The sensing of BOT and EOT is achieved by shining a small lamp at the tape's surface at an oblique angle. When the foil strip (glued to the tape) moves past the lamp a photo-receptor sees the reflected flash of light and triggers the system to halt tape motion. This is the main reason that photographic flash cameras are not allowed in data centers with 9-track tape drives since they can trick the tape drives into falsely sensing BOT and EOT.