Position determining apparatus for coordinate positioning machine

Described is a device which enables toolsetting on a machine tool. The device includes a light emitting unit (10) and a light detecting unit (14). A light source (30) causes a light beam (12) to propagate from the emitting unit (10) to the detecting unit (14). A light detector (40) detects the presence or absence of the beam. A trigger signal is produced when occlusion occurs and so the position of an object can be determined by reference to the machine's coordinate readings. The beam (12) may be uncollimated and thus provide a device with an easier set-up and a greater resistance to vibration in use. The light detector (40) and/or light source may be protected from contamination by windows (34,44) or a protector having an aperture therein. The light emitting and detecting units each have an aperture for the light beam. The apertures may be oblique conduits (104) for pressurised air flow, thereby keeping the conduits free from contamination, while minimising turbulence in the path of the beam which could cause noise in the resulting signal.