An infra-red motion detector for monitoring motion in a monitored space is described. The motion detector includes, within a housing, an infra-red radiation sensor sensitive to infra-red radiation incident through a window. A refractive optical system within the housing selects incident infra-red radiation for redirection onto the infra-red sensor. An external light source mounted externally of the housing directs external light through the window providing a recognizable light input. A refractive Fresnel patch mounted externally to the window selects incident light from the external light source and redirects it to a tamper sensor within the housing. A detection controller processes signals output by both sensors in monitoring the space. The detection controller detects incident light from the external light source and trips a tamper alarm in failing detection, and further detects changes in infra-red radiation in the monitored space and trips an intrusion alarm upon detecting changes.