You could a get a cheap motion detector connected to the barn lamp, not that expensive.
Then set up a web camera to your PC with a security program that starts recording when something moves.
there are some freeware I belive.
The problem is that movement occurs often outside without it being a thief. But I belive the sensitivity can be adjusted so that it only starts when the light switches on.
And a webcam, altough cheap isn't the best quality to try to identify someone with.
Better is a digital video camera connected to the PC so that it starts the camera, a bit more expensive.
And cameras directed outside isn't legal (Here we are again) everywhere.
EDIT Quick google search
Thief jailer
[ June 17, 2004, 09:28: Message edited by: Ruatha ]