An apparatus for enabling and disabling a lookdown zone mask in an intrusion detector unit. The unit may include a passive infrared motion detector, a lens assembly, and a mask or cover which selectively enables and disables a lookdown zone associated with the detector. The lens assembly provides a lens proximate the detector. The lens provides the lookdown zone. When the mask substantially covers the lens, the lookdown zone is disabled because the path of radiant energy to the detector is blocked and prevents the detector from detecting any motion in the lookdown zone. When the mask does not cover the lens, the lookdown zone is enabled because the lens permits the path of radiant energy to the detector through the lens and allows the detector to detect any motion in the lookdown zone.