The Birdi Smart Detector use a photoelectric sensor, which uses a beam of light to detect smoke. These alarm types are most effective for typical residential fires, that originate from a smoldering source, which can also fill a home with dangerous gases even before the fire ever erupts.
Older smoke detectors used ionization sensors, which use radioactive material to detect small particles entering the chamber. These ionization sensors were originally designed to detect heat, not smoke, and so do not respond fast or at all to slow smoldering, smoky fires which are the most common types of residential fires. These were typically used due to their significantly lower cost. Common belief is that one of these ionization sensors is necessary for flaming fires, but research done by the National Institute of Standards and Technology (NIST) has shown that during a flame ignition fire, the photoelectric smoke detector has only a 3.99' probability of a failure to detect the fire whereas the ionization detector probability of failure is as high as 19.8'. Besides that, ionization sensors have been the largest cause of nuisance alarms (in one case, 97' of devices tested for nuisance alarms were ionization-type). This frustration encourages homeowners to remove or disable their smoke alarms. Given the significant amount of research testifying to their poor performance and unreliable nature, we believed that the future of smoke detection should not rely on using cheap sensors that use radioactive material. http://fire.nist.gov