The multi-criteria multi-sensor HM-PTSE-S2 Photo Thermal detector uses thermal assistance to the core photoelectric smoke detector to give enhanced false alarm immunity and faster response to a wide range of incipient fires. The plug-in unit combines two separate sensing elements that are managed by embedded software to act as a single unit. The MI-PTSE-S2 conforms to EN54-5, a 58°C fixed temperature and rate of rise thermal assistance conforming to EN54-7. The thermal detection function combines thermistor technology with a software corrected linear temperature response.