Optical smoke detectors have always been recognised as good detectors for general use. They are regarded as particularly suitable for smouldering fires and escape routes. The performance of Orbis optical detectors is good with black as well as with white smoke. In this respect Orbis detectors are different from traditional optical smoke detectors which perform far better in white smoke than in black. Orbis Optical Smoke Detectors are also designed to reduce significantly the incidence of false alarms through oversensitivity to transient phenomena.