is ratcheting up the human drama at this year's U.S. Open. Rather than simply relying on officials to make line calls, the entire crowd now acts as referee, hollering "Challenge!" after controversial judgements.