A roller shutter door (10) is mounted on a barrel (12) above a doorway opening (14). A motor (not shown) is used to turn the barrel (12) to allow the roller shutter door (10) to be rolled up and down to either open or close the doorway (14). A leading edge (16) of the roller shutter door (10) comprises a compressible rubber strip. The leading edge (16) is secured to a lower lath (18) of the roller shutter door (10). There is a channel (20) that extends inside the length of the leading edge (16). The channel is enclosed to prevent the ingress of external light that could give undesired light detection. The passage/non-passage of light along the channel (20) is used to determine the presence of obstructions.