Abstract: A detector (101) for detecting smoke from a fire and generating an alarm has a detection chamber (105) in which a light source (111) and a light detector (113) are arranged. In normal use, the light detector (113) only receives light from the light source that has been scattered by the presence of smoke in the chamber (105). When testing the detector (101), the light detector (113) receives light from the light source (111) by presenting a reflecting surface (129a) of a rotor member (129) to reflect light from the light source (111) towards the light detector (113). In normal use, the rotor member (129) is positioned so that the reflecting surface (129a) no longer reflects light from the light source (111) towards the light detector (113).