A location system for locating workers comprising: a plurality of light detectors mounted at known locations and configured to detect light from one or more workers ; and a processing system configured to determine locations of the workers using the light detected by the light detectors ; characterised in that the location system further comprises: wearable devices to be worn by one or more of the workers who are authorised, the wearable devices including wireless transceivers configured to transmit radio-frequency signals; and a plurality of wireless receivers mounted at known locations and configured to detect the radio-frequency signal transmissions from the wearable devices , and the processing system is further configured to: determine locations of the authorised workers using the radio-frequency signal transmissions from the wearable devices detected by the wireless receivers ; and identify as unauthorised workers any workers whose location is determined using the light detected by the light detectors who are not authorised workers whose location is determined using the radio-frequency signal transmissions from the wearable device . a plurality of light detectors mounted at known locations and configured to detect light from one or more workers ; and a processing system configured to determine locations of the workers using the light detected by the light detectors ; characterised in that the location system further comprises: wearable devices to be worn by one or more of the workers who are authorised, the wearable devices including wireless transceivers configured to transmit radio-frequency signals; and a plurality of wireless receivers mounted at known locations and configured to detect the radio-frequency signal transmissions from the wearable devices , and the processing system is further configured to: determine locations of the authorised workers using the radio-frequency signal transmissions from the wearable devices detected by the wireless receivers ; and identify as unauthorised workers any workers whose location is determined using the light detected by the light detectors who are not authorised workers whose location is determined using the radio-frequency signal transmissions from the wearable device . wearable devices to be worn by one or more of the workers who are authorised, the wearable devices including wireless transceivers configured to transmit radio-frequency signals; and a plurality of wireless receivers mounted at known locations and configured to detect the radio-frequency signal transmissions from the wearable devices , and the processing system is further configured to: determine locations of the authorised workers using the radio-frequency signal transmissions from the wearable devices detected by the wireless receivers ; and identify as unauthorised workers any workers whose location is determined using the light detected by the light detectors who are not authorised workers whose location is determined using the radio-frequency signal transmissions from the wearable device . determine locations of the authorised workers using the radio-frequency signal transmissions from the wearable devices detected by the wireless receivers ; and identify as unauthorised workers any workers whose location is determined using the light detected by the light detectors who are not authorised workers whose location is determined using the radio-frequency signal transmissions from the wearable device . <EOS>