Orbis operates on the well-established light scatter principle. The remarkable optical design of the Orbis optical smoke detector allows it to respond to a wide spectrum of fires. The sensing chamber of the Orbis optical smoke detector contains an optical sensor which measures back-scattered light as well as the more usual forward-scattered light. Sensitivity to black smoke is greatly improved. The detector is calibrated so that Orbis is highly reliable in detecting fires but is much less likely to generate false alarms than earlier smoke detectors. The stability of the detector‚ high reliability, low false alarm rate‚ is further increased by the use of algorithms to decide when the detector should change to the alarm state. This removes the likelihood of a detector producing an alarm as a result of smoke from smoking materials or from another non-fire source.